JPH09165620A - Production of building use thick fire resistant steel tube low in yield ratio - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an excellent building use thick fire resistant steel tube with high productivity by rolling a steel having a specified compsn. under specified conditions to form a steel plate, executing heating to a two phase region, subjecting the same to bending from the edge parts of the steel plate, forming it into a tubular shape at the center part and thereafter executing cooling. SOLUTION: A slab contg., by weight, 0.05 to 0.25% C, 0.10 to 2.00% Si, 0.5 to 2.0% Mn, 0.10 to 0.60% Mo, 0.01 to 0.1% V, 0.002 to 0.20% sol.Al and 0.001 to 0.02% Ni or furthermore contg. specified small amounts of Nb, Ti, Cr, B, Ca or the like is subjected to hot rolling at >=30% cumulative draft from the Ar3 to the recrystallization temp. region. This hot rolled plate is heated to the two phase region of the Ac1 to Ac3 , working into a tubular shape is started from the edge parts of the steel plate in the temp. range of the Ar1 or above, and the tubular working is finished at the center part of the steel plate. Next, cooling is executed at a cooling rate of P deg.C/sec to 100×P<2> deg.C/sec, where the value of Pcm in the steel shown by the formula 1 is defined as P to form its microstructure into the one essentially consisting of ferrite + bainite and in which insular martensite is present by <=5%, by which the building use thick fire resistant steel tube low in a yield ratio can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thick-walled construction refractory steel pipe of about 40 to 150 mm which has a low yield ratio and is used for high-rise buildings and marine structures, etc.

[0002]

2. Description of the Related Art Thick-walled steel pipes used for high-rise buildings and marine structures are required to have high strength, high toughness, low yield ratio and high weldability. Therefore, in the case of relatively thin-walled steel pipes, a low-component composition is used to secure weldability, and technologies such as controlled rolling and controlled cooling are used to manufacture high-strength and high-toughness steel plates, It is formed into a tubular shape by processing. In this case, since the material changes due to work hardening during forming into a tubular shape, a heat treatment such as a stress relieving process is required after the forming in order to achieve predetermined characteristics.

On the other hand, at the present time when the height of the building is increasing, the steel pipe column for construction tends to be thickened. In the case of thick-walled steel pipe,
Cold forming is impossible from the viewpoint of the load of the pressing device during processing, and warm forming or hot forming is adopted.

However, when hot forming is adopted for the pipe making, the effect on the strength obtained by controlled rolling or the like disappears, so that a material of high component type is used, A high component system causes deterioration in toughness and weldability, which is not preferable in this respect.

Regarding warm forming, JP-A-62-5
No. 4018 discloses reheating to a temperature range of Ac ₁ or less of 750 to 400 ° C. and immediately or after cooling to 750 to 2
It is disclosed that by processing in a temperature range of 50 ° C., toughness and the like are made excellent.

On the other hand, as a method for obtaining a low yield ratio, which is one of the important characteristics of steel pipe columns for construction, the following method has been proposed. JP-A-3-87318, proposed a method to temper the steel Ac ₃ -250 ℃ ~Ac ₃ -20 ℃ if subsequently cooled to a temperature range, or to impart working strain after these heat treatments Has been done. Further, in Japanese Patent Laid-Open No. 3-87317, a steel pipe is heated to Ac ₃ or more and then air-cooled, and then water-cooled from a temperature range of Ar ₃ −250 ° C. to Ar ₃ −20 ° C., or a processing strain is generated after the heat treatment. A method of applying and tempering is proposed. Further, JP-A-4-321, Ac ₃ -20 steel tube
A method of heating to a temperature of 0 ° C. or higher, starting strain application at Ac ₃ −200 ° C. or higher, ending strain application at a temperature range of Ac ₃ −200 ° C. to Ac ₃ −20 ° C., and then tempering after water cooling. Is proposed. However, in these methods, the steel pipe is subjected to complicated treatment, and economical efficiency and productivity are significantly impaired.

Further, in recent years, a reduction in the refractory coating has been demanded from the viewpoint of construction cost reduction, and a refractory property having a yield stress at 600 ° C. of ⅔ or more of room temperature is also required. As means for ensuring the fire resistance, Japanese Patent Laid-Open No. 4-128
315, JP-A-4-168218, and JP-A-4-16.
8219, JP-A-4-176821, and JP-A4-2.
Mo as disclosed in each publication of 28521,
A method has been proposed in which component elements such as V and Nb are added, strain is applied at a predetermined temperature, and cooling is performed at a predetermined rate.

[0008]

As described above, in the production of thick-walled steel pipe columns for construction, it is difficult to perform the cold working from the viewpoint of the capability of the forming machine, and in the hot forming, the controlled rolling is performed. Since the effect of (1) is lost, the composition must be a high-strength, high-component system, which causes inconveniences such as deterioration of toughness and weldability.

On the other hand, even in the case of warm forming, when warm forming is performed after heating to a temperature of Ac ₁ or lower, the deformation resistance tends to increase due to the temperature drop during processing, and the yield ratio increases as the temperature drops. There is a problem that it tends to rise and a low yield ratio is not achieved. Further, various heat treatments have been attempted on a steel pipe in order to achieve a low yield ratio, but all of these methods are complicated and disadvantageous in economical efficiency.

Further, as a means for ensuring fire resistance, Japanese Patent Laid-Open No. Hei 4 (1999) -4
-128315, JP-A-4-168218, JP-A-4-168219, JP-A-4-176821, and JP-A-4-228521, and other components such as Mo, V, and Nb. In the case of a method in which an element is added, strain is applied at a predetermined temperature, and cooling is performed at a predetermined rate, a large amount of elements such as Mo that enhance hardenability and easily form island martensite. Addition causes disadvantages such as high yield ratio and reduction in toughness.

Further, when forming a steel pipe from a steel plate, a tensile stress acts on the outer surface of the steel pipe and a compressive stress acts on the inner surface of the steel pipe, and the stress does not work at the center of the plate thickness, which is the neutral axis of the stress. In the case of hot or hot forming, even at the same working temperature, the center of thickness has a lower yield ratio than the inner and outer surfaces, but the strength is remarkably low. There is a tendency not to achieve.
Also, in the case of warm forming or hot forming, the temperature to be processed differs depending on each part in the steel sheet, and the part that is processed at relatively high temperature has a low yield ratio but low strength, and conversely it is processed at low temperature. It is possible to achieve a predetermined strength at the site receiving the stress, but a low yield ratio tends not to be achieved. For this reason,
In pipe forming by warm forming or hot forming, the distribution of mechanical properties in the plate thickness direction becomes maximum at the final processing part where the processing temperature is the lowest.

The present invention has been made in view of such circumstances, and it is possible to achieve high strength and a low yield ratio in each portion of the plate thickness without impairing the economical efficiency and the productivity, and also at a high temperature. An object of the present invention is to provide a method for manufacturing a thick-walled construction fire-resistant steel pipe with little strength reduction.

[0013]

The inventors of the present application have achieved high strength and low yield ratio in each part of the plate thickness without requiring a facility having a large working capacity for pipe production and causing deterioration of toughness. And, as a result of a detailed examination of the method for manufacturing a steel pipe with little strength reduction even at high temperature,
A steel with a specific composition is rolled under specific rolling conditions, the steel sheet is heated to a two-phase region, and bending processing for pipe manufacturing is started from that temperature region from the plate edge, and at the plate central portion. It was found that the above problems can be solved by controlling the microstructure by terminating and then cooling at a predetermined cooling rate. That is, it has been conventionally known that a low yield ratio can be obtained by heating in the two-phase region, but at the same time, it causes deterioration of toughness, so that processing in this temperature region has not been performed. However, as described above, by controlling the composition and rolling conditions, as well as the pipe forming order and the cooling rate during the warm period, the microstructure is adjusted and toughness deterioration is suppressed, and high strength at room temperature and high temperature is achieved in each part of the plate thickness. , As well as a low yield ratio can be achieved.

The present invention has been completed on the basis of such findings. Firstly, C: 0.05-by weight%.
0.25%, Si: 0.10 to 2.00%, Mn: 0.
5 to 2.0%, Mo: 0.10 to 0.60%, V: 0.
01-0.1%, sol. Al: 0.002 to 0.20
%, N: 0.01 to 0.02%, is subjected to hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of recrystallization temperature or lower and Ar ₃ or higher to obtain a steel plate. A
Heating to a two-phase region of c ₁ or more and Ac ₃ or less, Ar ₁
From the above temperature range, the processing from the end of the plate to the end of the plate is started, and at the center of the plate, after that, when the value of Pcm of the steel shown below is P, P ° C / sec or more and 100 or more. × P
Cooling at a rate of ² ° C / sec or less to make the microstructure a structure mainly composed of ferrite + bainite and having an island-like martensite of 5% or less, a thick wall refractory steel pipe for construction with a low yield ratio The present invention provides a method for manufacturing the same.

Secondly, C: 0.05 to 0.2 by weight%.
5%, Si: 0.10 to 2.00%, Mn: 0.5 to
2.0%, Mo: 0.10 to 0.60%, V: 0.01
~ 0.1%, sol. Al: 0.002 to 0.20%,
N: 0.01 to 0.02%, and (i) Nb
+ V + Ti Nb and Ti in a range of 0.2% or less
At least one of (ii) Cu, Ni and Cr of 0.01 to 1.5%, (iii) B: 0.
0005-0.005%, and (iV) Ca: 0.00
Steel containing at least one of (i) to (iV) of 05 to 0.005% is subjected to hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of recrystallization temperature or lower and Ar ₃ or higher. To form a steel plate, heat the steel plate to a two-phase region of Ac ₁ or more and Ac ₃ or less, and start processing from the temperature region of Ar ₁ or more into a tubular shape at the plate end and finish at the plate center. Then, when the value of Pcm of the steel shown below is set to P, P ° C
Yield ratio characterized by making the microstructure mainly ferrite + bainite and island martensite 5% or less by cooling at a rate of not less than / sec and not more than 100 x P ² ° C / sec. The present invention provides a method for manufacturing a thick wall refractory steel pipe for construction.

Pcm = [C] + [Si] / 30 + [M
n] / 20 + [Cu] / 20 + [Ni] / 60 + [C
r] / 20 + [Mo] / 15 + [V] / 10 + 5 ×
[B] (However, [] indicates the concentration expressed by weight% of each element.)

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a thick-walled refractory steel pipe according to the present invention will be described below in detail in terms of composition, rolling conditions, forming conditions, and microstructure together with its action. (Composition) In the present invention, C, Si, and M are used as basic constituent elements.
n, Mo, V, solAl, and N, and (i) at least one of Nb and Ti as a selective component, (ii)
At least one of Cu, Ni and Cr, (iii) B, and at least one of (i) to (iV) of (iV) Ca are added.

The reasons for limiting these constituent elements will be described below. In the following description, all percentages are% by weight. C: To secure the strength of this type of steel inexpensively and effectively, C is required to be 0.05%. However, 0.2
If it exceeds 5%, low temperature cracking, high temperature cracking, etc. occur, and the weldability and toughness are impaired. Therefore, the C content is 0.05 to 0.
The range is 25%.

Si: Si is added as a deoxidizing agent, but if it is less than 0.10%, a sufficient deoxidizing effect cannot be obtained, while if it exceeds 2.00%, toughness and weldability deteriorate. Invite. Therefore, the Si content is 0.10 to 2.00%
Range.

Mn: Mn is added as a basic element of steel effective for improving the strength and toughness of steel, but if it is less than 0.5%, its effect is small, and if it exceeds 2.0%, weldability and toughness are improved. Is significantly deteriorated. Therefore, the Mn content is set to the range of 0.5 to 2.0%.

Mo: Mo has the effect of achieving high strength at room temperature and high temperature in each part of the plate thickness by solid solution strengthening, and when added in a small amount, it suppresses deterioration of toughness due to structural change due to increase in hardenability. There is. However, if the content is less than 0.10%, these effects are not sufficient, and the increase in strength and suppression of deterioration of toughness become insufficient. Further, if it is added in a large amount exceeding 0.60%, not only the weldability and toughness are impaired, but also the economical efficiency is impaired. Therefore, the Mo content is set to the range of 0.10 to 0.60%.

V: V forms a nitride to improve the high temperature strength, but if it is less than 0.01%, its effect is small, and if it exceeds 0.1%, toughness is reduced. For this reason,
The V content is set to the range of 0.01 to 0.1%.

Sol. Al: sol. Al is added as a deoxidizing agent, but if it is less than 0.002%, a sufficient deoxidizing effect cannot be obtained, and if it is added at about 0.20%, the effect is saturated, and it is economical to add more than that. It is not desirable from the standpoint of physical aspects. Therefore, sol. Al content 0.00
The range is 2 to 0.20%.

N: N forms a nitride to improve high temperature strength, but if less than 0.001%, its effect is small,
On the other hand, if it exceeds 0.02%, the toughness decreases. Therefore, the N content is set in the range of 0.001 to 0.02%.

The above are the basic components, but the following N
By adding b, Ti, Cu, Ni, Cr, B, and Ca as selective components, it becomes possible to improve the high temperature strength, improve the toughness, control the morphology of inclusions, and improve the weldability.

Nb, Ti: Nb and Ti, like V, have the effect of achieving high strength at high temperature by precipitation hardening. However, if the content of Nb and Ti and V added as an essential component exceeds 0.20% in Nb + V + Ti, the strength becomes extremely high at room temperature, and the toughness decreases. Therefore, Nb and Ti are contained in a range such that Nb + 2V + 1.5Ti is 0.20% or less.

Cu, Ni, Cr: Cu, Ni, and Cr can be strengthened by solid solution strengthening or improving hardenability without significantly impairing toughness, but from the viewpoint of weldability and economical efficiency, The content of at least one of Cu, Ni and Cr is in the range of 0.01 to 1.5%.

Ca: Ca has the effect of spheroidizing the form of inclusions, thereby preventing hydrogen-induced cracking, lamellatea, etc. However, if it is less than 0.0005%, that effect cannot be obtained, and about 0.005%. The effect is saturated by the addition of, and addition of a larger amount is not preferable from the economical point of view. Therefore, the Ca content is 0.0005 to 0.005.
% Range.

B: B can be strengthened by improving the hardenability without significantly impairing the toughness, but if its content is less than 0.0005%, its effect cannot be sufficiently obtained. The effect is saturated with the addition of about 005%,
It is not preferable from the economical viewpoint to add a larger amount than that. Therefore, the B content is 0.0005 to 0.005.
% Range.

(Rolling conditions) In the present invention, for the steel having the above composition, the recrystallization temperature is not higher than Ar ₃
In the above temperature range, hot rolling with a cumulative reduction of 30% or more is performed to obtain a steel sheet.

Here, the reason why hot rolling with a cumulative reduction of 30% or more at the recrystallization temperature or lower is carried out is that the effect of the controlled rolling is sufficiently achieved by performing sufficient rolling in the unrecrystallized region. This is because the fine austenite grains are obtained by exerting them. Further, since hot rolling is performed in a temperature range of Ar ₃ or higher,
The rolling end temperature is naturally Ar ₃ or higher, but the reason for doing this is to suppress the formation of a structure elongated by rolling.

The recrystallization temperature varies depending on the component system, but as a guide, it is about 900 ° C. in the Nb-free system and about 950 ° C. in the Nb-containing system. Further, Ar ₃ also varies depending on the C and Mn contents, but a guideline is about 750 ° C.

(Forming Conditions) In the present invention, the steel sheet rolled as described above is heated to a temperature range of a two-phase region of Ac ₁ or more and Ac ₃ or less, and a temperature range of Ar ₁ or more is converted into a tubular shape. When the processing starts from the edge of the plate and ends at the center of the plate, and then the value of Pcm of steel is P, P ° C /
By cooling at a rate of not less than sec and not more than 100 × P ² ° C./sec, the microstructure is made mainly of ferrite + bainite and has an island martensite structure of 5% or less.

Here, Pcm = [C] + [Si] /
30+ [Mn] / 20 + [Cu] / 20 + [Ni] / 6
0+ [Cr] / 20 + [Mo] / 15 + [V] / 10 +
5 × [B] (where [] indicates the concentration expressed by weight% of each element).

Here, the steel plate is heated in the temperature range of the two-phase region of Ac ₁ or more and Ac ₃ or less, and the processing into the tubular form is started from the temperature region of Ar ₁ or more. This is because of lowering and forming into a tubular shape without requiring a large facility capacity, and for achieving a low yield ratio by processing in the two-phase region.

After that, P ° C./sec or more and 100
The reason for cooling at a rate of × P ² ° C / sec or less is that cooling in this range has the effect of adjusting the microstructure of the steel sheet and suppressing higher yield ratio and lower toughness. . If the cooling rate is less than P ° C./sec, the structure becomes coarse and the toughness becomes low. If the cooling rate is more than 100 × P ² ° C./sec, a full bainite structure is formed, resulting in a high yield ratio.

Further, the reason why the processing is started from the plate end and finished at the plate central part is that the plate end which has a rapid temperature decrease during processing is initially formed and the temperature decrease is relatively low. This is because the effect of reducing the temperature difference at the time of processing in each part of the plate can be obtained by forming the slow plate center part later.

(Microstructure) The microstructure is controlled to have a structure mainly composed of ferrite + bainite and an island martensite content of 5% or less. This is because the effect of securing The full bainite structure has a high yield ratio, and if the island martensite content is more than 5%, the toughness is significantly reduced. Therefore, the microstructure is controlled to be a ferrite + bainite structure containing 5% or less of island martensite. In particular, in order to reduce the yield ratio, the island martensite content is preferably 1% or more.

The tubular molding is preferably carried out by using a method which is usually used for processing into a cylinder such as a press bend, but the method is not limited. (Operation) By using the present invention, it is possible to achieve high strength at room temperature and high temperature and low yield ratio at room temperature in each part of the plate thickness by the following operation.

As basic components, C, Si, Mn, Mo,
V, sol. Containing Al and N, or (i) at least one of Nb and Ti, (ii) Cu, Ni
And at least one of Cr, (iii) B, and (iV)
By containing at least one of (i) to (iV) of Ca as a selective component, controlling these to a specific content, and further controlling the conditions and order of pipe making during warm, the plate thickness High strength is achieved at room temperature and high temperature in each part. That is, in the production of the steel pipe column, due to the change in material due to the strain distribution during forming into a tubular shape, lower strength occurs in the central portion of the plate thickness and higher yield ratio occurs in the inner and outer surfaces, but in the present invention, , A large effect of solid solution strengthening by Mo and V among the basic components or elements selectively added,
Also, by reducing the distribution of the working temperature in the steel sheet by controlling the order of pipe making during warming, it is possible to achieve a small change in material in the sheet thickness direction and to achieve high strength.

Further, heating the steel sheet to a temperature range of a two-phase region of Ac ₁ or more and Ac ₃ or less and starting the processing into a tubular shape from the temperature region of Ar ₁ or more lowers the deformation resistance, It has a function of achieving a low yield ratio by forming into a tubular shape without requiring a large facility capacity and processing in the two-phase region. After that, P ° C / sec or more and 100 ×
The microstructure is cooled at a rate of P ² ° C / sec or less,
Controlling the structure mainly composed of ferrite + bainite and containing island martensite at 5% or less has the effect of achieving a low yield ratio by refining the structure and using island martensite + ferrite + bainite. . In addition,

[0042]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described. (Example 1) A steel having a composition shown in Table 1 was heated to 1150 ° C, rough-rolled, and then finish-rolled at a cumulative reduction of 40% in a temperature range of 900 to 800 ° C to produce a steel plate having a thickness of 70 mm. did. After these steel plates were heated to 800 ° C., they were immediately pressed into a tubular shape by press bending from the plate end to the plate center, and then cooled at a rate of 0.5 ° C./sec. The ratio D / t of the outer diameter D of the steel pipe and the plate thickness t at this time was set to 10.

In the steel pipe formed as described above, the mechanical properties in the final processed portion having the lowest working temperature and the large distribution of mechanical properties in the plate thickness direction, the yield strength (YS ), And the volume fraction of island martensite near the surface layer are shown in Table 2.

[0044]

[Table 1]

[0045]

[Table 2]

As shown in Table 2, with the component compositions within the range of the present invention, high toughness with Charpy absorbed energy vE at 0 ° C. of 200 J or more and low yield ratio of 80% or less were obtained. Further, a high yield strength (YS) of 200 MPa or more was obtained even at 600 ° C. at the center of the plate thickness.

On the other hand, those having a component composition outside the scope of the present invention show low toughness with Charpy absorbed energy vE at 0 ° C. of less than 100 J at the surface layer portion or central portion of the sheet thickness and at the sheet thickness center. The yield strength (YS) at 600 ° C. was low strength of less than 100 MPa.

Example 2 A steel having a composition of A05 in Table 1 was heated to 1150 ° C. and rough-rolled, and then, as shown in Table 3, in a temperature range of 1000 to 650 ° C., a cumulative reduction rate of 20 to 5 was obtained.
0% finish rolling was performed to manufacture a steel plate having a plate thickness of 70 mm. Immediately after heating the steel plate to 800 ° C., press-bending was started to form a tubular form from the plate end portion to the plate central portion, and then cooled at a rate of 0.5 ° C./sec. At this time, the ratio D / t between the outer diameter D of the steel pipe and the plate thickness t is 1
It was set to 0.

In the steel pipe thus formed, the mechanical properties at the final working portion, the yield strength (YS) near 600 ° C. at the center of the sheet thickness, and the volume fraction of island martensite near the surface layer were measured. It shows in Table 3.

[0050]

[Table 3]

As shown in Table 3, under the manufacturing conditions within the range of the present invention and the island-like martensite volume fraction is 5% or less, the Charpy absorbed energy vE at 0 ° C. in each part of the plate thickness is 200 J or more. High toughness, and 80%
The following low yield ratios were obtained. Furthermore, 60 at the center of the plate thickness
A high yield strength (YS) of 200 MPa or more was obtained at 0 ° C.

On the other hand, under the conditions out of the range of the present invention, the Charpy absorbed energy vE at 0 ° C. in the surface layer portion or the central portion of the plate thickness shows low toughness of less than 100 J or exceeds 80%. It has a high yield ratio.

(Example 3) Steel having a composition of A05 in Table 1 was heated to 1150 ° C and rough-rolled, and then 900-800 ° C.
Finish rolling with a cumulative rolling reduction of 40% was performed in the temperature range of 1 to produce a steel sheet having a thickness of 70 mm. As shown in Table 4, after heating this steel plate to 1000 to 700 ° C., forming into a tubular shape by press bend starts from the plate end portion at 850 to 500 ° C. and ends at the plate central portion, and then 0 0.01 to 100 ° C
It was cooled at a rate of / sec. The ratio D / t of the outer diameter D of the steel pipe and the plate thickness t at this time was set to 10.

In the steel pipe thus formed, the mechanical properties at the final processed portion, the yield strength (YS) near the center of the plate thickness of 600 ° C., and the volume fraction of island martensite near the surface layer were measured. It shows in Table 4.

[0055]

[Table 4]

As shown in Table 4, under the manufacturing conditions within the range of the present invention and the island-like martensite volume fraction is 5% or less, the Charpy absorbed energy vE at 0 ° C. in each part of the plate thickness is 200 J or more. High toughness, and 80%
The following low yield ratios were obtained. Furthermore, 60 at the center of the plate thickness
A high yield strength (YS) of 200 MPa or more was obtained at 0 ° C.

On the other hand, under the conditions out of the range of the present invention, the Charpy absorbed energy vE at 0 ° C. in the surface layer portion or the central portion of the plate thickness shows low toughness of less than 100 J or exceeds 80%. It has a high yield ratio. In the above embodiment, the ratio D / t of the outer diameter D of the steel pipe to the plate thickness t
Was set to 10, but it goes without saying that it is not limited to this.

[0058]

As described above, according to the present invention,
With a highly economical process that does not require large equipment capacity, it has a high strength at room temperature and high temperature in each part of the plate thickness without compromising toughness, and a low yield ratio at room temperature. It becomes possible to manufacture refractory steel pipes for construction.

Claims (2)

[Claims] 1. C: 0.05 to 0.25% by weight,
Si: 0.10 to 2.00%, Mn: 0.5 to 2.0
%, Mo: 0.10 to 0.60%, V: 0.01 to 0.
1%, sol. Al: 0.002 to 0.20%, N:
The steel containing 0.001 to 0.02%, the recrystallization temperature or less and Ar ₃ or more cumulative rolling reduction in a temperature range of 30%
The above hot rolling is applied to form a steel plate, the steel plate is heated to a two-phase region of Ac ₁ or more and Ac ₃ or less, and processing from an Ar ₁ or more temperature region into a tubular shape is started from the plate end, After finishing at the center, the value of Pcm of steel shown below is set to P
And P × C / sec or more and 100 × P ² ° C. /
Cooling at a rate of sec or less, the microstructure is mainly composed of ferrite + bainite, and island martensite is 5%.
A method for manufacturing a thick wall refractory steel pipe for construction having a low yield ratio, which has the following structure. Pcm = [C] + [Si] / 30 + [Mn] / 20 +
[Cu] / 20 + [Ni] / 60 + [Cr] / 20 +
[Mo] / 15 + [V] / 10 + 5 × [B] (where [] indicates the concentration expressed by weight% of each element.) 2. C: 0.05 to 0.25% by weight,
Si: 0.10 to 2.00%, Mn: 0.5 to 2.0
%, Mo: 0.10 to 0.60%, V: 0.01 to 0.
1%, sol. Al: 0.002 to 0.20%, N:
0.001 to 0.02%, and (i) Nb +
At least one of Nb and Ti in a range where V + Ti is 0.2% or less, (ii) 0.01 to 1.5% of at least one of Cu, Ni and Cr, (iii) B: 0.0
005-0.005%, and (iV) Ca: 0.000
At least 1 out of 5 to 0.005% of (i) to (iV)
A steel containing seeds is subjected to hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of recrystallization temperature or lower and Arc ₃ or higher to obtain a steel sheet, and the steel sheet has two phases of Ac ₁ or more and Ac ₃ or less. When the area is heated, the processing from the temperature range of Ar ₁ or higher to the tubular shape is started from the plate end and finished at the plate center, and then when the value of Pcm of the steel is P, P ° C / Cool at a rate of not less than sec and not more than 100 x P ² ° C / sec,
A method for producing a thick-walled refractory steel pipe for construction with a low yield ratio, characterized in that a microstructure is mainly composed of ferrite + bainite and an island martensite is 5% or less. Pcm = [C] + [Si] / 30 + [Mn] / 20 +
[Cu] / 20 + [Ni] / 60 + [Cr] / 20 +
[Mo] / 15 + [V] / 10 + 5 × [B] (where [] indicates the concentration expressed by weight% of each element.)