Heat- treated steel and heat treatment method for steel

Abstract

To provide a heat-treated steel excellent in both tensile strength and toughness.A heat-treated steel contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, wherein an average crystal grain size at a grain boundary setting angle of 15° is 10×C+7 (μm) or less (wherein, C represents a carbon content (%)).

1 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 an average crystal grain size at a grain boundary setting angle of 15° is 10×C+7 (μm) or less (wherein C represents a carbon content (%)).   
     
     
         2 . The heat-treated steel according to  claim 1 , characterized in that
 (average crystal grain size at a surface portion at a grain boundary setting angle of 15°)/(average crystal grain size at a center portion at a grain boundary setting angle of) 15° is 0.70 or more and 1.10 or less.   
     
     
         3 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 (the number of crystal grains at a grain boundary setting angle of 5°)/(the number of crystal grains at a grain boundary setting angle of 15°) is 5.4×C−0.95 or less (wherein C represents a carbon content (%)).   
     
     
         4 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 (the number of crystal grains at a grain boundary setting angle of 2°)/(the number of crystal grains at a grain boundary setting angle of 15°) is 9.8×C−1.9 or less (wherein C represents a carbon content (%)).   
     
     
         5 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 a GOS value at a grain boundary setting angle of 15° is 11×(C−0.42)+5.3 or less (wherein C represents a carbon content (%)).   
     
     
         6 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 a cumulative frequency of a GOS value in a range of 0° to 10° at a grain boundary setting angle of 15° is −0.1C 3 −1.3C 2 +1.1C+0.7 or more (wherein C represents a carbon content (%)).   
     
     
         7 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 when a structure is observed with a backscattered electron (BSE) image, in a layered structure of ferrite and an iron carbide, an area fraction of the branched, bent, or curved iron carbide is 9% or more in a field of view.   
     
     
         8 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 when a structure is observed with a scanning electron microscope (SEM), in a layered structure of ferrite and an iron carbide, a spherical protrusion is observed on a surface of the iron carbide.   
     
     
         9 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 when a structure is observed with a scanning electron microscope (SEM), in a layered structure of ferrite and an iron carbide, a surface of the iron carbide has unevenness, and a rod-shaped or plate-shaped relatively isotropic iron carbide, which is three-dimensionally comb-shaped or mesh-shaped, is produced.   
     
     
         10 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 when a tensile strength is TS (MPa), a reduction of area is −0.000064TS 2 +0.09TS+46 (%) or more.   
     
     
         11 . A heat-treated steel which contains 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities, characterized in that:
 a difference in proof stress obtained by subtracting a 0.2% proof stress obtained in a S-S curve from a 0.4% proof stress obtained in the S-S curve is 45×C−3 (MPa) or less (wherein C represents a carbon content (%)).   
     
     
         12 . A heat treatment method for a steel, characterized by comprising the steps of:
 preparing s steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities;   causing the steel itself to generate heat to directly heat the steel; and   passing the heated steel through a bath in which a cooling medium capable of isothermal transformation is stored to cool the steel, wherein   a temperature gradient in a final stage of heating of the heating step is the largest, and the heated steel is allowed to enter the cooling medium immediately after the steel reaches a predetermined maximum heating temperature in the final stage of heating of the heating step to start the cooling without maintaining the predetermined maximum heating temperature.   
     
     
         13 . A heat treatment method for a steel, comprising:
 heating a steel from room temperature to 820° C. or more within a few seconds, and cooling the heated steel to 620° C. or less within a few seconds without maintaining a maximum heating temperature, the steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities.