Extra-thick rack steel EQ70 for ocean platform and production method of extra-thick rack steel EQ70

Abstract

The invention discloses extra-thick rack steel EQ70 for an ocean platform and a production method of the extra-thick rack steel EQ70, the thickness of a steel plate ranges from 210 mm to 254 mm, the steel comprises, by weight, 0.10%-0.12% of C, 0.10%-0.20% of Si, 1.60%-1.70% of Mn, smaller than or equal to 0.010% of P, smaller than or equal to 0.003% of S, 0.030%-0.050% of Als, 2.00%-2.50% of Ni, 0.40%-0.50% of Cr, 0.50%-0.60% of Mo, 0.030%-0.040% of Nb, 0.0015%-0.0020% of B and the balance Fe and inevitable impurities, and the components of the steel meet the conditions that Ceq is smaller than or equal to 0.76%, Pcm A novel alloy component system is combined with clean steel smelting, water-cooling die casting, a heating temperature control process, a high-pressure rolling and after-rolling cooling process, a high-temperature rapid quenching process and an after-tempering rapid cooling process, and finally the steel plate performance is obtained, ReH is larger than or equal to 690 MPa, Rm is 810-890 MPa, the minus 40 DEG C longitudinal and transverse Charpy impact energy unit value is larger than or equal to 120 J at the 1 / 4 thickness position and the 1 / 2 thickness position of the steel plate, and flaw detection meets the GB / T 2970 first level. By adopting the scheme, the alloy is low in cost, high in yield and high in production efficiency, and has good market competitiveness.

Description

Technical Field

[0001] The invention relates to the technical field of extra-thick plate production, and in particular to an extra-thick rack steel EQ70 for an offshore platform and a production method thereof. Background Art

[0002] With the increasing global efforts to develop deep-sea oil, gas, minerals and other resources and the construction of marine engineering infrastructure such as deep-sea ports, submarine tunnels, and offshore wind power, the demand for extra-thick rack steel is increasing, and the maximum thickness actually used has reached 254mm. While the thickness has been greatly increased, strict requirements are still imposed on performance, including: first, high strength is required, and its index does not decrease with increasing thickness to meet the requirements of large loads and stresses; second, good low-temperature impact toughness to meet the stability and safety of the structure in a complex and changeable marine environment; third, excellent seawater corrosion resistance to meet the long-term use in harsh marine environments without being easily damaged; fourth, excellent wear resistance, fatigue resistance, and impact resistance to meet the ability to maintain good performance stability and extend service life during dynamic operation; fifth, good processing and weldability to meet the manufacturing needs of complex structures of marine engineering equipment.

[0003] At present, there are many bottleneck problems in the process of manufacturing extra-thick rack steel plates. First, high-quality alloy raw materials need to be used, and the cost of these raw materials is relatively high. At the same time, due to strict quality requirements, the scrap rate and loss rate in the production process are relatively high, which greatly increases the production cost. Second, the companies that can currently produce extra-thick 254mm rack steel mainly produce it by forging (or forging + rolling), which has low production efficiency and seriously affects the construction period of downstream customers.

[0004] The patent with announcement number CN107974638B discloses a method for manufacturing a rack steel plate with a thickness of 180 mm made by continuous casting. The chemical composition of the steel plate is C: 0.11-0.15%, Si: 0.15-0.35%, Mn: 0.95-1.25%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.4-0.6%, Ni: 1.3-2.6%, Cu: 0.2-0.4%, Al: 0.06-0.09%, V: 0.03-0.06%, Nb: ≤0.04%, N: ≤0.006%, B: 0.001-0.002%, and the remainder is iron and unavoidable impurity elements; a continuous casting billet with a thickness of 370 mm is used as the billet for manufacturing. The biggest advantage of this invention is that it uses continuous casting billets with low compression ratio to produce 180mm rack steel with excellent performance, which greatly reduces production costs and delivery cycles. However, the internal flaw detection guarantee level is not mentioned. In addition, its applicable thickness is limited to ≤180mm, and its capacity is insufficient for thicknesses exceeding this range.

[0005] The patent with announcement number CN112226687B discloses a low rolling compression ratio rack steel plate and a manufacturing method thereof, and the production thickness thereof is 200-260 mm. The chemical composition of the steel is as follows by mass percentage: C 0.12%-0.15%, Si 0.1%-0.3%, Mn 1.0%-1.3%, P≤0.02%, S≤0.01%, Als 0.01%-0.03%, Ni 2.5%-3.0%, Cr 0.5%-0.8%, Mo 0.5%-0.8%, Cu 0.2%-0.5%, Nb 0.02%-0.04%, V0.03%-0.06%, Ti 0.005%-0.03%, B 0.001%-0.0015%, and the balance is iron and unavoidable impurities. After electroslag remelting and forging, the first-stage rolling temperature is 1150-1250℃, the second-stage rolling temperature is 950-1000℃, the final rolling temperature is 850-900℃, and the rolling reduction ratio is ≤2; then high-temperature quenching + sub-temperature quenching + tempering treatment is adopted, and the single value of the -40℃ Charpy impact energy of the core of the finished steel plate is ≥90J. This patent adopts electroslag remelting and forging, and the production process is complicated and the production cost is high. In addition, the -40℃ Charpy impact energy of the core is unstable and the quality surplus is small.

[0006] The patent with announcement number CN109234643B discloses a method for rolling ultra-high strength rack steel for offshore platforms, with a production thickness of 150 to 210 mm. The steel contains the following chemical components by mass percentage: C 0.14% to 0.21%, Si 0.15% to 0.35%, Mn 0.95% to 1.30%, P ≤ 0.035%, S ≤ 0.035%, Cr 1.00% to 1.50%, Ni 1.50% to 2.00%, Mo 0.40% to 0.60%, V 0.03% to 0.08%, Cu ≤ 0.15%, Nb 0.020% to 0.040%, and the rest is Fe and unavoidable impurities. According to the patent embodiment, rack steels with yield strengths of 543 MPa and 566 MPa were obtained. Its yield strength is low and does not meet the current performance requirements of rack steel yield strength ≥ 690 MPa. Summary of the invention

[0007] In order to solve the above-mentioned technical defects, the purpose of the present invention is to provide an extra-thick rack steel EQ70 for offshore platforms, overcome the shortcomings of the prior art, and prepare a 210-254mm thick EQ70 rack steel plate suitable for the field of offshore engineering by rolling. Its strength performance, low-temperature impact toughness, seawater corrosion resistance, wear resistance, fatigue resistance and impact resistance, machinability and weldability meet the service requirements of offshore platforms.

[0008] Another object of the present invention is to provide a method for producing extra-thick rack steel EQ70 for offshore platforms.

[0009] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: the inventors have finally determined the alloy element ratio and production process that can meet the purpose of the present invention by designing a composition that matches low C, high Mn with Cr, Ni, Mo, Nb, and B elements, combined with clean steel smelting, water-cooled die casting, heating, rolling, and heat treatment process optimization. The specific technical solution is:

[0010] The invention discloses an extra-thick rack steel EQ70 for an offshore platform. The steel contains, by weight percentage, the following: C 0.10-0.12%, Si 0.10-0.20%, Mn 1.60-1.70%, P≤0.010%, S≤0.003%, Als 0.030-0.050%, Ni2.00-2.50%, Cr 0.40-0.50%, Mo 0.50-0.60%, Nb 0.030-0.040%, B 0.0015-0.0020%, and the rest is Fe and unavoidable impurities. The steel meets the following requirements: Ceq≤0.76%, Pcm≤0.32%, and I≥8.30.

[0011] The reasons for designing the chemical composition of steel grades are as follows:

[0012] (1) C can increase the stability of supercooled austenite, which is conducive to obtaining a deeper hardened layer, thereby improving the hardenability of steel. C forms a solid solution structure and a carbide structure with iron, which significantly improves the strength and hardness of the steel and significantly improves the wear resistance. However, steel with a high C content is more susceptible to electrochemical corrosion and oxidative corrosion in a humid or corrosive environment, resulting in a decrease in the corrosion resistance of the steel; cracks and embrittlement are easily generated during the welding process, resulting in poor welding performance of the steel and reduced weldability; when the C content is greater than 0.30%, the lattice distortion of the steel will be aggravated, thereby increasing the brittleness of the steel, deteriorating the plasticity and toughness of the steel, and prone to cold brittleness at low temperatures. In the present invention, the C content is controlled at 0.10% to 0.12%.

[0013] (2) Si is a ferrite-forming element and can exist in ferrite or austenite in the form of solid solution, with a strong solid solution strengthening effect. In hypoeutectoid steel, Si can reduce the austenite phase area, prevent ferrite nucleation and growth, and shift the "C" curve to the right, thereby improving the hardenability of the steel. During tempering, Si can inhibit the nucleation, growth and transformation of carbides, thereby improving the low-temperature tempering stability of the steel. However, too much Si will cause the lattice distortion of the steel to increase, increase the brittleness of the steel, and reduce the plasticity and toughness of the steel. In addition, Si has a stronger ability to combine with O than iron, and it is easy to generate low-melting silicates during the welding process. These silicates will increase the fluidity of the slag and molten metal, causing splashing, thereby affecting the welding quality. The Si content in the present invention is 0.10% to 0.20%.

[0014] (3) Mn improves the strength of steel mainly through solid solution strengthening and fine grain strengthening, and can also improve the toughness of steel to a certain extent. The addition of Mn can improve the hardenability of steel, making it easier for steel to form high-strength structures such as martensite during quenching. Considering the effect of excessive addition on welding performance, the Mn content in the present invention is 1.60% to 1.70%.

[0015] (4) P and S will seriously affect the mechanical properties of steel, mainly including reducing toughness, causing segregation, promoting cold brittleness and hot brittleness, reducing impact toughness and fatigue strength, and causing welding cracks, etc. In the present invention, the P and S contents are controlled to P≤0.010% and S≤0.003%.

[0016] (5) Al is a strong deoxidizing and nitrogen fixing agent, which can effectively remove oxygen and nitrogen impurities in steel and improve the purity of steel. It can refine the intrinsic grains of steel, increase the strength and toughness of steel, and improve the mechanical properties of steel. However, steel with high Al content is prone to defects such as cracks during welding, and Al oxide and nitride inclusions are easily formed during the welding process. The Als content in the present invention is 0.030% to 0.050%.

[0017] (6) Ni and iron can form an infinite solid solution, which can significantly expand the austenite zone and increase the stability of austenite. This stability helps to maintain the austenite structure at a slower cooling rate, thereby providing the possibility for the subsequent formation of a high-strength martensite structure. At the same time, Ni can reduce the critical transformation temperature and critical cooling rate of steel, which makes it easier for steel to form high-strength structures such as martensite during quenching. This means that steel can also form high-hardness and high-strength structures at a slower cooling rate, which is particularly important for extra-thick steel plates because they are often difficult to obtain uniform quenching effects at a faster cooling rate. Cr-free steel with a Ni content of 3.5% can be air quenched, and Cr steel with a Ni content of 8% can also be transformed into M-body at a very low cooling rate. Since Ni is a relatively scarce resource, the Ni content in the present invention is 2.00% to 2.50%.

[0018] (7) The main function of Cr in quenched and tempered steel is to improve hardenability, making it easier for the steel to form high-strength structures such as martensite during the quenching process, so that the steel has better comprehensive mechanical properties after quenching and tempering. The dense and stable passivation film formed by Cr on the surface of the steel can improve the corrosion resistance of the steel. When used in combination with elements such as Ni and Mn with an appropriate Cr content, the toughness of the steel can be improved by forming a stable austenite phase. When the Cr content is too high, it will promote the precipitation of brittle phases (such as σ phases) in the steel. These brittle phases can easily become crack sources under stress, causing the toughness of the steel to drop sharply. In addition, it may also increase the temper brittleness tendency of the steel, further reducing the toughness of the steel. The Cr content in the present invention is 0.40% to 0.50%.

[0019] (8) Mo can significantly improve the hardenability of steel, so that the steel can obtain a deeper hardened layer after quenching, thereby improving the overall performance. Mo can form a defensive layer on the surface of steel, acting as an antioxidant and rust inhibitor, effectively preventing the corrosion of corrosive media such as oxygen, chlorine, and sulfur on the steel, thereby improving the corrosion resistance of the steel. The Mo element can inhibit the brittleness of steel during the tempering process, so that the steel has good toughness while maintaining high strength. When the Mo content in the steel is too high, it may cause the brittleness of the steel to increase, thereby reducing its toughness and impact strength. The Mo element may promote the decarburization of the steel during the heating process, which will affect the structure and performance of the steel. Since Mo is a relatively scarce resource, the Mo content in the present invention is 0.50% to 0.60%.

[0020] (9) Nb plays a variety of important roles in extra-thick quenched and tempered steel: Nb in the solid solution state segregates to the grain boundaries, producing a solute drag effect, reducing the migration rate of the grain boundaries, and thus hindering the growth of austenite grains; Nb in the precipitated state forms fine precipitate phase particles (such as NbC, (Nb, Ti, V) (C, N), etc.), which can pin the grain boundaries and dislocations, hinder the growth of grains, and thus refine the grains. In addition, Nb can improve the grain boundary stability of steel, reduce the brittleness of the heat-affected zone, and thus improve the strength and toughness of the welded joint; the stable compounds formed with other elements in the steel can form a strong protective film on the steel surface, reduce surface wear, and improve the wear resistance of the steel; and form stable compounds such as iron nitride with the iron element in the steel to enhance the corrosion resistance of the steel. The Nb content in the present invention is 0.030% to 0.040%.

[0021] (10) Boron can significantly improve the hardenability of steel, thus saving other relatively rare and precious metal elements, such as nickel, chromium, molybdenum, etc. to a certain extent. However, it should be noted that boron has a tendency to promote temper brittleness and increase welding crack sensitivity. In the present invention, the B element is controlled to 0.0015% to 0.0020%.

[0022] The production method of extra thick rack steel EQ70 for offshore platforms includes the following steps:

[0023] 1) Clean steel smelting: In order to avoid slag during the casting process, which leads to non-conformity due to inclusions, the converter tapping volume is greater than the total ingot weight of a single furnace by 8-10 tons; C ≥ 0.06%, P ≤ 0.015% when tapping the converter; LF refining temperature ≥ 1550℃, LF refining time ≥ 90min, white slag holding time ≥ 25min; slag must be poured before entering VD, and the amount of slag poured is controlled to be more than half of the total slag; the vacuum degree in the VD tank is ≤ 45pa, and the VD pressure holding time is ≥ 18min;

[0024] The molten steel is smelted through a converter, LF furnace, and VD furnace to reduce the content of P, S, and non-metallic inclusions. The ingredients are adjusted in the LF stage to obtain clean molten steel with the above ingredients.

[0025] 2) Water-cooled mold casting: Select steel ingots with a thickness greater than 2 times the thickness of the finished steel plate, use argon protection for casting to reduce the degree of oxidation of molten steel. Casting temperature 1560℃~1570℃, single ingot casting speed 1.5~1.9t / min, in order to accelerate solidification, water is passed through the bottom and surrounding cavities of the ingot mold to cool the ingot mold during the casting and solidification process, the inlet and outlet water temperature difference ≤30℃, flow ≥300m 3 / h; Due to the high alloy content and large internal structural stress of this steel, in order to avoid cracking, the total cooling time is 6 to 10 hours, the demoulding ingot temperature is 450 to 600℃, and the hot cleaning is adopted. After the cleaning is qualified, the furnace is loaded;

[0026] In order to solve the problems of looseness, severe segregation and coarsening of solidification structure in steel ingots cast with conventional cast iron ingot molds, the production method is innovated and water-cooled ingot molds that can achieve accelerated cooling during the solidification process are used for casting. Accelerated cooling helps to inhibit the growth of dendrites, and more molten steel solidifies in the form of equiaxed crystals. The growth of equiaxed crystals helps to fill the gaps between dendrites and reduce the formation of looseness. In addition, rapid cooling prevents the solute elements in the molten steel from diffusing and enriching in time, thereby reducing the formation of segregation and looseness, while refining the cast grains.

[0027] 3) Heating: When charging the furnace, the temperature of the steel ingot is 300℃~500℃, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, when the ingot temperature reaches 1160℃~1180℃, the temperature is raised to 1200~1240℃ at the maximum speed, and the timing of heat preservation is started when the ingot temperature reaches 1160℃, and the heat preservation time is controlled at 6~8min / cm;

[0028] In order to prevent the steel ingot from cracking due to excessive thermal stress during the heating process, slow heating is adopted to extend the low temperature stage time before heat preservation, reduce the temperature gradient in the thickness direction of the steel ingot, and prevent cracking under the combined influence of temperature stress and organizational stress. In order to avoid improper heat preservation and coarsening of the internal organization, the process innovation of calculating the heat preservation time in advance and continuing to heat up to the heat preservation stage is adopted.

[0029] 4) Controlled rolling and cooling: the starting rolling temperature is ≥950℃, the final rolling temperature is not limited, and the pass reduction is ≥60mm. In order to control the flatness of the plate, a flattening pass reduction margin is reserved before the end of rolling. The number of flattening times is ≥3 times, and the pass reduction is 5mm~10mm. Online water cooling is performed after rolling, the return temperature is ≤450℃, and the cooling speed is unlimited;

[0030] By selecting water-cooled steel ingots with excellent internal quality as raw materials, and adopting strong pressure rolling and post-rolling cooling processes, steel plates with low compression ratio and excellent flaw detection quality can be produced. During the rolling process, the strong pressure reduction process can significantly increase the deformation of the extra-thick plate during the rolling process, promote the plastic deformation and recrystallization process inside the metal, which helps to eliminate or reduce defects in the original steel ingot, such as looseness and cracks, and helps to promote the diffusion and discharge of hydrogen, reduce the formation of hydrogen-induced cracks, and thus improve the flaw detection quality of the extra-thick plate. The strong pressure reduction process can introduce greater plastic deformation in the core and 1 / 4 position of the extra-thick plate, promote grain refinement in these areas, increase dislocation density, and then form more dislocation entanglements and dislocation cells. At the same time, the distribution and morphology of precipitates (such as carbides, nitrides, etc.) can also be optimized, and the uniformity of the organization in the thickness direction is improved, which helps to improve the strength, hardness and corrosion resistance of the extra-thick plate. Cooling after rolling can reduce the grain size in the extra-thick plate and control the phase change process, thereby improving the mechanical properties of the extra-thick plate, such as tensile strength, yield strength and impact toughness. The specific production methods adopted are as follows:

[0031] 5) Heat treatment: including two process stages of quenching and tempering. First, high-temperature rapid quenching is adopted, the quenching temperature is 900-940℃, the holding time is 2.0-2.5min / mm, the water pressure of the quenching machine is ≥0.8MPa, the quenching cooling rate is ≥8℃ / min, and tempering is carried out after quenching to room temperature, the tempering temperature is 600-650℃, the holding time is 3.0-3.5min / mm, and after being taken out of the furnace, the quenching water pool is used to cool to below 400℃, and then air-cooled to room temperature;

[0032] The biggest problem faced by heat treatment of extra-thick quenched and tempered steel is the control of full-thickness microstructure uniformity and obtaining the ideal microstructure type. To achieve this goal, this patent adopts a new heat treatment process of high-temperature quenching and accelerated cooling after tempering. By reasonably controlling the temperature and cooling rate of high-temperature quenching, the austenite phase in the extra-thick quenched and tempered steel undergoes a phase transformation under the action of rapid cooling and transforms into martensite. This transformation process involves the rearrangement of atoms and the reconstruction of the lattice, and the pinning of precipitates to grain boundaries and dislocations, resulting in a significant increase in the hardness and strength of the material. At the same time, the type, quantity and distribution of precipitates will also affect the corrosion resistance of the material. During the tempering process, a certain amount of precipitates such as carbides will precipitate inside the steel. Accelerated cooling after tempering can stabilize these precipitates, prevent them from re-dissolving or undergoing adverse transformations, thereby maintaining the stability of the performance of the steel. At the same time, the generation of temper brittleness can be reduced or avoided, thereby maintaining the toughness of the steel. The specific production methods adopted are as follows:

[0033] Compared with the prior art, the beneficial effects of the present invention include:

[0034] 1) The innovative alloy component system of the present invention realizes the reduction of alloys such as Ni, Mo, and Cr, and combines the key technology of water-cooled die casting to greatly increase the Mn element content in the steel, thereby improving the hardenability of the steel. Combined with the heat treatment process, the thickness cross-section structure is uniform tempered troostite and tempered bainite structure, which can ensure the performance of the steel plate: ReH ≥ 690MPa, Rm 810 ~ 890MPa, and the single value of longitudinal and transverse Charpy impact energy at -40°C at 1 / 4 and 1 / 2 of the thickness of the steel plate is ≥ 120J.

[0035] 2) Combine clean steel smelting, water-cooled die casting, heating temperature control process, large-pressure rolling and cooling after rolling, high-temperature rapid quenching process, and rapid cooling after tempering to refine the original grain structure of the cast state, stabilize and evenly heat the temperature, fully deform during the rolling process, and press the micro defects of the full-thickness section to ensure the internal quality. The lower limit of the compression ratio of the production of extra-thick rack steel plates can be reduced to more than 2 times. It can ensure that the steel plate meets the first-level flaw detection quality of GB / T2970. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of grain size detection at the surface, 1 / 4 thickness, and 1 / 2 thickness of the steel plate in Example 3.

[0037] Figure 2 This is a schematic diagram of sulfur print detection of a steel plate in Example 3 where no segregation or sulfide aggregation was found.

[0038] Figure 3 This is a schematic diagram of the cold acid etching method for testing the steel plate in Example 3. After acid etching, the steel plate has no visible defects such as cracks, white spots, and inclusions.

[0039] Implementation examples:

[0040] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[0041] The production thickness of Example 1 of the present invention is 210 mm, the production thickness of Example 2 is 230 mm, and the production thickness of Example 3 is 254 mm.

[0042] The embodiments are all produced using water-cooled steel ingots with a thickness of 510 mm.

[0043] The following process flow is adopted in all embodiments: converter → LF → VD → water-cooled mold casting → heating → rolling → water cooling after rolling → quenching → tempering, wherein

[0044] In clean steel smelting, in order to avoid slag during the casting process, which leads to non-conformity in flaw detection due to inclusions, the converter tapping volume is greater than the total ingot weight of a single furnace by 8 to 10 tons; when tapping the converter, C ≥ 0.06%, P ≤ 0.015%, LF refining temperature ≥ 1550℃, LF refining time ≥ 90min, white slag holding time ≥ 25min, slag must be poured before entering VD, and the slag pouring volume is controlled to be more than half of the total slag volume, the vacuum degree in the VD tank ≤ 45pa, and the VD pressure holding time ≥ 18min;

[0045] In water-cooled mold casting, choose ingots with a thickness greater than 2 times the thickness of the finished steel plate, the casting temperature is 1560℃~1570℃, and the casting speed of a single ingot is 1.5~1.9t / min. In order to accelerate solidification, water is passed through the bottom and surrounding cavities of the ingot mold to cool the bottom during the casting and solidification process. The temperature difference between the inlet and outlet water is ≤30℃ and the flow rate is ≥300m 3 / h. Because this type of steel has a high alloy content and large internal structural stress, in order to avoid cracking, the total cooling time is 6 to 10 hours, the demoulding ingot temperature is 450 to 600℃, and hot cleaning is adopted. After cleaning is qualified, it is loaded into the furnace.

[0046] In the heating process, the temperature of the ingot is 300℃~500℃ when charging the furnace, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, and when the ingot temperature reaches 1160℃~1180℃, it is heated to 1200~1240℃ at the maximum speed. At the same time, the timing of insulation starts when the ingot temperature reaches 1160℃, and the insulation time is controlled at 6~8min / cm.

[0047] In controlled rolling and controlled cooling, the starting rolling temperature is ≥950℃, the final rolling temperature is not limited, the reduction per pass is ≥60mm, and a reduction margin for the leveling pass is reserved before the end of rolling. The number of leveling passes is ≥3 times, the reduction per pass is 5mm~10mm, and online water cooling is performed after rolling, and the red-return temperature is ≤450℃.

[0048] In the heat treatment, including quenching and tempering, high-temperature rapid quenching is first adopted, the quenching temperature is 900-940℃, the holding time is 2.0-2.5min / mm, the water pressure of the quenching machine is ≥0.8MPa, the quenching cooling rate is ≥8℃ / min, and tempering is carried out after quenching to room temperature. The tempering temperature is 600-650℃, and the holding time is 3.0-3.5min / mm. After taking out of the furnace, it is water-cooled to below 400℃ in the quenching water pool, and then air-cooled to room temperature.

[0049] The chemical composition of the steel of the embodiment of the present invention is shown in Table 1, the water-cooled mold casting process of the steel ingot of the embodiment of the present invention is shown in Table 2, the heating process of the steel ingot of the embodiment of the present invention is shown in Table 3, the rolling and post-rolling cooling process of the embodiment of the present invention is shown in Table 4, the heat treatment process of the steel plate of the embodiment of the present invention is shown in Table 5, and the mechanical properties of the steel plate of the embodiment of the present invention are shown in Table 6.

[0050] Table 1 Chemical composition of steel according to the present invention (wt%)

[0051] Example C Si Mn P S Als Cr Ni Mo Nb B Ceq Pc I 1 0.11 0.13 1.61 0.006 0.001 0.048 0.46 2.13 0.57 0.037 0.0017 0.726 0.300 9.00 2 0.11 0.18 1.62 0.007 0.001 0.035 0.45 2.35 0.53 0.038 0.0019 0.733 0.305 9.90 3 0.12 0.12 1.67 0.006 0.001 0.035 0.46 2.30 0.58 0.032 0.0019 0.760 0.319 9.63

[0052] Table 2 Water-cooled mold casting process of steel ingots according to the present invention

[0053]

[0054]

[0055] Table 3 Ingot heating process of the embodiment of the present invention

[0056]

[0057] Table 4 Rolling and post-rolling cooling process of the embodiment of the present invention

[0058]

[0059] Table 5 Heat treatment process of steel plate in the embodiment of the present invention

[0060]

[0061] Table 6 Mechanical properties of steel plates in the present invention

[0062]

[0063]

[0064] The finished steel plates are tested for flaws and all meet GB / T 2970 Level 1.

Claims

1. A special thick rack steel EQ70 for offshore platform, characterized in that: The thickness of the steel plate is 210-254 mm. The steel contains, by weight percentage, C 0.10-0.12%, Si 0.10-0.20%, Mn 1.60-1.70%, P≤0.010%, S≤0.003%, Als 0.030-0.050%, Ni 2.00-2.50%, Cr 0.40-0.50%, Mo 0.50-0.60%, Nb0.030-0.040%, B 0.0015-0.0020%, and the rest is Fe and unavoidable impurities, and the composition satisfies Ceq≤0.76%, Pcm≤0.32%, and I≥8.30; The ReH of the steel is ≥690MPa, Rm is 810-890MPa, and the single values ​​of longitudinal and transverse Charpy impact energy at -40°C at 1 / 4 and 1 / 2 of the thickness of the steel plate are ≥120J.

2. The method for producing extra thick rack steel EQ70 for offshore platforms according to claim 1, characterized in that: Including clean steel smelting, water-cooled mold pouring, heating, controlled rolling and controlled cooling, and heat treatment, specifically: 1) Clean steel smelting: In order to avoid slag during the casting process, which leads to non-conformity in flaw detection due to inclusions, the converter tapping volume is greater than the total ingot weight of a single furnace by 8 to 10 tons; when tapping the converter, C ≥ 0.06%, P ≤ 0.015%, LF refining temperature ≥ 1550℃, LF refining time ≥ 90min, white slag holding time ≥ 25min, slag must be poured before entering VD, and the slag pouring volume is controlled to be more than half of the total slag volume, the vacuum degree in the VD tank ≤ 45pa, and the VD pressure holding time ≥ 18min; 2) Water-cooled mold casting: Select steel ingots with a thickness greater than 2 times the thickness of the finished steel plate, the casting temperature is 1560℃~1570℃, and the casting speed of a single steel ingot is 1.5~1.9t / min. In order to accelerate solidification, water is passed through the bottom and surrounding cavities of the ingot mold to cool the bottom during the casting and solidification process. The temperature difference between the inlet and outlet water is ≤30℃ and the flow rate is ≥300m 3 / h, because this type of steel has high alloy content and large internal structural stress, in order to avoid cracking, the total cooling time is 6 to 10 hours, the demoulding ingot temperature is 450 to 600℃, and the hot cleaning is adopted. After the cleaning is qualified, the furnace is loaded; 3) Heating: When charging the furnace, the temperature of the steel ingot is 300℃~500℃, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, when the ingot temperature reaches 1160℃~1180℃, the temperature is raised to 1200~1240℃ at the maximum speed, and the timing of heat preservation is started when the ingot temperature reaches 1160℃, and the heat preservation time is controlled at 6~8min / cm; 4) Controlled rolling and controlled cooling: the starting rolling temperature is ≥950℃, the final rolling temperature is not limited, the pass reduction is ≥60mm, and a flattening pass reduction margin is reserved before the end of rolling. The number of flattening times is ≥3 times, and the pass reduction is 5mm~10mm. Online water cooling is performed after rolling, and the return temperature is ≤450℃; 5) Heat treatment: including quenching and tempering, first use high temperature rapid quenching, quenching temperature 900 ~ 940 ℃, holding time 2.0 ~ 2.5min / mm, quenching machine water pressure ≥ 0.8MPa, quenching cooling rate ≥ 8℃ / min, quenching to room temperature and then tempering, tempering temperature 600 ~ 650℃, holding time 3.0 ~ 3.5min / mm, after taking out of the furnace, use quenching water pool to cool to below 400℃, and then air cool to room temperature.

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