A high-strength and tough low-carbon steel for ocean engineering with a strength level of 1 GPa and its preparation method

Abstract

The present invention belongs to the field of steel materials and their preparation, and discloses a 1GPa grade high-strength and tough low-carbon steel for ocean engineering and its preparation method. The steel plate has the following chemical components 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. A heat treatment process of air cooling after two-stage controlled rolling + tempering after quenching is adopted, and through Nb-Ti microalloying, nano-sized Cu-rich phases and (Cr,Mo)2C precipitation phases are obtained, and finally a structure of tempered martensite matrix and nano-sized precipitation phases is obtained.

Description

Technical Field

[0001] The present invention relates to the field of steel materials and their preparation, and particularly relates to a high-strength and tough low-carbon steel for offshore engineering with a yield strength of 1 GPa 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 efficiently utilize these resources, humans have continuously promoted the progress of ship and offshore engineering equipment technologies. These equipments not only need to carry heavy loads, but also must operate stably in harsh marine environments. Therefore, the technical requirements for the equipments have become more stringent. First of all, ships and offshore engineering equipments need to bear huge loads. Especially during deep-sea operations, the steel must have high yield strength and fatigue resistance to ensure long-term stable operation. Secondly, the complexity of the marine environment requires the steel to have properties of low-temperature resistance, corrosion resistance and pressure resistance. In low-temperature environments, the steel will undergo a brittle transition, and the corrosiveness of seawater affects the durability of the steel. Therefore, the steel should not only have strength, but also ensure good toughness and corrosion resistance. In addition, the weldability and formability of the steel are equally important to ensure structural reliability and convenient processing. High-strength steels usually have a relatively high carbon content, which may affect their weldability, resulting in cracks or poor connections. Therefore, optimizing the weldability and formability of the steel is the key. To sum up, the steel for ships and offshore engineering equipments must have high strength, good toughness, corrosion resistance, excellent weldability and formability. These requirements are the basis for the long-term reliable operation of the equipments and the key to the development of marine resources and the sustainable development of offshore engineering.

[0003] At present, the yield strength of the steel used for traditional ships and offshore engineering equipments has been increased from the 550 MPa level to the 900 MPa level. The heat treatment process usually first adopts quenching to form a high-density lath martensite structure, and then uses the tempering process to form precipitation phases on the martensite matrix to achieve precipitation strengthening. At present, the steel used for ships and offshore engineering equipments has the following deficiencies:

[0004] (1) Insufficient yield strength: With the deepening of marine resource development, ships and offshore engineering equipments require higher-strength steels. At present, most of the yield strengths can only reach 900 MPa, which is difficult to meet the future demand for high-strength steels above 1 GPa.

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

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

[0007] In order to solve the above problems existing in the prior art, a high-strength and tough low-carbon steel for offshore engineering with a strength level of 1 GPa and its preparation method are proposed. By microalloying to control the microstructure and precipitation phases, a martensite structure with excellent low-temperature toughness is formed. The final steel plate obtained has excellent welding properties, and at the same time has the characteristics of a yield strength greater than 1000 MPa, a tensile strength greater than 1100 MPa, and an impact energy at -40 °C greater than 150 J.

[0008] In order to achieve the above object, systematic experiments have been carried out on the steel plate of the present invention in terms of composition design, rolling process parameters, heat treatment process control, etc. The technical solutions include:

[0009] A high-strength and tough low-carbon steel for offshore engineering with a strength level of 1 GPa has 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 high-strength and tough low-carbon steel for offshore engineering with a strength level of 1 GPa is tempered martensite matrix, nano-scale Cu-rich phase and (Cr, Mo)2C precipitation phase.

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

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

[0013] A preparation method of a high-strength and tough low-carbon steel for offshore engineering with a strength level of 1 GPa includes the following steps:

[0014] S1. After smelting and continuous casting according to the chemical composition of the high-strength and tough low-carbon steel for offshore engineering with a strength level of 1 GPa, a continuous casting billet is obtained; the smelting is treated by LF and RH refining furnaces, and the superheat of the tundish molten steel is less than 20 °C; the whole process of continuous casting is carried out under protective casting;

[0015] S2. Heat the continuous casting billet obtained in step S1 to 1150°C - 1200°C and hold for 2 - 4 hours, then carry out controlled rolling. After rolling, air cool to room temperature to obtain a rolled steel plate.

[0016] S3. Quench the rolled steel plate obtained in step S2, cool it to room temperature in water after holding, to obtain a quenched steel plate.

[0017] S4. Temper the quenched steel plate obtained in step S3, cool it to room temperature in air after holding, to obtain the final steel plate.

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

[0019] In step S2, two - stage rolling is adopted for the continuous casting billet. The rough rolling temperature in the first stage is 1100°C - 1150°C, and the deformation amount is greater than or equal to 50%; after the rough rolling in the first stage, wait until the temperature reaches 920°C - 960°C for the second - stage finish rolling, the deformation amount is greater than or equal to 70%, and air cool to room temperature after rolling.

[0020] In step S3, the quenching temperature range is 880°C - 920°C, the holding time coefficient is 1.5 min / mm - 2.5 min / mm, and it is cooled to room temperature by 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 - 560°C, and the holding time coefficient is 4.0 min / mm - 6.0 min / mm.

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

[0023] Element C is the most important strengthening element in the steel plate of the present invention. It 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 preferably controlled that the C content is 0.03% - 0.05%.

[0024] Element Ni provides a solid - solution strengthening effect in the steel plate of the present invention. It affects the cross - 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, element Ni is a strong hardenability element and also helps to improve the strength. However, the price of element Ni is relatively high. Therefore, it is preferably that the Ni content is 3.0% - 5.0%.

[0025] Elements Mo and Cr mainly provide precipitation strengthening effect in the steel plate of the present invention. Due to the low C content, 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, the content of Mo is preferably 0.5% - 0.7%; when the content of Ni is 3.0% - 5.0%, elements Cr and Mo are added in combination, and the ratio of the mass percentage of element Mo to the mass percentage of element Cr is 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 temperature. At the same time, the addition of element Cr can also significantly improve the precipitation strengthening effect. Tempering between 540°C and 560°C can precipitate nano-scale (Cr, Mo)2C to achieve the best precipitation strengthening effect.

[0026] Element Cu, as an important element of the steel plate of the present invention, provides solid solution strengthening and precipitation strengthening effects. When element Cu is solid dissolved in the matrix, it will cause lattice distortion, thereby improving the strength of the steel plate. However, too high content of element Cu will lead to a decrease in toughness. Therefore, the content of Cu is preferably 1.0% - 1.5%; in addition, during tempering, nano-scale Cu-rich phases will also precipitate to produce precipitation strengthening. At the same time, the Cu-rich phases will also inhibit dislocation recovery, thereby promoting the precipitation of nano-scale (Cr, Mo)2C, accelerating the dissolution of cementite, and being beneficial to the simultaneous improvement of strength and toughness.

[0027] Elements Nb and Ti are important microalloying elements of the steel plate of the present invention. The ratio of the mass percentage of element Nb to the mass percentage of element Ti is controlled to be 1:1. Through the synergistic effect of microalloying, the martensite matrix is refined to increase the dislocation density, providing more nucleation sites for the Cu-rich phases. The previously precipitated Cu-rich phases serve as heterogeneous nucleation sites for (Cr, Mo)2C, promoting the precipitation of nano-scale (Cr, Mo)2C. Under the synergistic effect of nano-scale Cu-rich phases and (Cr, Mo)2C precipitation phases, dislocation recovery is hindered, which not only ensures the strength of the steel plate but also significantly improves 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 the synergistic effect from multiple angles. The main sources of strength are the solid solution strengthening of element C, the solid solution strengthening of element Cu, and the precipitation strengthening of coherent nano-scale Cu-rich phases and (Cr, Mo)2C; the main sources of toughness are the solid solution toughening of element Ni, the precipitation toughening of coherent nano-scale Cu-rich phases and (Cr, Mo)2C, and the improvement of toughness brought about by the reduction of dislocation density due to long-term tempering.

[0030] (2) The steel plate of the present invention ensures excellent welding performance and impact performance by reducing the C content. At the same time, Cu element is added to make up for the strength loss caused by the reduction of C element. When the Ni content is 3.0 - 5.0%, the ratio of Cr and Mo contents is adjusted to 1:1, and the resulting Cu-rich phase will promote the formation of coherent nano-scale (Cr, Mo)₂C precipitation phases, thereby synchronously 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 1:1, and uses the synergistic effect of Nb-Ti for microalloying to refine the martensite matrix and increase the dislocation density, providing 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 precipitation phase is coherent with the martensite matrix and can provide double strengthening and toughening effects; finally, under the synergistic effect of the nano-scale Cu-rich phase and (Cr, Mo)₂C precipitation phase, the dislocation recovery is hindered during the long-term tempering process, which not only ensures the strength of the steel plate but also significantly improves the low-temperature toughness.

[0032] (4) The steel plate of the present invention has excellent welding performance, with 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 at -40°C greater than 150 J. Description of the Drawings

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

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

[0035] Figure 3 It is an atomic probe reconstruction diagram of the finished steel plate prepared in Example 4 of the present invention;

[0036] Figure 4 It is an atomic probe reconstruction diagram of the finished steel plate prepared in Comparative Example 3 of the present invention. Specific Embodiments

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

[0038] (1) Tensile test: The execution standard is "GB / T 228.1 - 2021 Metallic materials - Tensile testing - Part 1: Method of test at room temperature";

[0039] (2) Impact test: The execution standard is "GB / T 229 - 2020 Metallic materials - Charpy pendulum impact test method";

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

[0041] Examples 1 - 3

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

[0043] Smelting was carried out according to the chemical composition contents described in Table 1. The treatment time in the LF and RH refining furnaces was 20 min, and the superheat temperature of the tundish molten steel was 15 °C. The 400-mm-thick continuous casting billet was heated to 1200 °C, held for 4 h and then rolled. The rough rolling temperature was 1150 °C, the total rough rolling reduction was 50%, and after the rough rolling was completed, it was held at a temperature of 930 °C for finish rolling. The total finish rolling reduction was 70%. After rolling, it was air-cooled to room temperature. The rolled steel plate was quenched, heated to 890 °C and held for 120 min, and then cooled to room temperature using a fast laminar cooling system with an average cooling rate greater than 15 °C / s. Subsequently, the quenched steel plate was tempered. The quenched steel plate was heated in the furnace to 550 °C, held for 300 min and then air-cooled to room temperature to obtain a 60-mm-thick finished plate.

[0044] Example 4

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

[0046] Smelting was carried out according to the chemical composition contents described in Table 1. The treatment time in the LF and RH refining furnaces was 20 min, and the superheat temperature of the tundish molten steel was 15 °C. The 400-mm-thick continuous casting billet was heated to 1200 °C, held for 4 h and then rolled. The rough rolling temperature was 1150 °C, the total rough rolling reduction was 75%, and after the rough rolling was completed, it was held at a temperature of 930 °C for finish rolling. The total finish rolling reduction was 90%. After rolling, it was air-cooled to room temperature. The hot-rolled steel plate was quenched, heated to 890 °C and held for 20 min, and then cooled to room temperature using a fast laminar cooling system with an average cooling rate greater than 15 °C / s. Subsequently, the quenched steel plate was tempered. The quenched steel plate was heated in the furnace to 550 °C, held for 50 min and then air-cooled to room temperature to obtain a 10-mm-thick finished plate.

[0047] Comparative Examples 1 - 3

[0048] Prepare a high-strength and tough low-carbon steel for ocean engineering with a strength level of 1 GPa. The preparation processes of Comparative Examples 1 to 3 are the same as that of Example 4, but the chemical composition contents are different. The chemical compositions are shown in Table 1, the rolling processes are shown in Table 2, the heat treatment processes are shown in Table 3, and the mechanical properties are shown in Table 4.

[0049] Compared with Example 4, for the steel plate of Comparative Example 1, the ratio of the mass percentage of Cr element to the mass percentage of Mo element is not 1:1, and other conditions are the same. Under this condition, the formed (Cr,Mo)2C precipitates are coarse, which have an adverse effect on both the strength and toughness of the steel plate.

[0050] Compared with Example 4, for the steel plate of Comparative Example 2, the ratio of the mass percentage of Nb element to the mass percentage of Ti element is not 1:1, and other conditions are the same. Under this condition, due to the increase in Ti content, more large-sized TiC are 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 nanoscale precipitates are coarsened, resulting in a decrease in the strength and toughness of the steel plate.

[0051] Compared with Example 4, for the steel plate of Comparative Example 3, Ti element is not added, and other conditions are the same. Under this condition, without the effect of Nb-Ti microalloying, the nanoscale Cu-rich phase and (Cr,Mo)2C precipitates become coarse, resulting in a significant decrease in 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 other conditions are the same. Under this condition, the tempering temperature is relatively low, the internal residual stress of the steel plate is not completely released, the dislocation density of martensite is larger, and the stress concentration is serious. Although the strength is also greater than 1 GPa, the toughness is significantly reduced. At this time, although there is the effect of Nb-Ti microalloying, the tempering temperature is low, which does not promote the precipitation of Cu-rich phase and (Cr,Mo)2C, and the toughening effect of the coherent nano-precipitates is not fully exerted.

[0054] From Figures 1 to 4 the microstructure pictures, it can be seen that the steel plate of Example 4 ( Figure 1 ) contains more carbides, while the steel plate of Comparative Example 3 ( Figure 2 ) has fewer carbides. Further combining with the atom probe reconstruction diagram shows that the steel plate of Example 4 ( Figure 3 ) has a large number of nanoscale Cu-rich phases and (Cr,Mo)2C. These precipitates are coherent with the martensite matrix and belong to the BCC structure. The precipitation strengthening is good, so it has excellent strength and toughness. Without the effect of Nb-Ti microalloying, the number of (Cr,Mo)2C in the steel plate of Comparative Example 3 ( Figure 4 ) is less, and at the same time, the size of the Cu-rich phase is larger, and the precipitation strengthening effect is poor. Therefore, the strength and toughness are lower.

[0055] Table 1 Chemical Compositions of the Steel Plate Examples and Comparative Examples of the Present Invention

[0056]

[0057] Table 2 Rolling Processes of the Steel Plate Examples and Comparative Examples of the Present Invention

[0058]

[0059] Table 3 Heat Treatment Processes of the Steel Plate Examples and Comparative Examples of the Present Invention

[0060]

[0061] Table 4 Mechanical Properties of the Steel Plate Examples and Comparative Examples of the Present Invention

[0062]

[0063] By adjusting the process parameters and component contents according to the content of the present invention, the preparation of the 1GPa-grade high-strength and tough low-carbon steel for offshore engineering 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 replacement 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 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% to 0.05%, Si: 0.10% to 0.30%, Mn: 0.8% to 1.2%, Ni: 3.0% to 5.0%, Cu: 1.0% to 1.5%, Cr: 0.5% to 0.7%, Mo: 0.5% to 0.7%, Nb: 0.01% to 0.03%, Ti: 0.01% to 0.03%, S≤0.002%, P≤0.003%, the balance being Fe and unavoidable impurities; The ratio of the mass percentage of Cr element to the mass percentage of Mo element is 1:1, the ratio of the mass percentage of Nb element to the mass percentage of Ti element is 1:1, and the Nb-Ti synergistic microalloying refines the martensitic matrix; The microstructure of the 1 GPa-level high-strength and tough low-carbon steel for marine engineering is a tempered martensite matrix, a coherent nano-scale Cu-rich phase, and a coherent nano-scale (Cr, Mo) 2 C precipitation phase.

2. The 1GPa-level high-strength and tough low-carbon steel for marine engineering according to claim 1, 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.

3. 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.

4. The preparation method according to claim 3, 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.

5. The preparation method according to claim 3, characterized in that: In step S2, the continuous casting billet is subjected to two-stage rolling, wherein the first-stage rough rolling temperature is 1100°C to 1150°C, and the deformation is greater than or equal to 50%. After the first-stage rough rolling is completed, the second-stage finish rolling is carried out when the temperature reaches 920°C to 960°C, and the deformation is greater than or equal to 70%. After rolling, the billet is air-cooled to room temperature.

6. The preparation method according to claim 3, 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.

7. The preparation method according to claim 3, 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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