Steel near-net-shape material and method for producing same

Abstract

A steel near-net-shape material having high fatigue strength and high tensile strength is provided. The steel near-net-shape material has a chemical composition containing, in mass %, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%, and N: 0.003 to 0.030%. The steel near-net-shape material is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and satisfies Formula (1). A diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. [ V in precipitates]/[ V ]≥0.30  (1)

1 . A steel near-net-shape material, comprising:
 a chemical composition consisting of, in mass %,   C: 0.03 to 0.25%,   Si: 0.02 to 0.50%,   Mn: more than 0.70 to 2.50%,   P: 0.035% or less,   S: 0.050% or less,   Al: 0.005 to 0.050%,   V: more than 0.10 to 0.40%,   N: 0.003 to 0.030%,   Cr: 0 to 0.70%,   Nb: 0 to 0.100%,   B: 0 to 0.0100%,   Cu: 0 to 0.30%,   Ni: 0 to 0.30%,   Ca: 0 to 0.0050%,   Bi: 0 to 0.100%,   Pb: 0 to 0.090%,   Mo: 0 to 0.05%,   Ti: 0 to 0.005%,   Zr 0 to 0.010%,   Se: 0 to 0.10%,   Te: 0 to 0.10%,   rare earth metal: 0 to 0.010%,   Sb: 0 to 0.10%,   Mg: 0 to 0.0050%,   W: 0 to 0.050%, and   the balance: Fe and impurities,   wherein:   a microstructure of the steel near-net-shape material is composed of:   polygonal ferrite having an area fraction of 20 to 90%, and   a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;   when a content of V in the chemical composition is defined as [V](mass %), and a total content of V in V precipitates in the steel near-net-shape material is defined as [V in precipitates] (mass %), the steel near-net-shape material satisfies Formula (1); and   a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more:
   [ V  in precipitates]/[ V ]≥0.30  (1).
 
   
     
     
         2 . The steel near-net-shape material according to  claim 1 , wherein:
 the chemical composition contains, in lieu of a part of Fe, one or more elements selected from the group consisting of:   Cr: 0.01 to 0.70%,   Nb: 0.001 to 0.100%,   B: 0.0001 to 0.0100%,   Cu: 0.01 to 0.30%,   Ni: 0.01 to 0.30%,   Ca: 0.0001 to 0.0050%,   Bi: 0.001 to 0.100%,   Pb: 0.001 to 0.090%,   Mo: 0.01 to 0.05%,   Ti: 0.001 to 0.005%,   Zr 0.002 to 0.010%,   Se: 0.01 to 0.10%,   Te: 0.01 to 0.10%,   rare earth metal: 0.001 to 0.010%,   Sb: 0.01 to 0.10%,   Mg: 0.0005 to 0.0050%, and   W: 0.001 to 0.050%.   
     
     
         3 . A method for producing the steel near-net-shape material according to  claim 1 , comprising:
 a steel material preparation process of preparing a steel material having a chemical composition consisting of, in mass %,   C: 0.03 to 0.25%,   Si: 0.02 to 0.50%,   Mn: more than 0.70 to 2.50%,   P: 0.035% or less,   S: 0.050% or less,   Al: 0.005 to 0.050%,   V: more than 0.10 to 0.40%,   N: 0.003 to 0.030%,   Cr: 0 to 0.70%,   Nb: 0 to 0.100%,   B: 0 to 0.0100%,   Cu: 0 to 0.30%,   Ni: 0 to 0.30%,   Ca: 0 to 0.0050%,   Bi: 0 to 0.100%,   Pb: 0 to 0.090%,   Mo: 0 to 0.05%,   Ti: 0 to 0.005%,   Zr 0 to 0.010%,   Se: 0 to 0.10%,   Te: 0 to 0.10%,   rare earth metal: 0 to 0.010%,   Sb: 0 to 0.10%,   Mg: 0 to 0.0050%,   W: 0 to 0.050%, and   the balance: Fe and impurities,   wherein:   a microstructure of the steel material is composed of:   polygonal ferrite having an area fraction of 20 to 90%, and   a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%, and   when a content of V in the chemical composition is defined as [V](mass %), and a total content of V in V precipitates in the steel material is defined as [V in precipitates] (mass %), [V in precipitates]/[V] is 0.05 to less than 0.30;   a cold working process of subjecting the steel material to cold working;   an age hardening treatment process of subjecting the steel material after cold working to an age hardening treatment in which a treatment temperature is set in a range of 500° C. to an A c1  point, and a holding time at the treatment temperature is set in a range of 15 to 150 minutes;   wherein:   the cold working process includes:   a first-direction cold working process of subjecting the steel material to, from a first direction, cold working in which a working strain amount is 0.05 or more, and   a second-direction cold working process of subjecting the steel material to, from a second direction which is different from the first direction, cold working in which a working strain amount is 0.05 or more; and   a total of a working strain amount generated in the steel material in the first-direction cold working process and a working strain amount generated in the steel material in the second-direction cold working process is 0.20 or more.