Control method for large-particle inclusions in medium-carbon alloy sulfur-containing steel

Abstract

The invention specifically discloses a control method for large-particle inclusions in medium-carbon alloy sulfur-containing steel, which sequentially comprises the steps of KR desulfurization, LD converter, LF external refining, RH vacuum degassing and continuous casting, only silicon carbide deoxidation is used in the LF external refining, a calcium line is forbidden to be used for treating molten steel, and meanwhile, the S component is finely adjusted in the LF, so that the content of impurities in the molten steel is reduced, and the content of impurities in the molten steel is reduced; feeding an S line according to a component analysis result of the cake sample taken by the LF furnace, and adjusting the S content to 0.020-0.025%; the RH vacuum degassing continuous soft blowing time is longer than or equal to 20 minutes; the number of continuous casting furnaces of a continuous casting tundish is not more than 5, the insertion depth of a crystallizer nozzle is 100-130 mm, and the remaining steel amount after casting is not less than 5 t. 42CrMoS4 round steel for automobile crankshafts and connecting rods with the specification of phi 40-80 mm and without large-particle inclusions can be produced, and high-quality and high-performance steel products are provided for the field of high-end manufacturing.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and specifically relates to a method for controlling large inclusions in medium-carbon alloy sulfur-containing steel 42CrMoS4 used for manufacturing automobile engine crankshafts and connecting rods. Background Technique

[0002] 42CrMoS4 steel belongs to ultra-high-strength steel, which has high strength and toughness, good hardenability, no obvious temper brittleness, has a relatively high fatigue limit and impact resistance after quenching and tempering treatment, and also has good low-temperature impact toughness. It is often used to manufacture automobile engine crankshafts and connecting rods. In order to reduce tool wear, 42CrMoS4 steel often contains 0.020 - 0.040% sulfur to facilitate subsequent machining. Since engine crankshafts and connecting rods are safety components, high quality requirements are imposed on the round steel base material, and large inclusions are not allowed in the steel. Large inclusions in steel usually refer to inclusions with a size larger than 50μm. If large inclusions exist in 42CrMoS4 steel, it will have a fatal impact on the safe operation of engine crankshafts and connecting rods. Therefore, large inclusions are not allowed in 42CrMoS4 steel used to manufacture engine crankshafts and connecting rods.

[0003] When producing 42CrMoS4 steel with a Φ40 - 80mm specification by the conventional process of Al deoxidation method, ultrasonic flaw detection is carried out according to the requirements (the flaw detection standard is: SEP19203 C / c), and it is found that the qualified rate of ultrasonic flaw detection is very low, only about 30%. Through sampling analysis, large inclusions are found, and the size of the inclusions is about 110 - 500μm. The main components of the inclusions are CaS, Al 2 O 3 、MgO, and most of the inclusions are located 5 - 15mm under the skin, all of which are subcutaneous large inclusions.

[0004] In order to meet the requirement that there are no large inclusions in 42CrMoS4 round steel for engine crankshafts and connecting rods, it is extremely urgent to develop a new method for controlling large inclusions in medium-carbon alloy sulfur-containing steel. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for controlling large inclusions in medium-carbon alloy sulfur-containing steel to solve the problem of large inclusions existing in the existing 42CrMoS4 steel with a Φ40 - 80mm specification for crankshafts and connecting rods.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] A method for controlling large-sized inclusions in medium-carbon alloy sulfur-containing steel, successively including the steps of KR desulfurization → LD converter → LF secondary refining → RH vacuum degassing → continuous casting. Only silicon carbide is used for deoxidation in the LF secondary refining, adding aluminum pellets and calcium carbide for deoxidation is strictly prohibited, and treating molten steel with calcium wire is prohibited. Meanwhile, the S component is finely adjusted in the LF furnace. According to the component analysis result of the cake sample taken from the LF furnace, S wire is fed to adjust the S content within the range of 0.020 - 0.025%. In the RH vacuum degassing, the continuous soft blowing time is ≥ 20 minutes. The number of continuous casting heats in the tundish does not exceed 5 heats, the immersion depth of the mold nozzle is 100 - 130 mm, and when the casting is finished, the operation of leaving residual steel in the tundish is carried out, and the required amount of residual steel is ≥ 5 t.

[0008] Preferably, in the KR desulfurization, the S content is not more than 0.005%.

[0009] Preferably, the bottom blowing of the LD converter blows argon throughout the process. In order to reduce the O content in the steel, the end-point C is required to be ≥ 0.12%. When 1 / 3 of the steel is tapped, a low-nitrogen carburizer is added, and then ferro-silicon, ferro-silico-manganese, ferro-chrome, and ferro-molybdenum alloys are added in sequence. Adding ferro-aluminum is not allowed.

[0010] Preferably, 400 kg of lime + 600 kg of refining slag are added to the LF refining furnace to make a white slag and keep it all the time, and at the same time, the fluidity of the top slag is ensured. In order to prevent FeO and MnO in the slag from adding oxygen to the molten steel, the requirement for FeO + MnO in the slag is ≤ 0.50%.

[0011] Preferably, alloy addition and returning to the LF furnace for temperature raising are strictly prohibited in the RH. The ultimate vacuum degree is ≤ 67 Pa, and the vacuum treatment time is ≥ 10 minutes.

[0012] Preferably, an integral tundish is used in the continuous casting. The tundish is cleaned thoroughly, purged with argon before casting, and the argon blowing time is ≥ 2 minutes. The whole process is protected casting, and casting starts outside the tundish.

[0013] Compared with the prior art, the beneficial effects of the present invention are:

[0014] During smelting, the S content is controlled at the lower limit allowed by the standard, aluminum and calcium carbide deoxidation are cancelled, only silicon carbide is used for deoxidation, calcium wire is not used to treat molten steel, the lime dosage of the refining slag is reduced, the soft blowing time is increased, the number of continuous casting heats is reduced, the operation of leaving residual steel in the tundish is carried out when the continuous casting is finished, and at the same time, the immersion depth of the mold nozzle is optimized, so that round steel for automotive crankshafts and connecting rods with a specification of Φ40 - 80 mm and 42CrMoS4 without large-sized inclusions can be produced, providing high-quality and high-performance steel products for the high-end manufacturing field. Specific Embodiments

[0015] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0016] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways than those specifically described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0017] Secondly, the so-called "one embodiment" or "embodiment" refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or alternative embodiments that exclude each other.

[0018] Embodiment

[0019] The present invention will be further described below in conjunction with embodiments. This embodiment provides a method for controlling large particle inclusions in medium-carbon alloy sulfur-containing steel.

[0020] In the embodiment of the present invention, the range of chemical composition requirements for 42CrMoS4 steel with a specification of Φ40 - 80 mm is shown in Table 1.

[0021] Table 1 Chemical Composition (wt%)

[0022]

[0023] Example 1:

[0024] Four heats of 42CrMoS4 steel were continuously cast. During the KR desulfurization process, the S content of the four heats of steel was between 0.002% and 0.004%. Argon was blown throughout the bottom blowing of the LD converter. The end-point C content of the four heats of steel was between 0.15% and 0.18%. A low-nitrogen carburizer was added when 1 / 3 of the steel was tapped, and then ferrosilicon, silicomanganese, ferrochrome, and ferromolybdenum alloys were added in sequence; 400 kg of lime + 600 kg of refining slag were added to the LF refining furnace to make a white slag and keep it all the time, while ensuring the fluidity of the top slag. Silicon carbide was used for deoxidation. The content of FeO + MnO in the slag of the four heats of steel was between 0.38% and 0.41%. The S component was finely adjusted by feeding the S wire in the LF furnace. The S content of the four heats of steel fluctuated between 0.020% and 0.023%; RH vacuum degassing was carried out. The soft blowing time of the four heats of steel was between 22 and 25 minutes, the ultimate vacuum degree was 60 Pa, and the vacuum treatment time was 15 minutes; an integral tundish was used during continuous casting. The tundish was cleaned thoroughly, purged with argon before casting, and the argon blowing time was ≥2 minutes. The whole process was protected casting, and casting was started outside the ladle. In order to ensure the uniformity of composition and the quality of the billet, electromagnetic stirring was used in the mold and at the end of solidification. The insertion depth of the mold nozzle was 110 mm, and 6 t of remaining steel was left in the tundish at the end of casting. The continuous casting billets were drawn into 240 mm × 300 mm billets, and the four heats of steel were simultaneously rolled into 42CrMoS4 round bars with a diameter of Φ80 mm. Ultrasonic flaw detection was carried out on these four heats of Φ80 mm 42CrMoS4 round bars according to the requirements of SEP19203 C / c, and no large particle inclusions were found. The specific test results are shown in Table 2.

[0025] Example 2:

[0026] Continuous casting production of 5 heats of 42CrMoS4 steel. During the KR desulfurization process, the S content of the 5 heats of steel is between 0.001% and 0.003%. Argon is blown throughout the bottom blowing of the LD converter. The end-point C content of the 5 heats of steel is between 0.17% and 0.21%. A low-nitrogen carburizer is added when 1 / 3 of the steel is tapped, and then ferrosilicon, silicomanganese, ferrochrome, and ferromolybdenum alloys are added sequentially. 400 kg of lime + 600 kg of refining slag are added to the LF refining furnace to produce and maintain a white slag, while ensuring the fluidity of the top slag. Silicon carbide is used for deoxidation. The FeO + MnO content in the slag of the 5 heats of steel is between 0.35% and 0.39%. The S composition is fine-tuned by feeding an S wire in the LF furnace. The S content of the 5 heats of steel fluctuates between 0.022% and 0.024%. RH vacuum degassing is carried out. The soft blowing time of the 5 heats of steel is between 20 and 23 minutes, the ultimate vacuum degree is 65 Pa, and the vacuum treatment time is 13 minutes. The continuous casting uses an integral tundish. The tundish is cleaned thoroughly, purged with argon before casting, and the argon blowing time is ≥2 minutes. The whole process is protected casting, and casting starts outside the ladle. To ensure the uniformity of composition and the quality of the billet, electromagnetic stirring is used in the mold and at the solidification end. The immersion depth of the mold nozzle is 100 mm. When casting ends, 7 t of remaining steel is left in the tundish. The continuous casting is made into 240 mm × 300 mm billets, and the 5 heats of steel are simultaneously rolled into Φ40 mm 42CrMoS4 round bars. The 5 heats of Φ40 mm 42CrMoS4 round bars are ultrasonically inspected according to the requirements of SEP19203 C / c, and no large particle inclusions are found. The specific test results are shown in Table 2.

[0027] Example 3:

[0028] Continuously cast and produced 5 heats of 42CrMoS4 steel. During the KR desulfurization process, the S content of the 5 heats of steel was between 0.002% and 0.005%. Argon was blown throughout the bottom blowing of the LD converter. The final C content of the 5 heats of steel was between 0.17% and 0.23%. A low-nitrogen carburizer was added when 1 / 3 of the steel was tapped, and then ferrosilicon, silicomanganese, ferrochrome, and ferromolybdenum alloys were added in sequence. 400 kg of lime + 600 kg of refining slag were added to the LF refining furnace to create a white slag and maintain it, while ensuring the fluidity of the top slag. Silicon carbide was used for deoxidation. The FeO + MnO content in the slag of the 5 heats of steel was between 0.37% and 0.41%. The S component was fine-tuned by feeding an S wire in the LF furnace. The S content of the 5 heats of steel fluctuated between 0.021% and 0.025%. RH vacuum degassing was carried out. The soft blowing time of the 5 heats of steel was between 23 and 26 minutes, the ultimate vacuum degree was 67 Pa, and the vacuum treatment time was 10 minutes. The continuous casting used an integral tundish. The tundish was cleaned thoroughly, purged with argon before casting, and the argon blowing time was ≥2 minutes. The whole process was protected by casting, and casting started outside the ladle. To ensure the uniformity of composition and the quality of the billet, electromagnetic stirring was used in the mold and at the solidification end. The immersion depth of the mold nozzle was 130 mm, and 5 t of remaining steel was left in the tundish at the end of casting. The continuous casting was drawn into billets with a size of 240 mm × 300 mm, and the 5 heats of steel were simultaneously rolled into 42CrMoS4 round bars with a Φ70 mm specification. Ultrasonic flaw detection was carried out on these 5 heats of 42CrMoS4 round bars with a Φ70 mm specification according to the requirements of SEP19203 C / c, and no large particle inclusions were found. The specific test results are shown in Table 2.

[0029] Table 2 Ultrasonic Flaw Detection Results of the Examples

[0030]

[0031]

[0032] As can be seen from the above examples, by using the present invention, when smelting 42CrMoS4 steel for crankshafts and connecting rods, the S content is controlled at the lower limit allowed by the standard, aluminum and calcium carbide deoxidation are cancelled, only silicon carbide is used for deoxidation, calcium wire is not used to treat the molten steel, the lime consumption of the refining slag is reduced, the soft blowing time is increased, the number of continuous casting heats is reduced, the operation of leaving remaining steel in the tundish at the end of continuous casting is carried out, and at the same time, the immersion depth of the mold nozzle is optimized, solving the problem of large particle inclusions existing in 42CrMoS4 steel in the conventional process, and producing high-quality 42CrMoS4 steel for crankshafts and connecting rods with Φ40 - 80 mm specifications without large particle inclusions.

[0033] It should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel, comprising the steps of KR desulfurization → LD converter → LF refining → RH vacuum degassing → continuous casting, characterized in that: The LF refining outside the furnace only uses silicon carbide for deoxidation, and it is strictly forbidden to add aluminum particles and calcium carbide for deoxidation, and it is forbidden to use calcium wire to treat molten steel. At the same time, the S component is fine-tuned in the LF furnace. According to the component analysis results of the cake samples taken from the LF furnace, the S wire is fed to adjust the S content to within the range of 0.020-0.025%; the RH vacuum degassing, the continuous soft blowing time is ≥20 minutes; the number of continuous casting furnaces of the continuous casting tundish does not exceed 5 furnaces, the insertion depth of the crystallizer water nozzle is 100-130mm, and the tundish residual steel operation is performed at the end of pouring, and the residual steel quantity is required to be ≥5t.

2. The method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel according to claim 1, characterized in that: In the KR desulfurization, the S content is not more than 0.005%.

3. The method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel according to claim 1, characterized in that: The LD converter bottom blowing is carried out with argon blowing throughout the whole process. In order to reduce the O content in the steel, the end point C is required to be ≥0.12%. A low nitrogen carburizer is added when 1 / 3 of the steel is discharged, and then ferrosilicon, silicon manganese, ferrochrome, and ferromolybdenum alloy are added in sequence. The addition of ferroaluminum is not allowed.

4. The method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel according to claim 1, characterized in that: The LF refining furnace adds 400kg lime + 600kg refining slag to produce white slag and keep it all the time, while ensuring the fluidity of the top slag. In order to prevent FeO and MnO in the slag from adding oxygen to the molten steel, the FeO+MnO in the slag is required to be ≤0.50%.

5. The method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel according to claim 1, characterized in that: The RH is strictly prohibited from adding alloys and returning to the LF furnace to increase the temperature. The ultimate vacuum degree is ≤67Pa and the vacuum treatment time is ≥10 minutes.

6. The method for controlling large particle inclusions in medium carbon alloy sulfur-containing steel according to claim 1, characterized in that: The continuous casting adopts an integral tundish, which is cleaned and emptied by filling with argon before pouring. The argon blowing time is ≥ 2 minutes, and the casting is protected throughout the whole process. The pouring is started outside the tank.

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