KR100325706B1 - High stress steel for suspension spring and manufacture method thereof - Google Patents

Abstract

In the present invention, the chemical composition is 0.4-0.6% carbon, 2.8-4.0% silicon, 0.1-0.3% manganese, 0.3-0.6% chromium, 0.0015% or less oxygen, 0.005-0.01% nitrogen, 0.01% or less sulfur. Low alloy type high stress spring steel, wherein the vanadium or niobium 0.01-0.1%, nickel 0.1-0.3% is added one or two kinds, and is composed of the balance Fe and other unavoidable impurities. Austenitic heat treatment of the chemical composition of the steel is a low alloy high stress spring, characterized in that the heating in a temperature range of 900-1050 ℃ and maintained for 10-60 minutes and then oil-cooled and tempered 30-60 minutes in the range 350-430 ℃ It is a main idea to provide a method for producing molten steel.

Description

Low alloy high stress spring steel and its manufacturing method

The present invention relates to a spring steel used in automobile suspension coils and leaf springs, and more particularly, to a high stress spring steel capable of low alloying.

In the early '90s, the severity of air pollution rose around the world, and oil spills from other large tankers began to raise the voice of concern in the United States, reducing the consumption of oil and emissions. Efforts have been made to reduce the weight of automobiles. Among them, suspension springs are one of the components that contribute to the reduction of weight.

Against this backdrop, the high stress spring steels with the maximum design stress of 130kg / mm ² (25% lighter than ordinary materials) have been developed and put into practical use. This is in pursuit of price priority considering manufacturing cost in light weight priority that ignores manufacturing cost. Therefore, in reality, it is necessary to pursue spring weight reduction, but it can be considered that it is impossible to use it if it is not competitive in terms of price compared to conventional materials.

Problems in the commercialization of the high-stress spring steel developed in advance include the high cost of manufacturing due to the high alloying in terms of alloy design, and the low temperature structure due to the lack of slow cooling ability in the manufacture of wire rods due to high alloying (bainite + martensite complex). Tissue) occurs. When such a structure is formed, the surface hardness of the wire is increased, and thus, a peeling process for adjusting the wire diameter and improving the surface quality of the wire before forming the spring (a process for improving the surface quality of the wire and adjusting the wire diameter for each user's use) ), Softening heat treatment should be inevitably given to secure the surface hardness required for peeling processing. This means that the additional cost of heat treatment increases the manufacturing cost, which leads to a decrease in price competitiveness when commercialized.

Based on these backgrounds, it is possible to reduce the weight of the spring compared to conventional materials (SAE 9254, spring design maximum stress 110 kg / mm2) and cost-competitive low cost type compared to previously developed high stress spring steel (spring design maximum stress 130kg / mm2). The need for the development of spring steel has begun to emerge.

Conventional techniques for high stress materials include U.S. Patent Nos. 5575973A, 4795609 A, German Patent Publications EP 265 273 A2, Japanese Patent Application Laid-Open Nos. 5-59431, 4-4-88123, 44-247824, Hei 1-184259, So 64-39353.

The U.S. Patent No. 5575973A contains a large amount of silicon components effective for the spring characteristics, solves the problem of decarburization in the manufacturing process according to the high silicon content by adding nickel, and achieves high spring stress by the precipitation strengthening effect by the addition of vanadium. However, there is a problem of price increase due to the addition of high alloy to existing materials. In U.S. Patent No. 4795609A, molybdenum and vanadium were added to distribute stable precipitates at high temperature, and thus, the spring stress was improved by improving the permanent strain resistance effect, the toughness improvement by the addition of nickel, and the cold forming. , German Patent Publication No. EP 0 265 273 A2, Japanese Patent Publication No. Hei 5-59431, Hei 4-88123, Hei 1-184259, So 64-39353, So 60-89553 Although it is possible, however, the manufacturing cost is increased compared to the existing material even if the spring weight is achieved by high alloy treatment due to the characteristics of the alloy component.

The present invention has been made to solve the above problems, the present inventors in the manufacture of the design maximum stress 130kg / mm2 class high stress spring steel capable of reducing the weight of the spring 30% high stress by low alloy design without deterioration of the spring characteristics As a result of various studies on the method for producing spring steel, when the silicon content is added 2.8-4.0%, the addition of trace alloy elements and expensive alloy elements is omitted while satisfying the spring characteristics required for high spring stress. Another object of the present invention is to propose a method for producing a low alloy type high stress spring steel.

In order to achieve the above object, the present invention has a chemical composition of 0.4-0.6% carbon, 2.8-4.0% silicon, 0.1-0.3% manganese, 0.3-0.6% chromium, 0.0015% or less oxygen, 0.005-0.01% nitrogen, Low alloy type high stress spring steel containing 0.01% or less of phosphorus and sulfur, wherein 0.01-0.1% of vanadium or niobium, 0.1-0.3% of nickel are added, and are composed of balance Fe and other unavoidable impurities. It characterized in that to provide.

Hereinafter, the present invention will be described in detail.

The chemical composition of the present invention is 0.4% by weight of carbon, 2.8-4.0% of silicon, 0.1-0.3% of manganese, 0.3-0.6% of chromium, 0.0015% or less of oxygen, 0.005-0.01% of nitrogen, 0.01% or less of sulfur and sulfur. Containing 0.01-0.1% vanadium or niobium, 0.1-0.3% nickel and one or two kinds thereof, and relates to a method for producing a low-alloy type high stress spring steel composed of remainder Fe and other unavoidable impurities will be.

Hereinafter, the reason for limitation of the component and the component range will be described.

When the carbon content is 0.4-0.6% by weight, the required strength, fatigue characteristics, permanent deformation resistance, precipitate control, residual austenite content, and machinability of the spring steel by quenching and annealing at 0.4% or less. This is because it is difficult to secure graphitized structure for improvement, and it is difficult to secure the wire surface hardness suitable for peeling process due to difficulty in securing toughness due to high strength and increase of ferrite fraction during wire cooling at 0.6% or more. This is because it is difficult to avoid the occurrence of hardening cracks.

The silicon content of 2.8-4.0% is less than 2.8%, which makes it difficult to secure spring characteristics, namely fatigue characteristics and permanent deformation resistance, as low alloying high stress spring steel, and reduce nickel and chromium elements, which are decarbonization inhibitors. In case of wire rod heating, the surface decarburization is likely to be intensified, and low alloy type high stress spring steel is produced due to difficulty in controlling residual austenite decomposition time, temper brittleness control, and precipitates (epsilon cementite, vanadium or niyoboom-based precipitates). This is because it is impossible to secure the spring characteristics, the softening rate of the structure decreases during softening heat treatment to improve the cutting process, and the softening heat treatment is inevitable due to the decrease of ferrite fish rate during wire cooling, which increases the wire hardness. At 4.0% or more, there is a disadvantage that high surface defects occur during steelmaking and heat treatment time is required for a long time due to spheroidization or graphitization solid-solution delay in heat treatment to impart spring characteristics. This is because the compression residual stress distribution affects the shot peening to improve the fatigue characteristics.

The more preferable silicone component range in the present invention is 3.2-3.6%, which is the critical shooter section of the component phase showing the maximum value in this component range in the case of fatigue characteristics, and in the case of permanent strain resistance (resistance shear strain), it is continuously It is effective in improving the strength of the base material, the distribution of precipitates (epsilon carbide precipitates, vanadium or niyobul precipitates), surface decarburization, ferrite formation time, fatigue characteristics, permanent deformation, machinability according to deep processing, temper brittleness, residual austerity This is because the nit decomposition time, surface defects during steelmaking, and graphitization fraction during heat treatment can be controlled very effectively.

The content of manganese (Mn) in the range of 0.1-0.3% is less than 0.1% because it has no effect of quenching, deoxidation, and solid solution, and in 0.3% or more, when wire rod is cooled due to expansion of austenite region (raising pearlite transformation temperature). It affects the cornerstone ferrite fraction, which makes it difficult to control the wire surface hardness, and the tissue heterogeneity caused by manganese segregation is more harmful to the spring characteristics than the solid solution strengthening effect. Therefore, limiting the content of manganese to 0.1-0.3% takes into account the strength, deoxidation, solid solution strengthening, microstructure control of wire rod cooling, spring characteristics (fatigue characteristics, permanent deformation resistance), and harmful effects of segregation zones. Range.

The content of chromium (Cr) in the range of 0.3-0.6% is less than 0.3% in improving the corrosion resistance, it is difficult to control the surface decarburization during heat treatment that imparts the spring characteristics, and it is difficult to control the cementite thickness of pearlite. This is because it affects the visualization time. If it is more than 0.6%, it is difficult to secure the strength by increasing the softening rate of the microstructure, and it is detrimental to the spring characteristics due to early deposition of cementite during tempering.

Vanadium (V) or niobium (Nb) is an element that improves the spring characteristics due to precipitation hardening, and its content is 0.01-0.1%, and the vanadium and niobium-based precipitates are distributed less than 0.01%. (Strain resistance) is not sufficient, the effect of improving the spring characteristics due to precipitates is more than 0.1%, and the coarsened alloy carbide that does not dissolve in the base material increases during austenite heat treatment, such as non-metal inclusions Because of this, the fatigue characteristics are lowered.

The Ni content of 0.1-0.3% makes it difficult to improve the corrosion fatigue resistance under fatigue load at 0.1% or less, and improves decarburization control, toughness and cold workability, residual austenite control, It is also because there is no graphitization promoting effect during the graphitization heat treatment to improve the machinability. Above 0.3%, it is difficult to control the surface hardness of wire rod because it affects the formation of cornerstone ferrite during wire cooling, and the ferrite layer formation temperature on the surface to prevent decarburization is increased by increasing the ferrite layer formation temperature to prevent decarburization. This is because it has a harmful effect on.

The content of oxygen (O) is 0.0015% or less because coarse oxide-based nonmetallic inclusions are easily formed at 0.0015% or more, and fatigue life is reduced.

The content of nitrogen (N) in the range of 0.005-0.01% is because carbide production rate due to the formation of vanadium and niobium-based nitride is very small at 0.005% or less, and the effect is saturated at 0.01% or more.

The content of phosphorus (P) and sulfur (S) of 0.01% or less is limited to 0.01% because phosphorus segregates at grain boundaries and degrades toughness, and sulfur is segregated at grain boundaries as a low melting point element. It is preferable to limit the upper limit to 0.01% because it lowers and forms an emulsion, which adversely affects the spring characteristics.

The reason for limiting heat treatment conditions (hardening, consideration) imparting spring characteristics in the present invention is as follows.

Limiting the austenite temperature and time to 10-15 minutes in the 900-950 ° C range is not sufficient austenitization below 900 ° C, making it difficult to secure sufficient martensite structure during quenching, and coarse precipitated vanadium during wire fabrication. This is because it is difficult to expect the employment strengthening effect due to the difficulty in reusing the base or niobium-based precipitates in the base metal. Above 950 ° C, the vanadium-based or niobium-based precipitates that are coarsened during wire rod manufacturing are fully employed in the base material, and the employment strengthening effect is expected. This is because the precipitation strengthening effect cannot be expected with full employment of precipitates. On the other hand, the heat treatment time is limited to 10-15 minutes because complete austenitization is possible within this range, and productivity deterioration is caused by decarburization promotion and increase of rework time by heating over 15 minutes.

Limiting the tempering temperature and time from the range of 350-430 ° C to the range of 60-90 minutes is difficult to secure toughness, proper amount of retained austenite (4-7%), fatigue characteristics, etc. at 350 ° C or lower, and permanent deformation resistance. It is difficult to expect the effect of imparting a pre-setting for improvement, and the distribution of epsilon carbide effective for the spring characteristics is indiscriminate, leading to deterioration of the permanent deformation resistance. At 430 ℃ or higher, the toughness is lowered and the toughness is lowered.It is difficult to expect the effect of improving fatigue characteristics due to the complete decomposition of residual austenite produced during quenching, and it is possible to reduce the yield strength and decrease the permanent deformation resistance due to spheroidal cementation. Because it causes. On the other hand, the heat treatment time is a time in consideration of workability.

Next, the present invention will be described in detail with reference to Examples.

(Example)

Table 1 shows the chemical composition of the present invention and the comparative materials for the effect of the present invention. The present invention materials were cast in 50kg ingot and the steels of Table 1 were homogenized and heat treated at 1250 ° C. for 48 hours, and then hot forged into a cross section of 60 × 60 mm. At this time, the finishing temperature was 950 ° C. or higher, followed by air cooling. Thereafter, after maintaining the heating temperature at 1100 ° C. for 3 hours, hot rolling was performed using a round bar having a diameter of 13 mm.

Heat treatment for imparting the spring characteristics of the present invention was heated for 20 minutes in the range of 930 ℃ and hot-cooled after hot forming the spring to the spring design specifications as shown in Table 2, and then tempered at 390 ℃. After the shot peening warm using a steel ball of 0.8mm size at 250 ℃ or more, and then cold shot peening using 0.6mm steel balls. After the cold compression (cold setting) to a stress of 140kg / mm 2 and then to the coating to prepare a spring.

Fatigue tests were performed on the springs prepared as described above under the conditions shown in Table 3 below, and the fatigue properties were evaluated. The test speed was 1.3 Hz, and the fatigue life values were evaluated by performing ten tests per steel grade.

The calculation of the spring test stress is as follows.

τ = (8PD / πd3) K

τ: Spring test stress (kg / mm2)

P: Load Load (kg)

D: mean diameter of coil (mm)

d: wire diameter (mm)

K: stress correction coefficient of coil

Where K = [(4C-1) / (4C-4)] + [0.615 / C], (C = D / d)

The residual shear strain measurement test was carried out under the test conditions of Table 4 above, and measured after 72 hours at room temperature under the measured stress. The high temperature residual shear strain measurement temperature was tested at 80 ℃.

The criterion for measuring the residual shear strain is based on the amount of load change required when the spring is compressed to the same free height (spring height) before and after the test (△ P: load before test-load after test). The calculation is made as follows.

γ = (8D / πd ³ G) △ P

γ: residual shear strain

D: diameter of coil (mm)

d: wire diameter (mm)

G: Lateral modulus of elasticity (8000kg / mm2)

ΔP: Load reduction (kg)

Fatigue characteristics and permanent strain resistance (residual shear strain) were measured for the springs of the inventive materials and the comparative materials manufactured as described above, and the results are shown in Table 5. As shown in Table 5, the present invention materials are low alloyed steels and exhibit spring characteristics equal to or greater than the fatigue life and the permanent strain resistance (residual shear strain) compared to the comparative materials. On the other hand, as the silicon content increases, the residual shear strain tends to be gradually improved, and in the case of the fatigue properties, it can be seen that the most excellent fatigue life value can be obtained when the silicon content is in the range of 3.2-3.6%.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

In view of the above results, the present invention can provide a low alloy type high stress spring steel bar that can achieve a significant cost reduction in terms of manufacturing cost by achieving a high stress of the spring alloy system composed of a low alloy system. It is.

Claims (3)

Chemical composition contains 0.4-0.6% of carbon, 2.8-4.0% of silicon, 0.1-0.3% of manganese, 0.3-0.6% of chromium, 0.0015% of oxygen, 0.005-0.01% of nitrogen, 0.01% of phosphorus and sulfur , 0.01-0.1% vanadium, 0.1-0.3% nickel is added one or two kinds, low alloy type high stress spring steel, characterized in that the composition is composed of the remaining Fe and other unavoidable impurities. Chemical composition contains 0.4-0.6% of carbon, 2.8-4.0% of silicon, 0.1-0.3% of manganese, 0.3-0.6% of chromium, 0.0015% of oxygen, 0.005-0.01% of nitrogen, 0.01% of phosphorus and sulfur , Where one or two of 0.01-0.1% vanadium and 0.1-0.3% nickel are added, and the austenitic heat treatment of steel composed of the balance Fe and other unavoidable impurities is heated in a temperature range of 900-950 ° C. and 10-15 The process for producing a low alloy high stress spring steel, characterized in that the oil is cooled and then tempered for 60-90 minutes in the range of 350-430 ° C. The low alloy high stress spring steel according to claim 1, wherein the silicon content is 3.2-3.6% by weight.