Method for controlling grain size of low-carbon bearing steel through cooperation of nitrogen circulation and niobium

Abstract

According to the method for controlling the grain size of the low-carbon bearing steel through cooperation of nitrogen circulation and niobium, the smelted raw material ferroniobium is expensive, by means of nitrogen circulation, the grain size is optimized, meanwhile, the adding amount of ferroniobium is reduced, and low-cost smelting is achieved; the niobium element can be combined with carbon and nitrogen to form niobium (carbon and nitrogen) particles, and the undissolved niobium (carbon and nitrogen) particles become second-phase particle pinning grain boundaries to improve the resistance of grain growth, so that the migration of the grain boundaries is inhibited, the grains are refined, the hardness after quenching is improved, and the wear resistance is also improved. The method disclosed by the invention has the beneficial effects that by adopting the method disclosed by the invention, the cost is reduced, an obvious local grain coarse area does not appear on a 20Cr sample along with the prolonging of carburizing time, the grain size is always relatively stable at 7-8 levels, the average quenching hardness is 43HB, and the wear resistance is greatly improved.

Description

Technical Field

[0001] The invention belongs to the field of steel smelting technology, and specifically relates to a smelting method which adopts nitrogen circulation and rare element niobium to effectively control the grain size of hot-rolled bars for low-carbon carburized bearings. Background Art

[0002] 20Cr is carburized bearing steel. Its chemical composition complies with GB / T3077-2015 standard and should meet the requirements in Table 1.

[0003] Table 1. Chemical composition of 20Cr

[0004]

[0005] 20Cr steel is the most widely used alloy steel in my country. The traditional smelting method is: only control carbon, silicon, manganese, chromium, phosphorus, sulfur and residual elements during the refining process, and use argon circulation in vacuum. That is, no niobium is added during the refining process, and no nitrogen circulation is performed during the vacuum process. During the simulated carburizing at 930℃, a large area of ​​mixed crystals appeared on the inspection surface. The grain size was rated as level 3 and level 4 according to the GB / T6394-2017 standard. The customer required a grain size greater than level 6. The simulated carburizing method was used to inspect the grains, and the problem of coarse grains seriously affected the product quality. Summary of the invention

[0006] The invention discloses a method for controlling the grain size of low-carbon bearing steel by utilizing nitrogen circulation and niobium, and optimizes the smelting method to control the grain size of 20Cr bearing steel by utilizing nitrogen circulation and niobium.

[0007] The following specific technical solutions are adopted:

[0008] 1. Process route

[0009] The smelting process route is optimized as follows: KR molten iron pretreatment → converter → LF furnace → RH furnace → billet continuous casting.

[0010] 2. Critical Control Points

[0011] ⑴KR molten iron pretreatment, use KR to desulfurize the molten iron, and remove the sulfur in the molten iron to no less than 0.010%.

[0012] ⑵ The converter adopts high carbon drawing operation, and the carbon of steel tapping is controlled at 0.08% to 0.10% to prevent overoxidation of molten steel, reduce the pressure of subsequent LF refining and deoxidation outside the furnace, and improve the purity of molten steel. The converter uses front and rear slide plates to block slag + slag block mark to control the amount of slag to reduce the pollution of converter slag to molten steel; deoxidation and alloying of converter steel tapping: the converter steel tapping volume is 105±5 tons. When the converter steel tapping volume is about 1 / 5, according to the chemical composition standard requirements, add aluminum ingots (blocks), low-titanium ferrosilicon, medium-carbon ferromanganese, low-titanium high chromium, 300kg of lime, and 1000kg of low-titanium refined slag to add alloy and slag in order. The addition of alloy and slag is completed before 4 / 5 of the steel is tapped to ensure the alloying and slag-removing effects. At the same time, aluminum ingots are used for pre-deoxidation, and aluminum inclusions can get sufficient floating time.

[0013] Note: The amount of aluminum ingot (block) added is calculated using the aluminum addition model based on the converter end carbon. The amount of other alloys added is calculated based on the converter steel output, the yield rate of 100%, and the range of the lower limit of element content - (lower limit of element content - 0.1%).

[0014] ⑶After the LF furnace enters the station, it is powered on and the temperature is raised to no less than 1550℃. After stirring with argon gas, an analysis is carried out to ensure that the alloy is melted evenly, and then a sample is taken for analysis. The composition is fine-tuned according to the analysis results. The LF furnace uses white slag operation, and the slag composition is CaO: 52%~55%, SiO2: 6%~10%, Al2O3: 28%~32%, basicity R: 6±1, CaO / Al2O3: 1.8±0.2. This slag has good adsorption capacity for inclusions while ensuring good desulfurization and deoxidation capabilities. After the LF furnace enters the station, ferroniobium is used for alloying, according to the niobium element content range of 0.02%~0.04%.

[0015] (4) The key components of the RH furnace are the vacuum degassing chamber and the vacuum pump system. When the equipment is working, the vacuum circulation uses nitrogen instead of the traditional argon circulation. Two immersion tubes are inserted into the molten steel to form a closed system. The ladle and the vacuum chamber are connected. The vacuum pump is turned on to evacuate the air, and nitrogen is blown into the riser for circulation. The molten steel circulates between the riser and the downcomer to form extremely fine bubble nuclei and realize continuous degassing. The vacuum circulation time is set to 15 minutes, and the nitrogen content control target is (70-110) × 10 -6 .

[0016] Points of the Invention

[0017] ⑴ The raw material ferroniobium for smelting is relatively expensive. By utilizing nitrogen circulation, the grain size can be optimized while reducing the amount of ferroniobium added, thus achieving low-cost smelting.

[0018] (2) Niobium element can combine with carbon and nitrogen to form niobium (carbon, nitrogen) particles. These undissolved niobium (carbon, nitrogen) particles become second phase particles to pin the grain boundaries, increase the resistance to grain growth, thereby inhibiting the migration of grain boundaries, achieving grain refinement, and improving the hardness after quenching, which also improves wear resistance.

[0019] The benefits of the present invention are as follows: after the smelting process is optimized, the cost is reduced, and as the carburizing time increases, no obvious local coarse grain area appears in the 20Cr sample, the grain size has been relatively stable at level 7 to 8, the average quenching hardness is 43HB, and the wear resistance is greatly improved. DETAILED DESCRIPTION

[0020] The present invention is further described in detail with reference to Examples 1 to 5.

[0021] 1. Execute the smelting process (Examples 1 to 2 are conventional processes, and Examples 3 to 5 are the processes of the present invention)

[0022] Example 1: Execute the smelting process without adding niobium and nitrogen circulation: KR hot metal pretreatment → converter tapping 105±5 tons → LF furnace refining silicon, manganese, and chromium alloying treatment according to chemical composition standards → RH furnace vacuum treatment, vacuum process using nitrogen circulation, after nitrogen circulation, nitrogen target value (70-90)×10 -6 →Continuous casting of 380mm×490mm square billets.

[0023] Example 2: Execute the niobium addition and argon circulation smelting process: KR molten iron pretreatment → converter steel tapping 105±5 tons → LF furnace refining silicon, manganese, and chromium, alloying treatment according to the chemical composition standard requirements, adding niobium iron according to the niobium element range of 0.020% to 0.030% → RH furnace vacuum treatment, using argon circulation in the vacuum process → continuous casting and drawing 380mm×490mm square billet.

[0024] Example 3: Execute the smelting process of adding niobium and nitrogen circulation of the present invention: KR hot metal pretreatment → converter tapping 105±5 tons → LF furnace refining silicon, manganese, and chromium alloying treatment according to the chemical composition standard requirements, add niobium iron according to the niobium element range of 0.020% to 0.030% → RH furnace vacuum treatment using nitrogen circulation, after nitrogen circulation, the nitrogen is required to reach the target value (70 to 90)×10 -6 →Continuous casting of 380mm×490mm square billets.

[0025] Example 4: The smelting process of adding niobium and nitrogen circulation according to the present invention is implemented: KR hot metal pretreatment → converter steel tapping 105±5 tons → LF furnace refining silicon, manganese, and chromium are alloyed according to the chemical composition standard requirements, and niobium iron is added according to the niobium element range of 0.030% to 0.035% → RH furnace vacuum treatment using nitrogen circulation, and the nitrogen is required to reach the target value (70 to 90)×10-6 →Continuous casting of 380mm×490mm square billets.

[0026] Example 5: Execute the smelting process of adding niobium and nitrogen circulation of the present invention: KR hot metal pretreatment → converter tapping 105±5 tons → LF furnace refining silicon, manganese, and chromium alloying treatment according to the chemical composition standard requirements, add niobium iron according to the niobium element range of 0.030% to 0.035% → RH furnace vacuum treatment using nitrogen circulation, after nitrogen circulation, nitrogen is required to reach the target value (90 to 110)×10 -6 →Continuous casting of 380mm×490mm square billets.

[0027] 2. Comparison of smelting components using different smelting methods is shown in Table 2.

[0028] Table 2

[0029]

[0030] Example 1 is a traditional smelting method using nitrogen circulation without adding niobium. The process requires RH vacuum treatment with nitrogen circulation. The nitrogen content after circulation is required to be (70-90)×10 -6 The principle of subsequent grain size control in this smelting method is that titanium that is not completely dissolved in austenite will combine with nitrogen and carbon to form titanium (nitrogen, carbon) precipitation phase, pinning the austenite grain boundaries, thereby preventing the growth of austenite grains, thereby avoiding large-area mixed crystals and coarse grains. However, the effect is not very good. The simulated carburizing method was kept warm for 1h, 3h, and 6h, and the grain size was coarse. The average grain size of 4.8 to 3.5 did not meet the standard requirements.

[0031] In order to improve the grain size, Example 2 adopts the smelting method of adding niobium and using argon circulation.

[0032] The process requires that niobium iron should be added according to the target niobium content of 0.020% to 0.030% during LF furnace refining, and argon gas circulation should be used for RH furnace vacuum treatment. The principle of controlling grain size by the optimized smelting method is that niobium, as an element with the function of refining grains, will combine with carbon in steel to form NbC, and the NbC precipitation phase will be pinned at the grain boundary to play a hindering role, thereby effectively preventing the growth of austenite grains and achieving grain refinement. The 20Cr smelted by this method simulates the carburizing method and is kept warm for 1h, 3h, and 6h. The grain size tested after keeping warm for 1h and 3h is higher than that of Example 1, and the average grain size is 6.4 to 6.3. However, when the holding time is further extended to 6h, the grain size is obviously coarsened, and the average grain size is 3.7, which does not meet the standard requirements. Although this method has been improved, niobium iron and argon are expensive, which increases the smelting cost and cannot meet the requirement of long-term carburizing without coarsening of grains.

[0033] On the basis of Examples 1 and 2, in order to further optimize the smelting method and reduce the smelting cost, Example 3 adopts a smelting method of adding niobium and using nitrogen circulation. This process requires that during LF furnace refining, ferroniobium is added according to the target niobium content of 0.020% to 0.030%, and RH furnace vacuum treatment uses nitrogen circulation. The nitrogen content after circulation is required to be (70 to 90) × 10 -6 The cost of smelting 20Cr using this method is lower than that of Example 2. The average grain size is 6.5 to 5.7 when the simulated carburizing method is kept warm for 1h, 3h, and 6h. The average grain size is higher than that of Example 2 when the insulation is 6h, but it still does not meet the standard requirements.

[0034] Example 4 Based on Example 3, during the alloying treatment in LF furnace refining, the amount of ferroniobium added is increased, and the niobium content is required to be in the range of 0.030% to 0.035%. Nitrogen circulation is used in the RH furnace vacuum treatment, and the nitrogen content after circulation is required to be (70 to 90) × 10 -6 The cost of smelting 20Cr using this method is higher than that of Example 3. The simulated carburizing method is kept warm for 1h, 3h, and 6h, and the average grain size is 8.0 to 7.1. The average grain size is higher than that of Example 3 after keeping warm for 1h, 3h, and 6h. Although it meets the standard requirements, the smelting cost increased more than that of Example 3.

[0035] Example 5 is based on Example 4, but the amount of niobium added is reduced. This process requires that the amount of niobium added is reduced during the LF furnace refining and alloying treatment, and the niobium content is required to be in the range of 0.015% to 0.020%. The RH furnace vacuum treatment uses nitrogen circulation, and the nitrogen content after circulation is required to be (90 to 110) × 10 -6 The cost of smelting 20Cr using this method is lower than that of Example 4. The simulated carburizing method is kept warm for 1h, 3h, and 6h, and the average grain size is 8.0 to 7.1. The average grain size is almost the same as that of Example 4 after keeping warm for 1h, 3h, and 6h, which meets the standard requirements and reduces the smelting cost compared with Examples 2, 3, and 4.

[0036] 3. Impact of improvements on grain size and hardness

[0037] ⑴ According to GB / T6394-2017 standard, the influence of different smelting methods on the grain size and hardness of 20Cr at 930℃ simulated carburizing for 1h, 3h and 6h was studied. After 930℃ simulated carburizing for 1h, 3h and 6h, it was taken out of the furnace and water-cooled, and then ground and polished with four sandpapers. The austenite grains were corroded by supersaturated picric acid + detergent solution. According to GB / T6394-2017 standard X100 metallographic photography was carried out, and professional software was used to perform average grain size rating and average grain size calculation. The average austenite grain size and grain size of 20Cr smelted by different smelting methods at different holding carburizing times are shown in Table 2.

[0038] Table 2

[0039]

[0040] It can be seen from the above table that Example 5 has the best improvement on grain size.

[0041] ⑵ According to the national standard GB / T5216-2014 for hardenability, 6 points were randomly selected for hardenability test of 20Cr with different smelting methods after simulated carburizing at 930℃ for 1h, 3h and 6h.

[0042] The Rockwell hardness values ​​are shown in Table 3.

[0043] Table 3

[0044]

[0045]

[0046] It can be seen from the above table that after the improvement, the hardness is increased and the wear resistance of the material is improved.

Claims

1. A method for controlling the grain size of low-carbon bearing steel by using nitrogen circulation and niobium, characterized in that: Optimize the smelting method to use nitrogen circulation combined with niobium to control the grain size of 20Cr bearing steel. The process route is optimized as follows: KR hot metal pretreatment → converter → LF furnace → RH furnace → bloom continuous casting; The KR molten iron pretreatment uses KR to desulfurize the molten iron to reduce the sulfur content of the molten iron to less than 0.010%; The converter adopts high carbon drawing operation, and the carbon of steel tapping is controlled at 0.08% to 0.10%. The converter tapping adopts front and rear slide plates to block slag + slag block mark to control the amount of slag; deoxidation and alloying of converter tapping: the converter tapping amount is 105±5 tons. When the converter tapping amount is about 1 / 5, according to the chemical composition standard requirements, aluminum ingots (blocks), low-titanium ferrosilicon, medium-carbon ferromanganese, low-titanium high-chromium, 300kg of lime, and 1000kg of low-titanium refined slag are added in order to add alloy and slag. The addition of alloy and slag is completed before 4 / 5 of the steel is tapped, and aluminum ingots are used for pre-deoxidation at the same time; The LF furnace is powered on and heated to no less than 1550°C, and after nitrogen stirring, an analysis is performed to ensure uniform melting of the alloy, and then sampling and analysis are performed. The composition is fine-tuned according to the analysis results. The LF furnace is operated with white slag, and the slag components are CaO: 52% to 55%, SiO2: 6% to 10%, Al2O3: 28% to 32%, basicity R: 6±1, CaO / Al2O3: 1.8±0.

2. After the LF furnace enters the station, niobium iron is used for alloying, and the niobium element content ranges from 0.02% to 0.04%. The RH furnace is mainly composed of a vacuum degassing chamber and a vacuum pump system. When the equipment is working, nitrogen is used to replace the traditional argon circulation in the vacuum cycle. Two immersion pipes are inserted into the molten steel to form a closed system. The ladle and the vacuum chamber are connected. The vacuum pump is turned on to evacuate the air. Nitrogen is blown into the riser for circulation. The molten steel circulates between the riser and the downcomer to form extremely fine bubble nuclei, thereby achieving continuous degassing. The vacuum circulation time is set to 15 minutes, and the nitrogen content control target is (70-110)×10 -6 .

2. According to claim 1, a method for controlling the grain size of low-carbon bearing steel by using nitrogen circulation and niobium, characterized in that: KR hot metal pretreatment → converter steel tapping 105±5 tons → LF furnace refining silicon, manganese, chromium alloying treatment according to chemical composition standard requirements, add niobium iron according to the niobium element range of 0.030% ~ 0.035% → RH furnace vacuum treatment using nitrogen circulation, after nitrogen circulation, nitrogen is required to reach the target value (90 ~ 110) × 10 -6 →Continuous casting of 380mm×490mm square billets.

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