An eh690 grade marine steel and a method of manufacturing the same

Abstract

The application relates to an EH690-grade marine steel and a manufacturing method thereof, and the chemical composition of the steel plate is as follows: C: 0.10%-0.16%; Si: 0.15%-0.40%; Mn: 1.00%-1.50%; P<=0.012%; S<=0.002%; Ni: 0.25%-0.50%; Cr: 0.50%-0.90%; Nb: 0.015%-0.04%; Mo: 0.25%-0.50%; Ti: 0.007%-0.015%; B: 0.0007%-0.002%; Als: 0.04%-0.08%; O<=0.001%; N<=0.004%; H<=0.002%; and the balance is Fe and impurities. Through coupling design of the alloy composition-deformation-heat treatment process, the product has good comprehensive mechanical properties, the finished steel plate has the characteristics of ultrahigh strength, excellent low-temperature toughness, excellent Z-direction performance, uniform structure performance, excellent low-temperature fracture toughness and good plate shape.

Description

Technical Field

[0001] This invention relates to the field of marine engineering steel production technology, and in particular to a low-cost, high-performance EH690 grade marine engineering steel and its manufacturing method. Background Technology

[0002] In recent years, with the continuous exploration and development of the ocean, the research and development of steel required for ships and marine engineering has received increasing attention. As requirements for larger marine engineering equipment, greater safety of ultra-deep-water operations on offshore platforms, and lighter marine equipment increase, the performance requirements for steel used in marine engineering equipment are also constantly rising. For example, there is a need to design thinner and lighter load-bearing structures to lower the platform's center of gravity, improve stability, reduce welding, and simultaneously increase the effective load of important equipment and improve equipment efficiency. To meet the demand for high-performance steel plates in marine engineering, there is an urgent need to develop low-cost, high-performance EH690 grade marine engineering steel.

[0003] Chinese invention patent CN 102226255 B discloses a "high-strength and tough steel plate with a yield strength of 690MPa and its preparation process." The yield strength of this ship plate steel is not less than 690MPa, it possesses good low-temperature (-80℃) impact toughness, and a thickness of 50-100mm. It is suitable for ships and offshore oil platforms, particularly in ship plate manufacturing processes requiring high strength and low-temperature toughness, as well as good weldability. However, the Cu content of this steel plate is 1.00%-1.50%, exceeding the design specifications for existing ship-grade steel plates, and the high Ni content increases production costs, hindering its widespread application.

[0004] Chinese invention patent CN 109652733 B discloses "a 690MPa grade extra-thick steel plate and its manufacturing method". It adopts a die casting followed by forging of slab blanks and rolling and heat treatment of the forged slab blanks. The resulting extra-thick steel plate has excellent core mechanical properties such as high strength, high plasticity, and high toughness, as well as resistance to lamellar tearing. It can meet the requirements of high-performance extra-thick steel plates for harsh service environments such as marine engineering. However, its process is complex and not suitable for mass production.

[0005] Chinese invention patent CN 102888571 B discloses "a 690MPa grade low welding crack sensitivity steel and its manufacturing method". Through reasonable composition design, Pcm.20 is guaranteed. By adopting two-stage controlled rolling, controlled cooling and reasonable heat treatment process, the steel plate produced has a tensile strength of ≥90MPa. However, its finished thickness is only 15-30mm, which cannot meet the demand for large thickness specifications of ultra-high strength marine engineering steel for marine engineering platform equipment.

[0006] As can be seen from the above existing technologies, the currently available high-performance EH690 grade marine engineering steel has the following shortcomings:

[0007] 1. The product thickness specifications are too small to meet the needs of large thickness specifications;

[0008] 2. High content of precious metals or complex heat treatment processes result in higher production costs;

[0009] To address the above shortcomings, this invention optimizes the alloy content and employs a coupled design of alloy composition with smelting, controlled rolling, controlled cooling, and tempering heat treatment processes, ultimately obtaining a low-cost, high-performance EH690 grade marine steel with a maximum thickness of 60mm. Summary of the Invention

[0010] This invention provides an EH690 grade marine engineering steel and its manufacturing method. Through the coupled design of alloy composition, deformation, and heat treatment processes, the product possesses excellent comprehensive mechanical properties, solving the problem of the difficulty in matching and harmonizing the strength, plasticity, and low-temperature toughness (-40℃) of 690MPa grade steel plates. The finished steel plate has the characteristics of ultra-high strength (yield strength ≥690MPa, tensile strength 770~940MPa, elongation after fracture ≥14%), excellent low-temperature toughness (core impact energy ≥100J at -40℃), excellent low-temperature fracture toughness (CTOD characteristic value of low-temperature fracture ≥0.5mm at -10℃), excellent Z-direction properties (Z-direction reduction of area ≥65%), uniform microstructure, and good plate shape (steel plate unevenness ≤4mm / 2m).

[0011] To achieve the above objectives, the present invention employs the following technical solution:

[0012] An EH690 grade marine engineering steel plate has the following chemical composition by mass percentage: C: 0.10%–0.16%; Si: 0.15%–0.40%; Mn: 1.00%–1.50%; P≤0.012%; S≤0.002%; Ni: 0.25%–0.50%; Cr: 0.50%–0.90%; Nb: 0.015%–0.04%; Mo: 0.25%–0.50%; Ti: 0.007%–0.015%; B: 0.000%–0.002%; Mo: 0.000%–0.002%; S: 0.000%–0.002%; Ni: 0.25%–0.50%; Cr: 0.50%–0.90%; Nb: 0.015%–0.04%; Mo: 0.25%–0.50%; Ti: 0.007%–0.015%; B ... 7%~0.002%; Als: 0.04%~0.08%; O≤0.001%; ​​N≤0.004%; H≤0.002%; balance is Fe and unavoidable impurities; Ceq≤0.64%, Pcm≤0.33%; where Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5, Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0013] A manufacturing method for EH690 grade marine engineering steel, the process route including smelting and slow cooling of cast billets, controlled rolling, controlled cooling and offline quenching and tempering heat treatment; wherein the following processes are controlled:

[0014] 1) Slow cooling during smelting and casting;

[0015] LF refining and RH refining each last 20–40 minutes. The superheat of the molten steel in the tundish is 22–28°C. The gas content is controlled as follows: [N] ≤ 40 ppm, [O] ≤ 10 ppm, [H] ≤ 2.0 ppm. The entire casting process is protected. All types of inclusions in the steel meet the following requirements: Class A inclusions ≤ 1.0 grade, Class B inclusions ≤ 1.0 grade, Class C inclusions ≤ 1.0 grade, and Class D inclusions ≤ 1.0 grade. The continuous casting billet forming process adopts light reduction, with a reduction of 6–8 mm. After the continuous casting billet is removed from the line, it is stacked and slowly cooled, with a stacking temperature ≥ 600°C and a time ≥ 48 hours.

[0016] 2) Controlled rolling;

[0017] The continuously cast billet is heated in four stages, including a first heating stage, a second heating stage, a third heating stage, and a soaking stage; the upper and lower heating temperatures of each heating stage and the soaking stage are controlled separately; the total heating time of the continuously cast billet is 10-12 min / mm; the initial rolling temperature is 950-1000℃, and the single-pass reduction rate is ≥15% except for the widening pass; the second-stage initial rolling temperature is 720-760℃, the single-pass reduction rate is ≥11%, and the final rolling temperature is 680-720℃;

[0018] 3) Control cooling;

[0019] After final rolling, the steel plate is directly cooled by the DQ+ACC rapid cooling system, with an average cooling rate of ≥10℃ / s and a reddening temperature of 200~300℃.

[0020] 4) Offline tempering heat treatment;

[0021] Quenching: The steel plate is subjected to quenching heat treatment. The quenching heating temperature is 860~910℃, the heating rate is 1.3~2min / mm, the holding time is 20~40min, and the plate is then water-quenched to room temperature after being taken out of the furnace.

[0022] Tempering: The steel plate is tempered at a temperature of 580-630℃ for 3-4.5 min / mm. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate.

[0023] In step 2), the lower heating temperature of the first heating section is 1000–1120℃, and the upper heating temperature of the first heating section is 1050–1180℃; the lower heating temperature of the second heating section is 1130–1180℃, and the upper heating temperature of the second heating section is 1180–1220℃; the lower heating temperature of the third heating section is 1150–1200℃, and the upper heating temperature of the third heating section is 1200–1250℃; the lower heating temperature of the soaking section is 1050–1150℃, and the upper heating temperature of the soaking section is 1150–1200℃.

[0024] In step 2), after the continuously cast billet is taken out of the furnace, it is dephosphorized by high-pressure water 2 to 3 times.

[0025] In step 2), the thickness of the intermediate billet is 2 to 2.5 times the thickness of the finished steel plate, and the intermediate billet is water-cooled at a cooling rate of ≥4℃ / s.

[0026] The microstructure of the finished steel plate is tempered martensite + bainite, with a grain size ≥ 7.

[0027] The finished steel plate has a yield strength ≥690MPa, tensile strength 770~940MPa, elongation at section ≥14%, Z-direction reduction of area ≥65%, Charpy impact energy of steel plate core at -40℃ ≥100J, and CTOD characteristic value of low temperature fracture at -10℃ ≥0.5mm.

[0028] The maximum thickness of the finished steel plate is 60mm.

[0029] The unevenness of the finished steel plate is ≤4mm / 2m.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) This invention uses a coupled design of alloy composition, deformation and heat treatment process to make the product have good comprehensive mechanical properties, and solves the problem of the difficulty in matching and harmonizing the strength, plasticity and low temperature toughness (-40℃) of 690MPa grade steel plate. The yield strength of the finished steel plate is ≥690MPa, the tensile strength is 770~940MPa, the elongation of the section is ≥14%, the Z-direction reduction of area is ≥65%, the Charpy impact energy of the steel plate core at -40℃ is ≥100J, and the low temperature fracture toughness is excellent (the CTOD characteristic value of low temperature fracture at -10℃ is ≥0.5mm).

[0032] (2) This invention fully leverages the technical equipment advantages of the heavy plate rolling mill and heat treatment unit, and combines 250-300mm thick continuous casting slabs to develop ultra-high strength and high toughness marine engineering steel plates with a maximum thickness of 60mm.

[0033] (3) By combining a reasonable rolling process with an offline tempering (quenching + high temperature tempering) process, compared with the TMCP or TMCP+T process commonly used for existing steel plates of the same strength, offline quenching can accurately control the starting temperature of quenching and the temperature of the whole plate is uniform; and it is easier to control the flatness of the plate shape, with the flatness of the finished steel plate within 2 meters ≤4mm.

[0034] (4) The microstructure of the steel plate is tempered martensite / bainite with a grain size ≥ 7. Attached Figure Description

[0035] Figure 1 This is a metallographic photograph (magnified 500 times) of the finished steel plate in Embodiment 2 of the present invention.

[0036] Figure 2 This is a grain size photograph (magnified 500 times) of the finished steel plate in Embodiment 2 of the present invention. Detailed Implementation

[0037] This invention discloses an EH690 grade marine engineering steel and its manufacturing method. The design considers several aspects, including alloy element screening and proportioning, steel cleanliness control, continuous casting process control, heating and controlled rolling / cooling, optimization of tempering heat treatment processes, and matching of microstructure strength and toughness. Ultimately, a composition system and manufacturing process that meet the objectives of this invention were determined. Details are as follows:

[0038] Low-cost, high-performance EH690 grade marine engineering steel is characterized by the following chemical composition by mass percentage: C: 0.10%–0.16%; Si: 0.15%–0.40%; Mn: 1.00%–1.50%; P≤0.012%; S≤0.002%; Ni: 0.25%–0.50%; Cr: 0.50%–0.90%; Nb: 0.015%–0.04%; Mo: 0.25%–0.50%; Ti: 0.007%–0.015%; B : 0.0007%~0.002%; Als: 0.04%~0.08%; O≤0.001%; ​​N≤0.004%; H≤0.002%; balance is Fe and unavoidable impurities; Ceq≤0.64%, Pcm≤0.33%; where, Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5, Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0039] The mechanisms of action of each alloy component in the steel of this invention are as follows:

[0040] Carbon (C) is an essential and inexpensive element for ensuring strength and hardenability. It plays a significant role in improving the strength of steel through solid solution strengthening and precipitation strengthening. However, as the carbon content increases, the plasticity, low-temperature toughness, and resistance to weld cracking of the steel plate decrease significantly. Low carbon content results in lower hardness and strength after quenching and tempering, but higher plasticity and toughness. Considering both economic efficiency and product performance, this invention controls the C content in the steel to be between 0.10% and 0.16%.

[0041] Si: As a solid solution strengthening element, although Si is beneficial to improving the strength of steel plates and their oxidation resistance at high temperatures, Si will promote the coarsening of packet size, which will seriously impair the low-temperature toughness, elongation and weldability of ultra-high strength steel plates. Taking into account the economy and operability of steelmaking, this invention controls the Si content in steel at 0.15% to 0.40%.

[0042] Mn: As the most important alloying element, in addition to improving the strength of steel plates, it also has the functions of expanding the austenite phase region, lowering the Ar3 point temperature, and refining ferrite grains, thereby improving the low-temperature toughness of steel plates. However, when the Mn content is too high, Mn segregation and strip-shaped MnS are easily formed, resulting in poor low-temperature toughness and resistance to lamellar tearing in the core of thick plates, and a decrease in the performance of the weld heat-affected zone. Therefore, this invention controls the Mn content in steel at 1.00% to 1.50%.

[0043] P: is an element that has an adverse effect on impact value. It will segregate in the center of the slab and accumulate at the grain boundaries, thus impairing low-temperature toughness. This invention controls the P content in the steel to be no higher than 0.012%.

[0044] S: is an element that has an adverse effect on impact value. It can form sulfide inclusions and become crack initiation sites. This invention controls the S content in steel to be no higher than 0.002%.

[0045] Ni: Nickel dissolves in austenite, inhibiting austenite recrystallization, refining austenite grains, and improving the low-temperature toughness of steel plates. Ni can reduce the diffusion rate of various elements in steel, thereby delaying the austenite decomposition transformation and improving the steel's permeability. When Ni and Mo are added simultaneously, they can improve the steel's toughness while increasing its strength. However, excessive Ni content will increase Ceq and Pcm, affecting weldability; therefore, this invention controls the Ni content in the steel to be between 0.25% and 0.50%.

[0046] Cr: Cr improves the hardenability and strength of steel plates. The presence of Cr enhances the tempering stability of steel, ensuring a tempered bainitic / martensite structure during high-temperature tempering. Cr has similar solid solution strengthening effects to Mn and is less prone to segregation. Excessive Cr content increases temper brittleness and welding difficulty; conversely, insufficient Cr content fails to effectively exert its strengthening effect. This invention controls the Cr content in the steel to be between 0.50% and 0.90%.

[0047] The addition of niobium (Nb) refines the grain size after quenching, thereby effectively refining the microstructure and strengthening the matrix through precipitation. The combined addition of Nb and boron effectively inhibits austenite recrystallization during rolling, refining the grains while suppressing carbon diffusion. This effectively inhibits the precipitation of M23(C,B)6 at austenite grain boundaries, ensuring effective boron segregation at ferrite nucleation sites, improving the hardenability of the steel, and giving it higher strength and better low-temperature toughness. The addition of Nb and the formation of Nb(C,N) in the steel suppress the appearance of coarse M23(C,B)6 structures. While reducing carbon diffusion and improving boron utilization efficiency, it also promotes microstructure homogenization. However, when the Nb content exceeds a certain range, MA islands will form in the welded HAZ, which is detrimental to the steel's toughness. This invention controls the Nb content in the steel to 0.015%–0.04%.

[0048] Mo (Mo) can shift the C-curve of steel, thus significantly improving its hardenability. It promotes the formation of martensite or bainite with numerous dislocations within the grains over a wider cooling range, resulting in phase transformation strengthening and dislocation strengthening, significantly improving the strength and microstructure uniformity of the steel. In this invention, the combined addition of Mo and Nb not only provides the strengthening effect of adding molybdenum or niobium alone, but also allows molybdenum to segregate at the NbC matrix interface, preventing the coarsening of NbC particles. The combined addition of Mo and B increases the content of effective (solid-dissolved) B in the steel, inhibits the diffusion of C to grain boundaries, suppresses the precipitation and growth of M23(C,B)6 at grain boundaries, promotes the segregation of B at grain boundaries, reduces the nucleation sites of ferrite, and improves the hardenability of the steel. The combined addition of Mo and Ni can improve the toughness of steel while increasing its strength. However, with the increase of Mo, the amount of Mo-containing carbides also increases. The Mo-containing carbides distributed at the grain boundaries disrupt the continuity of the matrix and deteriorate the toughness of the steel. Therefore, the present invention controls the Mo content in steel to be between 0.25% and 0.50%.

[0049] Ti: Inhibits excessive austenite grain growth during slab heating, improving steel toughness. Simultaneously, Ti acts as a solidifier for nitrogen, ensuring a certain level of dissolved boron (B) in the steel and improving hardenability. However, excessive Ti content leads to the precipitation of TiC as TiC on martensite / bainite laths and grain boundaries, severely degrading the low-temperature toughness of the steel. Therefore, this invention controls the Ti content in the steel to between 0.007% and 0.015%.

[0050] B: It can improve the hardenability and strength of steel plates, but if the B content is too high, it will form coarse BN particles that are detrimental to hardenability and toughness. It will also affect the weldability and surface quality of steel plates. Therefore, this invention controls the Mo content in steel to be between 0.0007% and 0.002%.

[0051] Als (Al) is a deoxidizing and grain-refining element. At high temperatures, Al forms fine AlN precipitates, which inhibit austenite grain growth during the austenitization of slabs / steel plates, thereby refining the austenite grains and improving the toughness of steel at low temperatures. Simultaneously, excess Al in the steel can fix free [N], ensuring a certain amount of dissolved boron (B) and improving the hardenability of the steel plate. Therefore, the Al content should be above 0.04%. However, excessive Al content leads to the formation of larger Al oxides, reducing the low-temperature impact performance of the steel plate. Furthermore, it makes the slab prone to edge and corner cracks during continuous casting. Therefore, this invention controls the Al content in the steel to 0.04%–0.08%.

[0052] O: An element that has an adverse effect on impact toughness. It combines with other elements in steel to form non-metallic inclusions, which become crack sources. This invention controls the O content in steel to below 10 ppm.

[0053] N: N element will form BN with B element, and form coarse ALN with AL, which precipitates along the original austenite grain boundary, affecting the hardenability and low temperature impact toughness of steel. This invention controls the N content in steel to below 40ppm.

[0054] H: The most harmful element in steel. When the cooling rate is too fast, high-pressure hydrogen cannot escape from the steel in time. The stress of high-pressure hydrogen, combined with other stresses, may exceed the material's tensile strength (σ). b This results in numerous microcracks, which appear as circular or elliptical silvery-white spots on the longitudinal section of the steel, known as "white spots." These are actually intersecting fine cracks. This invention controls the H content in the steel to below 2 ppm.

[0055] The key to preparing the EH690 grade marine steel described in this invention lies in the following steps:

[0056] 1) Slow cooling during smelting and casting;

[0057] Select high-quality smelting raw materials, primarily molten iron or high-quality recycled steel, with P and S content as low as possible. Control the smelting composition according to target values, strictly control the residual element content, and avoid exceeding the carbon equivalent limit. Perform LF refining and RH refining for 20–40 minutes each, with the molten steel in the ladle superheated at 22–28°C. Strictly control the residual element content, with gas content controlled as follows: [N] ≤ 40 ppm, [O] ≤ 10 ppm, [H] ≤ 2.0 ppm. Use full-process protective casting. Inclusions of types A, B, C, and D in the steel must meet the following requirements: Type A inclusions ≤ 1.0 grade, Type B inclusions ≤ 1.0 grade, Type C inclusions ≤ 1.0 grade, and Type D inclusions ≤ 1.0 grade. The continuous casting billet forming process adopts a light reduction technology with a reduction of 6-8mm, which can effectively solve the center segregation of the continuous casting billet. After the continuous casting billet is removed from the line, it is stacked and slowly cooled with a stacking temperature ≥600℃ and a time ≥48h. The slow cooling of the continuous casting billet by stacking improves the cracks in the continuous casting billet on the one hand, and reduces the hydrogen embrittlement sensitivity of the product on the other hand, thereby improving the product performance.

[0058] 2) Controlled rolling;

[0059] The continuously cast billet employs a four-stage heating system: a first heating stage, a second heating stage, a third heating stage, and a soaking stage. The upper and lower temperatures of each stage are controlled separately. Specifically: the lower heating temperature of the first heating stage is 1000–1120℃, and the upper heating temperature is 1050–1180℃; the lower heating temperature of the second heating stage is 1130–1180℃, and the upper heating temperature is 1180–1220℃; the lower heating temperature of the third heating stage is 1150–1200℃, and the upper heating temperature is 1200–1250℃; the lower heating temperature of the soaking stage is 1050–1150℃, and the upper heating temperature is 1150–1200℃. The total heating time for the continuously cast billet is 10–12 min / mm. This segmented heating control prevents uneven internal heating of the billet due to excessively rapid heating, while also ensuring sufficient dissolution of C and N compounds, thus preventing abnormal growth of the as-cast microstructure.

[0060] After exiting the furnace, the continuously cast billet undergoes 2-3 high-pressure water descaling passes to remove surface iron oxide scale and lower the billet temperature. The initial rolling temperature is 950-1000℃, with a single-pass reduction rate ≥15% except for the widening pass, improving the as-cast microstructure of the slab, reducing the billet's waiting thickness for heating, and shortening the slab's waiting time. The intermediate billet thickness is 2-2.5 times the finished product thickness, and it is water-cooled at a cooling rate ≥4℃ / s. The second-stage initial rolling temperature is 720-760℃, with a single-pass reduction rate ≥11%, and the final rolling temperature is 680-720℃. High-temperature rolling improves the as-cast microstructure of the slab. Two-stage controlled rolling promotes the flattening and refinement of austenite grains. By controlling the phase transformation microstructure through two-stage rolling, initial microstructure preparation is provided for tempering heat treatment.

[0061] 3) Control cooling;

[0062] After final rolling, the steel plate is directly cooled by the DQ+ACC rapid cooling system, with an average cooling rate of ≥10℃ / s and a reddening temperature of 200~300℃. The original microstructure is refined by combining the large deformation rolling process with the post-rolling cooling process.

[0063] 4) Offline tempering heat treatment;

[0064] Quenching; the steel plate is subjected to quenching heat treatment, wherein the quenching heating temperature is controlled between 860 and 910℃, the heating rate is 1.3 to 2 min / mm, the holding time is 20 to 40 min, and the plate is then water-quenched to room temperature. The quenching process achieves the effect of uniformizing the original austenite grain size on the full thickness section of the thick steel plate.

[0065] Tempering: The steel plate is tempered at a temperature of 580-630℃ for 3-4.5 min / mm in the furnace. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate. By controlling the tempering process, the composition, uniformity and refinement of the phase transformation structure and the precipitation behavior of multiphase particles are controlled and adjusted, thereby improving the steel plate's resistance to low-temperature impact toughness and resistance to low-temperature fracture (CTOD).

[0066] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0067]

Example

[0068] The chemical composition of the steel in each embodiment is shown in Table 1. The carbon equivalent and welding crack sensitivity coefficient of the steel in each embodiment are shown in Table 2. The smelting process parameters of the steel in each embodiment are shown in Table 3. The slab heating process parameters of each embodiment are shown in Table 4. The rolling and cooling process parameters of the steel plate in each embodiment are shown in Table 5. The quenching and tempering heat treatment process parameters of the steel plate in each embodiment are shown in Table 6. The mechanical properties of the finished steel plate in each embodiment are shown in Table 7. The low-temperature fracture properties of the finished steel plate in each embodiment are shown in Table 8. The unevenness and grain size of the finished steel plate in each embodiment are shown in Table 9.

[0069] Table 1 Chemical composition of steel (wt%)

[0070] Example C Si Mn P S Ni Cr Nb Mo Ti B Als O N 1 0.151 0.35 1.26 0.011 0.0018 0.35 0.77 0.026 0.35 0.011 0.002 0.076 0.0003 0.0014 2 0.124 0.29 1.44 0.01 0.002 0.31 0.69 0.019 0.41 0.015 0.0011 0.051 0.0008 0.0005 3 0.129 0.17 1.41 0.009 0.0016 0.39 0.56 0.034 0.44 0.008 0.001 0.044 0.0007 0.0024 4 0.121 0.19 1.49 0.01 0.0015 0.41 0.81 0.031 0.26 0.011 0.0007 0.061 0.0004 0.0019 5 0.111 0.25 1.31 0.008 0.002 0.49 0.85 0.039 0.31 0.014 0.0009 0.074 0.0001 0.0022 6 0.141 0.34 1.21 0.012 0.0019 0.31 0.59 0.029 0.32 0.012 0.0016 0.061 0.0005 0.0008 7 0.159 0.21 1.12 0.01 0.002 0.36 0.61 0.027 0.29 0.007 0.0013 0.055 0.0004 0.0036 8 0.149 0.27 1.18 0.012 0.0014 0.47 0.55 0.015 0.42 0.009 0.0017 0.052 0.0008 0.0015 9 0.131 0.31 1.33 0.009 0.0016 0.32 0.64 0.022 0.31 0.009 0.0014 0.049 0.0003 0.0027 10 0.139 0.28 1.23 0.01 0.002 0.26 0.66 0.032 0.39 0.012 0.0012 0.069 0.0002 0.0032

[0071] Table 2 Carbon equivalent and welding crack sensitivity coefficient of steel

[0072] Example Ceq / % Pcm / % 1 0.61 0.30 2 0.60 0.28 3 0.59 0.27 4 0.61 0.27 5 0.59 0.26 6 0.55 0.28 7 0.55 0.28 8 0.57 0.29 9 0.56 0.27 10 0.57 0.28

[0073] Table 3 Steel smelting process parameters

[0074]

[0075]

[0076] Table 4 Heating process parameters for continuously cast billets

[0077]

[0078] Table 5 Rolling and Cooling Process Parameters for Steel Plates

[0079]

[0080] Table 6. Heat treatment process parameters for steel plates

[0081]

[0082]

[0083] Table 7 Mechanical Properties of Finished Steel Plates

[0084]

[0085] Table 8 Low-Temperature Fracture Properties of Finished Steel Plates

[0086]

[0087] Table 9. Unevenness and Grain Size of Finished Steel Plates

[0088]

[0089]

[0090] Metallographic photographs of the finished steel plate in Example 2 are shown below. Figure 1 (Magnified 500x), see grain size photo. Figure 2 (Magnified 500 times).

[0091] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing EH690 grade marine engineering steel, characterized in that, The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.111%–0.16%; Si: 0.27%–0.40%; Mn: 1.00%–1.18%; P≤0.012%; S≤0.002%; Ni: 0.25%–0.50%; Cr: 0.61%–0.90%; Nb: 0.031%–0.04%; Mo: 0.31%–0.50%. Ti: 0.007%~0.009%; B: 0.0007%~0.0009%; Als : 0.04%~0.08%; O≤0.001%; ​​N≤0.004%; H≤0.002%; balance is Fe and unavoidable impurities; Ceq≤0.64%, Pcm≤0.33%; where, Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5, Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B; The process route includes smelting and slow cooling of the billet, controlled rolling, controlled cooling, and offline tempering heat treatment; among which the following processes are controlled: 1) Slow cooling during smelting and casting; LF refining and RH refining each last 20–40 minutes. The superheat of the molten steel in the tundish is 22–28°C. The gas content is controlled as follows: [N] ≤ 40 ppm, [O] ≤ 10 ppm, [H] ≤ 2.0 ppm. The entire casting process is protected. All types of inclusions in the steel meet the following requirements: Class A inclusions ≤ 1.0 grade, Class B inclusions ≤ 1.0 grade, Class C inclusions ≤ 1.0 grade, and Class D inclusions ≤ 1.0 grade. The continuous casting billet forming process adopts light reduction, with a reduction of 6–8 mm. After the continuous casting billet is removed from the line, it is stacked and slowly cooled, with a stacking temperature ≥ 600°C and a time ≥ 48 hours. 2) Controlled rolling; The continuously cast billet is heated in four stages: a first heating stage, a second heating stage, a third heating stage, and a soaking stage. The upper and lower heating temperatures of each heating stage and the soaking stage are controlled separately. The lower heating temperature of the first heating stage is 1000–1120℃, and the upper heating temperature is 1050–1180℃. The lower heating temperature of the second heating stage is 1130–1180℃, and the upper heating temperature is 1180–1220℃. The lower heating temperature of the third heating stage is 1150–1200℃, and the upper heating temperature is 1200–1250℃. The lower heating temperature of the soaking stage is 1050–1150℃, and the upper heating temperature is 1150–1200℃. The total heating time for the continuously cast billet is 10–12 min / mm; the initial rolling temperature is 950–991℃, and the single-pass reduction rate is ≥15% except for the widening pass; the thickness of the intermediate billet is 2–2.5 times the thickness of the finished steel plate, and the intermediate billet is water-cooled at a cooling rate of ≥4℃ / s; the initial rolling temperature for the second stage is 720–760℃, the single-pass reduction rate is ≥11%, and the final rolling temperature is 680–720℃. 3) Control cooling; After final rolling, the steel plate is directly cooled by the DQ+ACC rapid cooling system, with an average cooling rate of 16-20℃ / s and a reddening temperature of 200-300℃. 4) Offline tempering heat treatment; Quenching: The steel plate is subjected to quenching heat treatment. The quenching heating temperature is 860~910℃, the heating rate is 1.3~2min / mm, the holding time is 20~40min, and the plate is then water-quenched to room temperature after being taken out of the furnace. Tempering: The steel plate is tempered at a temperature of 610-630℃ for 3-4.5 min / mm. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate. The finished steel plate has a yield strength ≥690MPa, tensile strength 770~940MPa, elongation at section ≥14%, Z-direction reduction of area ≥65%, Charpy impact energy of steel plate core at -40℃ ≥100J, and CTOD characteristic value of low temperature fracture at -10℃ ≥0.5mm.

2. The manufacturing method of EH690 grade marine steel according to claim 1, characterized in that, In step 2), after the continuously cast billet is taken out of the furnace, it is dephosphorized by high-pressure water 2 to 3 times.

3. The manufacturing method of EH690 grade marine steel according to claim 1, characterized in that, The microstructure of the finished steel plate is tempered martensite + bainite, with a grain size ≥ 7.

4. The manufacturing method of EH690 grade marine steel according to claim 1, characterized in that, The maximum thickness of the finished steel plate is 60mm.

5. The manufacturing method of EH690 grade marine steel according to claim 1, characterized in that, The unevenness of the finished steel plate is ≤4mm / 2m.

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