Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-10-02
2003-05-13
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C148S327000
Reexamination Certificate
active
06562153
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a strain-induced type martensitic steel having high fatigue strength and high hardness which is suitably used for members, which are required to have both of high hardness and high fatigue strength, such as a power transmission belt etc. used in a continuously variable transmission etc., and a steel strip made of this strain-induced type martensitic steel.
Hitherto, for forming members required to have high strength such as members for rockets, members for a centrifugal separator, members for aircraft, members for a continuously variable transmission of an automobile, metal mold and etc., there has been mainly used a maraging steel having a very high tensile strength of about 2000 MPa, the representative composition of which is, for example, 18%Ni—8%Co—5%Mo—0.4%Ti—0.1%Al—bal. Fe. The maraging steel contains, as strengthening elements, much amount of each of Co and Mo which are expensive, so that the price of it becomes very high with the result that the maraging steel is used only for such particular uses as explained above.
In general, in a steel of high tensile strength used in a case where high strength is required, there are required not only both high hardness and high tensile strength but also both high fatigue strength and high toughness. In a case where the tensile strength is not more than 1200 MPa, there is such a tendency as the fatigue strength increases in proportion to the increase of the tensile strength, however, in a high strength steel having hardness not less than about 400 Hv and tensile strength not less than about 1200 MPa, the fatigue strength does not increase even in a case where both of the hardness and the tensile strength increase. This is also applicable to the maraging steel, that is, the fatigue strength thereof is not high although the maraging steel has high tensile strength. Thus, there has been desired a novel, high tensile strength steel cheap in price and having high fatigue strength which steel can be used in place of the conventional maraging steel.
SUMMARY OF THE INVENTION
The present inventors researched various high tensile steels in order to obtain a novel, high tensile steel which is usable in place of the conventional maraging steel.
First, as a relatively cheap, high hardness material, a quenched, martensite steel of 13% Cr type containing carbon of about 0.5% is researched. The steel of this type is produced by a process comprising the steps of cold-working the steel softened by annealing so that a predetermined dimensions may be obtained, and subjecting the cold-worked steel into a heat treatment including quenching and tempering. By this heat treatment is obtained a hard, martensite phase containing carbon, so that it becomes possible to obtain very high hardness regarding this steel.
However, in the steel, since it is necessary to perform the heat treatment of quenching and tempering for obtaining the high hardness, there are such problems as many steps are needed for obtaining an intended article, as the production steps are complicated, and as heat treatment deformation occurring during the quenching performed from a high temperature is large. Further, since the steel contains a relatively much amount of carbon, the weldability thereof is not necessarily good.
Then, the research is performed regarding Steel of JIS-SUS631 which is well known as a steel of a type in which martensite transformation occurs by cold working. In the steel of JIS-SUS 631, it is possible to obtain a hardness of about 490 Hv by performing solution heat-treatment, cold working, and aging treatment. However, in the steel of JIS-SUS 631, there is such a problem as the characteristics thereof such as hardness and etc. are very sensitive to the composition and the conditions of the heat treatment, so that the variation of the characteristics are apt to occur.
Further, it is possible to obtain high hardness by cold-working austenitic steels such as steels of JIS-SUS304 and JIS-SUS201. However, in the austenitic steels, since the austenite phase is a stable phase, only a part of the austenite phase is transformed into a strain-induced martensite even in a case where intensive cold-working is performed, and much of the austenite phase remains to be work-hardened austenite phase, so that there is such a problem as it is impossible to obtain a sufficiently high hardness.
The object of the invention is to provide a strain-induced type martensitic steel and a steel strip made of the strain-induced type martensitic steel.
In general, in a high tensile steel, fatigue fracture occurs due to the occurrence of cracks which commence at the surface of the steel and due to the propagation of the cracks, in a case of fatigue fracture occurring at a low cycle range, as disclosed in Japanese Mechanical Society Theses Vol. A64, pages 2536 to 2541. Further, in a very high cycle range exceeding 10
7
cycles which are conventionally deemed to be fatigue limit, it is known that the fatigue fracture does not commence at the surface of the steel but commences at inclusions included in the interior of the steel.
In the conventional maraging steel, it is known that inclusions which become the internal initiating points of the fatigue fracture are TiN (or Ti(C, N)). Thus, the reducing of the TiN (or Ti(C, N)) as little of possible is deemed to be effective to enhance the level of the fatigue strength, and it is deemed that a high strength steel containing no Ti has high fatigue strength.
Thus, the present inventors aimed at strain-induced type martensitic steels in which high hardness can be obtained without using Ti which is a precipitation-strengthening element. However, in the strain-induced type martensitic steel, the characteristics are apt to vary similarly to the case of the JIS-SUS 631 steel explained above. Particularly, in the JIS-SUS 631 steel, there is such a shortcoming as the weldability thereof is inferior due to aluminum of 1 mass % contained therein, and there is such an advantage as high hardness can be obtained without adding expensive Co which is used in the maraging steel, with the result that the price thereof can be greatly lowered. Further, in the JIS-SUS 631 steel, since cold plastic working is used to obtain high hardness and a finished shape, there is such another advantage as no quenching which is performed from a high temperature in the case of the quenched, martensite steel explained above is needed, with the result that no heat treatment deformation occurs.
Thus, the present inventors researched regarding various alloying elements and the amount of each of them which alloying elements can remove the above-explained shortcoming occurring in the JIS-SUS 631 and which can maximize the above-explained advantages, and have found out that, by adding particular alloying elements each with an appropriate amount in the strain-induced type martensitic steel, by adding therein age-hardening elements such as Mo and Cu etc., and by performing aging treatment after cold plastic working, it becomes possible to obtain further improved, high strength.
The prevent inventors have further researched so, that there may be obtained a strain-induced type martensitic steel which can provide such high strength, high hardness and high fatigue strength as to be usable, for example, for producing a power transmission belt used in a continuously variable transmission of an automobile, and have achieved the present invention.
According to the first aspect of the invention, there is provided a strain-induced type martensitic steel having high hardness and high fatigue strength, consisting essentially, by mass, of 0.01 to 0.10% carbon, not more than 3.0% silicon, more than 5.0 but not more than 10.0% manganese, 1.0 to 12.0% nickel, 4 to 18% chromium, at least one kind of 0.1 to 4.0% in total in terms of Mo+1/2 W which at least one kind is selected from the group consisting of molybdenum and tungsten, from 0 inclusive but not more than 5.0% copper, from 0 inclusive but not more than 0.15% nitrogen, not more than 0.10% al
Fujita Etsuo
Minagi Yoshihiro
Uehara Toshihiro
Hitachi Metals Ltd.
King Roy
Sughrue & Mion, PLLC
Wilkins, III Harry D.
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