JP3414855B2 - High speed tool steel for precision casting - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to high speed tool steels used in precision casting.

[0002]

BACKGROUND OF THE INVENTION High speed tool steels include carbon and Cr, Mo,
It is a kind of special steel that contains carbide forming metal components such as W, and by increasing the hardness and wear resistance at high temperatures by crystallizing and precipitating those carbides, for manufacturing gears. Widely used for metal or wood processing tools such as gear cutting cutters, cutting tools, chips, drills, end mills, etc.

[0003]

In the conventional high-speed tool steel, since carbides (primary carbides) that crystallize during solidification in the cast state are formed in a coarse mesh shape, the toughness required for the high-speed tool steel is high. I can't get enough. Therefore, it is necessary to repeat hot working etc. after casting to homogenize the coarse primary carbide structure finely, and it is practically impossible to achieve almost the final product shape (so-called near net shape) as cast. is there. Therefore, the final tool shape needs to be separately formed by cutting, hot forging, or the like after the hot working, and there is a problem that the number of steps increases and the material yield decreases. On the other hand, hot isostatic pressing (HIP) is performed on high-speed tool steel powder produced by gas atomization etc. without melting the ingot.
Then, although a sintered body in which carbides are finely homogenized is also used, the HIP method is more expensive than the casting method,
In addition, since the block-shaped sintered body obtained by HIP is used after hot working, there is a problem that a near net shape cannot be obtained in the same manner.

The object of the present invention is to eliminate the need for processing for refining carbides after casting, to enable formation of near net shapes by precision casting, and to reduce the number of tool manufacturing steps and to achieve high material yield in precision casting. To provide high speed tool steel for use.

[0005]

MEANS FOR SOLVING THE PROBLEMS, ACTIONS AND EFFECTS The high speed tool steel of the present invention is used for precision casting,
Fe is a main component and is characterized by containing the following components: N: 0.025 wt% or more, C: 0.7 wt% or more and 2.2 wt% or less, Si: 3 wt% or less, Mn : 1.5
% By weight, Cr: 3.0% by weight or more and 6.0% by weight or less, W: 27% by weight or less, Mo: 13.5% by weight or less, where W + 2Mo is 14% by weight or more and 27% by weight or less, V: 6.0 wt% or less, and inevitable impurities.

In the cast structure of the high speed tool steel, a matrix phase mainly composed of Fe and primary carbides etc. formed in the matrix phase in a mesh shape during casting are generated. In N., N (nitrogen) in the above composition range is contained in the component, and the network spacing (for example, secondary dendrite arm spacing) of the primary carbide to be crystallized is smaller than that in the case where nitrogen is not contained. As a result, the mesh-shaped primary carbide becomes fine and uniform in the cast state,
It is not necessary to process the primary carbide refining by subsequent hot working, etc., and it is possible to form a product shape (near net shape) close to the final by precision casting, and the fine mesh-like primary carbide itself is also a material. It contributes to an increase in hardness as a whole and an improvement in wear resistance. By subjecting the cast material to heat treatment such as quenching and tempering, a structure is formed in which, in addition to the primary carbides, secondary carbides and the like precipitated by the heat treatment are mixed.

If the nitrogen content is 0.025% by weight or less, the effect of refining the primary carbides cannot be sufficiently obtained, leading to a decrease in toughness. Therefore, the nitrogen content is 0.0
It is set to 25% by weight or more. On the other hand, the maximum content of nitrogen is almost determined according to the chemical composition of the material, and is about 0.1% by weight in the case of high speed tool steel.

In the high speed tool steel of the present invention, it is preferable that the network-like carbides present in the matrix phase have a mesh size as fine as possible in order to improve the toughness. For example, the average mesh size is 50 μm or less. be able to. The average mesh spacing is desirably 35 μm or less.

Next, the role of contained components other than nitrogen will be described. C (carbon) is Cr, W, and
It forms carbides with Mo and V, crystallizes the primary carbides during casting, and precipitates fine secondary carbides in the matrix phase by tempering to strengthen the precipitation and also to form a solid solution in the matrix phase. Also acts as a solid solution strengthener.
All of these effects contribute to the improvement of the hardness (strength) of the high speed tool steel of the present invention.

Here, the high speed tool steel of the present invention has a hardness of 64 or more, preferably 65 or more in Rockwell C scale hardness in order to secure sufficient wear resistance and strength as a tool material. However, if the content of carbon is less than 0.70% by weight, the effect of strengthening the material by crystallization and precipitation of carbide, and further solid solution strengthening cannot be sufficiently obtained, and the hardness is within the above range. Will be less than. Further, if the carbon content exceeds 2.2% by weight, the amount of coarse carbide formed increases and the toughness decreases. Therefore, the carbon content is within the above range. The carbon content is preferably set within the range of 0.80 to 2.0% by weight.

Si (silicon) not only functions as a deoxidizing agent that binds to and removes dissolved oxygen components in the molten metal, and also functions as a solid solution in the matrix to strengthen it. When Si exceeds 3% by weight, the toughness decreases, so the content is set within the range below, preferably 0.5
The amount is 0% by weight or less, and more preferably 0.10% by weight or less.

Mn (manganese) also functions as a deoxidizer like Si. If the content exceeds 1.5% by weight, the toughness decreases, so the content is set within the range below, preferably 0.50% by weight or less.

Cr (chromium) is one of the carbide forming components, and precipitates fine M ₂₃ C ₆ type carbides in the matrix phase during tempering to increase the hardness of the material. Also, C
Since r moves the nose of the isothermal transformation curve of steel to the side for a long time, it also has the effect of enhancing hardenability. If the Cr content is less than 3.0% by weight, the effect of precipitation strengthening by the carbide cannot be sufficiently obtained, and if it exceeds 6.0% by weight, the amount of coarse carbide formed increases and the toughness decreases. Therefore, the Cr content is within the above range.

W (tungsten) and Mo (molybdenum)
Has the same effect on the formation of carbides and is the main component of M ₂ C and M ₆ C type primary carbides formed in a fine mesh during casting, and at the time of tempering, M ₂ C type fine carbides. As a carbide, it precipitates in the matrix phase and contributes to increase the hardness and wear resistance of the material. The upper limits of the W and Mo contents are 27% by weight for the former and 13.5% by weight for the latter, but when both components are co-added, the value of (W content + Mo content × 2) Is set to 27% by weight. Mo has almost half the atomic weight of W, and it gives almost twice as many atoms as W at the same content.
Gives the same effect as. Therefore, in setting the upper limit of the content at the time of the co-addition, the content of Mo is doubled to be converted into the equivalent amount of W which gives the same effect.

If both of these components are contained in excess of the above upper limits, the amount of coarse carbides formed will increase and the toughness of the material will decrease. Also, (W content + Mo content × 2) is 14
If it is less than wt%, the amount of crystallization or the amount of precipitation of carbides decreases, and the hardness and wear resistance of the material become insufficient. In addition,
Only one of W and Mo may be added alone.

V (vanadium) forms MC type carbide having high hardness and increases the hardness of the material. If the V content exceeds 6.0% by weight, the toughness of the material decreases, so V
The content of is within the above range.

The high speed tool steel of the present invention may contain 13% by weight or less of Co. Since Co improves the heat resistance of the material, it is possible to obtain a high-speed steel suitable for a tool used in an environment in which the temperature easily rises due to friction, such as a drill for heavy grinding or high-speed cutting. In addition, Co has a solid solution in the matrix phase to strengthen it, and also has an effect of increasing the solid solution limit of carbon in the matrix phase and accelerating the solid solution strengthening by carbon. Even when tool steel is required, its addition is effective. When Co is contained in an amount of more than 13% by weight, the above effect is saturated and the Co component is wasted. Further, in order to obtain the above effects remarkably, it is desirable to contain Co in an amount of 3% by weight or more.

Further, the high speed tool steel of the present invention can be formed by adding Al, N
Any one or two or more of b and Ti can be contained within the range of the total of 3% by weight or less. By incorporating the above-mentioned components, fine nitrides or carbides of these components are dispersed and precipitated in the matrix phase, and a fixing effect on the movement of grain boundaries is produced. As a result, it is possible to prevent the crystal grains from growing and coarsening during quenching and heating, and consequently to prevent the toughness of the material from decreasing. If the above-mentioned components are contained excessively, the above-mentioned effects will be saturated, and the waste of the components will increase, so the content is set within the above range. On the other hand, in order to sufficiently obtain the effect of addition, it is desirable that the total content is at least about 0.04% by weight or more.
Here, the total content is preferably set within the range of 0.04 to 1.0% by weight.

The high speed tool steel of the present invention may contain 0.15% by weight or less of S and 0.40% by weight or less of Pb, or both. Both of these elements improve the machinability of the material. Therefore, when it is necessary to slightly process a precision-cast casting by cutting or the like, its addition is effective. Note that if these elements are excessively contained, the toughness of the material is lowered, so the content is set within the above range. On the other hand, in order to sufficiently obtain the effect of addition, the content should be set within the range of 0.03 to 0.10% by weight for S and 0.05 to 0.30% by weight for Pb. Is desirable.

The high speed tool steel of the present invention may contain a rare earth component in an amount of 0.60% by weight or less. The inclusion of rare earth components narrows the crystallization temperature range (solid-liquid coexistence range) of carbides during solidification by casting, and the networked carbides that crystallize are further refined to improve the toughness and transverse rupture strength of the material. . The type of rare earth component used is not particularly limited, but non-separated rare earth metal such as misch metal is preferably used because it is relatively inexpensive.

If the rare earth component is excessively contained, the above effect is saturated, the component is wasted, and the rare earth metal is expensive, resulting in high cost. Therefore, the content of the rare earth component is set within the above range. On the other hand, in order to sufficiently obtain the effect of the addition, it is desirable that the total content of the rare earth components is at least about 0.05% by weight or more. Here, the total content is preferably set within the range of 0.05 to 0.10% by weight.

Fe is the main component of the high speed tool steel of the present invention and the main component of the matrix phase.

In addition to the above-mentioned components, unavoidable impurities such as P, Cu, Ni, and O which are mixed in from the raw materials for compounding may be contained in a total amount of, for example, 0.7% by weight or less.

A method of manufacturing various members such as tools using the high speed tool steel of the present invention will be described below. First,
A predetermined amount of starting materials are mixed so as to obtain the above alloy composition, and this is melted by a known melting method such as high frequency induction melting. Here, the nitrogen component is blended in the form of a solid substance containing nitrogen such as chromium nitride and manganese nitride. Then, the molten metal is vacuum suction casting method, top pouring vacuum casting method,
By a precision casting method such as a top pouring casting method, a cast body having a near net shape, which does not require post-processing except for a cutting edge portion, is manufactured.

FIG. 1 shows an example of an apparatus for performing vacuum suction casting. The entire casting apparatus 1 is placed in the atmosphere or in an inert gas atmosphere, a crucible 2 made of alumina or the like is placed in the lower portion, and an induction heating coil 9 is placed outside the crucible 2. A mold chamber 3 is provided on the upper side of the crucible 2, and a breathable mold 4 such as a ceramic shell mold is arranged inside the mold chamber 3. Inside the breathable mold 4, there is a casting space 5 for casting a high speed tool steel member having a predetermined shape.
Is formed and communicates with the inside of the crucible 2 via the suction passage portion 6. The inner space of the mold chamber 3 has a suction port 7
It is adapted to be sucked under reduced pressure by a pump (not shown). In addition, 8 is a seal member for maintaining airtightness between the mold chamber 3 and the mold 4.

When a high-frequency current is supplied to the induction heating coil 9 from a power source (not shown), the raw material in the crucible 2 is induction-heated and melted. In this state, if the inner space of the mold chamber 3 is decompressed and sucked from the suction port 7, the pressure in the casting space 5 is also reduced through the wall portion of the air-permeable mold 4, so that the molten metal M in the crucible 2 is sucked into the suction passage portion 6 It is sucked up into the casting space 5 through and is cast into a member. The molten metal M sucked up into the casting space 5 is uniformly supplied to every corner of the space 5 by the suction force due to the decompression, so that a near net shape cast body with high dimensional accuracy can be obtained.

In the cast body obtained as described above,
The above-mentioned fine mesh-like carbide (primary carbide) is formed. In addition, when it is necessary to slightly process the cast body by cutting or the like, the annealing treatment for softening the cast body to improve the machinability is performed by, for example, 0.5 to 800 at 900 to 900 ° C.
It is performed for about 2 hours, and after annealing, the cast body is gradually cooled.

Then, the obtained cast body is quenched. The heating for quenching is performed using a vacuum furnace, a salt bath, or the like, and the cast body is heated to a predetermined temperature, for example, 1150 to 1250.
By holding at about 0 ° C. for a predetermined time, for example, for about 3 to 30 minutes, coarse carbides and the like precipitated during cooling after solidification are redissolved in the matrix phase, and the precipitation is performed so that the coarse carbides do not re-precipitate. Temperature range (eg 900-10
(00 ° C) is quenched and quenched. For cooling, usually, when a vacuum furnace is used, pressurized gas cooling with an inert gas or oil cooling is used, and when a salt bath is used, oil cooling or salt bath quenching is used. A matrix phase (for example, a martensite phase and a retained austenite phase) containing supersaturated carbon and the primary carbide phase and the like are formed in the cast body after quenching.

The cast body after quenching is subjected to a predetermined temperature, for example, 500 to 600 ° C. by using an atmospheric furnace, a vacuum furnace, a salt bath or the like.
Tempered in the temperature range. By this tempering treatment, C
The hardness of the material is greatly increased by the precipitation of fine secondary carbides such as r, W, Mo and V, and the decomposition of the retained austenite phase. The tempering treatment is preferably performed twice or more for increasing the decomposition rate of the retained austenite phase, and three times or more for a thick material or the like in which component segregation easily occurs. The cast body that has been hardened by tempering is subjected to finishing processing such as sharpening of the cutting edge to obtain a final product.

[0030]

EXAMPLES Examples of the present invention will be described below. Sample numbers 1 to 21 in Tables 1 and 2 (18 to 21 are comparative examples)
The alloys having the respective compositions of 1 are melted by using a high frequency induction melting furnace, and a gear cutting cutter (outer diameter 30 mm, inner diameter 28 mm, height 28 mm) is cast by a vacuum suction casting method to obtain a test product. It was Next, the cast body is placed in a vacuum furnace for 8 hours.
Annealing was performed at 70 ° C. for 1 hour. The cooling after annealing was performed by gradually cooling to 600 ° C. at a rate of 15 ° C./hr, and then air-cooling in the atmosphere. Then, the cast body after annealing was subjected to quenching and tempering using a salt bath under the five conditions of A to E shown in Table 3. Note that quenching was oil quenching, and tempering was repeated three times under the same conditions. The cooling after tempering was air hollow cooling.

Test pieces (gear cutters) hardened by tempering were set on gear cutting machines after measuring Rockwell hardness (C scale), and helical gears made of Cr alloy steel (SCr420 normalizing material). (Diameter 22 mm, height 15 m
The gear cutting process of m) was repeated, and the life was evaluated by the number of times of processing until cutting became impossible. For comparison, commercially available powder high speed tool steel materials with different hardness (HRC
67 and 69), the life evaluation of two types of gear cutting cutters (Sample Nos. 22 and 23, Table 2) was also performed at the same time. The results are shown in Tables 1 and 2. In addition, the test product of each number is sorted into two hardness levels corresponding to the comparative product, and the tool life of the comparative product is set to 100 for each hardness level, and the relative service life is displayed.

Next, a width of 5 m from the test product after the end of life evaluation.
A test piece of m, a thickness of 3 mm, and a length of 30 mm was cut out, and a bending test was performed by bending at three points with a lower span of 20 mm. The surface of the same test piece was polished and etched, and the formed network carbide structure was observed with an optical microscope and a microstructure photograph was taken. Then, a straight line is appropriately drawn on the structure photograph with ink or the like, and from the average value of the mesh spacing of the carbide image cut by the straight line,
The actual size average value of the mesh spacing of the carbide was estimated. Specifically, for example, 60 mm and 45 mm of 2 per 1 field of view of a photograph
If a straight line is drawn and each straight line crosses the mesh of 21 and 12 carbide images, the average interval on the photograph of these meshes is (60 ÷ 21 + 45 ÷ 12) ÷ 2 =
It becomes 3.31 mm. And the magnification of the picture is, for example, 10
In the case of 0 times, the actual mesh size is 3.31 mm ÷ 100 =
It becomes 33.1 μm. Tables 1 and 2 show the network spacing of the carbides thus estimated.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

As is clear from the results shown in Tables 1 and 2, the samples having a nitrogen content of 0.025% by weight or less (Comparative Examples, Sample Nos. 18 to 21) all have a carbide network spacing of 50 μm. The cutter life is as low as about 60 to 90 and the transverse rupture strength is 200 kgf when the powder high speed tool steel product (sample numbers 22 and 23) is set to 100.
/ Mm ² or less. On the other hand, in the high-speed tool steels of the present invention (Sample Nos. 1 to 17) in which the nitrogen content exceeds 0.025% by weight, the carbide network intervals are all smaller than those of the samples of Comparative Examples, and any hardness level. Also, it is understood that, regardless of the heat treatment conditions, the cutter life and the transverse rupture strength are higher than those of the powder high speed tool steel steel products, and the durability and toughness are excellent. Among these, particularly in the samples having a nitrogen content of more than 0.04% by weight (Sample Nos. 2, 4, 6, 8 to 17), the carbide mesh spacing was reduced to around 30 μm, and the bending strength and the cutter were reduced. The lifespan is increasing.

2, 3 and 4 are photographs of the optical microscope structures of Sample Nos. 2, 1 and 18 (magnification: 100).
Times). In the photograph, the white portions are mesh-like carbides, and the slightly dark portions are the matrix phase.
Sample 18 (FIG. 4) with a low nitrogen content has a large carbide network, but samples 2 and 1 with a low nitrogen content (FIG. 2).
It can be seen that the mesh spacing in FIG. 3) is smaller. The secondary carbides precipitated during tempering are extremely fine and are not shown in these photographs with low magnification.

FIG. 5 is a plot of the transverse rupture strength of the high speed tool steel samples of the above Examples and Comparative Examples, with respect to the hardness of the samples for each nitrogen content level. In the figure, the plot points surrounded by a solid line are samples with a nitrogen content of 0.04 wt% or more, the plot points surrounded by a dot-dash line are samples with a nitrogen content of 0.25 to 0.04 wt%, and a broken line. Plot points surrounded by ∘ correspond to samples having a nitrogen content of less than 0.025% by weight, respectively. As is clear from this figure, the samples containing 0.025% by weight or more of nitrogen have a larger transverse rupture strength than those of the samples having a nitrogen content of less than 0.025% by weight and have a nitrogen content of any hardness. It can be seen that when the content is 0.04% by weight or more, particularly excellent transverse rupture strength is exhibited. Also,
It can be seen that these samples with a high nitrogen content have a bending strength equal to or higher than that of the powder high-speed tool steel steel products (plotted with ● in the figure), although they are cast products.

[Brief description of drawings]

1 for precision casting of high speed tool steel of the invention,
The schematic diagram which shows an example of a casting apparatus.

FIG. 2 is an optical micrograph of a high speed tool steel sample of an example.

FIG. 3 is an optical micrograph of another high speed tool steel sample of Example.

FIG. 4 is an optical micrograph of a high speed tool steel sample of a comparative example.

FIG. 5 is a diagram showing a relationship between hardness and resistance of high speed tool steel samples of Examples and Comparative Examples.

[Explanation of symbols]

1 casting equipment 3 Mold chamber 4 Breathable mold

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI C22C 38/60 C22C 38/60

Claims (6)

(57) [Claims] 1. A group comprising Fe as a main component, containing the following components and having carbides in a matrix in a matrix.
Have a woven structure and the average mesh spacing of the mesh-like carbide is 50 μm
High-speed tool steel for precision casting characterized by the following : N: 0.025 wt% or more, C: 0.7 wt% or more and 2.2 wt% or less, Si: 3 wt% or less, Mn: 1 0.5 wt% or less, Cr: 3.0 wt% or more and 6.0 wt% or less, W: 27 wt% or less, Mo: 13.5 wt% or less, where W + 2Mo is 14 wt% or more and 27 wt% or less, V: 6.0 wt% or less, and inevitable impurities. 2. A composition containing 13% by weight or less of Co.
High-speed tool steel for precision casting according to 1. 3. Any one or two of Al, Nb and Ti
A contract containing the above in the range of 3% by weight or less in total.
High-speed tool steel for precision casting according to claim 1 or 2. 4. S and 0.40 weight of 0.15 wt% or less
A Pb containing one or both of Pb in an amount of not more than%.
High speed tool steel for precision casting according to any one of 1 to 3. 5. A rare earth component in the range of 0.60% by weight or less.
Precision contained in any one of claims 1 to 4.
High speed tool steel for casting. 6. A Rockwell C scale hardness of 64 or more.
For precision casting according to any one of claims 1 to 5,
High speed tool steel.