JP3694967B2 - Method for producing martensitic stainless steel seamless steel pipe - Google Patents

Description

【００３１】
【表２】

【００３２】
得られた鋼管について、管端から長手方向に３ｍおきの３カ所およびこれらの各位置について円周方向に４等分した位置の合計12カ所から管軸方向に引張試験片、シャルピー衝撃試験片および耐食性試験片を採取し、下記の試験を行い機械的性質および耐食性を調べた。
【００３３】
引張試験は、直径４mm、平行部 34mm の丸棒試験片を用いて行った。シャルピー衝撃試験は、５mm×10mm×55mmの２mmＶノッチ試験片を用い、０℃での衝撃値で評価した。耐食性 (耐ＳＳＣ性）は、NACE TM 0177 METHOD-A に規定された定荷重試験に従い、45kgf/mm² の応力を負荷し、「30atm. CO₂＋0.01 atm. H₂S ＋ 5％NaCl」の溶液に浸漬し、200 時間後の割れの有無によって評価した。試験結果を表３に示す。
【００３４】
表３では、降伏強度と衝撃値は上記の12カ所の試験片による最大値（Ｍ）、最小値（ｍ）、平均値、およびバラツキ（Ｍ−ｍ）で示した。また、異方性は衝撃試験片の破面にセパレーションが発生しているか否かで示した。耐ＳＳＣ性は、12カ所からの試験片のうち、何本が合格（割れ発生無し) であったか、により評価した。
【００３５】
【表３】

【００３６】
表３から次の事実が明らかである。
【００３７】
1) 本発明例である試番１から14までは、強度、靱性ともに良好であり、鋼管の部位によるそれらの値の差異は極めて小さい。即ち、バラツキが小さい。また、上記本発明例の衝撃試験片の破面には異方性の指標となるセパレーションが見られない。
【００３８】
2) 耐ＳＳＣ性試験では、12本の試験片の全てに割れが無く、耐ＳＳＣ性も良好である。
【００３９】
3) 従来例である試番22〜28では、鋼管の部位による機械的性質、特に強度のバラツキが大きい。また、耐ＳＳＣ性試験でも全数合格には到っていない。
【００４０】
4) 試番15〜21は、製管および熱処理の条件のどれかが本発明で定める条件を満たしていない比較例である。これらのうち、試番15は延伸加工と仕上げ加工の合計加工度が 5％と小さく、試番16は仕上げ温度が高過ぎて、いずれもオーステナイト結晶粒が粗大になり、靱性および耐ＳＳＣ性が劣る。
【００４１】
5) 試番17は、補熱温度が低く過ぎて炭化物が粗大に成長し、かつフェライト変態が起きたために強度が低く、靱性および耐ＳＳＣ性が劣る。
【００４２】
6) 試番18は、ｆｎの値が小さ過ぎたために再結晶が十分でなくセパレーションが観察された。即ち、異方性が大きい。他方、試番19は、ｆｎの値が大き過ぎてオーステナイト結晶粒が粗大化したため、靱性および耐ＳＳＣ性が劣る。
【００４３】
7) 試番20は、補熱後の 600℃までの冷却速度が小さいので、粗大炭化物の析出によって靱性および耐ＳＳＣ性が低下している。一方、試番21は、冷却終了温度が高過ぎたためにマルテンサイト変態が完了しない状態で焼戻されてしまい、強度が低く、靱性および耐ＳＳＣ性も劣っている。
【００４４】
【発明の効果】
実施例からも明らかなとおり、本発明方法によって製造したマルテンサイト系ステンレス鋼の継目無鋼管には、従来の直接焼入れ法で製造された鋼管の難点であった特性のバラツキが殆どなく、かつ異方性もない。本発明方法は、製管から熱処理まで、連続的にオンラインで実施できるので、継目無鋼管の製造における生産性の向上と製造コストの低減にも大きく寄与する。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a seamless steel pipe having high strength and excellent toughness and corrosion resistance using stainless steel having a substantially martensitic structure at room temperature.
[0002]
[Prior art]
A martensitic stainless steel seamless steel pipe is conventionally made into a product after pipe-making and then subjected to quenching-tempering heat treatment. In this method, since the steel pipe once cooled after pipe making must be reheated and quenched, there are many steps and energy consumption is also large. Therefore, an attempt has been made to apply so-called “direct quenching” employed in the production of seamless steel pipes of ordinary steel and low alloy steel to the production of martensitic stainless steel seamless steel pipes.
[0003]
For example, Japanese Patent Publication No. 5-45651 discloses a method in which a steel pipe after being manufactured by the mandrel mill method is cooled to room temperature as it is and then tempered under specific conditions. Discloses a method in which martensitic stainless steel (steel plate, steel pipe, etc.) after hot working is immediately cooled in two stages as it is.
[0004]
However, in steel pipes obtained by these methods, the formation of texture is significant, and it overlaps with the effect of chromium carbide precipitated at the grain boundaries, resulting in mechanical properties such as toughness, sulfide stress crack resistance (SSC resistance). ) And other corrosion resistance are markedly anisotropic.
[0005]
The present inventor proposed a method of performing hot working under conditions of complete recrystallization after finish rolling and directly quenching as a method of solving the above-described anisotropy problem (Japanese Patent Laid-Open No. 4104420, See). However, since the recrystallization temperature of martensitic stainless steel is significantly higher than that of low alloy steel, it may be difficult to completely recrystallize depending on the product size in a normal seamless steel pipe making mill. Further, even a steel pipe of a size that can be finished at a high temperature above the recrystallization temperature has temperature unevenness depending on the part of the steel pipe, and this has an undesirable effect on the product.
[0006]
In the production process of the seamless steel pipe, the hollow shell or the rolled steel pipe does not come into uniform contact with the entire surface of the rolling roll, the beam of the conveying line, or the like. Therefore, the cooling situation differs depending on the part (position in the longitudinal direction and the circumferential direction) of one steel pipe, and a considerable temperature difference occurs. If such a steel pipe is quenched as it is, depending on the part, it will be quenched without recrystallization, and as a result, an anisotropic part or a part with different mechanical properties and corrosion resistance is generated in one steel pipe. Resulting in. In other words, the product steel pipe cannot withstand practical use with many variations in characteristics.
[0007]
[Problems to be solved by the invention]
The object of the present invention is to perform the conventional quenching-tempering process, which is performed by reheating in a separate line after the pipe making, following the pipe making in the pipe making line, and anisotropy in all product sizes. Another object of the present invention is to provide a method for producing a martensitic stainless steel seamless steel pipe excellent in strength and corrosion resistance.
[0008]
[Means for Solving the Problems]
The present invention is a method for producing a seamless steel pipe using stainless steel having a substantially martensitic structure at room temperature, wherein the following steps (1) to (3) are sequentially performed. The gist is a method for manufacturing a steel pipe.
[0009]
(1) Stretching and finishing of the hollow shell are performed at a total processing degree of both processes at 40% or more at a finishing temperature of 800 to 1100 ° C to make a seamless steel pipe.
(2) The above seamless steel pipe is charged into a reheating furnace, and the temperature T (° C.) and the time t (hr) at which the value of fn defined by the following equation (a) is between 22000 and 27000. ) Process of performing supplementary heat in
(3) A step of tempering the seamless steel pipe taken out from the reheating furnace to 200 ° C. or less at a cooling rate of 10 ° C./min or more up to 600 ° C. and then tempering at 500 to 780 ° C.
[0010]
fn = (T + 273) × (21 + logt) (a)
However, T ≧ 800 (° C.).
[0011]
The “stainless steel that has a substantially martensitic structure at room temperature” as a target of the method of the present invention is a stainless steel that has a martensite-based structure (may contain δ ferrite of less than 50%) at room temperature. is there. Although there is no restriction | limiting in particular in the chemical composition, it will be as follows if a general component and its content are illustrated (% is weight%).
[0012]
C: 0.001 to 1.2%, Si: 1% or less,
Mn: 2% or less, Cr: 8-17%,
sol.Al: 0.005 to 0.1%,
P and S: 0.05% or less,
Mo: 0 to 3%, Ni: 0 to 8%,
Cu: 0 to 5%, N: 0.001 to 0.15%,
B: 0 to 0.01%
Ti, Nb, V: 0-0.5% each
Ca, Mg, Y, rare earth elements (La, Ce, etc.): 0 to 0.01% each.
[0013]
In addition to these alloy elements, an appropriate amount of other alloy elements may be contained.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, each process of the method of the present invention will be described in order. The pipe making material (billet) may be a billet manufactured by ingot or continuous casting slab, bloom, etc., through ingot rolling or forging. Can be attached.
[0015]
Production of a hollow shell (hollow shell) to be subjected to drawing and rolling, that is, drilling may be performed by any method. For example, it can be performed by a so-called piercer such as a tilt roll mill. The drilling conditions may be basically the same as in the case of manufacturing a normal martensitic stainless steel seamless steel pipe. However, a large mill power is required to roll a thick hollow shell with a high degree of processing in the next process of drawing and rolling. Therefore, in order to increase the workability of the rolling process in the next process, it is preferable to make it as thin as possible in the drilling process. For example, a method of punching using a piercer of a type in which the piercer has a cone shape and can be expanded and thinned with a roll having a crossed angle is recommended.
[0016]
(1) Stretching and finishing process:
There are various types of equipment for performing this processing, such as a Mannesmann mandrel mill method and a Mannesmann plug mill method. Any method can be adopted in the method of the present invention. For example, in the mandrel mill system, stretching is performed by a mandrel mill, and finishing is performed by a sizer or a reducer.
[0017]
Drawing and finishing are low-temperature processing compared to drilling, and are important processing steps for crystal grain refinement. When the finishing temperature in these processes is lower than 800 ° C., coarse chromium carbide that does not sufficiently dissolve even with subsequent supplemental heat is deposited, and the SSC resistance and toughness of the product steel pipe are deteriorated. On the other hand, when the temperature exceeds 1100 ° C., the crystal grains become coarse, and the SSC resistance and toughness are also lowered. Therefore, the finishing temperature must be 800-1100 ° C. The finishing temperature is preferably as low as about 800 to 900 ° C. from the viewpoint of the refinement of the structure.
[0018]
If the total degree of drawing and finishing is less than 40%, the crystal grains are not sufficiently refined. The upper limit of the degree of processing is not particularly limited, but if it exceeds 90%, the load on the tool is large, so it is preferable to set it in the range of 40 to 90%.
[0019]
From the viewpoint of crystal grain refinement, the interval between the drawing process and the finishing process should be as short as possible. That is, if the finishing process is performed before the dislocations introduced at the time of the stretching process are recovered, and the strain is sufficiently accumulated, the effect of refining by recrystallization is great.
[0020]
In the processing as described above, the distance between the rolling mill (for example, a mandrel mill) that performs stretching processing and the rolling mill (for example, a sizer or reducer) that performs finishing processing is shorter than the length of the hollow element tube processed in the former. It can be implemented using equipment installed at intervals. For example, a rolling process is preferred in which finish rolling is immediately performed by an extractor sizer and at the same time the bar is pulled out from the hollow shell.
[0021]
(2) Heating process:
The supplementary heat is performed by charging the seamless steel pipes that have been produced into a supplementary heating furnace provided in the production line. This supplementary heat can be achieved by recrystallizing the steel pipe that has been strained in the previous processing to form a fine structure, solidifying the chromium carbide precipitated during the rolling process, and heating the steel pipe uniformly to vary the characteristics. There are many purposes to reduce local anisotropy. By using the auxiliary heat furnace, not only the temperature of the entire tube is made uniform, but also the temperature can be accurately adjusted, and there is an advantage that the heat treatment conditions can be selected in accordance with the characteristics desired for the product.
[0022]
When the temperature of supplementary heat is lower than 800 ° C, chromium carbide precipitates and becomes coarse.
[0023]
Therefore, it is necessary to perform supplementary heat above 800 ℃. That is, T (° C.) in the above formula (a) must be 800 or more.
[0024]
In the case of recrystallization at a high temperature, since the growth and coarsening of crystal grains start immediately after recrystallization, it is necessary to supplement heat for a short time. The relationship between the temperature T (° C.) for supplementary heat and the time t (hr) needs to be adjusted so that the value of fn in the formula (a) derived from the activation energy of recrystallization is 22000 to 27000. . Reheating is not completed completely when the fn value is less than 22000, while recrystallization is not complete when the fn value exceeds 27000, and the SSC resistance and toughness of the product steel pipe deteriorate.
[0025]
The finishing temperature in the step (1) can be any of a case where the finishing temperature is higher than, a case where the finishing temperature is equal to or a case where the finishing temperature is lower. Accordingly, the supplementary heating in the method of the present invention can be any of slow cooling from the rolling finishing temperature, holding at substantially the same temperature as the finishing temperature, and heating from the finishing temperature (temperature increase). As long as the condition of the formula (a) is satisfied, there is no restriction on the heat pattern. If the heat is supplemented under the condition that satisfies the equation (a), the temperature of the entire tube can be made uniform.
[0026]
(3) Cooling process:
Since the martensitic stainless steel has high hardenability, it is sufficient to cool the steel pipe that has exited the auxiliary heat furnace so long as it does not precipitate chromium carbide. Cool at a cooling rate of 10 ° C / min or more to at least 600 ° C, where chromium carbide is likely to precipitate. Thereby, the precipitation of carbides can be suppressed to an extent that does not cause a problem in practice. At a temperature lower than 600 ° C., it may be cooled to 200 ° C. or lower, which is a substantial Mf point, at an arbitrary cooling rate. However, it is preferable to completely cool down to room temperature in order to minimize residual austenite.
[0027]
Tempering is performed to temper the martensite structure generated by quenching and improve the toughness and SSC resistance of the product steel pipe. At temperatures lower than 500 ° C, the tempering effect is not sufficient, and at temperatures above 780 ° C, the strength decreases. Tempering time may be about 5 minutes to 1 hour.
[0028]
【Example】
Steels A to G shown in Table 1 were melted to produce billets having an outer diameter of 225 mm and rolled using a Mannesmann-mandrel mill to produce a steel pipe having an outer diameter of 273.1 mm and a wall thickness of 9.3 mm. The production conditions are as shown in Table 2. In the heat treatment after supplementary heating, cooling in the lower temperature range than 600 ° C. was air cooling, and the tempering temperature was reheated from each cooling end temperature. Since the steel pipe strength varies depending on the steel type, the tempering temperature was changed to adjust the yield strength (yield strength) to around 60 kgf / mm ² for any steel type.
[0029]
The conventional example in Table 2 is the same as the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-104420. The steel pipe after the recrystallization is finished by directly quenching the steel pipe after being processed at a sufficiently high temperature and subjected to tempering treatment. This is an example.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
About the obtained steel pipe, a tensile test piece, a Charpy impact test piece in the pipe axis direction from a total of 12 places of 3 places every 3 m in the longitudinal direction from the pipe end and a position obtained by dividing each of these positions into 4 parts in the circumferential direction, Corrosion resistance test specimens were collected and subjected to the following tests to examine mechanical properties and corrosion resistance.
[0033]
The tensile test was performed using a round bar test piece having a diameter of 4 mm and a parallel part of 34 mm. The Charpy impact test was evaluated using an impact value at 0 ° C. using a 2 mm V notch test piece of 5 mm × 10 mm × 55 mm. Corrosion resistance (SSC resistance) is measured according to the constant load test specified in NACE TM 0177 METHOD-A, with a stress of 45 kgf / mm ² and “30 atm. CO ₂ +0.01 atm. H ₂ S + 5% NaCl. And was evaluated by the presence or absence of cracks after 200 hours. The test results are shown in Table 3.
[0034]
In Table 3, the yield strength and the impact value are shown by the maximum value (M), the minimum value (m), the average value, and the variation (M−m) by the above 12 test pieces. Anisotropy was indicated by whether or not separation occurred on the fracture surface of the impact test piece. The SSC resistance was evaluated based on how many of the test pieces from 12 locations passed (no cracking occurred).
[0035]
[Table 3]

[0036]
From Table 3, the following facts are clear.
[0037]
1) Sample Nos. 1 to 14, which are examples of the present invention, have good strength and toughness, and the difference in their values depending on the portion of the steel pipe is extremely small. That is, the variation is small. Further, no separation as an anisotropy index is observed on the fracture surface of the impact test piece of the present invention.
[0038]
2) In the SSC resistance test, all 12 specimens are not cracked and the SSC resistance is good.
[0039]
3) In the conventional samples Nos. 22 to 28, the mechanical properties, particularly the strength, vary greatly depending on the part of the steel pipe. In addition, the SSC resistance test has not passed all the tests.
[0040]
4) Test numbers 15 to 21 are comparative examples in which any of the conditions for pipe making and heat treatment does not satisfy the conditions defined in the present invention. Of these, trial No. 15 has a small total degree of drawing and finishing of 5%, and trial No. 16 has a finishing temperature that is too high, both of which have coarse austenite grains, and have good toughness and SSC resistance. Inferior.
[0041]
5) Test No. 17 has a low heat resistance temperature, carbides grow coarsely, and ferrite transformation has occurred, resulting in low strength and poor toughness and SSC resistance.
[0042]
6) In Test No. 18, separation was observed because the value of fn was too small and recrystallization was not sufficient. That is, the anisotropy is large. On the other hand, since the austenite crystal grains coarsened in the trial number 19 because the value of fn was too large, the toughness and the SSC resistance were inferior.
[0043]
7) In Test No. 20, the cooling rate to 600 ° C. after the supplemental heating is small, so the toughness and SSC resistance are reduced by the precipitation of coarse carbides. On the other hand, the trial number 21 was tempered in a state where the martensitic transformation was not completed because the cooling end temperature was too high, the strength was low, and the toughness and the SSC resistance were inferior.
[0044]
【The invention's effect】
As is apparent from the examples, the martensitic stainless steel seamless steel pipe produced by the method of the present invention has almost no variation in characteristics, which is a difficulty of the steel pipe produced by the conventional direct quenching method. There is no direction. Since the method of the present invention can be continuously performed on-line from pipe making to heat treatment, it greatly contributes to the improvement of productivity and the reduction of manufacturing cost in the production of seamless steel pipes.

Claims (1)

A method for producing a seamless steel pipe using stainless steel having a substantially martensitic structure at room temperature, wherein the following steps (1) to (3) are sequentially performed: .
(1) Stretching and finishing of the hollow shell are performed at a total processing degree of both processes at 40% or more at a finishing temperature of 800 to 1100 ° C to make a seamless steel pipe.
(2) The above seamless steel pipe is charged into a reheating furnace, and the temperature T (° C.) and the time t (hr) at which the value of fn defined by the following equation (a) is between 22000 and 27000. ) Process of performing supplementary heat in
(3) A step of tempering the seamless steel pipe taken out from the reheating furnace to 200 ° C. or less at a cooling rate of 10 ° C./min or more up to 600 ° C. and then tempering at 500 to 780 ° C.
fn = (T + 273) × (21 + logt) (a)
However, T ≧ 800 (° C.).