A tubular spring, such as a coil spring, a torsion-rod spring, and/or a stabilizer for a motor vehicle, may include at least one metal tube element having a tube internal cross section, a tube internal diameter, a tube external diameter, a tube internal wall, and a tube wall thickness. At least one metal foam may be disposed in the tube internal cross section of the at least one metal tube element of the tubular spring in at least one part-region. In particular, the metal foam may be connected in an at least partially materially integral manner to the tube internal wall of the metal tube element. The at least one metal tube element may have an at least partially martensitic structure.

A hollow stabilizer (1) according to the invention is formed in a hollow shape and includes a bent portion (1c1, 1c2, m1-m5). With a thickness of an inner side of the bent portion (1c1, 1c2, m1-m5) being larger than a thickness of an outer side of the bent portion (1c1, 1c2, m1-m5), a hardness of the outer side of the bent portion (1c1, 1c2, m1-m5) is higher than a hardness of the inner side of the bent portion (1c1, 1c2, m1-m5) through quenching by cooling down after heating by electrical heating.

A hollow stabilizer has a tubular shape and is provided with a torsion section that is provided to a vehicle and that extends in the vehicle width direction; an arm section that extends in the front-back direction of the vehicle; and bent sections that connect the torsion section and the arm section. The hardness of the outer surface of the bent inner sides of the bent sections of the hollow stabilizer is 70% or more with respect to the hardness of the outer surface of the arm section.

A method of manufacturing a hollow stabilizer includes a forming step of subjecting an element pipe to a bending process, to form a product shape including bent portions, and a quenching step of quenching the element pipe subjected to the bending process. In the quenching step, a cooling process is performed by immersing the element pipe made of steel in coolant and by spraying the coolant to an outer surface of the bent portion.

Leaf springs, and methods of manufacturing thereof, having first and second sections, spaced apart along the length of said leaf spring, said sections are through hardened and tempered to achieve, respectively different levels of finished through hardness, are disclosed.

An electric resistance welded steel pipe for manufacturing a hollow stabilizer has a Lankford value in a pipe longitudinal direction of from 0.7 to less than 1.0. The electric resistance welded steel pipe is subjected to cold bending and then to a heat treatment including quenching and tempering to manufacture a stabilizer. The cold bending is cold rotary draw bending. When bent with a bend radius of from 1.0 times to 3.0 times an outer diameter of the pipe before cold bending, a flattening ratio is from 0% to 10%, a thickness reduction rate on a bending outside and a thickness increase rate on a bending inside are from 0% to 10%, and additionally, a circumferential length change of a bending center portion is from 0% to 10%. A Vickers hardness of the stabilizer after the heat treatment is adjusted to from 400 HV to less than 580 HV.

Leaf springs, and methods of manufacturing thereof, having first and second sections, spaced apart along the length of said leaf spring, said sections are through hardened and tempered to achieve, respectively different levels of finished through hardness, are disclosed.

Leaf springs, and methods of manufacturing thereof, having first and second sections, spaced apart along the length of said leaf spring, said sections are through hardened and tempered to achieve, respectively different levels of finished through hardness, are disclosed.

A hollow stabilizer (1) according to the invention is formed in a hollow shape and includes a bent portion (1c1, 1c2, m1-m5). With a thickness of an inner side of the bent portion (1c1, 1c2, m1-m5) being larger than a thickness of an outer side of the bent portion (1c1, 1c2, m1-m5), a hardness of the outer side of the bent portion (1c1, 1c2, m1-m5) is higher than a hardness of the inner side of the bent portion (1c1, 1c2, m1-m5) through quenching by cooling down after heating by electrical heating.

A steel, having a high corrosion resistance and low-temperature toughness, for a vehicle suspension spring part, the steel includes 0.21 to 0.35% by mass of C, more than 0.6% by mass but 1.5% by mass or less of Si, 1 to 3% by mass of Mn, 0.3 to 0.8% by mass of Cr, 0.005 to 0.080% by mass of sol. Al, 0.005 to 0.060% by mass of Ti, 0.005 to 0.060% by mass of Nb, not more than 150 ppm of N, not more than 0.035% by mass of P, not more than 0.035% by mass of S, 0.01 to 1.00% by mass of Cu, and 0.01 to 1.00% by mass of Ni, the balance being Fe and unavoidable impurities, with Ti+Nb0.07% by mass, wherein crystal grains of the steel after hardening have a prior austenite grain size number of 7.5 to 10.5, and the steel having a tensile strength of not less than 1,300 MPa.