KR20080109351A

KR20080109351A - Micro alloyed steel for manufacturing steering rack bar

Info

Publication number: KR20080109351A
Application number: KR1020070057552A
Authority: KR
Inventors: 박진우
Original assignee: 현대자동차주식회사
Priority date: 2007-06-13
Filing date: 2007-06-13
Publication date: 2008-12-17

Abstract

The present invention relates to an uncoated steel for manufacturing a steering rack bar of a vehicle, and more particularly, by adding a trace amount of V (vanadium) and precipitation strengthening thereof, it is possible to enhance the strength of the material without omitting the post-heat treatment step. The present invention relates to a non-manufactured steel for manufacturing a steering rack bar of a vehicle which has improved the disadvantage of lowering the impact toughness of the non-manufactured steel through the addition of Mn (manganese) and Cr (chromium).

To this end, the present invention is based on iron (Fe), carbon (C) 0.37 to 0.43% by weight, silicon (Si) 0.15 to 0.35% by weight, manganese (Mn) 1.00 to 1.50% by weight, phosphorus (P) ) 0.03% by weight or less, sulfur (S) 0.015 to 0.030% by weight, chromium (Cr) 0.40 to 0.70% by weight, vanadium (V) 0.12 to 0.18% by weight of the crude steel for manufacturing a steering rack bar To provide.

Description

Micro alloyed steel for manufacturing steering rack bar

1 is a photograph illustrating a steering rack bar for a vehicle,

Figure 2 is an electron micrograph showing a non-coated steel and conventional non-coated steel structure according to the present invention.

Steering Rack Bar, which is one of the chassis parts for automobiles, is a component as shown in FIG. 1 attached, and currently uses mostly carbon steel as a material, and its required strength through heat treatment (Quenching & Tempering). Is satisfying.

However, there are disadvantages such as cost increase, productivity decrease, and quality deviation caused by the additional heat treatment process.

In order to remedy this disadvantage, there is a demand for the development of non-manufactured steel for manufacturing a steering rack bar that can improve the strength of a material without performing a heat treatment process.

The present invention has been made in view of the above, and the improvement of the material properties of the steel in parts requiring high load is mainly dependent on the alloy design and the secondary heat treatment, the present invention is hot forging S45C carbon steel Unlike the conventional steering rack bar manufacturing process that enhances the mechanical strength of the material by changing the structure through heat treatment (quenching & tempering), a small amount of V (vanadium) is added and precipitation strengthened to omit the heat treatment process. The strength of the material can be strengthened, and the addition of Mn (manganese) and Cr (chromium) can be used to improve the disadvantages of impact toughness reduction of the non-alloyed steel. , Steering rack of vehicle that can get positive effect such as securing quality and improving productivity by shortening process Its purpose is to provide non-manufactured steel for bar manufacture.

The present invention for achieving the above object is based on iron (Fe), 0.37 to 0.43% by weight of carbon (C), 0.15 to 0.35% by weight of silicon (Si), 1.00 to 1.50 weight of manganese (Mn) %, 0.03% by weight or less of phosphorus (P), 0.015 to 0.030% by weight of sulfur (S), 0.40 to 0.70% by weight of chromium (Cr), 0.12 to 0.18% by weight of vanadium (V) Provided are crude steel for rack bar manufacture.

Hereinafter, the present invention will be described in more detail.

Table 1 below is a comparison table of the chemical composition for the conventional rack bar material and the non-prepared steel according to the present invention.

As described in Table 1, the conventional rack bar manufacturing material is based on iron (Fe), carbon (C) 0.42 to 0.48% by weight, silicon (Si) 0.15 to 0.35% by weight, manganese (Mn) Using a material containing 0.60 to 0.90% by weight, phosphorus (P) 0.03% or less, and sulfur (S) 0.035% or less, hot rolling was performed at 1200 ° C together with the material heating, followed by quenching and tempering. And a rack bar for steering with enhanced mechanical strength while changing the structure through a process such as room temperature oil cooling.

On the other hand, according to the present invention, in order to compensate the tendency of the impact toughness of the existing material is less than half compared to the carbon steel, so as to secure the toughness equivalent to the existing carbon steel through the addition of Mn (manganese) and Cr (chromium) Based on iron (Fe), 0.37 to 0.43% by weight of carbon (C), 0.15 to 0.35% by weight of silicon (Si), 1.00 to 1.50% by weight of manganese (Mn), 0.03% or less by weight of phosphorus (P), Steering rack bar is made at 1200 ° C without using the same heat treatment process by using non-alloyed steel containing 0.015 to 0.030% by weight of sulfur (S), 0.40 to 0.70% by weight of chromium (Cr) and 0.12 to 0.18% by weight of vanadium (V). After performing only hot rolling, it can be manufactured through a process of air cooling.

Here, the reasons for the addition and content ratio limitation for each component of the non-coated steel according to the present invention are as follows.

(1) 0.37 to 0.43% by weight of carbon (C)

First, carbon (C) increases the strength and hardness of the material, the fine alloying element is added as an essential element to precipitate carbides, the tensile strength is lowered when 0.37% by weight or less, the impact toughness when 0.43% by weight or more Since it lowers, the quantity is limited to 0.37-0.43 weight%.

(2) 0.15 to 0.35% by weight of silicon (Si)

Silicon (Si) increases the hardness, modulus of elasticity, etc., and reinforces the ferrite phase, but decreases the elongation and impact value. When the content exceeds 0.35% by weight, silicon (Ferrite) transformation occurs. Defects which promote and lower toughness occur, which is not preferable, and less than 0.15% by weight is not preferable due to the decrease in strength.

(3) Manganese (Mn) 1.00-1.50 wt%

In the case of manganese (Mn), the content was higher than that of the existing material to improve the material strength and toughness. When the content is 1.50% by weight or more, the machinability is lowered, so the content is limited to 1.00 to 1.50% by weight.

(4) Chromium (Cr) 0.40 to 0.70 wt%

In the case of chromium (Cr), it is an important factor that contributes to the improvement of impact toughness, which is an important characteristic of chassis parts. If the content is 0.70% by weight or more, the impact strength effect is insignificant, and a cost increase factor occurs, and the content is 0.40 to 0.70% by weight. It is good to limit it.

(5) 0.03% by weight or less of phosphorus (P)

Phosphorus is an impurity concept, i.e. it is absent but generally only the upper limit is regulated due to steelmaking technology problems.

(6) 0.015 to 0.030 wt% of sulfur (S),

Sulfur is recognized as an inclusion in ordinary alloy steels, and the content thereof is preferably suppressed to a minimum, and MnS in combination with Mn improves machinability, but when the content is increased, strength decreases, so the content of sulfur (S) is 0.015. Limit to 0.030% by weight.

(7) Vanadium (V) 0.12 to 0.18 wt%

Vanadium was added in a small amount in order to maintain the same strength as the existing method by the martensite strengthening method, for the purpose of increasing the strength due to carbide precipitation.

At this time, the strength decreases when the content is 0.12% by weight or less, and the impact toughness decreases when the content is 0.18% by weight or more, and therefore the content is limited to 0.12 to 0.18% by weight.

Hereinafter, the embodiment of the present invention will be described in more detail with a comparative example, but the present invention is not limited to the following examples.

Example

As an embodiment of the present invention, based on iron (Fe), carbon (C) 0.40% by weight, silicon (Si) 0.30% by weight, manganese (Mn) 1.50% by weight, phosphorus (P) 0.03% by weight Hereinafter, only hot rolling is performed at 1200 ° C. on an untreated steel material containing 0.020% by weight of sulfur (S), 0.50% by weight of chromium (Cr), and 0.15% by weight of vanadium (V), followed by air cooling without additional heat treatment. Through the steering rack bar was prepared.

Comparative example

As a comparative example, a material containing 0.40% by weight of carbon (C), 0.30% by weight of silicon (Si), 0.70% by weight of manganese (Mn), 0.03% by weight or less of phosphorus (P), and 0.035% by weight of sulfur (S) Hot rolling at 1200 ℃ with material heating, then heat treatment [Quenching (30 minutes at 850 ℃) & Tempering (30 minutes at 580 ℃)], then the rack for steering while changing the structure through the room temperature cooling process The bar was prepared.

Test Example 1

Various strength and elongation tests for the steering rack bar according to the above-described examples and comparative examples were measured using a conventional measuring equipment, and the results are shown in Table 2 below.

As shown in Table 2 above, it can be seen that the non-alloyed steel according to the embodiment has excellent strength and equivalent impact toughness compared to the existing material according to the comparative example, and through this, the non-alloyed steel of the present invention can replace the existing carbon steel. I could see that.

Test Example 2

The specimen structure of the steering rack bar according to the above-described Examples and Comparative Examples was observed with an electron microscope. As shown in FIG. 2, the structure of the specimen was heat treated after hot forging in the case of the existing material according to the Comparative Example. Through the martensite structure, on the other hand, the non-coated steel according to the embodiment was found to have a ferrite + pearlite structure because the heat treatment step after the hot forging is omitted, the hardness was measured by the usual equipment after such a structure analysis Bar, it can be seen that the rough steel hardness of the present invention is represented by the HRC 20 ~ 26 level.

As seen above, according to the non-coarse steel for manufacturing a steering rack bar of the vehicle according to the present invention, unlike the conventional to improve the strength through the heat treatment process after hot forging carbon steel, the strength of the material by adding a small amount of vanadium By increasing the amount of manganese and chromium, and by ensuring the same toughness as existing materials, the existing heat treatment process can be omitted, reducing the cost of production process, ensuring equal quality and improving productivity. You can get it.

Claims

Based on iron (Fe), 0.37 to 0.43% by weight of carbon (C), 0.15 to 0.35% by weight of silicon (Si), 1.00 to 1.50% by weight of manganese (Mn), 0.03% or less by weight of phosphorus (P), A crude steel for manufacturing a steering rack bar of a vehicle, characterized by containing 0.015 to 0.030 wt% of sulfur (S), 0.40 to 0.70 wt% of chromium (Cr), and 0.12 to 0.18 wt% of vanadium (V).