JPH0372002A - Alloy steel powder for parts having high strength, high toughness and high accuracy and manufacture of sintered alloy steel using this - Google Patents

Abstract

PURPOSE:To give high strength and high toughness and to reduce dispersion of dimensional variation at the time of heat treatment by sticking graphite powder to steel powder with specified contents of Ni, Mo and C, forming, sintering and applying quenching and tempering treatment. CONSTITUTION:The alloy steel powder having composition of 0.5-3.5wt.% Ni, 0.7-3.5% Mo, 0.3-0.8% C and the balance of substantially Fe, is manufactured. C in the alloy steel powder is stuck on surface of steel powder particles with eutectic material of lubricant and binder in the form of graphite powder. To this alloy steel powder, one or more times of the formings and one or more times of the sinterings are executed. To this sintered alloy steel, the quenching and annealing treatments are applied. This alloy steel is available to machining parts for car needing the high strength, high toughness and high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field 1] The present invention belongs to the field of iron-based powder metallurgy technology, and relates to an alloy steel powder for high strength, high toughness, and high precision parts, and a method for producing this alloy steel powder. The present invention relates to a method for manufacturing sintered parts with less variation in dimensional change during heat treatment using the present invention.

[Conventional Technology J] Powder metallurgy technology has advanced significantly in recent years, and sintered parts have been increasingly applied to transportation equipment and industrial equipment.

Automotive gears and the like are parts that require high strength, high toughness, and high precision. When these parts are manufactured by powder metallurgy, it is necessary to add alloying elements and further heat treatment in order to increase the strength.

As described in Japanese Patent Application Laid-Open No. 62-146203, attempts have been made to heat treat the alloy sintered body, but the tensile strength is only 120 kgf/mrn", and higher strength is not required. I can't deal with it (/X.

In addition, in JP-A-61-231102, Ni, Mo
Attempts have been made to increase the alloying element and C using a binder to maintain high compressibility and high strength, but in this case, the tensile strength is 120
kgf/mrn''.Additionally, the addition of alloy components such as Ni and Mo has the effect of lowering the martensitic transformation (Ms) point, and when heat treatment such as quenching and tempering is performed, austenite is generated and the dimensions are This may cause variation in changes.

[Problem to be Solved by the Invention 1] The present inventors have previously proposed a composite alloy powder and a method for producing the same for achieving high strength and high toughness of a sintered body (Japanese Patent Application No. 137400/1982).

In this method, with the reduction of Nil, the formation of austenite in the sintered body is suppressed, and high strength and high toughness are achieved. On the other hand, when carburizing and quenching is performed to increase strength, there is a problem that dimensional accuracy varies due to overcarburizing.

The present invention further improves the dimensional accuracy during heat treatment, and provides (1) an alloy steel powder for sintered bodies with high strength, high toughness, and excellent dimensional accuracy during heat treatment, and (2) a method for producing sintered alloy steel. It gives

[Means to solve the problem] The present inventors have attempted to improve the strength and toughness of a sintered body.

As a result of research into high precision, the following findings were made and the present invention was completed. In other words, (1) Chemical composition of alloyed steel powder: Ni: 0.5-3.5% by weight, Mo: 0.7-3.5% by weight, C: 0.3-0.8% insectic acid, balance Fe. (2) Attach graphite powder to M powder using a co-melt of lubricant and binder to suppress C segregation in the sintered body, (3) High strength by quenching and tempering. It was found that a sintered alloy steel with high toughness and little variation in dimensional change during heat treatment can be obtained.

By limiting the chemical composition as described in (1) above,
This prevents a large amount of austenite from forming in the sintered body, which causes a decrease in strength and variation in dimensional changes during heat treatment.

In addition, the segregation of C in the sintered body causes non-uniform transformation in the sintered body during quenching, increasing the variation in dimensional changes. Suppresses C segregation in the body.

Furthermore, quenching and tempering treatments are necessary to toughen the sintered body.

[Operation] The reasons for limiting the components, heat treatment conditions, etc. of the present invention are as follows. The content of alloy components is determined based on requirements for mechanical properties.

Nil is set at 0.5 to 3.5% by weight. In order to obtain the effect of improving strength and toughness, it is necessary to add 0.5% by weight or more. On the other hand, if the content exceeds 3.5i, excessive austenite will be produced in the sintered body.

Excessive production of austenite increases the variation in dimensional changes during heat treatment and also causes a decrease in strength.

The amount of Mo is 0.7 to 3.5% by weight. Like Ni, 0.7% by weight or more is required to achieve high strength and toughness, and if it exceeds 3.5%, the toughness of the sintered body is adversely affected.

The content of C is 0.3 to 0.8% by weight. If it is less than 0.3% by weight, it does not contribute to improving the strength of the sintered body. Moreover, if it exceeds 0.8% by weight, toughness will be significantly impaired.

Regarding the method of adding Ni and Mo, production can be performed by either a pre-alloying method in which alloying is performed at the molten metal stage, or a composite alloying method in which Ni and MoFL particles are diffusion bonded to iron powder.

Next, a method for attaching graphite powder to the above-mentioned steel powder will be described. The lubricant may be one commonly used in powder metallurgy, such as metal soaps such as zinc stearate, and the binder may be, for example, vinyl acetate, ethyl cellulose, methyl cellulose,
The lubricant and binder, which may be thermoplastic such as phenol resin, cellulose derivative, thermosetting, etc., are thoroughly mixed with the steel powder and graphite powder at the same time or in stages until they are homogeneous. . 9 in a uniformly mixed state
Heating to 0°C to 150°C creates a co-melt of lubricant and binder, which fixes the graphite powder to the copper powder. Heating temperature is 90
Adhesion is insufficient below ℃. However, the heating temperature is 15
There is no improvement in adhesion even at temperatures above 0°C.

Powder molding is performed at a molding pressure of 4 to Lo, as in normal powder metallurgy.
The number of times of molding and sintering performed at t/cm'' may be one or more times, but higher strength and toughness can be obtained by increasing the density.

In the present invention, it is also essential to subject the sintered body to quenching and tempering treatments. Quenching treatment refers to a treatment in which steel is heated to a predetermined temperature and then rapidly cooled. Tempering treatment refers to a treatment in which steel is heated to a predetermined temperature and then air cooled or slowly cooled. The quenching temperature is a process in which a uniform austenite structure is formed during heating. Therefore, the temperature is desirably 800°C or higher.If the temperature exceeds 950°C, the austenite becomes coarse and the toughness decreases, so the upper limit temperature is set at 950°C.

If the tempering temperature is less than 100°C, the toughness will be low, and if it is higher than 250°C, the strength will be reduced.

100-250°C is preferred.

[Example 1 Example 1 180 mesh water atomized Fe, 1325 mesh carbonyl Ni powder and Mo trioxide (Mo03) were mixed at a predetermined ratio, and the mixture was heated at 875°C in a hydrogen gas atmosphere.
Annealed for 60 minutes, crushed, Ni: 1.76i amount%, M
A composite alloy steel powder containing o: 1.0 Offi amount % was obtained.

Above 'steel powder, graphite powder 0.6% by weight, zinc stearate 1
．． 0% by weight and 0.2% by weight of oleic acid were mixed and stirred at a temperature of 120° C. for 10 minutes to fix graphite powder on the surface of the steel powder.

Next, the above steel powder was molded using a cemented carbide mold at a pressure of 7t/cm', and sintered at 850°C for 30 minutes in a 75%H2-25%N2 gas (dry) atmosphere. It was recompressed at a pressure of "crn".

Furthermore, 1 in a 75% H2-25% N2 gas (dry) atmosphere.
Re-sintering was performed at 250°C for 60 minutes, and the resulting sintered body had a
After heating at 0℃ for 60 minutes, quenching in oil at 60℃, then 180℃
It was tempered by holding it in oil at ℃ for 60 minutes, and then subjected to a tensile test and a Charpy impact test.

In addition, the outer diameter is 60 mm, the inner diameter is 25 mm, and the height is 5.5 m.
10 m rings were molded, and at each step, the outer diameter (X, Y) of each ring was measured in two directions perpendicular to each other.
The standard deviation of the absolute value of the difference in change (ΔX−ΔY) in each step was determined and the one-dimensional accuracy was evaluated. Table 1 shows the tensile test, Charpy impact value, sintered body density, and dimensional accuracy before and after heat treatment.

In addition, the same table shows the results of an experiment conducted under the same conditions as in this example using the same composite steel powder as in this example, mixed with 0.6% by weight of graphite powder and 1.0% by weight of zinc stearate, as a comparative example. Shown as

There is no obvious difference in tensile strength, Charpy impact value, or sintered compact density between the two, but the dimensional accuracy of C with segregation prevention is 0.012%, or when simply mixed powder is 0.012%.
．． 025%, and by applying segregation prevention treatment to C, the variation in dimensional change is reduced to approximately 1/2.

Example 2 The addition amount of 180 mesh water atomized Fe layer and 1325 mesh carbonyl Ni layer was changed, MoO3 layer was mixed for a certain period, and Ni: 0.31 to 5 was added in the same manner as in Example 1.
．． A composite alloy steel powder of lO heavy duty was obtained.

0.6% of graphite powder was fixed to these steel powders in the same manner as in Example 1, and then a heat-treated sintered body was produced in the same manner as in Example 1, and the tensile strength, Charpy impact value, The density and dimensional accuracy of the sintered body were set at 1l11. The results are shown in Table 2.

The one containing 0.31i% Ni has little variation in dimensional change, but has low strength and toughness.

Furthermore, it can be seen that when the Nl content is 3.82% by weight or more, the strength is somewhat lowered and the dimensional accuracy is significantly lowered.

Example 3 Here, the influence of the amount of C added is shown.

The same composite alloy steel powder as in Example 1 was added with 0.3% by weight of graphite powder.
．． 6% by weight and 0.9% by weight of steel powder fixed in the same manner as in Example 11, and graphite powder of 0.2% by weight, 0.4% by weight, 0.6% by weight, and 0.9% by weight. A steel powder was produced by simply mixing 1.0% by weight of zinc stearate.

Using these steel powders, sintered bodies were produced in the same manner as in Example 1, subjected to heat treatment, and measured for tensile strength, Charpy impact value, sintered body density, and 1-dimensional accuracy.

The results are shown in Table 3.

When the amount of graphite added is 0.3% and 0.6%.

The variation in dimensional changes is significantly smaller in the segregation-prevention treatment than in the simple mixed powder.

Similar results can be seen when the amount of graphite added is 0.9% by weight, but both the strength and toughness decrease rapidly. Also, 0
．． At 2% by weight, the strength is low.

Example 4 Here, the influence of the amount of Mo added will be shown.

-80 meshes of water atomized Fe coatings were mixed with a certain amount of =325 meshes of carbonyl Ni coatings and MOO3 coatings in varying amounts, and the same method as in Example 1 was carried out, with Ni: 1.76% by weight, Mo: 0. Composite alloy steel powder containing 50% to 4.50% by weight was obtained.

Graphite was added to these steel powders in the same manner as in Example 2, and the tensile strength, Charpy impact value, and variation in 1 dimensional change in sintered body density were measured. The results are shown in Table 4. The tensile strength of the one containing 0.5% by weight of Mo was 142.2 kg.
f/mrw? If the MO content exceeds 3.5% by weight, the Charpy impact value is extremely low, and a sharp decrease in toughness is observed. Moreover, the dimensional accuracy has also decreased rapidly.

Example 5 Ni: 1.94% by weight by water atomization.

Pre-alloyed steel powder containing Mo: 1.05% by weight was produced, subjected to reduction treatment in a hydrogen gas atmosphere, and then classified using a -80 mesh to obtain pre-alloyed steel powder having the above composition.

Regarding the above steel powder, experiments similar to those in Example 1 were conducted to measure various properties of the simple mixed powder and the graphite-fixed powder. Table 5 shows the results.

For the pre-alloyed steel powder, the variation in dimensional accuracy was 0.022% for the simple mixed powder, but it was 0.0IQ% for the graphite-fixed powder, and the same effect as the composite alloyed steel powder was obtained.

[The Power of the Invention] Based on the results described above, the present invention provides a sintered alloy steel that has extremely high strength and toughness, and has little variation in dimensional change during heat treatment, and has high strength and I. ';;A for automatic iris that requires toughness and high precision! It is for ff on mechanical parts.

11 applicants Kawasaki Steel Co., Ltd. teenager Reason Man excuse small Thirteen residence lf

Claims (1)

[Claims] 1. As alloy components: Ni: 0.5% to 3.5% by weight Mo: 0.7% to 3.5% by weight C: 0.3% to 0.8% by weight An alloy steel powder containing % by weight or less, the remainder being substantially Fe,
An alloy steel powder for use in high-strength, high-toughness, and high-precision parts, characterized in that the C is fixed to the surface of copper powder particles in the form of graphite powder by a co-melt of a lubricant and a binder. 2. Using the alloy steel powder described in paragraph 1, producing a sintered alloy steel by performing one or more forming operations and one or more sintering operations,
A method for producing a sintered alloy steel for high strength, high toughness, and high precision parts, which further comprises subjecting the sintered alloy steel to quenching and tempering treatments.