JPS6058776B2 - high speed tool steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high speed tool steel that has significantly superior heat resistance, wear resistance and toughness compared to conventional high speed tool steels.

It is a well-known fact that the biggest drawback of conventional high-speed tool steels, compared to cemented carbides, is their lack of heat resistance and wear resistance.

The reason for this is that the heat resistance of high-speed tool steel is limited to C, Cr, W, and Mo.
, V, etc., and hardening due to martensitic transformation from a base containing V, etc., and precipitation hardening of alloy carbides. The absolute value of the degree of hardening is determined by C, Cr, w,
-C is mostly determined by the amount of solid solution of Mo and V and the amount of precipitation, and the degree of decrease in hardness at high temperatures (softening resistance) is controlled by the amount of solid solution of the above elements and the type of precipitated carbide. Since carbides of Cr, Mo, W, and V precipitate in the order of low temperature, in other words, the higher the content of W and V in the matrix, the better the softening resistance becomes. From the above, in order to improve the heat resistance of high-speed tool steel, it is necessary to
It is preferable to increase the amount of solid solution of elements such as o, W, and V, and at this time, increase the amount of solid solution of alloys such as W and V as much as possible.

As a measure to this end, AIS1M7 is a high-C type that increases the amount of carbide precipitation by increasing the amount of C added relative to the content of alloying elements such as Cr, Mo, W, and V.
A series of steel types such as M4l and M42 have been developed, and it is common to add Co, which has the effect of increasing the solid solution amount of the alloying elements and suppressing the agglomeration and softening of precipitated carbides during tempering.
Copper grades have come to constitute the current wide variety of high-speed tool steels.

However, with such conventional methods, there is a limit to which the amount of C and alloying elements in the base cannot be increased due to the following reasons. (
1) In the phase diagram, in the composition range of conventional high-speed tool steel, the reaction of decomposition from melt into austenite and eutectic carbide [L-
γ+M6C] (■4.

C) exists, and a high austenitizing temperature higher than this reaction temperature cannot be adopted. This reaction temperature is determined by Steven [(G, S, Steven, A, E, Nehrenb
erg: TransASM57 (1964) P, 925
]According to Te(°F)■2310-200(%C)+
It is expressed by the formula: 40(%V)+8(%W)+5(%Mo).

This formula is C0.7-1.3%, Cr3.5-4.5%,
V1.0-5.0%, W0-18%, M00-9%, C
It is said that it can be applied to materials containing 00 to 15%, for example, SKH9 has a temperature of 1256°C. If austenitization occurs above this eutectic reaction temperature, a liquid phase will be generated in the material, and the amount of solid solution of the alloy in the matrix is subject to an absolute limit at this temperature. (2) The softening resistance of the base changes depending on the type of tempered carbide, and Cr23c6 MO2C<W2C<V
The precipitation temperature increases in the order of 4C3. In other words, the more V is contained in the base, the more the softening resistance increases. However, VC carbide, which is a source of V, is difficult to form a solid solution in the matrix, and in conventional steel types, the amount of V dissolved in the matrix at a temperature below the above equation is said to be 1.6% or less at maximum. For the above reasons, the base composition of conventional steel types is 0.5 regardless of the steel composition.
~0.6%C, 3~8%W, 4~5%Cr, O. 9-1
．． 6%V (Tomoo Sato, Taiji Nishizawa, Hirosuke Murai, Tetsu-to-Hagane No.4 Misaki NO5P.2l) As a result, it was thought that the heat resistance of high-speed tool steel could not be significantly improved. .

The present invention differs from such conventional concepts and ideas, and one is that L→γ, which naturally occurs in conventional steel types, during the solidification process.
By using a chemical component that inhibits the eutectic reaction of +M6C, all existing M6C carbides are dissolved in the matrix, and at the same time, the eutectic reaction is eliminated, making high-temperature austenitization possible. The purpose of this method is to increase the amount of solid solution of MC type carbide in the matrix and provide a material with excellent heat resistance.

The high speed tool steel of the present invention has a weight ratio of Cl. O~1.8,
%, Cr3. O~6.0%, W+2M08~12%, V
O. 8-2.2%, Nb2. O~7.0%, V+Nb2
．． 6-9.0%, V/NbO. 7 or less, CO1 or less, 2% or less, and 1% or less of S], Mn, and unavoidably mixed impurities, and the remainder.

But! The most distinctive feature of the present invention is that w+2
The rV10 (W equivalent) is suppressed at 8 to 12%, which is lower than that of conventional steels (according to the current JIS standard, the w equivalent is low at 11.0% for SKH6;
This comes next at 12.5%. In the AISI standard, AISI
8% of M5O is very low. S3-3 according to German standards
12 is 9%. The above-mentioned M5O and S3-3-2 are low-alloy high-speed steel usually called semi-high-speed steel. ). and■
The total content of +Nb is 2.6 to 9% and V/Nb'' is specified to be 0.7 or less, and the combination of these components substantially eliminates corner C-type carbides in the austenitic state. In other words, the reaction of γ ten corner C-+L is suppressed during the temperature raising process during quenching, making high-temperature austenitization possible.As a result, most of the VC carbides are dissolved in the base, and the remaining M℃ The undissolved carbide provides an effect of suppressing the growth of crystal grains and an effect of imparting wear resistance.
Examples of the present invention will be described in detail below. Table 1 shows the chemical components of the invention steel, conventional steel, and comparative steel.

Sample A in Table 1 is the basic component of the steel of the present invention. Sample B-1 in the table is Sample A (7) in which the W equivalent, V/Nb ratio, V+Nb amount, and CO content were varied within the composition of the present invention. Samples J, K, and L are comparative steels. Sample J is conventional steel type SKH.
1O, Samples K and L were obtained by varying V/Nb of Samples A to 1 (7). The above raw materials were melted in the atmosphere using a small high-frequency melting furnace to obtain a 10 kg steel ingot. This was forged into a square shape of 20 wrm under the same hot working conditions as normal SKHlO, completely annealed at 860'C, and then subjected to various tests. Table 2 shows the X-ray diffraction results of residual carbides in each sample oil-cooled after austenitizing at 1180, 1220, and 1260°C, and 560, 600, and 640
This figure shows the results of observing the hardness after tempering at 1 hour x 3 times at 1 hour and the melting state of eutectic carbide after high temperature austenitization up to 1270°C. The residual carbide in the X-ray diffraction results is also shown in parentheses. In this case, a slight diffraction phenomenon was observed, and it was determined that a small amount of residual carbide was present.

Furthermore, according to the X-ray diffraction results, it is clear that the steel of the present invention is austenitized at a temperature of 11,800 C or higher, and almost no M6C type carbide exists. In addition, if Samples D and I are excluded, there is only one type of residual carbide at M°C, and when samples D and I are austenitized at 1260°C, there is only one type of residual carbide at X°C. Compared to these, M6C carbide remains in all of Samples J, K, and L, and VC carbide also remains. In sample K containing 2% Nb, M°C carbides also remain as a matter of course. Thus, compared to conventional steels, the steel of the present invention has a distinctive metallographic feature in that M6C carbides and type C carbides do not exist due to high-temperature austenitization.

Further, while the conventional steel and comparative steel showed melting of eutectic carbides at temperatures below 1260°C, no significant melting of eutectic carbides was observed in the steel of the present invention even at 1270°C. Next, it is clear that the steel according to the invention has a very high absolute value of tempering hardness and a high resistance to tempering softening.

For example, sample A steel is 560 compared to sample J steel (SKHlO).
Provides tempering hardness higher than HRC2 in the range of °C to 640 °C. W and M are expensive alloying elements in high-speed tool steel.
Having such high tempering hardness despite the low O content is also effective from the viewpoint of resource saving. It is also noteworthy that Sample G, which contains almost no CO, has a tempering hardness equivalent to that of Sample J (SKHlO). From the above, it has become clear that the steel of the present invention has higher tempering hardness, that is, higher heat resistance than conventional steel.

Table 3 shows the toughness evaluation test results.

The test method was a bending test using a single point loading method on a 5φ x 70e steel bar. In comparison with conventional steel J (SKHlO) and comparative steels K and L, it is clear that the steel of the present invention has high hardness, equivalent fracture resistance, and superior toughness.

Table 4 shows sample A steel and sample J steel (SKHlO
) is the result of measuring the base composition of

Sample A steel has higher amounts of alloying elements of W, MO, V, and CO in the base compared to sample J steel. As mentioned at the beginning, this is the reason for the high heat resistance of the steel of the present invention. There is clearly a strength. Table 5 shows S45C annealed steel bars ([18175) at 53rT1/Min, 6Or
The measurement results of the tool life when continuous cutting is performed in the longitudinal direction of the outer circumference at a cutting speed of Tl/Min are shown.

All depth of cut is 1.0TIr1n1 feed is 0.3TWt/
Rev. All of the steels of the present invention exhibit superior cutting durability compared to comparative steels. Even sample G steel, which contains almost no CO, has the same durability as sample J steel. Next, we will discuss the reasons for limiting the ingredients.

In addition to the base martensite hardening, C is Cr, W, MO,
Source of C to temper precipitated carbides such as ■ and residual M
It is an essential element as a source of C in carbides.

The amount of C can be based on the chemical equivalent value shown by the following formula. The maximum amount of C should be determined appropriately depending on the amount of other elements added, and the maximum amount of C should be determined appropriately depending on the amount of other elements added.
In relation to the content of Nb, etc., the lower limit is 1% and the upper limit is 1.8
%is necessary.

Cr is an essential element in the same way as conventional high-speed tool steel in order to improve the hardenability of the material.

If it is less than 3%, the hardenability will be poor, and if it exceeds 6%, the absolute value of hardness will decrease, so it was set at 3 to 6%.

W (5M0 is W2C, MO2 at the time of tempering in the present invention)
It is essential as a supply source for precipitated carbides, which are the main cause of secondary hardening of C, but unlike conventional high-speed tool steels, it is hardly necessary for the formation of undissolved carbides.

The effect of W (5M0 is defined by the W equivalent expressed as W+2M0 in the sense that 1% MO has the same effect as 0.5% W. When the amount of W+?0 is less than 8%, the W
+2 The amount of M0 decreases, the tempering hardness is insufficient, and if it exceeds 12%, r! Since 4Gc carbide remains as undissolved carbide, the upper limit is set at 12%.

(2) and Nb have similar tendencies in that they have a very strong tendency to form MC type carbides.

Claims (1)

[Claims] 1 C1.0-1.8%, Cr3.0-6.0 in weight ratio
%, W+2Mo8-12%, V0.8-2.2%, Nb
2.0-7.0%, V+Nb is 2.6-9.0%, V/
Nb is 0.7 or less, Co is 12% or less, Si, Mn is 1% or less, the remainder is unavoidably mixed impurities and Fe, 1
A high-speed tool steel characterized by having excellent heat resistance, wear resistance, and toughness without substantially remaining M_6C type carbides at an austenitizing temperature of 180°C or higher.