Method for producing non-quenched and tempered steel by LF-VD-RH refining process

Abstract

The present invention discloses a method for producing non-quenched and tempered steel using an LF-VD-RH refining process, which belongs to the technical field of smelting and production of non-quenched and tempered steel. The method comprises the following steps: a primary smelting process; an LF refining process; a VD furnace vacuum refining process; an RH furnace treatment process; a continuous casting process; and a slow cooling process. The present invention utilizes the LF-VD-RH refining process to produce non-quenched and tempered steel. This process not only fully utilizes the LF furnace's heating and slag-forming functions, and the VD furnace's dehydrogenation and inclusion removal functions, but also achieves stable control of nitrogen, sulfur, and other components in the steel through RH light treatment, reduces deoxidation inclusions in the steel, improves purity, and ensures uniform composition across furnaces, thus meeting the quality requirements of non-quenched and tempered steel for the content of elements such as sulfur and nitrogen, as well as the control of inclusion morphology.

Description

Technical Field

[0001] The invention relates to a method for producing non-quenched and tempered steel by using an LF-VD-RH refining process, belonging to the technical field of smelting production of non-quenched and tempered steel. Background Art

[0002] Non-quenched and tempered steel refers to a type of high-quality or special-quality steel that achieves or approaches the mechanical properties of quenched and tempered steel by eliminating the quenching and tempering heat treatment through strengthening and toughening methods such as microalloying (primarily adding alloying elements such as V, Nb, and Ti), controlled rolling and controlled cooling, etc. Because non-quenched and tempered steel omits the heat treatment process, it can significantly save energy compared to quenched and tempered steel, reducing energy consumption by approximately 30% to 40%. It also avoids defects such as quenching cracks, deformation, oxidation, decarburization, and uneven hardness caused by the quenching and tempering process. In recent years, non-quenched and tempered steel has been promoted in engineering machinery and automobiles. Automobile crankshafts, connecting rods, axles and other parts are widely produced using non-quenched and tempered steel. With the rapid development of my country's automobile and machinery industries, the demand for non-quenched and tempered steel is increasing.

[0003] Non-quenched and tempered steel improves cutting performance by adding a certain amount of sulfur (w(S) = 0.02% to 0.08%). At the same time, to maintain the steel's mechanical properties, nitrogen is permitted, with a recommended nitrogen content of 0.0080% to 0.0200%. Downstream industries demand stable steel composition, controllable inclusion levels and sulfide inclusion morphology, and excellent mechanical properties. Therefore, non-quenched and tempered steel faces challenges in production, such as maintaining a stable composition and controlling inclusions.

[0004] The traditional LF+VD process offers excellent dehydrogenation, deoxidation, and inclusion removal, but due to the intense slag mixing reaction, the content of microalloying elements such as sulfur and nitrogen in the steel is difficult to control. Furthermore, large inclusions are easily introduced into the steel, severely impacting its performance. The LF+RH process offers advantages in dehydrogenation and composition adjustment, but lacks diffusion deoxidation, resulting in low steel cleanliness. Therefore, the development of high-quality non-quenched and tempered steel requires consideration of these issues. Summary of the Invention

[0005] In order to solve the above problems, the present invention proposes a method for producing non-quenched and tempered steel by the LF-VD-RH refining process, which can fully give play to the functions of LF furnace heating and slag formation, VD dehydrogenation and inclusion removal, and can also achieve stable control of nitrogen, sulfur and other components in the steel through RH light treatment, reduce deoxidation inclusions in the steel, improve purity, and uniform composition of the furnace, thereby meeting the quality requirements of non-quenched and tempered steel for the content of elements such as sulfur and nitrogen and the control of inclusion morphology.

[0006] The technical solution adopted by the present invention to solve its technical problems is:

[0007] An embodiment of the present invention provides a method for producing non-quenched and tempered steel by an LF-VD-RH refining process, comprising the following steps:

[0008] Primary smelting process: adopt converter / electric furnace smelting, scrap steel + hot-charged molten iron ratio is not less than 60wt%, the molten iron is low-phosphorus and low-sulfur molten iron, and the arsenic content of the molten iron is not more than 0.006wt%, the carbon content of the molten steel is controlled to be not less than 0.15wt%, the phosphorus content is not more than 0.015wt%, and the tapping temperature is ≥1620℃;

[0009] LF refining process: Use high-purity silicon carbide and aluminum particles for diffusion deoxidation. The ratio of CaO to Al2O3 in LF refining slag is controlled to be above 3.0. The final refining slag has Al2O3 = 20wt% to 30wt%, FeO + MnO ≤ 0.5wt%. After the white slag is produced, the white slag is kept for more than 10 minutes.

[0010] VD furnace vacuum refining process: adopt VD process, slag is skimmed and silica raw material is added before entering the VD furnace to adjust the slag composition and control the slag amount. The slag basicity is not higher than 2.0, and the bottom blowing argon flow rate is controlled in stages from large to small to medium. The vacuum degree during the vacuum treatment process is less than 67Pa and the time is maintained for ≥15 minutes. After the VD is broken, oxygen and hydrogen are determined, requiring [O] ≤ 2.0ppm and [H] ≤ 1.5ppm.

[0011] RH furnace treatment process: adopt light treatment mode, degassing, temperature and composition adjustment in a short time at a relatively low vacuum degree of 3-20KPa, using nitrogen as the RH treatment boost gas, the gas flow rate is 1500-2000NL / min; the treatment time is not less than 20 minutes, the vacuum degree is controlled below 20KPa before 7 minutes, and the vacuum degree is controlled below 6KPa after 7 minutes; the pure degassing time is ≥5 minutes;

[0012] Continuous casting process: A tundish is used to protect pouring. Argon is blown into the tundish throughout the production process, and the tundish cover is sealed. Asbestos gaskets and argon gas are used to seal and protect the long shroud of the large ladle during pouring. The crystallizer liquid level fluctuation is ≤±3mm. The steel liquid is induction heated in the tundish, and the superheat of the tundish in normal heats is controlled at a target of 15-20℃. The secondary cooling water ratio is reduced, weak cooling mode is used, and the electromagnetic stirring intensity is adjusted to ensure that the temperature of the billet entering the straightening machine is ≥900℃.

[0013] Slow cooling process: The cold cast billet is slowly cooled on the ground, and the slow cooling time should be greater than 36 hours.

[0014] As a possible implementation of this embodiment, the chemical composition of the non-quenched and tempered steel includes: C: 0.55wt% to 0.75wt%, Si: 0.17wt% to 0.23wt%, Mn: 0.54wt% to 0.64wt%, P: 0.01wt% to 0.03wt%, S: 0.06wt% to 0.08wt%, Al: 0.02wt% to 0.04wt%, Ca: 0.001wt% to 0.003wt%, Nb: 0.01wt% to 0.04wt%, V: 0.03wt% to 0.04wt%, Cr: 0.10wt% to 0.20wt%, Mo: 0.01wt% to 0.05wt%, Ni: 0.04wt% to 0.08wt%, N: 0.01wt% to 0.02wt%, and the rest are Fe and unavoidable impurity elements.

[0015] As a possible implementation of this embodiment, among the unavoidable impurities, O≤0.002wt% and Ca≤0.004wt%.

[0016] As a possible implementation of this embodiment, during the primary smelting process, the slag basicity and the iron oxide content in the slag are controlled to produce foamed slag, to control the balance of carbon-oxygen reaction, and to achieve uniform decarburization, ensuring that there is no back-drying or splashing throughout the process.

[0017] The ladle must be clean and free of residue. The ladle mouth, cold steel inside the ladle, and residue must be cleaned up. The baking temperature must be ≥800°C. During ladle alloying, ferrosilicon and silicomanganese are added according to the lower limit of the composition, and slag-making material is added at 8-15 kg / t. The order of charging for steel tapping is: aluminum ingot → ferromanganese → ferrosilicon → ferrochrome → slag-making material.

[0018] As a possible implementation of this embodiment, during the LF refining process, after white slag is produced, sampling and analysis are performed, and the contents of C, Si, Mn, Cr, Ni, and V are adjusted according to the target composition requirements based on the test results. Then, after bottom blowing and soft stirring with argon for 5 to 10 minutes, a second sample is taken for analysis. If the chemical composition meets the standard target requirements and the white slag is maintained for more than 10 minutes, the LF refining process is terminated.

[0019] As a possible implementation of this embodiment, during the RH furnace treatment process, alloys are added to adjust the composition according to the molten steel composition and the target composition. The order of alloy addition is ferrosulfur → ferromanganese nitride → silicon-calcium alloy; if other alloys are added, the order of alloy addition is: Mn → Al → Si.

[0020] As a possible implementation method of this embodiment, after the RH furnace treatment process is completed, a weak stirring operation of the ladle is performed, and the argon flow rate is controlled at 30-120Nl / min, with the slag surface slightly fluctuating and the molten steel not exposed, and the duration is ≥5min; during soft argon blowing, it is strictly prohibited to expose the molten steel and stir and cool down with a large amount of argon.

[0021] As a possible implementation of this embodiment, the method further includes the following steps:

[0022] Controlled rolling and controlled cooling process: The steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled to be 1150-1200℃, the starting rolling temperature is 1050-1100℃, the final rolling temperature is ≥800℃, and controlled cooling is performed after rolling;

[0023] Slow cooling process in the pit: the temperature entering the slow cooling pit is ≥450℃, and the temperature leaving the slow cooling pit is ≤150℃.

[0024] As a possible implementation of this embodiment, in the controlled rolling and controlled cooling process, the steel billet is descaled by high-pressure water after leaving the heating furnace and then begins rolling. After rolling, air cooling, wind cooling or pile cooling is adopted.

[0025] As a possible implementation method of this embodiment, during the controlled rolling and controlled cooling process, the steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled to be 1193-1194°C, the starting rolling temperature is 1091-1095°C, the finishing rolling temperature is 944-947°C, and the cooling rate of the steel after rolling is controlled to be no higher than 2°C / s, thereby finally obtaining a medium-carbon free-cutting non-quenched and tempered steel containing sulfur and tellurium.

[0026] The beneficial effects of the technical solutions of the embodiments of the present invention are as follows:

[0027] The present invention adopts the LF-VD-RH refining process to produce non-quenched and tempered steel, which can fully utilize the functions of LF furnace heating and slag formation, VD dehydrogenation and inclusion removal, and can also achieve stable control of nitrogen, sulfur and other components in the steel through RH light treatment, reduce deoxidation inclusions in the steel, improve purity, and achieve uniform composition across heats, thus meeting the quality requirements of non-quenched and tempered steel for the content of elements such as sulfur and nitrogen and the control of inclusion morphology. The present invention successfully produces non-quenched and tempered steel materials that meet "T / CSAE 47-2016 Non-Quenched and Tempered Mechanical Structural Steel for Automobiles" by optimizing the function distribution and process design of the refining process and using smelting and continuous casting technologies.

[0028] To improve the product quality of non-quenched and tempered steel while ensuring smooth production, the present invention proposes a triple LF-VD-RH refining process. This refining process design fully utilizes the intense slag-metal reaction in the VD furnace to achieve deep deoxidation and dehydrogenation and remove large inclusions. Simultaneously, alloying elements such as sulfur, nitrogen, and vanadium are added during the RH treatment. The added alloys react directly with the molten steel, avoiding intense reactions with the slag, ensuring the stability of the sulfur, nitrogen, and vanadium content in the steel, and achieving high cleanliness control for non-quenched and tempered steel. At the same time, the triple refining process places high demands on production organization and the allocation of metallurgical processes across the various steps, particularly challenging logistics and transportation.

[0029] Under the condition of controlling the end point of the primary refining furnace to reduce the oxygen content of the molten steel, the present invention ensures that the free oxygen content in the non-quenched and tempered steel is not higher than 2ppm through measures such as refined slag control, vacuum treatment and soft blowing process optimization, and controls the morphology of inclusions in the steel through measures such as calcium and rare earth treatment to adjust the morphology of inclusions in the non-quenched and tempered steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flow chart of a method for producing non-quenched and tempered steel by a LF-VD-RH refining process according to an exemplary embodiment;

[0031] Figure 2 is a schematic diagram showing the morphology of sulfide inclusions in steel according to an exemplary embodiment;

[0032] Figure 3 yes Figure 2 Morphological images of the sulfides shown;

[0033] Figure 4 yes Figure 3 Energy spectrum analysis diagram of sulfide in Figure 1 );

[0034] Figure 5 yes Figure 3 Energy spectrum analysis diagram of sulfide in Figure 2 ). DETAILED DESCRIPTION

[0035] In order to more clearly illustrate the technical features of the present invention, the present invention is described in detail below through specific implementation methods and in conjunction with the accompanying drawings.

[0036] like Figure 1 As shown, an embodiment of the present invention provides a method for producing non-quenched and tempered steel using a LF-VD-RH refining process, comprising the following steps:

[0037] Primary smelting process: adopt converter / electric furnace smelting, scrap steel + hot-charged molten iron ratio is not less than 60wt%, the molten iron is low-phosphorus and low-sulfur molten iron, and the arsenic content of the molten iron is not more than 0.006wt%, the carbon content of the tapped steel is controlled to be not less than 0.15wt%, the phosphorus content is not more than 0.015wt%, and the tapping temperature is ≥1620℃;

[0038] LF refining process: Use high-purity silicon carbide and aluminum particles for diffusion deoxidation. The ratio of CaO to Al2O3 in LF refining slag is controlled to be above 3.0. The final refining slag has Al2O3 = 20wt% to 30wt%, FeO + MnO ≤ 0.5wt%. After the white slag is produced, the white slag is kept for more than 10 minutes.

[0039] VD furnace vacuum refining process: adopt VD process, slag is skimmed and silica raw material is added before entering the VD furnace to adjust the slag composition and control the slag amount. The slag basicity is not higher than 2.0, and the bottom blowing argon flow rate is controlled in stages from large to small to medium. The vacuum degree during the vacuum treatment process is less than 67Pa and the time is maintained for ≥15 minutes. After the VD is broken, oxygen and hydrogen are determined, requiring [O] ≤ 2.0ppm and [H] ≤ 1.5ppm.

[0040] RH furnace treatment process: adopt light treatment mode, degassing, temperature and composition adjustment in a short time at a relatively low vacuum degree of 3-20KPa, using nitrogen as the RH treatment boost gas, the gas flow rate is 1500-2000NL / min; the treatment time is not less than 20 minutes, the vacuum degree is controlled below 20KPa before 7 minutes, and the vacuum degree is controlled below 6KPa after 7 minutes; the pure degassing time is ≥5 minutes;

[0041] Continuous casting process: A tundish is used to protect pouring. Argon is blown into the tundish throughout the production process, and the tundish cover is sealed. Asbestos gaskets and argon gas are used to seal and protect the long shroud of the large ladle during pouring. The crystallizer liquid level fluctuation is ≤±3mm. The steel liquid is induction heated in the tundish, and the superheat of the tundish in normal heats is controlled at a target of 15-20℃. The secondary cooling water ratio is reduced, weak cooling mode is used, and the electromagnetic stirring intensity is adjusted to ensure that the temperature of the billet entering the straightening machine is ≥900℃.

[0042] Slow cooling process: The cold cast billet is slowly cooled on the ground, and the slow cooling time should be greater than 36 hours.

[0043] As a possible implementation of this embodiment, the chemical composition of the non-quenched and tempered steel includes: C: 0.55wt% to 0.75wt%, Si: 0.17wt% to 0.23wt%, Mn: 0.54wt% to 0.64wt%, P: 0.01wt% to 0.03wt%, S: 0.06wt% to 0.08wt%, Al: 0.02wt% to 0.04wt%, Ca: 0.001wt% to 0.003wt%, Nb: 0.01wt% to 0.04wt%, V: 0.03wt% to 0.04wt%, Cr: 0.10wt% to 0.20wt%, Mo: 0.01wt% to 0.05wt%, Ni: 0.04wt% to 0.08wt%, N: 0.01wt% to 0.02wt%, and the rest are Fe and unavoidable impurity elements.

[0044] As a possible implementation of this embodiment, among the unavoidable impurities, O≤0.002wt% and Ca≤0.004wt%.

[0045] As a possible implementation of this embodiment, during the primary smelting process, the slag basicity and the iron oxide content in the slag are controlled to produce foamed slag, to control the balance of carbon-oxygen reaction, and to achieve uniform decarburization, ensuring that there is no back-drying or splashing throughout the process.

[0046] The ladle should be clean and free of residue. The ladle mouth, cold steel and residue inside the ladle should be cleaned up. The baking temperature should be ≥800℃. When alloying the ladle, ferrosilicon and silicomanganese should be added according to the lower limit of the composition, and slag-making material should be added at a rate of about (8-15) kg / t. The order of charging materials for steel tapping should be: aluminum ingot → ferromanganese → ferrosilicon → ferrochrome → slag-making material.

[0047] As a possible implementation of this embodiment, during the LF refining process, after white slag is produced, sampling and analysis are performed, and the contents of C, Si, Mn, Cr, Ni, and V are adjusted according to the target composition requirements based on the test results. Then, after bottom blowing and soft stirring with argon for 5 to 10 minutes, a second sample is taken for analysis. If the chemical composition meets the standard target requirements and the white slag is maintained for more than 10 minutes, the LF refining process is terminated.

[0048] As a possible implementation of this embodiment, during the RH furnace treatment process, alloys are added to adjust the composition according to the molten steel composition and the target composition. The order of alloy addition is ferrosulfur → ferromanganese nitride → silicon-calcium alloy; if other alloys are added, the order of alloy addition is: Mn → Al → Si.

[0049] As a possible implementation method of this embodiment, after the RH furnace treatment process is completed, a weak stirring operation of the ladle is performed, and the argon flow rate is controlled at 30-120Nl / min, with the slag surface slightly fluctuating and the molten steel not exposed, and the duration is ≥5min; during soft argon blowing, it is strictly prohibited to expose the molten steel and stir and cool down with a large amount of argon.

[0050] As a possible implementation of this embodiment, the method further includes the following steps:

[0051] Controlled rolling and controlled cooling process: The steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled to be 1150-1200℃, the starting rolling temperature is 1050-1100℃, the final rolling temperature is ≥800℃, and controlled cooling is performed after rolling;

[0052] Slow cooling process in the pit: the temperature entering the slow cooling pit is ≥450℃, and the temperature leaving the slow cooling pit is ≤150℃.

[0053] As a possible implementation of this embodiment, in the controlled rolling and controlled cooling process, the steel billet is removed from the heating furnace and descaled by high-pressure water before rolling begins. After rolling, air cooling, wind cooling or pile cooling is adopted.

[0054] As a possible implementation method of this embodiment, during the controlled rolling and controlled cooling process, the steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled to be 1193-1194°C, the starting rolling temperature is 1091-1095°C, the finishing rolling temperature is 944-947°C, and the cooling rate of the steel after rolling is controlled to be no higher than 2°C / s, thereby finally obtaining a medium-carbon free-cutting non-quenched and tempered steel containing sulfur and tellurium.

[0055] In order to overcome the shortcomings of the existing technology, the present invention provides a refining process for high-quality non-quenched and tempered steel, which adopts the LF-VD-RH refining process to produce non-quenched and tempered steel. By optimizing the function distribution and process design of the refining process, and combining smelting and continuous casting technology, a non-quenched and tempered steel material suitable for meeting the "T / CSAE 47-2016 Non-quenched and tempered mechanical structural steel for automobiles" is successfully produced.

[0056] The present invention utilizes a smelting process for producing high-quality non-quenched and tempered steel by LF-VD-RH refining. The process can fully utilize the functions of LF furnace heating and slag formation, VD dehydrogenation and inclusion removal, and can also achieve stable control of nitrogen, sulfur and other components in the steel through RH light treatment, reduce deoxidation inclusions in the steel, improve purity, and achieve uniform composition among furnaces, thus meeting the quality requirements of non-quenched and tempered steel for the content of elements such as sulfur and nitrogen and the control of inclusion morphology.

[0057] The LF-VD-RH refining process for producing non-quenched and tempered steel using the method described herein is as follows: batching (molten iron + scrap) → converter / electric furnace → LF furnace refining → VD vacuum degassing → RH vacuum treatment → continuous casting (electromagnetic stirring) → slow cooling → controlled rolling and controlled cooling – pit entry and slow cooling. The following uses C70S6 non-quenched and tempered steel and 45MnVS non-quenched and tempered steel as examples to illustrate the effects of the present invention.

[0058] 1. C70S6 non-quenched and tempered steel

[0059] Production process: batching → 100t electric furnace → LF furnace refining → VD vacuum degassing → RH vacuum treatment → continuous casting (electromagnetic stirring) → slow cooling → controlled rolling and controlled cooling → slow cooling in the pit, smelting furnace number: T221-06873, specification φ70mm.

[0060] 1) Gas content

[0061] The oxygen content is 12.8 ppm, the nitrogen content is 167 ppm, and the hydrogen content is 0.7 ppm.

[0062] 2) Low-magnification tissue (2 pieces)

[0063] No shrinkage cavities, bubbles, cracks, inclusions, peeling, white spots, or intergranular cracks were observed on the acid-etched macrostructure specimens of the round steel cross section. The acid-etched macrostructure is shown in Table 1.

[0064] Table 1: Low-magnification tissue

[0065]

[0066] 3) Mechanical properties (2 pieces)

[0067] A Φ25mm specimen blank was taken longitudinally at the 1 / 2 radius of the round steel for normalizing treatment. The normalizing temperature was 920℃±10℃ and the holding time was 40min. After holding, it was placed vertically for air cooling. The mechanical properties were tested after normalizing. The results are shown in Table 2.

[0068] Table 2: Mechanical properties

[0069]

[0070] 4) Non-metallic inclusions (6 pieces)

[0071] Rating according to DIN 50602 standard, qualified level K4(O) ≤ 25. Also tested according to ASTM E45 Method A. The qualified levels and test results are shown in Table 3.

[0072] Table 3: Non-metallic inclusions

[0073]

[0074] 5) Grain size (1 piece)

[0075] The austenite grain size of the steel measured according to GB / T 6394 is grade 7.0.

[0076] 6) Decarburized layer (1 piece)

[0077] The decarburization layer of steel is tested according to GB / T 224, and the total decarburization layer depth is 0.33%D.

[0078] 7) Delivery hardness (3 pieces)

[0079] The steel is delivered in hot rolled state with delivery hardness of 252, 255 and 262HB respectively.

[0080] 8) NDT

[0081] The round steel passed the magnetic flux leakage + ultrasonic flaw detection test.

[0082] 9) Hairline (3 pieces)

[0083] The round steel grain test was carried out in accordance with JIS G0556 and the test results were qualified.

[0084] 10) Photo of inclusions in steel, short rod-shaped and small in size, see Figure 2-Figure 5 .

[0085] 2. 45MnVS non-quenched and tempered steel

[0086] Production process: batching → 100t electric furnace → LF furnace refining → VD vacuum degassing → RH vacuum treatment → continuous casting (electromagnetic stirring) → slow cooling → controlled rolling and controlled cooling → slow cooling in the pit, smelting furnace number: T222-07315, specification φ65mm.

[0087] 1) Gas content:

[0088] Oxygen content: 11.7 ppm, nitrogen content: 146 ppm, hydrogen content: 0.9 ppm.

[0089] 2) Mechanical properties (2 pieces)

[0090] For the inspection of the longitudinal mechanical properties in the hot-rolled state, refer to Table 4.

[0091] Table 4: Mechanical properties

[0092]

[0093] 3) Low-magnification tissue (2 pieces)

[0094] According to GB / T 1979, cross-sectional acid-etched macrostructure specimens of the steel are free of visible shrinkage cavities, bubbles, cracks, inclusions, peeling, white spots, or intergranular cracks. The steel is also free of subcutaneous inclusions exceeding the allowable depth for surface defects. Macrostructure is shown in Table 5.

[0095] Table 5: Macroscopic tissue

[0096] Generally loose Loose center Ingot segregation General spot segregation, edge spot segregation, subcutaneous bubbles Level 0 Level 0.5 Level 0 none

[0097] 4) Non-metallic inclusions (1 piece)

[0098] Non-metallic inclusions in steel are tested in accordance with GB / T 10561 and evaluated using Method A. The test results are shown in Table 6.

[0099] Table 6: Non-metallic inclusions

[0100]

[0101] 5) Grain size (1 piece)

[0102] The austenite grain size of the steel tested is grade 7.

[0103] 6) Microstructure (1 piece)

[0104] The microstructure of steel is ferrite + pearlite (F+P).

[0105] 7) Decarburized layer (1 piece)

[0106] The total decarburized layer depth (ferrite + transition layer) on each side tested by microstructure method is 0.27% of the steel diameter or thickness.

[0107] 8) NDT

[0108] Each steel piece is subjected to magnetic flux leakage + ultrasonic flaw detection and passes the inspection.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for producing non-quenched and tempered steel using a LF-VD-RH refining process, characterized in that: The steps include: Primary smelting process: adopt converter / electric furnace smelting, scrap steel + hot charged molten iron ratio is not less than 60wt%, the molten iron is low phosphorus and low sulfur molten iron, and the arsenic content of the molten iron is not more than 0.006wt%, the carbon content of the tapped steel is controlled to be not less than 0.15wt%, the phosphorus content is not more than 0.015wt%, and the tapping temperature is ≥1620℃; LF refining process: Use high-purity silicon carbide and aluminum particles for diffusion deoxidation. The ratio of CaO to Al2O3 in LF refining slag is controlled to be above 3.

0. The final refining slag has Al2O3 = 20wt% ~ 30wt%, FeO + MnO ≤ 0.5wt%. After the white slag is produced, the white slag is kept for more than 10 minutes. VD furnace vacuum refining process: adopt VD process, slag is skimmed and silica raw material is added before entering the VD furnace to adjust the slag composition and control the slag amount. The slag basicity is not higher than 2.0, and the bottom blowing argon flow rate is controlled in stages from large to small to medium. The vacuum degree during the vacuum treatment process is less than 67Pa and the time is maintained for ≥15 minutes. After the VD is broken, oxygen and hydrogen are determined, requiring [O] ≤ 2.0ppm and [H] ≤ 1.5ppm. RH furnace treatment process: Use light treatment mode, degassing, temperature and composition adjustment for a short time at a relatively low vacuum degree of 3-20kPa, use nitrogen as the RH treatment boost gas, and the gas flow rate is 1500-2000NL / min; the treatment time is not less than 20 minutes, and the vacuum degree is controlled below 20kPa before 7 minutes, and below 6kPa after 7 minutes; the pure degassing time is ≥5 minutes; Continuous casting process: A tundish is used to protect pouring. Argon is blown into the tundish throughout the production process, and the tundish cover is sealed. Asbestos gaskets and argon gas are used to seal the long nozzle of the large ladle for protection during pouring. The fluctuation of the liquid level in the crystallizer is controlled within the range of ±3mm. The steel liquid is induction heated in the tundish, and the superheat of the tundish in normal heats is controlled at a target of 15-20℃. The water content of the secondary cooling is reduced, weak cooling mode is used, and the electromagnetic stirring intensity is adjusted to ensure that the temperature of the billet entering the straightening machine is ≥900℃. Slow cooling process: The cold cast billet is slowly cooled on the ground, and the slow cooling time should be greater than 36 hours.

2. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 1, characterized in that: The chemical composition of the non-quenched and tempered steel includes: C: 0.55wt% to 0.75wt%, Si: 0.17wt% to 0.23wt%, Mn: 0.54wt% to 0.64wt%, P: 0.01wt% to 0.03wt%, S: 0.06wt% to 0.08wt%, Al: 0.02wt% to 0.04wt%, Ca: 0.001wt% to 0.003wt%, Nb: 0.01wt% to 0.04wt%, V: 0.03wt% to 0.04wt%, Cr: 0.10wt% to 0.20wt%, Mo: 0.01wt% to 0.05wt%, Ni: 0.04wt% to 0.08wt%, N: 0.01wt% to 0.02wt%, and the rest are Fe and unavoidable impurity elements.

3. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 2, characterized in that: Among the unavoidable impurities, O≤0.002wt%, Ca≤0.004wt%.

4. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 1, characterized in that: During the primary smelting process, the slag basicity and iron oxide content in the slag are controlled to create foamy slag, balance the carbon-oxygen reaction, and achieve uniform decarburization, ensuring no back-drying or splashing throughout the process. The ladle must be clean and free of residue. The ladle mouth, cold steel inside the ladle, and residue must be cleaned up. The baking temperature must be ≥800°C. During ladle alloying, ferrosilicon and silicomanganese are added according to the lower limit of the composition, and slag-making material is added at 8-15 kg / t. The order of charging for steel tapping is: aluminum ingot → ferromanganese → ferrosilicon → ferrochrome → slag-making material.

5. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 1, characterized in that: During the LF refining process, after the white slag is produced, sampling is performed for analysis. Based on the test results, the contents of C, Si, Mn, Cr, Ni and V are adjusted according to the target composition requirements. Then, after bottom blowing with argon and soft stirring for 5 to 10 minutes, a second sample is taken for analysis. If the chemical composition meets the standard target requirements and the white slag is kept for more than 10 minutes, the LF refining process is terminated.

6. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 1, characterized in that: During the RH furnace treatment process, alloys are added to adjust the composition according to the molten steel composition and the target composition. The order of alloy input is ferrosulfur → ferromanganese nitride → silicon-calcium alloy; if other alloys are added, the order of alloy input is: Mn → Al → Si.

7. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 1, characterized in that: After the RH furnace treatment process is completed, the ladle is weakly stirred, and the argon flow rate is controlled at 30-120NL / min, with the slag surface slightly fluctuating and the molten steel not exposed, and the duration is ≥5min; it is strictly forbidden to expose the molten steel and stir and cool down with a large amount of argon during soft argon blowing.

8. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to any one of claims 1 to 7, characterized in that: The following steps are also included: Controlled rolling and controlled cooling process: The steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled to be 1150-1200℃, the starting rolling temperature is 1050-1100℃, the final rolling temperature is ≥800℃, and controlled cooling is performed after rolling; Slow cooling process in the pit: the temperature entering the slow cooling pit is ≥450℃, and the temperature leaving the slow cooling pit is ≤150℃.

9. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 8, characterized in that: In the controlled rolling and controlled cooling process, the steel billet is descaled by high-pressure water after leaving the heating furnace and then begins rolling. After rolling, air cooling, wind cooling or pile cooling is adopted.

10. The method for producing non-quenched and tempered steel by the LF-VD-RH refining process according to claim 8, characterized in that: In the controlled rolling and controlled cooling process, the steel billet obtained after continuous casting is heated in a heating furnace, the soaking temperature is controlled at 1193-1194°C, the starting rolling temperature is 1091-1095°C, the finishing rolling temperature is 944-947°C, and the cooling rate of the steel after rolling is controlled to be no higher than 2°C / s, finally obtaining sulfur-containing medium-carbon free-cutting non-quenched and tempered steel.

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