Smelting method of high-cleanliness gear steel

Abstract

The invention discloses a smelting method of high-cleanliness gear steel, and belongs to the technical field of special steel smelting. According to the method, the BOF-LF-RH-CCM technological process is adopted, the cleanliness of molten steel is optimally controlled through the whole-process technology, the end point carbon and phosphorus content is controlled through converter smelting, and aluminum deoxidation and synthetic refining slag washing are conducted; in LF refining, high-alkalinity low-oxidability white slag (FeO + MnO < = 0.8%) is produced through combination of precipitation and diffusive deoxidation; the vacuum degree of RH vacuum treatment is controlled to be smaller than or equal to 67 Pa and kept larger than or equal to 15 min; carrying out calcium treatment on silicon-calcium wire-feeding denatured inclusions; and an alkaline covering agent and pre-melting type casting powder are adopted for whole-course protection during continuous casting. The process is high in compatibility, the purity and comprehensive performance of the gear steel can be remarkably improved, and the process is suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of special steel smelting, and particularly relates to a smelting method of high-cleanliness gear steel SAE8620H, which is suitable for the fields of automobile and mechanical industry with strict requirements on the cleanliness and comprehensive performance of gear steel. BACKGROUND

[0002] With the development of automobile and mechanical industry, higher requirements are put forward for the strength and toughness, wear resistance and fatigue resistance of gear steel. The cleanliness of SAE8620H as Cr-Ni-Mo gear steel directly affects the service life and reliability of gears. However, in the traditional smelting process, it is difficult to control the non-metallic inclusions (such as Al2O3) and harmful elements (P, S, N, H, O) in the steel, which easily leads to gear fatigue failure. Therefore, it is of great significance to develop a smelting process which can effectively reduce the content of harmful elements and improve the cleanliness of molten steel. SUMMARY

[0003] The purpose of the present application is to provide a smelting method of high-cleanliness gear steel SAE8620H, which reduces the content of non-metallic inclusions and harmful elements in the steel through process optimization, improves the cleanliness and comprehensive performance of gear steel, and meets the demand of high-end manufacturing industry for high-quality gear steel.

[0004] To solve the above technical problems, the present application adopts the following technical scheme:

[0005] The smelting method of high-cleanliness gear steel of the present application adopts the process flow of BOF→LF→RH→CCM, and specifically includes the following steps:

[0006] 1) Converter smelting: control the end point temperature to be 1600-1630℃, [C] to be 0.06-0.10%, [P] to be ≤0.013%, add 1.0-1.3kg / t aluminum cake for deoxidation when tapping, and simultaneously add 4-5kg / t active lime and 3.0-4.0kg / t pre-melted synthetic refining slag for slag washing, and control the back phosphorus to be ≤0.002% by blocking slag;

[0007] 2) LF refining: the acid-melted aluminum [Al] is 0.015-0.040%, which is adjusted to 0.035-0.040% by feeding aluminum wire, white slag is made by using reducing agent and maintained for ≥15min, the final slag basicity R is 5.0-8.0, and the composition includes CaO 52-56%, SiO2 6-10%, MgO 3-8%, and Al2O3 26-33%;

[0008] 3) RH vacuum treatment: the vacuum degree is ≤67Pa, the holding time is ≥15min, [N] is controlled to be ≤45×10 -6 , and [H] is controlled to be ≤1.3×10 -6 ;

[0009] 4) Calcium treatment: feeding calcium-silicon wire ≤100 m, soft-blowing argon gas ≥10 min, bottom-blowing flow 30-60 NL / min;

[0010] 5) Continuous casting: superheating degree 15-25℃, using alkaline covering agent and pre-melted type of protective slag for full-process protection pouring, cooperating with M+F electromagnetic stirring: M-EMS stirring frequency controlled at 4-6 Hz, current intensity 250-300 A, stirring time throughout the whole pouring of the liquid steel in the tundish, forming stable transverse flow, promoting the floating of inclusions and the expansion of isothermal zone; F-EMS stirring frequency controlled at 3-5 Hz, current intensity 300-350 A, forming spiral flow in the crystallizer, improving the uniformity of the solidification shell, reducing segregation and center porosity; the joint stirring start-stop point is controlled through the linkage control of the automatic control system of the continuous casting machine, ensuring that the stirring action covers the whole pouring period, optimizing the internal quality of the casting blank; controlling the low-multiple center porosity of the casting blank ≤1.5 level.

[0011] Further, 4.5 kg / t of active lime is added.

[0012] Further, the reducing agent at least includes one of CaC2 and aluminum pellets.

[0013] Further, the gear steel has the following mass percentage chemical components: C 0.18-0.22%, Si 0.20-0.30%, Mn 0.60-0.80%, P ≤0.015%, S ≤0.003%, Cr 0.40-0.50%, Al 0.020-0.040%, Mo 0.020-0.040%, Ni 0.40%-0.70%, and the rest is Fe and inevitable impurities.

[0014] Further, the CaO content of the alkaline covering agent in the tundish during the continuous casting process is ≥35%, the viscosity of the crystallizer protective slag at 1300℃ is 0.5-0.6 Pa·s, and the half-sphere temperature is 1100-1150℃.

[0015] Further, through full-process process optimization, the [P] in the finished steel is ≤0.015%, the [S] is ≤0.003%, the [N] is ≤45×10 -6 , the [H] is ≤1.3×10 -6 , the T[O] is ≤12×10 -6 , the average value is 7.6×10 -6 , and the total of harmful elements is ≤260×10 -6 , which is far more than the requirements of ASTM A304 standard.

[0016] Further, based on the existing BOF-LF-RH-CCM process, without large-scale equipment modification, it is suitable for industrial production and has significant economic benefits.

[0017] Design principles of each chemical component and element:

[0018] Carbon (C) 0.18% to 0.22%: Carbon is the core element for strengthening the matrix. If the content is too low, it will lead to insufficient strength, and if it is too high, the toughness will decrease. Ensure that the carbon content of the finished product is stable in the low-carbon range to meet the requirements of the core toughness of gear steel during carburizing process.

[0019] Silicon (Si) 0.20% to 0.30%: Silicon, as a deoxidizer, can effectively reduce the initial oxygen content of molten steel when the content is ≥0.20%, and can also improve the strength through solid solution strengthening. Controlling the upper limit to ≤0.30% can avoid the increase of steel brittleness caused by excessive silicon, especially the decrease of low-temperature impact toughness.

[0020] Manganese (Mn) 0.60% to 0.80%: Manganese can expand the austenite region and improve the hardenability, ensuring that the hardness gradient between the surface and the core of the gear after carburizing is reasonable. At the same time, it can form MnS with sulfur to reduce the tendency of thermal embrittlement. The content of [S] needs to be controlled to ≤0.003% to avoid excessive MnS inclusions.

[0021] Phosphorus (P) ≤0.015%: Phosphorus easily segregates to cause grain boundary embrittlement, especially when the gear is subjected to alternating loads, it is easy to cause fatigue cracks. Therefore, its content is strictly limited to ≤0.015%.

[0022] Sulfur (S) ≤0.003%: High sulfur content easily forms long strip-shaped MnS inclusions, which become fatigue sources. Through LF high basicity slag (R = 5.0 to 8.0) desulfurization, combined with calcium treatment to change the sulfide into short rod or spherical shape, reduce the damage to the continuity of the matrix, ensure that [S] is ≤0.003%, meet the high cleanliness requirements.

[0023] Chromium (Cr) 0.40% to 0.50%: Chromium forms carbides (such as Cr23C6) to improve surface hardness and wear resistance; at the same time, it improves the hardenability, so that the gear after carburizing obtains uniform martensite structure, and the core maintains toughness.

[0024] Aluminum (Al) 0.020% to 0.040%: Aluminum, as a strong deoxidizer, preferentially forms Al2O3 inclusions, which are absorbed by the LF refining slag; at the same time, it inhibits austenite recrystallization in the form of AlN to refine the grains.

[0025] Molybdenum (Mo) 0.15% to 0.25%: Molybdenum can effectively inhibit the grain boundary embrittlement of gear steel during tempering, especially suitable for gear parts that need high-frequency induction heating; combined with chromium and nickel, it can still obtain sufficient hardenability at a lower carbon content, ensuring the core strength of large-size gears.

[0026] Ni 0.40%~0.70%: nickel expands austenite zone, improves toughness and low temperature impact performance of the steel, adapts to the anti-fracture requirement of the gear under complex load; meanwhile, enhances corrosion resistance, prolongs service life in marine or humid environment; cooperates with chromium and molybdenum, promotes formation of bainite or martensite structure, avoids insufficient strength caused by single ferrite.

[0027] Compared with the prior art, the beneficial technical effects of the present application are:

[0028] (1) Through whole-process process optimization, [P]≤0.015%, [S]≤0.003%, [N]≤45×10 -6 , [H]≤1.3×10 -6 , T[O]≤12×10 -6 (average value 7.6×10 -6 ) in the finished steel, and the total of harmful elements is ≤260×10 -6 , which is far more than the requirement of ASTM A304 standard.

[0029] (2) The various non-metallic inclusions (A, B, C, D, Ds) in the steel are ≤1.0 grade, effectively reducing the origin point of fatigue damage, and improving the fatigue resistance of the gear steel.

[0030] (3) Based on the existing BOF-LF-RH-CCM process, without large-scale equipment modification, it is suitable for industrial production, and the economic benefit is remarkable. DETAILED DESCRIPTION

[0031] The smelting method of the high-cleanliness gear steel of the present application is further described in detail below.

[0032] Embodiment: The present embodiment is one preferred embodiment in various embodiments of the present application.

[0033] Further, converter smelting: the raw material is 80% desulfurized molten iron ([S]=0.003%) + 20% scrap steel, the end point temperature is 1620℃, [C]=0.08%, [P]=0.011%. 1.2kg / t aluminum cake is added for deoxidation, 4.5kg / t active lime + 3.5kg / t synthetic refining slag is used for slag washing, and the phosphorus is 0.001% after blocking slag.

[0034] Further, LF refining: [Al]=0.025% at the inlet, adjusted to [Al]=0.038% by feeding aluminum wire, CaC2 and aluminum pellets are added for diffusion deoxidation, white slag (FeO+MnO=0.6%) is formed and maintained for 20min, and the final slag composition is: CaO 54%, SiO2 8%, MgO 5%, Al2O3 30%.

[0035] Further, RH vacuum degassing: vacuum degree ≤67 Pa, holding time 18 min, [N] = 38 x 10 -6 , [H] = 1.1 x 10 -6 .

[0036] Further, calcium treatment: feeding of 80 m of silicon-calcium wire, soft argon blowing for 12 min, and degeneration rate of inclusions ≥90%. Further, continuous casting: superheat 20°C, casting speed 1.3 m / min, alkaline covering agent and protective slag in tundish for whole process protection, and center porosity of casting blank ≤1.0 grade.

[0037] Product testing: chemical composition: C = 0.20%, Si = 0.25%, Mn = 0.70%, P = 0.012%, S = 0.002%, Cr = 0.45%, Al = 0.028%. Cleanliness: T[O] = 8 x 10 -6 , non-metallic inclusions: class A ≤0.5 grade, class B ≤0.5 grade, class C 0 grade, class D ≤1.0 grade, class Ds ≤1.0 grade, fully meeting the ASTM A304 standard.

[0038] Comparative Example 1:

[0039] No CaC2, aluminum pellets or other reducing agents were added for diffusion deoxidation in the refining step, and only aluminum cake was used for sedimentation deoxidation, with the final slag FeO + MnO = 2.5% (significantly higher than ≤0.8% in the example), and no white slag was formed, and the refining time was only 10 min. Except for this, the other processes were the same as in the example.

[0040] Test results: [N] = 58 x 10 -6 (over standard, standard ≤45 x 10 -6 ), [H] = 1.5 x 10 -6 (near upper limit), T[O] = 9 x 10 -6 (qualified but higher than in the example). Class A (sulfide) fine line 1.5 grade, class D (spherical oxide) fine line 1.5 grade, and the total amount of non-metallic inclusions increased.

[0041] Comparative Example 2:

[0042] Vacuum degree ≤67 Pa but holding time was only 8 min (18 min in the example), and the other processes were the same as in Example 1.

[0043] Test results: [N] = 58 x 10 -6 (over standard, standard ≤45 x 10 -6 ), [H] = 1.5 x 10 -6 (near upper limit), T[O] = 9 x 10 -6Class A (sulfide) fine line 1.5 level, class D (spherical oxide) fine line 1.5 level, and the total amount of non-metallic inclusions increased.

[0044] Summary of the comparison between the examples and the comparative examples

[0045]

[0046] From the comparison between the examples and the comparative examples, it can be obviously seen that:

[0047] (1) In the comparative example 1, the slag oxidizability was not controlled, which led to secondary oxidation of the molten steel, increase of T[O], aggregation of Al2O3 inclusions and ineffective absorption, and decrease of desulfurization efficiency (high basicity but strong oxidizability of the slag, and decrease of sulfur distribution ratio Ls), resulting in excessive [S].

[0048] (2) Insufficient vacuum holding time led to insufficient removal of [N] (diffusion of nitrogen in the molten steel needs sufficient time), and insufficient molten steel circulation limited the calcium treatment effect, incomplete modification of inclusions, and part of Al2O3 not converted into low-melting-point calcium aluminate, resulting in increase of the number of residual inclusions.

[0049] The above examples only describe the preferred modes of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements of the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for smelting high-purity gear steel, characterized in that, The process flow adopted is BOF→LF→RH→CCM, which specifically includes the following steps: 1) Converter smelting: Control the final temperature to 1600~1630℃, [C] 0.06~0.10%, [P] ≤0.013%, add 1.0~1.3kg / t aluminum cake for deoxidation when tapping steel, and add 4-5kg / t quicklime and 3.0~4.0kg / t pre-melted synthetic refining slag for washing. Control the phosphorus return to ≤0.002% when blocking slag. 2) LF refining: The aluminum [Al] content of the molten aluminum entering the station is 0.015-0.040%, which is adjusted to 0.035-0.040% by the aluminum feeding line. A reducing agent is used to make white slag and maintain it for ≥15 minutes. The final slag basicity R = 5.0-8.0, and the composition includes CaO 52-56%, SiO2 6-10%, MgO 3-8%, and Al2O3 26-33%. 3) RH vacuum treatment: vacuum degree ≤ 67 Pa, holding time ≥ 15 min, control [N] ≤ 45 × 10 -6 [H]≤1.3×10 -6 ; 4) Calcium treatment: Feed in ≤100m of silicon-calcium wire, gently blow argon gas for ≥10min, and bottom blow flow rate of 30~60NL / min; 5) Continuous casting: Superheated to 15-25℃, alkaline covering agent and pre-melted protective slag are used for full-process protection during casting, combined with M+F electromagnetic stirring, to control the low-magnification center porosity of the billet to ≤1.5 grade.

2. The smelting method for high-purity gear steel according to claim 1, characterized in that, Add 4.5 kg / t of active lime.

3. The smelting method for high-purity gear steel according to claim 1, characterized in that, The reducing agent includes at least one of CaC2 and aluminum pellets.

4. The smelting method for high-purity gear steel according to claim 1, characterized in that, The chemical composition of the gear steel by mass percentage is as follows: C 0.18-0.22%, Si 0.20-0.30%, Mn 0.60-0.80%, P≤0.015%, S≤0.003%, Cr 0.40-0.50%, Al 0.020-0.040%, Mo 0.020-0.040%, Ni 0.40%-0.70%, with the remainder being Fe and unavoidable impurities.

5. The smelting method for high-purity gear steel according to claim 1, characterized in that, The alkaline covering agent in the tundish during continuous casting has a CaO content of ≥35%, the viscosity of the mold flux at 1300℃ is 0.5~0.6Pa·s, and the hemispherical temperature is 1100~1150℃.

6. The smelting method for high-purity gear steel according to claim 1, characterized in that, Through process optimization, the finished steel contains [P] ≤ 0.015%, [S] ≤ 0.003%, and [N] ≤ 45 × 10⁻⁶. -6 [H]≤1.3×10 -6 T[O]≤12×10 -6 The average value is 7.6 × 10 -6 The total amount of harmful elements is ≤260×10 -6 It far exceeds the requirements of the ASTM A304 standard.

7. The smelting method for high-purity gear steel according to claim 1, characterized in that, Based on the existing BOF-LF-RH-CCM process, no large-scale equipment modification is required, making it suitable for industrial production and yielding significant economic benefits.

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