Low-cost and good-wear-resistance steel for grinding balls and production method of low-cost and good-wear-resistance steel

Abstract

The invention belongs to the technical field of metallurgy, and particularly relates to steel for grinding balls with low cost and good wear resistance and a production method of the steel, and the steel comprises the following chemical components in percentage by weight: 0.72%-0.80% of C, 0.70%-0.80% of Mn, 0.70%-0.95% of Si, 0.70%-0.90% of Cr, less than or equal to 0.025% of S, less than or equal to 0.030% of P, 0.025%-0.040% of Alt and the balance of Fe and inevitable impurities. According to the method, the influence of carbon and all alloy elements on hardenability, hardenability, thermal conductivity, heat resistance and linear expansion coefficient is comprehensively considered, and through low-cost alloy design and process design, the flaw detection qualification rate of base metal round steel is increased, meanwhile, the surface hardness and the average volume hardness of the grinding balls are increased, hollow grinding balls are reduced, the wear resistance of the grinding balls is improved, and the cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a grinding ball steel with low cost and good wear resistance and a production method thereof. Background Art

[0002] With the rapid development of the mining industry, cast iron balls are gradually being replaced by wear-resistant steel balls. The demand for grinding ball steel is increasing, and the comprehensive performance requirements of steel balls, such as wear resistance and toughness, are becoming increasingly higher. Therefore, not only are the raw materials required to have good internal quality, high hardenability and hardenability, but also the basic requirements of ensuring that the steel balls do not crack during quenching during the manufacturing process, and the ball breakage rate during service is no more than 1%, and the out-of-round rate is no more than 5%. At the same time, the ore crushing effect must be guaranteed and the grinding ball consumption must be low.

[0003] The chemical composition of the grinding ball steel produced by a steel plant is shown in Table 1. Although the chemical composition design takes into account the requirements of high surface hardness and average volume hardness of the grinding balls, it only meets the requirements of surface hardness of grinding balls above φ90mm being above 56HRC (YB / T091 requires ≥55HRC) and core hardness being above 50HRC (YB / T091 requires ≥46HRC). The wear resistance effect is not very ideal, and the silicon content is unstable when it is above 1.5%.

[0004] Table 1 Chemical composition design of the original grinding ball steel grade of a steel plant

[0005] The aforementioned chemical composition design includes a Si content as high as 1.75%, which dramatically reduces thermal conductivity. This makes controlling the heating rate after charging the steel mill and easily leads to breakage during the rolling process. Furthermore, the grinding ball manufacturing process is prone to hollow balls after quenching, which significantly inconveniences production. Ultrasonic flaw detection of the core of the raw material (round steel) used for grinding ball production results in a low yield (85%), and the finished grinding balls have a high rate of surface inspection failures (over 4% for hollow balls, etc.). Furthermore, the finished grinding balls exhibit low hardenability and hardenability, with low surface and core hardnesses of 58.7 HRC and 57 HRC, respectively, and an average volume hardness of 58 HRC. Overall, wear resistance is insufficient, but the alloy content is high, resulting in high cost. Summary of the Invention

[0006] The present invention aims to provide a low-cost, wear-resistant steel for grinding balls and a production method thereof. By comprehensively considering the effects of carbon and various alloying elements on hardenability, hardenability, thermal conductivity, thermal strength, and linear expansion coefficient, and through low-cost alloy design and process engineering, the core ultrasonic flaw detection pass rate of the raw material (round steel) for the grinding balls produced is increased to over 95%, the hollow ball ratio of the finished grinding balls is reduced to below 0.20%, the surface and core hardnesses are increased to over 61HRC and 58HRC, respectively, and the average volume hardness is increased to over 59HRC. This improves the wear resistance of the grinding balls while reducing costs.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: A low-cost steel for grinding balls with good wear resistance. The chemical composition of the steel is as follows by weight: C: 0.72%-0.80%, Mn: 0.70%-0.80%, Si: 0.70%-0.95%, Cr: 0.70%-0.90%, S≤0.025%, P≤0.030%, Alt: 0.025%-0.040%, and the remainder is Fe and unavoidable impurities.

[0008] Preferably, the chemical composition of the steel is as follows by weight: C: 0.75% to 0.80%, Mn: 0.72% to 0.78%, Si: 0.75% to 0.85%, Cr: 0.80% to 0.86%, S≤0.010%, P≤0.020%, Alt: 0.030% to 0.040%, and the rest is Fe and unavoidable impurities.

[0009] The specifications of round steel for grinding balls are φ95~110mm.

[0010] The JMatPro software calculates the end hardenability of the steel as J1.5 value is 62.0~64.4HRC; the J25 value is 54.0~55.8HRC.

[0011] A method for producing grinding ball steel with good wear resistance at low cost, the production process includes: converter smelting, deoxidation and alloying, LF refining, RH refining, continuous casting, steel rolling, and grinding balls, specifically including: Raw material conditions: molten iron Si content 0.15wt% to 0.55wt%; molten iron P content 0.065wt% to 0.110wt%; molten iron S content 0.010wt% to 0.050wt%; molten iron temperature 1270-1400°C. Scrap steel is self-circulating scrap steel.

[0012] 1) Converter smelting: a) Charging quantity: 110-115 tons of molten iron / furnace + 15-20 tons of scrap steel / furnace.

[0013] b) Oxygen supply conditions: oxygen supply pressure 0.80~0.85MPa, blowing time 13~18min.

[0014] c) The converter end point C ≥ 0.08%, P ≤ 0.015%, and the end point temperature is 1610 ~ 1650 ° C.

[0015] d) Slag formation: Lime and dust balls are added in batches in the early stage of blowing to ensure the early slagging and dephosphorization effects. Dust balls and medium stones are added in the middle and late stages of blowing according to the slag state and temperature.

[0016] e) Deoxidation and alloying: Using the aluminum ingot deoxidation process, aluminum ingots, ferrosilicon, silicon-manganese alloy, and high-carbon ferrochrome are added in sequence for alloying.

[0017] f) Top slag modification: Pre-melted calcium aluminate and refined lime are added to modify the top slag during steel tapping.

[0018] 2) LF Refining: Ensure good ladle top slag melting after refining. The refining temperature is 1493-1513°C. The basicity of the slag sample after argon and LF refining is 5.0-7.5. The power treatment time is 45-60 minutes, and the refining cycle is 110-125 minutes.

[0019] 3) RH refining: RH working vacuum degree ≤67Pa, holding time ≥12min, [H] in molten steel ≤2.0ppm; the discharge temperature for pouring or tank changing is 1540-1570℃ for the first furnace, and 1500-1521℃ for the second and subsequent furnaces.

[0020] 4) Continuous casting: During production, full protection pouring is adopted. The ladle casing is equipped with a sealing gasket and argon protection. When pouring, the tundish adopts argon replacement to prevent secondary oxidation of molten steel. The tundish is covered with low-carbon covering agent, composite covering agent and carbonized rice husk. The tundish liquid level is controlled above 800mm. The wide range of the crystallizer water flow is 80-86m 3 / h, narrow side 68~74m 3 / h, secondary cooling water volume ratio 0.11~0.13L / kg; crystallizer electromagnetic stirring current 370~410A, frequency 1.4~1.6Hz; end electromagnetic stirring current 430~470A, frequency 5.2~5.8Hz; superheat 17~30℃, pulling speed 0.38~0.40m / min; using soft reduction technology, the total reduction is 14~20mm.

[0021] The size of continuous casting billet is 390mm*510mm.

[0022] 5) Steel rolling: When the billet is put into the furnace, the burner is not turned on in the heating section 1 (preheating section), the temperature of each section of the furnace is 720-780℃ for heating section 1, 980-1050℃ for heating section 2, 1190-1250℃ for heating section 3, and 1180-1240℃ for soaking section. The time in the furnace is not less than 450 minutes. The target temperature for starting rolling is 1070-1150℃, the temperature for entering continuous rolling is not less than 1000℃, and the target temperature for finishing rolling is 950-990℃. After sawing, the cooling bed advances quickly, and after collection, it enters the slow cooling pit for slow cooling. The pit cooling time is not less than 36 hours, and the temperature out of the pit is not more than 100℃. After leaving the pit, it is straightened and internal ultrasonic flaw detection is performed.

[0023] 6) Grinding balls: a) Rolling: The grinding balls are produced by medium frequency induction heating, with the heating temperature set at 1080-1120°C, the rolling temperature at 1060-1100°C, and the ball rolling speed at 1.49-1.51 m / min, with a constant speed throughout the entire process.

[0024] b) Quenching: Use residual temperature quenching. According to the phase change point calculated by JMatPro software, the water inlet temperature is controlled at 790-810℃. The quenching medium is turbid water. The water temperature is controlled between 10-35℃. The quenching time in water is 5.0-6.0min. The surface temperature of the steel ball is 40-60℃ when it comes out of water. The surface temperature of the steel ball is 70-100℃ 14-15 minutes after it comes out of water.

[0025] c) Tempering: Tempering in an online pit furnace, with a tempering temperature not lower than 180°C and keeping warm for 4 to 6 hours.

[0026] The reasons for designing chemical elements in the present invention are as follows: 1) [C] content: Considering the high surface hardness, that is, the hardenability requirement of the steel ball product, the chemical composition C content is designed. Considering that the hardness increase is no longer obvious when [C] is above 0.80%, and the higher the carbon content, the more obvious the segregation is, the design carbon content target is 0.72%~0.80%.

[0027] 2) [Si], [Mn], and [Cr] Content: Considering the rapid heating characteristics of high-frequency induction heating during steel ball manufacturing, the alloy composition must be rationally designed to ensure crack-free quenching and high core hardness after quenching and tempering. The effects of [Si], [Mn], and [Cr] on thermal conductivity, decarburization, hardenability, and the critical cooling rate of pearlite steel should also be considered. The process window for slab heating, ball heating, and quenching and tempering should be maximized to minimize slab fracture and ball hollowing. The target values for [Si] and [Cr] are 0.70% to 0.80% and 0.70% to 0.90%, respectively, with preferred values of 0.72% to 0.78% and 0.80% to 0.86%, respectively. Calculations using JMatPro software show significant improvements in thermal conductivity, linear expansion coefficient, and hot strength.

[0028] 3) [Al] content optimization: Considering the impact of vacuum process on the reduction of N content, the original Al content target is increased to 0.025%~0.040%, and particularly preferably Al: 0.030%~0.040%, to ensure the amount of aluminum nitride formed under low nitrogen (30ppm) conditions and uniformly refine the grains.

[0029] 4) Calculation of hardenability, etc.: According to the above target values, the critical quenching diameter DI value can reach 114mm, and the wear-resistant steel ball with a diameter of 100mm can be fully hardened. Figure 1 、 Figure 3The good thermal conductivity reduces both thermal stress and structural stress, making the steel ball less prone to cracking. Calculated using JMatPro software, the hardenability J1.5 and J25 values increased to 64.4HRC and 55.8HRC, respectively. The critical cooling rate of 4°C / s remained unchanged, while the Ms value (199°C) and Mf value (approximately 70°C) decreased. These results indicate that the downstream production process fully meets the requirements for quenching with a water cooling medium at 10-35°C (the actual cooling rate of the quenching medium water should be no less than 100°C / s).

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. The overall alloy content of the present invention is low, the thermal conductivity is good, and the heating rate requirement is wide (see JMatPro software Figure 1 and Figure 3 ),from Figure 1 and Figure 3 In contrast, the lowest point of the thermal conductivity of the present invention moves toward the low-temperature side. Since the temperature is low and the strength is high, it is beneficial to alleviate internal cracks, improve production efficiency, prevent billet breakage, and reduce hollow ball phenomena.

[0031] 2. The carbon content is significantly increased, the surface hardness of the steel ball is increased, and the wear resistance of the steel ball is improved.

[0032] 3. The present invention comprehensively considers the effects of carbon and various alloying elements on hardenability, hardenability, thermal conductivity, thermal strength, and linear expansion coefficient, thereby eliminating the problem of billet fracture during the heating process of the cold-charging furnace when rolling large square billets (390mm*510mm) into round steel for φ100mm grinding balls. The qualified rate of ultrasonic flaw detection of the core of the raw material (round steel) for the produced grinding balls is increased to over 95%, the proportion of hollow balls in the finished grinding balls is reduced to below 0.20%, the surface and core hardness are increased to over 61HRC and 58HRC respectively, and the average volume hardness is increased to over 59HRC.

[0033] 4. Increasing carbon and reducing silicon can reduce alloy costs by more than RMB 30 per ton; the qualified rate of round steel flaw detection is expected to increase by 10%, which can reduce costs by RMB 20 / ton. Therefore, without considering the impact of improved wear resistance of grinding balls and reduced grinding ball consumption on cost reduction, the cost reduction is expected to reach RMB 50 / ton. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a graph showing the thermal conductivity of grinding balls produced using existing technology.

[0035] Figure 2 It is a linear expansion coefficient curve of grinding balls produced by existing technology.

[0036] Figure 3 It is a thermal conductivity curve diagram of the grinding balls produced by the present invention.

[0037] Figure 4It is a linear expansion coefficient curve diagram of the grinding balls produced by the present invention. DETAILED DESCRIPTION

[0038] The specific embodiments of the present invention will be further described below in conjunction with examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0039] The production process of the embodiment of the present invention is as follows: molten iron - converter smelting - deoxidation and alloying - LF refining - RH refining - continuous casting - slow cooling - delivery - charging and heating - cogging - continuous rolling - sawing - cooling and collection - loading and slow cooling - delivery - straightening - flaw detection - packaging - weighing - delivery. The production process of grinding balls is as follows: heating - rolling - quenching - tempering - inspection - delivery.

[0040] The raw material conditions for converter smelting are as follows: molten iron Si content 0.32 wt %; molten iron P content 0.079 wt %; molten iron S content 0.032 wt %; molten iron temperature 1310° C. The oxygen supply conditions for converter smelting are: oxygen supply pressure 0.80 MPa, blowing time 15 min.

[0041] The chemical composition design of the grinding ball steel in the embodiment is shown in Table 2, the production process parameters of the grinding ball steel in the embodiment are shown in Tables 3-5, the process parameters of the grinding balls in the embodiment are shown in Table 6, and the product performance of the grinding balls in the embodiment are shown in Table 7.

[0042] Table 2 Chemical composition of steel for grinding balls in the embodiment (mass fraction %)

[0043] Table 3 Smelting process of grinding ball steel

[0044] LF refining parameters in this example: refining temperature was 1503°C. Post-argon and LF furnace refining lime consumption was approximately 3.75 kg / t, pre-melted calcium aluminate consumption was approximately 10.0 kg / t, calcium carbide consumption was approximately 1.20 kg / t, and aluminum pellets consumption was approximately 0.19 kg / t. The final slag was white slag. The LF refining slag sample was designed to contain approximately 52% CaO, approximately 8% SiO₂, approximately 30% Al₂O₃, and approximately 0.4% FeO, resulting in a basicity R of approximately 6.5. The power treatment time was 55 minutes, and the refining cycle was 120 minutes.

[0045] Table 4 Continuous casting process of grinding ball steel

[0046] Table 5 Rolling process of grinding ball steel

[0047] Table 6 Example grinding process

[0048] Table 7 Performance indicators of the embodiment products (φ100mm) .

Claims

1. A low-cost grinding ball steel with good wear resistance, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.72%~0.80%, Mn: 0.70%~0.80%, Si: 0.70%~0.95%, Cr: 0.70%~0.90%, S≤0.025%, P≤0.030%, Alt: 0.025%~0.040%, and the rest is Fe and unavoidable impurities.

2. The low-cost and wear-resistant grinding ball steel according to claim 1, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.75%~0.80%, Mn: 0.72%~0.78%, Si: 0.75%~0.85%, Cr: 0.80%~0.86%, S≤0.010%, P≤0.020%, Alt: 0.030%~0.040%, and the rest is Fe and unavoidable impurities.

3. The low-cost and wear-resistant grinding ball steel according to claim 1, characterized in that: The specifications of round steel for grinding balls are φ95~110mm.

4. A low-cost grinding ball steel with good wear resistance according to any one of claims 1 to 3, characterized in that: The end hardenability J1.5 value of the steel is 62.0~64.4HRC; the J25 value is 54.0~55.8HRC.

5. A method for producing the low-cost, wear-resistant grinding ball steel according to any one of claims 1 to 4, the production process comprising: Converter smelting, deoxidation alloying, LF refining, RH refining, continuous casting, steel rolling, grinding balls, characterized by specifically including: 1) Converter smelting: converter end point C ≥ 0.08%, P ≤ 0.015%, end point temperature is 1610 ~ 1650 ° C; 2) LF refining: the refining temperature is 1493-1513℃; the basicity R of the slag sample after argon and LF furnace refining is 5.0-7.5; 3) RH refining: RH working vacuum degree ≤ 67Pa, holding time ≥ 12min, the outlet temperature of the first furnace is 1540-1570℃, and the second and subsequent furnaces are 1500-1521℃; 4) Steel rolling: The furnace temperature of each section is 720-780℃ for heating section 1, 980-1050℃ for heating section 2, 1190-1250℃ for heating section 3, and 1180-1240℃ for soaking section. The furnace time is not less than 450 minutes. The starting rolling temperature is 1070-1150℃, the continuous rolling temperature is not less than 1000℃, and the finishing rolling temperature is 950-990℃. The pit cooling time is not less than 36 hours, and the pit exit temperature is not more than 100℃. 5) Grinding balls: a) Rolling: The grinding balls are produced by medium frequency induction heating, with the heating temperature set at 1080-1120°C, the rolling temperature at 1060-1100°C, and the ball rolling speed at 1.49-1.51 m / min, with a uniform speed throughout the entire process. b) Quenching: adopt residual temperature quenching, water inlet temperature 790~810℃, water temperature controlled at 10~35℃, quenching time in water 5.0~6.0min, surface temperature of steel ball when out of water 40~60℃, surface temperature of steel ball 70~100℃ 14~15min after out of water; c) Tempering: The tempering temperature should not be lower than 180℃ and the temperature should be kept for 4 to 6 hours.

6. The method for producing grinding ball steel with low cost and good wear resistance according to claim 5, characterized in that: The raw material conditions for converter smelting are: molten iron Si content 0.15wt% to 0.55wt%; molten iron P content 0.065wt% to 0.110wt%; molten iron S content 0.010wt% to 0.050wt%; molten iron temperature 1270 to 1400°C.

7. The method for producing grinding ball steel with low cost and good wear resistance according to claim 5, characterized in that: The oxygen supply conditions for the converter smelting are: oxygen supply pressure 0.80-0.85 MPa, and blowing time 13-18 minutes.

8. The method for producing grinding ball steel with low cost and good wear resistance according to claim 5, characterized in that: The continuous casting includes: a crystallizer electromagnetic stirring current of 370 to 410 A and a frequency of 1.4 to 1.6 Hz; an end electromagnetic stirring current of 430 to 470 A and a frequency of 5.2 to 5.8 Hz; a superheat of 17 to 30° C. and a casting speed of 0.38 to 0.40 m / min; and a soft reduction technology with a total reduction of 14 to 20 mm.

9. A method for producing grinding ball steel with low cost and good wear resistance according to claim 5 or 8, characterized in that: The size of continuous casting billet is 390mm*510mm.

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