GB1583695A - Nitrogen containing high speed steel obtained by powder metallurgical process - Google Patents

Description

(54) NITROGEN CONTAINING HIGH SPEED STEEL OBTAINED BY POWDER METALLURGICAL PROCESS (71) We, KABUSHIKI KAISHA KOBE SEIKOSHO, also known as KOBE STEEL LTD., a corporation organised under the laws of Japan, of 3-18, 1-chome, Wakinohamacho, Fukiai-ku, Kobe-city, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and bv the following statement: The present invention relates to a high-speed steel produced by a powder metallurgical process, more particularly to a nitrogen-containing, high-speed steel produced by a powder metallurgical process, wherein the amounts of C, N and V are properly adjusted to improve the continuous cutting propertv of the steel.

It is known that the properties of high-speed steels containing alloying elements such as Cr, W and V can be improved by the incorporation of nitrogen into the steels (see, for example, Kobe Steel Technical Bulletin, R & D, Vol. 24, No. 3, pages 11 to 15, and Japanese Patent Applications laid open Nos. 78606/74, 49109/75 and 49156/75). By the nitriding treatment, a nitride of the type MX or M6X (in which M represents an alloying element and X represents nitrogen) is formed, and this nitride is more stable than a carbide of the type MC or M6C. Accordingly, the appropriate quenching temperature range is broadened and control of the heat treatment can be facilitated.

Further, the temper hardening characteristic is improved and a fine austenitic crystal structure can be obtained to improve the mechanical properties. The machinability of the steels is therefore improved. By virtue of these effects, the properties of such high-speed steels can be improved by the incorporation of nitrogen into the steels.

Most conventional nitrogen-containing, high-speed steels have heretofore been prepared by a smelting process. When the smelting process is adopted for the production of nitrogen-containing, high-speed steels, it is necessary to perform complicated steps such as the step of melting steel in a high-pressure nitrogen atmosphere or the step of throwing a nitride into molten steel. Further, according to the smelting process, since the amount of nitrogen included in the steel is small and it is difficult to form a fine carbonitride and to distribute it uniformly in steel, it has hitherto proved impossible to improve the properties to desirable levels.

As a means of overcoming the defects or limitations involved in the smelting process, methods have recently been proposed for obtaining nitrogen-containing, high-speed steels by the powder metallurgical process or the powder forging process. In these methods, by utilizing the fact that powder has a large specific surface area (surface area/volume), and the fact that a powder sintered body has a porous structure, an optional amount of nitrogen can be included in steel by a simple means, for example. by adding nitrogen in advance to the starting powder or by adjusting the heating temperature, the heating time or the nitrogen partial pressure in the treatment atmosphere at the sintering step. It is expected that nitrogen will be finely and uniformly distributed in steels according to these methods.

In conventional nitrogen-containing, high-speed steels produced by a powder metallurgical process, the machinability is not as highly improved as might be expected. Instead, the machinability is degraded by the incorporation of nitrogen into the steels. Accordingly, it is often said that the value of nitrogen-containing, high-speed steels produced by a powder metallurgical process is questionable. However, several nitrogen-containing, high-speed steels produced by a powder metallurgical process, which have recently been put to practical use, have exhibited a good machanability and a good wear resistance in combination. The reason for this has not been elucidated. In particular, the relation between the amounts of alloying elements which impart excellent machinability to steel and the amount of nitrogen enrichment has not been clarified. Therefore, the kinds of steels which are enriched with nitrogen for the production of high-speed steels by a powder metallurgical process and which are applicable are drastically limited. For example. Kobe Steel Technical Bulletin. R & D. Vol. 24, No. 3. page 10 discloses that, when 0.4-0.5% nitrogen is added to Mo-type. high-speed steels (JIS SKH t3 and modified JIS SKH 55) by a powder metallurgical process. the machinability of intermittent cutting tools such as pinion cutters and bobs. is remarkably improved.

However. cutting test results in which a continuous cutting tool such as a drill or a bit (single point tool) composed of nitrogen-containing. high-speed steel produced by a powder metallurgical process has been adopted as a cutter have only rarely been reported. The machinability of a continuous cutting tool closely relates to the wear resistance and heat resistance of the tool. It is generally known that an increase in the amounts of carbide-forming elements such as V. W and Mo is advantageous in improving the wear resistance of the steels by increasing carbide formation and strengthening the matrix of the steel structure. An increase in Co is also known to be advantageous for the improvement of the heat resistance of the steel. So far as the wear resistance is concerned, the hardest carbide (MC) is effective to improve the wear resistance of the steel. In this sense, the V content is known to be closelv related to wear resistance. as V is an alloving element which forms carbides of this type. However. amongst the high-speed steels used in practice. even the highest alloying steels include at most 5% V. If still more V is included, the workability of the steel, e.g. its forgeability, mechanical workability or grindability, deteriorates. In these circumstances, the development of a high-speed steel produced by a powder metallurgical process which can solve the above-mentioned defects has long been desired.

(see @TETSU TO HAGANE@ vol. 65 (1975) No. 11 p.2629).

It is an object of the present invention to resolve problems involved with conventional nitrogen-containing, high-speed steels produced by powder metallurgical processes. One object of the present invention is to provide a nitrogen-containing, high-speed steel produced by a powder metallurgical process. which has good continuous cutting properties.

A second object of the present invention is to provide a nitrogen-contlining. high-speed steel produced by a powder metallurgical process which has a long service life.

In accordance with a first aspect of the present invention, a nitrogen-containing high-speed steel produced by a powder metallurgical process, comprises at least 0.40% N, 3.0-15% V, carbon (C) in an amount satisfying the relationship of 1.0 + 0.2V (%) < @ (C + N) < =1.5 + 0.2V (%), and a proportion of at least one element consisting of up to 15% Cr, up to 10% Mo, up to 20% W or up to 15% Co, with the balance being iron and the inevitable impurities.

In accordance with a second aspect of the present invention, a nitrogen-containing, high-speed steel as set forth under the first aspect above is provided in which the steel comprises C in an amount satisfying the relationship of 1.1 + 0.2V (%) < @ (C + N) < @1.5 + 0.2V (%).

In accordance with a third aspect of the present invention, a nitrogen-containing, high-speed steel as set forth in the first and the second aspects is provided wherein the steel comprises at least one element consisting of up to 2% Zr, up to 5% Nb, and up to 1% B.

Reference is now made to the accompanying drawnings in which: Figure 1 is a graph illustrating a comparison of the cutting life-time of certain powder metallurgical steels and the steel obtained by a smelting process; Figure 2 is a graph illustrating the relationship of the (C + N) content of JIS SKH 1() type high-speed steels versus the cutting life-time, at a cutting speed of 30 m./min.; Figure 3 is a graph illustrating the relationship of the (C + N) content of JIS SKH 57 type high-speed steels versus the cutting life-time, at a cutting speed of 30 m/min.; Figure 4 is a graph illustrating the relationship of the (C + N) content of the high-speed steels containing approximately 12% V versus the cutting life-time at a cutting speed of 3() m/min.; FigureS is a graph illustrating the relationship of the (C + N) content of JIS SKH 9 type high-speed steels versus the cutting life-time, at a cutting speed of 40 m/min.; Figure 6 is a graph illustrating the relationship of the (C + N) content of JIS SKH 10 type high-speed steels versus the cutting life-time, at a cutting speed of 40 m/min.; Figure 7 is a graph illustrating the relationship of the (C + N) content of JIS SKH 57 type high-speed steels versus the cutting life-time at a cutting speed of 40 m./min.; and Figure 8 is a graph illustrating the relationship of the (C + N) content of the nitrogen-containing high-speed steel containing approximately 12% V versus the cutting life-time when the cutting speed is higher (40 m./min.).

The nitrogen containing high-speed steels produced by the powder metallurgical process according to the present invention will now be described in detail with reference to the accompanying drawings.

A typical example of a steel powder heretofore used for the production of a nitrogen-containing, high-speed steel by a powder metallurgical process, is a powder of a steel corresponding to JIS SKH 10 (comprising 1.5% C, 4.0% Cr, 5.7% Co, 11.8% W, 4.5% V, with the balance iron). Nitrogen was incorporated into this steel and high-speed steels differing in their nitrogen contents were prepared. In these high-speed steels, the influence of the nitrogen content on the machinability was examined and the results shown in Figure 1 were obtained.

As is apparent from the results shown in Figure 1, the machinability is remarkably improved when the nitrogen content is at least 0.40%, a maximum value is obtained when the nitrogen content is approximately ().(r and, when the nitrogen content is above ().9%, the machinability deteriorates. In contrast, in the case of a nitrogen-containing, high-speed steel ciontaining 1.45% C and 0.05% N, which is produced by a smelting process, it was confirmed that the machinability is not good.

Carbon which is an essential element of high-speed steels has general properties quite similar to those of nitrogen. which is an additive element. Each of these elements has a vcry small atomic number of 6 or 7 and is an atom of the interstitial type having a tendency readilv to form an alloy compound. Accordingly. it is deemed reasonable to adjust or to regulate the nitrogen content in combination with the carbon content. for example, relving on such factors as the (C + N) content irrespective of the carbon content. Moreover. it is desired to regulate or to adjust the nitrogen content after due consideration of the contents of elements which have been admitted in the art to be elements capable of forming carbides with carbon in high-speed steels, in particular the element vanadium.

In viewing of the foregoing. as illustrated in the Examples hereinafter, steel powders corresponding to JIS SKH 1() or 57. which differ in nitrogen content. were prepared and nitrogen was incorporated in these steel powders in an amount of at least ().4()Y? necessary for improving the machinability of the steels. High-speed steels were then prepared from these powders by a powder metallurgical process, and they were tested with respect to machinability. The results obtained are shown in Figures 2-5, respectively.

Figure 2 illustrates the results obtained with steels corresponding to JIS SKH 10 containing 4.45-4.53% V. It is seen from Figure 2 that, when the (C + N) content is in the range of 1.9-2.4%, the machinability is remarkably improved. Thus, in a nitrogencontaining. high-speed steel produced by a powder metallurgical process. which corres- ponds to JIS SKIl 1(). a suitable range of the (C + N) content for improving the machinability is 1.().-9 Figure 3 illustrates the results obtained with steels corresponding to JIS SKH 57 containing 3.52-3.53% V. From Figure 3, it is apparent that a suitable range of (C + N) content is 1.7-2.2%.

Figure 4 illustrates the results obtained with steels having an increased V content, namely 4% Cr-3.5% Mo-10% W-12% V steels. In this case, a suitable range of (C + N) content is 3.4-3.9%.

If the foregoing experimental results obtained with respect to various high-speed steels produced by the powder metallurgical process are collectively considered mainly with respect to the (C + N) and V contents, it is apparent that, in order to improve the machinability of the steel, the following requirement must be satisfied: 1.0 + ().2V (%) # (C + N) # 1.5 + 0.2V (%) According to this requirement. if the V content excceds 15%, the toughness ordinarily decreases drastically because a vandium-type carbonitride is coarsened, and, in such a case, the above relationship which defines a range of (C + N) content suitable for machinability, is not satisfied. Moreover, if the vanadium content is higher than 15%, since a vanadium type carbonitride is coarsened, the grindability and foregoing property are very substantially degraded. If the vanadium content is below 3.0%, as can be seen from Figure 5 illustrating the machinability test results with respect to JIS SKH 9 containing I .95-2.()()% V, a substantial change in the machinability cannot be observed regardless of the (C + N) content. The test conditions were as shown below in connection with Example I except that the cutting speed was 4() m./min. Therefore, the V content must be at least 3.0% No significant improvemcnt of the machinability is attained if the nitrogen content is below 0.40%. According to the present invention. it is accordingly preferred that the nitrogen content should be at least 0.459? As is apparent from the foregoing experimental results, the above-mentioned relationship, namely an appropriate range of the (C + N) content. is not changed in various high-speed steels which differ from each other in the in rcspective contents of such metals as Cr. Mo. W and Co. In general. in high-speed steels, Cr is added in an amount of up to 15%, Mo is added in an amount of up to 10%, W is added in an amount of up to 20% and Co is added in an amount of up to 15%. Further, according to need, up to 2% Zr, up to 5% Nb and up to 1% B may be added.

The function of the additive elements will now be described. Tungsten (W) is an element which is important for imparting the required properties to high-speed steels. It combines with C, N and Fe to form a nitride of the Mf,X type and is dissolved in the matrix to improve the temper-hardening property and the high temperature hardness. and thereby enhances the wear resistance. Therefore, W makes a great contribution to the improvement of the machinability of the steel. However, if the W content exceeds 20%, no substantial increase in such effects is attained. Therefore, in the practice of the present invention. W is incorporated in an amount of up to 20%. In high-speed steels. Mo exerts a similar effect to those of W, but Mo is different from W in that molybdenum inhibits the growth of the crystal grains and does not greatly reduce the toughness. If the Mo content exceeds 10%, however, these effects are not substantially attained but the hot workability is degraded.

Accordingly. Mo is incorporated in an amount of up to 10%. Cr is present in the matrix as carbonitrides. which improve the quenching property and enhance the temper hardening property and high temperature hardness. However, if the Cr content exceeds 15%, the retained austenite content is drastically increased. Accordingly, Cr is incorporated in an amount of up to 15%. When Co is used in combination with W, Mo, V and the like, the cobalt efficiently improves the high temperature hardness; cobalt is also an additive element which is important for a tool steel for bird cutting materials. However, if the Co content exceeds 156/,. the quenching property and the hot workability are degraded.

Accordingly, Co is incorporated in an amount of up to 15%. Among impurities, Al is not preferred. The reason is that Al is present in the form of AlN which reduces the effects of the nitrogen. Accordingly. it is necessary to suppress the Al content below 0.4% The present invention will now be described with reference to the following Examples.

Example 1 Gas-atomized steel powders corresponding to JIS SKH 10 and differing in carbon content were packed in mild steel cans, subjected to degasfication and nitriding treatments and then compression-formed by a hot isostatic press subjecting the posders to a heist treatment. The preparation conditions and the tests for determining g the machinability are given below. For comparison, a steel product prepared by subjecting a steel produced by a smelting process to a beat treatment was similarly tested. and the results obtained are described below.

(1) Preparation conditions (a) Chemical composition and grain size of starting powder: The starting powders used are shown in Table 1 (below).

(b) Nitriding treatment: The nitriding treatment was conducted at 1150 C for 2 hours in a nitrogen atmosphere. The pressure of the atmosphere was appropriately controlled to adjust the nitrogen content in the product steel.

(c) Hot Isostatic Press Treatment: Hardening . 1200 C x 3 minutes (Oil Quenching) TABLE 1 Kind of Composition (%) Grain Steel Size C Si Mn P S Cr W V Co O N A (1.8% C) 1.79 0.18 0.27 0.01 0.02 4.02 12.1 4.48 4.81 0.028 0.038 smaller than 28 mesh B (1.5% C) 1.52 0.15 0.29 0.02 0.03 3.98 11.8 4.51 4.71 0.031 0.040 ditto C (1.2% C) 1.20 0.21 0.31 0.01 0.02 4.05 11.9 4.45 4.61 0.030 0.050 ditto D (0.9% C) 0.91 0.25 0.25 0.02 0.03 3.91 12.3 4.53 4.85 0.035 0.031 ditto Tempering : repeated 2-4 times with a heating pattern of 5600C x 1.5 hours. In the case of the comparative steel produced by a smelting process, the oil quenching was conducted at 1200 C for 3 minutes and the tempering was repeated twice, with a heating pattern of 560 C x 1.5 hours.

(2) Test conditions (a) Machinability Test .

Cutting speed : 3() m/min.

Cut depth : 1.5 mm Fced rate . 0.2mm/revolution Cutting oil : not used Tool shape : 0 , 15 , 6 , 6 , 15 , 15 , 1.0 Material machined : JIS SCM 4 (Quenched and Tempered) HB 300 - 350 (3) Results of Test Test results are shown in Figure 2. As is apparent from the results shown in Figure 2, in nitrogen-containing, high-speed steels containing approximately 4.5% vanadium, produced by a powder metallurgical process, in order to improve the machinability, the nitrogen content must be at least 0.40%, and an appropriate (C + N) content is in the range of 1.92.4%. Even in cases where the (C + N) content is 1.9-2.4%, if the nitrogen content is 0.2%, a significant improvement of the machinability is not observed.

Example II Atomized steel powders corresponding to JIS SKH 57 and differing in carbon content as shown in Table 2 were used as the starting powders and formed into nitrogen-containing, high-speed steels by the powder metallurgical process in the same manner as described in Example I. The machinability was tested and the results obtained are shown in Figure 3.

As is apparent from the results shown in Figure 3, a (C + N) content effective to improve the machinability is in the range of 1.7-2.2%.

Example III Gas-atomized steel powders containing approximately 12% vanadium and differing in carbon content as shown in Table 3 were used as the starting powders and formed into nitrogen-containing, high-speed steels by the powder metallurgical process in the same manner as described in Example I. The machinability was tested and the results obtained are shown in Figure 4.

As is apparent from Figure 4, a suitable (C + N) content effective to improve the machinability is in the range of 3.4-3.9%.

In accordance with the results obtained by the above Examples I - III, wherein the cutting speed is 30m/min, the contents of C, N and V must satisfy the following requirements: N # 0.40 %, (preferably N # 0.45%) 3.0% # V # 15%, and 1.0 + 0.2V (%) # (C + N) # 1.5 + 0.2V(%) whereby excellent machinability, in particular an excellent cutting life-time, is obtained.

Further examples will now be described wherein the cutting speed is 40 m/min.

TABLE 2 Kind of Composition (%) Grain Steel Size C Si Mn P S Cr Mo W V Co O N E(1.3%) 1.32 0.16 0.21 0.01 0.02 4.05 3.61 10.5 3.52 10.2 0.030 0.030 smaller C than 28 mesh F(1.0% C) 1.03 0.20 0.28 0.02 0.02 4.08 3.56 9.8 3.50 10.6 0.028 0.025 ditto G (0.7% C) 0.71 0.18 0.30 0.01 0.02 3.95 3.55 10.3 3.53 9.9 0.035 0.023 ditto TABLE 3 Kind of Composition (%) Grain Steel Size C Si Mn P S Cr Mo W V O N H (3.0% C) 2.98 0.15 0.28 0.01 0.02 4.05 3.59 10.4 12.0 0.038 0.15 smaller than 30 mesh I (2.5% C) 2.50 0.29 0.31 0.01 0.02 4.01 3.56 10.3 12.2 0.041 0.16 ditto J (2.0% C) 2.01 0.29 0.30 0.01 0.02 4.04 3.61 9.8 12.3 0.036 0.18 ditto Example IV Gas-atomized steel powders corresponding to JIS SKH 10 shown in Table 1 were used as the starting powders and formed into nitrogen-containing high-speed steels by a powder metallurgical process in the same manncr as described in Example I except for the cutting speed. The machinability was tested and the results obtained are shown in Figure 6.

As is apparent from Figure 6, a suitable (C + N) content effective for improving the machinability is in the range of 1.9 - 2.4% more preferably, 2.0 - 2.4%.

Example V Gas-atomized steel powders corresponding to JIS SKH 57 shown in Table 2 were used as the starting powders and formed into nitrogen-containing, high-speed steels by a powder metallurgical process in the same manner as desscribed in Example I except for the cutting speed. The machinability was tested and the results obtained are shown in Figure 7.

As is apparent from Figure 7, a suitable (C + N) content effective for improving the machinability is 1.7 - 2.2%, more preferably, 1.8 - 2.2%.

Example VI Gas-atomized steel powders containing approximately 12% vanadium shown in Table 3 were used as the starting powders and prepared into nitrogen-containing, high-speed steels by a powder metallurgical process in the same manner as described in Example I except for the cutting speed. The machainability was tested and the results obtained are shown in Figure 8.

As is apparent from Figure 8, a suitable (C + N) content effective for improving the machinability is 3.4 - 4.0%, more preferably, 3.5 - 3.9%.

As is readily apparent from the foregoing illustration, in the nitrogen-containing high-speed steel produced by the powder metallurgical process, according to the present invention, an excellent machinability, in particular an excellent cutting life-time can be obtained by adjusting and controlling the contents of C'. N and V so that the following requirements are satisfied: N # 0.40%, more preferably N # 0.45% 3.0% # V # 15% and 1.0 + 0.2V(%) # (C + N) # 1.5 + 02V (%), more preferably 1 + 0.2V (%) # (C + N) # 1.5 + 0.2V (%).

Further. the steel comprises a proportion of at least one element consisting of up to 15% Cr, up to 10% Mo, up to 20% W or up to 15% Co, with the balance iron and impurities. In addition, according to need, the steel may contain up to 2% Zr, up to 5% Nb, and up to 1% B.

Claims (5)

WHAT WE CLAIM IS:

1. A nitrogen-containing, high-speed steel produced by a powder metallurgical process, which comprises at least 0.40%, N, - 15% V, C in an amount satisfying the relationship of 1.0 + 0.2V (%) # (C + N) # 1.5 + 0.2V (%), and at least one element consisting of up to 15% Cr, up to 10% Mo, up to 20% W or up to 15% Co, the balance being iron together with any incidental injurities.

2. A nitrogen-containing, high-speed steel as set forth in Claim 1, wherein the C content is in an amount satisfying the relationship of 1.1 + 0.2V (%) # 1.5 + 0.2V (%).

3. A nitrogen-containing, high-speed steel as set forth in Claim 1, wherein the N content is at least 0.45%.

4. A modification of a nitrogen-containing, high-speed steel as set forth in Claim 1, which further comprises: at least one element consisting of up to 2% Zr, up to 5% Nb or up to 1% B.

5. A nitrogen-containing high-speed steel as claimed in Claim 1 substantially as herein described with reference to the accompanying drawings and/or any of the specific examples.