A production process for improving the castability of bearing steel based on magnesium treatment

Abstract

The present invention belongs to the field of iron and steel metallurgy, and is a production process for improving the castability of bearing steel based on magnesium treatment. The present invention first controls the timing of adding ferrosilicon and the S content in the refining process to ensure that S<0.0020% in the steel after the refining is completed, and controls the vacuum treatment. After the RH breaks through the air, the Mg‑Si‑Fe alloy cored wire is fed. The composition and mass percentage of the Mg‑Si‑Fe alloy cored wire are Mg: 16-20%, Si: 38-42%, and the rest are Fe and unavoidable impurities. The present invention can control the inclusions to MgO through magnesium treatment, and the proportion of MgO inclusions with a size of less than 2 microns is controlled to be more than 90%, and the number density of MgO inclusions is less than 260 / 100mm. 2 MgO inclusions do not sinter at the pouring temperature (1470-1495°C), which can greatly increase the number of continuous pouring furnaces and the qualified rate of water immersion flaw detection.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy and relates to a production process for improving the castability of bearing steel based on magnesium treatment. Background Art

[0002] To ensure ultra-high purity, bearing steel is typically deoxidized with metallic aluminum during the refining process, along with a high-basicity refining slag refining treatment. This process can control the total oxygen content in the steel to less than 6 ppm, achieving ultra-high cleanliness in bearing steel. However, under current process conditions, the predominant inclusion in the steel is magnesia-alumina spinel. Magnesia-alumina spinel inclusions have a melting point of 2053°C, are smaller than 10 microns in size, and are solid at steelmaking temperatures. During the casting process, these tiny solid spinel inclusions can accumulate and sinter on the tip of the stopper rod and the inner wall of the submerged nozzle, causing the stopper rod to rise and the nozzle to clog, leading to interruptions in continuous casting production. Furthermore, these accumulated and sintered spinel inclusions can flake off and enter the molten steel under the erosion of the molten steel, ultimately remaining in the molten steel as large inclusions that cause the product to fail water immersion testing. The current deoxidation and refining process can achieve ultra-low oxygen content control in bearing steel, but the generated spinel inclusions will cause interruption of continuous casting and failure of the final rolled material to pass water immersion flaw detection. The low number of continuous casting furnaces and the low pass rate of water immersion flaw detection of rolled materials are difficult problems faced by bearing steel production. Studies have shown that Mg treatment in the refining process is conducive to the formation of magnesium-containing inclusions, but bearing steel generally contains 0.002-0.004% S. If the Mg and S contents are not properly controlled, a large amount of CaS and MgS inclusions will be generated in the steel. These inclusions will also cause the stopper rod to rise and the nozzle to be blocked, deteriorating the purity and castability of the bearing steel. How to moderately modify the inclusions in the steel while avoiding the formation of CaS and MgS inclusions is the key to be solved by the present invention. Summary of the Invention

[0003] To address the issues of low continuous casting furnace counts and low water immersion flaw detection pass rates in bearing steel production, this paper proposes a production process that improves the castability of bearing steel using magnesium treatment. By modifying inclusions into MgO with an ultra-high melting point and controlling the size and quantity of these MgO inclusions, the process significantly increases the number of continuous casting furnaces and the water immersion flaw detection pass rate for rolled products.

[0004] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a production process for improving the castability of bearing steel based on magnesium treatment.

[0005] (1) First, the timing of adding ferrosilicon and the control of S content during the refining process

[0006] The production process of bearing steel adopts BOF steelmaking - LF refining - RH vacuum treatment - CC continuous casting. Ferrosilicon is added in full at one time during the steel tapping process, and no ferrosilicon is added during the LF refining process. The refining slag volume is 10-12kg / ton of steel, CaO / SiO2>8.0, Al2O3=25-30%, ensuring that S in the steel is less than 0.0020% after the refining.

[0007] (2) Control of vacuum treatment

[0008] The RH vacuum degree is required to be less than 30Pa, the processing time is not less than 18 minutes, the TO content after vacuum breaking is required to be less than 5.5ppm, and the acid-soluble aluminum content in the steel is controlled at 0.010% to 0.025%.

[0009] (3) Magnesium treatment feed rate

[0010] After the RH is broken, Mg-Si-Fe alloy cored wire is fed in. The composition and weight percentage of the Mg-Si-Fe alloy cored wire are: Mg: 16-20%, Si: 38-42%, and the remainder is Fe and inevitable impurities. The parameters of the Mg-Si-Fe alloy cored wire are: diameter φ16.0-16.5mm, iron sheet thickness 0.45-0.48mm, core powder particle size 0.01-3mm, and unit weight 160g / m.

[0011] Feed different lengths of alloy cored wire according to the actual acid-soluble aluminum content in the steel. The feeding amount of alloy cored wire follows the following empirical formula:

[0012] Feeding length of cored wire (m) = 15000-20000 × (acid-soluble aluminum content × molten steel weight)

[0013] Wherein, the acid-soluble aluminum content is expressed as mass fraction (%), and the molten steel weight is expressed in tons. For example, if the Al content in the steel after RH degassing is 0.015% and the molten steel weight is 100 tons, the feed length of the cored wire is: 15,000-20,000 × (0.015% × 100) = 225-300 m.

[0014] Ensure that the Mg content in the steel is 12-25ppm and the wire feeding speed is 3.5-5.0m / s.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The present invention can control inclusions to MgO through magnesium treatment, and the proportion of MgO inclusions with a size of less than 2 microns is controlled to be above 90%, and the number density of MgO inclusions is less than 260 / 100mm 2 MgO inclusions do not sinter at the pouring temperature (1470-1495°C), which can greatly increase the number of continuous pouring furnaces and the qualified rate of water immersion flaw detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is an analysis diagram of inclusion composition in the example. DETAILED DESCRIPTION

[0018] Example

[0019] The production process is "120t converter - LF refining - RH vacuum treatment - 240mm×260mm square billet continuous casting". LF refining heating, alloying, high basicity slag deep desulfurization, and continuous casting are carried out according to conventional production processes (steel grade is GCr15).

[0020] (1) Ferrosilicon is added in full at one time during the steel tapping process. No ferrosilicon is added during the LF refining process. The amount of refining slag is 10-12 kg / ton of steel, CaO / SiO2>8.0, Al2O3=25-30%, ensuring that S<0.0020% in the steel after refining.

[0021] (2) Control of vacuum treatment

[0022] The RH vacuum degree is required to be less than 30Pa, the processing time is not less than 18 minutes, the TO content after vacuum breaking is required to be less than 5.5ppm, and the acid-soluble aluminum content in the steel is controlled at 0.010% to 0.025%.

[0023] (3) Magnesium treatment feed rate

[0024] After the RH is broken, Mg-Si-Fe alloy cored wire is fed in. The composition and weight percentage of the Mg-Si-Fe alloy cored wire are: Mg: 16-20%, Si: 38-42%, with the remainder being Fe and unavoidable impurities. The Mg-Si-Fe alloy cored wire parameters are: diameter φ16.0-16.5mm, iron sheet thickness 0.45-0.48mm, core powder particle size 0.01-3mm, and unit weight 160g / m. The alloy cored wire is fed to ensure a Mg content of 12-25ppm in the steel, and the feed speed is 3.5-5.0m / s.

[0025] After RH breaking, molten steel samples were taken and the acid-soluble aluminum content in the steel was analyzed online. Mg-Si-Fe alloy cored wire was fed according to the aluminum content in the steel. Multiple furnace tests were arranged for each casting. The aluminum content, wire feeding amount and wire feeding speed of each furnace are shown in Table 1.

[0026] Comparative Example

[0027] The production process is "120t converter - LF refining - RH vacuum treatment - 240mm×260mm square billet continuous casting". LF refining, heating, alloying, RH vacuum treatment, and continuous casting are carried out according to the conventional production process. A total of 3 castings are carried out, and the number of continuous casting furnaces in each casting is counted.

[0028] Test Example 1

[0029] Bearing steel was continuously produced according to the comparative example and the embodiment described above until the stopper rod expanded to its travel limit, halting the molten steel flow. The number of times the liquid level fluctuated by more than ±5° in a single furnace and the change in stopper rod position during the pouring process under each process condition were counted. The tundish steel was sampled for each pour and analyzed for inclusion type and number density under each process condition. Mg content was determined using ICP, and inclusion number density was determined using an automated scanning electron microscope with a scanning area of 100 mm. 2 .

[0030] Test Example 2

[0031] The ingots obtained in the examples and comparative examples were rolled into bars with a diameter of 60 mm. The obtained bars were inspected using an ultrasonic water immersion tester with a frequency of 10 MHz. Ten bars were inspected from each furnace, and the qualified rates under various process conditions were calculated.

[0032] Table 1 Key parameter control of examples and comparative examples, %

[0033]

[0034]

[0035] Table 2 Comparison of production performance between examples and comparative examples

[0036]

[0037] As shown in Examples 1-8, under the process conditions proposed by the present invention, the Mg content in the steel can be controlled at 12 to 25 ppm, the inclusions in the casting process are MgO inclusions, the number of fluctuations in each casting process does not exceed 1, the stopper rod position changes within +0.8 cm, the casting performance is good, and the water immersion inspection pass rate is 100%.

[0038] Comparative Examples 1-3 did not undergo Mg treatment. The inclusions during the casting process consisted of CA2 + MgO·Al2O3. The number of fluctuations during each casting process ranged from 12 to 16, and the stopper rod rose by +2.3 to +3.6 cm. The casting performance was poor, and the water immersion test pass rate was below 50%. Comparative Examples 4-5 did undergo Mg treatment, but the S and TO content in the steel was improperly controlled, and the inclusion modification was incomplete. The inclusions during the casting process consisted of MgO + MgS + CaS. The number of fluctuations during each casting process was 23 and 25, respectively. The stopper rod rose by +4.1 and +4.6 cm. The casting performance was poor, and the water immersion test pass rate was only 10% and 20%. Comparative Example 6 met the process requirements of the present invention in terms of S and TO control, but the amount of Mg treatment was insufficient. Consequently, more spinel (MgO·Al2O3) inclusions were formed during the casting process. The stopper rod rose by +3.9 cm and fluctuated 19 times. The water immersion test pass rate was only 20%. The control of S and TO in the steel of Comparative Example 7 also met the process requirements of the present invention, but the Mg treatment was excessive, and the inclusions in the steel during the casting process were mainly MgS and CaS. The stopper rod rose by +4.3 cm and fluctuated 23 times, and the water immersion inspection pass rate was only 30%.

[0039] The above comparison results show that the control proposed in the present invention can stably obtain MgO inclusions while avoiding the formation of CaS and MgS inclusions, significantly improving the casting performance and water immersion flaw detection pass rate of bearing steel.

[0040] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A production process for increasing the number of continuous casting furnaces for bearing steel based on magnesium treatment, characterized by: (1) The production process adopts BOF steelmaking - LF refining - RH vacuum treatment - CC continuous casting, in which ferrosilicon is added once in the steel-making process, and no ferrosilicon is added during the LF refining process. At the end of refining, the S content in the steel is less than 0.0020%; (2) The RH vacuum degree is required to be less than 30 Pa, the processing time is not less than 18 minutes, the TO content is required to be less than 5.5ppm after vacuum breaking, the acid-soluble aluminum content in the steel is controlled at 0.010%~0.025%, and the Mg-Si-Fe alloy cored wire is fed, wherein the composition and mass percentage of the Mg-Si-Fe alloy cored wire are Mg: 16~20%, Si: 38~42%, and the rest are Fe and unavoidable impurities, ensuring that the Mg content in the steel is 12~25ppms.

2. The production process for increasing the number of continuous casting furnaces for bearing steel based on magnesium treatment according to claim 1, characterized in that: Feed alloy cored wires of different lengths according to the actual acid-soluble aluminum content in the steel. The formula for calculating the feeding amount of alloy cored wire is: feeding length of cored wire = (15000~20000) × (acid-soluble aluminum content × molten steel weight) m; where acid-soluble aluminum content is mass fraction, %; molten steel weight, ton.

3. The production process for increasing the number of continuous casting furnaces for bearing steel based on magnesium treatment according to claim 1, characterized in that: The amount of refining slag in LF refining is 10~12kg / ton of steel, CaO / SiO2>8.0, Al2O3=25-30%, ensuring that S in the steel is less than 0.0020% after refining.

4. The production process for increasing the number of continuous casting furnaces for bearing steel based on magnesium treatment according to claim 1, characterized in that: The feeding speed of the Mg-Si-Fe alloy cored wire is 3.5~5.0m / s.

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