Method for improving the quantity and length of dendrites of macrostructure of 42crmoa hot-rolled round steel

Abstract

The application belongs to the technical field of steelmaking, and discloses a method for improving the dendrite quantity and length of macrostructure of 42CrMoA hot-rolled round steel, the steel grade is 42CrMoA, and the production process is converter smelting+LF refining+RH vacuum treatment+continuous casting+rolling. By reasonably designing the component, adding appropriate Ni and Cu elements, optimizing the converter tapping temperature and deoxidation slagging process, controlling the content of easily-segregating S element, optimizing the continuous casting secondary cooling intensity, optimizing the electromagnetic stirring current and frequency of the crystallizer, and adopting the light pressing down process, the dendrite quantity and length of macrostructure of 42CrMoA hot-rolled round steel can be greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking technology, and specifically relates to a method for improving the number and length of dendrites in the low magnification structure of 42CrMoA hot-rolled round steel. Background Technology

[0002] Dendrites are a characteristic of solidified structures, but well-developed dendrites imply severe compositional and structural inhomogeneity, with systemic consequences. The main hazards include: Anisotropy of properties: the inheritance of dendritic direction leads to significantly lower transverse (radial) plasticity and toughness in steel compared to the longitudinal direction, resulting in directional weaknesses in product performance; Deterioration of processing and heat treatment: the accumulation of low-melting-point elements and inclusions between dendrites easily becomes crack initiation points during rolling or forging. Dendritic segregation is inherited in the final heat-treated state, leading to banded structures, excessively high or low local hardness, and increased quenching distortion, severely affecting the uniformity after tempering; Reduced fatigue life and reliability: the interdendritic region is a weak zone, prone to fatigue crack initiation under alternating stress. For critical moving parts such as shafts and connecting rods commonly used in 42CrMoA, this is a fatal defect; Increased risk of non-compliance in flaw detection: severe interdendritic segregation and porosity may appear as internal defects in ultrasonic testing, leading to product rejection.

[0003] Existing measures to reduce dendrite formation are relatively simple, such as optimizing continuous casting speed, electromagnetic stirring intensity, and high-temperature diffusion during steel rolling. These methods have limited effectiveness in improving low-dendrite formation in overall round steel. For 42CrMoA hot-rolled round steel, especially round steel with large billet cross-sections and large rolling specifications, improving low-dendrite formation in round steel presents a significant challenge in terms of cost and technological limits, given the inherent deficiencies in solidification and the persistent segregation of alloying elements.

[0004] To control low-dendritic dendrites in round steel, it is essential to control the source, namely the dendrites in the continuously cast billet. This technology aims to suppress dendrite growth through precise control of the entire steelmaking process. In terms of composition design, elements that improve the corrosion resistance of the steel matrix are added, and harmful elements that agglomerate at grain boundaries are purified to the extreme. In terms of continuous casting process, electromagnetic stirring technology that acts on the initial dendrite germination zone is explored. Combined with optimization of cooling process and light reduction process, the goal is to suppress dendrite growth. Summary of the Invention

[0005] This invention aims to provide a method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel. This is achieved through rational composition design and the addition of appropriate amounts of Ni and Cu elements, optimization of converter tapping temperature and deoxidation slag-forming process, control of easily segregating S element content, optimization of continuous casting secondary cooling intensity, optimization of crystallizer electromagnetic stirring current and frequency, and the adoption of a light reduction process. This significantly reduces the number of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel, with a random dendrite zone of 200mm in the rolled material. 2The number of dendrites in the area should not exceed 5, and the dendrite length should be controlled to ≤4.2mm to improve the processing and use performance of 42CrMoA hot-rolled round steel.

[0006] The technical solution adopted to achieve the purpose of this invention is as follows:

[0007] A method for improving the number and length of dendrites in the low-magnification structure of 42CrMoA hot-rolled round steel includes the following steps performed in sequence: converter smelting, LF refining, RH vacuum treatment, continuous casting, and rolling. After rolling, a hot-rolled round steel product is obtained. Samples are taken, milled, ground, and then acid-etched to check the low-magnification structure.

[0008] (1) The tapping temperature of the converter process is controlled to be ≥1600℃. After tapping 1 / 4 of the steel, aluminum blocks 1.8~2.2kg / t, low sulfur calcium ferrosilicon 1.2~1.9kg / t, low sulfur calcium low carbon ferrochrome 15~18kg / t, active lime 750~1000kg / furnace, and cryolite 80~120kg / furnace are added in sequence.

[0009] The low-sulfur, calcium-containing ferrosilicon has the following composition by mass percentage: Si≥69%, Ca≥2%, S≤0.01%, with the balance being Fe and trace impurities. The low-sulfur, calcium-containing, low-carbon ferrochrome has the following composition by mass percentage: Cr≥52%, Ca≥0.5%, S≤0.01%, with the balance being Fe and trace impurities. The active lime has the following composition by mass percentage: CaO≥85%, S≤0.05%. The cryolite has the following composition by mass percentage: F≥40%, Al≥12%.

[0010] This process employs a higher tapping temperature, significantly reducing the viscosity of both slag and molten steel. The aluminum deoxidation product Al2O3 and the alloy deoxidation product CaO can rapidly enter the slag along with the steel slag. Combined with the addition of cryolite, this increases the dissolution rate of CaO in the slag, rapidly forming slag and providing favorable desulfurization prerequisites for LF refining. Simultaneously, the sulfur content in the main raw materials, alloys, and lime is limited to prevent further sulfur enrichment in the molten steel.

[0011] (2) After the LF refining station is filled with gas, the power is turned on and the temperature is raised. In the early stage, the bottom blowing argon pressure is controlled at 0.5-0.62MPa and the argon flow rate is 350-430m³. 3 / h, calcium carbide (CaC2) and aluminum granules (Al) are added to the slag surface to create high-alkalinity reducing foam slag. The slag composition is monitored in real time through a slag rapid separation system to control FeO+MnO≤0.4% and slag alkalinity between 9 and 14, ultimately controlling the S at the LF refining station to be ≤0.003%;

[0012] This process utilizes a large bottom-blown argon flow rate to create an upward bubble flow, providing favorable kinetic conditions for LF refining and desulfurization. No additional lime is needed (as this would affect the slag formation rate and the LF's heating rhythm). Calcium carbide is used to adjust the slag alkalinity and form CO bubbles within the slag, ensuring good slag fluidity and reducibility. A rapid slag separation system monitors slag composition in real time, precisely controlling slag reducibility and alkalinity to ensure efficient deep desulfurization, maintaining an LF refining outlet sulfur content of ≤0.003%.

[0013] (3) LF refining adjusts the composition, controlling Ni to 0.15-0.25% and Cu to 0.03-0.05%;

[0014] This process optimizes and controls the Ni and Cu content to narrow the solidification temperature range of molten steel (liquidotherm temperature - solidus temperature), thereby reducing dendrite segregation. Simultaneously, small amounts of Ni and Cu increase the corrosion resistance of the steel matrix, preventing over-corrosion during subsequent acid leaching testing, which could lead to grain boundary corrosion being misdiagnosed as dendritic structure.

[0015] (4) The cross-sectional dimensions of the crystallizer billet in the continuous casting process are 300×325mm. 2 The second cooling zone adopts the "Hard" cooling mode, with a total water flow of 165~195L / min and a four-stage distribution ratio of 40%, 24%, 18%, and 18%.

[0016] This process uses strong secondary cooling water, controls the total water volume, and employs four-stage cooling, starting with strong cooling and then decreasing it to reduce the surface temperature difference of the continuously cast billet.

[0017] (5) The electromagnetic stirring parameters of the crystallizer in the continuous casting process are current 280~320A and frequency 4~7Hz;

[0018] This process selects an appropriate electromagnetic stirring current for the crystallizer to give the molten steel a certain stirring flow rate and kinetic energy; a medium-range working frequency is selected so that the center of the magnetic field acts on the center position between the quenching layer and the mushy zone of the billet, effectively breaking the dendrite tips and interfering with their growth path, thus promoting compositional homogenization.

[0019] (6) The light reduction process in the continuous casting process is 12mm (4-4-2-2), and the billet size after cooling is 281×321mm. 2 Width and thickness deviation range ≤ ±2.5mm.

[0020] This process employs a 4-4-2-2 reduction technique, meaning a total reduction of 12mm. The reduction amounts of the first four reduction rollers are 4, 4, 2, and 2mm respectively, appropriately compensating for the shrinkage at the end of the billet's solidification and inhibiting the flow of molten steel rich in solute elements towards the center. Precise control of the cross-sectional dimensional deviation range ensures that the light reduction technique is effective.

[0021] The chemical composition of the 42CrMoA described in this invention, by mass percentage, is: C 0.38-0.45%, Si 0.17-0.37%, Mn 0.50-0.80%, Cr 0.90-1.20%, Mo 0.15-0.25%, Ni 0.15-0.25%, Cu 0.03-0.05%, P≤0.020%, S≤0.020%, with the remainder being Fe and unavoidable impurities.

[0022] Compared with existing technologies, the process of this invention optimizes the converter tapping temperature and deoxidation slag-forming process by rationally designing the composition and adding appropriate amounts of Ni and Cu elements, controlling the content of easily segregated S elements, optimizing the secondary cooling intensity of continuous casting, optimizing the electromagnetic stirring current and frequency of the crystallizer, and adopting a light reduction process. This can significantly reduce the number of dendrites in the low magnification structure of 42CrMoA hot-rolled round steel and control the dendrite length to ≤4.2mm. Attached Figure Description

[0023] Figure 1 The number of dendrites was measured at low magnification for dendrite corrosion in the φ65mm rolled material produced in Example 1.

[0024] Figure 2 The dendrite quantity was measured at low magnification to examine the dendrite corrosion of the φ65mm rolled material produced in Comparative Example 1.

[0025] Figure 3 The dendrite length was inspected at low magnification for dendrite corrosion in the φ65mm rolled material produced in Example 5.

[0026] Figure 4 The dendrite length was inspected at low magnification for dendrite corrosion of the φ65mm rolled material produced in Comparative Example 10. Detailed Implementation

[0027] This invention is not limited to the specific embodiments listed below. Those skilled in the art can implement this invention using various other specific embodiments based on the content disclosed herein. Any modifications or alterations made to the design structure and concept of this invention fall within the protection scope of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0028] This embodiment and comparative example illustrate the comprehensive control effect of the present invention using the smelting and continuous casting process of 42CrMoA hot-rolled round steel.

[0029] The present invention will be further described in detail below with reference to the embodiments:

[0030] The low-sulfur calcium-containing ferrosilicon used in the following embodiments of the present invention has Si≥69%, Ca≥2%, S≤0.01%; low-sulfur calcium-containing low-carbon ferrochrome has Cr≥52%, Ca≥0.5%, S≤0.01%; active lime has CaO≥85%, S≤0.05%; cryolite has F≥40%, Al≥12%; other raw materials or methods not specifically mentioned are conventional raw materials or conventional methods in the art.

[0031] Unless otherwise specified, all percentages of contents, ingredients, etc. in this invention represent mass percentages.

[0032] Example 1

[0033] A method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel. The chemical composition of 42CrMoA, by mass percentage, is C 0.38-0.45%, Si 0.17-0.37%, Mn 0.50-0.80%, Cr 0.90-1.20%, Mo 0.15-0.25%, Ni 0.15-0.25%, Cu 0.03-0.05%, P≤0.020%, and S≤0.020%. The method includes the following sequential steps: converter smelting, LF refining, RH vacuum treatment, continuous casting, and rolling, resulting in a φ65mm hot-rolled round steel product. Specific process control parameters are as follows:

[0034] (1) The tapping temperature of the converter process is 1610℃. After tapping 1 / 4 of the steel, 2.1 kg / t of aluminum blocks, 1.8 kg / t of low sulfur calcium ferrosilicon, 17 kg / t of low sulfur calcium low carbon ferrochrome, 900 kg / furnace of active lime, and 100 kg / furnace of cryolite are added in sequence.

[0035] (2) After the LF refining station is powered on and heated, the bottom blowing argon pressure is controlled at 0.58 MPa and the argon flow rate is 400 m³ / h. 3 / h, calcium carbide (CaC2) and aluminum granules (Al) are added to the slag surface to create high-alkalinity reducing foam slag. The slag composition is monitored in real time through a slag rapid separation system to control FeO+MnO at 0.31% and slag basicity at 10.5, ultimately controlling the S at the LF refining station to be ≤0.003%;

[0036] (3) LF refining adjusts the composition to control Ni 0.20% and Cu 0.04%;

[0037] (4) After RH vacuum treatment, the product enters the continuous casting process. The cross-sectional dimensions of the mold billet are 300×325mm. 2 The second cooling zone adopts the "Hard" cooling mode, with a total water flow of 180L / min and a four-stage distribution ratio of 40%, 24%, 18%, and 18%.

[0038] (5) The electromagnetic stirring parameters of the crystallizer in the continuous casting process are 300A current and 5.5Hz frequency;

[0039] (6) The light reduction process in the continuous casting process is 12mm (4-4-2-2), and the size of the billet after cooling is 282×320mm. 2 .

[0040] (7) Hot-rolled round steel is obtained by conventional hot rolling process.

[0041] Example 2

[0042] In step (1), after 1 / 4 of the steel is tapped, 1.8 kg / t of aluminum blocks are added sequentially, and the remaining operations are the same as in Example 1.

[0043] Example 3

[0044] In step (1), 750 kg of active lime was added per furnace, and the rest of the operation was the same as in Example 1.

[0045] Example 4

[0046] In step (2), FeO+MnO is controlled to be 0.20% and slag basicity is controlled to be 13.5. Finally, the LF refining outlet S is controlled to be 0.001%. The remaining operations are the same as in Example 1.

[0047] Example 5

[0048] In step (3), the composition of LF refining is adjusted to control Ni 0.16% and Cu 0.03%, and the remaining operations are the same as in Example 1.

[0049] Example 6

[0050] In step (5), the electromagnetic stirring parameters of the crystallizer are 280A current and 4.5Hz frequency, and the rest of the operation is the same as in Example 1.

[0051] Comparative Example 1

[0052] In Example 1, step (1) "converter tapping temperature 1610℃" is modified to "converter tapping temperature 1575℃", and other conditions are the same as in Example 1.

[0053] Comparative Example 2

[0054] In Example 1, step (1) "add 2.1 kg / t of aluminum blocks sequentially after tapping 1 / 4 of the steel" is modified to "add 1.6 kg / t of aluminum blocks sequentially after tapping 1 / 4 of the steel", with other conditions remaining the same as in Example 1.

[0055] Comparative Example 3

[0056] In Example 1, step (1) "1.8 kg / t of low-sulfur calcium-containing ferrosilicon and 17 kg / t of low-sulfur calcium-containing low-carbon ferrochrome" is modified to "1.8 kg / t of ordinary ferrosilicon (Si≥69%) and 17 kg / t of ordinary low-carbon ferrochrome (Cr≥52%)", with other conditions the same as in Example 1.

[0057] Comparative Example 4

[0058] In Example 1, step (1) "100kg / furnace of cryolite" is modified to "60kg / furnace of cryolite", and other conditions are the same as in Example 1.

[0059] Comparative Example 5

[0060] In step (2) of Example 1, the initial control of the bottom blowing argon pressure is 0.58 MPa and the argon flow rate is 400 m³ / s. 3 " / h" should be changed to "Preliminary control of bottom blowing argon pressure: 0.40MPa, argon flow rate 280m³ / h". 3 " / h", other conditions are the same as in Example 1.

[0061] Comparative Example 6

[0062] In Example 1, step (2) "controlling FeO+MnO to 0.31%, slag basicity to 10.5, and finally controlling LF refining output S≤0.003%" is modified to "controlling FeO+MnO to 0.6%, slag basicity to 6.3, and finally controlling LF refining output S≤0.005%", with other conditions the same as in Example 1.

[0063] Comparative Example 7

[0064] In Example 1, step (3) "LF refining and adjusting composition to control Ni 0.20% and Cu 0.04%" is modified to "LF refining and adjusting composition to control Ni 0.02% and Cu 0.04%", with other conditions the same as in Example 1.

[0065] Comparative Example 8

[0066] In Example 1, step (3) "LF refining and adjusting composition to control Ni 0.20% and Cu 0.04%" is modified to "LF refining and adjusting composition to control Ni 0.20% and Cu 0.01%", with other conditions the same as in Example 1.

[0067] Comparative Example 9

[0068] In Example 1, step (4) "the 4-segment allocation ratio is 40%, 24%, 18%, 18%" is modified to "the 3-segment allocation ratio is 44%, 33%, 23%", and other conditions are the same as in Example 1.

[0069] Comparative Example 10

[0070] In Example 1, step (5) "the frequency of electromagnetic stirring of the crystallizer is 5.5 Hz" is modified to "the frequency of electromagnetic stirring of the crystallizer is 3.5 Hz", and other conditions are the same as in Example 1.

[0071] Comparative Example 11

[0072] In Example 1, step (5) "the frequency of electromagnetic stirring of the crystallizer is 5.5 Hz" is modified to "the frequency of electromagnetic stirring of the crystallizer is 8.5 Hz", and other conditions are the same as in Example 1.

[0073] Comparative Example 12

[0074] In Example 1, step (6) "the continuous casting process with light reduction is 12mm (4-4-2-2), and the size of the cast billet after cooling is 282×320mm" 2 The original text was revised to read: "The continuous casting process uses a light reduction technique of 12mm (2-2-4-4), and the billet size after cooling is 286×316mm." 2 Other conditions are the same as in Example 1.

[0075] The rolled materials obtained in Examples 1-6 and Comparative Examples 1-12 were sampled and milled, and then acid-etched to examine their low-magnification microstructure. The number and length of dendrites examined by low-magnification acid etching are shown in Table 1.

[0076] Figure 1 The φ65mm rolled material produced in Example 1 was subjected to low-magnification dendrite corrosion inspection, and it was found that almost no dendrites were present. Figure 2 As a comparative example, the φ65mm rolled material produced was subjected to low-magnification dendrite corrosion inspection, which revealed a large number of dendrites. Figure 3 The φ65mm rolled material produced in Example 5 was subjected to low-magnification dendrite corrosion inspection. The typical dendrite No. 1 has a length of 3.16mm and the dendrite No. 2 has a length of 4.12mm. Figure 4 For comparative example 10, the φ65mm rolled steel was inspected under low magnification for dendritic corrosion. The typical dendrite lengths were: dendrite 1 (6.87mm), dendrite 2 (6.23mm), and dendrite 3 (6.83mm). The scale bar in the figure is 5.00mm.

[0077] Table 1. Dendrite number and length of rolled materials obtained from Examples 1-6 and Comparative Examples 1-12, determined by low-magnification acid leaching.

[0078]

[0079] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel, characterized in that: The chemical composition of the 42CrMoA steel, by mass percentage, includes: C 0.38-0.45%, Si 0.17-0.37%, Mn 0.50-0.80%, P ≤0.020%, S ≤0.003%, Cr 0.90-1.20%, Ni 0.15-0.25%, Mo 0.15-0.25%, Cu 0.03-0.05%, with the balance being Fe and unavoidable impurities; The method comprises the following sequential steps: converter smelting, LF refining, RH vacuum treatment, continuous casting, and rolling; the main process control methods are as follows: (1) In the converter smelting process, the tapping temperature is controlled to be ≥1600℃. After tapping 1 / 4 of the steel, aluminum blocks, low-sulfur calcium-containing ferrosilicon, low-sulfur calcium-containing low-carbon ferrochrome, active lime, and cryolite are added in sequence. (2) LF refining: control the slag composition FeO+MnO≤0.4wt%, slag basicity 9~14, and LF refining outlet S≤0.003wt%; (3) LF refining adjusts the composition of molten steel, controlling Ni to 0.15-0.25wt and Cu to 0.03-0.05wt; (4) Continuous casting process, crystallizer blank mold section size 300x325mm 2 , two cold zone using "Hard" cooling mode, total water 165~195L / min; (5) In the continuous casting process, the electromagnetic stirring parameters of the crystallizer are 280~320A current and 4~7Hz frequency; (6) Continuous casting process, light reduction process; (7) Rolling to obtain hot-rolled round steel.

2. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (1), the amount of aluminum blocks added is 1.8~2.2 kg / t, the amount of low sulfur calcium ferrosilicon added is 1.2~1.9 kg / t, the amount of low sulfur calcium low carbon ferrochrome added is 15~18 kg / t, the amount of active lime added is 750~1000 kg / furnace, and the amount of cryolite added is 80~120 kg / furnace.

3. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: The low-sulfur calcium-containing ferrosilicon contains Si ≥ 69 wt%, Ca ≥ 2 wt%, and S ≤ 0.01 wt%; the low-sulfur calcium-containing low-carbon ferrochrome contains Cr ≥ 52 wt%, Ca ≥ 0.5 wt%, and S ≤ 0.01 wt%; the active lime contains CaO ≥ 85 wt% and S ≤ 0.05 wt%; and the cryolite contains F ≥ 40 wt% and Al ≥ 12 wt%.

4. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (2), the ladle is heated after being seated, and the bottom argon blowing pressure is controlled at 0.5-0.62 MPa and the argon flow rate is controlled at 350-430 m 3 / h.

5. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (2), calcium carbide and aluminum particles are added to the slag surface to create high-alkalinity reducing foam slag.

6. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (2), the slag composition is detected in real time through the slag rapid separation system.

7. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (4), the secondary cooling water is distributed in four sections at a ratio of 40%, 24%, 18%, and 18%.

8. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: In process (6), the light pressing adopts the "4-4-2-2" pressing process, with a total pressing amount of 12mm.

9. The method for improving the number and length of dendrites in the low-magnification microstructure of 42CrMoA hot-rolled round steel according to claim 1, characterized in that: The 42CrMoA hot-rolled round steel is subjected to dendrite corrosion low-power inspection, and the dendrite area of the rolled material is 200 mm 2 The number of dendrites in the area is not more than 5, and the length of the dendrites is ≤4.2 mm.

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