Disclosed is a steel whose composition includes specified wt % of: Ni, Mo, Co, Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Y+Ta+Cr+C+Al+B+Ti+N, Ni+Co+Mo, Al, Ti, N, Si, Mn, C, S, P, B, H, O, Cr, Cu, W, Zr, Ca, Mg, Nb, V, Ta, Y, the remainder being iron and impurities resulting from production. The inclusion population, observed by image analysis on a polished surface of 650 mm.sup.2 if the steel is in the form of a hot-formed part or a hot-rolled sheet and 800 mm.sup.2 if the steel is in the form of a cold-rolled sheet, does not include non-metal inclusions of an equivalent diameter greater than 10 m. Also disclosed are a product created from the steel, and a manufacturing method.

A high strength forged steel comprises composition which comprises predetermined amounts of predetermined chemical elements and a balance being Fe and inevitable purities. A major constituent of a metallographic structure of the steel comprises bainite, martensite or a combination thereof. An average crystal grain size in steel microstructures between a near-surface level below a surface of the steel at a distance of 5 mm and a mid level below the surface at a distance of one quarter of the thickness of the steel is less than or equal to 50 m. A difference between peak values of crystal grain size distributions at the near-surface level and the mid level is greater than or equal to 10 m and less than or equal to 30 m. The total index as determined in accordance with DIN 50602-1985, method K0 for evaluating non-metallic inclusions is preferably less than or equal to 10.

Provided is a large crankshaft comprising a pin fillet portion, wherein: an average initial compression stress in a surface layer region from a surface of the pin fillet portion to a depth of 500 m is 500 Mpa or more; an average Vickers hardness of the surface of the pin fillet portion is 600 or more; an arithmetic average roughness Ra of the surface of the pin fillet portion is 1.0 m or less; and an average prior austenite grain size of a metallographic structure is 100 m or less. The large crankshaft has composition comprising C: 0.2% by mass to 0.4% by mass, Si: 0% by mass to 1.0% by mass, Mn: 0.2% by mass to 2.0% by mass, Al: 0.005% by mass to 0.1% by mass, N: 0.001% by mass to 0.02% by mass, and a balance being Fe and inevitable impurities.

Provided is a large crankshaft comprising a pin fillet portion, wherein: an average initial compression stress in a surface layer region from a surface of the pin fillet portion to a depth of 500 m is 500 Mpa or more; an average Vickers hardness of the surface of the pin fillet portion is 600 or more; an arithmetic average roughness Ra of the surface of the pin fillet portion is 1.0 m or less; and an average prior austenite grain size of a metallographic structure is 100 m or less. The large crankshaft has composition comprising C: 0.2% by mass to 0.4% by mass, Si: 0% by mass to 1.0% by mass, Mn: 0.2% by mass to 2.0% by mass, Al: 0.005% by mass to 0.1% by mass, N: 0.001% by mass to 0.02% by mass, and a balance being Fe and inevitable impurities.

The invention relates to a method for producing martensitic steel that comprises a content of other metals such that the steel can be hardened by an intermetallic compound and carbide precipitation, with an Al content of between 0.4% and 3%, comprising the following steps: (a) heating the entirety of the steel above its austenizing temperature, (b) cooling said steel approximately to ambient temperature, (c) placing said steel in a cryogenic medium. The temperature T.sub.1 is substantially lower than the martensitic transformation temperature Mf, and the time t during which said steel is kept in said cryogenic medium at a temperature T.sub.1 from the moment when the hottest part of the steel reaches a temperature lower than the martensitic transformation temperature Mf is at least equal to a non-zero time t.sub.1, the temperature T.sub.1 (in C.) and the time t.sub.1 (in hours) being linked by the equation T.sub.1=(t.sub.1), the first derivative of the function relative to t, (t), being positive, and the second derivative of relative to t, (t), being negative.

The invention relates to a method for producing martensitic steel that comprises a content of other metals such that the steel can be hardened by an intermetallic compound and carbide precipitation, with an Al content of between 0.4% and 3%, comprising the following steps: (a) heating the entirety of the steel above its austenizing temperature, (b) cooling said steel approximately to ambient temperature, (c) placing said steel in a cryogenic medium. The temperature T.sub.1 is substantially lower than the martensitic transformation temperature Mf, and the time t during which said steel is kept in said cryogenic medium at a temperature T.sub.1 from the moment when the hottest part of the steel reaches a temperature lower than the martensitic transformation temperature Mf is at least equal to a non-zero time t.sub.1, the temperature T.sub.1 (in C.) and the time t.sub.1 (in hours) being linked by the equation T.sub.1=(t.sub.1), the first derivative of the function relative to t, (t), being positive, and the second derivative of relative to t, (t), being negative.

A method of laser hardening of a surface area of a workpiece, such as a surface of a journal of a crankshaft, including the steps of generating a relative movement between the surface of the workpiece and a laser source to allow a laser spot to subsequently be projected onto different portions of the surface area, and during the relative movement, repetitively scanning the laser beam so as to produce a two-dimensional equivalent effective laser spot on the surface area. The energy distribution of the effective laser spot is adapted so that it is different in a more heat sensitive subarea, such as in an area adjacent to an oil lubrication opening, than in a less heat sensitive subarea, so as to prevent overheating of the more heat sensitive subarea.

A method of laser hardening of a surface area of a workpiece, such as a surface of a journal of a crankshaft, including the steps of generating a relative movement between the surface of the workpiece and a laser source to allow a laser spot to subsequently be projected onto different portions of the surface area, and during the relative movement, repetitively scanning the laser beam so as to produce a two-dimensional equivalent effective laser spot on the surface area. The energy distribution of the effective laser spot is adapted so that it is different in a more heat sensitive subarea, such as in an area adjacent to an oil lubrication opening, than in a less heat sensitive subarea, so as to prevent overheating of the more heat sensitive subarea.

A method of crankshaft laser hardening includes grinding one or more surfaces of a green crankshaft to produce a green ground crankshaft and to define journal geometry thereon prior to hardening of the surfaces to avoid loss of compressive stresses associated with grinding a hardened crankshaft. The method also includes laser hardening the surfaces of the green ground crankshaft to induce compressive stresses.

A method of crankshaft laser hardening includes grinding one or more surfaces of a green crankshaft to produce a green ground crankshaft and to define journal geometry thereon prior to hardening of the surfaces to avoid loss of compressive stresses associated with grinding a hardened crankshaft. The method also includes laser hardening the surfaces of the green ground crankshaft to induce compressive stresses.