Application of Niobium in Spring Steel

Application of Niobium in Spring Steel
Development of Microalloy Steel for Vehicle Suspension Spring

With the continuous emphasis on reducing the weight of passenger cars, more and more attention has been paid to the development of lightweight suspension components. The focus is on developing spring steel that can withstand higher working stress, allowing the use of thinner metal wires to reduce the weight of the coiled spring. A spring steel containing trace alloys has been developed. Its tensile strength is 2000MPa, and its fracture toughness is the same as that of ordinary spring steel with a tensile strength of 1725MPa. The fatigue test shows that under higher working stress than the ordinary spring steel currently used, very good fatigue resistance and sag resistance can be obtained. The improvement of these conditions can reduce the carbon content and obtain a fine-grained austenite structure. At the same time, the dispersion of microalloy carbonitrides can be improved through reasonable thermomechanical treatment.

(1) Chemical composition and technology of steel

Table 1 shows the calibrated composition of the steel and detects the influence of various alloy additives on the structure and properties. It can be seen that the significant difference between this new steel grade and other types of steel is its lower carbon content, which greatly improves the fracture toughness. The steel is produced through electric furnace smelting, ladle refining and continuous casting. Hot rolling is carried out on a continuous rolling mill equipped with a Stymore controlled cooling device.

(2) Microstructure and performance under heat treatment conditions

The heat treatment conditions are austenitizing at 925°C, oil quenching and tempering at 340°C for 2 hours. Martensite block structure is found in the microalloyed spring steel structure, and fine and dispersed carbides are found in the twin boundary zone, which may be Nb and V that have not been dissolved during reheating and hot rolling. The tensile properties of the carbides are shown in Table 2. It can be seen that the microalloyed steel has significantly high yield strength and tensile strength.

(3) Fatigue performance

The fatigue limit of the microalloyed steel is higher than SAE9259.

(4) Fracture toughness

For the fracture toughness of the two steels, microalloyed steel shows higher toughness.

In short, the developed microalloy spring steel can be effectively used as a high-stress coil spring in automobile suspension devices. Its higher tensile strength and better fatigue resistance can be used to reduce weight. Achieving weight reduction will be a major task of spring design and steel performance, and shows that good fatigue resistance and sag resistance can be obtained.

Nb, V precipitation-strengthened spring steel for suspension springs reduces weight by 25%

Most Japanese cars use SAE9260 to make suspension springs, which brings many safety problems. It is urgent to develop spring steel with high elasticity resistance. The typical composition of SAE9260 is as follows:

Co.59, Si2.17, Mn0.86, P0.025, S0.010, Cr0.15, Al0.018.

Adding Nb or Nb, V to SAE9260 steel can achieve the purpose of grain refinement and precipitation strengthening. Thereby improving the spring resistance of the spring.

(1) The characteristics of SAE9260 plus Nb-V treatment refine the grain, and the grain size is 1/4 of the original steel.

The austenite grain size of chemical composition A3-A6 is obviously better than SAE9260, and the lower the austenitizing temperature, the better.

(2) Hardenability

Hardenability is an important factor in the production of hot-coiled springs.

The hardenability of Nb-V treated steel, as shown in the curve, is actually equivalent to SAE9260, only showing the effect of the carbon content when the Nb-V carbide is completely dissolved. It seems that it has nothing to do with Nb, V, but when it is far from the end point, it clearly shows the superiority of adding Nb, V.

(3) Performance after tempering

1. Hardness

9260-Cb with only Nb and 9260b without Nb are at the same level, while 9260V-Cb3 with V and Nb and 9260-V2 with V only have the same hardening tendency. Add V0.29 to 9260-V2. Only 9260-V-Cb3 is the best.

2. Yield strength

The hardness and 0.05% yield strength, 0.2% yield strength, 0.2% torsional yield strength of steel with different Nb and V content after tempering, 9260b with Nb and V added to the same hardness level are much higher, about 100MPa higher, and The torsional yield strength plus Nb, V is more superior.

(4) Resilience of niobium and vanadium composite treatment

Under the stress conditions of 1030MPa and 1128MPa, the elasticity resistance is determined by stress relaxation test. The spring is rolled in a large production line and quenched at 900°C. After tempering, shot blasting on the production line.

The residual shear strain appears to be a function of hardness and decreases with increasing hardness. Although SAE9260 has the highest elasticity reduction among all spring steels specified by the standard, the elastic reduction of steel treated with Nb and V is exciting. This steel can be used under high stress, reducing the weight of the spring.

(5) Nb in SAE9260+Nb

The role of V precipitation hardening hot-coiled springs is to implement the production of SAE9260Nb, V-treated steel springs without changing the original SAE9260 suspension spring production line.

The effect of austenitizing and precipitation hardening at a temperature lower than 1100℃ by adding only Nb is minimal, and only by refining the grain to improve the elasticity resistance. But only adding V can produce precipitation strengthening and grain refinement effects, but it is difficult for vanadium carbides to dissolve in austenite in a short time on the production line. Experiments show that it takes 10 minutes for vanadium carbides with vanadium only to be added during austenitization at 900°C, while it takes only 1 minute for Nb-V composite addition. From the standpoint of not changing the production line, adding Nb-V makes sense. It seems that the presence of NbC promotes the dissolution of vanadium.

(6) Other conclusions

1. SAE9260 plus V is unfavorable for elasticity reduction and fracture toughness.

2. After SAE9260-V treatment, it has no effect on corrosion resistance and corrosion fatigue.

3. The fatigue performance of Nb-V treated steel is the same as the original SAE9260, and the fatigue limit is 745MPa. The results of shot peening or grinding surface test show that there is no difference between steel types.

4. Add B, Nb, V to treat steel, because B further improves hardenability. The steel torsion bar has been put into practical use.

(7) The production process of SAE9260 plus Nb, V treated steel and the production of hot coil springs can be operated by a series of SAE9260 processes without any difficulty.