600MPa-grade low-carbon-equivalent high-toughness hydroelectric steel and manufacturing method thereof

Abstract

The invention relates to 600MPa-grade hydroelectric steel with low carbon equivalent and high strength and toughness and a manufacturing method thereof. The steel plate comprises the following chemical components: 0.03-0.05% of C; 0.15% to 0.35% of Si; mn: 1.10% to 1.50%; p is less than or equal to 0.01%; s < = 0.002%; 0.02% to 0.15% of Cr; 0.15% to 0.40% of Mo; 0.05% to 0.30% of Ni; b: 0.0001% to 0.0005%; ti: 0.005% to 0.015%; 0.01% to 0.035% of Nb; 0.02% to 0.035% of Al; less than or equal to 0.025% of Cu; and the balance Fe and impurities. Through chemical component proportioning, the key element B exists in the form of effective B, so that the hardenability of the steel is greatly improved; the production process of hot rolling, sectional quenching and thermal refining is adopted, grain refinement of the steel plate is facilitated, and meanwhile the technical problems that the steel plate prepared through off-line quenching is low in strength and the steel plate prepared through conventional tempering is poor in plate shape are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel production for hydropower, and particularly relates to a 600MPa grade low carbon equivalent high strength and toughness steel for hydropower and a manufacturing method thereof. Background Art

[0002] Hydropower generation has become the first choice for countries around the world to obtain clean energy due to its advantages such as high efficiency, cleanliness, and sustainability. At present, the development of hydropower in China has advanced by leaps and bounds. Especially in the past decade, the number of large-scale conventional hydropower stations and large-capacity pumped storage power stations that have been built and are under construction is considerable. The service environment of steel for hydropower is relatively harsh, requiring it to have a good strength-toughness ratio and good welding performance, and it belongs to a "double high" product with high technology and high added value in steel materials.

[0003] With the development of the hydropower industry and downstream enterprises, more stringent requirements are put forward for 600MPa grade steel for hydropower, that is, while the strength level and the content of other alloying elements remain unchanged, it is required to significantly reduce the carbon equivalent of the steel, and the carbon equivalent is the most critical factor to ensure the strength of the steel plate. As a new type of steel, the existing technology has not fully studied its preparation process for such low carbon equivalent 600MPa grade steel for hydropower.

[0004] Chinese patent application with publication number CN115094298A discloses "a production method of 600MPa grade low carbon equivalent steel for hydropower". The mass percentage of chemical components of the steel is C = 0.05% - 0.07%, Si = 0.15% - 0.30%, Mn = 1.50% - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Alt = 0.020% - 0.050%, Nb = 0.020% - 0.030%, V = 0.040% - 0.050%, Ti = 0.012% - 0.020%, Ni = 0.20% - 0.30%, Cr = 0.10% - 0.15%, Mo = 0.30% - 0.40%, Cu ≤ 0.05%, and the rest are Fe and inevitable residual elements, CE ≤ 0.45%; its process steps are: converter smelting → LF refining → VD vacuum treatment → continuous casting → slab heating → rolling → heat treatment. By adding Nb, V, Ti, Ni, Cr, Mo and adopting the LF+VD process to ensure the cleanliness of the steel quality, it produces steel plates with a thickness of 30 - 60mm, a yield strength ≥ 550MPa, a tensile strength of 640 - 830Mpa, an elongation rate ≥ 17%, and impact values at -20°C and strain aging impact ≥ 100J. It adopts a production process of "hot rolling + off-line quenching and tempering", and the quenching temperature is 910 - 930°C. Although the mechanical properties of the steel plate better meet the relevant standard requirements, off-line quenching and tempering result in the tensile properties of the steel plate approaching the lower limit of the standard, with a small performance margin and a slow production rhythm.

[0005] Chinese Patent Application with Publication No. CN104962837A discloses a "Steel Plate for Hydroelectric Penstock with 600MPa Grade and Low Crack Sensitivity and Its Production Method". The weight percentages of the chemical components of the steel plate are as follows: C: 0.070 - 0.090%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P ≤ 0.012%, S ≤ 0.005%, Cu ≤ 0.06%, Ni: 0.15 - 0.25%, Cr ≤ 0.06%, Mo: 0.10 - 0.25%, V: 0.05 - 0.06%, Nb: 0.015 - 0.025%, Ti: 0.007 - 0.020%, N ≤ 0.008%, Als: 0.015 - 0.040%, As ≤ 0.010%, and the rest is Fe and inevitable inclusions. The steel plate is produced through hot metal pretreatment, converter smelting, soft blowing in the argon station, deoxidation alloying, LF furnace refining, RH furnace refining, argon-sealed protection slab continuous casting, slab heating, high-pressure water descaling, two-stage controlled rolling, ACC cooling, offline intercritical quenching and tempering. The mechanical properties of the 600MPa grade steel plate produced can meet the relevant standards, and the carbon equivalent is 0.42. However, its production steps are relatively complex, involving hot rolling, ACC cooling after hot rolling, offline intercritical quenching and tempering, and the economy is poor.

[0006] Chinese Patent Application with Publication No. CN103484766A discloses "A Quenched and Tempered High Strength Steel Plate with Low Welding Crack Sensitivity for 600MPa Grade Hydroelectric Engineering and Its Preparation Method". The steel plate is prepared from the following components by mass percentage: C 0.07 - 0.09%, Si 0.20 - 0.40%, Mn 1.50 - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Mo 0.12 - 0.25%, Nb 0.027 - 0.050%, Ni 0.15 - 0.30%, V 0.035 - 0.060%, Ti 0.010 - 0.020%, Alt 0.020 - 0.050%, and the rest is Fe and inevitable impurities, and the carbon equivalent CE ≤ 0.42% and the welding crack sensitivity index Pcm ≤ 0.20% are controlled. It adopts the production process of "online quenching + offline tempering". All properties of the steel plate meet the technical requirements. However, for such thin-specification high-strength and tough steel plates, shape control is a major problem and hot and cold straightening is required.

[0007] The Chinese patent application with the publication number of CN103103455A discloses "A Steel Plate for 600MPa Hydropower Penstock and Its Manufacturing Method". The chemical composition of the steel plate by weight percentage is: C: 0.06 - 0.09%, Si: 0.20 - 0.40%, Mn: 1.4 - 1.6%, P: ≤0.015%, S: ≤0.005%, Nb: 0.02 - 0.04%, Ni: 0.20 - 0.40%, Cr: 0.10 - 0.30%, Mo: 0.10 - 0.30%, V: 0.03 - 0.05%, Ti: 0.01 - 0.02%, and the balance is Fe and inevitable impurities. The manufacturing method is: using hot metal and scrap steel after desulfurization pretreatment as raw materials, and obtaining the steel plate for 600MPa hydropower penstock through processes such as converter smelting, LF refining, RH vacuum treatment, continuous casting, heating, rolling and cooling, and quenching and tempering heat treatment. The mechanical properties of the produced steel plate are relatively excellent, but its production process of "offline quenching + tempering" with a tempering temperature of 600 - 630°C is difficult to meet the requirements of high tempering temperature in the current technical conditions, and the production rhythm is slow and the economic efficiency is low.

[0008] The Chinese patent application with the publication number of CN108823489A discloses "A Steel Plate for 600MPa Hydropower and Its Production Method". The chemical composition and mass percentage of the steel plate are: C: 0.04 - 0.07%, Si: 0.20 - 0.30%, Mn: 1.30 - 1.60%, P ≤ 0.012%, S ≤ 0.002%, Als: 0.015 - 0.03%, Ti: 0.01 - 0.02%, Nb: 0.02 - 0.03%, V: 0.05 - 0.06%, Cr: 0.20 - 0.30%, Mo: 0.15 - 0.25%, and the balance is iron and inevitable impurities; the production method includes hot metal pretreatment, converter smelting, refining, continuous casting, rolling, cooling, and tempering processes. The mechanical property indexes of its steel plate are qualified, and the production process of "hot rolling + online quenching + offline tempering" has the problem that the plate shape is difficult to control.

[0009] The Chinese patent application with the publication number of CN106222557A discloses "A high-efficiency and low-cost 610MPa hydroelectric steel and its production method". The chemical composition of the hydroelectric steel in mass percentage is as follows: C: 0.07 - 0.09%, Si: 0.15 - 0.35%, Mn: 1.40 - 1.60%, P ≤ 0.15%, S ≤ 0.08%, Nb: 0.015 - 0.035%, V: 0.030 - 0.050%, Ti: 0.008 - 0.030%, Ni: 0.10 - 0.30%, Mo: 0.10 - 0.30%, and the rest is Fe and unavoidable impurities. The process route is: KR hot metal pretreatment - converter smelting - LF, RH refining - continuous casting - slow cooling - heating of continuous casting billet - rolling - cooling - straightening - finishing - tempering. It adopts the production process of "hot rolling + online quenching + offline tempering". Each index of the steel plate meets the standard requirements, but its lowest carbon element content is 0.07%, and it has relatively high requirements for straightening equipment.

[0010] The Chinese patent application with the publication number of CN113444975A discloses "A weldable preheating-free low-carbon equivalent 600MPa grade high-strength hydroelectric steel and its manufacturing method". The composition of this hydroelectric steel by mass percentage includes: C: 0.06 - 0.10, Si: 0.05 - 0.20, Mn: 1.40 - 1.70, P: ≤ 0.013, S: ≤ 0.008, Nb: 0.02 - 0.06, Cr: 0.05 - 0.15, Mo: 0.10 - 0.30, Ni: 0.10 - 0.30, V: 0.02 - 0.04, Cu: 0.05 - 0.2, Ti: 0.005 - 0.02, B: 0.0005 - 0.0013, Mg 0.0010 - 0.0040, Al: 0.003 - 0.008, and the rest is Fe and unavoidable impurities, and its carbon equivalent Ceq ≤ 0.42%; each mechanical property index of the steel plate can meet the requirements of relevant standards, and its welding performance is relatively excellent, but it adopts the production process of "hot rolling + offline hardening and tempering", and there are also problems of slow production rhythm and high cost.

[0011] To solve the above problems, the present invention conducts systematic research on the preparation process of low-carbon equivalent 600MPa grade hydroelectric steel, and regulates its microstructure and comprehensive properties through process improvement, and develops a low-carbon equivalent 600MPa grade high-strength and tough hydroelectric steel plate, filling the gap of such steel plates in China, which is beneficial to promoting the independent innovation level in China and has great scientific research and economic value. Summary of the Invention

[0012] The present invention provides a 600 MPa - grade low - carbon - equivalent high - strength and tough hydroelectric steel and its manufacturing method. The thickness of the produced steel plate is 18 - 95 mm, and the lowest carbon equivalent is 0.27%. Through chemical composition ratio, the key element B exists in the form of effective B, greatly improving the hardenability of the 600 MPa - grade hydroelectric steel. The production process of "hot rolling + sectional quenching + tempering treatment" can greatly reduce the grain size before quenching of the steel plate, and thus is conducive to grain refinement of the finished steel plate. This method also solves the problem of low strength of the steel plate produced by off - line quenching and the technical problem of poor plate shape caused by the conventional on - line rolling + tempering preparation method.

[0013] To achieve the above - mentioned purpose, the present invention is realized by adopting the following technical solutions:

[0014] A 600 MPa - grade low - carbon - equivalent high - strength and tough hydroelectric steel, the chemical composition of the steel plate is by mass percentage: C: 0.03% - 0.05%; Si: 0.15% - 0.35%; Mn: 1.10% - 1.50%; P ≤ 0.01%; S ≤ 0.002%; Cr: 0.02% - 0.15%; Mo: 0.15% - 0.40%; Ni: 0.05% - 0.30%; B: 0.0001% - 0.0005%; Ti: 0.005% - 0.015%; Nb: 0.01% - 0.035%; Al: 0.02% - 0.035%; Cu ≤ 0.025%; the balance is Fe and unavoidable impurities; the carbon equivalent Ceq: 0.27% - 0.45%, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14; the hardenability index DI ≥ 0.65 × plate thickness.

[0015] The microstructure at 1 / 4 thickness of the finished steel plate is granular bainite + lath bainite. And by volume percentage, the proportion of granular bainite is 25% - 90%, and the proportion of lath bainite is 10% - 75%.

[0016] The properties of the finished steel plate are: yield strength Rp 0.2 ≥ 490 MPa, tensile strength 610 ≤ R m ≤ 730 MPa, elongation A ≥ 17%, reduction of area in the thickness direction Z ≥ 70%; impact energy KV2 at - 20 °C ≥ 100 J.

[0017] A manufacturing method of a 600 MPa - grade low - carbon - equivalent high - strength and tough hydroelectric steel, comprising the following steps:

[0018] (1) Smelting:

[0019] The smelting operation is carried out using high-purity molten iron and the double slag method. After deep desulfurization treatment, the slag is removed thoroughly. LF and RH refining are adopted. When refining in the LF furnace, white slag is formed, and the inclusions in the steel are modified by adding a composite deoxidizer.

[0020] (2)Continuous casting:

[0021] During the whole process of continuous casting, protective casting and soft reduction technology at the end of continuous casting are adopted.

[0022] (3)Rolling and the first-stage quenching:

[0023] The heating temperature of the continuous casting billet is 1100 - 1200 °C, and the net heat preservation time is 8.5 - 10 h. The rolling adopts two-stage rolling methods of rough rolling and finish rolling. The starting rolling temperature of rough rolling is 1100 - 1150 °C, and the final rolling temperature of rough rolling is 1050 - 1000 °C. The starting rolling temperature of finish rolling is 980 - 960 °C, and the final rolling temperature of finish rolling is ≥ 900 °C. The total reduction ratio of rolling is 70% - 92%, and the reduction ratio of at least 3 passes is ≥ 20%. Immediately after finish rolling, the first-stage quenching of the steel plate is carried out by laminar flow cooling. After quenching, the temperature of the steel plate drops by 30 - 110 °C.

[0024] (4)Tempering treatment:

[0025] The quenching temperature is 900 - 940 °C, and the net heat preservation time is 0.5 - 2.5 min / mm.

[0026] The tempering temperature is 540 - 600 °C, and the net heat preservation time is 2.0 - 4.0 min / mm.

[0027] In the step (1), the high-purity molten iron is molten iron with Fe ≥ 99.99%, P ≤ 0.03%, and S ≤ 0.02% by mass percentage.

[0028] In the step (1), deep desulfurization treatment is carried out through multiple slag-making and slag-pouring operations; the [S] content at the end point of molten steel is ≤ 0.001%, and the [H] content in the steel is controlled to be ≤ 3 ppm, the total oxygen content T[O] is ≤ 50 ppm, and [N] is ≤ 70 ppm; the net circulation time of RH refining is ≥ 10 min, and the standing time of molten steel before casting is ≥ 15 min.

[0029] In the step (2), the superheat of molten steel is controlled below 30 °C, and the casting speed of the continuous casting billet is controlled at 0.8 - 1.1 m / min.

[0030] The thickness of the finished steel plate is 18 - 95 mm.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1) The minimum carbon equivalent of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate of the present invention is 0.27. On this basis, the functions of trace B (1-7 ppm) and other micro-alloying elements are fully exerted. The B element can shift the austenite isothermal transformation curve to the right and delay the transformation start curve within the pearlite formation temperature range, which is conducive to the formation of upper bainite. In addition, supplemented with Ti (0.02%-0.04%) element, the effective B content in the steel can be increased, and through chemical composition ratio, the key element B exists in the form of effective B, thereby greatly improving the hardenability of the 600 MPa grade hydroelectric steel.

[0033] (2) According to the material characteristics of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate of the present invention, the production process of "hot rolling + step quenching + tempering treatment" is selected, which can greatly reduce the grain size before quenching of the steel plate, and thus is conducive to the grain refinement of the finished steel plate. This method also solves the problem of low strength of the steel plate produced by off-line quenching, and the technical problem of poor plate shape caused by the conventional on-line rolling + tempering preparation method.

[0034] (3) Compared with the manufacturing method of the conventional 600 MPa grade hydroelectric steel, the manufacturing method of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate of the present invention can solve the problems of slow production rhythm and low strength of off-line quenching, and the problem of poor plate shape caused by the conventional "on-line rolling + tempering" production process at the same time. After implementation, the first-pass rate of the plate shape is higher than 96%, the production efficiency is high, and the production cost is low, which can create greater economic value for the enterprise. Description of the Drawings

[0035] Figure 1 It is a metallographic structure photo at 1 / 4 thickness of the 18 mm thick steel plate produced in Example 1 of the present invention.

[0036] Figure 2 It is a metallographic structure photo at 1 / 4 thickness of the 40 mm thick steel plate produced in Example 2 of the present invention.

[0037] Figure 3 It is a metallographic structure photo at 1 / 4 thickness of the 58 mm thick steel plate produced in Example 3 of the present invention.

[0038] Figure 4 It is a metallographic structure photo at 1 / 4 thickness of the 65 mm thick steel plate produced in Example 4 of the present invention.

[0039] Figure 5 It is a metallographic structure photo at 1 / 4 thickness of the 95 mm thick steel plate produced in Example 5 of the present invention. Detailed Embodiments

[0040] A 600MPa - grade low - carbon - equivalent high - strength and tough hydroelectric steel according to the present invention is characterized in that the chemical composition of the steel plate is as follows by mass percentage: C: 0.03% - 0.05%; Si: 0.15% - 0.35%; Mn: 1.10% - 1.50%; P ≤ 0.01%; S ≤ 0.002%; Cr: 0.02% - 0.15%; Mo: 0.15% - 0.40%; Ni: 0.05% - 0.30%; B: 0.0001% - 0.0005%; Ti: 0.005% - 0.015%; Nb: 0.01% - 0.035%; Al: 0.02% - 0.035%; Cu ≤ 0.025%; the balance is Fe and inevitable impurities; the carbon equivalent Ceq: 0.27% - 0.45%, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14; the hardenability index DI ≥ 0.65 × plate thickness.

[0041] In the 600MPa - grade low - carbon - equivalent high - strength and tough hydroelectric steel according to the present invention, the reasons for the selection of the contents of key elements and their functions are as follows:

[0042] Element C: It is the most effective strengthening element in steel. It is dissolved in the matrix to play a solid - solution strengthening role, and at the same time can improve the hardenability of the steel plate. During the heat treatment process, C can form carbides with strong carbide - forming elements to play a precipitation strengthening role. For 600MPa - grade low - carbon - equivalent hydroelectric steel, it is required to significantly reduce the carbon content. Therefore, in the present invention, the C content is controlled at 0.03% - 0.05%.

[0043] Element Mn: It is an austenite - stabilizing element, which can significantly improve the hardenability of the steel plate, slow down the decomposition and transformation speed of the structure during the heat treatment process, and increase the tempering resistance. To a certain extent, it can hinder the formation of MC - type carbides inside the structure and reduce the consumption of elements such as Ti and Nb. However, if the Mn element content is too high, it will be unfavorable for the weldability and toughness of the steel plate. Therefore, in the present invention, the Mn content is controlled at 1.10% - 1.50%.

[0044] Element Cr: It can improve the hardenability of the steel plate. Although the effect is not as good as that of Mn and Mo elements, the cost is relatively low. Cr can significantly improve the corrosion resistance and oxidation resistance of the steel plate. In the present invention, the Cr content is controlled at 0.02% - 0.15%.

[0045] Element Mo: It is a strong hardenability element, which helps to improve the hardenability in the thickness direction of the steel plate and the later - stage tempering stability, enables the steel plate to be tempered at a higher temperature, and thus improves the plasticity and toughness of the steel plate. Mo can also increase the solubility of micro - alloying elements in austenite, reduce the precipitation of carbonitrides of micro - alloying elements, and make the micro - alloying elements precipitate from ferrite at a lower temperature, enhancing the precipitation strengthening effect. Therefore, in the present invention, the Mo content is controlled at 0.15% - 0.40%.

[0046] Element Ni: It is a good solution strengthening and hardenability enhancing element in steel. It can expand the austenite phase region, lower the austenite transformation temperature, prevent the transformation of austenite to pearlite, and thus reduce the critical transformation temperature of steel. Ni and Fe exist in a mutually soluble form, which can effectively improve the toughness of the steel plate, especially suitable for improving the plasticity and toughness of the core of extra-thick plates. However, its cost is relatively high. Therefore, in this invention, the Ni content is controlled within 0.05% - 0.30%.

[0047] Element B: It is a powerful hardenability enhancing element. It is easy to adsorb on the grain boundaries to reduce the grain boundary energy, making it difficult for proeutectoid ferrite to nucleate, prolonging the incubation period of the proeutectoid ferrite and upper bainite transformations, suppressing and delaying the ferrite transformation, and thus improving the hardenability, especially suitable for improving the hardenability of extra-thick plates. However, the solubility of B in steel is relatively low, and affected by its existing form, it is easy to form B embrittlement phenomenon, which is not conducive to the plasticity of the steel. Therefore, in this invention, the B content is controlled within 0.0001% - 0.0005%.

[0048] The manufacturing method of a 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel described in this invention adopts the process of "deep desulfurization of hot metal + converter smelting + secondary refining + vacuum treatment + continuous casting", strictly prepares continuous casting billets according to the target chemical composition, and combines the process of "online rolling + sectional quenching + tempering treatment" to prepare low carbon equivalent 600MPa grade high strength and toughness hydroelectric steel plates with different thicknesses. The specific process is as follows:

[0049] I. Smelting:

[0050] In the smelting process, high-purity hot metal (by mass percentage, Fe ≥ 99.99%, P ≤ 0.03%, S ≤ 0.02%) and the double slag method are used for operation. Through multiple slag-making and slag-pouring operations, deep desulfurization treatment is achieved and the slag is removed cleanly. The LF + RH process is used for secondary refining of the molten steel to ensure that the LF furnace makes white slag, ensure that [S] in the steel at the end of smelting ≤ 0.001%, strictly control the content of gases such as H, O, N in the steel, require that the net circulation time of RH refining is not less than 10 minutes, the standing time of the molten steel before pouring is not less than 15 minutes, keep the slag surface in the ladle in a surging state without exposing the molten steel surface, and at the same time, modify the inclusions in the steel by adding a composite deoxidizer (purchased finished product). Elements such as calcium and aluminum in the composite deoxidizer can reduce the interfacial tension between the inclusions and the molten steel, making it easier for the inclusions to precipitate and float from the molten steel. This reduction in interfacial tension helps to accelerate the deoxidation reaction and improve the removal efficiency of inclusions, thereby maximizing the purity of the steel quality.

[0051] II. Continuous casting:

[0052] During the continuous casting process, protective casting is adopted throughout to prevent the molten steel from being re-oxidized. The superheat of the molten steel is controlled below 30°C to minimize the occurrence of defects such as central porosity and central segregation. To ensure the internal quality of the continuous casting billet, the casting speed is controlled at 0.8 - 1.1 m / min, and the soft reduction technology at the end of continuous casting is used to fully improve the internal quality of the billet.

[0053] III. Rolling and the first-stage quenching:

[0054] The continuous casting billet with a set thickness is sent into the heating furnace, where the heating temperature is 1100 - 1200°C and the net holding time is 8.5 - 10 h. A two-stage rolling method of rough rolling and finish rolling is adopted, and different rolling passes are set according to the finished plate thickness. The starting rolling temperature in the rough rolling stage is 1100 - 1150°C, and the final rolling temperature is 1050 - 1000°C. The starting rolling temperature in the finish rolling stage is 980 - 960°C, and the final rolling temperature is above 900°C. The total rolling reduction rate is 70% - 92%, and it is ensured that the reduction rate of at least 3 passes is ≥20% per pass. Immediately after finish rolling, the first-stage quenching of the steel plate is carried out by laminar flow cooling, and the temperature of the steel plate drops by 30 - 110°C after quenching.

[0055] IV. Tempering treatment:

[0056] To ensure excellent comprehensive mechanical properties of the finished steel plate along the thickness direction, corresponding heat treatment processes are adopted according to the finished plate thickness to give full play to the roles of key elements such as B, Ni, Ti in the steel, so that excellent strength-ductility matching can also be obtained at 1 / 4 of the thickness of the 600 MPa grade low-carbon equivalent hydroelectric steel plate. The specific process parameters are as follows:

[0057] Quenching (the second-stage quenching) temperature: 900 - 940°C, net holding time: 0.5 - 2.5 min / mm;

[0058] Tempering temperature: 540 - 600°C, net holding time: 2.0 - 4.0 min / mm.

[0059] The microstructure at 1 / 4 of the thickness of the finished steel plate is granular bainite + lath bainite, and by volume percentage, the proportion of granular bainite is 25% - 90%, and the proportion of lath bainite is 10% - 75%.

[0060] The properties of the finished steel plate are: yield strength Rp 0.2 ≥490 MPa, tensile strength 610 ≤ R m ≤730 MPa, elongation A ≥ 17%, reduction of area in the thickness direction Z ≥ 70%; impact energy KV2 at -20°C ≥ 100 J.

[0061] The thickness of the finished steel plate is 18 - 95 mm.

[0062] To more intuitively illustrate the present invention, the implementation manners of the present invention will be further described in combination with embodiments. The following embodiments are only preferred specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention, including simple changes or equivalent replacements, are within the protection scope of the present invention.

[0063]

Embodiment

[0064] The chemical compositions of the steel in each embodiment are shown in Table 1.

[0065] Table 1 Chemical Compositions of the Steel in Each Embodiment (wt%)

[0066]

[0067] In this embodiment, the process of "deep desulfurization of hot metal + converter smelting + secondary refining + vacuum treatment + continuous casting" is adopted. The continuous casting billets are prepared strictly according to the components in Table 1. High-purity hot metal is required in the smelting process, and the double slag method is used for deep desulfurization treatment; for secondary refining, it is required that the LF furnace makes white slag to refine the molten steel, ensuring that [S] in the molten steel at the end of smelting ≤ 0.001%, the net circulation time of RH refining is not less than 10 minutes, the standing time before casting is not less than 15 minutes, the slag surface in the ladle is kept in a surging state and the molten steel surface is not exposed, and the inclusions in the steel are modified by treatment with a composite deoxidizer to maximize the purity of the steel quality; protective casting is adopted to avoid secondary oxidation of the molten steel.

[0068] Embodiments 1 to 3 all use continuous casting billets prepared according to the components in Table 1, and after selectively undergoing different hot working and sectional quenching, 600 MPa grade low carbon equivalent high strength and toughness hydroelectric steel plates are obtained.

[0069]

Embodiment 1

[0070] In this embodiment, the chemical compositions and mass percentages of the 600 MPa grade low carbon equivalent high strength and toughness hydroelectric steel plate are shown in Table 1, and the preparation method of the steel plate is carried out according to the following steps:

[0071] Step 1: The heating temperature of the continuous casting billet is 1180 ± 10 °C, and the net holding time is 9 h. The two-stage rolling method of rough rolling and finish rolling is adopted. The starting rolling temperature in the rough rolling stage is set at 1100 °C, and the final rolling temperature is set at 1050 °C; the starting rolling temperature in the finish rolling stage is set at 980 °C, and the final rolling temperature is set at 900 °C; the total rolling reduction rate is 94%, and the reduction rate of each pass is allocated by the automatic control system of the rolling mill. At the same time, it is necessary to ensure that the reduction rate of at least 3 passes in the rough rolling stage is ≥ 20%, and the finished thickness of the steel plate is 18 mm; the rolling reduction rate and the remaining thickness of the steel plate in the rough rolling stage in this embodiment are shown in Table 2:

[0072] Table 2 Rolling Reduction Rate and Remaining Thickness of the Steel Plate in the Rough Rolling Stage

[0073] Pass Reduction ratio (%) Remaining thickness (mm) 1 26.0 222.0 2 26.0 165.0 3 26.0 122.0 4 18.0 100.0

[0074] Step 2: The hot-rolled steel plate is subjected to the first-stage quenching by laminar flow cooling, and the temperature of the steel plate decreases by 30 °C after quenching;

[0075] Step 3: The steel plate after the first-stage quenching is successively subjected to quenching (second-stage quenching) at 940 °C with a net holding time of 1.5 min / mm, and tempering treatment at 600 °C with a net holding time of 2.0 min / mm.

[0076] The metallographic microstructure at 1 / 4 thickness of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate prepared in this example is as Figure 1 shown. The microstructure at 1 / 4 thickness of the steel plate is a "granular bainite + lath bainite" structure, where the volume percentage of lath bainite is 10%, and carbides are precipitated uniformly and dispersedly.

[0077] In this example, the tensile properties and impact properties at 1 / 4 thickness of the finished steel plate are shown in Table 3 and Table 4 respectively.

[0078] Table 3 Tensile properties of the finished steel plate at 1 / 4 thickness

[0079]

[0080] Table 4 Impact properties of the finished steel plate at 1 / 4 thickness

[0081]

[0082]

Example 2

[0083] In this example, the chemical composition and mass percentage of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate are shown in Table 1, and its preparation method is carried out according to the following steps:

[0084] Step 1: The continuous casting billet is heated to a temperature of 1180 ± 10 °C with a net holding time of 9 h. The two-stage rolling method of rough rolling and finish rolling is adopted. The starting rolling temperature in the rough rolling stage is set at 1100 °C, and the final rolling temperature is set at 1050 °C. The starting rolling temperature in the finish rolling stage is set at 980 °C, and the final rolling temperature is set at 900 °C. The total rolling reduction rate is 87%. The reduction rate of each pass is distributed by the automatic control system of the rolling mill. At the same time, it is necessary to ensure that the reduction rate of at least 3 passes in the rough rolling stage is ≥ 20%. The thickness of the finished steel plate is 40 mm. The rolling reduction rate and the remaining thickness of the steel plate in the rough rolling stage in this example are shown in Table 5:

[0085] Table 5 Rolling reduction rate and remaining thickness of the steel plate in the rough rolling stage

[0086] Pass Reduction ratio (%) Remaining thickness (mm) 1 26.0 222.0 2 26.0 165.0 3 26.0 122.0 4 18.0 100.0

[0087] Step 2: The hot-rolled steel plate is subjected to the first-stage quenching by laminar flow cooling, and the temperature of the steel plate decreases by 50 °C after quenching;

[0088] Step 3: The segmented quenched steel plate is successively subjected to quenching (second-stage quenching) at 920 °C with a net holding time of 1.5 min / mm; tempering treatment at 560 °C with a net holding time of 4.0 min / mm.

[0089] The metallographic microstructure at 1 / 4 thickness of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate prepared in this example is as Figure 2 shown. The microstructure at 1 / 4 thickness of the steel plate is a "granular bainite + lath bainite" structure, in which the volume percentage of lath bainite is 65%, and the average grain size is 18 μm.

[0090] In this example, the tensile properties and impact properties at 1 / 4 thickness of the finished steel plate are shown in Table 6 and Table 7 respectively.

[0091] Table 6 Tensile properties at 1 / 4 thickness of the finished steel plate

[0092]

[0093] Table 7 Impact properties at 1 / 4 thickness of the finished steel plate

[0094]

[0095]

Example 3

[0096] In this example, the chemical composition and mass percentage of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate are shown in Table 1, and its preparation method is carried out according to the following steps:

[0097] Step 1: The continuous casting billet is heated to a temperature of 1180 ± 10 °C with a net holding time of 9.5 h. The two-stage rolling method of rough rolling and finish rolling is adopted. The rough rolling stage sets the starting rolling temperature at 1100 °C and the final rolling temperature at 1050 °C. The finish rolling stage sets the starting rolling temperature at 960 °C and the final rolling temperature at 900 °C; the total rolling reduction rate is 81%. The reduction rate of each pass is distributed by the automatic control system of the rolling mill. At the same time, it is necessary to ensure that the single-pass reduction rate of at least 3 passes in the rough rolling stage is ≥ 20%. The thickness of the finished steel plate is 58 mm. The rolling reduction rate and the remaining thickness of the steel plate in the rough rolling stage in this example are shown in Table 8:

[0098] Table 8 Rolling reduction rate and remaining thickness of the steel plate in the rough rolling stage

[0099] Pass Reduction ratio (%) Remaining thickness (mm) 1 23.0 233.0 2 23.0 180.0 3 23.0 140.0 4 18.0 116.0

[0100] Step 2: The hot-rolled steel plate is subjected to the first-stage quenching by laminar flow cooling, and the temperature of the steel plate decreases by 70 °C after quenching;

[0101] Step 2: The steel plate after sectional quenching is successively subjected to quenching (second-stage quenching) at 920 °C with a net holding time of 1.0 min / mm; tempering treatment at 540 °C with a net holding time of 2.4 min / mm.

[0102] The metallographic microstructure at 1 / 4 thickness of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate prepared in this example is as Figure 3 shown. The microstructure at 1 / 4 thickness of the steel plate is a "granular bainite + lath bainite" structure, where the volume percentage of lath bainite is 70%, and the average grain size is 25 μm.

[0103] In this example, the tensile properties and impact properties at 1 / 4 thickness of the steel plate are shown in Table 9 and Table 10 respectively.

[0104] Table 9 Tensile properties at 1 / 4 thickness of the finished steel plate

[0105]

[0106] Table 10 Impact properties at 1 / 4 thickness of the finished steel plate

[0107]

[0108]

Example 4

[0109] In this example, the chemical composition and mass percentage of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate are shown in Table 1, and its preparation method is carried out according to the following steps:

[0110] Step 1: The continuous casting billet is heated to a temperature of 1180 ± 10 °C with a net holding time of 9.5 h. The two-stage rolling method of rough rolling and finish rolling is adopted. The starting rolling temperature in the rough rolling stage is set at 1100 °C, and the final rolling temperature is set at 1050 °C. The starting rolling temperature in the finish rolling stage is set at 960 °C, and the final rolling temperature is set at 900 °C; the total rolling reduction rate is 78%. The reduction rate of each pass is allocated by the automatic control system of the rolling mill. At the same time, it is necessary to ensure that the reduction rate of at least 3 passes in the rough rolling stage is ≥ 20%. The thickness of the finished steel plate is 65 mm. The rolling reduction rate and the remaining thickness of the steel plate in the rough rolling stage in this example are shown in Table 11:

[0111] Table 11 Rolling reduction rate and remaining thickness of the steel plate in the rough rolling stage

[0112] Pass Reduction ratio (%) Remaining thickness (mm) 1 23.0 233.0 2 23.0 180.0 3 23.0 140.0 4 18.0 116.0

[0113] Step 2: The steel plate after finish rolling is subjected to the first-stage quenching by laminar flow cooling, and the temperature of the steel plate decreases by 80°C after quenching;

[0114] Step 2: The steel plate after the first-stage quenching is successively subjected to quenching (second-stage quenching) at 920°C with a net holding time of 1.5 min / mm; tempering treatment at 580°C with a net holding time of 3.0 min / mm.

[0115] The metallographic microstructure at 1 / 4 thickness of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate prepared in this example is as Figure 4 shown. The microstructure at 1 / 4 thickness of the steel plate is a "granular bainite + lath bainite" structure, in which the volume percentage content of lath bainite reaches 75%, and the average grain size is 24 μm.

[0116] In this example, the tensile properties and impact properties at 1 / 4 thickness of the steel plate are shown in Table 12 and Table 13 respectively.

[0117] Table 12 Tensile properties at 1 / 4 thickness of the finished steel plate

[0118]

[0119] Table 13 Impact properties at 1 / 4 thickness of the finished steel plate

[0120]

[0121]

Example 5

[0122] In this example, the chemical composition and mass percentage content of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate are shown in Table 1, and its preparation method is carried out according to the following steps:

[0123] Step 1: The continuous casting billet is heated to a temperature of 1180 ± 10°C with a net holding time of 9.5 h. The two-stage rolling method of rough rolling and finish rolling is adopted. The starting rolling temperature in the rough rolling stage is set at 1100°C, and the final rolling temperature is set at 1050°C. The starting rolling temperature in the finish rolling stage is set at 960°C, and the final rolling temperature is set at 900°C; the total rolling reduction rate is 68%, and the reduction rate of each pass is allocated by the automatic control system of the rolling mill. At the same time, it is necessary to ensure that the reduction rate of at least 3 passes in the rough rolling stage is ≥ 20%, and the finished thickness of the steel plate is 95 mm; the rolling reduction rate and the remaining thickness of the steel plate in the rough rolling stage in this example are shown in Table 14:

[0124] Table 14 Rolling reduction rate and remaining thickness of the steel plate in the rough rolling stage

[0125] Pass Reduction ratio (%) Remaining thickness (mm) 1 20.0 240.0 2 20.0 192.0 3 20.0 153.6 4 20.0 122.9

[0126] Step 2: The steel plate after finish rolling is subjected to the first-stage section quenching by laminar flow cooling. After quenching, the temperature of the steel plate drops by 110 °C;

[0127] Step 2: The steel plate after the first-stage quenching is successively subjected to quenching (second-stage quenching) at 900 °C with a net holding time of 2.5 min / mm; and tempering at 600 °C with a net holding time of 4.0 min / mm.

[0128] The metallographic microstructure at 1 / 4 thickness of the 600 MPa grade low-carbon equivalent high-strength and tough hydroelectric steel plate prepared in this embodiment is as Figure 5 shown. The microstructure at 1 / 4 thickness of the steel plate is a "granular bainite + lath bainite" structure, in which the volume percentage content of lath bainite reaches 73%, and the average grain size is 27 μm.

[0129] In this embodiment, the tensile properties and impact properties at 1 / 4 thickness of the steel plate are shown in Table 15 and Table 16 respectively.

[0130] Table 15 Tensile properties at 1 / 4 thickness of the finished steel plate

[0131]

[0132] Table 16 Impact properties at 1 / 4 thickness of the finished steel plate

[0133]

[0134] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel, characterized in that, The chemical composition of the steel plate by mass percentage is C: 0.03% - 0.05%; Si: 0.15% - 0.35%; Mn: 1.10% - 1.50%; P ≤ 0.01%; S ≤ 0.002%; Cr: 0.02% - 0.15%; Mo: 0.15% - 0.40%; Ni: 0.05% - 0.30%; B: 0.0001% - 0.0005%; Ti: 0.005% - 0.015%; Nb : 0.01% - 0.035%; Al: 0.02% - 0.035%; Cu ≤ 0.025%; the balance is Fe and inevitable impurities; carbon equivalent Ceq: 0.27% - 0.45%, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14; hardenability index DI ≥ 0.65 × plate thickness.

2. A 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 1, characterized in that, The microstructure at 1 / 4 thickness of the finished steel plate is granular bainite + lath bainite, and by volume percentage, the proportion of granular bainite is 25% - 90%, and the proportion of lath bainite is 10% - 75%.

3. A 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 1, characterized in that, The properties of the finished steel plate are as follows: yield strength Rp 0.2 ≥ 490 MPa, tensile strength 610 ≤ R m ≤ 730 MPa, elongation A ≥ 17%, reduction of area in the thickness direction Z ≥ 70%; The impact energy KV2 at -20°C ≥ 100 J.

4. A manufacturing method of a 600MPa - grade low - carbon equivalent high - strength and tough hydroelectric steel as described in any one of claims 1 to 3, characterized in that, It includes the following steps: (1) Smelting: The smelting is carried out by using high-purity hot metal and the double slag method, with deep desulfurization treatment and slag skimming; LF and RH refining are adopted. When refining in the LF furnace, white slag is made, and the inclusions in the steel are modified by adding a composite deoxidizer. (2) Continuous casting: The whole process of continuous casting adopts protective casting and soft reduction technology at the end of continuous casting. (3) Rolling and the first-stage quenching: The heating temperature of the slab is 1100 - 1200°C, and the net holding time is 8.5 - 10 h; the rolling adopts two-stage rolling of rough rolling and finish rolling. The starting rolling temperature of rough rolling is 1100 - 1150°C, and the final rolling temperature of rough rolling is 1050 - 1000°C; the starting rolling temperature of finish rolling is 980 - 960°C, and the final rolling temperature of finish rolling ≥ 900°C; the total reduction ratio of rolling is 70% - 92%, and the reduction ratio of at least 3 passes is ≥ 20%; immediately after finish rolling, the steel plate is subjected to the first-stage quenching by laminar flow cooling, and the temperature of the steel plate drops by 30 - 110°C after quenching. (4) Tempering treatment: The quenching temperature is 900 - 940°C, and the net holding time is 0.5 - 2.5 min / mm; The tempering temperature is 540 - 600°C, and the net holding time is 2.0 - 4.0 min / mm.

5. The manufacturing method of a 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 4, characterized in that, In the step (1), the high-purity hot metal is hot metal with Fe ≥ 99.99%, P ≤ 0.03%, and S ≤ 0.02% by mass percentage.

6. The manufacturing method of a 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 4, characterized in that, In the step (1), deep desulfurization treatment is carried out through multiple slag-making and slag-dumping operations; the [S] at the end point of the molten steel ≤ 0.001%, and [H] in the steel is controlled ≤ 3 ppm, the total oxygen content T[O] ≤ 50 ppm, [N] ≤ 70 ppm; the net circulation time of RH refining ≥ 10 min, and the standing time of the molten steel before pouring ≥ 15 min.

7. The manufacturing method of a 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 4, characterized in that, In the step (2), the superheat of the molten steel is controlled below 30°C, and the casting speed of the continuous casting billet is controlled at 0.8 - 1.1 m / min.

8. The manufacturing method of a 600MPa grade low carbon equivalent high strength and toughness hydroelectric steel according to claim 4, characterized in that, The thickness of the finished steel plate is 18 - 95 mm.

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