JPH0512424B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a sintered tool steel that has excellent wear resistance and heat resistance and is suitable as a high speed tool steel or a hot/cold work tool steel.

[Conventional technology]

In order to increase the wear resistance and heat resistance required for tool steel, hard and heat-resistant carbides, nitrides, or carbonitrides (carbides and nitrides usually exist as a composite, so below) Carbonitrides (hereinafter referred to collectively as "carbonitrides") must be present in as large a quantity as possible in the steel. Even if it is attempted to manufacture such a tool steel by a melting process, there are restrictions on processing and use of the tool steel in terms of hot workability, grindability, or toughness, so a sintering process is preferably employed. Powder metallurgy also has the advantage that fine carbides can be uniformly dispersed compared to melting methods. However, even in powder steel, the particle size and distribution state of carbides are influenced by the cooling rate during solidification during powder production, and if cooling is slow, carbide particles will grow. This means that when manufacturing powdered steel, in order to obtain fine carbides that are uniformly dispersed, only fine powder can be used, and large grains must be sieved out. This means that the yield will be lower. The grain size of carbonitrides also varies depending on the amount of added alloy, and tends to become larger as the alloy becomes higher. In general, the larger the amount of carbonitride, the lower the forging workability, but if the amount is the same, the decrease in forging workability is more significant as the grain size becomes larger. For this reason, the sintered steels that have been produced and used in the past have not been able to significantly exceed that of molten steel in terms of degree of alloying and carbide content.

[Problem to be solved by the invention]

The present invention solves these problems and obtains a powder alloy steel in which the particles do not become large even when a large amount of carbonitrides are present in the steel and are evenly distributed by making the steel higher in alloy than before. In addition, there are fewer restrictions due to the particle size of the powder, increasing the yield of usable powder, and the material obtained by sintering this material has excellent forging workability, and the product tool has high wear resistance and heat resistance. The object of the present invention is to provide a sintered tool steel that exhibits the following properties.

[Means to solve the problem]

The basic composition of the sintered tool steel of the present invention is as follows:
C: 0.35~10.0%, Si: 0.1~2.0%, Mn: 0.1~
Contains 1.5%, Cr: 3.0 to 20.0%, one or more REM (rare earth elements) (total amount if two or more): 0.001 to 0.60%, N: 0.30% or less,
The alloy steel is made by sintering the powder of O: 0.03% or less, carbonitride: 5 to 75%, and the balance consisting of Fe and impurities. Here, "REM" means La, Ce, Pr, Nd,
It means a lanthanide rare earth element represented by Sm. The sintered tool steel of the present invention may have a composition in which various alloying elements are added in addition to the basic composition described above. The first embodiment has Mo: 0.10 with respect to the basic composition.
~20.0%, W: 0.10~20.0%, V: 0.01~30.0%,
Co: 0.01~30.0%, Nb: 0.01~30.0%, Ta: 0.01
~10.0%, Zr: 0.001~10.0%, Ti: 0.001~10.0
%, Hf: 0.001 to 2.0%, Sc: 001 to 2.0%, and Y: 0.001 to 2.0%. A second modification of the sintered tool steel of the present invention is that the basic composition is Ni: 0.25 to 2.0% and Cu:
0.25-2.0% and B: 0.001-0.050% type 1, 2
A species or a combination of three species. A third variant of the sintered tool steel of the invention is a combination of the first and second variants described above. That is, in addition to the above basic composition, Mo:
0.10~20.0%, W: 0.10~20.0%, V: 0.01~30.0
%, Co: 0.01-30.0%, Nb: 0.01-30.0%,
Ta: 0.01~10.0%, Zr: 0.001~10.0%, Ti:
0.001~10.0%, Hf: 0.001~2.0%, Sc: 0.001~
2.0% and Y: 0.001~2.0%, one or more types, and Ni: 0.25~2.0%, Cu: 0.25~2.0
% and B: 0.001 to 0.050% of one type, two types, or three types of alloy steel powder is sintered. The production of the alloy steel powder forming the sintered tool steel of the present invention is carried out by spraying molten steel containing the desired alloying elements with water spray, gas (argon, nitrogen, etc.) spray, or both. The atomization process may be carried out by selecting an arbitrary technique such as a combination of the above. Sintering of the alloy steel powder thus obtained also
This can be done by known means. The preferred method is
After filling the powder into a mild steel capsule and deaerating it, it is subjected to nitriding treatment if necessary, and then compression molded to give the product shape. For compression molding, hot isostatic pressing, which has become popular in recent years, is suitable, but die pressing or forging may also be used. Sintering and subsequent heat treatment should be selected depending on the alloy steel composition. In addition, if it is desired to impart free machinability to the sintered tool steel of the present invention, one or more free machining elements such as S, Al, Ca, Mg, Pb, and Te may be contained in an amount of 0.2% or less. and such sintered tool steels are also included within the scope of the present invention.

[Effect]

The role of each alloy component in the basic composition described above and the reason for its limitation are as follows. C: 0.35-10.0% C combines with elements such as Cr, Mo, W, V, Ti, Zr, and REM to form hard double carbides, contributing to improving the wear resistance required for tools, It forms a solid solution in the base and gives the tool the required hardness. The appropriate amount of addition varies depending on the carbonitride-forming element, but if it is lower than 0.35%, the amount of C dissolved in the matrix during quenching will be insufficient, making it difficult to obtain a tempered hardness of HRC58 or higher. become. On the other hand, the larger the amount added, the higher the wear resistance, but the lower the forgeability and toughness, so it should be limited to 10% or less. Si: 0.1-2.0% Si is added as a deoxidizing agent and is normally contained at 0.1-0.5%, but if a larger amount is added, it can accelerate the precipitation reaction of carbonitrides and make them finer. . In addition, it improves the hardening process, strengthens the solid solution base, increases the yield point, prevents surface oxidation at high temperatures, and improves the fatigue limit. However, if it becomes too large, the decrease in thermal conductivity and toughness will shorten the tool life, so 2.0%
was set as the upper limit. Mn: 0.1-1.5% Like Si, this is also added as a deoxidizing agent, but it also contributes to improving hardenability. In order to achieve a deoxidizing effect, it is necessary to add at least 0.1%, but if the amount is too large, the toughness and temper softening resistance will decrease due to the precipitation of Mn compounds, and the work hardening will become significant, making it difficult to cut the workpiece. Since the quality decreases, it was set to 1.5% or less. Cr: 3.0 to 20.0% Cr combines with C to form composite carbides, which greatly contributes to improving wear resistance, and is also dissolved in large quantities in the base, improving hardenability and improving oxidation resistance. Make things better too. These effects can be obtained with a minimum addition of 3.0%, but of course the higher the amount, the better. However, if the content becomes too high, the increase in toughness and temper softening resistance will reach a plateau, and there will be a tendency for embrittlement. According to the invention, REM
By adding Cr, even if a large amount of Cr is added, embrittlement is alleviated and hot workability is clearly improved. this is,
The presence of REM usually results in giant needle-like crystals M ₇ C ₃
This seems to be because the type eutectic carbide is made very fine and dispersed uniformly. The upper limit of the Cr content at which this effect is observed is 20.0%. REM: 0.001~0.60% REM is an important element that forms rare earth carbonitrides. This carbonitride is extremely finely and uniformly dispersed and stable, and is highly resistant to MC, M ₆ C and M ₂₃
It also affects the precipitation reaction of _C6 type carbonitride,
It serves as the nucleus for the precipitation. As a result, the entire carbonitride is finely and uniformly dispersed, improving wear resistance and preventing a decrease in toughness and hardness. This result can be obtained by adding at least 0.005% of one or more REMs (in the case of two or more, in total). Since hot workability deteriorates when added in large amounts, a limit of 0.60% was set. REM forms carbonitrides directly from molten steel in the temperature range of 1400 to 1500℃, where the steel is in a molten state.
As mentioned above, it precipitates extremely finely. This carbonitride does not segregate in a part of the steel,
Moreover, this serves as a nucleus for the carbonitride production reaction when the powder is solidified, and the eutectic reaction is completed in a short period of time. Therefore, even when a large amount of carbonitride is contained, the distance between the dendrite arms and the average intergranular distance of the granular crystals can be reduced. This is considered to be the mechanism by which the effect of REM addition is expressed. In addition, the eutectic carbonitride with rare earth carbonitride as a core formed in the above manner is easily decomposed and becomes finer again through subsequent processing and heat treatment, so a large amount of carbonitride remains in the sintered tool steel. exist finely and uniformly dispersed. This fact was confirmed by microscopic examination of the product's microstructure. The applicant has discovered and already disclosed that the addition of REM has the effect of refining carbonitrides in molten tool steel (Patent Application No. 1983-
27611, 56-27612 and 56-80062). This time, the effect of this REM addition was demonstrated in sintered tool steel.
By avoiding the inherent problem of the influence of cooling rate, in other words, the restriction of powder particle size, we are able to improve powder manufacturability, enable the use of higher alloy steel, and limit the limits of sintered alloy tool steel. The present invention was completed knowing that it would overcome this problem and provide a further improved tool product. Carbonitride: 5-75% Carbonitride is a component responsible for the hardness and wear resistance required for tool steel, and must be present in an amount of at least 5%. On the other hand, the presence of more than 75% significantly reduces toughness and renders the material unusable as a tool. N and O as impurities must be controlled within the limits described above for the following reasons. N: 0.30% or less N forms nitrides, and if this limit is exceeded, huge carbonitrides will be present in the steel.
It will impair the performance of the tool. O: 0.03% or less O forms an oxide, and if this limit is exceeded, the sinterability deteriorates, which also impairs the performance of the tool. The elements added in the first modification of the sintered tool steel of the present invention all have the common effect of strengthening the steel base and improving heat resistance, wear resistance, toughness, etc. It is convenient to understand it by dividing it into several groups, so they will be explained below. Mo and W: 0.10-20.0% Mo and W combine with C to form fine M ₂ C-type or M ₆ C-type composite carbides, and solidly dissolve in the matrix to strengthen it. In addition to increasing wear resistance and high-temperature hardness, it also contributes to improving resistance to temper softening and heat check resistance. These additions make wear resistance and hardenability equal to or better than that of high carbon, high chromium steel. 0.10%
The above range was chosen because the effect was observed even with a small amount, but the effect did not increase any further when added in excess of 20%. V: 0.01-30.0% Combines with C to form MC type carbides that are hard and difficult to form solid solutions, contributing to improved wear resistance and increased tempering hardness, and further refines crystal grains.
It also has a positive effect on improving toughness. To achieve this effect, an addition of at least 0.01% is required.
The decrease in machinability and toughness that tends to accompany the improvement in wear resistance due to MC type carbides is due to the presence of Si and REM.
Since the effect of fineness and uniform dispersion of MC type carbides greatly alleviates the problem, V can be added in a fairly large amount up to about 30%. Co: 0.01 to 30.0% Co dissolves in the matrix to strengthen it, delay precipitation and aggregation of carbides, and improve hardness and yield strength at high temperatures. Therefore, it has a remarkable effect on improving heat resistance and abrasion resistance. This effect can be seen from as little as 0.01%. When the amount is large, from solid solution
The upper limit of 30.0% was set because crystallization of a single Co phase increases internal strain and adversely affects toughness. Both Nb: 0.01-30.0% and Ta: 0.01-10.0% form fine special carbides with high melting points, thus preventing coarsening of crystal grains caused by temperature increases during forging, rolling, and quenching. Combined addition with REM is
High melting point fine Nb/REM-carbide or Ta/
REM - results in the formation of carbides. These carbides also participate in the precipitation reactions of M ₇ C ₃ type, MC type, M ₆ C type, and M ₂₃ C ₆ type carbides, and as a result of playing the role of nuclei for their formation, the carbides are fine and uniform. A distribution is obtained. 0.01% or more of Nb or Ta gives this effect. The upper limit was determined based on temper softening resistance and toughness reduction. Zr and Ti: 0.001-10.0%, Hf, Sc and Y:
0.001 to 2.0% These elements fix N and indirectly contribute to the fine precipitation of MC type carbides, and also have the effect of refining crystal grains, thereby improving toughness. The effects can be obtained from as little as 0.001%.
However, if the amount is too large, the MC type carbide becomes huge and these elements preferentially precipitate at grain boundaries, resulting in embrittlement. The upper limit was determined from this perspective. All of the additive elements used in the second modification of the sintered tool steel of the present invention improve hardenability and toughness, but if the unique properties are shown together with the reason for limiting the composition range, It is as follows. Ni: 0.25-2.0% In addition to improving hardenability, it also has the effect of refining grains, so it has a great effect of increasing toughness, but for this purpose it is necessary to add at least 0.25%. If the amount is too large, the amount of retained austenite will rapidly increase, resulting in a decrease in temper softening resistance and toughness, and at the same time, machinability will deteriorate, so the limit is set at 2.0%. Cu: 0.25-2.0% In addition to improving hardenability, it increases toughness by suppressing the precipitation of initial carbides. This effect is 0.25
% or more. On the other hand, Cu in excess of 2.0% segregates in the surface layer of the material and embrittles grain boundaries. B: 0.001 to 0.050% Even in a very small amount, such as the above 0.001%, B significantly improves hardenability. This is due to the effect of suppressing the precipitation of pro-eutectoid carbides at austenite grain boundaries during the cooling process during quenching. Addition of a large amount of B
Since it causes the formation of a large amount of borides and impairs forgeability, the upper limit is set at 0.050%.

[Examples and comparative examples]

Alloy composition shown in table (remainder is Fe and impurities)
of steel is melted and prepared, and this melt is atomized by a gas atomization method to obtain a powder with a particle size of 5×10 ¹ to 5×10 ³
An alloy steel powder was obtained containing powder in the μ range. In the table, Nos. 1 to 15 are examples of the present invention, and Nos. 16* and 17* are comparative examples. For each sample, the amount of carbide contained therein was quantified, and the distance between the secondary dendrite arms of the carbide was measured. The relationship between the powder particle size and the secondary dendrite arm spacing is shown in FIG. 1, and the relationship between the amount of carbide and the secondary dendrite arm spacing in a powder with a particle size of 10 ³ μm is shown in FIG. 2, respectively. It can be seen that the alloy steel with the composition according to the invention has a shorter secondary dendrite arm spacing and smaller carbide particles than the comparative steel.
The dispersion of carbide fine particles according to the present invention was highly uniform in all cases. Next, samples No. 15, 18*, 7, 17*, 11, 16* were placed in a mild steel capsule and heated to a temperature of 1170 to 1200.
It was molded into a cylindrical shape with a diameter of 350 mm by hot isostatic press molding at a temperature of 1000 Kg/cm ² and sintered at the same time. It was then hot forged and rolled to create a final product, a round bar with a diameter of 28 mm. This product was subjected to a bending test and the following results were obtained. No. HRC resistance (Kgf/mm ² ) 15 65 550 18* 60 430 7 69 580 17* 64 380 11 62 410 16* 58 350

【table】

【table】

【Effect of the invention】

The alloy steel powder forming the sintered tool steel of the present invention is
Even if a large amount of carbonitride is present in a high alloy composition, the particles are fine and uniformly distributed, so the material obtained by sintering it has excellent forging workability, and the product tool is Shows high wear resistance and heat resistance.
Since it is not necessary to make the solidification rate extremely high in producing the powder, the restriction that the powder particle size must be made extremely fine can be avoided. This means that the yield of usable powder produced by spraying is high.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the particle size of the powder steel used for manufacturing the sintered tool steel of the present invention and the secondary dendrite arm spacing of carbonitrides therein, along with that of comparative steel, for each example. It is a graph. Figure 2 shows
It is also a graph showing the relationship between the amount of carbonitride and the secondary dendrite arm spacing of the powder steel used in the sintered tool steel of the present invention, together with that of the comparative steel.
In both figures, numbers represent sample numbers.

Claims (1)

[Claims] 1 C: 0.35-10.0%, Si: 0.1-2.0%, Mn: 0.1
~1.5%, Cr: 3.0~20.0%, REM (rare earth elements)
Contains one or more of the following (total amount if two or more): 0.001 to 0.60%, N: 0.30% or less,
A sintered tool steel obtained by sintering alloy steel powder containing O: 0.03% or less, carbonitride: 5 to 75%, and the remainder being Fe and impurities. 2 C: 0.35-10.0%, Si: 0.1-2.0%, Mn: 0.1
~1.5%, Cr: 3.0~20.0%, REM type 1 or 2
Species or more (total amount if 2 or more types): 0.001 to 0.60
In addition to %, Mo: 0.10~20.0%, W: 0.10~20.0
%, V: 0.01-30.0%, Co: 0.01-30.0%, Nb:
0.01~30.0%, Ta: 0.01~10.0%, Zr: 0.001~
10.0%, Ti: 0.001~10.0%, Hf: 0.001~2.0%,
Contains one or more selected from Sc: 001-2.0% and Y: 0.001-2.0%, N: 0.30% or less, O: 0.03% or less, and carbonitride: 5-75
%, and the remainder is Fe and impurities. 3 C: 0.35-10.0%, Si: 0.1-2.0%, Mn: 0.1
~1.5%, Cr: 3.0~20.0%, REM type 1 or 2
Species or more (total amount if 2 or more types): 0.001 to 0.60
%, contains one, two or three of Ni: 0.25-2.0%, Cu: 0.25-2.0% and B: 0.001-0.050%, N: 0.30% or less, O: 0.03% or less. Carbonitride: 5 to 75% is present, and the remainder is Fe.
and sintered tool steel made by sintering impurity alloy steel powder. 4 C: 0.35-10.0%, Si: 0.1-2.0%, Mn: 0.1
~1.5%, Cr: 3.0~20.0%, REM type 1 or 2
Species or more (total amount if 2 or more types): 0.001 to 0.60
In addition to %, Mo: 0.10~20.0%, W: 0.10~20.0
%, V: 0.01-30.0%, Co: 0.01-30.0%, Nb:
0.01~30.0%, Ta: 0.01~10.0%, Zr: 0.001~
10.0%, Ti: 0.001~10.0%, Hf: 0.001~2.0%,
One or more of Sc: 0.001 to 2.0% and Y: 0.001 to 2.0%, and Ni: 0.25 to 2.0%,
One type of Cu: 0.25-2.0% and B: 0.001-0.05%,
Contains 2 or 3 types, N: 0.30% or less, 0.03
% or less, carbonitride: 5 to 75% is present,
Sintered tool steel made by sintering alloy steel powder with the remainder being Fe and impurities. 5. The sintered tool steel according to any one of claims 1 to 4, which is a high-speed tool steel or a tool steel for cold or hot forging.