JP2004027351A - Manufacturing method of high strength and high toughness martensitic stainless steel seamless pipe - Google Patents

Abstract

A method for producing a martensitic stainless steel seamless pipe having high strength, high toughness, and low anisotropy is proposed.
SOLUTION: A martensitic stainless steel material is heated to an austenite region, and is made into a base tube by piercing and elongation rolling, and then cooled to have a substantially martensitic structure. After reheating, finish rolling is performed to set the rolling start temperature T to a temperature within the range of the Ac ₁ transformation point to the Ac ₃ transformation point, and then cooled to obtain a product tube, and then tempered at a temperature equal to or lower than the Ac ₁ transformation point. . In addition, the rolling start temperature T is a temperature within the range of the Ac ₁ transformation point to the Ac ₃ transformation point, the cross-section reduction rate R is 10 to 90%, and the rolling start temperature T and the cross-section reduction rate R are 800 ≦ T− It is preferable to perform finish rolling that satisfies 0.625R ≦ 850.
[Selection diagram] Fig. 1

Description

【００４０】
【表２】

【００４１】
【表３】

【００４２】
本発明例はいずれも、高い強度（ＹＳ：５５０ＭＰａ以上）と高い靭性（Ｌ方向の（Ｅ_−４０）_Ｌ：１８０　Ｊ／ｃｍ^２以上）を有し、さらに靭性（衝撃値）のＣ方向とＬ方向の比、（Ｅ_−４０）_Ｃ／（Ｅ_−４０）_Ｌが０．８０以上を示しており、従来例（鋼管Ｎｏ．　８）、比較例に比べて、異方性の少ない高強度高靭性鋼管となっている。一方、本発明の範囲から外れる比較例では、Ｌ方向の靭性（衝撃値）が劣化しているか、あるいはＣ方向の靭性（衝撃値）向上が少なく、また（Ｅ_−４０）_Ｃ／（Ｅ_−４０）_Ｌが０．８０未満と靭性の異方性が大きくなっている。
【００４３】
【発明の効果】
本発明によれば、高強度・高靭性でかつ異方性の少ないマルテンサイト系ステンレス鋼継目無管が安価にかつ安定して製造でき、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】靭性および靭性の異方性におよぼす仕上圧延開始温度と断面減少率との影響を示すグラフである。
【図２】本発明の製造工程の概略を示す説明図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a martensitic stainless steel seamless pipe having excellent corrosion resistance and suitable for oil country tubular goods, and more particularly to improvement in toughness and improvement in toughness anisotropy.
[0002]
[Prior art]
In recent years, in view of soaring crude oil prices, the depletion of petroleum resources expected in the near future, in consideration of deep oil fields that were not previously excluded, and highly corrosive sour gas fields that were once abandoned, etc. Has become active on a global scale.
Such oil fields and gas fields generally have extremely deep depths, high temperatures, and severe corrosive environments including CO ₂ , Cl ⁻ and the like. Therefore, oil country tubular goods used in such oil fields and gas fields are required to be made of a material having high strength, high toughness, and corrosion resistance. Generally, a martensitic stainless steel seamless tube containing 13% Cr excellent in CO ₂ corrosion resistance is often used in a severe corrosive environment containing CO ₂ , Cl ^{− and the} like.
[0003]
Conventionally, a martensitic stainless steel seamless pipe is heated to a temperature at which the steel material can be pierced, then pierced and rolled by a piercer mill, and stretched and rolled by a mandrel mill or plug mill to obtain a raw tube, and then reheated to an austenite region. For example, it was usual that finish rolling was performed by a stretch reducer or a sizer to obtain a product tube. While air-cooled after finish rolling, the seamless pipe has a martensitic structure, but after finish rolling, the seamless pipe is usually quenched from the austenite region and given a temperature not higher than the Ac ₁ transformation point in order to impart necessary strength and toughness. Tempered.
[0004]
However, with the recent deterioration of the oil well environment, the required characteristics of the oil well pipe used have been further enhanced, and in particular, there has been a strong demand for steel pipes for oil well pipes having excellent low-temperature toughness and sulfide stress corrosion cracking resistance. I have.
In response to such a demand, for example, Japanese Patent Application Laid-Open No. Hei 1-23025 discloses a process in which a martensitic stainless steel slab is heated to a temperature of 1050 to 1250 ° C. to perform piercing and rolling, and a step of heating at least 500 ° C. to 30 ° C. / Cooling to a temperature below the martensite transformation start temperature as a cooling rate per minute to form a structure in which 80% by volume or more is occupied by martensite, and (Ac ₁ transformation point) to (Ac ₁ transformation point -200 ° C.) A step of reheating to a temperature range and performing finish rolling at a cross-sectional reduction rate of 5% or more, and holding at the finish rolling end temperature, or immediately after finishing finish rolling, reheating to a temperature below the Ac ₁ transformation point, and then air cooling. Alternatively, a method for producing a martensitic stainless steel seamless pipe in which a step of forcibly cooling is sequentially performed is described. According to the technique described in Japanese Patent Application Laid-Open No. 1-123025, after the step of forming a structure occupied by martensite, reheating is performed to a temperature range of (Ac ₁ transformation point) to (Ac ₁ transformation point -200 ° C.). It is stated that a step of performing finish rolling at a cross-sectional reduction rate of 5% or more and air cooling or forcibly cooling, and a step of heating to an Ac ₁ transformation point or less and then air cooling or forced cooling may be performed.
[0005]
[Problems to be solved by the invention]
However, in the seamless pipe manufactured by the technique described in Japanese Patent Application Laid-Open No. 1-123025, the structure is elongated in the rolling direction because the rolling is performed in the non-recrystallization temperature range, and therefore, the characteristics in the rolling direction are excellent. However, there is a problem that properties in the circumferential direction are deteriorated, and remarkable anisotropy is observed in toughness and corrosion resistance.
[0006]
An object of the present invention is to advantageously solve the above-mentioned problems of the prior art, and to provide a method for producing a martensitic stainless steel seamless pipe having high strength, high toughness, and low anisotropy in properties. In the present invention, “high strength” means that the yield strength YS has 551 MPa or more, and “high toughness” means that the Charpy impact value (absorbed energy per unit cross-sectional area) at −40 ° C. is 90%. J / cm ² or more.
[0007]
[Means for Solving the Problems]
The present inventors have diligently studied the effect of finish rolling conditions on toughness in order to achieve the above-mentioned object. As a result, before the finish rolling, the raw tube having a martensitic structure was reheated to the two-phase region of ferrite (α) + austenite (γ), and then the rolling start temperature and the area reduction rate (reduction rate) were determined. It was newly found that a product pipe having a fine and less anisotropic martensitic structure can be obtained by subjecting finish rolling to cooling under appropriate conditions with the relationship within a specific range, followed by cooling and then tempering.
[0008]
The present invention has been completed based on the above findings, with further investigations.
That is, the present invention is to heat the martensitic stainless steel material to the austenite region, and to perform a raw tube production step of forming a raw tube by piercing rolling and elongating rolling, and then perform finish rolling by reheating the raw tube, a rolling process finishing the product pipe having a predetermined size was cooled, the finish after rolling process, a tempering step of subjecting the tempering treatment at a temperature below the Ac ₁ transformation point to said product tube, sequentially subjected martensitic stainless steel In the method for manufacturing a seamless pipe, the raw pipe is cooled to a substantially martensitic structure after the elongation rolling, and the reheating temperature of the raw pipe in the finish rolling step is (Ac ₁ transformation point) to (Ac ₍₃ transformation points), and the rolling start temperature T of the finish rolling is a temperature in the range of (Ac ₁ transformation point) to (Ac ₃ transformation point). Ma It is a manufacturing method of the ten sites stainless steel seamless pipe. In the present invention, the cross-sectional reduction rate R of the finish rolling is set to 10 to 90%, and the relationship between the rolling start temperature T of the finish rolling and the cross-sectional reduction rate R is expressed by the following equation (1): 800 ≦ T−0 .625R ≦ 850 (1)
(Where, T: rolling start temperature of finish rolling (° C.), R: reduction ratio of finish rolling section (%))
Is preferable.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
As the martensitic stainless steel used as the steel material in the present invention, all commonly known martensitic stainless steels can be suitably used. The preferred composition of the martensitic stainless steel material suitable for the present invention is as follows. In addition, the mass% in the composition is simply described as%. As a martensitic stainless steel material, C: 0.30% or less, Si: 1.00% or less, Mn: 0.05 to 2.00%, P: 0.03% or less, S: 0.005% Hereinafter, it is preferable that the composition contains Cr: 10.0 to 15.0% and Al: 0.05% or less, and has a composition including the balance of Fe and inevitable impurities. In addition to the above composition, one or more of Ni: 7.0% or less, Mo: 3.0% or less, Cu: 3.0% or less, and / or Nb: 0.2% or less , V: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005 to 0.01%, N: 0.07% or less Or more, and / or one or two of Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.01%.
[0010]
Next, the reasons for limiting the preferable composition of the steel material will be described.
C: 0.30% or less C is an element necessary for securing the strength of the martensitic stainless steel pipe. In the present invention, it is preferable that the content of C is 0.005% or more, but more than 0.30%. When it is contained, coarse carbides increase and the toughness decreases, and the corrosion resistance decreases. Therefore, in the present invention, C is preferably limited to 0.30% or less. In order to obtain more stable corrosion resistance, C is more preferably set to 0.22% or less.
[0011]
Si: 1.00% or less Si is an element necessary as a deoxidizing agent in a normal steelmaking process, and it is preferable to contain 0.10% or more, but if it exceeds 1.00%, the toughness is reduced. Also reduces hot workability. For this reason, Si is preferably limited to 1.00% or less. In addition, more preferably, it is 0.10 to 0.50%.
[0012]
Mn: 0.05-2.00%
Mn is an element necessary for securing the strength of the martensitic stainless steel pipe. In the present invention, Mn needs to be contained at 0.05% or more. However, if it exceeds 2.00%, the toughness is adversely affected. Exert. For this reason, Mn is preferably limited to the range of 0.05 to 2.00%. In addition, more preferably, it is 0.30 to 1.60%.
[0013]
P: 0.03% or less P is an element that deteriorates both corrosion resistance, sulfide stress corrosion cracking resistance and hot workability, and it is desirable to reduce it as much as possible. However, extreme reduction leads to a rise in manufacturing costs. . Therefore, P is set to 0.03% or less, which is a range that can be implemented industrially at relatively low cost and does not deteriorate the corrosion resistance and the sulfide stress corrosion cracking resistance.
[0014]
S: 0.010% or less S is an element that significantly degrades hot workability, and is reduced as much as possible to improve productivity in steel pipe production, or to further improve toughness and stress corrosion cracking resistance. However, an extreme reduction leads to an increase in manufacturing costs. If it is reduced to 0.010% or less, steel pipe can be manufactured in a normal process. Therefore, in the present invention, the upper limit of S is set to 0.010%. Incidentally, the content is preferably 0.005% or less.
[0015]
Cr: 10.0 to 15.0%
Cr is a main element for maintaining corrosion resistance and stress corrosion cracking resistance. From the viewpoint of corrosion resistance, Cr needs to be contained at least 10.0%, but if it exceeds 15.0%, it becomes hot. Workability deteriorates. For this reason, Cr was limited to the range of 10.0 to 15.0%.
Al: 0.05% or less Al is an element having a strong deoxidizing effect, and is preferably contained in the present invention in an amount of 0.001% or more. Increases, adversely affecting toughness. For this reason, Al was limited to 0.05% or less.
[0016]
In addition to the components described above, one or more of Ni, Mo, and Cu, and one or more of Nb, V, Ti, Zr, B, and N can be further contained as necessary.
One or more of Ni: 7.0% or less, Mo: 3.0% or less, and Cu: 3.0% or less Ni, Mo, and Cu are elements having an effect of improving corrosion resistance. Yes, it can be selected and contained as needed.
[0017]
Ni is an element that enhances corrosion resistance and greatly improves strength and toughness. Such an effect is remarkably recognized at a content of 1.0% or more, but is included at a content exceeding 7.0%. However, an effect commensurate with the content cannot be expected.
Mo is an element that increases resistance to pitting corrosion and improves corrosion resistance. Such an effect is remarkably recognized at a content of 0.1% or more, while a content exceeding 3.0% causes generation of δ-ferrite, corrosion resistance, stress corrosion cracking resistance and hot working. Decreases workability.
[0018]
Cu is an element that strengthens the protective coating and enhances the corrosion resistance. Such an effect is remarkably recognized at a content of 0.1% or more, but when the content exceeds 3.0%, hot working is performed. Is reduced.
Nb: 0.2% or less, V: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005 to 0.01%, N: 0.07% or less One or more of the above,
Nb, V, Ti, Zr, B, and N all have the effect of improving strength and toughness, and can be contained as necessary. However, when the content exceeds Nb: 0.2%, V: 0.2%, Ti: 0.3%, Zr: 0.2%, B: 0.01%, and N: 0.07%, the toughness increases. And the corrosion resistance is reduced.
[0019]
One or two of Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.01%. Each of Ca and REM has an effect of spheroidizing inclusions. 0.0005% or more, REM: 0.0005% or more. On the other hand, if the content of Ca exceeds 0.01% and the content of REM exceeds 0.01%, the toughness and the corrosion resistance decrease.
The balance other than the components described above consists of Fe and inevitable impurities.
[0020]
In the present invention, preferably, the martensitic stainless steel having the above composition is melted by a generally known melting method such as a converter, and then cast into a slab (slab) by a continuous casting method. The slab is preferably rolled into a billet to be used as a raw material for pipe production. The billet may be directly formed into a billet by a continuous casting method, and may be used as a raw material for producing a pipe.
[0021]
FIG. 2 shows an outline of the manufacturing process of the present invention.
In the present invention, preferably, a martensitic stainless steel material (billet) having the above-described composition is first heated to the austenite region, and subjected to a blank tube manufacturing step of forming a blank tube by piercing rolling and elongating rolling.
The heating of the martensitic stainless steel material is preferably performed in the austenitic range of 1100 to 1300 ° C. If the heating temperature is lower than 1100 ° C., the deformation resistance in the next step, piercing rolling and elongation rolling, increases, while if it exceeds 1300 ° C., δ ferrite is generated, and the hot workability and toughness are remarkably reduced. The generation of scale is remarkable, leading to a decrease in yield and a decrease in surface properties.
[0022]
For piercing and rolling, any of known piercer mills such as inclined rolling method (Mannesmann method) and press piercing method can be applied, and the method of piercing and rolling is not particularly limited. The perforated steel material is then drawn and rolled into a raw tube. For the elongation rolling, any of the generally known methods such as a mandrel mill and a plug mill can be applied, and the elongation rolling method is not particularly limited. The elongation rolling is preferably completed at a temperature of 800 ° C. or higher.
[0023]
In the present invention, after the elongation-rolling is completed, the raw tube is cooled to a martensite transformation start temperature (Ms point) or lower to substantially change the structure to a martensite structure. Here, “substantially have a martensite structure” refers to a state where the raw tube structure after cooling is composed of a martensite phase having an area ratio of 90% or more. The structure other than the martensite phase is an austenite phase up to about 10% or a ferrite phase up to about 2%. By making the structure of the raw tube before the finish rolling substantially a martensite structure, a fine recrystallized structure can be obtained by reheating after that. If a structure other than the martensite structure is used as the main phase, the subsequent reheating cannot make the structure of the raw tube into a fine recrystallized structure, and eventually a remarkable improvement in toughness cannot be obtained, or , Anisotropy of toughness becomes remarkable.
[0024]
The raw tube having a substantially martensitic structure is then reheated, finished into a product tube having a predetermined size by finish rolling, and then subjected to a finish rolling step of cooling.
For finish rolling, the tube is reheated to a two-phase region temperature of (Ac ₁ transformation point) to (Ac ₃ transformation point). By heating to the two-phase region, the martensite structure can be made into a fine recrystallized structure. In the present invention, the raw tube having such a fine recrystallized structure is finish-rolled into a product tube having a predetermined size. When the reheating temperature exceeds the Ac ₃ transformation point, the crystal grains eventually become coarse and no improvement in toughness is observed. On the other hand, when the reheating temperature is less than the Ac ₁ transformation point, the anisotropy of toughness becomes remarkable.
[0025]
In the present invention, at the time of finish rolling, the rolling start temperature T is set to a temperature within the range of (Ac ₁ transformation point) to (Ac ₃ transformation point). If the rolling start temperature T is lower than (Ac ₁ transformation point), the rolling temperature becomes too low, recrystallization becomes insufficient, and the rolling texture tends to remain, and the anisotropy of the characteristics tends to be remarkable. On the other hand, if the rolling start temperature T exceeds the (Ac ₃ transformation point), the rolling temperature is too high, and recrystallization proceeds excessively even after rolling. unacceptable. For this reason, the rolling start temperature T is limited to a temperature within the range of (Ac ₁ transformation point) to (Ac ₃ transformation point).
[0026]
In the finish rolling, the area reduction rate R is set to 10 to 90%, and the relationship between the rolling start temperature T and the area reduction rate R is expressed by the following equation (1): 800 ≦ T−0.625R ≦ 850 (1) )
(Here, T: rolling start temperature of finish rolling (° C.), R: finish rolling section reduction rate (%)) are preferably performed under conditions that satisfy the conditions. The cross-sectional reduction rate R (%) is the ratio of the difference between the cross-sectional area of the tube before finish rolling and the cross-sectional area of the product tube after finish rolling to the cross-sectional area of the tube before finish rolling, ｛(cross-sectional area of the tube before finish rolling) -(Cross-sectional area of product tube after finish rolling) / (cross-sectional area of raw tube before finish rolling).
[0027]
When the cross-sectional reduction rate in the finish rolling is less than 10%, the strain introduced by the rolling process is small, the degree of microstructure refinement after rolling is small, and the desired strength increase and toughness improving effect are small. On the other hand, when the cross-sectional reduction rate exceeds 90%, the tissue elongates and the anisotropy of the characteristics becomes remarkable. For this reason, it is preferable that the rolling reduction (cross-sectional reduction rate) of the finish rolling is limited to the range of 10 to 90%. In addition, it is more preferably 30% to 70%.
[0028]
In the finish rolling in the present invention, the rolling start temperature T is set within the above-mentioned range, the cross-section reduction rate R is further set within the above-described range, and the cross-section reduction rate R is adjusted so as to satisfy the formula (1). It is preferable to perform rolling while adjusting the rolling start temperature T.
FIG. 1 shows the toughness of the martensitic stainless steel seamless pipe in relation to the cross-sectional reduction rate R and the rolling start temperature T.
[0029]
The rolling start temperature T of the finish rolling is within the range of the present invention ((Ac ₁ transformation point) to (Ac ₃ transformation point)), and the relationship between the rolling start temperature T and the area reduction rate R satisfies the formula (1). In the region C, the Charpy impact value (absorbed energy per unit sectional area) at −40 ° C. in the tube axis direction (L direction) (E ₋₄₀ ) _L and −40 ° C. in the tube circumferential direction (C direction). Impact energy (absorbed energy per unit cross-sectional area) (E- ₄₀ ) _C is 180 J / cm ² or more, and anisotropy (E- ₄₀ ) _C / (E- ₄₀ ) _{L of} toughness is 0. .80 or more. That is, in the region C, the Charpy impact value (absorbed energy per unit cross-sectional area) at −40 ° C. is high, and high toughness with little anisotropy is shown.
[0030]
When the rolling start temperature T is within the range of the present invention ((Ac ₁ transformation point) to (Ac ₃ transformation point)), the left side of the expression (1) is not satisfied, that is, T−0.625R <800. In the area B, the Charpy impact value (absorbed energy per unit cross-sectional area) (E- ₄₀ ) _L in the pipe axis direction (L direction) is 180 J / cm ² or more, but the pipe circumferential direction (C Direction), the Charpy impact value (absorbed energy per unit cross-sectional area) (E- ₄₀ ) _C slightly decreases to a range of 90 to 180 J / cm ² . As a result, (E- ₄₀ ) _C / (E- ₄₀ ) _L becomes less than 0.80, and the anisotropy increases. However, a sufficient Charpy impact value (absorbed energy per unit cross-sectional area) for use is ensured.
[0031]
Further, the rolling start temperature T is within the range of the present invention ((Ac ₁ transformation point) to (Ac ₃ transformation point)), but the right side of the expression (1) is not satisfied, that is, 850 <T−0.625R. In the region D, the Charpy impact value in the pipe axis direction (L direction) (absorbed energy per unit cross-sectional area) (E- ₄₀ ) _L and the Charpy impact value in the pipe circumferential direction (C direction) (unit cutoff) Both (absorbed energy per area) (E- ₄₀ ) and _C are slightly reduced to less than 180 J / cm ² , but all are 90 J / cm ² or more, and sufficient toughness in use is secured.
[0032]
Further, in a region A where the rolling start temperature T is lower than the range of the present invention ((Ac ₁ transformation point) to (Ac ₃ transformation point)), the Charpy impact value (per unit cross-sectional area) in the tube axis direction (L direction). Absorbed energy) (E- ₄₀ ) _L is 180 J / cm ² or more, but Charpy impact value (absorbed energy per unit cross-sectional area) in the circumferential direction (C direction) of the tube (E- ₄₀ ) _C is 90 J / cm2. cm ² , and when (E− ₄₀ ) _C / (E− ₄₀ ) _L is less than 0.80, the anisotropy of toughness increases.
[0033]
In a region E where the rolling start temperature T is outside the range of the present invention, the Charpy impact value (absorbed energy per unit cross-sectional area) (E- ₄₀ ) _L in the tube axis direction (L direction) and the tube circumferential direction ( (C direction) Charpy impact value (absorbed energy per unit cross-sectional area) (E- ₄₀ ) When both _C are less than 90 J / cm ² , toughness is reduced.
That is, in a region where the rolling start temperature T falls within the range of the present invention ((Ac ₁ transformation point) to (Ac ₃ transformation point)), the Charpy impact value (absorbed energy per unit cross-sectional area) in the tube axis direction (L direction). ) (E- ₄₀ ) _L and Charpy impact value (absorbed energy per unit cross-sectional area) in the circumferential direction (C direction) of the tube (E- ₄₀ ) _C are both 90 J / cm ² or more, and sufficient toughness in use is obtained. Can be secured. Further, by performing the finish rolling under the condition satisfying the expression (1), the Charpy impact value (absorbed energy per unit sectional area) (E- ₄₀ ) _{L in} the pipe axis direction (L direction) and the pipe circumferential direction ( Charpy impact value C direction) (becomes a unit absorbed _{energy) (E} -40 per cross sectional _{area) C} together 180 J / cm ² or more, and _{(E -40) C / (E} -40) L is 0.80 A seamless steel pipe having a small anisotropy and high toughness, which exceeds the above, is obtained.
[0034]
In the present invention, the finish rolling start temperature is set within the above range, and preferably, as finish rolling satisfying the expression (1), after finish rolling, air cooling or cooling at a cooling rate higher than air cooling. By the subsequent tempering treatment, the structure becomes a fine martensitic structure with little anisotropy, and a product tube having high strength, high toughness and low anisotropy characteristics can be obtained.
[0035]
The finish rolling is preferably performed by a continuous rolling mill such as a stretch reducer or a sizer.
[0036]
【Example】
A martensitic stainless steel melt having the composition shown in Table 1 was melted in a converter, the molten steel was converted into a slab by a continuous casting method, and then the slab was rolled into a billet (raw material for raw pipe production). These billets (martensitic stainless steel material) were pierced and rolled by a Mannesmann piercer mill, and then drawn and rolled by a mandrel mill to obtain raw tubes having the dimensions shown in Table 2. After the elongation rolling, the raw tube was cooled to a temperature equal to or lower than the Ms point, and the structure was substantially changed to a martensite structure. A test piece was collected from a part of the tube, and the tube structure was observed with an optical microscope. In addition, an example in which reheating was performed immediately after elongation rolling without cooling to a temperature equal to or lower than the Ms point was adopted as a conventional example.
[0037]
Next, after the raw tube was reheated to the temperature shown in Table 2, finish rolling was performed under the conditions shown in Table 2 to obtain a product tube having the size shown in Table 2. The finish rolling was performed using a stretch reducer. After finish rolling, the product tube was air-cooled (cooled). After cooling, the product tubes were further tempered at the temperatures shown in Table 2.
From each of the obtained product pipes, a test piece was sampled from the pipe axis direction (L direction), and a tensile test was performed in the pipe axis direction in accordance with the provisions of ASTM A370 to obtain tensile properties (yield strength YS, tensile strength). TS). In addition, a Charpy impact test in a tube axis direction and a circumferential direction at −40 ° C. was performed at −40 ° C. in accordance with the provisions of ASTM A370, and an impact value (absorbed energy per unit sectional area) E− ₄₀ at −40 ° C. was obtained. I asked. The impact test piece was a sub-size having a thickness of 5 mm, and the test piece from the pipe circumferential direction (C direction) was tested by correcting the end of the test piece. From the obtained results, the impact value in the pipe circumferential direction (absorbed energy per unit sectional area) (E- ₄₀ ) _C and the impact value in the pipe axis direction (absorbed energy per unit sectional area) (E- ₄₀ ) _L , And (E- ₄₀ ) _C / (E- ₄₀ ) _L was calculated.
[0038]
Table 3 shows the obtained results.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
[Table 3]

[0042]
Each of the examples of the present invention has high strength (YS: 550 MPa or more) and high toughness ((E- ₄₀ ) _{L in the} L direction: 180 J / cm ² or more). The ratio in the L direction, (E- ₄₀ ) _C / (E- ₄₀ ) _L, is 0.80 or more, which is high strength with less anisotropy as compared with the conventional example (steel pipe No. 8) and the comparative example. It is a high toughness steel pipe. On the other hand, in the comparative examples departing from the scope of the present invention, either the L direction of toughness (impact value) is degraded, or C in toughness (impact value) increased less, also (E _{-40) C} / _{(E - 40} ) When _L is less than 0.80, the anisotropy of toughness is large.
[0043]
【The invention's effect】
According to the present invention, a martensitic stainless steel seamless pipe having high strength, high toughness, and low anisotropy can be manufactured stably at low cost, and has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of finish rolling start temperature and cross-sectional reduction rate on toughness and toughness anisotropy.
FIG. 2 is an explanatory view showing an outline of a manufacturing process of the present invention.

Claims (2)

A martensitic stainless steel material is heated to an austenitic region, a raw tube production process of forming a raw tube by piercing and elongation rolling, and the raw tube is reheated to finish rolling, and then cooled to a predetermined size. Manufacturing a martensitic stainless steel seamless pipe in which a finish rolling step for producing a product pipe of the above and a tempering treatment step of subjecting the product pipe to a tempering treatment at a temperature not higher than the Ac ₁ transformation point after the finish rolling step are sequentially performed. In the method, after the elongation rolling, the raw tube is cooled to have a substantially martensitic structure, and the reheating temperature of the raw tube in the finish rolling step is (Ac ₁ transformation point) to (Ac ₃ transformation point). a two-phase region temperature, rolling start temperature T (Ac ₁ transformation point) of the finish rolling - high strength and high toughness martensitic stainless, characterized in that a temperature in the range of (Ac ₃ transformation point) Manufacturing method of the scan steel seamless pipe. The cross-sectional reduction rate R of the finish rolling is set to 10 to 90%, and the relationship between the rolling start temperature T of the finish rolling and the cross-sectional reduction rate R satisfies the following expression (1). 2. A method for producing a high-strength and high-toughness martensitic stainless steel seamless tube according to item 1.
800 ≦ T− 0.625R ≦ 850 (1)
Here, T: rolling start temperature of finish rolling (° C.)
R: Finish rolling cross-section reduction rate (%)

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