Production method of round steel for preparing sleeve, sleeve prepared from round steel and connecting method of sleeve and reinforcing steel bar

Abstract

The invention provides a production method of round steel, a sleeve and a connecting method of the sleeve and a steel bar. The round steel in the production method comprises the following chemical components in percentage by mass: 0.42 to 0.52 percent of C, 0.90 to 1.30 percent of Si, 1.30 to 1.60 percent of Mn, 0.025 to 0.050 percent of Nb, 0.01 to 0.02 percent of N and the balance of iron and inevitable impurities. According to the production method, the cooling speed of a casting blank formed after continuous casting is smaller than or equal to 1 DEG C / s, the casting blank is heated at the temperature of 1200-1250 DEG C, then finish rolling is started at the temperature of 800-850 DEG C to obtain round steel, the temperature of the round steel on a cooling bed is 700-800 DEG C, and the cooling speed of the round steel is 10 DEG C / s or below after the round steel is put on the cooling bed. According to the production method provided by the invention, the high-strength round steel is obtained through cooperation of high carbon content and rolling and cooling processes, and the high-strength round steel can be used for machining sleeves capable of being matched with high-strength steel bars.

Description

Technical Field

[0001] The present application relates to the technical field of steel smelting, and in particular to a method for producing round steel for preparing sleeves for mechanical connection of steel bars, sleeves prepared from the round steel, and a method for connecting the sleeves for mechanical connection of steel bars and steel bars. Background Art

[0002] High reinforcement of steel bars is a development trend. By adding alloy elements, the strength of steel bars can be improved, but it also deteriorates the welding performance of steel bars. Mechanical connection has become an important way to connect steel bars. However, with the increase in the strength of steel bars, the feasibility, stability and economy of mechanical connection of steel bars have become increasingly prominent. The tensile strength of the sleeve does not match the tensile strength of the steel bar. The tensile strength of the existing sleeve is lower than that of the steel bar. The sleeve of the steel bar sleeve connector breaks during the tensile test, which does not meet the relevant requirements; the tensile strength of the sleeve can be improved by alloying or heat treatment process, but the high cost and complex process limit its promotion. Summary of the invention

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

[0004] 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 a sleeve for mechanical connection of steel bars, wherein the chemical composition of the round steel comprises, 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; The production method comprises the following steps: converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling, wherein: In the continuous casting process, the cooling rate of the ingot formed after continuous casting is ≤1℃ / s; In the heating process, the ingot is heated at 1200~1250℃; In the rolling process, the heated ingot begins to be finished rolled at 800-850℃, and round steel is obtained after the finish rolling; 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.

[0005] In one embodiment of the present application, 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 to be 50~150L / min during the waiting period, the argon flow rate is controlled to be 150~350L / min during the heating deoxidation and desulfurization period, the argon flow rate is controlled to be 350~600L / min during the alloying period, the argon flow rate is controlled to be 30~80L / min during the soft stirring period, and the soft stirring time is ≥10min.

[0006] 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 amount of ferroniobium alloy added is 0.6~1.2kg / t, the niobium content in the ferroniobium alloy is 40~45%, and the rest is iron and inevitable impurities; the amount of ferrosilicon nitride alloy added is 1~2kg / 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.

[0007] In one embodiment of the present application, in the converter / electric furnace smelting process, the steel tapping temperature is 1585~1625°C, and alloys and slag are added in the order of silicon-manganese alloy, ferrosilicon alloy, and lime. Among them, the amount of silicon-manganese alloy added is 15~25kg / t, the silicon-manganese alloy has a silicon content of 17~20%, the manganese content is 65~70%, and the rest is iron and unavoidable impurities; the amount of ferrosilicon alloy added is 9~13kg / t, the silicon-content of ferrosilicon alloy is 70~75%, and the rest is iron and unavoidable impurities.

[0008] In one embodiment of the present application, 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 entire tapping process. The argon pressure is controlled at 0.4~0.5MPa in the 1 / 3 section before tapping, and the argon pressure is controlled at 0.3~0.4MPa in the 2 / 3 section after tapping.

[0009] In one embodiment of the present application, in the continuous casting process, the superheat is controlled to be 25-35° C. and the pulling speed is 2-2.5 m / min.

[0010] One embodiment of the present application also provides a sleeve for mechanical connection of steel bars, which is obtained by using the round steel obtained by the aforementioned production method of round steel for preparing the sleeve for mechanical connection of steel bars, and the round steel is processed into 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 unavoidable impurities; the structure of the sleeve includes ferrite, pearlite and bainite, wherein the volume proportion of ferrite is ≥30%, and the volume proportion of bainite is 15~30%.

[0011] In one embodiment of the present application, the tensile strength of the sleeve is ≥850 MPa, and the elongation after fracture is ≥25%.

[0012] An embodiment of the present application also provides a method for connecting a steel bar mechanical connection sleeve and a steel bar, comprising the following steps: Rebar processing: The end of the rebar is processed into an external thread by stripping the ribs and rolling the straight thread. The thread bottom is arc-shaped, and the arc radius R is 0.1~0.3mm; the yield strength of the rebar is ≥650MPa, the tensile strength is ≥820MPa, the strength-to-yield ratio is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%; Sleeve processing: the aforementioned sleeve for mechanical connection of steel bars is used, and the internal thread of the sleeve matches the external thread of the steel bar; Mechanical connection between steel bar and sleeve: Tighten the steel bar and sleeve through threads. The number of threads of the steel bar not screwed into the sleeve shall not exceed 1.5 threads, and the torque shall be 300~400N·m.

[0013] 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 is less than 25 mm, the pitch P is 2.5 mm, and when the nominal diameter of the steel bar d is greater than or equal to 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°.

[0014] 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.

[0015] One or more technical solutions provided by this application have at least the following technical effects or advantages: In the production method of round steel for preparing sleeves for mechanical connection of steel bars provided in the present application, a medium-carbon high-silicon niobium microalloying component system is designed. Through the controlled rolling and controlled cooling process, the strengthening effects such as solid solution strengthening, precipitation strengthening and phase transformation strengthening are fully exerted, the strength of the round steel is improved at a low cost, and the plasticity of the round steel is also guaranteed, so that the sleeve made of the round steel obtained by the production method can coordinate the mechanical properties of high-strength earthquake-resistant steel bars. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0017] 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; The production method includes the following steps: converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling, wherein: In the continuous casting process, the cooling rate of the ingot formed after continuous casting is ≤1℃ / s; In the heating process, the ingot is heated at 1200~1250℃; In the rolling process, the heated ingot begins to be finished rolled at 800-850℃, and round steel is obtained after the finish rolling; 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.

[0018] 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.

[0019] The effects of each chemical component have the following characteristics: 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.

[0020] 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%.

[0021] 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%.

[0022] 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%.

[0023] 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%.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Furthermore, in the LF refining process, ferroniobium alloy and ferrosilicon nitride alloy are added during the alloying period, and the addition amount of ferroniobium alloy and ferrosilicon nitride alloy is controlled according to the niobium content, nitrogen content and silicon content in the target component. The addition amount of ferroniobium alloy is 0.6~1.2kg / t, the niobium content in ferroniobium alloy is 40~45%, and the rest is iron and inevitable impurities; the addition amount of ferrosilicon nitride alloy is 1~2kg / 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 the addition amount varies according to the content of each alloying element in the alloy.

[0028] In one embodiment of the present application, in the converter / electric furnace smelting process, the tapping temperature is 1585~1625℃, and alloys and slag are added in the order of silicon-manganese alloy, ferrosilicon alloy, and lime. Among them, the amount of silicon-manganese alloy added is 15~25kg / t, the silicon content in the silicon-manganese alloy is 17~20%, the manganese content is 65~70%, and the rest is iron and inevitable impurities; the amount of ferrosilicon alloy added is 9~13kg / t, the silicon content in the ferrosilicon alloy is 70~75%, and the rest is iron and inevitable impurities. First, add the relatively cheap silicon-manganese alloy to make the manganese content in the molten steel reach the target content, and then supplement the silicon element through the ferrosilicon alloy. The silicon nitride ferroalloy in the subsequent LF refining process can also add a certain amount of silicon element.

[0029] 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 pressure is controlled at 0.4~0.5MPa in the 1 / 3 section before tapping, and 0.3~0.4MPa in the 2 / 3 section after tapping. A large amount of alloy needs to be added in the converter / electric furnace smelting process, and argon is blown at the bottom to provide soft stirring during tapping. A relatively large air pressure is used in the early stage, which can have a certain stirring effect on the alloy, and a relatively gentle pressure is required in the later stage.

[0030] In one embodiment of the present application, in the continuous casting process, the superheat is controlled to be 25~35°C and the casting speed is 2~2.5m / min. The water flow rate of the crystallizer is 1750~1850L / min, the electromagnetic stirring current of the crystallizer is 350A, and the electromagnetic stirring frequency is 3-7Hz; the terminal electromagnetic stirring current is 500A, and the terminal electromagnetic stirring frequency is 10-15Hz. The secondary cooling water volume is automatically distributed according to the parameters (L / m) in the following table:

[0031] 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 = casting speed × m; c and co are the value ranges of m, where co is the upper limit and c is the lower limit.

[0032] An embodiment of the present application also provides a sleeve for mechanical connection of steel bars, which is obtained by using the round steel obtained by the aforementioned production method of round steel for preparing the sleeve for mechanical connection of steel bars, and the round steel is processed into 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 unavoidable impurities; the structure of the sleeve includes ferrite, pearlite and bainite, wherein the volume proportion of ferrite is ≥30%, and the volume proportion of bainite is 15~30%.

[0033] The round steel obtained by the above-mentioned production method of round steel for preparing sleeves for mechanical connection of steel bars is cut and segmented into the required length of the sleeve, and its outer surface is processed to obtain a better surface quality. The center of the round steel is then hollowed out and the internal thread is processed to finally obtain the sleeve.

[0034] Since the sleeve is processed from the round steel obtained by the aforementioned production method, its organization and mechanical properties are the same as those of the round steel. The tensile strength of the sleeve is ≥850MPa, and the elongation after fracture is ≥25%.

[0035] The present application also provides a method for connecting a steel bar mechanical connection sleeve to a steel bar, comprising the following steps: Rebar processing: The end of the rebar is processed into an external thread by stripping the ribs and rolling the straight thread. The thread bottom is arc-shaped, and the arc radius R is 0.1~0.3mm; the yield strength of the rebar is ≥650MPa, the tensile strength is ≥820MPa, the strength-to-yield ratio is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%; Sleeve processing: the aforementioned sleeve for mechanical connection of steel bars is used, and the internal thread of the sleeve matches the external thread of the steel bar; Mechanical connection between steel bar and sleeve: Tighten the steel bar and sleeve through threads. The number of threads of the steel bar not screwed into the sleeve shall not exceed 1.5 threads, and the torque shall be 300~400N·m.

[0036] The steel bar with yield strength ≥650MPa and tensile strength ≥820MPa is used in combination with the sleeve with tensile strength ≥850MPa, which meets the matching requirements of steel bar and sleeve strength in the industry. The thread bottom of the external thread of the steel bar is processed into an arc shape to avoid stress concentration at the thread bottom during tension, which may cause cracks and brittle fracture at the root of the thread. The torque is controlled to avoid the steel bar being unable to be screwed into the sleeve or to damage the threaded connection between the sleeve and the steel bar surface.

[0037] Furthermore, 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 is less than 25 mm, the pitch P is 2.5 mm, and when the nominal diameter of the steel bar d is ≥ 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°.

[0038] Furthermore, the outer diameter of the sleeve is 30-60 mm, the length is 60-90 mm, and the thickness is 6-10 mm.

[0039] The technical solution of the present application is further described below in conjunction with some specific embodiments.

[0040] The present application provides sleeves of the following embodiments 1 to 6 and comparative examples 1 to 5, the chemical composition of the sleeves is shown in Table 1, and after converter / electric furnace smelting (parameters are shown in Table 2), LF refining (parameters are shown in Table 3), continuous casting (parameters are shown in Table 4), heating, rolling, and cooling processes (parameters are shown in Table 5) to obtain round steel having the organization and mechanical properties as shown in Table 6, the round steel is cut, surface processed, center hollowed out, and internally threaded to finally obtain a sleeve, and the steel bars in Table 7 are used, and the steel bars and sleeves are processed and connected according to the processing and connection process in Table 8, and the strength tests in Tables 9 and 10 are obtained with reference to JGJ107 "Technical Code for Mechanical Connection of Steel Bars".

[0041] Among them, the structures and properties of the sleeves of Examples 1 to 6 and Comparative Examples 1 to 5 obtained by referring to the standards GB / T13298 "Metallic Microstructure Examination Methods" and GB / T228.1 "Metallic Material Tensile Test Part 1 Room Temperature Test Methods" are shown in Table 6.

[0042] Table 1 Chemical composition of sleeve (wt%)

[0043] Table 2 Smelting process

[0044] Table 3 LF refining process

[0045] Table 4 Continuous casting process

[0046] Table 5 Heating, rolling and cooling processes

[0047] Table 6 Organization and mechanical properties of sleeve

[0048] Note: In the column of organizational form in Table 6, F stands for ferrite, P stands for pearlite, B stands for bainite, and M stands for martensite.

[0049] Table 7 Mechanical properties of steel bars connected to sleeves

[0050] Table 8 Rebar-sleeve connection process

[0051] Table 9 Performance test of steel bar-sleeve connector 1

[0052] Table 10 Performance test of steel bar-sleeve connector 2

[0053] It should be understood that although this specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description 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 mode may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0054] The series of detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present application. They are not intended to limit the scope of protection of the present application. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present application should be included in the scope of protection of the present application.

Claims

1. A method for producing 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 rest is Fe and unavoidable impurities; The production method comprises the following steps: converter / electric furnace smelting, LF refining, continuous casting, heating, rolling, and cooling, wherein: In the continuous casting process, the cooling rate of the ingot formed after continuous casting is ≤1℃ / s; In the heating process, the ingot is heated at 1200~1250℃; In the rolling process, the heated ingot begins to be finished rolled at 800-850℃, and round steel is obtained after the finish rolling; 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.

2. The method for producing round steel for preparing sleeves for mechanical connection of steel bars according to claim 1, characterized in that: In the LF refining process, after the ladle is in place, the argon blowing control valve at the bottom of the ladle is opened. The argon flow rate is controlled at 50~150L / min during the waiting period, 150~350L / min during the heating deoxidation and desulfurization period, 350~600L / min during the alloying period, and 30~80L / min during the soft stirring period. The soft stirring time is ≥10min.

3. The method for producing round steel for preparing sleeves 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 amount of ferroniobium alloy added is 0.6~1.2kg / t, the niobium content in the ferroniobium alloy is 40~45%, and the rest is iron and inevitable impurities; the amount of ferrosilicon nitride alloy added is 1~2kg / 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.

4. The method for producing round steel for preparing sleeves for mechanical connection of steel bars according to claim 2, characterized in that: In the converter / electric furnace smelting process, the steel tapping temperature is 1585~1625℃, and alloys and slag are added in the order of silicon-manganese alloy, ferrosilicon alloy and lime. Among them, the addition amount of silicon-manganese alloy is 15~25kg / t, the silicon content in the 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 9~13kg / t, the silicon content in the ferrosilicon alloy is 70~75%, and the rest is iron and inevitable impurities.

5. The method for producing round steel for preparing sleeves for mechanical connection of steel bars according to claim 4, characterized in that: 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 entire tapping process. The argon pressure is controlled at 0.4~0.5MPa in the 1 / 3 section before tapping, and at 0.3~0.4MPa in the 2 / 3 section after tapping.

6. The method for producing round steel for preparing sleeves for mechanical connection of steel bars according to claim 1, characterized in that: During the continuous casting process, the superheat is controlled at 25~35℃ and the casting speed is 2~2.5m / min.

7. A sleeve for mechanical connection of steel bars, characterized in that: The invention discloses a round steel obtained by the production method of round steel for preparing a sleeve for mechanical connection of steel bars according to claim 1, and the round steel is processed into an internal thread. 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 unavoidable impurities; the structure of the sleeve includes ferrite, pearlite and bainite, wherein the volume proportion of ferrite is ≥30%, and the volume proportion of bainite is 15-30%.

8. The sleeve for mechanical connection of steel bars according to claim 7, characterized in that: The tensile strength of the sleeve is ≥850MPa, and the elongation after fracture is ≥25%.

9. A method for connecting a sleeve for mechanical connection of steel bars to steel bars, characterized in that: The steps include: Rebar processing: The end of the rebar is processed into an external thread by stripping the ribs and rolling the straight thread. The thread bottom is arc-shaped, and the arc radius R is 0.1~0.3mm; the yield strength of the rebar is ≥650MPa, the tensile strength is ≥820MPa, the strength-to-yield ratio is ≥1.25, the elongation after fracture is ≥20%, and the total elongation at maximum force is ≥11%; Sleeve processing: using the sleeve for mechanical connection of steel bars as described in claim 8, and the internal thread of the sleeve matches the external thread of the steel bar; Mechanical connection between steel bar and sleeve: Tighten the steel bar and sleeve through threads. The number of threads of the steel bar not screwed into the sleeve shall not exceed 1.5 threads, and the torque shall be 300~400N·m.

10. The method for connecting a steel bar mechanical connection sleeve to a steel bar according to claim 9, 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 is less than 25 mm, the pitch P is 2.5 mm. When the nominal diameter of the steel bar d is ≥ 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°.

11. The method for connecting a steel bar mechanical connection sleeve to a steel bar according to claim 9, characterized in that: The outer diameter of the sleeve is 30~60mm, the length is 60~90mm, and the thickness is 6~10mm.

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