A rolled steel bar for hot forging consisting, by mass percent, of C: 0.25-0.50%, Si: 0.40-1.0%, Mn: 1.0-1.6%, S: 0.005-0.035%, Al: 0.005-0.050%, V: 0.10-0.30%, and N: 0.005-0.030%, and the balance of Fe and impurities, i.e., P: 0.035% or less and O: 0.0030% or less, wherein Fn1=C+Si/10+Mn/5+5Cr/22+1.65V5S/7 is 0.90 to 1.20. The predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing by taking the width of nonmetallic inclusion in an R.sub.1/2 part of a longitudinal cross section of the steel bar as W (m) is 99.99% is 100 m or narrower. The number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 m observed per unit area of the R.sub.1/2 part of a transverse cross section of the steel bar is 500 pieces/mm.sup.2 or higher.

A hybrid induction welded piston including an upper piston part welded to a lower piston part is provided. The piston is produced by induction heating the upper piston part and the lower piston part, and bringing the parts together to a part growth compensated position. The method then includes rotating the upper piston part 17 to 34 degrees clockwise and then 17 to 34 degrees counterclockwise. In addition to controlling the axial position and degree of rotation, the force applied to the piston parts is controlled so that preferably no flash is formed in a narrow cooling chamber of the piston. During the rotating steps, the pressure gradually increases to a maximum level which occurs while the upper piston part is rotating in the second direction. The piston includes a homogenous metallurgical bond across the weld and no indentation on the outer surface at the weld prior to machining.

A hybrid induction welded piston including an upper piston part welded to a lower piston part is provided. The piston is produced by induction heating the upper piston part and the lower piston part, and bringing the parts together to a part growth compensated position. The method then includes rotating the upper piston part 17 to 34 degrees clockwise and then 17 to 34 degrees counterclockwise. In addition to controlling the axial position and degree of rotation, the force applied to the piston parts is controlled so that preferably no flash is formed in a narrow cooling chamber of the piston. During the rotating steps, the pressure gradually increases to a maximum level which occurs while the upper piston part is rotating in the second direction. The piston includes a homogenous metallurgical bond across the weld and no indentation on the outer surface at the weld prior to machining.

A rolled steel bar for hot forging consisting, by mass percent, of C: 0.25-0.50%, Si: 0.40-1.0%, Mn: 1.0-1.6%, S: 0.005-0.035%, Al: 0.005-0.050%, V: 0.10-0.30%, and N: 0.005-0.030%, and the balance of Fe and impurities, i.e., P: 0.035% or less and O: 0.0030% or less, wherein Fn1=C+Si/10+Mn/5+5Cr/22+1.65V5S/7 is 0.90 to 1.20. The predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing by taking the width of nonmetallic inclusion in an R.sub.1/2 part of a longitudinal cross section of the steel bar as W (mm) is 99.99% is 100 mm or narrower. The number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 mm observed per unit area of the R.sub.1/2 part of a transverse cross section of the steel bar is 500 pieces/mm.sup.2 or higher.

A rolled steel bar for hot forging consisting, by mass percent, of C: 0.25-0.50%, Si: 0.40-1.0%, Mn: 1.0-1.6%, S: 0.005-0.035%, Al: 0.005-0.050%, V: 0.10-0.30%, and N: 0.005-0.030%, and the balance of Fe and impurities, i.e., P: 0.035% or less and O: 0.0030% or less, wherein Fn1=C+Si/10+Mn/5+5Cr/22+1.65V5S/7 is 0.90 to 1.20. The predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing by taking the width of nonmetallic inclusion in an R.sub.1/2 part of a longitudinal cross section of the steel bar as W (mm) is 99.99% is 100 mm or narrower. The number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 mm observed per unit area of the R.sub.1/2 part of a transverse cross section of the steel bar is 500 pieces/mm.sup.2 or higher.

A drive plate having a plate portion and a ring portion is shaped integrally by pressing using dies. The ring gear portion has inner recessed portions positioned on the inner side of a plurality of outer teeth. The inner peripheral surface of each of the inner recessed portions includes an inner bottom surface having a pair of curved corner surfaces, a pair of first side surfaces inclined so as to become closer to each other as the first side surfaces extend from the opening end of the inner recessed portion toward the inner bottom surface, and a pair of second side surfaces formed in a band shape between the corner surfaces and the first side surfaces. The degree of inclination of the second side surfaces with respect to a center line is smaller than the degree of inclination of the first side surfaces with respect to the center line.

A drive plate having a plate portion and a ring portion is shaped integrally by pressing using dies. The ring gear portion has inner recessed portions positioned on the inner side of a plurality of outer teeth. The inner peripheral surface of each of the inner recessed portions includes an inner bottom surface having a pair of curved corner surfaces, a pair of first side surfaces inclined so as to become closer to each other as the first side surfaces extend from the opening end of the inner recessed portion toward the inner bottom surface, and a pair of second side surfaces formed in a band shape between the corner surfaces and the first side surfaces. The degree of inclination of the second side surfaces with respect to a center line is smaller than the degree of inclination of the first side surfaces with respect to the center line.

A mold for forging a scroll rotor capable of controlling material flow speed by adjusting a land section, a molding device applying same, and a method for manufacturing the mold. In the mold, a die includes an extrusion unit having a molding space therein and a spiral-shaped extrusion passage provided in the lower portion thereof so as to correspond to a wrap portion disposed in the scroll rotor. A punch is connected to the die to be in close contact with the inner circumference of the molding space and to be able to slide up and down. The lower portion has a shape corresponding to the upper surface of the flange portion of the scroll rotor and a boss portion. The extrusion passage has a land section in direct contact with the extrusion material. The height of a land of the land section varies along the spiral direction.

A mold for forging a scroll rotor capable of controlling material flow speed by adjusting a land section, a molding device applying same, and a method for manufacturing the mold. In the mold, a die includes an extrusion unit having a molding space therein and a spiral-shaped extrusion passage provided in the lower portion thereof so as to correspond to a wrap portion disposed in the scroll rotor. A punch is connected to the die to be in close contact with the inner circumference of the molding space and to be able to slide up and down. The lower portion has a shape corresponding to the upper surface of the flange portion of the scroll rotor and a boss portion. The extrusion passage has a land section in direct contact with the extrusion material. The height of a land of the land section varies along the spiral direction.

A method for producing a camshaft module and a camshaft module are provided. The camshaft module includes a camshaft and a bearing frame having bearing devices with cut-outs. The camshaft includes a shaft and cams having cut-outs. The shaft has at least two different outside diameters. At least some of the cams and/or some of the bearing devices have cut-outs of differing diameters. The bearing frame is supported during shaft insertion to be movable along a movement axis during production of the camshaft module and to be substantially rigid along an insertion direction of the shaft. The shaft is inserted into the bearing frame in the insertion direction in such a way that interference fit is obtained in each case at least between the shaft and at least some of the cams. A predefined target distance between two elements such as a gear located outside of the bearing frame and a cam or bearing device located inside of the bearing frame adjacent to the element outside of the bearing frame may be obtained by partially inserting the shaft, determining a distance between the two elements, and applying an insertion force to the shaft based on the determined distance to advance the shaft to result in setting the distance between the two elements to the target distance.