An eh550 steel plate for offshore engineering and a manufacturing method thereof

Abstract

An EH550 marine engineering steel plate and a manufacturing method thereof, the steel plate having a chemical composition by mass percent of C=0.06%~0.12%, Si=0.10%~0.50%, Mn=1.20%~1.80%, Cr=0.20%~0.50% or Mo=0.20%~0.50%, Nb≤0.03%, V=0.02~0.04%, Ti=0.01~0.03%, Al=0.015%~0.045%, P≤0.015%, S≤0.006%, and the rest being Fe and inevitable impurities; the steel plate having a thickness of 6~50mm, a yield strength not lower than 550MPa, a uniform elongation not lower than 10%, a yield strength ratio not higher than 0.80, and an impact energy at-40℃ not lower than 200J. Through high-temperature controlled rolling and controlled cooling, a bainite structure with uniform distribution of dislocation density is obtained, and the problems of insufficient uniform elongation and excessively high yield strength ratio of marine engineering steel plates produced by traditional quenching+tempering process are solved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of low-alloy high-strength steel production, and relates to an EH550 marine engineering steel plate with high uniform elongation and low yield ratio and a manufacturing method thereof. BACKGROUND

[0002] With the progress of deep-sea oil and gas exploration technology, there is an increasing demand for large-scale marine engineering structures. In order to achieve the design requirements of lightweight and gravity balance of large structures, the strength grade of structural steel is becoming higher and higher. With the increase of the strength grade of steel plate, different processes must be used to produce to meet the requirements of strength, toughness and other indicators. For example, the quenched and tempered heat treatment process is used to produce marine engineering steel with a yield strength of 550 MPa or above. The tempered martensite or tempered bainite structure formed by the quenching and tempering process can effectively improve the strength while meeting the requirement of high impact toughness. However, there are other important factors that affect the safety of steel service, such as uniform deformation ability and yield ratio, i.e. the material can uniformly distribute stress during the dynamic load process of the structure, and whether it still has high plastic deformation ability after yielding. However, with the increase of yield strength, the uniform elongation is lower, and the yield ratio is higher, and the safety is decreased.

[0003] Uniform elongation and yield ratio are important parameters that jointly affect the safety performance of material service. Improving the uniform elongation can help to improve the uniformity of stress distribution of marine engineering structures under marine dynamic load, avoid stress concentration and improve fatigue performance. A proper yield ratio helps to balance the strength and plasticity of the material to ensure that it still has sufficient deformation ability after yielding to ensure safety.

[0004] Chinese patent CN109112419A discloses a method for preparing 80-100mm thick EH550 steel alloy components including 0.13%-0.15% C and 0.50%-0.55% Ni and other high alloy components, using a long process of twice quenching heat treatment, which has poor weldability and high production cost.

[0005] Chinese patent CN114134414A is directed to a 500MPa strength grade bridge and building structure steel, using a 730℃ low-temperature final rolling controlled rolling and controlled cooling process, which requires high rolling capacity and the rolling mill bears excessive load, which is prone to failure. The patent with publication number CN111455287A adds a high content of Ni, Cr, Mo, Nb and V, and uses an offline tempering process to produce, resulting in high cost and failing to meet the requirement of 550MPa yield strength.

[0006] The patent CN114134416A has the following deficiencies: the low carbon content of 0.04%-0.06% and the high Nb content of 0.030-0.080% are adopted, the converter smelting is difficult, and the micro-alloy cost is high. The patent CN114032459A has the following deficiencies: a very complex heat treatment process route is adopted, the hot-rolled steel plate needs to be heated to 300-650 DEG C for pre-heat preservation, then heated to the two-phase region for heat preservation and quenching, and then heated to 200-450 DEG C for low-temperature tempering, which is completely unable to adapt to the industrialized production equipment of the medium-thick steel plate at present. SUMMARY

[0007] The present application aims to provide an EH550 marine engineering steel plate with high uniform elongation and low yield ratio and a manufacturing method thereof, and produce a marine engineering steel plate with a thickness of 6-50 mm, a yield strength of not less than 550 MPa, a uniform elongation of not less than 10%, a yield ratio of not higher than 0.80, and an impact energy at-40 DEG C of not less than 200 J.

[0008] The technical scheme of the present application is as follows:

[0009] An EH550 marine engineering steel plate, the chemical composition of the steel is as follows: C=0.06%-0.12%, Si=0.10%-0.50%, Mn=1.20%-1.80%, Cr=0.20%-0.50% or Mo=0.20%-0.50%, Nb≤0.03%, V=0.02-0.04%, Ti=0.01-0.03%, Al=0.015%-0.045%, P≤0.015%, S≤0.006%, and the rest is Fe and inevitable impurities; the thickness of the steel plate is 6-50 mm, the yield strength is not less than 550 MPa, the uniform elongation is not less than 10%, the yield ratio is not higher than 0.80, and the impact energy at-40 DEG C is not less than 200 J.

[0010] A manufacturing method of an EH550 marine engineering steel plate, comprising the following steps:

[0011] (1) Converter smelting: after the molten iron produced by the blast furnace is pre-desulfurized, it is poured into a 100-300 t converter for top and bottom combined blowing oxygen treatment, the molten iron is oxidized and decarburized to C=0.05%-0.10%, dephosphorized and desulfurized, and the calculated weight of Mn and Ni ferroalloy raw materials is added, the oxygen blowing is continued to adjust the temperature of the molten steel to 1620-1660 DEG C, then according to the [O] content in the molten steel, aluminum ingot is added for static deoxidization, and argon bottom blowing is performed before tapping;

[0012] (2) Furnace refining: the molten steel with adjusted temperature and preliminary chemical composition is poured into a ladle, transported to a LF refining furnace, and the molten steel is stirred by bottom blowing argon to make the solid inclusions float up fully; 200 meters of pure calcium wire is fed before the LF treatment is completed; then the ladle is sent to a RH vacuum refining furnace for vacuum degassing treatment, the treatment time is greater than or equal to 15 minutes, and the gas content of the molten steel is removed to [H]≤2ppm, [O]≤20ppm, [N]≤50ppm;

[0013] (3) Slab continuous casting: after the molten steel is subjected to secondary refining, the temperature reaches 20~45℃ of overheat for casting, and then the molten steel is transported to a casting machine platform, and is continuously cast into a 200~400mm thick and 1650~2650mm wide slab under the protection of a protective atmosphere and a protective slag covering through an intermediate ladle;

[0014] (4) Slab heating: after the high-temperature continuous casting slab is subjected to flame cutting, the slab is stacked and slowly cooled for 46~50h in a slab yard to prevent slab cracking, and then is transported to a heating furnace and heated to a temperature of 1150~1220℃, the heating time is 4~8h, and the austenite grain size is more than 6 levels;

[0015] (5) Steel plate controlled rolling: after the surface of the billet is removed of the iron oxide scale by high-pressure water, the billet is expanded and rolled in 3~7 passes along the width direction, the rolling temperature is 1100~1150℃, after rolling to the required width, the billet is turned by 90°, and then is rolled to the required steel plate thickness at 900~1050℃, so that the austenite grains are kept in the recrystallized equiaxed shape, and the grain size is more than 8 levels;

[0016] (6) Steel plate controlled cooling: the rolled steel plate is subjected to accelerated cooling by water spraying through a laminar flow header, the cooling rate is 10~30℃ / s, the final cooling temperature is 200~300℃, and the proportion of lath bainite in the microstructure of the steel plate is more than 90%.

[0017] Further, in step (5), the final rolling temperature is 880~950℃, the initial cooling temperature is 850~920℃, and the final cooling temperature is 200~350℃; the high-temperature controlled rolling + controlled cooling function makes the steel plate form not less than 90% of lath bainite and not more than 10% of martensite / austenite components, and the dislocation density is uniformly distributed, and VC or (Nb, V)C composite nanoparticles are precipitated.

[0018] The alloy composition and content of the present application are set based on the following action mechanism:

[0019] C element is a basic strengthening element of ultra-high strength steel, which can effectively improve the bainite formation ability during rolling and cooling. However, too high C content is easy to form martensite, which is not conducive to uniform deformation ability, weldability and low temperature toughness. In order to balance the strength, uniform elongation, low temperature toughness and welding performance of the steel, the C content is controlled to be 0.06~0.12%.

[0020] Si element is one of the main deoxidizing elements in the steelmaking process, and is also a solid solution strengthening element, but the Si content is too high, which will form Fe2SiO4 difficult to remove in the heating process, so that the steel plate surface scale is difficult to peel off, and the surface quality of the steel plate is affected, therefore, the Si content is controlled at 0.10~0.35%.

[0021] Mn is the most important solid solution strengthening element, and is also a strong stable element of austenite, which improves the ability of bainite formation of steel in the cooling process, and plays a role of fine grain strengthening. If the Mn content is too low, the strength is insufficient, and if it is too high, it is easy to segregate and form banded structure, therefore, the Mn content of the present application is controlled at 1.20~1.80%.

[0022] Cr and Mo are both strong hardenability elements of steel, which promote the formation of lath bainite in the controlled cooling process after rolling, and are also strong carbide forming elements, which promote the formation of M 23 C6 and MC type carbide, reduce the pearlite banded structure of the steel plate. Since the effects of the two are similar, the present application adds either one of Cr or Mo to achieve the expected effect, and the content is controlled at 0.20~0.50%.

[0023] Nb is a strong carbide forming element, which precipitates NbC nanoscale particles during slab casting, and effectively inhibits the growth of austenite grains during slab heating. At the same time, Nb can effectively reduce the non-recrystallization temperature of austenite during rolling, inhibit the austenite recrystallization process, and refine the austenite grains in the high temperature range of hot rolling. To play the above role, the Nb content is controlled at 0.030% or less.

[0024] V is a strong carbide forming element, which precipitates nanoscale VC during hot rolling and cooling, thereby improving the strength of the steel. The V content of the present application is controlled at 0.02~0.04%.

[0025] Ti element is a strong oxide and nitride forming element, which effectively removes O and N gas from the molten steel during steel refining, forms TiO and TiN, and inhibits the growth of austenite grains during subsequent heating process. The Ti content of the present application is controlled at 0.01~0.03%.

[0026] Al is a strong deoxidizing and fine-grained element in the steelmaking process, which effectively removes O gas from the molten steel, and forms stable AlN precipitates with N, pinning the austenite grain boundary and effectively inhibiting the growth of austenite grains. The Al content of the present application is controlled at 0.015~0.045%.

[0027] P and S are both harmful elements and should be removed as much as possible in the steelmaking process. P is easy to segregate on the original austenite grain boundaries, forming a high P content enrichment zone at the end of solidification in the center of the slab, thereby reducing toughness. In the steelmaking process, S forms MnS inclusions with Mn, especially in the Mn segregation area of the slab center, a large number of coarse MnS inclusions are formed and transformed into strips during rolling. Therefore, the P and S contents of the present application should be controlled in the low level range of P≤0.015% and S≤0.006%.

[0028] Advantages of the present application: The present application adopts a simple composition design of C-Mn-Cr / Mo alloying and V or Nb-V micro-alloying with strong bainite forming ability, adapts to the characteristics of industrialized mainstream production technology and equipment of steel plate, utilizes the advantages of high temperature zone austenite recrystallization controlled rolling and accelerated cooling, forms dislocation density uniformly distributed lath bainite and residual austenite from uniform equiaxed recrystallized austenite, precipitates high density nanoscale VC or (Nb, V) C particles, improves uniform deformation ability to more than 10%, reduces the yield ratio to less than 0.80, while maintaining the yield strength to more than 550 MPa, and the impact energy at-40℃ is more than 200J. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The complete martensite microstructure diagram of the comparative steel.

[0030] Figure 2 (a), (b), (c) correspond to the optical microstructure of the steel plate after controlled rolling and direct quenching of examples 1, 2 and 3 respectively.

[0031] Figure 3 (a), (b), (c) correspond to the kernel average orientation error map obtained by electron backscatter diffraction data of the steel plate after controlled rolling and controlled cooling direct quenching of examples 1, 2 and 3 respectively, reflecting the local distribution of dislocation density.

[0032] Figure 4 (a), (b), (c) correspond to the transmission electron microscope photos of the steel plate after tempering of examples 1, 2 and 3 respectively, high density of nanoscale VC-(Nb, Ti) C composite carbide precipitates. DETAILED DESCRIPTION

[0033] The technical scheme of the present application will be further illustrated below in combination with examples and comparative steels. The yield strength, tensile strength, uniform elongation, yield ratio and elongation after fracture data are obtained by room temperature tensile according to the national standard "GB / T 228.1-2021 Metal Materials Tensile Test", and the-40℃ Charpy impact energy is obtained by test under low temperature conditions according to the national standard "GB / T 229-2020 Metal Materials Charpy Pendulum Impact Test Method". Example 1

[0034] The present embodiment prepares a marine engineering steel plate of EH550 grade with thickness of 6 mm, uniform elongation of more than 10%, and yield ratio of 0.80 or less. The molten steel is smelted according to the set composition and cast into a 200 mm-thick slab. The chemical composition is shown in Table 1 in terms of percentage by weight, and the balance is Fe and inevitable impurities.

[0035] Process steps: The 200 mm-thick cast slab is heated to 1150℃, held for 3 h, and then pushed out of the heating furnace. The high-pressure water with a pressure of 19 MPa is used to remove the iron oxide scale on the upper and lower surfaces of the slab. Then, two-stage controlled rolling is performed: the first-stage rolling temperature is 1150~950℃, the cumulative reduction is 80% to the intermediate slab thickness of 40 mm, to ensure full recrystallization and refinement of austenite; the second-stage rolling temperature is 920~880℃, the reduction is 80%, and finally the steel plate is hot-rolled to 6 mm at 880℃. After hot rolling, the steel plate passes through 12 groups of header laminar cooling rollers at a speed of 1.2 mm / s, and the final cooling temperature is 260℃, to obtain 95% bainite and 5% martensite-austenite components.

[0036] The yield strength is 578 MPa, the tensile strength is 728 MPa, the elongation after fracture is 18%, the uniform elongation is 10.5%, the yield ratio is 0.79, the impact energy of the half-size specimen at -40℃ is 132 J, and the equivalent impact energy of the full-size specimen is 264 J. Example 2

[0037] The present embodiment prepares a marine engineering steel plate of EH550 grade with thickness of 20 mm, uniform elongation of more than 10%, and yield ratio of 0.80 or less. The molten steel is smelted according to the set composition and cast into a 250 mm-thick slab. The chemical composition is shown in Table 1 in terms of percentage by weight, and the balance is Fe and inevitable impurities.

[0038] Process steps: The 250 mm-thick cast slab is heated to 1200℃, held for 4 h, and then pushed out of the heating furnace. The high-pressure water with a pressure of 19 MPa is used to remove the iron oxide scale on the upper and lower surfaces of the slab. Then, two-stage controlled rolling is performed: the first-stage rolling temperature is 1150~950℃, the cumulative reduction is 64% to the intermediate slab thickness of 90 mm, to ensure full recrystallization and refinement of austenite; the second-stage rolling temperature is 910~870℃, the reduction is 75%, and finally the steel plate is hot-rolled to 20 mm at 870℃. After hot rolling, the steel plate passes through the laminar cooling roller table with 15 groups of headers opened at an acceleration cooling roller table at a speed of 1.0 mm / s, and the final cooling temperature is 290℃, to obtain 92% bainite and 8% martensite-austenite components.

[0039] The yield strength is 575 MPa, the tensile strength is 732 MPa, the elongation after fracture is 20%, the uniform elongation is 11.2%, the yield ratio is 0.78, and the impact energy of the full-size specimen at -40℃ is 272 J. Example 3

[0040] This example prepares a 50 mm thick, 10% or more uniform elongation, 0.80 or less yield ratio EH550 grade steel plate for ocean engineering, smelts the molten steel according to the set composition and casts a 360 mm thick slab, the chemical composition is shown in Table 1 in terms of percentage by weight, and the balance is Fe and inevitable impurities.

[0041] Process steps: heat the 360 mm thick cast slab to 1200℃, hold for 3h and push out of the heating furnace, remove the iron oxide scale on the upper and lower surfaces of the slab with 19 MPa high pressure water, and then carry out two-stage controlled rolling: the first stage rolling temperature is 1150~950℃, the cumulative reduction is 58% to the intermediate slab thickness of 150 mm, to ensure full recrystallization and refinement of austenite; the second stage rolling temperature is 900~850℃, the reduction is 67%, and finally the steel plate is hot rolled to 50 mm at 850℃. After hot rolling, the steel plate passes through the accelerated cooling roller at a speed of 0.80 mm / s, opens 21 groups of cluster laminar flow water for controlled cooling, and the final cooling temperature is 310℃, obtaining 90% bainite and 10% martensite-austenite components.

[0042] The yield strength is 563 MPa, the tensile strength is 727 MPa, the elongation after fracture is 21%, the uniform elongation is 10.5%, the yield ratio is 0.77, and the impact energy of the full-size sample at -40℃ is 234 J.

[0043] Comparative steel:

[0044] A 20 mm thick 550 MPa grade comparative steel is prepared, the molten steel is smelted and cast into a 250 mm thick slab, the chemical composition is shown in Table 1 in terms of percentage by weight, and the balance is Fe and inevitable impurities.

[0045] Process steps: heat the 250 mm thick cast slab to 1200℃, hold for 4h and push out of the heating furnace, remove the iron oxide scale on the upper and lower surfaces of the slab with 19 MPa high pressure water, and then carry out two-stage controlled rolling: the first stage rolling temperature is 1150~950℃, the cumulative reduction is 64% to the intermediate slab thickness of 90 mm, to ensure full recrystallization and refinement of austenite; the second stage rolling temperature is 910~870℃, the reduction is 75%, and finally the steel plate is hot rolled to 20 mm at 870℃. After hot rolling, the steel plate passes through the accelerated cooling roller at a speed of 1.0 mm / s, opens 15 groups of cluster laminar flow cooling roller, and the final cooling temperature is 286℃, obtaining 100% martensite.

[0046] The yield strength is 685 MPa, the tensile strength is 967 MPa, the elongation after fracture is 15.3%, the uniform elongation is 7.1%, the yield ratio is 0.71, and the impact energy of the full-size sample at -40℃ is 36 J.

[0047] Table 1. Chemical composition and measured properties of the example and comparative steels

[0048] .

Claims

1. A method of manufacturing an EH550 marine engineering steel plate, characterized by: The steel has the following chemical composition in mass percent: C = 0.06% to 0.12%, Si = 0.10% to 0.50%, Mn = 1.20% to 1.80%, Cr = 0.20% to 0.50% or Mo = 0.20% to 0.50%, Nb ≤ 0.03%, V = 0.02 to 0.04%, Ti = 0.01 to 0.03%, Al = 0.015% to 0.045%, P ≤ 0.015%, S ≤ 0.006%, and the balance of Fe and unavoidable impurities; the steel plate has a thickness of 6 to 50 mm, a microstructure containing not less than 90% by volume fraction of lath bainite and not more than 10% of martensite-austenite constituent, a yield strength of not less than 550 MPa, a uniform elongation of not less than 10%, a yield strength ratio of not more than 0.80, and an impact energy at -40°C of not less than 200 J; Key process steps include: (1) Converter smelting: After pre-desulfurization treatment of molten iron produced by a blast furnace, the molten iron is poured into a 100-300 t converter for top and bottom combined blowing oxygen treatment, to complete oxidation decarburization of the molten iron to C = 0.05% to 0.10%, dephosphorization and desulfurization, and addition of Mn and Ni iron alloy raw materials with a calculated weight, and then oxygen blowing is continued to adjust the molten iron temperature to 1620-1660°C, and then aluminum ingots are added for static deoxidization according to the [O] content in the molten iron, and argon bottom blowing is performed before tapping; (2) Secondary refining: The molten iron with adjusted temperature and preliminary chemical composition is poured into a ladle and transported to an LF refining furnace for continued argon bottom blowing to stir the molten iron to make solid inclusions float fully; 200 meters of pure calcium wire is fed before the end of LF treatment; and then the ladle is sent to an RH vacuum refining furnace for vacuum degassing treatment for a treatment time of ≥15 minutes to remove gas content in the molten iron to [H] ≤ 2 ppm, [O] ≤ 20 ppm, and [N] ≤ 50 ppm; (3) Slab continuous casting: After secondary refining of the molten iron, the temperature reaches a casting superheat of 20-45°C, and then the molten iron is transported to a casting platform to be continuously cast into a 200-400 mm thick and 1650-2650 mm wide slab under the protection of a protective atmosphere and a protective slag covering; (4) Slab heating: After flame cutting of the high-temperature continuous casting slab, the slab is stacked and slowly cooled in a slab yard for 46-50 hours to prevent slab cracking, and then the slab is transported to a heating furnace to be heated to a temperature of 1150-1220°C for a heating time of 4-8 hours, and the austenite grain size is grade 6 or higher; (5) Steel plate controlled rolling: After the removal of iron oxide scale on the surface of the slab by high-pressure water, the slab is expanded and rolled in 3-7 passes along the width direction at a rolling temperature of 1100-1150°C, and then the slab is rotated by 90°, and then rolled at 900-1050°C, and the final rolling temperature is 880-950°C, so as to keep the austenite grains in a recrystallized equiaxed shape with a grain size of grade 8 or higher; (6) Steel plate controlled cooling: after rolling, the steel plate is accelerated by water spray with laminar flow header, the initial cooling temperature is 850-920 °C, the cooling rate is 10-30 °C / s, and the final cooling temperature is 200-300 °C; high temperature controlled rolling + controlled cooling makes the steel plate form not less than 90% lath bainite and not more than 10% martensite-austenite component, the dislocation density is uniformly distributed, and VC or (Nb, V) C composite nanoparticles are precipitated.

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