Ni-free fine alloy high-strength steel with ultra-low temperature toughness and method for preparing the same

Abstract

To provide a Ni-free micro-alloyed high-strength steel with ultra-low temperature toughness, and preparation method thereof.SOLUTION: A Ni-free micro-alloyed high-strength steel includes the following chemical components by mass%: 0.011 to 0.099% of C, 0.051 to 0.24% of Si, 1.21 to 1.49% of Mn, 0.030 to 0.059% of Nb, 0.009 to 0.016% of Ti, 0.001 to 0.018% of Zr, 0.001 to 0.018% of rare earth (RE), and the balance of Fe and inevitable impurities, and satisfies the following formulas in terms of mass%: 0.21%<C+Si<0.24%, and Si / C=1 to 8; 0.02%<Nb+Ti<0.05%, and Nb / Ti=1 to 3; 0.010%<Zr+RE<0.019%, and Zr / RE=1 to 6.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and specifically, to a Ni-free fine alloy high-strength steel having ultra-low temperature toughness and a method for preparing the same.

Background Art

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

[0003] Low-temperature steels are generally classified into nickel-free steels and nickel steels. Nickel-free steels generally refer to fine-grained steels and low-temperature high-strength steels, and their use temperature is above -60°C. Nickel steels are steels to which an alloy element Ni soluble in ferrite is added. In order to change the general phenomenon of low-temperature brittleness transition of a metal material having a body-centered cubic lattice, the low-temperature toughness of the matrix is significantly improved, and its use temperature may reach below -196°C.

[0004] With the development of the petrochemical industry, new processes and equipment are constantly emerging, and the liquefaction, separation, storage, transportation, and utilization of gases have become widespread in every country. The development of cryogenic technologies and equipment, such as those mentioned above, is driving the development of steel for cryogenic pressure vessels. According to the Japanese JIS G 3127 (1977) standard for nickel steel sheets for cryogenic pressure vessels, typical steel grades include SL3N255, SL3N275, and SL3N440. According to the US SA-203 / SA-203M standard for nickel alloy steel sheets for pressure vessels, typical steel grades are SA203Gr.D, SA203Gr.E, and SA203GR.F. Cryogenic steel containing 3.5-9% Ni has corresponding standards in the US, Japan, and Europe. In the late 1960s, China developed low-temperature steel for temperatures from -40°C to -253°C, but it did not become widespread. In 1983, the Standardization Administration of the People's Republic of China issued GB3531-83, "Technical Specifications for Thick Plates of Low-Alloy Steel for Low-Temperature Pressure Vessels," which specified that four types of nickel-free low-temperature steel could be used from -30°C to -90°C. 16MnDR steel is an economical and mature steel grade for the manufacture of low-temperature equipment up to -40°C, and can be used, for example, in the manufacture of liquid ammonia equipment. 15MnNiDR and 09MnNiDR are nickel-based low-temperature steels with excellent low-temperature toughness and weldability. 09MnNiDR low-temperature steel, suitable for temperatures up to -70°C, is widely applied to low-temperature equipment for ethylene, chemical fertilizers, city gas, carbon dioxide, etc., and is gradually succeeding in replacing imported low-temperature steel in the manufacture of ammonia separators (-28°C), high-pressure nitrogen storage tanks (-28°C), carbon dioxide low-temperature storage tanks (-50°C), ethylene low-temperature storage tanks (-60°C), etc.

[0005] Regarding low-temperature steel with high nickel content, the materials section of the China CCS 1996 "Steel Ship Construction and Classification Rules" stipulates that nickel alloy steel with a thickness of 50 mm or less is suitable for the manufacture of liquid cargo tanks and hull structures near liquid cargo tanks of liquefied gas carriers, and there are three types of steel: 3.5Ni, 5Ni, and 9Ni (Table 1). In China, there is a consensus in the pressure vessel industry that 3.5Ni low-temperature steel is used for the manufacture of -100°C class low-temperature vessels, and 9Ni low-temperature steel is used for the manufacture of -196°C low-temperature vessels. 3.5Ni steel is widely applied to low-temperature vessels from -101°C to -80°C. [Table 1]

[0006] Nickel-based cryogenic steel, a type of low-temperature steel, is widely used in developed countries such as the United States and Japan because it offers high strength, excellent low-temperature toughness, and is less expensive than Cr-Ni stainless steel for the corresponding temperature range. In the case of cryogenic containers, the lower the temperature of the storage medium, the lower the pressure exerted on the container, resulting in higher safety and reliability. Currently, 9Ni (-196℃) cryogenic steel is widely used for liquefied natural gas, while in cryogenic equipment for the petrochemical and fertilizer industries, where gas needs to be liquefied at around -80℃, 3.5Ni (-100℃) cryogenic steel is typically used.

[0007] The above explanation primarily describes the applications and requirements of cryogenic steel for cryogenic pressure vessels and equipment. From the perspectives of polar oil and gas development, polar shipping, and polar icebreaking equipment, it introduces the requirements for the polar environment, the extremely cold environment, and its low-temperature capabilities. The natural conditions are harsh, presenting challenges such as low-temperature testing, sea ice damage, iceberg attacks, blizzard attacks, fragile ecological environments, polar night damage, and poor visibility, creating numerous demands on related equipment and material performance. The ultra-low temperature environment of the Arctic Circle is driving the demand for cryogenically resistant steel. According to a 40-year tracking study of approximately 700 polar vessels by Lloyd's Register, 57% of polar vessels experienced cracks or failures in their hull steel structures after an average of 13 years. Large icebreakers typically use special cryogenic steel, requiring comprehensive properties such as low-temperature toughness, strength, and fatigue strength.

[0008] In recent years, polar vessels have gradually evolved from low-grade ice-strengthened types to high-grade ice-strengthened types with self-icebreaking capabilities, and the demand for newly built commercial icebreakers with icebreaking capabilities, such as polar oil tankers, polar LNG carriers, and polar container ships, is rapidly increasing. Accordingly, research and development of materials for polar vessel steel plates, deck machinery, and core components that possess cold resistance and high toughness must be strengthened. The development of low-temperature steel materials such as low-temperature high-tensile steel plates for ships and high-tensile steel plates for low-temperature vessels, and breakthroughs in key technologies such as low-temperature resistance, excellent weldability, and high toughness, are urgently needed for the construction of polar vessels and equipment.

[0009] Currently, to meet the specific requirements described above, conventional low-temperature steel technology typically involves adding Ni to improve low-temperature toughness. This is mainly because (1) Ni is the main alloying element that forms and stabilizes austenite without forming carbides with carbon, (2) Ni is an element that dissolves purely in steel, strengthening the ferrite matrix and significantly lowering the ductile-brittle transition temperature, (3) a fine grain structure can be obtained through controlled rolling, and (4) a stable structure can be obtained through heat treatment. Secondly, carbide-forming elements such as Nb, Ti, and Mo are added to the steel, improving the overall properties of the steel through precipitation strengthening and grain refinement. Thirdly, weldability is improved by fine alloying of Nb, Ti, and V. In some cases, it may be necessary to add alloying elements such as Cr and Cu to improve corrosion resistance. However, the above conventional technologies result in high-cost and expensive low-temperature steel.

[0010] However, the minimum temperature in polar and extremely cold regions is below -70°C, and steel plates used in these regions need to satisfy impact toughness at lower temperatures in order to improve safety reserve. Therefore, there is an urgent need to develop high-strength steel plates that are high in strength, have excellent low-temperature toughness, and are inexpensive. This invention has been made in view of these circumstances and provides a Ni-free fine alloy high-strength steel with ultra-low temperature toughness and a method for preparing the same. [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The technical problem that this invention aims to solve is to provide a Ni-free fine alloy high-strength steel with ultra-low temperature toughness and a method for preparing the same. The objective is to solve the problems of high cost, insufficient weldability, and complex manufacturing processes of conventional low-temperature steels. [Means for solving the problem]

[0012] The present invention solves the above technical problems. On the one hand, it provides a Ni-free fine alloy high-strength steel with ultra-low temperature toughness. In terms of mass%, it has a chemical composition of C: 0.011 to 0.099%, Si: 0.051 to 0.24%, Mn: 1.21 to 1.49%, Nb: 0.030 to 0.059%, Ti: 0.009 to 0.016%, Zr: 0.001 to 0.018%, RE: 0.001 to 0.018%, and the balance of Fe and inevitable impurities.

[0013] The mass% of element C and element Si further simultaneously satisfy the equations 0.21% < C + Si < 0.24% and Si / C = 1 to 8.

[0014] The mass% of element Nb and element Ti further simultaneously satisfy the equations 0.02% < Nb + Ti < 0.05% and Nb / Ti = 1 to 3.

[0015] The mass% of element Zr and element RE further simultaneously satisfy the equations 0.010% < Zr + RE < 0.019% and Zr / RE = 1 to 6.

[0016] Each element in the Ni-free fine alloy high-strength steel with ultra-low temperature toughness provided by the present invention plays the following roles.

[0017] Carbon: The content of element C has a great influence on the mechanical properties, welding characteristics, and corrosion characteristics of the steel. At the same temperature, when the content of C increases, the number of C atoms moving due to diffusion-controlled interface movement increases, and diffusion phase transformations, such as ferrite phase transformation and pearlite phase transformation, are suppressed. When C is contained excessively, it is easy to generate bainite phase or martensite phase during cooling. The bainite phase and martensite phase are hard and brittle, and the low-temperature impact characteristics are also relatively poor. In the present invention, C is appropriately added to obtain polygonal ferrite and finely dispersed pearlite by appropriate diffusion phase transformation without forming bainite phase or martensite phase during cooling. Therefore, the mass% of carbon in the fine alloy high-strength steel according to the present invention is 0.011% < C < 0.099%.

[0018] Silicon (Si) does not form carbides with carbon (C) but exists in steel in the form of a solid solution. It interacts with the stress field of migrating dislocations, hindering dislocation movement and improving the strength of steel and iron materials. High Si content is detrimental to the weldability of steel and iron materials. Furthermore, high Si content reduces the ductility and toughness of steel. Therefore, the mass percentage of silicon in the fine alloy high-strength steel according to the present invention is 0.051%. <Si<0.24%である。

[0019] Manganese (Mn) is an austenite-forming element that expands the austenite phase region. During the cooling process, Mn dissipates free energy through the solvent drag effect, suppressing diffusion transformation. By adding an appropriate amount of Mn, the microstructure of the steel sheet can be controlled under appropriate technical conditions, and a fine bainite lath structure with high strength and toughness can be formed. Excessive Mn content can cause cracking in the slab during continuous casting or subsequent cooling. Mn also achieves deoxidation and eliminates the influence of sulfur (S) during the smelting process. Mn in the steel combines with S to form MnS, preventing thermal brittleness caused by S. Generally, an appropriate amount of Mn is added to low-alloy high-strength steel to improve the strength of the steel. Therefore, the mass % of Mn in the fine-alloy high-strength steel according to the present invention is 1.21% to 1.49%.

[0020] Phosphorus: While phosphorus has a strong solid solution strengthening effect in steel and can improve the strength and atmospheric corrosion resistance of low-alloy structural steel when added as an alloying element, it also has the most harmful effect of causing severe segregation, increasing temper brittleness, and significantly impairing the plasticity and toughness of steel. Phosphorus also adversely affects weldability. Thus, phosphorus is a harmful element and must be strictly controlled. Therefore, the mass percentage of phosphorus in the fine-alloy high-strength steel according to the present invention is P ≤ 0.0049%.

[0021] Sulfur: Sulfur is a harmful element because it causes serious segregation in steel, degrading the internal and surface quality of the steel, and also reduces the plasticity of the steel. It exists in the form of FeS, which has a low melting point. The melting point of elemental FeS is only 1,190°C, but the eutectic temperature formed by FeS and iron in steel is even lower, at only 988°C. When the steel solidifies, iron sulfide precipitates at the primary grain boundaries. When the steel is rolled at 1,100°C to 1,200°C, the FeS present at the grain boundaries melts, significantly weakening the bonding force between crystal grains and leading to hot brittleness of the steel. Therefore, sulfur must be strictly controlled, and the mass percentage of sulfur in the fine alloy high-strength steel according to the present invention is S ≤ 0.0010%.

[0022] Niobium: When the carbon content is low, appropriately incorporating Nb can refine the ferrite grains and improve the strength and low-temperature toughness of the steel. When the Nb content is high, the precipitation of eutectoid ferrite is delayed, the timing of the decomposition of austenite into pearlite is extremely delayed, and there is almost no effect on the transformation from austenite to bainite. In this case, bainite is formed in the steel sheet, but the impact toughness of the steel sheet deteriorates. Therefore, the mass % of niobium in the fine-grained alloy high-strength steel according to the present invention should be 0.031% to 0.059%.

[0023] Titanium: Titanium has a strong affinity for oxygen, nitrogen, and carbon, is a good deoxidizer, and is an effective element for fixing nitrogen and carbon. TiN particles can effectively prevent austenite grain coarsening during the welding thermal cycle, which is beneficial for improving toughness. TiN particles can effectively promote the formation of acicular ferrite, effectively improving the welding performance of steel. However, if the Ti content is excessive, it does not contribute to improving the properties of the steel, and coarse titanium carbonitrides, which are a source of cracks, are more likely to form, reducing toughness. Therefore, the mass content of Ti in this invention is set to 0.009% to 0.016%.

[0024] Zirconium: Zirconium is a strong carbide-forming element, a strong deoxidizing element, and also a composite sulfide-forming element. By adding a small amount of zirconium, the functions of degassing, purification, and grain refinement can be achieved, which is beneficial for improving the low-temperature performance and stamping performance of low-alloy high-strength steel. When zirconium is dissolved in austenite, the hardenability of the steel is significantly improved. Therefore, the mass percentage of zirconium in the fine-grained high-strength steel according to the present invention is 0.001% - 0.018%.

[0025] Rare earths: The element RE mainly plays the following roles in steel. (1) The purity of the molten steel is improved, and harmful elements are removed from the steel, that is, strong deoxidation and desulfurization are achieved. By adding rare earth elements, the contents of oxygen and sulfur in the molten steel can be reduced to a very low level. Therefore, it is necessary to perform other deoxidation treatments before adding rare earths as a deoxidizer to the molten steel, and its utilization rate can be increased. (2) Rare earth elements can act as modifiers for inclusions. Compared with conventional Al deoxidation inclusions, the added rare earths can react with Al elements in the molten steel to generate REAlO3, and the added rare earths can refine and spheroidize the inclusions. (3) A fine alloying function can be obtained. It is considered that the added rare earths can suppress the temper embrittlement of the steel. Elements such as phosphorus and sulfur in the steel tend to segregate towards the austenite primary grain boundary. As a result, the steel and iron materials change from toughness to brittleness with increasing temperature. The added rare earth elements can combine with elements such as oxygen, sulfur, and phosphorus distributed at the grain boundary to form inclusions, and the influence of these elements on the temper embrittlement of the steel and iron materials can be removed or reduced. When the addition amount of rare earths exceeds the critical amount, the rare earths segregate at the grain boundary, the orientation relationship of pearlite is destroyed, the interlamellar spacing widens, and the impact toughness of pearlite decreases. Therefore, the mass percentage of rare earths in the fine-grained high-strength steel according to the present invention is 0.001% - 0.018%.

[0026] In the above formula, Si / C = 1 to 8 represents the mass percentages of Si and C respectively, and the value substituted into the above formula is the value before the percent symbol. For example, if the mass percentage of Si is 0.17% and the mass percentage of C is 0.05%, the above formula becomes Si / C = 0.17 / 0.06 = 2.83. Also, the formulas Nb / Ti = 1 to 3 and Zr / RE = 1 to 6 are the same as Si / C = 1 to 8.

Advantages of the Invention

[0027] The present invention has the following beneficial effects.

[0028] (1) In order to achieve ultra-low temperature toughness, the fine alloy high-strength steel according to the present invention adopts a composition design completely different from the prior art, that is, a simple low-cost Ni-free composition design, and realizes ultra-low temperature toughness of -100 to -120 °C with the help of Zr + RE composite deoxidation and Nb + Ti composite micro-alloying technology.

[0029] (2) The fine alloy high-strength steel plate of the present invention uses an inexpensive chemical composition design of low carbon, low silicon, and medium manganese, and does not contain any precious metal elements such as Cr, Ni, and Cu, so the material cost can be significantly reduced.

[0030] (3) Instead of the conventional Al deoxidation technology, the present invention uses Si-Mn deoxidation supplemented by Zr-Ti-RE compound deoxidation to form fine and dispersed uniform composite oxysulfides, significantly improving plasticity and toughness.

[0031] (4) Due to the low carbon equivalent design, the fine alloy high-strength steel plate according to the present invention has excellent welding performance (CEV ≤ 0.39, Ceq ≤ 0.17). By using Nb, Ti, Zr, and RE compound fine alloys together with the control of rolling parameters in TMCP, the steel plate has fine crystal grains and high strength and toughness. This Ni-free fine alloy 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-temperature and ultra-low temperature (-20 to -120 °C) environments. This material has excellent low-temperature toughness, and in addition to its advantages, it has large characteristics such as high strength and excellent weldability.

[0032] Based on the technical solutions described above, the present invention can also be improved as follows.

[0033] Furthermore, it consists of 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%, with the remainder being Fe and unavoidable impurities.

[0034] Furthermore, it consists of C:0.05%, Si:0.17%, Mn:1.4%, Nb:0.03%, Ti:0.015%, Zr:0.008%, RE:0.007%, with the remainder being Fe and unavoidable impurities.

[0035] Furthermore, among the unavoidable impurities, the mass percentages of elements P, S, O, N, and H satisfy the following conditions: P ≤ 0.0049%, S ≤ 0.0010%, O ≤ 0.0049%, N ≤ 0.0039%, and H ≤ 0.00019%.

[0036] Furthermore, the element RE includes lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

[0037] Furthermore, the microstructure of the fine alloy high-strength steel does not have a ferrite-pearlite band structure, and the effective grain size of the microstructure of the fine alloy high-strength steel is 5 mm or less.

[0038] Furthermore, the aforementioned fine-grained alloy high-strength steel exhibits a V-notch impact absorption energy exceeding 300 J under conditions of -120°C.

[0039] Furthermore, the ductile-brittle transition temperature of the aforementioned fine-grained alloy high-strength steel is -110°C to -130°C.

[0040] A second aspect of the present invention provides a method for preparing a Ni-free fine alloy high-strength steel having ultra-low temperature toughness, the method being:

[0041] 1) The steps include sequentially smelting and refining molten steel, performing vacuum treatment, and continuously casting to obtain slag,

[0042] 2) A step of heating and immersing the slag to obtain heat-treated slag,

[0043] 3) The process includes the steps of continuously rolling the heat-treated slag, controlling the final rolling temperature to 750-850°C, water-cooling it to 410-550°C after rolling, and then naturally cooling it to room temperature to obtain fine alloy high-strength steel.

[0044] Furthermore, the smelting and refining method in step 1) specifically includes: using a rotary furnace or electric furnace to steel molten iron and / or scrap, adjusting the temperature and composition to obtain molten steel, adjusting the tapping temperature of the molten steel to 1549-1689°C, and ensuring that the free oxygen content in the molten steel is 99-398 ppm; charging the molten steel into a ladle, pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or Fe-Si-Mn alloy under fine argon bubbling to adjust the free oxygen content in the molten steel to 10-98 ppm; performing final deoxidation using a composite additive under fine argon bubbling, and performing LF refining, VD refining, or RH refining on the molten steel after final deoxidation.

[0045] The above-mentioned composite additive is added to the molten steel in the form of block alloy or core wire, with a particle size of 4 mm to 20 mm, and the amount of composite additive added is 0.49 kg to 4.8 kg per ton of molten steel. Next, the molten steel is subjected to LF refining, VD refining, or RH refining according to a conventional process, and finally, the refined molten steel is continuously cast according to a conventional process. The composite additive is a composition of zirconium, titanium, and rare earth elements, with a weight ratio of zirconium, titanium, and rare earth elements in the composite additive of 7:20:6, and the rare earth elements include lanthanum and cerium, with a weight ratio of lanthanum to cerium of 80:20. [Brief explanation of the drawing]

[0046] [Figure 1]This is the low-temperature ductile-brittle transition temperature (DBTT) curve of the fine-grained alloy high-strength steel of the present invention. [Figure 2] This is the oscillometric impact curve of the fine alloy high-strength steel and the control steel of the present invention. [Figure 3] This is an Ashby diagram showing the Charpy impact energy at -100°C as a function of the yield strength at room temperature for the fine alloy high-strength steel and the control steel of the present invention. [Figure 4] These are front view diagrams of the low-temperature Charpy impact fracture of the fine alloy high-strength steel of the present invention, with the left diagram showing -100°C and the right diagram showing -110°C. [Figure 5] This invention presents the micromorphology of different parts of Charpy impact fracture at -100°C in a fine alloy high-strength steel. [Modes for carrying out the invention]

[0047] The principles and features of the present invention are described below, but the examples provided are for illustrative purposes only and do not limit the scope of the invention. Examples in which no specific techniques or conditions are shown should follow the techniques or conditions described in the literature in the relevant art or the product specifications. Reagents or equipment whose manufacturers are not indicated are all conventional products that are commercially available through normal channels.

[0048] The following composite additive is a composition of zirconium, titanium, and rare earth elements, with a weight ratio of zirconium, titanium, and rare earth elements in the composite additive of 7:20:6, and the rare earth elements include lanthanum and cerium, with a weight ratio of lanthanum to cerium of 80:20. The control steel is bulky, clustered alumina and its composite oxides formed by the final deoxidation of conventional aluminum blocks, aluminum particles, or aluminum wires.

[0049] Example 1

[0050] This embodiment relates to a Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and has a chemical composition in mass%, of 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%, with the remainder being Fe and unavoidable impurities. The RE contains lanthanum and cerium, and the weight ratio of the lanthanum and cerium elements is 70:30.

[0051] This embodiment relates to a method for preparing a Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and includes the following steps.

[0052] 1) Molten steel is sequentially smelted and refined, then vacuum-treated, and continuously cast to obtain slag.

[0053] The smelting and refining method specifically involves using a rotary furnace or electric furnace to steel molten iron and / or scrap, adjusting the temperature and composition to obtain molten steel, adjusting the tapping temperature to 1620°C, and ensuring the free oxygen content in the molten steel is 250 ppm; charging the molten steel into a ladle, bubbling it with fine argon for 6 minutes, pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or Fe-Si-Mn alloy to adjust the free oxygen content in the molten steel to 55 ppm, bubbling it with fine argon for 6 minutes, and then performing final deoxidation with a composite additive; adding the composite additive to the molten steel in the form of a block alloy or core wire, with a particle size of 12 mm, and adding 2.7 kg of the composite additive per ton of molten steel; and then performing LF refining and RH refining of the molten steel according to conventional methods.

[0054] LF Refinement:

[0055] The viscosity of the smelting slag is controlled to 1.517-1.933 Pa·s to improve the adsorption capacity of inclusions in the slag system and improve the purity of the molten steel. The alkalinity of the white slag in the smelting furnace is controlled to 5.15≦R≦7.47 to increase the desulfurization rate, improve the purity of the molten steel, and reduce oxide inclusions in the molten steel. The MI slag index (=CaO / SiO2:Al2O3 ratio) is controlled to MI>0.147 to significantly increase the sulfur distribution coefficient, control the appropriate fluidity of the smelting slag under a certain alkalinity, set the white slag holding time to ≥14.46 min, the smelting cycle to ≥39.39 min, and the soft blow time to >4.51 min to control the tapped [O] content.

[0056] RH vacuum treatment:

[0057] The pressure in the vacuum chamber is set to 66.69 kPa or less and maintained for 12.35 to 14.47 minutes, the argon bottom blow flow rate is set to 10.26 to 19.38 m3 / h, four cycles of molten steel are performed, the type and weight of additive alloys are strictly controlled, higher-grade alloys such as low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, and ferrotitanium are used to ensure that the composition of the molten steel is perfectly suitable, and the vacuum is maintained for 5.28 minutes or more to obtain cleaner molten steel, while simultaneously ensuring a molten steel temperature suitable for continuous casting, such that the superheat of the tundish is 10.17°C to 29.46°C above the liquidus line.

[0058] 2) The slag is heated and heat-treated at 1195°C for 3.5 hours to obtain heat-treated slag.

[0059] 3) The heat-treated slag is continuously rolled, the final rolling temperature is controlled to 750-850°C, the slag is water-cooled to 410-550°C after rolling, and then naturally cooled to room temperature to obtain fine alloy high-strength steel.

[0060] The rolling method typically involves heating and immersing the slag, then continuously rolling it to produce steel sheets. The final rolling temperature is controlled to 800°C, followed by water cooling to 480°C, and then natural cooling to room temperature.

[0061] The steel sheet obtained by the above process does not have a ferrite-pearlite band structure, the effective grain size of the steel sheet microstructure is 4.5 μm, and the V-notch impact absorption energy of the steel sheet under -120°C conditions exceeds 315 J.

[0062] Example 2

[0063] This embodiment relates to a Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and has a chemical composition in mass%, of 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%, with the remainder being Fe and unavoidable impurities. The RE contains lanthanum and cerium, and the weight ratio of the lanthanum and cerium elements is 80:20.

[0064] This embodiment relates to a method for preparing Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and the method includes the following steps.

[0065] 1) Molten steel is sequentially smelted and refined, then vacuum-treated, and continuously cast to obtain slag.

[0066] The smelting and refining method specifically involves using a rotary furnace or electric furnace to steel molten iron and / or scrap, adjusting the temperature and composition to obtain molten steel, adjusting the tapping temperature to 1680°C, and ensuring the free oxygen content in the molten steel is 380 ppm; charging the molten steel into a ladle, bubbling it with fine argon for 8 minutes, pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or Fe-Si-Mn alloy to adjust the free oxygen content in the molten steel to 90 ppm, bubbling it with fine argon for 7 minutes, and then performing final deoxidation with a composite additive; adding the composite additive to the molten steel in the form of a block alloy or core wire, with a particle size of 19 mm, and adding 4.5 kg of the composite additive per ton of molten steel; and then performing LF refining and RH refining of the molten steel according to conventional methods.

[0067] LF Refinement:

[0068] The viscosity of the smelting slag is controlled to 1.531-1.964 Pa·s to improve the adsorption capacity of slag-based inclusions and enhance the purity of molten steel. The alkalinity of the white slag in the smelting furnace is controlled to 5.15≦R≦7.67 to increase the desulfurization rate, enhance the purity of molten steel, and reduce oxide inclusions in the molten steel. The MI slag index (=CaO / SiO2:Al2O3 ratio) is controlled to MI>0.149 to significantly increase the sulfur distribution coefficient, thereby controlling the appropriate fluidity of the smelting slag under a certain alkalinity. The white slag holding time is set to ≥14.23 min, the smelting cycle to ≥39.36 min, and the soft blow time is set to >4.58 min to control the tapped [O] content.

[0069] RH vacuum treatment:

[0070] The pressure in the vacuum chamber is set to 66.59 kPa or less and maintained for 12.27 to 14.48 minutes, the argon bottom blow flow rate is set to 10.21 to 19.39 m3 / h, five cycles of molten steel are performed, the type and weight of the added alloys are strictly controlled, higher-grade alloys such as low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, and ferrotitanium are used to ensure that the composition of the molten steel is perfectly suitable, and the vacuum is maintained for 5.33 minutes or more to obtain cleaner molten steel, while simultaneously ensuring a molten steel temperature suitable for continuous casting, such that the superheat of the tundish is 10.17°C to 29.49°C above the liquidus line.

[0071] 2) The slag is heated and heat-treated at 1205°C for 3.4 hours to obtain heat-treated slag.

[0072] 3) The heat-treated slag is continuously rolled, the final rolling temperature is controlled to 750-850°C, the slag is water-cooled to 410-550°C after rolling, and then naturally cooled to room temperature to obtain fine alloy high-strength steel.

[0073] The rolling method typically involves heating and immersing the slag, then continuously rolling it to produce steel sheets. The final rolling temperature is controlled to 840°C, followed by water cooling to 530°C, and then natural cooling to room temperature.

[0074] The steel sheet obtained by the above process does not have a ferrite-pearlite band structure, the effective grain size of the steel sheet microstructure is 4.8 μm, and the V-notch impact absorption energy of the steel sheet under -120°C conditions exceeds 309 J.

[0075] Example 3

[0076] This embodiment relates to a Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and has a chemical composition in mass%, of 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%, with the remainder being Fe and unavoidable impurities. The RE contains lanthanum and cerium, and the weight ratio of the lanthanum and cerium elements is 90:10.

[0077] This embodiment relates to a method for preparing Ni-free fine alloy high-strength steel having ultra-low temperature toughness, and the method includes the following steps.

[0078] 1) Molten steel is sequentially smelted and refined, then vacuum-treated, and continuously cast to obtain slag.

[0079] The smelting and refining method specifically involves using a rotary furnace or electric furnace to steel molten iron and / or scrap, adjusting the temperature and composition to obtain molten steel, adjusting the tapping temperature to 1580°C, and ensuring the free oxygen content in the molten steel is 150 ppm; charging the molten steel into a ladle, bubbling it with fine argon for 5 minutes, pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or Fe-Si-Mn alloy to adjust the free oxygen content in the molten steel to 90 ppm, bubbling it with fine argon for 5 minutes, and then performing final deoxidation with a composite additive; adding the composite additive to the molten steel in the form of a block alloy or core wire, with a particle size of 9 mm, and adding 0.59 kg of the composite additive per ton of molten steel; and then performing LF refining and RH refining of the molten steel according to conventional methods.

[0080] LF Refinement:

[0081] The viscosity of the smelting slag is controlled to 1.526-1.953 Pa·s to improve the adsorption capacity of the slag system for inclusions and improve the purity of the molten steel. The alkalinity of the white slag in the smelting furnace is controlled to 5.16≦R≦7.63 to increase the desulfurization rate, improve the purity of the molten steel, and reduce oxide inclusions in the molten steel. The MI slag index (=CaO / SiO2:Al2O3 ratio) is controlled to MI>0.153 to significantly increase the sulfur distribution coefficient, control the appropriate fluidity of the smelting slag under a certain alkalinity, set the white slag holding time ≥14.35 min, the smelting cycle ≥39.47 min, and the soft blow time >4.58 min to control the tapped [O] content.

[0082] RH vacuum treatment:

[0083] The pressure in the vacuum chamber is set to 66.69 kPa or less and maintained for 12.21 to 14.47 minutes, the argon bottom blow flow rate is set to 10.23 to 19.46 m3 / h, the molten steel is circulated six times, the type and weight of the added alloys are strictly controlled, higher quality alloys such as low carbon ferromanganese, metallic manganese, low carbon ferrosilicon, and ferrotitanium are used to ensure that the composition of the molten steel is perfectly suitable, and the vacuum is maintained for 5.33 minutes or more to obtain cleaner molten steel, while simultaneously ensuring a molten steel temperature suitable for continuous casting, such that the superheat of the tundish is 10.35°C to 29.47°C above the liquidus line.

[0084] 2) The slag is heated and heat-treated at 1225°C for 3.1 hours to obtain heat-treated slag.

[0085] 3) The heat-treated slag is continuously rolled, the final rolling temperature is controlled to 750-850°C, the slag is water-cooled to 410-550°C after rolling, and then naturally cooled to room temperature to obtain fine alloy high-strength steel.

[0086] The rolling method typically involves heating and immersing the slag, then continuously rolling it to produce steel sheets. The final rolling temperature is controlled to 760°C, followed by water cooling to 430°C, and then natural cooling to room temperature.

[0087] The steel sheet obtained by the above process does not have a ferrite-pearlite band structure, the effective grain size of the steel sheet microstructure is 4.4 μm, and the V-notch impact absorption energy of the steel sheet under -120°C conditions exceeds 311 J.

[0088] Experimental example

[0089] The following low-temperature impact properties were tested and analyzed with reference to the Ni-free fine alloy high-strength steel (R&D steel) with ultra-low temperature toughness prepared in Example 1, and the following results were obtained.

[0090] (1) Test method for low-temperature shock performance

[0091] The low-temperature shock test will be conducted in accordance with the Chinese national standards GB / T229-2020 "Metallic Materials - Shear Pendulum Shock Test Method" and GB / T19748-2019 "Metallic Materials - Instrumented Test Method for Shear V-Notch Pendulum Shock Test". The sample specifications are a standard sample of 55 × 10 × 10 mm, with a V-notch, notch depth of 2 mm, and root radius of 0.25 mm. The radius of curvature of the pendulum's striking end is 2 mm. The test temperatures are 20, -20, -40, -60, -80, -100, -110, -120, -130, -140, -160, and -196°C. During the pendulum shock process, the force and displacement values ​​are recorded using a resistance strain gauge and optical method at the striking end, respectively, and a force-displacement curve is obtained during the shock process. The shock absorption energy is obtained by integral calculation.

[0092] (2) Analysis results of low-temperature impact toughness

[0093] Figure 1 shows the low-temperature ductile-brittle transition temperature (DBTT) curve of the fine-grained alloy high-strength steel according to the present invention. As shown in Figure 1, the ductile-brittle transition temperature of the fine-grained alloy high-strength steel according to the present invention is in the range of approximately -110°C to -130°C, which is more than 50°C lower than that of the same strength FH36 (standard requirement is a ductile-brittle transition temperature of less than -60°C).

[0094] Figure 2 shows a comparison of the -100°C oscillometric impact curves of the fine alloy high-strength steel of the present invention and a control steel of the same type. The fine alloy high-strength steel of the present invention has a crack initiation-propagation-passivation process in the -100°C impact process, while the control steel shows a rapid decline in the force-displacement curve after yielding. This indicates that the control steel undergoes instantaneous brittle fracture at crack initiation without a stable crack propagation and passivation process.

[0095] Figure 3 is an Ashby diagram of the impact energy at -100°C against the yield strength at room temperature of the fine-grained alloy high-strength steel of the present invention and other steel types. The dotted line connects the coordinates (200 MPa, 300 J) and (1000 MPa, 0 J). Steel types to the right of the dotted line are considered to have achieved excellent trace-off in low-temperature impact toughness and room-temperature yield strength. As can be seen from Figure 3, the fine-grained alloy high-strength steel of the present invention has the highest impact energy at -100°C, which is higher than conventional low-carbon fine-grained alloy steels [1-3], medium-manganese steels [6-9], chromium-manganese stainless steels [13-14], duplex stainless steels

[15] , nickel-based low-temperature steels [8,16,17], maraging steels [18,19], manganese-nickel fine-grained alloy steels [4,5] and high-manganese steels [10-12].

[0096] Figure 4 shows the macroscopic morphology of impact fracture of the fine alloy high-strength steel of the present invention at two low temperatures, -100°C and -110°C. As can be seen from Figure 4, at -100°C, the fine alloy high-strength steel according to the present invention undergoes complete ductile fracture, and at -120°C, the fine alloy high-strength steel according to the present invention changes completely to brittle fracture.

[0097] Figure 5 shows the micromorphology of different parts of Charpy impact fracture at -100°C of the fine alloy high-strength steel according to the present invention, where both crack initiation and stable crack propagation appear as dimple morphology stretched in the fracture direction, with many tear marks present at the ends of the dimples, and crack passivation appears as a mixed morphology of oblique and normal dimples.

[0098] The requirements for low-temperature steel performance include, firstly, ensuring sufficient impact toughness at the operating temperature, and from the perspective of fracture mechanics, having sufficient resistance to brittle cracking at the operating temperature. In particularly critical structures, it is necessary to have crack-blocking properties against the propagation of brittle cracks in order to avoid accidents. Furthermore, from a safety standpoint, it is desirable that the yield ratio of low-temperature steel be low, as a high yield ratio reduces the margin of plastic deformation capacity and the stress redistribution capacity at stress concentration points, making it more susceptible to brittle fracture.

[0099] The technical requirements for low-temperature steel generally include sufficient strength and toughness at low temperatures, excellent technical properties, workability, and corrosion resistance. Low-temperature toughness, that is, the ability to avoid the occurrence and propagation of brittle fracture at low temperatures, is the most important factor. Therefore, a certain value for impact toughness at the lowest temperature is usually specified in every country.

[0100] Currently, conventional techniques involve alloying nickel to meet the above-mentioned low-temperature toughness requirements. The main reasons for this are: (1) Ni is a major alloying element that does not form carbides with carbon but forms and stabilizes austenite; (2) Ni is an element that dissolves purely in steel, strengthening the ferrite matrix and significantly lowering the ductile-brittle transition temperature; (3) a fine grain structure can be obtained through controlled rolling; and (4) a stable structure can be obtained through heat treatment.

[0101] However, in order to achieve ultra-low temperature toughness, the present invention employs a completely different composition design and a matched manufacturing process, achieving ultra-low temperature toughness from -100 to -120°C with the help of Zr+RE composite deoxidation and Nb+Ti composite fine alloying technology, i.e., a simple low-cost Ni-free composition design. The fine alloy high-strength steel sheet of the present invention adopts an inexpensive chemical composition design of low carbon, low silicon, and medium manganese, and does not contain any precious metal elements such as Cr, Ni, and Cu, so material costs can be greatly reduced. Instead of conventional Al deoxidation technology, the present invention uses Si-Mn deoxidation supplemented by Zr-Ti-RE compound deoxidation, forming a fine, dispersed, and uniform composite oxysulfide, which can significantly improve plasticity and toughness. Due to the low-carbon equivalent design, the steel sheet according to the present invention has excellent weldability, and by using Nb, Ti, Zr, and RE compound fine alloys together with rolling parameter control in TMCP, the steel sheet has fine crystalline grains and high strength and toughness. This nickel-free fine alloy high-strength low-temperature steel is particularly suitable for structural materials used in polar regions, containers, pipelines, refining, storage and transportation, and equipment in low-temperature and ultra-low-temperature (-20 to -120°C) environments. This material boasts excellent low-temperature toughness, and in addition to this advantage, it also possesses significant features such as high strength and excellent weldability.

[0102] References [1]-

[19] for low-carbon fine alloy steels [1-3], medium-manganese steels [6-9], chromium-manganese stainless steels [13-14], duplex stainless steels

[15] , nickel-based low-temperature steels [8,16,17], maraging steels [18,19], manganese-nickel fine alloy steels [4,5], and high-manganese steels [10-12] are listed below. [Prior art documents] [Non-patent literature]

[0103] [Non-Patent Document 1] [1]H. Tervo , A. Kaijalainen , S. Pallaspuro , S. Anttila , S. Mehtonen , D. Porter , J. Komi . Low-temperature toughness properties of 500MPa offshore steels and their simulated coarse-grained heat-affected zones. Materials Science and Engineering: A, 2020, 773:138719.

Outdoor Tool2

Outdoor Tools3

Outdoor Tools 4

Direct Environment 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Non-Patent Document 9

Non-Patent Document 10

[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.

Non-Patent Document 11

[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.

Non-Patent Document 12

[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. [Non-Patent Document 13]

[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. [Non-Patent Document 14]

[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. [Non-Patent Document 15]

[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. [Non-Patent Document 16]

[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.

Non-patent Document 17

[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.

Non-patent Document 18

[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.

Non-patent Document 19

[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.

[0104] In this specification, reference terms such as “one embodiment,” “several embodiments,” “example,” “specific example,” or “several examples” mean that the specific features, structures, materials, or properties described in combination with such 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. Furthermore, the specific features, structures, materials, or properties described may be combined in any suitable manner in any one or more embodiments or examples. In addition, a person skilled in the art may combine and combine different embodiments or examples, and features of different embodiments or examples, as described herein, as long as they do not conflict with each other.

[0105] Although embodiments of the present invention have been illustrated and described, these embodiments are merely illustrative and should not be interpreted as limiting the present invention. Those skilled in the art can modify, alter, substitute, and transform these embodiments within the scope of the present invention.

Claims

1. It has a chemical composition of the following in mass percent: 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%, with the remainder being Fe and unavoidable impurities. The mass percentages of elements C and Si also satisfy the following conditions simultaneously: 0.21% < C + Si < 0.24%, and Si / C = 1 to 8. The mass percentages of elements Nb and Ti also satisfy the following conditions simultaneously: 0.02% < Nb + Ti < 0.05%, and Nb / Ti = 1 to 3. A Ni-free alloy steel characterized in that the mass percentages of elements Zr and RE simultaneously satisfy the equations 0.010% < Zr + RE < 0.019% and Zr / RE = 1 to 6.

2. The Ni-free alloy steel according to claim 1, characterized in that it consists of, by mass%, 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%, with the remainder being Fe and unavoidable impurities.

3. The Ni-free alloy steel according to claim 1, characterized in that it consists of, by mass%, C: 0.05%, Si: 0.17%, Mn: 1.4%, Nb: 0.03%, Ti: 0.015%, Zr: 0.008%, RE: 0.007%, with the remainder being Fe and unavoidable impurities.

4. The Ni-free alloy steel according to any one of claims 1 to 3, characterized in that, in the unavoidable impurities, the mass percent of elements P, S, O, N, and H satisfy P ≤ 0.0049%, S ≤ 0.0010%, O ≤ 0.0049%, N ≤ 0.0039%, and H ≤ 0.00019%, respectively.

5. The Ni-free alloy steel according to any one of claims 1 to 3, characterized in that the element RE comprises lanthanum and cerium, and the weight ratio of the lanthanum element to the cerium element is (70-90):(10-30).

6. The Ni-free alloy steel according to any one of claims 1 to 3, characterized in that the microstructure of the alloy steel does not have a ferrite-pearlite band structure.

7. The Ni-free alloy steel according to any one of claims 1 to 3, characterized in that the alloy steel has a V-notch shock absorption energy of more than 300 J under the condition of a temperature of -120°C.

8. The Ni-free alloy steel according to any one of claims 1 to 3, characterized in that the ductile-brittle transition temperature of the alloy steel is -110°C to -130°C.

9. A method for preparing Ni-free alloy steel according to any one of claims 1 to 3, 1) The steps of sequentially smelting and refining molten steel, performing vacuum treatment, and continuously casting to obtain slag, 2) A step of heating and immersing the slag to obtain heat-treated slag, 3) A method for preparing Ni-free alloy steel, comprising the steps of: continuously rolling the heat-treated slag, controlling the final rolling temperature to 750 to 850°C, water-cooling it to 410 to 550°C after rolling, and then naturally cooling it to room temperature to obtain alloy steel.

10. The method for preparing Ni-free alloy steel according to claim 9 is characterized in that the smelting and refining method in step 1) specifically includes: using a rotary furnace or an electric furnace to steel molten iron and / or scrap, adjusting the temperature and composition to obtain molten steel, adjusting the tapping temperature of the molten steel to 1549 to 1689°C, and ensuring that the free oxygen content in the molten steel is 99 to 398 ppm; charging the molten steel into a ladle, pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or Fe-Si-Mn alloy under fine argon bubbling to adjust the free oxygen content in the molten steel to 10 to 98 ppm; performing final deoxidation using a composite additive under fine argon bubbling, and performing LF refining, VD refining, or RH refining on the molten steel after final deoxidation.

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