690mpa grade ship plate with excellent isotropic properties and manufacturing method

Abstract

The present application belongs to the field of metal material production, and particularly relates to a 690MPa-grade ship plate with excellent isotropic performance and a manufacturing method. The composition is as follows in terms of percentage by weight: C: 0.020-0.070%, Si: 0.030-0.250%, Mn: 0.60-1.40%, P: 0.005% or less, S: 0.003% or less, Nb: 0.050-0.100%, V: 0.120-0.220%, N: 0.0280-0.0380%, Cu: 0.50-1.00%, Cr: 0.50-1.50%, Ni: 2.80-3.80%, Ti: 0.010-0.030%, Als: 0.020-0.045%, Ce: 0.0150-0.0850%, and the balance of Fe and inevitable impurities. The advantage is that the isotropy of the steel plate performance is improved.

Description

Technical Field

[0001] This invention belongs to the field of metal material production technology, and in particular relates to a 690MPa 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 CN202310906921.3 discloses a 690MPa grade steel plate with excellent low-temperature ductility and its manufacturing method. Its chemical composition is as follows: C: 0.100%~0.180%, Si: 0.15%~0.75%, Mn: 0.80%~1.60%, V: 0.060%~0.200%, Cu: 0.40%~0.80%, Ni: 2.50%~3.50%, Cr: 0.80%~1.80%, Mo: 0.60%~1.60%, La: 0.020%~0.070%, B: 0.0020~0.0040%, P≤0.008%, S≤0.003%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of over 700 MPa, a tensile strength of over 800 MPa, and an impact absorption energy of over 200 J at -60℃. However, the patent does not evaluate its isotropic properties, and its production process uses a three-stage rolling + tempering process, which is complex. Patent application number CN202310162225.6 discloses an EH40 grade marine high-strength steel and its production method. Its chemical composition is: 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.

[0007] 3) The steel plate has a low strength and toughness level, which cannot meet the requirements for use in the harsh service environment of ships. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a 690MPa grade ship plate with reasonable composition design, simple process, and excellent isotropic properties, as well as a manufacturing method. The invention improves the low-temperature toughness of the steel and the isotropic properties of the steel plate through chemical composition design; and optimizes continuous casting, rolling and other processes to improve the mechanical properties and isotropic properties of the steel plate.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A type of 690MPa grade ship plate with excellent isotropic properties, the composition of which is as follows by weight percentage:

[0011] C: 0.020%~0.070%, Si: 0.030%~0.250%, Mn: 0.60%~1.40%, P≤0.005%, S≤0.003%, Nb: 0.050%~0.100%, V: 0.120%~0.220%, N: 0.0280%~0.0380%, Cu: 0.50%~1.00%, Cr: 0.50%~1.50%, Ni: 2.80%~3.80%, Ti: 0.010%~0.030%, Als: 0.020%~0.045%, Ce: 0.0150%~0.0850%, balance Fe and unavoidable impurities.

[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, and will also affect the properties in the thickness direction. Therefore, considering factors such as cost and performance, the range of C is controlled to be 0.020%~0.070%.

[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 increase the ductile-brittle transition temperature of steel, reduce the low-temperature toughness of the steel plate, and affect the surface quality of the steel plate. Considering factors such as cost and performance, the Si content is controlled within the range of 0.030% to 0.250%.

[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, enhancing steel strength through solid solution strengthening and positively impacting grain refinement. It can also delay the ferrite-pearlite transformation in steel, significantly increasing hardenability, lowering the ductile-brittle transition temperature, altering impact toughness, eliminating the influence of sulfur (S), and improving hot workability. However, excessive Mn content can easily lead to segregation in steel, negatively affecting both plasticity and toughness, particularly in the thickness direction. Considering all factors, the recommended Mn content is controlled within the range of 0.60% to 1.40%.

[0016] Nitrogen (Nb): A grain-refining element. When heated, undissolved Nb carbon and nitride particles are distributed on the austenite grain boundaries, which can hinder the growth of austenite grains in steel during heating. Upon cooling, it can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, and refine ferrite grains, thus improving the strength and toughness of steel. Therefore, the range of Nb should be controlled between 0.050% and 0.100%.

[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 0.120%~0.220%.

[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 c and ni, reducing the carbon equivalent of the steel, reducing the alloy cost of the steel plate, reducing the tendency of core segregation in the steel plate, and promoting the uniform distribution of mechanical properties. Considering both performance and cost, the range of nitrogen is controlled at 0.0280%~0.0380%.

[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 0.50%~1.00%.

[0020] Cr: Increases the hardenability of steel and enhances its strength and toughness, especially in the preparation of extra-thick steel plates, where it plays a positive role in improving the strength and toughness of the steel core. Adding a small amount of Cr can also effectively delay the initial corrosion of the steel plate. Considering factors such as cost and performance, the range of Cr is controlled at 0.50%~1.50%.

[0021] 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 2.80%~3.80%.

[0022] 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 concentration of Ti is controlled within the range of 0.010% to 0.030%.

[0023] 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 0.020% to 0.045%.

[0024] Ce has excellent deoxidizing and desulfurizing effects, improves the fluidity of steel, reduces non-metallic inclusions, makes the steel structure denser and purer, and improves the anisotropic properties of steel, which is beneficial to improving the comprehensive mechanical properties of steel. Therefore, the range of Ce should be controlled at 0.0150%~0.0850%.

[0025] The transverse yield strength at 1 / 4 thickness of the ship plate is 732~773MPa, the tensile strength is 801~854MPa, the elongation after fracture is ≥22.5%, the impact absorption energy at -60℃ is ≥280J, and the impact absorption energy at -80℃ is ≥220J.

[0026] The transverse yield strength of the core is 730~768MPa, the tensile strength is 798~845MPa, the elongation after fracture is ≥23.0%, the impact absorption energy at -60℃ is ≥260J, and the impact absorption energy at -80℃ is ≥215J.

[0027] The longitudinal yield strength at 1 / 4 thickness is 730~771MPa, the tensile strength is 802~847MPa, the elongation after fracture is ≥23.5%, the impact absorption energy at -60℃ is ≥280J, and the impact absorption energy at -80℃ is ≥215J.

[0028] The longitudinal yield strength of the core is 721~764MPa, and the tensile strength is 793~838MPa; the elongation after fracture is ≥23.0; the impact energy absorbed at -60℃ is ≥280J; and the impact energy absorbed at -80℃ is ≥220J.

[0029] Z-direction properties: tensile strength: 790~830MPa; reduction of area: 75%~85%; impact energy absorbed at -60℃ ≥265J, impact energy absorbed at -80℃ ≥212J;

[0030] Lateral NDTT ≤ -80℃, longitudinal NDTT ≤ -85℃, thickness NDTT ≤ -80℃.

[0031] A method for manufacturing 690MPa grade ship plates with excellent isotropic properties includes smelting, continuous casting, furnace heating, rolling, online accelerated cooling, and stacking slow cooling, as detailed below:

[0032] 1) Smelting:

[0033] a) Adjust the content of elements such as C, Si, Mn, P, and S during converter smelting to the set range, and add other alloying components for smelting;

[0034] b) Adjust the content of other alloying elements to within a set range during steel refining;

[0035] c) The refined molten steel is subjected to RH treatment for 50-70 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 640-660 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.

[0036] To obtain molten steel with a chemical composition within a specified range;

[0037] 2) Continuous casting:

[0038] Step 1) The molten steel obtained is continuously cast to produce the required billet. During continuous casting, a high superheat is used in the tundish, controlling the superheat to 45~65℃, with full-process protective pouring. The secondary cooling water is controlled in sections, with strong secondary cooling water control in the vertical section, and the secondary cooling water specific volume is 0.60~0.90m³. 3 / t, the curved section uses weak secondary cooling water control, with a secondary cooling water specific flow rate of 0.40~0.70m³. 3 / t; the billet pulling speed is 0.45~0.95m / min; electromagnetic stirring is used during continuous casting, with a stirring current of 500~600A, and a light reduction process is used at the end of continuous casting, with a reduction of 13.0~18.0mm.

[0039] 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 700~750℃. It is then stacked and slowly cooled with an initial stacking temperature of 650~700℃ and a slow cooling time of ≥36h.

[0040] 3) Heating in a heating furnace:

[0041] The billet is heated to 1100~1250℃ and held for 1.0~2.0h, with a total furnace time of 4.5~6.5h.

[0042] 4) Rolling:

[0043] The cast billet is rolled into hot-rolled steel plate in two stages:

[0044] 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 descaling, the billet is heated to 930~980℃ before rolling. The roll speed is controlled at 1.80~2.80m / s. After widening the continuously cast billet, it is longitudinally rolled in one pass for warming. The reduction in the longitudinal rolling pass is 50~60mm, the reduction rate is >18%, and the final rolling temperature is 860~900℃. The thickness of the warmed billet after longitudinal rolling is (1.5~2.0)t, where t is the final product thickness.

[0045] The second stage rolling temperature is 760~810℃, and the final rolling temperature is 660~710℃.

[0046] 5) Online accelerated cooling:

[0047] To maintain fine grains after rolling and prevent grain growth, the rolled steel plate adopts ultra-fast cooling (UFC cooling) + laminar accelerated cooling (ACC cooling). The UFC cooling rate is 15.0~25.0℃ / s, and the final cooling temperature is 450~550℃. The ACC cooling rate is 8.0~18.0℃ / s, and the final cooling temperature is 300~400℃.

[0048] 6) Stacking and slow cooling:

[0049] 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 200~300℃ and a stacking time of ≥20h.

[0050] Compared with the prior art, the beneficial effects of the present invention are:

[0051] 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. By adding elements such as Ni, Cr, and Cu to improve hardenability, the thickness-direction strength and toughness of the steel plate are increased, and the addition of Ce element improves the purity of molten steel and the quality of continuously cast billets, thereby enhancing the isotropic properties of the steel plate.

[0052] This invention employs a continuous casting process with high superheat, strong secondary cooling water in the vertical section, weak secondary cooling water in the curved section, electromagnetic stirring, and light pressure to control center segregation in the billet and obtain fine initial austenite grains. The continuously cast billet utilizes 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 steel plate with excellent mechanical properties and isotropic properties. The steel plate has a bainitic microstructure, with tensile strengths in the transverse, longitudinal, and Z directions all exceeding 790 MPa, a transverse-to-longitudinal strength difference ≤15 MPa, impact absorption energy at -60℃ ≥260 J, impact absorption energy at -80℃ ≥212 J, and NDTT ≤-80℃. Attached Figure Description

[0053] Figure 1 This is a metallographic diagram of a 690MPa grade ship plate with excellent isotropic properties. Detailed Implementation

[0054] 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.

[0055] 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 1The 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.

[0056] Table 1. Smelting process and chemical composition (wt%) of the steel in the examples

[0057]

[0058] Table 2. Steelmaking process in the examples

[0059]

[0060] Table 3. Casting billet cooling and reheating process

[0061]

[0062] Table 4. Rolling process of steel in the examples

[0063]

[0064] Table 5 Cooling process of steel in the examples

[0065]

[0066] 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.

[0067] Table 6-1 Mechanical properties of the steel in the examples

[0068]

[0069] Table 6-2 Mechanical properties of steel in the examples

[0070]

[0071] 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.

[0072] Table 7. Z-axis properties of the steels in the examples.

[0073]

[0074] 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.

[0075] Table 8 NDTT of steel in the examples

[0076] .

Claims

1. A 690MPa grade ship plate with excellent isotropic properties, characterized in that, The composition of the ship plate, by weight percentage, is as follows: C: 0.020%~0.070%, Si: 0.030%~0.250%, Mn: 0.60%~1.40%, P≤0.005%, S≤0.003%, Nb: 0.050%~0.100%, V: 0.120%~0.220%, N: 0.0280%~0.0380%, Cu: 0.50%~1.00%, Cr: 0.50%~1.50%, Ni: 2.80%~3.80%, Ti: 0.010%~0.030%, Als: 0.020%~0.045%, Ce: 0.0150%~0.0850%, balance Fe and unavoidable impurities; The manufacturing method for the 690MPa grade ship plate with excellent isotropic properties includes smelting, continuous casting, heating in a heating furnace, rolling, online accelerated cooling, and slow cooling in stacking, 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 produce the required billet. During continuous casting, the superheat of the tundish is controlled at 45~65℃, and the entire process is protected during pouring. The secondary cooling water is controlled in sections, with the vertical section using strong secondary cooling water control. The specific flow rate of the secondary cooling water is 0.60~0.90m³. 3 / t, the curved section uses weak secondary cooling water control, with a secondary cooling water specific flow rate of 0.40~0.70m³. 3 / t; the billet pulling speed is 0.45~0.95m / min; electromagnetic stirring is used during continuous casting, with a stirring current of 500~600A, and a light reduction process is used at the end of continuous casting, with a reduction of 13.0~18.0mm; The continuously cast billet is rapidly cooled, with an initial cooling temperature of 1000~1100℃ and a final cooling temperature of 700~750℃. It is then stacked for slow cooling, with an initial stacking temperature of 650~700℃ 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 930~980℃ before rolling. The roll speed is controlled at 1.80~2.80m / 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 50~60mm, the reduction rate is >18%, and the final rolling temperature is 860~900℃. 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 760~810℃, and the final rolling temperature is 660~710℃. 5) Online accelerated cooling: The rolled steel plate adopts ultra-fast cooling + laminar flow accelerated cooling. The ultra-fast cooling rate is 15.0~25.0℃ / s, and the final cooling temperature is 450~550℃. The ACC cooling rate is 8.0~18.0℃ / s, and the final cooling temperature is 300~400℃. 6) Stacking and slow cooling: After cooling, the steel plates are stacked for slow cooling at a temperature of 200~300℃ for a duration of ≥20h.

2. The 690MPa grade ship plate with excellent isotropic properties according to claim 1, characterized in that, The transverse yield strength at 1 / 4 thickness of the ship plate is 732~773MPa, the tensile strength is 801~854MPa, the elongation after fracture is ≥22.5%, the impact absorption energy at -60℃ is ≥280J, and the impact absorption energy at -80℃ is ≥220J. The transverse yield strength of the core is 730~768MPa, the tensile strength is 798~845MPa, the elongation after fracture is ≥23.0%, the impact absorption energy at -60℃ is ≥260J, and the impact absorption energy at -80℃ is ≥215J. The longitudinal yield strength at 1 / 4 thickness is 730~771MPa, the tensile strength is 802~847MPa, the elongation after fracture is ≥23.5%, the impact absorption energy at -60℃ is ≥280J, and the impact absorption energy at -80℃ is ≥215J. The longitudinal yield strength of the core is 721~764MPa, and the tensile strength is 793~838MPa; the elongation after fracture is ≥23.0; the impact energy absorbed at -60℃ is ≥280J; and the impact energy absorbed at -80℃ is ≥220J. Z-direction properties: tensile strength: 790~830MPa; reduction of area: 75%~85%; impact energy absorbed at -60℃ ≥265J, impact energy absorbed at -80℃ ≥212J; Lateral NDTT ≤ -80℃, longitudinal NDTT ≤ -85℃, thickness NDTT ≤ -80℃.

3. The 690MPa grade ship plate with excellent isotropic properties according to claim 1, 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 50-70 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 640-660 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.

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