KR101965148B1 - Super high strength austenitic lihgt-weight steel and method of manufacturing the same - Google Patents

Abstract

Disclosed are austenitic light-weight steel having 1 GPa class super high strength and a method of manufacturing the same. According to the present invention, the method of manufacturing the super high strength austenitic light-weight steel comprises the following steps of: (a) hot rolling steel comprising 27-33 wt% of manganese (Mn), 10-12 wt% of aluminum (Al), 1.0-1.2 wt% of carbon (C), 3.0-5.0 wt% of molybdenum (Mo), and the remainder consisting of iron (Fe) and inevitable impurities; (b) homogenizing and heat treating the hot rolled steel under a condition of 1,050 ± 25°C for one to three hours; and (c) cooling the homogenized and heat treated steel.

Description

TECHNICAL FIELD [0001] The present invention relates to a super high strength austenitic lightweight steel material and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to austenitic lightweight steel and a method of manufacturing the same, and more particularly, to an austenitic lightweight steel having an ultra-high strength of 1 GPa and a manufacturing method thereof.

In recent years, the strength of automotive materials has been steadily increasing to improve passenger safety and fuel efficiency of vehicles.

Particularly, parts such as a bumper and a front side member are required to have a very high strength as a part abutting against a collision of a vehicle. Accordingly, Martensite steel and TRIP (Transformation-Induced Plasticity) steel are applied.

Among them, martensitic steel has a martensitic matrix structure through quenching to a temperature lower than the martensite transformation starting temperature (Ms temperature) after hot rolling. As a result, the martensitic steel has an ultrahigh strength of 1 GPa or more, while having an elongation of 10% or less. TRIP steel is a phase in which residual austenite is formed in a ferrite matrix and has a relatively low elongation of less than 1 GPa while having excellent elongation due to retained austenite during component forming.

It is an object of the present invention to provide an ultra-high strength austenitic lightweight steel material having an ultra-high strength of 1 GPa class and capable of securing excellent formability by improvement of an elongation rate, and a manufacturing method thereof.

In order to achieve the above object, an ultra-high strength austenitic light weight steel according to an embodiment of the present invention comprises 27 to 33% by weight of manganese (Mn), 10 to 12% by weight of aluminum (Al) (Fe) and inevitable impurities, and has a tensile strength (TS) of 1 GPa or more and an elongation (EL) of 15% or more.

(A) 27 to 33% by weight of manganese (Mn), 10 to 12% by weight of aluminum, 10 to 12% by weight of aluminum (Al) C): 1.0 to 1.2% by weight, molybdenum (Mo): 3.0 to 5.0% by weight, and the balance of iron (Fe) and unavoidable impurities; (b) subjecting the hot-rolled steel to homogenization heat treatment at 1,050 占 폚 to 25 占 폚 for 1 to 3 hours; And (c) cooling the homogenized heat treated steel material.

The ultra-high strength austenitic light weight steel according to the present invention and its manufacturing method can be obtained by adding Mo at an optimum content ratio to an Fe-C-Mn-Al alloy as a basic component system, thereby obtaining a steel having an ultrahigh strength of 1 GPa, And it is possible to achieve weight reduction by having low density through addition of Al by 10 wt% or more.

As a result, the ultra-high strength austenitic light weight steel according to the present invention and the method of producing the same have a tensile strength (TS) of 1 GPa or more and an elongation (EL) of 15% or more, and a tensile strength (MPa) % Or more, and exhibits a density reduction ratio of 15% or more with respect to pure iron.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow chart showing a method for producing an ultra-high strength austenitic lightweight steel according to the present invention. FIG.
2 is a SEM photograph of the specimen according to Comparative Examples 1 to 4;
3 is a SEM photograph of a specimen according to Comparative Examples 5 to 8. Fig.
4 is a SEM photograph of specimens according to Examples 1 and 2 and Comparative Examples 9 to 10;
5 is a TEM photograph of a specimen according to Comparative Example 5. Fig.
6 is a photograph showing SEM and EDS analysis results of a specimen according to Comparative Example 6. Fig.
7 is a TEM photograph of a specimen according to Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent by reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an ultra-high strength austenitic light weight steel according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Ultra high strength Austenitic system  Lightweight steel

The ultra high strength austenitic light weight steel according to the present invention has a tensile strength (TS) of not less than 1 GPa and an elongation (EL) of not less than 15%, and satisfies a tensile strength (MPa) x elongation (%) of not less than 15,000 MPa% And a density of 7.0 g / cm 3 or less.

To this end, the ultra high strength austenitic light weight steel according to the present invention comprises 27 to 33 wt% of manganese (Mn), 10 to 12 wt% of aluminum (Al), 1.0 to 1.2 wt% of carbon (C) Mo): 3.0 to 5.0% by weight, and balance iron (Fe) and unavoidable impurities.

Further, the steel material may further contain at least one of sulfur (S): 0.01 wt% or less and phosphorus (P): 0.02 wt% or less.

Hereinafter, the role and content of each component contained in the ultra-high strength austenitic lightweight steel according to the present invention will be described.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element, which serves to stabilize an austenitic matrix to improve ductility. At this time, the austenitic lightweight steel according to the present invention contains a large amount of Al, which is a ferrite stabilizing element, compared with conventional TWIP (TWIN Induced Plasticity) steels. As a result, austenitic- In order to produce lightweight steels, the manganese (Mn) content should be increased to 27 wt% or more as compared to TWIP steels. However, when the addition amount of manganese (Mn) is over 33% by weight, the formation of a weak? -Mn phase is promoted, thereby deteriorating ductility and toughness.

Therefore, manganese (Mn) is preferably added at a content ratio of 27 to 33% by weight of the total weight of the austenitic light-weight steel according to the present invention, more preferably 29 to 31% by weight.

Aluminum (Al)

Aluminum (Al) is an indispensable element for lighter weight, and is lighter in weight than Fe atoms, and has an effect of lowering the density of the steel because the volume per mole is large.

The aluminum (Al) is preferably added in an amount of 10 to 12% by weight of the total weight of the austenitic light weight steel according to the present invention, and more preferably 10.5 to 11.0% by weight. When the addition amount of aluminum (Al) is less than 10% by weight, the addition amount thereof is insufficient and it may be difficult to exhibit the effect of lightening properly. On the contrary, when the addition amount of aluminum (Al) is more than 12% by weight, there is a problem of not only forming an AlN article but also inhibiting the formation of the austenite phase and promoting ferrite generation to lower the elongation.

Carbon (C)

Carbon (C) is an austenite stabilizing element and is required for the production of austenitic lightweight steels. In addition, carbon combines with Fe and Al to form κ-carbide, which is a regular phase, and binds with Mo to form carbide, thereby greatly improving the strength of steel.

The carbon (C) is preferably added in an amount of 1.0 to 1.2% by weight based on the total weight of the austenitic lightweight steel according to the present invention, and more preferably 1.05 to 1.15% by weight. When the addition amount of carbon (C) is less than 1.0% by weight, the addition amount thereof is insufficient and it may be difficult to exhibit the effect of improving the strength properly. On the contrary, if the addition amount of carbon (C) exceeds 1.2% by weight and is added excessively, it may act as a cause of lowering the ductility.

molybdenum( Mo )

Molybdenum (Mo) is a carbide-forming element that bonds with C to form precipitates such as Mo ₂ C, Mo ₆ C, and Mo ₂₃ C ₆ to suppress crystal growth during homogenization heat treatment in austenite region, . ≪ / RTI > In addition, molybdenum can also help to secure ductility by delaying κ-carbide formation.

The molybdenum (Mo) is preferably added in an amount of 3.0 to 5.0% by weight of the total weight of the austenitic light-weight steel according to the present invention, more preferably 3.5 to 4.95% by weight. When the addition amount of molybdenum (Mo) is less than 3.0% by weight, it is difficult to exhibit the above effect. On the contrary, when the addition amount of molybdenum (Mo) is excessively over 5.0 wt%, a large amount of coarse precipitate is promoted to lower the precipitation strengthening effect and to deteriorate the ductility.

Sulfur (S), phosphorus (P)

Sulfur (S) and phosphorus (P) induce segregation in the ingot during steelmaking and performance, thereby lowering the toughness and ductility of the steel. Particularly in the case of sulfur (S), MnS inclusions are formed by binding with manganese The ductility can be lowered. Therefore, it is preferable that sulfur (S) and phosphorus (P) are not contained, and even if it is inevitably included as an impurity, it is preferably limited to 0.01 wt% or less of sulfur (S) and 0.02 wt% or less of phosphorus .

Ultra high strength Austenitic system  How to make lightweight steel

1 is a process flow chart showing a method for manufacturing an ultra-high strength austenitic lightweight steel according to the present invention.

Referring to FIG. 1, a method of manufacturing an ultra-high strength austenitic lightweight steel according to the present invention includes a hot rolling step (S110), a homogenizing heat treatment step (S120), and a cooling step (S130).

Hot rolling

In the hot rolling step S110, 27 to 33 wt% of manganese (Mn), 10 to 12 wt% of aluminum (Al), 1.0 to 1.2 wt% of carbon (C), 3.0 to 5.0 wt% of molybdenum (Mo) And the remaining iron (Fe) and inevitable impurities are hot-rolled. At this time, the steel material may further contain 0.01 wt% or less of sulfur (S) and 0.02 wt% or less of phosphorus (P).

Before the hot rolling step (S110), a step of reheating at about 1,150 to 1,250 DEG C for one to three hours may be further performed.

In this step, the hot rolling is preferably carried out at a finish rolling temperature of 900 ° C or higher, more preferably 900-1150 ° C. If the finish rolling temperature is lower than 900 占 폚, there is a problem that an uncrosslinked structure due to abnormal reverse rolling occurs. On the other hand, if the finish rolling temperature exceeds 1,150 占 폚, the austenite grains may become coarse and the strength may become difficult to secure.

Homogenization  Heat treatment

In the homogenization heat treatment step (S120), the hot-rolled steel material is subjected to homogenization heat treatment at 1,050 占 25 占 폚 for 1 to 3 hours. At this time, when the homogenization treatment temperature is less than 1,025 DEG C, the homogenization effect is insufficient. On the other hand, when the homogenization heat treatment temperature exceeds 1,075 DEG C, the strength and toughness may be lowered due to crystal grain coarsening.

Cooling

In the cooling step (S130), the homogenized heat treated steel is cooled to room temperature at a rate of 300 to 350 DEG C / sec. At this time, when the cooling rate is less than 300 ° C / sec, a large amount of coarse carbide may be generated upon cooling. On the contrary, when the cooling rate exceeds 350 ° C / sec, the strength increases, but it may be difficult to obtain the target toughness. The cooling in this step is preferably conducted by a water-cooling method, and the room temperature may be 1 to 40 캜, but is not limited thereto.

The ultra-high strength austenitic lightweight steel produced by the above-described processes (S110 to S130) has an ultra-high strength of 1 GPa by adding Mo to the Fe-C-Mn-Al alloy as the basic component at an optimum content ratio An elongation of not less than 15% can be ensured, and light weight can be achieved by having a low density through addition of Al by 10 wt% or more.

As a result, the ultrahigh-strength austenitic light-weight steel produced by the method according to the present invention has a tensile strength (TS) of 1 GPa or more and an elongation (EL) of 15% or more and a tensile strength (MPa) % Or more, and exhibits a density reduction ratio of 15% or more with respect to pure iron.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Specimen Manufacturing

The ingot having the chemical composition shown in Table 1 was produced in a vacuum induction melting furnace, and then the ingot was reheated at 1,200 占 폚 for 2 hours and hot rolled to a thickness of 12 mm under the condition of finish hot rolling at 1,000 占 폚. Thereafter, homogenization heat treatment was performed at 1,050 占 폚 for 2 hours and then cooled to room temperature (15 占 폚) to prepare specimens according to Examples 1 to 2 and Comparative Examples 1 to 10. At this time, water cooling was performed after homogenization heat treatment and the cooling rate was measured at 310 ± 10 ° C / sec.

[Table 1] (unit:% by weight)

2. Evaluation of microstructure and mechanical properties

Table 2 shows the microstructure and mechanical properties of the samples according to Examples 1 to 2 and Comparative Examples 1 to 10. FIG. 2 is a SEM image of a specimen according to Comparative Examples 1 to 4, FIG. 3 is a SEM image of a specimen according to Comparative Examples 5 to 8, FIG. SEM photograph of the specimen.

[Table 2]

As can be seen from Tables 1 to 2 and FIGS. 2 to 4, all of the specimens according to Examples 1 to 2 and Comparative Examples 1 to 10 have a base structure of austenite and the following characteristics Respectively.

As the ferrite stabilizing element Mo was added, the ordered-BCC phase D03 was developed and the fraction increased with the increase of Mo addition.

Here, when a large amount of C added is added in an amount of 1.0 wt% or more as in the specimens according to Comparative Examples 6 to 8, κ-carbide is developed in the austenite matrix, but κ- The precipitation was delayed, and the yield strength decreased as Mo increased at the same amount of C added.

On the other hand, as shown in Table 1 and Fig. 4, when the addition amount of Mo is increased to 3.5 wt% or more in the specimen in which C is added at 1.0 wt% or more, a large amount of Mo is contained in the austenite grain boundary and the matrix at the time of homogenization heat treatment Mo-enriched carbide is formed. This carbide interferes with the growth of austenite grains during the homogenization heat treatment so as to have fine grains, and the strength of the steel can be greatly improved through strengthening of precipitation and strengthening of grain boundaries.

Accordingly, it is confirmed that the specimens according to Examples 1 and 2 exhibit a tensile strength (TS) of not less than 1,000 MPa, an elongation (EL) of not less than 15% and a tensile index (TS EL) of not less than 15,000 MPa 揃% .

However, in the case of the specimens according to Comparative Examples 9 to 10, the addition of Mo in an amount exceeding 5 wt% accelerated the formation of a large amount of coarse precipitates, thereby deteriorating the precipitation strengthening effect and lowering the ductility. (TS EL) value did not satisfy the target value.

FIG. 5 is a TEM photograph of the specimen according to Comparative Example 5, FIG. 6 is a photograph showing SEM and EDS analysis results of the specimen according to Comparative Example 6, and FIG. 7 is a TEM photograph of the specimen according to Example 1 . FIG. 5A shows a dark-field image, and FIG. 5B shows a SAD (selected area electron diffraction) pattern analysis result. 6 (a) shows an SEM image, and FIG. 6 (b) shows an EDS analysis result of Mo-enriched carbide. 7A shows a dark-field image, and FIG. 7B shows a SAD (selected area electron diffraction) pattern analysis result.

As shown in FIG. 5, it was confirmed that the specimen according to Comparative Example 5 was composed of austenite single-phase microstructure after the homogenization heat treatment, and κ-carbide was regularly precipitated in the austenite phase in the lattice structure have.

Also, as shown in FIG. 6, as can be seen from the SEM image and EDS analysis results of the specimen according to Comparative Example 6, it can be confirmed that the Mo-enriched carbide is precipitated by the Mo addition after the homogenization heat treatment.

7, in the specimen according to Example 1, the microstructure after the homogenization heat treatment showed austenite phase and κ-carbide in the austenitic phase, as can be seen from TEM image and SAD analysis results, D03 and Mo carbide were precipitated.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. These changes and modifications may be made without departing from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Hot rolling step
S120: homogenization heat treatment step
S130: cooling step

Claims (8)

(a) 27 to 33 wt% of manganese (Mn), 10 to 12 wt% of aluminum (Al), 1.0 to 1.2 wt% of carbon (C), 3.50 to 4.95 wt% of molybdenum Fe) and inevitable impurities;
(b) subjecting the hot-rolled steel to homogenization heat treatment at 1,050 占 폚 to 25 占 폚 for 1 to 3 hours; And
(c) cooling the homogenized heat treated steel material;
Based lightweight steel material.
The method according to claim 1,
In the step (c)
The cooling
Wherein the heat treatment is carried out at a temperature of 300 to 350 DEG C / sec to room temperature.
The method according to claim 1,
The steel material
0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P), based on the total weight of the steel.
(Fe) and the balance of iron (Fe) and molybdenum (Mo), 27 to 33 wt% of manganese (Mn), 10 to 12 wt% Including unavoidable impurities,
An ultra-high strength austenitic light weight steel having a tensile strength (TS) of 1 GPa or more and an elongation (EL) of 15% or more.
delete 5. The method of claim 4,
The steel material
0.01% by weight or less of sulfur (S) and 0.02% by weight or less of phosphorus (P).
5. The method of claim 4,
The steel
And a density of 7.0 g / cm < 3 > or less.
5. The method of claim 4,
The steel
And a tensile index (TS x EL) of 15,000 MPa.% Or more.

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