A method for improving the thickness direction microstructure and properties of a steel for offshore engineering

Abstract

This invention discloses a method for improving the microstructure and properties of steel used in marine engineering, belonging to the field of steel material hot working and microstructure control technology. This method constructs a synergistic process system of "deep-penetration high-reduction rough rolling—three-stage gradient finish rolling—dual-zone red-heat steady-state temperature control—regional strengthening quenching—low-temperature self-tempering homogenization," ensuring consistency in temperature, deformation, and phase transformation processes between the surface and core of thick-gauge marine engineering steel. Ultra-low speed high-reduction rolling achieves sufficient core deformation, and dual-zone red-heat and regional quenching technologies increase the core cooling rate by 60–100%, followed by short-time self-tempering to form an ultrafine ferrite-bainite composite microstructure. 60–100 mm thick marine engineering steel treated by this method exhibits a core impact value exceeding 80 J at -40°C and maintains 50–65 J at -60°C, with a 55–62% improvement in seawater corrosion resistance, significantly enhancing the consistency of properties in the thickness direction and reliability in extreme environments.

Description

Technical Field

[0001] This invention belongs to the field of microstructure regulation and controlled rolling and cooling technology of steel materials, and particularly relates to a method for improving the microstructure and properties of steel in the thickness direction for marine engineering. Background Technology

[0002] As marine engineering equipment develops towards larger scale, deeper water, and polar applications, the uneven microstructure in the thickness direction of thick-gauge marine engineering steel plates (≥40mm) has become a significant bottleneck restricting their performance. In existing processes, as the steel plate thickness increases, rough rolling deformation struggles to penetrate to the core, resulting in significantly coarser austenite grains (up to 24–30 μm) and a markedly lower dislocation density. During quenching, the cooling rate cannot penetrate the thick section, leading to the formation of larger and more numerous brittle carbides. These factors result in a typical "hard surface, soft core; strong surface, weak core" gradient microstructure, making the core's strength, hardness, and corrosion resistance insufficient for the marine environment. Simultaneously, the microstructure difference between the surface and core significantly reduces the crack resistance and impact resistance of marine engineering steel in low-temperature environments ranging from -20℃ to -60℃. Although studies have proposed methods such as reducing rolling temperature and increasing cooling capacity, overall uniformity of the microstructure in the thickness direction remains elusive. Therefore, a novel synergistic process that can precisely control the microstructure and properties in the thickness direction is urgently needed. Summary of the Invention

[0003] The purpose of this invention is to provide a method for improving the microstructure and properties of steel used in marine engineering along its thickness. The core idea is to solve the microstructure gradient problem from the surface to the core of thick marine engineering plates by constructing five control modules: "deep-penetration large-deformation rough rolling—three-stage gradient finish rolling—double-layer differential temperature warming—regional hardening strengthening—low-temperature self-tempering homogenization." Ultra-low speed, high-pressure rough rolling ensures sufficient deformation energy storage in the steel plate core; dynamic recrystallization control through multi-stage finish rolling yields a uniform and refined austenite parent phase; differential temperature warming eliminates temperature and stress inhomogeneity in the thick plate section; non-uniform water pressure "core-strengthened cooling" technology is used during quenching to increase the cooling rate of the thick section steel plate center by 60-100%, achieving consistent hardening penetration between the surface and core; finally, short-time low-temperature self-tempering homogenizes the carbide size distribution. This method, without altering the chemical composition of the steel plate, makes the grain refinement, dislocation density, carbide morphology and distribution in the thickness direction of marine steel more uniform, thereby significantly improving the comprehensive performance of the steel plate under harsh marine corrosion environment and low-temperature impact load.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for improving the microstructure and properties of steel used in marine engineering along the thickness direction, comprising the following steps:

[0006] (1) Homogenization heating: The continuous casting billet with a thickness of 40-120 mm is heated at 1180-1220℃ for 2.5-3.5 hours to completely austenitize the segregated area and eliminate the temperature gradient;

[0007] (2) Deep penetration rough rolling: The rolling strategy of "high-speed bite in front and ultra-low speed deep deformation in the back" is adopted. The rolling speed of the first two passes is 1.2 to 1.5 m / s, and the speed of the subsequent 3 to 5 passes is reduced to 0.25 to 0.45 m / s. The cumulative reduction rate of the last three passes is 60 to 70%, so that the deformation energy penetrates into the core and forms a high-density dislocation and deformation zone.

[0008] (3) Three-stage gradient finishing rolling: The finishing rolling stage adopts a multi-stage speed reduction of 0.9→0.6→0.3 m / s, and controls the final rolling temperature to 800~820℃, which is in the austenite incomplete recrystallization zone, so that dynamic recrystallization (DRX) and deformation-induced ferrite phase transformation (DIFT) are carried out simultaneously.

[0009] (4) Dual-zone red-heat control: After rolling, the steel plate passes through two red-heat zones in sequence. The temperature of the first red-heat zone is 720-740℃ and the time is 10-30 s, which is used to stabilize the dynamic recrystallized grains; the temperature of the second red-heat zone is 600-620℃ and the time is 15-60 s, which is used to promote the refinement and nucleation of ferrite in the core.

[0010] (5) Zoned strengthening quenching: The surface is cooled by conventional cooling and the core is cooled by water pressure. The surface cooling rate is 20-30℃ / s, the core cooling rate is 35-45℃ / s, and the cooling equalization time is 10-20 s, so that the surface and the core undergo phase transformation simultaneously.

[0011] (6) Low-temperature self-tempering homogenization: Hold at 260-320℃ for 5-15 min for a short time to release residual stress, disperse carbides and homogenize the structure;

[0012] The chemical composition of the marine engineering steel by mass percentage is as follows: C 0.08–0.12%, Si 0.20–0.30%, Mn 1.20–1.40%, Cr 0.40–0.50%, Ni 0.30–0.40%, Mo 0.15–0.25%, Cu 0.20–0.30%, Nb 0.020–0.040%, V 0.030–0.060%, Ti 0.010–0.020%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the heating process employs a zoned temperature control strategy to keep the temperature difference between the surface and core of the billet within ±10℃, in order to prevent center segregation and surface overheating.

[0014] Furthermore, the total reduction in the last three passes of rough rolling accounts for 50-60% of the total reduction in the entire process, in order to form fine austenite deformation nuclei in the core and increase the density of potential nucleation sites.

[0015] Furthermore, the final rolling temperature is controlled within the region where austenite is not fully recrystallized, by controlling the strain rate to 0.5–1.0 s⁻¹. -1 This refines the grain size to 35-50% while retaining some deformation energy for subsequent phase transformation.

[0016] Furthermore, the temperature difference between the two incandescent zones is controlled at 100–140°C, and the incandescent time is dynamically adjusted according to the plate thickness to achieve temperature consistency and microstructure stabilization in the thickness direction.

[0017] Furthermore, the partitioned enhanced cooling system features an independent water pressure channel in the core, with a spray pressure 1.5 to 2.0 times that of a conventional water curtain, keeping the cooling rate difference in the thickness direction within 10℃ / s.

[0018] Furthermore, the low-temperature self-tempering stage employs multi-segment gas protection or air circulation, which reduces the residual stress in the core by 40-60% after tempering and controls the carbide size to 50-120 nm.

[0019] Furthermore, after treatment with this method, the grain size of the steel plate reaches ASTM 11-12 grade across the entire thickness range, and the difference between the surface and the core is ≤0.5 grade.

[0020] Furthermore, after treatment with this method, the steel plate absorbs ≥80 J of core impact energy at −40℃ and still maintains ≥50 J at −60℃, with a surface-core strength difference ≤2%.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0022] (1) Significantly improve the consistency of microstructure in the thickness direction: Through the synergistic process of "ultra-low speed deep penetration rough rolling + three-stage speed reduction fine rolling + double red zone temperature control", the cumulative deformation, temperature history and dynamic recrystallization process of the steel plate from the surface to the core are highly consistent, and the core grain size is improved from ASTM 7-9 grade of traditional process to ASTM 11-12 grade, realizing one-time overall refinement of the microstructure of marine engineering thick plate;

[0023] (2) Completely solve the problem of insufficient core hardening in thick plates: The partitioned quenching technology of "conventional surface quenching + core enhanced water pressure cooling" is adopted, which increases the core cooling rate by 60-100%, effectively avoiding the generation of inferior structures such as tempered sorbite and coarse ferrite. The core hardness and strength are almost the same as the surface layer, and the difference can be controlled within 2%, which is far superior to the existing technology;

[0024] (3) The method of the present invention enables 60-100 mm thick steel plates to still have excellent toughness under -40℃ and -60℃ conditions, and the core impact energy can reach more than 80 J and 50-65 J respectively, which is 2 to 4 times higher than the comparative example, and can meet the service requirements of harsh environments such as polar oil and gas platforms and deep-sea wind power foundations.

[0025] (4) Significantly enhances resistance to seawater corrosion and chloride ion attack: By refining grains, uniformly precipitating carbides and eliminating the thickness gradient, the seawater corrosion rate is reduced by 55-62%, effectively extending the service life of marine engineering structural components, reducing maintenance costs, and having significant economic benefits. Detailed Implementation

[0026] The specific steps and key control points of a method for improving the microstructure and properties of steel in the thickness direction for marine engineering are as follows:

[0027] Heating and homogenization of continuously cast billets: Place the continuously cast billets (1200-2200 mm wide, 250-320 mm thick) into a walking beam or box furnace and heat to 1180-1220℃, holding for 2.5-3.5 hours (longer holding times are preferred for thicker billets). The purpose is to eliminate chemical segregation and release internal stress, ensuring uniform austenitization. Multi-point infrared or thermocouple online temperature measurement is used, and furnace temperature fluctuations are controlled within ±8-10℃. The heating rate should be controlled to avoid surface overheating (surface temperature should not exceed 1230℃).

[0028] Roughing – Deep Penetration and Large Reduction Deformation: The roughing mill is operated at two speeds: the first 1-2 passes at 1.2-1.5 m / s (to ensure stable bite); the subsequent 3-5 passes at 0.25-0.45 m / s to achieve deep penetration deformation. The total reduction rate in the last three passes is controlled at 60-70% (for thicknesses of 40-120 mm), achieved by increasing the reduction per pass or reducing the number of passes. Purpose: To ensure that the rolling force effectively penetrates to the core, generating a high-density deformation band and storing deformation energy. The exit temperature of the roughing mill is maintained at ≥950℃, and the temperature profile is recorded online to ensure that the core does not cool prematurely and enter the recrystallization or tempering zone.

[0029] Finishing Rolling – Gradient Dynamic Recrystallization Control: The finishing rolling process employs a three-stage speed reduction: 0.9 → 0.6 → 0.3 m / s (adjustable to the unit's capacity). The final rolling temperature is controlled at 800-820℃ (located in the incomplete austenite recrystallization zone or critical zone) to achieve synergy between dynamic recrystallization (DRX) and deformation-induced phase transformation (DIFT). Online strain rate monitoring (encoder / tension sensor) is implemented for each pass to ensure a smooth strain rate gradient and avoid localized overheating or undercooling. Differential rolling can be used if necessary: ​​the surface temperature is slightly lower by 10-30℃ to enhance surface-to-core temperature difference control.

[0030] Dual-zone reddening (segmented heat preservation): Two reddening zones are set after finishing rolling: Reddening I (720-740℃) for a short time (10-30 s) to stabilize dynamically recrystallized grains and release high-density dislocations; Reddening II (600-620℃) for a short time (15-60 s) to promote ferrite nucleation and inhibit coarsening bainite / carbide growth. The reddening zones, in conjunction with inter-roll insulation covers and online temperature control, allow for controllable dwell time at various points along the thickness direction. The temperature and speed of the reddening zones can be controlled in a closed loop via PLC and thermocouples.

[0031] Quenching – Surface / Core Zone Cooling and Core Enhanced Cooling Technology: A composite quenching strategy is adopted: Conventional spraying (or water curtain) on the outer layer achieves rapid surface cooling (cooling rate 20-30℃ / s), while a high-pressure water curtain or water jet "enhanced zone" is set up in the post-rolling quenching section for the core. By adjusting the water pressure and nozzle distribution, the core cooling rate is increased to ≥35℃ / s (for 40-80 mm sections) or ≥30℃ / s (for thicker sections). The cooling medium pressure, flow rate, spray distance, and nozzle diameter need to be calibrated in the process. The quenching goal is to achieve consistent quenching penetration in both the surface and core, and to suppress the formation of coarse carbides in the core. A quenching water recovery and temperature monitoring system should be activated simultaneously.

[0032] Short-time low-temperature self-tempering and homogenization treatment: After quenching, short-time self-tempering (5-15 min) is performed in the range of 260-320℃. The time-temperature window is optimized according to the thickness and alloy element content to promote the refinement and dispersed precipitation of carbides, reduce martensite brittleness, and adjust dislocation density. The self-tempering stage is held at a short time with nitrogen or a protective atmosphere to ensure minimal surface oxidation.

[0033] The following is a detailed description of a method for improving the microstructure and properties of steel for marine engineering in the thickness direction according to the present invention.

[0034] The overall process flow of this invention is built around the core objective of "uniform microstructure in the thickness direction." Through systematic and continuous control of the entire process of heating, rough rolling, finish rolling, red-heating, quenching, and self-tempering of thick plates, the surface and core maintain similar temperature trajectories, phase transformation rates, and deformation energy distribution throughout the entire hot working and phase transformation process, thereby achieving microstructure homogenization and performance consistency. Specifically, this invention first addresses the issue at the initial stage of hot working by improving heating uniformity and appropriately extending the holding time, significantly reducing the temperature gradient inside the thick-section continuous casting billet and decreasing the degree of segregation in the central area, laying the foundation for subsequent deep penetration deformation. During rough rolling, a "high-speed bite-ultra-low-speed deep penetration deformation" strategy is adopted, ensuring that deformation energy is truly transferred to half the thickness or even the core of the steel plate, increasing the deformation energy storage and dislocation density in the central region, and forming numerous refined nuclei in the austenite. One of the key measures of this invention is to use a high reduction rate of ≥60% in the last three passes, so that the austenite in the core of the thick plate can obtain a cumulative strain similar to that of the surface layer. This measure is the fundamental way to solve the difference in microstructure in the thickness direction.

[0035] Following the finishing rolling stage, this invention employs a gradient speed reduction mode of 0.9→0.6→0.3 m / s to allow austenite to enter a critical range across the entire thickness direction where dynamic recrystallization can occur while retaining a certain amount of deformation energy. This ensures that the parent phase austenite grains are refined and stabilized in a highly consistent manner, avoiding the excessive recrystallization of the surface layer and insufficient recrystallization of the core that occurs in traditional processes. Furthermore, this invention introduces a dual red-heat treatment system to allow the steel plate to quickly return to a temperature equilibrium state after rolling. Red-heat zone I (720–740℃) is mainly used to stabilize the fine grains generated by dynamic recrystallization, while red-heat zone II (600–620℃) promotes the early nucleation of ferrite in the core, resulting in a more natural and thorough microstructure transition and significantly reducing the tendency for coarsening of the microstructure in the core of thick plates.

[0036] In the phase transformation control stage, the significant innovation of this invention compared to existing processes lies in its adoption of a dual-zone cooling system of "conventional surface quenching + enhanced core cooling." High-pressure water curtains, water jets, and controllable spray arrays are used to apply higher cooling intensity to the core region, increasing the core cooling rate by 60-100%. This promotes the complete phase transformation of austenite in the core towards martensite / bainite, completely avoiding defects commonly found in traditional thick steel plates, such as "core tempered sorbite" or "localized ferrite network structure in the core." This measure is a key technology for achieving consistent hardenability across high thickness.

[0037] Ultimately, short-duration low-temperature self-tempering stabilizes the martensite / bainite microstructure formed after rapid phase transformation, while simultaneously promoting the dispersed precipitation of fine carbides, further simulating the similarity in hardness, dislocation density, and carbide size distribution between the surface and core layers. Throughout the process, this invention establishes a quantitative correlation between temperature, reduction rate, strain rate, cooling rate, and microstructure evolution. Through synergistic process control, the differences in microstructure along the thickness direction are significantly reduced, ultimately achieving high consistency between the surface and core layers in terms of grain size, strength, hardness, impact toughness, and corrosion resistance. This overall process route is not only applicable to ultra-thick steel plates of 40–120 mm for marine engineering, but also to the production of thick-gauge high-performance steels such as wind turbine towers, high-strength bridge steel, and ultra-low temperature storage tank steel, demonstrating significant versatility and promotional value.

[0038] Example 1:

[0039] Marine engineering steel plates with a thickness of 80 mm were selected, with the following chemical composition: C 0.10%, Si 0.25%, Mn 1.35%, Cr 0.45%, Ni 0.35%, Mo 0.20%, Cu 0.25%, Nb 0.030%, V 0.040%, Ti 0.015%, and the balance being Fe. This embodiment follows the method of the present invention for full-process control: the continuously cast billet is held at 1200℃ for 3 hours to ensure complete homogenization of austenite; the rough rolling adopts a "high-speed bite-ultra-low-speed deep penetration deformation" strategy, with the rolling speed of 1.3 m / s in the first two passes and decreasing to 0.32 m / s thereafter, and achieving 65% cumulative reduction in the last three passes to effectively transfer the deformation energy to the core of the steel plate; the finish rolling adopts a three-stage speed reduction rolling (0.9→0.6→0.3 m / s), with the final rolling temperature stabilized at 815℃ to ensure that dynamic recrystallization occurs consistently in the thickness direction; after rolling, it enters the double red zone of 730℃ and 610℃ in sequence to stabilize the recrystallized grains and promote the nucleation of fine ferrite in the core; then, through a partitioned quenching method of "conventional surface cooling + enhanced core water pressure cooling", the surface cooling rate is about 23℃ / s and the core cooling rate reaches 40℃ / s; finally, it is self-tempered at 290℃ for 10 minutes. After this process, the grain size of the steel plate surface and core reaches ASTM 12.0 and 11.5-12.0 respectively. The impact value of the core at −40℃ is increased to 80 J, and the impact value at −60℃ is still 50 J. At the same time, the difference in tensile strength in the thickness direction of the steel plate is less than 2%, the seawater corrosion rate is reduced by 55%, and the overall microstructure and performance are extremely superior.

[0040] Example 2:

[0041] A 100 mm thick marine steel plate for ultra-low temperature service was selected. With the chemical composition consistent with Example 1, low-temperature toughness was further optimized through process strengthening. The continuously cast billet was heated to 1190℃ and held for 3.2 h. The cumulative reduction rate of the last three passes of rough rolling was increased to 68%, and a lower subsequent rolling speed was used to further enhance the core deformation penetration effect. The finishing rolling temperature was controlled at 805℃, which is in the incomplete recrystallization zone of austenite, conducive to retaining an appropriate amount of deformation energy in the core. The dual red-heat zones were maintained at 725℃ and 605℃ respectively, and the red-heat I residence time was appropriately extended to fully stabilize the fine-grained austenite. During the quenching stage, the water pressure in the core strengthening zone was increased by about 20%, so that the core cooling rate reached 45℃ / s, thereby forming a finer bainite / martensite composite structure. The self-tempering temperature was slightly increased to 300℃ to improve ultra-low temperature impact toughness. Ultimately, the core grain size of the steel plate in this embodiment reaches ASTM 11–12 grade, the impact value at −60℃ can reach 65J, the difference between yield strength and tensile strength is minimal (yield-to-tensile ratio is less than 0.80), and the resistance to chloride ion stress corrosion life is increased by 60% compared with the comparative example, making it suitable for polar oil and gas platforms and low-temperature marine environments.

[0042] Example 3:

[0043] For 60 mm thick high-strength, corrosion-resistant marine engineering steel, the chemical composition was appropriately adjusted, increasing the Mo content to 0.25% and the Cu content to 0.30%. The angle and coverage area of ​​the quenching water curtain nozzles were optimized, improving the core strengthening cooling capacity by approximately 30%. The continuously cast billet was held at 1180℃ for 2.8 h before rolling. The finishing rolling temperature was controlled at 810℃, and the red-heat dwell time was shortened to promote the formation of finer ferrite and bainite in the initial stage of phase transformation. During quenching, the core cooling rate reached 38℃ / s, while the surface cooling rate was maintained at around 25℃ / s. The self-tempering temperature was set at 285℃ for 8 min. The final test plate showed a surface-to-core hardness difference of less than 5 HV, a tensile strength of 950–980 MPa, a 62% reduction in seawater immersion corrosion rate, and a 35% improvement in low-cycle fatigue life compared to the control group. This design is suitable for components such as the main body of offshore wind turbine towers and pile leg node plates that are exposed to chlorinated seawater environments for extended periods.

[0044] Comparative Example 1:

[0045] The conventional two-stage rolling process and single-zone quenching method commonly used by enterprises were adopted. After the continuously cast billet was heated to 1180℃, the rough rolling was carried out at a medium-high speed of 1.5–2.0 m / s throughout, and the cumulative reduction rate of the last three passes was only about 40%, resulting in almost no deformation penetration in the core of the steel plate. The finishing rolling speed changed little, and dynamic recrystallization mainly occurred in the surface layer, while the core still maintained coarse austenite. After rolling, a single red-hot tempering was carried out only at around 700℃, which did not have the effect of ferrite refinement. Quenching was carried out using a common wide-angle water curtain, and the core cooling rate was only about 12℃ / s, which made it difficult to achieve complete quenching. The final core microstructure of the steel plate was coarse ferrite + tempered sorbite, with a grain size of 22–28 μm, an impact value of 25 J at −40℃, a surface-core strength difference of more than 10%, and corrosion resistance was also significantly lower than that of the embodiment of this invention.

[0046] Comparative Example 2:

[0047] The only change was the quenching method, without employing core-strengthening cooling technology. Although the initial heating and rolling were essentially the same as in the previous example, the quenching method was merely ordinary spray cooling, resulting in insufficient core cooling rate (approximately 15°C / s). The surface layer transformed into fine acicular bainite, while the core formed incompletely quenched tempered sorbite and island-like ferrite, leading to significant differences in strength along the thickness direction. The final surface tensile strength was approximately 930 MPa, while the core only reached 820 MPa, a difference exceeding 100 MPa. Furthermore, the corrosion rate increased by 30% compared to the previous example, and the microstructure was severely uneven, failing to meet the service conditions for marine engineering.

[0048] Comparative Example 3:

[0049] The rolling process remained consistent with Example 1, but only one temperature zone was set during the red-heat stage, with the red-heat temperature fixed at 680℃ and the time short. This resulted in insufficient dynamic recrystallization in the core and weak ferrite nucleation ability. During quenching, due to the uneven microstructure, the phase transformation in the core was delayed, forming coarse bainite and chain carbides, significantly reducing low-temperature toughness. The final steel plate had an impact value of less than 20 J at -60℃, and the core hardness was more than 20 HV lower than the surface layer, showing a clear performance gradient and failing to meet the requirements for high-grade marine engineering steel.

[0050] Table 1. Comparison of chemical composition between the examples and comparative examples (mass percentage, %)

[0051] Example C Si Mn Cr Ni Mo Cu Nb V Ti Remark Example 1 0.10 0.25 1.35 0.45 0.35 0.20 0.25 0.030 0.040 0.015 Standard marine steel composition Example 2 0.10 0.25 1.35 0.45 0.35 0.20 0.25 0.030 0.040 0.015 Same as Example 1, but with process optimization. Example 3 0.10 0.25 1.35 0.45 0.35 0.25 0.30 0.030 0.040 0.015 The Mo and Cu contents are slightly increased to enhance corrosion resistance. Comparative Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same ingredients, different processes Comparative Example 2 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 The ingredients are the same, only the quenching process is different. Comparative Example 3 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same ingredients, only different redness

[0052] Table 2 Comparison of process parameters between the examples and comparative examples

[0053] Example Heating temperature / ℃ Insulation time / h Roughing speed / m·s⁻¹ Last three pressure reduction rate / % Finishing rolling speed reduction / m·s⁻¹ Final rolling temperature / ℃ Redness I / ℃ Redness II / ℃ Cardiac cooling rate / °C·s⁻¹ Self-tempering temperature / °C Feature Description Example 1 1200 3.0 1.3→0.32 65 0.9→0.6→0.3 815 730 610 40 290 Standard process route Example 2 1190 3.2 1.2→0.30 68 0.9→0.6→0.3 805 725 605 45 300 Enhanced core cooling for higher low-temperature resilience Example 3 1180 2.8 1.3→0.35 60 0.8→0.5→0.3 810 720 600 38 285 Improved Cu and Mo content results in superior corrosion resistance. Comparative Example 1 1180 2.5 1.5–2.0 40 0.9–1.2 820 Single zone 700 — 12 — Non-penetration rolling and double red-return Comparative Example 2 Comparative Example 3

[0054] Table 3 Comparison of tissue properties between the examples and comparative examples

[0055] Example Grain size (ASTM) Impact energy (J) at -40℃ Impact energy (J) at -60℃ Difference in tensile strength between surface and core / % Surface / core hardness difference (HV) Seawater corrosion rate decreases by % Organizational characteristics Example 1 11.5–12.0 150 (table) / 80 (heart) 95 (table) / 50 (heart) ≤2 ≤5 55 Ultrafine ferrite + granular bainite Example 2 11–12 160 / 85 100 / 65 ≤3 ≤6 60 Refine the bainitic complex structure, with the core fine grains fully hardened. Example 3 11.5–12.0 155 / 82 95 / 55 ≤2 ≤5 62 Fine bainite + micro-fine carbides, Cu-Mo enhances corrosion resistance Comparative Example 1 7–9 90 / 25 60 / <20 ≥10 ≥20 0 The core contains ferrite and sorbite, with severely coarse grains. Comparative Example 2 8–9 100 / 35 65 / 25 ≥8 ≥15 -30 (accelerated corrosion) The core was not fully quenched, resulting in a large difference in strength. Comparative Example 3 8–10 95 / 30 60 / <20 ≥9 ≥18 10 Chain-like carbide precipitation, low toughness

[0056] As can be seen from the embodiments and comparative examples, the present invention has the following advantages:

[0057] (1) In the embodiment, by combining "ultra-low speed high reduction rough rolling" with "three-stage speed reduction fine rolling", deformation penetrates to the core of the steel plate, and the core grains are refined to ASTM grade 11-12, while the comparative example is only grade 7-9. The difference between the surface layer and the core structure is significantly reduced, and the consistency of properties in the thickness direction is significantly improved.

[0058] (2) The present invention adopts the "partition quenching + core strengthening water pressure cooling" technology, which increases the core cooling rate to 35-45℃ / s, and achieves complete quenching of the core and the surface. In the comparative example, ordinary quenching leads to the formation of a mixed structure of sorbite and ferrite in the core, while the present invention obtains a fine bainite / ferrite composite structure, and the strength gradient is basically eliminated.

[0059] (3) The impact value of the core of the embodiment reaches more than 80 J at -40°C and remains at 50-65 J at -60°C, while the comparative examples are generally below 25 J. It can be seen that the present invention still has high energy absorption and crack resistance under extreme low temperature conditions, which is significantly better than conventional marine engineering steel.

[0060] (4) After the steel plate in the example was refined and homogenized through multiple layers, the corrosion rate was reduced by 55-62% compared with ordinary marine steel, which effectively suppressed local pitting corrosion and stress corrosion cracking induced by chloride ions; the comparative example still had corrosion-sensitive structures, and the difference in corrosion resistance was obvious.

[0061] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for improving the microstructure and properties of steel in the thickness direction for marine engineering, characterized in that, Includes the following steps: (1) Homogenization heating: The continuous casting billet with a thickness of 40-120 mm is heated at 1180-1220℃ for 2.5-3.5 hours to completely austenitize the segregated area and eliminate the temperature gradient; (2) Deep penetration rough rolling: The rolling strategy of "high-speed bite in front and ultra-low speed deep deformation in the back" is adopted. The rolling speed of the first two passes is 1.2 to 1.5 m / s, and the speed of the subsequent 3 to 5 passes is reduced to 0.25 to 0.45 m / s. The cumulative reduction rate of the last three passes is 60 to 70%, so that the deformation energy penetrates into the core and forms a high-density dislocation and deformation zone. (3) Three-stage gradient finishing rolling: The finishing rolling stage adopts a multi-stage speed reduction of 0.9→0.6→0.3 m / s, and controls the final rolling temperature to 800~820℃, which is in the austenite incomplete recrystallization zone, so that dynamic recrystallization and deformation-induced ferrite phase transformation are carried out simultaneously; (4) Dual-zone red-heat control: After rolling, the steel plate passes through two red-heat zones in sequence. The temperature of the first red-heat zone is 720-740℃ and the time is 10-30 s, which is used to stabilize the dynamic recrystallized grains; the temperature of the second red-heat zone is 600-620℃ and the time is 15-60 s, which is used to promote the refinement and nucleation of ferrite in the core. (5) Zoned strengthening quenching: The surface is cooled by conventional cooling and the core is cooled by water pressure. The surface cooling rate is 20-30℃ / s, the core cooling rate is 35-45℃ / s, and the cooling equalization time is 10-20 s, so that the surface and the core undergo phase transformation simultaneously. (6) Low-temperature self-tempering homogenization: Hold at 260-320℃ for 5-15 min for a short time to release residual stress, disperse carbides and homogenize the structure; The chemical composition of the marine engineering steel by mass percentage is as follows: C 0.08–0.12%, Si 0.20–0.30%, Mn 1.20–1.40%, Cr 0.40–0.50%, Ni 0.30–0.40%, Mo 0.15–0.25%, Cu 0.20–0.30%, Nb 0.020–0.040%, V 0.030–0.060%, Ti 0.010–0.020%, with the balance being Fe and unavoidable impurities.

2. The method for improving the microstructure and properties of marine engineering steel in the thickness direction according to claim 1, characterized in that, The heating process employs a zoned temperature control strategy to keep the temperature difference between the surface and core of the billet within ±10℃, thereby preventing center segregation and surface overheating.

3. The method for improving the microstructure and properties of steel for marine engineering according to claim 1, characterized in that, The total reduction in the last three passes of rough rolling accounts for 50-60% of the total reduction in the entire process, in order to form fine austenite deformation nuclei in the core and increase the density of potential nucleation sites.

4. The method for improving the microstructure and properties of steel for marine engineering according to claim 1, characterized in that, The final rolling temperature is controlled within the region where austenite is not fully recrystallized, by controlling the strain rate from 0.5 to 1.0 s⁻¹. -1 This refines the grain size to 35-50% while retaining some deformation energy for subsequent phase transformation.

5. The method for improving the microstructure and properties of marine engineering steel in the thickness direction according to claim 1, characterized in that, The temperature difference between the two incandescent zones is controlled at 100-140℃, and the incandescent time is dynamically adjusted according to the plate thickness to achieve temperature consistency and microstructure stability in the thickness direction.

6. The method for improving the microstructure and properties of steel for marine engineering according to claim 1, characterized in that, The partitioned enhanced cooling system features an independent water pressure channel in the core, with a spray pressure 1.5 to 2.0 times that of a conventional water curtain, keeping the thickness-direction cooling rate difference within 10℃ / s.

7. The method for improving the microstructure and properties of marine engineering steel in the thickness direction according to claim 1, characterized in that, During the low-temperature self-tempering stage, multi-segment gas protection or air circulation is adopted. After tempering, the residual stress in the core is reduced by 40-60%, and the carbide size is controlled at 50-120 nm.

8. The method for improving the microstructure and properties of steel for marine engineering according to claim 1, characterized in that, After treatment by this method, the grain size of the steel plate reaches ASTM 11-12 grade across the entire thickness range, and the difference between the surface and the core is ≤0.5 grade.

9. The method for improving the microstructure and properties of marine engineering steel in the thickness direction according to claim 1, characterized in that, After treatment by this method, the steel plate absorbs ≥80 J of core impact energy at −40℃ and still maintains ≥50 J at −60℃, with a surface-core strength difference ≤2%.