KR102222244B1 - Martensit-based precipitation hardening type lightweight steel and manufacturing method for the same - Google Patents

Abstract

The present invention contains components of manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight and aluminum (Al): 4.0-7.0% by weight, and the balance iron (Fe) and other inevitable It contains impurities, and the lightweight steel is subjected to hot rolling and cold rolling processes, and then annealing process is performed at 900-1100°C, which is an austenite single phase region of _{Ac 3 or higher, and 100°C- after passing through the annealing process.} Tempering process is performed at 150℃,
The lightweight steel is cooled after the annealing process so that the volume fraction of martensite is 60% or more, and the lightweight steel with a volume fraction of martensite of 60% or more has kappa precipitates, cementite, and epsilon inside the martensite after the tempering process. It is a martensitic precipitation hardening type lightweight steel characterized in that carbide is formed as a precipitate.
The lightweight steel according to the present invention has the effect of innovatively increasing the strength by using the main structure as martensite.

Description

Martensitic precipitation hardening type lightweight steel and its manufacturing method {MARTENSIT-BASED PRECIPITATION HARDENING TYPE LIGHTWEIGHT STEEL AND MANUFACTURING METHOD FOR THE SAME}

The present invention relates to a high-strength lightweight steel and a method for manufacturing the same, and more particularly, to a high-strength martensitic lightweight steel that forms kappa precipitation and martensite as a main structure at a low temperature.

The present invention relates to an alloy design of martensitic lightweight steel to replace high-strength carbon steel used in shock absorbers such as automobile steel plates, bumper impact beams and pillars of automobiles requiring low specific gravity and high strength. More specifically, it relates to a martensitic lightweight steel having a yield strength of 1.0 GPa or more and a tensile strength of 1.5 GPa or more by precipitation hardening, and a method of manufacturing the same.

Recently, as environmental problems such as air pollution have emerged, many methods have been proposed to increase the fuel efficiency of automobiles. In particular, as the weight reduction of automobiles emerges as an important part, high-strength steel sheets having high strength as well as high formability are required.

In addition, since automobile parts such as bumper reinforcement or shock absorbers in doors are parts directly related to passenger safety, ultra-high-strength steel plates having a tensile strength of 980 MPa or more are used and must have high strength. As for these parts, as the rate of using high-strength steel increases, research on commercialization of high-strength steel is increasing.

In accordance with these social demands, studies on lightweight steels with aluminum added to the middle manganese steel are being actively conducted. Lightweight steel is mainly composed of austenite or ferrite, and has excellent elongation and low specific gravity due to the addition of aluminum, but low strength is emerging as a problem. Although research is being conducted to complement the low strength of such lightweight steel, there is no clear plan for obtaining strength of 1 GPa or more, and there is no research in which the main structure of lightweight steel is martensite.

(Document 1) Korean Patent Application Publication No. 10-2018-0070940 (2018.06.27)

The martensitic precipitation hardening lightweight steel according to the present invention has the following challenges.

First, unlike existing lightweight steel, it has the effect of innovatively increasing the strength by using the main structure as martensite.

Second, the addition of aluminum has the effect of reducing weight and generating precipitates (kappa precipitates, cementite, epsilon carbide).

Third, there is an effect of improving the strength by finely depositing the precipitates through low temperature tempering.

Fourth, due to the presence of retained austenite, metamorphic organic plasticity (trip) occurs during deformation, thereby increasing the elongation.

The problem of the present invention is not limited to those mentioned above, and other problems that are not mentioned will be clearly understood by those skilled in the art from the following description.

The martensitic precipitation-hardening lightweight steel according to the present invention contains components of manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight, and aluminum (Al): 4.0-7.0% by weight, It contains the balance iron (Fe) and other inevitable impurities.

In the present invention, at least one selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and vanadium (V) in the lightweight steel is 0.001-5.0 weight % May contain more.

In the present invention, the lightweight steel may be subjected to a hot-rolling and cold-rolling process, and then an annealing process at 900-1100°C, which is an austenite single-phase region of _{Ac 3 or higher.}

In the present invention, the annealing process may be performed in a temperature range in which the volume fraction of austenite is 80% or more.

In the present invention, the volume fraction of martensite may be 60% or more by cooling after annealing.

In the present invention, the volume fraction of retained austenite may be formed up to 20%.

In the present invention, it is preferable that the ferrite formed during annealing remains and has a volume fraction of at most 20%.

In the present invention, the lightweight steel having a volume fraction of martensite of 60% or more may have kappa precipitates, cementite, and epsilon carbide precipitates formed inside martensite after a tempering process.

In the present invention, it is preferable that the yield strength of the lightweight steel is 1.0 GPa or more, the tensile strength is 1.5 GPa or more, and the elongation is 15% or more.

The manufacturing method of martensitic precipitation-hardening lightweight steel according to the present invention comprises components of manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight, and aluminum (Al): 4.0-7.0% by weight. Containing, and remaining iron (Fe) and other unavoidably contained impurities or a composition alloy containing titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and Step S1 of homogenizing by heating a composition alloy further containing 0.001-5.0% by weight of at least one selected from the group consisting of vanadium (V); The homogenized composition alloy is reheated for a preset time at 1000℃-1200℃, which is the temperature range of the single-phase austenite region, and _{hot finish rolling was performed at Ac 3} temperature ~ 1000℃, and then wound at _{Ms temperature ~ Ac 3 temperature.} Taking, slow cooling and S2 step of manufacturing a hot-rolled steel sheet; Step S3 of producing a cold-rolled steel sheet by performing cold rolling at room temperature on the steel sheet that has undergone the step S2; Step S4 in which the steel sheet passing through the step S3 is heated and annealed at 900°C-1100°C for a preset time; And it may include the step S5 of the steel sheet subjected to the step S4 is tempered at 100 ℃ -150 ℃.

In the present invention, the homogenization of step S1 may be performed at 1000° C.-1400° C.

In the present invention, the martensite transformation starts below the temperature Ms during the slow cooling of the step S2, and the final microstructure at room temperature of the steel sheet passed through the step S2 is the four phases of tempered martensite, bainite, ferrite and austenite. It is desirable.

In the present invention, cementite may precipitate inside martensite during slow cooling.

In the present invention, the annealing process of step S4 may be performed in a temperature range in which the volume fraction of austenite is 80% or more.

In the present invention, the heat treatment time of the annealing process in step S4 is preferably a time during which the alloying elements are uniformly dissolved.

In the present invention, the cooling rate of the annealing process in step S4 is 50 to 200°C/sec, and it can be cooled to room temperature.

In the present invention, it is preferable that the volume fraction of martensite is 60% or more after cooling.

In the present invention, the volume fraction of retained austenite may be formed up to 20%.

In the present invention, it is preferable that the ferrite formed in the annealing process of step S4 remains, and the volume fraction is at most 20%.

In the present invention, kappa precipitates, cementite, and epsilon carbide may be formed as precipitates in martensite during tempering in step S5.

The martensitic precipitation hardening lightweight steel according to the present invention has the following challenges.

First, unlike existing lightweight steel, it has the effect of innovatively increasing the strength by using the main structure as martensite.

Second, the addition of aluminum has the effect of reducing weight and generating precipitates (kappa precipitates, cementite, epsilon carbide).

Third, there is an effect of improving the strength by finely depositing the precipitates through low temperature tempering.

Fourth, due to the presence of retained austenite, metamorphic organic plasticity (trip) occurs during deformation, thereby increasing the elongation.

The problem of the present invention is not limited to those mentioned above, and other problems that are not mentioned will be clearly understood by those skilled in the art from the following description.

1 is a diagram schematically illustrating the composition and manufacturing and heat treatment process of a martensitic precipitation hardening type lightweight steel according to the present invention.
2 is a diagram showing the result of thermodynamically calculating the fraction of microstructure according to temperature using the TCFE 9 program of the martensitic precipitation hardening type lightweight steel according to the present invention.
3 is a photograph showing the microstructure of the martensitic precipitation hardening light steel (Fe-8.8Mn-0.31C-5.0Al) immediately after reheating at 1100 degrees according to the present invention.
4 to 6 are photographs showing the microstructure according to tempering of the martensitic precipitation hardening type lightweight steel according to the present invention.
7 to 9 are SEM photographs showing the precipitates according to the tempering temperature of the martensitic precipitation-hardening lightweight steel according to the present invention.
10 to 11 are TEM photographs showing precipitates according to the tempering temperature of the martensitic precipitation-hardening lightweight steel according to the present invention.
12 is a graph showing the tensile properties according to the tempering temperature of the martensitic precipitation hardening light steel according to the present invention.
13 is a photograph showing a change in microstructure according to a strain rate in a tempered specimen at 100° C. of a martensitic precipitation hardening type lightweight steel according to the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. As those of ordinary skill in the art to which the present invention pertains can be easily understood, the following embodiments may be modified in various forms without departing from the concept and scope of the present invention. As far as possible, the same or similar parts are indicated using the same reference numerals in the drawings.

The terminology used in this specification is for referring only to specific embodiments and is not intended to limit the present invention. Singular forms as used herein also include plural forms unless the phrases clearly indicate the opposite.

As used herein, the meaning of "comprising" specifies a particular characteristic, region, integer, step, action, element and/or component, and other specific characteristic, region, integer, step, action, element, component and/or It does not exclude the presence or addition of the military.

All terms including technical and scientific terms used in the present specification have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are further interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal meaning unless defined.

In general, in order to lighten steel, aluminum (Al) is widely used. However, aluminum (Al) inherently poses a problem of weak strength.

Accordingly, the prior art satisfies a certain strength by utilizing kappa carbide formed from an aluminum alloy. To this end, the strength has been supplemented by forming kappa carbide while maintaining austenite and ferrite formed during phase transformation.

On the other hand, it is a well-known fact that the'martensite' structure plays an important role in high strength from the viewpoint of the strength of alloy steel. However, the prior art did not know how to form martensite as a main structure in an aluminum-manganese alloy. More precisely, although some of the martensite was formed in the prior art, it was not formed in a significant proportion to affect the strength. Therefore, the prior art chose to phase change into a single tissue of Ostaynat.

However, the present invention proposes a technique for realizing ultra-high strength by forming martensite as a main structure and further forming precipitates.

In the present specification,% means% by weight (wt%).

In this specification, an ultra-high strength martensitic precipitation hardening type lightweight steel according to the precipitation of kappa precipitates and the trip phenomenon of retained austenite, which replaces the existing high-strength carbon steel, is presented. Hereinafter, the present invention will be described in detail.

(1) Medium manganese alloy design

The alloy according to the present invention contains components of manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight, and aluminum (Al): 4.0-7.0% by weight, and the balance iron (Fe ) And other inevitable impurities.

The alloy may further contain 0.001-5.0% by weight of at least one selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and vanadium (V). have.

Hereinafter, the reason for limiting the chemical composition range of the above-described steel will be described.

Manganese (Mn): 8-10% by weight

By improving the stability of austenite at high temperature through 8-10% by weight of Mn content, it is possible to suppress ferrite transformation during cooling, thereby obtaining a martensite structure having a volume fraction of 60% or more at room temperature.

If the Mn content is less than the specified 8% by weight, the austenite stability is deteriorated, and ferrite is formed during cooling after hot rolling, and the martensite fraction falls below 60% at room temperature. On the other hand, if the Mn content exceeds 10% by weight, high-temperature austenite formed in annealing becomes stable, and martensite transformation does not occur during cooling. When the martensite fraction decreases, the strength of the steel material decreases. In addition, not only increases the raw material cost and manufacturing cost, but also causes a problem in that Mn oxide is formed on the surface, resulting in a decrease in weldability and formation of a large amount of MnS, which is a publication. MnS inclusions are strong brittle inclusions and can cause rapid destruction during molding when a large amount is formed. Therefore, in the present invention, it is preferable to limit the Mn content to 8-10% by weight.

Carbon (C): 0.2-0.4% by weight

Through the C content of 0.2-0.4% by weight, the stability of austenite can be secured at high temperature, and the strength can be improved by being dissolved in martensite at room temperature.

If the C content is less than the specified 0.2%, there is a possibility that ferrite may be formed during cooling due to a decrease in the stability of austenite, and the strength may be lowered due to less solid solution C in martensite. On the other hand, if the C content exceeds 0.4%, the strength of the hot-rolled steel sheet increases, and the cold-rolling property may decrease, and the weldability may decrease. Therefore, in the present invention, it is preferable to limit the C content to 0.2-0.4% by weight.

Aluminum (Al): 4.0-7.0% by weight

Although Al is a ferrite stabilizing element, it is added for the purpose of weight reduction. When 1% is added, it has the effect of reducing the specific gravity of about 1.6%. Al combines with carbon and manganese to form a kappa precipitate, which is a perovskite structure precipitate. It is also added as an element that improves the cleanliness of steel by acting as a deoxidation.

If the Al content is less than 4.0% by weight, the reduction in density is small, thus reducing the weight reduction effect, and it is difficult to produce kappa precipitates. On the other hand, if the Al content exceeds 7.0% by weight, problems such as an increase in austenizing temperature, an increase in raw material costs and manufacturing costs, and difficulties in continuous casting, welding and plating may occur. Therefore, in the present invention, it is preferable to limit the Al content to 4.0-7.0% by weight.

Sum of at least one of Ti, Nb, Mo, Cr, Ni, and V: 0.001-5.0% by weight

Titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni), and vanadium (V) are elements with a great effect of securing high strength with hardenability and precipitation strengthening effect. . If it is less than 0.001% by weight, it is difficult to expect sufficient hardenability and precipitation strengthening effect, and if it exceeds 5.0% by weight, a large amount of precipitate is formed and the content of dissolved carbon in martensite decreases, and eventually the strength of martensite Will degrade. In addition, manufacturing costs increase due to the high price of trace alloying elements. Therefore, in the present invention, it is preferable to limit the content to 0.001-5.0% by weight.

The lightweight steel according to the present invention may be subjected to a hot-rolling and cold-rolling process, and then an annealing process at 900-1100°C, which is an austenite single-phase region of _{Ac 3 or higher.}

The annealing process according to the present invention may be performed in a temperature range in which the volume fraction of austenite is 80% or more.

In the present invention, it is preferable that the volume fraction of martensite is 60% or more by cooling (quenching) after annealing. At this time, it is preferable that the volume fraction of retained austenite is at most 20%.

In the present invention, it is preferable that the ferrite formed during annealing remains and has a volume fraction of at most 20%.

It is preferable that the yield strength of the lightweight steel according to the present invention is 1.0 GPa or more, and the tensile strength is 1.5 GPa or more.

(2) Manufacturing method

Hereinafter, a method of manufacturing a lightweight steel according to the present invention will be described.

In the present invention, the method for manufacturing ultra-high strength martensitic precipitation hardening lightweight steel includes hot and cold rolling conditions, annealing conditions, and tempering conditions.

The manufacturing method of martensitic precipitation-hardening lightweight steel according to the present invention comprises components of manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight, and aluminum (Al): 4.0-7.0% by weight. Containing, and remaining iron (Fe) and other unavoidably contained impurities or a composition alloy containing titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and Step S1 of homogenizing by heating a composition alloy further containing 0.001-5.0% by weight of at least one selected from the group consisting of vanadium (V); The homogenized composition alloy is reheated at 1000-1200°C, which is the temperature range of the single-phase austenite region, for a preset time _{, hot finish rolling at Ac 3} temperature ~ 1000°C, and then wound at _{Ms temperature ~ Ac 3 temperature.} , S2 step of cooling and manufacturing a hot-rolled steel sheet; Step S3 of producing a cold-rolled steel sheet by performing cold rolling at room temperature on the steel sheet that has undergone the step S2; Step S4 in which the steel sheet passed through the step S3 is heated and annealed at 900-1100°C for a preset time; And it includes a step S5 of the steel sheet subjected to the step S4 is tempered at 100-150 ℃.

(2.1) Hot-rolled and cold-rolled conditions

After dissolving the composition of the alloy of the present invention, the cast ingot is homogenized at 1000-1400° C. for 12 hours for uniform distribution of alloy elements in the steel (step S1). If the heating temperature is less than 1000° C., homogenization of the playing structure is not sufficiently ensured. If it exceeds 1400°C, an increase in manufacturing cost occurs.

After that, reheating is carried out for 1 hour in the austenite single phase region of 1000-1200 degrees. Hot finish rolling is carried out at the temperature of Ac _{3 or higher and below 1000 degrees Celsius.} Thereafter, it is wound up and cooled (slow cooling) at a temperature of _{Ac 3 or less Ms (martensite generation start temperature) or higher.}

Upon cooling (slow cooling), austenite begins to transform into martensite as it passes the Ms temperature. At this time, martensite has a body centered tetragonal (bct) crystal structure, and a large amount of dislocation is formed therein, and carbon diffusion is accelerated. Therefore, during cooling (slow cooling), cementite is deposited inside martensite. The final microstructure at room temperature of the steel sheet after step S2 is composed of four phases of tempered martensite, bainite, ferrite and austenite.

Thereafter, the hot-rolled sheet may be additionally cold-rolled at room temperature to manufacture a cold-rolled steel sheet. Subsequently, the cold-rolled steel sheet may include annealing heat treatment for 1-5 minutes at 550-750°C.

(2.2) Annealing conditions

The alloy is heated at 900-1100° C. for a period of time so that the alloying elements are sufficiently dissolved. This is a heat treatment condition for obtaining the maximum value of the martensite microstructure, which plays the most important role in improving the strength, and is based on a temperature range in which the volume fraction of austenite is 80% or more through thermodynamic calculation results. The heat treatment time is set until all alloying elements are uniformly dissolved, and cooling (quick cooling) is performed at a cooling rate of 50-200℃/sec to room temperature. In the case of less than 50° C./sec, high-temperature austenite cannot be transformed into martensite, and thus the fraction of martensite decreases, and the strength may decrease. If it exceeds 200℃/sec, the manufacturing cost increases, so it is preferable to limit the cooling rate to 50-200℃/sec after the main annealing. Austenite that cannot be transformed into martensite exists as residual austenite at room temperature, and ferrite formed outside of austenite during annealing remains during cooling and is present in a maximum volume fraction of 20% at room temperature.

(2.3) Tempering conditions

The precipitates are precipitated in the temperature range of 100-300°C, which is the production temperature range of the next precipitates (kappa precipitates, cementite, epsilon carbide) and tempered until they reach equilibrium. If the temperature is less than 100°C, the precipitate may not be sufficiently precipitated. If the temperature exceeds 300°C, mechanical properties may decrease due to coarsening of the precipitate and increase in manufacturing cost.

Therefore, the tempering conditions are limited to 100-300℃.

In prior studies, a high tempering (aging) temperature of 400-600°C was required to precipitate kappa precipitates, but the present invention is limited to a lower tempering (aging) temperature of 100-150°C in order to see the cementite effect as well. desirable.

In previous studies, the main results were that the tensile strength was 1 GPa or more and the elongation was 7% or more. However, the present invention is characterized in that martensite is the main structure and has a tensile strength of 1.5 GPa and an elongation of 15% or more at which precipitation of cementite and kappa precipitates occur.

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

The steel slab composition as shown in Table 1 below was heated for 2 hours at a reheating temperature of 1150-1250°C for 2 hours, and hot rolling was performed. At this time, the hot rolling was terminated at 900-1100°C, and cooled by furnace. The prepared hot-rolled sheet was reheated at 1100°C for 15 minutes and cooled with water. These processes and heat treatment conditions are shown in detail in FIG. 1. The reheating temperature was performed at 1100° C. showing the maximum austenite fraction region through thermodynamic calculation using the TCFE9 program, and the thermodynamic calculation results are shown in FIG. 2.

The microstructure of the water-cooled hot-rolled sheet is shown in FIG. 3.

Steel grade Fe Mn C Al S P Invention River Base 8.8 0.31 5.0 0.002 0.003

Tempering was performed to precipitate precipitates using the thus prepared hot-rolled sheet. The tempering temperature range was specified from 60°C to 400°C, and the time was the same as 100 minutes. The microstructure according to the tempering temperature is shown in FIGS. 4 to 6. Unlike the microstructure before tempering, it was confirmed that as the tempering temperature increased, the precipitated area was clearly distinguished and widened inside the martensite.

On the other hand, the tempering was performed while changing the temperature to 60-400°C, and it was confirmed that the area in which the physical properties required in the present invention are satisfied is 100-150°C.

SEM analysis was performed to analyze the precipitate according to the tempering temperature. SEM observation pictures are shown in FIGS. 7 to 9. Before tempering, it was confirmed that no precipitate was present, and in the case of a 100°C tempered specimen, a 15-30 nm thick precipitate was precipitated in the martensite at 10-20 nm intervals. On the other hand, in the case of the tempered specimen at 300°C, it was confirmed that precipitates of 150-200 nm were precipitated in the martensite at intervals of 400 nm or more.

TEM analysis was performed for more detailed analysis of the precipitate. TEM observation photographs are shown in FIGS. 10 and 11. In the case of the tempered specimen at 100°C, fine precipitates having a thickness of 15-30 nm were observed as cementite through diffraction pattern analysis. In the case of the 300°C tempered specimen, it was confirmed that kappa carbide was formed in the precipitates having a thickness of 150-200 nm.

Tensile tests were conducted to confirm the mechanical properties. Fig. 12 shows a tensile curve according to tempering for 100 minutes at each temperature of the inventive steel. The 100℃ tempered specimen of Inventive Steel showed the best mechanical properties with a tensile strength of 1557 MPa and an elongation of 17%. It can be seen that the strength is improved by precipitation of fine precipitates due to tempering.

In order to analyze the metamorphic organic plasticity (trip) phenomenon in the retained austenite, an in-situ EBSD experiment was conducted. A photograph of the EBSD observation is shown in Figure 13. It was confirmed that the residual austenite volume fraction of 15.8% before stretching decreased with the strain rate to 10.5%, 6.2%, and 4.1%. Through this, it was confirmed that metamorphic organic plasticity occurred slowly depending on the strain rate.

The embodiments described in the present specification and the accompanying drawings are merely illustrative of some of the technical ideas included in the present invention. Accordingly, it is obvious that the embodiments disclosed in the present specification are not intended to limit the technical idea of the present disclosure, but to explain the technical idea, and thus the scope of the technical idea of the present disclosure is not limited by these embodiments. Modification examples and specific embodiments that can be easily inferred by those skilled in the art within the scope of the technical idea included in the specification and drawings of the present invention should be interpreted as being included in the scope of the present invention.

Claims (20)

Manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight and aluminum (Al): 4.0-7.0% by weight, the balance iron (Fe) and other inevitably contained Contains impurities,
The lightweight steel is subjected to hot-rolling and cold-rolling processes, _{and an annealing process is performed at 900-1100°C, which is an austenite single phase region of Ac 3} or higher, and a tempering process is performed at 100°C-150°C after the annealing process. ,
The lightweight steel is cooled after the annealing process so that the volume fraction of martensite is 60% or more, and the lightweight steel with a volume fraction of martensite of 60% or more has kappa precipitates, cementite, and epsilon inside the martensite after the tempering process. Martensitic precipitation hardening lightweight steel, characterized in that carbide is formed as a precipitate. The method according to claim 1,
0.001-5.0% by weight of at least one selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and vanadium (V) is further contained in the lightweight steel. Martensitic precipitation hardening type lightweight steel, characterized in that. delete The method according to claim 1,
The annealing process is a martensitic precipitation-hardening lightweight steel, characterized in that the austenite volume fraction is carried out in a temperature range of 80% or more. delete The method according to claim 1,
Martensitic precipitation hardening lightweight steel, characterized in that the volume fraction of retained austenite is formed up to 20%. The method of claim 4,
Martensitic precipitation-hardening lightweight steel, characterized in that ferrite formed during annealing remains at a maximum volume fraction of 20%. delete The method according to claim 1,
Martensitic precipitation hardening lightweight steel, characterized in that the yield strength of the lightweight steel is 1.0 GPa or more, tensile strength is 1.5 GPa or more, and elongation is 15% or more. Manganese (Mn): 8-10% by weight, carbon (C): 0.2-0.4% by weight and aluminum (Al): 4.0-7.0% by weight, the balance iron (Fe) and other inevitably contained A composition alloy containing impurities or at least one selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), nickel (Ni) and vanadium (V) is 0.001- Step S1 of homogenizing by heating the composition alloy further contained 5.0% by weight;
The homogenized composition alloy is reheated for a preset time at 1000℃-1200℃, which is the temperature range of the single-phase austenite region, and _{hot finish rolling was performed at Ac 3} temperature ~ 1000℃, and then wound at _{Ms temperature ~ Ac 3 temperature.} Taking, cooling, and S2 step of manufacturing a hot-rolled steel sheet;
Step S3 of producing a cold-rolled steel sheet by performing cold rolling at room temperature on the steel sheet that has undergone the step S2;
Step S4 in which the steel sheet passing through the step S3 is heated and annealed at 900°C-1100°C for a preset time; And
Including the step S5 of the steel sheet subjected to the step S4 is tempered at 100 ℃-150 ℃,
After the S4 step is cooled, the martensite volume fraction is 60% or more by cooling after the annealing process, and the steel sheet having a martensite volume fraction of 60% or more has kappa precipitates, cementite and A method of manufacturing a martensitic precipitation hardening type lightweight steel, characterized in that epsilon carbide is formed as a precipitate. The method of claim 10,
Homogenization of step S1 is a method of manufacturing a martensitic precipitation hardening type lightweight steel, characterized in that the homogenization is carried out at 1000 ℃-1400 ℃. The method of claim 10,
When cooling in the S2 stage, martensite transformation starts below the Ms temperature,
The final microstructure at room temperature of the steel sheet after step S2 is a method of manufacturing a martensitic precipitation hardening lightweight steel, characterized in that the four phases of tempered martensite, bainite, ferrite and austenite. The method of claim 12,
A method of manufacturing a martensitic precipitation-hardening type lightweight steel, characterized in that cementite is precipitated in the martensite upon cooling. The method of claim 10,
The annealing process of step S4 is a method of manufacturing a martensitic precipitation hardening lightweight steel, characterized in that it is performed in a temperature range in which the volume fraction of austenite is 80% or more. The method of claim 14,
The heat treatment time of the annealing process in step S4 is a time during which the alloying elements are uniformly dissolved. A method of manufacturing a martensitic precipitation hardening type lightweight steel. The method of claim 14,
The cooling rate of the annealing process in step S4 is 50 to 200°C/sec, and the method of manufacturing a martensitic precipitation hardening lightweight steel, characterized in that it is cooled to room temperature. delete The method of claim 15,
A method of manufacturing a martensitic precipitation hardening type lightweight steel, characterized in that the volume fraction of retained austenite is formed up to 20%. The method of claim 14,
A method of manufacturing a martensitic precipitation-hardening lightweight steel, characterized in that the ferrite formed in the annealing process of step S4 remains and has a volume fraction of at most 20%. delete

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