High-toughness low-carbon steel for 1GPa-grade ocean engineering and preparation method of high-toughness low-carbon steel

Abstract

The invention belongs to the field of steel and iron materials and preparation thereof, and discloses high-toughness low-carbon steel for 1GPa-grade ocean engineering and a preparation method of the high-toughness low-carbon steel. The steel plate comprises the following chemical components in percentage by mass: 0.03 to 0.05 percent of C, 0.10 to 0.30 percent of Si, 0.8 to 1.2 percent of Mn, 3.0 to 5.0 percent of Ni, 1.0 to 1.5 percent of Cu, 0.5 to 0.7 percent of Cr, 0.5 to 0.7 percent of Mo, 0.01 to 0.03 percent of Nb, 0.01 to 0.03 percent of Ti, 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 inevitable impurities. A heat treatment process of air cooling after two-stage control rolling and tempering after quenching is adopted, a nanoscale Cu-rich phase and a (Cr, Mo) 2C precipitated phase are microalloyed through Nb-Ti, and finally a structure of a tempered martensite matrix and a nanoscale precipitated phase is obtained.

Description

Technical Field

[0001] The present invention relates to the field of steel materials and preparation thereof, and in particular to a 1GPa-level high-strength and low-carbon steel for marine engineering and a preparation method thereof. Background Art

[0002] The ocean covers about 71% of the earth's surface and contains rich natural resources, including oil, natural gas, minerals, etc. In order to make efficient use of these resources, humans continue to promote the advancement of ship and marine engineering equipment technology. These equipment not only need to bear heavy loads, but also must operate stably in harsh marine environments, so the technical requirements for equipment have become more stringent. First, ships and marine engineering equipment need to bear huge loads, especially in deep-sea operations. Steel must have high yield strength and fatigue resistance to ensure long-term stable operation; secondly, the complexity of the marine environment requires steel to have low temperature resistance, corrosion resistance and pressure resistance. In low temperature environments, steel will undergo brittle transformation, and the corrosiveness of seawater affects the durability of steel. Therefore, steel must not only have strength, but also ensure good toughness and corrosion resistance; in addition, the welding performance and formability of steel are equally important to ensure structural reliability and convenient processing. High-strength steel usually has a high carbon content, which may affect its welding performance and cause cracks or poor connections. Therefore, optimizing the welding performance and formability of steel is the key. In summary, steel for ships and marine engineering equipment must have high strength, good toughness, corrosion resistance, and excellent weldability and formability. These requirements are the basis for the long-term reliable operation of equipment and are also the key to the sustainable development of marine resource development and marine engineering.

[0003] At present, the yield strength of traditional steel for shipbuilding and marine engineering equipment has been increased from 550MPa to 900MPa. The heat treatment process usually first uses quenching to form a high-density lath martensite structure, and then uses the tempering process to form a precipitation phase on the martensite matrix to achieve precipitation strengthening. At present, the steel for shipbuilding and marine engineering equipment has the following shortcomings:

[0004] (1) Insufficient yield strength: With the deepening of marine resource development, ships and marine engineering equipment require higher strength steel. The current yield strength can only reach 900MPa, which is difficult to meet the future demand for high-strength steel above 1GPa.

[0005] (2) Poor low-temperature toughness: The increase in the yield strength of steel is usually accompanied by a decrease in toughness, especially a decrease in low-temperature toughness. This poses a safety hazard to the use of steel in extreme marine environments, especially as brittle fracture is prone to occur at low temperatures.

[0006] (3) Poor welding and forming performance: High-strength steel often requires an increased carbon content, but the increase in carbon content will lead to a decrease in welding and forming performance, affecting the assembly and processing efficiency of ships and marine engineering equipment, and restricting its further application. Summary of the invention

[0007] In order to solve the above-mentioned problems existing in the prior art, a 1GPa-level high-strength and tough low-carbon steel for marine engineering and a preparation method thereof are proposed. The microstructure and precipitation phase are regulated by microalloying to form a martensitic structure with excellent low-temperature toughness. The final steel plate obtained has excellent welding performance, and has the characteristics of yield strength greater than 1000MPa, tensile strength greater than 1100MPa, and impact energy greater than 150J at -40℃.

[0008] In order to achieve the above-mentioned purpose, the steel plate of the present invention has been systematically tested in terms of composition design, rolling process parameters, heat treatment process control, etc. The technical solution includes:

[0009] A 1 GPa-level high-strength and tough low-carbon steel for marine engineering, having the following chemical composition by mass percentage: C: 0.03%-0.05%, Si: 0.10%-0.30%, Mn: 0.8%-1.2%, Ni: 3.0%-5.0%, Cu: 1.0%-1.5%, Cr: 0.5%-0.7%, Mo: 0.5%-0.7%, Nb: 0.01%-0.03%, Ti: 0.01%-0.03%, S≤0.002%, P≤0.003%, and the balance is Fe and unavoidable impurities;

[0010] The microstructure of the 1 GPa-level high-strength and tough low-carbon steel for marine engineering is a tempered martensite matrix, a nano-scale Cu-rich phase and a (Cr, Mo)2C precipitation phase.

[0011] 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 Nb element to the mass percentage of the Ti element is 1:1.

[0012] The steel plate thickness range of 1GPa-grade high-strength and tough low-carbon steel for marine engineering is 10mm-60mm, the yield strength is greater than 1000MPa, the tensile strength is greater than 1100MPa, and the impact energy at -40℃ is greater than 150J.

[0013] A method for preparing 1GPa-level high-strength and low-carbon steel for marine engineering, comprising the following steps:

[0014] S1. Smelting and continuous casting are performed according to the chemical composition of the 1GPa-grade high-strength and tough low-carbon steel for marine engineering to obtain a continuous casting billet; the smelting is treated by LF and RH refining furnaces, and the superheat of the molten steel in the middle package is less than 20° C. The continuous casting is performed with full protection casting;

[0015] S2, heating the continuous casting billet in step S1 to 1150° C. to 1200° C. and then keeping the temperature for 2 to 4 hours, and then performing controlled rolling. After the rolling is completed, the billet is air-cooled to room temperature to obtain a rolled steel plate;

[0016] S3, quenching the rolled steel plate in step S2, keeping it warm and then cooling it to room temperature to obtain a quenched steel plate;

[0017] S4, tempering the quenched steel plate in step S3, keeping the temperature and then air-cooling it to room temperature to obtain a final steel plate.

[0018] In step S1, the treatment time of LF and RH refining furnaces is 10min to 30min respectively, and the superheat temperature of molten steel in the tundish is greater than or equal to 10°C and less than 20°C; during the smelting process, the gas content [H] is less than 1.2ppm, [N] is less than 30ppm, and [O] is less than 15ppm.

[0019] In step S2, the continuous casting billet is subjected to two-stage rolling, wherein the rough rolling temperature of the first stage is 1100°C~1150°C, and the deformation is greater than or equal to 50%; after the first stage rough rolling is completed, the second stage finishing rolling is carried out when the temperature reaches 920°C~960°C, and the deformation is greater than or equal to 70%, and air-cooling is performed to room temperature after rolling.

[0020] In step S3, the quenching temperature range is 880°C to 920°C, the holding time coefficient is 1.5 min / mm to 2.5 min / mm, and the steel is cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15°C / s.

[0021] In step S4, the tempering temperature range is 540°C to 560°C, and the holding time coefficient is 4.0 min / mm to 6.0 min / mm.

[0022] The key elements in the steel plate of the present invention are as follows:

[0023] The C element is the most important strengthening element in the steel plate of the present invention, and can significantly improve the strength of the steel plate through solid solution strengthening and precipitation strengthening. However, too high a C content will have an adverse effect on plasticity, toughness, especially formability and welding performance. Therefore, it is preferred that the C content be controlled within a range of 0.03% to 0.05%.

[0024] The Ni element provides a solid solution strengthening effect in the steel plate of the present invention, affects the lateral slip of dislocations during plastic deformation, reduces the ductile-brittle transition temperature of the steel, and can significantly improve the low-temperature toughness of the steel; at the same time, the Ni element is a strong hardenability element and also helps to improve the strength, but the Ni element is relatively expensive, so the preferred Ni content is 3.0%~5.0%.

[0025] Mo and Cr elements mainly provide precipitation strengthening effect in the steel plate of the present invention. Due to the low C content, the precipitation strengthening effect is poor when the Mo element is too low, and the fracture toughness will be reduced when the Mo element is too high. Therefore, the preferred Mo content is 0.5%~0.7%; when the Ni content is 3.0%~5.0%, Cr and Mo elements are added in combination, and the ratio of the mass percentage of the Mo element to the mass percentage of the Cr element is controlled to be 1:1. At this time, the Cr element can reduce the precipitation temperature and effectively avoid the coarsening of the precipitate caused by excessive tempering. At the same time, the addition of the Cr element can also significantly improve the precipitation strengthening effect. Tempering between 540°C~560°C can precipitate nano-level (Cr, Mo)2C, achieving the best precipitation strengthening effect.

[0026] Cu element is an important element of the steel plate of the present invention, providing solid solution strengthening and precipitation strengthening effects. Cu element solid solution in the matrix will cause lattice distortion, thereby improving the strength of the steel plate, but too high Cu element will lead to a decrease in toughness, so the preferred Cu content is 1.0%~1.5%; in addition, tempering will also precipitate a nano-scale Cu-rich phase to produce precipitation strengthening, and the Cu-rich phase will also inhibit dislocation recovery, thereby promoting the precipitation of nano-scale (Cr, Mo)2C, accelerating the dissolution of cementite, and is beneficial to the simultaneous improvement of strength and toughness.

[0027] Nb and Ti elements are important microalloying elements for the steel plate of the present invention. The ratio of the mass percentage of the Nb element to the mass percentage of the Ti element is controlled to be 1:1. The dislocation density is increased by synergistic microalloying and refining of the martensitic matrix, providing more nucleation sites for the Cu-rich phase. The Cu-rich phase precipitated first serves as a heterogeneous nucleation site for (Cr, Mo)2C, promoting the precipitation of nano-level (Cr, Mo)2C. Under the synergistic effect of the nano-level Cu-rich phase and the (Cr, Mo)2C precipitated phase, dislocation recovery is hindered, thereby ensuring the strength of the steel plate and significantly improving the low-temperature toughness.

[0028] Compared with the prior art, the beneficial effects of the present invention include:

[0029] (1) The strengthening and toughening mechanism of the steel plate of the present invention is a synergistic effect from multiple angles. The main sources of strength are solid solution strengthening of C elements, solid solution strengthening of Cu elements, and precipitation strengthening of coherent nano-scale Cu-rich phase and (Cr, Mo)2C; the main sources of toughness are solid solution toughening of Ni elements, precipitation toughening of coherent nano-scale Cu-rich phase and (Cr, Mo)2C, and improvement of toughness due to reduction of dislocation density caused by long-term tempering.

[0030] (2) The steel plate of the present invention ensures excellent welding performance and impact resistance by reducing the C content, and at the same time adds the Cu element to compensate for the strength loss caused by the reduction of the C element. When the Ni content is 3.0-5.0%, the ratio of Cr and Mo contents is adjusted to 1:1. The generated Cu-rich phase will promote the formation of coherent nano-level (Cr, Mo)2C precipitation phase, thereby simultaneously improving the strength and toughness of the steel plate.

[0031] (3) First, the steel plate of the present invention controls the content ratio of Nb and Ti elements to be 1:1, utilizes the synergistic effect of Nb-Ti to microalloy and refine the martensitic matrix to increase the dislocation density, and provide more nucleation sites for the Cu-rich phase; secondly, the precipitated Cu-rich phase serves as a heterogeneous nucleation site for (Cr, Mo)2C, promoting the precipitation of nano-scale (Cr, Mo)2C. The nano-scale (Cr, Mo)2C precipitate phase is coherent with the martensitic matrix, and can provide a dual effect of strengthening and toughening; finally, under the synergistic effect of the nano-scale Cu-rich phase and the (Cr, Mo)2C precipitate phase, the dislocation recovery is hindered during the long tempering process, which ensures the strength of the steel plate and significantly improves the low-temperature toughness.

[0032] (4) The steel plate of the present invention has excellent welding performance, a thickness range of 10-60 mm, a yield strength greater than 1000 MPa, a tensile strength greater than 1100 MPa, and an impact energy greater than 150 J at -40°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a microstructure picture of the finished steel plate prepared in Example 4 of the present invention;

[0034] Figure 2 This is a microstructure picture of the finished steel plate prepared in Comparative Example 3 of the present invention;

[0035] Figure 3 This is an atom probe reconstruction image of the finished steel plate prepared in Example 4 of the present invention;

[0036] Figure 4 This is an atom probe reconstruction image of the finished steel plate prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described below in conjunction with specific embodiments, and the following performance test related standards are used for testing:

[0038] (1) Tensile test: The implementation standard is "GB / T 228.1-2021 Tensile test of metallic materials Part 1: Room temperature test method";

[0039] (2) Impact test: The implementation standard is "GB / T 229-2020 Metal Materials Charpy Pendulum Impact Test Method";

[0040] (3) Microstructure observation: Metallographic characterization was performed using a field emission scanning microscope.

[0041] Embodiment 1~3

[0042] A 1 GPa-grade high-strength and tough low-carbon steel for marine engineering was prepared. The preparation processes of Examples 1 to 3 were the same, but the chemical composition contents were different. The chemical composition was shown in Table 1, the rolling process was shown in Table 2, the heat treatment process was shown in Table 3, and the mechanical properties were shown in Table 4. The specific preparation process was as follows:

[0043] The smelting was carried out according to the chemical composition content described in Table 1. The treatment time of LF and RH refining furnace was 20min, and the superheat temperature of the molten steel in the tundish was 15℃; the 400mm thick continuous casting billet was heated to 1200℃, kept warm for 4h and then rolled, the rough rolling temperature was 1150℃, the total deformation of rough rolling was 50%, and the temperature was heated to 930℃ for finishing rolling after the rough rolling, and the total deformation of finishing rolling was 70%, and it was air-cooled to room temperature after the rolling; the rolled steel plate was quenched, heated to 890℃ and kept warm for 120min, and then cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s; then the quenched steel plate was tempered, and the quenched steel plate was heated to 550℃ with the furnace and kept warm for 300min, and then air-cooled to room temperature to obtain a 60mm thick finished plate.

[0044] Example 4

[0045] A 1GPa-grade high-strength and tough low-carbon steel for marine engineering was prepared. The chemical composition is shown in Table 1, the rolling process is shown in Table 2, the heat treatment process is shown in Table 3, and the mechanical properties are shown in Table 4. The specific preparation process is as follows:

[0046] The smelting was carried out according to the chemical composition content described in Table 1. The treatment time of LF and RH refining furnace was 20min, and the superheat temperature of the molten steel in the tundish was 15℃; the 400mm thick continuous casting billet was heated to 1200℃, kept warm for 4h and then rolled, the rough rolling temperature was 1150℃, the total deformation of rough rolling was 75%, and the temperature was heated to 930℃ for finishing rolling after the rough rolling, and the total deformation of finishing rolling was 90%, and it was air-cooled to room temperature after the rolling; the hot-rolled steel plate was quenched, heated to 890℃ and kept warm for 20min, and then cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s; then the quenched steel plate was tempered, and the quenched steel plate was heated to 550℃ with the furnace and kept warm for 50min, and then air-cooled to room temperature to obtain a 10mm thick finished plate.

[0047] Comparative Examples 1 to 3

[0048] A 1 GPa-level high-strength and tough low-carbon steel for marine engineering was prepared. The preparation process of Comparative Examples 1 to 3 was the same as that of Example 4, but the chemical composition content was different. The chemical composition is shown in Table 1, the rolling process is shown in Table 2, the heat treatment process is shown in Table 3, and the mechanical properties are shown in Table 4.

[0049] The chemical composition of the steel plate in Comparative Example 1 is different from that in Example 4, in that the mass percentage of Cr element and the mass percentage of Mo element are not 1:1, and other conditions are the same. Under this condition, the formed (Cr, Mo)2C precipitates are coarse, which has an adverse effect on the strength and toughness of the steel plate.

[0050] Compared with Example 4, the chemical composition of the steel plate of Comparative Example 2 is that the ratio of the mass percentage of the Nb element to the mass percentage of the Ti element is not 1:1, and other conditions are the same. Under this condition, due to the increase in the Ti content, more large-sized TiC is formed, which is not conducive to the toughness of the steel plate. At the same time, the effect of Nb-Ti microalloying is reduced, and the nano-scale precipitation phase is coarsened, which reduces the strength and toughness of the steel plate.

[0051] The chemical composition of the steel plate of Comparative Example 3 is the same as that of Example 4, except that no Ti element is added and other conditions are the same. Under these conditions, there is no Nb-Ti microalloying effect, resulting in coarse nano-scale Cu-rich phase and (Cr, Mo)2C precipitation phase, which significantly reduces the strength and toughness of the steel plate.

[0052] Comparative Example 4

[0053] The chemical composition of the steel plate of Comparative Example 4 is the same as that of the steel plate of Example 4, the tempering temperature is 500°C, and the other conditions are the same. Under this condition, the tempering temperature is low, the residual stress inside the steel plate is not completely released, the martensite dislocation density is larger, the stress concentration is serious, and although the strength is also greater than 1GPa, the toughness is significantly reduced. At this time, although there is a role of Nb-Ti microalloying, the tempering temperature is low, and the precipitation of Cu-rich phase and (Cr, Mo)2C is not promoted, and the toughening effect of the coherent nano-precipitated phase is not fully exerted.

[0054] Depend on Figure 1~Figure 4 It can be seen from the organizational picture that the steel plate of Example 4 ( Figure 1 ) contains more carbides, while the comparative example 3 steel plate ( Figure 2 ) has less carbides, and the atomic probe reconstruction image further shows that the steel plate of Example 4 ( Figure 3 ) has a large number of nano-scale Cu-rich phases and (Cr, Mo)2C. These precipitates are coherent with the martensite matrix and belong to the BCC structure. They have good precipitation strengthening and thus have excellent strength and toughness. However, there is no effect of Nb-Ti microalloying. The steel plate of comparative example 3 ( Figure 4 ) has a small amount of (Cr,Mo)2C, and the size of the Cu-rich phase is large, so the precipitation strengthening effect is poor, so the strength and toughness are low.

[0055] Table 1 Chemical composition of the steel plate examples of the present invention and the comparative examples

[0056]

[0057] Table 2 Rolling process of steel plate examples of the present invention and comparative examples

[0058]

[0059] Table 3 Heat treatment process of steel plate examples of the present invention and comparative examples

[0060]

[0061] Table 4 Mechanical properties of steel plate examples of the present invention and comparative examples

[0062]

[0063] According to the content of the present invention, the process parameters and component contents are adjusted to achieve the preparation of the 1GPa-level high-strength and low-carbon steel for marine engineering of the present invention, and the performance is basically consistent with 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 replacement that can be made by other technicians in this field without spending creative labor falls within the protection scope of the present invention without departing from the core of the present invention.

Claims

1. A 1GPa-grade high-strength and low-carbon steel for marine engineering, characterized in that: Having the following chemical composition in mass percentage: C: 0.03%~0.05%, Si: 0.10%~0.30%, Mn: 0.8%~1.2%, Ni: 3.0%~5.0%, Cu: 1.0%~1.5%, Cr: 0.5%~0.7%, Mo: 0.5%~0.7%, Nb: 0.01%~0.03%, Ti: 0.01%~0.03%, S≤0.002%, P≤0.003%, the balance is Fe and unavoidable impurities; The microstructure of the 1 GPa-level high-strength and tough low-carbon steel for marine engineering is a tempered martensite matrix, a nano-scale Cu-rich phase and a (Cr, Mo)2C precipitation phase.

2. The 1GPa-level high-strength and tough low-carbon steel for marine engineering 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 Nb element to the mass percentage of the Ti element is 1:

1.

3. The 1 GPa-grade high-strength and tough low-carbon steel for marine engineering according to claim 1 or 2, characterized in that: The steel plate thickness range of 1GPa-grade high-strength and tough low-carbon steel for marine engineering is 10mm-60mm, the yield strength is greater than 1000MPa, the tensile strength is greater than 1100MPa, and the impact energy at -40℃ is greater than 150J.

4. A method for preparing 1GPa-grade high-strength and tough low-carbon steel for marine engineering according to claim 1 or 2, characterized in that: The steps include: S1. Smelting and continuous casting are performed according to the chemical composition of the 1GPa-grade high-strength and tough low-carbon steel for marine engineering to obtain a continuous casting billet; the smelting is treated by LF and RH refining furnaces, and the superheat of the molten steel in the middle package is less than 20° C. The continuous casting is performed with full protection casting; S2, heating the continuous casting billet in step S1 to 1150° C. to 1200° C. and then keeping the temperature for 2 to 4 hours, and then performing controlled rolling. After the rolling is completed, the billet is air-cooled to room temperature to obtain a rolled steel plate; S3, quenching the rolled steel plate in step S2, keeping it warm and then cooling it to room temperature to obtain a quenched steel plate; S4, tempering the quenched steel plate in step S3, keeping the temperature and then air-cooling it to room temperature to obtain a final steel plate.

5. The preparation method according to claim 4, characterized in that: In step S1, the treatment time of LF and RH refining furnaces is 10min to 30min respectively, and the superheat temperature of molten steel in the tundish is greater than or equal to 10°C and less than 20°C; during the smelting process, 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 continuous casting billet is subjected to two-stage rolling, wherein the rough rolling temperature of the first stage is 1100°C~1150°C, and the deformation is greater than or equal to 50%; after the first stage rough rolling is completed, the second stage finishing rolling is carried out when the temperature reaches 920°C~960°C, and the deformation is greater than or equal to 70%, and air-cooling is performed to room temperature after rolling.

7. The preparation method according to claim 4, characterized in that: In step S3, the quenching temperature range is 880°C to 920°C, the holding time coefficient is 1.5 min / mm to 2.5 min / mm, and the steel is cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15°C / s.

8. The preparation method according to claim 4, characterized in that: In step S4, the tempering temperature range is 540°C to 560°C, and the holding time coefficient is 4.0 min / mm to 6.0 min / mm.

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