JP6245139B2 - Thick steel plate for sour welded steel pipe excellent in scale peel resistance, method for producing the same, and welded steel pipe - Google Patents

Description

  The present invention is a steel plate that is a suitable material for a welded steel pipe used as a process pipe installed in a line pipe for transporting natural gas, oil, or the like that requires HIC resistance and SSC resistance, or an oil refining plant. And a manufacturing method thereof, and a welded steel pipe made of the thick steel plate.

Welded steel pipes used for process pipes installed in various plants such as natural gas and oil, or oil refining plants, form thick steel plates by various methods such as UOE, press bend, and 3-roll bend. However, it is manufactured by welding straight seams (so-called straight seams). These straight seam welded steel pipes are formed without removing the scale generated on the surface of the thick steel plate (hereinafter referred to as descaling). However, because the scale formed on the surface of the thick steel plate is easy to peel off, when it is formed without descaling, the scale peels off due to the deformation of the thick steel plate,
(a) The scale is pressed against the thick steel plate to generate surface defects (for example, indentation defects).
(b) The marking printed on the scale disappears.
(c) When welding a seam, a problem arises in that a scale is involved and welding defects (for example, blow) occur.

So to solve these problems,
(x) Remove the scales that have been peeled off during the thick steel plate forming process.
(y) Build a traceability system that enables tracking without relying on marking,
(z) Techniques such as washing the formed thick steel plate before welding are being studied. However, both cause a significant increase in the manufacturing cost of welded steel pipes and a significant decrease in productivity.

  Therefore, the above-described (a) to (c) can be achieved by making the scale difficult to peel (hereinafter also referred to as improving the scale peel resistance) while suppressing an increase in the manufacturing cost of the welded steel pipe and a decrease in productivity. There is a need for technology that can solve these problems.

  On the other hand, line pipes and process pipes that may be exposed to a wet hydrogen sulfide environment are required to have a characteristic that combines HIC resistance and SSC resistance (hereinafter referred to as sour resistance). Therefore, by adjusting the start temperature and cooling rate of accelerated cooling performed after hot rolling in the manufacturing process of the thick steel plate used as the material of the welded steel pipe (hereinafter referred to as sour-resistant welded steel pipe) used for those applications. It is necessary to increase the sour resistance by forming an appropriate microstructure.

  However, by performing accelerated cooling, a tendency is observed that the scale peel resistance of the thick steel plate deteriorates (that is, the scale is easily peeled). In view of this, a technique for achieving both scale peeling resistance and sour resistance has been studied.

  For example, Patent Document 1 discloses a technique for preventing the scale from peeling from a thick steel plate by adding Si in an amount of 0.50% by mass or more. However, in this technique, Si facilitates the formation of island martensite in the weld heat affected zone. As a result, the obtained welded steel pipe is not necessarily suitable as a line pipe or a process pipe.

In Patent Documents 2 and 3, the ratio of the cooling time to the total rolling time is increased and the scale is thinned to increase the ratio of Fe ₃ O ₄ in the scale so that the scale is in close contact with the thick steel plate. It is disclosed. However, in this technique, the surface temperature of the thick steel plate is excessively lowered, so that a ferrite phase is easily generated on the surface layer. As a result, a significant improvement in sour resistance (particularly HIC resistance) of the thick steel sheet cannot be expected.

  Patent Document 4 discloses a technique for suppressing the generation of scale by heating in a non-oxidizing heating furnace before hot rolling, and a technique for performing descaling using shot blasting. However, since this technique manufactures a thick steel plate through a non-oxidation heating furnace and shot blasting, the manufacturing process becomes complicated, resulting in an increase in manufacturing cost and a decrease in productivity.

  Patent Document 5 discloses a technique for generating a black scale that is difficult to peel off by optimizing descaling conditions and adjusting the surface roughness of a thick steel plate. However, with this technique, it is difficult to obtain good scale peeling resistance in severe bending processes such as the formation of welded steel pipes.

  Patent Document 6 discloses a technique for making separation difficult by adjusting the color tone of a scale. However, with this technique, it is difficult to obtain good scale peeling resistance in severe bending processes such as the formation of welded steel pipes.

  Patent Document 7 discloses a technique in which a scale is brought into close contact with a thick steel plate by appropriately performing heating before hot rolling and cooling after hot rolling. However, with this technique, it is necessary to perform slow cooling, which leads to a decrease in productivity. In addition, when producing a high-strength material by performing accelerated cooling, it becomes difficult to perform slow cooling under appropriate conditions after stopping the accelerated cooling, and as a result, good scale peel resistance can be obtained. Becomes difficult.

  Patent Document 8 discloses a technique for generating a scale that hardly peels by starting accelerated cooling immediately after hot rolling is finished and stopping accelerated cooling at a predetermined temperature. However, with this technique, since the temperature at which accelerated cooling is stopped is high, the microstructure becomes uneven, making it difficult to obtain good sour resistance (particularly HIC resistance).

In Patent Document 9, after hot rolling is completed, molten salt is injected immediately to cool the thick steel plate, thereby suppressing variation in scale thickness and increasing the ratio of Fe ₃ O _{4 in} the scale. And the technique which makes a scale contact | adhere to a thick steel plate is disclosed. However, in this technique, it is necessary to install a device for injecting the molten salt, which causes an increase in manufacturing cost.

  Patent Document 10 discloses a technique for optimizing the rolling end temperature, the cooling condition, and the descaling condition in hot rolling so that the scale is in close contact with the thick steel plate. However, in this technique, since the rolling end temperature is low, a ferrite phase is easily generated on the surface layer, and good sour resistance (particularly HIC resistance) cannot be ensured.

  Patent Document 11 discloses a technique for optimizing the rolling reduction per pass, the rolling end temperature, and descaling conditions in hot rolling, and bringing the scale into close contact with the thick steel plate. However, in this technique, since the rolling end temperature is low, a ferrite phase is easily generated on the surface layer, and it becomes difficult to obtain good sour resistance (particularly, HIC resistance).

  Patent Document 12 discloses a technique in which, after hot rolling is completed, a scale is brought into close contact with a thick steel plate by charging into a heat-retaining furnace. However, in this technique, since a thick steel plate is manufactured through a heat-retaining furnace, the manufacturing process becomes complicated, resulting in an increase in manufacturing cost and a decrease in productivity.

  Patent Document 13 discloses a technique for optimizing the rolling reduction ratio, rolling end temperature, and cooling conditions in hot rolling, and bringing the scale into close contact with the thick steel plate. However, with this technique, it is necessary to perform slow cooling, which leads to a decrease in productivity. In addition, when producing a high-strength material by performing accelerated cooling, it becomes difficult to perform slow cooling under appropriate conditions after stopping the accelerated cooling, and as a result, good scale peel resistance can be obtained. Becomes difficult.

  Patent Document 14 discloses a technique for suppressing the generation of scale by performing descaling in advance of heating prior to hot rolling and further applying an antioxidant. However, in this technique, it is necessary to install a device for applying the antioxidant, which causes an increase in manufacturing cost.

  Patent Document 15 discloses a technique for optimizing the descaling and cooling conditions in hot rolling and bringing the scale into close contact with a thick steel plate. However, with this technique, it is necessary to perform slow cooling, which leads to a decrease in productivity. In addition, when producing a high-strength material by performing accelerated cooling, it becomes difficult to perform slow cooling under appropriate conditions after stopping the accelerated cooling, and as a result, good scale peel resistance can be obtained. Becomes difficult.

  Patent Document 16 discloses a technique for optimizing the heating conditions before hot rolling and the conditions of reduction and accelerated cooling in hot rolling so that the scale is in close contact with the thick steel plate. However, with this technique, since the temperature at which accelerated cooling is stopped is high, the microstructure becomes uneven, making it difficult to obtain good sour resistance (particularly HIC resistance).

  Patent Document 17 discloses a technique for optimizing heating conditions before hot rolling and conditions of reduction and accelerated cooling in hot rolling so that the scale is in close contact with the thick steel plate. However, with this technique, it is difficult to obtain good scale peeling resistance in severe bending processes such as the formation of welded steel pipes.

  Patent Document 18 discloses a technique in which a scale is brought into close contact with a thick steel plate by repeating a thermal cycle of cooling and recuperation two or more times after hot rolling is completed. However, in this technique, a thick steel plate is manufactured through a thermal cycle, so that the manufacturing process becomes complicated, resulting in an increase in manufacturing cost and a decrease in productivity.

  Patent Document 19 discloses a technology for suppressing scale generation by performing descaling after hot rolling is completed and further performing accelerated cooling. However, in this technique, since it is necessary to install equipment for performing descaling between hot rolling and accelerated cooling, the manufacturing cost increases.

  In Patent Document 20, the scale is thickened by optimizing the average thickness of the scale, the thickness of the subscale layer formed between the scale and the steel plate, and the Cu concentration and Ni concentration of the subscale layer. A technique for closely contacting a steel sheet is disclosed. However, in this technique, accelerated cooling is not performed, so that a ferrite phase is easily generated on the surface layer, and it becomes difficult to obtain good sour resistance (particularly, HIC resistance). Further, since accelerated cooling is not performed, it is difficult to manufacture a high strength material.

  Patent Document 21 discloses a technique for bringing a scale into close contact with a thick steel plate by reducing the thickness and porosity of the scale and the interface peeling rate between the scale and the thick steel plate. However, in this technique, since it is necessary to install a device for adjusting the porosity and the interface peeling rate, the manufacturing cost increases.

  That is, a technique for obtaining a thick steel plate having both excellent scale peeling resistance and sour resistance while suppressing an increase in manufacturing cost and a decrease in productivity has not yet been established.

JP-A-5-39523 Japanese Patent Laid-Open No. 5-195055 JP-A-6-73504 JP-A-62-262243 Japanese Unexamined Patent Publication No. 7-252593 JP-A-9-87799 Japanese Patent Laid-Open No. 9-209036 Japanese Patent Laid-Open No. 9-239428 JP-A-9-271806 Japanese Patent Laid-Open No. 9-272917 JP-A-9-272918 Japanese Patent Laid-Open No. 11-117018 Japanese Patent Laid-Open No. 11-277105 JP 2000-290727 A JP 2003-231918 A JP 2004-204346 A JP 2006-249469 A JP 2009-203532 A JP 2010-99725 A JP 2013-82979 A Japanese Unexamined Patent Publication No. 2014-4610

  The present invention eliminates the problems of the prior art, a steel plate having excellent scale peeling resistance and sour resistance, a manufacturing method for efficiently producing the steel plate at low cost, and the steel plate It aims at providing the welded steel pipe which becomes.

The present inventor, since the thick steel plate that is the material of the sour-resistant welded steel pipe naturally requires sour resistance, earnestly researches on the technology to improve the sour resistance of the thick steel plate,
(A) It is important to make the microstructure uniform, Pcm value is 0.11 or more,
(B) It is important to reduce the center segregation, PHIC value is 0.95 or less,
(C) It is important to suppress MnS crystallization caused by center segregation, and the ACRM value is 0 or more.
(D) It was important to suppress the formation of Ca-based clusters in the vicinity of the surface layer, and it was found that it was effective to set the ratio of calcium content to oxygen content to 2.5 or less.

In addition, about technology to improve scale peel resistance, especially technology to suppress scale peeling when forming thick steel plates in the manufacturing process of straight seam sour welded steel pipes,
(E) In order to improve the scale peel resistance, the average grade evaluated by the cross cut method defined by JIS standard K5600-5-6 is 2 or less.
(F) In order to make the average grade evaluated by the cross-cut method 2 or less, it was found that it is effective to appropriately control the rolling end temperature within the range of Ar3 point to FT value.

  The present invention has been made based on these findings.

That is, the present invention, C: 0.02 to 0.08 wt%, Si: 0.45 wt% or less, Mn: 1.00-1.70 wt%, Al: 0.001-0.060 mass%, Ca: 0.0010 to .0060 containing mass%, further Cu : 0.50 mass % or less, Ni: 1.00 mass % or less, Cr: 0.50 mass % or less, Mo: 0.50 mass % or less, Nb: 0.06 mass% or less, V: 0.10 mass% or less, Ti: 0.03 mass% or less, B: It contains one or more selected from 0.0030 mass% or less, the balance consists of Fe and inevitable impurities, and the Pcm value calculated by the following formula (1) is 0.110 or more, calculated by formula (2) The PHIC value is 0.95 or less, the ACRM value calculated by the formula (3) is 0 or more, and the ratio of the calcium content and the oxygen content calculated by the formula (4) is 2.5 or less, and the bainite phase Has a structure with an area ratio of 95% or more, the average thickness of the scale formed on the surface is more than 10μm and less than 40μm, and the scale peelability is specified by JIS standard K5600-5-6 The average grade was evaluated by the cross-cut method is a steel plate for excellent sour welded steel pipe resistance to scale peeling resistance of 2 or less.
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20) + ([% Ni] / 60) + ([% Cr] / 20 ) + ([% Mo] / 15) + ([% V] / 10) +5 [% B] (1)
PHIC = 4.46 [% C] + (2.37 [% Mn] / 6) + (1.74 [% Cu] +1.7 [% Ni]) / 15+ (1.18 [% Cr] +1.95 [% Mo] +1.74 [% V]) / 5 + 22.36 [% P] (2)
ACRM = [[% Ca] − (1.23 [% O] −0.000365)] / (1.25 [% S]) (3)
Ratio of calcium content to oxygen content = [% Ca] / [% O] (4)
Where [% C], [% Si], [% Mn], [% Cu], [% Ni], [% Cr], [% Mo], [% V], [% B], [% [P], [% Ca], [% O] and [% S] are the contents (mass) of C, Si, Mn, Cu, Ni, Cr, Mo, V, B, P, Ca, O and S, respectively. %), And zero if not contained.

In addition, the present invention heats a steel slab having the above composition to 1000 to 1250 ° C., performs high-pressure water on the steel slab and performs descaling to remove scales on both sides, and then performs hot processing with N passes. Rolling is performed, and descaling is performed twice or more during the rolling pass from the (N-4) th pass to the final Nth pass of hot rolling, and below the FT value calculated by the following equation (5) In addition, after the hot rolling is completed at a temperature higher than the Ar3 point, the accelerated cooling is performed at a cooling rate of 10 ° C / second or higher from the temperature higher than the Ar3 point to less than 600 ° C. This is a method for producing a thick steel plate for a sour-resistant welded steel pipe having excellent properties.
FT = 845− [500/3] × [(1.5 [% Si] −2 [% Cu] −2 [% Ni] − [% Cr])] (5)
Here, [% Si], [% Cu], [% Ni], and [% Cr] represent the contents (mass%) of Si, Cu, Ni, and Cr, respectively, and are zero when not contained.

  N is an integer of 5 or more.

  Furthermore, this invention is a welded steel pipe which consists of said thick steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the thick steel plate which has the outstanding scale peeling resistance and sour resistance, suppressing the increase in manufacturing cost and the fall of productivity, and there exists a remarkable industrial effect.

  First, the components of the thick steel plate will be described.

C: 0.02 to 0.08 mass%
C has an effect of enhancing the hardenability and is an important element for securing the strength of the thick steel plate. If the C content is less than 0.02% by mass, this effect cannot be obtained. On the other hand, if the C content exceeds 0.08% by mass, a large amount of a hard second phase (for example, a martensite phase) is generated, so that the toughness of the thick steel plate deteriorates. Therefore, C is in the range of 0.02 to 0.08 mass%. Preferably it is 0.03-0.06 mass%.

Si: 0.45 mass% or less
Si is an element added as a deoxidizer in the process of melting a steel slab, which is a material for a thick steel plate. However, if the Si content is too large, island-shaped martensite is generated in the weld heat affected zone (hereinafter referred to as HAZ) by welding of thick steel plates, leading to deterioration of HAZ toughness. Therefore, Si is 0.45 mass% or less. Since Si is an element having an effect of degrading the scale peel resistance, the smaller the content, the better. However, in order to reduce the Si content to less than 0.05% by mass, it takes a long time to melt the steel slab, which causes various industrial problems (for example, a decrease in productivity, an increase in manufacturing cost, etc.). . Therefore, 0.05-0.45 mass% is preferable, More preferably, it is 0.05-0.35 mass%.

Mn: 1.00 to 1.70% by mass
Mn is an element having an effect of improving the strength and toughness of the thick steel plate. If the Mn content is too small, the effect cannot be obtained. On the other hand, when there is too much Mn content, Mn will concentrate in a center segregation part, the hardness of a center segregation part will increase, and the toughness of a thick steel plate will deteriorate. In addition, MnS is generated in the HAZ by welding thick steel plates, leading to deterioration of the HAZ toughness. Therefore, the Mn content is in the range of 1.00 to 1.70% by mass. Preferably it is 1.00-1.50 mass%.

Al: 0.001 to 0.060 mass%
Al is an element added as a deoxidizer in the process of melting a steel slab, which is a material for a thick steel plate. If the Al content is too small, the effect cannot be obtained. On the other hand, when there is too much Al content, it will become easy to produce Al system inclusions, and sour resistance will deteriorate. Therefore, the Al content is in the range of 0.001 to 0.060 mass%. Preferably it is 0.010-0.050 mass%.

Ca: 0.0010 to 0.0060 mass%
Ca is an element that has the effect of improving the toughness of thick steel plates and HAZ by spheroidizing the form of acicular MnS formed in the central segregation part. If the Ca content is too small, the effect cannot be obtained. On the other hand, when there is too much Ca content, clusters of Ca-based oxysulfides (for example, CaOS) are generated, and sour resistance (particularly HIC resistance) is deteriorated. Therefore, when it contains Ca, it shall be in the range of 0.0010-0.0060 mass%. Preferably it is 0.0010-0.0040 mass%.

O: 0.0030 mass% or less
O is an element inevitably contained in steel, and is usually present as an oxide combined with Al or Ca. If O is contained excessively, the content of these Al and Ca-based oxides in the steel will increase too much, forming clusters and degrading the HIC resistance, so the O content should be 0.0030% by mass or less. Is preferred.

  The thick steel plate according to the present invention contains one or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, and B in addition to the above components.

Cu: 0.50 mass% or less
Cu is an element which has the effect | action which improves the toughness of a thick steel plate, and improves scale peeling resistance. However, when there is too much Cu content, it will cause deterioration of weldability and HAZ toughness. Therefore, when it contains Cu, it is 0.50 mass% or less. Preferably it is 0.35 mass% or less, More preferably, it is 0.30 mass% or less.

Ni: 1.00 mass% or less
Ni is an element having an effect of improving the toughness and strength of the thick steel plate and improving the resistance to scale peeling. However, if the Ni content is too high, cracks occur in the steel slab during continuous casting, so that it is necessary to clean the surface before hot rolling, resulting in a significant reduction in productivity. Therefore, when it contains Ni, it is 1.00 mass% or less. Preferably it is 0.50 mass% or less, More preferably, it is 0.30 mass% or less.

Cr: 0.50 mass% or less
Cr is an element having an effect of improving the strength of the thick steel plate and improving the resistance to scale peeling by a low C component design. However, when there is too much Cr content, deterioration of weldability and HAZ toughness will be caused. Therefore, when it contains Cr, it is 0.50 mass% or less. Preferably it is 0.45 mass% or less, More preferably, it is 0.30 mass% or less.

Mo: 0.50 mass% or less
Mo is an element that has the effect of improving the hardenability of the thick steel plate and improving the strength. However, if the Mo content exceeds 0.50 mass%, the HAZ toughness is deteriorated. Therefore, when it contains Mo, it is 0.50 mass% or less. Preferably it is 0.35 mass% or less.

Nb: 0.06 mass% or less
Nb is an element that has the effect of refining the structure of a thick steel plate to improve toughness and strength. However, if the Nb content is too high, the HAZ toughness is significantly deteriorated. Therefore, when it contains Nb, it is 0.06 mass% or less. Preferably it is 0.05 mass% or less.

V: 0.10% by mass or less
V is an element having an action of improving the hardenability of the thick steel plate and improving the strength. However, if the V content is too large, precipitation embrittlement is caused, leading to toughness deterioration of the thick steel plate or HAZ. Therefore, when it contains V, it is 0.10 mass% or less. More preferably, it is 0.05 mass% or less.

Ti: 0.03 mass% or less
Ti is an element that has the effect of suppressing the coarsening of austenite crystals during heating before hot rolling and improving the toughness of thick steel plates and HAZ due to the pinning effect of TiN. However, when there is too much Ti content, coarse TiN will produce | generate and will cause deterioration of HAZ toughness. Therefore, when it contains Ti, it is 0.03 mass% or less.

B: 0.0030% by mass or less
B is an element having an effect of remarkably enhancing the hardenability, and the strength of the thick steel plate can be improved by adding a small amount (that is, inexpensively). However, when there is too much B content, deterioration of weldability and HAZ toughness will be caused. Therefore, when it contains B, it is made into 0.0030 mass% or less. Preferably it is 0.020 mass% or less.

Pcm: 0.110 or more
Pcm is a value calculated by the equation (1) and is an index for evaluating the weld cold cracking property, but can also be used as an index for evaluating the strength and structure of the thick steel plate. That is, if Pcm is too low, a uniform structure mainly composed of a bainite phase cannot be formed even if accelerated cooling described later is performed, resulting in deterioration of sour resistance. Therefore, Pcm is 0.110 or more. Preferably it is 0.12 or more.
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20) + ([% Ni] / 60) + ([% Cr] / 20 ) + ([% Mo] / 15) + ([% V] / 10) +5 [% B] (1)
Here, [% C], [% Si], [% Mn], [% Cu], [% Ni], [% Cr], [% Mo], [% V], [% B] are respectively It represents the content (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B. If any element is not contained, it is zero.

PHIC: 0.95 or less
PHIC is a value calculated by equation (2) .In consideration of the influence of P in addition to the alloy elements used in general carbon equivalent calculations, the properties of the final solidified part due to central segregation are determined. It is an index for evaluation. If the PHIC value is too high, even if the steel slab does not generate coarse MnS in the final solidified part, HIC cracking is likely to occur starting from the NbTi-CN compound. Therefore, PHIC is 0.95 or less.
PHIC = 4.46 [% C] + (2.37 [% Mn] / 6) + (1.74 [% Cu] +1.7 [% Ni]) / 15+ (1.18 [% Cr] +1.95 [% Mo] +1.74 [% V]) / 5 + 22.36 [% P] (2)
Here, [% C], [% Mn], [% Cu], [% Ni], [% Cr], [% Mo], [% V], [% P] are C, Mn, Cu, respectively. , Ni, Cr, Mo, V, P content (% by mass), zero if not contained.

ACRM: 0 (zero) or more
ACRM is a value calculated by equation (3), and is an index for evaluating the phenomenon that MnS generated in the final solidified portion of the steel slab is spheroidized by Ca. If the ACRM value is too low, coarse MnS remains in the final solidified part, resulting in deterioration of sour resistance. Therefore, ACRM is 0 or more.
ACRM = [[% Ca] − (1.23 [% O] −0.000365)] / (1.25 [% S]) (3)
Here, [% Ca], [% O], and [% S] represent the contents (mass%) of Ca, O, and S, respectively, and are zero when not contained.

Ratio of calcium content to oxygen content: 2.5 or less The ratio of calcium content to oxygen content is a value calculated by equation (4), and is used to quantify CaO clusters that accumulate near the surface layer of thick steel plates. It is an indicator. If the ratio is too high, sour resistance (particularly HIC resistance) near the surface layer deteriorates. Therefore, the ratio of calcium content to oxygen content is 2.5 or less. Preferably it is 2.2 or less.
Ratio of calcium content to oxygen content = [% Ca] / [% O] (4)
Here, [% Ca] and [% O] represent the contents (mass%) of Ca and O, respectively.

  In the present invention, P and S are inevitably contained impurity elements, and the smaller the number, the better. However, the following ranges are acceptable.

P: 0.015 mass% or less
P is an element that easily segregates and concentrates in the center. Even if contained in a small amount, P increases the hardness of the center segregation significantly and degrades sour resistance. However, up to 0.015% by mass is acceptable. More preferably, it is 0.010 mass% or less.

S: 0.0015% by mass or less
S combines with Mn to generate MnS. Further, since S is an element that is easily concentrated in the central portion, as in Mn, if the amount of S is large, a large number of central segregations of MnS are generated, and sour resistance is significantly deteriorated. Therefore, it is desirable to reduce S as much as possible, but 0.0015% by mass is acceptable. More preferably, it is 0.0010 mass% or less.

  The preferable content of the element added to the thick steel plate according to the present invention and the reason for limiting it are as described above, and the balance other than the above components is Fe and inevitable impurities.

  Next, the structure of the thick steel plate will be described.

Bainitic phase: 95% or more In order to ensure sour resistance (particularly HIC resistance) of a thick steel plate, it is necessary to form a uniform structure. In order to develop the strength required for welded steel pipes used as line pipes and process pipes (that is, tensile strength of 530 MPa or more), it is necessary to form a structure mainly composed of bainite phase. When the area ratio of the bainite phase is less than 95%, cracks caused by HIC propagate between the bainite phase and other phases, resulting in deterioration of HIC resistance. Therefore, the bainite phase is 95% or more in area ratio. The ratio of the bainite phase is a value measured from the surface layer at a position in the plate thickness direction (1/8) × t to (7/8) × t. Fine cementite and island martensite formed between laths of the bainite phase are regarded as part of the bainite phase. That is, the remaining structure of less than 5% other than the bainite phase is coarse cementite generated in places other than the ferrite phase, the pearlite phase, the martensite phase, and the bainite phase.

  Next, the properties of the scale will be described.

Average thickness of scale: more than 10 μm and not more than 40 μm As the scale generated on the surface of the thick steel plate is smaller, the scale peel resistance is improved. However, when the average thickness of the scale is 10 μm or less, rust is generated during storage and transportation in the field, especially during marine transportation by ship, and problems such as loss of marking as well as deterioration of surface properties occur. On the other hand, when the average thickness exceeds 40 μm, even when the component design contains Cr, Cu, and Ni, the scale is easily peeled when the steel pipe is formed from the thick steel plate. Therefore, the average thickness of the scale is more than 10 μm and 40 μm or less.

Average grade evaluated by the cross-cut method: 2 or less A scale obtained by evaluating the scale of the surface of a thick steel plate by the cross-cut method defined by JIS standard K5600-5-6 is used as an index indicating the resistance to scale peeling. The grade obtained by the cross-cut method varies depending on the part to be measured on the surface of the thick steel plate. Therefore, by carrying out the evaluation by the cross-cut method at three or more locations and setting the average grade to 2 or less, scale peeling that occurs when forming a thick steel plate into a steel pipe can be suppressed.

  Next, the manufacturing method of a thick steel plate is demonstrated.

Steel slab heating temperature: 1000-1250 ℃
Although the manufacturing method of the steel slab used as the raw material of hot rolling is not specifically limited, For example, it is preferable to melt by a converter method and to make a slab by a continuous casting method.

  If the temperature at which the steel slab is heated before hot rolling is less than 1000 ° C., the strength required for the thick steel plate (that is, the tensile strength of 530 MPa or more) cannot be expressed. On the other hand, when the heating temperature exceeds 1250 ° C., the toughness of the thick steel plate deteriorates. Therefore, the heating temperature of the steel slab is set within a range of 1000 to 1250 ° C.

Descaling before hot rolling: High-pressure water is sprayed on both sides of the heated steel slab By heating the steel slab, a scale (hereinafter referred to as primary scale) is generated on the surface. When hot rolling is performed with the primary scale remaining on the surface of the steel slab, the above-mentioned problems (a) to (c) occur, and when the resulting thick steel plate is formed, the scale peeling occurs. Is likely to occur. Therefore, it is necessary to remove the primary scale before hot rolling. In the descaling, high-pressure water is sprayed on both sides of the heated steel slab. The pressure and flow rate of the high-pressure water are not particularly limited, and are appropriately set so that the primary scale can be removed.

Descaling in hot rolling: The steel slab is heated twice or more during the rolling pass from the (N-4) pass to the final N pass, and after further descaling of the primary scale, A scale (hereinafter referred to as a secondary scale) is generated again during rolling (number of passes: N). Since the secondary scale contains pores, it is easy to peel off and grows quickly, and therefore, descaling is performed before the reduction of the final hot rolling pass (that is, the Nth pass) is completed. However, even if the descaling is performed at an early stage of hot rolling (for example, after the first pass), a secondary scale is generated thereafter. Therefore, descaling is performed from the (N-4) th pass to the final pass (ie, the Nth pass). In addition, since a small amount of the primary scale may remain, descaling is performed twice or more in order to completely remove the secondary scale and the primary scale. Therefore, the descaling is performed twice or more during the rolling pass from the (N-4) pass to the final N pass of the hot rolling. Here, N is the total number of passes of rough rolling and finish rolling, and is an integer of 5-50.

  When the descaling is not performed twice or more during the rolling pass from the (N-4) pass to the final N pass of the hot rolling, the secondary scale and the primary scale remain. The scale on the surface becomes thick, and scale peeling occurs when forming into a welded steel pipe. The descaling is preferably performed at least from the entrance side where the material to be rolled bites into the rolling mill during the rolling pass, and more preferably from both sides of the rolling mill during the rolling pass.

  If the descaling is performed twice or more during the rolling pass from the (N-2) th pass to the Nth pass of the hot rolling, the effect of removing the secondary scale and the primary scale is further enhanced. Here, N is an integer of 3 to 50.

Rolling end temperature of hot rolling: FT value or less and Ar3 point or more The rolling end temperature (° C.) of hot rolling is a factor that greatly affects the strength, toughness, and scale peel resistance of thick steel plates. And by the component design to ensure the strength and toughness as described above, and by terminating hot rolling at a temperature below the FT value calculated by Equation (5), the average by the cross-cut method A grade can be made into 2 or less, and scale peeling resistance can be improved. Note that the lower limit of the rolling end temperature is set to the Ar3 point or higher in relation to accelerated cooling described later. It is desirable to actually measure the Ar3 point using a test piece of the same component. Alternatively, a value estimated by the following equation (6) may be used.
FT = 845− [500/3] × [(1.5 [% Si] −2 [% Cu] −2 [% Ni] − [% Cr])] (5)
Here, [% Si], [% Cu], [% Ni], and [% Cr] represent the contents (mass%) of Si, Cu, Ni, and Cr, respectively. If any element is not contained, it is zero.
Ar3 = 910−310 [% C] −80 [% Mn] −20 [% Cu] −55 [% Ni] −15 [% Cr] −80 [% Mo] (6)
Here, [% C], [% Mn], [% Cu], [% Ni], [% Cr] and [% Mo] are the contents of C, Mn, Cu, Ni, Cr and Mo, respectively ( Mass%). If any element is not contained, it is zero.

Accelerated cooling start temperature: Ar3 point or higher Accelerated cooling is performed after hot rolling is completed. If the start temperature (° C.) of accelerated cooling is lower than the Ar3 point, a ferrite phase is generated, and sour resistance (particularly HIC resistance) cannot be ensured. Therefore, the starting temperature of accelerated cooling is set to the Ar3 point or higher.

Cooling rate of accelerated cooling: 10 ° C / second or more The cooling rate of accelerated cooling is an index for controlling the structure of the thick steel plate. In the above component design, the bainite phase is set to 10 ° C / second or more. Can be obtained as a main component (95% or more in area ratio). When the cooling rate is less than 10 ° C./second, a ferrite phase is easily generated, and sour resistance (particularly HIC resistance) cannot be ensured.

Accelerated cooling stop temperature: less than 600 ° C Accelerated cooling stop temperature is an index for controlling the structure of thick steel sheets. In the above component design, the bainite phase is mainly (by area) by making it less than 600 ° C. A ratio of 95% or more) can be obtained.

  In the method for producing a thick steel plate according to the present invention, accelerated cooling is stopped and then cooled to room temperature in the atmosphere (so-called air cooling). When the accelerated cooling stop temperature is 600 ° C. or higher, a ferrite phase is generated by the air cooling. Therefore, sour resistance (particularly HIC resistance) cannot be ensured.

  Further, tempering may be performed after air cooling to adjust the balance between strength and toughness of the thick steel plate.

  According to the production conditions described above, the thick steel plate for welded steel pipes excellent in scale peel resistance and material uniformity of the present invention can be obtained. Furthermore, the manufacturing method of the welded steel pipe of this invention is demonstrated.

  This invention makes a steel pipe using said thick steel plate. Examples of the method for forming a steel pipe include a method for forming a steel pipe into a shape by cold forming such as a UOE process or a press bend (also called a bending press).

  In the UOE process, after performing groove processing on the width direction end of a thick steel plate as a material, the end of the width direction end of the thick steel plate is bent using a press machine, and then the thickness is increased using a press machine. By forming the steel plate into a U shape and an O shape, the thick steel plate is formed into a cylindrical shape so that the width directions of the thick steel plates face each other. Next, the opposite ends in the width direction of the thick steel plates are butted and welded. This welding is called seam welding. In this seam welding, a cylindrical thick steel plate is constrained, and the end portions in the width direction of opposing thick steel plates are butt-welded to each other and tack welding is performed. A method having a two-step process, that is, a main welding process for welding the outer surface, is preferable. After seam welding, pipe expansion is performed to remove residual welding stress and improve roundness of the steel pipe. In the pipe expansion process, the pipe expansion ratio (ratio of the outer diameter change amount before and after the pipe expansion to the outer diameter of the pipe before the pipe expansion) is usually performed in the range of 0.3 to 1.5%. From the viewpoint of the balance between the roundness improvement effect and the capacity required for the pipe expansion device, the pipe expansion ratio is preferably in the range of 0.5 to 1.2%. Thereafter, a coating treatment can be carried out for the purpose of preventing corrosion. As the coating treatment, for example, a known resin may be applied to the outer surface after heating to a temperature range of 200 to 300 ° C., for example.

  In the case of a press bend, a steel pipe having a substantially circular cross-sectional shape is manufactured by sequentially forming a thick steel plate by repeating three-point bending. After that, seam welding is performed in the same manner as the above UOE press. Also in the case of press bend, tube expansion may be performed after seam welding, and coating may also be performed.

  Molten steel having the components shown in Table 1 was melted and formed into a steel slab by continuous casting. And after heating a steel slab, it hot-rolled and made it the thick steel plate, continued cooling water to inject accelerated cooling, and also air-cooled and cooled to room temperature. Table 2 shows the calculated values of Pcm, PHIC, ACRM, [% Ca] / [% O], FT, Ar3 from the contents of each element shown in Table 1. Table 3 shows the steel slab heating temperature, hot rolling descaling, rolling end temperature, accelerated cooling start temperature, stop temperature, cooling rate, and steel plate thickness.

  The rolling end temperature of hot rolling was determined by measuring the surface temperature of the thick steel plate immediately after the final pass (that is, the Nth pass). The start temperature and stop temperature of accelerated cooling were determined by measuring the surface temperature at a position 500 mm from the tail end side crop in the traveling direction (ie, the longitudinal direction) of the thick steel plate. The cooling rate was calculated by a heat conduction simulation at a cooling rate of 700 → 650 ° C. at the center in the thickness direction.

  The position in the plate thickness direction (1/8) × t to (7/8) × t is photographed with an optical microscope from the surface layer of the thick steel plate thus obtained, and the area ratio of the bainite phase is analyzed by image analysis using the photograph. Asked. Moreover, the cross section of the thick steel plate was image | photographed with the optical microscope, the thickness of five scales was measured using the photograph, and the average value was calculated | required. The cross-cut test was conducted in accordance with JIS standard K5600-5-6. The front and back surfaces of the thick steel plate were photographed with an optical microscope, and the average value of five grades measured using the photographs was obtained. Furthermore, a full-thickness tensile test piece conforming to API standard 5L was taken perpendicularly to the longitudinal direction of the thick steel plate, and a tensile test was performed to obtain a tensile strength. The target value of tensile strength was set to 530 MPa or more.

  The HIC test was performed according to NACE TM0284. As the solution, NACE-A solution was used. The target of the HIC test was CLR ≦ 10%. The reason is to satisfy CLR ≦ 15% in a welded steel pipe state.

  Next, the thick steel plate was formed by the press bend method, and the seam portion was welded to obtain a straight seam welded steel pipe (outer diameter 457 to 914 mm), and thereafter, pipe expansion was performed for the purpose of improving roundness. Then, the surface of the welded steel pipe was visually observed to investigate scale peeling. As a result, it was evaluated as good (○) that the markings applied by spray coating on the thick steel plate that was the material was not exposed and the marking was lost. What is being evaluated is evaluated as defective (×), and is shown in Table 3. In addition, about scale peeling, it made the target that marking does not lose | disappear (namely, evaluation (circle)).

  As is clear from Table 4, in the inventive examples, the tensile strength of the thick steel plate, CLR, and scale peeling after being processed into a welded steel pipe all achieved the target, and good results were obtained. On the other hand, one of the comparative examples does not meet the target.

Claims (3)

C: 0.02 to 0.08 wt%, Si: 0.45 wt% or less, Mn: from 1.00 to 1.70 wt%, Al: .001-0.060 mass%, Ca: from .0010 to .0060 containing mass%, further Cu: 0.50 wt% or less Ni: 1.00 mass % or less, Cr: 0.50 mass % or less, Mo: 0.50 mass % or less, Nb: 0.06 mass% or less, V: 0.10 mass% or less, Ti: 0.03 mass% or less, B: 0.0030 mass% or less It contains one or more selected from among them, the balance is Fe and inevitable impurities, and the Pcm value calculated by the following formula (1) is 0.110 or more, and the PHIC value calculated by the formula (2) is 0.95 Hereinafter, the ACRM value calculated by the formula (3) is 0 or more, the ratio of the calcium content and the oxygen content calculated by the formula (4) is 2.5 or less, the bainite phase is 95 by area ratio. %, The average thickness of the scale formed on the surface is more than 10μm and less than 40μm, and the scale peelability is a cross-cut method specified by JIS standard K5600-5-6. Valence and sour welded steel pipe for thick steel plate having excellent scale exfoliation resistance, wherein the average rating of 2 or less were.
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20) + ([% Ni] / 60) + ([% Cr] / 20 ) + ([% Mo] / 15) + ([% V] / 10) +5 [% B] (1)
PHIC = 4.46 [% C] + (2.37 [% Mn] / 6) + (1.74 [% Cu] +1.7 [% Ni]) / 15+ (1.18 [% Cr] +1.95 [% Mo] +1.74 [% V]) / 5 + 22.36 [% P] (2)
ACRM = [[% Ca] − (1.23 [% O] −0.000365)] / (1.25 [% S]) (3)
Ratio of calcium content to oxygen content = [% Ca] / [% O] (4)
Where [% C], [% Si], [% Mn], [% Cu], [% Ni], [% Cr], [% Mo], [% V], [% B], [% [P], [% Ca], [% O] and [% S] are the contents (mass) of C, Si, Mn, Cu, Ni, Cr, Mo, V, B, P, Ca, O and S, respectively. %), And zero if not contained. A steel slab having the composition according to claim 1 is heated to 1000 to 1250 ° C., subjected to descaling by spraying high-pressure water onto the steel slab to remove scales on both sides, and then hot rolling with N passes. And performing the descaling twice or more during the rolling pass from the (N-4) pass to the final N pass of the hot rolling, and the FT value calculated by the following equation (5) The hot rolling is terminated at a temperature not lower than the Ar3 point and not lower than the Ar3 point, and then accelerated cooling is performed at a cooling rate of 10 ° C / second or higher from the temperature not lower than the Ar3 point to less than 600 ° C. It has an excellent scale peel resistance and has a structure in which the bainite phase has an area ratio of 95% or more. The average thickness of the scale formed on the surface exceeds 10 μm and the scale peelability is 40 μm or less. The cross-cut method specified by JIS standard K5600-5-6 The manufacturing method of the thick steel plate for sour-resistant welded steel pipes whose average grade evaluated in (2) is 2 or less .
FT = 845− [500/3] × [(1.5 [% Si] −2 [% Cu] −2 [% Ni] − [% Cr])] (5)
Here, [% Si], [% Cu], [% Ni], and [% Cr] represent the contents (mass%) of Si, Cu, Ni, and Cr, respectively, and are zero when not contained.
N is an integer of 5 or more.   A welded steel pipe comprising the thick steel plate according to claim 1.