Method for preparing steel plate for hydropower

Abstract

The present application discloses a method for preparing a steel plate for hydropower, comprising: steelmaking; continuous casting, the superheat of the molten steel in the tundish is 15-25°C, the crystallizer uses a low-melting-point alkaline protective slag with a melting point of 1120-1180°C, the pulling speed v is 0.60-1.20 m / min, and the taper of the crystallizer is 1.05-1.25%; after the slab leaves the crystallizer, it is cooled in a secondary cooling zone, the water volume of the 1st to 5th sections of the secondary cooling zone decreases successively, the water volume of the 6th to 8th sections is greater than the water volume of the 5th section and gradually decreases, and the water volume of the 9th to 10th sections is greater than the water volume of the 8th section; the slab is controlled to pass through the 9th section of the continuous casting machine fan-shaped section. # The central solid fraction fs is 0.3~0.7 during the 10th stage. # The solid phase ratio fs>0.7 in the middle of the slab # , 10 # The 9th stage is pressed lightly. # The reduction rate of the segment>10 # Reduction rate of the segment; rolling; heat treatment.

Description

Technical Field

[0001] The present application belongs to the technical field of steel material preparation, and relates to a method for preparing a steel plate for hydropower. Background Art

[0002] Hydropower steel is a special steel designed specifically for hydropower projects. It needs to withstand huge water pressure and impact during service, so it has extremely high requirements on the strength, toughness and flaw detection of the steel.

[0003] In the related art, when preparing high-strength hydropower steel with large thickness and 800MPa and above, there are usually problems such as high alloy content, complex composition structure, sensitivity to ingot quality, poor low-temperature impact performance at the center of the steel plate thickness, and low flaw detection pass rate. As a result, the mechanical properties of hydropower steel plates are poor and the service life is limited, which cannot meet the construction needs of large-capacity pumped-storage power stations. Summary of the invention

[0004] The purpose of this application is to provide a method for preparing a steel plate for hydropower.

[0005] In order to achieve the above application purpose, an embodiment of the present application provides a method for preparing a hydropower steel plate, comprising the following steps:

[0006] Steelmaking, the chemical composition of the molten steel obtained includes, by mass percentage, C 0.07-0.11%, Si 0.15-0.25%, Mn 0.90-1.40%, Cr 0.50-0.65%, Ni 1.0-2.0%, Mo 0.50-0.75%, Cu 0.20-0.30%, Nb 0.01-0.04%, V 0.030-0.045%, Ti 0.01-0.02%, Al 0.04-0.05%, P≤0.010%, S≤0.002%, N 0.003-0.004%, O≤0.002%, H≤0.00015%, B 0.0006-0.0016, and the rest is Fe and unavoidable impurities;

[0007] The molten steel is continuously casted, and the obtained molten steel is poured by a slab continuous casting machine to obtain a slab, the superheat of the molten steel in the tundish is 15-25°C, the crystallizer uses a low-melting-point alkaline protective slag with a melting point of 1120-1180°C, the pulling speed v is 0.60-1.20 m / min, and the taper of the crystallizer is 1.05-1.25%; after the slab exits the crystallizer, it is cooled in a secondary cooling zone, and the secondary cooling zone is divided into 10 sections according to different water volumes, the water volumes of the 1st to 5th sections decrease in sequence, the water volumes of the 6th to 8th sections are greater than the water volumes of the 5th section and gradually decrease, and the water volumes of the 9th to 10th sections are greater than the water volumes of the 8th section; the slab is controlled to pass through the 9th section of the continuous casting machine fan-shaped section. #The central solid fraction fs is 0.3~0.7 during the 10th stage. # The solid phase ratio fs>0.7 in the middle of the slab # , 10 # The 9th stage is pressed lightly. # The reduction rate of the segment>10 # The reduction rate of the segment; the central carbon segregation level of the slab obtained by continuous casting is C0.5 level and below, and the central looseness level is not higher than 0.5 level;

[0008] Rolling, heating the slab obtained by continuous casting and then rolling it in two stages to obtain steel plates;

[0009] The steel plate obtained by rolling is subjected to heat treatment in sequence by quenching and tempering, so that the structure of the steel plate is a tempered martensite structure.

[0010] In this way, based on the design of the chemical composition of the molten steel obtained by steelmaking, the water volume in different sections of the secondary cooling zone is controlled according to the cross-sectional size of the slab continuous casting machine and the continuous casting speed, so as to avoid steel leakage while ensuring the cooling uniformity of the slab, and then control the slab passing through the 9th sector of the continuous casting machine. # , 10 # The central solid phase rate at the 9th stage # , 10 # The 9th # The use of a smaller reduction rate in the first stage is beneficial to promote the flow of molten steel to compensate for shrinkage and control segregation, while preventing the formation of intermediate cracks in the slab, which will cause the deterioration of the slab quality. # The use of a larger reduction rate in the first stage is beneficial to controlling the central porosity and shrinkage cavity, thereby controlling the central carbon segregation level of the slab obtained by continuous casting to C0.5 and below, and the central porosity level not higher than 0.5. Further combined with two-stage rolling and heat treatment, the structure of the steel plate is tempered martensite structure, which effectively improves the product's flaw detection pass rate and improves the low-temperature impact performance of the core of the slab.

[0011] In a further improvement of an embodiment, the water ratios of the wide sides of the first to fifth sections are 2.3: 1.7: 1.25: 1.25: 1. By controlling the water ratios of the wide sides of the first to fifth sections, while ensuring that leakage is avoided, internal cracks caused by overcooling during straightening, cutting and stacking of the slab can also be avoided.

[0012] In a further improvement of one embodiment, the water volume ratio of the outer arc to the inner arc of the 6th to 10th sections is gradually increased. In this way, the uniformity of the blank cooling can be ensured, and the edges and corners can be prevented from being overcooled and cracked.

[0013] In a further improvement of an embodiment, the water volume ratio of the outer arc to the inner arc of the 6th to 10th sections is 1.3, 1.5, 1.7, 1.85 and 1.85, respectively. By precisely controlling the water volume ratio of the inner arc and the outer arc of the 6th to 10th sections, the uniformity of the cooling of the blank can be ensured, and the cracks caused by overcooling of the edges and corners can be prevented.

[0014] In a further improvement of one embodiment, when the thickness d of the slab is 220 mm, the unit of the water volume T of each section of the secondary cooling zone is L / min and satisfies:

[0015] The water volume on the broad side of the 5th section is Tk(5)=148.7·v±5;

[0016] The inner arc water volume of the 6th section is Tn(6)=190·v±5;

[0017] The inner arc water volume of the 7th section is Tn(7)=120·v±5;

[0018] The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

[0019] In this way, by setting the water volume in sections 5 to 10 according to the thickness of the slab and the continuous casting speed v, and controlling the relationship between the water volume in each section and the continuous casting speed v, the surface temperature of the slab can be evenly distributed, reducing the temperature gradient difference between the core and the surface of the slab, and effectively avoiding internal and external cracks in the slab.

[0020] In a further improvement of one embodiment, when the thickness d of the slab is 220 mm, the narrow side water volume Tn(1) of the first section is 61·v±5.

[0021] In a further improvement of one embodiment, when the thickness d of the slab is 320 mm, the unit of the water volume T of each section of the secondary cooling zone is L / min and satisfies:

[0022] The water volume on the broad side of the 5th section is Tk(5)=264.7·v±5;

[0023] The inner arc water volume of the 6th section is Tn(6)=338·v±5;

[0024] The inner arc water volume of the 7th section is Tn(7)=213·v±5;

[0025] The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

[0026] In this way, by setting the water volume in sections 5 to 10 according to the thickness of the slab and the continuous casting speed v, and controlling the relationship between the water volume in each section and the continuous casting speed v, the surface temperature of the slab can be evenly distributed, reducing the temperature gradient difference between the core and the surface of the slab, and effectively avoiding internal and external cracks in the slab.

[0027] In a further improvement of one embodiment, when the thickness d of the slab is 320 mm, the narrow side water volume Tn(1) of the first section is 108·v±5.

[0028] In a further improvement of one embodiment, when the thickness d of the slab is 220 mm, 1.05 m / min≤v≤1.20 m / min, and 1.15%<the taper of the crystallizer≤1.25%;

[0029] When the thickness d of the slab is 320 mm, 0.60 m / min≤v≤0.70 m / min, and the taper of the crystallizer is 1.05%<1.15%.

[0030] In this way, the continuous casting drawing speed v is set according to the thickness of the slab, and the taper of the crystallizer is accurately controlled based on the drawing speed v, which can reduce the problem of uneven cooling caused by the shrinkage of the slab during the cooling process.

[0031] In a further improvement of one embodiment, in the continuous casting process, the basicity of the low-melting-point alkaline protective slag is 1.1-1.5, and its chemical composition includes by mass percentage: F 6-7%, C 6-7%, Fe2O3 2-3%, MnO2 4.5-5.5%, Na2O 6-7%, MgO 1-1.5%, Al2O3 5-6%; the consumption of the low-melting-point alkaline protective slag is 0.45-0.55 kg / t. In this way, by designing the composition of the low-melting-point alkaline protective slag and controlling the consumption, the protective slag can be melted at a lower rate, so that the crust of the slab has excellent protection and lubrication, thereby improving the thermal conductivity and further improving the cooling uniformity of the slab.

[0032] In a further improvement of one embodiment, the particle size of the low-melting-point alkaline protective slag is ≤0.3 mm, and the mass of particles with a particle size of ≤0.2 mm accounts for more than 90%. Such a particle size can ensure the melting rate of the low-melting-point alkaline protective slag in the crystallizer, thereby improving its protective effect.

[0033] In a further improvement of one embodiment, the covering agent of the tundish is double-layer slag, the bottom layer is alkaline protective slag with a slag basicity of 4.0-8.0, and the top layer is carbonized rice husk. The alkaline protective slag of the bottom layer has high alkalinity, which is conducive to absorbing large inclusions in the molten steel and improving the cleanliness of the molten steel; the carbonized rice husk of the top layer does not directly contact the molten steel while keeping warm, reducing environmental pollution.

[0034] In a further improvement of one embodiment, in the alkaline protective slag, the mass percentage of Al2O3 and MgO is 35-60% in total, and F is less than 5.0%. Al2O3, as an amphoteric oxide, is acidic in the alkaline protective slag, and after the steel slag reacts, the Al2O3 content increases, and Al2O3 will change from acidic to alkaline, which helps to reduce the viscosity of the slag and improve the ability of the protective slag to absorb inclusions; MgO can reduce the viscosity and solidification temperature of the protective slag, thereby improving the heat transfer of the crystallizer, improving the quality of the slab, and reducing the adhesion of the slab; the fluorine (F) content is controlled at a low level below 5.0%, which can avoid the unstable performance of the protective slag caused by excessive fluorine content, such as avoiding the increase of the viscosity and melting point of the slag, and avoiding the reduction of the ability to absorb inclusions.

[0035] In a further improvement of one embodiment, the continuous casting machine has a foot roll segment, a zero segment, and an 11th segment of a fan-shaped segment. # , 12 # The segment does not perform light pressing, and the roller gap shrinkage between the full roller segment and the zero segment is 0.1~0.2mm. # , 12 # The roller gap shrinkage of the segment is 0.3~0.5mm;

[0036] The slab is in the 9th sector # The reduction rate of the segment is 0.7~1.2mm / m. # The reduction rate of the segment is 1.2~1.7mm / m.

[0037] In this way, by controlling the shrinkage of the roll gap, the slab can be prevented from bulging and deforming in these areas, internal cracks and segregation can be reduced, and the internal quality of the ingot can be improved; by controlling the reduction of the slab and regulating the distribution of the reduction amount, the internal quality and segregation problems of the slab can be optimized.

[0038] In a further improvement of one embodiment, in the rolling process, the temperature of the soaking section during heating is 1130-1170° C., and the heating time of the soaking section is t1=a1·d1, wherein the heating coefficient a1=0.1-0.3 min / mm, and d1 is the slab thickness in mm;

[0039] After heating, the slab is first rough-rolled into an intermediate billet, the final rolling temperature of the rough rolling is 1000~1040℃, and the thickness of the intermediate billet is 1.2~3.0 times the thickness of the final prepared steel plate; then, finish rolling is carried out to obtain a steel plate, the start rolling temperature of the finish rolling is ≤890℃, and the final rolling temperature of the finish rolling is 800~820℃; after rolling, it is cooled to below 400℃, and the cooling rate is 0.5~20℃ / s.

[0040] In this way, the heating time of the soaking section is controlled according to the slab thickness and the heating coefficient. Combined with the control of the temperature of the soaking section, the alloy in the slab can be dissolved and the core can be completely heated, so that the slab is heated evenly, avoiding cracks caused by uneven temperature inside and outside the slab during subsequent rolling. Further controlling the high temperature and rapid rolling in the rough rolling stage, as well as controlling the ratio of the intermediate slab thickness to the thickness of the final prepared steel plate, can ensure that the deformation penetrates into the core of the slab. By controlling the finishing rolling temperature, the cumulative effect of the deformation can be guaranteed.

[0041] In a further improvement of one embodiment, in the heat treatment process, the quenching temperature is 870-890°C, the quenching time is t2=a2·d2, wherein the heating coefficient a2=1.6-1.8min / mm, and d2 is the thickness of the steel plate, in mm; after quenching, the steel plate is rapidly cooled to below 200°C, with a cooling rate of ≥20°C / s, and the structure of the steel plate after quenching is a lath martensite structure; then the steel plate is tempered, the tempering temperature is 590-610°C, the tempering time is t3=a3·d2, wherein the heating coefficient a3=2.8-3.2min / mm, and the structure of the steel plate after tempering is a tempered martensite structure. The quenching time and tempering time are controlled according to the thickness of the steel plate and the heating coefficient, and combined with the control of the quenching temperature and the tempering temperature, the phase change can be controlled to ensure the uniformity of the structure, while ensuring the strength and low-temperature toughness of the steel plate.

[0042] In a further improvement of one embodiment, the steelmaking process includes KR pre-desulfurization, converter smelting, LF refining, and RH vacuum refining steps performed in sequence;

[0043] In the KR pre-desulfurization step, the S content in the molten iron after desulfurization is ≤0.0015%, and the molten iron temperature at the end point of desulfurization is 1320-1380°C;

[0044] In the converter smelting step, the desulfurized molten iron, nickel plate, copper plate, ferromolybdenum and scrap steel are added to the converter for smelting, wherein the scrap steel accounts for 15-20%; when 20-30% of the steel is tapped, aluminum particles, ferrosilicon, metallic manganese, low-carbon ferrochrome, ferrovanadium and ferroniobium are added to the molten steel for deoxidation and alloying, when 70-80% of the steel is tapped, lime and fluorite are added to make slag, and calcium aluminate is added to the slag surface after the steel is tapped to synthesize slag, the ladle is bottom blown and stirred during the steel tapping process, and the bottom blowing flow rate is 600-800NL / min, and stirring is continued for 3-5min after the steel is tapped;

[0045] In the LF refining step, after the molten steel enters the station, it is powered on to increase the temperature, calcium carbide, lime and fluorite are added to adjust the slag, bottom blowing and stirring are turned on for desulfurization, and alloys are added according to the composition of the molten steel entering the station, until the molten steel composition, slag composition and molten steel temperature meet the standards, and then it is transported to the RH furnace for vacuum refining;

[0046] In the RH vacuum refining step, vacuuming is started after the molten steel enters the station, and vacuuming is continued for 5 minutes after the vacuum degree reaches below 2 mbar, and then ferrotitanium is added, and the net circulation treatment is more than 15 minutes, and the air is broken to tap the steel. After tapping the steel, pure calcium wire is fed into the molten steel to control the Ca content in the molten steel to be 15-25 ppm.

[0047] In this way, after steelmaking through the above steps, the S and P contents in the molten steel can be controlled, and the RH vacuum treatment can reduce the gas content in the molten steel and improve the cleanliness of the molten steel. Further, by feeding the calcium wire, molten steel with high purity can be obtained, and the inclusions in the molten steel are low-melting-point plastic inclusions.

[0048] In a further improvement of one embodiment, in the converter smelting step, double slag smelting is adopted, and the first slag pouring is carried out when the molten steel temperature reaches 1340°C and the slag basicity is 2.5-3.0, and the T.Fe content in the slag is 20-30%; the second slag pouring is carried out when the molten steel temperature reaches 1580-1620°C, the P content in the molten steel is ≤0.008%, the C content is 0.03-0.05%, and the slag basicity is 4.0-6.5, and the T.Fe content in the slag is 25-35%; then the steel is tapped. By adopting double slag smelting in the converter and using high-basicity, high-oxidizing slag, the P content in the molten steel can be controlled at a lower level.

[0049] In a further improvement of an implementation mode, the thickness of the obtained hydropower steel plate is ≥40mm, the tensile strength is ≥950MPa, the elongation after fracture is ≥15.5%, the yield strength ratio is ≥0.8, the low-temperature impact energy Akv of the core of the steel plate at -40°C is >80J, and the flaw detection of the steel plate meets the first-level standard of NB 47013.3. The steel plate not only has a large thickness and high strength, but also has excellent low-temperature impact performance at the center of the steel plate thickness, qualified flaw detection, and excellent comprehensive performance. It is not only suitable for the construction of large-capacity pumped storage power stations, but also greatly extends its service life. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in conjunction with the specific implementation methods of the present application. Obviously, the described implementation methods are only part of the implementation methods of the present application, not all of the implementation methods. Based on the implementation methods in the present application, all other implementation methods obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0051] The present application provides a method for preparing a hydropower steel plate, which comprises the following steps:

[0052] (1) Steelmaking process

[0053] Through steelmaking, the chemical composition of the molten steel obtained, in terms of mass percentage, includes: C 0.07~0.11%, Si 0.15~0.25%, Mn 0.90~1.40%, Cr 0.50~0.65%, Ni 1.0~2.0%, Mo 0.50~0.75%, Cu 0.20~0.30%, Nb0.01~0.04%, V 0.030~0.045%, Ti 0.01~0.02%, Al 0.04~0.05%, P≤0.010%, S≤0.002%, N0.003~ 0.004%, O≤0.002%, H≤0.00015%, B 0.0006~0.0016, and the rest are Fe and unavoidable impurities.

[0054] The role of each element in this chemical composition is introduced below.

[0055] C: It is a strengthening element of steel and also an austenite stabilizing element. Reversing the austenite enriched with C will significantly reduce the Ms point and improve its stability. However, too high a C content will increase the ductile-brittle transition temperature, which is harmful to the low-temperature toughness of the HAZ. Therefore, under the premise of ensuring strength, the lower the C content, the better.

[0056] Si: It is a deoxidizing element in the steelmaking process and is very important for reducing the content of harmful element O in nickel-based steels. At the same time, Si can improve strength. In addition to existing in steel in a certain proportion with Mn to inhibit Mn segregation, Si can also inhibit P segregation at grain boundaries. Too high Si content is not conducive to welding performance. Reducing Si content can improve the low-temperature toughness of the base material and the welding heat affected zone (HAZ).

[0057] Mn: It is an austenite stabilizing element. Its enrichment in austenite is conducive to reversing the stability of austenite. Mn is also a matrix strengthening element. It can form fine MnS particles through solid solution strengthening and precipitation strengthening, thereby improving the strength. Mn can also significantly improve the hardenability of the material. If the Mn content is too low, the strength will not meet the requirements. If the Mn content is too high, large-sized MnS inclusions will easily form, which will deteriorate the toughness.

[0058] Cr: In structural steel and tool steel, chromium can significantly improve strength, hardness and wear resistance, while reducing plasticity and toughness, and can also improve the steel's oxidation resistance and corrosion resistance.

[0059] Ni: It can improve the strength of steel and maintain good plasticity and toughness; nickel has high corrosion resistance to acids and alkalis, and has excellent rust prevention and heat resistance at high temperatures.

[0060] Mo: It can refine the grain of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures. Creep refers to the deformation of materials caused by stress under high temperature for a long time.

[0061] Cu: It can improve strength and toughness, especially atmospheric corrosion resistance; the disadvantage is that it is easy to become hot brittle during hot working, and too high copper content will lead to a significant decrease in plasticity.

[0062] Nb: It can improve the strength of steel. The Nb element can form high-hardness carbides with C, enhance the strength and hardness of steel, and improve the wear resistance and tensile strength of steel. Nb can stabilize the formation of carbides, limit corrosion and hydrogen-induced cracking at the grain boundaries, and improve the corrosion resistance and weather resistance of steel.

[0063] V: It can significantly improve the strength and toughness of steel. At the same time, the carbides formed can refine the grains and reduce the grain size of steel, thereby improving the strength and toughness of steel. Vanadium can also improve the welding performance of steel and reduce the crack sensitivity during welding, which is especially important for steel plate components that need to be welded in large hydropower projects.

[0064] Ti: It can refine grains, prevent austenite grain growth, delay recrystallization during thermal deformation, and change recrystallization dynamics, thereby increasing the strength of the material and improving toughness. Ti is a strong nitrogen-fixing element, and through the nitrogen-fixing effect of Ti, the strain aging properties of hydropower steel can be effectively improved.

[0065] Al: It is used as a deoxidizing and nitrogen fixing agent in steelmaking to refine grains, inhibit the aging of low-carbon steel, improve the toughness of steel at low temperatures, and especially reduce the brittle transition temperature of steel and improve the oxidation resistance of steel.

[0066] P, S: P is easy to segregate at the grain boundary and form Fe3P with iron, which weakens the bonding force between Fe atoms and surrounding Fe atoms, reduces the grain boundary crack growth resistance, and deteriorates low-temperature toughness; S is easy to form precipitate MnS with the metal element Mn, reducing low-temperature toughness. Therefore, S and P are both harmful elements to low-temperature toughness.

[0067] O, N: O, N and Al easily form high melting point precipitates Al2O3 and AlN; and the precipitates are relatively large in diameter, reaching several microns, which can easily cause stress concentration near the precipitates and become crack sources, seriously affecting the low-temperature toughness of the matrix. The content of these elements should be reduced as much as possible; oxygen, like hydrogen, will have an adverse effect on the mechanical properties of steel. Not only the oxygen concentration, but also the amount, type and distribution of oxygen-containing inclusions have a very important impact.

[0068] H: When the hydrogen content in steel is greater than 2ppm, hydrogen plays an important role in the so-called "scaling" phenomenon. This scaling phenomenon is generally more obvious when internal cracks and fractures occur during the cooling process after rolling and forging, and is more often found in large sections or high carbon steels.

[0069] B: A trace amount of boron can significantly improve the hardenability of steel, which is very beneficial to improving the density and hot rolling performance of steel and increasing strength; an appropriate amount of boron can improve hot ductility and reduce the possibility of quenching crack formation during heat treatment; but too much B will segregate at the grain boundary, which can easily lead to a decrease in impact toughness.

[0070] In a specific embodiment, the molten steel can be refined through the process of KR pre-desulfurization, converter smelting, LF refining, and RH vacuum refining. The following is a detailed description of each step of the steelmaking process.

[0071] a. KR pre-desulfurization step

[0072] The blast furnace molten iron is desulfurized by KR desulfurization technology. The S content in the molten iron after desulfurization is ≤0.0015%, and the molten iron temperature at the end of desulfurization is 1320~1380℃. Specifically, the KR desulfurization technology can be implemented by any feasible technology disclosed in the art, which will not be elaborated here.

[0073] b. Converter smelting steps

[0074] The desulfurized molten iron, nickel plate, copper plate, ferromolybdenum and scrap steel are added into the converter for smelting, of which the scrap steel accounts for 15-20%; when the steel tapping rate is 20-30%, aluminum particles, ferrosilicon, metallic manganese, low-carbon ferrochrome, ferrovanadium and ferroniobium are added to the molten steel for deoxidation and alloying; when the steel tapping rate is 70-80%, lime and fluorite are added to make slag; after the steel tapping is completed, calcium aluminate is added to the slag surface to synthesize slag; during the steel tapping process, the ladle is bottom blown and stirred, and the bottom blowing flow rate is 600-800NL / min; after the steel tapping is completed, stirring is continued for 3-5 minutes.

[0075] Preferably, the converter smelting adopts double slag smelting, which means two slag pourings. Specifically, the first slag pouring is carried out when the molten steel temperature reaches 1340°C and the slag basicity is 2.5-3.0, and the T.Fe content in the slag is 20-30%; the second slag pouring is carried out when the molten steel temperature reaches 1580-1620°C, the P content in the molten steel is ≤0.008%, the C content is 0.03-0.05%, and the slag basicity is 4.0-6.5, and the T.Fe content in the slag is 25-35%; then the steel is tapped.

[0076] c. LF refining steps

[0077] After the molten steel enters the station, electricity is turned on to increase the temperature, calcium carbide, lime and fluorite are added to adjust the slag, bottom blowing and stirring are turned on for desulfurization, and alloys are added according to the composition of the molten steel entering the station. When the composition of the molten steel, the slag composition and the temperature of the molten steel meet the standards, it is transported to the RH furnace for vacuum refining.

[0078] d. RH vacuum refining step

[0079] After the molten steel enters the station, vacuuming begins. After the vacuum degree reaches below 2mbar, vacuuming continues for 5 minutes. Then ferrotitanium is added and the net circulation treatment lasts for more than 15 minutes. The steel is then discharged. After the steel is discharged, pure calcium wire is fed into the molten steel to control the Ca content in the molten steel to 15~25ppm.

[0080] The molten steel produced in the RH vacuum refining step is hoisted onto a platform of a slab continuous casting machine and allowed to stand for more than 10 minutes to obtain molten steel for pouring to prepare slabs.

[0081] That is to say, the chemical composition of the molten steel produced in the RH vacuum refining step includes, by mass percentage, C 0.07-0.11%, Si 0.15-0.25%, Mn 0.90-1.40%, Cr 0.50-0.65%, Ni 1.0-2.0%, Mo 0.50-0.75%, Cu 0.20-0.30%, Nb 0.01-0.04%, V 0.030-0.045%, Ti 0.01-0.02%, Al 0.04-0.05%, P≤0.010%, S≤0.002%, N 0.003- 0.004%, O≤0.002%, H≤0.00015%, B 0.0006-0.0016, and the rest is Fe and unavoidable impurities.

[0082] (2) Continuous casting process

[0083] The obtained molten steel is casted by a slab continuous casting machine to obtain a slab. The chemical composition of the obtained slab is consistent with the chemical composition of the molten steel produced in the RH vacuum refining step, and will not be described in detail here.

[0084] The continuous casting machine includes a foot roll segment, a zero segment and a fan segment arranged in sequence, and the fan segment includes a first # Section ~ 12 # part.

[0085] Specifically, multiple ladles containing molten steel are sequentially poured into a tundish, and the molten steel in the tundish is poured into a crystallizer of a slab continuous casting machine, and then pulled out of the crystallizer to obtain a billet having a width of 1500-2300 mm and a thickness of 220 mm or 320 mm.

[0086] The superheat of the molten steel in the tundish is 15-25°C, the crystallizer adopts a low-melting-point alkaline protective slag with a melting point of 1120-1180°C, the pulling speed v is 0.60-1.20 m / min, and the taper of the crystallizer is 1.05-1.25%; the slab is cooled in the secondary cooling zone after exiting the crystallizer, and the inlet temperature of the secondary cooling water is 28-32°C. The secondary cooling zone is divided into 10 sections according to the amount of water, and the amount of water in sections 1-5 decreases successively, the amount of water in sections 6-8 is greater than that in section 5 and gradually decreases, and the amount of water in sections 9-10 is greater than that in section 8; the slab is controlled to pass through the 9th section of the continuous casting machine fan-shaped segment. # The central solid fraction fs is 0.3~0.7 during the 10th stage. # The solid phase ratio fs>0.7 in the middle of the slab # , 10 # The 9th stage is pressed lightly. # The reduction rate of the segment>10 # The reduction rate of the segment; the central carbon segregation level of the slab obtained by continuous casting is C0.5 and below, and the central porosity level is not higher than 0.5.

[0087] The so-called central solid phase ratio refers to the content of solid structure in the central area (i.e., the core) of the slab. During the continuous casting process, the molten steel gradually solidifies under the cooling effect of the secondary cooling zone. During the cooling process, the central temperature of the slab is higher than the surface temperature, resulting in the solidification rate of the molten steel in the central area of ​​the slab being lower than that on the surface, making the central area of ​​the slab in some sections of the continuous casting machine sector in a semi-solidified state.

[0088] The slab continuous casting machine is provided with a secondary cooling electromagnetic stirring device. During the casting process, the secondary cooling electromagnetic stirring device continuously performs electromagnetic stirring to control the internal quality of the slab, reduce segregation, and thus improve the quality of the final slab.

[0089] Preferably, the slab is controlled to pass through the 10th # When the solid phase ratio fs is 1.0 at the 3 / 4 position along the billet drawing direction, the slab is controlled to pass through the 10th # When the segment is at 3 / 4 of the direction of the billet drawing, its center is completely solidified.

[0090] Preferably, the water ratio of the wide side of the first to fifth sections is controlled to be 2.3: 1.7: 1.25: 1.25: 1. By controlling the water ratio of the wide side of the first to fifth sections, while ensuring that leakage is avoided, internal cracks caused by overcooling during straightening, cutting and stacking of the slab can also be avoided.

[0091] Preferably, the water volume ratio between the outer arc and the inner arc of the 6th to 10th sections is controlled to gradually increase.

[0092] Preferably, the water volume ratio of the outer arc to the inner arc of the 6th to 10th sections is controlled to be 1.3, 1.5, 1.7, 1.85 and 1.85 in sequence. By accurately controlling the water volume ratio of the inner arc and the outer arc of the 6th to 10th sections, the uniformity of the cooling of the blank can be ensured, and the edges and corners can be prevented from being overcooled and cracked.

[0093] The unit of the water volume T in each section of the secondary cooling zone is L / min.

[0094] When the thickness d of the slab is 220 mm, the water volume in each section of the secondary cooling zone satisfies:

[0095] The narrow side water volume of section 1 is Tn(1)=61·v±5;

[0096] The water volume on the broad side of the 5th section is Tk(5)=148.7·v±5;

[0097] The inner arc water volume of the 6th section is Tn(6)=190·v±5;

[0098] The inner arc water volume of the 7th section is Tn(7)=120·v±5;

[0099] The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

[0100] When the thickness d of the slab is 320 mm, the water volume in each section of the secondary cooling zone satisfies:

[0101] The narrow side water volume of section 1 is Tn(1)=108·v±5;

[0102] The water volume on the broad side of the 5th section is Tk(5)=264.7·v±5;

[0103] The inner arc water volume of the 6th section is Tn(6)=338·v±5;

[0104] The inner arc water volume of the 7th section is Tn(7)=213·v±5;

[0105] The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

[0106] Preferably, when the thickness d of the slab is 220 mm, 1.05 m / min≤v≤1.20 m / min, and the taper of the crystallizer is 1.15%<1.25%;

[0107] When the thickness d of the slab is 320 mm, 0.60 m / min≤v≤0.70 m / min, and the taper of the crystallizer is 1.05%<1.15%.

[0108] Preferably, the continuous casting machine is in the foot roll section, the zero section, and the 11th section of the fan section. # , 12 # The segment does not perform light pressing, and the roller gap shrinkage between the full roller segment and the zero segment is 0.1~0.2mm. # , 12 # The roller gap shrinkage of the section is 0.3~0.5mm; the slab is in the 9th sector # The reduction rate of the segment is 0.7~1.2mm / m. # The reduction rate of the segment is 1.2~1.7mm / m.

[0109] In a preferred embodiment, the basicity of the low-melting-point alkaline protective slag is 1.1-1.5, and its chemical composition includes by mass percentage: F 6-7%, C 6-7%, Fe2O3 2-3%, MnO2 4.5-5.5%, Na2O 6-7%, MgO 1-1.5%, Al2O3 5-6%. In addition to the above components, the remaining components of the low-melting-point alkaline protective slag are unavoidable impurities. The water content of the low-melting-point alkaline protective slag is ≤0.5%.

[0110] The consumption of the low-melting-point alkaline mold slag is 0.45-0.55 kg / t. That is, the low-melting-point alkaline mold slag is added to the crystallizer at a ratio of 0.45-0.55 kg per ton of molten steel.

[0111] Among them, the particle size of the low-melting-point alkaline protective slag is ≤0.3mm, and the mass of the particle size ≤0.2mm accounts for more than 90%.

[0112] Preferably, the covering agent of the tundish is double-layer slag, the so-called double layer refers to a bottom layer and a top layer covering the bottom layer, the bottom layer is alkaline protective slag with a slag basicity of 4.0-8.0, and the top layer is carbonized rice husk.

[0113] Preferably, in the alkaline protective slag used in the bottom layer, the total mass percentage of Al2O3 and MgO is 35-60%, and F is less than 5.0%.

[0114] Furthermore, after the slab continuous casting process is completed, the stacking is carried out for slow cooling, the slow cooling time is 48 hours, and the stacking is removed after the slow cooling to below 200°C.

[0115] (3) Rolling process

[0116] The slab obtained by continuous casting is heated and then rolled in two stages to obtain a steel plate with a thickness of d2 ≥ 40 mm.

[0117] Preferably, the temperature of the soaking section during heating is 1130~1170°C, and the heating time of the soaking section is t1=a1·d1, wherein the heating coefficient a1=0.1~0.3min / mm, and d1 is the slab thickness in mm.

[0118] After heating, the slab is first rough-rolled into an intermediate billet, the final rolling temperature of the rough rolling is 1000~1040℃, and the thickness of the intermediate billet is 1.2~3.0 times the thickness of the final prepared steel plate; then, finish rolling is carried out to obtain a steel plate, the start rolling temperature of the finish rolling is ≤890℃, and the final rolling temperature of the finish rolling is 800~820℃; after rolling, it is cooled to below 400℃, and the cooling rate is 0.5~20℃ / s.

[0119] (4) Heat treatment process

[0120] The rolled steel plate is sequentially quenched and tempered to make the structure of the steel plate a tempered martensite structure.

[0121] Preferably, the quenching temperature is 870~890℃, the quenching time is t2=a2·d2, wherein the heating coefficient a2=1.6~1.8min / mm, d2 is the thickness of the steel plate, in mm; after quenching, it is rapidly cooled to below 200℃, and the cooling rate is ≥20℃ / s. After testing, the structure of the steel plate after quenching is lath martensite structure; then the steel plate is tempered, the tempering temperature is 590~610℃, and the tempering time is t3=a3·d2, wherein the heating coefficient a3=2.8~3.2min / mm. After testing, the structure of the steel plate after tempering is tempered martensite structure.

[0122] The obtained steel plate was subjected to performance testing, wherein the tensile strength was ≥950MPa, the elongation after fracture was ≥15.5%, the yield strength ratio was ≥0.8, the low-temperature impact energy Akv of the core of the steel plate at -40°C was >80J, and the flaw detection of the steel plate met the first-level standard of NB 47013.3.

[0123] Example

[0124] Provided below are six examples of a method for preparing a hydropower steel plate according to one embodiment of the present application.

[0125] (1) Steelmaking process

[0126] Molten steel is refined through the process route of KR pre-desulfurization, converter smelting, LF refining and RH vacuum refining as described above.

[0127] Among them, in the KR pre-desulfurization step, the S content in the molten iron after desulfurization and the molten iron temperature at the end point of desulfurization are shown in Table 1 respectively.

[0128] In the converter smelting step, the slag basicity and T.Fe content in the slag during the first slag pouring are shown in Table 1; the molten steel temperature, P content, C content and slag basicity during the second slag pouring are shown in Table 1.

[0129] Table 1

[0130]

[0131] The chemical composition of the molten steel obtained in the steelmaking process is shown in Table 2 in mass percentage, with the unit being %. In addition to the chemical components in Table 2, the rest are Fe and unavoidable impurities.

[0132] Table 2

[0133]

[0134] (2) Continuous casting process

[0135] Among them, the superheat of the molten steel in the tundish, the melting point of the low-melting-point alkaline protective slag used in the crystallizer, the pulling speed, the taper of the crystallizer, and the inlet temperature of the secondary cooling water are shown in Table 3; the water volume on the narrow side of the first section of the secondary cooling zone, the water volume on the wide side of the first section, the water volume on the wide side of the second section, the water volume on the wide side of the third section, the water volume on the wide side of the fourth section, the water volume on the wide side of the fifth section, the inner arc water volume of the sixth section, and the inner arc water volume of the seventh section are shown in Table 4.

[0136] Table 3

[0137]

[0138] Table 4

[0139]

[0140] Among them, the chemical composition of the low-melting point alkaline protective slag includes, by mass percentage: F 6.5%, C 6.2%, Fe2O32.3%, MnO24.8%, Na2O 6.27%, MgO 1.2%, Al2O35.6%; the basicity of the low-melting point alkaline protective slag is 1.4, the water content is 0.4%, and the consumption is 0.50kg / t.

[0141] In Examples 1 to 6, the slag basicities of the alkaline protective slag used in the bottom layer of the covering agent of the tundish are 4.0, 4.5, 5.0, 6.2, 7.5, and 7.9, respectively; the total mass percentages of Al2O3 and MgO are 35%, 40%, 45%, 55%, 58%, and 38%, respectively; and the mass percentages of F are 2.0%, 2.2%, 2.8%, 3.7%, 4.9%, and 4.4%, respectively.

[0142] According to YB / T 4003-2016, the macrostructural defects of the slabs obtained by continuous casting are graded.

[0143] After testing, the central carbon segregation level of the slab obtained by continuous casting is C0.5, and the central porosity level is 0.5.

[0144] (3) Rolling process

[0145] The soaking zone temperature during heating, the heating time t1 of the soaking zone, the slab thickness d1, the final rolling temperature of rough rolling, the thickness of the intermediate slab, the start rolling temperature of finishing rolling, the final rolling temperature of finishing rolling, and the steel plate thickness d2 are shown in Table 5.

[0146] Table 5

[0147]

[0148] (4) Heat treatment process

[0149] Among them, the quenching temperature is 880°C, and the heating coefficient a2=1.7min / mm; rapid cooling is performed after quenching, and the temperatures to which the steel plates are cooled in Examples 1 to 6 are 200°C, 180°C, 190°C, 170°C, 200°C, and 195°C, respectively.

[0150] The structure of the obtained steel plate was observed by an optical microscope. The structure of the steel plate after quenching was lath martensite structure; the structure of the steel plate after tempering was tempered martensite structure.

[0151] Mechanical properties

[0152] According to the standard test method and definition of GB228.1 "Tensile test of metallic materials Part 1: Test method at room temperature", the yield strength and tensile strength of the obtained steel plate were tested by a tensile testing machine, and then the yield strength ratio was calculated. The yield strength, tensile strength, elongation after fracture and yield strength ratio were shown in Table 6;

[0153] According to GB229 "Charpy pendulum impact test method for metallic materials", the low-temperature impact energy Akv of the core of the steel plate at -40°C was measured and the results are shown in Table 6.

[0154] According to the NB 47013.3-2015 standard, the first-level flaw detection of Examples 1 to 6 all passed.

[0155] Table 6

[0156]

[0157] It should be understood that although this specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each implementation mode may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0158] The detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present invention. They are not intended to limit the scope of protection of the present invention. All equivalent implementation methods or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a steel plate for hydropower, characterized in that: Including the following processes: Steelmaking, the chemical composition of the obtained molten steel includes, by mass percentage, C 0.07-0.11%, Si 0.15-0.25%, Mn 0.90-1.40%, Cr 0.50-0.65%, Ni 1.0-2.0%, Mo 0.50-0.75%, Cu 0.20-0.30%, Nb 0.01-0.04%, V 0.030-0.045%, Ti 0.01-0.02%, Al 0.04-0.05%, P≤0.010%, S≤0.002%, N 0.003-0.004%, O≤0.002%, H≤0.00015%, B 0.0006-0.0016, and the rest is Fe and unavoidable impurities; The molten steel is continuously casted, and the obtained molten steel is poured by a slab continuous casting machine to obtain a slab, the superheat of the molten steel in the tundish is 15-25°C, the crystallizer uses a low-melting-point alkaline protective slag with a melting point of 1120-1180°C, the pulling speed v is 0.60-1.20 m / min, and the taper of the crystallizer is 1.05-1.25%; after the slab exits the crystallizer, it is cooled in a secondary cooling zone, and the secondary cooling zone is divided into 10 sections according to different water volumes, the water volumes of the 1st to 5th sections decrease in sequence, the water volumes of the 6th to 8th sections are greater than the water volumes of the 5th section and gradually decrease, and the water volumes of the 9th to 10th sections are greater than the water volumes of the 8th section; the slab is controlled to pass through the 9th section of the continuous casting machine fan-shaped section. # The central solid fraction fs is 0.3~0.7 during the 10th stage. # The solid phase ratio fs>0.7 in the middle of the slab # , 10 # The 9th stage is pressed lightly. # The reduction rate of the segment>10 # The reduction rate of the segment; the central carbon segregation level of the slab obtained by continuous casting is C0.5 level and below, and the central looseness level is not higher than 0.5 level; Rolling, heating the slab obtained by continuous casting and then rolling it in two stages to obtain steel plates; The steel plate obtained by rolling is subjected to heat treatment in sequence by quenching and tempering, so that the structure of the steel plate is a tempered martensite structure.

2. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The water volume ratio on the wide side of sections 1 to 5 is 2.3:1.7:1.25:1.25:1 respectively.

3. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The water volume ratio between the outer arc and the inner arc in sections 6 to 10 gradually increases.

4. The method for preparing a steel plate for hydropower according to claim 3, characterized in that: The water volume ratio between the outer arc and the inner arc of sections 6 to 10 is 1.3, 1.5, 1.7, 1.85 and 1.85 respectively.

5. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: When the thickness d of the slab is 220 mm, the unit of the water volume T of each section of the secondary cooling zone is L / min and satisfies: The water volume on the broad side of the 5th section is Tk(5)=148.7·v±5; The inner arc water volume of the 6th section is Tn(6)=190·v±5; The inner arc water volume of the 7th section is Tn(7)=120·v±5; The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

6. The method for preparing a steel plate for hydropower according to claim 5, characterized in that: When the thickness d of the slab is 220 mm, the narrow side water volume Tn(1) of the first section is 61·v±5.

7. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: When the thickness d of the slab is 320 mm, the unit of the water volume T of each section of the secondary cooling zone is L / min and satisfies: The water volume on the broad side of the 5th section is Tk(5)=264.7·v±5; The inner arc water volume of the 6th section is Tn(6)=338·v±5; The inner arc water volume of the 7th section is Tn(7)=213·v±5; The inner arc water volumes of sections 8, 9, and 10 are 110±5L / min, 180±5L / min, and 180±5L / min, respectively.

8. The method for preparing a steel plate for hydropower according to claim 7, characterized in that: When the thickness d of the slab is 320 mm, the narrow side water volume Tn(1) of the first section is 108·v±5.

9. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: When the thickness d of the slab is 220 mm, 1.05 m / min≤v≤1.20 m / min, and the taper of the crystallizer is 1.15%<≤1.25%; When the thickness d of the slab is 320 mm, 0.60 m / min≤v≤0.70 m / min, and the taper of the crystallizer is 1.05%<1.15%.

10. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: In the continuous casting process, the basicity of the low-melting-point alkaline protective slag is 1.1-1.5, and its chemical composition, in mass percentage, includes: F 6-7%, C 6-7%, Fe2O3 2-3%, MnO2 4.5-5.5%, Na2O 6-7%, MgO 1-1.5%, and Al2O3 5-6%; the consumption of the low-melting-point alkaline protective slag is 0.45-0.55 kg / t.

11. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The particle size of the low-melting-point alkaline protective slag is ≤0.3 mm, and the mass of particles ≤0.2 mm accounts for more than 90%.

12. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The covering agent of the tundish is double-layer slag, the bottom layer is alkaline protective slag with a slag basicity of 4.0-8.0, and the top layer is carbonized rice husk.

13. The method for preparing a steel plate for hydropower according to claim 12, characterized in that: In the alkaline protective slag, the total mass percentage of Al2O3 and MgO is 35-60%, and F is less than 5.0%.

14. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The continuous casting machine has the 11th section of the foot roll section, the zero section, and the fan section. # , 12 # The segment does not perform light pressing, and the roller gap shrinkage between the full roller segment and the zero segment is 0.1~0.2mm. # , 12 # The roller gap shrinkage of the segment is 0.3~0.5mm; The slab is in the 9th sector # The reduction rate of the segment is 0.7~1.2mm / m. # The reduction rate of the segment is 1.2~1.7mm / m.

15. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: In the rolling process, the temperature of the soaking section during heating is 1130-1170°C, and the heating time of the soaking section is t1=a1·d1, wherein the heating coefficient a1=0.1-0.3min / mm, and d1 is the slab thickness in mm; After heating, the slab is first rough-rolled into an intermediate billet, the final rolling temperature of the rough rolling is 1000~1040℃, and the thickness of the intermediate billet is 1.2~3.0 times the thickness of the final prepared steel plate; then, finish rolling is carried out to obtain a steel plate, the start rolling temperature of the finish rolling is ≤890℃, and the final rolling temperature of the finish rolling is 800~820℃; after rolling, it is cooled to below 400℃, and the cooling rate is 0.5~20℃ / s.

16. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: In the heat treatment process, the quenching temperature is 870~890℃, the quenching time is t2=a2·d2, wherein the heating coefficient a2=1.6~1.8min / mm, d2 is the thickness of the steel plate, and the unit is mm; after quenching, it is quickly cooled to below 200℃, and the cooling rate is ≥20℃ / s. The structure of the steel plate after quenching is lath martensite structure; then the steel plate is tempered, the tempering temperature is 590~610℃, and the tempering time is t3=a3·d2, wherein the heating coefficient a3=2.8~3.2min / mm, and the structure of the steel plate after tempering is tempered martensite structure.

17. The method for preparing a steel plate for hydropower according to claim 1, characterized in that: The steelmaking process includes KR pre-desulfurization, converter smelting, LF refining, and RH vacuum refining steps performed in sequence; In the KR pre-desulfurization step, the S content in the molten iron after desulfurization is ≤0.0015%, and the molten iron temperature at the end point of desulfurization is 1320-1380°C; In the converter smelting step, the desulfurized molten iron, nickel plate, copper plate, ferromolybdenum and scrap steel are added to the converter for smelting, wherein the scrap steel accounts for 15-20%; when 20-30% of the steel is tapped, aluminum particles, ferrosilicon, metallic manganese, low-carbon ferrochrome, ferrovanadium and ferroniobium are added to the molten steel for deoxidation and alloying, when 70-80% of the steel is tapped, lime and fluorite are added to make slag, and calcium aluminate is added to the slag surface after the steel is tapped to synthesize slag, the ladle is bottom blown and stirred during the steel tapping process, and the bottom blowing flow rate is 600-800NL / min, and stirring is continued for 3-5min after the steel is tapped; In the LF refining step, after the molten steel enters the station, it is powered on to increase the temperature, calcium carbide, lime and fluorite are added to adjust the slag, bottom blowing and stirring are turned on for desulfurization, and alloys are added according to the composition of the molten steel entering the station, until the molten steel composition, slag composition and molten steel temperature meet the standards, and then it is transported to the RH furnace for vacuum refining; In the RH vacuum refining step, vacuuming is started after the molten steel enters the station, and vacuuming is continued for 5 minutes after the vacuum degree reaches below 2 mbar, and then ferrotitanium is added, and the net circulation treatment is more than 15 minutes, and the air is broken to tap the steel. After tapping the steel, pure calcium wire is fed into the molten steel to control the Ca content in the molten steel to be 15-25 ppm.

18. The method for preparing a steel plate for hydropower according to claim 17, characterized in that: In the converter step, double slag smelting is adopted, and the first slag pouring is carried out when the molten steel temperature reaches 1340°C and the slag basicity is 2.5-3.0, and the T.Fe content in the slag is 20-30%; the second slag pouring is carried out when the molten steel temperature reaches 1580-1620°C, the P content in the molten steel is ≤0.008%, the C content is 0.03-0.05%, and the slag basicity is 4.0-6.5, and the T.Fe content in the slag is 25-35%; then the steel is tapped.

19. The method for preparing a steel plate for hydropower according to any one of claims 1 to 18, characterized in that: The obtained hydropower steel plate has a thickness of ≥40mm, a tensile strength of ≥950MPa, an elongation after fracture of ≥15.5%, a yield strength ratio of ≥0.8, a low-temperature impact energy Akv of the core of the steel plate at -40°C>80J, and the flaw detection of the steel plate meets the first-level standard of NB 47013.3.

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