Free-cutting tool steel

Details Free-cutting tool steel Free-cutting tool steel

2002-02-28

2004-07-13

Yee, Deborah (Department: 1742)

Alloys or metallic compositions

Ferrous

Nine percent or more chromium containing

C420S042000, C420S084000, C420S087000

Reexamination Certificate

active

06761853

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tool steel used as source materials for tools and dies, and in particular to a tool steel having machinability.
2. Related Art
It is a general practice for dies and tools that they are fabricated first using an annealed steel material, then subjected to roughing, quench-and-temper for adjusting hardness, and finishing. In some cases, the tools and dies are fabricated using an already quenched-and-tempered steel material and then directly finished aiming at shorter period before delivery date. This relates to process sharing in order to fabricate the dies and tools between the material supplier and the user fabricating such dies and tools. That is, the material supplier supplies the user with an annealed steel material in the former case, so that the user is responsible for roughing, quench-and-temper and finishing. On the other hand, the steel material is supplied in a quenched and tempered form in the latter case, so that the user is responsible only for the final processing. The final processing herein requires a somewhat larger amount of processing since no roughing has been effected.
In either case, the processing is mainly aimed at removal operation such as cutting and grinding. Processing of tool steel is however not so easy as compared with other steel materials, since the tool steel necessarily has hardness and toughness to a level enough to overwhelm the material to be processed. The processing will be more difficult after quench-and-temper. There is a growing demand for shorter period before the delivery and an expanded range of unmanned processing of the die in order to reduce production costs of such dies and tools, so that, to cope with such situation, a tool steel having a better machinability than the previous materials have has been desired.
Known elements for improving the machinability of iron-base materials include S, Pb, Se, Bi, Te and Ca. Recent trends in environmental preservation in a global scale have been repelling use of Pb, and there is a growing number of instruments and parts limiting the use thereof. So that there are proposed substitutive materials using S and Te as major elements for improving the machinability. Such materials are successful in improving the machinability and grinding property since inclusions such as MnS and MnTe generated therein can exhibit stress concentration effect during chip formation, and lubricating effect between the tool and chip.
A problem however resides in the steel materials using S and Te as elements for improving the machinability that such materials tend to be elongated along the direction of rolling or forging to thereby cause undesirable anisotropy in the mechanical properties thereof, although the inclusions such as MnS and MnTe can improve the machinability. More specifically, the crack resistance will be ruined due to degradation of the toughness in the direction normal to the direction of forging and rolling (referred to as T-direction, hereinafter). This raises another problem that it will always be necessary to consider material orientation depending on mode of use of the tools and dies, which tends to result in degrading production efficiency and yield ratio from a viewpoint of effective use of the material.
It is also undesirable that the inclusions are generally as long as exceeding 50 &mgr;m. Such large inclusions will undesirably roughen the polished surface of the material during mirror polishing if they drop to thereby scratch the polished surface, or to thereby form large pits where they were embedded, which makes it difficult to obtain a desired level of smoothness of the mirror-polished surface. The large inclusion of the sulfide-base is even causative of degraded corrosion resistance of the material. This is apparent for example from Japanese Laid-Open Patent Publication No. 7-188864 which discloses that the corrosion resistance can be improved by controlling the length of such sulfide-base inclusion so that 80% of which have a length of 50 &mgr;m or less.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a free-cutting steel which has an excellent machinability, and is less causative of anisotropy in mechanical properties, particularly in toughness, depending on forging-and-rolling direction.
To solve the foregoing problems, a first aspect of a free-cutting tool steel of the present invention contains Fe as a major component and C in an amount of 0.1 to 2.5 wt %;
containing Ti and/or Zr so that W
Ti
+0.52W
Zr
amounts to 0.03 to 3.5 wt %, where W
Ti
represents Ti content (wt %) and W
Zr
represents Zr content (wt %);
contains at least any one of S, Se and Te so that W
S
+0.4W
Se
+0.25W
Te
amounts to 0.01 to 1.0 wt %, and so that (W
Ti
+0.52W
Zr
)/(W
S
+0.4W
Se
+0.25W
Te
) amounts to 1 to 4, where W
S
represents S content (wt %), W
Se
represents Se content (wt %) and W
Te
represents Te content (wt %); and
has dispersed in a texture thereof a machinability improving compound phase within a range from 0.1 to 10% in terms of area ratio in a section; wherein
such machinability improving compound phase comprises a metallic element component having Ti and/or Zr as major components, and a binding component for such metallic element component essentially containing C and also containing any one of S, Se and Te. It is to be noted that “major component” in the context of this specification means a component (also including “phase” in conception) having a highest content on the weight basis in a material or texture in consideration (the same will apply to other expressions such as “mainly comprises”).
By adding C, Ti, Zr, S, Se and Te in the foregoing compositional ranges, the machinability improving compound phase is produced in a dispersed manner in the texture of the steel material, which compound phase comprising a metallic element component having Ti and/or Zr as major components, and a binding component for such metallic element component essentially containing C and also containing any one of S, Se and Te. The formation of such compound can successfully add an excellent machinability to the steel material. The present inventors suppose that, when the material is processed by cutting or grinding in order to remove a portion thereof, the machinability improving compound phase finely dispersed in the texture functions just like a perforation to thereby facilitate formation of the sectional plane, which is responsible for improved machinability.
An essential point is that the machinability improving compound phase does not elongate in the forging-and-rolling direction even after such processing and can keep the grain form. So that the compound phase is successful in considerably suppressing the degradation of toughness in the T direction, unlike MnS or so which tends to elongate along the forging-and-rolling direction. The free-cutting tool steel of the present invention is excellent in the machinability not only in the annealed state but also in the quenched-and-tempered state, and thus can desirably cope with the repetitive processing in the quenched and tempered state which is beneficial in reducing the period before the delivery.
The machinability improving compound phase is necessarily formed so as to be dispersed in the texture within a range from 0.1 to 10% in terms of area ratio in a section. The area ratio less than 0.1% results in only a poor improving effect of the machinability, and exceeding 10% results in a degraded toughness. The area ratio is more preferably 0.2 to 4%. In order to raise the improving effect of the machinability, it is preferable to control an average size of the machinability improving compound phase observed in the polished sectional texture (maximum width between two parallel tangential lines which are drawn in some different directions so as to circumscribe the outer contour of the compound grain) within a range from 1 to 5 &mgr;m or around.
The machinability improving compound phase can typically be such

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