Production method of round steel for preparing sleeves, sleeves prepared from the round steel, and connection method between sleeves and steel bars

Abstract

The present application provides a production method of round steel, a sleeve, and a connection method between the sleeve and a reinforcing bar. The chemical composition of the round steel in the production method, by mass percentage, includes: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the balance is iron and unavoidable impurities; in the production method, the cooling rate of the continuously cast billet formed is ≤ 1 °C / s, the billet is heated at 1200 - 1250 °C, and then finish rolling is started at 800 - 850 °C to obtain round steel, and the temperature of the round steel on the cooling bed is 700 - 800 °C. After being on the cooling bed, the cooling rate of the round steel is below 10 °C / s. The production method provided by the present application adopts a relatively high carbon content and the coordination of rolling and cooling processes to obtain high-strength round steel, which can be used to process sleeves that can match high-strength grade reinforcing bars.

Description

Technical Field

[0001] This application relates to the technical field of steel smelting, and in particular 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. By adding alloying elements, the strength of steel bars can be improved, but the welding performance of steel bars is deteriorated, and mechanical connection has become an important way of steel bar connection. However, with the increase of the strength of steel bars, problems such as the feasibility, stability and economy of mechanical connection of steel bars are becoming increasingly prominent. The tensile strength of the sleeve does not match that of the steel bar. The tensile strength of the existing sleeve is lower than that of the steel bar, and the sleeve breaks during the tensile test of the steel bar sleeve connector, which does not meet the relevant requirements; adopting alloying or heat treatment processes can improve the tensile strength of the sleeve, but the cost is high and the process is complex, which limits the popularization. Summary of the Invention

[0003] The purpose of this application is to provide a production method of round steel for preparing sleeves for mechanical connection of steel bars, which solves the problem that the strength of the sleeve in the prior art does not match that of the steel bar.

[0004] In order to achieve one of the above-mentioned invention purposes, an embodiment of this application provides a production method of round steel for preparing sleeves for mechanical connection of steel bars. The chemical composition of the round steel includes, by mass percentage: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the rest is Fe and inevitable impurities;

[0005] The production method includes the converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling processes carried out in sequence. Among them,

[0006] In the continuous casting process, the cooling rate of the cast slab formed after continuous casting is ≤1°C / s;

[0007] In the heating process, the cast slab is heated at 1200 - 1250°C;

[0008] In the rolling process, the heated cast slab starts finish rolling at 800 - 850°C, and round steel is obtained after finish rolling;

[0009] In the cooling process, the temperature of the round steel on the cooling bed is 700 - 800°C. After being on the cooling bed, the cooling rate of the round steel is controlled below 10°C / s.

[0010] 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 50 - 150 L / min, during the heating, deoxidizing, and desulfurizing period, the argon gas flow rate is controlled at 150 - 350 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 30 - 80 L / min, and the time of soft stirring is ≥ 10 min.

[0011] In one embodiment of the present application, in the LF refining process, ferroniobium alloy and ferrosilicon nitride alloy are added during the alloying period. The addition amount of ferroniobium alloy is 0.6 - 1.2 kg / t, the niobium content in the ferroniobium alloy is 40 - 45%, and the rest is iron and inevitable impurities; 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.

[0012] In one embodiment of the present application, in the converter / electric furnace smelting process, the tapping temperature is 1585 - 1625 °C, and alloys and slag materials are added in the order of silicomanganese alloy, ferrosilicon alloy, and lime during tapping. Among them, the addition amount of silicomanganese alloy is 15 - 25 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 9 - 13 kg / t, the silicon content in the ferrosilicon alloy is 70 - 75%, and the rest is iron and inevitable impurities.

[0013] 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 pressure is controlled at 0.4 - 0.5 MPa in the first 1 / 3 section before tapping, and the argon gas pressure is controlled at 0.3 - 0.4 MPa in the last 2 / 3 section after tapping.

[0014] In one embodiment of the present application, in the continuous casting process, the superheat is controlled at 25 - 35 °C, and the drawing speed is controlled at 2 - 2.5 m / min.

[0015] One 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 of round steel for preparing the sleeve for mechanical connection of steel bars, and the inner thread of the round steel is processed. The chemical composition of the sleeve includes, by mass percentage: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the rest is Fe and inevitable impurities; the structure of the sleeve includes ferrite, pearlite, and bainite. Among them, the volume fraction of ferrite is ≥ 30%, and the volume fraction of bainite is 15 - 30%.

[0016] In one embodiment of the present application, the tensile strength of the sleeve is ≥850 MPa, and the elongation after fracture is ≥25%.

[0017] 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:

[0018] Steel bar processing: The end of the steel bar is processed with external threads by the method of rib peeling and rolling straight threads. The bottom of the thread is arc-shaped, and the arc radius R is 0.1 - 0.3 mm; the yield strength of the steel bar is ≥650 MPa, the tensile strength is ≥820 MPa, the ratio of yield strength to tensile strength is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%;

[0019] 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;

[0020] Mechanical connection between the steel bar and the sleeve: Tighten the steel bar and the sleeve through threads. The number of thread turns of the steel bar that are not screwed into the sleeve does not exceed 1.5 turns, and the torque is 300 - 400 N·m.

[0021] In one embodiment of the present application, the nominal diameter of the thread M = the nominal diameter of the steel bar d + (0.5 - 1) mm. When the nominal diameter of the steel bar d < 25 mm, the pitch P is 2.5 mm. When the nominal diameter of the steel bar d ≥ 25 mm, the pitch P is 3.0 mm; the tooth profile of the external thread of the steel bar is triangular, and the tooth profile angle is 55 - 70°.

[0022] In one embodiment of the present application, the outer diameter of the sleeve is 30 - 60 mm, the length is 60 - 90 mm, and the thickness is 6 - 10 mm.

[0023] One or more technical solutions provided by the present application have at least the following technical effects or advantages:

[0024] In the production method of the round steel for preparing the sleeve for mechanical connection of steel bars provided by the present application, a medium-carbon high-silicon niobium microalloying composition system is designed. Through the controlled rolling and controlled cooling process, the full play of strengthening effects such as solid solution strengthening, precipitation strengthening, and phase transformation strengthening is realized, the strength of the round steel is improved at low cost, and at the same time, the plasticity of the round steel is ensured, so that the sleeve made of the round steel obtained by this production method can coordinate the mechanical properties of high-strength earthquake-resistant steel bars. Specific Embodiments

[0025] 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 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.

[0026] The embodiment of the present application provides a production method of round steel for preparing a sleeve for mechanical connection of steel bars, wherein the chemical composition of the round steel includes, by mass percentage: C: 0.42-0.52%, Si: 0.90-1.30%, Mn: 1.30-1.60%, Nb: 0.025-0.050%, N: 0.01-0.02%, and the rest is Fe and unavoidable impurities;

[0027] The production method includes the following steps: converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling, wherein:

[0028] In the continuous casting process, the cooling rate of the ingot formed after continuous casting is ≤1℃ / s;

[0029] In the heating process, the ingot is heated at 1200~1250℃;

[0030] In the rolling process, the heated ingot begins to be finished rolled at 800-850℃, and round steel is obtained after the finish rolling;

[0031] During the cooling process, the temperature of the round steel on the cooling bed is 700~800℃. After being placed on the cooling bed, the cooling rate of the round steel is controlled below 10℃ / s.

[0032] This application designs a medium-carbon high-silicon niobium microalloying composition system, and cooperates with the rolling process and cooling process design to give full play to the solid solution strengthening, precipitation strengthening and phase transformation strengthening of carbon elements, significantly improving the tensile strength of steel. At the same time, the role of silicon elements in promoting the formation of ferrite is used to adjust the volume ratio of the soft phase (ferrite) and hard phase (pearlite, bainite) of the steel, and the coordinated matching of strength and plasticity of the steel is achieved at a low cost.

[0033] The effects of each chemical component have the following characteristics:

[0034] Carbon (C): A strengthening element that can significantly improve the strength of steel, but too high a carbon content is detrimental to the plasticity of steel. In order to ensure that the steel has good comprehensive properties, the carbon content is controlled at 0.42~0.52% after comprehensive consideration.

[0035] Silicon (Si): a ferrite-forming element that promotes ferrite formation and helps regulate the problem of insufficient plasticity caused by high carbon content. Taking all factors into consideration, the silicon content is controlled to be 0.90~1.30%.

[0036] Manganese (Mn): Manganese is a commonly used solid solution strengthening element that can significantly improve the strength and toughness of steel. A high manganese content can ensure that the steel has a high seismic performance and coordinates with the mechanical properties of seismic 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. Taking all factors into consideration, the manganese content is controlled at 1.30~1.60%.

[0037] Niobium (Nb): Niobium improves the strength and toughness of steel through precipitation strengthening and fine grain strengthening; steel containing niobium can promote the formation of hard phase structure (lowering the transformation temperature of ferrite and increasing the transformation temperature of pearlite and bainite) through rolling process control, which is beneficial to adjust the strength-plasticity and seismic resistance (yield ratio) of steel; but 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 0.025~0.050%.

[0038] Nitrogen (N): a solid solution strengthening and precipitation strengthening element, combined with the alloying element niobium to form carbonitrides, which improves strength; however, too high a nitrogen content is detrimental to plasticity; too low a nitrogen content increases the difficulty of smelting, and the role of the strengthening element cannot be fully exerted. Taking all factors into consideration, the nitrogen content is controlled at 0.01~0.02%.

[0039] A higher carbon content is used to improve the strength of the steel. After continuous casting, the continuous casting billet is cooled at a lower rate to avoid segregation of carbon. A higher temperature is used in the heating process to fully dissolve carbon and niobium elements, improve the stability of austenite, and delay the phase transformation of pearlite and bainite. The rolling process and cooling process are controlled to regulate the hard phase and soft phase of the round steel, thereby achieving a coordinated combination of strength and plasticity of the round steel.

[0040] Furthermore, in the LF refining process, after the ladle is in place, the argon blowing control valve at the bottom of the ladle is opened, wherein the argon flow rate is controlled at 50~150L / min during the waiting period, the argon flow rate is controlled at 150~350L / min during the heating deoxidation and desulfurization period, the argon flow rate is controlled at 350~600L / min during the alloying period, and the argon flow rate is controlled at 30~80L / min during the soft stirring period. The soft stirring time is ≥10min.

[0041] During the LF refining process, the argon control valve is opened throughout the process for soft stirring to make the molten steel uniform. The waiting period is a short period of time from the arrival of the ladle to the deoxidation and desulfurization. At this time, there is no other action in the preparation stage, and the argon flow rate can be controlled at a small level; in the heating deoxidation and desulfurization period, a relatively large argon flow rate is used to ensure the deoxidation and desulfurization efficiency; in the alloying period after the addition of the alloy, a larger argon flow rate is required for stirring because the alloy needs to be melted and dispersed. After alloying, soft stirring can maintain the uniformity of the molten steel, and a longer time and a smaller argon flow rate are used.

[0042] Further, in the LF refining process, ferroniobium alloy and ferrosilicon nitride alloy are added during the alloying period. The addition amounts of ferroniobium alloy and ferrosilicon nitride alloy are controlled according to the niobium content, nitrogen content, and silicon content in the target components. The addition amount of ferroniobium alloy is 0.6 - 1.2 kg / t, and the niobium content in ferroniobium alloy is 40 - 45%, with the rest being iron and inevitable impurities; 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. Of course, other types of ferroniobium alloy and / or ferrosilicon nitride alloy can also be used, and their addition amounts vary according to the contents of various alloying elements in the alloy.

[0043] In an embodiment of the present application, in the converter / electric furnace smelting process, the tapping temperature is 1585 - 1625 °C. Alloys and slag materials are added in the order of silicomanganese alloy, ferrosilicon alloy, and lime during tapping. Among them, the addition amount of silicomanganese alloy is 15 - 25 kg / t, the silicon content in 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 9 - 13 kg / t, the silicon content in ferrosilicon alloy is 70 - 75%, and the rest is iron and inevitable impurities. First, the relatively inexpensive silicomanganese alloy is added to make the manganese content in the molten steel reach the target content, and then the silicon element is supplemented by ferrosilicon alloy. A certain amount of silicon element can also be added by ferrosilicon nitride alloy in the subsequent LF refining process.

[0044] Further, 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 pressure is controlled at 0.4 - 0.5 MPa in the first 1 / 3 section before tapping, and the argon gas pressure is controlled at 0.3 - 0.4 MPa in the last 2 / 3 section after tapping. A large amount of alloy needs to be added in the converter / electric furnace smelting process. Bottom blowing argon gas provides soft stirring during tapping. A relatively large gas pressure is used in the early stage, which can play a certain stirring role for the alloy, and a relatively gentle condition is needed in the later stage.

[0045] In an embodiment of the present application, in the continuous casting process, the superheat is controlled at 25 - 35 °C, and the drawing speed is 2 - 2.5 m / min. The water flow rate of the mold is 1750 - 1850 L / min, the electromagnetic stirring current of the mold is 350 A, and the electromagnetic stirring frequency is 3 - 7 Hz; the electromagnetic stirring current at the end is 500 A, and the electromagnetic stirring frequency at the end is 10 - 15 Hz. The secondary cooling water ratio is automatically distributed according to the following table parameters (L / m):

[0046]

[0047] Among them, m, c, and co are correction coefficients related to the steel grade, which are used to adjust the secondary cooling water ratio. The set value of the secondary cooling water ratio = drawing 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.

[0048] The embodiment of the present application also provides a sleeve for mechanical connection of steel bars. The sleeve is made of the round steel obtained by the aforementioned production method for the round steel used to prepare the sleeve for mechanical connection of steel bars, and the round steel is processed to form internal threads. The chemical composition of the sleeve includes, by mass percentage: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the rest is Fe and inevitable impurities; the structure of the sleeve includes ferrite, pearlite and bainite, wherein the volume fraction of ferrite is ≥30%, and the volume fraction of bainite is 15 - 30%.

[0049] The round steel obtained by the aforementioned production method for the round steel used to prepare the sleeve for mechanical connection of steel bars is cut into segments of the required length for the sleeve, and its outer surface is processed to obtain better surface quality. Then, the round steel is hollowed out at the center and processed with internal threads, and finally the sleeve is obtained.

[0050] Since the sleeve is processed from the round steel obtained by the aforementioned production method, its structure and mechanical properties are the same as those of the round steel. The tensile strength of the sleeve is ≥850 MPa, and the elongation after fracture is ≥25%.

[0051] 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:

[0052] Steel bar processing: The end of the steel bar is processed with external threads by the method of rib-removing and rolling straight threads. The bottom of the thread is arc-shaped, and the arc radius R is 0.1 - 0.3 mm; the yield strength of the steel bar is ≥650 MPa, the tensile strength is ≥820 MPa, the ratio of tensile strength to yield strength is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%.

[0053] 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;

[0054] Mechanical connection between the steel bar and the sleeve: The steel bar and the sleeve are tightened by threads. The number of thread turns of the steel bar not screwed into the sleeve does not exceed 1.5 turns, and the torque is 300 - 400 N·m.

[0055] Using steel bars with a yield strength ≥650 MPa and a tensile strength ≥820 MPa, and matching with the aforementioned sleeve with a tensile strength ≥850 MPa meets the industry's matching of the strengths of steel bars and sleeves. The bottom of the external thread of the steel bar is processed into an arc shape to avoid stress concentration at the bottom of the thread during tension, which may cause cracks and the risk of brittle fracture at the root of the thread. Controlling the torque size can avoid the situation where the steel bar cannot be screwed into the sleeve or damage the thread connection state of the sleeve and the steel bar surface.

[0056] Further, the nominal diameter of the thread M = the nominal diameter of the steel bar d + (0.5 - 1) mm. When the nominal diameter of the steel bar d < 25 mm, the pitch P is 2.5 mm. When the nominal diameter of the steel bar d ≥ 25 mm, the pitch P is 3.0 mm. The tooth profile of the external thread of the steel bar is triangular, and the tooth profile angle is 55 - 70°.

[0057] Further, the outer diameter of the sleeve is 30 - 60 mm, the length is 60 - 90 mm, and the thickness is 6 - 10 mm.

[0058] Next, in combination with some specific embodiments, the technical solutions of the present application will be further described.

[0059] The present application provides sleeves of Examples 1 - 6 and Comparative Examples 1 - 5 as follows. 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, center hollowed, and internally threaded processed. Finally, sleeves are obtained. Steel bars in Table 7 are used, and the steel bars and sleeves are processed and connected according to the processes in Table 8, and the strength tests in Tables 9 and 10 are obtained with reference to JGJ107 "Technical Specification for Mechanical Connection of Reinforced Bars".

[0060] Among them, for the sleeves of Examples 1 - 6 and Comparative Examples 1 - 5, the microstructures and properties of Examples 1 - 6 and Comparative Examples 1 - 5 obtained with reference to GB / T13298 "Test Methods for Metallographic Microstructure of Metals" and GB / T228.1 "Metallic Materials - Tensile Testing - Part 1: Method of Test at Room Temperature" are shown in Table 6.

[0061] Table 1 Chemical Composition of the Sleeve (wt%)

[0062]

[0063] Table 2 Smelting Process

[0064]

[0065] Table 3 LF Refining Process

[0066]

[0067] Table 4 Continuous Casting Process

[0068]

[0069] Table 5 Heating, Rolling, and Cooling Processes

[0070]

[0071] Table 6 Microstructure and Mechanical Properties of the Sleeve

[0072]

[0073] Note: In the column of microstructure morphology in Table 6, F represents ferrite, P represents pearlite, B represents bainite, and M represents martensite.

[0074] Table 7 Mechanical Properties of the Reinforcing Bars Connected to the Sleeve

[0075]

[0076] Table 8 Reinforcing Bar-Sleeve Connection Process

[0077]

[0078] Table 9 Performance Test 1 of Reinforcing Bar-Sleeve Connectors

[0079]

[0080] Table 10 Performance Test 2 of Reinforcing Bar-Sleeve Connectors

[0081]

[0082] 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.

[0083] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of this application, and they are not intended to limit the protection scope of this application. Any equivalent embodiments or changes made without departing from the technical spirit of this application should be included in the protection scope of this application.

Claims

1. A production method of round steel for preparing sleeves for mechanical connection of steel bars, characterized in that, The chemical composition of the round steel includes, by mass percentage: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the balance is Fe and inevitable impurities; The production method includes the following processes in sequence: converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling processes. Among them, In the continuous casting process, the cooling rate of the billet formed after continuous casting is ≤ 1 °C / s; In the heating process, the billet is heated at 1200 - 1250 °C; In the rolling process, the billet after heating starts finish rolling at 800 - 850 °C, and round steel is obtained after finish rolling; In the cooling process, the temperature of the round steel when it enters the cooling bed is 700 - 800 °C. After entering the cooling bed, the cooling rate of the round steel is controlled below 10 °C / s; The tensile strength of the round steel obtained by the above production method is ≥ 850 MPa, and the elongation after fracture is ≥ 25%; 2. The production method of the round steel for preparing the sleeve used for mechanical connection of steel bars according to claim 1, characterized in that, In the LF refining process, after the ladle arrives, the argon gas control valve at the bottom of the ladle is opened. Among them, the argon gas flow rate is controlled at 50 - 150 L / min during the waiting period, 150 - 350 L / min during the heating, deoxidizing, and desulfurizing period, 350 - 600 L / min during the alloying period, and 30 - 80 L / min during the soft stirring period, and the soft stirring time is ≥ 10 min.

3. The production method of the round steel for manufacturing the sleeve used for mechanical connection of steel bars according to claim 2, characterized in that, In the LF refining process, ferroniobium alloy and ferrosilicon nitride alloy are added during the alloying period. The addition amount of ferroniobium alloy is 0.6 - 1.2 kg / t, the niobium content in the ferroniobium alloy is 40 - 45%, and the balance is Fe and inevitable impurities; 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 balance is Fe and inevitable impurities.

4. The production method of the round steel for preparing the sleeve for mechanical connection of steel bars according to claim 2, characterized in that, In the converter / electric furnace smelting process, the tapping temperature is 1585 - 1625 °C. Alloys and slag materials are added in the order of silicomanganese alloy, ferrosilicon alloy, and lime during tapping. Among them, the addition amount of silicomanganese alloy is 15 - 25 kg / t, the silicon content in the silicomanganese alloy is 17 - 20%, the manganese content is 65 - 70%, and the balance is Fe and inevitable impurities; the addition amount of ferrosilicon alloy is 9 - 13 kg / t, the silicon content in the ferrosilicon alloy is 70 - 75%, and the balance is Fe and inevitable impurities.

5. The production method of the round steel for preparing the sleeve for mechanical connection of steel bars according to claim 4, characterized in that, 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 pressure is controlled at 0.4 - 0.5 MPa during the first 1 / 3 section before tapping, and 0.3 - 0.4 MPa during the last 2 / 3 section after tapping.

6. The production method of the round steel for preparing the sleeve used 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 2 - 2.5 m / min.

7. A sleeve for mechanical connection of steel bars, characterized in that, The sleeve is made of round steel obtained by using the production method of round steel for preparing the sleeve for mechanical connection of steel bars described in claim 1, and the inner thread of the round steel is processed. The chemical composition of the sleeve includes, by mass percentage: C: 0.42 - 0.52%, Si: 0.90 - 1.30%, Mn: 1.30 - 1.60%, Nb: 0.025 - 0.050%, N: 0.01 - 0.02%, and the rest is Fe and inevitable impurities; the structure of the sleeve includes ferrite, pearlite and bainite, wherein the volume fraction of ferrite is ≥30%, and the volume fraction of bainite is 15 - 30%; the tensile strength of the sleeve is ≥850 MPa, and the elongation after fracture is ≥25%.

8. A connection method between a sleeve for mechanical connection of steel bars and steel bars, characterized in that, It includes the following steps: Steel bar processing: The outer thread is processed at the end of the steel bar by the method of rib-removing and rolling straight thread, and the bottom of the thread is arc-shaped, with the arc radius R being 0.1 - 0.3 mm; the yield strength of the steel bar is ≥650 MPa, the tensile strength is ≥820 MPa, the strength ratio is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%; Sleeve processing: Use the sleeve for mechanical connection of steel bars described in claim 7, and the inner thread of the sleeve matches the outer thread of the steel bar; Mechanical connection of the steel bar and the sleeve: Tighten the steel bar and the sleeve by thread, and the number of thread turns of the steel bar not screwed into the sleeve does not exceed 1.5 turns, and the torque is 300 - 400 N·m.

9. The connection method between the sleeve for mechanical connection of steel bars and the steel bars according to claim 8, characterized in that, The nominal diameter of the thread M = the nominal diameter of the steel bar d + (0.5 - 1) mm. When the nominal diameter of the steel bar d < 25 mm, the pitch P is 2.5 mm. When the nominal diameter of the steel bar d ≥ 25 mm, the pitch P is 3.0 mm; the tooth profile of the outer thread of the steel bar is triangular, and the tooth profile angle is 55 - 70°.

10. The connection method between the sleeve for mechanical connection of steel bars and the steel bars according to claim 8, characterized in that, The outer diameter of the sleeve is 30 - 60 mm, the length is 60 - 90 mm, and the thickness is 6 - 10 mm.

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