CN113265586B - Sleeve for mechanically connecting low-temperature-resistant steel bars for LNG storage tank and manufacturing method - Google Patents

The application provides a sleeve for mechanically connecting low-temperature-resistant steel bars for an LNG storage tank and a manufacturing method, and relates to the field of low-temperature steel. The chemical components of the sleeve comprise the following components in percentage by mass: c: 0.09% -0.15%, Si: 0.15-0.35%, Mn: 0.60-0.80%, Cr: 14.0% -16.0%, Ni: 1.45% -1.65%, V: 0.03% -0.05%, Cu: 0.12% -0.16%, Co: 0.015% -0.03%, Als: 0.01-0.02%, P is less than or equal to 0.015%, S is less than or equal to 0.01%, N: 0.008-0.015 percent, and the balance of Fe and inevitable impurities; the microstructure of the sleeve is uniform sorbite. The sleeve manufactured by the method has the advantages of good tensile strength, elongation, corrosion resistance and other properties at low temperature, meets the design and construction requirements of the LNG storage tank, and is low in cost.

Sleeve for mechanically connecting low-temperature-resistant steel bars for LNG storage tank and manufacturing method

Technical Field

The application relates to the field of low-temperature steel, in particular to a sleeve for mechanically connecting low-temperature-resistant steel bars for an LNG storage tank and a manufacturing method of the sleeve.

Background

Natural Gas (LNG) is used as a clean, efficient and high-quality energy source, and plays an increasingly important role in optimizing energy consumption structures, controlling greenhouse gas emission, improving atmospheric environment, and the like. The demand is getting bigger and bigger, the output and the import of the Liquefied Natural Gas (LNG) in China are steadily increased, and the import of the LNG is 6959 million tons in 2020, which is the largest world for importing LNG. LNG needs special storage tank to receive and store, and in order to adapt to the increase of demand, the state is building liquefied natural gas receiving station in coastal area vigorously, and the LNG storage tank is as the core of receiving station, and its biggest characteristics are cryogenic at low temperature, and in order to guarantee low temperature stability and security in the jar, the reinforcing bar that the storage tank was built and is used need to bear the ultra-low temperature service environment, still possesses performance characteristics such as high strength, high toughness and incision sensitivity are little under-165 ℃ cryogenic condition.

Because welding can affect the low-temperature mechanical property of the steel bars, the ultra-low-temperature steel bars used for building the LNG storage tank cannot be welded and connected, the steel bars must be connected mechanically, and the steel bar connecting sleeve matched with the low-temperature steel bars has good low-temperature resistance, so that the steel bars are not brittle at the subzero temperature of-165 ℃ after being connected, and the overall low-temperature property is not lower than the performance of the steel bars.

After the two ends of the applicable low-temperature steel bar mechanical connecting sleeve are connected with steel bars, the tensile property, the high-stress repeated tension and compression property and the large-deformation repeated tension and compression property under the normal-temperature condition can meet the standard requirement of JGJ107 'steel bar mechanical connection technical regulation', and the low-temperature connection performance requirement under the service low-temperature environment test condition can be further met. The mechanical connection research on the steel bars used at the normal temperature of HRB400E or HRB500E in China is reported more, most of the steel bars are straight thread sleeve joints processed and manufactured by medium and low carbon steel, the sleeve is simple in material, small in processing and manufacturing difficulty, capable of guaranteeing the mechanical property at the normal temperature, and suitable for being applied to common building engineering. However, because the material used by the sleeve does not have good low-temperature resistance, the strength of the material is improved in a low-temperature environment, but the plasticity index sum is sharply reduced, the notch sensitivity is sharply increased, the sleeve or the joint is easily brittle-broken, and the requirement of high toughness in a cryogenic service environment at the temperature of-165 ℃ cannot be ensured; in addition, the adopted high-manganese straight thread structure has the defects of insecure connection, unsealed connection, easy generation of water seepage along threads to the interior of a joint sleeve in the construction process, corrosion, serious influence on the low-temperature mechanical property and the service life of the reinforcing steel bar, serious influence on the operation safety of the low-temperature storage tank and the like.

Patent CN111041361A discloses a preparation process of a mechanical connecting sleeve of a steel bar resisting a low temperature of-165 ℃ for an LNG storage tank, wherein the sleeve comprises the following components: 0.06-0.12% of C, 0.30-0.50% of Si, 1.30-1.80% of Mn, 1.00-2.50% of Ni, 0.060-1.000% of V, less than or equal to 0.010% of P, less than or equal to 0.010% of S, less than or equal to 0.00015% of H, less than or equal to 0.0015% of O, and the balance of Fe and impurity elements; low-temperature quenching and tempering are adopted in the quenching and tempering treatment. Although the performance of the sleeve prepared by the method meets the standard requirements of JGJ107 technical code for mechanical connection of steel bars, the method has the following defects: the tensile strength, the elongation, the corrosion resistance and other properties are not high at low temperature, and the components adopt high manganese and high vanadium, so the cost is higher.

In addition, the prior art also discloses a large amount of steel for low-temperature steel bars, ocean platforms and the like, which is not suitable for the sleeve for mechanically connecting the low-temperature resistant steel bars for the LNG storage tank on the requirements of application environment, performance indexes and the like.

Disclosure of Invention

In order to overcome the above disadvantages of the prior art, an object of the present invention is to provide a sleeve for mechanically connecting low temperature resistant steel bars for an LNG storage tank and a manufacturing method thereof, wherein the sleeve has good tensile strength, elongation, corrosion resistance and other properties at low temperature, and after the sleeve is connected with the low temperature steel bars, the properties of the sleeve meet the design and construction requirements of the LNG storage tank in a unidirectional tensile test under the cryogenic low temperature test condition of-165 ℃.

In a first aspect, the application provides a LNG storage tank is with low temperature resistant steel bar sleeve for mechanical connection, its characterized in that, its chemical composition includes by mass percent: c: 0.09% -0.15%, Si: 0.15-0.35%, Mn: 0.6-0.8%, Cr: 14.0% -16.0%, Ni: 1.45% -1.65%, V: 0.03% -0.05%, Cu: 0.12% -0.16%, Co: 0.015% -0.03%, Als: 0.01-0.02%, P is less than or equal to 0.015%, S is less than or equal to 0.01%, N: 0.008-0.015 percent, and the balance of Fe and inevitable impurities;

the microstructure of the sleeve is uniform sorbite.

The application improves the low-temperature resistance of the sleeve by designing chemical components, and the action mechanism of each main chemical element and the content is explained as follows:

the C element and the C play a role in solid solution strengthening in the steel, and form alloy carbide with alloy elements such as chromium, vanadium and the like in the steel, so that the strength performance of the product is effectively improved. The C element improves the toughness and the brittle transition temperature of the steel and deteriorates the low-temperature performance of the steel while improving the strength of the steel, and the lower the carbon content is, the lower the toughness and the brittle transition temperature of the steel is, the better the toughness of the material is. In addition, the high carbon content also affects the rust prevention performance of the sleeve. The lower the carbon content, the better the antirust performance, and in combination with the requirements of the strength, low-temperature toughness and antirust performance of the sleeve, the invention C: 0.09% -0.15%; specifically, the C content is 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, or the like.

Mn element, effective strengthening element in the steel, manganese content is higher, the more favorable to improving the tensile strength of steel, the prior art mostly adopts the steel of high Mn content to prepare the sleeve, but the inventor finds that too high manganese easily causes the structure thick, and the banding is obvious, reduces the low temperature toughness of steel. In summary, the Mn: 0.60 to 0.80%, specifically 0.60%, 0.63%, 0.65%, 0.67%, 0.70%, 0.73%, 0.75%, 0.77%, or 0.80%, and preferably 0.64% to 0.76%.

Cr element, an effective strengthening element in steel, can be combined with carbon to form alloy carbide, so that the tensile strength of the steel is improved; the nickel-based alloy is added in a matching way with nickel element, so that the strength performance of the steel can be synergistically improved. In addition, when the chromium content in the steel is increased to be more than or equal to 12 percent, the rust resistance of the steel is greatly improved; but the Cr content cannot be too high, otherwise, excessive large-particle-size chromium carbide type eutectic carbides are formed in the steel, cannot be completely dissolved into high-temperature austenite in the heating process of subsequent heat treatment, and remain after quenching and tempering, a single tempered sorbite structure cannot be formed, so that the strength and low-temperature toughness of the steel are reduced, and the antirust performance of the material is greatly reduced. In summary, the present invention provides a Cr: 14.0% -16.0%; specifically, the Cr content is 14.3%, 14.5%, 14.7%, 15.0%, 15.3%, 15.5%, 15.7%, 16.0%, or the like. The content design can improve the strength and the corrosion resistance of the sleeve and prolong the service life.

The Ni element has the functions of improving the strength of the steel, reducing the low-temperature ductile-brittle transition temperature of the steel and maintaining the good plasticity and toughness of the steel, and can obviously improve the strength and corrosion resistance of the steel and prolong the service life of the steel by being matched with chromium. The nickel element has the most obvious influence on the low-temperature performance of steel, the sleeve is required to meet the low-temperature toughness of-165 ℃, but the nickel is a precious alloy, and the comprehensive consideration on the Ni content is as follows: 1.45% -1.65%; specifically, the Ni content is 1.45%, 1.47%, 1.50%, 1.53%, 1.55%, 1.57%, 1.60%, 1.63%, 1.65%, or the like.

In one possible implementation, the chemical composition of the sleeve is such that Cr: 14.5-15.5%, Ni: 1.5-1.65%, wherein the range of Cr + Ni is 16.0-17.0%. According to the synergistic effect of nickel and chromium, the content of the nickel and chromium is limited within the range, so that the low-temperature resistance and the corrosion resistance of the steel can be obviously considered, and the service life of the sleeve is prolonged; if the sum of the Cr + Ni contents is higher than 17.0 percent, the tempering brittleness of the steel is increased, and the low-temperature toughness of the steel is influenced; if less than 16.0%, the strength properties of the steel will be insufficient, and the low-temperature toughness and rust-preventing properties of the steel will also be affected; the sum of the contents is preferably 16.5 to 17.0%.

The V element can obviously refine the grain structure of the steel and improve the strength, toughness and wear resistance of the steel; the content is 0.03-0.05%; specifically, the V content is 0.03%, 0.032%, 0.035%, 0.037%, 0.04%, 0.043%, 0.045%, 0.048%, 0.05%, or the like; in the invention, V is a micro-strengthening alloy element, vanadium is a strong carbon bonding element, and forms a compound with carbon (nitrogen), fine and dispersedly distributed vanadium-carbon (nitrogen) compounds are precipitated at low temperature, so that the strength of the material is improved, in addition, the vanadium can obviously improve the tempering stability of steel and has a strong secondary hardening effect, and the addition of vanadium can properly improve the tempering temperature and improve the strength and toughness of the sleeve. The steel of the invention is added with a certain content of nitrogen element, and is combined with vanadium to separate out a large amount of vanadium-nitrogen compounds with nano-scale size, refine crystal grains, generate precipitation hardening effect, further improve the toughness and low-temperature toughness of the steel, and reduce the notch sensitivity of the steel. V is a precious metal element, and high vanadium can significantly increase the cost; preferably, the V content is 0.035% to 0.045%.

The N element belongs to beneficial elements, and the content is 0.008-0.015 percent; specifically, the N content is 0.008%, 0.009%, 0.01%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, or the like; in the invention, nitrogen is combined with vanadium, and in the low-temperature phase change process, a certain content of nitrogen in steel can promote to separate out a large amount of nano-scale fine and uniform vanadium-nitrogen compounds, refine crystal grains, generate a precipitation hardening effect, and improve the separation strengthening and fine grain strengthening effects of vanadium to the maximum extent, wherein the two effects have the advantages of improving the strength, not sacrificing the plastic property of materials, further improving the toughness and low-temperature toughness of the steel, and reducing the low-temperature notch sensitivity of the steel; however, too high N content may cause free nitrogen in the steel, deteriorating the low temperature toughness of the steel; in combination with the content of V added to steel, the N content is preferably 0.009% to 0.013%.

In one possible implementation, the ratio of V/N in the sleeve chemistry is in the range of 3-4.5. According to the synergistic effect of V and N, the strength and toughness and low-temperature toughness of the steel can be obviously improved by limiting the proportion in the range; if the ratio of V/N is less than 3, a certain content of free nitrogen exists in the steel, and the low-temperature toughness of the steel is deteriorated; if the content of vanadium and nitrogen in the steel is higher than 4.5, the vanadium and nitrogen in the steel are not completely combined, and a large amount of vanadium and nitrogen compounds cannot be effectively separated out in the subsequent tempering process, so that the strength performance and the low-temperature toughness of the steel are influenced; specifically, V/N is 3.2, 3.4, 3.5, 3.8, 4, 4.2, 4.4, etc.; the ratio of V/N is preferably 3.8 to 4.2%.

Cu element, which is a beneficial alloy element in the invention, and the content is 0.12-0.16%; specifically, 0.12%, 0.13%, 0.14%, 0.15%, or 0.16%, etc. The steel is added with copper with proper content and nickel for combined action, so that the austenite stability can be improved, the strength and the toughness of the steel can be improved, and the notch sensitivity of the steel can be reduced; but the copper brittleness phenomenon can be caused by the over-high Cu content, and the low-temperature toughness of the steel is reduced; preferably, the Cu content is 0.13% to 0.015%.

Co element, which belongs to beneficial elements in the invention, the content is 0.015% -0.03%; specifically, 0.015%, 0.017%, 0.019%, 0.02%, 0.022%, 0.025%, 0.028%, 0.03%, or the like; in the invention, a small amount of cobalt element is added, and the function is embodied in the following two aspects: on the one hand, the cobalt can refine grains and can effectively reduce the overheating tendency of the steel. The quenching temperature of the invention is higher (the purpose is to make the carbon-chromium compound completely dissolved in the high-temperature austenite), and a certain amount of cobalt element is added, thus avoiding the growth of crystal grains caused by high-temperature quenching and reducing the overheating tendency. On the other hand, the cobalt element can promote the formation of tempered carbide, the invention uses the high-temperature tempering process, and a small amount of cobalt element can effectively promote the high-temperature tempered carbide (M)_xC_y) The strength and toughness of the material are improved; however, too high a Co content leads to the precipitation of hard and brittle metallic compounds, and cobalt is a precious alloying element, too highThe cobalt of (a) can significantly increase the cost of the alloy; preferably, the content of Co is 0.019% -0.026%, more preferably, the content of Co is 0.02% -0.025%.

After the sleeve is subjected to quenching and tempering, the microstructure of the sleeve is as follows: the single uniform sorbite has high strength, high toughness and low notch sensitivity under the ultralow temperature environment through the control of the microstructure, and the single uniform tempered sorbite has the best antirust performance under the condition of ensuring the strength performance.

In one possible implementation, the sleeve structure of the present application is: the inside cavity of sleeve, the internal thread symmetry in both ends, the internal thread is circular cone thread structure.

In above-mentioned technical scheme, this application telescopic internal thread has adopted conical thread, compares with straight thread, has good sealing performance. The sleeve has the characteristic of being screwed more and more tightly in the process of mechanically connecting the sleeve and the reinforcing steel bar through the conical threads, and has good sealing performance after being screwed; moreover, the conical threads can ensure that water vapor cannot permeate into the threaded sleeve in the construction process, the corrosion resistance of the steel bar and the sleeve is protected, the service life of the steel bar and the sleeve is prolonged, and the safety performance of the LNG low-temperature storage tank is ensured.

In one possible implementation mode, the outer diameter, the inner diameter, the length and the thread depth of the sleeve are matched with the specification of the matched low-temperature steel bar; illustratively, the outer diameter of the sleeve is 30mm to 40mm, and the inner diameter: 12 mm-25 mm; the length is 80mm-90 mm. Taper thread size, taper angle: 12.5-13.5 degrees; the thread pitch is 2 mm; the tooth form angle is 59-61 degrees.

In the technical scheme, the structural design of the steel bar sleeve can ensure that the sleeve is suitable for the mechanical connection performance of the low-temperature steel bar used under the cryogenic condition with the strength level of 500MPa not lower than-165 ℃.

In one possible implementation, the hardness of the sleeve is 230-250 HBW, preferably 235-245 HBW.

In above-mentioned technical scheme, the hardness control of this application can guarantee that the barrel casing has certain wearability, guarantees the surface quality of a protection barrel in the work progress, guarantees low temperature performance.

In a possible implementation mode, after the two ends of the sleeve are mechanically connected with the reinforcing steel bar, the mechanical property of the unidirectional stretching at the normal temperature is as follows:

In one possible implementation, after the two ends of the sleeve are mechanically connected with the reinforcing steel bar, the mechanical properties of the unidirectional stretching at the low temperature of-165 ℃ are as follows:

In a second aspect, the present application provides a method for manufacturing the above sleeve, comprising the steps of:

(1) selecting a spheroidizing annealed steel bar with the components meeting the requirements, wherein the hardness of the steel bar is less than or equal to 185 HBW;

(2) according to the requirements of the overall dimensions of sleeves of different specifications, steel bars of different diameters are selected for rough machining, and sleeve blanks are manufactured;

(3) carrying out quenching and tempering heat treatment on the sleeve blank, wherein the quenching and tempering special treatment comprises quenching and tempering, wherein the quenching temperature is as follows: 1050-1070 ℃, heat preservation time: the method is characterized by comprising the following steps of (1) determining according to the thickness of a sleeve blank and the charging amount during heating, and calculating according to a formula t ═ d × δ (min), wherein t in the formula is quenching heat preservation time (unit min), and d is the thickness of the cylinder wall of the sleeve blank (unit mm); delta is a coefficient, and is determined according to the charging amount, and the value is 4-6; quenching medium: cooling with oil; tempering temperature: 650-690 ℃, tempering time: air cooling for 120-150 min;

(4) and (4) performing finish machining on the sleeve blank subjected to quenching and tempering heat treatment according to the final size requirement of the sleeve.

In the technical scheme, the steel bar with the hardness of less than or equal to 185HBW in the spheroidizing annealing state is selected in the step (1), and on one hand, rough machining in the step (2) of the sleeve is facilitated, such as: turning, namely turning the end face, the outer diameter of the sleeve and the inner hole of the sleeve; on the other hand, the spheroidized structure provides good uniform structure preparation for subsequent quenching treatment, and a uniform sorbite structure is obtained after quenching and tempering treatment.

In the above technical solution, the rough machining in step (2) exemplarily includes the following steps: and (3) processing and blanking the steel bar with the corresponding length, roughly turning the steel bar to process an inner hole and two end planes, reserving the subsequent finish machining size allowance, and processing the steel bar into a sleeve blank with the corresponding size.

In the technical scheme, the quenching temperature in the step (3) is 1050-1070 ℃, the quenching heating temperature is selected to be more than or equal to 1050 ℃, complex chromium-containing carbide in the steel is completely dissolved in high-temperature austenite, if the temperature is lower than 1050 ℃, the chromium-containing carbide is not easily and completely dissolved in the high-temperature austenite, the chromium-containing carbide remains in a martensite matrix in the subsequent quenching process, and single and uniform sorbite cannot be obtained after tempering, so that the strength performance and the low-temperature toughness of the material can be reduced, and the corrosion resistance of the steel can be strongly reduced. However, if the quenching heating temperature is too high, which is higher than 1070 ℃, two major disadvantages are caused: on one hand, under the high-temperature heating condition, austenite grains grow to be large, coarse martensite is obtained, and the low-temperature toughness of the steel is reduced; on the other hand, under the high-temperature quenching condition, more delta ferrite exists in the steel, and the low-temperature toughness and the corrosion resistance of the steel are strongly reduced. Therefore, the quenching heating temperature of 1050-1070 ℃ is selected; specifically, the quenching temperature is 1050 ℃, 1055 ℃, 1060 ℃, 1065 ℃, 1070 ℃ or the like.

In one possible implementation, the quench heating temperature range is 1058-.

In the technical scheme, the tempering temperature in the step (3) is 650-690 ℃, and by adopting the high-temperature tempering condition, on one hand, martensite formed by quenching is fully converted into a uniform tempered sorbite structure, the strength and the toughness of the steel are optimally matched, and chromium-containing carbide is fully converted into Cr_xC_yUnder the condition of high-temperature tempering, the equal carbide is beneficial to the diffusion of alloy elements such as chromium and the like, so that the components and the structure are more uniform, the tempering brittleness is eliminated, the good comprehensive performance of mutual matching of strength and toughness is obtained, and the low-temperature toughness and the corrosion resistance of the steel are improved; on the other hand, the hardness of the sleeve can be controlled to 230 to 250HBW as required by the tempering treatment; if the tempering temperature is lower than 650 ℃, the chromium alloy element is not favorably diffused, the alloy carbide is incompletely converted, and correspondingly, the low-temperature brittleness is increased, so that the thread brittle fracture can be caused during the low-temperature stretching; if the tempering temperature is higher than 690 ℃, the hardness value of the sleeve is low, on one hand, the strength performance of the sleeve thread is influenced to be smaller than the design requirement, and the thread part may be subjected to the thread stripping phenomenon in the tensile test process, so that the normal-temperature and low-temperature tensile properties cannot meet the design requirement; therefore, the tempering temperature of 650-690 ℃ is selected; specifically, the tempering temperature is 650 ℃, 660 ℃, 670 ℃, 680 ℃ or 690 ℃, etc.

In one possible implementation, the tempering heating temperature range is 650-.

In the technical scheme, in the step (4), the finish machining is performed, for example, the outer diameter, the conical thread and the end part of the sleeve blank after the heat treatment are finished, so that the finish degree of the outer diameter and the finish degree of the end part are less than or equal to 0.8 μm; preferably, the smoothness is less than or equal to 0.3 μm.

The application has the following beneficial technical effects:

in composition, the chemical composition and the corresponding content of the sleeve are controlled, and particularly the Mn: 0.6-0.8%, Cr: 14.0% -16.0%, Ni: 1.45-1.65%, Ni + Cr: 16.0-17.0% and the content and proportion of corresponding components such as Cu, Co, N and the like; structurally, the internal thread of the sleeve adopts a conical thread; in the process, a modulation heat treatment process of high-temperature quenching and high-temperature tempering is adopted to obtain a microstructure of a uniform sorbite; through the control of the components, the structure and the process, the sleeve has good antirust and anticorrosive performance, and a finished sleeve can be kept free of rust for a long time when placed in a natural environment; the sleeve has good tensile strength, elongation, corrosion resistance and other properties at low temperature, after the sleeve is connected with the low-temperature steel bar, the properties meet the design and construction requirements of the LNG storage tank in a unidirectional tensile test carried out under the subzero cryogenic test condition at-165 ℃, the operation safety of the LNG storage tank is ensured, namely the sleeve can be applied to the mechanical connection performance requirements of the low-temperature steel bar used under the subzero cryogenic test condition at not lower than-165 ℃, and the cost is low. In addition, the technology can also be used for producing other low-temperature connecting pieces used at the temperature of not lower than-165 ℃ for reference.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are required to be used in the embodiments of the present application will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and that those skilled in the art can also obtain other related drawings based on the drawings without inventive efforts.

FIG. 1 is a gold phase diagram of the microstructure of the sleeve of example 1;

FIG. 2 is a schematic structural view of the sleeve;

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The sleeve having low temperature resistance and the manufacturing method of the present embodiment will be described in detail below.

The application provides a LNG storage tank is with low temperature resistant reinforcing bar sleeve for mechanical connection, its characterized in that, its chemical composition includes according to mass percent: c: 0.09% -0.15%, Si: 0.15-0.35%, Mn: 0.6-0.8%, Cr: 14.0% -16.0%, Ni: 1.45% -1.65%, V: 0.03% -0.05%, Cu: 0.12% -0.16%, Co: 0.015% -0.03%, Als: 0.01-0.02%, P is less than or equal to 0.015%, S is less than or equal to 0.01%, N: 0.008-0.015 percent, and the balance of Fe and inevitable impurities; the microstructure of the sleeve is uniform sorbite.

FIG. 1 shows a gold phase diagram of the microstructure of the sleeve of example 1 below, which is a single homogeneous sorbite.

The application also provides a manufacturing method of the sleeve for mechanically connecting the low-temperature-resistant steel bars for the LNG storage tank, which comprises the following steps:

(1) selecting a spheroidizing annealed steel bar with the components meeting the requirements, wherein the hardness of the steel bar is less than or equal to 185 HBW;

(2) according to the requirements of the overall dimensions of sleeves of different specifications, steel bars of different diameters are selected for rough machining, and sleeve blanks are manufactured;

(3) carrying out quenching and tempering heat treatment on the sleeve blank, wherein the quenching and tempering special treatment comprises quenching and tempering, wherein the quenching heating temperature is as follows: 1050-1070 ℃, heat preservation time: the method is characterized by comprising the following steps of (1) determining according to the thickness of a sleeve blank and the charging amount during heating, and calculating according to a formula t ═ d × δ (min), wherein t in the formula is quenching heat preservation time (unit min), and d is the thickness of the cylinder wall of the sleeve blank (unit mm); delta is a coefficient, and is determined according to the charging amount, and the value is 4-6; quenching medium: cooling with oil; tempering temperature: 650-690 ℃, tempering time: air cooling for 120-150 min;

(4) and (4) performing finish machining on the sleeve blank subjected to quenching and tempering heat treatment according to the final size requirement of the sleeve.

As shown in fig. 2, the sleeve structure after finishing is: the inside cavity of sleeve, the internal thread symmetry in both ends, the internal thread is circular cone thread structure.

The sleeve produced according to the above manufacturing method has the following performance characteristics:

microstructure: homogeneous sorbite.

Hardness: 230 to 250 HBW.

Mechanical properties at normal temperature: after the two ends of the sleeve are mechanically connected with the reinforcing steel bar, the mechanical property of the unidirectional stretching at normal temperature is as follows:

Mechanical properties at low temperature: after the two ends of the sleeve are mechanically connected with the reinforcing steel bar, the mechanical property of the unidirectional stretching at the low temperature of-165 ℃ is as follows:

Corrosion resistance: the finished sleeve is placed in a natural environment and can be kept free of corrosion for a long time.

The features and properties of the present application are described in further detail below with reference to examples.

Examples 1 to 9

Each embodiment provides a sleeve which is one of three specifications of 12mm, 16mm and 25mm suitable for low-temperature steel bars, and is manufactured by the manufacturing method of the sleeve in the embodiment, but the chemical composition of the copper sleeve and/or the technological parameters of the quenching and tempering heat treatment are different among the embodiments.

Comparative examples 1 to 7

Each comparative example provides a sleeve of one of three specifications, namely a 12mm specification sleeve, a 16mm specification sleeve and a 25mm specification sleeve, and each example is manufactured by the manufacturing method of the sleeve of the embodiment, but the chemical composition, structure or technological parameters of the quenching and tempering heat treatment of the sleeve are different among the various embodiments.

The chemical composition and process parameters of the sleeves of different specifications of the different embodiments and the comparative examples are shown in the following tables 1 to 5.

TABLE 1 chemical composition of sleeves (wt%) of different examples and comparative examples

The related quenching and tempering heat treatment process parameters of the sleeves of the examples and the comparative examples and the microstructure and hardness of the obtained products are shown in Table 2.

TABLE 2 comparison of hardening and tempering heat treatment process parameters and structure and hardness of examples and comparative examples

From the above results, it can be seen that comparative examples 4 to 7 adopt the sleeve prepared by the same composition and specification as those of example 2, wherein in comparative example 4, the quenching temperature is lower than 1050 ℃, and the organization structure of the sleeve is tempered sorbite + a small amount of carbide, which affects the anti-rust performance of the sleeve; in the comparative example 5, the quenching temperature is too high, and the organization structure of the sleeve is tempered sorbite and a small amount of ferrite, so that the antirust performance of the sleeve is influenced, and the low-temperature toughness of the sleeve is reduced; in the comparative example 6, the tempering temperature is too low, the tissue structure is tempered sorbite, the hardness of the sleeve is higher, and the low-temperature toughness of the sleeve is influenced; in comparative example 7, the tempering temperature is too high, the texture tempered sorbite is low, and the hardness and strength properties are low.

The parameters related to the specific structure of the sleeves of the examples and comparative examples are shown in Table 3.

TABLE 3 structural parameters of the examples and comparative examples

After the sleeves of the embodiment and the comparative example are mechanically connected with the LNG low-temperature steel bars with the strength level of 500MPa through threads, normal-temperature mechanical property tests are carried out.

The method comprises the steps of taking a 500MPa strength-level steel bar with mechanical properties meeting the LNG storage tank construction design requirements, processing one end of the steel bar into a thread matched with the sleeve thread, connecting a steel bar to each end of each sleeve, and then integrally performing a normal-temperature tensile test, wherein the related mechanical property test results of the embodiment and the comparative example are shown in the following table 4.

TABLE 4 mechanical properties at room temperature of examples and comparative examples

As can be seen from the table above, after the sleeve is connected with a common conventional sleeve and a low-temperature steel bar, the mechanical property test at normal temperature is far higher than the mechanical property requirement at normal temperature.

After the sleeves of the embodiment and the comparative example are mechanically connected with the LNG low-temperature steel bar with the strength level of 500MPa through threads, a mechanical property test at a low temperature of-165 ℃ is carried out.

The method comprises the steps of taking a 500MPa strength-level steel bar with mechanical properties meeting the LNG storage tank construction design requirements, processing one end of the steel bar into a thread matched with the sleeve thread, connecting a steel bar to each end of each sleeve, and performing a-165 ℃ low-temperature tensile test on the whole, wherein the related mechanical property test results of the embodiment and the comparative example are shown in the following table 5.

TABLE 5 Low-temp. mechanical Properties at-165 ℃ of examples and comparative examples

As can be seen from the table above, after the sleeve is connected with the low-temperature steel bar, the mechanical property of the sleeve at the low temperature of-165 ℃ meets the design requirement of the LNG storage tank; comparative examples 1-3 after the common sleeve low-temperature steel bar is connected, the mechanical property of the common sleeve at the low temperature of-165 ℃ does not meet the design requirement of the LNG storage tank, which indicates that the low-temperature property of the common sleeve can not meet the construction requirement of the LNG storage tank; comparative examples 5 to 7 although the sleeve has the same composition and structure as example 2, the parameters of the quenching and tempering heat treatment do not fall within the range defined in the present application, and the low temperature properties have a problem of breaking at the joint in the low temperature tensile test, resulting in a low total elongation at maximum force.

The above description is only an example of the present application and is not intended to limit the scope of the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.