500mpa grade ship plate with excellent isotropic properties and manufacturing method

Abstract

The application belongs to the technical field of metal material production and relates to a 500MPa-grade ship plate with excellent isotropic performance and a manufacturing method. The composition of the ship plate is as follows in terms of percentage by weight: C: 0.030%-0.080%, Si: 0.050%-0.200%, Mn: 0.70%-1.20%, P: 0.005% or less, S: 0.003% or less, Nb: 0.040%-0.090%, V: 0.080%-0.180%, N: 0.0250%-0.0300%, Cu: 0.20%-0.50%, Ni: 0.30%-1.00%, Ti: 0.006%-0.025%, Als: 0.015%-0.040%, La: 0.0100%-0.0800%, and the balance of Fe and inevitable impurities. The ship plate has the advantages that the isotropy of the performance of the steel plate is improved.

Description

Technical Field

[0001] This invention belongs to the field of metal material production technology, and in particular relates to a 500MPa grade ship plate with excellent isotropic properties and its manufacturing method. Background Technology

[0002] Ships operate in the marine environment and must withstand harsh conditions such as wind, waves, rain, snow, and low temperatures for extended periods, requiring the steel used in shipbuilding to possess excellent strength and toughness. For thick-gauge hull steel, not only is good strength and toughness required, but it is also necessary to have good strength and toughness in the transverse, longitudinal, and thickness directions, meaning that the steel plate must exhibit good isotropic properties.

[0003] Patent application number CN202011054673.7 discloses an isotropic EH690 steel plate and its manufacturing method. Its chemical composition is as follows: C: 0.12%-0.14%, Si: 0.3%-0.4%, Mn: 0.8%-1.2%, P≤0.02%, S≤0.01%, Als: 0.01%-0.03%, Ni: 1.0%-2.0%, Cr: 0.5%-1.0%, Mo: 0.3%-0.4%, Cu: 0.2%-0.4%, Nb: 0.02%-0.04%, V: 0.03%-0.05%, Ti: 0.01%-0.02%, B: 0.001%-0.0015%, with the balance being iron and unavoidable impurities. The production process is as follows: initial rolling temperature of the cast billet is 1000-1100℃, final rolling temperature is 850-950℃, normalizing temperature is 850-900℃, holding time is 1.0-1.5 min / mm, quenching temperature is 850-880℃, holding time is 1.0-2.0 min / mm, tempering temperature is 580-630℃, holding time is 2.5-3.5 min / mm. The difference in transverse and longitudinal impact toughness of the steel plate is small. However, the process is complex, and only the difference in transverse and longitudinal impact toughness is evaluated, without evaluating the isotropic strength of the steel plate. Patent application number CN202110392427.0 discloses an economical low-temperature toughness grade E high-strength marine steel plate and its manufacturing method. Its chemical composition is: C 0.11%-0.16%, Si 0.05%-0.14%, Mn 1.45%-1.65%, S≤0.002%, P≤0.008%, Als0.02%-0.04%, N 0.003%-0.015%, Nb 0.01%-0.03%, Ti 0.008%-0.014%, with the balance being Fe and unavoidable impurities. The production process is as follows: a two-stage controlled rolling technology is adopted, involving both the austenitic recrystallization and non-recrystallization zones. The initial rolling temperature is 1000-1050℃, with a single-pass deformation rate ≥16%. The intermediate billet thickness is 3-3.5 times the finished steel plate thickness. The second-stage rolling temperature is 750-850℃, with a single-pass deformation rate ≥12%. The final rolling temperature is 700-780℃, the initial cooling temperature is 660-740℃, and the final cooling temperature is 480-560℃. This steel exhibits excellent tensile strength and impact properties, but its isotropic properties were not evaluated.Patent application number CN202310162225.6 discloses an EH40 grade marine high-strength steel and its production method. Its chemical composition is as follows: C: 0.13%-0.16%, Si: 0.35%-0.50%, Mn: 1.45%-1.60%, P≤0.015%, S≤0.005%, Als: 0.015%-0.045%, Nb: 0.035%-0.050%, V: 0.050%-0.065%, Ti: 0.008%-0.020%, N≤40ppm. The production process is as follows: initial rolling temperature 1080-1120℃, finishing rolling initial rolling temperature 860-920℃, finishing rolling final rolling temperature 800-840℃, initial cooling temperature ≥760℃, final cooling temperature 500±20℃, normalizing temperature 860℃, holding time 1.6±0.1min / mm, using weak cooling during normalizing, and a final cooling temperature of 670℃±15℃. This patent only describes the production process of EH40 steel plates and does not evaluate its isotropy; its strength level is also relatively low, failing to meet the requirements of harsh marine service environments.

[0004] In summary, the following problems exist in the current production of isotropic high-strength ship plates.

[0005] 1) The production process of steel plates is complex, the production cycle is long, and the production cost is relatively high;

[0006] 2) The isotropic properties of the steel plates are insufficient and cannot meet the requirements for use in high-tech ships. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a 500MPa grade ship plate with excellent isotropic properties, which has a reasonable composition design and simple process, and a manufacturing method thereof. The low-temperature toughness of the steel and the isotropic properties of the steel plate are improved by chemical composition design; and the mechanical properties and isotropic properties of the steel plate are improved by optimizing continuous casting, rolling and other processes.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A 500MPa grade ship plate with excellent isotropic properties, the composition of which is as follows by weight percentage:

[0010] C: 0.030%~0.080%, Si: 0.050%~0.200%, Mn: 0.70%~1.20%, P≤0.005%, S≤0.003%, Nb: 0.040%~0.090%, V: 0.080%~0.180%, N: 0.0250%~0.0300%, Cu: 0.20%~0.50%, Ni: 0.30%~1.00%, Ti: 0.006%~0.025%, Als: 0.015%~0.040%, La: 0.0100%~0.0800%, balance Fe and unavoidable impurities.

[0011] The metallographic structure of the ship plate is ferrite + bainite, and the proportion of ferrite is 3% to 8% by area ratio.

[0012] The role of each chemical component in this invention will be described in detail below.

[0013] C: An important strengthening element in steel, in this invention it is the main element for controlling the microstructure of the steel plate, optimizing the precipitation of microalloying elements, and ensuring the strength and hardness of the rolled steel plate. A low content will reduce the formation of carbides, affecting the grain refinement effect during rolling. A high content will increase the cementite content in the steel, which is detrimental to the low-temperature toughness, plasticity, and weldability of the steel plate. Therefore, considering factors such as cost and performance, the range of C is controlled to be 0.030%~0.080%.

[0014] Si (Si): It can improve the strength and hardness of steel and plays a deoxidizing role in the steelmaking process. However, excessive Si content can cause an increase in the ductile-brittle transition temperature, reduce the low-temperature toughness of the steel plate, and affect its surface quality. Considering factors such as cost and performance, the Si content is controlled within the range of 0.050% to 0.200%.

[0015] Mn (Mn): An element that expands the austenite phase region, forming substitutional solid solutions in steel. It can dissolve extensively in the Fe matrix, strengthening the steel through solid solution and positively impacting grain refinement. It can also delay the ferrite-pearlite transformation, significantly increase hardenability, lower the ductile-brittle transition temperature, alter impact toughness, eliminate the influence of sulfur (S), and improve hot workability. However, excessive Mn content can easily lead to segregation, negatively affecting both plasticity and toughness, especially in the thickness direction. Considering all factors, the recommended Mn content is controlled within the range of 0.70% to 1.20%.

[0016] Nitrogen (Nb): A grain-refining element. When heated, undissolved Nb carbon and nitride particles are distributed along the austenite grain boundaries, hindering austenite grain growth during heating. Upon cooling, it effectively delays the recrystallization of deformed austenite, preventing austenite grain growth and refining ferrite grains, thus improving both the strength and toughness of the steel. Therefore, the range of Nb should be controlled between 0.040% and 0.090%.

[0017] Vanadium (V): A key strengthening and toughening element in this invention, it has a strong affinity for nitrogen and carbon, and is a strong carbide and nitride forming element, capable of forming extremely stable carbonitrides. In steel, it improves the strength through grain refinement, precipitation strengthening, and solid solution strengthening. As the V content increases, the ductile-brittle transition temperature of the steel decreases. However, excessively high V content can actually increase the ductile-brittle transition temperature. Vanadium carbonitrides can precipitate at lower temperatures, hindering dislocation movement and thus playing a strengthening and toughening role, significantly improving the overall mechanical properties of steel plates. In the production of thick steel plates, by utilizing the cross-sectional temperature gradient of the steel plate, the differentiated precipitation of V phases in the cross-section is controlled, achieving isotropic mechanical properties. Considering all factors, the controlled V range is 0.080%~0.180%.

[0018] Nitrogen (N): Another important strengthening and toughening element in this invention. The addition of nitrogen promotes the formation of a large amount of v (CN) in the steel, thereby refining its grains and improving its plasticity and toughness. Nitrogen-containing steel not only eliminates the cost increase caused by degassing and refining denitrification during steelmaking, but also allows the addition of nitrogen to fully utilize the role of microalloying elements, saving the amount of alloying elements used, thus greatly reducing production costs. It can optimize the precipitation behavior of v in the steel cross-section, making its properties more uniform. In addition, the addition of nitrogen can partially replace the role of carbon, reducing the carbon equivalent of the steel, thereby reducing core segregation in the steel plate and promoting uniform distribution of properties. Considering both performance and cost, the range of nitrogen is controlled at 0.0250%~0.0300%.

[0019] Cu can improve the stability of austenite in steel, increase the hardenability of steel, and improve the strength, plasticity and low-temperature toughness of steel when added in appropriate amounts. During low-temperature treatment, it can produce ε-Cu precipitation, thereby improving the strength of steel. However, if the content is too high, the hot brittleness of steel will deteriorate and hot cracking will easily occur. Therefore, the range of Cu should be controlled at 0.20%~0.50%.

[0020] Ni has no adverse effects on the hardening and toughness of the weld heat-affected zone of steel, and can improve the plasticity and low-temperature toughness of steel. In addition, the addition of Ni can reduce the tendency for hot cracking when the Cu content is high. Taking into account factors such as cost and performance, the range of Ni is controlled at 0.30%~1.00%.

[0021] Ti (TiO2) exhibits strong precipitation strengthening, increasing the strength of steel and preventing austenite recrystallization. Simultaneously, it refines the grain size, improving the yield strength of the steel. The high dissolution temperature of Ti carbides and nitrides inhibits austenite grain growth during heating, thus refining the grain size of the steel plate, improving its strength and toughness, and promoting the overall mechanical properties of the steel plate. Considering all factors, the Ti content should be controlled within the range of 0.006% to 0.025%.

[0022] Al: A strong deoxidizer in steel; adding a small amount can generate highly fine, ultra-microscopic oxides, which has a beneficial effect on improving the purity of steel. In this invention, the Al content is controlled within the range of 0.015% to 0.040%.

[0023] La has excellent deoxidizing and desulfurizing effects, improves the fluidity of steel, reduces non-metallic inclusions, makes the steel structure dense and pure, and improves the anisotropic properties of steel, which is beneficial to improving the comprehensive mechanical properties of steel. In this invention, the range of La is controlled to be 0.0100%~0.0800%.

[0024] The transverse yield strength at 1 / 4 thickness of the ship plate is 530~580MPa, the tensile strength is 618~670MPa, the elongation after fracture is ≥23.5%, the impact absorption energy at -60℃ is ≥260J, and the impact absorption energy at -80℃ is ≥200J.

[0025] The transverse yield strength of the core is 520~560MPa, the tensile strength is 612~660MPa, the elongation after fracture is ≥23.0%, the impact absorption energy at -60℃ is ≥250J, and the impact absorption energy at -80℃ is ≥200J.

[0026] The longitudinal yield strength at 1 / 4 thickness is 530~580MPa, the tensile strength is 619~660MPa, the elongation after fracture is ≥24.5%, the impact absorption energy at -60℃ is ≥270J, and the impact absorption energy at -80℃ is ≥200J.

[0027] The longitudinal yield strength of the core is 520~564MPa, and the tensile strength is 600~650MPa; the elongation after fracture is ≥23.0, the impact energy absorbed at -60℃ is ≥260J, and the impact energy absorbed at -80℃ is ≥200J.

[0028] Z-direction properties: Z-direction tensile strength: 600~630MPa; reduction of area: 75%~85%; impact energy absorbed at -60℃ ≥250J, impact energy absorbed at -80℃ ≥200J;

[0029] Transverse NDTT ≤ -75℃, longitudinal NDTT ≤ -80℃, thickness NDTT ≤ -75℃.

[0030] A method for manufacturing 500MPa grade ship plates with excellent isotropic properties includes smelting, continuous casting, furnace heating, rolling, online accelerated cooling, and stacking slow cooling, as detailed below:

[0031] 1) Smelting:

[0032] a) Adjust the content of C, Si, Mn, P and S elements during converter smelting and add other alloying components for melting;

[0033] b) Adjusting the content of other alloying elements during steel refining;

[0034] c) The refined molten steel is subjected to RH treatment for 40-60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 620-640 Pa to ensure that the final N content is within the set range and to control [H] ≤ 1.5 ppm and [O] ≤ 10 ppm in the steel.

[0035] 2) Continuous casting:

[0036] Step 1) The molten steel obtained is continuously cast to obtain the required billet. During continuous casting, a high superheat is used in the tundish, and the superheat is controlled at 45~65℃, with full-process protective pouring; the billet pulling speed is controlled at 0.40~0.90m / min, and the secondary cooling water ratio is 0.60~0.90m³ / min. 3 / t; Electromagnetic stirring is used during continuous casting with a stirring current of 480~580A, and a light reduction process is used at the end of continuous casting with a reduction of 10.0~15.0mm;

[0037] To control the grain size of the continuously cast billet, the billet is rapidly cooled with an initial cooling temperature of 1000~1100℃ and a final cooling temperature of 750~800℃. It is then stacked and slowly cooled with an initial stacking temperature of 700~730℃ and a slow cooling time of ≥36h.

[0038] 3) Heating in a heating furnace:

[0039] The billet is heated to 1100~1250℃ and held for 1.0~2.0h, with a total furnace time of 4.5~6.5h.

[0040] 4) Rolling:

[0041] The cast billet is rolled into hot-rolled steel plate in two stages:

[0042] In the first stage, in order to fully break down the columnar crystals of the continuously cast billet and prepare for subsequent grain refinement, a high-temperature fast rolling + large reduction method is adopted. After the billet is descaled after exiting the furnace, it is heated to 900~950℃ before rolling. The roll speed is controlled at 1.50~2.50m / s. After the continuously cast billet is widened, it is longitudinally rolled in one pass for warming. The reduction of the longitudinal rolling pass is 45~55mm, the reduction rate is >15%, and the final rolling temperature is 830~870℃. The thickness of the warmed billet after longitudinal rolling is (1.5~2.0)t, where t is the final product thickness.

[0043] The second stage rolling temperature is 750~800℃, and the final rolling temperature is 680~730℃.

[0044] 5) Online accelerated cooling:

[0045] To maintain fine grains after rolling and prevent grain growth, the rolled steel plate is subjected to ultra-fast cooling (UFC cooling) with a cooling rate of 8.0~20.0℃ / s and a reddening temperature of 370~420℃.

[0046] 6) Stacking and slow cooling:

[0047] In order to release the internal stress formed during the rolling and cooling process of steel plates and to further form fine precipitates, the cooled steel plates are stacked for slow cooling at a stacking temperature of 250~350℃ and a stacking time of ≥20h.

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] This invention employs a low-carbon chemical composition system to improve the low-temperature toughness of steel, and adds grain-refining elements such as Nb, VN, and Ti to promote the formation of ferrite in the steel. The addition of La enhances the purity of the molten steel and the quality of the continuously cast billet, thereby improving the isotropic properties of the steel plate. The continuous casting process utilizes high superheat, strong secondary cooling water, electromagnetic stirring, and light pressure to control center segregation in the billet and obtain fine initial austenite grains. The continuously cast billet employs a rapid cooling followed by slow stacking cooling process to control grain growth and adjust the precipitated phases. The rolling process uses a two-stage controlled rolling TMCP process, resulting in a final steel plate with excellent mechanical properties and isotropic properties. The metallographic structure of the steel plate is ferrite + bainite, with ferrite accounting for 3-8%. The tensile strength in the transverse, longitudinal, and Z directions all reach 600MPa or more, the difference in strength between the transverse and longitudinal directions is ≤15MPa, the impact absorption energy at -60℃ is ≥250J, the impact absorption energy at -80℃ is ≥200J, and the NDTT is ≤-75℃. Attached Figure Description

[0050] Figure 1 This is a metallographic diagram of a 500MPa grade ship plate with excellent isotropic properties. Detailed Implementation

[0051] The present invention will be described in more detail below through embodiments. These embodiments are merely descriptions of the best implementation of the present invention and do not limit the scope of the present invention in any way.

[0052] Smelting was carried out according to the chemical composition range designed according to the present invention. The chemical composition is shown in Table 1. The metallographic structure of the ship plate is shown in Table 1. Figure 1 The molten steel obtained is continuously cast, heated, rolled and cooled to obtain the steel plate of the present invention. The smelting process is shown in Table 2, the heating process is shown in Table 3, the rolling process is shown in Table 4 and the cooling process is shown in Table 5.

[0053] Table 1. Smelting process and chemical composition (wt%) of the steel in the examples

[0054]

[0055] Table 2. Steelmaking process in the examples

[0056]

[0057] Table 3. Casting billet cooling and reheating process

[0058]

[0059] Table 4. Rolling process of steel in the examples

[0060]

[0061] Table 5 Cooling process of steel in the examples

[0062]

[0063] The mechanical properties of the steel in the embodiments of the present invention were tested, and the results are shown in Tables 6-1 and 6-2.

[0064] Table 6-1 Mechanical properties of the steel in the examples

[0065]

[0066] Table 6-2 Mechanical properties of steel in the examples

[0067]

[0068] The Z-axis properties of the steel in this invention embodiment are shown in Table 7. The crack propagation direction of the impact specimen is consistent with the rolling direction.

[0069] Table 7. Z-axis properties of the steels in the examples.

[0070]

[0071] The NDTT of the steel in this embodiment is shown in Table 8. The NDT specimen is a P3 specimen, and the NDT in the thickness direction is such that the crack propagation direction is consistent with the rolling direction.

[0072] Table 8 NDTT of steel in the examples

[0073] .

Claims

1. A 500MPa grade ship plate with excellent isotropic properties, characterized in that, The composition of the ship plate, by weight percentage, is as follows: C: 0.030%~0.080%, Si: 0.050%~0.200%, Mn: 0.70%~1.20%, P≤0.005%, S≤0.003%, Nb: 0.040%~0.090%, V: 0.080%~0.180%, N: 0.0250%~0.0300%, Cu: 0.20%~0.50%, Ni: 0.30%~1.00%, Ti: 0.006%~0.025%, Als: 0.015%~0.040%, La: 0.0100%~0.0800%, balance Fe and unavoidable impurities; The metallographic structure of the ship plate is ferrite + bainite, and the proportion of ferrite is 3% to 8% by area ratio; The transverse yield strength at 1 / 4 thickness of the ship plate is 530~580MPa, the tensile strength is 618~670MPa, the elongation after fracture is ≥23.5%, the impact absorption energy at -60℃ is ≥260J, and the impact absorption energy at -80℃ is ≥200J. The transverse yield strength of the core is 520~560MPa, the tensile strength is 612~660MPa, the elongation after fracture is ≥23.0%, the impact absorption energy at -60℃ is ≥250J, and the impact absorption energy at -80℃ is ≥200J. The longitudinal yield strength at 1 / 4 thickness is 530~580MPa, the tensile strength is 619~660MPa, the elongation after fracture is ≥24.5%, the impact absorption energy at -60℃ is ≥270J, and the impact absorption energy at -80℃ is ≥200J. The longitudinal yield strength of the core is 520~564MPa, the tensile strength is 600~650MPa; the elongation after fracture is ≥23.0, the impact energy absorbed at -60℃ is ≥260J, and the impact energy absorbed at -80℃ is ≥200J. Z-direction properties: Z-direction tensile strength: 600~630MPa; reduction of area: 75%~85%; impact energy absorbed at -60℃ ≥250J, impact energy absorbed at -80℃ ≥200J; Transverse NDTT ≤ -75℃, longitudinal NDTT ≤ -80℃, thickness NDTT ≤ -75℃.

2. A method for manufacturing 500MPa grade ship plates with excellent isotropic properties as described in claim 1, characterized in that, This includes smelting, continuous casting, furnace heating, rolling, online accelerated cooling, and slow cooling in stacks, as detailed below: 1) Smelting: To obtain molten steel with a chemical composition within a specified range; 2) Continuous casting: Step 1) The molten steel obtained is continuously cast to obtain the required billet. During continuous casting, the superheat of the tundish is controlled at 45~65℃, and the casting process is protected throughout; the billet pulling speed is controlled at 0.40~0.90m / min, and the secondary cooling water ratio is 0.60~0.90m³ / min. 3 / t; Electromagnetic stirring is used during continuous casting with a stirring current of 480~580A, and a light reduction process is used at the end of continuous casting with a reduction of 10.0~15.0mm; The continuously cast billet is rapidly cooled, with an initial cooling temperature of 1000~1100℃ and a final cooling temperature of 750~800℃. It is then stacked for slow cooling, with an initial stacking temperature of 700~730℃ and a slow cooling time of ≥36h. 3) Heating in a heating furnace: The billet is heated to 1100~1250℃ and held for 1.0~2.0h, with a total furnace time of 4.5~6.5h. 4) Rolling: The cast billet is rolled into hot-rolled steel plate in two stages: The first stage adopts a high-temperature fast rolling + large reduction method. After the billet is descaled after exiting the furnace, it is heated to 900~950℃ before rolling. The roll speed is controlled at 1.50~2.50m / s. After the continuous casting billet is widened, it is longitudinally rolled in one pass for warming. The reduction of the longitudinal rolling pass is 45~55mm, the reduction rate is >15%, and the final rolling temperature is 830~870℃. The thickness of the warmed billet after longitudinal rolling is (1.5~2.0)t, where t is the final product thickness. The second stage rolling temperature is 750~800℃, and the final rolling temperature is 680~730℃. 5) Online accelerated cooling: The rolled steel plate is subjected to ultra-fast cooling, with a cooling rate of 8.0~20.0℃ / s and a red-hot temperature of 370~420℃; 6) Stacking and slow cooling: After cooling, the steel plates are stacked for slow cooling at a temperature of 250~350℃ for a duration of ≥20h.

3. The method for manufacturing a 500MPa grade ship plate with excellent isotropic properties according to claim 2, characterized in that, The specific smelting process in step 1) is as follows: a) Adjust the content of C, Si, Mn, P and S elements during converter smelting and add other alloying components for melting; b) Adjusting the content of other alloying elements during steel refining; c) The refined molten steel is subjected to RH treatment for 40-60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 620-640 Pa to ensure that the final N content is within the set range and that [H] ≤ 1.5 ppm and [O] ≤ 10 ppm in the steel are controlled.

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