KR950013193B1 - Low decorburizing & high toughness spring steel - Google Patents

Abstract

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Description

Low Telting Toughness Spring Molten Steel

1 is a graph showing the spring characteristics change according to the Si content change in the spring molten steel.

The present invention relates to a spring molten steel used in automobile suspension coils and leaf springs, and more particularly, according to the high strength while significantly reducing the decarburization layer generated during the hot working and the heat treatment operation to impart the spring characteristics of the spring molten steel The present invention relates to a low decarburized high toughness spring molten steel capable of preventing toughness deterioration.

The improvement of competitiveness by energy saving in the automobile industry is an urgent problem to save fuel by reducing the weight of automobiles. Among them, the suspension coil spring is used to reduce the weight of the automobile parts. It has been considered one of the high contributing parts.

As a means to reduce the weight of the coil spring, a material having excellent deformation resistance is required to prevent contact with the bumper due to the decrease in the permanent deformation (strain resistance or residual shear strain) of the coil spring due to the weight of the vehicle. In response to this trend, Si-added steel, which has higher strain resistance than SAE 6150 (Cr-V) alloy steel, which has been widely used, has been attracting attention. Among them, SAE 9260 (1.8-2.2% Si, SUP7) has excellent strain resistance. However, problems such as fatigue life reduction due to surface decarburization and surface processing cost due to surface decarburization in the manufacturing process were caused. In order to solve this problem, SAE 9254 has been developed in which silicon (Si), which promotes surface decarburization, is added and chromium (Cr), an element effective in preventing decarburization, has been developed in a component range that does not significantly reduce deformation resistance. However, due to the ever-increasing weight of automobiles, there is an urgent need for materials that have better strain resistance than existing spring steels.In response, a small amount of V is added to SAE 9254 steel grades, which is effective for strain resistance of springs. SRS 60 steel (Japanese Patent Publication 57-27956, Japanese 57-169062, Japanese 57-13148) has been developed, but the development of new steel grades is urgently needed in light of the increasing weight of automobiles. .

In this situation, in order to meet the trend of light weight of automobiles, in order to reduce the weight of suspension coil springs, spring materials with excellent deformation resistance are urgently needed, and therefore, high silicon-added steel is required. There is a need to solve the problems of decarburization occurring on the surface of the material during the hot working or heat treatment to impart the characteristics of the spring during the spring manufacturing process and the problems of deterioration of the toughness of the molten steel due to high strength.

Conventional techniques for suppressing the decarburized layer include Japanese Patent Laid-Open Nos. 2-301541, 1-31960, S. 63-216591, S. 63-153240, S. 58 -67847, (S) 58-27956, etc. are mentioned.

Japanese Patent Application Laid-Open Nos. 2-301541 and 63-153240 disclose a method of increasing the content of chromium (1.5-3.0%) or adding lead, sulfur, calcium, and the like. There is a disadvantage in that the deformation resistance characteristic of the spring decreases with the increase, and also in the case of Japanese Patent Application Laid-Open No. 62-274058 of a similar alloy component, there is a disadvantage in that it is not maximized in the amount of silicon effective for deformation resistance.

On the other hand, in (S) 63-216591 and (S) 58-67847, a method of adding copper, molybdenum, tin, antimony, arsenic, copper, etc. while lowering the carbon content has been proposed, but adding expensive alloying elements and deteriorating toughness There are disadvantages. Japanese Patent Application Laid-Open Nos. 1-31960 and 58-27956 disclose methods for lowering the content of silicon, but improvement of deformation resistance due to the decrease in the content of silicon is not expected.

As a conventional technique for improving toughness, Japanese Patent Laid-Open No. 3-2354 is mentioned.

Japanese Patent Application Laid-Open No. 3-2354 has had an effect of improving the toughness by adjusting the carbon content and adding nickel, but there is a problem in terms of improving the deformation resistance due to the decrease in yield strength due to the reduction of the carbon content. .

Accordingly, an object of the present invention is to solve the problem of material decarburization and toughness deterioration in the manufacturing process of the spring due to the addition of high silicon while maximizing the effect of the chemical composition (silicon) effective for the deformation resistance of the spring steel without reducing the carbon content Decarburized high toughness spring molten steel.

The present inventors have conducted in-depth studies on the effects of various elements on the spring characteristics (fatigue characteristics, strain resistance), toughness and decarburization. When nickel is added, the effects of chemical components (silicon) effective on the deformation resistance of spring steel It was able to solve the problem of material decarburization in the manufacturing process of the spring due to the addition of high silicon and maximized the decarburization and toughness of the molten steel. It was possible to produce new steel grades that could achieve the improvement of.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is in weight%, C (carbon): 0.5-0.7%, Si (silicon): 2.60-3.50%, Mn (manganese): 0.3-1.5%, Cr (chromium): 0.3-0.8%, V (vanadium ) And or Nb (niobium): 0.05-0.5%, P: 0.02% or less, S: 0.02% or less, Ni: 0.05-5.0%, balance Fe and other unavoidable impurities. .

Hereinafter, the reason for limitation of the said component and a component range is demonstrated.

The content of the carbon is 0.50-0.70% because the carbon content is less than 0.50%, it is difficult to secure sufficient strength as the high stress spring molten steel due to hardening and condensation. This is because it is difficult to suppress material decarburization derived from difficulty and high silicon content.

If the silicon content is less than 2.0%, the silicon content must be contained more than 2.0% because the silicon is not dissolved in the ferrite to strengthen the base material and improve the deformation resistance. In order to reduce the weight of the material, it is more preferable to contain more than 2.50%. Even more preferred, the silicon content of 2.60% or more may have excellent fatigue properties. However, when silicon is more than 3.5%, it is not preferable because the improvement effect of the deformation resistance is saturated and the possibility of decarburization during heat treatment is high.

The content of the manganese is selected as 0.3-1.5% because the strength and hardenability as spring molten steel is insufficient when the content of manganese is 0.3% or less, and the toughness is lowered when the content of manganese is 1.5% or more.

The content of chromium is 0.3-0.8% because the chromium content is less than 0.3% because it does not have sufficient quenching and decarburization effects, and at 0.8% or more, the deformation resistance is reduced.

The vanadium or niobium is an element added alone or in combination in order to improve the deformation resistance of the present invention, respectively, and the content of vanadium or niobium is selected as 0.05-0.5%, respectively, to improve the deformation resistance when the content is 0.05% or less. This is because the effect is not sufficient, and if it is 0.05% or more, the effect is saturated to increase the amount of coarse alloy carbides that are not dissolved in austenite, which acts like a nonmetallic inclusion, leading to a decrease in fatigue properties.

Since the phosphorus (P) is segregated at the grain boundary to reduce the toughness, the upper limit is preferably limited to 0.02%, and the sulfur (S) has a harmful effect on the spring characteristics by reducing the toughness and forming an emulsion, It is preferable to limit the upper limit to 0.02%.

The nickel is an element added in order to reduce the ferrite decarburization layer and improve the toughness in the present invention, the nickel content of 0.5-5.0% is selected when the content is 0.5% or less. This is because it is not sufficient, and if it is more than 5.0%, the additive effect is saturated, and the amount of retained austenite increases, which is detrimental to the fatigue characteristics.

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

Example 1

After casting the steel composition as shown in Table 1 as a sample and heated at 1200 ℃ for 2 hours, the hot rolling was carried out to a rolling finish temperature of 950 ℃, the rolling ratio was at least 70%.

A decarburization test was carried out using a test piece cut into the hot-rolled material as described above in a size of 20 × 30 × 10 mm, and the depth of the ferrite decarburized layer was measured. The results are shown in Table 2 below.

In addition, the hardness and impact value for each considered temperature were measured, and the results are shown in Table 2 below.

The decarburization test was heat-treated in the air at a heat treatment temperature of 900 ° C., 1000 ° C., and 1100 ° C. for 2 hours, respectively, and the furnace was cooled after the heat treatment to measure the depth of the ferrite decarburization layer.

The depth of the decarburized layer for the test piece was measured by the KS standard (KS D 0216). According to this standard, an optical microscope observation method and a microscopic hardness measurement method have been proposed. In the present invention, the depth measurement of the ferrite decarburized layer was used by the optical microscope observation method.

The heat treatment conditions for the evaluation of impact toughness were heat-treated at 850 ° C. for 30 minutes, followed by oil cooling, followed by heat treatment for 30 minutes in a salt bath for each soaking temperature (300 ° C., 400 ° C.). The hardness measurement was measured using a Rockwell (150 kg) hardness tester, the impact test was measured using a Charpy tester (notch condition was 2mm-U notch).

TABLE 1

(Unit: weight%)

TABLE 2

As shown in Table 2, the conventional material (1), which is currently being melted, is a steel having a silicon content of 1.5% level, and has a depth of 0.19-0.44mm of ferrite decarburized layer in the test temperature range and the impact toughness of 1.5-. Whereas 2.6kg / ㎠, in the case of the present invention (1-3) has a depth of the ferrite decarburization layer of 0.06-0.31mm, impact toughness having a distribution of 2.6-4.1kg-m / ㎠, In the case of the present invention material (1-3) it can be seen that the production of the ferrite decarburized layer is significantly suppressed, and also excellent in impact toughness while maximizing the silicon content as compared to the conventional material (1). On the other hand, the hardness value was similar, and the decarburized layer was slightly superior.

In addition, in the case of the comparative material (1-5), the silicon component range is 1.0-2.5% level, while in the case of the present invention material (1-3), the Si content effective for the spring characteristics is maximized to the level of 2.51-3.5%. It can be seen that it is possible to maximize the spring characteristics (strain resistance) and the spring material in comparison with the ash (1-5).

In addition, as shown in the case of the comparative material (6-7), when the silicon component exceeds 3.5%, the toughness is lowered and decarburization is intensified, so it can be seen that it is not preferable to add more than this.

As such, the improvement of material decarburization and toughness according to the maximization of the silicon component effective for the spring characteristics depends on the addition degree of nickel, and as shown in Table 2 above, in particular, the invention having a silicon content of more than 2.50% (1--1). In the case of 3), the characteristics of spring are the best and it shows that decarburization and toughness of the same level or higher than those of conventional materials (1) and comparative materials (1-7) solve the problems of manufacturing process during spring molten steel and spring manufacturing. Can be.

Example 2

Using a steel grade having the same composition as Comparative materials (1-4) (6) (7) and Inventive Materials (2-3) of Example 1, a wire rod having a diameter of 13 mm having a different silicon content was produced, and then a diameter of 11 mm and a height. A coil spring of 355 mm and an effective number of turns 3.69 was produced. The spring properties (residual shear strain, fatigue properties) were measured for each spring manufactured, and the results are shown in FIG. At this time, the residual shear strain was measured after maintaining 72 hours at room temperature under a stress of 130kg / ㎜, and the fatigue test was carried out six times per steel grade with a test stress of 93 ± 36kg / ㎜ and then averaged.

As shown in FIG. 1, as the silicon component increases in the spring molten steel, the residual shear strain becomes smaller. In particular, when the silicon content exceeds 2.50%, there is almost no change in the residual shear strain, which is the content of silicon in the spring steel. This is in accordance with the results of Example 1 in which formation of the ferrite decarburized layer is suppressed by addition of Ni.

On the other hand, the fatigue characteristics test results for the spring, the silicon content of the spring steel in the range of 2.60-3.50% fatigue life is more than 400,000 times, it can be seen that the maximum fatigue characteristics when containing about 3% Si. As a result, the spring molten steel of the present invention can be seen that not only the strain resistance is extremely excellent but also the fatigue life through the maximization of the silicon component.

As described above, it is expected to improve the deformation resistance with a higher silicon content than the conventional steel of the present invention. Since it can be significantly reduced, there is an effect that can produce an improved spring molten steel that can suppress the ferrite decarburization layer generated in the manufacturing process of high strength spring molten steel and achieve an improvement in toughness.

Claims (1)

By weight%, C: 0.5-0.7%, Si: 2.60-3.50%, Mn: 0.3-1.5%, Cr: 0.3-0.8%, V and Nb: 0.05-0.5%, P: 0.02% or less, S: 0.02 Low molten carbon toughness spring molten steel, characterized by the composition of% or less, Ni: 0.5-5.0%, balance Fe and other unavoidable impurities.