KR100352607B1 - Manufacturing method of high stress spring steel wire - Google Patents

Abstract

The present invention relates to a method for manufacturing a high stress spring steel wire, the object of which is to control the surface decarburization layer of the spring steel by appropriately controlling the conditions during heating for wire rod rolling of high silicon added steel It is to provide a manufacturing method.

The present invention for achieving the above object is by weight, carbon: 0.4-0.6%, silicon: 2.8-4.0%, manganese: 0.1-0.3%, chromium: 0.3-0.6%, oxygen: 0.0015% or less, nitrogen: 0.005 -0.01%, phosphorus: 0.01% or less, sulfur: 0.01% or less, wherein one or two selected from vanadium: 0.01-0.1%, niobium: 0.01-0.1%, and nickel: 0.1-0.3% are added , The remainder Fe and other unavoidable impurities are heated to a heating rate of 15 ± 5 ° C./minute to Ac1 (ideal phase transformation start temperature), and the range of Ac1 to Ac3 in an ideal range of 6 ± 3 ° C./minute After heating at a heating rate, and then heated to a heating rate of 10 ± 5 ℃ / min to 1050 ± 50 ℃, maintained for 30-60 minutes, the wire rod rolling to a diameter of 16 mm or less in the method of manufacturing a high stress spring steel wire Let that point be about that.

Description

Manufacturing method of high stress spring steel wire

The present invention relates to a high stress spring steel wire for use in automotive suspension coils and leaf springs, torsion bars, stabilizers, etc. More specifically, the improved high silicon-added high stress spring steel wire for reducing decarburization on the wire surface It relates to a manufacturing method of.

In the early 1990s, as the seriousness of air pollution rose around the world, and oil spill accidents of large tankers continued, voices of concern began to rise worldwide. To reduce oil consumption and emissions, the automotive industry Efforts have been made to reduce the weight of automobiles. Suspension spring is one of the components that contribute to the weight reduction, so spring weight reduction has emerged as an urgent problem.

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.However, the problem in the practical application of the developed high stress spring steel is alloy. In terms of design, it is known that the reason for pursuing high alloying is that the manufacturing cost is high and that low-temperature structure (bainite + martesite complex structure) occurs due to the lack of slow cooling ability when wire rod is produced by high alloying. When the low temperature structure is formed as above, the surface hardness of the wire is increased, and thus, the peeling processing for improving the wire diameter and surface quality before the spring molding (improving the surface quality of the spring wire rod and adjusting the wire diameter for each user's use). Softening heat treatment must be inevitably given to secure the surface hardness required for peeling. This means that manufacturing cost increases due to additional heat treatment costs, and there is a problem that the price competitiveness is lowered during the practical use.

Based on these backgrounds, it is possible to reduce the weight of the spring compared to conventional materials (SAB9254, spring design maximum stress 110kg / mm ² ) and at low cost with competitive cost than developed high stress spring steel (spring design maximum stress 130kg / mm ² ). The need to develop (low alloy) spring steel has begun to grow significantly.

Conventional techniques for high stress materials include US Patent Publications US005575973A, US004795609A, German Patent Publication EP0265273 A2, Japanese Patent Publications (Pyung) 5-59431, (Pyung) 4-88123, (Pyung) 4 -247824, (Pyeong) 1-184259, (S) 64-39353, and the like.

The U.S. Patent Publication No. US005575973A contains a large amount of silicone components effective for spring characteristics, solves the problem of decarburization in the manufacturing process due to the high silicon content by adding nickel, and achieves high stress of the spring by 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. The U.S. Patent Publication US004795609A adds molybdenum and vanadium components to distribute stable precipitates at high temperatures, thereby improving the stress resistance and cold forming properties, especially the permanent deformation resistance effect among the spring properties, and the formation of high stress by the addition of nickel. German Patent Publications EP 0265273 A2, Japanese Patent Publications (Pyeong) 5-59431, (Pyeong) 4-88123, (Pyeong) 4-247824, (Pyeong) 1-184259, (Small) 60-89553 is also possible to increase the stress of the spring, but due to the characteristics of the alloy composition system, even if the weight of the spring is achieved by high alloy treatment, the manufacturing cost is increased compared to the existing material.

On the other hand, the most important characteristics in automotive spring steel are classified into fatigue characteristics and permanent deformation resistance, and the degree of surface decarburization layer of wire rod adversely affects the fatigue characteristics of spring final products. This is because there is almost no shot peening effect in the surface decarburized layer which gives surface compressive residual stress for the purpose of improving fatigue characteristics. In addition, according to the degree of surface decarburization during wire rolling, the possibility of surface grooves during rolling is very high, and there is a problem that the amount of processing is inevitably increased during surface processing (peeling) before spring production.

Among the low alloy type high stress spring steels, high silicon-added steel is very advantageous in terms of spring characteristics and price, but the surface decarburization is inevitably deepened when wires are manufactured because the silicon content, which is the surface decarburization promoting element during heating, is inevitably increased. As a result, a significant amount of metal loss occurs in a peeling process, which is a surface cutting process that is given to improve surface quality during spring manufacturing. In addition, in recent years, in view of simplifying the process for spring steel, there is an urgent situation in which the thickness of the spring wire decarburizing layer should be further reduced due to the omission of the conventional surface processing process or the reduction of the processing amount.

Therefore, if the wire surface decarburization layer can be improved during the manufacture of wire rod for high silicon-added spring steel, it is possible to secure both the price side and the excellent spring characteristics which are advantages of the high silicon-added steel.

Conventional techniques for suppressing decarburization of spring steel wire rods include Korean Patent Publication Nos. 92-24974, 92-24163, 92-24161, Japanese Patent Publications (Pyeong) 2-301514, (Pyeong) 2-301514 (Pyeong) 1-31960, (S) 63-26591, etc. are mentioned.

In the above-mentioned Korean Patent Publication Nos. 92-24974 and 92-24161, nickel is added to suppress decarburization and increase the amount of silicon, which is a decarburization promoting element, to improve the spring characteristics very effectively, but the manufacturing cost increases due to the addition of nickel. In the Republic of Korea Patent Publication No. 92-24163, the addition of lead, tin has significantly suppressed the occurrence of surface decarburization, but has a disadvantage in that high temperature impact toughness is lowered.

In Japanese Patent Publication No. 2-301514, a method of increasing the content of chromium to 1.5-3.0% or adding lead, sulfur, calcium, etc. has been proposed, but the increase in chromium has a permanent deformation resistance of the spring. The disadvantages of deterioration and the addition of lead, sulfur, calcium, etc. are effective in improving decarburization but have the disadvantage of deteriorating impact toughness. However, Japanese Patent Laid-Open No. 1-31960 suggests a method of lowering the content of silicon. It is difficult to expect the improvement of spring permanent strain resistance by decreasing the content of. In addition, Japanese Patent Publication No. 63-216591 discloses a method of adding copper, molybdenum, tin, antimony, and arsenic while lowering carbon content, but there are disadvantages of adding expensive alloying elements and reducing toughness. .

Accordingly, the present invention is to solve the problems of the prior art, the present invention can control the surface decarburization layer of the spring steel by appropriately controlling the conditions during heating for wire rods of high silicon added steel To provide a manufacturing method, the object is.

1 is a cross-sectional optical micrograph of a wire rod obtained by satisfying the conditions of the present invention

In the present inventors, in the manufacture of a design maximum stress 130kg / mm class ² high stress spring steel, which can reduce the weight of the spring by 30%, with respect to a method that can produce high stress spring steel by low alloy design without deterioration of the spring characteristics, When the silicon content is added 2.8-4.0%, it is proposed to reduce or considerably reduce the addition amount of trace alloy element and expensive alloy element while satisfying the spring characteristics required for high stress of spring (Korea Patent Publication) 97-73725), and based on this, the present inventors conducted a multi-angle study to reduce the decarburization of high-silicon-added spring steel wire, resulting in abnormality of the ferrite during the heating of the billet for the wire rod (heating of the wire furnace). When the temperature increase rate passing through the austenite mixture is maintained at 6 ± 3 ℃ / min from 15 ± 5 ℃, the carbon solubility of the billet surface is very high. The silver ferrite layer can be induced uniformly to a thickness of 0.2-0.4mm, and since the surface ferrite layer has very low carbon solubility, diffusion of carbon atoms through this region can hardly be expected because of the very low carbon solubility. We found that we could significantly reduce the speed.

Starting from the above point of view, the present invention is in terms of weight%, carbon: 0.4-0.6%, silicon: 2.8-4.0%, manganese: 0.1-0.3%, chromium: 0.3-0.6%, oxygen: 0.0015% or less, Nitrogen: 0.005-0.01%, phosphorus: 0.01% or less, sulfur: 0.01% or less, wherein one or two selected from vanadium: 0.01-0.1%, niobium: 0.01-0.1%, nickel: 0.1-0.3% Is added, and the steel billet composed of the balance Fe and other unavoidable impurities is heated to Ac1 (ideal phase transformation start temperature) at a heating rate of 15 ± 5 ° C / min, and the range of Ac1 to Ac3, which is an ideal range, is 6 ± 3 ° C. Of the high stress spring steel wire, which is heated to a heating rate of 10 minutes per minute, and then heated at a heating rate of 10 ± 5 ° C./minute to 1050 ± 50 ° C. for 30 to 60 minutes, and then the wire is rolled to a diameter of 16 mm or less. It relates to a manufacturing method.

Hereinafter, the present invention will be described in detail. First, the reason for limitation of the components and the range of components for showing the effects of the present invention will be described.

The content of carbon (C) is 0.4-0.6%, but less than 0.4% improves the required strength, fatigue properties, permanent deformation resistance, precipitate control, residual austenite amount, and machinability as spring steel by hardening and thinning. This is because it is difficult to secure graphitized tissue for the purpose.If it exceeds 0.6%, it is difficult to secure the wire surface hardness suitable for peeling due to difficulty in securing toughness due to high strength and increase of ferrite fraction during wire cooling. This is because it is difficult to avoid the occurrence of hardening cracks due to the formation of the plate martensite.

The content of silicon (Si) is 2.8-4.0%. When the silicon content is less than 2.8%, it is difficult to secure spring characteristics, ie fatigue characteristics and permanent deformation resistance, as a low alloying high stress spring steel, and when the nickel and chromium elements, which are decarburization inhibiting elements, are reduced, the surface decarburization occurs when the wire is heated. Due to the high possibility of deepening, difficulty in controlling residual austenite decomposition time, temper brittleness control, and control of precipitates (epsilon cementite, vanadium or niobium-based precipitates), it is impossible to secure spring characteristics when manufacturing low alloy high stress spring steel. This is because softening heat treatment is inevitable due to a decrease in the rate of tissue softening during softening heat treatment for improvement and a decrease in ferrite production rate during wire cooling, which leads to an increase in surface hardness of wire. In addition, if it exceeds 4.0%, there is a disadvantage that high occurrence rate of surface defects during steelmaking is required, and heat treatment time is required for a long time due to the delay of spheroidization or graphitization solid solution time during heat treatment to impart spring characteristics. This is because the deterioration and the impact of the compressive residual stress distribution upon giving shot peening to improve the fatigue characteristics.

The more preferable component range of the silicon is 3.2-3.6%, which is the grain boundary cross section of the component phase showing the maximum value in the bicomponent range in the case of fatigue characteristics, and in the case of the permanent strain resistance (resistance shear strain), it is continuously improved as the amount added is increased. Strength of the base material, distribution of precipitates (epsilon carbide precipitates, vanadium or niyo boom precipitates), surface decarburization, ferrite formation time, fatigue characteristics, permanent deformation, machinability according to deep processing, temper embrittlement, residual austenite decomposition This is because timing, surface defects during steelmaking, and graphitization fraction during heat treatment can be controlled very effectively.

The content of manganese (Mn) is 0.1-0.3%, but less than 0.1% because it has no effect of hardening, deoxidation and solid solution, and if it exceeds 0.3%, the austenite region is expanded (perlite transformation temperature is increased). It is difficult to control the surface hardness of wire rod because it affects the cornerstone ferrite content during cooling of wire rod, and the structure heterogeneity caused by manganese segregation has more detrimental effect on the spring characteristics than the solid solution strengthening effect. Therefore, limiting the content of manganese to 0.1-0.3% means controlling the strength, deoxidation, solid solution strengthening, microstructure control of wire rod cooling, spring characteristics (fatigue characteristics, permanent strain resistance), and harmful effects of segregation. Considered range.

The content of chromium (Cr) is limited to 0.3-0.6%. It is less effective to improve corrosion resistance when the content of chromium is less than 0.3%, and it is difficult to control surface decarburization at the heat treatment to impart spring characteristics, and it is difficult to control the cementite thickness of the ferrite, affecting the nodularization time during nodular heat treatment. Because. In addition, if the content exceeds 0.6%, it is difficult to secure the strength by increasing the rate of softening of the microstructure, and it is harmful to spring characteristics due to early deposition of cementite during tempering.

The vanadium (V) or niobium (Nb) is a spring property improvement element due to precipitation hardening, and the content thereof is limited to 0.01-0.1%. If the content is less than 0.01%, vanadium or niobium-based precipitates are distributed less and have spring characteristics. (Fatigue property and permanent strain resistance) is not enough to improve the effect, if it exceeds 0.1% the effect of improving the spring characteristics due to precipitates is saturated and the amount of coarse alloy carbide that does not dissolve in the base material during austenite heat treatment increases This is because it causes the same effect as the non-metallic inclusions, resulting in a decrease in fatigue characteristics.

The content of nickel (Ni) is limited to 0.1-0.3%. It is difficult to improve the corrosion fatigue resistance under fatigue load when the nickel content is less than 0.1% .It is difficult to improve the corrosion fatigue resistance under fatigue load, and to improve the decarburization control, toughness and cold workability, residual austenite control, and graphitization heat treatment to improve machinability. This is because there is no graphitization promoting effect. In addition, if the content exceeds 0.3%, it affects the formation of cornerstone ferrite during wire rod cooling, which makes it difficult to control the surface hardness of wire rod, and affects the precipitation temperature of ferrite when billet reheats in wire rod furnace to form the surface ferrite layer to prevent decarburization. This is because raising the temperature has a detrimental effect on decarburization control.

The content of oxygen (O) is limited to 0.0015% or less, because when the content exceeds 0.0015% coarse oxide-based non-metallic inclusions are easily formed to reduce the fatigue life.

The content of nitrogen (N) is limited to 0.005-0.01%, because less than 0.005% carbide production rate due to the formation of vanadium and niobium-based nitride is very small, and if the content exceeds 0.01%, the effect is saturated.

The content of phosphorus (P) and sulfur (S) is limited to 0.01% or less. The phosphorus segregates at grain boundaries and lowers toughness, so the upper limit thereof is limited to 0.01%. It is preferable to limit the upper limit to 0.01% because it lowers the temperature and forms an emulsion, which adversely affects the spring characteristics.

Next, a method for producing a wire rod using the steel billet having the composition described above will be described in detail.

In the present invention, heating is carried out at a heating rate of 15 ± 5 ° C./minute to Ac1 (ideal region transformation start temperature).

If the heating rate up to Ac1 is less than 10 ℃ / minute, the decarburization reaction on the surface of the billet due to the increase in the amount of oxidation and the distribution of heterogeneous oxide layer is difficult to achieve the effect of the present invention, 20 ℃ / It is not preferable to exceed the minutes because the temperature deviation inside and outside the billet is deepened and billet bending occurs.

Moreover, in this invention, it heats at the heating rate of 6 +/- 3 degreeC / min from Ac1 to Ac3 which is an ideal range range.

If the temperature increase rate is less than 3 ℃ / min when passing the above ideal temperature range, the thickness of the surface ferrite layer is increased more than necessary (≥0.5mm), rather it is difficult to obtain a decarburization improvement effect, and also the reheat time (charge time) of the furnace This is because there is a disadvantage. In addition, it is because it is difficult to form the appropriate surface ferrite layer (0.2-0.4mm) necessary for suppressing the decarburization reaction when the temperature increase rate exceeds 9 ° C / min.

In the present invention, the wire is heated to 1050 ± 50 ℃ at a heating rate of 10 ± 5 ℃ / min and maintained for 50-70 minutes before rolling the wire.

The temperature rise rate is limited from the Ac ₃ temperature, which is the abnormal station termination temperature, to the heating holding temperature. After the abnormal station termination temperature, the microstructure of the base material is transformed from the abnormal structure (ferrite + austenite) to the austenite single phase structure. Therefore, at this time, if the temperature increase rate is less than 5 ℃ / min, the material time increases, and if it exceeds 15 ℃ / min, the heating rate is 10 ± 5 ℃ / min, because the billet bends easily occur due to the deep temperature deviation inside and outside the billet. It is limited.

The heating rate of 10 ± 5 ℃ / min is performed to the heating temperature by the wire rod heating, the heating temperature of the wire heating is limited to 1050 ± 50 ℃ range. This is a problem in controlling the proper thickness of the billet surface ferrite layer for the decarburization control when the heating temperature is less than 1000 ℃ by the wire heating, it is not easy to re-use the coarse precipitated vanadium-based or niobium-based precipitates, hot This is because the workability becomes poor due to the overload during rolling due to the increase in the deformation resistance, and if the ferrite layer for the decarburization control cannot be deposited on the surface when it exceeds 1100 ° C. That is, it is possible to have a surface ferrite layer with a very low carbon solubility. However, if the heating temperature exceeds 1100 ° C, the surface ferrite layer is rapidly decarburized because the surface ferrite layer transforms into austenite, thereby increasing the surface decarburization. There is a problem.

The heating holding time at the heating temperature of the wire heating furnace as described above is limited to the range of 30-60 minutes, because in less than 30 minutes, it is difficult to secure a uniform temperature distribution inside and outside the billet for rolling the wire, and exceeds 60 minutes. This is because the amount of oxidation rapidly increases, making it difficult to secure an appropriate ferrite layer for billet surface decarburization control.

In the billet heated by the above method, the decarburized area during extraction by wire heating is the same as the decarburized area of the wire rod after the wire rolling, which means that the wire diameter increases. Therefore, when considering the decarburization layer when extracting the billet according to the present invention, the wire diameter (diameter) is preferably 16 mm or less.

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

Example

Billet of steel grade, containing 0.55% carbon, 3.53% silicon, 0.3% manganese, 0.5% chromium, 0.12% vanadium, 0.25% nickel, 0.001% oxygen, 0.009% nitrogen and consisting of balance Fe and other unavoidable impurities (160 sq.) Were prepared several.

Each prepared billet of high-silicon-added spring steel was heated to Ac1 transformation point at the heating rate as shown in Table 1, and then heated from Ac1 to Ac3 in the ideal range at the heating rate as shown in Table 1, and then to the furnace temperature. It was heated at the heating rate as shown in Table 1. At this time, the heating furnace holding temperature was as shown in Table 1, the holding time was also as shown in Table 1.

Thus, the heated billate was wire-rolled to diameter 13mm by the conventional method. After rolling at a high speed, the rolled diameter 13 mm wire product was rapidly cooled by water spraying at 830 ° C., respectively, and wound up, and slowly cooled at 1.0 ° C./sec to 600 ° C., followed by air cooling.

The surface decarburized layer depth of the wire rod manufactured as described above was measured by the KS standard (KD D 0216). According to this standard, an optical microscope observation method and a microhardness measuring method are proposed, but the microhardness measuring method was used in the present Example.

The decarburized layer depth measured as described above is shown in Table 1 below. At this time, the measurement position was measured at the position where the wire rod section was divided into 8 and the measured value was based on the average value.

Wire heating furnace heating pattern Wire Stripping Depth (mm) Heating rate up to Ac1 (℃ / min) Ideal temperature rise rate (℃ / min) Temperature rise rate after Ac3 (℃ / min) Heating temperature (℃) Holding time (min) Inventive Example 1 15 4 11 1050 30 0.06 Inventive Example 2 15 5 13 1100 60 0.07 Inventive Example 3 15 6 12 1100 30 0.04 Inventive Example 4 16 7 12 1050 60 0.06 Inventive Example 5 17 8 12 1050 50 0.05 Inventive Example 6 15 10 11 1050 50 0.06 Comparative Example 1 16 12 12 1050 50 0.12 Comparative Example 2 16 15 13 1050 40 0.13 Comparative Example 3 16 15 13 1050 60 0.15 Comparative Example 4 15 19 11 1050 30 0.18 Comparative Example 5 15 19 11 1050 50 0.20

As can be seen in Table 1, the surface decarburization depth of the wire rod of the invention example (1-6) that satisfies the conditions of the present invention shows a range of 0.04-0.07mm, while the comparative example outside the conditions of the present invention (1--1). 5) showed wire decarburization in the range of 0.12-0.20mm. Therefore, the present invention was found to be very effective in improving the wire decarburization. Here, Ac1 is an ideal range starting temperature and Ac3 is an austenite single phase starting temperature range.

On the other hand, Figure 1 is a photograph of observation of the wire cross-section with an optical microscope in Example 2, which satisfies the conditions of the present invention by water cooling immediately after wire rod rolling (13mmØ). The white part on the wire surface is a ferrite layer exhibiting the effect of the present invention and the martensite structure inside.

As can be seen in the photograph as shown in Figure 1, by uniformly dispersing the ferrite layer pursued in the present invention, the total depth of the ferrite decarburization layer partial decarburization layer was significantly reduced.

As described above, according to the present invention, by lowering the pass-through temperature rise rate during heating by the wire rod to deposit a uniform and thin ferrite layer on the surface of the billet, it is possible to suppress the decarburization reaction during heating. It can provide.

Claims (1)

By weight, carbon: 0.4-0.6%, silicon: 2.8-4.0%, manganese: 0.1-0.3%, chromium: 0.3-0.6%, oxygen: 0.0015% or less, nitrogen: 0.005-0.01%, phosphorus: 0.01% or less , Sulfur: 0.01% or less, wherein one or two selected from vanadium: 0.01-0.1%, niobium: 0.01-0.1%, nickel: 0.1-0.3% is added, and the composition is composed of the balance Fe and other unavoidable impurities. Heating the steel billet to Ac1 at a heating rate of 15 ± 5 ° C./minute, and heating the Ac1 to Ac3 in the ideal range at a heating rate of 6 ± 3 ° C./minute, and then heating speed to 101050 ± 50 ° C. Method for producing a high stress spring steel wire, characterized in that the wire is rolled to a diameter of 16mm or less after heating at 5 ℃ / min 30-60 minutes