KR102464609B1 - Non-tempered steel for steering rack bar and manufacturing method thereof - Google Patents

Abstract

The present invention is to provide non-tempered steel for a steering rack bar and a manufacturing method thereof to achieve excellent mechanical properties and toughness without a heat treatment process. Non-tempered steel for a steering rack bar according to an aspect of the present invention comprises 0.40 to 0.55 wt% of carbon (C), 0.10 to 0.40 wt% of silicon (Si), 1.00 to 1.50 wt% of manganese (Mn), 0.01 to 0.20 wt% of vanadium (V), 0.05 to 0.30 wt% of chromium (Cr), 0.01 to 0.050 wt% of aluminum (Al), 0.002 wt% or less of titanium (Ti), 0.05 to 0.30 wt% of nickel (Ni), 0.006 to 0.020 wt% of nitrogen (N), and the balance iron (Fe) and other unavoidable impurities, and includes a ferrite and pearlite composite structure.

Description

NON-TEMPERED STEEL FOR STEERING RACK BAR AND MANUFACTURING METHOD THEREOF

The present invention relates to a non-tempered steel material for a steering rack bar and a manufacturing method thereof, and more particularly, to a non-tempered steel material for a steering rack bar having excellent mechanical properties and toughness without a heat treatment process through improvement of the steel composition and process. and to a method for manufacturing the same.

The steering rack bar is a component connected to the vehicle's steering system, and since it is a component directly receiving the vehicle's load, high strength and toughness are required. Therefore, as a material of the rack bar, tempered steel that improves the strength and toughness of the material by heat-treating medium-carbon steel in the austenite region has generally been used. However, as described above, the temper steel manufacturing process involves processes such as quenching, annealing, and induction heat treatment, which makes the process complicated and requires high cost and time.

On the other hand, in the case of non-tempered steel, in which the heat treatment process is omitted, the process is simplified and it is preferable in that it is free from bending defects that may occur during the heat treatment process.

Accordingly, there is a demand for an invention capable of improving the impact toughness and strength of non-tempered steel while using non-tempered steel with higher process efficiency than tempered steel.

As a related technology, there is Korean Patent Publication No. 10-2008-0109351 (published on December 17, 2008, title of the invention: non-tempered steel for manufacturing a steering rack bar of a vehicle).

The problem to be solved by the present invention is to provide a non-tempered steel material having excellent strength and impact toughness and a method for manufacturing the same.

Non-tempered steel material according to an aspect of the present invention, by weight, carbon (C): 0.40 to 0.55, silicon (Si): 0.10 to 0.40, manganese (Mn): 1.00 to 1.50, vanadium (V): 0.01 to 0.20, chromium (Cr): 0.05 to 0.30, aluminum (Al): 0.01 to 0.050, titanium (Ti): 0.002 or less, nickel (Ni): 0.05 to 0.30, nitrogen (N): 0.006 to 0.020, remainder iron (Fe) ) and other unavoidable impurities, and has a ferrite and pearlite composite structure.

In one embodiment, the ferrite fraction of the non-tempered steel may be 18 to 30%.

In one embodiment, the non-tempered steel may have an impact strength of 55 J/cm ² or more, a tensile strength (TS, Tensile strength) of 820 MPa or more, and an elongation ratio (EL) of 12% to 23%.

In one embodiment, the non-tempered steel includes at least one or more precipitates from the group consisting of VC, VN, AlN, TiN and BN on the surface, and the precipitates have an average diameter of 10 nm or less, and the non-tempered steel It may be included in an average of 3 to 10 or less per 100 x 100 nm of the surface area.

Disclosed is a method for manufacturing a non-tempered steel material for a steering rack bar according to another aspect of the present invention. The manufacturing method is, in weight %, carbon (C): 0.40 to 0.55, silicon (Si): 0.10 to 0.40, manganese (Mn): 1.00 to 1.50, vanadium (V): 0.01 to 0.20, chromium (Cr): 0.05 to 0.30, aluminum (Al): 0.01 to 0.050, titanium (Ti): 0.002 or less, nickel (Ni): 0.05 to 0.30, nitrogen (N): 0.006 to 0.020, the remainder including iron (Fe) and other unavoidable impurities Reheating the material at 1,200° C. or higher to less than 1,300° C.; and controlling the reheated material at 700°C to 900°C.

According to the present invention, it is possible to efficiently provide a high-strength and high-toughness non-tempered steel having a microstructure of ferrite and pearlite without a heat treatment process by appropriately controlling alloy components and their content, heating and high-temperature and high-speed rolling.

1 is a process flow diagram schematically showing a method of manufacturing a non-refined steel material according to an embodiment of the present invention.
2 is a graph schematically showing process conditions of rolling according to an embodiment and a comparative example of the present invention.
3 is a photograph showing the microstructure of a non-tempered steel specimen according to an embodiment and a comparative example of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily practice it. The present invention may be embodied in several different forms, and is not limited to the embodiments described herein. The same reference numerals are given to the same or similar components throughout this specification. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

In an embodiment of the present invention, by omitting the heat treatment process, bending defects of the steel are prevented, and at the same time, even if the heat treatment process is omitted through the component and content design of the steel and the rolling process, a non-tempered steel material having high impact toughness and a method for manufacturing the same to provide.

According to an embodiment of the present invention, by adding a certain amount of vanadium (V) to medium carbon steel, VC, VN, V(C,N) type precipitates are effectively formed on the steel, and by adding a certain amount of nickel (Ni) to the steel, non-refining The low-temperature impact toughness and fatigue properties of steel are improved, and the formation of precipitates that act as ferrite nucleation sites at a relatively early point during the rolling process after reheating by including a large amount of nitrogen (N), which has little effect on the increase in ferroalloy cost, is prevented. It is possible to improve the strength and impact toughness of non-tempered steel without increasing the cost. The precipitates can also maximize the ferrite fraction by effectively refining the hard grain in the steel and at the same time acting as a ferrite nucleation site during rolling.

High strength and high toughness non-tempered steel

High-strength and high-toughness non-tempered steel according to an embodiment of the present invention is, by weight, carbon (C): 0.40 to 0.55, silicon (Si): 0.10 to 0.40, manganese (Mn): 1.00 to 1.50, vanadium (V) : 0.01 to 0.20, Chromium (Cr): 0.05 to 0.30, Aluminum (Al): 0.01 to 0.050, Titanium (Ti): 0.002 or less, Nickel (Ni): 0.05 to 0.30, Nitrogen (N): 0.006 to 0.020, the rest iron (Fe) and other unavoidable impurities. The non-tempered steel material has a ferrite and pearlite composite structure.

The non-tempered steel material according to an embodiment of the present invention may include ferrite in a fraction of 18 to 30%.

The non-tempered steel material according to an embodiment of the present invention may have an impact strength of 55 J/cm ² or more, a tensile strength (TS, Tensile strength) of 820 MPa or more, and an elongation ratio (EL) of 12% to 23%.

The non-tempered steel material according to an embodiment of the present invention includes at least one precipitate from the group consisting of VC, VN, AlN, TiN and BN on the surface, and the precipitate has an average diameter of 10 nm or less, and the non-tempered steel material It may be included in an average of 3 to 10 or less per 100 x 100 nm of the surface area.

Hereinafter, the role and content of each component included in the high-strength and high-toughness non-tempered steel material according to an embodiment of the present invention will be described in detail. The component content indicated below is a weight % with respect to the total steel material, and the portion indicated by simply % below means weight % unless otherwise specified.

Carbon (C): 0.40 to 0.55 wt%

Carbon (C) is added to secure the strength of the steel. In one embodiment, the carbon (C) is preferably added in an amount of 0.40 to 0.55% by weight of the total weight of the steel, for example, 0.41 to 0.46% by weight. When the carbon is included in less than 0.40 weight %, it is not possible to secure sufficient strength of the steel, and when included in more than 0.55 weight %, the impact toughness and ductility of the steel are reduced.

Silicon (Si): 0.10 to 0.40 wt%

Silicon (Si) is included to secure deoxidation and fatigue resistance of steel, and has the effect of increasing strength. In one embodiment, the silicon (Si) is preferably added in an amount of 0.10 to 0.40% by weight, for example, 0.20 to 0.30% by weight of the total weight of the steel. The silicon is an element for improving the deformation resistance, and when it is added in excess of 0.40%, the impact toughness and ductility of the steel may be greatly reduced, and when it is added in less than 0.10%, it is difficult to secure sufficient strength of the steel.

Manganese (Mn): 1.00 to 1.50 wt%

Manganese (Mn) contributes to securing the strength and toughness of non-tempered steel through refining of pearlite between layers by inducing a decrease in the A1 transformation point of non-tempered steel. In one embodiment, manganese is included in an amount of 1,00 to 1.50% by weight, for example, 1.30 to 1.60% by weight, thereby increasing the fraction of the low-temperature phase and used as an element for increasing the strength of the steel with a solid solution strengthening effect. When the manganese content is less than 1.00%, it is difficult to sufficiently secure the strength of the steel, and when it exceeds 1.50%, the toughness of the steel may be reduced due to the improvement of the strength.

Vanadium (V): 0.010 to 0.200%

Vanadium (V) is used as an element that reacts with carbon and nitrogen in the steel material to form V(C, N) composite precipitates. In one embodiment, vanadium (V) is included in an amount of 0.010 to 0.200, for example, 0.050 to 0.10% by weight, so that the V-based precipitates contribute to precipitation strengthening and preventing grain boundary coarsening, and act as a ferrite nucleation site in the steel rolling process. Increasing the ferrite fraction ultimately improves the strength and toughness of the steel. However, when vanadium is contained in an amount of less than 0.010%, it is difficult to expect a sufficient precipitation strengthening effect, and when it is added in an amount exceeding 0.20%, the effect of refining grains in the steel is reduced, leading to an excessive increase in strength, which can reduce the impact toughness of non-tempered steel. have.

Chromium (Cr): 0.05 to 0.30 wt%

Chromium (Cr) is included in the non-refining steel and increases the A3 transformation point, thereby contributing to the improvement of low-temperature rolling efficiency and strength of steel. In one embodiment, chromium is included in an amount of 0.05 to 0.30% by weight, for example, 0.12 to 0.25% by weight, based on the total weight of the steel. When chromium is less than 0.05%, the effect of securing strength is low, and when it is added in excess of 0.30%, the impact toughness of the steel may be greatly reduced.

Aluminum (Al): 0.010 to 0.050 wt%

Aluminum (Al) may be included as a deoxidation element, and reacts with nitrogen in the steel material to form AlN, thereby contributing to grain refinement and impact toughness improvement in the steel material. In one embodiment, the aluminum (Al) is the steel sheet 0.010 to 0.050% by weight, for example, 0.030 to 0.045% by weight based on the total weight. When the aluminum is included in less than 0.010%, the deoxidation effect is insignificant, and it is not possible to form sufficient AlN, so the improvement of the impact toughness of the steel may be insufficient. When the aluminum is included in an amount exceeding 0.050%, mechanical properties may be reduced due to an increase in Al-based inclusions.

Nickel (Ni): 0.05 to 0.30 wt%

Nickel (Ni) contributes to improving the low-temperature impact toughness and fatigue properties of non-tempered steel. In one embodiment, nickel is included in an amount of 0.05 to 0.30% by weight based on the total weight of the steel. When the nickel content is less than 0.05% by weight, it is difficult to expect an effect of securing strength, and when it is added in an amount of more than 0.3% by weight, it may significantly decrease the stability during cold drawing.

Nitrogen (N): 0.004 to 0.020 wt%

Nitrogen (N) combines with aluminum (Al) and vanadium (V) at high temperatures to form Al and V composite nitrides that lead to grain refinement and an increase in ferrite fraction in the steel, ultimately contributing to the improvement of the impact toughness of the steel. In one embodiment, the nitrogen may be included in an amount of 0.0080 to 0.020 wt%, for example, 0.010 to 0.015 wt%, based on the total weight of the steel. When the nitrogen is included in less than 0.004 wt%, sufficient complex nitride (AlN, VN) cannot be formed, making it difficult to expect high toughness of the steel, and when added in excess of 0.020 wt%, coarsened precipitates according to the Al content It may rather reduce the impact toughness.

The relationship between the nitrogen input amount and the AlN precipitate production amount can be expressed by the following equation.

AlN = 0.21 x exp(0.012 x [N])

In the above formula,

AlN = aluminum nitride content (Vol.%)

[N] = nitrogen content (ppm)

‘0.021’ and ‘0.012’ = means the correction factor.

The remaining component of the high-tensile hot-rolled steel sheet of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steel manufacturing process, it cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing process, all of them are not specifically mentioned in this specification.

The high-strength and high-tensile non-tempered steel of the present invention described above may be manufactured by the following method as an embodiment thereof.

Manufacturing method of high-strength, high-tensile non-tempered steel

1 is a process flow diagram schematically illustrating a method of manufacturing a high-strength high-tensile non-tempered steel material according to an embodiment of the present invention. Referring to FIG. 1 , the method for manufacturing a non-tempered steel material according to the present invention includes a slab reheating step (S110) of a steel material, and a control rolling step (S120). Hereinafter, the manufacturing method of the hot-rolled steel sheet of the present invention will be described in more detail step by step.

Bloom and billet reheating step (S110)

In the bloom and billet reheating step (S110), the non-refined steel is in weight %, carbon (C): 0.40 to 0.55, silicon (Si): 0.10 to 0.40, manganese (Mn): 1.00 to 1.50, vanadium (V): 0.01 to 0.20, Chromium (Cr): 0.05 to 0.30, Aluminum (Al): 0.01 to 0.050, Titanium (Ti): 0.002 or less, Nickel (Ni): 0.05 to 0.30, Nitrogen (N): 0.006 to 0.020, remaining iron This is a step of re-dissolving segregated components during casting by heating a steel material containing (Fe) and other unavoidable impurities. The process temperature of the reheating step is a factor that has an important influence on the grain growth of steel.

The reheating condition according to an embodiment of the present invention is heating at a temperature (SRT) of 1,000 to 1,300 °C, and in particular, may be in the range of 1,150 to 1,250 °C. If the heating temperature is less than 1,000℃, there is a possibility of poor formation due to rolling overload during rolling of the semi-finished product.

Control rolling step (S120)

The hot rolling step ( S120 ) is a step of manufacturing a wire rod product by hot rolling the reheated steel billet. In one embodiment of the present invention, the control rolling step may be performed after rough or intermediate rolling of the reheated billet.

As shown in FIG. 2, when the steel of the composition according to the present invention is heated and then rolled, precipitates such as V(C,N) and AlN start to be generated at a higher temperature than before, so that during the rolling process, The ferrite fraction is increased. Specifically, in the case of the non-refined steel according to the present invention, the formation of precipitates starts at a temperature of about 30 to 50° C. higher than that of the prior art due to the high nitrogen content, and for example, when the nitrogen content is 50 ppm under the same conditions, V When the nitrogen content is 100 ppm, the formation temperature of V(C,N) is about 865°C, whereas the formation temperature of (C,N) is about 840°C. Since the precipitates generated in the non-refined steel act as a ferrite nucleation site, the ferrite fraction increases during the rolling process, thereby increasing the impact toughness of the non-tempered steel.

Furthermore, as shown in FIG. 3 , in the case of the steel material according to the present invention, the production temperature of AlN-based precipitates is also increased, so that the fraction of AlN precipitates is also increased. Based on the time when the rolling process is finished, the fraction of AlN precipitates is about twice that of the prior art, and the increase in the AlN fraction soon leads to a grain pinning effect, thus preventing the mixing of grains in non-refined steel and at the same time refinement intensifies.

Based on the finish rolling, the controlled rolling temperature may be performed between the A1 transformation point and the A3 transformation point. In the case of a steel material according to an embodiment of the present invention, the temperature of the controlled rolling may be performed at 780 to 820 °C. When the finish rolling temperature is less than 780° C., it is difficult to secure the workability of the wire rod due to the generation of a mixed grain structure due to rolling in an abnormal region, which may cause a load on the rolling process.

Hereinafter, the present invention will be described in detail through examples, but these are only preferred examples of the present invention, and the scope of the present invention is not limited by the scope of the description of these examples. Content not described here will be omitted because it can be technically inferred sufficiently by those skilled in the art.

Example

A steel material having the component system (unit: wt%) of Table 1 below was prepared, and heating, rolling, and cooling were performed under the process conditions of Table 2 for the steel material to prepare specimens of Comparative Examples and Examples, respectively.

division C Si Mn V Cr Al Ni N Comparative Example 1 0.45 0.25 1.33 0.054 0.18 0.035 0.02 0.0041 Comparative Example 2 0.45 0.25 1.35 0.055 0.19 0.040 0.02 0.0215 Comparative Example 3 0.45 0.25 1.35 0.055 0.19 0.045 0.11 0.0054 Example 1 0.44 0.25 1.36 0.055 0.19 0.030 0.11 0.0085 Example 2 0.45 0.25 1.35 0.054 0.19 0.042 0.10 0.0093 Example 3 0.45 0.25 1.32 0.054 0.19 0.033 0.09 0.0104 The content of each component means the weight % with respect to the total weight of the steel.

division Reheating temperature (℃) Controlled rolling temperature (℃) cooling rate
(℃/s) impact strength
(J/cm ² ) The tensile strength
(MPa) elongation
(%) ferrite
Fraction (%) Comparative Example 1 1230 802 1　 or less 52.2 825 12.3 16.5 Comparative Example 2 813 62.5 849 9.1 27.1 Comparative Example 3 785 56.5 822 12.0 17.4 Example 1 802 63.6 902 18.6 19.8 Example 2 808 64.2 884 18.9 20.9 Example 3 785 71.2 900 19.6 26.3

After reheating the specimens of Examples and Comparative Examples having the component system of Table 1 to 1230° C., after rough rolling, intermediate rolling, and control rolling, natural cooling, and then evaluating the physical properties of the specimens of Comparative Examples and Examples was performed, and the The results are shown in Table 2.

Next, the number and size of the precipitates and the ferrite fraction were measured for the specimens of Examples and Comparative Examples, and the results are shown in Table 3.

division group number of precipitates
(100 x 100 nm ² ) Average particle size of precipitates (nm) Ferrite fraction (%) Example 1 F+P 3.4 3.3 19.8 Example 2 F+P 4.6 4.0 20.9 Example 3 F+P 6.7 3.3 26.3 Comparative Example 1 F+P 1.4 4.3 16.5 Comparative Example 2 F+P 6.2 7.3 27.1 Comparative Example 3 F+P 1.8 3.9 17.4 In the specimen structure, F stands for ferrite and P stands for pearlite.

For the comparative examples and examples, a cold drawing test was performed by manufacturing a steering rack bar, and whether cracks occurred in the product was visually observed, and the results are shown in Table 4.

Assessment Methods

Mechanical properties: As for the tensile test for the steels manufactured in Examples 1 to 3, the tensile strength and elongation of the test piece made in KS 14A according to the KS B 0802 standard are shown in Table 2.

Precipitation measurement method: The precipitate of the steel material is TEM (transmission electron microscope) carbide extraction

Replication analysis was performed.

Cold drawing: The above steels are reduced by 12% using a Schmag drawing machine.

Cold drawing was performed.

division Is there any breakage during cold drawing? No cracks in the product Defect occurrence evaluation Example 1 doesn't exist doesn't exist fitness Example 2 doesn't exist doesn't exist fitness Example 3 doesn't exist doesn't exist fitness Comparative Example 1 doesn't exist doesn't exist fitness Comparative Example 2 Occur doesn't exist incongruity Comparative Example 3 doesn't exist doesn't exist fitness <Evaluation criteria>
Unsuitable: Cracks in the final product or breakage during cold drawing
Suitable: No breakage during cold drawing and no cracks in the final product

Referring to Tables 1 to 4, it was confirmed that the higher the nitrogen content in the specimen, the higher the ferrite fraction was formed. It was confirmed that this is because, as the nitrogen content increases, the temperature at which the precipitates acting as ferrite-generating nuclei are generated increases, so that the formation of ferrite on the surface of the non-refined steel material is accelerated during the rolling process. Such a difference leads to an increase in the amount of precipitates produced during the rolling process and the resulting ferrite fraction in the case of steel products that undergo a high-speed rolling process such as wire rod and BLC rolling, and as a result, the physical properties of non-refined steel are also affected. go crazy

However, as confirmed in Comparative Example 2, when the nitrogen content exceeds 0.02%, it was confirmed that breakage occurred during cold drawing, and the precipitates were coarsened and the impact toughness was rather reduced.

On the other hand, it was confirmed that when nickel (Ni) was included in an appropriate amount, it contributed to the improvement of the impact toughness and mechanical properties of the steel compared to the case where it was not.

Component analysis of the precipitates through an EDS component analyzer was performed on the specimen of Example 4, and as a result, it was confirmed that VC, VN, AlN, TiN and BN precipitates were formed on the surface of the specimen.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Accordingly, the scope of the present invention should be judged by the claims set forth below.

Claims (5)

A non-tempered steel material for a steering rack bar, comprising:
In wt%, carbon (C): 0.40 to 0.55, silicon (Si): 0.10 to 0.40, manganese (Mn): 1.00 to 1.50, vanadium (V): 0.01 to 0.20, chromium (Cr): 0.05 to 0.30, aluminum (Al): 0.01 to 0.050, nickel (Ni): 0.05 to 0.30, nitrogen (N): 0.01 to 0.02, remaining iron (Fe) and other unavoidable impurities,
The ferrite fraction of the non-tempered steel is 18 to 30%,
The non-tempered steel includes at least one or more precipitates from the group consisting of VC, VN, AlN, TiN and BN on the surface,
The non-tempered steel is reheated at 1,150° C. or higher to less than 1,250° C., characterized in that it is controlled-rolled at 780° C. to 820° C.,
The precipitates include AlN precipitates having an average diameter of 10 nm or less,
The number of precipitates is an average of 3 to 10 per 100 x 100 nm of the surface area of the non-refined steel,
The nitrogen content is a non-refined steel comprising a ferrite and pearlite composite structure that satisfies the formula;
AlN = 0.21 x exp(0.012 x [N])
during the meal,
AlN is aluminum nitride content (Vol.%)
[N] is the nitrogen content (ppm).
delete The non-tempered steel material according to claim 1, wherein the non-tempered steel has an impact strength of 55 J/cm ² or more, a tensile strength of 820 MPa or more, and an elongation of 12% to 23%. delete delete

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