An ultra-low-carbon 600mpa-grade high-strength and high-toughness hydroelectric steel plate and a preparation method thereof

Abstract

The application relates to the technical field of water and electricity steel plates, in particular to an ultra-low-carbon 600MPa-grade high-strength and high-toughness water and electricity steel plate and a preparation method thereof. The steel plate is composed of the following chemical components with the weight percentage: C: 0.03%-0.05%, Si: 0.15%-0.35%, Mn: 1.40%-1.60%, P<=0.01%, S<=0.002%, Cr: 0.02%-0.09%, Mo: 0.15%-0.40%, Ni: 0.15%-0.35%, B: 0.0001%-0.0007%, La: 0.01%-0.04%, Ti: 0.005%-0.025%, Nb: 0.01%-0.035%, Al: 0.02-0.045%, Cu<=0.03%, and the balance is Fe. The steel plate satisfies DI>=0.65H; wherein DI is a hardenability index, and the unit is mm; and H is the thickness of the steel plate, and the unit is mm. The thickness of the finished steel plate is 20-90 mm, and the minimum carbon content is 0.03%. The carbon content of the application reaches the ultra-low-carbon level (0.03%), the maximum thickness reaches 90 mm, and the comprehensive performance satisfies the requirements of relevant standards.

Description

Technical Field

[0001] This invention relates to the field of hydropower steel plate technology, specifically to an ultra-low carbon 600MPa grade high-strength and tough hydropower steel plate and its preparation method. Background Technology

[0002] Hydropower is a method of generating electricity that uses the kinetic energy of water flow to drive a turbine. As a clean energy source, hydropower has become the preferred choice for energy acquisition due to its advantages such as high efficiency, cleanliness, and sustainability.

[0003] The service environment of hydropower steel is harsh, requiring a good strength-toughness ratio and excellent weldability. It is considered a high-tech, high-value-added product among steel materials. With the development of the hydropower industry and downstream enterprises, more stringent requirements have been placed on 600MPa grade hydropower steel. These requirements demand a significant reduction in the carbon equivalent and carbon content of the steel plate while maintaining the same strength level, as these are the most critical factors in ensuring the steel plate's strength. Therefore, how to significantly reduce the carbon equivalent and carbon content of the steel plate while ensuring its strength and overall performance meet relevant standards has become a pressing technical challenge in this field.

[0004] Chinese patent document CN115094298A discloses "A method for producing 600MPa grade low carbon equivalent hydropower steel." Although the mechanical properties of this steel plate meet the relevant standard requirements well, the minimum carbon content in its embodiments is 0.07%, proving that 0.05% is only the lower limit of the smelting composition, not the actual composition of the steel plate. Furthermore, its carbon equivalent is 0.45, which is relatively high and cannot meet the ultra-low carbon requirements. Chinese patent document CN103484766A discloses "A 600MPa grade tempered high-strength low-weld-crack-sensitivity steel plate for hydropower projects and its preparation method." Similarly, although all mechanical properties meet the standard requirements, its minimum carbon content is 0.07%, which cannot meet the ultra-low carbon requirements for this steel grade. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides an ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate and its preparation method. The carbon content reaches an ultra-low carbon level (0.03%), the maximum thickness reaches 90mm, and the comprehensive performance meets the relevant standard requirements.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A type of ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is composed of the following chemical composition by weight percentage:

[0008] C: 0.03%–0.05%, Si: 0.15%–0.35%, Mn: 1.40%–1.60%, P≤0.01%, S≤0.002%, Cr: 0.02%–0.09%, Mo: 0.15%–0.40%, Ni: 0.15%–0.35%, B: 0.0001%–0.0007%, Ti: 0.005%–0.025%, Nb: 0.01%–0.035%, Al: 0.02%–0.045%, Cu≤0.03%, balance Fe.

[0009] A type of ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is composed of the following chemical composition by weight percentage:

[0010] C: 0.03%–0.05%, Si: 0.15%–0.35%, Mn: 1.40%–1.60%, P≤0.01%, S≤0.002%, Cr: 0.02%–0.09%, Mo: 0.15%–0.40%, Ni: 0.15%–0.35%, B: 0.0001%–0.0007%, La: 0.01%–0.04%, Ti: 0.005%–0.025%, Nb: 0.01%–0.035%, Al: 0.02%–0.045%, Cu≤0.03%, balance Fe.

[0011] The steel plate must meet the hardenability index (DI) ≥ 0.65H; where DI is the hardenability index in mm, and H is the plate thickness in mm. The finished steel plate thickness is 20–90 mm, and the minimum carbon content is 0.03%.

[0012] The reasons for using the above-mentioned components and their weight percentages in this invention are explained in detail below:

[0013] C element: The most effective strengthening element in steel. It is dissolved in the matrix to play a solid solution strengthening role and can improve the hardenability of steel plates. During heat treatment, it can form carbides with strong carbide combining elements to play a precipitation strengthening role. However, the new 600MPa grade hydroelectric steel requires a significant reduction in carbon content, so the C content of this invention is 0.03% to 0.05%.

[0014] Mn is an austenite stabilizing element that can significantly improve the hardenability of steel plates, slow down the decomposition and transformation rate of the microstructure during heat treatment, and increase tempering resistance. To a certain extent, it can inhibit the formation of MC-type carbides in the microstructure and reduce the consumption of elements such as Ti and Nb. However, if its content is too high, it will be detrimental to the weldability and toughness of the steel plate. Therefore, the Mn content in this invention is 1.40% to 1.60%.

[0015] Cr element: can improve the hardenability of steel plate, although the effect is not as good as Mn and Mo elements, but the cost effect is better; it can significantly improve the corrosion resistance and oxidation resistance of steel plate; the Cr content in this invention is 0.02% to 0.09%.

[0016] Mo (Mo) is a highly hardenable element that helps improve hardenability and subsequent tempering stability in the thickness direction of steel plates. It allows steel plates to be tempered at higher temperatures, thereby improving their plasticity and toughness. It can also increase the solubility of microalloying elements in austenite, reduce the precipitation of carbonitrides of microalloying elements, and allow microalloying elements to precipitate from ferrite at lower temperatures, thus enhancing the precipitation strengthening effect. Therefore, the Mo content in this invention is 0.15% to 0.40%.

[0017] Ni element: A good solid solution strengthening and hardenability enhancing element in steel. It can expand the austenite phase region, reduce the austenite transformation temperature, prevent the transformation of austenite to pearlite, and thus reduce the critical transformation temperature of steel. Ni and Fe elements exist in a mutually soluble form, which can effectively improve the toughness of steel plates. It is particularly suitable for improving the plasticity and toughness of the core of extra-thick plates, but its cost is high. Therefore, the Ni content in this invention is 0.15% to 0.35%.

[0018] Element B: A strong hardenability-enhancing element, it is easily adsorbed at grain boundaries to reduce grain boundary energy, making it difficult for proeutectoid ferrite to nucleate, prolonging the incubation period of the transformation of proeutectoid ferrite and upper bainite, inhibiting and delaying the ferrite transformation, thereby improving hardenability, especially suitable for improving the hardenability of extra-thick plates; however, its solubility in steel is low, and due to the influence of its existence form, it is easy to form B brittleness, which is not conducive to the plasticity of steel. Therefore, its content in this invention is 0.0001% to 0.0007%.

[0019] La: a rare earth element, which purifies steel and alters inclusions in the steel, thereby improving the purity of the steel; it has a certain ability to capture hydrogen, reducing defects such as white spots in ultra-thick plates; the precipitates formed can serve as a nucleation basis, thereby playing a role in grain refinement and strengthening. Therefore, its content in this invention is 0.01% to 0.04%.

[0020] This invention employs a process of deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment, and continuous casting. Continuous casting billets are prepared strictly according to the target chemical composition, and appropriate rolling and heat treatment regimes are selected to prepare new 600MPa grade hydroelectric steel plates of different thicknesses.

[0021] A method for preparing an ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate specifically includes the following steps:

[0022] 1. Smelting

[0023] High-purity molten iron is used in the smelting process, and the double-slag method is used. Deep desulfurization treatment and slag removal are required. LF and RH are used for ladle refining of molten steel to ensure that white slag is produced in the LF furnace and that [S] in the steel is ≤0.001%. The content of gases such as H, O, and N in the steel is strictly controlled. The net RH circulation is required to be no less than 10 minutes, and the settling time before pouring is no less than 15 minutes. The slag surface in the ladle is kept in a surging state without being exposed above the molten steel surface. At the same time, the inclusions in the steel are modified by the treatment with composite deoxidizer to maximize the purity of the steel.

[0024] 2. Pouring

[0025] Protective casting is used throughout the continuous casting process to prevent secondary oxidation of the molten steel. The superheat of the molten steel is controlled below 30°C to minimize defects such as central porosity and central segregation. To ensure the internal quality of the continuously cast billet, the casting speed is controlled at 1.0 m / min, and a light reduction technique is used at the end of the continuous casting process to fully improve the internal quality of the billet.

[0026] 3. Heated rolling

[0027] A continuously cast billet of appropriate thickness is fed into a heating furnace and heated to 1100–1200℃. The net holding time is 8.5–12 hours. A two-stage rolling process of roughing and finishing is adopted. Different rolling passes are set according to the thickness of the finished plate. The starting rolling temperature of the roughing stage is 1050℃, and the finishing rolling temperature is 950–970℃. The starting rolling temperature of the finishing stage is 910–950℃, and the finishing rolling temperature is above 860℃. The total reduction rate is 70%–92%, ensuring that at least three passes have a reduction rate of ≥20%.

[0028] 4. Heat treatment

[0029] To ensure that the finished steel plate has excellent comprehensive mechanical properties along the thickness direction, the corresponding heat treatment process is adopted according to the thickness of the finished plate to give full play to the role of key elements such as B, Ni, and Ti in the steel, so that the new 600MPa grade hydropower steel plate can also achieve excellent strength and toughness matching at 1 / 4.

[0030] The specific process is as follows:

[0031] Quenching process: Temperature 900~940℃, net holding time 0.5~2.5min / mm;

[0032] Sub-critical quenching process: temperature 850~890℃, net holding time 0.5~1.0min / mm;

[0033] Tempering process: temperature 570~600℃, net holding time 2.0~4.0min / mm.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention achieves an ultra-low carbon content (0.03%). Based on this, it fully utilizes the role of trace amounts of B (1-7 ppm) and other microalloying elements. B is a surface-active element that readily adsorbs at grain boundaries, reducing grain boundary energy and making proeutectoid ferrite less prone to nucleation. This prolongs the incubation period of proeutectoid ferrite and upper bainite transformation, inhibiting and delaying ferrite transformation to improve hardenability. Furthermore, B can shift the isothermal transformation curve of austenite to the right and delay the transformation initiation curve within the pearlite formation temperature range, thus promoting upper bainite formation. B has a strong affinity for O and N; combining with either will reduce the effective B content in the steel, rendering it ineffective. Supplementing with Ti (0.02-0.04%) can increase the effective B content in the steel. A Ti / N ratio greater than 6 can result in an effective B content exceeding 90%. This composition ratio ensures that the key element B exists in an effective form.

[0036] 2. Compared with conventional 600MPa grade hydroelectric steel, this invention adds trace amounts of rare earth element La. During the smelting process, La can significantly reduce the content of O and S, improve the purity of the steel, reduce defects such as white spots in ultra-thick plates, reduce the harmful effects of low-melting-point elements, and improve the utilization rate of alloying elements such as Ti and Cr. After adding La, the long strip-shaped MnS inclusions in the steel are transformed into spherical MnLa inclusions, eliminating their deterioration on the performance of the steel plate. In addition, the atomic radius of La is much larger than that of Fe atoms, and when dissolved in the matrix, it can cause more severe lattice distortion, resulting in a strong solid solution strengthening effect.

[0037] 3. The forming process and heat treatment process of this invention are matched. A two-stage rolling process is adopted. The first stage is carried out in the austenite recrystallization zone. The rolling force penetrates into the core of the steel plate through low speed and high pressure, breaking down the coarse austenite grains. After a suitable time of waiting at the temperature, recrystallization occurs inside the steel plate to refine the grains. The second stage rolling is carried out in the non-recrystallization zone of austenite. It is required that the starting rolling temperature of the second stage is 50°C lower than the recrystallization temperature, and the grains are further broken down with the same rolling requirements. During the sub-temperature quenching process, new grains will nucleate at the original grain boundaries, which will further refine the grains. Through the appropriate matching of the above technologies, the problem of significantly reducing the carbon content while maintaining the strength level of 600MPa grade hydroelectric steel is successfully solved.

[0038] In summary, the novel ultra-low carbon 600MPa grade electric steel sheet manufactured using this invention has an ultra-low carbon content (0.03%), a maximum thickness of 90mm, a yield strength at 1 / 4 of the steel sheet ≥540MPa, a tensile strength ≥634MPa, and an impact energy value at -20℃ ≥294J. Its comprehensive performance meets the relevant standard requirements. Attached Figure Description

[0039] Figure 1This invention relates to the metallographic structure of a 20mm thick steel plate at 1 / 2 of its thickness.

[0040] Figure 2 This invention relates to the metallographic structure of a 20mm thick steel plate at 1 / 4 of its thickness.

[0041] Figure 3 This is another metallographic structure of a 20mm thick steel plate at 1 / 2.

[0042] Figure 4 This is another metallographic structure of a 20mm thick steel plate at 1 / 4 of its thickness, as described in this invention.

[0043] Figure 5 This invention relates to the metallographic structure of a 40mm thick steel plate at half its length.

[0044] Figure 6 This invention relates to the metallographic structure of a 40mm thick steel plate at 1 / 4 of its thickness.

[0045] Figure 7 This invention relates to the metallographic structure of a 60mm thick steel plate at half its length.

[0046] Figure 8 This invention relates to the metallographic structure of a 60mm thick steel plate at 1 / 4 of its thickness.

[0047] Figure 9 This invention relates to the metallographic structure of a 68mm thick steel plate at half its length.

[0048] Figure 10 This invention relates to the metallographic structure of a 68mm thick steel plate at 1 / 4 of its thickness.

[0049] Figure 11 This invention relates to the metallographic structure of a 90mm thick steel plate at half its length.

[0050] Figure 12 This invention relates to the metallographic structure of a 90mm thick steel plate at 1 / 4 of its thickness. Detailed Implementation

[0051] This invention discloses an ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate and its preparation method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0052]

Example

[0053] The chemical composition (Wt%) of the embodiments of the present invention is shown in Table 1.

[0054] Table 1. Chemical composition (wt%) of the steel in each embodiment.

[0055] element C Si Mn P S Ni Cr Mo Al La Nb Ti B Example 1 0.03 0.17 1.51 0.005 0.0007 0.16 0.52 0.24 0.035 0 0.025 0.016 0.0002 Example 2 0.03 0.20 1.52 0.005 0.0008 0.15 0.050 0.24 0.03 0.011 0.015 0.015 0.0002 Example 3 0.03 0.16 1.51 0.005 0.0007 0.20 0.054 0.28 0.03 0.015 0.025 0.016 0.0004 Example 4 0.04 0.20 1.49 0.007 0.0010 0.20 0.050 0.30 0.025 0.027 0.035 0.015 0.0004 Example 5 0.04 0.23 1.51 0.007 0.0005 0.25 0.050 0.34 0.03 0.026 0.035 0.017 0.0006 Example 6 0.05 0.22 1.51 0.007 0.0005 0.24 0.050 0.34 0.03 0.039 0.035 0.020 0.0007

[0056] Note: The steel plates in all embodiments meet the following requirements.

[0057] 1. CEV(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14

[0058] 2. Pcm(%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Mo / 15+V / 10+5B

[0059] 3. The above components are smelting and melting components; H in steel ≤ 3ppm; TO ≤ 50ppm, N ≤ 70ppm.

[0060] The process employs deep desulfurization of molten iron + converter smelting + ladle refining + vacuum treatment + continuous casting. Continuous casting billets are prepared strictly according to the composition in Table 1. The smelting process requires high-purity molten iron, and deep desulfurization is performed using a double-slag method. Ladle refining requires the use of white slag in an LF furnace to refine the molten steel, ensuring that [S] in the steel is ≤0.001%, the net RH circulation is no less than 10 minutes, and the settling time before casting is no less than 15 minutes. The slag surface in the ladle is kept in a surging state and does not expose the molten steel. A composite deoxidizer is used to modify inclusions in the steel to maximize steel purity. Protective casting is employed to prevent secondary oxidation of the molten steel.

[0061] like Figure 1-12 As shown, the materials used in Examples 1 to 6 below are all continuously cast billets prepared according to the composition in Table 1. After selectively undergoing different hot working and heat treatment, examples of mechanical properties and microstructures at 1 / 2 and 1 / 4 of their thickness are presented.

[0062] Example 1:

[0063] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0064] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0065] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 9h. It adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is above 940℃; the initial rolling temperature of the finishing stage is 920℃, and the final rolling temperature is above 850℃. The total reduction rate is 92%, ensuring that the reduction rate is ≥20% in at least three passes during the roughing stage. The thickness of the finished steel plate is 20mm.

[0066] Step 2: The obtained steel plate is subjected to quenching at 940℃ with a net holding time of 1.5 min / mm; tempering at 600℃ with a net holding time of 2.0 min / mm.

[0067] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 1 , 2 As shown, the microstructure of the steel plate at 1 / 4 and 1 / 2 sections is granular bainite and dispersed alloy cementite. The tensile and impact properties of the steel plate at 1 / 4 and 1 / 2 sections in this embodiment are shown in Tables 2 and 3, respectively.

[0068] Table 2 Tensile properties of 20mm thick steel plate at various locations

[0069]

[0070] Table 3 Impact performance of 20mm thick steel plate at various locations

[0071]

[0072]

[0073] Example 2:

[0074] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0075] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0076] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 9h. It adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is above 960℃; the initial rolling temperature of the finishing stage is 950℃, and the final rolling temperature is above 860℃. The total reduction rate is 92%, ensuring that the reduction rate is ≥20% in at least three passes during the roughing stage. The thickness of the finished steel plate is 20mm.

[0077] Step 2: The obtained steel plate is subjected to quenching at 940℃ with a net holding time of 1.5 min / mm; tempering at 600℃ with a net holding time of 2.0 min / mm.

[0078] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 3 , 4 As shown, the microstructure at 1 / 4 and 1 / 2 of the steel plate is granular bainite, with carbides uniformly dispersed and precipitated. The tensile and impact properties at 1 / 4 and 1 / 2 of the steel plate in this embodiment are shown in Tables 4 and 5, respectively.

[0079] Table 4 Tensile properties of 20mm thick steel plate at various locations

[0080]

[0081] Table 5 Impact performance of 20mm thick steel plate at various locations

[0082]

[0083] Example 3:

[0084] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0085] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0086] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 11h. It adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is above 960℃; the initial rolling temperature of the finishing stage is 950℃, and the final rolling temperature is above 860℃. The total reduction rate is 87%, ensuring that the reduction rate is ≥20% in at least three passes during the roughing stage. The thickness of the finished steel plate is 40mm.

[0087] Step 2: The obtained steel plate is subjected to quenching at 900℃ with a net holding time of 1.5 min / mm; tempering at 630℃ with a net holding time of 4.0 min / mm.

[0088] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 5 , 6As shown, the microstructure at 1 / 4 and 1 / 2 of the steel plate is granular bainite + lath bainite, with average grain sizes of 15 and 20 μm, respectively. The lath bainite content is much higher than that of granular bainite. The tensile and impact properties of the steel plate at 1 / 4 and 1 / 2 of this embodiment are shown in Tables 6 and 7, respectively.

[0089] Table 6 Tensile properties of 40mm thick steel plate at various locations

[0090]

[0091] Table 7 Impact performance of 40mm thick steel plate at various locations

[0092]

[0093] Example 4:

[0094] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0095] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0096] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 11h. It adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is above 960℃; the initial rolling temperature of the finishing stage is 950℃, and the final rolling temperature is above 860℃. The total reduction rate is 80%, ensuring that the reduction rate is ≥20% in at least three passes during the roughing stage. The thickness of the finished steel plate is 60mm.

[0097] Step 2: The obtained steel plate is subjected to quenching at 920℃ with a net holding time of 1.0 min / mm; tempering at 630℃ with a net holding time of 2.0 min / mm.

[0098] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 7 , 8 As shown, the microstructure at 1 / 4 and 1 / 2 of the steel plate is granular bainite + lath bainite, with average grain sizes of 18 and 22 μm, respectively. The lath bainite content is much higher than that of granular bainite. The tensile and impact properties of the steel plate at 1 / 4 and 1 / 2 of this embodiment are shown in Tables 8 and 9, respectively.

[0099] Table 8 Tensile properties of 60mm thick steel plate at various locations

[0100]

[0101] Table 9 Impact performance of 60mm thick steel plate at various locations

[0102]

[0103] Example 5:

[0104] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0105] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0106] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 10.5h. A two-stage rolling method of roughing and finishing is adopted. The initial rolling temperature of the roughing stage is 1080℃, and the final rolling temperature is above 950℃; the initial rolling temperature of the finishing stage is 870℃, and the final rolling temperature is above 810℃. The total reduction rate is 77%, ensuring that the reduction rate is ≥20% in at least three passes during the roughing stage. The thickness of the finished steel plate is 68mm.

[0107] Step 2: The obtained steel plate is subjected to quenching at 920℃ with a net holding time of 1.5 min / mm; sub-temperature quenching at 870℃ with a net holding time of 0.7 min / mm; and tempering at 570℃ with a net holding time of 3.0 min / mm.

[0108] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 9 , 10 As shown, the microstructure at 1 / 4 and 1 / 2 of the steel plate is lath bainite with trace amounts of blocky ferrite, with average grain sizes of 21 and 24 μm, respectively. The tensile and impact properties at 1 / 4 and 1 / 2 of the steel plate in this embodiment are shown in Tables 10 and 11, respectively.

[0109] Table 10 Tensile properties of 68mm thick steel plate at various locations

[0110]

[0111] Table 11 Impact performance of 68mm thick steel plate at various locations

[0112]

[0113] Example 6:

[0114] A novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, the composition and mass percentage of which are shown in Table 1.

[0115] The preparation method of this novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate is carried out according to the following steps:

[0116] Step 1: The continuous casting billet is heated to 1180±10℃, and the net holding time is 11h. It adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is above 950℃; the initial rolling temperature of the finishing stage is 870℃, and the final rolling temperature is above 810℃. The total reduction rate is 70%, ensuring that the reduction rate is ≥20% in at least four passes in the roughing stage. The thickness of the finished steel plate is 90mm.

[0117] Step 2: The obtained steel plate is subjected to quenching at 920℃ with a net holding time of 1.0 min / mm; sub-temperature quenching at 890℃ with a net holding time of 1.0 min / mm; and tempering at 600℃ with a net holding time of 3.0 min / mm.

[0118] The metallographic microstructures of the novel ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate prepared in this embodiment at 1 / 2 and 1 / 4 of the plate are as follows: Figure 11 , 12 As shown, the microstructure at 1 / 4 and 1 / 2 of the steel plate is lath bainite with trace amounts of blocky ferrite, with average grain sizes of 24 and 28 μm, respectively. The tensile and impact properties at 1 / 4 and 1 / 2 of the steel plate in this embodiment are shown in Tables 12 and 13, respectively.

[0119] Table 12 Tensile properties of 90mm thick steel plate at various locations

[0120]

[0121] Table 13 Impact performance of 90mm thick steel plate at various locations

[0122]

[0123] The carbon content of this invention reaches an ultra-low carbon level (0.03%), the maximum thickness reaches 90mm, the yield strength at 1 / 4 of the steel plate is ≥540MPa, the tensile strength is ≥634MPa, the impact energy at -20℃ is ≥294J, and the comprehensive performance meets the relevant standard requirements.

[0124] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-low carbon 600MPa grade high-strength and tough hydroelectric steel plate, characterized in that, The steel plate has the following chemical composition by weight percentage. composition: C: 0.03%~0.05%, Si: 0.15%~0.35%, Mn: 1.40%~1.60%, P≤0.007%, S≤0.0008%, Cr: 0.02%~0.09%, Mo: 0.15%~0.40%, Ni: 0.15%~0.35%, B: 0.0001%~0.0007%, Ti: 0.005%~0.025%, Nb: 0.01%~0.035%, Al: 0.02~0.045%, La: 0.01%~0.04%, Cu≤0.03%, balance Fe; CEV(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14; The steel plate satisfies DI≥0.65H; where: DI is the hardenability index, in mm; H is the steel plate thickness, in mm; The microstructure at 1 / 4 and 1 / 2 of the steel plate consists of granular bainite and dispersed alloy cementite. Specifically, the steps include the following: 1) Smelting High-purity molten iron is used in the smelting process, and the double-slag method is used for operation. Deep desulfurization treatment is carried out and the slag is removed cleanly. LF and RH are used to refine the molten steel outside the ladle to ensure that white slag is produced in the LF furnace, and that the [S] in the steel is ≤0.0008%. The net circulation of RH is not less than 10 minutes, and the standing time before pouring is not less than 15 minutes. The slag surface in the ladle is kept in a surging state and does not expose the molten steel surface. The inclusions in the steel are modified by the treatment of composite deoxidizer. 2) Casting The continuous casting process employs protective casting, controls the superheat of molten steel to below 30°C, controls the billet casting speed to 1.0 m / min, and uses a light reduction technique at the end of the continuous casting process. 3) Heated rolling The heating temperature is 1100~1200℃, and the net heat preservation time is 8.5~12h; The rolling process adopts a two-stage rolling method of roughing and finishing. The initial rolling temperature of the roughing stage is 1050℃, and the final rolling temperature is 950~970℃. The initial rolling temperature for finishing rolling is 910~950℃, and the final rolling temperature is above 860℃. The total reduction rate is 70%~92%, ensuring that at least three passes have a reduction rate of ≥20%; 4) Heat treatment Quenching process: temperature 900~940℃, net holding time 0.5~2.5min / mm; Sub-temperature quenching process: temperature 850~890℃, net holding time 0.5~1.0min / mm; Tempering process: temperature 570~600℃, net holding time 2.0~4.0min / mm.

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