Rare earth steel for 355MPa-grade hull structure and preparation method of rare earth steel

Abstract

The invention relates to rare earth steel for a 355MPa-grade hull structure and a preparation method of the rare earth steel, belongs to the technical field of metal materials, and solves the problems that the existing steel for the 355MPa-grade hull structure is poor in comprehensive performance and cannot meet the service requirements of high-end ships. The steel comprises the following chemical components in percentage by weight: 0.10%-0.15% of C, 0.20%-0.50% of Si, 0.70%-1.00% of Mn, 0.03%-0.10% of V, 0.10%-0.30% of Cu, 0.04%-0.08% of Mo, 0.10%-0.20% of Cr, 0.30%-0.40% of Ni, 0.003%-0.01% of Ce, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.003% of O and the balance of iron and inevitable impurities. The prepared 355MPa-grade rare earth steel for the hull structure has obdurability and corrosion resistance at the same time.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and in particular to a rare earth steel for ship hull structures of grade 355MPa and its preparation method. Background Technology

[0002] Structural steel for shipbuilding and marine engineering is a key material for constructing hulls and marine structures. Its performance directly affects the safety, durability, and service life of the equipment. Currently, 355MPa grade (such as E36) hull structural steel is widely used in shipbuilding. At present, large marine vessels are increasingly being deployed to extreme environments such as polar regions and deep seas, posing unprecedented challenges to the comprehensive performance of hull structural steel. The low-temperature impact toughness (~40℃) and seawater corrosion resistance of existing 355MPa grade hull structural steel (such as E36 steel) are no longer sufficient to meet the service requirements of the next generation of high-end ships.

[0003] To address the insufficient low-temperature impact toughness and seawater corrosion resistance at ~40℃ of existing 355MPa grade ship structural steel, methods such as increasing the content of precious metal alloying elements (e.g., increasing Nb and Ti content) or performing complex subsequent heat treatments are commonly employed. However, these approaches have significant limitations: on the one hand, the substantial addition of alloying elements significantly increases costs and may impair the weldability and machinability of the material; on the other hand, the simple alloying approach often reaches a bottleneck in its ability to simultaneously and synergistically improve toughness and corrosion resistance, making it difficult to achieve a qualitative breakthrough.

[0004] Metallurgical research has shown that rare earth elements have the potential to purify molten steel, remove impurities, and microalloy. However, applying rare earth elements to ship hull steel still faces significant challenges: First, the process window for rare earth addition is narrow, and improper control can easily lead to the formation of large inclusions, which in turn deteriorates performance; second, the interaction between rare earth elements and other elements in steel is complex. How to achieve a "dual benefit" effect of improving low-temperature toughness and corrosion resistance simultaneously and efficiently, while ensuring strength, through precise composition design and strict process control, remains a long-standing technical problem that has not been effectively solved in this field.

[0005] Therefore, developing a new type of 355MPa grade ship hull structural steel with reasonable composition design, controllable process, economical cost, and simultaneously possessing ultra-high and low temperature toughness and excellent seawater corrosion resistance has become an urgent need in the industry. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a 355MPa grade rare earth steel for ship hull structures and its preparation method, at least to solve one of the following problems existing in the 355MPa grade ship hull structural steel: 1. The existing 355MPa grade ship hull structural steel (such as E36 steel) has low-temperature impact toughness and seawater corrosion resistance at ~40℃ that are difficult to meet the service requirements of the new generation of high-end ships; 2. Existing methods to improve the comprehensive performance of 355MPa grade ship hull structural steel mainly involve increasing the content of precious metal alloying elements or carrying out complex subsequent heat treatment, resulting in high composition.

[0007] The objective of this invention is mainly achieved through the following technical solutions:

[0008] This invention provides a 355MPa grade rare earth steel for ship hull structures, the chemical composition of which, by weight, includes: C: 0.10%–0.15%, Si: 0.20%–0.50%, Mn: 0.70%–1.00%, V: 0.03%–0.10%, Cu: 0.10%–0.30%, Mo: 0.04%–0.08%, Cr: 0.10%–0.20%, Ni: 0.30%–0.40%, Ce: 0.003%–0.01%, P≤0.01%, S≤0.01%, O≤0.003%, with the remainder being iron and unavoidable impurities.

[0009] Furthermore, the mass percentage of Cu to Ni satisfies Ni / Cu≥1.5.

[0010] Furthermore, the mass percentage of Ce to S satisfies Ce / S≥1.5.

[0011] Furthermore, the sum of the mass contents of Cr and Mo satisfies: Cr + Mo ≤ 0.25%.

[0012] Furthermore, the microstructure of the rare earth steel used for the 355MPa grade hull structure is ferrite and pearlite, wherein the ferrite grain size is 9-12μm.

[0013] This invention also provides a method for preparing 355MPa grade rare earth steel for ship hull structures, comprising the following steps:

[0014] S1: Hot metal pretreatment: Deep desulfurization of hot metal using the KR method to ensure that the S content after treatment is ≤0.002%;

[0015] S2: Converter smelting: Top and bottom blowing converter smelting is adopted, and argon gas is blown from the bottom throughout the steel tapping process;

[0016] S3: LF Refining: The LF furnace produces white slag to refine molten steel, adjusting the composition of the molten steel to achieve the target value;

[0017] S4: Continuous casting: The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the tundish is ≤25℃, and after continuous casting, it enters the slow cooling pit for treatment.

[0018] S5: Controlled rolling and cooling: The continuous casting billet is heated and then subjected to controlled rolling and cooling.

[0019] Furthermore, in step S2, the tapping temperature of the converter smelting is ≥1600℃, and the oxygen content of the tapped steel is ≥500ppm.

[0020] Furthermore, in step S5, the heating temperature of the continuously cast billet is 1100-1150℃, and the heating time is 200-250min.

[0021] Furthermore, in step S5, the controlled rolling is a two-stage rolling process. The first stage is roughing, and the second stage is finishing. The roughing start temperature is 1050-1100℃, and the roughing finish temperature is 950-980℃. The roughing is divided into three passes. The deformation amount of the first pass is 15%-17%, the deformation amount of the second pass is 23%-25%, and the deformation amount of the third pass is 20%-22%.

[0022] Furthermore, the initial rolling temperature of the finishing mill is 860–930°C, and the final rolling temperature is 750–850°C;

[0023] Finishing rolling is performed in three passes: the first pass has a deformation of 19% to 21%, the second pass has a deformation of 24% to 26%, and the third pass has a deformation of 33% to 34%.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] 1. The rare earth steel for 355MPa grade ship hull structure and its preparation method of the present invention adopt a rare earth microalloying design. By controlling elements such as carbon, manganese, and vanadium within an optimized range to ensure basic strength, and strictly limiting the sulfur and oxygen content to purify the steel to the extreme, a method is adopted. On this basis, 0.003-0.01% cerium (Ce) is precisely added to effectively achieve spheroidization of sulfides and fundamentally eliminate crack sources. At the same time, in terms of process, a two-stage controlled rolling and cooling is adopted. In the finishing rolling stage, large deformation rolling is carried out in the non-recrystallization zone, which greatly increases the density of austenite grain boundaries and deformation bands, providing a large number of nucleation sites for phase transformation, and finally obtaining an ultra-fine ferrite-pearlite structure, thereby achieving the purpose of improving strength and toughness. The strength and toughness matching of the steel no longer relies solely on increasing the content of precious metal alloying elements, but on the microstructure refinement brought about by rare earth microalloying.

[0026] 2. The rare earth steel for 355MPa grade ship hull structure and its preparation method of the present invention utilize the synergistic effect of alloying elements such as copper, nickel, and chromium with the surface segregation effect of rare earth cerium in the composition design. Rare earth segregates at the interface, exposing grain boundaries, phase boundaries, and free surfaces, thereby reducing the interfacial energy and avoiding localized corrosion. Rare earth microalloying modifies the elongated inclusions with a high pitting corrosion tendency into round inclusions with a lower pitting corrosion tendency, reducing the potential difference between the inclusions and the matrix, and significantly enhancing the seawater corrosion resistance.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 This is the microstructure of the rare earth steel used in the 355MPa grade ship hull structure in Example 1. Detailed Implementation

[0030] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] This invention provides a 355MPa grade rare earth steel for ship hull structures, the chemical composition of which, by weight, comprises: C: 0.10%–0.15%, Si: 0.20%–0.50%, Mn: 0.70%–1.00%, V: 0.03%–0.10%, Cu: 0.10%–0.30%, Mo: 0.04%–0.08%, Cr: 0.10%–0.20%, Ni: 0.30%–0.40%, Ce: 0.003%–0.01%, P: ≤0.01%, S: ≤0.01%, O ≤0.003%, with the remainder being iron and unavoidable impurities;

[0032] The reasons for limiting the chemical composition of the rare earth steel for 355MPa grade ship hull structure and its preparation method in this invention will be explained. Hereinafter, only the percentage of mass in the composition is expressed as %.

[0033] Carbon (C): Carbon is an essential element for improving strength, but it is also an element that reduces the weldability of materials. Increasing the C content can improve the strength of steel plates, but excessive carbon content will have an adverse effect on the weldability and low-temperature toughness of steel plates. Taking both aspects into consideration, the C content in this invention is controlled at 0.10 to 0.15%.

[0034] Silicon (Si): Silicon is a deoxidizing element and a solid solution strengthening element, which can improve the strength of steel. When the silicon content is too high, it will reduce the low-temperature toughness of steel and worsen its weldability. Therefore, the Si content in this invention is controlled at 0.20-0.50%.

[0035] Manganese (Mn): Manganese is a basic element for the formation of bainite, causing obvious bends in the supercooled austenite transformation curve and significantly delaying the high-temperature transformation, resulting in the separation of the upper and lower C curves of steel; Mn easily combines with S in steel to form MnS impurities, which impair the properties of the steel; therefore, the Mn content in this invention is controlled at 0.70% to 1.00%.

[0036] Vanadium (V): Vanadium is a commonly used microalloying element with significant precipitation strengthening effect; it can form V(C,N) precipitates with carbon, which play a role in precipitation strengthening; in this invention, the V content is controlled at 0.03-0.10%.

[0037] Copper (Cu): Copper is a non-carbide-forming element that can be used to improve the strength and corrosion resistance of steel. In easily weldable ship hull steel, a certain amount of Cu is usually added, and Cu particles are fully precipitated through aging to improve strength. In this invention, Cu aging is completed through tempering heat treatment, which helps to precipitate copper-rich phases and improves strength while ensuring toughness. Therefore, the Cu content in this invention is controlled at 0.10-0.30%.

[0038] Molybdenum (Mo): Molybdenum has a significant solute dawd effect, which causes obvious bends in the supercooled austenite transformation curve and significantly delays the high-temperature transformation, causing the upper and lower C curves of the steel to separate; therefore, the Mo content in this invention is controlled at 0.04% to 0.08%.

[0039] Chromium (Cr): Chromium plays an important role in the solid solution strengthening mechanism of microalloyed steel and can significantly improve corrosion resistance; however, if the amount added is too high, it will not only increase the cost, but also deteriorate the processing and welding properties of the steel; therefore, the Cr content in this invention is controlled at 0.10-0.20%.

[0040] Nickel (Ni): Nickel plays an important role in the phase transformation strengthening and solid solution strengthening mechanism of microalloyed steel, and can also improve the low-temperature toughness and corrosion resistance of steel; however, if the amount added is too high, it will not only increase the cost, but also deteriorate the processing and welding performance of steel; therefore, the Ni content in this invention is controlled at 0.30 to 0.40%.

[0041] Cerium (Ce): As a microalloying element, cerium can improve the corrosion resistance and strength-toughness balance of steel through inclusion modification, grain boundary segregation, and microstructure refinement. Therefore, the cerium content is controlled between 0.003% and 0.01%.

[0042] Phosphorus (P): Phosphorus is an impurity element in steel that can impair the toughness of steel plates and weld heat-affected zones; therefore, the P content in this invention is controlled below 0.01%.

[0043] Sulfur (S): Sulfur is an impurity element in steel, which can form sulfide inclusions and become crack sources; therefore, the S content in this invention is controlled below 0.01%.

[0044] Oxygen (O): Oxygen is the main impurity gas in steel. It can form oxide inclusions, which seriously impair the toughness, fatigue performance and weldability of steel. Therefore, the O content should be controlled below 0.003%.

[0045] Preferably, the mass percentages of Cu and Ni satisfy the relationship: Ni / Cu≥1.5; this design utilizes Ni to suppress the hot brittleness that may be caused by Cu enrichment at grain boundaries, while the two work synergistically to improve the corrosion resistance and low-temperature toughness of the steel.

[0046] Preferably, the mass percentages of Ce and S satisfy the following relationship: Ce / S ≥ 1.5. When Ce / S is less than 1.5, the amount of Ce is insufficient, and not all sulfides can be degraded. Some harmful MnS will still exist, resulting in poor toughness improvement. When Ce / S is too high, excessive Ce may form coarse rare earth oxides or intermetallic compounds, which may become crack sources. This design ensures that Ce reacts sufficiently with S and O to form spherical rare earth inclusions, reducing pitting corrosion sensitivity, improving corrosion resistance, avoiding rare earth waste, and improving inclusion control efficiency.

[0047] Preferably, the sum of the mass contents of Cr and Mo satisfies: Cr + Mo ≤ 0.25%, controlling the total amount to avoid a decrease in welding performance. Excessive content will significantly increase the hardening and cold cracking tendency of the weld heat-affected zone, which is not conducive to large-scale welding construction of the hull structure; and improve the synergistic effect of corrosion resistance.

[0048] This invention also provides a method for preparing 355MPa grade rare earth steel for ship hull structures, comprising the following steps:

[0049] S1: Hot metal pretreatment: Deep desulfurization of hot metal using the KR method to ensure that the S content after treatment is ≤0.002%;

[0050] Specifically, by pre-treating molten iron, impurity elements in the steel are reduced, ensuring that the sulfur content is ≤0.002% after KR desulfurization; high sulfur content will produce inclusions, affecting the performance of the steel.

[0051] S2: Converter smelting: Top and bottom blowing converter smelting is adopted, and argon gas is blown from the bottom throughout the steel tapping process;

[0052] Specifically, to ensure sufficient temperature drop space for molten steel during subsequent LF refining, calcium treatment, and continuous casting, a high starting temperature is necessary to prevent condensation of the molten steel before casting. The tapping temperature of the converter smelting should be ≥1600℃. A high tapping oxygen content is beneficial for generating a large number of fine CO bubbles during the deoxidation and alloying process after tapping. Through the "carbon-oxygen boiling" effect, gases (such as [H] and [N]) in the molten steel are effectively removed, and non-metallic inclusions are promoted to float, thereby purifying the molten steel and creating clean molten steel conditions for subsequent rare earth treatment. The tapping oxygen content should be ≥500ppm.

[0053] S3: LF Refining: The LF furnace produces white slag to refine molten steel, adjusting the composition of the molten steel to achieve the target value;

[0054] Specifically, the LF furnace produces white slag to refine molten steel, adjusting the composition of the molten steel to the target value; the content of inclusions in the steel is strictly controlled through operations such as degassing, desulfurization and argon blowing in the later stage of refining; calcium treatment is carried out at the end of refining, with a pure calcium feed rate of 200±50m.

[0055] S4: Continuous casting: The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the tundish is ≤25℃, and after continuous casting, it enters the slow cooling pit for treatment.

[0056] Excessive superheat can lead to an increase in inclusions in the molten steel, affecting the purity and performance of the casting. For example, the target superheat of the molten steel in the tundish is 25°C, 24°C, 23°C, 22°C, 21°C, 20°C, 19°C, 18°C, 17°C, 16°C, and 15°C.

[0057] S5: Controlled rolling and cooling: The continuous casting billet is heated and then subjected to controlled rolling and cooling.

[0058] Specifically, the continuous casting billet is heated to 1100–1150℃ (exemplary heating temperatures are 1105℃, 1110℃, 1115℃, 1120℃, 1125℃, 1130℃, 1135℃, 1140℃, and 1145℃), and the heating time is 200–250 min (exemplary heating times are 205 min, 210 min, 215 min, 220 min, 225 min, 230 min, 235 min, 240 min, and 245 min). This ensures that the high content of alloying elements is fully dissolved in the matrix, providing a uniform pre-structure for subsequent "precipitation strengthening" and "microstructure refinement" through rolling and cooling. If the temperature is too low or the time is insufficient, the alloying elements cannot be completely dissolved, which will reduce the final strengthening effect; if the temperature is too high or the time is too long, it will lead to abnormal growth of austenite grains, which will damage the toughness.

[0059] The controlled rolling process employs a two-stage rolling method: the first stage is roughing, with an initial rolling temperature of 1050–1100℃ (exemplarily, initial rolling temperatures of 1055℃, 1060℃, 1065℃, 1070℃, 1075℃, 1080℃, 1085℃, 1090℃, and 1095℃), and a final rolling temperature of 950–980℃ (exemplarily, final rolling temperatures of 955℃, 960℃, 965℃, 970℃, and 975℃). The roughing is performed in three passes, with the first pass deformation ranging from 15% to 17% (exemplarily, the first pass deformation is 15.2%, 15.4%, 15.5%, 15.6%, 15.8%, and 1...). The deformation amounts for the first pass are 6.0%, 16.2%, 16.4%, 16.5%, 16.6%, and 16.8%, respectively. The deformation amount for the second pass is 23% to 25% (exemplarily, the deformation amounts for the second pass are 23.2%, 23.4%, 23.5%, 23.6%, 23.8%, 24.0%, 24.2%, 24.4%, 24.6%, and 24.8%), and the deformation amount for the third pass is 20% to 22% (exemplarily, the deformation amounts for the third pass are 20.2%, 20.4%, 20.5%, 20.6%, 20.8%, 21.0%, 21.2%, 21.4%, 21.5%, 21.6%, and 21.8%). During the rolling deformation process, the flattened and elongated austenite grains will immediately undergo dynamic recrystallization to form new, fine equiaxed austenite grains; breaking the original coarse cast structure and refining the austenite grains.

[0060] The second stage is finishing rolling, with an initial rolling temperature of 860–930℃ (exemplary initial rolling temperatures are 865℃, 870℃, 875℃, 880℃, 885℃, 890℃, 895℃, 900℃, 905℃, 910℃, 915℃, 920℃, and 925℃), and a final rolling temperature of 750–850℃ (exemplary final rolling temperatures are 755℃, 760℃, 765℃, 770℃, 775℃, 780℃, 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃, 820℃, 825℃, 830℃, 835℃, 840℃, and 845℃). Finishing rolling is performed in three passes, with the first pass having a deformation of 19... The deformation amounts are 24% to 21% (for example, the deformation amounts for the first pass are 19.2%, 19.4%, 19.5%, 19.6%, 19.8%, 20%, 20.2%, 20.4%, 20.5%, 20.6%, and 20.8%), the deformation amounts for the second pass are 24% to 26% (for example, the deformation amounts for the second pass are 24.2%, 24.4%, 24.5%, 24.6%, 24.8%, 25.0%, 25.2%, 25.4%, 25.5%, 25.6%, and 25.8%), and the deformation amounts for the third pass are 33% to 34% (for example, the deformation amounts for the third pass are 33.2%, 33.4%, 33.5%, 33.6%, and 33.8%). The cooling rate after rolling is controlled at 0.1–5.0 °C / s to room temperature. For example, the cooling rates are 0.5 °C / s, 1.0 °C / s, 1.5 °C / s, 2.0 °C / s, 2.5 °C / s, 3.0 °C / s, 3.5 °C / s, 4.0 °C / s, and 4.5 °C / s. The finishing rolling process strictly controls the initial and final rolling temperatures within the non-recrystallization zone and implements a large deformation amount that increases with each pass (especially the third pass, which reaches 33%–34%). Its core function is to greatly increase the density of austenite grain boundaries and deformation bands through intense plastic deformation without inducing recrystallization, thereby providing sufficient nucleation sites for ferrite during subsequent cooling and achieving ultra-fine final microstructure. After rolling, the cooling rate is controlled at 0.1–5.0 °C / s to room temperature to ensure that the refined austenite has enough time to completely transform into fine ferrite and pearlite; this provides kinetic conditions for the dispersed precipitation of carbonitrides of microalloying elements such as V and Mo in ferrite, making full use of "precipitation strengthening"; excessively fast cooling rates may generate hard and brittle phases such as bainite or martensite, which are detrimental to toughness and weldability.

[0061] In terms of composition, this application ensures basic strength by controlling elements such as carbon, manganese, and vanadium within optimized ranges, and strictly limits the content of sulfur (≤0.005%) and oxygen (≤0.003%) to purify the steel to the extreme. Based on this, 0.003–0.01% cerium (Ce) is precisely added to effectively achieve spheroidization of sulfides, fundamentally eliminating crack initiation points. In terms of process, a two-stage controlled rolling and cooling method is adopted. The finishing rolling stage involves large deformation rolling in the non-recrystallization zone (especially the third pass reaching 33%–34%), which greatly increases the density of austenite grain boundaries and deformation bands, providing numerous nucleation sites for phase transformation. This ultimately yields an ultra-fine ferrite-pearlite microstructure, with ferrite grain size of 9–12 μm, achieving the goal of improving strength and toughness. Meanwhile, the synergistic effect of alloying elements such as copper, nickel, and chromium with the surface segregation effect of rare earth cerium causes rare earth to segregate at the interface, exposing grain boundaries, phase boundaries, and free surfaces, thereby reducing the interfacial energy and preventing localized corrosion. Rare earth microalloying modifies the elongated inclusions with a high pitting corrosion tendency into round inclusions with a lower pitting corrosion tendency, reducing the potential difference between the inclusions and the matrix, and significantly enhancing the seawater corrosion resistance.

[0062] The rare earth steel for ship hull structures prepared by this invention has a yield strength ≥355MPa (e.g., 379~401MPa), tensile strength ≥500MPa (e.g., 509~544MPa), elongation ≥25% (e.g., 25%~26%), impact energy at ~40℃ ≥250J (e.g., 273~296J), and annual corrosion rate ≤0.2mm / a (e.g., 0.147~0.173mm / a), meeting the service requirements of high-end ships.

[0063] The advantages of precise control of the elemental chemical composition, content, and preparation process parameters of the present invention will be demonstrated below with specific examples and comparative examples.

[0064] Example

[0065] This embodiment provides four types of rare earth steels (1# to 4# steel) for ship hull structures and their preparation methods, and selects three types of steel (5# to 7#) as comparative steels.

[0066] Steels #1 through #4 are all prepared using the same preparation steps:

[0067] S1: Hot metal pretreatment: Deep desulfurization of hot metal using the KR method to ensure that the S content after treatment is ≤0.002%;

[0068] S2: Converter smelting: Top and bottom blowing converter smelting is adopted, and argon is blown into the bottom throughout the tapping process; the tapping temperature of the converter smelting is ≥1600℃, and the oxygen content of the tapping steel is ≥500ppm;

[0069] S3: LF Refining: The LF furnace produces white slag to refine the molten steel and adjusts the composition of the molten steel to the target value; the content of inclusions in the steel is strictly controlled through operations such as degassing, desulfurization and argon blowing in the later stage of refining; calcium treatment is carried out at the end of refining, with a pure calcium wire feed rate of 200±50m.

[0070] S4: Continuous casting: The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the tundish is ≤25℃, and after continuous casting, it enters the slow cooling pit for treatment.

[0071] S5: Controlled rolling and cooling: The continuous casting billet is heated and then subjected to controlled rolling and cooling.

[0072] The continuous casting billet is heated at 1100–1150℃ for 200–250 min.

[0073] The controlled rolling adopts a two-stage rolling method: the first stage is rough rolling, with an initial rolling temperature of 1050-1100℃ and a final rolling temperature of 950-980℃. The rough rolling is divided into three passes, with the first pass having a deformation of 15%-17%, the second pass having a deformation of 23%-25%, and the third pass having a deformation of 20%-22%.

[0074] The second stage is finishing rolling, with an initial rolling temperature of 860–930℃ and a final rolling temperature of 750–850℃. Finishing rolling is divided into three passes: the first pass has a deformation of 19%–21%, the second pass has a deformation of 24%–26%, and the third pass has a deformation of 33%–34%.

[0075] After rolling, the cooling rate is controlled at 0.1–5.0 °C / s to room temperature.

[0076] The elemental composition mass percentages of steel #1 to #4 and #6 all meet the requirements of this invention, while the elemental composition mass percentages of steel #5 and #7 do not meet the requirements of this invention. The differences in their elemental composition are shown in Table 1.

[0077] The preparation process parameters of steels 1 to 4, 5 and 7 all meet the requirements of this invention, while the process parameters of steel 6 do not meet the requirements of this invention. The differences in their process parameters are shown in Table 2.

[0078] The types and contents of inclusions in steels #1 to #7 are shown in Table 3; the microstructure, mechanical properties, and corrosion resistance of steels #1 to #7 are shown in Table 4; the microstructure of steel #1 is shown in the figure. Figure 1 As shown.

[0079] Table 1 Chemical composition (wt, %) of steel #1 to #7

[0080]

[0081]

[0082] Table 2. Preparation process parameters for steel #1 to #7

[0083] 1# 2# 3# 4# 5# 6# 7# Superheat of continuous casting tundish / °C 25 25 23 24 22 25 25 Heating temperature / ℃ 1100 1150 1150 1150 1150 1200 1150 Furnace time / min 250 200 230 230 230 200 250 Roughing rolling temperature / ℃ 1050 1100 1050 1050 1050 1150 1050 Roughing and finishing rolling temperatures / ℃ 980 980 950 960 950 900 980 Deformation amount in the first pass of rough rolling / % 15 17 16 16 16 18 16 Deformation amount in the second pass of rough rolling / % 24 23 25 24 24 25 24 Deformation amount in the third pass of rough rolling / % 22 21 21 20 21 21 21 Finishing rolling start temperature / ℃ 900 930 860 900 860 850 880 Finishing rolling temperature / ℃ 750 850 810 830 810 730 780 Deformation amount in the first pass of finishing rolling / % 20 21 20 19 20 18 20 Deformation amount in the second pass of finishing rolling / % 25 25 24 26 25 23 25 Deformation amount in the third pass of finishing rolling / % 33 34 33 33 33 32 33 Cooling rate after rolling, ℃ / s 0.1 2.7 5.0 3.6 5.0 5.0 3.0

[0084] Table 3. Types and contents of inclusions in steels #1 to #7

[0085]

[0086]

[0087] Table 4. Microstructure, mechanical properties and corrosion properties of steels #1 to #7

[0088]

[0089] Comparison reveals that the smelting methods and process parameters for steels #1 to #5 and #7 are the same or similar. However, the elemental composition mass percentages of steels #1 to #4 all meet the requirements of this invention, while those of steels #5 and #7 do not. Furthermore, the low-temperature toughness and corrosion resistance of steels #5 and #7 are significantly lower than those of steels #1 to #4. Similarly, the elemental composition mass percentages of steels #1 to #4 and #6 meet the requirements of this invention. However, while the smelting methods and process parameters for steels #1 to #4 all meet the requirements, some process parameters for steel #6 do not. The strength, toughness, and corrosion resistance of steel #6 are significantly lower than those of steels #1 to #4.

[0090] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 355 MPa grade rare earth steel for ship hull structures, characterized in that, The chemical composition comprises, by weight: C: 0.10%-0.15%, Si: 0.20%-0.50%, Mn: 0.70%-1.00%, V: 0.03%-0.10%, Cu: 0.10%-0.30%, Mo: 0.04%-0.08%, Cr: 0.10%-0.20%, Ni: 0.30%-0.40%, Ce: 0.003-0.01%, P≤0.01%, S≤0.01%, O≤0.003%, and the rest is iron and inevitable impurities.

2. The 355 MPa grade rare earth steel for hull structure according to claim 1, characterized by, The mass percentage of Cu and Ni satisfies Ni / Cu≥1.

5.

3. The 355 MPa grade rare earth steel for hull structure according to claim 1, characterized by, The mass percentage of Ce and S satisfies Ce / S≥1.

5.

4. The 355 MPa grade rare earth steel for hull structure according to claim 1, characterized by, The sum of the mass contents of Cr and Mo satisfies Cr+Mo≤0.25%.

5. The 355 MPa grade rare earth steel for hull structure according to claim 1, characterized by, The microstructure of the 355MPa-grade rare earth steel for ship structure is ferrite and pearlite, wherein the ferrite grain size is 9-12μm.

6. A preparation method of a 355MPa-grade rare earth steel for ship structure, for preparing the 355MPa-grade rare earth steel for ship structure according to any one of claims 1-5, comprising the following steps: S1: hot metal pretreatment: deep desulfurization of hot metal by KR method, ensuring that the S content after treatment is ≤0.002%; S2: converter smelting: top and bottom combined blowing converter smelting, argon blowing throughout the tapping process; S3: LF refining: white slag refining of steel liquid in LF furnace, adjusting the steel liquid composition to reach the target value; S4: continuous casting: full-process protective casting during continuous casting, the target superheat of the molten steel in the tundish is ≤25℃, and the continuous casting enters a slow cooling pit for treatment after continuous casting; S5: controlled rolling and controlled cooling: heating the continuous casting billet and performing controlled rolling and controlled cooling on the continuous casting billet.

7. The preparation method according to claim 6, characterized in that, In step S2, the converter smelting temperature is ≥1600℃, and the tapping oxygen content is ≥500ppm.

8. The preparation method according to claim 7, characterized in that, In step S5, the heating temperature of the continuous casting billet is 1100-1150℃, and the heating time is 200-250min.

9. The preparation method according to claim 8, characterized in that, In step S5, the controlled rolling is two-stage rolling, the first stage is rough rolling, and the second stage is finish rolling, the rough rolling opening rolling temperature is 1050-1100℃, the rough rolling final rolling temperature is 950-980℃, and the rough rolling is divided into three passes, the first pass deformation is 15%-17%, the second pass deformation is 23%-25%, and the third pass deformation is 20%-22%.

10. The preparation method according to claim 8, characterized in that, The finish rolling opening rolling temperature is 860-930℃, and the final rolling temperature is 750-850℃; The finish rolling is divided into three passes, the first pass deformation is 19%-21%, the second pass deformation is 24%-26%, and the third pass deformation is 33%-34%.

Images