Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-04-11
2001-09-04
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000, C148S645000, C148S663000
Reexamination Certificate
active
06284064
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a processing method for strengthening materials such as thin plates and rods made of carbon steel, and specifically relates to a strengthening process for improving fatigue strength.
2. Background Arts
Until recently, it had been believed that the fatigue strength of mechanical parts, specifically in carbon steel materials used for parts in automobiles, among mechanical properties of the materials, greatly depends on tensile strength. Therefore, fatigue limits such as the number of repeated rotating and bending cycles, the number of repeated tensioning and compressing cycles, the number of repeated twisting cycles, and the like have been evaluated based on tensile strength. However, it is well seen that even if tensile strength is improved, fatigue strength cannot be easily improved, or there may be a region with no improvement in fatigue strength. Therefore, in order to improve fatigue strength, methods for hardening surfaces of materials by providing residual compressive stress thereto through several types of plastic processing, such as rolling, form rolling, drawing, and shot-peening have been generally applied.
However, in a method such as the above in which fatigue strength is improved, although fatigue properties of a material can be somewhat improved, fatigue strength of the overall material could not be improved. As a result, a portion with low fatigue strength may be the initiation site of a fracture, and this readily results in premature failure. Furthermore, in a shot-peening processing, a number of conditions such as shot diameter, incident speed, frequency of shot-peening processing (number of steps), and shot washing vary for each type of material, so that extensive experimentation was required to determine the optimal conditions for a specific case. The compressing processing such as rolling, form rolling, and drawing similarly require extensive experimentation to select processing conditions.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a processing method for strengthening carbon steel materials, which can relatively easily improve the fatigue strength of the overall material.
The inventors have intensively researched the phenomena in the region in which there is no improvement in fatigue strength even if tensile strength is improved. As a result, the inventors have discovered that fatigue strength is more closely related to yield point in measuring tensile strength than tensile strength. That is, when the tensile load is removed after increasing to an optional load which exceeds the yield point and is lower than the fracture load, and is then again increased, the yield point increases approximately up to the previous tensile load (the load just before the removal thereof). The increase in the yield point is due to the residual strain (residual tensile stress) provided to the entire material. Thus, the fatigue strength of the material with residual strain is improved. The inventors have discovered that a carbon steel having an average Rockwell hardness on the C scale (hereinafter referred to as “HRC”) in the range of 50 to 57 and a bainite structure is a material to which residual strain can easily be provided, and that one having a residual strain of at least 0.3% is effective for significantly improving fatigue strength. The present invention is made based on such knowledge. The invention provides a processing method for strengthening carbon steel materials, the method comprises performing a tension working to a carbon steel material having an average hardness in the range of HRC 50 to 57 and a bainite structure so as to provide a residual strain of at least 0.3%. It should be noted that the yield point refers to a stress at which plastic deformation of the material can be clearly observed to some extent, and the yield point includes the stress at which a predetermined stress, 0.2%, occurs, namely it includes proof stress at 0.2%.
The reason for limiting the average hardness is described below. When the average hardness is less than HRC 50, the fracture load is low and sufficient tensile load cannot be exerted on the material beyond the yield point. As a result, residual strain cannot be provided and improvement in fatigue strength cannot be expected. In contrast, when the average hardness is more than HRC 57, a martensite structure and a austenite structure may extensively precipitate in a bainite structure. When a tensile load is exerted on the structure, the residual austenite may be transformed into induced martensite, and the entire material may be hard and brittle. As a result, the material may be extended only up to a certain point, at which its ability to extend suddenly decreases, and elongation at a tensile load range above the yield point may be difficult to obtain. Therefore, an increase in the yield point may not be expected, and improvement in fatigue strength may not be expected. Carbon steels generally include four structures which are a ferrite structure, a pearlite structure, a martensite structure, and a bainite structure. Among these structures, the ferrite structure and the pearlite structure are soft, so that sufficient residual strain cannot be provided even if a tensile load is exerted thereon. The martensite structure is not suitable since it is hard and brittle, as mentioned above. In contrast, the bainite structure has good ductility, and sufficient residual strain may be provided thereon by exerting a tensile load.
In the invention, a material is subjected to a tensile working instead of compressing working as in conventional methods, so that residual strain, which is a positive residual stress, is provided to the material.
FIG. 1
shows a stress-strain diagram. When the tensile load is removed after increasing from A
0
and exceeding the yield point B
0
to B
1
, the strain does not return to A
0
, but returns along the line B
1
to A
1
, so that the strain corresponding to A
0
to A
1
remains in the material. The amount of the strain is a residual strain provided by the tension working and contributes to hardening of the entire material and to improving fatigue strength thereof. The size of the tensile load exerted again on the material after removing the load is chosen from the range exceeding the yield point and less than the fracture load, in which the entire material is evenly elongated. The size of the load is preferably 95% or less of the fracture load to avoid large deformation. The residual strain provided to the material by tension working is 0.3% or more, and is preferably 1.0% or more.
There were disadvantages in that surfaces of materials may be damaged and roughened in compressing working, but the invention is free from such disadvantages since a tension working is performed instead. Moreover, only a tensile load for providing residual strain needs to be set as a condition for improving fatigue strength, and is obtained by measuring a yield point and a fracture load through a tensile test. Therefore, the amount of experimentation to conditions can be remarkably small, and the process for improving fatigue strength can be efficient.
The chemical composition of the carbon steel material may be, for example, 0.5 to 0.65 weight % of C; 0.1 to 1.5 weight % of Si; 0.5 to 1.2 weight % of Mn; 0.5 to 0.8 weight % of Cr; no more than 0.15 weight % of Mo; no more than 0.5 weight % of V; no more than 0.2 weight % of Ni; and the balance of Fe. The carbon steel may be subjected to a heat treatment such as martempering in which the carbon steel is quenched from, for example, 880° C. to a temperature just above the Ms point and maintained at that temperature so as to cause isothermal transformation. By such a treatment, a carbon steel having an average hardness in the range of HRC 50 to 57 and a bainite structure as a main structure can be obtained. The carbon steel having such characteristics is subsequently subjected to the aforementioned tension working, so that fatigue strength can be improved over the ent
Fujiwara Akira
Shirakawa Atsushi
Yakubo Kazushige
Arent Fox Kintner & Plotkin & Kahn, PLLC
Honda Giken Kogyo Kabushiki Kaisha
Yee Deborah
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