KR101683987B1 - Precipitation hardening steels having low density, high strength and elongation and manufacturing method thereof - Google Patents

Abstract

Precipitation hardening type high strength and high oriented low specific gravity steel sheet and a manufacturing method thereof are introduced.
The precipitation hardening type high strength and high oriented low specific gravity steel sheet production method of the present invention is characterized by comprising 0.15 to 0.3% of C, 9 to 10.5% of Mn, 5 to 7% of Al, and 0.03% or less of P ), S: not more than 0.03% (excluding 0), the remainder being Fe and other unavoidable impurities; Annealing at 850 to 950 ° C and air cooling; And air cooling at a temperature of 500 to 600 DEG C for 4 to 8 hours after aging.

Description

TECHNICAL FIELD [0001] The present invention relates to precipitation hardening type high strength and high elongation low specific gravity steel sheets,

More particularly, the present invention relates to a precipitation hardening type high strength and high oriented low specific gravity steel sheet having a high strength and a high elongation and a low specific gravity property and a process for producing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardening type high strength and high stretched low specific gravity steel sheet, .

In recent years, automobiles are becoming lighter in weight in order to enhance fuel efficiency, in addition to environmental problems such as CO2 reduction and exhaust gas regulation.

As the application ratio of super high tensile strength steadily increases to satisfy this trend, Xinjiang paper which satisfies high strength and high elongation at the same time as Twinning Induced Plasticity Steel is being developed.

However, in case of super high tensile steel, the part applicable to the body parts is very limited due to the low formability. In the case of high manganese TWIP steel, there is a limit in obtaining a weight reduction effect due to a small specific gravity reduction rate compared to the existing automotive steel sheet. There is a limit in obtaining an automobile part which simultaneously satisfies the high tensile properties.

Therefore, there is an urgent need to develop a low specific gravity steel capable of realizing tensile properties of high strength and high elongation and lightweight automotive parts.

Low specific gravity steel sheets have been developed in two directions when classified in terms of alloy composition.

At the beginning of the development, an austenitic high alloy low specific gravity steel having 18 to 32 wt% manganese (Mn), 8 to 12 wt% aluminum (Al) and 0.7 to 2 wt% carbon (C) alloy composition was developed, And an elongation of 50% or more at the same time, which is an alloy satisfying both high strength and high elongation at the same time.

This is because the precipitate called k-carbide having a (Fe, Mn) 3AlC composition in the austenite grains is precipitated in a fine size of 20 to 40 nm to increase the work hardening rate. However, such an alloy has disadvantages in that it has a problem of expensive casting and manufacturing process due to a high alloy element content, and thus there is a disadvantage that it is limited in practical use.

Thereafter, a ferrite based low alloyed steel having a composition of 3 to 6 wt% manganese, 5 to 7 wt% of aluminum and 0.1 to 0.5 wt% of a carbon alloy has been developed. This is because the ratio of the alloy element is relatively low, However, there is a problem that cracks are generated in the steel sheet even during hot rolling because the ductility is very high.

This is because the coarse-sized k-carbides precipitated along the grain boundaries act as the epicenter of the cracks during deformation, thereby reducing ductility.

In order to overcome this, a product having a mid-level elongation rate was developed through heat treatment at a specific temperature, but it is at a level comparable to the elongation of a high alloy low-strength steel.

Juornal of Alloys and Compounds vol. Isothermal precipitation behavior of k-carbide in the Fe-9Mn-6Al-0.15C lightweight steel with a multiphase microstructure ", which is listed in Journal of the Korean Society of Precision Engineering, 574, 299p-304p The present inventor has found that the present inventors have found that there is no description about how the phase transformation behavior progresses according to time, and the present inventor has found that the tensile properties of high strength and high stretching And a method for realizing reduction in weight of automobile parts.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

Korean Patent Publication No. 2002-0046988 (Jun. 2002)

Juornal of Alloys and Compounds vol. 574, 299p-304p "Isothermal precipitation behavior of k-carbide in the Fe-9Mn-6Al-0.15C lightweight steel with a multiphase microstructure"

In order to solve such conventional problems, the present invention has been made to solve the problems of the prior art, and it is an object of the present invention to provide a precipitation hardening type high strength and high stretching low- And a method of manufacturing the same.

The precipitation hardening type high strength and high oriented low specific gravity steel sheet according to the present invention comprises 0.15 to 0.3% of C, 9 to 10.5% of Mn, 5 to 7% of Al, 0.03 to 0.3% of P, (Excluding 0), S: not more than 0.03% (excluding 0), and the balance of Fe and other unavoidable impurities, and has a tensile strength of 787 MPa or more and an elongation of 40% or more.

0.15 to 0.25% of C, 9 to 10% of Mn and 5 to 6% of Al.

0.2 to 0.3% of C, 9.5 to 10.5% of Mn and 6 to 7% of Al.

The precipitation hardening type high strength and high oriented low specific gravity steel sheet of the present invention is characterized in that K carbide exists along the ferrite crystal grains.

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Wherein the K carbide is formed in a layered structure with a ferrite structure, and the precipitation hardening type high strength and highly oriented low specific gravity steel.

In order to achieve the above object, the precipitation hardening type high strength and high oriented low specific gravity steel sheet manufacturing method according to the present invention comprises 0.15 to 0.3% of C, 9 to 10.5% of Mn, 5 to 7% of Al, % Or less (excluding 0), S: 0.03% or less (excluding 0), and the remainder is Fe and other unavoidable impurities; Annealing at 850 to 950 ° C and air cooling; And air cooling at a temperature of 500 to 600 DEG C for 4 to 8 hours after aging.

0.15 to 0.25% of C, 9 to 10% of Mn and 5 to 6% of Al.

0.2 to 0.3% of C, 9.5 to 10.5% of Mn and 6 to 7% of Al.

According to the present invention, the following effects can be realized.

First, there is an advantage in that a lightweight steel having a low density compared to a conventional steel sheet material can be produced through a high lightweight element content.

Second, precipitation hardening through proper time aging is realized to increase the yield and tensile strength, thereby achieving high strength of steel and securing a high level of elongation.

Third, there is an advantage that it can be applied to parts which are difficult to apply due to the lack of formability of ultra high tensile steel.

1 is a view showing a precipitation hardening type high strength and highly oriented low specific gravity steel sheet manufacturing method,
2 is a view showing a microstructure of a conventional steel sheet before aging heat treatment,
As shown in FIG. 3 and FIG. 4, it can be seen that the K carbide precipitates along the ferrite grain boundaries after the aging heat treatment within the temperature and time range described above.
Fig. 3 is a view showing the precipitation of K carbide along the ferrite grain boundaries after the aging heat treatment within the temperature and time range of the present invention
FIG. 4 is a view showing a state in which K carbide is precipitated along the ferrite grain boundaries at a high magnification after aging heat treatment within the temperature and time range of the present invention. FIG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a precipitation hardening type high strength and highly oriented low specific gravity steel sheet according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

The precipitation hardening type high strength and high oriented low specific gravity steel sheet according to the present invention comprises 0.15 to 0.3% of C, 9 to 10.5% of Mn, 5 to 7% of Al, 0.03% or less of P (excluding 0) S: 0.03% or less (excluding 0), and the balance includes Fe and other unavoidable impurities.

Manganese (Mn) is an austenite stabilizing element and when a large amount of manganese is added to steel, the austenite fraction is increased to increase the elongation. In addition, manganese is a substitutional element of iron and plays a role of increasing the strength by being substituted solid solution in the lattice structure. In the present invention, manganese content is increased compared to conventional low alloyed low alloy steel (3 to 6 wt%) Alloy.

If the content of manganese is less than 9%, the elongation rate decreases. If the content exceeds 10.5%, the cost increases and at the same time, the content of manganese is lowered to 9 to 10.5%.

Aluminum (Al) has a specific gravity of about 2.7 and is a representative lightweight element corresponding to about 1/3 of iron. This aluminum increases the ferrite fraction as a strong ferrite stabilizing element and binds iron (Fe) and carbon (C) to form precipitates and increase strength at the applied temperature range. Therefore, it is desirable to realize weight reduction and high strength by maximizing aluminum content.

When aluminum is added in an amount of less than 5%, the effect on weight reduction and high strength is insignificant. When added in an amount exceeding 7%, there is a problem of forming a mild intermetallic compound such as FeAl or Fe ₂ Al ₅ and causing brittleness, Taking this into consideration, aluminum content should be 5 ~ 7%.

Carbon (C) is an element that is used to impart strength to steel, and plays a role of interstitial solid solution, or plays a role of precipitating carbide to increase strength. In the present invention, carbon is added in the range of 0.15 to 0.3%. When the carbon content is less than 0.15%, there arises a problem that the strength is decreased or the fraction of austenite, which is the main structure which plays a role of increasing the elongation, is decreased. If it exceeds 0.3%, the problem of weldability is caused. Therefore, the concentration of carbon is adjusted within the above-mentioned range.

Phosphorus (P) and sulfur (S) are typical impurity elements present in the steel. When they are present in the steel, they have an adverse effect on the ductility causing brittleness, so their content is limited to less than 0.03%.

The precipitation hardening type high strength and high oriented low specific gravity steel sheet of the present invention may contain 0.2 to 0.3% of carbon, 9.5 to 10.5% of manganese and 6 to 7% of aluminum.

As described above, manganese is an element that plays an important role in the elongation and strength, but when the content is increased, the cost is increased and the melt temperature is lowered. Considering this point, if it is necessary to maximize the elongation, the content of manganese should be adjusted to 9.5% or more, but not exceed 10.5% in consideration of the disadvantage.

Aluminum can also be added in an amount of 7% in order to achieve weight reduction and high strength in consideration of light weight, high strength, and brittleness.

Carbon may be adjusted to a lower limit of 0.2% or more in consideration of strength and elongation.

The precipitation hardening type high strength and high oriented low specific gravity steel of the present invention may be added in a content of 0.15 to 0.25% carbon, 9-10% manganese, and 5 to 6% aluminum.

Considering the control of the temperature of the molten metal and the economical efficiency over the elongation, the manganese content may be adjusted in the range of 9 to 10%. However, the 9% range is the minimum lower limit for achieving the desired elongation.

It is also desirable to limit the content of aluminum to 6% or less when it is necessary to more strictly control the criteria relating to light weight, high strength and brittleness.

The carbon content may be adjusted in the range of 0.15 to 0.25% in consideration of problems such as the austenite fraction and weldability.

Hereinafter, a precipitation hardening type high strength and highly oriented low specific gravity steel sheet producing method of the present invention will be described.

As shown in FIG. 1, the precipitation hardening type high strength and high oriented low specific gravity steel sheet producing method of the present invention comprises 0.15 to 0.3% of C, 9 to 10.5% of Mn, 5 to 7% of Al, : 0.03% or less (excluding 0), S: 0.03% or less (excluding 0), and the remainder is Fe and other unavoidable impurities. The steel is continuously rolled, hot rolled, annealed at 850 to 950 ° C, Air cooling, aging at 500 to 600 ° C for 4 to 8 hours, followed by air cooling.

As described above, carbon can be added in a controlled amount of 0.2 to 0.3%, manganese 9.5 to 10.5% and aluminum 6 to 7%, carbon 0.15 to 0.25%, manganese 9 to 10%, aluminum 5 To 6% by weight.

Annealing is carried out at a temperature of 850 ° C or higher in order to increase the elongation and to maximize the fraction of austenite which is a structure that forms a precipitate. In order to prevent the grain size from excessively increasing to adversely affect strength and elongation, Do not exceed 950 ℃.

The annealed steel is again aged at 500 to 600 ° C for 4 to 8 hours and then air-cooled.

When the aging time is less than 4 hours, the precipitation hardening effect is insignificant. When the aging time exceeds 8 hours, the elongation rate is decreased and the formability is lowered. Therefore, the aging time is in the range of 4 to 8 hours.

If the aging heat treatment is carried out at a temperature lower than 500 ° C or higher than 600 ° C, precipitates are not formed properly. Therefore, the aging heat treatment temperature should be within the above-mentioned range.

As shown in Fig. 2, the microstructure before aging heat treatment includes ferrite, austenite, and martensite.

As shown in FIG. 3 and FIG. 4, it can be seen that the K carbide precipitates along the ferrite grain boundaries after the aging heat treatment within the temperature and time range described above.

It can also be seen that a new phase is formed in a black form in the area consisting of austenite and martensite, in which K carbide and ferrite exist in a layered structure.

The present inventors have shown the physical properties of the steel sheet produced by the precipitation hardening type high strength and high oriented low specific gravity steel sheet manufacturing method of the present invention through a tensile test.

Tensile specimens were made with ASTM E8M subsize specimens and tensile tests were carried out with a strain rate of 10 ^-3 / S using an INTRON 3382 tester.

As shown in Examples 1 to 3, the tensile strength at the level of 787 to 833 MPa and the elongation at the level of 40 to 50% can be secured at the same time.

In the case of Comparative Example 1 in which the aging heat treatment was not carried out, it was possible to secure an elongation of about 60%, but the tensile strength was found to be 633 MPa, compared to the Example. In Comparative Example 2 in which the aging time was less than 4 hours, the precipitation hardening did not proceed sufficiently, and the strength was 700 MPa, which was lower than that in the Example.

As shown in Comparative Example 3 and Comparative Example 4, when the aging heat treatment was performed for more than 8 hours, the tensile strength was maintained at 900 MPa or more, but the elongation was drastically reduced.

When the content of manganese or aluminum is different from the numerical range of the present invention as in Comparative Examples 5 to 8, the tensile strength and the elongation are both lower than those of the Examples. When the carbon content is low as in Comparative Example 9, When the carbon content was high as in Comparative Example 10, the strength was formed at the same level as in Example, but the elongation was decreased.

Considering the physical properties of the steel sheet applied to the present part as in the comparative example 11, it is expected that the steel sheet which can satisfy both the high strength and the light weight can be applied to the parts.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be apparent to those of ordinary skill in the art.

Claims (9)

(Excluding 0), S: not more than 0.03% (excluding 0), and the remainder being Fe (Al), 0.03% And other unavoidable impurities, and has a tensile strength of 787 MPa or more and an elongation of 40% or more.

The method according to claim 1,
Wherein the precipitation hardening type high strength and high elongation low specific gravity type steel has the composition of C: 0.15 to 0.25%, Mn: 9 to 10%, and Al: 5 to 6%.
The method according to claim 1,
Wherein the precipitation hardening type high strength and high orientation low specific gravity steel sheet is characterized by comprising 0.2 to 0.3% of C, 9.5 to 10.5% of Mn, and 6 to 7% of Al.
The method according to any one of claims 1 to 3,
A precipitation hardening type high strength and high stretch low specific gravity steel sheet characterized by presence of K carbide along the ferrite crystal grains.
delete The method of claim 4,
Wherein the K carbide is formed in a layered structure with a ferrite structure, and the precipitation hardening type high strength and highly oriented low specific gravity steel.
(Excluding 0), S: not more than 0.03% (excluding 0), and the remainder being Fe (Al), 0.03% And other unavoidable impurities are continuously cast and rolled after hot rolling;
Annealing at 850 to 950 ° C and air cooling; And
And a step of air cooling after aging at a temperature of 500 to 600 ° C for 4 to 8 hours, and a method of producing a precipitation hardening type high strength and highly oriented low specific gravity steel sheet.
The method of claim 7,
Wherein the precipitation hardening type high strength and high oriented low specific gravity steel sheet comprises 0.15 to 0.25% of C, 9 to 10% of Mn, and 5 to 6% of Al.
The method of claim 7,
Wherein the content of C is 0.2 to 0.3%, Mn is 9.5 to 10.5%, and Al is 6 to 7%.

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