A method for refining and controlling nitrogen in steel for fracture-resistant connecting rods

Abstract

This invention belongs to the field of metallurgical refining technology, specifically disclosing a method for refining and controlling nitrogen in steel for fracture-resistant connecting rods. The refining and nitrogen control method provided by this invention includes: subjecting molten steel to differential flow nitrogen blowing at 1550~1580℃ to obtain first-stage LF refined molten steel; subjecting the alloyed molten steel to nitrogen enrichment at a rate of 3~7ppm / min to obtain LF refined molten steel; and subjecting the LF refined molten steel to nitrogen blowing at 1640~1645℃ and a vacuum of 70~80Pa in four stages with progressively increasing flow rates to obtain RH refined steel for fracture-resistant connecting rods. The steel for fracture-resistant connecting rods produced using the method of this invention has a total oxygen content ≤3.5 ppm and a nitrogen content of 160~200 ppm, achieving the goal of low-cost production of high-quality nitrogen-containing steel.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical refining technology, specifically relating to a method for refining and controlling nitrogen in steel for fracture-resistant connecting rods. Background Technology

[0002] As a core load-bearing component of the engine, the connecting rod of an automobile requires stringent standards for material strength, toughness, fatigue life, and microstructure uniformity. The high-end nitrogen-containing steel used in this component must maintain a nitrogen content precisely within a narrow compositional window of 150–200 ppm under ultra-low oxygen clean conditions (total oxygen content T[O] ≤ 4 ppm). However, in actual production, there is a significant technical contradiction between nitrogen addition and deep deoxidation: nitrogen addition easily exacerbates secondary oxidation of the molten steel, increasing the oxygen content; while deep deoxidation reduces the oxygen content. Liquid-liquid interface reactivity inhibits nitrogen absorption, and the synergistic control of these two factors is extremely difficult, directly determining product performance stability and production costs.

[0003] In current industrial production, nitrogen enrichment of nitrogen-containing steel mainly adopts methods such as adding nitride alloys, feeding manganese nitride wires, and bottom blowing nitrogen, or a simple combination thereof. Although nitride alloy enrichment is simple to operate, the raw material cost is high, the alloy is easily burned off, the nitrogen recovery rate is only 60%~80%, with significant fluctuations, and foreign inclusions can also reduce the cleanliness of molten steel; feeding manganese nitride wires is a local point-source enrichment method, which can easily cause macroscopic segregation of nitrogen in large-capacity ladles, making it difficult to meet the stringent requirements for compositional uniformity in fracture-resistant connecting rod steel; bottom blowing nitrogen is cheaper, but under ultra-low oxygen conditions of T[O]≤4ppm, the interfacial reaction is severely affected by factors such as temperature and sulfur content, resulting in low mass transfer efficiency, poor nitrogen control accuracy, large fluctuations in nitrogen content, and easy occurrence of insufficient or excessive nitrogen absorption, making it impossible to stably lock the target window.

[0004] A more prominent problem is that existing processes mostly rely on single methods or physical superposition, failing to form a systematic and coordinated control approach, especially in LF. The RH duplex refining process suffers from severe process fragmentation. After nitrogen addition is completed in the LF furnace, the RH vacuum treatment triggers a significant denitrification effect, rendering the initial nitrogen addition ineffective and requiring repeated adjustments, which greatly increases costs and the risk of composition fluctuations. Furthermore, the RH process often employs constant or extensive adjustment modes to boost gas flow, making it difficult for nitrogen addition, denitrification, and inclusion removal reactions to proceed under optimal conditions, resulting in poor process stability.

[0005] Due to the aforementioned technical limitations, existing production processes struggle to achieve precise control of nitrogen content within a narrow window under ultra-low oxygen clean conditions. This results in problems such as high cost, low yield, insufficient component hit rate, and difficulty in ensuring microstructure uniformity, severely hindering the high-performance, mass production, and stable production of steel for automotive connecting rods. Therefore, developing a nitrogen-controlled refining method for connecting rod steel that can control the T[O] content of molten steel below 4 ppm and stabilize the nitrogen content within the range of 160–200 ppm, while simultaneously achieving low cost and high yield, has become a key technological bottleneck that urgently needs to be overcome for the industrial production of this type of high-end steel. Summary of the Invention

[0006] In view of this, the present invention provides a method for nitrogen control in the refining of steel for fracture-resistant connecting rods. This invention abandons a single nitrogen-increasing method and constructs a dual-synergistic nitrogen control system of LF+RH. First, precise pre-nitrogen increase is performed during the LF refining process, and then nitrogen stabilization is achieved using a nitrogen gradient under RH vacuum conditions, avoiding repeated fluctuations between nitrogen increase and denitrification. Simultaneously, the flow field of the RH molten steel is precisely controlled through a four-stage stepped flow rate, achieving precise control of the nitrogen content in the molten steel under ultra-low oxygen conditions. The nitrogen control method for refining steel for fracture-resistant connecting rods provided by this invention combines process stability, low cost, and high cleanliness.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a refining and nitrogen control method for steel used in fracture-resistant connecting rods, comprising the following steps: S1. At 1550~1580℃, the molten steel is subjected to differential flow nitrogen blowing treatment at flow rates of 300~500NL / min and 700~900NL / min to obtain the first stage LF refined molten steel. S2. After the first stage of LF refining steel alloying is completed, the steel is subjected to nitrogen addition treatment at a rate of 3~7ppm / min to obtain LF refining steel with a nitrogen content of 200~250ppm. S3. Under conditions of 1640~1645℃ and vacuum degree of 70~80Pa, the LF refined steel is subjected to nitrogen blowing treatment at flow rates of 800~1200NL / min, 1400~1500NL / min, 1550~1600NL / min and 1700~1800NL / min in sequence to obtain RH refined steel for expansion fracture connecting rod.

[0008] Compared to existing technologies, the nitrogen control method for refining steel for fracture-resistant connecting rods provided by this invention limits the temperature conditions during differential flow nitrogen blowing in S1. The inventors discovered that when the temperature is too low, the viscosity of the molten steel easily increases due to subsequent nitrogen absorption, affecting nitrogen enrichment efficiency. Under specific temperature conditions, the molten steel has good fluidity, which is conducive to nitrogen dissolution and diffusion, thus improving nitrogen enrichment efficiency. This invention further controls the nitrogen flow rate range of differential flow nitrogen blowing, enabling the molten steel to form asymmetric turbulent motion within the ladle, generating a larger velocity gradient, strengthening the entrainment effect of the molten steel, and further shearing the nitrogen bubbles into smaller bubbles, increasing the gas-liquid contact area, and ensuring the nitrogen recovery rate during LF refining. Furthermore, controlling the flow rate range within 700~900 NL / min avoids the molten steel surface being exposed due to excessive flow, thereby preventing secondary oxidation and excessively rapid cooling of the molten steel.

[0009] In S2, by limiting the nitrogen enrichment treatment to after alloying, the problem of dilution or interference of the alloy addition on the local nitrogen content of the molten steel can be avoided, thus improving the nitrogen recovery rate. During the research process, the inventors discovered that when the nitrogen enrichment rate is too high, it can easily cause local nitrogen supersaturation in the molten steel, leading to bubbling or slag entrapment. When the nitrogen enrichment rate is too low, it will prolong the refining cycle and reduce production efficiency. Controlling the nitrogen enrichment rate at 3~7 ppm / min can further stabilize the nitrogen recovery rate, obtaining LF refined molten steel with a nitrogen content of 200~250 ppm, providing a uniform and stable steel composition foundation for precise nitrogen control in subsequent RH refining.

[0010] In S3, under specific temperature and vacuum conditions, a low flow rate of nitrogen (800-1200 NL / min) ensures the dissolution and mass transfer of nitrogen molecules on the surface of LF refined steel, achieving stable nitrogen addition. Increasing the nitrogen flow rate to 1400-1500 NL / min enhances steel circulation, accelerating the dissolution and homogenization of alloying elements. Further increasing the nitrogen flow rate to 1550-1600 NL / min strengthens steel circulation, promotes inclusion flotation, and significantly improves steel cleanliness. When the nitrogen flow rate is increased to 1700-1800 NL / min, it further enhances steel circulation and uniform mixing, ensuring uniform nitrogen distribution in RH refined steel. During RH refining, this invention optimizes the kinetics of nitrogen dissolution and mass transfer, alloy diffusion, and inclusion collision and flotation through stepped flow control, allowing the denitrification reaction to smoothly transition to equilibrium. Ultimately, under ultra-low oxygen content, precise and stable control of nitrogen content in RH refined steel for fracture-resistant connecting rods is achieved.

[0011] Preferably, in S1, during the differential flow nitrogen blowing treatment, the nitrogen flow ratio is 1:(1.8~2.5).

[0012] Preferably, in S1, the nitrogen blowing time of the differential flow nitrogen blowing treatment is 30~40 min.

[0013] Preferably, in S1, the conditions for differential flow nitrogen blowing treatment further include: dissolved oxygen ≤ 20 ppm and sulfur content ≤ 0.005%.

[0014] This invention, by controlling dissolved oxygen concentration and sulfur content parameters, can prevent surface-active elements such as oxygen and sulfur from occupying active sites on the surface of molten steel and hindering nitrogen adsorption and dissociation; under this cleanliness condition, differential flow bottom blowing of nitrogen can effectively guarantee and improve the initial nitrogen addition efficiency.

[0015] It should be further explained that in S1, before the molten steel undergoes differential flow nitrogen blowing treatment, argon gas is blown at an equal flow rate of 400~410 NL / min.

[0016] For example, in S1, the differential flow nitrogen blowing process is differential flow bottom blowing nitrogen.

[0017] Preferably, in S2, the molten steel is subjected to nitrogen enrichment treatment at a rate of 5 ppm / min.

[0018] For example, in S2, after the nitrogen enrichment treatment is completed, argon gas is blown into the bottom of the LF refined steel at a flow rate of 400~410 NL / min.

[0019] After nitrogen enrichment is completed, bottom-blown argon gas can remove the tiny inclusions formed during the nitrogen enrichment process, while ensuring the smooth operation of the continuous casting process.

[0020] For example, in S3, before the nitrogen blowing treatment, the molten steel is further subjected to argon blowing treatment at a flow rate of 40-45 NL / min under a vacuum of 70-80 Pa, and the argon blowing treatment time is 15-30 min.

[0021] For example, in S3, the argon blowing process is carried out in the form of soft-blown argon gas.

[0022] Preferably, in S3, when the flow rate is 800~1200 NL / min, the nitrogen blowing time is 1~2 min.

[0023] Preferably, in S3, when the flow rate is 1400~1500 NL / min, the nitrogen blowing time is 2~3 min.

[0024] Preferably, in S3, when the flow rate is 1550~1600 NL / min, the nitrogen blowing time is 2.5~3.5 min.

[0025] Preferably, in S3, when the flow rate is 1700~1800 NL / min, the nitrogen blowing time is 2.5~3.5 min.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention abandons the traditional process separation mode and adopts the relay nitrogen control idea of ​​LF precise pre-nitrogen increase and RH gradient nitrogen stabilization. In the LF stage, the nitrogen content of the molten steel is controlled at 200-250ppm to reserve a reasonable denitrification margin for subsequent RH refining. In the RH refining stage, nitrogen is used as the lifting gas and the denitrification driving force is finely controlled by step flow rate to make the denitrification reaction stable and close to the equilibrium state, effectively avoiding drastic fluctuations in nitrogen content. Finally, under ultra-low oxygen conditions, the nitrogen content of the molten steel is stably controlled at 160-200ppm with a fluctuation range of ±20ppm. The nitrogen control accuracy is significantly better than the ±30ppm level of the existing technology.

[0027] (2) This invention controls the flow rate in four stages during the RH refining process, gradually increasing the nitrogen flow rate in stages to optimize the nitrogen mass transfer environment. Simultaneously, the stepwise increase in flow rate also gradually enhances turbulence intensity, thereby optimizing the kinetic conditions for nitrogen dissolution, diffusion, and inclusion flotation, ensuring stable nitrogen mass transfer. Experimental verification shows that the total oxygen content in molten steel can be stably controlled below 3.5 ppm, achieving the goal of ultra-low oxygen clean steel and solving the technical problem of difficulty in simultaneously controlling nitrogen and oxygen in traditional processes, which often leads to fluctuations.

[0028] (3) By controlling the dissolved oxygen concentration and sulfur content during the LF refining process and adopting the differential flow nitrogen blowing treatment mode, the present invention avoids the occupation of active sites on the surface of molten steel by oxygen and sulfur, and achieves stable control of predictable and repeatable nitrogen increase effect.

[0029] (4) The present invention uses nitrogen to replace nitride alloys and manganese nitride wires. As the main nitrogen-enhancing medium, it has the following advantages: ① Nitrogen is widely available and can significantly reduce the cost of nitrogen control; ② Nitrogen as a nitrogen-enhancing medium can avoid the fluctuation of nitrogen element recovery rate in alloy nitrogen enhancement. The nitrogen element recovery rate using nitrogen is close to 100%; ③ Nitrogen as a nitrogen-enhancing medium can eliminate the risk of inclusions brought in by foreign alloys. Combined with the optimization of the flow field during RH refining, the endpoint T[O] can be stably controlled below 3.5ppm, achieving the goal of low-cost production of high-quality nitrogen-containing steel. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0031] Example 1 This embodiment provides a refining and nitrogen control method for steel used in fracture-resistant connecting rods, comprising the following steps: S1. After the molten steel enters the ladle, quicklime, refining slag and silicon carbide with a purity of 45% are added. Argon is blown at a constant flow rate of 400 NL / min. When the molten steel temperature is 1550℃, the dissolved oxygen is 13 ppm and the sulfur content is 0.001%, nitrogen is blown at a differential flow rate. The main control gas flow rate is set to 400 NL / min, the auxiliary control gas flow rate is set to 800 NL / min, and the nitrogen blowing time is 35 min, to obtain the first stage LF refining molten steel. S2. When the alloying of the first stage LF refining molten steel is completed and the white slag refining period is carried out, that is, when the slag color is white, nitrogen addition treatment is carried out at a rate of 5 ppm / min. After the nitrogen addition treatment is completed, argon gas is blown under the flow rate at 400 NL / min to obtain LF refining molten steel. S3. Evacuate the LF refined molten steel until the vacuum degree reaches 70Pa. Softly blow argon gas into the molten steel at a flow rate of 40NL / min for 15min. When the molten steel temperature reaches 1640℃, switch the argon gas to nitrogen gas and blow nitrogen gas at a flow rate of 800NL / min for 1min. Adjust the nitrogen gas flow rate to 1500NL / min and maintain for 2min. Adjust the nitrogen gas flow rate to 1550NL / min and maintain for 2.5min. Adjust the nitrogen gas flow rate to 1700NL / min and maintain for 3min to obtain RH refined molten steel for fracture-resistant connecting rods.

[0032] Example 2 This embodiment provides a refining and nitrogen control method for steel used in fracture-resistant connecting rods, comprising the following steps: S1. After the molten steel enters the ladle, quicklime, refining slag and silicon carbide with a purity of 45% are added. Argon is blown at a constant flow rate of 400 NL / min. When the molten steel temperature is 1580℃, the dissolved oxygen is 15 ppm and the sulfur content is 0.001%, nitrogen is blown at a differential flow rate. The main control gas flow rate is set to 300 NL / min, the auxiliary control gas flow rate is set to 750 NL / min, and the nitrogen blowing time is 40 min, to obtain the first stage LF refined molten steel. S2. When the alloying of the first stage LF refining molten steel is completed and the white slag refining period is carried out, that is, when the slag color is visually white, nitrogen addition treatment is carried out at a rate of 7ppm / min. After the nitrogen addition treatment is completed, argon gas is blown under the bottom at a flow rate of 400NL / min to obtain LF refining molten steel. S3. Evacuate the LF refined molten steel until the vacuum degree reaches 80Pa. Softly blow argon gas into the molten steel at a flow rate of 45NL / min for 20min. When the molten steel temperature reaches 1645℃, switch the argon gas to nitrogen gas and blow nitrogen gas at a flow rate of 1000NL / min for 2min. Adjust the nitrogen gas flow rate to 1400NL / min and maintain for 3min. Adjust the nitrogen gas flow rate to 1600NL / min and maintain for 3min. Adjust the nitrogen gas flow rate to 1800NL / min and maintain for 3.5min to obtain RH refined molten steel for fracture-resistant connecting rods.

[0033] Example 3 This embodiment provides a refining and nitrogen control method for steel used in fracture-resistant connecting rods, comprising the following steps: S1. After the molten steel enters the ladle, quicklime, refining slag and silicon carbide with a purity of 45% are added. Argon is blown at a constant flow rate of 400 NL / min. When the molten steel temperature is 1560℃, the dissolved oxygen is 10 ppm and the sulfur content is 0.001%, nitrogen is blown at a differential flow rate. The main control gas flow rate is set to 400 NL / min, the auxiliary control gas flow rate is set to 720 NL / min, and the nitrogen blowing time is 30 min, to obtain the first stage LF refining molten steel. S2. When the alloying of the first stage LF refining molten steel is completed and the white slag refining period is carried out, that is, when the slag color is visually white, nitrogen addition treatment is carried out at a rate of 3ppm / min. After the nitrogen addition treatment is completed, argon gas is blown under the bottom at a flow rate of 400NL / min to obtain LF refining molten steel. S3. Evacuate the LF refined molten steel until the vacuum degree reaches 75 Pa. Softly blow argon gas into the molten steel at a flow rate of 45 NL / min for 25 min. When the molten steel temperature reaches 1645℃, switch the argon gas to nitrogen gas and blow nitrogen gas at a flow rate of 1200 NL / min for 1 min. Adjust the nitrogen gas flow rate to 1400 NL / min and maintain for 3 min. Adjust the nitrogen gas flow rate to 1600 NL / min and maintain for 3.5 min. Adjust the nitrogen gas flow rate to 1750 NL / min and maintain for 3 min to obtain RH refined molten steel for fracture-resistant connecting rods.

[0034] Comparative Example 1 This comparative example provides a refining nitrogen control method for steel used in fracture-resistant connecting rods. The difference from Example 1 is that in S1, nitrogen is blown under the bottom using a constant flow rate of 800 NL / min. The remaining steps are the same as in Example 1, and will not be repeated here.

[0035] Comparative Example 2 This comparative example provides a refining nitrogen control method for steel used in fracture-resistant connecting rods. The difference from Example 1 is that in S1, the dissolved oxygen concentration is 35 ppm, the sulfur content is 0.038%, and bottom blowing of nitrogen is carried out using a constant flow rate of 800 NL / min. The remaining steps are the same as in Example 1, and will not be repeated here.

[0036] Comparative Example 3 This comparative example provides a refining nitrogen control method for steel for fracture-resistant connecting rods. The difference from Example 1 is that in S3, the LF refined molten steel is evacuated to a vacuum degree of 70 Pa, and argon gas is gently blown into the molten steel at a flow rate of 40 NL / min for 15 min. When the temperature of the molten steel is 1640℃, argon gas is used as the lifting gas and argon gas is blown at a flow rate of 1200 NL / min for 8.5 min to obtain RH refined molten steel for fracture-resistant connecting rods. The remaining steps are the same as in Example 1, and will not be repeated here.

[0037] Comparative Example 4 This comparative example provides a refining nitrogen control method for steel used in fracture-resistant connecting rods. The difference from Example 1 is that: LF refined molten steel is evacuated to a vacuum level of 70 Pa, and argon gas is gently blown into the molten steel at a flow rate of 40 NL / min for 15 min. When the temperature of the molten steel reaches 1640℃, the argon gas is switched to nitrogen gas, and nitrogen blowing is performed at a flow rate of 1800 NL / min for 8.5 min to obtain RH refined steel for fracture-resistant connecting rods. The remaining steps are the same as in Example 1, and will not be repeated here.

[0038] Comparative Example 5 This comparative example provides a refining nitrogen control method for steel used in fracture-resistant connecting rods. The difference from Example 1 is that: LF-refined molten steel is evacuated to a vacuum level of 70 Pa, and argon gas is gently blown into the molten steel at a flow rate of 40 NL / min for 15 min. When the temperature of the molten steel reaches 1640℃, the argon gas is switched to nitrogen gas, and nitrogen gas is blown at a flow rate of 1400 NL / min for 5 min. The nitrogen gas flow rate is then adjusted to 1800 NL / min and maintained for 3.5 min to obtain RH-refined molten steel for fracture-resistant connecting rods. The remaining steps are the same as in Example 1, and will not be repeated here.

[0039] Comparative Example 6 This comparative example provides a method for refining and controlling nitrogen in steel for fracture-resistant connecting rods. The difference from Example 1 is as follows: The LF-refined molten steel is evacuated to a vacuum level of 70 Pa. Argon gas is then gently blown into the molten steel at a flow rate of 40 NL / min for 15 minutes. When the molten steel temperature reaches 1640°C, the argon gas is switched to nitrogen gas, and nitrogen is blown at a flow rate of 800 NL / min for 1 minute. The nitrogen gas flow rate is then adjusted to 1200 NL / min and maintained for 2 minutes. The nitrogen gas flow rate is then adjusted to 1400 NL / min and maintained for 2.5 minutes. Finally, the nitrogen gas flow rate is adjusted to 1600 NL / min and maintained for 3 minutes, yielding RH-refined steel for fracture-resistant connecting rods. The remaining steps are the same as in Example 1, and will not be repeated here.

[0040] Comparative Example 7 This comparative example provides a refining nitrogen control method for steel used in fracture-resistant connecting rods. The method differs from Example 1 in that: LF-refined molten steel is evacuated to a vacuum level of 70 Pa. Argon gas is then gently blown into the molten steel at a flow rate of 40 NL / min for 15 min. When the molten steel temperature reaches 1640°C, the argon gas is switched to nitrogen gas, and nitrogen is blown at a flow rate of 1400 NL / min for 1 min. The nitrogen gas flow rate is then adjusted to 1600 NL / min and maintained for 2 min. The nitrogen gas flow rate is then adjusted to 1800 NL / min and maintained for 2.5 min. Finally, the nitrogen gas flow rate is adjusted to 1200 NL / min and maintained for 3 min, yielding RH-refined steel for fracture-resistant connecting rods. The remaining steps are the same as in Example 1, and will not be repeated here.

[0041] Example of effect The nitrogen content of LF refined steel, the nitrogen content of RH refined steel for expansion-break connecting rods and the total oxygen content (T[O]) were detected in all embodiments and comparative examples of the present invention. The testing standards for nitrogen content and total oxygen content are as follows: after sampling the molten steel in the ladle using a molten steel sampler, the total oxygen (O) and nitrogen (N) content of the steel sample is tested using an oxygen-nitrogen analyzer.

[0042] The specific test results are shown in Table 1.

[0043] Table 1

[0044] The refining and nitrogen control method for steel used in fracture-resistant connecting rods provided in this invention can ensure that the total oxygen content of the refined molten steel is ≤3.5ppm and the nitrogen content is stably controlled within the range of 160 to 200ppm.

[0045] In Comparative Examples 1 and 2, using the method of bottom blowing nitrogen at the same flow rate, or changing the LF refining conditions while using bottom blowing nitrogen at the same flow rate, the nitrogen and oxygen content in the molten steel for the RH refining fracture-resistant connecting rod could not reach the expected results.

[0046] In Comparative Examples 3-7 of this invention, argon or nitrogen were used as the boosting gas, a two-stage incremental flow rate method was adopted, and a four-stage boosting gas flow rate method was adjusted. All of these methods resulted in an excessively high total oxygen content at the endpoint, and the nitrogen content also failed to achieve the expected effect.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for refining and controlling nitrogen in steel for fracture-resistant connecting rods, characterized in that, Includes the following steps: S1. At 1550~1580℃, the molten steel is subjected to differential flow nitrogen blowing treatment at flow rates of 300~500NL / min and 700~900NL / min to obtain the first stage LF refined molten steel. S2. After the first stage of LF refining steel alloying is completed, the steel is subjected to nitrogen addition treatment at a rate of 3~7ppm / min to obtain LF refining steel with a nitrogen content of 200~250ppm. S3. Under conditions of 1640~1645℃ and vacuum degree of 70~80Pa, the LF refined molten steel is subjected to nitrogen blowing treatment at flow rates of 800~1200NL / min, 1400~1500NL / min, 1550~1600NL / min and 1700~1800NL / min in sequence to obtain RH refined molten steel for expansion fracture connecting rod; In S1, during the differential flow nitrogen blowing treatment, the nitrogen flow rate ratio is 1:(1.8~2.5).

2. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S1, the nitrogen blowing time for the differential flow nitrogen blowing treatment is 30~40 min.

3. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S1, the conditions for differential flow nitrogen blowing treatment also include: dissolved oxygen ≤ 20 ppm and sulfur content ≤ 0.005%.

4. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S2, the molten steel is subjected to nitrogen enrichment treatment at a rate of 5 ppm / min.

5. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In step S3, before the nitrogen blowing treatment, the molten steel is further subjected to argon blowing treatment at a flow rate of 40-45 NL / min under a vacuum of 70-80 Pa.

6. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S3, when the flow rate is 800~1200 NL / min, the nitrogen blowing time is 1~2 min.

7. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S3, when the flow rate is 1400~1500 NL / min, the nitrogen blowing time is 2~3 min.

8. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S3, when the flow rate is 1550~1600 NL / min, the nitrogen blowing time is 2.5~3.5 min.

9. The refining and nitrogen control method for steel used in fracture-resistant connecting rods as described in claim 1, characterized in that, In S3, when the flow rate is 1700~1800 NL / min, the nitrogen blowing time is 2.5~3.5 min.

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