Stainless steel powder, metallurgical powder composition, preparation processes for compacted parts and sintered parts containing them

Abstract

STAINLESS STEEL DUST, METALLURGICAL DUST COMPOSITION, PREPARATION PROCESSES FOR COMPACTED PIECES AND SINTERED PIECES CONTAINING THE SAME. The invention relates to a stainless steel powder and compositions comprising at least 10% by weight of chromium. Vanadium is present in an amount of at least 4 times the amount of carbon and nitrogen. The steel powder comprises 10 to 30% chromium, 0.1 to 1.0 vanadium, 0.5 to 1.5% silica, less than 0.1% carbon and less than 0, 07% nitrogen. A process for preparing an agglomerated part and an agglomerated part is also claimed.

Description

Descriptive Report of the Invention Patent for "STAINLESS STEEL POWDER". Field of Invention The present invention relates to a novel stainless steel powder and to stainless steel powder compositions that include this novel powder. Specifically, the invention relates to stainless steel powder compositions for the manufacture of agglomerated powder metallurgical parts having high densities. Background of the Invention A primary goal in powder metallurgy is to achieve high density in agglomerated and compacted bodies. Several methods exist to increase density; one such method is hot compaction, which increases the compressibility of the powder, resulting in a green body with high green density. Applying lubrication to the mold wall, which minimizes the amount of internal lubricants used, can also increase green density. Using high compression pressures combined with low amounts of lubricants also results in high green densities. Mild quenching of a stainless steel powder, where the material is released from stress and recrystallized, also increases compressibility. After compaction, the green body undergoes an agglomeration operation to achieve an agglomerated body.High temperatures during agglomeration, i.e., above approximately 1180 to 1200°C, lead to increased shrinkage during agglomeration and a high density of the body. However, agglomeration at elevated temperatures requires specially equipped agglomeration furnaces. Furthermore, energy consumption will have to be increased. Special problems are encountered when high-density stainless steel PM parts are manufactured due to the presence of chromium, which makes the steel corrosion-resistant. Stainless steels contain approximately over 10% chromium. More frequently, carbon is present in steels and will cause the formation of chromium carbides. The formation of chromium carbides... This lowers the chromium content in the matrix, which in turn causes lower corrosion resistance. To prevent the chromium content in the matrix from being reduced, carbide formation stabilizers, such as niobium, are almost always used. In this way, the formation of chromium carbides can be avoided and niobium carbides are formed instead, resulting in maintained corrosion resistance. However, a problem with the use of niobium is that high agglomeration temperatures are required to obtain high agglomeration densities, and the energy consumption is considerable. It has now been discovered that, by using the new powder according to the present invention, the energy costs related to the production of PM sintered stainless steel parts can be reduced. Another significant advantage of using the new powder is that a comparatively higher sintering density can be obtained. The agglomerated parts manufactured using the new powder are of particular interest within the automotive industry where the demand for both cost and performance of the parts is high. The new powder can also be used for agglomerated parts in exhaust systems, and especially for flanges in exhaust systems. The present invention relates to stainless steel powder, stainless steel powder compositions, as well as agglomerated and compacted parts obtained therefrom having high densities. Specifically, the invention relates to stainless steel powder compositions for the manufacture of powder metallurgical parts. Summary of the Invention It has now been surprisingly discovered that by adding vanadium as a stabilizer to stainless steel powder, the agglomeration temperature and consequently energy consumption can be reduced, while the agglomerated density is similar to or even increased compared to the niobium stabilizer currently used. Furthermore, it was found that vanadium must be present in an amount at least four times the combined amounts of carbon. Carbon and nitrogen, which is why the amount of nitrogen should be less than 0.07% by weight and the amount of carbon should be less than 0.1% by weight. The amount of vanadium should be in the range of 0.1 to 1% by weight. Stainless steel compositions that include vanadium are described in publication WO 03 / 106077 and US Patent 5,856,625. WO 03 / 106077 does not describe any effects or any actual examples of powder containing vanadium. According to US Patent 5,856,625, the stainless steel powder preferably comprises 1.5 to 2.5% vanadium. This known stainless steel powder is intended for materials with high wear resistance and requires a high carbon content to achieve an appropriate amount of hard carbides in the matrix formed primarily from strong carbide-forming elements such as Mo, V, and W. Also, patent publication JP 59-47358 describes a steel powder composed of chromium, silica, carbon, and nitrogen. This powder may also contain nickel and / or copper and vanadium. The purpose of steel powder according to JP 59-47358 is the manufacture of, for example, a sliding surface. Detailed Description of the Invention Specifically, the stainless steel powder according to the invention comprises 10 to 30% chromium, 0.1 to 1% vanadium, 0.5 to 1.5% silica, less than 0.1% carbon, and less than 0.05% nitrogen. Preferably, the stainless steel powder comprises 10 to 20% chromium, 0.15 to 0.8% vanadium, 0.7 to 1.2% silica, less than 0.05% carbon, and less than 0.05% nitrogen. Since corrosion resistance in stainless steels is of great interest, the vanadium content should be chosen in such a way that vanadium carbides and nitrides are formed instead of chromium carbides and nitrides. Preferably, the vanadium content will be chosen in relation to the actual carbon and nitrogen content in the sintered component to be capable of forming vanadium carbides and nitrides. It is believed that the vanadium carbides and nitrides formed are of the VC and NC type and according to Current knowledge suggests that the vanadium content should preferably be at least four times the carbon and nitrogen content in the powder. The precise carbon and nitrogen content in the agglomerated component may be higher than the content of these elements in the powder, due to uptake during delubrication. 5. The amount of silica should be between 0.5% and 1.5%. Silica is An important element as it creates a thin, coherent oxide layer during the atomization of the molten stainless steel material, that is, the silica content should be 0.5% by weight or above. The oxide layer prevents further oxidation. A very high silica level will lead to a reduction in compressive strength, and for this reason the silica content should be 1.5% by weight or lower. The amount of nitrogen should be as low as possible, since nitrogen can have the same influence as carbon, that is, sensitizing the material through the formation of chromium nitrides or chromium carbonitrides. Nitrogen also has a precipitation hardening effect, which will decrease the compressive strength. For this reason, the nitrogen content should not exceed 0.07%, preferably no more than 0.05% by weight. In practice, it is difficult to obtain nitrogen contents lower than 0.001%. 20 Other alloying elements are added to To enhance certain properties, such as strength, hardness, etc., the alloying elements are selected from the group consisting of molybdenum, copper, manganese, and nickel. According to the present invention, ferritic stainless steels are preferred. Ferritic stainless steels are no less expensive than austenitic stainless steels that are alloyed with nickel. Compared to an austenitic matrix, a ferritic matrix has a lower coefficient of thermal expansion, which is advantageous, for example, in flanges in a stainless steel exhaust system. For this reason, a preferred embodiment of the stainless steel according to the invention is essentially nickel-free. Specifically, the ferritic stainless steel may comprise 10 to 20% by weight of chromium, and 0 to 5% by weight of nickel. of molybdenum, less than 1% by weight of nickel, less than 0.2% by weight of manganese. Other possible additives include flow agents, machining improvers such as calcium fluoride, manganese sulfide, boron nitrite, or combinations thereof. Stainless steel powder can be a gas- or water-atomized powder, pre-formed into an alloy having an average particle size above about 20 µm, depending on the powder consolidation method. Typically, the average particle size is above about 50 µm. More frequently, a lubricant is added before compaction in order to increase the compressibility of the powder and to facilitate the ejection of the green component. The amount of lubricant is typically between 0.1% and 2%, preferably between 0.3% and 1.5%. Lubricants can be chosen from the group consisting of metal stearates, such as zinc or lithium stearates, Kenolube®, amide polymers or amide oligomers, ethylene bis-esteramide, fatty acid derivatives or other suitable substances with a lubricating effect. Mold wall lubrication alone or in combination with internal lubricants can also be used. After optional quenching, the stainless steel powder is mixed with the lubricant and other optional additives. The powder mixture is compacted at 400 to 1200 MPa and agglomerated at 1150 to 1350°C for 5 minutes to one hour to obtain a density of at least 7.20 g / cm³. 3 However, the powder according to the invention can be used for the production of parts that have a lower agglomeration density in order to reduce processing costs. The compaction step can be performed as cold compaction or hot compaction. The high agglomeration density is obtained through increased shrinkage during agglomeration, and without being tied to any specific theory, it is believed that this shrinkage is a consequence of the promoted volume diffusion. The vanadium carbides that are formed In the presence of carbon, they will dissolve at high temperatures, especially at agglomeration temperatures, but also at lower temperatures such as during the quenching of metal powder. Typically, the agglomeration temperature for stainless steel powders is around 1150 to 1300°C. Example 1 Three different cast materials were produced, each with a chemical composition as per Table 1, containing niobium and vanadium as carbide-forming elements. Several mixtures were prepared for cold and hot compaction according to Tables 2 and 3. Lubricants were used for both cold and hot compaction. Degussa® Aerosil A-200 was used as the fluxing agent for hot compaction. Table 1. Chemical analysis of untempered powders. 15 (1 a part) Lot Cr% Nb% V% Si% Mn% A 11.85 — 0.29 0.68 0.23 B 11.94 0.39 --- 0.68 0.23 C 11.79 0.58 — 0.73 0.23 (2 a part) Lot Ni% P% C% N% 0% S% A 0.053 0.008 0.024 0.014 0.144 0.0033 B 0.051 0.010 0.025 0.011 0.152 0.0027 C 0.056 0.009 0.026 0.011 0.143 0.0030 Table 2. Mixtures for cold compaction. Mixture No. Composition 4* A + 1% lubricant 5 B + 1% lubricant 6 C + 1% lubricant * = composition according to the invention. Table 3. Mixtures for hot compaction Mixture No. Composition 10* A + 1% lubricant + 0.1% A-200 11 B + 1% lubricant + 0.1% A-200 12 C + 1% lubricant + 0.1% A-200 * = composition according to the invention. The powder mixtures according to Tables 2 and 3 were compacted, and the green properties were determined in relation to various compaction pressures. The results are presented in Table 4. The compacted bodies were agglomerated at 1250°C in a hydrogen atmosphere for 45 minutes, and the agglomeration densities and mechanical properties were determined. The results are shown in Table 5. Table 4 Mixture No. Compaction Pressure Green Strength (MPa) Green Density (g / cm³) 3 ) 4* 600 15.3 6.57 700 18.0 6.69 800 19.3 6.79 5 600 15.4 6.55 700 18.1 6.68 800 19.5 6.80 6 600 15.3 6.55 700 18.1 6.68 800 19.4 6.78 10* 600 31.3 6.73 700 37.5 6.87 800 39.9 6.96 11 600 30.1 6.71 700 36.7 6.86 800 40.4 6.96 12 600 29.4 6.71 700 34.9 6.86 800 39.4 6.96 * = composition according to the invention. Table 5 Mixture No. Compaction pressure (MPa) Agglomeration density (g / cm³) 3) Dimensional change (%) Yield strength (MPa) Tensile strength (MPa) 4* 600 7.36 -3.87 222 390 700 7.42 -3.29 216 409 800 7.45 -2.71 215 405 5 600 7.24 -3.48 204 366 700 7.31 -3.09 208 375 800 7.38 -2.82 228 384 6 600 7.10 -2.85 202 356 700 7.20 -2.55 208 366 800 7.26 -2.30 213 376 10* 600 7.42 -3.38 221 420 700 7.47 -2.67 230 434 800 7.49 -2.20 234 431 11 600 7.28 -2.93 206 371 700 7.36 -2.52 210 386 800 7.43 -2.20 216 400 12 600 7.16 -2.36 203 361 700 7.27 -2.05 212 377 800 7.33 -1.79 214 389 * = composition according to the invention From Table 4 and Table 5, it can be clearly identified that the agglomeration densities of the samples produced from the material according to the invention are improved, while the green densities of the material according to the invention are similar to the comparison materials. The mechanical properties of the agglomerated components are also improved with the material according to the invention when compared to known materials. Example 2 In order to evaluate the influence of agglomeration temperatures and agglomeration times, powder mixtures 4, 5, and 6 were compacted into tensile strength test samples according to ISO 2740 was subjected to a uniaxial compaction process at room temperature and 600 MPa. The resulting green samples were agglomerated at 1200°C, 1250°C, and 1300°C in a hydrogen atmosphere for 20 and 45 minutes, respectively. 5 After agglomeration, the clustered density of the samples The agglomerated densities were measured according to ISO 3369. The results are shown in Table 6. From Table 6, it can be concluded that agglomerated densities above 7.2 g / cm³ 3 Higher values ​​can be obtained with respect to a ferritic stainless steel powder, provided vanadium is added, even at an agglomeration temperature as low as 1200°C. An agglomeration time of 20 minutes at an agglomeration temperature of 1250°C produces an agglomerated density of 7.35 g / cm³. 3 , while the corresponding density with respect to niobium-stabilized ferritic stainless steel powder is 7.15 g / cm³. 3 and 7.03 g / cm³ 3 respectively, from -15 depending on the amount of niobium added. The example reveals a surprisingly large impact on concentration during the agglomeration of a green body produced from ferritic stainless steel powder according to the invention. Table 6 Mixture No. Agglomeration Time (minutes) Agglomeration Densities (g / cm³) 3 ) at different agglomeration temperatures 1200°C 1250°C 1300°C 4* 45 7.29 7.36 7.46 5 45 7.03 7.24 7.47 6 45 6.92 7.1 7.38 4* 20 - 7.35 - 5 20 - 7.16 - 6 20 - 7.03 - 20 * = compositions according to the invention. Example 3 In order to evaluate the influence of the nitrogen content of stainless steel powder, a molten material was atomized and samples of the powder, which had different nitrogen contents, were prepared from the powder. Atomized powder was quenched in a nitrogen-containing atmosphere. As a reference material, powder quenched in a 100% hydrogen atmosphere was used. The powder samples were mixed with 1% lubricant, and the resulting compositions were cold-packed at different pressures into specimens. The specimens were agglomerated at 1250°C in a hydrogen atmosphere for 45 minutes. The chemical analysis of the different powder samples is presented in Table 7, except for the nitrogen content, which was determined after quenching as shown in Table 8. Table 8 shows the density of the agglomeration for different specimens. Table 7 (1 a part) Lot Cr% Nb% V% Si% Mn% D 12.14 0.01 0.29 0.83 0.13 (2 a part) Lot Ni%P%c%S%D 0.05 0.001 0.017 0.012 Table 8 Lot Compaction pressure (MPa) %N Agglomeration density (g / cm³) 3) D1 600 0.056 7.18 D1 700 7.28 D1 800 7.36 D2 600 0.072 7.13 D2 700 7.24 D2 800 7.31 D(ref) 600 0.019 7.23 D (ref) 700 7.34 D (ref) 800 7.39 It can be observed from example 3 that a nitrogen content above 0.07% will result in an undesirable agglomeration density.

Claims

• • • tf tf • p» tf • tf • • • • * • • • • tf tf • • tf • • • »• • • • O • • • O • tf • tf • • tf • • •• tf • • • • • tf • tf tf CLAIMS 1. Pre-formed stainless steel alloy powder comprising at least 10% by weight of chromium, less than 0.1% by weight of carbon and less than 0.07% by weight of nitrogen, said powder further comprising vanadium in an amount of at least 4 times the combined amounts of carbon and nitrogen, wherein the amount of vanadium is 0.1 to 1% by weight.

2. Stainless steel powder according to claim 1, wherein the steel powder further comprises 10 to 30% chromium and 0.5 to 1.5% silica.

3. Stainless steel powder according to claims 1 or 2, wherein the steel powder comprises 10 to 20% chromium, 0.15 to 0.8% vanadium, 0.7 to 1.2% silica, less than 0.05% carbon and less than 0.05% nitrogen.

4. Stainless steel powder according to claim 1, 2 or 3, wherein the steel powder is essentially nickel-free.

5. Metallurgical powder composition comprising a stainless steel powder as defined in any of the preceding claims and additives selected from the group consisting of lubricants, flow agents, machining improvers, and alloying elements.

6. Process for the preparation of compacted stainless steel powder parts, comprising the following steps: - submit a steel powder according to any one of claims 1 to 4 optionally mixed with a lubricant; - agglomerating the compacted piece at a temperature of 1150 to 1350°C.

7. Process according to claim 6, in which the agglomeration is carried out to a density of at least 7.20 g / cm³. 3 .

8. Agglomerated part of stainless steel powder as defined in any one of claims 1 to 4, having an agglomeration density of at least 7.20 g / cm³. 3 . SUMMARY Invention Patent: "STAINLESS STEEL POWDER". The invention relates to a stainless steel powder and compositions comprising at least 10% by weight of chromium. Vanadium is present in an amount at least four times the amount of carbon and nitrogen. The steel powder comprises 10 to 30% chromium, 0.1 to 1.0% vanadium, 0.5 to 1.5% silica, less than 0.1% carbon, and less than 0.07% nitrogen. A process for preparing an agglomerated part and an agglomerated piece is also claimed. New page 1 of the descriptive report, new claims table (total of 22 claims) and new summary for processing in the Brazilian national phase. Descriptive Report of the Invention Patent for "STAINLESS STEEL POWDER, METALLURGICAL POWDER COMPOSITION, PROCESSES FOR PREPARING COMPACTED PARTS AND SINTERED PARTS CONTAINING THE SAME". Field of Invention The present invention relates to a novel stainless steel powder and to stainless steel powder compositions that include this novel powder. Specifically, the invention relates to stainless steel powder compositions for the manufacture of agglomerated powder metallurgical parts having high densities. Background of the Invention A primary goal in powder metallurgy is to achieve high density in agglomerated and compacted bodies. Several methods exist to increase density; one such method is hot compaction, which increases the compressibility of the powder, resulting in a green body with high green density. Applying lubrication to the mold wall, which minimizes the amount of internal lubricants used, can also increase green density. Using high compression pressures combined with low amounts of lubricants also results in high green densities. Mild quenching of a stainless steel powder, where the material is released from stress and recrystallized, also increases compressibility. After compaction, the green body undergoes an agglomeration operation to achieve an agglomerated body.High temperatures during agglomeration, i.e., above approximately 1180 to 1200°C, lead to increased shrinkage during agglomeration and a high density of the body. However, agglomeration at elevated temperatures requires specially equipped agglomeration furnaces. Furthermore, energy consumption will have to be increased. Special problems are encountered when high-density stainless steel PM parts are manufactured due to the presence of chromium, which makes the steel corrosion-resistant. Stainless steels contain approximately over 10% chromium. More frequently, carbon is present in steels and will cause the formation of chromium carbides. The formation of chromium carbides... CLAIMS 1. Pre-formed stainless steel alloy powder, characterized in that it comprises at least 10% by weight of chromium, less than 0.1% by weight of carbon and less than 0.07% by weight of nitrogen, the re- 5. Wounded powder further comprising vanadium in an amount of at least 4 times the amounts of carbon and nitrogen, wherein the amount of vanadium is 0.1 to 1% by weight.

2. Stainless steel powder according to claim 1, characterized in that it further comprises 10 to 30% chromium and 0.5 10 to 1.5% silicon.

3. Stainless steel powder according to claim 1, characterized in that it comprises 10 to 20% chromium, 0.15 to 0.8% vanadium, 0.7 to 1.2% silicon, less than 0.05% carbon and less than 0.05% nitrogen. 1-5 4. Stainless steel powder according to claim 2, character characterized by the fact that it comprises 10 to 20% chromium, 0.15 to 0.8% vanadium, 0.7 to 1.2% silicon, less than 0.05% carbon and less than 0.05% nitrogen.

5. Stainless steel powder according to claim 1, characterized by the fact that it is essentially nickel-free.

6. Stainless steel powder according to claim 2, characterized in that it is essentially nickel-free.

7. Stainless steel powder according to claim 3, characterized in that it is essentially nickel-free. 25 8. Stainless steel powder according to claim 4, character It is characterized by the fact that it is essentially nickel-free.

9. Metallurgical powder composition, characterized in that it comprises a stainless steel powder, as defined in claim 1, which includes at least one additive selected from the group consisting of lubricants. 30 fillers, flow agents, machinability improvers and alloying elements.

10. Metallurgical powder composition, characterized by the fact that comprising a stainless steel powder, as defined in claim 2, which includes at least one additive selected from the group consisting of lubricants, flow agents, machinability improvers and alloying elements.

11. Metallurgical powder composition, characterized in that it comprises a stainless steel powder, as defined in claim 2, which includes at least one additive selected from the group consisting of lubricants, flow agents, machinability improvers and alloying elements.

12. Metallurgical powder composition, characterized in that it comprises a stainless steel powder, as defined in claim 5, which includes at least one additive selected from the group consisting of lubricants, flow agents, machinability improvers and alloying elements.

13. Process for preparing compacted stainless steel powder parts, characterized in that it comprises the following steps: - subjecting a stainless steel powder, as defined in claim 1, to compaction, and - Sinter the compacted part at a temperature of 1150 to 1350 °C.

14. Process for preparing compacted stainless steel powder parts, characterized in that it comprises the following steps: - subjecting a stainless steel powder, as defined in claim 2, to compaction, and - Sinter the compacted part at a temperature of 1150 to 1350 °C.

15. Process for preparing compacted stainless steel powder parts, characterized in that it comprises the following steps: - subjecting a stainless steel powder, as defined in claim 3, to compaction, and - Sinter the compacted part at a temperature of 1150 to 1350 °C.

16. Process for preparing compacted stainless steel powder parts, characterized in that it comprises the following steps: - subjecting a stainless steel powder, as defined in claim 5, to compaction, and 5 - Sinter the compacted piece at a temperature of 1150 to 1350 °C.

17. Process according to claim 13, characterized in that said stainless steel powder is mixed with a lubricant. 10 18. Process according to claim 13, characterized because the sintering is done at a density of at least 7.20 g / cm³. 3 .

19. Sintered part, characterized by being made of stainless steel powder, as defined in claim 1, having a density 15 sintering of 7.20 g / cm 3 .

20. Sintered part, characterized by being made of stainless steel powder, as defined in claim 2, having a sintering density of 7.20 g / cm³. 3 .

21. Sintered part, characterized by being made of steel powder. 20 stainless steel, as defined in claim 3, having a sintering density of 7.20 g / cm³. 3 .

22. Sintered part, characterized by being made of stainless steel powder, as defined in claim 5, having a sintering density of 7.20 g / cm³. 3 . SUMMARY Invention Patent: "STAINLESS STEEL POWDER, METALLURGICAL POWDER COMPOSITION, PROCESSES FOR PREPARING COMPACTED PARTS AND SINTERED PARTS CONTAINING THE SAME". The invention relates to a stainless steel powder and compositions comprising at least 10% by weight of chromium. Vanadium is present in an amount at least four times the amount of carbon and nitrogen. The steel powder comprises 10 to 30% chromium, 0.1 to 1.0% vanadium, 0.5 to 1.5% silica, less than 0.1% carbon, and less than 0.07% nitrogen. A process for preparing an agglomerated part and an agglomerated piece is also claimed.