JP6805639B2 - Manufacturing method of stainless steel pipe - Google Patents

Description

The present invention relates to a method for manufacturing a stainless steel pipe, and more particularly to a method for manufacturing a stainless steel pipe for an oil well. In this specification, an oil well and a gas well are collectively referred to as an "oil well". Therefore, in the present specification, the "stainless steel pipe for oil wells" includes a stainless steel pipe for oil wells and a stainless steel pipe for gas wells.

Traditionally, oil and natural gas have been mined from oil fields. However, recently, the amount of oil and natural gas mined from oil fields has decreased. Along with this, the development of deep oil wells is progressing. The temperature of the oil well rises as its depth increases. Therefore, deep wells often have a high temperature environment. The "high temperature" here means a temperature of 150 ° C. or higher. Deep wells in high temperature environments contain CO ₂ gas, or CO ₂ gas and hydrogen sulfide gas. These gases are corrosive gases.

Conventional oil well environment, CO ₂ gas (CO ₂₎ and chlorine ions ^- containing (Cl). Therefore, in the conventional oil well environment, martensitic stainless steel (hereinafter referred to as 13% Cr steel) containing 13% by mass of Cr, which is excellent in CO ₂ gas corrosion resistance, has been used.

However, the steel pipes for oil wells used in the above-mentioned deep oil wells are required to have higher strength and higher corrosion resistance than 13% Cr steel. Duplex stainless steel has a high Cr content and has higher strength and higher corrosion resistance than 13% Cr steel. Duplex stainless steel is, for example, 22% Cr steel containing 22% by mass Cr and 25% Cr steel containing 25% by mass Cr. However, duplex stainless steel is expensive.

Therefore, as a steel pipe having excellent CO ₂ gas corrosion resistance in a high temperature environment of 150 ° C. or higher, stainless steel for oil wells and oil wells containing 15.5 to 18.0% by mass of Cr, Ni, Mo, etc. Stainless steel pipes have been proposed in JP-A-2002-409 (Patent Document 1) and International Publication No. 2004/001082 (Patent Document 2).

The high-strength martensitic stainless steel pipe for oil wells disclosed in Patent Document 1 contains Cr: 11.0 to 17.0% and Ni: 2.0 to 7.0% by mass, and further It has a chemical composition that satisfies Cr + Mo + 0.3Si-40C-10N-Ni-0.3Mn ≦ 10. The stainless steel pipes of this document further have a tempered martensite structure containing less than 10% retained austenite. It is described that the stainless steel pipe in this document has a yield strength of 860 MPa or more and excellent CO ₂ gas corrosion resistance in an environment of 150 ° C.

The stainless steel pipe for oil wells disclosed in Patent Document 2 has Cr: 14.0 to 18.0%, Ni: 5.0 to 8.0%, Mo: 1.5 to 3.5%, Cu by mass. : Contains 0.5 to 3.5%, Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 18.5, and Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≦ 11 Meet. It is described that the steel pipe in this document has a yield strength of 550 MPa or more, preferably 654 MPa or more, and exhibits excellent corrosion resistance even in a corrosive environment of high-pressure CO ₂ gas at 230 ° C.

JP-A-2002-409 International Publication No. 2004/001082

However, in the stainless steel pipes of Patent Document 1 and Patent Document 2, in the case of a large and thick stainless steel pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more, the toughness may be low.

An object of the present invention is to provide a stainless steel pipe containing Cr of less than 11.0 to 15.5% by mass and having excellent toughness even if it is large and thick.

The method for producing a stainless steel pipe according to the present invention is, in mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.05 to 2.0%, P: 0.05% or less, S. : 0.005% or less, Cr: 11.0 to less than 15.5%, Ni: 4.5 to 8.0%, Mo: 1.5 to 3.5%, Al: 0.001 to 0.1 %, N: 0.001 to 0.10%, O: 0.01% or less, Cu: 0 to 3.5%, V: 0 to 0.20%, Ti: 0 to 0.20%, Nb: It contains 0 to 0.20%, Co: 0 to 2.0%, W: 0 to 2.0%, Ca: 0 to 0.01%, and Mg: 0 to 0.01%, and the balance is A step of hot-working a steel material composed of Fe and impurities to make a raw pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more, a step of quenching the raw pipe from 800 to 1000 ° C., and a step of tempering the hardened raw pipe. In the step of tempering, the step of holding the raw pipe at 500 to 650 ° C. and the step of cooling the raw pipe after holding and setting the average cooling rate in the temperature range of 500 to 400 ° C. Includes a step of ℃ / min or more.

The stainless steel pipe according to the present invention contains Cr of 11.0 to 15.5% by mass, and has excellent toughness even in a large thick steel pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more.

FIG. 1 is an overall configuration diagram showing an example of a manufacturing facility used in the method for manufacturing a stainless steel pipe according to the first embodiment. FIG. 2 is an overall configuration diagram showing an example of a manufacturing facility used in the method for manufacturing a stainless steel pipe according to the second embodiment. FIG. 3 is an overall configuration diagram showing an example of a manufacturing facility used in the method for manufacturing a stainless steel pipe according to a third embodiment.

The present inventors investigated and examined the toughness of a large-sized thick stainless steel pipe containing Cr of 11.0 to 15.5% by mass or more and having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more. As a result, the following items were newly discovered. Hereinafter, in the present specification, the “large-sized thick-walled” stainless steel pipe means a stainless steel pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more.

Of the stainless steel pipes containing Cr of less than 11.0 to 15.5% by mass, large thick stainless steel pipes tend to have reduced toughness. Therefore, the present inventors have investigated and examined the manufacturing conditions that affect the toughness in the manufacturing process of a large thick stainless steel pipe. As a result, the following findings were obtained.

Quenching and tempering are performed on stainless steel pipes containing Cr of less than 11.0 to 15.5% by mass. A stainless steel plate having the above chemical composition or a stainless steel pipe having a wall thickness of less than 10 mm is unlikely to have a decrease in toughness after tempering. On the other hand, in a large thick stainless steel pipe having the above chemical composition, the toughness tends to decrease after tempering. In a large thick stainless steel pipe, the cooling rate in the tempering process is slower than the cooling rate in the tempering process of a stainless steel plate or a stainless steel pipe having a wall thickness of less than 10 mm. Compared to steel sheets, steel pipes are suppressed from cooling from the inner surface of the pipe due to the influence of radiant heat on the inner surface of the pipe. Furthermore, the wall thickness of the steel pipe also suppresses the cooling rate. Therefore, in a large thick stainless steel pipe containing Cr of less than 11.0 to 15.5% by mass, the cooling rate in the tempering step becomes slow.

The following are possible reasons for the decrease in toughness when the cooling rate is slow. In stainless steel having the above-mentioned chemical composition, it is considered that a phenomenon in which two-phase separation of body-centered cubic crystals occurs in ferrite, that is, embrittlement at 475 ° C. occurs. Therefore, among the steps of manufacturing a large-sized thick stainless steel pipe having the above chemical composition, when the raw pipe is cooled in the tempering step, it is preferable to pass through a region near 475 ° C. in the shortest possible time. In the tempering step, if the average cooling rate in the embrittlement temperature range of 500 to 400 ° C is set to 10 ° C / min or more, the occurrence of embrittlement at 475 ° C can be suppressed in a large thick stainless steel pipe, which is excellent. Obtains toughness.

The method for producing a stainless steel pipe according to the present invention completed based on the above findings is, in terms of mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.05 to 2.0%, P. : 0.05% or less, S: 0.005% or less, Cr: 11.0 to less than 15.5%, Ni: 4.5 to 8.0%, Mo: 1.5 to 3.5%, Al : 0.001 to 0.1%, N: 0.001 to 0.10%, O: 0.01% or less, Cu: 0 to 3.5%, V: 0 to 0.20%, Ti: 0 ~ 0.20%, Nb: 0 to 0.20%, Co: 0 to 2.0%, W: 0 to 2.0%, Ca: 0 to 0.01%, and Mg: 0 to 0. A step of hot-working a steel material containing 01% and the balance of Fe and impurities to make a raw pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more, and a step of quenching the raw pipe from 800 to 1000 ° C. It is provided with a step of tempering the raw pipe after quenching. The steps of tempering are a step of holding the raw tube at 500 to 650 ° C. and a step of cooling the raw tube after holding, and the average cooling rate in the temperature range of the raw tube temperature of 500 to 400 ° C. is set to 10 ° C./min or more. Including the process.

In the step of forming the raw tube, the raw tube after hot working may be cooled, and in the step of quenching, the cooled raw tube may be heated to 800 ° C. or higher and then quenched.

In the above quenching step, the raw pipe having a temperature of 800 ° C. or higher after hot working may be directly quenched. In this case, the manufacturing cost can be suppressed and the productivity is increased.

In the process of making the raw pipe, the steel material is hot-processed using the hot pipe making equipment, and the method for manufacturing the stainless steel pipe further uses a quenching furnace connected to the hot pipe making equipment via a transfer line. A step of heating the raw tube after hot working to 800 ° C. or higher is provided, and in the step of quenching, the raw tube heated by the heat supplementary furnace may be quenched.

Hereinafter, the method for manufacturing a stainless steel pipe according to the present invention will be described in detail.

[First Embodiment]
The method for manufacturing a martensite-based stainless steel pipe according to the present embodiment is a step of hot-working a steel material to make a raw pipe (hot working step) and a step of quenching the raw pipe after hot-working (hot working step). It includes a quenching step) and a step of tempering the raw pipe after quenching (tempering step). Each step is carried out using, for example, the manufacturing equipment shown in FIG.

[production equipment]
With reference to FIG. 1, the manufacturing equipment includes a heating furnace 10, a hot pipe making equipment 20, a quenching device 30, and a tempering device (heat treatment furnace) 40.

The heating furnace 10 and the hot pipe making facility 20 are connected to each other via a transfer line 50. The transport line 50 is, for example, a plurality of transport rollers. That is, these facilities (heating furnace 10, hot pipe making facility 20) are included in one production line (also referred to as in-line).

The hot pipe making facility 20 hot-processes a steel material to manufacture a raw pipe. In FIG. 1, the hot pipe making facility 20 includes a drilling machine 21, a drawing rolling mill 22, and a constant diameter rolling mill 23 in this order from upstream to downstream. The facilities 21, 22 and 23 are connected via a transport line 50. The drilling machine 21 includes a pair of tilt rolls and a plug arranged between the pair of tilt rolls. The drilling machine 21 drills and rolls a steel material into a raw pipe. The draw-rolling machine 22 stretch-rolls the raw pipe. The constant diameter rolling mill 23 rolls the raw pipe in a constant diameter. The draw rolling mill 22 is, for example, a mandrel mill. The constant diameter rolling mill 23 is, for example, a sizer or a reducer.

The quenching apparatus 30 includes a heat treatment furnace and a cooling facility located downstream of the heat treatment furnace. The quenching device 30 reheats and quenches the raw pipe produced by hot processing in the hot pipe making equipment 20. The quenching method is, for example, water cooling, oil cooling, or the like. The tempering device 40 performs tempering on the hardened raw pipe. The tempering method will be described later.

In FIG. 1, the hot pipe making facility 20 includes a drilling machine 21, a drawing rolling mill 22, and a constant diameter rolling mill 23. However, the hot pipe making equipment 20 may be a hot extruder or a hot forging machine.

[Hot working process]
In the hot working step, the steel material is hot worked to produce a large thick raw pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more. The chemical composition of steel contains the following elements. Hereinafter, "%" in the description of the element means mass% unless otherwise specified.

C: 0.05% or less Carbon (C) is inevitably contained. C produces Cr carbides during tempering and reduces the corrosion resistance of steel to high temperature CO ₂ gas. Therefore, in the present invention, it is preferable that the C content is low. The C content is 0.05% or less. The preferred C content is 0.04% or less. Considering the decarburization cost, the lower limit of the C content is preferably 0.001%, more preferably 0.002%, still more preferably 0.005%.

Si: 1.0% or less Silicon (Si) is inevitably contained. Si deoxidizes steel. However, if the Si content is too high, delta ferrite (δ ferrite) is contained in the structure, and the proof stress and toughness are lowered. Therefore, the Si content is 1.0% or less. The preferred upper limit of the Si content is 0.8%, more preferably 0.6%, still more preferably 0.4%. The preferable lower limit of the Si content for further effectively enhancing the deoxidizing effect is 0.01%, more preferably 0.02%, still more preferably 0.05%, still more preferably 0. It is 1%. However, even if the Si content is less than 0.01%, Si deoxidizes the steel to some extent.

Mn: 0.05 to 2.0%
Manganese (Mn) deoxidizes and desulfurizes steel and enhances hot workability. Mn further suppresses the excessive formation of δ ferrite as an austenite stabilizing element. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the corrosion resistance in a high temperature environment is lowered. Further, when the content of alloying elements such as Ni and Cu is high and the Mn content is also high, the Ms point is excessively lowered. In this case, the retained austenite after quenching increases, a sufficient amount of martensite cannot be secured, and the strength (proof stress) of the steel decreases. Therefore, the Mn content is 0.05 to 2.0%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%. The preferred upper limit of the Mn content is 1.0%, more preferably 0.8%, still more preferably 0.5%.

P: 0.05% or less Phosphorus (P) is an impurity. P reduces the corrosion resistance of steel to high temperature CO ₂ gas. Therefore, it is preferable that the P content is low. The P content is 0.05% or less. The preferred upper limit of the P content is 0.03%, more preferably 0.025%, and even more preferably 0.020%.

S: 0.005% or less Sulfur (S) is an impurity. S lowers the hot workability. If the S content is too high, the hot workability is significantly reduced. Therefore, it is preferable that the S content is low. The S content is 0.005% or less. The preferred upper limit of the S content is 0.002%, more preferably 0.001%.

Cr: 11.0 to less than 15.5% Chromium (Cr) improves corrosion resistance to high temperature CO ₂ gas. Specifically, Cr improves SCC resistance in a high-temperature CO ₂ gas environment by a synergistic effect with other elements that improve corrosion resistance. If the Cr content is too low, the above effect cannot be obtained. However, Cr is a ferrite forming element. Therefore, if the Cr content is too high, ferrite in the steel remains during quenching, and the strength of the steel decreases. Therefore, the Cr content is less than 11.0 to 15.5%. The lower limit of the Cr content is preferably 11.5%, more preferably 12.5%, still more preferably 14.0%, still more preferably 14.5%. The preferred upper limit of the Cr content is 15.2%, more preferably 15.0%.

Ni: 4.5-8.0%
Nickel (Ni) is an austenite-forming element that stabilizes austenite at high temperatures and increases the amount of martensite at room temperature. Therefore, Ni increases the strength of steel. Ni also enhances the corrosion resistance of steel in high temperature corrosive environments and also enhances toughness at low temperatures. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, the Ms point is greatly reduced. In this case, the amount of retained austenite after quenching increases. A small amount of retained austenite increases the toughness of the steel. However, if the Ni content is too high, a large amount of retained austenite is generated and the strength of the steel is lowered. Therefore, the Ni content is 4.5-8.0%. The preferred lower limit of the Ni content is 4.8%, more preferably 5.2%, still more preferably 5.6%. The preferred upper limit of the Ni content is 7.0%, more preferably 6.8%.

Mo: 1.5-3.5%
When fluid production is suspended in the well, the temperature of the fluid in the well pipe drops. At this time, the sulfide stress corrosion cracking sensitivity of the high-strength material is generally high. Molybdenum (Mo) improves sulfide stress corrosion cracking susceptibility. If the Mo content is too low, this effect cannot be obtained. On the other hand, Mo is a ferrite forming element. Therefore, if the Mo content is too high, the amount of ferrite in the steel increases and the strength of the steel decreases. Therefore, the Mo content is 1.5-3.5%. The lower limit of the Mo content is preferably 1.8%, more preferably 2.0%. The preferred upper limit of the Mo content is 3.2%, more preferably 3.0%, still more preferably 2.8%.

Al: 0.001 to 0.1%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, inclusions are likely to occur in the steel, and the toughness and SSC resistance are lowered. Further, the amount of ferrite increases and the strength of the steel decreases. Therefore, the Al content is 0.001 to 0.1%. The lower limit of the Al content is preferably 0.005%, more preferably 0.01%. The preferred upper limit of the Al content is 0.08%, more preferably 0.06%. The Al content referred to in the present specification is sol. It means the content of Al (acid-soluble Al).

N: 0.001 to 0.10%
Nitrogen (N) is inevitably contained. N increases the strength of steel. However, if the N content is too high, inclusions in the steel will increase, corrosion resistance will decrease, and the Ms point will also decrease. On the other hand, excessive reduction of N content increases refining cost. Therefore, the N content is N: 0.001 to 0.10%. The preferable lower limit of the N content is 0.002%, more preferably 0.005%, still more preferably 0.008%. The preferable upper limit of the N content is 0.08%, more preferably 0.05%, still more preferably 0.03% or less.

O: 0.01% or less Oxygen (O) is an impurity. O reduces the toughness and corrosion resistance of steel. Therefore, it is preferable that the O content is low. The O content is 0.01% or less. The preferred upper limit of the O content is 0.008%.

The balance of the chemical composition of the steel material used as the material of the stainless steel pipe of the present invention consists of Fe and impurities. Here, impurities are those that are mixed in from ore, scrap, manufacturing environment, etc. as raw materials when steel materials are industrially manufactured, and are allowed as long as they do not adversely affect the stainless steel pipe of the present invention. Means what is done.

The chemical composition of the above-mentioned steel material may further contain Cu instead of a part of Fe.

Cu: 0-3.5%
Copper (Cu) is an optional element and may not be contained. When contained, Cu is an austenite-forming element that stabilizes austenite at high temperatures and increases the amount of martensite at room temperature. Cu further increases the strength of steel by age hardening. However, if the Cu content is too high, the hot workability is reduced. If the Cu content is too high, the Ms point is further lowered. In this case, it is difficult to obtain a stable martensite structure during quenching. Therefore, the Cu content is 0 to 3.5%. The lower limit of the Cu content is preferably 0.1%, more preferably 0.2%, still more preferably 0.5%. The preferred upper limit of the Cu content is 2.5%, more preferably 2.0%, still more preferably 1.5%.

The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of V, Ti and Nb instead of a part of Fe.

V: 0 to 0.20%
Ti: 0-0.20%
Nb: 0 to 0.20%
Vanadium (V), titanium (Ti) and niobium (Nb) are all optional elements and may not be contained. When contained, these elements form carbides to increase the strength and toughness of the steel. However, if the content of these elements is too high, the carbides will be coarse. In this case, the toughness and corrosion resistance of the steel are reduced. Therefore, the V content is 0 to 0.20%, the Ti content is 0 to 0.20%, and the Nb content is 0 to 0.20%. The preferable lower limit of the V content for obtaining the above effect particularly effectively is 0.005%, the preferable lower limit of the Ti content is 0.005%, and the preferable lower limit of the Nb content is 0.005%. .. However, even if the content of these elements is less than 0.005%, the above effect can be obtained to some extent. Further, when two or more kinds selected from the group consisting of V, Ti and Nb for obtaining the above effect particularly effectively are contained, the preferable content is 0.01 to 0.20% in total.

The chemical composition of the above-mentioned steel material may further contain Co instead of a part of Fe.

Co: 0-2.0%
Cobalt (Co) is an optional element and may not be contained. When contained, Co enhances toughness at low temperatures. However, if the Co content is too high, the alloy price will increase and the economic efficiency will be significantly reduced. Therefore, the Co content is 0 to 2.0%. The preferable lower limit of the Co content for obtaining the above effect particularly effectively is 0.10%, more preferably 0.20%, and further preferably 0.50%. The preferred upper limit of the Co content is 1.8%, more preferably 1.5%.

The chemical composition of the above-mentioned steel material may further contain W instead of a part of Fe.

W: 0-2.0%
Tungsten (W) is an optional element and may not be contained. When contained, W enhances SCC resistance in high temperature environments. However, if the W content is too high, the alloy price will increase and the economic efficiency will be significantly reduced. Therefore, the W content is 0 to 2.0%. The preferable lower limit of the W content for obtaining the above effect particularly effectively is 0.10%, more preferably 0.20%, and further preferably 0.50%. The preferred upper limit of the W content is 1.8%, more preferably 1.5%.

The chemical composition of the steel material may further contain one or more selected from the group consisting of Ca and Mg instead of a part of Fe.

Ca: 0-0.01%
Mg: 0-0.01%
Both calcium (Ca) and magnesium (Mg) are optional elements and may not be contained. In stainless steel containing a large amount of alloying elements as shown in the present invention, hot working may cause scratches and defects on the stainless steel. Ca and Mg suppress the formation of scratches and defects during hot working. However, if the Ca and Mg contents are too high, inclusions in the steel will increase. In this case, the toughness and corrosion resistance of the steel are reduced. Therefore, the Ca content is 0 to 0.01% and the Mg content is 0 to 0.01%.

The preferable lower limit of the Ca content for obtaining the above effect particularly effectively is 0.0002%, and the preferable lower limit of the Mg content is 0.0002%. Further, in order to obtain the above effect particularly effectively, the preferable content when Ca and Mg are contained in combination is 0.0002 to 0.01% in total.

Steel materials having the above-mentioned chemical composition are produced by a well-known method. The steel material may be, for example, a slab produced by a continuous casting method (including round CC). The steel material may also be a piece of steel produced by hot-working an ingot produced by the ingot method. The steel material may also be a piece of steel made from slabs.

The steel material is charged into the heating furnace 10 and heated. Subsequently, the heated steel material is hot-processed using the hot pipe making facility 20 to manufacture a raw pipe. Hot working is, for example, hot pipe making by the Mannesmann method. In the Mannesmann method, a steel material is drilled and rolled by a drilling machine 21 to form a raw pipe. Subsequently, the raw pipe is stretch-rolled and constant-diameter rolled by the draw-rolling machine 22 and the constant-diameter rolling mill 23. Hot working is not limited to the Mannesmann method. The hot working may be, for example, hot extrusion or hot forging.

Cool the raw tube after hot working. For example, the raw tube is cooled to room temperature. The cooling method may be air cooling or water cooling. In the steel material of the present invention, martensitic transformation occurs even if it is air-cooled if it is cooled below the Ms point. Therefore, the structure of the raw tube at room temperature is mainly martensite.

[Quenching process]
Subsequently, quenching is performed on the cooled raw pipe. Specifically, the quenching device 30 reheats the raw tube to a quenching temperature of 800 to 1000 ° C. Subsequently, the heated raw tube is quenched. The quenching method is, for example, water cooling such as a dipping method or a spraying method. By the above quenching step, most of the portion that was austenite at high temperature is transformed into martensite, and the yield strength of the stainless steel pipe after production becomes 758 MPa or more, more preferably 862 MPa or more. The quenching temperature is preferably 850 ° C. or higher and 900 ° C. or lower.

In order to stably obtain high strength, it is preferably cooled by water quenching until the surface temperature of the raw tube becomes at least 100 ° C. or lower, preferably 60 ° C. or lower. That is, preferably, the raw pipe after hot working is water-cooled, and the water cooling stop temperature is set to 60 ° C. or lower. A more preferable water cooling stop temperature is 50 ° C. or lower, and even more preferably 40 ° C. or lower.

[Tempering process]
Tempering is performed on the hardened raw pipe. Specifically, the tempering device 40 is used to equalize the heat of the raw tube at a tempering temperature of 500 ° C. to 650 ° C.

If the tempering temperature exceeds 650 ° C., the volume fraction of retained austenite rapidly increases, and the strength tends to decrease. On the other hand, if the tempering temperature is less than 400 to 500 ° C., the tempering heat treatment itself makes the raw tube embrittlement. If the tempering temperature is less than 400 ° C., the tempering will be insufficient. When the tempering temperature is 500 ° C. to 650 ° C., the yield strength can be adjusted to 758 MPa or more while suppressing embrittlement even in a thick raw tube having a wall thickness of 10 mm or more.

After soaking the raw pipe at the above tempering temperature, the raw pipe is cooled. At this time, the average cooling rate in the temperature range of 500 to 400 ° C. (hereinafter referred to as the embrittlement temperature range) is set to 10 ° C./min or more. A large thick stainless steel pipe having the above-mentioned chemical composition and having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more becomes brittle due to a long residence time in the embrittlement temperature range in the case of natural cooling or the like. Therefore, it is preferable that the staying time in the embrittlement temperature range is as short as possible. As described above, when the average cooling rate in the embrittlement temperature range is 10 ° C./min or more, the residence time in the embrittlement temperature range is short, so that the embrittlement of the stainless steel pipe is suppressed and excellent toughness can be obtained. The average cooling rate in the embrittlement temperature range is preferably 15 ° C./min or higher, more preferably 20 ° C./min or higher, and even more preferably 30 ° C./min or higher. This cooling can be achieved, for example, by oil cooling or water cooling.

Through the above manufacturing process, a stainless steel pipe which is a seamless steel pipe is manufactured. The manufactured stainless steel pipe has high strength and excellent toughness.

[Second Embodiment]
In the first embodiment, the hot-worked raw tube is once cooled to room temperature, and then the raw tube is reheated to perform quenching. However, the raw pipe immediately after hot working may be directly quenched.

FIG. 2 is a schematic view showing an example of a manufacturing facility used in the manufacturing method of the second embodiment. Compared with FIG. 1, the manufacturing facility of FIG. 2 newly includes a water cooling device 60 instead of the quenching device 30. The water cooling device 60 is connected to the hot pipe making facility 20 via the transfer line 50. That is, the water cooling device 60 is an in-line facility.

In the manufacturing method of the second embodiment, the hot working step and the quenching step are different from those of the first embodiment.

[Hot working process]
In the hot working step, the raw pipe is manufactured by hot working as in the first embodiment. However, the quenching step is carried out as it is without cooling the raw tube after production to room temperature.

[Quenching process]
In the quenching step, the water cooling device 60 on the same line as the hot pipe making facility 20 is used to directly quench the raw pipe immediately after the hot working in-line. That is, in the present embodiment, the raw pipe is quenched in-line. At this time, the raw tube temperature is 800 to 1000 ° C., and the preferable raw tube temperature is 850 to 900 ° C. After the above quenching step is completed, the same tempering step as in the first embodiment is carried out.

In the production method using direct quenching of the present embodiment, it is not necessary to reheat the raw tube at the time of quenching. Therefore, not only the cost required for reheating after pipe production can be suppressed, but also the productivity can be increased.

[Third Embodiment]
When direct quenching is carried out, the temperature of the raw tube after hot working may be less than 800 ° C. In this case, it is preferable to reheat the raw pipe to 800 ° C. or higher using a reheating furnace and then quench it.

FIG. 3 is a schematic view showing an example of a manufacturing facility used in the manufacturing method of the third embodiment. The manufacturing equipment of FIG. 3 includes a reheating furnace 70 between the hot pipe making equipment 20 and the water cooling device 60 as compared with FIG. The reheating furnace 70 is connected to the hot pipe making facility 20 and the water cooling device 60 via a transfer line 50. That is, the reheating furnace 70 is an in-line facility.

The manufacturing method of the third embodiment includes the hot working step, the quenching step, and the tempering step of the above-mentioned second embodiment, and further, a new method is provided between the hot working step and the quenching step. Is equipped with a heating process.

[Heat replenishment process]
In the heat supplementing step, the raw pipe after hot working is reheated to 800 ° C. or higher using a heating furnace 70. Then, the reheated raw pipe is quenched by the water cooling device 60. In this case, the temperature of the raw tube can be adjusted to 800 ° C. or higher by the in-line reheating furnace 70. Therefore, as compared with the manufacturing method using the manufacturing equipment of FIG. 1, it is possible to manufacture a stainless steel pipe having desired characteristics while suppressing the manufacturing cost and increasing the productivity.

Preferably, the heating furnace 70 heats the raw pipe to 850 ° C. or higher and 900 ° C. or lower. In this case, precipitation of harmful intermetallic compounds and carbides is suppressed, and desired properties can be obtained.

[Manufactured stainless steel pipe]
The stainless steel pipe produced by the production method of the first to third embodiments described above has the same chemical composition as the above-mentioned steel material. Further, this stainless steel pipe has a microstructure mainly composed of martensite, or mainly composed of martensite and retained austenite, or mainly composed of martensite, ferrite and retained austenite, and yields at least 758 MPa. Has strength.

Preferably, in the microstructure of the stainless steel, the volume fraction of ferrite is 9% or less, the volume fraction of retained austenite is 15% or less, and the balance is martensite (however, non-metal inclusions and precipitates). except for). The microstructure of the steel material having the above chemical composition becomes austenite single-phase during hot working, or becomes a structure composed of 9% or less of ferrite and retained austenite. Since ferrite does not transform even when cooled, it remains ferrite even at room temperature. Although austenite transforms into martensite when cooled, some may become retained austenite. Therefore, the microstructure of the stainless steel pipe of the present invention is mainly composed of martensite, or mainly composed of martensite and retained austenite, or mainly composed of martensite, ferrite and retained austenite.

As described above, the preferable ferrite volume fraction is 9% or less. In the case of the above chemical composition, the ferrite volume fraction is 9% or less. If the ferrite volume fraction exceeds 9%, the yield strength of the stainless steel pipe is lowered to less than 758 MPa. If the ferrite volume fraction exceeds 9%, the low temperature toughness is further lowered. Therefore, the preferable ferrite volume fraction is 9% or less.

The above manufacturing method is effective when manufacturing a large-sized thick stainless steel pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more, particularly a wall thickness of 15 mm or more. As described above, it is difficult to obtain high toughness in a large thick stainless steel pipe having the above-mentioned chemical composition. This is because embrittlement at 475 ° C is likely to occur in large thick stainless steel. Therefore, in the present invention, the cooling rate in the embrittlement temperature range of 400 to 500 ° C., where embrittlement is likely to occur at 475 ° C., is increased, and the average cooling rate in the embrittlement temperature range is set to 10 ° C./min or more. As a result, in a large-sized thick stainless steel pipe having the above-mentioned chemical composition, toughness can be enhanced while maintaining high strength.

If the stainless steel pipe has an outer diameter of less than 175 mm or a wall thickness of less than 10 mm, the toughness can be ensured by natural air cooling after tempering even when the pipe has the above chemical composition.

Molten steels of steel grades A to M shown in Table 1 were produced.

A slab was produced by a continuous casting method using the produced molten steel. Each slab was rolled by a block matrix rolling mill to produce a round billet. The round billet was heated to 1150 to 1250 ° C. using a heating furnace. After heating, the round billet was hot rolled. Specifically, a raw pipe was manufactured by drilling and rolling a round billet with a drilling machine. The raw pipe was stretched and rolled with a mandrel mill, and further subjected to constant diameter rolling (reduced diameter rolling) with a sizer to produce a raw pipe having an outer diameter of 140 to 273 mm and a wall thickness of 9 to 26 mm. The raw pipe after hot rolling was cooled to room temperature. The cooling of the raw pipes was natural cooling.

Quenching was carried out on the raw pipe after allowing it to cool. Specifically, the raw tube was placed in a heat treatment furnace of a quenching apparatus and heated at 900 ° C. for 15 minutes. The raw tube after soaking was water-cooled by a spray method and quenched. The raw tube after quenching was tempered under the conditions shown in Table 2.

Specifically, the raw pipes of each test number were kept at the tempering temperature (° C.) shown in Table 2 for a soaking time (minutes). The raw pipe after soaking was cooled by the cooling method shown in Table 2. The cooling rate (° C./min) when the tube temperature was 500 to 400 ° C. (embrittlement temperature range) was as shown in Table 2. The average value of the cooling rate in the embrittlement temperature range was defined as the cooling rate (° C./min). Through the above production, a stainless steel pipe (seamless steel pipe) having an outer diameter (mm) and a wall thickness (mm) shown in Table 2 was produced.

[Tensile test]
A round bar tensile test piece was collected from the central part of the wall thickness of each steel pipe. The longitudinal direction of the round bar tensile test piece was parallel to the tube axis direction. The diameter of the parallel portion of the round bar tensile test piece was 6 mm, and the distance between the gauge points was 40 mm. A tensile test based on JIS Z2241 (2011) was carried out on the collected round bar tensile test pieces at room temperature and in the air to determine the yield strength (0.2% proof stress, unit is MPa).

[Charpy impact test]
In accordance with JIS Z2242 (2005), a full-size V-notch test piece was taken from each steel pipe and a Charpy impact test was carried out. The Charpy impact test was carried out three times at -10 ° C, and the minimum value of the absorbed energy obtained in the three tests was defined as the absorbed energy _v E _-10 (J) at that test number. For the test numbers 7, 8 and 9 for which full-size test pieces could not be collected due to the thickness of the steel pipe, half-size test pieces were collected and a Charpy impact test was conducted. Twice the absorbed energy obtained in the Charpy impact test was replaced with full size energy.

[Test results]
The test results are shown in Table 2. With reference to Table 2, the chemical composition was appropriate for the steel pipes of all test numbers. Therefore, the yield strength was 758 MPa or more in all the test numbers.

Further, in test numbers 1, 2, 4, 5, 10 to 21, the outer diameter was 175 mm or more, the wall thickness was 10 mm or more, and the steel pipe was a large thick steel pipe. The production conditions of these test numbers were appropriate, and in particular, the cooling rate in the embrittlement temperature range (500 to 400 ° C.) during tempering was 10 ° C./min or more. Therefore, the absorbed energy _v E _-10 (J) of these test numbers at −10 ° C. was 80 J or more, and excellent toughness was obtained.

On the other hand, in Test Nos. 3 and 6, since the steel pipe was a large thick steel pipe and the cooling method for tempering was natural air cooling, the cooling rate in the embrittlement temperature range was less than 10 ° C./min. Therefore, the absorbed energy _v E _-10 (J) was as low as less than 80J.

In the steel pipes of test numbers 7 to 9, the wall thickness was less than 10 mm, which did not correspond to the thick steel pipe referred to in the present specification. Therefore, the cooling rate in the embrittlement temperature range was 10 ° C./min or more, including the test number 9 in which the cooling method for tempering was natural air cooling. Therefore, the absorbed energy _v E _-10 (J) was 80 J or more.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

10 Heating furnace 20 Hot pipe making equipment 21 Drilling machine 22 Stretching rolling mill 23 Fixed diameter rolling mill 30 Quenching equipment 40 Tempering equipment 50 Conveying line 60 Water cooling equipment 70 Reheating furnace

Claims (4)

By mass%
C: 0.05% or less,
Si: 1.0% or less,
Mn: 0.05-2.0%,
P: 0.05% or less,
S: 0.005% or less,
Cr: 11.0 to less than 15.5%,
Ni: 4.5-8.0%,
Mo: 1.5-3.5%,
Al: 0.001 to 0.1%,
N: 0.001 to 0.10%,
O: 0.01% or less,
Cu: 0-3.5%,
V: 0-0.20%,
Ti: 0-0.20%,
Nb: 0 to 0.20%,
Co: 0-2.0%,
W: 0-2.0%,
Ca: 0-0.01% and
A process in which a steel material containing 0 to 0.01% of Mg and the balance is composed of Fe and impurities is hot-processed to obtain a raw pipe having an outer diameter of 175 mm or more and a wall thickness of 10 mm or more.
The step of quenching the raw tube from 800 to 1000 ° C.
It is provided with a step of tempering the raw tube after quenching.
The step of tempering is
The step of holding the raw tube at 500 to 650 ° C.
A method for producing a stainless steel pipe, which comprises a step of cooling the raw pipe after holding and setting the average cooling rate of the raw pipe in a temperature range of 500 to 400 ° C. to 10 ° C./min or more. The method for manufacturing a stainless steel pipe according to claim 1.
In the step of making the raw pipe, the raw pipe after hot working is cooled.
In the quenching step, a method for producing a stainless steel pipe, in which the cooled raw pipe is heated to 800 ° C. or higher and then quenched. The method for manufacturing a stainless steel pipe according to claim 1.
In the quenching step, a method for producing a stainless steel pipe, which directly quenches a raw pipe having a temperature of 800 ° C. or higher after hot working. The method for manufacturing a stainless steel pipe according to claim 2.
In the process of making the raw pipe, the steel material is hot-processed using a hot pipe making facility.
The method for manufacturing the stainless steel pipe further
A step of heating the raw pipe after hot working to 800 ° C. or higher by using a heating furnace connected to the hot pipe making facility via a transfer line is provided.
In the quenching step, a method for producing a stainless steel pipe, in which the raw pipe heated by the heating furnace is quenched.

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