Abstract: A gear keeping the bending stress at the dedendum from becoming locally high at the time of power transmission and thereby raising the dedendum bending strength. The radius of curvature is maximum at a critical section position determined by the Hofer's 30° tangent method. Both radii of curvature from the critical section position to the first and second connecting points X1 and Y1 are constant or decreasing. In dedendum line segment 23, there are points A1 and B1 where radius of curvature is smaller than the critical section position. In the dedendum line segment 23, the maximum radius of curvature is 3 times or less the minimum radius of curvature. By a cross-sectional view, the critical section position is part of an arc, and the arc extends to the two sides of the critical section position. Thus, the maximum bending stress becomes smaller and dedendum bending strength is improved.

Abstract: A gear keeping the bending stress at the dedendum from becoming locally high at the time of power transmission and thereby raising the dedendum bending strength is provided. The radius of curvature is maximum at a critical section position determined by the Hofer's 30° tangent method. Both of the radii of curvature from the critical section position to the first and second connecting points X1 and Y1 are constant or decreasing. In the dedendum line segment 23, there are the points A1 and B1 where the radius of curvature is smaller than the critical section position. In the dedendum line segment 23, the maximum radius of curvature is 3 times or less the minimum radius of curvature. By a cross-sectional view, the critical section position is part of an arc, and the arc extends to the two sides of the critical section position. Due to this, the maximum bending stress becomes smaller and improvement of the dedendum bending strength can be realized.

Abstract: A carburized steel part having excellent low cycle bending fatigue strength which is comprised of a steel material which contains, by mass %, C: 0.1 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.3 to 2.0, P: 0.02% or less, S: 0.001 to 0.15%, N: 0.001 to 0.03%, Al: 0.001 to 0.06%, and O: 0.005% or less and has a balance of substantially iron and unavoidable impurities and which is carburized and quenched, and then tempered, which steel part has a surface hardness of HV550 to HV800 and a core hardness of HV400 to HV500.

Abstract: A carburized steel part having excellent low cycle bending fatigue strength which is comprised of a steel material which contains, by mass %, C: 0.1 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.3 to 2.0, P: 0.02% or less, S: 0.001 to 0.15%, N: 0.001 to 0.03%, Al: 0.001 to 0.06%, and O: 0.005% or less and has a balance of substantially iron and unavoidable impurities and which is carburized and quenched, and then tempered, which steel part has a surface hardness of HV550 to HV800 and a core hardness of HV400 to HV500.

Abstract: A carburized steel part having excellent low cycle bending fatigue strength which is comprised of a steel material which contains, by mass %, C: 0.1 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.3 to 2.0, P: 0.02% or less, S: 0.001 to 0.15%, N: 0.001 to 0.03%, Al: 0.001 to 0.06%, and O: 0.005% or less and has a balance of substantially iron and unavoidable impurities and which is carburized and quenched, and then tempered, which steel part has a surface hardness of HV550 to HV800 and a core hardness of HV400 to HV500.

Abstract: A process of hot forging a steel at an ultrahigh temperature, including the steps of: heating a steel containing 0.1 wt % or more and less than 1 wt % carbon, in a non-oxidizing gas atmosphere, at a heating rate of from 3.degree. to 20.degree. C./sec, in a differential manner such that the steel shell having a thickness of from 0.5 mm to 1/5 of a maximum diameter of the steel is heated to a temperature within a range from a higher value selected from a temperature 45.degree. C. below a solidus line and a temperature of 1250.degree. C. to a temperature 20.degree. C. below a liquidus line while the steel core is heated to a temperature 20.degree. C. below the liquidus line or higher; blowing a cooling medium onto the surface of the heated steel to remove an oxide film from the steel furnace while cooling the steel shell having a thickness of from 1 mm to 1/5 of the maximum diameter of the steel, at a high cooling rate of 10.degree. C./sec or more to a hot forging temperature of 1200.degree. C.