Large-thickness high-toughness 1.1 GPa-grade ocean engineering steel and preparation method thereof

Abstract

The invention belongs to the field of steel and iron materials and preparation thereof, and discloses large-thickness high-toughness 1.1 GPa grade steel for ocean engineering and a preparation method of the large-thickness high-toughness 1.1 GPa grade steel. The steel comprises the following components in percentage by weight: 0.09 to 0.12 percent of C, 0.2 to 0.4 percent of Si, 0.6 to 1.0 percent of Mn, 7.0 to 9.0 percent of Ni, 0.8 to 1.2 percent of Cr, 0.8 to 1.2 percent of Mo, 0.04 to 0.08 percent of V, 0.02 to 0.04 percent of Ti, 0.005 to 0.01 percent of B, less than or equal to 0.002 percent of S, less than or equal to 0.003 percent of P and the balance of Fe and impurities. Ni-Cr-Mo-V composite strengthening is adopted, the alloy element proportion is regulated and controlled, the quenching and tempering heat treatment process after rolling is controlled, and the 1.1 GPa grade ocean engineering steel with excellent low-temperature toughness is obtained. The thickness of the steel plate ranges from 10 mm to 60 mm, excellent low-temperature toughness is guaranteed, meanwhile, the steel plate has ultrahigh strength, and the percentage elongation after fracture is larger than 16%.

Description

Technical Field

[0001] The invention relates to the field of steel materials and preparation thereof, and in particular to a thick and high-toughness 1.1 GPa grade marine engineering steel and a preparation method thereof. Background Art

[0002] The ocean occupies 71% of the total area of ​​the earth, and contains extremely rich marine mineral resources, seawater chemical resources, marine biological resources and marine power resources. In order to develop and utilize these resources, humans continue to improve the technical level of ships and marine engineering equipment, and steel materials account for more than 70% of ships and marine engineering equipment. Therefore, steel materials play a very important role in ships and marine engineering equipment. However, the marine environment is extremely complex. During the service process, ships and marine engineering equipment not only need to withstand hundreds of tons of gravity loads, but also face severe service environments such as low temperature, waves, corrosion, and pressure. Therefore, thick plate steel with high strength plasticity, excellent low-temperature toughness and welding performance has important application prospects in the field of marine engineering.

[0003] At present, the chemical composition of traditional steel for shipbuilding and marine engineering is usually composed of low carbon and a small amount of alloying elements. The quenching + tempering heat treatment process is used to obtain a single martensitic structure with a high dislocation density. Taking the 980 series high-strength steel 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 close to 1GPa, the toughness and welding performance are poor. The 921 series high-strength steel ensures excellent welding performance by reducing the carbon content. Although it has good impact toughness, the yield strength is only about 600MPa. Therefore, steel with high strength greater than 1GPa, excellent low-temperature toughness and welding performance is an important direction for the future development of steel for shipbuilding and marine engineering equipment.

[0004] The invention patent with publication number CN113430458B discloses an ultra-high strength steel plate with a yield strength of 1040MPa or more and a manufacturing method thereof. The patent adds Cu: 0.76~1.40%, Nb≤0.1%, Al≤0.04% and rare earth element RE, adopts an online quenching + tempering heat treatment process to form a martensitic structure, and uses NiAl phase + Cu-rich phase precipitation strengthening to obtain a high-strength steel with a yield strength of more than 1040MPa, but the yield strength ratio of this steel is greater than 0.95, and the impact toughness at a low temperature of -80℃ is poor.

[0005] The invention patent with publication number CN111041329B discloses a high-strength and high-toughness steel plate for marine engineering and its production method. The patent adds Cu: 0.20~0.30%, Nb: 0.01~0.02%, Al: 0.02~0.04%, and also uses the precipitation strengthening method of NiAl phase + Cu-rich phase. After two-stage hot rolling, hot stacking treatment + quenching and tempering treatment are performed. The process is complicated, and the yield strength of the obtained steel plate is less than 900MPa.

[0006] The invention patent with publication number CN115558863B discloses a low yield ratio marine steel with a yield strength ≥750MPa and its production process. The patented steel contains 0.62~1.20% Cu, ≤0.06% Nb, and ≤0.04% Al. Through Cu-Mo-Nb-V-Ti composite strengthening and adjustment of controlled rolling and controlled cooling parameters, a mixed structure of martensite-bainite-nanoscale precipitates is formed, and the yield strength is less than 800MPa. Summary of the invention

[0007] 1. Technical issues to be resolved

[0008] In order to solve the problem of insufficient strength and toughness in the prior art, a thick and high-toughness 1.1GPa marine engineering steel and its preparation method are proposed, which are suitable for low-temperature service environments and have a yield strength of up to 1.1GPa. Through microalloy regulation, controlled rolling and heat treatment process optimization, a martensitic matrix with excellent toughness and blocky martensite-austenite components are formed, accompanied by (Cr, Mo)2C and VC precipitation phases, and finally the steel plate of the present invention is obtained, with a yield strength of >1100MPa, a tensile strength of >1200MPa, a Charpy V-notch impact energy of >130J at -80℃, an elongation after fracture of more than 16%, and a thickness range of 10mm~60mm.

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

[0010] In order to achieve the above-mentioned purpose, the steel plate of the present invention has been systematically tested in terms of alloy composition ratio, rolling process parameters, microstructure control, etc. The technical solution includes:

[0011] A thick and high-toughness 1.1 GPa grade marine engineering steel having the following chemical composition 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 being Fe and unavoidable impurities;

[0012] The microstructure of the thick and high-toughness 1.1 GPa-grade 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 to 60 mm.

[0013] 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.

[0014] The thick and high-toughness 1.1 GPa-grade marine engineering steel has a yield strength of >1100 MPa, a tensile strength of >1200 MPa, a Charpy V-notch impact energy at -80°C of >130 J, and an elongation after fracture of more than 16%.

[0015] A method for preparing thick and high-toughness 1.1 GPa grade marine engineering steel comprises the following steps:

[0016] 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. The continuous casting is fully protected during casting.

[0017] S2, rolling: heating the continuous casting billet in step S1 to 1100°C-1200°C, keeping the temperature for 2-4 hours, performing controlled rolling, and air cooling to room temperature to obtain a hot-rolled steel plate;

[0018] 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;

[0019] 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.

[0020] In step S1, the smelting temperature is 1600°C to 1700°C, the treatment time of the 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.

[0021] In step S2, the controlled rolling is a two-stage rolling; the first stage rough rolling temperature is 1100°C~1150°C, the rough rolling is performed for 3~5 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~870°C for the second stage finishing rolling, the finishing rolling is performed for 2~4 passes, and the reduction rate of a single pass is greater than 20%, and air cooling is performed to room temperature after rolling.

[0022] In step S3, a furnace heating method is adopted, the heating rate is controlled at 100°C / h~150°C / h, the quenching temperature is 800°C~830°C, the uniform temperature holding time coefficient is 1.5min / mm~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.

[0023] 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.

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

[0025] The C element provides solid solution strengthening and precipitation strengthening effects in the present invention. Too high a carbon content has an adverse effect on toughness, formability and welding performance, while too low a carbon content is not conducive to the formation of martensite to provide strength, so it is preferred that the C element content is controlled within 0.09% to 0.12%.

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

[0027] Mo and Cr elements provide precipitation strengthening effects in the present invention. Mo element mainly provides precipitation strengthening effects during tempering. When Mo element is too low, the precipitation strengthening effect is poor. When Mo element is too high, the fracture toughness will be reduced. Therefore, the preferred Mo content is 0.8% to 1.2%. When the Ni content is between 7.0% and 9.0%, Mo and Cr elements are added in combination, and the mass percentage of Mo element and the mass percentage of Cr element are controlled to be 1:1. At this time, Cr element can reduce the precipitation temperature, effectively avoiding the coarsening of precipitates caused by excessive tempering. At the same time, the addition of Cr element can also significantly improve the precipitation strengthening effect. Tempering between 520°C and 560°C can precipitate nano-level (Cr, Mo)2C, achieving the best precipitation strengthening effect.

[0028] The V element provides a further precipitation strengthening effect in the present invention. When tempered between 520 and 560°C, nano-sized VC can be formed to hinder dislocation movement, and synergize with (Cr, Mo)2C to further produce a precipitation strengthening effect, thereby improving the strength of the steel. Therefore, the preferred V content is 0.04% to 0.08%.

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

[0030] The B element provides the effect of inhibiting grain boundary precipitates in the present invention. The B element can combine with N to form BN, and by absorbing the N element, the precipitation of coarse Ti(C,N) particles at the grain boundary during tempering is further reduced, and (Cr,Mo)2C and VC are evenly dispersed inside the grains, effectively improving the strength, toughness and plasticity of the steel. However, too high a B content will lead to grain boundary embrittlement and hot cracking tendency, so the preferred B content is 0.005% to 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 C element and Cr, Mo, V, and Ti alloy elements, so that C element plays a dual role in solid solution strengthening and precipitation strengthening, thereby avoiding the adverse effect of excessive C content on toughness and welding performance; the addition of Ni element improves the low-temperature toughness and thick plate uniformity of steel, ensuring the excellent performance of steel in harsh low-temperature environments; the addition of Ti element produces a fine grain strengthening effect, which is beneficial to the improvement of the overall performance of steel; the addition of B element effectively inhibits the formation of coarse Ti (C, N) particles at the grain boundaries during tempering, avoids grain boundary embrittlement caused by tempering, and improves the plasticity and toughness of steel.

[0033] (2) When the Ni content is between 7.0% and 9.0%, the composite addition of Cr and Mo effectively reduces the precipitation temperature, avoids the coarsening of the precipitates, and fully realizes the uniform dispersion and precipitation of (Cr, Mo)2C at the nanoscale. At the same time, by controlling the mass percentage of V to the mass percentage of Ti 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, thereby avoiding the precipitation of Ti(C, N) particles during the tempering process. The synergistic effect of V and Ti makes the nano-precipitation phase 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] A 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 process steps are as follows:

[0047] 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 10mm thick hot-rolled steel plate through 8 passes, rough rolling 5 passes, and the starting rolling temperature is 1140℃; finishing rolling 3 passes, 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 15min, and then cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s; then the quenched plate is tempered, the quenched plate is heated to 560℃ with the furnace, kept warm for 25min, and then air-cooled to room temperature to obtain a finished plate.

[0048] Comparative Examples 1 to 6

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

[0050] The composition of the steel plate of Comparative Example 2 is that the content of Cr and Mo in the steel plate of Comparative Example 2 is higher than that in the steel plate of Example 3, and other conditions are the same. Under this condition, more (Cr, Mo)2C precipitates are formed, resulting in coarse precipitates, and although the strength is improved, the toughness is reduced.

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

[0052] The composition of the steel plate of Comparative Example 4 is higher in V and Ti content than that of the steel plate of Example 3, and other conditions are the same. Under this condition, more Ti(C,N) and VC precipitates will be produced during tempering, and the size is larger, which is not conducive to toughness.

[0053] The composition of the steel plates of Comparative Examples 5 and 6 is the same as that 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, and a higher tempering temperature will decompose the martensite structure, thereby affecting the mechanical properties.

[0054] Table 1 Chemical composition of the steel plates of the present invention and the comparative examples

[0055]

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

[0057]

[0058] According to the content of the present invention, the process parameters and component contents are adjusted to achieve the preparation of a thick and high-toughness 1.1GPa marine engineering steel of the present invention, and the steel exhibits performance substantially consistent with the present invention. The present invention is described above by way of example. It should be noted that any simple deformation, modification or equivalent replacement that can be made by other technicians in this field without inventive 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%~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%, the balance is Fe and unavoidable impurities; The microstructure of the thick and high-toughness 1.1 GPa-grade 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 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.

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

4. 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-1200°C, keeping the temperature for 2-4 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.

5. The preparation method according to claim 4, characterized in that: In step S1, the smelting temperature is 1600°C to 1700°C, the treatment time of the 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.

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

7. The preparation method according to claim 4, characterized in that: In step S3, a furnace heating method is adopted, the heating rate is controlled at 100°C / h~150°C / h, the quenching temperature is 800°C~830°C, the uniform temperature holding time coefficient is 1.5min / mm~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.

8. The preparation method according to claim 4, 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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