STEEL WIRE ROD, STEEL WIRE, AND PART
Steel wire rod etc. enabling elimination the heat treatment processes of spheroidizing annealing and of quenching and tempering are provided. A predetermined chemical composition is provided, 90% or more of the microstructure by area ratio is bainite, the mean bainite block size of the surface layer measured at the cross-section is 15 μm or less, the ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part is less than 1.0, and the mean particle size of cementite dispersed in bainite is 0.1 μm or less.
1 . Steel wire rod comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which steel wire rod,
90% or more of the microstructure by area ratio is bainite, a mean bainite block size of the surface layer measured at the cross section is 15 μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
2 . The steel wire rod according to claim 1 , wherein said steel wire rod further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
3 . Drawn steel wire comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which drawn steel wire,
90% or more of the microstructure by area ratio is bainite, at the surface layer of the drawn steel wire, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross section is (15/R)μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
4 . The drawn steel wire according to claim 3 , wherein said drawn steel wire further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
5 . The drawn steel wire according to claim 3 , wherein a critical upsetting rate is 80% or more.
6 . A part comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which part,
90% or more of the microstructure by area ratio is bainite, at a surface layer of the part, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross-section is (15/R) μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
7 . The part according to claim 6 , wherein said part further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
8 . The drawn steel wire according to claim 4 , wherein a critical upsetting rate is 80% or more.