Steel for mechanical structure for cold working, and method for manufacturing same
The present invention is a steel for a mechanical structure for cold working, the steel characterized in containing C, Si, Mn, P, S, Al, N, and Cr, the remainder being iron and inevitable impurities; the metal composition having pearlite and pro-eutectoid ferrite; the combined area of the pearlite and pro-eutectoid ferrite being 90% or more of the total composition; the area percentage A of the pro-eutectoid ferrite having the relationship A>Ae, where Ae=(0.8−Ceq)×96.75 (Ceq=[C]+0.1×[Si]+0.06×[Mn]−0.11×[Cr], and “(element names)” indicates the element content (percent in mass); and the mean grain size of the pro-eutectoid ferrite and the ferrite in the pearlite being 15 to 25 μm.
1. A steel, having a chemical composition comprising:
by mass %,
iron;
C: 0.2-0.6%;
Si: 0.01-0.5%;
Mn: 0.2-1.5%;
P: a positive amount of 0.03% or less;
S: 0.001-0.05%;
Al: 0.01-0.1%;
N: a positive amount of 0.015% or less; and
Cr: more than 0.5% and 2.0% or less, and
a metal microstructure comprising: pearlite and pro-eutectoid ferrite with a combined area percentage of the pearlite and the pro-eutectoid ferrite being 90% or more,
wherein
an average grain size of the pro-eutectoid ferrite and ferrite in the pearlite is 15-25 μm, and
a ratio of an area percentage A of the pro-eutectoid ferrite to Ae satisfies A/Ae>1, where Ae is calculated by expression (1):
Ae =(0.8- Ceq )×96.75 (1)
where Ceq=[C]+0.1×[Si]+0.06×[Mn]+0.11×[Cr], and [C], [Si], [Mn], and [Cr] represent mass % of C, Si, Mn, Cr in the steel, respectively.
2. The steel according to claim 1 , further comprising one or more elements selected from the group consisting of:
Mo: a positive amount of 1% or less;
Ni: a positive amount of 3% or less;
Cu: a positive amount of 0.25% or less;
B: a positive amount of 0.010% or less;
Ti: a positive amount of 0.2% or less;
Nb: a positive amount of 0.2% or less; and
V: a positive amount of 0.5% or less.
3. The steel according to claim 1 , wherein A≧Ae+0.5.
4. The steel according to claim 1 , wherein A≧Ae+1.5.
5. The steel according to claim 1 , wherein A≦Ae+5.
6. The steel according to claim 1 , wherein the steel further comprises Cu in a positive amount of 0.25% or less.
7. The steel according to claim 1 , wherein the steel further comprises Nb in a positive amount of 0.2% or less.
8. The steel according to claim 1 , wherein A≧Ae+1.0.
9. A method for manufacturing the steel according to claim 1 , the method comprising
(i) finish rolling the steel at 850-1,100° C.;
(ii) cooling thereafter to 720-780° C. with an average cooling rate of 10° C./s or more;
(iii) cooling thereafter to 680° C. or above with an average cooling rate of 1° C./s or less; and
(iv) further cooling to 640° C. or below with an average cooling rate of 0.5° C./s or less.
10. The method according to claim 9 , wherein
the steel is cooled at an average cooling rate of 20° C./s or more to 740 to 760° C. in the cooling (ii),
the steel is cooled at an average cooling rate of 0.6° C./s or less to 690 to 700° C. in the cooling (iii), and
the steel is cooled at an average cooling rate of 0.3° C./s or less to 600 to 620° C. in the cooling (iv).
11. The method according to claim 9 , further comprising
(v) cooling the steel to room temperature, and
(vi) drawing and/or spheroidizing annealing, wherein the spheroidizing annealing is done after the finish rolling and the drawing.
12. The method according to claim 9 , wherein the steel further comprises one or more elements selected from the group consisting of:
Mo: a positive amount of 1% or less;
Ni: a positive amount of 3% or less;
Cu: a positive amount of 0.25% or less;
B: a positive amount of 0.010% or less;
Ti: a positive amount of 0.2% or less;
Nb: a positive amount of 0.2% or less; and
V: a positive amount of 0.5% or less.