JPS6237109B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a spring steel with excellent resistance to settling. Conventionally, SUP6 and SUP9 have been the main spring steels used in suspension systems for automobiles and the like. In recent years, there has been a demand for lighter automobiles, and there has also been a strong demand for lighter suspension systems themselves. In response to this problem, various measures have been attempted for suspension systems in general, and among these measures, a measure of increasing the spring design stress is considered to be effective. With this high stress design, when springs are manufactured using the above-mentioned conventional spring steel as a material, a problem arises in that the set-off increases. Particularly when used in passenger cars, increased sag leads to a reduction in bumper height, posing a major safety problem. As a result of various studies, it was discovered that increasing the Si content in spring steel improves the fatigue resistance. SUP7, which has the highest Si among all spring steels, has come to be widely used as a steel for passenger car suspension springs. However, there are strict requirements for reducing the weight of suspension springs, and there has been a strong desire to develop a spring steel with even better fatigue resistance than SUP7. Against this background, the present invention was developed as a result of repeated research by the present inventors, and was developed by adding an appropriate amount of V to high-Si spring steel.
Add Nb and then add B or B depending on the purpose.
and Ni, one of the rare earth elements, or also Ti
By adding , we have developed a spring steel that has better fatigue resistance, hardenability, and toughness than SUP7, and also has the same fatigue resistance as SUP7, which is necessary for spring steel. be. V and Nb form carbides in steel, and vanadium carbide and niobium carbide (hereinafter referred to as alloy carbides) are dissolved in austenite during heating during quenching. When this is rapidly cooled and quenched, martensite containing these elements in a supersaturated solid solution is obtained. When this is tempered, fine alloy carbides begin to re-precipitate in the process, which prevents the movement of dislocations in the steel, causing secondary hardening, making the steel harder than spring steel without the addition of V and Nb. It also works to improve the resistance to fatigue. In addition, alloy carbides that are not dissolved in austenite during heating during quenching can refine austenite crystal grains and prevent them from becoming coarser. Moreover, such fine crystal grains improve the resistance to settling by reducing the amount of movement of dislocations. The reason why the amount of Mn is lowered in the steel of the present invention is to improve the toughness of the steel by suppressing the formation of retained austenite during quenching. Recently, there has been a trend to increase the hardness of springs in order to reduce spring fatigue, but increasing the hardness of springs causes a new problem of reduced toughness. Therefore, the steel of the present invention attempts to solve this new problem by limiting the amount of Mn to a low level. To clarify this, the 0.38%
Table 1 shows the results of impact tests conducted on A2 steel containing Mn and B1 steel, which is a conventional steel containing 0.86% Mn and is substantially SUP7. As is clear from Table 1, it is the steel of the present invention.
It can be seen that A2 steel with a small amount of Mn has higher toughness than B1 steel, which is a conventional steel.

[Table] In addition, B, Ni, and rare earth elements are elements that improve the hardenability of steel.In particular, low Mn steel such as the steel of the present invention may have insufficient hardenability, so these elements are This not only works effectively in the sense of supplementing the above, but also enables the steel of the present invention to be applied to even larger and thicker springs. In order to clarify this, the 0.24% V described below
A4 steel containing 0.11% Nb and 0.0022% B,
0.20% V, 0.13% Nb, 0.48% Ni, 0.0027%
A6 steel containing B, 0.21% V, 0.12%
Table 1 shows the results of comparing the hardenability of A8 steel containing Nb, 0.0029% B, and 0.05% rare earth elements and B1 steel, which is a conventional steel SUP7. As is clear from FIG. 1, it can be seen that the addition of hardenability-improving elements makes it possible to obtain hardenability higher than that of conventional steels. Next, Ti often combines with N to form nitrides in steel, which refines austenite grains during hot rolling and prevents austenite grains from coarsening when heated to the austenitizing temperature. It has the function of Since the amount of movement of dislocations in a structure with finer grains is smaller, the fatigue resistance of the steel can be improved. Figure 2 shows the size of austenite crystal grains when heated and held at each austenitizing temperature of 850 to 1100°C for A12 steel with Ti added and conventional steel B1, which will be described later. The effect of the addition of chemical elements is clearly recognized. The chemical composition of the steel of the present invention is C0.50-0.80%, Si1.50
Contains ~2.50%, Mn0.45% or less, and V0.05
~0.50%, Nb0.11~0.50%, and depending on the purpose of use, it may further contain B0.0005~0.0100% or B0.0005~0.0100%, Ni0.20~0.60%, and rare earth elements 0.10% or less. It contains one of these, or it also contains one of 0.02 to 0.10% Ti, and the rest is essentially Fe. The reasons for limiting the composition of the steel of the present invention will be explained below. The reason why the C content is set at 0.50 to 0.80% is that if it is less than 0.50%, sufficient strength cannot be obtained as a steel for high stress springs through quenching and tempering. This is because the decrease in toughness becomes significant. The reason why the amount of Si is set at 1.50 to 2.50% is because if it is less than 1.50%, the effect of increasing the strength of the base material and improving the resistance to settling cannot be obtained sufficiently by solid solution of Si in the ferrite. This is because even if the content exceeds 2.50%, the effect of improving the resistance to settling is saturated, and there is a risk that free carbon may be generated by heat treatment. The reason why the Mn amount is 0.45% or less is that the Mn amount is 0.45%.
This is to provide toughness to the steel by suppressing the formation of retained austenite by limiting it to the following. Both V and Nb are elements that improve the sag resistance in the steel of the present invention. The content of V, which plays this role, is 0.05~
The reason why the Nb content was set at 0.50% is that the above effects cannot be obtained sufficiently below 0.05%.
The reason for setting it to 0.11 to 0.50% is that the above effects cannot be sufficiently obtained below the lower limit. Even if the content exceeds 0.50%, the effect is saturated, and the amount of alloy carbides that are not dissolved in austenite increases, forming large lumps that reduce the fatigue strength of steel by acting like nonmetallic inclusions. This is because there is fear. These V and Nb can be added individually or by adding the two in combination.
Compared to when Nb is added alone, dissolution into austenite starts at a lower temperature, and the precipitation of fine alloy carbides during the tempering process is
By further promoting secondary curing, the sagging resistance is further improved. In addition, B, Ni, which are elements that improve the hardenability of steel,
The amount of rare earth elements is B0.0005~0.0100%, respectively.
The reason for setting Ni to 0.20 to 0.60% and rare earth elements to 0.10% or less is because B is not sufficient to improve hardenability when it is contained at 0.0005% or less, and even if it is contained in excess of 0.0100%, the effect is saturated. This is also because the formation of the B compound may cause hot embrittlement. Regarding Ni, if it is less than 0.20%, the effect of improving hardenability and toughness cannot be sufficiently obtained, and if it is contained in an amount exceeding 0.60%, the effect is not improved much. The reason why the amount of rare earth elements is set to 0.30% or less is that if it is contained more than that, the crystal grains may become coarse. In addition, the content of Ti, which refines crystal grains and improves resistance to settling, was set at 0.02 to 0.10% because if it is less than that, the distribution of nitrides will be sparse and will not contribute to grain refinement. This is because if the content exceeds 0.10%, it may cause cracks during hot rolling or deteriorate the toughness of the steel as nonmetallic inclusions. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 2 shows the chemical composition of these test steels.

[Table] In Table 2, A2 steel is the first invention steel, A4 steel is the second invention steel, A6 and A8 steel are the third invention steel, A12 steel is the fourth invention steel, and B1 steel is the conventional steel with SUP7 It is. All of the sample steels were hot-rolled at a rolling ratio of 50 or higher after casting, and were subjected to the sag test described below. To carry out the fatigue test, a coil spring with the specifications shown in Table 3 is formed, quenched and tempered to a final hardness of HRC45 to 55, and then A setting test piece was prepared by setting the wire so that the shear stress τ=115 Kg/mm ² . Then, a load was applied to this test piece at a constant temperature of 20°C so that the shear stress of the wire was τ = 105Kg/ ^mm2 ,
The amount of fatigue of the coil spring was measured after 96 hours (hereinafter referred to as long-term loading).

[Table] However, A4, A6, which added hardenability improving elements,
For A8 steel, we created a torsion bar with the specifications shown in Table 4, and the final hardness was HRC45~
After quenching and tempering so that it becomes 55, τ
Setting was performed with a stress of =110 Kgf/mm ² , and then a stress of τ = 100 Kgf/mm ² was applied for 96 hours, and the amount of settling that occurred during that time was measured.

[Table] The amount of set is the load required to compress the coil spring to a certain height before applying the long-term load mentioned above.
Measure P ₁ and the load P ₂ required to compress to the same height after applying the long-term load, and calculate the difference △P
(=P ₁ −P ₂ ) using the following formula, and has a unit of shear strain, and was evaluated using a value called residual shear strain. γR=1/G・K8D/πd ³ △PG G: Transverse elastic modulus (Kgf/mm ² ) D: Coil center diameter (mm) d: Wire diameter (mm) K: Whirl correction coefficient (shape of coil spring In addition, the amount of setback from the torsion bar was determined by converting the amount of decrease in the torsion angle Δθ into the amount of residual shear strain according to YR=Δθ·d/2l. d: Wire diameter (mm) l: Effective length (mm) The amount of set in relation to the hardness of the test piece is shown in Figures 3 to 6. As is clear from FIGS. 3 to 6, it is recognized that all of the steels of the present invention have superior sag resistance compared to B1 steel, which is a conventional steel. In addition, A2 and B1 steels of the present invention are coil springs having the same specifications as above, and A4,
A fatigue test was conducted on A6 and A8 by repeatedly applying a stress of 10 to 110 Kgf/mm ² using the above-mentioned torsion bar, but none of the samples broke even after 200,000 repetitions. As mentioned above, the steel of the present invention is a high-Si spring steel with V,
By adding precipitation-strengthening elements such as Nb, hardenability-improving elements such as B, Ni, and rare earth elements, and grain-refining elements such as Al and Ti, conventional high-Si spring steels can be improved. This product has succeeded in further improving its excellent fatigue resistance, and the fatigue resistance required for spring steel is also comparable to that of conventional steel, and is particularly high as a steel for passenger car suspension springs. It is practical.

[Brief explanation of the drawing]

Figure 1 is a diagram comparing the hardenability of the present invention steel with added hardenability-improving elements and conventional steel.
Figure 2 is a diagram showing the austenite grain size when the present invention steel and conventional steel are heated and held at various austenitizing temperatures of 850 to 1100°C, and Figures 3 to 5 are
FIG. 2 is a diagram showing the amount of settling of test pieces when the steel of the present invention and the conventional steel are hardened to HRC45 to HRC55 after being quenched and tempered.

Claims (1)

[Claims] 1. C0.50-0.80%, Si1.50-2.50 in terms of weight ratio
%, Mn0.45% or less, V0.05~0.50%, Nb0.11~
A spring steel with excellent fatigue resistance characterized by containing 0.50% Fe with the remainder essentially consisting of Fe. 2 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.45% or less, V0.05~0.50%, Nb0.11~
A spring steel with excellent fatigue resistance, characterized in that it contains 0.50% B, further contains 0.0005 to 0.0100% B, and the remainder substantially consists of Fe. 3 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.45% or less, V0.05~0.50%, Nb0.11~
Contains 0.50% and further B0.0005 to 0.0100%,
1 of Ni 0.20-0.60%, rare earth elements 0.10% or less
A spring steel with excellent fatigue resistance, which is characterized by containing seeds and the remainder being essentially Fe. 4 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.45% or less, V0.05~0.50%, Nb0.11~
A spring steel with excellent fatigue resistance, characterized by containing 0.50% of Ti, further containing 0.020 to 0.10% of Ti, and the remainder substantially consisting of Fe.