JPWO2015005119A1 - Manufacturing method of high Cr steel pipe - Google Patents

Abstract

Provided is a production method capable of producing a low hardness, high Cr steel pipe in order to facilitate processing such as pipe expansion or drawing performed in a subsequent process. The manufacturing method of the high Cr steel pipe by this invention is the mass%, C: 0.05-0.15%, Si: 0.02-0.70%, Mn: 0.10-1.0%, P: 0.025% or less, S: 0.010% or less, Cr: 8.0 to 10%, Mo: 0.15 to 1.25%, V: 0.08 to 0.35%, Nb: 0.02 -0.12%, Al: 0.05% or less, N: 0.01-0.10%, W: 0-2.50%, B: 0-0.01%, Ti: 0-0.1 %, Ni: 0 to 0.8%, Ca and / or Mg total: 0 to 0.01%, and the balance is obtained by cooling a billet composed of Fe and impurities after hot working A first heat treatment step in which the tube temperature is maintained at a first temperature higher than the AC1 point and not higher than 950 ° C., and after the first heat treatment step, the tube temperature is lowered to a temperature below the Ms point. Rukoto without and a second heat treatment step of holding the blank tube temperature continues second temperature below AC1 point.

Description

  The present invention relates to a method for manufacturing a steel pipe, and more particularly to a method for manufacturing a high Cr steel pipe used in an oil plant or a thermal power plant.

  Various heat-resistant steel pipes are used in oil plants and thermal power plants. Among the equipment constituting the plant, high Cr steel pipes containing 8.0 to 10% by mass of Cr are used for pipes used under high temperature and high pressure, reaction pipes, and steel pipes used for heat exchangers. A typical example of such a high Cr steel pipe is a steel pipe having a chemical composition defined in ASTM P91 and P92.

  Examples of the high Cr steel pipe include a forged steel pipe manufactured by forging, a welded steel pipe manufactured by welding, and a seamless steel pipe manufactured by piercing and rolling by the Mannesmann method.

  For example, a connection pipe that connects a turbine and a boiler of a thermal power plant has a large outer diameter of 450 to 900 mm. As such a large-diameter high Cr steel pipe, a forged steel pipe or a welded steel pipe can also be used. However, the forged steel pipe has low productivity and it is difficult to produce a thin steel pipe. Also, welded steel pipes may have low mechanical properties at the welds.

  If a large-diameter high Cr steel pipe is produced by expanding the pipe using a seamless steel pipe, it is possible to prevent the productivity from being lowered, and it is also possible to produce a thin steel pipe. Further, there is no welded portion extending in the axial direction such as a welded steel pipe. Therefore, it is preferable to use a seamless steel pipe to manufacture a high Cr steel pipe.

  An example of the manufacturing method of the high Cr steel pipe product using the seamless steel pipe is as follows. A high Cr steel pipe intermediate product (seamless steel pipe) is manufactured by piercing and rolling. The manufactured high Cr steel pipe is cold or warm processed (expanded or expanded) to a predetermined size. Heat treatment (normalizing and tempering, so-called norten treatment) is performed on the processed high Cr steel pipe to produce a high Cr steel pipe product.

  Since the high Cr steel pipe has a high Cr content of 8.0 to 10%, the hardness of the high Cr steel pipe (base pipe) after piercing and rolling is high. Therefore, when cold or warm processing is performed on the raw tube, softening treatment is performed on the raw tube before processing.

  As described above, the general heat treatment method for ASTM P91 standard high Cr steel pipe products is the norten treatment. Therefore, this norten treatment can be used as a softening treatment method for high Cr steel pipe (element tube). However, if the same tube is subjected to the same nortening treatment as that of the high Cr steel pipe product as the final product, the normalizing temperature is high and the manufacturing cost is increased. Furthermore, the amount of scale formed on the surface of the base tube after norten treatment increases. For this reason, there is a case where descaling (such as shot blasting) must be performed before expanding or expanding the pipe.

JP-A-10-30121 contains C: 0.20%, Cr: 8-10%, Mo: 1.5% or less, W: 2.0% or less Of CrMo steel that is kept at a temperature range of A _c1 transformation point to A _c3 transformation point for 5 minutes or more, then cooled to a constant temperature holding temperature of 660 to 800 ° C., and held at a high temperature for a predetermined holding time. A softening heat treatment method is disclosed. However, it is said that this method cannot be applied to steels containing alloy elements that form carbonitrides such as V and Nb. Japanese Patent Application Laid-Open No. 2004-285432 proposes a method for softening a high Cr steel pipe (base pipe). JP-A-2004-285432 discloses, after manufacturing the high-Cr steel by hot rolling, a high Cr steel _{(A C1} transformation temperature _{+ A C3} transformation temperature) / 2 or more, _{(A C3} transformation temperature + 50 ° C.) below the temperature After heating at 700 ° C., hold at a temperature of 700 to 800 ° C. for 30 minutes or more and cool. In Japanese Patent Application Laid-Open No. 2004-285432, the parent phase is austenitized by heat treatment in the first stage, and coarse carbides and carbonitrides are precipitated in the steel as much as possible. Then, C and N dissolved in austenite are sufficiently coarsely precipitated by the second stage heat treatment. That is, in Japanese Patent Application Laid-Open No. 2004-285432, both the first-stage heat treatment and the second-stage heat treatment aim to coarsely precipitate carbides and carbonitrides.

  According to Japanese Patent Laid-Open No. 2004-285432, C and N dissolved in austenite are sufficiently coarsely precipitated in the second stage heat treatment (see paragraph [0024] of Japanese Patent Laid-Open No. 2004-285432). Therefore, in the steel pipe disclosed in Japanese Patent Application Laid-Open No. 2004-285432, residual austenite should be present in the steel during the second heat treatment. Since retained austenite is present in quenched steel, it is considered in JP-A-2004-285432 that the structure of the steel after the first heat treatment is composed of martensite and retained austenite. In the case of such a structure, the steel pipe may not be sufficiently softened even after performing the second stage heat treatment.

  An object of the present invention is to provide a production method capable of producing a low-hardness high Cr steel pipe in order to facilitate processing such as pipe expansion or drawing performed in a subsequent process.

The manufacturing method of the high Cr steel pipe by this invention is the mass%, C: 0.05-0.15%, Si: 0.02-0.70%, Mn: 0.10-1.0%, P: 0.025% or less, S: 0.010% or less, Cr: 8.0 to 10%, Mo: 0.15 to 1.25%, V: 0.08 to 0.35%, Nb: 0.02 -0.12%, Al: 0.05% or less, N: 0.01-0.10%, W: 0-2.50%, B: 0-0.01%, Ti: 0-0.1 %, Ni: 0 to 0.8%, Ca and / or Mg total: 0 to 0.01%, and the balance is obtained by cooling a billet composed of Fe and impurities after hot working preparing a lower base tube temperature and a first heat treatment step of holding at a first temperature of 950 ° C. or less higher than the _C1 point a, after the first heat treatment step, the raw tube temperature to a temperature below Ms point Without the gel continued and a second heat treatment step of holding at a second temperature below point _C1 A.

  In the method for producing a high Cr steel pipe according to the present invention, a low hardness high Cr steel pipe can be produced.

FIG. 1 is a diagram showing a heat pattern (heat history) of first and second heat treatment steps in a method for producing a high Cr steel pipe according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an example of a heat pattern used in the example (comparative example). FIG. 3 is an example of a heat pattern used in the example (comparative example), and is a diagram illustrating an example of another heat pattern different from FIG. FIG. 4 is a diagram showing an example of a heat pattern according to the present invention used in an example (invention example). FIG. 5 is an example of a heat pattern used in the example (comparative example), and is a diagram illustrating an example of a heat pattern different from those in FIGS. 2 and 3. FIG. 6 is an example of a heat pattern according to the present invention used in an example (invention example), and is a diagram showing an embodiment different from FIG. FIG. 7 is an example of a heat pattern used in another example (comparative example) different from FIGS. 2 to 6, and is a diagram illustrating an example of a heat pattern different from FIGS. 2, 3, and 5. FIG. 8 is a diagram showing the Vickers hardness of the high Cr steel pipe when the heat treatment of each heat pattern of FIGS. 2 to 7 is performed. FIG. 9 is a diagram showing the Vickers hardness of a high Cr steel pipe when heat treatment of each heat pattern of FIGS. 2 to 7 is performed on a high Cr steel pipe of a steel type different from FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. In the following description, “%” means mass% unless otherwise specified.

  The present inventors examined a method of reducing the hardness of a high Cr steel pipe that is subjected to processing such as pipe expansion or drawing in a later process. As a result, the following knowledge was obtained.

  If the formation of martensite can be suppressed, the hardness of the high Cr steel pipe will be low and it will be easy to process. High Cr steel containing 8 to 10% Cr has high self-hardness. Therefore, if a raw pipe (seamless steel pipe) is manufactured by piercing and rolling a material made of high Cr steel, martensite is generated when the raw pipe is cooled.

Therefore, in order to eliminate or reduce the generated martensite, as the first heat treatment step, the blank tube temperature was heated to a first temperature of 950 ° C. or less higher than the _C1 point A, holds. In this case, part or all of martensite is transformed into austenite, so that martensite in the steel can be eliminated or reduced.

However, as described above, high Cr steel has high self-hardness, so that martensite may be generated again during cooling after the first heat treatment step. In this case, the second heat treatment step, if tempering the blank tube at a temperature of _C1 points A, high Cr steel is to some extent soften.

  However, if martensite remains in the base tube before the second heat treatment step, even if tempering is performed in the second heat treatment step, tempered martensite remains in the steel, and the hardness is unlikely to be sufficiently low. .

  Therefore, in the present invention, as shown in FIG. 1, the tempering process is continuously started as the second heat treatment step without lowering the tube temperature after the first heat treatment step below the Ms point. In this case, it is possible to suppress the formation of martensite by cooling in the raw tube after the first heat treatment step. Therefore, tempered martensite is not easily generated in the high Cr steel pipe in the second heat treatment step. For this reason, the hardness of the high Cr steel pipe can be lowered, and it becomes easy to perform processing such as pipe expansion or pipe extension in a subsequent process.

  Preferably, the cooling rate of the raw tube temperature is 90 ° C./min or less after the first heat treatment step is completed and before the second heat treatment step is started. In this case, the hardness of the high Cr steel pipe can be further reduced.

  The gist of the present invention completed based on the above findings is as follows.

The manufacturing method of the high Cr steel pipe by this invention is the mass%, C: 0.05-0.15%, Si: 0.02-0.70%, Mn: 0.10-1.0%, P: 0.025% or less, S: 0.010% or less, Cr: 8.0 to 10%, Mo: 0.15 to 1.25%, V: 0.08 to 0.35%, Nb: 0.02 -0.12%, Al: 0.05% or less, N: 0.01-0.10%, W: 0-2.50%, B: 0-0.01%, Ti: 0-0.1 % Ni: 0 to 0.8% total Ca and / or Mg: containing 0 to 0.01%, the balance being a step of preparing a mother tube made of Fe and impurities, the blank tube temperature _{a C1} a first heat treatment step of holding at a first temperature of 950 ° C. or less higher than the point, after the first heat treatment step, without lowering the raw tube temperature to a temperature below Ms point, continued a following point _C1 And a second heat treatment step of holding the blank tube temperature at 2 temperatures.

  The second temperature is preferably 700 ° C. or higher.

  In this case, the manufactured high Cr steel pipe is easily softened.

  In the above chemical composition, the N content is preferably less than 0.05%.

  In this case, the occurrence of blow holes in the high Cr steel pipe material is suppressed. Therefore, wrinkles are not easily formed on the surface of the high Cr steel pipe. In addition, since the N content is low, the ratio of nitride formation and nitrogen solid solution is small, and the steel is easily softened.

  In the said manufacturing method, it is preferable that the cooling rate of the raw tube after a 1st heat treatment process until it starts a 2nd heat treatment process is 90 degrees C / min or less.

  In this case, the hardness of the high Cr steel pipe is further reduced.

  In the above manufacturing method, the first heat treatment step is performed in the first heat treatment furnace, the second heat treatment step is performed in a second heat treatment furnace different from the first heat treatment furnace, and the manufacturing method includes: It is preferable that the method further comprises a step of extracting from the above and a step of charging the extracted raw tube into the second heat treatment furnace.

  In this case, productivity can be improved compared with the case where heat treatment is performed in a single heat treatment furnace. The reason will be described below.

  Conventionally, when isothermally transforming a steel pipe at a predetermined holding temperature, it has been considered that the temperature history from the austenite state to the holding temperature must be strictly managed. However, according to the investigation by the present inventors, it is found that the influence of the temperature history between the first heat treatment and the second heat treatment is surprisingly small unless the temperature of the tube is equal to or lower than the Ms point. It was.

  When the heat treatment is performed in a single heat treatment furnace, it is necessary to lower the temperature of the heat treatment furnace from the first temperature to the second temperature. On the other hand, as described above, large-diameter high Cr steel pipes having an outer diameter of 450 to 900 mm are used in petroleum plants and thermal power plants. Even when a seamless steel pipe is expanded to produce a high Cr steel pipe, a seamless steel pipe (base pipe) having a large diameter and a wall thickness of 185 to 450 mm is required. It takes a long time to lower the temperature of such a large diameter and large heat capacity pipe from the first temperature to the second temperature. Alternatively, special equipment for forced cooling is required.

  On the other hand, when the first heat treatment furnace and the second heat treatment furnace are used, the temperature of the raw pipe is quickly increased by extracting the raw pipe outside the furnace after holding the raw pipe at the first temperature in the first heat treatment furnace. Can be lowered. Moreover, since the temperature of each heat treatment furnace is constant and there is no need to lower the temperature or reheat, the heat treatment can be performed continuously. Therefore, productivity can be improved compared with the case where heat treatment is performed in a single heat treatment furnace.

  In the manufacturing method, it is preferable that the extracting step lowers the tube temperature to a temperature higher than the Ms point and lower than the second temperature. In this case, temperature management becomes easier.

  The present invention having the above gist is described in detail below.

[Production method]
The manufacturing method of the high Cr steel pipe by this embodiment is provided with the process of preparing a raw pipe, and the 1st and 2nd heat treatment process of implementing heat processing with respect to a raw pipe. Hereinafter, each process is explained in full detail.

[Preparation process]
In the preparation process, a raw tube is prepared. The raw tube is made of high Cr steel, and the chemical composition of the high Cr steel is as follows.

C: 0.05 to 0.15%
Carbon (C) increases the high-temperature strength of the steel by forming carbides and carbonitrides in the heat treatment step (norten treatment) after processing (expanding or drawing) the high Cr steel pipe produced in this embodiment. . If the C content is too low, the above effect cannot be obtained. On the other hand, if C content is too high, the weldability of steel will fall. Therefore, the C content is 0.05 to 0.15%. The minimum with preferable C content is 0.07%, More preferably, it is 0.08%. The upper limit with preferable C content is 0.13%, More preferably, it is 0.12%.

Si: 0.02 to 0.70%
Silicon (Si) deoxidizes steel. Si further increases the oxidation resistance of the steel. If the Si content is too low, the above effect cannot be obtained. On the other hand, if the Si content is too high, the toughness of the steel decreases. Therefore, the Si content is 0.02 to 0.70%. The minimum with preferable Si content is 0.05%, More preferably, it is 0.20%. The upper limit with preferable Si content is 0.55%, More preferably, it is 0.50%.

Mn: 0.10 to 1.0%
Manganese (Mn) desulfurizes steel. Mn further increases the strength of the steel. If the Mn content is too low, the above effect cannot be obtained. On the other hand, if the Mn content is too high, the toughness of the steel decreases. Therefore, the Mn content is 0.10 to 1.0%. The minimum with preferable Mn content is 0.25%, More preferably, it is 0.28%, More preferably, it is 0.30%. The upper limit with preferable Mn content is 0.70%, More preferably, it is 0.60%, More preferably, it is 0.45%.

P: 0.025% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and embrittles the steel. As a result, the creep strength of the steel decreases. Therefore, the P content is preferably as low as possible. The P content is 0.025% or less. P content is preferably 0.018% or less, more preferably 0.012% or less.

S: 0.010% or less Sulfur (S) is an impurity. S segregates at the grain boundary and causes grain boundary embrittlement. Therefore, it is preferable that the S content is as small as possible. S content is 0.010% or less. A preferable S content is 0.008% or less, and more preferably 0.005% or less.

Cr: 8.0 to 10%
Chromium enhances the steam oxidation resistance and hot corrosion resistance of steel. Further, Cr forms fine carbides such as M ₂₃ C ₆ and M ₆ C to increase the high temperature strength of the steel. Cr further enhances the corrosion resistance of steel in oil well environments. On the other hand, if the Cr content is too high, the weldability, toughness and hot workability of the steel are reduced. Therefore, the Cr content is 8.0 to 10%. The minimum with preferable Cr content is 8.2%, More preferably, it is 8.5%. The upper limit with preferable Cr content is 9.5%.

Mo: 0.15-1.25%
Molybdenum (Mo) increases the high temperature strength of steel as a solid solution strengthening element and a carbide forming element. If the Mo content is too low, the above effect cannot be obtained. On the other hand, if the Mo content is too high, the weldability and toughness of the steel decrease. Therefore, the Mo content is 0.15 to 1.25%. The minimum with preferable Mo content is 0.70%, More preferably, it is 0.85%. The upper limit with preferable Mo content is 1.15%, More preferably, it is 1.05%.

V: 0.08 to 0.35%
Vanadium (V) forms carbonitrides and increases the high temperature strength and creep rupture strength of the steel. If the V content is too low, the above effect cannot be obtained. On the other hand, if the V content is too high, coarse carbides are generated and the creep rupture strength of the steel is reduced. Therefore, the V content is 0.08 to 0.35%. The minimum with preferable V content is 0.15%, More preferably, it is 0.18%. The upper limit with preferable V content is 0.30%, More preferably, it is 0.25%.

Nb: 0.02 to 0.12%
Niobium (Nb), like V, forms carbonitrides and increases the high temperature strength and creep rupture strength of the steel. If the Nb content is too low, the above effect cannot be obtained. On the other hand, if the Nb content is too high, carbonitrides aggregate and coarsen, and the strength of the steel decreases. Therefore, the Nb content is 0.02 to 0.12%. The minimum with preferable Nb content is 0.04%, More preferably, it is 0.06%. The upper limit with preferable Nb content is 0.10%, More preferably, it is 0.09%.

Al: 0.05% or less Aluminum (Al) deoxidizes steel. If Al is contained even a little (that is, if more than 0% is contained), the above effect can be obtained. On the other hand, if the Al content is too high, the high temperature strength of the steel decreases. Therefore, the Al content is 0.05% or less. The minimum with preferable Al content is 0.001%, More preferably, it is 0.003%. A preferable upper limit of the Al content is 0.03%. Al content in this specification is Total. It means the content of Al (total Al).

N: 0.01-0.10%
Nitrogen (N) forms carbonitrides with V or Nb and increases the creep rupture strength of the steel. If the N content is too low, the above effect cannot be obtained. On the other hand, if the N content is too high, blow holes are likely to occur. Blow holes become a cause of product surface flaws. Further, when the N content is excessive, the steel is easily hardened due to the formation of nitrides and an increase in solute N. Therefore, the N content is 0.01 to 0.10%. The minimum with preferable N content is 0.025%, More preferably, it is 0.038%. The upper limit with preferable N content is 0.060%, More preferably, it is 0.048%.

  The high Cr steel of the present invention may further contain W, B, Ti, and Ni. These elements are selective elements, and all are common in increasing the high temperature strength.

W: 0 to 2.50%
Tungsten (W) is a selective element. W, like Mo, enhances the high-temperature strength of steel as a solid solution strengthening element and a carbide-type element. If W is contained even a little, the above effect can be obtained. If W is twice the weight ratio of Mo, it is effective in improving the creep strength at high temperatures. On the other hand, if the W content is too high, the strength of the base metal becomes too high, so that the strength of the welded joint portion relative to the base material is relatively lowered. Therefore, the W content is 0 to 2.50%.

  As described above, W has almost the same effect as Mo. When W is contained, Mo + W / 2 is preferably 1.0 to 1.6%.

  A preferable lower limit of the W content is 1.5%, and a preferable upper limit of the W content is 2.0%.

B: 0 to 0.01%
Boron (B) is a selective element. B disperses and stabilizes carbides in the steel. If B is contained even a little, the above effect can be obtained. On the other hand, if the B content is too high, the weldability and workability of the steel deteriorate. Therefore, the B content is 0 to 0.01%. The minimum with preferable B content is 0.0003%, More preferably, it is 0.001%. The upper limit with preferable B content is 0.008%, More preferably, it is 0.005%.

Ti: 0 to 0.1%
Titanium (Ti) is a selective element. Ti forms a carbide that is more stable up to a higher temperature than Cr and increases the creep strength of the steel. On the other hand, if the Ti content is too high, a large amount of coarse carbide precipitates and the toughness of the steel decreases. Therefore, the Ti content is 0 to 0.1%. The minimum with preferable Ti content is 0.003%, More preferably, it is 0.007%. The upper limit with preferable Ti content is 0.03%, More preferably, it is 0.022%.

Ni: 0 to 0.8%
Nickel (Ni) is a selective element. Ni is an austenite stabilizing element and suppresses the formation of delta (δ) ferrite. In particular, when the content of W, which is a ferrite-forming element, is large, Ni is preferably contained. On the other hand, if the Ni content is too high, the creep rupture strength of the steel decreases. Therefore, the Ni content is 0 to 0.8%. A preferable lower limit of the Ni content is 0.2%. In addition, when there is no need to suppress δ ferrite, such as when the W content is low, the preferred Ni content is less than 0.2%.

  The high Cr steel of the present invention may further contain at least one of Ca and Mg. All of these elements are selective elements and are common in that they increase the hot workability of steel.

Total of at least one of Ca and Mg: 0 to 0.01%
Calcium (Ca) and magnesium (Mg) are both selective elements. These elements increase the hot workability of the steel. On the other hand, if the total content of these elements is too high, the cleanliness of the steel decreases. Therefore, the total content of at least one or more of Ca and Mg (hereinafter referred to as Ca and / or Mg total amount) is 0 to 0.01%. The minimum with preferable Ca and / or Mg total amount is 0.0005%, More preferably, it is 0.001%, More preferably, it is 0.0015%. The upper limit with preferable Ca and / or Mg total amount is 0.008%, More preferably, it is 0.006%.

  The balance of the high Cr steel according to the present invention is Fe and impurities. An impurity means an element mixed from ore and scrap used as a raw material of steel, or an environment in the manufacturing process, etc., and is allowed as long as it does not adversely affect the high Cr steel of the present invention.

  A high Cr steel base tube having the above chemical composition is manufactured, for example, by the following method.

  A steel having the above chemical composition is melted and refined by a well-known method. Subsequently, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, a slab, bloom, or round billet. Moreover, you may make molten steel into an ingot by an ingot-making method.

  Slabs, blooms, and ingots are hot-worked into billets. The billet may be formed by hot rolling or may be formed by hot forging.

  A billet obtained by continuous casting or hot working is hot-worked to manufacture a raw pipe. For example, Mannesmann piercing and rolling is performed as hot working to produce a seamless steel pipe that is a raw pipe. The base tube after piercing and rolling may be stretch-rolled using a mandrel mill, or may be subjected to constant-diameter rolling using a sizer or a stretch reducer after stretching and rolling. The raw tube manufactured by the above hot working is cooled. The raw tube may be cooled to room temperature. A preferred cooling method is air cooling or standing cooling.

[Heat treatment process]
A first heat treatment is performed on the prepared tube, and then a second heat treatment is performed. As described above, in this heat treatment step, the amount of martensite in the high Cr steel pipe after the heat treatment is suppressed as much as possible.

[About the first heat treatment step]
The elementary tube having the above chemical composition has high self-hardness. Therefore, martensite is generated even when it is air-cooled or allowed to cool after being hot worked by the manufacturing method described above. Therefore, heat treatment for the purpose of softening is performed on the prepared raw tube.

First, heating the prepared raw pipe to a first temperature of 950 ° C. or less higher than the _C1 point A. Then, the raw tube is held at the first temperature. For example, the raw tube is inserted into a heat treatment furnace having a furnace temperature of the first temperature, and after the raw tube temperature reaches the first temperature, the raw tube is held in the heat treatment furnace for a predetermined time. By the first heat treatment, martensite in the raw tube is transformed into austenite, and martensite in the structure is reduced.

If the first temperature is not more than the _AC1 point, the martensite in the structure is not transformed into austenite. On the other hand, when the first temperature exceeds 950 ° C., the amount of scale generation on the outer surface of the blank tube becomes excessive. If a large amount of scale is generated on the outer surface of the raw pipe, a descaling process (a process for removing the scale from the outer surface) must be performed on the high Cr steel pipe after the heat treatment process. Therefore, the first temperature is higher than the _AC1 point and not higher than 950 ° C.

A preferable lower limit of the first temperature is 840 ° C. or higher, more preferably 860 ° C. or higher, and further preferably _{AC 3} points or higher. In this case, the amount of martensite remaining in the blank after the first heat treatment step is reduced or eliminated.

  After heating the raw tube to the first heat treatment temperature, the raw tube is preferably held at the first heat treatment temperature for 5 minutes or more. In this case, martensite in the raw tube structure is reduced. A more preferable lower limit of the holding time is 8 minutes. If the holding time is too long, the amount of scale generated on the surface of the raw tube increases. Therefore, the upper limit with preferable holding time is 40 minutes, More preferably, it is 30 minutes.

[After the first heat treatment step to about the second heat treatment step]
After the first heat treatment step, a second heat treatment step is performed. At this time, as shown in FIG. 1, the second heat treatment step is continuously started without lowering the tube temperature after the first heat treatment step below the Ms point. Therefore, the second heat treatment step is continuously started while the tube temperature after the first heat treatment step is maintained at a temperature higher than the Ms point.

In the second heat treatment step, with respect to base tube, out the heat treatment at a second temperature below point _C1 A. Specifically, after heating the raw tube to the second temperature, the raw tube is held at the second temperature. A preferable lower limit of the second temperature is 700 ° C, and a preferable upper limit is 800 ° C.

  At this time, it is preferable that the first heat treatment step is performed in a first heat treatment furnace and the second heat treatment step is performed in a second heat treatment furnace different from the first heat treatment furnace. In this case, the raw tube is extracted out of the first heat treatment furnace, and the raw tube is charged into the second heat treatment furnace. By extracting the raw tube from the first heat treatment furnace, it is possible to quickly lower the temperature of the raw tube. The raw tube temperature immediately before being charged into the second heat treatment furnace can be adjusted by the time from the extraction from the first heat treatment furnace to the charging into the second heat treatment furnace.

  It is preferable that the raw tube temperature immediately before being charged into the second heat treatment furnace is a temperature lower than the second temperature. That is, in the extracting step, it is preferable to lower the raw tube temperature to a temperature higher than the Ms point and lower than the second temperature. The raw tube temperature is more preferably higher than the Ms point and Ms point + 200 ° C. or less, and more preferably higher than the Ms point and Ms point + 100 ° C. or less. The one where the raw tube temperature just before being charged into the second heat treatment furnace is lower becomes easier to manage the temperature in operation. For example, in the case of a thin-walled steel pipe, in order to increase the temperature of the raw pipe immediately before being charged into the second heat treatment furnace to the second temperature or higher, a special treatment for keeping the temperature, management of the conveyance speed to the furnace, etc. Severe response is required.

  On the other hand, if the raw tube temperature is cooled to below the Ms point after extraction from the first heat treatment furnace and before charging into the second heat treatment furnace, martensite is formed in the fine structure of the raw tube. . In the second heat treatment step, martensite does not transform into austenite, so martensite remains in the high Cr steel pipe in the second heat treatment step. In this case, the hardness and strength of the high Cr steel pipe become high, and processing such as pipe expansion and drawing becomes difficult.

  In the method for producing a high Cr steel pipe according to the present invention, as described above, the raw pipe temperature between the first heat treatment step and the second heat treatment step is maintained higher than the Ms point. Therefore, it can suppress that a martensite is produced | generated in a high Cr steel pipe, and can reduce the hardness of a high Cr steel pipe.

  After the completion of the first heat treatment step, the cooling rate of the raw tube temperature is preferably 140 ° C./min or less, more preferably 90 ° C./min or less, further preferably 70 ° C./min. Less than minutes. Since the ferrite precipitation amount increases as the cooling rate decreases, the hardness can be further reduced.

  The lower limit value of the cooling rate is not particularly limited. A preferable lower limit of the cooling rate is 3 ° C./min.

  High Cr steel pipes were produced under various production conditions, and the Vickers hardness of the high Cr steel pipes was measured.

[Method of manufacturing high Cr steel pipe]
Steels A and B shown in Table 1 were melted.

  Referring to Table 1, Steel A had a chemical composition corresponding to ASTM P91, and Steel B had a chemical composition corresponding to ASTM P92.

  Raw pipes of each steel type were manufactured by Mannesmann drilling. Specifically, a plurality of round billets of each steel type were manufactured by a continuous casting method. A round billet was pierced and rolled using a piercer, and further, constant diameter rolling was performed using a mandrel mill, a sizer, or a stretch reducer to produce a blank tube. The outer diameter of each element tube of steel A was 406.4 mm, and the wall thickness was 37.0 mm. The outer diameter of each elementary tube of steel B was 219.1 mm, and the wall thickness was 23.0 mm.

  Heat treatment was performed under the production conditions shown in Table 2 using the produced elementary tube.

  2-7 is a figure which shows the heat pattern of each manufacturing conditions. In each manufacturing condition, the first heat treatment step and the second heat treatment step were performed using the same heating furnace or different heating furnaces.

  Specifically, in manufacturing condition 1, as shown in Table 2 and FIG. 2, as a first heat treatment step, the material was charged into a heating furnace and heated to 1060 ° C. Thereafter, the raw tube was held at 1060 ° C. for 10 minutes. Subsequently, the raw tube was extracted from the first heating furnace and cooled to room temperature (25 ° C.) outside the furnace. The cooling rate at this time was as shown in Table 2. Thereafter, the raw tube was charged into another heating furnace different from the heating furnace used in the first heat treatment step, the raw tube was heated to 780 ° C., and the raw tube was held at 780 ° C. for 60 minutes. The high Cr steel pipe was manufactured by the above process.

  In production condition 2, as shown in Table 2 and FIG. 3, as a first heat treatment condition, the raw tube was held at 780 ° C. for 60 minutes using a heating furnace. Thereafter, the tube was extracted from the heating furnace and cooled to room temperature (25 ° C.) at the cooling rate shown in Table 2. That is, in the manufacturing condition 2, the heat treatment process was performed only once.

  In manufacturing condition 3-1, as shown in Table 2 and FIG. 4, it implemented on the manufacturing conditions which become in the scope of the present invention. Specifically, as a first heat treatment step, the raw tube was charged into a heating furnace and heated to 920 ° C. Thereafter, the raw tube was held at 920 ° C. for 10 minutes. Subsequently, the raw tube was extracted from the first heating furnace, and the raw tube temperature was maintained at a temperature higher than the Ms point (435 ° C. or higher) outside the furnace. The cooling rate at this time was 120 ° C./min. The raw tube was charged into the second heating furnace without lowering the raw tube temperature after the first heat treatment step below the Ms point. The raw tube was heated to 780 ° C. in the second heating furnace, and the raw tube was held at 780 ° C. for 60 minutes. The high Cr steel pipe was manufactured by the above process.

  In the production condition 3-2, only the cooling rate was different from that in the production condition 3-1. Specifically, the cooling rate under production condition 3-2 was 20 ° C./min. Other conditions were the same as manufacturing conditions 3-1. In the production condition 3-3, only the cooling rate was different from the production conditions 3-1, 3-2. Specifically, the cooling rate under production condition 3-3 was 90 ° C./min. Other conditions were the same as manufacturing conditions 3-1, 3-2.

  In the production condition 4, as shown in Table 2 and FIG. 5, only the cooling condition was different from the production condition 3-1. Specifically, in the manufacturing condition 4, after completion of the first heat treatment step, the raw tube was cooled to 150 ° C. or lower. Conditions (first and second heat treatment conditions) other than the cooling conditions were the same as the manufacturing conditions 3-1.

  In production condition 5, as shown in Table 2 and FIG. 6, the heat treatment temperature in the first heat treatment step is lower than that in production condition 3-1, which is 850 ° C., and the tube cooling after the first heat treatment step is completed. The rate was 140 ° C./min. Other conditions were the same as manufacturing conditions 3-1.

  In manufacturing condition 6, as shown in Table 2 and FIG. 7, compared with manufacturing condition 5, only the cooling condition was different. Specifically, in manufacturing condition 6, after completion of the first heat treatment step, the raw tube was cooled to 150 ° C. or lower. Conditions (first and second heat treatment steps) other than the cooling conditions were the same as the manufacturing conditions 5.

  The raw tubes of Test Nos. 1 to 13 were heat-treated under the production conditions shown in Table 2 to produce high Cr steel tubes.

[Vickers test]
Of the cross sections of the high Cr steel pipes of each test number, an arbitrary point at the center of the wall thickness was selected. At each measurement point, a Vickers hardness test according to JIS Z2244 (2009) was performed. At this time, the test force was 10 kgf. The average of the values obtained at the three measurement points was defined as the hardness (HV) of the high Cr steel pipe of that test number.

[Test results]
Table 3 shows the hardness (HV) of the high Cr steel pipe of each test number. FIG. 8 illustrates the results of test numbers 1 to 7 (test numbers using steel A) in Table 3. FIG. 9 illustrates test numbers 8 to 13 (using steel B in Table 3). The result of (test number) is illustrated.

  Referring to Table 3, FIG. 8, and FIG. 9, the chemical compositions of the steels of test numbers 1 to 3, 8, and 9 were within the scope of the present invention. Furthermore, since it manufactured using manufacturing conditions 3-1, 3-2, 3-3, and 5, both the 1st temperature and the 2nd temperature were in the scope of the present invention. Furthermore, the raw tube temperature (cooling stop temperature) during the transition from the first heat treatment step to the second heat treatment step was higher than the Ms point. Therefore, in any of the test numbers, the Vickers hardness was as low as 190 HV or less. Moreover, since the first temperature was not excessively high in any of the test numbers, the amount of scale formed on the surface of the high Cr steel pipe after the second heat treatment step was small.

  Furthermore, since the high Cr steel pipe of test number 2 was manufactured under manufacturing condition 3-2, the cooling rate of the raw pipe after the first heat treatment step was 90 ° C./min or less. Therefore, the Vickers hardness HV was further lower as compared with other examples of the present invention (test numbers 1 and 3) using the same steel A. Similarly, since the high Cr steel pipe of test number 8 was manufactured under manufacturing condition 3-3, the cooling rate of the master after the first heat treatment step was 90 ° C./min or less. Therefore, the Vickers hardness HV was further lower than that of another example of the present invention (test number 9) using the same steel B.

  On the other hand, high Cr steel pipes with test numbers 4 and 5 were produced under production conditions 4 and 6, respectively. Therefore, although the 1st heat treatment temperature and the 2nd heat treatment temperature were within the limits of the present invention, the cooling stop temperature was below Ms point (150 ° C or less). Therefore, the Vickers hardness was high exceeding 190 HV. It is considered that martensite was formed in the raw tube after the first heat treatment step because the raw tube temperature was too low.

  The high Cr steel pipe of test number 6 was manufactured under manufacturing condition 1. Since the cooling stop temperature was below the Ms point (25 ° C.), the Vickers hardness was higher than 190 HV. Furthermore, since the first temperature was excessively high at 1060 ° C., the amount of scale formed on the surface of the high Cr steel pipe was larger than that in the inventive examples (test numbers 1 to 3).

The high Cr steel pipe of test number 7 was manufactured under manufacturing condition 2. Since only simple annealing treatment (heat treatment at AC ₁ point or less) was performed, the Vickers hardness exceeded 190 HV.

  With reference to Table 3 and FIG. 9, the high Cr steel pipe of the test number 10 was manufactured on the manufacturing conditions 4 using the steel B, and the test number 11 was manufactured on the manufacturing conditions 6. FIG. Therefore, the Vickers hardness of these test numbers was higher than 190 HV.

  Since the high Cr steel pipe of test number 12 was manufactured under manufacturing condition 1, the Vickers hardness exceeded 190 HV and the amount of scale was large.

  Since the high Cr steel pipe of test number 13 was manufactured under manufacturing condition 2, the Vickers hardness exceeded 190 HV.

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

Claims (6)

In mass%, C: 0.05 to 0.15%, Si: 0.02 to 0.70%, Mn: 0.10 to 1.0%, P: 0.025% or less, S: 0.010 %: Cr: 8.0-10%, Mo: 0.15-1.25%, V: 0.08-0.35%, Nb: 0.02-0.12%, Al: 0.05 %: N: 0.01 to 0.10%, W: 0 to 2.50%, B: 0 to 0.01%, Ti: 0 to 0.1%, Ni: 0 to 0.8%, A total of Ca and / or Mg: containing 0 to 0.01%, the balance is a step of preparing a blank obtained by cooling a billet composed of Fe and impurities after hot working;
The raw tube temperature and the first heat treatment step of holding at a first temperature of 950 ° C. or less higher than the _C1 point A,
After the first heat treatment step, without reducing the raw tube temperature to a temperature below Ms point, it continued and a second heat treatment step of holding the raw tube temperature at a second temperature below _C1 points A, high Cr steel tube Manufacturing method. The manufacturing method according to claim 1,
The manufacturing method, wherein the second temperature is 700 ° C. or higher. It is a manufacturing method of Claim 1 or Claim 2, Comprising:
The manufacturing method whose N is less than 0.05% in the said chemical composition. It is a manufacturing method given in any 1 paragraph of Claims 1-3,
The manufacturing method in which the cooling rate of the raw tube is 90 ° C./min or less after the first heat treatment step until the second heat treatment step is started. It is a manufacturing method of any one of Claims 1-4, Comprising:
The first heat treatment step is performed in a first heat treatment furnace,
The second heat treatment step is performed in a second heat treatment furnace different from the first heat treatment furnace,
The manufacturing method includes:
Extracting the raw tube from the first heat treatment furnace;
And a step of charging the extracted raw pipe into the second heat treatment furnace. It is a manufacturing method of Claim 5, Comprising:
The extracting step is a manufacturing method in which the raw tube temperature is lowered to a temperature higher than the Ms point and lower than the second temperature.

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