Steel for machine structural use

Details Steel for machine structural use Steel for machine structural use

2002-11-15

2004-09-28

Yee, Deborah (Department: 1742)

Alloys or metallic compositions

Ferrous

Lead, bismuth, selenium, tellurium or calcium containing

C420S087000, C420S126000, C420S110000, C420S128000, C420S109000

Reexamination Certificate

active

06797231

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a steel for machine structural use, which is to be subjected to machining for use as industrial machinery or automotive parts, among others. More particularly, the invention relates to a steel for machine structural use excellent in chip disposability and effective in prolonging the cutting tool life (hereinafter referred to as “tool life improvement”).
PRIOR ART
Among the steels for machine structural use, which are used as industrial machinery or automotive parts, among others, there are steels for machine structural use as defined in the Japanese Industrial Standard JIS G 4051, and such alloy steels as nickel-chromium steels according to JIS G 4102, nickel-chromium-molybdenum steels according to JIS C 4103, chromium steels according to JIS G 4104 and manganese and manganese-chromium steels for machine structural use according to JIS G 4106. Also in use are steels improved in hardenability by modifying the amount of addition of the specified components of these steels or by adding B (boron) or the like and/or improved in metallurgical structure by addition of Ti, Nb: V and/or the like.
In many cases, these steels are subjected, after rolling or after further forging or other working, to machining to desired forms or shapes, followed by heat treatment according to the required characteristics, to give final products. For improving the productivity in this machining step, it is strongly desired that the steels be excellent in machinability. Good machinability means that the period between exchanges of tools for use in machining due to wear is long, that is, that the tool life is long, that chips generated during machining can be finely torn and separated, that the cutting force is not so great, and that good machined or ground surfaces can be obtained.
With the advancement in automation of machining, not only the tool life but also the separability of chips, namely “chip disposability”, becomes very important. Since the tool life is influenced by the characteristics of the material steel as well as the performance characteristics of the tool, tool selection is also important. On the contrary, good chip disposability means that chips generated during machining are finely torn or divided and separated but will not entwine the tool. The chip disposability greatly depends on the characteristics of the material steel. For improving the machinability of steel, it is very important to improve this chip disposability.
The machinability of steel can be improved by addition of Pb. However, addition of Pb not only increases the cost of steel but also may lead to environmental contamination. Therefore, investigations have been carried out in search of technologies of improving the machinability of steel without adding Pb. A typical one is the technology of improving the machinability by utilizing MnS inclusions. This technology has been studied in various aspects and already put to practical use.
Thus, for example, the steel disclosed in Japanese Patent Publication (JP Kokoku) H05-15777 contains Mn—Ca—S type inclusions with a Ca content of 3—55% as uniformly dispersed therein. As for their sizes, the major axis L is not longer than 20 &mgr;m and the ratio thereof to the minor axis W (L/W) is not more than 3. In this steel, however, individual sulfide inclusions become coarse, hence the number of sulfide inclusions at the same S concentration decreases. Therefore, the improvement in chip disposability is not entirely satisfactory. In addition, because the steel is Al-killed steel, even after treatment with Ca, the oxide inclusions are of the CaO—Al
2
O
3
type, hence the improving effects on the machinability such as tool life are not very satisfactory. When an attempt is made to disperse a large number of sulfide inclusions containing a high concentration of CaS by increasing the S concentration, addition of a large amount of Ca is required, and this disadvantageously causes an increase in cost.
Laid-open Japanese Patent Application (JP Kokai) 2001-131684 discloses steels for machine structural use, in which manganese sulfide-based inclusions have an average oxygen content of not more than 10%. The steels have the following principal composition, in % by mass: C: 0.05-0.7%, Si: not more than 2.5%, Mn: 0.1-3.0%, Al: not more than 0.1%, S: 0.003-0.2%, N: 0.002-0.025%, and O (oxygen): not more than 0.003%, with the balance being Fe. In addition to these components, the steels may contain not more than 0.01%, in total, of one or more elements selected from the group consisting of rare earth elements, Ca and Mg.
However, the steels according to the invention disclosed in JP Kokai 2001-131684, as described in the example section thereof, contain not less than 0.018% of Al used as a deoxidizer clement so that the average oxygen concentration in sulfides may be reduced to 10% or less for obtaining such a sulfide form as effective in improving the chip disposability. In such a case, the oxides in steel are mainly hard Al
2
O
3
type oxides, and the tool life is improved only to an unsatisfactory extent. Thus, the invention disclosed in the above-cited publication is not an invention made in an attempt to improve the chip disposability and, at the same time, improve the tool life.
In JP Kokai 2000-34538, there is disclosed a steel for machine structural use which contains C, Si, Mn, P, S, Al, Ca and N each in a specified amount and is excellent in machinability in turning. This steel has the following characteristic features. Namely, the following two relations are satisfied:
A
/(
A+B+C
)≦0.3 and
B
/(
A+B+C
)≧0.1
wherein A is the area percentage of sulfide inclusions having a Ca content exceeding 40% relative to the total area of an investigation field of view, B is the area percentage of sulfide inclusions having a Ca content of 0.3-40% relative to the total area of the investigation field view, and C is the area percentage of sulfide inclusions having a Ca content of less than 0.3% relative to the total area of the investigation field of view. The steel of JP Kokai 2000-34538 is characterized by increasing sulfide containing 0.3-40% of Ca. However, increase of such sulfide of high Ca content makes the sulfide coarse and makes improvement of chip disposability difficult.
JP Kokai 2000-282169 discloses a steel, which contains C, Si, Mn, P and S and further contains one or more elements selected from among Zr, Te, Ca and Mg and satisfies the conditions: Al≦0.01%, total O≦0.2% and total N≦0.02%. This steel is excellent in forgeability owing to spheroidizing of sulfide inclusions and has good machinability. Thus, on the premise that Ca is added, it is intended that Ca solutes in MnS and lowers the deforming ability of MnS for spheroidizing the same in this steel. In this case, however, individual sulfide inclusions become coarse, whereby that sulfide morphology suited for providing good chip disposability cannot he obtained, hence the improvement in chip disposability is not yet satisfactory.
All the steels disclosed in the above mentioned publications may contain Ca and are improved primarily in machinability. However, it cannot be said that sufficient considerations have been given to the level of addition of Ca, the timing of addition thereof and the dissolved oxygen content in the steel. Thus, they are not satisfactorily improved simultaneously in chip disposability and in tool life.
It is an object of the present invention to provide a steel for machine structural use, which is improved in machinability, especially in chip disposability and, at the same time, can prolong the tool life, without containing Pb.
SUMMARY OF THE INVENTION
It is well known that the machinability of steel is greatly influenced by the state of sulfide and/or oxide inclusions in the steel. For improving the machinability of Pb-free steels for machine structural use, the present inventors made close investigations concerning the relationship between the form and distribution of inclusions in the steels and the machinability thereof, and st

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