WO2005052205A1

WO2005052205A1 - High tensile steel excellent in toughness of welded zone and offshore structure

Info

Publication number: WO2005052205A1
Application number: PCT/JP2004/017575
Authority: WO
Inventors: Takahiro Kamo; Takeshi Urabe; Hirofumi Nakamura; Kazushi Ohnishi; Masahiko Hamada
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2003-11-27
Filing date: 2004-11-26
Publication date: 2005-06-09
Also published as: CN1886530A; JPWO2005052205A1; US20100226813A1; KR20060090287A; US20070051433A1; JP4432905B2; CN100422370C; KR100776470B1

Abstract

A high tensile steel, characterized in that it has a chemical composition, in mass %, that 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 to 1.5 %, Ni: 0.2 to 1.5 %, Al: 0.001 to 0.05 %, N: 0.0030 to 0.0080 %, O: 0.0005 to 0.0035 %, optionally one or two of Ti: 0.005 to 0.03 %, Nb: 0.003 to 0.03 % and Mo: 0.1 to 0.8 %, and the balance: Fe or impurities, with the proviso that N/Al is 0.3 to 3.0. The high tensile steel can be welded at a high heat input and also is excellent in the toughness at a low temperature.

Description

Specification

High strength steel and offshore structures with excellent weld toughness

Technical field

The present invention relates to a high-strength steel and an offshore structure, particularly to a high-strength steel for welding and an offshore structure having excellent weld toughness.

More specifically, the present invention is applied to high-strength steel for welding used as a welding structure in buildings, civil engineering structures, construction machines, ships, pipes, tanks, marine structures, etc., and particularly to marine structures. Related to high-strength steel for welding and offshore structures, for example

The present invention relates to a 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.

Background art

[0002] In recent years, energy demand has been increasing more and more, and the search for offshore petroleum resources has been activated. In the offshore structures used for these, for example, platforms and jack-up rigs are becoming larger, and with this, steel materials such as steel plates are being made thicker, and ensuring more safety has become an important issue. You.

[0003] For ordinary marine structures, medium strength steel materials with a yield stress of 300-360MPa class are used.

1S For large structures such as those mentioned above, high-strength steel with a high strength of 460-70 OMPa class and a plate thickness exceeding 100 mm may be used.

[0004] In recent years, the search area for offshore petroleum resources has shifted to cold regions and deep water areas, and offshore structures operating in those areas or in the sea area are exposed to extremely severe weather and ocean conditions.

For this reason, the steel materials used in these offshore structures are required to have toughness in a very severe low-temperature range of, for example, 40 ° C or less, and also to have weldability.

[0005] Further, from the viewpoint of safety, the user's inspection standards are strict. For both base metal and welded parts, in addition to the conventional Charpy impact value specification, the CTOD value at the lowest operating temperature is also specified to evaluate toughness. It is becoming. In other words, even if a micro test piece cut and sampled to a size of 10 mm X 10 mm obtains stable characteristics in the Charpy test, which is an evaluation test for all, In many cases, the required characteristics cannot be satisfied with the CTOD characteristics evaluated using a test piece with the actual thickness of the structure, and even more severe CTOD characteristics are required today.

[0006] As described above, not only steel materials used for offshore structures installed in icy waters, but also line pipes for cold regions used in milder environments, or ships and LNG tanks, etc. There is a strong demand for steel materials used in large-scale welded structures to improve the low-temperature toughness of the heat affected zone (HAZ).

[0007] On the other hand, to obtain a toughness of -40 ° C or lower! / ヽぅ low temperature range! ヽ Welding must be performed under poor welding efficiency and low heat input welding conditions. Welding costs are a large part of offshore structure construction costs. The most direct way to reduce welding work costs is to use a high-efficiency welding method capable of high heat input welding and reduce the number of weld layers.

[0008] Therefore, today, it is important to perform welding for a structure for a cold region where low temperature toughness is strictly required, in consideration of the toughness of the HAZ, with welding work cost being as low as possible.

Disclosure of the invention

Problems to be solved by the invention

[0009] It is known that lowering the C content is effective in dramatically improving the toughness of HAZ in conventional steel materials. Strengthening and high strength utilizing the aging precipitation hardening effect are being attempted. For example, ASTM A710 discloses a steel utilizing the aging precipitation hardening effect of Cu, and based on such a concept, some reports have been made! /.

[0010] For example, Japanese Patent Publication No. Hei 7-81164, Japanese Patent Laid-Open No. 5-186820, Japanese Patent Laid-Open No. 5-17934

No. 4 proposes a Cu-precipitated steel having excellent toughness of a weld.

However, Japanese Patent Publication No. 7-81164 only evaluated the Charpy characteristics of a welded joint obtained with a plate thickness of 30 mm and a welding heat input of 40 kjZcm, and is unlikely to be a material compatible with large heat input welding.

Japanese Patent Application Laid-Open No. 5-186820 proposes a high-strength steel having a tensile strength of 686 MPa or more obtained by adding 0.5 to 4.0% Cu and adding copper. Since the transition temperature of the Charpy test is even 30 ° C, it is thought that low-temperature CTOD characteristics of extra-thick steel plates can be secured. hard.

[0012] Japanese Patent Application Laid-Open No. 5-179344 proposes a Cu-precipitated steel excellent in the Charpy toughness of a weld, but only evaluates the Charpy characteristics of a welded joint obtained at a welding heat input of 5 kJ, mm. Therefore, it is difficult to imagine that it is a technology that can sufficiently satisfy the safety of structures during adult thermal welding.

[0013] Here, an object of the present invention is to provide a high-strength steel for welding which generally improves the low-temperature toughness of a weld, especially the HAZ low-temperature toughness.

Means for solving the problem

[0014] The present inventors have conducted various experiments on steel components and a method for producing the same in order to develop a thick high-strength steel sheet having excellent weld toughness, and have obtained the following findings.

(i) Control the NZA1 ratio in addition to adjusting the N and A1 contents based on the Cu-added steel.

[0015] To improve the high heat input HAZ toughness of high Cu component materials, fine dispersion of carbonitrides such as TiN, Ti (C, N) and A1N is effective. Therefore, when we investigated using high Cu-Ti additives, we found that controlling the NZA1 ratio in addition to adjusting the N and A1 contents was effective. This is thought to be because if the NZA1 ratio is too small, coarse A1N precipitates, which itself has an adverse effect on toughness and, at the same time, disperses fine / large amounts of TiN. On the other hand, when the NZA1 ratio is excessive, it is considered that the solid solution N increases, but the dispersion density of A1N and TiN decreases.

[0016] In order to increase the GO yield strength, it is necessary to disperse as many fine Cu particles as possible.

(m) In order to ensure toughness, especially low temperature CTOD characteristics, it is necessary to coarsen Cu particles to some extent and to suppress the amount of dispersion.

(Iv) In order to make the dispersion state of the Cu particles uniform, generation of Cu particles before the aging treatment is suppressed as much as possible, and the dispersion state of the Cu particles is controlled by controlling the conditions of the aging treatment.

(v) The strength and toughness balance can be controlled by arranging the distribution state of the Cu particles by the average value of the equivalent circle diameter obtained from the TEM photograph and the area equivalent area ratio. (Vi) Cu particles are easily generated on crystal defects (mainly dislocations) in steel. If the dislocation density is high, the precipitation of Cu particles is promoted. Also, Cu particles on dislocations hinder dislocation movement and increase the yield strength.

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

(Viii) Large heat input welding HAZ toughness can be stabilized by controlling the hardenability by adjusting the amounts of C, Mn and Mo based on the high Cu component.

In other words, the HAZ toughness can be improved by lowering the weld crack susceptibility index Pcm value with high Cu component materials, and it was a component that low C and low Mn-Dani were effective for that purpose. However, in order to ensure high strength, it is necessary to supplement with other elements, and by controlling the amount of added Mo, it was also a component that the strength can be stabilized.

[0021] The present invention has been constituted based on such knowledge, and the gist thereof is as follows.

(1) At mass ⁰ / o, C: 0.01-0.10%, Si: 0.5% or less, Mn: 0.8-1.8%, P: 0.020% or less, S: 0.01% or less, Cu: 0.8-1.5%, Ni: 0.2-1.5%, A1: 0.001-0.05%, N: 0.0030-0.0080%, 0: 0.0005-0.0035%, Fe and impurity balance, and NZA1 of 0.3-3.0 Characterized by high strength steel.

(2) The high tensile strength steel according to the above (1), wherein the steel contains 0.005 to 0.03% by mass%.

(3) The high-strength steel according to the above (1) or (2), containing 0.003 to 0.03% by mass of Nb.

(4) The high tensile strength steel according to any one of (1) to (3) above, which contains 0.1 to 0.8% of Mo by mass%.

(5) the mass ^{_{0/0, Cr: 0.03-0.80%}} , V: 0.001-0.05%, B: 0.0002-0.0020 above, characterized by containing one or more (1) any one (4) High strength steel described in

(6) Weight ^{_{0/0, Ca: 0.0005-0.005%}} , Mg: 0.0001-0.005%, REM: characterized in that it contains 0.000 1-0.01% one or more, the (1) i (5) Written in one of High strength steel.

(7) For Cu particles having a Pcm of 0.25 or less as shown in the following formula (I) and a long diameter dispersed in steel of lnm or more, the average value of the equivalent circle diameters is 425 nm and the plane ratio The high-tensile steel according to any one of the above (1) to (6), wherein the converted distribution amount is 3 to 20%.

[0022] Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (ν / 10) + 5Β · · (Ι)

(8) An offshore structure using the high-tensile steel according to any one of (1) to (7).

The invention's effect

[0023] According to the present invention, a yield stress excellent in weldability, which enables welding with a welding heat input of 300 KJ / cm or more by a welding method such as, but not limited to, gas arc welding at an electoral port, is excellent in weldability. Production of ^two or more high-strength steels became possible. As a result, the efficiency and safety of on-site welding were significantly improved. In addition, it has become possible to provide high-strength steel that can be used in extremely harsh environments such as offshore structures.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] The present invention will be described in detail. First, the reason why the present invention is limited to the above steel composition 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 the effect of refining the structure when adding Nb, V, and the like. Below 0.01%, these effects are not sufficient. If too much force is applied, a hardened structure called martensite-austenite constituent (M-A) will be formed in the welded area, deteriorating the HAZ toughness and reducing the toughness and weldability of the base metal. Also have an adverse effect. Therefore, C should be 0.10% or less. Preferably 0.02-0.08. More preferably ί or 0. 02-0. 05 ^_0/0.

[0025] Si does not form a solid solution in force cementite, which is an effective element for pre-deoxidation of molten steel, so if added in a large amount, untransformed austenite grain force inhibits decomposition into S ferrite grains and cementite, Promotes the formation of island martensite. For these reasons, the content of Si in the steel is 0.5% or less. Preferably it is 0.2% or less, more preferably 0.15% or less. [0026] Mn is an element necessary for ensuring strength, and is also an effective element as a deoxidizing agent. Therefore, the content of Mn must be 0.8% or more. Excessive addition of Mn excessively increases the hardenability and degrades the weldability and HAZ toughness. Furthermore, since Mn is considered as an element that promotes central segregation, its content in terms of suppressing central segregation should not exceed 1.8%. Therefore, the content of Mn should be 0.8-1.8% or less. Preferably it is 0.9-1.5%.

[0027] P is an impurity element inevitably contained in steel, and is a grain boundary segregation element, which causes grain boundary cracking in HAZ. In order to further improve the toughness of the base metal, the toughness of the weld metal and the HAZ, and reduce the segregation at the center of the slab, the P content is set to 0.0020% or less. Preferably it is 0.015% or less, more preferably 0.01% or less.

[0028] When S is present in a large amount, it forms a precipitate of MnS alone serving as a welding crack initiation point. Therefore, the content of S should be 0.01% or less. It is preferably 0.008% or less, more preferably 0.005%.

[0029] Cu has the 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 required in order to expect the effect of increasing strength by ε-Cu precipitation during aging treatment. However, the higher the Cu content, the higher the susceptibility to welding hot cracking, which complicates welding work such as preheating. Therefore, the Cu content was set to 1.5% or less. Preferably, it is 0.9-1.1%.

[0030] Ni is an effective element for increasing the strength and toughness of the steel material and for further increasing the HAZ toughness. However, if the content is less than 0.2%, these effects will be lost.If the content exceeds 1.5%, it is not possible to obtain an effect worth the cost increase. It was decided. Preferably it is 0.4-1.2%.

[0031] A1 is an essential element for deoxidation. However, as the content increases, the toughness tends to deteriorate, especially in HAZ. This is presumably because coarse cluster-like alumina-based inclusion particles are easily formed. Therefore, the content of A1 is set to 0.001-0.05%. Preferably ί or 0. 001 0.03 ^_0/0. In addition there is preferably ί or 0. 001 0.015 ^_0/0

[0032] Ν contributes to fine graining of the structure by forming a nitride, but when excessively added, In addition, the toughness is deteriorated through the aggregation of nitride. Therefore, the content of N is set to 0.003-0.008%. Preferably it is 0.0035-0.0065%.

[0033] By controlling the NZA1 ratio to 0.3-3.0, large heat input HAZ toughness, especially for joints

CTOD characteristics can be improved.

This is presumably because when the NZA1 ratio is smaller than 0.3, coarse A1N precipitates, which itself has an adverse effect on toughness and, in addition, disperses fine / large amounts of TiN. On the other hand, if the NZA1 ratio exceeds 3.0, it is considered that the solute N increases and the HAZ toughness is deteriorated, and the dispersion density of A1N and TiN becomes sparse. The preferred range for making the effect more effective is 0.4-2.5.

[0034] O (oxygen) is effective for forming an oxidized product serving as a ferrite generation nucleus. On the other hand, if present in a large amount, the cleanliness deteriorates significantly, and it is difficult to secure practical toughness for the base metal, weld metal and HAZ. Therefore, the content of O is set to 0.0005 to 0.0035%. Preferably <is 0.0008-0.0018%.

[0035] Ti has an action of generating nitrides to suppress the coarsening of crystal grains and to refine the transformed structure. However, the addition of less than the specified amount does not exert the above-mentioned effects, and the addition of a large amount adversely affects the toughness of the base metal and the toughness of the welded portion. Therefore, the content of Ti is set to 0.005 to 0.03%. Preferably, it is 0.007-0.015%.

[0036] Nb improves the strength and toughness of the base material by grain refinement and carbide precipitation. On the other hand, if it is added excessively, the effect of improving the performance of the base material saturates and the toughness of HAZ is significantly impaired. The Nb content is set to 0.003—0.03%. Preferably, it is 0.003% to 0.015%.

[0037] Mo has the effect of securing quenchability and improving HAZ toughness. Mo, when added excessively, causes significant hardening in HAZ and deteriorates toughness. Therefore, the content of Mo is set to 0.1 to 0.8%. Preferably it is 0.1-0.5%.

[0038] Cr enhances the hardenability of steel materials and is effective in ensuring strength. However, when added in a small amount, Cr does not show an improvement effect. When added excessively, it prevents the hardening of the weld metal and HAZ and prevents low-temperature cracking of the weld. Tends to increase sensitivity. Therefore, when Cr is added, the Cr content is set to 0.03-0.80%. Preferably it is 0.05-0.60%. [0039] B has the effect of improving hardenability and increasing strength. On the other hand, if it is added excessively, the effect of increasing the strength is saturated, and the tendency of toughness deterioration of both the base material and HAZ becomes remarkable. Therefore, when adding B, the content of B shall be 0.0002-0.002%. Preferably it is 0.003-0.0015%.

[0040] V has a function of generating carbonitrides to suppress the coarsening of the crystal grains and to refine the transformed structure. However, the addition of less than the specified amount does not exert the above-mentioned effects, and the addition of a large amount adversely affects the toughness of the base metal and the toughness of the welded portion. Therefore, when adding V, the content of V should be 0.001-0.05%. Preferably it is 0.005-0.04%.

[0041] Ca, Mg, and REM are elements that generate oxides and sulfides that serve as precipitation nuclei for intragranular ferrite. In addition, it controls the form of the sulphide and improves the low-temperature toughness. In order to obtain such an effect of Ca, Mg, and REM, the content of Ca must be 0.0005% or more, and in the case of Mg or REM, the content must be 0.001% or more. On the other hand, if Ca exceeds 0.005%, Mg and REM exceed 0.01%, Ca and Mg-based large inclusions and clusters are formed, deteriorating the purity of steel. . Therefore, when Ca is added, the Ca content is 0.0005-0.005%, and when Mg and REM are added, the Mg and REM contents are 0.0001-0.01%.

[0042] The steel of the present invention has a Pcm represented by the following formula (I) of 0.25 or less, and the average force of the circle-equivalent diameter of Cu particles dispersed in the steel and having a major axis of 1 nm or more is not less than 1 nm. It is preferable that the thickness is 25 nm and the distribution amount in terms of plane ratio is 3 to 20%.

[0043] Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (ν / 10) + 5Β · · (Ι)

Pcm is an index that indicates the susceptibility to weld cracking. If its value is 0.25 or less, weld cracks will not occur under normal welding conditions. Therefore, Pcm should be 0.25 or less. Preheating during welding can be omitted by reducing Pcm. Preferably it is 0.22 or less, more preferably 0.20 or less.

Next, the average value of the equivalent circle diameter of the Cu precipitate and the distribution amount in terms of flatness will be described. The reason for targeting Cu particles with a diameter of 1 mm or more is that particles smaller than 1 mm contribute to increasing strength. This is because the force S is small. The upper limit of the major axis of the Cu particles is not particularly defined, but no particles exceeding 100 nm will appear if the average value is in the range of 425 nm. Although the precipitation form of the Cu particles is approximately spherical, it is not easy to measure the three-dimensional shape, so the projected shape is measured.

Here, the equivalent circle diameter is the diameter of a circle having the same area as the projected area of the particle, and specifically,

d = ^ (4a / pai) a: Projected area (nm ² ), d: Circle equivalent diameter (nm pai: 3.14

Ask by.

[0046] Regarding the distribution in terms of flatness, a steel material is processed into a thin film, and a portion having a thickness of about 0.2 micrometer is observed with a TEM, and is distributed three-dimensionally in the thin film test piece. It is calculated by measuring the area ratio when the Cu particles are planarly projected on a TEM photograph with a magnification of 100,000.

Here, the reason why the equivalent circle diameter and the distribution amount in terms of flatness are defined as described above will be described in more detail.

One of the characteristics of steel used in offshore structures is that, in order to withstand the external forces of storm waves, extremely high-strength steel with a maximum plate thickness of nearly 100 mm is often used, and because it will be used in severe conditions in the future. It is required to meet even more stringent CTOD values.

[0048] If the strength is too high due to Cu precipitation, the CTOD value will be low, and if the Cu precipitation is insufficient,

Even if the CTOD value is high, the strength will be insufficient.

In conventional Cu-added steel, the demand for strict CTOD values has hardly been met, as there are almost no applications for offshore structures.Therefore, it is not necessary to strictly control the average diameter / distribution amount of such Cu precipitate particles. Helped.

[0049] Therefore, in a preferred embodiment of the present invention, the average diameter / distribution amount of the Cu precipitated particles is specified as described above in order to balance the increase in strength due to Cu precipitation and the decrease in CTOD value. The reason why the equivalent circle diameter is set to 425 is to balance strength and toughness, and the reason why the flatness equivalent distribution is set to 3 to 20% is to balance strength and toughness.

The following factors can be considered as factors for controlling the Cu particle diameter and the distribution amount.

(1) The distribution amount increases as the Cu addition amount increases. Appropriate effect on particle size Within the addition range, the average particle size is determined mainly by the structure before the aging treatment, the temperature and the time of the aging treatment. If the amount is less than the proper amount, the precipitation of Cu particles is insufficient and the particle size is small. If the amount is large, the particle size tends to be large.

(2) The influence of the structure before aging is so great that the structure before aging is preferably a fine structure mainly composed of ferrite and bainite.

Since dislocations or crystal grain boundaries serve as Cu particle precipitation sites, a structure containing a large number of such precipitation sites increases the distribution of Cu particles by reducing the particle diameter. For this purpose, it is necessary to appropriately control the steel composition and rolling conditions, and then select the water cooling conditions so that the ferrite-bainite-based microstructure is obtained.

(3) The aging temperature and time are important factors. The target particle dispersion state is controlled by strictly adjusting the Cu diffusion rate and the particle growth rate according to the aging treatment conditions.

It is not difficult for those skilled in the art to implement the present invention based on the above disclosure if the above three factors are appropriately adjusted to produce the steel of the present invention.

Next, a method for producing a high-tensile steel according to the present invention will be described.

Even with the steel composition as described above, the precipitation hardening of Cu is sufficiently exhibited, and the strength and toughness of each position in the thickness direction of the thick material having a thickness of 50 mm or more are uniformly and highly tough. Moreover, in order to improve the yield strength, a manufacturing method must be appropriate.

[0054] The present invention is not particularly limited as long as it is performed by a conventional method up to the steel making process. Although slabs are obtained following the steelmaking process, slabs (slabs) are preferably manufactured by a continuous sintering method from the viewpoint of cost reduction.

[0055] Here, the heating, hot rolling, cooling and tempering conditions of the slab will be described. First, a steel slab having the above composition is heated to 900-1120 ° C and hot rolled. In the present invention, in order to obtain high toughness, it is necessary to sufficiently reduce austenite grains in the center of the thickness of a thick-walled material even if an upper bainite structure is generated. Therefore, it is important to refine the austenite grains in the thick slab. At a low temperature of less than 900 ° C, sufficient precipitation hardening cannot be expected in the tempering treatment in which this solution action is not sufficient. However, at a heating temperature exceeding 1120 ° C, austenite grains before rolling cannot be kept fine and sized, and even after rolling, austenite grains are not uniformly fine-grained. . Therefore, the caloric heat temperature of Maokagata was set to 900-1120 ° C. Preferably 900-1050 ° C, more preferably 900-1000. C.

[0056] In rolling, the total reduction at 900 ° C or less is desirably 50% or more.

After hot rolling, start water cooling at a temperature above the Ar point and stop at a temperature of 600 ° C or less.

1

Perform the insertion process. This is for the purpose of miniaturizing the structure and suppressing the precipitation of Cu particles before the aging treatment as much as possible. Water cooling from temperatures below the Ar point is

1

Otherwise, if the cooling is air cooling, the processing strain disappears, which causes a decrease in strength * toughness.

[0057] It is preferable that the rolling finish temperature is 700 ° C or more, the cooling start temperature is 680 ° C to 750 ° C, and the cooling rate to the cooling stop temperature is 1 to 50 ° C / s. If the water cooling stop temperature exceeds 600 ° C, the precipitation strengthening effect in the tempering treatment will be insufficient.

[0058] The Ar point is determined by a method of measuring a change in volume of the minute test piece.

1

Next, after hot rolling, the water-cooled steel is heated, if necessary, subjected to an aging treatment at a temperature of 540 or more and an Ac point or less, and then cooled.

1

Here, when heating to the aging temperature, the average heating rate up to the aging temperature of 100 ° C. and the average cooling rate up to 500 ° C. are controlled. This aging treatment is to sufficiently precipitate and harden the precipitates of Cu, and the control of the heating Z cooling rate is performed to make the dispersion of the Cu particles uniform. Therefore, the heating rate is 5-50 ° CZ for the average heating rate up to the target temperature of 100 ° C, the holding time is 1 hour or more, and the cooling rate is 5-60 ° CZ for the cooling rate up to 500 ° C. It is preferable to do so.

[0060] 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 layer temperature of the steel material, The average heating / cooling rate during reheating shall be calculated from the temperature calculation at the steel material thickness l / 2t position.

[0061] In order to construct a large offshore structure from the high-tensile steel according to the present invention, a force to assemble steel materials such as plates, pipes, and furthermore, shapes and the like by welding is generally used as a steel plate. In this specification, the term “excellent in weldability” usually means that arc welding with a welding heat input of 300 kj / cm or more is possible.Other welding methods include submerged arc welding and coating. Arc welding or the like may be used. [0062] Here, the offshore structure includes a platform laid on the sea floor, a semi-sub rig (a semi-submersible oil digging ij rig) that is not only a jack-up rig, etc. There are no particular restrictions on the offshore structure required. In addition, “large” means that the thickness of the steel used for it is 50 mm or more.

Example

[0063] 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 controlling inclusions at the center of the sheet thickness, in the continuous forming process, the temperature of the molten steel is not excessively increased, and the difference between the solidification temperature determined by the composition force of the molten steel and the solidification temperature is set within 50 ° C. Under the control, electromagnetic stirring immediately before coagulation and reduction during coagulation were performed.

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

[0065] The slab having a thickness of 300 mm was heated at each heating temperature and each heating time, and then subjected to hot rolling, and then cooled to a water cooling start temperature at an average cooling rate of 5 ° C / s to a water cooling stop temperature. The steel plate was 77 mm thick. (These conditions are shown in Tables 3 and 4 as initial heating and rolling conditions.)

Then, it was heated again to each aging temperature and held for each holding time. Here, the heating rate is controlled so that the average heating rate up to the aging temperature-100 ° C is 10 ° C / min, and the cooling rate is 10 ° C / min up to 500 ° C. Was controlled as follows. (These conditions are referred to as aging conditions in Tables 3 and 4.)

The tensile test of the steel obtained in this manner was performed by taking a tensile test specimen having a diameter of 12.5 mm in the parallel part from the center of the thickness in the direction perpendicular to the rolling direction in accordance with the ASTM standard.

[0066] The CTOD test of the steel obtained in the same manner was conducted at 40 ° C by taking a three-point bending test piece of full thickness from a direction perpendicular to the rolling direction in accordance with the BS7448 standard.

The welded joints were obtained by performing FCAW welding (Flux Cored Arc Welding) of 10. Okj / cm on the butt joints of the K-grooves in accordance with BS7448 standard. The joint obtained in this way was processed at 40 ° C on a specimen obtained by processing the CTOD specimen so that the fatigue notch of the specimen became the weld line on the straight part side of the V-shaped groove. A CTOD test was performed. [0067] In addition, in order to confirm compatibility with large heat input welding, the same steel was subjected to 20 ° V groove processing, then butt-joined, and welded joints were produced by electorifice gas arc welding (EGW) with a heat input of 350kjZcm. did. The welded joints produced at this time were subjected to a CTOD test according to ASTM E1290. The CTOD test piece was applied so that the fatigue notch became the weld line, and the critical CTOD value was measured at a test temperature of -10 ° C.

[0068] Furthermore, the average value of the circle equivalent diameter of the Cu particles was determined using a transmission electron microscope with a magnification of 100,000.

(TEM) The equivalent circle diameter was measured for each precipitate having a major axis of 1 mm or more observed in the photograph, and the diameter was calculated by obtaining an average value for each visual field of the photograph. In addition, in order to reduce the dispersion of the measurement, 10 fields of view (one field of view is a rectangle of 900 X 700 nm) of the TEM photograph were observed at a position 1/4 of the original steel thickness, and the average value was used.

Table 1 shows test materials satisfying the chemical components specified in the present invention. As shown in Table 5, when the test steels were manufactured and processed under the processing conditions shown in Table 3, the deviations also satisfied the dispersion state of the Cu particles within the specified range. Therefore, the base metal strength, base metal CTOD characteristics, and joint CTOD characteristics (-40 ° C and -10 ° C) of all test steels were high.o

[0070] In Table 2, No. 40 shows the test materials satisfying the chemical components specified in the present invention, and No. 41 to No. 60 show that any one of the chemical components ranges The test materials outside the range specified in the invention are shown. When these test steels were manufactured and processed under the processing conditions shown in Table 4, the dispersed state of Cu particles as shown in Table 6 was obtained.

[0071] For No. 40, the chemical composition defined by the present invention was satisfied, but the dispersed state of the Cu particles did not satisfy the specified range, so that 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 specified in the present invention.

[0072] Since No. 41 to No. 60 do not satisfy the chemical composition defined in the present invention, the base metal strength, base metal CTOD characteristics, and joint CTOD characteristics (-40 ° C and -10 ° C) Was not able to satisfy at the same time.

[0073] [Table 1]

/ vu / O / J0sa oifcld sossosooiAV ¾u0

Component C Si n P S Cu Ni Nb Mo AI N Ti 0 N / AI Cr V B Ca Ms REM Pcm

40 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003 0.0052 0.015 0.0013 1.7333 0.31 one one one one 0.188

41 0.1 10 0.15 1.01 0.007 0.005 0.97 0.89 one 0.30 0.012 0.0049 0.010 0.0015 0.4083 one one-one--0.249

42 0.061 0.05 1.94 0.005 0.003 0.94 0.95 one 0.20 0.003 0.0052-0.0013 1.7333 0.15 one-one one one 0.243

43 0.052 0.08 1.21 0.008 0.002 0.98 1.21--0.019 0.0006-0.0015 0.0316 0.2 0.01 1 1-0.195

44 0.041 0.16 1.05 0.009 0.003 0.95 1.03 0.019 0.26 0.005 0.0055 0.017 0.0051 1.1000------0.181

45 0.029 0.21 1.24 0.021 0.006 0.47 0.52 0.012 one 0.031 0.0071 0.015 0.0071 0.2290--one--one 0.130

46 0.042 0.35 1.02 0.009 0.006 0.87 0.86 one 0.30 0.112 0.0065 one 0.0065 0.0580 0.2 one one one-one 0.193

47 0.1 10 0.06 1.28 0.010 0.002 0.93 0.88 0.008 0.50 0.006 0.0041 0.013 0.0016 0.6833 one 0.035--0.0021-0.274

48 0.033 0.04 1.39 0.008 0.002 1.94 0.64 0.013 0.42 0.009 0.0052 0.012 0.0017 0.5778 0.22 one 0.0009 one one-0.255

49 0.043 0.09 1.39 0.010 0.002 0.98 0.10 0.005 0.42 0.003 0.0063 0.008 0.0017 2.1000 0.40----One 0.214

50 0.030 0.11 1.39 0.009 0.003 0.90 1.00 0.005 0.34 0.003 0.0041 0.011 0.0014 1.3667 0.85 0.008 0.0012 one 0.0025-0.237

51 0.041 0.14 1.06 0.005 0.003 0.93 0.88 0.015 0.85 0.004 0.0046 0.008 0.0017 1.1500 one 0.035 ― 0.0007-one 0.220

52 0.030 0.06 1.39 0.010 0.002 0.90 0.88 0.013 0.18 0.009 0.0057 0.009 0.0016 0.6333-0.061 0.0010 one-one-one 0.184

53 0.030 0.06 1.50 0.008 0.003 0.90 0.64 0.052 0.42 0.003 0.0063 0.009 0.0017 2.1000-one-0.0012--0.191

54 0.040 0.14 1, 06 0.005 0.003 1.00 0.76 0.005 0.42 0.007 0.0041 0.034 0.0016 0.5857-0.035 0.0021 One-one-one 0.202

55 0.030 0.09 1.17 0.009 0.002 0.90 0.52 0.015 0.26 0.003 0.0052 0.008 0.0014 1.7333 0.16 one 0.0045--0.0021 0.193

56 0.047 0.04 1.17 0.008 0.002 0.90 0.52 0.013 0.18 0.062 0.0032 0.012 0.0018 0.0516--0.0026 One-One 0.186

57 0.040 0.04 1.39 0.009 0.002 0.90 0.64 0.008 0.50 0.007 0.01 10 0.013 0.0017 1.5714-one 0.0012-0.0007 one 0.206

58 0.035 0.1 1 1.06 0.008 0.002 0.90 0,52 0.013 0.26 0.006 0.0046 0.008 0.0041 0.7667 one 0.015 0.0023-one 0.176

59 0.051 0,04 1.17 0.005 0.002 0.90 0.52 0.005 0.26 0.034 0.0031 0.012 0.0014 0.0912------0.182

60 0.033 0.06 1.39 0.009 0.003 0.95 0.52 0.005 0.42 0.002 0.0078 0.008 0.0013 3.9000 one 0.015-one one 0.190

[Table 3]

[Table 4] Initial heating and rolling conditions Aging treatment conditions Heating temperature Heating time Finishing temperature Water cooling start Water cooling stop Aging temperature Holding time Component (° C) (hour) (° C) Temperature (° C) Temperature (° c) (.C) (hour )

40 950 10 730 720 350 720 5

41 950 10 750 705 350 600 5

42 950 10 740 710 350 580 5

43 950 10 715 710 350 550 5

44 950 10 720 720 350 620 5

45 950 10 725 710 350 550 5

46 950 10 730 715 350 590 5

47 1000 5 730 720 250 600 5

48 950 5 730 710 250 580 5

49 1000 5 730 720 250 600 5

50 1000 5 720 700 250 600 5

51 950 5 730 710 250 600 5

52 950 5 730 720 250 590 5

53 950 5 730 700 250 590 5

54 1000 5 730 700 250 600 5

55 1000 5 720 700 250 600 5

56 1000 5 730 700 250 600 5

57 950 5 730 700 250 600 5

58 1000 5 730 710 250 600 5

59 950 5 730 700 250 600 5

60 950 5 730 700 250 600 5]

I Small heat input welding Large heat input welding Flatness Base material CTO [Joint CTOD Joint CTOD component (nm) (%) (N / mm2) (N / mm2) (mm) (mm) (mm)

1 16 5 510 571> 1.3 0.61 0.31

2 14 16 580 627> 1.3 0.98 0.41

3 15 17 589 640 1.10 0.79 0.32

4 15 14 563 637 1.09 0.61 0.40

5 15 15 571 641> 1.3 1.10 0.42

6 16 10 512 572 0.89 0.62 0.29

7 14 16 592 646> 1.3 0.83 0.31

8 17 17 509 578> 1.3 1.12 0.28

9 16 15 506 564 1.20 0.62 0.29

10 15 16 499 552 1.00 0.70 0.31

1 1 16 15 520 579> 1.3 0.56 0.30

12 14 14 497 552> 1.3 0.80 0.42

13 14 16 521 578> 1.3 0.72 0.42

14 15 16 493 572 1.10 0.37 0.41

15 13 14 523 589> 1.3 0.46 0.31

16 14 15 512 584> 1.3 0.53 0.37

17 12 13 530 600> 1.3 0.26 0.27

18 14 14 524 591> 1.3 0.53 0.32

19 15 17 519 587 1.20 0.41 0.33

20 13 16 541 612 1.30 0.63 0.41

21 17 9 497 558 1.10 0.35 0.29

22 18 δ 489 553> 1.3 0.31 0.41

23 13 14 531 603 1.10 0.56 0.36

24 14 13 528 600 1.20 0.71 0.31

25 16 15 512 574> 1.3 0.38 0.29

26 14 16 510 581> 1.3 0.42 0.31

27 15 14 507 572> 1.3 0.61 0.36

28 13 13 482 551> 1.3 0.31 0.29

29 12 14 500 571> 1.3 0.42 0.31

30 13 16 510 581> 1.3 0.35 0.31

31 12 15 502 573> 1.3 0.32 0.36

32 14 14 501 569> 1.3 0.31 0.34

33 14 16 580 627 1.10 0.94 0.40

34 15 17 584 642 0.94 0.79 0.35

35 15 14 560 637 1.09 0.54 0.41

36 15 15 569 640 1.10 0.63 0.42

6]

Small heat input welding Large heat input welding Flatness Base material CTO [Joint CTOD Joint CTOD

Cu particle size conversion distribution Ys Ts -40 ° C -40 ° C-10 ° C Component (nm) (%) (N / mm2) (N / mm2) (mm) (mm) (mm)

40 28 2 417 472> 1.3 0.67 0.30

42 1 7 16 626 71 1 0.12 0.09 0.006

43 14 14 642 720 0.19 0.12 0.006

44 14 13 632 691 0.41 0.06 0.007

45 17 17 578 632 0.08 0.09 0.007

46 10 15 384 420 0.84 0.71 0.006

47 17 15 574 623 0.12 0.04 0.006

48 16 15 621 716 0.10 0.03 0.007

49 18 23 619 701 0.08 0.02 0.007

50 14 16 512 589 0.08 0.03 0.006

51 15 15 623 712 0.08 0.04 0.007

52 14 16 621 700 0.09 0.01 0.008

53 13 14 552 636 0.08 0.02 0.008

54 13 15 541 617 0.09 0.01 0.006

55 17 13 543 675 0.09 0.03 0.008

56 14 17 567 648 0.08 0.04 0.009

57 14 13 51 1 652 0.04 0.03 0.008

58 14 14 498 612 0.06 0.02 0.008

59 16 15 470 530 0.06 0.03 0.009

60 16 16 502 580 0.21 0.16 0.018

61 15 15 491 565 0.22 0.17 0.019

Claims

The scope of the claims

[1] in a weight ^{_{0/0, C:. 0.01-0}} 10%, Si: 0.5% or less, Mn: 0.8- 1.8%, P : 0.020% or less, S: 0.01% or less, Cu: 0.8-1.5%, Ni: 0.2-1.5%, A1: 0.001-0.05%, N: 0.003-0.008%, 0: 0.0005-0.0035%, Fe and impurities remaining, and NZA1 0.3-3.0 A high-tensile steel characterized by the following.

[2] The high tensile strength steel according to claim 1, wherein the steel contains 0.005 to 0.03% by mass%.

[3] The high-strength steel according to claim 1 or 2, wherein the steel contains 0.003 to 0.03% by mass of Nb.

[4] The high-strength steel according to any one of claims 1 to 3, wherein the high-strength steel contains 0.1 to 0.8% by mass of Mo.

In [5] Mass ^{_{0/0, Cr: 0.03-0.80%}} , B: claim of 1, characterized in that it contains 0.0002-0.002 one or more! High-strength steel according to any one of 4 above.

[6] The high-tensile steel according to any one of claims 1 to 5, wherein the steel contains V: 0.001 to 0.05% by mass%.

In [7] Mass ^{_{0/0, Ca: 0.0005-0.005%}} , Mg: 0.0001-0.005%, REM: 0.000

The high-strength steel according to any one of claims 1 to 6, comprising one or more of 1 to 0.01%.

[8] For Cu particles with a Pcm of 0.25 or less and a long diameter of lnm or more dispersed in steel, the average value of the equivalent circle diameters is 425 nm and the distribution in terms of flatness is as follows. The high-tensile steel according to any one of claims 1 to 7, wherein the amount is 3 to 20%.

Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5Β Ι)

[9] An offshore structure using the high-tensile steel according to any one of claims 18 to 18.