1.1GPa Grade Marine Engineering Steel with Large Thickness and High Toughness and Its Preparation Method

Abstract

The present invention belongs to the field of steel materials and their preparation, and discloses a 1.1GPa-grade marine engineering steel with large thickness and high toughness and its preparation method. C: 0.09 - 0.12%, Si: 0.2 - 0.4%, Mn: 0.6 - 1.0%, Ni: 7.0 - 9.0%, Cr: 0.8 - 1.2%, Mo: 0.8 - 1.2%, V: 0.04 - 0.08%, Ti: 0.02 - 0.04%, B: 0.005 - 0.01%, S ≤ 0.002%, P ≤ 0.003%, and the balance is Fe and impurities. By adopting Ni-Cr-Mo-V composite strengthening and regulating the alloy element ratio, and controlling the quenching + tempering heat treatment process after rolling, a 1.1GPa-grade marine engineering steel with excellent low-temperature toughness is obtained. The steel plate of the present invention has a thickness range of 10 - 60mm, ensuring excellent low-temperature toughness while having ultra-high strength, and the elongation after fracture > 16%.

Description

Technical Field

[0001] The present invention relates to the field of steel materials and their preparation, and particularly to a high-strength and high-toughness 1.1GPa-grade steel for offshore engineering with large thickness and a preparation method thereof. Background Art

[0002] The ocean covers 71% of the Earth's total area and contains extremely rich marine mineral resources, seawater chemical resources, marine biological resources, marine power resources, etc. In order to develop and utilize these resources, humans have continuously improved the technical level of ships and offshore engineering equipment. Since steel materials account for more than 70% of ships and offshore engineering equipment, steel materials play a very important role in ships and offshore engineering equipment. However, the marine environment is extremely complex. During the service process, ships and offshore engineering equipment not only need to bear a gravity load of hundreds of tons, but also face severe service environments such as low temperature, sea waves, corrosion, and pressure. Therefore, thick steel plates with high strength and plasticity, excellent low-temperature toughness, and welding performance have important application prospects in the offshore engineering field.

[0003] At present, the chemical composition of traditional steels for ships and offshore engineering usually consists of low carbon and a small amount of alloying elements, and a heat treatment process of quenching + tempering is used to obtain a single martensite structure with a high dislocation density. Taking the 980 series high-strength steels as an example, in order to improve the strength, the carbon content of this type of steel plate is up to 0.18%. Although the yield strength is nearly 1GPa, the toughness and welding performance are poor. The 921 series high-strength steels ensure excellent welding performance by reducing the carbon content. Although they have good impact toughness, the yield strength is only about 600MPa. Therefore, steel materials with a strength greater than 1GPa, excellent low-temperature toughness, and welding performance are an important development direction for future steels used in ships and offshore engineering equipment.

[0004] The invention patent with the publication number CN113430458B discloses a super high-strength steel plate with a yield strength of more than 1040MPa and a manufacturing method thereof. The composition of this patent adds Cu: 0.76 - 1.40%, Nb ≤ 0.1%, Al ≤ 0.04%, and rare earth element RE. An online quenching + tempering heat treatment process is used to form a martensite structure, and precipitation strengthening methods such as NiAl phase + Cu-rich phase are utilized to obtain high-strength steel with a yield strength above 1040MPa. However, the yield ratio of this steel grade is greater than 0.95, and the impact toughness at -80°C is poor.

[0005] The invention patent with the publication number CN111041329B discloses a high-strength and high-toughness steel plate for ocean engineering and its production method. The composition of this patent adds Cu: 0.20 - 0.30%, Nb: 0.01 - 0.02%, Al: 0.02 - 0.04%. Also using precipitation strengthening methods such as NiAl phase + Cu-rich phase, after two-stage hot rolling, hot stacking treatment + quenching and tempering treatment are carried out. The process is complex, and the yield strength of the obtained steel plate is less than 900 MPa.

[0006] The invention patent with the publication number CN115558863B discloses a low yield ratio offshore steel with a yield strength ≥750 MPa and its production process. The composition of this patent adds Cu: 0.62 - 1.20%, Nb ≤ 0.06%, Al ≤ 0.04%. Through Cu-Mo-Nb-V-Ti composite strengthening and adjusting controlled rolling and controlled cooling parameters, a mixed structure of martensite-bainite-nano-scale precipitates is formed, and the yield strength is less than 800 MPa. Summary of the Invention

[0007] (I) Technical problems to be solved

[0008] In order to solve the problem of insufficient strength and toughness in the prior art, a large-thickness high-toughness 1.1 GPa grade steel for ocean engineering and its preparation method are proposed. It is applicable to low-temperature service environments, and the yield strength can reach 1.1 GPa grade. Through microalloy control, optimization of controlled rolling and heat treatment processes, a martensite matrix with excellent toughness and massive martensite-austenite components are formed, and at the same time, (Cr,Mo)2C and VC precipitation phases are accompanied. Finally, the steel plate of the present invention is obtained, with a yield strength > 1100 MPa, a tensile strength > 1200 MPa, a Charpy V-notch impact energy at -80 °C > 130 J, an elongation after fracture greater than 16%, and a thickness range of 10 mm - 60 mm.

[0009] (II) Technical solution of the present invention

[0010] In order to achieve the above object, the steel plate of the present invention has carried out systematic tests in terms of alloy composition ratio, rolling process parameters, microstructure control, etc. The technical solution includes:

[0011] A large-thickness high-toughness 1.1 GPa grade steel for ocean engineering, having the following chemical components by mass percentage: C: 0.09% - 0.12%, Si: 0.2% - 0.4%, Mn: 0.6% - 1.0%, Ni: 7.0% - 9.0%, Cr: 0.8% - 1.2%, Mo: 0.8% - 1.2%, V: 0.04% - 0.08%, Ti: 0.02% - 0.04%, B: 0.005% - 0.01%, S ≤ 0.002%, P ≤ 0.003%, and the balance is Fe and unavoidable impurities;

[0012] The microstructure of the large-thickness high-toughness 1.1GPa-grade steel for ocean engineering is a martensite matrix and massive martensite-austenite components, accompanied by precipitation phases of (Cr, Mo)2C and VC, and the thickness ranges from 10 to 60 mm.

[0013] The ratio of the mass percentage of Cr element to the mass percentage of Mo element is 1:1, and the ratio of the mass percentage of V element to the mass percentage of Ti element is 2:1.

[0014] The yield strength of the large-thickness high-toughness 1.1GPa-grade steel for ocean engineering is >1100 MPa, the tensile strength is >1200 MPa, the Charpy V-notch impact energy at -80 °C is >130 J, and the elongation after fracture is greater than 16%.

[0015] A preparation method of a large-thickness high-toughness 1.1GPa-grade steel for ocean engineering includes the following steps:

[0016] S1. Smelting: After smelting and continuous casting according to the chemical composition, a continuous casting billet is obtained. The smelting is processed by an LF and RH refining furnace, and the superheat of the tundish molten steel is less than 20 °C; during continuous casting, full protection casting is carried out;

[0017] S2. Rolling: The continuous casting billet in step S1 is heated to 1100 °C - 1200 °C, held for 2 - 4 hours, then controlled rolling is carried out, and air-cooled to room temperature to obtain a hot-rolled steel plate;

[0018] S3. Quenching: The hot-rolled steel plate in step S2 is fully austenitized, held, and then water-quenched to room temperature to obtain a quenched steel plate;

[0019] S4. Tempering: The quenched steel plate in step S3 is tempered, held, and then air-cooled to room temperature to obtain the final steel plate.

[0020] In step S1, the smelting temperature is 1600 °C - 1700 °C, the treatment time of the LF and RH refining furnace is 10 min - 30 min, and the superheat of the tundish molten steel is greater than 10 °C and less than 20 °C; during the full protection casting process of the continuous casting billet, the gas content [H] < 1.2 ppm, [N] < 30 ppm, [O] < 15 ppm.

[0021] In step S2, the controlled rolling is two-stage rolling; the rough rolling temperature in the first stage is 1100 °C - 1150 °C, rough rolling is carried out for 3 - 5 passes, and the single-pass reduction rate > 20%; after the rough rolling in the first stage is completed and the temperature reaches 830 °C - 870 °C, the second-stage finish rolling is carried out, finish rolling is carried out for 2 - 4 passes, and the single-pass reduction rate > 20%, and after rolling, it is air-cooled to room temperature.

[0022] In step S3, the furnace heating method is adopted, the heating rate is controlled at 100°C / h to 150°C / h, the quenching temperature is 800°C to 830°C, the soaking and heat preservation time coefficient is 1.5 min / mm to 2.0 min / mm, and after heat preservation, it is cooled to room temperature by a rapid laminar cooling system with an average cooling rate greater than 15°C / s.

[0023] In step S4, the tempering temperature range is 520°C to 560°C, the soaking and heat preservation time coefficient is 2.0 min / mm to 3.0 min / mm, and it is air-cooled to room temperature.

[0024] The principles of the key elements in the steel plate of the present invention are as follows:

[0025] Element C provides solid solution strengthening and precipitation strengthening effects in the present invention. Excessively high carbon content has an adverse effect on toughness, formability and welding performance, while too low carbon content is not conducive to forming martensite to provide strength. Therefore, it is preferably that the content of element C is controlled at 0.09% to 0.12%.

[0026] Element Ni provides solid solution strengthening effect in the present invention. Element Ni is beneficial to the formation of fine martensite structure, thereby improving the strength and toughness of the steel. At the same time, element Ni affects the lateral slip of dislocations, reduces the ductile-brittle transition temperature of the steel, and significantly improves the low-temperature toughness of the steel. Element Ni is also a strong hardenability element, which shifts the CCT curve to the right, being beneficial to the uniformity of the thick plate structure and the low-temperature toughness. Therefore, it is preferably that the content of element Ni is 7.0% to 9.0%.

[0027] Elements Mo and Cr provide precipitation strengthening effects in the present invention. Element Mo mainly provides precipitation strengthening effect during tempering. When the content of element Mo is too low, the precipitation strengthening effect is poor, and when the content of element Mo is too high, the fracture toughness will be reduced. Therefore, it is preferably that the content of Mo is 0.8% to 1.2%; when the content of Ni is between 7.0% and 9.0%, elements Mo and Cr are added in combination, and the mass percentage of element Mo and the mass percentage of element Cr are controlled to be 1:1. At this time, element Cr can reduce the precipitation temperature, effectively avoid the coarsening of precipitates caused by too high tempering, and at the same time, the addition of element Cr can also significantly improve the precipitation strengthening effect. Nano-scale (Cr, Mo)2C can be precipitated during tempering between 520°C and 560°C to achieve the best precipitation strengthening effect.

[0028] Element V provides further precipitation strengthening effect in the present invention. Nano-scale VC can be formed during tempering between 520 and 560°C, which hinders the movement of dislocations and synergistically acts with (Cr, Mo)2C to further produce precipitation strengthening effect, thereby improving the strength of the steel. Therefore, it is preferably that the content of V is 0.04% to 0.08%.

[0029] The Ti element provides a fine-grain strengthening effect in the present invention. During high-temperature melting and rolling, high-stability Ti(C,N) particles can be formed, which can significantly inhibit the growth of the original austenite grains, producing a fine-grain strengthening effect and also providing nucleation cores for the precipitation of (Cr,Mo)2C and VC; in addition, the Ti element can also improve the toughness of the steel by reducing grain boundary segregation and improving the morphology of inclusions. However, if the Ti element content is too high, coarse Ti(C,N) particles will precipitate during the tempering process. Therefore, V and Ti elements are added in combination. By controlling the mass percentage of the V element to the Ti element to be 2:1, Ti(C,N) is formed during high-temperature melting and rolling, thereby reducing the Ti element in the matrix. During tempering, the precipitation of VC can be preferentially promoted, thus avoiding the precipitation of Ti(C,N) particles during the tempering process. Therefore, the preferred Ti content is 0.02~0.04%.

[0030] The B element provides an effect of inhibiting grain boundary precipitates in the present invention. The B element can combine with N to form BN. By absorbing the N element, the precipitation of coarse Ti(C,N) particles at the grain boundaries during the tempering process can be further reduced, making (Cr,Mo)2C and VC uniformly and dispersedly distributed inside the grains, effectively improving the strength, toughness and plasticity of the steel. However, if the B content is too high, it will lead to grain boundary embrittlement and hot cracking tendency. Therefore, the preferred B content is 0.005%~0.01%.

[0031] (III) Compared with the prior art, the beneficial effects of the present invention include:

[0032] (1) The present invention utilizes the action mechanism of the C element and the Cr, Mo, V, and Ti alloy elements, enabling the C element to play a dual role in solid solution strengthening and precipitation strengthening, avoiding the adverse effects of too high C content on toughness and welding performance; the addition of the Ni element improves the low-temperature toughness and thick plate uniformity of the steel, ensuring excellent performance of the steel in harsh low-temperature environments; the addition of the Ti element produces a fine-grain strengthening effect, which is beneficial to the improvement of the overall performance of the steel; the addition of the B element effectively inhibits the formation of coarse Ti(C,N) particles at the grain boundaries during the tempering process, avoiding grain boundary embrittlement caused by tempering and improving the plasticity and toughness of the steel.

[0033] (2) When the Ni content is between 7.0% and 9.0%, the Cr element and the Mo element are added in equal amounts in combination, effectively reducing the precipitation temperature, avoiding the coarsening of precipitates, and fully realizing the uniform and dispersed precipitation of nano-level (Cr,Mo)2C; at the same time, by controlling the mass percentage of the V element to the Ti element to be 2:1, Ti(C,N) is formed during high-temperature melting and rolling, thereby reducing the Ti element in the matrix. During tempering, the precipitation of VC can be preferentially promoted, thus avoiding the precipitation of Ti(C,N) particles during the tempering process. The synergistic effect of V and Ti makes the nano-precipitates during the tempering process more uniform, further improving the precipitation strengthening effect.

[0034] (3) The thickness of the steel plate of the present invention ranges from 10 mm to 60 mm, the yield strength is greater than 1100 MPa, the tensile strength is greater than 1200 MPa, the Charpy V-notch impact energy at -80°C is greater than 130 J, and the elongation after fracture is greater than 16%.

[0035] (4) The manufacturing process of the steel plate of the present invention is simple and easy to implement, the product performance is stable, and the yield rate is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a microstructure picture of the finished steel plate prepared in Example 3 of the present invention.

[0037] Figure 2 This is a microstructure picture of the finished steel plate prepared in Comparative Example 6 of the present invention. DETAILED DESCRIPTION

[0038] Example 1

[0039] A thick and high-toughness 1.1 GPa grade marine engineering steel is prepared, which is suitable for low-temperature service environment. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2. The process steps are as follows:

[0040] According to the composition content described in Table 1, the smelting temperature is 1650℃, the LF and RH refining furnace treatment time is 20min, and the superheat temperature of the molten steel in the tundish is 15℃; the 370mm thick continuous casting billet is heated to 1200℃, kept warm for 4h, and then hot-rolled into a 60mm thick hot-rolled steel plate through 7 passes; the first stage is rough rolling with 4 passes, and the starting rolling temperature is 1140℃; the second stage is finishing rolling with 3 passes, and the starting rolling temperature is 850℃, and air-cooled to room temperature after the hot rolling is completed; the hot-rolled plate is then quenched, heated to 820℃ and kept warm for 90min, and then cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s; the quenched plate is then tempered, and the quenched plate is heated to 520℃ with the furnace, kept warm for 2h, and then air-cooled to room temperature to obtain a finished plate.

[0041] Example 2

[0042] Thick and high-toughness 1.1 GPa grade marine engineering steel was prepared. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2. The preparation process is consistent with the preparation process of the steel plate in Example 1.

[0043] Example 3

[0044] A thick and high-toughness 1.1 GPa grade marine engineering steel was prepared. The chemical composition is shown in Table 1, the mechanical properties are shown in Table 2, and the preparation process is consistent with the preparation process of the steel plate in Example 1.

[0045] Example 4

[0046] Prepare a marine engineering steel with a large thickness and high toughness at the level of 1.1 GPa. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2. The technological steps are as follows:

[0047] Smelt according to the component contents described in Table 1. The smelting temperature is 1650 °C, the treatment time in the LF and RH refining furnaces is 20 min, and the superheat temperature of the tundish molten steel is 15 °C; heat the 370-mm-thick continuous casting billet to 1200 °C, keep it warm for 4 h, and then hot-roll it into a hot-rolled steel plate with a thickness of 10 mm through 8 passes. There are 5 rough rolling passes, and the starting rolling temperature is 1140 °C; there are 3 finish rolling passes, and the starting rolling temperature is 850 °C. After hot rolling, air-cool it to room temperature; then perform quenching treatment on the hot-rolled plate. After heating to 820 °C and keeping it warm for 15 min, cool it to room temperature using a fast laminar cooling system with an average cooling rate greater than 15 °C / s; subsequently, perform tempering treatment on the quenched plate. Heat the quenched plate in the furnace to 560 °C, keep it warm for 25 min, and then air-cool it to room temperature to obtain the finished plate.

[0048] Comparative Examples 1 - 6

[0049] For the steel plate composition of Comparative Example 1, compared with the steel plate of Example 3, the mass percentage of Cr element and Mo element in the steel plate of Comparative Example 1 is 2:1, and other conditions are the same. Under this condition, there is more Cr element in the formed (Cr,Mo)₂C, resulting in coarser precipitates and reduced toughness.

[0050] For the steel plate composition of Comparative Example 2, compared with the steel plate of Example 3, the contents of Cr element and Mo element in the steel plate of Comparative Example 2 are higher, and other conditions are the same. Under this condition, more (Cr,Mo)₂C precipitates are formed, resulting in coarser precipitate sizes. Although the strength is improved, the toughness is reduced.

[0051] For the steel plate composition of Comparative Example 3, compared with the steel plate of Example 3, the mass percentage of V element and Ti element in the steel plate of Comparative Example 3 is 1:1, and other conditions are the same. Under this condition, the V ratio decreases, and more coarse Ti(C,N) particles will precipitate during tempering, reducing the toughness.

[0052] For the steel plate composition of Comparative Example 4, compared with the steel plate of Example 3, the contents of V element and Ti element in the steel plate of Comparative Example 4 are higher, and other conditions are the same. Under this condition, more Ti(C,N) and VC precipitates will be generated during tempering, and their sizes are relatively large, which is not conducive to toughness.

[0053] The steel plate compositions of Comparative Examples 5 - 6 are the same as those of the steel plate of Example 3. Compared with Example 3, the tempering temperature of the steel plate of Comparative Example 5 is 480 °C, and the tempering temperature of the steel plate of Comparative Example 6 is 600 °C, and other conditions are the same. A lower tempering temperature cannot produce sufficient precipitation strengthening, while a higher tempering temperature will decompose the martensite structure, thus affecting the mechanical properties.

[0054] Table 1 Chemical compositions of the steel plates in the embodiments and comparative examples of the present invention

[0055]

[0056] Table 2 Mechanical properties of the steel plates in the embodiments and comparative examples of the present invention

[0057]

[0058] By adjusting the process parameters and component contents according to the content of the present invention, the preparation of a 1.1GPa-grade high-toughness offshore engineering steel with large thickness of the present invention can be achieved, and the performance is basically the same as that of the present invention. The above is an exemplary description of the present invention. It should be noted that any simple deformation, modification or equivalent substitution that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention without departing from the core of the present invention.

Claims

1. A thick and high-toughness 1.1GPa grade marine engineering steel, characterized in that: Having the following chemical composition in mass percentage: C: 0.09% to 0.12%, Si: 0.2% to 0.4%, Mn: 0.6% to 1.0%, Ni: 7.0% to 9.0%, Cr: 0.8% to 1.2%, Mo: 0.8% to 1.2%, V: 0.04% to 0.08%, Ti: 0.02% to 0.04%, B: 0.005% to 0.01%, S≤0.002%, P≤0.003%, the balance being Fe and unavoidable impurities; The ratio of the mass percentage of the Cr element to the mass percentage of the Mo element is 1:1, and the ratio of the mass percentage of the V element to the mass percentage of the Ti element is 2:1; The microstructure of the thick and high-toughness 1.1 GPa marine engineering steel is a martensite matrix and blocky martensite-austenite components, accompanied by (Cr, Mo)2C and VC precipitation phases, and the thickness ranges from 10 mm to 60 mm.

2. The thick, high-toughness 1.1 GPa grade marine engineering steel according to claim 1, characterized in that: The thick and high-toughness 1.1 GPa-grade marine engineering steel has a yield strength greater than 1100 MPa, a tensile strength greater than 1200 MPa, a Charpy V-notch impact energy at -80°C greater than 130 J, and an elongation after fracture greater than 16%.

3. A method for preparing the thick, high-toughness 1.1 GPa grade marine engineering steel according to claim 1 or 2, characterized in that: The steps include: S1. Smelting: Smelting and continuous casting are performed according to the chemical composition to obtain continuous casting billets; the smelting is performed by LF and RH refining furnaces, and the superheat of the molten steel in the tundish is less than 20°C; during continuous casting, the casting is protected throughout the entire process; S2, rolling: heating the continuous casting billet in step S1 to 1100° C. to 1200° C., keeping the temperature for 2 to 4 hours, performing controlled rolling, and air cooling to room temperature to obtain a hot-rolled steel plate; S3, quenching: the hot-rolled steel plate in step S2 is completely austenitized, and then water-quenched to room temperature after heat preservation to obtain a quenched steel plate; S4, tempering: tempering the quenched steel plate in step S3, keeping the temperature and then air-cooling to room temperature to obtain a final steel plate.

4. The preparation method according to claim 3, characterized in that: In step S1, the smelting temperature is 1600°C to 1700°C, the treatment time of LF and RH refining furnaces is 10min to 30min, the superheat of the molten steel in the tundish is greater than 10°C and less than 20°C; during the whole-process protective casting of the continuous casting billet, the gas content [H] is less than 1.2ppm, [N] is less than 30ppm, and [O] is less than 15ppm.

5. The preparation method according to claim 3, characterized in that: In step S2, the controlled rolling is a two-stage rolling; the first stage rough rolling temperature is 1100°C to 1150°C, the rough rolling is performed for 3 to 5 passes, and the reduction rate of a single pass is greater than 20%; after the first stage rough rolling is completed, the temperature is waited to reach 830°C to 870°C for the second stage finishing rolling, the finishing rolling is performed for 2 to 4 passes, and the reduction rate of a single pass is greater than 20%, and after rolling, it is air-cooled to room temperature.

6. The preparation method according to claim 3, characterized in that: In step S3, a furnace heating method is adopted, the heating rate is controlled at 100°C / h to 150°C / h, the quenching temperature is 800°C to 830°C, the uniform temperature holding time coefficient is 1.5min / mm to 2.0min / mm, and after the holding is completed, a rapid laminar cooling system with an average cooling rate greater than 15°C / s is used to cool to room temperature.

7. The preparation method according to claim 3, characterized in that: In step S4, the tempering temperature range is 520°C to 560°C, the temperature holding time coefficient is 2.0 min / mm to 3.0 min / mm, and the steel is air-cooled to room temperature.

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