A smelting method for low-cost nitrogen-increasing of non-quenched and tempered steel for crankshaft

Abstract

This invention discloses a low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel used in crankshafts. The process employs a converter smelting → tapping → LF refining → RH vacuum → continuous casting. Through a rationally designed smelting process, the converter increases the nitrogen content of the tapped steel by controlling the top-blown oxygen intensity and bottom-blown nitrogen intensity in stages. The RH is controlled in stages to ensure the removal of inclusions and meet nitrogen-enhancing requirements. This eliminates the need for vanadium-nitrogen alloys and the use of nitrogen-manganese lines after vacuum treatment, improving steel cleanliness while reducing production costs. When this invention is applied to the smelting of non-quenched and tempered steel for crankshafts, both Class B inclusions and Class Ds inclusions can be controlled within 0.5 grade, while production costs can be reduced by more than 30 yuan / ton of steel.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel used in crankshafts. Background Technology

[0002] Non-quenched and tempered steel for crankshafts is a type of steel that uses micro-alloying and controlled forging and cooling processes to replace traditional quenching and tempering heat treatment. It eliminates the need for quenching and high-temperature tempering, reducing energy consumption and production costs by approximately 20%-30%. Through controlled forging and cooling and micro-alloying, non-quenched and tempered steel directly achieves a fine-grained ferrite-pearlite microstructure with uniform mechanical properties. This reduces the risk of deformation and cracking during heat treatment, achieving tensile strength ≥800MPa, yield strength ≥500MPa, and elongation ≥15%, meeting the requirements for medium- and high-strength crankshafts. Adding a certain amount of nitrogen to non-quenched and tempered steel strengthens the precipitation of elements such as V, Ti, and Nb, refining the grain size, providing solid solution strengthening, and improving fatigue and toughness. This allows non-quenched and tempered steel for crankshafts to achieve high strength, high toughness, and high fatigue performance without the need for quenching and tempering, while simultaneously reducing costs, saving energy, and simplifying the process.

[0003] Generally, increasing the nitrogen content of molten steel by adding nitrogen-containing alloys is relatively expensive. In addition, because non-quenched and tempered steel for crankshafts has high requirements for inclusion and gas content, vacuum treatment is required. However, vacuum treatment will reduce the nitrogen content of the molten steel, so nitrogen-manganese wire needs to be added after vacuum treatment to compensate for the nitrogen content of the molten steel. However, adding nitrogen-manganese wire will cause slight splashing of molten steel, resulting in secondary oxidation and contamination of the molten steel.

[0004] Therefore, it is necessary to develop a low-cost nitrogen-enhancing method for non-quenched and tempered steel that requires less alloy and has a simple smelting process. Summary of the Invention

[0005] In response to the problems mentioned in the background art, this invention develops a low-cost nitrogen-increasing smelting method for non-quenched and tempered steel for crankshafts. It uses inexpensive nitrogen gas instead of expensive nitrogen-containing alloys, and combines it with suitable converter smelting and vacuum furnace smelting processes to increase the nitrogen content of molten steel. Furthermore, it avoids nitrogen feeding operations after vacuum treatment, improves the quality of molten steel, and reduces production costs.

[0006] This invention provides a low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel used in crankshafts. The process flow is as follows: converter smelting → tapping → LF refining → RH vacuum → continuous casting. The specific method steps and control parameters are as follows: Step 1: Converter smelting. Nitrogen is used as the bottom blowing gas and oxygen is used as the top blowing gas in the converter blowing process. The carbon content of the molten steel tapped from the converter is 0.08~0.15%, the oxygen content is 0.010~0.025%, the phosphorus content is ≤0.018%, the nitrogen content is 60~100ppm, and the tapping temperature is 1620~1650℃.

[0007] Specifically, nitrogen is used as the bottom-blowing gas in the converter blowing process. The nitrogen flow rate for bottom blowing is controlled in stages according to the percentage of bottom blowing time to total blowing time. From the start of blowing to <80% of blowing time, the bottom blowing intensity is controlled at 0.03~0.05 Nm. 3 / t / min; from 80% to the end of the blowing process, the bottom blowing intensity is controlled at 0.10~0.15Nm. 3 / t / min.

[0008] The top-blown oxygen flow rate of the converter is controlled in stages, based on the percentage of top-blown time to the total blowing time. From the start of blowing to <80% of blowing time, the top-blown intensity is controlled at 5.0~5.5 Nm. 3 / t / min; from 80% to the end of blowing, the top blowing intensity is controlled at 3.0~3.5 Nm. 3 / t / min.

[0009] Furthermore, as a preferred embodiment, in step one, the carbon content of the molten steel tapped from the converter is 0.08~0.15%, the oxygen content is 0.010~0.025%, the phosphorus content is ≤0.018%, the nitrogen content is 60~100ppm, and the tapping temperature is 1620~1650℃.

[0010] Step 2: Tapping. No nitriding alloys such as vanadium nitride are added during the tapping process. At the end of tapping, a slag-blocking plug and a sliding plate are used for dual-stage operation. The slag discharge is ≤3.0kg / t. At the end of tapping, the nitrogen content of the molten steel is controlled at 80~120ppm.

[0011] Specifically, aluminum blocks are used as deoxidizers during the tapping process; then, silicon manganese, high-carbon ferrochrome, ferrovanadium alloy, carbon raiser, slag remover, and lime are added in sequence, without adding nitriding alloys such as vanadium nitride, silicon nitride, manganese nitride, titanium nitride, or chromium nitride; at the end of tapping, a slag-blocking plug and sliding plate are used for dual-stage operation, with slag discharge ≤3.0kg / t, and the nitrogen content of the molten steel is controlled at 80~120ppm at the end of tapping.

[0012] Step 3: LF refining. Argon gas is used for stirring, and deoxidizer is added for slag surface deoxidation. The alloy composition is finely adjusted. Nitriding alloys are not used. At the end of refining, the nitrogen content of the molten steel is controlled at 90~130ppm.

[0013] Specifically, argon gas is used for stirring during the refining process to prevent cross-contamination; silicon carbide and aluminum particles are added for slag surface deoxidation, and the alloy composition is finely adjusted according to the first batch of refining to ensure that the steel composition meets the requirements; aluminum wire and ferrous wire are fed in before the end of refining to control the aluminum content of the steel to 0.020~0.030% and the sulfur content of the steel to 0.02~0.035%; the nitrogen content of the steel is controlled at 90~130ppm after refining.

[0014] Step 4: RH vacuum treatment, using nitrogen as the lifting gas. RH vacuum treatment is divided into two stages: Stage 1: Vacuum degree controlled below 67 Pa, vacuum treatment for 10-15 minutes, mainly to remove inclusions in the molten steel; Stage 2: Simultaneous top-blowing of nitrogen, vacuum degree controlled below 8k-10kPa, vacuum treatment for 5-10 minutes, mainly to enhance inclusion removal and simultaneously increase nitrogen content in the molten steel, with the nitrogen content controlled at 0.013%-0.016%. After vacuum treatment, the vacuum is broken, and pure calcium wire is fed in to adjust the calcium content of the molten steel. The calcium content of the RH-exit molten steel is controlled at 4-6 ppm. The molten steel is allowed to stand for 10 minutes before being poured.

[0015] Furthermore, as a preferred embodiment, in step four, in the first stage, nitrogen is used as the boosting gas, with a boosting gas flow rate of 80~100 Nm³. 3 The vacuum degree is controlled below 67 Pa per hour, and the vacuum treatment time is 10-15 minutes. After the first stage of treatment, the nitrogen content of the molten steel is controlled at 50-80 ppm. The second stage is mainly used to enhance the removal of inclusions and simultaneously increase nitrogen in the molten steel. Nitrogen gas is used as the booster gas, with a booster gas flow rate of 80-100 Nm³. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 4.8~5.0m, nitrogen flow rate 800~1200Nm 3 / h, nitrogen blowing rate 80~120m³ 3 The vacuum degree is controlled within 8k~10kPa, and the vacuum treatment time is 5~10min; the nitrogen content of the molten steel can be controlled within 0.013%~0.016%.

[0016] Step 5: Continuous casting.

[0017] The chemical composition of the non-quenched and tempered steel for crankshafts described in this invention, calculated by weight percentage, is as follows: C: 0.38~0.42%, Si: 0.55~0.70%, Mn: 1.55~1.70%, P≤0.025%, S: 0.020~0.035%, Cr: 0.15~0.25%, Ni≤0.15%, Cu≤0.20%, Al: 0.010~0.030%, V: 0.03~0.06%, Ti: 0.010~0.020%, N: 0.012~0.018%, with the remainder being Fe and unavoidable impurities.

[0018] The beneficial effects of this invention are as follows: 1. In converter smelting, the intensity of top blowing and bottom blowing is controlled in stages. From the beginning of converter blowing to 80% of the blowing is completed, due to the high carbon content in the molten pool and the rapid decarburization rate, it is difficult to effectively increase nitrogen. Therefore, a high top blowing intensity and a low bottom blowing intensity are adopted to accelerate smelting and shorten the smelting cycle. From 80% of the blowing is completed to the end of the blowing, the carbon content in the molten pool decreases and the decarburization rate slows down, which is the rapid nitrogen increase period. At this time, the top blowing flow rate is reduced and the blowing time is extended, while the bottom blowing nitrogen intensity is increased simultaneously to increase the nitrogen content of the molten steel. At the end of the converter smelting, the steel is tapped with high carbon and low oxidation, which can increase the solubility of nitrogen in the molten steel and further increase the nitrogen content of the molten steel.

[0019] 2. During the refining process, nitrogen is not blown to prevent cross-contamination and its impact on the production of other steel grades.

[0020] 3. The RH process is controlled in stages. In the first stage, high vacuum is used to quickly remove inclusions from the molten steel. In the second stage, low vacuum is used to continue the treatment to enhance the inclusion removal effect. Top-blown nitrogen and bottom-blown nitrogen are used simultaneously to increase the nitrogen content of the molten steel. This achieves the goal of removing inclusions while increasing the nitrogen content of the molten steel.

[0021] 4. This invention utilizes a dual-station nitrogen enhancement system in both the converter and RH furnace, combined with controlled smelting process parameters. This eliminates the need for nitrogen alloys, reducing production costs. It also eliminates the need for vacuum-fed nitrogen-manganese wire, preventing secondary oxidation and contamination of the molten steel during wire feeding. Both Class B and Class D inclusions in the finished steel can be controlled to within 0.5 levels, while production costs can be reduced by more than 30 yuan per ton of steel. Detailed Implementation

[0022] The present invention will be described in detail below with reference to the embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0023] The following examples use steel with the following composition as an example, and employ a process route of converter → tapping → refining furnace → RH furnace → continuous casting for smelting.

[0024] The chemical composition of the non-quenched and tempered steel for crankshafts, calculated by weight percentage, is as follows: C: 0.38~0.42%, Si: 0.55~0.70%, Mn: 1.55~1.70%, P≤0.025%, S: 0.020~0.035%, Cr: 0.15~0.25%, Ni≤0.15%, Cu≤0.20%, Al: 0.010~0.030%, V: 0.03~0.06%, Ti: 0.010~0.020%, N: 0.012~0.018%, with the remainder being Fe and unavoidable impurities.

[0025] Example 1 A low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel used in crankshafts, the specific steps of which are as follows: 1. A 120t converter is used for smelting, with 110t of molten iron and 20t of scrap steel as raw materials. Nitrogen is used as the bottom blowing gas during the converter blowing process. From the start of blowing to <80% of the blowing is completed, the bottom blowing intensity is controlled at 230 Nm. 3 / h; from 80% to the end of the blowing process, the bottom blowing intensity is controlled at 1000 Nm. 3 / h. The top-blown oxygen flow rate of the converter is controlled in stages: from the start of blowing to <80% of blowing, the top-blown intensity is controlled at 39000 Nm. 3 / h; from 80% to the end of blowing, the top blowing strength is controlled at 23000 Nm. 3 / h.

[0026] The converter steel has a phosphorus content of 0.016%, a carbon content of 0.12%, an oxygen content of 0.02%, a sulfur content of 0.016%, a tapping temperature of 1638℃, and a nitrogen content of 96ppm.

[0027] 2. During tapping, add 100kg of aluminum blocks for deoxidation when 1 / 3 of the steel has been tapped. Then, add 2800kg of ferrosilicon manganese, 80kg of ferrosilicon, 240kg of high-carbon ferrochrome, 100kg of ferrovanadium alloy, 195kg of carbon raiser, 400kg of slag remover, and 600kg of lime in sequence. At the end of tapping, use a slag-blocking plug and sliding plate for dual-stage operation, with a slag discharge of about 230kg. During the tapping process, the nitrogen bottom blowing flow rate is controlled at 450L / min.

[0028] 3. LF furnace smelting: After refining, bottom blowing argon gas is started for stirring, with the flow rate controlled at 200 L / min. Then, electrodes are inserted and energized for heating. 200 kg of silicon carbide and 50 kg of aluminum granules are added for slag surface deoxidation. Simultaneously, the alloy composition is fine-tuned according to the first refining step to ensure the steel composition meets requirements. Afterwards, the bottom blowing flow rate is set to 130 L / min, and aluminum wire and ferrous sulfate wire are fed in, controlling the aluminum content in the molten steel at 0.025% and the sulfur content at 0.026%. The final steel composition after refining is: C: 0.40%, Si: 0.62%, Mn: 1.62%, P: 0.015%, S: 0.026%, Cr: 0.18%, Ni: 0.03%, Cu: 0.03%, Al: 0.025%, V: 0.042%, Ti: 0.014%, N: 0.011%.

[0029] 4. RH furnace smelting: After the molten steel reaches the RH furnace, it undergoes vacuum treatment. In the first stage, nitrogen is used as the lifting gas, with a flow rate of 100 Nm³. 3 / h, then the vacuum pump is turned on to evacuate the vacuum, controlling the vacuum degree below 67Pa, and the vacuum treatment time is 13min; the composition of the molten steel after the first stage of treatment is C: 0.395%, Si: 0.61%, Mn: 1.61%, P: 0.015%, S: 0.025%, Cr: 0.18%, Ni: 0.03%, Cu: 0.03%, Al: 0.020%, V: 0.042%, Ti: 0.014%, N: 0.006%. In the second stage, the vacuum degree is first adjusted to 9.6kPa, and nitrogen is used as the boosting gas, with a boosting gas flow rate of 100Nm. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 4.8m, nitrogen flow rate 1200Nm 3 / h, nitrogen blowing rate 108m 3 Vacuum treatment time: 5.4 min. After vacuum breaking, 50 m of pure calcium wire was fed in, and a steel sample was taken for analysis of the steel composition: C: 0.392%, Si: 0.61%, Mn: 1.60%, P: 0.015%, S: 0.025%, Cr: 0.18%, Ni: 0.03%, Cu: 0.03%, Al: 0.018%, V: 0.042%, Ti: 0.014%, N: 0.015%, Ca: 5.8 ppm; after soft blowing for 15 min, it was continuously cast.

[0030] The molten steel produced by this process is rolled into 85mm bars and inspected for non-metallic inclusions according to ISO 4967. The test results are: Class B inclusions grade 0 and Class Ds inclusions grade 0.

[0031] Example 2 A low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel used in crankshafts, the specific steps of which are as follows: 1. A 120t converter is used for smelting, with 110t of molten iron and 20t of scrap steel as raw materials. Nitrogen is used as the bottom blowing gas during the converter blowing process. From the start of blowing to <80% of the blowing is completed, the bottom blowing intensity is controlled at 320 Nm. 3 / h; from 80% to the end of the blowing process, the bottom blowing intensity is controlled at 900 Nm. 3 / h. The top-blown oxygen flow rate of the converter is controlled in stages, from the start of blowing to 80% of the blowing volume, with the top-blown intensity controlled at 38000 Nm. 3 / h; from 80% to the end of blowing, the top blowing strength is controlled at 25000 Nm. 3 / h.

[0032] The converter steel has a phosphorus content of 0.015%, a carbon content of 0.10%, an oxygen content of 0.022%, a sulfur content of 0.015%, a tapping temperature of 1642℃, and a nitrogen content of 89ppm.

[0033] 2. During tapping, 100 kg of aluminum blocks are added to deoxidize 1 / 3 of the tapped steel. Then, 2810 kg of silicon manganese, 78 kg of ferrosilicon, 236 kg of high-carbon ferrochrome, 96 kg of ferrovanadium alloy, 200 kg of carbon raiser, 400 kg of slag remover, and 600 kg of lime are added in sequence. At the end of tapping, a slag-blocking plug and sliding plate are used for dual-stage operation, with a slag discharge of about 240 kg. The nitrogen bottom blowing flow rate is controlled at 450 L / min during the tapping process.

[0034] 3. LF furnace smelting: After refining, bottom blowing argon gas is started for stirring, with the flow rate controlled at 200 L / min. Then, electrodes are inserted and energized for heating. 200 kg of silicon carbide and 50 kg of aluminum granules are added for slag surface deoxidation. Simultaneously, the alloy composition is fine-tuned according to the first refining step to ensure the steel composition meets requirements. Afterwards, the bottom blowing flow rate is set to 130 L / min, and aluminum wire and ferrous sulfate wire are fed in, controlling the aluminum content in the molten steel at 0.026% and the sulfur content at 0.025%. The final steel composition after refining is: C: 0.396%, Si: 0.61%, Mn: 1.60%, P: 0.016%, S: 0.025%, Cr: 0.19%, Ni: 0.032%, Cu: 0.033%, Al: 0.026%, V: 0.041%, Ti: 0.014%, N: 0.010%.

[0035] 4. RH furnace smelting: After the molten steel reaches the RH furnace, it undergoes vacuum treatment. In the first stage, nitrogen is used as the lifting gas, with a flow rate of 80 Nm³. 3 / h, then the vacuum pump is turned on to evacuate the vacuum, controlling the vacuum degree below 67Pa, and the vacuum treatment time is 14min; the composition of the molten steel after the first stage of treatment is C: 0.39%, Si: 0.60%, Mn: 1.59%, P: 0.016%, S: 0.025%, Cr: 0.18%, Ni: 0.03%, Cu: 0.03%, Al: 0.020%, V: 0.041%, Ti: 0.014%, N: 0.0052%. In the second stage, the vacuum degree is first adjusted to 9.2kPa, and nitrogen is used as the boosting gas, with a boosting gas flow rate of 100Nm. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 4.8m, nitrogen flow rate 1200Nm 3 / h, nitrogen blowing rate 120m³ 3 Vacuum treatment time: 6.0 min. After vacuum breaking, 50 m of pure calcium wire is fed in, and a steel sample is taken for analysis. The steel composition is: C: 0.39%, Si: 0.60%, Mn: 1.59%, P: 0.016%, S: 0.025%, Cr: 0.18%, Ni: 0.03%, Cu: 0.03%, Al: 0.019%, V: 0.041%, Ti: 0.014%, N: 0.016%, Ca: 5.8 ppm. After soft blowing for 15 min, it is continuously cast.

[0036] The molten steel produced by this process is rolled into 85mm bars and inspected for non-metallic inclusions according to ISO 4967. The test results are: Class B inclusions grade 0.5 and Class Ds inclusions grade 0.

[0037] Comparative Example 1 Compared with Example 1, the converter was top-blown with a high flow rate throughout the process, resulting in a relatively low nitrogen content in the converter.

[0038] The 120t converter smelting process uses 110t of molten iron and 20t of scrap steel as raw materials. Nitrogen is used as the bottom blowing gas during the converter blowing process. From the start of blowing to <80% of the blowing is completed, the bottom blowing intensity is controlled at 230 Nm. 3 / h; from 80% to the end of the blowing process, the bottom blowing intensity is controlled at 1000 Nm. 3 / h. Converter top-blown oxygen, from the start to the end of blowing, with the top-blown intensity controlled at 39000 Nm. 3 / h.

[0039] The converter steel has a phosphorus content of 0.015%, a carbon content of 0.121%, an oxygen content of 0.019%, a sulfur content of 0.016%, a tapping temperature of 1639℃, and a nitrogen content of 67ppm.

[0040] Comparative Example 2 Compared with Example 1, the low-flow bottom blowing in the later stage of the converter resulted in lower nitrogen content in the tapped steel.

[0041] The 120t converter smelting process uses 110t of molten iron and 20t of scrap steel as raw materials. Nitrogen is used as the bottom blowing gas during the converter blowing process. The bottom blowing intensity is controlled at 230 Nm from the start to the end of the blowing process. 3 / h. The top-blown oxygen flow rate of the converter is controlled in stages: from the start of blowing to <80% of blowing, the top-blown intensity is controlled at 39000 Nm. 3 / h; from 80% to the end of blowing, the top blowing strength is controlled at 23000 Nm. 3 / h.

[0042] The converter steel has a phosphorus content of 0.015%, a carbon content of 0.119%, an oxygen content of 0.0195%, a sulfur content of 0.0162%, a tapping temperature of 1641℃, and a nitrogen content of 58ppm.

[0043] Comparative Example 3 The second stage of RH blowing uses a high vacuum of less than 100Pa. After the second stage of treatment, there is basically no nitrogen addition to the molten steel. Nitrogen and manganese wire is added, and the steel has a high content of inclusions.

[0044] The composition of the molten steel after the first stage of RH treatment was C: 0.40%, Si: 0.60%, Mn: 1.61%, P: 0.014%, S: 0.026%, Cr: 0.176%, Ni: 0.031%, Cu: 0.032%, Al: 0.021%, V: 0.042%, Ti: 0.014%, N: 0.0062%. In the second stage, the vacuum level was controlled below 100 Pa, and nitrogen was used as the boosting gas, with a boosting gas flow rate of 100 Nm³. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 4.8m, nitrogen flow rate 1200Nm 3 / h, nitrogen blowing rate 108m 3 Vacuum treatment time: 5.4 min. After vacuum breaking, 50 m of pure calcium wire was fed in, and a steel sample was taken for analysis of the steel composition: C: 0.391%, Si: 0.60%, Mn: 1.61%, P: 0.014%, S: 0.025%, Cr: 0.18%, Ni: 0.031%, Cu: 0.031%, Al: 0.018%, V: 0.042%, Ti: 0.014%, N: 0.0069%, Ca: 5.8 ppm. The nitrogen content of the steel was insufficient, so 200 m of nitrogen-manganese wire was added, and after soft blowing for 15 min, it was continuously cast.

[0045] The molten steel produced by this process is rolled into 85mm bars and inspected for non-metallic inclusions according to ISO 4967. The test results are: Class B inclusions grade 2.0 and Class Ds inclusions grade 1.5.

[0046] Comparative Example 4 Compared with Example 1, the second stage of RH blowing only used bottom blowing nitrogen and did not use top blowing nitrogen. The molten steel had almost no nitrogen addition. Nitrogen and manganese wire was added, resulting in high inclusion content in the steel.

[0047] The composition of the molten steel after the first stage of RH treatment was C: 0.41%, Si: 0.62%, Mn: 1.63%, P: 0.015%, S: 0.024%, Cr: 0.172%, Ni: 0.035%, Cu: 0.036%, Al: 0.022%, V: 0.042%, Ti: 0.0141%, N: 0.0065%. In the second stage, the vacuum level was adjusted to 9.6 kPa, and nitrogen was used as the boosting gas, with a boosting gas flow rate of 100 Nm³. 3 / h, vacuum treatment time 5.4min. After breaking the vacuum, 50m of pure calcium wire was fed in, and a steel sample was taken for analysis of the steel composition: C: 0.408%, Si: 0.61%, Mn: 1.61%, P: 0.015%, S: 0.024%, Cr: 0.17%, Ni: 0.035%, Cu: 0.035%, Al: 0.019%, V: 0.041%, Ti: 0.0142%, N: 0.068%, Ca: 5.6ppm; the nitrogen content of the steel was insufficient, so 200m of nitrogen-manganese wire was fed in, and after soft blowing for 15min, it was continuously cast.

[0048] The molten steel produced by this process is rolled into 85mm bars and inspected for non-metallic inclusions according to ISO 4967. The test results are: Class B inclusions grade 1.5 and Class Ds inclusions grade 2.0.

[0049] Comparative Example 5 Compared with Example 1, the top-blown nitrogen lance position in the second stage of RH blowing is higher than 5.0m, resulting in poor nitrogen enhancement effect in molten steel, requiring supplemental feeding of nitrogen and manganese wire, and high inclusion content in the steel.

[0050] The composition of the molten steel after the first stage of RH treatment was C: 0.399%, Si: 0.613%, Mn: 1.63%, P: 0.016%, S: 0.026%, Cr: 0.183%, Ni: 0.034%, Cu: 0.034%, Al: 0.022%, V: 0.041%, Ti: 0.0144%, N: 0.0062%. In the second stage, the vacuum level was adjusted to 9.6 kPa, and nitrogen was used as the boosting gas, with a boosting gas flow rate of 100 Nm³. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 5.1m, nitrogen flow rate 1200m³ / h. 3 / h, nitrogen blowing rate 108m 3 Vacuum treatment time: 5.4 min. After vacuuming, 50 m of pure calcium wire was fed in, and a steel sample was taken for analysis of the steel composition: C: 0.395%, Si: 0.608%, Mn: 1.60%, P: 0.016%, S: 0.026%, Cr: 0.181%, Ni: 0.034%, Cu: 0.034%, Al: 0.020%, V: 0.042%, Ti: 0.014%, N: 0.0108%, Ca: 5.5 ppm. The nitrogen content of the steel was insufficient, so 80 m of nitrogen-manganese wire was added, and after soft blowing for 15 min, it was continuously cast.

[0051] The molten steel produced by this process is rolled into 85mm bars and inspected for non-metallic inclusions according to ISO 4967. The test results are: Class B inclusions grade 1.0 and Class Ds inclusions grade 1.0.

[0052] Comparative Example 6 Compared with Example 1, the top-blown nitrogen lance position in the second stage of RH blowing was lower than 4.8m, the slag at the top of the ladle was severely foamed, and slag overflowed from the ladle opening, posing a safety hazard.

[0053] The steel composition after the first stage of treatment in the RH furnace was: C: 0.396%, Si: 0.615%, Mn: 1.619%, P: 0.014%, S: 0.025%, Cr: 0.182%, Ni: 0.03%, Cu: 0.03%, Al: 0.021%, V: 0.0417%, Ti: 0.0143%, N: 0.0062%. In the second stage, the vacuum level was adjusted to 9.6 kPa, and nitrogen was used as the boosting gas, with a boosting gas flow rate of 100 Nm³. 3 / h; Synchronous top-blown nitrogen, nitrogen lance position 4.7m, nitrogen flow rate 1200Nm 3 / h, nitrogen blowing rate 108m 3 Vacuum treatment time: 5.4 min; During the second stage of treatment, slag overflow was found at the ladle opening, posing a safety hazard.

[0054] Unless otherwise specified, all raw materials used in this invention are commercially available materials commonly used in the field. The process steps not specifically described are conventional process steps in non-quenched and tempered steel smelting. The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in this invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of this invention.

Claims

1. A low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts, characterized in that: The process flow is as follows: converter smelting → tapping → LF refining → RH vacuum → continuous casting; the specific steps and control parameters are as follows: Step 1: Converter smelting. Nitrogen is used as the bottom blowing gas and oxygen is used as the top blowing gas in the converter blowing process. The carbon content of the molten steel tapped from the converter is 0.08~0.15%, the oxygen content is 0.010~0.025%, the phosphorus content is ≤0.018%, the nitrogen content is 60~100ppm, and the tapping temperature is 1620~1650℃. Step 2: Tapping. No nitriding alloy is added during the tapping process. At the end of tapping, a slag-blocking plug and sliding plate are used for dual-stop operation. The slag discharge is ≤3.0kg / t. At the end of tapping, the nitrogen content of the molten steel is controlled at 80~120ppm. Step 3, LF refining, uses argon gas stirring, adds deoxidizer for slag surface deoxidation, fine-tunes alloy composition, does not use nitriding alloys, and controls the nitrogen content of molten steel at 90~130ppm after refining. Step 4, RH vacuum, using nitrogen as the lifting gas: In the first stage, the vacuum degree is controlled within 67 Pa, and the vacuum treatment lasts for 10~15 min; In the second stage, nitrogen is blown simultaneously from the top, and the vacuum degree is controlled within 8k~10kPa. The vacuum treatment lasts for 5~10 minutes, and the nitrogen content of the molten steel is controlled at 0.013%~0.016%. After the vacuum is completed, the vacuum is broken, and pure calcium wire is fed in to adjust the calcium content of the molten steel. The calcium content of the RH-leaving molten steel is controlled at 4~6ppm. After the molten steel has been allowed to stand, it is then cast. Step 5: Continuous casting.

2. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In step one, the converter bottom blowing nitrogen flow is controlled in stages according to the percentage of the bottom blowing time in the total blowing time, and the blowing intensity is controlled at 0.03~0.05Nm 3 / t / min; and the blowing intensity is controlled at 0.10~0.15Nm 3 / t / min.

3. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In step one, the converter top blowing oxygen flow is controlled in stages according to the percentage of top blowing time in total blowing time, and the blowing intensity is controlled at 5.0~5.5 Nm 3 / t / min; and the blowing intensity is controlled at 3.0~3.5 Nm 3 / t / min.

4. The nitrogen enrichment method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In step two, aluminum blocks are used as deoxidizers during the tapping process; then silicon manganese, high-carbon ferrochrome, ferrovanadium alloy, carbon raiser, slag remover, and lime are added in sequence.

5. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In step three, silicon carbide and aluminum particles are added to deoxidize the slag surface. At the same time, the alloy composition is finely adjusted according to the first refining process to ensure that the composition of the molten steel meets the requirements.

6. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In the fourth step, in the first stage, nitrogen is used as the lifting gas, and the flow rate of the lifting gas is 80-100 Nm 3 / h, and the nitrogen content of the molten steel at the end of the first stage treatment is controlled at 50-80 ppm.

7. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: In the fourth step, the second stage, the top blowing nitrogen is synchronized, the nitrogen lance position is 4.8~5.0m, the nitrogen flow is 800~1200Nm 3 / h, and the nitrogen amount is 80~120m 3 .

8. The low-cost nitrogen-enhancing smelting method for non-quenched and tempered steel for crankshafts according to claim 1, characterized in that: The non-quenched and tempered steel used for the crankshaft has the following chemical composition by weight percentage: C: 0.38~0.42%, Si: 0.55~0.70%, Mn: 1.55~1.70%, P≤0.025%, S: 0.020~0.035%, Cr: 0.15~0.25%, Ni≤0.15%, Cu≤0.20%, Al: 0.010~0.030%, V: 0.03~0.06%, Ti: 0.010~0.020%, N: 0.012~0.018%, with the remainder being Fe and unavoidable impurities.