Ni-FREE MICRO-ALLOYED HIGH-STRENGTH STEEL WITH ULTRA-LOW TEMPERATURE TOUGHNESS, AND PREPARATION METHOD THEREOF

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 metallic materials, in particular to a Ni-free microalloyed high strength steel with ultra-low temperature toughness and a method for preparing the same. [Background technology]

[0002] Low-temperature steel is an important type of low-alloy high-strength steel. Generally, various production and storage vessels for liquefied petroleum gas, liquid ammonia, liquid oxygen, liquid nitrogen, etc., transportation pipelines, and equipment used in cold regions are called low-temperature vessels, and the steel materials used to manufacture these vessels are collectively called low-temperature steel. In China, the design temperature is usually -20℃ or lower (GB150-1998《Steel Pressure Vessels》Appendix C Definition of low-temperature 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, whose service temperatures are above -60°C. Nickel steels refer to steels that contain the alloying element Ni, which is soluble in ferrite, and have a significantly improved low-temperature toughness of the matrix, in order to change the general phenomenon of low-temperature brittle transition of metal materials with body-centered cubic lattice, and whose service temperatures can 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 gas are widespread in all countries. The development of low-temperature technologies and equipment such as these promotes the development of steel for low-temperature pressure vessels. According to the Japanese JIS G 3127 (1977) 《Nickel Steel Plate for Low-Temperature Pressure Vessels》 standard, the representative steel types are SL3N255, SL3N275, SL3N440, etc. According to the US SA-203 / SA-203M 《Nickel Alloy Steel Plate for Pressure Vessels》 standard, the representative steel types are SA203Gr.D, SA203Gr.E, SA203GR.F. Low-temperature steels containing 3.5-9% Ni have corresponding standards in the US, Japan and Europe. In the late 1960s, China developed low-temperature steels for temperatures from -40℃ to -253℃, but they did not become widespread. In 1983, the Standardization Administration of the People's Republic of China issued GB3531-83 "Technical Specifications for Low-Alloy Steel Plates for Low-Temperature Pressure Vessels", stipulating that the service temperatures of four nickel-free low-temperature steels are from -30℃ to -90℃. 16MnDR steel is an economical and mature steel grade for the manufacture of low-temperature equipment at -40℃, such as the manufacture of liquid ammonia equipment. 15MnNiDR and 09MnNiDR are nickel-based low-temperature steels with excellent low-temperature toughness and weldability. The -70℃-class 09MnNiDR low-temperature steel is widely used in low-temperature equipment for ethylene, chemical fertilizer, city gas, carbon dioxide, etc., and has gradually succeeded in replacing imported low-temperature steels in the manufacture of ammonia separators (-28℃), high-pressure nitrogen storage tanks (-28℃), carbon dioxide low-temperature storage tanks (-50℃), ethylene low-temperature storage tanks (-60℃), etc.

[0005] Regarding low-temperature steels with high Ni content, the material section of China CCS 1996 "Rules for Steel Shipbuilding and Classification" stipulates that nickel alloy steels with a thickness of 50 mm or less are 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, the consensus has been reached in the pressure vessel industry to use 3.5Ni low-temperature steel for the manufacture of -100℃-class cryogenic vessels and 9Ni low-temperature steel for the manufacture of -196℃-class cryogenic vessels. 3.5Ni steel is widely applied to cryogenic vessels from -101℃ to -80℃. [Table 1]

[0006] Nickel-based low-temperature steels are widely used in developed countries such as the United States and Japan because they have high strength, excellent low-temperature toughness, and are cheaper than Cr-Ni stainless steels at the corresponding temperature level. For cryogenic containers, the lower the temperature of the storage medium, the lower the pressure the container bears, and the higher the safety and reliability. Currently, 9Ni (-196℃) low-temperature steels are widely used for liquefied natural gas, and 3.5Ni (-100℃) low-temperature steels are usually used in low-temperature equipment in the petrochemical and fertilizer industries, as it is necessary to liquefy gas at about -80℃.

[0007] The above explanation mainly describes the applications and requirements of low-temperature steels for cryogenic pressure vessels and equipment. From the perspective of polar oil and gas development, polar shipping, and polar icebreaking equipment, the polar environment, the extreme cold environment, and their low temperature requirements are introduced. The natural conditions in the Arctic are harsh, and there are challenges such as low-temperature testing, sea ice damage, iceberg attacks, snowstorm attacks, fragile ecological environment, polar night damage, and poor visibility, which place many requirements on related equipment and material performance. The ultra-low temperature environment in the Arctic has spurred the demand for low-temperature resistant steels. According to a 40-year tracking survey of about 700 polar ships by Lloyd's Register, 57% of polar ships have cracks or damage in the hull steel structure after an average of 13 years. Special low-temperature steels are usually used for large icebreakers, and comprehensive properties such as low-temperature toughness, strength, and fatigue strength are required.

[0008] In recent years, polar ships have gradually developed from low-grade ice-strengthened to high-grade reinforced with self-icebreaking capability, and the demand for new commercial icebreakers with icebreaking capability, such as polar oil tankers, polar LNG carriers, and polar container ships, is rapidly increasing. Accordingly, research and development of materials for steel plates, deck machinery, and key components for polar ships with cold resistance and high toughness must also 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 ships, and breakthroughs in key technologies such as low-temperature resistance, excellent weldability, and high toughness are urgently needed for the construction of polar ships and polar equipment.

[0009] At present, in response to the above-mentioned special requirements, first, in the conventional technology of low-temperature steel, Ni element is usually added to improve low-temperature toughness. This is mainly because (1) Ni is a main alloying element that does not form carbides with carbon but forms and stabilizes austenite, (2) Ni is an element that dissolves purely in steel, which can strengthen the ferrite matrix and obviously lower the ductile-brittle transition temperature, (3) a fine grain structure can be obtained by controlled rolling, and (4) a stable structure can be obtained by heat treatment. Secondly, carbide-forming elements such as Nb, Ti, and Mo are added to the steel, which improves the overall properties of the steel through the precipitation strengthening and grain refinement functions that result from them. Thirdly, weldability is improved by fine alloying of Nb, Ti, and V. In order to improve corrosion resistance, it may be necessary to add alloying elements such as Cr and Cu. The above conventional technology results in high-cost and expensive low-temperature steel.

[0010] However, the minimum temperature in polar regions and extremely cold regions is -70°C or lower, and steel plates used in polar regions and extremely cold 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 and excellent in low-temperature toughness, and are inexpensive. The present invention has been made in view of the above circumstances, and provides a Ni-free fine-grained high-strength alloy steel having ultra-low-temperature toughness and a method for preparing the same. Summary of the Invention [Problem to be solved by the invention]

[0011] The technical problem to be solved by the present invention is to provide a Ni-free fine-alloyed high-strength steel with ultra-low temperature toughness and a preparation method thereof, which aims to solve the problems of high cost, poor weldability and complicated manufacturing process 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, which has a chemical composition in mass percentage of 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 balance of Fe and inevitable impurities.

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

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

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

[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 in excess, bainite phase or martensite phase is likely to be generated 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 percentage of carbon in the fine alloy high-strength steel according to the present invention is 0.011% < C < 0.099%.

[0018] Silicon: Silicon does not form carbides with carbon, but exists in steel in the form of a solid solution, and by interacting with the stress field of moving dislocations, it impedes the movement of dislocations, improving the strength of steel and steel materials. A high content of silicon is detrimental to the weldability of steel and iron materials. In addition, a high content of silicon reduces the ductility and toughness of steel. Therefore, the mass percent of silicon in the fine alloyed 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 and expands the austenite phase region. In the cooling process, Mn dissipates free energy through the solvent drag effect and inhibits diffusion transformation. By adding an appropriate amount of Mn, the structure of the steel plate can be controlled under appropriate technological conditions, and a fine bainitic lath structure with high strength and toughness can be formed. If Mn is contained in excess, cracks may occur in the slab during continuous casting or subsequent cooling. Mn can also achieve deoxidation and eliminate the effect of S in the smelting process, and Mn in the steel can combine with S to form MnS, preventing thermal embrittlement 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: Phosphorus has a strong solid solution strengthening effect in steel, and by adding it to low alloy structural steel as an alloying element, it can improve the strength and atmospheric corrosion resistance of the steel. However, phosphorus has the most harmful effect of serious segregation, and also increases temper brittleness, significantly impairing the plasticity and toughness of the steel. Phosphorus also has a negative effect on weldability. Thus, phosphorus is a harmful element and needs to be strictly controlled. Therefore, the mass percent 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, deteriorating the internal and surface quality of steel, and sulfur also reduces the plasticity of steel. It exists in the form of FeS, which has a low melting point. The melting point of FeS alone is only 1,190°C, but the eutectic temperature at which FeS and iron form in steel is even lower, at only 988°C, and when steel solidifies, iron sulfide precipitates at the primary grain boundaries. When steel is rolled at 1,100°C to 1,200°C, 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 percent of sulfur in the fine-alloyed high-strength steel according to the present invention is S≦0.0010%.

[0022] Niobium: When the carbon content is low, the ferrite grains can be refined by appropriately adding Nb, and the strength and low-temperature toughness of the steel can be improved. When the Nb content is high, the precipitation of eutectoid ferrite is delayed, the time when austenite starts to decompose 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 plate, but the impact toughness of the steel plate is deteriorated. Therefore, the mass% of niobium in the fine alloy high-strength steel according to the present invention is 0.031% to 0.059%.

[0023] Titanium: Titanium has a strong affinity with oxygen, nitrogen and carbon, is a good deoxidizer, and is an effective element for fixing nitrogen and carbon. TiN particles can effectively prevent austenite grains from coarsening during the welding heat cycle, which is beneficial to improving toughness. TiN particles can effectively promote the formation of acicular ferrite, which effectively improves the welding performance of steel. Therefore, if the Ti content is excessive, it will not contribute to improving the properties of steel, and will easily generate coarse titanium carbonitrides that are the source of cracks, which will reduce toughness. Therefore, the mass content of Ti in the present invention is 0.009% to 0.016%.

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

[0025] Rare earth: The element RE mainly plays the following roles in steel: (1) The purity of molten steel is improved, and harmful elements are removed from steel, that is, strong deoxidation and desulfurization are achieved. By adding rare earth elements, the oxygen and sulfur content in molten steel can be reduced to an extremely low level, so that rare earth elements as deoxidizers need to undergo other deoxidation treatment before entering molten steel, and their utilization rate can be increased. (2) Rare earth elements can play the role of inclusion modifier. Compared with the traditional Al deoxidization inclusion, the added rare earth can react with the Al element in molten steel to produce REAlO3, and the added rare earth can refine and spheroidize the inclusions. (3) The fine alloying function is obtained. It is believed that the added rare earth can suppress the temper embrittlement of steel, and elements such as phosphorus and sulfur in steel tend to segregate toward the austenite primary grain boundaries, which results in the temperature rise of steel or iron from tough to brittle, and the added rare earth combines with elements such as oxygen, sulfur, and phosphorus distributed at the grain boundaries to form inclusions, which can eliminate or reduce the effect of these elements on the temper embrittlement of steel or iron. If the amount of added rare earth exceeds a critical amount, the rare earth segregates to the grain boundaries, destroying the orientation relationship of pearlite, widening the lamellar spacing, and reducing the impact toughness of pearlite. Therefore, the mass% of rare earth in the fine alloy high strength steel according to the present invention is 0.001% to 0.018%.

[0026] In the above formula, Si / C=1-8 indicates the mass% of Si and C, respectively, and the value substituted in the above formula is the value before the percentage symbol. For example, if the mass% of Si is 0.17% and the mass% of C is 0.05%, the above formula becomes Si / C=0.17 / 0.06=2.83. In addition, the formulas of Nb / Ti=1-3 and Zr / RE=1-6 are the same as Si / C=1-8. Effect of the Invention

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

[0028] (1) In order to achieve ultra-low temperature toughness, the fine-alloyed high-strength steel of the present invention adopts a composition design that is completely different from that of conventional techniques, i.e., a simple, low-cost Ni-free composition design, and with the help of Zr+RE composite deoxidization and Nb+Ti composite fine alloying techniques, achieves ultra-low temperature toughness at -100 to -120°C.

[0029] (2) The fine alloyed 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 that the material cost can be significantly reduced.

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

[0031] (4) Due to the low carbon equivalent design, the fine alloyed high strength steel plate of the present invention has excellent weldability (CEV≦0.39, Ceq≦0.17), and by using Nb, Ti, Zr and RE compound fine alloys together with the rolling parameter control in TMCP, the steel plate has fine grains and high strength and toughness. This Ni-free fine alloyed high strength low temperature steel is especially suitable for structural materials such as arctic, containers, pipelines, refining, storage and transportation, equipment, etc. used in low and ultra-low temperature (-20 to -120℃) environments, and this material has excellent low temperature toughness, and in addition to its advantages, it has great characteristics such as high strength and excellent weldability.

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

[0033] Further, it is composed 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%, and the balance is Fe and unavoidable impurities.

[0034] Further, it is composed of 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.

[0035] Furthermore, with regard to unavoidable impurities, the mass percentages of the 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.

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

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

[0038] Furthermore, the fine alloyed high strength steel has a V-notch impact absorption energy of more than 300J under the condition of -120°C temperature.

[0039] Furthermore, the fine alloyed high strength steel has a ductile-brittle transition temperature of -110°C to -130°C.

[0040] A second aspect of the present invention provides a method for preparing a Ni-free finely alloyed high strength steel with ultra-low temperature toughness, the method comprising the steps of:

[0041] 1) smelting and refining molten steel in sequence, vacuum treating it, and continuously casting it to obtain slag;

[0042] 2) heating and soaking the slag to obtain a heat-treated slag;

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

[0044] Furthermore, the smelting and refining method in step 1) specifically includes: using a rotary furnace or an electric furnace to make steel from molten pig 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 adjusting the free oxygen content in the molten steel to 99-398 ppm; charging the molten steel into a ladle, pre-deoxidizing the molten steel in the ladle using an Fe-Si alloy or an Fe-Si-Mn alloy under fine argon bubbling, and adjusting 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 that has been final deoxidized.

[0045] The above composite additive is added into molten steel in the form of block alloy or core wire, the particle size of the composite additive is 4mm-20mm, the amount of the composite additive added is 0.49kg-4.8kg per ton of molten steel, 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 earth, the weight ratio of the elements zirconium, titanium and rare earth in the composite additive is 7:20:6, the rare earth includes lanthanum and cerium, the weight ratio of the lanthanum and cerium is 80:20. [Brief description of the drawings]

[0046] [Figure 1]1 is a low temperature ductile brittle transition temperature (DBTT) curve of the fine alloyed high strength steel of the present invention. [Diagram 2] 1 is an oscillometric impact curve of a microalloyed high strength steel of the present invention and a control steel. [Diagram 3] FIG. 2 is an Ashby diagram of Charpy impact energy at −100° C. versus yield strength at room temperature for the microalloyed high strength steel of the present invention and a control steel. [Figure 4] 1 is a front view of low-temperature Charpy impact fracture of the fine alloyed high strength steel of the present invention, the left side shows −100° C., and the right side shows −110° C. [Diagram 5] 1 shows the micromorphology of different parts of the Charpy impact fracture of the microalloyed high strength steel of the present invention at −100° C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The principles and features of the present invention are explained below, but the illustrated examples are for the purpose of illustrating the present invention and are not intended to limit the scope of the present invention. Examples in which specific techniques or conditions are not shown should follow the techniques or conditions described in the literature in the field or follow 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, the weight ratio of element zirconium, titanium and rare earth in the composite additive is 7:20:6, the rare earth includes lanthanum and cerium, the weight ratio of lanthanum and cerium is 80:20. The control steel is bulky and cluster-like alumina and its composite oxides formed by the final deoxidation of conventional aluminum block, aluminum particle or aluminum wire.

[0049] Example 1

[0050] This embodiment relates to a Ni-free fine alloyed high strength steel having ultra-low temperature toughness, and has a chemical composition, in mass%, of 0.05% C, 0.17% Si, 1.4% Mn, 0.03% Nb, 0.015% Ti, 0.008% Zr, 0.007% RE, 0.0025% P, 0.008% S, 0.0025% O, 0.0030% N, 0.00016% H, and the balance of Fe and unavoidable impurities. The RE includes lanthanum and cerium, and the weight ratio of the lanthanum element and the cerium element is 70:30.

[0051] This embodiment relates to a method for preparing a Ni-free microalloyed high strength steel with ultra-low temperature toughness, which includes the following steps:

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

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

[0054] LF Refining:

[0055] Control the viscosity of the refining slag to 1.517~1.933 Pa·s, improve the inclusion adsorption capacity of the slag system, improve the cleanliness of molten steel, control the alkalinity of the white slag in the refining furnace to 5.15≦R≦7.47, increase the desulfurization rate, improve the cleanliness of molten steel, reduce the oxide inclusions in molten steel, control the MI slag index (=CaO / SiO2:Al2O3 ratio) to MI>0.147, significantly increase the distribution coefficient of sulfur, control the appropriate fluidity of the refining slag under a certain alkalinity, make the white slag retention time ≧14.46min, the refining cycle ≧39.39min, the soft blow time ≧4.51min, and control the tapping [O] content.

[0056] RH Vacuum Treatment:

[0057] The vacuum chamber pressure is below 66.69kPa and maintained for 12.35~14.47min. The argon bottom blow flow rate is 10.26~19.38m3 / h. The molten steel is circulated four times. The type and weight of the added alloys are strictly controlled. Higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys are used to ensure that the composition of the molten steel is fully qualified. The vacuum is maintained for more than 5.28min to obtain cleaner molten steel. At the same time, the molten steel temperature is suitable for continuous casting, so that the superheat of the tundish is 10.17℃~29.46℃ higher than the liquidus.

[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, and the final rolling temperature is controlled to 750-850°C. After rolling, the slag is water-cooled to 410-550°C, and then naturally cooled to room temperature to obtain a fine alloy high-strength steel.

[0060] The rolling method usually involves heating and soaking the slag, then continuously rolling it to produce the product steel plate. The final rolling temperature is controlled at 800°C, and after rolling, the plate is water-cooled to 480°C and naturally cooled to room temperature.

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

[0062] Example 2

[0063] This embodiment relates to a Ni-free fine alloyed high strength steel having ultra-low temperature toughness, and has a chemical composition, in mass%, of 0.09% C, 0.13% Si, 1.48% Mn, 0.035% Nb, 0.012% Ti, 0.014% Zr, 0.004% RE, 0.0048% P, 0.0010% S, 0.0048% O, 0.0038% N, 0.00019% H, and the balance of Fe and unavoidable impurities. The RE includes lanthanum and cerium, and the weight ratio of the lanthanum element and the cerium element is 80:20.

[0064] This embodiment relates to a method for preparing a Ni-free microalloyed high strength steel with ultra-low temperature toughness, which includes the following steps:

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

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

[0067] LF Refining:

[0068] The viscosity of the refining slag is controlled to 1.531~1.964 Pa·s, the slag inclusion adsorption capacity is improved, the cleanliness of the molten steel is improved, the alkalinity of the white slag in the refining furnace is controlled to 5.15≦R≦7.67, the desulfurization rate is increased, the cleanliness of the molten steel is improved, the oxide inclusions in the molten steel are reduced, the MI slag index (=CaO / SiO2:Al2O3 ratio) is controlled to MI>0.149, the distribution coefficient of sulfur is significantly increased, the fluidity of the refining slag is controlled appropriately under a certain alkalinity, the retention time of the white slag is ≧14.23min, the refining cycle is ≧39.36min, the soft blow time is ≧4.58min, and the tapping [O] content is controlled.

[0069] RH Vacuum Treatment:

[0070] The vacuum chamber pressure is below 66.59kPa and maintained for 12.27~14.48min. The argon bottom blow flow rate is 10.21~19.39m3 / h. The molten steel is circulated five times. The type and weight of the added alloys are strictly controlled. Higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys are used to ensure that the composition of the molten steel is fully qualified. The vacuum is maintained for more than 5.33min to obtain cleaner molten steel. At the same time, the molten steel temperature is suitable for continuous casting, so that the superheat of the tundish is 10.17℃~29.49℃ higher than the liquidus.

[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, and the final rolling temperature is controlled to 750-850°C. After rolling, the slag is water-cooled to 410-550°C, and then naturally cooled to room temperature to obtain a fine alloy high-strength steel.

[0073] The rolling method usually involves heating and soaking the slag, then continuously rolling it to produce the product steel plate. The final rolling temperature is controlled at 840°C, and after rolling, the plate is water-cooled to 530°C and naturally cooled to room temperature.

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

[0075] Example 3

[0076] This embodiment relates to a Ni-free fine alloyed high strength steel with ultra-low temperature toughness, and has a chemical composition, in mass%, of 0.03% C, 0.20% Si, 1.4% Mn, 0.035% Nb, 0.013% Ti, 0.010% Zr, 0.002% RE, 0.0020% P, 0.0007% S, 0.0020% O, 0.0030% N, 0.00016% H, and the balance of Fe and unavoidable impurities. The RE includes lanthanum and cerium, and the weight ratio of the lanthanum element and the cerium element is 90:10.

[0077] This embodiment relates to a method for preparing a Ni-free microalloyed high strength steel with ultra-low temperature toughness, which includes the following steps:

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

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

[0080] LF Refining:

[0081] Control the viscosity of the refining slag to 1.526~1.953 Pa·s, improve the inclusion adsorption capacity of the slag system, improve the cleanliness of molten steel, control the alkalinity of the white slag in the refining furnace to 5.16≦R≦7.63, increase the desulfurization rate, improve the cleanliness of molten steel, reduce the oxide inclusions in molten steel, control the MI slag index (=CaO / SiO2:Al2O3 ratio) to MI>0.153, significantly increase the distribution coefficient of sulfur, control the appropriate fluidity of the refining slag under a certain alkalinity, make the white slag retention time ≧14.35min, the refining cycle ≧39.47min, the soft blow time >4.58min, and control the tapping [O] content.

[0082] RH Vacuum Treatment:

[0083] The vacuum chamber pressure is below 66.69kPa and maintained for 12.21~14.47min. The argon bottom blow flow rate is 10.23~19.46m3 / h. The molten steel is circulated six times. The type and weight of the added alloys are strictly controlled. Higher-grade low-carbon ferromanganese, metallic manganese, low-carbon ferrosilicon, ferrotitanium and other alloys are used to ensure that the composition of the molten steel is fully qualified. The vacuum is maintained for more than 5.33min to obtain cleaner molten steel. At the same time, the molten steel temperature is suitable for continuous casting, so that the superheat of the tundish is 10.35℃~29.47℃ higher than the liquidus.

[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, and the final rolling temperature is controlled to 750-850°C. After rolling, the slag is water-cooled to 410-550°C, and then naturally cooled to room temperature to obtain a fine alloy high-strength steel.

[0086] The rolling method usually involves heating and soaking the slag, then continuously rolling it to produce the product steel plate. The final rolling temperature is controlled at 760°C, and after rolling, the plate is water-cooled to 430°C and naturally cooled to room temperature.

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

[0088] Experimental Example

[0089] Referring to the Ni-free fine alloyed high strength steel (R&D steel) having ultra-low temperature toughness prepared in Example 1, a series of low temperature impact properties were tested and analyzed, and the following results were obtained.

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

[0091] The low-temperature impact test is carried out in accordance with the Chinese national standards GB / T229-2020 "Metallic Materials-Shear Pendulum Impact Test Method" and GB / T19748-2019 "Metallic Materials-Shear V-notch Pendulum Impact Test Instrumentation Test Method". The sample specifications are 55×10×10mm standard specimens with V-notches, notch depths of 2mm, and root radius of 0.25mm. The curvature radius of the impact end of the pendulum is 2mm. The test temperatures are 20, -20, -40, -60, -80, -100, -110, -120, -130, -140, -160 and -196℃. During the pendulum impact process, the force and displacement values ​​are recorded respectively by the resistance strain gauge of the impact end and the optical method, the force-displacement curve during the impact process is obtained, and the impact absorption energy is obtained by integral calculation.

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

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

[0094] FIG. 2 is a comparison of the -100℃ oscillometric impact curves of the microalloyed high strength steel of the present invention and the same type of control steel, in which the microalloyed high strength steel of the present invention has a crack initiation-propagation-passivation process in the -100℃ impact process, while the control steel has a rapid drop after yielding in the force-displacement curve, which indicates that the control steel undergoes instantaneous brittle fracture upon crack initiation without a stable crack propagation and passivation process.

[0095] Figure 3 is an Ashby diagram of impact energy at -100°C versus yield strength at room temperature for the microalloyed high strength steel of the present invention and other steels, where the dotted line is the line connecting the coordinates (200MPa, 300J) and (1000MPa, 0J), and the steels on the right side of the dotted line are considered to have achieved an excellent trade-off between low-temperature impact toughness and room-temperature yield strength. As can be seen from Figure 3, the microalloyed high strength steel of the present invention has the highest impact energy at -100°C, which is higher than that of conventional low-carbon microalloyed 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 microalloyed steels [4,5] and high manganese steels [10-12].

[0096] Figure 4 shows the macroscopic morphology of impact fracture of the microalloyed 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 microalloyed high strength steel of the present invention undergoes completely ductile fracture, while at -120°C, the microalloyed high strength steel of the present invention changes completely to brittle fracture.

[0097] FIG. 5 shows the micromorphology of different parts of the Charpy impact fracture at −100°C of the fine-alloyed high-strength steel according to the present invention, in which both the crack initiation and the stable crack growth appear as elongated dimple morphologies in the fracture direction, and there are many tear marks at the ends of the dimples, while the crack passivation appears as a mixed morphology of oblique dimples and normal dimples.

[0098] The first requirement for low-temperature steel performance is that sufficient impact toughness is ensured at the service temperature, and from the viewpoint of fracture mechanics, sufficient brittle crack resistance is required at the service temperature. In particularly important structures, it is necessary to have crack arrest properties against the development of brittle cracks in order to avoid accidents. Also, from the viewpoint of safety, it is desirable that the yield ratio of low-temperature steel is not high. If the yield ratio is high, the margin of plastic deformability is small and the stress redistribution ability of the stress concentration part is small, making it more susceptible to brittle fracture.

[0099] The technical requirements for low temperature steels are generally sufficient strength and sufficient toughness at low temperatures, good technical properties, workability, corrosion resistance, etc. Low temperature toughness, i.e. the ability to avoid the initiation 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] At present, in order to meet the above-mentioned low-temperature toughness requirements, nickel is alloyed in the conventional technology, mainly because (1) Ni does not form carbides with carbon, but is a major alloying element that forms and stabilizes austenite, (2) Ni is an element that dissolves purely in steel, and can strengthen the ferrite matrix and significantly reduce the ductile-brittle transition temperature, (3) a fine grain structure can be obtained by controlled rolling, and (4) a stable structure can be obtained by heat treatment.

[0101] However, in the present invention, in order to realize ultra-low temperature toughness, a completely different composition design and a matching manufacturing process are adopted, and with the help of Zr+RE composite deoxidization, Nb+Ti composite fine alloying technology, that is, a simple low-cost Ni-free composition design, ultra-low temperature toughness of -100 to -120 ° C is realized. The fine-alloyed high-strength steel plate 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 that the material cost can be greatly reduced. Instead of the traditional Al deoxidation technology, the present invention uses Si-Mn deoxidation supplemented with Zr-Ti-RE compound deoxidation, which forms fine, dispersed, uniform composite oxysulfides, which can significantly improve the plasticity and toughness. With the low carbon equivalent design, the steel plate according to the present invention has excellent welding performance. By using Nb, Ti, Zr and RE compound fine alloying together with the rolling parameter control in TMCP, the steel plate has fine grains and has high strength and toughness. This Ni-free, finely alloyed, high-strength, low-temperature steel is particularly suitable for use in polar regions, containers, pipelines, refining, storage and transportation, equipment, and other structural materials used in low and ultra-low temperature (-20 to -120°C) environments. In addition to its excellent low-temperature toughness, this material also has other major features such as high strength and excellent weldability.

[0102] References [1]-

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

[15] , nickel-base low temperature steels [8,16,17], maraging steels [18,19], manganese nickel fine alloyed steels [4,5] and high manganese steels [10-12] are given 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 the description of this specification, the description of a reference term such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in combination with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, unless mutually inconsistent, a person skilled in the art can combine or combine different embodiments or examples described herein, features of different embodiments or examples.

[0105] Although embodiments of the present invention have been illustrated and described, the above embodiments are merely illustrative and should not be construed as limiting the present invention. Those skilled in the art may make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. The chemical composition is, in mass%, 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 balance being Fe and unavoidable impurities; The mass percentages of the element C and the element Si further satisfy the formulas 0.21%<C+Si<0.24% and Si / C=1 to 8 at the same time; The mass percentages of the elements Nb and Ti further satisfy the formulas 0.02%<Nb+Ti<0.05% and the sum Nb / Ti=1 to 3 at the same time; The mass percentages of the element Zr and the element RE further satisfy the formulas 0.010%<Zr+RE<0.019% and Zr / RE=1 to 6 at the same time.

2. 2. The Ni-free fine alloyed high strength steel having ultra-low temperature toughness according to claim 1, characterized in that the steel consists of, in 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%, and the balance being Fe and unavoidable impurities.

3. 2. The Ni-free fine alloyed high strength steel having ultra-low temperature toughness according to claim 1, characterized in that the steel consists of, in mass%, 0.05% C, 0.17% Si, 1.4% Mn, 0.03% Nb, 0.015% Ti, 0.008% Zr, 0.007% RE, and the balance being Fe and unavoidable impurities.

4. The Ni-free fine alloyed high strength steel with ultra-low temperature toughness according to any one of claims 1 to 3, characterized in that the mass percentages of the elements P, S, O, N and H in the unavoidable impurities satisfy P≦0.0049%, S≦0.0010%, O≦0.0049%, N≦0.0039%, and H≦0.00019%, respectively.

5. The Ni-free fine alloyed high strength steel with ultra-low temperature toughness 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 fine alloyed high strength steel with ultra-low temperature toughness according to any one of claims 1 to 3, characterized in that the microstructure of the fine alloyed high strength steel does not have a ferrite-pearlite band structure, and the effective grain size of the microstructure of the fine alloyed high strength steel is 5 μm or less.

7. The Ni-free fine alloyed high strength steel having ultra-low temperature toughness according to any one of claims 1 to 3, characterized in that the fine alloyed high strength steel has a V-notch impact absorption energy of more than 300 J under a temperature of -120 ° C.

8. The Ni-free fine-alloyed high-strength steel having ultra-low temperature toughness according to any one of claims 1 to 3, characterized in that the fine-alloyed high-strength steel has a ductile-brittle transition temperature of -110°C to -130°C.

9. A method for preparing the Ni-free fine-grained alloyed high strength steel having ultra-low temperature toughness according to any one of claims 1 to 3, comprising the steps of: 1) smelting and refining molten steel in sequence, vacuum treating the molten steel, and continuously casting the molten steel to obtain slag; 2) heating and soaking the slag to obtain a heat-treated slag; 3) continuously rolling the heat-treated slag, controlling the final rolling temperature to 750-850°C, water-cooling to 410-550°C after rolling, and then naturally cooling to room temperature to obtain a fine-alloyed high-strength steel.

10. The method for preparing a Ni-free fine-alloyed high strength steel with ultra-low temperature toughness according to claim 9, characterized in that the smelting and refining method in step 1) specifically includes: using a rotary furnace or an electric furnace to make steel from hot metal 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 making the free oxygen content in the molten steel 99-398ppm; charging the molten steel into a ladle, and pre-deoxidizing the molten steel in the ladle with an Fe-Si alloy or an Fe-Si-Mn alloy under fine argon bubbling, and adjusting the free oxygen content in the molten steel to 10-98ppm; performing final deoxidation with a composite additive under fine argon bubbling, and performing LF refining, VD refining, or RH refining on the molten steel that has been final deoxidized.

Images