JP2011001625A - High tensile strength steel having excellent corrosion resistance and weld zone toughness, and marine structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel for welding in which low temperature toughness in the weld zone, particularly low temperature toughness in the HAZ is improved, also, corrosion resistance is improved, and a coating life can be elongated.SOLUTION: The high tensile strength steel has a composition containing 0.01 to 0.10%C, ≤0.5% Si, 0.8 to 1.8% Mn, ≤0.030% P, ≤0.02% S, 0.8 to 1.5% Cu, 0.2 to 1.5% Ni, 0.01 to 0.30% Sn, 0.001 to 0.05% sol.Al, 0.003 to 0.008% N and ≤0.0035% O, and the balance Fe with impurities, and in which N/Al is 0.3 to 3.0; Sn/Cu is ≥0.025; and further, the value of Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B is ≤0.25; and, regarding the Cu particles with a major axis of ≥1 nm dispersed into the steel, the average value of a circle equivalent diameter is 4 to 25 nm; and also, the distribution amount expressed in terms of a flat rate is 3 to 20%. The steel has excellent corrosion resistance and weld zone toughness.

Description

The present invention relates to high-strength steel and offshore structures, and particularly to high-strength steel and offshore structures excellent in corrosion resistance and 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. For example, offshore structures such as platforms and jack-up rigs that are used for this purpose are becoming larger, and as a result, steel materials such as steel plates become thicker, and ensuring further safety is an important issue. . For ordinary marine structures, medium-strength steel materials with a yield stress of 300 to 360 MPa are used. For the large structure as described above, an extremely thick high-tensile steel material having a high strength of 460 to 700 MPa class and a plate thickness exceeding 100 mm may be used.

  In recent years, search areas for submarine oil resources have moved to cold regions and deep waters. Offshore structures operating in these regions and seas are exposed to extremely severe weather and ocean conditions. For this reason, 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 lower, and of course weldability.

  Moreover, the inspection standard of the user is strict from the viewpoint of safety. The toughness of steel materials has been evaluated for both the base metal and the welded part in addition to the conventional definition of Charpy impact value, in addition to the definition of the CTOD value at the minimum operating temperature. Even when stable characteristics can be obtained by the Charpy test, which is an evaluation test for a micro test piece cut and sampled to a size of 10 mm × 10 mm, the CTOD characteristics evaluated by the test piece of the actual thickness of the structure This is because there are many cases where desired characteristics cannot be satisfied. Today, more stringent CTOD characteristics are required.

  Thus, the desire to improve the low temperature toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) is not limited to steel materials used in offshore structures installed in ice seas, but in milder environments than this. It is also strong against steel pipes used for large-scale welded structures such as line pipes for cold regions or ships and LNG tanks.

  On the other hand, in order to obtain high toughness in a low temperature range of −40 ° C. or lower, it is necessary to perform welding 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 welding under welding conditions with a large heat input.

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 HAZ.
In addition, offshore structures are exposed to corrosive environments from seawater. In particular, the sea is in an extremely severe corrosive environment where it is repeatedly subjected to wet and dry conditions caused by tidal and seawater splashes. For this reason, the marine structure is painted as a countermeasure against corrosion. However, corrosion progresses rapidly at portions where the coating thickness is thin, such as edges and coating scratches, and portions where the steel surface is exposed. Although the portion where corrosion has progressed is repaired, the repair cost increases and the repair may not be possible depending on the portion. Therefore, extending the coating life in a corrosive environment is also an important issue.

  Conventionally, low carbon content is known as a means for dramatically improving the toughness of HAZ of steel materials. In order to compensate for the strength reduction due to the low carbon content, the strength can be increased by adding various alloy elements and the strength can be increased by utilizing the aging precipitation hardening action. For example, ASTM A710 discloses steel utilizing the aging precipitation hardening action of Cu, and several inventions based on such a concept have been made. For example, Patent Documents 1 to 3 disclose inventions related to Cu precipitation steel having excellent weld toughness.

Japanese Examined Patent Publication No. 7-81164 Japanese Patent Laid-Open No. 5-186820 Japanese Patent Laid-Open No. 5-179344

  However, since Patent Document 1 merely evaluates the Charpy characteristics of a welded joint obtained with a plate thickness of 30 mm and a welding heat input of 40 kJ / cm, the invention related to the Cu precipitation steel disclosed in Patent Document 1 is a large heat input. It is difficult to imagine that welding under the welding conditions described above is possible.

  Patent Document 2 discloses a tensile strength of 686 MPa or more to which Cu is added in an amount of 0.5 to 4.0% (in this specification, “%” means “% by mass” unless otherwise specified). An invention relating to a high-strength steel is disclosed. However, since the low temperature toughness is −30 ° C. even at the transition temperature of the Charpy test, it is difficult to think that the low temperature CTOD characteristic can be secured in the extra heavy steel plate.

  Furthermore, since Patent Document 3 merely evaluates the Charpy characteristics of a welded joint obtained with a welding heat input of 5 kJ / mm, the invention disclosed in Patent Document 3 is a Cu precipitation type steel excellent in Charpy toughness of a welded portion. Although it is such an invention, it is unlikely that the safety of a structure that is welded and assembled under welding conditions with a large heat input can be sufficiently satisfied.

  Furthermore, although steel containing Cu is generally considered to be excellent in corrosion resistance in a salt environment, the corrosion resistance is deteriorated in a corrosive environment where salt is accumulated. For this reason, the invention concerning Cu precipitation type steel indicated by patent documents 1-3 may become a problem in terms of corrosion resistance.

  It is an object of the present invention to generally improve the low-temperature toughness of welds, particularly HAZ low-temperature toughness, as well as high-strength steel and offshore structures that can improve corrosion resistance and extend coating life Is to provide.

  The present inventors conducted various experiments on steel components and production methods for the purpose of developing a thick-walled high-strength steel sheet excellent in weld zone toughness. As a result, the following findings (i) to (viii) Got.

  (I) Based on Cu-added steel, in addition to adjusting the N content and Al content, the (N / Al) ratio is controlled. That is, in order to improve the HAZ toughness when welding with high heat input welding conditions in steel with a high Cu content, fine dispersion of carbonitrides such as TiN, Ti (C, N), AlN, etc. Is effective. In a steel containing a high Cu content and Ti, it is effective to control the (N / Al) ratio in addition to the adjustment of the N content and the Al content. When the (N / Al) ratio is too small, coarse AlN is precipitated, which itself adversely affects toughness, and it is considered that fine and large-scale dispersion of TiN is inhibited. On the other hand, if the (N / Al) ratio is excessive, it is considered that in addition to the increase in solid solution N, 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) In order to ensure toughness, particularly low temperature CTOD characteristics, it is necessary to coarsen the 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, the generation of Cu particles in the stage 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 balance of strength and toughness can be controlled by organizing the distribution state of Cu particles using the average value of equivalent circle diameters obtained from TEM photographs and the plane conversion area ratio.

  (Vi) Cu particles are easily generated on crystal defects (mainly dislocations) in steel, and precipitation of Cu particles is promoted when the dislocation density is high. Further, the Cu particles on the dislocation inhibit the movement of the dislocation and increase the yield strength.

  (Vii) The dislocation density in the steel can be controlled by rolling conditions and water cooling conditions. Further, a decrease in rolling temperature, an increase in total rolling reduction, an increase in water cooling start temperature, an increase in cooling rate, and a decrease in water cooling stop temperature all increase the dislocation density.

  (Viii) Stabilization of HAZ toughness when welding under high heat input welding conditions by controlling hardenability by adjusting C content, Mn content and Mo content based on steel with high Cu content Is possible. That is, in steel with a high Cu content, the HAZ toughness can be improved as the weld crack sensitivity index Pcm value decreases. For that purpose, reduction of the C content and the Mn content is effective. However, supplementation with other elements is necessary to ensure high strength. By adjusting the Mo content in addition to the decrease in the C content and the Mn content, the strength and toughness can be stabilized.

The present inventors have further obtained the following findings (ix) to (xi) regarding the corrosion resistance of the bare steel material and the painted steel material.
(Ix) In general, Cu improves corrosion resistance in repeated wet and dry environments with seawater. However, in an environment where the adhering salt is accumulated without being washed away, the corrosion resistance is deteriorated by containing Cu.

  (X) At this time, by containing Sn, corrosion resistance is greatly improved both in an environment where salt is washed away and in an environment where washing is not carried out, and corrosion at coating film scratches is remarkably suppressed in coated steel materials. Furthermore, even when repainting is performed with rust remaining during repair, corrosion of the scratches on the coating film is remarkably suppressed.

The mechanism by which the corrosion resistance improvement effect by containing Sn is obtained is considered as follows. In a salt environment, Cl ions locally concentrate at the corrosion site and the pH is lowered. In such an environment, Sn melts and precipitates on the steel material. Since Sn is an element having a large hydrogen overvoltage, the hydrogen generation reaction, which is a cathode reaction in an environment having a low pH, is remarkably suppressed in a portion where Sn is deposited, and as a result, the corrosion resistance is improved. Moreover, even when Sn is present as ions, there is an effect of suppressing the anode reaction that is a dissolution reaction of the steel material. This is because the action of Sn ions suppresses the adsorption of OH ⁻ and / or Cl ⁻ ions to the iron surface, which is the iron dissolution path, and suppresses the dissolution of iron itself.

  (Xi) Corrosion resistance due to Sn cannot be obtained unless a certain amount or more of Sn is contained. In an environment where the adhering salt content is accumulated without being washed away, the corrosion resistance is more severely deteriorated as the Cu content is larger. Therefore, it is necessary to contain Sn sufficient to compensate for the deterioration caused by Cu. Specifically, it is necessary to contain Sn in an amount satisfying Sn / Cu ≧ 0.025.

This invention is made | formed based on such knowledge (i)-(xi), The summary is as follows.
(1) C: 0.01% to 0.10%, Si: 0.5% or less, Mn: 0.8% to 1.8%, P: 0.030% or less, S: 0.02 %: Cu: 0.8% to 1.5%, Ni: 0.2% to 1.5%, Sn: 0.01% to 0.30%, sol. Al: 0.001% or more and 0.05% or less, N: 0.003% or more and 0.008% or less, O: 0.0035% or less, the balance being Fe and impurities, and N / Al Is 0.3 or more and 3.0 or less, Sn / Cu is 0.025 or more, Pcm represented by the following formula (I) is 0.25 or less, and the major axis dispersed in the steel is 1 nm or more. High-tensile steel excellent in corrosion resistance and weld zone toughness, characterized in that the mean value of equivalent circle diameter of Cu particles is 4 nm or more and 25 nm or less, and the plane rate conversion distribution amount is 3% or more and 20% or less .
Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B (I) )
(2) The high-strength steel excellent in corrosion resistance and weld toughness as described in (1) above, containing Ti: 0.03% or less.

(3) Nb: 0.03% or less. The high-strength steel excellent in corrosion resistance and weld toughness described in (1) or (2) above.
(4) Mo: High-tensile steel excellent in corrosion resistance and weld toughness according to any one of items (1) to (3), characterized by containing 0.8% or less.

  (5) From (1) to (4) above, characterized by containing one or two selected from the group consisting of Cr: 0.80% or less and B: 0.002% or less High tensile strength steel excellent in corrosion resistance and weld toughness according to any one of the above.

(6) V: A high-tensile steel excellent in corrosion resistance and weld toughness according to any one of (1) to (5) above, characterized by containing 0.05% or less.
(7) One or more selected from the group consisting of Ca: 0.005% or less, Mg: 0.01% or less, and REM: 0.01% or less (1) The high-tensile steel excellent in corrosion resistance and weld zone toughness according to any one of items (6) to (6).

  (8) An offshore structure excellent in corrosion resistance and weld toughness using the high-tensile steel described in any one of (1) to (7).

By the present invention, although not particularly limited thereto, for example, by a welding method such as electrogas arc welding, it is excellent in weldability that welding can be performed under welding conditions of large heat input with a welding heat input of 300 KJ / cm or more, It has become possible to produce a high-strength steel that has a yield stress of 420 N / mm ² or more and can be used even in extremely severe environments such as offshore structures.

  For this reason, according to the present invention, for example, the on-site welding efficiency and safety of large offshore structures can be significantly increased.

FIG. 1 is an explanatory view showing an outline of a test piece for corrosion test.

Hereinafter, embodiments for carrying out the present invention will be described in detail. First, the reason for limiting the composition of the high-strength steel according to the present invention will be described.
[C: 0.01% or more and 0.10% or less]
C is contained for securing strength and for producing an effect of refining the structure when Nb, V, or the like is contained. If the C content is less than 0.01%, these effects are not sufficient. However, when the C content exceeds 0.10%, a hardened structure called island-like martensite (MA) is generated in the welded portion to deteriorate the HAZ toughness, and the toughness of the base material and It also has an adverse effect on weldability. For this reason, C content shall be 0.01% or more and 0.10% or less. Preferably they are 0.02% or more and 0.08% or less, More preferably, they are 0.02% or more and 0.05% or less.

[Si: 0.5% or less]
Si is an effective element for preliminary deoxidation of molten steel. Since Si does not dissolve in cementite, if the Si content exceeds 0.5%, untransformed austenite grains are inhibited from being decomposed into ferrite grains and cementite, and the formation of island martensite is promoted. For this reason, Si content shall be 0.5% or less. Preferably it is 0.2% or less, More preferably, it is 0.15% or less.

[Mn: 0.8% to 1.8%]
Mn is an element effective for ensuring strength and is also an effective element as a deoxidizer. For this reason, content of Mn shall be 0.8% or more. However, if the Mn content is excessive, the hardenability is excessively increased and the weldability and the HAZ toughness are deteriorated. Furthermore, Mn is an element that promotes center segregation, and in order to suppress center segregation, the Mn content should not exceed 1.8%. Therefore, the Mn content is 0.8% or more and 1.8% or less. Preferably they are 0.9% or more and 1.5% or less.

[P: 0.030% or less]
P is an impurity element inevitably contained in the steel, and is a grain boundary segregation element, and therefore causes grain boundary cracking in the HAZ. Furthermore, in order to improve the base material toughness, the toughness of the weld metal part and the HAZ, and to reduce slab center segregation, the P content is set to 0.030% or less. Preferably it is 0.015% or less, More preferably, it is 0.01% or less.

[S: 0.02% or less]
S is an impurity element inevitably contained in the steel, and when it is present in a large amount, it forms a precipitate of MnS as a starting point for weld cracking. Therefore, the S content is 0.02% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less.

[Cu: 0.8% to 1.5%]
Cu has the effect of increasing the strength and toughness of the steel material, and has little adverse effect on the HAZ toughness. In particular, it contains 0.8% or more in order to obtain the effect of increasing the strength due to ε-Cu precipitation during aging treatment. However, if the Cu content exceeds 1.5%, the sensitivity to welding hot cracking becomes high, the welding work such as preheating becomes complicated, and the corrosion resistance is remarkably deteriorated in an environment where salt is attached and is not washed away. Therefore, the Cu content is 1.5% or less. Preferably they are 0.9% or more and 1.1% or less.

[Ni: 0.2% to 1.5%]
Ni is an effective element for increasing the strength and toughness of the steel material and further increasing the HAZ toughness. However, if the Ni content is less than 0.2%, this effect cannot be obtained, and even if the Ni content exceeds 1.5%, it is not possible to obtain an effect commensurate with the cost increase. Therefore, the Ni content is 0.2% to 1.5%. Preferably they are 0.4% or more and 1.2% or less.

[Sn: 0.01% or more and 0.30% or less]
Sn is an element that enhances the corrosion resistance of bare steel and painted steel under a salt environment. This effect can be obtained when the Sn content is 0.01% or more, but when the Sn content exceeds 0.30%, the toughness deteriorates. Therefore, the Sn content is set to 0.01% or more and 0.30% or less. Preferably they are 0.03% or more and 0.25% or less.

[Sol. Al: 0.001% to 0.05%]
Al is an essential element for deoxidation. However, as the Al content increases, the toughness tends to deteriorate particularly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, sol. Al content shall be 0.001% or more and 0.05% or less. Preferably it is 0.001% or more and 0.03% or less, More preferably, it is 0.001% or more and 0.015% or less.

[N: 0.003% to 0.008%]
N contributes to the refinement of the structure by forming nitrides, but if contained excessively, N deteriorates toughness through aggregation of nitrides. Therefore, the N content is set to 0.003% or more and 0.008% or less. Preferably it is 0.0035% or more and 0.0065% or less.

[O: 0.0035% or less]
O (oxygen) is present as an impurity, and if it is present in a large amount, the cleanliness deteriorates remarkably, making it difficult to ensure practical toughness for both the base material, the weld metal part and the HAZ. Therefore, the O content is 0.0035% or less. Preferably it is 0.0018% or less.

[N / Al: 0.3 to 3.0]
By making the ratio of N content to Al content (N / Al) 0.3 or more and 3.0 or less, HAZ toughness when welded under welding conditions with a large heat input, particularly joint CTOD characteristics, is improved. It is possible. This is because when the ratio (N / Al) is smaller than 0.3, coarse AlN precipitates, and the coarse Al itself adversely affects toughness, and the fine and large amount of dispersion of TiN is inhibited. Conceivable. On the other hand, when the ratio (N / Al) exceeds 3.0, the solid solution N increases, and the HAZ toughness deteriorates. In addition, the dispersion density of AlN and TiN becomes sparse. Therefore, the ratio (N / Al) is set to 0.3 or more and 3.0 or less. A preferable range for achieving the effect is 0.4 or more and 2.5 or less.

[Sn / Cu: 0.025 or more]
By setting the ratio of Sn content to Cu content (Sn / Cu) to be 0.025 or more, it is sufficient even in an environment where the adhering salt is accumulated without being washed away, more specifically in an environment such as under the eaves. Can exhibit excellent corrosion resistance. As described above, when steel containing Cu is used in places such as under the eaves, corrosion proceeds at an early stage. In order to compensate for this, the Sn content is determined corresponding to the Cu content. When the ratio (Sn / Cu) is 0.025 or more, corrosion due to Cu can be avoided, and sufficient corrosion resistance can be obtained even in a salt accumulation environment. The ratio (Sn / Cu) is preferably 0.05 or more.

Next, arbitrary elements will be described.
[Ti: 0.03% or less]
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 Ti content exceeds 0.03%, it adversely affects the base metal toughness and weld zone toughness. Therefore, the Ti content is set to 0.03% or less. Preferably it is 0.015% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable that Ti content is 0.005% or more, More preferably, it is 0.007% or more.

[Nb: 0.03% or less]
Nb improves the strength and toughness of the base material by refining and carbide precipitation. However, when the Nb content exceeds 0.03%, the performance improvement effect of the base material is saturated and the toughness of the HAZ is significantly impaired. Therefore, the Nb content is 0.03% or less. Preferably it is 0.015% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable to contain 0.003% or more.

[Mo: 0.8% or less]
Mo has the effect of ensuring hardenability and improving HAZ toughness. However, if the Mo content exceeds 0.8%, the HAZ is markedly hardened and the toughness is deteriorated. Therefore, the Mo content is set to 0.8% or less. Preferably it is 0.5% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable that Mo content is 0.1% or more.

[One or two selected from the group consisting of Cr: 0.80% or less and B: 0.002% or less]
Cr increases the hardenability of the steel material and is effective in securing the strength. However, if the Cr content exceeds 0.80%, the weld metal part and the HAZ tend to be hardened and increase the weld cold cracking susceptibility. Therefore, the Cr content is set to 0.80% or less. Preferably it is 0.60% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable that Cr content is 0.03% or more, and it is further more preferable that it is 0.05% or more.

  B has the effect of improving the hardenability and increasing the strength. However, when the B content exceeds 0.002%, the effect of increasing the strength is saturated, and the tendency of deterioration in toughness becomes remarkable in both the base material and HAZ. Therefore, the B content is 0.002% or less. Preferably it is 0.0015% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable that B content is 0.0002% or more, and it is further more preferable that it is 0.0003% or more.

Cr and B can be contained alone or in combination.
[V: 0.05% or less]
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 V content exceeds 0.05%, the base metal toughness and weld zone toughness are adversely affected. Therefore, the V content is 0.05% or less. The V content is preferably 0.04% or less. Moreover, in order to acquire the effect mentioned above reliably, it is preferable that V content is 0.001% or more, and it is further more preferable that it is 0.005% or more.

[One or more selected from the group consisting of Ca: 0.005% or less, Mg: 0.01% or less, and REM: 0.01% or less]
Ca, Mg, and REM are elements that generate oxides and sulfides as precipitation nuclei for intragranular ferrite, and improve the low-temperature toughness by controlling the form of sulfides. However, when the Ca content exceeds 0.005%, the Mg content exceeds 0.01%, and the REM content exceeds 0.01%, large inclusions and clusters of Ca and Mg are generated. Degrading the cleanliness of steel. Therefore, the Ca content is 0.005% or less, the Mg content is 0.01% or less, and the REM content is 0.01% or less. In order to reliably obtain the above-described effects, the Ca content is 0.0005% or more, the Mg content is 0.0001% or more, and the REM content is 0.0001% or more. preferable.

The balance other than those described above is Fe and impurities.
In the high-strength steel according to the present invention, Pcm represented by the formula (I) is 0.25 or less, and the average value of equivalent circle diameters is 4 nm or more and 25 nm for Cu particles having a major axis of 1 nm or more dispersed in the steel. And the plane rate conversion distribution amount is 3% or more and 20% or less. These will be described.

[Formula (I); Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + Pcm shown by 5B: 0.25 or less]
Pcm is an index representing weld crack sensitivity. If the value of Pcm is 0.25 or less, weld cracks do not occur under normal welding conditions. Therefore, Pcm is set to 0.25 or less. When Pcm is lowered, preheating during welding can be omitted. Preferably it is 0.22 or less, More preferably, it is 0.20 or less.

[For Cu particles having a major axis of 1 nm or more dispersed in steel, the average value of equivalent circle diameters: 4 nm to 25 nm, and the plane rate conversion distribution amount: 3% to 20%]
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 nm to 25 nm. In addition, although the precipitation form of Cu particle | grains is a spherical shape, since it is not easy to measure a solid | 3D shape, the projected shape is measured.

Here, the circle-equivalent diameter is a diameter of a circle having the same area as the projected area of the particles, and is specifically obtained as d = √ (4a / pai). Here, a is the projected area (nm ² ), d is the equivalent circle diameter (nm), and pai is 3.14.

  Flatness conversion distribution amount is the case where a steel material is processed into a thin film shape, a TEM observation is performed on a portion having a thickness of about 0.2 micrometers, and Cu particles distributed three-dimensionally in the thin film specimen are projected on a plane. The area ratio is calculated by measuring a TEM photograph with a magnification of 100,000.

Here, the reason for prescribing the equivalent circle diameter and the flat rate conversion distribution amount as described above will be described in more detail.
As a feature of steel used in offshore structures, it is often very heavy high-tensile steel with a maximum thickness of 100mm to withstand external forces caused by storm waves. Further, it is required to satisfy strict CTOD values.

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

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

The following can be considered as factors controlling the Cu particle diameter and the distribution amount.
(A) The distribution amount increases as the Cu content increases. 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, as long as the content is in the proper range. If the content is less than the appropriate content, Cu particles are not sufficiently precipitated and the particle size is small, and if the content is large, the particle size tends to be large.

(B) The influence of the pre-aging structure is large, and the pre-aging structure 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 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, make the rolling conditions appropriate, and select the subsequent water-cooling conditions so that the microstructure is mainly composed of ferrite and bainite.

(C) Aging temperature and time are important factors. The target particle dispersion state is controlled by strictly adjusting the Cu diffusion rate and particle growth rate according to the aging treatment conditions.
The above-described three factors may be appropriately adjusted to produce the high-strength steel according to the present invention, and those skilled in the art can easily implement the present invention based on the above description.

Next, the manufacturing method of the high strength steel which concerns on this invention is demonstrated.
Even in the case of having the steel composition specified in the present invention described above, the precipitation hardening of Cu is sufficiently exhibited, and the strength and toughness at each position in the plate thickness direction of a thick material having a thickness of 50 mm or more are uniformly increased. Suitable manufacturing conditions for toughening and improving yield strength will be described below.

  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 steel making process, it is preferable to produce a slab (steel slab) by a continuous casting method in order to reduce manufacturing costs. Next, the heating, hot rolling, cooling and tempering conditions of the steel slab will be described.

  First, a steel slab having the above steel composition is heated to 900 ° C. or higher and 1120 ° C. or lower, and then 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 is set to 900 ° C. or higher and 1120 ° C. or lower. Preferably they are 900 degreeC or more and 1050 degrees C or less, More preferably, they are 900 degreeC or more and 1000 degrees C or less.

In rolling, it is desirable that the total reduction amount at 900 ° C. or less is 50% or more. After the hot rolling, water cooling is started from a temperature of _one or more points of Ar, and quenching is performed at a temperature of 600 ° C. or lower. This is because the structure is refined and the precipitation of Cu particles in the pre-aging stage is suppressed as much as possible. In the case of water cooling from a temperature less than _one Ar point, or when the cooling is air cooling, loss of processing strain occurs, which causes a decrease in strength and toughness.

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

In addition, Ar ₁ point is calculated | required by the method of measuring the volume change of a micro test piece.
Next, after hot rolling, the water-cooled steel is then heated if 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 an aging temperature, it is preferable to control about the average heating rate to aging temperature-100 degreeC, and the average cooling rate to 500 degreeC. This is because the aging treatment sufficiently precipitates and hardens the Cu precipitate, and the control of the heating rate and the cooling rate is performed in order to make the dispersion of Cu particles uniform. Therefore, the heating rate is 5 ° C./min to 50 ° C./min, 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 500 ° C./average cooling rate up to 500 ° C. It is preferable to set it to 60 ° C./min.

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

In order to construct a large marine structure from the high-tensile steel according to the present invention, steel materials such as a plate material, a pipe material, and a shape material are assembled by welding, but generally used as a steel plate.
In this specification, “excellent weldability” 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 covered arc welding. It may be.

  “Ocean structures” include not only platforms laid on the seabed and jack-up rigs, but also semi-sub rigs (semi-submersible oil drilling rigs), etc. If it is a structure, there will be no restriction | limiting in particular. In addition, the term “large” means an offshore structure having a steel material thickness of 50 mm or more.

In this way, according to the present invention, for example, by a welding method such as electrogas arc welding, it is possible to perform welding under welding conditions with a large heat input at a welding heat input of 300 KJ / cm or more, and the yield stress is 420 N. a is / mm ² or more, and has enabled the production of high strength steel that can be used in extremely harsh environments, such as marine structures.

The present invention will be described more specifically with reference to examples.
In this example, a 300 mm thick steel slab having the composition shown in Tables 1 and 2 was produced by a continuous casting method. Here, from the viewpoint of 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 within 50 ° C. with respect to the solidification temperature determined from the molten steel composition. In addition, electromagnetic stirring immediately before solidification and reduction during solidification were performed.

  Tables 3 and 4 show the processing conditions of the steel slabs having the chemical components shown in Tables 1 and 2.

  A 300 mm thick slab is heated at each heating temperature and each heating time, and then hot-rolled, and 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 shown in Tables 3 and 4 as initial heating / rolling conditions.

  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 shown in Tables 3 and 4 as aging treatment conditions.

The tensile test of the steel thus obtained was carried out in accordance with the ASTM standard 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 the steel sheet butted portion subjected to K groove processing in accordance with the 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. went. In addition, in order to confirm the compatibility with high heat input welding, the same steel was subjected to 20 ° V groove processing, and then a butt joint was produced 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. In the present invention, the weld toughness was determined based on whether both the critical CTOD values at −40 ° C. and −10 ° C. were 0.25 mm or more.

  Furthermore, the average value of the equivalent circle diameter of 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 addition, in order to reduce the variation of the measurement, the measurement was carried out by observing 10 fields of view of the TEM photograph (one field is a rectangle of 900 × 700 nm) at the position of (¼) t of the steel material original thickness, and using the average value It was.

  Furthermore, the corrosion resistance was evaluated as follows. A test piece having a width of 60 mm, a length of 100 mm, and a thickness of 3 mm was collected from the surface of the steel plate, and subjected to shot blasting on the entire surface, and then the modified epoxy paint was coated with a dry film thickness of 200 μm. One surface of the coated surface was scratched (x mark) with a cutter knife having a width of 1 mm and a length of 100 mm to expose the steel material surface, and a corrosion test piece simulating a coating film scratched portion and a steel plate edge portion was obtained. FIG. 1 is an explanatory view showing an outline of the test piece for corrosion test.

The SAE J2334 test was conducted as a corrosion test in an environment where seawater was attached and was washed away, and a salt-attached dry and wet test was conducted as a corrosion test in an environment where seawater was attached and was not washed away. The test conditions of the SAE J2334 test and the repeated test for the adhesion and drying of salt are shown below.
(SAE J2334 test)
Wet: 50 ° C. 100% RH / 6 hours Salt deposition: 0.5% NaCl + 0.1% CaCl ₂ + 0.075% NaHCO ₃ /0.25
Time drying: 60 ° C., 50% RH / 17.75 hours (salt-attached wet-dry repeat test)
NaCl adhesion: 1 mg / cm ²
Wet: 40 ° C. 80% RH / 4 hours Dry: 40 ° C. 40% RH / 4 hours Here, RH means relative humidity. For the SAE J2334 test, 40 cycles were performed, and for the salt adhesion wet / dry cycle test, 200 cycles were performed. After removing the coating film and the corrosion products from each test piece after the test, the maximum corrosion depth in the corroded portion was measured. . In the present invention, the corrosion resistance was determined based on whether the maximum corrosion depth of the corroded portion in the SAE J2334 test and the salt-attached dry / wet repeated test was 0.20 mm or less and 0.15 mm or less, respectively.

  Table 5 shows the test materials 1 to 36 that satisfy the chemical components defined in the present invention. In each of the test materials 1 to 36 produced by processing the test steels 1 to 36 having the composition shown in Table 1 under the processing conditions shown in Table 3, the dispersion state of the Cu particles satisfied the specified range. . 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 addition, since the test materials 1 to 36 contain Sn, they exhibited good corrosion resistance in both the environment where the salt content is washed away and the environment where the salt material is not washed away.

  On the other hand, among the test steels 37 to 58 having the composition shown in Table 2, the test steel 37 indicates a test steel that satisfies the chemical components defined in the present invention, and the test steels 38 to 58 are used. No. 58 shows a test steel whose chemical component range is outside the range defined in the present invention. Specimens 37 to 58 produced by processing these test steels under the processing conditions shown in Table 4 were in a dispersed state of Cu particles shown in Table 6.

  Although the test material 37 satisfies the chemical composition specified in the present invention, the dispersion state of Cu particles does not satisfy the conditions specified in the present invention, and thus the strength of the base material is low. It is necessary to satisfy the dispersion state of the Cu particles defined in the present invention in order to make both the high heat input welding characteristics and the base material strength compatible by comparing the test material 37 and the test materials 1 to 36. I know that there is.

  In addition, since the test materials 38 to 59 do not satisfy the chemical composition defined in the present invention, the base material strength, the base material CTOD characteristics, the joint CTOD characteristics (−40 ° C. and −10 ° C.), and the corrosion resistance must be satisfied at the same time. I could not.

  In particular, since the test material 58 does not contain Sn, the corrosion resistance in an environment where salt was not washed away was insufficient. Further, since the test material 59 is plain steel not containing not only Sn but also Cu and Ni, the corrosion resistance in not only the environment where salt is not washed away but also the environment where the salt is washed away is insufficient.

  According to the present invention, in an offshore structure, corrosion resistance can be improved while securing toughness of a welded portion, so that maintenance costs due to member switching and repainting can be greatly reduced.

Claims (8)

In mass%, C: 0.01% to 0.10%, Si: 0.5% or less, Mn: 0.8% to 1.8%, P: 0.030% or less, S: 0.0. 02% or less, Cu: 0.8% to 1.5%, Ni: 0.2% to 1.5%, Sn: 0.01% to 0.30%, sol. Al: 0.001% or more and 0.05% or less, N: 0.003% or more and 0.008% or less, O: 0.0035% or less, the balance being Fe and impurities, and N / Al Is 0.3 or more and 3.0 or less, Sn / Cu is 0.025 or more, Pcm represented by the following formula (I) is 0.25 or less, and the major axis dispersed in the steel is 1 nm or more. High-tensile steel excellent in corrosion resistance and weld zone toughness, characterized in that the mean value of equivalent circle diameter of Cu particles is 4 nm or more and 25 nm or less, and the plane rate conversion distribution amount is 3% or more and 20% or less .
Pcm = C + (Si / 30) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B (I) )   The high-strength steel excellent in corrosion resistance and weld toughness according to claim 1, characterized by containing Ti: 0.03% or less in mass%.   The high-strength steel excellent in corrosion resistance and weld toughness according to claim 1 or 2, characterized by containing Nb: 0.03% or less in mass%.   The high-strength steel excellent in corrosion resistance and weld toughness according to any one of claims 1 to 3, characterized by containing, by mass%, Mo: 0.8% or less.   5. One or more of elements selected from the group consisting of Cr: 0.80% or less and B: 0.002% or less are contained by mass%. High-strength steel excellent in corrosion resistance and weld toughness as described in item 1.   The high-strength steel excellent in corrosion resistance and weld toughness according to any one of claims 1 to 5, characterized by containing, in mass%, V: 0.05% or less.   The composition contains one or more selected from the group consisting of Ca: 0.005% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass%. The high tensile steel excellent in corrosion resistance and weld toughness according to any one of claims 1 to 6.   An offshore structure excellent in corrosion resistance and weld toughness using the high-strength steel according to any one of claims 1 to 7.

Images