WO2010143433A1 - 高強度鋼管及びその製造方法 - Google Patents
高強度鋼管及びその製造方法 Download PDFInfo
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- WO2010143433A1 WO2010143433A1 PCT/JP2010/003866 JP2010003866W WO2010143433A1 WO 2010143433 A1 WO2010143433 A1 WO 2010143433A1 JP 2010003866 W JP2010003866 W JP 2010003866W WO 2010143433 A1 WO2010143433 A1 WO 2010143433A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
Definitions
- the present invention relates to a high-strength steel pipe excellent in deformation characteristics as it is manufactured (before aging) and after aging, and a manufacturing method thereof.
- This application claims priority based on Japanese Patent Application No. 2009-140280 filed in Japan on June 11, 2009, the contents of which are incorporated herein by reference.
- the line pipe is required to have a steel pipe for a line pipe that is excellent in internal pressure resistance, does not easily buckle against bending deformation, and has excellent strength and deformability.
- Patent Documents 2 and 3 In order to suppress such strain aging caused by molding and heating, steel pipes utilizing Ni, Cu, and Mo have been proposed (for example, see Patent Documents 2 and 3).
- the strength is increased by hard bainite, and the deformability is improved by soft ferrite. Therefore, it was necessary to control the amount of ferrite produced by the control cooling start temperature and the cooling rate after hot rolling.
- the present inventors have found that in order to improve the deformation performance of a steel pipe having a bainite structure, it is effective to stop accelerated cooling at a high temperature before the bainite transformation is completed. Furthermore, the present inventors have found that the deformation performance of the steel pipe is improved and the deformation performance after aging is excellent due to recovery of strain caused by accelerated cooling and bainite transformation, that is, reduction of the dislocation density of the steel. When accelerated cooling is stopped at a high temperature, bainite transformation has not been completed, so austenite remains in the remainder of the bainite structure.
- the remaining austenite is transformed into bainite, and the bainite transformation is carried out in a range from the accelerated cooling stop temperature to a temperature about 50 ° C. lower than the stop temperature.
- the bainite generated during the accelerated cooling is relatively soft.
- generated after the stop of accelerated cooling is harder than the bainite produced
- the present invention has been made based on such knowledge, and the gist thereof is as follows.
- the high-strength steel pipe according to one aspect of the present invention is, in mass%, C: 0.02 to 0.09%, Mn: 0.4 to 2.5%, Cr: 0.1 to 1.0 %, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.3%, the balance containing iron and inevitable impurities, Si: 0.6% or less, Al: 0.1 % Or less, P: 0.02% or less, S: 0.005% or less, N: 0.008% or less, and a bainite transformation index BT determined by the formula (2) described later is 650 ° C.
- the metal structure is a simple bainite structure including a first bainite and a second bainite, the first bainite is a texture of bainitic ferrite containing no carbide, and the second bainite is Bainitic ferrite containing no carbide and the bainitic It is a mixed structure of cementite during ferrite.
- the high-strength steel pipe described in the above (1) is in mass%, Ni: 0.65% or less, Cu: 1.5% or less, Mo: 0.3% or less, V: 0.2% or less You may further contain at least 1 sort (s) of these.
- the total amount of the first bainite and the second bainite may be 95% or more of the entire structure.
- the product of the tensile strength in the pipe axis direction and the n value at a tensile strain of 1 to 5% may be 60 or more.
- a steel slab satisfying the steel components described in the above (1) or (2) is heated, and the steel slab is heated to 750 to 870 ° C.
- Hot rolling finish rolling is performed within the range of, and accelerated cooling with a cooling rate of 5 to 50 ° C./s is started at 750 ° C. or higher, and the accelerated cooling is stopped within the range of 500 to 600 ° C., and air cooling is performed.
- a steel plate is produced, the steel plate is cold-formed into a tubular shape, and the butt portion is welded.
- a high-strength steel pipe having a predetermined simple bainite structure advantageous for productivity and having sufficient deformation performance even after aging by heating such as a coating process, and a method for producing the same. And the industrial contribution is very significant.
- the inventors first studied the relationship between the accelerated cooling stop temperature and the mechanical characteristics of a steel material whose components were adjusted so that the metal structure of the steel material had a bainite structure.
- the product [TS ⁇ n] of the tensile strength TS and the n value was used as an index representing the balance between strength and ductility.
- the n value is a general index for evaluating work hardening characteristics, and is obtained from the relationship (stress-strain curve) between the true stress ⁇ and the true strain ⁇ in the following equation (1).
- n K ⁇ n (1) Since the correlation between the n value obtained by the tensile test within the range of 1 to 5% and the buckling characteristics of the steel pipe is remarkable, in the present invention, within the range of 1 to 5% strain.
- the n value is obtained. That is, the relationship between the true stress ⁇ and the true strain ⁇ is obtained by a tensile test, and the exponent part (n) of the equation (1) is calculated from the relationship between the true stress ⁇ and the true strain ⁇ within the range of 1 to 5% of the strain amount. Value).
- the parameter K in the above equation (1) is a constant determined by the material.
- Fig. 1 shows the relationship between the accelerated cooling stop temperature (cooling stop temperature) and the strength-ductility balance [TS x n].
- the strength-ductility balance [TS ⁇ n] increases. That is, the balance between the strength and ductility of the steel material having a simple bainite structure is improved by increasing the cooling stop temperature.
- the balance between the strength and ductility of this steel material is considered to improve for the following reasons.
- the remaining austenite is transformed into bainite, and the bainite transformation is completed in a range from the acceleration cooling stop temperature to a temperature about 50 ° C. lower than the stop temperature.
- the strain generated by the accelerated cooling and the bainite transformation is recovered, so that the bainite generated during the accelerated cooling is relatively soft.
- generated after the stop of accelerated cooling is harder than the bainite produced
- the present inventors examined the influence of aging when applying anticorrosion coating to a steel pipe.
- the temperature range for coating heating is about 150 to 300 ° C.
- the present inventors examined changes in the strength-ductility balance [TS ⁇ n] with respect to aging temperature using three types of steel pipes having a simple bainite structure. The results are shown in FIG.
- the strength-ductility balance [TS ⁇ n] of three types of steel pipes indicated by white circles “ ⁇ ”, white triangles “ ⁇ ”, and white squares “ ⁇ ” is as follows. The lowest aging temperature was found to be 200 ° C.
- the stop temperature of accelerated cooling is increased to 500 ° C. or higher, in order to complete the bainite transformation, it is necessary to adjust the steel component composition to an appropriate range.
- the present inventors examined the influence of steel components on the bainite transformation. As a result, it has been found that if the bainite transformation index BT obtained by the following equation (2) is 650 ° C. or less, the bainite transformation is completed even if accelerated cooling is stopped at 500 ° C. or more.
- C 0.02 to 0.09% C is an extremely effective element for improving the strength of steel. In order to obtain sufficient strength, 0.02% or more of C is added to the steel. On the other hand, if the amount of C is more than 0.09%, the low temperature toughness of the base metal and the weld heat affected zone is lowered, and the on-site weldability is deteriorated. Therefore, the upper limit of the C amount is 0.09%. Therefore, the C content is 0.02% or more and 0.09% or less.
- Mn 0.4 to 2.5% Mn is an extremely important element for improving the balance between strength and low temperature toughness. Therefore, 0.4% or more of Mn is added to the steel. On the other hand, if the amount of Mn is more than 2.4%, segregation (center segregation) at the center of the plate thickness parallel to the steel plate surface becomes significant. In order to suppress the deterioration of the low temperature toughness due to the center segregation, the upper limit of the Mn amount is set to 2.4%. Therefore, the amount of Mn is 0.4% or more and 2.5% or less.
- Cr 0.1 to 1.0% Cr increases the strength of the base material and the weld. Therefore, 0.1% or more of Cr is added to the steel. However, if the Cr content is more than 1.0%, the HAZ toughness and on-site weldability deteriorate significantly, so the upper limit of the Cr content is set to 1.0% or less. Therefore, the Cr content is 0.1% or more and 1.0% or less.
- Ti 0.005 to 0.03%
- Ti forms fine TiN, refines the structure of the base material and the weld heat affected zone, and contributes to improved toughness. This effect appears very remarkably by the combined addition with Nb. In order to sufficiently exhibit this effect, 0.005% or more of Ti needs to be added to the steel.
- the amount of Ti is more than 0.03%, TiN coarsening and precipitation hardening due to TiC occur, so the low-temperature toughness decreases. Therefore, the upper limit of Ti content is limited to 0.03%. Therefore, the Ti content is 0.005% or more and 0.03% or less.
- Nb 0.005 to 0.3%
- Nb not only suppresses recrystallization of austenite during controlled rolling to refine the structure, but also increases hardenability and improves steel toughness. In order to obtain this effect, 0.005% or more of Nb needs to be added to the steel. On the other hand, if the Nb amount is more than 0.3%, the toughness of the weld heat affected zone is lowered, so the upper limit of the Nb amount is made 0.3% or less. Therefore, the Nb amount is 0.005% or more and 0.3% or less.
- Si 0.6% or less (including 0%) Si is an element that acts as a deoxidizer and contributes to strength improvement. If Si is added to the steel in an amount of more than 0.6%, the on-site weldability deteriorates significantly, so the upper limit of Si content is limited to 0.6%. Moreover, it is preferable to add 0.001% or more of Si for deoxidation. Furthermore, it is more preferable to add 0.1% or more of Si in order to increase the strength.
- Al 0.1% or less (excluding 0%) Al is an element that is generally used as a deoxidizer and refines the structure. However, when the Al content exceeds 0.1%, Al-based non-metallic inclusions increase and the cleanliness of the steel is impaired. Therefore, the upper limit of Al content is limited to 0.1%. Moreover, in order to fix the solid solution N which influences age hardening by precipitation of AlN, it is preferable to add 0.001% or more of Al.
- P 0.02% or less (including 0%)
- P is an impurity.
- the upper limit of the P content is limited to 0.02% or less.
- the amount of P is reduced, grain boundary fracture is prevented and low temperature toughness is improved.
- the amount of P is so preferable that it is small, from a balance with a characteristic and cost, 0.001% or more of P is usually contained in steel.
- S 0.005% or less (including 0%) S is an impurity.
- the upper limit of the amount of S is made 0.005% or less.
- the amount of S is preferably as small as possible, but usually contains 0.0001% or more of S in the steel from the balance between characteristics and cost.
- N 0.008% or less (including 0%)
- N is an impurity. Since the low temperature toughness decreases due to the coarsening of TiN, the upper limit of the N content is limited to 0.008% or less. Moreover, N forms TiN and suppresses the coarsening of crystal grains in the base material and the weld heat affected zone. In order to improve low temperature toughness, it is preferable to contain 0.001% or more of N in the steel.
- Bainite transformation index BT 650 ° C. or less
- the content of C, Mn, Mo, Ni, Cr in the steel is adjusted, and the bainite transformation index BT determined by the above-described equation (1) is 650 ° C. or less. It is extremely important to do.
- the bainite transformation index BT is set to 650 ° C. or lower, the bainite transformation is completed even if the accelerated cooling is stopped at 500 ° C. or higher.
- the dislocation density decreases due to the recovery at the time of air cooling after the stop of the accelerated cooling, and the deformability as produced (before aging) and the deformability after aging, that is, the deformation characteristics are increased.
- BT is calculated by setting the contents of Mo and Ni to zero.
- the upper limit of BT is not prescribed
- regulated 780.3 degreeC or less may be sufficient from the lower limit of content of C, Mn, and Cr.
- one or more of Ni, Cu, Mo, and V may be added to the steel.
- Ni 0.65% or less (including 0%) Ni is an element that improves strength without degrading low-temperature toughness. When the addition amount of Ni exceeds 0.65%, the HAZ toughness decreases. Therefore, it is preferable to set the upper limit of the Ni amount to 0.65% or less.
- Cu 1.5% or less (including 0%) Cu is an element that improves the strength of the base material and the weld heat affected zone. If the added amount of Cu exceeds 1.5%, the on-site weldability decreases. Therefore, it is preferable that the upper limit of the amount of Cu is 1.5% or less.
- Mo 0.3% or less (including 0%) Mo is an element that improves hardenability and increases strength. If the amount of Mo exceeds 0.3%, the HAZ toughness deteriorates. Therefore, it is preferable that the upper limit of the Mo amount be 0.3% or less.
- V 0.2% or less (including 0%) V, like Nb, contributes to refinement of the structure and increase of hardenability, and increases the toughness of the steel. However, the effect of adding V is small compared to Nb. V is effective for suppressing softening of the weld. From the viewpoint of ensuring the toughness of the welded portion, the upper limit of the V amount is preferably 0.2% or less.
- FIG. 3 is an example of a mixed structure of ferrite and bainite
- FIG. 4 is an example of a simple bainite structure.
- ferrite is defined as ferrite crystal grains (ferrite phase) that do not contain lath grain boundaries and carbides, as indicated by arrows in FIG.
- This ferrite is, for example, pro-eutectoid ferrite.
- the steel structure is, for example, a simple bainite structure shown in FIG.
- the components of the steel are adjusted in order to increase the strength and toughness of the weld heat affected zone. Therefore, with this steel component, it is difficult to generate ferrite as shown by the arrows in FIG.
- the ferrite (ferrite fraction) contained in this simple bainite structure is limited to 5% or less of the entire structure, the strength characteristics due to aging Can be ignored. Therefore, 5% or less of ferrite may be contained in the steel.
- the ferrite and the bainite structure can be distinguished using an optical microscope.
- the simple bainite structure may contain 3% or less of martensite-austenite composite, so-called MA (Martensite-Austenite constituents). However, if the MA is 3% or less, the influence on the mechanical properties can be ignored, and therefore 3% or less of MA may be contained in the steel.
- the simple bainite structure mainly includes a first bainite and a second bainite among the following three types of bainite.
- the first bainite (high-temperature bainite) 10 is a structure in which elongated bainitic ferrites 2a mainly grown from the prior austenite grain boundaries 1 are gathered.
- residual austenite 3 may exist between the bainitic ferrites 2a. Since this first bainite 10 has a small amount of C and is susceptible to strain recovery due to holding at a high temperature, it contains almost no carbide and is relatively soft. Therefore, this first bainite 10 can enhance the deformation performance of the steel pipe. Further, as shown in FIG.
- the second bainite (medium temperature bainite) 11 is a mixed structure of elongated bainitic ferrite 2a and cementite 4 between bainitic ferrite 2a.
- the second bainite 11 is harder than the first bainite 10.
- the bainitic ferrite 2a contained in the first bainite 10 and the second bainite 11 does not contain carbide. That is, the simple bainite structure contains bainitic ferrite 2a that does not contain carbide. Further, as shown in FIG.
- the third bainite (low-temperature bainite) 12 is a mixed structure of elongated bainitic ferrite 2b in which carbides 5 are formed in grains and cementite 4 between the bainitic ferrite 2b. is there.
- the third bainite 12 is present, the strain of the first bainite 10 is not sufficiently recovered, so that the structure non-uniformity in strength is less likely to occur, and the deformation performance of the steel pipe is difficult to improve. Therefore, it is preferable that the third bainite 12 is as few as possible. In order to sufficiently recover the strain of the first bainite 10, it is necessary to limit the bainitic ferrite 2b containing the third bainite 12 or carbide to 1% or less.
- the cementite 4 may contain a carbide such as niobium carbide as an impurity. Therefore, in the present invention, the simple bainite structure mainly contains the first bainite and the second bainite. The total amount of the first bainite and the second bainite is preferably 95% or more of the entire structure. In this simple bainite structure, a third bainite may be generated unexpectedly. Therefore, 1% or less of the third bainite may be included in the simple bainite structure. In order to distinguish the three types of bainite, a transmission microscope (TEM) can be used.
- TEM transmission microscope
- the steel pipe having the above-described steel components and structure is excellent in deformation characteristics, particularly strength-ductility balance after aging.
- a steel pipe for a line pipe manufactured by controlled rolling and accelerated cooling is heated to 150 to 300 ° C. when a resin coating is applied.
- the aging temperature at which the strength-ductility balance decreases most is 200 ° C.
- the product of the tensile strength TS in the tube axis direction and the n value (work hardening coefficient) at a tensile strain of 1 to 5% is 60 or more.
- This steel pipe is excellent in deformation characteristics after aging even when heat treatment is performed at an aging temperature at which the strength-ductility balance decreases most.
- the manufacturing method of the steel pipe in one Embodiment of this invention is demonstrated.
- a steel pipe according to the present embodiment after melting steel, it is cast to produce a steel slab, this steel slab is heated and hot rolled, then cooled to produce a steel sheet, and the steel sheet is cooled.
- a steel pipe is manufactured by forming the tube into a cylindrical shape and welding the ends together. The manufactured steel pipe is heated to a temperature of 150 to 350 ° C. when coating the surface of the steel pipe with a resin film or the like for corrosion prevention.
- the heating temperature of the hot-rolled steel slab is not specified, it is preferably 1000 ° C. or higher in order to reduce the deformation resistance. Moreover, in order to make Nb and Cr carbides dissolve in steel, it is more preferable to heat the steel piece to 1050 ° C. or higher. On the other hand, when the heating temperature exceeds 1300 ° C., the crystal grains become coarse and the toughness may be lowered. Therefore, the heating temperature is preferably 1300 ° C. or lower.
- finish rolling of hot rolling is performed at less than 750 ° C.
- ferrite is generated before rolling, and processed ferrite is generated during rolling.
- the hot rolling finish rolling is performed at 750 ° C. or higher in order to impair the deformation performance of the steel pipe.
- finish rolling is performed at 870 ° C. or lower.
- start temperature of finish rolling is 870 ° C. or lower
- end temperature is 750 ° C. or higher.
- Accelerated cooling starts immediately after hot rolling.
- the start temperature of accelerated cooling is significantly lower than 750 ° C.
- layered ferrite is generated in the steel, and the strength and toughness are reduced.
- the start of accelerated cooling is delayed, the dislocations introduced by the non-recrystallized zone rolling recover and the strength decreases.
- Acceleration cooling stop temperature is extremely important to obtain a steel pipe with excellent deformation characteristics. As shown in FIG. 1 described above, generally, as the cooling stop temperature increases, the strength-ductility balance [TS ⁇ n] increases. FIG. 1 shows that when the cooling stop temperature is set to 500 ° C. or higher, the strength-ductility balance [TS ⁇ n] rapidly increases. In this example, in order to reduce the dislocation density in the steel, the lower limit of the stop temperature of accelerated cooling is set to 500 ° C. or higher. After the accelerated cooling is stopped, air cooling (for example, less than 5 ° C./s) is performed to produce a steel plate.
- air cooling for example, less than 5 ° C./s
- the density of dislocations introduced during bainite transformation decreases, dislocations (strains) recover during air cooling, and the deformation characteristics of a steel pipe having a simple bainite structure can be improved.
- the upper limit of the stop temperature of accelerated cooling exceeds 600 ° C., layered ferrite is generated in the steel, and the strength and toughness are lowered. Therefore, the accelerated cooling stop temperature is 500 to 600 ° C.
- the cooling rate of this accelerated cooling is 5 to 50 ° C./s.
- the cooling rate of this accelerated cooling is preferably 10 to 50 ° C./s.
- the first bainite is mainly generated, and the second bainite is mainly generated immediately before the stop of the accelerated cooling and after the stop of the accelerated cooling. Therefore, by controlling the cooling rate and the cooling stop temperature in this way, a mixed structure of the first bainite and the second bainite can be obtained as described above.
- a 3rd bainite produces
- the steel plate after manufacture is formed into a tubular shape in the cold, and the butt portion is welded to manufacture a steel pipe. From the viewpoint of productivity, the UOE process or the bend process is preferable. Moreover, it is preferable to use submerged arc welding for welding of a butt
- Steel pipes are usually subjected to anticorrosion coating such as resin coating.
- the temperature range for coating heating of the steel pipe is 150 ° C. to 300 ° C.
- the metal structure of the manufactured steel pipe was observed with an optical microscope to confirm the presence or absence of ferrite. Moreover, the kind of bainite was confirmed using the scanning electron microscope (SEM) or the transmission electron microscope (TEM). Furthermore, after cutting out a part of steel pipe and performing an aging treatment at 200 degreeC using a salt bath, the arc-shaped full thickness tensile test piece (API specification) was extract
- SEM scanning electron microscope
- TEM transmission electron microscope
- the work hardening coefficient (n value) is calculated by using the formula (1) from the relationship (stress-strain curve) between the true stress ⁇ and the true strain ⁇ at a tensile strain of 1 to 5%. Calculated. Further, the strength-ductility balance [TS ⁇ n] was calculated from the product of the tensile strength TS and the work hardening coefficient (n value).
- Table 1 shows the chemical composition of the steel
- Table 2 shows a method for manufacturing the steel pipe.
- the steel pipes of Examples 1 to 10 had a simple bainite structure having the first bainite (B1) and the second bainite (B2) described above. Further, in this simple bainite structure, ferrite (F) and third bainite (B3) were not confirmed.
- Comparative Example 1 not only the first bainite (B1) and the second bainite (B2) but also the third bainite (B3) was generated in the metal structure.
- Comparative Example 2 ferrite (F) was also generated in the metal structure in addition to the above three types of bainite (B1, B2, B3).
- the bainite transformation index BT exceeds 650 ° C.
- the strength-ductility balance [TS ⁇ n] was less than 60, and ferrite (F) and third bainite (B3) were generated in the metal structure.
- the bainite transformation index BT is 650 ° C. or less and that the amount of ferrite (F) and third bainite (B3) produced is limited.
- the steel pipes of these comparative examples 3 to 5 satisfy the composition of the present invention with respect to the conditions regarding the chemical components excluding the bainite transformation index BT.
- the steel pipes of Comparative Examples 6 to 9 are steels (A, E, B) satisfying the composition of the present invention shown in Table 1 and the stop temperature of accelerated cooling is less than 500 ° C. as shown in Table 2. It is a steel pipe manufactured under certain manufacturing conditions (production Nos. 16 to 19).
- the strength-ductility balance [TS ⁇ n] was less than 60, and the third bainite (B3) was generated in the metal structure. Therefore, it can be seen that in these Comparative Examples 6 to 9, good characteristics (deformation performance) cannot be obtained. Therefore, it can be seen that it is important to limit the amount of third bainite (B3) produced in order to sufficiently secure the deformation performance. Furthermore, the steel pipes of Comparative Examples 1 to 9 had a strength-ductility balance [TS ⁇ n] of less than 60 when aging treatment was performed at 200 ° C.
- the symbol “B” in Table 3 is a structure including the first bainite (B1), the second bainite (B2), and the third bainite (B3).
Abstract
Description
本願は、2009年6月11日に、日本に出願された特願2009-140280号に基づき優先権を主張し、その内容をここに援用する。
σ=Kεn・・・(1)
引張試験によって歪み量が1~5%の範囲内で求められたn値と、鋼管の座屈特性との相関が顕著であるため、本発明では、1~5%の歪み量の範囲内でn値を求めている。すなわち、真応力σと真歪みεとの関係を引張試験によって求め、歪み量が1~5%の範囲内における上記真応力σと真歪みεとの関係から(1)式の指数部(n値)が求められる。なお、上記(1)式におけるパラメータKは、材料により定まる定数である。
BT=830-270[C]-90[Mn]-37[Mo]-70[Ni]-83[Cr]・・・(2)
なお、[C]、[Mn]、[Mo]、[Ni]、[Cr]は、それぞれ、C、Mn、Mo、Ni、Crの含有量である。
まず、鋼管の成分について説明する。なお、成分の量(%)は、いずれも質量%である。
Cは、鋼の強度向上に極めて有効な元素である。十分な強度を得るためには、鋼中に0.02%以上のCを添加する。一方、C量が0.09%よりも多いと、母材及び溶接熱影響部の低温靭性が低下して、現地溶接性が劣化する。そのため、C量の上限は、0.09%である。したがって、C量は、0.02%以上0.09%以下である。
Mnは、強度と低温靭性とのバランスを向上させるために極めて重要な元素である。そのため、鋼中に0.4%以上のMnを添加する。一方、Mn量が2.4%よりも多いと、鋼板表面に平行な板厚中心部の偏析(中心偏析)が顕著になる。この中心偏析による低温靭性の劣化を抑制するために、Mn量の上限を2.4%にする。したがって、Mn量は、0.4%以上2.5%以下である。
Crは、母材及び溶接部の強度を増加させる。そのため、鋼中に0.1%以上のCrを添加する。しかし、Cr量が1.0%よりも多いと、HAZ靱性及び現地溶接性が著しく劣化するため、Cr量の上限を1.0%以下にする。したがって、Cr量は、0.1%以上1.0%以下である。
Tiは、微細なTiNを形成して、母材および溶接熱影響部の組織を微細化し、靭性向上に寄与する。この効果は、Nbとの複合添加により極めて顕著に現れる。この効果を十分に発現させるためには、0.005%以上のTiを鋼中に添加する必要がある。一方、Ti量が0.03%より多いと、TiNの粗大化及びTiCによる析出硬化が生じるため、低温靭性が低下する。そのため、Ti量の上限を0.03%に限定する。したがって、Ti量は、0.005%以上0.03%以下である。
Nbは、制御圧延時にオーステナイトの再結晶を抑制して組織を微細化するだけでなく、焼入れ性を増大させて鋼の靭性を向上させる。この効果を得るためには、鋼中にNbを0.005%以上添加する必要がある。一方、Nb量が0.3%よりも多いと、溶接熱影響部の靭性が低下するため、Nb量の上限を0.3%以下にする。したがって、Nb量は、0.005%以上0.3%以下である。
Siは、脱酸剤として作用し、強度向上に寄与する元素である。Siを鋼中に0.6%より多く添加すると現地溶接性が著しく劣化するので、Si量の上限を0.6%に制限する。また、脱酸のために、0.001%以上のSiを添加することが好ましい。更に、強度を高めるために、Siを0.1%以上添加することがより好ましい。
Alは、脱酸剤として一般的に使用され、組織を微細化する元素である。しかし、Al量が0.1%を超えるとAl系非金属介在物が増加して鋼の清浄度を害する。そのため、Al量の上限を0.1%に制限する。また、時効硬化に影響を及ぼす固溶NをAlNの析出によって固定するために、0.001%以上のAlを添加することが好ましい。
Pは、不純物である。母材及び溶接熱影響部の低温靭性を向上させるために、P量の上限を0.02%以下に制限する。P量を低減すると、粒界破壊が防止され、低温靭性が向上する。なお、P量は、少ないほど望ましいが、特性とコストとのバランスから、通常、鋼中に0.001%以上のPを含有する。
Sは、不純物である。母材及び溶接熱影響部の低温靭性を向上させるために、S量の上限を0.005%以下にする。S量を低減すると、熱間圧延によって延伸されるMnSの量を低減し、延性と靭性とを向上させることができる。S量は、少ないほど望ましいが、特性とコストとのバランスから、通常、鋼中に0.0001%以上のSを含有する。
Nは、不純物である。TiNの粗大化によって低温靭性が低下するため、N量の上限を0.008%以下に制限する。また、Nは、TiNを形成し、母材及び溶接熱影響部の結晶粒の粗大化を抑制する。低温靭性を向上させるためには、鋼中に0.001%以上のNを含有させることが好ましい。
本発明では、鋼中のC、Mn、Mo、Ni、Crの含有量を調節して、上述した(1)式によって求められるベイナイト変態指標BTを650℃以下にすることが極めて重要である。上述のように、ベイナイト変態指標BTを650℃以下にすれば、加速冷却を500℃以上で停止しても、ベイナイト変態が完了する。その結果、加速冷却の停止後の空冷時の回復によって転位密度が低下し、製造したまま(時効前)での変形能及び時効後の変形能、即ち、変形特性が高まる。なお、Mo、Niを含有しない場合には、Mo、Niの含有量を0としてBTを求める。BTの上限は、規定されないが、C、Mn、Crの含有量の下限値から、780.3℃以下であってもよい。
更に、強度を向上させるために、鋼中にNi、Cu、Mo、Vの1種以上を添加してもよい。
Niは、低温靭性を劣化させることなく強度を向上させる元素である。Niの添加量が、0.65%を超えると、HAZ靭性が低下する。そのため、Ni量の上限を0.65%以下にすることが好ましい。
Cuは、母材及び溶接熱影響部の強度を向上させる元素である。Cuの添加量が、1.5%を超えると、現地溶接性が低下する。そのため、Cu量の上限を1.5%以下にすることが好ましい。
Moは、焼入れ性を向上させ、強度を高める元素である。Moの添加量が、0.3%を超えると、HAZ靭性が劣化する。そのため、Mo量の上限を0.3%以下にすることが好ましい。
Vは、Nbと同様、組織の微細化及び焼入れ性の増大に寄与し、鋼の靭性を高める。しかしながら、Vを添加する効果は、Nbと比べると小さい。また、Vは、溶接部の軟化の抑制に有効である。溶接部の靭性確保の観点から、V量の上限を0.2%以下にすることが好ましい。
したがって、本発明では、単純ベイナイト組織は、第一のベイナイトと、第二のベイナイトとを主に含有する。この第一のベイナイトと第二のベイナイトとを合計した組織の量は、組織全体の95%以上であることが好ましい。なお、この単純ベイナイト組織中には、予期せず第三のベイナイトが生成する場合もある。そのため、単純ベイナイト組織中に、第三のベイナイトが、1%以下含まれてもよい。3種のベイナイトを区別するためには、透過型顕微鏡(TEM)を用いることができる。
本実施形態による鋼管の製造方法では、鋼を溶製後、鋳造して鋼片を作製し、この鋼片を加熱して熱間圧延した後、冷却して鋼板を作製し、その鋼板を冷間で筒状に成形して端部同士を溶接し、鋼管を製造する。なお、製造後の鋼管は、防食のために樹脂等の皮膜を鋼管表面にコーティングする際に、150~350℃の温度に加熱される。
Claims (5)
- 質量%で、
C:0.02~0.09%、
Mn:0.4~2.5%、
Cr:0.1~1.0%、
Ti:0.005~0.03%、
Nb:0.005~0.3%
を含有し、残部が鉄および不可避的不純物を含み、
Si:0.6%以下、
Al:0.1%以下、
P:0.02%以下、
S:0.005%以下、
N:0.008%以下
に制限し、
下記(3)式によって求められるベイナイト変態指標BTが650℃以下であり、
金属組織が、第一のベイナイトと第二のベイナイトとを含む単純ベイナイト組織であり、前記第一のベイナイトが、炭化物を含まないベイニティックフェライトの集合組織であり、前記第二のベイナイトが、前記炭化物を含まないベイニティックフェライトとこのベイニティックフェライトの間のセメンタイトとの混合組織である
ことを特徴とする高強度鋼管。
BT=830-270[C]-90[Mn]-37[Mo]-70[Ni]-83[Cr]・・・(3)
ここで、[C]、[Mn]、[Mo]、[Ni]、[Cr]は、それぞれ、C、Mn、Mo、Ni、Crの含有量である。 - 質量%で、
Ni:0.65%以下、
Cu:1.5%以下、
Mo:0.3%以下、
V:0.2%以下
の少なくとも1種をさらに含有することを特徴とする請求項1に記載の高強度鋼管。 - 前記第一のベイナイトと前記第二のベイナイトとを合計した組織の量が、組織全体の95%以上であることを特徴とする請求項1又は2に記載の高強度鋼管。
- 200℃で時効処理を行った場合に、管軸方向の引張強度と、1~5%の間の引張歪みにおけるn値との積が60以上になることを特徴とする請求項1又は2に記載の高強度鋼管。
- 請求項1又は2に記載の鋼成分を満足する鋼片を加熱し、この鋼片に対して750~870℃の範囲内で熱間圧延の仕上圧延を行い、冷却速度が5~50℃/sである加速冷却を750℃以上で開始し、500~600℃の範囲内で前記加速冷却を停止し、空冷して鋼板を作製し、この鋼板を冷間で管状に成形し、突合せ部を溶接することを特徴とする高強度鋼管の製造方法。
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BRPI1012964A BRPI1012964A2 (pt) | 2009-06-11 | 2010-06-10 | tubo de aço de alta resistência e método de produção do mesmo |
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JP2010541624A JP4741715B2 (ja) | 2009-06-11 | 2010-06-10 | 高強度鋼管及びその製造方法 |
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