Gear steel continuous cast billet and method for producing same

Abstract

This invention relates to a continuously cast steel billet for gears and its preparation method, belonging to the field of iron and steel metallurgy technology. To address the problems of uneven composition, central porosity and shrinkage cavities, imbalanced crystal ratios, and surface and internal cracks in the preparation of continuously cast steel billets for gears using traditional smelting and casting processes, this invention provides a continuously cast steel billet for gears and its preparation method, comprising the steps of primary refining and converter processes, LF refining and vacuum treatment processes, and continuous casting processes. This invention solves defects such as segregation and cracks in the continuously cast billet by precisely controlling the steel composition and coordinating the parameters of each process. Primary refining, refining, and vacuum treatment improve the cleanliness of the molten steel, and dual electromagnetic stirring ensures that the central carbon segregation index is ≤1.10, improving crystal uniformity and effectively preventing crack formation. The final billet has no surface cracks, defects such as depressions ≤2mm, central porosity ≤1 level, and shrinkage cavities ≤0.5 level, fully meeting the stringent requirements for billets in high-performance gear steel. The process is simple and low-cost.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and particularly relates to a continuous casting billet for gears and its preparation method. Background Technology

[0002] As a core component of modern mechanical transmission systems, gears are widely used in many key fields such as automobile manufacturing, wind power generation, and engineering machinery equipment. Their service environment is harsh, requiring them to withstand high loads, high speeds, and complex multiaxial stresses for a long time. Therefore, extremely high requirements are placed on the strength, toughness, wear resistance, and microstructure uniformity of gear materials.

[0003] Gear steel is a key structural material for power transmission, and its internal quality directly determines the service life, operational stability, and overall reliability of the gears. As an important intermediate product in the gear steel production process, the uniformity of its internal structure, chemical composition, and overall density of the continuously cast billet not only affect the smooth progress of subsequent rolling or forging processes but also form the core foundation for determining the performance and service life of the final gear product.

[0004] Currently, while traditional continuous casting processes can meet the basic production requirements for gear steel, the shortcomings of existing continuous casting billet preparation technologies are becoming increasingly prominent as high-end equipment demands higher gear lifespan and reliability. These shortcomings have become a bottleneck restricting the development of high-performance gear steel. The main problems are: poor compositional uniformity, with elements such as C, Mn, and S easily accumulating at the solidification end to form segregation bands, leading to uneven hardness and toughness after subsequent rolling, and a tendency for stress concentration cracking; central porosity and shrinkage cavities, with micropores formed by insufficient steel feeding during solidification significantly reducing material density and affecting gear fatigue performance; an imbalance in the ratio of columnar to equiaxed crystals, with significant differences between inner and outer arc-shaped columnar crystals, resulting in anisotropic mechanical properties; frequent surface quality defects, with peritectic reactions easily occurring during gear steel solidification, leading to surface depressions, transverse cracks, and other defects; and prominent internal cracking risks, with uneven secondary cooling strength or billet bulging easily causing subcutaneous cracks and corner transverse cracks. These defects can be inherited by the final gear product, severely shortening its service life. Summary of the Invention

[0005] To address the problems of uneven composition, central porosity and shrinkage cavities, unbalanced crystal ratios, and surface and internal cracks in the preparation of gear steel continuous casting billets using traditional casting processes, this invention provides a gear steel continuous casting billet and its preparation method.

[0006] The technical solution of the present invention:

[0007] A method for preparing a continuously cast steel billet for gears includes the following steps:

[0008] Step 1: Primary Refining and Converter Processes

[0009] The initial smelting furnace molten iron is controlled as follows: Si 0.60~0.85%, S≤0.030%, P≤0.120%; the converter final C is 0.06~0.12%. Aluminum blocks are added during tapping for pre-deoxidation, and lime and fluorite are added to form slag.

[0010] Step 2, LF refining and vacuum treatment process:

[0011] The LF refining time shall not be less than 45 min, the white slag holding time shall not be less than 20 min, and the S in the refined molten steel shall be ≤0.005% and Als shall be 0.020%~0.040%; the vacuum treatment shall adopt deep vacuum ≤67 Pa and the vacuum holding time shall not be less than 15 min.

[0012] Step 3, Continuous casting process:

[0013] The superheat is 25~30℃, and the pulling speed is constant at 0.74m / min; the water flow rate in the second cooling section is 0.24L / min, and the cooling is divided into three sections with a water flow distribution ratio of 45 / 35 / 20%. The first section uses full water cooling, the second section uses air mist cooling, and the third section uses weak water cooling; the electromagnetic stirring parameters of the crystallizer are 150A / 2Hz, and the final electromagnetic stirring parameters are 300A / 3Hz.

[0014] Furthermore, in step one, the temperature of the molten iron in the primary refining furnace is controlled at 1300~1350℃, and the tapping temperature in the converter is 1620~1650℃.

[0015] Furthermore, the amount of aluminum blocks added during the tapping of steel in step one is 8 kg / t.

[0016] Furthermore, in step two, the temperature of the molten steel after LF refining is controlled at 1590~1610℃.

[0017] Furthermore, after vacuum treatment in step two, the H content of the molten steel is ≤1.5ppm and the Al content is ≤0.035%.

[0018] Furthermore, the temperature of the tundish in the third continuous casting step is controlled at 1520~1540℃.

[0019] Furthermore, in step three, the water flow rate in the crystallizer is 2350 L / min.

[0020] Furthermore, after the billet obtained in step three is kept warm in a slow cooling pit for 24 hours, it is either put into rolling or the surface temperature of the billet is reduced to 200°C before rolling.

[0021] The gear steel continuous casting billet prepared by the preparation method provided by the present invention has the following chemical composition by weight percentage: C 0.17~0.23%, Si 0.17~0.37%, Mn 0.80~1.10%, Cr 1.00~1.30%, Ti 0.04~0.10%, P≤0.020%, S≤0.020%, Alt 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

[0022] The beneficial effects of this invention are:

[0023] This invention effectively solves the core defects of traditional gear steel continuous casting billets, such as segregation, porosity, crystal imbalance, and cracks, through composition control, process coordination, and multi-technology linkage, thereby improving the quality and performance stability of the billets. In terms of composition, it strictly controls the core components of the steel grade, precisely regulates the C content to reduce the carbon equivalent, strictly controls easily segregating elements such as P and S, and adds Ti to form fine carbonitrides, refining the grains and inhibiting element diffusion. This reduces segregation and coarse microstructure from the source, laying the foundation for high-quality billets.

[0024] At the process coordination level, the primary refining and converter processes ensure the purity and sufficient deoxidation of the primary refining process; LF refining deeply desulfurizes to ≤0.005%, reducing low-melting-point inclusions; vacuum treatment controls [H] to ≤1.5ppm, achieving efficient degassing and impurity removal, reducing the risk of hydrogen-induced cracking, and significantly improving the cleanliness of the molten steel. In the continuous casting stage, low superheat and constant casting speed suppress columnar crystal growth; weak cooling in the crystallizer and gradient weak water mode in the secondary cooling reduce cracks and promote equiaxed crystal transformation; dual electromagnetic stirring keeps the central carbon segregation index ≤1.10, improving crystal uniformity; and the slow cooling process of the billet avoids stress superposition, effectively preventing crack formation.

[0025] The final cast billet has no cracks on its surface, and defects such as depressions are ≤2mm, with central porosity ≤1 grade and shrinkage cavities ≤0.5 grade, fully meeting the stringent requirements for billets in high-performance gear steel. This process is highly collaborative, easy to operate, and requires no additional complex equipment. It can not only consistently improve the quality of cast billets but also reduce subsequent processing costs, effectively enhancing the product's market competitiveness. Attached Figure Description

[0026] Figure 1 A low-magnification transverse photograph of the continuously cast steel billet for gears prepared in Example 2;

[0027] Figure 2 A low-magnification longitudinal photograph of the continuously cast steel billet for gears prepared in Example 2;

[0028] Figure 3 The graph shows the trend of segregation index variation across the entire cross section of the steel continuous casting billet for gears prepared by five-strand continuous casting in Example 2. Detailed Implementation

[0029] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0030] Example 1

[0031] This embodiment provides a continuous casting billet for gear steel and its preparation method. It adopts a continuous casting process for a 250mm×280mm cross-section square billet. The process route is: converter / electric furnace → refining → vacuum treatment → five-machine five-strand continuous casting. The parameters of each process are precisely matched and work together to systematically solve the problems of compositional segregation, central porosity and shrinkage cavities, crystal imbalance and surface and internal cracks from the aspects of compositional purity, solidification structure, cooling regime and electromagnetic stirring. The overall uniformity and density of the gear steel continuous casting billet are improved.

[0032] Step 1: Primary Refining and Converter Processes

[0033] This embodiment uses 20CrMnTi steel, with strict control over the chemical composition by weight percentage: C 0.17~0.23%, Si 0.17~0.37%, Mn 0.80~1.10%, Cr 1.00~1.30%, Ti 0.04~0.10%, P≤0.020%, S≤0.020%, Alt 0.020~0.050%, with the remainder being Fe and unavoidable impurities. By precisely controlling the C content to reduce the carbon equivalent, the enrichment of solutes between dendrites is alleviated; strict control of easily segregating elements such as P and S, and the addition of Ti to form fine carbonitrides, refines the grains and inhibits element diffusion, thereby reducing segregation and coarse microstructure from the source of composition.

[0034] The primary smelting furnace hot metal is controlled as follows: Si 0.60~0.85%, S≤0.030%, P≤0.120%, temperature 1300~1350℃; the converter final temperature is C 0.06~0.12%, tapping temperature 1620~1650℃, steel output 100t±5t, with 8kg / t aluminum blocks added for pre-deoxidation, 400kg lime, and 100kg fluorite slag. This ensures the purity and sufficient deoxidation of the primary smelting furnace, laying the foundation for subsequent deep desulfurization and low inclusion reduction in refining.

[0035] Step 2, LF refining and vacuum treatment process:

[0036] The LF refining process employs deep desulfurization and composition fine-tuning, with a refining time of no less than 45 minutes and a white slag holding time of no less than 20 minutes. After refining, S ≤ 0.005%, Als 0.020%~0.040%, and the temperature is 1590~1610℃. This significantly reduces sulfur content, decreases low-melting-point inclusions and segregation tendency, and the white slag atmosphere further enhances the purity of the molten steel.

[0037] Vacuum treatment employs a deep vacuum ≤67Pa, held for at least 15 minutes, resulting in Als ≤0.035% and [H] ≤1.5ppm after vacuum treatment. This efficient degassing removes fine inclusions, reduces the risk of hydrogen-induced cracking, and further improves compositional uniformity and material cleanliness.

[0038] Step 3, Continuous casting process:

[0039] During the continuous casting stage, the tundish temperature is 1520~1540℃, the superheat is 25~30℃, and the casting speed is constant at 0.74m / min. The low superheat can suppress the excessive growth of columnar crystals, and the constant casting speed stabilizes the solidification front, avoiding the aggravation of segregation, porosity and cracks due to temperature and casting speed fluctuations.

[0040] The crystallizer has a water flow rate of 2350 L / min and employs a relatively weak cooling intensity. This ensures more uniform primary cooling, matches the peritectic reaction characteristics of gear steel, reduces surface depressions and initial cracks, and guarantees uniform outlet billet shell thickness.

[0041] The secondary cooling section has a water flow rate of 0.24 L / min, divided into three stages with a distribution ratio of 45 / 35 / 20%. The first stage uses all water, the second stage uses air mist, and the third stage uses weak water. The weak cooling mode with decreasing water flow rate avoids premature solidification of the billet shell, which hinders the core feeding process. It also smooths the surface temperature gradient, reduces structural stress and microcracks, while simultaneously cooling the core to promote the transformation of columnar crystals to equiaxed crystals and improve the central density.

[0042] The crystallizer is electromagnetically stirred at 150A / 2Hz, with a tail-end electromagnetic stirrer at 300A / 3Hz. The crystallizer stirring breaks up the initial columnar crystals, promotes uniform solute distribution, and suppresses segregation; the tail-end stirring breaks up dendrites and increases equiaxed crystal nuclei, keeping the central carbon segregation index stable below 1.10, balancing the inner and outer arc structures, and significantly improving the crystal ratio and overall uniformity.

[0043] After the billet comes off the production line, it is kept at a low temperature in a slow cooling pit for 24 hours, or the surface temperature is reduced to below 200℃ before it enters the rolling mill. This avoids the superposition of thermal stress and structural stress, effectively preventing the generation of surface and internal cracks, and ensuring that the stable quality of the billet is inherited by the subsequently rolled products.

[0044] This embodiment achieves precise composition control, constant temperature and speed casting, gradient weak cooling regime and dual electromagnetic stirring through the coordinated cooperation of the above steps. The parameters of each process are matched and supported by each other, and the defects of segregation, porosity, crystal imbalance and cracks in gear steel continuous casting billets are solved in an integrated manner, providing a stable and reliable billet basis for the preparation of high-performance gear steel.

[0045] Example 2

[0046] This embodiment provides a continuous casting billet for gear steel and its preparation method. It employs a 250mm×280mm cross-section square billet continuous casting process. The process route is: converter / electric furnace → refining → vacuum treatment → five-machine five-strand continuous casting. The specific method is as follows:

[0047] This embodiment uses 20CrMnTi steel, with the following chemical composition by weight percentage: C 0.20%, Si 0.27%, Mn 0.95%, Cr 1.15%, Ti 0.07%, P 0.015%, S 0.015%, Alt 0.035%, and the remainder being Fe and unavoidable impurities.

[0048] Step 1: Primary Refining and Converter Processes

[0049] The primary furnace molten iron contained 0.72% Si, 0.025% S, and 0.100% P at a temperature of 1325℃; the converter final stage contained 0.09% C at a tapping temperature of 1635℃, with a tapping capacity of 100t. Pre-deoxidation was performed with 8kg / t aluminum blocks, and 400kg of lime and 100kg of fluorite were added for slag formation.

[0050] Step 2, LF refining and vacuum treatment process:

[0051] LF refining time 45 min, white residue holding time 20 min, after refining S 0.004%, Als 0.030%, temperature 1600℃; vacuum treatment vacuum degree 60 Pa, holding time 15 min, after vacuum Als 0.030%, [H] 1.3 ppm.

[0052] Step 3, Continuous casting process:

[0053] The tundish temperature is 1530℃, the superheat is 27℃, and the casting speed is constant at 0.74m / min; the water flow rate in the crystallizer is 2350L / min; the water flow rate in the second cooling section is 0.24L / min; the water distribution ratio in the three sections is 45 / 35 / 20%, with the first section using all water, the second section using air mist, and the third section using weak water; the electromagnetic stirring in the crystallizer is 150A / 2Hz, and the electromagnetic stirring at the end is 300A / 3Hz; after the billet is removed from the production line, it is kept warm in a slow cooling pit for 24 hours.

[0054] Figure 1 This is a low-magnification transverse photograph of the continuously cast steel billet for gears prepared in Example 2; (The image is from...) Figure 1 It can be seen that the macroscopic structure of the continuously cast billet cross section is uniform and dense, with no obvious shrinkage cavities, porosity, or V-shaped segregation bands in the central region, and no defects such as cracks or depressions on the surface, which meets the requirements of gear steel for billet density and surface quality. This indicates that the weak cooling + stirring process in the continuous casting process effectively inhibits the excessive growth of columnar crystals and improves the proportion of equiaxed crystals in the center and the uniformity of the cross section.

[0055] Figure 2 This is a low-magnification longitudinal photograph of the continuously cast steel billet for gears prepared in Example 2; (The image is from...) Figure 2 It can be seen that the longitudinal structure of the continuously cast billet is continuous and smooth, without penetrating porosity, internal cracks or coarse dendrites, and no obvious inclusions or flow lines, and the overall density is high. This indicates that the end electromagnetic stirring and gradient weak cooling process successfully broke up the dendritic skeleton, ensured core feeding, avoided longitudinal defects, and provided a stable billet for subsequent rolling.

[0056] Figure 3 This is a trend chart showing the segregation index variation of the full cross-section of the five-strand continuous casting billet for gears prepared by five-strand continuous casting in Example 2. Different colored curves represent the segregation index of different strands of continuous casting billets; from Figure 3 It can be seen that the carbon segregation index of the entire cross-section of the continuously cast billet is controlled at <1.10, with most points close to 1.00, showing small fluctuations and stability. This demonstrates the effectiveness of the continuous casting billet preparation method of this invention in controlling solute segregation: low superheat + dual electromagnetic stirring + precise composition control effectively suppresses solute enrichment between dendrites and achieves the goal of compositional uniformity.

[0057] Example 3

[0058] This embodiment provides a continuous casting billet for gear steel and its preparation method. It employs a 250mm×280mm cross-section square billet continuous casting process. The process route is: converter / electric furnace → refining → vacuum treatment → five-machine five-strand continuous casting. The specific method is as follows:

[0059] This embodiment uses 20CrMnTi steel, with the following chemical composition by weight percentage: C 0.17%, Si 0.17%, Mn 0.80%, Cr 1.00%, Ti 0.04%, P 0.020%, S 0.010%, Alt 0.020%, and the remainder being Fe and unavoidable impurities.

[0060] Step 1: Primary Refining and Converter Processes

[0061] The primary furnace molten iron contains 0.60% Si, 0.030% S, and 0.120% P at a temperature of 1300℃; the converter final temperature is 0.06% C, the tapping temperature is 1620℃, the tapping volume is 95t, 8kg / t aluminum blocks are added for pre-deoxidation, 400kg of lime is added, and 100kg of fluorite is added for slag formation.

[0062] Step 2, LF refining and vacuum treatment process:

[0063] LF refining time 50 min, white residue holding time 25 min, after refining S 0.003%, Als 0.020%, temperature 1590℃; vacuum treatment vacuum degree 55 Pa, holding time 18 min, after vacuum Als 0.032%, [H] 1.1 ppm.

[0064] Step 3, Continuous casting process:

[0065] The tundish temperature is 1520℃, the superheat is 25℃, and the casting speed is constant at 0.74m / min; the water flow rate in the crystallizer is 2350L / min; the water flow rate in the second cooling section is 0.24L / min; the water distribution ratio in the three cooling sections is 45 / 35 / 20%, with the first section using all water, the second section using air mist, and the third section using weak water; the electromagnetic stirring in the crystallizer is 150A / 2Hz, and the electromagnetic stirring at the end is 300A / 3Hz; after the billet leaves the production line, it is cooled to below 200℃ before entering the rolling mill.

[0066] Example 4

[0067] This embodiment provides a continuous casting billet for gear steel and its preparation method. It employs a 250mm×280mm cross-section square billet continuous casting process. The process route is: converter / electric furnace → refining → vacuum treatment → five-machine five-strand continuous casting. The specific method is as follows:

[0068] This embodiment uses 20CrMnTi steel, with the following chemical composition by weight percentage: C 0.23%, Si 0.37%, Mn 1.10%, Cr 1.30%, Ti 0.10%, P 0.010%, S 0.020%, Alt 0.050%, and the remainder being Fe and unavoidable impurities.

[0069] Step 1: Primary Refining and Converter Processes

[0070] The primary furnace molten iron has Si 0.85%, S 0.020%, P 0.090%, and a temperature of 1350℃; the converter final temperature is C 0.12%, the tapping temperature is 1650℃, the tapping volume is 105t, 8kg / t aluminum blocks are added for pre-deoxidation, 400kg of lime is added, and 100kg of fluorite is added for slag formation.

[0071] Step 2, LF refining and vacuum treatment process:

[0072] The LF refining time was 55 min, the white residue holding time was 30 min, and the S content after refining was 0.005%, Al content was 0.040%, and the temperature was 1610℃. The vacuum treatment was performed at a vacuum degree of 67 Pa for 20 min, and the Al content after vacuum treatment was 0.035%, and the [H] content was 1.5 ppm.

[0073] Step 3, Continuous casting process:

[0074] The tundish temperature is 1540℃, the superheat is 30℃, and the casting speed is constant at 0.74m / min; the water flow rate in the crystallizer is 2350L / min; the water flow rate in the second cooling section is 0.24L / min; the water distribution ratio in the three cooling sections is 45 / 35 / 20%, with the first section using all water, the second section using air mist, and the third section using weak water; the electromagnetic stirring in the crystallizer is 150A / 2Hz, and the electromagnetic stirring at the end is 300A / 3Hz; after the billet is removed from the production line, it is kept warm in a slow cooling pit for 24 hours.

Claims

1. A method for preparing a continuously cast steel billet for gears, characterized in that, Includes the following steps: Step 1: Primary Refining and Converter Processes The initial smelting furnace molten iron is controlled as follows: Si 0.60~0.85%, S≤0.030%, P≤0.120%; the converter final C is 0.06~0.12%. Aluminum blocks are added during tapping for pre-deoxidation, and lime and fluorite are added to form slag. Step 2, LF refining and vacuum treatment process: The LF refining time shall not be less than 45 min, the white slag holding time shall not be less than 20 min, and the S in the refined molten steel shall be ≤0.005% and Als shall be 0.020%~0.040%; the vacuum treatment shall adopt deep vacuum ≤67 Pa and the vacuum holding time shall not be less than 15 min. Step 3, Continuous casting process: The superheat is 25~30℃, and the pulling speed is constant at 0.74m / min; the water flow rate in the second cooling section is 0.24L / min, and the cooling is divided into three sections with a water flow distribution ratio of 45 / 35 / 20%. The first section uses full water cooling, the second section uses air mist cooling, and the third section uses weak water cooling; the electromagnetic stirring parameters of the crystallizer are 150A / 2Hz, and the final electromagnetic stirring parameters are 300A / 3Hz.

2. The method for preparing a continuously cast steel billet for gears according to claim 1, characterized in that, In step one, the temperature of molten iron in the primary refining furnace is controlled at 1300~1350℃, and the tapping temperature in the converter is 1620~1650℃.

3. The method for preparing a continuously cast steel billet for gears according to claim 1 or 2, characterized in that, The amount of aluminum blocks added during the first step of steel tapping is 8 kg / t.

4. The method for preparing a continuously cast steel billet for gears according to claim 3, characterized in that, Step 2: After LF refining, the temperature of the molten steel is controlled at 1590~1610℃.

5. The method for preparing a continuously cast steel billet for gears according to claim 4, characterized in that, After vacuum treatment in step two, the H content of the molten steel is ≤1.5ppm and the Al content is ≤0.035%.

6. The method for preparing a continuously cast steel billet for gears according to claim 5, characterized in that, In step three, the temperature of the tundish in continuous casting is controlled at 1520~1540℃.

7. The method for preparing a continuously cast steel billet for gears according to claim 6, characterized in that, Step 3: The water flow rate in the crystallizer is 2350 L / min.

8. The method for preparing a continuously cast steel billet for gears according to claim 7, characterized in that, After the billet obtained in step three is kept warm in the slow cooling pit for 24 hours, it is either put into rolling or the surface temperature of the billet is reduced to 200℃ before rolling.

9. A continuously cast steel billet for gears prepared by the preparation method according to any one of claims 1-8, characterized in that, The chemical composition by weight percentage includes: C 0.17~0.23%, Si 0.17~0.37%, Mn 0.80~1.10%, Cr 1.00~1.30%, Ti 0.04~0.10%, P≤0.020%, S≤0.020%, Alt 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

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