Preparation method of high-strength impact-resistant round steel

Abstract

The invention relates to a preparation method of high-strength impact-resistant round steel. The preparation method comprises the step of strictly controlling chemical components of C, Si, Mn, Cr, Cu, Ni, Mo, V, N, P, S and Nb according to a design process. The heating process strictly controls the high-temperature section time and the high-temperature section temperature, and coarse grain size is prevented. And the rolling reduction of front and back passes is controlled during cogging. And continuously rolling until the temperature reaches 860 DEG C, then rolling, and controlling the final rolling temperature. Before and on a cooling bed, the cooling speed within the phase transition temperature of the steel is reasonably controlled, and the yield strength, the tensile strength and the impact energy are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of round steel hot rolling. BACKGROUND

[0002] The automobile axle head has high requirements on yield strength, tensile strength and impact work. To improve the yield strength and tensile strength, the grain size must be refined. Adjusting the rolling parameters is an effective method to improve the grain size of the material, which usually includes adjusting the rolling temperature, rolling speed and parameters. After the composition of the material is determined, the grain refinement of the material can be effectively controlled by adjusting different processing parameters. The higher the rolling temperature, the larger the grain size, and the lower the rolling temperature, the smaller the grain size. The rolling speed and the number of rolling also have a significant effect on the grain size. High speed and large deformation times will make the grain finer.

[0003] The main reasons for the unqualified impact work are: ①the center segregation of the billet, and whether there are defects such as slag inclusion, segregation, white spots, cracks and non-metallic inclusions exceeding the standard, which all affect the impact performance of the finished round steel; ②the billet heating temperature is too high, and the heating time is too long, which will make the austenite grains grow too much, weaken the bonding force between the grains, and reduce the plasticity of the steel; ③the pass reduction amount and the change of pass reduction schedule change, the reduction amount, speed and other factors change, different pressures are generated, thereby affecting the uniformity of the internal organization; ④the finish rolling temperature, the finish rolling temperature has a more obvious effect on the impact work of medium and low carbon alloy steel, the higher the finish rolling temperature, the lower the impact work, and the lower the finish rolling temperature, the higher the impact work.

[0004] At present, most non-adjustable steels use water cooling devices to increase the plasticity and toughness of the steel, but the initial investment is large and is limited by the site. SUMMARY

[0005] The present application aims to solve the above-mentioned problems of the prior art, and provides a preparation method of high-strength and impact-resistant round steel to meet the use requirements of automobile axle heads.

[0006] The technical scheme adopted by the present application to solve the above-mentioned problems is: a preparation method of high-strength and impact-resistant round steel, characterized by comprising Step one, element composition design: according to the mass percentage, it comprises C: 0.19~0.25%, Si: 0.20~0.50%, Mn: 0.9~1.6%, Cr: 0.12~0.25%, Cu: 0.09~0.2%, Ni: 0.02~0.14%, Mo: 0.02~0.5%, V: 0.12~0.25%, N: 0.012~0.035%, P: ≤0.03%, S: 0.015~0.025%, Nb: 0.02~0.06%, and the balance is Fe and unavoidable impurity elements; Step two, casting and heating: smelt the molten steel according to the element composition design, cast the molten steel into a billet, reheat the billet after cooling to room temperature, and then tap the molten steel, remove the surface scale, and then perform blooming to control the blooming temperature at 1000-1050℃; Step three, reciprocating blooming: refine the grain and simultaneously compress the shrinkage cavity in the billet, dynamic recrystallization occurs in the austenite phase region during rolling, and the final structure is fine equiaxed austenite grains, and the intermediate bloom is rolled into a round billet through 11 passes using a reciprocating mill, the reduction amount of each pass is controlled at 30-50mm, and the total blooming rolling time is controlled at ≥3min to complete; Step four, cutting and continuous rolling: the intermediate bloom after blooming is cut before entering the continuous rolling mill, and is rolled at a temperature of 860-870℃, the exit line speed of the finished product is controlled, and speed reduction rolling is performed to ensure that the finished product temperature is below 820℃; the intermediate bloom is rolled into a round steel bar through 6-10 passes after entering the continuous rolling mill, the deformation amount of each pass is ≥12.5%, and the total deformation amount of the intermediate bloom to the round steel is ≥81%; after the continuous rolling is completed, the round steel is moved to the sawing machine through the conveying roller way for fixed-length sawing; the purpose of low-temperature rolling is to improve the yield strength and toughness; Step five, after the round steel is sawed, the red steel at a temperature of 710-730℃ is subjected to water mist spraying cooling before the upper cooling bed to rapidly cool the red steel, and the temperature drop speed is 15-20℃ / min; the technical purpose of the spraying cooling is to improve the toughness of the finished round steel, i.e., to improve the impact work; Step six, after the temperature of the red steel is reduced to 660-680℃, the red steel is subjected to air cooling by blowing from below the cooling bed when stepping, the cooling speed is 10-15℃ / min, the austenite is transformed into pearlite during the air cooling, and part of ferrite is precipitated; Step seven, when the air cooling is completed to a temperature of 600-630℃, the red steel is uniformly stepped to the lower cooling bed collection frame, the temperature of the lower cooling bed is controlled at 300-350℃, and then the round steel is packaged and cooled, and the final metallographic structure of the round steel is 80-85% of pearlite, 15-20% of ferrite, and a grain size of 7.5 or above.

[0007] Preferably, steps five and six, the red steel is rapidly cooled in the temperature range of 730-600℃ to inhibit the generation of widmanstatten structure and promote the generation of pearlite.

[0008] Preferably, step two, the billet is heated and kept at a temperature of 1200-1260℃ in the furnace, the high-temperature stage stays for 1.5 hours, and the time in the furnace is controlled within 5.5 hours.

[0009] Preferably, step three, the reduction amount of each pass of the first 7 passes before blooming rolling is controlled at 35-60mm, and the reduction amount of the last 4 passes is controlled at 30mm or below.

[0010] Compared with the prior art, the advantages of the present application are: The present application strictly controls the steelmaking component, controls the furnace time and production rhythm, and controls the finish rolling temperature of continuous rolling, and strictly controls the cooling speed in the phase transformation range, so that the high yield strength, high tensile strength and high impact energy of the round steel are realized.

[0011] Wait for 3 minutes before cogging, control the cogging temperature, control the reduction of each pass between 35-60mm in the first 7 passes, and control the reduction below 30mm in the last 4 passes.

[0012] Wait for 4-5 minutes before entering the continuous rolling, start biting when the temperature drops to 860℃, and control the exit line speed, provided that the rolling mill load does not exceed 90%, so that a relatively low finish rolling temperature is realized.

[0013] After sawing, the upper cooling bed is controlled, the cooling speed in the cooling bed is controlled, and the cooling bed temperature is controlled by controlling the air volume and water volume and the cooling bed running speed.

[0014] Taking the production specification of φ85mm of the round steel as an example, the metallographic structure is 83% pearlite and 17% ferrite, the grain size is 7.5 level, the yield strength is 550MPa, the tensile strength is 722MPa, and the room temperature impact energy is 93J.

[0015] Using the existing upper cooling bed space, a set of water mist spraying devices are installed at the upper cooling bed, and air blowers are arranged below the cooling bed, the cooling speed can be controlled by controlling the water volume and air volume, the cooling speed of the upper and lower cooling beds is strictly controlled, and the round steel is quickly cooled between 730-600 to avoid the generation of widmanstatten structure (as much as possible to generate pearlite), and the cooling speed is controlled at 550℃-350℃ to improve the plasticity and impact energy. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The figure is a schematic diagram of the water mist spraying position of the cooling bed of the embodiment of the present application; Figure 2 The figure is a schematic diagram of the fan position below the cooling bed of the embodiment of the present application; In the figure, 1 is a cooling bed, 2 is a fan, 3 is a round steel, 4 is a conveying roller, 5 is a lifting baffle, and 6 is a water mist spraying device. DETAILED DESCRIPTION

[0017] The present application will be further described in detail below in combination with the embodiments, which are exemplary and intended to explain the present application, and cannot be understood as a limitation of the present application.

[0018] The steelmaking is controlled according to the set chemical element composition, and the content of special components is strictly controlled according to the set percentage of quality. The actual content of part of element components in the steelmaking part is as follows: C (0.22%), Si (0.45%), Mn (1.6%), Cr (0.16%), Cu (0.12%), Ni (0.14%), Mo (0.4%), V (0.14%), N (0.01%), S (0.017%), P (0.016%), and Nb (0.0023%).

[0019] The embodiment adopts the following technical scheme to realize the manufacturing method of the application. A manufacturing method of high-yield strength and impact-resistant round steel for automobile axle head, comprising the following steps: 1) The blank 300mm×340mm enters the step heating furnace, and the temperature is controlled at about 5.5h in the furnace time. The temperature of the soaking section 1, 2 and 3 is controlled at 1230℃. 2) The temperature is warmed up to 1000~1050℃ before cogging, and then the automatic reciprocating cogging is carried out according to the set table 1 to obtain a size of 170 mm×175mm.

[0020] Table 1 cogging process

[0021] 3) After the hydraulic shearing head, wait for 4~5 minutes before continuous rolling, and bite in when the temperature is displayed as 860℃, and automatically carry out rolling according to the continuous rolling program of table 2. At this time, the finish rolling temperature is about 820℃.

[0022] Table 2 continuous rolling process

[0023] 4) After being cut into 7-meter length by hot saw, the round steel is conveyed on the roller way in front of the cooling bed. At this time, the red steel temperature is 710℃~730℃. At this time, 5 water mist sprayers with a spacing of 1.75m are placed beside the conveying roller way. The horizontal distance of the water mist sprayer from the red steel is about 1m, and the height direction is about 0.8m higher than the red steel. The water mist spraying is adjusted to the second gear (i.e. the maximum spraying pressure, 6MPa). The water mist spraying position is shown in Figure 1 After the cooling bed, the red steel temperature is reduced to 660℃~680℃ after water mist spraying.

[0024] 6) After the red steel is conveyed on the cooling bed, 5 air fans are placed at a horizontal distance of 2m from the conveying roller way and a height direction distance of 2m from the cooling bed. At the same time, the cooling bed moving speed gear 3 (the lowest cooling bed moving speed 31mm / s) is adjusted. After the cooling bed moves at an interval of 5m, the temperature is reduced to about 600℃. The cooling bed air fan position is shown in Figure 2 . The air cooling speed is 10~15℃ / min.

[0025] 7) The subsequent cooling bed moves the red steel according to speed gear 2 to 350°C, and then the automatic packing machine is used to bundle and stack the red steel, and the red steel is naturally cooled in an open place.

[0026] 8) After the above processes, the mechanical properties of the round steel reach good data, the grain size is refined to 7.5, and the yield strength and tensile strength are respectively improved by 136 MPa and 100 MPa on average, and the impact energy is improved by 52 J on average; the performance comparison of the round steel of the example and the comparative example is shown in Table 3 (in the table, the process of the comparative example: the breakdown temperature is not controlled, the breakdown temperature is 1100-1150°C, the continuous rolling temperature is 950-980°C, the final rolling temperature is 930-950°C, the spray is not used on the cooling bed, the fan is not used for cooling, and the pile temperature is 400-450°C. In the table, the process of the example: slow breakdown, the breakdown temperature is 1000-1050°C, dynamic recrystallization, grain refinement, yield strength and tensile strength improvement, continuous rolling temperature 870-860°C, low temperature breakdown and control of the final rolling temperature below 820°C, improvement of yield strength and toughness (impact energy); spray cooling + fan cooling, further precipitation of pearlite, grain refinement, increase of toughness, and improvement of impact energy. The performance of the round steel controlled by rolling and cooling is obviously improved, which verifies the innovation and operability of the application.

[0027] Table 3 Performance of round steel of example and comparative example .

Claims

1. A method for preparing high-strength impact-resistant round steel, characterized in that: include Step 1. Element composition design: in terms of mass percentage, it includes C: 0.19-0.25%, Si: 0.20-0.50%, Mn: 0.9-1.6%, Cr: 0.12-0.25%, Cu: 0.09-0.2%, Ni: 0.02-0.14%, Mo: 0.02-0.5%, V: 0.12-0.25%, N: 0.012-0.035%, P: ≤0.03%, S: 0.015-0.025%, Nb: 0.02-0.06%, and the balance is Fe and unavoidable impurity elements; Step 2, billet casting and heating: smelt molten steel according to the elemental composition design, cast the molten steel into billets, cool the billets to room temperature and then re-heat them in the furnace, tap them, and remove the surface oxide scale. After descaling, the billets are opened, and the opening temperature is controlled at 1000-1050℃; Step 3, reciprocating blanking: Refine the grains while pressing the shrinkage cavities in the blank. Dynamic recrystallization occurs in the austenite phase during rolling, and the final structure is refined equiaxed austenite grains. The intermediate square billet is rolled using a reciprocating rolling mill through 11 passes, with the reduction of each pass controlled at 30-50 mm. The total blanking and rolling time is controlled to be ≥3 minutes. Step 4: Cropping and continuous rolling: The intermediate billet after slab cutting is cropped before entering the continuous rolling mill and is rolled in the continuous rolling mill at a temperature between 860°C and 870°C. The outlet speed of the finished product is controlled and the rolling speed is reduced to ensure that the finished product temperature is below 820°C. After entering the continuous rolling mill, the intermediate billet is rolled into round steel bars in 6 to 10 passes, with the deformation of each pass being ≥12.5%, and the total deformation from the intermediate billet to the round steel being ≥81%. After the continuous rolling, the intermediate billet is transferred to the sawing machine via a conveyor roller for cut-to-length sawing; Step 5: After the round steel is sawn, the red steel at a temperature of 710℃~730℃ is sprayed with water before being put on the cooling bed to rapidly cool the red steel at a temperature drop rate of 15~20℃ / min; Step 6: After the red steel is placed on the cooling bed at a temperature of 660-680°C, it is blown and cooled from under the cooling bed at a cooling rate of 10-15°C / min. During the cooling period, austenite transforms into pearlite and some ferrite precipitates. Step 7. After the air-cooled red steel is cooled to between 600 and 630°C, it is then moved to the lower cooling bed collection frame at a uniform speed. The temperature of the lower cooling bed is controlled at 300 to 350°C. After that, it is packaged and stacked for cooling. The final metallographic structure of the round steel is 80 to 85% pearlite, 15 to 20% ferrite, and a grain size of 7.5 or above.

2. The method according to claim 1, characterized in that: In steps five and six, the red steel is rapidly cooled in the range of 730-600°C to inhibit the formation of Widmanstätten structure and promote the formation of pearlite.

3. The method according to claim 1, wherein: Step 2: The casting is heated in the furnace at a temperature of 1200-1260°C, with a residence time of 1.5 hours in the high temperature stage and a furnace time of less than or equal to 5.5 hours.

4. The method according to claim 1, wherein: Step 3: The reduction of each of the first 7 passes of billet rolling is controlled between 35 and 60 mm, and the reduction of the last 4 passes is controlled below 30 mm.

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