Super-high strength marine steel with excellent bauschinger effect and manufacturing method thereof

Abstract

The application relates to the field of steel material preparation, in particular to an ultrahigh-strength marine steel with excellent Bauschinger effect resistance and a manufacturing method thereof. The ultrahigh-strength marine steel is composed of the following chemical components with the weight percentage: 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%, and the rest is Fe and inevitable impurities. In combination with the special production process of high cleanliness and alloying smelting+continuous casting heavy pressing+forging+high-efficiency rolling+high-temperature normalizing, the thickness of the steel plate finished product can reach 80 mm at most, meanwhile, the strength, toughness and Bauschinger effect resistance of the thick-gauge steel plate are considered, and the uniformity of the core structure of the thick plate is effectively controlled.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultra-high strength marine steel, in particular to an ultra-high strength marine steel with excellent anti-Bauschinger effect and a manufacturing method thereof. BACKGROUND

[0002] With the development of polar resources and the expansion of marine engineering, the polar marine environment has put forward more stringent requirements on the performance of marine steel. The traditional high-strength marine steel is prone to Bauschinger effect under the conditions of polar low temperature, high stress and dynamic load, that is, the yield strength of the material significantly decreases during the reverse loading process, resulting in a decrease in plastic deformation capacity and affecting the load-bearing safety and fatigue life of the structure. In particular, under the polar low temperature environment, the mechanical property deterioration caused by the Bauschinger effect is more significant, which seriously restricts the application of ultra-high strength marine steel in key equipment such as polar ships and offshore platforms.

[0003] At present, although the yield strength of ≥620MPa grade ultra-high strength marine steel can be realized by alloy design and controlled rolling and controlled cooling process, its anti-Bauschinger effect is generally insufficient. Specifically, under the condition of cyclic loading or pre-strain, the yield strength decreases greatly (usually >10%), and the uniform elongation is low (<8%), resulting in easy failure of the structure due to local plastic deformation concentration during service. In addition, the existing technology cannot simultaneously consider the strength, toughness and anti-Bauschinger effect performance of thick steel plates, and the control of the uniformity of the core structure of thick plates becomes a technical difficulty.

[0004] The Chinese patent application with the application number CN202211664907.9 "A super-thick high-strength steel for nuclear reactor containment vessel and a manufacturing method thereof" proposes a super-thick quenched and tempered steel plate for nuclear power, which adopts a low C, low Si and high Mn composition system. The low carbon equivalent composition design improves the welding performance, but without the combined action of Co, Ni and other elements, it cannot produce ultra-high strength steel plates with anti-Bauschinger effect. The Chinese patent application with the application number CN202310778326.6 "Thick gauge 800MPa hydroelectric steel with excellent core toughness and manufacturing method thereof" proposes a low C and low alloy ultra-high strength quenched and tempered steel. Although this composition can produce thick steel plates, the low alloy composition and quenching and tempering process cannot guarantee the -60℃ low temperature toughness of the core of the steel plate, and cannot produce anti-Bauschinger effect steel plates. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the present application provides an ultra-high strength marine steel with excellent anti-Bauschinger effect and a manufacturing method thereof. The thickness of the steel plate finished product can reach 80mm, and the strength, toughness and anti-Bauschinger effect performance of thick steel plates are considered at the same time, and the uniformity of the core structure of thick plates is effectively controlled.

[0006] In order to achieve the above object, the present application adopts the following technical solutions:

[0007] An ultra-high-strength marine steel with excellent Bauschinger effect is composed of the following chemical components with 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%, and the rest is Fe and inevitable impurities.

[0009] The roles of selecting the types and contents of the above alloy elements are as follows:

[0010] 1. C is the basic strengthening element in steel, which can be dissolved in the ferrite austenite matrix to improve the strength of the steel plate. If the content of C is too low, the solid solution content and carbide content of C will be reduced, the grain refinement effect will be low, the strength of the steel plate will be insufficient, the steel plate rigidity cannot be increased by grain boundary strengthening, and serious Bauschinger effect will be caused. However, if the content of C is too high, a large amount of hardened structure will be produced, and the crack tendency will be large during deformation, which will affect the low-temperature impact toughness. Therefore, the content of C should be accurately controlled. Therefore, the content of C in the present application is accurately controlled at 0.2%~0.3%.

[0011] 2. Si can improve the strength and rigidity of the steel plate through solid solution strengthening effect. When the content of Si is less than 0.2%, the deoxidizing effect is not obvious, the strength and rigidity are reduced, the steel plate is prone to plastic deformation, and the Bauschinger effect is obvious. In order to avoid segregation and inclusions in large-thickness billets, the content of Si should not be greater than 0.4%. Therefore, the content of Si in the present application is accurately controlled at 0.2%~0.4%.

[0012] 3. Mn element and Co element are dissolved in Fe matrix in large amounts to improve the strength of the steel plate. Since the content of C element in the present application is too high, in order to avoid the segregation of billets and the decrease of low-temperature toughness of the core, which will cause serious Bauschinger effect, the content of Mn should be appropriately reduced. When the content of Mn is less than 0.4%, it is not beneficial to the deoxidation of the steel plate. When the mass percentage of Mn element is greater than 0.75%, the low-temperature toughness of the core of the thick plate will be reduced. Therefore, the content of Mn in the present application is accurately controlled at 0.4%~0.75%.

[0013] 4. P and S elements have no benefit to the mechanical properties of the steel plate, especially the elongation. The content of P should be controlled at ≤0.02%, and the content of S should be controlled at ≤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 ultra-high strength marine steel with excellent Bauchinger effect has a maximum thickness of 80 mm, a yield strength of ≥620 MPa, a tensile strength of 740-890 MPa, an elongation of ≥18%, a Charpy impact energy of ≥150 J at -60 ℃ in the core of the steel plate, a yield strength reduction of ≤10% at 2% residual strain, excellent Bauchinger effect, and a uniform elongation of ≥8%.

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

[0023] The manufacturing method of the ultra-high strength marine steel with excellent Bauchinger effect specifically comprises the following steps:

[0024] 1) Steel refining:

[0025] The molten steel is refined by a converter, an LF furnace, an RH or VD furnace to further reduce the contents of P, S and non-metallic inclusions.

[0026] 2) Continuous casting:

[0027] The tundish superheat is 8-15 ℃, the single-roller weight pressing amount of the caster is ≥10 mm, and the billet is offline stacked and slowly cooled for 24-48 h. The forging heating temperature of the continuous casting billet is 1250-1300 ℃, the heating time is 5-8 h, the forging compression ratio is 30%-40%, the first-fire material is obtained, the final forging temperature is ≥1000 ℃, and the stacking slow cooling time is 36-48 h.

[0028] 3) Rolling:

[0029] The electroslag ingot is loaded into a heating furnace at a furnace temperature of 650-750 ℃, and the low-temperature holding time is 30-90 min, which aims to keep the thickness direction temperature of the steel billet consistent in the low-temperature stage and prepare for the uniformity of the high-temperature stage structure. The heating rate is 4-7 ℃ / min, the heating temperature is 1250-1280 ℃, and the holding time is 60-120 min. The purpose of low-temperature short-time holding is to ensure the sufficient dissolution of C / N compounds while avoiding the abnormal growth of the as-cast structure.

[0030] The opening rolling temperature is 1180-1250 ℃, the single-pass average reduction is 20-35 mm, and the final rolling temperature is 1050-1150 ℃. The high-temperature hot rolling aims to increase the single-pass reduction and improve the as-cast structure of the slab. Increasing the single-pass reduction in the recrystallization temperature region can maximize the breaking of the as-cast structure, refine the grains, ensure the steel plate to have high strength and high stiffness, and organize the plastic deformation of the steel plate, thereby improving the Bauchinger effect of the steel plate. The fine austenite structure after rolling and the residual deformation stress are used to reduce the effective grain size of the steel plate structure.

[0031] 4) normalizing:

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

[0033] The normalizing process aims at obtaining fine dispersed precipitated strengthening phase and fine effective grain in the high C high alloy component system of the application, increasing the number of high angle grain boundaries, and further improving the low temperature toughness and the resistance to Bauschinger effect of the steel plate.

[0034] Compared with the prior art, the application has the beneficial effects that:

[0035] 1. The application adopts the component design of high C, low Mn and Ni, Cr, Mo, Co, V, Ti and other alloy elements. The application improves the strength of the steel plate and the ability to resist Bauschinger effect by precisely controlling the C content, and ensures that the core -60 DEG C Charpy impact energy is greater than or equal to 150 J. Since the C element of the application is too high, in order to avoid segregation of the billet and cause the core low temperature toughness to decrease and produce serious Bauschinger effect, the Mn content is appropriately reduced. The role of Ni is to improve the low temperature toughness and the resistance to Bauschinger effect of the steel plate through dispersion strengthening and fine grain strengthening. The Ni element is largely solid-solved in the matrix with Mn and Co elements, which improves the strength, reduces the ductile-brittle transition temperature, prevents plastic deformation of the steel plate, and at the same time, the Ni element can improve the corrosion fatigue resistance of the steel plate. Cr is an element that improves the hardness and stiffness of the steel plate. In the case of low C content, the addition of appropriate amount of Cr element can ensure that the steel plate reaches the required stiffness, prevents plastic deformation of the steel plate, and improves the resistance to Bauschinger effect. Cr element can improve the corrosion resistance of the steel plate to a certain extent. The addition of Mo element can improve the stiffness and hardness. The addition of appropriate amount of Mo element can also improve the temper brittleness of the steel plate. Mo element can also cooperate with Ni to improve the creep resistance under stress and improve the corrosion fatigue resistance. These beneficial effects can improve the strength and toughness of the steel plate and the resistance to Bauschinger effect. Ti, V, N and C can form fine dispersed C, N compound second phase, effectively control the growth of original austenite grains, and further improve the low temperature toughness of the steel plate.

[0036] 2. The electroslag ingot is loaded into the heating furnace at a furnace temperature of 650-750 DEG C, and the low temperature holding time is 30-90 min. The purpose is to keep the temperature uniform in the thickness direction of the steel billet in the low temperature stage, and to prepare for the uniform organization in the high temperature stage. The heating rate is 4-7 DEG C / min, the heating temperature is 1250-1280 DEG C, and the holding time is 60-120 min. The purpose of low temperature short time holding is to ensure that the C / N compound is fully dissolved, while avoiding abnormal growth of the as-cast structure.

[0037] 3, the open mill temperature of the application is 1180~1250℃, the single pass average reduction is 20~35mm, and the finish rolling temperature is 1050~1150℃. The purpose of high temperature hot rolling is to increase the single pass reduction, and to improve the as-cast structure of the slab. Increasing the single pass reduction in the recrystallization temperature region can maximize the crushing of the as-cast structure, refine the grains, ensure that the steel plate has high strength and high stiffness, and organize the plastic deformation of the steel plate, thereby improving the resistance to Bauschinger effect of the steel plate. Make good organizational preparation for normalizing treatment. Use the fine austenite structure and residual deformation stress after rolling to reduce the effective grain size of the steel plate structure.

[0038] 4, the application obtains fine and dispersed precipitated strengthening phase and fine effective grain by normalizing process for high C high alloy component system, increases the number of high angle grain boundaries, and further improves the low temperature toughness and Bauschinger effect performance of the steel plate.

[0039] In summary, the application uses the composition design of high C, low Mn, and Ni, Cr, Mo, Co, V, Ti and other alloy elements, combined with high cleanliness and alloying smelting + continuous casting heavy reduction + forging + high efficiency rolling + high temperature normalizing special production process, to produce 620MPa grade anti-Bauschinger effect ultra-high strength offshore steel with a maximum thickness of 80mm. The yield strength of the steel plate is ≥620MPa, the tensile strength is 740~890MPa, the elongation is ≥18%, the Charpy impact energy of the core of the steel plate at -60℃ is ≥150J, the yield strength of the steel plate at 2% residual strain is reduced by ≤10%, the anti-Bauschinger effect is excellent, and the uniform elongation is ≥8%. The microstructure of the steel plate at 1 / 2 thickness is bainite + 20%~30% pearlite, the original austenite grain size is 15~25μm, the effective grain size is 3~6μm, the proportion of high angle grain boundaries is ≥30%, and the mechanical properties are good. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is the metallographic structure diagram of example 1 of the application. DETAILED DESCRIPTION

[0041] The application discloses an ultra-high strength offshore steel with excellent anti-Bauschinger effect and a manufacturing method thereof. Those skilled in the art can refer to the content herein and appropriately improve the process parameters to realize. It is particularly pointed out that all similar replacements and changes are obvious to those skilled in the art, and they are regarded as included in the application. The method and application of the application have been described through the preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the method and application described herein without departing from the content, spirit and scope of the application, to realize and apply the technical content of the application.

[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, the application is a super-high-strength marine steel with excellent Bauschinger effect resistance, with yield strength ≥620 MPa, tensile strength 740~890 MPa, elongation ≥18%, Charpy impact energy of the core of the steel plate ≥150 J at-60 ℃, yield strength reduction ≤10% at 2% residual strain, excellent Bauschinger effect resistance, and uniform elongation ≥8%. The finished thickness of the steel plate can reach 80 mm, and the strength, toughness and Bauschinger effect resistance of the thick steel plate are considered at the same time, and the uniformity of the core structure of the thick plate is effectively controlled.

[0056] The above is only the preferred specific embodiment of the application, but the protection scope of the application is not limited to this. Any person skilled in the art can make equivalent replacements or changes according to the technical solution and the inventive concept of the application within the technical range disclosed by the application, which should be covered in the protection scope of the application.

Claims

1. A method for producing an ultrahigh-strength marine steel excellent in Bauschinger effect, characterized by, The offshore steel is composed of the following chemical components by weight percentage 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%, and the rest is Fe and inevitable impurities; The offshore steel manufacturing method specifically comprises the following steps: 1) Steel refining; 2) Continuous casting: The tundish superheat is 8~15℃, the single roller heavy reduction of the caster is ≥10mm, and the billet is offline stacked and slowly cooled for 24~48h; 3) Rolling: The open rolling temperature is 1180~1250℃, the single pass average reduction is 20~35mm, and the finish rolling temperature is 1050~1150℃; 4) Normalizing: The normalizing temperature is 880~940℃, and the normalizing holding time is 2.5~3.5min / mm.

2. The offshore steel manufacturing method according to claim 1, wherein The maximum thickness of the steel plate finished product is 80mm, the yield strength is ≥620MPa, the tensile strength is 740~890MPa, the elongation is ≥18%, the Charpy impact energy of the steel plate core at -60℃ is ≥150J, the yield strength reduction of the steel plate at 2% residual strain is ≤10%, and the uniform elongation is ≥8%.

3. The offshore steel manufacturing method according to claim 1, wherein The structure at the 1 / 2 thickness of the steel plate is bainite+pearlite, the pearlite is 20%~30%, the original austenite grain size is 15~25μm, the effective grain size is 3~6μm, and the proportion of high-angle grain boundaries is ≥30%.

4. The offshore steel manufacturing method according to claim 1, wherein 1) The molten steel is refined by a converter, an LF furnace, an RH or a VD furnace.

5. The offshore steel manufacturing method according to claim 1, wherein 2) The forging heating temperature of the continuous casting billet is 1250~1300℃, the total heating time is 5~8h, the forging compression ratio is 30%~40%, the first heating material is formed, the final forging temperature is ≥1000℃, and the stacking slow cooling time is 36~48h.

6. The offshore steel manufacturing method according to claim 1, wherein 3) The casting billet is loaded into the heating furnace at the furnace temperature of 650~750℃ and is kept for 30~90min.

7. The offshore steel manufacturing method according to claim 6, wherein 3) The heating temperature is 1250~1280℃, the holding time is 60~120min, and the heating rate is controlled at 4~7℃ / min. ​

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