KR100349148B1 - A method for manufacturing wire roe for high silicon spring having high strength - Google Patents

Abstract

PURPOSE: A method for manufacturing a wire rod for high silicon suspension spring having high strength is provided. CONSTITUTION: In a manufacture method of a wire rod for suspension spring that includes the steps of pouring a molten steel comprising C 0.48 to 0.52 wt.%, Si 2.90 to 3.10 wt.%, Mn 0.40 to 0.60 wt.%, 0.015 wt.% or less of P, 0.008 wt.% or less of S, 0.010 wt.% or less of sol-Al, Cr 0.55 to 0.75 wt.%, Ni 0.25 to 0.35 wt.%, V 0.05 to 0.15 wt.%, N 0.0080 to 0.0150 wt.% into a mold for continuous casing, followed by hot rolling obtained bloom, the method is characterized in that the superheat degree of tundish is kept between 20 and 25 deg.C; casting rate in continuous caster is regulated within 0.7 and 0.75 m/min; the amount of secondary cooling water to be sprayed right below the mold is controlled within 0.2 and 0.3 L/kg.

Description

A method for manufacturing wire roe for high silicon spring having high strength}

The present invention relates to the production of automotive suspension spring steel, and more particularly to a method of manufacturing a high-stress high-silicon spring steel wire production is suppressed generation of ferrite decarburized layer.

In the automotive industry, one of energy savings is accelerating fuel economy savings by lightening automobiles. Such lightening of the vehicle means that the weight of a large number of material parts is increased, and a suspension spring is a major contributor to the lightening of the automobile.

In order to reduce the weight of an automobile spring, a material having excellent resistance to permanent deformation of the spring is required. As a result, Si-added steel, which has excellent strain resistance compared to Cr-V-containing SAE6150 alloy steel, has been attracting attention. As a representative example, SAE9260 alloy steel containing about 1.8 to 2.2% of Si may be mentioned.

However, spring steel is usually hot-rolled, and then heat-treated to impart hot workability and spring characteristics of the spring. In the case of SAE9260 alloy steel, fatigue life is reduced due to the generation of material surface decarburization, that is, ferrite decarburization layer during the heat treatment. And the surface processing is very difficult disadvantages.

In order to compensate for these drawbacks in recent years, the addition of a small amount of V to the SAE9254 steel, as well as SAE9254 containing chromium, which is an element effective in preventing surface decarburization and reducing the decarburization in the component range which does not significantly deteriorate the deformation resistance, leads to deformation resistance. Significantly improved SRS60 steels were developed.

On the other hand, despite the advantages of high-silicon-containing spring steel, the production of high-silicon spring steel wire through the reprocessing process is not only a variety of alloying elements but also anxiety due to the high silicon content is added to the bulging (bulging), Stable production conditions have not yet been established due to the frequent occurrence of central segregation, and significant operational disturbances such as internal inclusions as well as even break-out. Specifically, high stress high silicon spring wire should be free from surface defects of slabs and minimize internal inclusions in order to secure the quality of continuous fatigue shocks, as well as the center of accumulation of solute atoms in the slabs. Segregation should be minimized. Particularly, if there are many inclusions in the river or the central segregation is severe, the cracks in the corners of the cast steel are latent. Accordingly, when continuous fatigue stress is applied, the corner cracks act as a starting point of destruction from the periphery and the segregation. Cause. Therefore, above all, in order to manufacture high-stress high-silicon, a technique for minimizing inclusions in steel, preventing bulging of cast steel and improving degree of segregation is required.

Therefore, the present invention not only contains various alloying elements for high stress but also prevents internal defects such as center segregation and cast slab punishment so as to maximize the silicon content effective for deformation resistance, and enables high-silicon high stress spring steel wire rod The purpose is to provide a method of manufacturing.

The present invention for achieving the above object by weight, C: 0.48 ~ 0.52%, Si: 2.90 ~ 3.10%, Mn: 0.40 ~ 0.60%, P: 0.015% or less, S: 0.008% or less, sol. Molten steel, including Al: 0.010% or less, Cr: 0.55 ~ 0.75%, Ni: 0.25 ~ 0.35%, V: 0.05 ~ 0.15%, N: 0.0080 ~ 0.0150%, is injected into the mold from the tundish of the continuous casting machine. In the method of manufacturing a wire rod for spring steel, including continuously rolling, then casting the cast slab (rollom),

The superheat degree of the tundish is maintained in the range of 20 to 25 ° C., the casting speed of the continuous casting machine is in the range of 0.7 to 0.75 m per minute, and the amount of the second coolant water directly under the mold is 0.2 to 0.30. It relates to a method for producing a high-stress high-silicon spring wire rod for cooling by pouring in the range of l / Kg.

Hereinafter, the present invention will be described in detail.

First, the spring steel in accordance with the present invention is by weight%, C: 0.48 ~ 0.52%, Si: 2.90 ~ 3.10%, Mn: 0.40 ~ 0.60%, P: 0.015% or less, S: 0.008% or less, sol. Molten steel containing Al: 0.010% or less, Cr: 0.55-0.75%, Ni: 0.25-0.35%, V: 0.05-0.15%, and N: 0.0080-0.0150% is the object. Springs made of this type of high silicon-containing molten steel contain alloying elements such as Cr and V to maintain a tensile strength of ²⁰⁰ Kg / mm ² , while the decarburized layer is largely suppressed by Ni, and the silicon content is 2.5 It contains a large amount of more than%, which is extremely excellent in deformation resistance.

However, molten steel having the composition described above requires stable continuous casting conditions because it contains a large amount of components that adversely affect the surface and the internal quality during the performance operation.

First, in the present invention, the continuous superheat of the tundish (difference between the reference temperature of the tundish and the theoretical solidification temperature of the molten steel) during continuous casting of the molten steel is maintained in the range of about 20 ~ 25 ℃ higher than normal playing conditions need. That is, when the superheat degree of the tundish is in the range, the molten steel can be completed in a stable casting time for a molten steel of one charge (charge) without falling below the theoretical solidification temperature. Specifically, when the superheat is below 20 ° C, the nozzle of the tundish is clogged and it is difficult to perform stable performance. When the temperature exceeds 25 ° C, segregation is severe in the center of the cast steel, which may seriously cause large operation disturbances such as bursting of cast iron. have.

Casting speed, together with tundish superheat, is a major factor in the production of cast steel during continuous casting. In general, the casting speed of continuous casting machine is affected by the thickness of cast steel, the width of mold, etc., but the casting speed and the superheat degree of tundish have some correlation because the casting speed is determined by the discharge flow rate of tundish nozzle.

The casting speed is important to select the casting speed suitable for the steel type characteristics so that molten steel can be continuously cast within a certain amount of time without solidification. In the present invention, it is preferable to maintain in the range of 0.7 to 0.75 m / min slower than the 0.8 ~ 1.0 m / min casting speed of the conventional molten steel. If the casting speed is faster than 0.75m / min, productivity is good, but it adversely affects the inner quality and center segregation of the cast, and if it is slower than 0.7m / min, the cast may be supercooled to cause corner cracks.

On the other hand, internal defects such as center segregation and internal cracks among the defects occurring in the cast steel have a deep relationship with the casting solidification phenomenon as well as the condition of the equipment of the instrument and the secondary cooling conditions poured directly under the mold. In addition, the surface cracks of cast steels are related to the solidification in the molds, and the overall temperature history of the secondary cooling stage, cooling of spray nozzles or rollers, etc. also have a large effect on the surface temperature history of the cast steels. do. As such, secondary cooling affects the interior and surface quality of the cast. Therefore, it is essential to control the appropriate secondary cooling conditions in order to secure cast quality. Particularly, in the case of spring steel of the present invention, since it contains high silicon, it is easy to cause not only the corner cracks of the cast parts but also operation disturbances such as cast cracks due to the phenomenon of cast parts coming out of the mold. Control of secondary cooling conditions is very important. Secondary cooling in normal performance is faster in the case of high-carbon molten steel, so the corner cracks easily occur in the cast steel when the cooling is cold, so the higher the carbon content of the molten steel is weak cooling, the higher the silicon content is required cold cooling. However, in the case of the present invention, it takes advantage of the softness of silicon to bring it faster than the normal secondary cooling rate. The present invention prevents the cast iron parts of high-silicon molten steel by pouring the cooling amount of the secondary cooling water directly in the mold in the range of 0.2 ~ 0.30ℓ / Kg.

The cast slab (bloom) as described above does not have internal defects such as corner cracks and center segregation, and when the slab is rolled into steel sheets, it is possible to obtain a high stress spring wire rod having low decarburization and excellent deformation resistance.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

For the steel grades (ac) formed as shown in Table 1, a 250x330mm sized slab was manufactured through the manufacturing conditions as shown in Table 2, and the prepared slabs were rolled into steel sheets to form a billet of 160x160mm, followed by wire rolling. Through the wire rod 7mm in diameter was prepared. The mechanical properties of the wire rods were observed, and the results are shown in Table 2. In addition, Table 3 shows the continuous casting operation conditions and experimental results for each step to reduce the crack by the bulging of the invention steel (steel type a).

TABLE 1

TABLE 2

TABLE 3

As shown in Tables 2 and 3, in the case of the wire rod manufactured within the conditions of the present invention, not only the mechanical properties are excellent, but also the cracking of the corner of the cast slab due to the upper bulging of the mold through the hardening during the secondary cooling. As a result, a wire rod having no cracking and good surface quality was obtained even during rolling of the steel sheet.

On the other hand, in the case of the conventional steel, in the case of the conventional example (1) and (2) manufactured by a general playing method, cracks occurred above the corner surface layer of the cast steel, and surface burst occurred in a straight line during rolling of the steel sheet.

As described above, according to the present invention, despite the fact that the molten steel contains a large amount of various alloying elements, especially silicon, which is effective for deformation resistance, internal defects such as central segregation and cast slab punishment can be prevented and stable operation is possible. Not only is the mass production of high stress springs with long deformation resistance and long life, but also contributes to the weight reduction of the vehicle itself.

Claims (1)

By weight%, C: 0.48-0.52%, Si: 2.90-3.10%, Mn: 0.40-0.60%, P: 0.015% or less, S: 0.008% or less, sol. Molten steel, including Al: 0.010% or less, Cr: 0.55 ~ 0.75%, Ni: 0.25 ~ 0.35%, V: 0.05 ~ 0.15%, N: 0.0080 ~ 0.0150%, is injected into the mold from the tundish of the continuous casting machine. In the method of manufacturing a wire rod for spring steel, including continuously rolling, then casting the cast slab (rollom), The superheat degree of the tundish is maintained in the range of 20 to 25 ° C., the casting speed of the continuous casting machine is in the range of 0.7 to 0.75 m per minute, and the amount of the second coolant water directly under the mold is 0.2 to 0.30. Manufacturing method of wire rod for high stress high silicon spring, characterized by cooling by pouring in the range of ℓ / Kg