CN114107812B - High-fracture-toughness 420 MPa-grade heat-treated steel plate for marine platform and preparation method thereof - Google Patents
High-fracture-toughness 420 MPa-grade heat-treated steel plate for marine platform and preparation method thereof Download PDFInfo
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Abstract
The invention relates to a high-fracture-toughness 420 MPa-grade heat-treated steel plate for a marine platform and a preparation method thereof, belongs to the technical field of steel material production, and solves the problem of insufficient fracture toughness of the existing steel plates such as NDT (non-oriented ductile), CTOD (thermal conductivity) and CTOD (thermal conductivity) in a welding heat affected zone. The steel plate comprises the following components in percentage by mass: 0.07 to 0.15%, si: 0.10-0.50%, mn:1.00% -1.55%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr:0.01% -0.65%, mo:0.01 to 0.50 percent, ni:0.01% -2.00%, cu:0.01 to 0.45 percent, nb:0.005% -0.04%, V:0.02% -0.06%, al: 0.04-0.12%, N: less than or equal to 0.012 percent, ti:0.007 to 0.015 percent, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 20-150 mm. The steel plate for the 420 MPa-level marine platform provided by the invention has high fracture toughness in both the base metal and the welding heat affected zone, can be applied to the construction of a marine platform structure, and is particularly applied to the construction of a deepwater jacket platform with the scale of more than 2 ten thousand tons.
Description
Technical Field
The invention relates to the technical field of steel material production, in particular to a heat-treated steel plate for a 420 MPa-grade marine platform with high fracture toughness in both a base metal and a welding heat affected zone and a preparation method thereof.
Background
The demand for the development of marine industry and the localization of equipment in the marine oil industry has raised new demands for high-performance steel for marine engineering structures. At present, most of steel for marine engineering equipment structures still mainly comprises DH36 and EH36 high-strength steel with yield strength of 355MPa, for example, the mainstream jacket platforms in China are large jackets with the yield strength of more than 15000 tons, the use amount of the DH36 high-strength steel of each platform exceeds ten thousand tons, the delivery state mainly comprises normalizing and thermomechanical rolling (TMCP), the thickness mainly comprises 16-80 mm, and the small amount reaches 100mm.
As the exploration and development of marine oil and gas in China are promoted to deep water, ultra-deep water and polar region environments, the overall design reasonability and safety are comprehensively considered, and the requirement on steel for the marine platform structure is more severe. On the other hand, the large scale of the platform requires an increase in the grade of structural steel and an increase in the thickness of steel sheet. For example, in an oil field of 11-1 ocean streamers in south China, the depth of water in an operating sea area reaches 300-400 m, if all constructed deepwater jacket structures adopt 355 MPa-level steel plates, the total design weight of the jacket reaches more than 38000 tons, and the water launching capacity of the existing equipment cannot be met. If the key structure and the parts adopt higher-level high-strength steel plates, the total weight of the jacket can be saved by thousands of tons, the maximum thickness of the structural steel plates can be effectively reduced, and the limit requirements of structural design and steel plate production specifications are met. On the other hand, the development of the oil and gas exploration and exploitation field in China to two poles is urgent to adapt to the demand of a maritime work platform in an extremely cold region, so that the demand for higher toughness of structural steel is brought. In summary, the requirements for the performance of high strength steel plates required by marine platforms and equipment are gradually increasing, and especially the application performance greatly affecting the construction of the equipment, such as the fracture toughness of steel plates NDT, steel plates CTOD, welding heat affected zone CTOD, etc., are receiving more and more attention in recent years, and are also the key factor for good application of the steel plates.
Disclosure of Invention
In view of the above analysis, the embodiment of the invention aims to provide a high-fracture-toughness 420 MPa-level heat-treated steel plate for a marine platform and a preparation method thereof, so as to solve the problem of insufficient fracture toughness of the existing steel plates for the marine platform, such as NDT, CTOD, welding heat affected zone CTOD, and the like.
On one hand, the invention provides a high-fracture-toughness 420 MPa-grade heat-treated steel plate for a marine platform, which comprises the following components in percentage by mass: 0.07 to 0.15%, si: 0.10-0.50%, mn:1.00% -1.55%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr: 0.01-0.65%, mo:0.01 to 0.50 percent, ni: 0.01-2.00%, cu:0.01 to 0.45 percent, nb:0.005% -0.04%, V:0.02% -0.06%, al: 0.04-0.12%, N: less than or equal to 0.012 percent, ti:0.007 to 0.015 percent, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 20-150 mm.
Further, the Ni content of the steel plate and the thickness t of the steel plate also meet the following requirements: when t is less than or equal to 100mm, 100Ni is more than or equal to 0.3 (t/50) +0.05 (t/50) 2 -0.15; when t is more than 100mm and less than or equal to 150mm, 100Ni is more than or equal to 1.5 (t/100) -0.6; wherein Ni means a mass percentage of Ni element.
Further, when the content of N in the steel plate is more than or equal to 0.0030 percent, 100Ti (14/48)/a 1 +100Al(14/27)/a 2 Not less than 100N-0.0005, wherein a 1 ＝1，a 2 And =3,the Ti, al and N represent the mass percent of the elements Ti, al and N.
Further, the steel plate comprises the following components in percentage by mass: 0.08 to 0.13 percent, si: 0.15-0.40%, mn:1.10% -1.45%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr: 0.05-0.35%, mo:0.03% -0.30%, ni: 0.35-1.25%, cu: 0.05-0.38%, nb:0.01% -0.032%, V: 0.035-0.055%, al: 0.05-0.074%, N: 0.004-0.007%, ti: 0.008-0.012%, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 50-120mm.
Further, the steel plate comprises the following components in percentage by mass: 0.11-0.15%, si: 0.15-0.40%, mn:1.20% -1.50%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr: 0.25-0.60%, mo: 0.25-0.40%, ni:0.90% -1.80%, cu:0.15% -0.42%, nb:0.015% -0.035%, V:0.040% -0.06%, al:0.05% -0.065%, N: 0.004-0.007%, ti:0.008 to 0.014 percent, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 100-150 mm.
Furthermore, the metallographic structure of the steel plate is tempered martensite plus a small amount of granular bainite, the metallographic structure at the position of 1/4 of the thickness of the steel plate is completely martensite, the content of the martensite structure in the metallographic structure at the position of 1/2 of the thickness of the steel plate is 85% -100%, the size of a maroe island in the granular bainite at the position of 1/2 of the thickness of the steel plate is 0.2-3 mu m, and the average size of the maroe island is less than or equal to 1.1 mu m.
The invention also provides a preparation method of the heat-treated steel plate for the maritime work platform with the high fracture toughness of 420MPa, which is used for preparing the steel plate and comprises the following steps:
step 1, heating a steel billet to 1100-1200 ℃, and carrying out heat preservation and homogenization;
step 2, cooling after two-stage rolling;
and 3, quenching and tempering the steel plate obtained in the step 2.
Further, in the step 2, in the two-stage rolling process, the first stage is recrystallization controlled rolling, and the rolling temperature is not lower than 950 ℃; the second stage is non-recrystallization controlled rolling, and the finishing rolling temperature is not higher than (830-0.25 x t) DEG C.
Further, in the step 3, the quenching and tempering treatment comprises quenching and high-temperature tempering, the quenching and heat preservation temperature is 890-940 ℃, a rolling type quenching mode is adopted, the water pressure of a high-pressure section quenching nozzle is 0.7-1.0 MPa, and the water pressure of a low-pressure section quenching nozzle is 0.35-0.5 MPa; the high-temperature tempering temperature is 630-680 ℃.
Further, in the step 3, the size of the prior austenite crystal grains after the steel plate is quenched is fully refined, the size of the prior austenite crystal grains at the position of 1/4 of the thickness of the steel plate with the thickness of 100mm or less is 7.5-9 grades, and the size of the prior austenite crystal grains at the position of 1/2 of the thickness is 7-8.5 grades; the original austenite grain size at the 1/4 position of the thickness of the steel plate with the thickness more than 100mm is 7-9 grades, and the original austenite grain size at the 1/2 position of the thickness of the steel plate is 6.5-8.5 grades.
Further, the fracture toughness of the heat-treated steel sheet is-10 ℃ CTOD value of 1.35mm or more; NDT temperature is-50 to-90 ℃; the CTOD value of the welding heat affected zone at minus 10 ℃ is more than 0.50mm, and the CTOD value at minus 20 ℃ is more than 0.36 mm.
Compared with the prior art, the invention can realize at least one of the following beneficial effects:
(1) The high-fracture-toughness 420 MPa-grade heat-treated steel plate for the marine platform is matched with a rolling process through components, and provides refined microstructure preparation for subsequent quenching and tempering heat treatment. Specifically, the addition of Nb with the content of 0.005-0.04% is matched, so that a non-recrystallization region of steel is obviously moved upwards, austenite recrystallization in the steel plate rolling process is inhibited, austenite recrystallization does not occur in the steel rolled at a lower temperature, so that the austenite of the steel is flattened in the rolling process, the area of deformed austenite is increased, the area of heterogeneous phase change is increased, the deformation energy storage of austenite increases the driving force of phase change, the nucleation core of gamma → alpha phase change is improved, the ferrite phase change generation and the phase change ratio thereof are promoted, the ferrite grain size of the phase change is refined, and refined microstructure preparation is provided for the subsequent quenching process; meanwhile, nb microalloying also promotes the precipitation of Nb (C, N) second phase particles, pins austenite grain boundaries and prevents grain growth; meanwhile, a certain amount of Cu, ni, cr and Mo elements are added into the steel, so that the ferrite phase transformation refinement in the rolling process is facilitated, the phase transformation CCT curve of the steel is shifted to the right in the quenching process, the high-temperature transformation of the steel is delayed, the hardenability of the steel is improved, and the steel can obtain a martensite structure as much as possible; the addition of Ni element is beneficial to the homogenization of the structure and performance of the whole thickness section of the steel plate, and the higher the Ni content is, the larger the thickness of the steel plate which can realize the homogenization of the structure and performance is.
(2) Most of Ti exists in a precipitation form, and most of N elements in the steel are fixed; the slightly excessive added Al content also has the function of fixing the N content, and the separated AlN second phase is fine and dispersed due to the lower separation temperature; most of Nb also exists as precipitates Nb (CN), which plays an austenite refining role and a partial precipitation strengthening role. Compared with the prior art, the invention obtains the second-phase precipitates through controlling elements such as Ti, al, N and the like, and the second-phase precipitates play a role in preventing austenite grains in a coarse crystal region from growing and are an important reason for improving the CTOD toughness in a welding heat affected zone.
(3) The invention obtains a refined original austenite structure through a quenching heating process matched with components, the total grain size of the original austenite is 6.5-9 grades, and the difference between the 1/4 position and the 1/2 position is not large. The size of the prior austenite grain is not increased remarkably along with the increase of the thickness, and the prior austenite grain size of the core part of the steel plate can reach 6.5 to 7.5 grades even if the thickness is 150mm. For example, the prior austenite grain size at 1/4 of the thickness of a steel sheet of 100mm or less is on the order of 7.5 to 9, and the prior austenite grain size at 1/2 of the thickness is on the order of 7 to 8.5; the original austenite grain size of the steel plate with the thickness of more than 100mm at the position of 1/4 of the thickness is 7-9 grades, and the original austenite grain size of the steel plate with the thickness of 1/2 of the thickness is 6.5-8.5 grades.
(4) The microstructure of the quenched and tempered steel plate is mainly martensite, the martensite structure is completely at the position of 1/4 of the thickness of the steel plate, the content of the martensite structure at the position of 1/2 of the thickness is 85% -100%, the size of a martensite island in granular bainite at the position of 1/2 of the thickness is 0.2-3 mu m, and the average size is less than or equal to 1.1 mu m. The content of granular bainite in the microstructure of the welding heat affected zone is not higher than 25%.
(5) The steel plate obtained by the invention has good obdurability and good uniformity of structure and performance. For example, the mechanical properties at 1/4 of the thickness of the plate are as follows: the yield strength is more than 455MPa, the tensile strength Rm is more than 550MPa, the elongation percentage A is more than 24 percent, and the impact energy KV at minus 40 ℃ is more than 267J. The mechanical properties of the steel plate at the position of 1/2 of the thickness of the steel plate are as follows: the yield strength is more than 436MPa, the tensile strength Rm is more than 545MPa, the elongation percentage is more than A25 percent, and the impact energy KV is more than-40 ℃ and is more than 245J. The yield strength difference between 1/4 and 1/2 of the steel plate in the thickness direction is not higher than (15 +0.20 multiplied by t) MPa; the CTOD value of the steel plate at the fracture toughness of-10 ℃ is not less than 1.35mm, and both crack propagation and fracture are in a stable mode; the NDT temperature of the steel plate is in the range of-50 to-90 ℃, and the steel plate has higher fracture toughness level; especially has good low-temperature impact energy level, and the impact energy at-60 ℃ is higher than 160J.
(6) The CTOD value of a welding heat affected zone of the welded steel plate obtained by the invention at minus 10 ℃ is more than 0.50mm, the CTOD value at minus 20 ℃ is more than 0.36mm, and compared with the technical index of high-strength steel for an electrode zone specified by EN10225-2019 standard, the steel plate also has good fracture toughness level. The steel plate can be applied to the construction of a marine platform structure, in particular to the construction of a deepwater jacket platform with the scale of more than 2 ten thousand tons.
In the invention, the technical schemes can be combined with each other to realize more preferable combination schemes. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.
Drawings
The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, wherein like reference numerals are used to designate like parts throughout.
FIG. 1a is the thermodynamic calculation of the second phase precipitation in example 3;
FIG. 1b is the thermodynamic calculation of the second phase precipitation in example 4;
FIG. 2a is a metallographic structure of a steel sheet of example 3 at a thickness of 1/4;
FIG. 2b is the metallographic structure of the steel sheet of example 3 taken at 1/2 of the thickness thereof;
FIG. 2c shows the metallographic structure of the weld heat affected zone of the steel sheet of example 3.
Detailed Description
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate preferred embodiments of the invention and together with the description, serve to explain the principles of the invention and not to limit the scope of the invention.
On one hand, the invention discloses a high-fracture-toughness 420 MPa-grade heat-treated steel plate for a marine platform, which comprises the following components in percentage by mass: 0.07 to 0.15%, si: 0.10-0.50%, mn:1.00% -1.55%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr:0.01% -0.65%, mo:0.01 to 0.50 percent, ni:0.01% -2.00%, cu:0.01 to 0.45 percent, nb:0.005% -0.04%, V:0.02% -0.06%, al: 0.04-0.12%, N: less than or equal to 0.012 percent, ti:0.007 to 0.015 percent, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 20-150 mm.
It should be noted that, by adopting a lower C content and a higher Mn content, not only is good low-temperature toughness and weldability ensured, but also the improvement of hardenability of the steel is promoted; the Nb element with the content of 0.005-0.04% is added to inhibit austenite recrystallization in the steel plate rolling process, so that the austenite is flattened in the steel plate rolling process, the area of deformed austenite is increased, gamma → alpha phase transformation is promoted, and good original structure preparation is provided for quenching and tempering heat treatment. Meanwhile, nb microalloying also plays a certain role in precipitation strengthening. And a small amount of Cu, ni, cr and Mo elements are added into the steel, so that the hardenability of the steel is improved. The addition of the Ni element is beneficial to the homogenization of the structure and the performance of the whole thickness section of the steel plate, and the higher the Ni content is, the larger the thickness of the steel plate which can realize the homogenization of the structure and the performance is.
The inventors have intensively studied and found that 50mm is an important critical steel plate thickness t for achieving homogenization by in-line rolling, and in order to achieve structure homogenization and good toughness of the entire thickness section of the steel plate, in the present invention, when t.ltoreq.100 mm, 100 Ni.gtoreq.0.3 (t/50) +0.05 (t/50) 2 -0.15; when t is more than 100 and less than or equal to 150mm, 100Ni is more than or equal to 1.5 (t/100) -0.6, wherein Ni refers to the mass percent of Ni element.
Preferably, when the content of N in the steel is more than or equal to 0.0030 percent, the content of Ti, al and N elements should meet 100Ti (14/48)/a 1 +100Al(14/27)/a 2 Not less than 100N-0.0005, in the formula, a 1 ＝1，a 2 And =3,the Ti, al and N represent the mass percentages of the Ti, al and N elements.
It should be noted that in order to obtain good fracture toughness of the heat affected zone of the steel sheet, it is also necessary to form a favorable microstructure of the heat affected zone, and in particular, the coarse grain region should prevent austenite grains from growing excessively large.
According to the invention, solid N elements such as Ti and Al are added to form second phase particles such as TiN and AlN, and the ductile damage effect of free N on a heat affected zone is prevented. The inventor finds that when the elements of Ti, al and N satisfy 100Ti (14/48)/a 1 +100Al(14/27)/a 2 When the relation of more than or equal to 100N-0.0005, the steel has good N fixing effect, and the formed TiN and AlN second phase particles have good function of preventing austenite grains from growing. Wherein, a 1 And a 2 The solid N coefficients of Ti and Al, respectively. Because the precipitation temperature of Ti and N is as high as more than 1350 ℃, the N fixing effect is good, a 1 The coefficient is low, and the needed additional Ti content is low; the precipitation temperature of Al and N is about 1100 ℃, a 2 Coefficient is relatively higher thanA of Ti 1 Coefficient, more Al content is required to achieve the solid N effect. Adding a little excessive Al content to promote the precipitation of AlN second phase, matching with TiN, refining austenite grains in a coarse grain region and improving the toughness level of a heat affected zone.
In order to further improve the performance of the steel plate for the marine platform, the steel plate preferably comprises the following components in percentage by mass: 0.08-0.13%, si: 0.15-0.40%, mn:1.10% -1.45%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr: 0.05-0.35%, mo:0.03% -0.30%, ni: 0.35-1.25%, cu: 0.05-0.38%, nb:0.01% -0.032%, V: 0.035-0.055%, al: 0.05-0.074%, N: 0.004-0.007%, ti:0.008% -0.012% and the rest is Fe and other unavoidable impurities; the thickness t of the steel plate is 50-120mm.
Alternatively, preferably, the ingredient comprises C: 0.11-0.15%, si: 0.15-0.40%, mn:1.20% -1.50%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr: 0.25-0.60%, mo:0.25% -0.40%, ni:0.90% -1.80%, cu:0.15% -0.42%, nb:0.015% -0.035%, V:0.040% -0.06%, al:0.05% -0.065%, N: 0.004-0.007%, ti:0.008 to 0.014 percent, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 100-150 mm.
The reason why the composition of the steel sheet for a 420MPa grade marine platform is limited in the present invention will be described, and hereinafter, the composition is expressed in% by mass.
C: carbon is an essential element for improving strength and also an element for reducing weldability of the material. When the carbon content is less than 0.07%, the yield strength of the steel sheet is lowered, but too high carbon content adversely affects weldability and low temperature toughness of the steel sheet. Considering two aspects comprehensively, the content of C should be controlled between 0.07 percent and 0.15 percent.
Si: silicon is a deoxidizing element and also a solid-solution strengthening element, and also plays a role in assisting in improving hardenability during quenching, and can improve the strength of steel. If the content of silicon is more than 0.6%, the low-temperature toughness of the steel is lowered and the weldability is deteriorated. Therefore, the Si content is controlled to be 0.10-0.50%.
Mn: manganese is an essential element for ensuring the strength and the toughness of the steel, the hardenability of the steel is obviously improved, and the manganese can be combined with S to generate MnS, so that FeS is prevented from being formed at a crystal boundary, and further, the generation of hot cracks is avoided. When the manganese content is too high, center segregation is caused, and the toughness and weldability are lowered. Therefore, the Mn content is controlled to be 1.00-1.55%.
Cu: copper is a non-carbide forming element and is used to improve the strength and corrosion resistance of steel. In the welding process, the solid solution of TiN in a delta region can be reduced, the high-temperature nail rolling effect is increased, and the austenite grain size is reduced. When the Cu content is too low, the effect is not obvious; when the content is too large, segregation of copper is easily caused, and workability and weldability are deteriorated. Therefore, the Cu content is controlled to be 0.01 to 0.45 percent.
Ni: the addition of Ni can improve the toughness of steel, and especially for thick plates, the addition of Ni can effectively improve the core toughness level of steel. In order to ensure the section uniformity of the steel and good toughness level of the core, the Ni content and the thickness t also meet the condition that when t is less than or equal to 100, 100Ni is more than or equal to 0.3 (t/50) +0.05 (t/50) 2 -0.15; when t is more than 100 and less than or equal to 150, 100Ni is more than or equal to 1.5 (t/100) -0.6, and the content control is an important guarantee for realizing tissue homogenization and good toughness of the whole thickness section.
Nb: the Nb element is added, so that austenite recrystallization in the steel plate rolling process can be inhibited, austenite in the steel plate rolling process is flattened, the area of deformed austenite is increased, and gamma → alpha phase transformation is promoted, and the refining process provides good microstructure preparation for the quenching and tempering heat treatment of the steel plate. Meanwhile, nb microalloying also plays a certain role in precipitation strengthening. Therefore, the Nb content is controlled to be in the range of 0.005% to 0.04%.
Al: al element is added to promote the AlN second phase to be separated out, and is matched with TiN to refine austenite grains in a coarse crystal area, so that the toughness level of a heat affected zone is improved. Therefore, the Al content is controlled to be in the range of 0.04% to 0.12%.
Ti: adding trace Ti element, and combining with N element, on one hand, fixing free N in steel, reducing aging sensitivity of steel, and improving toughness level of steel; on the other hand, the formed TiN functions to improve weldability. Therefore, the control range of the Ti content is 0.007 to 0.015 percent.
N: n with a certain content can form TiN with Ti to improve the toughness of a steel plate and a welding heat affected zone, and form carbonitrides with Ti, al and the like to improve the strength, but the low-temperature toughness of the material is affected by the excessively high content of N, so that the content of N is controlled to be less than or equal to 0.0120%; when the content of N in the steel is more than or equal to 0.0030 percent, the content of Ti, al and N elements should meet 100Ti (14/48)/a 1 +100Al(14/27)/a 2 Not less than 100N-0.0005, wherein a 1 ＝1，a 2 =3, to optimize weld heat to affect the texture and grain size of the macrocrystalline region.
P: phosphorus is an impurity element in steel and may impair the toughness of steel sheets and weld heat affected zones. Therefore, the P content is controlled to be less than 0.011%.
S: sulfur is an impurity element in steel, and sulfide inclusions can be formed and become a crack source. Therefore, the S content is controlled to be less than 0.003%.
On the other hand, the invention also provides a preparation method of the heat-treated steel plate for the 420 MPa-grade marine platform with high fracture toughness, which comprises the following steps:
step 1, heating: heating the steel billet to 1100-1200 ℃, and carrying out heat preservation and homogenization;
step 2, rolling: performing a thermomechanical control process (TMCP) to provide microstructural preparation for subsequent thermal treatment;
step 3, quenching and tempering heat treatment: and (3) carrying out quenching and tempering heat treatment on the steel plate obtained in the step (2).
Specifically, the steel billet in the step 1 is prepared through the processes of molten steel purification smelting, LF refining, vacuum treatment and continuous casting billet.
Specifically, in step 2, the TMCP process includes the following steps:
s21, carrying out two-stage rolling, wherein the first stage is recrystallization controlled rolling, the rolling temperature is not lower than 950 ℃ (such as 950-1060 ℃), the single-pass deformation of at least two passes is not lower than 18 percent in the rolling process, and the rolling speed is not higher than 1.5m/S; the second stage is non-recrystallization controlled rolling, the finishing temperature is not higher than (830-0.25 x t) DEG C (for example 800-845 ℃), during rolling, the single-pass deformation of at least three passes is not lower than 12 percent, wherein the single-pass deformation of two passes reaches more than 15 percent;
and S22, cooling the rolled steel plate in water at the cooling speed of 5-15 ℃/S and the re-reddening temperature of 480-600 ℃, naturally cooling to 400 ℃, and then slowly cooling.
Specifically, in the step 2, a TMCP process is adopted, and rolling and cooling are controlled to obtain a uniformly refined ferrite structure, so that good structure preparation is provided for quenching and tempering heat treatment.
Specifically, in step 3, the quenching and tempering heat treatment includes quenching and high-temperature tempering.
Specifically, in the step 3, the quenching process parameters are as follows: the quenching heat preservation temperature is 890-940 ℃, and the heat preservation time is 1-5 h; a roll-pressing quenching mode is adopted, the water pressure of a high-pressure section quenching nozzle is 0.7-1.0 MPa, and the water pressure of a low-pressure section quenching nozzle is 0.35-0.5 MPa; the high-temperature tempering temperature is 630-680 ℃, and the high-temperature tempering heat preservation time is 2-10 h.
Specifically, in the step 3, a relatively refined original austenite structure is obtained through a quenching heating process matched with the components, in the quenching process, parameters such as nozzle water pressure and the like are matched, so that a steel plate is ensured to obtain a good through quenching effect along the whole thickness section, a microstructure mainly comprising martensite is obtained, and good toughness matching is obtained through proper high-temperature tempering.
Specifically, the thickness of the steel sheet obtained by the above preparation method is 20 to 150mm, in the above step 3, the grain size of the prior austenite after quenching of the steel sheet is sufficiently refined, the grain size of the prior austenite at the thickness of 1/4 of the steel sheet with a thickness of 100mm or less is 7.5 to 9 grades, and the grain size of the prior austenite at the thickness of 1/2 is 7 to 8.5 grades; the size of the prior austenite crystal grain at the position of 1/4 of the thickness of the steel plate with the thickness more than 100mm is 7-9 grades, and the size of the prior austenite crystal grain at the position of 1/2 of the thickness of the steel plate is 6.5-8.5 grades.
Specifically, in the step 3, the metallographic structure of the steel plate after the quenching and tempering heat treatment includes tempered martensite and a small amount of granular bainite, the entire 1/4 part of the thickness of the steel plate is a martensite structure, the martensite structure content at the 1/2 part of the thickness of the steel plate is 85% -100%, the mohno island size in the granular bainite at the 1/2 part of the thickness of the steel plate is 0.2-3 μm, and the average size is less than or equal to 1.1 μm.
The mechanical properties of the 1/4 part of the thickness of the heat-treated steel plate for the high-fracture-toughness 420 MPa-level marine platform are as follows: a yield strength of 455MPa or more (e.g., 455-475 MPa), a tensile strength Rm of 550MPa or more (e.g., 550-582 MPa), an elongation of 24% or more (e.g., 24% -28.5%), an impact energy at-40 ℃ of KV2 or more (e.g., 267-326J), and an impact energy at 60 ℃ of KV2 or more 243J (e.g., 243-263J). The mechanical properties of the steel plate at the position of 1/2 of the thickness are as follows: yield strength of 436MPa or more (e.g., 436 to 486 MPa), tensile strength Rm545MPa or more (e.g., 545 to 579 MPa), elongation A25% or more (e.g., 25% to 26.5%), impact energy at-40 ℃ of KV2 245J or more (e.g., 245 to 319J), impact energy at 60 ℃ of KV2 177J or more (e.g., 177 to 236J). The difference between the yield strengths at 1/4 and 1/2 in the thickness direction of the steel plate is 0-15 +0.20 × t) MPa, for example, the difference between the yield strengths at 1/4 and 1/2 in the thickness direction of the steel plate is less than 40MPa (for example, the difference is 7-29 MPa); the steel sheet has a fracture toughness of-10 ℃ and a CTOD value of not less than 1.35mm, and an NDT temperature of-50 to-90 ℃ (for example, -55 to-80 ℃).
Specifically, welding the prepared steel plate by adopting two modes of flux-cored wire gas shield welding and submerged arc automatic welding, wherein the energy of a welding line is 7-10 kJ/cm (gas shield welding) and 35-50 kJ/cm (submerged arc welding), performing a-10 ℃ full-thickness CTOD test on a welding line/coarse grain zone (CGHAZ) after welding, and selecting part of welding joints to perform a-20 ℃ CTOD test: after 7-10 kJ/cm gas shield welding and 35-50 kJ/cm submerged arc welding, the CTOD value at-10 ℃ in a welding heat affected zone is 0.50-1.55 mm, the CTOD value at-20 ℃ is 0.36-0.87 mm (for example, 0.38-0.73 mm), and the content of granular bainite in a microstructure of the welding heat affected zone is less than or equal to 25 percent (for example, 8-16 percent).
It should be noted that, according to the latest edition of classification society, steel for ships and ocean engineering generally requires that the CTOD value of a steel plate at-10 ℃ in a welding heat affected zone is not less than 0.15mm or 0.25mm; in some ultra-large structural members, the CTOD value is required to be not less than 0.38mm; in the technical protocols of some maritime work users in China, the CTOD value of material authentication is even required to be not less than 0.46mm; according to the european standard EN10225 of the latest edition (2019), a steel sheet for a polar region is specified, and a weld heat affected zone CTOD test is required to be performed in an environment of not higher than-20 ℃. In conclusion, the steel plate has excellent performance and meets the requirements of CTOD tests on welding heat affected zones in the specifications.
The strength performance and the toughness level of the steel plate are improved simultaneously through original austenite grain refinement and martensite structure toughening; the special second phase particles are pinned at the position of the welding heat affected zone, so that the growth of crystal grains is prevented, and the fracture toughness level of the welding heat affected zone is improved. The invention adopts multiple technologies to realize the effects of grain refinement, precipitation strengthening and toughening of a heat affected zone on the aspects of component design and heat treatment process.
The invention adopts low-C and high-Mn components and small amounts of Cu, ni, cr and Mo elements to reduce the phase transition temperature of steel, delay the critical cooling rate of the phase transition of the steel and optimize the cooling characteristic of the steel, thereby improving the hardenability of the steel and improving the microstructure of the steel plate after heat treatment; nb microalloying is adopted to be matched with rolling in a non-recrystallization area, and a uniformly refined ferrite structure is obtained after rolling through a flat austenitizing and phase transformation refining way, so that good structure preparation is provided for heat treatment; nb microalloying is also the primary mode of precipitation strengthening; the element coordination of Ti, al, N and the like controls the precipitation behavior of a second phase, plays a role in pinning grains, has obvious effect on grain refinement of the steel plate, and is one of the main technical means for controlling high fracture toughness of the welding heat affected zone.
The high-fracture-toughness 420 MPa-grade heat-treated steel plate for the marine platform is obtained by the preparation method provided by the invention, and meets the CTOD (thermal deformation OD) test requirements of welding heat affected zone in the specifications.
The heat-treated steel plate with high fracture toughness and 420MPa grade for the marine platform can be applied to the construction of the marine platform structure, and is particularly applied to the construction of deepwater jacket platforms with the scale of more than 2 ten thousand tons.
The heat-treated steel plate with high fracture toughness and 420MPa grade for the marine platform provided by the invention can be applied to the construction of the marine platform in extremely cold areas and polar ring area environments.
Example 1
The embodiment provides a heat-treated steel plate with high fracture toughness and 420MPa grade for a marine platform and a preparation method thereof, and the specific details are as follows:
the steel billet is obtained by the processes of molten steel purification smelting, LF refining, vacuum treatment, continuous casting billet and the like.
Step 1, heating: heating the steel billet to 1150 ℃, and carrying out heat preservation and homogenization;
step 2, rolling: performing a thermomechanical control process (TMCP) to provide microstructural preparation for subsequent thermal treatment;
step 3, quenching and tempering heat treatment: the steel plate is produced by adopting a quenching and tempering heat treatment (quenching and high-temperature tempering) process, the quenching heat preservation temperature is 930 ℃, the heat preservation time is 100min, a roll-press type quenching mode is adopted, the water pressure of a quenching nozzle in a high-pressure section is 0.75MPa, the water pressure of a quenching nozzle in a low-pressure section is 0.38MPa, the tempering temperature is 660 ℃, and the tempering heat preservation time is 160min.
The thickness of the steel plate obtained in example 1 is 32mm, and the steel plate comprises the following chemical components in percentage by mass: 0.08%, si:0.25%, mn:1.15%, cr:0.05%, mo:0.02%, ni:0.27%, cu:0.08%, nb:0.013%, V:0.043%, al:0.068%, N:0.0048%, ti:0.009%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, and the balance of Fe and other inevitable impurities.
Example 2
The embodiment provides a heat-treated steel plate with high fracture toughness and 420MPa grade for a marine platform and a preparation method thereof, and the specific details are as follows:
the steel billet of the embodiment is obtained through the processes of molten steel purification smelting, LF refining, vacuum treatment, continuous casting billet and the like.
Step 1, heating: heating the steel billet to 1180 ℃, and carrying out heat preservation and homogenization;
step 2, rolling: performing a thermomechanical control process (TMCP) to provide microstructural preparation for subsequent thermal treatment;
step 3, quenching and tempering heat treatment: the steel plate is produced by adopting a quenching and tempering heat treatment (quenching and high-temperature tempering) process, the quenching heat preservation temperature is 910 ℃, the heat preservation time is 165min, a roll-pressing type quenching mode is adopted, the water pressure of a high-pressure section quenching nozzle is 0.8MPa, the water pressure of a low-pressure section quenching nozzle is 0.4MPa, the tempering temperature is 655 ℃, and the tempering heat preservation time is 300min.
The thickness of the steel plate obtained in example 2 is 55mm, and the steel plate comprises the following chemical components in percentage by mass: 0.10%, si:0.29%, mn:1.23%, cr:0.11%, mo:0.08%, ni:0.45%, cu:0.16%, nb:0.024%, V:0.039%, al:0.074%, N:0.0054%, ti:0.011%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, and the balance of Fe and other inevitable impurities.
Example 3
The embodiment provides a high-fracture-toughness 420 MPa-grade heat-treated steel plate for a marine platform and a preparation method thereof, and the specific details are as follows:
the steel billet of the embodiment is obtained through the processes of molten steel purification smelting, LF refining, vacuum treatment, continuous casting billet and the like.
Step 1, heating: heating the steel billet to 1140 ℃, and preserving heat and homogenizing;
step 2, rolling: performing a thermomechanical control process (TMCP) to provide microstructural preparation for subsequent thermal treatment;
step 3, quenching and tempering heat treatment: the steel plate is produced by adopting a quenching and tempering heat treatment (quenching and high-temperature tempering) process, wherein the quenching heat preservation temperature is 900 ℃, the heat preservation time is 250min, a rolling type quenching mode is adopted, the water pressure of a quenching nozzle at a high pressure section is 0.8MPa, the water pressure of a quenching nozzle at a low pressure section is 0.4MPa, the tempering temperature is 655 ℃, and the tempering heat preservation time is 450min.
The thickness of the steel plate obtained in example 3 is 90mm, and the steel plate comprises the following chemical components in percentage by mass: 0.13%, si:0.21%, mn:1.42%, cr:0.28%, mo:0.27%, ni:0.73%, cu:0.38%, nb:0.032%, V:0.052%, al:0.057%, N:0.0049%, ti:0.012%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, and the balance of Fe and other inevitable impurities.
Example 4
The embodiment provides a high-fracture-toughness 420 MPa-grade heat-treated steel plate for a marine platform and a preparation method thereof, and the specific details are as follows:
the steel billet of the embodiment is obtained through the processes of molten steel purification smelting, LF refining, vacuum treatment, continuous casting billet and the like.
Step 1, heating: heating the steel billet to 1130 ℃, and preserving heat and homogenizing;
step 2, rolling: performing a thermomechanical control process (TMCP) to provide microstructural preparation for subsequent thermal treatment;
step 3, quenching and tempering heat treatment: the steel plate is produced by adopting a quenching and tempering heat treatment (quenching and high-temperature tempering) process, the quenching heat preservation temperature is 895 ℃, the heat preservation time is 270min, a roll-press type quenching mode is adopted, the water pressure of a quenching nozzle at a high-pressure section is 0.9MPa, the water pressure of a quenching nozzle at a low-pressure section is 0.45MPa, the tempering temperature is 655 ℃, and the tempering heat preservation time is 560min.
The thickness of the steel plate obtained in example 4 was 115mm, and the steel plate had a chemical composition comprising, in mass percent, C:0.14%, si:0.34%, mn:1.53%, cr:0.37%, mo:0.29%, ni:1.32%, cu:0.42%, nb:0.028%, V:0.058%, al:0.064%, N:0.0063%, ti:0.014%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, and the balance of Fe and other inevitable impurities.
For example 3 and example 4, alN, tiN, nbCN and VCN precipitates in the steel were extracted by an electrochemical extraction method, and quantitative statistics were performed, and the content of the TiN precipitates in example 3 and example 4 was 0.0134% and 0.0157% in percentage by mass, respectively, indicating that most of Ti in the steel was present in a precipitated form and most of N element in the steel was fixed. The AlN precipitates in examples 3 and 4 were 0.0024% and 0.0035%, respectively, which shows that the slightly excessive Al content also has the effect of fixing the N content, and the precipitated AlN second phase is finely dispersed due to the low precipitation temperature. The chemical phase analysis results also show that in examples 3 and 4, some NbCN and VCN precipitates were present and acted as precipitation strengthening. The second phase precipitates in the present invention play a role of preventing austenite grains in the coarse grain region from growing large by controlling elements such as Ti, al, N, etc., and are an important factor for improving the CTOD toughness in the welding heat affected zone. The thermodynamic calculation results and the chemical phase analysis results of fig. 1a and 1b are matched, and the function of the precipitated phase in the invention is better verified.
In the chemical composition analysis of the embodiment of the invention, the C-Mn alloy system is taken as the basis in the embodiments 1 to 4, a small amount of Cr, mo, ni, cu, V and other elements are added, the Ni content is increased along with the increase of the thickness of a designed product, and micro Ti and Al alloying treatment is carried out.
TABLE 1 main chemical composition (wt%) of inventive examples 1-4
Table 2 shows the mechanical property results of the examples of the invention, and the results show that the examples 1-4 all obtain good toughness matching, meet the performance requirements of 420 MPa-grade steel plates, and particularly have good low-temperature impact energy level and impact energy at-60 ℃ higher than 160J. The base metal-10 ℃ CTOD values of all the steel plates are higher than 1.3mm, and the crack propagation and the fracture are in a steady state mode (delta m). The NDT temperature of the steel plate is in the range of-50 to-90 ℃, and the steel plate has higher fracture toughness level.
TABLE 2 mechanical Properties of inventive examples 1 to 4
Table 3 shows the microstructure characteristics of examples 1 to 4 of the present invention. As can be seen from the results, the prior austenite grain size of the steel sheet after quenching is sufficiently fine, the prior austenite grain size at 1/4 of the steel sheet with the thickness of 100mm or less is 7.5 to 9 grades, and the prior austenite grain size at 1/2 is 7 to 8.5 grades; the grain size of the prior austenite at 1/4 position of the steel plate with the thickness of more than 100mm is 7-9 grades, and the grain size of the prior austenite at 1/2 position is 6.5-8.5 grades. The metallographic structure of the steel plate is mainly tempered martensite, the martensite structure is completely at 1/4 part, the martensite structure content at 1/2 part is 85-100%, and the granular bainite structure content in a welding heat affected zone (coarse crystal zone) is not higher than 25%. FIGS. 2a and 2b are microstructures at 1/4 and 1/2 of the thickness of the steel sheet of example 3, respectively, and FIG. 2c is a microstructure of a weld heat-affected coarse grain region.
TABLE 3 microstructural characteristics of inventive examples 1-4
The steel plates of the embodiment of the invention are welded by adopting two modes of flux-cored wire gas shielded welding and submerged arc automatic welding, the welding line energy is respectively 7kJ/cm (gas shielded welding) and 45kJ/cm (submerged arc welding), after welding, a welding line/coarse grain zone (CGHAZ) is subjected to a-10 ℃ full-thickness CTOD test, and the 45kJ/cm submerged arc welding head of the embodiment 3 is selected to be subjected to a-20 ℃ CTOD test. From the CTOD results of the tests, as shown in Table 4, the CTOD values at-10 ℃ in the weld heat affected zone of examples 1 to 4 were in the range of 0.50 to 1.55mm, and they had better fracture toughness.
The CTOD value of the welding heat affected zone at-20 ℃ in the example 3 is in the range of 0.36-0.87 mm, and compared with the technical index of high-strength steel for the polar region specified by EN10225-2019 standard, the CTOD value also has a good fracture toughness level.
TABLE 4 examples 1-4 of the invention weld HAZ fracture toughness (coarse grain zone)
In conclusion, the steel sheets obtained in examples 1 to 4 of the present invention had toughness matching, good uniformity of structural properties and excellent level of fracture toughness in the weld heat affected zone.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Claims (6)
1. A high fracture toughness 420MPa grade marine platform uses the steel plate of heat treatment state, characterized by that, the composition of the said steel plate is according to the weight percent: c:0.07% -0.15%, si:0.10% -0.50%, mn: 1.00-1.55%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr:0.01% -0.65%, mo:0.01% -0.50%, ni:0.01% -2.00%, cu:0.01% -0.45%, nb:0.005% -0.04%, V:0.02% -0.06%, al:0.04% -0.12%, N:0.0048% -0.012%, ti: 0.007-0.015% of the total weight of the alloy, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 20 to 150mm;
the Ni content of the steel plate and the thickness t of the steel plate meet the following requirements: when t is less than or equal to 100mm, 100Ni is more than or equal to 0.3 (t/50) +0.05 (t/50) 2 -0.15; when t is more than 100mm and less than or equal to 150mm, 100Ni is more than or equal to 1.5 (t/100) -0.6; wherein, ni refers to the mass percentage of Ni element;
100Ti (14/48)/a 1 +100Al (14/27)/a 2 not less than 100N-0.0005, wherein a 1 =1，a 2 =3, ti, al, N are the mass percent of Ti, al, N elements;
the metallographic structure of the steel plate is tempered martensite and a small amount of granular bainite, the metallographic structure at the position of 1/4 of the thickness of the steel plate is completely a martensite structure, the content of the martensite structure in the metallographic structure at the position of 1/2 of the thickness of the steel plate is 85% -100%, the size of a martensite island in the granular bainite at the position of 1/2 of the thickness of the steel plate is 0.2 to 3 mu m, and the average size of the martensite island is less than or equal to 1.1 mu m.
2. The steel sheet according to claim 1, wherein the steel sheet has a composition comprising, in mass percent, C:0.08% -0.13%, si:0.15% -0.40%, mn:1.10% -1.45%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr:0.05% -0.35%, mo: 0.03-0.30%, ni:0.35% -1.25%, cu:0.05% -0.38%, nb:0.01% -0.032%, V:0.035% -0.055%, al:0.05% -0.074%, N:0.0048% -0.007%, ti:0.008% -0.012%, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 50-120mm.
3. The steel sheet according to claim 1, wherein the steel sheet has a composition comprising, in mass percent, C:0.11% -0.15%, si:0.15% -0.40%, mn:1.20% -1.50%, P: less than or equal to 0.011 percent, S: less than or equal to 0.003 percent, cr:0.25% -0.60%, mo:0.25% -0.40%, ni:0.90% -1.80%, cu:0.15 to 0.42%, nb:0.015% -0.035%, V:0.040% -0.06%, al:0.05% -0.065%, N: 0.0048-0.007%, ti:0.008% -0.014%, and the balance of Fe and other inevitable impurities; the thickness t of the steel plate is 100 to 150mm.
4. The steel plate according to claim 1, wherein the steel plate has a fracture toughness-10 ℃ CTOD value of 1.35mm or more; NDT temperature is-50 to-90 ℃; the CTOD value of the welding heat affected zone at minus 10 ℃ is more than 0.50mm, and the CTOD value at minus 20 ℃ is more than 0.36 mm.
5. A method for preparing a high fracture toughness 420MPa grade steel plate for a maritime work platform in a heat treated state according to any one of claims 1 to 3, comprising the following steps:
step 1, heating a steel blank to 1100-1200 ℃, and carrying out heat preservation and homogenization;
step 2, cooling after two-stage rolling;
step 3, carrying out quenching and tempering heat treatment on the steel plate obtained in the step 2;
in the step 3, quenching and tempering heat treatment comprises quenching and high-temperature tempering, wherein the quenching heat preservation temperature is 890-940 ℃, a roll-in type quenching mode is adopted, the water pressure of a high-pressure section quenching nozzle is 0.7-1.0 MPa, and the water pressure of a low-pressure section quenching nozzle is 0.35-0.5 MPa; the high-temperature tempering temperature is 630 to 680 ℃.
6. The method according to claim 5, wherein in the step 3, the grain size of the prior austenite after the steel plate is quenched is fully refined, the grain size of the prior austenite at the position of 1/4 of the thickness of the steel plate with the thickness of 100mm or less is 7.5 to 9 grades, and the grain size of the prior austenite at the position of 1/2 of the thickness of the steel plate is 7 to 8.5 grades; the size of an original austenite grain at a position 1/4 of the thickness of the steel plate with the thickness of more than 100mm is 7 to 9 grades, and the size of the original austenite grain at a position 1/2 of the thickness of the steel plate is 6.5 to 8.5 grades.
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