JP2004225132A - High strength cold rolled steel sheet and plated steel sheet having excellent deep drawability, steel tube having excellent workability, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet which has excellent deep drawability, to provide a steel tube which has excellent workability, and to provide their production methods. <P>SOLUTION: The high strength cold rolled steel sheet having excellent deep drawability has a composition comprising, by mass, 0.025 to 0.25% C, 0.001 to 3.0% Si, 0.01 to 3.0% Mn, 0.001 to 0.15% P, ≤0.05% S, 0.0005 to 0.010% N, <0.008% Al and 0.0005 to 0.010% B, and the balance iron with inevitable impurities, has a metallic structure comprising one or more kinds selected from bainite, austenite, martensite and pearlite by a volume ratio of ≥2%, and has a mean r value of ≥1.1. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

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【表２】

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【表３】

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【表４】

【０１０１】
【発明の効果】
本発明は、深絞り性に優れた高強度鋼板と鋼管、及び、その製造方法を提供するものであり、地球環境保全などに貢献するものである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel plate and a steel pipe used for, for example, automobile panels, suspensions, members, frames, and the like, and a method for manufacturing the same.
[0002]
The steel sheet of the present invention includes both a cold-rolled steel sheet that has not been subjected to a surface treatment and a steel sheet that has been subjected to a surface treatment such as galvanizing and electroplating for rust prevention. Zinc plating includes plating of an alloy whose main component is zinc, in addition to pure zinc. Further, the steel pipe of the present invention is also suitable as a steel pipe material for hydroforming.
[0003]
According to the present invention, since a high-strength steel sheet and a steel pipe excellent in formability can be obtained at low cost, it is considered that they can contribute to global environmental conservation.
[0004]
[Prior art]
With the need to reduce the weight of automobiles, higher strength of steel sheets is desired. By increasing the strength of the steel sheet, it is possible to reduce the weight by reducing the thickness of the steel sheet and to improve the safety in a collision.
[0005]
However, in order to obtain a steel sheet having high strength and excellent formability, especially deep drawability, conventionally, for example, as disclosed in Patent Document 1, ultra-low carbon aluminum killed steel with significantly reduced carbon content has been used. , Mn, P and the like had to be strengthened.
[0006]
In order to reduce the amount of C, vacuum degassing must be performed in the steel making process, and a large amount of CO ₂ is generated in the manufacturing process, which is not always the best in terms of global environmental protection.
[0007]
On the other hand, a steel sheet having a relatively large C content and good deep drawability is also disclosed. For example, it is disclosed in Patent Documents 2 and 3. However, these are premised on box annealing of aluminum-killed steel, and are inferior in productivity as compared with continuous annealing or continuous hot-dip galvanizing process.
[0008]
Further, in box annealing, high-temperature annealing is difficult, and since a forced cooling device is not generally provided, it is difficult to obtain an austenite phase, a martensite phase, and the like, and it is difficult to utilize structure strengthening. Therefore, there is also a problem that the strength is low compared to the amount of the alloy added.
[0009]
Regarding the amount of Al, the inventions disclosed in Patent Literature 1 and Patent Literature 2 require that 0.02% or more be added in order to fix N as AlN.
[0010]
Further, the invention described in Patent Document 3 contains 0.005 to 0.100% of Al, but the purpose is to fix N as AlN as in the inventions disclosed in Patent Documents 1 and 2. This is different from the technical idea of the present invention in which the amount of Al is set to less than 0.008% to suppress the precipitation of AlN and to improve the r value by adding B.
[0011]
[Patent Document 1]
JP-A-56-139654 [Patent Document 2]
Japanese Patent Publication No. 57-47746 [Patent Document 3]
Japanese Patent Publication No. 37456/1993
[Problems to be solved by the invention]
The present invention provides a good deep drawability of a steel having a relatively high C content without increasing the cost of a high-strength steel sheet and a steel pipe having good formability and without imposing an excessive load on the global environment. It is an object of the present invention to provide a steel sheet having the same, a steel pipe having a good workability, and a method for producing the same.
[0013]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, it has been found that a steel sheet or a steel pipe having good deep drawability can be obtained even if the C content is relatively large. Moreover, there is no need to rely on the conventional box annealing process.
[0014]
That is, by using a steel type in which Al is reduced and B and N are added in trace amounts, it is possible to improve the deep drawability after cold rolling annealing even if the final annealing is performed in a continuous line with good productivity. I found something.
[0015]
The reason for this is not necessarily clear, but is considered as follows.
[0016]
Generally, in steel having a relatively high C content, shear strain is likely to be introduced during cold rolling. For this reason, when this is annealed, recrystallization occurs preferentially from a portion having a large shear strain. At this time, the crystal orientation of the recrystallized grains is often unfavorable for deep drawability. Therefore, it is considered that the r value of the steel having a large C content is 1.0 or less.
[0017]
By reducing Al and adding a small amount of B and N, nucleation and growth in an orientation unfavorable for deep drawing at the initial stage of recrystallization are suppressed, and nucleation and growth of recrystallized grains in an orientation preferable for deep drawing are suppressed. It is thought to encourage.
[0018]
The gist of the present invention is:
(1) In mass%, C: 0.025 to 0.25%, Si: 0.001 to 3.0%, Mn: 0.01 to 3.0%, P: 0.001 to 0.15% , S: 0.05% or less, N: 0.0005 to 0.010%, Al: less than 0.008%, B: 0.0005 to 0.010%, the balance being iron and inevitable. Characterized in that the metal structure contains one or more of bainite, austenite, martensite, and pearlite in a volume ratio of 2% or more and has an average r value of 1.1 or more. High-strength cold-rolled steel sheet with excellent heat resistance.
[0019]
(2) When the r value in the direction perpendicular to the rolling direction is rC and the r value in the rolling direction is rL, rC ≧ rL, and the deep drawability according to (1) is excellent. High strength cold rolled steel sheet.
[0020]
(3) The high-strength cold-rolled steel sheet according to (1) or (2), wherein Mn and C are contained in a range satisfying Mn + 11 × C> 1.5.
[0021]
(4) The random intensity ratio of the {111} and {100} X-ray reflecting surfaces of the plate surface at a steel plate thickness of 1/2 is 2.0 or more and 3.0 or less, respectively. The high-strength cold-rolled steel sheet excellent in deep drawability according to any one of the above (1) to (3).
[0022]
(5) The deep drawability according to any of (1) to (4), wherein one or more of Zr, Ce, and Mg are contained in a total of 0.0001 to 0.5% by mass. High strength cold rolled steel sheet.
[0023]
(6) The deep drawability according to any one of (1) to (5), wherein one or more of Ti, Nb, and V are contained in a total of 0.001 to 0.2% by mass. High strength cold rolled steel sheet.
[0024]
(7) The method according to (1) to (6), wherein one or more of Sn, Cr, Cu, Ni, Co, W, and Mo are contained in a total of 0.001 to 2.5% by mass. A high-strength cold-rolled steel sheet excellent in deep drawability according to any of the above.
[0025]
(8) The high-strength cold-rolled steel sheet according to any one of (1) to (7), which contains 0.0001 to 0.01% by mass of Ca.
[0026]
(9) A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability according to any one of the above (1) to (8), wherein the (1), (3), (5) to (5) 8) and at least 1/4 to 3/4 of the plate thickness, the volume fraction of one or two of the bainite phase and the martensite phase is 70 in total. % Of hot-rolled steel sheet having a structure of not less than 25% by cold rolling at a rolling reduction of 25% or more and 95% or less and continuous annealing at a recrystallization temperature of 1000 ° C or less. Manufacturing method of cold rolled steel sheet.
[0027]
(10) A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability according to any one of (1) to (8), wherein the (1), (3), (5) to (5) 8) The steel having the chemical composition according to any one of the above items is hot-rolled, hot rolling is completed at (Ar3-50) ° C. or more, and the average cooling rate from the hot-rolling finishing temperature to 550 ° C. is 30 ° C. / A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, characterized by cooling at a temperature of s or more, winding at a temperature of 550 ° C. or less, and then performing cold rolling and continuous annealing.
[0028]
(11) A method for producing a high-strength galvanized steel sheet excellent in deep drawability, characterized by applying galvanization subsequent to the continuous annealing according to (9) or (10).
[0029]
(12) A high-strength steel pipe excellent in workability, characterized by being a steel pipe made of the steel sheet according to any of (1) to (8).
[0030]
(13) A steel pipe formed by joining steel sheets manufactured according to the method for manufacturing a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet having excellent deep drawability according to any of (9) to (11) above. The method for producing a high-strength steel pipe excellent in workability characterized by the following.
It is in.
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0032]
C: Since it is an element effective for achieving both high strength and good moldability, it is positively added. The addition is made 0.025% by mass or more, but excessive addition is not preferable in order to obtain a good r value and weldability, and the upper limit is made 0.25% by mass. 0.05 to 0.17% by mass is a desirable range. More preferably, it is 0.08 to 0.16% by mass.
[0033]
Si: The mechanical strength can be increased at low cost, and is added according to the required strength level. Further, Si has an effect of increasing the r value by reducing the amount of carbide present in the hot-rolled sheet or reducing the size of the carbide.
[0034]
On the other hand, excessive addition not only deteriorates the wettability and workability of the plating but also makes it difficult for the main phase of the hot-rolled sheet structure to be bainite or martensite, so the upper limit is 3.0 mass%. %, And preferably 2.5% by mass or less.
[0035]
The lower limit is set to 0.001% because it is difficult to make the lower limit from the viewpoint of steelmaking technology. From the viewpoint of improving the r value, 0.4 to 2.3% by mass is a more preferable range.
[0036]
Mn: an element that is effective not only for increasing the strength but also for changing the hot-rolled structure to a structure mainly composed of bainite or martensite. On the other hand, excessive addition deteriorates the r value, so the upper limit is 3.0% by mass. If the content is less than 0.01% by mass, steelmaking cost increases and hot rolling cracks caused by S are induced. Therefore, the lower limit is set to 0.01% by mass. 0.8 to 2.4% by mass is a preferable range for obtaining good deep drawability.
[0037]
P: an element effective for increasing the strength, so that 0.001% by mass or more is added. If more than 0.15% by mass is added, the weldability, the fatigue strength of the welded portion, and the resistance to secondary working brittleness deteriorate, so the upper limit is 0.15% by mass. Preferably, the upper limit is 0.06% by mass. Further, when particularly good fatigue strength of the welded portion is required, the upper limit is 0.015% by mass.
[0038]
S: It is an impurity, and the lower the better, the better. In order to prevent hot cracking, the content is set to 0.05% by mass or less, preferably 0.03% by mass or less. More preferably, the content is 0.015% by mass or less. Further, in relation to the amount of Mn, it is preferable that Mn / S> 10.
[0039]
N: An important element that combines with B and controls nucleation and growth at the initial stage of recrystallization. If it is too large, it precipitates during hot rolling, impairing the crystal orientation control action at the initial stage of recrystallization, and further deteriorates the aging resistance at room temperature. Therefore, the upper limit is made 0.010% by mass.
[0040]
If N is less than 0.0005% by mass, not only the above-described control of the crystal orientation becomes difficult, but also a significant increase in steel making cost is caused. Therefore, the lower limit is 0.0005% by mass. 0.0015 to 0.0050% by mass is a more preferable range for deep drawability.
[0041]
Al: The component range of Al is a feature of the present invention. That is, in the present invention, it has been found that not adding Al or keeping the content to a very small amount is extremely important for obtaining good workability.
[0042]
The reason for this is not clear, but it is considered that if the amount of Al is too large, it bonds with N to precipitate AlN, thereby suppressing the effect of B described later.
[0043]
Al is usually used as a deoxidizing element. However, for the reasons described above, the workability is reduced, and the scratches due to the alumina (Al ₂ O ₃ ) cluster are likely to be generated. Preferably reduces Al as much as possible, and sets the upper limit to less than 0.008% by mass.
[0044]
The lower limit is not particularly specified, but is preferably set to 0.0001% by mass in consideration of the level inevitably mixed during refining. A preferable upper limit is less than 0.005% by mass in view of processability and stable productivity.
[0045]
B: B is an important element in the present invention, and is added in an amount of 0.0005% by mass or more in order to stably obtain a good r value irrespective of production conditions such as a heating rate during annealing and a cold rolling rate. I do. However, an excessive addition lowers the workability and raises the recrystallization temperature, so the upper limit is made 0.010% by mass. Preferably it is 0.0008-0.0035 mass%.
[0046]
The structure of the steel sheet of the present invention is as follows. That is, the structure preferably contains at least 2% in total of one or more of bainite, austenite, martensite, and pearlite. 5% or more is more preferable. The remainder is desirably made of ferrite. Bainite, austenite, martensite, and pearlite are effective in increasing the mechanical strength of steel.
[0047]
Further, as is well known, bainite has an effect of improving burring workability and hole expanding property, austenite has an effect of improving n value and elongation, and martensite has an effect of reducing YR (yield strength / tensile strength). Therefore, the volume ratio of each of the above phases may be appropriately changed according to the required characteristics of the product plate.
[0048]
However, if the volume ratio is less than 2%, a very clear effect cannot be expected. For example, in order to improve burring characteristics, a structure composed of 90% or more of bainite and 10% or less of ferrite is preferable, and in order to improve elongation, 5% or more of retained austenite and 95% or less of ferrite. Is preferred.
[0049]
Here, the bainite includes upper bainite and lower bainite, as well as acicular ferrite and bainitic ferrite. Although pearlite does not particularly improve workability, it may be inevitably obtained, and is arbitrarily specified to distinguish it from carbide obtained by box annealing. When determining the structure, precipitates and carbides are excluded.
[0050]
It is preferable that the volume ratio of the tissue is measured by a so-called point count method in which an arbitrary 500 points are selected from the optical microscope structure to determine the tissue. However, since it is difficult to judge austenite from the structure, it is preferable to judge from a crystal structure by X-ray diffraction.
[0051]
The average r value of the steel sheet obtained by the present invention is 1.1 or more. More preferably, the average r value is greater than 1.2. Note that the average r value is given by (rL + 2 × rD + rC) / 4. Here, rL is the r value in the rolling direction, rD is the r value at 45 degrees to the rolling direction, and rC is the r value in the direction perpendicular to the rolling direction.
[0052]
The r value may be calculated according to the definition of the r value from a change in the gauge length and a change in the plate width after 10% or 15% tension after performing a tensile test using a JIS No. 13B or JIS No. 5B test piece. . When the uniform elongation is less than 10%, it may be evaluated by giving a tensile deformation of 3% or more and less than the uniform elongation.
[0053]
In general, the workability in the C direction is lower than that in the L direction, but rC ≧ rL is preferable because workability anisotropy can be reduced.
[0054]
The Mn content and the C content are preferably contained so as to satisfy Mn + 11 × C> 1.5. By satisfying this condition, the hot-rolled structure can be easily made into a structure mainly composed of bainite or martensite. More preferably, Mn + 11 × C> 2.0.
[0055]
In the steel sheet obtained by the present invention, the X-ray reflection surface random intensity ratio of the plate surface at least at the center of the plate thickness is 2.0 or more and 3.0 or less for the {111} surface and the {100} surface, respectively. It is preferable that More preferably, they are 2.0 or more and 2.0 or less, respectively.
[0056]
The random intensity ratio is a relative intensity based on the X-ray intensity of a random sample. The plate thickness center indicates a range of 3/8 to 5/8 of the plate thickness, and the measurement may be performed on any surface in this range. Intensities of (111) [1-10], (111) [1-21], and (554) [-2-25] on the φ2 = 45 ° cross section of the three-dimensional texture calculated by the series expansion method Is preferably 2.0 or more, 2.5 or more, and 2.5 or more, respectively.
[0057]
In the present invention, the X-ray intensity of the {110} plane may be 0.1 or more, and at this time, the intensity of (110) [001] is 1.0 or more in the above φ2 = 45 ° cross section. It may be. Therefore, rC is often larger than rL.
[0058]
Zr, Ce and Mg are effective as deoxidizing elements. On the other hand, excessive addition causes a large amount of crystallization or precipitation of oxides, sulfides or nitrides, deteriorating cleanliness, lowering ductility, and impairing plating properties. Therefore, if necessary, it is preferable to add one or more of these in an amount of 0.0001 to 0.5% by mass.
[0059]
Ti, Nb, and V are also added as needed. These improve the r-value by making the hot-rolled structure a bainite or martensitic structure, similarly to B, and also increase the strength of steel by forming carbide, nitride or carbonitride. One or more of these are added in a total amount of 0.001% by mass or more because they are effective in improving workability such as hole-expandability.
[0060]
If the sum exceeds 0.2% by mass, a large amount of carbides, nitrides or carbonitrides precipitate in ferrite grains or grain boundaries, which are the parent phase, and reduce ductility. The range is 0.001 to 0.2% by mass. More preferably, it is 0.01 to 0.03% by mass.
[0061]
Sn, Cr, Cu, Ni, Co, W, and Mo are strengthening elements, and if necessary, one or more of these are added in a total amount of 0.001% by mass or more, if necessary. . Excessive addition leads to an increase in cost and a reduction in ductility, so the content was made 2.5 mass% or less.
[0062]
Ca: an element effective for deoxidation in addition to controlling inclusions. Addition of an appropriate amount improves hot workability, but excessive addition conversely promotes hot embrittlement. 0001-0.01% by mass is added.
[0063]
The effects of the present invention are not lost even if O, Zn, Pb, As, Sb, and the like are included as unavoidable impurities in a range of 0.02% by mass or less.
[0064]
The structure of the hot-rolled sheet to be subjected to cold rolling has a volume ratio of one or two of the bainite phase and the martensite phase of at least 70% in a range of at least 1/4 to 3/4 of the sheet thickness. Is preferred.
[0065]
This can promote the effect of improving the r value achieved by the components. The volume ratio is preferably 80% or more, and more preferably 90% or more. Needless to say, it is preferable to have such a structure over the entire range of the plate thickness.
[0066]
The reason why the hot-rolled structure is made to be bainite or martensite improves the deep drawability after cold-rolling annealing is not necessarily clear, but it is necessary to make the carbide in the hot-rolled sheet finer, and furthermore, to reduce the crystal grain size. It is presumed to be due to the effect of making finer.
[0067]
Here, the bainite includes upper bainite and lower bainite, as well as acicular ferrite and bainitic ferrite. It goes without saying that from the viewpoint of making the carbide finer, lower bainite is more preferable than upper bainite.
[0068]
Further, the bainite of the cold-rolled steel sheet and the galvanized steel sheet, which is the final structure, includes not only upper bainite and lower bainite but also acicular ferrite and bainitic ferrite.
[0069]
In production, following smelting in a blast furnace, converter, electric furnace, etc., various secondary smelting is performed, ingot casting and continuous casting are performed, and in the case of continuous casting, hot cooling is performed without cooling to around room temperature. Rolling may be performed in combination with a manufacturing method such as CC-DR. It goes without saying that hot rolling may be performed by reheating the cast ingot or cast slab.
[0070]
The heating temperature of the hot rolling is not particularly limited, but is preferably 1100 ° C. or higher in order to secure a finishing temperature described later.
[0071]
Lubrication may be performed for one or more passes of hot rolling. Further, the rough rolling bars may be joined to each other, and the hot rolling may be continuously performed. The rough rolling bar may be wound once, rewound again, and then subjected to finish hot rolling.
[0072]
The finishing temperature of hot rolling is preferably performed at (Ar3-50) ° C or higher. It is more preferably at least the Ar3 transformation temperature. If the hot-rolling finishing temperature is lower than this, it is difficult to make the hot-rolled structure bainite or martensite. The upper limit of the finishing temperature is not particularly limited, but is preferably 1000 ° C. or lower in order to obtain a uniform hot-rolled structure.
[0073]
It is preferable that the cooling rate after hot rolling is up to 550 ° C. at an average cooling rate of 30 ° C./s or more. As a result, a structure in which the hot-rolled structure is mainly composed of bainite or martensite is obtained. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 200 ° C./s or less in consideration of the capacity of existing equipment.
[0074]
The winding temperature is preferably 550 ° C. or lower. More preferably, it is 400 ° C. or lower. This is because the hot rolled sheet has a structure of bainite or martensite as a main phase and suppresses precipitation of coarse carbides, thereby obtaining a good r value after cold rolling annealing. On the other hand, although the lower limit of the winding temperature can be obtained without any particular effect, the temperature is preferably set to 200 ° C. or higher from the viewpoint of reducing solid solution C.
[0075]
After hot rolling, it is desirable to perform pickling.
[0076]
The rolling reduction in cold rolling after hot rolling is preferably 25 to 95%. If the cold rolling reduction is less than 25% or more than 95%, the r-value will be low, so it is limited to 25 to 95%. 35 to 70% is a more preferable range.
[0077]
It is preferable that the annealing temperature is not lower than the recrystallization temperature and not higher than 1000 ° C. The recrystallization temperature in the present invention indicates a temperature at which recrystallization starts when annealing is performed in a continuous annealing line with a retention of 10 s. If the annealing temperature is lower than the recrystallization temperature, a favorable texture is not developed, and the r-value tends to be poor.
[0078]
Further, in the case of annealing in a continuous annealing or continuous hot-dip galvanizing step, if the annealing temperature is higher than 1000 ° C., a heat buckle or the like is induced, which may cause a sheet break or the like.
[0079]
When it is desired to obtain a second phase of bainite, austenite, and martensite after annealing, the annealing temperature is heated in the α + γ2 phase region or γ single phase region, and a cooling rate and overaging conditions suitable for obtaining each phase. Needless to say, when hot-dip galvanizing is performed, it is necessary to select a plating bath temperature and a subsequent alloying temperature. In the present invention, annealing is limited to a continuous line and does not include box annealing.
[0080]
After annealing, plating mainly using zinc may be performed. It is preferable that the galvanizing is performed continuously by annealing and plating in a continuous hot-dip galvanizing line. After immersion in the hot-dip galvanizing bath, heating may be performed to promote alloying between the galvanizing and the base iron. It goes without saying that various electroplating mainly composed of zinc may be performed in addition to hot-dip galvanizing.
[0081]
The skin pass after annealing or galvanizing is performed as necessary from the viewpoint of shape correction and strength adjustment, and further ensuring non-aging at room temperature. 0.3 to 5.0% is a preferable rolling reduction.
[0082]
The steel sheet obtained by the present invention has a tensile strength of 340 MPa or more.
[0083]
The steel plates manufactured in this manner can be joined to form a steel pipe. It is desirable that the rolling direction of the steel plate coincides with the tube axis direction. If the pipe axis direction is other than the rolling direction, for example, the direction perpendicular to the rolling direction is the pipe axis direction, it is not particularly inferior for hydroforming, but this is because the productivity of steel pipe production decreases.
[0084]
In the production of steel pipes, usually, electric resistance welding is used, but welding and pipe forming techniques such as TIG, MIG, laser welding, UO and forging can also be used. In the production of these welded steel pipes, the weld heat-affected zone may be subjected to local solution heat treatment alone or in combination depending on the required properties, and in some cases, may be performed a plurality of times. Further enhance.
[0085]
This heat treatment is intended to be applied only to the welded portion and the weld heat affected zone, and can be performed online or offline during manufacturing.
[0086]
The r value of the steel pipe has the same characteristics as that of the steel plate. The measurement of the r value of the steel pipe is performed by cutting a test piece from the steel pipe, forming a flat plate by pressing, and further processing the tensile test piece.
[0087]
Depending on the diameter of the steel pipe and the direction of sample collection, it may be difficult to collect JIS No. 13 B test pieces. In this case, use small test pieces such as JIS No. 6 and JIS No. 14 B test pieces. , Within the range of uniform elongation. When cutting out the test piece from the steel pipe, care is taken that the welded portion of the steel pipe is not included in the parallel part of the tensile test piece.
[0088]
Since X-ray measurement cannot be performed with a steel pipe itself, it is performed as follows. First, a steel pipe is appropriately cut and formed into a plate shape by a press or the like. The thickness is reduced by mechanical polishing or the like to the thickness of the measurement plate, and finally finished by chemical polishing so as to reduce the thickness by about 30 to 100 μm with the average crystal grain size not less than 1 as a standard.
[0089]
The steel pipe of the present invention has a small surface roughness. That is, Ra defined by JIS B0601 is preferably 0.8 μm or less. This is in contrast to the case where the Ra of the steel pipe produced by the high-temperature warping is greater than 0.8 μm. More preferably, it is 0.6 μm or less.
[0090]
【Example】
Each steel having the components shown in Table 1 was melted and heated to 1250 ° C., then hot-rolled at a finishing temperature of Ar3 transformation temperature or higher, cooled under the conditions shown in Table 2, and wound up. The hot rolled structure obtained at that time is also shown in Table 2. Furthermore, cold rolling was performed under the conditions shown in Table 2 to obtain a steel sheet of 1.0 mm. Next, continuous annealing was performed for an annealing time of 60 s, and overaging treatment (180 s) was performed depending on the type of steel. The annealing temperature and the overaging temperature are as shown in Table 2. An additional 0.8% skin pass was applied.
[0091]
The r value of the obtained steel sheet was evaluated by a JIS No. 13 B test piece, and the other tensile properties were evaluated by a tensile test using a JIS No. 5 test piece. The sample to be subjected to the X-ray measurement was manufactured by reducing the thickness to near the center of the plate thickness by mechanical polishing and finishing by chemical polishing.
[0092]
The obtained steel plate was pipe-formed by electric resistance welding to obtain a steel pipe having a diameter of 60 mm. The workability of the obtained steel pipe was evaluated by the following method. A scribed circle having a diameter of 10 mm was transferred to a steel pipe in advance, and the inner pressure and the amount of axial pressing were controlled to perform overhang forming in the circumferential direction.
[0093]
The strain εΦ in the axial direction and the strain εθ in the circumferential direction of the portion showing the maximum expansion rate immediately before the burst (expansion rate = maximum perimeter after molding / perimeter of the mother pipe) were measured.
[0094]
The ratio ρ = εΦ / εθ of the two strains and the maximum expansion ratio were plotted, and the expansion ratio Re at which ρ = −0.5 was used as the formability index of the hydroform. Evaluation of tensile strength and elongation was performed using JIS No. 12 arc-shaped test pieces.
[0095]
The r value of the steel pipe was measured by cutting a test piece from the steel pipe, forming a flat plate by pressing, processing the test piece into a tensile test piece, and using a JIS No. 13B test piece.
[0096]
As is clear from Table 3 (continuation of Table 2) and Table 4, according to the examples of the present invention, good workability can be obtained. When bainite or martensite remains in the structure of the hot-rolled sheet, the r-value is further improved. Moreover, a composite structure steel in which an appropriate amount of austenite and martensite were dispersed in addition to ferrite was obtained.
[0097]
[Table 1]

[0098]
[Table 2]

[0099]
[Table 3]

[0100]
[Table 4]

[0101]
【The invention's effect】
The present invention provides a high-strength steel sheet and a steel pipe excellent in deep drawability, and a method for producing the same, and contributes to global environmental protection and the like.

Claims (13)

In mass%,
C: 0.025 to 0.25%,
Si: 0.001 to 3.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.05% or less,
N: 0.0005 to 0.010%,
Al: less than 0.008%,
B: 0.0005 to 0.010%
And the balance consists of iron and unavoidable impurities, and the metal structure contains at least 2% by volume of one or more of bainite, austenite, martensite, and pearlite, and an average r value of 1 A high-strength cold-rolled steel sheet excellent in deep drawability, characterized in that it is at least 1. 2. The high-strength cold steel with excellent deep drawability according to claim 1, wherein rC ≧ rL, where rC in the direction perpendicular to the rolling direction is rC and r-value in the rolling direction is rL. Rolled steel sheet. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein Mn and C are contained in a range satisfying Mn + 11 x C> 1.5. The X-ray reflection surface random intensity ratio of {111} and {100} of the plate surface at a steel plate thickness of 1/2 is 2.0 or more and 3.0 or less, respectively. Item 4. A high-strength cold-rolled steel sheet excellent in deep drawability according to any one of Items 1 to 3. The high drawability excellent in deep drawability according to any one of claims 1 to 4, wherein one or more of Zr, Ce, and Mg are contained in a total of 0.0001 to 0.5% by mass. Strength cold rolled steel sheet. 6. The high drawability excellent in deep drawability according to any one of claims 1 to 5, wherein one or more of Ti, Nb, and V are contained in a total of 0.001 to 0.2% by mass. Strength cold rolled steel sheet. 7. The composition according to claim 1, wherein one or more of Sn, Cr, Cu, Ni, Co, W and Mo are contained in a total amount of 0.001 to 2.5% by mass. High strength cold rolled steel sheet with excellent deep drawability. The high-strength cold-rolled steel sheet according to any one of claims 1 to 7, comprising 0.0001 to 0.01% by mass of Ca. A method for producing a high-strength cold-rolled steel sheet having excellent deep drawability according to any one of claims 1 to 8, wherein the method is any one of claims 1, 3, and 5 to 8. And a structure in which at least 1/4 to 3/4 of the plate thickness has a total volume fraction of one or two of the bainite phase and the martensite phase of 70% or more. Production of a high-strength cold-rolled steel sheet excellent in deep drawability, characterized by subjecting a hot-rolled steel sheet having cold rolling to a rolling reduction of 25% or more and 95% or less and continuously annealing at a recrystallization temperature or more and 1000 ° C or less. Method. A method for producing a high-strength cold-rolled steel sheet having excellent deep drawability according to any one of claims 1 to 8, wherein the method is any one of claims 1, 3, and 5 to 8. The steel having the chemical composition described in (1) is hot-rolled, hot rolling is completed at (Ar3-50) ° C. or more, and the steel is cooled from the hot-rolling finishing temperature to 550 ° C. at an average cooling rate of 30 ° C./s or more. A method for producing a high-strength cold-rolled steel sheet having excellent deep drawability, comprising winding at a temperature of 550 ° C. or lower, followed by cold rolling and continuous annealing. A method for producing a high-strength galvanized steel sheet having excellent deep drawability, wherein galvanizing is performed subsequent to the continuous annealing according to claim 9 or 10. A high-strength steel pipe excellent in workability, characterized by being a steel pipe made of the high-strength steel sheet excellent in deep drawability according to any one of claims 1 to 8. A steel pipe formed by joining steel sheets manufactured according to the method for manufacturing a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet having excellent deep drawability according to any one of claims 9 to 11. A method for manufacturing high-strength steel pipe with excellent workability.