JPS6366361B2 - - Google Patents

Description

[Detailed description of the invention]

This invention contains carbon 0.1wt% or more, oxygen 0.2wt% or less, Mn 0.5 to 7wt%, and Cr, V, Nb,
The present invention relates to a method for decarburizing steel powder containing one or more easily oxidizable elements having lower oxide formation free energy than Fe, such as B and Si. Steel powder, which is a raw material for powder metallurgy products, is required to have a low oxygen content and, except for very special uses, a low carbon content. This is because oxygen and carbon in the steel powder adversely affect the compressibility, formability, sinterability, etc. of the steel powder. By the way, water atomization method, gas atomization method, and oil atomization method are known as methods for producing steel powder. The steel powder produced by the water atomization method is easily oxidized by water, which is a spray medium, and has a high oxygen content. In particular, Cr,
Steel powder containing easily oxidizable elements such as Mn and V is easily oxidized, and even when the obtained oxidized steel powder is reduced, it is difficult to reduce the amount of oxygen to the required level. 1000
Even if the reduction treatment is carried out at temperatures above ℃ for 5 hours, the oxygen content can only be reduced to about 0.4 wt%. Another deoxidizing method is to intentionally add carbon as a reducing agent to the atomized steel powder itself, and then heat it at high temperature in a vacuum to remove oxygen and carbon through the C+O→CO reaction. A special method has been proposed. However, although this method can reduce the oxygen content considerably, it is necessary to maintain the amount of carbon remaining in the final steel powder above a certain level in order to lower the oxygen content.
Their equipment and operations are becoming more complex. The gas atomization method is a method of manufacturing steel powder using so-called inert gases such as N ₂ and Ar as atomizing medium, and it is possible to reduce the content of carbon and oxygen, but on the other hand, it produces large amounts of There were problems in that the required inert gas was expensive, and the particles of the obtained product were spherical, resulting in poor sinterability. The oil atomization method is a method for producing steel powder using oil as a spray medium, and compared to the water atomization method,
Although the obtained steel powder is excellent in that no oxidation occurs (low oxygen content), there is a problem in that decarburization treatment must be performed because it is carburized during atomization. Moreover, at present, no continuous and efficient decarburization treatment method has been found after oil atomization. This invention contains 0.1 wt% or more of carbon, 0.2 wt% or less of oxygen, 0.5 to 7 wt% of Mn, and Cr, V, Nb, B,
In order to obtain steel powder containing one or more types of Si elements, steel powder with a relatively low oxygen content of 0.2wt% or less and a relatively high carbon content is used, and decarburization is completed before oxidation progresses. This method proposes a method, and its characteristics are: 750℃≦t≦1250℃, _PH2O / _{PH2 ≦} 0.04, where t _: atmosphere Temperature (°C) PH ₂ O: Partial pressure of water vapor PH ₂ : Partial pressure of hydrogen, and the treatment time is 15 minutes or less, preferably 10 minutes or less. Here, "atmospheric pressure or higher" means that the pressure is slightly more positive than atmospheric pressure in order to prevent outside air from entering the equipment. This invention method will be explained in detail below. First, the inventors discovered that the steel powder contains
We investigated the atmosphere for decarburizing 0.1wt% or more of carbon. In order to decarburize the carbon in the steel powder, it is conceivable to create an atmosphere in which as much oxygen as possible, which reacts with carbon, is added as much as possible. However, in an atmosphere containing oxygen, Fe, the main component of steel powder, or Cr, Mn, V, Nb, which is more easily oxidized than Fe,
This results in oxidation of easily oxidizable elements such as B and Si. Therefore, it is important to select an atmosphere that promotes decarburization and is relatively non-oxidizing. By the way, dry pure hydrogen does not have much decarburization property, but in a humid atmosphere, the decarburization reaction is promoted by the action of water vapor. Furthermore, since water vapor has oxidizing properties, if the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is appropriately selected, efficient decarburization can be performed without oxidizing Fe and easily oxidizable elements. Therefore, the inventors conducted an experiment on a decarburization atmosphere that does not cause effective oxidation in an atmosphere consisting of water vapor, H ₂ , and N ₂ . Figure 1 shows the results of an experiment to investigate the atmospheric conditions.
%, Mn: 1.5wt%) in a stainless steel container.
H ₂ O at a slightly positive pressure above atmospheric pressure, maintained at a bed height of mm,
The values of PH ₂ O/PH ₂ were varied in an atmosphere of H ₂ and N ₂ , and the carbon ( C ) content and oxygen ( O ) content after 10 minutes were determined.
The content was investigated. However, the atmospheric temperatures at this time were 800°C and 1050°C. From this experimental result, PH ₂ O/ It is best to keep the PH ₂ value below 0.04. Therefore, in this invention method, the atmospheric condition was set to PH ₂ O/PH ₂ ≦0.04. Figure 2 shows the results of an experiment investigating the ambient temperature. :1.5wt%) was kept at a layer height of 3mm in a stainless steel container, and the atmosphere inside the furnace was
PH ₂ O / PH ₂ = 0.03, PH ₂ = 70%, the remaining N ₂ gas was used to make the pressure slightly positive than atmospheric pressure, and the ambient temperature was varied to determine the treatment time and the content of carbon ( C ) and oxygen ( O ). We looked at changes in the rate. From FIG. 2, it can be seen that decarburization is more effective as the ambient temperature is raised, but oxidation progresses as time passes. Due to both decarburization and oxidation, the carbon content can be easily decarburized to 0.1 wt% or less at temperatures above 750°C within 10 minutes, and extreme oxidation can also be prevented. Furthermore, from Figure 2, it can be seen that the higher the ambient temperature is, the slower the oxidation progresses, but the more rapidly decarburization progresses. It can be seen that it is. However, as a result of investigating the phenomenon of particles adhering to each other when steel powder was used under the same conditions as in FIG. 2 and the ambient temperature was variously changed in the atmosphere, the following was found. That is, FIG. 3 illustrates the adhesion force when the ambient temperature is maintained for 20 minutes. This third
The figure shows that when the temperature exceeds 1250°C, the mutual adhesion of the steel powder rapidly becomes strong, making it difficult to crush the steel powder after decarburization, which is not a good idea. From the results shown in FIGS. 2 and 3 above, the atmospheric temperature (t°C) for steel powder treatment is preferably in the range of 750°C≦t≦1250°C. From these experimental results, it was found that 0.1wt% or more of carbon and 0.2wt% of oxygen were produced from molten metal by the spray method.
The following contains Mn0.5 to 7wt%, and Cr, V,
Steel powder containing one or more easily oxidizable elements such as Nb, B, and Si is heated in an atmosphere above atmospheric pressure at 750℃≦t≦1250℃, PH ₂ O/PH ₂ ≦0.04, where t: atmosphere Temperature (°C) PH ₂ O: Water vapor partial pressure PH ₂ : By decarburizing the treatment time under the conditions of 15 minutes or less, preferably 10 minutes or less, the harmful effects of steel powder sticking together can be avoided. It becomes possible to reduce the carbon content to the desired 0.1 wt% or less while maintaining the oxygen content at a predetermined value in a short period of time. Next, an example of this invention method will be described.
First, in presenting examples, an overview of the test equipment used will be explained. Figure 4 shows the outline of the test equipment, and the supply systems for H ₂ O (steam), H ₂ , and N ₂ gases are supplied into the decarburization furnace 1 in order to adjust the atmospheric gas. The respective gases from these supply systems are uniformly mixed in a gas mixer 10, and while being heated by a heating heater 8, a desired decarburizing atmosphere gas is supplied to the decarburizing furnace 1. It is now supplied internally. On the other hand, the sample steel powder 7 is taken from the sample outlet 4 at the right end of the figure in the decarburization furnace 1.
It is charged into the furnace 1 and heated to a predetermined temperature by a radiant tube 2 provided on the furnace ceiling. In this way, decarburization is performed while maintaining the atmospheric gas and temperature,
The generated exhaust gas is discharged from the exhaust gas outlet 3, and after a certain period of time has passed, the heating is stopped and only _N2 gas is supplied from near the sample extraction port 4 to cool it to near room temperature, and the cooled sample steel powder is sampled. The structure is such that it is taken out from the furnace through outlet 4. In addition,
Reference numeral 6 indicates a sample container with an axle, and 5 indicates a sample transport rail used for loading and unloading the sample container. Example 1 Having the chemical composition and particle size distribution shown in Table 1
Cr-Mn based low alloy steel powder was decarburized using the testing apparatus shown in Figure 4. The treatment conditions at that time are shown in Table 2, and the chemical composition and particle size distribution of the steel powder after decarburization are shown in Table 3.
Shown in the table. As is clear from the results in Table 3, the oxygen content can be kept below 0.1wt% without increasing significantly, and the carbon content can be reduced to 0.51wt% without any negative effect on other chemical components or particle size distribution. % to 0.05wt%.

【table】

【table】

[Table] Example 2 This example concerns low-alloy steel powder containing V. Steel powder having the chemical composition and particle size distribution shown in Table 4 was decarburized using the testing apparatus shown in Figure 4. Processed. The treatment conditions at that time are shown in Table 5, and the chemical composition and particle size distribution of the steel powder after decarburization treatment are shown in Table 6. As is clear from Table 6, the carbon content can be reduced from 0.47wt% without significantly increasing the oxygen content, keeping it below 0.1wt%, and without having any adverse effects on other chemical components or particle size distribution. It was possible to decarburize down to 0.04wt%.

【table】

【table】

【table】

[Table] Example 3 This example concerns SUS304 steel powder, in which steel powder having the chemical composition and particle size distribution shown in Table 7 was decarburized using the testing apparatus shown in FIG. 4.
The treatment conditions at that time are shown in Table 8, and the chemical composition and particle size distribution of the steel powder after decarburization treatment are shown in Table 9. As is clear from Table 9, the oxygen content can be kept below 0.1wt% without increasing significantly, and the carbon content can be lowered from 0.57wt% without any adverse effects on other chemical components or particle size distribution. It was possible to decarburize down to 0.09wt%.

【table】

【table】

[Table] As is clear from the above examples, carbon
Steel powder containing 0.1wt% or more of Mn, a relatively low oxygen content, and one or more easily oxidizable elements such as Cr, V, Nb, B, and Si can be shortened by the method of the present invention. It became possible to perform decarburization treatment in a short amount of time. Regarding easily oxidizable elements such as Nb and B,
Although no examples have been given, it goes without saying that the same effects as in the previous examples can be obtained.

[Brief explanation of drawings]

Figure 1 shows PH ₂ O/
A chart showing the relationship between PH ₂ and carbon and oxygen content. Figure 2 is a chart showing the relationship between time and carbon and oxygen content when various temperatures are changed to investigate the ambient temperature. Figure 3 is a chart showing the relationship between PH 2 and carbon and oxygen content. 4 is a diagram showing the relationship between the atmospheric temperature and the adhesion force of steel powder for investigating the upper limit of the atmospheric temperature, and FIG. 4 is an explanatory diagram showing the outline of a test apparatus in an embodiment of the present invention. 1... Decarburization furnace, 2... Radiant tube, 3
...Exhaust gas outlet, 4...Sample extraction port, 5...
Sample transport rail, 6...Sample container with axle, 7...Sample, 8...Heating heater, 9
...Pressure gauge, 10...Gas mixer.

Claims (1)

[Claims] 1 Carbon 0.1wt% or more, oxygen 0.2wt% or less, Mn 0.5
~7wt% and at least one of the elements Cr, V, Nb, B, and Si.
In an atmosphere above atmospheric pressure consisting of _H2O and _H2 , 750℃≦t≦1250℃, _PH2O / _PH2 ≦0.04, where t: ambient temperature (℃) _PH2O : water vapor partial pressure _PH2 : A method for processing steel powder characterized by processing under hydrogen partial pressure conditions in a processing time of 15 minutes or less.