A 785mpa grade corrosion resistant offshore steel plate and a manufacturing method thereof

Abstract

The present application relates to the field of steel material preparation, in particular to a yield strength 785MPa grade corrosion resistant offshore steel plate and a manufacturing method thereof. The steel plate is composed of the following chemical components by weight percentage: C: 0.15%-0.22%, Si: 0.15%-0.25%, Mn: 0.8%-1.2%, P≤0.02%, S≤0.01%, Als: 0.02%-0.05%, Ni: 1.5%-2.8%, Cr: 0.7%-1.2%, Mo: 0.2%-0.45%, Cu: 0.1%-0.3%, Co: 0.25%-0.55%, Sn: 0.1%-0.25%, Nb: 0.03%-0.06%, Ti: 0.008%-0.013%, N: 0.004%-0.006%, and the rest is Fe and inevitable impurities. The thickness of the finished steel plate can reach 80mm, while maintaining high strength and high toughness, the corrosion resistance of the steel plate in the extremely cold deep sea environment is significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of 785MPa-grade corrosion-resistant marine steel plate, in particular to a 785MPa-grade yield strength corrosion-resistant marine steel plate and a manufacturing method thereof. BACKGROUND

[0002] In the field of extremely cold deep-sea ocean engineering, equipment materials need to withstand the severe challenges of low-temperature environment, high-pressure corrosion and complex mechanical load. Traditional marine engineering steels (such as 355MPa~550MPa grade) have been difficult to meet the comprehensive demands of high strength, high corrosion resistance and low-temperature toughness for deep-sea underwater structures, polar drilling platforms and LNG storage tanks and other facilities. In particular, as oil and gas resource development extends to the Arctic Circle and ultra-deep sea, the demand for high-performance corrosion-resistant marine steel plates with a yield strength of 785MPa is increasingly urgent.

[0003] Currently, marine steel plates of this strength level face the following technical bottlenecks: (1) the contradiction between strength and toughness: after increasing the yield strength to 785MPa, the low-temperature toughness of the steel (below-60℃) is easily deteriorated, leading to an increased risk of brittle fracture in extremely cold environments. (2) insufficient corrosion resistance: the deep-sea high-salt and high-pressure environment accelerates local corrosion (such as pitting and stress corrosion cracking), and traditional Ni-Cr-Mo alloy design is difficult to balance strength and seawater corrosion resistance. (3) poor welding adaptability: high-carbon equivalent component design easily causes welding cold cracks, and the strength of the heat-affected zone (HAZ) decreases significantly, restricting engineering applications.

[0004] Chinese patent application CN116043120A "1000MPa grade cold rolled multi-phase steel with excellent forming performance and preparation method thereof" proposes a 1000MPa grade cold rolled steel plate using a high Mn low alloy component system. Since the steel plate is processed by cold rolling and continuous annealing, the strength and formability of 4mm steel plate can be guaranteed, but the combined effect of elements such as Ni, Cr, Co and Sn is not considered, and the strength and corrosion resistance of large-thickness steel plates cannot be guaranteed. Chinese patent application CN202311690116.8 "Production method of 150~200mm thick low-cost EH690 marine platform steel" proposes a 690MPa yield strength super-high-strength thick rack steel plate using a low-Ni, Cr, Mo and Cu alloy component design, without utilizing the effects of Co, Sn and other elements on the low-temperature toughness and corrosion resistance of the steel plate, and the strength of the steel plate cannot meet the requirements of marine environment corrosion resistance. SUMMARY

[0005] In order to overcome the prior art, the present application provides a yield strength 785MPa grade corrosion-resistant offshore steel plate and a manufacturing method thereof, the thickness of the finished steel plate can reach 80mm, while maintaining high strength and high toughness, the corrosion resistance in extremely cold deep sea environment is significantly improved.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0007] A yield strength 785MPa grade corrosion-resistant offshore steel plate is composed of the following weight percentage of chemical components:

[0008] C: 0.15%~0.22%, Si: 0.15%~0.25%, Mn: 0.8%~1.2%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Ni: 1.5%~2.8%, Cr: 0.7%~1.2%, Mo: 0.2%~0.45%, Cu: 0.1%~0.3%, Co: 0.25%~0.55%, Sn: 0.1%~0.25%, Nb: 0.03%~0.06%, Ti: 0.008%~0.013%, N: 0.004%~0.006%, the rest is Fe and inevitable impurities.

[0009] The role of selecting the above alloy element types and contents:

[0010] 1. C is the basic strengthening element in steel, which is used to improve the hardenability of the steel plate in the present application. If the C content is too low, the solid solution content and carbide content will be reduced, which cannot guarantee the strength and hardenability of the steel plate. If the C content is too high, a large amount of hardened structure will be produced, which will reduce the elongation and low temperature impact toughness of the steel plate, and the corrosion resistance will be reduced. Therefore, the C content is accurately controlled to be 0.15%~0.22% in the present application.

[0011] 2. Si plays a solid solution strengthening role in steel, but Si elements can also produce hard and brittle phases, causing the toughness and plasticity of the steel plate to decrease. Adding a certain amount of Si to the steel can form hard alloy, inhibit crystal growth, improve the hardness and strength of the steel, and improve the corrosion resistance. Therefore, in order to maximize the corrosion resistance and low temperature toughness of the steel plate, the Si content needs to be accurately controlled. Therefore, the Si content is accurately controlled to be 0.15%~0.25% in the present application.

[0012] 3. Mn element can be solid-solved with Ni and Co elements in Fe matrix to improve the strength and low temperature toughness of the steel plate. Appropriate amount of Mn element can improve the oxidation resistance and corrosion resistance of the steel plate. When the Mn content is less than 0.8%, the contribution to the strength of the steel plate is small. In order to improve the low temperature toughness of the steel plate, the influence of the segregation of Mn element on the performance of the thick plate core needs to be reduced. Therefore, the Mn content is accurately controlled to be 0.8%~1.2% in the present application.

[0013] 4. P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to ≤0.02% and S to ≤0.01%.

[0014] 5. Al is the main deoxidizing element in steel. However, when steel contains a large amount of nitrogen, excessively high Al content can lead to the formation of large inclusions, affecting the low-temperature impact toughness of the steel plate. Therefore, the Al content should be minimized. Thus, this invention precisely controls the Al content to 0.02%~0.05%.

[0015] 6. Ni can significantly improve the low-temperature toughness and corrosion resistance of steel plates. Adding a large amount can lower the ductile-brittle transition temperature of the steel plate; Ni has certain corrosion resistance and good resistance to acidic marine environments. When used in combination with elements such as Cr and Mo, it can improve the corrosion resistance of steel. At the same time, Ni can reduce intergranular corrosion and improve the corrosion fatigue performance of steel plates. Therefore, this invention precisely controls the Ni content to 1.5%~2.8%.

[0016] 7. Cr (Cr) can improve the strength of steel plates. Cr's corrosion resistance in steel is mainly due to its ability to form a dense passivation film. This film effectively prevents further oxidation of the metal surface, thus significantly improving the corrosion resistance of the steel. Combined with elements such as Ni and Sn, excellent resistance to marine corrosion can be achieved even with a Cr content below 13%. Cr can also effectively improve the strength of steel plates, and the Cr content can be appropriately increased. However, excessively high Cr content will reduce the impact toughness of the steel plate. Therefore, this invention precisely controls the Cr content to be between 0.7% and 1.2%.

[0017] 8. Mo can improve the hardenability of steel plates. Simultaneously, Mo can form fine carbides in steel, effectively improving the strength and heat treatment stability of the steel plate. Adding an appropriate amount of Mo to heat-treated steel plates can enhance their corrosion resistance and pitting corrosion resistance. Mo can also combine with carbides to reduce Cr precipitation, thereby improving the corrosion resistance of the steel plate. The combination of Mo, Ni, Cr, and other elements is a targeted design of this invention for specific effects. Therefore, this invention precisely controls the Mo content to 0.2%~0.45%.

[0018] 9. Cu and Cr in steel work together to form a dense oxide layer on the steel surface, which significantly improves the steel's resistance to atmospheric corrosion. In marine environments, the addition of Cu can inhibit corrosion and biofouling in saline and humid environments. However, excessive Cu content can cause cracks in the steel billet during heating. Therefore, this invention precisely controls the Cu content to 0.1%~0.3%.

[0019] 10、Co element is an important element added in the application, which can improve the strength and wear resistance of the steel plate, and Co element can improve the fatigue resistance of the steel plate together with Ni element in the steel. Co element can form a dense Co oxide film in a humid environment, effectively preventing oxygen and moisture from eroding, thereby improving the corrosion resistance of the steel plate. Therefore, the content of Co element is accurately controlled to be 0.25%-0.55%.

[0020] 11、Sn element can form a solid solution with iron atoms, which can improve the hardness and strength of the steel, and effectively inhibit the grain boundary corrosion of the steel, thereby enhancing the corrosion resistance of the steel. Sn element cooperates with other corrosion-resistant elements in acid-resistant steel to form a dense oxide film, which significantly hinders the transmission of corrosive ions, thereby improving the corrosion resistance of the steel in harsh environments. The corrosion resistance of Sn element is more obvious in the steel used in seawater environment. Therefore, the content of Sn is accurately controlled to be 0.1%-0.25%.

[0021] 12、Nb is a strong carbonitride forming element. Nb element can improve the strength and hardness of the steel plate; Nb can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, refine the grain, and improve the strength and toughness; during the cooling process of the steel plate, a large amount of Nb(CN) is precipitated, which further promotes dislocation entanglement and grain refinement. Nb element can also stabilize CN compounds and improve the corrosion resistance of the steel plate. Therefore, the content of Nb is accurately controlled to be 0.03%-0.06%.

[0022] 13、Ti can combine with CN to form TiCN, which has high stability and can refine the grain, thereby improving the strength and hardness of the steel; Ti can combine with carbon to form stable TiC, preventing the formation of grain boundary chromium carbide, thereby avoiding intergranular corrosion. Therefore, the content of Ti is accurately controlled to be 0.008%-0.013%.

[0023] 14、N and Cr, Nb, Ti and other elements can generate extremely stable nitrides, which can act as surface hardening and strengthening elements. At the same time, N can improve the strength and impact toughness of high-Cr steel and high-CrNi steel without reducing plasticity. Therefore, the content of N is accurately controlled to be 0.004%-0.006%.

[0024] The yield strength of the 785 MPa grade corrosion-resistant marine steel plate is greater than or equal to 785 MPa, the tensile strength is 865-980 MPa, the elongation is greater than or equal to 17%, and the Charpy impact energy of the core of the steel plate at -60°C is greater than or equal to 150 J. The maximum thickness of the finished steel plate is 80 mm. The corrosion resistance is excellent, the seawater corrosion resistance rate of the steel plate is less than 40% of the conventional 785 MPa marine steel performance, and the marine atmospheric corrosion resistance rate is less than 50% of the conventional 785 MPa marine steel performance.

[0025] The microstructure of the steel plate at a thickness of 1 / 2 is tempered sorbite (20%-35%) + tempered martensite, and the effective grain size of the tempered structure is 2-5 μm.

[0026] The manufacturing method of the above-mentioned 785 MPa grade corrosion-resistant marine steel plate adopts high cleanliness and alloying smelting + casting machine heavy reduction + two-stage controlled rolling + quenching heat treatment + tempering heat treatment process, and the manufacturing method specifically comprises the following steps:

[0027] 1) Steel refining:

[0028] The molten steel is refined by converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions.

[0029] 2) Continuous casting:

[0030] During the casting process of the casting machine, two sets of compression rollers are used for heavy reduction at the front end of the core solidification zone 1-2 meters, the reduction amount of a single set of compression rollers is greater than or equal to 7 mm, the water quantity of the secondary cooling is controlled to make the surface cooling rate of the continuous casting billet 0.1-0.3 ℃ / s, and the continuous casting billet is directly loaded online.

[0031] The continuous casting billet is loaded into a heating furnace at 550-650 ℃ for heat preservation, the heating furnace temperature is 500-600 ℃, and the heat preservation time is 60-120 min, so that the billet maintains the same temperature in the thickness direction in the low temperature stage.

[0032] The continuous casting billet is heated, the heating temperature is 1200-1300 ℃, the heating rate is controlled at 2.5-5 ℃ / min, and the heat preservation time is 90-150 min.

[0033] The purpose of high-temperature soaking is to ensure that C / N compounds are fully dissolved, and at the same time, the hardened steel plate formed by elements such as Co is fully softened, and the rapid heating avoids the occurrence of brittle defects in the internal of the billet.

[0034] 3) Two-stage controlled rolling:

[0035] The first-stage rolling temperature is 1100-1150 ℃, the first-stage deformation amount is 45%-55%, and the deformation rate is less than or equal to 1 s -1 The intermediate billet cooling rate is 3-6 ℃ / s.

[0036] High temperature and large reduction rolling process in the stage of low deformation resistance, by applying large rolling force, quickly rolling the steel plate to the target thickness. The advantage of this process is that it can complete large-scale batch production in a short time. However, due to the presence of a large amount of Co elements in the steel, the material may form a hard phase structure, thereby increasing the risk of cracks during processing. In order to reduce the cracks caused by the hard phase, the deformation rate is strictly controlled.

[0037] The two-stage roughing temperature is 750-800 DEG C, the last three passes are greater than 8mm, and the finish rolling temperature is 720-770 DEG C. The purpose of the low temperature stage rolling process design is to utilize the surface temperature drop more, increase the deformation of the core of the steel plate, improve the grain size of the core of the steel plate, promote the flattening and fine of austenite grain, and prepare for the deformation energy storage of the quenched and tempered structure. The purpose of controlling the single pass reduction rate of the last three passes near the finish rolling temperature is to avoid the deterioration of the flatness of the steel plate caused by the increase of the hardness of the steel plate.

[0038] 4) quenching and tempering:

[0039] The quenching temperature is 780-840 DEG C, and the quenching holding time is 1.0-1.6 min / mm. The tempering temperature is 420-500 DEG C, and the tempering holding time is 3-4.5 min / mm. The purpose of quenching and tempering heat treatment can obtain uniform and fine size of tempering sorbite (20%-35%) and tempering martensite structure, the effective grain size of the tempering structure is 2-5 mu m, which significantly improves the low temperature impact toughness of the steel plate, adjusts the strength and hardness of the steel plate, and avoids the precipitation of hard and brittle phase.

[0040] Compared with the prior art, the beneficial effects of the present application are:

[0041] 1. The present application precisely controls the C content, which ensures the strength and hardenability of the steel plate, and does not reduce the elongation, low temperature impact toughness and corrosion resistance of the steel plate. The addition of Mn elements and Ni, Co elements in the Fe matrix, combined with Cr, Mo and other elements, improves the strength and low temperature toughness of the steel plate, reduces the intergranular corrosion of the steel plate, improves the corrosion fatigue performance of the steel plate, and improves the oxidation resistance and corrosion resistance of the steel plate. Sn element in acid-resistant steel cooperates with other corrosion-resistant elements to form a dense oxide film, which significantly hinders the transmission of corrosive ions, improves the corrosion resistance of the steel in harsh environment. In the steel used in seawater environment, the corrosion resistance of Sn element is more obvious. The effective combination of alloy elements and process can make the steel plate obtain excellent mechanical properties while improving the corrosion resistance of the steel plate.

[0042] 2、The continuous casting billet of the present application adopts a straight loading process, and the continuous casting billet with a surface temperature of 550-650℃ is loaded into a heating furnace at a furnace temperature of 500-600℃ for heat preservation for 60-120min, so as to keep the thickness direction temperature of the steel billet consistent in the low temperature stage and make preparation for the uniform structure in the high temperature stage, and the straight loading reduces the heating cost. The purpose of the heat preservation in the high temperature stage is to ensure that the C / N compounds are fully dissolved and the hardened steel plate formed by Co and other elements is fully softened.

[0043] 3、The one-stage rolling temperature of the present application is 1100-1150℃, the one-stage deformation amount is 45%-55%, and the deformation rate is ≤1s -1 . The intermediate billet cooling rate is 3-6℃ / s. In the stage with low deformation resistance, the high temperature large rolling force is applied to quickly roll the steel plate to the target thickness. The advantage of this process is that it can complete large-scale batch production in a short time. In order to avoid the risk of cracks caused by the hard phase structure of the material formed by Co elements, the deformation rate is strictly controlled.

[0044] 4、The two-stage opening rolling temperature of the present application is 750-800℃, the last three passes are rolled by ≥8mm, and the finish rolling temperature is 720-770℃. The purpose of the low temperature stage rolling process design is to utilize the more surface temperature drop, increase the deformation amount of the core of the steel plate, improve the grain size of the core of the steel plate, and make preparation for the deformation energy storage of the quenched and tempered structure.

[0045] 5、The present application adopts quenching and tempering heat treatment process, and the quenching and tempering can obtain a tempered sorbite (20%-35%) + tempered martensite structure with uniform structure and small size, the effective grain size of the tempered structure is 2-5μm, which significantly improves the low temperature impact toughness of the steel plate, adjusts the strength and hardness of the steel plate, and avoids the precipitation of hard and brittle phases.

[0046] In summary, the present application adopts C, Mn, Ni, Cr, Mo, Co, Sn elements and Ni, Ti and other micro-alloy phase matching component design, combined with high cleanliness and alloying smelting + casting machine heavy reduction + two-stage controlled rolling + quenching heat treatment + tempering heat treatment process, to produce an ultra-high strength 785MPa corrosion-resistant offshore steel with a maximum thickness of 80mm.

[0047] The yield strength of the steel plate after the quenching and tempering treatment is ≥785MPa, the tensile strength is 865-980MPa, the elongation is ≥17%, and the Charpy impact energy of the core of the steel plate at-60℃ is ≥150J. The corrosion resistance is excellent, the seawater corrosion resistance rate of the steel plate is 40% or less of the performance of the conventional 785MPa offshore steel, and the marine atmospheric corrosion resistance rate is 50% or less of the performance of the conventional 785MPa offshore steel. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is the metallographic structure diagram of Example 1 of the present application. Detailed Implementation

[0049] This invention discloses a corrosion-resistant marine engineering steel plate with a yield strength of 785 MPa and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the same result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0050] To address the compositional performance requirements of corrosion-resistant marine engineering steel with a yield strength of 785 MPa in extremely cold and deep-sea environments, this invention utilizes a compositional design combining elements such as C, Mn, Ni, Cr, Mo, Co, and Sn, along with microalloying elements like Ni and Ti, and key production technologies for corrosion-resistant steel plates in such environments. The inventors conducted extensive and systematic experimental research in several aspects, including alloy element screening and proportioning, steel cleanliness control, and optimization and parameter selection of efficient rolling processes. Ultimately, they determined the alloy element proportions and production processes that meet the objectives of this invention. The chemical composition of the steel in this invention's embodiments is shown in Table 1; the manufacturing processes of the steel plates in this invention's embodiments are shown in Tables 2 and 3; the mechanical properties of the steel plates in this invention's embodiments are shown in Table 4; and the corrosion resistance properties of the steel plates in this invention's embodiments are shown in Table 5.

[0051] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention

[0052]

[0053] Table 2 Steel plate manufacturing process of embodiments of the present invention

[0054]

[0055] Table 3 Steel plate manufacturing process of embodiments of the present invention

[0056]

[0057] Table 4 Mechanical properties of steel plates in embodiments of the present invention

[0058]

[0059] Table 5 Corrosion resistance of embodiments of the present invention

[0060]

[0061] Note: The full immersion test reference standard JBT 7901, and the salt spray test reference standard GBT 10125. The comparison steel composition is 0.013C~0.3Si~1.40Mn~1.0Ni~0.55Cr~0.55Mo~0.04Nb~0.04V~0.01Ti.

[0062] like Figure 1 As shown, the metallographic structure of Example 1 is tempered sorbite (20%~35%) and tempered martensite at 1 / 2 thickness of the steel plate. The effective grain size of the tempered structure is 2~5μm, and it has good mechanical properties.

[0063] As shown in Table 5, this invention is a corrosion-resistant marine engineering steel plate with a yield strength of 785 MPa. Its yield strength is ≥785 MPa, tensile strength is 865~980 MPa, elongation is ≥17%, and the Charpy impact energy of the steel plate core at -60℃ is ≥150 J. It exhibits excellent corrosion resistance; the seawater corrosion rate is less than 40% of that of conventional 785 MPa marine engineering steel, and the marine atmospheric corrosion rate is less than 50% of that of conventional 785 MPa marine engineering steel. The finished steel plate can be up to 80 mm thick, significantly improving its corrosion resistance in extremely cold deep-sea environments while maintaining high strength and high toughness.

[0064] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A corrosion resistant offshore steel plate of yield strength 785 MPa grade, characterized in that, Composed of the following weight percentage chemical components: C: 0.15%~0.22%, Si: 0.15%~0.25%, Mn: 0.8%~1.2%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Ni: 1.5%~2.8%, Cr: 0.7%~1.2%, Mo: 0.2%~0.45%, Cu: 0.1%~0.3%, Co: 0.25%~0.55%, Sn: 0.1%~0.25%, Nb: 0.03%~0.06%, Ti: 0.008%~0.013%, N: 0.004%~0.006%, the rest is Fe and inevitable impurities; The yield strength is ≥785MPa, the tensile strength is 865~980MPa, the elongation is ≥17%, and the Charpy impact energy of the core of the steel plate at -60℃ is ≥150J; The maximum thickness of the finished steel plate is 80mm, the microstructure at 1 / 2 of the thickness of the steel plate is tempered sorbite+tempered martensite, the tempered sorbite is 20%~35%, and the effective grain size of the tempered structure is 2~5μm. The manufacturing method specifically comprises the following steps:

2. A method of producing a corrosion-resistant marine steel plate of a yield strength of 785 MPa according to claim 1, characterized in that, 1) Steel refining; 2) Continuous casting: During the casting process of the caster, two sets of compression rollers are used for heavy reduction at the front end of the core solidification zone 1~2m; The continuous casting billet is directly loaded into the heating furnace at a surface temperature of 550~650℃ and a furnace temperature of 500~600℃, and is kept for 60~120min; 3) Two-stage controlled rolling: The first-stage rolling temperature is 1100~1150℃, and the intermediate billet cooling rate is 3~6℃ / s; The second-stage opening rolling temperature is 750~800℃, and the finish rolling temperature is 720~770℃; 4) Quenching and tempering: The quenching temperature is 780~840℃, and the quenching holding time is 1.0~1.6min / mm; The tempering temperature is 420~500℃, and the tempering holding time is 3~4.5min / mm.

3. The manufacturing method of the 785MPa-grade corrosion-resistant marine steel plate according to claim 2, characterized in that, 1) The steel is refined by a converter, an LF furnace, an RH or a VD furnace.

4. The manufacturing method of the 785MPa-grade corrosion-resistant marine steel plate according to claim 2, characterized in that, 2) In the single set of compression rollers, the reduction amount is ≥7mm.

5. The manufacturing method of the 785MPa-grade corrosion-resistant marine steel plate according to claim 2, characterized in that, 2) In the single set of compression rollers, the reduction amount is ≥7mm.

6. The manufacturing method of the 785MPa-grade corrosion-resistant marine steel plate according to claim 2, characterized in that, 7. The manufacturing method of the 785MPa-grade corrosion-resistant marine steel plate according to claim 2, characterized in that, 3) in the first stage, the deformation amount is 45%~55%, and the deformation rate is ≤1s -1 . 3) In the second stage, the reduction amount of the last three passes is ≥8mm. ​

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