WO2015092916A1 - 電縫溶接鋼管 - Google Patents
電縫溶接鋼管 Download PDFInfo
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- WO2015092916A1 WO2015092916A1 PCT/JP2013/084255 JP2013084255W WO2015092916A1 WO 2015092916 A1 WO2015092916 A1 WO 2015092916A1 JP 2013084255 W JP2013084255 W JP 2013084255W WO 2015092916 A1 WO2015092916 A1 WO 2015092916A1
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
- F16L9/00—Rigid pipes
- F16L9/17—Rigid pipes obtained by bending a sheet longitudinally and connecting the edges
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
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Definitions
- the present invention relates to an electric resistance welded steel pipe having a high strength and a high yield ratio that is optimal for oil well pipe applications such as shale gas mining.
- Shale gas a kind of unconventional natural gas produced from other than ordinary oil or gas fields, is a natural material confined in a shale layer, which is a very hard rock layer that is several hundred to several thousand meters underground. Gas. In order to extract shale gas, it is necessary to hydraulically crush this very hard shale layer and collect the gas confined inside the rock layer from deep underground. Therefore, steel pipes used for shale gas mining have high strength. It has been demanded.
- a steel pipe having an API standard 5CT P110 (hereinafter referred to as “P110”) equivalent strength (yield stress YS: 758 to 965 MPa, tensile strength TS: 862 MPa or more) is generally used. It has been. In order to ensure such strength, the entire steel pipe is quenched and tempered after pipe forming, but with the aim of reducing shale gas mining costs, it has high strength and does not perform heat treatment after pipe forming. There is an increasing demand for ERW steel pipes that have been formed into pipes (quenching and tempering omitted).
- ERW molding becomes difficult to manufacture as the tensile strength (TS) increases. Therefore, it is desirable to ensure the formability by increasing the yield ratio (YS / TS: hereinafter referred to as “YR”), which is the ratio between the yield strength (YS) and TS, and obtaining the desired YS with a low TS.
- YiR yield ratio
- the yield ratio in the rolling direction (L direction) tends to decrease due to the Bauschinger effect.
- the Baudinger effect appears remarkably, so that the yield ratio tends to decrease.
- Patent Document 1 discloses an electric-welded steel pipe that uses work hardening in order to ensure the strength equivalent to P110 and can omit the heat treatment after pipe making.
- This is not a duplex steel but a steel sheet having a uniform structure of bainite. That is, by using the cooling rate V C90 that becomes a hardness corresponding to a 90% martensitic structure estimated from the C content as an index of hardenability, controlling V C90 to an appropriate range, and making the uniform structure of bainite, A high strength and high yield ratio electric resistance welded steel pipe is disclosed.
- a low carbon boron-added steel has a uniform bainite structure, a low Bausinger effect, a high YR, and a YS that satisfies P110 with a low TS hot-rolled steel sheet. It has been demonstrated that the strength of
- the electric resistance welded steel pipe disclosed in Patent Document 1 controls VC90 , which is an index of hardenability, and lowers the coiling temperature after hot rolling, thereby suppressing the formation of polygonal ferrite and forming a uniform bainite structure. It has gained.
- the electric resistance welded steel pipe requires the addition of a small amount of boron (B) (0.0005 mass% to 0.0030 mass%).
- B has an effect of improving the hardenability and strength of the steel pipe, but the effect is saturated even when a certain amount or more is added.
- B is inexpensive, the manufacturing condition range for obtaining stable characteristics is narrow, and careful attention is required for use. In particular, steel that realizes characteristics while being hot rolled without quenching and tempering needs to be manufactured under appropriate hot rolling conditions.
- the present invention has been made in view of such circumstances, and has a strength equivalent to P110, without containing B, and without performing heat treatment after pipe making, while keeping the C content relatively high to ensure strength. It is another object of the present invention to provide an electric resistance welded steel pipe having a yield stress and a method for producing the electric resistance welded steel pipe.
- ⁇ Dual phase steel undergoes work hardening by introducing dislocations into the soft phase around the hard phase during plastic deformation. Therefore, if the deformation of the hard phase is suppressed, the accumulation of dislocations into the soft phase is promoted, and the work hardening rate can be increased. Further, the refinement of ferrite, which is a soft phase, can increase the work hardening rate and suppress the Bauschinger effect, so that the strength of the steel pipe after ERW molding can be increased. Further, the cooling control after hot rolling to obtain the above structure can be applied to a steel plate having a relatively large thickness.
- Component composition is mass%, C: 0.08 to 0.18%, Si: 0.01% to 0.50%, Mn: 1.30 to 2.1%, Al: 0.001 to 0.10%, Nb: 0.005 to 0.08% Ti: 0.005 to 0.03% Each containing N 0.008% or less, P: 0.020% or less, S: 0.010% or less, Limited to Steel with the balance being Fe and inevitable impurities,
- the structure in the central portion of the thickness contains a ferrite phase with an equivalent circle diameter of 1.0 ⁇ m to 10.0 ⁇ m in an area ratio of 40% to 70%, and the balance is a low-temperature transformation generation phase containing a bainite phase.
- Ceq C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
- C, Mn, Cr, Mo, V, Cu, and Ni in (Equation 1) are values representing the content of each element in mass%, and when these elements are not included, the elements are Calculate as 0.
- the component composition is further mass%, V: 0.08% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Ca: 0.005% or less, REM: The ERW welded steel pipe according to [1], containing one or more of 0.005% or less.
- the component composition is further mass%, B: The electric resistance welded steel pipe according to [1] or [2], which is limited to 0.0004% or less.
- (A) is an observation result of the ERW steel pipe of the present invention by a high resolution crystal orientation analysis method, and (b) is a distribution state diagram of ferrite obtained by image analysis of the observation result.
- (A) is an observation result of the ERW steel pipe of the present invention by a high resolution crystal orientation analysis method, and (b) is a distribution state diagram of ferrite obtained by image analysis of the observation result.
- (A) is an observation result of the ERW steel pipe of the present invention by a high resolution crystal orientation analysis method, and (b) is a distribution state diagram of ferrite obtained by image analysis of the observation result.
- the components of the ERW welded steel pipe of the present invention will be described.
- the components of the hot rolled steel sheet, which is the material of the ERW steel pipe, are the same as the components of the ERW welded steel pipe.
- “%” represents “% by mass” unless otherwise specified.
- ⁇ C: 0.08 to 0.18%> C is effective for improving the strength. Since the strength of the steel can be increased by increasing the amount of C added to the steel, the lower limit of the C content is 0.08%. On the other hand, if the C content exceeds 0.18%, the strength of the steel becomes too high and the toughness is deteriorated, so the upper limit is made 0.18%. Further, from the viewpoint of ensuring the strength equivalent to P110, the lower limit of the C amount is preferably 0.1% or more. From the viewpoint of ensuring toughness without excessively increasing the strength, the upper limit of the C content is preferably 0.17%, more preferably 0.16%, and 0.15% or less to ensure toughness reliably. It is preferable to make it.
- Si is effective as a deoxidizer.
- addition of 0.01% or more is preferable.
- Si is an element that increases the strength by solid solution strengthening, addition of 0.05% or more is more preferable, and addition of 0.10% or more is more preferable. If Si is added in an amount exceeding 0.50%, the low temperature toughness and further the electric resistance weldability are impaired, so the upper limit is made 0.50%. From the viewpoint of ensuring toughness, the Si content is preferably 0.40% or less, and more preferably 0.30% or less.
- Mn is an element that enhances the hardenability of steel.
- 1.30% or more of Mn is added to ensure strength.
- the upper limit is made 2.10%.
- the Mn content is preferably 1.40% or more, more preferably 1.50% or more.
- the Mn content is preferably 2.0% or less, and more preferably 1.90% or less.
- Al is effective as a deoxidizer.
- 0.001% or more of addition is preferable.
- 0.005% or more of Al is preferably added, and 0.01% or more of addition is more preferable.
- Al is added in excess of 0.10%, inclusions increase and ductility and toughness are impaired, so the content is limited to 0.10% or less. From the viewpoint of ensuring toughness, the Al content is preferably 0.06% or less.
- Nb is an element that lowers the recrystallization temperature, and suppresses the recrystallization of austenite and contributes to the refinement of the structure during hot rolling, so 0.005% or more is added. If Nb is added in excess of 0.08%, the toughness deteriorates due to coarse precipitates, so the content is made 0.08% or less. From the viewpoint of securing toughness, the upper limit is preferably 0.07%, and more preferably 0.05%. On the other hand, the lower limit is preferably 0.008%, more preferably 0.010%, and still more preferably 0.015% in order to ensure the effect of refining the structure.
- Ti forms fine nitrides (TiN), suppresses the coarsening of austenite grains during slab heating, and contributes to the refinement of the structure.
- TiN fine nitrides
- 0.005% or more of Ti is added. If Ti is added excessively exceeding 0.030%, TiN coarsening and precipitation hardening due to TiC occur and toughness deteriorates, so 0.030% is made the upper limit.
- the Ti content is preferably 0.008% or more, and more preferably 0.010% or more.
- the Ti content is preferably 0.025% or less, and more preferably 0.020% or less.
- N is inevitably present in the steel, but if the amount of N is too large, TiN and AlN will increase excessively, which may cause adverse effects such as surface defects and deterioration of toughness. Therefore, the upper limit is made 0.008%. Furthermore, from the viewpoint of suppressing the formation of inclusions, the upper limit of the N amount is preferably 0.007%, and a more preferable upper limit is 0.006%. Although the lower limit is not particularly set, it is preferably set to 0.002% in consideration of the cost of removing N and the economic efficiency.
- P is an impurity, and the upper limit of the content is 0.02%. Since the toughness is improved by reducing the amount of P, the amount of P is preferably 0.015% or less, and more preferably 0.010% or less. Since it is preferable that the amount of P is small, there is no lower limit. Usually, 0.001% or more is contained from the balance between characteristics and cost.
- S is an impurity, and the upper limit of the content is 0.010%.
- the amount of S is preferably 0.003% or less, and more preferably 0.002% or less. Since a smaller amount of S is preferable, no lower limit is provided. Usually, 0.0001% or more is contained from the balance between characteristics and cost.
- one or more of V, Ni, Cu, Cr, Mo, Ca, and REM can be added in order to further improve the hardenability of the steel and increase the strength.
- a preferable lower limit value is described. This is a preferable lower limit value for obtaining the effect of improving the hardenability and increasing the strength by adding each element. Even if the content of each element is less than the preferred lower limit, the steel is not adversely affected.
- V is an element that generates carbides and nitrides and improves the strength of the steel by precipitation strengthening. In order to effectively increase the strength, it is preferable to add 0.01% or more. If V is added excessively, carbides and nitrides are coarsened and toughness is deteriorated, so the upper limit of V content is 0.08%, more preferably 0.05%.
- Cu is an element that improves the hardenability of steel and contributes to solid solution strengthening, so 0.05% or more may be added. Since excessive addition of Cu may impair the surface properties of the steel sheet, the upper limit is made 0.50% or less. From the economical viewpoint, the more preferable upper limit of the amount of Cu is 0.30% or less. When adding Cu, it is preferable to add Ni simultaneously from the viewpoint of preventing deterioration of surface properties.
- Ni is an element that improves the hardenability of steel and contributes to the improvement of toughness.
- the Ni content is preferably 0.05% or more.
- the upper limit is 0.50% or less, preferably 0.30% or less.
- Cr is an element effective for improving the strength, and it is preferable to add 0.05% or more. If Cr is added excessively, the electric resistance weldability may be deteriorated, so the upper limit is 0.5%, and preferably 0.2% or less.
- Mo is an element that contributes to increasing the strength of steel, and it is preferable to add 0.05% or more.
- Mo is an expensive element, and the upper limit is 0.50%.
- a more preferable upper limit of the Mo amount is 0.30% or less, and further preferably 0.10% or less.
- REM controls the form of sulfide inclusions, improves low temperature toughness, and further refines the oxide of the ERW weld to improve the toughness of the ERW weld, so either or both Is preferably added in an amount of 0.001% or more. If Ca and REM are added excessively, oxides and sulfides increase and adversely affect toughness. Therefore, the upper limit of the addition amount is set to 0.005%.
- REM is a general term for Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
- B is not an intentionally added element but is mixed as an inevitable impurity contained in the raw material, and its content is limited to 0.0004% or less.
- Ceq is an index of hardenability and may be used as an index of strength. It calculates
- Ceq needs to be 0.32 or more.
- Ceq needs to be 0.60 or less.
- the lower limit of Ceq is preferably 0.35 or more, and more preferably 0.4 or more.
- the upper limit of Ceq is preferably 0.50 or less, more preferably 0.45 or less.
- C, Mn, Cr, Mo, V, Ni, and Cu are the content [% by mass] of each element.
- Cr, Mo, V, Ni, and Cu are elements that are selectively added in the present invention. When these elements are not included, the calculation is performed with 0 as the element in the above (formula 1).
- the remainder of the component composition of the electric resistance welded steel pipe according to the present invention other than those described above is iron and inevitable impurities.
- Inevitable impurities are components contained in raw materials or mixed in during the manufacturing process, and are components not intentionally contained in steel.
- P and S need to be controlled to be 0.02% or less and 0.010% or less, respectively, as described above. It is preferable to control O to be 0.006% or less.
- Sb, Sn, W, Co, and As are usually unavoidable 0.1% or less, Mg, Pb, and Bi are 0.005% or less, and B and H are 0.0004% or less. There may be contamination as an impurity, but there is no need to control in the normal range.
- Si, Al, Ni, Cu, Cr, Mo, V, Ca, and REM which are optional elements or optional elements in the steel pipe of the present invention, are mixed as inevitable impurities even if they are not intended to be contained.
- the steel pipe of the present invention is not adversely affected as long as it is below the upper limit of the content when intentionally contained.
- N may be treated as an inevitable impurity in steel, but in the electric resistance welded steel pipe of the present invention, it is necessary to control it within a certain range as described above.
- the ERW welded steel pipe according to the present invention has a structure mainly composed of ferrite and a low-temperature transformation generation phase represented by bainite.
- ferrite since B is not added, ferrite is likely to be generated, but the ferrite phase is refined in order to ensure yield strength while utilizing the properties of the ferrite to ensure toughness.
- the overall strength is ensured by controlling the C content and Ceq.
- tissues point out the structure
- the wall thickness central portion refers to a portion corresponding to a depth of 1 ⁇ 4 to 3 ⁇ 4 of the plate thickness from the surface of the steel pipe in the cross section of the steel pipe.
- the ferrite phase constituting the ERW welded steel pipe of the present invention has a circle equivalent diameter of 1.0 ⁇ m to 10.0 ⁇ m.
- the lower limit is preferably 2.0 ⁇ m or more.
- the equivalent circle diameter of the ferrite phase exceeds 10.0 ⁇ m, the Bausinger effect becomes remarkable, YR after ERW molding becomes low, and low temperature toughness is deteriorated.
- the upper limit of the equivalent circle diameter of the ferrite phase is preferably 7 ⁇ m, more preferably 6 ⁇ m, and further preferably 5.0 ⁇ m to ensure the effect.
- the area ratio of the ferrite phase needs to be 40% or more in order to ensure toughness and improve the yield ratio.
- a preferable lower limit of the area ratio of the ferrite phase is 45%, and a more preferable lower limit is 50%.
- the upper limit is set to 70% in comparison with the P110 standard. From the viewpoint of securing strength, the preferred upper limit is 65%, more preferably 60%.
- the ERW welded steel pipe of the present invention is composed of a low-temperature transformation generation phase mainly composed of bainite, with the remainder excluding ferrite. Further, it may contain a retained austenite phase or martensite.
- the area ratio of the bainite phase is preferably 90% or more of the balance excluding the ferrite phase.
- the retained austenite is unstable and lowers the yield stress.
- the upper limit of the area ratio of each of retained austenite and martensite is 1%. More preferably, it is 0.5% or less, and it is desirable that it is not present if possible.
- the distribution state and area ratio of each of the ferrite phase, bainite phase, retained austenite phase, martensite phase, and the like are obtained by high resolution crystal orientation analysis (hereinafter referred to as “EBSP method”) and image analysis by software such as KAM. be able to.
- EBSP method high resolution crystal orientation analysis
- KAM KAM
- FIG. 1 (a), FIG. 2 (a) and FIG. 3 (a) show the results of observing the ERW steel pipes T1 to T3 of the present invention by the EBSP method.
- FIG. 1B, FIG. 2B, and FIG. 3B show the results of image analysis using software “KAM” with respect to FIG. 1A, FIG. 2A, and FIG. 3A. . From this image analysis, the area ratio of ferrite can be obtained.
- the ferrite phase corresponds to that having an orientation difference of less than 1 ° by the KAM method, and is expressed by being color-coded in the image.
- FIGS. 1 (a), 2 (a) and 3 (a) respectively show the electro-sewing of the present invention produced under the production conditions of Mn amount and hot rolling finishing temperature shown in Table 1-1.
- 3 is a photographed image of welded steel pipes T1 to T3 by the EBSP method.
- FIGS. 1B, 2B, and 3B show the results of image analysis of FIGS. 1A, 2A, and 3A by KAM, respectively.
- the area ratio of the martensite phase and residual austenite of FIG. 1 (b), FIG. 2 (b) and FIG. 3 (b) was measured. As a result, the area ratio of both the martensite phase and the retained austenite was 1% or less. It was also confirmed that if the martensite phase and retained austenite had an area ratio of 1% or less, the properties of the ERW steel pipe of the present invention were not affected.
- the yield ratio was obtained from a yield strength YS and a tensile strength TS by performing a tensile test.
- the yield ratio exceeded 95% and the toughness was significantly reduced.
- the area ratio of the ferrite phase exceeds 70%, the yield strength decreases and the yield ratio decreases to less than 85%.
- the area ratio of the ferrite phase exceeds 70%, not only the yield strength but also the tensile strength is lowered, and the strength equivalent to P110 cannot be obtained.
- the ERW steel pipe of the present invention can suppress the Bauschinger effect by containing a fine ferrite phase having an equivalent circle diameter of 1.0 ⁇ m to 10.0 ⁇ m in an area ratio of 40% to 70%, and yield strength can be reduced. And a yield ratio of 85 to 95% can be secured. Moreover, it has been confirmed that the ERW steel pipe of the present invention has no yield elongation in the stress strain curve of the tensile test.
- the steel having the above-mentioned components is heated and hot-rolled, and then controlled cooling and winding are performed to produce a hot-rolled steel sheet.
- the heating temperature of the steel is preferably 1150 ° C. or higher in order to dissolve the elements that form carbides such as Nb in the steel.
- 1000 to 1250 ° C. is desirable to obtain a fine grain structure. If the heating temperature is too high, the structure becomes coarse. Therefore, 1250 ° C. or lower is preferable in order to prevent coarsening of the ferrite grain size.
- Hot rolling needs to be performed in a temperature range where the steel structure is an austenite phase. This is because if the rolling is performed after the ferrite transformation has started, processed ferrite is generated and the anisotropy of the characteristics is increased. Therefore, the finishing temperature of hot rolling is preferably Ar 3 or higher at which ferrite transformation during cooling starts. If the finishing temperature is too high, the structure becomes coarse, so the upper limit of the finishing temperature for hot rolling is preferably 1000 ° C.
- Ar 3 can be obtained from the thermal expansion behavior when heated and cooled using a test material having the same composition as the hot-rolled steel sheet. Moreover, it is also possible to obtain
- C, Mn, Ni, Cu, Cr, and Mo are the content [% by mass] of each element.
- Ni, Cu, Cr, and Mo are arbitrary additive elements in the present invention. When these elements are not added intentionally, the calculation is made as 0 in the above (Formula 2).
- the amount of reduction at 950 ° C. or less is reduced to 70% or more.
- the reduction amount of 950 ° C. or less is obtained as a percentage by dividing the difference between the plate thickness at 950 ° C. and the plate thickness after finish rolling by the plate thickness after finish rolling.
- FT rolling finish temperature
- the average cooling rate from 650 ° C. to 300 ° C. is preferably 15 ° C./s or more.
- the transformation of bainite can be promoted and strength can be secured.
- the upper limit of the cooling rate is set to 50 ° C./s.
- the temperature is preferably 40 ° C./s, more preferably 30 ° C./s.
- the subsequent stage cooling rate is 1.5 times or more, preferably 2 times or more, that of the preceding stage.
- the end temperature of the cooling step is 300 ° C. or lower which is the bainite transformation temperature or lower. This is to obtain an appropriate amount of bainite phase.
- the steel sheet is wound at 300 ° C. or lower.
- the coiling temperature of the hot-rolled steel sheet in the present invention is 300 ° C. or less.
- the lower limit may be room temperature.
- the obtained hot-rolled steel sheet is air-cooled, formed into a tubular shape in the cold, and the end portions are butted together and electro-welded to produce an electro-welded steel pipe.
- the present invention does not particularly define the thickness and outer shape of the ERW steel pipe, but the ratio t / D of the thickness t of the steel sheet and the outer diameter D of the ERW steel pipe is about 2.0 to 6.0%. And t can be suitably applied to those having a thickness of 7 mm to 12.7 mm.
- a seam heat treatment may be applied in which only the ERW weld is heated and accelerated.
- ERW welding the butt portion is heated and melted, pressure is applied, and solid phase bonding is performed. Therefore, the vicinity of the ERW weld portion is plastically deformed at a high temperature and then rapidly cooled. Therefore, the ERW welded portion is harder than the steel plate, and the low temperature toughness and deformation performance of the ERW steel pipe can be further improved by performing seam heat treatment.
- Steels A to L and AA to AD having chemical components shown in Table 2-1 and Table 2-2 were cast into steel pieces. These steel slabs were heated to the heating temperatures shown in Table 3-1 and Table 3-2, and the rolling amount was 1000 ° C. or less and the rolling finishing temperature (FT in Tables 3-1 and 3-2). Hot rolling was performed and cooled to obtain a hot rolled steel sheet.
- the cooling process is performed by two-stage cooling in which the cooling rate is changed at the intermediate temperature (MT in Table 3-1 and Table 3-2) as a boundary, and the cooling rate in the latter stage (MT or lower) is changed from the first stage (from the cooling start temperature).
- the average cooling rate (up to MT) was 1.5 times or more.
- the steel sheet after the cooling step was wound at a winding temperature (CT) described in Table 3-1 and Table 3-2 to obtain a hot rolled steel sheet.
- CT winding temperature
- the obtained hot-rolled steel sheet was air-cooled, it was formed into a tubular shape in a continuous roll forming process, and the ends of the hot-rolled steel sheet were butted together and subjected to electric resistance welding. Thereafter, if necessary, the ERW weld was heated, cooled at an accelerated rate, and subjected to seam heat treatment.
- the amount of reduction” in Table 3-1 and Table 3-2 is the amount of reduction at 950 ° C. or less in the hot rolling process.
- T indicates the thickness (mm) of the steel sheet, and “D” indicates the outer diameter (mm) of the steel pipe after pipe making.
- Ar 3 in Table 2-1 and Table 2-2 was determined from the content [% by mass] of C, Mn, Ni, Cu, Cr, and Mo. Ni, Cu, Cr, and Mo are optional additive elements in the present invention. As shown in blanks in Tables 2-1 and 2-2, when not intentionally added, the following (formula In 2), it was calculated as 0.
- a sample for observing the structure was taken from the obtained electric resistance welded steel pipe, and a cross section parallel to the longitudinal direction of the steel pipe was subjected to nital etching, and the structure was observed and photographed with an optical microscope. The observation position was 2t / 5 from the outer surface. Using these structure photographs, it was confirmed that structures other than ferrite phase or bainite phase such as pearlite and martensite were not generated. Thereafter, the area ratio of the ferrite phase was obtained by image analysis of the image observed by the EBSP method. About the area ratio of the ferrite phase, ten fields of view of 100 ⁇ m ⁇ 200 ⁇ m were measured, and an average value was obtained. Furthermore, the area ratio of austenite was measured by an X-ray diffraction method and confirmed to be 1% or less.
- Each of 1 to 14 is a metal structure composed of ferrite and bainite having an appropriate area ratio.
- the tensile strength of these electric resistance welded steel pipes is all 758 MPa or higher, and the yield ratio is 85% to 95%.
- No. No. 22 has a C content lower than the range of the present invention, so that the area ratio of the ferrite phase becomes excessive, and the average ferrite equivalent grain size exceeds 10 ⁇ m, so that the ferrite structure of the steel is not refined. It was enough. Therefore, no. In No. 22, YS strength equivalent to P110 was not obtained, and the yield ratio in the rolling direction was less than 85%.
- No. 23 is an example in which the strength is insufficient because Ceq is lower than the range of the present invention.
- No. No. 24 is an example in which the strength is excessively increased because Ceq is higher than the range of the present invention, and the yield ratio in the rolling direction exceeds 95%.
- No. No. 25 is an example in which the C content is higher than the range of the present invention and the strength is excessively increased.
- the present invention aims to produce an ERW steel pipe at a low cost, and defines the conditions for satisfying the required characteristics as it is formed by ERW. If tempering is performed after ERW molding, there is a material change in which YR increases significantly and yield elongation appears.
- an ERW welded steel pipe having strength equivalent to API standard 5CT P110, which is optimal for applications such as shale gas mining, without heat treatment after pipe making, that is, at low cost.
- the availability is great.
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Abstract
Description
C :0.08~0.18%、
Si:0.01%~0.50%、
Mn:1.30~2.1%、
Al:0.001~0.10%、
Nb:0.005~0.08%
Ti:0.005~0.03%
をそれぞれ含有し、
N 0.008%以下、
P:0.020%以下、
S:0.010%以下、
に制限され、
残部がFe及び不可避的不純物である鋼であって、
肉厚中央部の組織が、円相当径1.0μm~10.0μmのフェライト相を面積率で40%~70%含有し、残部はベイナイト相を含有する低温変態生成相であり、
下記(式1)で表わされるCeqが、0.32≦Ceq≦0.60を満たすことを特徴とする電縫溶接鋼管。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15…(式1)
ここで、(式1)におけるC、Mn、Cr、Mo、V、Cu、Niは、各元素の含有量を質量%で表した値であり、これらの元素を含まない場合は、その元素は0として計算する。
[2]成分組成が、さらに、質量%で、
V :0.08%以下、
Cu:0.5%以下、
Ni:0.5%以下、
Cr:0.5%以下、
Mo:0.5%以下、
Ca:0.005%以下、
REM:0.005%以下
の1種または2種以上を含有することを特徴とする[1]に記載の電縫溶接鋼管。
[3]成分組成が、さらに、質量%で、
B :0.0004%以下
に制限されることを特徴とする[1]又は[2]に記載の電縫溶接鋼管。
[4]全厚試験片による管軸方向引張試験による降伏強度が758MPa以上、965MPa以下の強度を有することを特徴とする[1]又は[2]に記載の電縫溶接鋼管。
[5]全厚試験片による管軸方向引張試験による降伏比が85~95%であることを特徴とする[1]又は[2]に記載の電縫溶接鋼管。
[6]引張り試験の応力歪曲線において、降伏伸びが無いことを特徴とする[1]又は[2]に記載の電縫溶接鋼管。
[7]板厚が7~12.7mmであることを特徴とする[1]又は[2]に記載の電縫溶接鋼管。
Cは、強度の向上に有効である。鋼に添加するC量を増やすことによって、鋼の強度を高めることができるため、Cの含有量の下限を0.08%とする。一方、C量が0.18%を超えると、鋼の強度が高くなりすぎ、靭性を劣化させるので、上限を0.18%とする。また、P110相当の強度を確保する観点から、C量の下限を好ましくは0.1%以上にするとよい。強度を過剰に上昇させず、靱性を確保する観点からは、C量の上限を好ましくは0.17%、より好ましくは0.16%、確実に靭性を確保するためには0.15%以下にすることが好ましい。
Siは、脱酸剤として有効である。脱酸剤としての効果を得るためには、0.01%以上の添加が好ましい。また、Siは固溶強化によって強度を高める元素であるので、0.05%以上の添加がより好ましく、0.10%以上の添加がより好ましい。Siは、0.50%を超えて添加すると、低温靭性、さらには、電縫溶接性を損なうので、上限を0.50%とする。靱性を確保する観点からは、Si量を0.40%以下にすることが好ましく、0.30%以下がより好ましい。
Mnは、鋼の焼入れ性を高める元素である。本発明では、強度を確保するために、1.30%以上のMnを添加する。しかし、Mnを過度に添加すると、マルテンサイトの生成を助長し、靱性が劣化するので、上限を2.10%とする。強度を確保する観点からは、Mn量を1.40%以上にすることが好ましく、1.50%以上がより好ましい。靱性を確保する観点からは、Mn量を2.0%以下にすることが好ましく、1.90%以下がより好ましい。
Alは、脱酸剤として有効である。脱酸剤としての効果を得るためには、0.001%以上の添加が好ましい。脱酸の効果を高めるためには、0.005%以上のAlの添加が好ましく、0.01%以上の添加がより好ましい。Alは、0.10%を超えて添加すると、介在物が増加して、延性や靭性を損なうので、0.10%以下に制限する。靱性を確保する観点からは、Al量を0.06%以下にすることが好ましい。
Nbは、再結晶温度を低下させる元素であり、熱間圧延を行う際に、オーステナイトの再結晶を抑制して組織の微細化に寄与するので、0.005%以上を添加する。Nbを0.08%を超えて添加すると粗大な析出物によって靭性が劣化するので、その含有量は0.08%以下とする。靭性確保の観点から、上限は0.07%にすることが好ましく、より好ましい上限は、0.05%である。一方、下限は組織微細化効果を確実にするため、下限は、好ましくは0.008%、より好ましくは0.010%、さらに好ましくは0.015%とするとよい。
Tiは、微細な窒化物(TiN)を形成し、スラブ加熱時のオーステナイト粒の粗大化を抑制し組織の微細化に寄与する。その効果を得るため、0.005%以上のTiを添加する。Tiを0.030%を超えて過剰に添加するとTiNの粗大化や、TiCによる析出硬化が生じ、靭性が劣化するので、0.030%を上限とする。組織を微細化して靱性を確保する観点からは、Ti量を0.008%以上にすることが好ましく、0.010%以上がより好ましい。析出物に起因する靭性の低下を抑制する観点からは、Ti量は0.025%以下が好ましく、0.020%以下がより好ましい。
Nは不可避的に鋼中に存在するが、N量が多すぎると、TiNやAlNが過度に増大して表面疵、靱性劣化等の弊害が生じるおそれがある。そのため、上限を0.008%とする。さらに、介在物の生成を抑制する観点から、N量の上限は、好ましくは0.007%であり、より好ましい上限は0.006%である。下限は特に設定しないが、脱Nのコストや経済性を考慮し、0.002%とすることが好ましい。
Pは、不純物であり、含有量の上限を0.02%とする。P量の低減により、靭性が向上することから、P量は0.015%以下が好ましく、0.010%以下がより好ましい。P量は少ない方が好ましいので、下限は設けない。特性とコストのバランスから、通常は、0.001%以上が含有される。
Sは、不純物であり、含有量の上限を0.010%とする。S量の低減により、熱間圧延によって延伸化するMnSを低減し、靭性を向上させることができることから、S量は0.003%以下が好ましく、0.002%以下がより好ましい。S量は少ない方が好ましいので、下限は設けない。特性とコストのバランスから、通常は、0.0001%以上が含有される。
Vは、炭化物、窒化物を生成し、析出強化によって鋼の強度を向上させる元素であり、強度を効果的に上昇させるために、0.01%以上を添加することが好ましい。Vを過剰に添加すると、炭化物及び窒化物が粗大化し、靭性の劣化をもたらすため、V量の上限は0.08%とし、さらに好ましくは0.05%とする。
Cuは、鋼の焼入れ性を向上させる元素であり、固溶強化にも寄与するので、0.05%以上を添加しても良い。Cuを過度に添加すると鋼板の表面性状を損なうことがあるため、上限は0.50%以下とする。経済性の観点から、Cu量のより好ましい上限は0.30%以下である。Cuを添加する場合は、表面性状劣化防止の観点から、同時にNiを添加することが好ましい。
Niは、鋼の焼入れ性を向上させる元素であり、靭性の向上にも寄与する。強度を向上させるためには、Ni量を0.05%以上にすることが好ましい。また、Niは高価な元素であるため、上限は0.50%以下とし、0.30%以下とすることが好ましい。
Crは、強度の向上に有効な元素であり、0.05%以上を添加することが好ましい。Crを過度に添加すると、電縫溶接性が劣化することがあるので、0.5%を上限とし、好ましくは0.2%以下である。
Moは、鋼の高強度化に寄与する元素であり、0.05%以上を添加することが好ましい。ただし、Moは高価な元素であり、0.50%を上限とする。より好ましいMo量の上限は0.30%以下であり、さらに好ましくは0.10%以下とする。
Ca、REMは、硫化物系介在物の形態を制御し、低温靭性を向上させ、さらに、電縫溶接部の酸化物を微細化して電縫溶接部の靭性を向上させるので、一方、又は双方を0.001%以上添加することが好ましい。Ca、REMを過剰に添加すると、酸化物・硫化物が大きくなり靭性に悪影響を及ぼすので、添加量の上限は0.005%とする。ここでREMとは、Sc、Y、La、Ce、Pr、Nd、Pm、Sm、Eu、Gd、Tb、Dy、Ho、Er、Tm、Yb、Luの総称である。
炭素当量Ceqは、焼入れ性の指標であり、強度の指標としても使用されることがある。C、Mn、Cr、Mo、V、Ni、Cuの含有量[質量%]から、下記(式1)によって求める。強度を確保するためには、Ceqを0.32以上にすることが必要である。靱性を確保するためには、Ceqを0.60以下にすることが必要である。これらの効果を確実にするため、Ceqの下限は、0.35以上が好ましく、0.4以上がより好ましい。Ceqの上限は、0.50以下が好ましく、0.45以下がより好ましい、
まず、本発明の電縫溶接鋼管の素材である熱延鋼板の製造条件について説明する。
-15Cr-80Mo … (式2)
まず前段の冷却は、熱間圧延の仕上げ圧延終了後、Ar3温度から650℃までの平均冷却速度を10~25℃/sにて冷却を行うのが望ましい。熱間圧延後、温度が低下しすぎると、粗大なポリゴナルフェライトが生成し、強度が低下したり、靭性が劣化することがあるので、FT-50℃以上から水冷するのが望ましい。
-15Cr-80Mo ・・・ (式2)
Claims (7)
- 成分組成が質量%にて、
C :0.08~0.18%、
Si:0.01%~0.50%、
Mn:1.30~2.1%、
Al:0.001~0.10%、
Nb:0.005~0.08%、
Ti:0.005~0.03%、
をそれぞれ含有し、
N:0.008%以下、
P:0.020%以下、
S:0.010%以下、
に制限され、
残部がFe及び不可避的不純物である鋼であって、
肉厚中央部の組織が、面積率で40%~70%の円相当径1.0μm~10.0μmのフェライト相と、残部はベイナイト相を含有する低温変態生成相であり、
下記(式1)で表わされるCeqが、0.32≦Ceq≦0.60を満たすことを特徴とする電縫溶接鋼管。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15…(式1)
ここで、(式1)におけるC、Mn、Cr、Mo、V、Cu、Niは、各元素の含有量を質量%で表した値であり、これらの元素を含まない場合は、その元素は0として計算する。 - 前記成分組成が、さらに、質量%で、
V :0.08%以下、
Cu:0.5%以下、
Ni:0.5%以下、
Cr:0.5%以下、
Mo:0.5%以下、
Ca:0.005%以下、
REM:0.005%以下
の1種または2種以上を含有することを特徴とする請求項1に記載の電縫溶接鋼管。 - 前記成分組成が、さらに、質量%で、
B :0.0004%以下
に制限されることを特徴とする請求項1又は請求項2に記載の電縫溶接鋼管。 - 全厚試験片による管軸方向引張試験による降伏強度が758MPa以上、965MPa以下の強度を有することを特徴とする請求項1又は2に記載の電縫溶接鋼管。
- 全厚試験片による管軸方向引張試験による降伏比が85~95%であることを特徴とする請求項1又は2に記載の電縫溶接鋼管。
- 引張り試験の応力歪曲線において、降伏伸びが無いことを特徴とする請求項1又は2に記載の電縫溶接鋼管。
- 板厚が7~12.7mmであることを特徴とする請求項1又は2に記載の電縫溶接鋼管。
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US15/022,004 US10738366B2 (en) | 2013-12-20 | 2013-12-20 | Electric-resistance welded steel pipe |
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EP3085800A4 (en) | 2017-07-05 |
KR101795979B1 (ko) | 2017-11-08 |
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CN105612267A (zh) | 2016-05-25 |
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CA2923586A1 (en) | 2015-06-25 |
CA2923586C (en) | 2020-10-06 |
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US10738366B2 (en) | 2020-08-11 |
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