A production method for reducing the hardness of 20CrMnTi gear steel hot-rolled material

Abstract

The application discloses a production method for reducing the hardness of 20CrMnTi gear steel hot-rolled materials, belongs to the field of metallurgical technology, and comprises heating, rolling and post-rolling cooling processes; the rolling process adopts two-stage rolling, the first stage is rolling in a recrystallization temperature interval, the rolling temperature is greater than 900 DEG C, and rapid cooling is carried out after rolling; the second stage is rolling in a non-recrystallization interval, the rolling temperature is less than 820 DEG C; the post-rolling cooling process adopts the mode of air cooling on a cooling bed to cool to below 200 DEG C, the cooling rate is controlled to be less than or equal to 1.2 DEG C / s, and no heat preservation cover is added on the cooling bed during the cooling process. In the 20CrMnTi gear steel hot-rolled structure obtained through the controlled rolling and controlled cooling process, the volume percentage of ferrite is greater than 60%, the hardness reaches the national standard of 221HV or below, and no additional annealing or other heat treatment processes are needed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy, and particularly relates to a production method for reducing the hardness of 20CrMnTi gear steel hot-rolled material. BACKGROUND

[0002] Due to the continuous development of the automobile and heavy industry, the demand for parts such as gears gradually increases. For many years, 20CrMnTi has been dominant in the production of gear steel in China as a low-carbon alloy carburizing steel.

[0003] Because the gear steel bar needs to go through multiple processes such as shearing, turning and other processes in the processing process of the gear enterprise, the gear enterprise has requirements for the hardness and cutting performance of the alloy steel bar supplied by the steel plant. For 20CrMnTi gear steel, the national standard GB / T 3077-2015 "Alloy Structural Steel" stipulates that the hardness of the bar supplied should be controlled below 221HV. However, in the actual production of the steel enterprise, there is often a problem of hardness exceeding the standard caused by too high volume fraction of bainite in 20CrMnTi gear steel hot-rolled material, which cannot be directly delivered to the customer and needs additional annealing heat treatment of 20CrMnTi gear steel hot-rolled material to reduce the hardness, resulting in a series of problems such as increased production cost, deteriorated surface of the rolled material, increased burning loss and increased carbon emissions.

[0004] The patent application with application number 202111001919.9 and the name "A rolling method for reducing the hardness and bending degree of 20CrMnTi hot-rolled state" proposes a rolling method for reducing the hardness of 20CrMnTi hot-rolled state, but the realization of the hardness control target still depends on the use of a cooling bed heat preservation cover to achieve a low cooling speed (cooling rate in the heat preservation cover is 0.18-0.22℃ / s) after rolling, and most of the ferrite transformation is completed in the cooling bed heat preservation cover (temperature out of the heat preservation cover is 580-600℃). This process has high requirements for the cooling bed heat preservation equipment and the running speed of the cooling bed, and is not suitable for high-efficiency production without a cooling bed heat preservation cover. SUMMARY

[0005] To solve the above technical problems, the application provides a production method for reducing the hardness of 20CrMnTi gear steel hot-rolled material, which can make the hardness of 20CrMnTi gear steel hot-rolled material meet the national standard requirements through controlled rolling and controlled cooling process, without the need for additional annealing and other heat treatment processes.

[0006] To achieve the above application purposes, the technical solution adopted by the application is as follows:

[0007] A production method for reducing the hardness of 20CrMnTi gear steel hot-rolled material, including heating, rolling and post-rolling cooling processes;

[0008] The rolling process adopts two-stage rolling, the first stage is rolled in the recrystallization temperature interval, the rolling temperature is greater than 900 DEG C, and rapid cooling is carried out after rolling; the second stage is rolled in the non-recrystallization interval, and the rolling temperature is less than 820 DEG C;

[0009] The post-rolling cooling process adopts the mode of air cooling on the cooling bed to cool to below 200 DEG C, and the cooling rate is controlled to be less than or equal to 1.2 DEG C / s, and the cooling process does not add a cooling bed heat preservation cover.

[0010] The heating process in the application controls the heating 1 segment heating temperature to be 950-1150 DEG C, the heating 2 segment heating temperature to be 1100-1250 DEG C, and the soaking segment temperature to be 1150-1250 DEG C, and the total heating time is 120-180 min.

[0011] The rolling process in the application controls the opening rolling temperature to be 1050-1180 DEG C, and the final rolling temperature to be 780-810 DEG C.

[0012] The rolling process in the application controls the rapid cooling after the first stage rolling, and the cooling speed is greater than or equal to 10 DEG C / s, and the cooling is to the second stage rolling temperature.

[0013] The rolling process in the application controls the total reduction of the second stage rolling to be not less than 20%.

[0014] The microstructure of the steel material obtained after rolling in the application is ferrite, pearlite and bainite, wherein the volume percentage of ferrite is greater than or equal to 60%, and the hardness of the steel material is less than or equal to 221 HV.

[0015] The beneficial effects generated by the above technical scheme are: the application reduces the hardness of the 20CrMnTi gear steel hot-rolled material through the controlled rolling and controlled cooling process, the hot-rolled structure is significantly optimized, the hardness of the rolled material after hot rolling is significantly reduced, the original required annealing heat treatment process is saved, the energy is saved, and the production efficiency is improved.

[0016] (1) The first stage rolling temperature is controlled to be greater than or equal to 900 DEG C, so that the rolled material can be fully recrystallized to form uniform small austenite grains during rolling, and the deformation storage energy introduced by subsequent deformation in the non-recrystallization zone is further increased; after the first stage rolling, the rapid cooling inhibits the growth of the fine austenite grains.

[0017] (2) The second stage rolling temperature is controlled to be less than or equal to 820 DEG C, which can improve the strain storage energy of the rolled material during rolling, thereby increasing the phase change driving force of austenite to ferrite, and at the same time, since the temperature is close to the Ae3 temperature of the 20CrMnTi gear steel, the strain-induced ferrite phase change during rolling can be promoted, and the temperature at which the ferrite phase change begins is increased, thereby promoting the full transformation of ferrite during the structure transformation.

[0018] (3) After rolling, air cooling is adopted on the cooling bed, the cooling rate is controlled below 1.2 ℃ / s, and a cooling bed heat preservation cover is not needed, so that the ferrite volume fraction in the phase change structure of the hot-rolled material can be increased to more than 60%, thereby reducing the hardness of the hot-rolled material.

[0019] In the hot-rolled structure of the 20CrMnTi gear steel obtained by the method, the volume percentage of ferrite is greater than 60%, and the sum of the volume percentages of hard phases such as bainite and pearlite is less than 40%, so that the hardness of the 20CrMnTi hot-rolled material reaches the control below 221HV specified in the national standard GB / T3077-2015 "Alloy Structural Steel". BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 1 is shown in the figure.

[0021] Figure 2 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 2 is shown in the figure.

[0022] Figure 3 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 3 is shown in the figure.

[0023] Figure 4 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 4 is shown in the figure.

[0024] Figure 5 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 5 is shown in the figure.

[0025] Figure 6 The microstructure of the 20CrMnTi gear steel hot-rolled material in Example 6 is shown in the figure.

[0026] Figure 7 The microstructure of the 20CrMnTi gear steel hot-rolled material in Comparative Example 1 is shown in the figure.

[0027] Figure 8 The microstructure of the 20CrMnTi gear steel hot-rolled material in Comparative Example 2 is shown in the figure. DETAILED DESCRIPTION Example 1

[0028] In this embodiment, the composition of the 20CrMnTi gear steel sample is as follows in terms of weight percentage: C 0.2%, Cr 1.22%, Mn 1.0%, Si 0.25%, Ti 0.06%, Al 0.02%, S 0.002%, P 0.002%, and the balance is Fe and unavoidable impurities.

[0029] The production method for reducing the hardness of the 20CrMnTi gear steel hot-rolled material in this embodiment comprises the following process steps:

[0030] (1) The 20CrMnTi gear steel 300mmx350mm continuous casting billet is sent into a step-by-step heating furnace for heating, the heating temperature of the first heating section is controlled to be 1150℃, the heating temperature of the second heating section is controlled to be 1250℃, the temperature of the soaking section is controlled to be 1250℃, and the total heating time is 120min;

[0031] (2) In the rolling process, two-stage rolling is adopted, the open rolling temperature is 1180℃, the first stage is rolled in the recrystallization temperature range, and the 20CrMnTi intermediate material with a diameter of 89mm is obtained through 13 passes, and the rolling temperature of the 13th pass is controlled to be 950℃, so as to fully recrystallize in the rolling process to form uniform small austenite grains and further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized zone.

[0032] (3) After the first stage rolling, rapid cooling is carried out to cool the 20CrMnTi intermediate material to 819℃, and the cooling speed is 10.2℃ / s, so as to inhibit the growth of the fine austenite grains.

[0033] (4) The second stage is rolled in the unrecrystallized zone, the intermediate material is rolled into a rod with a diameter of 65mm through 4 passes, and the final rolling temperature is 810℃, and the total reduction is 27%, so as to increase the strain storage energy of the rolled material in the rolling process, thereby increasing the phase change driving force of the austenite to ferrite transformation, and at the same time, since the temperature is close to the Ae3 temperature of the 20CrMnTi gear steel, the strain-induced ferrite phase change in the rolling process can be promoted, and the temperature at which the ferrite phase change starts can be increased.

[0034] (5) After the rolling is completed, the rolled material is air-cooled to below 200℃ on the cooling bed, and the cooling rate is 1.1℃ / s, and the cooling bed is not covered with a heat preservation cover during the cooling process.

[0035] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 1 The microstructure of the 20CrMnTi hot-rolled material observed under an optical microscope in this embodiment is shown in the figure, in which the volume percentage of the relatively soft ferrite structure reaches 61%, the sum of the volume percentages of the pearlite and the bainite is 39%, and the hardness of the hot-rolled material is 216HV, which meets the national standard. Example 2

[0036] In this embodiment, the composition of the 20CrMnTi gear steel sample is as follows in terms of weight percentage: C 0.21%, Cr 1.20%, Mn 1.1%, Si 0.22%, Ti 0.06%, Al 0.021%, S 0.003%, P 0.003%, and the balance is Fe and unavoidable impurities.

[0037] The production method of the present embodiment for reducing the hardness of 20CrMnTi gear steel hot-rolled material comprises the following process steps:

[0038] (1) The 300mm x 350mm continuous casting billet of 20CrMnTi gear steel is sent into a walking beam heating furnace for heating, the heating temperature of the first heating section is controlled to be 950℃, the heating temperature of the second heating section is controlled to be 1100℃, the temperature of the soaking section is controlled to be 1150℃, and the total heating time is 180min.

[0039] (2) In the rolling process, two-stage rolling is adopted, the open rolling temperature is 1050℃, the first stage is rolled in the recrystallization temperature range, and the 20CrMnTi intermediate material with a diameter of 89mm is obtained through 13 passes, and the rolling temperature of the 13th pass is controlled to be 905℃, so as to fully recrystallize in the rolling process to form uniform and fine austenite grains, and further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized zone.

[0040] (3) After the first stage rolling, rapid cooling is carried out to cool the 20CrMnTi intermediate material to 790℃, and the cooling speed is 10.5℃ / s, so as to inhibit the growth of fine austenite grains.

[0041] (4) The second stage is rolled in the unrecrystallized zone, the intermediate material is rolled into a rod with a diameter of 60.8mm through 4 passes, and the finish rolling temperature is 780℃, and the total reduction is 32%, so as to increase the strain storage energy of the rolled material in the rolling process, thereby increasing the phase change driving force of austenite to ferrite, and at the same time, since the temperature is close to the Ae3 temperature of the deformed 20CrMnTi gear steel, the strain-induced ferrite phase change in the rolling process can be promoted, and the temperature at which the ferrite phase change starts is increased.

[0042] (5) After rolling is completed, the rolled material is air-cooled to below 200℃ on the cooling bed, and the cooling rate is 1.2℃ / s, and no cover cooling bed heat preservation cover is added during the cooling process.

[0043] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 2 The microstructure of the 20CrMnTi hot-rolled material of the present embodiment observed under an optical microscope is shown in the figure, in which the volume percentage of the relatively soft ferrite structure reaches 63%, the sum of the volume percentages of the pearlite and bainite is 37%, the hardness of the hot-rolled material is 214HV, and it meets the national standard. Example 3

[0044] In the present embodiment, the composition of the 20CrMnTi gear steel sample is as follows in terms of weight percentage: C 0.21%, Cr 1.21%, Mn 1.10%, Si 0.25%, Ti 0.066%, Al 0.022%, S 0.003%, P 0.002%, and the balance is Fe and unavoidable impurities.

[0045] The production method of the present embodiment for reducing the hardness of 20CrMnTi gear steel hot-rolled material includes the following process steps:

[0046] (1) The 20CrMnTi gear steel 300mm x 350mm continuous casting billet is sent into a step-by-step heating furnace for heating, with the heating 1 segment temperature controlled at 1020°C, the heating 2 segment temperature controlled at 1145°C, the soaking segment temperature controlled at 1180°C, and the total heating time controlled at 156 min;

[0047] (2) During the rolling process, two-stage rolling is adopted, with the open rolling temperature controlled at 1090°C, the first stage rolled in the recrystallization temperature range to form an intermediate material with a diameter of 89mm through 13 passes, with the 13th pass rolling temperature controlled at 920°C, so as to fully recrystallize to form uniform small austenite grains during the rolling process and further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized zone.

[0048] (3) After the first stage rolling, rapid cooling is performed to cool the 20CrMnTi intermediate material to 803°C at a cooling speed of 10.8°C / s, so as to suppress the growth of the fine austenite grains.

[0049] (4) The second stage is rolled in the unrecrystallized zone, with the intermediate material rolled into a rod with a diameter of 65mm through 4 passes, with the finish rolling temperature controlled at 791°C and the total reduction controlled at 27%, so as to increase the strain storage energy of the rolled material during the rolling process, thereby increasing the phase change driving force of the austenite to ferrite transformation, and at the same time, since the temperature is close to the Ae3 temperature of the 20CrMnTi gear steel, the strain-induced ferrite phase change during the rolling process can be promoted, and the temperature at which the ferrite phase change begins can be increased.

[0050] (5) After the rolling is completed, the rolled material is air-cooled to below 200°C on the cooling bed at a cooling speed of 1.15°C / s, and no cover cooling bed heat preservation cover is added during the cooling process.

[0051] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 1 The microstructure of the 20CrMnTi hot-rolled material of the present embodiment observed under an optical microscope is shown in the figure, in which the volume percentage of the relatively soft ferrite structure reaches 62%, the sum of the volume percentages of the pearlite and bainite is 38%, and the hardness of the hot-rolled material is 215HV, which meets the national standard. Example 4

[0052] The composition of the 20CrMnTi gear steel sample in percentage by weight is: C 0.22%, Cr 1.10%, Mn 1.12%, Si 0.23%, Ti 0.062%, Al 0.026%, S 0.003%, P 0.004%, and the balance of Fe and inevitable impurities.

[0053] The production method for reducing the hardness of the 20CrMnTi gear steel hot-rolled material in this embodiment comprises the following process steps:

[0054] (1) The 20CrMnTi gear steel 300mmx350mm continuous casting billet is sent into a walking beam heating furnace for heating, the heating temperature of the first heating section is controlled to be 1070°C, the heating temperature of the second heating section is controlled to be 1190°C, the soaking section temperature is controlled to be 1210°C, and the total heating time is 144 min.

[0055] (2) In the rolling process, two-stage rolling is adopted, the opening rolling temperature is 1130°C, the first stage is rolled in the recrystallization temperature range, and the 20CrMnTi intermediate material with a diameter of 89mm is obtained through 13 passes, and the rolling temperature of the 13th pass is controlled to be 931°C, the purpose is to fully recrystallize to form uniform and fine austenite grains in the rolling process, and further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized zone.

[0056] (3) After the first stage rolling, rapid cooling is carried out to cool the 20CrMnTi intermediate material to 807°C, and the cooling speed is 10.3°C / s, the purpose is to inhibit the growth of fine austenite grains.

[0057] (4) The second stage is rolled in the unrecrystallized zone, and the intermediate material is rolled into a rod with a diameter of 60.8mm through 4 passes, and the final rolling temperature is 798°C, and the total reduction is 32%, the purpose is to increase the strain storage energy of the rolled material in the rolling process, so as to increase the phase change driving force of austenite to ferrite, and at the same time, since the temperature is close to the Ae3 temperature of the deformed 20CrMnTi gear steel, the strain-induced ferrite phase change in the rolling process can be promoted, and the temperature at which the ferrite phase change starts is increased.

[0058] (5) After rolling is completed, the rolled material is air-cooled to below 200°C on the cooling bed, and the cooling rate is 1.13°C / s, and no cover cooling bed heat preservation cover is added during the cooling process.

[0059] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 2 The microstructure of the 20CrMnTi hot-rolled material in this embodiment observed under an optical microscope is shown in the figure, in which the volume percentage of the relatively soft ferrite structure reaches 61.8%, the sum of the volume percentages of the pearlite and bainite is 38.2%, and the hardness of the hot-rolled material is 215HV, which meets the national standard. Embodiment 5

[0060] In this embodiment, the composition of the 20CrMnTi gear steel sample is, in percentage by weight: C 0.21%, Cr 1.21%, Mn 1.13%, Si 0.30%, Ti 0.05%, Al 0.022%, S 0.004%, P 0.004%, and the balance being Fe and unavoidable impurities.

[0061] The production method of this embodiment for reducing the hardness of 20CrMnTi gear steel hot-rolled material includes the following process steps:

[0062] (1) The 20CrMnTi gear steel 300mm x 350mm continuous casting billet is sent into a walking beam heating furnace for heating, the heating temperature of the first heating section is controlled to be 1110℃, the heating temperature of the second heating section is controlled to be 1220℃, the soaking section temperature is controlled to be 1230℃, and the total heating time is controlled to be 132min;

[0063] (2) In the rolling process, two-stage rolling is adopted, the opening rolling temperature is 1154℃, the first stage is rolled in the recrystallization temperature range, and the 20CrMnTi intermediate material with a diameter of 89mm is obtained after 13 passes, and the rolling temperature of the 13th pass is controlled to be 941℃, the purpose is to fully recrystallize to form uniform and small austenite grains in the rolling process, and further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized zone.

[0064] (3) After the first stage rolling, rapid cooling is carried out to cool the 20CrMnTi intermediate material to 813℃, and the cooling speed is 10.5℃ / s, the purpose is to suppress the growth of fine austenite grains.

[0065] (4) The second stage is rolled in the unrecrystallized zone, the intermediate material is rolled into a rod with a diameter of 65mm through 4 passes, and the final rolling temperature is 804℃, and the total reduction is 27%, the purpose is to increase the strain storage energy of the rolled material in the rolling process, so as to increase the phase change driving force of austenite to ferrite, at the same time, since the temperature is close to the Ae3 temperature of 20CrMnTi gear steel, it can promote the strain-induced ferrite phase change in the rolling process, and increase the temperature at which the ferrite phase change starts.

[0066] (5) After rolling is completed, the rolled material is air-cooled to below 200℃ on the cooling bed, and the cooling rate is 1.1℃ / s, and no cover cooling bed heat preservation cover is added during the cooling process.

[0067] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 1 The microstructure of the 20CrMnTi hot-rolled material of this embodiment observed under an optical microscope is shown in the figure, in which the volume percentage of the relatively soft ferrite structure reaches 61%, the sum of the volume percentages of pearlite and bainite is 39%, and the hardness of the hot-rolled material is 215HV, which meets the national standard. Example 6

[0068] In this example, the composition of the 20CrMnTi gear steel sample is as follows in terms of weight percentage: C 0.21%, Cr 1.22%, Mn 1.10%, Si 0.26%, Ti 0.055%, Al 0.021%, S 0.003%, P 0.006%, and the balance being Fe and unavoidable impurities.

[0069] The production method of this example for reducing the hardness of a 20CrMnTi gear steel hot-rolled material includes the following process steps:

[0070] (1) The 20CrMnTi gear steel 300mm x 350mm continuous casting billet is sent to a walking beam heating furnace for heating, with the heating 1 segment temperature controlled at 990°C, the heating 2 segment temperature controlled at 1130°C, the soaking segment temperature controlled at 1170°C, and the total heating time controlled at 168 minutes.

[0071] (2) During rolling, two-stage rolling is used, with the rolling temperature controlled at 1080°C, the first stage rolled in the recrystallization temperature range to form an intermediate material with a diameter of 89mm through 13 passes, with the 13th pass rolling temperature controlled at 910°C. The purpose is to fully recrystallize to form uniform and fine austenite grains during rolling, and to further increase the deformation storage energy introduced in the subsequent deformation in the unrecrystallized region.

[0072] (3) After the first stage of rolling, rapid cooling is performed to cool the 20CrMnTi intermediate material to 796°C at a cooling rate of 10.5°C / s. The purpose is to suppress the growth of the fine austenite grains.

[0073] (4) The second stage is rolled in the unrecrystallized region, with the intermediate material rolled into a bar with a diameter of 60.8mm through 4 passes, with the finish rolling temperature controlled at 784°C and the total reduction controlled at 32%. The purpose is to increase the strain storage energy of the rolled material during rolling, thereby increasing the phase transformation driving force of the austenite to ferrite transformation. At the same time, since the temperature is close to the Ae3 temperature of the 20CrMnTi gear steel after deformation, it can promote the strain-induced ferrite phase transformation during rolling and increase the temperature at which the ferrite phase transformation begins.

[0074] (5) After rolling is completed, the rolled material is air-cooled to below 200°C on a cooling bed at a cooling rate of 1.2°C / s, and no cover cooling bed heat preservation cover is added during the cooling process.

[0075] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness testing. Figure 2The microstructure of the hot-rolled material of Example 20CrMnTi observed under an optical microscope, wherein the volume percentage of the soft ferrite structure is 62%, the sum of the volume percentages of pearlite and bainite is 38%, and the hardness of the hot-rolled material is 216 HV, which meets the national standard.

[0076] Comparative Example 1

[0077] In the present comparative example, the composition of the 20CrMnTi gear steel sample is as follows: C 0.2%, Cr 1.22%, Mn 1.0%, Si 0.25%, Ti 0.06%, Al 0.02%, S 0.002%, P 0.002%, and the balance is Fe and inevitable impurities.

[0078] The process steps of the present comparative example are as follows:

[0079] (1) The 20CrMnTi gear steel 300 mm x 350 mm continuous casting billet is sent into a walking beam heating furnace for heating, the heating temperature of the first heating stage is controlled at 1150°C, the heating temperature of the second heating stage is controlled at 1250°C, the soaking temperature is controlled at 1250°C, and the heating time is 176 min.

[0080] (2) During the rolling process, two-stage rolling is adopted, the open rolling temperature is 1100°C, the first stage is rolled in the recrystallization temperature range, and the intermediate material with a diameter of 89 mm is obtained after 13 passes, and the rolling temperature of the 13th pass is 920°C.

[0081] (3) No cooling measures are adopted between the first stage and the second stage rolling, and the second stage rolling is carried out by 4 passes to roll the intermediate material into a rod with a diameter of 65 mm, the finish rolling temperature is 870°C, and the total reduction is 27%.

[0082] (4) After the rolling is completed, the rolled material is air-cooled on the cooling bed.

[0083] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 3 The microstructure of the hot-rolled 20CrMnTi observed under an optical microscope in the present comparative example, wherein the volume percentage of the soft ferrite structure is only 30%, the sum of the volume percentages of pearlite and bainite is 70%, and the hardness of the hot-rolled material is 286 HV, which exceeds the national standard.

[0084] Comparative Example 2

[0085] In the present comparative example, the composition of the 20CrMnTi gear steel sample is as follows: C 0.21%, Cr 1.20%, Mn 1.1%, Si 0.22%, Ti 0.06%, Al 0.021%, S 0.003%, P 0.003%, and the balance is Fe and inevitable impurities.

[0086] The process steps of the present comparative example are as follows:

[0087] (1) The 20CrMnTi gear steel 300mmx350mm continuous casting billet is sent into a step-by-step heating furnace for heating, the heating temperature of heating stage 1 is controlled to be 950℃, the heating temperature of heating stage 2 is controlled to be 1100℃, the temperature of the soaking stage is 1150℃, and the heating time is 177min.

[0088] (2) In the rolling process, two-stage rolling is adopted, the open rolling temperature is 1080℃, the first stage is rolled in the recrystallization temperature range, and the intermediate material with a diameter of 89mm is obtained through 13 passes, and the rolling temperature of the 13th pass is 905℃.

[0089] (3) No cooling measures are adopted between the first stage and the second stage rolling, the second stage rolling is carried out through 4 passes, the intermediate material is rolled into a rod with a diameter of 60.8mm, the finish rolling temperature is 850℃, and the total reduction is 32%.

[0090] (4) After the rolling is completed, the rolled material is air-cooled on the cooling bed.

[0091] After the rolled material is cooled to room temperature, samples are taken for metallographic structure observation and Vickers hardness detection. Figure 4 The microstructure of the 20CrMnTi hot rolling observed under the optical microscope is shown in the present comparative example, the volume percentage of the relatively soft ferrite structure is 33%, the sum of the volume percentages of the pearlite and the bainite is 67%, the hardness of the hot rolled material is 291HV, which exceeds the national standard.

[0092] From the comparative examples 1 and 2, it can be seen that without adopting the rolling process of the present application, the volume percentage of the ferrite structure in the rolled material after being cooled to room temperature is low, which leads to the hardness of the hot rolled material exceeding the national standard requirement.

Claims

1. A production method for reducing the hardness of a hot-rolled material of 20CrMnTi gear steel, characterized in that, The heating, rolling and post-rolling cooling processes are included; The rolling process is carried out at an open rolling temperature of 1050-1180 DEG C and a finish rolling temperature of 780-810 DEG C, and two-stage rolling is adopted, the first stage is carried out at a recrystallization temperature interval, the rolling temperature is > 900 DEG C, and rapid cooling is carried out after rolling; the second stage is carried out at a non-recrystallization interval, the rolling temperature is < 820 DEG C, and the total reduction in the second stage is not less than 20%; The post-rolling cooling process is carried out by air cooling on a cooling bed to below 200 DEG C, the cooling rate is controlled to be ≤ 1.2 DEG C / s, and no cooling bed heat preservation cover is added during the cooling process.

2. The production method of reducing the hardness of a 20CrMnTi gear steel hot-rolled material according to claim 1, characterized in that, In the heating process, the heating temperature of the first heating section is controlled to be 950-1150 DEG C, the heating temperature of the second heating section is controlled to be 1100-1250 DEG C, the soaking section temperature is controlled to be 1150-1250 DEG C, and the total heating time is 120-180 min.

3. The production method of reducing the hardness of a 20CrMnTi gear steel hot-rolled material according to claim 2, characterized in that, In the rolling process, rapid cooling is carried out after the first stage rolling, the cooling rate is ≥ 10 DEG C / s, and the second stage rolling temperature is obtained.

4. The production method for reducing the hardness of hot-rolled 20CrMnTi gear steel according to any one of claims 1-3, characterized in that, The microstructure of the obtained steel after rolling is ferrite, pearlite and bainite, and the volume percentage of ferrite is ≥ 60%.

5. The production method of reducing the hardness of a 20CrMnTi gear steel hot-rolled material according to claim 4, characterized in that, The hardness of the obtained steel after rolling is ≤ 221 HV.

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