JP6197850B2 - Method for producing duplex stainless steel seamless pipe - Google Patents

Description

The present invention relates to an austenitic / ferritic duplex stainless steel seamless steel pipe excellent in corrosion resistance under an environment containing carbon dioxide CO ₂ , chlorine ions Cl ^{−, and the} like, particularly deformability during hot rolling, and low temperature toughness And improve yield strength.

  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. The oil well steel pipe 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.

  Conventionally known austenitic and ferritic duplex stainless steels (hereinafter also referred to as duplex stainless steels) such as 22% Cr steel and 25% Cr steel are known as steel materials having such materials, particularly hydrogen sulfide. It is used for oil well seamless steel pipes and the like used in severe corrosive environments containing a large amount of and high temperatures. In addition, the duplex stainless steel is a high Cr-based ultra-low carbon of about 21 to 28%, and various steel materials including Mo, Ni, N and the like have been developed. The JIS standard also includes JIS G 4303 to 4305, It is defined as SUS329J1, SUS329J3L, SUS329J4L, or the like.

  However, since duplex stainless steel contains a large amount of alloy elements such as Cr and Mo, it generates hard and brittle intermetallic compounds (embrittlement phase) during cooling from the normal hot working temperature range. The hot workability is inferior, and if the embrittlement phase is precipitated at the use temperature, the mechanical properties and corrosion resistance are greatly reduced. Therefore, normally, heating is performed at a temperature equal to or higher than the precipitation temperature of the embrittlement phase, the hot working is finished before the embrittlement phase precipitation, and the alloying element concentrated in the intermetallic compound precipitated during cooling is dissolved in the base material. Therefore, a solution treatment is performed in which heating is performed at a temperature higher than the precipitation temperature of the embrittlement phase and quenching is performed. In addition, steels containing a large amount of these alloy elements are often in a multi-phase region even in a hot temperature region where no embrittlement phase precipitates. For example, in the above-described SUS329J4L, a ferrite is formed in the hot working temperature region. -Since it is a two-phase region of an austenite phase, strain concentrates on a ferrite phase having a relatively low deformation resistance, and breakage easily occurs. Therefore, especially when manufacturing thick-walled seamless steel pipes, it is necessary to finish the processing at a high temperature or to reduce the amount of strain in order to suppress defects during hot working, and a large strain is accumulated in the center of the thickness. It becomes difficult. If the application of strain is insufficient during hot working, it is difficult to refine crystal grains due to strain, and mechanical properties, particularly low-temperature toughness and yield strength, at the time of the product are lowered.

For example, Patent Document 1 proposes a method for manufacturing a high-strength stainless steel pipe. The technique described in Patent Document 1 is mass%, C: 0.03% or less, Si: 1% or less, Mn: 0.1 to 4%, Cr: 20 to 35%, Ni: 3 to 10% , Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to 3%, N: 0.15 to 0.60%, with the balance being a chemical composition consisting of Fe and impurities After producing a cold-working blank tube by hot working or by further solution heat treatment of steel, cold-rolling produces a duplex stainless steel pipe, which is processed at the cross-sectional reduction rate in the final cold rolling process. A method for producing a high-strength duplex stainless steel pipe, characterized in that cold rolling is performed under a condition where the degree Rd is in the range of 10 to 80% and the following expression (1) is satisfied.
Rd = exp [{ln (MYS) -ln (14.5 × Cr + 48.3 × Mo + 20.7 × W + 6.9 × N)} / 0.195] (1)
Rd: cross-sectional reduction rate (%), MYS: target yield strength (MPa), Cr, Mo, W, and N: element content (% by mass).

  According to the technique described in Patent Document 1, a high-strength duplex stainless steel seamless steel pipe can be obtained by strictly managing the content of the proper composition element and the cold working degree.

  For example, Patent Document 2 proposes a method for producing a high-strength duplex stainless steel material. The technique described in Patent Document 2 is that a solution treatment material of austenite-ferritic duplex stainless steel containing Cu is subjected to cold working with a cross-section reduction rate of 35% or more, and then once at 50 ° C./s. After heating to the temperature range of 800 to 1150 ° C. at the above heating rate, quenching is performed, and then warm processing is performed at 300 to 700 ° C. and then cold processing is performed again, or further aging at 450 to 700 ° C. It is a manufacturing method of the high intensity | strength duplex stainless steel material processed. In the technique described in Patent Document 2, by combining processing and heat treatment, the structure can be refined and the amount of processing can be significantly reduced even when cold processing is performed, so that deterioration of corrosion resistance can be prevented. .

Further, in Patent Document 3, a steel pipe material having a composition containing at least 9% by mass and Cr: 9% or more is heated, pierced by rolling by a tilt rolling method, and drawn into a seamless pipe having a predetermined dimension. A manufacturing method of a high Cr steel seamless pipe is disclosed in which the heating is set to (Ac ₄ transformation point + 10 ° C.) or higher and lower than the liquidus generation temperature to suppress the occurrence of rolling defects. In the technique described in Patent Document 3, the heating temperature of the steel pipe material is equal to or higher than the temperature at which δ-ferrite precipitates in γ grains other than the γ grain boundary, and less than the temperature at which the grain boundary liquefies (liquid phase generation temperature). By setting the temperature, the occurrence of grain boundary fracture is suppressed, and the rolling wrinkles are remarkably reduced.

  Patent Document 4 discloses a seamless steel pipe rolling method in which a heated material is perforated and then rolled by a rolling mill such as an elongator, a plug mill, and a mandrel mill. A rolling method of a seamless steel pipe material such as alloy steel with high work heat generation that cools the inside of the raw pipe before the rolling after piercing in advance so as not to reach the embrittlement region due to the above is disclosed. In the technique described in Patent Document 4, the entire raw tube is uniformly cooled, and it is possible to avoid brittle zone rolling while securing a sufficient processing temperature for a material having a material such as alloy steel having a large processing heat generation. Therefore, it is said that a seamless steel pipe can be rolled while achieving both rolling processability and quality.

Japanese Patent No. 4462454 Japanese Patent Application Laid-Open No. 07-207337 JP-A-2005-14031 Japanese Unexamined Patent Publication No. 01-278902

  However, in the technique described in Patent Document 1, it is necessary to increase the degree of processing at the cross-section reduction rate by final cold working, and a powerful cold working apparatus for working duplex stainless steel having high deformation resistance. Expensive capital investment is required. In addition, in the technique described in Patent Document 2, it has been pointed out that the corrosion resistance is lowered particularly in a high-temperature wet environment by increasing the degree of processing by cold working. It is said that it is effective to improve the strength by optimizing and to reduce the workability in cold working. On the other hand, the technique described in Patent Document 2 as a technique for that purpose requires multiple heat treatments including solution treatment and heat treatment after cold working, which complicates the process and reduces productivity. However, there is a problem that the amount of energy used increases and the manufacturing cost increases. In addition, there is a problem that hot working flaws occur in warm working at 300 to 700 ° C.

  Further, the technique described in Patent Document 3 is intended for rolling mills generated in seamless steel pipes of martensitic Cr steel or ferritic Cr steel containing 9% by mass or more, preferably 22% or less of Cr. The mechanical properties of the steel pipe after rolling are not taken into consideration at all, and even when applied to the duplex stainless steel seamless steel pipe targeted by the present invention, the corrosion resistance and hole resistance possessed by the duplex structure It becomes a problem that hot workability and mechanical properties cannot be improved while obtaining food properties.

  Moreover, the technique described in Patent Document 4 is forcibly cooling the inner surface of the raw tube after piercing and rolling by a press piercing mill in rolling of Ni-based alloy steel (Cr ≧ 20%, Ni ≧ 20%) having a large work heat generation. As a result, the cooling capacity equivalent to the inner and outer surfaces is ensured, and the entire pipe is uniformly cooled to a predetermined temperature.When rolling with a rolling mill such as an elongator, plug mill or mandrel mill, the pipe temperature rises due to processing heat. However, it is a technique for preventing the occurrence of internal flaws by suppressing the temperature below the embrittlement temperature range, and the structure of the raw pipe during rolling and the mechanical characteristics of the steel pipe after rolling are not considered at all.

  The present invention advantageously solves the problems of the prior art and stabilizes a high-strength duplex stainless steel seamless steel pipe that has been improved in strength by refining the structure without complicated heat treatment and cold working without the occurrence of cracks and the like. An object of the present invention is to provide an inexpensive manufacturing method that can be manufactured. Here, “high strength” refers to the case where the yield strength is YS: 588 MPa or more. The “two-phase” here refers to a structure including an austenite phase and a ferrite phase, a ferrite phase and a martensite phase, or a ferrite phase, an austenite phase, and a martensite phase.

  In order to achieve the above-described object, the present inventors diligently studied various factors that affect the strength of the duplex stainless steel seamless steel pipe. As a result, it came to mind that the most effective method for increasing the strength and improving the low temperature toughness of the duplex stainless steel pipe is to refine the structure.

  Therefore, further research was conducted, and in order to refine the structure of the duplex stainless steel seamless steel pipe, once it was heated to a temperature range with a large amount of ferrite precipitation, it was rapidly cooled to produce a supercooled ferrite phase. It has been found that it is effective to form a structure mainly composed of a strain-induced austenite phase by processing the supercooled ferrite phase.

The present invention has been completed based on 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 seamless pipe having a structure including a ferrite phase and an austenite phase in a hot working temperature range, and in mass%, C: 0.05% or less, Si: 2.0% Hereinafter, Mn: 2.0% or less, P: 0.05% or less, S: 0.03% or less, Cr: 16.0 to 35.0%, Ni: 3.0 to 12.0%, Mo: A steel material having a composition including 1.1 to 5.0% and N: 0.50% or less, the balance being Fe and inevitable impurities, is heated with a heating device, and then subjected to piercing and rolling to form a hollow material. When the material is processed into a duplex stainless steel pipe having a predetermined size, the heating is a process of heating to a temperature in the δ ferrite single phase region, and the hollow material after the piercing and rolling is subjected to a thickness center temperature. Temperature difference from the cooling start temperature is small at an average cooling rate of 1.0 ° C / s or more. Both are subjected to a cooling treatment for cooling to a cooling stop temperature of 50 ° C. or higher and 600 ° C. or higher, and then subjected to the processing, and have a structure including a ferrite phase, an austenite phase and / or a martensite phase. A method for producing a high-strength duplex stainless steel seamless steel pipe, characterized in that it is a seamless steel pipe.
(2) In addition to the above composition, Nb: 0.01-3.0%, Ti: 0.01-0.1%, V: 0.01-3.0% or one selected from The method for producing a high-strength seamless steel pipe according to (1), comprising two or more kinds.
(3) In addition to the above composition, W: 0.01 to 3.5%, Cu: 0.01 to 3.5%, Zr: 0.01 to 0.5%, REM: 0.005 to 0 The method for producing a high-strength seamless steel pipe according to (1) or (2), comprising one or more selected from 0.05%.
(4) The method for producing a high-strength seamless steel pipe according to any one of (1) to (3), further comprising Al: 0.50% or less in addition to the composition.
(5) The method for producing a high-strength seamless steel pipe according to any one of (1) to (4), further including Ca: 0.0005 to 0.01% in addition to the composition.
(6) The process according to (1) to (5), wherein the process is a process including a plurality of stages, and the cooling treatment is performed before at least one stage of the plurality of stages. The manufacturing method of the high intensity | strength duplex stainless steel seamless steel pipe in any one.
(7) The cooling after the processing is adjusted so as to have an average cooling rate of 20 ° C./s or less at the tube thickness center temperature, (1) to (6) Manufacturing method of high strength duplex stainless steel seamless steel pipe.
(8) The method for producing a high-strength duplex stainless steel seamless steel pipe according to (7), further comprising quenching and tempering treatment or solution treatment after cooling after the processing.

  According to the present invention, it is not necessary to perform complicated heat treatment and cold working, and there is no occurrence of piercing and rolling, cracking during processing, etc., and stable duplex stainless steel pipes having high strength and excellent low temperature toughness Therefore, it can be manufactured easily and has a remarkable industrial effect.

It is explanatory drawing which shows typically an example of the preferable apparatus row | line | column used by this invention.

  The present invention is a method for producing a duplex stainless steel seamless pipe, in which a steel material having a duplex stainless steel composition is subjected to heat treatment and piercing and rolling, and then subjected to cooling treatment and processing to obtain a seamless steel pipe having a predetermined size. is there.

As the steel material to be used, any of the steel materials having a duplex stainless steel composition defined as SUS329J1, SUS329J3L, and SUS329J4L in JIS G 4303 to 4305 can be applied. 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.03% or less, Cr: 16.0 to 35.0%, Ni : 3.0 to 12.0%, Mo: 1.1 to 5.0%, N: 0.50% or less, and a duplex stainless steel composition comprising the balance Fe and unavoidable impurities.
The reason for limiting the composition of the steel material will be described. Hereinafter, mass% is simply expressed as%.

C: 0.05% or less C is an element that increases strength, but it is desirable to reduce it as much as possible in order to reduce corrosion resistance. However, excessive reduction leads to an increase in manufacturing cost. For this reason, in this invention, it limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

Si: 2.0% or less Si is an element that acts as a deoxidizer and improves the strength. In order to obtain such an effect, it is desirable to contain 0.01% or more. If contained in a large amount exceeding%, the ductility is lowered and precipitation of intermetallic compounds is promoted, and the corrosion resistance is lowered. For this reason, Si was limited to 2.0% or less. In addition, Preferably it is 0.5 to 1.5%.

Mn: 2.0% or less Mn is an austenite stabilizing element, and appropriately adjusts the fraction of the austenite phase and the ferrite phase and contributes to the improvement of the corrosion resistance and workability of the duplex stainless steel seamless pipe. In order to obtain such an effect, the content is preferably 0.01% or more, but the content exceeding 2.0% lowers the corrosion resistance and hot workability. For this reason, Mn was limited to 2.0% or less. In addition, Preferably it is 0.5 to 1.5%.

P: 0.05% or less P is an element mixed as an impurity, and is easily segregated at grain boundaries and the like, and causes a decrease in corrosion resistance and hot workability. % Is acceptable. However, excessive reduction leads to an increase in material cost, so 0.002% or more is preferable. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.02% or less.

S: 0.03% or less S, like P, is an element mixed as an impurity, and exists as sulfide inclusions in steel, reducing ductility, corrosion resistance, and hot workability. However, it is acceptable up to 0.03%. However, excessive reduction leads to an increase in material cost, so 0.002% or more is preferable. For this reason, S is limited to 0.03% or less. In addition, Preferably it is 0.005% or less.

Cr: 16.0 to 35.0%
Cr is an element that improves the corrosion resistance. In order to obtain such an effect, the content of 16.0% or more is required. On the other hand, if the content exceeds 35.0%, the formation of intermetallic compounds such as σ phase and χ phase is promoted, and the corrosion resistance is reduced. For this reason, Cr was limited to the range of 16.0 to 35.0%. In addition, Preferably it is 16.0-28.0%.

Ni: 3.0 to 12.0%
Ni is an austenite stabilizing element, which appropriately adjusts the fraction of the austenite phase and the ferrite phase, and contributes to the improvement of the corrosion resistance and workability of the duplex stainless steel pipe. In order to obtain such an effect, the content of 3.0% or more is required. On the other hand, when the content exceeds 12.0%, an excessive increase in austenite phase is caused, and it becomes difficult to maintain a desired two-phase structure. For this reason, Ni was limited to the range of 3.0 to 12.0%. In addition, Preferably it is 3.0 to 9.0%.

Mo: 1.1-5.0%
Mo is an element that controls the ferrite phase fraction of the duplex stainless steel, and improves the corrosion resistance when added. In order to acquire such an effect, it is desirable to contain 1.1% or more. On the other hand, when it contains exceeding 5.0%, precipitation of an intermetallic compound is promoted and corrosion resistance and hot workability are reduced. For this reason, Mo was limited to 1.1 to 5.0%. In addition, Preferably it is 2.0 to 4.0%.

N: 0.50% or less N is a strong austenite stabilizing element and contributes to the improvement of corrosion resistance. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.50%, an excessive increase in austenite phase is caused, and it becomes difficult to maintain a desired two-phase structure. For this reason, N was limited to 0.50% or less.

  In addition to the above composition, Nb: 0.01 to 3.0%, Ti: 0.01 to 0.1%, V: 0.01 to 3.0%, or one or two selected More than seeds and / or W: 0.01-3.5%, Cu: 0.01-3.5%, Zr: 0.01-0.5%, REM: 0.005-0.05% 1 type (s) or 2 or more types selected from among them may be contained.

  Nb, Ti, and V are all elements that effectively contribute to an increase in strength, and can be selected and contained by one or more as necessary. In order to obtain such an effect, it is desirable to contain Nb: 0.01% or more, Ti: 0.01% or more, and V: 0.01% or more. On the other hand, even if it contains exceeding Nb: 3.0%, Ti: 0.1%, V: 3.0%, toughness and hot workability will fall. For this reason, when it contains, it is preferable to limit to Nb: 0.01-3.0%, Ti: 0.01-0.1%, V: 0.01-3.0%.

  Each of W, Cu, Zr, and REM is an element that contributes effectively to the improvement of corrosion resistance, and can be selected and contained as needed, if necessary. In order to acquire such an effect, it is desirable to contain W: 0.01% or more, Cu: 0.01% or more, Zr: 0.01% or more, REM: 0.005% or more. On the other hand, when it contains exceeding W: 3.5%, Cu: 3.5%, Zr: 0.5%, REM: 0.05%, toughness will fall. For this reason, when it contains, W: 0.01-3.5%, Cu: 0.01-3.5%, Zr: 0.01-0.5%, REM: 0.005-0. It is preferable to limit each to 05%.

  Furthermore, Al: 0.50% or less and / or Ca: 0.0005-0.01% can be contained.

  Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is desirable to contain 0.001% or more, but if it contains more than 0.50%, cleanliness Incurs a decline. For this reason, when it contains, it is preferable to limit Al to 0.01 to 0.50%.

  Ca is an element having an action of fixing S as CaS and detoxifying sulfide inclusions. Such an effect becomes remarkable when the content is 0.0005% or more. However, when the content exceeds 0.01%, the cleanliness is deteriorated. For this reason, when it contains, it is preferable to limit Ca to 0.0005 to 0.01%. In addition, More preferably, it is 0.0005 to 0.005% or less.

  The balance other than the components described above consists of Fe and inevitable impurities. As an unavoidable impurity, O (oxygen): 0.0050% or less is acceptable.

Any conventional method can be applied to the method for producing the steel material used in the present invention, and there is no particular need to limit it. For example, molten steel with a predetermined duplex stainless steel composition is melted in a converter, electric furnace, melting furnace or the like, or further refined by an AOD apparatus, VOD apparatus, etc., and then slab, billet, etc. by a continuous casting method It is preferable to make steel slabs such as slabs and billets by the slabs of slabs or the ingot-bundling rolling method. The steel material may be subjected to homogenization annealing at a high temperature in advance.
First, the steel material is subjected to heat treatment by a heating device.

The heat treatment is a treatment in which a steel material is charged into a heating device and heated to a temperature (heating temperature) in the δ ferrite single phase region. The “temperature in the single phase region of δ ferrite” here refers to a temperature that is equal to or higher than the lower limit temperature (hereinafter also referred to as δ _A point) at which the single phase of δ ferrite is formed and lower than the melting point. The δ _A point may be obtained from, for example, equilibrium state calculation using Thermocalc or the like, or may be obtained from analysis of a thermal expansion curve.

  The heating device 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 cast 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.

Heating temperature: δ ferrite single phase region temperature (δ _A point or more and less than the melting point)
By setting the heating temperature to the δ _A point or higher, the ferrite phase in a non-equilibrium state can be obtained by subsequent rapid cooling, and the microstructure can be remarkably refined by processing. Further, the lower deformation resistance is advantageous in reducing the hot working load and suppressing wrinkles. On the other hand, when the heating temperature is equal to or higher than the melting point, accumulation of strain due to processing becomes difficult. Therefore, the heating temperature of the steel material is limited to a temperature in the range of less than the melting point or higher [delta] _A point. In addition, Preferably it is 1100-1300 degreeC.
The heat-treated steel material is made into a hollow material by piercing and rolling, and then subjected to cooling treatment and processing.

  The cooling process is performed using a cooling device at an average cooling rate of 1.0 ° C./s or more at the thickness center temperature of the hollow material, a temperature difference from the cooling start temperature is at least 50 ° C., and 600 ° C. or more It is set as the process cooled to the cooling stop temperature which becomes. The wall thickness center temperature is calculated by heat transfer calculation based on the measured surface temperature.

  The type of the cooling device used in the present invention is not particularly limited as long as it is a device that can cool a high-temperature hollow material (material to be cooled) at a desired cooling rate or higher. As a cooling device that can secure a desired cooling rate relatively easily, cooling water, compressed air, or mist is injected or supplied to the outer surface or outer surface of a material being processed (including a hollow material) that is a material to be cooled. Thus, it is preferable to use a cooling system.

Average cooling rate: 1.0 ° C./s or more In the present invention, the cooling treatment is performed at least at the center of the thickness of the material to be cooled in order to obtain a supercooled ferrite phase (non-equilibrium phase distribution). The cooling is performed at an average cooling rate of 0 ° C./s or more. If cooling can only be slower than the average cooling rate described above, the austenite phase precipitates from the ferrite phase grain boundaries and within the grains, it is not possible to obtain a non-equilibrium phase distribution, Cannot be refined. The upper limit of the cooling rate is not particularly limited, but is preferably 50 ° C./s from the viewpoint of preventing cracking and bending due to thermal stress. In addition, Preferably it is 3-30 degrees C / s.

Cooling temperature range: 50 ° C. or higher The cooling temperature range, that is, the temperature difference between the cooling start temperature and the cooling stop temperature is at least 50 ° C. at the wall thickness center temperature. When the cooling temperature range is less than 50 ° C., the fraction of the supercooled ferrite phase is small, and a remarkable non-equilibrium phase fraction cannot be ensured, and the desired structure refinement cannot be achieved by subsequent processing. For this reason, the temperature range of cooling was limited to 50 degreeC or more. The larger the cooling temperature range, the easier it is to secure a non-equilibrium phase fraction. In addition, Preferably it is 100 degreeC or more. The cooling start temperature is the thickness center temperature of the material to be cooled before the start of cooling.

Cooling stop temperature: 600 ° C or more When the cooling stop temperature is less than 600 ° C, the diffusion of elements is slow, and the phase transformation (α → γ transformation) by the subsequent processing is delayed, and it takes a long time to secure the desired microstructure. In addition, productivity is reduced, and processing load is increased and hot workability is reduced. For this reason, the cooling stop temperature is limited to 600 ° C. or more at the thickness center temperature. In addition, Preferably it is 700 degreeC or more.

  The hollow material cooled to a predetermined cooling stop temperature is then processed (hot processing) to obtain a seamless steel pipe having a predetermined size. The time from the end of cooling to hot working is preferably within 600 s. If the time from the end of cooling to the start of processing exceeds 600 s, the phase transformation proceeds and a non-equilibrium state cannot be secured.

  The piercing-rolling and processing (hot processing) applied to the steel material only need to be a seamless steel pipe having a predetermined size, and any conventional processing conditions can be applied, and there is no particular limitation. In the present invention, a desired structure refinement is possible even with a relatively low processing amount (rolling rate). However, from the viewpoint of structure refinement, it is preferable that the cross-sectional reduction rate is at least 20% or more cumulatively.

  The cooling rate after processing is not particularly limited. However, when the average cooling rate at the tube thickness center position exceeds 20 ° C./s, the cooling rate is set on the outlet side of the processing apparatus. It is preferable to charge the heat retaining device and adjust the average cooling rate to 20 ° C./s or less. When cooling after processing exceeds 20 ° C./s and becomes too fast, precipitation of the second phase becomes insufficient, a desired microstructure cannot be obtained, and strength and low-temperature toughness decrease. For this reason, it is preferable that the cooling rate after processing is limited to an average cooling rate of 20 ° C./s or less at the tube thickness center position by installing a heat retaining device on the outlet side of the processing device.

  The apparatus row used in the present invention is a piercing-rolling of a heated steel material, and after cooling the hollow material after piercing-rolling to a temperature within an appropriate temperature range at an appropriate cooling rate, it is processed and seamlessly having a predetermined dimension. It is a device row that can be a steel pipe. An example of a preferable apparatus row used in the present invention is shown in FIG. In FIG. 1 (a), it is an apparatus row | line | column which has arrange | positioned the cooling device 3 between the piercing-rolling apparatus 2 and the hot processing apparatus 4, and the thermal insulation apparatus 5 is arrange | positioned on the exit side of the hot processing apparatus 4. Yes.

  In the case where a plurality of hot working apparatuses 4 (41, 42,...) Are arranged, as shown in FIG. You may arrange | position in the entrance side of the inter-processing apparatus 42. FIG. In addition, the apparatus which arrange | positioned between the piercing-rolling apparatus 2 and the hot processing apparatus 4 when the apparatus 3 arrange | positioned between the hot working apparatus 41 and the hot working apparatus 42 is used for the cooling device 3 You can expect the same effect as using columns. This is because, in the present invention, it is confirmed that the influence of the processing mode of the hot processing apparatus is small.

  The steel material obtained by subjecting the steel material having the above-described duplex stainless steel composition to the above-described heating and cooling treatment after piercing and rolling, and further processing the steel material, the ferrite phase, the balance is the austenite phase and / or martensite It has a refined structure consisting of phases. The term “miniaturized structure” as used herein refers to a case where the number of grain boundaries in a unit length is 1.1 times or more compared to the case where rapid cooling is not performed after piercing and rolling. .

  In the present invention, in order to improve strength adjustment and toughness, it is preferable to perform quenching and tempering (QT) treatment or solution treatment after cooling after processing. The quenching temperature or solution temperature is a temperature range in which intermetallic compounds such as σ phase and χ phase, which are embrittlement phases, are dissolved, that is, 900 ° C. or higher, preferably 1150 ° C. or lower. It has been confirmed that even when such a heat treatment is performed, no significant change is observed in the microstructure obtained by the method of the present invention.

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

  Molten steel having the steel composition shown in Table 1 was melted in a vacuum melting furnace and formed into round steel pieces having a diameter of 70 mm by hot rolling and machining. Next, using the seamless steel pipe manufacturing apparatus row shown in FIG. 1 (a), these steel materials are charged into the heating apparatus 1 and heated to the heating temperatures shown in Table 2 for a certain time (60 min). After being held, the barrel type Mannesmann type piercing and rolling device 2 is used to perform piercing and rolling to form a hollow material (thickness 20 mm), and the cooling device 3 using coolant by spraying as a coolant at the average cooling rate shown in Table 2. It cooled to the cooling stop temperature shown in Table 2, and immediately rolled with the elongator shown in the hot processing apparatus 4 by the cumulative reduction shown in Table 2, and it was set as the predetermined seamless steel pipe (outer diameter 74 mm x thickness 13-16 mm). . The cooling after rolling was allowed to cool (0.1 to 0.5 ° C./s), and the heat retaining device 5 was not particularly used. The obtained seamless steel pipe was subjected to an appropriate quenching and tempering treatment (QT treatment) or a solid solution heat treatment that was heated to 1050 to 1150 ° C. and then rapidly cooled.

  In some of the seamless steel pipes, the cooling device 3 in FIG. 1A was not used, and cooling before processing was allowed to cool (0.8 ° C./s), which was a comparative example (reference).

  About the obtained seamless steel pipe, first, the presence or absence of the crack generation | occurrence | production in the steel pipe edge part, and the grade, when the crack has generate | occur | produced, 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”.

From the obtained steel plate, a test piece was collected and subjected to a structure observation and a tensile test. The test method was as follows.
(1) Microstructure observation From the obtained seamless steel pipe, a specimen for microstructural observation was collected, and the cross section (C cross section) perpendicular to the tube axis direction was polished and corroded (corrosive liquid: Villera liquid). The tissue was observed with a scanning electron microscope (magnification: 200 times) or a scanning electron microscope (magnification: 1000 times), imaged, and the type of tissue and the fraction thereof were measured using image analysis. As an index of refinement, the number of phase / grain boundaries intersecting the unit length straight line was measured from the structure photograph. Note that the number of phases / grain boundaries per unit length is the same steel type, the same heating temperature, and the cooling rate before processing under the same cooling stop temperature condition is allowed to cool (0.8 ° C./s The value of the steel pipe that is) is shown as a ratio with respect to the reference value as the reference (1.00).
(2) Tensile test From the obtained seamless steel pipe, a round bar tensile test piece (parallel part 6 mmφ x GL20 mm) was sampled so that the pipe axis direction would be the tensile direction, and pulled in accordance with the provisions of JIS Z 2241. A test was conducted to determine the yield strength YS. The yield strength was 0.2% elongation. The yield strength of the steel sheet with the same steel type and the cooling rate before processing is allowed to cool (0.8 ° C / s) is defined as the (basic yield strength), and the difference between the yield strength and the standard yield strength. The value divided by the reference yield strength (%), ΔYS (%) (= (yield strength−reference yield strength) × 100 / (reference yield strength) was calculated. The yield strength YS was 588 MPa. Those less than were marked with x, and those above were marked with.
(3) Charpy test A V-notch full-size Charpy test piece was cut out from the obtained seamless steel pipe so as to be parallel to the cross section (C cross section) perpendicular to the pipe axis direction, cooled to -10 ° C, and then subjected to the Charpy test. The absorbed energy (vE- ₁₀ ) was determined. The absorption energy of a steel plate of the same steel type with a cooling rate before processing is allowed to cool (0.8 ° C / s) is taken as the reference absorption energy value, and the difference between the obtained absorption energy value and the reference absorption energy value is used as the reference absorption energy. A value (%) divided by the value, ΔE (%) (= (absorbed energy value−reference absorbed energy value) × 100 / (reference absorbed energy value)) was calculated.

  The obtained results are shown in Table 3.

  In all of the examples of the present invention, the phase / grain boundary number ratio exceeds 1.00, the structure can be refined, and a yield strength of 588 MPa or more (evaluation of yield strength) is ensured at −10 ° C. A duplex stainless steel seamless pipe capable of ensuring low temperature toughness with an absorption energy improved by 40% or more without the occurrence of cracks during hot working than the conventional comparative example can be manufactured. On the other hand, in the comparative examples outside the scope of the present invention, it was confirmed that the structure was not refined, the desired high strength and toughness were not ensured, or cracks were generated.

DESCRIPTION OF SYMBOLS 1 Heating apparatus 2 Punching and rolling apparatus 3 Cooling apparatus 4 Hot working apparatus 5 Thermal insulation apparatus

Claims (8)

A method for producing a duplex stainless steel pipe having a structure including a ferrite phase and an austenite phase in a hot working temperature range,
In mass%, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.03% or less, Cr: 16.0 35.0%, Ni: 3.0-12.0%, Mo: 1.1-5.0%, N: 0.50% or less, and a steel material having a composition comprising the balance Fe and inevitable impurities After heating with a heating device, it is subjected to piercing and rolling to a hollow material, and the hollow material is processed into a duplex stainless steel pipe of a predetermined size.
The heating is a treatment for heating to a temperature in the δ ferrite single phase region,
Cooling stop temperature at which the temperature difference from the cooling start temperature is at least 50 ° C. and 600 ° C. or more at an average cooling rate of 1.0 ° C./s or more at the wall thickness center temperature in the hollow material after the piercing and rolling. Cooling process to cool to
After the hollow material has a non-equilibrium ferrite phase, the processing is performed within 600 s to obtain a seamless steel pipe having a structure including a ferrite phase, an austenite phase and / or a martensite phase. A method for producing a high-strength duplex stainless steel seamless steel pipe. In addition to the above composition, one or more selected from Nb: 0.01 to 3.0%, Ti: 0.01 to 0.1%, V: 0.01 to 3.0% The manufacturing method of the high intensity | strength duplex stainless steel seamless steel pipe of Claim 1 characterized by the above-mentioned. In addition to the above composition, W: 0.01 to 3.5%, Cu: 0.01 to 3.5%, Zr: 0.01 to 0.5%, REM: 0.005 to 0.05% The manufacturing method of the high intensity | strength duplex stainless steel seamless steel pipe of Claim 1 or Claim 2 containing 1 type, or 2 or more types chosen from these. The method for producing a high-strength duplex stainless steel seamless steel pipe according to any one of claims 1 to 3, further comprising Al: 0.50% or less in addition to the composition. The method for producing a high-strength duplex stainless steel seamless steel pipe according to any one of claims 1 to 4, further comprising Ca: 0.0005 to 0.01% in addition to the composition.   The said process is a process which consists of multiple steps | paragraphs, The said cooling process is given before the process of at least 1 step | paragraph among the said multi-stage process. Manufacturing method of high strength duplex stainless steel seamless steel pipe.   The high-strength duplex stainless steel according to any one of claims 1 to 6, wherein the cooling after the processing is adjusted so as to obtain an average cooling rate of 20 ° C / s or less at a tube wall thickness center temperature. A method for producing seamless steel pipes. The method for producing a high-strength duplex stainless steel seamless pipe according to claim 7, further comprising a quenching and tempering treatment or a solution treatment after the cooling after the processing.

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