Steel for suspension spring and method of manufacturing same

1. Steel for a suspension spring comprising, as a chemical composition, by % by mass:

C: more than 0.40% and 0.65% or less;

Si: 1.00% to 3.50%;

Mn: 2.10% to 3.00%;

Cr: 0.01% to 2.00%;

V: 0.02% to 0.50%;

Al: 0% to 0.060%;

Mo: 0% or more and less than 0.50%;

Cu: 0% to 1.00%;

Ni: 0% to 1.50%;

Nb: 0% to 0.015%;

Ti: 0% to 0.200%;

B: 0% to 0.0050%;

P: limited to 0.020% or less;

S: limited to 0.020% or less:

N: limited to 0.0100% or less; and

a remainder of Fe and impurities,

wherein Kf defined by the following Equation (1) is 280 or more,

wherein a structure contains a tempered martensite of which an area ratio is 90% or more,

wherein a Fe-based carbide precipitated in the tempered martensite is a cementite,

wherein an average of an area ratios of prior austenite grains which stretch in a longitudinal direction of the steel and of which an aspect ratio exceeds 3.0, is 80% or more in a case of depths of 0.1 mm, 0.2 mm, and 0.3 mm in a sheet thickness direction from a surface of the steel, and

wherein a tensile strength is 1900 MPa or more,

Kf= 10 (3.288×C (%)−0.168×Si (%)+1.068×Mn (%)+0.3×Ni (%)+1.266×Cr (%)+0.626×Cu (%)+2.086×Mo (%)−1.931)   (1)

here, C (%), Si (%), Mn (%), Ni (%), Cr (%), Cu (%), and Mo (%) in the equation (1) indicate the amounts of C, Si, Mn, Ni, Cr, Cu, and Mo by % by mass, and in a case where the elements are not contained, the value is 0.

2. The steel for a suspension spring according to claim 1 , comprising, as a chemical composition, by % by mass:

one of or two or more of Mo: 0.02% or more and less than 0.50%;

Cu: 0.02% to 1.0%;

Ni: 0.02% to 1.5%; and

Nb: 0.002% to 0.015%.

3. The steel for a suspension spring according to claim 1 or 2 , comprising, as a chemical composition, by % by mass:

one of or both of Ti: 0.010% to 0.200%; and

B: 0.0005% to 0.0050%,

in case the steel comprises Ti and B, the amount of Ti and the amount of N satisfy Ti≥3.5×N.

4. A method of manufacturing the steel for a suspension spring according to claim 1 , the method comprising:

heating a steel piece containing the chemical composition within a temperature range of 950° C. or more and less than 1150° C.;

a wire rod rolling including a finish rolling in which a reduction of an area is 30% or more and a finish rolling temperature is 750° C. to 850° C., with respect to the heated steel piece to obtain a wire rod;

winding with respect to the wire rod to make the wire rod into a coil;

cooling the coil so as to start cooling in 5 to 30 seconds after the performing of the winding and to set an average cooling rate when the temperature is 800 to 300° C. to be 5 to 50° C./s to cool the coil until reaching 300° C. or less; and

tempering of retention for 10 to 1800 seconds at 410° C. to 550° C. after the cooling.

5. A method of manufacturing the steel for a suspension spring according to claim 2 , the method comprising:

heating a steel piece containing the chemical composition within a temperature range of 950° C. or more and less than 1150° C.;

a wire rod rolling including a finish rolling in which a reduction of an area is 30% or more and a finish rolling temperature is 750° C. to 850° C., with respect to the heated steel piece to obtain a wire rod;

winding with respect to the wire rod to make the wire rod into a coil;

cooling the coil so as to start cooling in 5 to 30 seconds after the performing of the winding and to set an average cooling rate when the temperature is 800 to 300° C. to be 5 to 50° C./s to cool the coil until reaching 300° C. or less; and

tempering of retention for 10 to 1800 seconds at 410° C. to 550° C. after the cooling.

6. A method of manufacturing the steel for a suspension spring according to claim 3 , the method comprising:

heating a steel piece containing the chemical composition within a temperature range of 950° C. or more and less than 1150° C.;

a wire rod rolling including a finish rolling in which a reduction of an area is 30% or more and a finish rolling temperature is 750° C. to 850° C., with respect to the heated steel piece to obtain a wire rod;

winding with respect to the wire rod to make the wire rod into a coil;

cooling the coil so as to start cooling in 5 to 30 seconds after the performing of the winding and to set an average cooling rate when the temperature is 800 to 300° C. to be 5 to 50° C./s to cool the coil until reaching 300° C. or less; and

tempering of retention for 10 to 1800 seconds at 410° C. to 550° C. after the cooling.