JPH032220B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing iron-based sintered materials, such as tool materials, which particularly require high strength and wear resistance. BACKGROUND OF THE INVENTION In recent years, the production of high-speed tool steel, cold-worked tool steel, etc. by the sintering method has become popular. These are prepared by press-forming pre-alloyed powder using a water atomization method and then sintering it. That is, a water atomized powder with a relatively large amount of oxygen but excellent moldability is used as a starting material, and this surface oxide is reduced by adding graphite powder to the powder, and the eutectic carbide in the powder material is reduced. This is due to the combination of two conditions: sintering in a vacuum or inert gas in a melting temperature range to achieve high density close to true density. These are high-speed tool steels such as end mills, drills, taps, bit tips, etc., and automobile-related products.
It is applied to Rotscar arms, camshaft rings, etc. Compared to ingot materials such as rolled materials, this sintered material has the advantage that it can be manufactured with a high material yield and reduced man-hours by setting the shape at the time of completion of sintering close to that of the final product. However, conventional sintered products have not been fully satisfactory in terms of performance. Next, using Experimental Example 1, problems with the performance of the conventional method will be illustrated with a concrete example. First, the weight ratio equivalent to JIS SKH9 is C0.91%,
Si0.23%, Mn0.20%, Cr4.10%, W5.95%,
A 100 mesh dry water atomized powder with an O ₂ content of 3200 PPm was prepared consisting of 5.02% Mo, 1.89% V, Fe Bal, and unavoidable impurities. Graphite powder is added to this powder as the stoichiometric amount of carbon to reduce the oxygen content to CO gas.
0.24% was added and subjected to reduction treatment in vacuum at 1000°C for 3 hours. The amount of oxygen after reduction is 960ppm, and the amount of carbon is
It was 0.83%. This reduced powder was used in the experiments described below. Experimental example 1 0.7% zinc stearate was added as a lubricant to this reduced powder, and after dry mixing, it was molded with a normal press at a pressure of 6 tonf/cm ² and heated at a temperature increase rate of 200°C in a vacuum of 10 ^-2 Torr. The temperature was increased to 1255°C at a rate of 1255°C at a rate of 1255°C, maintained at this temperature for 1 hour, and then cooled in the furnace for sintering.
The half-cooling time at this time was 2 hours, and this treatment followed the conventional manufacturing method. The half-cooling time refers to the time required for cooling to t/2°C, where the quenching temperature is t°C. The density of this sintered sample is 8.06 g/ ^cm3 , which is 400 g/cm3.
As a result of the microscopic examination at double magnification, it was determined that there were no residual pores and that the density was substantially true, and the austenite grain size was
It was 9.0 according to JIS method. Next, this sintered material was completely annealed, quenched, and tempered, along with a test piece cut from a 30mmφ steel bar of SKH9 with almost the same composition by the melting method as a comparison material. A bending test using a 40× specimen, hardness measurement, and microscopic examination were performed. The processing conditions are standard conditions for ingot lumber, annealing is 860℃ x 3 hours.
After heating and holding, cooling at 20℃/hr and quenching in a vacuum furnace.
After heating and holding at 1200°C for 20 min, cooling with 3 bar _N2 gas injection (half-cooling time 3 min), tempering at 560°C in the air
1hr x 2 times. The hardness and transverse rupture strength of the sintered material and ingot material are HRC63.1, 270Kgf/mm ² and
HRC66.1, 410Kgf/ ^mm2 . This result shows that the sintered material has significantly lower hardness and transverse rupture strength than the ingot material. The reason for this is that, as shown in the microstructure of the sintered material in Figure 1, the eutectic carbides are coarser than the standard structure of the sintered material, making it unstable in shape and prone to stress concentration, and the austenite crystal grains. This is presumed to be due to the roughness of the surface. It has been reported that the eutectic initiation temperature of eutectic carbides in high-speed tool steel is expressed by the following empirical formula: Te(〓)=2310−200(%C)+40(%V)+8(%
W) + 5 (%Mo) Substituting the components of the experimental example into this formula, Te = 2292
(〓) = 1256 (℃). On the other hand, the above raw material powder was heated to 1300°C at a heating rate of 200°C/hr using a differential thermal analyzer, and the eutectic initiation temperature was measured to be 1256°C.
The result almost coincides with the result of the above equation, and it was found that the above equation can be used to select the sintering temperature. The sintering temperature in the above-mentioned experimental example was also close to this temperature, and it is thought that the densification of the high-speed tool steel by sintering is progressing due to the formation of a partial liquid phase due to eutectic. When the densification of the material progresses in the presence of a liquid phase, the eutectic carbide becomes coarser and the shape of the carbide becomes amorphous. In addition, the coarsening of the eutectic carbide naturally coarsens the austenite grain size, and during the coarsening process, alloying elements such as C, W, Mo, and V in the matrix are absorbed by the carbide and alloyed with C in the matrix. The amount will decrease and the tempering hardness will decrease. Therefore, the conclusion is that the coarsening of carbides causes a decrease in strength and hardness of the sintered material. In other words, sintered high speed steel within the scope of the prior art is
This shows that it is not possible to obtain satisfactory performance compared to ingot lumber. Problems to be Solved by the Invention The present invention aims to provide an iron-based sintered material with improved performance by improving the conventional manufacturing method. Means for Solving the Problems The present invention sinters the eutectic carbide at a temperature higher than the eutectic start temperature in the temperature raising process, and from this temperature the eutectic carbide is sintered at a rate of 6°C/min from this temperature.
Iron characterized by being cooled to below the temperature at which eutectic carbide solidification ends in the cooling process at a cooling rate of 64°C/min or less, and then quenched by inert gas injection at a cooling rate of 10 min or more for half-cooling time. This is a method for producing a base sintered material. Effects and Examples The effects and effects of the present invention will be described below using experimental examples. Experimental Example 2 After molding under the same conditions as Experimental Example 1, similarly,
The sample, which has been sintered by holding it at 1255℃ for 1 hour in a vacuum furnace, is heated at 3bar from the top at this temperature.
Forced cooling was performed by blowing N ₂ gas. The half-cooling time was about 3 minutes. After cooling to room temperature, 560℃×1hr×2 without annealing or quenching.
A second tempering process was performed. The microstructure of this sample is shown in FIG. Compared to the structure in Experimental Example 1, the grain size of the eutectic carbide was slightly refined, and the austenite crystal grain size also showed a tendency to become finer in JIS9.5. Also, the density measurement result is 8.08g/
No residual pores were observed at cm ³ and the density was substantially the same. The transverse rupture strength of the same specimen as in Experimental Example 1 was 370.
Kgf/mm ² , hardness is HRC65.5, and compared to Experimental Example 1, by rapidly cooling and quenching immediately from the sintering temperature, the structure becomes finer and the mechanical strength and hardness are significantly improved. It turned out that. However, large distortions had occurred in the sample, which was expected to pose a serious obstacle to practical application. Since a partial liquid phase is present, probably due to a eutectic reaction, quenching is presumed to be accompanied by large strain. Experimental Example 3 After molding under the same conditions as Experimental Example 1, in the same way, after holding in a vacuum furnace at 1255°C for 1 hour, each was heated at approximately 8°C/min.
1245℃, 1235℃, 1225℃ and 1215℃ with cooling rate of
Same as Experimental Example 2 from each temperature.
Cooled with _N2 gas. The half-cooling time at this time is approximately 3 minutes.
It was hot. After this hardened sample was tempered under the same conditions as in Experimental Example 1, strain measurements, austenite grain size measurements, bending tests, and hardness measurements were performed. The results in Table 1 are shown together with the results for the sintered material and ingot material conducted in Experimental Example 1 and the sintered material conducted in Experimental Example 2.

[Table] From this table, it can be seen that strain decreases rapidly by setting the quenching cooling start temperature to 20 to 30°C lower than the sintering temperature. Since the eutectic solidification end temperature in the temperature decreasing process is lower than the eutectic start temperature in the temperature increasing process, in Experimental Examples 2 and 3, D and E, there was a large quenching strain because the phase liquid was rapidly cooled. seems to have occurred. Experimental Example 4 Next, an experiment was conducted to investigate the influence of the cooling rate from the sintering temperature to the eutectic extinction temperature. After molding under the same conditions as Experimental Example 1, after holding at 1255℃ for 1 hour.
Up to 1225℃, 2, 4, 16, 32 and 64℃/
After cooling at a cooling rate of min, quenching was performed by injecting N ₂ gas at 3 bar, tempering was performed under the same conditions as in Experimental Example 1, and the characteristics were investigated under the same conditions as in Experimental Example 3. The half-cooling time of each quenching was 3 min. The results are shown in Table 2, and the cooling rate of 8° C./min is shown together with F in Table 1.

【table】

[Table] From this table, it can be seen that as the temperature decreases to 1225℃, that is, the eutectic disappearance temperature, the austenite crystal grain size becomes slightly coarser, the hardness and transverse rupture strength decrease slightly, and the quenching strain increases. I understand. The microstructure observation results confirmed that as the cooling rate decreased, the eutectic carbides became somewhat coarser and became amorphous. FIG. 3 shows the microstructure of sample J. 1225
By setting the cooling rate to 16°C/min relatively fast, the microstructure of the eutectic carbide can be made extremely fine. Considering the results of Experimental Example 3 as well, it can be seen that cooling at an extremely high rate in the presence of a liquid phase causes quenching distortion, and conversely, when the cooling rate is extremely reduced, the structure becomes coarser and distortion increases. Intermediate 4-64℃/
It can be seen that there is an appropriate cooling rate around min. Therefore, in the present invention, the lower limit of this cooling rate is set to 6° C./min. Experimental example 5 Weight ratio: C1.6%, Si0.6%, Mn0.5%, Cr12.6
%, Mo1.0%, V0.4%, JIS consisting of Fe Ba
Graphite powder was added to -100 mesh water atomized powder equivalent to SKD11, and reduced in vacuum at 1000°C for 3 hours. The amount of residual O ₂ was 820 ppm, and the amount of C was 1.5%. Differential thermal analysis of this powder revealed that the eutectic initiation temperature of the eutectic carbide during the heating process was 1235°C, and the liquid phase disappearance temperature during the cooling process was 1202°C. 6tonf of the same powder/
After cold forming at ^cm2 , the temperature was raised to 1235℃ at a temperature increase rate of 200℃/hr, and after holding the temperature for 1 hour, it was quenched under the following conditions and tempered twice at 550℃ x 1hr. , austenite grain size, bending test,
Hardness measurements and microscopic examination were performed. (M) After temperature maintenance, half-cooling time to room temperature: Furnace cooling for about 2 hours, annealing at 850℃ x 3 hours, and after holding at 1050℃ x 1 hour, half-cooling time by 3 bar _N2 gas injection for about 3 minutes.
Quenching treatment (N) Immediately after holding the temperature, forced cooling to room temperature with a half-cooling time of approximately 3 minutes using _N2 gas injection at 3 bar (O) After holding the temperature, the temperature is lowered at a rate of 8°C/min to 1200°C, and from this temperature (N) Similarly, by _N2 gas injection, forced cooling to room temperature in about 3 minutes (P) After temperature maintenance, the temperature is lowered at 8℃/min to 1050℃, and from this temperature, by _N2 gas injection, like (N), it is half cooled. Forced cooling to room temperature in about 3 minutes Table 3 shows the results. This table also shows melted materials with almost the same composition that were held at 1050°C for 1 hour, then quenched with _N2 gas injection at 3 bar for a half-cooling time of about 3 minutes, and then tempered under the same conditions as above. The measurement results (Q) of the vines are also listed.

[Table] According to this table and the microscopic results, the above experimental examples 1 to 4
It can be seen that exactly the same phenomenon as in the case of SKH9 obtained in . In other words, the conventional method of furnace cooling and re-quenching after sintering can achieve true density, but it results in coarsening of eutectic carbides, coarsening of austenite crystal grains, and a decrease in transverse rupture strength and heat treatment hardness ( On the other hand, when quenching directly from the sintering temperature, the hardness and transverse rupture strength increase, but the strain is large (N), and the eutectic carbide solidification completion temperature ( With this component, by cooling to below 1202℃ and quenching from this state, hardness and transverse rupture strength comparable to that of ingot material can be obtained (O,
P) It turns out that. Effects of the Invention As described above, the present invention can be fired into a shape close to the final product shape, and has advantages in terms of yield, finishing efficiency, etc. However, in terms of performance, it has traditionally been inferior to ingot material. After sintering, iron-based sintered material is cooled to below the eutectic solidification completion temperature at an appropriate temperature cooling rate and then quenched to prevent excessive distortion and improve its performance to match that of molten material. This is particularly effective for tool applications. Further, the present invention utilizes the temperature decreasing process of sintering for quenching cooling, and has the effects of energy saving and process shortening.

[Brief explanation of the drawing]

Figures 1 and 2 are photographs of the micrometallic structure of a test piece produced by a conventional manufacturing method and quenched and tempered, and a test piece that was quenched directly from the sintering temperature and then tempered, respectively. . Figure 3 is a photograph of the micrometallic structure of a sample obtained by the method of the present invention, which was cooled at a cooling rate of 16°C/min to the solidification end temperature of the eutectic carbide, then quenched for a half-cooling time of 10 min, and then tempered. .

Claims (1)

[Claims] 1. In a method for producing an iron-based sintered material containing eutectic carbide and imparting strength and wear resistance through martensitic hardening, sintering is performed at a temperature higher than the eutectic start temperature of the eutectic carbide in the heating process, and this sintering 6 from temperature
After cooling at a cooling rate of ℃/min to 64℃/min to below the temperature at which eutectic carbide solidification is completed in the cooling process, inert gas is directly injected and half-cooling time is 10 mm.
A method for producing an iron-based sintered material, characterized by cooling and quenching at a rate equal to or higher than the above.