High-strength steel material and production method therefor
A high-strength steel material that has a chemical composition containing, by mass %, C: 0.30 to 1.0%, Si: 0.05 to 1.0%, Mn: 16.0 to 35.0%, P: 0.030% or less, S: 0.030% or less, Al: 0.003 to 0.06%, N: 0.1% or less, V: 0 to 3.0%, Ti: 0 to 1.5%, Nb: 0 to 1.5%, Cr: 0 to 5.0%, Mo: 0 to 3.0%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0.02%, Zr: 0 to 0.5%, Ta: 0 to 0.5%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, and the balance: Fe and impurities, and that satisfies [V+Ti+Nb>2.0], in which: a number density of carbides/carbo-nitrides having a circle-equivalent diameter of 5 to 30 nm precipitating in the steel is 50 to 700/μm 2 , and a number density of carbides/carbo-nitrides having a circle-equivalent diameter of more than 100 nm precipitating in the steel is less than 10/μm 2 ; a yield stress is 758 MPa or more; and a K ISSC value obtained in a DCB test is 33.7 MPa·m 0.5 or more.
1. A high-strength steel material having a chemical composition consisting of, by mass %,
C: 0.30 or more and less than 0.60%,
Si: 0.05 to 0.33%,
Mn: 16.0 to 35.0%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.003 to 0.06%,
N: 0.1% or less,
V: 0 to 3.0%,
Ti: 0 to 1.5%,
Nb: 0 to 1.5%,
Cr: 0 to 5.0%,
Mo: 0 to 3.0%,
Cu: 0 to 1.0%,
Ni: 0 to 1.0%,
B: 0 to 0.02%,
Zr: 0 to 0.5%,
Ta: 0 to 0.5%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%, and
the balance: Fe and impurities,
and satisfying formula (i) hereunder,
wherein:
a number density of carbides and/or carbo-nitrides having a circle-equivalent diameter of 5 nm to 30 nm precipitating in the steel is 50 to 700/μm 2 , and a number density of carbides and/or carbo-nitrides having a circle-equivalent diameter of more than 100 nm precipitating in the steel is less than 10/μm 2 ,
a yield stress is 758 MPa or more, and
a K ISSC value obtained in a DCB test is 33.7 MPa·m 0.5 or more;
V+Ti+Nb>2.0 (i)
where, V, Ti and Nb in formula (i) above represent a content (mass %) of the respective elements contained in the steel, with the value thereof being set to zero in a case where the corresponding element is not contained.
2. The high-strength steel material according to claim 1 , wherein the chemical composition contains, by mass %, one or more elements selected from the group consisting of
V: 0.1 to 3.0%,
Ti: 0.003 to 1.5%,
Nb: 0.003 to 1.5%,
Cr: 0.1 to 5.0%,
Mo: 0.5 to 3.0%,
Cu: 0.1 to 1.0%,
Ni: 0.1 to 1.0%,
B: 0.0001 to 0.02%,
Zr: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ca: 0.0003 to 0.005%, and
Mg: 0.0003 to 0.005%.
3. A method for producing a high-strength steel material according to claim 1 ,
the method comprising performing steps of (a) to (f) described hereunder in sequence on a steel material having a chemical composition described in claim 1 :
(a) a hot working step of heating to a temperature in a range of 900° C. to 1200° C., and thereafter finishing into a predetermined shape;
(b) a cooling step of cooling to a temperature of 100° C. or less;
(c) a solid solution heat treatment step of heating to a temperature in a range of 800° C. to 1200° C. and holding at the temperature for not less than 10 minutes, and thereafter quenching;
(d) a cold working step of performing working with a reduction of area in a range of 5% to 20%;
(e) an aging treatment steps of holding at a temperature of 600° C. to 750° C. for 0.5 hours to 2 hours; and
(f) a cooling step of cooling to a temperature of 100° C. or less.
4. A method for producing a high-strength steel material according to claim 1 ,
the method comprising performing steps of (g) to (k) described hereunder in sequence on a steel material having a chemical composition described in claim 1 :
(g) a hot working step of heating to a temperature in a range of 900° C. to 1200° C., and thereafter finishing into a predetermined shape at a temperature of 800° C. or more;
(h) a solid solution heat treatment step of quenching immediately following the step of (g);
(i) a cold working step of performing working with a reduction of area in a range of 5% to 20%;
(j) an aging treatment steps of holding at a temperature of 600° C. to 750° C. for 0.5 hours to 2 hours; and
(k) a cooling step of cooling to a temperature of 100° C. or less.
5. A method for producing a high-strength steel material according to claim 2 ,
the method comprising performing steps of (a) to (f) described hereunder in sequence on a steel material having a chemical composition described in claim 2 :
(a) a hot working step of heating to a temperature in a range of 900° C. to 1200° C., and thereafter finishing into a predetermined shape;
(b) a cooling step of cooling to a temperature of 100° C. or less;
(c) a solid solution heat treatment step of heating to a temperature in a range of 800° C. to 1200° C. and holding at the temperature for not less than 10 minutes, and thereafter quenching;
(d) a cold working step of performing working with a reduction of area in a range of 5% to 20%;
(e) an aging treatment steps of holding at a temperature of 600° C. to 750° C. for 0.5 hours to 2 hours; and
(f) a cooling step of cooling to a temperature of 100° C. or less.
6. A method for producing a high-strength steel material according to claim 2 ,
the method comprising performing steps of (g) to (k) described hereunder in sequence on a steel material having a chemical composition described in claim 2 :
(g) a hot working step of heating to a temperature in a range of 900° C. to 1200° C., and thereafter finishing into a predetermined shape at a temperature of 800° C. or more;
(h) a solid solution heat treatment step of quenching immediately following the step of (g);
(i) a cold working step of performing working with a reduction of area in a range of 5% to 20%;
(j) an aging treatment steps of holding at a temperature of 600° C. to 750° C. for 0.5 hours to 2 hours; and
(k) a cooling step of cooling to a temperature of 100° C. or less.