CN116607076A - A thin-gauge low-yield ratio 690MPa-grade marine engineering steel and its manufacturing method - Google Patents

Abstract

A thin-gauge, low-yield-ratio 690MPa grade marine engineering steel and a manufacturing method thereof. The chemical composition of steel is C=0.06%~0.15%, Si=0.10%~0.80%, Mn=0.50%~1.60%, P≤0.015%, S≤0.006%, Nb=0.015%~0.05%, V=0.02%~0.10%, Ti=0.01%~0.03%, Ni=0.30%~1.00%, Cu=0.20%~1.50%, Cr=0.30%~1.20%, Mo=0.20%~0.70%, Al= 0.015%~0.045%, Ca=0.001%~0.015%, the rest is Fe and other unavoidable impurities. Through smelting, continuous casting, controlled rolling, controlled cooling, direct quenching + tempering, the production process is regulated through the whole process of low yield ratio, and the chemical composition design of hardenability matching the thin gauge thickness and the controlled quenching of relaxation after rolling The dual-phase ratio of soft phase ferrite and hard phase martensite is adjusted to produce a 690MPa ultra-high-strength steel plate for marine engineering with a yield ratio lower than 0.94 and other mechanical properties that meet the requirements of classification societies.

Description

A thin-gauge low-yield ratio 690MPa-grade marine engineering steel and its manufacturing method

technical field

The invention belongs to the technical field of low-alloy high-strength steel production, relates to a thin-gauge steel plate and a manufacturing method thereof, and is especially suitable for producing steel for marine engineering structures with low yield ratio.

Background technique

With the large-scale development of engineering structures such as marine equipment, ships, bridges, and machinery, the requirements for the strength level of structural steel are getting higher and higher, in order to achieve the design goals of lightweight structures and overall structural center of gravity balance. For example, the ultra-high-strength level in the International Classification Society has increased the highest level of yield strength from 690MPa to 960MPa. However, the higher the strength grade of the steel plate, the higher the ratio of yield strength to tensile strength (yield strength ratio). The Structural Integrity Evaluation Project (SINTAP) funded by the European Union conducted a statistical analysis of a large amount of data on the yield strength and yield ratio of steel, and the regression showed that the upper limit relationship between the two is Y/T=1/[1+2(150/YS)2.5 ]. The SINTAP project concluded that too high yield strength ratio is not good for the integrity of engineering structures, and should be included in relevant standards and specifications as an important parameter for engineering structure safety design. The higher the yield ratio of the steel plate, the smaller the allowable margin for deformation and fracture of the fabricated structural parts, which will increase the safety risk of the offshore engineering structure during service. Therefore, the material specifications of classification societies such as Det Norske Veritas (DNV) and Lloyd's Register (LR) stipulate that the yield strength ratio of steel plates with a yield strength exceeding 420MPa cannot exceed 0.94.

Due to the characteristic that the yield ratio increases with the increase of the strength level, the application of ultra-high-strength steel in marine engineering equipment is limited. EH690 is currently the highest grade that has been applied in batches. Yield strength ratio lower than 0.94 is a recognized technical problem for quenching + tempering quenching and tempering process to produce 690MPa grade and above steel plates, especially thin steel plates with a thickness of 6~20mm. This is because the thin-gauge steel plate obtains a complete lath martensitic structure in the full thickness direction after quenching, and complex changes such as internal stress release, carbide precipitation, and lath softening occur during tempering, resulting in processing from yield to necking failure stage. Insufficient hardening ability.

Chinese patent CN114231714A discloses a heat treatment method for 890MPa-grade ultra-high-strength low-yield ratio marine engineering steel. After rolling, the process of online cooling and quenching plus sub-temperature quenching and tempering water cooling is adopted. The final rolling temperature of 10~40mm thick steel plate is 820~ 860°C, cooling to ≤200°C, then heating the steel plate to the temperature of the two-phase zone of 820-850°C, keeping it warm for 0.3-0.5min/mm and then quenching quickly, and finally heating the steel plate after sub-temperature quenching to 560-620 °C tempering, water cooling to ≤80°C when out of the furnace, to obtain ultra-high-strength low-yield ratio marine engineering steel plates with a yield ratio not higher than 0.93 and a yield strength not lower than 890MPa. However, since the temperature in the two-phase zone is limited to a narrow temperature range of 820-850 °C, the requirements for the control of the chemical composition of the steel are relatively high, and the water cooling process after tempering is required, which is not suitable for the existing industrial production equipment conditions.

Chinese patent CN114134414A discloses a low-yield ratio high-toughness steel and its preparation method. Through the two-stage rolling process of the continuous casting billet, after the first stage of horizontal and vertical rolling, the intermediate billet is accelerated to 730 ° C, and the second stage is started. Stage low-temperature final rolling, the steel plate after rolling is water-cooled and stacked and slowly cooled, and the thickness of the steel plate with low yield ratio and high toughness is ≤80mm, yield strength ≥500MPa, tensile strength ≥640MPa, elongation after fracture ≥20.0%, yield strength Ratio ≤0.80, -60°C Charpy impact energy >200J. This patent is aimed at 500MPa-level strength steel for bridges and building structures, and adopts the controlled rolling and controlled cooling process of low-temperature final rolling, which cannot meet the requirements of 890MPa ultra-high-strength level.

Chinese patent CN114134416A discloses a low-yield ratio high-strength medium-thick steel plate and its short-process manufacturing method. On the basis of low-carbon composition design, a small amount of micro-alloying elements are added to reduce costs. At the same time, it is developed through rolling and water-cooling processes to form a Bainite structure with soft and hard substructure, no need for quenching and tempering heat treatment, water cooling after rolling, direct delivery, steel plate thickness 15-60mm, yield strength 580-680MPa, tensile strength 700-820MPa, elongation after fracture The rate is 16-23%, the longitudinal Charpy impact absorption energy at the test temperature of -40°C is 80-260J, and the yield ratio is ≤0.82. However, its manufacturing method is suitable for the production of steel with a yield strength of 580-680 MPa.

Chinese patent CN114058793A discloses a heat treatment method for reducing the yield ratio of ultra-high-strength marine steel EH890, which adopts the quenching and tempering process of quenching + sub-temperature quenching + tempering, the primary quenching process is 910-940 ° C, and the holding time is PLC + ( 30-40) min, water cooled to room temperature after coming out of the furnace, sub-temperature quenching process quenching temperature 810-830°C, holding time PLC+(10-30) min, water cooled to room temperature after coming out of the furnace, tempering temperature 630-660°C, total Heating time 4.0-5.0min/mm, air-cooled to room temperature, can produce steel plates with a thickness specification of 15-50mm, yield strength ≥ 890MPa, tensile strength 940-1100MPa, yield ratio ≤ 0.94, V-shaped transverse direction -40°C Impact energy ≥ 50J. However, this patent uses multiple heat treatment processes of quenching + sub-temperature quenching + tempering, which has a long production cycle and high process costs. At the same time, the surface quality of the steel plate deteriorates after multiple high-temperature heat treatments, making it difficult to meet the coating requirements of marine engineering equipment.

Chinese patent CN114032459A discloses a method for preparing a medium-thick steel plate with a yield strength of 690 MPa, high strength toughness and low yield ratio, which can be used to produce steel plates with a thickness of 10-50 mm. The hot-rolled steel plate is heated to 300-650 ° C for pre-insulation and heat preservation. The time is not less than 60 minutes, so that M-A is fully decomposed to obtain uniform and fine cementite, and then heated to a certain temperature in the (α+γ) two-phase region at a heating rate of not less than 1°C/s, and after holding for 30 to 120 minutes Water quenching, reheating to 200-450°C, tempering at medium and low temperatures, and holding for 30-120 minutes to obtain precipitation-strengthened ferrite (soft phase) and tempered martensite (hard phase) structures. However, this patent adopts a very complicated heat treatment process route, which requires heating the hot-rolled steel plate to 300-650°C for pre-insulation, then heating to the two-phase zone for heat preservation, then quenching, and then heating to 200-450°C for medium-low temperature tempering , completely unable to adapt to the industrialized production equipment of medium and thick steel plates.

Chinese patent CN111705268B discloses a low-yield-ratio ultra-high-strength high-toughness pressure-resistant shell steel and its preparation method. It adopts secondary quenching heat treatment, the first quenching is completely austenitized, and then the second quenching and tempering are carried out. Finally, a multi-phase structure such as tempered martensite, critical ferrite, and retained austenite is obtained, and an 890MPa-class ultra-high-strength, high-toughness pressure-resistant shell steel with a low yield ratio of no higher than 0.90 is realized. This patent is aimed at steel for pressure shells, adding 5.00%~10.00% Ni to obtain enough retained austenite to reduce the yield ratio. At the same time, the long-term heat treatment process of two quenchings is used, and the alloy and process costs are extremely high.

Contents of the invention

The object of the present invention is to provide a low-yield-ratio marine engineering structural steel and its manufacturing method, which can produce a structure with a thickness of 6-20mm, a yield strength of not less than 690MPa, and a ratio of yield strength to tensile strength of not higher than 0.94 Use thin gauge steel.

Technical scheme of the present invention:

A thin-gauge, low-yield-ratio 690MPa grade marine engineering steel, the chemical composition of the steel is C=0.06%~0.15%, Si=0.10%~0.80%, Mn=0.50%~1.60%, P≤0.015 %, S≤0.006%, Nb=0.015%~0.05%, V=0.02%~0.10%, Ti=0.01%~0.03%, Ni=0.30%~1.00%, Cu=0.20%~1.50%, Cr=0.30 %~1.20%, Mo=0.20%~0.70%, Al=0.015%~0.045%, Ca=0.001%~0.015%, the rest is Fe and other unavoidable impurities; steel plate thickness 6~20mm, yield strength not less than 690MPa, the ratio of yield strength to tensile strength is not higher than 0.94.

A method for manufacturing thin-gauge, low-yield-ratio 690MPa grade marine engineering steel, comprising the following steps:

1) Converter smelting: After pre-desulfurization treatment of molten iron produced in blast furnace, it is poured into 100~300 tons of converter smelting for top-bottom reblowing oxygen treatment to complete oxidative decarburization, dephosphorization and desulfurization of molten iron, and then add Mn and Ni ferroalloy raw materials Continue to blow oxygen, adjust the temperature of molten steel to 1620~1660°C, and then add aluminum ingots for calm deoxidation according to the oxygen content in molten steel, and blow argon at the bottom before tapping;

2) Secondary refining: pour the molten steel into the ladle, transport it to the LF refining furnace, and continue to blow the argon gas to stir the molten steel; during the refining process, take samples to check the composition of the molten steel, fine-tune the alloy content to make all the components meet the above requirements, and the processing time is 20~30 minutes; Feed 200 meters of pure calcium wire before the end of LF treatment; then send the ladle to RH vacuum refining furnace for vacuum degassing treatment, the time is ≥15 minutes, and the gas content of molten steel is removed to [H]≤2 ppm, [O]≤20 ppm, [N]≤60ppm;

3) Continuous casting of slabs: continuous casting into slabs with a thickness of 150~320mm and a width of 1650~2650mm through the intermediate ladle under the protective atmosphere and protective slag;

4) Slab heating: after the continuous casting high-temperature slabs are flame-cut, they are stacked and cooled slowly for 45~50 hours in the blank yard, and then transported to the heating furnace to be heated to a temperature of 1150°C~1220°C, and the heating time is 4 ~ 8 h, the austenite grain size is above grade 6;

5) Steel plate rolling: After the surface of the steel billet is removed with high-pressure water, the iron scale is removed, and then it is stretched and rolled along the width direction for 3~7 times at a rolling temperature of 1100~1150°C. After rolling to the required width, turn the steel 90° and then Rolling at a temperature of 900~1050°C above the austenite non-recrystallization zone to the required steel plate thickness, keeping the austenite grains in a recrystallized equiaxed shape, and the grain size is above grade 8;

6) Cooling of the steel plate: The laminar flow header is used to spray water on the steel plate after rolling to accelerate the cooling rate, the cooling rate is 10~30°C/s, and the final cooling temperature is 200~450°C. The proportion is above 60%;

7) Steel plate heat treatment: conduct a tempering heat treatment on the steel plate.

In the step 3), the non-metallic inclusions in the slab meet the following grade requirements: Type A inclusions ≤ 0.5, Type B inclusions ≤ 0.5, Type C inclusions (silicates) ≤ 0.5, Type D inclusions ≤0.5.

Invention principle:

In the above-mentioned technical scheme, the microstructure of 690MPa grade marine engineering steel with a low yield ratio of 6~20mm is tempered martensite, proeutectoid ferrite, retained austenite and other multiphase structures, and there are a large number of Nano-sized Nb-V-Ti composite precipitation strengthening phase, so as to achieve low yield strength ratio while maintaining the performance index requirements of yield strength above 690MPa.

In the above technical scheme, the above-mentioned low yield ratio 690MPa grade marine engineering steel has a yield strength ≥ 690MPa, which can actually reach 710~850MPa; tensile strength ≥ 760 MPa; elongation after fracture ≥ 15%; yield ratio ≤ 0.92 ; −40°C impact energy ≥ 100J, actually up to 120~230J.

In the above technical scheme, when performing controlled rolling, the finish rolling temperature is 750-850°C, the start cooling temperature is 680-750°C, and the end cooling temperature is 200-350°C. During tempering treatment, the tempering temperature is 550~620°C, and the tempering time is 30~60 min. The effect of controlled rolling + controlled cooling on-line quenching + tempering is to make the steel plate form a matrix structure of not less than 60% martensite and not more than 40% bainitic ferrite, and at the same time precipitate (Nb,Ti)C+ VC composite nano-particles, reduce the yield strength ratio with dual-phase matrix structure, and use high-density composite nano-phase precipitation strengthening to ensure that the yield strength is not less than 690MPa.

The setting of each alloy composition and mass percentage content in the steel plate for marine engineering with a yield strength not lower than 690 MPa, a yield ratio not higher than 0.94, and a thickness specification of 6-20 mm is based on the following mechanism:

C element is a basic strengthening element of ultra-high-strength steel, which can significantly improve the formation ability of martensitic phase in the quenching process, thereby improving hardenability. However, high C content is unfavorable to the weldability and low-temperature toughness of the quenched and tempered ultra-high-strength steel. In order to ensure the balance of ultra-high strength, low-temperature toughness and weldability of the steel of the present invention, the C content is controlled at 0.06% to 0.15%.

Si element is one of the main deoxidizing elements in the steelmaking process, and it is also a solid solution strengthening element. However, if the Si content is too high, it is easy to form ferrous silicate Fe2SiO4 that is difficult to remove during the heating process of the steel billet and steel plate, which makes the surface of the steel plate oxidized. It is difficult to peel off and affects the surface quality of the steel plate, so the Si content is controlled at 0.10%~0.80%.

Mn element is the most important strengthening element, and it is also an austenite stabilizing element, which improves the hardenability of steel during quenching, and plays the role of solid solution strengthening and fine grain strengthening. However, Mn is very easy to segregate in the center of the continuous casting slab to form a banded structure, and forms easily deformable MnS inclusions with S, and it is easy to interact with hydrogen to cause hydrogen-induced cracks in the marine environment, so the Mn content is controlled at 0.50%~1.60%.

Nb element is a strong carbon-nitrogen compound forming element, and Nb(C,N) nano-sized particles are dispersed and precipitated in the steel, which effectively refines the growth of austenite grains during the heating process of the slab. At the same time, Nb can effectively reduce the austenite non-recrystallization temperature, inhibit the austenite recrystallization process, and refine the austenite grains in the high temperature range of hot rolling. Combined with the controlled rolling process, it can effectively refine the final ferrite structure of the steel plate , thereby improving the strength and toughness of the steel plate. During quenching, part of Nb remains in solid solution state, and precipitates during tempering, which increases the contribution of precipitation strengthening. To play the above role, the Nb content should be controlled at 0.015~0.050%.

V element is a carbide-forming element in steel, which exists in solid solution during hot rolling and quenching, and precipitates nano-scale VC or (MoV)C during tempering to increase the strength of steel. Therefore, the V content is controlled at 0.02%~0.10%.

Ti element is a strong oxide and nitride forming element. During the steel refining process, O and N gases in molten steel are effectively removed to form TiO and TiN, which inhibits the growth of austenite grains in the subsequent heating process. However, adding too much Ti tends to precipitate large-sized square TiCN particles at the austenite grain boundaries during the solidification process of the billet, resulting in cracks on the billet surface. Therefore, the Ti content is controlled at 0.01%~0.03%.

Ni element is a strong austenite stabilizing element, which improves the hardenability and martensite formation ability of steel, and forms high thermally stable retained austenite in the Ni micro-segregation zone during quenching and tempering, thus significantly Improve the low temperature toughness of high strength steel produced by quenching and tempering process. Another function of Ni element is to inhibit Cu embrittlement caused by low melting point Cu melting during high temperature heating of Cu-containing steel, and improve the surface quality of steel. However, due to the high cost of Ni, the Ni content is controlled at 0.30%~1.00%.

Cu element can improve the corrosion resistance of steel in marine environment, and at the same time, Cu-rich nano-scale particles are precipitated during the quenching and tempering process, which plays a role in precipitation strengthening. However, the melting point of Cu is low, and it is easy to melt at the austenite grain boundary to cause cracks during the heating process of the billet, which needs to be suppressed by adding Ni, so the Cu content is controlled at 0.20%~0.50%.

Both Cr and Mo elements are strong hardenability elements of steel, which promote the formation of martensite during the quenching process, and are also strong carbide forming elements, which promote the formation of M23C6 and MC type carbides, effectively inhibit temper brittleness, and reduce steel plate pearlite. Banded structure, so the content of Cr and Mo is controlled at 0.30%~1.20% and 0.30%~0.70%, respectively.

Al is a strong deoxidizing and fine-grained element in the steelmaking process, which effectively removes the O gas content in molten steel, and forms a stable AlN precipitate with N, which plays a role in the growth of austenite grains during the pinning heating process, but the added Too much Al will keep too many Al2O3 inclusions in the molten steel, and agglomerate at the casting nozzle to cause accidents, so the Al content is controlled at 0.015%~0.045%.

Ca is a strong sulfide-forming element, forming Ca-Mn-S composite inclusions, which changes the shape of MnS inclusions and the formability of the hot deformation process, especially in the Mn segregation zone at the core of the steel plate, keeping the sulfide inclusions spherical and inhibiting the tensile strength. The delamination phenomenon of the tensile and impact fractures, and improve the uniformity of the transverse and longitudinal properties of the steel plate. The Ca content should be kept at the ratio of Ca/S, so it should be controlled at 0.001%~0.015%.

Both P and S are harmful elements and should be removed through the steelmaking process as much as possible. During the solidification process of the continuous casting slab, P is very prone to segregation, especially at the end of the solidification of the core of the slab to form a high P content enrichment zone; in the steelmaking process, S and Mn form manganese sulfide inclusions, especially in the slab A large number of MnS inclusions are formed at the central Mn segregation position, which is deformed into strips in the subsequent rolling process, especially for thin-gauge steel plates. Therefore, the content of P and S should be controlled at a low level as far as possible, considering the difficulty of steelmaking process and the method of improving inclusions by Ca treatment in the present invention, so as to control P≤0.015% and S≤0.006% in steel.

The outstanding features and remarkable effects of the present invention are mainly reflected in:

(1) The present invention produces 6~20mm thickness thin specifications, yield Low-yield-ratio thin-gauge steel plates for marine engineering with strength higher than 690MPa, elongation after fracture greater than 17%, yield ratio lower than 0.92, and impact energy at -40 °C higher than 100J.

(2) The present invention is a short-flow process adapted to the existing technical route of industrial production of steel plates, and avoids the long-period and high-energy consumption process of the current low-yield-ratio double-quenching technology. Taking advantage of the advantages of high cooling rate and good uniformity of thin-gauge steel plates, the design of low-carbon and low-alloy components with matching hardenability is adopted, and the shape and state of austenite are adjusted by controlling the final rolling temperature after rolling, and the cooling temperature is controlled by the transmission time of the roller table and cooling rate, so that the deformed austenite of the steel plate can be properly relaxed, and part of the grain boundary ferrite soft phase is precipitated first, and then the remaining relaxed austenite is transformed into a martensitic hard phase by forced water cooling, reducing the yield of the thin steel plate. The strength ratio is below 0.92, while maintaining the solid solution state of Nb, Ti, and V microalloying elements.

(3) Through the optimized heat treatment of tempering temperature and time, high-density nanoscale composite microalloy carbide (Nb,Ti)C+VC is obtained, and the yield strength is increased to above 690 MPa. At the same time, tempering makes high dislocation density and high C The quenched martensite recovery.

Description of drawings

(a), (b) and (c) in Fig. 1 correspond to the optical microstructure of the steel plate after controlled rolling and controlled cooling and direct quenching in Example 1, Example 2 and Example 3, respectively.

(a), (b) and (c) in Fig. 2 correspond to the average orientation error diagram of the kernel obtained from the electron backscatter diffraction data of the steel plate after controlled rolling and controlled cooling and direct quenching respectively in Example 1, Example 2 and Example 3. The figure reflects the local distribution of dislocation density. Blue indicates extremely low dislocation density, which corresponds to the ferrite region in Figure 1; yellow indicates the high dislocation density region, which corresponds to the martensite region in Figure 1.

(a), (b), and (c) in Figure 3 correspond to the transmission electron micrographs of steel plates after tempering in Example 1, Example 2, and Example 3, respectively. In the figure, it can be seen that high-density precipitation of nanoscale VC-(Nb, Ti) C composite carbides.

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with specific embodiments. The following relevant standards are used for performance testing of the steel products produced in the examples: (1) Yield strength, tensile strength, elongation after break: GB/T 228.1-2021 Metal material tensile test part 1: room temperature test method; ( 2) Charpy impact energy at -40 °C: GB/T 229-2020 Charpy pendulum impact test method for metal materials; (3) Yield ratio is the ratio of yield strength to tensile strength, according to GB/T 228.1-2021 Test methods were obtained separately.

Example 1

Manufacture EH690-grade steel plates for marine engineering with a thickness of 6mm and a yield ratio ≤0.92. Smelt molten steel according to the set composition and cast it into a slab with a thickness of 200 mm. The chemical composition by weight percentage is: 0.06%C, 0.25%Si, 0.80%Mn , 0.25%Mo, 0.012%P, 0.005%S, 0.030%Nb, 0.03%V, 0.015%Ti, 0.30%Ni, 0.25%Cu, 0.35%Cr, 0.25%Mo, 0.03%Al, 0.008%Ca, other The amount is Fe and unavoidable impurities.

Process steps: heat the billet with a thickness of 200mm to 1200°C, hold it for 3 hours and push it out of the heating furnace, use 19MPa high-pressure water to remove the oxide scale on the upper and lower surfaces of the billet, and then carry out two-stage controlled rolling: the rolling temperature of the first stage is 1150~950°C, The cumulative reduction rate is 80% to the thickness of the intermediate billet of 40 mm to ensure that the austenite is fully recrystallized and refined; the rolling temperature in the second stage is 920-850 ° C, the reduction rate is 80 %, and finally the steel plate is hot rolled to 6 mm at 850 ° C . After hot rolling, the steel plate is air-cooled to 750°C by a conveying roller with a speed of 0.5 mm/s, so that the deformation and work-hardened austenite can fully recover and relax. The steel plate passes through 12 sets of header laminar flow sections at a speed of 1.5 mm/s on the accelerated cooling roller table, and the final cooling temperature is 260 ° C to obtain 80% martensite. After controlled rolling and controlled cooling, the steel plate is sent to the tempering furnace after direct quenching, and is soaked at 600°C for 30 minutes. The yield strength of the obtained steel is 795MPa, the tensile strength is 873MPa, the elongation after fracture is 20%, the yield strength ratio is 0.91, and the impact energy of a half-size sample at −40°C is 122J.

Example 2

Manufacture EH690-grade steel plates for marine engineering with a thickness of 15mm and a yield ratio ≤0.92. Smelt molten steel according to the set composition and cast it into a slab with a thickness of 200mm. The composition by weight percentage is: 0.06%C, 0.25%Si, 1.10%Mn, 0.35%Mo, 0.012%P, 0.004%S, 0.035%Nb, 0.04%V, 0.015%Ti, 0.45%Ni, 0.30%Cu, 0.42%Cr, 0.30%Mo, 0.03%Al, 0.009%Ca, balance It is Fe and unavoidable impurities.

The process steps include: heating the slab with a thickness of 200mm to 1200°C, holding it for 3 hours and pushing it out of the heating furnace, using 19MPa high-pressure water to remove the oxide scale on the upper and lower surfaces of the billet, and then carrying out two-stage controlled rolling: the rolling temperature of the first stage is 1150~950°C , the cumulative reduction rate is 70% to the thickness of the intermediate billet 60mm, to ensure that the austenite is fully recrystallized and refined; the rolling temperature in the second stage is 900-820°C, the reduction rate is 75%, and finally the steel plate is hot-rolled at 790°C to 15 mm. After hot rolling, the steel plate is air-cooled to 720°C by a conveying roller with a speed of 0.3 mm/s, so that the deformation and work-hardened austenite can fully recover and relax. The steel plate passes through 15 sets of header laminar flow sections at a speed of 1.0 mm/s on the accelerated cooling roller table, and the final cooling temperature is 300 ° C to obtain 75% martensite. After controlled rolling and controlled cooling, the steel plate is sent to the tempering furnace after direct quenching, and is soaked at 600°C for 45 minutes. The yield strength of the obtained steel is 757MPa, the tensile strength is 842MPa, the elongation after fracture is 22%, the yield strength ratio is 0.90, and the impact energy of the full-scale sample at −40°C is 172J.

Example 3

Manufacture an EH690 steel plate for marine engineering with a thickness of 20mm and a yield ratio of ≤0.92, smelt molten steel according to the set composition and cast it into a slab with a thickness of 200mm, and the composition is by weight: 0.06%C, 0.25%Si, 1.20% Mn, 0.45%Mo, 0.012%P, 0.004%S, 0.035%Nb, 0.06%V, 0.015%Ti, 0.70%Ni, 0.35%Cu, 0.55%Cr, 0.45%Mo, 0.03%Al, 0.009%Ca, The balance is Fe and unavoidable impurities.

The process steps include: heating the slab with a thickness of 200mm to 1200°C, holding it for 3 hours and pushing it out of the heating furnace, using 19MPa high-pressure water to remove the oxide scale on the upper and lower surfaces of the billet, and then carrying out two-stage controlled rolling: the rolling temperature of the first stage is 1150~950°C , the cumulative reduction rate is 60% to the intermediate billet thickness of 80mm, to ensure the recrystallization and refinement of austenite; the rolling temperature of the second stage is 880-810°C, the reduction rate is 75%, and finally the steel plate is hot-rolled to 20mm at 760°C . After hot-rolling, the steel plate is air-cooled to 700°C by a conveying roller at a speed of 0.25 mm/s, so that the deformation and work-hardened austenite can fully recover and relax. The steel plate passes through 21 sets of header laminar flow sections at a speed of 0.80 mm/s on the accelerated cooling roller table, and the final cooling temperature is 320 ° C to obtain 70% martensite. After controlled rolling and controlled cooling, the steel plate is sent to the tempering furnace after direct quenching, and is soaked at 600°C for 60 minutes. The yield strength of the obtained steel is 731MPa, the tensile strength is 821MPa, the elongation after fracture is 24%, the yield strength ratio is 0.89, and the impact energy of the full-scale sample at −40°C is 212J.

Claims (3)

1. A thin-gauge, low-yield ratio 690MPa grade marine engineering steel, characterized in that: the chemical composition of the steel is C=0.06%~0.15%, Si=0.10%~0.80%, Mn=0.50%~ 1.60%, P≤0.015%, S≤0.006%, Nb=0.015%~0.05%, V=0.02%~0.10%, Ti=0.01%~0.03%, Ni=0.30%~1.00%, Cu=0.20%~ 1.50%, Cr=0.30%~1.20%, Mo=0.20%~0.70%, Al=0.015%~0.045%, Ca=0.001%~0.015%, the rest is Fe and other unavoidable impurities; steel plate thickness 6~20mm , The yield strength is not lower than 690MPa, and the ratio of yield strength to tensile strength is not higher than 0.94. 2. A method for manufacturing thin-gauge, low-yield ratio 690MPa steel for ocean engineering, characterized in that it comprises the following steps: 1) Converter smelting: After pre-desulfurization treatment of molten iron produced in blast furnace, it is poured into 100~300 tons of converter smelting for top-bottom reblowing oxygen treatment to complete oxidative decarburization, dephosphorization and desulfurization of molten iron, and then add Mn and Ni ferroalloy raw materials Continue to blow oxygen, adjust the temperature of molten steel to 1620~1660°C, and then add aluminum ingots for calm deoxidation according to the oxygen content in molten steel, and blow argon at the bottom before tapping; 2) Secondary refining: pour the molten steel into the ladle, transport it to the LF refining furnace, and continue to blow the argon gas to stir the molten steel; during the refining process, take samples to check the composition of the molten steel, fine-tune the alloy content to make all the components meet the above requirements, and the processing time is 20~30 minutes; Feed 200 meters of pure calcium wire before the end of LF treatment; then send the ladle to RH vacuum refining furnace for vacuum degassing treatment, the time is ≥15 minutes, and the gas content of molten steel is removed to [H]≤2 ppm, [O]≤20 ppm, [N]≤60ppm; 3) Continuous casting of slabs: continuous casting into slabs with a thickness of 150~320mm and a width of 1650~2650mm through the intermediate ladle under the protective atmosphere and protective slag; 4) Slab heating: after the continuous casting high-temperature slabs are flame-cut, they are stacked and cooled slowly for 45~50 hours in the blank yard, and then transported to the heating furnace to be heated to a temperature of 1150°C~1220°C, and the heating time is 4 ~ 8 h, the austenite grain size is above grade 6; 5) Steel plate rolling: After the surface of the steel billet is removed with high-pressure water, the iron scale is removed, and then it is stretched and rolled along the width direction for 3~7 times at a rolling temperature of 1100~1150°C. After rolling to the required width, turn the steel 90° and then Rolling at a temperature of 900~1050°C above the austenite non-recrystallization zone to the required steel plate thickness, keeping the austenite grains in a recrystallized equiaxed shape, and the grain size is above grade 8; 6) Cooling of the steel plate: The laminar flow header is used to spray water on the steel plate after rolling to accelerate the cooling rate, the cooling rate is 10~30°C/s, and the final cooling temperature is 200~450°C. The proportion is above 60%; 7) Steel plate heat treatment: conduct a tempering heat treatment on the steel plate. 3. A method for manufacturing thin-gauge, low-yield-ratio 690MPa grade marine engineering steel according to claim 2, characterized in that: in step 3), the non-metallic inclusions in the slab meet the following grade requirements: Class A Inclusions ≤ 0.5, Type B inclusions ≤ 0.5, Type C inclusions (silicates) ≤ 0.5, Type D inclusions ≤ 0.5.