GB1583777A - 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 by the following statement:- This invention relates to nitrogen-containing high-speed steels obtained by powder metallurgical processes.

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 (as laid-open) Nos. 78606/74, 49109/75 and 49156/75). By the nitriding treatment, a nitride of the type MX or MX (in which M represents an alloy element and X represents nitrogen) is formed, and this nitride is stabler than a carbide of the MC or MC type. 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 finer austenite crystal structure can be obtained to improve the mechanical properties. The machinability of the steel can also be improved. By virtue of these effects, it is believed that 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 the 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 the steel, it is impossible to improve the properties to desirable levels.

As a means of overcoming the defect or limitations involved in the smelting process, methods have recently beec proposed for obtaining nitrogen-containing, bigh- 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 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 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 obtained by the 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 steel.

Accordingly, it is often said that the value of nitrogen-containing, high-speed steels obtained by the powder metallurgical process is questionable.

On the other hand, several nitrogen-containing, high-speed steels obtained by a powder metallurgical process, which have recently been put into practical use, have exhibited good machinability and good wear resistance in combination. The reason for this has not been elucidated. In particular, the relationship between amounts of alloy elements which impart excellent machinability to steels and the amount of nitrogen enrichment is not clarified. Therefore, the kinds of steels which are enriched with nitrogen for the production of high-speed steels by the 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 to 0.5% of nitrogen is added to Mo-type, high-speed steels (JIS SKH 9 and modified JIS SKH 55) by a powder metallurgical process, the machinability is remarkably improved. Indeed, improved machinability can be attained by such technique. However, the addition of such a large amount of nitrogen to high-speed steels in which the contents of alloy elements are standardized results in a disorder of the stoichiometric balance among the alloy elements in the high-speed steels. Therefore, various problems are caused by this technique. For example, the residual austenite content at the quenching step is increased and it is necessary to increase the number of repetitive tempering steps.

Accordingly, the size change during the heat treatment is enhanced. Moreover, the toughness is degraded and the applicable range of cutting tools of these steels is considerably limited.

The above advantage of the powder metallurgical process is that an optional amount of nitrogen can be included into the steel and a fine carbonitride can be distributed uniformly in the steel was observed, and consequently, research was conducted with a view to improving various properties of high-speed steels, especially the machinability, by nitrogen enrichment of high-speed steels comprising various alloy components according to a powder metallurgical process. As a result, we have found that, in order to improve the machinability without degradation of the heat treatment characteristics and mechanical properties of high-speed steels, a specific relationship must be established among C, N and V as specific ingredients of high-speed steels.

The present invention has been completed on the basis of these findings.

An object of the present invention is to solve the problems involved with conventional nitrogen-containing, high-speed steels obtained by a powder metallurgical process. It is, therefore, a major object of the present invention to provide a nitrogencontaining, high-speed steel by means of a powder metallurgical process in which the machinability is improved without degradation of the heat treatment characteristics and the transverse bending strength.

In accordance with a first aspect of the present invention in which the above and other objects are attained, a nitrogen-containing, high-speed steel obtained by a powder metallurgical process which comprises at least Cur.40% of N, 1.6 to 15% of V, C in an amount satisfying the relationship of 0.5+0.2V(%)~(C+N)~0.8+0.2V(%) and a proportion of at least one element consisting of up to 15% of Cr, up to 10% of Mo, up to 20% of W and up to 15 of Co, or any combination therefore, 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 in the first aspect is provided wherein the content of N is at least 0.45%.

In accordance with a third aspect of the present invention, a nitrogen-containing, high-speed steel as set forth in the first and second aspects is provided wherein the content of V is 2.5 to 15%.

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

Reference is now made to the accompanying drawings, in which: Fig. 1 is a graph illustrating the relationship of the nitrogen content in highspeed steels versus the cutting life time, the size change during heat-treatment and the bending strength; Fig. 2 is a graph illustrating the relationship of the (C+N) content of highspeed steels versus the cutting life time, the size change during heat-treatment and the bending strength; Fig. 3 is a graph illustrating the relationship of the (C+N) content of highspeed steels versus the cutting life time, the size change during heat treatment and the bending strength; and Fig. 4 is a graph illustrating the relationship of the (C+N) content of highspeed steels versus the cutting life-time, the size change during heat treatment and the bending strength.

The nitrogen-containing, high-speed steels obtained 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 nitrogen-containing, high-speed steels by the powder metallurgical process, is a powder of a steel corresponding to JIS SKH 9 (comprising 0.8% of C, 4.1% of Cr, 5.2% of Mo, 6.2% of W and 2.019b of V). Nitrogen was incorporated into this steel and high-speed steels differing in their nitrogen content were prepared. In these highspeed steels, the influence of the nitrogen content on the machinability, the size change during heat treatment and the transverse bending strength were examined and the results shown in Fig. 1 were obtained.

As will be apparent from the results shown in Fig. 1, the machinability is remarkably improved when the nitrogen content is at least 0.40% and a maximum value is obtained when the nitrogen content is substantially 0.5%. The heat treatment strain is lower and better than in steels prepared by the melting process when the nitrogen content is below 0.40%. However, if the nitrogen content exceeds 0.40%, the heat treatment strain is increased to a level substantially equal to the size change during heat-treatment in steels obtained by the smelting process. Furthermore, a better bending strength is obtained when the nitrogen content is below 0.40%, but the bending strength apparently diminishes if the nitrogen content exceeds 0.40%.

Thus, the machinability cannot be improved if the nitrogen content does not exceed 0.40%. On the other hand, better results are not obtained in connection with the size change during heat treatment and the bending strength if the nitrogen content is not lower than 0.40%. In short, a nitrogen content giving a better machinability adversely affects the heat treatment characteristic and the transverse bending strength of the steel.

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 very small atomic number of 6 or 7 and is an atom of the interstitial type having a tendency readily to form an alloy compound. Accordingly, in high-speed steels having a high nitrogen-content, it is deemed reasonable to adjust or to regulate the nitrogen content in combination, with the carbon content, for example, relying on such factors as the (C+N) content and the C/N ratio, instead of adjusting or regulating the nitrogen content irrespective of the carbon content only. 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 as elements capable of forming carbides together with C and N in high-speed steels, especially vanadium.

In view of the foregoing, as illustrated in the Examples hereinafter, steel powders corresponding to JIS SKH 9 or 10, which differ in carbon content, were prepared and nitrogen was incorporated in these steel powders in an amount greater than 0.35%, which is a critical level necessary for improving the machinability. Then, high-speed steels were prepared from these powders by a powder metallurgical process, and they were tested with respect to the machinability, the heat treatment strain and the traverse bending strength and the results obtained are shown in Figs. 2 to 4.

Fig. 2 illustrates the results obtained with respect to steels corresponding to SKH 9 (V=1.95 to 2.04%). It is seen from Fig. 2 that, if the (C+N) content is higher than 0.9"j,, the machinability is remarkably improved and either the size change during heat treatment or the bending strength is maintained at a desirable level when the (C+N) content is less than 1.2%. Namely, in a nitrogen-containing, high-speed steel obtained by a powder metallurgical process, which corresponds to JIS SKH 9, a suitable range of the (can) content for improving the machinability without degradation of the heat treatment characteristic and mechanical properties is from 0.9% to 1.2%.

Fig. 3 illustrates the results obtained with respect to steels corresponding to SKH 10 (V=4.45 to 4.53%). From Fig. 3, it is apparent that a suitable range of the (C+N) content is from 1.4% to 1.7%.

Fig. 4 illustrates the results obtained with respect to steels having an increased V content, namely 4%Cr3.5%Mo10%W12%V steels. In this case, a suitable range of (C+N) content is from 2.9% to 3.2%.

If the foregoing experimental results obtained with respect to various high-speed steels obtained by the powder metallurgical process are collectively considered mainly in view of the (C+N) and V contents, it is apparent that, in order to improve the machinability of the steel without degradation of the heat-treatment characteristic and the transverse bending strength, the following requirement must be satisfied: 0.5+0.2V(%) < (C+N)'~0.8+0.2V(%) If the V content exceeds 15%, the toughness ordinarily decreases drastically because a vanadium-type carbonitride is coarsened, and, in such a case, the above relationship which defines a range of (C+N) content suitable for machinability, the heat treatment characteristics and the mechanical properties are not satisfied. Moreover, if the vanadium content is higher than 15%, since a vanadium type carbonitride is coarsened, the grindability and forging property are very substantially degraded. If the V content is lower than 1.6%, it is difficult to include at least 0.4% of N in the form of a nitride. Of course, enrichment of N can be accomplished if the nitriding pressure is made higher than the atmospheric pressure. However, in that case, nitrides of Cr, Fe and the like are formed, resulting in a remarkable reduction of the machinability and transverse bending strength. Therefore, in the present invention, the lower limit of the V content is defined as 1.6%. In high-speed steels, a higher wear resistance is obtained as the amount of a carbonitride cf the MX type comprising V as the main alloy element, which is hardest amcng carbcnitrides, is larger. Therefore, in the present invention, in order to enhance the effect by the addition of N, it is preferred that the V.content should be at least 2.5%. No significant improvement of the machinability is attained if the nitrogen content is lower than 0.40%. In the present invention, it is preferred that the nitrogen content should be at least 0.45%.

As will be 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 differing in the content 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% of Zr, up to 5% of Nb and up to 1% of B may be added. The function of the additive elements will now be described.

W is an element important for imparting the required properties to high-speed steels. It combines with C, N and Fe to form a nitride of the M,X type and is dissolved in the substrate to improve the temper-hardening property and the high temperature hardness, and thereby to enhance 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 of such effects is attained.

Therefore, in the present invention, W is incorporated in amount of up to 20%. In high-speed steels, Mo exerts effects similar to those of W, but Mo is different from W from the viewpoint that it inhibits the growth of the crystal grain 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 substrate as carbonitrides and improves the quenching property and enhances the temper hardening property and high temperature hardness. However, if the Cr content exceeds 15%, the residual 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, it efficiently improves the high-temperature hardness, and it is an additive element important for a tool steel for hard cutting materials. However, if the Co content exceeds 15%, the quenching property and the hot workability are degraded. Accordingly, Co is incorporated in an amount of up to 150%. Among irnpuritios Al is not preferred. The reason is that Al is present in the form of AlN which reduces the effects of N. Accordingly, it is necessary to reduce 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 9 and differing in carbon content were packed in mild steel cans, subjected to degasification and nitriding treatments and then compression-formed by a hot isostatic press to yield steel ingots.

Products were obtained by subjecting these ingots to a heat treatment. The preparation conditions and the rests for determining the machinability, size change during heat treatment and traverse bending strength are illustrated below. For comparison, products prepared by subjecting steels obtained by the smelting process to a heat treatment were similarly tested, and the results are described below.

(1) Preparation Conditions (a) Chemical Composition and Grain Size of Starting Powder: The starting powders used are shown in Table 1 as follows: TABLE 1 Composition (%) Kind of Steel C Si Mn P S Cr Mo W V O N Grain Size A (0.9% C) 0.91 0.30 0.30 0.01 0.03 4.15 4.91 6.03 1.98 0.025 0.025 smaller than 80 mesh B (0.7% C) 0.70 0.29 0.27 0.01 0.03 4.30 5.01 6.12 1.95 0.010 0.021 ditto C (0.5% C) 0.49 0.25 0.24 0.01 0.04 4.35 5.12 6.06 2.00 0.030 0.015 ditto D (0.3% C) 0.32 0.31 0.32 0.01 0.03 4.11 4.97 6.15 2.04 0.035 0.018 ditto (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.

(c) Hot Isostatic Press Treatment: The treatment was conducted at 11000C. for 2 hours under 2000 atmospheres.

(dj Heat Treatment: Hardening: 12000cox 3 minutes (oil quenching) Tempering: repeated 2 to 4 times with a heating pattern of 560"C.X1.5 hours.

In the case of comparative steels obtained by a smelting process, the oil quench ing 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: 30 m/min Cut depth: 1.5 mm Feed rate: 0.2 mm/revolution Cutting oil: not used Tool shape: 0 , 150, 60, 60, 15 , 150, 1.0 Material machined: JIS SCM 4 (quenched and tempered), H3 of 250270, 4 slots.

(b) Heat Treatment Strain: A non-heat treated material having a diameter of 80 mm and a height of 100 mm was used as the specimen, and after the heat treatment, the (size change) from the circle (maximum diameter-minimum diameter) was measured.

(c) Transverse Bending Strength: A specimen of 5 mmxl0 mmx 30 mm was subjected to a static bending test in which the distance between fulcra was 20 mm and the load was imposed on the center alone.

(3) Results of Tests Test results are shown in Fig. 2 of the accompanying drawings. As is apparent from the results shown in Fig. 2, in nitrogen-containing, high-speed steels containing 2% vanadium, obtained by a powder metallurgical process, in order to improve the machinability without degradation of the heat treatment characteristics and the transverse bending strength, the nitrogen content must be at least 0.4%, preferably at least 0.45%, and an appropriate (C+N) content is in the range of from 0.9 to 1.2%.

If the nitrogen content is below 0.4%, a sufficient nitriding effect canriot be obtained, and if the (C tN) content is below 0.9%, the amount of precipitated nitrides is small and the wear resistance is degraded. If the (C+N) content exceeds 1.2%, the residual austenite content is increased at the hardening step and its amount beccmes unstable. Therefore, the size change during heat treatment becomes conspicuous and the bending strength is lowered.

Example 2.

Atomized steel powders corresponding to JIS SKH 10 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 a powder metallurgical process in the same manner as described in Example 1. The machinability, size change during heat treatment and bending strength were tested and the results obtained are shown in Fig. 3 of the acn-Irpanyin,, drawings.

TABLE 2 Composition (%) Kind of Steel C Si Mn P S Cr W V Co O N Grain Size E (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 smaller than 28 mesh F (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 G (0.6% C) 0.59 0.31 0.29 0.01 0.03 4.12 12.8 4.48 4.92 0.021 0.063 ditto As is apparent from the results shown in Fig. 3, a (C+N) content effective for improving the machinability without degradation of the heat treatment characteristic and the bending strength is in the range of from 1.4 to 1.7%. Fig. 3 also indicates that, in nitrogen-containing, high-speed steels obtained by the powder metallurgical process, even if the (C+N) content is in the range of 1.4#1.7%, when N is added in such a small amount as substantially 0.3%, a significant improvement of the machinability cannot be attained.

Example 3.

Gas atomized steel powders containing 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 a powder metallurgical process in the same manner as described in Example 1. The machinability, size change during heat treatment and bending strength were tested and the results obtained are as shown in Fig. 4 of the accompanying drawings.

TABLE 3 Composition (%) Kind of Steel C Si Mn P S Cr Mo W V O N Grain Size H (2.8% C) 2.81 0.28 0.30 0.01 0.02 4.05 3.51 10.5 12.1 0.035 0.15 smaller than 80 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 0.18 ditto J (2.0% C) 2.01 0.29 0.30 0.01 0.02 4.04 3.61 9.8 12.3 0.030 0.18 ditto As will be apparent from the results shown in Fig. 4, a (C+N) content effective to improve the machinability without degradation of the heat treatment characteristic and the bending strength of the steel is in the range of from 2.9 to 3.2%.

As is readily apparent from the foregoing illustration, in the nitrogen-containing, high-speed steel obtained by the powder metallurgical process according to the present invention, the machinability, heat treatment characteristics and mechanical properties can be remarkably improved by adjusting and controlling the contents of carbon, nitrogen and vaanadium so that the following requirements are satisfied.

N#0.40%, 1.6%#V#15%, and 0.5+0.2V(%)#(C+N)#0.8+0.2V(%).

Claims (5)

1. WHAT WE CLAIM IS:1. A nitrogen-containing, high-speed steel obtained by a powder metallurgical process, which comprises at least 0.40% N, 1.6 to 15% V, C in an amount satisfying the relationship of 0.5+0.2V(%)#(C+N)#0.8+0.2V(%), and a proportion of at least one element consisting of up to 15% of Cr, up to 10% of Mo, up to 20% of W and up to 15% of Co, the balance being iron and inevitable impurities.
2. 2. A nitrogen-containing, high-speed steel as claimed in claim 1, wherein the content of N is at least 0.45%.
3. 3. A nitrogen-containing high-speed steel as claimed in claim 1, wherein the content of V is 2.5 to 15%.
4. 4. A modification of a nitrogen-containing, high-speed steel as claimed in claim 1, which further comprises at least one element consisting of up to 2% of Zr, up to 5% of Nb and up to 1% of B.
5. 5. A nitrogen-containing high-speed steel according to claim 1, substantially as herein described with reference to the accompanying drawings and/or any of the specific examples.