Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-03-04
2003-05-06
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S334000, C148S335000
Reexamination Certificate
active
06558484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a steel for the manufacture of high-strength screws and to a high-strength screw made from said steel. More specifically, the present invention relates: to a steel for the manufacture of high-strength screws having a tapping ability for joining a member (in which a prepared hole has been formed) whilst forming a large diameter (M8 or larger) internal thread and having a strength of 800 N/mm
2
or more; and to a high-strength screw made from such a steel.
2. Description of the Prior Art
A tapping screw joins members together through forming an internal thread through the members. This can only be achieved if a prepared hole is formed in the members that are to be joined together. In order to be able to use tapping screws to join members together by forming an internal thread the tapping screws must be harder than the members. The tapping screw must be sufficiently harder than the members to be joined in order to cut the thread in the members. This is also important for the joint to be mechanically sound.
For these reasons, a conventional screw, for example, a cross-recessed tapping screw (in accordance with JIS B1122) has been manufactured from carbon-steel wires of SWRCH 12A to 22A (aluminum killed steel) or from SWRCH 12K to 22K (killed steel) (in accordance with JIS G3539) through the processes of forming a screw through rolling the steel, and refining the formed screw by using the techniques of cementation, hardening, and tempering.
One important factor of steel for use in the manufacture of tapping screws is its' toughness after hardening, therefore aluminum killed steels are used as they have fine crystal grains. However, properties that conflict with toughness (such as hardness and strength) must also reach satisfactory levels as well. Japanese Patent Laid Open No. 9-67625 discloses a tapping screw manufactured from a steel that has a high-magnesium (Mn) content and a low carbon (C) content by a process of cementation, hardening and tempering, that has a surface hardness Hv of 560 to 600 and an internal hardness Hv of 320 to 360. Hereafter, this type of tapping screw is referred to as prior art 1.
Japanese Patent Laid Open No. 10-196627 (1998) discloses a screw manufactured from a low carbon-high Mn steel that has a surface hardness Hv of 550 or higher and an internal hardness Hv of 320 to 400. Hereafter, this type of tapping screw is referred to as prior art 2.
In order to be able to join high strength members, an even higher surface hardness and internal toughness is required in the screw in order to be able to form an internal thread in the members. At present however, the materials and the method for manufacturing such a screw have not been established.
Both prior art 1 and prior art 2 are intended to be used for the manufacture of relatively small diameter screws (for example, smaller than M6). Therefore, if screws or bolts of M8 or larger are manufactured from these materials it is difficult to obtain the well-balanced surface hardness and internal hardness (after cementation) and the required strength.
The object of the present invention is to provide a steel for use in the manufacture of high-strength tapping screws(having a strength of 800 N/mm
2
or higher) and for tapping screws or bolts of large diameters (M8 or larger) and also to provide a high-strength screw manufactured from such a steel.
SUMMARY OF THE INVENTION
The inventors of the present invention conducted intensive studies in order to solve the above-described problems and obtained the following findings.
The hardness balance of screws and bolts of large diameters after cementation can be controlled and the desired strength can be obtained by:
(1) the addition of a large quantity of Cr,
(2) the adjustment of the ingredients to the adequate DI-value range,
(3) the adequate control of the surface hardness internal hardness and effective depth of the hardened layer, and
(4) the adequate control of the tempering temperature after cementation hardening.
The present invention is based on such findings and is characterized by the following;
The invention is characterized by a steel for high-strength screws comprising(by % mass): C: 0.05 to 0.20, Si: 0.20 or less (not including 0), Mn: 0.5 to 2.0, P: 0.015 or less, S: 0.015 sol. Al: 0.020 to 0.080, N: 0.0060 or less, Cr: more than 0.80 to 2.0 and the balance being iron and unavoidable impurities.
The invention is also characterized by the steel for high-strength screws further comprising (by % mass) of at least one selected from a group consisting of: Ni: 3.5 or less, Cu: 1.0 or less, Mo: 0.30 or less, and B: 0.0005 to 0.0050: and at least one selected from a group consisting of: Ti: 0.005 to 0.050 and Nb: 0.005 to 0.050.
The invention is also characterized by the steel for high-strength screws wherein the DI value represented by the following equation (1) is within a range of between 17 mm and 43 mm.
The invention is also characterized by the steel for high-strength screws wherein the DI value represented by the above equation (1) is within a range between 17 mm and 43 mm.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm
2
or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm
2
or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm
2
or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm
2
or more.
The invention is also characterized by the high-strength screw wherein tempering is carried out within a temperature range between 200° C. and 400° C. after cementation.
The invention is also characterized by the high-strength screw wherein tempering is carried out within a temperature range between 200° C. and 400° C. after cementation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The reason for limiting the values in the present invention will be described below.
(1) C: 0.05 to 0.20% by mass
“C” is an important element in the manufacture of strong steel. If the content of C is less than 0.05% by mass high strength cannot be obtained and cementation-hardenability lowers. If the content of C exceeds 0.20% by mass the internal hardness of the screw becomes too high and the toughness of the steel lowers. Therefore, the content of C was limited to the range between 0.05 and 0.20% by mass.
(2) “Si”: 0.20% by mass or less (not including 0)
Since Si plays an important role as a deoxidizing agent it is always added to steel in the manufacturing process. It also improves the resistance of the steel to softening (due to tempering and hardenability) and increases the strength of the steel. If the content of Si is too high the resistance to deformation increases and therefore the ability to cold-forge the steel is lowered. The upper limit of the Si content was determined to be 0.20% by mass.
(3) Mn: 0.5 to 2.0% by mass
Similarly to Si, Mn is an element required in the deoxidizing process of steel. It also increases the hardenability of steel. The addition of at least 0.5% Mn by mass is necessary for the steel to reach the required strength. Since Mn (as does P and S) separates on the crystal grain boundary of steel (and therefore increases the brittlement at the grain boundary) the upper li
Kikuchi Katsuhiko
Onoe Hiroshi
Shiraga Tetsuo
Armstrong Westerman & Hattori, LLP
Onoe Hiroshi
Yee Deborah
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