JP2795665B2 - High speed tool steel and method for manufacturing the same - Google Patents

Description

The present invention relates to a high-speed tool steel used for a cutting tool such as a tap, a reamer, and a broach having a cutting edge on the surface of the tool, and a method for manufacturing the same. In particular, the present invention relates to miniaturization of a steel surface structure.

[Prior Art] In the field of tool steel, it is desirable that the microstructure is fine and the segregation is small, and it has been conventionally focused on the homogenization and refinement of the cast structure.

Japanese Patent Application Laid-Open
There is a technique disclosed in -207455.

This method uses a thin slab continuous caster such as Hesley,
It is characterized by producing a rapidly solidified slab, followed by hot working and annealing to form carbide into fine spheres. According to this method, it is possible to obtain a high-speed tool steel having a fine carbide structure without performing a step of breaking eutectic carbide by casting as in the related art.

By the way, in recent years, application of a cutting tool having a cutting edge on an outer peripheral portion such as a tap to the processing of difficult-to-cut materials has progressed, and a higher performance of the tool is required.

It has been found that cutting tool performance can be improved by adding a large amount of V and increasing the amount of hard MC-type carbide. However, in this case, in a normal manufacturing method such as melting-casting-forging-rolling, there is a problem that the MC-type carbide is easily coarsened and segregated.

As a method for solving this problem, a method of adding a rare earth element of Ce or La (Japanese Patent Application Laid-Open No. 57-143468, etc.), a method of reducing the shape of a steel ingot, application of powder metallurgy, or the aforementioned continuous casting method The law can be considered.

[Problems to be solved by the invention]

The method of adding a rare earth element as a method of refining the outer peripheral structure by the conventional melting method is difficult in the technique of adding a rare earth element, and the presence of inclusions such as sulfides and oxides of the rare earth element, and further hot heating There has been a problem that cracks sometimes occur at the corners of the steel material.

In addition, miniaturization of the steel ingot to make the microstructure of the outer peripheral portion fine has a problem in terms of manufacturing cost and is not suitable for mass production. The production method by the powder metallurgy method has a problem that although a fine and uniform structure can be obtained over the entire material, the production process is more than the melting method and the production cost is high.

Further, in the continuous casting method disclosed in JP-A-63-207455, it was difficult to sufficiently reduce the carbide of high-speed tool steel having a high V content.

In view of the above background, an object of the present invention is to provide a high-speed tool steel having a fine surface structure at low cost.

[Means for solving the problem]

The present inventor has conducted various systems to achieve the above-mentioned object by using a continuous casting method, and as a result, regulates the working temperature of hot working performed after continuous casting, or further performs diffusion heat treatment before hot working. It has been found that a sufficiently fine structure can be obtained by this, and that the tool cutting life is greatly affected by the mean free path of the MC type carbide of the high-speed tool steel thus obtained, and the present invention has been completed. It was reached.

 Hereinafter, the present invention will be described in detail.

A more specific configuration of the high-speed tool steel of the present invention is in terms of weight ratio.
C 1.1 ~ 2.0%, Si 2.0% or less, Mn 1.5% or less, Cr 2.0 ~
10.0%, one or two types of W and Mo W + 2Mo 8.0 ~ 2
A high-speed tool steel comprising 0.0%, V 2.5 to 5.5%, balance Fe and unavoidable impurities, and 1 / th of the wall thickness from the surface of the steel.
Mean free path of MC type carbide at up to 8 sites is 15μ
m or less, which is a high-speed tool steel.

The high-speed tool steel of the present invention belongs to high-V-type high-speed steel. The MC carbide mainly composed of V which precipitates in such steel is hard to thermally decompose, and it is difficult to decompose the huge MC carbide crystallized at the time of casting in the subsequent steps such as hot working. .
Therefore, in order to obtain a high-speed tool steel having fine MC-type carbide, it is necessary to reduce the amount of MC-type carbide crystallized during casting. In order to achieve this requirement, the present invention employs a continuous casting method having a specific casting size.

That is, if the cooling rate of the part from the slab surface to 1/8 of the minimum vertical or horizontal dimension in the slab cross section is less than 100 ° C / min, a huge MC type carbide crystallizes, so the cooling element in this part The temperature must be 100 ° C / min or more. Further, when the minimum size of the cross section of the slab exceeds 150 mm, the cooling rate of the portion becomes less than 100 ° C./min. Therefore, the minimum cross section size of the slab is 150 mm or less.

However, the high-speed tool steel of the present invention cannot be obtained only by a continuous casting method under specific conditions.

It is important that the fine MC-type carbides crystallized by continuous casting are not grown by subsequent hot working. According to the study of the present inventors, in a temperature range exceeding 1180 ° C.,
It was found that hot working at 1180 ° C or less is good because MC-type carbides on the surface layer of steel material start to coarsen rapidly when hot working is performed.

In addition, it is desirable to perform high-temperature heating and holding in order to decompose the cast structure before hot working and promote the solid-phase transformation of the somewhat coarse rod-shaped M ₂ C-type carbide generated during casting. The holding temperature is
When the temperature exceeds 1180 ° C, coarsening of carbides remarkably progresses, and 1100
If the temperature is lower than 0 ° C, even if it is maintained for a long time, almost no structural change is recognized, so the temperature is set to 1100 to 1180 ° C. The holding time is determined in consideration of the temperature. The higher the holding temperature, the shorter the heating time.
To 10 hours.

According to the present invention described above, a high-speed tool steel having a fine structure at the surface can be obtained, but it also has the effect of reducing the amount of steel surface removal performed during the manufacturing process.

In other words, in the production of high-speed tool steel, the scale on the steel surface and the abnormal parts on the steel surface are removed by grinding or the like in accordance with the amount of decarburization. While the present invention does not perform forging to form a billet while performing heating twice, there is little occurrence of scale and decarburization, and conventionally it was necessary to remove at least about 3 mm of surface abnormalities. In the present invention, it is possible to set the thickness to 2 mm or less.

By the way, it has recently been found that the following characteristics are particularly required as characteristics required for a high-speed tool steel for a cutting tool having a cutting edge on an outer surface of the tool.

(1) Hardness after heat treatment must be at least HRC65.

(2) Presence of a sufficient amount of hard carbide (MC type carbide).

The inventor further quantitatively views the microstructure of the tool edge,
There is a clear correlation between the average free path of the MC type carbide and the tool cutting life.Specifically, the average free path of the MC type carbides must be finely and densely dispersed to be 15 μm or less. It has been found that the cutting life can be improved by the method. The high-speed tool steel of the present invention having such a structure can be manufactured by the above-described manufacturing method of the present invention.

In the high-speed tool steel of the present invention, the reason for limiting the mean free path between the MC type carbides to a portion from the steel surface to 1/8 of the thickness of the steel is that this portion becomes a cutting tool blade and this portion This is because controlling the properties of the carbides is extremely important.

The chemical composition of the high-speed tool steel of the present invention can satisfy the above-mentioned required characteristics (1) and (2). The reason for limiting the components will be described more specifically below.

C: 1.1 to 2% C forms a carbide together with Cr, W, Mo, and V, and also partially dissolves in a matrix, and has a hardness required as a tool.
Provides abrasion resistance. If it is less than 1.1%, the effect is insufficient, and if it exceeds 2%, toughness is deteriorated and microstructure segregation is increased. Therefore, C is set to 1.1 to 2%.

Si: 2% or less Si is mainly added as a deoxidizing agent, but has the effect of further promoting the solid-phase transformation of M ₂ C-type carbides to form fine carbides composed of M ₆ C and MC. However, if added in a large amount, the thermal conductivity decreases and the toughness deteriorates, so the content was limited to 2.0% or less.

Mn: 1.5% or less It is added as a deoxidizing agent like Si. When added in excess of 1.5%, the precipitation of Mn compounds reduces toughness and tempering softening resistance, and also has high work hardening ability and deteriorates machinability, so Mn was set to 1.5% or less.

Cr: 2.0 to 10.0% Cr combines with C to form a carbide, which is effective in improving wear resistance and hardenability. To achieve this effect, 2.0% or more is required.
Since Cr-based giant eutectic carbides are easily formed and it is difficult to refine the structure aimed at by the present invention, the Cr content is set to 2.0 to 10.0%.

W + 2Mo: 8.0 to 20.0% W and Mo combine with C to form carbides and are effective in improving the wear resistance required as a cutting tool. However, if a large amount is added, a giant eutectic carbide is likely to be formed, and forgeability and toughness are greatly deteriorated. Therefore, one or two of W and Mo is used, and W + 2Mo is set in the range of 8.0 to 20.0%.

V: 2.5-5.5% V is the most important element in the present invention. V combines with C to form a very hard MC-type carbide, which has a great effect on improving wear resistance. In the conventional production method, when V is added in excess of 2.5%, a huge MC type carbide is formed also on the surface layer of the steel material, and the toughness and grindability are remarkably deteriorated. But,
By applying the manufacturing method according to the present invention, the MC type carbide at the outer peripheral portion of the steel material becomes fine and uniform. However, if it exceeds 5.5%, a fine carbide structure cannot be obtained even by the present invention, so the upper limit was made 5.5%.

Co: 1.0 to 20.0% Co is an element that forms a solid solution in the matrix and significantly improves the hardness and proof stress at high temperatures. Therefore, when higher cutting durability is desired, it is necessary to add 1.0% or more. However, if a large amount is added, the crystallization of the Co single phase due to solid solution occurs, and the toughness is reduced.
% Or less.

〔Example〕

 Hereinafter, the present invention will be described based on examples.

Example 1 High-speed tool steels having the chemical components shown in Table 1 were manufactured under various manufacturing conditions.

Nos. 1 to 3 were manufactured by manufacturing a billet of 110 mm x 110 mm in cross section using the same chemical composition by continuous casting and changing the hot rolling temperature to 1100 ° C, 1150 ° C, and 1190 ° C. The cooling rate in continuous casting was about 500 ° C / min on the billet surface, and 200 ° C /
min.

Nos. 4 to 6 were obtained by continuous casting as in Nos. 1 to 3, but before hot rolling, they were 1140 ° C x 2 hours and 1170 ° C, respectively.
× 2 hours, 1190 ° C × 2 hours.
In addition, the cooling rate in continuous casting is the same as that of Nos. 1 to 3, and the temperature of hot rolling is 1100 ° C. in all of Nos. 4 to 6.

For No.7, 0.1% by weight of mesh metal was added, melted in a vacuum melting furnace, cast into a small 200kg mold, disassembled with a hammer at 1140 ° C, made a 110mm angle, and hot-rolled at 1100 ° C to a 15mmφ. It is made of steel.

No. 8 was manufactured under the same conditions as No. 7 after canning the powder obtained by the gas atomizing method.

Nos. 9 to 11 have the same manufacturing method as No. 1, Nos. 12 and 13 have the same manufacturing method as No. 5, and Nos. 14 and 15 have the same manufacturing method as No. 4.

The above steel was cold drawn and ground to form a 6.0 mmφ steel bar, and from these materials, a hand tap of equal diameter for Japanese Industrial Standard coarse thread was manufactured. The heat treatment conditions were 12
After quenching in a 20 ° C salt bath, tempering twice at 560 ° C x 1hr (however,
Nos. 9 and 14 are 3 times). Using this tap, a cutting test was performed using S45C as a work material. S45C is 10mm thick
Was prepared by drilling a pilot hole with an inner diameter of 5.0 mm in the plate.

The cutting performance is 10 times longer than that of No. 7 mesh metal additive steel.
The life index was evaluated as 0. The calculation of the life index was performed by the following equation.

A: Number of perforations up to the end of life (average of 3 times) A ': Number of perforations for which No. 7 steel reached the end of life (3516) On the other hand, for the purpose of quantifying the microstructure, the same as a tap on a 6.0mmφ material After heat treatment, diamond polishing is performed on a position corresponding to the root of the tap blade portion in the vertical section, that is, around 1/8 of the material diameter from the surface layer of the tap (see FIG. 1).
Corroded with 10% nitric alcohol and further corroded with Murakami reagent, leaving MC-type carbides, corroding other carbides and bases. Using this sample, the total visual field area measured was 94,000.
The total number, average particle size, and area ratio of MC type carbide in μm ² are measured. The formula of 94,000 · {1- (area ratio)} / {(average particle size) · (total number)} was used for calculating the mean free path.

Table 1 shows the life index, MC type carbide mean free path and hardness (HRC).

It can be seen from Nos. 1 to 3 that when the hot working temperature exceeds 1180 ° C., the MC type carbide mean free path becomes large and the life is remarkably reduced.

Nos. 4 and 5 show that the cutting life is improved by performing the diffusion heat treatment before hot working. However, when the temperature of the diffusion heat treatment is high as in No. 6, the alloy becomes coarse and the life is shortened.
Although the MC type carbide mean free path of Nos. 4 and 5 is longer than that of Nos. 1 and 2, the life is improved because of the diffusion heat treatment.
Although there is growth of MC-type carbides, coarse M ₂ C-type carbides become MC and M ₆ C-carbides that have become spherical by solid-phase transformation,
Also, this is because the striped segregation was reduced. No. 7 is a steel to which a rare earth element has been added, and although the MC type carbide mean free path is almost satisfactory, cracks occur at the corners of the steel material during hot working and the surface removal amount is 3 to 6 mm. It had to be larger than in the present invention.

No. 9 had a high V content and was also a material to which Co was added, and the highest cutting performance was obtained in this experiment. In Nos. 10 to 13, the mean free path of the MC type carbide changes by about 10 to 14 μm depending on the V content and the manufacturing conditions, and the cutting life tends to be slightly shorter as the mean free path becomes longer. No. 14 is a CO additive and has a longer cutting life than No. 11. No. 15 has a low V content, a large mean free path of MC type carbide, and a short cutting life.

FIG. 2 summarizes the relationship between the mean free path of the MC type carbide, the tap and the cutting life, and shows that a good cutting life can be obtained when the mean free path is 15 μm or less.

Example 2 C 1.31%, Si 0.45%, Mn 0.3%, Cr 3.8%,
After melting in a vacuum melting furnace with the target components of W 5.0%, Mo 7.5%, and V 3.3%, hot working (1100 ° C), cold drawing,
A 6.0mmφ steel bar was formed by grinding (conventional example). The cooling rate at the time of solidification from the surface of the ingot to 1/8 or less of the ingot diameter was 50 to 80 ° C / min.

In addition, aiming at the same components as those described above,
After being formed into a billet having a size of 0 mm x 110 mm, it was heated and maintained at 1140 ° C for 4 hours, and then hot-rolled (1100 ° C), cold drawn and ground to obtain a 6.0 mmφ bar (the present invention).

These samples were subjected to salt bath quenching at 1220 ° C.
After tempering at ℃, the microstructure was observed and quantified. The same quantification method as in Example 1 was used.

3 (1) and 3 (2) show a conventional example, and (3) and (4) show microstructures near the depth of 1/8 from the surface layer of the material diameter of the present invention, hardness after heat treatment and MC type. The measurement results of the mean free path of carbide are shown. In the method according to the conventional manufacturing method, the carbide is coarse and the mean free path is large. In comparison, in the continuous casting method according to the method of the present invention, the microstructure is fine and uniform,
The mean free path is also small.

〔The invention's effect〕

According to the present invention, the carbide microstructure from the outer periphery to the position corresponding to the used portion of the surface layer is so fine that it cannot be easily achieved by the conventional production process, and the tool life is greatly improved in a tool having a cutting edge on the outer periphery. Can be achieved.

[Brief description of the drawings]

FIG. 1 (1) is a photograph of a metal microstructure corresponding to the portion A of (3) in the portion where the microstructure was quantified in the example,
(2) is a schematic view of the tap, (3) is a partially enlarged view of the tap of (2), and FIG. 2 is a mean free path of MC type carbide and S45C.
FIG. 3 is a diagram showing the relationship between the life of the material when tapping and tapping, and FIG. 4).

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00-38/60 C21D 6 / 00,8 / 00 B22D 11/00

Claims (6)

(57) [Claims] Claims: 1. C 1.1-2.0% by weight, Si 2.0% or less, Mn
A high-speed tool steel comprising 1.5% or less, Cr 2.0 to 10.0%, W and Mo in one or two kinds, W + 2Mo 8.0 to 20.0%, V 2.5 to 5.5%, balance Fe and unavoidable impurities. A high-speed tool steel characterized in that the mean free path of MC type carbide in a region from the surface to 1/8 of the wall thickness is 15 μm or less. 2. The high-speed tool steel according to claim 1, containing 1.0 to 20.0% of Co. 3. A slab having a minimum vertical or horizontal dimension of 150 mm or less in a cross section of the slab by the continuous casting method, wherein the slab having a minimum dimension of 150 mm
A method for producing high-speed tool steel, comprising: producing a portion up to / 8 at a cooling rate of 100 ° C./min or more, and lowering a hot working temperature performed later to 1180 ° C. or less. 4. The method for producing a high-speed tool steel according to claim 3, wherein before the hot working, a diffusion heat treatment is performed in a temperature range of 1100 to 1180 ° C. for 1 to 10 hours. 5. The method for producing a high-speed tool steel according to claim 3, wherein the high-speed tool steel contains 2.5 to 5.5% of V. 6. The method for producing a high-speed tool steel according to claim 3, wherein an amount of surface removal of the cast slab obtained by the continuous casting method is 2 mm or less on one side.