JP4432905B2 - High-strength steel and offshore structures with excellent weld toughness - Google Patents

Description

The present invention relates to high-strength steel and offshore structures, and particularly to high-strength steel for welding and offshore structures having excellent weld toughness.
More specifically, the present invention is used for welding high-tensile steel used as a welded structure in buildings, civil engineering structures, construction machinery, ships, pipes, tanks, offshore structures, etc., particularly offshore structures. The present invention relates to a high-strength steel for welding and an offshore structure, for example, a thick high-strength steel plate having a yield strength of 420 N / mm ² or more and a plate thickness of 50 mm or more, and an offshore structure using the same.

  In recent years, energy demand has been increasing more and more, and search for offshore oil resources has been activated. As for the offshore structures used for these, for example, platforms and jack-up rigs have become larger, and accordingly, steel materials such as steel plates have become thicker, and ensuring safety is an important issue.

  For ordinary marine structures, medium strength steel with a yield stress of 300 to 360 MPa is used, but for large structures such as those mentioned above, the strength is 460 to 70 OMPa and the plate thickness is more than 100 mm. Tensile steel may be used.

In addition, search areas for submarine oil resources have recently moved to cold regions and deep water areas, and offshore structures operating in those regions or sea areas are exposed to extremely severe weather and ocean conditions.
For this reason, steel materials used for these offshore structures are required to have toughness in a very severe low temperature range of, for example, −40 ° C. or lower, and of course weldability.

  In addition, from the standpoint of safety, user inspection standards are strict, and in addition to the conventional Charpy impact values for both the base metal and welded parts, the CTOD value at the minimum operating temperature is also defined to evaluate toughness. ing. In other words, even when stable characteristics are obtained in the Charpy test, which is an evaluation test for a small test piece cut and sampled to a size of 10 mm x 10 mm, the CTOD characteristics evaluated with the actual thickness test piece are required. There are many cases where the characteristics cannot be satisfied, and more severe CTOD characteristics are now required.

  In this way, not only steel materials used in offshore structures installed in ice seas, but also line pipes for cold regions used in milder environments than these, or large welded structures such as ships and LNG tanks There is a strong demand for improving the low-temperature toughness of the weld heat-affected zone (hereinafter referred to as HAZ) also for steel materials used in products.

  On the other hand, in order to obtain high toughness in a low temperature range of −40 ° C. or lower, welding must be performed under welding conditions of low heat input with poor welding efficiency. The welding construction cost accounts for the construction cost of offshore structures. The most direct method for reducing the welding construction cost is to reduce the number of weld layers by adopting a high efficiency welding method capable of high heat input welding.

  Therefore, today, it is important that a structure for a cold district, where the requirement for low-temperature toughness is severe, be welded with a welding construction cost as low as possible considering the toughness of the HAZ.

  Conventionally, it has been known that low C is effective in dramatically improving the HAZ toughness of steel materials. To compensate for the decrease in strength caused by low C, high strength and aging by adding various alloys are known. Strengthening using the precipitation hardening action is achieved. For example, ASTM A710 discloses a steel that uses the aging precipitation hardening action of Cu, and some reports based on this concept have been made.

For example, Japanese Patent Publication No. 7-81164, Japanese Patent Application Laid-Open No. 5-186820, and Japanese Patent Application Laid-Open No. 5-179344 propose Cu-precipitation steel having excellent weld toughness.
However, in Japanese Patent Publication No. 7-81164, the Charpy characteristics of a welded joint obtained with a plate thickness of 30 mm and a welding heat input of 40 kJ / cm are merely evaluated, and it is difficult to think of a material that supports high heat input welding.

  JP-A-5-186820 proposes a high-tensile steel with a tensile strength of 686 MPa or more to which Cu is added in an amount of 0.5 to 4.0%, but the low-temperature toughness is -30 even at the transition temperature of the Charpy test. Since it is at ℃, it is difficult to think that low-temperature CTOD characteristics can be secured with extra-thick steel plates.

  In Japanese Patent Laid-Open No. 5-179344, although a Cu precipitation type steel having excellent Charpy toughness of a welded portion is proposed, only Charpy characteristics of a welded joint obtained with a welding heat input of 5 kJ / mm were evaluated. It is unlikely that the technology can sufficiently satisfy the safety of the structure during heat welding.

  Here, the subject of this invention is providing the high tensile steel for welding which improved the weld part low temperature toughness generally, especially HAZ low temperature toughness.

As a result of conducting various experiments on steel components and manufacturing methods for the purpose of developing a thick high-strength steel sheet having excellent weld toughness, the present inventors have obtained the following knowledge.
(i) Based on Cu-added steel, in addition to adjusting the N and Al contents, the N / Al ratio should be controlled.

  In high Cu component materials, fine dispersion of carbonitrides such as TiN, Ti (C, N), and AlN is effective for improving high heat input HAZ toughness. Then, when it examined using the high Cu-Ti additive material, in addition to adjustment of N and Al content, the effectiveness of controlling N / Al ratio was discovered. This is presumably because when the N / Al ratio is too small, coarse AlN precipitates, which adversely affects the toughness, and that fine / large amount dispersion of TiN is inhibited. On the other hand, it is considered that when the N / Al ratio is excessive, the solute N increases and the dispersion density of AlN and TiN becomes sparse.

(ii) In order to increase the yield strength, it is necessary to disperse as many fine Cu particles as possible.
(iii) To ensure toughness, particularly low temperature CTOD characteristics, it is necessary to coarsen the Cu particles to some extent and suppress the amount of dispersion.

(iv) In order to make the dispersed state of Cu particles uniform, the formation of Cu particles in the pre-aging process is suppressed as much as possible, and the dispersed state of Cu particles is controlled by controlling the conditions of the aging process.
(v) The strength and toughness balance can be controlled by organizing the distribution state of Cu particles by the average value of equivalent circle diameters obtained from TEM photographs and the plane conversion area ratio.

  (vi) Cu particles are likely to form on crystal defects (mainly dislocations) in steel, and precipitation of Cu particles is promoted when the dislocation density is high. Also, Cu particles on dislocations inhibit dislocation movement and increase yield strength.

  (vii) The dislocation density in the steel should be controllable under rolling and water cooling conditions. Also, decrease in rolling temperature, increase in total rolling reduction, increase in water cooling start temperature, increase in cooling rate, decrease in water cooling stop temperature, all of which increase dislocation density.

(viii) Based on the high Cu component, it is possible to stabilize the high heat input HAZ toughness by controlling the hardenability by adjusting the amounts of C, Mn, and Mo.
In other words, it was found that HAZ toughness can be improved with a high Cu component material as the weld cracking susceptibility index Pcm value is reduced, and for that purpose, low C and low Mn are effective. However, in order to ensure high strength, it was necessary to supplement with other elements, and it was also found that the strength / toughness can be stabilized by controlling the amount of Mo added.

The present invention is configured based on such knowledge, and the gist thereof is as follows.
(1) By mass%, C: 0.01 to 0.10%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.020% or less, S: 0.01 %: Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.003-0.008%, O: 0 .0005 to 0.0035%, the balance being Fe and impurities, N / Al being 0.3 to 3.0, and Pcm represented by the following formula (I) being 0.25 or less for dispersed diameter is 1nm or more Cu particles in the steel, an average value of equivalent circle diameter of 4～25Nm, and thickness 50mm or more, wherein the flat rate conversion distribution amount is 3-20% high-tensile steel material.
Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B (I )
(2) The high-tensile steel material according to (1) above, which contains Ti: 0.005 to 0.03% by mass.
(3) The high-tensile steel material according to (1) or (2) above, which contains Nb: 0.003 to 0.03% by mass%.
(4) The high-tensile steel material as described in any one of (1) to (3) above, which contains Mo: 0.1 to 0.8% by mass.
(5) Any one of (1) to (4) above, characterized by containing at least one of Cr: 0.03 to 0.80% and B: 0.0002 to 0.002 by mass% The high-strength steel materials described in 1.
(6) The high-tensile steel material as described in any one of (1) to (5) above, which is contained by mass and contains V: 0.001 to 0.05%.
(7) It is characterized by containing at least one of Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, REM: 0.0001 to 0.01% by mass%. The high-tensile steel material according to any one of (1) to (6) above.
(8) An offshore structure using the high-tensile steel material according to any one of (1) to (7).

According to the present invention, the yield stress is 420 N / mm ² or more, which is excellent in weldability, and can be welded at a welding heat input of 300 KJ / cm or more by a welding method such as electrogas arc welding. High-strength steel can be manufactured. As a result, on-site welding work efficiency and safety were significantly improved. In addition, it has become possible to provide high-tensile steel that can be used in extremely severe environments such as offshore structures.

The present invention will be described in detail. First, the reason why the present invention is limited to the steel composition as described above will be described. In this specification, “%” indicating the steel composition is indicated by “mass%”.
C is added to secure the strength of the steel and to produce an effect of refining the structure when adding Nb, V or the like. If it is less than 0.01%, these effects are not sufficient. However, if there is too much C, a hardened structure called island martensite (MA) is formed in the welded portion to deteriorate the HAZ toughness and adversely affect the toughness and weldability of the base metal. . Therefore, C is 0.10% or less. Preferably it is 0.02-0.08%, More preferably, it is 0.02-0.05%.

  Si is an effective element for preliminary deoxidation of molten steel, but since it does not dissolve in cementite, when added in a large amount, it inhibits the decomposition of untransformed austenite grains into ferrite grains and cementite, resulting in island martensite. Contributes to the generation of For these reasons, the Si content should be 0.5% or less. Preferably it is 0.2% or less, More preferably, it is 0.15% or less.

  Mn is an element necessary for ensuring strength and is also an effective element as a deoxidizer. For this reason, the content of Mn needs to be 0.8% or more. However, excessive addition of Mn excessively increases hardenability and degrades weldability and HAZ toughness. Further, since Mn is an element that promotes center segregation, its content should not exceed 1.8% from the viewpoint of suppressing center segregation. Therefore, the Mn content is set to 0.8 to 1.8% or less. Preferably it is 0.9 to 1.5%.

  P is an impurity element inevitably contained in the steel, and is a grain boundary segregation element, which causes grain boundary cracking in the HAZ. Furthermore, in order to improve the toughness of the base metal, the weld metal part and the HAZ, and to reduce the slab center segregation, the P content is set to 0.020% or less. Preferably it is 0.015% or less, More preferably, it is 0.01% or less.

When a large amount of S is present, a precipitate of MnS simple substance that becomes a weld crack starting point is generated. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less .

  Cu has an effect of increasing the strength and toughness of the steel material, but has little adverse effect on the HAZ toughness. In particular, 0.8% or more is necessary in order to expect the effect of increasing the strength due to ε-Cu precipitation during the aging treatment. However, when the Cu content increases, the sensitivity to hot cracking of the weld increases and the welding work such as preheating becomes complicated, so the Cu content is set to 1.5% or less. Preferably it is 0.9 to 1.1%.

  Ni is an effective element for increasing the strength and toughness of the steel material and further increasing the HAZ toughness. However, if it is 0.2% or less, those effects are not obtained, and if it exceeds 1.5%, it is not possible to obtain an effect sufficient for cost increase. It was. Preferably it is 0.4 to 1.2%.

  Al is an essential element for deoxidation. However, when the content increases, the toughness tends to deteriorate particularly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. For this reason, the content of Al is set to 0.001 to 0.05%. Preferably it is 0.001 to 0.03%. More preferably, it is 0.001 to 0.015%.

  N contributes to the refinement of the structure by forming nitrides, but when added excessively, the toughness deteriorates through the aggregation of nitrides. Therefore, the N content is set to 0.003 to 0.008%. Preferably it is 0.0035 to 0.0065%.

By controlling the N / Al ratio to 0.3 to 3.0, it is possible to improve large heat input HAZ toughness, particularly joint CTOD characteristics.
This is presumably because when the N / Al ratio is less than 0.3, coarse AlN precipitates, which adversely affects the toughness and inhibits the fine / large amount of dispersion of TiN. On the other hand, when the N / Al ratio exceeds 3.0, it is considered that the solid solution N increases and the HAZ toughness deteriorates, and the dispersion density of AlN and TiN becomes sparse. A preferable range for further exhibiting the effect is 0.4 to 2.5.

  O.sub.2 (oxygen) is effective in generating an oxide serving as a ferrite forming nucleus. On the other hand, when it is present in a large amount, the cleanliness deteriorates remarkably, and it becomes difficult to ensure practical toughness for both the base metal, the weld metal part and the HAZ. Therefore, the content of O is set to 0.0005 to 0.0035%. Preferably it is 0.0008 to 0.0018%.

  Ti has the effect | action which refines | miniaturizes a transformation structure | tissue while producing | generating a nitride and suppressing the coarsening of a crystal grain. However, if the addition is less than a specific amount, the above-mentioned effect is not exhibited, and if it is added in a large amount, the base material toughness and weld zone toughness are adversely affected. Therefore, the Ti content is 0.005 to 0.03%. Preferably it is 0.007 to 0.015%.

  Nb improves the strength and toughness of the base material by refining and carbide precipitation. On the other hand, if added excessively, the performance improvement effect of the base material is saturated and the toughness of the HAZ is significantly impaired. Therefore, the Nb content is set to 0.003 to 0.03%. Preferably it is 0.003 to 0.015%.

  Mo has the effect of ensuring hardenability and improving the HAZ toughness, but if added excessively, it causes significant hardening in the HAZ and degrades the toughness. Therefore, the Mo content is set to 0.1 to 0.8%. Preferably it is 0.1 to 0.5%.

  Cr increases the hardenability of steel and is effective in securing strength. However, when added in a small amount, Cr cannot be effective, and when added in excess, it prevents hardening of the weld metal and HAZ and increases the sensitivity to cold cracking in welding. It tends to make it. Therefore, when adding Cr, the content of Cr is set to 0.03 to 0.80%. Preferably it is 0.05 to 0.60%.

  B has the effect of improving the hardenability and increasing the strength. On the other hand, if added excessively, the effect of increasing the strength is saturated, and the tendency of deterioration of toughness becomes remarkable in both the base material and HAZ. Therefore, when adding B, content of B shall be 0.0002 to 0.002%. Preferably it is 0.003 to 0.0015%.

  V has the effect | action which refine | miniaturizes a transformation structure | tissue while producing | generating carbonitride and suppressing the coarsening of a crystal grain. However, if the addition is less than a specific amount, the above-mentioned effect is not exhibited, and if it is added in a large amount, the base material toughness and weld zone toughness are adversely affected. Therefore, when adding V, the content of V is set to 0.001 to 0.05%. Preferably it is 0.005 to 0.04%.

  Ca, Mg, and REM are elements that generate oxides and sulfides that serve as precipitation nuclei for intragranular ferrite. It also controls sulfide morphology and improves low temperature toughness. In order to obtain such effects of Ca, Mg, and REM, it is necessary to contain 0.0005% or more in the case of Ca and 0.0001% or more in the case of Mg and REM. On the other hand, in the case of Ca, if it exceeds 0.005%, in the case of Mg and REM, if it exceeds 0.01%, Ca and Mg-based large inclusions and clusters are generated to deteriorate the cleanliness of the steel. Therefore, when adding Ca, the content of Ca is 0.0005 to 0.005%, and when adding Mg and REM, the contents of Mg and REM are 0.0001 to 0.01%.

  The steel according to the present invention has a Pcm represented by the following formula (I) of 0.25 or less, and an average equivalent circle diameter of 4 to 25 nm for Cu particles having a major axis of 1 nm or more dispersed in the steel. And, it is preferable that the plane rate conversion distribution amount is 3 to 20%.

Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B (I )
Pcm is an index representing the susceptibility to weld cracks. If the value is 0.25 or less, no weld cracks occur under normal welding conditions. Therefore, Pcm is set to 0.25 or less. If Pcm is lowered, preheating during welding can be omitted. Preferably it is 0.22 or less, More preferably, it is 0.20 or less.

  Next, the circle equivalent diameter average value of the Cu precipitate and the flatness equivalent distribution amount will be described. The reason for targeting Cu particles having a major axis of 1 nm or more is that particles smaller than 1 nm have a small contribution to increasing the strength. The upper limit of the major axis of the Cu particles is not particularly defined, but particles exceeding 100 nm do not appear when the average value is in the range of 4 to 25 nm. In addition, although the precipitation form of Cu particle | grains is about spherical shape, since it is not easy to measure a solid | 3D shape, the projected shape is measured.

Here, the equivalent circle diameter is a diameter of a circle having the same area as the projected area of the particle. Specifically, d = √ (4a / pai) a: projected area (nm ² ), d: equivalent circle diameter. (nm), pai: 3.14
Ask for.

  For flatness conversion distribution, the steel material is processed into a thin film, and TEM observation is performed on a portion with a thickness of approximately 0.2 micrometers, and Cu particles distributed three-dimensionally in the thin film specimen are projected on a plane. Is calculated by measuring a TEM photograph with a magnification of 100,000 times.

Here, the reason why the equivalent circle diameter and the flatness conversion distribution amount are defined as described above will be described in more detail.
As a feature of steel used in offshore structures, in order to withstand external forces due to storm waves, it is often very heavy steel with a maximum thickness of 100mm, and it will be used in severe conditions in the future. It is required to meet even more stringent CTOD values.

If the strength becomes too high due to Cu precipitation, the CTOD value decreases, and if Cu precipitation is insufficient, the strength is insufficient even if the CTOD value is high.
In conventional Cu-added steels, there are almost no applications for offshore structures, and there was no strict CTOD value requirement, so it was not necessary to strictly control the average diameter and distribution of such Cu precipitate particles. .

Therefore, in the preferred embodiment of the present invention, the average diameter and the distribution amount of the Cu precipitation particles are defined as described above in order to balance the strength increase due to Cu precipitation and the decrease in CTOD value.
The reason why the equivalent circle diameter is 4 to 25 nm is for the balance between strength and toughness, and the reason why the plane rate conversion distribution amount is 3 to 20% is for the balance between strength and toughness.

The following factors can be considered as factors controlling the Cu particle size and distribution.
(1) The greater the amount of Cu added, the greater the distribution amount. Regarding the influence on the particle size, the average particle size is determined mainly by the structure before the aging treatment, the temperature and time of the aging treatment in the appropriate addition range. If the amount is less than the appropriate addition amount, Cu particles are not sufficiently precipitated and the particle size is small, and if it is large, the particle size tends to be large.

(2) The influence of the pre-aging structure is large, and it is preferable that the pre-aging structure is a fine structure mainly composed of ferrite and bainite.
Since dislocations or crystal grain boundaries serve as Cu particle precipitation sites, a structure containing many such precipitation sites reduces the Cu particle diameter and increases the amount of distribution. For this purpose, it is necessary to appropriately control the steel components, to make the rolling conditions appropriate, and to select the subsequent water cooling conditions so as to have a fine structure mainly composed of ferrite and bainite.

(3) Aging temperature and time are important factors. The target particle dispersion state is controlled by precisely adjusting the Cu diffusion rate and particle growth rate according to the aging conditions.
The steel of the present invention may be manufactured by appropriately adjusting the above three factors, and according to the above disclosure, it is not difficult for those skilled in the art to implement the present invention.

Next, the manufacturing method of the high strength steel concerning this invention is demonstrated.
Even with the steel composition as described above, Cu precipitation hardening can be fully exerted, and the strength and toughness at each position in the thickness direction of a thick material having a thickness of 50 mm or more can be uniformly and toughened. In order to improve strength, the manufacturing method must be appropriate.

  The steelmaking process may be performed by a conventional method and is not particularly limited in the present invention. Although a steel slab is obtained following the steelmaking process, it is preferable to produce a slab (steel slab) by a continuous casting method from the viewpoint of cost reduction.

  Here, the heating, hot rolling, cooling and tempering conditions of the steel slab will be described. First, a steel slab having the above component composition is heated to 900 to 1120 ° C. and hot-rolled. In the present invention, in order to obtain high toughness, it is necessary to make the austenite grains fine enough even if the upper bainite structure is generated at the center of the plate thickness of the thick-walled material. It is important to refine the austenite grains in the thick wall. If the temperature is lower than 900 ° C., this solid solution action is not sufficient, and sufficient precipitation hardening cannot be expected in the tempering treatment. However, when the heating temperature exceeds 1120 ° C., the austenite grains before rolling cannot be kept fine and sized, and the austenite grains are not uniformly refined even in subsequent rolling. Therefore, the heating temperature of the steel slab was set to 900 to 1120 ° C. Preferably it is 900-1050 degreeC, More preferably, it is 900-1000 degreeC.

In rolling, the total reduction amount at 900 ° C. or lower is desirably 50% or more. After hot rolling, water cooling is started from a temperature of ₁ Ar or higher, and quenching is performed at a temperature of 600 ° C or lower. This is for the purpose of refining the structure and suppressing Cu particle precipitation in the pre-aging treatment stage as much as possible. When water cooling is performed at a temperature lower than ₁ Ar or when cooling is performed by air cooling, the work strain is lost, which causes a decrease in strength and toughness.

  The rolling finish temperature is preferably 700 ° C. or higher, the cooling start temperature is 680 ° C. to 750 ° C., and the cooling rate to the cooling stop temperature is preferably 1 to 50 ° C./s. When the water cooling stop temperature exceeds 600 ° C., the precipitation strengthening effect in the tempering process becomes insufficient.

The Ar ₁ point is obtained by a method of measuring the volume change of the micro test piece.
Next, after hot rolling, the water-cooled steel is then heated as necessary, subjected to an aging treatment at a temperature of 540 ° C. or higher and Ac ₁ point or lower, and then cooled.

  Here, when heating to the aging temperature, the average heating rate up to the aging temperature −100 ° C. and the average cooling rate up to 500 ° C. are controlled. This is because the aging treatment sufficiently precipitates and hardens the Cu precipitate, and the heating / cooling rate is controlled in order to make the dispersion of Cu particles uniform. Therefore, the average heating rate up to the target temperature of −100 ° C. is 5 to 50 ° C./min, the holding time is 1 hour or more, and the cooling rate is 5 to 60 ° C./min or more up to 500 ° C. Is preferred.

  Here, the heating temperature in the present specification is the furnace atmosphere temperature, the holding time after heating is the holding temperature at the furnace atmosphere temperature, the rolling end temperature and the water cooling start / stop temperature are the surface temperature of the steel material, and during reheating The heating / cooling average speed is calculated from the temperature calculation at the 1 / 2t thickness position of the steel material.

In order to construct a large marine structure from the high-strength steel according to the present invention, steel materials such as plates, pipes, and shapes are assembled by welding, but they are generally used as steel plates.
In this specification, when it is said that “weldability” is excellent, it usually means that arc welding with a welding heat input of 300 kJ / cm or more is possible, but other welding methods include submerged arc welding and coating. Arc welding or the like may be used.

  Here, offshore structures include not only platforms laid on the seabed and jack-up rigs, but also semi-sub rigs (semi-submersible oil drilling rigs), and offshore structures that require weldability and low-temperature toughness. If it is a thing, there is no restriction in particular. In addition, the term “large” means that the thickness of the steel used for it is 50 mm or more.

  In this example, a 300 mm-thick steel slab having the chemical components shown in Tables 1 and 2 was produced by a continuous casting method. Here, from the viewpoint of inclusion inclusion control at the center of the plate thickness, in the continuous casting process, the temperature of the molten steel is not excessively increased, and the difference is controlled within 50 ° C with respect to the solidification temperature determined from the molten steel composition. Further, electromagnetic stirring immediately before solidification and reduction during solidification were performed.

  Tables 3 and 4 show the processing conditions of steel slabs having the chemical components shown in Tables 1 and 2. Here, the processing conditions shown in Tables 3 and 4 are the processing conditions of the steel pieces having the chemical components shown in Tables 1 and 2, respectively.

300mm thick slab is heated at each heating temperature and each heating time, then hot rolled, then cooled at an average cooling rate of 5 ° C / s from the water cooling start temperature to the water cooling stop temperature. It was. (These conditions are described as initial heating / rolling conditions in Tables 3 and 4)
Then, it reheated to each aging temperature and hold | maintained for each holding time. Here, the heating rate was controlled so that the average heating rate up to an aging temperature of −100 ° C. was 10 ° C./min, and the cooling rate was controlled so that the average cooling rate up to 500 ° C. was 10 ° C./min. (These conditions are indicated as aging treatment conditions in Tables 3 and 4)
The tensile test of the steel thus obtained was carried out in accordance with ASTM standards by taking a tensile test piece having a parallel part diameter of 12.5 mm from the center of the plate thickness in the direction perpendicular to the rolling direction.

Similarly, the CTOD test of the obtained steel was performed at −40 ° C. in accordance with the BS7448 standard, and a three-point bending test piece having a full thickness was taken from a direction perpendicular to the rolling direction.
The welded joint portion was obtained by performing 10.0 kJ / cm FCAW welding (Flux Cored Arc Welding) on a steel plate butted portion subjected to K groove processing in accordance with BS7448 standard. For the joint thus obtained, the CTOD test piece was subjected to a CTOD test at −40 ° C. on the test piece obtained by processing so that the fatigue notch of the CTOD test piece was a weld line on the straight part side of the V-shaped groove. Carried out.

  In addition, in order to confirm the compatibility with high heat input welding, the same steel was subjected to groove processing at 20 ° V, and then joined together to produce a welded joint by electrogas arc welding (EGW) with a heat input of 350 kJ / cm. A CTOD test according to ASTM E1290 was performed on the welded joint produced at this time. The CTOD specimen was processed so that the fatigue notch became a weld line, and the critical CTOD value was measured at a test temperature of −10 ° C.

  Furthermore, the average value of the equivalent circle diameter of the Cu particles is observed in a transmission electron microscope (TEM) photograph with a magnification of 100000 times. The equivalent circle diameter is measured for each precipitate having a major axis of 1 nm or more, and the diameter is photographed. It calculated by calculating | requiring the average value for every visual field. In order to reduce the variation in measurement, 10 fields of TEM photographs (one field is a rectangle of 900 x 700 nm) were observed at the position of 1/4 of the original steel thickness, and the average value was used.

  Table 1 shows test materials that satisfy the chemical components defined in the present invention. As shown in Table 5, all of these test steels manufactured and processed under the processing conditions shown in Table 3 satisfied the specified range of the dispersed state of Cu particles. For this reason, all of the test steels had high base material strength, base material CTOD characteristics, and joint CTOD characteristics (−40 ° C. and −10 ° C.).

  In Table 2, No. 40 indicates a test material that satisfies the chemical components specified in the present invention, and No. 41 to No. 60 indicate that any of the chemical component ranges are specified in the present invention. This shows the specimens that are outside the range. When these test steels were manufactured under the processing conditions shown in Table 4, Cu particles were dispersed as shown in Table 6.

  For No. 40, the chemical composition defined in the present invention was satisfied, but the dispersion state of the Cu particles did not satisfy the specified range, so the base material strength was low. Therefore, in order to achieve both high heat input welding characteristics and base metal strength, it is desirable to satisfy the dispersed state of Cu particles defined in the present invention.

  No. 41 to No. 60 do not satisfy the chemical composition specified in the present invention, and therefore satisfy the base material strength, the base material CTOD characteristic, and the joint CTOD characteristic (−40 ° C. and −10 ° C.) at the same time. I could not.

Claims (8)

In mass%, C: 0.01 to 0.10%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.020% or less, S: 0.01% or less, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.003-0.008%, O: 0.0005 0.0035% is contained, the balance is Fe and impurities, N / Al is 0.3 to 3.0, Pcm represented by the following formula (I) is 0.25 or less, Cu particles having a major axis of 1 nm or more dispersed in the plate are characterized by an average equivalent circle diameter of 4 to 25 nm and a plane rate conversion distribution of 3 to 20%. steel material.
Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B (I )   2. The high-tensile steel material according to claim 1, which contains Ti: 0.005 to 0.03% by mass.   The high-tensile steel material according to claim 1 or 2, characterized by containing Nb: 0.003 to 0.03% by mass%.   The high-tensile steel material according to any one of claims 1 to 3, characterized by containing Mo: 0.1 to 0.8% in mass%.   The high-tensile steel material according to any one of claims 1 to 4, characterized by containing at least one of Cr: 0.03-0.80% and B: 0.0002-0.002 in mass%. .   The high-tensile steel material according to any one of claims 1 to 5, characterized by containing, in mass%, V: 0.001 to 0.05%.   It contains at least one of Ca: 0.0005-0.005%, Mg: 0.0001-0.005%, REM: 0.0001-0.01% by mass%. The high-tensile steel material according to any one of 1 to 6.   An offshore structure using the high-tensile steel material according to claim 1.