KR101695263B1 - High strength steel sheet with excellent productivity, combination of strength and ductility, method of manufacturing the same - Google Patents

Abstract

A high strength steel sheet excellent in productivity and excellent in combination of strength and ductility, and a method of manufacturing the same.
The method for manufacturing a high strength steel sheet according to the present invention is characterized in that it comprises 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 and the balance of Fe and unavoidable impurities Heating the steel sheet to austenitize the steel sheet; Firstly cooling the austenitized steel sheet to T1 corresponding to the bainite region, and subjecting the steel sheet to first-order thermophilic transformation; And secondarily cooling the first steel sheet to a temperature T2 corresponding to the bainite region and lower than the T1 by at least 50 deg. C and subjecting the first steel sheet to constant temperature transformation.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel sheet excellent in productivity, a combination of strength and ductility, and a manufacturing method thereof. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

TECHNICAL FIELD The present invention relates to a high strength steel sheet manufacturing technique, and more particularly, to a high strength steel sheet having excellent productivity and excellent combination of strength and ductility, and a manufacturing method thereof.

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

In the case of a general steel sheet, the elongation decreases as the strength increases, while the strength decreases as the elongation increases. In the case of TRIP steel, 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 sheet composed of ferrite and pearlite and having a proper amount of Mn and Si is maintained at a proper temperature between Ac1 and Ac3 in which ferrite and austenite coexist, the austenite stabilizing element in steel sheet, particularly carbon, 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 of the steel sheet containing such retained austenite is applied, 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 the increase of the work hardening rate due to the martensitic 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.

It is an object of the present invention to provide a method of manufacturing a high strength steel sheet which can increase the film-like retained austenite fraction using the cascade constant temperature transformation in the bainite region and improve the productivity.

Another object of the present invention is to provide a high strength steel sheet having a high film-like retained austenite fraction and thus having excellent strength and ductility combination.

A method of manufacturing a high strength steel plate for achieving the above object, comprising: 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, A step of heating and austenitizing a steel sheet made of unavoidable impurities; Firstly cooling the austenitized steel sheet to T1 corresponding to the bainite region, and subjecting the steel sheet to first-order thermophilic transformation; And secondarily cooling the first steel sheet to a temperature T2 corresponding to the bainite region and lower than the T1 by at least 50 deg. C and subjecting the first steel sheet to constant temperature transformation.

At this time, the steel sheet is more preferably Si? Al and Si + Al? 2.5% by weight.

In addition, austenite in the form of a film and austenite in the form of a block remain as the austenite is transformed into bainite in the primary thermostatic transformation, and a block-shaped austenite formed in the primary thermostatic transformation at the secondary thermostatic transformation The residual austenite fraction in the form of a film is increased while being further transformed into bainite.

The austenitization may be carried out by maintaining Ac3 to Ac3 + at 200 DEG C for 1 minute or more.

Further, each of the primary cooling and the secondary cooling may be performed at an average cooling rate of 20 DEG C / sec or more.

In addition, the primary thermostatic transformation may be performed such that the bainite transformation is 30 to 70% in area ratio.

Also, T1 may be 400 DEG C or higher, and the primary thermostatic transformation may be performed for 3 seconds to 25 seconds.

Further, the secondary thermostatic transformation may be performed for 40 seconds or more.

In order to achieve the other object, the high strength steel sheet 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 and the balance of Fe and unavoidable impurities Wherein the retained austenite has a microstructure including bainite and retained austenite, wherein the area percentage of the retained austenite is 10% or more, the retained austenite has a film-like retained austenite having a length three times or more of the width, Shaped retained austenite, wherein the area of the film-shaped retained austenite is larger than the retained austenite area of the block shape.

At this time, the steel sheet is more preferably Si? Al and Si + Al? 2.5% by weight.

The area of the film-like retained austenite may be 60% or more of the total area of the retained austenite.

The high-strength steel sheet may have a tensile strength of 1000 MPa or more and a product of tensile strength and elongation of 25,000 MPa ·% or more. Furthermore, the high-strength steel sheet can exhibit an elongation of 25% or more.

According to the method for manufacturing a high strength steel sheet according to the present invention, a high-strength steel sheet having excellent strength and ductility can be produced by forming a large amount of film-like retained austenite through a two-step constant temperature heat treatment process.

In addition, according to the method for manufacturing a high strength steel sheet according to the present invention, addition of Al in an amount of 0.5 wt% or more in addition to C, Si, and Mn can shorten the heat treatment time at a constant temperature, thereby improving productivity.

1 is a flowchart schematically showing a method of manufacturing a high-strength steel sheet 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.

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.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a high strength steel sheet excellent in strength and ductility 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 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 above example 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 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.

Hereinafter, the role and content of the alloy components included in the high-strength steel sheet according to the above example will be described.

Carbon (C)

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)

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.

The silicon is preferably contained in an amount of 1.0 wt% or less based on the total weight of the steel sheet. If the content of silicon exceeds 1.0% by weight, the weldability and plating ability may be deteriorated.

Manganese (Mn)

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.

Aluminum (Al)

Aluminum (Al) generally acts as a deoxidizer, but aluminum in the high-strength steel sheet according to the present invention 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, the above silicon and aluminum are more preferable in view of surface quality and plating ability because Si? Al and Si + Al? 2.5% by weight.

Other

(P), sulfur (S), nitrogen (N), chromium (Cr), molybdenum (Mo), boron (B) and niobium (Nb) are added to the high strength steel sheet according to the present invention, ), Vanadium (V), titanium (Ti), calcium (Ca), and the like.

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 and precipitation hardening, 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 preferred embodiment of the present invention having the above alloy composition can exhibit a tensile strength of 1000 MPa or more and a product of tensile strength and elongation of 25,000 MPa ·% or more, Further, the high strength steel sheet according to the present invention can exhibit an elongation of 25% or more.

Further, the high-strength steel sheet according to the present invention has a microstructure including bainite and retained austenite. At this time, the retained austenite has an area ratio of 10% or more. The residual austenite is composed of residual austenite in film form and retained austenite in block form.

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

At this time, the high-strength steel sheet according to the present invention is characterized in that the area of the film-shaped retained austenite is larger than that of the block-shaped retained austenite.

More specifically, the area of the film-shaped retained austenite may be 60% or more of the total area of the retained austenite.

The microstructural characteristics of the high strength steel sheet 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.

1 is a flowchart schematically showing a method of manufacturing a high-strength steel sheet 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).

In the austenitizing step (S110), the steel sheet having the above-described alloy composition is heated and austenitized. Through this, the microstructure can be fully austenitized.

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.

The first thermostatic transformation may be performed at T1 but may be performed at a temperature lower than about T1 by about 10 캜 depending on process equipment 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.

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, the first thermostatic 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. When aluminum is not added, the secondary thermostatic transformation is required for about 100 seconds or more, but in the case of the present invention, the secondary thermostatic transformation can also be reduced to more than 40 seconds through the effect of aluminum addition.

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.

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 specimen having the alloy components listed in Table 1 was austenitized at 900 ° C for 10 minutes, first cooled to the first temperature-constant transformation temperature described in Table 2 at an average cooling rate of 60 ° C / sec, Secondarily 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 at 25 DEG C To thereby prepare steel plate specimens 1 to 6.

[Table 1]

[Table 2]

2. Microstructure and property evaluation

SEM photographs and TEM photographs of the prepared steel plate specimens were used to calculate the retained austenite fraction. The residual austenite fraction 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 3.

In Table 3, the gamma fraction means the residual austenite fraction, and the f-gamma fraction means the fraction of retained austenite in film form in retained austenite.

[Table 3]

Referring to Table 1, in the case of the specimens 2 and 4 satisfying the alloy composition and the first-order constant-temperature transformation and the second-order constant-temperature transformation conditions proposed in the present invention, the product of tensile strength of 1000 MPa or more and tensile strength and elongation of 25,000 MPa ·% And an elongation of 25% or more.

On the other hand, when the alloy composition is not satisfied (Sample 1), the alloy composition is satisfied but the first constant temperature transformation temperature exceeds 25 seconds (Specimens 3 and 5), the tensile strength is less than 1000 MPa or the product of tensile strength and elongation It was less than 25,000 MPa ·%.

Fig. 4 shows the thermostatic transformation diagram of the steel type 1, Fig. 5 shows the constant temperature transformation diagram of the steel type 2, and Fig. 6 shows the constant temperature transformation diagram of the steel type 3. 4 to 6, it can be seen that the transformation time is remarkably reduced in the case of the steel type 2 and the steel type 3 to which Al is added in an amount of 0.5% by weight or more, compared with the steel type 1 to which no Al is added.

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 (15)

A steel sheet comprising 0.2 to 0.5% of C, 1.0% or less of Si (excluding 0%), 1.0 to 3.0% of Mn, and 0.5 to 2.0% of Al and containing the remaining Fe and unavoidable impurities is heated Austenitizing;
The austenitized steel sheet is firstly cooled to T1 corresponding to the bainite region, and the austenite in the form of a film and the austenite in block form remain while the austenite is transformed into bainite by the first thermostatic transformation;
The primary thermostat-transformed steel sheet is secondarily cooled to T2, which is in the bainite region but is lower than the above-mentioned T1 by 50 占 폚 or more, and the austenite in the block form formed in the first thermostatic transformation is further transformed into bainite So that the residual austenite fraction in the form of a film is increased,
Wherein martensite and carbide are not produced by the above steps.
The method according to claim 1,
Wherein the steel sheet has Si? Al and Si + Al? 2.5 weight%.
The method according to claim 1,
The steel sheet 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% by weight, Cr: not more than 3.0%, Mo: not more than 1.0% : 0.005% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.1% or less, and Ca: 0.005% or less.
delete The method according to claim 1,
Wherein the austenitization is carried out by maintaining at a temperature of Ac 3 to Ac 3 + 200 ° C for at least 1 minute.
The method according to claim 1,
Wherein each of the primary cooling and the secondary cooling is performed at an average cooling rate of 20 DEG C / sec or more.
The method according to claim 1,
Wherein the primary thermostatic transformation is performed such that the bainite transformation is 30 to 70% in area ratio.
The method according to claim 1,
Wherein the T1 is 400 DEG C or more and the primary thermostatic transformation is performed for 3 to 25 seconds.
The method according to claim 1,
Wherein the secondary thermostatic transformation is performed for at least 40 seconds.
The steel sheet according to any one of claims 1 to 3, which comprises 0.2 to 0.5% of C, 1.0% or less of Si (excluding 0%), 1.0 to 3.0% of Mn and 0.5 to 2.0% of Al,
Does not include martensite and carbide formed during cooling,
Bainite and residual austenite, wherein the area ratio of the retained austenite is 10% or more,
Wherein the retained austenite is composed of a residual film of austenite having a length three times or more of the width and a residual austenite having a shape of a block having a length of less than 3 times the width of the retaining austenite, Is larger than the austenite area.
11. The method of claim 10,
Wherein the high-strength steel sheet comprises Si? Al and Si + Al? 2.5% by weight.
11. The method of claim 10,
The steel sheet 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% by weight, Cr: not more than 3.0%, Mo: not more than 1.0% : 0.005% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.1% or less and Ca: 0.005% or less
11. The method of claim 10,
And the area of the film-shaped retained austenite is 60% or more of the total area of the retained austenite.
11. The method of claim 10,
Wherein the high-strength steel sheet has a tensile strength of 1000 MPa or more and a product of tensile strength and elongation of 25,000 MPa ·% or more.
15. The method of claim 14,
Wherein the high-strength steel sheet exhibits an elongation of 25% or more.

Images