Preparation and use method of heat-treatment-free high-strength stainless steel powder for MIM

Abstract

The invention discloses a preparation and use method of heat-treatment-free high-strength stainless steel powder for MIM, and belongs to the field of powder metallurgy. The traditional MIM stainless steel is difficult to consider excellent wear resistance, compressive strength and corrosion resistance at the same time. The stainless steel powder comprises Fe (12%-13%) Al (5%-6%) Cr (5%-6%) Ni (2%-3%) Ti (1.8%-2%) Mn (1.8%-2%) Co, formic acid is added into an atomized water medium, powder can be remarkably refined, and meanwhile precipitation of strengthening elements is avoided. When the powder is formed by adopting an MIM process, an Al42Ni28Fe14Co8Ti8 phase separated out during degreasing can prevent crystal grains from growing up in the sintering process, the separated-out phase is subjected to solid solution in a matrix during sintering, a small amount of liquid phase is formed to promote sintering densification, heat preservation is conducted at the temperature of 600 DEG C in the subsequent cooling process, and the separated-out phase is dispersed and separated out from the matrix again. The fine grain and second phase dispersion strengthening greatly improve the strength and wear resistance of the matrix, meanwhile, high corrosion resistance is kept, and excellent comprehensive performance can be obtained without heat treatment.

Description

TECHNICAL FIELD

[0001] The present application relates to a raw material powder for preparing high-strength stainless steel by powder injection molding, and belongs to the field of powder metallurgy. BACKGROUND

[0002] In recent years, with the rapid development of emerging fields such as 3C electronics and humanoid robots, the demand for special-shaped metal parts has increased dramatically. A large part of these parts are small in size and require some special properties, such as high wear resistance, high compressive strength, and excellent corrosion resistance, such as the gears of humanoid robots, mobile phone charging interfaces, etc. Traditional corrosion-resistant materials such as austenitic stainless steel have excellent corrosion resistance but poor wear resistance. Martensitic stainless steel has high strength and wear resistance, but it needs to be heat treated, which can easily cause deformation of the parts and affect the dimensional accuracy. It is also difficult to mass-produce complex-shaped parts at low cost based on traditional materials using traditional machining processes.

[0003] Metal powder injection molding (MIM) has unique advantages in molding three-dimensional complex structure metal parts, and has been widely used in the preparation of various complex-shaped metal components. Currently, most of the parts prepared by MIM process still use traditional material grades, and specific component design is needed for specific mechanical property requirements. There are few metal materials suitable for MIM process that have high wear resistance, corrosion resistance, and high compressive strength, and there is no mature grade available. This has seriously hindered the rapid development of the above-mentioned emerging fields. Therefore, it is imperative to develop a metal powder that can be used for MIM process and has high wear resistance, corrosion resistance, and compressive strength after sintering without the need for heat treatment. SUMMARY

[0004] In view of the current situation that there is no mature MIM material that can meet the performance requirements of high wear resistance and corrosion resistance, a stainless steel powder suitable for MIM process is invented. The main components of the powder are Fe-(12-13%) Al-(5-6%) Cr-(5-6%) Ni-(2-3%) Ti-(1.8-2%) Mn-(1.8-2%) Co. The powder is prepared by water atomization process, and through control of the atomization process and sintering process, excellent wear resistance, corrosion resistance, and high compressive strength can be obtained without the need for heat treatment.

[0005] The application discloses a preparation method of a heat-treatment-free high-strength stainless steel powder for MIM, and has the characteristics that: firstly, an alloy powder is prepared by using a water atomization powder preparation technology, and the composition of the alloy powder is Fe-(12-13%)Al-(5-6%)Cr-(5-6%)Ni-(2-3%)Ti-(1.8-2%)Mn-(1.8-2%)Co; during atomization, a non-vacuum induction melting mother alloy is firstly used, then the mother alloy is poured into a tundish with an atomization nozzle at the bottom, the nozzle is controlled by a stopper, and high-pressure water containing a certain content of formic acid is used as an atomization medium during atomization; by controlling the alloy temperature and the atomization process, the alloy powder with a medium diameter (D50) of 5-7 microns can be obtained, and the yield reaches more than 85%.

[0006] Further, the tundish is provided with an atomization nozzle at the bottom and the alloy flow is controlled by a stopper, the tundish is wound with an induction coil, on one hand, the alloy is continuously heated during the atomization process, so that the temperature drop is reduced, on the other hand, the alloy can be stirred by induction, so that the inclusions are floated and the purity of the alloy is improved; the tundish is pre-filled with slag, and the slag is floated with the injection of the alloy, on one hand, the alloy is protected from oxidation, on the other hand, the alloy inclusions are adsorbed by bottom argon blowing.

[0007] Further, the composition of the tundish slag is (9-10%)TiO2+(69-70%)CaF2+(4-5%)SiO2+(4-5%)Al2O3+(9-10%)FeO, and the temperature of the alloy during atomization is 1580-1600 DEG C.

[0008] Further, the pressure of the atomization water is 30-40 MPa, and the content of the formic acid is 15-18%. Further, when the powder is used for injection molding, the debinding can adopt a conventional process of solvent debinding and then thermal debinding, the thermal debinding is followed by sintering in a furnace, the heating rate is 5-6 DEG C / min when the debinding temperature is below 700 DEG C, the temperature is kept at 700 DEG C for 60-90 min, then the temperature is increased to 1260-1280 DEG C at a speed of 10 DEG C / min for sintering, and the sintering is kept for 60-90 min; after the temperature is kept, the temperature is decreased to 600 DEG C in the furnace for 50-60 min, and then the temperature is continuously decreased to the temperature for discharging.

[0009] Further, the powder injection molding process is used for the molding of parts, the binder can be wax-based or plastic-based, by controlling the sintering process, a stainless steel material with a density greater than 99% can be prepared, the material forms submicron precipitates during the sintering process, high compressive strength, wear resistance and corrosion resistance can be obtained without heat treatment, and the material is suitable for gears, frames and other parts of 3C electronic products and humanoid robots.

[0010] The mechanism of the present application is that a certain amount of Ni, Al, Co and Ti elements are added in Fe, and about 15wt% of Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase, which is small in size, high in hardness and uniformly distributed in the matrix, can greatly improve the compressive strength and wear resistance of the alloy, and the addition of appropriate amount of Cr can ensure excellent corrosion resistance of the alloy together with Al 42 Ni 28 Fe 14 Co8Ti8 from the Fe matrix.

[0011] Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase, on the one hand, as a second phase to strengthen the matrix, on the other hand, it can prevent the growth of the matrix grains during sintering, can significantly refine the grains, and play the role of fine-grain strengthening. Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase is easily precipitated out during water atomization with the solidification of the powder, which will affect the inhibition of the grain during the subsequent sintering process, and the size of the strengthening phase precipitated during the solidification process is large and cannot play the role of dispersion strengthening, therefore, a proper amount of formic acid is added in the high-pressure water during the water atomization process, and the high-pressure water impacts the alloy liquid stream at high speed during atomization, and the formic acid in the water preheats and rapidly evaporates and decomposes into hydrogen and carbon dioxide, on the one hand, the cooling capacity of the water is enhanced, and the Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase, which is small in size, high in hardness and uniformly distributed in the matrix, can greatly improve the compressive strength and wear resistance of the alloy, and the addition of appropriate amount of Cr can ensure excellent corrosion resistance of the alloy together with Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase, the precipitation temperature is 500-700℃, and after the debinding, the sintering densification continues in the furnace, Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase can prevent the growth of the grains and refine the grains, and when the temperature reaches 1260-1280℃, Al 42 Ni 28 Fe14 Co8Ti8 strengthening phase will dissolve into the matrix, and instantaneously produce 0.2% liquid phase to promote densification sintering. After high-temperature densification sintering is completed, the furnace is cooled down, and when the temperature is lowered to 500-700℃, Al 42 Ni 28 Fe 14 Co8Ti8 strengthening phase is re-precipitated from the matrix, and plays a role of second-phase strengthening. Finally, through second-phase strengthening and fine-grain strengthening, the compressive strength of the MIM sintered body exceeds 1900MPa, which is much higher than the strength of the current heat-treated MIM 17-4PH, the wear resistance is more than 2 times higher than that of 17-4PH and other conventional stainless steels, and meanwhile, good corrosion resistance is maintained.

[0012] The advantages of the present application are: (1) by adding formic acid in high-pressure water, the particle size of the atomized powder can be significantly reduced, while maintaining high tap density, the median particle size of the atomized powder is 5-7 microns, and the fine powder yield is more than 80%, which is much higher than the current industry level; (2) through component design, excellent comprehensive performance can be obtained without heat treatment, especially the compressive strength and wear resistance are much better than those of traditional MIM martensitic stainless steel, which simplifies the process and reduces the cost; (3) the slag composition designed according to the alloy composition of the present application can effectively absorb inclusions, improve the purity of the powder, and ensure that the alloy finally has excellent performance. DETAILED DESCRIPTION

[0013] Example 1: Preparation of MIM high-strength stainless steel based on the powder of the present application using wax-based binder

[0014] First, the stainless steel powder with a median particle size of 5-7 microns is prepared by water atomization according to the component ratio of Fe-12%Al-5%Cr-6%Ni-3%Ti-2%Mn-1.8%Co; then the powder is mixed with a wax-based binder to obtain a feedstock, the binder composition is 70% paraffin+15% high-density polyethylene+10% polypropylene+5% stearic acid, and the proportion of the powder is 63vol%; then the feedstock is formed into a part of the required shape by an injection molding machine, and then solvent debinding is carried out, and after drying, thermal debinding and sintering are carried out in a debinding and sintering furnace, the debinding heating rate is 5℃ / min, and 700℃ is maintained for 60min; then the temperature is raised to 1260℃ at a rate of 10℃ / min for sintering, and the sintering holding time is 60min; after holding, the furnace is cooled down to 600℃ for 50-60min, and then the temperature is continuously lowered to the discharge temperature. The mechanical properties are tested, the compressive strength reaches 1950MPa, the wear resistance is 2 times higher than that of 17-4PH stainless steel under the same test conditions, and the corrosion resistance is equivalent to that of conventional stainless steel.

[0015] Example 2: Preparation of MIM high-strength stainless steel based on the powder of the present application using plastic-based binder

[0016] First, the water atomization powdering is carried out according to the component proportioning Fe-13%Al-6%Cr-5%Ni-2%Ti-1.8%Mn-2%Co to obtain the stainless steel powder with the median diameter particle size of 5-7 microns; then the powder is mixed with the plastic-based binder to obtain the feedstock, the binder component is 50% polyoxymethylene + 20% microcrystalline wax + 15% high-density polyethylene + 10% dibutyl sebacate + 5% stearic acid, and the proportion of the powder is 63vol%; then the feedstock is formed into the required shape of the part through the injection molding machine, then catalytic debinding is carried out, and then thermal debinding and sintering are carried out in the debinding sintering furnace, the debinding heating rate is 6℃ / min, 700℃ is kept for 90min; then the sintering is carried out at the speed of 10℃ / min to 1280℃, the sintering holding time is 90min; after the holding, the furnace is cooled to 600℃ for 60min, and then the cooling continues until the ejection. The mechanical properties are tested, the compressive strength reaches 1930MPa, the wear resistance is increased by 2 times compared with 17-4PH stainless steel under the same test conditions, and the corrosion resistance is equivalent to that of the traditional stainless steel.

Claims

1. A method for preparing heat-free high-strength stainless steel powder for MIM, characterized in that: First, alloy powder is prepared using water atomization powder preparation technology. Its composition is Fe-(12-13%), Al-(5-6%), Cr-(5-6%), Ni-(2-3%), Ti-(1.8-2%), Mn-(1.8-2%), and Co. During atomization, the master alloy is first melted using non-vacuum induction melting, and then the master alloy is poured into an intermediate ladle with an atomizing nozzle at the bottom. High-pressure water containing a certain amount of formic acid is used as the atomizing medium. By controlling the alloy temperature and atomization process, alloy powder with a median diameter (D50) of 5-7 micrometers can be obtained, and the yield can reach more than 85%.

2. The method for preparing heat-free high-strength stainless steel powder for MIM according to claim 1, characterized in that: The tundish has an atomizing nozzle at the bottom, and the alloy flow is controlled by a stopper rod. The tundish is wound with an induction coil, which continuously heats the alloy during atomization to reduce temperature drop. On the other hand, the alloy can be stirred by induction, which helps to float inclusions and improve the purity of the alloy. The tundish is pre-filled with slag. As the alloy is injected, the slag floats up, which protects the alloy from oxidation. In addition, bottom blowing of argon helps to adsorb inclusions in the alloy.

3. The method for preparing heat-free high-strength stainless steel powder for MIM according to claim 2, characterized in that: The composition of the tundish slag is (9-10%)TiO2+(69-70%)CaF+(4-5%)SiO2+(4-5%)Al2O3+(9-10%)FeO, and the temperature during alloy atomization is 1580℃~1600℃.

4. The method for preparing heat-free high-strength stainless steel powder for MIM according to claim 1, characterized in that: The pressure of the atomized water is 30-40 MPa, and the formic acid content is 15-18%.

5. The method of using the heat-free high-strength stainless steel powder for MIM prepared according to claim 1, characterized in that: When using the powder of this invention for injection molding, the degreasing process adopts the conventional process of solvent degreasing followed by hot degreasing. After hot degreasing, sintering is carried out in the furnace. The heating rate during degreasing below 700°C is 5-6°C / min, and the temperature is held at 700°C for 60-90 min. Then, the temperature is increased to 1260-1280°C at a rate of 10°C / min for sintering, and the sintering holding time is 60-90 min. After holding, the temperature is lowered to 600°C in the furnace and held for 50-60 min, and then the temperature is further lowered until the product is removed from the furnace.

6. The method of using the heat-free high-strength stainless steel powder for MIM according to claim 5, characterized in that: Parts are formed using powder injection molding, with wax-based or plastic-based binders. By controlling the sintering process, stainless steel materials with a density greater than 99% can be prepared. During sintering, this material forms submicron-sized Al precipitates. 42 Ni 28 Fe 14 Co8Ti8 has fine grains and can achieve high compressive strength, wear resistance and corrosion resistance without heat treatment, making it suitable for gears and frame components in 3C electronics and humanoid robots.