34crni mo6 steel for shield machine bolt and heat treatment process thereof

Abstract

The present application relates to a kind of heat treatment process of shield machine bolt steel 34CrNiMo6, specifically, steel is treated by sectional annealing+strengthening quenching+high temperature tempering, using sectional annealing steel preliminary organization as uniform ferrite+pearlite, Brinell hardness is 215HBW;After strengthening quenching+high temperature tempering, steel organization is uniform dispersion tempering sorbite, tensile strength 1100MPa, yield strength 990MPa, elongation after break 15%, area reduction 58%, impact energy 124J, can satisfy shield machine bolt performance requirement, with good strength and plasticity.The 34CrNiMo6 bolt produced by the heat treatment process can be stably applied in batch shield engineering, and has a positive role in promoting the high-quality development and construction of underground engineering in China.

Description

Technical Field

[0001] This invention relates to the heat treatment process of steel, specifically to a 34CrNiMo6 steel for tunnel boring machine bolts and its heat treatment process, belonging to the technical field of metallic materials. Background Technology

[0002] With the rapid development of my country's social economy, infrastructure construction has entered a stage of high-quality development. The construction of tunnel projects such as urban subways, highways, and high-speed railways all rely heavily on tunnel boring machines (TBMs), often referred to as the "king of engineering machinery." A TBM is a tunneling machine that uses the shield tunneling method, capable of excavating and cutting soil, transporting excavated material, and lining tunnels. Bolts, as crucial connecting elements in TBM construction, must ensure the secure connection of the tunnel segments, guaranteeing construction safety and quality. During construction, bolts must withstand high loads, strong vibrations, and humid environments, requiring bolt materials to possess high strength, high toughness, fatigue resistance, and high corrosion resistance.

[0003] Currently, the commonly used bolt material for tunnel boring machines (TBMs) in my country is 18Cr2Ni4WA. To reduce production costs and enhance their market competitiveness, downstream manufacturers plan to use 34CrNiMo6 steel to replace 18Cr2Ni4WA for TBM bolts. To ensure the requirements for TBM bolts, the raw material must have a Brinell hardness ≤248HBW, and the overall heat-treated mechanical properties must be tensile strength ≥1080MPa, yield strength ≥981MPa, elongation ≥14%, reduction of area ≥55%, and impact energy ku2 ≥80J. The plasticity of the steel after ordinary heat treatment does not meet the performance requirements for TBM bolts. Summary of the Invention

[0004] To address the aforementioned problems, this invention discloses a 34CrNiMo6 steel for tunnel boring machine bolts and its heat treatment process, the specific technical solution of which is as follows:

[0005] A heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts, the process flow is as follows: steel → segmented annealing in a bell-type furnace → strengthening quenching → high-temperature tempering, specifically:

[0006] Step (1) Segmented annealing: Heat the steel to 400℃ and hold for 2 hours, heat to 600℃ and hold for 2 hours, heat to 680℃ and heat evenly for 12-15 hours. The heating time is controlled according to the number of layers of steel loaded into the furnace. The temperature drop rate is no more than 30℃ / h. Cool the steel with the furnace to 550℃ and then air cool it.

[0007] Step (2) Strengthening quenching: Quenching heating temperature 880℃, quenching holding time 115min-125min, water cooling to room temperature;

[0008] Step (3) High-temperature tempering: Tempering temperature is 550℃, tempering holding time is 205min-215min, and water cooling is performed to room temperature.

[0009] Furthermore, the chemical composition and mass percentage of the steel are: C 0.30%-0.38%, Si 0.15%-0.40%, Mn 0.40%-0.80%, P≤0.020%, S≤0.020%, Cr 1.40%-1.70%, Ni 1.40%-1.70%, Mo 0.15%-0.30%, with the remainder being Fe and unavoidable impurities; the steel specification is φ60mm; the mechanical properties of the steel require a Brinell hardness ≤248HBW, and overall heat-treated mechanical properties of tensile strength ≥1080MPa, yield strength ≥981MPa, elongation ≥14%, reduction of area ≥55%, and impact energy ku2 ≥80J.

[0010] Furthermore, in step (1), the segmented annealing involves loading the steel into a bell-type bogie furnace, slowly heating it to 400°C at a heating rate not exceeding 80°C / h, holding it at that temperature for 2 hours, slowly heating it to 600°C at a heating rate not exceeding 100°C / h, holding it at that temperature for 2 hours, and then slowly heating it to 680°C at a heating rate not exceeding 60°C / h for uniform through-firing.

[0011] Furthermore, in step (1), the uniform temperature and thorough firing time for 1-2 layers of furnace loading is 12 hours, and the uniform temperature and thorough firing time increases by 1 hour for each additional layer of furnace loading, with a maximum of 5 layers of furnace loading.

[0012] Furthermore, the enhanced quenching involves loading the steel into a box-type resistance furnace, slowly heating it to 880°C at a heating rate not exceeding 60°C / h, holding it at that temperature, opening the furnace door, and quickly placing the steel into a heat treatment water bath for cooling, while stirring to ensure uniform quenching of the steel.

[0013] Furthermore, the high-temperature tempering involves loading the steel into a box-type resistance furnace, slowly heating it to 550°C at a rate not exceeding 60°C / h, holding it at that temperature, opening the furnace door, quickly placing the steel into a heat treatment water bath for cooling, and stirring to ensure the steel is fully tempered.

[0014] Furthermore, in the segmented annealing process, the furnace is cooled to 550°C with the furnace, the combustion medium is turned off, the furnace door of the hood-type trolley is opened, and the steel is hoisted to room temperature.

[0015] A heat treatment process for the aforementioned 34CrNiMo6 steel used for tunnel boring machine bolts.

[0016] Furthermore, 34CrNiMo6 steel has a Brinell hardness of 215 HBW, a metallographic structure of uniformly dispersed sorbite, and mechanical properties including tensile strength of 1100 MPa, yield strength of 990 MPa, elongation after fracture of 15%, reduction of area of ​​58%, and impact energy of 124 J. The hardness difference between the surface and the core is only 2-3 HBW.

[0017] The working principle of this invention is:

[0018] (1) Segmented annealing of the present invention: heating to 400℃ and holding for 2 hours to effectively preheat the steel, perform dehydrogenation treatment, reduce strength, and further reduce bolt crack sensitivity.

[0019] Heating to 600℃ and holding for 2 hours eliminates residual internal stress in the steel, avoids local overheating of the steel, creates favorable conditions for microstructure transformation, and can reduce deformation and cracking in subsequent bolt processing.

[0020] Heating to 680℃ and holding for 12-15 hours transforms the microstructure into pearlite + ferrite, reduces the carbide diffusion rate, further disperses and distributes the carbides evenly, reduces the Brinell hardness, and prepares the microstructure for the next quenching step.

[0021] In summary, segmented annealing can reduce the hydrogen content of steel, reduce the sensitivity of bolts to cracking, eliminate residual stress in steel, reduce deformation and cracking in subsequent bolt processing, reduce the carbide diffusion rate, further disperse and uniformly distribute carbides, and reduce Brinell hardness. After segmented annealing, the steel has a uniform microstructure and excellent properties, which prepares the microstructure for the next quenching step.

[0022] (2) Strengthening quenching: The heating temperature is mainly determined based on the phase transformation critical point of the steel. Hypoeutectoid steel is usually heated to 30℃~50℃ above Ac3. If the heating temperature is too low, austenitization will be insufficient, and if the temperature is too high, the austenite grains will coarsen, both of which are not conducive to obtaining the target microstructure and properties.

[0023] The holding time is 120 minutes. If the holding time is too short, the austenitization will be insufficient, and if it is too long, the austenite grains will coarsen, both of which are not conducive to obtaining the target microstructure and properties.

[0024] (3) High-temperature tempering: The heating temperature is 550℃. The matrix material of the shield machine bolt is used in a quenched and tempered state, which can ensure good impact toughness. Therefore, high-temperature tempering is adopted. If the tempering temperature is too low, the toughness of the steel is insufficient. If the tempering temperature is too high, the hardness after quenching and tempering will decrease. Neither of these can meet the target performance of the bolt.

[0025] The holding time is 210 minutes. If the holding time is too short, it will not be conducive to tissue transformation, and if the holding time is too long, the toughness will decrease.

[0026] The beneficial effects of this invention are:

[0027] This invention employs a reasonable heat treatment process to ensure the comprehensive performance of tunnel boring machine (TBM) bolts. The steel undergoes a segmented annealing + strengthening quenching + high-temperature tempering heat treatment. After heat treatment using this method, the Brinell hardness and mechanical properties of the steel meet the requirements for TBM bolts. The overall mechanical properties of the heat-treated bolts are: tensile strength ≥1080MPa, yield strength ≥981MPa, elongation ≥14%, reduction of area ≥55%, and impact energy ku2 ≥80J. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the segmented annealing process of the present invention;

[0029] Figure 2 This is a schematic diagram of the enhanced quenching + high-temperature tempering process of the present invention;

[0030] Figure 3 This is a metallographic diagram of the segmented annealing structure of the present invention;

[0031] Figure 4 This is a microstructure diagram of the enhanced quenching + high-temperature tempering process of this invention. Detailed Implementation

[0032] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] A heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts, the process flow is as follows: steel → segmented annealing in a bell-type furnace → strengthening quenching → high-temperature tempering, specifically:

[0034] Step (1) Segmented annealing: The steel is loaded into a bell-type bogie hearth furnace and slowly heated to 400℃ at a rate not exceeding 80℃ / h, held for 2 hours, then slowly heated to 600℃ at a rate not exceeding 100℃ / h, held for 2 hours, and then slowly heated to 680℃ at a rate not exceeding 60℃ / h for uniform heating for 12-15 hours. The uniform heating time is controlled according to the number of layers of steel loaded into the furnace (1-2 layers of steel require 12 hours of uniform heating time, and each additional layer increases the uniform heating time by 1 hour, with a maximum of 5 layers). The temperature drop rate is not greater than 30℃ / h. The steel is cooled in the furnace to 550℃ and then air-cooled.

[0035] The chemical composition and mass percentage of the steel used are: C 0.30%-0.38%, Si 0.15%-0.40%, Mn 0.40%-0.80%, P≤0.020%, S≤0.020%, Cr 1.40%-1.70%, Ni 1.40%-1.70%, Mo 0.15%-0.30%, with the remainder being Fe and unavoidable impurities; the steel specification is φ60mm; the mechanical properties of the steel are required to be Brinell hardness ≤248HBW, and the overall heat-treated mechanical properties are tensile strength ≥1080MPa, yield strength ≥981MPa, elongation ≥14%, reduction of area ≥55%, and impact energy ku2 ≥80J.

[0036] Step (2) Intensive quenching: Put the steel into a box-type resistance furnace and slowly heat it to 880℃ at a heating rate of no more than 60℃ / h. The quenching holding time is 115min-125min. Open the furnace door and quickly put the steel into the heat treatment water tank for cooling. Stir the steel to ensure that the quenching is uniform.

[0037] Step (3) High-temperature tempering: Put the steel into the box-type resistance furnace and slowly heat it to 550℃ at a heating rate of no more than 60℃ / h. The tempering holding time is 205min-215min. Open the furnace door and quickly put the steel into the heat treatment water tank for cooling. Stir the steel to ensure that it is fully tempered.

[0038] A specific embodiment of the present invention is given below.

[0039] A type of 34CrNiMo6 steel for tunnel boring machine bolts has the following chemical composition by mass percentage: 0.35% C, 0.26% Si, 0.60% Mn, 1.48% Cr, 1.46% Ni, 0.01% Cu, 0.20% Mo, 0.011% P, 0.006% S, with the remainder being Fe and other unavoidable impurities.

[0040] The heat treatment process used in this method is as follows: Figure 1 , Figure 2 As shown. Steel is loaded into the bell-type furnace, according to... Figure 1 The Brinell hardness and metallographic structure of the steel were tested after heat treatment. The Brinell hardness after segmented annealing is shown in Table 1, and the annealed microstructure is as follows: Figure 3 As shown, according to Figure 2 The mechanical properties after process-strengthened quenching and high-temperature tempering are shown in Table 2, and the metallographic structure is as follows: Figure 4As shown in the table, the segmented annealing of the steel results in a Brinell hardness of 215 HBW, with a uniform ferrite + pearlite microstructure and good hardness uniformity across the cross-section. The mechanical properties of the steel after strengthening quenching and high-temperature tempering are shown in Table 2, with a uniform tempered sorbite microstructure. The steel's strength, plasticity, and toughness all meet the performance requirements for tunnel boring machine bolts. In summary, the 34CrNiMo6 material produced using this heat treatment process fully meets the comprehensive performance requirements for tunnel boring machine bolts.

[0041] Table 1. Brinell hardness after heat treatment in the examples.

[0042]

[0043] Table 2 Mechanical properties of the embodiments after heat treatment

[0044]

[0045] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts, characterized in that, The process flow is as follows: steel → bell-type furnace segmented annealing → strengthening quenching → high-temperature tempering, specifically: Step (1) Segmented annealing: The steel is heated to 400℃ and held for 2 hours, then heated to 600℃ and held for 2 hours, then heated to 680℃ and heated evenly for 12-15 hours. The heating time is controlled according to the number of layers of steel loaded into the furnace. The temperature drop rate is no more than 30℃ / h. The steel is cooled to 550℃ with the furnace and then air-cooled. The segmented annealing process involves loading the steel into a bell-type bogie hearth furnace, slowly heating it to 400℃ at a rate not exceeding 80℃ / h, holding it at that temperature for 2 hours, slowly heating it to 600℃ at a rate not exceeding 100℃ / h, holding it at that temperature for 2 hours, and then slowly heating it to 680℃ at a rate not exceeding 60℃ / h for uniform through-firing. Step (2) Strengthening quenching: Quenching heating temperature 880℃, quenching holding time 115min-125min, water cooling to room temperature; The enhanced quenching process involves loading the steel into a box-type resistance furnace, slowly heating it to 880°C at a heating rate of no more than 60°C / h, holding it at that temperature, opening the furnace door, quickly placing the steel into a heat treatment water bath for cooling, and stirring to ensure uniform quenching of the steel. Step (3) High-temperature tempering: Tempering heating temperature 550℃, tempering holding time 205 min - 215 min, water cooling to room temperature; The high-temperature tempering involves loading the steel into a box-type resistance furnace, slowly heating it to 550°C at a rate not exceeding 60°C / h, holding it at that temperature, opening the furnace door, and quickly placing the steel into a heat treatment water bath for cooling, while stirring to ensure the steel is fully tempered.

2. The heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts according to claim 1, characterized in that, The chemical composition and mass percentage of the heat-treated steel are as follows: C 0.30%-0.38%, Si 0.15%-0.40%, Mn 0.40%-0.80%, P≤0.020%, S≤0.020%, Cr 1.40%-1.70%, Ni 1.40%-1.70%, Mo 0.15%-0.30%, with the remainder being Fe and unavoidable impurities. The specification of the heat-treated steel is φ60mm. The mechanical properties of the heat-treated steel are as follows: Brinell hardness ≤248HBW, tensile strength ≥1080MPa, yield strength ≥981MPa, elongation ≥14%, reduction of area ≥55%, and impact energy ku2 ≥80J.

3. The heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts according to claim 1, characterized in that, In step (1), the uniform temperature and thorough firing time for 1-2 layers of furnace loading is 12 hours. For each additional layer of furnace loading, the uniform temperature and thorough firing time increases by 1 hour, with a maximum of 5 layers of furnace loading.

4. The heat treatment process for 34CrNiMo6 steel used in tunnel boring machine bolts according to claim 1, characterized in that, The segmented annealing process involves air cooling after exiting the furnace to 550°C, then shutting off the combustion medium, opening the hood-type trolley furnace door, and hoisting the steel to room temperature.

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