Steel for mechanical structure for cold working, and method for manufacturing same

1. A steel for mechanical structure for cold working, comprising:

C: 0.3 to 0.6% by mass,

Si: 0.005 to 0.5% by mass,

Mn: 0.2 to 1.5% by mass,

P: a positive amount of 0.03% or less by mass,

S: a positive amount of 0.03% or less by mass,

Al: 0.01 to 0.1% by mass,

N: a positive amount of 0.015% or less by mass, and

iron,

wherein

the steel has a metallic microstructure comprising pearlite and pro-eutectoid ferrite, in a total area proportion of 90% or more based on area of the entire metallic microstructure;

area proportion A of the pro-eutectoid ferrite satisfies A>Ae, where Ae is calculated by equation (1):

Ae =(0.8− Ceq 1 )×96.75  (1)

wherein Ceq 1 =[C]+0.1×[Si]+0.06×[Mn], wherein [C], [Si] and [Mn] represent the respective contents of C, Si and Mn by mass percentage; and

bcc-Fe crystal grains each surrounded by a high angle grain boundary through which two crystal grains are adjacent to each other at a misorientation larger than 15° have an average circular equivalent diameter of 15 to 35 μm, and an average circular equivalent diameter of the largest and the second largest bcc-Fe crystal grains is 50 μm or less, where the bcc-Fe crystal grains contain crystal grains of the pro-eutectoid ferrite and ferrite contained in the pearlite.

2. The steel according to claim 1 , further comprising, one or more elements selected from the group consisting of:

Cr: a positive amount of 0.5% or less by mass,

Cu: a positive amount of 0.25% or less by mass,

Ni: a positive amount of 0.25% or less by mass,

Mo: a positive amount of 0.25% or less by mass, and

B: a positive amount of 0.01% or less by mass.

3. The steel according to claim 2 , further comprising, one or more elements selected from the group consisting of:

Ti: a positive amount of 0.2% or less by mass,

Nb: a positive amount of 0.2% or less by mass, and

V: a positive amount of 0.5% or less by mass.

4. The steel according to claim 1 , further comprising, one or more elements selected from the group consisting of:

Ti: a positive amount of 0.2% or less by mass,

Nb: a positive amount of 0.2% or less by mass, and

V: a positive amount of 0.5% or less by mass.

5. The steel according to claim 1 , wherein the total area proportion of pearlite and pro-eutectoid ferrite based on area of the entire metallic microstructure is 95% or more.

6. The steel according to claim 1 , wherein the total area proportion of pearlite and pro-eutectoid ferrite based on area of the entire metallic microstructure is 97% or more.

7. The steel according to claim 1 , wherein the total area proportion of pearlite and pro-eutectoid ferrite based on area of the entire metallic microstructure is 100%.

8. The steel according to claim 1 , wherein the average circular equivalent diameter of the bcc-Fe crystal grains each surrounded by a high angle grain boundary through which two crystal grains are adjacent to each other at a misorientation larger than 15° is 18 to 32 μm.

9. The steel according to claim 1 , wherein the average circular equivalent diameter of the bcc-Fe crystal grains each surrounded by a high angle grain boundary through which two crystal grains are adjacent to each other at a misorientation larger than 15° is 20 to 30 μm.

10. The steel according to claim 1 , wherein the average circular equivalent diameter of the largest and the second largest bcc-Fe crystal grains is 45 μm or less.

11. The steel according to claim 1 , wherein the average circular equivalent diameter of the largest and the second largest bcc-Fe crystal grains is 40 μm or less.

12. A steel for mechanical structure for cold working, comprising:

C: 0.3 to 0.6% by mass,

Si: 0.005 to 0.5% by mass,

Mn: 0.2 to 1.5% by mass,

P: a positive amount of 0.03% or less by mass,

S: a positive amount of 0.03% or less by mass,

Al: 0.01 to 0.1% by mass,

N: a positive amount of 0.015% or less by mass, and

iron,

wherein the steel has a metallic microstructure in which an average circular equivalent diameter of bcc-Fe crystal grains is from 15 to 35 μm, cementite inside the bcc-Fe crystal grains has an aspect ratio of 2.5 or less, and a K value calculated by equation (2) is 1.3×10 −2 or less:

K value=( N×L )/ E   (2)

wherein E is the average circular equivalent diameter of the bcc-Fe crystal grains by μm; N is number density of the cementite inside the bcc-Fe crystal grains per μm 2 ; and L is the aspect ratio of the cementite inside the bcc-Fe crystal grains.

13. A method for manufacturing the steel according to claim 1 the method comprising:

subjecting a working steel to finish rolling at a temperature higher than 950° C. and 1100° C. or lower to obtain a resultant steel,

cooling the resultant steel to a temperature of 700° C. or higher and lower than 800° C. at an average cooling rate of 10° C./second or more, and

subsequently cooling the resultant steel at an average cooling rate of 0.2° C./second or less for 100 seconds or more.

14. A method for manufacturing the steel according to claim 1 , the method comprising:

subjecting a working steel to finish rolling at a temperature of 1050° C. or higher and 1200° C. or lower to obtain a resultant steel,

cooling the resultant steel to a temperature of 700° C. or higher and lower than 800° C. at an average cooling rate of 10° C./second or more,

subsequently cooling the resultant steel at an average cooling rate of 0.2° C./second or less for 100 seconds or more,

subsequently cooling the resultant steel to a temperature ranging from 580 to 660° C. at an average cooling rate of 10° C./second or more, and

subsequently cooling the resultant steel at an average cooling rate of 1° C./second or less for 20 seconds or more or keeping the resultant steel at the temperature of from 580 to 660° C.