Low-yield-ratio super-thick maritime work steel with excellent core low-temperature toughness and manufacturing method of low-yield-ratio super-thick maritime work steel

Abstract

The invention provides low-yield-ratio super-thick maritime work steel with excellent core low-temperature toughness and a manufacturing method of the low-yield-ratio super-thick maritime work steel, and belongs to the field of steel and iron material preparation. The low-yield-ratio super-thick marine steel with the excellent core low-temperature toughness comprises the following chemical components in percentage by mass: 0.10%-0.15% of C, 0.35%-0.45% of Si, 1.20%-1.50% of Mn, 3.00%-3.50% of Ni, 0.30%-0.50% of Mo, 0.15%-0.3% of Cr, 0.01%-0.02% of Ti, 0.030%-0.040% of V, 0.050%-0.060% of Nb, 0.0016%-0.0020% of B, 0.15%-0.25% of Cu, 0.060%-0.070% of Al, less than or equal to 0.010% of P, less than or equal to 0.004% of S, less than or equal to 0.39% of CET and the balance of Fe and inevitable impurities. According to the invention, a component design thought of a low-C, Ni-Nb-Mo-Al-B system is adopted, and a three-section rolling and quenching and tempering heat treatment process is combined. Through strict control of smelting continuous casting, controlled rolling and controlled cooling processes and quenching and tempering heat treatment parameters, the super-thick marine steel with excellent comprehensive mechanical properties is successfully prepared. The steel shows good low-temperature toughness in the core part, and meanwhile, the steel has a relatively low yield ratio. According to the low-yield-ratio super-thick maritime work steel with the excellent core low-temperature toughness, the thickness range is 80-150 mm, the yield strength is larger than or equal to 690 MPa, the tensile strength is larger than or equal to 920 MPa, the percentage elongation after fracture is larger than or equal to 20%, the yield ratio is smaller than or equal to 0.77, and the impact energy KV2 at the temperature of-60 DEG C is larger than or equal to 80 J.

Description

Technical Field

[0001] The invention belongs to the field of steel material preparation, and particularly relates to a low yield ratio ultra-thick marine engineering steel with excellent core low-temperature toughness and a manufacturing method thereof. Background Art

[0002] In recent years, with the continuous development of deep-sea oil and gas resources, marine equipment is increasingly developing towards large-scale and extreme service environments such as deep sea and extremely cold environment. General strength marine engineering steel has gradually been unable to meet the structural requirements of marine equipment, and the demand for ultra-high strength marine engineering steel with lightweight, high load-bearing and excellent low-temperature toughness is increasing. Therefore, the development of ultra-high strength and ultra-thick marine engineering steel with low yield ratio, excellent impact performance at low temperature of -60℃, and thickness of 80-150mm, strength grade ≥690MPa, yield ratio ≤0.77, and low-temperature impact performance at -60℃ plays an important role and significance in ensuring the safety of marine equipment in service.

[0003] The invention patent with publication number CN113549827ACN113549827B, "A FH690 grade marine engineering steel with excellent low-temperature toughness and its manufacturing method", adopts three-stage controlled rolling technology and DQ+ACC rapid cooling system, and high-temperature tempering after cooling to produce low-yield and high-thick marine engineering steel with excellent low-temperature toughness. However, the thickness of the 40mm steel plate produced is relatively small, only 40-50mm; the elongation after fracture of the thick plate is low, at 16%-20%, all below 20%; and the yield strength is relatively high, all greater than 0.92, and the plasticity is poor.

[0004] The invention patent with publication number CN117187684A, "A large-thickness FH690 marine engineering steel and its preparation method", adopts a screened alloy element ratio and a double quenching and tempering heat treatment process to obtain FH690 marine engineering steel with a maximum thickness of 100mm and excellent low-temperature performance. However, its manufacturing process and procedures are relatively complicated, and the elongation after fracture is low, at 16%-20%, both below 20%; and the yield strength is relatively high, both greater than 0.94, and the plasticity is poor.

[0005] The invention patent with publication number CN109161791B, "690MPa grade ship and marine engineering steel with excellent low temperature toughness and its manufacturing method", adopts low C, low Mn, Cr-Ni-Mo-Cu-B alloying and V, Ti micro-alloying composition system. After smelting, refining casting outside the furnace, controlled rolling, heat treatment and other processes, ship and marine engineering steel with high strength and excellent low temperature impact toughness is manufactured. However, the thickness of the steel plate produced is relatively small, ranging from 20-90mm; the 40mm thick plate has a high yield strength ratio, both greater than 0.94, and the plasticity is poor.

[0006] The invention patent with publication number CN117551945A, "A low-cost FH690 steel plate for marine engineering and its production method", adopts low C, medium Mn, Cr-Ni-Mo-Cu-B alloying component system and online quenching two-stage controlled cooling and two-phase short-time tempering process to obtain a 60-120mm thick steel plate with excellent performance and tempered troostite and reversed austenite structure. However, its Mn content is too high, reaching 4%-5%, which is easy to cause segregation and is not conducive to welding and continuous casting process control.

[0007] The invention patent with publication number CN109161791ACN113637917B, "A 690MPa grade ultra-high strength and extra-thick ship plate steel with excellent low-temperature impact performance and its production method", adopts a two-stage controlled rolling and quenching and tempering heat treatment process to obtain an ultra-high strength and extra-thick ship plate steel with excellent low-temperature impact performance and tempered martensite structure. However, the thickness of the steel plate produced is relatively small, 50-80mm; its yield strength ratio is relatively high, all above 0.83, and the plasticity is relatively poor.

[0008] In view of the above shortcomings, how to produce ultra-thick marine engineering steel with a thickness of 80-150mm and excellent low-temperature performance and low yield strength ratio has become the focus of research. Summary of the invention

[0009] The present invention aims to provide a method for producing ultra-thick marine steel with excellent core low-temperature toughness and low yield ratio. The method adopts the basic design idea of ​​low carbon, Ni-Nb-Mo-Al-B system, and strictly controls the content of alloy components. The ultra-high strength and excellent low-temperature toughness of the steel are ensured by the balanced ratio of low carbon and high nickel. At the same time, 0.15-0.25% of copper is added to play the role of solid solution strengthening and precipitation strengthening, improve the strength grade of the steel plate and improve its low-temperature toughness. In order to optimize hot workability, the manganese content is strictly controlled at 1.20% to 1.50%. Reasonable addition of Nb, Mo, Ti, B and other elements that enhance hardenability to support the effect of nickel ensures that the steel plate has high toughness. In addition, the aluminum content is controlled in the range of 0.060% to 0.070% to ensure that most of the boron atoms are dissolved in austenite to avoid cracks caused by excessive addition, thereby affecting the quality of the steel plate. At the same time, the process parameters of smelting, continuous casting, controlled rolling and controlled cooling, and heat treatment are reasonably designed to obtain the required lower bainite and granular bainite structures, thereby ensuring comprehensive mechanical properties such as ultra-high strength, excellent low-temperature toughness and low yield ratio. By shortening the quenching and holding time in the quenching and tempering heat treatment to control the grain size, and setting high-temperature tempering close to the critical temperature to achieve maximum precipitation strengthening, a 690MPa-grade low yield ratio ultra-thick marine steel with excellent low-temperature toughness is prepared, and its core structure is uniform and has excellent performance. The production process is simple, economical and applicable.

[0010] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0011] The invention discloses an ultra-thick marine engineering steel with excellent low-temperature toughness in the core and low yield strength ratio. The ultra-thick marine engineering steel with excellent low-temperature toughness in the core and low yield strength ratio contains the chemical components of the steel and their corresponding mass percentage contents as follows: C: 0.10% to 0.15%, Si: 0.35% to 0.45%, Mn: 1.20% to 1.50%, Ni: 3.00% to 3.50%, Mo: 0.30% to 0.50%, Cr: 0.1 5%~0.3%, Ti: 0.01%~0.02%, V: 0.030%~0.040%, Nb: 0.050%~0.060%, B: 0.0016%~0.0020%, Cu: 0.15%~0.25%, Al: 0.060%~0.070%, P: ≤0.010%, S: ≤0.004%, and CET is controlled ≤0.39%, and the balance is Fe and unavoidable impurities.

[0012] The mechanism of action of each alloy component in the low yield strength and high-thickness marine engineering steel with excellent low-temperature toughness in the core of the present invention is as follows:

[0013] C: The content determines the equilibrium structure of the iron-carbon alloy, which in turn affects the mechanical properties of the material. The mass fraction of carbon in steel, as well as the form and distribution of carbon, affect the plasticity of steel at room temperature. During the thermal deformation of the steel plate, the C element will cause banded segregation, thereby reducing the isotropy of the steel and deteriorating the room temperature plasticity; as the carbon content increases, carbides will aggregate in large quantities in the aggregate, producing coarse precipitates, which will deteriorate the plasticity and low-temperature toughness of the material, and deteriorate the welding performance. Therefore, the present invention controls the C content range to be 0.10% to 0.15%.

[0014] Si: It is a commonly used deoxidizer, used to remove oxygen from molten steel. It exists in the form of SiO2 and can also play a role in solid solution strengthening. However, when the mass fraction of Si is high, the toughness and plasticity of the steel will be reduced. Adding an appropriate amount of Si to steel can also inhibit the growth of carbides, thereby improving the toughness of the steel. However, the higher the Si content, the better. Too much Si content deteriorates the surface quality of the steel plate and affects the toughness of the steel. Therefore, the present invention controls the Si content range to be 0.35% to 0.45%.

[0015] Mn: It is also one of the deoxidizers, which can effectively improve the hot working performance and tear resistance of steel. Mn has the effect of delaying the transformation of austenite to ferrite in steel, which is beneficial to refining ferrite and improving strength and toughness. When the content of Mn is low, the above effect is not significant, and the strength and toughness of the steel plate are low. If the content is too high, it will cause segregation of the continuous casting billet and poor toughness. Therefore, the preferred Mn content range of the present invention is 1.20% to 1.50%.

[0016] Ni: It is a solid solution strengthening element in steel and also a good element for enhancing hardenability. It can effectively improve the low temperature performance of steel, especially the low temperature toughness. In addition, the compounding effect with the residual Cr and P in the steel will help to improve the corrosion resistance of the steel. Therefore, the content of Ni in the present invention is controlled within the range of 3.00% to 3.50%.

[0017] Cr is a strong carbide-forming element. When added to steel, it will combine with carbon atoms to precipitate fine alloy cementite or alloy carbide. In addition to improving the hardenability of steel, it can also significantly improve the corrosion resistance and oxidation resistance of steel, and can improve wear resistance and high temperature performance. However, too high a content will affect the toughness of steel, cause temper brittleness, and also affect the surface quality. Therefore, the present invention controls the Cr content range to be 0.15% to 0.3%.

[0018] Mo: It is a strong hardenability additive. Under high temperature tempering conditions, Mo can increase the resistance to temper softening and improve tempering stability. In addition, Mo can also improve the corrosion resistance of stainless steel. The composite addition of Mo and Ni can improve the toughness of steel while increasing the strength of steel. However, as the Mo element increases, the number of Mo-containing carbides also increases. The carbides distributed at the grain boundaries destroy the continuity of the matrix and deteriorate the toughness of the steel. Therefore, the present invention controls the Mo content range to be 0.30% to 0.50%.

[0019] Nb: It can effectively prevent the recovery and recrystallization of deformed austenite during the rolling stage, and has the effect of refining the grains. It can also play a role in precipitation strengthening. The solute drag effect of Nb and the pinning effect of Nb (C, N) on the austenite grain boundaries both inhibit the recrystallization of deformed austenite, expand the non-recrystallization range of austenite, and reduce the waiting time for ultra-thick plate production. Precipitates are formed during cooling or tempering, thereby improving both the strength and toughness, and can also improve the corrosion resistance of steel. The present invention controls the Nb content range to be 0.050% to 0.060%.

[0020] Al: It is an important deoxidizing element in the steelmaking process. Adding a small amount of Al to steel can also effectively reduce the inclusion content in the steel and refine the grains. However, too much Al will promote surface cracks in the continuous casting billet, produce internal aluminum inclusions, and reduce the quality of the continuous casting billet. Therefore, the content of Al in the present invention is controlled to be in the range of 0.060% to 0.070%.

[0021] Ti: It also has the effect of grain refinement and precipitation strengthening. Ti and C have a strong affinity. Ti and C combine to form stable TiC particles, which can withstand high temperatures above 1300°C without dissolving, and play a role in pinning austenite grain boundaries and maintaining austenite grain refinement, thereby improving the inhomogeneity of the structure and improving the impact toughness of the steel. Therefore, the content of Ti in the present invention is controlled within the range of 0.01% to 0.02%.

[0022] V: It mainly forms V (C, N) in steel, thus affecting the structure and performance of steel. It can inhibit the recrystallization of austenite and prevent the growth of grains during rolling. It precipitates during the cooling process after rolling, which can refine the grains and improve the strength and toughness of steel. However, the amount of V added should not be too much. Therefore, the content of V in the present invention is controlled to be 0.030% to 0.040%.

[0023] B: Aggregates at the austenite grain boundary, effectively inhibits ferrite nucleation, delays the ferrite-pearlite transformation, and thus improves the hardenability of the steel. Adding a very low content of B can significantly improve the hardenability of the steel. To a large extent, it can replace the addition of alloying elements such as Ni, Cr, and Mo to achieve the same hardenability effect; B can also be used to control phase transformation and microstructure in low-alloy high-strength steel. However, excessive B will deteriorate the hardenability of the steel, change its deformation behavior, cause brittleness during heat treatment, and reduce the low-temperature toughness of the material. Therefore, the present invention controls the B content range to be 0.0016% to 0.0020%.

[0024] Cu: As an important additive element for improving the mechanical properties, corrosion resistance and cold working properties of steel plates, Cu exists in the form of solid solution and precipitation in steel, so the strengthening mechanism of Cu on steel is mainly solid solution strengthening and precipitation strengthening. However, too high Cu will deteriorate the welding performance of steel, so the present invention controls the Cu content range to 0.15% to 0.25%.

[0025] P and S are unfavorable elements in steel, which seriously affect the strength, toughness and corrosion resistance of steel, and their content needs to be controlled as low as possible. Therefore, the present invention controls the content of P to be below 0.010%, and the content of S to be below 0.0040%.

[0026] The finished product of the ultra-thick marine steel plate with excellent low-temperature toughness in the core and low yield strength ratio has a thickness range of 80-150mm, a core yield strength ≥690MPa, a tensile strength ≥920MPa, an elongation after fracture ≥20%, a yield strength ratio ≤0.77, and an impact absorption energy KV2 ≥80J at -60°C.

[0027] The method for manufacturing a low yield ratio ultra-thick marine engineering steel with excellent low-temperature toughness in the core of the present invention adopts a technical solution comprising the following steps:

[0028] (1) Smelting and continuous casting:

[0029] The raw materials are weighed according to the chemical composition of low yield strength and high-thickness marine engineering steel with excellent low-temperature toughness in the core, melted, desulfurized, smelted in a converter, LF refined, and RH vacuum treated, and then continuously cast into billets;

[0030] Among them, the sulfur content of molten iron after desulfurization treatment is controlled at ≤0.0040%, the converter adopts double slag method to control P≤0.010%, LF refining adopts white slag operation for desulfurization and deoxidation, the vacuum degree of RH vacuum treatment is ≤3.0mbar, the vacuum holding time is ≥15min, calcium treatment is carried out after the vacuum ends, and then the static stirring time is ≥12min, and the continuous casting controls the overheating degree of the tundish at 10-15℃. Continuous casting implements full protection casting, adopts light pressure reduction process to improve the internal quality of continuous casting billet, and uses end-stage electric stirring to control segregation. The thickness of continuous casting billet is 300-600mm, and the stacking slow cooling is ≥36h.

[0031] (2) Heating:

[0032] The continuous casting billet with a thickness of 300 to 600 mm is heated in a soaking furnace to obtain a heated continuous casting billet; the soaking time is ≥ 120 min, the time in the furnace is 10-15 min / cm, preferably 12 min / cm; the heating temperature is 1150 to 1250°C;

[0033] (3) Controlled rolling:

[0034] The heated continuous casting billet must go through three stages of rolling, including two stages of rough rolling and a stage of finishing rolling, to obtain a uniform core structure of the ultra-thick marine steel after rolling;

[0035] Among them, the starting temperature of rough rolling is 1040-1060°C, the reduction rate of the middle two passes is ≥20%, and large reduction rolling is adopted; the reduction rate of the last 1-3 passes is ≥15%, preferably 4-6 rough rolling passes; the cumulative reduction ratio of rough rolling is ≥2.3, and the thickness of the intermediate billet is 130-260mm.

[0036] The starting rolling temperature in the finishing rolling stage is 820-840°C. The finishing rolling is preferably performed in three passes, with a pass reduction rate of 13%-18%, and the pass reduction rate gradually decreases with the number of rolling passes. The final rolling temperature is ≥790°C.

[0037] (4) Controlled cooling:

[0038] The rolled steel is cooled by laminar flow to 600-660° C. and then air-cooled to room temperature to obtain a cooled steel plate.

[0039] (5) Quenching and tempering process: After cooling, the rolled steel is subjected to quenching and tempering heat treatment, and is heated in a soaking furnace. The quenching temperature is 890-920°C, the holding time is 30-45 minutes, and the steel is quickly water-cooled to room temperature after the holding is completed; the tempering temperature is 660-680°C, the holding time is 1-2 hours, and the steel is air-cooled to room temperature to obtain a low yield strength and high-thickness marine steel with excellent low-temperature toughness.

[0040] In the manufacturing method of the present invention, compared with the prior art, the mechanism of performance improvement is:

[0041] (1). Rolling process

[0042] The heating temperature is determined to be 1150-1250°C, which not only enables titanium nitride to inhibit the growth of austenite grains, but also ensures the solid solution of carbides such as niobium and vanadium.

[0043] The starting temperature of rough rolling is 1040-1060℃, and the first stage is controlled rolling in the austenite recrystallization zone. Austenite deformation and recrystallization occur simultaneously, and static recrystallization is repeated during the interval between passes, thereby refining the grains. The second stage is controlled rolling in the austenite non-recrystallization zone. When the rolling temperature is reduced to below 950℃, the recrystallization of austenite is suppressed. Not only does recrystallization not occur during the deformation process, but static recrystallization is also difficult to occur during the interval between passes after deformation. At this time, the austenite grains are elongated, and a large number of deformation bands and deformation twins are generated in the grains. The effective grain boundary area as the ferrite nucleation point increases, which ultimately leads to the refinement of ferrite grains. The reduction rate of the rough rolling pass increases gradually, which is convenient for uniform organization and the rolling force is transmitted to the core to the maximum extent. The reduction rates of the last 1-3 passes are all above 15% to maximize the density and driving force of the nucleation zone and refine the grains. Increasing the deformation in the non-recrystallized zone can form high-density dislocations in austenite and store more energy, providing more nucleation points for phase transformation. High-density dislocations can also promote the precipitation of carbides during tempering to pin dislocations and precipitation strengthen. However, the deformation in the non-recrystallized zone of austenite has a decreasing marginal effect on the improvement of the impact toughness of the steel plate, and will not increase linearly. Excessive deformation will cause the quenching martensite phase transformation to be biased towards a certain orientation, which is not conducive to low-temperature toughness. The cumulative compression ratio of rough rolling is more suitable at 2 to 2.5, and the present invention adopts a control of more than 2.3.

[0044] The finishing rolling stage is rolled in the two-phase region of 790-840℃, the purpose is to fully flatten the austenite grains, and at the same time form a large number of dislocations in the grains, increase the nucleation energy when austenite transforms to bainite, and make the grains more uniform and fine after the phase transformation. The finishing rolling start temperature is reduced, so that the deformation occurs at a lower temperature, which promotes the ferrite phase transformation and increases the deformation energy. Studies have shown that the dislocation density of ferrite will increase significantly when the reduction rate is 10-20%, thereby increasing the strength of the steel. The finishing rolling can maintain a large reduction rate of 13% to 18% in the initial stage, and a small reduction rate in the last few passes. It can avoid excessive grain growth, improve the surface quality of the steel plate, and reduce warping deformation.

[0045] Strict control of process parameters in the rough rolling and finishing rolling stages rationally regulates the high-density dislocations and precipitation inside the steel plate, ensures the ultra-high strength and low yield ratio of the steel plate, and lays the foundation for obtaining high toughness in subsequent heat treatment.

[0046] (2) Cooling process

[0047] After controlled rolling, ultra-rapid cooling to 600-660°C allows the rolled piece to quickly pass through the austenite phase region, providing a large degree of undercooling for the phase transformation. At the same time, the growth of ferrite after the phase transformation is suppressed, and a higher cooling rate can even form bainite and acicular ferrite structures. For the low-C, Ni-Nb-Mo-Al-B system microalloy steel of the present invention, in addition to fine grain strengthening, the increase in cooling rate prevents or delays the premature precipitation of carbides during cooling, making it easy to form more fine and dispersed precipitates, and ultimately producing a better precipitation strengthening effect. After rapid cooling, air cooling to room temperature allows the carbides to be fully dispersed and precipitated to obtain a bainite structure with high strength.

[0048] Compared with the prior art, the invention provides an ultra-thick marine engineering steel with excellent low-temperature toughness in the core and a low yield strength ratio and a manufacturing method thereof, and its beneficial effects are:

[0049] (1) In terms of composition design, the basic design concept of low C and Ni-Nb-Mo-Al-B system is adopted, which greatly improves the hardenability of the core of the steel plate, refines the microstructure of the steel plate, and ensures that the steel plate has good strength and low-temperature toughness, as well as weldability.

[0050] (2) In terms of technology, the present invention adopts a three-stage rolling process. The reduction rate in the rough rolling stage increases gradually, so that the rolling force is transmitted to the core and the structure is homogenized. The reduction rate is reduced in the last pass to reduce the warping deformation of the steel plate. The overall reduction rate in the finishing rolling stage is less than that in the rough rolling, which is conducive to obtaining high strength and improving the surface quality of the steel plate. At the same time, the cooling process parameters are strictly controlled to obtain fine and dispersed precipitates, which play a role in precipitation strengthening and grain refinement. After controlled rolling and controlled cooling, the steel plate undergoes a suitable tempering heat treatment process, which can eliminate the internal stress generated during the rolling of the steel plate and improve the processing performance. At the same time, it changes the internal structure of the steel plate, refines the grains, and ensures the uniformity and low-temperature toughness of the core structure of the steel plate.

[0051] (3) In terms of performance, the steel plate produced by the present invention has excellent comprehensive mechanical properties, with a thickness range of 80 to 150 mm, a central yield strength ≥ 690 MPa, a tensile strength ≥ 920 MPa, an elongation after fracture ≥ 20%, a yield strength ratio ≤ 0.77, and an impact absorption energy KV2 ≥ 80 J at -60°C. It can produce ultra-thick ultra-high strength marine steel with a low yield strength ratio and a thickness specification of 80 to 150 mm. The ultra-thick marine steel plate with a low yield strength ratio produced by the present invention has excellent strength-plasticity matching and can be used for key components of ship and marine engineering structures, which meets the current development needs of my country for steel for ship and marine engineering equipment and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is the microstructure morphology of the steel plate at the 1 / 2 position in the tempered state in Example 1 of the present invention.

[0053] Figure 2This is the microstructure morphology of the steel plate at the 1 / 2 position in the tempered state in Example 2 of the present invention. DETAILED DESCRIPTION

[0054] The present invention is further described in detail below in conjunction with embodiments.

[0055] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are partial embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0056] Table 1 shows the chemical composition (wt.%) of the ultra-thick marine engineering steel plate with low yield ratio and excellent low-temperature toughness in the steel core according to the embodiment of the present invention.

[0057] Table 2 shows the chemical composition (wt.%) of the low yield ratio ultra-thick marine engineering steel plate with excellent low-temperature toughness in the core of the comparative steel of the present invention.

[0058] Table 3 shows the core mechanical properties of low yield ratio ultra-thick marine engineering steel plates with excellent low-temperature toughness in the core of the steel plates of the embodiments of the present invention and the comparative examples.

[0059] Table 4 shows the surface mechanical properties of the low yield ratio ultra-thick marine engineering steel plate with excellent low-temperature toughness in the core of the steel plate according to the embodiment of the present invention.

[0060] Example 1

[0061] A method for manufacturing ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness, wherein the thickness of the steel plate is 80-150 mm, and the chemical composition and mass percentage thereof are shown in Table 1, and the remainder is Fe and unavoidable impurities. The method includes smelting, continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0062] (1) Smelting and continuous casting: The raw materials are weighed according to the chemical composition of the ultra-thick marine steel with low yield ratio and excellent core low-temperature toughness. After melting, they are subjected to desulfurization treatment, converter smelting, LF refining and RH vacuum treatment, and then continuous casting into billets. The sulfur content of the molten iron is controlled at ≤0.0040% after desulfurization treatment. The converter adopts the double slag method to control P≤0.010%. LF refining adopts white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to 10-15℃. The continuous casting implements full protection casting, adopts the light pressure process to improve the internal quality of the continuous casting billet, and uses the end-stage electric stirring to control segregation. The thickness of the continuous casting billet is 300mm, and the stacking and slow cooling of the billet off the line is ≥36h.

[0063] (2) Heating: The 300 mm thick continuous casting slab is heated in a soaking furnace, the soaking time is ≥ 120 min, the furnace time is 12 min / cm, and the heating temperature is 1150-1250 °C;

[0064] (3) Controlled rolling: The heated continuous casting billet is subjected to a three-stage rolling process, including two-stage rough rolling and finishing rolling. The starting rolling temperature of the rough rolling is 1040-1060°C. After four rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%. The cumulative reduction ratio of the rough rolling is not less than 2.3, and the thickness of the intermediate billet is 130mm. The starting rolling temperature of the finishing rolling stage is 820-840°C. After three rolling passes, the reduction rate of each pass is 13%-18%, and the reduction rate of each pass gradually decreases with the number of rolling passes. The final rolling temperature is ≥790°C.

[0065] (4) Controlled cooling: The finished plate has a thickness of 80 mm after rolling. After rolling, it is laminar cooled to 660 °C and then air cooled to room temperature.

[0066] (5) Quenching and tempering: The cooled plate is subjected to quenching and tempering heat treatment, and is heated in a soaking furnace. The quenching temperature is 890-920°C, the holding time is 30-45 minutes, and then it is quickly cooled to room temperature by water; the tempering temperature is 660-680°C, the holding time is 1-2 hours, and then it is air-cooled to room temperature.

[0067] The core microstructure of the low yield strength and high-thickness marine engineering steel plate obtained in this embodiment is as follows Figure 1 As shown in the figure, it can be seen that the center is lower bainite and granular bainite structure with fine grains.

[0068] Example 2

[0069] The implementation method is the same as that of Example 1, except that the continuous casting billet thickness is 480mm, the rough rolling start temperature is 1040-1060℃, after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the rough rolling cumulative compression ratio is not less than 2.3, and the intermediate billet thickness is 200mm. The start rolling temperature of the finishing rolling stage is 820-840℃, after 3 rolling passes, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥800℃. Roll to a finished product thickness of 120mm, and laminar cooling to 630℃ after rolling is completed. The quenching temperature is 890-920℃, the holding time is 30-45min, and the insulation is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680℃, the holding time is 1-2h, and air cooling to room temperature.

[0070] The core microstructure of the low yield strength and high-thickness marine steel plate obtained in this embodiment is as follows. Figure 2As shown in the figure, compared with Example 1, the granular bainite structure of the low-yield ultra-thick marine engineering steel with excellent low-temperature toughness in this example is obviously the largest, because the core cooling rate is relatively slow due to the increase in thickness, which makes the grain size increase compared with Example 1.

[0071] Example 3

[0072] The implementation method is the same as that of Example 1, except that the continuous casting billet has a thickness of 400 mm, the starting temperature of rough rolling is 1040-1060°C, and after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the cumulative compression ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 170 mm. The starting temperature of the finishing rolling stage is 820-840°C, and after 3 rolling passes, the reduction rate of each pass is 13%-18%, and the final rolling temperature is ≥800°C. Roll to a finished product thickness of 100 mm, and laminar cooling to 610°C after rolling is completed. The quenching temperature is 890-920°C, the holding time is 30-45 minutes, and the insulation is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680°C, the holding time is 1-2 hours, and air cooling to room temperature.

[0073] Example 4

[0074] The implementation method is the same as that of Example 1, except that the continuous casting billet thickness is 520mm, the rough rolling start temperature is 1040-1060℃, after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the cumulative compression ratio of rough rolling is not less than 2.3, and the intermediate billet thickness is 220mm. The start rolling temperature of the finishing rolling stage is 820-840℃, after 3 rolling passes, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥800℃. Roll to a finished product thickness of 130mm, and laminar cooling to 600℃ after rolling is completed. The quenching temperature is 890-920℃, the holding time is 30-45min, and the holding is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680℃, the holding time is 1-2h, and air cooling to room temperature.

[0075] Example 5

[0076] The implementation method is the same as that of Example 1, except that the continuous casting billet thickness is 600mm, the rough rolling start temperature is 1040-1060℃, after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the cumulative compression ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 260mm. The start rolling temperature of the finishing rolling stage is 820-840℃, after 3 rolling passes, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥800℃. Roll to a finished product thickness of 150mm, and laminar cooling to 660℃ after rolling is completed. The quenching temperature is 890-920℃, the holding time is 30-45min, and the insulation is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680℃, the holding time is 1-2h, and air cooling to room temperature.

[0077] Example 6

[0078] The implementation method is the same as that of Example 1, except that the continuous casting billet thickness is 440mm, the rough rolling start temperature is 1040-1060℃, after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the cumulative compression ratio of rough rolling is not less than 2.3, and the intermediate billet thickness is 190mm. The start rolling temperature of the finishing rolling stage is 820-840℃, after 3 rolling passes, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥800℃. Roll to a finished product thickness of 110mm, and laminar cooling to 630℃ after rolling is completed. The quenching temperature is 890-920℃, the holding time is 30-45min, and the insulation is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680℃, the holding time is 1-2h, and air cooling to room temperature.

[0079] Example 7

[0080] The implementation method is the same as that of Example 1, except that the continuous casting billet thickness is 360mm, the rough rolling start temperature is 1040-1060℃, after 4 rolling passes, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last pass is ≥15%; the cumulative compression ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 150mm. The start rolling temperature of the finishing rolling stage is 820-840℃, after 3 rolling passes, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥800℃. Roll to a finished product thickness of 90mm, and laminar cooling to 620℃ after rolling is completed. The quenching temperature is 890-920℃, the holding time is 30-45min, and the insulation is completed. Rapid water cooling to room temperature; the tempering temperature is 660-680℃, the holding time is 1-2h, and air cooling to room temperature.

[0081] Comparative Example 1

[0082] The marine engineering steel of this comparative example adopts the chemical composition of the low yield ratio ultra-thick marine engineering steel with excellent low-temperature toughness in the center of Example 1, and the implementation method is the same as Example 1, except that, in the heating process of step (2), the heating temperature adopted is 1400°C, so the austenite grains grow, and the final steel grains are coarse, and compared with the performance of the steel in the embodiment of the present invention, the strength and toughness are reduced.

[0083] Comparative Example 2

[0084] The marine engineering steel of this comparative example adopts the chemical composition of the low yield ratio ultra-thick marine engineering steel with excellent low-temperature toughness in the center of Example 1, and the implementation method is the same as Example 1, except that: during the rough rolling process, the reduction rate of each pass is 10%, and the obtained steel has lower strength and low-temperature impact toughness than the steel performance of the embodiment of the present invention.

[0085] Comparative Example 3

[0086] The marine engineering steel of this comparative example adopts the chemical composition of the low yield ratio ultra-thick marine engineering steel with excellent low-temperature toughness in the center of Example 1, and the implementation method is the same as Example 1, except that: in step 4, the rolled steel is directly cooled naturally to room temperature after rolling, and the toughness of the obtained steel is lower than that of the steel performance of the embodiment of the present invention.

[0087] Comparative Example 4

[0088] The marine engineering steel of this comparative example is different from the chemical composition of the low yield ratio ultra-thick marine engineering steel with excellent low temperature toughness in the center of Example 1 in that the mass percentage of Ni is 0.94%, the mass percentage of Mo is 0.25%, the mass percentage of Mo is 0.21%, and no Nb is contained. The implementation method is exactly the same as that of Example 1. Compared with the steel performance of the steel in the embodiment of the present invention, the low temperature impact toughness of the obtained steel is slightly increased, and the strength is significantly reduced.

[0089] Table 1 Chemical composition of steel plates in the examples (wt.%)

[0090] Element Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 C 0.1 0.15 0.13 0.1 0.12 0.12 0.11 Si 0.35 0.39 0.41 0.44 0.39 0.39 0.43 Mn 1.21 1.16 1.25 1.23 1.24 1.27 1.29 Ni 3.02 3.05 3.01 3.50 3.00 3.07 3.16 Mo 0.33 0.30 0.3 0.31 0.46 0.33 0.32 Cr 0.16 0.16 0.17 0.28 0.21 0.18 0.19 Nb 0.051 0.052 0.055 0.058 0.053 0.051 0.055 V 0.035 0.033 0.039 0.039 0.031 0.035 0.039 Ti 0.014 0.011 0.019 0.012 0.015 0.017 0.013 Al 0.062 0.069 0.061 0.065 0.067 0.06 0.063 Cu 0.15 0.17 0.17 0.18 0.17 0.20 0.19 B 0.0016 0.0016 0.0018 0.0017 0.0019 0.0017 0.0017 P ≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01 S ≤0.004 ≤0.004 ≤0.004 ≤0.004 ≤0.004 ≤0.004 ≤0.004 CET 0.345 0.389 0.377 0.355 0.384 0.376 0.369

[0091] Table 2 Chemical composition of steel plates in comparative examples (wt.%)

[0092]

[0093]

[0094] Table 3 Core mechanical properties of steel plates of the embodiments of the present invention and comparative examples

[0095]

[0096] Table 4 Surface mechanical properties of steel plates in the embodiments of the present invention

[0097]

[0098] By comparing the data in the above tables, it can be seen that the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness prepared by the present invention has uniform core structure and excellent performance.

[0099] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness, characterized in that: The chemical composition and mass percentage of the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness are as follows: C: 0.10%-0.15%, Si: 0.35%-0.45%, Mn: 1.20%-1.50%, Ni: 3.00%-3.50%, Mo: 0.30%-0.50%, Cr: 0.15%-0.3%, Ti: 0.01%-0.02%, V: 0.030%-0.040%, Nb: 0.050%-0.060%, B: 0.0016%-0.0020%, Cu: 0.15%-0.25%, Al: 0.060%-0.070%, P: ≤0.010%, S: ≤0.004%, the balance is Fe and unavoidable impurities, and CET is controlled to be ≤0.39%, which conforms to GB / T 712-2022 Standard requirements for structural steel for ships and offshore engineering; Among them, CET=C+(Mo+Mn) / 10+(Cr+Cu) / 20+Ni / 40.

2. The ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness according to claim 1, characterized in that: The ultra-thick marine engineering steel plate with low yield strength ratio and excellent core low-temperature toughness has a thickness range of 80 to 150 mm, a yield strength ≥690 MPa, a tensile strength ≥920 MPa, an elongation after fracture ≥20%, a yield strength ratio ≤0.77, and an impact energy KV2 ≥80 J at -60°C.

3. The method for manufacturing the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness according to claim 1 or 2, characterized in that: The main steps are as follows: S1: Smelting and continuous casting The raw materials are weighed according to the chemical composition of the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness, and after desulfurization treatment, converter smelting, LF refining and RH vacuum treatment, continuous casting is performed into billets to obtain 300-600mm thick continuous casting billets; S2: Heating The continuous casting billet with a thickness of 300 to 600 mm is heated in a soaking furnace to obtain a heated continuous casting billet; S3: Controlled rolling The heated continuous casting billet is subjected to a three-stage rolling process, which includes two stages of rough rolling and one stage of finishing rolling, to obtain rolled steel of the continuous casting billet; the rolled steel thickness of the continuous casting billet is the thickness of low yield strength and high-thickness marine engineering steel with excellent low-temperature toughness; Among them, the starting temperature of rough rolling is 1040-1060℃, the reduction rate of the middle two passes is ≥20%, and the reduction rate of the last 1-3 passes is ≥15%; the cumulative reduction ratio of rough rolling is ≥2.3, and the thickness of the intermediate billet is 130-260mm; The starting rolling temperature in the finishing rolling stage is 820-840℃, the pass reduction rate is 13%-18%, and the final rolling temperature is ≥790℃; S4: Controlled Cooling The rolled steel plate is laminar-cooled to 600-660°C and then air-cooled to room temperature to obtain a cooled steel plate; S5: Quenching and Tempering Process The rolled steel plate after cooling is subjected to quenching and tempering heat treatment, and then rapidly cooled to room temperature by water after quenching and heat preservation; and then air-cooled to room temperature after tempering and heat preservation to obtain low yield strength and high specific thickness marine engineering steel with excellent low-temperature toughness.

4. The method for manufacturing the low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness according to claim 3, characterized in that: After the desulfurization treatment, the sulfur content is controlled at ≤0.0040%; in the converter smelting, the double slag method is used to control P ≤0.010%.

5. The method for manufacturing the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness according to claim 3, characterized in that: The LF refining adopts white slag operation for desulfurization and deoxidation; the vacuum degree of the RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min; after the vacuum is finished, calcium treatment is carried out, and then the static stirring time is ≥12min.

6. The method for manufacturing the ultra-thick marine engineering steel with low yield ratio and excellent core low-temperature toughness according to claim 3, characterized in that: In the continuous casting, the overheat of the tundish is controlled at 10-15°C; the continuous casting is performed with full protection casting, a light reduction process is adopted to improve the internal quality of the continuous casting billet, and the end-stage electric stirring is used to control segregation.

7. The method for manufacturing the low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness according to claim 3, characterized in that: The slow cooling of the 300-600mm thick continuous casting billet stacking off the line is ≥36h.

8. The method for manufacturing the low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness according to claim 3, characterized in that: In the above-mentioned S2, the soaking time of heating is ≥120min, the time in the furnace is 10-15min / cm, and the heating temperature is 1150-1250°C.

9. The method for manufacturing the low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness according to claim 3, characterized in that: In the S3, the quenching temperature is 890-920° C., and the holding time is 30-45 minutes.

10. The method for manufacturing the low yield ratio ultra-thick marine engineering steel with excellent core low temperature toughness according to claim 3, characterized in that: In the S3, the tempering temperature is 660-680° C., and the holding time is 1-2 hours.

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