JP6202010B2 - Manufacturing method of high-strength duplex stainless steel seamless steel pipe - Google Patents

Description

  The present invention relates to a method for producing a seamless steel pipe, and more particularly to a method for producing a high-strength duplex stainless steel seamless steel pipe excellent in low temperature toughness. High strength means that the yield strength is 654 MPa or more and that the low temperature toughness is excellent means that the absorbed energy of the Charpy test at −10 ° C. is 50 J or more.

  In recent years, from the viewpoint of soaring energy prices such as crude oil due to an increase in global energy consumption, and the depletion of petroleum resources, deep oil fields (deep oil fields) and hydrogen sulfide that have not been previously excluded Energy resources are being actively developed in oil fields and gas fields in severe corrosive environments under a so-called sour environment, and in oil fields and gas fields in the extreme north of severe weather environments. A steel material used in such an environment is required to have a material having high strength and excellent corrosion resistance (sour resistance) and excellent low temperature toughness. Also, various steel pipe thicknesses are required from thin to thick.

Conventionally, carbon dioxide CO _2, chloride ion Cl ^- oilfield environment and the like, in the gas field, 13% Cr martensitic stainless steel is often used as a steel for use in mining. However, since 13% Cr martensitic stainless steel does not have sufficient corrosion resistance in a sour environment, recently, the use of duplex stainless steel with reduced C content and increased Cr content and Ni content has been expanded. .

For example, Patent Document 1 describes a method for producing a high-strength stainless steel pipe for oil wells having excellent corrosion resistance. In the technique described in Patent Document 1, in mass%, C: 0.005 to 0.050%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, Cr: 15 5 to 18%, Ni: 1.5 to 5%, Mo: 1 to 3.5%, V: 0.02 to 0.20%, N: 0.01 to 0.15%, O: 0.0. 006% or less and having a composition satisfying Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 and Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 The steel pipe material is heated and formed by hot working. After the pipe formation, the steel pipe is cooled to room temperature at a cooling rate higher than air cooling to obtain a seamless steel pipe of a predetermined size, and then the seamless steel pipe is heated to a temperature of 850 ° C. or higher. Reheat and cool to 100 ° C or lower at a cooling rate higher than air cooling, then 700 ° C By applying a quenching-tempering treatment that heats to a lower temperature, the structure has a structure in which a ferrite phase of 10 to 60% by volume and the balance is a martensite phase, and the yield strength is 654 MPa or more. Strength stainless steel pipes can be obtained. As a result, it has high strength and has sufficient corrosion resistance even under severe corrosive environments up to 230 ° C., including CO ₂ and Cl ⁻ , and has high toughness with an absorbed energy at −40 ° C. of 50 J or more. It is said to be a steel pipe.

Japanese Patent No. 5109222

  Recently, duplex stainless steel pipes are frequently used as member steel pipes used in deep oil wells. In the production of duplex stainless steel pipes, in many cases, it is often in the two-phase region state of the ferrite phase and austenite phase even during hot working, and the phase fraction varies depending on the heating holding temperature and the heating time, but particularly at high temperatures. In the region, the phase fraction of ferrite is often high. Ferrite grains grow rapidly when held at high temperatures, and crystal grains divided by the initial crystal grain size or hot working tend to grow and become coarse. The coarsened structure deteriorates the deformability during hot working. In particular, with thick materials, it is difficult to give strain to the center of the thickness, so the ferrite grains cannot be divided, and grains that are coarsened when held at high temperatures due to heat generated during high-temperature heating or hot rolling are products. Sometimes prone to persist. Since the connected coarse ferrite grains serve as a propagation path of cracks, the low temperature toughness is reduced at the central portion (low strain portion) of the steel pipe rolled at a high temperature with many ferrite phases. Further, the coarsening of ferrite grains also affects the strength of the steel pipe, and in particular, the yield strength of the steel pipe is reduced. Therefore, in the manufacture of a stainless steel pipe that is hot-rolled in a two-phase region, desired characteristics cannot be obtained unless the hot rolling conditions and the temperature control in the subsequent heat treatment are made appropriate.

  The technique described in Patent Document 1 does not refer to temperature management at the time of high-temperature heating or hot working of a material. In particular, in the technique described in Patent Document 1, there is no mention of improving the properties of the steel pipe by temperature control during high-temperature heating or during hot rolling, in particular, improving low-temperature toughness.

  In view of the state of the prior art, an object of the present invention is to provide a method for producing a high-strength duplex stainless steel seamless steel pipe excellent in low-temperature toughness.

  In order to achieve the above-described object, the present inventors have intensively studied various factors affecting the toughness of the duplex stainless steel material. As a result, the inventors have come up with the idea that the most effective method for improving toughness is to refine the ferrite phase structure.

Then, the grain growth of the ferrite phase which arises when a two-phase stainless steel which becomes a two-phase region including a δ ferrite phase is heated by using a high-frequency heating device was investigated. FIG. 2 shows that the temperature of the duplex stainless steel is maintained at (δ _A −100 ° C.) for 600 s during the temperature rising process, and then rapidly (10 ° C./s) to a temperature exceeding the temperature (δ _A ) at which it becomes a single phase of δ ferrite phase. This is a result of measuring the δ ferrite particle size by rapidly cooling and freezing the structure after holding at 15 ° C. for 15 seconds. As shown in FIG. 2, two-phase stainless steel is heated to a temperature above the [delta] _A, just 15s and was also [delta] ferrite grains rapidly grain growth. FIG. 3 shows the results of measuring the δ ferrite particle size by similarly heating the duplex stainless steel to a temperature of δ _A or less, holding the heated temperature for 3600 s, and then rapidly cooling the structure to quench the structure. As shown in FIG. 3, in the duplex stainless steel held at a temperature of (δ _A −100 ° C.) or higher for 3600 s or more, δ ferrite grains grow rapidly, whereas the heating holding temperature is (δ _A Below -100 ° C, very fine grains were maintained. In addition, when the holding time was shorter than 3600 s, element diffusion necessary for grain growth could not catch up, and a relatively fine structure was maintained even at a temperature higher than (δ _A -100 ° C.).

That is, in a two-phase stainless steel, which is a two-phase region including a δ ferrite phase in the hot state, the fraction of the δ ferrite phase generally increases in a high temperature region, and especially the δ and ferrite crystals that are kept at a high temperature for a long time. Grain grows rapidly. On the other hand, when heated to a low temperature in a shorter time, the growth of δ ferrite crystal grains can be suppressed. In particular, in a high temperature range of (δ _A −100 ° C.) or higher, the δ ferrite crystal grains become coarse when held for more than 3600 s, and in a high temperature range where the single phase of δ ferrite phase is obtained, the δ ferrite phase is further shortened. Therefore, if the temperature of the steel material (hereinafter also referred to as the workpiece) and the heating and holding time are appropriately controlled, the δ ferrite grains of the steel tube after the subsequent hot rolling can be kept fine. I was able to achieve a finer structure.

Therefore, in order to refine the structure of the two-phase stainless steel seamless steel pipe by further research, when heating the steel material with the heating device 1 in the seamless steel pipe manufacturing process shown in FIG. The maximum temperature of the steel material during heating and hot working does not exceed the temperature (δ _A ) at which the δ ferrite phase becomes a single phase during the temperature rising process, and the temperature of the steel material (workpiece) is ( δ _A −100 ° C.) or more is set to 3600 s or less, and the final hot working by the hot working apparatus 2 is finished at a temperature of 1200 ° C. or less, so that the ferrite grains can be controlled to a fine grain, and then appropriately It was found that the low temperature toughness is remarkably improved even at the center of the wall thickness of the stainless steel seamless pipe by applying a quenching and tempering heat treatment.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) A method for producing a duplex stainless steel pipe having a structure including a ferrite phase and the balance including an austenite phase, a martensite phase, or both, and after heating the steel material, it is hot-worked to have a predetermined shape When the steel pipe is made into a seamless steel pipe, the maximum reached temperature of the steel material during the heating and hot working is set to 1000 ° C. or more and δ _A (temperature at which a δ ferrite phase becomes a single phase in the temperature rising process) or less, and the steel The time for the temperature of the material to become (δ _A −100 ° C.) or more is set to 3600 s or less, and the final processing of the hot working is performed in a temperature range of 1200 ° C. or less at the outer surface temperature of the steel material. A method for producing a high-strength duplex stainless steel seamless pipe excellent in low-temperature toughness.
(2) The steel material is mass%,
C: 0.050% or less, Si: 1.00% or less,
Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, N: 0.01 to 0.15%,
The method for producing a high-strength dual-phase stainless steel seamless steel pipe according to (1), characterized in that the composition contains O 6: 0.006% or less, the balance being Fe and inevitable impurities.
(3) In addition to the said composition, the said steel raw material is further the mass%, and the following A group-D group A group: Al: 0.002-0.050%
Group B: Cu: 3.5% or less, W: 3.5% or less, REM: 0.3% or less C group: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2 % Selected from 1% or 2 or more selected from the group D: Ca: 0.01% or less, B: 0.01% or less selected from 1 or 2 The method for producing a high-strength dual-phase stainless steel seamless steel pipe according to (2), comprising a group or two or more groups.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength duplex stainless steel seamless steel pipe excellent in low temperature toughness can be manufactured easily, and there exists a remarkable effect on industry. Further, according to the present invention, by preventing the coarsening of the initial structure, the ferrite grain size of the duplex stainless steel seamless steel pipe can be kept fine even with a relatively small amount of processing, and the amount of processing at the thickness center portion can be reduced. Even in a thick-walled steel pipe that cannot be made large, there is an effect that the low-temperature toughness at the center of the wall thickness can be improved.

It is a figure which shows the process which manufactures a seamless steel pipe. It is a figure which shows the influence which heating temperature has on the growth of (delta) ferrite grain, when duplex stainless steel is heated to the temperature exceeding (delta) _A and hold | maintained for 15 s. It is a figure which shows the influence which heating temperature has on the growth of (delta) ferrite grain, when duplex stainless steel is heated to the temperature below (delta) _{A and} hold | maintained for 3600 s.

  The seamless steel pipe according to the present invention uses a device row in which the heating device 1 and the hot working device 2 shown in FIG. It is manufactured through a process.

  The heating device 1 used in the present invention is not particularly limited as long as it is a heating furnace capable of heating a steel material such as a slab and a steel slab to a predetermined temperature. For example, any conventional heating furnace such as a rotary hearth type heating furnace or a walking beam type heating furnace can be applied. Alternatively, an induction heating type heating furnace may be used.

  As the hot working apparatus 2 used in the present invention, any of the hot working apparatuses that are usually applied when a steel material is a seamless steel pipe having a predetermined size can be applied. Examples thereof include a piercing and rolling device and a generally known rolling device such as a reduced diameter rolling and straightening rolling. An example of a preferred device row is shown in FIG.

  The piercing and rolling device 21 that is one of the hot working devices 2 may be any piercing and rolling device that can pierce and roll a heated steel material to form a hollow material. For example, a barrel-type roll and a cone-type roll Any generally known piercing and rolling apparatus such as a Mannesmann tilting piercing machine, a hot extrusion piercing machine, or the like using any of the above can be applied. In addition, the rolling device 22 that is one of the hot working devices 2 may be any device that can process a hollow material into a seamless steel pipe having a predetermined shape. For example, an elongator 221, A rolling mill arranged in the order of a plug mill 222 for extending the hollow hollow tube thinly and long, a reeler (not shown) for smoothing the inner and outer surfaces of the hollow tube, a sizing mill 223 for adjusting a predetermined dimension, or a hollow tube is predetermined. Normally known hot working devices such as a mandrel mill (not shown) with a steel pipe of a size, a rolling device provided with a reducer (not shown) that adjusts the outer diameter and thickness by performing a slight reduction Applicable.

In the manufacturing method according to the present invention, in the heating by the heating device 1 and the hot working process by the hot working device 2, the temperature of the steel material (hereinafter also referred to as a workpiece in the hot working step) is δ _A (temperature rise). (The temperature at which the δ ferrite phase becomes a single phase in the process) is not exceeded. When the temperature of the steel material (work material) reaches the single phase of δ ferrite phase, there is no second phase with a different crystal structure existing between adjacent ferrite crystal grains, and the adjacent ferrite grains rearrange atoms. Just start grain growth easily. Therefore, the maximum temperature reached by the steel material (work material) is set to δ _A or less. In addition, from the viewpoint of energy cost reduction, it is preferably (δ _A -50 ° C) or less.

  On the other hand, if the maximum reached temperature of the steel material (work material) falls below 1000 ° C and is subjected to subsequent hot working, the fraction of the second phase, which is stronger than δ ferrite, becomes too large, and the rolling load Not only increases, but also strain concentrates in the soft δ-ferrite phase, causing ductile fracture during hot working and causing product defects. Therefore, the lower limit of the maximum attainable temperature of the steel material (work material) is limited to 1000 ° C. In addition, Preferably it is 1100 degreeC or more.

In addition, when the time when the temperature of the steel material (work material) is (δ _A -100 ° C.) or more is longer than 3600 s, austenite existing at the grain boundary of δ ferrite, which is effective in suppressing the growth of δ ferrite grains, etc. The second phase is reduced and δ ferrite grains grow easily. That is, the amount of the second phase that exists at the triple point of the δ ferrite grain boundary or the δ ferrite grain boundary and plays an important role in suppressing the grain growth of the δ ferrite, that is, the fraction forms the second phase. Therefore, the fraction of the second phase of the steel material (workpiece) during temperature rise is larger than that in the equilibrium state. However, if the time for the temperature of the steel material (work material) to become (δ _A -100 ° C.) or longer is increased, the diffusion of the elements forming the second phase catches up and the fraction of the second phase decreases. In addition, the second phase existing at the δ ferrite grain boundary, which is particularly effective for suppressing the grain growth of δ ferrite, is reduced, and the remaining second phase is unevenly distributed in a spherical form at the triple point of the δ ferrite grain boundary. The interfaces of adjacent δ ferrite grains are in contact with each other, and grain growth is facilitated. Therefore, the time when the temperature of the steel material (work material) is (δ _A −100 ° C.) or more is limited to 3600 s or less. In addition, Preferably it is 900 s or less.

  The heated steel material is then processed into a predetermined shape by a hot processing apparatus. At this time, the final hot working is finished after the average temperature of the surface of the workpiece is lowered to 1200 ° C. or lower.

  When the hot working finish temperature exceeds 1200 ° C, the low temperature toughness of the steel pipe after hot working decreases due to the relaxation of strain imparted by hot working during the subsequent cooling and the grain growth of fine δ ferrite phase. To do. For this reason, the final hot working temperature is limited to 1200 ° C. or less. In addition, since it will lead to the increase in apparatus load by generation | occurrence | production of deformation resistance, and generation | occurrence | production of a flaw if final hot processing temperature becomes low temperature, Preferably, final hot processing temperature is 900-1200 degreeC.

Note that δ _A (the temperature at which the δ ferrite single phase forms during the temperature rise process) may be calculated by thermal equilibrium calculation, or the thermal expansion that occurs when the δ ferrite single phase is obtained by measuring the thermal expansion curve during heating. The inflection point of the curve may be measured.

  Next, the reason for limiting the composition of the high-strength duplex stainless steel seamless steel pipe according to the present invention will be described.

The steel material that is effective by the application of the present invention is a steel material that has a composition in which a δ ferrite phase becomes a single phase at a relatively low temperature and the δ ferrite phase remains in a product at room temperature,
“In mass%,
C: 0.050% or less, Si: 1.00% or less,
Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, N: 0.01 to 0.15%,
O: Including 0.006% or less,
Or further A group-D group A group: Al: 0.002-0.050%
Group B: Cu: 3.5% or less, W: 3.5% or less, REM: one or more selected from 0.3% or less Group C: Nb: 0.2% or less, Ti : 0.3% or less, Zr: One or more selected from 0.2% or less Group D: Ca: 0.01% or less, B: Selected from 0.01% or less It is a “steel material having a composition comprising one group or two or more groups selected from one or two types, the balance being Fe and inevitable impurities”.

  Hereinafter, the reasons for limiting the composition will be described. Unless otherwise specified, mass% is simply expressed as%.

C: 0.050% or less C is an important element that affects the amount of martensite phase produced, and is preferably contained in an amount of 0.005% or more. On the other hand, if the content exceeds 0.050%, sensitization during tempering due to Ni inclusion increases. From the viewpoint of corrosion resistance, it is desirable that C is small. For these reasons, C is limited to 0.050% or less. In addition, Preferably it is 0.030 to 0.050%.

Si: 1.00% or less Si is an element that acts as a deoxidizing agent, and it is desirable to contain 0.05% or more. If the content exceeds 1.00%, the corrosion resistance is lowered, and the hot workability is also lowered. For this reason, Si was limited to 1.00% or less. In addition, Preferably it is 0.10 to 0.30%.

Mn: 0.20 to 1.80%
Mn is an element having an action of increasing the austenite phase fraction, and in order to obtain such an effect, the content of 0.20% or more is required. On the other hand, if the content exceeds 1.80%, the toughness is adversely affected. For this reason, Mn was limited to 0.20 to 1.80%. In addition, Preferably it is 0.20-1.00%.

Cr: 15.5 to 18.0%
Cr is a main element that forms a protective film and improves corrosion resistance, and at the same time, an element that has an effect of increasing the phase fraction of the ferrite phase. In order to obtain such an effect, the content of 15.5% or more is required. On the other hand, if the content exceeds 18.0%, the strength decreases. For this reason, Cr was limited to 15.5 to 18.0%. In addition, Preferably it is 16.0-18.0%.

Ni: 1.5-5.0%
Ni is an element having an action of repairing the protective film and improving the corrosion resistance, and at the same time, an element having an action of increasing the phase fraction of the austenite phase. It is also an element that improves toughness. Such an effect is recognized when the content is 1.5% or more. On the other hand, if the content exceeds 5.0%, the material cost increases and the strength decreases. For this reason, Ni was limited to 1.5 to 5.0%. In addition, Preferably it is 2.5 to 4.5%.

Mo: 1.0-3.5%
Mo is an element that increases resistance to pitting corrosion caused by Cl ⁻ . In order to acquire such an effect, it is desirable to contain 1.0% or more. On the other hand, if the content exceeds 3.5%, the strength decreases and the material cost increases. For this reason, Mo was limited to 1.0 to 3.5%. In addition, Preferably it is 2.0 to 3.5%.

V: 0.02 to 0.20%
V is an element that increases the strength and improves the corrosion resistance. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, when it contains exceeding 0.20%, toughness will fall. For this reason, V was limited to 0.02 to 0.20%. In addition, Preferably it is 0.02-0.08%.

N: 0.01 to 0.15%
N is an element that remarkably improves the pitting corrosion resistance. To obtain such an effect, N is required to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.15%, various nitrides are formed and the toughness is lowered. In addition, Preferably it is 0.02-0.08%.

O: 0.006% or less O exists as an oxide in steel and adversely affects various properties. For this reason, it is desirable to reduce as much as possible. In particular, when O is contained in a large amount exceeding 0.006%, the hot workability, toughness and corrosion resistance are remarkably lowered. For this reason, O was limited to 0.006% or less.

The above-mentioned components are basic components, but in addition to the basic components, the following groups A to D: Group A: Al: 0.002 to 0.050%
Group B: Cu: 3.5% or less, W: 3.5% or less, REM: one or more selected from 0.3% or less Group C: Nb: 0.2% or less, Ti : 0.3% or less, Zr: One or more selected from 0.2% or less Group D: Ca: 0.01% or less, B: Selected from 0.01% or less One group or two or more groups selected from one or two types can be contained.

Group A: Al: 0.002 to 0.050%
Group A: Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is preferably contained in an amount of 0.002% or more, but if contained in excess of 0.050%, the toughness is increased. Adversely affect. For this reason, when it contains, it is preferable to limit to 0.002 to 0.050%. When Al is not added, an inevitable impurity of about 0.002% is allowed.

Group B: Cu: 3.5% or less, W: 3.5% or less, REM: 0.3% or less Group B: Cu, W, and REM strengthen the protective film and prevent hydrogen from entering the steel. Suppress and improve resistance to sulfide stress corrosion cracking. Such an effect becomes remarkable when Cu: 0.5% or more, W: 0.5% or more, and REM: 0.001% or more. However, when it contains exceeding Cu: 3.5%, W: 3.5%, and REM: 0.3%, toughness will fall. For this reason, when it contains, it is preferable to limit Cu and W to 3.5% or less and REM to 0.3% or less, respectively. More preferably, Cu is 0.8 to 1.2%, W is 0.8 to 1.2%, and REM is 0.001 to 0.010.

Group C: Nb: 0.2% or less, Ti: 0.3% or less, Zr: one or more selected from 0.2% or less Group C: Nb, Ti and Zr are all It is an element that improves strength and hot workability, and can be selected and contained as necessary. Such an effect is recognized by containing Nb: 0.03% or more, Ti: 0.03% or more, and Zr: 0.03% or more. On the other hand, inclusions exceeding Nb: 0.2%, Ti: 0.3%, and Zr: 0.2% respectively reduce toughness. For this reason, when it contains, it is preferable to limit to Nb: 0.2% or less, Ti: 0.3% or less, and Zr: 0.2% or less, respectively.

Group D: Ca: 0.01% or less, B: One or two selected from 0.01% or less Group D: Ca and B improve hot workability during multi-phase rolling. , Has the effect of suppressing product wrinkles, and can contain one or two as required. Such an effect becomes remarkable when Ca is contained at 0.0005% or more and B: 0.0005% or more. However, when Ca is contained in an amount exceeding 0.01% and B: 0.01%, the corrosion resistance is improved. descend. For this reason, when it contains, it is preferable to limit to Ca: 0.01% or less and B: 0.01% or less.

  The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include P: 0.03% or less and S: 0.005% or less.

  The method for producing a steel material having the above composition need not be particularly limited. Using a conventional smelting furnace such as a converter or electric furnace, the molten steel having the composition described above is melted, and a slab (round slab) is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a raw material. In addition, it is good also as a steel raw material as a steel slab of a predetermined dimension by hot-rolling a slab. Moreover, it is set as a steel slab by the ingot-making-slab rolling method, and there is no problem as a steel raw material.

  In addition, the above-mentioned explanation is about the heating temperature by the heating device, the holding time, the maximum temperature of the steel material (work material) during the hot working by the hot working device, the holding time in the high temperature range, and the final hot time. Although the temperature at which the processing is performed has been described, it is described in the means (1) according to the present invention from the viewpoint of suppressing grain growth even when the steel pipe is heated to a temperature of 1000 ° C. or higher by heat treatment or the like after the final hot processing. It is desirable to apply limiting conditions to limit the maximum temperature and holding time. For example, in the case where the seamless steel pipe manufactured by the manufacturing method of the present invention is subjected to quenching and tempering heat treatment, when the temperature is raised to a temperature of 1000 ° C. or higher, the heating temperature and the holding time satisfy the above condition (1). It has been confirmed that by heating and holding, grain growth can be suppressed and desired mechanical properties can be obtained.

  The temperature may be measured using a direct contact type or a non-contact type thermometer represented by a radiation thermometer. Further, temperature management may be performed based on the maximum attained temperature and temperature history derived from the measured temperature by heat transfer analysis or the like. As the temperature control method, any one of heating furnace temperature management, rolling speed control, and a method using a refrigerant may be used.

A duplex stainless steel pipe obtained by the above-described production method is a duplex stainless steel pipe having the above composition and a δ ferrite phase, and having a martensite phase or a structure composed of a retained austenite phase. Become. The residual austenite phase is 25% or less in terms of area ratio. A duplex stainless steel pipe having such a structure has a yield strength of 654 MPa or higher and a Charpy impact test temperature at the center of the wall thickness: absorbed energy at −10 ° C. (vE ₋₁₀ ) Is a high-strength duplex stainless steel seamless steel pipe having excellent low-temperature toughness of 50 J or more.

  Next, the present invention will be further described based on examples.

Molten steel having the composition shown in Table 1 was melted in an electric furnace and subjected to hole rolling to obtain round steel pieces (steel material) having a diameter of 58 mm. Next, using the apparatus row shown in FIG. 1, these round steel pieces are charged into the heating apparatus 1 and heated, and then the cumulative area reduction rate is obtained by a hot working apparatus 2 (a piercing rolling apparatus, a regular rolling apparatus). 60% hot working (finished wall thickness: 7.5 mm) was applied and allowed to cool to obtain a seamless steel pipe. The obtained seamless steel pipe was subjected to quenching and tempering heat treatment, and then a test piece was collected and subjected to structure observation, tensile test, and impact test. The temperature measurement of the steel material (work material) during heating and hot working and the test method of the obtained seamless steel pipe were as follows.
(1) Temperature measurement The temperature at the time of seamless steel pipe manufacture measured three points, the temperature in a heating furnace, the steel pipe inner surface temperature immediately after completion | finish of rolling, and the final hot working temperature. The temperature history during heating and hot working was derived by heat transfer analysis based on the temperature measured by a contact thermocouple. For the temperature (δ _A ) at which the δ ferrite single phase is obtained, the thermal expansion curve in the heating process was measured in advance, and the point where the transformation to the δ ferrite was completed and the curvature of the expansion curve changed was used.
Table 2 shows δ _{A of} each round steel piece (steel material), the maximum temperature reached by the steel material at the time of manufacturing each steel pipe, the holding time in the temperature range of (δ _A -100 ° C) or higher, and the final processing temperature.

(2) Microstructure observation From the obtained seamless steel pipe, a specimen for microstructural observation was collected, and after polishing the thickness center of the cross section parallel to the cross section perpendicular to the pipe axis direction, the structure after virera corrosion was observed. And confirmed to be a ferrite martensite structure.
Moreover, the presence or absence of the crack generation | occurrence | production in the steel pipe inner and outer surface, and the extent to which the crack has occurred were evaluated. The case where the crack occurrence depth was 1.0 mm or more was 5 or more was evaluated as “present / many”, and the case where it was less than that was evaluated as “present / small”.

(3) Tensile test A round bar tensile test piece (parallel part 6mmφ × GL20mm) was taken from the thickness center of the obtained seamless steel pipe so that the pipe axis direction was the tensile direction, and stipulated in JIS Z 2241 In accordance with the tensile test, yield strength YS (yield strength is strength at 0.2% elongation) was determined.
(4) Impact test V notch test so that the direction (L direction) parallel to the direction (C direction) perpendicular to the tube axis direction is the specimen longitudinal direction from the thickness center of the obtained seamless steel pipe A piece (half size) was collected and subjected to a Charpy impact test in accordance with the provisions of JIS Z 2242, and the absorbed energy (vE ₋₁₀ ) at a test temperature of −10 ° C. was measured. The number of test pieces was three each, and the average value thereof was the absorbed energy of the steel sheet.

  The obtained results are shown in Table 3.

  Any of the seamless steel pipes (herein referred to as examples of the present invention) manufactured under the preferable manufacturing conditions by applying the heating and hot working conditions proposed in the present invention can be refined in the structure even at the center of the wall thickness. Despite the high strength of yield strength: 654 MPa or more, the absorption energy at the test temperature: −10 ° C. has excellent low temperature toughness of 50 J or more. On the other hand, since the heating and hot working conditions of the present invention are not used, a seamless steel pipe that is out of the range of preferable manufacturing conditions cannot have a fine structure, and cannot secure desired high toughness and high strength.

DESCRIPTION OF SYMBOLS 1 Heating apparatus 2 Hot working apparatus 21 Punching and rolling apparatus 22 Rolling apparatus 221 Elongator 222 Plug mill 223 Sizing mill

Claims (2)

A method for producing a duplex stainless steel pipe having a structure including a ferrite phase and the balance of an austenite phase, a martensite phase, or both,
% By mass
C: 0.050% or less, Si: 1.00% or less,
Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, N: 0.01 to 0.15%,
O: 0.006% or less
And heating the steel material having a composition composed of the remaining Fe and inevitable impurities, and then performing hot working to obtain a seamless steel pipe of a predetermined shape, the heating and the hot working of the steel material during the hot working The temperature is measured with a direct contact or non-contact thermometer, and the maximum temperature derived by heat transfer analysis from the temperature at the measured timing is 1000 ° C or higher δ _A (δ ferrite phase becomes a single phase during the temperature rising process) Temperature) and the time during which the temperature of the steel material is (δ _A -100 ° C.) or more from the temperature history derived by the heat transfer analysis is 3600 s or less, and the final processing of the hot working is further performed by the steel material A method for producing a high-strength duplex stainless steel seamless pipe excellent in low-temperature toughness, characterized by being applied at a temperature range of 1200 ° C. or less at an outer surface temperature of the steel. In addition to the said composition, the said steel raw material is the mass%, and the following A group-D group A group: Al: 0.002-0.050%
Group B: Cu: 3.5% or less, W: 3.5% or less, REM: 0.3% or less C group: Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2 % Selected from 1% or 2 or more selected from the group D: Ca: 0.01% or less, B: 0.01% or less selected from 1 or 2 The method for producing a high-strength dual-phase stainless steel seamless steel pipe according to claim 1 , comprising a group or two or more groups.

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