JP2002275577A - Steel tube having excellent workability and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a steel tube which has better formability since the request for a steel tube having high strength to hydroforming increases, and its workability is made into a more serious problem compared with the conventional case, and to provide a production method therefor without spending a high cost. SOLUTION: The steel tube having excellent workability has a composition containing, by mass, 0.08 to 0.25% C, 0.001 to 1.5% Si, 0.01 to 2.0% Mn, 0.001 to 0.06% P, 0.008 to 0.2% Al and 0.001 to 0.007% N, and in which the content of S is controlled to <=0.05%, and the rest iron and inevitable impurities. The steel tube has the average crystal grain size of >=5 μm, and has an r* value of >=1.2: r*=ln (C0 /C)÷ln (C×L/C0 ×L0 ); wherein, C0 and C are the other circumference (mm) of the steel tube before and after testing, and L0 and L are the distance (mm) between the gauges in the longitudinal direction at each circumferential position of the steel tube before and after testing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe used for, for example, automobile panels, suspensions, members, and the like, and a method of manufacturing the same. Especially hydropho
This method is suitable for use in the memory processing (see JP-A-10-175027). The steel sheet of the present invention includes both a steel sheet not subjected to a surface treatment and a steel sheet 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. INDUSTRIAL APPLICABILITY The steel pipe according to the present invention is particularly excellent in hydroformability in which an axial pressing force acts, and can improve the production efficiency of automotive parts during hydroform processing. further,
Since the present invention can be applied to high-strength steel pipes, the thickness of parts can be reduced, and it is considered that the present invention can contribute to global environmental protection.

[0002]

2. Description of the Related Art With the need for lighter automobiles, higher strength of steel sheets is desired. By increasing the strength, it is possible to reduce the weight by reducing the plate thickness and to improve the safety in the event of a collision. Also,
Recently, an attempt has been made to machine a part having a complicated shape from a high-strength steel pipe by using a hydroform method. This is intended to reduce the number of parts and the number of welding flanges in accordance with the need for weight reduction and cost reduction of automobiles. As described above, if a new processing method such as a hydroform is actually adopted, great merits such as reduction in cost and expansion of design freedom are expected. In order to make full use of the merits of such hydroforming, materials suitable for these new processing methods are required. The present inventors filed an application in Japanese Patent Application No. 2000-52574 for a steel pipe having a controlled texture and excellent workability. However, a steel pipe obtained by finishing such a steel pipe by high-temperature processing often contains a large amount of solid solution C and solid solution N, which may cause cracking during hydroform processing, and may cause surface damage such as stretcher strain. May induce defects. Further, applying a thermomechanical treatment at a high temperature after winding a steel sheet into a tube has a problem in that productivity is poor, which imposes a burden on the global environment and increases costs.

[0003]

As the global environmental problem becomes more and more serious, it is considered inevitable that the demand for higher strength steel pipes for hydroforming is inevitable. There is no doubt that workability will be more problematic than before. The present invention provides a steel pipe having better formability and a method for manufacturing the same without increasing costs.

[0004]

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.08 to 0.25%, Si: 0.2%
001 to 1.5%, Mn: 0.01 to 2.0%, P: 0.0
01 to 0.06%, Al: 0.008 to 0.2%, N: 0.2.
001-0.007%, S: not more than 0.05%, the balance being iron and unavoidable impurities, having an average crystal grain size of not less than 5 μm, and having an r * value of 1. A steel pipe excellent in workability characterized by being 2 or more. r * = ln (C ₀ / C) ÷ ln (C × L / C ₀ × L ₀ ) C ₀ , C: outer circumference (mm) of the steel pipe before and after the test L ₀ , L: each circumference of the steel pipe before and after the test Distance between the evaluation points in the longitudinal direction of the position (mm) (2) Further, in mass%, Cr: 0.05 to 10%, N
i: 0.05 to 20%, Cu: 0.05 to 20%, M
o: 0.05 to 1.0%, Co: 0.05 to 1.0%, W:
0.05 to 1.0%, Sn: 0.05 to 1.0%, Zr:
0.0001 to 0.5%, Mg: 0.0001 to 0.5%,
Ti: 0.001 to 0.2%, Nb: 0.001 to 0.2
%, V: 0.001 to 0.2%, B: 0.0001 to 0.0
The steel pipe having excellent workability according to the above (1), characterized in that it contains one or more of 1% and Ca: 0.0001 to 0.01%. (3) In mass%, C: 0.08 to 0.25%, Si: 0.2%
001 to 1.5%, Mn: 0.01 to 2.0%, P: 0.0
01 to 0.06%, Al: 0.008 to 0.2%, N: 0.2.
Containing from 001 to 0.007%, S: is 0.05% or less, the steel balance of iron and inevitable impurities, (A _r3
Hot rolling is completed at a transformation point of -50 ° C or higher, winding is performed at a temperature of 700 ° C or lower, cold rolling is performed at a rolling reduction of 25% to less than 70%, and heating is performed at an average heating rate of 4 to 200 ° C / hour. Then, annealing is performed at a maximum temperature of 600 to 800 ° C., and after cooling at a rate of 5 to 100 ° C./hr, the pipe is formed so that the rolling direction is in the pipe axis direction. Manufacturing method of steel pipe. (4) Further, in mass%, Cr: 0.05 to 10%, N
i: 0.05 to 20%, Cu: 0.05 to 20%, M
o: 0.05 to 1.0%, Co: 0.05 to 1.0%, W:
0.05 to 1.0%, Sn: 0.05 to 1.0%, Zr:
0.0001 to 0.5%, Mg: 0.0001 to 0.5%,
Ti: 0.001 to 0.2%, Nb: 0.001 to 0.2
%, V: 0.001 to 0.2%, B: 0.0001 to 0.0
The method for producing a steel pipe having excellent workability according to the above (3), wherein one or more of Ca is contained in 1% and 0.0001 to 0.01% of Ca. (5) The lubricating oil applied surface is uniformly dried on the surface of the steel pipe, and the friction coefficient of the surface is 70% or less as compared with the friction coefficient immediately after applying the lubricating oil.
Or the steel pipe excellent in workability as described in (2). (6) Dry the lubricating oil applied to the surface of the steel pipe evenly,
The method for producing a steel pipe excellent in workability according to the above (3) or (4), wherein the friction coefficient of the surface is 70% or less as compared with the friction coefficient immediately after the application of the lubricating oil.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The steel pipe excellent in workability of the present invention and a method for manufacturing the same will be described in detail below. First, the reasons for limiting the chemical components will be described.

C: Addition of 0.08% or more is effective for increasing the strength and increasing the cost to reduce the amount of C. On the other hand, in order to obtain a good r value, excessive addition is not preferable, and the upper limit is set to 0.25%.
It goes without saying that if the C content is less than 0.08%, the r-value will be improved, but the reduction of C is not the object of the present invention and was therefore excluded. More than 0.10 to 0.18% is a desirable range.

Si: It is possible to increase the mechanical strength at low cost, and it is sufficient to add Si according to the required strength level. However, excessive addition not only causes deterioration of the wettability and workability of the plating but also r. Since the value deteriorated, the upper limit was set to 1.5%.
The lower limit is set to 0.001% because it is difficult for steelmaking technology to make the lower limit less than 0.001%. 0.5% or less is a more preferable upper limit.

Mn: Mn is effective for increasing the strength, and may be added as needed. However, excessive addition degrades the r value, so the upper limit is 2.0%. If the content is less than 0.01%, the steelmaking cost increases, and hot rolling cracks caused by S are induced. 0.04 to 0.8% is preferred. In order to further increase the r value, the lower the Mn content, the better. Therefore, the Mn content is preferably in the range of 0.04 to 0.12%.

P: 0.
001% or more is added. If more than 0.06% is added, the weldability, the fatigue strength of the welded portion, and the resistance to secondary working brittleness will be deteriorated. Preferably it is less than 0.04%.

[0010] S: impurities, the lower the more, the more preferable, in order to prevent hot cracking, the content is 0.05% or less. Preferably it is 0.015% or less.

Al: 0.008% or more is added because it is necessary to obtain a good r value. However, excessive addition not only reduces the effect but also induces surface defects, so the upper limit is made 0.2%. Preferably 0.015
To 0.07%.

N: 0.001 to obtain a good r value
% Or more is essential. If the amount is too large, the aging property is deteriorated, and a large amount of Al needs to be added.
07%. 0.002 to 0.005% is a more preferable range.

Cr: strengthening element, if necessary, 0.0
Add 5% or more. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 1.0%. Ni: a strengthening element, and 0.05% or more is added as necessary. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 2.0%. Cu: a strengthening element, and 0.05% or more is added as necessary. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 2.0%. Mo: strengthening element, added at 0.05% or more as necessary. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 1.0%. Co: a strengthening element, and 0.05% or more is added as necessary. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 1.0%. W: strengthening element, if necessary added at 0.05% or more. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 1.0%. Sn: a strengthening element, and 0.05% or more is added as necessary. Excessive addition increases the cost and lowers the ductility, so the upper limit is made 1.0%. Zr: Effective as a deoxidizing element. 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, 0.0001 to 1.5
%. Mg: Effective as a deoxidizing element. 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, 0.0001 to 0.5
%. Ti, Nb, V: These can form a carbide, a nitride, or a carbonitride to increase the strength of a steel material and improve workability. On the other hand, excessive addition causes precipitation as a large amount of carbides, nitrides or carbonitrides in ferrite grains or grain boundaries which are the parent phase, and lowers ductility. 2%
And B: Since it is effective for improving the r value and improving the brittleness resistance to secondary workability, it is added as necessary. If it is less than 0.0001% by mass, the effect is slight, and even if it exceeds 0.01%, no remarkable effect can be obtained. 0.0002 to 0.0030
% Is a preferable range. 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. ~ 0.
01% range.

Further, in the production, ingot casting or continuous casting is performed by performing various secondary smelting following smelting in a blast furnace, a converter, an electric furnace, or the like. In the case of continuous casting, heat is produced without cooling to near room temperature. It may be manufactured by combining manufacturing methods such as CC-DR for cold rolling. It goes without saying that hot rolling may be performed by reheating the cast ingot or cast slab. The heating temperature of the hot rolling is not particularly limited, but is set to 11 in order to bring AlN into a solid solution state.
The temperature is preferably set to 00 ° C. or higher.

The hot rolling is performed at a finishing temperature of (A _r3 -50) ° C. or higher. The temperature is preferably (A _r3 +30) ° C or more, more preferably (A _r3 +70) ° C. or more. In the present invention, the texture of the hot rolled sheet is made as random as possible, and the crystal grain size of the hot rolled sheet is made to grow as much as possible.
This is because it is preferable for improving the value.

The cooling rate after hot rolling is not particularly specified, but the average cooling rate up to the winding temperature is preferably less than 30 ° C./s.

The winding temperature is set to 700 ° C. or less. Al
This is because a good r value is ensured by suppressing the coarsening of N. Preferably it is 620 ° C or lower. Hot rolling 1
Lubrication may be applied to more than the pass. Further, the rough rolling bars may be joined to each other and the finish hot rolling may be continuously performed. The rough rolling bar may be wound once, rewound again, and then subjected to finish hot rolling.

Although the lower limit of the winding temperature can be obtained without any particular effect, the temperature is preferably set to 350 ° C. or higher from the viewpoint of reducing solid solution C. After hot rolling, it is desirable to perform pickling.

The rolling reduction in cold rolling after hot rolling is important in the present invention. That is, the rolling reduction is set to 25 to less than 70%. In the prior art, it is fundamental to improve the r-value by cold rolling under high rolling reduction, but it has been newly found that it is more important to lower the rolling reduction in the steel sheet of the present invention. Things. Reduction rate less than 25% or 70
%, The r-value becomes low, so that it is limited to 25 to 70%. 30-55% is a more preferable range.

The annealing is basically box annealing, but is not limited to the above if the following requirements are satisfied. In order to obtain a good r value, the heating rate needs to be 4 to 200 ° C./hr.
Furthermore, 10 to 40 ° C./hr is preferable. It is desirable that the highest temperature be 600 to 800 ° C. from the viewpoint of securing the r value. If the temperature is lower than 600 ° C., recrystallization is not completed and workability deteriorates. On the other hand, when the temperature exceeds 800 ° C., the workability is sometimes deteriorated because the γ fraction in the α + γ region is on the higher side. The holding time at the highest temperature is not particularly specified, but the holding time at (highest temperature -20) ° C. or more is 2 hours.
It is preferably at least r from the viewpoint of improving the r value. The cooling rate is determined from the viewpoint of sufficiently reducing solid solution C. That is, the range is 5 to 100 ° C./hr. The skin pass after the annealing is performed as necessary from the viewpoint of forcing the shape, adjusting the strength, and further ensuring the non-aging property at room temperature. 0.5-5.0%
Is a preferable rolling reduction.

Using the steel plate manufactured in this way, pipes are formed so that the rolling direction is in the tube axis direction, and welded. In the production of steel pipes, ERW is usually used, but TI
G, MIG, laser welding, welding and pipe forming techniques such as UO and forging can also be used. In the production of these welded steel pipes, the heat-affected zone of the weld may be subjected to local solution heat treatment alone or in combination depending on the required characteristics, and may be performed multiple times depending on the case. Further enhance. This heat treatment is intended to be applied only to the welded portion and the heat affected zone, and can be performed online or offline during manufacturing. The same heat treatment may be performed on the entire steel pipe for the purpose of improving workability.

The r * value of the steel pipe obtained by the present invention is 1.
2 or more. For the measurement of the r * value, a tensile test was performed using a JIS No. 11 tubular test piece, and after 15% tension, the r * value represented by the following equation r * = ln (C ₀ / C) ÷ ln (C × L / C ₀ × L ₀ ) C ₀ , C: Outer circumference (mm) of the steel pipe before and after the test L ₀ , L: Distance between the evaluation points (mm) in the longitudinal direction of each circumferential position of the steel pipe before and after the test, and the r * value is calculated. If the uniform elongation is less than 15%, it may be evaluated at 10%.

The average crystal grain size of the crystal grains constituting the steel pipe is 5
μm or more. If the crystal grain size is smaller than this, a good r value cannot be obtained. If the thickness is 60 μm or more, a problem such as rough skin may occur at the time of molding. Therefore, the thickness is desirably less than 60 μm. The crystal grain size may be measured by a point calculation method or the like in a range of a sheet thickness (3/8 to 5/8) of a cut surface (L cross section) perpendicular to the sheet surface and parallel to the rolling direction. In order to reduce the measurement error, it is necessary to measure the area where 100 or more crystal grains exist. Etching is preferably performed with nitral. The crystal grains are ferrite grains, and the average crystal grain size is an arithmetic average (simple average) of all data of the crystal grain sizes measured as described above.

As the lubricating oil applied to the surface of the steel pipe,
Any lubricating oil that is commonly used, for example, a machine oil, a rust preventive oil, a lubricating oil containing a rust inhibitor, and the like can be used. As a method of applying to the surface of the steel pipe, any method such as application using a brush, dip coating, spray coating and the like can be used. The surface of the steel pipe immediately after the application of the lubricating oil is wet by the lubricating oil.

When the steel pipe whose surface is wet with the lubricating oil immediately after application of the lubricating oil or immediately after unpacking the steel pipe from the binding is exposed to normal air or the like and dried, the coefficient of friction of the surface of the steel pipe becomes dry. Decrease with progress. Assuming that the friction coefficient of the surface immediately after the application of the lubricating oil is 100%, the friction coefficient decreases to 60% after 1 hour in the air, 55% after 3 hours, and 30% after 8 hours.
Therefore, in the present invention, the drying time is preferably set to 1 hour or more. The drying time is more preferably 3 hours or more. More preferably, the drying time is at least 8 hours. It is possible to determine whether the surface of the steel pipe to which the lubricating oil has been applied is in a wet state or in a state in which drying has progressed by touch. When the surface of the steel pipe is touched by hand, it is judged that the lubricating oil is dry to the extent that it does not stick. Further, quantitatively, it is possible to confirm that the steel pipe is dry by measuring the friction coefficient of the surface of the steel pipe to which the lubricating oil is applied. That is, the friction coefficient of the surface is 70% or less as compared with the friction coefficient immediately after the application of the lubricating oil,
If it is preferably 60% or less, more preferably 40% or less, it can be confirmed that the present invention is in a dry state. The measurement of the coefficient of friction of the surface of the steel pipe coated with the lubricating oil is performed as follows. The test apparatus uses a bead drawing apparatus. A 30 × 300 (mm) steel plate is cut out, and lubricating oil is applied to the surface of the steel plate. Thereafter, the steel plate is sandwiched between two mirror-finished flat molds (SKD 11), and the steel plate is pulled out. The friction coefficient is determined from the pressing load of the mold and the pulling load of the steel sheet.

[0026]

EXAMPLE Each steel having the components shown in Table 1 was melted at 1230 ° C.
, And then hot-rolled at the finishing temperature shown in Table 1 and wound up. After pickling, the steel sheet was cold-rolled at the rolling reduction shown in Table 2, then annealed at a heating rate of 20 ° C./hr and a maximum temperature of 690 ° C., kept for 12 hours, and cooled at 17 ° C./hr. An additional 1.5% skin pass was applied. The plate was formed by electric resistance welding.

[0027]

[Table 1]

[0028]

[Table 2]

The properties of the obtained steel pipe were evaluated by the following methods. Evaluation of the mechanical properties was performed using a JIS No. 12 arc-shaped test piece. The r * value was determined by performing a tensile test using a JIS No. 11 tubular test piece and, after 15% tension, the r * value given by the following equation: r * = ln (C ₀ / C) ÷ ln (C × L / C ₀ × L ₀ ) C ₀ , C: Outer circumference (mm) of the steel pipe before and after the test L ₀ , L: The r * value is calculated from the distance (mm) between the longitudinal grades of each circumferential position of the steel pipe before and after the test. If the uniform elongation is less than 15%, it may be evaluated at 10%.

The coefficient of friction of a steel pipe is 30 × 30
After cutting into a 0 (mm) steel plate, the test apparatus uses a bead drawing apparatus. A steel plate is sandwiched between two mirror-finished flat molds (SKD 11), and the steel plate is pulled out. The friction coefficient is determined from the pressing load of the mold and the pulling load of the steel sheet.

As is clear from Table 2, all of the examples of the present invention have a good r * value and a good surface lubricity,
In the examples outside the present invention, these properties were inferior.

[0032]

Industrial Applicability The present invention provides a steel pipe excellent in workability and a method for producing the same, which is suitable for hydroform workability and contributes to global environmental protection and the like.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/06 C22C 38/06 38/60 38/60 (72) Inventor Koji Sakuma 1st Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works (72) Inventor Hirosaku Shioda 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works (72) Inventor Naoki Yoshinaga 20-1 Shintomi Futtsu City Nippon Steel Corporation (72) Nobuhiro Fujita, Inventor 20-1 Shintomi, Futtsu-shi Nippon Steel Corporation Technology Development Headquarters (72) Inventor Jun Itami 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation F term in Tsu Works (reference) E02 FE03 FJ01 FJ04 FJ05 FK01 GA03 HA02

Claims (6)

[Claims] 1. A mass%, C: 0.08 to 0.25%, S
i: 0.001 to 1.5%, Mn: 0.01 to 2.0%,
P: 0.001 to 0.06%, Al: 0.008 to 0.2
%, N: 0.001 to 0.007%, S: 0.0
5% or less, the balance being iron and unavoidable impurities, having an average crystal grain size of 5 μm or more, and r *
A steel pipe excellent in workability, characterized in that the value is 1.2 or more. r * = ln (C ₀ / C) ÷ ln (C × L / C ₀ × L ₀ ) C ₀ , C: outer circumference (mm) of the steel pipe before and after the test L ₀ , L: each circumference of the steel pipe before and after the test Distance between longitudinal scores of position (mm) 2. Cr: 0.05 to 10 in mass%.
%, Ni: 0.05-20%, Cu: 0.05-20
%, Mo: 0.05 to 1.0%, Co: 0.05 to 1.0%
%, W: 0.05 to 1.0%, Sn: 0.05 to 1.0%,
Zr: 0.0001-0.5%, Mg: 0.0001-0.00%
5%, Ti: 0.001 to 0.2%, Nb: 0.001 to
0.2%, V: 0.001 to 0.2%, B: 0.0001 to
The steel pipe excellent in workability according to claim 1, characterized in that the steel pipe contains one or more of 0.01% and Ca: 0.0001 to 0.01%. 3. In mass%, C: 0.08 to 0.25%, S
i: 0.001 to 1.5%, Mn: 0.01 to 2.0%,
P: 0.001 to 0.06%, Al: 0.008 to 0.2
%, N: 0.001 to 0.007%, S: 0.0
Hot rolling is completed at a temperature of not more than ( _Ar3 transformation point −50 ° C.), and is rolled at a temperature of not more than 700 ° C., and a reduction rate of not less than 25% and not more than 5%. % Cold rolling at an average heating rate of 4 to 200%.
C./hour, annealing at a maximum temperature of 600 to 800.degree. C., cooling at a rate of 5 to 100.degree. C./hr, and pipe forming so that the rolling direction becomes the tube axis direction. Manufacturing method of steel pipe with excellent workability. 4. Further, in mass%, Cr: 0.05 to 10
%, Ni: 0.05-20%, Cu: 0.05-20
%, Mo: 0.05 to 1.0%, Co: 0.05 to 1.0%
%, W: 0.05 to 1.0%, Sn: 0.05 to 1.0%,
Zr: 0.0001-0.5%, Mg: 0.0001-0.00%
5%, Ti: 0.001 to 0.2%, Nb: 0.001 to
0.2%, V: 0.001 to 0.2%, B: 0.0001 to
The method for producing a steel pipe having excellent workability according to claim 3, wherein one or more of 0.01% and Ca: 0.0001 to 0.01% are contained. 5. The lubricating oil applied surface is uniformly dried on the surface of the steel pipe, and the friction coefficient of the surface is 70% or less as compared with the friction coefficient immediately after applying the lubricating oil. 3. A steel pipe excellent in workability according to 1 or 2. 6. The method according to claim 3, wherein the lubricating oil applied to the surface of the steel pipe is uniformly dried, and the friction coefficient of the surface is made 70% or less as compared with the friction coefficient immediately after the application of the lubricating oil. 5. The method for producing a steel pipe having excellent workability according to 4.