WO2020003499A1 - Steel pipe and steel sheet - Google Patents
Steel pipe and steel sheet Download PDFInfo
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- WO2020003499A1 WO2020003499A1 PCT/JP2018/024839 JP2018024839W WO2020003499A1 WO 2020003499 A1 WO2020003499 A1 WO 2020003499A1 JP 2018024839 W JP2018024839 W JP 2018024839W WO 2020003499 A1 WO2020003499 A1 WO 2020003499A1
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Definitions
- the present invention relates to a steel pipe and a steel sheet suitable as a material of the steel pipe.
- Patent Documents 1 and 2 Steel pipes having excellent HIC resistance have hitherto been disclosed, for example, in Patent Documents 1 and 2, by purifying steel, reducing inclusions, controlling the form of sulfide-based inclusions by adding Ca, It has been manufactured using technologies such as reducing the center segregation by light pressure during casting and accelerated cooling.
- Patent Document 3 discloses a method for producing a thin sour-resistant steel plate having a thickness of 15 mm or less.
- the production method of Patent Document 3 defines the conditions of finish rolling from the viewpoint of improving low-temperature toughness.
- Patent Documents 1 to 4 there is a problem that the steel sheet is subjected to accelerated cooling and the surface layer of the steel sheet is hardened. As a result of the investigation by the present inventors, it was found that SSC resistance may be reduced in such a steel sheet having a hardened surface layer.
- the sheet when the sheet thickness is small, as described in Non-Patent Document 1, the sheet may be manufactured by air cooling after rolling without applying accelerated cooling. However, when manufactured by air cooling, ferrite (polygonal ferrite) is generated, and the SSC resistance may be reduced.
- ferrite polygonal ferrite
- the present invention does not use expensive, easily segregated elements such as V, Cu, Ni, and / or Mo as essential elements for securing strength,
- a steel pipe having a strength of X60 grade, excellent DWTT characteristics at ⁇ 30 ° C., and excellent SSC resistance and HIC resistance, and having a base material steel plate thickness (wall thickness of steel pipe) of 15 mm or less. And a steel plate as a material of the steel pipe.
- a hot-rolled steel sheet obtained by hot rolling a steel slab having a predetermined chemical composition at a finish rolling temperature of 830 to 1000 ° C. is accelerated and cooled in two stages, and then restored to a required temperature. It has been found that by heating, a steel pipe having a strength of X60 to X70 according to the API standard, excellent DWTT characteristics, SSC resistance and HIC resistance and a wall thickness of 15 mm or less can be manufactured.
- the steel pipe according to the present embodiment has a predetermined strength by controlling the rolling and cooling conditions in the thick plate process after suppressing the Ceq low in a steel plate having a thickness of 15 mm or less used as a material of the base material portion. DWTT characteristics, SSC resistance and HIC resistance are obtained. This idea is greatly different from the technology of manufacturing a steel pipe by Asroll (as-rolled) or normalizing (normalizing) by adding a large amount of alloying elements.
- the present invention has been made based on the above findings, and the gist is as follows.
- a steel pipe includes a base material portion formed of a cylindrical steel plate, and a welded portion provided at a butt portion of the steel plate and extending in a longitudinal direction of the steel plate.
- the steel sheet has a chemical composition of 0.030 to 0.070% by mass, Si: 0.05 to 0.50%, Mn: 1.05 to 1.65%, and Al: 0.010% by mass.
- Ti 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0.0010 to 0.0050%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0 0.0100%, REM: 0 to 0.0100%, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less
- the balance is composed of Fe and impurities, and the steel sheet has a Ceq defined by the following formula (I) of 0.250 to 0.350, and the steel sheet has a Ceq of 1.50 in the depth direction from the surface of the base material portion.
- the internal metal structure which is a metal structure in the range from more than 0 mm to the center of the plate thickness, contains 85% or more of granular bainite and / or bainite in total area ratio, and contains 1.0% or less of MA in area ratio.
- the maximum hardness is 248 Hv or less and the average hardness is 170 to 220 Hv
- the base material portion is parallel to the plate surface at a position of 1/4 of the plate thickness in the plate thickness direction from the surface.
- Metal structure is granular Wherein one or both of bainite and tempered bainite in a total area of 95% or more, the maximum hardness of the surface layer portion metallographic structure is not more than 250 Hv, a plate thickness of the steel sheet is 15mm or less.
- Ceq [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 15 (I)
- [C], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] in the formula (I) represent C, Mn, Ni, Cu, Cr and Mo in the steel sheet. , V in mass%.
- the chemical composition is Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0 in mass%. .50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100% It may include one or more selected from the group.
- the remainder of the internal metal structure may be made of ferrite.
- a steel sheet according to another aspect of the present invention is used for the base material portion of the steel pipe according to any one of the above (1) to (3).
- an additional element such as V, Cu, Ni, and / or Mo
- it has a strength of X60 to X70 (tensile strength of 520 MPa to 760 MPa) according to API standards, and has a DWTT characteristic.
- a high-strength steel sheet for line pipes excellent in DWTT characteristics, sulfide stress cracking resistance, and hydrogen-induced cracking resistance which is suitable as a line pipe for transporting oil, natural gas, and the like, and this steel sheet as a base metal
- a steel pipe for a line pipe excellent in DWTT characteristics, sulfide stress cracking resistance and hydrogen-induced cracking resistance which is suitable as a line pipe for transporting oil, natural gas, and the like, and this steel sheet as a base metal
- a steel pipe for a line pipe excellent in DWTT characteristics, sulfide stress cracking resistance and hydrogen-induced cracking resistance which is suitable as a line pipe for transporting oil, natural gas, and the like
- tissue photograph observed by the scanning electron microscope which shows the internal metallographic structure which is a metal structure in the range from more than 1.0 mm in the depth direction to the center of the thickness from the surface of the base material portion of the steel pipe according to the present embodiment.
- tissue photograph observed with the scanning electron microscope which shows the surface layer part metal structure which is a metal structure in the range from the surface of the base material part of the steel pipe which concerns on this embodiment to depth direction 1.0mm.
- a steel pipe according to an embodiment of the present invention (hereinafter referred to as a steel pipe according to the present embodiment) is provided in a base material portion formed of a tubular steel plate and a butt portion of the steel plate, and is a weld extending in a longitudinal direction of the steel plate.
- the steel sheet has a chemical composition of C: 0.030 to 0.070%, Si: 0.05 to 0.50%, Mn: 1.05 to 1.65 by mass%. %, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0.
- Ni 0.50% or less
- Mo 0.50% or less
- Cr 0.50% or less
- Cu 0.50% or less
- V 1 selected from the group consisting of 0.100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less Or two or more kinds, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less, and the balance: Fe and impurities.
- Ceq defined in 1) is 0.250 to 0.350
- An internal metal structure that is a metal structure in a range from more than 1.0 mm to a plate thickness center in a depth direction from the surface of the base material portion includes one or both of granular bainite and bainite in a total area ratio of 85% or more,
- MA may contain 1.0% or less in area ratio and may contain ferrite as the balance
- the maximum hardness is 248 Hv or less
- the average hardness is 170 to 220 Hv
- the base material portion has a texture in which the degree of integration of ⁇ 100 ⁇ ⁇ 110> is 1.5 or more in a plane parallel to the plate surface at a position 1 / of the plate thickness in the plate thickness direction from the surface.
- a surface layer metal structure that is a metal structure in a range from the surface of the base material portion to 1.0 mm in the depth direction includes one or both of granular bainite and tempered bainite in a total area ratio of 95% or more,
- the maximum hardness of the surface layer metal structure is 250 Hv or less,
- the plate thickness of the steel plate is 15 mm or less.
- the steel sheet according to the present embodiment is used for the base material of the steel pipe according to the present embodiment.
- C 0.030-0.070%
- C is an element that improves the strength of steel. If the C content is less than 0.030%, the effect of improving strength cannot be sufficiently obtained. Therefore, the C content is set to 0.030% or more. Preferably it is 0.040% or more. On the other hand, when the C content exceeds 0.070%, the strength is excessively increased, and the HIC resistance is reduced. Therefore, the C content is set to 0.070% or less.
- the C content is preferably 0.060% or less from the viewpoint of suppressing a decrease in weldability and toughness.
- Si 0.05 to 0.50% Si is an element that functions as a deoxidizer during steelmaking. If the Si content is less than 0.05%, this effect cannot be sufficiently obtained. Therefore, the Si content is set to 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the toughness of the heat affected zone (HAZ) decreases. Therefore, the Si content is set to 0.50% or less. Preferably it is 0.35% or less.
- Mn 1.05 to 1.65%
- Mn is an element that contributes to improving the strength and toughness of steel. If the Mn content is less than 1.05%, the effect of improving strength and toughness cannot be sufficiently obtained. Therefore, the Mn content is set to 1.05% or more. It is preferably at least 1.15%.
- Mn is also an element that forms MnS and reduces the HIC resistance. If the Mn content exceeds 1.65%, the HIC resistance decreases, so the Mn content is set to 1.65% or less. Preferably it is 1.50% or less.
- Al 0.010 to 0.070%
- Al is an element that functions as a deoxidizing agent. If the Al content is less than 0.010%, this effect cannot be sufficiently obtained. Therefore, the Al content is set to 0.010% or more. Preferably it is 0.020% or more. On the other hand, if the Al content exceeds 0.070%, Al oxides accumulate to form clusters, and the HIC resistance decreases. Therefore, the Al content is set to 0.070% or less. Preferably it is 0.040% or less, more preferably 0.030% or less.
- Ti 0.005 to 0.020%
- Ti is an element that forms a nitride and contributes to refinement of crystal grains. If the Ti content is less than 0.005%, the above effects cannot be sufficiently obtained. Therefore, the Ti content is set to 0.005% or more. Preferably it is 0.008% or more. On the other hand, if the Ti content exceeds 0.020%, coarse nitrides are generated, and the HIC resistance decreases. Therefore, the Ti content is set to 0.020% or less. Preferably it is 0.015% or less.
- Nb 0.005 to 0.045%
- Nb is an element that expands the non-recrystallization temperature range to make crystal grains fine and forms carbides and nitrides, thereby contributing to improvement in the strength of steel. If the Nb content is less than 0.005%, the above effects cannot be sufficiently obtained. Therefore, the Nb content is set to 0.005% or more. Preferably it is 0.010% or more. On the other hand, when the Nb content exceeds 0.045%, coarse carbides and nitrides are generated, and the HIC resistance decreases. In addition, elongation and toughness are also reduced. Therefore, the Nb content is set to 0.045% or less. Preferably it is 0.035% or less.
- Ca 0.0010 to 0.0050%
- Ca is an element that generates CaS and suppresses the generation of MnS extending in the rolling direction, thereby contributing to an improvement in HIC resistance. If the Ca content is less than 0.0010%, the above effects cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0010% or more. Preferably it is 0.0020% or more. On the other hand, when the Ca content exceeds 0.0050%, Ca oxide accumulates, and the HIC resistance decreases. Therefore, the Ca content is set to 0.0050% or less. Preferably it is 0.0040% or less.
- N 0.0010 to 0.0050%
- N is an element that contributes to the refinement of the structure by forming a nitride that suppresses coarsening of austenite grains during heating. If the N content is less than 0.0010%, the effect of refining the structure cannot be sufficiently obtained. Therefore, the N content is set to 0.0010% or more. Preferably it is 0.0020% or more. On the other hand, when the N content exceeds 0.0050%, coarse nitrides are generated, and the HIC resistance is reduced. Therefore, the N content is set to 0.0050% or less. Preferably it is 0.0040% or less.
- Ni, Mo, Cr if necessary, in order to improve strength, toughness, and other properties.
- Cu, V, Mg, and REM may be contained in the following range. However, each of these elements is an arbitrary element that is not an essential element, and thus the lower limit is 0%.
- Ni is an element that contributes to improving the toughness, strength, and corrosion resistance of steel. If the Ni content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, to obtain these effects, the Ni content is preferably set to 0.05% or more. More preferably, it is 0.10% or more. On the other hand, when the Ni content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even when Ni is contained, the Ni content is set to 0.50% or less. Preferably it is 0.35% or less.
- Mo 0 to 0.50%
- Mo is an element that contributes to improving the hardenability of steel. If the Mo content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Mo content is preferably set to 0.05% or more. More preferably, it is 0.10% or more. On the other hand, when the Mo content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even when Mo is contained, the Mo content is set to 0.50% or less. Preferably it is 0.35% or less.
- Cr 0 to 0.50% Cr is an element that contributes to improving the strength of steel. If the Cr content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Cr content is preferably set to 0.05% or more. More preferably, it is 0.10% or more. On the other hand, if the Cr content exceeds 0.50%, the strength is excessively increased, and the toughness is reduced. Therefore, even when it is contained, the Cr content is set to 0.50% or less. Preferably it is 0.35% or less.
- Cu 0 to 0.50%
- Cu is an element that contributes to increasing the strength of steel and improving corrosion resistance. If the Cu content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when the above effects are obtained, the Cu content is preferably set to 0.05% or more. More preferably, it is 0.10% or more. On the other hand, when the Cu content exceeds 0.50%, the maximum hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, the Cu content is set to 0.50% or less even when it is contained. Preferably it is 0.35% or less.
- V 0 to 0.100%
- V is an element that forms carbides and nitrides and contributes to improving the strength of steel. If the V content is less than 0.010%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the V content is preferably set to 0.010% or more. It is more preferably at least 0.030%. On the other hand, if the V content exceeds 0.100%, the toughness of the steel decreases. Therefore, the V content is set to 0.100% or less. Preferably it is 0.080% or less.
- Mg 0 to 0.0100%
- Mg is an element that forms a fine oxide that contributes to improvement in toughness by suppressing coarsening of crystal grains. If the Mg content is less than 0.0001%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Mg content is preferably set to 0.0001% or more. More preferably, it is 0.0010% or more. On the other hand, when the Mg content exceeds 0.0100%, the oxides are agglomerated and coarsened, and the HIC resistance and toughness are reduced. Therefore, even if it is contained, the Mg content is set to 0.0100% or less. Preferably it is 0.0050% or less.
- REM 0-0.0100% REM is an element that controls the form of sulfide-based inclusions and contributes to improvement in toughness. If the REM content is less than 0.0001%, the above effects cannot be sufficiently obtained. Therefore, in order to obtain the above effects, the REM content is preferably set to 0.0001% or more. More preferably, it is 0.0010% or more. On the other hand, if the REM content exceeds 0.0100%, oxides are generated, and the cleanliness of the steel is reduced, and as a result, the toughness is reduced. Therefore, even when it is contained, the REM content is set to 0.0100% or less. Preferably it is 0.0060% or less. In this embodiment, REM means a rare earth element and is a general term for 17 elements of Sc, Y and lanthanoid, and the REM content indicates the total content of these 17 elements.
- the base material portion (steel plate according to the present embodiment) of the steel pipe according to the present embodiment basically includes the above essential elements, optionally includes the above optional elements, and the balance is composed of Fe and impurities.
- impurities are components that are mixed from raw materials such as ore or scrap or the like from various environments in the manufacturing process when steel products are manufactured industrially, and do not adversely affect the properties of the steel. Means acceptable.
- P, S, O, Sb, Sn, Co, As, Pb, Bi, and H are preferably controlled in a range described later.
- P 0.015% or less
- P is an impurity element. If the P content exceeds 0.015%, the HIC resistance is significantly reduced. Therefore, the P content is set to 0.015% or less. Preferably it is 0.010% or less. Since the smaller the content, the better, the lower limit includes 0%. However, reducing the P content to less than 0.003% significantly increases manufacturing costs. Therefore, 0.003% is a substantial lower limit of the P content.
- S 0.0015% or less
- S is an element that generates MnS that elongates in the rolling direction during hot rolling and reduces the HIC resistance. If the S content exceeds 0.0015%, the HIC resistance is significantly reduced. Therefore, the S content is set to 0.0015% or less. Preferably it is 0.0010% or less. The lower the S content, the better, so the lower limit contains 0%. However, reducing the S content to less than 0.0001% significantly increases manufacturing costs. Therefore, 0.0001% is a substantial lower limit of the S content.
- O 0.0040% or less
- O is an element inevitably remaining in steel after deoxidation. If the O content exceeds 0.0040%, an oxide is generated, and the HIC resistance is reduced. Therefore, the O content is set to 0.0040% or less. Preferably it is 0.0030% or less. The lower the O content, the better, so the lower limit contains 0%. However, when the O content is reduced to less than 0.0010%, the production cost is significantly increased. Therefore, 0.0010% is a practical lower limit of the O content in practical steel sheets.
- Ceq 0.250-0.350 Ceq (carbon equivalent) is an index indicating the hardenability of a steel sheet.
- Ceq defined by the following equation (1) is set to 0.250 to 0.350.
- Ceq [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 5 (1)
- [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in the formula (1) are C, Mn, Ni, Cu, This is the content of Cr, Mo, and V in mass%.
- Ceq is set to 0.250 or more. Preferably it is 0.260 or more.
- Ceq exceeds 0.350, the hardenability becomes too high, and the maximum hardness in the internal metal structure exceeds 248 Hv and / or the maximum hardness in the surface layer metal structure exceeds 250 Hv. As a result, the HIC resistance and / or SSC resistance decreases. Therefore, Ceq is set to 0.350 or less. Preferably it is 0.340 or less, more preferably 0.330 or less.
- Metal structure ranging from more than 1.0 mm to the center of the thickness in the depth direction (thickness direction) from the surface of the steel sheet of the base material portion: one of granular bainite and bainite having a total area ratio of 85% or more. Or both, and the area ratio of MA is 1.0% or less.
- the metal structure in the range from more than 1.0 mm to the center of the sheet thickness in the depth direction from the steel sheet surface. (Hereinafter, it may be simply referred to as “internal metal structure.”) Is a metal structure containing one or both of granular bainite and bainite in a total area ratio of 85% or more.
- the total area ratio of granular bainite and / or bainite is less than 85%, it becomes difficult to secure required mechanical properties and HIC resistance. Therefore, the total area ratio of one or both of granular bainite and bainite is 85% or more. Preferably it is 90% or more. Since the area ratio depends on the type of steel and the cooling rate, the upper limit may be 100%, but 95% is a substantial upper limit.
- the area ratio of MA (Martensite-Austenite Constituent) exceeds 1.0%, the DWTT characteristics deteriorate. Therefore, in the internal metal structure, the area ratio of MA is set to 1.0% or less. MA may be 0%.
- the remainder of the internal metal structure may be made of ferrite.
- Metal structure up to 1.0 mm in the depth direction from the surface of the steel sheet: One or both of granular bainite and tempered bainite in an area ratio of 95% or more. It is preferable that the surface layer metal structure contains granular bainite and tempered bainite in a total area ratio of 95% or more, because the SSC resistance is improved.
- the measurement of the area ratio in the metal structure can be obtained by observing the metal structure with a scanning electron microscope at, for example, 1000 times magnification. Since the structure at a position (t / 4) of the plate thickness from the surface of the steel plate indicates a representative structure of the internal metallographic structure, in this embodiment, the t / 4 of the base material (steel plate) of the steel pipe is used. Is observed, and if the structure at t / 4 is the above-described structure, it is determined that the internal metallographic structure is within the above-described range.
- the structure of the surface layer portion is obtained by measuring positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel sheet and averaging the area ratio at each position.
- bainite is a structure in which the former austenite grain boundaries are clear, fine lath structures are developed in the grains, and fine carbides and MA are scattered in the laths and between the laths.
- Tempered bainite is a structure having a lath shape in which carbides are dispersed in the lath and at the lath boundary.
- Granular bainite is formed at an intermediate transformation temperature between acicular ferrite and bainite and has an intermediate texture.
- FIG. 3A shows an example of a metal structure imaged by a scanning electron microscope at a position of t / 4 of a steel plate as a base material of the steel pipe according to the present embodiment
- FIG. 3B shows a base metal part of the steel pipe according to the present embodiment.
- 1 shows an example of a metal structure imaged by a scanning electron microscope at a surface of a steel sheet of 0.5 mm.
- Hardness of internal metal structure Maximum hardness: 248 Hv or less Average hardness: 170 to 220 Hv In the steel pipe according to the present embodiment, in order to secure excellent strength, SSC resistance and HIC resistance, the maximum hardness is 248 Hv or less and the average hardness is 170 to 220 Hv in the internal metal structure of the base metal part.
- the maximum hardness exceeds 248 Hv, the HIC resistance decreases, so the maximum hardness is set to 248 Hv or less. Preferably it is 230 Hv. If the average hardness is less than 170 Hv, required strength cannot be secured, so the average hardness is 170 Hv or more. Preferably it is 180 Hv or more. On the other hand, if the average hardness exceeds 220 Hv, HIC resistance and toughness decrease. Therefore, the average hardness is set to 220 Hv or less. Preferably it is 210 Hv or less.
- the maximum hardness of the surface layer metal structure 250 Hv or less If the maximum hardness of the surface layer metal structure is more than 250 Hv, the SSC resistance decreases. Therefore, the maximum hardness of the surface layer metal structure is set to 250 Hv or less. Preferably, it is 240 Hv or less.
- the maximum hardness and the average hardness in the internal metal structure can be measured by the following methods. Using a Vickers hardness tester (load: 100 g), starting at a depth of 1.1 mm from the surface of the steel sheet and extending to the center of the thickness at intervals of 0.1 mm in the thickness direction and 1.0 mm in the width direction for the same depth. Measure hardness at 20 points at intervals. As a result of the above measurement, if two or more measurement points exceeding 248 Hv do not appear continuously in the thickness direction, it is determined that the maximum hardness of the internal metal structure is Hv 248 or less. In the base material of the steel pipe according to the present embodiment, a high hardness value (abnormal value) may appear locally due to inclusions or the like.
- HIC resistance and SSC resistance can be ensured even if such abnormal values appear.
- the HIC resistance and / or the SSC resistance are not due to inclusions but are unacceptable. Therefore, in the present embodiment, even if there is one measurement point exceeding 248Hv, if two or more points do not appear continuously in the sheet thickness direction, the point is not adopted as an abnormal point, and is not adopted as an abnormal point. Is the maximum hardness.
- the highest value is adopted as the maximum hardness. The average hardness is calculated by averaging the hardness of all the measurement points.
- the measurement of the maximum hardness of the surface layer metal structure from the surface of the steel sheet to a depth of 1.0 mm is performed as follows. First, from the end in the width direction of the steel sheet (corresponding to a butt portion in the case of a steel pipe), a position of 4, ⁇ ⁇ , and ⁇ in the width direction of the steel sheet (in the case of a steel pipe, the welded portion is 0 mm) In the case where it is time, a 300 mm square (300 mm ⁇ 300 mm) steel plate is cut out from the 3 o'clock, 6 o'clock and 9 o'clock positions by gas cutting, and a block test of 20 mm in length and 20 mm in width is performed from the center of the cut out steel plate.
- Pieces are collected by mechanical cutting and polished by mechanical polishing. Using a Vickers hardness tester (load: 100 g), one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points are measured. That is, a total of 300 points are measured with three block test pieces. As a result of the above measurement, when two or more measurement points exceeding 250 Hv do not appear continuously in the thickness direction, it is determined that the maximum hardness of the surface layer portion is 250 Hv or less.
- the degree of integration of ⁇ 100 ⁇ ⁇ 110> is 1.5 or more. Without hot rolling, cooling and reheating. Therefore, the internal metal structure has the above-described texture. Having a texture improves the DWTT characteristics of the steel sheet. Such a texture cannot be obtained when a steel sheet is manufactured by quenching and tempering or when a steel sheet is manufactured by normalizing.
- the texture can be obtained by the following method. Assuming that the thickness of the steel sheet of the base metal part is t, a 2.0 mm ⁇ 2.0 mm area is defined as 0 mm by using EBSP with respect to a plane parallel to the plate surface at a depth of t / 4 from the surface. The crystal orientation analysis is performed at 1 mm intervals to determine the degree of integration of the (100) ⁇ 110> texture.
- the thickness of the steel sheet of the base metal part (wall thickness of the steel pipe): 15 mm or less
- the steel pipe according to the present embodiment is hardened so as to have DWTT characteristics, SSC resistance, and HIC resistance, which have been difficult to satisfy at the same time.
- This is a steel pipe manufactured without performing a tempering process (rolled and cooled) and using a steel plate having a thickness of 15 mm or less as a base material.
- the steel pipe according to the present embodiment has excellent SSC resistance and HIC resistance even when the thickness of the steel plate is 12 mm or less.
- the strength of the base material portion (steel plate according to the present embodiment) of the steel pipe according to the present embodiment is a strength equivalent to 5L-X60 to X70 in the API standard (tensile strength 520 MPa to) in order to ensure the strength as a steel pipe. 760 MPa).
- the upper limit of the tensile strength is preferably 650 MPa or less in order to secure an overmatch of the welded portion during on-site welding.
- the steel pipe according to the present embodiment is obtained by processing the steel plate according to the present embodiment into a tubular shape, butting and welding both ends of the tubular steel plate. Therefore, it has the welding part provided in the butting part of a steel plate and extending in the longitudinal direction of a steel plate.
- the welded portion is constructed so as to be thicker than the base material portion.
- the weld metal is a higher alloy than the base metal and has high corrosion resistance. Therefore, the weld is rarely the starting point of the destruction. Therefore, the welded portion of the steel pipe according to the present embodiment is not particularly limited as long as it is obtained under normal conditions by SAW welding or the like.
- the steel pipe according to the present embodiment has the above-described configuration, and the effect can be obtained.
- the following manufacturing method it is preferable because it can be obtained stably.
- the steel sheet according to the present embodiment is: (I) A slab satisfying a predetermined chemical composition and Ceq is heated to 1050 to 1250 ° C. and subjected to hot rolling, and finish rolling at 830 to 1000 ° C. to obtain a steel sheet having a thickness of 15 mm or less.
- Process hot rolling process
- Ii-1) a step of cooling the steel sheet after rolling from above 750 to 950 ° C. to a temperature range of 660 to 750 ° C. at an average cooling rate of 25 to 100 ° C./sec (first cooling step);
- the steel pipe according to the present embodiment is: (Iv) a step of forming the steel sheet obtained through the steps (i) to (iii) into a cylindrical shape (forming step); (V) a process of welding both ends of the tubular steel plate by butt welding (welding process); Is obtained by a manufacturing method including: The above temperature is managed by the surface temperature.
- preferable conditions of each step will be described.
- the billet heating temperature be 1050 ° C. or higher. More preferably, the temperature is 1100 ° C. or higher.
- the slab heating temperature exceeds 1250 ° C., the crystal grains become coarse and the low-temperature toughness decreases. Therefore, it is preferable that the billet heating temperature be 1250 ° C. or less. More preferably, it is 1200 ° C or lower. Casting of molten steel and production of billets prior to the hot rolling step may be performed according to a conventional method.
- Finish rolling temperature 830 to 1000 ° C
- the heated steel slab is hot-rolled to a steel sheet of 15 mm or less.
- the finish rolling temperature is preferably set to 830 to 1000 ° C.
- the finish rolling temperature is 850 ° C. or higher.
- the finish rolling temperature is preferably set to 1000 ° C. or less. The temperature is more preferably 980 ° C or lower.
- Cooling start temperature Ts more than 750 to 950 ° C
- Average cooling rate Vc1 25 to 100 ° C./sec.
- Cooling stop temperature Tm 660 to 750 ° C.
- the steel sheet having a surface temperature of Ts (cooling start temperature) in a temperature range of more than 750 to 950 ° C. at an average cooling rate Vc of 25 to 50 ° C./sec.
- the cooling start temperature Ts is 750 ° C. or less at the surface temperature, the area ratio of ferrite exceeds 15%. In this case, the area ratio of one or both of granular bainite and bainite is less than 85%, and the HIC resistance is reduced. Therefore, the cooling start temperature Ts is preferably set to be higher than 750 ° C. at the surface temperature. It is more preferably at least 800 ° C.
- the cooling start temperature Ts exceeds 950 ° C., the crystal grains become coarse and the low-temperature toughness decreases. Further, the maximum hardness of the surface layer may be too high. Therefore, it is preferable that the cooling start temperature Ts be 950 ° C. or less at the surface temperature. The temperature is more preferably 930 ° C or lower.
- the average cooling rate Vc1 is less than 25 ° C./sec, the cooling rate is too slow, and a large amount of ferrite is generated in the surface layer and the internal metal structure, and one or both of granular bainite and bainite having an area ratio of 85% or more. Cannot be obtained, and the SSC resistance and the HIC resistance decrease. Therefore, the average cooling rate Vc1 is preferably set to 25 ° C./sec or more. More preferably, it is 30 ° C./sec or more.
- the average cooling rate Vc1 exceeds 100 ° C./sec, the maximum hardness exceeds 248 Hv in the internal metal structure, and the HIC resistance decreases. Therefore, the average cooling rate Vc1 is preferably set to 100 ° C./sec or less. More preferably, it is 50 ° C./sec or less, further preferably 45 ° C./sec or less.
- the cooling stop temperature Tm of the first cooling step is lower than 660 ° C., a large amount of ferrite is generated, and one or both of granular bainite and bainite having an area ratio of 85% or more cannot be obtained. SSC property and HIC resistance decrease. Therefore, the cooling stop temperature Tm is preferably set to 660 ° C. or higher. It is more preferably at least 680 ° C. On the other hand, if the cooling stop temperature Tm exceeds 750 ° C., there is a concern that the surface layer is hardened and the SSC resistance is reduced. Therefore, the cooling stop temperature Tm is preferably set to 750 ° C. or lower. More preferably, it is 720 ° C. or lower.
- Cooling start temperature Tm 660-750 ° C Average cooling rate Vc2: more than 50 ° C./sec.
- Cooling stop temperature Tf 400 ° C. or less
- the first stage cooling stop temperature Tm 660 to 750 ° C. Cool to a cooling stop temperature Tf of 400 ° C. or less.
- the average cooling rate Vc2 In accelerated cooling from the cooling start temperature Tm of 660 to 750 ° C., if the average cooling rate Vc2 is 50 ° C./sec or less, the internal maximum hardness is increased, and there is a concern that the HIC resistance is reduced. Therefore, it is preferable that the average cooling rate Vc2 be more than 50 ° C./sec. More preferably, it is 60 ° C./sec or more.
- the upper limit of the average cooling rate Vc2 is not particularly limited, but is about 200 ° C./sec at present because the cooling capacity of the cooling equipment is a practical upper limit.
- the cooling stop temperature Tf is preferably set to 400 ° C. or lower. More preferably, it is 380 ° C or lower. Since the cooling stop temperature Tf is determined according to the type of steel and the cooling rate, the lower limit is not particularly set. However, it is preferably 250 ° C. or higher from the viewpoint of sufficiently recovering the heat to obtain a required structure and hardness.
- two-stage accelerated cooling with different cooling rates is performed.
- Such cooling is performed by adjusting the amount of cooling water injected into the steel sheet for each cooling zone in a cooling facility in which the cooling zone is divided into a plurality of pieces in the longitudinal direction (transport direction) of the steel sheet.
- the cooling rate is obtained by dividing the temperature difference between the cooling start temperature and the cooling stop temperature by the cooling time.
- the recuperation rate Vr is less than 50 ° C./sec, there is a concern that the surface layer is hardened and the SSC resistance is reduced. Therefore, the recuperation rate is set to 50 ° C./sec or more.
- the recuperation rate may be appropriately set in consideration of the time required for the surface temperature of the steel sheet to exceed 550 to 650 ° C., and the upper limit is not particularly limited.
- the recuperation speed is obtained by dividing the recuperation temperature width by the time required for recuperation.
- the steel sheet surface temperature after recuperation is 550 ° C. or less, the maximum hardness of the internal structure exceeds 248 Hv, so the steel sheet surface temperature after recuperation is preferably more than 550 ° C. More preferably, it is 580 ° C or higher.
- the steel sheet surface temperature after reheating exceeds 650 ° C., the average hardness does not reach 170 Hv. Therefore, it is preferable that the surface temperature of the steel sheet after reheating is 650 ° C. or less. The temperature is more preferably 620 ° C or lower.
- the recuperation rate and the quantity of recuperation vary depending on the temperature difference between the surface and the inside when cooling is stopped.
- the temperature difference between the surface and the inside is not simply determined by the cooling rate, but changes depending on the water density in water cooling, the collision pressure, and the like. Therefore, the cooling conditions may be determined so that the recuperation rate is 50 ° C./sec or more and the surface temperature after recuperation is more than 550 to 650 ° C. For example, if an experiment for determining conditions is performed in advance, appropriate conditions can be set.
- FIG. 2 schematically shows an example of a cooling curve of the steel sheet after the finish rolling (change in the steel sheet surface temperature in the first cooling step, the second cooling step, and the recuperation step).
- the steel sheet after the recuperation step is preferably cooled to an average cooling rate of 0.01 ° C./sec or more and 300 ° C. or less. If the average cooling rate is less than 0.01 ° C./sec, the desired strength cannot be obtained.
- a steel plate used for the base material portion of the steel pipe according to the present embodiment can be manufactured. That is, the steel sheet according to the present embodiment is a non-heat treated steel.
- the steel sheet according to the present embodiment obtained in the above process is formed into a tubular shape, and butted portions (both ends in the width direction of the steel plate) of the tubular steel plate are welded to form a steel pipe.
- the forming of the steel sheet according to the present embodiment into a steel pipe is not limited to a specific forming. Although warm working may be performed, cold working is preferred in terms of dimensional accuracy.
- the welding is not limited to a specific welding, but a submerged arc welding is preferable.
- the welding condition may be a known condition according to the thickness of the steel sheet or the like.
- a heat treatment may be performed on the welded portion in order to improve the toughness of the welded portion.
- the heat treatment temperature may be in a normal temperature range, but is particularly preferably in a range of 300 to Ac1 point.
- the pipe according to the present embodiment is a steel pipe having sufficient mechanical properties as a steel pipe for a line pipe in both the base material portion and the welded portion.
- the conditions in the examples are one condition examples adopted for confirming the feasibility and effects of the present invention, and the present invention is not limited to these one condition examples.
- the present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
- a test piece was sampled from the manufactured steel sheet, and the internal metallographic structure was observed at a magnification of 1000 times from the surface of the steel sheet at a position (t / 4) 1/4 of the plate thickness using a scanning electron microscope. I decided. Further, the surface layer metal structure was obtained by observing and measuring 0.1 mm, 0.2 mm, and 0.5 mm positions from the surface of the steel sheet and averaging the area ratio at each position. Further, a JIS No. 5 tensile test piece was prepared, a tensile test specified in JIS Z 2241 was performed, and the yield strength and the tensile strength were measured.
- the hardness of the internal metal structure and the surface metal structure was measured with a Vickers hardness tester.
- a Vickers hardness tester load: 100 g
- a Vickers hardness tester load: 100 g
- the highest value is defined as the maximum hardness.
- the average hardness was calculated by averaging the hardness of all the measurement points.
- a 300 mm square (300 mm ⁇ 300 mm) steel sheet is cut out from the end in the width direction of the steel sheet by gas cutting, and a block test piece having a length of 20 mm and a width of 20 mm is mechanically cut from the center of the cut steel sheet. And polished by mechanical polishing.
- one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points were measured. That is, a total of 300 points were measured with three block test pieces.
- the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did.
- the highest value is defined as the maximum hardness.
- the NACE test saturates hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, immerses the steel sheet in the solution, and observes whether cracks occur after 96 hours. It is a test.
- the DWTT property (ductile fracture rate at ⁇ 30 ° C.) was evaluated by the following method. DWTT test pieces were sampled from the steel sheet so that the width direction of the steel sheet was parallel to the longitudinal direction of the test piece. The sampling position was 1/4 position in the width direction of the steel sheet. The DWTT test piece was a full thickness test piece with a press notch. The test piece was subjected to a DWTT test at ⁇ 30 ° C. in accordance with API 5L, and the ductile fracture ratio occupying the entire fracture surface was measured. The higher the numerical value of the fracture surface ratio (%), the better the DWTT characteristics. In the present invention, it was determined that the DWTT characteristics were excellent when the ductile fracture ratio was 85% or more.
- a test piece was collected from the base material of the manufactured steel pipe, and the fraction (area ratio) of each of the surface layer metal structure and the internal metal structure was calculated.
- the internal metallographic structure was determined by observing the structure at a position (t / 4) 1/4 of the plate thickness from the surface of the steel plate using a scanning electron microscope at a magnification of 1000 times. The remaining structure not described in the table was ferrite.
- the surface layer metal structure was obtained by measuring positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel sheet and averaging the area ratio at each position. Further, a JIS No. 5 tensile test piece was prepared, a tensile test specified in JISZ2241 was performed, and the yield strength and the tensile strength were measured.
- the hardness of the internal metal structure and the surface metal structure was measured with a Vickers hardness tester.
- a Vickers hardness tester load: 100 g
- a Vickers hardness tester load: 100 g
- the highest value is defined as the maximum hardness.
- the average hardness was calculated by averaging the hardness of all the measurement points.
- a 300 mm square (300 mm ⁇ 300 mm) steel plate is cut out from the positions of 3 o'clock, 6 o'clock, and 9 o'clock when the welded portion is set to 0 o'clock from the butt portion of the steel pipe by gas cutting. From the center of the steel plate, a block test piece having a length of 20 mm and a width of 20 mm is collected by mechanical cutting and polished by mechanical polishing.
- one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points were measured. That is, a total of 300 points were measured with three block test pieces.
- the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did.
- the highest value is defined as the maximum hardness.
- test piece was sampled from the base material portion of the manufactured steel pipe, and the following test was performed to evaluate the HIC resistance and the SSC resistance.
- HIC resistance A test based on NACE (National Association of Corrosion and Engineer) TM0284 was performed, and the presence or absence of HIC (hydrogen induced cracking) was observed. The case where the HIC property was excellent (OK) was evaluated, and the case where the HIC fracture ratio was more than 5% was evaluated as poor (NG) when the HIC resistance was poor.
- NACE National Association of Corrosion and Engineer
- the NACE test saturates hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, immerses the steel sheet in the solution, and observes whether cracks occur after 96 hours. It is a test.
- the DWTT characteristics were evaluated by the following method. DWTT test pieces were sampled from the steel pipe such that the circumferential direction of the steel pipe was parallel to the longitudinal direction of the test piece. The sampling position was 90 ° from the seam position of the steel pipe. Here, the DWTT test piece was a full thickness test piece with a press notch. The test piece was subjected to a DWTT test at ⁇ 30 ° C. in accordance with API 5L, and the ductile fracture ratio occupying the entire fracture surface was measured. The higher the numerical value of the fracture surface ratio (%), the better the DWTT characteristics. In the present invention, it was determined that the DWTT characteristics were excellent when the ductile fracture ratio was 85% or more.
- ADVANTAGE OF THE INVENTION even if it does not use additional elements, such as V, Cu, Ni, and / or Mo, it has the intensity
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Abstract
Description
本実施形態に係る鋼管は、その母材部の素材として用いる板厚15mm以下の鋼板において、Ceqを低く抑えた上で、厚板工程における圧延及び冷却条件を制御することによって、所定の強度、DWTT特性、耐SSC性及び耐HIC特性を得ている。これは、合金元素を多量に添加して、Asroll(圧延まま)や焼準(ノルマライジング)によって鋼管を製造する技術とはその思想が大きく異なる。 The present inventors have diligently studied a technique for solving the above problem. As a result, a hot-rolled steel sheet obtained by hot rolling a steel slab having a predetermined chemical composition at a finish rolling temperature of 830 to 1000 ° C. is accelerated and cooled in two stages, and then restored to a required temperature. It has been found that by heating, a steel pipe having a strength of X60 to X70 according to the API standard, excellent DWTT characteristics, SSC resistance and HIC resistance and a wall thickness of 15 mm or less can be manufactured.
The steel pipe according to the present embodiment has a predetermined strength by controlling the rolling and cooling conditions in the thick plate process after suppressing the Ceq low in a steel plate having a thickness of 15 mm or less used as a material of the base material portion. DWTT characteristics, SSC resistance and HIC resistance are obtained. This idea is greatly different from the technology of manufacturing a steel pipe by Asroll (as-rolled) or normalizing (normalizing) by adding a large amount of alloying elements.
Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/15・・・(I)
式(I)中の[C]、[Mn]、[Ni]、[Cu]、[Cr]、[Mo]、[V]は、前記鋼板中のC、Mn、Ni、Cu、Cr、Mo、Vの質量%での含有量である。
(2)上記(1)に係る鋼管では、前記化学組成が、質量%で、Ni:0.05~0.50%、Mo:0.05~0.50%、Cr:0.05~0.50%、Cu:0.05~0.50%、V:0.010~0.100%、Mg:0.0001~0.0100%、REM:0.0001~0.0100%、からなる群から選択される1種又は2種以上を含んでもよい。
(3)上記(1)または(2)に記載の鋼管では、前記内部金属組織の残部が、フェライトからなってもよい。
(4)本発明の別の態様に係る鋼板は、上記(1)~(3)のいずれか一項に記載の鋼管の前記母材部に用いる。 (1) A steel pipe according to one embodiment of the present invention includes a base material portion formed of a cylindrical steel plate, and a welded portion provided at a butt portion of the steel plate and extending in a longitudinal direction of the steel plate. The steel sheet has a chemical composition of 0.030 to 0.070% by mass, Si: 0.05 to 0.50%, Mn: 1.05 to 1.65%, and Al: 0.010% by mass. To 0.070%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0.0010 to 0.0050%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0 0.0100%, REM: 0 to 0.0100%, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less The balance is composed of Fe and impurities, and the steel sheet has a Ceq defined by the following formula (I) of 0.250 to 0.350, and the steel sheet has a Ceq of 1.50 in the depth direction from the surface of the base material portion. The internal metal structure, which is a metal structure in the range from more than 0 mm to the center of the plate thickness, contains 85% or more of granular bainite and / or bainite in total area ratio, and contains 1.0% or less of MA in area ratio. In the internal metal structure, the maximum hardness is 248 Hv or less and the average hardness is 170 to 220 Hv, and the base material portion is parallel to the plate surface at a position of 1/4 of the plate thickness in the plate thickness direction from the surface. A surface layer portion having a texture in which the degree of integration of {100} <110> is 1.5 or more on the surface, and a metal structure ranging from the surface of the base material portion to 1.0 mm in the depth direction. Metal structure is granular Wherein one or both of bainite and tempered bainite in a total area of 95% or more, the maximum hardness of the surface layer portion metallographic structure is not more than 250 Hv, a plate thickness of the steel sheet is 15mm or less.
Ceq = [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 15 (I)
[C], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] in the formula (I) represent C, Mn, Ni, Cu, Cr and Mo in the steel sheet. , V in mass%.
(2) In the steel pipe according to the above (1), the chemical composition is Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0 in mass%. .50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100% It may include one or more selected from the group.
(3) In the steel pipe according to (1) or (2), the remainder of the internal metal structure may be made of ferrite.
(4) A steel sheet according to another aspect of the present invention is used for the base material portion of the steel pipe according to any one of the above (1) to (3).
前記母材部の表面から深さ方向に1.0mm超から板厚中心までの範囲の金属組織である内部金属組織が、グラニュラーベイナイト及びベイナイトの一方又は両方を合計面積率で85%以上含み、かつ、MAを面積率で1.0%以下含み、残部として、フェライトを含む場合があり、
前記内部金属組織において、最大硬度が248Hv以下、かつ平均硬度が170~220Hvであり、
前記母材部が、前記表面から板厚方向に板厚の1/4の位置の板面に平行な面において{100}<110>の集積度が1.5以上である集合組織を有し、
前記母材部の前記表面から前記深さ方向に1.0mmまでの範囲の金属組織である表層部金属組織が、グラニュラーベイナイト及び焼戻しベイナイトの一方または両方を合計面積率で、95%以上含み、
前記表層部金属組織の最大硬度が、250Hv以下であり、
前記鋼板の板厚が15mm以下である。 A steel pipe according to an embodiment of the present invention (hereinafter referred to as a steel pipe according to the present embodiment) is provided in a base material portion formed of a tubular steel plate and a butt portion of the steel plate, and is a weld extending in a longitudinal direction of the steel plate. And the steel sheet has a chemical composition of C: 0.030 to 0.070%, Si: 0.05 to 0.50%, Mn: 1.05 to 1.65 by mass%. %, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0. 0010 to 0.0050%, Ni: 0.50% or less, Mo: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, V: 1 selected from the group consisting of 0.100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less Or two or more kinds, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less, and the balance: Fe and impurities. Ceq defined in 1) is 0.250 to 0.350,
An internal metal structure that is a metal structure in a range from more than 1.0 mm to a plate thickness center in a depth direction from the surface of the base material portion includes one or both of granular bainite and bainite in a total area ratio of 85% or more, In addition, MA may contain 1.0% or less in area ratio and may contain ferrite as the balance,
In the internal metal structure, the maximum hardness is 248 Hv or less, and the average hardness is 170 to 220 Hv;
The base material portion has a texture in which the degree of integration of {100} <110> is 1.5 or more in a plane parallel to the plate surface at a
A surface layer metal structure that is a metal structure in a range from the surface of the base material portion to 1.0 mm in the depth direction includes one or both of granular bainite and tempered bainite in a total area ratio of 95% or more,
The maximum hardness of the surface layer metal structure is 250 Hv or less,
The plate thickness of the steel plate is 15 mm or less.
Cは、鋼の強度を向上させる元素である。C含有量が0.030%未満であると、強度向上効果が十分に得られない。そのため、C含有量は0.030%以上とする。好ましくは0.040%以上である。
一方、C含有量が0.070%を超えると、強度が上昇しすぎて、耐HIC性が低下する。そのため、C含有量は0.070%以下とする。溶接性、靱性等の低下を抑制する点で、C含有量は0.060%以下が好ましい。 C: 0.030-0.070%
C is an element that improves the strength of steel. If the C content is less than 0.030%, the effect of improving strength cannot be sufficiently obtained. Therefore, the C content is set to 0.030% or more. Preferably it is 0.040% or more.
On the other hand, when the C content exceeds 0.070%, the strength is excessively increased, and the HIC resistance is reduced. Therefore, the C content is set to 0.070% or less. The C content is preferably 0.060% or less from the viewpoint of suppressing a decrease in weldability and toughness.
Siは、製鋼時、脱酸剤として機能する元素である。Si含有量が0.05%未満であると、この効果が十分に得られない。そのため、Si含有量は0.05%以上とする。
一方、Si含有量が0.50%を超えると、溶接熱影響部(HAZ)の靱性が低下する。そのため、Si含有量は0.50%以下とする。好ましくは0.35%以下である。 Si: 0.05 to 0.50%
Si is an element that functions as a deoxidizer during steelmaking. If the Si content is less than 0.05%, this effect cannot be sufficiently obtained. Therefore, the Si content is set to 0.05% or more.
On the other hand, if the Si content exceeds 0.50%, the toughness of the heat affected zone (HAZ) decreases. Therefore, the Si content is set to 0.50% or less. Preferably it is 0.35% or less.
Mnは、鋼の強度及び靱性の向上に寄与する元素である。Mn含有量が1.05%未満であると、強度及び靱性の向上効果が十分に得られない。そのため、Mn含有量は1.05%以上とする。好ましくは1.15%以上である。
一方、Mnは、MnSを形成して、耐HIC性を低下させる元素でもある。Mn含有量が1.65%を超えると、耐HIC性が低下するので、Mn含有量は1.65%以下とする。好ましくは1.50%以下である。 Mn: 1.05 to 1.65%
Mn is an element that contributes to improving the strength and toughness of steel. If the Mn content is less than 1.05%, the effect of improving strength and toughness cannot be sufficiently obtained. Therefore, the Mn content is set to 1.05% or more. It is preferably at least 1.15%.
On the other hand, Mn is also an element that forms MnS and reduces the HIC resistance. If the Mn content exceeds 1.65%, the HIC resistance decreases, so the Mn content is set to 1.65% or less. Preferably it is 1.50% or less.
Alは、脱酸剤として機能する元素である。Al含有量が0.010%未満であると、この効果が十分に得られない。そのため、Al含有量は0.010%以上とする。好ましくは0.020%以上である。
一方、Al含有量が0.070%を超えると、Al酸化物が集積してクラスターが生成し、耐HIC性が低下する。そのため、Al含有量は0.070%以下とする。好ましくは0.040%以下、より好ましくは0.030%以下である。 Al: 0.010 to 0.070%
Al is an element that functions as a deoxidizing agent. If the Al content is less than 0.010%, this effect cannot be sufficiently obtained. Therefore, the Al content is set to 0.010% or more. Preferably it is 0.020% or more.
On the other hand, if the Al content exceeds 0.070%, Al oxides accumulate to form clusters, and the HIC resistance decreases. Therefore, the Al content is set to 0.070% or less. Preferably it is 0.040% or less, more preferably 0.030% or less.
Tiは、窒化物を形成し、結晶粒の微細化に寄与する元素である。Ti含有量が0.005%未満であると、上記効果が十分に得られない。そのため、Ti含有量は0.005%以上とする。好ましくは0.008%以上である。
一方、Ti含有量が0.020%を超えると、粗大な窒化物が生成し、耐HIC性が低下する。そのため、Ti含有量は0.020%以下とする。好ましくは0.015%以下である。 Ti: 0.005 to 0.020%
Ti is an element that forms a nitride and contributes to refinement of crystal grains. If the Ti content is less than 0.005%, the above effects cannot be sufficiently obtained. Therefore, the Ti content is set to 0.005% or more. Preferably it is 0.008% or more.
On the other hand, if the Ti content exceeds 0.020%, coarse nitrides are generated, and the HIC resistance decreases. Therefore, the Ti content is set to 0.020% or less. Preferably it is 0.015% or less.
Nbは、未再結晶温度域を拡大して結晶粒を微細にするとともに、炭化物や窒化物を形成して、鋼の強度の向上に寄与する元素である。Nb含有量が0.005%未満であると、上記効果が十分に得られない。そのため、Nb含有量は0.005%以上とする。好ましくは0.010%以上である。
一方、Nb含有量が0.045%を超えると、粗大な炭化物や窒化物が生成し、耐HIC性が低下する。また、伸びや靭性も低下する。そのため、Nb含有量は0.045%以下とする。好ましくは0.035%以下である。 Nb: 0.005 to 0.045%
Nb is an element that expands the non-recrystallization temperature range to make crystal grains fine and forms carbides and nitrides, thereby contributing to improvement in the strength of steel. If the Nb content is less than 0.005%, the above effects cannot be sufficiently obtained. Therefore, the Nb content is set to 0.005% or more. Preferably it is 0.010% or more.
On the other hand, when the Nb content exceeds 0.045%, coarse carbides and nitrides are generated, and the HIC resistance decreases. In addition, elongation and toughness are also reduced. Therefore, the Nb content is set to 0.045% or less. Preferably it is 0.035% or less.
Caは、CaSを生成し、圧延方向に伸長するMnSの生成を抑制することによって、耐HIC性の向上に寄与する元素である。Ca含有量が0.0010%未満では、上記効果が十分に得られない。そのため、Ca含有量は0.0010%以上とする。好ましくは0.0020%以上である。
一方、Ca含有量が0.0050%を超えると、Ca酸化物が集積し、耐HIC性が低下する。そのため、Ca含有量は0.0050%以下とする。好ましく0.0040%以下である。 Ca: 0.0010 to 0.0050%
Ca is an element that generates CaS and suppresses the generation of MnS extending in the rolling direction, thereby contributing to an improvement in HIC resistance. If the Ca content is less than 0.0010%, the above effects cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0010% or more. Preferably it is 0.0020% or more.
On the other hand, when the Ca content exceeds 0.0050%, Ca oxide accumulates, and the HIC resistance decreases. Therefore, the Ca content is set to 0.0050% or less. Preferably it is 0.0040% or less.
Nは、加熱時のオーステナイト粒の粗大化を抑制する窒化物を形成することによって、組織の微細化に寄与する元素である。N含有量が0.0010%未満であると、組織微細化効果が十分に得られない。そのため、N含有量は0.0010%以上とする。好ましくは0.0020%以上である。
一方、N含有量が0.0050%を超えると、粗大な窒化物が生成し、耐HIC性が低下する。そのため、N含有量は0.0050%以下とする。好ましくは0.0040%以下である。 N: 0.0010 to 0.0050%
N is an element that contributes to the refinement of the structure by forming a nitride that suppresses coarsening of austenite grains during heating. If the N content is less than 0.0010%, the effect of refining the structure cannot be sufficiently obtained. Therefore, the N content is set to 0.0010% or more. Preferably it is 0.0020% or more.
On the other hand, when the N content exceeds 0.0050%, coarse nitrides are generated, and the HIC resistance is reduced. Therefore, the N content is set to 0.0050% or less. Preferably it is 0.0040% or less.
Niは、鋼の靱性、強度、及び、耐食性の向上に寄与する元素である。Ni含有量が0.05%未満では、上記効果が十分に得られない。そのため、これらの効果を得る場合、Ni含有量は0.05%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Ni含有量が0.50%を超えると、母材部の硬度が248Hvを超え、耐HIC性が劣化する。そのため、含有させる場合でも、Ni含有量は0.50%以下とする。好ましくは0.35%以下である。 Ni: 0 to 0.50%
Ni is an element that contributes to improving the toughness, strength, and corrosion resistance of steel. If the Ni content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, to obtain these effects, the Ni content is preferably set to 0.05% or more. More preferably, it is 0.10% or more.
On the other hand, when the Ni content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even when Ni is contained, the Ni content is set to 0.50% or less. Preferably it is 0.35% or less.
Moは、鋼の焼入れ性の向上に寄与する元素である。Mo含有量が0.05%未満では、上記効果が十分に得られない。そのため、上記効果を得る場合、Mo含有量は0.05%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Mo含有量が0.50%を超えると、母材部の硬度が248Hvを超え、耐HIC性が劣化する。そのため、含有させる場合でも、Mo含有量は0.50%以下とする。好ましくは0.35%以下である。 Mo: 0 to 0.50%
Mo is an element that contributes to improving the hardenability of steel. If the Mo content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Mo content is preferably set to 0.05% or more. More preferably, it is 0.10% or more.
On the other hand, when the Mo content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even when Mo is contained, the Mo content is set to 0.50% or less. Preferably it is 0.35% or less.
Crは、鋼の強度の向上に寄与する元素である。Cr含有量が0.05%未満では、上記効果が十分に得られない。そのため上記効果を得る場合、Cr含有量は0.05%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Cr含有量が0.50%を超えると、強度が上昇しすぎて、靱性が低下する。そのため、含有させる場合でも、Cr含有量は0.50%以下とする。好ましくは0.35%以下である。 Cr: 0 to 0.50%
Cr is an element that contributes to improving the strength of steel. If the Cr content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Cr content is preferably set to 0.05% or more. More preferably, it is 0.10% or more.
On the other hand, if the Cr content exceeds 0.50%, the strength is excessively increased, and the toughness is reduced. Therefore, even when it is contained, the Cr content is set to 0.50% or less. Preferably it is 0.35% or less.
Cuは、鋼の強度の上昇と、耐食性の向上とに寄与する元素である。Cu含有量が0.05%未満では、上記効果が十分に得られない。そのため、上記効果を得る場合、Cu含有量は0.05%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Cu含有量が0.50%を超えると、母材部の最大硬度が248Hvを超え、耐HIC性が劣化する。そのため、含有させる場合でもCu含有量は0.50%以下とする。好ましくは0.35%以下である。 Cu: 0 to 0.50%
Cu is an element that contributes to increasing the strength of steel and improving corrosion resistance. If the Cu content is less than 0.05%, the above effects cannot be sufficiently obtained. Therefore, when the above effects are obtained, the Cu content is preferably set to 0.05% or more. More preferably, it is 0.10% or more.
On the other hand, when the Cu content exceeds 0.50%, the maximum hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, the Cu content is set to 0.50% or less even when it is contained. Preferably it is 0.35% or less.
Vは、炭化物、窒化物を形成し、鋼の強度の向上に寄与する元素である。V含有量が0.010%未満では、上記効果が十分に得られない。そのため、上記効果を得る場合、V含有量は0.010%以上とすることが好ましい。より好ましくは0.030%以上である。
一方、V含有量が0.100%を超えると、鋼の靱性が低下する。そのため、V含有量は0.100%以下とする。好ましくは0.080%以下である。 V: 0 to 0.100%
V is an element that forms carbides and nitrides and contributes to improving the strength of steel. If the V content is less than 0.010%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the V content is preferably set to 0.010% or more. It is more preferably at least 0.030%.
On the other hand, if the V content exceeds 0.100%, the toughness of the steel decreases. Therefore, the V content is set to 0.100% or less. Preferably it is 0.080% or less.
Mgは、結晶粒の粗大化を抑制することによって靭性の向上に寄与する微細な酸化物を形成する元素である。Mg含有量が0.0001%未満では、上記効果が十分に得られない。そのため、上記効果を得る場合、Mg含有量は0.0001%以上とすることが好ましい。より好ましくは0.0010%以上である。
一方、Mg含有量が0.0100%を超えると、酸化物が凝集、粗大化して、耐HIC性や靱性が低下する。そのため、含有させる場合でも、Mg含有量は0.0100%以下とする。好ましくは0.0050%以下である。 Mg: 0 to 0.0100%
Mg is an element that forms a fine oxide that contributes to improvement in toughness by suppressing coarsening of crystal grains. If the Mg content is less than 0.0001%, the above effects cannot be sufficiently obtained. Therefore, when obtaining the above effects, the Mg content is preferably set to 0.0001% or more. More preferably, it is 0.0010% or more.
On the other hand, when the Mg content exceeds 0.0100%, the oxides are agglomerated and coarsened, and the HIC resistance and toughness are reduced. Therefore, even if it is contained, the Mg content is set to 0.0100% or less. Preferably it is 0.0050% or less.
REMは、硫化物系介在物の形態を制御して、靭性の向上に寄与する元素である。REM含有量が0.0001%未満では、上記効果が十分に得られない。そのため、上記効果を得る場合、REM含有量は0.0001%以上とすることが好ましい。より好ましくは0.0010%以上である。
一方、REM含有量が0.0100%を超えると、酸化物が生成して、鋼の清浄度が低下し、その結果、靱性が低下する。そのため、含有させる場合でも、REM含有量は0.0100%以下とする。好ましくは0.0060%以下である。本実施形態において、REMとは、希土類元素を意味し、Sc、Yおよびランタノイドの17元素の総称であり、REM含有量は、これらの17元素の合計含有量を示す。 REM: 0-0.0100%
REM is an element that controls the form of sulfide-based inclusions and contributes to improvement in toughness. If the REM content is less than 0.0001%, the above effects cannot be sufficiently obtained. Therefore, in order to obtain the above effects, the REM content is preferably set to 0.0001% or more. More preferably, it is 0.0010% or more.
On the other hand, if the REM content exceeds 0.0100%, oxides are generated, and the cleanliness of the steel is reduced, and as a result, the toughness is reduced. Therefore, even when it is contained, the REM content is set to 0.0100% or less. Preferably it is 0.0060% or less. In this embodiment, REM means a rare earth element and is a general term for 17 elements of Sc, Y and lanthanoid, and the REM content indicates the total content of these 17 elements.
不純物のうち、P、S、O、Sb、Sn、Co、As、Pb、Bi、Hについては、後述する範囲に制御することが好ましい。 The base material portion (steel plate according to the present embodiment) of the steel pipe according to the present embodiment basically includes the above essential elements, optionally includes the above optional elements, and the balance is composed of Fe and impurities. . In addition, impurities are components that are mixed from raw materials such as ore or scrap or the like from various environments in the manufacturing process when steel products are manufactured industrially, and do not adversely affect the properties of the steel. Means acceptable.
Of the impurities, P, S, O, Sb, Sn, Co, As, Pb, Bi, and H are preferably controlled in a range described later.
Pは、不純物元素である。P含有量が0.015%を超えると、耐HIC性が著しく低下する。そのため、P含有量は0.015%以下とする。好ましくは0.010%以下である。含有量は少ないほど好ましいので、下限は0%を含む。しかしながら、P含有量を0.003%未満に低減すると、製造コストが大幅に上昇する。そのため、0.003%がP含有量の実質的な下限である。 P: 0.015% or less P is an impurity element. If the P content exceeds 0.015%, the HIC resistance is significantly reduced. Therefore, the P content is set to 0.015% or less. Preferably it is 0.010% or less. Since the smaller the content, the better, the lower limit includes 0%. However, reducing the P content to less than 0.003% significantly increases manufacturing costs. Therefore, 0.003% is a substantial lower limit of the P content.
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。S含有量が0.0015%を超えると、耐HIC性が著しく低下する。そのため、S含有量は0.0015%以下とする。好ましくは0.0010%以下である。S含有量は少ないほど好ましいので、下限は0%を含む。しかしながら、S含有量を0.0001%未満に低減すると、製造コストが大幅に上昇する。そのため、0.0001%がS含有量の実質的な下限である。 S: 0.0015% or less S is an element that generates MnS that elongates in the rolling direction during hot rolling and reduces the HIC resistance. If the S content exceeds 0.0015%, the HIC resistance is significantly reduced. Therefore, the S content is set to 0.0015% or less. Preferably it is 0.0010% or less. The lower the S content, the better, so the lower limit contains 0%. However, reducing the S content to less than 0.0001% significantly increases manufacturing costs. Therefore, 0.0001% is a substantial lower limit of the S content.
Oは、脱酸後、鋼中に不可避的に残留する元素である。O含有量が0.0040%を超えると、酸化物が生成して、耐HIC性が低下する。そのため、O含有量は0.0040%以下とする。好ましくは0.0030%以下である。O含有量は少ないほど好ましいので下限は0%を含む。しかしながら、O含有量を0.0010%未満に低減すると、製造コストが大幅に上昇するので、実用鋼板上、0.0010%がO含有量の実質的な下限である。 O: 0.0040% or less O is an element inevitably remaining in steel after deoxidation. If the O content exceeds 0.0040%, an oxide is generated, and the HIC resistance is reduced. Therefore, the O content is set to 0.0040% or less. Preferably it is 0.0030% or less. The lower the O content, the better, so the lower limit contains 0%. However, when the O content is reduced to less than 0.0010%, the production cost is significantly increased. Therefore, 0.0010% is a practical lower limit of the O content in practical steel sheets.
Ceq(炭素当量)は、鋼板の焼入れ性を表示する指標である。本実施形態に係る鋼管において所要の強度を確保するため、下記式(1)で定義するCeqを0.250~0.350とする。
Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/5・・・(1)
ここで、式(1)中の[C]、[Mn]、[Ni]、[Cu]、[Cr]、[Mo]、[V]は、前記鋼板中のC、Mn、Ni、Cu、Cr、Mo、Vの質量%での含有量である。 Ceq: 0.250-0.350
Ceq (carbon equivalent) is an index indicating the hardenability of a steel sheet. In order to secure required strength in the steel pipe according to the present embodiment, Ceq defined by the following equation (1) is set to 0.250 to 0.350.
Ceq = [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 5 (1)
Here, [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in the formula (1) are C, Mn, Ni, Cu, This is the content of Cr, Mo, and V in mass%.
優れた機械特性と耐HIC性とを確保するため、鋼板表面から深さ方向に1.0mm超から板厚中心までの範囲の金属組織(以下、単に「内部金属組織」ということがある。)を、合計面積率で85%以上のグラニュラーベイナイト及びベイナイトの一方又は両方を含む金属組織とする。 Metal structure (internal metal structure) ranging from more than 1.0 mm to the center of the thickness in the depth direction (thickness direction) from the surface of the steel sheet of the base material portion: one of granular bainite and bainite having a total area ratio of 85% or more. Or both, and the area ratio of MA is 1.0% or less. In order to ensure excellent mechanical properties and HIC resistance, the metal structure in the range from more than 1.0 mm to the center of the sheet thickness in the depth direction from the steel sheet surface. (Hereinafter, it may be simply referred to as “internal metal structure.”) Is a metal structure containing one or both of granular bainite and bainite in a total area ratio of 85% or more.
表層部金属組織が、面積率で合計95%以上のグラニュラーベイナイト及び焼戻しベイナイトを含むと、耐SSC性が向上するので、好ましい。 Metal structure (surface layer metal structure) up to 1.0 mm in the depth direction from the surface of the steel sheet: One or both of granular bainite and tempered bainite in an area ratio of 95% or more.
It is preferable that the surface layer metal structure contains granular bainite and tempered bainite in a total area ratio of 95% or more, because the SSC resistance is improved.
また、表層部の組織は、鋼板の表面から0.1mm、0.2mm及び0.5mmの位置を測定し、それぞれの位置での面積率を平均することによって得られる。 The measurement of the area ratio in the metal structure can be obtained by observing the metal structure with a scanning electron microscope at, for example, 1000 times magnification. Since the structure at a position (t / 4) of the plate thickness from the surface of the steel plate indicates a representative structure of the internal metallographic structure, in this embodiment, the t / 4 of the base material (steel plate) of the steel pipe is used. Is observed, and if the structure at t / 4 is the above-described structure, it is determined that the internal metallographic structure is within the above-described range.
The structure of the surface layer portion is obtained by measuring positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel sheet and averaging the area ratio at each position.
焼戻しベイナイトは、ラス形状をしていて、ラス内、ラス境界に炭化物が分散している組織である。
グラニュラーベイナイトは、アシキュラーフェライトとベイナイトとの中間の変態温度で生成し、中間の組織的特徴を有する。具体的には、部分的に旧オーステナイト粒界が見え、粒内に粗いラス組織が存在し、ラス内、ラス間に細かい炭化物およびオーステナイト・マルテンサイト混成物が散在してする部分と、旧オーステナイト粒界が不明瞭で針状または不定形のフェライトの部分が混在する組織である。
フェライトは、粒内の内部微視組織がほとんどなく、粒内が平滑な組織である。光学顕微鏡で観察した場合には、白く見える組織である。
MAは、レペラーエッチにより着色されるので、判別可能である。
図3Aに本実施形態に係る鋼管の母材部である鋼板のt/4の位置における走査型電子顕微鏡で撮像した金属組織の一例を示し、図3Bに本実施形態に係る鋼管の母材部である鋼板の表面0.5mmにおける走査型電子顕微鏡で撮像した金属組織の一例を示す。 In the present embodiment, bainite is a structure in which the former austenite grain boundaries are clear, fine lath structures are developed in the grains, and fine carbides and MA are scattered in the laths and between the laths.
Tempered bainite is a structure having a lath shape in which carbides are dispersed in the lath and at the lath boundary.
Granular bainite is formed at an intermediate transformation temperature between acicular ferrite and bainite and has an intermediate texture. Specifically, old austenite grain boundaries are partially visible, coarse lath structures are present in the grains, and fine carbides and austenitic-martensite hybrids are scattered in and between the laths, and the former austenite This is a structure in which the grain boundaries are unclear and acicular or amorphous ferrite portions are mixed.
Ferrite has almost no internal microstructure in the grains and has a smooth structure in the grains. When observed with an optical microscope, the structure looks white.
The MA is colored by the Repeller etch and can be distinguished.
FIG. 3A shows an example of a metal structure imaged by a scanning electron microscope at a position of t / 4 of a steel plate as a base material of the steel pipe according to the present embodiment, and FIG. 3B shows a base metal part of the steel pipe according to the present embodiment. 1 shows an example of a metal structure imaged by a scanning electron microscope at a surface of a steel sheet of 0.5 mm.
最大硬度:248Hv以下
平均硬度:170~220Hv
本実施形態に係る鋼管では、優れた強度、耐SSC性及び耐HIC性を確保するため、母材部の内部金属組織において、最大硬度を248Hv以下、平均硬度を170~220Hvとする。 Hardness of internal metal structure Maximum hardness: 248 Hv or less Average hardness: 170 to 220 Hv
In the steel pipe according to the present embodiment, in order to secure excellent strength, SSC resistance and HIC resistance, the maximum hardness is 248 Hv or less and the average hardness is 170 to 220 Hv in the internal metal structure of the base metal part.
また、平均硬度が170Hv未満であると、所要の強度を確保できないので、平均硬度は170Hv以上とする。好ましくは180Hv以上である。
一方、平均硬度が220Hvを超えると、耐HIC及び靱性が低下する。そのため、平均硬度は220Hv以下とする。好ましくは210Hv以下である。 If the maximum hardness exceeds 248 Hv, the HIC resistance decreases, so the maximum hardness is set to 248 Hv or less. Preferably it is 230 Hv.
If the average hardness is less than 170 Hv, required strength cannot be secured, so the average hardness is 170 Hv or more. Preferably it is 180 Hv or more.
On the other hand, if the average hardness exceeds 220 Hv, HIC resistance and toughness decrease. Therefore, the average hardness is set to 220 Hv or less. Preferably it is 210 Hv or less.
表層部金属組織の最大硬度が250Hv超であると、耐SSC性が低下する。そのため、表層部金属組織の最大硬度は250Hv以下とする。好ましくは、240Hv以下である。 Maximum hardness of the surface layer metal structure: 250 Hv or less If the maximum hardness of the surface layer metal structure is more than 250 Hv, the SSC resistance decreases. Therefore, the maximum hardness of the surface layer metal structure is set to 250 Hv or less. Preferably, it is 240 Hv or less.
ビッカース硬度計(荷重:100g)で、鋼板の表面から1.1mmの深さ位置を始点として、板厚方向に0.1mm間隔で板厚中心まで、かつ、同一深さについて幅方向1.0mm間隔で20点硬さを測定する。
上記測定の結果、248Hvを超える測定点が板厚方向に2点以上連続して現れなければ、内部金属組織の最大硬度はHv248以下であると判断する。
本実施形態に係る鋼管の母材では、局所的には、介在物等によって高い硬度の値(異常値)が現れる場合がある。しかしながら、介在物は割れの原因とならないので、このような異常値が現れても、耐HIC性、耐SSC性は確保できる。一方、板厚方向に連続して2点以上248Hvを超える測定点が存在する場合、介在物起因ではなく、耐HIC性及び/または耐SSC性が低下するので許容されない。したがって、本実施形態では、248Hvを超える測定点が1点存在しても、板厚方向に2点以上連続して現れなければ、その点は異常点であるとして採用せず、次に高い値を最大硬硬度とする。一方、板厚方向に連続して2点以上248Hvを超える測定点が存在する場合には、それらの最も高い値を最大硬度として採用する。
また、平均硬度は、全ての測定点の硬さを平均して算出する。 The maximum hardness and the average hardness in the internal metal structure can be measured by the following methods.
Using a Vickers hardness tester (load: 100 g), starting at a depth of 1.1 mm from the surface of the steel sheet and extending to the center of the thickness at intervals of 0.1 mm in the thickness direction and 1.0 mm in the width direction for the same depth. Measure hardness at 20 points at intervals.
As a result of the above measurement, if two or more measurement points exceeding 248 Hv do not appear continuously in the thickness direction, it is determined that the maximum hardness of the internal metal structure is Hv 248 or less.
In the base material of the steel pipe according to the present embodiment, a high hardness value (abnormal value) may appear locally due to inclusions or the like. However, since inclusions do not cause cracking, HIC resistance and SSC resistance can be ensured even if such abnormal values appear. On the other hand, if there are two or more measurement points continuously exceeding 248 Hv in the thickness direction, the HIC resistance and / or the SSC resistance are not due to inclusions but are unacceptable. Therefore, in the present embodiment, even if there is one measurement point exceeding 248Hv, if two or more points do not appear continuously in the sheet thickness direction, the point is not adopted as an abnormal point, and is not adopted as an abnormal point. Is the maximum hardness. On the other hand, when there are two or more measurement points continuously exceeding 248 Hv in the thickness direction, the highest value is adopted as the maximum hardness.
The average hardness is calculated by averaging the hardness of all the measurement points.
まず、鋼板の幅方向の端部(鋼管の場合には、突合せ部に相当)から鋼板の幅方向に1/4、1/2及び3/4の位置(鋼管でいうと、溶接部を0時とした場合の、それぞれ3時、6時及び9時の位置)から300mm角(300mm×300mm)の鋼板をガス切断で切り出し、切り出した鋼板の中心から、長さ20mm、幅20mmのブロック試験片を機械切断によって採取し、機械研磨で研磨する。1つのブロック試験片について、ビッカース硬度計(荷重:100g)で、表面から0.1mmを始点として、板厚方向に0.1mm間隔で10点、同一深さについて幅方向1.0mm間隔で10点、合計100点測定する。すなわち、3つのブロック試験片で合計300点測定する。
上記測定の結果、250Hvを超える測定点が板厚方向に2点以上連続して現れなければ、表層部の最大硬度は250Hv以下であると判断する。 The measurement of the maximum hardness of the surface layer metal structure from the surface of the steel sheet to a depth of 1.0 mm is performed as follows.
First, from the end in the width direction of the steel sheet (corresponding to a butt portion in the case of a steel pipe), a position of 4, 及 び, and に in the width direction of the steel sheet (in the case of a steel pipe, the welded portion is 0 mm) In the case where it is time, a 300 mm square (300 mm × 300 mm) steel plate is cut out from the 3 o'clock, 6 o'clock and 9 o'clock positions by gas cutting, and a block test of 20 mm in length and 20 mm in width is performed from the center of the cut out steel plate. Pieces are collected by mechanical cutting and polished by mechanical polishing. Using a Vickers hardness tester (load: 100 g), one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points are measured. That is, a total of 300 points are measured with three block test pieces.
As a result of the above measurement, when two or more measurement points exceeding 250 Hv do not appear continuously in the thickness direction, it is determined that the maximum hardness of the surface layer portion is 250 Hv or less.
本実施形態に係る鋼板は、焼入れ焼戻し処理を行わずに、熱間圧延、冷却、復熱等の工程を経て製造される。そのため、内部金属組織が上記のような集合組織を有する。集合組織を有することで、鋼板のDWTT特性が向上する。
焼入れ焼戻しによって鋼板を製造した場合、及び、焼準によって鋼板を製造した場合には、このような集合組織は得られない。 In a plane parallel to the plate surface at a
Such a texture cannot be obtained when a steel sheet is manufactured by quenching and tempering or when a steel sheet is manufactured by normalizing.
母材部の鋼板の板厚をtとしたとき、表面からt/4の深さにおける板面に平行な面に対して、EBSPを用いて、2.0mm×2.0mmの領域を0.1mm間隔で結晶方位解析を行い、(100)<110>集合組織の集積度を求める。 The texture can be obtained by the following method.
Assuming that the thickness of the steel sheet of the base metal part is t, a 2.0 mm × 2.0 mm area is defined as 0 mm by using EBSP with respect to a plane parallel to the plate surface at a depth of t / 4 from the surface. The crystal orientation analysis is performed at 1 mm intervals to determine the degree of integration of the (100) <110> texture.
本実施形態に係る鋼管は、従来同時に満足させることが難しかった、DWTT特性、耐SSC性、耐HIC特性を備えるように、焼入れ焼戻し処理を行わず(圧延-冷却まま)に製造した、板厚が15mm以下の鋼板を母材部に用いた鋼管である。本実施形態に係る鋼管は、鋼板の板厚が12mm以下であっても、優れた耐SSC性、耐HIC特が得られる。 The thickness of the steel sheet of the base metal part (wall thickness of the steel pipe): 15 mm or less The steel pipe according to the present embodiment is hardened so as to have DWTT characteristics, SSC resistance, and HIC resistance, which have been difficult to satisfy at the same time. This is a steel pipe manufactured without performing a tempering process (rolled and cooled) and using a steel plate having a thickness of 15 mm or less as a base material. The steel pipe according to the present embodiment has excellent SSC resistance and HIC resistance even when the thickness of the steel plate is 12 mm or less.
通常、鋼管溶接において、溶接部は母材部よりも厚みが大きくなるように施工される。また、溶接金属は母材よりも高合金であり、耐食性も高い。そのため、溶接部が破壊の起点になることはほとんどない。したがって、本実施形態に係る鋼管の溶接部は、SAW溶接等で、通常の条件で得られたものであれば、特に限定されない。 The steel pipe according to the present embodiment is obtained by processing the steel plate according to the present embodiment into a tubular shape, butting and welding both ends of the tubular steel plate. Therefore, it has the welding part provided in the butting part of a steel plate and extending in the longitudinal direction of a steel plate.
Usually, in steel pipe welding, the welded portion is constructed so as to be thicker than the base material portion. Further, the weld metal is a higher alloy than the base metal and has high corrosion resistance. Therefore, the weld is rarely the starting point of the destruction. Therefore, the welded portion of the steel pipe according to the present embodiment is not particularly limited as long as it is obtained under normal conditions by SAW welding or the like.
本実施形態に係る鋼管は、製造方法によらず、上述の構成を有していれば、その効果が得られるが、例えば以下のような製造方法によれば、安定して得られるので好ましい。 Next, a preferred method for manufacturing the steel pipe according to the present embodiment will be described.
Regardless of the manufacturing method, the steel pipe according to the present embodiment has the above-described configuration, and the effect can be obtained. However, according to, for example, the following manufacturing method, it is preferable because it can be obtained stably.
(i)所定の化学組成とCeqとを満たす鋼片を、1050~1250℃に加熱して熱間圧延に供し、830~1000℃で仕上げ圧延を終了して、板厚15mm以下の鋼板を得る工程(熱間圧延工程)と、
(ii-1)圧延終了後の鋼板を、750超~950℃から、25~100℃/秒の平均冷却速度で660~750℃の温度域まで冷却する工程(第1の冷却工程)と、
(ii-2)表面温度で、660~750℃の温度域から、50℃/秒超の平均冷却速度で400℃以下まで冷却する工程(第2の冷却工程)と、
(iii)表面温度が550超~650℃に達するまで、50℃/秒以上の復熱速度で復熱させる工程(復熱工程)と、
を含む製造方法によって得られる。
また、本実施形態に係る鋼管は、
(iv)上記(i)~(iii)の工程を経て得られた鋼板を筒状に成形する工程(成形工程)と、
(v)筒状鋼板の両端部を突き合せて溶接する工程(溶接工程)と、
を含む製造方法によって得られる。
上記温度は、表面温度による管理である。
以下、各工程の好ましい条件について説明する。 The steel sheet according to the present embodiment is:
(I) A slab satisfying a predetermined chemical composition and Ceq is heated to 1050 to 1250 ° C. and subjected to hot rolling, and finish rolling at 830 to 1000 ° C. to obtain a steel sheet having a thickness of 15 mm or less. Process (hot rolling process);
(Ii-1) a step of cooling the steel sheet after rolling from above 750 to 950 ° C. to a temperature range of 660 to 750 ° C. at an average cooling rate of 25 to 100 ° C./sec (first cooling step);
(Ii-2) a step of cooling from a temperature range of 660 to 750 ° C. at a surface temperature to 400 ° C. or lower at an average cooling rate of more than 50 ° C./second (second cooling step);
(Iii) a step of recuperating at a recuperation rate of 50 ° C./second or more until the surface temperature reaches more than 550 to 650 ° C. (recuperation step);
Is obtained by a manufacturing method including:
Further, the steel pipe according to the present embodiment is:
(Iv) a step of forming the steel sheet obtained through the steps (i) to (iii) into a cylindrical shape (forming step);
(V) a process of welding both ends of the tubular steel plate by butt welding (welding process);
Is obtained by a manufacturing method including:
The above temperature is managed by the surface temperature.
Hereinafter, preferable conditions of each step will be described.
鋼片加熱温度:1050~1250℃
熱間圧延を行うため、上述した化学組成を有する鋼片を加熱する。鋼片加熱温度が1050℃未満では、未固溶の粗大なNb及びTiの炭窒化物が生成し、耐HIC性が低下する。そのため、鋼片加熱温度は1050℃以上とすることが好ましい。より好ましくは1100℃以上である。
一方、鋼片加熱温度が1250℃を超えると、結晶粒が粗大化し、低温靭性が低下する。そのため、鋼片加熱温度は1250℃以下とすることが好ましい。より好ましくは1200℃以下である。
熱間圧延工程に先立つ溶鋼の鋳造及び鋼片の製造は常法に従って行えばよい。 <Hot rolling process>
Billet heating temperature: 1050 to 1250 ° C
To perform hot rolling, a steel slab having the above-described chemical composition is heated. If the slab heating temperature is less than 1050 ° C., undissolved coarse Nb and Ti carbonitrides are generated, and the HIC resistance is reduced. Therefore, it is preferable that the billet heating temperature be 1050 ° C. or higher. More preferably, the temperature is 1100 ° C. or higher.
On the other hand, if the slab heating temperature exceeds 1250 ° C., the crystal grains become coarse and the low-temperature toughness decreases. Therefore, it is preferable that the billet heating temperature be 1250 ° C. or less. More preferably, it is 1200 ° C or lower.
Casting of molten steel and production of billets prior to the hot rolling step may be performed according to a conventional method.
加熱した鋼片を、熱間圧延して15mm以下の鋼板とする。その際、仕上げ圧延温度を830~1000℃とすることが好ましい。仕上げ圧延温度が830℃未満であると、フェライトが多量に生成し所定の内部金属組織を得ることができなくなることが懸念される。好ましくは、仕上げ圧延温度は850℃以上である。
一方、仕上げ圧延温度が1000℃を超えると、結晶粒が粗大化しDWTT特性などの低温靭性が低下する。また、再結晶、粒成長が生じ、集合組織が得られない。そのため、仕上げ圧延温度は1000℃以下とすることが好ましい。より好ましくは980℃以下である。 Finish rolling temperature: 830 to 1000 ° C
The heated steel slab is hot-rolled to a steel sheet of 15 mm or less. At that time, the finish rolling temperature is preferably set to 830 to 1000 ° C. When the finish rolling temperature is lower than 830 ° C., there is a concern that a large amount of ferrite is generated and a predetermined internal metal structure cannot be obtained. Preferably, the finish rolling temperature is 850 ° C. or higher.
On the other hand, when the finish rolling temperature exceeds 1000 ° C., the crystal grains become coarse, and the low-temperature toughness such as DWTT characteristics is reduced. In addition, recrystallization and grain growth occur, and a texture cannot be obtained. Therefore, the finish rolling temperature is preferably set to 1000 ° C. or less. The temperature is more preferably 980 ° C or lower.
冷却開始温度Ts:750超~950℃
平均冷却速度Vc1:25~100℃/秒
冷却停止温度Tm:660~750℃
圧延終了後の1段目の加速冷却にて、表面温度で、750超~950℃の温度域の温度Ts(冷却開始温度)の鋼板を、平均冷却速度Vc1:25~50℃/秒で、660~750℃の温度域の温度Tm(冷却停止温度)まで冷却する。 <First cooling step>
Cooling start temperature Ts: more than 750 to 950 ° C
Average cooling rate Vc1: 25 to 100 ° C./sec. Cooling stop temperature Tm: 660 to 750 ° C.
In the first-stage accelerated cooling after the end of the rolling, the steel sheet having a surface temperature of Ts (cooling start temperature) in a temperature range of more than 750 to 950 ° C. at an average cooling rate Vc of 25 to 50 ° C./sec. Cool to a temperature Tm (cooling stop temperature) in a temperature range of 660 to 750 ° C.
冷却開始温度Tm:660~750℃
平均冷却速度Vc2:50℃/秒超
冷却停止温度Tf:400℃以下
第2の冷却工程では、1段目の冷却停止温度Tm:660~750℃から、平均冷却速度50℃/秒超で、400℃以下の冷却停止温度Tfまで冷却する。 <Second cooling step>
Cooling start temperature Tm: 660-750 ° C
Average cooling rate Vc2: more than 50 ° C./sec. Cooling stop temperature Tf: 400 ° C. or less In the second cooling step, the first stage cooling stop temperature Tm: 660 to 750 ° C. Cool to a cooling stop temperature Tf of 400 ° C. or less.
冷却速度は、冷却開始温度と冷却停止温度との温度差を、冷却時間で除することで得られる。 As described above, in the method for manufacturing a steel pipe according to the present embodiment, two-stage accelerated cooling with different cooling rates is performed. Such cooling is performed by adjusting the amount of cooling water injected into the steel sheet for each cooling zone in a cooling facility in which the cooling zone is divided into a plurality of pieces in the longitudinal direction (transport direction) of the steel sheet. Can be.
The cooling rate is obtained by dividing the temperature difference between the cooling start temperature and the cooling stop temperature by the cooling time.
復熱速度Vr:50℃/秒以上
復熱後の鋼板表面温度Tr:550超~650℃
上述のように鋼板を400℃以下の冷却停止温度Tfまで加速冷却した後、50℃/秒以上の復熱速度Vrで、鋼板表面温度Trが550超~650℃に達するまで復熱させる。 <Recuperation process>
Heat recovery speed Vr: 50 ° C./sec or more Steel sheet surface temperature after heat recovery Tr: Over 550 to 650 ° C.
After the steel sheet is accelerated and cooled to the cooling stop temperature Tf of 400 ° C. or less as described above, the steel sheet is reheated at a recuperation speed Vr of 50 ° C./sec or more until the steel sheet surface temperature Tr exceeds 550 to 650 ° C.
復熱速度は、復熱温度幅を復熱に要した時間で除することで得られる。 If the recuperation rate Vr is less than 50 ° C./sec, there is a concern that the surface layer is hardened and the SSC resistance is reduced. Therefore, the recuperation rate is set to 50 ° C./sec or more. The recuperation rate may be appropriately set in consideration of the time required for the surface temperature of the steel sheet to exceed 550 to 650 ° C., and the upper limit is not particularly limited.
The recuperation speed is obtained by dividing the recuperation temperature width by the time required for recuperation.
上記工程によって、本実施形態に係る鋼管の母材部に使用される鋼板が製造できる。すなわち、本実施形態に係る鋼板は、非調質鋼である。 The steel sheet after the recuperation step is preferably cooled to an average cooling rate of 0.01 ° C./sec or more and 300 ° C. or less. If the average cooling rate is less than 0.01 ° C./sec, the desired strength cannot be obtained.
Through the above steps, a steel plate used for the base material portion of the steel pipe according to the present embodiment can be manufactured. That is, the steel sheet according to the present embodiment is a non-heat treated steel.
<溶接工程>
上記工程で得られた本実施形態に係る鋼板を、筒状に成形し、筒状鋼板の突合せ部(鋼板の幅方向両端部)を溶接して鋼管とする。
本実施形態に係る鋼板の鋼管への成形は、特定の成形に限定されない。温間加工でもよいが、寸法精度の点で、冷間加工が好ましい。溶接も、特定の溶接に限定されないが、サブマージドアーク溶接が好ましい。溶接条件は、鋼板の厚み等に応じて、公知の条件とすればよい。 <Molding process>
<Welding process>
The steel sheet according to the present embodiment obtained in the above process is formed into a tubular shape, and butted portions (both ends in the width direction of the steel plate) of the tubular steel plate are welded to form a steel pipe.
The forming of the steel sheet according to the present embodiment into a steel pipe is not limited to a specific forming. Although warm working may be performed, cold working is preferred in terms of dimensional accuracy. The welding is not limited to a specific welding, but a submerged arc welding is preferable. The welding condition may be a known condition according to the thickness of the steel sheet or the like.
また、表層部金属組織は、鋼板の表面から0.1mm、0.2mm及び0.5mmの位置を観察、測定し、それぞれの位置での面積率を平均することによって得た。
また、JIS5号引張試験片を作製し、JIS Z 2241に規定の引張試験を行い、降伏強度と引張強度を測定した。 A test piece was sampled from the manufactured steel sheet, and the internal metallographic structure was observed at a magnification of 1000 times from the surface of the steel sheet at a position (t / 4) 1/4 of the plate thickness using a scanning electron microscope. I decided.
Further, the surface layer metal structure was obtained by observing and measuring 0.1 mm, 0.2 mm, and 0.5 mm positions from the surface of the steel sheet and averaging the area ratio at each position.
Further, a JIS No. 5 tensile test piece was prepared, a tensile test specified in JIS Z 2241 was performed, and the yield strength and the tensile strength were measured.
内部金属組織については、ビッカース硬度計(荷重:100g)で、鋼板の表面から1.1mmの深さ位置を始点として、板厚方向に0.1mm間隔で板厚中心まで、かつ、同一深さについて幅方向1.0mm間隔で20点硬さを測定した。上記測定の結果、248Hvを超える測定点が1点存在しても、板厚方向に2点以上連続して現れなければ、その点は異常点であるとして、次に高い値を最大硬硬度とした。一方、板厚方向に連続して2点以上248Hvを超える測定点が存在する場合には、それらの最も高い値を最大硬度とした。また、平均硬度は、全ての測定点の硬さを平均して算出した。
表層部金属組織においては、鋼板の幅方向の端部から300mm角(300mm×300mm)の鋼板をガス切断で切り出し、切り出した鋼板の中心から、長さ20mm、幅20mmのブロック試験片を機械切断によって採取し、機械研磨で研磨する。1つのブロック試験片について、ビッカース硬度計(荷重:100g)で、表面から0.1mmを始点として、板厚方向に0.1mm間隔で10点、同一深さについて幅方向1.0mm間隔で10点、合計100点測定した。すなわち、3つのブロック試験片で合計300点測定した。上記測定の結果、250Hvを超える測定点が1点存在しても、板厚方向に2点以上連続して現れなければ、その点は異常点であるとして、次に高い値を最大硬硬度とした。一方、板厚方向に連続して2点以上250Hvを超える測定点が存在する場合には、それらの最も高い値を最大硬度とした。 The hardness of the internal metal structure and the surface metal structure was measured with a Vickers hardness tester.
Regarding the internal metallographic structure, using a Vickers hardness tester (load: 100 g), starting at a depth of 1.1 mm from the surface of the steel plate, starting at a depth of 1.1 mm and extending in the thickness direction at intervals of 0.1 mm to the center of the thickness and at the same depth. Was measured for hardness at 20 points at intervals of 1.0 mm in the width direction. As a result of the measurement, even if there is one measurement point exceeding 248 Hv, if two or more points do not appear continuously in the thickness direction, the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did. On the other hand, when there are two or more measurement points continuously exceeding 248 Hv in the plate thickness direction, the highest value is defined as the maximum hardness. The average hardness was calculated by averaging the hardness of all the measurement points.
In the surface layer metal structure, a 300 mm square (300 mm × 300 mm) steel sheet is cut out from the end in the width direction of the steel sheet by gas cutting, and a block test piece having a length of 20 mm and a width of 20 mm is mechanically cut from the center of the cut steel sheet. And polished by mechanical polishing. Using a Vickers hardness tester (load: 100 g), one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points were measured. That is, a total of 300 points were measured with three block test pieces. As a result of the above measurement, even if there is one measurement point exceeding 250 Hv, if two or more points do not appear continuously in the thickness direction, the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did. On the other hand, when there are two or more measurement points continuously exceeding 250 Hv in the plate thickness direction, the highest value is defined as the maximum hardness.
鋼板から、鋼板の幅方向が試験片の長手方向と平行となるようにDWTT試験片を採取した。採取位置は、鋼板の幅方向1/4位置とした。DWTT試験片は、プレスノッチ付の全厚試験片とした。
この試験片に対し、API 5Lに準拠して、-30℃でDWTT試験を行い、破面全体に占める延性破面率を測定した。破面率(%)の数値が高いほど、DWTT特性に優れることを示す。本発明では延性破面率が85%以上である場合にDWTT特性に優れると判断した。 The DWTT property (ductile fracture rate at −30 ° C.) was evaluated by the following method.
DWTT test pieces were sampled from the steel sheet so that the width direction of the steel sheet was parallel to the longitudinal direction of the test piece. The sampling position was 1/4 position in the width direction of the steel sheet. The DWTT test piece was a full thickness test piece with a press notch.
The test piece was subjected to a DWTT test at −30 ° C. in accordance with API 5L, and the ductile fracture ratio occupying the entire fracture surface was measured. The higher the numerical value of the fracture surface ratio (%), the better the DWTT characteristics. In the present invention, it was determined that the DWTT characteristics were excellent when the ductile fracture ratio was 85% or more.
表では、鋼板No.S-x(x=1~54)を成形したものを、鋼管No.P-x(x=1~54)とした。 The steel sheets shown in Tables 1 to 3 were formed into a tube by C press, U press, and O press, and the end faces were tack-welded, and the main and inner surfaces were welded. . In addition, submerged arc welding was applied to the main welding.
In the table, the steel sheet No. Sx (x = 1 to 54) was molded into a steel pipe No. Px (x = 1 to 54).
また、JIS5号引張試験片を作製し、JISZ 2241に規定の引張試験を行い、降伏強度と引張強度を測定した。 A test piece was collected from the base material of the manufactured steel pipe, and the fraction (area ratio) of each of the surface layer metal structure and the internal metal structure was calculated. Specifically, the internal metallographic structure was determined by observing the structure at a position (t / 4) 1/4 of the plate thickness from the surface of the steel plate using a scanning electron microscope at a magnification of 1000 times. The remaining structure not described in the table was ferrite. The surface layer metal structure was obtained by measuring positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel sheet and averaging the area ratio at each position.
Further, a JIS No. 5 tensile test piece was prepared, a tensile test specified in JISZ2241 was performed, and the yield strength and the tensile strength were measured.
内部金属組織については、ビッカース硬度計(荷重:100g)で、鋼板の表面から1.1mmの深さ位置を始点として、板厚方向に0.1mm間隔で板厚中心まで、かつ、同一深さについて幅方向1.0mm間隔で20点硬さを測定した。上記測定の結果、248Hvを超える測定点が1点存在しても、板厚方向に2点以上連続して現れなければ、その点は異常点であるとして、次に高い値を最大硬硬度とした。一方、板厚方向に連続して2点以上248Hvを超える測定点が存在する場合には、それらの最も高い値を最大硬度とした。また、平均硬度は、全ての測定点の硬さを平均して算出した。
表層部金属組織においては、鋼管の突合せ部から溶接部を0時とした場合の、それぞれ3時、6時及び9時の位置から300mm角(300mm×300mm)の鋼板をガス切断で切り出し、切り出した鋼板の中心から、長さ20mm、幅20mmのブロック試験片を機械切断によって採取し、機械研磨で研磨する。1つのブロック試験片について、ビッカース硬度計(荷重:100g)で、表面から0.1mmを始点として、板厚方向に0.1mm間隔で10点、同一深さについて幅方向1.0mm間隔で10点、合計100点測定した。すなわち、3つのブロック試験片で合計300点測定した。上記測定の結果、250Hvを超える測定点が1点存在しても、板厚方向に2点以上連続して現れなければ、その点は異常点であるとして、次に高い値を最大硬硬度とした。一方、板厚方向に連続して2点以上250Hvを超える測定点が存在する場合には、それらの最も高い値を最大硬度とした。 The hardness of the internal metal structure and the surface metal structure was measured with a Vickers hardness tester.
Regarding the internal metallographic structure, using a Vickers hardness tester (load: 100 g), starting at a depth of 1.1 mm from the surface of the steel plate, starting at a depth of 1.1 mm and extending in the thickness direction at intervals of 0.1 mm to the center of the thickness and at the same depth. Was measured for hardness at 20 points at intervals of 1.0 mm in the width direction. As a result of the measurement, even if there is one measurement point exceeding 248 Hv, if two or more points do not appear continuously in the thickness direction, the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did. On the other hand, when there are two or more measurement points continuously exceeding 248 Hv in the plate thickness direction, the highest value is defined as the maximum hardness. The average hardness was calculated by averaging the hardness of all the measurement points.
In the surface layer metal structure, a 300 mm square (300 mm × 300 mm) steel plate is cut out from the positions of 3 o'clock, 6 o'clock, and 9 o'clock when the welded portion is set to 0 o'clock from the butt portion of the steel pipe by gas cutting. From the center of the steel plate, a block test piece having a length of 20 mm and a width of 20 mm is collected by mechanical cutting and polished by mechanical polishing. Using a Vickers hardness tester (load: 100 g), one block test piece was measured at a point of 0.1 mm from the surface as a starting point, at 10 points at intervals of 0.1 mm in the thickness direction, and at the same depth at intervals of 1.0 mm in the width direction. Points, a total of 100 points were measured. That is, a total of 300 points were measured with three block test pieces. As a result of the above measurement, even if there is one measurement point exceeding 250 Hv, if two or more points do not appear continuously in the thickness direction, the point is regarded as an abnormal point, and the next highest value is regarded as the maximum hardness. did. On the other hand, when there are two or more measurement points continuously exceeding 250 Hv in the plate thickness direction, the highest value is defined as the maximum hardness.
NACE(National Association of Corrosion and Engineer)のTM0284に準拠した試験を行い、HIC(水素誘起割れ)の発生の有無を観察し、HIC破面率が5%以下の場合を、耐HIC性が優れている(OK)と評価し、HIC破面率が5%超の場合を耐HIC性が劣っている(NG)と評価した。 Evaluation of HIC resistance A test based on NACE (National Association of Corrosion and Engineer) TM0284 was performed, and the presence or absence of HIC (hydrogen induced cracking) was observed. The case where the HIC property was excellent (OK) was evaluated, and the case where the HIC fracture ratio was more than 5% was evaluated as poor (NG) when the HIC resistance was poor.
鋼管から、鋼管の周方向が試験片の長手方向と平行となるようにDWTT試験片を採取した。採取位置は、鋼管のシーム位置から90°位置とした。ここで、DWTT試験片は、プレスノッチ付の全厚試験片とした。
この試験片に対し、API 5Lに準拠して、-30℃でDWTT試験を行い、破面全体に占める延性破面率を測定した。破面率(%)の数値が高いほど、DWTT特性に優れることを示す。本発明では延性破面率が85%以上である場合にDWTT特性に優れると判断した。 The DWTT characteristics (ductile fracture rate at −30 ° C.) were evaluated by the following method.
DWTT test pieces were sampled from the steel pipe such that the circumferential direction of the steel pipe was parallel to the longitudinal direction of the test piece. The sampling position was 90 ° from the seam position of the steel pipe. Here, the DWTT test piece was a full thickness test piece with a press notch.
The test piece was subjected to a DWTT test at −30 ° C. in accordance with API 5L, and the ductile fracture ratio occupying the entire fracture surface was measured. The higher the numerical value of the fracture surface ratio (%), the better the DWTT characteristics. In the present invention, it was determined that the DWTT characteristics were excellent when the ductile fracture ratio was 85% or more.
Claims (4)
- 筒状の鋼板からなる母材部と、
前記鋼板の突合せ部に設けられ、前記鋼板の長手方向に延在する溶接部と、
を有し、
前記鋼板は、化学組成として、質量%で、
C :0.030~0.070%、
Si:0.05~0.50%、
Mn:1.05~1.65%、
Al:0.010~0.070%、
Ti:0.005~0.020%、
Nb:0.005~0.045%、
Ca:0.0010~0.0050%、
N :0.0010~0.0050%、
Ni:0~0.50%、
Mo:0~0.50%、
Cr:0~0.50%、
Cu:0~0.50%、
V :0~0.100%、
Mg:0~0.0100%、
REM:0~0.0100%、
を含み、
P :0.015%以下、
S :0.0015%以下、
O :0.0040%以下、
に制限し、
残部:Fe及び不純物からなり、
前記鋼板は、下記式(1)で定義するCeqが0.250~0.350であり、
前記母材部の表面から深さ方向に1.0mm超から板厚中心までの範囲の金属組織である内部金属組織が、グラニュラーベイナイト及びベイナイトの一方又は両方を合計面積率で85%以上含み、かつ、MAを面積率で1.0%以下含み、
前記内部金属組織において、最大硬度が248Hv以下、かつ平均硬度が170~220Hvであり、
前記母材部が、前記表面から板厚方向に板厚の1/4の位置の板面に平行な面において{100}<110>の集積度が1.5以上である集合組織を有し、
前記母材部の前記表面から前記深さ方向に1.0mmまでの範囲の金属組織である表層部金属組織が、グラニュラーベイナイト及び焼戻しベイナイトの一方または両方を合計面積率で95%以上含み、
前記表層部金属組織の最大硬度が250Hv以下であり、
前記鋼板の板厚が15mm以下である
ことを特徴とする鋼管。
Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/15・・・(1)
式(1)中の[C]、[Mn]、[Ni]、[Cu]、[Cr]、[Mo]、[V]は、前記鋼板中のC、Mn、Ni、Cu、Cr、Mo、Vの質量%での含有量である。 A base member made of a tubular steel plate;
A welded portion provided at a butt portion of the steel plate and extending in a longitudinal direction of the steel plate;
Has,
The steel sheet has a chemical composition in mass%,
C: 0.030 to 0.070%,
Si: 0.05 to 0.50%,
Mn: 1.05 to 1.65%,
Al: 0.010 to 0.070%,
Ti: 0.005 to 0.020%,
Nb: 0.005 to 0.045%,
Ca: 0.0010 to 0.0050%,
N: 0.0010 to 0.0050%,
Ni: 0 to 0.50%,
Mo: 0 to 0.50%,
Cr: 0 to 0.50%,
Cu: 0 to 0.50%,
V: 0 to 0.100%,
Mg: 0 to 0.0100%,
REM: 0-0.0100%,
Including
P: 0.015% or less,
S: 0.0015% or less,
O: 0.0040% or less,
Limited to
The balance is composed of Fe and impurities,
The steel sheet has a Ceq defined by the following formula (1) of 0.250 to 0.350,
An internal metal structure that is a metal structure in a range from more than 1.0 mm to a plate thickness center in a depth direction from a surface of the base material portion includes one or both of granular bainite and bainite in a total area ratio of 85% or more, And containing MA of 1.0% or less in area ratio,
In the internal metal structure, the maximum hardness is 248 Hv or less, and the average hardness is 170 to 220 Hv;
The base material portion has a texture in which the degree of integration of {100} <110> is 1.5 or more on a plane parallel to the plate surface at a position 1 / of the plate thickness in the plate thickness direction from the surface. ,
A surface layer metal structure that is a metal structure ranging from the surface of the base material portion to 1.0 mm in the depth direction includes one or both of granular bainite and tempered bainite in a total area ratio of 95% or more,
The maximum hardness of the surface layer metal structure is 250 Hv or less,
A steel pipe having a thickness of 15 mm or less.
Ceq = [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 15 (1)
[C], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] in the formula (1) represent C, Mn, Ni, Cu, Cr and Mo in the steel sheet. , V in mass%. - 前記化学組成が、質量%で、
Ni:0.05~0.50%、
Mo:0.05~0.50%、
Cr:0.05~0.50%、
Cu:0.05~0.50%、
V :0.010~0.100%、
Mg:0.0001~0.0100%、
REM:0.0001~0.0100%、
からなる群から選択される1種又は2種以上を含む
ことを特徴とする請求項1に記載の鋼管。 The chemical composition is, in mass%,
Ni: 0.05 to 0.50%,
Mo: 0.05 to 0.50%,
Cr: 0.05 to 0.50%,
Cu: 0.05 to 0.50%,
V: 0.010 to 0.100%,
Mg: 0.0001 to 0.0100%,
REM: 0.0001-0.0100%,
The steel pipe according to claim 1, wherein the steel pipe comprises one or more kinds selected from the group consisting of: - 前記内部金属組織の残部が、フェライトからなることを特徴とする請求項1又は2に記載の鋼管。 The steel pipe according to claim 1 or 2, wherein the remainder of the internal metal structure is made of ferrite.
- 請求項1~3のいずれか一項に記載の鋼管の前記母材部に用いることを特徴とする鋼板。 (4) A steel sheet used for the base material portion of the steel pipe according to any one of (1) to (3).
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JP2018556509A JP6460297B1 (en) | 2018-06-29 | 2018-06-29 | Steel pipe and steel plate |
CN201880094993.XA CN112313357B (en) | 2018-06-29 | 2018-06-29 | Steel pipe and steel plate |
EP18923989.0A EP3816311B1 (en) | 2018-06-29 | 2018-06-29 | Steel pipe and steel sheet |
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JP (1) | JP6460297B1 (en) |
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JP7396551B1 (en) | 2022-06-21 | 2023-12-12 | Jfeスチール株式会社 | High-strength steel plate for sour-resistant line pipe and its manufacturing method, and high-strength steel pipe using high-strength steel plate for sour-resistant line pipe |
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