JP3243659B2 - Manufacturing method of coiled steel pipe with excellent cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a coiled steel pipe used for automobile or industrial machine parts, geothermal, chemical plants, oil well pipes for petroleum or natural gas, and general pipes. .

[0002]

2. Description of the Related Art A coiled steel pipe is produced by rolling a forged steel pipe, an electric resistance welded steel pipe, a seamless steel pipe, or the like as a raw pipe by a hot stretch reducer, and then winding it into a coil. Thereafter, the coiled steel pipe is subjected to secondary processing such as drawing, rolling, and screw cutting, and is used for various pipes and machine parts.

[0003] The metallographic structure of the coiled steel pipe is disclosed in
This is an annealed structure having ferrite and pearlite as described in JP-A-16657, JP-A-60-125326, and JP-A-60-137510. The material having the ferrite + pearlite structure has poor cold work resistance. Therefore, automobiles that require a high degree of cold workability,
As materials for industrial machine parts and the like, spheroidizing annealing materials are usually used.

[0004] As the spheroidizing method, a method of heating to more than _one point of Ac and then gradually cooling, and a method of repeating heating and cooling to a temperature above and below _one point of Ac are implemented. However, the above-described spheroidizing annealing requires a long processing time of several tens of hours, and has a problem of quality increase in heat treatment cost and decarburization and oxidation. Conventionally, cold drawing is performed before annealing to reduce the annealing time. Despite shortening, satisfactory results have not been obtained.

[0005]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is to improve the cold workability by spheroidizing the metal structure of a coiled steel pipe, and to reduce the cost of the treatment efficiently. The goal is to get in.

[0006]

The present invention is characterized in that the metal structure of the coiled steel pipe obtained by hot drawing and rolling exhibits a spheroidized structure, and the cost of the spheroidizing treatment is low. It is an object of the present invention to provide a method for producing a coiled steel pipe having excellent cold workability.

[0007] The coiled steel pipe of the present invention has a rolling end temperature,
By controlling the cooling temperature after rolling to a specific range, fine pearlite , fine bainite, or a mixed structure thereof is formed. When the fine structure is heat-treated in the reheating furnace 4, the carbide in the finely processed structure becomes spheroidized And easily become a spheroidized tissue in a short time. Therefore, the cost of spheroidizing heat treatment is reduced, and the material properties such as cold workability are remarkably excellent.

The gist of the present invention is that when a coiled steel pipe is manufactured by hot drawing and rolling, the raw tube for hot drawing and rolling is heated to 900 to 1100 ° C. _Three
Point below (hereinafter simply referred to as the following ₃ points Ar), (Ar ₃ -10
Performed hot reducing rolling at 0 ° C.) or higher temperature range, followed by a temperature range of Ar ₁ -200 ° C. from the hot reducing rolling end temperature of 10
Cooling at a cooling rate of to 40 ° C. / sec, followed by Ac ₁ or less,
A method for producing a coiled steel pipe having excellent cold workability, characterized by reheating to a temperature range of (Ac ₁ -50 ° C.) or higher and winding the coil into a coil.

Hereinafter, the production method and the equipment configuration of the present invention will be described. The method for manufacturing a coiled steel pipe of the present invention is shown in FIG.
As shown in FIG.
° C. to heating in a heating furnace 1, a hot reducing rolling of the rolling pipe for 11 Ar ₃ point or less, it is terminated in a temperature range of not lower than (Ar ₃ -100 ℃).

At this time, proeutectoid ferrite grains are precipitated at _three Ar points. The pro-eutectoid ferrite grains become elongated ferrite grains in which strain is accumulated inside by hot-draw rolling between (Ar ₃ -100 ° C.). The stretched ferrite grains serve as transformation nuclei when the untransformed austenite phase is transformed into Ar ₁ , and a fine precipitate and a fine pearlite structure are obtained. Subsequently, the rolling tube 12 is cooled by the cooling device 3 from the hot-rolling end temperature to Ar _1-2.
The temperature range of 00 ° C. is cooled at a cooling rate of 10 to 40 ° C./sec, and the forcibly cooled rolling tube 12 is re-heated to a temperature range of _one point Ac or less and (Ac ₁ -50 ° C.) or more in the reheating furnace 4. The coiled steel pipe is manufactured by heating and winding it into a coil.

As for the Ar ₃ and Ar ₁ points, the present inventors have conducted a number of experiments and found that the equations (1) and (2)
As shown in Table ₂ , the relationship between the chemical composition of the steel and Ar ₃ and Ar ₁ point was determined. Ar ₃ = 827−64C−33Si−19Mn + 17Cr−48Mo (1) Formula Ar ₁ = 686−2C−0.4Si−30Mn + 40Cr−85Mo (2) Formula Ac ₁ = 723 (・ ・ ・) 3) Formula

The rolled raw pipe 11 is a steel pipe made of carbon steel, and may be a seamless steel pipe, a hot extruded steel pipe, an electric resistance welded steel pipe, a forged steel pipe, or the like manufactured by various Mannesmann methods. As the heating furnace 1 for heating the rolling element tube 11, a barrel furnace, a walking beam furnace, an induction heating furnace, or the like can be used. An induction heating furnace or the like can be used as the reheating furnace 4 that heats the hot drawn pipe to aggregate carbides in steel.

The stretch reducer 2 is a rolling mill in which roll housings of 2 rolls or 3 rolls are continuously arranged in 10 to 30 stands, and the raw tube 11 has a peripheral speed between adjacent stands while the outer diameter is sequentially rolled. Rolled tube 1 which has been stretched in the axial direction by the difference in
The rolled tube 2 is forcibly cooled by the cooling device 3, and subsequently the rolled tube 12 that has been forcibly cooled is reheated by the reheating furnace 4, guided by the pinch roller 5, wound up by the winder 6, and wound into the coiled steel tube 13. Becomes

The reasons for limiting the configuration of the present invention are as follows. When the hot rolled steel pipe is hot-rolled and drawn to produce a coiled steel pipe, the raw rolled tube is heated to 900 to 1100 ° C., and when the hot-rolled steel is subsequently hot-rolled, the hot-rolled rolling is completed. The reason why the temperature was limited to Ar ₃ points or less is that, as shown in the hot-rolling rolling pattern in FIG. 2, when the hot-rolling rolling was completed at Ar ₃ points or more, the recrystallized and sized austenite phase, and then Ar ₃ and Ar _{In one} transformation, it is transformed into a coarse ferrite structure and a pearlite structure, and even if it is heat-treated in a reheating furnace 4 for agglomerating carbide in steel thereafter, the desired spheroidized structure cannot be obtained.

The reason why the temperature is limited to (Ar ₃ -100 ° C.) or higher is that when the hot-rolling end temperature is equal to or lower than (Ar ₃ -100 ° C.), austenite is reduced during hot-rolling rolling or during cooling after hot-rolling rolling. The phase transforms into a martensite structure or a bainite structure, and the transformation resistance increases, which hinders hot-drawing rolling. For the above reason, the hot-rolling rolling end temperature is set to (Ar ₃
(−100 ° C.) or higher.

Next, as for the supercooled austenite temperature for cooling the hot-rolled tube, the inventors conducted a number of experiments and found that a mixed structure of a fine pearlite structure and a fine bainite structure that can be easily spheroidized in the next step. In order to obtain a (Zweisssen structure), (Ar _1-2 ) is determined from the hot-rolling end temperature.
(00 ° C.) at a cooling rate of 10 ° C./sec to 40 ° C./sec or less.

That is, the cooling end temperature is (Ar ₁ -200
When the cooling rate is higher than 40 ° C./sec, the untransformed austenite phase is transformed into a martensitic phase structure when the cooling rate is lower than 10 ° C./sec. , Fine bainite or a mixed structure thereof cannot be obtained. For this reason, when forcibly cooling the hot rolled tube, the cooling rate is 1
0 ° C / sec to 40 ° C / sec or less, the cooling end temperature is (A
r ₁ -200 ° C.)

Next, after the hot drawing rolling, the temperature range in which the forcibly cooled rolled steel pipe is reheated is set to ₁ point or less of Ac, (Ac ₁ -50
C)) or more, the carbide in steel is decomposed and solid-dissolved when heated to more than ₁ point of Ac, and does not bring about the effect of spheroidization. At a temperature below (Ac ₁ -50 ° C.), diffusion of carbon in the steel does not occur sufficiently, so that carbides are not agglomerated. Therefore, the heating temperature is limited to below the Ac ₁ point and above (Ac ₁ -50 ° C.). .

[0019]

The effects of the embodiments of the present invention will be described below. Quality characteristics of coiled steel pipes manufactured using carbon steels of various compositions shown in Table 1 by changing the hot rolling temperature, the rolling end temperature, the cooling temperature after the rolling end, and the reheating temperature, ie, metal structure, compression working The results of evaluating the characteristics are shown.

[Table 1] According to the production method of the present invention, the spheroidized structure can be processed in a short time without requiring a long time treatment as in the conventional method, and the obtained coiled steel pipe has a high critical compression ratio. Cold workability is greatly improved compared to coiled steel pipes.

[0020]

According to the present invention, the coiled steel pipe manufactured according to the present invention can be subjected to spheroidizing treatment in a short time, and the spheroidizing treatment cost is reduced. It also has excellent cold workability, and the material properties of the coiled steel pipe in the longitudinal and circumferential directions are uniform, and the quality characteristics are dramatically improved as compared with the conventional coiled steel pipe. And the industrial value is enormous.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a facility for manufacturing a coiled steel pipe of the present invention.

FIG. 2 is a diagram showing hot reduction rolling patterns of the present invention and a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Stretch reducer 3 Cooling device 4 Reheating furnace 5 Pinch roll 6 Winder 11 Rolling tube 12 Rolling tube 13 Coiled steel tube

[Table 1]

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-125326 (JP, A) JP-A-1-283321 (JP, A) JP-A-5-247532 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C21D 8/10 B21B 17/14

Claims (1)

(57) [Claims] When a coiled steel pipe is manufactured by hot-rolling, a raw pipe for hot-rolling is heated to 900 to 1100 ° C., and then the Ar of the raw pipe is reduced to ₃ points or less (Ar ₃ -100
° C.) to exit the hot reducing rolling at temperatures above and subsequently the temperature range of Ar ₁ -200 ° C. from the hot grain <br/> Ri rolling end temperature 10
Cooling at a cooling rate of ４０40 ° C./sec followed by Ac ₁
Re-heated to a temperature range of (Ac ₁ -50 ° C.) or higher,
Subsequently, a method for producing a coiled steel pipe having excellent cold workability, which is wound in a coil shape.