Ultrahigh-strength maritime work steel with excellent Bauschinger effect resistance and manufacturing method of ultrahigh-strength maritime work steel

Abstract

The invention relates to the field of steel and iron material preparation, in particular to ultrahigh-strength maritime work steel with the excellent Bauschinger effect resistance and a manufacturing method of the ultrahigh-strength maritime work steel. The high-strength steel is composed of the following chemical components in percentage by weight: 0.2%-0.3% of C, 0.2%-0.4% of Si, 0.4%-0.75% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 1.0%-3.0% of Ni, 0.1%-0.3% of Cr, 0.1%-0.3% of Mo, 0.2%-0.45% of Co, 0.02%-0.05% of V, 0.01%-0.015% of Ti, 0.004%-0.008% of N and the balance of Fe and inevitable impurities. By combining the special production process of high cleanliness and alloying smelting, continuous casting and heavy pressing, forging, efficient rolling and high-temperature normalizing, the maximum thickness of the finished steel plate can reach 80 mm, meanwhile, the strength, toughness and Bauschinger effect resistance of the thick steel plate are considered, and the uniformity of the core structure of the thick plate is effectively controlled.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength marine engineering steel technology, specifically to an ultra-high strength marine engineering steel with excellent resistance to the Bauschinger effect and its manufacturing method. Background Technology

[0002] With the development of polar resources and the expansion of marine engineering, the polar marine environment places more stringent demands on the performance of marine engineering steel. Traditional high-strength marine engineering steel is prone to the Bauschinger effect under polar low temperatures, high stress, and dynamic load conditions. This effect occurs when the yield strength of the material decreases significantly during reverse loading, leading to a decline in plastic deformation capacity and consequently affecting the structural load-bearing safety and fatigue life. Particularly in the low-temperature environment of polar regions, the mechanical property degradation caused by the Bauschinger effect is more pronounced, severely restricting the application of ultra-high-strength marine engineering steel in critical equipment such as polar ships and offshore platforms.

[0003] Currently, while ultra-high-strength marine steel with a yield strength ≥620MPa can achieve high strength and low-temperature toughness through alloy design and controlled rolling and cooling processes, its resistance to the Bauschinger effect is generally insufficient. Specifically, under cyclic loading or pre-strain conditions, the yield strength decreases significantly (typically >10%), and the uniform elongation is low (<8%), making the structure prone to failure due to concentrated local plastic deformation during service. Furthermore, existing technologies struggle to simultaneously achieve the strength, toughness, and resistance to the Bauschinger effect in thick steel plates, making the control of the uniformity of the microstructure in the core of thick plates a key technical challenge.

[0004] Chinese patent application CN202211664907.9, entitled "An Ultra-Thick High-Strength Steel for Nuclear Reactor Containment and Its Manufacturing Method," proposes an ultra-thick tempered steel plate for nuclear power plants. It employs a low-C, low-Si, high-Mn composition system. The low-carbon equivalent composition design improves weldability, but without the combined effect of elements such as Co and Ni, it cannot produce ultra-high-strength steel plates resistant to the Bauschinger effect. Chinese patent application CN202310778326.6, entitled "Thick 800MPa Hydropower Steel with Excellent Core Toughness and Its Manufacturing Method," proposes an ultra-high-strength tempered steel with low C and low alloy composition. While this composition can produce thick steel plates, its low-alloy composition and tempering process cannot guarantee the -60℃ low-temperature toughness of the steel plate core, making it impossible to produce steel plates resistant to the Bauschinger effect. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides an ultra-high strength marine engineering steel with excellent resistance to the Bauschinger effect and its manufacturing method. The finished steel plate thickness can reach 80mm, while taking into account the strength, toughness and resistance to the Bauschinger effect of the thick steel plate, and effectively controlling the uniformity of the microstructure in the core of the thick plate.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] An ultra-high strength marine steel with excellent resistance to the Bauschinger effect is composed of the following chemical composition by weight percentage:

[0008] C: 0.2%~0.3%, Si: 0.2%~0.4%, Mn: 0.4%~0.75%, P≤0.02%, S≤0.01%, Ni: 1.0%~3.0%, Cr: 0.1%~0.3%, Mo: 0.1%~0.3%, Co: 0.2%~0.45%, V: 0.02%~0.05%, Ti: 0.01%~0.015%, N: 0.004%~0.008%, with the remainder being Fe and unavoidable impurities.

[0009] The effect of selecting the above alloying elements and their contents:

[0010] 1. Carbon (C), as a fundamental strengthening element in steel, can dissolve in the ferrite-austenite matrix to form a solid solution structure, thereby increasing the strength of the steel plate. Too low a C content leads to a decrease in C solid solution content and carbide content, resulting in less grain refinement, insufficient steel plate strength, and an inability to increase steel plate stiffness through grain boundary strengthening, leading to a severe Bauschinger effect. However, too high a C content will produce a large amount of hardened structure, increasing the tendency to crack during deformation and affecting low-temperature impact toughness. Therefore, the C content should be precisely controlled. Thus, this invention precisely controls the C content to 0.2%~0.3%.

[0011] 2. Si can improve the strength and stiffness of steel plates through solid solution strengthening. Simultaneously, Si acts as a deoxidizer, reducing the oxygen content. When the Si content is below 0.2%, the deoxidation effect is not significant, strength and stiffness decrease, and the steel plate is prone to plastic deformation, exhibiting a pronounced Bauschinger effect. To avoid segregation and inclusions in thick billets, the Si content should not exceed 0.4%. Therefore, this invention precisely controls the Si content between 0.2% and 0.4%.

[0012] 3. Like Co, Mn is dissolved in the Fe matrix in large quantities, increasing the strength of the steel plate. However, due to the excessively high C content in this invention, to avoid segregation in the billet leading to a decrease in low-temperature toughness in the core and a severe Bauschinger effect, the Mn content should be appropriately reduced. An Mn content below 0.4% is detrimental to steel plate deoxidation, while an Mn mass percentage above 0.75% reduces the low-temperature toughness of the core in thick plates. Therefore, this invention precisely controls the Mn content between 0.4% and 0.75%.

[0013] 4. P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to ≤0.02% and S to ≤0.01%.

[0014] 5. The role of Ni is to improve the low-temperature toughness and resistance to Baosinger in steel plates through dispersion strengthening and grain refinement. Ni, along with Mn and Co, is largely dissolved in the matrix, which can increase strength, lower the ductile-brittle transition temperature, and prevent plastic deformation of the steel plate. Simultaneously, Ni can improve the corrosion fatigue resistance of the steel plate. Therefore, this invention precisely controls the Ni content to 1.0%~3.0%.

[0015] 6. Cr is an element that improves the hardness and stiffness of extra-thick steel plates. When the carbon content is low, adding an appropriate amount of Cr can ensure the steel plate achieves the required stiffness, prevent plastic deformation, and improve its resistance to the Boushinger effect. However, adding excessive Cr will reduce the material's low-temperature toughness and fatigue resistance. Cr also improves the corrosion resistance of the steel plate to some extent. Therefore, this invention precisely controls the Cr content to 0.1%~0.3%.

[0016] 7. The addition of Mo can improve stiffness and hardness. Adding an appropriate amount of Mo can also improve the temper brittleness of the steel plate. Mo can also work with Ni to provide some creep resistance under stress and improve corrosion fatigue resistance. These beneficial effects can enhance the strength and toughness of the steel plate and its resistance to the Bouschinger effect. Therefore, this invention precisely controls the Mo content to 0.1%~0.3%.

[0017] 8. The role of Co is to improve the hardness and high-temperature thermal stability of steel. During the normalizing process, Co can inhibit grain growth, and at the same time, Co can effectively improve the strength and anti-Bosenger properties of the steel plate. Therefore, this invention precisely controls the Co content to 0.2%~0.45%.

[0018] 9. V can form V(C,N) particles in the matrix, which can refine and strengthen the grains. Adding V to heat-treated steel plates can significantly improve the strength and toughness of the steel plates. Therefore, this invention precisely controls the V content to 0.02%~0.05%.

[0019] 10. Ti exhibits a strong precipitation strengthening effect, inhibiting austenite recrystallization and growth, and refining grain size to improve the yield strength of steel. Ti can form fine and dispersed C and N compound second phases with elements such as V, N, and C, effectively controlling the growth of the original austenite grains, thereby significantly improving the low-temperature toughness of the steel plate. During service, Ti causes plastic deformation in the steel plate. Therefore, this invention precisely controls the Ti content to be between 0.01% and 0.015%.

[0020] 11. Nitrogen (N) can play a role in solid solution strengthening. Combined with elements such as V and Ti, it can improve the strength and resistance to plastic deformation of steel plates, as well as enhance resistance to the Bauschinger effect. Therefore, this invention precisely controls the N content to be between 0.004% and 0.008%.

[0021] The above-mentioned ultra-high strength marine steel with excellent resistance to the Bauschinger effect has a maximum thickness of 80mm for finished steel plates. Its yield strength is ≥620MPa, tensile strength is 740~890MPa, elongation is ≥18%, Charpy impact energy of the steel plate core at -60℃ is ≥150J, yield strength decreases by ≤10% when the steel plate has 2% residual strain, excellent resistance to the Bauschinger effect, and uniform elongation is ≥8%.

[0022] The microstructure at 1 / 2 thickness of the steel plate is bainite + 20%~30% pearlite, with the original austenite grain size of 15~25μm, the effective grain size of 3~6μm, and the proportion of large-angle grain boundaries ≥30%.

[0023] The manufacturing method of the aforementioned ultra-high strength marine steel with excellent resistance to the Bauschinger effect specifically includes the following steps:

[0024] 1) Steel refining:

[0025] Molten steel is refined in a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions.

[0026] 2) Continuous casting:

[0027] The tundish superheat is 8~15℃, the single roll reduction on the casting machine is ≥10mm, and the billet is stacked and slowly cooled for 24~48h after leaving the line. The continuous casting billet forging heating temperature is 1250~1300℃, heating time is 5~8h, forging compression ratio is 30%~40%, one-fire forming, final forging temperature is ≥1000℃, and stacked and slowly cooled for 36~48h.

[0028] 3) Rolling:

[0029] The electroslag ingot is loaded into the heating furnace at a temperature of 650-750℃, and held at a low temperature for 30-90 minutes. The purpose is to maintain a uniform temperature in the thickness direction of the billet during the low-temperature stage, preparing for a uniform microstructure in the high-temperature section. The heating rate is 4-7℃ / min, the heating temperature is 1250-1280℃, and the holding time is 60-120 minutes. The purpose of the short low-temperature holding time is to ensure the full dissolution of C / N compounds while avoiding abnormal growth of the as-cast microstructure.

[0030] The initial rolling temperature is 1180~1250℃, with an average reduction of 20~35mm per pass, and the final rolling temperature is 1050~1150℃. The purpose of high-temperature hot rolling is to increase the reduction per pass and improve the as-cast microstructure of the slab. Increasing the reduction per pass in the recrystallization temperature range allows for maximum fragmentation of the as-cast microstructure under heavy pressure and, based on forging, refinement of the grains, ensuring high strength and rigidity of the steel plate, and inhibiting plastic deformation, thereby improving the steel plate's resistance to the Bauschinger effect. It also prepares the microstructure for normalizing treatment. Utilizing the fine austenite microstructure and residual deformation stress after rolling, the effective grain size of the steel plate is reduced.

[0031] 4) Positive heat:

[0032] The normalizing temperature is 880~940℃, and the normalizing holding time is 2.5~3.5min / mm.

[0033] The purpose of the normalizing process is to obtain fine and dispersed precipitated strengthening phases and fine effective grains in the high-C, high-alloy composition system of this invention, increase the number of large-angle grain boundaries, and further improve the low-temperature toughness and resistance to the Bauschinger effect of the steel plate.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. This invention employs a composition design that combines high C, low Mn with alloying elements such as Ni, Cr, Mo, Co, V, and Ti. By precisely controlling the C content, this invention improves the strength and resistance to the Bauschinger effect of the steel plate, ensuring a Charpy impact energy of ≥150J at -60℃ in the core. Due to the excessively high C content, to avoid segregation in the billet leading to a decrease in low-temperature toughness in the core and a severe Bauschinger effect, this invention appropriately reduces the Mn content. Ni improves the low-temperature toughness and resistance to Bauschinger through dispersion strengthening and grain refinement. Ni, along with Mn and Co, dissolves extensively in the matrix, increasing strength, lowering the ductile-brittle transition temperature, and preventing plastic deformation of the steel plate. Simultaneously, Ni enhances the corrosion fatigue resistance of the steel plate. Cr increases the hardness and stiffness of extra-thick steel plates. With a low C content, adding an appropriate amount of Cr ensures the steel plate achieves the required stiffness, prevents plastic deformation, and improves resistance to the Bauschinger effect. Cr also improves the corrosion resistance of the steel plate to a certain extent. The addition of molybdenum (Mo) can improve stiffness and hardness. Adding an appropriate amount of Mo can also improve the temper brittleness of steel plates. Mo can also work with Ni to provide some creep resistance under stress and improve corrosion fatigue resistance. These beneficial effects can enhance the strength, toughness, and resistance to the Bauschinger effect of steel plates. Ti, along with elements such as V, N, and C, can form fine and dispersed C and N compound second phases, effectively controlling the growth of the original austenite grains, thereby significantly improving the low-temperature toughness of steel plates.

[0036] 2. In this invention, the electroslag ingot is loaded into the heating furnace at a furnace temperature of 650~750℃, and held at a low temperature for 30~90 minutes. The purpose is to maintain a uniform temperature in the thickness direction of the billet during the low-temperature stage, preparing for a uniform microstructure in the high-temperature section. The heating rate is 4~7℃ / min, and the heating temperature is 1250~1280℃, held for 60~120 minutes. The purpose of the short low-temperature holding is to ensure the full dissolution of C / N compounds while avoiding abnormal growth of the as-cast microstructure.

[0037] 3. The initial rolling temperature of this invention is 1180~1250℃, the average reduction per pass is 20~35mm, and the final rolling temperature is 1050~1150℃. The purpose of high-temperature hot rolling is to increase the reduction per pass and improve the as-cast microstructure of the slab. Increasing the reduction per pass in the recrystallization temperature range can, on the basis of heavy pressing and forging, maximize the fragmentation of the as-cast microstructure, refine the grains, ensure that the steel plate has high strength and high rigidity, and prevent plastic deformation of the steel plate, thereby improving the steel plate's resistance to the Bauschinger effect. It also prepares the microstructure for normalizing treatment. By utilizing the fine austenite microstructure and residual deformation stress after rolling, the effective grain size of the steel plate microstructure is reduced.

[0038] 4. This invention uses a normalizing process to obtain fine and dispersed precipitated strengthening phases and fine effective grains in a high-C, high-alloy system, increasing the number of large-angle grain boundaries and further improving the low-temperature toughness and resistance to the Bauschinger effect of the steel plate.

[0039] In summary, this invention utilizes a compositional design combining high C, low Mn, and alloying elements such as Ni, Cr, Mo, Co, V, and Ti. Combined with a special production process involving high cleanliness, alloying smelting, continuous casting under heavy pressure, forging, efficient rolling, and high-temperature normalizing, it produces 620MPa grade ultra-high-strength marine engineering steel with a maximum thickness of 80mm and resistance to the Bauschinger effect. This ensures a yield strength ≥620MPa, tensile strength 740~890MPa, elongation ≥18%, Charpy impact energy at -60℃ ≥150J in the core of the steel plate, a yield strength reduction ≤10% at 2% residual strain, excellent resistance to the Bauschinger effect, and a uniform elongation ≥8%. The microstructure at half the thickness of the steel plate is bainite + 20%~30% pearlite, with an original austenite grain size of 15~25μm, an effective grain size of 3~6μm, a large-angle grain boundary ratio ≥30%, and good mechanical properties. Attached Figure Description

[0040] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0041] This invention discloses an ultra-high strength marine engineering steel with excellent resistance to the Bauschinger effect and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0042] To address the compositional performance requirements of ultra-high-strength marine engineering steel with excellent resistance to the Bauschinger effect, this invention utilizes a compositional design that combines high C, low Mn, and alloying elements such as Ni, Cr, Mo, Co, V, and Ti. This is coupled with a unique production process involving high cleanliness and alloying smelting, continuous casting under heavy pressure, forging, efficient rolling, and high-temperature normalizing to produce 620MPa grade ultra-high-strength marine engineering steel with a maximum thickness of 80mm. This invention involved extensive and systematic experimental research in several aspects, including alloying element screening and proportioning, steel cleanliness control, and optimization and parameter selection of efficient rolling processes. Ultimately, the alloying element proportions and production process that meet the objectives of this invention were determined.

[0043] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1, the steel refining and continuous casting process in the embodiments of the present invention is shown in Table 2, the rolling process in the embodiments of the present invention is shown in Table 3, the normalizing process in the embodiments of the present invention is shown in Table 4, and the mechanical properties of the steel plate in the embodiments of the present invention are shown in Table 5.

[0044] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention

[0045]

[0046] Table 2 Steel refining and continuous casting processes in embodiments of the present invention

[0047]

[0048] Table 3 Rolling process of embodiments of the present invention

[0049]

[0050] Table 4 Normalizing process of embodiments of the present invention

[0051]

[0052] Table 5 Mechanical properties of steel plates in embodiments of the present invention

[0053]

[0054] like Figure 1 As shown, the metallographic structure of Example 1 is bainite + 20%~30% pearlite at 1 / 2 thickness of the steel plate. The original austenite grain size is 15~25μm, the effective grain size is 3~6μm, the proportion of large-angle grain boundaries is ≥30%, and the mechanical properties are good.

[0055] As shown in Table 5, this invention is an ultra-high-strength marine steel with excellent resistance to the Bauschinger effect. Its yield strength is ≥620MPa, tensile strength is 740~890MPa, elongation is ≥18%, Charpy impact energy of the steel plate core at -60℃ is ≥150J, yield strength decreases by ≤10% at 2% residual strain, exhibiting excellent resistance to the Bauschinger effect and uniform elongation ≥8%. The finished steel plate thickness can reach 80mm, simultaneously considering the strength, toughness, and resistance to the Bauschinger effect of thicker steel plates, effectively controlling the uniformity of the microstructure in the core of thick plates.

[0056] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A high-strength marine steel with excellent resistance to the Bauschinger effect, characterized in that, Composed of the following chemical components in weight percentage composition: C: 0.2%~0.3%, Si: 0.2%~0.4%, Mn: 0.4%~0.75%, P≤0.02%, S≤0.01%, Ni: 1.0%~3.0%, Cr: 0.1%~0.3%, Mo: 0.1%~0.3%, Co: 0.2%~0.45%, V: 0.02%~0.05%, Ti: 0.01%~0.015%, N: 0.004%~0.008%, with the remainder being Fe and unavoidable impurities.

2. The ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 1, characterized in that, The maximum thickness of the finished steel plate is 80mm, with a yield strength ≥620MPa, tensile strength 740~890MPa, elongation ≥18%, Charpy impact energy of the steel plate core at -60℃ ≥150J, yield strength reduction ≤10% when the steel plate has 2% residual strain, and uniform elongation ≥8%.

3. The ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 1, characterized in that, The microstructure at 1 / 2 thickness of the steel plate is bainite + pearlite, with pearlite accounting for 20%~30%, the original austenite grain size being 15~25μm, the effective grain size being 3~6μm, and the proportion of large-angle grain boundaries being ≥30%.

4. A method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect as described in any one of claims 1 to 3, characterized in that, The manufacturing method specifically includes the following steps: 1) Steel refining; 2) Continuous casting: The tundish superheat is 8~15℃, the single roll pressure reduction of the casting machine is ≥10mm, and the billet is stacked and slowly cooled for 24~48h after leaving the line. 3) Rolling: The initial rolling temperature is 1180~1250℃, the average reduction per pass is 20~35mm, and the final rolling temperature is 1050~1150℃. 4) Positive heat: The normalizing temperature is 880~940℃, and the normalizing holding time is 2.5~3.5min / mm.

5. The method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 4, characterized in that, In step 1), molten steel is refined through a converter, LF furnace, RH or VD furnace.

6. The method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 4, characterized in that, 2) In the continuous casting billet forging heating temperature is 1250~1300℃, the total heating time is 5~8h, the forging compression ratio is 30%~40%, the billet is formed in one firing, the final forging temperature is ≥1000℃, and the billet is stacked and cooled slowly.

7. The method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 6, characterized in that, Stack and cool slowly for 36-48 hours.

8. The method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 4, characterized in that, In step 3), the billet is loaded into the heating furnace at a furnace temperature of 650~750℃ and held for 30~90 minutes.

9. The method for manufacturing ultra-high strength marine steel with excellent resistance to the Bauschinger effect according to claim 8, characterized in that, The heating rate is controlled at 4~7℃ / min, the heating temperature is 1250~1280℃, and the holding time is 60~120 min.

Images