APPARATUS LINE FOR MANUFACTURING SEAMLESS STEEL PIPE AND TUBE AND METHOD OF MANUFACTURING DUPLEX SEAMLESS STAINLESS STEEL PIPE

Abstract

An apparatus line for manufacturing seamless steel pipes and tubes includes: a heating apparatus for heating a steel raw material; a piercing apparatus for piercing the heated steel raw material thus forming a hollow material; and a rolling apparatus for applying working to the hollow material to form a seamless steel pipe having a predetermined shape. A cooling apparatus is arranged on an exit side of the rolling apparatus. A heated steel raw material is worked by the rolling apparatus after being pierced by the piercing apparatus, and thereafter, using a surface temperature of a hollow piece before being cooled by the cooling apparatus as a cooling start temperature, the hollow piece is cooled to a cooling stop temperature differing by 50° C. or more from the cooling start temperature and being equal to or above 600° C. at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature.

Claims

1. An apparatus line for manufacturing seamless steel pipes and tubes comprising: a heating apparatus for heating a steel raw material; a piercing apparatus for piercing the heated steel raw material to form a hollow material; a rolling apparatus for applying hot working to the hollow material to form a seamless steel pipe having a predetermined size; and a cooling apparatus arranged on an exit side of the rolling apparatus.

2. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 1, wherein a heat-retention apparatus having a heating function is arranged on an exit side of the cooling apparatus.

3. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 1, wherein the cooling apparatus has a cooling capability where an average cooling speed at a position on an outer surface of a material to be cooled is set to 1.0° C./s or more.

4. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 2, wherein the heat retention apparatus has a heat retention capability where an average cooling speed at a position on an outer surface of a material to be cooled is set to 1.0° C./s or less.

5. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 2, wherein the heat retention apparatus has a heating capability where an average heating speed at a position on an outer surface of a material to be heated is set to 1.0° C./s or more in heating.

6. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 4, wherein the heat retention apparatus has a heating capability where an average heating speed at a position on an outer surface of a material to be heated is set to 1.0° C./s or more in heating.

7. A method of manufacturing a duplex seamless stainless steel pipe using the apparatus line for manufacturing seamless steel pipes and tubes described in any claim 1, wherein a steel raw material is heated by the heating apparatus, the hollow material is formed by piercing the steel raw material by the piercing apparatus, the hollow material is formed into a hollow piece by applying hot working by the rolling apparatus, the hollow piece is cooled by the cooling apparatus, and the steel material has the composition which contains, by mass %, 0.050% or less C, 2.00% or less Si, 5.00% or less Mn, 0.05% or less P, 0.03% or less S, 16.0 to 35.0% Cr, 3.0 to 12.0% Ni, 5.0% or less Mo, 0.1% or less Al, 0.5% or less N, and Fe and an unavoidable impurities as a balance, wherein the steel raw material is heated to a temperature of (δ.sub.A−300° C.) to (δ.sub.A+100° C.) by the heating apparatus, hot working is applied to the rolling apparatus, and using a surface temperature of the hollow piece before being cooled by the cooling apparatus as a cooling start temperature, the cooling apparatus cools the hollow piece to a cooling stop temperature having a temperature difference of at least 50° C. or more from the cooling start temperature and being equal to or above 600° C. at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature.

8. The method of manufacturing a duplex seamless stainless steel pipe according to claim 7, wherein the hollow piece after cooling is made to pass through a heat retention apparatus having a heating function.

9. The method of manufacturing a duplex seamless stainless steel pipe according to claim 8, wherein cooling is performed in the heat retention apparatus at an average cooling speed of 1.0° C./s or less at a position on an outer surface of the hollow piece.

10. The method of manufacturing a duplex seamless stainless steel pipe according to claim 8, wherein heating is performed in the heat retention apparatus is at an average heating speed of 1.0° C./s or more at a position on an outer surface of the hollow piece.

11. The method of manufacturing a duplex seamless stainless steel pipe in claim 7, wherein the steel raw material further contains, in addition to the composition, by mass %, one, two or more kinds of elements selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0.5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

12. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 2, wherein the cooling apparatus has a cooling capability where an average cooling speed at a position on an outer surface of a material to be cooled is set to 1.0° C./s or more.

13. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 12, wherein the heat retention apparatus has a heat retention capability where an average cooling speed at a position on an outer surface of a material to be cooled is set to 1.0° C./s or less.

14. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 12, wherein the heat retention apparatus has a heating capability where an average heating speed at a position on an outer surface of a material to be heated is set to 1.0° C./s or more in heating.

15. The apparatus line for manufacturing seamless steel pipes and tubes according to claim 13, wherein the heat retention apparatus has a heating capability where an average heating speed at a position on an outer surface of a material to be heated is set to 1.0° C./s or more in heating.

16. The method of manufacturing a duplex seamless stainless steel pipe according to claim 9, wherein the average heating speed at the position on the outer surface of the hollow piece by the heat retention apparatus is set to 1.0° C./s or more.

17. The method of manufacturing a duplex seamless stainless steel pipe in claim 8, wherein the steel material further contains, in addition to the composition, by mass %, one, two or more kinds of elements selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0.5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

18. The method of manufacturing a duplex seamless stainless steel pipe in claim 9, wherein the steel material further contains, in addition to the composition, by mass %, one, two or more kinds of elements selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0.5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

19. The method of manufacturing a duplex seamless stainless steel pipe in claim 10, wherein the steel material further contains, in addition to the composition, by mass %, one, two or more kinds of elements selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0. 5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

20. The method of manufacturing a duplex seamless stainless steel pipe in claim 16, wherein the steel material further contains, in addition to the composition, by mass %, one, two or more kinds of elements selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0.5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0052] The drawing is an explanatory view schematically showing one example of an apparatus line for manufacturing seamless steel pipes and tubes according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0053] The apparatus line used in the present invention is an apparatus line which can apply working to a heated steel raw material, and thereafter, can cool the steel raw material within a proper temperature range thus manufacturing a seamless steel pipe having a predetermined size. One preferred example of the apparatus line used in the present invention is shown in the drawing. The apparatus line for manufacturing seamless steel pipes and tubes of the present invention is either (a) an apparatus line where a heating apparatus 1, a piercing apparatus 2, a rolling apparatus 3 and a cooling apparatus 4 are arranged in this order, and (b) an apparatus line where a heating apparatus 1, a piercing apparatus 2, a rolling apparatus 3, a cooling apparatus 4 and a heat retention apparatus 5 are arranged in this order.

[0054] As the heating apparatus 1 which can be used in the present invention, it is possible to adopt any one of ordinary heating furnaces which can heat a steel raw material such as round cast billets or round steel billets to a predetermined temperature, for example, a rotary hearth type heating furnace or a walking-beam type heating furnace. Further, an induction heating type heating furnace may be used as the heating apparatus 1.

[0055] As the piercing apparatus 2 used in the present invention, one can use any piercing apparatus which can form a hollow material by applying piercing to a heated steel raw material. For example, any usually known piercing apparatus such as a Mannesmann type skew rolling type piercing apparatus which uses barrel type rolls or a hot-extruded type piercing apparatus can be used.

[0056] As the rolling apparatus 3 used in the present invention, any rolling apparatus can be used provided that the rolling apparatus can form a seamless steel pipe having a predetermined shape (hereinafter referred to also as “hollow piece”) by applying working to the hollow material. It is preferable to use the rolling apparatus 3 depending on a purpose of use. For example, any ordinary known rolling apparatus can be used including a rolling apparatus where an elongator 31, a plug mill 32 which stretches the pierced hollow material into a thin and elongated shape, a reeler which makes both outer and inner surfaces of the hollow piece smooth(not shown in the drawing) and a sizing mill 33 which shapes the hollow piece into a predetermined size are arranged in this order and a rolling apparatus where a mandrel mill for forming a hollow material into a hollow piece having a predetermined size (not shown in the drawing), and a stretch reducing mill (not shown in the drawing) for adjusting an outer diameter and a wall thickness by applying a slight reduction are arranged. It is preferable to use the elongator or the mandrel mill which allows the applying of a large amount of working.

[0057] The cooling apparatus 4 used in the present invention is arranged on an exit side of the rolling apparatus 3 for cooling the hollow piece to a proper temperature range by suppressing recovery and a phase transformation of a ferrite phase in which strain is cumulated. It is unnecessary to limit the type of the cooling apparatus 4 used in the present invention provided that the cooling apparatus 4 can cool the hollow piece immediately after rolling at a desired cooling speed or more. As the cooling apparatus which can relatively easily ensure a desired cooling speed, it is desirable to provide an apparatus of a type which cools a hollow piece by spraying or supplying cooling water, compressed air or mist to outer and inner surfaces of the hollow piece which is a material to be cooled.

[0058] The cooling apparatus 4 used in the present invention may preferably be a cooling apparatus which has, in the manufacture of a steel pipe having a duplex stainless steel composition, cooling capability by which it is possible to acquire at least an average cooling speed of 1.0° C./s or more at a position on an outer surface of a material to be cooled (hollow piece) to acquire phase distributions in a nonequilibrium state. When cooling capability of the cooling apparatus is insufficient so that only cooling with a cooling speed lower than the average cooling speed is possible, the recovery and a phase change of a ferrite phase where strain is cumulated progresses and hence, the phase distributions in a nonequilibrium state cannot be acquired whereby the refinement of microstructure cannot be acquired. Although it is unnecessary to particularly limit an upper limit of a cooling speed, it is preferable to set the upper limit of the cooling speed to 30° C./s from a viewpoint of prevention of cracks and bend caused by a thermal stress.

[0059] In the present invention, it is preferable that the apparatus line where the heat retention apparatus 5 is disposed on an exit side of the cooling apparatus 4. In the present invention, the heat retention apparatus 5 is disposed for lowering a cooling speed after a material to be cooled (hollow piece) is cooled to a predetermined temperature by the cooling .apparatus 4. In case of the duplex stainless steel pipe, when cooling of the steel pipe in an austenite generation temperature region is too fast, a nonequilibrium ferrite phase is cooled without generating an α.fwdarw.γ transformation so that the formation of fine austenite grains is not acquired whereby desired refinement of microstructure cannot be achieved. It is preferable that the heat retention apparatus 5 have heat insulation capacity capable of adjusting an average cooling speed at a position on an outer surface of a material to which heat retention is applied (hollow piece) to at least 1° C./s or less. Further, it is preferable that the heat retention apparatus 5 have heating property capable of adjusting an average heating speed at a position on an outer surface of a material to which heating is applied (hollow piece) to 1.0° C./s or more.

[0060] Next, the explanation is made with respect to the method of manufacturing a heavy-wall-thickness duplex seamless stainless steel pipe for an oil well having high-strength, excellent corrosion resistance, and excellent low-temperature toughness by using the above-mentioned apparatus line for manufacturing seamless steel pipes and tubes.

[0061] The steel raw material is heated by the heating apparatus 1, and thereafter, a hollow material is formed by piercing the steel raw material, and then, a hollow piece is formed by applying hot working to the hollow material by the rolling apparatus 3. Then, the hollow piece is cooled by the cooling apparatus 4, or further, treatment where the hollow piece is made to pass through the heat retention apparatus 5 is applied to the hollow piece after such cooling thus forming a seamless steel pipe having a predetermined size.

[0062] As a steel raw material to be used in the method, any one of steel raw materials having the duplex stainless steel composition stipulated as SUS329J1, SUS329J3L, SUS329J4L and the like in JIS G 4303-4305 is applicable. The steel material has the composition which contains, by mass %, 0.05% or less C, 2.0% or less Si, 5.0% or less Mn, 0.05% or less P, 0.03% or less S, 3.0 to 12.0% Ni, 16.0 to 35.0% Cr, 5.0% or less Mo, 0.1% or less Al, 0.5% or less N, and Fe and unavoidable impurities as a balance.

[0063] Firstly, the reasons for limiting the steel raw material to the preferred composition are explained. Unless otherwise specified, mass % in the composition is simply indicated by “%” hereinafter.

[0064] C: 0.05% or less

[0065] Although C is an element which increases strength of steel, C lowers the corrosion resistance of steel. Accordingly, it is desirable to set the content of C as small as possible. However, the excessive reduction of C brings about a sharp increase of a manufacturing cost.

[0066] Accordingly, in the present invention, the content of C is limited to 0.05% or less. The content of C is preferably limited to 0.03% or less.

[0067] Si: 2.0% or less

[0068] Si is an element which functions as a deoxidant and has a function of increasing strength of steel. To enable the steel pipe to acquire such an effect, it is desirable to set the content of Si to 0.01% or more. On the other hand, when the content of Si is large and exceeds 2.00%, ductility is lowered or the precipitation of an intermetallic compound is accelerated thus lowering corrosion resistance. Accordingly, the content of Si is limited to 2.0% or less. The content of Si is preferably limited to a value which falls within a range of 0.5% to 1.5%.

[0069] Mn: 5.0% or less

[0070] Mn is an austenite stabilizing element, and properly adjusts fractions of duplex microstructure, and contributes to the enhancement of corrosion resistance and workability of duplex stainless steel material. To acquire such advantageous effects, it is desirable that the content of Mn be 0.01% or more. However, when the content of Mn exceeds 5.0%, hot workability and corrosion resistance are lowered. Accordingly, the content of Mn is limited to 5.0% or less. The content of Mn is preferably limited to a value which falls within a range of 0.5 to 2.0%.

[0071] P: 0.05% or less

[0072] P is an element which is mixed into steel as impurities, and is liable to generate segregation in a grain boundary or the like thus causing lowering of corrosion resistance and hot workability. Accordingly, although it is desirable to decrease the content of P as small amount as possible, the presence of less than or equal to 0.05% of P is permissible. However, the excessive reduction of P causes a sharp rise of a material cost and hence, it is desirable to set the content of P to 0.002% or more. Accordingly, the content of P is limited to 0.05% or less. The content of P is preferably limited to 0.02% or less.

[0073] S: 0.03% or less

[0074] S is, in the same manner as P, an element which is mixed in steel as impurities, and is present in steel as a sulfide-based inclusion and lowers ductility, corrosion resistance and hot workability of steel. Accordingly, although it is preferable to decrease the content of S as small amount as possible, the presence of less than or equal to 0.03% of S is permissible. However, the excessive reduction of S causes a sharp rise of a material cost and hence, it is desirable to set the content of S to 0.002 or more. Accordingly, the content of S is limited to 0.03% or less. The content of S is preferably set to 0.005% or less.

[0075] Ni: 3.0 to 12.0%

[0076] Ni is an austenite stabilizing element, and properly adjusts fractions of duplex microstructure, and contributes to the enhancement of corrosion resistance and workability of duplex stainless steel material. To acquire such advantageous effects, it is necessary to set the content of Ni to 3.0% or more. On the other hand, when the content of Ni exceeds 12.0%, an austenite phase is excessively increased so that the maintenance of desired duplex phase microstructure becomes difficult. Accordingly, the content of Ni is limited to a value which falls within a range of 3.0 to 12.0%. The content of Ni is preferably limited to a value which falls within a range of 5.0 to 9.0%.

[0077] Cr: 16.0 to 35.0%

[0078] Cr is an element which enhances the corrosion resistance. Cr is also a ferrite stabilizing element and is a main element for deciding fractions of duplex phase microstructure consisting of a ferrite phase and an austenite phase. It is necessary for steel to set the content of Cr to 16.0% or more to acquire such an advantageous effect. On the other hand, when the content of Cr becomes large and exceeds 35.0%, the formation of an intermetallic compound such as a σ phase or a χ phase is accelerated thus giving rise to lowering of corrosion resistance of steel. Accordingly, the content of Cr is limited to a value which falls within a range of 16.0 to 35.0%. The content of Cr is preferably set to a value which falls within a range of 16.0 to 28.0%.

[0079] Mo: 5.0% or less

[0080] Mo is an element which enhances corrosion resistance of steel, and it is desirable that the content of Mo is set to 1.0% or more to acquire such an advantageous effect. On the other hand, when the content of Mo exceeds 5.0%, the precipitation of an intermetallic compound is accelerated and hence, corrosion resistance and hot workability are lowered. Accordingly, the content of Mo is limited to 5.0% or less. The content of Mo is preferably set to a value which falls within a range of 2.0 to 4.0%.

[0081] Al: 0.1% or less

[0082] Al is an element which functions as a deoxidant, and it is desirable for steel that the content of Al be set to 0.001% or more to acquire such an advantageous effect. However, when the content of Al is large and exceeds 0.1%, an amount of oxide-based inclusion is increased and hence, cleanliness is lowered. Accordingly, the content of. Al is limited to a 0.1% or less. The content of Al is preferably set to a value which falls within a range of 0.001 to 0.050%.

[0083] N: 0.5% or less

[0084] N is a strong austenite stabilizing element, and also contributes to the enhancement of corrosion resistance. It is desirable for steel that the content of N be set to 0.050% or more to acquire such an advantageous effect. However, when the content of N exceeds 0.5%, an austenite phase is excessively increased so that the maintenance of the desired duplex phase microstructure becomes difficult. Accordingly, the content of N is limited to 0.5% or less.

[0085] In addition to the above-mentioned composition, steel raw material may contain one or two or more kinds selected from a group consisting of 3.0% or less Nb, 0.1% or less Ti, 3.0% or less V, 0.5% or less Zr, 3.5% or less W, 3.5% or less Cu, 0.05% or less REM, 0.01% or less B, and 0.1% or less Ca.

[0086] All of Nb, Ti, V and Zr are elements which effectively contribute to the enhancement of strength and toughness as well as the enhancement of corrosion resistance. Steel raw material can selectively contain one kind or two or more kinds of these elements when desired. To acquire these advantageous effects, it is desirable that steel raw material contain 0.01% or more Nb, 0.01% or more Ti, 0.01% V, 0.01% or more Zr. On the other hand, even when the content of Nb exceeds 3.0%, the content of Ti exceeds 0.1%, the content of V exceeds 3.0% or the content of Zr exceeds 0.5%, toughness and hot workability are lowered. Accordingly, when steel raw material contains these elements, it is preferable to limit the contents of these elements such that the content of Nb is 3.0% or less, the content of Ti is 0.1% or less, the content of V is 3.0% or less, and the content of Zr is 0.5% or less.

[0087] All of W, Cu and REM are elements which effectively contribute to the enhancement of corrosion resistance. Steel raw material can selectively contain one kind or two or more kinds of these elements when desired. To acquire these advantageous effects, it is desirable that steel raw material contain 0.01% or more W, 0.01% or more Cu, 0.005% or more REM. On the other hand, when the content of W exceeds 3.5%, the content of Cu exceeds 3.5% or the content of REM exceeds 0.05%, toughness is lowered. Accordingly, when steel raw material contains these elements, it is preferable to limit the contents of these elements respectively such that the content of W is 3.5% or less, the content of Cu is 3.5% or less, and the content of REM is 0.05% or less.

[0088] Both B and Ca are elements which contribute to the suppression of formation of a defect during hot working, and steel raw material can selectively contain one kind or two or more kinds of these elements in addition to the above-mentioned composition. To acquire such an advantageous effect, it is desirable that steel raw material contain 0.0001% B and 0.001% Ca. On the other hand, when the content of B exceeds 0.01% or the content of Ca exceeds 0.1%, toughness is lowered. Accordingly, when steel raw material contains these elements, it is preferable to limit the contents of these elements such that the content of B is 0.01% or less, and the content of Ca is 0.1% or less.

[0089] A balance other than the above-mentioned components is formed of Fe and unavoidable impurities. As unavoidable impurities, 0.0050% or less 0 (oxygen) is permissible.

[0090] Any ordinary methods can be used as the method of manufacturing steel raw material used in the present invention, and it is unnecessary to particularly limit the method of manufacturing steel raw material used in the present invention. For example, it is preferable that molten steel having the predetermined duplex stainless steel composition is made by a converter, an electric furnace, a melting furnace or the like, or molten steel is further subjected to secondary refining by an AOD apparatus, a VOD apparatus or the like and, thereafter, molten steel is formed into cast pieces such as slabs or billets by a continuous casting method or molten steel is formed into cast pieces such as slabs or billets by an ingot molding-rolling. Homogenizing annealing at a high temperature may be applied to steel raw material in advance.

[0091] Firstly, heat treatment is applied to steel raw material.

[0092] In the heat treatment, steel raw material is loaded into the heating apparatus 1, and steel raw material is heated to a temperature (heating temperature) of (δ.sub.A−300° C.) to (δ.sub.A+100° C.)

[0093] Heating temperature: (δ.sub.A−300° C.) to ((δ.sub.A+100° C.)

[0094] When a heating temperature is below (δ.sub.A−300° C.), the refinement of microstructure which makes use of the transformation from a ferrite phase cannot be achieved. Further, an austenite phase fraction is increased and hence, working of steel raw material becomes difficult due to the increase of a load or lowering of hot ductility. On the other hand, when the heating temperature is ((δ.sub.A+100° C.) or above, cumulation of strain by working becomes difficult. Accordingly, the heating temperature of a steel raw material is limited to a temperature which falls within a range of (δ.sub.A−300° C.) to (δ.sub.A+100° C.). The heating temperature of steel raw material may preferably falls within a range of 1100 to 1300° C. δ.sub.A may be obtained using general-use equilibrium state calculation software, or δ.sub.A may be obtained such that a thermal expansion curve is measured, and δ.sub.A may be obtained from an inflection point of the thermal expansion curve due to the completion of a δ ferrite phase transformation.

[0095] The steel raw material to which heat treatment is applied is formed into a hollow material by piercing the steel raw material by the piercing apparatus 2 and, thereafter, hot working is applied to the hollow material by the rolling apparatus 3 thus manufacturing a seamless steel pipe (hollow piece) having a predetermined size. It is sufficient for hot working applied to the steel raw material that a hollow piece having a predetermined size can be formed, and any ordinary working conditions are applicable and, it is unnecessary to particularly limit the working condition. In the present invention, although the desired refinement of microstructure can be acquired even with a relatively small amount of working (reduction), it is preferable to set a cumulative working amount to at least 10% or more from a viewpoint of the refinement of microstructure.

[0096] The hollow piece is subjected to cooling treatment immediately after hot working is applied to the steel raw material.

[0097] In the cooling treatment, by making use of the cooling apparatus 4, the hollow piece is cooled to a cooling stop temperature having a temperature difference of at least 50° C. or more from a cooling start temperature and being 600° C. or above at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature of the hollow piece.

[0098] Average cooling speed: 1.0° C./s or more

[0099] In an aspect of the present invention, in the cooling treatment, to acquire a ferrite phase in a super-cooled state (phase distribution in a non-equilibrium state) where strain is cumulated, a material to be cooled (hollow piece) is cooled at an average cooling speed of at least 1.0° C./s or more at a position on an outer surface of the material to be cooled (hollow piece). When the material to be cooled cannot be cooled only at a cooling speed lower than the above-mentioned average cooling speed, the above-mentioned strain is restored, and at the same time, an austenite phase and other precipitation phases precipitate so as to approach an equilibrium state from a ferrite phase grain boundary or grains and hence, a phase distribution in a non-equilibrium state cannot be acquired whereby the refinement of microstructure cannot be achieved. Although it is unnecessary to particularly limit an upper limit of cooling speed, from a viewpoint of preventing cracks or bending caused by a thermal stress, it is preferable to set the upper limit of the cooling speed to 50° C./s. The cooling speed is preferably set to a value which falls within a range from 3 to 30° C./s.

[0100] Cooling temperature range: 50° C. or more

[0101] A cooling temperature range, that is, a temperature difference between a cooling start temperature and a cooling stop temperature is set to at least 50° C. or more in terms of a temperature on an outer surface of a material to be cooled (hollow piece). When the cooling temperature range is below 50° C., a fraction of a super-cooled ferrite phase is small and hence, phase fractions in a remarkable non-equilibrium state cannot be ensured whereby the desired refinement of microstructure cannot be achieved. Accordingly, the cooling temperature range is limited to 50° C. or more. The larger the cooling temperature range, the more easily the fractions in a non-equilibrium state can be ensured. It is preferable that the cooling temperature range is set to 100° C. or above. The cooling start temperature is a temperature of an outer surface of the material to be cooled (hollow piece) before cooling is started.

[0102] Cooling stop temperature: 600° C. or above

[0103] When the cooling stop temperature is below 600° C., the diffusion of elements is delayed and hence, a phase transformation (α.fwdarw.γ transformation) which occurs during a period where the temperature is held thereafter is delayed whereby a long time is required to ensure desired refined microstructure thus lowering productivity. Accordingly, the cooling stop temperature is limited to 600° C. or above at a wall thickness center temperature of a material to be cooled (hollow piece). It is preferable that the cooling stop temperature be set to 700° C. or above.

[0104] As described previously, it is necessary that the cooling stop temperature is 600° C. or above and the temperature difference between the cooling start temperature and the cooling stop temperature is 50° C. or above and hence, a lower limit of the cooling start temperature is 650° C. or above, preferably 900° C. or above, and more preferably 1150° C. or above.

[0105] Cooling speed after stopping cooling: 1.0° C./s or less

[0106] When cooling of a material to be cooled (hollow piece) becomes cooling where an average cooling speed at a position on an outer surface of the material to be cooled (hollow piece) after cooling by the cooling apparatus 4 is stopped exceeds 1.0° C./s, it is preferable to adjust the average cooling speed to 1.0° C./s or less by loading the material to be cooled (hollow piece) into the heat retention apparatus 5 provided on an exit side of the cooling apparatus 4. When the average cooling speed at the position on the outer surface of the material to be cooled (hollow piece) after stopping cooling becomes excessively fast exceeding 1.0° C./s, the precipitation of a second phase becomes insufficient so that desired fractions cannot be acquired when products are manufactured.

[0107] Heating speed after stopping cooling: 1.0° C./s or more

[0108] When a cooling stop temperature becomes lower than 600° C., by heating a material to be heated (hollow piece) to a temperature region of 600° C. or above and below 1150° C. at a heating speed of 1.0° C./s or more at a temperature on an outer surface of the material to be heated (hollow piece) using the heat retention apparatus 5, it is possible to acquire the substantially same advantageous effects as the case where the cooling is performed under a condition that cooling stop temperature does not become lower than 600° C. Although it is unnecessary to particularly define an upper limit of the heating speed, it is preferable to set the heating speed to 50° C./s or below for uniformly heating the whole material to be heated (hollow piece).

[0109] It is sufficient that cooling treatment according to the present invention which is performed after hot working is performed after hot working performed by at least one rolling mill mounted on the rolling apparatus 3. It is ascertain that, provided that cooling treatment is performed within a temperature region below 1150° C. where acquired minute grain structure does not become coarse, there arises no problem even when reheating is performed and, further, hot working (size determining working by a sizing mill, a stretch reducing mill or the like) is performed.

[0110] Next, the present invention is described further based on an example.

Example

[0111] Molten steels having the compositions shown in Table 1 were prepared by melting in a vacuum melting furnace, and round billets having a diameter of 63 mm were produced through hot rolling and machining. Next, using the apparatus line for manufacturing a seamless steel pipe shown in the drawing, these steel raw materials were loaded into the heating apparatus 1 thus heating the steel raw materials at heating temperatures shown in Table 2. After holding the steel raw materials in the heating apparatus 1 for a fixed time (60 min) and, thereafter, hollow materials (wall thickness: 20 mm) were produced by applying piercing to the steel raw materials using a barrel type Mannesmann piercing apparatus 2. Then, hot working is applied to the steel raw materials using the rolling apparatus 3, and thereafter, using the cooling apparatus 4 where cooling water supplied by spraying is used as a refrigerant, the hollow materials were cooled to cooling stop temperatures shown in Table 2 at average cooling speeds shown in Table 2 thus manufacturing seamless steel pipes (outer diameter: 74 mm, wall thickness: 13 to 16 mm). After cooling by the cooling apparatus 4, the seamless steel pipes were cooled by natural cooling (0.1 to 0.5° C./s). When the cooling stop temperatures became lower than predetermined temperatures, the seamless steel pipes were inserted into the heat retention apparatus 5 and were heated to predetermined temperatures at a heating speed of 1.2° C./s. The obtained seamless steel pipes were subjected to proper quenching and tempering treatment (QT treatment) or were subjected to solution treatment where the seamless steel pipes were heated at a temperature which falls within a range of 1050 to 1150° C., and thereafter, were rapidly cooled.

[0112] Specimens were sampled from the acquired seamless steel pipes and the structure observation and a tensile test were carried out. The following testing methods were used.

(1) Structure Observation

[0113] The presence or the non-presence of cracks in end portions of the steel pipes was observed by naked eye using the obtained seamless steel pipes. When cracks were generated, the degree of cracks was evaluated. The evaluation “present (large)” was given when the number of places where the cracks occurred was 5 or more, and the evaluation “present (small)” was given when the number of places where the cracks occurred was less than 5.

[0114] Next, specimens for structure observation were sampled from the obtained steel pipes, cross sections (C cross sections) orthogonal to the pipe longitudinal direction were polished and corroded (corrosion liquid: Villella liquid). Next, the microstructures were observed using an optical microscope (magnification: 200 times) or a scanning electron microscope (magnification: 1000 times), and the microstructures were imaged and kinds of microstructures were measured using an image analysis. As an index for determining whether or not the microstructures were refined, the number of phase boundaries which intersect with a straight line of a unit length was measured from the microstructure photographs. In Table 3, numerical values of the phase boundaries of the obtained respective steel pipes were indicated as ratios with respect to reference values (phase boundary number ratios) using the numerical values of the phase boundaries of the steel pipes where cooling after hot working was performed by natural cooling (cooling speed: 0.8° C./s) among pipes of the same kind as references (1.00).

(2) Tensile Test

[0115] Round bar type tensile specimens (parallel portion: 6 mmφ×G.L. 20 mm) were sampled from the obtained seamless steel pipes such that the pipe-axis direction is aligned with the pulling direction. The tensile test was carried out in accordance with the provision of JIS Z 2241, and yield strength YS was obtained with respect to each specimen. Strength at the elongation of 0.2% was set as the yield strength. A value (%) obtained by dividing the difference between the obtained yield strength and yield strength (reference yield strength) of a steel pipe of the same kind where cooling after hot working is natural cooling (cooling speed: 0.8° C./S) by the reference yield strength, that is, ΔYS (%) (=(yield strength-reference yield strength)×100/(reference yield strength)) was calculated, and a strength enhancement ratios of the respective steel pipes were evaluated. The evaluation “x bad” was given to the seamless pipes where the yield strength YS became lower than 588 MPa, and the evaluation “o good” was given to the seamless steel pipes where the yield strength YS became larger than 588 MPa.

(3) Charpy Impact Test

[0116] Charpy impact specimens (V-notched specimens) were sampled from the obtained seamless steel pipes such that the longitudinal direction of the specimen becomes the direction (C direction) orthogonal to the pipe axis direction, and the Charpy impact test was carried out in accordance with the provision stipulated in JIS Z 2242, and absorbed energy vE.sub.−10 (J) at a test temperature of −10° C. were acquired. Three specimens were used in each test, an arithmetic mean of absorbed energy of the respective specimens was obtained, and the arithmetic mean value was set as a value of the steel pipe. A value (%) obtained by dividing the difference between the absorbed energy value of each steel pipe obtained as the result of the test and an absorbed energy value (reference absorbed energy value) of a steel pipe of the same kind where cooling after hot working was natural cooling (cooling speed: 0.8° C./s) by reference absorbed energy value, that is, ΔE (%) (=(absorbed energy value−reference absorbed energy value) ×100/(reference absorbed energy value)) was calculated, and absorbed energy enhancement ratios of the respective steel pipes were evaluated.

[0117] The obtained result is shown in Table 3.

[0118] All present invention examples succeeded in manufacturing the duplex seamless stainless steel pipes which can realize the refinement of the microstructure thereof, can acquire an effect of enhancing strength thereof by 2.5% or more compared to the a steel pipe manufactured by performing cooling by natural cooling after hot working, can acquire an effect of enhancing absorbed energy thereof by 20% or more compared to a steel pipe manufactured by performing cooling by natural cooling after hot working, and possess high strength (yield strength YS: 588 MPa or more) without causing the occurrence of cracks. On the other hand, the comparative examples which do not fall within the scope of the present invention cannot realize the refinement of microstructure and hence, the comparative examples cannot ensure desired strength and desired low-temperature toughness or the occurrence of cracks was recognized in the comparative examples.

TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) No. C Si Mn P S Cr Ni Mo Al N Nb, Ti, V, Zr W, Cu, B, Ca, REM A 0.025 0.28 0.25 0.0021 0.0004 16.2 4.2 2.2 0.040 0.038 Nb: 0.11, V: 0.22, Zr: 0.03 W: 1.0, Cu: 1.1, Ca: 0.001 B 0.018 0.85 0.50 0.0018 0.0003 23.2 5.9 2.8 0.010 0.071 Ti: 0.03, Nb: 0.05 W: 2.5, Cu: 1.5, B: 0.005 C 0.018 0.86 0.52 0.0019 0.0003 25.5 6.9 3.3 0.030 0.500 Ti: 0.03, Nb: 0.05 W: 2.8, Cu: 1.5, Ca: 0.001 D 0.011 0.25 0.45 0.0015 0.0003 35.0 12.0 5.0 0.030 0.115 Nb: 0.10, V: 0.10 W: 3.0, REM: 0.006

TABLE-US-00002 TABLE 2 Hot working Wall Cooling after hot working Heating thickness Cooling Average Cooling Steel Heating reduction Wall start cooling stop pipe Steel δ.sub.A temperature ratio thickness temperature speed temperature No. No. (° C.) (° C.) (%) (mm) (° C.) (° C./s) (° C.) 1 A 1345 1250 20 16 1200 2.5 1155  2 A 1250 35 13 1200 3.3 1140  3 A 1250 10 18 1200 4.4 950 4 A 1250 20 16 1200 5.3 615 5 A 1250 20 16 1200 5.5 580 6 A 1250 20 16 1200 4.3  25 7 A 1250 20 16 1200 2.3 1145  8 A 1250 20 16 1100 3.4 990 9 A 1250 20 16 750 4.1 605 10 A 1250 20 16 1200 0.8 950 11 B 1280 1350 20 16 1270 3.8 950 12 B 1350 20 16 1270 0.8 925 13 C 1290 1250 20 16 1200 3.5 955 14 C 1250 20 16 1200 0.8 935 15 D 1260 1250 20 16 1200 3.6 1080  16 D 1250 20 16 1200 0.8 1075  Cooling after Retention/heating Heat treatment hot working Presence or non Quenching Cooling presence Solution and Steel temperature (cooling or heating treatment tempering pipe range average speed temperature temperatures No. (° C.) (° C./s)) (° C.) Q/T(° C.) Remarks 1  45 not present 900/600 comparative example 2  60 not present 900/600 present invention example 3 250 not present 900/600 present invention example 4 585 not present 900/600 present invention example 5 620 not present 900/600 comparative example 6 1175  heating (1.2) 900/600 present invention example 7  55 not present 900/600 present invention example 8 110 not present 900/600 present invention example 9 145 not present 900/600 present invention example 10 250 not present 900/600 comparative example 11 320 not present 1030 present invention example 12 345 not present 1030 comparative example 13 245 not present 1050 present invention example 14 265 not present 1050 comparative example 15 120 not present 1150 present invention example 16 125 not present 1150 comparative example * Underlined values fall outside the scope of the present invention.

TABLE-US-00003 TABLE 3 Steel Microstructure pipe Steel Phase boundary Tensile property Toughness No. No. Kinds* number ratio YS ≧ 558 MPa ΔYS** ΔE*** Cracks during working Remarks 1 A F + M 0.45 ∘ −2.0  −33.3  not present comparative example 2 A F + M 1.65 ∘ 2.7 57.1 not present present invention example 3 A F + M 3.95 ∘ 6.1 81.0 not present present invention example 4 A F + M 1.85 ∘ 4.7 69.0 not present present invention example 5 A F + M 0.95 ∘ 3.4 −11.9  present (small) comparative example 6 A F + M 2.18 ∘ 6.1 81.0 not present present invention example 7 A F + M 1.25 ∘ 3.4 40.5 not present present invention example 8 A F + M 3.50 ∘ 6.2 76.2 not present present invention example 9 A F + M 1.55 ∘ 3.4 64.3 not present present invention example 10 A F + M 1.00 ∘ reference reference present (large) comparative example 11 B F + A 4.95 ∘ 4.4 22.7 not present present invention example 12 B F + A 1.00 x reference reference present (large) comparative example 13 C F + A 3.91 ∘ 6.3 22.2 not present present invention example 14 C F + A 1.00 x reference reference present (large) comparative example 15 D F + A 4.91 ∘ 6.7 93.3 not present present invention example 16 D F + A 1.00 ∘ reference reference present (large) comparative example *F: ferrite, A: austenite, M: martensite (containing residual γ) **ΔYS (%) = (yield strength − reference yield strength) × 100/(reference yield strength) ***ΔE (%) = (absorbed energy value − reference absorbed energy value) × 100/(reference absorb energy value)

REFERENCE SIGNS LIST

[0119] 1: heating apparatus [0120] 2: piercing apparatus [0121] 3: rolling apparatus [0122] 4: cooling apparatus [0123] 5: heat retention apparatus [0124] 31: elongator [0125] 32: plug mill [0126] 33: sizing mill