Spring steel for suspension and method for producing same

Abstract

Provided is spring steel for suspension suppressing or not requiring addition of expensive alloy elements, having a large tensile strength, and excellent in cold formability and delay fracture resistance, wherein at a cross-section parallel to a rolling direction, 90% or more of the metal microstructures by area fraction is tempered martensite, and at a cross-section parallel to the rolling direction, in a range of 10% of diameter or thickness from the surface, a ratio of a length in a long axis direction of prior austenite grains and a length in a direction perpendicular to the long axis direction of the prior austenite grains is 1.5 or more and a ratio of <011> fraction/<111> fraction of martensite texture as seen from the rolling direction is 3.0 or more.

1 . Spring steel for suspension containing, by mass %:
 C: 0.40 to 0.70%;   Si: 0.80 to 2.20%;   Mn: 0.05 to 1.50%;   Cr: 0.05 to 1.00%;   P: limited to 0.020% or less;   S: limited to 0.020% or less, and
 a balance of Fe and unavoidable impurities, wherein 
 at a cross-section parallel to a rolling direction, 90% or more of the metal microstructures by area fraction is tempered martensite, and 
 at a cross-section parallel to the rolling direction, in a range of 10% of diameter or thickness from the surface, a ratio of a length in a long axis direction of prior austenite grains and a length in a direction perpendicular to the long axis direction of the prior austenite grains is 1.5 or more and a ratio of <011> fraction/<111> fraction of martensite texture as observed from the rolling direction is 3.0 or more. 
   
     
     
         2 . The spring steel for suspension according to  claim 1 , further containing, by mass %, one or more of:
 Mo: 0.50% or less;   V: 0.50% or less;   Ni: 1.00% or less;   Cu: 0.50% or less; and   B: 0.0050% or less.   
     
     
         3 . The spring steel for suspension according to  claim 1 , further containing, by mass %, one or more of:
 Al: 0.50% or less;   Ti: 0.20% or less; and   Nb: 0.10% or less.   
     
     
         4 . The spring steel for suspension according to  claim 1 , wherein a tensile strength is 1800 MPa or more, a reduction of area in tensile test is 40% or more, and a critical hydrogen content measured by applying a test load of 30% of the tensile strength to a test piece provided with a 60° annular notch down to a depth of 10% of a rod diameter is 0.30 ppm or more. 
     
     
         5 . A method for producing spring steel for suspension according to  claim 1 , comprising:
 hot rolling a wire rod having steel components so as to give a 15% or more to less than 30% reduction of area in rolling in a less than 1150° C. to 950° C. or more temperature region, then give a 20% or more reduction of area in rolling in a less than 950° C. to 750° C. or more temperature region and a 40% or more cumulative reduction of area in rolling with the reduction of area in rolling in the less than 1150° C. to 950° C. or more temperature region,   quenching the hot rolled steel wire to transform 90% or more of the metal microstructure by area fraction at a cross-section parallel to the rolling direction to martensite, and   tempering the quenched steel wire,   wherein a time from an end of the hot rolling to a start of the quenching is within 3 seconds; and   wherein said steel component contains, by mass %:   C: 0.40 to 0.70%;   Si: 0.80 to 2.20%;   Mn: 0.05 to 1.50%;   Cr: 0.05 to 1.00%;   P: limited to 0.020% or less;   S: limited to 0.020% or less, and
 a balance of Fe and unavoidable impurities. 
   
     
     
         6 . The spring steel for suspension according to  claim 2 , further containing, by mass %, one or more of:
 Al: 0.50% or less;   Ti: 0.20% or less; and   Nb: 0.10% or less.   
     
     
         7 . The spring steel for suspension according to  claim 2 , wherein a tensile strength is 1800 MPa or more, a reduction of area in tensile test is 40% or more, and a critical hydrogen content measured by applying a test load of 30% of the tensile strength to a test piece provided with a 60° annular notch down to a depth of 10% of a rod diameter is 0.30 ppm or more. 
     
     
         8 . The spring steel for suspension according to  claim 3 , wherein a tensile strength is 1800 MPa or more, a reduction of area in tensile test is 40% or more, and a critical hydrogen content measured by applying a test load of 30% of the tensile strength to a test piece provided with a 60° annular notch down to a depth of 10% of a rod diameter is 0.30 ppm or more. 
     
     
         9 . A method for producing spring steel for suspension according to  claim 2 , comprising:
 hot rolling a wire rod having steel components so as to give a 15% or more to less than 30% reduction of area in rolling in a less than 1150° C. to 950° C. or more temperature region, then give a 20% or more reduction of area in rolling in a less than 950° C. to 750° C. or more temperature region and a 40% or more cumulative reduction of area in rolling with the reduction of area in rolling in the less than 1150° C. to 950° C. or more temperature region,   quenching the hot rolled steel wire to transform 90% or more of the metal microstructure by area fraction at a cross-section parallel to the rolling direction to martensite, and   tempering the quenched steel wire,   wherein a time from an end of the hot rolling to a start of the quenching is within 3 seconds; and   wherein said steel component contains, by mass %:   C: 0.40 to 0.70%;   Si: 0.80 to 2.20%;   Mn: 0.05 to 1.50%;   Cr: 0.05 to 1.00%;   P: limited to 0.020% or less;   S: limited to 0.020% or less, and
 a balance of Fe and unavoidable impurities. 
   
     
     
         10 . A method for producing spring steel for suspension according to  claim 3 , comprising:
 hot rolling a wire rod having steel components so as to give a 15% or more to less than 30% reduction of area in rolling in a less than 1150° C. to 950° C. or more temperature region, then give a 20% or more reduction of area in rolling in a less than 950° C. to 750° C. or more temperature region and a 40% or more cumulative reduction of area in rolling with the reduction of area in rolling in the less than 1150° C. to 950° C. or more temperature region,   quenching the hot rolled steel wire to transform 90% or more of the metal microstructure by area fraction at a cross-section parallel to the rolling direction to martensite, and   tempering the quenched steel wire,   wherein a time from an end of the hot rolling to a start of the quenching is within 3 seconds; and   wherein said steel component contains, by mass %:   C: 0.40 to 0.70%;   Si: 0.80 to 2.20%;   Mn: 0.05 to 1.50%;   Cr: 0.05 to 1.00%;   P: limited to 0.020% or less;   S: limited to 0.020% or less, and
 a balance of Fe and unavoidable impurities. 
   
     
     
         11 . A method for producing spring steel for suspension according to  claim 4 , comprising:
 hot rolling a wire rod having steel components so as to give a 15% or more to less than 30% reduction of area in rolling in a less than 1150° C. to 950° C. or more temperature region, then give a 20% or more reduction of area in rolling in a less than 950° C. to 750° C. or more temperature region and a 40% or more cumulative reduction of area in rolling with the reduction of area in rolling in the less than 1150° C. to 950° C. or more temperature region,   quenching the hot rolled steel wire to transform 90% or more of the metal microstructure by area fraction at a cross-section parallel to the rolling direction to martensite, and   tempering the quenched steel wire,   wherein a time from an end of the hot rolling to a start of the quenching is within 3 seconds; and   wherein said steel component contains, by mass %:   C: 0.40 to 0.70%;   Si: 0.80 to 2.20%;   Mn: 0.05 to 1.50%;   Cr: 0.05 to 1.00%;   P: limited to 0.020% or less;   S: limited to 0.020% or less, and
 a balance of Fe and unavoidable impurities.