KR20180001704A - Steel having film type retained austenite - Google Patents

Abstract

The present invention relates to steel having a microstructure composed of bainite and retained austenite, wherein the area ratio of the retained austenite is 10% or more, and the retained austenite is composed of a film type retained austenite whose length is more than 3 times the width and a block type retained austenite whose length is less than 3 times the width, and wherein the area of the film type retained austenite is 60% or more of the total area of the retained austenite. The steel of the present invention can exhibit excellent strength and elongation.

Description

[0001] STEEL HAVING FILM TYPE RETAINED AUSTENITE [0002]

The present invention relates to a steel material, and more particularly, to a steel material containing a large amount of film-like retained austenite.

Transformed Organic Plasticity (TRIP) steel is a steel in which the metastable austenite structure remaining in the steel is transformed into martensite by plastic deformation applied from the outside, and the ductility is improved with the strength.

In general steels, the elongation decreases as the strength increases, while the strength decreases as the elongation increases. In the case of TRIP steels, both strength and elongation are favorable.

The process of formation of metastable retained austenite, which is indispensable for transformational organic calcination, is as follows. When a steel body made of ferrite and pearlite and having a proper amount of Mn and Si is maintained at a proper temperature between Ac1 and Ac3 where ferrites and austenite coexist, the austenite stabilizing elements in steel, especially carbon, are mostly contained in austenite It becomes employment. When the steel is quenched into a bainite transformation region having a lower temperature than that of the pearlitic transformation region and then subjected to a constant temperature transformation treatment for several minutes, protonic ferrite is formed in the austenite, and carbon is diffused from the ferrite to the austenite to increase the carbon concentration in the austenite Accordingly, the Ms point as the martensitic transformation start temperature of austenite can be lowered to room temperature or lower, and the austenite can remain stably without being transformed into martensite even at room temperature. When plastic deformation is applied to a steel material containing such a retained austenite, the plastic deformation at this time acts as a mechanical driving force, and the retained austenite is transformed into martensite, and the necking is delayed due to an increase in work hardening rate due to martensite transformation Ductility increases with strength.

The background art related to the present invention is a high-strength cold-rolled steel sheet excellent in tensile strength, yield strength and elongation, disclosed in Korean Patent Laid-Open Publication No. 10-2011-0100868 (published on September 15, 2011) and a method for producing the same.

One object of the present invention is to provide a steel comprising a large amount of film-like retained austenite.

In order to achieve the above object, the steel material according to the present invention has microstructure composed of bainite and retained austenite, wherein the area ratio of the retained austenite is 10% or more, and the retained austenite has a length of 3 times or more Shaped retained austenite having a length of less than 3 times the width of the retained austenite, wherein the area of the film-like retained austenite is 60% or more of the total area of the retained austenite.

The residual austenite can be classified into block-like retained austenite and film-like retained austenite. In this specification, the retained austenite having a length less than three times the width is defined as a block-like retained austenite, and the film-type retained austenite having a width not less than three times the width is defined as a film-like retained austenite.

The retained austenite by the constant temperature transformation process in the above-described bainite region is mostly block-like retained austenite although it contains some film-like retained austenite.

The block-like retained austenite is relatively coarse and the supersaturated carbon concentration is also relatively low. Therefore, there is a problem that even a small deformation is transformed into martensite. On the other hand, since the film-like retained austenite is relatively fine and the supersaturated carbon concentration is relatively high, the effect of retarding the martensitic transformation is obtained even under large deformation.

In addition, the steel material may include 0.2 to 0.5% of C, 1.0 to 3.0% of Si, and 1.0 to 3.0% of Mn, and the balance of Fe and unavoidable impurities.

The steel material contains 0.2 to 0.5% of C, 1.0 to 3.0% of Si, 1.0 to 3.0% of Mn, 0.1% or less of P, 0.1% or less of S, , At most one of N and 0.02% or less of Cr, at most 3.0% of Cr, at most 1.0% of Mo, at most 0.005% of B, at most 0.1% of Nb, at most 0.5% of V and at most 0.1% And Ca: 0.005% or less, and the balance of Fe and unavoidable impurities.

The steel material may contain 0.2 to 0.5% of C, 1.0 to 4.0% of Si, 1.0 to 3.0% of Mn, and 0.5 to 2.0% of Al, and the balance of Fe and unavoidable impurities.

The steel material preferably contains 0.2 to 0.5% of C, 1.0 to 3.0% of Si, 1.0 to 3.0% of Mn, and 0.5 to 2.0% of Al in terms of weight%, P: 0.1% At least one of Cr, 3.0% or less, Mo: at most 1.0%, B: at most 0.005%, Nb: at most 0.1%, V: at most 0.5%, Ti: at most 0.1% And Ca: 0.005% or less, and the balance of Fe and unavoidable impurities.

The steel material according to the present invention comprises a large amount of film-like retained austenite. Accordingly, the steel according to the present invention is advantageous in strength and ductility in comparison with a conventional steel mainly containing block-like retained austenite.

1 is a flowchart schematically showing a method of manufacturing a steel material according to the present invention.
Fig. 2 is a view conceptually showing that bainite is formed as a result of first-order constant temperature transformation.
Fig. 3 is a view conceptually showing that bainite is additionally formed as a result of secondary thermostatic transformation.
Fig. 4 shows the thermostatic transformation diagram of the steel grade 1. Fig.
Fig. 5 shows a thermostatic transformation diagram of the steel type 2. Fig.
Fig. 6 shows a thermostatic transformation diagram of the steel grade 3. Fig.
Fig. 7 shows the microstructure of the specimen 11 and specimen 12. Fig.
Fig. 8 shows the strain-stress curves of the specimen 11 and specimen 12. Fig.
Figure 9 shows EBSD results for various variations of specimen 11 and specimen 12.
Figure 10 shows the normalized residual austenite volume fraction for various strains.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a steel material containing a film-like retained austenite according to the present invention will be described in detail with reference to the accompanying drawings.

The high strength steel sheet according to the present invention has a microstructure composed of bainite and retained austenite. At this time, the area ratio of retained austenite is 10% or more, and the area ratio of bainite is 90% or less.

The residual austenite is composed of a film-like retained austenite and a block-type retained austenite.

In the present invention, when the film-like retained austenite is longer than the width, it means a retained austenite whose length is at least three times the width, more specifically at least three times the maximum width. In addition, the block-like retained austenite means residual austenite other than film-like retained austenite, that is, retained austenite having a length less than three times the width.

The block-like retained austenite is relatively coarse and the supersaturated carbon concentration is also relatively low. Therefore, there is a problem that even a small deformation is transformed into martensite. On the other hand, since the film-like retained austenite is relatively fine and the supersaturated carbon concentration is relatively high, the effect of retarding the martensitic transformation is obtained even under large deformation.

At this time, the area of the film-like retained austenite is larger than the area of the block-type retained austenite, and particularly, the area of the film-like retained austenite is 60% or more of the total area of the retained austenite. As described above, since the steel sheet according to the present invention contains more than 60% of the total retained austenite in the film-like retained austenite, the martensite transformation can be delayed even in a large deformation compared to the conventional block-type retained austenite- Accordingly, the strength and elongation can be greatly improved.

Such microstructural characteristics of the steel material according to the present invention can be achieved by a manufacturing method including a multi-step constant temperature transformation in the bainite region described later.

The steel material according to the present invention can be applied to any steel sheet having an alloy composition capable of containing residual austenite in the final microstructure, but it is more preferable that the area ratio of retained austenite can be stably maintained at 10% A steel material having an alloy composition can be presented. The shape of the steel sheet prior to the heat treatment may be a hot rolled steel sheet or a cold rolled steel sheet, and more preferably a cold rolled steel sheet.

The high strength steel sheet according to the first preferred embodiment of the present invention may contain 0.2 to 0.5% of C, 1.0 to 3.0% of Si, and 1.0 to 3.0% of Mn in terms of% by weight and the balance of Fe and unavoidable impurities .

The high-strength steel sheet according to the first embodiment further contains at least one of P: not more than 0.1%, S: not more than 0.1%, Al: not more than 0.5%, and N: not more than 0.02% can do. The high-strength steel sheet according to the above example may further contain, by weight%, at most 3.0% of Cr, at most 1.0% of Mo, at most 0.005% of B, at most 0.1% of Nb, at most 0.5% of V, 0.1% or less and Ca: 0.005% or less.

Carbon (C) is an element for forming a large amount of retained austenite in the steel sheet. The carbon is preferably contained in an amount of 0.2 to 0.5% by weight based on the total weight of the steel sheet. If the content of carbon is less than 0.2% by weight, it may be difficult to secure a residual austenite of 10% or more in the final microstructure. On the other hand, if the content of carbon exceeds 0.5% by weight, the weldability may be deteriorated.

Silicon (Si) contributes to increasing the thermal mechanical stability of austenite by contributing to carbon enrichment in retained austenite by inhibiting carbide formation. And acts as a deoxidizer in the steel. Silicon also stabilizes the ferrite and contributes to strength. Silicon also serves to increase the ferrite fraction by promoting austenite-ferrite transformation. In the present embodiment, the silicon is preferably contained in an amount of 1.0 to 3.0% by weight based on the total weight of the steel sheet. When the content of silicon exceeds 1.0% by weight, the effect of the addition is insufficient. On the contrary, when the content of silicon exceeds 3.0% by weight, the weldability and plating ability may be lowered.

Manganese (Mn) contributes to austenite stabilization and strength enhancement. The manganese is preferably added in an amount of 1.0 to 3.0% by weight based on the total weight of the steel sheet. If the addition amount of manganese is less than 1.0% by weight, the effect of addition thereof is insufficient. On the other hand, if the addition amount of manganese exceeds 3.0% by weight, an oxide scale problem and a plating problem may occur.

(P), sulfur (S), nitrogen (N), aluminum (Al), chromium (Cr), molybdenum (Mo), boron (B), niobium (Nb), vanadium (V), titanium (Ti), calcium (Ca) Phosphorus (P), sulfur (S) and nitrogen (N) contribute partially to strength, workability and grain refinement, but if contained in large amounts, toughness and cracks can be caused. When these elements are included, the content thereof is limited to P: not more than 0.1% by weight, S: not more than 0.1% by weight, Al: not more than 0.5% and N: not more than 0.02% by weight based on the total weight of the steel sheet. Elements such as chromium (Cr), molybdenum (Mo), boron (B), niobium (Nb), vanadium (V) and titanium (Ti) contribute to the improvement of steel strength through work hardening, precipitation hardening, Calcium (Ca) contributes to the purification of steel by spheroidizing the inclusions. However, if these components are excessive, the combination of strength and elongation can be lowered or the effect can be saturated due to a decrease in elongation. Therefore, it is preferable that Cr: not more than 3.0% by weight, Mo: not more than 1.0% 0.005 wt% or less, Nb: 0.1 wt% or less, V: 0.5 wt% or less, Ti: 0.1 wt% or less, and Ca: 0.005 wt% or less.

The high strength steel sheet according to the second preferred embodiment of the present invention contains 0.2 to 0.5% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, and 0.5 to 2.0% of Al in terms of% by weight. The remainder can be made of Fe and unavoidable impurities.

The high-strength steel sheet according to the second embodiment may further contain at least one of P: not more than 0.1%, S: not more than 0.1%, and N: not more than 0.02% in weight% instead of Fe. The high-strength steel sheet according to the above example may further contain, by weight%, at most 3.0% of Cr, at most 1.0% of Mo, at most 0.005% of B, at most 0.1% of Nb, at most 0.5% of V, 0.1% or less and Ca: 0.005% or less.

The high-strength steel sheet according to the second embodiment contains no silicon or 1.0 wt% or less of aluminum, and 0.5-2.0 wt% of aluminum (Al).

In the high strength steel sheet according to the second embodiment, the silicon is preferably contained in an amount of 1.0 wt% or less based on the total weight of the steel sheet. Considering that aluminum (Al) is contained in an amount of 0.5 to 2.0% by weight in this embodiment, when the content of silicon exceeds 1.0% by weight in this embodiment, the weldability and plating ability may be lowered.

Aluminum (Al) generally acts as a deoxidizer, but aluminum in the high-strength steel sheet according to the second embodiment promotes the austenite-bainite phase transformation to improve the productivity. The aluminum is preferably contained in an amount of 0.5 to 2.0% by weight based on the total weight of the steel sheet. If the addition amount of aluminum is less than 0.5% by weight, the productivity improvement effect may be insufficient. On the contrary, when the addition amount of aluminum exceeds 2.0% by weight, the steel sheet surface quality may be a problem.

On the other hand, in the high-strength steel sheet according to the second embodiment, silicon and aluminum are more preferable in terms of surface quality and plating property, with Si = Al and Si + Al = 2.5% by weight.

(P), sulfur (S), nitrogen (N), chromium (Cr), molybdenum (Mo), boron (B), or the like is used as the impurity ), Niobium (Nb), vanadium (V), titanium (Ti), calcium (Ca) The phosphorus (P), sulfur (S) and nitrogen (N) contribute to the strength, workability and grain refinement. However, when they are contained in large amounts, they may cause toughness and cracks. When these elements are included, P: not more than 0.1% by weight, S: not more than 0.1% by weight, and N: not more than 0.02% by weight based on the total weight of the steel sheet. Elements such as chromium (Cr), molybdenum (Mo), boron (B), niobium (Nb), vanadium (V) and titanium (Ti) contribute to the improvement of steel strength through work hardening, precipitation hardening, Calcium (Ca) contributes to the purification of steel by spheroidizing the inclusions. However, if these components are excessive, the combination of strength and elongation can be lowered or the effect can be saturated due to a decrease in elongation. Therefore, it is preferable that Cr: not more than 3.0% by weight, Mo: not more than 1.0% 0.005 wt% or less, Nb: 0.1 wt% or less, V: 0.5 wt% or less, Ti: 0.1 wt% or less, and Ca: 0.005 wt% or less.

The high-strength steel sheet having the alloy composition according to the first or second embodiment has a tensile strength of 1000 MPa or more and a product of tensile strength and elongation of 25,000 MPa ·% or more, in some cases of 30,000 MPa ·% or more, Or more. Further, the high strength steel sheet according to the present invention can exhibit an elongation of 25% or more.

However, in the case of a steel having a C content of less than 0.2% by weight and a Mn content of less than 1.0% by weight, the hardenability is low and a high temperature phase such as ferrite is likely to be generated during cooling after austenitization, It may become difficult. Therefore, a steel material having a C content of 0.2% by weight or more and a Mn content of 1.0% by weight or more can be preferably applied to the present invention as in the above-mentioned examples.

1 is a flowchart schematically showing a method of manufacturing a steel material according to the present invention.

Referring to FIG. 1, a method of manufacturing a high strength steel sheet according to an embodiment of the present invention includes austenitization step (S110), a primary thermostat transformation step (S120), and a second thermostat transformation step (S130). A feature of this method is that both primary and secondary thermostatic transformation are performed at temperatures higher than the bainite region, i.e., the martensitic transformation temperature.

In the austenitizing step (S110), the steel sheet is heated and austenitized. Through this, the microstructure can be fully austenitized.

The steel sheet to be used is not particularly limited as long as it is a steel sheet having an alloy composition capable of containing retained austenite in the final microstructure. More preferably, A steel sheet having an alloy composition according to one embodiment or the second embodiment can be presented. The shape of the steel sheet prior to the heat treatment may be a hot rolled steel sheet or a cold rolled steel sheet, more preferably a cold rolled steel sheet.

The austenitization can be carried out at a temperature of Ac 3 to Ac 3 + 200 ° C for 1 minute or more, for example, for 1 to 30 minutes. When the austenitizing temperature is lower than Ac3, a large amount of ferrite remains, and when the austenitizing temperature exceeds Ac3 + 200 deg. C, the grain size may excessively increase. Also, if the austenitization is less than one minute, austenitization may be insufficient.

Next, in the first thermostatic transformation step (S120), the austenitized steel sheet is first cooled to T1 corresponding to the bainite region, and subjected to primary thermostatic transformation. Here, the bainite region means a temperature region not less than Bs, which is the bainite transformation start temperature, and Ms or more, which is the martensitic transformation start temperature.

Here, the first-order constant temperature transformation may be performed at T1, but it is not necessarily limited thereto, and may be performed at a temperature lower by about 10 ° C than T1 depending on process facility conditions and the like. This concept can be similarly applied to the second-order constant temperature transformation described later.

As a result of the first-order constant temperature transformation in the bainite region, a part of the austenite is transformed into bainite, more specifically, lath-shaped bainite as in the example shown in Fig. Austenite remains in the form of a film between the bainites, but austenite generally remains in the form of blocks in areas where bainites are not formed.

The primary thermostatic transformation can be performed so that the bainite transformation is 30 to 70% in area ratio. This considers the formation of retained austenite in the form of a film between lath-shaped bainites and the formation of retained austenite in an area ratio of 10% or more after secondary thermostatic transformation.

The average cooling rate during the primary cooling may be 20 ° C / sec or more, more preferably 50 to 100 ° C / sec, in order to minimize the occurrence of phase transformation such as ferrite.

Next, in the secondary thermostatic transformation step (S130), the steel sheet subjected to the first thermostat transformation is cooled to an average temperature of not less than 20 占 폚 / sec, for example, 20 to 100 占 폚 / sec, corresponding to the bainite region but lower than 50 占 폚 Second rate, and secondarily thermostable. After the secondary thermostatic transformation, final cooling may be performed by air cooling, water cooling, or the like, and the final cooling may be performed at room temperature.

As a result of secondary thermostatic transformation in the bainite region, a part of the remaining austenite is further transformed into bainite as in the example shown in Fig. In this process, the bainite is formed in the block-shaped austenite, and the residual austenite fraction in the form of the film is increased.

The reason why the second-order constant-temperature transformation temperature is lower than the first-order constant-temperature transformation temperature by 50 ° C or more is that when the temperature difference between the second-order constant-temperature transformation temperature and the first-order constant-temperature transformation temperature is less than 50 ° C , The strength was greatly reduced and the resultant combination of strength elongation was not good.

That is, in the case of the present invention, the austenite in the form of a film and the austenite in the form of a block remain while the austenite is transformed into bainite in the first thermostatic transformation, and in particular, Type austenite is further transformed into bainite so that the residual austenite fraction in the form of a film is increased.

On the other hand, in the case of the steel sheet having the alloy composition according to the first embodiment, the first-order constant temperature transformation can be performed at 400 to 600 ° C for 20 to 100 seconds. In a steel sheet containing the above alloy composition, if T1 is less than 400 DEG C, secondary thermostatic transformation at Ms or more may be difficult. If the primary thermostatic transformation time is less than 20 seconds, bainite may not be sufficiently formed. If 100 seconds have elapsed, formation of retained austenite at an area ratio of 10% or more after secondary thermostatic transformation may become difficult.

Further, in order to form sufficient bainite at the time of secondary thermostatic transformation, it is preferable to perform secondary thermostatic transformation for 100 seconds or more. In addition, the second-order constant temperature transformation is performed at a temperature 50 ° C lower than the first constant-temperature transformation temperature. The secondary thermostatic transformation can be performed for 100 seconds or more, more preferably 100 to 150 seconds. The film-type austenite fraction of the retained austenite can be increased to the maximum through the bainite additional transformation in the form of a lass through the second-order constant-temperature transformation of 100 seconds or more.

On the other hand, in the case of the steel sheet having the alloy composition according to the second embodiment, the first-order constant temperature transformation can be performed at 400 to 600 ° C for 3 to 25 seconds. In the case of the present invention, when the aluminum is added in an amount of 0.5 wt% or more, the austenite-bainite phase transformation is promoted and the phase transformation time can be reduced to 25 seconds or less. If the first thermostatic transformation time is less than 3 seconds, bainite may not be sufficiently formed. On the other hand, when the primary thermostatic transformation time exceeds 25 seconds, formation of retained austenite at an area ratio of 10% or more after the secondary thermostatic transformation may become difficult.

In addition, in order to form sufficient bainite during the second-stage constant temperature transformation, it is preferable to perform secondary thermostatic transformation for 40 seconds or more, more preferably 40 to 80 seconds. In the case of the alloying composition according to the first embodiment, the second-order constant temperature transformation is required for at least about 100 seconds, while in the case of the alloy composition according to the second embodiment, .

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. Preparation of steel plate specimens

The cold-rolled steel sheet specimens having the alloy components described in Table 1 were austenitized at 900 ° C for 10 minutes, cooled to an average cooling rate of 60 ° C / sec to the first temperature-constant transformation temperature shown in Table 2, Secondarily cooled to the secondary thermostatic transformation temperature as shown in Table 2 at an average cooling rate of 25 DEG C / sec, subjected to secondary thermostatic transformation for 100 seconds, and then cooled at an average cooling rate of 30 DEG C / sec at 25 DEG C To prepare steel plate specimens 1 to 8.

[Table 1]

[Table 2]

The cold-rolled steel sheet specimen having the alloy component described in Table 3 was austenitized at 900 ° C for 10 minutes, cooled to an average cooling rate of 60 ° C / sec to the primary heat-resistant transformation temperature listed in Table 4, And then secondary cooled to the secondary thermostatic transformation temperature described in Table 2 at an average cooling rate of 25 DEG C / sec, subjected to secondary thermostatic transformation for 60 seconds, and then cooled at an average cooling rate of 30 DEG C / sec And finally cooled to 25 캜 to prepare steel plate specimens 9 to 10.

[Table 3]

[Table 4]

2. Microstructure and property evaluation

The residual austenite fraction of the prepared steel plate specimens 1 to 10 was analyzed by SEM photograph and TEM photograph analysis, and the residual austenite having a maximum length of 3 times or more of the maximum width was made into a retained austenite film. The tensile strength and elongation of the prepared steel sheet specimens were measured by tensile test.

The results are shown in Table 5.

In Table 5, the? Fraction means the residual austenite fraction, and the f-? Fraction means the fraction of retained austenite in film form in retained austenite.

 [Table 5]

Referring to Table 5, in the case of specimens 2, 6, 7, 9 and 10 in which the retained austenite fraction was not less than 10% by area ratio and the film retained austenite fraction in retained austenite was not less than 60% And a product of tensile strength and elongation of 25,000 MPa ·% or more, and elongation of 25% or more.

On the other hand, in the case of specimens 1, 3, 4, 5 and 8 in which the residual austenite fraction is less than 10% in area ratio or the film retained austenite fraction in retained austenite is less than 60%, the tensile strength is less than 1000 MPa, And the elongation ratio was less than 25,000 MPa ·%. This is due to the fact that the transformation temperature difference is less than 50 占 폚 even if the alloy composition is such that the retained austenite can not be produced by more than 10%, or the two-step constant temperature transformation is not carried out or the two-step constant temperature transformation is carried out.

Fig. 4 shows the thermostatic transformation diagram of the steel type 2, Fig. 5 shows the constant temperature transformation diagram of the steel type 5, and Fig. 6 shows the constant temperature transformation diagram of the steel type 6. Fig. 4 to 6, it can be seen that the transformation time is remarkably reduced in the case of the steel type 5 and the steel type 6 to which Al is added in an amount of 0.5 wt% or more, compared with the steel type 2 to which no Al is added.

As a result, in the case of specimens 9 and 10, although the constant temperature transformation time is relatively shorter than that of specimens 2, 6 and 7, the prepared steel sheet can exhibit the same or higher physical properties, have.

Fig. 7 shows the microstructure of specimen 11 and specimen 12, and Fig. 8 shows the strain-stress curve of specimen 11 and specimen 12. Fig.

The specimen 11 and the specimen 12 of Figs. 7 and 8 were manufactured by the following procedure.

The cold-rolled steel sheet specimen having the composition of steel grade 2 was austenitized at 950 ° C for 5 minutes, cooled to 400 ° C at an average cooling rate of 20 ° C / sec and then subjected to constant temperature transformation for 100 seconds and water quenching to prepare specimen 11 Respectively.

The cold-rolled steel sheet specimen having the composition of steel grade 2 was austenitized at 950 ° C for 5 minutes, primary cooled to 500 ° C at an average cooling rate of 20 ° C / sec, subjected to primary thermostatic transformation for 30 seconds, / sec to 400 DEG C, secondary thermocouple transformation was performed for 100 seconds, and water quenching was performed to prepare specimen 12.

7, most of the block-like retained austenite (γ _B ) is observed in the microstructure of the specimen 11 (FIG. 11A), but the microstructure of the specimen 12 (FIG. 11B) It can be seen that the residual austenite is film-like retained austenite ((? _F ).

8, the specimen 11 and the specimen 12 show similar tensile strengths, while the specimen 11 containing a large amount of the block-like retained austenite shows elongation of about 25%, whereas the film-like retained austenite The elongation of the specimen 12 is about 30%.

Figure 9 shows the EBSD results for various variations of specimen 11 and specimen 12, and Figure 10 shows the normalized residual austenite volume fraction for various variations.

Figures 9 (a), 9 (b) and 9 (c) are for sample 11, and Figures 9 (d), 9 (e), 9 (f) and 9 (g) are for sample 12. Fig. 9 (a) shows a 0% strain, Fig. 9 (b) shows a 6% strain, % Strain, and (g) is for 24% strain. In Fig. 9, green means bainite, red means retained austenite, and black means martensite.

9 (a) and 9 (d), there are many block-like retained austenite in the case of the specimen 11, but a large number of film-like retained austenite in the case of the specimen 12 can be seen.

9 and 10, it can be seen that, in the case of the specimen 11, most of the retained austenite was transformed into martensite with respect to deformation of up to 14% (FIGS. 9 (b) and 9 (c)). On the other hand, in the case of the specimen 12, it is seen that the residual austenite phase remains almost unchanged as martensite with respect to the deformation of up to 14% (Figs. 9E and 9F) (Fig. 9 (g)).

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.

Claims (5)

Bainite and residual austenite, wherein the area ratio of the retained austenite is 10% or more,
Wherein the residual austenite is composed of a film-like retained austenite having a length three times or more of the width and a block-type retained austenite having a width less than 3 times the width, wherein the area of the film-like retained austenite is 60 % Or more.
The method according to claim 1,
Wherein the steel comprises, by weight, 0.2 to 0.5% of C, 1.0 to 3.0% of Si, and 1.0 to 3.0% of Mn and the balance of Fe and unavoidable impurities.
The method according to claim 1,
Wherein the steel contains 0.2 to 0.5% of C, 1.0 to 3.0% of Si, 1.0 to 3.0% of Mn, 0.1% or less of P, 0.1% or less of S, : Not more than 0.02%, or not more than 3.0% of Cr, not more than 1.0% of Mo, not more than 0.005% of B, not more than 0.1% of Nb, not more than 0.5% of V, not more than 0.1% of Ti, not more than 0.1% : 0.005% or less, and the balance of Fe and unavoidable impurities.
The method according to claim 1,
Wherein the steel comprises, by weight, 0.2 to 0.5% of C, 1.0 to 4.0% of Si, 1.0 to 3.0% of Mn, and 0.5 to 2.0% of Al, with the balance of Fe and unavoidable impurities.
The method according to claim 1,
Wherein the steel contains 0.2 to 0.5% of C, 1.0 to 3.0% of Si, 1.0 to 3.0% of Mn and 0.5 to 2.0% of Al, 0.1% or less of P, 0.1% or less of S, : Not more than 0.02%, or not more than 3.0% of Cr, not more than 1.0% of Mo, not more than 0.005% of B, not more than 0.1% of Nb, not more than 0.5% of V, not more than 0.1% of Ti, not more than 0.1% : 0.005% or less, and the balance of Fe and unavoidable impurities.

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