A high-strength, high-hardness, fatigue-resistant marine steel plate and its manufacturing method

Abstract

The present invention relates to a high-strength, high-hardness, fatigue-resistant marine steel plate and a manufacturing method thereof. The chemical composition of the steel plate includes: C 0.25%-0.30%, Si 0.25%-0.35%, Mn 0.5%-0.7%, P ≤ 0.02%, S ≤ 0.01%, Ni 0.85%-2.0%, Cr 0.5%-0.9%, Mo 0.25%-0.55%, Co 0.1%-0.3%, V 0.04%-0.07%, N 0.002%-0.006%, with the remainder being Fe and impurities. A unique composition design combined with corresponding production processes ensures that the mechanical properties and service safety performance of the steel plate meet the service conditions of marine engineering equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore steel production, and in particular to a high-strength, high-hardness, fatigue-resistant offshore steel plate and a manufacturing method thereof. Background Art

[0002] Since the beginning of the 21st century, the shipbuilding and marine engineering sectors have experienced rapid development. The accelerated development of shipping routes in cold oceans and the Arctic Circle has placed higher demands on related technologies and equipment. Simultaneously, with the deepening of research into offshore steel materials for use in extreme environmental conditions, new offshore steels are emerging to meet increasingly stringent requirements. The Arctic region, rich in resources such as oil and natural gas, holds enormous development potential. Arctic shipping demands not only technical performance in cryogenics, corrosion, and fatigue resistance, but also environmental protection, such as ship emissions and the prevention of marine pollution, which also require relevant technologies and equipment. To adapt to these changes and challenges, research into new offshore steel materials is currently focused on microalloying, controlled rolling, controlled cooling, and heat treatment processes, aiming to improve the materials' frost resistance, corrosion resistance, and other application properties.

[0003] Microalloying technology is currently a hot topic in the research and development of marine 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 heat treatment process optimization are also an important research direction. The use of controlled rolling technology can effectively control the structure and phase transformation of steel, and improve the toughness and fatigue resistance of steel. The fatigue resistance of steel plates is of great significance to marine engineering equipment, and can significantly improve its reliability, safety and economic benefits. By adding appropriate trace alloying elements and combining with advanced heat treatment processes, the fatigue resistance of steel plates can be effectively improved. In response to the new requirements for the fatigue resistance of marine steel plates, fatigue crack growth tests, fatigue life tests and low-cycle fatigue strength tests can be used for verification.

[0004] When selecting steel materials and processing techniques, factors such as performance, cost, and feasibility must be comprehensively considered. While new marine steel materials are constantly emerging, they are generally expensive, requiring a rational selection based on specific needs and economic viability for specific applications. Furthermore, in addition to micro-alloying and heat treatment processes, emerging technologies such as ultrasonic surface treatment can also be used to enhance material fatigue resistance.

[0005] Existing marine engineering steels can already meet most market demands, but specialized steels with superior combined performance, including thick thickness, ultra-high strength, and fatigue resistance, remain a research and development goal for countries worldwide. The development of high-strength steel plates with high service safety is challenging, requiring rigorous production processes and high equipment requirements. For example, the Chinese invention patent with authorization publication number CN115323251B discloses an "ultra-thick, high-strength, high-toughness, and high-homogeneity steel plate for hydropower and its manufacturing method." The steel plate utilizes a composition system with low carbon, high manganese, and rare earth elements. The addition of rare earth elements improves steel cleanliness and impact toughness, but high manganese content in steel ingots can lead to severe center segregation, significantly reducing core toughness and fatigue resistance. In particular, the core mechanical properties of ultra-thick steel plates rely on through-hardening and strengthening with alloying elements such as nickel and chromium. However, the low alloying element content in such steel plates makes it difficult to achieve ultra-high strength, high toughness, and fatigue fracture resistance with only manganese and rare earth elements. A Chinese invention patent with authorization announcement number CN105452511B discloses a "thick steel plate with excellent fatigue properties and its manufacturing method." The steel plate has low contents of elements such as C, Mn, and Ni. Although the steel plate has excellent fatigue performance, its conventional composition and process design can only produce thin-gauge steel plates. As the thickness of the steel plate increases, its composition and process cannot guarantee the steel plate's toughness and fatigue resistance. Summary of the Invention

[0006] The present invention provides a high-strength, high-hardness, fatigue-resistant marine steel plate and a manufacturing method thereof. The unique composition design is combined with the corresponding production process to ensure that the mechanical properties and service safety performance of the steel plate meet the service conditions of marine engineering equipment. The impact energy at -60°C is ≥120J, the Brinell hardness HBW is ≥250, and the fatigue crack growth performance is: when Lg(ΔK)=2MPa·m 0.5 When Lg(da / dN)≤-3mm / cycle, a marine steel plate with large thickness, high strength, high hardness and fatigue resistance is obtained.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A high-strength, high-hardness, fatigue-resistant marine steel plate. The chemical composition of the steel plate comprises, by weight percentage, C 0.25%-0.30%, Si 0.25%-0.35%, Mn 0.5%-0.7%, P ≤ 0.02%, S ≤ 0.01%, Ni 0.85%-2.0%, Cr 0.5%-0.9%, Mo 0.25%-0.55%, Co 0.1%-0.3%, V 0.04%-0.07%, N 0.002%-0.006%, and the remainder is Fe and unavoidable impurities.

[0009] The yield strength of the finished steel plate is ≥690MPa, the tensile strength is 770~940MPa, the elongation is ≥18%, the average Charpy impact energy at -60℃ is ≥120J; the Brinell hardness HBW is ≥250, and the fatigue crack growth performance is: when Lg(ΔK)=2MPa·m 0.5 When, Lg(da / dN)≤-3mm / cycle.

[0010] The maximum thickness of the finished steel plate is 100mm.

[0011] The metallographic structure at 1 / 2 of the thickness of the finished steel plate is tempered martensite + ferrite, with the volume ratio of ferrite being 3% to 10% and the size of ferrite being 2 to 5 μm.

[0012] A method for manufacturing a high-strength, high-hardness, fatigue-resistant marine steel plate comprises the following steps:

[0013] 1) Smelting: After the molten steel is smelted in a converter, it is further subjected to LF refining, RH refining or VD refining to reduce the content of P, S and non-metallic inclusions;

[0014] 2) Mold casting: The casting superheat is 15-25℃, and the slow cooling time after the ingot is demoulded is ≥72h;

[0015] 3) Heating: The billet is loaded into the heating furnace when the furnace temperature is 550-750℃ and kept warm for 7-9 hours. The billet is heated at a rate of 3-4℃ / min in the subsequent heating process. After heating to the low-temperature section heating temperature of 900-1050℃, it is kept warm for 3-4 hours. The secondary heating rate is controlled at 2-4℃ / min. After heating to the high-temperature section heating temperature of 1200-1250℃, it is kept warm for 4-5 hours. The heating temperature of the soaking section is 1180-1230℃ and kept warm for 4-5 hours. The thickness of the intermediate billet after splitting is 270-450mm, and the slow cooling time of the intermediate billet off the line is ≤48 hours. The intermediate billet is re-loaded into the furnace when the temperature is above 400℃, and the heating temperature is 1200-1250℃, and the soaking temperature is 1200-1300℃.

[0016] 4) Rolling: The starting rolling temperature is 1200-1300℃, the average pass reduction rate is 8%-20%, and the final rolling temperature is 1000-1150℃;

[0017] 5) Quenching and tempering: The sub-temperature quenching temperature is 600-750℃, the quenching temperature is 800-950℃, the quenching cooling rate at 1 / 4 of the steel plate thickness is 5-20℃ / s, the tempering temperature is 400-650℃, and the tempering holding time is 3-5min / mm.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1) The composition design combines C, Mn, Ni, Cr, Mo with Co, N, and V elements, and is combined with a production process specifically designed for thick, high-strength, high-hardness, and fatigue-resistant offshore steel to produce offshore steel plates with a maximum thickness of 100 mm that feature thick, high-strength, high-hardness, and fatigue resistance.

[0020] 2) The unique alloy composition system and production process are used to ensure that the mechanical properties and service safety performance of the quenched and tempered steel plate meet the service conditions of marine engineering equipment. Specifically, the yield strength is ≥690MPa, the tensile strength is 770~940MPa, the Charpy impact energy at -60℃ is ≥120J; the Brinell hardness HBW is ≥250, and the fatigue crack growth performance is: when Lg(ΔK)=2MPa·m 0.5 When, Lg(da / dN)≤-3mm / cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a metallographic photograph of the finished steel plate of the present invention at 1 / 2 of the thickness. DETAILED DESCRIPTION

[0022] The present invention is aimed at the performance requirements of high-strength, high-hardness and fatigue-resistant marine engineering steel plates. It utilizes a composition design that combines C, Mn, Ni, Cr, Mo with Co, N, and V elements, and cooperates with a production process suitable for large-thickness, high-strength, high-hardness and fatigue-resistant marine engineering steel. A large number of systematic experimental studies have been carried out in many aspects, including alloy element screening and proportioning, steel cleanliness control, efficient rolling process optimization and parameter selection, and ultimately determined the alloy element proportioning and manufacturing process that can meet the purpose of the present invention.

[0023] The high-strength, high-hardness, and fatigue-resistant marine steel plate disclosed in the present invention comprises, by weight percentage, the following chemical components: C 0.25% to 0.30%, Si 0.25% to 0.35%, Mn 0.5% to 0.7%, P ≤ 0.02%, S ≤ 0.01%, Ni 0.85% to 2.0%, Cr 0.5% to 0.9%, Mo 0.25% to 0.55%, Co 0.1% to 0.3%, V 0.04% to 0.07%, N 0.002% to 0.006%, and the remainder being Fe and unavoidable impurities.

[0024] The composition design reasons of the high-strength, high-hardness, and fatigue-resistant marine steel plate described in the present invention are as follows:

[0025] 1) Carbon, a fundamental strengthening element in steel, plays a key role in ensuring strength and hardness in the marine steel plates described herein. Excessive carbon content will produce a large amount of hardened structure, affecting low-temperature impact toughness and fatigue resistance. Excessive carbon content will result in reduced steel plate strength. The present invention limits the carbon content to 0.25% to 0.30%.

[0026] 2) Si can improve the strength of the steel plate. At the same time, Si can reduce the O content as a deoxidizer. When the Si content is lower than 0.35%, the low-temperature toughness of the steel plate can be prevented from decreasing. In the present invention, the Si content is controlled to be 0.25% to 0.35%.

[0027] 3) Mn, similar in atomic radius to Fe, can be dissolved in large quantities in the Fe matrix, thereby increasing the strength of the steel plate. Mn refines the steel plate's grain structure, increases the number of grain boundaries, improves its low-temperature impact toughness, and enhances its resistance to fatigue crack growth. However, the strength-enhancing alloy content in this invention is relatively high, resulting in greater steel plate strength. When the Mn content exceeds 0.7%, the hardening effect of Mn reduces the low-temperature toughness of the thick plate's core. Therefore, this invention limits the Mn content to 0.5% to 0.7%.

[0028] 4) P and S elements have no benefit on the mechanical properties of the steel plate, especially the elongation. The present invention controls P≤0.02% and S≤0.01%.

[0029] 5) Ni significantly improves the toughness and corrosion resistance of steel plates. Large additions can lower the ductile-brittle transition temperature. Ni and Mn reduce the notch sensitivity of steel plates and improve fatigue performance. Ni, in combination with microalloying elements such as V and Nb, further enhances the strength, toughness, and fatigue resistance of steel plates. The Ni content in this invention is controlled to 0.85% to 2.0%.

[0030] 6) Cr (Cr) in steel can effectively increase the strength of steel plates. Appropriate additions of Cr can improve the cooling control of steel plates and enhance the hardenability of extra-thick steel plates. However, excessive Cr content can produce a large amount of Cr carbides, reducing the impact toughness and fatigue resistance of the steel plates. Furthermore, excessive amounts of Cr can cause temper brittleness in the steel plates after quenching and tempering. Therefore, the present invention controls the Cr content to 0.5% to 0.9%.

[0031] 7) Mo improves the hardenability of the steel plate and forms fine carbides in the steel, effectively increasing its strength. The strengthening precipitates formed by Mo also hinder the displacement and propagation of defects, improving the steel plate's fatigue resistance. Based on the alloy system of the present invention, a Mo content below 0.55% does not significantly reduce low-temperature toughness. Therefore, the present invention limits the Mo content to 0.25% to 0.55%.

[0032] 8) Co can refine the grains, improve the durability and fatigue resistance of the steel plate, and work together with Ni and Cr to improve the low-temperature toughness of the steel plate. However, if the Co content is too high, the hardened phase will precipitate and reduce the mechanical properties of the steel plate. The Co content is 0.1% to 0.3%.

[0033] 9) V is a key alloying element in the steel plate of this invention. It refines the microstructure and improves the steel's strength and toughness. V, combined with carbon and nitrogen, forms V(C,N) particles in the matrix, which refine and strengthen the grains. Adding V to the steel plate during tempering significantly improves its strength, toughness, and fatigue resistance. However, excessive V content can reduce surface quality, lead to fine cracks beneath the surface, and reduce toughness. Therefore, the V content in this invention is controlled to 0.04% to 0.07%.

[0034] 10) The combination of nitrogen and vanadium forms fine, dispersed nitride precipitates, which effectively promote the nucleation and growth of intragranular ferrite, thereby effectively controlling the growth of prior austenite grains. However, excessive dissolved nitrogen content can reduce the hot plasticity of the steel, the toughness and fatigue resistance of the steel plate, and the formation of numerous microcracks on the steel plate surface. Therefore, the present invention limits the nitrogen content to 0.002% to 0.006%.

[0035] The manufacturing process of the high-strength, high-hardness, fatigue-resistant marine steel plate described in the present invention adopts a production process of "high-cleanliness and alloying smelting + die casting + four-stage heating + cogging + controlled rolling + two quenching + tempering", which specifically includes the following steps:

[0036] (1) High cleanliness and alloy smelting;

[0037] After smelting in a converter, the molten steel is refined in an LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions to obtain molten steel that meets the composition requirements.

[0038] (2) mold casting;

[0039] The casting superheat is 15-25℃, and the slow cooling time after the ingot is demoulded is ≥72h.

[0040] (3) rolling;

[0041] The ingot is loaded into the heating furnace at a furnace temperature of 550-750°C and held at this temperature for 7-9 hours. This ensures a consistent temperature throughout the thickness of the ingot during the low-temperature stage, paving the way for uniform microstructure in the high-temperature stage. The subsequent heating rate is controlled at 3-4°C / min. When the ingot reaches the low-temperature stage, 900-1050°C, it is held at this temperature for 3-4 hours to prevent excessive heating and uneven internal heating. The secondary heating rate is controlled at 2-4°C / min, with the high-temperature stage heated to 1200-1250°C for 4-5 hours, and the soaking stage heated to 1180-1230°C for 4-5 hours. The four-stage heating and low-temperature soaking and holding process ensures the full dissolution of C / N compounds, preventing uneven heating of extremely thick mold-cast ingots and abnormal growth of localized as-cast microstructure.

[0042] After slab opening, the intermediate bar thickness is 270-450mm, and the intermediary bar's cooling time after rolling is ≤48h. Controlling the intermediate bar thickness ensures that the compression ratio of the finished steel plate meets the core mechanical property requirements. Controlling the stacking cooling time aims to ensure intermediate bar quality while avoiding the brittle zone and allowing for rapid furnace loading and rolling, improving efficiency and saving heating energy.

[0043] The intermediate billet is re-charged at a temperature above 400°C, with a heating temperature of 1200-1250°C and an immersion temperature of 1200-1300°C. The starting rolling temperature is 1200-1300°C, the average pass reduction is 8-20%, and the final rolling temperature is 1000-1150°C. The temperature of the intermediate billet when re-charging the furnace is limited to avoid hot brittle cracks in the intermediate billet and save energy. The purpose of using high-reduction rolling in the high-temperature section is to maximize the pass reduction under relatively high temperature and low steel plate hardness conditions, further break up the cast grains of the electroslag billet, and increase deformation energy storage and grain nucleation sites.

[0044] (4) Tempering;

[0045] The sub-tempering temperature is 600-750°C, the quenching temperature is 800-950°C, the quenching cooling rate at 1 / 4 of the steel plate thickness is 5-20°C / s, the tempering temperature is 400-650°C, and the tempering holding time is 3-5 min / mm. The purpose of sub-tempering is to refine the as-rolled microstructure, prepare it for high-temperature quenching, further increase the grain size of the tempered microstructure, and enhance the fatigue resistance of the steel plate. Increasing the tempering temperature and tempering holding time maximizes the steel plate's low-temperature toughness and fatigue resistance while maintaining strength.

[0046] The yield strength of the finished steel plate is ≥690MPa, the tensile strength is 770~940MPa, the elongation is ≥18%, the average Charpy impact energy at -60℃ is ≥120J. The Brinell hardness HBW is ≥250, and the fatigue crack growth performance is Lg(ΔK)=2MPa·m 0.5 When Lg(da / dN)≤-3mm / cycle. The maximum thickness of the finished steel plate is 100mm.

[0047] like Figure 1 As shown in the figure, the metallographic structure of the finished steel plate at 1 / 2 of the thickness is "tempered martensite + ferrite", the volume ratio of ferrite is 3% to 10%, the size of ferrite is 2 to 5 μm, and the mechanical properties of the finished product are good.

[0048] In order to more intuitively embody the present invention, the embodiments of the present invention are further described in conjunction with examples. The following examples are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solution that can be obviously obtained by a person skilled in the art within the technical scope disclosed in the present invention, including simple changes or equivalent replacements, is within the scope of protection of the present invention.

[0049] [Example]

[0050] The chemical composition of the steel in each example is shown in Table 1, the main production process parameters are shown in Table 2-1 and Table 2-2, and the properties of the finished steel plate are shown in Table 3.

[0051] Table 1 Chemical composition of steel (weight percentage, %)

[0052]

[0053] Table 2-1 Production process parameters (I)

[0054]

[0055] Table 2-2 Production process parameters (II)

[0056]

[0057] Table 3 Finished steel plate properties

[0058]

[0059] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for manufacturing high-strength, high-hardness, fatigue-resistant marine steel plates, characterized in that: The chemical composition of the steel plate includes, by weight percentage, C: 0.25% to 0.30%, Si: 0.25% to 0.35%, Mn: 0.5% to 0.7%, P≤0.02%, S≤0.01%, Ni: 0.85% to 2.0%, Cr: 0.5% to 0.9%, Mo: 0.25% to 0.55%, Co: 0.1% to 0.3%, V: 0.04% to 0.07%, N: 0.002% to 0.006%, and the rest is Fe and unavoidable impurities; The method for manufacturing the high-strength, high-hardness, fatigue-resistant marine steel plate comprises the following steps: 1) Smelting: After the molten steel is smelted in a converter, it is further subjected to LF refining, RH refining or VD refining to reduce the content of P, S and non-metallic inclusions; 2) Mold casting: The casting superheat is 15-25℃, and the slow cooling time after the ingot is demoulded is ≥72h; 3) Heating: The billet is loaded into the heating furnace when the furnace temperature is 550-750℃ and kept warm for 7-9 hours. The billet is heated at a rate of 3-4℃ / min in the subsequent heating process. After heating to the low-temperature section temperature of 900-1050℃, it is kept warm for 3-4 hours. The secondary heating rate is controlled at 2-4℃ / min. After heating to the high-temperature section temperature of 1200-1250℃, it is kept warm for 4-5 hours. The soaking section temperature is 1180-1230℃ and kept warm for 4-5 hours. The thickness of the intermediate billet after billet opening is 270-450mm, and the slow cooling time of the intermediate billet off the line is ≤48 hours. The intermediate billet is re-loaded into the furnace when the temperature is above 400℃, and the heating temperature is 1200-1250℃, and the soaking temperature is 1200-1300℃. 4) Rolling: The starting rolling temperature is 1200-1300℃, the average pass reduction rate is 8%-20%, and the final rolling temperature is 1000-1150℃; 5) Quenching and tempering: The sub-temperature quenching temperature is 600-750℃, the quenching temperature is 800-950℃, the quenching cooling rate at 1 / 4 of the steel plate thickness is 5-20℃ / s, the tempering temperature is 400-650℃, and the tempering holding time is 3-5min / mm.

2. The method for manufacturing a high-strength, high-hardness, fatigue-resistant marine steel plate according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥690MPa, the tensile strength is 770~940MPa, the elongation is ≥18%, the average Charpy impact energy at -60℃ is ≥120J; the Brinell hardness HBW is ≥250, and the fatigue crack growth performance is: when Lg(ΔK)=2MPa·m 0.5 When, Lg(da / dN)≤-3mm / cycle.

3. The method for manufacturing a high-strength, high-hardness, fatigue-resistant marine steel plate according to claim 1, characterized in that: The maximum thickness of the finished steel plate is 100mm.

4. The method for manufacturing a high-strength, high-hardness, fatigue-resistant marine steel plate according to claim 1, characterized in that: The metallographic structure at 1 / 2 of the thickness of the finished steel plate is tempered martensite + ferrite, with the volume ratio of ferrite being 3% to 10% and the size of ferrite being 2 to 5 μm.

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