Hollow stabilizer, and steel pipe for hollow stabilizers and method of producing the same

Abstract

A hollow stabilizer having an excellent fatigue property and higher strength compared to the conventional ones has a chemical composition containing 0.26% to 0.30% of C, 0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005% to 0.005% of Ca, wherein; Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively, a remainder of the chemical composition consists of Fe and unavoidable impurities, a value of a product of the Mn content and the S content is 0.0025 or less, and a critical cooling rate Vc90 represented by a predetermined equation is 40° C./s or less; and wherein a metallic structure comprises a tempered martensite, a length of elongated MnS present at a center part in a thickness direction of the hollow stabilizer is 150 μm or less, a HRC is from 40 to 50, a thickness to outer diameter ratio is 0.14 or more, and a depth of a decarburized layer at an inner surface part is 20 μm or less from the inner surface; and a steel pipe for hollow stabilizers used as a material for the hollow stabilizer.

1 . A hollow stabilizer having a chemical composition comprising, as chemical components, in terms of % by mass:
 0.26% to 0.30% of C,   0.05% to 0.35% of Si,   0.5% to 1.0% of Mn,   0.05% to 1.0% of Cr,   0.005% to 0.05% of Ti,   0.0005% to 0.005% of B, and   0.0005% to 0.005% of Ca,   wherein:   Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively,   a remainder of the chemical composition consists of Fe and unavoidable impurities,   a value of a product of the Mn content and the S content is 0.0025 or less, and   a critical cooling rate Vc90 represented by the following Equation (1) is 40° C./s or less:
   log  Vc 90=2.94−0.75β  Equation (1):
 
 wherein 13=2.7C+0.4Si+Mn+0.8Cr; 
   and wherein:   a metallic structure comprises a tempered martensite,   a length of elongated MnS present at a center part in a thickness direction of the hollow stabilizer is 150 μm or less,   a Rockwell C hardness (HRC) is from 40 to 50,   a wall thickness/outer diameter ratio is 0.14 or more, and   a depth of a decarburized layer at an inner surface part of the hollow stabilizer is 20 μm or less from the inner surface.   
     
     
         2 . The hollow stabilizer according to  claim 1 , further comprising, in terms of % by mass, one or more of:
 0.05% to 0.5% of Mo,   0.01% to 0.1% ofNb,   0.01% to 0.1% of V, or   0.1% to 1.0% of Ni,   wherein, in Equation (1), β=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni.   
     
     
         3 . The hollow stabilizer according to  claim 1 , wherein a maximum compressive residual stress on an outer surface is 400 MPa or more. 
     
     
         4 . The hollow stabilizer according to  claim 3 , wherein the outer surface and the inner surface are subjected to shot peening. 
     
     
         5 . A steel pipe for a hollow stabilizer used as a material for the hollow stabilizer according to  claim 1 , the steel pipe having a chemical composition comprising, as chemical components, in terms of % by mass:
 0.26% to 0.30% of C,   0.05% to 0.35% of Si,   0.5% to 1.0% of Mn,   0.05% to 1.0% of Cr,   0.005% to 0.05% of Ti,   0.0005% to 0.005% of B, and   0.0005% to 0.005% of Ca,   wherein:   Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively,   the chemical composition optionally comprises one or more of:   0.05% to 0.5% of Mo,   0.01% to 0.1% of Nb,   0.01% to 0.1% of V, or   0.1% to 1.0% of Ni,   a remainder of the chemical composition consists of Fe and unavoidable impurities,   a value of a product of the Mn content and the S content is 0.0025 or less, and   a critical cooling rate Vc90 represented by the following Equation (1) is 40° C./s or less:
   log  Vc 90=2.94−0.75β  Equation (1):
 
 wherein β=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni; 
   and wherein:   a metallic structure comprises a mixed structure of ferrite and perlite,   a length of elongated MnS present at a center part in a thickness direction of the steel pipe is 150 μm or less,   a Rockwell B hardness (HRB) is 95 or less,   a wall thickness/outer diameter ratio is 0.14 or more, and   a depth of a decarburized layer at an inner surface part of the steel pipe is 20 μm or less from the inner surface.   
     
     
         6 . A method of producing the steel pipe for a hollow stabilizer according to  claim 5 , wherein:
 the steel pipe is an electric resistance-welded steel pipe; and   the method comprises a process of subjecting the electric resistance-welded steel pipe to heating, after electric resistance-welding, to a temperature of from 800° C. to 1200° C. and diameter-reduction hot rolling to a reduction in cross sectional area of from 40% to 80%.   
     
     
         7 . A method of producing the steel pipe for a hollow stabilizer according to  claim 5 , wherein:
 the steel pipe is an electric resistance-welded steel pipe; and   the method comprises a process of elongating the electric resistance-welded steel pipe by cold-drawing after electric resistance-welding.   
     
     
         8 . The hollow stabilizer according to  claim 2 , wherein a maximum compressive residual stress on an outer surface is 400 MPa or more. 
     
     
         9 . The hollow stabilizer according to  claim 8 , wherein the outer surface and the inner surface are subjected to shot peening. 
     
     
         10 . A steel pipe for a hollow stabilizer used as a material for the hollow stabilizer according to  claim 2 , the steel pipe having a chemical composition comprising, as chemical components, in terms of % by mass:
 0.26% to 0.30% of C,   0.05% to 0.35% of Si,   0.5% to 1.0% of Mn,   0.05% to 1.0% of Cr,   0.005% to 0.05% of Ti,   0.0005% to 0.005% of B, and   0.0005% to 0.005% of Ca,   wherein:   Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively,   the chemical composition optionally comprises one or more of:   0.05% to 0.5% of Mo,   0.01% to 0.1% of Nb,   0.01% to 0.1% of V, or   0.1% to 1.0% of Ni,   a remainder of the chemical composition consists of Fe and unavoidable impurities,   a value of a product of the Mn content and the S content is 0.0025 or less, and   a critical cooling rate Vc90 represented by the following Equation (1) is 40° C./s or less:
   log  Vc 90=2.94−0.75β  Equation (1):
 
 wherein β=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni; 
   and wherein:   a metallic structure comprises a mixed structure of ferrite and perlite,   a length of elongated MnS present at a center part in a thickness direction of the steel pipe is 150 μm or less,   a Rockwell B hardness (HRB) is 95 or less,   a wall thickness/outer diameter ratio is 0.14 or more, and   a depth of a decarburized layer at an inner surface part of the steel pipe is 20 μm or less from the inner surface.   
     
     
         11 . A method of producing the steel pipe for a hollow stabilizer according to  claim 10 , wherein:
 the steel pipe is an electric resistance-welded steel pipe; and   the method comprises a process of subjecting the electric resistance-welded steel pipe to heating, after electric resistance-welding, to a temperature of from 800° C. to 1200° C. and diameter-reduction hot rolling to a reduction in cross sectional area of from 40% to 80%.   
     
     
         12 . A method of producing the steel pipe for a hollow stabilizer according to  claim 10 , wherein:
 the steel pipe is an electric resistance-welded steel pipe; and   the method comprises a process of elongating the electric resistance-welded steel pipe by cold-drawing after electric resistance-welding.