A cold-weather 420MPa grade marine engineering steel plate and its manufacturing method

Abstract

This invention relates to a cold-weather, ice-impact resistant 420MPa grade marine engineering steel plate and its manufacturing method. The chemical composition of the steel plate includes C: 0.05%–0.08%, Si: 0.2%–0.35%, Mn: 1.35%–1.55%, P≤0.02%, S≤0.01%, Als: 0.015%–0.045%, Ni: 0.2%–0.48%, Mo: 0.1%–0.25%, Cu: 0.22%–0.35%, Co: 0.01%–0.1%, Nb: 0.02%–0.06%, Ti: 0.008%–0.015%, N: 0.003%–0.005%, with the remainder being Fe and impurities. Through unique composition design and corresponding production processes, a steel plate with mechanical properties and service safety meeting the service conditions of marine engineering equipment is obtained.

Description

Technical Field

[0001] This invention relates to the field of marine engineering steel production technology, and in particular to a cold-weather 420MPa grade marine engineering steel plate and its manufacturing method. Background Technology

[0002] Since the beginning of the 21st century, the shipbuilding and marine engineering field has developed rapidly. With the accelerated development of shipping routes in cold-water oceans and the Arctic Circle, higher demands have been placed on related technologies and equipment. Simultaneously, with the continuous deepening of research on marine engineering steel materials for extreme environments, new types of marine engineering steel are constantly emerging to meet increasingly stringent usage requirements. The Arctic region possesses abundant resources such as oil and natural gas, and has enormous development potential. Arctic shipping places demands on related ships and marine engineering equipment not only in terms of technical performance such as low-temperature resistance, corrosion resistance, and fatigue resistance, but also involves environmental protection issues, such as ship emissions and prevention of marine pollution, which also require the support of relevant technologies and equipment. To adapt to these changes and challenges, research on new marine engineering steel materials is currently focusing on microalloying, controlled rolling, controlled cooling, and heat treatment processes, with the aim of improving the materials' freeze resistance, corrosion resistance, and other application properties.

[0003] Microalloying technology is currently a hot research area in marine engineering steel materials. By adding appropriate trace alloying elements, the fatigue resistance and service life of steel plates can be significantly improved. Controlled rolling, controlled cooling, and optimized heat treatment processes are also important research directions. Controlled rolling processes can effectively control the microstructure and phase transformation of steel, improving its toughness and fatigue resistance. The fatigue resistance of steel plates is crucial for marine engineering equipment, significantly enhancing its reliability, safety, and economic efficiency. Adding appropriate trace alloying elements in conjunction with advanced heat treatment processes can effectively improve the fatigue resistance of steel plates. To address the new requirements for the fatigue resistance of marine engineering steel plates, verification can be performed using fatigue crack propagation testing, fatigue life testing, and low-cycle fatigue strength testing.

[0004] When selecting steel materials and processing techniques, factors such as performance, cost, and feasibility need to be comprehensively considered. Although new types of marine engineering steel materials are constantly emerging, they are generally expensive, requiring rational selection based on specific needs and economic considerations for practical applications. Furthermore, in addition to micro-alloying technology and heat treatment processes, emerging technologies such as ultrasonic surface treatment are available to improve the fatigue resistance of materials.

[0005] Existing marine engineering steels can basically meet most market demands, but special steels with lower ductile-brittle transition temperatures and excellent comprehensive performance remain the research and development goals of countries worldwide. The development of high-strength steel plates with high service safety is challenging, requiring stringent production processes and sophisticated equipment. For example, the Chinese patent application CN116162869A, entitled "Anti-fatigue steel plate for marine environments and its production method," produces a ferritic + bainitic anti-fatigue steel plate. While employing a low-C, low-Mn composition design and incorporating elements such as Ni, Mo, Cr, V, Sn, and Re, which helps improve the steel plate's corrosion fatigue resistance, few elements effectively enhance its strength and toughness. Furthermore, it only utilizes conventional controlled rolling processes, making it impossible to produce a 120mm thick, 420MPa grade anti-fatigue low-temperature toughness marine engineering steel. Chinese invention patent CN111676425B discloses "a bridge steel with excellent toughness and fatigue resistance at extreme low temperatures and its manufacturing method". It adopts an alloy composition with high Si, high Mn and high Cr to produce steel plates with a thickness of 10-60mm. The finished steel plates have high strength and high hardness, but the steel plates contain a lot of hard phases. They have insufficient ability to resist fatigue crack initiation and propagation, and cannot produce fatigue-resistant low-temperature tough steel plates with a maximum thickness of 120mm. Summary of the Invention

[0006] This invention provides a cold-weather ice-impact resistant 420MPa grade marine engineering steel plate and its manufacturing method. Through unique composition design and corresponding production processes, a steel plate with mechanical properties and service safety meeting the service conditions of marine engineering equipment is obtained. The maximum thickness of the finished steel plate is 120mm, and the Charpy impact energy at -60℃ is ≥120J. The fatigue crack propagation performance of the base metal and the weld heat-affected zone is as follows: when Lg(ΔK) = 3.5MPa·m 0.5 When, Lg(da / dN)≤-2.4mm / cycle.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A cold-weather, ice-resistant, 420MPa grade marine engineering steel plate, the chemical composition of the steel plate by weight percentage includes C: 0.05%~0.08%, Si: 0.2%~0.35%, Mn: 1.35%~1.55%, P≤0.02%, S≤0.01%, Als: 0.015%~0.045%, Ni: 0.2%~0.48%, Mo: 0.1%~0.25%, Cu: 0.22%~0.35%, Co: 0.01%~0.1%, Nb: 0.02%~0.06%, Ti: 0.008%~0.015%, N: 0.003%~0.005%, with the remainder being Fe and unavoidable impurities.

[0009] The finished steel plate has a yield strength ≥420MPa, a tensile strength of 520~680MPa, and a transverse elongation ≥26%; the average Charpy impact energy at -60℃ is ≥120J; fatigue crack propagation performance of the base metal and weld heat-affected zone: when Lg(ΔK)=3.5MPa·m 0.5 When, Lg(da / dN)≤-2.4mm / cycle.

[0010] The maximum thickness of the finished steel plate is 120mm.

[0011] The metallographic structure at 1 / 2 thickness of the finished steel plate consists of acicular ferrite, bainite, and polygonal ferrite, with the volume ratio of acicular ferrite being 60%–80%, the volume ratio of bainite being 15%–30%, and the remainder being polygonal ferrite.

[0012] A method for manufacturing a cold-weather-resistant 420MPa grade marine engineering steel plate includes the following steps:

[0013] 1) Smelting and continuous casting: After being smelted in a converter, the molten steel is further refined by LF, RH or VD to reduce the content of P, S and non-metallic inclusions; continuous casting is carried out under full protection.

[0014] 2) Heating: The billet is loaded into the heating furnace at 500-650℃ and held for ≥60min; then the temperature is raised to 1120-1200℃ at a heating rate of 3-6℃ / min, the soaking temperature is 1150-1180℃, and the total time in the furnace is 6-9h.

[0015] 3) Two-stage rolling: The initial rolling temperature of the first stage is 1100-1150℃, the single-pass reduction of the first 3 passes is ≥30mm, and the total reduction rate of the first stage rolling is ≥60%; the initial rolling temperature of the second stage is 800-900℃, the single-pass reduction of the second stage is 6-10mm, and the final rolling temperature is ≥800℃.

[0016] 4) Cooling control: The water temperature of the steel plate is 680-750℃, the reddening temperature is 250-450℃, the water volume ratio of the upper and lower spray pipes is 1-1.4, and the roller speed is 0.3-1.5m / s;

[0017] 5) Tempering: The tempering temperature of the steel plate is 325~450℃, and the tempering time is 2~4min / mm.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1) By adopting a composition design that incorporates elements such as C, Mn, Ni, Co, Mo, and Cu, and combined with appropriate production processes, we can produce 420MPa grade marine engineering steel plates with a maximum thickness of 120mm that are resistant to ice impact in cold regions.

[0020] 2) A unique alloy composition system and production process are employed to ensure that the mechanical properties and service safety of the finished steel plates meet the service conditions for marine engineering equipment, namely, yield strength ≥ 420 MPa, tensile strength 520~680 MPa, transverse elongation ≥ 26%, Charpy impact energy at -60℃ ≥ 120 J; fatigue crack propagation performance of the base metal and weld heat-affected zone: when Lg(ΔK) = 3.5 MPa·m 0.5 When, Lg(da / dN)≤-2.4mm / cycle. Attached Figure Description

[0021] Figure 1 This is a metallographic photograph of the steel plate at 1 / 2 thickness of the finished product according to the present invention. Detailed Implementation

[0022] This invention addresses the performance requirements of ultra-high strength marine engineering steel in extremely cold marine environments. It utilizes a composition design combining C, Mn, Ni, Co, Mo, Cu, and other microalloying elements, along with a production process suitable for 420MPa grade marine engineering steel plates designed for ice impact resistance in cold regions. Extensive and systematic experimental research was conducted on various aspects, including alloy element screening and proportioning, steel cleanliness control, two-stage rolling, precise controlled cooling, tempering process optimization, and parameter selection. Ultimately, the alloy element proportions and production process that meet the objectives of this invention were determined.

[0023] The present invention discloses a cold-weather resistant 420MPa grade marine engineering steel plate, which, by weight percentage, comprises C: 0.05%–0.08%, Si: 0.2%–0.35%, Mn: 1.35%–1.55%, P≤0.02%, S≤0.01%, Als: 0.015%–0.045%, Ni: 0.2%–0.48%, Mo: 0.1%–0.25%, Cu: 0.22%–0.35%, Co: 0.01%–0.1%, Nb: 0.02%–0.06%, Ti: 0.008%–0.015%, N: 0.003%–0.005%, with the remainder being Fe and unavoidable impurities.

[0024] The rationale for the composition design of the 420MPa grade marine engineering steel plate for low-altitude ice impact resistance described in this invention is as follows:

[0025] 1) Carbon (C), as a fundamental strengthening element in steel, is the main element ensuring strength and hardness in the marine engineering steel plate described in this invention. Too low a C content will lead to a decrease in C solid solution content and carbide content, reducing grain refinement and resulting in insufficient steel plate strength. Too high a C content will produce a large amount of hardened structure, reducing impact toughness. Therefore, the C content in the steel must be precisely controlled. This invention controls the C content to be 0.05%–0.08%.

[0026] 2) Si can improve the strength of steel plates and refine the grain size, thereby improving the fatigue resistance of steel plates; at the same time, Si can reduce the O content as a deoxidizer. When the Si content is less than 0.2%, the deoxidation effect is not obvious, and when the Si content is greater than 0.35%, it will lead to coarsening of the structure and reduction of toughness. Therefore, the present invention controls the Si content to be 0.2% to 0.35%.

[0027] 3) Mn has a similar atomic radius to Fe and can dissolve in large quantities in the Fe matrix, improving the strength and wear resistance of the steel plate. Simultaneously, Mn can increase the heat transfer capacity of extra-thick steel plates. When the Mn content is below 1.35%, the core heat transfer rate is low; when the Mn content is above 1.55%, Mn segregation leads to poor low-temperature toughness in the core of the thick plate. Therefore, this invention controls the Mn content to be between 1.35% and 1.55%.

[0028] 4) P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. In this invention, P is controlled to be ≤0.02% and S to be ≤0.01%.

[0029] 5) Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is not good. Microalloying elements such as Ti cannot play a role in refining grains because they are oxidized. In particular, for thick high-strength steel plates, it is necessary to appropriately increase the Al content in the steel. Conversely, if the Al content is too high, large inclusions will be formed. Therefore, this invention controls the Al content to be 0.015% to 0.045%.

[0030] 6) The role of Ni is to improve the toughness of steel plates. Adding a large amount can achieve a lower ductile-brittle transition temperature and improve the low-temperature toughness of the steel plate. Adding a certain amount of Ni to steel can also refine ferrite grains, prevent intergranular corrosion, and thus improve fatigue resistance. At the same time, the addition of Ni can also reduce the hot cracking tendency of Cu in steel. In this invention, the Ni content is controlled at 0.2% to 0.48%.

[0031] 7) Mo can form fine carbides in steel, effectively improving the strength of steel plates. Adding a certain amount of Mo can also improve the cooling control effect of extra-thick steel plates. Mo can also work with Ni to provide some corrosion resistance. Furthermore, Mo can improve the tempering stability of steel plates, and enhance their low-temperature toughness and fatigue resistance after tempering. In this invention, the Mo content is controlled at 0.1%–0.25%.

[0032] 8) Cu can improve the wear resistance and fatigue resistance of steel plates. Cu, in combination with Ni, can lower the ductile-brittle transition temperature of the steel plate and improve its low-temperature toughness. However, adding excessive Cu alone will lead to a decrease in the low-temperature toughness of the steel plate and cause hot brittleness. This invention controls the Cu content to be 0.22%–0.35%.

[0033] 9) Co can work synergistically with Mn to significantly strengthen the steel plate through solid solution, improving its strength and fatigue resistance. However, excessive addition can reduce its low-temperature toughness. Co can also improve the tempering stability of the steel plate; this invention controls the Co content to be 0.01%–0.1%.

[0034] 10) Nb is an important additive element in the steel of this invention. Nb can form fine compounds with C and N, improving the strength and hardness of the steel plate. During heating, undissolved Nb C and N compound particles are distributed on the austenite grain boundaries, which can hinder the growth of austenite grains in the steel. Nb can also effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the fatigue resistance of the steel. During the controlled cooling process of the steel plate, a large amount of Nb (CN) precipitates, further promoting dislocation entanglement and refining the grains. The Nb content in this invention is controlled at 0.02% to 0.06%.

[0035] 11) The addition of Ti is to form TiN with N, which prevents grain growth in the billet during heating, rolling, and welding, thereby improving the toughness of the steel plate. A certain content of Ti can improve the steel's resistance to intergranular corrosion. In this invention, the Ti content is controlled at 0.008% to 0.015%.

[0036] 12) Nitrogen (N) can combine with Nb and Ti to form fine, dispersed nitrogen oxide precipitates, effectively promoting the nucleation and growth of ferrite within the grains and effectively controlling the growth of the original austenite grains. Increased N content leads to an increase in TiN in the steel. However, when the dissolved N content is too high, the hot plasticity of the steel decreases, the toughness of the steel plate decreases, and a large number of microcracks easily appear on the surface of the steel plate. Therefore, this invention controls the N content to be 0.003%–0.005%.

[0037] The present invention discloses a cold-weather-resistant 420MPa grade marine engineering steel plate, which adopts a production process of high-purity alloying smelting + low-temperature heating + two-stage low-temperature controlled rolling + controlled cooling + low-temperature tempering. The specific process is as follows:

[0038] (1) High-purity alloying smelting and continuous casting;

[0039] After being smelted in a converter, the molten steel is further refined in an LF furnace, RH furnace, or VD furnace to further reduce the content of phosphorus, sulfur, and non-metallic inclusions. This results in molten steel with the required chemical composition, which is then continuously cast under protective conditions.

[0040] (2) Two-stage rolling and controlled cooling;

[0041] The continuously cast billet is charged into the heating furnace at 500–650℃ and held for ≥60 min. It is then heated to 1120–1200℃ at a rate of 3–6℃ / min, with a homogenization temperature of 1150–1180℃. The total furnace time is 6–9 hours. Ensuring the total heating time ensures that the core of the thick billet reaches the target temperature. Controlling the homogenization temperature prevents it from becoming too high, and controlling the holding time prevents abnormal growth of the billet microstructure, which could affect the fatigue resistance and low-temperature toughness of the steel plate core during rolling.

[0042] The initial rolling temperature for the first stage is 1100–1150℃, and the reduction per pass for at least the first three passes is ≥30mm. The total reduction rate for the first stage rolling is ≥60%. The initial rolling temperature for the second stage is 800–900℃, the reduction per pass for the second stage is 6–10mm, and the final rolling temperature is ≥800℃.

[0043] The water temperature of the rolled steel plate is 680-750℃, the red-hot temperature is 250-450℃, the water volume ratio of the upper and lower spray pipes is 1-1.4, and the roller speed is 0.3-1.5m / s.

[0044] The purpose of two-stage controlled rolling is to maximize the reduction per pass under conditions of relatively high temperature and low hardness of the steel plate, further break up the as-cast grains, increase deformation energy storage and grain nucleation sites, and improve the low-temperature toughness of the core of the extra-thick plate and its ability to resist fatigue crack initiation and propagation. The purpose of controlled cooling after rolling is to rapidly cool the steel plate below the dynamic recrystallization temperature to prevent the grains inside the steel plate from growing again.

[0045] (3) Tempering;

[0046] The tempering temperature is 325–450℃, and the tempering time is 2–4 min / mm.

[0047] The finished steel plate has a yield strength ≥420MPa, tensile strength of 520~680MPa, transverse elongation ≥26%, and average Charpy impact energy at -60℃ ≥120J. Fatigue crack propagation performance of the base metal and weld heat-affected zone: when Lg(ΔK)=3.5MPa·m 0.5 At this time, Lg(da / dN)≤-2.4mm / cycle. The maximum thickness of the finished steel plate is 120mm.

[0048] like Figure 1 As shown, the metallographic structure at 1 / 2 thickness of the finished steel plate consists of acicular ferrite + bainite + polygonal ferrite, with the volume ratio of acicular ferrite being 60% to 80%, the volume ratio of bainite being 15% to 30%, and the remainder being polygonal ferrite. The finished product has good mechanical properties.

[0049] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0050]

Example

[0051] The chemical composition of the steel in each embodiment is shown in Table 1, the production process parameters are shown in Tables 2-1 and 2-2, and the properties of the finished steel plate are shown in Table 3.

[0052] Table 1 Chemical composition of steel (weight percentage, %)

[0053]

[0054] Table 2-1 Production Process Parameters (I)

[0055]

[0056] Table 2-2 Production Process Parameters (II)

[0057]

[0058] Table 3 Mechanical Properties of Finished Steel Plates

[0059]

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a cold-weather 420MPa grade marine engineering steel plate, characterized in that, The chemical composition of the steel plate, by weight percentage, includes: C 0.05%–0.08%, Si 0.2%–0.34%, Mn 1.35%–1.55%, P≤0.02%, S≤0.01%, Als 0.015%–0.045%, Ni 0.38%–0.48%, Mo 0.19%–0.25%, Cu 0.24%–0.32%, Co 0.06%–0.1%, Nb 0.044%–0.06%, Ti 0.008%–0.015%, N 0.0031%–0.005%, with the remainder being Fe and unavoidable impurities; The manufacturing method of the cold-weather ice-impact 420MPa grade marine engineering steel plate includes the following steps: 1) Smelting and continuous casting: After being smelted in a converter, the molten steel is further refined by LF, RH or VD to reduce the content of P, S and non-metallic inclusions; continuous casting is carried out under full protection. 2) Heating: The billet is loaded into the heating furnace at 500-650℃ and held for ≥60min; then the temperature is raised to 1120-1200℃ at a heating rate of 3-6℃ / min, the soaking temperature is 1150-1180℃, and the total time in the furnace is 6-9h. 3) Two-stage rolling: The initial rolling temperature of the first stage is 1100-1140℃, the single-pass reduction of the first 3 passes is ≥30mm, and the total reduction rate of the first stage rolling is ≥60%; the initial rolling temperature of the second stage is 800-900℃, the single-pass reduction of the second stage is 6-10mm, and the final rolling temperature is ≥800℃. 4) Cooling control: The water temperature of the steel plate is 680-750℃, the reddening temperature is 250-450℃, the water volume ratio of the upper and lower spray pipes is 1-1.4, and the roller speed is 0.3-1.5m / s; 5) Tempering: The tempering temperature of the steel plate is 325~450℃, and the tempering time is 2~4min / mm.

2. The manufacturing method of a cold-weather ice-impact 420MPa grade marine engineering steel plate according to claim 1, characterized in that, The finished steel plate has a yield strength ≥420MPa, a tensile strength of 520~680MPa, and a transverse elongation ≥26%; the average Charpy impact energy at -60℃ is ≥120J; fatigue crack propagation performance of the base metal and weld heat-affected zone: when Lg(ΔK)=3.5MPa·m 0.5 When, Lg(da / dN)≤-2.4mm / cycle.

3. The manufacturing method of a cold-weather ice-impact 420MPa grade marine engineering steel plate according to claim 1, characterized in that, The maximum thickness of the finished steel plate is 120mm.

4. The manufacturing method of a cold-weather ice-impact 420MPa grade marine engineering steel plate according to claim 1, characterized in that, The metallographic structure at 1 / 2 thickness of the finished steel plate consists of acicular ferrite, bainite, and polygonal ferrite, with the volume ratio of acicular ferrite being 60%–80%, the volume ratio of bainite being 15%–30%, and the remainder being polygonal ferrite.

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