Patent Application
20180179618
The present invention relates to a tool steel alloy, and more particularly to a powder metallurgy wear and corrosion resistance alloy.
In some special working conditions, tools or parts are worn due to direct contact with hard abrasive particles of moving parts or working mediums, while corroded by moisture, corrosive acids or other corrosive agents. Under such typical operating conditions, parts such as screw, screw head and screw sleeve used in plastic injection molding machine are fiercely worn due to a lot of hard grains such as glass fibers and carbon fibers added in the plastic; while being chemically corroded by corrosion components in the plastic. For obtaining a long part service life under such special conditions, a tool steel used must have high wear resistance and corrosion resistance. In addition, to bear loading and shock of working stress, the tool steel needs certain hardness and toughness. Wear resistance of the tool steel depends on the matrix hardness, as well as content, morphology and particle size distribution of the second hard phase in the steel. The second hard phase in the steel comprises M6C, M2C, M23C6, M7C3 and MX carbides, wherein microhardness of the MX carbides are higher than other carbides, for providing better matrix protection during operation, thereby reducing wear and improving the service life of molds. Corrosion resistance increase of the tool steel mainly depends on chromium dissolved in the matrix, and it is considered that 11% chromium should be dissolved in the matrix. Toughness of the tool steel depends on the matrix hardness and particle size distribution of the second hard phase in the steel. Coarse carbides in the steel will cause stress concentration, which reduces the toughness of the tool steel, resulting in fracture under a relatively low external load. In order to improve the toughness of the tool steel, it is important to reduce or refine the carbides. In order to avoid plastic deformation during utilization, hardness of the tool steel is usually required to be HRC60 or more.
Conventionally, the tool steel is mainly casted and forged by traditional production processes, wherein the tool steel prepared by casting and forging processes is limited by liquid steel which is slowly cooled during the processes. As a result, alloy components are easy to be segregated during consolidation and to form the coarse carbides. Even after subsequent forging and rolling processes, such bad structure will still adversely affect the performance of the alloy, resulting in low performances of the tool steal in strength, toughness, wear resistance and grinding performance, which is difficult to meet material performance and life stability requirements of high-end manufacturing. Tool steel prepared by a powder metallurgy method avoids the segregation problem of alloy elements, wherein the powder metallurgy method comprises steps of: preparing powder by atomization, and consolidating the powder. In the step of preparing powder by atomization, the liquid steel is rapidly cooled into powder. Therefore, the alloy elements in the liquid steel are completely consolidated before segregation. A structure is fine and even after powder consolidation, wherein compared with casting and forging, alloy performance is significantly improved. Conventionally, only the powder metallurgy method is able to satisfy extremely high performance requirements of high alloy tool steel. Tool steel prepared by powder metallurgy has been reported, but components of some kinds of steel are not reasonably designed, so structure and performance should be further improved.
An object of the present invention is to solve at least one of the above technical problems to some extent. Therefore, the present invention provides a powder metallurgy wear and corrosion resistance alloy with excellent performances.
Accordingly, in order to accomplish the above objects, the present invention provides a powder metallurgy wear and corrosion resistance alloy, comprising chemical components by mass percent of: C: 2.36%-3.30%, W: 0.1%-1.0%, Mo: ≤1.8%, Cr: 12.6%-18.0%, V: 6.0%-12.5%, Nb: 0.5%-2.1%, Co: 0.1%-0.5%, Si: ≤1.0%, Mn: 0.2%-1.0%, N: 0.05%-0.35%, with balance iron and impurities; wherein a carbide component of the powder metallurgy wear and corrosion resistance alloy is an MX carbide and a M7C3 carbide, wherein the MX carbide has a NaCl type face-centered cubic lattice structure; an M element of the MX carbide comprises V and Nb, and an X element of the MX carbide comprises C and N.
According to the powder metallurgy wear and corrosion resistance alloy of an embodiment of the present invention, alloy components are designed and powder metallurgy is adapted, for obtaining an alloy with high wear resistance and high corrosion resistance. According to the present invention, the MX carbide comprises alloy elements such as C, N, V and Nb. A type of the MX carbide is (V, Nb) (C, N). Under a rapid cooling condition of liquid steel, Nb and N are involved in formation of the MX carbide, so as to increase a nucleation rate and refine the MX carbide, in such a manner that toughness of the alloy is improved.
Preferably, the impurities comprise O, wherein an O content is no more than 0.01%.
Preferably, the powder metallurgy wear and corrosion resistance alloy comprises the chemical components by mass percent of: C: 2.40%-3.18%, W: 0.1%-0.8%, Mo: ≤1.8%, Cr: 13.0%-18.0%, V: 6.2%-12.5%, Nb: 1.0%-2.0%, Co: 0.1%-0.4%, Si: ≤0.8%, Mn: 0.2%-0.8%, N: 0.05%-0.30%, O: ≤0.008%, with balance iron and impurities.
Preferably, the impurities comprise S, wherein a S content is no more than 0.1%.
Preferably, the impurities comprise P, wherein a P content is no more than 0.03%.
Preferably, a volume fraction of the MX carbide is 12%-20%.
Preferably, a size of at least 80% of the MX carbide is no more than 1.3 μm judging from volume percentage.
Preferably, a maximum size of the MX carbide is no more than 5.0 μm.
Preferably, the M7C3 carbide is a Cr-enriched carbide.
Preferably, a volume fraction of the M7C3 carbide is 12%-19%.
Preferably, a size of at least 80% of the M7C3 carbide is no more than 5 μm judging from volume percentage.
Preferably, a maximum size of the M7C3 carbide is no more than 10 μm.
Preferably, the M7C3 carbide has a hexagonal lattice structure.
Preferred embodiments of the present invention are illustrated as follows. Examples of the embodiments are shown in drawings. The same or similar elements and the elements having same or similar functions are denoted by like reference numerals throughout the descriptions. The embodiments described herein with reference to drawings are explanatory, and used to generally understand the present disclosure, and not intended to be limiting.
The present invention provides a powder metallurgy wear and corrosion resistance alloy with significant performances. According to the present invention, the powder metallurgy wear and corrosion resistance alloy comprises chemical components by mass percent of: C: 2.36%-3.30%, W: 0.1%-1.0%, Mo: ≤1.8%, Cr: 12.6%-18.0%, V: 6.0%-12.5%, Nb: 0.5%-2.1%, Co: 0.1%-0.5%, Si: ≤1.0%, Mn: 0.2%-1.0%, N: 0.05%-0.35%, with balance iron and impurities; wherein a carbide component of the powder metallurgy wear and corrosion resistance alloy is an MX carbide and a M7C3 carbide, wherein the MX carbide has a NaCl type face-centered cubic lattice structure; an M element of the MX carbide comprises V and Nb, and an X element comprises C and N.
According to the embodiments of the present invention, alloy components are designed and powder metallurgy is adapted, for obtaining an alloy with high wear resistance and high corrosion resistance.
C is dissolved in the matrix for enhancing matrix strength. Meanwhile, C is one of forming elements of carbide, whose content should be no less than 2.36%, so as to ensure that the alloy elements are fully involved in carbide precipitation. A maximum C content is no more than 3.30%, for avoiding toughness decrease due to excessive C dissolved in the matrix. A C content is at 2.36%-3.30% for optimizing a combination of wear resistance and toughness.
W and Mo are dissolved in the matrix for increasing hardenability thereof. According to the present invention, a W content is 0.1%-1.0%, and a Mo content is Mo≤1.8%.
On one hand, Cr is dissolved in the matrix for improving corrosion resistance and hardenability thereof; on the other hand, Cr is precipitated as the M7C3 carbide. In consideration of a balance between Cr dissolved in the matrix and precipitated as carbide, a Cr content of the present invention is 12.6%-18.0%.
V is used for forming the MX carbide and improving alloy wear resistance, whose content is controlled between 6.0%-12.5%.
Nb and V have similar functions, both of which are involved in forming the MX carbide. According to the present invention, Nb is dissolved in the MX carbide for increasing a nucleation rate when the MX carbide is precipitated, so as to improve precipitating and refining of the MX carbide, and improve the wear resistance. A Nb adding limit prevents Nb-enriched MX carbide from precipitation. According to the present invention, a Nb content is controlled at 0.5%-2.1%.
Co is mainly dissolved in the matrix for promoting carbide precipitation and refining particle size of the carbide. According to the present invention, a Co content is 0.1%-0.5%.
Si is not involved in carbide formation, but as a deoxidizing agent and a strengthening element of the matrix. Excessive Si will lower the toughness of the matrix, so a Si content is controlled at Si≤1.0%.
Mn is added as a deoxidizing agent, for fixing sulfur and reducing hot brittleness. In addition, manganese increases a hardenability. According to the present invention, a Mn content is controlled within 0.2%-1.0%.
N is involved in MX carbide formation. Under a rapid cooling condition, N promotes nucleation precipitation of the MX carbide while the MX carbide never excessively grows, which is conducive to improvement of the wear resistance as well as corrosion resistance. A N content is limited within 0.05%-0.35%.
According to the embodiments of the present invention, the MX carbide comprises alloy elements such as C, N, V and Nb. A type of the MX carbide comprises (V, Nb) (C, N). Under a rapid cooling condition of liquid steel, Nb and N are involved in formation of the MX carbide, so as to increase a nucleation rate and refine the MX carbide, in such a manner that toughness of the alloy is improved.
Preferably, the impurities comprise O, wherein an O content is no more than 0.01%.
Excessive O will lower the toughness of the alloy. According to the embodiments of the present invention, the O content is no more than 0.01% for ensuring an outstanding steel performance.
Preferably, the powder metallurgy wear and corrosion resistance alloy comprises the chemical components by mass percent of: C: 2.40%-3.18%, W: 0.1%-0.8%, Mo: ≤1.8%, Cr: 13.0%-18.0%, V: 6.2%-12.5%, Nb: 1.0%-2.0%, Co: 0.1%-0.4%, Si: ≤0.8%, Mn: 0.2%-0.8%, N: 0.05%-0.30%, O: ≤0.008%, with balance iron and impurities.
For obtaining a better combination performance, the chemical components of the powder metallurgy wear and corrosion resistance alloy should be controlled within a certain range.
Preferably, a volume fraction of the MX carbide is 12%-20%.
Preferably, a size of at least 80% of the MX carbide is no more than 1.3 μm judging from volume percentage, and a maximum size of the MX carbide is no more than 5 μm.
Preferably, the M7C3 carbide has a complex hexagonal lattice structure, a main element M in the M7C3 carbide is Cr.
Preferably, a volume fraction of the M7C3 carbide is 12%-19%.
Preferably, a size of at least 80% of the M7C3 carbide is no more than 5 μm judging from volume percentage, and a maximum size of the M7C3 carbide is no more than 10 μm.
According to the embodiments of the present invention, the powder metallurgy wear and corrosion resistance alloy is prepared by a method comprising steps of:
a) preparing a liquid tool steal with the above components and loading the liquid tool steal into a ladle;
b) electrically heating covering slag at a top surface of the liquid steel in the ladle for maintaining superheat, injecting an inert gas from a hole at a bottom of the ladle for stirring the liquid steel;
c) moving the liquid steel into a tundish which is pre-heated through the guiding tube at the bottom of the ladle, adding covering slag to the top surface of the liquid steel when the liquid steel enters into the tundish and buries a bottom end face of the guiding tube;
d) continuously additional heating the tundish for maintaining the superheat;
e) moving the liquid steel into an atomization chamber from the tundish for atomization with an inert gas, wherein metal powder obtained is deposited at a bottom of the atomization chamber; then entering a powder storage with a protective atmosphere; after atomization, screening with a protective screening device before storing in the powder storage; and
f) loading the metal powder in the powder storage into a hot isostatic pressing capsule with inert gas protection; after fully vibration filled, evacuate-degassing the hot isostatic pressing capsule; and then sealing welding a capsuling end; finally providing a hot isostatic pressing treatment, in such a manner that the metal powder is fully consolidated, and completing powder metallurgy.
The above powder metallurgy comprises non-vacuum melting atomization and hot isostatic pressing processes with full-process protection to control the oxygen content and carbide morphology, and optimize an alloy performance. The covering slag of the ladle is able to cut off the air and conductively heat. The inert gas is injected into the bottom of the ladle through the hole, so that temperatures at different positions of the liquid steel equals to each other, and harmful inclusions rapidly floats, thus being removed. The guiding tube at the bottom of the ladle guides the liquid steel as well as reduces turbulence fluid generated during flowing, so as to keep slag and inclusion out. Furthermore, the guiding tube prevents the liquid steel from being exposed to air, avoiding increase of an oxygen content of the liquid steel. The covering slag of the tundish prevents the liquid steel from being exposed to air when the liquid steel flows through the tundish, avoiding increase of the oxygen content. The tundish is pre-heated before the liquid steel enters, so as to avoid local condensation or early precipitation of a second phase when the liquid steel enters into the tundish. The powder storage has the protection atmosphere inside and a forced cooling function. The protective screening device protects a screening process and prevents the powder from flying. The powder storage is connected to the hot isostatic pressuring capsule in a sealed form, and the inert gas is injected into the hot isostatic pressing capsule before loading powder for discharging air, so as to control the oxygen content.
In summary, according to the present invention, a powder metallurgy tool steel with high wear resistance and high corrosion resistance is obtained, which has an excellent overall performance, especially in the wear resistance and the corrosion resistance, so as to be used under wearing and corroding working conditions. According to the embodiments of the present invention, the alloy adapts certain chemical components and rapid cooling-consolidation process of the powder metallurgy, wherein a type of the MX carbide is (V, Nb) (C, N), in such a manner that the MX carbide precipitated is finer and a distribution is evener. With a higher carbide content, it is easy to obtain high toughness and grinding performance. After heat treatment, the hardness is more than HRC60, so as to satisfy different application requirements with a wide range of uses. The alloy of the present invention is prepared according to the powder metallurgy, wherein a plurality of effective protection methods are used for keeping the liquid steel and the powder clean. An increase of the oxygen content is ≤30 ppm, which ensures a high-performance alloy.
For better understanding by the skilled person in the art, preferred embodiments of the present invention are illustrated as follows.
The preferred embodiment 1 refers to a group of powder metallurgy wear and corrosion resistance alloys, whose chemical components are listed in Table 1.1:
The powder metallurgy wear and corrosion resistance alloys are prepared with a method comprising steps of:
a) loading liquid tool steal of the present invention into a smelting ladle with a load weight of 1.5-8 ton;
b) electrically heating covering slag at a top surface of the liquid steel in the smelting ladle by graphite electrodes, injecting argon or nitrogen gas from a hole at a bottom of the smelting ladle for stirring the liquid steel, opening a guiding tube when a liquid steel overheated temperature is 100° C.-150° C.;
c) moving the liquid steel into a tundish, which is pre-heated to 800° C.-1200° C., through the guiding tube at the bottom of the smelting ladle, controlling a size of an inlet of the guiding tube, in such a manner that a flow rate of the liquid steel is 10 kg/min-50 kg/min, adding a covering slag when the liquid steel enters into the tundish and buries a bottom end face of the guiding tube;
d) forming powder by atomization while continuously heating the tundish for maintaining the liquid steel temperature at 100° C.-150° C.;
e) moving the liquid steel into an atomization chamber through an opening at a bottom of the tundish, opening an atomizing gas nozzle, using nitrogen as an atomizing gas for atomization, wherein a nitrogen purity is ≥99.999%, an oxygen content is ≤2 ppm, a gas pressure is 1.0 MPa-5.0 MPa; cracking the liquid steel into drops by impact of an inert gas, while rapidly cooling into metal powder and depositing at a bottom of the atomization chamber; then entering a powder storage through the bottom of the atomization chamber; after atomization, waiting until the powder in the powder storage is cooled to a room temperature, and screening with a protective screening device; wherein an inert protective gas with a positive pressure is injected into a screening device chamber, and the powder storage has a protective atmosphere with a positive pressure inert gas; and
f) loading the metal powder in the powder storage into a hot isostatic pressing capsule, firstly injecting an inert gas into the hot isostatic pressing capsule for excluding air, then connecting the hot isostatic pressuring capsule and the powder storage in a sealed form; providing a vibration operation during loading for increasing filling density of the metal powder; then evacuate-degassing the hot isostatic pressing capsule while keeping a temperature at 200° C.-600° C.; degassing to 0.01 Pa and continuously heating for ≥2 h, and then sealing welding a capsuling end; finally providing a hot isostatic pressing treatment, with a temperature of 1100° C.-1160° C., and keeping a pressure of ≥100 MPa for ≥1 h, naturally cooling after the metal powder is fully consolidated.
According to requirements, the alloy of the present invention are further forged for obtaining certain shapes and sizes, and are treated with different heat treatments for obtaining different performances, wherein the heat treatments comprises annealing, quenching and tempering. Annealing comprises steps of heating a forging piece to 860° C.-900° C. and keeping the temperature for 2 h; cooling to 530° C. with a rate of ≤15° C./h; then cooling to below 50° C. by furnace cooling or static air cooling. Quenching comprises steps of pre-heating the forging piece after annealing at a temperature at 815° C.-845° C.; keeping the temperature at 1000° C.-1200° C. for 15-40 min after the temperature is even, then quenching to 530° C.-550° C., and cooling to below 50° C. Tempering comprises steps of heating the forging piece after quenching to 540° C.-670° C. and keeping the temperature for 1.5-2 h, then air-cooling to below 50° C.; repeating for 2-3 times.
According to embodiments 1.1-1.4, the powder metallurgy wear and corrosion resistance alloys are obtained, wherein increase of the oxygen content during process is ≤30 ppm. After hot working, a fully dense alloy with a relative density of 100% is obtained, which is prepared into columns with a diameter of 50 mm.
Preferred Embodiment 2
The preferred embodiment 2 proves carbide content and particle size, heat treatment hardness, wear resistance, and corrosion resistance of the powder metallurgy wear and corrosion resistance alloy of the preferred embodiment 1, wherein the carbide content and the particle size is analyzed based on structure images obtained by scanning electron microscope; and the heat treatment hardness and the wear resistance are tested referring to GB/T 230.1, and GB/T 12444-2006. The corrosion resistance is tested by immersing in a corrosive agent of 5% HNO3+1% HCl at a room temperature.
The powder metallurgy wear and corrosion resistance alloys of the embodiments 1.1 and 1.2 are compared with a forged tool steel (alloy A) and a powder metallurgy tool steel (alloy B) bought, wherein results are as follows:
According to the powder metallurgy wear and corrosion resistance alloys of the embodiments 1.1 and 1.2, the oxygen content is 50-60 ppm before preparing and 60-80 ppm after preparing, which means the increase of the oxygen content is ≤30 ppm.
According to carbide analysis of the powder metallurgy wear and corrosion resistance alloy, carbide components are the MX carbide and the M7C3 carbide. The type of the MX carbide is (V, Nb) (C, N), which is mainly formed by V, Nb, C, N and a few alloy elements such as Fe and Cr. The M7C3 carbide is a Cr-enriched carbide. According to Table 2.2, the MX carbide of the alloy of the present invention is extremely small, wherein a size of at least 80% of the MX carbide is no more than 1.3 μm. After further measurement, a maximum size of the MX carbide is no more than 5 μm. Due to a high hardness of the MX carbide, the alloy of the present invention is excellent in grinding performance and toughness is easy to increase. The volume fraction of the MX carbide is up to 12%-20%, so as to provide excellent wear resistance. According to the present invention, a volume fraction of the M7C3 carbide is 12%-19%. A size of at least 80% of the M7C3 carbide is no more than 5 μm judging from volume percentage, and a maximum size of the MX carbide is no more than 10 μm. The M7C3 carbide is bigger than the MX carbide in particle size, but still smaller than M7C3 carbide in the forged alloy A. The alloy B adapts a powder metallurgy method, wherein carbide particle size thereof is extremely small. Most MX carbide of the alloy B is 0.5-1.5 μm with a volume fraction of 3%-6%.
According to Table 2.3, after heat treatment, the alloy hardness is more than HRC60, so as to satisfy application requirements of the alloy according to the present invention. Accordingly, wear resistance of the alloy of the present invention is the best.
The corrosion resistance is tested by immersing the alloy of the present invention in the corrosive agent of 5% HNO3+1% HCl at the room temperature. The alloy A with a high content of Cr is tested as a contrast, wherein corrosion resistance comparison results are listed in Table 2.4.
It should be noticed that different applications requires different wear and corrosion resistances, so proper heat treatment should be selected. That is to say, with the same quenching conditions, lower tempering will dissolve more Cr in the matrix, so as to obtain higher corrosion resistance. Higher tempering will precipitate more Cr in a carbide form, so as to obtain lower corrosion resistance but higher wear resistance. In general, within a wide heat treatment range, the alloy of the present invention is excellent in both wear and corrosion resistances, so as to satisfy application requirements under wearing and corroding occasions.
In summary, according to the present invention, a powder metallurgy wear and corrosion resistance alloy is obtained, which has an excellent overall performance, especially with both high wear resistance and high corrosion resistance, so as to be used under wearing and corroding working conditions. According to the present, the alloy adapts certain chemical components and powder metallurgy. With a higher carbide content, the carbide particles are still fine and even-distributed, which is conducive to obtain high toughness and grinding performance. After heat treatment, the hardness is more than HRC60, so as to satisfy different application requirements with a wide range of uses. For example, the present invention is applicable to squeezing plastic machinery parts such as screws, screw sleeves, screw head as well as check rings, and food industry, surgical instruments, industrial cutting blades, wear and corrosion resistant parts, etc. According to the present invention, a plurality of effective protection methods are used for keeping the liquid steel and the powder clean. The increase of the oxygen content is ≤30 ppm, which ensures a high-performance alloy.
During description, words such as “first” and “second” are describing only without indicating importance or numbers of technical features. Therefore, “first” or “second” may refer to one or more features. During description, “a plurality of” refers to no less than two except for detailed illustration.
During description, references such as “one embodiment”, “some embodiments”, “an example”, “specific example”, or “some examples” mean that a particular feature, structure, material, or characteristic of the described embodiments or examples are included in at least one embodiment or example of the present invention. In the specification, the terms of the above schematic representation is not necessarily for the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described in any one or more of the embodiments or examples are able to be combined in a suitable manner. One skilled in the art will understand that features in different embodiments or examples may be combined if not conflicting to each other.
One skilled in the art will understand that the embodiment of the present invention as shown in the drawings and described above is exemplary only and not intended to be limiting. It will thus be seen that the objects of the present invention have been fully and effectively accomplished. Its embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the present invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201510248007.X | May 2015 | CN | national |
This is a U.S. National Stage under 35 U.S.C 371 of the International Application PCT/CN2015/091274, filed on Sep. 30, 2015, which claims priority under 35 U.S.C. 119(a-d) to CN 201510248007.X, filed on May 15, 2015.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2015/091274 | 9/30/2015 | WO | 00 |
