KR100775254B1 - Suspension spring having excellent impact properties and method for producing the same - Google Patents

Abstract

A spring and a method for producing the spring are provided, wherein the spring has high strength and high toughness by controlling components of the spring, forming a fine precipitate phase in the spring, and properly treating the spring. A suspension spring having excellent impact properties has a composition comprising, by weight percent, 0.4 to 0.7% of C, 1.5 to 3.5% of Si, 0.3 to 1.0% of Mn, 0.01 to 1.5% of Cr, 0.01 to 1.0% of Ni, 0.01 to 1.0% of Cu, 0.005 to 0.02% of B, 0.1% or less of Al, 0.0015% or less of O, 0.02% or less of P, 0.02% or less of S, 0.005 to 0.02% of N, and the balance of Fe and other inevitable impurities, wherein a spherical austenite grain in the internal structure of the spring has a grain size of 5 to 30 mum. The composition for the spring further comprises, by weight percent, 0.5% or less of V and 0.5% or less of Ti.

Description

Suspension spring with excellent impact value and manufacturing method {SUSPENSION SPRING HAVING EXCELLENT IMPACT PROPERTIES AND METHOD FOR PRODUCING THE SAME}

1 is a tissue photograph of the internal structure of the spring made of Comparative Materials 1 and 2 and Inventive Materials 1 and 2, and

2 is a graph comparing old austenite average grain sizes of Comparative Materials 1 and 2 and Inventive Materials 1 and 2;

The present invention relates to a suspension spring having excellent impact value and a method of manufacturing the same, and more particularly, a spring used in automotive coil springs, leaf springs, torsion bars, stabilizers, etc. And to a method for producing the same.

Recently, the seriousness of air pollution caused by the pollution sources caused by the combustion of the petroleum fuels has increased worldwide as the use of fossil fuels, especially petroleum fuels, and in addition to the oil spill accident of large tankers, In order to avoid the harm caused by petroleum fuels, research on technologies to reduce the use of petroleum fuels has been conducted in various angles.

As a demand source for using a large amount of petroleum fuel, an automobile may be cited, and the automobile manufacturer is continuously conducting various attempts and studies to reduce the amount of petroleum fuel used. One of the most traditional ways to reduce the use of petroleum fuel is to improve the fuel efficiency of automobiles, which is the mainstream of the technology currently being developed and applied. The method of improving is one of them, and another method is a method of reducing the amount of energy required to move the unit distance by reducing the weight of the vehicle body.

In order to reduce the weight of the vehicle body, there may be a method of replacing the parts in the car with a light weight material having a low specific gravity, but there are not many fields that can replace the superiority of steel parts. Therefore, steel parts are still often used as automobile parts, and it is common to attempt to improve automobile fuel efficiency by lightening the steel parts.

The simple weight reduction of steel parts can cause a fatal problem for the safety of automobiles because the load that can be supported per unit weight is determined. Therefore, the weight reduction of parts becomes a feasible task after solving the task of increasing the strength of parts.

In particular, automotive spring is a concept similar to high strength is a component that strongly demands excellent permanent deformation resistance. Permanent deformation resistance means resistance to the phenomenon of permanent deformation in which the spring height does not change completely after the spring is used for a long time. In order to increase the permanent deformation resistance of the spring, steel wire rods in which Si is added a large amount It has been used as a spring material. The Si serves to prevent permanent deformation by increasing the yield strength of the steel.

In addition, Si is an element belonging to Group 4 on the periodic table and is thermodynamically similar to C. As described above, the high strength (high tensile strength) of the component is not an exception to the spring, but the element added essentially for the high strength is C. C is easy to add and improves the strength of the steel through the action of strengthening the solid solution or causing precipitation strength together with other alloying elements added together. However, when C is added to the alloy together with a large amount of Si, the two elements compete with each other by the similar thermodynamic behavior of C and Si, and as a result, decarburization occurs in which C is removed from the alloy.

Conventional Si-added spring steel, such as SUP7, the Si content in the steel for this spring reaches 1.8 to 2.0% by weight, the surface decarburization of C in the steel species becomes more severe, as a result Steel grades have a problem such as fatigue life degradation due to surface decarburization layer, which makes it difficult to use them in springs.

In order to solve this problem, by lowering the overall carbon content and adding Ni to prevent the presence of decarburized portion in the surface layer, further adjust the Si content to compensate for the decrease in strength as the carbon content decreases, Mo High-strength spring steels (Japanese Patent Application Nos. JP1998-110247 and JP1996-176737, Korean Patent Application No. 1997-0073576 and Korean Patent Application No. 1999-0048929) have been developed that further increase the maximum design stress to 1200 MPa.

However, it was pointed out that this development steel had Si content in order to improve yield strength and deformation resistance. Since the Si segregation zone is mainly formed in the center of the wire rod, the formation of such segregation zone is a major cause of ferrite formation and encourages nonuniformity of the central microstructure, and it is a major cause of large change in physical properties and deterioration of spring toughness. Cause.

In addition, the strength and toughness in general, it is another concept that is to be solved in the high-strength spring steel is difficult to secure them at a time because it is a concept of being disposed with each other. In other words, in order to improve the strength of the spring, it is essential to form a hard tissue such as martensite or bainite inside the steel, and such a hard tissue such as martensite or bainite is weak in general. This is because the impact toughness is poor due to its brittle characteristics.

Accordingly, an object of the present invention is to provide a spring having both high strength and high toughness and a method of manufacturing the spring.

Suspension spring, which is one aspect of the present invention for achieving the above object, in weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01 to 1.0% or less, Cu: 0.01 to 1.0% or less, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.005 to 0.02 It is characterized by having a composition containing%, the balance Fe and other unavoidable impurities, the spherical austenite grain size of the internal structure is 5 ~ 30㎛.

At this time, it is more preferable that the composition of the spring further includes V: 0.5% or less and Ti: 0.5% or less by weight.

Another aspect of the present invention is a method for producing a suspension gas spring, in weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01 ~ 1.0% or less, Cu: 0.01 ~ 1.0% or less, B: 0.005 ~ 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.005 ~ 0.02% Performing a spring forming after the austenitic treatment of the steel wire having a composition including the balance Fe and other unavoidable impurities at a temperature of 900 to 1000 ° C .; Oil-cooling; And a step of soaking at 350 to 450 ° C.

At this time, it is more preferable that the composition of the steel wire further include V: 0.5% or less and Ti: 0.5% or less by weight.

Hereinafter, the present invention will be described in detail.

In general, tensile strength and impact toughness, which are the most important properties in the development of spring steel, exhibit opposite properties. Therefore, it is the most important development goal to maximize the opposite impact toughness while keeping the tensile strength value at a minimum. Therefore, the composition of the spring of the present invention described below is a composition capable of maximizing the impact toughness while maintaining a high tensile strength.

The inventors of the present invention control the composition of the steel wire as described below, in order to realize the technical idea, the acid / carbon of Al, B, V and Ti in the inside when manufacturing the steel wire having the following composition as a spring / Nitride-based precipitates were formed to secure strength and toughness, while at the same time, annealing enhancement element B was used to enhance hardenability and grain boundary strengthening to improve high strength and toughness simultaneously.

Hereinafter, the component system which comprises the spring of this invention is demonstrated.

C: 0.4-0.7 wt%

C is an essential element added to secure the strength of the spring. When the content of C is less than 0.4% by weight, hardenability is not secured, and thus the strength required for the spring cannot be secured. In addition, when the C content is more than 0.7% by weight, the fatigue strength is significantly reduced because twin martensite structures are formed during the hardening and annealing treatment to cause material cracks. In addition, it is preferable to limit the C content to 0.4 to 0.7% by weight because it is difficult to secure sufficient toughness due to the high strength and suppress the material decarburization caused by high Si addition.

Si: 1.5-3.5 wt%

Si is dissolved in ferrite and has the effect of strengthening the base material strength and improving the deformation resistance. By the way, when the Si content is less than 1.5% by weight, the lower limit of Si needs to be limited to 1.5% by weight because Si is not sufficiently effective in solid solution in the ferrite to strengthen the base material strength and improve the deformation resistance. In addition, when the Si content exceeds 3.5% by weight, the effect of improving the deformation resistance is saturated, so that the effect of addition may not be obtained, and the content of Si is limited to 1.5 to 3.5% by weight because it promotes surface decarburization during heat treatment. It is desirable to.

Mn: 0.3 ~ 1.0 wt%

Mn is an element that is beneficial to secure strength by improving the hardenability of steel when present in steel. Therefore, when the Mn content is less than 0.3% by weight, it is difficult to obtain sufficient strength and hardenability required as a material for high strength springs. On the contrary, when the Mn content exceeds 1.0% by weight, the toughness decreases, so the Mn content is 0.3 to 1.0% by weight. It is desirable to limit to%.

Cr: 0.01 ~ 1.5 wt%

Cr is a useful element for securing oxidation resistance, temper softening, surface decarburization prevention and quenching. However, when the Cr content is less than 0.01% by weight, it is difficult to secure sufficient oxidation resistance, temper softening property, surface decarburization and quenching effect. In addition, when the content exceeds 1.5% by weight, the deformation resistance may be lowered, which may lead to a decrease in strength. Therefore, the addition amount of preferable Cr is 0.01 to 1.5 weight%.

Ni: 0.01-1.0 wt%

Ni is an element added to improve the hardenability and toughness. If the content of Ni is less than 0.01% by weight, the effect of improving the hardenability and toughness is not sufficient. If the content of Ni is more than 1% by weight, the residual austenite content is increased to reduce the fatigue life. Since it causes an increase in the amount of addition, it is necessary to limit the amount to 0.01 to 1% by weight.

Cu: 0.01 ~ 1.0 wt%

The addition of Cu is effective for preventing decarburization and improving corrosion resistance. The decarburized layer significantly reduces fatigue life after spring processing. This effect is weak at less than 0.01% by weight, and more than 1.0% by weight is likely to cause rolling defects due to embrittlement.

B: 0.005 to 0.02 wt%

The addition of B has an effect of densifying rust generated on the surface, increasing corrosion resistance and enhancing grain boundary strength by improving hardenability. If less than 0.005% by weight, the hardenability is not secured, and thus the strength required for the spring cannot be secured. If it exceeds 0.02% by weight, the carbonitride-based precipitates are coarsened, which adversely affects the fatigue characteristics.

O: 0.0015 wt% or less

The content of O is limited to 0.0015% by weight or less. When it exceeds 0.0015% by weight, oxide-based nonmetallic inclusions are coarsened, and fatigue life is drastically reduced.

Al: 0.1 wt% or less

The addition of Al refines the grain size and improves toughness. When the Al content exceeds 0.1% by weight, the amount of oxide precipitates is increased and the size thereof is coarsened, which adversely affects the fatigue characteristics.

P and S: 0.02% by weight or less

The content of P and S is limited to 0.02% by weight or less. Since P segregates at grain boundaries to reduce toughness, the upper limit thereof is limited to 0.01% by weight. It is preferable to limit the upper limit to 0.02% by weight since it has a detrimental effect on the spring characteristics.

N: 0.005 to 0.02 wt%

The content of N is precipitated by Al, V, Ti and the like and carbonitrides to have the effect of grain refinement and precipitation strengthening. If it is less than 0.005% by weight, the carbonitride precipitate may not be sufficiently precipitated, and thus the strength and toughness required for the spring may not be secured at the same time. In addition, if the content exceeds 0.02% by weight, large nitrides such as BN may be precipitated by reacting with B added for grain strengthening, thereby reducing the strength and toughness of the spring at the same time. Therefore, the upper limit is limited to 0.02% by weight. desirable.

A sufficient effect can be obtained only by the above composition, but in addition to the advantageous spring composition, the strength and toughness of the steel can be further improved by adding V and Ti as necessary as described below.

V: 0.5 wt% or less, Ti: 0.5 wt% or less

The V or Ti is one of the more preferable elements of the spring composition of the present invention, as an element for improving the spring characteristics by forming a carbon / nitride by a single or complex addition to cause precipitation hardening action, the content of each 0.5 weight It is limited to the range of% or less and 0.5 weight%. If the content is excessively high, the manufacturing cost rises sharply, the spring property improvement effect due to the precipitate is saturated, and the coarse alloy carbide which does not dissolve in the base material increases during austenite heat treatment, thus acting as a nonmetal inclusion. Fatigue characteristics and precipitation strengthening effect is reduced.

The above-mentioned composition is a composition suitable for the spring which is excellent in strength and toughness. That is, the composition not only improves the strength of the spring, but also allows the carbon / nitride-based precipitates of elements such as Al, B, V, and Ti to be formed in a large amount in the spring, thereby simultaneously obtaining a precipitation strengthening effect and a grain refinement effect. Both toughness and security can be secured. When a large amount of carbon / nitride-based precipitates of elements such as Al, B, V, and Ti are formed inside as a spring having the composition, the average grain size of the internal structure of the spring is determined to be about 5 to 30 μm based on the former austenite particle size. Springs with this particle size have excellent impact toughness.

Hereinafter, a method of manufacturing a spring having the above preferable conditions will be described in detail.

In order to manufacture a spring having an advantageous effect intended in the present invention, after producing a steel wire having the same composition as the above spring composition by a conventional wire rod manufacturing method, and performing austenite treatment at a temperature of 900 ~ 1000 ℃, this After the spring molding, it is preferable to use a method of oil-cooled and then treated at 350 ~ 450 ℃.

Each step condition of the manufacturing method will be described in more detail.

Since martensite is formed by austenite structure without diffusion diffusion by quenching, in order to form a hard structure, such as martensite, on a spring, an austenitization treatment is required in which all of the steel structure is transformed into austenite. At this time, the austenitization is preferably carried out at a temperature of 900 ~ 1000 ℃ to prevent the crystal grains from becoming too coarse by recrystallization. That is, when the austenitization treatment temperature is less than 900 ° C., it is not preferable that the superfine ferrite is generated during cooling because of its low temperature, and when it is above 1000 ° C., it is not preferable because it promotes decarburization and grain growth.

After that, the austenitic steel wire is preferably molded in a spring shape. This process is called hot forming, and the specific conditions of hot forming follow the conventional hot forming method.

The hot-formed steel material manufactured in the form of a spring is then subjected to quenching by quenching (oil-cooling) to transform the internal structure into a hard structure such as martensite.

Since the quenched spring is high in strength but martensite structure is not desirable for improving toughness, it is preferable that the soaking process is followed. The internal structure is changed from martensite to sour martensite by the soaking process.

Preferred consideration temperature is 350-450 degreeC. If the soaking temperature is less than 350 ° C., the martensite effect may not be sufficient, and the toughness of the spring may deteriorate. If the soaking temperature exceeds 450 ° C., the martensite may transform into a higher temperature structure. Therefore, it is preferable that the soaking temperature is 350-450 degreeC.

By the above-described preferred process it is possible to obtain a spring having both high strength and high toughness intended in the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

To cast steels having the composition as shown in Table 1 and to produce a steel strip, and then subjected to hot rolling to prepare a wire. Thus, the hot-rolled steel wire was processed into a spring shape, and then heat-treated at 950 ° C., then oil-cooled to perform heat treatment at a temperature of 390 ° C. and 420 ° C. to prepare a specimen.

Tensile test pieces were taken in the rolling direction and processed to ASTM-Sub size. Tensile tests were conducted at a cross head speed of 2 mm / min, and the detailed values are shown in Table 2. In addition, the impact value shown in Table 2 was the value measured using the Charpy test machine, and the notch conditions were 2 mm-U notch at this time.

division C Si Mn Ni Cr V Ti Cu Al P S B N Comparative Material 1 0.55 3.0 0.50 0.25 0.70 0.05 0.05 0.10 0.001 0.01 0.01 0.001 0.005 Comparative Material 2 0.50 2.2 0.50 0.25 0.70 0.20 0.20 0.10 0.01 0.008 0.008 0.03 0.006 Invention 1 0.55 2.2 0.50 0.25 0.70 0.20 0.20 0.10 0.03 0.008 0.008 0.007 0.006 Invention 2 0.50 2.2 0.70 0.30 1.00 0.20 0.20 0.30 0.06 0.009 0.007 0.007 0.006

※ The content unit of each component is weight%.

Consideration temperature division Tensile Strength (MPa) Elongation (%) Cross section reduction rate (%) Hardness (HRC) Impact value (Kg-m / cm ² ) 390 ℃ Comparative Material 1 2101 8.0 33.4 55.4 3.20 Comparative Material 2 2030 8.6 36.2 55.1 3.12 Invention 1 2167 8.1 37.7 55.5 4.08 Invention 2 2125 8.0 34.3 56.3 6.54 420 ℃ Comparative Material 1 1950 10 37 54.5 3.90 Comparative Material 2 1943 9.9 39.6 54.2 4.21 Invention 1 2021 10.5 39.2 54.6 5.20 Invention 2 2057 11.3 41.8 55.2 6.68

From the results of Table 2, the invention materials 1 and 2 according to the conditions of the present invention, although the tensile strength and impact toughness excellent in all the considered temperature range compared to the comparative materials 1 and 2, the elongation and cross-sectional area reduction rate is almost the same. Able to know. As a result supporting this, the photograph of FIG. 1 and the graph of FIG. 2 are shown. 1 and 2 are results of the spring produced by setting the soaking temperature to 420 ° C. In FIG. 1, in the case of Comparative Material 1 (a) and Comparative Material 2 (b), coarse grains of 40 μm or more are equivalent. In the case of the invention material 1 (c) and the invention material 2 (d), it can be seen that the fine austenite grains having a size of about 5 to 30 μm are formed in comparison with the partial formation.

In addition, even when viewing the average austenite grain size of each case shown in Figure 2 it was confirmed that the average grain size of the invention material 1 and the invention material 2 is significantly fine compared to the average grain size of the comparative material 1 and the comparative material 2. Such fine grain size contributes greatly to the toughness improvement of the steel.

Therefore, by adding elements such as Al, V, Ti, etc. as in the conditions of the present invention, their carbon / nitrides (Al (C, N), V (C, N) or Ti (C, N)) are formed to be tough. It can be confirmed that it can contribute to the improvement.

As described above, according to the present invention, it is possible to provide a spring having both high strength and high toughness and a method of manufacturing the spring by controlling the components and forming a fine precipitated phase therein and performing appropriate treatment.

Claims (4)

By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: Internal structure of 0.005 ~ 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.005 ~ 0.02%, balance Fe and other unavoidable impurities Suspension spring with excellent impact value, characterized in that the old austenite grain size of 5 ~ 30㎛. The suspension spring having excellent impact value according to claim 1, wherein the spring composition further comprises V: 0.5% or less and Ti: 0.5% or less by weight. By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: A steel wire with a composition containing 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.005 to 0.02%, balance Fe and other unavoidable impurities Forming austenite treatment and spring at a temperature of 900-1000 ° C .; Oil-cooling; And Soaking at 350˜450 ° C .; Method of producing a suspension spring excellent impact, characterized in that consisting of. The method of claim 3, wherein the composition of the steel wire further comprises V: 0.5% or less and Ti: 0.5% or less by weight.

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