A CrMnTi gear steel with good low temperature toughness and its production method

Abstract

The invention discloses a CrMnTi gear steel with good low-temperature toughness and a production method thereof, belonging to the field of gear steel bar steel. The gear steel comprises: C: 0.18%-0.22%, Si: 0.20%-0.30%, Mn: 0.90%-1.10%, Cr: 1.00%-1.20%, Mo: 0.10%-0.20%, Ti: 0.040%-0.065%, W: 0.05%-0.15%; Ni: 0.05%-0.10%; Al: 0.020%-0.040%, B: 0.0020%-0.0040%, N: 0.010%-0.016%, Mg: 0.0035%-0.0065%, Ca: 0.0040%-0.0080%; the balance is Fe and unavoidable impurities. The gear steel of the invention can have good toughness at low temperatures.

Description

Technical Field

[0001] The invention belongs to the field of steel for gear steel bars, and in particular relates to a CrMnTi gear steel with good low-temperature toughness and a production method thereof. Background Art

[0002] In recent years, the automotive industry has increasingly higher requirements for the quality and performance of gear steel. Many automotive gear manufacturers have further put forward strict requirements for the quality and technical indicators of gear steel, mainly including: narrow end hardenability band, low oxygen content and inclusion content, fine grains, good machinability, excellent surface quality, etc. For example: the technical indicators of carburized gear steel for FAW stipulate that the oxygen content is less than 20ppm, the hardenability band is ≤6HRC, and the grain size is required to be better than grade 5.

[0003] In the prior art, the patent application with publication number CN101096742 discloses a high-strength gear steel for automobiles: its composition mass percentage is: C: 0.20-0.40, Si: 0.20-0.50, Mn: 0.50-1.00, Cr: 0.80-1.30, Nb: 0.015-0.080, V: 0.030-0.090, Mo: 0.15-0.55, Al: 0.015-0.050, and the rest is Fe and unavoidable impurities. After adding a small amount of Nb and V, the grain size, hardenability and bandwidth of the gear steel are significantly optimized in this patent application, so that the heat treatment deformation of the mass-produced gears is small and the matching rate is improved; at the same time, the comprehensive mechanical properties of the gear steel are increased, the service life is extended, and the cost is low.

[0004] The patent application with publication number CN108342640B discloses a high hardenability gear steel and its manufacturing method: the chemical elements mass percentage is: C: 0.19%-0.25%, Si: 0.15%-0.35%, Mn: 0.70%-1.05%, Cr: 0.70%-0.80%, Ni: 0.40%-0.65%, 0<Cu≤0.20%, Mo: 0.33-0.40%, Al: 0.020-0.050%, Ti: 0.003-0.010%, b: 0.05-0.08%, B: 0.0001-0.0003%, N: 0.0080%-0.0150%, the balance is Fe and other inevitable impurities. The gear steel of the patent application has high strength and good toughness. However, the segregation control of steel components such as C, Mn, S, and the modification of non-metallic inclusions are not taken into account, which may easily lead to steel quality defects such as excessive banded structure and poor deformation ability of non-metallic inclusions, resulting in microcrack defects in the steel.

[0005] The gear steels disclosed in the above patent applications do not have good low temperature toughness. Summary of the invention

[0006] In view of the above problems, the present invention provides a CrMnTi gear steel with good low-temperature toughness and a production method thereof. The gear steel has good low-temperature toughness.

[0007] The first aspect of the present invention protects a CrMnTi gear steel with good low temperature toughness, comprising the following chemical components in percentage by mass:

[0008] C: 0.18%-0.22%, Si: 0.20%-0.30%, Mn: 0.90%-1.10%, Cr: 1.00%-1.20%, Mo: 0.10%-0.20%, Ti: 0.040%-0.065%, W: 0.05%-0.15%, Ni: 0.05%-0.10%, Al: 0.020%-0.040%, B: 0.0020%-0.0040%, N: 0.010%-0.016%, Mg: 0.0035%-0.0065%, Ca: 0.0040%-0.0080%, the balance is Fe and unavoidable impurities;

[0009] Mg and Ca in steel satisfy the relationship: 0.0080%≤Mg+Ca≤0.0120%;

[0010] Ti and N in steel satisfy the relationship: Ti / N≥4.

[0011] Furthermore, the gear steel produces bars with specifications of Ø90mm-Ø200mm, with a hardenability J9 of 34-40HRC and a hardenability J15 of ≥22HRC.

[0012] Furthermore, the non-metallic inclusions in the gear steel meet the following requirements: Class A ≤ Grade 2.0, Class B ≤ Grade 2.0, Class C ≤ Grade 1.0, and Class D ≤ Grade 1.0.

[0013] Furthermore, the gear steel has a tensile strength greater than 960 MPa, an elongation after fracture greater than 6%, a shrinkage after fracture greater than 26%, and a low-temperature impact energy of -40°C greater than 30J.

[0014] The second aspect of the present invention protects the above-mentioned method for producing gear steel, comprising the following steps:

[0015] (1) LF refining: During the LF refining, the electrode heating time is controlled to be 20-25 min, and the composition is adjusted twice, namely, coarse adjustment and fine adjustment. The coarse adjustment is to add 0.5-0.8 kg / t of aluminum powder for deoxidation, and the fine adjustment is to control the target contents of C, Si, Mn and Cr.

[0016] (2) VD vacuum degassing: The VD vacuum degassing includes a VD pressure holding time of 10-20 minutes, a soft blow of argon for 5-15 minutes after the VD breaks the air, and a standing time of 10-15 minutes after the soft blow is completed;

[0017] (3) Continuous casting: The continuous casting includes protective pouring, in which the electromagnetic stirring voltage of the crystallizer is 100-200V, the current intensity is 150-280A, and the frequency is 3-8Hz; the electromagnetic stirring voltage at the end of solidification is 280-380V, the current intensity is 200-350A, and the frequency is 2-8Hz;

[0018] (4) Rolling: The rolling includes a preheating section temperature of ≤850°C, a heating section temperature of 1050-1150°C, and a soaking section temperature of 1140-1180°C;

[0019] The high-pressure water descaling control of the continuous casting billet before rolling: single-pass multi-nozzle water high-pressure descaling, water descaling pressure ≥ 20MPa, water descaling rate ≥ 95%;

[0020] Process temperature control: starting rolling temperature ≥ 1000℃, final rolling temperature 900-960℃;

[0021] (5) Slow cooling of steel: The slow cooling of steel includes a slow cooling time of more than 24 hours, and the temperature of the steel leaving the slow cooling pit is lower than 200°C.

[0022] Furthermore, the cross-sectional size of the continuous casting billet in the continuous casting is 250-350mm×350-450mm, the superheat of the tundish is controlled at 20-30°C, the continuous casting billet is pulled at a constant speed, and the speed controlled by the constant pulling speed is 0.50-0.70m / min.

[0023] The principles for adding each element in the present invention are as follows:

[0024] C: The C element is used to ensure the strength of the steel material and improve the hardenability of the steel material. If the mass percentage of carbon is lower than 0.18%, the hardenability and strength of the gear steel cannot be improved. If the mass percentage of carbon is higher than 0.22%, the fatigue performance and cutting performance of the gear steel will be reduced. Therefore, the mass percentage of C is controlled at 0.18%-0.22%;

[0025] Si: Si element acts as a deoxidizer in steel. Since Si does not form carbides in steel, but exists in ferrite in the form of solid solution, it has a significant solid solution strengthening effect on ferrite, thereby reducing the plasticity of steel materials. In addition, Si can promote the segregation of P and S elements at the grain boundaries, making the grain boundaries appropriately brittle. Therefore, the Si content is appropriately reduced, and the mass percentage of Si is controlled at 0.20%-0.30%;

[0026] Mn: In addition to acting as a deoxidizer and desulfurizer, the Mn element also has a significant solid solution strengthening effect. In addition, Mn and S can form MnS inclusions, which play a notch effect and lubricant role during gear processing, improving the cutting performance of steel materials. Therefore, the mass percentage of Mn is controlled at 0.90%-1.10%;

[0027] Cr: Cr is a medium-strong carbide forming element, which can significantly improve the strength, wear resistance and other comprehensive properties of steel materials. In addition, Cr can also reduce the activity of C to prevent decarburization during heating, rolling and heat treatment. However, too high a Cr content will deteriorate the machinability of steel materials and increase the cost of alloy addition. Therefore, the mass percentage of Cr needs to be controlled at 1.00%-1.20%;

[0028] Mo: As a strong carbide-forming element in steel, molybdenum can strongly hinder the nucleation and growth of carbides, and can also effectively improve the hardenability. The combined effect of molybdenum and manganese can significantly improve the stability of austenite and the hardenability of steel. The Mo element can delay the transformation of pearlite and promote the formation of acicular ferrite structure. By adding an appropriate amount of Mo, the formation of banded structure can be effectively inhibited. Therefore, the mass percentage of Mo is controlled at 0.10%-0.20%;

[0029] Ti: The addition of Ti is mainly to refine the austenite grain size to ensure that the austenite grain size of the steel is finer than level 5. In addition, titanium has a complex effect on hardenability. On the one hand, it can combine with C to form TiC, which is difficult to dissolve in austenite, reducing the stability of supercooled austenite, thereby reducing the hardenability. At the same time, it can also refine the grains and improve the hardenability of steel. The Ti element has little effect on the hardenability of steel within the range of 0.040%-0.065%, and can avoid the TiN inclusions being too large in size and too much in content; at the same time, the Ti in the steel preferentially combines with N to form TiN, resulting in the B element in the steel existing in the form of "effective B", thereby affecting the stability of the hardenability of the steel.

[0030] W: W can significantly hinder the precipitation of carbides at the austenite grain boundaries, refine the grains, and improve the strength, plasticity, impact toughness, hydrogen embrittlement sensitivity and other properties of steel. The W content is controlled at 0.05%-0.15%;

[0031] Ni: It can enhance the cross-slip ability of the matrix, reduce the cold-brittle transition temperature, and improve toughness. It can also make the carburized steel obtain a surface layer with a small carbon gradient and a certain toughness, so the Ni content is controlled at 0.05%-0.10%;

[0032] Al: Add a certain amount of Al (0.020%-0.040%) to refine the grains and deoxidize to ensure that the austenite grain size of the steel is finer than grade 5, and obtain an extremely low oxygen content, reducing the number and size of non-metallic inclusions in the steel;

[0033] B: B element exists in the steel in the form of acid-soluble boron, which improves the hardenability of steel and replaces some precious alloy elements; acid-insoluble boron in the form of carbon and nitride can refine the grains and improve the toughness of steel, so the B content is controlled at 0.0020%-0.0040%;

[0034] N: N combines with C, Al, B, Ti and other elements and precipitates to form corresponding precipitates, which are used to refine the grains. Considering the relative content control of Al and B with positive effects, the N content is controlled at 0.010%-0.016%;

[0035] Mg: It is often used as a deoxidizer and desulfurizer in steelmaking. It reacts with O and S in the molten steel to generate magnesium oxide or sulfide, which reduces [O] and [S] in the molten steel to a lower level. The generated inclusions float up or are adsorbed by the protective slag and removed from the molten steel, playing a role in deoxidation and desulfurization. The addition of trace amounts of Mg can improve the composition, size and morphology of inclusions in steel, and enhance the deformation and machinability of inclusions in steel. Therefore, the Mg content is controlled at 0.0035%-0.0065%;

[0036] The main inclusions in Ca:Al deoxidized steel are Al 2 O 3 , and Al 2 O 3 Inclusions are hard and have sharp edges, which can easily scratch the matrix during subsequent processing of steel. In addition, due to their high melting point, they can easily adhere to the inner wall of the nozzle, affecting the smooth casting process and even causing serious accidents such as nozzle blockage. 2 O 3 The formation of inclusions, the Ca treatment process is used to make Al 2 O 3 The inclusions react with CaO to form low melting point calcium aluminate inclusions. At the same time, the MnS inclusions in the steel are converted into CaS or CaO·Al 2 O 3 ·CaS composite inclusions improve the isotropy of the steel plate and the performance of the steel matrix. Therefore, the Ca content is designed to be 0.0040% to 0.0080%.

[0037] Beneficial effects:

[0038] The present invention adopts the Ca treatment process to make Al 2 O 3The inclusions react with CaO to form low-melting-point calcium aluminate inclusions, which prevent the nozzle from floccules during the pouring of molten steel. At the same time, the MnS inclusions in the steel are converted into CaS or CaO·Al 2 O 3 ·CaS composite inclusions improve the isotropy of the steel plate and the performance of the steel matrix; the size of the oxide core in magnesium-treated steel is smaller than that in calcium-treated steel, indicating that magnesium treatment is conducive to the formation of finer oxide inclusions in steel and reduces the precipitation of MnS at grain boundaries. Through the unique and composite effects of Mg and Ca elements on non-metallic inclusions in steel, non-metallic inclusions can be removed and modified, so that gear steel can have good toughness at low temperatures. By further limiting the Ti and N ratios, the austenite grain size of the steel is adjusted to improve hardenability.

[0039] The Ø90 mm-Ø200 mm specification bars produced by the present invention have a hardenability J9 of 34-40 HRC and a J15 of ≥22 HRC; the non-metallic inclusions in the steel are of Class A≤2.0 level, Class B≤2.0 level, Class C≤1.0 level, Class D≤1.0 level, the austenite grain size is finer than Class 5, and the banded structure is ≤2 level; the mechanical properties of the gear steel measured after quenching and tempering heat treatment include the following: tensile strength greater than 960 MPa, elongation after fracture greater than 6%, shrinkage after fracture greater than 26%, and low-temperature impact energy of -40°C greater than 30 J. DETAILED DESCRIPTION

[0040] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. 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.

[0041] The first aspect of the present invention protects a CrMnTi gear steel with good low temperature toughness, comprising the following chemical components in percentage by mass:

[0042] C: 0.18-0.22%, Si: 0.20-0.30%, Mn: 0.90-1.10%, Cr: 1.00-1.20, Mo: 0.10-0.20%, Ti: 0.040-0.065%, W: 0.05-0.15%, Ni: 0.05-0.10%, Al: 0.020-0.040%, B: 0.0020-0.0040%, N: 0.010-0.016%, Mg: 0.0035-0.0065%, Ca: 0.0040-0.0080%, the balance is Fe and unavoidable impurities;

[0043] Mg and Ca in steel satisfy the relationship: 0.0080%≤Mg+Ca≤0.0120%;

[0044] Ti and N in steel satisfy the relationship: Ti / N≥4.

[0045] The second aspect of the present invention protects a production method of CrMnTi gear steel with good low temperature toughness, which is: molten iron pretreatment - converter smelting - LF furnace refining - VD refining - billet continuous casting (referred to as continuous casting) - heating - rolling - steel slow cooling - finishing - inspection - storage. Among them, many methods are the same as the existing methods, which will not be further described in this application, and the focus is on explaining the different methods.

[0046] The method of the invention comprises LF refining, VD vacuum degassing, continuous casting, rolling and slow cooling of steel materials.

[0047] LF Refining:

[0048] The main tasks of LF refining are composition fine-tuning, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. Use lime and fluorite for slag-making operation, and control the electrode heating time to 20-25min. Perform two composition adjustments. Add 0.5-0.8kg / t aluminum powder for deoxidation during rough adjustment, and adjust the C, Si, Mn and Cr content in the steel according to the target during fine adjustment. After the treatment, do appropriate thick slag treatment to ensure the stability of the VD process.

[0049] VD vacuum degassing:

[0050] The use of high basicity and low oxidation top slag for control, combined with VD vacuum treatment, soft argon blowing and other processes, can ensure the cleanliness of molten steel and reduce the total oxygen and hydrogen content in the steel. The VD pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after VD breaks the air. After the soft blowing is completed, it is left to stand for 10-15 minutes to ensure that the inclusions float fully, and the ladle is moved out to achieve the target composition control.

[0051] Continuous Casting:

[0052] The cross-sectional size of the continuous casting billet is (250-350) mm × (350-450) mm, the superheat of the tundish is controlled at 20-30°C, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.50-0.70 m / min); protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); crystallizer + electromagnetic stirring at the end of solidification: the electromagnetic stirring voltage of the crystallizer is 100-200V, the current intensity is 150-280A, and the frequency is 3-8Hz; the electromagnetic stirring voltage at the end of solidification is 280-380V, the current intensity is 200-350A, and the frequency is 2-8Hz. The electromagnetic stirring process can promote the floating of non-metallic inclusions in molten steel, reduce the particle size of inclusions in steel, and make the inclusions evenly distributed; reduce the element segregation of steel, thereby uniformly organizing the steel and reducing the banded structure defects of steel; reduce the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks in steel billets, and provide high-quality raw materials for rolling bars without scratches on the surface;

[0053] Rolling:

[0054] Continuous casting billet heating system: preheating section temperature ≤ 850℃, heating section temperature 1050-1150℃, soaking section temperature 1140-1180℃;

[0055] High-pressure water descaling control of continuous casting billet before rolling: single-pass multi-nozzle water high-pressure descaling, water descaling pressure ≥ 20MPa, water descaling rate ≥ 95%;

[0056] Process temperature control: starting rolling temperature ≥ 1000℃, final rolling temperature 900-960℃;

[0057] Slow cooling of steel: slow cooling in the pit, the slow cooling pit must be dry, slow cooling in the pit at more than 500℃, slow cooling time more than 24h, and ensure that the temperature out of the slow cooling pit is below 200℃, finally CrMnTi gear steel is produced.

[0058] The following are specific embodiments.

[0059] Example 1

[0060] LF refining: The main tasks of LF refining are composition fine-tuning, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. Use lime and fluorite for slag-making operation, and control the electrode heating time to 20 minutes. Perform two composition adjustments. During the rough adjustment, add 0.8kg / t aluminum powder for deoxidation operation, and during the fine adjustment, adjust the C, Si, Mn and Cr contents in the steel according to the target. After the treatment, do appropriate thick slag treatment to ensure the stability of the VD process.

[0061] VD vacuum degassing: high basicity, low oxidation top slag is used for control, and combined with VD vacuum treatment, soft argon blowing and other processes, it can ensure the cleanliness of molten steel and reduce the total oxygen and hydrogen content in the steel. The VD pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after VD is broken. After the soft blowing is completed, it is left to stand for 13 minutes to ensure that the inclusions are fully floated, and the ladle is moved out to achieve the target control of composition.

[0062] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.55m / min); protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); crystallizer + electromagnetic stirring at the end of solidification: the electromagnetic stirring voltage of the crystallizer is 100V, the current intensity is 200A, and the frequency is 4Hz; the electromagnetic stirring voltage at the end of solidification is 300V, the current intensity is 250A, and the frequency is 5Hz;

[0063] Rolling: continuous casting billet heating system: preheating section temperature 840℃, heating section temperature 1130℃, soaking section temperature 1165℃; high-pressure water descaling control of continuous casting billet before rolling: single-pass multi-nozzle water spraying high-pressure descaling, water descaling pressure 23MPa, water descaling rate 97%; process temperature control: starting rolling temperature 1030℃, final rolling temperature 930℃; slow cooling of steel: slow cooling in the pit, the slow cooling pit must be dry, slow cooling in the pit at 550℃, slow cooling time 26h, and ensure that the temperature is below 200℃ when leaving the slow cooling pit, and finally CrMnTi gear steel is obtained.

[0064] Example 2

[0065] LF refining: The main tasks of LF refining are composition fine-tuning, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. Use lime and fluorite for slag-making operation, and control the electrode heating time to 22 minutes. Perform two composition adjustments. During the rough adjustment, add 0.7kg / t aluminum powder for deoxidation operation, and during the fine adjustment, adjust the C, Si, Mn and Cr contents in the steel according to the target. After the treatment, do appropriate thick slag treatment to ensure the stability of the VD process.

[0066] VD vacuum degassing: high basicity, low oxidation top slag is used for control, and combined with VD vacuum treatment, soft argon blowing and other processes, it can ensure the cleanliness of molten steel and reduce the total oxygen and hydrogen content in the steel. The VD pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after VD is broken. After the soft blowing is completed, it is left to stand for 13 minutes to ensure that the inclusions are fully floated, and the ladle is moved out to achieve the target control of composition.

[0067] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.55m / min); protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); crystallizer + electromagnetic stirring at the end of solidification: the electromagnetic stirring voltage of the crystallizer is 120V, the current intensity is 230A, and the frequency is 6Hz; the electromagnetic stirring voltage at the end of solidification is 330V, the current intensity is 280A, and the frequency is 6Hz;

[0068] Rolling: continuous casting billet heating system: preheating section temperature 830℃, heating section temperature 1125℃, soaking section temperature 1160℃; high-pressure water descaling control of continuous casting billet before rolling: single-pass multi-nozzle water high-pressure descaling, water descaling pressure 22MPa, water descaling rate 98%; process temperature control: starting rolling temperature 1020℃, final rolling temperature 950℃; slow cooling of steel: slow cooling in the pit, the slow cooling pit must be dry, slow cooling in the pit at 535℃, slow cooling time 24h, and ensure that the temperature is below 200℃ when leaving the slow cooling pit, and finally CrMnTi gear steel is obtained.

[0069] Example 3

[0070] LF refining: The main tasks of LF refining are composition fine-tuning, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. Use lime and fluorite for slag-making operation, and control the electrode heating time to 25 minutes. Perform two composition adjustments. During the rough adjustment, add 0.6kg / t aluminum powder for deoxidation operation. During the fine adjustment, adjust the C, Si, Mn and Cr content in the steel according to the target. After the treatment, do appropriate thick slag treatment to ensure the stability of the VD process.

[0071] VD vacuum degassing: high basicity, low oxidation top slag is used for control, and combined with VD vacuum treatment, soft argon blowing and other processes, it can ensure the cleanliness of molten steel and reduce the total oxygen and hydrogen content in the steel. The VD pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after VD is broken. After the soft blowing is completed, it is left to stand for 14 minutes to ensure that the inclusions float fully, and the ladle is moved out to achieve the target control of composition.

[0072] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 27℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.60m / min); protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); crystallizer + electromagnetic stirring at the end of solidification: the electromagnetic stirring voltage of the crystallizer is 180V, the current intensity is 240A, and the frequency is 7Hz; the electromagnetic stirring voltage at the end of solidification is 350V, the current intensity is 320A, and the frequency is 5Hz;

[0073] Rolling: continuous casting billet heating system: preheating section temperature 850℃, heating section temperature 1120℃, soaking section temperature 1150℃; high-pressure water descaling control of continuous casting billet before rolling: single-pass multi-nozzle water spraying high-pressure descaling, water descaling pressure 24MPa, water descaling rate 98%; process temperature control: starting rolling temperature 1030℃, final rolling temperature 930℃; slow cooling of steel: slow cooling in the pit, the slow cooling pit must be dry, slow cooling in the pit at 540℃, slow cooling time greater than 24h, and ensure that the temperature out of the slow cooling pit is below 200℃, and finally CrMnTi gear steel is obtained.

[0074] Example 4

[0075] LF refining: The main tasks of LF refining are composition fine-tuning, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. Use lime and fluorite for slag-making operation, and control the electrode heating time to 23 minutes. Perform two composition adjustments. During the rough adjustment, add 0.7kg / t aluminum powder for deoxidation operation, and during the fine adjustment, adjust the C, Si, Mn and Cr contents in the steel according to the target. After the treatment, do appropriate thick slag treatment to ensure the stability of the VD process.

[0076] VD vacuum degassing: high basicity, low oxidation top slag is used for control, and combined with VD vacuum treatment, soft argon blowing and other processes, it can ensure the cleanliness of molten steel and reduce the total oxygen and hydrogen content in the steel. The VD pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after VD is broken. After the soft blowing is completed, it is left to stand for 15 minutes to ensure that the inclusions float fully, and the ladle is moved out to achieve the target composition control.

[0077] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.60m / min); protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); crystallizer + electromagnetic stirring at the end of solidification: the electromagnetic stirring voltage of the crystallizer is 200V, the current intensity is 260A, and the frequency is 7Hz; the electromagnetic stirring voltage at the end of solidification is 350V, the current intensity is 300A, and the frequency is 5Hz;

[0078] Rolling: continuous casting billet heating system: preheating section temperature 830℃, heating section temperature 1060℃, soaking section temperature 1170℃; high-pressure water descaling control of continuous casting billet before rolling: single-pass multi-nozzle water spraying high-pressure descaling, water descaling pressure 23MPa, water descaling rate 96%; process temperature control: starting rolling temperature 1025℃, final rolling temperature 940℃; slow cooling of steel: slow cooling in the pit, the slow cooling pit must be dry, slow cooling in the pit at 545℃, slow cooling time 25h, and ensure that the temperature is below 200℃ when leaving the slow cooling pit, and finally CrMnTi gear steel is obtained.

[0079] Steelmaking was carried out according to the chemical compositions and contents designed in the present invention and comparative example 1, and gear steel bars were produced through LF refining, VD vacuum treatment, continuous casting, heating, rolling, slow cooling and other processes. The measured chemical compositions are shown in Table 1.

[0080] Table 1 Chemical composition of steel (wt%)

[0081]

[0082] The inspection results of non-metallic inclusions, austenite grain size and banded structure of the steel are shown in Table 2. After normalizing at 910℃ for 1 hour and end quenching at 880℃ for 30 minutes, the quenching properties of the steel are shown in Table 2.

[0083] Table 2 Test results of non-metallic inclusions, austenite grain size, banded structure and hardenability of steel

[0084]

[0085] The mechanical properties of the Ø90mm-Ø200mm bars produced by the present invention after quenching and tempering heat treatment are shown in Table 3 (the heat treatment system is in accordance with the provisions of Table 4).

[0086] Table 3 Mechanical properties of steel after heat treatment

[0087]

[0088] Table 4 Steel heat treatment process system

[0089]

[0090] The gear steel bar produced by the present invention has non-metallic inclusions, austenite grain size, banded structure, hardenability and mechanical property indicators that are better than the national standard control range, has a significant improvement in low-temperature impact performance, and can meet the use requirements of low-temperature environments.

[0091] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A CrMnTi gear steel with good low temperature toughness, characterized in that: The chemical composition includes the following mass percentages: C: 0.18%-0.22%, Si: 0.20%-0.30%, Mn: 0.90%-1.10%, Cr: 1.00%-1.20%, Mo: 0.10%-0.20%, Ti: 0.051%-0.065%, W: 0.05%-0.15%, Ni: 0.05%-0.10%, Al: 0.020%-0.040%, B: 0.0020%-0.0040%, N: 0.011%-0.016%, Mg: 0.0035%-0.0065%, Ca: 0.0040%-0.0080%, the balance is Fe and unavoidable impurities; Mg and Ca in steel satisfy the relationship: 0.0080%≤Mg+Ca≤0.0120%; Ti and N in steel satisfy the relationship: Ti / N≥4; The gear steel has a bar size of Ø90mm-Ø200mm, a hardenability J9 of 34-40HRC, and a hardenability J15 of ≥22HRC; The non-metallic inclusions in the gear steel meet the following requirements: Class A ≤ Grade 2.0, Class B ≤ Grade 2.0, Class C ≤ Grade 1.0, Class D ≤ Grade 1.0; The gear steel has a tensile strength greater than 960 MPa, an elongation after fracture greater than 6%, a shrinkage after fracture greater than 26%, and an impact energy at -40°C greater than or equal to 37 J.

2. A method for producing CrMnTi gear steel with good low temperature toughness as claimed in claim 1, characterized in that: The steps include: (1) LF refining: During the LF refining, the electrode heating time is controlled to be 20-25 min, and the composition is adjusted twice, namely, coarse adjustment and fine adjustment. The coarse adjustment is to add 0.5-0.8 kg / t of aluminum powder for deoxidation, and the fine adjustment is to control the target contents of C, Si, Mn and Cr. (2) VD vacuum degassing: The VD vacuum degassing includes a VD pressure holding time of 10-20 minutes, a soft blow of argon for 5-15 minutes after the VD breaks the air, and a standing time of 10-15 minutes after the soft blow is completed; (3) Continuous casting: The continuous casting includes protective pouring, in which the electromagnetic stirring voltage of the crystallizer is 100-200V, the current intensity is 150-280A, and the frequency is 3-8Hz; the electromagnetic stirring voltage at the end of solidification is 280-380V, the current intensity is 200-350A, and the frequency is 2-8Hz; (4) Rolling: The rolling includes a preheating section temperature of ≤850°C, a heating section temperature of 1050-1150°C, and a soaking section temperature of 1140-1180°C; The high-pressure water descaling control of the continuous casting billet before rolling: single-pass multi-nozzle water high-pressure descaling, water descaling pressure ≥ 20MPa, water descaling rate ≥ 95%; Process temperature control: starting rolling temperature ≥ 1000℃, final rolling temperature 900-960℃; (5) Slow cooling of steel: The slow cooling of steel includes a slow cooling time of more than 24 hours, and the temperature of the steel leaving the slow cooling pit is lower than 200°C.

3. The method for producing CrMnTi gear steel with good low temperature toughness according to claim 2, characterized in that: The cross-sectional size of the continuous casting billet in the continuous casting is 250-350mm×350-450mm, the superheat of the tundish is controlled at 20-30°C, the continuous casting billet is pulled at a constant speed, and the speed controlled by the constant pulling speed is 0.50-0.70m / min.

Images