Zirconium corundum abrasive grains with high SiO2 content

Abstract

The present invention relates to a zirconium corundum abrasive grain based on Al2O3 and ZrO2 melted in an electric arc furnace, comprising: -52% to 62% by weight of Al2O3; -35.0% to 45.0% by weight of ZrO2 (+HfO2), wherein at least 80% by weight of the total ZrO2 is in the tetragonal and / or cubic high-temperature phase, based on the total ZrO2 content; more than -0.8% by weight of Si compounds expressed as SiO2; -0.03% to 0.5% by weight of carbon; -0.5% to 10% by weight of additives; and less than -3% by weight of raw material-related impurities, and wherein the raw material base of the abrasive grain comprises aluminum oxide, baddeleyite, and zircon sand.

Description

Technical Field

[0001] The present invention relates to abrasive grains based on Al2O3 and ZrO2 melted in an electric arc furnace, with an Al2O3 content of 52% to 62% by mass and a ZrO2(+HfO2) ratio of 35% to 45% by mass, and the raw material base of the abrasive grains includes aluminum oxide, baddeleyite, and zircon sand. By definition, the term baddeleyite includes all natural and artificial ZrO2 concentrates with a ZrO2 content of at least 96% by mass.

Background Art

[0002] Abrasive grains based on zirconium corundum have been known for many years and have been successfully used, especially as grinding wheels or abrasive papers for cutting high-alloy steels. In addition to the fine crystal structure, the proportion of the high-temperature phase of zirconium oxide has a great influence on the performance of the abrasive grains. This is particularly true for so-called eutectic zirconium corundum, which preferably contains 35 to 50 weight percent of zirconium oxide in addition to aluminum oxide and other oxides present as impurities or intentionally introduced additives. Therefore, conventionally, attempts have been repeatedly made to improve the performance of zirconium corundum by refining the structure and / or increasing the proportion of the high-temperature phase. The structure can be improved by efficiently and rapidly quenching the molten liquid, but a high proportion of the high-temperature phase can be achieved mainly by targeted use of stabilizers, and titanium oxide and / or yttrium oxide are often used as stabilizers for the high-temperature phase of zirconium oxide. Zirconium oxide exists in three different phases. The monoclinic phase, stable at room temperature, transforms into the tetragonal phase at a temperature of approximately 800-1200°C, which remains stable up to approximately 2300°C, after which it transforms into the cubic phase. The above temperatures apply to pure zirconium oxide. In mixtures or doped materials, the temperature changes. The reversible phase transformation is accompanied by a volume change, with the tetragonal high-temperature phase having the smallest volume. The transition from tetragonal to the monoclinic phase, which has the largest volume, is associated with a volume increase of 4.5%. Those skilled in the art will explain the favorable effect of the high-temperature phase on abrasive grains by the fact that the heat generated during grinding causes a phase transformation from tetragonal to monoclinic, and the increase in volume generates stress accompanied by the formation of microcracks due to the favorable fracture of small fragments, thereby forming a new cutting edge. This process is often called self-polishing.

[0003] U.S. Patent No. 5,525,135 (European Patent No. 0595,081) describes abrasive grains based on zirconium corundum, in which more than 90 mass percent zirconium oxide is in a tetragonal high-temperature phase. In this case, titanium oxide is added in the presence of carbon as a reducing agent, and the high-temperature phase is stabilized by rapidly cooling the molten material. As a result, it is presumed that the titanium compound, reduced in the form of a suboxide, stabilizes the high-temperature phase of zirconium oxide. U.S. Patent No. 7,122,064 (European Patent No. 1,341,866) relates to abrasive grains based on zirconium corundum, in which the high-temperature phase of zirconium oxide is similarly stabilized with a reduced titanium compound. The abrasive grains described in this document have a silicon compound content expressed as 0.2 to 0.7 mass percent SiO2. The addition of SiO2 significantly reduces the stabilizing effect of the reduced titanium compound, but at the same time, the viscosity of the molten material is greatly reduced, making it easier to rapidly cool the molten material by pouring the liquid material between the metal plates. This rapid cooling has a good effect on the structure of the finished abrasive grains, and this method makes it possible to achieve a particularly fine crystalline and uniform structure, which, in addition to the high proportion of the high-temperature phase of zirconium oxide, is another important criterion for determining the quality of the product. U.S. Patent No. 4,457,767 describes zirconium corundum abrasive grains containing 0.1 to 2 mass percent yttrium oxide, where yttrium oxide is used as a stabilizer for the high-temperature phase of zirconium oxide. The stabilizing effect of Y2O3 on the high-temperature phase of zirconium oxide is known to be more pronounced than that of reduced TiO2, and relatively small amounts of Y2O3 must be used to obtain a comparable proportion of the high-temperature phase.

[0004] Abrasive grains based on zirconium corundum are one of the most important conventional abrasive grains for steel processing, and significant efforts are being made worldwide to further improve the performance of these abrasive grains. However, it is believed that simply increasing the proportion of the high-temperature phase will not yield sufficient performance improvements. International Publication No. 2011 / 141037 describes grinding tests using zirconium corundum abrasive grains, some of which contain only the high-temperature phase of zirconium oxide. However, the improvement in grinding performance is not clear compared to abrasive grains with a high-temperature phase of approximately 90 mass percent of the total proportion of zirconium oxide. In contrast, International Publication No. 2011 / 141037 is the first to consistently distinguish between the tetragonal high-temperature phase and the cubic high-temperature phase, describing the optimization of grinding performance when the cubic high-temperature phase contains more than 20% by mass of zirconium oxide relative to the total proportion of zirconium oxide, and the tetragonal high-temperature phase contains more than 50% by mass of zirconium oxide. This is achieved by using Y2O3 and TiO2 as stabilizers in the presence of a small amount of SiO2 as a flux. U.S. Patent Application Publication No. 2012 / 0186161 describes abrasive grains based on fused eutectic zirconium corundum, which have a proportion of 60-90 mass percent of tetragonal zirconium oxide phase relative to the total proportion of zirconium oxide. The phase distribution with a relatively low proportion of the tetragonal phase is achieved by a chemical composition using yttrium oxide and titanium oxide in the presence of SiO2 with a Y2O3 / SiO2 ratio of 0.8-2.0 as a stabilizer for the high-temperature phase. Due to its lower toughness, the product is said to be particularly suitable for machining alloy steels with low contact pressure. Since the self-polishing of the abrasive grains occurs under relatively mild conditions, thermal damage to the workpiece can be avoided, while simultaneously achieving a high removal rate.

[0005] Generally, the SiO2 content in zirconium corundum abrasive grains is always viewed critically because SiO2 limits the stabilizing effect of additives or hinders the stabilization of the high-temperature phase of ZrO2. Therefore, all high-performance eutectic zirconium corundum abrasive grains described in the prior art have an SiO2 content of less than 0.8% by mass. Consequently, conventionally, relatively pure raw materials based on Al2O3 and ZrO2 have always been used to avoid excessive SiO2 contamination. Mainly, pure aluminum oxide and baddeleyite were used, along with stabilizing additives such as TiO2 and Y2O3, and silica sand or zircon sand as a small source of SiO2 necessary to improve the fluidity of the molten liquid. Recently, due to the limited resources of natural baddeleyite, artificially produced ZrO2 concentrate has also been used.

[0006] Furthermore, with the aim of optimizing the production of eutectic zirconium corundum abrasive grains, attempts were made to use less expensive raw materials. For example, a series of tests were conducted in which zircon sand, which had previously only been used as an SiO2 source, was directly used as a ZrO2 raw material for zirconium corundum, in addition to baddeleyite. As expected, the proportion of SiO2 in the product increased, and in most cases, this also led to the expected degradation of the product. However, surprisingly, when stabilized with a combination of TiO2 and Y2O3, the amount of zircon sand could be tripled without degrading the quality of the product. For those skilled in the art, a high proportion of SiO2 was previously synonymous with product degradation, so the raw material composition of the molten material was changed, and quartz was used in addition to zircon sand as an SiO2 source, and tests were frequently repeated and adjusted as needed. It was shown that when the use of zircon sand as a raw material source was increased and ZrO2 was stabilized with a mixture of TiO2 and Y2O3 in a ratio of 2:1 to 4:1, the proportion of SiO2 in the product increased, but it was found that even when the proportion of SiO2 in the product exceeded 1 mass%, no adverse effects on product quality were observed. On the other hand, in comparative tests using the standard formulation as raw material and adding silica sand to increase the SiO2 content, it was found that if the SiO2 content in the product exceeded 0.6% by mass, the product quality always deteriorated significantly. Product deterioration was also consistently observed when the high-temperature phase of zirconium oxide was stabilized using TiO2 or Y2O3 alone, or when the ratio of TiO2 to Y2O3 was other than 2:1 to 4:1. [Overview of the Initiative]

[0007] Therefore, the subject of the present invention relates to abrasive grains based on Al2O3 and ZrO2, which are melted in an electric arc furnace and have a content of 52% to 62% by mass of Al2O3 and 35% to 45% by mass of ZrO2 (+HfO2). In these abrasive grains, at least 80% by mass of the total ZrO2 content is a tetragonal and / or cubic high-temperature phase. Since the abrasive grains are manufactured under reducing conditions in which carbon is used as a reducing agent, the abrasive grains contain 0.03 to 0.5% by mass of carbon. The high-temperature phase of zirconium oxide is stabilized by the addition of rutile (TiO2) and yttrium oxide, and the abrasive grains have a reduced titanium oxide content expressed as TiO2 of 1.0% to 4.0% by mass and Y2O3 of 0.2% to 1.5% by mass, with a TiO2 to Y2O3 ratio of 2:1 to 6:1. Furthermore, the abrasive grains have less than 3% by mass of raw material-related impurities. The proportion of Si compounds represented as SiO2 in the abrasive grains according to the present invention is greater than 0.8% by mass, preferably greater than 1.0% by mass. In advantageous embodiments of the present invention, the SiO2 content is 1.1% to 1.5% by mass. The raw material base for the abrasive grains comprises aluminum oxide, baddeleyite, and zircon sand, with a baddeley to zircon sand ratio of 3:1 to 1:2, preferably 1.5:1 to 1:1.5. Grinding tests are typically conducted to evaluate the quality of abrasive grains. Such grinding tests are relatively complex and time-consuming. Therefore, in the abrasives industry, it is common practice to pre-evaluate the quality of abrasive grains based on mechanical properties, which are more readily available and serve as indicators of their behavior in subsequent grinding tests. In addition to the structure and high-temperature phase ratio already mentioned, the breakdown of fine particles during grinding, particularly in ball mills, is used to evaluate the quality of abrasive grains.

[0008] Micrograin disintegration (MKZ) To measure the disintegration properties of the fine particles, 10 g of corundum (grain size 36) is ground in a ball mill filled with 12 steel balls (15 mm in diameter, 330-332 g in mass) at 188 revolutions per minute for a predetermined time. The ground abrasive particles are then sieved through a 250 μm sieve (Haver Boecker EML200 sieve) for 5 minutes, and the fine powder is weighed. The MKZ value is calculated as follows: MKZ(%) = (250 μm passing through sieve / total mass) × 100 In this example, the proportion of the high-temperature phase of zirconium oxide was determined as a further criterion for product quality. However, no distinction was made between the cubic and tetragonal phases; only the T-factor encompassing both phases was determined. T-Factor The quantitative measurement of the proportion of the high-temperature phase of ZrO2 relative to the total proportion of ZrO2 is performed using an X-ray diffractometer in a two-theta measurement range of 27.5° to 32.5°. The proportion of the high-temperature phase (T-factor) is determined according to the following formula:

number

[0009] The present invention will be described in detail below, without limitation, using several selected examples. These examples are used to demonstrate some general relationships in a molten Al2O3 / ZrO2 system using stabilizers TiO2 and Y2O3 in the presence of SiO2 as a flux, thereby providing those skilled in the art with clues on how to optimize the production of abrasive grains based on eutectic zirconium corundum molten in an electric arc furnace without product degradation.

[0010] The sample for investigation was prepared by conventional means by adding rutile sand and / or Y2O3 to a mixture of alumina, baddeleyite concentrate, zircon sand, and petroleum coke and melting it in an electric arc furnace. After the entire raw material mixture had completely melted, the molten material was poured into a gap of approximately 3-5 mm between metal sheets in accordance with European Patent No. 0593977. After complete cooling, the zirconium corundum sheets thus quenched were crushed in a conventional manner using a jaw crusher, roller crusher, roller mill, or cone crusher and sieved to obtain the desired particle size fraction. In Comparative Example H, quartz was used instead of zircon sand as the SiO2 source.

[0011] First, the raw materials are listed in Table 1 below, and the proportion of coal that burns in a molten state is not included in the total raw material mixture. In the product composition, the remainder relative to 100% is the value of Al2O3. In addition to the MKZ and T values, the proportion (percent) of Zr sand in the raw material mixture, which is extremely important for the desired product optimization, is again listed separately. [Table 1]

[0012] Examples A and B are comparative examples, equivalent to commercially available products. In Comparative Example A, the zirconium oxide was stabilized solely with reduced titanium oxide, while in Comparative Example B, the high-temperature phase was stabilized with a combination of titanium oxide and yttrium oxide. The difference in stabilization is most evident in Comparative Example B, mainly in the increase in the T value. At the same time, an improvement in the MKZ value was clearly observed compared to Comparative Example A, which indicates that an improvement in grinding performance can be expected, and this was confirmed in subsequent grinding tests. In both comparative examples, 8% by mass of zircon sand was used as the SiO2 source, resulting in SiO2 content of 0.4% by mass and 0.37% by mass in the products, respectively. Example C is equivalent to Example A in terms of stabilization, but the proportion of zircon sand is doubled, while the proportion of baddeleyite concentrate is reduced accordingly to maintain a constant overall zirconium oxide content in the product. Doubling the zircon sand increased the SiO2 proportion in the product to 1.1 mass%. The MKZ value of 6.9 is within the range of Product A. Similar to Example B, Example D was stabilized with a combination of TiO2 and Y2O3, and the proportion of zircon sand was increased as in Example C. In Product D, the corresponding SiO2 proportion was measured to be 1.1 mass%. The MKZ value of 4.6 is remarkably low, suggesting attractive grinding performance.

[0013] In Example E, baddeleyite and zircon sand were used in a 1:1 ratio, further increasing the proportion of zircon sand in the raw material mixture. The baddeleyite content was appropriately reduced to maintain the same level of zirconium oxide content in Product E. In this way, the raw material mixture contained a total of 22% by mass of zircon sand. Product E had an SiO2 content of 1.3% by mass and an MKZ value of 5.0. Example F is another comparative example using the conventional proportion of zircon sand, where the high-temperature phase of zirconium oxide is stabilized only with Y2O3. The T-factor and MKZ values ​​are comparable to those of product A, which can be seen as indicating that when only one type of stabilizer is used, the type of stabilizer may play only a small role. In Example G, where the proportion of zircon sand is doubled compared to Example F, deterioration of key values ​​is observed, but it is relatively small compared to the individual stabilization by TiO2 in Example C. Comparative example H is equivalent to Examples B, D, and E in terms of stabilization, but uses only quartz as the SiO2 source. The proportion of zirconium oxide in the product was adjusted by increasing the amount of baddeleyite used. In the case of product H, the adverse effect on product quality due to the high proportion of SiO2 is clearly evident in the key values ​​(MKZ value, T-factor), and this is also shown in the grinding test. Furthermore, scanning electron microscopy (SEM) of the polished surface of product H revealed a significant increase in porosity, with a high ratio of micropores to macropores. This is also considered another possible cause for the poor results in the grinding test of the polished surface.

[0014] Grinding Test 1 (Cutting Disk Test) For the cutting disk test, a cutting disk with the specification R-T1 180×3×22.23 was selected. For this purpose, a press mixture consisting of 75% by mass of zirconium corundum, 5% by mass of liquid resin, 12% by mass of powder resin from HEXION specialty chemicals GmBH, 4% by mass of pyrite, and 4% by mass of cryolite was prepared. To manufacture the disk, 160 g of the press mixture was formed on a commercially available cloth, pressed at 200 bar, and then cured according to the resin manufacturer's instructions. For the cutting test itself, a round bar made of stainless steel (CrNi) with a diameter of 20 mm was used. The cutting operation was carried out at a disk speed of 8,000 revolutions per minute and a cutting time of 3 seconds. After cutting 20 times, the disk loss was determined from the decrease in the diameter of the disk. Then, the G ratio was determined from the quotient of the material removal amount and the disk loss. Cutting Disk: 180×3×22.23 mm Abrasive Grain: F24 (40%); F30 (40%); F36 (30%) Material: Cr-Ni Stainless Steel Bar, 20 mm in Diameter Grinder: Fein WSB 25-180X, Speed 8000 rpm

[0015] Test Method: For conditioning of the system, cutting was performed 3 times in advance. Then, the initial diameter of the disk was measured. After further cutting 40 times, the final diameter of the disk was determined. The decrease in the disk diameter after 40 cuts was determined to evaluate the performance of the abrasive grain. Three disks were manufactured and tested for each abrasive grain. Table 2 below shows the average values of each of the three cutting disks.

Table 2

[0016] Grinding Test 2 (Cutting Disk Test) Commercially available synthetic resin-bonded and glass fiber-reinforced cutting disks were manufactured and tested using the produced abrasive grains (Examples A to H according to Table 1): Cutting disk: 125×1.2×22.23 mm Abrasive grains: F46 (60%); F60 (40%) Material: Stainless steel bar, diameter 20 mm Grinder: Fein WS 14 1, 2 kW, speed approximately 10,000 rpm Test method: For conditioning of the system, three cuts were made in advance. Then, the initial diameter of the disk was determined. After further 25 cuts, the final diameter of the disk was determined.

[0017] The performance of the abrasive grains was determined by measuring the reduction in the disk diameter after 25 cuts. Three disks were manufactured and tested for each abrasive grain. Table 3 below shows the average values of three measurements.

Table 3

[0018] Grinding test 3 (grinding belt) Commercially available grinding belts were manufactured (Examples A to H according to Table 1) by electrostatic spraying the produced abrasive grains onto an impregnated polyester / cotton blended fabric. Grinding belt: length 2000 mm, width 50 mm Grain size: NP40 Abrasive grain coating: see below

[0019] The surface of a stainless steel bar was polished using the grinding belt. Workpiece: Stainless steel bar (CrNi steel) diameter 20 mm Contact disk: diameter 250 mm, hardness 90 Shore Contact force: 68.7 N Cutting speed: 30 m / s Grinding cycle: grinding time 10 s - cooling time 20 s The performance of the abrasive grains in the belt was determined by the total removal rate after 24 grinding cycles with a total grinding time of 12 minutes. The results are summarized in Table 4 below.

[0020] [Table 4]

[0021] As can be seen from the grinding tests in Tables 2-4, no performance improvement was achieved in Examples D and E compared to Comparative Example B. Rather, product optimization consists of the fact that the proportion of inexpensive zircon sand in the raw material mixture can be increased compared to the prior art (Comparative Example B) without any loss of performance, by adjusting the ratio and amount of the stabilizer selected. Inexpensive zircon sand partially replaces the expensive and rare raw material baddeleyite or artificial ZrO2 concentrate. The cost reduction is approximately 30% for the raw materials baddeleyite and zircon sand, and approximately 8-10% for the final product (abrasive grains), which will give it a very strong competitive advantage in the highly competitive abrasive market.

Claims

1. Al 2 O 3 and ZrO 2 Abrasive grains based on, - Al 2 O 3 is 52% by mass to 62% by mass, - ZrO 2 (+HfO 2 ) is 35.0 mass% to 45.0 mass%, and with respect to the total content of ZrO 2 , at least 80 mass% of the total of ZrO 2 is the tetragonal and / or cubic high-temperature phase, - Carbon content: 0.03% to 0.5% by mass, - TiO 2 Reduced titanium oxide, expressed as such, is present in an amount of 1.0% to 4.0% by mass. - Y 2 O 3 is 0.2% by mass to 1.5% by mass, - SiO 2 The Si compound expressed as is exceeds 0.8% by mass. - Raw material-related impurities are less than 3% by mass. It contains, The abrasive grain is characterized in that the raw material base of the abrasive grain contains aluminum oxide, baddeleyite, and zircon sand, and the ratio of baddeleyite to zircon sand is 3:1 to 1:

2.

2. The abrasive grain according to claim 1, characterized in that the raw material base of the abrasive grain comprises aluminum oxide, baddeleyite, and zircon sand, and the ratio of baddeleyite to zircon sand is 1.5:1 to 1:1.

5.

3. In the abrasive grains, SiO 2 The abrasive grain according to claim 1 or 2, characterized in that the proportion of the Si compound expressed as is is greater than 1.0% by mass.

4. In the abrasive grains, SiO 2 The abrasive grain according to claim 3, characterized in that the proportion of the Si compound expressed as is 1.1% by mass to 1.5% by mass.

5. TiO 2 and Y 2 O 3 Abrasive grains according to any one of claims 1 to 4, characterized in that the ratio of is 2:1 to 6:

1.

6. A method for producing abrasive grains according to any one of claims 1 to 5, - A step of mixing abrasive starting materials in the following proportions to obtain a mixture, a) 52% to 62% by mass of Al 2 O 3 , b) 35.0% to 45.0% by mass of ZrO 2 (+HfO 2 ) and ZrO 2 In relation to the total content, ZrO 2 At least 80% by mass of the total is a tetragonal and / or cubic high-temperature phase, the ZrO 2 (+HfO 2 ), c) More than 0.8 mass% of SiO 2 Si compounds represented as, d) 0.03% to 0.5% by mass of carbon, e) 1.0% to 4.0% by mass of TiO 2 Reduced titanium oxide, which is expressed as f) TiO 2 and Y 2 O 3 Y has a ratio of 2:1 to 6:1 and is 0.2 mass% to 1.5 mass%. 2 O 3 , and g) Raw material-related impurities of less than 3% by mass, - A step of melting the mixture in an electric arc furnace to obtain a molten mixture, - A step of rapidly cooling the molten mixture to obtain a solid product, and - A step of crushing the solid product and then sieving it to obtain abrasive grains. Includes, The method, characterized in that the raw material base for the abrasive grains contains aluminum oxide, baddeleyite, and zircon sand, and the ratio of baddeleyite to zircon sand is 3:1 to 1:

2.

7. The method according to claim 6, characterized in that the ratio of baddeleyite to zircon sand is 1.5:1 to 1:1.5.

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