A method by controlling t 8 / 5 Welding process that improves the bainitic structure in the heat-affected zone within a time window

Abstract

This invention discloses a method for controlling t 8 / 5 This invention relates to the field of welding metallurgical technology for marine engineering steel, specifically providing a method for controlling the welding process of 355MPa grade high-strength marine engineering structural steel for applications such as offshore platforms, subsea pipelines, and port machinery. This method improves the welding process by optimizing the time window to improve the bainitic microstructure in the heat-affected zone. t 8 / 5 This invention establishes a welding process based on the thickness of the steel plate to be welded, which improves the bainitic microstructure of the heat-affected zone through a time window. t 8 / 5 A time-graded precision control model is used to match the welding heat input range corresponding to the plate thickness. This model simultaneously constructs a comprehensive process control system encompassing preheating, interpass temperature management, and post-weld slow cooling, and is deeply coupled with a composite microalloying composition system for Nb-V-Ti toughening and Cr-Ni-Cu corrosion resistance. Through this invention, the coarse-grained zone of the weld heat-affected zone can be transformed into a homogeneous structure dominated by refined lath bainite or granular bainite, effectively avoiding the formation of martensitic hardened structures and coarse ferrite structures. This results in excellent strength-toughness matching, adaptability to on-site construction, and industrial operability.

Description

Technical Field

[0001] This invention belongs to the field of steel welding metallurgy technology for marine engineering, and particularly relates to a method for controlling... t 8 / 5 The time window improves the welding process for bainitic microstructure in the heat-affected zone. Background Technology

[0002] Marine engineering equipment (such as deep-sea drilling platforms and oil and gas pipelines) operates for extended periods in extreme environments characterized by low temperatures, high salt spray, and complex alternating loads, placing near-stringent demands on the quality of its critical load-bearing structure—the welded joint. The weld heat-affected zone, especially the coarse-grained region experiencing peak temperatures exceeding 1100℃, is the most microstructure and property-unstable area within the welded joint and the origin of ductile failure. Therefore, controlling the microstructure and properties of the weld heat-affected zone is crucial for improving the safety and service life of the entire welded structure.

[0003] t 8 / 5 Time, specifically the time it takes for the heat-affected zone to cool from 800℃ to 500℃ during welding, is a core parameter characterizing the welding cooling rate. This temperature range precisely corresponds to the critical phase transformation range in low-carbon microalloyed steel, where austenite transforms into ferrite, bainite, and martensite, directly determining the final microstructure and mechanical properties of the heat-affected zone. For low-carbon microalloyed steel used in marine engineering, an ideal heat-affected zone microstructure possesses high strength, high and low temperature toughness, and good crack resistance; while when... t 8 / 5 When the cooling time is too short (i.e., the cooling rate is too fast), the supercooled austenite is prone to undergo martensitic transformation, forming a hard and brittle martensitic structure, leading to a sharp increase in joint hardness and a significant increase in sensitivity to cold cracking; when t 8 / 5 When the cooling time is too long (i.e., the cooling rate is too slow), austenite is prone to undergo high-temperature phase transformation, forming coarse proeutectoid ferrite and pearlite structures, which leads to a simultaneous decrease in joint strength and toughness, making it impossible to meet the service requirements of marine engineering structures.

[0004] Chinese patent CN116536579A discloses a high-toughness, easily weldable steel for wind power and its preparation method. Its core focus is on the design of the steel's chemical composition, mentioning only the different heat inputs in the embodiments. t 8 / 5 Time, but no refined system has been established for marine engineering steel of different plate thicknesses. t 8 / 5 The control window is also not clearly defined. t 8 / 5 The correlation between time and organizational control makes it impossible to achieve precise directional control of the bainitic structure in the heat-affected zone.

[0005] Chinese patent CN121267314A discloses a method for improving the welding performance of low-alloy high-strength wind power steel, proposing to... t 8 / 5 A technical solution to optimize the time to 10-100 seconds, but t 8 / 5 The control scope is too broad, failing to consider the differences in heat dissipation characteristics of steel with different plate thicknesses, and failing to integrate with the steel micro-alloying system for collaborative design. In actual construction, it is very easy to cause organizational loss of control problems. At the same time, the scheme has not been specifically optimized for the low-temperature toughness requirements of steel used in marine engineering, and cannot be adapted to the harsh service conditions of marine engineering. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention proposes a method by controlling t 8 / 5 The time window improves the welding process for bainitic microstructure in the heat-affected zone. This invention aims to improve the welding process by precisely controlling the welding time. t 8 / 5 The cooling time was synergistically optimized with the microalloying composition of the steel and multidimensional process parameters to solve the problems in welding existing marine engineering steels. t 8 / 5 To address the problems of uncontrolled microstructure in the heat-affected zone, poor bainite refinement, large fluctuations in mechanical properties, insufficient low-temperature toughness, and high sensitivity to cold cracking caused by a wide control range, insufficient precision, and a single process, a [system / mechanism] was established. t 8 / 5 The quantitative correlation between time, bainitic microstructure characteristics and weld mechanical properties provides a set of welding processes with controllable microstructure, stable performance and reproducibility.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for controlling t 8 / 5 The welding process for improving the bainitic microstructure in the heat-affected zone within a time window includes the following steps: The first step is to determine the composition and thickness of the marine engineering steel plate to be welded (the marine engineering steel plate to be welded). h Confirm and clarify the thickness (i.e., plate thickness), chemical composition, and carbon equivalent (CEV) of the steel plate. The second step is to determine the type of steel plate to be welded for marine engineering applications. h Determine the target t 8 / 5 Time window, the t 8 / 5 The time is the time it takes for the heat-affected zone to cool from 800℃ to 500℃ during the welding process; The third step is based on the target. t 8 / 5 Time window, matching the welding method and welding heat input corresponding to the plate thickness (Q ), establish heat input and t 8 / 5 The time correspondence is as follows: preheating is performed before welding heat input; the welding method is selected from submerged arc welding or gas shielded welding. The fourth step is to immediately perform post-weld slow cooling treatment after welding to control the cooling rate and ensure... t 8 / 5 The time falls within the target window; Wherein, the target t 8 / 5 Time window based on h Determined according to the following hierarchical control rules: When h When ≤20mm, t 8 / 5 The time should be controlled between 8 and 25 seconds; when 20mm < h When ≤50mm, t 8 / 5 The time should be controlled between 20 and 45 seconds; when h When >50mm t 8 / 5 The time should be controlled between 35 and 70 seconds; The CEV is ≤0.45, preferably CEV ≤0.40; The chemical composition of the marine engineering steel plate to be welded, by mass percentage, includes 0.015%~0.030% Nb, 0.08%~0.15% V, and 0.015%~0.025% Ti.

[0008] The core technical principle of this invention lies in... t 8 / 5 A time window tiered control system is deeply synergistic with the Nb-V-Ti composite microalloying system. Composition and thickness are measured in the steel plates to be welded for marine engineering, clarifying the plate thickness, mass percentage of each alloying element, calculating the carbon equivalent, and confirming the delivery condition and original mechanical properties of the base material. Based on the continuous cooling transformation (CCT) curve of the experimental steel, combined with two-dimensional welding heat conduction numerical simulation results for different plate thicknesses, the system ensures that the corresponding welding heat conduction is achieved within the specified time window. t 8 / 5 Within the time window, supercooled austenite can completely avoid the martensitic and proeutectoid ferrite transformation regions, stably entering the bainitic transformation region, achieving targeted microstructural control, and according to the target... t 8 / 5 Time window, matching welding method and welding heat input corresponding to plate thickness, establishing welding heat input and... t 8 / 5 The linear correlation over time ensures precise control over heat input adjustments. t 8 / 5The timeframe falls within the target window; the preheating temperature is determined based on the carbon equivalent of the steel, and a contact thermometer is used throughout the preheating process to ensure uniform preheating without localized overheating or underheating. The core function of preheating before welding is to reduce the cooling rate of the weld joint and prolong the welding process. t 8 / 5 Time, while reducing welding stress, avoids cold cracking, and forms a combination with welding heat input. t 8 / 5 The first line of defense in time control: multi-layer, multi-pass welding is performed according to the determined welding heat input. Before each pass, the interpass temperature is measured with a thermometer, and the next pass can only be performed after the temperature meets the standard. The core function of interpass temperature control is to stabilize the welding heat cycle and prevent the interpass temperature from being too low, which would cause the subsequent weld to cool too quickly. t 8 / 5 Shortening the time while preventing excessively high interlayer temperatures from causing continuous coarsening of austenite grains is... t 8 / 5 Time-based process control is a core aspect; immediately after welding, the weld and heat-affected zone are fully covered with an aluminum silicate refractory insulation blanket (i.e., slow post-weld cooling). Slow post-weld cooling is... t 8 / 5 The time control is the last line of defense, effectively preventing martensitic phase transformation caused by rapid cooling of the joint after welding, while promoting sufficient bainitic phase transformation and ensuring microstructure uniformity. Through the synergistic effect of the above technical solutions, this invention can achieve directional control of the microstructure in the coarse-grained zone of the weld heat-affected zone. The final microstructure is dominated by refined lath bainite or granular bainite with a volume fraction of ≥85%, the bainite lath bundle size is stably controlled at 5~15μm, there is no blocky martensite hardened structure, no coarse proeutectoid ferrite structure, and the original austenite grain size is ≤80μm, ensuring the excellent performance of the welded joint from the root of the microstructure.

[0009] Furthermore, the chemical composition of the marine engineering steel plate to be welded, by mass percentage, is as follows: C: 0.03%~0.07%, Si: 0.08%~0.15%, Mn: 1.50%~1.60%, Nb: 0.015%~0.030%, V: 0.08%~0.15%, Ti: 0.015%~0.025%, Cr: 0.15%~0.20%, Ni: 0.12%~0.18%, Cu: 0.20%~0.30%, P≤0.008%, S≤0.006%, with the remainder being Fe and unavoidable impurities.

[0010] The carbon equivalent (CEV) is calculated as follows: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. In the composition system of the marine engineering steel plate of this invention, no Mo element is actively added; therefore, the Mo term is set to 0 during the carbon equivalent calculation, and it will not affect the final carbon equivalent calculation result.

[0011] In the technical solution of this invention, the role of alloying elements is as follows: C is a basic element for improving the strength of steel, but excessive content will significantly reduce toughness and weldability; Mn improves strength through solid solution strengthening, while lowering the phase transformation temperature and refining the microstructure, which is conducive to bainite formation. It can work synergistically with microalloying elements to promote bainite formation during welding cooling; Nb forms nanoscale Nb(C,N) precipitates during welding heating, pinning austenite grain boundaries and effectively preventing austenite grain coarsening at high welding temperatures. Simultaneously, it provides a large number of nucleation sites for bainite phase transformation during cooling. t 8 / 5 Within the window, the size of bainite lath bundles can be refined to 5~15μm; V precipitates fine V(C,N) during welding cooling, producing a significant precipitation strengthening effect, while improving the stability of supercooled austenite and promoting bainite phase transformation. Controlling V at 0.08%~0.15% ensures strength while improving low-temperature toughness; Ti forms high-temperature stable TiN particles, which remain stable even at welding temperatures above 1300℃, continuously pinning austenite grain boundaries and forming a composite microalloying system with Nb and V. This achieves full-process microstructure control, including grain refinement during heating and phase transformation regulation during cooling, along with hierarchical... t 8 / 5 Window control creates synergistic effects; Cr enhances resistance to pitting and crevice corrosion, forming a composite passivation film to resist chloride ion attack; simultaneously, it widens the bainitic phase transformation range, improving the tolerance of the t8 / 5 control window; Ni synergistically enhances the stability of the passivation film, refines the bainitic structure, and improves low-temperature toughness, while controlling Ni / Cu ≥ 0.5 to avoid the risk of Cu hot brittleness. Ni content is controlled at 0.12%~0.18%, balancing synergistic effects, cost, and weldability; Cu is a core corrosion-resistant element, reducing the uniform corrosion rate through a Cu-rich passivation film; simultaneously, precipitation strengthening supplements strength, ensuring the steel's strength grade. Cu content is controlled at 0.20%~0.30%, balancing corrosion resistance and weld crack resistance; P and S are harmful elements that cause steel to become brittle, severely reducing the steel's toughness and plasticity.

[0012] For steel plates used in marine engineering, this invention precisely controls the thickness of the plate. t 8 / 5 By combining the time window with microalloying composition design and carbon equivalent control, the bainitic microstructure in the heat-affected zone can be improved in a targeted manner, which is of great significance for improving the welding quality and service reliability of marine engineering steel.

[0013] This invention provides a plate thickness-based classification t 8 / 5The precise control method of the time window, coupled with the specific Nb-V-Ti microalloying composition design, enables directional control of the morphology, size, distribution, and effective grain size of the weld heat-affected zone. This is a welding process that significantly improves the low-temperature toughness and strength-toughness matching of the weld joint.

[0014] Furthermore, in the second step, when the welding method is submerged arc welding, Q according to h Match according to the following range: when h When ≤20mm, Q Control it to 20~30kJ / cm; When 20mm < h When ≤50mm, Q Control it to 28~40kJ / cm; when h When >50mm Q The concentration should be controlled between 35 and 50 kJ / cm.

[0015] Furthermore, in the third step, when the welding method is gas shielded welding, Q according to h Match according to the following range: when h When ≤20mm, Q Control it to 10~20kJ / cm; When 20mm < h When ≤50mm, Q Control it within 15~28 kJ / cm; when h When >50mm Q The concentration should be controlled between 22 and 35 kJ / cm.

[0016] Furthermore, in the third step, the preheating temperature is determined based on CEV. When CEV ≤ 0.40, the preheating temperature is 80~100℃; when 0.40 < CEV ≤ 0.45, the preheating temperature is 100~150℃.

[0017] Furthermore, in the third step, multi-layer, multi-pass welding is adopted during the welding process, and the interpass temperature is controlled throughout the process. The penetration depth of a single weld is controlled at 3~6mm, and the weld width is controlled at 8~15mm. The interpass temperature is controlled at 100~180℃ throughout the process, and the interpass temperature does not exceed the preheating temperature +50℃. The welding interval between adjacent weld passes does not exceed 2min.

[0018] Furthermore, the post-weld slow cooling treatment involves fully covering the weld and heat-affected zone with an aluminum silicate refractory insulation blanket, covering the weld and an area on both sides at least twice the plate thickness, and slowly cooling to room temperature with a cooling rate controlled at 0.5~2℃ / s.

[0019] The present invention also provides a marine engineering steel welded joint prepared according to the above method, wherein the heat-affected zone of the marine engineering welded joint is mainly composed of refined lath bainite or granular bainite with a volume fraction of ≥85%, the size of the bainite lath bundles is controlled at 5~15μm, and the original austenite grain size is ≤80μm.

[0020] The microstructure of the coarse-grained zone of the welded heat-affected zone of the marine engineering steel welded joint obtained by the welding process of the present invention is mainly composed of refined lath bainite or granular bainite, without blocky martensite or coarse proeutectoid ferrite.

[0021] Furthermore, the welded joint for marine engineering exhibits an impact absorption energy ≥85J in the coarse-grained region of the heat-affected zone at -40℃, and a cold crack sensitivity coefficient ( Pcm ≤0.20%, hardness 170~210HV 10 It has a tensile strength of 420~520MPa, a yield strength of ≥355MPa, and no welding cold cracks.

[0022] The present invention also provides the application of the above-mentioned steel welded joints for marine engineering in marine engineering equipment.

[0023] Compared with the prior art, the present invention has the following advantages and technical effects: (1) This invention addresses different plate thicknesses of steel and will t 8 / 5 Window divided into h ≤20mm: 8~25s; 20mm< h ≤50mm, 20~45s h For thicknesses greater than 50mm, the three precise intervals of 35-70s represent a significant improvement in control precision compared to the broader range of 10-100s in existing technologies. This hierarchical model allows for stable control of the bainite lath bundle size in the heat-affected zone within the ideal range of 5-15μm, while the comparative example shows a microstructure size exceeding 20μm or the presence of martensite, demonstrating the precise control capability of this invention over the microstructure of the heat-affected zone.

[0024] (2) This invention combines Nb-V-Ti composite microalloying design with t 8 / 5 The process combines window control with advanced technology. Nanoscale carbonitrides precipitated during welding cooling pin grain boundaries and promote bainite nucleation. Simultaneously, excellent weldability is ensured through carbon equivalent control (CEV≤0.45), allowing the preheating temperature to be reduced to 80~150℃. The synergistic effect of composition and process enhances impact toughness compared to methods that rely solely on single control. t 8 / 5 The process has been significantly improved. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The continuous cooling transformation (CCT) curve of the marine engineering steel plate used in Example 2 is shown. Figure 2 This is a schematic diagram of the X-shaped butt joint bevel and weld arrangement of the steel plate for marine engineering in Embodiment 2 of the present invention. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] Embodiments of the present invention provide a method for controlling t 8 / 5 The welding process for improving the bainitic microstructure in the heat-affected zone within a time window includes the following steps: The first step is to determine the composition and thickness of the steel plates to be welded for marine engineering. h Confirmation, clarifying the steel plate h Chemical composition (i.e., mass percentage of each alloying element), calculate CEV, and confirm the delivery condition and original mechanical properties of the base material; The second step is to determine the type of steel plate to be welded for marine engineering applications. h Based on the heat dissipation characteristics of steel plates and the dynamics of austenite phase transformation, the target was determined. t 8 / 5 Time window t 8 / 5 The time is the time it takes for the heat-affected zone to cool from 800℃ to 500℃ during the welding process; The third step is based on the target. t 8 / 5 Time window, matching welding method with plate thickness and Q ,Establish Q and t 8 / 5 The time correlation ensures precise control over welding heat input adjustments. t 8 / 5 The timeframe falls within the target window, and the welding method is selected from submerged arc welding (the main welding method for steel plates used in marine engineering) or gas shielded welding (for on-site repair welding and vertical welding conditions). The fourth step is to determine the preheating temperature based on the steel's CEV. When CEV ≤ 0.40, the preheating temperature is 80~100℃; when CEV > 0.40, the preheating temperature is 100~150℃. Preheating is performed using flame preheating or electric heating. A contact thermometer is used to monitor the entire preheating process to ensure uniform preheating temperature and prevent localized overheating or underheating. The core function of preheating before welding is to reduce the cooling rate of the weld joint and prolong the welding time. t 8 / 5 Time, while reducing welding stress, avoids cold cracking, and forms a combination with welding heat input. t 8 / 5 The first line of defense for time regulation; The fifth step involves multi-layer, multi-pass welding according to the determined welding heat input. The penetration depth of a single weld pass is controlled at 3-6 mm, and the weld width at 8-15 mm to avoid excessive grain coarsening caused by high heat input in a single pass. Interpass temperature is strictly controlled throughout the process, maintaining it between 100-180℃ and not exceeding the preheating temperature +50℃. The interval between adjacent weld passes should not exceed 2 minutes. The interpass temperature is measured with a thermometer before each pass, and the next pass can only proceed after the interpass temperature meets the standard. The core function of interpass temperature control is to stabilize the welding heat cycle and prevent excessively low interpass temperatures from causing subsequent weld passes to cool too quickly. t 8 / 5Shortening the time while preventing excessively high interlayer temperatures from causing continuous coarsening of austenite grains is... t 8 / 5 The core aspect of time-based process management; Step 6: Immediately after welding, perform post-weld slow cooling treatment to control the cooling rate and ensure... t 8 / 5 The time falls within the target window; Among them, the target t 8 / 5 Time window based on h Determined according to the following hierarchical control rules: When h For thicknesses ≤20mm, medium-thickness materials dissipate heat quickly. To prevent excessively rapid cooling and the formation of martensite, and to prevent excessively slow cooling and the resulting coarsening of the microstructure, t 8 / 5 The timing should be precisely controlled between 8 and 25 seconds, with 10 to 20 seconds being the optimal range; when 20mm < h For thicknesses ≤50mm, the heat dissipation rate of thicker plates is moderate, which helps to balance microstructure refinement and phase transformation sufficiency. t 8 / 5 The timing should be precisely controlled between 20 and 45 seconds, with 25 to 40 seconds being the optimal range; when h For thicknesses greater than 50mm, extra-thick plates have slow heat dissipation and high restraint. To avoid slow cooling leading to microstructure coarsening and to reduce welding stress, t 8 / 5 The time is precisely controlled within 35~70s, preferably 40~60s. The above-mentioned graded control window settings are based on the continuous cooling transformation (CCT) curve of the test steel, combined with the two-dimensional welding heat conduction numerical simulation results for different plate thicknesses, ensuring that the corresponding... t 8 / 5 Within the time window, supercooled austenite can completely avoid the martensitic and proeutectoid ferrite transformation regions and stably enter the bainitic transformation region, thus achieving directional control of the microstructure. CEV≤0.45, preferably CEV≤0.40; The steel plate to be welded for marine engineering is Nb-V-Ti composite microalloyed low-carbon steel. Its chemical composition, by mass percentage, includes 0.015%~0.030% Nb, 0.08%~0.15% V, and 0.015%~0.025% Ti.

[0032] In a preferred embodiment of the present invention, the chemical composition of the steel plate to be welded for marine engineering, by mass percentage, is as follows: C: 0.03%~0.07%, Si: 0.08%~0.15%, Mn: 1.50%~1.60%, Nb: 0.015%~0.030%, V: 0.08%~0.15%, Ti: 0.015%~0.025%, Cr: 0.15%~0.20%, Ni: 0.12%~0.18%, Cu: 0.20%~0.30%, P≤0.008%, S≤0.006%, with the remainder being Fe and unavoidable impurities.

[0033] The carbon equivalent (CEV) is calculated as follows: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. In the composition system of the marine engineering steel plate of this invention, no Mo element is actively added; therefore, the Mo term is set to 0 during the carbon equivalent calculation, and it will not affect the final carbon equivalent calculation result.

[0034] In a preferred embodiment of the present invention, in the second step, when the welding method is submerged arc welding, Q according to h Match according to the following range: when h When ≤20mm, Q Control it to 20~30kJ / cm; When 20mm < h When ≤50mm, Q Control it to 28~40kJ / cm; when h When >50mm Q The concentration should be controlled between 35 and 50 kJ / cm.

[0035] In a preferred embodiment of the present invention, in the third step, when the welding method is gas shielded welding, Q according to h Match according to the following range: when h When ≤20mm, Q Control it to 10~20kJ / cm; When 20mm < h When ≤50mm, Q Control it within 15~28 kJ / cm; when h When >50mm Q The concentration should be controlled between 22 and 35 kJ / cm.

[0036] In a preferred embodiment of the present invention, the post-weld slow cooling treatment involves immediately covering the weld and heat-affected zone with an aluminum silicate refractory insulation blanket, covering the weld and an area on both sides at least twice the plate thickness, and then slowly cooling to room temperature. The cooling rate in the 800°C to 500°C range is controlled at 0.5~2°C / s to ensure... t 8 / 5 The timeframe ultimately fell within the target window. Post-weld slow cooling is... t 8 / 5 The time control is the last line of defense, which can effectively avoid the martensitic phase transformation caused by rapid cooling of the joint after welding, while promoting the full bainitic phase transformation and ensuring the uniformity of the microstructure.

[0037] An embodiment of the present invention also provides a marine engineering steel welded joint prepared according to the above method. The coarse grain region of the heat-affected zone of the marine engineering welded joint is mainly composed of refined lath bainite or granular bainite with a volume fraction of ≥85%. The size of the bainite lath bundles is controlled at 5~15μm, and the original austenite grain size is ≤80μm.

[0038] The microstructure of the coarse-grained zone of the welded heat-affected zone of the marine engineering steel welded joint obtained by the welding process of the present invention is mainly composed of refined lath bainite or granular bainite, without blocky martensite or coarse proeutectoid ferrite.

[0039] In a preferred embodiment of the present invention, the impact absorption energy of the coarse-grained zone in the heat-affected zone of the welded joint for marine engineering is ≥85J at -40℃, and the cold crack sensitivity coefficient is ( Pcm ≤0.20%, hardness 170~210HV 10 It has a tensile strength of 420~520MPa, a yield strength of ≥355MPa, and no welding cold cracks.

[0040] Embodiments of the present invention also provide the application of the above-mentioned steel welded joints for marine engineering in marine engineering equipment.

[0041] Example 2 of this invention: The continuous cooling transformation (CCT) curve of the marine engineering steel plate used in the experiment is shown below. Figure 1 As shown.

[0042] The base material (i.e., the steel plate to be welded for marine engineering) used in the embodiments and comparative examples of this invention is 355MPa grade low carbon microalloyed marine engineering steel, covering three kinds of industrial production steel plates with different compositions and different plate thicknesses. Their chemical compositions by mass percentage are shown in Table 1. All steel plates are in TMCP+ tempered state, and the original microstructure is ferrite + pearlite + a small amount of bainite.

[0043] Table 1. Chemical composition (wt.%) of the marine steel used in the experiment Submerged arc welding uses H08MnA welding wire with a diameter of 3.2mm / 4.0mm, and is equipped with SJ101 sintering flux.

[0044] This invention provides a method for controlling the welding and processing of 355MPa-grade high-strength marine engineering structural steel for offshore platforms, subsea pipelines, port machinery, etc. t 8 / 5 This invention establishes a welding process based on the thickness of the steel plate to be welded, which improves the bainitic microstructure of the heat-affected zone through a time window. t 8 / 5 A time-graded precision control model is used to match the welding heat input range corresponding to the plate thickness. This model simultaneously constructs a comprehensive process control system encompassing preheating, interpass temperature management, and post-weld slow cooling, and is deeply coupled with a composite microalloying composition system for Nb-V-Ti toughening and Cr-Ni-Cu corrosion resistance. Through this invention, the coarse-grained zone of the weld heat-affected zone can be transformed into a homogeneous structure dominated by refined lath bainite or granular bainite, effectively avoiding the formation of martensitic hardened structures and coarse ferrite structures. This results in excellent strength-toughness matching, adaptability to on-site construction, and industrial operability.

[0045] Unless otherwise specified, the room temperature in this invention is 25±2℃.

[0046] All raw materials used in the embodiments of the present invention were obtained through commercial purchase.

[0047] Unless otherwise specified, the term "parts" used in the embodiments of this invention refers to "parts by weight".

[0048] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0049] The technical solution of the present invention will be further illustrated by the following embodiments.

[0050] Example 1 Plate Thickness h =12mm (medium-thick plate) This embodiment uses steel 1, plate thickness... h =12mm ( h ≤20mm), the bevel type is a V-shaped bevel, the bevel angle is 60°, and the blunt edge is 2mm. Based on the graded control rules of this invention, the target is determined. t 8 / 5 The time window is 8~25s, with a preferred target value of 12s; the matching welding method is submerged arc welding, and the heat input is controlled within the range of 20~30kJ / cm. This embodiment controls... t 8 / 5 The welding process that improves the bainite microstructure in the heat-affected zone within a specific time window involves the following steps: 1. Pre-welding preparation: Grind the bevel and 20mm on both sides with a grinding wheel to remove rust, oil, and oxide scale, exposing the metal luster; Based on the steel carbon equivalent CEV=0.39≤0.40, determine the preheating temperature to be 80℃ before welding, use electric heating for preheating, and measure the temperature throughout the process to ensure that the preheating temperature is uniform and stable at 80℃±5℃. 2. Welding Construction: Submerged arc welding is used with a welding wire diameter of 3.2mm and flux SJ101. The flux is dried at 350℃ for 2 hours and kept at that temperature for later use. Welding parameters: welding current 450A, arc voltage 28V, welding speed 35cm / min, actual heat input 20.53kJ / cm. Multi-layer, multi-pass welding is used, with a total of 3 passes. The penetration depth of each pass is 4mm, and the width of each pass is 10mm. The interpass temperature is controlled at 120℃ throughout the process, with an interval of 1 minute between adjacent passes. The interpass temperature is measured before each pass to confirm that it meets the standard. 3. Post-weld slow cooling: Immediately after welding, cover the weld and 150mm on both sides with a 50mm thick aluminum silicate insulation blanket to allow it to cool slowly to room temperature, with the cooling rate controlled at 1.2℃ / s. 4. Post-weld inspection: 24 hours after welding is completed, the welded joint is subjected to 100% ultrasonic non-destructive testing. The test results meet the requirements and there are no welding defects.

[0051] Measurements showed that the actual coarse-grained region of the weld heat-affected zone in this embodiment... t 8 / 5 The time was 13.2 s, falling entirely within the target control window. The microstructure of the coarse-grained zone in the heat-affected zone was dominated by fine lath bainite, with a bainite volume fraction of approximately 90%. The bainite lath bundle size was approximately 8–12 μm, and the original austenite grain size was approximately 50 μm. No martensite or coarse ferrite was present. The mechanical property test results of the welded joint are shown in Table 2.

[0052] Example 2 Plate Thickness h =35mm (thick plate) This embodiment uses steel 2, plate thickness... h =35mm (20mm < h (≤50mm), the bevel type is a symmetrical X-shaped bevel, the bevel angle is 60°, and the blunt edge is 2mm. Based on the graded control rules of this invention, the target is determined. t 8 / 5 The time window is 20~45s, with a preferred target value of 30s; the matching welding method is submerged arc welding, and the heat input is controlled within the range of 28~40kJ / cm.

[0053] The schematic diagram of the X-shaped butt joint groove and weld arrangement of steel plates for marine engineering in this embodiment is shown below. Figure 2 The specific welding process parameters and implementation steps are shown below: 1. Pre-welding preparation: Grind the bevel and 30mm on both sides with a grinding wheel to remove rust, oil, and oxide scale, exposing the metal luster; Based on the steel's carbon equivalent CEV=0.40, determine the preheating temperature to be 100℃, use electric heating for preheating, and measure the temperature throughout the process to ensure that the preheating temperature is uniform and stable at 100℃±5℃. 2. Welding Construction: Submerged arc welding is used with a welding wire diameter of 4.0mm and flux SJ101. The flux is dried at 350℃ for 2 hours and kept at that temperature for later use. Welding parameters: welding current 550A, arc voltage 30V, welding speed 28cm / min, actual heat input 35.40kJ / cm. Multi-layer, multi-pass welding is used, with a total of 8 passes on both sides. The penetration depth of each pass is 5mm, and the width of each pass is 12mm. The interpass temperature is controlled at 140℃ throughout the process, and the interval between adjacent passes is 1.5min. The temperature is measured and confirmed before each pass. 3. Post-weld slow cooling: Immediately after welding, cover the weld and 200mm on both sides with a 50mm thick aluminum silicate insulation blanket to allow it to cool slowly to room temperature, with the cooling rate controlled at 1.5℃ / s. 4. Post-weld inspection: 24 hours after welding is completed, the welded joint is subjected to 100% ultrasonic non-destructive testing. The test results meet the requirements and there are no welding defects.

[0054] According to the measurement, the actual performance of this embodiment is... t 8 / 5 The time was 32.5 s, falling entirely within the target control window. The microstructure of the coarse-grained zone in the heat-affected zone was dominated by uniform granular bainite, with a bainite volume fraction of approximately 88%. The bainite lath bundle size was approximately 10–14 μm, and the original austenite grain size was approximately 65 μm. There was no hardened microstructure or coarse ferrite. The mechanical property test results of the welded joint are shown in Table 2.

[0055] Example 3 Plate Thickness h =60mm (Extra-thick plate) This embodiment uses steel 3, plate thickness... h =60mm ( h >50mm), the bevel type is a symmetrical X-shaped bevel, the bevel angle is 45°, and the blunt edge is 2mm. Based on the graded control rules of this invention, the target is determined. t 8 / 5 The time window is 35~70s, with a preferred target value of 50s; the matching welding method is submerged arc welding, and the heat input is controlled within the range of 35~50kJ / cm.

[0056] Specific welding process parameters and implementation steps: 1. Pre-welding preparation: Grind the bevel and 50mm on both sides to remove impurities; Based on the steel carbon equivalent CEV=0.41≥0.40, determine the preheating temperature to be 150℃, use electric heating for preheating, and control the temperature throughout the process to ensure that the preheating temperature is uniform and stable at 150℃±5℃. 2. Welding Construction: Submerged arc welding is used with a welding wire diameter of 4.0mm and flux SJ101. The flux is dried at 350℃ for 2 hours and kept at that temperature for later use. Welding parameters: welding current 600A, arc voltage 32V, welding speed 22cm / min, actual heat input 49.70kJ / cm. Multi-layer, multi-pass welding is used, with a total of 12 passes on both sides. The penetration depth of each pass is 5mm, and the width of the pass is 14mm. The interpass temperature is controlled at 160℃ throughout the process, and the interval between adjacent passes is 1.5min. The temperature is measured and confirmed before each pass. 3. Post-weld slow cooling: Immediately after welding, cover the weld and 300mm on both sides with an 80mm thick aluminum silicate insulation blanket to allow it to cool slowly to room temperature. The cooling rate should be controlled at 0.8℃ / s. 4. Post-weld inspection: 24 hours after welding is completed, the welded joint is subjected to 100% ultrasonic non-destructive testing. The test results meet the requirements and there are no welding defects.

[0057] According to the measurements, the actual performance of this embodiment is... t 8 / 5 The time was 51.8 s, falling entirely within the target control window. The microstructure of the coarse-grained zone in the heat-affected zone was dominated by refined lath bainite, with a bainite volume fraction of approximately 86%, bainite lath bundle size of approximately 12–15 μm, and the original austenite grain size of approximately 75 μm, showing no abnormal microstructure. The mechanical property test results of the welded joint are shown in Figure 2.

[0058] Example 4 Plate Thickness h =20mm (grading boundary value) This embodiment uses steel 1, plate thickness... h =20mm, which is the boundary value between medium-thick and thick plates. The bevel type is a double-sided V-shaped bevel with a bevel angle of 60° on each side and a blunt edge of 2mm. According to the rules of this invention, the target... t 8 / 5 The time window is 8~25s, preferably 20s; the matching welding method is gas shielded welding, and the heat input is controlled in the range of 10~20kJ / cm.

[0059] Specific welding process parameters and implementation steps: 1. Pre-welding preparation: Grind the bevel and 20mm on both sides with a grinding wheel to remove rust, oil, and oxide scale, exposing the metal luster; Based on the steel carbon equivalent CEV=0.39≤0.40, determine the preheating temperature to be 80℃ before welding, use electric heating for preheating, and measure the temperature throughout the process to ensure that the preheating temperature is uniform and stable at 80℃±5℃. 2. Welding Construction: Gas shielded welding uses ER50-6 solid welding wire with a diameter of 1.2mm. The shielding gas is a mixture of 80% Ar and 20% CO2. Welding parameters: welding current 280A, arc voltage 30V, welding speed 30cm / min, actual heat input 16.80kJ / cm; multi-layer, multi-pass welding is adopted, with a total of 5 passes, single pass penetration depth 4mm, and weld width 10mm; the interpass temperature is controlled at 130℃ throughout the process, the interval between adjacent weld passes is 1min, and the interpass temperature is measured before each pass to confirm that it meets the standard. 3. Post-weld slow cooling: Immediately after welding, cover the weld and 150mm on both sides with a 50mm thick aluminum silicate insulation blanket to allow it to cool slowly to room temperature. The cooling rate should be controlled at 1.8℃ / s. 4. Post-weld inspection: 24 hours after welding is completed, the welded joint is subjected to 100% ultrasonic non-destructive testing. The test results meet the requirements and there are no welding defects.

[0060] Upon testing, the actual implementation of this embodiment... t 8 / 5 The time was 12.8s, falling within the target window; the microstructure of the coarse-grained zone of the heat-affected zone was mainly refined lath bainite, with a bainite volume fraction of about 89%, and the coarse-grained zone of the heat-affected zone was mainly lath bainite, with a bainite lath bundle size of 9~13μm and a original austenite grain size of about 60μm, without hardened microstructure and coarse ferrite; the mechanical property test results of the welded joint are shown in 2.

[0061] Comparative Example 1 t 8 / 5 Too short (cools down too quickly) This comparative example uses the exact same base material (steel 2, 35mm thick plate), bevel shape, and welding method as Example 2. It is a common case of rapid cooling in the prior art. The welding heat input is controlled at 15kJ / cm. No preheating or slow cooling is performed before welding. The interpass temperature is allowed to cool naturally to room temperature.

[0062] According to thermocouple testing, the actual comparative example... t 8 / 5 The time was only 9.2 seconds, far below the 20-second lower limit set in this invention. Metallographic observation showed that the coarse-grained region of the heat-affected zone was mainly composed of blocky martensite, accompanied by a small amount of coarse bainite. The volume fraction of martensite was approximately 65%, indicating severe hardening of the microstructure. The mechanical property test results are shown in Table 2. Although the tensile strength increased to 500 MPa, the impact absorption energy at -40℃ was only 28 J, a decrease of 71.4% compared to Example 2, while the hardness reached as high as 242 HV. 10 It is extremely sensitive to cold cracking. Microcracks were found in the welded joints after non-destructive testing, which completely fails to meet the service requirements of marine engineering.

[0063] Comparative Example 2 t 8 / 5Too long (cooling down too slowly) The comparative example used the exact same base material (steel 2, 35mm thick plate), bevel shape, and welding method as Example 2, which is a common slow cooling condition in the prior art. The welding heat input was controlled at 60kJ / cm, and the weld was naturally air-cooled without any heat preservation or slow cooling measures.

[0064] According to thermocouple testing, the actual comparative example... t 8 / 5 The time reached 58.5s, far exceeding the 45s upper limit set in this invention. Metallographic observation showed that the coarse-grained region of the heat-affected zone was dominated by coarse proeutectoid ferrite, accompanied by a small amount of pearlite and degenerate bainite. The ferrite grain size was over 40μm, indicating severe microstructural coarsening. The mechanical property test results are shown in Table 2. Its yield strength was only 335MPa, and its tensile strength was 412MPa, both lower than the lower limit of the standard requirement. The impact absorption energy at -40℃ was only 38J, a decrease of 61.2% compared to Example 2, which completely fails to meet the strength and toughness requirements of marine engineering structures.

[0065] Comparative Example 3 only lacks interlayer temperature control. Using the exact same base material (steel 2, 35mm thick), symmetrical X-groove, welding method, welding heat input, preheating temperature, and number of weld passes as in Example 2, only the interpass temperature control was removed. After the previous weld was completed, the weld was allowed to cool naturally to room temperature before the next weld was performed. All other conditions were exactly the same as in Example 2.

[0066] Thermocouple testing showed that the t8 / 5 time fluctuation range for each weld pass in this comparative example was 12.4 s. Metallographic observation revealed poor uniformity of the coarse-grained region in the heat-affected zone, with a small amount of proeutectoid ferrite appearing in some areas. The size of the bainite lath bundles fluctuated between 8 and 22 μm, and the volume fraction of bainite was only 82%. The mechanical property test results are shown in Table 2. The low-temperature impact absorption energy at -40℃ was only 72 J, indicating poor quality stability. No macroscopic cracks were found during nondestructive testing.

[0067] Comparative Example 4 only lacks post-weld slow cooling. Using the exact same base material (steel 2, 35mm thick), symmetrical X-groove, welding method, welding heat input, preheating temperature, interpass temperature control, and number of weld passes as in Example 2, only the post-weld slow cooling step was omitted. After welding, the material was naturally air-cooled to room temperature. All other conditions were exactly the same as in Example 2.

[0068] Thermocouple testing showed that the t8 / 5 time of the last weld pass in this comparative example was only 18.5 s, slightly lower than the 20 s control limit of this invention. Metallographic observation showed that a small amount of blocky martensite appeared in the coarse-grained region of the heat-affected zone, the bainite lath bundle size was 7~18 μm, and the bainite volume fraction was only 76%. The mechanical property test results are shown in Table 2. The low-temperature impact absorption energy at -40℃ was only 65 J, and the maximum hardness reached 218 HV. 10 After 24 hours of non-destructive testing, no macroscopic cold cracks were found in the welded joint, but there was a risk of micro-defects in the locally hardened area.

[0069] Cold crack sensitivity coefficient ( P cm The calculation formula is: P cm =C+Si / 30+(Cr+Mn+Cu) / 20+(Ti+Nb+) / 10+Ni / 60+Mo / 15+5B.

[0070] The testing standards of this invention are as follows: tensile testing is performed according to GB / T 2651-2008 "Tensile Testing Method for Welded Joints"; impact testing is performed according to GB / T 2650-2008 "Impact Testing Method for Welded Joints", with sampling location in the coarse-grained region of the heat-affected zone, test temperature -40℃, 3 parallel specimens per group, and the average value of the results is taken; hardness testing is performed according to GB / T 2654-2008 "Hardness Testing Method for Welded Joints", with a test load of 10 kgf and a holding time of 10 s.

[0071] Table 2 Mechanical properties of each embodiment and comparative example The experimental results from the above embodiments and comparative examples clearly show that: All embodiments of the present invention t 8 / 5 The timing was precisely within the target grading window, and a homogeneous structure dominated by refined lath bainite or granular bainite was formed in the coarse-grained zone of the heat-affected zone. The size of the bainite lath bundles was stably controlled at 5~15μm, with no abnormal structure generated, thus achieving directional and precise control of the structure. The mechanical properties of the welded joints in all embodiments of the present invention fully meet the standard requirements for marine engineering steel. The low-temperature impact absorption energy at -40℃ is ≥85J, with a maximum of 112J, which is greatly improved compared with Comparative Example 1 and Comparative Example 2. At the same time, it achieves an excellent match of strength, plasticity, toughness and hardness, and solves the problem of "strength and toughness cannot be taken into account" in the prior art. when t 8 / 5When the time deviates from the graded window set by this invention, whether it is too short or too long, it will lead to tissue loss of control and a sharp deterioration of mechanical properties, which fully proves the scientific nature and necessity of the graded control window of this invention. This invention provides a comprehensive collaborative management and control system that can effectively safeguard... t 8 / 5 With stable time control, the non-destructive testing pass rate of welded joints is 100%, with no cold cracks generated, and the quality stability is far superior to existing technical solutions.

[0072] In summary, this invention achieves hierarchical control. t 8 / 5 By combining the time window with the synergistic design of microalloying composition and the coupling of the entire process, the precise directional improvement of the bainitic structure in the heat-affected zone of welded steel for marine engineering was achieved, which significantly improved the low-temperature toughness and crack resistance of the welded joint. The technical solution is complete, controllable, and industrially applicable, and completely solves many of the shortcomings of existing technologies, providing an advanced and reliable technical solution for welding marine engineering structures.

[0073] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations 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 method of controlling t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, Includes the following steps: The first step is to confirm the composition and thickness of the steel plate to be welded for marine engineering, and to clarify the thickness, chemical composition and carbon equivalent of the steel plate. The second step is to determine the target based on the thickness of the steel plate to be welded for marine engineering. t 8 / 5 Time window, the t 8 / 5 The time is the time it takes for the heat-affected zone to cool from 800℃ to 500℃ during the welding process; The third step is based on the target. t 8 / 5 A time window is established to match the welding method and welding heat input corresponding to the plate thickness, and to set the welding heat input in relation to the plate thickness. t 8 / 5 The time correspondence is as follows: preheating is performed before welding heat input; the welding method is selected from submerged arc welding or gas shielded welding. The fourth step is to immediately perform post-weld slow cooling treatment after welding to control the cooling rate and ensure... t 8 / 5 The time falls within the target window; Wherein, the target t 8 / 5 The time window is determined according to the following graded control rules based on the plate thickness: when the plate thickness is ≤20mm... t 8 / 5 The time should be controlled between 8 and 25 seconds; when the plate thickness is between 20 mm and 50 mm, t 8 / 5 The time should be controlled between 20 and 45 seconds; when the plate thickness is > 50 mm, t 8 / 5 The time should be controlled between 35 and 70 seconds; The carbon equivalent is ≤0.45; The chemical composition of the marine engineering steel plate to be welded, by mass percentage, includes 0.015%~0.030% Nb, 0.08%~0.15% V, and 0.015%~0.025% Ti.

2. The method of controlling according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, The chemical composition of the marine engineering steel plate to be welded, by mass percentage, is as follows: C: 0.03%~0.07%, Si: 0.08%~0.15%, Mn: 1.50%~1.60%, Nb: 0.015%~0.030%, V: 0.08%~0.15%, Ti: 0.015%~0.025%, Cr: 0.15%~0.20%, Ni: 0.12%~0.18%, Cu: 0.20%~0.30%, P≤0.008%, S≤0.006%, with the remainder being Fe and unavoidable impurities.

3. The method of control according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, In the third step, when the welding method is submerged arc welding, the welding heat input is matched according to the plate thickness within the following range: When the plate thickness is ≤20mm, the welding heat input should be controlled at 20~30kJ / cm; When the plate thickness is 20mm < 50mm, the welding heat input should be controlled at 28~40kJ / cm. When the plate thickness is greater than 50 mm, the welding heat input should be controlled at 35~50 kJ / cm.

4. The method of control according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, In the third step, when the welding method is gas shielded welding, the welding heat input is matched according to the plate thickness within the following range: When the plate thickness is ≤20mm, the welding heat input should be controlled at 10~20kJ / cm; When the plate thickness is 20mm < 50mm, the welding heat input should be controlled at 15~28kJ / cm. When the plate thickness is >50mm, the welding heat input should be controlled at 22~35kJ / cm.

5. The method of control according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, In the third step, the preheating temperature is determined based on the carbon equivalent. When the carbon equivalent is ≤0.40, the preheating temperature is 80~100℃; when 0.40 < carbon equivalent ≤0.45, the preheating temperature is 100~150℃.

6. The method of control according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, In the third step, multi-layer, multi-pass welding is adopted during the welding process. The interpass temperature is controlled throughout the process. The penetration depth of a single weld is controlled at 3~6mm, and the weld width is controlled at 8~15mm. The interpass temperature is controlled at 100~180℃ throughout the process, and the interpass temperature does not exceed the preheating temperature +50℃. The welding interval between adjacent weld passes does not exceed 2min.

7. The method of control according to claim 1 t 8 / 5 The welding process with a time window that improves the bainitic microstructure in the heat-affected zone is characterized by, In the fourth step, the post-weld slow cooling treatment involves fully covering the weld and heat-affected zone with an aluminum silicate refractory insulation blanket. The coverage area includes the weld and an area on both sides with a thickness of no less than twice the plate thickness. The weld is then slowly cooled to room temperature, with the cooling rate controlled at 0.5~2℃ / s.

8. A steel welded joint for marine engineering, characterized in that, The welded joint for marine engineering is obtained by welding according to any one of claims 1 to 7. The coarse grain region of the heat-affected zone is mainly composed of refined lath bainite or granular bainite with a volume fraction of ≥85%, the size of the bainite lath bundles is controlled at 5~15μm, and the original austenite grain size is ≤80μm.

9. The steel welded joint for marine engineering according to claim 8, characterized in that, The welded joint for marine engineering exhibits an impact absorption energy ≥85J in the coarse-grained zone of the heat-affected zone at -40℃, a cold crack sensitivity coefficient ≤0.20%, and a hardness of 170~210HV. 10 The tensile strength is 420~520MPa, and the yield strength is ≥355MPa.

10. The application of a steel welded joint for marine engineering as described in any one of claims 8 to 9 in marine engineering equipment.

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