JP2003301219A - Method for manufacturing steel pipe with high strength and low yield ratio superior in weld part toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an advantageous method for improving the weld part toughness, in a welded steel pipe which satisfies mechanical properties of a high strength and a low yield ratio. <P>SOLUTION: This manufacturing method comprises heating a steel sheet, having a composition including 0.05-0.15 mass% C, 0.10-0.50 mass% Si, 0.5-2.0 mass% Mn, 0.005-0.10 mass% Al, 0.05-0.50 mass% Cr, 0.05-0.50 mass% Mo, further 0.002-0.03 mass% Ti and 0.001-0.01 mass% REM (rare earth metals), to a range between the A<SB>c3</SB>transformation temperature and 1000°C, and quenching it, or heating and rolling a steel slab having the same composition, and quenching it from the A<SB>r3</SB>transformation temperature or higher, consequently heating it again at a range between the A<SB>c1</SB>transformation temperature and the A<SB>c3</SB>transformation temperature and quenching it, further tempering it in a temperature range of 460-650°C, forming the steel sheet into a shape of a pipe, welding the seam to manufacture the steel pipe, and annealing the steel pipe at a temperature range of 500-700°C. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength low-yield ratio steel pipe for use in the fields of construction and civil engineering, and more specifically, to a steel having a high yield stress.
The present invention relates to a method for producing a welded steel pipe having various properties of 440 to 590 MPa and tensile strength of 590 to 740 MPa and a yield ratio of 85% or less, especially a high strength and low yield ratio steel pipe having good weld toughness.

[0002]

2. Description of the Related Art Conventionally, a box-shaped column welded with four strips of steel was often used as a column for a high-rise building, but in recent years, rigidity, deformation performance, and fire resistance have been used. Steel pipe columns with excellent cross-section and small column cross-sectional area to increase the effective space in the building, especially the so-called CFT (Co
ncrete Filled Steel Tube) has come into use.

This CFT type steel pipe is formed by bending a steel plate into a tubular shape, and then welding the joints to form a pipe, which is then tempered for use. Therefore, it is necessary to have the above-mentioned mechanical properties after tempering. However, its production was not easy. That is, the steel for steel pipes used as columns for high-rise buildings has a tensile strength of 590 MPa mainly as columns for high-rise buildings.
Grades are required, and for construction the yield ratio is limited to 85% or less, these mechanical properties especially low yield ratio,
It was difficult to realize under high strength.

[0004] Here, in Patent Document 1, the steel which is adjusted to a predetermined component After hot rolling, heat quenching to a temperature range of A _c3 transformation point to 1000 ° C., subsequently the temperature range of 750 to 850 ° C. It has been proposed to realize a low yield ratio by reheating and quenching to form a two-phase mixed structure. This technology has made it possible to achieve a low yield ratio under high strength.

By the way, as described above, since this kind of steel pipe is manufactured by bending a steel plate into a tubular shape and welding its joints, it is unavoidable that a welded portion is formed. The toughness of the welded portion requires Charpy absorbed energy of 47 J or more at 0 ° C. at least as in the base metal. Recently, due to intensifying competition among steel pipe manufacturing assembly processors, there has been a strong demand for improvement in welding efficiency of steel pipes. Further, as the strength of steel becomes higher, good toughness is strongly demanded not only in the base metal but also in the welded portion. For example, in the corner welding of the box type column, it is required that the welded portion having a large heat input of 600,000 J / cm has absorbed energy of 70 J or more. Therefore, it is certain that similar requirements will be made in the above CFT for welded parts with a heat input of 100,000 J / cm in the near future.

[0006] In this respect, in Patent Document 1, the weld toughness is Ti
It is described that the addition of
There is a limit to the improvement of the weld toughness by adding Ti. For example, when manufacturing a large-diameter steel pipe, the weld toughness is guaranteed especially when welding with a large heat input exceeding 70,000 J / cm. Not as long as.

[0007]

[Patent Document 1] Japanese Patent Laid-Open No. 5-117746

[0008]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an advantageous way of improving the toughness of a welded steel pipe in a welded steel pipe satisfying the mechanical properties of high strength and low yield ratio.

[0009]

[Means for Solving the Problems]
As a result of earnest efforts in the development of steel for use in steel, in order to achieve the desired high strength and low yield ratio, it is necessary to carry out a two-phase quenching treatment at the stage of the steel sheet to obtain the concentration distribution of C and Mn, Mo and Ti.
It was confirmed that it is effective to secure the strength after strain relief annealing by refining the precipitates by adding and to reduce the yield ratio by fixing the solid solution C by adding Cr. Moreover,
As a result of diligent research into means for improving the toughness of the welded part, they have found that the combined addition of REM and Ti is an extremely effective measure, and have completed the present invention.

That is, the gist of the present invention is as follows. (1) C: 0.05 to 0.15 mass%, Si: 0.10 to 0.50 mass%, M
n: 0.5 to 2.0 mass%, Al: 0.005 to 0.10 mass%, Cr:
0.05 to 0.50 mass% and Mo: 0.05 to 0.50 mass% are included,
Furthermore, Ti: 0.002-0.03 mass% and REM: 0.001--0.
A steel sheet containing 01 mass% and having a composition of Ceq according to the following formula of 0.47 or less is heated to a temperature range of A _c3 transformation point to 1000 ° C. for quenching, and then A _c1 transformation point or more A
_After reheating to a temperature range below the _c3 transformation point and quenching, and further tempering in a temperature range of 460 to 650 ° C, the steel sheet is formed into a tubular shape and its seam is welded to produce a steel pipe, A method for producing a high-strength low-yield-ratio steel pipe having good weld toughness, characterized by annealing the steel pipe in a temperature range of 500 to 700 ° C. Note Ceq = C [mass%] + Mn [mass%] / 6+ Si [mass%] / 24+
Ni [mass%] / 40 + Cr [mass%] / 5 + Mo [mass%] / 4 + V
[mass%] ／ 14

(2) C: 0.05 to 0.15 mass%, Si: 0.10 to 0.
50mass%, Mn: 0.5-2.0 mass%, Al: 0.005-0.10ma
ss%, Cr: 0.05 to 0.50 mass% and Mo: 0.05 to 0.50 mass
%, And further Ti: 0.002-0.03 mass% and REM:
Ceq according to the above formula containing 0.001 to 0.01 mass%
The steel slab having the component composition is 0.47 or less, and heated to a temperature range of A _c3 transformation point to 1300 ° C., subjected to rolling in a temperature range of more than A _r3 transformation point, directly quenching the steel sheet after the rolling,
Then, it is reheated to a temperature range of A _c1 transformation point or more and A _c3 transformation point or less to quench it, and further tempered in a temperature range of 460 to 650 ° C, and then a steel sheet is formed into a tubular shape and its seam is welded. A method for producing a high-strength, low-yield-ratio steel pipe having good weld toughness, which comprises producing a steel pipe by annealing at a temperature range of 500 to 700 ° C.

(3) In the above (1) or (2), the steel sheet further contains Cu: 0.05 to 1.0 mass% and Ni: 0.05 to 1.0 mass.
%, V: 0.005 to 0.05 mass%, Nb: 0.005 to 0.05 mass%
And Ca: 0.0005 to 0.005 mass% of one or two or more of them are contained in the composition, and a method for producing a high-strength, low-yield ratio steel pipe having good weld toughness.

[0013]

First, the experimental results leading to the present invention will be described in detail. That is, C: 0.11 mass
%, Si: 0.30 mass%, Mn: 1.45 mass%, Al: 0.027 mass
%, Cr: 0.16mass% and Mo: 0.25mass%, and steel sheets prepared by adding various amounts of Ti and REM individually or in combination, and heating these steel sheets to 900 ° C. And quenching, then 790 ℃
After reheating to quench and quenching, and further tempering at 540 ℃, the steel sheet is formed into a tube and its joints are heat-input by submerged arc welding 20000, 50000, 70000 and 100000.
Welded under the condition of J / cm to produce a steel pipe,
Annealed in the temperature range of 700 ° C.

Low temperature toughness _v E ₀ (1 mm from weld bond to weld heat affected zone) in the weld of the steel pipe thus obtained
Table 1 shows the results of an investigation on the Charpy absorbed energy at 0 ° C .: J) at the separated positions. As shown in Table 1, in comparison with the weld zone toughness of the steel pipe A using the steel having the above basic composition, Ti or REM
Steel pipes B and C made of the steel with only added have improved toughness at the welded parts, but both have a absorbed energy of less than 70 J after welding with a large heat input exceeding 70,000 J / cm. Will fall.

[0015]

[Table 1]

On the other hand, the weld toughness of the steel pipe C made of the steel in which Ti and REM are added to the basic components is improved to 70000 as a matter of course under the welding conditions of heat input of 20,000 and 50,000 J / cm. Even when welding is performed with a large heat input of 100,000 J / cm, the Charpy absorbed energy of the welded portion can be maintained at 70 J or more.

Further, the effect of the combined addition of Ti and REM will be described in detail. That is, as shown in the concept of addition of Ti and REM in Fig. 1, Ti is first precipitated as TiN and strongly pinned the initial γ grains to suppress grain growth and improve toughness. Since it melts, it is difficult to exert the effect of improving toughness in the temperature range exceeding 1400 ° C, such as during high heat input welding. On the other hand, REM is RE
It remains as M (O, S) without melting even at 1400 ° C. or higher, and has the effect of pinning initial γ grains and suppressing grain growth, although not to the extent of TiN. Therefore, if both are added in combination, the temperature range over 1400 ° C, such as during high heat input welding, expands, and even if it becomes difficult to suppress grain growth by TiN, it is possible to suppress grain growth by REM. Since it is continuously maintained, it becomes possible to obtain sufficient toughness. In other words, the combined addition of Ti and REM complements each other's deficiency, resulting in an effect that is not merely a sum of the two.

The present invention is derived from the above new finding, and in particular, by providing Ti and REM in combination to secure weld toughness, it is possible to provide a steel pipe suitable for CFT for columns of high-rise buildings. , Realized here.

The manufacturing method of the present invention will be described in detail below for each step. First, the reasons for limiting the amount of each component in the component composition of the steel sheet will be described. C: 0.05 to 0.15 mass% C is required to be at least 0.05 mass% in order to obtain desired strength and to precipitate carbides, while the upper limit is 0.15 mass% from the viewpoint of deterioration of toughness and weldability. .

Si: 0.10-0.5 mass% Si requires 0.10 mass% or more in steelmaking, while 0.5ma
If it exceeds ss%, the toughness of the base material deteriorates, so 0.10
The range is to 0.5 mass%.

Mn: 0.5 to 2.0 mass% Mn needs to be 0.5 mass% or more in order to secure the strength of the base metal. On the other hand, if it exceeds 2.0 mass%, the toughness of the welded portion is significantly deteriorated. : 0.5 to 2.0 mass%

Al: 0.005 to 0.10 mass% Al requires 0.005 mass% or more as a deoxidizing agent,
The effect is saturated even if 0.10 mass% or more is added.

Cr: 0.05 to 0.50 mass% Cr is effective for reducing the yield stress and the yield ratio of the steel pipe after the stress relief annealing in the manufacturing process described later by adding 0.05 mass% or more. , The effect is saturated even if added over 0.50mass%, so the range is 0.05 to 0.50mass%.

Mo: 0.05 to 0.50 mass% Mo has the effect of increasing the tensile strength of the steel pipe after stress relief annealing in the manufacturing process described later by achieving the refinement of precipitates by the addition of 0.05 mass% or more. . On the other hand, if added over 0.50 mass%, the weldability will deteriorate, so 0.50 mass%
Is the upper limit.

Based on the above component composition, Ti:
0.002-0.030 mass% and REM: 0.001-0.01 mass%
It is essential to include a complex of. That is, as described above, by adding Ti and REM in combination, the toughness of the weld zone can be significantly improved.

Ti: 0.002 to 0.030 mass% Ti precipitates as TiN and is useful for improving the toughness of the welded portion by suppressing the growth of initial γ grains in the temperature range up to 1400 ° C. For that, 0.002 mass%
The above content is required, but even if added over 0.030 mass%, the effect is saturated, so 0.002 to 0.030 mass%
The range is.

REM: 0.001 to 0.01 mass% REM has the effect of pinning the initial γ grains and suppressing grain growth, similar to Ti, especially as REM (O, S) even in the high temperature range of 1400 ° C or higher. Since it remains, the effect can be exhibited even when large heat input welding is performed. For that purpose, it is necessary to contain 0.001 mass% or more,
Even if added in excess of 0.01 mass%, the effect will be saturated, so the range is 0.001 to 0.01 mass%.

Further, in the above component composition, it is necessary to regulate Ceq according to the above formula to 0.47 or less. That is, if Ceq exceeds 0.47, the weldability becomes extremely poor and cracks easily occur after welding, so Ceq is set to 0.47 or less.

In addition to the above essential components, Cu: 0.05
~ 1.0 mass%, Ni: 0.05-1.0 mass%, V: 0.005-0.0.
05mass%, Nb: 0.005-0.05mass% and Ca: 0.0005-
Any one kind or two kinds or more of 0.005 mass% can be contained.

Cu: 0.05 to 1.0 mass% Cu can be expected to be solid solution strengthened by adding 0.05 mass% or more, but if added in excess of 1.0 mass%, toughness deteriorates. Preferably.

Ni: 0.05-1.0 mass% When Ni is added in an amount of 0.05 mass% or more, it contributes to the increase in strength without impairing the high toughness of the base material.
If added over 5, the effect will saturate, so 0.05-
It is preferably in the range of 1.0 mass%.

V: 0.005 to 0.05 mass% When V is added in an amount of 0.005 mass% or more, V is precipitated as V (CN), which is useful for ensuring the tensile strength after strain relief annealing in the manufacturing process described later. %, The toughness is lowered. Therefore, it is preferable to set the content in the range of 0.005 to 0.05 mass%.

Nb: 0.005 to 0.05 mass% Nb precipitates with addition of 0.005 mass% or more, and helps secure the tensile strength after strain relief annealing in the manufacturing process described later,
Even if added in excess of 0.05 mass%, the effect will be saturated, so the range of 0.005 to 0.05 mass% is preferable.

The steel sheet having the above-described composition can be advantageously manufactured into a cylindrical steel tubular column by the following two methods. That is, one is reheating and quenching,
Two-phase region tempering, tempering, pipe forming, strain relief annealing (hereinafter referred to as A method), the remaining one is direct quenching after rolling, two-phase region tempering, tempering, pipe forming, strain relief annealing (Hereinafter referred to as method B).

First, the method A will be described. That is, in the steel sheet having the above-mentioned composition, the steel melted in the predetermined composition range is heated to the γ single phase region of the _{Ac 3} transformation point or higher, and rolled in the γ recrystallization region to the predetermined plate thickness and air-cooled. Although it can be obtained through the process, it is not particularly limited to this process.

Then, the above steel sheet is subjected to an A _c3 transformation point to 1000 ° C.
To the temperature range of A _c1 transformation point or more and A _c3 transformation point or less, and quenching is performed.
After tempering in the temperature range of 460 to 650 ℃,
The steel sheet is formed into a tubular shape, the seams thereof are welded to produce a steel pipe, and the steel pipe is annealed in a temperature range of 500 to 700 ° C.

That is, the steel sheet before pipe _forming has an A _c3 transformation point of about 10
The reason for heating and quenching to 00 ° C is that the steel is completely austenitized by heating in this temperature range, and then it is quenched to ensure sufficient strength. If it is not heated to A _c3 point or more, it will not be fully austenitized and sufficient strength cannot be obtained. On the other hand, if the temperature exceeds 1000 ° C, the initial austenite grain size becomes coarse and the toughness deteriorates.

Next, 2 above the A _c1 transformation point and below the A _c3 transformation point
The reason for heating and quenching in the phase region is that C and Mn are sufficiently separated into two phases to form a concentration distribution. This allows
The concentration distribution is maintained even after the subsequent pipe forming and strain relief annealing, and the yield ratio can be easily lowered by the two-phase formation of the hard region and the soft region. Such a concentration distribution of C and Mn is unlikely to occur unless it is maintained in the two-phase region between the A _c1 transformation point and the A _c3 transformation point. Optimally, such distribution of hard and soft regions occurs at 1: 1. Also,
It is easy to secure the final strength by quenching after heating the two-phase region.

Further, the reason for tempering to 460 to 650 ° C. is 46
If it is not heated above 0 ° C, the strength of the steel plate will increase and the subsequent load on the pipe making will increase. On the other hand, if the temperature exceeds 650 ° C and is heated, the strength is remarkably reduced, and the strength cannot be finally secured.

Finally, the annealing after pipe making is carried out in the temperature range of 500 to 700 ° C. This is because the strength becomes too high below 500 ° C, while the strength becomes low above 700 ° C.

Next, the method B will be described. That is, the steel melted in the above-mentioned predetermined composition range is heated to a γ single-phase region above the A _c3 transformation point and rolled in a temperature region above the _{Ar 3} transformation point, and the steel sheet after this rolling is immediately rolled. After quenching, then reheating to a temperature range of A _c1 transformation point or more and A _c3 transformation point or less for quenching, and further tempering in a temperature range of 460 to 650 ° C, the steel sheet is formed into a tubular shape. A seam is welded to produce a steel pipe, and the steel pipe is annealed in a temperature range of 500 to 700 ° C.

Here, the reason for first heating to the γ single phase region above the A _c3 transformation point and rolling in the temperature region above the A _r3 transformation point and immediately quenching the rolled steel sheet is this A _The steel is completely austenitized by heating to the γ single-phase region above the _c3 transformation point, and then rolling is performed in the temperature region above the _Ar3 transformation point to _{refine the} coarsened γ grains, and the steel sheet This is to adjust the shape. Further, the rolled steel sheet is quenched as it is, that is, at least immediately after rolling, A
By cooling from the _r3 transformation point or higher, sufficient strength can be secured. In addition, this method B is
Since the old γ grain size is likely to be coarse, the yield stress (YS) becomes low, and the yield ratio (YR) can be made lower.

[0043]

Example 1 A steel slab having the chemical composition shown in Table 2 was prepared by hot rolling, in which method A: heating to 1200 ° C. and then rolling up to 1100 ° C. was completed, and the steel sheet was reheated to 900 ° C. After heating and quenching or Method B: After heating to 1150 ° C, rolling was performed while the temperature dropped to 930 ° C, and immediately water cooling from 900 ° C or higher to room temperature was performed.

Subsequently, after quenching in the two-phase region of 790 ° C. and tempering at 540 ° C., the steel sheet was subjected to a pipe forming treatment of plate thickness (t) / outer diameter (d) = 8%. Molded into a tubular shape, the seam of which is input by submerged arc welding with heat input of 20000
, 50000, 70000 and 100000 J / cm were welded to produce a steel pipe, and the steel pipe was subjected to strain relief annealing at a temperature of 550 ° C.

[0045]

[Table 2]

Table 3 shows the results of evaluation of the mechanical properties of the base material of the steel pipe thus obtained. For some steels, weld toughness was evaluated to investigate the effect of Ti and REM additions. The evaluation results are also shown in Table 3. From Table 3, the steel pipe having the composition according to the present invention has high strength and low yield ratio, and in particular, steel Nos. 15 and 21 in which Ti and REM are compounded in a predetermined range are added.
It is clear that the weld toughness is particularly high in the range of ~ 23, especially after the high heat input welding.

[0047]

[Table 3]

Example 2 A steel sheet having the composition of steel No. 15 shown in Table 2 was used in Example 1
Produced by hot rolling in the same manner as described above, and after subjecting the steel sheet to heat treatment under various conditions shown in Table 4, the steel sheet was formed into a pipe having a thickness (t) / outer diameter (d) = 4 to 8%. The tube was treated to form a tube, and the joints were welded under various conditions shown in Table 4 to prepare a steel tube, and the steel tube was subjected to strain relief annealing at the temperatures shown in Table 4.

Table 4 also shows the results of evaluating the mechanical properties of the base material of the steel pipes thus obtained. From Table 4, it can be seen that the steel sheet obtained by performing the heat treatment according to the present invention has a high yield strength and a low yield ratio. Further, it can be seen that a low yield ratio can be obtained by manufacturing according to the method B using direct quenching.

[0050]

[Table 4]

[0051]

EFFECTS OF THE INVENTION According to the present invention, since the weld toughness of welded steel pipe satisfying high strength and low yield ratio is remarkably improved, a steel pipe most suitable for CFT which is a pillar of a high-rise building is provided. can do.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the effect of adding Ti and REM.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/38 C22C 38/38 38/58 38/58 F term (reference) 4K037 EA01 EA05 EA06 EA11 EA13 EA15 EA17 EA20 EA27 EA31 EA32 EA36 FA02 FC04 FC05 FF02 4K042 AA06 BA01 CA03 CA05 CA06 CA08 CA10 CA12 CA13 CA14 DA03 DC02

Claims (3)

[Claims] 1. C: 0.05 to 0.15 mass%, Si: 0.10 to 0.50
mass%, Mn: 0.5 to 2.0 mass%, Al: 0.005 to 0.10 mass
%, Cr: 0.05 to 0.50 mass% and Mo: 0.05 to 0.50 mass%
In addition, Ti: 0.002-0.03 mass% and REM: 0.
Ceq in accordance with the following formula is 0.
A steel sheet having a chemical composition of 47 or less is _c3Transformation point ~ 10
Heated to a temperature range of 00 ° C to quench it, then A_c1transformation
Point or above A_c3Reheat to a temperature range below the transformation point and quench
And tempered in the temperature range of 460 to 650 ℃.
Then, the steel plate is formed into a tubular shape and the seams are welded to form a steel pipe.
And annealing the steel pipe in the temperature range of 500 to 700 ° C.
Manufacture of high strength and low yield ratio steel pipe with good weld toughness
Build method. Record Ceq = C [mass%] + Mn [mass%] / 6+ Si [mass%] / 24+
 Ni [mass%] / 40 + Cr [mass%] / 5 + Mo [mass%] / 4 + V
[mass%] ／ 14 2. C: 0.05 to 0.15 mass%, Si: 0.10 to 0.50
mass%, Mn: 0.5 to 2.0 mass%, Al: 0.005 to 0.10 mass
%, Cr: 0.05 to 0.50 mass% and Mo: 0.05 to 0.50 mass%
In addition, Ti: 0.002-0.03 mass% and REM: 0.
Ceq in accordance with the following formula is 0.
A steel slab having a component composition of 47 or less is heated to a temperature range of A _c3 transformation point to 1300 ° C., rolled in a temperature range of A _r3 transformation point or more, and the steel sheet after the rolling is quenched as it is, After reheating to a temperature range of A _c1 transformation point or more and A _c3 transformation point or less and quenching, and further tempering in a temperature range of 460 to 650 ° C, the steel sheet is formed into a tubular shape and its seam is welded. A method for producing a high-strength low-yield-ratio steel pipe having good weld toughness, which comprises producing a steel pipe and annealing the steel pipe in a temperature range of 500 to 700 ° C. Note Ceq = C [mass%] + Mn [mass%] / 6+ Si [mass%] / 24+
Ni [mass%] / 40 + Cr [mass%] / 5 + Mo [mass%] / 4 + V
[mass%] ／ 14 3. The steel sheet according to claim 1, further comprising Cu: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0 mass%,
V: 0.005 to 0.05 mass%, Nb: 0.005 to 0.05 mass%, and Ca: 0.0005 to 0.005 mass% Any one or two or more kinds of component compositions are contained, and good weld toughness is characterized. Manufacturing method of high strength and low yield ratio steel pipe.