Ultra-thick 460MPa-grade steel plate for ocean engineering and preparation method of ultra-thick 460MPa-grade steel plate

Abstract

The invention belongs to the technical field of ultrahigh strength maritime work steel plate production, and particularly relates to an extra-thick specification 460 MPa grade steel plate for ocean engineering and a preparation method thereof.The steel plate comprises, by mass, 0.08%-0.10% of C, 0.20%-0.40% of Si, 1.50%-1.65% of Mn, smaller than or equal to 0.010% of P, smaller than or equal to 0.005% of S, 0.030%-0.040% of Nb, 0.010%-0.020% of Ti, 0.26%-0.35% of Cr, 0.35%-0.45% of Ni, 0.15%-0.25% of Cu and 0.020%-0.050% of Al, Pcm is controlled to be smaller than or equal to 0.22%, and Ceq is controlled to be smaller than or equal to 0.46%. According to the 460MPa-grade ultrahigh-toughness maritime work steel plate, the upper yield strength ReH is larger than or equal to 460MPa, the tensile strength Rm ranges from 540 MPa to 720MPa, the percentage elongation after fracture A is larger than or equal to 17%, the core transverse KV2 at the temperature of minus 60 DEG C is larger than or equal to 150J, the CTOD characteristic value of base metal at the temperature of minus 40 DEG C is larger than or equal to 0.25 mm, and the CTOD characteristic value of a welding heat affected zone at the temperature of minus 20 DEG C is larger than or equal to 0.45 mm after 50kJ / cm heat input submerged-arc welding.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultra-high-strength marine engineering steel production, and specifically relates to a 460MPa grade steel plate with extra-thick specifications for marine engineering and a preparation method thereof. Background Art

[0002] As the exploitation of marine resources develops from shallow waters to deep seas and polar high-cold areas, the service environment of marine engineering equipment has become more severe, the service temperature of the required steel plates has become lower and lower, and the possibility of brittle failure of the structure has continued to increase. It is of great significance to develop marine engineering steel with high strength, low temperature resistance and high crack arrest performance. At present, the annual demand for polar ships and marine engineering steel has reached 600,000 tons, and the delivery status is mainly quenched and tempered and TMCP, among which specifications of 80mm and above are basically required to be delivered in quenched and tempered state.

[0003] There are many deficiencies in the current preparation technology of extra-thick 460MPa grade marine steel plates. Due to the insufficient deformation of the core of the extra-thick steel plate during the rolling process and the difficulty in hardening to the core during the heat treatment process, the size effect of the steel plate is obvious, and the core is prone to become a weak point of the steel plate, which in turn affects the overall performance of the steel plate.

[0004] After searching, patent application CN118374736A discloses an economical EH460 steel plate with large thickness and low compression ratio for marine engineering and its preparation method. The core structure is granular bainite + acicular ferrite structure, and pearlite structure exists in the segregation zone, which easily causes core performance fluctuations. Patent CN114107812B provides a heat-treated steel plate with high fracture toughness of 420MPa grade for offshore platforms and its manufacturing method. The content of martensite structure in the core is 85% to 100%, which is extremely difficult to achieve for extra-thick steel plates. Patent application CN118581401A discloses an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation and its manufacturing method. The finished product is in TMCP state, with a final cooling temperature of 380 to 450°C. After welding and reheating, the performance is prone to significant attenuation. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a 460MPa grade extra thick steel plate for marine engineering and its preparation method. The 80-100mm thick steel plate provided by the invention has excellent core performance, and the base material and welding heat affected zone have good crack arrest performance.

[0006] In order to achieve the above-mentioned object, the first aspect of the present invention provides a composition design of a 460MPa grade steel plate with extra-thick specifications for marine engineering, which adopts the following technical scheme:

[0007] A 460MPa grade steel plate with extra-thick specifications for marine engineering, wherein the chemical composition mass percentage of the steel plate is: C: 0.08%-0.10%, Si: 0.20%-0.40%, Mn: 1.50%-1.65%, P≤0.010%, S≤0.005%, Nb: 0.030%-0.040%, Ti: 0.010%-0.020%, Cr: 0.26%-0.35%, Ni: 0.35%-0.45%, Cu: 0.15%-0.25%, Al: 0.020%-0.050%, and the rest is Fe and unavoidable impurity elements. The thickness of the steel plate is 80-100 mm.

[0008] Furthermore, the carbon equivalent Ceq and welding crack sensitivity index Pcm of the steel plate should also satisfy:

[0009] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.46%;

[0010] Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.22%.

[0011] Furthermore, the inevitable impurity elements are, by mass percentage, H≤0.0002%, O≤0.003%, N≤0.004%, B≤0.0005%, As≤0.006%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, and Bi≤0.010%.

[0012] Furthermore, the structure of 1 / 2 of the thickness of the steel plate is ferrite + pearlite + a small amount of granular bainite, the average grain size of ferrite is 10.05-12.16 μm, ferrite accounts for about 75%, the structure at 1 / 4 of the thickness is tempered bainite + granular bainite + a small amount of pearlite, and the structure near the surface is tempered bainite + a small amount of bainite.

[0013] A second aspect of the present invention provides a method for preparing a 460 MPa grade extra thick steel plate for marine engineering, comprising the following steps:

[0014] Step S1: smelting and refining molten steel, and casting to obtain a continuous casting billet;

[0015] Step S2: reheating the cast billet;

[0016] Step S3: obtaining a steel plate through two-stage rolling;

[0017] Step S4: heat treating the rolled steel plate;

[0018] Furthermore, in the step S1, LF+RH refining is performed after smelting; the macrostructure center segregation of the continuous casting billet is ≤C class 1.5 level, and the center porosity is ≤1.0 level.

[0019] Furthermore, in step S2, the reheating temperature of the ingot is 1170-1200° C., and the time in the furnace is 280-310 min.

[0020] Furthermore, in step S3, the rolling process is a two-stage rolling process, the first stage is recrystallization rolling, the rolling temperature is not less than 980°C, and the reduction rate of at least two passes is guaranteed to be ≥16%; the second stage is non-recrystallization rolling, the finishing rolling start temperature is 840-860°C, and the finishing rolling final temperature is ≥820°C.

[0021] Furthermore, in step S4, the heat treatment method is normalizing + quenching + tempering, the normalizing holding temperature is 860-890°C, the furnace time is ≥150min, the quenching temperature is 910-930°C, and the tempering temperature is 640-670°C.

[0022] Furthermore, after heat treatment, there is a composite precipitation of (Ti, Nb)C in the core of the steel plate, and the precipitate phase size is mainly concentrated in 10-60nm, of which 18-36nm accounts for about 50%, and the alloy cementite in the core is polygonal and has a size between 100-250nm.

[0023] Furthermore, relative to the Ag / AgCl reference electrode, the saturation corrosion current density of the steel plate at -300mV is ≤1.50mA / cm 2 , density of corrosion active inclusions ≤ 8 pieces / mm 2 .

[0024] Furthermore, the yield strength ReH of the obtained steel plate is ≥460MPa, the tensile strength Rm is 540~720MPa, the elongation after fracture A is ≥17%, the transverse KV2 (-60℃) of the core is ≥150J, the characteristic value of the CTOD (-40℃) of the parent material is ≥0.25mm, and the characteristic value of the CTOD (-20℃) of the welding heat affected zone after submerged arc welding with a heat input of 50kJ / cm is ≥0.45mm.

[0025] Compared with the prior art, the advantages of the present invention are:

[0026] (1) Aiming at the common technical problem in the industry that the core of thick-gauge steel plates is difficult to fully harden, the present invention improves the toughness of the core of the steel plate by regulating and refining the core ferrite + pearlite microstructure and controlling the second phase precipitates. For quenched and tempered steel plates, the traditional strategy is to improve the hardenability of the steel plate so that the core of the steel plate obtains a quenched and tempered structure. However, due to the limitations of existing equipment and technology, it is difficult to achieve complete hardening of steel plates with a thickness of more than 80 mm. The present invention controls the quenched and tempered structure for the near-surface and 1 / 4 thickness positions that can be hardened, and refines the core ferrite + pearlite microstructure morphology and controls the second phase precipitates for the core position that is difficult to harden, thereby achieving high toughness control in the entire thickness direction.

[0027] (2) The present invention obtains a high-toughness structure in the core by combining component design with rolling and heat treatment processes. Specifically, the addition of Nb element causes the non-recrystallized zone of the steel to move upward, while strictly controlling the rolling passes to ensure the pass reduction rate in the rolling stage, so that the rolling effect penetrates into the core, thereby increasing the proportion of deformed austenite. The deformation energy storage of the deformed austenite increases the driving force for the phase transformation of the core structure, promotes the nucleation core for the transformation of austenite to ferrite, and significantly refines the ferrite grains in the core. A normalizing heat treatment process is added before the quenching and tempering heat treatment, and the furnace time is ensured, so that the Mn, Cr, and Ni-rich alloy cementite in the core is fully refined, and the pearlite structure is further homogenized. In this way, even if the core is not completely hardened in the subsequent quenching and tempering process, the fine and uniform ferrite + pearlite structure formed in the early stage, combined with the precipitation of the second phase particles formed after the heat treatment, can still ensure the high toughness of the core.

[0028] (3) The present invention improves the fracture toughness of steel plates from two aspects: inhibiting the initiation of cracks and preventing the expansion of cracks. By controlling the composition, purity and gas content during the steelmaking process, high-quality ingot raw materials are provided, effectively reducing the defects that cause crack initiation. The second phase precipitates formed by Nb, Ti, Al, N, etc., combined with the large number of grain boundaries provided by fine grains, effectively prevent the expansion of cracks, and significantly improve the CTOD characteristic values ​​of the base material and the heat-affected zone of welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The near-surface metallographic structure of the steel plate obtained in Example 1 of the present invention;

[0030] Figure 2 The metallographic structure at the position of 1 / 2 thickness of the steel plate obtained in Example 1 of the present invention;

[0031] Figure 3 This is the SEM image of the steel plate at the 1 / 2 thickness position obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0032] The present invention provides a 460MPa grade steel plate with extra-thick specifications for marine engineering, which comprises, by mass percentage, C: 0.08%-0.10%, Si: 0.20%-0.40%, Mn: 1.50%-1.65%, P≤0.010%, S≤0.005%, Nb: 0.030%-0.040%, Ti: 0.010%-0.020%, Cr: 0.26%-0.35%, Ni: 0.35%-0.45%, Cu: 0.15%-0.25%, Al: 0.020%-0.050%, and the rest is Fe and unavoidable impurity elements. The thickness of the steel plate is 80-100 mm.

[0033] It should be noted that the use of lower C content and higher Mn content not only ensures good low-temperature toughness and weldability, but also promotes the improvement of the hardenability of steel; the addition of Nb elements inhibits the recrystallization of austenite during the rolling process of the steel plate, flattens the austenite during the rolling process, increases the area of ​​deformed austenite, promotes the γ→α phase transformation, and provides a good original organization preparation for tempering heat treatment. At the same time, Nb microalloying also plays a certain precipitation strengthening role. Adding a small amount of Cu, Ni, and Cr elements to steel is also beneficial to improving the corrosion resistance and hardenability of steel. The addition of Ni elements is also beneficial to the homogenization of the organization and performance of the entire thickness section of the steel plate. The higher the Ni content, the greater the thickness of the steel plate that can achieve homogenization of organization and performance.

[0034] The effects of the main alloying elements in the extra-thick 460MPa grade steel plate for marine engineering are as follows:

[0035] Cr can effectively improve the hardenability of extra-thick steel plates to compensate for the strength loss caused by the thickness. However, if the content is too high, it will lead to the formation of Cr-Mn composite oxides with low melting points, which will easily cause surface cracks to form during hot working of the steel plates and also deteriorate the welding performance. Therefore, the Cr content in the present invention is controlled to be 0.26% to 0.35%.

[0036] Ni is the element that most significantly improves the low-temperature toughness of extra-thick steel plates. Adding appropriate amounts of Ni can reduce the stacking fault energy of the crystal, facilitate the slip movement of dislocations, and improve impact toughness. However, too high a Ni content is not conducive to ensuring welding performance. Therefore, the Ni content in the present invention is controlled at 0.35% to 0.45%.

[0037] Cu is a non-carbide forming element that can be used to improve the strength and corrosion resistance of steel. During welding, it increases the high temperature pinning effect and reduces the austenite grain size. When the Cu content is too little, the effect is not obvious; when the content is too much, it is easy to cause copper segregation, which deteriorates the processability and weldability. Therefore, the Cu content is controlled at 0.15% to 0.25%.

[0038] The addition of Nb element can inhibit the austenite recrystallization during the rolling process of the steel plate, flatten the austenite during the rolling process, increase the area of ​​the deformed austenite, and promote the γ→α phase transformation. This refinement process provides a good microstructure preparation for the quenching and tempering heat treatment of the steel plate. At the same time, Nb microalloying also plays a certain precipitation strengthening role. Therefore, the Nb content in the present invention is controlled at 0.030% to 0.040%.

[0039] Ti can also play the role of grain refinement and precipitation strengthening, significantly improving the low-temperature impact toughness of steel plates. However, when the content is too high, it is easy to form large particles of TiN and lose the grain refinement effect. Therefore, the present invention controls the Ti content to 0.010% to 0.020%.

[0040] Al can fix free N in steel, improve the low temperature toughness of steel plate and welded HAZ, and the dispersion precipitation of AlN can inhibit the growth of austenite grains during heating, evenly refine the austenite grain size, and improve impact toughness. However, too much Al content will increase the number of inclusions in the steel, increase the size of the inclusions, reduce the internal quality of the steel plate, and affect the hot working performance, welding performance and cutting performance of the steel. Therefore, the present invention controls the Al content to 0.020% to 0.050%.

[0041] Ceq: Controlling the carbon equivalent index is beneficial to ensuring the strength and weldability of the steel plate. The present invention controls Ceq≤0.46%.

[0042] Pcm: Controlling the cold crack sensitivity coefficient is beneficial to ensuring the welding performance of the product. The present invention controls Pcm≤0.22%.

[0043] Impurity elements in steel, such as S and P, will increase the segregation degree of continuous casting billet and deteriorate the uniformity of the structure and performance of the steel plate in the thickness direction. Therefore, the S and P contents are controlled below 0.005% and 0.010% respectively; B is easy to enrich at the grain boundary, reducing the low-temperature impact performance and fatigue performance of the steel plate, and will also significantly increase the tendency of welding cracks, so B is controlled to be ≤ 0.0005%; at the same time, other inevitable impurity elements As ≤ 0.006%, Sb ≤ 0.010%, Sn ≤ 0.020%, Pb ≤ 0.010%, Bi ≤ 0.010%, H ≤ 0.0002%, O ≤ 0.003%, N ≤ 0.004%.

[0044] On the other hand, the present invention also provides a method for preparing a 460 MPa grade extra thick steel plate for marine engineering, comprising the following steps:

[0045] Step S1: smelting and refining molten steel, and casting to obtain a continuous casting billet;

[0046] Step S2: reheating the cast billet;

[0047] Step S3: obtaining a steel plate through two-stage rolling;

[0048] Step S4: heat treating the rolled steel plate;

[0049] Specifically, in the above step S1, top and bottom combined oxygen converter smelting is adopted, hot metal KR desulfurization is performed before smelting, and double slag deep desulfurization is performed during smelting. LF+RH refining is adopted, LF refining time is ≥50min, RH refining time is ≥40min, continuous casting process is fully protected casting, and the macrostructure center segregation of continuous casting billet is ≤C class 1.5 level, and the center looseness is ≤1.0 level.

[0050] Specifically, in the above step S2, the reheating temperature of the ingot is 1170-1200°C, and the time in the furnace is 280-310 minutes.

[0051] Specifically, the above step S3 includes the following steps:

[0052] S31, two-stage rolling, the first stage is recrystallization rolling, the rolling temperature is not less than 980℃, the rolling passes and reduction rate are strictly controlled, except for the widening pass, the reduction of the other longitudinal rolling passes is not less than 35mm, and the reduction rate of at least two passes is guaranteed to be ≥16%; the second stage is non-recrystallization rolling, the finishing rolling start temperature is 840~860℃, and the finishing rolling final temperature is ≥820℃.

[0053] S32, cooling after rolling: the steel plate is water-cooled after rolling, with the initial cooling temperature of 770-820°C, the final cooling temperature of 580-640°C, and the cooling rate of 5-8°C / s.

[0054] S33, slow cooling of steel plate. After rolling, the steel plate is transferred to the slow cooling pit for slow cooling for more than 48 hours.

[0055] Specifically, in the above step S4, the heat treatment method is normalizing + quenching + tempering, the normalizing holding temperature is 860-890°C, the furnace time is ≥150min, the heating temperature will not cause excessive grain coarsening, and the core of the alloy cementite rich in Mn, Cr, and Ni is fully refined and homogenized by extending the furnace time. The quenching temperature is 910-930°C, the holding time is 20-30min, the tempering temperature is 640-670°C, the holding time is 20-30min, and the tempering is air-cooled to room temperature after tempering.

[0056] Specifically, in the above step S4, the structure of 1 / 2 thickness of the steel plate obtained after heat treatment is ferrite + pearlite + a small amount of granular bainite, the average ferrite grain size is 10.05-12.16 μm, ferrite accounts for about 75%, the structure at 1 / 4 thickness is tempered bainite + granular bainite + a small amount of pearlite, and the structure near the surface is tempered bainite + a small amount of bainite.

[0057] Specifically, according to the preparation method of steps S1-S4, there is a composite precipitation of (Ti, Nb)C in the core of the steel plate, and the size of the precipitated phase is mainly concentrated in 10-60nm, of which 18-36nm accounts for about 50%, and the alloy cementite in the core is polygonal and has a size between 100-250nm. The precipitates are pinned to the grain boundaries, which can effectively inhibit the expansion of cracks on the one hand, and prevent excessive coarsening of grains during welding on the other hand, thereby ensuring the fracture toughness of the base material and the welding heat affected zone.

[0058] Specifically, according to the preparation method of steps S1-S4 above, the saturated corrosion current density of the steel plate at -300mV (relative to the Ag / AgCl reference electrode) is ≤1.50mA / cm 2 , density of corrosion active inclusions ≤ 8 pieces / mm 2 .

[0059] The ultra-thick 460MPa grade steel plate for marine engineering of the present invention has a yield strength ReH≥460MPa, a tensile strength Rm=540-720MPa, an elongation after fracture A≥17%, a core transverse KV2 (-60°C)≥150J, a base material CTOD (-40°C) characteristic value≥0.25mm, and a welding heat affected zone CTOD (-20°C) characteristic value≥0.45mm after 50kJ / cm heat input submerged arc welding.

[0060] The present invention aims to solve the common technical problem in the industry that the core of thick-gauge steel plates is difficult to fully harden, and improves the toughness of the core of the steel plate by adjusting and refining the core ferrite + pearlite microstructure and controlling the second phase precipitates. For quenched and tempered steel plates, the traditional strategy is to improve the hardenability of the steel plate so that the core of the steel plate obtains a quenched and tempered structure. However, due to the limitations of existing equipment and technology, it is difficult to achieve complete hardening of steel plates with a thickness of more than 80 mm. The present invention controls the quenched and tempered structure for the near-surface and 1 / 4 thickness positions that can be hardened, refines the core ferrite + pearlite microstructure morphology for the core position that is difficult to harden, and controls the second phase precipitates, thereby achieving high toughness control in the entire thickness direction.

[0061] The key technical points of the present invention are specifically described below through 3 embodiments and 2 comparative examples.

[0062] Through the process of molten steel smelting, refining and casting, high-clean and high-homogeneity continuous casting billets are obtained, providing qualified billet raw materials for the subsequent rolling process. The continuous casting billet thickness is 300mm, and the chemical composition by mass percentage includes C: 0.085%, Si: 0.35%, Mn: 1.63%, P: 0.008%, S: 0.003%, Nb: 0.036%, Ti: 0.014%, Cr: 0.28%, Ni: 0.39%, Cu: 0.18%, Al: 0.033%. Ceq = 0.45%, Pcm = 0.21%. The billet is slow-cooled in the slow cooling pit for 48 hours, then hoisted out of the slow cooling pit and stacked for slow cooling to below 200℃. The billet is tested by low-power microstructure, and the center segregation is C-type 1.0 and the center looseness is 0.5.

[0063] Example 1: 80 mm thick

[0064] Step S1: smelting and refining molten steel, and then casting to obtain the above-mentioned continuous casting billet.

[0065] Step S2: The ingot is reheated to 1190°C, with a furnace time of 310 min.

[0066] Step S3: The steel plate is obtained by two-stage rolling. The rough rolling adopts a large reduction mode, with 4 rough rolling passes, and the reduction rates of the 3rd and 4th passes are 19.3% and 23.7% respectively. The thickness of the intermediate billet after the rough rolling is 150mm. The finishing rolling stage begins when the temperature reaches 845℃, and the finishing rolling is performed for 6 passes; the cooling is controlled at a cooling rate of 8℃ / s, the starting cooling temperature is 815℃, and the final cooling temperature is 620℃.

[0067] Step S4: heat treatment of the rolled steel plate: normalizing at a holding temperature of 880°C, a furnace time of 170 min, quenching at a heating temperature of 910°C, a holding time of 25 min, tempering at a heating temperature of 660°C, and a holding time of 30 min.

[0068] Figure 1 The near-surface metallographic structure of the steel plate obtained in Example 1 of the present invention; Figure 2 The metallographic structure at the position of 1 / 2 thickness of the steel plate obtained in Example 1 of the present invention; Figure 3 The SEM image of the steel plate at 1 / 2 thickness obtained in Example 1 of the present invention. Figure 1-3 It can be seen that the structure of 1 / 2 thickness of the steel plate is ferrite + pearlite + a small amount of granular bainite, the average ferrite grain size is 10.05-12.16 μm, ferrite accounts for about 75%, and the structure near the surface is tempered troostite + a small amount of bainite.

[0069] Example 2: 90 mm thick

[0070] Step S1: smelting and refining molten steel, and then casting to obtain the above-mentioned continuous casting billet.

[0071] Step S2: The ingot is reheated to 1200° C. and the furnace time is 300 min.

[0072] Step S3: The steel plate is obtained by two-stage rolling. The rough rolling adopts a large reduction mode, with 4 rough rolling passes, and the reduction rates of the 3rd and 4th passes are 18.3% and 24.1% respectively. The thickness of the intermediate billet after the rough rolling is 160mm. The finishing rolling stage begins when the temperature reaches 850℃, and the finishing rolling is performed for 6 passes; the cooling is controlled at a cooling rate of 7℃ / s, the starting cooling temperature is 815℃, and the final cooling temperature is 608℃.

[0073] Step S4: heat treatment of the rolled steel plate: normalizing at a holding temperature of 860°C, a furnace time of 165 min, quenching at a heating temperature of 920°C, a holding time of 25 min, tempering at a heating temperature of 640°C, a holding time of 25 min.

[0074] Example 3: 100 mm thick

[0075] Step S1: smelting and refining molten steel, and then casting to obtain the above-mentioned continuous casting billet.

[0076] Step S2: The ingot is reheated to 1190°C, and the furnace time is 280 minutes.

[0077] Step S3: The steel plate is obtained by two-stage rolling. The rough rolling adopts a large reduction mode, with 4 rough rolling passes, and the reduction rates of the 3rd and 4th passes are 19.2% and 23.9% respectively. The thickness of the intermediate billet after the rough rolling is 155mm. The finishing rolling stage begins when the temperature reaches 855℃, and the finishing rolling is performed for 5 passes; the cooling is controlled at a cooling rate of 8℃ / s, the starting cooling temperature is 820℃, and the final cooling temperature is 590℃.

[0078] Step S4: heat treatment of the rolled steel plate: normalizing at a holding temperature of 890°C, a furnace time of 150 min, quenching at a heating temperature of 930°C, a holding time of 20 min, tempering at a heating temperature of 670°C, and a holding time of 30 min.

[0079] Comparative Example 1: 90mm thick

[0080] The steps S1, S2 and S3 of comparative example 1 are the same as those of example 2. The heat treatment method of step S4 is normalizing+quenching+tempering, but the normalizing time in the furnace is 60 minutes.

[0081] Comparative Example 2: 90mm thick

[0082] Steps S1, S2, and S4 of Comparative Example 2 are the same as those of Example 2. In step S3, the rolling process does not control the rolling passes and the pass reduction rate, and is controlled according to the conventional product rolling regulations. Specifically, the rough rolling adopts a large reduction mode, 6 rough rolling passes, and the pass reduction rate is less than 16%. After the rough rolling is completed, the thickness of the intermediate billet is 160 mm. When the temperature reaches 850°C, the finishing rolling stage begins, and the finishing rolling is performed for 8 passes; the cooling is controlled at a cooling rate of 7°C / s, the start cooling temperature is 815°C, and the final cooling temperature is 608°C.

[0083] Table 1 shows the parent material performance results of the embodiments of the present invention and the comparative examples. It can be seen from the results that embodiments 1 to 3 have achieved good strength-toughness matching, especially the average values ​​of the transverse impact energy of the core at -60°C are all above 150J, the characteristic value of the parent material CTOD (-40°C) is close to 1mm, the fracture toughness is excellent, and the fracture toughness decreases with the increase of thickness. The normalizing heat treatment time in the furnace of comparative example 1 is 90min, which is more than 60min shorter than that of the embodiment. The alloy cementite rich in Mn, Cr, and Ni in the core cannot be fully homogenized, and the subsequent quenching process cannot be effectively quenched, resulting in low impact energy in the core. In comparative example 2, the rolling passes and pass reduction rates were not controlled during the rolling process, the proportion of deformed austenite was low, the γ→α phase transformation energy storage was insufficient, the core nucleation core was small, and the ferrite structure could not be effectively refined, resulting in poor impact in the core. At the same time, when the comparative examples 1 to 2 were tested for the CTOD characteristic value at -40°C throughout the thickness, crack sources were prone to occur at the weak points in the core and crack propagation could not be prevented to a limited extent, and the fracture toughness was also poor.

[0084] Submerged arc automatic welding was used to weld the 100 mm steel plate in Example 3 of the present invention, with a welding heat input of 50±3 kJ / cm. The weld joints were sampled for -20°C CTOD test, and the CTOD characteristic values ​​of the welding heat affected zone were 0.48 mm, 0.59 mm, and 0.51 mm, which were much higher than the CTOD characteristic value requirements for 460 MPa grade marine steel in various classification societies and the European standard EN10225.

[0085] The composition of the artificial seawater solution for testing the corrosion resistance of the steel plate is shown in Table 2, and Table 3 is the corrosion resistance index of the steel plate of Example 3. The saturated corrosion current density was measured using an Autolab electrochemical workstation, using a three-electrode system, with the Ag / AgCl electrode as the reference electrode and the Pt electrode as the auxiliary electrode. The steel plate was sampled to make an exposure area of ​​1 cm 2 The sample was used as the working electrode, in which the sample was anodic polarized at a constant potential of -300mV, and the change of polarization current and the corrosion current density after stabilization were recorded. When measuring the density of corrosion active inclusions, the steel plate was sampled to make a 10mm*10mm*5mm sample, which was polished and immersed in artificial seawater for 20 minutes. After being taken out, it was rinsed with alcohol, dried with cold air, placed under a 50x optical microscope, and several photos were taken continuously to calculate the density of corrosion active inclusions.

[0086] Table 1 Parent material properties of steel plates of the embodiments of the present invention and the comparative examples

[0087]

[0088] Table 2 Composition of artificial seawater

[0089]

[0090] Table 3 Corrosion resistance index of steel plate in Example 3 of the present invention

[0091]

[0092] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed one by one here.

[0093] Any contents not described in detail in the present invention can be based on the conventional technical knowledge in the art.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A 460MPa grade extra thick steel plate for marine engineering, characterized in that: The chemical composition mass percentage of the steel plate is: C: 0.08% to 0.10%, Si: 0.20% to 0.40%, Mn: 1.50% to 1.65%, P≤0.010%, S≤0.005%, Nb: 0.030% to 0.040%, Ti: 0.010% to 0.020%, Cr: 0.26% to 0.35%, Ni: 0.35% to 0.45%, Cu: 0.15% to 0.25%, Al: 0.020% to 0.050%, and the rest is Fe and unavoidable impurity elements; At the same time, Ceq≤0.46%, Pcm≤0.22%, and the steel plate thickness is 80-100mm.

2. The extra-thick 460MPa grade steel plate for marine engineering according to claim 1, characterized in that: The structure of the steel plate at 1 / 2 of the thickness is ferrite + pearlite + a small amount of granular bainite, the average ferrite grain size is 10.05-12.16 μm, and ferrite accounts for about 75%; the structure at 1 / 4 of the thickness is tempered bainite + granular bainite + a small amount of pearlite; the structure near the surface is tempered bainite + a small amount of bainite.

3. The extra-thick 460MPa grade steel plate for marine engineering according to claim 1, characterized in that: The yield strength ReH on the steel plate is ≥460MPa, the tensile strength Rm is 540~720MPa, the elongation after fracture A is ≥17%, the transverse KV2 at the core at -60℃ is ≥150J, the CTOD characteristic value of the base material at -40℃ is ≥0.25mm, and the CTOD characteristic value of the heat affected zone at -20℃ after submerged arc welding with a heat input of 50kJ / cm is ≥0.45mm.

4. A method for preparing the extra-thick 460MPa grade steel plate for marine engineering according to any one of claims 1 to 3, the method comprising the following steps: Step S1: smelting and refining molten steel, and casting to obtain a continuous casting billet; Step S2: reheating the cast billet; Step S3: obtaining a steel plate through two-stage rolling; Step S4: heat treating the rolled steel plate.

5. The preparation method according to claim 4, characterized in that: In the step S1, LF+RH refining is performed after smelting; the macrostructure center segregation of the continuous casting billet is ≤C class 1.5 level, and the center porosity is ≤1.0 level.

6. The preparation method according to claim 4, characterized in that: In step S2, the reheating temperature of the cast billet is 1170-1200° C., and the time in the furnace is 280-310 min.

7. The preparation method according to claim 4, characterized in that: In step S3, the rolling process is a two-stage rolling process, wherein the first stage is recrystallization rolling, the rolling temperature is not less than 980°C, and the reduction rate of at least two passes is guaranteed to be ≥16%; the second stage is non-recrystallization rolling, the finishing rolling start temperature is 840-860°C, and the finishing rolling final temperature is ≥820°C.

8. The preparation method according to claim 4, characterized in that: In step S4, the heat treatment method is normalizing + quenching + tempering, the normalizing holding temperature is 860-890°C, the furnace time is ≥150min, the quenching temperature is 910-930°C, and the tempering temperature is 640-670°C.

9. The preparation method according to claim 4, characterized in that: After heat treatment, there is a composite precipitation of (Ti, Nb)C in the core of the steel plate. The precipitate phase size is mainly concentrated in 10-60nm, of which 18-36nm accounts for about 50%. The alloy cementite in the core is polygonal and the size is between 100 and 250nm.

10. The preparation method according to claim 4, characterized in that: Relative to Ag / AgCl reference electrode, steel plate -300mV saturation corrosion current density ≤1.50mA / cm 2 , density of corrosion active inclusions ≤ 8 pieces / mm 2 .

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