Production method of fire-resistant steel bar mechanical connection sleeve

Abstract

The invention discloses a production method of a fire-resistant steel bar mechanical connection sleeve. The sleeve is prepared by drawing and straightening a wire rod, extruding, threading, gradient quenching and low-temperature tempering. The wire rod contains C 0.35-0.42%, Si 0.12-0.25%, Mn 1.6-2.0%, Cr 0.25-0.35%, Nb 0.10-0.15%, V 0.04-0.08%, Ti 0.02-0.05%, Cu 0.15-0.25%, B 0.001-0.003%, P≤0.02%, S≤0.02%, N 0.01~0.02%, the rest Fe and impurities; FRE=[Nb]+1.5[Cr]+0.8[V]+0.5[Ti]+0.1[Cu], FRE is 0.60~0.75%, and the obtained sleeve has excellent mechanical properties and fire resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production method of a fire-resistant steel bar mechanical connection sleeve. Background Art

[0002] With the development of society and advancements in technology, reinforced concrete structures are gradually becoming larger and taller. For safety reasons, the requirements for their fire resistance are also increasing. Reinforced concrete structures typically use a large amount of hot-rolled steel bars. Currently, GB / T 37622-2019, "Hot-rolled fire-resistant steel bars for reinforced concrete," specifies the fire resistance performance of hot-rolled steel bars in reinforced concrete structures.

[0003] However, simply using hot-rolled steel bars with fire-resistant properties is far from enough. In actual construction, hot-rolled steel bars are often mechanically connected using steel bar mechanical connection sleeves. When a fire occurs, once the sleeve fails, it will seriously affect the stability and safety of reinforced concrete structures. Therefore, it is necessary to prepare steel bar mechanical connection sleeves with fire-resistant properties and excellent strength and toughness. Summary of the Invention

[0004] The purpose of the present invention is to provide a production method of a fire-resistant steel bar mechanical connection sleeve.

[0005] To achieve one of the above-mentioned purposes, an embodiment of the present invention provides a method for producing a fire-resistant steel bar mechanical connection sleeve, wherein the wire rod is subjected to drawing and straightening, extrusion molding, thread tapping, gradient quenching and low-temperature tempering processes to prepare the sleeve; wherein,

[0006] The chemical composition of the wire rod includes, by mass percentage, C 0.35-0.42%, Si 0.12-0.25%, Mn 1.6-2.0%, Cr 0.25-0.35%, Nb 0.10-0.15%, V 0.04-0.08%, Ti 0.02-0.05%, Cu 0.15-0.25%, B 0.001-0.003%, P≤0.02%, S≤0.02%, N 0.01-0.02%, and the remainder is Fe and unavoidable impurities; the fire resistance index FRE=[Nb]+1.5[Cr]+0.8[V]+0.5[Ti]+0.1[Cu], and the FRE is 0.60-0.75%;

[0007] In the drawing and straightening process, the wire rod is drawn to a set size and then sent to a straightening machine for straightening, and then cut and finished according to the set sleeve length to obtain a sleeve blank;

[0008] In the extrusion molding process, the sleeve blank after drawing and straightening is perforated, and then extrusion molding is performed according to the sleeve diameter using equal 6-angle, equal 8-angle or equal 12-angle;

[0009] In the thread tapping process, an automatic tapping machine is used to tap the sleeve blank after extrusion, and the thread angle is 75°.

[0010] As a further improvement of one embodiment of the present invention, in the gradient quenching process, the sleeve after threading is induction heated at a heating temperature of 900-950°C and a heating time of 3-5 minutes; then enters a first salt bath furnace for isothermal quenching at a quenching temperature of 600-680°C and a quenching time of 10-25 minutes; then enters a second salt bath furnace for isothermal quenching at a quenching temperature of 400-460°C and a quenching time of 8-15 minutes; then oil quenching is performed, the quenching oil temperature is 30-50°C, and the cooling rate is 0.1-0.5°C / s.

[0011] As a further improvement of one embodiment of the present invention, in the low-temperature tempering process, the sleeve after gradient quenching is sent to a muffle furnace for tempering, the tempering temperature is 200-250°C, the tempering time is 20-40 minutes, and after tempering, it is left in the muffle furnace to cool to room temperature, with a cooling rate of ≤1°C / s.

[0012] As a further improvement of one embodiment of the present invention, the wire rod is prepared by sequentially performing the steps of molten iron pre-desulfurization, electric furnace smelting, LF refining, continuous casting, heating, controlled rolling, and controlled cooling.

[0013] As a further improvement of one embodiment of the present invention, in the molten iron pre-desulfurization process, the blast furnace molten iron is subjected to KR method pre-desulfurization, the S in the molten iron at the end of desulfurization is ≤0.01%, and the slag skimming rate of the desulfurization slag is ≥98%;

[0014] In the electric furnace smelting process, scrap steel and pre-desulfurized blast furnace molten iron are sequentially added into the electric furnace, wherein the scrap steel accounts for ≥75%, the C in the molten steel at the end of the electric furnace smelting is ≤0.15%, the P is ≤0.015%, the tapping temperature is 1595-1625°C, argon is blown from the bottom of the ladle throughout the tapping process, and the argon flow rate of the ladle bottom blowing is 200-250L / min; after 1 / 5 of the steel is tapped, ferrosilicon nitride, silicomanganese, ferromanganese, high carbon ferrochrome, copper blocks and lime are sequentially added for deoxidation and alloying.

[0015] As a further improvement of one embodiment of the present invention, in the LF refining process, after the molten steel produced by the electric furnace smelting process is injected into the LF furnace, 2.4-3.8 kg of lime and 0.8-1.2 kg of fluorite are added per ton of molten steel to adjust the slag to yellow, and then the power is turned on to increase the temperature and soft stirring is performed. The soft stirring time is 5-8 minutes, and the argon flow rate of the bottom of the ladle during the soft stirring is 150-200 L / min; after the basicity of the refined slag is adjusted to 2.0-2.5, borax, vanadium-nitrogen alloy, ferroniobium and ferrotitanium are added in sequence for alloying. The argon flow rate of the bottom of the ladle during the alloying period is 250-300 L / min, and the steel tapping temperature at the end point of LF refining is 1550-1580°C.

[0016] As a further improvement of one embodiment of the present invention, in the continuous casting process, the tundish temperature is 1525-1550°C, and the continuous casting process adopts a large ladle long water nozzle and a sealing gasket, an immersed water nozzle, and an alkaline tundish covering agent for full protection casting. Argon is blown through the long water nozzle throughout the process, and the protective slag adopts low-carbon protective slag, and the slag layer thickness is 8-10mm; the water distribution flow rate of the crystallizer in the first solidification cooling zone is 1800-2400L / min, and the temperature difference between the water outlet and the water inlet of the crystallizer is <10°C; electromagnetic stirring is adopted in the second solidification cooling zone, and the electromagnetic stirring frequency is 3-5Hz, and the liquid level fluctuation is controlled within ±2mm. The water distribution flow rate of the crystallizer in the second solidification cooling zone is 500-800L / min, and the continuous casting speed is 2.2-2.5m / min.

[0017] As a further improvement of one embodiment of the present invention, in the heating process, the heating temperature is 1150° C. to 1220° C., the total heating time is 60 to 90 minutes, and the soaking period time is ≥40 minutes.

[0018] As a further improvement of one embodiment of the present invention, in the controlled rolling process, the continuous casting billet after the heating process is rolled into wire rods and coiled into coils, the starting rolling temperature is ≥1080°C, the finishing rolling inlet temperature is 980-1020°C, the final rolling speed is 12-15m / s, the coiling temperature is ≥950°C, and all fans are turned off during the coiling process.

[0019] As a further improvement of one embodiment of the present invention, in the controlled cooling process, the obtained coil is sent to an insulation pit for pile cooling, the insulation pit is covered with an insulation cover, the cooling rate is ≤0.7℃ / s, and after cooling to below 300℃, it is discharged from the pit for air cooling.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) In the chemical composition design, under the comprehensive consideration of the effects of different elements on the high temperature strength and creep strength of steel, not only the addition and content of each element are accurately selected and controlled, but also the content relationship of each element is coordinated by controlling the fire resistance index FRE. On the one hand, it is beneficial to the organizational control of the steel bar mechanical connection sleeve, improving the strength, toughness and fire resistance of the sleeve, so as to comprehensively improve the safety of the steel bar mechanical connection sleeve in reinforced concrete structure buildings, making it suitable for key fire protection projects. On the other hand, it can also reduce production costs and simplify the production process.

[0022] (2) Based on the design of chemical composition and combined with the process design of the production process of the steel bar mechanical connection sleeve, it can be ensured that the sleeve prepared by this production method has excellent mechanical properties and excellent fire resistance, which can comprehensively improve the safety of the use of steel bar mechanical connection sleeve in reinforced concrete structure buildings and make it suitable for key fire protection projects. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is further described below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0024] One embodiment of the present invention provides a wire rod for a fire-resistant steel bar mechanical connection sleeve, the chemical composition of which includes, by mass percentage, C 0.35-0.42%, Si 0.12-0.25%, Mn 1.6-2.0%, Cr 0.25-0.35%, Nb 0.10-0.15%, V 0.04-0.08%, Ti 0.02-0.05%, Cu 0.15-0.25%, B 0.001-0.003%, P≤0.02%, S≤0.02%, N 0.01-0.02%, and the remainder is Fe and unavoidable impurities; wherein, the fire resistance index FRE=[Nb]+1.5[Cr]+0.8[V]+0.5[Ti]+0.1[Cu], and the FRE is 0.60-0.75%.

[0025] Here, [Nb] represents the mass percentage of Nb, [Cr] represents the mass percentage of Cr, [V] represents the mass percentage of V, [Ti] represents the mass percentage of Ti, and [Cu] represents the mass percentage of Cu.

[0026] One embodiment of the present invention further provides a fire-resistant steel bar mechanical connection sleeve, which is prepared using the above-mentioned wire rod for fire-resistant steel bar mechanical connection sleeve as a base material and has the same chemical composition as the wire rod for fire-resistant steel bar mechanical connection sleeve.

[0027] The role of each chemical component is explained below:

[0028] C: As the most economical strengthening element in steel, it plays a role in solid solution strengthening, which is beneficial to improving the connection strength of the steel bar mechanical connection sleeve. In addition, C can form fine carbide particles with Nb, V, Ti and Cr, etc., strengthening ferrite through precipitation strengthening, thereby improving fire resistance. However, excessive C is detrimental to the plasticity and toughness of steel. In the present invention, the C content is controlled within the range of 0.35-0.42%.

[0029] Si: It has a solid solution strengthening effect in steel, improving hardenability, inhibiting the diffusion of carbon and thus delaying phase transformation, which is beneficial to increasing the elastic limit and yield limit of steel, improving the strength and wear resistance of steel, and can also act as a deoxidizer during the steelmaking process. However, excessive Si is detrimental to the plasticity and toughness of steel, and can also reduce the creep strength of steel, which is detrimental to its fire resistance. In the present invention, the Si content is controlled within the range of 0.12-0.25%.

[0030] Mn: It is the most economical solid solution strengthening element besides C. It can stabilize austenite, enhance hardenability, improve the strength and low-temperature toughness of steel, and help lower the brittle-to-ductile transition temperature of steel. It can also reduce the diffusion of C, refine carbide particles, strengthen ferrite, and help improve refractory properties. In the present invention, the Mn content is controlled in the range of 1.6-2.0%.

[0031] Cr: is an important refractory element that can significantly enhance hardenability. A portion of Cr is dissolved in ferrite, and the other portion combines with C to form carbide particles, strengthening ferrite, thereby effectively improving the high-temperature strength and creep strength of steel. In the present invention, the Cr content is controlled within the range of 0.25-0.35%.

[0032] Nb: It is an important grain-refining element and refractory-enhancing element in steel. It can not only delay the recrystallization of austenite, expand the recrystallization zone, postpone the ferrite phase transformation, lower the austenite-ferrite phase transformation point, and promote the formation of granular bainite with good toughness, but also has good grain refinement and precipitation strengthening effects. However, too high Nb content can easily cause cracks in the continuous casting slab. In the present invention, the Nb content is controlled within the range of 0.05-0.15%.

[0033] V: It can be fully dissolved during the heating process of the continuous casting billet and combine with C and N to form a large number of dispersed fine precipitated particles during the rolling deformation process. The precipitated particles hinder the growth of grains at high temperatures, thereby improving the high-temperature strength of the steel. In the present invention, the V content range is controlled to be 0.04-0.08%.

[0034] Ti: easily forms high-melting-point nitride particles with N, which not only refines the grains but also provides nucleation points for the precipitation of Nb and V, promotes the formation of composite particles, further improves the stability of precipitated particles at high temperatures, and enhances high-temperature strength. In the present invention, the Ti content is controlled within a range of 0.02 to 0.05%.

[0035] Cu: At high temperatures, a copper-rich phase can be precipitated inside the ferrite, thereby increasing strength and hardness in the form of precipitation strengthening, which helps to improve refractory properties. However, excessive Cu can easily cause hot brittleness of the steel and is not conducive to rolling. In the present invention, the Cu content is controlled within the range of 0.15-0.25%.

[0036] B: is a strong carbide-forming element. A small amount of B added can enhance hardenability, delay ferrite transformation, and promote the formation of high-strength bainite. In the present invention, the B content is controlled within the range of 0.001 to 0.003%.

[0037] P and S are impurity elements in steel. P is prone to segregation at grain boundaries, reducing grain boundary strength and low-temperature toughness of steel. S is prone to forming MnS inclusions, reducing low-temperature toughness of steel, and is easily distributed in the rolling direction, causing anisotropy. In the present invention, P is controlled to be ≤ 0.02% and S is ≤ 0.02%.

[0038] N: It can significantly enhance the precipitation effect of Ti and V and has an auxiliary effect on improving the high-temperature strength. However, too high N will combine with some alloy elements to form large-sized N-containing precipitates in the steel, affecting the plasticity and toughness. In the present invention, the N content is controlled in the range of 0.01-0.02%.

[0039] In addition, by comprehensively considering the differences in the improvement of the fire resistance performance of each element in the chemical composition of the wire rod for the mechanical connection sleeve of the fire-resistant steel bar, by controlling the fire resistance index FRE to 0.60-0.75%, the fire resistance performance of the wire rod and the sleeve prepared by further processing can be guaranteed, so that it has sufficient high-temperature strength and creep strength, and low cost can be guaranteed, and the production difficulty and quality control difficulty of the continuous casting billet can be reduced.

[0040] In this way, in the chemical composition design of the present invention, under comprehensive consideration of the effects of different elements on the high-temperature strength and creep strength of steel, not only the addition and content of each element are accurately selected and controlled, but the content relationship of each element is further coordinated by controlling the fire resistance index FRE. On the one hand, it is beneficial to the organizational control of the wire rod for mechanical connection sleeves of steel bars and the sleeves prepared by further processing, and to improve the strength, toughness and fire resistance of the wire rod and the sleeve, so as to comprehensively improve the safety of the use of mechanical connection sleeves of steel bars in reinforced concrete structures, making them suitable for key fire protection projects. On the other hand, it can also reduce production costs and simplify the production process.

[0041] Specifically, in terms of microstructure, the structure of the wire rod with a diameter of 16 to 40 mm is a two-phase structure of ferrite and pearlite, wherein the proportion of ferrite is ≥30% and the grain size is ≥9.5μm; the structure of the sleeve is a three-phase structure of ferrite, pearlite and bainite, wherein the proportion of ferrite is ≤10%, the proportion of pearlite is ≥70%, and the grain size is 7.5 to 9.2μm.

[0042] In terms of mechanical properties, the hardness of the wire rod is ≤200HV, the yield strength at room temperature is ≤500MPa, the tensile strength is ≤650MPa, and the elongation after fracture is ≥20%; the yield strength at 600℃ is ≥320MPa, the tensile strength is ≥400MPa, and the elongation after fracture is ≥25%; the hardness of the sleeve is ≥285HV, the yield strength at room temperature is ≥750MPa, the tensile strength is ≥920MPa, and the elongation after fracture is ≥16%; the yield strength at 600℃ is ≥480MPa, the tensile strength is ≥650MPa, and the elongation after fracture is ≥22%.

[0043] One embodiment of the present invention also provides a preferred production method for the wire rod for the mechanical connection sleeve of the refractory steel bar, which includes the steps of pre-desulfurization of molten iron, electric furnace smelting, LF refining, continuous casting, heating, controlled rolling, and controlled cooling in sequence to prepare the wire rod for the mechanical connection sleeve of the refractory steel bar.

[0044] The chemical composition of the wire rod includes, by mass percentage, C 0.35-0.42%, Si 0.12-0.25%, Mn 1.6-2.0%, Cr 0.25-0.35%, Nb 0.10-0.15%, V 0.04-0.08%, Ti 0.02-0.05%, Cu 0.15-0.25%, B 0.001-0.003%, P≤0.02%, S≤0.02%, N 0.01-0.02%, and the rest is Fe and unavoidable impurities; wherein, the refractoriness index

[0045] FRE=[Nb]+1.5[Cr]+0.8[V]+0.5[Ti]+0.1[Cu], FRE is 0.60~0.75%.

[0046] The production method is described in detail below according to the production sequence.

[0047] (1) Hot metal pre-desulfurization process

[0048] The blast furnace molten iron is fed into a ladle and a desulfurizer is added to perform KR method pre-desulfurization. At the end of desulfurization, the S in the molten iron is ≤ 0.01% and the slag skimming rate of the desulfurized slag is ≥ 98%.

[0049] (2) Electric furnace smelting process

[0050] Scrap steel and pre-desulfurized blast furnace iron are added into the electric furnace in sequence, wherein the scrap steel accounts for ≥75%, the C in the molten steel at the end of electric furnace smelting is ≤0.15%, P ≤0.015%, the tapping temperature is 1595-1625℃, and argon is blown from the bottom of the ladle throughout the tapping process, with an argon flow rate of 200-250L / min; after 1 / 5 of the steel is tapped, ferrosilicon nitride, silicomanganese, ferromanganese, high carbon ferrochrome, copper blocks and lime are added in sequence for deoxidation and alloying to reduce oxidation and burning loss and improve the utilization efficiency of the alloy.

[0051] (3) LF refining process

[0052] After the molten steel produced by the electric furnace smelting process is injected into the LF furnace, 2.4-3.8 kg of lime and 0.8-1.2 kg of fluorite are added per ton of molten steel to adjust the slag to yellow, then the power is turned on to increase the temperature and soft stirring is performed, the soft stirring time is 5-8 minutes, the argon flow rate of the ladle bottom blowing during the soft stirring is 150-200 L / min, and the total argon consumption is 10-20 L / t; after the basicity of the refined slag is adjusted to 2.0-2.5, borax, vanadium-nitrogen alloy, ferroniobium and ferrotitanium are added in sequence for alloying to adjust the chemical composition; during the alloying period, the argon flow rate of the ladle bottom blowing is 250-300 L / min, and the total argon consumption is 30-55 L / t; then sampling is taken and the chemical composition of the molten steel is fine-tuned before tapping, and the tapping temperature at the end point of LF refining is 1550-1580°C.

[0053] The chemical composition of the terminal molten steel in the LF refining process determines the chemical composition of the final refractory steel bar mechanical connection sleeve, that is, the chemical composition of the terminal molten steel is consistent with the chemical composition of the final refractory steel bar mechanical connection sleeve.

[0054] (4) Continuous casting process

[0055] The molten steel produced by the RH refining process is continuously cast into continuous casting billets, the tundish temperature is 1525-1550℃, and the continuous casting process adopts a large ladle long nozzle and sealing gasket, an immersed nozzle, and an alkaline tundish covering agent for full protection pouring. Argon is blown through the long nozzle throughout the process, and low-carbon protective slag is used as the protective slag, and the slag layer thickness is 8-10mm; the water distribution flow rate of the crystallizer in the first solidification cooling zone is 1800-2400L / min, and the temperature difference between the water outlet and the water inlet of the crystallizer is less than 10℃; electromagnetic stirring is adopted in the second solidification cooling zone, the electromagnetic stirring frequency is 3-5Hz, and the liquid level fluctuation is controlled within ±2mm, the water distribution flow rate of the crystallizer in the second solidification cooling zone is 500-800L / min, and the continuous casting speed is 2.2-2.5m / min.

[0056] Among them, the continuous casting billet is a small square billet with a cross-sectional size of 150mm×150mm.

[0057] (5) Heating process

[0058] After surface inspection, the continuous casting billet is loaded into a heating furnace for heating at a temperature of 1150°C to 1220°C, with a total heating time of 60 to 90 minutes and a soaking period of ≥40 minutes to ensure effective solid solution of the added alloying elements.

[0059] (6) Control the rolling process

[0060] The heated continuous casting billet is rolled into wire rod and then coiled into coil. The starting rolling temperature is ≥1080℃, the finishing rolling inlet temperature is 980-1020℃, the final rolling speed is 12-15m / s, the coiling temperature is ≥950℃, and all fans are turned off during the coiling process to achieve gradient cooling rolling of the wire rod.

[0061] Preferably, after the continuous casting billet leaves the heating furnace, it is rolled using a continuous wire mill to be rolled into a wire rod with a diameter of 16 to 40 mm.

[0062] (7) Control cooling process

[0063] The obtained coils are sent to the insulation pit for cooling. The insulation pit is covered with an insulation cover. The cooling rate is ≤0.7℃ / s. After cooling to below 300℃, they are discharged from the pit for air cooling.

[0064] In this way, through the control of the entire process of chemical composition design and production process, the microstructure of the prepared wire rod is a two-phase structure of ferrite and pearlite, among which the proportion of ferrite is ≥30%, the grain size is ≥9.5μm, and the hardness is ≤200HV; the yield strength at room temperature is ≤500MPa, the tensile strength is ≤650MPa, and the elongation after fracture is ≥20%. It can not only reduce the processing difficulty during the subsequent drawing and preparation of the sleeve, and can perform shaping processing without annealing, but also meet the strength requirements of the subsequent drawing and preparation of the sleeve; the yield strength at 600℃ is ≥320MPa, the tensile strength is ≥400MPa, and the elongation after fracture is ≥25%. It has certain strength and good plasticity, and excellent fire resistance, laying the foundation for the comprehensive performance of the finished product of the refractory steel bar mechanical connection sleeve prepared by further processing.

[0065] Furthermore, the fire-resistant steel bar mechanical connection sleeve can be prepared by further processing the wire rod for the fire-resistant steel bar mechanical connection sleeve through the following steps.

[0066] (8) Drawing and straightening process

[0067] The wire rod after controlled cooling is drawn to the set size and then sent to the straightening machine for straightening. It is then cut and finished according to the set sleeve length to obtain the sleeve blank.

[0068] (9) Extrusion molding process

[0069] The sleeve blank after drawing and straightening is perforated, and then extruded into equal 6-angle, equal 8-angle or equal 12-angle shapes according to the sleeve diameter.

[0070] (10) Thread tapping process

[0071] The sleeve blank after extrusion molding is threaded by an automatic tapping machine with a thread angle of 75° to obtain a sleeve.

[0072] (11) Gradient quenching process

[0073] The sleeve after thread tapping is induction heated at a temperature of 900-950°C for 3-5 minutes to completely austenitize it; then it enters the first salt bath furnace for austempering at a temperature of 600-680°C for 10-25 minutes to allow the austenite to quickly pass through the ferrite phase region and transform into pearlite; then it is transferred to the second salt bath furnace for austempering at a temperature of 400-460°C for 8-15 minutes to allow the retained austenite to fully transform into bainite; then it is oil quenched. The quenching oil temperature is 30-50°C, and the cooling rate is 0.1-0.5°C / s to refine the grains and improve the strength of the sleeve. By continuously quenching the wire rod with multiple temperature gradients, not only can the sleeve obtain a three-phase structure of pearlite + bainite + a small amount of ferrite, so that the sleeve can meet the strength and plasticity required for service at room temperature, but it can also ensure that the sleeve has a small strength loss when serving at a high temperature of 600°C, and has excellent fire resistance. It can also improve the degree of automation and avoid waiting time between quenching with different temperature gradients, thereby improving production efficiency.

[0074] (12) Low temperature tempering process

[0075] The sleeve after gradient quenching is sent to a muffle furnace for tempering at a tempering temperature of 200-250°C and a tempering time of 20-40 minutes. After tempering, it is left in the muffle furnace to cool to room temperature at a cooling rate of ≤1°C / s to eliminate the internal stress of the sleeve.

[0076] Thus, the production method of this embodiment, on the basis of the aforementioned chemical composition design, regulates a series of process means including molten iron pre-desulfurization, electric furnace smelting, LF refining, continuous casting, heating, controlled rolling, controlled cooling, drawing and straightening, extrusion molding, thread tapping, gradient quenching and low-temperature tempering, so that the microstructure of the sleeve finally prepared is a three-phase structure of ferrite, pearlite and bainite, wherein the proportion of ferrite is ≤10%, the proportion of pearlite is ≥70%, the grain size is 7.5~9.2μm, the hardness is ≥285HV, the yield strength at room temperature is ≥750MPa, the tensile strength is ≥920MPa, and the elongation after fracture is ≥16%; the yield strength at 600℃ is ≥480MPa, the tensile strength is ≥650MPa, and the elongation after fracture is ≥22%. It has excellent mechanical properties and plastic toughness both at room temperature and at 600℃, thereby ensuring that the sleeve has excellent mechanical properties and fire resistance when used in reinforced concrete structures.

[0077] The following six embodiments further illustrate the specific embodiments of the present invention. Of course, these six embodiments are only a portion of the numerous variations of this embodiment, and are not exhaustive. Other embodiments based on the aforementioned embodiments do not depart from the technical spirit of the present invention.

[0078] First, Examples 1 to 6 all provide a wire rod for a refractory steel bar mechanical connection sleeve, and a refractory steel bar mechanical connection sleeve prepared by further drawing and straightening, extrusion molding, thread tapping, gradient quenching and low-temperature tempering the wire rod. The chemical composition of the wire rod and the sleeve is shown in Table 1.

[0079] [Table 1]

[0080]

[0081] The wire rod production methods of each embodiment employ a process comprising, in sequence, pre-desulfurization of molten iron, electric furnace smelting, LF refining, continuous casting, heating, controlled rolling, and controlled cooling. The sleeves of each embodiment are further produced from the wire rod through drawing and straightening, extrusion molding, thread tapping, gradient quenching, and low-temperature tempering. The specific operations of each process are described above and will not be repeated here.

[0082] The wire rods and sleeves of Examples 1 to 6 were sampled and subjected to metallographic structure testing and mechanical property testing according to the same testing method. The diameters of the wire rods of Examples 1 to 6 are shown in Table 2, and the test results are as follows:

[0083] (1) In terms of structure, the wire rods of Examples 1 to 6 all have a two-phase structure of ferrite and pearlite, wherein the percentages of bainite and pearlite are shown in Table 2. In addition, the grain size of the wire rods of Examples 1 to 6 is ≥9.5 μm; while the sleeves of Examples 1 to 6 all have a three-phase structure of ferrite, pearlite and bainite, wherein the percentages of ferrite, pearlite and bainite are shown in Table 3. In addition, the grain size of the sleeves of Examples 1 to 6 is in the range of 7.5 to 9.2 μm.

[0084] (2) In terms of mechanical properties, the hardness, yield strength, tensile strength and elongation at room temperature, and the yield strength, tensile strength and elongation at 600°C of the wire rods of Examples 1 to 6 are shown in Table 2, respectively; the hardness, yield strength, tensile strength and elongation at room temperature, and the yield strength, tensile strength and elongation at 600°C of the sleeves of Examples 1 to 6 are shown in Table 3, respectively;

[0085] [Table 2]

[0086]

[0087] It can be seen from Table 2 that the wire rods in Examples 1 to 6 produced according to this embodiment have excellent comprehensive performance, and their structures are all two-phase structures of ferrite and pearlite, among which the proportion of ferrite is ≥30%; hardness ≤200HV, yield strength at room temperature ≤500MPa, tensile strength ≤650MPa, and elongation after fracture ≥20%; yield strength at 600°C ≥320MPa, tensile strength ≥400MPa, and elongation after fracture ≥25%.

[0088] [Table 3]

[0089]

[0090] It can be seen from Table 3 that the sleeves in Examples 1 to 6 produced and prepared according to this embodiment have excellent comprehensive performance, and their structures are all three-phase structures of pearlite + bainite + a small amount of ferrite, among which the proportion of ferrite is ≤10%, and the proportion of pearlite is ≥70%; the hardness is ≥285HV, the yield strength at room temperature is ≥750MPa, the tensile strength is ≥920MPa, and the elongation after fracture is ≥16%; the yield strength at 600°C is ≥480MPa, the tensile strength is ≥650MPa, and the elongation after fracture is ≥22%. It has excellent mechanical properties at room temperature and at a high temperature of 600°C, and excellent fire resistance, which can comprehensively improve the safety of the use of steel bar mechanical connection sleeves in reinforced concrete structure buildings, making it suitable for key fire protection projects.

[0091] It should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each implementation method can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for producing a refractory steel bar mechanical connection sleeve, characterized in that: The wire rod is prepared into a sleeve through the processes of drawing and straightening, extrusion forming, thread tapping, gradient quenching and low temperature tempering; wherein, The chemical composition of the wire rod includes, by mass percentage, C 0.35-0.42%, Si 0.12-0.25%, Mn 1.6-2.0%, Cr 0.25-0.35%, Nb 0.10-0.15%, V 0.04-0.08%, Ti 0.02-0.05%, Cu 0.15-0.25%, B 0.001-0.003%, P≤0.02%, S≤0.02%, N 0.01-0.02%, and the remainder is Fe and unavoidable impurities; the fire resistance index FRE=[Nb]+1.5[Cr]+0.8[V]+0.5[Ti]+0.1[Cu], and the FRE is 0.60-0.75%; In the drawing and straightening process, the wire rod is drawn to a set size and then sent to a straightening machine for straightening, and then cut and finished according to the set sleeve length to obtain a sleeve blank; In the extrusion molding process, the sleeve blank after drawing and straightening is perforated, and then extrusion molding is performed according to the sleeve diameter using equal 6-angle, equal 8-angle or equal 12-angle; In the thread tapping process, an automatic tapping machine is used to tap the sleeve blank after extrusion, and the thread angle is 75°; In the gradient quenching process, the sleeve after threading is subjected to induction heating at a temperature of 900-950°C for a heating time of 3-5 minutes; then enters a first salt bath furnace for isothermal quenching at a quenching temperature of 600-680°C for a quenching time of 10-25 minutes; then enters a second salt bath furnace for isothermal quenching at a quenching temperature of 400-460°C for a quenching time of 8-15 minutes; then oil quenching is performed at a quenching oil temperature of 30-50°C and a cooling rate of 0.1-0.5°C / s; In the low-temperature tempering process, the sleeve after gradient quenching is sent to a muffle furnace for tempering at a tempering temperature of 200-250°C for 20-40 minutes. After tempering, it is left in the muffle furnace to cool to room temperature at a cooling rate of ≤1°C / s.

2. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 1, characterized in that: The wire rod is prepared through the following steps: molten iron pre-desulfurization, electric furnace smelting, LF refining, continuous casting, heating, controlled rolling and controlled cooling.

3. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the molten iron pre-desulfurization process, the blast furnace molten iron is subjected to KR method pre-desulfurization, the S in the molten iron at the end of desulfurization is ≤0.01%, and the slag skimming rate of the desulfurization slag is ≥98%; In the electric furnace smelting process, scrap steel and pre-desulfurized blast furnace molten iron are added to the electric furnace in sequence, wherein the scrap steel accounts for ≥75%, the C in the molten steel at the end of the electric furnace smelting is ≤0.15%, and P is ≤0.015%. The tapping temperature is 1595~1625℃, and argon is blown from the bottom of the ladle throughout the tapping process, with an argon flow rate of 200~250L / min. After 1 / 5 of the steel is tapped, ferrosilicon nitride, silicomanganese, ferromanganese, high carbon ferrochrome, copper blocks and lime are added in sequence for deoxidation and alloying.

4. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the LF refining process, after the molten steel produced by the electric furnace smelting process is injected into the LF furnace, 2.4-3.8 kg of lime and 0.8-1.2 kg of fluorite are added per ton of molten steel to adjust the slag to yellow. Then, the slag is heated by power and softly stirred for 5-8 minutes. During the soft stirring, the argon flow rate of the ladle bottom is 150-200 L / min. After the basicity of the refined slag is adjusted to 2.0-2.5, borax, vanadium-nitrogen alloy, ferroniobium and ferrotitanium are added in sequence for alloying. During the alloying period, the argon flow rate of the ladle bottom is 250-300 L / min. The tapping temperature at the end point of LF refining is 1550-1580°C.

5. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the continuous casting process, the tundish temperature is 1525-1550°C, and the continuous casting process adopts a large ladle long nozzle and a sealing gasket, an immersed nozzle, and an alkaline tundish covering agent for full protection casting. Argon is blown through the long nozzle throughout the process, low-carbon protective slag is used as protective slag, and the slag layer thickness is 8-10 mm; the water distribution flow rate of the crystallizer in the first solidification cooling zone is 1800-2400 L / min, and the temperature difference between the water outlet and the water inlet of the crystallizer is less than 10°C; electromagnetic stirring is adopted in the second solidification cooling zone, the electromagnetic stirring frequency is 3-5 Hz, and the liquid level fluctuation is controlled within ±2 mm. The water distribution flow rate of the crystallizer in the second solidification cooling zone is 500-800 L / min, and the continuous casting speed is 2.2-2.5 m / min.

6. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the heating process, the heating temperature is 1150° C. to 1220° C., the total heating time is 60 to 90 minutes, and the soaking period time is ≥40 minutes.

7. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the controlled rolling process, the continuous casting billet after the heating process is rolled into wire rods and coiled into coils, the starting rolling temperature is ≥1080°C, the finishing rolling inlet temperature is 980~1020°C, the final rolling speed is 12~15m / s, the coiling temperature is ≥950°C, and all fans are turned off during the coiling process.

8. The method for producing a fire-resistant steel bar mechanical connection sleeve according to claim 2, characterized in that: In the controlled cooling process, the obtained coils are sent to an insulation pit for stack cooling, the insulation pit is covered with an insulation cover, the cooling rate is ≤0.7°C / s, and after cooling to below 300°C, they are discharged from the pit for air cooling.

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