EP0377307B1

EP0377307B1 - Powdered high speed tool steel

Info

Publication number: EP0377307B1
Application number: EP89313418A
Authority: EP
Inventors: Yoshitomo Hitachi; Kozo Ozaki; Yukinori Matsuda; Noriyoshi Shibata
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1988-12-27
Filing date: 1989-12-21
Publication date: 1994-04-06
Anticipated expiration: 2009-12-21
Also published as: DE68914429T2; JP2725333B2; JPH02175846A; DE68914429D1; ATE103993T1; EP0377307A1

Abstract

Disclosed is a powdered high speed tool steel of good wear resistance and toughness. The powdered high speed tool steel have the alloy composition essentially consisting of 1.2 - 4.5 % of C, up to 3.0 % of Si, up to 3.0 % of Mn, 3.0 - 6.0 % of Cr, 15.0 - 60.0 % of W (up to 30.0 % thereof may be replaced by Mo), 1.0 - 15.0 % of V and up to 20.0 % of Co, and the balance substantially of Fe, and the carbide particles in the steel is relatively large, and those having Heywood diameters 1 micron or more share at least 10 volume % of the whole carbide particles.

Description

The present invention relates to a powdered high-speed tool steel, and provides high-speed tools having both good wear-resistance and toughness.
Powdered high-speed tool steels have advantages, when compared with melted materials, that fine carbide particles precipitate therein uniformly without segregation. The steels thus have higher toughness, and it is possible to improve the wear-resistance by high alloying.
Nevertheless, our experience shows that, in the practical use of powdered high-speed tool steels, it is rather rare to get results better than those of melted materials with a lower degree of alloying. Therefore, little of the principal benefits of powdered metallurgy have been obtained.
JP-A-0119645 discloses that for powdered high-speed tool steel containing tungsten, molybdenum and chromium, in order to provide a material having both desired processability and cutting performance as a tool, the alloy composition should contain large quantities of vanadium and cobalt.
It has now been found that, not only the alloy composition, but also the size and quantity of the carbide particles therein greatly influence the performance of the tools.
According to a first aspect of the present invention, in a powdered high-speed tool steel having good wear-resistance and toughness, the powder of the steel comprises 1.2-4.5% C, up to 3.0% Si, up to 3.0% Mn, 3.0-6.0% Cr, 15.0-60.0% W, 1.0-15.0% V and up to 20.0% Co, the balance being Fe, apart from any impurities, and the steel comprises 10-30.2% by volume of carbide particles having Heywood diameters of at least 1 µm.
According to a second aspect of the present invention, such a steel is as defined above, except that the powder additionally comprises up to 30.0% Mo, provided that W + 2Mo = 15.0-60.0%. Preferably, 2Mo/Weq is not higher than 0.45 (wherein Weq = W + 2Mo).
The Heywood diameter (D) means, as understood from the term itself, a value determined by measuring the area (A) of the cross-section of the carbide particles (preferably by a measuring means of high performance such as a scanning electromicroscope), and calculated by the formula ${D = 2(A/π)}^{½}$
.
The powdered high-speed tool steel is produced by spraying the molten metal with a gas or water, to rapidly cool the sprayed drops, and sintering the thus-obtained powder by means of, e.g. HIP. The precipitated carbide particles are generally fine and distributed uniformly. It has been considered that the wear-resistance of the tool is high when the carbide particles in the powdered steel are relatively large, and also that the toughness of the tool is high when the carbide particles are fine. However, this invention is based on the discovery that, contrary to the prior common understanding, the existence of relatively large carbide particles in an appropriate amount is favourable not only for the wear-resistance but also the toughness.
The sizes of the carbide particles can be increased, as is well known, by soaking the powdered high-speed tool steel at a high temperature for a period of time, e.g. 1150-1250°C for 2-10 hours. The relatively large carbide particles content is sufficient at 20-30 volume %; at contents exceeding this level, the effect saturates. Over-soaking causes the formation of huge carbide particles, resulting in decreased toughness.
The alloy composition will now be explained briefly.
C : 1.2 - 4.5 %
For the purpose of forming a large amount of carbide to realize the high wear resistance, 1.2 % or more of C is to be contained. Because the toughness of this powdered steel is high, the upper limit of the C-content may be high, but cannot exceed 4.5 %.
Si: up to 3.0 %, Mn: up to 3.0%
Both the elements are used as the deoxidation agent, and the upper limits are determined from the view point of giving no unfavorable influence to the toughness.
Cr: 3.0 - 6.0 %, W: 15.0 - 60.0 %, V: 1.0 - 15.0 %, Co: up to 20.0 %
All the above elements form carbides and give toughness to the steel. The lower limits of Cr and W are those necessary for ensuring the amounts of the carbides. On the other hand, too much addition thereof may not be accompanied by expected effects, and the decrease of the toughness will be unbearable. The upper limits are thus determined. V further contributes to the toughness because it makes the crystal grains fine. To obtain this effect, the lower limit of the content, 1.0 %, is given. Co further increases the heat resistance of the tool.
Mo: up to 30.0 %, Weq: 15.0 - 60.0 %, 2Mo/Weq: up to 0.45
Mo exhibits nearly the same effect as W, but the extent of the influence on the wear resistance and the toughness is about half of that of W, and therefore, the formula, W + 2Mo = Weq was given. The purpose of adding relatively large amount of W to Mo is to obtain a sufficient hardness of quenching-tempering (in terms of HRC, 66 - 67 or higher), and to maintain the anti-breaking strength high (260 kgf/mm2).
The powdered high speed tool steel of the present invention can give tools having both the good wear resistance and the high toughness by chosing a particular alloy composition and controlling the sizes and quantities of the carbide particles therein. To date, usually, in case of producing high speed tool steel by powder metallurgy, sufficient merits of combination of the high alloy composition and the powder metallurgy technology has not been obtained, and the products of the conventional art are not so different to those made of the melted materials. The present invention solved this problem.

EXAMPLES

Steels of the compositions shown in Table 1 were prepared in the molten state and atomized by gas spraying. The obtained powder was sintered by HIP process to have the densities near 100 %, and the sintered bodies were forged, and then annealed by being heated to 870°C for 1 hour and subsequent slow cooling. For the steels of each compositions, some of the samples were subjected to soaking in accordance with the present invention after the HIP process or in the process of the forging to adjust the size of the carbide particles therein, but the remaining samples were used as they are for the purpose of comparison.
The sizes of the carbide particles in the forged sample of the annealed state were measured. Conditions of the soaking and the percentages of the relatively large carbide particles are shown in Table 2.
Then, the samples were subjected to quenching-anneling, and the HT hardness and bend fracture strength were measured. The wear resistance was also determined. The wear resistance test was conducted in accordance with Ohgoshi method for accellated abrasion test under the conditions below:

Mating material: SCM 415 (as annealed)

Turning distance: 200 m

Turning speed: 2.93m/sec

Weight: 6.3 kgf
Evaluation of the wear resistance is expressed by relative coefficients to the comparative examples in which the coefficient is 100.

Data of the conditions of quenching-annealing, HT-hardness, anti-breaking strength and the wear resistance are shown in Table 3.

Table 2

No.	Example	Steel	Soaking	Large Carbide Particles (volume %)
1	Invention	A	1220°C · 5 hrs.	30.2
2	Invention	A	1190°C · 5 hrs.	17.3
3	Control	A	none	4.1
4	Invention	B	1180°C · 5 hrs.	11.3
5	Control	B	none	5.8
6	Invention	C	1200°C · 5 hrs.	25.5
7	Invention	C	1180°C · 5 hrs.	13.7
8	Control	C	none	8.2
9	Invention	D	1180°C · 5 hrs.	10.6
10	Control	D	none	3.8
11	Invention	E	1200°C · 5 hrs.	18.8
12	Control	E	none	7.2
13	Invention	F	1200°C · 5 hrs.	26.1
14	Control	F	none	8.8

Claims

A powdered high-speed tool steel having good wear-resistance and toughness, wherein the powder of the steel comprises 1.2-4.5% C, up to 3.0% Si, up to 3.0% Mn, 3.0-6.0% Cr, 15.0-60.0% W, 1.0-15.0% V and up to 20.0% Co, the balance being Fe, apart from any impurities, and wherein the steel comprises 10-30.2% by volume of carbide particles having Heywood diameters of at least 1 µm.
A powdered high-speed tool steel having good wear-resistance and toughness, which is as defined in claim 1 except that the powder additionally comprises up to 30.0% Mo, provided that W + 2Mo = 15.0-60.0%.
A steel according to claim 2, wherein 2Mo/(W + 2Mo) is not higher than 0.45.