A hot rolling method for improving surface decarburization of gear steel

Abstract

The application discloses a kind of hot rolling methods for improving gear steel surface decarburization, including rolling preheating and rolling step;The preheating and rolling step includes preheating section, heating section, soaking section and carbon make-up section;The heating temperature of preheating section is 620-830 DEG C, and heating time is 60-90 min;The heating temperature of heating section is 1140-1180 DEG C, and heating time is 40-50 min;The heating temperature of soaking section is 1090-1130 DEG C, and heating time is 30-50 min;The heating temperature of carbon make-up section is 1080-1120 DEG C, and heating time is 40-60 min.This method greatly improves gear steel surface decarburization by optimizing heating process and furnace protective atmosphere, while improving steel surface quality;Hot-rolled state directly realizes decarburization depth ≤0.2% D, surface layer and core hardness ≤20 HBW, greatly improves yield, reduces gear steel grinding rate, ensures the various indexes of rolling product, realizes stable and efficient production.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and in particular to a hot rolling method for improving surface decarburization of gear steel. Background Technology

[0002] With the continuous upgrading of the aerospace, machinery manufacturing, and automotive industries, transmission gears, as core components of power transmission, are facing increasingly stringent service conditions. Under complex working conditions of high speed, heavy load, alternating stress, and impact loads, the fatigue life and transmission stability of gears largely depend on the surface quality and intrinsic properties of the raw materials. Therefore, the high-end equipment manufacturing industry has placed extremely stringent requirements on the metallurgical quality, surface decarburization layer control, and microstructure uniformity of gear steel.

[0003] Currently, the industry generally adopts the process route of "electric furnace smelting + billet opening + secondary rolling" to produce high-quality gear steel. Among them, the depth of the decarburized layer on the surface of the finished round steel is one of the key indicators for measuring product quality. Users usually require the decarburized layer thickness to be controlled within <0.8D% (D is the diameter of the round steel) to ensure subsequent heat treatment and machinability.

[0004] Patent application CN119056865A discloses a heating control method for reducing decarburization in high-temperature gear steel after two heating stages. By optimizing production scheduling, controlling the temperature of the heating section (e.g., 1050℃~1120℃ for the second heating stage), limiting the time of the high-temperature section (<80min), and closing the water tanks along the rolling line, the surface decarburization is reduced to 0.5~0.6d% (d is the diameter of the round steel). However, the invention is limited to suppressing decarburization in the heating furnace stage and reducing it to 0.5~0.6d%, so the effect is limited.

[0005] Patent application CN117488051A discloses a method for improving high-temperature grain size mixing and surface decarburization of gear steel used in cold forging. While this method improves the decarburized layer to some extent, it employs ultra-high temperature heating (1250–1280°C), significantly increasing the risk of abnormal austenite grain growth, which may exacerbate the grain size mixing problem. Furthermore, prolonged high-temperature heating may severely accelerate steel surface oxidation, increase oxide scale thickness and descaling difficulty, and deteriorate surface quality.

[0006] To address this issue, some manufacturers have resorted to peeling the decarburized layer from the billet before rolling. While this method removes the surface decarburized layer to meet delivery requirements, it also incurs significant economic costs: firstly, it increases the peeling and grinding process, along with associated equipment and labor costs; secondly, it reduces the metal yield by more than 2%, severely impacting production efficiency and resource utilization. Therefore, optimizing the heating regime and rolling process to control the decarburized layer depth at the source and reduce or even eliminate the peeling process has become a critical technical problem urgently needing to be solved in the gear steel production field. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a hot rolling method for improving the decarburization of the surface layer of gear steel, so as to effectively control the depth of the decarburized layer.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes pre-rolling heating and rolling steps; The pre-rolling heating step includes a preheating section, a heating section, a soaking section, and a carbonization section. The heating temperature of the preheating section is 620–830℃, and the heating time is 60–90 min. The heating temperature of the heating section is 1140–1180℃, and the heating time is 40–50 min. The heating temperature of the soaking section is 1090–1130℃, and the heating time is 30–50 min. The heating temperature of the carbonization section is 1080–1120℃, and the heating time is 40–60 min.

[0009] Furthermore, in the pre-rolling heating step, the furnace atmosphere requirements are as follows: from the preheating section to the middle of the heating section, the CO2 / CO ratio is 0.4 to 0.7; from the later stage of the heating section to the early stage of the soaking section, the CO / CO2 ratio is 10 to 20; and from the later stage of the soaking section and the carbon supplementation section, the CO / CO2 ratio is 16 to 30.

[0010] Furthermore, the furnace atmosphere requirements for the pre-rolling heating step are as follows: from the preheating section to the middle of the heating section: residual oxygen ≤0.3%, CO 8%~12%, CO2 0.5~1.5%, H2 8~12%, CH4 1~3%, N2 60~75%, H2O / H2 ratio ≤0.4~0.7; from the late heating section to the early soaking section: CO 10%~15%, CO2 0.5~1.5%, H2 8~12%, CH4 2~5%, N2 70~80%; from the late soaking section and the carbon supplementation section: CO 15%~25%, CO2 0.5~1.5%, H2 8~15%, CH4 3~8%, N2 45~65%.

[0011] Furthermore, the preheating section is controlled in two stages: first, it is kept at 620-680℃ for 30-40 minutes, and then it is raised to 700-830℃ and kept at 30-50 minutes.

[0012] Furthermore, the temperature control of the carbon replenishment section is as follows: first, the temperature is raised to 1110-1120℃ and held for 20-35 minutes; then, the temperature is lowered to 1080-1090℃ and held for 20-25 minutes.

[0013] Furthermore, the rolling steps are as follows: the initial rolling temperature is 1050-1100℃, the finishing rolling inlet temperature is 1000-1050℃, the finishing rolling outlet temperature is 850-950℃, and cooling is performed after finishing rolling.

[0014] Furthermore, the post-rolling cooling adopts water cooling + mist cooling, with a cooling rate of 30-50℃ / s.

[0015] Technical principles of the present invention: Preheating stage: The temperature window is narrower, locking in the medium temperature range of 620-830℃, avoiding the high-temperature preheating above 850℃ in existing technologies. The purpose is to form a dense FeO layer of a specific thickness of 30-50μm before the austenitic phase transformation, serving as the interface medium for subsequent "carburization". Secondly, the time is significantly shortened, reducing the preheating time from ≥3 hours in existing technologies to 60-90 minutes; especially when using a two-stage control of "low-temperature preheating + medium-temperature stabilization", the dual goals of fully releasing internal stress and controlling the growth of the surface oxide layer are achieved.

[0016] Heating Section: The heating temperature of 1140–1180℃ is lower than conventional temperatures and 40–100℃ lower than existing technologies. It utilizes a superheat slightly higher than Ac3 and phase transformation driving force to rapidly complete core austenitization, rather than relying on high-temperature heat conduction. Time Compression to the Limit: The residence time in the high-temperature section is drastically reduced from ≥2.5 hours in existing technologies to 40–50 minutes, reducing the risk of surface decarburization by more than 70% in terms of time.

[0017] The soaking zone has a significantly lower heating temperature of 1090–1130℃, which is 20–120℃ lower than the existing soaking temperature. It is used only to eliminate residual temperature differences and complete the final homogenization of the composition. Secondly, the time is shortened by 75%, from ≥2 hours to 30–50 minutes, to avoid grain coarsening and increased surface carbon diffusion caused by prolonged high-temperature exposure.

[0018] Carbon replenishment section: This is the first time a carbon replenishment process has been introduced, with an independently set carbon replenishment section during heating. This is a fundamental difference from all existing technologies and effectively achieves carbon replenishment. In particular, when using temperature cyclic fluctuations, the temperature is increased, held, and slowly decreased within the range of 1080℃ to 1120℃, dynamically controlling the adsorption-diffusion-solution kinetics of surface carbon atoms to achieve precise carbon replenishment.

[0019] Pulsed atmosphere conditioning: Carbon-rich gas (such as cracked natural gas) is introduced in a pulsed manner to restore the carbon content of the surface layer to 98% to 102% of the matrix.

[0020] In particular, the cooling at the exit of the finishing mill adopts water cooling + mist cooling in order to lock in the thin oxide scale structure and prevent the oxide scale from thickening due to subsequent slow cooling.

[0021] The beneficial effects of adopting the above technical solution are as follows: By optimizing the heating process and the protective atmosphere in the furnace, the present invention greatly improves the decarburization of the gear steel surface and improves the surface quality of the steel; the decarburization depth is directly achieved in the hot-rolled state with a depth of ≤0.2%D and the surface and core hardness is ≤20HBW, the yield is greatly improved, the grinding rate of gear steel is reduced, the various indicators of the rolled products are guaranteed, and stable and efficient production is achieved, while meeting the needs of users. Attached Figure Description

[0022] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 This is a decarburization detection diagram of the rolled product in Embodiment 2 of the present invention. Detailed Implementation

[0024] This hot rolling method for improving surface decarburization of gear steel includes the following steps; (1) Pre-rolling heating steps: A heating furnace is used for multi-section heating, which includes a preheating section, a heating section, a homogenizing section, and a carbon supplementation section in sequence; when the gear steel billet is pre-rolled, 3 to 5 steps are performed. The heating temperature of the preheating section is 620 to 830℃ and the heating time is 60 to 90 minutes; the preheating section is preferably controlled in two stages: first, it is held at 620 to 680℃ for 30 to 40 minutes, and then raised to 700 to 830℃ and held for 30 to 50 minutes. The heating temperature of the heating section is 1140 to 1180℃ and the heating time is 40 to 50 minutes. The heating temperature of the homogenizing section is 1090 to 1130℃ and the heating time is 30 to 50 minutes. The heating temperature of the carbon replenishment section is 1080-1120℃, and the heating time is 40-60min. The temperature control of the carbon replenishment section is preferably as follows: first heat up to 1110-1120℃ and hold for 20-35min; then cool down to 1080-1090℃ and hold for 20-25min.

[0025] Each stage is divided into early, middle, and late phases based on actual conditions. The early phase accounts for 1 / 4 to 1 / 3 of the total time of the stage, the late phase accounts for 1 / 4 to 1 / 3 of the total time of the stage, and the remainder is the middle phase. The furnace atmosphere requirements for each stage are as follows: From the preheating stage to the middle phase of the heating stage (vol): CO2 / CO ratio 0.4 to 0.7, residual oxygen ≤0.3%, CO content 8% to 12%, preferably meeting the following atmosphere requirements (vol): residual oxygen ≤0.3%, CO 8% to 12%, CO2 0.5% to 1.5%, H2 8% to 12%, CH4 1% to 3%, N2 60% to 75%, CO2 / CO ratio 0.4 to 0.7, H2O / H2 ratio ≤0.4 to 0.7.

[0026] From the late heating stage to the early soaking stage (vol): CO / CO2 ratio 10–20, preferably meeting the following atmosphere requirements (vol): CO 10%–15%, CO2 0.5–1.5%, H2 8–12%, CH4 2–5%, N2 70–80%, CO / CO2 ratio 10–20. This step controls carbon potential; CO, CO2, H2, and H2O (water vapor) reach dynamic equilibrium through the water-gas reaction (CO + H2O ⇌ CO2 + H2). By controlling the CO / CO2 ratio within the target range, decarbonization can be suppressed.

[0027] During the middle of the soaking zone, the furnace atmosphere is replaced from the atmosphere in the later stage of the heating zone to the early stage of the soaking zone, with the atmosphere in the later stage of the soaking zone and the carbon supplementation zone as described below.

[0028] In the later stages of the soaking bath and the carbonization stage: the CO / CO2 ratio should be 16–30, preferably meeting the following atmosphere requirements (vol): CO 15%–25%, CO2 0.5–1.5%, H2 8–15%, CH4 3–8%, N2 45–65%, CO / CO2 ratio 16–30. During this stage, carbon-rich gas is pulsed in to control the carbon potential (Cp) in the furnace. At this stage, the CH4 content significantly increases, and combined with the heating temperature and time, the carbon potential in the furnace exhibits periodic peaks, driving carbon atoms to diffuse to the surface of the billet, achieving layer-by-layer repair of the decarburized layer. The goal of the pulsed carbon-rich gas introduction is to control the CO / CO2 ratio at 16–30. The optimal pulsed carbon-rich gas process is as follows: total pulse cycle duration: 10–30 minutes / cycle; strong penetration period duration: 4–10 minutes; diffusion period duration (stop or reduced): 6–20 minutes; number of pulse repetitions: 2–4 times. The core of the carbon-rich gas pulse process is to periodically introduce carbon-rich gas CH4 into the furnace on the basis of a static base atmosphere, so that the carbon potential in the furnace exhibits periodic spikes. The temperature window of the carbon replenishment section is 1080-1120℃. At this temperature, the diffusion coefficient of carbon in austenite is about 2.5-3 times that at 925℃, which can complete the inward diffusion of carbon atoms in a short time. At the same time, this temperature range meets the thermodynamic conditions for methane cracking (CH4→[C]+2H2), which can efficiently generate active carbon atoms.

[0029] (2) Rolling steps: After the billet exits the heating furnace, it is descaled by high-pressure water with a descaling water pressure ≥30MPa; then the billet is directly hot-rolled into gear steel of the target specifications. In the rolling process, the initial rolling temperature is 1050~1100℃, the protective cover is opened at the finishing mill inlet for heat preservation, the finishing mill inlet temperature is 1000~1050℃, and the finishing mill outlet temperature is 850~950℃; after finishing milling, cooling is preferably achieved by water cooling + mist cooling. This cooling method is to protect the repaired surface layer and prevent re-oxidation and decarburization. The cooling rate is controlled at 30~50℃ / s.

[0030] (3) This hot rolling method is applicable to low carbon CrMo gear steel with a C content ≤0.2wt% and a round steel specification of 50~150mm. After adopting this hot rolling method, the decarburization depth of the gear steel in the hot-rolled state is directly achieved to ≤0.2%D, where D is the diameter of the round steel, and the surface and core hardness is ≤20HBW.

[0031] Example 1: The hot rolling method for improving surface decarburization of gear steel is described in detail below.

[0032] (1) Pre-rolling heating steps: A heating furnace is used for multi-zone heating, which includes a preheating zone, a heating zone, a homogenizing zone, and a carbonization zone in sequence; when heating the gear steel billet, a three-step process is performed. The preheating zone is controlled in two stages: first, it is held at 620℃ for 30 minutes, and then raised to 700℃ and held for 30 minutes. The heating temperature of the heating zone is 1140℃ and the heating time is 40 minutes. The heating temperature of the homogenizing zone is 1090℃ and the heating time is 30 minutes. The temperature of the carbonization zone is controlled in two stages: first, it is raised to 1110℃ and held for 20 minutes; then, it is lowered to 1080℃ and held for 20 minutes. Among them, the total duration of the pulse cycle is 10 minutes; the duration of the strong penetration period is 4 minutes; the duration of the diffusion period is 6 minutes; and the number of pulse repetitions is 4 times. The furnace atmosphere requirements for each section are as follows: From the preheating section to the heating section: residual oxygen ≤0.3%, CO content 8%, CO2 0.5%, H2 8%, CH4 1%, N2 60%, CO2 / CO ratio 0.06, H2O / H2 ratio ≤0.5; From the later stage of the heating section to the early stage of the soaking section: CO 10%, CO2 0.5%, H2 8%, CH4 2%, N2 70%, CO / CO2 20; From the later stage of the soaking section and the carbon supplementation section: CO 15%, CO2 0.5%, H2 8%, CH4 3%, N2 45%, CO / CO2 30.

[0033] (2) Rolling steps: After the billet exits the heating furnace, it is descaled by high-pressure water at a pressure of 32 MPa; then the billet is directly hot-rolled into gear steel of the target specifications. In the rolling process, the initial rolling temperature is 1050℃, the protective cover is opened at the finishing mill inlet for heat preservation, the finishing mill inlet temperature is 1000℃, and the finishing mill outlet temperature is 850℃; after finishing milling, cooling is preferably achieved by water cooling + mist cooling, with a cooling rate of 30℃ / s.

[0034] (3) This hot rolling method is applicable to low-carbon CrMo gear steel with a C content ≤0.2wt% and a round steel specification of 50mm. After adopting this hot rolling method, the decarburization depth of the gear steel in the hot-rolled state is 0.08mm, the edge hardness is 196HBW, and the core hardness is 184HBW.

[0035] Example 2: The hot rolling method for improving surface decarburization of gear steel is described in detail below.

[0036] (1) Pre-rolling heating steps: A heating furnace is used for multi-zone heating, which includes a preheating zone, a heating zone, a homogenizing zone, and a carbonization zone in sequence; when heating the gear steel billet, a four-step process is performed. The preheating zone is controlled in two stages: first, it is held at 650℃ for 35 minutes, and then raised to 760℃ and held for 40 minutes. The heating temperature of the heating zone is 1160℃, and the heating time is 45 minutes. The heating temperature of the homogenizing zone is 1110℃, and the heating time is 40 minutes. The temperature of the carbonization zone is controlled in two stages: first, it is raised to 1115℃ and held for 30 minutes; then, it is lowered to 1085℃ and held for 22 minutes. Among them, the total duration of the pulse cycle is 20 minutes; the duration of the strong penetration period is 7 minutes; the duration of the diffusion period is 13 minutes; and the number of pulse repetitions is 3 times. The furnace atmosphere requirements for each section are as follows: From the preheating section to the heating section: residual oxygen ≤0.3%, CO 9%, CO2 ≤1.5%, H2 10%, CH4 2%, N2 70%, CO2 / CO ≤0.17, H2O / H2 ≤0.6; From the later stage of the heating section to the early stage of the soaking section: CO 12%, CO2 1.0%, H2 10%, CH4 3.5%, N2 75%, CO / CO2 12; From the later stage of the soaking section to the carbon supplementation section: CO 20%, CO2 1.0%, H2 12%, CH4 5%, N2 50%, CO / CO2 20.

[0037] (2) Rolling steps: After the billet exits the heating furnace, it is descaled by high-pressure water at a pressure of 34 MPa; then the billet is directly hot-rolled into gear steel of the target specifications. In the rolling process, the initial rolling temperature is 1080℃, the protective cover is opened at the finishing mill inlet for heat preservation, the finishing mill inlet temperature is 1030℃, and the finishing mill outlet temperature is 900℃; after finishing milling, cooling is preferably achieved by water cooling + mist cooling, with a cooling rate of 40℃ / s.

[0038] (3) This hot rolling method is applicable to low-carbon CrMo gear steel with a C content ≤ 0.2wt% and a round steel specification of 100mm. The decarburization test diagram of the hot-rolled gear steel after using this hot rolling method is shown below. Figure 1 As shown, the decarburization depth is 0.17 mm, the edge hardness is 203 HBW, and the core hardness is 185 HBW.

[0039] Example 3: The hot rolling method for improving surface decarburization of gear steel is described in detail below.

[0040] (1) Pre-rolling heating steps: A heating furnace is used for multi-zone heating, which includes a preheating zone, a heating zone, a homogenizing zone, and a carbonization zone. When heating the gear steel billet, a single empty section is installed for 5 steps. The preheating zone is controlled in two stages: first, it is held at 680℃ for 40 minutes, and then raised to 830℃ and held for 50 minutes. The heating temperature of the heating zone is 1180℃ and the heating time is 50 minutes. The heating temperature of the homogenizing zone is 1130℃ and the heating time is 50 minutes. The temperature of the carbonization zone is controlled in two stages: first, it is raised to 1120℃ and held for 35 minutes; then, it is lowered to 1090℃ and held for 25 minutes. The total duration of the pulse cycle is 30 minutes; the duration of the strong penetration period is 10 minutes; the duration of the diffusion period is 20 minutes; and the number of pulse repetitions is 2. The furnace atmosphere requirements for each section are as follows: From the preheating section to the heating section: residual oxygen ≤0.3%, CO 10%, CO2 ≤1.5%, H2 12%, CH4 3%, N2 75%, CO2 / CO ratio ≤0.15, H2O / H2 ratio ≤0.7; From the later stage of the heating section to the early stage of the soaking section: CO 15%, CO2 1.5%, H2 12%, CH4 5%, N2 80%, CO / CO2 10; From the later stage of the soaking section and the carbon supplementation section: CO 25%, CO2 1.5%, H2 15%, CH4 8%, N2 65%, CO / CO2 17.

[0041] (2) Rolling steps: After the billet exits the heating furnace, it is descaled by high-pressure water at a pressure of 36 MPa; then the billet is directly hot-rolled into gear steel of the target specifications. In the rolling process, the initial rolling temperature is 1100℃, the protective cover is opened at the finishing mill inlet for heat preservation, the finishing mill inlet temperature is 1050℃, and the finishing mill outlet temperature is 950℃; after finishing milling, cooling is preferably achieved by water cooling + mist cooling, with a cooling rate of 50℃ / s.

[0042] (3) This hot rolling method is applicable to low carbon CrMo gear steel with a C content ≤ 0.2wt% and a round steel specification of 150mm. After adopting this hot rolling method, the decarburization depth of the gear steel in the hot-rolled state is 0.25mm, the edge hardness is 198HBW, and the core hardness is 186HBW.

Claims

1. A hot rolling method for improving surface decarburization of gear steel, characterized in that: This includes pre-rolling heating and rolling steps; The pre-rolling heating step includes a preheating section, a heating section, a soaking section, and a carbonization section. The heating temperature of the preheating section is 620–830℃, and the heating time is 60–90 min. The heating temperature of the heating section is 1140–1180℃, and the heating time is 40–50 min. The heating temperature of the soaking section is 1090–1130℃, and the heating time is 30–50 min. The heating temperature of the carbonization section is 1080–1120℃, and the heating time is 40–60 min.

2. The hot rolling method for improving surface decarburization of gear steel according to claim 1, characterized in that, The furnace atmosphere requirements for the pre-rolling heating step are as follows: from the preheating section to the middle of the heating section, the CO2 / CO ratio is 0.4 to 0.7; from the later stage of the heating section to the early stage of the soaking section, the CO / CO2 ratio is 10 to 20; and from the later stage of the soaking section and the carbon supplementation section, the CO / CO2 ratio is 16 to 30.

3. The hot rolling method for improving surface decarburization of gear steel according to claim 2, characterized in that, The furnace atmosphere requirements for the pre-rolling heating step are as follows: From the preheating section to the middle of the heating section: residual oxygen ≤0.3%, CO 8%~12%, CO2 0.5~1.5%, H2 8~12%, CH4 1~3%, N2 60~75%, H2O / H2 ratio ≤0.4~0.7; From the late heating section to the early soaking section: CO 10%~15%, CO2 0.5~1.5%, H2 8~12%, CH4 2~5%, N2 70~80%; From the late soaking section and the carbon supplementation section: CO 15%~25%, CO2 0.5~1.5%, H2 8~15%, CH4 3~8%, N2 45~65%.

4. The hot rolling method for improving surface decarburization of gear steel according to claim 1, characterized in that, The preheating section is controlled in two stages: first, it is kept at 620-680℃ for 30-40 minutes, and then it is raised to 700-830℃ and kept for 30-50 minutes.

5. The hot rolling method for improving surface decarburization of gear steel according to claim 1, characterized in that, The temperature control of the carbon supplementation section is as follows: first, raise the temperature to 1110-1120℃ and hold for 20-35 minutes; then lower the temperature to 1080-1090℃ and hold for 20-25 minutes.

6. A hot rolling method for improving surface decarburization of gear steel according to any one of claims 1-5, characterized in that, The rolling process is as follows: the initial rolling temperature is 1050-1100℃, the finishing rolling inlet temperature is 1000-1050℃, the finishing rolling outlet temperature is 850-950℃, and cooling is performed after finishing rolling.

7. The hot rolling method for improving surface decarburization of gear steel according to claim 6, characterized in that: The cooling after finishing rolling adopts water cooling + mist cooling, with a cooling rate of 30-50℃ / s.

Images