800MPa-grade hydroelectric steel with excellent strain aging impact toughness and production method of 800MPa-grade hydroelectric steel

Abstract

According to the 800MPa-grade hydroelectric steel with the excellent strain aging impact toughness and the production method thereof, through mutual cooperation of appropriate amounts of alloy elements, a synergistic effect can be achieved on the comprehensive performance of the strength, the toughness, the post-welding performance, the impact performance and the low-temperature performance of a steel plate, and the strength, the toughness, the post-welding performance, the impact performance and the low-temperature performance of the steel plate are improved; and the normalizing process is added, the structure of the steel plate before quenching is bainite + ferrite + pearlite, the dislocation density is reduced, the content of tempered sorbite is increased after the steel plate is quenched and tempered, and the obdurability of the steel plate is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to an 800MPa-grade hydropower steel with excellent strain aging impact toughness and a production method thereof. BACKGROUND

[0002] In recent years, hydropower stations are developing towards high water head, which puts forward higher requirements for key equipment materials. The core components such as giant water turbine runner, high-pressure water conduit and volute are subjected to high-speed water impact, sand erosion, huge water pressure (such as the water head of the machine set of the Yajiang Hydropower Station under construction is as high as 2000 meters) and complex alternating load for a long time. Ordinary steel materials are prone to wear, fatigue cracking and even catastrophic failure. Only by developing special hydropower steel with ultra-high strength (such as 800MPa), excellent toughness, excellent fatigue resistance and corrosion resistance can the reliable lifeline for key equipment of key projects be built to ensure the safe operation of the power station for a hundred years. SUMMARY

[0003] The purpose of the present application is to overcome the problems in the prior art that the existing 800MPa hydropower steel plate has poor strength and toughness matching, unstable strain aging impact toughness, low performance hit rate, and is difficult to meet the needs of large-scale pumped storage power station steel plate construction. The present application provides an 800MPa hydropower steel plate with excellent strain aging impact toughness and a production method. The quenched and tempered high-strength steel plate has good strength and toughness, strain aging performance and welding performance, can meet the technical requirements of customers, meet the harsh service conditions of large-scale hydropower station steel, has a wide thickness range and wide applicability.

[0004] The technical problem of the present application is solved by the following technical scheme.

[0005] The present application provides an 800MPa hydropower steel with excellent strain aging impact toughness, which comprises the following chemical components by mass percentage: C 0.07-0.09%, Mn 1.10-1.30%, Si 0.05-0.15%, S≤0.005%, P≤0.010%, Nb 0.005-0.02%, V 0.030-0.040%, Ti≤0.005%, Cr 0.40~0.50%, Ni 0.50-0.90%, Mo 0.30-0.50%, Alt 0.050-0.070%, Cu 0.15-0.25%, B 0.0009~0.0015%, N≤0.0040%, Mn / S>240, Alt / (B+N)>15, ∑(Cr+Mo)>0.75%, ∑(Ni+Cu)>0.8%, and the rest is Fe and unavoidable impurities.

[0006] The present invention provides a production method for the above-mentioned 800MPa grade hydroelectric steel, comprising the following smelting processes: steelmaking process, billet heating process, rolling process, cooling process and heat treatment process.

[0007] The present invention has the following beneficial effects: This invention employs a special compositional design, using appropriate amounts of C, Si, and Mn, combined with suitable amounts of grain-refining elements Nb, V, and Ti; adding appropriate amounts of Mo and trace amounts of B to improve the hardenability of thick steel plates; adding a small amount of Cu to improve the corrosion resistance of the steel plates; and Ni to improve the low-temperature toughness of the steel plates. This multi-microalloying element composite design also controls the content of non-metallic inclusions and harmful elements H, O, and N, as well as residual elements As and Sn. This approach helps improve the purity of molten steel, reduce inclusion content, increase hardenability, and after quenching, the steel plate obtains more lath martensite structure, and after tempering, it obtains tempered sorbite + bainite structure, exhibiting excellent comprehensive mechanical properties. The aforementioned hydropower steel possesses good mechanical and weldability properties, meeting the steel requirements for the construction of large-scale hydropower stations. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 This is a microstructure diagram of the surface of the hot-rolled steel plate in Embodiment 2 of the present invention; Figure 2 This is a microstructure diagram of the hot-rolled steel plate matrix in Embodiment 2 of the present invention; Figure 3 This is a microstructure diagram of the surface of the steel plate after normalizing, according to Embodiment 2 of the present invention; Figure 4 This is a microstructure diagram of the steel plate matrix after normalizing in Embodiment 2 of the present invention; Figure 5 This is a microstructure diagram of the steel plate surface after quenching and tempering in Embodiment 2 of the present invention; Figure 6 This is a microstructure diagram of the steel plate matrix after quenching and tempering in Embodiment 2 of the present invention. Detailed Implementation

[0010] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. If specific conditions are not specified in the embodiments, the conventional conditions or the conditions suggested by the manufacturers are adopted. If the manufacturers of the reagents or instruments are not specified, the conventional products that can be purchased in the market are adopted.

[0011] The 800MPa-grade hydroelectric steel with excellent strain aging impact toughness and the production method thereof provided by the embodiments of the present application will be described in detail below.

[0012] In a first aspect, the present application provides a 800MPa hydroelectric steel with excellent strain aging impact toughness, comprising the following chemical components in percentage by mass: C 0.07-0.09%, Mn 1.10-1.30%, Si 0.05-0.15%, S≤0.005%, P≤0.010%, Nb 0.005-0.02%, V 0.030-0.040%, Ti≤0.005%, Cr 0.40~0.50%, Ni 0.50-0.90%, Mo 0.30-0.50%, Alt 0.050-0.070%, Cu 0.15-0.25%, B 0.0009~0.0015%, N≤0.0040%, Mn / S>240, Alt / (B+N)>15, ∑(Cr+Mo)>0.75%, ∑(Ni+Cu)>0.8%, and the rest is Fe and inevitable impurities.

[0013] In the above scheme, the present application can play a synergistic effect on the comprehensive performance of the strength, toughness, post-weld performance, impact performance and low-temperature performance of the steel plate through the mutual cooperation of appropriate amounts of various alloying elements, which is especially beneficial to improving the low-temperature impact toughness of the steel plate after strain aging, increasing the normalizing process, and increasing the content of tempered sorbite of the steel plate after quenching and tempering, thereby improving the strength and toughness of the steel plate.

[0014] Among them, the roles of part of the chemical components and contents are as follows: Carbon has a significant impact on the yield strength, tensile strength and weldability of the steel, but too high carbon content will affect the weldability and toughness of the steel. The carbon content is controlled in the above range, which is beneficial to the comprehensive performance of the strength, weldability and toughness of the steel.

[0015] Manganese is low in cost, and its appropriate amount can increase the toughness, strength and hardness of the steel, improve the hardenability of the steel, and improve the hot working performance of the steel. Too high manganese content is easy to cause segregation in the center of the slab, which is not conducive to the toughness of the center of the steel plate.

[0016] The addition of appropriate amount of niobium is to promote the grain refinement of the steel rolling microstructure, which can improve the strength and toughness. Niobium can effectively refine the microstructure by inhibiting austenite recrystallization during controlled rolling, and precipitate strengthening matrix. Solid solution Nb prevents austenite grain growth during heating, and Nb(C, N) precipitates at high temperature during cooling.

[0017] The addition of appropriate amount of vanadium can refine the grain size of the microstructure, improve the strength and toughness. After controlled rolling, V(C, N) precipitates, which can improve the strength of the steel plate. V(C, N) precipitates at low temperature, which is beneficial to the good strength and toughness of the quenched and tempered steel plate and the performance after stress relief heat treatment.

[0018] Appropriate amount of nickel can reduce the total resistance of dislocation movement in the matrix metal at low temperature, increase the stacking fault energy, inhibit the formation of a large number of dislocations at low temperature, promote the screw dislocation cross slip at low temperature, increase the crack propagation power consumption, and thus improve the toughness, thereby reducing the ductile-brittle transition temperature of the steel. However, nickel is a precious metal, and excessive nickel will increase the cost, so it is controlled within the above range.

[0019] Molybdenum and boron can improve the hardenability of the steel plate, so that more lower bainite structure can be obtained after quenching of the thick steel plate, and good strength and toughness can be obtained after tempering of the steel plate. At the same time, appropriate amount of molybdenum is beneficial to reducing the temper brittleness of the steel plate.

[0020] Appropriate amount of copper not only improves the strength of the steel plate after tempering and after simulated welding and heat treatment, but also improves the corrosion resistance of the steel plate.

[0021] Aluminum can refine the grain size and strengthen the nitrogen element. During strain aging process, free nitrogen is segregated at the grain boundary, which reduces the strain aging sensitivity and improves the low temperature impact toughness of strain aging. Therefore, the content of aluminum is controlled at 0.05~0.07%, and N≤0.0040%.

[0022] Phosphorus and sulfur are harmful elements in steel, which can increase the cold brittleness and hot brittleness of the steel, reduce the plasticity, toughness, corrosion resistance, cold bending property of the steel plate, and increase the probability of crack formation. Therefore, the content of P and S in the steel should be reduced as much as possible.

[0023] Preferably, the quenched and tempered high-strength steel plate comprises the following chemical components in mass percentage: C 0.07-0.09%, Mn 1.10-1.30%, Si 0.05-0.15%, S≤0.005%, P≤0.010%, Nb 0.005-0.02%, V 0.030-0.040%, Ti≤0.005%, Cr 0.40~0.50%, Ni 0.50-0.90%, Mo 0.30-0.50%, Alt 0.050-0.070%, Cu 0.15-0.25%, B 0.0009~0.0015%, N≤0.0040%, Mn / S>240, Alt / (B+N)>15, ∑(Cr+Mo)>0.75%, ∑(Ni+Cu)>0.8%, and the rest is Fe and unavoidable impurities.

[0024] Preferably, the 800MPa-grade hydroelectric steel has a tempered sorbite structure and a bainite structure, and the content of the sorbite structure is 10%~15%, and the rest is bainite structure.

[0025] Preferably, the quenched and tempered high-strength steel plate has a thickness of 20-70mm. The thickness range of the steel plate of the present application is wider and has wide adaptability.

[0026] In various embodiments of the present application, the quenched and tempered high-strength steel plate has the following properties: yield strength Rel≥750MPa, tensile strength Rm is 800-850MPa, elongation A≥16%; average value of transverse impact energy at 1 / 4 thickness at-40℃≥200J, average value of transverse impact energy at core at-40℃ is greater than 150J, and average value of transverse impact energy at-40℃ of the sample after 5% strain aging is greater than 160J. Among them, the average value of transverse impact energy at core at-40℃ is greater than 150J, and the average value of transverse impact energy at-40℃ of the sample after 5% strain aging is greater than 160J. The average value of transverse impact energy of the present application refers to the average value of transverse V-shaped impact, and the average value refers to the average value of 3 test samples.

[0027] The steel plate of the present application belongs to a low welding crack sensitive steel grade, has low carbon equivalent, good comprehensive mechanical properties and welding performance, and can be used for large-scale hydropower station construction. The molten steel has high purity, and the internal structure of the steel plate is dense. The steel plate of the present application meets NB / T47013.3-2023, T1 grade qualified; yield strength Rel≥750MPa, tensile strength Rm is 800-850MPa, elongation A≥16%; average value of transverse impact energy at 1 / 4 thickness at-40℃≥200J, average value of transverse impact energy at core at-40℃ is greater than 150J, and average value of transverse impact energy at-40℃ of the sample after 5% strain aging is greater than 160J. The steel plate has good welding performance, and meets the demand of steel for large-scale hydropower station construction.

[0028] In various embodiments of the present application, the welding performance of the tempered high-strength steel plate satisfies: Pcm≤0.25%, Psr≤-1.25%, Ceq≤0.52%. Wherein: Ceq = C+Si / 24+Mn / 6+Cr / 5+Ni / 40+Mo / 4+V / 14(%) Psr=Cr+Cu+2Mo+5Ti+7Nb+10V-2(%) Pcm = C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+Mo / 15+V / 10+5B(%)。

[0029] In a second aspect, the present application provides a production method of the 800MPa hydroelectric steel with excellent strain aging impact toughness, comprising steelmaking, billet heating, rolling, cooling and heat treatment.

[0030] In some preferred embodiments, the steelmaking process comprises: taking top and bottom combined blowing converter production, tapping temperature 1580-1600℃, taking aluminum block deoxidation, large argon stirring to ensure Als≥0.015%; taking low nitrogen metal manganese, aluminum block further deoxidation and meeting the alloy content requirements of the steel grade in the refining process; adding fluorite ball slagging agent once to the position according to the requirement of 10-12.5kg / t, ensuring the thickness of the slag layer on the surface of the molten steel to be 10-12mm, at the same time adjusting the argon opening, the flow rate being higher than that of soft blowing state by 200-400NL / min, reducing nitrogen absorption. The vacuum degree is ≤80Pa, the vacuum time is ≥22min, the pure degassing time is ≥15min, the molten steel surface is not blown open, the soft blowing time target is ≥10 minutes, and the calcium line amount is controlled to be 80-100m. The superheat degree is 15-25℃, the withdrawal speed is 0.95-1.05m / min, and the slab into pit temperature is greater than 750℃, and the slow cooling is more than 48 hours.

[0031] In some preferred embodiments, the process of billet heating comprises: controlling the billet preheating section temperature to be 700-900℃, the first heating section temperature to be 1100-1200℃, the second heating section temperature to be 1200-1250℃, the soaking section temperature to be 1200-1230℃, and the furnace time to be (1.0-1.1)×H minutes, H being the slab thickness, unit: mm.

[0032] In some preferred embodiments, the process of rolling comprises: rough rolling opening temperature greater than 1000℃, adopting high temperature low speed large reduction rolling, roll speed ≤2.5m / s, pass reduction rate greater than 12%; finishing rolling opening temperature ≤880℃, and controlling the cumulative deformation rate below 880℃ to be 55-60%; and final rolling temperature being 810-830℃.

[0033] In some preferred embodiments, the cooling is ACC cooling, the cooling rate is controlled to be greater than 10℃ / s, and the re-drawing temperature is 660-700℃.

[0034] In some preferred embodiments, the heat treatment comprises normalizing + quenching + tempering.

[0035] More preferably, the normalizing process comprises: 870-890℃, a furnace time of (2.5-3.0)×H minutes, H is the thickness of the steel plate in mm; air cooling is used after the furnace to cool to room temperature.

[0036] The quenching process comprises: controlling the holding temperature to be 880-910℃, a furnace time of (2.5-3.8)×H minutes, H is the thickness of the steel plate in mm; water cooling is used after the furnace to cool to room temperature. The roll gap value of the quenching machine is set to H-(2-3) mm, and the roll gap pressure is used to reduce the water return on the surface of the steel plate during quenching, and to ensure that the water temperature at the tail of the steel plate is greater than 870℃. During quenching, the pressure in the high-pressure section is greater than 0.85MPa, and the water quantity is greater than 5000m 3 / h.

[0037] More preferably, the tempering process comprises: for a steel plate with a thickness of ≤40mm, controlling the holding temperature to be 630-650℃, a furnace time of (1.5H+40) minutes, H is the thickness of the steel plate in mm, and air cooling is used after the furnace; for a steel plate with a thickness of >40mm, controlling the holding temperature to be 610-630℃, a furnace time of (1.5H+60) minutes, H is the thickness of the steel plate in mm, and air cooling is used after the furnace.

[0038] The production method of the present application preferably controls the heating process, deformation amount and temperature of the metal during rolling to combine hot plastic deformation with solid phase transformation, and the reasonable pass reduction amount is allowed to penetrate the entire cross section of the large-thickness steel plate to obtain fine and uniform grain structure, so that the steel has more excellent comprehensive mechanical properties. The normalizing + quenching and tempering heat treatment can refine the grain and uniformize the structure, which is more beneficial to the excellent strength and toughness combination of the steel plate, and especially the good low-temperature toughness after strain aging.

[0039] The present application is described in detail below in combination with specific examples.

[0040] Example 1 An 800MPa hydroelectric steel plate with excellent strain aging impact toughness comprises the following chemical components by mass percentage: see Table 1 below; the rest is Fe and unavoidable impurities.

[0041] The production method of the 800MPa hydroelectric steel plate comprises the following processes: 1) Steelmaking process: when the weight of molten steel is 1 / 4, add 300 kg of lime into the steel stream, use a stopper (ball) + slide plate to stop slag, and the thickness of the slag layer is ≤50 mm; the tapping temperature is 1585℃, and the tapping adopts aluminum block deoxidation and large argon stirring to ensure Als≥0.015%; the refining process adopts low-nitrogen metal manganese and aluminum block for further deoxidation and meets the alloy content requirements of the steel grade; the fluorite ball slagging agent is added at one time according to the requirement of 11 kg / t, the thickness of the slag layer on the surface of the molten steel is ensured to be 10 mm, and the argon opening is adjusted, which is 230 NL / min higher than that in the soft blowing state to reduce nitrogen absorption. The vacuum degree is 76 Pa, the vacuum time is 23 min, the pure degassing time is 16 min, the molten steel surface is not blown open, the soft blowing time target is 12 minutes, and the calcium wire amount is controlled to be 86 m. The superheat degree is 18℃, the casting speed is 1.00 m / min, the slab into pit temperature is greater than 750℃, and the slow cooling is more than 48 hours.

[0042] 2) Billet heating process: the preheating section temperature of 300mm thick billet is 750-850℃, the first heating section is 1100-1200℃, the second heating section is 1200-1250℃, the soaking temperature is 1200-1230℃, the furnace time is 310 minutes, and the rolling is carried out after reaching the temperature; 3) Rolling process: two-stage rolling is adopted, the rough rolling opening temperature is 1060℃, high temperature, low speed and large reduction rolling is adopted, the roll speed is ≤2.5m / s, and the pass reduction rate is greater than 12%; the finish rolling opening temperature is 876℃, the steel plate thickness is 50mm, the finish rolling temperature is 825℃, and the rolling is carried out to 20mm thick steel plate; ACC cooling is adopted after rolling, the cooling speed is greater than 10℃ / s, and the re-red temperature is controlled to be 685℃; 4) Heat treatment process: the normalizing temperature is 870~890℃, the furnace time is 50 minutes, and the furnace is discharged after air cooling; the quenching temperature is 890±10℃, the furnace time is 60 minutes, the steel plate is discharged and water cooled to room temperature; during quenching, the roll gap is set to 2.0mm, the high pressure section pressure is 0.86MPa, and the water quantity is 5200m 3 / h; the tempering is carried out in the roller bottom type open flame combustion heat treatment furnace, the tempering temperature is 635℃, the furnace time is 70 minutes, and the furnace is discharged and air cooled.

[0043] After detection: the hot rolled steel plate organization is: the surface is tempered martensite, the matrix is sorbite + bainite. After normalizing, the surface organization of the steel plate is bainite + ferrite + pearlite, and the matrix organization is bainite + ferrite + pearlite, and the full thickness organization is uniform; the heat treated and quenched and tempered steel plate organization is tempered sorbite + bainite organization, and the content of sorbite organization is 12%.

[0044] Example 2 An 800MPa hydroelectric steel plate with excellent strain aging impact toughness, comprising the following mass percentages of chemical components: see Table 1 below; the rest is Fe and unavoidable impurities.

[0045] The production method of the 800MPa hydroelectric steel plate includes the following processes: 1) Steelmaking process: When 1 / 4 of the molten steel is tapped, 310 kg of lime is added with the steel stream. Slag is blocked using a slag-blocking plug (ball) and a sliding plate, ensuring a slag layer thickness of ≤50 mm. The tapping temperature is 1590℃. Deoxidation is achieved using aluminum blocks and large-volume argon agitation during tapping to ensure Als ≥0.015%. During refining, low-nitrogen manganese and aluminum blocks are used for further deoxidation and to meet the alloy content requirements of the steel grade. Fluorite ball slag-forming agent is added in one go at a rate of 12 kg / t to ensure a slag layer thickness of 11 mm on the molten steel surface. Simultaneously, the argon gas flow rate is adjusted to be 230 NL / min higher than during soft blowing to reduce nitrogen absorption. The vacuum degree is 76 Pa, the vacuum time is 23 min, the pure degassing time is 17 min, and the molten steel surface is not blown open. The target soft blowing time is 12 minutes, and the calcium wire content is controlled at 89 m. The superheat is 15℃, the drawing speed is 1.00 m / min, the slab entry temperature is greater than 750℃, and slow cooling is performed for at least 48 hours.

[0046] 2) Billet heating process: The preheating temperature of the 300mm thick billet is 750-850℃, the first heating section is 1100-1200℃, the second heating section is 1200-1250℃, the soaking temperature is 1200-1230℃, the furnace time is 316 minutes, and rolling is carried out after reaching the temperature. 3) Rolling process: Two-stage rolling is adopted. The roughing rolling starts at 1050℃ and adopts high temperature, low speed and large reduction rolling with a roll speed ≤2.5m / s and a reduction rate of more than 12% per pass. The finishing rolling starts at 860℃ with a steel plate thickness of 100mm and finishes at 820℃ to roll a steel plate with a thickness of 40mm. After rolling, ACC cooling is adopted with a cooling rate of more than 10℃ / s and the reddening temperature is controlled at 688℃. 4) Heat treatment process: Normalizing temperature 870~890℃, furnace time 105 minutes, air cooling after removal from the furnace; Quenching holding temperature 890±10℃, furnace time 120 minutes, water cooling after removal from the furnace, cooling to room temperature; during quenching, the roll gap is set at 2.5mm, the high-pressure section pressure is 0.86MPa, and the water flow rate is 5200m³. 3 / h; Tempering is carried out in a roller hearth open flame heat treatment furnace at a tempering temperature of 635℃ for 100 minutes, followed by air cooling after removal from the furnace.

[0047] Testing revealed that the microstructure of the hot-rolled steel plate consisted of tempered martensite on the surface and sorbite and bainite in the matrix. After normalizing, the surface microstructure consisted of bainite, ferrite, and pearlite, while the matrix microstructure consisted of bainite, ferrite, and pearlite, with uniform microstructure throughout the thickness. The microstructure of the heat-treated and tempered steel plate consisted of tempered sorbite and bainite, with the sorbite content at 13%.

[0048] Example 3 A strain-aged impact toughness of 800MPa hydroelectric steel plate comprises the following chemical composition by mass percentage: see Table 1 below; the remainder is Fe and unavoidable impurities.

[0049] The production method of the 800MPa hydroelectric steel plate includes the following processes: 1) Steelmaking process: When 1 / 4 of the molten steel is tapped, 310 kg of lime is added with the steel stream. Slag is blocked using a slag-blocking plug (ball) and a sliding plate, ensuring a slag layer thickness of ≤50 mm. The tapping temperature is 1595℃. Deoxidation is achieved using aluminum blocks and large-volume argon agitation during tapping to ensure Als ≥ 0.015%. During refining, low-nitrogen manganese and aluminum blocks are used for further deoxidation and to meet the alloy content requirements of the steel grade. Fluorite ball slag-forming agent is added in one go at a rate of 12 kg / t to ensure a slag layer thickness of 12 mm on the molten steel surface. Simultaneously, the argon gas flow rate is adjusted to be 230 NL / min higher than the soft-blowing flow rate to reduce nitrogen absorption. The vacuum degree is 76 Pa, the vacuum time is 23 min, the pure degassing time is 18 min, the molten steel surface is not blown open, the target soft-blowing time is 15 minutes, and the calcium wire content is controlled at 89 m. The superheat is 15℃, the casting speed is 1.00 m / min, the slab entry temperature is greater than 750℃, and slow cooling is performed for at least 48 hours.

[0050] 2) Billet heating process: The preheating temperature of the 300mm thick billet is 750-850℃, the first heating section is 1100-1200℃, the second heating section is 1200-1250℃, the soaking temperature is 1200-1230℃, the furnace time is 325 minutes, and rolling is carried out after reaching the temperature. 3) Rolling process: Two-stage rolling is adopted. The roughing rolling starts at 1050℃ and adopts high temperature, low speed and large reduction rolling with a roll speed ≤2.5m / s and a reduction rate of more than 12% per pass. The finishing rolling starts at 860℃ with a steel plate thickness of 160mm and finishes at 810℃ to roll a steel plate with a thickness of 70mm. After rolling, ACC cooling is adopted with a cooling rate of more than 10℃ / s and the reddening temperature is controlled at 695℃. 4) Heat treatment process: Normalizing temperature 870~890℃, furnace time 190 minutes, air cooling after removal from the furnace; Quenching holding temperature 890±10℃, furnace time 310 minutes, water cooling after removal from the furnace, cooling to room temperature; during quenching, the roll gap is set at 2.5mm, the high-pressure section pressure is 0.86MPa, and the water flow rate is 5200m³. 3 / h; Tempering is carried out in a roller hearth open flame heat treatment furnace at a tempering temperature of 619℃ for 165 minutes, followed by air cooling after removal from the furnace.

[0051] Testing revealed that the microstructure of the hot-rolled steel plate consisted of tempered martensite on the surface and sorbite and bainite in the matrix. After normalizing, the surface microstructure consisted of bainite, ferrite, and pearlite, while the matrix microstructure consisted of bainite, ferrite, and pearlite, with uniform microstructure throughout the thickness. The microstructure of the heat-treated and tempered steel plate consisted of tempered sorbite and bainite, with the sorbite content at 10.5%.

[0052] Comparative Example 1 Similar to the steps in Example 1, except that the heat treatment process does not include normalizing.

[0053] Testing revealed that the microstructure of the hot-rolled steel plate consisted of tempered martensite on the surface and a matrix of sorbite and bainite. The microstructure of the heat-treated and tempered steel plate was tempered sorbite and bainite, with the sorbite content at 6%.

[0054] Comparative Example 2 The steps are similar to those in Example 1, except that the tempering temperature of the heat treatment process is 620°C.

[0055] Testing revealed that the microstructure of the hot-rolled steel plate consisted of tempered martensite on the surface and sorbite and bainite in the matrix. After normalizing, the surface microstructure consisted of bainite, ferrite, and pearlite, while the matrix microstructure consisted of bainite, ferrite, and pearlite, with uniform microstructure throughout the thickness. The microstructure of the heat-treated and tempered steel plate consisted of tempered sorbite and bainite, with the sorbite content at 7%.

[0056] Comparative Example 3 The steps are similar to those in Example 1, except that the chemical composition of the steel plate is different from that in Example 1, as shown in Table 1.

[0057] Testing revealed that the microstructure of the hot-rolled steel plate consisted of tempered martensite on the surface and sorbite and bainite in the matrix. After normalizing, the surface microstructure consisted of bainite, ferrite, and pearlite, while the matrix microstructure consisted of bainite, ferrite, and pearlite, with uniform microstructure throughout the thickness. The microstructure of the heat-treated and tempered steel plate consisted of tempered sorbite and bainite, with the sorbite content at 9%.

[0058] Table 1. Chemical composition (wt%) of steel plates in each embodiment and comparative example.

[0059] Table 1 (continued)

[0060] Test Results The properties of the steel plates obtained in the above embodiments and comparative examples were tested according to GB / T228.1 and GB / T229, and are shown in Table 2 below. A 5% strain aging test was also conducted according to GB / T4160, and the results are shown in Table 3.

[0061] Table 2. Properties of the steel plates obtained in the examples and comparative examples.

[0062] Table 3. Properties of steel plates obtained after treatment with 5% strain and holding at 250℃ for 1 hour for samples from Examples 2-3 and Comparative Examples 1-3.

[0063] As can be seen from Tables 1-3 above, the steel plate prepared using the scheme provided in the embodiments of the present invention has excellent strain-aged low-temperature impact toughness, as well as plasticity and toughness. It also has a wide thickness range and broad adaptability, meeting the steel requirements for large-scale hydropower station construction. The heat treatment process of the steel plate in Comparative Example 1 did not include normalizing. The results showed that the product's microstructure was uneven, especially the surface grains were not fine enough. Under the same quenching and tempering process, the steel plate had a low sorbite content and low impact toughness, especially with even lower strain-aged impact. The tempering temperature of the steel plate in Comparative Example 2 was 620℃, which is 15℃ lower than the tempering temperature of the steel plate in Example 1. The results showed that the low tempering temperature resulted in insufficient tempering, inadequate recovery of lath martensite and bainite, leading to high strength but low low-temperature toughness. The chemical composition of Comparative Example 3, with Nb, Ni, Mo, and B elements exceeding the limits, showed that after the same heat treatment process as in Example 1, the 40mm thick steel plate had low strength and reduced low-temperature impact and aging impact at the core. The reason for this is speculated to be that the content of niobium, nickel, and molybdenum elements was low. Even with an increase in the boron content, the hardenability of the steel plate was poor. As the thickness of the steel plate increased, the martensitic ability of the obtained lath decreased, the cross-sectional structure was uneven, and the precipitation of boron-containing precipitates at the grain boundaries led to a decrease in toughness.

[0064] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A type of 800MPa grade hydroelectric steel with excellent strain-aged impact toughness, characterized in that, The chemical composition includes the following percentages by mass: C 0.07-0.09%, Mn 1.10-1.30%, Si 0.05-0.15%, S≤0.005%, P≤0.010%, Nb0.005-0.02%, V 0.030-0.040%, Ti≤0.005%, Cr 0.40-0.50%, Ni 0.50-0.90%, Mo 0.30-0.50%, Alt 0.050-0.070%, Cu 0.15-0.25%, B 0.0009-0.0015%, N≤0.0040%, Mn / S>240, Alt / (B+N)>15, ∑(Cr+Mo)>0.75%, ∑(Ni+Cu)>0.8%, with the remainder being Fe and unavoidable impurities.

2. The 800MPa grade hydroelectric steel according to claim 1, characterized in that, The mechanical properties of the 800MPa grade hydroelectric steel meet the following requirements: yield strength Rel≥750MPa, tensile strength Rm is 800-850MPa, elongation A≥16%; average transverse impact energy at -40℃ at 1 / 4 of the thickness is ≥200J, average transverse impact energy at -40℃ at the core is greater than 150J, and average transverse impact energy at -40℃ after 5% strain aging is greater than 160J.

3. The 800MPa grade hydroelectric steel according to claim 1, characterized in that, The welding performance of the 800MPa grade hydroelectric steel meets the following requirements: Pcm≤0.25%, Psr≤-1.25%, Ceq≤0.52%.

4. The 800MPa grade hydroelectric steel according to claim 1, characterized in that, The 800MPa grade hydroelectric steel has tempered sorbite and bainite structures, and the ferrite content is 10%~15%. The thickness of the 800MPa grade hydroelectric steel is 20-70mm.

5. A method for producing 800MPa grade hydroelectric steel according to any one of claims 1-4, characterized in that, It includes the following smelting processes: steelmaking, billet heating, rolling, cooling, and heat treatment.

6. The method for producing 800MPa grade hydroelectric steel according to claim 5, characterized in that, The steelmaking process includes: during converter smelting, a top-and-bottom blown converter is used for production, with a tapping temperature of 1580~1600℃. Aluminum ingots are used for deoxidation during tapping, and large-scale argon agitation is employed to ensure Als ≥ 0.015%. During LF refining, low-nitrogen metallic manganese and aluminum ingots are used for further deoxidation and to meet the alloy content requirements of the steel grade. Fluorite ball slagging agent is added in a single application at a rate of 10~12.5 kg / t to ensure a slag layer thickness of 10~12 mm on the molten steel surface, while simultaneously adjusting the argon gas flow rate. The flow rate in soft blowing mode is 200~400 NL / min higher to reduce nitrogen absorption. During VD vacuum refining, the vacuum degree is controlled to ≤80Pa, the vacuum time is ≥22min, the pure degassing time is ≥15min, the molten steel surface is not blown open, the soft blowing time is ≥10 minutes, the calcium wire is controlled at 80~100m, the superheat is 15~25℃, the casting speed is 0.95~1.05m / min, the slab entry temperature is greater than 750℃, and slow cooling is carried out for more than 48 hours.

7. The method for producing 800MPa grade hydroelectric steel according to claim 5, characterized in that, The billet heating process includes: controlling the billet preheating section temperature to 700-900℃, the first heating section temperature to 1100-1200℃, the second heating section temperature to 1200-1250℃, the soaking section temperature to 1200-1230℃, and the furnace time to (1.0-1.1)×H minutes, where H is the billet thickness in mm.

8. The method for producing 800MPa grade hydroelectric steel according to claim 5, characterized in that, The rolling process includes: controlling the roughing rolling temperature to be greater than 1000℃, using high-temperature low-speed high-reduction rolling, with a roll speed ≤2.5m / s and a pass reduction rate greater than 12%; the finishing rolling temperature to be ≤880℃, and controlling the cumulative deformation rate below 880℃ to be 55~60%; and the final rolling temperature to be 810-830℃.

9. The method for producing 800MPa grade hydroelectric steel according to claim 5, characterized in that, The cooling process includes: using ACC cooling, controlling the cooling rate to be greater than 10℃ / s, and controlling the reddening temperature to be 660-700℃.

10. The method for producing 800MPa grade hydroelectric steel according to claim 5, characterized in that, The heat treatment process includes normalizing, quenching, and tempering. Preferably, the normalizing process includes: controlling the holding temperature to 870~890℃, the furnace time to be (2.5~3.0)×H minutes, where H is the steel plate thickness in mm; and air cooling to room temperature after exiting the furnace. Preferably, the quenching process includes: controlling the holding temperature to 880-910℃, the furnace time to be (2.5-3.8)×H minutes, where H is the steel plate thickness in mm; water cooling after removal from the furnace to room temperature; setting the quenching machine roller gap value to H-(2~3) mm; reducing water backflow on the steel plate surface during quenching by roller gap pressure; ensuring the water entry temperature at the tail end of the steel plate is greater than 870℃; and ensuring the high-pressure section pressure is greater than 0.85MPa and the water volume is greater than 5000m³ during quenching. 3 / h; Preferably, the tempering process includes: for steel plates with a thickness ≤ 40 mm, controlling the holding temperature to 630-650℃, the furnace time to (1.5H+40) minutes, where H is the steel plate thickness in mm, and air cooling after removal from the furnace; for steel plates with a thickness greater than 40 mm, controlling the holding temperature to 610-630℃, the furnace time to (1.5H+60) minutes, where H is the steel plate thickness in mm, and air cooling after removal from the furnace.

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