Production method of round steel for preparing a sleeve, sleeve prepared from the round steel, and connection method between the sleeve and a steel bar

Abstract

The present application provides a production method of round steel, a sleeve, and a connection method between the sleeve and a steel bar. In the production method, the chemical composition of the round steel by mass percentage includes: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, and Ti, and the mass percentages of V, Nb, and Ti satisfy: V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the rest is iron and inevitable impurities; in the production method, the cooling rate of the continuous casting billet is ≤ 0.5 °C / s, the heating temperature is 1200 - 1300 °C, the finishing mill inlet temperature is 950 - 1050 °C, the finishing mill outlet temperature is 850 - 900 °C, the temperature for entering the cooling bed is 650 - 750 °C, and the cooling rate is 0.5 - 2 °C / s.

Description

Technical Field

[0001] The present application relates to the technical field of steel smelting, and particularly relates to a production method of round steel for preparing sleeves for mechanical connection of steel bars, a sleeve prepared from the round steel, and a connection method between the sleeve for mechanical connection of steel bars and the steel bars. Background Art

[0002] The high strengthening of steel bars is a development trend. The development of hot-rolled high-strength earthquake-resistant steel bars with a yield strength of 700 MPa grade has been achieved, but the lack of supporting mechanical connection technology has become the main factor restricting the wide application of 700 MPa grade high-strength earthquake-resistant steel bars.

[0003] The higher the strength of the steel bar, the higher the requirement for the co-deformation ability of the mechanical connector under external loads, that is, the greater the difficulty in keeping the deformation behavior of the sleeve for mechanical connection and the steel bar body consistent under external loads, resulting in the fracture of the sleeve or the brittle fracture of the steel bar (breaking at the thread processing part on the outer surface of the steel bar) during the tensile test of the steel bar mechanical connector, which cannot meet the requirements of relevant standards. The prior art has improved the comprehensive performance of the steel bar mechanical connector by alloying to increase the strength of the sleeve or improving the processing method of the external thread of the steel bar, but there are disadvantages such as low strength grade of the steel bar (yield strength below 700 MPa), high alloy cost, and complex processing and manufacturing process, which cannot be widely promoted and applied, nor can it be extended to 700 MPa grade high-strength earthquake-resistant steel bars. Summary of the Invention

[0004] The purpose of the present application is to provide a production method of round steel for preparing sleeves for mechanical connection of steel bars, which solves the problem that there is no sleeve steel matching the strength of 700 MPa grade high-strength earthquake-resistant steel bars in the prior art.

[0005] In order to achieve one of the above-mentioned invention purposes, an embodiment of the present application provides a production method of round steel for preparing sleeves for mechanical connection of steel bars. The chemical composition of the round steel in mass percentage includes: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, Ti, and the mass percentages of V, Nb, Ti satisfy V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the rest is Fe and inevitable impurities;

[0006] The production method includes the processes of converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling in sequence, wherein,

[0007] In the continuous casting process, the cooling rate of the slab formed after continuous casting ≤ 0.5 °C / s;

[0008] In the heating process, the continuous casting billet is heated at 1200 - 1300 °C;

[0009] In the rolling process, the continuously cast billet after heating starts finish rolling at 950 - 1050 °C, and round steel is obtained after finish rolling. The outlet temperature of finish rolling is 850 - 900 °C;

[0010] In the cooling process, the temperature of the round steel entering the cooling bed is 650 - 750 °C. After entering the cooling bed, the cooling rate of the round steel is controlled at 0.5 - 2 °C / s.

[0011] In one embodiment of the present application, in the converter / electric furnace smelting process, the tapping temperature is 1610 - 1650 °C. Alloys and slag materials are added in the order of silicomanganese alloy, ferrosilicon alloy, high-carbon ferrochrome alloy, and lime. Among them, the addition amount of silicomanganese alloy is 8 - 16 kg / t, the silicon content in the silicomanganese alloy is 17 - 20%, the manganese content is 65 - 70%, and the rest is iron and inevitable impurities; the addition amount of ferrosilicon alloy is 5 - 10 kg / t, the silicon content in the ferrosilicon alloy is 70 - 75%, and the rest is iron and inevitable impurities; the addition amount of high-carbon ferrochrome alloy is 6 - 10 kg / t, the carbon content in the high-carbon ferrochrome alloy is 8 - 12%, the chromium content is 53 - 57%, and the rest is iron and inevitable impurities.

[0012] In one embodiment of the present application, in the converter / electric furnace smelting process, the argon gas control valve at the bottom of the ladle is opened before tapping, and argon gas is blown throughout the tapping process. The argon gas flow rate is controlled at 800 - 1200 NL / min and the gas pressure is 0.4 - 0.5 MPa in the first 1 / 3 section before tapping, and the argon gas flow rate is controlled at 350 - 650 NL / min and the gas pressure is 0.3 - 0.4 MPa in the last 2 / 3 section after tapping.

[0013] In one embodiment of the present application, in the LF refining process, ferrosilicon nitride alloy is added when the ladle arrives at the station, and ferro-niobium alloy, vanadium nitride alloy, and ferro-titanium alloy are selectively added according to the foregoing components of the round steel. The addition amount of ferrosilicon nitride alloy is 1 - 2 kg / t, the nitrogen content in the ferrosilicon nitride alloy is 35 - 40%, the silicon content is 50 - 55%, and the rest is iron and inevitable impurities; the addition amount of ferro-niobium alloy is 0.24 - 1.2 kg / t, the niobium content in the ferro-niobium alloy is 40 - 45%, and the rest is iron and inevitable impurities; the addition amount of vanadium nitride alloy is 2 - 6 kg / t, the vanadium content in the vanadium nitride alloy is 75 - 80%, the nitrogen content is 5 - 10%, and the rest is iron and inevitable impurities; the addition amount of ferro-titanium alloy is 0.2 - 1 kg / t, the titanium content in the ferro-titanium alloy is 40 - 45%, and the rest is iron and inevitable impurities.

[0014] In one embodiment of the present application, in the LF refining process, after the ladle arrives, the argon gas control valve at the bottom of the ladle is opened. Among them, during the waiting period, the argon gas flow rate is controlled at 200 - 300 L / min, during the heating, deoxidizing and desulfurizing period, the argon gas flow rate is controlled at 300 - 400 L / min, during the alloying period, the argon gas flow rate is controlled at 350 - 600 L / min, during the soft stirring period, the argon gas flow rate is controlled at 50 - 100 L / min, and the soft stirring time is ≥ 10 min.

[0015] In one embodiment of the present application, in the continuous casting process, the superheat is controlled at 25 - 35 °C, and the casting speed is controlled at 2.3 - 2.6 m / min.

[0016] One embodiment of the present application also provides a sleeve for mechanical connection of steel bars, which is made of round steel obtained by the production method of round steel for preparing sleeves for mechanical connection of steel bars as described above, and the inner thread is processed on the round steel. The structure of the sleeve includes ferrite and pearlite. Among them, the volume ratio of pearlite is ≥ 90%, the pearlite lamellar spacing is 150 - 200 nm, and the average size of pearlite colonies is 5 - 10 μm.

[0017] In one embodiment of the present application, the chemical composition of the sleeve includes, by mass percentage: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, Ti, and the mass percentages of V, Nb, Ti satisfy V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the rest is Fe and inevitable impurities.

[0018] In one embodiment of the present application, the tensile strength of the sleeve is ≥ 900 MPa, and the elongation after fracture is ≥ 20%.

[0019] One embodiment of the present application also provides a connection method between a sleeve for mechanical connection of steel bars and steel bars, including the following steps:

[0020] Steel bar processing: The outer thread is processed on the end of the steel bar by the method of upsetting straight thread. When the diameter d of the steel bar is 10 - 25 mm, the upsetting pressure is 10 - 23 MPa, the upsetting base circle diameter is d + 4 - d + 6 mm, and the upsetting length is d + 2 mm; when the diameter d of the steel bar is 26 - 50 mm, the upsetting pressure is 30 - 55 MPa, the upsetting base circle diameter is d + 6.5 mm, and the upsetting length is d + 3 mm; the yield strength of the steel bar is ≥ 700 MPa, the tensile strength is ≥ 880 MPa, the strength ratio is ≥ 1.25, the elongation after fracture is ≥ 18%, and the total elongation at maximum force is ≥ 10%;

[0021] Sleeve processing: The sleeve for mechanical connection of steel bars as described above is adopted, and the internal thread of the sleeve matches the external thread of the steel bar;

[0022] Mechanical connection of steel bar and sleeve: The steel bar and the sleeve are tightened by threads, and the number of thread turns of the steel bar not screwed into the sleeve thread is no more than 1.0 turn, and the torque is 450 - 550 N·m.

[0023] In one implementation manner of this application, the tooth profile of the external thread of the steel bar is triangular, and the tooth profile angle is 60 - 70°; when the nominal diameter of the steel bar < 25 mm, the pitch P is 2.5 mm, and when the nominal diameter of the steel bar ≥ 25 mm, the pitch P is 3.0 mm.

[0024] One or more technical solutions provided by this application have at least the following technical effects or advantages:

[0025] The production method of the round steel for preparing the sleeve for mechanical connection of steel bars provided by this application adopts a medium carbon + a small amount of chromium + trace strengthening elements (niobium, vanadium, titanium) alloy composition system, combined with the controlled rolling and controlled cooling process (control of rolling temperature and cooling rate after rolling), giving full play to the effects of solid solution strengthening, refinement strengthening, and phase transformation strengthening of alloying elements, obtaining a ferrite and pearlite duplex structure, and simply and efficiently solving the technical problem that it is difficult to synergistically improve the strength and plasticity of the sleeve for mechanical connection of 700 MPa grade high-strength earthquake-resistant steel bars. Specific implementation manner

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] The embodiment of this application provides a production method of round steel for preparing the sleeve for mechanical connection of steel bars. The chemical composition of the round steel includes, by mass percentage: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, Ti, and the mass percentages of V, Nb, Ti satisfy V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the rest are Fe and inevitable impurities;

[0028] The production method includes the converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling processes carried out in sequence. Among them,

[0029] In the continuous casting process, the cooling rate of the slab formed after continuous casting ≤ 0.5℃ / s;

[0030] In the heating process, the billet is heated at 1200 - 1300 °C.

[0031] In the rolling process, the heated billet starts finish rolling at 950 - 1050 °C, and round steel is obtained after finish rolling. The outlet temperature of finish rolling is 850 - 900 °C.

[0032] In the cooling process, the temperature of the round steel on the cooling bed is 650 - 750 °C. After being on the cooling bed, the cooling rate of the round steel is controlled at 0.5 - 2 °C / s.

[0033] In this application, an alloy composition system of "medium carbon + a small amount of chromium + trace strengthening elements (niobium, vanadium, titanium)" is adopted. By controlling a relatively high heating temperature, the alloying elements are dissolved, the size of the original austenite grains is controlled, the rolling temperature and the temperature of the round steel on the cooling bed after rolling are controlled, so that there is a relatively fast cooling rate between finish rolling and being on the cooling bed, enabling the round steel to quickly pass through the ferrite phase transformation zone and controlling the content of ferrite. After being on the cooling bed, the cooling rate is controlled to be relatively small to control the pearlite phase transformation, regulate the volume fraction, lamellar spacing and the size of pearlite colonies of pearlite, and give full play to the effects of solid solution strengthening, grain refinement strengthening and phase transformation strengthening of alloying elements, obtaining a ferrite and pearlite duplex structure, simply and efficiently solving the technical problem that it is difficult to synergistically improve the strength and plasticity of the sleeve for mechanical connection of high-strength earthquake-resistant steel bars.

[0034] The functions of each chemical component have the following characteristics:

[0035] Carbon (C): It can significantly improve the strength of steel at low cost, but if the carbon content is too high, it is not beneficial to the plasticity of steel. Considering the strength-plasticity matching of the steel bars comprehensively, the carbon content is controlled at 0.5 - 0.6%.

[0036] Silicon (Si): A ferrite-forming element, which promotes the formation of ferrite and is beneficial to adjusting the problem of insufficient plasticity caused by high carbon content. Considering comprehensively, the Si content is controlled at 0.50 - 1.00%.

[0037] Manganese (Mn): A commonly used solid solution strengthening element, which can significantly improve the strength and toughness of steel. Designing the manganese content at a relatively high level can ensure that the steel has relatively high earthquake resistance performance and is coordinated and matched with the mechanical properties of earthquake-resistant steel bars; at the same time, manganese can improve the stability of austenite, promote the formation of hard phase structures such as pearlite or bainite, and improve the strength of steel. Considering comprehensively, the manganese content is controlled at 0.50 - 1.00%.

[0038] Chromium (Cr): A hardenability element, which combines with carbon to obtain structures such as pearlite, bainite, and martensite at a relatively low cooling rate, significantly improving the strength of steel; but if the content is too high, it is not beneficial to plasticity. Considering comprehensively, the chromium content is controlled at 0.30 - 0.50%.

[0039] Vanadium (V): A strengthening element that matches the rolling process and can give full play to the precipitation strengthening and fine grain strengthening effects of carbon and nitrides, and can significantly improve the comprehensive performance of steel. Taking all factors into consideration, the vanadium content is controlled to be 0.10~0.30%.

[0040] Niobium (Nb): A strengthening element that matches the rolling process and can improve the strength and toughness of steel through precipitation strengthening and fine grain strengthening. It can also promote the formation of hard phase structure (lower the ferrite transformation temperature and increase the transformation temperature of pearlite and bainite), which is beneficial to adjust the strength-plasticity and seismic resistance (yield ratio) of steel. However, if the niobium content is too high, the billet is prone to cracks during continuous casting. Taking all factors into consideration, the niobium content is controlled to 0.01~0.05%.

[0041] Titanium (Ti): A strengthening element that improves the strength and plasticity of steel through fine grain strengthening. However, if the titanium content is too high, it is easy to produce oxides during continuous casting and block the nozzle. Taking all factors into consideration, the titanium content is controlled to 0.01~0.05%.

[0042] Nitrogen (N): a solid solution strengthening and precipitation strengthening element, combined with vanadium, titanium and niobium to form carbonitrides, which improves strength; but too high a content is detrimental to plasticity; too low a content increases the difficulty of smelting, and the role of strengthening elements cannot be fully exerted. Taking all factors into consideration, the nitrogen content is controlled at 0.01~0.02%.

[0043] Furthermore, in the converter / electric furnace smelting process, the steel tapping temperature is 1610~1650℃, and alloys and slag are added in the order of silicon-manganese alloy, ferrosilicon alloy, high carbon ferrochrome alloy and lime. Among them, the addition amount of silicon-manganese alloy is 8~16kg / t, the silicon content of silicon-manganese alloy is 17~20%, the manganese content is 65~70%, and the rest is iron and inevitable impurities; the addition amount of ferrosilicon alloy is 5~10kg / t, the silicon content of ferrosilicon alloy is 70~75%, and the rest is iron and inevitable impurities; the addition amount of high carbon ferrochrome alloy is 6~10kg / t, the carbon content of high carbon ferrochrome alloy is 8~12%, the chromium content is 53~57%, and the rest is iron and inevitable impurities.

[0044] First, a relatively cheap silicon-manganese alloy is added to make the manganese content in the molten steel reach the target content, and then silicon is supplemented through ferrosilicon alloy. Finally, high-carbon ferrochrome containing chromium is added to add chromium on the one hand and increase the carbon content on the other.

[0045] Furthermore, in the converter / electric furnace smelting process, the argon blowing control valve at the bottom of the ladle is opened before tapping, and argon is blown throughout the tapping process. The argon flow rate in the first 1 / 3 section before tapping is controlled to be 800~1200NL / min, and the air pressure is 0.4~0.5MPa. The argon flow rate in the second 2 / 3 section after tapping is controlled to be 350~650NL / min, and the air pressure is 0.3~0.4MPa.

[0046] Since a large amount of alloy is added during the smelting process, argon gas is softly stirred throughout the tapping process. At the early stage of tapping, a relatively large flow rate and air pressure are adopted to evenly disperse the alloy in the molten steel. At the later stage, the argon gas flow rate and air pressure are reduced to maintain the temperature of the molten steel.

[0047] In an embodiment of the present application, during the LF refining process, ferro-silicon nitride alloy is added when the ladle arrives at the station, and ferro-niobium alloy, vanadium-nitrogen alloy, and ferro-titanium alloy are selectively added according to the aforementioned components of the round steel. The addition amount of ferro-silicon nitride alloy is 1 - 2 kg / t, the nitrogen content in the ferro-silicon nitride alloy is 35 - 40%, the silicon content is 50 - 55%, and the rest is iron and inevitable impurities; the addition amount of ferro-niobium alloy is 0.24 - 1.2 kg / t, the niobium content in the ferro-niobium alloy is 40 - 45%, and the rest is iron and inevitable impurities; the addition amount of vanadium-nitrogen alloy is 2 - 6 kg / t, the vanadium content in the vanadium-nitrogen alloy is 75 - 80%, the nitrogen content is 5 - 10%, and the rest is iron and inevitable impurities; the addition amount of ferro-titanium alloy is 0.2 - 1 kg / t, the titanium content in the ferro-titanium alloy is 40 - 45%, and the rest is iron and inevitable impurities.

[0048] Furthermore, during the LF refining process, after the ladle arrives at the position, the argon gas control valve at the bottom of the ladle is opened. Among them, the argon gas flow rate is controlled at 200 - 300 L / min during the waiting period, 300 - 400 L / min during the heating and deoxidation and desulfurization period, 350 - 600 L / min during the alloying period, and 50 - 100 L / min during the soft stirring period, and the soft stirring time ≥ 10 min.

[0049] During the LF refining process, the argon gas control valve is fully opened for soft stirring to make the molten steel uniform. Among them, the waiting period is a relatively short time after the ladle arrives at the station until before deoxidation and desulfurization; during the heating and deoxidation and desulfurization period, a relatively large argon gas flow rate is adopted to ensure the deoxidation and desulfurization efficiency; during the alloying period after adding the alloy, since the alloy needs to be melted and dispersed, a larger argon gas flow rate is required for stirring. After alloying, soft stirring can maintain the uniformity of the molten steel, and a longer time and a smaller argon gas flow rate are adopted.

[0050] In an embodiment of the present application, during the continuous casting process, the superheat is controlled at 25 - 35 °C, and the drawing speed is 2.3 - 2.6 m / min. The water flow rate of the mold is 1700 - 1750 L / min, the current of the mold electromagnetic stirrer is 400 A, and the electromagnetic stirring frequency is 3 - 7 Hz; the current of the final electromagnetic stirrer is 500 A, and the electromagnetic stirring frequency is 10 - 15 Hz. The secondary cooling water ratio is automatically distributed according to the following table parameters (L / m):

[0051]

[0052] Among them, m, c, and co are correction factors related to the steel type, which are used to adjust the secondary cooling water ratio. The set value of the secondary cooling water ratio = casting speed × m; c and co are the value ranges of m, where co is the upper limit value and c is the lower limit value.

[0053] The embodiment of the present application also provides a sleeve for mechanical connection of steel bars, which is made of the round steel obtained by the aforementioned production method for producing round steel for mechanical connection of steel bars, and the round steel is processed with internal threads. The structure of the sleeve includes ferrite and pearlite. Among them, the volume ratio of pearlite ≥ 90%, the pearlite lamellar spacing is 150 - 200 nm, and the average size of pearlite colonies is 5 - 10 μm.

[0054] The round steel obtained by the aforementioned production method for producing round steel for mechanical connection of steel bars is cut into segments of the required length of the sleeve, and its outer surface is processed to obtain better surface quality, and then the round steel is processed with central hollowing and internal threads, and finally the sleeve is obtained.

[0055] Since the sleeve is processed from the round steel obtained by the aforementioned production method, its components, structure and mechanical properties are the same as those of the round steel. The chemical composition of the sleeve in mass percentage includes: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, Ti, and the mass percentages of V, Nb, Ti satisfy V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the rest are Fe and inevitable impurities.

[0056] Furthermore, the tensile strength of the sleeve ≥ 900 MPa, and the elongation after fracture ≥ 20%.

[0057] The embodiment of the present application also provides a connection method between a sleeve for mechanical connection of steel bars and steel bars, including the following steps:

[0058] Steel bar processing: The end of the steel bar is processed with external threads by the method of upsetting straight threads. When the diameter d of the steel bar is 10 - 25 mm, the upsetting pressure is 10 - 23 MPa, the upsetting base circle diameter is d + 4 - d + 6 mm, and the upsetting length is d + 2 mm; when the diameter d of the steel bar is 26 - 50 mm, the upsetting pressure is 30 - 55 MPa, the upsetting base circle diameter is d + 6.5 mm, and the upsetting length is d + 3 mm; the yield strength of the steel bar ≥ 700 MPa, the tensile strength ≥ 880 MPa, the strength ratio ≥ 1.25, the elongation after fracture ≥ 18%, and the total elongation at maximum force ≥ 10%;

[0059] Sleeve processing: Use the aforementioned sleeve for mechanical connection of steel bars, and the internal threads of the sleeve match the external threads of the steel bars;

[0060] Mechanical connection of steel bars and sleeves: The steel bars and sleeves are tightened by threads. The number of thread turns of the steel bars not screwed into the sleeve threads does not exceed 1.0 turn, and the torque is 450 - 550 N·m.

[0061] Steel bars with a yield strength ≥ 700 MPa and a tensile strength ≥ 880 MPa are used, in combination with the aforementioned sleeves with a tensile strength ≥ 900 MPa, which meets the strength matching of steel bars and sleeves in the industry. The outer threads are processed on the ends of the steel bars by upsetting straight threads, making the diameter of the ends of the steel bars larger than the diameter in the middle of the steel bars, which can withstand greater tensile stress and avoid the risk of brittle fracture at the threads during tension. Control the torque to avoid the situation where the steel bars cannot be screwed into the sleeves or damage the thread bonding state of the sleeves and the steel bar surfaces.

[0062] Furthermore, the thread profile of the outer threads of the steel bars is triangular, and the thread angle is 60 - 70°; when the nominal diameter of the steel bars < 25 mm, the pitch P is 2.5 mm, and when the nominal diameter of the steel bars ≥ 25 mm, the pitch P is 3.0 mm.

[0063] Furthermore, the outer diameter of the sleeves after surface processing of the round steel is 28 - 76.5 mm, the length is 60 - 120 mm, and the thickness is 6 - 10 mm.

[0064] Next, in combination with some specific embodiments, the technical solutions of this application are further described.

[0065] This application provides the following sleeves of Embodiments 1 - 6 and Comparative Examples 1 - 5. The chemical components of the sleeves are shown in Table 1. After being smelted in a converter / electric furnace (parameters shown in Table 2), refined by LF (parameters shown in Table 3), continuously cast (parameters shown in Table 4), heated, rolled, and cooled (parameters shown in Table 5) to obtain round steel with the microstructure and mechanical properties shown in Table 6, the round steel is cut, surface processed, hollowed out in the center, and processed with internal threads. Finally, sleeves are obtained. The steel bars shown in Table 7 are used, and the steel bars and sleeves are subjected to the processing and connection processes shown in Table 8 for the strength tests shown in Tables 9 and 10.

[0066] Table 1 Chemical composition of the sleeves (wt%)

[0067]

[0068] Table 2 Smelting process

[0069]

[0070] Table 3 LF refining process

[0071]

[0072] Table 4 Continuous casting process

[0073]

[0074] Table 5 Rolling and Cooling Processes

[0075]

[0076] Table 6 Microstructure and Mechanical Properties of the Sleeve

[0077]

[0078] Table 7 Mechanical Properties of the Reinforcing Bars Connected to the Sleeve

[0079]

[0080] Table 8 Reinforcing Bar - Sleeve Connection Process

[0081]

[0082] Table 9 Performance Test 1 of Reinforcing Bar - Sleeve Connectors

[0083]

[0084] Table 10 Performance Test of Reinforcing Bar - Sleeve Connectors

[0085]

[0086] It should be understood that although this specification is described according to the embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0087] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of this application, and they are not used to limit the protection scope of this application. Any equivalent embodiments or modifications made without departing from the technical spirit of this application should be included within the protection scope of this application.

Claims

1. A sleeve for mechanical connection of steel bars, characterized in that, The chemical composition of the sleeve, by mass percentage, includes: C: 0.50 - 0.60%, Si: 0.50 - 1.00%, Mn: 0.50 - 1.00%, Cr: 0.30 - 0.50%, N: 0.01 - 0.02%, at least one of V, Nb, and Ti, and the mass percentages of V, Nb, and Ti satisfy V: 0.10 - 0.30%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, and the balance is Fe and unavoidable impurities; The sleeve is made by processing internal threads on round steel obtained through the processes of converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling in sequence. Among them, In the continuous casting process, the cooling rate of the billet formed after continuous casting is ≤ 0.5 °C / s; In the heating process, the billet is heated at 1200 - 1300 °C; In the rolling process, the billet after heating starts finish rolling at 950 - 1050 °C, and round steel is obtained after finish rolling. The outlet temperature of finish rolling is 850 - 900 °C; In the cooling process, the temperature of the round steel on the cooling bed is 650 - 750 °C. After being on the cooling bed, the cooling rate of the round steel is controlled at 0.5 - 2 °C / s; The structure of the sleeve includes ferrite and pearlite. Among them, the volume fraction of pearlite is ≥ 90%, the pearlite lamellar spacing is 150 - 200 nm, and the average size of pearlite colonies is 5 - 10 μm; The tensile strength of the sleeve is ≥ 900 MPa, and the elongation after fracture is ≥ 20%.

2. The sleeve for mechanical connection of steel bars according to claim 1, wherein In the converter / electric furnace smelting process, the tapping temperature is 1610 - 1650 °C. Alloys and slag materials are added in the order of silicomanganese alloy, ferrosilicon alloy, high-carbon ferrochrome alloy, and lime. Among them, the addition amount of silicomanganese alloy is 8 - 16 kg / t, the silicon content in the silicomanganese alloy is 17 - 20%, the manganese content is 65 - 70%, and the balance is Fe and unavoidable impurities; the addition amount of ferrosilicon alloy is 5 - 10 kg / t, the silicon content in the ferrosilicon alloy is 70 - 75%, and the balance is Fe and unavoidable impurities; the addition amount of high-carbon ferrochrome alloy is 6 - 10 kg / t, the carbon content in the high-carbon ferrochrome alloy is 8 - 12%, the chromium content is 53 - 57%, and the balance is Fe and unavoidable impurities.

3. The sleeve for mechanical connection of steel bars according to claim 2, wherein, In the converter / electric furnace smelting process, the argon gas control valve at the bottom of the ladle is opened before tapping, and argon gas is blown throughout the tapping process. In the first 1 / 3 section before tapping, the argon gas flow rate is controlled at 800 - 1200 NL / min, and the gas pressure is 0.4 - 0.5 MPa. In the last 2 / 3 section after tapping, the argon gas flow rate is controlled at 350 - 650 NL / min, and the gas pressure is 0.3 - 0.4 MPa.

4. The sleeve for mechanical connection of steel bars according to claim 2, characterized in that, In the LF refining process, ferrosilicon nitride alloy is added when the ladle arrives at the station, and ferro-niobium alloy, vanadium nitride alloy, and ferro-titanium alloy are selectively added according to the aforementioned chemical composition of the round steel. The addition amount of ferrosilicon nitride alloy is 1 - 2 kg / t, the nitrogen content in ferrosilicon nitride alloy is 35 - 40%, the silicon content is 50 - 55%, and the rest is iron and inevitable impurities; the addition amount of ferro-niobium alloy is 0.24 - 1.2 kg / t, the niobium content in ferro-niobium alloy is 40 - 45%, and the rest is iron and inevitable impurities; the addition amount of vanadium nitride alloy is 2 - 6 kg / t, the vanadium content in vanadium nitride alloy is 75 - 80%, the nitrogen content is 5 - 10%, and the rest is iron and inevitable impurities; the addition amount of ferro-titanium alloy is 0.2 - 1 kg / t, the titanium content in ferro-titanium alloy is 40 - 45%, and the rest is iron and inevitable impurities.

5. The sleeve for mechanical connection of steel bars according to claim 4, characterized in that, In the LF refining process, the argon gas control valve at the bottom of the ladle is opened after the ladle arrives. Among them, the argon gas flow rate is controlled at 200 - 300 L / min during the waiting period, 300 - 400 L / min during the heating, deoxidizing, and desulfurizing period, 350 - 600 L / min during the alloying period, and 50 - 100 L / min during the soft stirring period, and the soft stirring time is ≥10 min.

6. The sleeve for mechanical connection of steel bars according to claim 1, characterized in that, In the continuous casting process, the superheat is controlled at 25 - 35 °C, and the casting speed is controlled at 2.3 - 2.6 m / min.

7. A connection method between a sleeve for mechanical connection of steel bars and a steel bar, characterized in that, It includes the following steps: Steel bar processing: External threads are processed at the ends of the steel bars by upsetting straight threads. When the diameter d of the steel bar is 10 - 25 mm, the upsetting pressure is 10 - 23 MPa, the upsetting base circle diameter is d + 4 - d + 6 mm, and the upsetting length is d + 2 mm; when the diameter d of the steel bar is 26 - 50 mm, the upsetting pressure is 30 - 55 MPa, the upsetting base circle diameter is d + 6.5 mm, and the upsetting length is d + 3 mm; the yield strength of the steel bar is ≥700 MPa, the tensile strength is ≥880 MPa, the strength ratio is ≥1.25, the elongation after fracture is ≥18%, and the total elongation at maximum force is ≥10%; Socket processing: Use the sleeve for mechanical connection of steel bars described in claim 1, and the internal threads of the sleeve match the external threads of the steel bars; Mechanical connection of steel bars and sleeves: Tighten the steel bars and sleeves by threads, and the number of thread turns of the steel bar not screwed into the sleeve is not more than 1.0 turn, and the torque is 450 - 550 N·m.

8. The connection method between the sleeve for mechanical connection of steel bars and the steel bars according to claim 7, characterized in that, The tooth profile of the external threads of the steel bars is triangular, and the tooth profile angle is 60 - 70°; when the nominal diameter of the steel bar is <25 mm, the pitch P is 2.5 mm, and when the nominal diameter of the steel bar is ≥25 mm, the pitch P is 3.0 mm.

Images