STEEL WIRE FOR NON-HEAT TREATED MACHINE PART AND NON-HEAT TREATED MACHINE PART
A steel wire for non-heat treated machine parts, the steel wire contains, based on % by mass: C: from 0.20 to 0.40%, Si: from 0.05 to 0.50%, Mn: from 0.50 to 2.00%, Al: from 0.005 to 0.050%, and the balance being Fe and impurities, in which the microstructure contains bainite of (35×[C %]+50)% or more, and when the diameter is defined as D, the average aspect ratio of a bainite grain at a depth of 50 μm in the L cross section is defined as AR, and the average grain size of a bainite grain at a depth of 50 μM in the C cross section is defined as GD, AR is 1.4 or more, (AR)/(the average aspect ratio of a bainite grain at a depth of 0.25D in the L cross section) is 1.1 or more, GD is (15/AR) μm or less, and (GD)/(the average grain size of a bainite grain at a depth of 0.25D in the C cross section) is less than 1.0.
1 - 8 . (canceled)
9 . A steel wire for non-heat treated machine parts, the steel wire having a chemical composition, based on % by mass, of:
C: from 0.20 to 0.40%,
Si: from 0.05 to 0.50%,
Mn: from 0.50 to 2.00%,
Al: from 0.005 to 0.050%,
P: from 0 to 0.030%,
S: from 0 to 0.030%,
N: from 0 to 0.0050%,
Cr: from 0 to 1.00%,
Ti: from 0 to 0.050%,
Nb: from 0 to 0.05%,
V: from 0 to 0.10%,
B: from 0 to 0.0050%,
O: from 0 to 0.0030%, and
a balance being Fe and impurities, wherein:
a microstructure is composed of bainite having an area ratio of (35×[C %]+50)% or more, % by mass of C being defined as [C %], and a balance being at least one of proeutectoid ferrite or pearlite,
in a case in which a cross section parallel to an axial direction of the steel wire and including a central axis is defined as an L cross section, a cross section perpendicular to the axial direction of the steel wire is defined as a C cross section, a diameter of the steel wire is defined as D, an average aspect ratio of a bainite grain measured at a depth of 50 μm from a steel wire surface in the L cross section is defined as AR, and an average grain size of a bainite grain measured at a depth of 50 μm from the steel wire surface in the C cross section is defined as GD, AR is 1.4 or more, (AR)/(an average aspect ratio of a bainite grain measured at a depth of 0.25D from the steel wire surface in the L cross section) is 1.1 or more, GD is (15/AR) μm or less, and (GD)/(an average grain size of a bainite grain measured at a depth of 0.25D from the steel wire surface in the C cross section) is less than 1.0, and
a tensile strength is from 900 to 1,500 MPa.
10 . The steel wire for non-heat treated machine parts according to claim 9 , containing, based on % by mass, one, or two or more of:
Cr: more than 0 and 1.00% or less,
Ti: more than 0 and 0.050% or less,
Nb: more than 0 and 0.05% or less,
V: more than 0 and 0.10% or less, or
B: more than 0 and 0.0050% or less.
11 . The steel wire for non-heat treated machine parts according to claim 9 wherein D is from 3 to 30 mm.
12 . The steel wire for non-heat treated machine parts according to claim 10 , wherein D is from 3 to 30 mm.
13 . The steel wire for non-heat treated machine parts according to claim 9 , wherein a critical compression ratio is 75% or more.
14 . The steel wire for non-heat treated machine parts according to claim 10 , wherein a critical compression ratio is 75% or more.
15 . The steel wire for non-heat treated machine parts according to claim 11 , wherein a critical compression ratio is 75% or more.
16 . The steel wire for non-heat treated machine parts according to claim 12 , wherein a critical compression ratio is 75% or more.
17 . A non-heat treated machine part, comprising a cylindrical shaft, the non-heat treated machine part having a chemical composition, based on % by mass, of:
C: from 0.20 to 0.40%,
Si: from 0.05 to 0.50%,
Mn: from 0.50 to 2.00%,
Al: from 0.005 to 0.050%,
P: from 0 to 0.030%,
S: from 0 to 0.030%,
N: from 0 to 0.0050%,
Cr: from 0 to 1.00%,
Ti: from 0 to 0.050%,
Nb: from 0 to 0.05%,
V: from 0 to 0.10%,
B: from 0 to 0.0050%,
O: from 0 to 0.0030%, and
a balance being Fe and impurities, wherein:
a microstructure is composed of bainite having an area ratio of (35×[C %]+50)% or more, % by mass of C being defined as [C %], and a balance being at least one of proeutectoid ferrite or pearlite,
in a case in which a cross section parallel to an axial direction of the cylindrical shaft and including a central axis is defined as an L cross section, a cross section perpendicular to the axial direction of the cylindrical shaft is defined as a C cross section, a diameter of the cylindrical shaft is defined as D, an average aspect ratio of a bainite grain measured at a depth of 50 μm from a cylindrical shaft surface in the L cross section is defined as AR, and an average grain size of a bainite grain measured at a depth of 50 um from the cylindrical shaft surface in the C cross section is defined as GD, AR is 1.4 or more, (AR)/(an average aspect ratio of a bainite grain measured at a depth of 0.25D from the cylindrical shaft surface in the L cross section) is 1.1 or more, GD is (15/AR) um or less, and (GD/an average grain size of a bainite grain measured at a depth of 0.25D from the cylindrical shaft surface in the C cross section) is less than 1.0, and a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
18 . The non-heat treated machine part according to claim 17 containing, based on % by mass, one, or two or more of:
Cr: more than 0 and 1.00% or less,
Ti: more than 0 and 0.050% or less,
Nb: more than 0 and 0.05% or less,
V: more than 0 and 0.10% or less, or
B: more than 0 and 0.0050% or less.
19 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 9 , comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
20 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 10 , comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
21 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 11 , comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
22 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 12 , comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
23 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 13 comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
24 . A non-heat treated machine part, which is a cold-worked product of the steel wire for non-heat treated machine parts according to claim 14 comprising a cylindrical shaft, wherein a tensile strength of the cylindrical shaft is from 1,100 to 1,500 MPa.
25 . The non-heat treated machine part according to claim 17 , which is a non-heat treated bolt.
26 . The non-heat treated machine part according to claim 19 , which is a non-heat treated bolt.
27 . The non-heat treated machine part according to claim 21 , which is a non-heat treated bolt.
28 . The non-heat treated machine part according to claim 23 , which is a non-heat treated bolt.