Vanadium-niobium microalloyed steel for cutter head of shield tunneling machine and production method of vanadium-niobium microalloyed steel

Abstract

The invention relates to vanadium-niobium microalloyed steel for a shield tunneling machine cutterhead and a production method of the vanadium-niobium microalloyed steel, and belongs to the technical field of alloy steel preparation. The steel comprises the following chemical components in percentage by weight: 0.44%-0.49% of C, 1.4%-1.6% of Mo, 1.0%-1.4% of Si, 4.8%-5.2% of Cr, 0.4%-0.6% of Mn, 1.40%-1.70% of V + Nb and the balance of Fe and inevitable impurities. The method comprises the following steps: a, electric furnace smelting; b, refining outside a furnace; c, carrying out electroslag remelting to obtain a steel ingot; d, the steel ingot is subjected to element high-temperature uniform diffusion treatment; and e, combined modulation heat treatment. By optimizing alloy components, microelements niobium and vanadium are introduced to synergistically refine grains, and the high toughness index is shown after heat treatment. The problems that existing steel for the shield tunneling machine cutterhead is different in component, the heat treatment technology is complex, salt bath quenching and partitioning technologies are needed, and the technology for large-size cutters is difficult are solved.

Description

Technical Field

[0001] The invention relates to vanadium-niobium microalloyed steel for a shield machine cutter head and a production method thereof, belonging to the technical field of alloy steel preparation. Background Art

[0002] The construction of urban subways and high-speed railways cannot be separated from the construction of a large number of tunnels. As an important method of tunnel construction, shield machine excavation has gradually become popular due to its advantages such as fast excavation speed, good automation of construction process, safe and reliable operation, etc. Therefore, the huge scale of rail transit construction represented by subways and high-speed railways has put forward a huge market demand for shield machines and their cutterhead steel, and has put forward more stringent requirements on the performance of shield machine cutterhead steel.

[0003] The shield machine cutterhead and its tools are important functional components of the shield machine. A shield machine cutterhead is often equipped with hundreds of different types of tools. The roller cutter is the most core tool, and the tool cost accounts for about 30% of the entire excavation cost. When the shield machine breaks rock, the cutter ring not only bears a large radial rock breaking force, but also suffers from severe wear of the hard mineral phase of the rock. Once the tool is damaged, it will seriously affect the stress state of the shield machine excavation surface, accelerate the damage of other components, affect the operation of the shield machine and shield construction, and seriously affect the construction efficiency. Once the tool is found to be damaged, the entire shield machine will be shut down, exit the excavation working position, and replace all the tools. Therefore, it is very important to develop a steel material with high hardness, high strength and good impact toughness, so that the cutter ring is both wear-resistant during excavation and can avoid cracking or plastic deformation when subjected to huge impact.

[0004] At present, the medium alloy hot working die steel containing 5% Cr has very high hardenability, hardenability and red hardness, and can obtain higher wear resistance after heat treatment. Therefore, the current international manufacturing of shield machine cutterheads and their tools mostly adopts German standard 1.2345 (X50CrMoV5-1), which is equivalent to the high carbon version of 4Cr5MoSiV1 (H13). Its hardness, wear resistance and plasticity and toughness have reached a relatively high level. After heat treatment, it also shows high toughness indicators at a high hardness of 50-59HRC.

[0005] The steel grade of the steel for high-hardness shield cutters in the domestic patent CN112048668A and its manufacturing method includes the following alloying elements (mass percentage): C 0.40 - 0.60%, Si 0.80 - 1.20%, Mn 0.20 - 0.60%, Cr 4.00 - 6.00%, Mo 1.10 - 1.30%, V 0.45 - 1.0%, Ni 0.30%, Ti 0.10 - 0.60%, and the balance is Fe and inevitable impurity elements, where the content of impurity element S is ≤0.005% and the content of P is ≤0.020%. The steel for high-hardness shield cutters of the present invention is forged, and the carbon is partitioned into retained austenite by quenching and partitioning heat treatment process, and finally a duplex structure of stable martensite and retained austenite at room temperature is obtained, with a hardness of more than 55HRC, an impact energy at room temperature exceeding 28J, and a tensile strength of more than 1800MPa, having a good strength and toughness ratio. The heat treatment process of the invention is relatively complex and requires salt bath quenching and partitioning process. It is difficult to implement the above process for cutters with larger sizes. Summary of the Invention

[0006] The technical problem to be solved by the present invention is that the steel used for the cutter head of the existing shield machine has different components and a relatively complex heat treatment process, which requires salt bath quenching and partitioning process, and it is difficult to implement the process for cutters with larger sizes.

[0007] The technical solution adopted by the present invention to solve its technical problems is: a steel for the cutter head of a shield machine with vanadium and niobium microalloying, the chemical composition of which by weight percentage includes: C: 0.44 - 0.49%, Mo: 1.4 - 1.6%, Si: 1.0 - 1.4%, Cr: 4.8 - 5.2%, Mn: 0.4 - 0.6%, V + Nb: 1.40 - 1.70%, and the balance is Fe and inevitable impurities.

[0008] A production method of a steel for the cutter head of a shield machine with vanadium and niobium microalloying includes the following steps:

[0009] a. Electric furnace smelting, according to the above chemical composition ratio, use an electric arc furnace + LF furnace for smelting;

[0010] b. Secondary refining, through a VD furnace, make the oxygen content in the steel ≤15ppm and the N content ≤100ppm;

[0011] c. Electroslag remelting, after electrode bar casting → electrode bar annealing → electrode bar cleaning, carry out electroslag remelting smelting under a protective atmosphere to obtain an ingot;

[0012] d. Carry out high-temperature uniform diffusion treatment of elements on the ingot, using a box-type resistance furnace or a gas furnace, with the furnace temperature uniformity ≤ ±10°C;

[0013] e. Combined modulated heat treatment, using a box-type resistance furnace or a salt bath furnace, with the furnace temperature uniformity ≤ ±5°C.

[0014] Among them, in step a of the above method, pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium and ferrovanadium are added according to the components to obtain a ferroalloy raw material, and the above ferroalloy raw material accounts for 95% - 98% of the total raw material, the proportion of scrap steel is 2 - 5%, and in the ferroalloy raw material, Pb ≤ 0.01%, Sn ≤ 0.01%, As ≤ 0.01%, Sb ≤ 0.01% and Bi ≤ 0.01%.

[0015] Among them, in step a of the above method, the power is controlled between 14000 and 20000 KVA; in the melting stage, the V-Fe alloy is added in the later stage of melting, the addition temperature is 1580°C - 1600°C, and the stirring time is ≥ 10 minutes; the Nb-Fe alloy is added in the middle stage of melting, the addition temperature is 1550°C - 1570°C, and the stirring time is ≥ 8 minutes.

[0016] Among them, in step b of the above method, aluminum wire is used for deoxidation, and the addition amount is 0.02 - 0.03% to ensure that the oxygen content in the molten steel is ≤ 20 ppm; and the vacuum degree of the VD furnace is controlled below 50 Pa. After argon filling, the stirring time with argon is controlled between 35 and 45 minutes.

[0017] Among them, in step c of the above method, the melting rate is controlled between 5.5 and 7.0 kg / min, and the cooling water temperature is controlled between 30 and 50°C.

[0018] Among them, in step d of the above method, it is kept warm at a temperature of 1250 - 1270°C for 8 - 12 h to first ensure that the primary carbides in the ingot are fully dissolved back and ensure that the elements in the ingot are fully diffused; specifically, it can be determined according to the size of the ingot blank. For every 100 mm diameter, it is kept warm for 2 - 3 h, and the total holding time is ≥ 6 h. It is put into the furnace at room temperature, and the heating rate during heating with the furnace is ≤ 100°C / h. After forging, it is cooled in the furnace to 800 - 820°C, and the cooling rate is ≤ 50°C / h, and then air-cooled to room temperature.

[0019] Among them, in step e of the above method, the holding time in this temperature range is 1 - 10 h, and the holding time is determined according to the thickness of the workpiece. Usually, for every 25 - 30 mm thickness, it is kept warm for 1 - 1.5 h, and the heating rate is ≤ 100°C / h.

[0020] Furthermore, in step e of the above method, rapid quenching oil is used for oil quenching, and it is oil-cooled to room temperature of 20 - 30°C, and during oil cooling, a stirring or circulation system is adopted to ensure that the cooling rates of all parts of the workpiece are consistent.

[0021] Further, in step e of the above method, a tempering furnace is used for tempering. The temperature uniformity of the furnace chamber is ≤ ±5°C. A fan or natural convection is adopted, and tempering is carried out three times within the temperature range of 560 - 610°C, and the holding time is 2 - 3h. The holding time can be determined according to the thickness of the workpiece, usually 1 - 1.5h for every 25 - 30mm thickness; air-cooled to room temperature, and the cooling rate is ≤ 10°C / min.

[0022] The beneficial effects of the present invention are as follows: By optimizing the alloy composition, the present method innovatively proposes to use trace elements niobium and vanadium synergistically to refine the grains, and develops a steel material for shield machine cutter heads with high hardness, high strength and good impact toughness in terms of performance. After the heat treatment system, the steel prepared by this method has good impact toughness (20J - 25J) at a high hardness of 55 - 59HRC. The material exhibits good high hardness, high strength and impact toughness properties, effectively reducing the occurrence of cracking caused by impact or tool replacement due to plastic deformation, prolonging the service life and ensuring the construction efficiency. Specific embodiments

[0023] The present invention will be further described below in conjunction with embodiments.

[0024] The technical solution of the present invention is as follows: A steel for shield machine cutter head with vanadium and niobium microalloying, the chemical composition of which by weight percentage includes: C: 0.44 - 0.49%, Mo: 1.4 - 1.6%, Si: 1.0 - 1.4%, Cr: 4.8 - 5.2%, Mn: 0.4 - 0.6%, V+Nb: 1.40 - 1.70%, and the balance is Fe and inevitable impurities. Those skilled in the art can understand that C in this steel grade is the most important element to increase the hardness in the steel for shield machine cutter head. Controlling the C content higher than 0.44% is the key to ensuring its hardness. During the tunneling process of the shield machine, the cutter head needs to withstand large thrust and torque. The high strength of high-carbon steel can ensure that the cutter head does not deform or damage under these external forces. Although carbon elements can improve the hardness and strength of the steel, too high carbon content will also lead to a decrease in the toughness of the steel. Therefore, the preferred C content is 0.44 - 0.49%. Si can improve the wear resistance of the steel. Silicon can inhibit the movement of grain boundaries in the steel, making it have better anti-deformation ability, thus improving the wear resistance of the steel to a certain extent. During the tunneling process of the shield machine, the cutter head may encounter a high-temperature environment. This property of silicon helps to maintain the performance of the cutter head, but when its content is too high, it will reduce the plasticity and toughness of the steel. Therefore, the preferred Si content is 1.0 - 1.4%. Mn can improve the hardenability and strength of the steel, but too high Mn content will sharply reduce the Ms point, resulting in too much retained austenite content. Therefore, the preferred Mn content is 0.4 - 0.6%. Cr can improve the strength of the steel through solid solution strengthening and precipitation strengthening. Chromium can significantly improve the hardenability of the steel, making it easier to obtain a uniform hardness distribution during the heat treatment process of the steel. However, too high Cr content is likely to precipitate large-sized primary carbides, resulting in a significant decrease in impact toughness. Mo can form secondary carbides to improve the wear resistance of the steel, but Mo is expensive. Therefore, the preferred Cr content is 4.8 - 5.2%, and the Mo content is 1.4 - 1.6%. Adding V and Nb is to refine the grains and precipitate secondary carbides to further improve the wear resistance and hardness of the steel, and precipitate a certain amount of primary carbides. During the forging process, pinning the grain boundaries to hinder the movement of dislocations, thereby refining the grains and increasing the dislocation density. Therefore, the V+Nb content is preferably 1.40 - 1.70%.

[0025] A production method of a steel for shield machine cutter head with vanadium and niobium microalloying, comprising the following steps:

[0026] a. Electric furnace smelting, according to the above chemical composition ratio, using an electric arc furnace + LF furnace for smelting;

[0027] b. Secondary refining, making the oxygen content in the steel ≤ 15 ppm and the N content ≤ 100 ppm through a VD furnace;

[0028] c. Electro-slag remelting: After the electrode rods are cast, annealed, and cleaned, ingots are produced through electro-slag remelting under a protective atmosphere.

[0029] d. Subject the ingots to high-temperature uniform diffusion treatment of elements. Use a box-type resistance furnace or a gas furnace, with the temperature uniformity in the furnace chamber ≤ ±10°C.

[0030] e. Combined modulated heat treatment. Use a box-type resistance furnace or a salt bath furnace, with the temperature uniformity in the furnace chamber ≤ ±5°C. Those skilled in the art can understand that this method mainly includes smelting by the electric arc furnace + LF furnace + VD furnace + electro-slag remelting method, high-temperature homogenization treatment, and combined modulated heat treatment. Specifically, it is preferably that in step b, the oxygen content in the steel ≤ 15 ppm and the N content ≤ 100 ppm through the VD furnace. In step d, it is preferably to use a box-type resistance furnace or a gas furnace, with the temperature uniformity in the furnace chamber ≤ ±10°C, that is, the temperature change range in the furnace chamber ≤ ±10°C, to ensure high-temperature uniform diffusion of elements. In step e, it is preferably to use a box-type resistance furnace or a salt bath furnace, with the temperature uniformity in the furnace chamber ≤ ±5°C, to ensure the quality of heat treatment.

[0031] Preferably, in step a of the above method, pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium, and ferrovanadium are added according to the composition to obtain the ferroalloy raw materials, and the above ferroalloy raw materials account for 95% - 98% of the total raw materials, and the scrap steel accounts for 2 - 5%. And in the ferroalloy raw materials, Pb ≤ 0.01%, Sn ≤ 0.01%, As ≤ 0.01%, Sb ≤ 0.01%, and Bi ≤ 0.01%. Those skilled in the art can understand that this method actually preferably uses a better batching (adding ferroalloy raw materials such as pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium, and ferrovanadium according to the composition, and the above ferroalloy raw materials account for 95% - 98% of the total raw materials), reduces the incorporation of ordinary scrap steel (the scrap steel accounts for 2 - 5%), meets the five-hazard control requirements (lead (Pb) ≤ 0.01%, tin (Sn) ≤ 0.01%, arsenic (As) ≤ 0.01%, antimony (Sb) ≤ 0.01%, and bismuth (Bi) ≤ 0.01%), and ensures the quality of raw materials.

[0032] Preferably, in step a of the above method, the power is controlled between 14000 - 20000 KVA; in the melting stage, the V-Fe alloy is added in the later stage of melting, the addition temperature is 1580°C - 1600°C, and the stirring time ≥ 10 minutes; the Nb-Fe alloy is added in the middle stage of melting, the addition temperature is 1550°C - 1570°C, and the stirring time ≥ 8 minutes. Those skilled in the art can understand that by controlling the power between 14000 - 20000 KVA, a suitable melting rate is ensured to ensure the smooth progress of the melting process. In the melting stage, for the V-Fe alloy: it is added in the later stage of melting, the addition temperature is 1580°C - 1600°C, and the stirring time ≥ 10 minutes; for the Nb-Fe alloy: it is added in the middle stage of melting, the addition temperature is 1550°C - 1570°C, and the stirring time ≥ 8 minutes.

[0033] Preferably, in step b of the above method, aluminum wire is used for deoxidation, and the addition amount is 0.02-0.03%, ensuring that the oxygen content in the molten steel is ≤20 ppm; and the vacuum degree of the VD furnace is controlled below 50 Pa. After argon filling, the stirring time with argon is controlled within 35-45 minutes. Those skilled in the art can understand that in this method, aluminum wire is used for deoxidation, and the addition amount is 0.02-0.03%, ensuring that the oxygen content in the molten steel is ≤20 ppm. The vacuum degree of the VD furnace is controlled below 50 Pa. After argon filling, the stirring time with argon is controlled within 35-45 minutes. After VD degassing, the oxygen content in the steel is measured to be ≤15 ppm, and the N content is ≤100 ppm.

[0034] Preferably, in step c of the above method, the melting rate should be controlled at 5.5-7.0 kg / min, and the cooling water temperature should be controlled at 30-50 °C. Those skilled in the art can understand that in this method, electroslag remelting is carried out under a protective atmosphere. To ensure a suitable droplet size, the melting rate should be controlled at 5.5-7.0 kg / min, and the cooling water temperature should be controlled at 30-50 °C to ensure an appropriate cooling rate and control the segregation of the ingot.

[0035] Preferably, in step d of the above method, it is held at a temperature of 1250-1270 °C for 8-12 h. First, ensure that the primary carbides in the ingot are fully redissolved, and ensure that the elements in the ingot are fully diffused; specifically, it can be determined according to the ingot size. For every 100 mm diameter, it is held for 2-3 h, and the total holding time ≥6 h. It is placed in the furnace at room temperature, and the heating rate during heating with the furnace is ≤100 °C / h. After forging, it is furnace-cooled to 800-820 °C, and the cooling rate is ≤50 °C / h, and then air-cooled to room temperature. Those skilled in the art can understand that since this steel grade is to achieve excellent toughness, more V and Nb microalloying elements are added. Among them, the diffusion of Nb element is difficult and prone to segregation, and V element is easy to form primary carbides, which affects the mechanical properties. Therefore, special treatment is required. It is held at a temperature of 1250-1270 °C for 8-12 h. First, ensure that the primary carbides in the ingot are fully redissolved, and then ensure that the elements in the ingot are fully diffused. It is determined according to the ingot size, and for every 100 mm diameter, it is held for 2-3 h. Since the ingot is generally cylindrical, this is the diameter of the cylinder, and the total holding time ≥6 h. It is placed in the furnace at room temperature, and the heating rate during heating with the furnace is ≤100 °C / h to avoid cracking caused by thermal stress. After homogenization, after forging, it is furnace-cooled to 800-820 °C, and the cooling rate is ≤50 °C / h; then air-cooled to room temperature.

[0036] Preferably, in step e of the above method, the holding time in this temperature range is 1 to 10 h, and the holding time is determined according to the thickness of the workpiece. Generally, for every 25 to 30 mm thickness, the holding time is 1 to 1.5 h, and the heating rate ≤ 100 °C / h. Those skilled in the art can understand that the preferred heating temperature is 1050 to 1080 °C to ensure the full solution of alloying elements; the holding time in this temperature range is 1 to 10 h, and the holding time is determined according to the thickness of the workpiece. Generally, for every 25 to 30 mm thickness, the holding time is 1 to 1.5 h; the heating rate is ≤ 100 °C / h to avoid deformation or cracking caused by thermal stress.

[0037] Preferably, in step e of the above method, rapid quenching oil is used for oil quenching, and it is oil-cooled to room temperature of 20 to 30 °C, and a stirring or circulation system is adopted during oil cooling to ensure that the cooling rate of each part of the workpiece is consistent. Those skilled in the art can understand that using rapid quenching oil for oil quenching and oil-cooling to room temperature (20 to 30 °C) can ensure that the structure is completely transformed into martensite, obtaining high hardness and high strength; adopting a stirring or circulation system during oil cooling can ensure that the cooling rate of each part of the workpiece is consistent, obtaining a uniform and fine martensite structure.

[0038] Preferably, in step e of the above method, a tempering furnace is used for tempering, the temperature uniformity of the furnace chamber ≤ ±5 °C, a fan or natural convection is adopted, and tempering is carried out three times in the temperature range of 560 to 610 °C, and the holding time is 2 to 3 h. The holding time can be determined according to the thickness of the workpiece. Generally, for every 25 to 30 mm thickness, the holding time is 1 to 1.5 h; air-cooled to room temperature, and the cooling rate ≤ 10 °C / min. Those skilled in the art can understand that when tempering, a tempering furnace is used, the temperature uniformity of the furnace chamber ≤ ±5 °C, a fan or natural convection is adopted to ensure that the cooling rate of each part of the workpiece is consistent. Tempering is carried out three times in the temperature range of 560 to 610 °C to promote the precipitation of secondary carbides, further improve the hardness, and ensure the full tempering of martensite. The holding time is 2 to 3 h, and the holding time is determined according to the thickness of the workpiece. Generally, for every 25 to 30 mm thickness, the holding time is 1 to 1.5 h; air-cooled to room temperature, the cooling rate ≤ 10 °C / min; through the combination of quenching and tempering, the best balance of strength, toughness and wear resistance is achieved. At a high hardness of 55 to 59 HRC, it has good impact toughness (20 J to 25 J), and has good toughness while having high hardness. It is applicable to the cutter head of a shield machine under high load and complex geological conditions, and extends the service life.

[0039] Example 1

[0040] Step a, electric furnace smelting. According to the described chemical composition ratio, use an electric arc furnace + LF furnace for smelting. The chemical components are C: 0.49%, Mo: 1.48%, Si: 1.0%, Cr: 4.9%, Mn: 0.4%, V+Nb: 1.40%, and the balance is Fe and unavoidable impurities. By using better batching (adding pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium, ferrovanadium and other ferroalloy raw materials according to the composition, and the above ferroalloy raw materials account for 97% of the total raw materials), reduce the input of ordinary scrap steel (the proportion of scrap steel is 3%) to meet the requirements of controlling five harmful substances (Pb = 0.008%, Sn = 0.008%, As = 0.005%, Sb = 0.007%, and Bi = 0.009%). Ensure a suitable melting rate by controlling the power between 15800 KVA to ensure the smooth progress of the melting process. In the smelting stage, V-Fe alloy: added in the later stage of smelting, the adding temperature is 1590 °C, and the stirring time is 15 minutes; Nb-Fe alloy: added in the middle stage of smelting, the adding temperature is 1565 °C, and the stirring time is 10 minutes.

[0041] Step b, secondary refining. Use aluminum wire for deoxidation, and the addition amount is 0.028% to ensure that the oxygen content in the molten steel is 18 ppm. Control the vacuum degree of the VD furnace at 45 Pa. After argon charging, control the stirring time with argon within 38 minutes. After VD degassing, measure the oxygen content in the steel to be 13 ppm and the N content to be 98 ppm.

[0042] Step c, electroslag remelting. After electrode bar casting → electrode bar annealing → electrode bar cleaning, carry out electroslag remelting under a protective atmosphere. Control the melting rate at 6.0 kg / min, and the cooling water temperature is 45 °C.

[0043] Step d, perform high-temperature uniform diffusion treatment of elements on the ingot. Use a box-type resistance furnace or a gas furnace, and the temperature uniformity in the furnace chamber is ≤ ±8 °C. Insulate at 1255 °C for 10 h, cool the furnace to 800 °C, and the cooling rate is 45 °C / h; then air-cool to room temperature.

[0044] Step e, combined modulation heat treatment. Use a box-type resistance furnace or a salt bath furnace, and the temperature uniformity in the furnace chamber is ≤ ±5 °C. The heating temperature is 1060 °C; the holding time in this temperature range: 2 h; the heating rate: 80 °C / h to avoid deformation or cracking caused by thermal stress. Use fast quenching oil for oil quenching and cool to room temperature (25 °C). When tempering, use a tempering furnace, and the temperature uniformity in the furnace chamber is ≤ ±5 °C. Use a fan for convection to ensure that the cooling rate of each part of the workpiece is consistent. Temper three times in the temperature range of 560 °C, the holding time is 2 h, air-cool to room temperature, the hardness is 56 HRC, and the impact toughness is 25 J, having high hardness and good toughness at the same time.

[0045] Example 2

[0046] Step a, electric furnace smelting. According to the chemical composition ratio, use an electric arc furnace + LF furnace for smelting. The chemical components are C: 0.46%, Mo: 1.50%, Si: 1.10%, Cr: 5.10%, Mn: 0.55%, V+Nb: 1.66%, and the balance is Fe and inevitable impurities. By using better batching (adding pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium and ferrovanadium and other ferroalloy raw materials according to the composition, and the above ferroalloy raw materials account for 96% of the total raw materials), reduce the inclusion of ordinary scrap steel (the proportion of scrap steel is 4%) to meet the requirements of five-harm control (lead (Pb) = 0.009%, tin (Sn) = 0.007%, arsenic (As) = 0.006%, antimony (Sb) = 0.006% and bismuth (Bi) = 0.008%). Ensure a suitable melting rate by controlling the power between 17200 KVA to ensure the smooth progress of the melting process. In the smelting stage, V-Fe alloy: add it in the later stage of smelting, the addition temperature is 1585 °C, and the stirring time is 16 minutes; Nb-Fe alloy: add it in the middle stage of smelting, the addition temperature is 1560 °C, and the stirring time is 9 minutes.

[0047] Step b, secondary refining. Use aluminum wire for deoxidation, and the addition amount is 0.025% to ensure that the oxygen content in the molten steel is 16 ppm. Control the vacuum degree of the VD furnace at 47 Pa. After argon charging, control the stirring time of argon within 40 minutes. After VD degassing, measure the oxygen content in the steel to be 12 ppm and the N content to be 95 ppm.

[0048] Step c, electroslag remelting. After electrode bar casting → electrode bar annealing → electrode bar cleaning, carry out electroslag remelting under a protective atmosphere. Control the melting rate at 6.2 kg / min, and the cooling water temperature is 40 °C.

[0049] Step d, perform high-temperature uniform diffusion treatment of elements on the ingot. Use a box-type resistance furnace or a gas furnace, and the temperature uniformity in the furnace chamber is ≤ ±9 °C. Keep it at 1260 °C for 9.5 h, furnace cool to 810 °C, and the cooling rate is 40 °C / h; then air cool to room temperature.

[0050] Step e, combined modulation heat treatment. Use a box-type resistance furnace or a salt bath furnace, and the temperature uniformity in the furnace chamber is ≤ ±4 °C. The heating temperature is 1070 °C; the holding time in this temperature range: 1.5 h; the heating rate: 70 °C / h to avoid deformation or cracking caused by thermal stress. Use fast quenching oil for oil quenching and cool to room temperature (24 °C). When tempering, use a tempering furnace, and the temperature uniformity in the furnace chamber is ≤ ±4 °C. Use a fan for convection to ensure that the cooling rate of each part of the workpiece is consistent. Temper three times in the temperature range of 580 °C, the holding time is 2.5 h, air cool to room temperature, the hardness is 58 HRC, and the impact toughness is 22 J, having both high hardness and good toughness.

[0051] Example 3

[0052] Step a, electric furnace smelting. According to the chemical composition ratio, use an electric arc furnace + LF furnace for smelting. The chemical components are C: 0.44%, Mo: 1.52%, Si: 1.35%, Cr: 5.20%, Mn: 0.60%, V+Nb: 1.70%, and the balance is Fe and inevitable impurities. By using better batching (adding ferroalloy raw materials such as pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium, and ferrovanadium according to the composition, and the above ferroalloy raw materials account for 98% of the total raw materials), reducing the input of ordinary scrap steel (the proportion of scrap steel is 2%), meeting the requirements of controlling five harmful substances (lead (Pb) = 0.075%, tin (Sn) = 0.009%, arsenic (As) = 0.004%, antimony (Sb) = 0.0065%, and bismuth (Bi) = 0.008%). By controlling the power between 18500 KVA, ensure a suitable melting rate to ensure the smooth progress of the melting process. In the smelting stage, V-Fe alloy: added in the later stage of smelting, the addition temperature is 1600 °C, and the stirring time is 13 minutes; Nb-Fe alloy: added in the middle stage of smelting, the addition temperature is 1570 °C, and the stirring time is 8 minutes.

[0053] Step b, secondary refining. Use aluminum wire for deoxidation, and the addition amount is 0.03% to ensure that the oxygen content of the molten steel is 20 ppm. Control the vacuum degree of the VD furnace at 49 Pa. After argon charging, control the argon stirring time within 44 minutes. After VD degassing, measure the oxygen content in the steel to be 10 ppm and the N content to be 99 ppm.

[0054] Step c, electroslag remelting. After electrode bar casting → electrode bar annealing → electrode bar cleaning, carry out electroslag remelting under a protective atmosphere. Control the melting rate at 6.5 kg / min, and the cooling water temperature is 37 °C.

[0055] Step d, perform high-temperature uniform diffusion treatment of elements on the ingot. Use a box-type resistance furnace or a gas furnace, and the temperature uniformity of the furnace chamber is ≤ ±7 °C. Keep warm at 1265 °C for 8.5 h, cool the furnace to 815 °C, and the cooling rate is 38 °C / h; then air-cool to room temperature.

[0056] Step e, combined modulation heat treatment. Use a box-type resistance furnace or a salt bath furnace, and the temperature uniformity of the furnace chamber is ≤ ±4.5 °C. The heating temperature is 1075 °C; the holding time in this temperature range: 1.2 h; the heating rate: 75 °C / h to avoid deformation or cracking caused by thermal stress. Use fast quenching oil for oil quenching and cool to room temperature (22 °C). When tempering, use a tempering furnace, and the temperature uniformity of the furnace chamber is ≤ ±4.5 °C. Use a fan for convection to ensure that the cooling rate of each part of the workpiece is consistent. Carry out tempering three times in the temperature range of 600 °C, the holding time is 2.8 h, air-cool to room temperature, the hardness is 57 HRC, and the impact toughness is 24 J, having high hardness and good toughness at the same time.

[0057] Comparative Example 1

[0058] Step a, electric furnace smelting. According to the chemical composition ratio, an electric arc furnace + LF furnace is used for smelting. The chemical components are C: 0.55%, Mo: 0.9%, Si: 0.8%, Cr: 5.00%, Mn: 0.80%, V: 0.80%, and the balance is Fe and inevitable impurities. By using better batching (adding pig iron, ferronickel, ferrochrome, ferromanganese, ferrovanadium and other ferroalloy raw materials according to the composition, and the above ferroalloy raw materials account for 50% of the total raw materials), the proportion of scrap steel is 50%. The melting process is the same as that in Example 1. Without Nb

[0059] Step b, secondary refining. Deoxidize with aluminum wire, and the addition amount is 0.01%. The oxygen content of the molten steel is 25 ppm. Control the vacuum degree of the VD furnace at 49 Pa. After argon charging, the stirring time with argon is controlled at 15 minutes. After VD degassing, measure the oxygen content in the steel to be 25 ppm and the N content to be 130 ppm.

[0060] Steps c and d are the same as those in Example 1.

[0061] Step e, combined modulation heat treatment. The same as Example 1, the hardness is 55 HRC and the impact toughness is 8.8 J. Not only is the hardness lower than that in Example 1, but also the impact toughness is poorer.

[0062] Comparative Example 2

[0063] Step a, electric furnace smelting. The composition is the same as that in Example 2, and the melting process is the same as that in Example 3.

[0064] Step b, secondary refining. Control the vacuum degree of the VD furnace at 50 Pa. After argon charging, the stirring time with argon is controlled at 35 minutes. After VD degassing, measure the oxygen content in the steel to be 35 ppm and the N content to be 140 ppm, and there is no deoxidation with aluminum wire.

[0065] Steps c and d are the same as those in Example 2.

[0066] Step e, combined modulation heat treatment. Use a box-type resistance furnace or a salt bath furnace, and the temperature uniformity in the furnace chamber is ≤ ±4°C. The heating temperature is 1100°C; the holding time in this temperature range: 1 h; the heating rate: 70°C / h to avoid deformation or cracking caused by thermal stress. Use fast quenching oil for oil quenching and cool to room temperature (24°C). When tempering, use a tempering furnace, and the temperature uniformity in the furnace chamber is ≤ ±4°C. Temper twice in the temperature range of 550°C, the holding time is 2 h, and air-cool to room temperature. The hardness is 55 HRC and the impact toughness is 13 J. The hardness is on the low side and the toughness is worse than that in the example.

[0067] Comparative Example 3

[0068] Steps a and b are the same as those in Example 2, but electroslag remelting is not adopted. Steps c, d, and e are the same as those in Example 2, with a hardness of 58 HRC and an impact toughness of 10 J. Although the hardness meets the standard, the impact toughness is insufficient.

[0069] Comparative Example 4

[0070] Steps a, b, c, and d are the same as those in Example 3, but in step e, the combined modulation heat treatment stage, the heating temperature is 1120 °C; the holding time in this temperature range is 1 h, and then oil quenching is carried out using fast quenching oil, and the oil is cooled to room temperature (22 °C). Tempering is carried out twice within the temperature range of 400 °C, the holding time is 2 h, and it is air-cooled to room temperature. The hardness is 52 HRC and the impact toughness is 17 J. Both the hardness and the impact value are not high.

Claims

1. A steel for shield machine cutter head with vanadium and niobium microalloying, characterized in that: Its chemical composition by weight percentage includes: C: 0.44 - 0.49%, Mo: 1.4 - 1.6%, Si: 1.0 - 1.4%, Cr: 4.8 - 5.2%, Mn: 0.4 - 0.6%, V+Nb: 1.40 - 1.70%, and the balance is Fe and inevitable impurities.

2. A production method of steel for shield machine cutter head with vanadium and niobium microalloying, characterized in that: It includes the following steps: a. Electric furnace smelting, smelting using an electric arc furnace + LF furnace according to the above chemical composition ratio; b. Secondary refining, making the oxygen content in the steel ≤ 15 ppm and the N content ≤ 100 ppm through a VD furnace; c. Electroslag remelting, after electrode bar casting → electrode bar annealing → electrode bar cleaning, carrying out electroslag remelting under a protective atmosphere to obtain an ingot; d. Performing high-temperature uniform diffusion treatment of elements on the ingot, using a box-type resistance furnace or a gas furnace, with the furnace chamber temperature uniformity ≤ ±10°C; e. Combined modulated heat treatment, using a box-type resistance furnace or a salt bath furnace, with the furnace chamber temperature uniformity ≤ ±5°C.

3. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 2, characterized in that: In step a, pig iron, ferronickel, ferrochrome, ferromanganese, ferroniobium, and ferrovanadium are added according to the composition to obtain ferroalloy raw materials, and the above ferroalloy raw materials account for 95% - 98% of the total raw materials, the scrap steel accounts for 2 - 5%, and in the ferroalloy raw materials, Pb ≤ 0.01%, Sn ≤ 0.01%, As ≤ 0.01%, Sb ≤ 0.01%, and Bi ≤ 0.01%.

4. The production method of a steel for a shield machine cutter head with vanadium-niobium microalloying according to claim 2, characterized in that: In step a, the power is controlled between 14000 - 20000 KVA; in the melting stage, the V-Fe alloy is added in the later stage of melting, the addition temperature is 1580°C - 1600°C, and the stirring time ≥ 10 minutes; the Nb-Fe alloy is added in the middle stage of melting, the addition temperature is 1550°C - 1570°C, and the stirring time ≥ 8 minutes.

5. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 2, wherein: In step b, aluminum wire is used for deoxidation, the addition amount is 0.02 - 0.03%, ensuring that the oxygen content in the molten steel ≤ 20 ppm; and the vacuum degree of the VD furnace is controlled below 50 Pa, after argon filling, the argon stirring time is controlled between 35 - 45 minutes.

6. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 2, characterized in that: In step c, the melting rate should be controlled between 5.5 - 7.0 kg / min, and the cooling water temperature is controlled between 30 - 50°C.

7. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 2, characterized in that: In step d, it is kept warm at 1250 - 1270°C for 8 - 12 h, first ensuring that the primary carbides in the ingot are fully redissolved and ensuring sufficient diffusion of each element in the ingot; specifically, it can be determined according to the ingot blank size, for every 100 mm diameter, it is kept warm for 2 - 3 h, the total holding time ≥ 6 h, it is put into the furnace at room temperature, the heating rate with the furnace ≤ 100°C / h, after forging, it is furnace-cooled to 800 - 820°C, the cooling rate ≤ 50°C / h, and then air-cooled to room temperature.

8. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 2, characterized in that: In step e, the holding time in this temperature range is 1 - 10 h, and the holding time is determined according to the thickness of the workpiece, usually for every 25 - 30 mm thickness, it is kept warm for 1 - 1.5 h, and the heating rate ≤ 100°C / h.

9. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 8, characterized in that: In step e, rapid quenching oil is used for oil quenching, cooled to room temperature 20 - 30°C, and during oil cooling, a stirring or circulation system is used to ensure that the cooling rate of each part of the workpiece is consistent.

10. The production method of the steel for the cutter head of a shield machine with vanadium-niobium microalloying according to claim 9, characterized in that: In step e, tempering is carried out in a tempering furnace with the temperature uniformity in the furnace chamber ≤ ±5 °C. A fan or natural convection is used, and tempering is carried out three times within the temperature range of 560 - 610 °C, with the holding time being 2 - 3 h. The holding time can be determined according to the thickness of the workpiece, usually 1 - 1.5 h of holding for every 25 - 30 mm of thickness; air-cooled to room temperature, and the cooling rate ≤ 10 °C / min.