A method for controlling the uniformity of microstructure in gear steel

Abstract

This invention discloses a method for controlling the uniformity of gear steel microstructure, including hot metal desulfurization and vanadium extraction, semi-steel converter smelting, LF refining, RH refining, and continuous casting processes. The method controls the solidification process of the molten steel in the molten pool during continuous casting, ensuring solidification is completed within a 1 / 4 arc-shaped region of the slab, and implementing a specified cooling and solidification control process. This method effectively improves the macroscopic solute concentration uniformity in all directions of the cross-section of the gear steel continuous casting slab, improves the regional symmetry of the solidified microstructure, and promotes more uniform phase transformation and stress distribution in the gear microstructure during gear processing and subsequent heat treatment. The range of distance between the equiaxed crystal zone boundary and the slab surface is reduced from 30-40 mm to 10-15 mm. After rolling into a round bar, the range of distance between the equiaxed crystal trace zone boundary and the round bar surface is controlled to ≤5 mm, and the range of solute segregation (circumferential) at the same radial distance from the surface of the round bar is ≤0.02. The resulting gear steel grain size stabilizes from grade 6.0-7.0 to grade 7.0.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and specifically relates to a method for controlling the uniformity of the microstructure of gear steel. Background Technology

[0002] Gears are crucial power transmission mechanical components. The stress on gears varies depending on their operating conditions and location, leading to differences in failure modes. These failure modes are influenced by numerous factors, including gear material, manufacturing process, lubrication, friction, gear geometry, and load patterns. The main failure modes include: tooth root fracture, tooth surface deformation due to tooth surface concavity, fatigue spalling, and wear (adhesive wear and abrasive wear). Based on these main failure modes, the performance requirements for gear steel are high crush resistance and resistance to pitting and spalling; good impact resistance and bending resistance; suitable hardenability and hardened layer depth and core hardness; good processability and machinability; and good deformation and dimensional stability. Automotive gears play a vital role in power transmission and speed change, making them a major representative of gears. Gear steel must possess good strength, toughness, and wear resistance, effectively withstanding impact, bending, and contact stress, while also exhibiting minimal deformation. During gear manufacturing, in addition to reasonable design and machining, heat treatments such as carburizing, quenching, or tempering are required to harden the surface for wear resistance and enhance the impact resistance of the base material. The quality of gear blanks largely determines the efficiency and quality control of gear production.

[0003] With the continuous development of continuous casting production methods, the level of billet quality control methods has been continuously improved. Currently, the production of automotive gear billets in China is mainly based on small square billet continuous casting. As the quality requirements for gears continue to increase, the requirements for the density and homogeneity of gear steel billets are also higher. There is currently a considerable amount of research on the homogeneity control of continuously cast billets, mostly focusing on controlling center segregation of the billet, i.e., reducing the accumulation of solute elements at the center of the billet. This is largely achieved by increasing the equiaxed crystal ratio to promote synchronous nucleation and solidification of the molten steel in the central region. For example: CN107262689A discloses a large square billet secondary cold-pressing convex foot roll structure and pressing process; CN106735013A discloses a continuous casting process to improve the quality of the head billet of a large square billet; CN107225148A discloses a rolling method for 22CrMoH gear steel; CN106967931A discloses a 20Cr2Ni4 gear steel and its control technology; and CN106566998A discloses a manufacturing method for CrMo series gear round steel. However, some key specific details regarding "a technology for controlling the uniformity of gear steel microstructure" were not addressed. In production practice, during straight-arc or full-arc continuous casting, the well-developed equiaxed grain structure is accompanied by asymmetry between the inner and outer arcs. Furthermore, due to grain settling, the equiaxed grain region tends to favor the outer arc side. This asymmetric distribution of the solidification structure directly leads to uneven solute concentration distribution, particularly uneven distribution of semi-macroscopic segregation. The uneven solute distribution in the billet is inherited to varying degrees after rolling into round bars, undergoing different degrees of deformation under the rheological effects of the matrix during rolling, further exacerbating the uneven distribution of solute elements in the round bars. During gear machining, the uneven distribution of solute elements causes varying degrees of segregation in the CCT curve of the corresponding heat treatment region, resulting in differences in grain size and even microstructure type, ultimately leading to deformation and other problems that directly worsen product quality.

[0004] Based on the above analysis, in order to effectively improve the macroscopic solute concentration uniformity in all directions of the cross-section of the gear steel continuous casting billet, improve the regional symmetry of the solidification structure, and thus promote a more uniform multi-scale segregation distribution on the round bar, and promote a more uniform phase transformation and stress distribution of the gear structure during gear processing and subsequent heat treatment, and finally lay an important quality foundation for the high performance uniformity and high dimensional stability control of gears, it is necessary to propose a gear steel structure uniformity control method. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides a method for controlling the uniformity of gear steel microstructure, comprising the following steps: desulfurization and vanadium extraction of molten iron → semi-steel converter smelting → LF refining → RH vacuum refining → continuous casting. The converter process mainly completes dephosphorization and deoxidation alloying treatment during steel tapping to initially adjust the content of the main chemical components of the target grade gear steel. The LF process mainly completes refining and slag formation to promote the flotation and removal of non-metallic inclusions generated in the molten steel, and further finely adjusts the content of each alloy component based on the target composition. Simultaneously, the temperature of the molten steel is controlled by electrode heating to achieve precise and stable temperature control. Furthermore, based on the LF refining, the resulting molten steel undergoes real-time RH vacuum refining, and the vacuum degree and corresponding processing time are controlled during the refining process to effectively reduce the content of harmful gaseous elements in the molten steel to the target level.

[0006] To achieve the above-mentioned objective, this invention provides a method for controlling the uniformity of gear steel microstructure. The method includes hot metal desulfurization and vanadium extraction → semi-steel converter smelting → LF refining → RH vacuum refining → continuous casting process. The solidification process of the molten steel in the billet pool is controlled during the continuous casting process, and solidification is completed within the 1 / 4 arc-shaped area of ​​the billet. Based on this, a specified cooling and solidification control process is implemented step by step to control the growth, evolution and composition of the solidified microstructure.

[0007] Furthermore, in the converter smelting process, ferroaluminum is added during the converter tapping process to deoxidize the molten steel, wherein the amount of ferroaluminum added is 3.0~4.0 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 2.5~3.5 kg / t. 钢 .

[0008] Furthermore, the oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15m / ppm[O]. The diameter of the aluminum wire is Φ15mm-Φ16mm.

[0009] Furthermore, in the LF refining process, the Alt in the molten steel is controlled at 0.040%~0.060%, and even further, after the LF refining is completed, CaSi wire is fed at 1.5~2.0 m / ppm [S].

[0010] Furthermore, during the RH vacuum refining process, the vacuum level in the vacuum chamber is required to be reduced to ≤3mbar, and the vacuum level is maintained at <3mbar for ≥11 minutes. After vacuum depressurization, the CaSi line from the LF process is added based on the difference in [S]% between the RH process exiting and entering the molten steel. The feeding amount is controlled at 8~8.5m for every 1ppm increase in sulfur in the RH process. After the CaSi line is added, the molten steel is allowed to stand for >5 minutes before casting can begin.

[0011] The CaSi core powder content in the LF and RH processes is 130~140g / m, and the mass content of key components is: calcium content ≥28%, silicon content 45%~53%, and the balance is unavoidable impurities.

[0012] Furthermore, in the continuous casting process, the specified cooling and solidification control process of the present invention is implemented, and argon blowing protection is required throughout the casting process.

[0013] The "specified cooling and solidification control process" mentioned in the continuous casting process is as follows: the superheat of the cast steel is controlled at 30~35℃; the M-EMS stirring current intensity is set at 100~200A; the length of the secondary cooling coverage area is 7~10m from the bottom of the crystallizer, and the secondary cooling water ratio is controlled at 0.84~0.92L / kg steel; the F-EMS is stirred at a distance of 7.5~11m from the bottom of the crystallizer, and the stirring current intensity is set at 200~300A; the billet shell temperature in the straightening area is raised to 920~970℃.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] The gear steel microstructure uniformity control method provided by this invention effectively improves the macroscopic solute concentration uniformity in all directions of the cross-section of the gear steel continuous casting billet, improves the regional symmetry of the solidification structure, and thus promotes a more uniform multi-scale segregation distribution on the round bar. It also promotes a more uniform phase transformation and stress distribution of the gear microstructure during gear processing and subsequent heat treatment, ultimately laying an important quality foundation for the high performance uniformity and high dimensional stability control of gears.

[0016] By adopting the control method of this invention, the symmetry of the solidification structure of the gear steel billet is significantly improved. The difference in distance between the boundary of the equiaxed crystal region and the surface of the billet is reduced from 30-40 mm to 10-15 mm. After rolling into a round bar, the difference in distance between the boundary of the equiaxed crystal trace region of the solidification structure and the surface of the round bar is controlled to ≤5 mm, and the difference in solute segregation degree (circumferential direction) at the same radial distance from the surface of the round bar is ≤0.02. The resulting gear steel grain size stabilizes at grade 7.0, down from 6.0-7.0. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0018] This invention provides a method for controlling the uniformity of gear steel microstructure. The method includes hot metal desulfurization and vanadium extraction → semi-steel converter smelting → LF refining → RH vacuum refining → continuous casting process. The method controls the solidification process of the molten steel in the billet pool during continuous casting, and completes solidification within the 1 / 4 arc-shaped area of ​​the billet. Based on this, a specified cooling and solidification control process is implemented step by step to control the growth, evolution and composition of the solidified microstructure.

[0019] Example

[0020] A method for controlling the uniformity of microstructure in gear steel, the specific process being:

[0021] In the converter smelting process, ferrosilicon is added during the tapping process to deoxidize the molten steel, with an addition amount of 3.5 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 3.0 kg / t. 钢 .

[0022] Furthermore, the oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15m / ppm[O]; the diameter of the aluminum wire is Φ16mm.

[0023] Furthermore, in the LF refining process, the Alt in the molten steel is controlled at 0.052%; even further, after the LF refining is completed, CaSi wire is fed at 1.7m / ppm[S].

[0024] The CaSi wire core powder has a content of 136 g / m, and the key component mass content is: calcium content 28%, silicon content 53%, and the balance is unavoidable impurities.

[0025] Furthermore, during the RH vacuum refining process, the vacuum level in the vacuum chamber is required to be reduced to ≤3mbar, and the vacuum level is maintained at <3mbar for 13 minutes. After vacuum depressurization, the CaSi line from the LF process is fed according to the difference in [S]% in the molten steel when it leaves and enters the RH process. The feeding amount is controlled at 8.2m for every 1ppm increase in sulfur in the RH process. After the CaSi line is fed, the molten steel is allowed to stand for 6 minutes before casting can begin.

[0026] Furthermore, in the continuous casting process, the specified cooling and solidification control process of this method is implemented, and argon blowing protection is required throughout the casting process.

[0027] The "specified cooling and solidification control process" for the continuous casting process specifically refers to: controlling the superheat of the molten steel at 30~35℃; setting the M-EMS stirring current intensity to 100A; and setting the length of the secondary cooling coverage area to 10m from the bottom of the crystallizer, with a secondary cooling water ratio of 0.84L / kg. 钢 Control; F-EMS is stirred at a position 11m away from the bottom of the crystallizer, and the stirring current intensity is set at 200A; the billet temperature in the straightening zone rises to 970℃.

[0028] After adopting the above method, the symmetry of the solidification structure of the gear steel billet was significantly improved. The difference in distance between the boundary of the equiaxed crystal zone and the billet surface decreased from 30-40 mm to 15 mm. After rolling into a round bar, the difference in distance between the boundary of the equiaxed crystal trace zone and the surface of the round bar was controlled within 5 mm, and the difference in solute segregation (circumferential direction) at the same radial distance from the surface of the round bar was 0.02. The resulting gear steel grain size improved from grade 6.0 to grade 7.0.

[0029] Example 2

[0030] A method for controlling the uniformity of microstructure in gear steel, the specific process being:

[0031] In the converter smelting process, ferroaluminum is added during the tapping process to deoxidize the molten steel, with an addition amount of 3.0 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 3.5 kg / t. 钢 .

[0032] Furthermore, the oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15m / ppm[O]; the diameter of the aluminum wire is Φ15mm.

[0033] Furthermore, in the LF refining process, the Alt in the molten steel is controlled at 0.040%; even further, after the LF refining is completed, CaSi wire is fed at 2.0 m / ppm [S].

[0034] The CaSi wire core powder has a content of 130g / m, and the key component mass content is: calcium content 30%, silicon content 45%, and the balance is unavoidable impurities.

[0035] Furthermore, during the RH refining process, the vacuum level in the vacuum chamber is required to be reduced to ≤3mbar, and the vacuum level is maintained at <3mbar for 11 minutes of cyclic processing. After vacuum depressurization, the CaSi line from the LF process is fed according to the difference in [S]% in the molten steel when it leaves and enters the RH process. The feeding amount is controlled at 8.5m for every 1ppm increase in sulfur in the RH process. After the CaSi line is fed, the molten steel is allowed to stand for 5.5 minutes before casting can begin.

[0036] Furthermore, in the continuous casting process, the specified cooling and solidification control process of the present invention is implemented, and argon blowing protection is required throughout the casting process.

[0037] The "specified cooling and solidification control process" specifically refers to: controlling the superheat of the cast steel molten steel at 30~35℃; setting the M-EMS stirring current intensity to 150A; and setting the length of the secondary cooling coverage area to 9m from the bottom of the crystallizer, with a secondary cooling water ratio of 0.87L / kg. 钢 Control; the F-EMS is stirred at a position 9.5m away from the bottom of the crystallizer, and the stirring current intensity is set at 250A; the billet temperature in the straightening zone rises back to 950℃.

[0038] By employing the above method, the symmetry of the solidification structure of the gear steel billet was significantly improved. The range of distances between the equiaxed crystal zone boundary and the billet surface decreased from 30-40 mm to 10 mm. After rolling into a round bar, the range of distances between the equiaxed crystal trace zone boundary and the round bar surface was controlled within 3 mm, and the range of solute segregation (circumferential) at the same radial distance from the surface of the round bar was 0.014. The resulting gear steel grain size improved from grade 6.0 to grade 7.0.

[0039] Example 3

[0040] A method for controlling the uniformity of microstructure in gear steel, the specific process being:

[0041] In the converter smelting process, ferroaluminum is added during the tapping process to deoxidize the molten steel, with an addition amount of 4.0 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 2.5 kg / t. 钢 .

[0042] Furthermore, the oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15m / ppm[O]. The diameter of the aluminum wire is Φ16mm.

[0043] Furthermore, in the LF refining process, the Alt in the molten steel is controlled at 0.060%, and even further, after the LF refining is completed, CaSi wire is fed at 1.5 m / ppm [S].

[0044] The CaSi wire core powder has a content of 140g / m, and the key component mass content is: calcium content 32%, silicon content 49%, and the balance is unavoidable impurities.

[0045] Furthermore, during the RH vacuum refining process, the vacuum level in the vacuum chamber is required to be reduced to ≤3mbar, and the vacuum level is maintained at <3mbar for 15 minutes of cyclic processing. After vacuum depressurization, the CaSi line from the LF process is fed according to the difference in [S]% in the molten steel when it leaves and enters the RH process. The feeding amount is controlled at 8m for every 1ppm increase in sulfur in the RH process. After the CaSi line is fed, the molten steel is allowed to stand for 6.5 minutes before casting can begin.

[0046] Furthermore, in the continuous casting process, the specified cooling and solidification control process of the present invention is implemented, and argon blowing protection is required throughout the casting process.

[0047] The "specified cooling and solidification control process" for the continuous casting process specifically refers to: controlling the superheat of the molten steel at 30~35℃; setting the M-EMS stirring current intensity to 150A; and setting the length of the secondary cooling coverage area to 9m from the bottom of the crystallizer, with a secondary cooling water ratio of 0.87L / kg. 钢 Control; the F-EMS is stirred at a position 9.5m away from the bottom of the crystallizer, and the stirring current intensity is set at 250A; the billet temperature in the straightening zone rises back to 950℃.

[0048] By employing the above method, the symmetry of the solidification structure of the gear steel billet was significantly improved. The range of distances between the equiaxed crystal zone boundary and the billet surface decreased from 30-40 mm to 12 mm. After rolling into round bars, the range of distances between the equiaxed crystal trace zone boundary and the round bar surface was controlled at 4 mm, and the range of solute segregation (circumferential) at the same radial distance from the surface of the round bar was 0.018. The resulting gear steel grain size improved from grade 6.0 to grade 7.0.

[0049] Comparative Example

[0050] In the converter smelting process, ferroaluminum is added during the tapping process to deoxidize the molten steel, with the amount of ferroaluminum added being 3.0~4.0 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 2.5~3.5 kg / t. 钢 .

[0051] Furthermore, the oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15m / ppm[O]; the diameter of the aluminum wire is Φ15mm-Φ16mm.

[0052] Furthermore, in the LF refining process, the Alt in the molten steel is controlled at 0.040%~0.060%, and even further, after the LF refining is completed, CaSi wire is fed at 1.5~2.0 m / ppm [S].

[0053] The CaSi wire core powder has a content of 130~140g / m, and the mass content of key components is: calcium content ≥28%, silicon content 45%~53%, and the balance is unavoidable impurities.

[0054] Furthermore, during the RH vacuum refining process, the vacuum level in the vacuum chamber is required to be reduced to ≤3mbar, and the vacuum level is maintained at <3mbar for ≥11 minutes. After vacuum depressurization, the CaSi line from the LF process is added based on the difference in [S]% between the RH process exiting and entering the molten steel. The feeding amount is controlled at 8~8.5m for every 1ppm increase in sulfur in the RH process. After the CaSi line is added, the molten steel is allowed to stand for >5 minutes before casting can begin.

[0055] The distance difference between the equiaxed crystal zone boundary and the surface of the gear steel billet produced by this process is 30~40mm. After being rolled into a round bar, the distance difference between the equiaxed crystal trace zone boundary and the surface of the round bar is 7~9mm. The solute segregation degree difference (circumferential direction) at the same radial distance from the surface of the round bar reaches 0.04, and the resulting gear steel grain size is 6.0~7.0 grade.

[0056] Through comparison of embodiments and comparative examples, the gear steel microstructure uniformity control method provided by the present invention, when implemented in the continuous casting process, effectively improves the macroscopic solute concentration uniformity in all directions of the cross-section of the gear steel continuous casting billet, improves the regional symmetry of the solidification structure, and thus promotes a more uniform multi-scale segregation distribution on the round bar. It also promotes a more uniform phase transformation and stress distribution in the gear microstructure during gear processing and subsequent heat treatment, ultimately laying an important quality foundation for the high performance uniformity and high dimensional stability control of gears.

[0057] After adopting the control method of this invention, the symmetry of the solidification structure of the gear steel billet is significantly improved. The difference in distance between the boundary of the equiaxed crystal region and the surface of the billet is reduced from 30~40mm to 10~15mm. After rolling into a round bar, the difference in distance between the boundary of the equiaxed crystal trace region of the solidification structure and the surface of the round bar is controlled to ≤5mm, and the difference in solute segregation degree (circumferential direction) at the same radial distance from the surface of the round bar is ≤0.02. The resulting gear steel grain size stabilizes at grade 7.0 from 6.0~7.0.

[0058] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for controlling the uniformity of microstructure in gear steel, characterized in that, The method includes hot metal desulfurization and vanadium extraction → semi-steel converter smelting → LF refining → RH vacuum refining → continuous casting process, during which a specified cooling and solidification control process is implemented. In the continuous casting process, a specified cooling and solidification control process is implemented, and argon blowing is used to protect the casting process throughout. The "specified cooling and solidification control process" for the continuous casting process specifically refers to: controlling the superheat of the molten steel at 30~35℃; setting the M-EMS stirring current intensity to 100~200A; setting the length of the secondary cooling coverage area to 7~10m from the bottom of the crystallizer; and setting the secondary cooling water ratio to 0.84~0.92 L / kg. 钢 Control; F-EMS is stirred at a distance of 7.5~11m from the bottom of the crystallizer, and the stirring current intensity is set at 200~300A; the billet temperature in the straightening zone rises to 920~970℃.

2. The control method according to claim 1, characterized in that, In the converter smelting process, ferroaluminum is added during the converter tapping process to deoxidize the molten steel, wherein the amount of ferroaluminum added is 3.0~4.0 kg / t. 钢 After deoxidation, the molten steel is alloyed, and the content of key chemical components is initially adjusted to reach 70%~90% of the target component content. After alloying, active lime is added to the ladle at a rate of 2.5~3.5 kg / t. 钢 .

3. The control method according to claim 2, characterized in that, The oxygen activity of the molten steel in the ladle is measured, and aluminum wire is fed according to the oxygen activity measurement results. The feeding length is controlled at 15 m / ppm[O]; the diameter of the aluminum wire is Φ15mm-Φ16mm.

4. The control method according to claim 3, characterized in that, In the LF refining process, the Alt in the molten steel is controlled at 0.040%~0.060%; after the LF refining is completed, CaSi wire is fed at 1.5~2.0 m / ppm[S].

5. The control method according to claim 4, characterized in that, During the RH vacuum refining process, the vacuum level in the vacuum chamber is reduced to ≤3mbar and maintained at <3mbar for cyclic processing for ≥11min. After vacuum depressurization, the CaSi line from the LF process is fed according to the difference of [S]% in the molten steel when it leaves and enters the RH process. The feeding amount is controlled at 8~8.5m for every 1ppm increase in sulfur in the RH process. After the CaSi line is fed, the molten steel is allowed to stand for >5min before casting begins.

6. The control method according to claim 5, characterized in that, The CaSi core powder content during the LF refining and RH vacuum refining processes is 130~140 g / m. The key component mass content is: calcium content ≥28%, silicon content 45%~53%, and the balance is unavoidable impurities.