The quality of alloy spring steel is largely determined by the smelting process, including the chemical composition of the steel, the cleanliness of the molten steel (gas, harmful elements, inclusions) and the quality of the casting billet (component segregation, decarburization and surface conditions) These aspects are the key control points of smelting operations. In addition, spring steel is also required to have sufficient hardenability to ensure that the entire spring section obtains uniform microstructure and mechanical properties.
2.1 Improvement of non-metallic inclusions
The main cause of fatigue cracks is the oxide inclusions in steel, and the damage of type D inclusions to fatigue life is greater than that of type B inclusions. Therefore, foreign steel mills and automobile factories have put forward higher requirements for oxide inclusions in spring steel. For example, the Swedish SKF standard requires that the oxygen content in steel is less than 15×10-6, and the type D inclusions are lower than the type B inclusions. . In particular, Al2O3 and TiN inclusions are the most harmful to the fatigue life of the spring. In order to produce high-quality spring steel, special smelting methods such as electric furnace-electroslag remelting or vacuum arc remelting are usually used in the past.
With the development of out-of-furnace refining technology, the use of out-of-furnace refining technology can significantly reduce the inclusions in steel. For example, Japan uses RH vacuum degassing to achieve ultra-low oxygen steel (ULO) or ultra-pure steel (UCS) production. Killed steel with silicon deoxidation is used. During refining, refining synthetic slag with strictly controlled alkalinity is used to denature the non-deformable Al2O3-rich harmful inclusions. At the same time, the molten steel is stirred to float and remove the inclusions, reduce the content of inclusions and make The remaining inclusions are harmless, etc., thereby obtaining ultra-pure spring steel. The inspection confirmed that low-oxygen steel (LO) with an oxygen content of less than 15×10-6 can meet the requirements of 200MPa high-stress springs. The fatigue limit of ULO+ULTiN steel produced by ultra-low oxygen plus ultra-low titanium nitride process is the same as that of vacuum arc remelted steel, and the steel can be used to manufacture high-strength valve springs [4]. Xining Special Steel Plant and Jiangsu Huaiyin Steel Plant (super) high-power electric arc furnace primary refining + LF refining + alloy steel bloom or billet continuous casting to produce spring steel. Barium alloy deoxidation was carried out in 60 t and 80 t LF, respectively And the inclusion denaturation test study, the oxygen content in the steel is reduced to 15×10-6 or less, the residual aluminum content is controlled within 0.020%~0.025%, and the proportion of Al2O3 in the oxide inclusions is less than 40%, and the residual inclusions are small , Uniform and diffuse distribution of plastic inclusions [5]. Qingdao Iron and Steel Co., Ltd. adopts the pulling carbon method for smelting, blowing argon at the bottom of the ladle, feeding wire, continuous casting adopts protective casting, reasonable control of superheat, pulling speed and mold parameters, and secondary cooling water using gas-water atomization cooling methods, etc. Produced 60Si2Mn spring steel that meets the requirements of GB1222-84, which effectively solves the quality problems in the production process, and reduces the oxygen content and the level of inclusions in the steel [6].
2.2 Improvement of surface defects and decarburization layer
The surface strength of parts is an important factor affecting fatigue strength. Surface heat treatment and surface cold plastic deformation processing are very effective in improving fatigue strength, such as surface quenching, carburizing, carbonitriding, nitriding, shot peening and rolling. Improving the surface strength of parts can reduce the effective tensile stress and local uneven deformation on the surface of the parts, and reduce the formation of fatigue cracks.
Yamata et al. [7] studied the influence of different surface processing and surface treatment methods on the fatigue performance of spring steel SUP10. Table 2 shows the surface roughness, surface residual stress and fatigue limit measured by the test.
Table 2 Surface roughness, residual stress and fatigue limit of SUP10 steel fatigue specimen
Surface roughness of sample
Rrmax /μm surface residual stress
σw/MPa fatigue limit
Increase in fatigue limit of σw/MPa
Δσw/%
HT4.7-54200
HT+BF2.3-32447011.9
HT+SP5.7-47062047.6
HT+RG4.3-15663050.0
HT+FG1.1-13967560.7
HT+RG+SP3.8-62867560.7
HT+FG+SP1.8-62370067.7
FG+HT2.2-0349016.7
FG+HT+SP4.1-59164052.4
Note: ①Negative sign means compressive stress; ②HT means heat treatment, BF means barrel grinding, SP means shot peening, RG means rough grinding, FG means fine grinding.
It can be seen that: (1) Grinding off the surface decarburization layer produced by heat treatment can significantly increase the fatigue limit; (2) Direct shot peening without removing the surface decarburization layer produced after heat treatment increases the fatigue limit compared to removing the decarburization layer and then shot peening The range is large, the former is 30%-50%, and the latter is only 3%-6%.
In order to reduce the effect of surface decarburization, the surface of hot-rolled spring round steel should be stripped. To avoid surface decarburization, the carbon chemical potential gradient between the two should be eliminated or reduced. Heating with protective atmosphere is an effective measure to avoid or reduce surface decarburization. To shorten the heating time and reduce the depth of decarburization, rapid induction heating should be used. Due to the different effects of different alloying elements on the activity and diffusion of carbon, spring steels with different compositions under the same conditions will exhibit different decarburization behaviors. For example, Si can improve the elastic limit, strength, tempering stability, and resistance to elastic decline. However, attention must also be paid to the serious decarburization of the surface caused by the increase in the activity of carbon in austenite and the chemical potential gradient of Si. The low-carbon spring steel 28MnSiB trial-produced by Shigang Corporation reduces the carbon silicon content in the steel and effectively reduces the tendency of surface decarburization. The inspection results show that the actual carbon content is 0.10%-0.16%, with an average of 0.12%, which meets the standard The specified carbon content is less than the requirement of 0.23% [8].