Method for controlling the structure of round steel bars for medium carbon, high manganese and vanadium-containing alloy structures

Abstract

The present invention belongs to the technical field of metallurgy, specifically to a method for controlling the microstructure of round steel bar for medium carbon high manganese vanadium alloy structure. In the continuous casting process, pouring is performed at a low superheat and low casting speed, and pulse magnetic vibration solidification homogenization technology is used to crush dendrites with a large electromagnetic stirring current, increase the nucleation core of dendrites, expand the equiaxed crystal region, and reduce the columnar crystal region, thereby reducing the segregation of the billet, where the electromagnetic stirring current of the crystallizer is 350A and the electromagnetic stirring current of the terminal is 450A. In the rolling process, a high-temperature long-time diffusion heating process is used, and a controlled rolling and controlled cooling process is used. The microstructure of the produced round steel bar for medium carbon high manganese vanadium alloy structure is uniform, and the area ratio of the heat-affected zone (abnormal structure) after friction welding processing by the user is reduced to within 5%, which can reach the international advanced level.

Description

[Technical Field]

[0001] The present invention relates to the technical field of metallurgy, in particular to a method for controlling the microstructure of round steel bars for medium carbon high manganese vanadium-containing alloy structures, and more particularly to a continuous casting and rolling process for reducing the microstructure of round steel bars for medium carbon high manganese vanadium-containing alloy structures. [Background technology]

[0002] Medium carbon high manganese vanadium alloy structural steel has excellent processing and mechanical properties and is therefore widely used in fields such as the manufacture of construction machinery and automobile parts. However, this steel type is prone to the occurrence of abnormal structures such as band-like structures and central segregation bands, which ultimately affect the performance of the material in use.

[0003] Most metal materials must be pressed after smelting and pouring before being formed into profiles. However, the processed material is prone to forming a band-like structure, in which pearlite and ferrite are distributed in a band-like pattern along the deformation direction. In general carbon steels and alloy structural steels, the band-like structure is typically formed by stacking ferrite and pearlite bands along the rolling direction. The presence of the band-like structure creates anisotropy in the mechanical properties of the metal, significantly reducing the steel's plasticity and toughness, which are significantly superior along the band-like structure compared to the direction perpendicular to it. This also makes the material susceptible to boundary cracking during pressing. For parts requiring subsequent heat treatment, the band-like structure at the very least causes excessive thermal deformation and, in severe cases, stress concentrations and cracks, significantly shortening the material's service life.

[0004] Microsegregation of alloying elements is the primary cause of the formation of ferrite-pearlite bands. Microsegregation is caused by selective crystallization of alloying elements during the solidification process of molten steel. In high-manganese steels, the selective crystallization of elements results in a high interdendritic manganese concentration and a low intradendritic manganese concentration. Manganese is an austenite-stabilizing element that can lower the Ar3 transformation temperature. Therefore, the Ar3 transformation temperature in the high-manganese region (interdendritic) is lower than that in the low-manganese region (intradendritic). With decreasing temperature, ferrite preferentially nucleates within dendrites. Because the solubility of carbon in ferrite is much lower than that in austenite, carbon accumulates between dendrites. After rolling, pearlite bands form in the original interdendritic regions. The intradendritic ferrite bands and the interdendritic pearlite bands stack and intertwine to form a band-like structure, forming a broad, abnormal pearlite center and ultimately a martensite segregation band.

[0005] Therefore, how to reduce the problems of banded structure and abnormal central segregation band in medium carbon high manganese steel and improve the performance of the material has become a major challenge in the industry. Summary of the Invention [Problem to be solved by the invention]

[0006] The objective of the present invention is to provide a production process for reducing microsegregation, banding and central segregation zone in medium carbon high manganese vanadium-containing alloy structural round steel, thereby effectively improving the overall mechanical performance of the material. [Means for solving the problem]

[0007] The object of the present invention is achieved mainly by the following technical means.

[0008] In the method for controlling the structure of round steel material for medium carbon high manganese vanadium alloy structures, the process includes optimizing the composition, continuous casting, heating in a heating furnace, and rolling, and specifically includes the following steps (1) to (6):

[0009] In step (1), if the steel contains a high content of fine crystalline elements such as vanadium, titanium, and aluminum, and elements that are prone to segregation, such as Mn, P, and S, the corresponding rolling and forging will tend to produce ferrite-pearlite band structures, and large, abnormal pearlite in the center, and even martensite segregation bands. Therefore, on the premise that the material strength meets the standard requirements, the Mn and V contents are reduced by 10%, the aluminum content is reduced by 30%, and the original P≦0.015% is adjusted to P≦0.013%, and the original S≦0.010% is adjusted to S≦0.005%, and the grain size of the material is reduced from the original grade 7-8 to grade 6-7.

[0010] The elemental composition of medium carbon high manganese vanadium-containing alloy structural round steel is, in mass percentage, C 0.39-0.46%, Si 0.25-0.45%, Mn 0.90-1.1%, P≦0.013%, S≦0.005%, Ni≦0.20%, Cr 0.10-0.25%, Al 0.014-0.025%, V 0.07-0.11%, Cu≦0.20%, Mo≦0.05%, and the balance being Fe and unavoidable impurities.

[0011] In step (2), the high-temperature pouring causes a large temperature gradient to appear inside the billet during the solidification process, resulting in severe dendritic segregation. To prevent this, the superheat of the molten steel is strictly controlled to 15 to 30°C.

[0012] In step (3), dendrite segregation can be effectively controlled during the continuous casting process by rationally suppressing the growth of columnar crystals, increasing the proportion of equiaxed crystal regions, and reducing the secondary dendrite spacing. The relevant process measures include controlling the continuous casting speed, pouring the molten metal at a low casting speed, setting the casting speed at 0.60-0.68 m / min (target casting speed 0.65 m / min), controlling the secondary cooling specific water amount, using weak cooling, controlling the specific water amount to 0.18 L / kg, and setting the secondary cooling water distribution ratio to 35%:40%:25%.

[0013] In step (4), several auxiliary technical measures are used in the continuous casting production process. The electromagnetic stirring of the crystallizer and the electromagnetic stirring at the end are used to strengthen the electromagnetic stirring intensity and ensure that no white bands (negative segregation) appear in the low-magnification structure of the billet. The electromagnetic stirring current is maximized (crystallizer electromagnetic stirring current: 350±5A, frequency: 2±0.2Hz, end electromagnetic stirring current: 450±5A, frequency: 6±0.2Hz). This uses electromagnetic force to break up the dendrites, increase the dendrite nucleation core, and expand the equiaxed grain region. Pulsed magnetic vibration (PMO) solidification homogenization technology is used, with the voltage parameter set to 100V. This promotes heterogeneous nucleation and refines the solidification structure. The above measures achieve the goal of reducing microsegregation in the material.

[0014] In step (5), a high-temperature, long-time diffusion heating process is used to diffuse elements that are prone to segregation, such as Mn, C, S, and P, and prevent the appearance of band-shaped structures during the rolling process. Specific heating process parameters include a temperature of heating zone I of 900-1050°C, a temperature of heating zone II of 1255-1280°C, a temperature of soaking zone 1255-1280°C, a rolling start temperature of 1160-1190°C, a tapping time interval of 240-300 s, a total heating time of 420-550 min, and a high-temperature zone time of ≥ 180 min. To prevent the decarburization of the material from exceeding the standard due to high-temperature, long-time heating, the air-fuel ratio in the heating furnace is controlled to 0.40-0.75.

[0015] In step (6), a controlled rolling and controlled cooling process is used to achieve low-temperature rolling, preventing rolling in the two-phase region, and strong cooling is performed after rolling to prevent the appearance of pearlite bands, ferrite bands, and central segregation bands.The specific parameters of the controlled rolling and controlled cooling process are that when using KOCKS for round steel, the temperature is 850±15°C, and the bottom out temperature is 750±15°C, and this is mainly controlled by opening water tanks 1 to 7 to adjust the amount of water and adjusting the rolling speed. [Effects of the Invention]

[0016] The beneficial effects of the present invention are as follows: Considering that for medium carbon high manganese vanadium-containing alloy structural steel, pearlite segregation band structure is likely to appear in the material after rolling, and after friction welding by users, a large area of ​​heat-affected zone (martensite band) appears in the segregation band region, making the material embrittlement and ultimately affecting the service life of the material, the present invention takes the following measures.

[0017] (1) The chemical composition was optimized (reducing the Mn and V contents by 10%, reducing the aluminum content by 30%, adjusting the original P≦0.015% to P≦0.013%, and adjusting the original S≦0.010% to S≦0.005%). In order to prevent the crystal grains from becoming too fine and the appearance of band-like structures, the contents of fine crystal elements such as V and Al were reduced, while the Mn content was moderately reduced, and the contents of harmful elements such as P and S that are prone to segregation were reduced.

[0018] (2) In order to crush dendrites, increase the proportion of equiaxed crystals, refine the solidification structure, and ultimately reduce billet segregation, thereby lowering the grade of segregation and band structure in the rolled material, during the continuous casting production process, the superheat of molten steel is reduced, the casting speed is slowed, electromagnetic stirring in the crystallizer and electromagnetic stirring at the end are used, the electromagnetic stirring current parameters are optimized, and pulsed magnetic vibration (PMO) solidification homogenization technology is used.

[0019] (3) In the rolling process, a high-temperature diffusion heating process is used, and the segregated elements C, Mn, P, S, etc. in the billet are substantially uniformly diffused. In the subsequent cooling and recrystallization process, a high-temperature and long-term heat-holding process is required to prevent the formation of a ferrite-pearlite band structure and a central segregation band.

[0020] (4) By using controlled rolling and controlled cooling processes, the two-phase rolling is avoided, and low-temperature rolling is used to strengthen the cooling after rolling, thereby achieving the purpose of reducing the band structure of the rolled material.

[0021] The above efforts can effectively reduce material segregation, effectively control the center segregation zone width, and improve the performance of medium carbon, high manganese, and vanadium-containing steels used by users. [Brief explanation of the drawings]

[0022] [Figure 1] This is a photograph of the central segregation band width structure of round steel before optimization (central pearlite segregation band width 676 μm). [Figure 2] 1 is a photograph of the center segregation zone width structure of round steel of Example 1 (center pearlite segregation zone width: 82 μm). [Figure 3] This is a low-magnification photograph of the round steel before optimization (there is a clear black core of carbon segregation in the center). [Figure 4] 1 is a low-magnification photograph of the round steel bar of Example 1 (no obvious black core in the center). DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will now be described in detail with reference to specific examples.

[0024] Example 1 The production of medium carbon, high manganese, vanadium-containing steel F40MnV (C 0.41%, Si 0.31%, Mn 0.95%, P 0.011%, S≦0.003%, Ni 0.01%, Cr 0.15%, Al 0.018%, V 0.08%, Cu 0.04%, Mo 0.005%) (specification Φ62mm) will be explained as an example.

[0025] The production process involves five machines, five-flow arc-shaped continuous casting machines that use billets with a cross section of 220mm x 260mm, followed by billet finishing, heating in a regenerative heating furnace, rolling in a group of continuous rolling mills, rolling in a group of KOCKS finishing rolling mills, cooling, offline finishing, and packaging for storage.

[0026] 1. Continuous casting production The molten steel is poured at a low superheat, and the superheat of the molten steel is controlled at 28°C. The continuous casting speed is controlled at 0.65m / min. The secondary cooling specific water amount is controlled at 0.18L / Kg. The secondary cooling water distribution ratio is set to 35%:40%:25%. The electromagnetic stirring parameters of the crystallizer are set as follows: current 350A, frequency 2Hz, terminal electromagnetic stirring current 450A, frequency 6Hz. Pulse magnetic vibration (PMO) solidification homogenization technology is used, and its voltage parameters are set to 100V.

[0027] 2. Billet heating A continuous casting billet with a finished cross section of 220mm x 260mm is heated in a regenerative heating furnace, with the temperature of heating section I set to 980°C, the temperature of heating section II set to 1271°C, the temperature of the soaking section set to 1272°C, the tapping time interval set to 280s, the total heating time set to 515min, and the high-temperature section time set to 236min. In order to prevent decarburization of the material from exceeding the standard due to long-term heating at high temperatures, the air-fuel ratio in the heating furnace is controlled to 0.51.

[0028] 3. Controlled rolling Before rolling begins, the billet is descaled with high-pressure water, the pressure of the high-pressure water is set to 18 MPa, the rolling start temperature is controlled to 1172°C, the water tank is opened, the amount of water is adjusted, and the rolling speed is adjusted to control the temperature when using KOCKS on round steel at 855°C.

[0029] 4. Controlled cooling By opening water tanks No. 6 and No. 7 and adjusting the amount of water, the bottom-out temperature of the round steel bars is controlled to 752°C.

[0030] Example 2 In continuous casting production, the superheat of molten steel is controlled at 26°C.

[0031] In billet heating, the temperature of heating zone I is 975°C, the temperature of heating zone II is 1267°C, the temperature of the soaking zone is 1263°C, the tapping time interval is 270 s, the total heating time is 510 min, and the high temperature zone time is 237 min.

[0032] In controlled rolling, the rolling start temperature is set to 1169°C, and when using KOCKS for round steel, the temperature is set to 845°C.

[0033] In controlled cooling, the bottom out temperature of the round steel is set to 745°C.

[0034] Example 3 In continuous casting production, the superheat of molten steel is controlled at 27°C.

[0035] In billet heating, the temperature of heating zone I is 997°C, the temperature of heating zone II is 1262°C, the temperature of the soaking zone is 1258°C, the tapping time interval is 268 s, the total heating time is 507 min, and the high temperature zone time is 223 min.

[0036] In controlled rolling, the rolling start temperature is set to 1175°C, and when using KOCKS for round steel, the temperature is set to 849°C.

[0037] In controlled cooling, the bottom drop temperature of the round steel is set to 747°C.

[0038] Example 4 In continuous casting production, the superheat of molten steel is controlled at 28°C.

[0039] In billet heating, the temperature of heating zone I is 983°C, the temperature of heating zone II is 1269°C, the temperature of the soaking zone is 1261°C, the tapping time interval is 283 s, the total heating time is 542 min, and the high temperature zone time is 243 min.

[0040] In controlled rolling, the rolling start temperature is set to 1165°C, and when using KOCKS for round steel, the temperature is set to 837°C.

[0041] In controlled cooling, the bottom out temperature of the round steel is set to 740°C.

[0042] Example 5 In continuous casting production, the superheat of molten steel is controlled at 29°C.

[0043] In billet heating, the temperature of heating zone I is 1021°C, the temperature of heating zone II is 1266°C, the temperature of the soaking zone is 1265°C, the tapping time interval is 264 s, the total heating time is 499 min, and the high temperature zone time is 225 min.

[0044] In controlled rolling, the rolling start temperature is set to 1181°C, and when using KOCKS for round steel, the temperature is set to 844°C.

[0045] In controlled cooling, the bottom out temperature of the round steel is set to 754°C.

[0046] Using the above process, one batch of round steel bars is produced for each step, for a total of five batches of round steel bars.

[0047] (Comparative Example 1) Comparative Example 1 differs from Example 1 in that the molten metal is not poured at a low superheat degree, but is instead heated to 38°C, and the other operations are the same as those of the Example.

[0048] (Comparative Example 2) Comparative Example 2 differs from Example 1 in that the molten metal was not poured at a low casting speed, but the casting speed was controlled to 1.0 m / min, and the other operations were the same as those of Example.

[0049] (Comparative Example 3) In Comparative Example 3, compared to Example 1, a strong electromagnetic force was not used for stirring during the continuous casting pouring process, but rather a weak electromagnetic force was used for electromagnetic stirring, and the electromagnetic stirring parameters of the crystallizer were as follows: the current was controlled to 250±5A, the frequency was controlled to 2±0.2Hz, and the terminal electromagnetic stirring current was controlled to 200±5A, and the frequency was controlled to 6±0.2Hz; other operations were the same as those in Example 1.

[0050] Comparative Example 4 Comparative Example 4 differs from Example 1 in that the high-temperature diffusion heating process was not used, the temperature of heating section II was controlled to 1196°C, the temperature of the soaking section was controlled to 1202°C, the total heating time was controlled to 400 min, and the time of the high-temperature section was controlled to ≥ 100 min; other operations were the same as those of the Examples.

[0051] (Comparative Example 5) Comparative Example 5 differs from the Examples in that the controlled rolling and controlled cooling process was not used, the temperature when using KOCKS on round steel bars was controlled to 885°C, and the bottoming temperature was controlled to 835°C, but the other operations were the same as those of the Examples.

[0052] Table 1 below shows the comparison of the band-like structure, grain size, and low magnification structure of Examples 1 to 5 of the present invention and Comparative Examples 1 to 5.

[0053] [Table 1]

[0054] Table 1 shows the comparison of the physicochemical performance indexes of round steel and the user's usage (if the user uses the friction welding process on the material and the width of the pearlite band in the center is large, the material is likely to have a large heat-affected zone after friction welding, i.e., a large area of ​​abnormal martensite band structure, which will make the material brittle and reduce its plasticity and toughness, ultimately affecting the service life of the material).

[0055] The results show that by rationally optimizing the design of the components, optimizing the superheat, secondary cooling specific water amount, casting speed and other parameters during the continuous casting process, using large electromagnetic stirring current parameters, using a high-temperature, long-time heating process during the heating-by-rolling process, and using controlled rolling and controlled cooling processes, the structural detection indexes of medium-carbon, high-manganese, and vanadium-containing alloy round steel and the end-use performance of users can reach the international advanced level.

[0056] Unless otherwise specified, the raw materials and apparatus used in the present invention are all conventional raw materials and apparatus in this field, and the methods used in the present invention are all conventional methods in this field, unless otherwise specified. The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications made to the above examples based on the technical concept of the present invention are within the scope of protection of the present invention.

Claims

1. The specific process includes continuous casting, heating in a heating furnace, and rolling. Step (1) strictly controlling the superheat of the molten steel to 15-30°C in order to prevent a large temperature gradient from appearing inside the billet during the solidification process of the billet due to high-temperature pouring, which causes serious dendrite segregation and the appearance of segregation band structures during the subsequent rolling and elongation process; Step (2) in the continuous casting process, the casting speed is controlled to 0.60-0.68 m / min, the secondary cooling water specific amount is controlled to 0.18 L / Kg, and the secondary cooling water distribution ratio is set to 35%:40%:25%; Step (3) using the electromagnetic stirring of the crystallizer and the electromagnetic stirring of the billet solidification end to strengthen the electromagnetic stirring intensity, and maximize the electromagnetic stirring current under the premise of ensuring that no white band appears in the billet macrostructure, and using pulsed magnetic vibration (PMO) solidification homogenization technology to promote heterogeneous nucleation; Step (4) of diffusing C, S, P, and Mn using a high-temperature, long-time diffusion heating process, setting the temperature of the heating furnace in a heating I stage to 900-1050°C, the temperature of the heating furnace in a heating II stage to 1255-1280°C, the temperature of the heating furnace in a soaking stage to 1255-1280°C, the rolling start temperature to 1160-1190°C, the tapping time interval from the heating furnace to 240-300s, the total heating time to 420-550min, the time of the heating II stage and the soaking stage to ≧180min, and controlling the air-fuel ratio in the heating furnace to 0.40-0.75; and (5) using a controlled rolling and controlled cooling process to realize low-temperature rolling and perform intensive cooling after rolling, so that the temperature of the round steel entering the rolling mill is 850±15°C and the tailing temperature is 750±15°C; The elemental composition of the round steel material is, in mass percentage, C 0.39 to 0.46%, Si 0.25 to 0.45%, Mn 0.90 to 1.1%, P≦0.013%, S≦0.005%, Ni≦0.20%, Cr 0.10 to 0.25%, Al 0.014 to 0.025%, V 0.07 to 0.11%, Cu≦0.20%, Mo≦0.05%, and the balance being Fe and unavoidable impurities. A method for controlling the structure of round steel material for medium carbon high manganese vanadium containing alloy structures, characterized by the above.

2. In step (2), the continuous casting speed is set to 0.65 m / min.

2. The method for controlling the microstructure of a round steel material for medium carbon, high manganese, and vanadium-containing alloy structures according to claim 1.

3. In step (3), the electromagnetic stirring current of the crystallizer is 350±5 A, the frequency is 2±0.2 Hz, and the terminal electromagnetic stirring current is 450±5 A, the frequency is 6±0.2 Hz; 2. The method for controlling the microstructure of a round steel material for medium carbon, high manganese, and vanadium-containing alloy structures according to claim 1.

4. In step (3), the pulse magnetic vibration voltage is set to 100 V.

2. The method for controlling the microstructure of a round steel material for medium carbon, high manganese, and vanadium-containing alloy structures according to claim 1.

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