WO2013147197A1 - 耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管及びこれに用いる高強度ラインパイプ用鋼板、並びにこれらの製造方法 - Google Patents
耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管及びこれに用いる高強度ラインパイプ用鋼板、並びにこれらの製造方法 Download PDFInfo
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- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
Definitions
- HIC resistance hydrogen-induced cracking resistance
- the present invention relates to a steel pipe for a line pipe and a steel plate for a line pipe used therefor.
- sour environment An environment in which wet hydrogen sulfide (H 2 S) gas exists (hereinafter referred to as “sour environment”) exists in oil, natural gas drilling, production, and transportation, and steel pipes used therein are exposed to the sour environment. If transportation line pipes for petroleum, natural gas, etc. are exposed to a sour environment, there is a concern about the occurrence of hydrogen-induced cracking (hereinafter referred to as “HIC”). This is because in the sour environment, hydrogen easily enters the steel from the surface.
- HIC hydrogen-induced cracking
- HIC is hydrogen accumulated around defects in steel, such as stretched MnS present in the center segregation part of steel, accumulated Ti and Nb carbonitrides, and oxide inclusions in the oxide accumulation zone. caused by.
- Patent Documents 1 to 3 disclose a method for improving the HIC resistance by suppressing the segregation of Mn to the center of the steel sheet.
- Patent Document 1 proposes a steel sheet in which the ratio of the Mn content of the segregation part to the average Mn content in the steel is suppressed.
- Patent Documents 2 and 3 disclose a high-strength line pipe that limits the P concentration of the segregation part in addition to the size of the Mn segregation spot and further utilizes Ca.
- Patent Document 4 discloses a hot-rolled steel sheet having excellent HIC resistance, focusing on Nb segregation in addition to Mn segregation.
- Patent Documents 5 and 6 disclose methods for improving HIC resistance by suppressing inclusions such as carbides and nitrides of Ti and Nb.
- Patent Documents 7 and 8 disclose steel pipes that prevent the occurrence of HIC by suppressing segregation of Mn, Nb, and Ti, and further by setting the maximum hardness of the central segregation part to 300 Hv or less.
- the present invention has been made in view of the above circumstances, and is most suitable for steel pipes used for line pipes traversing the deep sea, and has an extremely high t / D and excellent HIC resistance as a whole steel pipe.
- An object of the present invention is to provide a steel pipe for a line pipe that prevents HIC on the surface layer of the steel sheet and a line pipe steel sheet used therefor.
- the present inventors have a steel pipe for a high-strength line pipe that has excellent HIC resistance near the inner and outer surfaces of the steel pipe even when t / D is high and has excellent resistance to hydrogen-induced cracking that can prevent HIC on the surface layer, and The earnest research was done about the conditions for obtaining the steel plate used for this.
- the hardness of the central segregation part is reduced as in conventional steel pipes for line pipes.
- the surface layer region has a high cooling rate and tends to become hard.
- the present inventors can reduce the hardness of the surface layer region of the steel sheet, which was conventionally about 350 Hv, to 300 Hv or less, resulting in a high t / It was found that even with the steel pipe of D, HIC generation from inclusions in the vicinity of the inner and outer surfaces can be suppressed, and a steel pipe having excellent HIC resistance near the inner and outer surfaces of the steel pipe can be obtained.
- the present invention has been made based on the above findings, and the gist thereof is as follows.
- the steel plate used as a base material is mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 1.2 to 1.8%, Nb: 0.00. 001 to 0.10%, Ca: 0.0005 to 0.0050%, N: 0.0010 to 0.0060%, O: 0.0001 to 0.0035%, and P: 0.01 %, S: 0.0020% or less, Al: 0.030% or less, Ti: 0.030% or less, S and Ca contents satisfy S / Ca ⁇ 0.5, the balance Fe and unavoidable impurities, maximum Mn segregation degree: 2.0 or less, Nb segregation degree: 4.0 or less, Ti segregation degree: 4.0 or less, length of unbonded part of center segregation part: 0.0.
- the total fraction of polygonal ferrite and processed ferrite having an aspect ratio of 3 or more in the surface layer region from the outermost surfaces of both the front and back plates to a depth of 5 mm is 0.1 to 20%
- the thickness t [ mm] and the outer diameter D [mm] of the steel pipe after pipe forming satisfy t ⁇ 25 and t / D ⁇ 0.035.
- the steel sheet is in% by mass, further Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.00. 01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050% , B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Y: 0.0001 to 0.005%, Hf: 0.0001
- the high-strength line pipe excellent in resistance to hydrogen-induced cracking as described in (1) above, containing one or more of 0.005% to 0.005% and Re: 0.0001 to 0.005% Steel pipe.
- a steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance used in the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance described in (1) or (2) above.
- a steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance used for the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance in (3) above.
- the start temperature of the first reheat treatment is 300 ° C. or more
- the end temperature of all the reheat treatments is less than 750 ° C.
- the molten steel is in% by mass, further Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.00. 01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050% , B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Y: 0.0001 to 0.005%, Hf: 0.0001
- the steel pipe for high-strength linepipe and steel plate for high-strength linepipe of the present invention has less segregation of Mn, Nb, Ti, the length of the non-crimped part of the center segregation part and the maximum hardness, and the surface layer region The hardness of is also suppressed. As a result, the HIC resistance is surely and sufficiently excellent, and it is extremely excellent as a material for a line pipe used in a sour environment.
- the steel pipe for line pipe of the present invention is such that the thickness t [mm] of the steel sheet and the outer diameter D [mm] of the steel pipe after pipe formation satisfy t ⁇ 25 and t / D ⁇ 0.035.
- Nb Ti segregation degree, length and maximum hardness of the non-crimped portion of the center segregation portion, maximum hardness and structure of the surface layer region from the outermost surface of both the front and back plate surfaces to a depth of 5 mm are appropriately defined. .
- C 0.02 to 0.08%
- C is an element that improves the strength of the steel and needs to be added in an amount of 0.02% or more. If the amount of C exceeds 0.08%, the formation of carbides is promoted and the HIC resistance is impaired. In order to secure more excellent HIC resistance and suppress deterioration of weldability and toughness, the C content is preferably 0.06% or less.
- Si 0.01-0.5%
- Si is a deoxidizing element, and addition of 0.01% or more is necessary.
- the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) decreases.
- Mn 0.8 to 1.8%
- Mn is an element that improves strength and toughness, and needs to be added in an amount of 0.8% or more. When the amount of Mn exceeds 1.8%, the HIC resistance decreases. In order to further suppress HIC, the Mn content is preferably 1.6% or less.
- Nb is an element that forms carbides and nitrides, promotes the refinement of the steel sheet as it is rolled, and contributes to the improvement of strength. In order to obtain the effect, it is necessary to add 0.0001% or more of Nb. If Nb is added excessively, the maximum degree of segregation of Nb is increased, the accumulation of Nb carbonitrides is caused, and the HIC resistance is lowered. Therefore, the upper limit of the Nb amount is 0.10%. When HIC resistance is more important, the Nb content is preferably 0.05% or less.
- Ca 0.0005 to 0.0050%
- Ca is an element that produces sulfide CaS, suppresses the production of MnS extending in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the addition amount of Ca is less than 0.0005%, the effect cannot be obtained. When the addition amount of Ca exceeds 0.0050%, oxides accumulate and the HIC resistance is impaired.
- N is an element that forms nitrides such as TiN and NbN.
- nitrides such as TiN and NbN.
- the N content exceeds 0.0060%, Ti and Nb carbonitrides are easily accumulated, and the HIC resistance is impaired.
- the N content is preferably 0.0035% or less in order to suppress TiN coarsening.
- O is an impurity and is limited to 0.0035% or less in order to suppress the accumulation of oxides and improve the HIC resistance.
- the O content is preferably 0.0035% or less, and more preferably 0.0020% or less. The smaller the amount of O, the better.
- the refining time becomes long and the cost becomes high, so the lower limit is made 0.0001%.
- P 0.01% or less
- P is an impurity. If the content exceeds 0.01%, the HIC resistance is impaired, and the toughness of the HAZ decreases. Therefore, the P content is limited to 0.01% or less.
- S is an element that reduces the HIC resistance by generating MnS that extends in the rolling direction during hot rolling.
- the S content is preferably 0.0010% or less. The smaller the amount of S, the better.
- the lower limit is preferably made 0.0001% or more from the viewpoint of manufacturing cost.
- Al 0.030% or less
- Al is a deoxidizing element, but when the addition amount exceeds 0.030%, an integrated cluster of Al oxides is generated.
- the Al content is preferably 0.017% or less.
- the lower limit of the amount of Al is not particularly limited, it is preferable to add Al in an amount of 0.0005% or more in order to reduce the amount of oxygen in the molten steel.
- Ti 0.030% or less
- Ti is an element usually used as a deoxidizer or nitride-forming element for refining crystal grains, but is an element that reduces HIC resistance and toughness by forming carbonitride. Therefore, the Ti content is limited to 0.030% or less.
- FIG. 1 shows the relationship between CLR (HIC length ratio) and S / Ca in the HIC test of 0.04% C-1.25% Mn steel.
- CLR HIC length ratio
- S / Ca ratio when the S / Ca ratio is 0.5 or more, HIC occurs. This is because when the S / Ca ratio is 0.5 or more, MnS is generated and MnS stretched during rolling is formed, and as a result, the HIC resistance deteriorates. Therefore, the S / Ca ratio needs to be less than 0.5.
- the steel pipe for line pipes and the steel sheet for line pipes of the present invention include Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B as elements that improve strength and toughness as necessary.
- One or two or more elements selected from may be added. The reason for limiting the addition amount of these optional additive elements is as follows.
- Ni 0.01-2.0%
- Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.01% or more is necessary. When the added amount of Ni exceeds 2.0%, the HIC resistance and weldability deteriorate.
- Cu 0.01 to 1.0%
- Cu is an element effective for increasing strength without reducing toughness, and in order to obtain the effect, addition of 0.01% or more is necessary. If the added amount of Cu exceeds 1.0%, cracks are likely to occur during heating of the steel slab or during welding.
- Cr 0.01 to 1.0%
- Cr is an element that improves the strength of steel by precipitation strengthening, and in order to obtain the effect, addition of 0.01% or more is necessary.
- the added amount of Cr exceeds 1.0%, the hardenability is increased and a bainite structure is formed. As a result, the HIC resistance and the toughness are lowered.
- Mo 0.01 to 0.60%
- Mo is an element that improves hardenability and at the same time forms carbonitrides and improves strength. In order to obtain the effect, addition of 0.01% or more is necessary. If the amount of Mo exceeds 0.60%, the cost increases. If the strength of the steel is excessively increased, the HIC resistance and toughness may be lowered. Therefore, the preferable addition amount of Mo is 0.20% or less.
- W 0.01 to 1.0%
- W is an element effective for improving the strength, and in order to obtain the effect, addition of 0.01% or more is necessary. If the addition amount of W exceeds 1.0%, the toughness may be lowered.
- Zr 0.0001 to 0.050%
- Zr is an element that contributes to the improvement of strength by forming carbides and nitrides like V, and in order to obtain the effect, it is necessary to add 0.0001% or more. If the amount of Zr added exceeds 0.050%, the toughness may decrease.
- Ta 0.0001 to 0.050%
- Ta is an element that forms carbides and nitrides and contributes to the improvement of strength. In order to obtain the effect, it is necessary to add 0.0001% or more. If the amount of Ta exceeds 0.050%, the toughness may decrease.
- B is an element that segregates at the grain boundaries of steel and contributes significantly to improving the hardenability. In order to obtain this effect, 0.0001% or more of B must be added. B is an element that generates BN, lowers the solid solution N, and contributes to the improvement of the toughness of the weld heat affected zone, so addition of 0.0005% or more is preferable. When the addition amount of B exceeds 0.0020%, segregation to the grain boundary becomes excessive, and the toughness may be lowered.
- the steel pipe for line pipes and the steel sheet for line pipes of the present invention include REM (rare earth element), Mg, Y, Hf, and One or more selected from Re may be added.
- REM rare earth element
- Mg Mg
- Y Y
- Hf Hf
- One or more selected from Re may be added. The reason for limiting the addition amount of these optional additive elements is as follows.
- REM rare earth element
- REM is an element added as a deoxidizing agent and a desulfurizing agent, and 0.0001% or more must be added to obtain the effect.
- the amount of REM added exceeds 0.010%, coarse oxides are generated, and the HIC resistance, the toughness of the base material and HAZ may be lowered.
- Mg is an element added as a deoxidizing agent and a desulfurizing agent.
- a fine oxide is generated and contributes to improvement of HAZ toughness.
- Y like Ca, is an element that generates sulfides, suppresses the generation of MnS elongated in the rolling direction, and contributes to improvement in HIC resistance. In order to obtain such an effect, it is necessary to add Y in an amount of 0.0001% or more. When the added amount of Y exceeds 0.005%, the oxides increase, aggregate and coarsen, and the HIC resistance is impaired.
- Hf 0.0001 to 0.005%
- Hf is an element that generates sulfides and suppresses the generation of MnS elongated in the rolling direction and contributes to improvement in HIC resistance. In order to obtain such an effect, it is preferable to add 0.0001% or more of Hf. On the other hand, if the amount of Hf exceeds 0.005%, the oxides increase, aggregate and coarsen, and the HIC resistance is impaired.
- Re like Ca, is an element that generates sulfides and suppresses the generation of MnS elongated in the rolling direction and contributes to improvement in HIC resistance. In order to obtain such an effect, it is necessary to add 0.0001% or more of Re. When the added amount of Re exceeds 0.005%, the oxide increases, aggregates and coarsens, and the HIC resistance is impaired.
- each element is Fe and inevitable impurities. It should be noted that the above-described Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B are all allowed to contain a trace amount less than the lower limit as an impurity. Also, REM, Mg, Y, Hf, and Re are allowed to contain trace amounts less than the respective lower limit values as impurities.
- the HIC is caused by hydrogen accumulated around the stretched MnS present in the center segregation portion of the steel, accumulated Ti, Nb carbonitride and the like.
- the maximum Mn segregation degree of the steel plate and the steel pipe needs to be 2.0 or less. Furthermore, by suppressing the accumulated Ti and Nb carbonitrides, the occurrence of HIC in the steel pipe for line pipe and the steel sheet for line pipe can be remarkably prevented.
- the N content is 0.0050% or less
- the C content is 0.06% or less
- the maximum segregation degree of Nb and Ti is 4.0 or less, respectively. do it.
- the maximum Mn segregation degree is the maximum Mn amount of the central segregation portion with respect to the average Mn amount in the Mn concentration distribution in the plate thickness direction of the steel sheet and the Mn concentration distribution in the thickness direction of the tube wall of the steel pipe.
- the Nb segregation degree and the Ti segregation degree are the average Nb amount (Ti amount) in the concentration distribution of Nb and Ti in the plate thickness direction of the steel sheet and the concentration distribution of Nb and Ti in the thickness direction of the tube wall of the steel pipe. It is the maximum Nb amount (Ti amount) averaged at the center segregation part.
- the maximum Mn segregation degree is obtained by measuring the Mn concentration distribution of steel plates and steel pipes by EPMA (Electron Probe Micro Analyzer) or CMA (Computer Aided Micro Analyzer) capable of image processing the measurement results by EPMA.
- the measurement object is an HIC test piece (20 mm width ⁇ 20 mm thickness ⁇ 100 mm length), and the measurement area is 20 mm width (test piece width) ⁇ 20 mm thickness (HIC test piece thickness) of the HIC test piece.
- the Nb segregation degree and the Ti segregation degree the same region may be measured by EPMA or CMA, and the Nb concentration distribution and the Ti concentration distribution may be measured, respectively.
- the probe diameter of EPMA (or CMA) is 2 ⁇ m.
- the concentration of Mn in the measurement area of 20 mm width (HIC test piece width) ⁇ 20 mm thickness (HIC test piece thickness) is measured in the plate thickness direction and plate width direction using EPMA with a beam diameter of 50 ⁇ m.
- the Mn concentration distribution is measured at regular intervals, and the average value in the measured Mn concentration distribution is defined as the average Mn concentration.
- the beam diameter was changed to 2 ⁇ m, and 50 points ⁇ 50 at equal intervals in the plate thickness direction and the plate width direction.
- the Mn concentration at the point is measured, and the maximum Mn concentration is obtained from the distribution.
- the ratio of the maximum Mn concentration obtained with a beam diameter of 2 ⁇ m and the average Mn concentration obtained with a beam diameter of 50 ⁇ m is defined as the maximum Mn segregation degree.
- the Nb segregation degree and the Ti segregation degree use EPMA, and the concentration of Nb and Ti in the thickness direction in a measurement region of 20 mm width (HIC test piece width) ⁇ 20 mm thickness (HIC test piece thickness) with a beam diameter of 50 ⁇ m. After the distribution is measured, the place where the amount of Nb and Ti is most concentrated is obtained by measuring the concentration of Nb and Ti in a region of 1 mm (width) ⁇ 1 mm (thickness) with a beam diameter of 2 ⁇ m.
- each segregation degree is obtained by excluding the measured value in that region.
- the uncrimped portion is a portion in which a void generated in a steel piece during solidification is not crimped by hot rolling and remains on the steel plate.
- the length of the non-bonded portion can be measured by nondestructive inspection such as ultrasonic waves.
- the reason why the uncompressed portion remains in the center segregation portion is mainly hydrogen contained in the steel slab before hot rolling.
- the steel is continuously cast after being melted by a converter and secondary refining, the steel is solidified, cooled, and contracted, so that a void is likely to be generated particularly at the center of the steel slab.
- this void has a negative pressure, if the amount of hydrogen contained in the steel slab is large, hydrogen gas enters the void.
- the hydrogen contained in the steel when melted by secondary refining remains almost intact in the voids in the steel slab after continuous casting.
- the structure of the steel slab is austenite having a large amount of hydrogen that can be dissolved in face-centered cubic crystals, so that hydrogen is not diffused out of the steel slab. If the steel slab is heated and subjected to reduction by hot rolling, the internal voids in the steel slab become smaller, but the pressure of hydrogen gas contained in the voids increases in inverse proportion to the size of the voids. For this reason, the gap cannot be crimped even by hot rolling, and an uncompressed portion remains in the steel sheet, particularly in the central segregation portion.
- the amount of hydrogen in the steel is a value obtained by measuring molten steel collected after secondary refining by a combustion method.
- the hydrogen partial pressure in the atmosphere during secondary refining may be reduced.
- the hydrogen partial pressure can be reduced by blowing inert gas, nitrogen, or the like into the atmosphere.
- the amount of hydrogen remaining in the steel sheet after hot rolling is such that hydrogen is dissipated to the outside if the steel sheet is cooled and the metal structure is transformed from austenite to ferrite, bainite, martensite, pearlite, etc. Reduced compared to amount.
- Maximum hardness of center segregation part 300 Hv or less
- it is effective to set the maximum hardness of the center segregation part to 300 Hv or less.
- the maximum hardness of the center segregation part is measured by corroding with 3% nitric acid + 97% nital solution and then performing a Vickers hardness test with a load of 25 g in accordance with JIS Z 2244.
- the center segregation part is a part where the concentration of Mn measured by EPMA or CMA is maximized.
- the maximum hardness of the surface layer region is determined by performing a Vickers hardness test at a predetermined interval (for example, 0.1 mm interval) in the depth direction from the outermost surface to a depth of 5 mm. To do.
- the steel structure of the base material is a uniform and fine acicular ferrite or bainite structure. Therefore, the structure of the steel base material for the high-strength line pipe of the present invention considering HIC resistance is basically preferably bainite or acicular ferrite.
- steel pipes for thick line pipes are often required to have characteristics that can withstand drop weight tests such as DWTT.
- the structure of a conventional steel pipe for line pipes does not have the characteristics to withstand a drop weight test such as DWTT, but if 0.1% or more of polygonal ferrite and processed ferrite are generated, DWTT characteristics are improved.
- DWTT drop weight test
- FIG. 2 shows the relationship between the total area ratio of polygonal ferrite and processed ferrite on the surface layer of the steel pipe and the HIC area ratio.
- 3 o'clock, 6 o'clock, and 9 o'clock are the positions in the circumferential direction of the steel pipe from which the test piece was taken, the welded part was taken as 0 o'clock, and 3 o'clock (90 °) and 6 o'clock as viewed from the bottom of the steel pipe ( 180 °), a specimen was taken from 9 o'clock (270 °), and the tissue was observed.
- the HIC area ratio greatly exceeds 3%. Therefore, even when polygonal ferrite and processed ferrite are included in the surface region, in order to reliably improve the HIC resistance, the total fraction of polygonal ferrite and processed ferrite is suppressed to 20% or less in terms of area ratio. It is desirable. From the viewpoint of HIC resistance, the smaller the processed ferrite, the better.
- the fraction of processed ferrite is preferably 10% or less in terms of area ratio, more preferably not present.
- the fraction of polygonal ferrite and processed ferrite by taking five 200 ⁇ optical micrographs, extracting polygonal ferrite and processed ferrite, and using the values obtained by image analysis.
- the white area is polygonal ferrite or processed ferrite
- the aspect ratio (the ratio of the horizontal length to the vertical length) is less than 3
- the polygonal ferrite is the one with an aspect ratio of 3 or more Defined as processed ferrite.
- the rolling end temperature and / or the water cooling start temperature is set to 750 ° C. or higher, the fraction of polygonal ferrite and processed ferrite in the surface layer region can be set to 20% or lower.
- the rolling end temperature and / or the water cooling start temperature is lower than 750 ° C., the polygonal ferrite and the processed ferrite in the surface layer region tend to increase by over 20%. It is preferable to set it as 750 degreeC or more.
- the rolling end temperature or the water cooling start temperature it is more preferable to set the rolling end temperature or the water cooling start temperature to 770 ° C. or higher.
- the above-mentioned fraction means the area ratio when observed in the L cross section (surface in the plate thickness direction and rolling direction).
- the structure other than the polygonal ferrite and the processed ferrite that is, the structure occupying an area of 80% or more of the surface layer region may be bainite and / or acicular ferrite pearlite.
- the structure on the inner side from the surface layer region is not particularly limited, but as a steel plate and steel pipe for high-strength line pipe with a tensile strength of 500 MPa or more, a high strength of the base material, base material toughness, HAZ toughness, weldability, etc. are secured.
- a structure mainly composed of acicular ferrite or bainite may be used.
- the steel having the above-described composition is melted by a conventional method so that the hydrogen content in the molten steel after the secondary refining is 2.5 ppm or less, and then steel slab is obtained by continuous casting. Heat to thick plate rolling to obtain a steel plate.
- continuous casting as described above, it is preferable to apply light reduction while controlling the amount of reduction in accordance with the distribution in the width direction of the central solid phase ratio at the final solidification position of the slab.
- the austenite particle size can be 20 ⁇ m or less.
- the steel sheet after rolling is subjected to water cooling at a start temperature of 750 ° C. or higher and a stop temperature of 400 to 600 ° C.
- the stop temperature of water cooling here refers to the maximum temperature of the steel sheet that has been raised by recuperation after stopping the cooling water.
- the recrystallization temperature range is a temperature range where recrystallization occurs after rolling, and is generally over 900 ° C. for the steel components of the present invention.
- the non-recrystallization temperature range is a temperature range in which recrystallization and ferrite transformation do not occur after rolling, and is generally 750 to 900 ° C. for the components of the steel of the present invention.
- Rolling in the recrystallization temperature range is called recrystallization rolling or rough rolling, and rolling in the non-recrystallization temperature range is called non-recrystallization rolling or finish rolling.
- the maximum hardness of the center segregation part can be suppressed to 300 Hv or less by starting water cooling from a temperature of 750 ° C. or higher and setting the water cooling stop temperature to 400 ° C. or higher.
- the water cooling start temperature is less than 750 ° C.
- C carbon
- C is discharged from the ferrite to austenite
- C is concentrated in the austenite phase.
- the austenite phase enriched with C is transformed into hard martensite containing a large amount of C during the cooling process.
- the water cooling start temperature is set to 750 ° C. or higher, the formation of hard martensite can be suppressed, so that the maximum hardness of the central segregation portion can be suppressed to 300 Hv or lower.
- the water cooling stop temperature is set to 400 ° C. or higher, the hard martensite after transformation is partially decomposed, so that the maximum hardness of the center segregation portion can be suppressed to 300 Hv or less. If the water cooling stop temperature is too high, the strength of the steel pipe is lowered, so the water cooling stop temperature is set to 600 ° C. or less.
- the maximum hardness from the outermost layer to 5 mm can be reduced to 300 Hv or less by performing recuperation at least twice. This is because the tempering effect is exhibited by recuperating and the hardness of the surface layer region can be lowered.
- the lower limit of the recuperation temperature is preferably 300 ° C.
- the upper limit temperature is preferably 750 ° C.
- recuperation temperature When the recuperation temperature is less than 300 ° C., 50% or more of martensite is generated and cured, and the hardness of the surface layer does not decrease. When the recuperation temperature exceeds 750 ° C., the hardness of the surface layer region is too low.
- 3A and 3B show examples of the cooling pattern of the cooling process in the present invention.
- 1 is a temperature change due to reheat treatment
- 2 is a recuperation start temperature
- 3 is a recuperation end temperature.
- the temperature of 4 in the graph is the water cooling stop temperature.
- Such a cooling pattern can be controlled by switching on and off a nozzle that discharges cooling water, and adjusting the amount of water.
- FIG. 3C is a cooling pattern by a conventional manufacturing method. Since the temperature of the steel plate rises after the cooling water is stopped, one recuperation is included.
- the total fraction of polygonal ferrite and processed ferrite can be suppressed to 20% or less in the structure of the surface layer region from the outermost surface to 5 mm.
- the water cooling start temperature is lower than 750 ° C., it falls below the ⁇ / ⁇ transformation temperature of steel of 500 MPa or more, so polygonal ferrite and processed ferrite are easily generated, and the total fraction of polygonal ferrite and processed ferrite is 20%. Over.
- the butt portion is arc-welded to form a welded steel pipe.
- the forming step is preferably a UOE step of C-pressing, U-pressing or O-pressing the steel plate.
- arc welding it is preferable to employ submerged arc welding from the viewpoint of the toughness and productivity of the weld metal.
- the heat input during arc welding is not particularly limited, but usually it is preferably 2.0 to 15.0 kJ / mm.
- Tables 1A to 1C Steels 1 to 35 having chemical components shown in Tables 1A to 1C were melted, and steel pieces having a thickness of 240 mm or 300 mm were obtained by continuous casting. Tables 1A to 1C also show analytical values of the hydrogen content of the molten steel.
- the first recuperation start and end temperatures and the second recuperation start temperature are shown in Table 2A.
- the second recuperation end temperature (or the first recuperation end temperature when the number of recuperations is one) is the cooling end temperature.
- the obtained steel sheet was formed into a tubular shape by a C press, U press, and O press, end surfaces were tack welded, main welding was performed from the inner and outer surfaces, and after pipe expansion, a steel pipe for a line pipe was obtained. Note that submerged arc welding was applied to the main welding.
- Tensile test pieces, HIC test pieces, and macro test pieces were sampled from the obtained steel plates and steel pipes and subjected to respective tests.
- the HIC test was performed according to NACETM0284.
- the segregation degree of Mn, Nb, and Ti was measured by EPMA using a macro test piece.
- the degree of segregation by EPMA was measured with probe diameters of 50 ⁇ m and 2 ⁇ m.
- the Vickers hardness of the central segregation part and the Vickers hardness in the surface layer region from the outermost surface of the steel plate and the steel pipe to the depth of 5 mm were measured according to JIS Z 2244. Vickers hardness was measured at a site where the load was 25 g and the Mn concentration was highest in the distribution of Mn concentration in the thickness direction measured by EPMA.
- Tables 2A to 2C show the steel sheet thickness, maximum Mn segregation degree, Nb segregation degree, Ti segregation degree, unbonded part length, center segregation part maximum hardness, surface area maximum hardness, tensile strength and HIC.
- the length ratio (CLR) of HIC obtained by the test and the total fraction of polygonal ferrite and processed ferrite in the surface layer region are shown.
- Table 3 shows the thickness of the steel pipe, the heat input of the main welding, and the length ratio (CLR) of the HIC determined by the HIC test.
- the maximum Mn segregation degree, Nb segregation degree, Ti segregation degree, the length of the non-crimped part, and the maximum hardness of the central segregation part in the steel pipe are all equivalent to the steel sheet, and the tensile strength of the steel pipe is Is about 10 to 20 MPa.
- Steel plates 1 to 23 are examples of the present invention. As shown in Tables 2A to 2C, the maximum Mn segregation degree of these steel sheets was 1.6 or less, the Nb segregation degree was 4.0 or less, and the Ti segregation degree was 4.0 or less. The maximum hardness from the outermost surface of the upper and lower surfaces of the steel plate and the inner and outer surfaces of the steel pipe to 5 mm thickness was 300 Hv or less, and the maximum hardness of the central segregation part was 300 Hv or less. Furthermore, the total of polygonal ferrite and processed ferrite The fraction was also 20% or less. Therefore, no HIC is generated by the HIC test. Similar results were obtained for steel pipes made of these steel plates 1 to 23 as shown in Table 3.
- Steel plates 24 to 43 are comparative examples that are outside the scope of the present invention.
- the steel plates 24 to 35 are out of the scope of the present invention in which any of the basic components or any of the additional elements is included.
- the steel plates 36 to 43 do not satisfy the production conditions of the present invention.
- HIC was generated in the HIC test, CLR exceeded 3%, or ductile fracture of DWTT at 0 ° C.
- the area ratio was less than 85%.
- FIG. 4A shows a hardness distribution from the outermost surface layer of steel 11 manufactured by the manufacturing method of the present invention to a thickness of 5 mm
- FIG. 4B shows a structural photograph of the surface layer of steel 11.
- FIG. 5A shows a hardness distribution from the outermost layer of steel 40 manufactured by a conventional method to a thickness of 5 mm
- FIG. 5B shows a structural photograph of the surface layer of steel 40.
- the hardness distribution of the steel sheet of the present invention shown in FIG. 4A is as low as 245 Hv in the maximum hardness, but the hardness distribution of the steel sheet manufactured by the conventional method shown in FIG. 5A has a portion where the hardness locally exceeds 300 Hv. Can be the starting point of HIC. Since the steel plate manufactured by the conventional method shown in FIG. 5B has one recuperation, the base material is not tempered sufficiently, and a hard structure is generated.
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Abstract
Description
Cは、鋼の強度を向上させる元素であり、0.02%以上の添加が必要である。C量が0.08%を超えると、炭化物の生成が促進されて耐HIC性を損なう。より優れた耐HIC性を確保し、溶接性、靱性の低下を抑制するためには、C量を0.06%以下とすることが好ましい。
Siは脱酸元素であり、0.01%以上の添加が必要である。Si量が0.5%を超えると、溶接熱影響部(HAZ)の靱性が低下する。
Mnは、強度及び靱性を向上させる元素であり、0.8%以上の添加が必要である。Mn量が、1.8%を超えると、耐HIC性が低下する。HICをさらに抑制するには、Mn量を1.6%以下とすることが好ましい。
Nbは、炭化物、窒化物を形成し、圧延まま鋼板の細粒化を促進し、強度の向上に寄与する元素である。その効果を得るためには、0.0001%以上のNbを添加することが必要である。Nbを過剰に添加すると、最大Nb偏析度が増加し、Nbの炭窒化物の集積を招いて、耐HIC性が低下するので、Nb量の上限は0.10%とする。耐HIC性をより重視する場合、Nb量は0.05%以下にすることが好ましい。
Caは、硫化物CaSを生成し、圧延方向に伸長するMnSの生成を抑制し、耐HIC性の改善に顕著に寄与する元素である。Caの添加量が0.0005%未満では、効果が得られない。Caの添加量が0.0050%を超えると、酸化物が集積し、耐HIC性を損なう。
Nは、TiN、NbNなどの窒化物を形成する元素である。窒化物を利用して加熱時のオーステナイト粒径を微細にするためには、N量の添加量を0.0010%以上とすることが必要である。Nの含有量が0.0060%を超えると、TiとNbの炭窒化物が集積しやすくなり、耐HIC性を損なう。靭性が要求される場合には、TiNの粗大化を抑制するため、N量を0.0035%以下にすることが好ましい。
Oは不純物であり、酸化物の集積を抑制して、耐HIC性を向上させるために、0.0035%以下に制限する。酸化物の生成を抑制して、母材及びHAZ靭性を向上させるためには、O量を0.0035%以下とすることが好ましく、0.0020%以下がより好ましい。O量は少ないほど好ましいが、0.0001%未満とするためには、精錬時間が長時間になってコストが高くなるので、下限を0.0001%とする。
Pは不純物であり、含有量が0.01%を超えると、耐HIC性を損ない、また、HAZの靱性が低下する。したがって、Pの含有量を0.01%以下に制限する。
Sは、熱間圧延時に圧延方向に延伸するMnSを生成して、耐HIC性を低下させる元素である。本発明では、S量を0.0020%以下に低減することが必要である。耐HIC特性を向上させるためには、S量を0.0010%以下とすることが好ましい。S量は、少ないほど好ましいが、0.0001%未満にすることは困難であり、製造コストの観点から、下限を0.0001%以上にすることが好ましい。
Alは脱酸元素であるが、添加量が0.030%を超えるとAl酸化物の集積クラスターが生成される。特に良好な靭性が要求される場合には、Al量を0.017%以下にすることが好ましい。Al量の下限値は特に限定しないが、溶鋼中の酸素量を低減させるためには、Alを0.0005%以上添加することが好ましい。
Tiは、通常、脱酸剤や窒化物形成元素として結晶粒の細粒化に利用される元素であるが、炭窒化物の形成によって耐HIC性や靱性を低下させる元素である。したがって、Tiの含有量は、0.030%以下に制限する。
本発明では、Caを添加して、CaSを形成させることにより、Sの固定を図りMnSの生成を抑制する。優れた耐HIC性を得るためには、さらに、S/Caの比を適切に規制することが必要である。図1に0.04%C-1.25%Mn鋼のHIC試験におけるCLR(HICの長さ率)とS/Caの関係を示す。図1に示すように、S/Caの比が0.5以上になると、HICが発生する。これは、S/Caの比が0.5以上になると、MnSが生成されて、圧延時に延伸化したMnSが形成され、その結果、耐HIC性が劣化するためである。したがって、S/Caの比は0.5未満とする必要がある。
Niは、靱性及び強度の改善に有効な元素であり、その効果を得るためには0.01%以上の添加が必要である。Niの添加量が2.0%を超えると、耐HIC性及び溶接性が低下する。
Cuは、靱性を低下させずに強度の上昇に有効な元素であり、その効果を得るためには0.01%以上の添加が必要である。Cuの添加量が1.0%を超えると、鋼片加熱時や溶接時に割れが生じやすくなる。
Crは、析出強化による鋼の強度を向上させる元素であり、その効果を得るためには0.01%以上の添加が必要である。Crの添加量が1.0%を超えると、焼入れ性が上昇して、ベイナイト組織が生じ、その結果、耐HIC性、靱性が低下する。
Moは、焼入れ性を向上させると同時に、炭窒化物を形成し強度を改善する元素であり、その効果を得るためには、0.01%以上の添加が必要である。Moの添加量が0.60%を超えると、コストが上昇する。鋼の強度が過度に上昇すると、耐HIC性及び靱性が低下することがあるので、好ましいMoの添加量は0.20%以下である。
Wは、強度の向上に有効な元素であり、その効果を得るためには0.01%以上の添加が必要である。Wの添加量が1.0%を超えると、靱性が低下することがある。
Zrは、Vと同様に炭化物、窒化物を形成して強度の向上に寄与する元素であり、その効果を得るためには、0.0001%以上を添加することが必要である。Zrの添加量が0.050%を超えると、靱性が低下することがある。
Taも、Vと同様に炭化物、窒化物を形成し強度の向上に寄与する元素であり、その効果を得るためには、0.0001%以上を添加する必要がある。Taの添加量が0.050%を超えると、靱性が低下することがある。
Bは、鋼の粒界に偏析して焼入れ性の向上に著しく寄与する元素である。この効果を得るには、0.0001%以上のBの添加が必要である。Bは、BNを生成し、固溶Nを低下させて、溶接熱影響部の靱性の向上にも寄与する元素であるため、0.0005%以上の添加が好ましい。Bの添加量が0.0020%を超えると、粒界への偏析が過剰になり、靱性が低下することがある。
REMは、脱酸剤及び脱硫剤として添加される元素であり、その効果を得るためには0.0001%以上の添加が必要である。REMの添加量が0.010%を超えると、粗大な酸化物を生じて、耐HIC性や、母材及びHAZの靱性が低下することがある。
Mgは、脱酸剤及び脱硫剤として添加される元素であり、特に、微細な酸化物を生じて、HAZ靭性の向上にも寄与する。この効果を得るには、0.0001%以上のMgを添加することが必要である。Mgの添加量が0.010%を超えると、酸化物が凝集、粗大化しやすくなり、耐HIC性や、母材及びHAZの靱性が低下することがある。
Yは、Caと同様、硫化物を生成し、圧延方向に伸長したMnSの生成を抑制し、耐HIC性の向上に寄与する元素である。このような効果を得るには、Yを0.0001%以上添加することが必要である。Yの添加量が0.005%を超えると、酸化物が増加して、凝集、粗大化し、耐HIC性を損なう。
Hfも、Caと同様、硫化物を生成し、圧延方向に伸長したMnSの生成を抑制し、耐HIC性の向上に寄与する元素である。このような効果を得るには、Hfを0.0001%以上添加することが好ましい。一方、Hfの量が0.005%を超えると、酸化物が増加して、凝集、粗大化し、耐HIC性を損なう。
Reも、Caと同様、硫化物を生成し、圧延方向に伸長したMnSの生成を抑制し、耐HIC性の向上に寄与する元素である。このような効果を得るには、Reを0.0001%以上添加する必要がある。Reの添加量が0.005%を超えると、酸化物が増加して、凝集、粗大化し、耐HIC性を損なう。
HICは、鋼の中心偏析部に存在する延伸化したMnSや、集積したTiやNbの炭窒化物などの周囲に集積した水素に起因する。
鋼板の中心偏析部に、長さ0.1mm以上の未圧着部があると、HICの起点となり、耐HIC性が劣化する。未圧着部とは、凝固時に鋼片に生じた空隙が、熱間圧延によって圧着されず、鋼板に残存したものである。未圧着部の長さは、超音波等の非破壊検査で測定できる。
耐HIC性改善には、中心偏析部の最高硬さを300Hv以下とすることが有効である。中心偏析部最高硬さの上限を300Hv以下とすることによって、確実にHICの発生を防止することができる。中心偏析部の最高硬さは、3%硝酸+97%ナイタール溶液で腐食した後、JIS Z 2244に準拠し、25gの荷重でビッカース硬さ試験を行って、測定する。中心偏析部とは、EPMAやCMAによって測定したMnの濃度が最大になる部位である。
鋼板における表裏の板面(圧延面)の最表面から板の厚み方向(深さ方向)へ5mmまでの領域(表層領域)、及び鋼管における内外の最表面から管壁の厚み方向(深さ方向)へ5mmまでの領域(表層領域)の最高硬さを、いずれも300Hv以下とすると、耐HIC性を確実に高めることができる。すなわち、最表面から5mmまでの表層領域の最高硬さの上限を300Hv以下とすることによって、t/Dが0.035以上と高いラインパイプ用鋼管においても、確実に介在物やブリスターなどの表層に起因するHICの発生を防止することができる。表層領域の最高硬さは、最表面から5mmの深さの位置まで、深さ方向に所定間隔(例えば0.1mm間隔)ごとにビッカース硬さ試験を行い、そのうちの最も高い値を最高硬さとする。具体的には、0.13%硝酸+97%ナイタール溶液で腐食した後、JIS Z 2244に準拠し、25gの荷重で、最表面から5mmの深さの位置まで、0.1mm間隔の50点×50点のビッカース硬さ試験を行って、調べる。
耐HIC性向上のためには、基本的には、母材の鋼組織が均一かつ微細なアシキュラーフェライト又はベイナイト組織であることが望ましい。したがって、耐HIC性を考慮した本発明の高強度ラインパイプ用の鋼母材の組織は、基本的にはベイナイトあるいはアシキュラーフェライトが好ましい。しかしながら、厚肉のラインパイプ用鋼管は、DWTTのような落重試験に耐え得る特性が要求される場合が多い。
2 復熱開始温度
3 復熱終了温度
4 水冷停止温度
Claims (8)
- 母材となる鋼板が質量%で、
C :0.02~0.08%、
Si:0.01~0.5%、
Mn:1.2~1.8%、
Nb:0.001~0.10%、
Ca:0.0005~0.0050%、
N :0.0010~0.0060%、
O :0.0001~0.0035%
を含有し、かつ、
P :0.01%以下、
S :0.0020%以下、
Al:0.030%以下、
Ti:0.030%以下
に制限し、S、Caの含有量が、
S/Ca<0.5
を満足し、残部がFe及び不可避的不純物であり、
最大Mn偏析度:2.0以下、
Nb偏析度:4.0以下、
Ti偏析度:4.0以下、
中心偏析部の未圧着部の長さ:0.1mm以下、
中心偏析部の最高硬度:300Hv以下、
表裏両板面の最表面から5mmの深さまでの表層領域の最高硬度:300Hv以下、
表裏両板面の最表面から5mmの深さまでの表層領域におけるポリゴナルフェライトとアスペクト比が3以上の加工フェライトの合計分率:0.1~20%
であり、
鋼板の厚さt[mm]と造管後の鋼管の外径D[mm]が、
t≧25
t/D≧0.035
を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。 - 前記鋼板が、質量%で、さらに、
Ni:0.01~2.0%、
Cu:0.01~1.0%、
Cr:0.01~1.0%、
Mo:0.01~0.60%、
W :0.01~1.0%、
V :0.01~0.10%、
Zr:0.0001~0.050%、
Ta:0.0001~0.050%、
B :0.0001~0.0020%、
REM:0.0001~0.01%、
Mg:0.0001~0.01%、
Y :0.0001~0.005%、
Hf:0.0001~0.005%、及び
Re:0.0001~0.005%
のうち1種又は2種以上を含有することを特徴とする請求項1に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。 - 表裏両板面の最表面から5mmの深さまでの表層領域においてアスペクト比が3以上の加工フェライトが存在しないことを特徴とする請求項1又は2に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。
- 請求項1又は2に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。
- 請求項3に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。
- 質量%で、
C :0.02~0.08%、
Si:0.01~0.5%、
Mn:1.2~1.8%、
Nb:0.001~0.10%、
Ca:0.0005~0.0050%、
N :0.0010~0.0060%、
O :0.0001~0.0035%
を含有し、かつ、
P :0.01%以下、
S :0.0020%以下、
Al:0.030%以下、
Ti:0.030%以下
に制限し、S、Caの含有量が、
S/Ca<0.5
を満足し、
2次精錬後の水素の含有量が2.5ppm以下であり、残部がFe及び不可避的不純物である溶鋼を溶製する工程と、
前記溶鋼を連続鋳造により鋼片とする工程と、
前記鋼片を1000℃以上に加熱する工程と、
加熱した鋼片を、再結晶温度域での圧下比を2以上、未再結晶温度域での圧下比を3以上の熱間圧延し、鋼板を得る工程と、
前記鋼板を、750℃以上の温度から400~600℃になるまで冷却する冷却工程を含み、
前記冷却工程は、前記鋼板の温度を上昇させる復熱処理を2回以上含み、
前記復熱処理において、1回目の復熱処理の開始温度が300℃以上であり、かつ、すべての復熱処理の終了温度が750℃未満である
ことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。 - 前記溶鋼が、質量%で、さらに、
Ni:0.01~2.0%、
Cu:0.01~1.0%、
Cr:0.01~1.0%、
Mo:0.01~0.60%、
W :0.01~1.0%、
V :0.01~0.10%、
Zr:0.0001~0.050%、
Ta:0.0001~0.050%、
B :0.0001~0.0020%、
REM:0.0001~0.01%、
Mg:0.0001~0.01%、
Y :0.0001~0.005%、
Hf:0.0001~0.005%、及び
Re:0.0001~0.005%
のうち1種又は2種以上を含有することを特徴とする請求項6に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。 - 請求項6又は7に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法により得られた鋼板から鋼管を製造する方法であって、
前記鋼板を管状に成形する工程と、
突き合わせ部を溶接する工程
を含み、
鋼板の厚さt[mm]と造管後の鋼管の外径D[mm]が、
t≧25
t/D≧0.035
を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管の製造方法。
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