Method for reducing surface bumps of sulfur-preserving steel 40CrA after hot forging

Abstract

The invention belongs to the technical field of iron and steel smelting, and relates to a method for reducing surface bumps of sulfur-preserving steel 40CrA after hot forging. According to the technical scheme, converter smelting is carried out, end point control target values are as follows: C is greater than or equal to 0.08%, P is less than or equal to 0.015%, a deoxidizer and an alloy are added along a steel flow to carry out deoxidation alloying when 1 / 4 of tapping is carried out, 3 / 4 of tapping is carried out, and bottom argon blowing is carried out in the whole process after the alloy is added; in an LF refining furnace, the total argon blowing time of molten steel in the LF furnace is larger than or equal to 40 min, aluminum particle deoxidation, rapid slagging deoxidation and desulfurization are used, after refining is completed, sulfur-preserving steel is fed to a Ca line, and other components except the Al content are finely adjusted according to target requirements; in a VD / RH refining furnace, when molten steel enters the VD / RH furnace, whole-course argon blowing and temperature measurement are started, the vacuumizing target is 0.5 torr or below, the vacuum keeping time is longer than or equal to 10 min, the soft blowing time is longer than or equal to 15 min after molten steel vacuum breaking, the number of furnaces is larger than or equal to 1 in each casting time, and the requirement that [H] is smaller than or equal to 1.5 ppm is met; continuous casting is conducted, pouring is protected in the whole process, large ladle slag tapping and large ladle steel remaining operation are strictly prohibited, and special medium carbon steel casting powder is adopted as crystallizer casting powder. By adopting the method, the bulging phenomenon of the forge piece is obviously reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and in particular relates to a method for reducing surface bulges of 40CrA after hot forging. Background Art

[0002] The purpose of sulfur retention is to facilitate cutting, reduce tool wear and improve processing efficiency; the disadvantage is that sulfide inclusions are easily formed during the smelting process, and bulging or cracking is easily formed after hot forging. Bulging exists on the surface during the forging process or in liquid form, solidifies and welds with the base metal during the subsequent cooling process. There is no obvious indentation on the surface morphology of the forging at the bulging location, and the bulging location is cast iron structure. The main source of bulging is steelmaking protective slag.

[0003] Chinese patent CN 117845124 A discloses a method for reducing the bulging defect of low-carbon boron steel continuous casting slab, which mainly solves the technical problem of the high incidence of bulging defects in existing low-carbon boron steel continuous casting slab. The technical solution is a method for reducing the bulging defect of low-carbon boron steel continuous casting slab, including: 1) converter smelting, using top and bottom double blowing converter smelting, the mass percentage of the metal raw material input is 75%-90% of molten iron, and the balance is scrap steel; converter tapping, the converter tapping immediately after the converter blowing is completed, the ladle is not a newly built ladle or a ladle that has not been repaired; continuous casting, the molten steel in the ladle is distributed to the crystallizer of the slab continuous casting machine through the tundish and then cast by the slab continuous casting machine to obtain a continuous casting slab; the casting speed of the continuous casting machine during continuous casting is 0.8-2.0m / min. This patent mainly reduces the hydrogen content in the molten steel in the tundish to avoid the bulging defect of the slab caused by high hydrogen content. However, it cannot solve the problem of bulges formed by slag sticking to the surface of the ingot during the continuous casting process. Summary of the invention

[0004] To achieve the above-mentioned purpose, the present invention provides a method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging, which can solve the problems existing in the prior art.

[0005] The main technical idea of ​​the present invention is to optimize casting process parameters, prevent slag from sticking on the surface of the ingot, and select protective slag with suitable physical and chemical performance indicators such as low viscosity, low melting point and fast melting speed.

[0006] The technical solution adopted by the present invention is: a method for reducing the surface bulging of sulfur-retaining steel 40CrA after hot forging, comprising the following steps:

[0007] 1) Converter smelting, endpoint control target value: C ≥ 0.08%, P ≤ 0.015%, slag blocking steel tapping, slag is strictly prohibited, deoxidizer and alloy are added along the steel flow for deoxidation and alloying when 1 / 4 of the steel is tapped, alloy is added for 3 / 4 of the steel, and argon is blown at the bottom of the furnace throughout the whole process;

[0008] 2) LF refining furnace, the total argon blowing time of molten steel in LF furnace is ≥40min, aluminum particles are used for deoxidation, and slag is quickly made for deoxidation and desulfurization. After refining, the sulfur-preserving steel is fed with Ca line. Except for Al content, other components of the outgoing composition are fine-tuned according to the target requirements to ensure that the finished product composition meets the internal control requirements. For steel types that need to be sulfur-added, the sulfur content target is controlled at 0.005% to 0.015%; for steel types that do not enter vacuum and steel types that undergo RH vacuum treatment, feed Ca line for soft blowing for more than 5min and then feed sulfur line for sulfur addition, and the soft blowing time before leaving the station is ≥10min; for molten steel that enters VD vacuum treatment, the sulfur line is not fed before leaving the LF furnace;

[0009] 3) VD / RH refining furnace: When the molten steel enters the VD / RH furnace, the whole process of argon blowing and temperature measurement is started, the vacuum target is below 0.5torr, the vacuum maintenance time is ≥10min, and the composition adjustment is strictly prohibited after the vacuum is broken. The soft blowing time of the molten steel after the vacuum is broken is ≥15min. [H] is set to ≥1 furnace for each pouring, and [H] is required to be ≤1.5ppm;

[0010] 4) Continuous casting, full protection pouring, strictly prohibited large ladle slag, large ladle steel retention operation, the mold protective slag uses medium carbon steel special protective slag, the superheat of the molten steel in the middle ladle exceeds the standard, the crystallizer liquid level fluctuation is ≤±5mm, if it exceeds the range, it will be picked out and changed to other steel types, take low-magnification samples for each pouring ≥1, the same pouring must cover each stream, if it exceeds the range, it will be picked out and changed to other steel types.

[0011] Furthermore, in step 2) of the present invention, slag is formed in the LF refining furnace within 20 minutes, and the desulfurization target is ≤0.005%.

[0012] Furthermore, in step 2) of the present invention, after refining, the sulfur-retaining steel is fed to the Ca line in a 120m open-casting furnace and a 80m continuous casting furnace.

[0013] Furthermore, in step 2) of the present invention, the target requirements are: C: 0.38% to 0.42%, Si: 0.17% to 0.37%, Mn: 0.80% to 1.10%, Cr: 0.80% to 1.10%, P≤0.020%; S≥0.005%; for steel types that need to increase sulfur, the sulfur content target is controlled at 0.005% to 0.015%.

[0014] Furthermore, in step 3) of the present invention, the soft blowing time of the molten steel in the VD / RH refining furnace after breaking through air is 20-25 minutes.

[0015] Furthermore, in step 4) of the present invention, the protective slag for medium carbon steel is 32%-38% calcium oxide (CaO), 27%-37% silicon dioxide (SiO2), 5%-10% aluminum oxide (Al2O3); 5%-15% calcium fluoride (CaF2), and 2%-3% carbon black. It mainly plays the role of heat insulation, which can reduce the heat loss of molten steel, keep the molten steel at a suitable temperature in the crystallizer, and facilitate the solidification and molding of the molten steel. It can also isolate the air, avoid oxidation of the molten steel, and prevent oxidation defects from forming on the surface of the ingot. At the same time, the protective slag can also lubricate the gap between the crystallizer wall and the ingot, reduce the resistance to billet drawing, so that the ingot can be smoothly pulled out of the crystallizer, and effectively improve the surface quality of the ingot. The melting rate of the protective slag can also be adjusted to play the role of heat preservation and air isolation.

[0016] The beneficial effects of the present invention are: the present invention optimizes the casting process parameters, eliminates slag sticking to the surface of the ingot, and selects protective slag with suitable physical and chemical performance indicators such as low viscosity, low melting point, and fast melting rate. Before adopting this technical solution, the company's 6 batches of 599.144 tons had a bulging ratio of more than 20%. After the improvement, the above-mentioned bulging situation no longer occurred. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 This is a macroscopic morphology of the product showing bulging after forging;

[0019] Figure 2 This is a macroscopic morphology of the product showing bulging after forging;

[0020] Figure 3 is a diagram of a steering knuckle forging of Example 1;

[0021] Figure 4 This is a diagram of the steering knuckle forging of Example 2. DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] 1. Correlation analysis of bulge samples

[0024] 40CrA forging surface bulge specimen, the product has bulge phenomenon after forging, the bulge position and macroscopic morphology are as follows Figure 1 , 2 As shown in the figure, there are obvious protrusions at the bulge, which are closely combined with the matrix. The larger bulge is the size of a finger, and there is no obvious regularity in its position on the forging.

[0025] Metallographic structure or inclusion analysis:

[0026] In order to understand the cause of the bulge, the longitudinal section sample at the bulge position was taken for analysis. According to the metallographic analysis results, the following conclusions were drawn:

[0027] 1. The metallographic structure of the bulge is ledeburite + a small amount of pearlite (Ld+P). According to the pearlite content and the lever law, it is inferred that it is a cast iron with a carbon content of 3.5% to 4.0%. The dendritic structure is obvious and contains defects such as bubbles. It is inferred that it should be a solidified structure.

[0028] 2. There is a clear dividing line between the bulging metal and the base metal, but the bonding is good. Except for defects such as bubbles and oxidation, it is basically welded together with the base metal.

[0029] 3. There is obvious carbon diffusion between the bulging metal and the matrix. The matrix diffusion depth is 0.4mm~0.5mm, and the metallographic structure at the diffusion point is pearlite + network cementite.

[0030] 4. The metallographic structure of the base metal is pearlite + broken network ferrite, the structure is relatively coarse, which is a typical forging structure. No obvious overheating or overburning was found at the bulge.

[0031] 5. EDS energy spectrum analysis In order to have a preliminary understanding of the composition of the blister metal, the INCA 250+ energy spectrometer was used to perform microscopic composition analysis. The results of EDS composition analysis showed that the main components of the blister metal were Fe, C, and Cr, of which the Cr content was about 0.62%. According to the results of metallographic analysis, the blister metal was considered to be cast iron with a carbon content of 3.5% to 4.0%.

[0032] Based on the above analysis, we can make the following judgment: there is no obvious indentation on the surface of the forging at the bulge position, but there is a carbon increase of 0.4mm to 0.5mm inside the base metal near the bulge. It is believed that the source of the bulge is the slag sticking to the surface of the ingot during the continuous casting process.

[0033] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0034] 1. Converter smelting: endpoint control target value: C ≥ 0.08%, P ≤ 0.015%, slag blocking for tapping, slag is strictly prohibited, deoxidizer and alloy are added along the steel flow for deoxidation and alloying when 1 / 4 of the steel is tapped, alloy is added for 3 / 4 of the steel, and argon is blown at the bottom of the furnace throughout the whole process.

[0035] 2. LF refining furnace: The total argon blowing time of molten steel in LF furnace is ≥40min, aluminum particles are used for deoxidation, rapid slag deoxidation and desulfurization, slag making within 20min, desulfurization target ≤0.005%, and sulfur-retaining steel is fed to Ca line after refining: open pouring furnace 120m, continuous pouring furnace 80m, except Al content, the outgoing composition is fine-tuned according to the target requirements to ensure that the finished product composition meets the internal control requirements, the target requirements are: C: 0.38%~0.42%; Si: 0.17%~ 0.37%; Mn: 0.80%~1.10%; Cr: 0.80%~1.10%: P: ≤0.020%; S: ≥0.005%; For steels that need to be sulfur-added, the sulfur content target is controlled at 0.005%~0.015%; For steels that do not enter vacuum and those that undergo RH vacuum treatment, feed the Ca line for soft blowing for more than 5 minutes and then feed the sulfur line for sulfur increase, and the soft blowing time before leaving the station is ≥10 minutes; For molten steel that enters VD vacuum treatment, the sulfur line is not fed before leaving the LF furnace.

[0036] 3. VD / RH refining furnace: When molten steel enters the VD / RH furnace, argon blowing and temperature measurement are started throughout the process. The vacuum target is below 0.5 torr (67 Pa). The vacuum maintenance time is ≥10 min. It is strictly forbidden to adjust the composition after the air is broken. The soft blowing time of the molten steel after the air is broken is ≥15 min, preferably 20-25 min. [H] is set to ≥1 furnace for each pouring, and [H] is required to be ≤1.5ppm.

[0037] 4. Continuous casting: Protect the pouring throughout the whole process. It is strictly forbidden to put slag in a large bag or keep steel in a large bag. The protective slag for the crystallizer adopts a special protective slag for medium carbon steel. It mainly plays the role of heat insulation and heat preservation, which can reduce the heat loss of molten steel, keep the molten steel at a suitable temperature in the crystallizer, and facilitate the solidification and molding of the molten steel. It can also isolate the air, prevent the molten steel from being oxidized, and prevent the formation of oxidation defects on the surface of the billet. At the same time, the protective slag can also lubricate the gap between the crystallizer wall and the billet, reduce the resistance of billet drawing, so that the billet can be smoothly pulled out of the crystallizer, and effectively improve the surface quality of the billet. Calcium oxide (CaO), the content is roughly 32%-38%; silicon dioxide (SiO2), the content is about 27%-37%; aluminum oxide (Al2O3), the content is 5%-10%; calcium fluoride (CaF2), usually accounts for 5%-15%; carbon black, the content is 2%-3%. The melting speed of the protective slag can be adjusted, and it also plays the role of heat preservation and air isolation to prevent the oxidation of molten steel. The superheat of the molten steel in the tundish exceeds the standard, and the liquid level fluctuation of the crystallizer is ≤±5mm. If it exceeds the range, it will be selected and judged as other steel grades. Take ≥1 low-magnification sample for each pouring, and the same pouring must cover each stream. If it exceeds the range, it will be selected and judged as other steel grades.

[0038] The present invention will be further described below in conjunction with the embodiments.

[0039] Embodiment 1:

[0040] This embodiment is a method for producing sulfur-retaining steel 40CrA, in which the chemical components of the steel are C: 0.39%, Si: 0.22%, Mn: 0.78%, P: 0.012%, S: 0.011%, Cr: 0.8723%, Ti: 0.0052%, Ca: 0.0007%, Al: 0.0089%, Ni: 0.013%, and Cu: 0.0258% in percentage by weight.

[0041] The key process steps include:

[0042] Smelting: The converter steel P=0.008%, C=0.06%, LF+VD refining process is adopted, LF furnace refining is argon-blown throughout the whole process, slag making and deoxidation, white slag holding time is 21min, refined slag basicity CaO / SiO2=3.5; VD vacuum degassing treatment, vacuum degree below 0.5tor, vacuum holding time 15min, outlet molten steel H=1.2ppm, O=3.4ppm.

[0043] Continuous casting: full-process protective casting is adopted, the mold protective slag adopts medium carbon steel special protective slag, and the tundish superheat is 23℃.

[0044] The steering knuckle forging obtained in this embodiment is shown in FIG. Figure 3 shown.

[0045] Embodiment 2:

[0046] This embodiment is a method for producing sulfur-retaining steel 40CrA, in which the chemical components in mass percentage are C: 0.39%, Si: 0.24%, Mn: 0.77%, P: 0.014%, S: 0.001%, Cr: 1.0981%, Ti: 0.0057%, Ca: 0.0006%, Al: 0.009%, Ni: 0.011%, and Cu: 0.0093%.

[0047] The key process steps include:

[0048] Smelting: The converter steel has P=0.010%, C=0.06%. The LF+VD refining process is adopted. Argon is blown throughout the LF furnace refining process, slag is deoxidized, the white slag is kept for 25 minutes, and the basicity of the refined slag is CaO / SiO2=3.6. VD vacuum degassing treatment, the vacuum degree is below 0.5tor, the vacuum is kept for 15 minutes, and the outlet molten steel has H=1.3ppm, O=3.1ppm.

[0049] Continuous casting: full-process protective casting is adopted, the mold protective slag adopts medium carbon steel special protective slag, and the tundish superheat is 21℃.

[0050] The steering knuckle forging obtained in this embodiment is shown in FIG. Figure 4 shown.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging, characterized in that: The method comprises the following steps: 1) Converter smelting, endpoint control target value: C ≥ 0.08%, P ≤ 0.015%, slag blocking steel tapping, slag is strictly prohibited, deoxidizer and alloy are added along the steel flow for deoxidation and alloying when 1 / 4 of the steel is tapped, alloy is added for 3 / 4 of the steel, and argon is blown at the bottom of the furnace throughout the whole process; 2) LF refining furnace, the total argon blowing time of molten steel in LF furnace is ≥40min, aluminum particles are used for deoxidation, and slag is quickly made for deoxidation and desulfurization. After refining, the sulfur-preserving steel is fed with Ca line. Except for Al content, other components of the outgoing composition are fine-tuned according to the target requirements to ensure that the finished product composition meets the internal control requirements. For steel types that need to be sulfur-added, the sulfur content target is controlled at 0.005% to 0.015%; for steel types that do not enter vacuum and steel types that undergo RH vacuum treatment, feed Ca line for soft blowing for more than 5min and then feed sulfur line for sulfur addition, and the soft blowing time before leaving the station is ≥10min; for molten steel that enters VD vacuum treatment, the sulfur line is not fed before leaving the LF furnace; 3) VD / RH refining furnace: When the molten steel enters the VD / RH furnace, the whole process of argon blowing and temperature measurement is started, the vacuum target is below 0.5torr, the vacuum maintenance time is ≥10min, and the composition adjustment is strictly prohibited after the vacuum is broken. The soft blowing time of the molten steel after the vacuum is broken is ≥15min. [H] is set to ≥1 furnace for each pouring, and [H] is required to be ≤1.5ppm; 4) Continuous casting, full protection pouring, strictly prohibited large ladle slag, large ladle steel retention operation, the mold protective slag uses medium carbon steel special protective slag, the superheat of the molten steel in the middle ladle exceeds the standard, the crystallizer liquid level fluctuation is ≤±5mm, if it exceeds the range, it will be picked out and changed to other steel types, take low-magnification samples for each pouring ≥1, the same pouring must cover each stream, if it exceeds the range, it will be picked out and changed to other steel types.

2. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging as claimed in claim 1, characterized in that: The LF refining furnace produces slag within 20 minutes, and the desulfurization target is ≤0.005%.

3. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging as claimed in claim 1, characterized in that: After refining, the sulfur-retaining steel is fed into the Ca line for a 120m pouring furnace and a 80m continuous pouring furnace.

4. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging as claimed in claim 1, characterized in that: The target requirements are: C: 0.38%~0.42%, Si: 0.17%~0.37%, Mn: 0.80%~1.10%, Cr: 0.80%~1.10%, P≤0.020%; S≥0.005%; for steel types that need to increase sulfur, the sulfur content target is controlled at 0.005%~0.015%.

5. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging as claimed in claim 1, characterized in that: The soft blowing time of molten steel in VD / RH refining furnace after breaking through air is 20 to 25 minutes.

6. A method for reducing surface bulging of sulfur-retaining steel 40CrA after hot forging as claimed in claim 1, characterized in that: The special protective slag for medium carbon steel is 32% to 38% calcium oxide (CaO), 27% to 37% silicon dioxide (SiO2), 5% to 10% aluminum oxide (Al2O3), 5% to 15% calcium fluoride (CaF2), and 2% to 3% carbon black.

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