Application of the hottest niobium in spring steel

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Application of niobium in spring steel

1 Development of microalloyed steel for car suspension springs

with the continuous emphasis on reducing the weight of passenger cars, people are also increasingly paying attention to the development of light suspension components with an income of 5.54 billion yuan. The focus is on developing spring steel that can withstand higher working stresses, thereby allowing the use of thinner wire to reduce the weight of coiled springs. A kind of spring steel containing trace alloy has been developed. Its tensile strength is 2000mPa, and its fracture toughness is the same as that of ordinary spring steel with tensile strength of 1725mpa. The fatigue test shows that under the working stress higher 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, obtain fine-grained austenite structure, and improve the dispersion of microalloyed carbonitrides through reasonable thermomechanical treatment

(1) chemical composition and process of steel

Table 1 gives the calibration composition of the steel, and detects the effects of various alloy additives on Microstructure and properties. It can be seen that the significant difference between this new steel and other types of steel is the low carbon content, which greatly improves the fracture toughness. The steel is produced by electric furnace smelting, ladle refining and continuous casting. The hot rolling is carried out on a set of continuous rolling mill equipped with stmo controlled cooling device

(2) microstructure and properties under heat treatment conditions

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

(3) fatigue performance

the fatigue limit of this microalloyed steel is higher than sae9259

(4) fracture toughness

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

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

2. Car suspension springs use Nb, V precipitation strengthened spring steel to reduce weight by 25%

most cars in Japan use sae9260 to make suspension springs, which brings many safety problems. It is urgent to develop spring steel with high bullet resistance. Typical components of sae9260 are as follows:

co.59, si2.17, mn0.86, p0.025, s0.010, cr0.15, al0.018

sae9260 steel causes foreign products to monopolize the market, and Nb or Nb, V compound can be added, and the purpose of grain refinement and precipitation strengthening can be achieved at the same time. So as to improve the spring's bullet resistance

(1) the characteristics of sae9260 plus Nb-V treatment refine the grain size, which is 1/4 of the original steel

the austenitic grain size of chemical composition a3-a6 is significantly better than sae9260, and the lower the austenitizing temperature, the better

(2) the corresponding speed of hardenability is fast

hardenability is an important factor in the production of hot-rolled springs

The hardenability of Nb-V treated steel, as shown in the curve, is actually equivalent to sae9260, which only shows the effect of carbon content when Nb-V carbonization is completely dissolved. It seems to have nothing to do with Nb and V, but when it is far away from the endpoint, it clearly shows the advantage of adding Nb and V

properties after tempering

1 Hardness

9260 CB with Nb only and 9260b without NB are at the same level, while 9260v CB3 with V and Nb and 9260-v2 with V only also have the same hardening trend. 9260-v2 plus v0.29. Only 9260-v-cb3 is the best

2. Yield strength

the tempering hardness and 0.05% yield strength, 0.2% yield strength and 0.2% torsional yield strength of steels with different Nb and V contents. Adding 9260b of Nb and V to the same hardness level is much higher, about 100MPa higher, while the torsional yield strength plus Nb and V is more superior

(4) bullet proof property of Nb and V composite treatment

the bullet proof property is determined by stress relaxation test under 1030mpa and 1128mpa stress conditions. Coil the spring in the large production line and quench it at 900 ℃. After tempering, shot blasting shall be conducted on the production line

residual shear strain seems to be a function of hardness, which decreases with the increase of hardness. Although sae9260 is the spring steel with the highest bullet resistance among all standards, the bullet resistance of steel treated with Nb and V is exciting. The company's brand reputation will be better. This steel can be used under high stress to reduce the weight of the spring

(5) the role of Nb in sae9260+nb V precipitation hardening hot rolled spring is to implement the production of sae9260nb and V treated steel springs without changing the original sae9260 suspension spring production line

the austenitizing and precipitation hardening effect of Nb at lower than 1100 ℃ is negligible, and the bullet resistance is improved only by refining the grain. However, only adding V can produce precipitation strengthening and grain refining effect, but it is difficult for vanadium carbides to dissolve in austenite in a short time on the production line. The test shows that it takes 10 minutes for vanadium carbide with only vanadium to dissolve completely when austenitizing at 900 ℃, while it takes only 1 minute when Nb-V is added. From the perspective of never changing the production line, adding Nb-V is reasonable. It seems that the presence of NBC promotes the dissolution of vanadium

(6) other conclusions

1 Sae9260 plus V is unfavorable for Anti Ballistic reduction and fracture toughness

2. Sae9260-v treatment has no effect on corrosion resistance and corrosion fatigue

3. The fatigue performance of Nb-V treated steel is no different from that of 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 treatment steel, because B further improves hardenability. The steel torsion bar has been put into practice

(7) the production process of sae9260 plus Nb and V treated steel and the production of hot coil spring can follow a series of process operations of sae9260 without any difficulties


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