JP2002020841A - Steel tube excellent in formability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube excellent in formability in hydroforming or the like and to provide its production method. SOLUTION: In this steel tube excellent in formability, in 0 deg. to ±25 deg. from the longitudinal direction of the steel tube, the average of the (r) value is >=1.5 and/or the lowest value of the (r) value is >=1.0. Preferably, the standard deviation of the distribution of the (r) value in 0 deg. to ±25 deg. from the longitudinal direction of the steel tube lies within ±50% of the average (r) value. Further, the above steel tube contains ferrite of >=50% by volume ratio, and each ferrite grain size is 0.1 to 200 μm. Preferably, the average aspect ratio (the grain length in the longitudinal direction/the grain thickness in the thickness direction) of the ferritic grains is 0.5 to 3.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material used for, for example, undercarriage and members of automobiles, and more particularly to a steel tube excellent in formability used for hydroforming and the like, and a method for producing the same.

[0002]

2. Description of the Related Art With the need to reduce the weight of automobiles, it is desired to increase the strength of steel sheets. 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. Recently, attempts have been made to form a complex-shaped portion from a high-strength steel sheet or steel pipe by using a hydroforming method. This is aimed at reducing the number of parts and reducing the number of welding flanges in response to the need for lighter and lower cost automobiles. As described above, if a new molding method such as hydroform (see Japanese Patent Application Laid-Open No. H10-175026) is actually adopted, great merits such as cost reduction and design flexibility are expected. .

In order to make full use of the merits of such hydroform molding, materials suitable for these new molding methods are required. For example, at the 50th Lecture Meeting on Plastic Working (1999, p. 447), the influence of the r value on hydroform molding was shown. However, the analysis here is mainly performed by simulation, and does not correspond one-to-one with the actual material. Also, JP-A-10-17502
In the 7th report, the T-shape height of the hydroform was improved by defining the relationship between the r values in the longitudinal and circumferential directions of the steel pipe as the material for the hydroform.

[0004] However, in such a simple T-shaped molding, it is possible to improve the relationship between the r values in the longitudinal direction and the circumferential direction. It is thought that hydroform molding in mode is performed, and in the essential points of the existing invention,
In many cases, it is not enough.

[0005]

As described above, the development of materials suitable for hydroforming has hardly been carried out on a practical level, and existing high r-value steel sheets and high ductility steel sheets have been developed for hydroforming. Being used. An object of the present invention is to provide a steel pipe excellent in formability such as hydroform by limiting the characteristic value of a material.

[0006]

According to the present invention, there is provided a hydrophobe.
The distribution of r-values for materials with good formability, such as
And molding of hydroforms in many deformation modes
To provide a steel pipe having excellent heat resistance and a method for producing the same.
You. That is, the gist of the present invention is as follows:
It is. (1) r value from 0 ° to ± 25 ° from the longitudinal direction of the steel pipe
Is 1.3 or more and / or the lowest value of r is
Steel excellent in formability characterized by being 1.0 or more
tube. (2) r value at 0 ° to ± 25 ° from the longitudinal direction of the steel pipe
That the standard deviation of the distribution is within ± 50% of the mean r value
The steel pipe excellent in formability according to the above (1), which is characterized in that: (3) Each ferrite contains 50% or more ferrite by volume ratio.
Characterized by a particle size of 0.1 to 200 μm
(1) The steel pipe excellent in formability according to (2). (4) Each ferrite contains 50% or more ferrite by volume ratio.
The particle size is 1 to 200 µm and the distribution of ferrite particle size
The standard deviation is within ± 40% of the average particle size.
(1) The steel excellent in formability according to any one of (1) to (3).
tube. (5) Ferrite containing 50% or more of ferrite by volume ratio
Average aspect ratio (grain length in the longitudinal direction / grain length in the thickness direction)
(Thickness) is 0.5 to 3.0 (1) to
(4) The steel pipe excellent in formability according to any of (4). (6) The steel pipe is expressed by mass% (the same applies hereinafter), and C: 0.0005 to
0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0%, P: 0.001
 ~ 0.20% and N: 0.0001 ~ 0.03%
It consists of iron and unavoidable impurities
(1) The steel pipe excellent in formability according to any of (5). (7) The steel pipe further contains: Ti: 0.001 to 0.5%, Zr: 0.001
~ 0.5%, Hf: 0.001 ~ 2.0%, Cr: 0.001 ~ 1.5%, Mo:
0.001 to 1.5%, W: 0.001 to 1.5%, V: 0.001 to 0.50
%, Nb: 0.001 to 0.5%, Ta: 0.001 to 2.0%, and C
o: 0.001 to 1.5%, containing one or more of the following
The feature according to any one of (1) to (6),
Steel pipe with excellent shape. (8) The steel pipe further contains B: 0.0001 to 0.01%, Ni: 0.001
~ 1.5% and Cu: 0.001 ~ 1.5%
Is characterized by containing two or more types (1) to (7)
The steel pipe excellent in formability according to any one of the above. (9) The steel pipe further contains Al: 0.001 to 0.5%, Ca: 0.000%
1 to 0.5%, Mg: 0.0001 to 0.5%, and Rem: 0.0001
0.5%, one or more of
Excellent moldability according to any one of (1) to (8)
Steel pipe. (10) Excellent moldability according to any one of (1) to (9)
When manufacturing steel pipes, hot rolled or cold rolled
After constructing the mother tube as_ThreeAbove the transformation point (Ac _Three+200)
° C or lower, and then at least (Ar_ThreePoint +5
0) Apply a diameter reduction of 20% or more at
And finish the hot diameter reduction at 700 ° C or more.
A method of manufacturing a steel pipe with excellent formability.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. R value in the longitudinal direction of steel pipe: For example, at the 50th Lecture Meeting on Plastic Working (1999, p. 447), the influence of the r value on the T-shaped forming height in hydroform forming was shown. In addition, JP-A-10-175027 reports that
It has been shown that the magnitude relationship between the tube axis direction and the direction perpendicular thereto is important in increasing the T-shaped molding height. In the T-shape forming, which is one of the basic forming modes of the hydroform, as described here, the r value in the tube axis direction is important.

However, the former is based on a simulation, and the latter uses only T-shaped molding as a hydroform property (evaluation index).
Good formability corresponding to many deformation modes cannot be obtained. In actual molding of parts, not only such a basic molding mode but also molding with a high degree of processing is often performed in a direction deviated several times from the circumferential direction or the longitudinal direction. In such a case, it is desired to improve the formability in all directions, but generally, from 0 ° to ± 25 ° from the longitudinal direction of the steel pipe.
It has been found that increasing the average r value in ° is suitable for various hydroforming processes.

That is, at various pushing amounts and internal pressures,
Hydroforming is performed until buckling or bursting, the maximum expansion ratio, and the ratio ρ of the longitudinal strain: εφ and the circumferential strain: εθ in the pipe in the vicinity of the fractured portion or the maximum thickness reduced portion; -0.9, -0.7,
Further, the pipe expansion ratio of -0.5 (becoming negative because the plate thickness is reduced) was obtained and used as an index of hydroform moldability.

As a result, 0 ° to ± 25 from the longitudinal direction of the steel pipe.
The average of the r values in ° is 1.5 or more, and / or
When the minimum value of the r value at 0 ° to ± 25 ° from the longitudinal direction of the steel pipe is 1.0 or more, it has been found that the pipe expansion ratio can be secured to 1.25 or more under any of the conditions. Of the invention. Further preferably, the average of the r values at 0 ° to ± 25 ° from the longitudinal direction of the steel pipe is 2.0 or more, and / or 0 ° to ± 25 ° from the longitudinal direction of the steel pipe.
It has also been found that when the maximum value of the r value at ± 25 ° is 3.0 or more, it is possible to secure a pipe expansion ratio of 1.35 or more under any of the conditions.

The r value in each direction is measured every 3 ° to 7 °, and is obtained as an average value or maximum / minimum values. Measurement at less than 3 ° is difficult in preparing a test piece, and at more than 7 °, there is a concern that the maximum and minimum values of r cannot be measured with high accuracy. Therefore, the r value is between 3 ° and 7
Measure every °. Furthermore, as defined in the invention of the above (2), when the standard deviation of the distribution of r values from 0 ° to ± 25 ° from the longitudinal direction of the steel pipe is within a range of ± 50% of the average r value, a higher expansion ratio is obtained. can get.

Crystal grain size: As specified in the inventions of (3) and (4), in controlling the r value distribution, it is important to control the ferrite crystal grain size. In particular, in order to further increase the average r value in the steel pipe longitudinal direction from 0 ° to ± 25 °, the grain size of the main phase ferrite is set to 0.1 to 200.
It is necessary to control to μm. It is difficult to industrially produce recrystallized grains of less than 0.1 μm,
1 μm was made the lower limit. On the other hand, if particles having a particle size of more than 200 μm are mixed, the r value distribution is disturbed. Therefore, the upper limit is set to 200 μm.

Here, the ferrite grain size is determined according to JIS G055.
2 according to the cutting method. Further, the standard deviation of the ferrite grain size and the aspect ratio of the ferrite grain are limited to improve the r value distribution. These values are 2 to 2 with an optical microscope of 100 to 1000 times.
Observation was carried out in a visual field of 0 or more, and for each particle size, the circle equivalent diameter was determined by image analysis to calculate the standard deviation.

The aspect ratio defined in the invention (5) is determined by the ratio of the number of ferrite grain boundaries intersecting a line segment parallel to the rolling direction and a line segment in the vertical direction having the same length. Ratio = (number of ferrite grain boundaries crossing a line segment in the vertical direction) / (number of ferrite grain boundaries crossing a line segment parallel to the rolling direction). When the standard deviation exceeds ± 40% of the average particle size, the aspect ratio exceeds 3.0, and the aspect ratio is less than 0.5, the r value distribution deviates from the range specified in the present invention. And the hydroformability tends to deteriorate. Therefore, the standard deviation of the ferrite grains is defined to be within ± 40% of the average grain size, and the average aspect ratio of the ferrite grains is 0.5%.
３3.0.

Next, the reasons for limiting the component composition will be described. C: C is an element effective for increasing the strength, and 0.0005%
The above addition is required. On the other hand, the addition of a large amount is not preferable for controlling the r value distribution, so the upper limit is set to 0.1.
30%. Si: Si is a strengthening element and also a deoxidizing element. In order to obtain these effects, 0.001% or more must be added, so the lower limit is made 0.001%. On the other hand, excessive addition causes deterioration of wettability and workability of plating, so the upper limit was made 2.0%.

Mn: Mn is an element effective for increasing the strength. To obtain this effect, 0.01% or more must be added, so the lower limit was made 0.01%. On the other hand, since excessive addition causes a decrease in ductility, the upper limit is set to 3.0%. P: P is an element effective for increasing the strength. However, P causes deterioration of weldability, slab crack resistance, fatigue characteristics, and toughness. 20%.

N: N is an element effective for increasing strength,
Addition of 0.0001% or more is required. On the other hand, the addition of a large amount is not preferable in terms of controlling welding defects, so the upper limit is made 0.03%. Ti, Zr, Hf, Cr, Mo, W, V, Nb, Ta and Co: Nb, Ti and V added as necessary
Is to form carbides, nitrides or carbonitrides by adding 0.001% to increase the strength of the steel material.
%, A large amount of carbides, nitrides, or carbonitrides precipitate in ferrite grains, which are the parent phase, or in the grain boundaries, thereby reducing ductility. Therefore, the respective addition amount ranges are set to 0.001 to 0.5%.

Zr is a deoxidizing element. Therefore, 0.
0001% or more is added. Excessive addition causes crystallization and precipitation of a large amount of oxides, sulfides and nitrides, deteriorating cleanliness and lowering ductility. Significantly impairs the performance. Therefore, the addition amount range is 0.0
01-0.5%. Hf and Ta are respectively
With the addition of 0.001% or more, carbides, nitrides or carbonitrides are formed to increase the strength of the steel material.
%, A large amount of carbides, nitrides, or carbonitrides precipitate in ferrite grains, which are the parent phase, or in the grain boundaries, thereby reducing ductility. Therefore, the respective addition amount ranges are 0.001 to 2.0%.

Cr, Mo, W and Co are strengthening elements, each of which is added at 0.001% or more as needed. On the other hand, since excessive addition causes a decrease in ductility, the upper limit of each is set to 1.5%. B, Ni and Cu: B, which is added as necessary, is effective for strengthening grain boundaries and increasing the strength of steel materials. However, if the added amount exceeds 0.01%, the effect is only saturated. In addition, it unnecessarily increases the strength of the steel sheet and lowers the workability. Therefore, the addition amount range is 0.0001 to 0.01%
And

Ni and Cu are strengthening elements, and each is added at 0.001% or more as needed. On the other hand, since excessive addition causes a decrease in ductility, the upper limit of each is set to 1.5%. Al, Ca, Mg and rare earth elements (Rem): Al and Mg are deoxidizing elements. In addition, Al contributes to improvement of the formability particularly when performing box annealing. On the other hand, excessive addition causes large amounts of crystallization and precipitation of oxides, sulfides and nitrides, deteriorating cleanliness and lowering ductility. Excessive addition significantly impairs plating properties. Therefore, if necessary, each addition amount range is 0.0001 to 0.50.
%.

In addition, Ca and rare earth elements (Rem)
Is an element effective for controlling inclusions.Addition of an appropriate amount improves hot workability, while excessive addition, on the other hand, promotes hot embrittlement. The addition amount range was 0.0001 to 0.5%. Here, the rare earth elements refer to Y, Sr and lanthanoid elements, and it is industrially advantageous to add them as misch metal, which is a mixture of these elements.

Further, the steel pipe of the present invention contains O, Sn, S, Zn, Pb, As, Sb, etc. as unavoidable impurities.
The effects of the present invention are not lost even if the content is 0.01% or less. Next, the invention (10) will be described. Heating temperature: In order to improve the formability of the welded part, the heating temperature before diameter reduction is set to the Ac ₃ transformation point or higher, and the heating temperature is set to (Ac ₃ +200) ° C. or lower to prevent coarsening of grains. Stipulated.

Diameter reduction processing temperature: In order to recover strain hardening after diameter reduction, the processing temperature at the time of diameter reduction is specified to be 700 ° C. or more, and in order to prevent grain coarsening, (Ar ₃ +5)
0) It is specified as below. The diameter reduction rate was set to 20% or more in order to increase the r value of 0 to ± 25 ° in the longitudinal direction of the steel pipe. In addition, the higher the diameter reduction ratio, the larger the 0 to ± 2
It is effective to increase the maximum value of the r value at 5 °, but if the tensile amount in the longitudinal direction of the steel pipe increases, the steel pipe will break at the time of diameter reduction. Therefore, the diameter reduction rate is desirably 200% or less.

The diameter reduction ratio is (the outer diameter of the original pipe−the outer diameter of the product pipe).
/ Defined as the outer diameter of the main pipe. Furthermore, the structure of the steel pipe of the present invention may include a structure such as pearlite, bainite, martensite, austenite, and carbonitride as a metal structure other than ferrite. Furthermore, in manufacturing, following smelting in a blast furnace, converter, and electric furnace, various secondary smelting is performed, ingot casting and continuous casting are performed, and in the case of continuous casting, hot rolling is performed as it is. Even if it produces by combining methods, the effect of this invention is not inhibited at all.

Further, 1050-1300 ° C. by heating a steel ingot to, by performing hot rolling at Ar ₃ transformation point -10 ° C. or higher Ar ₃ below transformation point tens 120 ° C. and, applying lubrication rolling to the hot rolling A method of manufacturing a steel sheet before pipe making, such as performing a winding process of a hot-rolled sheet at 750 ° C. or lower, further performing cold rolling, and then performing box annealing or continuous annealing. Even if they are manufactured in combination, the effects of the present invention are not impaired at all.

That is, a hot-rolled sheet, a cold-rolled sheet, or a cold-rolled annealed sheet can be used as the steel sheet for pipe making. Further, in the production of steel pipes, welding and pipe forming techniques such as electric resistance welding, TIG, MIG, racer welding, UO and forging and the like can be used. In the production of these welded steel pipes, the heat affected zone is subjected to local solution heat treatment according to the required properties,
It may be carried out singly or in combination, or may be repeated a plurality of times in some cases, and the effect of the present invention can be further enhanced. 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.

Further, the effect of the present invention is not impaired at all even if the homogenization heat treatment is performed before the diameter reduction or the diameter reduction. Further, lubrication at the time of diameter reduction is desirable from the viewpoint of improving formability through improvement of the r-value, and is particularly preferable in terms of controlling the texture of the surface layer, and promotes the effects of the present invention.

[0028]

EXAMPLES Each steel having the components shown in Tables 1 and 2 (continued from Table 1) was melted, heated to 1200 ° C., then hot-rolled, and the Ar ₃ transformation determined by the components of each steel and the cooling rate. Hot rolling is completed at a point of −10 ° C. or more and less than the Ar ₃ transformation point + 120 ° C. (approximately 900 ° C.), and a 2.2 mm thick hot rolled sheet and, for some steel types, a 6 mm thick hot rolled sheet are produced. Produced. The 6 mm material was then cold rolled and annealed to form a 2.2 mm thick cold rolled annealed sheet. Then, to the outer diameter of 205 to 89 mm, cold, TIG,
After forming tube using a laser or electric resistance welding, by heating to below Ac ₃ transformation point + 200 ° C. or higher Ac ₃ transformation point, (A
The outer diameter was reduced to 63.5 mm at a temperature not higher than r ₃ +50) ° C. and not lower than 700 ° C. to produce a high-strength steel pipe.

Hydroform molding is performed at various indentations and internal pressures until the hydroform molding buckles or bursts. The maximum expansion ratio and the longitudinal strain in the pipe near the fractured portion or at the portion where the maximum thickness is reduced are considered. : Εφ and circumferential strain: εθ ratio ρ; εφ / εθ is-
The maximum pipe expansion ratios of 0.9, -0.7, and -0.5 (becoming negative because the plate thickness decreases) were obtained, and this was evaluated as an index of hydroform moldability.

The r value from 0 ° to ± 25 ° from the longitudinal direction of the steel pipe is obtained by cutting an arc-shaped strip sample at every 5 ° and pressing it into a flat plate.
% Is 15%, 12% to 17% is 10%
Those of 12% or less were determined at the time of 7%.

[0031]

[Table 1]

[0032]

[Table 2]

Table 3 shows that 0 ° to ± 2 from the longitudinal direction of the steel pipe of each steel.
The r value at every 5 ° of 5 ° is shown. Table 4 (continuation of Table 3)
Table 3 shows the average value of the r values shown in Table 3, and the strain in the longitudinal direction of the tube: εφ
And the circumferential strain: the ratio ρ of εθ; εφ / εθ is -0.9,
The maximum expansion rate at which −0.7 and −0.5 are obtained is shown.
Inventive steels A to R show good values of r at each angle, and show an expansion ratio of 1.3 or more at any ρ.

On the other hand, in the case of S to W whose components are out of the range of the present invention, it is understood that the pipe expansion ratio has a low value or that there is a problem in the weldability at the time of pipe making. Table 5 shows the relationship between the volume ratio of ferrite, the particle size of each ferrite, the average particle size of ferrite, its standard deviation, the aspect ratio, and the expansion ratio. The aspect ratio takes a value close to 1, and the standard deviation is 4 of the average particle size.
The expansion ratio tends to increase as the ratio falls within the range of 0%. Further, those whose standard deviation and aspect ratio are out of the ranges specified in the invention of (4) or (5) above have low expansion ratios.

Table 6 shows the relationship between the heating temperature, the diameter reduction rate at (Ar ₃ +50) ° C. and 700 ° C. or higher, the diameter reduction end temperature, and the pipe expansion rate. Heating temperature is ₃ points or more of Ac (Ac ₃ +200)
C., not more than (Ar ₃ +50) .degree. C. and not less than 700.degree.
When the temperature is higher than ° C, a good expansion ratio is obtained.

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

[Table 5]

[0039]

[Table 6]

[0040]

According to the present invention, a steel pipe excellent in formability, such as a hydroform, can be obtained.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Yasuhiro Shinohara 20-1 Shintomi, Futtsu-shi, Chiba Made in New Japan (72) Inventor Toru Yoshida 20-1 Shintomi, Futtsu-shi, Chiba F-term (reference) 4N092 EA02 EA16 GA02 HA15 HA22 KA01 KA04 KA09 4K032 AA01 AA02 AA04 AA05 AA08 AA09 AA10 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA31 AA32 AA33 AA35 AA36 AA37 AA39 AA40 BA03 CA01 CA02 CB01 CB02 CC02 CC03 CC03

Claims (10)

[Claims] 1. Formability characterized by an average r value of 1.5 or more and / or a minimum r value of 1.0 or more from 0 ° to ± 25 ° from the longitudinal direction of the steel pipe. Excellent steel pipe. 2. The steel pipe having excellent formability according to claim 1, wherein the standard deviation of the distribution of r values at 0 ° to ± 25 ° from the longitudinal direction of the steel pipe is within ± 50% of the average r value. . 3. A ferrite containing at least 50% by volume of ferrite,
3. A steel pipe having excellent formability according to claim 1, wherein each ferrite has a grain size of 0.1 to 200 [mu] m. 4. A ferrite containing at least 50% by volume of ferrite,
The molding according to any one of claims 1 to 3, wherein each ferrite particle size is 1 to 200 µm, and the standard deviation of the ferrite particle size distribution is within ± 40% of the average particle size. Excellent steel pipe. 5. A ferrite containing at least 50% by volume of ferrite,
The steel pipe having excellent formability according to any one of claims 1 to 4, wherein an average aspect ratio of the ferrite grains (longitudinal grain length / thickness grain thickness) is 0.5 to 3.0. 6. The steel pipe according to claim 1, wherein:
0.0005 to 0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0
%, P: 0.001 to 0.20%, and N: 0.0001 to 0.03%, and the balance is iron and unavoidable impurities. The moldability according to any one of claims 1 to 5, wherein Excellent steel pipe. 7. The steel pipe further comprises: 0.001 to 0.5% Ti;
Zr: 0.001-0.5%, Hf: 0.001-2.0%, Cr: 0.001-1.5
%, Mo: 0.001 to 1.5%, W: 0.001 to 1.5%, V: 0.001 to 0.
50%, Nb: 0.001 to 0.5%, Ta: 0.001 to 2.0%, and
The steel pipe having excellent formability according to any one of claims 1 to 6, wherein one or more of Co: 0.001 to 1.5% is contained. 8. The steel pipe further comprises: B: 0.0001 to 0.01
%, Ni: 0.001 to 1.5%, and Cu: 0.001 to 1.5%, and one or more of these are contained, and the moldability according to any one of claims 1 to 7, Excellent steel pipe. 9. The steel pipe further comprises: Al: 0.001 to 0.5%;
Ca: 0.0001 to 0.5%, Mg: 0.0001 to 0.5%, and Re
The steel pipe excellent in formability according to any one of claims 1 to 8, wherein one or more kinds of m: 0.0001 to 0.5% are contained. 10. In producing the steel pipe excellent in formability according to any one of claims 1 to 9, after forming a mother pipe using a hot rolled plate or a cold rolled plate as a substrate, the Ac ₃ transformation is performed. (Ac ₃ +200) ° C or lower, and then at least (Ar
₃ +50) A method for producing a steel pipe excellent in formability, characterized in that a diameter reduction of 20% or more is performed at a temperature of 700 ° C. or less at a temperature of 700 ° C. or more, and a hot diameter reduction is completed at 700 ° C. or more.