Ultra-pure high-strength high-toughness easy-to-weld steel and preparation method thereof

Abstract

The invention relates to the field of alloy steel, and provides ultra-pure high-strength high-toughness easy-to-weld steel and a preparation method thereof.The ultra-pure high-strength high-toughness easy-to-weld steel comprises, by mass, 0.04%-0.09% of C, smaller than or equal to 0.5% of Si, 0.5%-1.0% of Mn, smaller than or equal to 0.005% of P, smaller than or equal to 0.0005% of S, 2.5%-4.0% of Cu, 9.0%-11.0% of Ni, 0.5%-1.0% of Cr, 0.5%-1.0% of Mo, 0.02%-0.05% of Nb, 0.02%-0.05% of Ti, 0.02%-0.05% of V, smaller than or equal to 0.05% of Al and the balance Fe and inevitable impurities. The ultra-pure high-strength high-toughness easy-to-weld steel can meet the requirements of ultrahigh strength, high toughness and good welding performance of a steel plate at the same time.

Description

Technical Field

[0001] The present invention relates to the field of alloy steel, and more specifically, to an ultra-pure high-strength, high-toughness and easily weldable steel and a preparation method thereof. Background Art

[0002] With the rapid development of the industrial society, the demand for the performance of steel materials in construction machinery and equipment is getting higher and higher, and it is required that the steel plate has ultra-high strength, high toughness and good weldability. The microstructure of traditional high-strength steel is mainly martensite with a relatively high carbon content. In order to ensure hardenability, usually a relatively high content of alloying elements such as Ni, Cr, Mo, V, etc. needs to be added, resulting in a high carbon equivalent of the material and poor weldability. At the same time, the control of impurity elements and inclusions in traditional smelting processes has reached its limit. In ultra-high-strength steel, as the stress concentration level increases, the influence of impurity elements and inclusions on toughness is further amplified, and traditional smelting is no longer applicable to ultra-high-strength steel. Currently, most high-strength steels above 1100 MPa still adopt the composition design of carbon elements plus micro-alloying elements and are smelted in the traditional way, resulting in unsatisfactory plastic toughness and weldability.

[0003] In patent documents such as CN110578095A, CN104513936B, and CN111910129A in the prior art, high-strength steels with a yield strength greater than 1100 MPa and their manufacturing methods are disclosed, but their carbon content is higher than 0.1% and they are not easy to weld. In the patent document with the publication number CN108486505A in the prior art, a 1200 MPa grade hot-rolled low-carbon steel of the silicon-manganese-chromium system and a preparation method thereof are disclosed, but the yield strength is less than 1200 MPa. In the patent document with the publication number CN110358971B in the prior art, a low-carbon ultra-high-strength steel with a yield strength of 1300 MPa grade and a preparation method thereof are disclosed. By adding 1.5 - 2.5% of copper for precipitation strengthening, the yield strength is greater than 1300 MPa, but the low-temperature toughness is poor, and the impact energy at -60 °C is less than 50 J. In the patent document with the publication number CN106636961A in the prior art, a Cu nanophase-reinforced easily weldable steel and a preparation method thereof are disclosed, which has a relatively high aluminum content and insufficient low-temperature toughness. In the patent document with the publication number CN114058815B in the prior art, an 1150 MPa grade high-strength, high-toughness and easily weldable nano steel and a preparation method thereof are disclosed, which adopts the traditional refining process, but the yield strength is less than 1200 MPa under the condition of ensuring toughness.

[0004] In summary, there is currently no steel material that can simultaneously meet ultra-high strength, high toughness and good weldability, which poses a great challenge to material composition design and smelting preparation processes.

[0005] In view of this, the present invention is specifically proposed. Summary of the Invention

[0006] The object of the present invention is to provide an ultra-pure high-strength, high-toughness and easily weldable steel and its preparation method, so as to solve the problem that there is currently no steel material in the prior art that can simultaneously meet the requirements of ultra-high strength, high toughness and good welding performance.

[0007] To achieve the above object, the technical solution of the present invention is realized as follows:

[0008] An ultra-pure high-strength, high-toughness and easily weldable steel, the chemical composition of the ultra-pure high-strength, high-toughness and easily weldable steel by mass percentage includes: C: 0.04 - 0.09, Si ≤ 0.5, Mn: 0.5 - 1.0, P ≤ 0.005, S ≤ 0.0005, Cu: 2.5 - 4.0, Ni: 9.0 - 11.0, Cr: 0.5 - 1.0, Mo: 0.5 - 1.0, Nb: 0.02 - 0.05, Ti: 0.02 - 0.05, V: 0.02 - 0.05, Al ≤ 0.05, and the balance is Fe and unavoidable impurities.

[0009] The ultra-pure high-strength, high-toughness and easily weldable steel of the present invention can simultaneously meet the requirements of ultra-high strength, high toughness and good welding performance for steel plates, with a plate thickness of 10 - 30 mm, yield strength R

[0010] , ,

[0009] , p0.2 , , m , p0.2 , , , m ,

[0014] , ,

[0013] , ,

[0012] ,

[0011] ≥ 1200 MPa, tensile strength R m ≥ 1220 MPa, elongation ≥ 15%, impact energy -80°C KV2 ≥ 100 J.

[0010] Furthermore, the microstructure of the ultra-pure high-strength, high-toughness and easily weldable steel is lath martensite, Cu-containing precipitation phase and austenite structure.

[0011] Furthermore, the plate thickness of the ultra-pure high-strength, high-toughness and easily weldable steel is 10 - 30 mm, yield strength R p0.2 ≥ 1200 MPa, tensile strength R m ≥ 1220 MPa, elongation ≥ 15%, impact energy -80°C KV2 ≥ 100 J.

[0012] In the second aspect of the present invention, a preparation method of an ultra-pure high-strength, high-toughness and easily weldable steel is provided. The preparation method is used to prepare any one of the above-mentioned ultra-pure high-strength, high-toughness and easily weldable steels, and the preparation method includes the following steps:

[0013] S1. Melting and refining: Using hot metal from a blast furnace or an electric furnace, blowing oxygen to remove phosphorus and carbon, deoxidizing with aluminum, then transferring to a ladle furnace for refining, and simultaneously adding alloy materials to adjust the composition to the target composition, and then carrying out dehydrogenation and deoxidation in a VD vacuum furnace;

[0014] S2. Electroslag remelting: The refined electrode billet is subjected to electroslag remelting, and after solidification, a dense and compositionally uniform ingot is formed.

[0015] S3. Rolling: The ingot is preheated before rolling. After preheating, rolling is carried out. Rolling includes rough rolling and finish rolling. The rough rolling temperature is controlled at 980 - 1150 °C, and the finish rolling temperature is 920 - 1000 °C.

[0016] S4. Heat treatment: First, quenching heat treatment is carried out. The temperature of the quenching heat treatment is 750 - 900 °C, and the quenching holding time is t1. The steel plate is held at the quenching temperature for t1 time and then ultra-rapidly cooled to room temperature. Subsequently, tempering heat treatment is carried out. The temperature of the tempering heat treatment is 500 - 600 °C, and the tempering holding time is t2. It is held at the tempering heat treatment temperature for t2 time and then air-cooled to room temperature.

[0017] Further, in step S4, the quenching holding time t1 = 30 + (H - 10) × 1.5, where the unit of t1 is min and the unit of H, the thickness of the finished steel plate, is mm.

[0018] Further, in step S4, the tempering holding time t2 = 60 + (H - 10) × 2.5, where the unit of t2 is min and the unit of H, the thickness of the finished steel plate, is mm.

[0019] Further, in step S3, the rough rolling starting temperature is 1100 - 1130 °C, and the rough rolling finishing temperature is 1020 - 1060 °C; the finish rolling starting temperature is 980 - 1000 °C, and the finish rolling finishing temperature is 940 - 950 °C.

[0020] Further, in step S3, the preheating temperature is 1150 - 1180 °C, and the holding time is 3 - 8 h.

[0021] Further, in step S3, the single-pass reduction ratio in the rough rolling process is ≥ 15%.

[0022] Further, in step S3, the scale on the ingot is removed by high-pressure water before rolling.

[0023] The present invention provides a super-pure high-strength, high-toughness and easily weldable steel and its preparation method. Compared with the prior art, the super-pure high-strength, high-toughness and easily weldable steel and its preparation method of the present invention have the following beneficial effects:

[0024] 1) A super-clean high-strength, high-toughness and easy-to-weld steel according to the present invention replaces traditional carbon strengthening with a high number density of Cu precipitation phases, has a low carbon content and good welding performance; through a variety of toughening mechanisms, high toughness is maintained while achieving high strength, improving the safety and stability of large and heavy steel structures; the good weldability saves the cost of component manufacturing. Especially for ultra-high-strength steel plates, the sensitivity to welding cold cracks is greatly reduced, the preheating and post-heating temperatures during welding are lowered, and the heat input range is wider, significantly reducing costs.

[0025] 2) The super-clean high-strength, high-toughness and easy-to-weld steel according to the present invention has a plate thickness of 10 - 30 mm, a yield strength R p0.2 ≥1200 MPa, a tensile strength R m ≥1220 MPa, an elongation rate ≥15%, and an impact energy -80°C KV2 ≥100 J.

[0026] 3) The preparation method of the super-clean high-strength, high-toughness and easy-to-weld steel of the present invention is simple, the process controllability is strong, and it is easy to realize industrial production.

[0027] 4) The super-clean high-strength, high-toughness and easy-to-weld steel of the present invention can meet the requirements of fields such as the shipbuilding industry, offshore engineering, mining, power and mechanical engineering, which have relatively high requirements for strength and low-temperature toughness. It is especially suitable for structural parts that need to be welded and is a technology worthy of popularization. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 s a metallographic diagram of a super-clean high-strength, high-toughness and easy-to-weld steel according to Embodiment 1 of the present invention;

[0029] Figure 2 is a metallographic diagram of a super-clean high-strength, high-toughness and easy-to-weld steel according to Embodiment 3 of the present invention;

[0030] Figure 3 is one of the test diagrams of a super-clean high-strength, high-toughness and easy-to-weld steel according to Embodiment 1 of the present invention using a transmission electron microscope;

[0031] Figure 4 is a test diagram of a super-clean high-strength, high-toughness and easy-to-weld steel according to Embodiment 1 of the present invention using TEM-EDS;

[0032] Figure 5 is the second test diagram of a super-clean high-strength, high-toughness and easy-to-weld steel according to Embodiment 1 of the present invention using a transmission electron microscope. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In order to make the technical means, objectives and effects of the present invention easy to understand, the embodiments of the present invention will be described in detail below with reference to specific drawings.

[0034] It should be noted that all the terms indicating direction and position in the present invention, such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "inner", "outer", "top", "bottom", "lateral", "longitudinal", "center", etc., are only used to explain the relative positional relationship and connection situation between components under a certain specific state (as shown in the attached drawings), and are only for the convenience of describing the present invention, rather than requiring the present invention to be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention. In addition, the descriptions involving "first", "second", etc. in the present invention are only for descriptive purposes, and should not be construed as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features.

[0035] In the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0036] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "schematic embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0037] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0038] In the prior art, there is currently no problem of a steel material that can simultaneously meet the requirements of ultra-high strength, high toughness, and good welding performance.

[0039] To solve the above technical problems, the applicant proposes a ultra-pure high-strength, high-toughness and easy-welding steel. The chemical composition of the ultra-pure high-strength, high-toughness and easy-welding steel by mass percentage includes: C: 0.04 - 0.09, Si ≤ 0.5, Mn: 0.5 - 1.0, P ≤ 0.005, S ≤ 0.0005, Cu: 2.5 - 4.0, Ni: 9.0 - 11.0, Cr: 0.5 - 1.0, Mo: 0.5 - 1.0, Nb: 0.02 - 0.05, Ti: 0.02 - 0.05, V: 0.02 - 0.05, Al ≤ 0.05, and the balance is Fe and unavoidable impurities.

[0040] The invention principle and chemical composition design basis of this ultra-pure, high-strength, high-toughness and easily weldable steel are as follows:

[0041] Invention principle: The microstructure of the ultra-pure, high-strength, high-toughness and easily weldable steel of the present invention is lath martensite, Cu-containing precipitate phase and a small amount of austenite structure. In order to ensure the weldability of the steel, the design idea of the present invention is to reduce the carbon content and the carbon equivalent of alloying elements as much as possible to improve the weldability. After the carbon content and the carbon equivalent of alloying elements are reduced, the strength will be significantly reduced. In order to meet the strength requirements, Cu-containing precipitate phases are precipitated during aging by adding copper, nickel and manganese elements, which are uniformly distributed in the matrix phase and play a strengthening role by hindering the movement of dislocations. When the strength of the steel plate is increased to more than 1200 MPa, it is generally difficult to maintain a high level of low-temperature toughness of the steel plate. In order to solve the problem of strength-toughness matching, the present invention realizes high toughness through the following three toughening mechanisms: First, through an ultra-clean smelting process, the content of impurity elements and inclusions in the steel is reduced to the lowest level, stress concentration and grain boundary embrittlement tendency are greatly reduced, and toughness is improved; Second, through multi-scale microalloying element design, an Nb-V-Ti composite microalloying element system is adopted, and the grain boundary pinning effect is realized by precipitating Nb(C, N), the V element controls austenite recrystallization, and the Ti element controls the morphology of sulfides, so as to fully refine the grain size and improve toughness; Third, the multi-phase tissue collaborative control technology forms a low-carbon lath martensite + reverse transformation austenite structure in the steel through composition design and thermo-mechanical treatment process. Lath martensite has an obvious hindering effect on crack propagation, and at the same time, reverse transformation austenite passivates the crack tip to inhibit crack initiation and propagation, ensuring that the toughness of the steel plate can reach a relatively high level.

[0042] Chemical composition design basis:

[0043] C: Carbon is a solid solution strengthening element and plays an important role in improving strength. Traditional steel materials mainly improve strength through the solid solution strengthening of carbon. However, excessive carbon will form large brittle cementite during tempering, seriously affecting toughness. At the same time, the increase in high carbon content will affect weldability. The present invention uses nano-phase strengthening instead of traditional carbon strengthening, so the mass percentage of carbon element is controlled at 0.04 - 0.09.

[0044] Cu: Copper is the main element for forming precipitate phases. It can improve strength without losing plasticity and toughness by forming nano-scale precipitate phases. At the same time, copper also has the effect of refining grains. Too low copper content will affect the strengthening effect, and too high content is easy to cause hot brittleness, affecting welding and hot working. Therefore, the mass percentage of copper element in the present invention is controlled at 2.5 - 4.0.

[0045] Ni: Nickel is one of the main elements for the formation of nano-precipitates. It forms a B2 ordered structure that wraps around the precipitate phase formed by copper elements, increasing the thermal stability of the precipitate phase. At the same time, nickel can strengthen the matrix and promote the formation of reverse-transformed austenite, significantly improving the low-temperature toughness. In this invention, the mass percentage of nickel element is controlled within 9.0 - 11.0.

[0046] Mn: Manganese is one of the main components of nano-precipitate phases. It can also refine grains, improve the strength and low-temperature toughness of steel. However, when the content is too high, it is prone to cause slab segregation, large tissue stress, and a decline in welding performance. In this invention, the mass percentage of manganese element is controlled within 0.5 - 1.0.

[0047] Si: Silicon mainly exists in the form of solid solution in steel. It can slightly increase the yield strength and fatigue strength of steel, and at the same time improve the hardenability of steel. However, when the content is too high, it will affect plasticity and weldability and form inclusions. Therefore, in this invention, the mass percentage of silicon element is controlled at ≤0.5.

[0048] Al: Aluminum is a strong deoxidizing element in the steel-making process and can also achieve the effect of refining grains. However, when the content is too high, it will promote the graphitization tendency of carbon in steel and reduce the effect of refining grains. In this invention, the mass percentage of aluminum element is controlled below 0.05.

[0049] Cr: Chromium can increase the corrosion resistance of steel, improve the hardenability and tempering stability of steel at the same time. In this invention, the mass percentage of chromium element is controlled within 0.5 - 1.0.

[0050] Mo: Molybdenum can increase the hardenability of steel, refine grains, and can also form carbides to increase strength. At the same time, it has a promoting effect on the nucleation of nano-precipitate phases. In this invention, the mass percentage of molybdenum element is controlled within 0.5 - 1.0.

[0051] Nb: Niobium can form carbonitrides to pin the austenite grain boundaries, prevent grain growth, and can also play a role in precipitation strengthening to increase strength. In this invention, the mass percentage of niobium element is controlled within 0.02 - 0.05.

[0052] Ti: Titanium can form carbonitrides to pin the grain boundaries and refine grains. In this invention, the mass percentage of titanium element is controlled within 0.02 - 0.05.

[0053] V: Vanadium can form carbonitrides to pin the grain boundaries, inhibit austenite recrystallization during rolling, and refine grains. In this invention, the mass percentage of vanadium element is controlled within 0.02 - 0.05.

[0054] P: Phosphorus is prone to segregation at grain boundaries during the solidification of molten steel, weakening the grain boundary bonding force, causing a sharp drop in impact toughness, a significant increase in the brittle transition temperature, and at the same time increasing the hardening tendency and cold cracking sensitivity in the heat affected zone of welding. In alloy steel, it will also cause temper brittleness. In the ultra-high strength steel of the present invention, with the increase in strength, the harm of P element is further amplified. Therefore, the mass percentage of P element in the present invention is ≤0.005.

[0055] S: Sulfur is likely to form sulfide inclusions in steel, which will lead to a decrease in transverse plastic toughness after rolling of the steel plate and cause the generation of welding hot cracks at the same time. Sulfide inclusions will also affect the toughness of the heat affected zone and deteriorate the welding performance. In the present invention, the mass percentage of S element is ≤0.0005.

[0056] For the ultra-pure high-strength, high-toughness and easily weldable steel described in the present invention, on the one hand, the chemical compositions of various elements are carefully designed, which not only ensures the strength but also avoids the decrease in toughness and the deterioration of weldability; on the other hand, the addition and proportion control of elements such as copper, nickel, and manganese form an effective precipitation strengthening mechanism, improving the strength and toughness of the steel; in addition, the addition of micro-alloying elements Nb, Ti, and V further improves the strength and toughness of the steel by refining grains and pinning grain boundaries.

[0057] The ultra-pure high-strength, high-toughness and easily weldable steel described in the present invention can simultaneously meet the requirements for ultra-high strength, high toughness and good welding performance of the steel plate. The plate thickness is 10 - 30 mm, and the yield strength R p0.2 ≥1200 MPa, the tensile strength R m ≥1220 MPa, the elongation ≥15%, and the impact energy -80°C KV2 ≥100 J.

[0058] Preferably, the chemical composition of the ultra-pure high-strength, high-toughness and easily weldable steel by mass percentage includes: C: 0.042 - 0.082, Si: 0.24 - 0.4, Mn: 0.81 - 0.93, P: 0.003 - 0.004, S: 0.0004, Cu: 2.6 - 3.8, Ni: 9.7 - 10.7, Cr: 0.65 - 0.95, Mo: 0.57 - 0.9, Nb: 0.03 - 0.05, Ti: 0.021 - 0.048, V: 0.02 - 0.05, Al: 0.022 - 0.042, and the balance is Fe and unavoidable impurities.

[0059] Specifically, the microstructure of the ultra-pure high-strength, high-toughness and easily weldable steel is lath martensite, Cu-containing precipitate phase and austenite structure.

[0060] Specifically, the plate thickness of the ultra-pure high-strength, high-toughness and easily weldable steel is 10 - 30 mm, and the yield strength R p0.2 ≥1200 MPa, the tensile strength Rm ≥1220 MPa, elongation ≥ 15%, impact energy -80°C KV2 ≥ 100 J.

[0061] In the second aspect of the present invention, a method for preparing the above-mentioned ultra-pure high-strength high-toughness weldable steel is proposed. The preparation method is used to prepare any one of the above-mentioned ultra-pure high-strength high-toughness weldable steels, and the preparation method includes the following steps:

[0062] S1. Melting and refining: Using hot metal from a blast furnace or an electric furnace, blowing oxygen to remove phosphorus and carbon, deoxidizing with aluminum, then transferring to a ladle furnace for refining, and adding alloy materials simultaneously to adjust the composition to the target composition, and then dehydrogenating and deoxidizing in a VD vacuum furnace;

[0063] S2. Electroslag remelting: Electroslag remelting the refined electrode billet, and forming a dense and uniform composition ingot after solidification;

[0064] S3. Rolling: Preheating the ingot before rolling, and performing rolling after preheating. The rolling includes rough rolling and finish rolling. The rough rolling temperature is 980 - 1150°C, and the finish rolling temperature is 920 - 1000°C;

[0065] S4. Heat treatment: First, perform quenching heat treatment. The temperature of the quenching heat treatment is 750 - 900°C, and the quenching holding time is t1. After the steel plate is held at the quenching temperature for t1 time, it is rapidly cooled to room temperature by ultra-fast cooling, and then tempering heat treatment is performed. The temperature of the tempering heat treatment is 500 - 600°C, and the tempering holding time is t2. After being held at the tempering heat treatment temperature for t2 time, it is air-cooled to room temperature.

[0066] In the method for preparing the ultra-pure high-strength high-toughness weldable steel of the present invention, steps S1 to S4 are interrelated and inseparable. Through the steps of blowing oxygen to remove phosphorus and carbon, deoxidizing with aluminum, ladle furnace refining, and dehydrogenating and deoxidizing in a VD vacuum furnace during the melting and refining process of step S1, the purity and composition uniformity of the steel are ensured; through the electroslag remelting process of step S1, the density and composition uniformity of the ingot are further improved; through the rolling process of step S3, the structure and properties of the steel plate are controlled through steps such as preheating, rough rolling, and finish rolling; through the heat treatment process of step S4, through steps such as quenching and tempering, and through a two-step heat treatment process of solution treatment plus aging treatment, the microstructure and properties of the steel plate are adjusted. In summary, the steel prepared by the method for preparing an ultra-pure high-strength high-toughness weldable steel of the present invention can simultaneously meet the requirements for ultra-high strength, high toughness, and good weldability of the steel plate. The plate thickness is 10 - 30 mm, and the yield strength R p0.2 ≥1200 MPa, tensile strength R m ≥1220 MPa, elongation ≥ 15%, impact energy -80°C KV2 ≥ 100 J.

[0067] Specifically, in step S4, the quenching holding time t1 = 30 + (H - 10)×1.5, where the unit of t1 is min and the unit of H, the thickness of the finished steel plate, is mm.

[0068] Specifically, in step S4, the tempering holding time t2 = 60 + (H - 10)×2.5, where the unit of t2 is min and the unit of H, the thickness of the finished steel plate, is mm.

[0069] Specifically, in step S3, the rough rolling starting temperature is 1100 - 1130°C, and the rough rolling finishing temperature is 1020 - 1060°C; the finish rolling starting temperature is 980 - 1000°C, and the finish rolling finishing temperature is 940 - 950°C.

[0070] Specifically, in step S3, the preheating temperature is 1150 - 1180°C, and the holding time is 3 - 8 h.

[0071] Specifically, in step S3, the single - pass reduction ratio in the rough rolling process is ≥15%.

[0072] Specifically, in step S3, the scale on the steel ingot is removed by high - pressure water before rolling.

[0073] Specifically, in step S3, high - pressure water descaling is adopted during the rolling process.

[0074] The ultra - pure, high - strength, high - toughness and easy - welding steel of the present invention and the preparation method act synergistically, are interrelated and inseparable, and have the following advantages:

[0075] 1. The prepared steel has ultra - high strength: Through precise chemical composition design and heat treatment process, the yield strength R p0.2 ≥1200 MPa, and the tensile strength R m ≥1220 MPa, meeting the requirements for ultra - high - strength materials.

[0076] 2. The prepared steel has high toughness: On the one hand, through the ultra - clean smelting process, the content of impurity elements and inclusions in the steel is reduced to the lowest level, reducing stress concentration and the tendency of grain - boundary embrittlement, thus improving toughness; on the other hand, the Nb - V - Ti composite micro - alloying element system is adopted. The precipitation of Nb(C, N) realizes the grain - boundary pinning effect, the V element controls austenite recrystallization, and the Ti element controls the morphology of sulfides, achieving sufficient refinement of grain size and further improving toughness; in addition, through composition design and thermo - mechanical treatment process, a low - carbon lath martensite + reverse - transformed austenite structure is formed in the steel. The lath martensite hinders crack propagation, and the reverse - transformed austenite passivates the crack tip, inhibiting crack initiation and propagation, ensuring high toughness of the steel plate, and the impact energy - 80°C KV2≥100 J.

[0077] III. The prepared steel has good weldability: on the one hand, by minimizing the carbon content and the carbon equivalent of alloying elements as much as possible, the weldability of the steel is improved; on the other hand, nano-phase strengthening replaces traditional carbon strengthening, avoiding the problem of deteriorated weldability caused by high carbon content.

[0078] IV. It can meet the requirements for ultra-high strength, high toughness and good weldability of steel plates simultaneously.

[0079] The chemical compositions of the embodiments of the present invention are shown in Table 1 (mass percentage), and the balance is Fe and inevitable impurities.

[0080] Table 1 Chemical Compositions of Embodiments

[0081]

[0082] The preparation method of the ultra-pure high-strength high-toughness weldable steel described in the above embodiments includes the following steps:

[0083] S1. Melting and refining: Using hot metal from a blast furnace or an electric furnace, blowing oxygen to remove phosphorus and carbon, deoxidizing with aluminum, then transferring to a ladle furnace for refining, adding alloy materials simultaneously, adjusting the composition to the target composition, and then performing dehydrogenation and deoxidation in a VD vacuum furnace;

[0084] S2. Electroslag remelting: Performing electroslag remelting on the refined electrode blank, and forming a dense and compositionally uniform ingot after solidification;

[0085] S3. Rolling: Preheating the ingot before rolling, and performing rolling after preheating. The rolling includes rough rolling and finish rolling. The rough rolling temperature is 980 - 1150 °C, and the finish rolling temperature is 920 - 1000 °C;

[0086] S4. Heat treatment: First performing quenching heat treatment, the temperature of the quenching heat treatment is 750 - 900 °C, the quenching holding time is t1, the steel plate is held at the quenching temperature for t1 time and then ultra-rapidly cooled to room temperature, and then performing tempering heat treatment. The temperature of the tempering heat treatment is 500 - 600 °C, the tempering holding time is t2, and it is air-cooled to room temperature after being held at the tempering heat treatment temperature for t2 time.

[0087] The rolling process parameters of each embodiment are shown in Table 2. During the rolling process in step S3, the rough rolling time and the finish rolling time of each embodiment are both 3 min; the single-pass reduction amount during rough rolling of each embodiment is 20%, and the scale on the ingot is removed by high-pressure water before rolling for each embodiment; high-pressure water is used for dephosphorization during the rolling process of each embodiment. The heat treatment process parameters of each embodiment are shown in Table 3.

[0088] Table 2 Rolling Process Parameters of Embodiments

[0089]

[0090] Table 3 Heat treatment process parameters of each embodiment

[0091]

[0092] Performance test:

[0093] I. Samples were taken from the steel plates prepared in Example 1 in the direction perpendicular to the rolling direction and tested by metallographic corrosion with 4% nitric acid alcohol. The results are shown in Figure 1 ; Samples were taken from the steel plates prepared in Example 3 in the direction perpendicular to the rolling direction and tested by metallographic corrosion with 4% nitric acid alcohol. The results are shown in Figure 2 ; Samples were taken from the steel plates prepared in Example 1 and tested using a transmission electron microscope. The results are shown in Figure 3 and Figure 5 ; Samples were taken from the steel plates prepared in Example 1 and tested using TEM-EDS. The results are shown in Figure 4 .

[0094] It can be seen from Figure 1 and Figure 2 that the microstructure of the steel prepared by Example 1 and Example 3 includes lath martensite structure. It can be seen from Figure 3 that the microstructure of the steel prepared by Example 1 includes Cu-containing precipitate phases. It can be seen from Figure 4 that the segregation of Cu element in the microstructure of the steel prepared by Example 1 under TEM-EDS also indicates that the microstructure of the steel includes Cu-containing precipitate phases. It can be seen from Figure 5 that the microstructure of the steel prepared by Example 1 includes lamellar austenite structure.

[0095] II. For the steel plates prepared in Examples 1 to 3, transverse samples were processed into tensile and impact specimens, and mechanical property tests were carried out. The results are shown in Table 4.

[0096] Table 4 Mechanical properties of each embodiment

[0097]

[0098] As can be seen from Table 4, the steel plates prepared in Examples 1 to 3 of the present invention have a plate thickness of 10 - 30 mm, a yield strength R p0.2 ≥1200 MPa, a tensile strength R m ≥1220 MPa, an elongation ≥15%, and an impact energy -80°C KV2≥100 J.

[0099] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. A super-pure high-strength, high-toughness and easily weldable steel, characterized in that, The chemical composition of the ultra-pure high-strength, high-toughness and weldable steel by mass percentage includes: C: 0.04 - 0.09, Si ≤ 0.5, Mn: 0.5 - 1.0, P ≤ 0.005, S ≤ 0.0005, Cu: 2.5 - 4.0, Ni: 9.0 - 11.0, Cr: 0.5 - 1.0, Mo: 0.5 - 1.0, Nb: 0.02 - 0.05, Ti: 0.02 - 0.05, V: 0.02 - 0.05, Al ≤ 0.05, and the balance is Fe and inevitable impurities.

2. The super-clean high-strength, high-toughness and easily weldable steel according to claim 1, characterized in that, The microstructure of the ultra-pure high-strength, high-toughness and weldable steel is lath martensite, Cu-containing precipitate phase and austenite structure.

3. The ultra-pure high-strength, high-toughness and easily weldable steel according to claim 1, wherein The plate thickness of the ultra-pure high-strength, high-toughness and easily weldable steel is 10 - 30 mm, and the yield strength R p0.2 ≥ 1200 MPa, the tensile strength R m ≥ 1220 MPa, the elongation ≥ 15%, and the impact energy -80°C KV2 ≥ 100 J.

4. A preparation method of an ultra-pure high-strength, high-toughness and easily weldable steel, characterized in that, The preparation method is used to prepare an ultra-pure high-strength, high-toughness and weldable steel according to any one of claims 1 - 3. The preparation method includes the following steps: S1. Melting and refining: Using hot metal from blast furnace or electric furnace, blowing oxygen to remove phosphorus and carbon, deoxidizing with aluminum, then transferring to ladle furnace for refining, adding alloy materials simultaneously to adjust the composition to the target composition, and then carrying out dehydrogenation and deoxidation in VD vacuum furnace; S2. Electroslag remelting: Electroslag remelting the refined electrode billet, and forming a dense and composition-uniform ingot after solidification; S3. Rolling: The ingot is preheated before rolling, and then rolled. The rolling includes rough rolling and finish rolling. The rough rolling temperature is 980 - 1150 °C, and the finish rolling temperature is 920 - 1000 °C; S4. Heat treatment: First, carry out quenching heat treatment. The temperature of quenching heat treatment is 750 - 900 °C, and the quenching holding time is t1. The steel plate is held at the quenching temperature for t1 time and then ultra-rapidly cooled to room temperature. Subsequently, carry out tempering heat treatment. The temperature of tempering heat treatment is 500 - 600 °C, and the tempering holding time is t2. It is held at the tempering heat treatment temperature for t2 time and then air-cooled to room temperature.

5. The preparation method of a super-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S4, the quenching holding time t1 = 30 + (H - 10) × 1.5, the unit of t1 is min, and H is the thickness of the finished steel plate, and the unit of H is mm.

6. The preparation method of a super-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S4, the tempering holding time t2 = 60 + (H - 10) × 2.5, where the unit of t2 is min, and H is the thickness of the finished steel plate, and the unit of H is mm.

7. The preparation method of an ultra-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S3, the rough rolling starting temperature is 1100 - 1130 °C, and the rough rolling finishing temperature is 1020 - 1060 °C; the finish rolling starting temperature is 980 - 1000 °C, and the finish rolling finishing temperature is 940 - 950 °C.

8. The preparation method of an ultra-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S3, the preheating temperature is 1150 - 1180 °C, and the holding time is 3 - 8 h.

9. The preparation method of a super-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S3, the single-pass reduction ratio in rough rolling process is ≥ 15%.

10. The preparation method of a super-pure high-strength, high-toughness and easily weldable steel according to claim 4, characterized in that, In step S3, the scale on the ingot is removed by high-pressure water before rolling.

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