A microalloyed high-strength steel with Ni-free ultra-low temperature toughness and its preparation method

Abstract

The present invention relates to a microalloyed high-strength steel with Ni-free ultra-low temperature toughness and a preparation method thereof, belonging to the technical field of metal materials. It is composed of the following chemical components by mass percentage: C: 0.011-0.099%, Si: 0.051-0.24%, Mn: 1.21-1.49%, Nb: 0.030-0.059%, Ti: 0.009-0.016%, Zr: 0.001-0.018%, RE (rare earth): 0.001-0.018%, and the rest is Fe and inevitable impurities; the mass percentages also simultaneously satisfy the formulas: 0.21% < C + Si < 0.24%, and Si / C = 1-8; 0.02% < Nb + Ti < 0.05%, and Nb / Ti = 1-3; 0.010% < Zr + RE < 0.019%, and Zr / RE = 1-6. The microalloyed high-strength steel of the present invention adopts a completely different composition design from the prior art to achieve ultra-low temperature toughness, that is, a Ni-free, low-cost and simplified composition design, supplemented by Zr+RE composite deoxidation and Nb+Ti composite microalloying technologies, to achieve ultra-low temperature toughness at -100 to -120 °C, and is suitable for structural materials such as polar regions, containers, pipelines, refining, storage and transportation, and equipment used in low-temperature and ultra-low temperature environments.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and particularly relates to a microalloyed high-strength steel with Ni-free ultra-low temperature toughness and a preparation method thereof. Background Art

[0002] Steel for low temperature use is an important variety of low-alloy high-strength steel. Generally, various production, storage containers and transportation pipelines for liquefied petroleum gas, liquid ammonia, liquid oxygen, liquid nitrogen, etc., and equipment serving in cold regions are called low-temperature containers, and the steel used to manufacture these containers is collectively called low-temperature steel. In China, the design temperature lower than or equal to -20°C is usually called low temperature (Definition of low-temperature pressure vessels in Appendix C of GB150-1998 "Steel Pressure Vessels").

[0003] Low-temperature steel is generally divided into Ni-free steel and Ni-containing steel. Ni-free steel generally refers to fine-grained steel and low-temperature high-strength steel, and its service temperature is above -60°C; Ni-containing steel refers to adding alloying element Ni into the steel, which is dissolved in ferrite, significantly improving the low-temperature toughness of the matrix, changing the common low-temperature brittle transition phenomenon of metal materials with body-centered cubic lattice, and its service temperature can reach below -196°C.

[0004] With the development of the petrochemical industry, new processes and new equipment have emerged continuously. The liquefaction, separation, storage and transportation, and application of gases have been very common in various countries. The development of these low-temperature technologies and equipment has promoted the development of steel for low-temperature pressure vessels. The Japanese JIS G3127(1977) "Nickel Steel Plates for Low-Temperature Pressure Vessels" standard, and the representative steel grades are SL3N255, SL3N275, SL3N440, etc. The American SA-203 / SA-203M "Nickel Alloy Steel Plates for Pressure Vessels" standard, and the representative grades are SA203Gr.D, SA203Gr.E, SA203Gr.F. Low-temperature steels with Ni content of 3.5% - 9% have corresponding standards in countries such as the United States, Japan, and Europe. In the late 1960s, China developed low-temperature steels from -40°C to -253°C, but they were not promoted and applied. In 1983, the National Bureau of Standards issued GB3531-83 "Technical Conditions for Low-Alloy Steel Heavy Plates for Low-Temperature Pressure Vessels", which specified 4 kinds of Ni-free low-temperature steels with service temperatures from -30°C to -90°C. 16MnDR steel is an economical and mature steel grade for manufacturing -40°C low-temperature equipment and can be used to manufacture equipment such as liquid ammonia. 15MnNiDR and 09MnNiDR belong to Ni-based low-temperature steels and have good low-temperature toughness and weldability. The -70°C grade 09MnNiDR low-temperature steel has been widely used in low-temperature devices such as ethylene, chemical fertilizers, city gas, and carbon dioxide, gradually replacing imported low-temperature steel, and successfully manufacturing ammonia separators (-28°C), high-pressure nitrogen storage tanks (-28°C), carbon dioxide low-temperature storage tanks (-50°C), and low-temperature ethylene storage tanks (-60°C).

[0005] Regarding low-temperature steel with a higher Ni content, in the material section of the CCS 1996 "Rules for the Classification and Construction of Steel Sea-Going Ships" in China, it is stipulated that nickel alloy steel with a thickness not exceeding 50 mm, which is applicable to the cargo tanks of liquefied gas carriers and the hull structures near the cargo tanks, includes 3 steel grades: 3.5Ni, 5Ni, and 9Ni (Table 1). In China, 3.5Ni low-temperature steel is used to manufacture low-temperature containers at -100°C, and 9Ni low-temperature steel is used to manufacture low-temperature containers at -196°C, which has reached a consensus in the pressure vessel industry. 3.5Ni steel is widely used in low-temperature containers from -101 to -80°C.

[0006] Table 1 Delivery Conditions and Mechanical Properties of Ni-Series Low-Temperature Steels in China

[0007]

[0008] Among low-temperature steels, nickel-based low-temperature steels are gradually widely adopted in developed countries such as the United States and Japan due to their high strength, excellent low-temperature toughness, and lower cost compared to Cr-Ni stainless steels of the corresponding temperature grades. For low-temperature containers, the lower the temperature of the stored medium, the lower the pressure the container needs to withstand, and the higher its safety and reliability. For liquefied natural gas, 9Ni (-196°C) low-temperature steel is mostly used at present; low-temperature devices in industries such as petrochemical and fertilizer require gas liquefaction at around -80°C, and 3.5Ni (-100°C) low-temperature steel is usually used.

[0009] The above mainly focuses on the application and requirements of low-temperature steel for low-temperature pressure vessels and equipment, etc. The following introduces the polar environment, alpine environment and their requirements for low temperature from aspects such as polar oil and gas development, polar ship transportation, and polar icebreaking equipment. The natural conditions in the polar regions are harsh, with challenges such as low-temperature tests, sea ice obstacles, iceberg attacks, blizzard attacks, fragile ecological environments, polar night disturbances, and poor visibility, which require high performance of relevant equipment and materials. The ultra-low temperature environment in the Arctic has put forward the demand for low-temperature-resistant steel materials, etc. According to the 40-year follow-up survey of nearly 700 polar ships by Lloyd's Register of Shipping in the UK, 57% of polar ships have cracks or fractures in the hull steel structure after an average ship age of 13 years. Heavy icebreakers usually use special low-temperature steel, which must have comprehensive properties such as sufficient low-temperature toughness, strength, and fatigue strength.

[0010] In recent years, polar ships have gradually developed from low-grade ice-strengthened types towards high-grade strengthened types with self-icebreaking performance, and the demand for new commercial icebreaking ships such as polar oil tankers, polar LNG ships, and polar container ships with icebreaking capabilities has increased rapidly. Correspondingly, the research and development of polar ship plates, deck machinery, and core component materials with cold resistance and high strength and toughness must also be strengthened. The construction of polar ships and equipment urgently needs to develop low-temperature steel materials for ships such as marine low-temperature high-strength steel and low-temperature container high-strength steel, and key technologies such as low-temperature resistance, easy welding, and high toughness need to be broken through.

[0011] Currently, for the above special requirements, first of all, the conventional technology of low-temperature steel usually improves the low-temperature toughness by adding Ni element. This is mainly because: (1) Ni does not form carbides with carbon, and it is the main alloying element for forming and stabilizing austenite; (2) Ni is a pure solid-solution element in steel, which can strengthen the ferrite matrix and has the effect of significantly reducing the ductile-brittle transition temperature. (3) Obtain a fine-grained structure through controlled rolling; (4) Obtain a stable structure through heat treatment. Secondly, add carbide-forming elements such as Nb, Ti, Mo, etc. to the steel, and utilize the precipitation strengthening and grain refinement effects they produce to improve the comprehensive performance of the steel. Thirdly, use Nb, Ti, V microalloying to improve the welding performance. In order to improve the corrosion resistance, alloying elements such as Cr and Cu are sometimes added. The above conventional technologies result in high costs for low-temperature steel alloys.

[0012] The lowest temperatures in polar and alpine regions reach -70°C and below. For steel plates applied in polar and alpine regions, in order to improve the safety reserve, they need to meet more low-temperature impact toughness. Therefore, it is urgent to develop a high-strength steel plate with high strength, excellent low-temperature toughness, and low cost. In view of this, the present invention provides a microalloyed high-strength steel with Ni-free ultra-low temperature toughness and a preparation method thereof. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to provide a microalloyed high-strength steel with Ni-free ultra-low temperature toughness and a preparation method thereof. The purpose is to solve the problems of high cost of traditional low-temperature steel alloys, insufficient weldability, and complex manufacturing processes.

[0014] To solve the above technical problems, the first aspect of the present invention is to provide a microalloyed high-strength steel with Ni-free ultra-low temperature toughness, which is composed of the following chemical components by mass percentage: C: 0.011 - 0.099%, Si: 0.051 - 0.24%, Mn: 1.21 - 1.49%, Nb: 0.030 - 0.059%, Ti: 0.009 - 0.016%, Zr: 0.001 - 0.018%, RE: 0.001 - 0.018%, and the rest is Fe and inevitable impurities;

[0015] The mass percentages of C element and Si element also simultaneously satisfy the formula: 0.21% < C + Si < 0.24%, and Si / C = 1 - 8;

[0016] The mass percentages of Nb element and Ti element also simultaneously satisfy the formula: 0.02% < Nb + Ti < 0.05%, and Nb / Ti = 1 - 3;

[0017] The mass percentage of Zr element and RE element also satisfies the formula: 0.010% <Zr+RE<0.019%,和Zr / RE=1~6。

[0018] The roles played by the various elements in the Ni-free ultra-low temperature tough microalloy high-strength steel provided by the present invention are as follows:

[0019] Carbon: The C element content has a great influence on the mechanical properties, welding properties, and corrosion properties of steel materials. Under the same temperature, as the C content increases, the number of C atoms required to migrate for diffusion-controlled interface movement increases, and diffusion-type phase transformations such as ferrite and pearlite phase transformations are inhibited. If the C content is too high, bainite and martensite phases are easily formed during the cooling process. Bainite and martensite phases are hard and brittle, and their low-temperature impact properties are poor. In the present invention, appropriate C is added to obtain polygonal ferrite and fine and dispersed pearlite through appropriate diffusion-type phase transformation, without forming bainite and martensite during the cooling process. Therefore, the carbon mass percentage of the microalloy high-strength steel of the present invention is 0.011% <C<0.099%。

[0020] Silicon: Si does not form carbides with C, but exists in the steel in a solid solution form. It interacts with the stress field of movable dislocations, hinders dislocation movement, and improves the strength of steel materials. When the Si content is high, it is not good for the welding performance of steel materials. In addition, when the Si content is too high, it will also reduce the plasticity and toughness of the steel. Therefore, the mass percentage of silicon in the microalloy high-strength steel of the present invention is 0.051% <Si<0.24%。

[0021] Manganese: Mn is an austenite-forming element that expands the austenite phase region. During the cooling process, Mn dissipates free energy through solute drag and inhibits diffusion-type phase transformation. By adding an appropriate amount of Mn, the microstructure of the steel plate can be controlled under appropriate process conditions to form a fine bainite lath structure with high strength and high toughness. If the Mn content is too high, cracks may appear in the steel billet during continuous casting and subsequent cooling. Mn can also deoxidize and eliminate the influence of S during the smelting process. The Mn in the steel combines with S to form MnS, which can prevent hot brittleness caused by S. Generally speaking, in low-alloy high-strength steel, an appropriate amount of Mn is added to improve the strength of the steel. Therefore, the mass percentage of manganese in the microalloy high-strength steel of the present invention is Mn1.21~1.49%.

[0022] Phosphorus: Phosphorus has a strong solid solution strengthening effect in steel. When added to low alloy structural steel as an alloying element, it can improve its strength and atmospheric corrosion resistance. However, the biggest harm of phosphorus is that it segregates severely, increases temper brittleness, and significantly damages the plasticity and toughness of steel. Phosphorus also has an adverse effect on weldability. It can be seen that phosphorus is a harmful element and should be strictly controlled. Therefore, the carbon mass percentage of the microalloy high-strength steel of the present invention is P≤0.0049%.

[0023] Sulfur: Sulfur segregates severely in steel, deteriorating the internal and surface quality of the steel. Sulfur also reduces the plasticity of the steel and is a harmful element, existing in the form of FeS with a relatively low melting point. The melting point of FeS alone is only 1190 °C, and the eutectic temperature of the eutectic formed with iron in the steel is even lower, only 988 °C. When the steel solidifies, iron sulfide segregates at the primary grain boundaries. When the steel is rolled at 1100 - 1200 °C, the FeS at the grain boundaries will melt, greatly weakening the bonding force between the grains and leading to the hot brittleness phenomenon of the steel. Therefore, sulfur should be strictly controlled. The sulfur mass percentage content of the microalloyed high-strength steel of the present invention is S ≤ 0.0010%.

[0024] Niobium: In the case of a relatively low carbon content, an appropriate amount of Nb content can refine the ferrite grains and improve the strength and low-temperature toughness of the steel. In the case of a relatively high Nb content, the precipitation of proeutectoid ferrite will be delayed, and the time for austenite to start decomposing into pearlite will be strongly delayed, while having little effect on the transformation from austenite to bainite. In this case, bainite will appear in the steel plate, and the impact toughness of the steel plate will deteriorate instead. Therefore, the niobium mass percentage content of the microalloyed high-strength steel of the present invention is 0.031 - 0.059%.

[0025] Titanium: Titanium has a very strong affinity with oxygen, nitrogen, and carbon and is a good deoxidizer and an effective element for fixing nitrogen and carbon. During the welding heat cycle process, TiN particles effectively prevent the coarsening of austenite grains, which is beneficial to the improvement of toughness. TiN particles can effectively promote the formation of acicular ferrite and effectively improve the welding performance of the steel. However, excessive Ti is not conducive to improving the performance of the steel and is likely to form coarse titanium carbonitrides, becoming a crack source and leading to a reduction in toughness. Therefore, the mass content of Ti in the present invention is 0.009 - 0.016%.

[0026] Zirconium: Zirconium is a strong carbide-forming element, as well as a strong deoxidizing element and a composite oxygen sulfide-forming element. Adding a small amount of zirconium has the effects of degassing, purifying, and refining grains, which is beneficial to improving the low-temperature performance of low-alloy high-strength steel, improving the stamping performance, and significantly increasing the hardenability of the steel when dissolved in austenite. Therefore, the zirconium mass percentage content of the microalloyed high-strength steel of the present invention is 0.001 - 0.018%.

[0027] Rare earth: The main functions of RE elements in steel are as follows: (1) Improving the purity of molten steel and removing harmful elements in the steel, that is, strong deoxidation and desulfurization. By adding rare earth elements, the oxygen content and sulfur content in the molten steel can be reduced to extremely low levels. Therefore, before the rare earth enters the molten steel as a deoxidizer, other deoxidation treatments are required to improve its utilization rate. (2) Modifier for inclusions. Compared with traditional Al-deoxidized inclusions, the addition of rare earth can react with the Al element in the molten steel and generate REAlO 3, adding rare earths can refine and spheroidize inclusions. (3) Microalloying effect. The addition of rare earth elements is believed to reduce the temper embrittlement of steel. Elements such as phosphorus and sulfur in steel are very likely to segregate to the original austenite grain boundaries, resulting in an increase in the temperature at which the steel material changes from toughness to brittleness. The addition of rare earth elements can be used to combine with oxygen, sulfur, phosphorus and other elements distributed on the grain boundaries to form inclusions to eliminate or mitigate the effects of such elements on the temper embrittlement of steel materials. When the rare earth addition exceeds the critical content, the rare earth will segregate at the grain boundaries, destroying the pearlite orientation relationship, resulting in an increase in the pearlite lamella spacing and a decrease in impact toughness. Therefore, the mass percentage of rare earth in the microalloyed high-strength steel of the present invention is 0.001 to 0.018%.

[0028] It should be noted that in the above formula Si / C=1-8, Si and C represent their respective mass percentages, and the value substituted into the above formula is the value before the percentage sign, for example, if the mass percentage of Si is 0.17% and the mass percentage of C is 0.05%, then Si / C=0.17 / 0.06=2.83 is substituted into the above formula. In addition, Nb / Ti=1-3 and Zr / RE=1-6 are the same as Si / C=1-8.

[0029] The beneficial effects of the present invention are:

[0030] (1) The microalloyed high-strength steel of the present invention adopts a composition design that is completely different from the prior art to achieve ultra-low temperature toughness, that is, a nickel-free, low-cost, simplified composition design, supplemented by Zr+RE composite deoxidation and Nb+Ti composite microalloying technology, to achieve ultra-low temperature toughness of -100 to -120°C;

[0031] (2) The microalloyed high-strength steel plate of the present invention is designed with a low-carbon, low-silicon, medium-manganese cheap chemical composition, and does not contain precious metal elements such as Cr, Ni, and Cu, which greatly reduces the material cost;

[0032] (2) The present invention does not use the traditional Al deoxidation technology, but uses Si-Mn deoxidation, and is supplemented by Zr-Ti-RE composite deoxidation to form fine, dispersed and uniform composite oxysulfides, which significantly improves plasticity and toughness;

[0033] (3) The present invention adopts low carbon equivalent design, and the microalloyed high-strength steel plate has excellent welding performance (CEV≤0.39, Ceq≤0.17); adopts composite microalloying of Nb, Ti, Zr, and RE, and cooperates with TMCP rolling parameter regulation to achieve fine grains and high toughness of the steel plate. This kind of nickel-free microalloyed high-strength low-temperature steel is particularly suitable for structural materials such as polar regions, containers, pipelines, refining, storage and transportation, and equipment used in low and ultra-low temperature (-20 to -120°C) environments. In addition to its superior low-temperature toughness, this material also has significant characteristics such as high strength and easy welding.

[0034] On the basis of the above technical solution, the present invention can be further improved as follows.

[0035] Further, C: 0.03 - 0.09%, Si: 0.13 - 0.20%, Mn: 1.4 - 1.48%, Nb: 0.035 - 0.055%, Ti: 0.009 - 0.016%, Zr: 0.010 - 0.015%, RE: 0.002 - 0.004%, and the balance is Fe and unavoidable impurities.

[0036] Further, C: 0.05%, Si: 0.17%, Mn: 1.4%, Nb: 0.03%, Ti: 0.015%, Zr: 0.008%, RE: 0.007%, and the balance is Fe and unavoidable impurities.

[0037] Further, among the unavoidable impurities, the mass percentage contents of P element, S element, O element, N element and H element respectively satisfy: P ≤ 0.0049%, S ≤ 0.0010%, O ≤ 0.0049%, N ≤ 0.0039%, H ≤ 0.00019.

[0038] Further, the RE element includes lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is (70 - 90):(10 - 30).

[0039] Further, the microstructure type of the microalloyed high-strength steel is free of ferrite-pearlite banded structure, and the effective grain size of the microstructure of the microalloyed high-strength steel is less than or equal to 5 μm.

[0040] Further, the V-notch impact absorption energy of the microalloyed high-strength steel at a temperature of -120°C is greater than 300 J.

[0041] Further, the ductile-brittle transition temperature of the microalloyed high-strength steel is -110°C to -130°C.

[0042] The second aspect of the present invention is to provide a preparation method of a microalloyed high-strength steel with Ni-free ultra-low temperature toughness, including the following steps:

[0043] 1) The molten steel is smelted and refined in sequence, then vacuum-treated, and finally continuously cast into a billet to obtain a cast billet;

[0044] 2) The cast billet is heated and soaked to obtain a heat-treated cast billet;

[0045] 3) The heat-treated cast billet is continuously rolled, and the temperature of the final rolling is controlled to be 750 - 850°C. After rolling, it is water-cooled to 410 - 550°C, and then naturally cooled to room temperature to obtain the microalloyed high-strength steel.

[0046] Further, the specific methods of smelting and refining in step 1) are as follows: Using a converter or an electric arc furnace, after melting iron and / or scrap steel in steelmaking, adjust the temperature and composition to obtain molten steel, adjust the tapping temperature of the molten steel to 1549 - 1689 °C, and the free oxygen content in the molten steel is 99 - 398 ppm; let the molten steel enter the ladle, and under the stirring of micro-submerged bubbles, pre-deoxidize the molten steel in the ladle with Fe-Si alloy or Fe-Si-Mn alloy, and adjust the free oxygen content in the molten steel to 10 - 98 ppm; under the stirring of micro-submerged bubbles, perform final deoxidation with a composite additive, and subject the finally deoxidized molten steel to LF refining, VD refining, or RH refining.

[0047] The above composite additive is added to the molten steel in the form of a massive alloy or a cored wire, and the particle size of the composite additive is 4 - 20 mm; the addition amount of the composite additive is 0.49 - 4.8 kg per ton of molten steel, and then the molten steel is subjected to LF refining, VD refining, or RH refining according to the conventional process; finally, the refined molten steel is continuously cast according to the conventional process. The composite additive is a composition of zirconium, titanium, and rare earths. The weight ratio of zirconium element, titanium element, and rare earth element in the composite additive is 7:20:6, where the rare earth is lanthanum and cerium, and the weight ratio of lanthanum and cerium is 80:20. Description of the Drawings

[0048] Figure 1 It is the low-temperature ductile-brittle transition temperature (DBTT) curve of the microalloyed high-strength steel of the present invention;

[0049] Figure 2 It is the oscilloscope impact curve of the microalloyed high-strength steel of the present invention and the comparative steel;

[0050] Figure 3 It is the Ashby diagram of the -100 °C Charpy impact energy - room temperature yield strength of the microalloyed high-strength steel of the present invention and the comparative steel;

[0051] Figure 4 It is the front morphology of the low-temperature Charpy impact fracture of the microalloyed high-strength steel of the present invention; the left figure is -100 °C, and the right figure is -110 °C;

[0052] Figure 5 It is the microscopic morphology of different parts of the -100 °C Charpy impact fracture of the microalloyed high-strength steel of the present invention. Detailed Embodiments

[0053] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention. For those not specifying specific techniques or conditions in the examples, they shall be carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. For those reagents or instruments not indicating the manufacturer, they are all conventional products that can be purchased through regular channels.

[0054] The composite additive in the following is a composition of zirconium, titanium, and rare earths. The weight ratio of zirconium element, titanium element, and rare earth element in the composite additive is 7:20:6. The rare earth is lanthanum and cerium, and the weight ratio of lanthanum and cerium is 80:20. The comparison steel is subjected to final deoxidation using conventional aluminum blocks, aluminum grains, or aluminum wires, forming coarse and clustered alumina and its composite oxides, etc.

[0055] Example 1

[0056] This example relates to a microalloyed high-strength steel with Ni-free ultra-low temperature toughness, which is composed of the following chemical components by mass percentage: C 0.05%, Si 0.17%, Mn 1.4%, Nb 0.03%, Ti 0.015%, Zr 0.008%, RE 0.007%, P 0.0025%, S 0.008%, O 0.0025%, N 0.0030%, H 0.00016%, and the rest is Fe and unavoidable impurities. The RE includes lanthanum and cerium, and the weight ratio of lanthanum element and cerium element is 70:30.

[0057] A preparation method of the microalloyed high-strength steel with Ni-free ultra-low temperature toughness involved in this example includes the following steps:

[0058] 1) The molten steel is smelted and refined in sequence, then subjected to vacuum treatment, and finally continuously cast into a billet to obtain a cast billet;

[0059] The smelting and refining method is as follows: Use a converter or an electric arc furnace to adjust the temperature and composition of the molten steel after smelting molten iron, or scrap steel, or molten iron and scrap steel. Adjust the tapping temperature to 1620°C, and the free oxygen content in the molten steel is 250 ppm. After the molten steel enters the ladle, it is stirred with micro-submerged bubbles for 6 minutes, and then pre-deoxidized with Fe-Si alloy or Fe-Si-Mn alloy in the ladle to adjust the free oxygen content in the molten steel to 55 ppm. After stirring with micro-submerged bubbles for 6 minutes, final deoxidation is carried out with a composite additive. The composite additive is added to the molten steel in the form of a massive alloy or a cored wire, and the particle size of the composite additive is 12 mm. The addition amount of the composite additive is 2.7 kg per ton of molten steel, and then the molten steel is subjected to LF refining and RH refining according to the conventional process.

[0060] LF refining:

[0061] Control the viscosity of the refining slag at 1.517 - 1.933 Pa·s to improve the ability of the slag system to adsorb inclusions, thereby improving the cleanliness of the molten steel; control the white slag alkalinity of the refining furnace at 5.15 ≤ R ≤ 7.47, which is beneficial to improving the desulfurization rate, and is beneficial to improving the cleanliness of the molten steel and reducing oxide inclusions in the molten steel; control the MI furnace slag index (=CaO / SiO 2 :Al2 O 3 When the ratio of CaO / SiO₂ > 0.147, the sulfur distribution coefficient increases significantly, thereby controlling the appropriate fluidity of the refining slag at a certain basicity; the white slag holding time ≥ 14.46 min, the refining cycle ≥ 39.39 min, and it is ensured that the soft blowing time > 4.51 min, thereby controlling the [O] content at the tapping.

[0062] RH vacuum treatment:

[0063] The pressure in the vacuum chamber is pumped to below 66.69 kPa and maintained for 12.35 - 14.47 min, and the bottom blowing argon gas flow rate is 10.26 - 19.38 m 3 / h to achieve 4 cycles of molten steel circulation; strictly control the types and weights of the added alloys, use higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys to ensure that the molten steel composition is completely qualified, and ensure that the vacuum is maintained for more than 5.28 min after the alloy is added to obtain purer molten steel; at the same time, provide appropriate molten steel temperature for continuous casting, and ensure that the tundish superheat is 10.17 - 29.46 °C above the liquidus.

[0064] 2) Heat and hold the slab at 1195 °C for 3.5 hours to obtain the heat-treated slab;

[0065] 3) Continuously roll the heat-treated slab, and control the final rolling temperature to be 750 - 850 °C, cool it by watering to 410 - 550 °C after rolling, and then naturally cool it to room temperature to obtain the microalloyed high-strength steel.

[0066] The rolling method is: conventionally heat and soak the slab; then continuously roll it into the product steel plate, control the final rolling temperature to be 800 °C, cool it by watering to 480 °C after rolling; naturally cool it to room temperature for standby.

[0067] The steel plate obtained through the above process has no ferrite-pearlite banded structure; the effective grain size of the microscopic structure of the steel plate is 4.5 μm; the V-notch impact absorption energy of the steel plate at -120 °C > 315 J.

[0068] Example 2

[0069] This embodiment relates to a microalloyed high-strength steel with Ni-free ultra-low temperature toughness, which is composed of the following chemical components by mass percentage: C 0.09%, Si 0.13%, Mn 1.48%, Nb 0.035%, Ti 0.012%, Zr 0.014%, RE 0.004%, P 0.0048%, S 0.0010%, O 0.0048%, N 0.0038%, H 0.00019%, and the rest is Fe and inevitable impurities. The RE includes lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is 80:20.

[0070] A preparation method of the microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to this embodiment includes the following steps:

[0071] 1) The molten steel is smelted and refined in sequence, then vacuum-treated, and finally continuously cast into a billet to obtain a cast billet;

[0072] The smelting and refining method is as follows: Use a converter or an electric arc furnace to adjust the temperature and composition of the molten steel after smelting molten iron, or scrap steel, or molten iron and scrap steel. Adjust the tapping temperature to 1680°C, and the free oxygen content in the molten steel is 380 ppm; After the molten steel enters the ladle, it is stirred with micro-submerged bubbles for 8 minutes, and then pre-deoxidized with Fe-Si alloy or Fe-Si-Mn alloy in the ladle to adjust the free oxygen content in the molten steel to 90 ppm. After stirring with micro-submerged bubbles for 7 minutes, final deoxidation is carried out with a composite additive; The composite additive is added to the molten steel in the form of a bulk alloy or a cored wire, and the particle size of the composite additive is 19 mm; The addition amount of the composite additive is 4.5 kg per ton of molten steel, and then the molten steel is subjected to LF refining and RH refining according to the conventional process.

[0073] LF refining:

[0074] Control the viscosity of the refining slag at 1.531 - 1.964 Pa·s to improve the ability of the slag system to adsorb inclusions, thereby improving the cleanliness of the molten steel; Control the white slag alkalinity of the refining furnace at 5.15 ≤ R ≤ 7.67, which is beneficial to improving the desulfurization rate, and is beneficial to improving the cleanliness of the molten steel and reducing oxide inclusions in the molten steel; Control the MI slag index (=CaO / SiO 2 :Al 2 O 3 ratio) MI > 0.149, and the sulfur distribution coefficient increases significantly, thereby controlling the appropriate fluidity of the refining furnace slag at a certain alkalinity; The white slag holding time ≥ 14.23 min, the refining cycle ≥ 39.36 min, and ensure that the soft blowing time > 4.58 min, thereby controlling the [O] content at the station.

[0075] RH vacuum treatment:

[0076] The air pressure in the vacuum chamber is pumped down to below 66.59 kPa and maintained for 12.27 - 14.48 min, and the flow rate of bottom-blown argon is 10.21 - 19.39 m 3 / h to achieve 5 cycles of molten steel circulation; strictly control the types and weights of added alloys, use higher-grade alloys such as low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium, etc., ensure that the composition of the molten steel is completely qualified, and ensure that the vacuum is maintained for more than 5.33 min after the addition of alloys to obtain purer molten steel; at the same time, provide a suitable molten steel temperature for continuous casting, and ensure that the superheat in the tundish is 10.17 - 29.49 °C above the liquidus line.

[0077] 2) Heat and hold the slab at 1205 °C for 3.4 hours to obtain a heat-treated slab;

[0078] 3) Continuously roll the heat-treated slab, and control the final rolling temperature to be 750 - 850 °C. After rolling, water-cool it to 410 - 550 °C, and then naturally cool it to room temperature to obtain microalloyed high-strength steel.

[0079] The rolling method is as follows: conventionally heat and soak the slab; then continuously roll it into a product steel plate, control the final rolling temperature to be 840 °C, water-cool it to 530 °C after rolling; naturally cool it to room temperature for standby.

[0080] The microalloyed high-strength steel obtained through the above process has no ferrite-pearlite banded structure; the effective grain size of the microstructure of the microalloyed high-strength steel is 4.8 μm; the V-notch impact absorption energy of the microalloyed high-strength steel at -120 °C > 309 J.

[0081] Example 3

[0082] This example relates to a microalloyed high-strength steel with Ni-free ultra-low temperature toughness, which is composed of the following chemical components by mass percentage: C 0.03%, Si 0.20%, Mn 1.4%, Nb 0.035%, Ti 0.013%, Zr 0.010%, RE 0.002%, P 0.0020%, S 0.0007%, O 0.0020%, N 0.0030%, H 0.00016%, and the rest is Fe and inevitable impurities. The RE includes lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is 90:10.

[0083] A preparation method of the microalloyed high-strength steel with Ni-free ultra-low temperature toughness involved in this example includes the following steps:

[0084] 1) Smelt and refine the molten steel in sequence, then conduct vacuum treatment, and finally continuously cast it into a slab to obtain a slab;

[0085] The smelting and refining method is as follows: Use a converter or an electric arc furnace to adjust the temperature and composition of the molten steel after smelting molten iron, or scrap steel, or molten iron and scrap steel. Adjust the tapping temperature to 1580 °C, and the free oxygen content in the molten steel is 150 ppm. After the molten steel enters the ladle, it is stirred with micro-submerged bubbles for 5 minutes, and then pre-deoxidized with Fe-Si alloy or Fe-Si-Mn alloy in the ladle to adjust the free oxygen content in the molten steel to 90 ppm. After stirring with micro-submerged bubbles for 5 minutes, final deoxidation is carried out with a composite additive. The composite additive is added to the molten steel in the form of a massive alloy or a cored wire, and the particle size of the composite additive is 9 mm. The addition amount of the composite additive is 0.59 kg per ton of molten steel, and then the molten steel is subjected to LF refining and RH refining according to the conventional process.

[0086] LF refining:

[0087] Control the viscosity of the refining slag at 1.526 - 1.953 Pa·s to improve the ability of the slag system to adsorb inclusions, thereby improving the cleanliness of the molten steel; control the white slag alkalinity of the refining furnace at 5.16 ≤ R ≤ 7.63, which is beneficial to improving the desulfurization rate, improving the cleanliness of the molten steel, and reducing oxide inclusions in the molten steel; control the MI slag index (=CaO / SiO 2 :Al 2 O 3 ratio) MI > 0.153, and the sulfur distribution coefficient increases significantly, thereby controlling the appropriate fluidity of the refining furnace slag at a certain alkalinity; the white slag holding time ≥ 14.35 min, the refining cycle ≥ 39.47 min, and ensure that the soft blowing time > 4.58 min, thereby controlling the [O] content at the outlet.

[0088] RH vacuum treatment:

[0089] The pressure in the vacuum chamber is pumped to below 66.69 kPa and maintained for 12.21 - 14.47 min, and the bottom blowing argon flow rate is 10.23 - 19.46 m 3 / h to achieve 6 cycles of molten steel circulation; strictly control the types and weights of the added alloys, use alloys with higher grades such as low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium, etc., ensure that the composition of the molten steel is completely qualified, and ensure that the vacuum is maintained for more than 5.33 min after the alloy is added to obtain purer molten steel; at the same time, provide a suitable molten steel temperature for continuous casting, and ensure that the tundish superheat is 10.35 - 29.47 °C above the liquidus.

[0090] 2) Heat and hold the cast billet at 1225 °C for 3.1 hours to obtain a heat-treated cast billet;

[0091] 3) Continuously roll the heat-treated cast billet, and control the final rolling temperature at 750 - 850 °C. After rolling, water-cool it to 410 - 550 °C, and then naturally cool it to room temperature to obtain a microalloyed high-strength steel.

[0092] The rolling method is as follows: The continuous casting billet is conventionally heated and soaked; then it is continuously rolled into a product steel plate, and the finish rolling temperature is controlled at 760 °C, and it is water-cooled to 430 °C after rolling; it is naturally cooled to room temperature for standby.

[0093] The microalloyed high-strength steel obtained through the above process has no ferrite-pearlite banded structure; the effective grain size of the microstructure of the microalloyed high-strength steel is 4.4 μm; the V-notch impact energy absorption of the microalloyed high-strength steel is > 311 J at -120 °C.

[0094] Experimental Example

[0095] The following is a test and analysis of the series of low-temperature impact properties using the microalloyed high-strength steel without Ni and with ultra-low temperature toughness (also known as the developed steel) prepared in Example 1. The results are as follows:

[0096] (1) Test method for low-temperature impact properties

[0097] The low-temperature impact test is carried out in accordance with the national standards GB / T229-2020 "Metallic materials - Charpy pendulum impact test method" and GB / T19748-2019 "Metallic materials - Instrumented Charpy V-notch pendulum impact test method". The specimen size is a standard specimen of 55×10×10 mm, with a V-notch, a notch depth of 2 mm, and a root radius of 0.25 mm. The radius of curvature of the edge of the pendulum hammer blade is 2 mm. The test temperatures include 20, -20, -40, -60, -80, -100, -110, -120, -130, -140, -160, and -196 °C. During the pendulum impact process, the force and displacement values are respectively recorded through the resistance strain gauge on the hammer blade and the optical method, and thus the force-displacement curve of the impact process is obtained, and the impact energy absorption is calculated by integration.

[0098] (2) Analysis of low-temperature impact toughness results

[0099] Figure 1 This is the ductile-brittle transition temperature (DBTT) curve of the microalloyed high-strength steel of the present invention. As Figure 1 shown, the ductile-brittle transition temperature of the microalloyed high-strength steel of the present invention is between about -110 °C and -130 °C, which is more than 50 °C lower than the standard requirement of the same strength FH36 (the standard requires a ductile-brittle transition temperature lower than -60 °C).

[0100] Figure 2The following is a comparison of the -100°C oscilloscope impact curves between the microalloyed high-strength steel of the present invention and a control steel of the same type. During the -100°C impact process, the microalloyed high-strength steel of the present invention has a complete crack initiation - propagation - passivation process, while the control steel rapidly drops after the force-displacement curve passes the yield point, indicating that the control steel undergoes instantaneous brittle fracture after crack initiation and has no stable crack propagation and passivation processes.

[0101] Figure 3 The following is an Ashby diagram of the impact energy of the microalloyed high-strength steel of the present invention and other steel grades at -100°C relative to the room temperature yield strength. The dashed line is the connection line between the coordinates (200 MPa, 300 J) and the coordinates (1000 MPa, 0 J). Steel grades to the right of the dashed line can be considered to achieve a relatively excellent trade-off between low-temperature impact toughness and room temperature yield strength. From Figure 3 it can be seen that the -100°C impact energy of the microalloyed high-strength steel of the present invention is the highest, higher than that of traditional low-carbon microalloyed steel [1～3] , medium manganese steel [6～9] , chromium-manganese stainless steel [13～14] , duplex stainless steel

[15] , nickel-based cryogenic steel [8,16,17] , maraging steel [18,19] , manganese-nickel microalloyed steel [4,5] and high manganese steel [10～12] .

[0102] Figure 4 The following is the macroscopic morphology of the impact fracture surface of the microalloyed high-strength steel of the present invention at two low temperatures of -100°C and -110°C. From Figure 4 it can be seen that at -100°C, the microalloyed high-strength steel of the present invention is all ductile fracture; the microalloyed high-strength steel of the present invention only completely transforms into brittle fracture at -120°C.

[0103] Figure 5 The following is the microscopic morphology of different parts of the Charpy impact fracture surface of the microalloyed high-strength steel of the present invention at -100°C. It can be seen that both the crack initiation site and the stable crack propagation site of the fracture surface show a dimple morphology of tensile along the fracture direction, and there are a large number of tearing marks at the edge of the dimples. The crack passivation site shows a mixed morphology of oblique tensile dimples and normal tensile dimples.

[0104] Regarding the performance requirements of cryogenic steel, first of all, it is necessary to ensure that it has sufficient impact toughness value at the service temperature. From the perspective of fracture mechanics, it is required that the material has sufficient ability to resist brittle cracking at the service temperature. In special important structures, in order to prevent accidents, it is also necessary to require the material to have a crack arrest performance against brittle crack propagation. In addition, from a safety perspective, it is desirable that the yield ratio of cryogenic steel is not high. The larger the yield ratio, the smaller the reserve of plastic deformation ability, and the lower the stress redistribution ability at the stress concentration site, thus being more likely to promote brittle fracture.

[0105] The general technical requirements for low-temperature steel are as follows: it should have sufficient strength and adequate toughness at low temperatures, as well as good technological properties, workability, and corrosion resistance. Among them, low-temperature toughness, that is, the ability to prevent brittle fracture from occurring and spreading at low temperatures, is the most important factor. Therefore, various countries usually stipulate a certain impact toughness value at the lowest temperature.

[0106] Currently, in order to meet the above requirements for low-temperature toughness, the conventional technology is to alloy with nickel. This is mainly because: (1) Ni does not form carbides with carbon, and it is the main alloying element for forming and stabilizing austenite; (2) Ni is a pure solid-solution element in steel, which can strengthen the ferrite matrix and has an obvious effect of reducing the ductile-brittle transition temperature; (3) Obtain a fine-grained structure through controlled rolling; (4) Obtain a stable structure through heat treatment.

[0107] However, the present invention adopts a completely different composition design and its matching manufacturing process to achieve ultra-low temperature toughness, that is, a nickel-free, low-cost, and simplified composition design, supplemented by Zr+RE composite deoxidation and Nb+Ti composite microalloying technology to achieve ultra-low temperature toughness at -100 to -120°C. The microalloyed high-strength steel plate of the present invention adopts a low-carbon, low-silicon, medium-manganese inexpensive chemical composition design, completely free of precious metal elements such as Cr, Ni, and Cu, greatly reducing the material cost; the present invention does not adopt the traditional Al deoxidation technology, but instead uses Si-Mn deoxidation, supplemented by Zr-Ti-RE composite deoxidation to form fine, dispersed, and uniform composite oxysulfides, significantly improving the plasticity and toughness; through low-carbon equivalent design, the steel plate has excellent welding performance; by using the composite microalloying of Nb, Ti, Zr, and RE, combined with the regulation of TMCP rolling parameters, the steel plate has fine grains, high strength, and high toughness. This nickel-free high-strength low-temperature steel is especially suitable for structural materials such as polar regions, containers, pipelines, refining, storage and transportation, and equipment used in low-temperature and ultra-low temperature (-20 to -120°C) environments. In addition to its excellent low-temperature toughness, this material also has remarkable characteristics such as high strength and easy welding.

[0108] Among them, low-carbon microalloyed steel [1～3] , medium-manganese steel [6～9] , chromium-manganese stainless steel [13～14] , duplex stainless steel

[15] , nickel-based low-temperature steel [8,16,17] , maraging steel [18,19] , manganese-nickel microalloyed steel [4,5] and high-manganese steel [10～12] The references [1] to

[15] are as follows:

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[0110] [2]X.Chen,A.Guo,H.Dong,S.Li.The properties of high toughness low~temperature-70℃ steel 09MnNiDR.International Journal of Pressure Vessels andPiping,1999,76(1):13~17.

[0111] [3]Y.J.Chao,J.D.Ward,Jr.R.G.Sands.Charpy impact energy,fracturetoughness and ductile–brittle transition temperature of dual~phase 590Steel.Materials&Design,2007,28(2):551~557.

[0112] [4]L.Jiang,J.Wang,T.Zhang,T.Dorin,X.Sun.Superior low temperaturetoughness in a newly designed low Mn and low Ni high strength steel.MaterialsScience and Engineering:A,2021,85:141899.

[0113] [5]J.Chen,W.Zhang,Z.Liu,G.Wang.The role of retained austenite on themechanical properties of a low carbon 3Mn~1.5Ni Steel.Metallurgical andMaterials Transactions A,2017,48:5849~5859.

[0114] [6]G.Su,X.Gao,D.Zhang,L.Du,J.Hu,Z.Liu.Impact of reversed austenite onthe impact toughness of the high~strength steel of low carbon mediummanganese.JOM,2018,70:672~679.

[0115] [7]H.Liu,L.X.Du,J.Hu,H.Y.Wu,X.H.Gao,R.D.K.Misra.Interplay betweenreversed austenite and plastic deformation in a directly quenched andintercritically annealed 0.04C~5Mn low~Alsteel.Journal of Alloys andCompounds,2017,695:2072~2082.

[0116] [8]H.W.Lee,T.M.Park,N.Seo,S.J.Lee,C.Lee,J.Han.Design of low~Nimartensitic steels with novel cryogenic impact toughnessexceeding 190J.Materials Science and Engineering:A,2022,840:142959.

[0117] [9]I.C.Yi,Y.Ha,H.Lee,A.Zargaran,N.J.Kim.Improvement of impacttoughness of 5Mn~1Al~0.5Ti steel by intercritical annealing.Metals andMaterials International,2017,23:283~289.

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[10] J. Lee, S. S. Sohn, S. Hong, B. C. Suh, S. K. Kim, B. J. Lee, N. J. Kim, S. Lee. Effects of Mn addition on tensile and charpy impact properties in austenitic Fe~Mn~C~Al~based steels for cryogenic applications. Metallurgical and Materials Transactions A, 2014, 45: 5419~5430.

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[11] S. S Sohn, S. Hong, J. Lee, B. C. Suh, S. K. Kim, B. J. Lee, N. J. Kim, S. Lee. Effects of Mn and Al contents on cryogenic~temperature tensile and Charpy impact properties in four austenitic high~Mn steels. Acta Materialia, 2015, 100: 39~52.

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[12] Q. Luo, H. H. Wang, G. Q. Li, C. Sun, D. H. Li, X. L. Wan. On mechanical properties of novel high~Mn cryogenic steel in terms of SFE and microstructural evolution. Materials Science and Engineering: A, 2019, 753: 91~98.

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[13] M. Milititsky, D. K. Matlock, A. Regully, N. Dewispelaere, J. Penning, H. Hanninen. Impact toughness properties of nickel~free austenitic stainless steels. Materials Science and Engineering: A, 2008, 496(1~2): 189~199.

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[14] Y. Tomota, Y. Xia, K. Inoue. Mechanism of low temperature brittle fracture in high nitrogen bearing austenitic steels. Acta Materialia, 1998, 46(5): 1577~1587.

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[15] C. Gennari, L. Pezzato, E. Piva, R. Gobbo, I. Calliari. Influence of small amount and different morphology of secondary phases on impact toughness of UNS S32205 Duplex Stainless Steel. Materials Science and Engineering: A, 2018, 729: 14~156.

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[16] M. Wang, Z. Y. Liu, C. G. Li. Correlations of Ni contents, formation of reversed austenite and toughness for Ni-containing cryogenic steels. Acta Metallurgica Sinica(English Letters), 2017, 30: 238-249.

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[17] J. Chen, Z. Liu. The combination of strength and cryogenic impact toughness in low carbon 5Mn–5Ni steel. Journal of Alloys and Compounds, 2020, 837: 155484.

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[18] U.K. Viswanathan, R. Kishore, M.K. Asundi. Effect of thermal cycling on the mechanical properties of 350-grade maraging steel. Metallurgical and Materials Transactions A, 1996, 27: 757-761.

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[19] H. Zhang, M. Sun, Y. Liu, D. Ma, B. Xu, M. Huang, D. Li, Y. Li. Ultrafine-grained dual-phase maraging steel with high strength and excellent cryogenic toughness. Acta Materialia, 2021, 211: 116878.

[0128] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means 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 representations 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. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0129] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A microalloyed high-strength steel with Ni-free ultra-low temperature toughness, characterized in that, it is composed of the following chemical components by mass percentage: C: 0.03 - 0.09%, Si: 0.13 - 0.20%, Mn: 1.4 - 1.48%, Nb: 0.035 - 0.055%, Ti: 0.009 - 0.016%, Zr: 0.010 - 0.015%, RE: 0.002 - 0.004%, and the rest is Fe and unavoidable impurities; The mass percentages of C element and Si element also simultaneously satisfy the formula: 0.21% < C + Si < 0.24%, and Si / C = 1 - 8; The mass percentages of Nb element and Ti element also simultaneously satisfy the formula: 0.02% < Nb + Ti < 0.05%, and Nb / Ti = 1 - 3; The mass percentages of Zr element and RE element also simultaneously satisfy the formula: 0.010% < Zr + RE < 0.019%, and Zr / RE = 1 - 6; The preparation method of the microalloyed high-strength steel with Ni-free ultra-low temperature toughness includes the following steps: 1) Smelt and refine the molten steel in sequence, then conduct vacuum treatment, and finally continuously cast it into a billet to obtain a cast billet; 2) Heat and soak the cast billet to obtain a heat-treated cast billet; 3) Continuously roll the heat-treated cast billet, and control the final rolling temperature to be 750 - 850 °C, cool it with water after rolling to 410 - 550 °C, and then naturally cool it to room temperature to obtain the microalloyed high-strength steel; The specific methods of smelting and refining in step 1) are: using a converter or an electric arc furnace, after melting iron and / or scrap steel, adjust the temperature and composition to obtain molten steel, adjust the tapping temperature of the molten steel to 1549 - 1689 °C, and the free oxygen content in the molten steel is 99 - 398 ppm; make the molten steel enter the ladle, and under the stirring of micro-submerged bubbles, pre-deoxidize the molten steel in the ladle with Fe-Si alloy or Fe-Si-Mn alloy, and adjust the free oxygen content in the molten steel to 10 - 98 ppm; under the stirring of micro-submerged bubbles, conduct final deoxidation with a composite additive, and subject the finally deoxidized molten steel to LF refining, VD refining, or RH refining.

2. The microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to claim 1, characterized in that, C: 0.05%, Si: 0.17%, Mn: 1.4%, Nb: 0.03%, Ti: 0.015%, Zr: 0.008%, RE: 0.007%, and the rest is Fe and unavoidable impurities.

3. The microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to any one of claims 1 to 2, characterized in that, Among the unavoidable impurities, the mass percentages of P element, S element, O element, N element, and H element respectively satisfy: P ≤ 0.0049%, S ≤ 0.0010%, O ≤ 0.0049%, N ≤ 0.0039%, H ≤ 0.00019.

4. The microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to any one of claims 1 to 2, characterized in that, The RE element described above includes lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

5. A microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to any one of claims 1 to 2, characterized in that the microstructure type of the microalloyed high-strength steel is a non-ferrite-pearlite banded structure, and the effective grain size of the microstructure of the microalloyed high-strength steel is less than or equal to 5 μm.

6. A microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to any one of claims 1 to 2, characterized in that the V-notch impact absorption energy of the microalloyed high-strength steel is greater than 300 J at a temperature of -120 °C.

7. A microalloyed high-strength steel with Ni-free ultra-low temperature toughness according to any one of claims 1 to 2, characterized in that the ductile-brittle transition temperature of the microalloyed high-strength steel is -110 °C to -130 °C.

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