JPH09276907A - Manufacture of austenitic stainless steel seamless tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture an austenitic stainless steel seamless tube by on-line solution heat treatment. SOLUTION: After heating a billet of austenitic stainless steel, hot piercing and successively executing finish rolling (elongating work + finishing work) in which average strain rate determined by the under-mentioned equation is >=0.01/sec and the compressibility of cross section is >=10% at finished temp. of 800-1050 deg.C, immediately the billet is reheated to 1000-1150 deg.C, held for 10sec to 30min and next, cooled at a cooling rate of >=1.5 deg.C/sec from the temp. of >=900 deg.C. The average strain rate = (the working strain in the elongating work + working strain in the finishing work)/(the time from the start of the elongating work to the completion of the finishing work at the tip of a hollow tube stock).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel seamless steel pipe excellent in strength, toughness, corrosion resistance and austenite stability, which is produced by a slim mill line with high productivity and effective heat treatment. It relates to a method of manufacturing.

[0002]

2. Description of the Related Art In the steel industry which requires huge equipment, various process simplifications have been studied by applying thermomechanical processing online from the viewpoint of process saving and energy saving. Particularly in the production of steel plates and thick plates, the production by off-line quenching and tempering has considerably decreased, and online heat-treated materials have become the majority.

However, in the production of an austenitic stainless seamless steel pipe, from the viewpoint of high reliability and high quality, it is the actual situation that the solution treatment of reheating and quenching is still performed offline. Generally, a heating device for solution treatment is installed separately from the pipe line, and reheating solution treatment is performed off-line.

On the other hand, if it is possible to manufacture the product by hot rolling without the off-line solution treatment, it is possible to reduce the heat treatment cost, omit the equipment, shorten the process, etc., and industrially reduce the cost. Therefore, there is a movement to introduce an online solution treatment process.

On the other hand, in the hot rolling process of austenitic stainless steel, 10% of austenitic stainless steel is used.
Rolling at a rolling reduction of 50% or more at a rolling reduction of 3% or more per pass in a temperature region of 50 ° C or more, and a cumulative rolling reduction of 20 at 950 ° C or more after 1050 ° C after rolling.
% Or more, and 900 to 500 after the rolling.
Solution treatment online by cooling the temperature range of ℃ in the condition that the average cooling rate V (℃ / sec) satisfies V ≧ C ³ × 10 ⁴ according to the carbon content C (wt%) in the steel. (JP-A-62-267418), a step of hot rolling an austenitic stainless steel in a temperature range of 900 to 1000 ° C. with a total reduction ratio of 30 to 50%, and the hot rolling. After 5 to 15 ℃ from 850 ℃ or more
A method (Japanese Patent Laid-Open No. 1-142025) of performing solution treatment online by cooling to a temperature range of 520 to 600 ° C. at a cooling rate of / sec has been proposed.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The processes disclosed in Japanese Patent No. 7418 and Japanese Unexamined Patent Publication No. 1-142025 are suitable for manufacturing an austenitic stainless seamless steel pipe, and are rolled in the axial direction of the pipe.
Since there is a difference in the rolling completion time at both ends and it is necessary to transfer it to the next process equipment, the temperature of the material to be treated drops significantly before the start of water cooling, and the cooling start temperature can be maintained at the solution temperature. However, it has a fatal defect that Cr carbides that deteriorate the corrosion resistance are deposited.

The object of the present invention is to solve the above-mentioned drawbacks of the prior art and to produce austenitic stainless steel seamless steel pipe by online solution treatment. It is an object of the present invention to provide a method for producing an austenitic stainless seamless steel pipe which can maintain the temperature at a solution heat treatment temperature and has performance equivalent to or better than that of a product subjected to conventional off-line solution heat treatment.

[0008]

Means for Solving the Problems The present inventors have intensively studied and studied to achieve the above object. As a result, in the production of the austenitic stainless seamless steel pipe, the cooling start temperature can be maintained at the solution temperature at which Cr carbide does not precipitate by defining the conditions of piercing, stretching and finish rolling, and by defining the solution treatment conditions. At the same time, it was clarified that an austenitic stainless steel seamless pipe having high productivity and performance equivalent to or better than the conventional off-line solution-treated product can be obtained, and the present invention was reached.

The present invention is a method for producing an austenitic stainless seamless steel pipe by heating austenitic stainless steel billet to hot piercing and rolling to obtain an austenitic stainless steel seamless pipe. Speed is 0.0
After finishing rolling for 1 / sec or more and a sectional compression rate of 10% or more at a finishing temperature of 800 ° C. or more and 1050 ° C., it is immediately reheated to 1000 ° C. or more and 1150 ° C. or less and held for 10 seconds or more and 30 minutes or less. 900 ° C or higher to 1.5 ° C /
It is supposed to cool at a cooling rate of more than a second. Average strain rate = (processing strain in stretching + processing strain in finishing) / (time from the start of stretching to the end of finishing at the tip of the hollow shell)

As described above, following the perforation, the average strain rate obtained by the above equation is 0.01 / sec or more, and the cross-section compression rate is 10%.
Finishing temperature above 800 ℃ above 1050 ℃
Immediately after reheating at 1000 ℃ to 1150 ℃, hold for 10 seconds to 30 minutes, then 900
By cooling at a cooling rate of ℃ or more to 1.5 ° C / sec or more, the cooling start temperature can be maintained at a solution temperature at which Cr carbide does not precipitate, and is high in productivity and comparable to the conventional off-line solution-processed products or It is possible to manufacture an austenitic stainless seamless steel pipe having higher performance.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The heating temperature of the billet before punching in the present invention is not particularly limited as long as it can be hot-punched by a punching machine in the subsequent stage, but the optimum temperature is in consideration of high temperature ductility and high temperature strength. , Usually from 1100 ° C to 1300 ° C
Between. In order to carry out billet heating with high efficiency, it is preferable that the billet length be as long as possible, and a tube cutting machine may be installed in the latter stage to cut the tube and then the tube is produced.

In the finish rolling in the present invention, the finishing process is carried out continuously before the processing strain applied to the hollow shell after perforation is recovered by the stretching process. Both the stretching process and the finishing process are integrated. It was made into finish rolling. In order to perform the finishing process before the processing strain imparted by the stretching process to the hollow shell after drilling is recovered, for example, a mandrel mill as a stretching machine that has been conventionally arranged independently, and a finishing machine. The extractor sizer can be used as an integrated continuous arrangement.
As described above, by immediately performing the finishing process by the extracting sizer before the processing strain imparted by the stretching process by the mandrel mill is recovered, the subsequent recrystallized grains can be made finer. That is,
Even when pipes are made on the same pass schedule, there is a difference in the grain size after recrystallization between when the stretching machine and finishing machine are placed separately and when they are integrated. Finer particles can be obtained.

In the present invention, the average strain rate of finish rolling is 0.01 / sec or more, the cross-section reduction rate is 10% or more, and the finishing temperature is 800 to 1050 ° C. If defined as (strain + processing strain in finishing) / (time from the start of stretching at the tip of the hollow shell to the end of finishing), it will recrystallize between each pass at 0.01 / sec or less. This is because strain is not accumulated, and the miniaturization effect by recrystallization in the subsequent process cannot be obtained. Further, when the cross-sectional reduction rate is 10% or less, recrystallization in the subsequent process does not proceed smoothly, and the miniaturization effect cannot be obtained. Furthermore, the finishing temperature is 800-1
When the temperature is 050 ° C., the recrystallization in the latter stage process proceeds smoothly, and the miniaturization effect is great.

The present invention is characterized in that recrystallization and solution treatment are carried out between a finish rolling machine composed of an integrated mandrel mill and an extracting sizer and a direct quenching apparatus. by this,
Recrystallization is induced by a combination of stretching and finishing with an integrated mandrel mill and an extracting sizer and slow cooling, heating, and heat retention, which makes it possible to reduce the crystal grain size. In the case of slow cooling, an air cooling rate or less is necessary, and preferably 0.5 ° C./second or less. As a method for gradually cooling the material, for example, a specific method may be used, such as covering the conveying path between the outlet of the extractor sizer and the heating device with a cover of a heat insulating material or a cover of a mirror surface that reflects radiant heat.

Further, the heating temperature for the recrystallization and solution treatment after the finish rolling of the stretching process and the finishing process is 1000 ° C. or higher and 1
The reason why the temperature is 150 ° C. or lower and the time is 10 seconds or longer and 30 minutes or shorter is that if the temperature is less than 1000 ° C. and less than 10 seconds, recrystallization does not proceed, and the cooling start temperature in the direct quenching device cannot be secured at the solution temperature. Carbide does not form a solid solution, and 1
This is because the crystal grains grow to become coarse grains by heating at more than 150 ° C. for more than 30 minutes. However, the heating temperature is 1150
Even if the temperature is less than or equal to ℃, if heating above 1100 ℃,
Since the crystal grains tend to coarsen slightly, it is desirable to set the heating temperature to 1100 ° C. or lower in order to have more preferable performance. By heating in this heating temperature range, the cooling start temperature can be ensured at a solution temperature at which Cr carbide does not precipitate, and further, the uniform temperature property between the longitudinal direction and the lot of the austenitic stainless seamless steel pipe is guaranteed, and the quality performance is improved. The variation can be significantly reduced.

Cooling after recrystallization and solution treatment requires cooling from a temperature of 900 ° C. or higher to a cooling rate of 1.5 ° C./sec or higher so that Cr carbide is not precipitated during cooling. The cooling end temperature is not particularly specified, but is preferably 500 ° C. or lower, which is the precipitation end temperature of Cr carbide. Although the cooling method is not particularly specified, water that is easy to regenerate after cooling in an online facility and is inexpensive is desirable as the cooling medium. It is desirable that the cooling device has a rotating mechanism effective for preventing bending of the austenitic stainless seamless steel pipe.

[0017]

EXAMPLES Austenitic stainless steels A to D having the chemical composition shown in Table 1 were melted by a usual method to obtain an inner diameter of 90 m.
The round billet obtained by casting in m
After inserting into a heating furnace at 1250 ° C and holding for 1 hour, piercing and rolling using a piercer to form a hollow shell, finish rolling (stretching and finishing) under the test conditions shown in Table 2, recrystallization solution treatment Treated and cooled to an outer diameter of 100 mm and a wall thickness of 10 m
m austenitic stainless seamless steel pipe was manufactured. A test piece was cut out from each obtained seamless steel pipe, a tensile test was performed, and the corrosion resistance was measured by an austenite grain size and a corrosion test. Table 3 shows the results. In addition, the asterisk * in each column in Table 2 indicates outside the range of the manufacturing conditions of the present invention, and the test No. The recrystallization solution treatment of 27 and 28 and the cooling conditions are offline conditions. Further, the tensile test is carried out according to the metal material tensile test method specified in JIS Z2241, the austenite grain size is measured according to the austenite grain size test method of steel specified in JIS G0551, and the corrosion test is J
The presence or absence of corrosion was measured according to the 10% oxalic acid etch test method for stainless steel specified in IS G0571.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

Austenitic stainless seamless steel pipes are mainly used for boilers and heat exchangers, and are required to have excellent high temperature strength and corrosion resistance. In order to deal with these, the test No. As shown in the conventional examples of Nos. 27 and 28, the yield point at room temperature is adjusted to around 300 N / mm ² and the tensile strength is adjusted to around 600 N / mm ² . As shown in Table 3, the test No. of the present invention example. Test Nos. 1 to 17 are Nos. Two
The strength and austenite grain size are the same as those of the conventional examples of Nos. 7 and 28, and there is no corrosion in the 10% oxalic acid etch test, which satisfies the requirements.

On the other hand, the test No. of the comparative example. 18, 2
Nos. 0 to 24 are insufficient in processing strain rate and cross-section reduction rate in finish rolling, high temperature finishing, recrystallization does not proceed by low temperature reheating, and crystal grains grow and coarsen due to high temperature heating to become coarse grains. ing. Test No. of the comparative example. 19 is
Since the finishing temperature was low and the deformation resistance of the material to be rolled was large, defects at a defective level occurred frequently. Further, the test No. of the comparative example. In Nos. 21, 22, 25 and 26, Cr carbide remained or precipitated before and after reheating, so that intergranular corrosion occurred.

[0023]

EFFECTS OF THE INVENTION The method of the present invention makes it possible to produce an austenitic stainless steel pipe having excellent strength, toughness and corrosion resistance by efficiently performing thermomechanical treatment in a slim and highly productive mill line. Reduction of heat treatment cost,
By omitting equipment, shortening the process, etc., it is possible to achieve a large industrial cost reduction.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shunji Abe 1850 Minato Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd. Wakayama Works

Claims (1)

[Claims] 1. A method for producing an austenitic stainless steel seamless pipe by heating and billeting an austenitic stainless steel billet in a hot manner, the method comprising the steps of: After finishing rolling of 0.01 / sec or more and 10% or more in sectional compression rate at a finishing temperature of 800 ° C. or more and 1050 ° C., immediately 1000
Manufacture of an austenitic stainless steel pipe characterized by being reheated to a temperature of not less than 1 ° C and not more than 1150 ° C, held for not less than 10 seconds and not more than 30 minutes, and then cooled at a cooling rate of not less than 900 ° C and not less than 1.5 ° C / sec. Method. Average strain rate = (processing strain in stretching + processing strain in finishing) / (time from the start of stretching to the end of finishing at the tip of the hollow shell)