Method for controlling the structure and hardness of small-sized 20CrMnTi bars

Abstract

The present invention relates to the field of rolling technology, and in particular to a method for controlling the structure and hardness of a small-sized 20CrMnTi bar, comprising the steps of: (1) measuring the actual composition of the elements contained in the bar; (2) calculating the DI value based on the actual composition measured in step (1); (3) rolling the bar so that the cumulative deformation of the rolled bar is ≥21% when the DI value calculated in step (2) is ≤47; and (4) placing the rolled bar on a cooling bed and cooling it to below 630°C when the temperature of the rolled bar is 800-850°C. The method of the present invention ensures the accuracy of the processing technology, so that the small-sized 20CrMnTi series bar can be stably controlled to obtain a structure of ferrite and pearlite and a hardness of 160-220 HBW.

Description

Technical Field

[0001] The invention relates to the technical field of rolling, in particular to a method for controlling the structure and hardness of a small-size 20CrMnTi bar. Background Art

[0002] In the rolling process of producing CrMnTi series gear steel, in order to facilitate subsequent turning processing, it is usually necessary to control the structure to ferrite and pearlite, and the hardness needs to be controlled at 160-220HBW. At present, the method of controlling the structure and hardness is mostly to control the temperature of the upper cooling bed and slow down the cooling, but this method is not suitable for small specifications. For example, the patent "CN110129653A A method for producing low-hardness 20CrMnTi round steel" mentions that after rolling, the round steel enters the cooling bed for natural cooling after passing through water. The temperature of the upper cooling bed of the round steel is controlled at 930±30℃. The specifications of 20CrMnTi round steel are Φ30mm-Φ42mm. When the specifications are larger, the cooling speed on the cooling bed is slow, which is suitable for this method. However, when the bar specifications are reduced, especially φ14-30mm, due to the fast cooling speed on the cooling bed, bainite and martensite structures often appear in the metallographic phase, and the hardness reaches more than 260HBW. Therefore, this method is not suitable for small-sized gear steel structure and hardness control.

[0003] As for the problem of high hardness caused by fast cooling speed after rolling of small-sized gear steel bars, most steel mills currently adopt low-temperature rolling. For example, "CN108906884B A method for producing high-performance 20CrMnTi gear steel at low temperature" mentions controlling the inlet temperature of the sizing unit or the finishing unit to 800-828°C. Although this method can control the structure and hardness of small-sized 20CrMnTi series bars, the 20CrMnTi series gear steel can be divided into multiple grades according to the composition, including 20CrMnTi, 20CrMnTiHL, 20CrMnTiH1, 20CrMnTiH3, 20CrMnTiH5, and 20CrMnTiHH, with different compositions. If all of them are rolled at such a low temperature, the load of equipment such as rolling mills and sizing units will increase, resulting in increased wear and reduced production efficiency and output. This method is not applicable to bar rolling lines dominated by domestic rolling mills. Therefore, a method of controlling the appropriate structure and hardness according to the composition of 20CrMnTi series gear steel is proposed, which is particularly important for bar rolling lines that are generally based on domestic equipment.

[0004] The existing technology adopts ultra-low temperature rolling for all small-sized 20CrMnTi series gear steels, and the final rolling temperature is as low as 800-828℃. The low rolling temperature will inevitably lead to increased loads on equipment such as rolling mills and sizing units, resulting in increased wear and tear, and reduced production efficiency and output. This method is not applicable to bar rolling lines dominated by domestic rolling mills. At the same time, the composition of 20CrMnTi series bars is different, and some low-composition 20CrMnTi does not need to be ultra-low temperature rolled. Summary of the invention

[0005] The technical problem to be solved by the present invention is: in order to overcome the problem in the prior art that small-sized gear steel bars have high hardness due to fast cooling rate after rolling, a method for controlling the structure and hardness of small-sized 20CrMnTi bars is provided.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a method for controlling the structure and hardness of small-sized 20CrMnTi bars,

[0007] Step (1): measuring the actual composition of the elements contained in the rod;

[0008] Step (2): Calculate the DI value based on the actual components measured in step (1);

[0009] Step (3): when the DI value calculated in step (2) is ≤47, the rolling speed is 6-12 m / s, and the rolling temperature is 870-900° C., rolling the bar for 3-5 passes so that the cumulative deformation of the rolled bar is ≥21%;

[0010] When 48≤DI value≤55, the bar is rolled for 3-5 times at a rolling temperature of 830-860°C, so that the cumulative deformation of the rolled bar is ≥25%;

[0011] Step (4): When the temperature of the rolled bar is 800-850 degrees Celsius, it is placed on a cooling bed and cooled to below 630 degrees Celsius at a cooling rate of 1.1 degrees Celsius / s.

[0012] The beneficial effects of the present invention are as follows: the present invention calculates the DI value according to the composition of the 20CrMnTi series bars, and then selects a suitable rolling process system, thereby avoiding the reduction of production efficiency caused by ultra-low temperature rolling, ensuring the accuracy of the processing technology, and thus being able to stably control the small-size 20CrMnTi series bars to obtain a structure of ferrite and pearlite and a hardness of 160-220 HBW, and being able to avoid the rolling mill, the sizing unit and other equipment from being subjected to a large load, reducing the aggravation of equipment wear, and improving production efficiency and output. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0014] Figure 1 It is a schematic diagram of the metallographic structure of Example 1 of the present invention magnified 100 times (200 μm);

[0015] Figure 2 It is a schematic diagram of the metallographic structure of Example 1 of the present invention magnified 500 times (50 μm);

[0016] Figure 3 It is a schematic diagram of the metallographic structure of Example 2 of the present invention magnified 100 times (200 μm);

[0017] Figure 4 It is a schematic diagram of the metallographic structure of Example 2 of the present invention magnified 500 times (50 μm);

[0018] Figure 5 It is a schematic diagram of the metallographic structure of Example 3 of the present invention magnified 100 times (200 μm);

[0019] Figure 6 It is a schematic diagram of the metallographic structure of Example 3 of the present invention magnified 500 times (50 μm);

[0020] Figure 7 It is a schematic diagram of the metallographic structure of Example 4 of the present invention magnified 100 times (200 μm);

[0021] Figure 8 It is a schematic diagram of the metallographic structure of Example 4 of the present invention magnified 500 times (50 μm);

[0022] Fig. 9 It is a schematic diagram of the metallographic structure of Example 5 of the present invention magnified 100 times (200 μm);

[0023] Fig.10 It is a schematic diagram of the metallographic structure of Example 5 of the present invention magnified 500 times (50 μm). DETAILED DESCRIPTION

[0024] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0025] The chemical composition of 20CrMnTi series is C: 0.17~0.23%, Si: 0.17~0.37%, Mn: 0.80~1.20%, P: ≤0.035%, S≤0.035%, Cr: 1.00~1.45%, Ni: ≤0.30, Cu: ≤0.20%, Ti: 0.04~0.10%, and the rest is Fe.

[0026] As a method for controlling the structure and hardness of small-sized 20CrMnTi bars of the present invention, the structure of small-sized 20CrMnTi bars of φ14-30mm can be stably obtained as bars with a structure of ferrite and pearlite and a hardness of 160-220HBW.

[0027] The control methods of 20CrMnTi bar structure include:

[0028] Step (1): measuring the actual composition of the elements contained in the rod;

[0029] Step (2): Calculate the DI value based on the actual composition measured in step (1). The DI value calculation formula is as follows: Di = 25.4 × 1.0 × (1 + 3.333 × [Mn]) × (1 + 0.7 × [Si]) × (1 + 0.363 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3.0 × [Mo]) × (1 + 0.365 × [Cu]) × (1 + 1.73 × [V]) × 0.540 [C] × (1 + 2.5 × [Zr]);

[0030] Step (3): when the DI value calculated in step (2) is ≤47, the rolling speed is 6-12 m / s, and the rolling temperature is 870-900° C., rolling the bar for 3-5 passes so that the cumulative deformation of the rolled bar is ≥21%;

[0031] When 48≤DI value≤55, the bar is rolled for 3-5 times at a rolling temperature of 830-860°C, so that the cumulative deformation of the rolled bar is ≥25%;

[0032] Step (4): When the temperature of the rolled bar is 800-850 degrees Celsius, it is placed on a cooling bed and cooled to below 630 degrees Celsius at a cooling rate of 1.1 degrees Celsius / s.

[0033] Embodiment 1:

[0034] The actual measured composition of low carbon steel ingot produced by 20CrMnTi steelmaking is as follows:

[0035] C Si Mn P S Cr Ti 0.18 0.22 0.90 0.016 0.002 1.14 0.05

[0036] Its DI value is 39 based on the ingredients;

[0037] The φ14mm bar was rolled and finished with 5 passes. The rolling temperature was 884℃, the cumulative deformation of finishing rolling was 28%, the upper cooling bed temperature was 823℃, and it was cooled to 610℃ at a rate of 1.0℃ / s on the cooling bed. After rolling, it was cooled in a windproof stack to obtain a bar with a structure of ferrite and pearlite and a hardness of 182HBW.

[0038] Embodiment 2:

[0039] The actual measured composition of low carbon steel ingot produced by 20CrMnTi steelmaking is as follows:

[0040] C Si Mn P S Cr Ti 0.21 0.22 1.03 0.016 0.002 1.14 0.06

[0041] Its DI value is 54 based on the ingredients;

[0042] The φ29mm bar was rolled, finished with 3 passes, the rolling temperature was 835℃, the cumulative deformation of finishing rolling was 26%, the upper cooling bed temperature was 806℃, and it was cooled to 610℃ at a rate of 0.8℃ / s on the cooling bed. After rolling, it was cooled in a windproof stack to obtain a bar with a structure of ferrite and pearlite and a hardness of 208HBW.

[0043] Embodiment 3:

[0044] The actual measured composition of low carbon steel ingot produced by 20CrMnTi steelmaking is as follows:

[0045] C Si Mn P S Cr Ti 0.18 0.22 0.90 0.016 0.002 1.14 0.05

[0046] Its DI value is 39 based on the ingredients;

[0047] The φ25mm bar was rolled, finished with 5 passes, the rolling temperature was 881℃, the cumulative deformation of finishing rolling was 18%, the upper cooling bed temperature was 819℃, and it was cooled to 615℃ at a rate of 0.7℃ / s on the cooling bed. After rolling, it was cooled in a windproof stack to obtain a bar with a small amount of ferrite and pearlite and a hardness of 238HBW.

[0048] Embodiment 4:

[0049] The actual measured composition of low carbon steel ingot produced by 20CrMnTi steelmaking is as follows:

[0050] C Si Mn P S Cr Ti 0.19 0.22 1.03 0.016 0.002 1.12 0.06

[0051] Its DI value is 49 based on the ingredients;

[0052] The φ30mm bar was rolled, finished rolling was performed for 3 passes, the rolling temperature was 832℃, the cumulative deformation of finishing rolling was 23%, the upper cooling bed temperature was 816℃, and it was cooled to 616℃ at a rate of 0.5℃ / s on the cooling bed. After rolling, it was cooled in a windproof stack to obtain a bar with a small amount of ferrite and bainite and a hardness of 261HBW.

[0053] Example 5

[0054] The actual measured composition of low carbon steel ingot produced by 20CrMnTi steelmaking is as follows:

[0055] C Si Mn P S Cr Ti 0.20 0.20 0.95 0.016 0.002 1.05 0.06

[0056] Its DI value is 47 based on the ingredients;

[0057] The φ22mm bar was rolled and finished with 5 passes. The rolling temperature was 874℃, the cumulative deformation of finishing rolling was 24%, the upper cooling bed temperature was 826℃, and it was cooled to 617℃ at a rate of 1.0℃ / s on the cooling bed. After rolling, it was cooled in a windproof stack to obtain a bar with a structure of ferrite and pearlite and a hardness of 196HBW.

[0058] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for controlling the structure and hardness of small-sized 20CrMnTi bars, characterized in that: Step (1): measuring the actual composition of the elements contained in the bar; Step (2): Calculate the DI value based on the actual composition measured in step (1), The DI value calculation formula is as follows: DI=25.4×1.0×(1+3.333×[Mn])×(1+0.7×[Si])×(1+0.363×[Ni])×(1+2.16×[Cr ])×(1+3.0×[Mo])×(1+0.365×[Cu])×(1+1.73×[V])×0.540[C]×(1+2.5×[Zr]); Step (3): when the DI value calculated in step (2) is ≤47, the bar is rolled for 3-5 times at a rolling temperature of 870-900° C., so that the cumulative deformation of the rolled bar is ≥21%; When 48≤DI value≤55, the bar is rolled for 3-5 times at a rolling temperature of 830-860°C, so that the cumulative deformation of the rolled bar is ≥25%; Step (4): When the temperature of the rolled bar is 800-850 degrees Celsius, place it on a cooling bed and cool it to below 630 degrees Celsius.

2. The method for controlling the structure and hardness of small-sized 20CrMnTi bars according to claim 1, characterized in that: The rolling speed in step (3) is 6 to 12 m / s.

3. The method for controlling the structure and hardness of small-sized 20CrMnTi bars according to claim 1, characterized in that: The cooling rate in step (4) is 1.1°C / s.

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