Low-cost fh690 steel plate for offshore engineering and production method thereof

Abstract

The application discloses a kind of low-cost FH690 steel plates for ocean engineering, using low C, middle Mn, Cr-Ni-Mo-Cu-B alloying component system, realize with Mn to replace Ni, can greatly reduce alloy cost;The production method of the steel plate includes using on-line quenching two-stage controlled cooling and two-phase region short-term tempering process, on the basis of shortening process flow, further reducing production cost, obtaining 60-120mm thick steel plate with tempered sorbite and reverse transformation austenite organization, yield strength ≥690MPa, tensile strength ≥770MPa, -60 ℃ V type transverse impact energy >200J, yield ratio ≤0.85, fully meet the requirements of deep sea and polar ocean platform to ultra-high strength steel safety performance, with good market value.

Description

Technical Field

[0001] This invention relates to the field of medium and heavy plate production, specifically to a low-cost, corrosion-resistant FH690 high-strength plate for marine engineering and its production method. Background Technology

[0002] Steel, as a key structural material for marine engineering equipment, is widely used in drilling platforms, production platforms, and subsea pipelines. Because marine equipment operates in harsh marine corrosive environments for extended periods, corrosion damage, abrasion failure, and biofouling of the equipment materials are unavoidable. In particular, high-strength steel accounts for 60% to 90% of the steel used in marine platforms. Currently available 690-grade high-strength steel in China incorporates large amounts of precious alloying elements such as Ni and Mo, resulting in high overall production costs and a high yield strength ratio.

[0003] For example, Chinese patent CN103938110B, FQ70 grade ultra-high strength extra-thick steel plate for marine engineering and its manufacturing method, uses Mn: 0.5-1.5%, Ni: 0.8-1.5%, Cr: 0.6-1.0%, and also adds 0.01-0.045% Nb. The overall alloy cost is relatively high. Although the quenching and tempering heat treatment process meets the 690 grade process performance, the yield strength ratio is high (0.92), the alloy cost and process cost are relatively high, and the production cycle is long.

[0004] For example, Chinese patent CN109161791B describes a 690MPa grade steel for shipbuilding and marine engineering with excellent low-temperature toughness and its manufacturing method. However, it lacks resistance to lamellar tearing performance indicators. It is smelted using a low Mn composition of 0.7-1.30%, Ni 1.0-2.5%, and V 0.01-0.05%. The heat treatment process involves quenching and tempering, or normalizing and quenching and tempering, or tempering and quenching and tempering. This results in high alloy costs, high process costs, and a long production cycle.

[0005] How to reduce alloy content while maintaining the performance of high-strength steel, thereby lowering the construction cost of offshore platforms, is a problem that needs to be solved. Therefore, this invention is proposed. Summary of the Invention

[0006] One of the objectives of this invention is to provide a low-cost FH690 steel plate for marine engineering. This steel plate has low alloy cost, a final microstructure of tempered sorbite and reverse-transformed austenite, excellent performance, and fully meets the requirements of marine steel for low temperature, high strength, and corrosion resistance.

[0007] Another objective of this invention is to provide a low-cost production method for FH690 steel plates used in marine engineering. This method has a reasonable process design, a short overall production process, and is conducive to reducing production costs.

[0008] To achieve the above objectives, the present invention employs the following technical solution: a low-cost FH690 steel plate for marine engineering, using a low-C, medium-Mn, Cr-Ni-Mo-Cu-B alloying system, wherein the steel plate has a thickness of 60-120mm and contains the following chemical composition by mass percentage (unit, wt%): C: 0.04~0.06, Si: 0.10~0.30, Mn: 4.0~5.0, P≤0.010, S≤0.0030, Cr: 0.15~0.35, Ni: 0.3~0.5, Mo: 0.20~0.40, Cu: 0.20-0.40, Alt: 0.01~0.04, B: 0.001-0.003, Ti<0.03, with the balance being Fe and unavoidable impurities.

[0009] In terms of composition design, alloy design allows for the substitution of Ni with Mn, significantly reducing alloy costs. Mn is a strong austenitic stabilizing element, and its price is only 1 / 10 that of Ni. High-Mn steel possesses excellent comprehensive properties of strength and plasticity, as well as superior low-temperature toughness. This can solve the problems of poor low-temperature toughness and high yield strength ratio of 690MPa grade ultra-high strength steel used in offshore platforms, meeting the safety performance and construction cost requirements of ultra-high strength steel for deep-sea and polar offshore platforms.

[0010] Furthermore, the microstructure of the steel plate consists of 76-79% tempered sorbite and 21-24% reverse-transformed austenite, wherein the reverse-transformed austenite is a soft phase distributed around the tempered sorbite, which can prevent the propagation of microcracks, thus exhibiting good strength and toughness. The steel plate has a yield strength ≥690MPa, tensile strength ≥770MPa, V-notch transverse impact energy ≥200J at -60℃, and a yield strength ratio ≤0.85. The steel plate not only possesses excellent comprehensive mechanical properties but also good corrosion resistance.

[0011] The aforementioned production method for low-cost FH690 steel plates used in marine engineering includes recrystallization zone rolling, two-stage controlled cooling with online quenching, and short-time tempering in the two-phase zone, as detailed below:

[0012] a. Recrystallization Zone Rolling: After heating the billet, rolling is carried out in the recrystallization zone at 950-1080℃. Through 5-9 rolling passes, a steel plate with a thickness of 60-120mm is obtained, with a final rolling temperature not lower than 930℃. It should be noted that repeated plastic deformation in the recrystallization zone causes the deformed austenite grains to recrystallize into equiaxed, finer austenite grains. During subsequent quenching, martensite laths with varying orientations grow from the grain boundaries. This microstructure is beneficial for a balance of strength and toughness after tempering.

[0013] b. Two-stage controlled cooling for online quenching: After rolling, when the online steel plate reaches 900-930℃, the first stage of controlled cooling begins. The ratio of top and bottom water is 1:2.3-2.5, the roll speed is controlled at 0.5-0.8 m / s, and the cooling rate is 15-25℃ / s. The steel plate temperature is rapidly cooled to 250-300℃. A reasonable roll speed and water ratio are used to ensure uniform cooling rate and uniform transformation of quenched martensite structure, while avoiding the formation of a thin layer of steam and ensuring good high-strength steel plate shape. The second stage of controlled cooling is carried out with the roll speed controlled at 1.2 m / s, the ratio of top and bottom water at 1:1.6, and the cooling rate at 3-8℃ / s. The steel plate is uniformly cooled to 120-200℃ to release the stress of the steel plate and prevent the steel plate from cracking. The above measures can refine the martensitic laths, obtain a fine and uniform microstructure, and allow carbon to exist in the microstructure in a solid solution state without forming needle-like cementite to reduce brittle crack initiation, thereby improving the strength and toughness of the steel plate.

[0014] c. Short-time tempering in the two-phase region: tempering temperature 630-680℃, holding time 25-40min. The matrix structure is mainly tempered sorbite. As the tempering temperature increases, reverse austenite appears, reaching its peak between 650℃ and 660℃. Obtaining the reverse austenite structure is beneficial to enhancing the low-temperature toughness of the steel plate.

[0015] Furthermore, the production of the steel plate also includes converter smelting before the recrystallization zone rolling process. When the temperature of the molten steel is raised to 1720-1780℃, the steel is tapped. The P content is controlled below 0.010% when tapping. When 1 / 3 of the molten steel has been tapped, 40% of the total amount of metallic manganese is added. The remaining metallic manganese is added before the end of tapping. The high temperature of the molten steel is used to melt the metallic manganese and fully alloy the molten steel.

[0016] The beneficial effects of this invention include: adopting a low-C, medium-Mn, Cr-Ni-Mo-Cu-B alloying composition system to replace Ni with Mn, which can significantly reduce alloy costs; combined with online quenching two-stage controlled cooling and short-time tempering in the two-phase region, the process is shortened and production costs are further reduced, resulting in steel plates with tempered sorbite and reverse-transformed austenite microstructures that can meet the safety performance requirements of ultra-high-strength steel for deep-sea and polar marine platforms. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to embodiments. These embodiments are merely descriptions of preferred implementations of the present invention.

[0018] Example 1

[0019] The steel plate involved in this embodiment has a thickness of 120mm and its chemical composition is as follows: C: 0.05%, Si: 0.25%, Mn: 4.8%, P: 0.009%, S: 0.0020%, Cr: 0.30%, Ni: 0.45%, Mo: 0.30%, Cu: 0.25%, Alt: 0.032%, B: 0.0015%, Ti: 0.015%, with the balance being Fe and unavoidable impurities.

[0020] The production process of the steel plate includes:

[0021] 1) Steelmaking: KR desulfurized molten iron is fed into the converter for smelting and tapped at a high temperature of 1770℃. When the molten steel is tapped to 1 / 3, 40% of the total amount of metallic manganese is added. The remaining metallic manganese is added before the end of tapping. The high temperature of the molten steel is used to melt the metallic manganese. Then, LF refining and VD vacuum degassing are carried out. The 350mm thick billet is cast by continuous casting.

[0022] 2) Billet heating: Heat the billet to 1250℃ and hold for 9 hours;

[0023] 3) Rolling: The initial rolling temperature is 1020℃. After 5 passes of rolling, a steel plate with a thickness of 120mm is obtained. The slab is cooled with strong descaling water in each pass. The final rolling temperature is not lower than 930℃.

[0024] 4) Online quenching: The rolled steel plate is quenched online. The first stage quenching temperature is 900-930℃, the roller speed is controlled at 0.5-0.8m / s, the water ratio is 1:2.3-2.5, and the cooling rate is 18-25℃ / s, which rapidly cools the steel plate to 250-300℃. The reasonable roller speed and water ratio ensure uniform cooling rate and uniform transformation of quenched martensite structure, while avoiding the formation of a thin layer of steam and ensuring good high-strength steel plate shape. The second stage is controlled cooling, with the roller speed controlled at 1.2m / s, the water ratio at 1:1.6, and the cooling rate at 3-8℃ / s, so that the steel plate is uniformly cooled to 120-160℃. The steel plate is then cooled to room temperature on a cooling bed.

[0025] 5) Tempering: Send the steel plate to the tempering furnace for tempering, set the heating temperature to 660℃, and hold for 35 minutes.

[0026] The steel plate obtained in the example was tested, and its microstructure was 77% tempered sorbite and 23% reverse-transformed austenite. Mechanical property testing was carried out according to GB / T 2975, low-temperature impact toughness testing was carried out according to GB / T 229, and tensile property testing was carried out according to GB / T 228. The test results are shown in Table 1.

[0027] Table 1

[0028]

[0029] Example 2

[0030] The steel plate involved in this embodiment has a thickness of 60mm and contains the following components: C: 0.06%, Si: 0.27%, Mn: 4.3%, P: 0.008%, S: 0.0010%, Cr: 0.28%, Ni: 0.35%, Mo: 0.29%, Cu: 0.27%, Alt: 0.030%, B: 0.0013%, Ti: 0.017%, with the balance being Fe and unavoidable impurities.

[0031] The production process of the steel plate includes:

[0032] 1) Steelmaking: KR desulfurized molten iron is fed into a converter for smelting and tapped at a high temperature of 1740℃. When 1 / 3 of the molten steel has been tapped, 40% of the total amount of metallic manganese is added. The remaining metallic manganese is added before the end of tapping. The high temperature of the molten steel is used to melt the metallic manganese. Then, LF refining and VD vacuum degassing are carried out. The 300mm thick billet is cast by continuous casting.

[0033] 2) Billet heating: Heat the billet to 1230℃ and hold for 8 hours;

[0034] 3) Rolling: The initial rolling temperature is 990℃. After 7 passes of rolling, a steel plate with a thickness of 60mm is obtained. The slab is cooled with strong descaling water in each pass. The final rolling temperature is not lower than 950℃.

[0035] 4) Online quenching: The rolled steel plate is quenched online. The first stage quenching temperature is 900-930℃, the roller speed is controlled at 0.5-0.8m / s, the water ratio is 1:2.3-2.5, and the cooling rate is 15-20℃ / s, which rapidly cools the steel plate to 250-300℃. The reasonable roller speed and water ratio ensure uniform cooling rate and uniform transformation of quenched martensite structure, while avoiding the formation of a thin layer of steam and ensuring good high-strength steel plate shape. The second stage is controlled cooling, with the roller speed controlled at 1.2m / s, the water ratio at 1:1.6, and the cooling rate at 3-8℃ / s, so that the steel plate is uniformly cooled to 140-180℃. The steel plate is then cooled to room temperature on a cooling bed.

[0036] 5) Tempering: Send the steel plate to the tempering furnace for tempering, set the heating temperature to 650℃, and hold for 25 minutes.

[0037] The steel plate obtained in the example was tested, and its microstructure was 76% tempered sorbite and 24% reverse-transformed austenite. Mechanical property testing was carried out according to GB / T 2975, low-temperature impact toughness testing was carried out according to GB / T 229, and tensile property testing was carried out according to GB / T 228. The test results are shown in Table 2.

[0038] Table 2

[0039]

[0040] The steel plate obtained by the above scheme has low alloy cost, yield strength ≥690MPa, tensile strength ≥770MPa, V-shaped transverse impact energy at -60℃ >200J, and yield strength ratio ≤0.85, which can meet the safety performance requirements of ultra-high strength steel for deep-sea and polar marine platforms.

[0041] Metallographic analysis: The microstructure of the steel plate is 76-79% tempered sorbite and 21-24% reverse-transformed austenite.

[0042] External inspection and flaw detection: The external inspection pass rate of the developed steel plate is 100%, and the final flaw detection of the steel plate meets the Class I flaw detection requirements of GB / T 2970 "Inspection Method for Thick Steel Plates".

[0043] The above description is only a preferred embodiment of the present invention. The above specific embodiments are not intended to limit the present invention. Any modifications, alterations or equivalent substitutions made by those skilled in the art based on the above description shall fall within the protection scope of the present invention.

Claims

1. A method for producing low-cost FH690 steel plates for marine engineering, characterized in that, The steel plate has a thickness of 60-120mm and contains the following chemical composition by mass percentage: C: 0.04-0.06, Si: 0.10-0.30, Mn: 4.0-5.0, P≤0.010, S≤0.0030, Cr: 0.15-0.35, Ni: 0.3-0.5, Mo: 0.20-0.40, Cu: 0.20-0.40, Alt: 0.01-0.04, B: 0.001-0.003, Ti<0.03, with the balance being Fe and unavoidable impurities. The steel plate production method includes recrystallization zone rolling, two-stage controlled cooling with online quenching, and short-time tempering in the two-phase zone, as detailed below: a. Recrystallization zone rolling: After the billet is heated, it is rolled in the recrystallization zone at 950-1080℃. Through 5-9 rolling passes, a steel plate with a thickness of 60-120mm is obtained, and the final rolling temperature is not lower than 930℃. b. Two-stage controlled cooling for online quenching: After rolling, when the online steel plate reaches 900-930℃, the first stage of controlled cooling begins, with an upper and lower water volume ratio of 1:2.3-2.5, a roll speed of 0.5-0.8m / s, and a cooling rate of 15-25℃ / s, rapidly cooling the steel plate to 250-300℃; the second stage of controlled cooling is performed, with the roll speed controlled at 1.2m / s, an upper and lower water volume ratio of 1:1.6, and a cooling rate of 3-8℃ / s, allowing the steel plate to cool uniformly to 120-200℃; c. Short-time tempering in the two-phase region: tempering temperature 630-680℃, holding time 25-40min.

2. The method for producing low-cost FH690 steel plates for marine engineering according to claim 1, characterized in that, The steel plate has a microstructure of 76-79% tempered sorbite and 21-24% reverse-transformed austenite, with a yield strength ≥690MPa, tensile strength ≥770MPa, V-notch transverse impact energy at -60℃ >200J, and a yield strength ratio ≤0.

85.

3. The method for producing low-cost FH690 steel plates for marine engineering according to claim 1, characterized in that: The production of the steel plate also includes converter smelting before the recrystallization zone rolling process. When the temperature of the molten steel is raised to 1720-1780℃, the steel is tapped. The P content is controlled below 0.010% when tapping. When 1 / 3 of the molten steel has been tapped, 40% of the total amount of metallic manganese is added. The remaining metallic manganese is added before the end of tapping.

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