KR101676128B1 - Quenched steel sheet having excellent strength and ductility and method for manufacturing the steel sheet using the same - Google Patents

Abstract

A heat-cured steel sheet and a method of manufacturing the same are disclosed. In one aspect of the present invention, there is provided a steel sheet comprising 0.12 to 0.25% of C, 0.5% or less of Si (excluding 0), 0.1 to 2.0% of Mn, 0.05% %, The balance Fe and other unavoidable impurities, wherein the average dislocation density determined by XRD measurement of the steel sheet is not less than 7 x 10 ¹⁵ / m ² , and the microstructure of the steel sheet is martensite having the first hardness Site and 90% by volume or more of martensite having a second hardness, wherein the first hardness has a hardness value larger than the second hardness, and the ratio of the difference between the first hardness and the second hardness to the first hardness is Wherein the heat treatment curing-type steel sheet satisfies the following relational expression (1).
[Relation 1]
5? (First hardness - second hardness) / (first hardness) * 100? 30

Description

TECHNICAL FIELD [0001] The present invention relates to a heat-curing type steel sheet having excellent strength and ductility, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a heat-treated curing-type steel sheet excellent in strength and ductility and a method for producing the same.

The strength and ductility are in inverse proportion, and the following conventional techniques are used as a method of obtaining a steel material excellent in both strength and ductility.

As a representative example, it is possible to control the phase fraction of ferrite, bainite, and martensite structure such as DP (Dual Phase) steel of Patent Document 1 and TRIP (Transformation Induced Plasticity) of Patent Document 2, To control the retained austenite fraction.

However, in the case of DP steel or TRIP steel, the strength is limited to be raised to 1500 MPa or more, and the technique of controlling the retained austenite fraction has a limitation in raising the strength to more than 1500 MPa, .

Therefore, there is an urgent need to develop a steel having excellent strength and ductility while minimizing the use of expensive alloying elements.

Korean Patent Registration No. 0782785 Korean Patent Registration No. 0270395 Korean Patent Registration No. 1054773

One aspect of the present invention is to provide a heat-treated curing type steel sheet excellent in strength and ductility without adding expensive alloying elements by appropriately controlling alloy composition and heat treatment conditions and a method for producing the same.

In order to achieve the above object, an embodiment of one aspect of the present invention is characterized by comprising 0.12 to 0.25% of C, 0.5% or less of Si (excluding 0), 0.1 to 2.0% of Mn, 0.1 to 2.0% of Mn, : 0.05% or less, S: 0.03% or less, the balance Fe and other unavoidable impurities, wherein the average dislocation density determined by XRD measurement of the steel sheet is 7 x 10 ¹⁵ / m ² or more, At least 90% by volume of martensite having a first hardness and martensite having a second hardness, wherein the first hardness has a hardness value larger than the second hardness, and the first hardness and the second hardness Wherein the ratio of the difference to the first hardness satisfies the following relational expression (1): " (1) "

[Relation 1]

5? (First hardness - second hardness) / (first hardness) * 100? 30

Another embodiment according to one aspect of the present invention is a steel sheet comprising 0.12 to 0.25% of C, 0.5% or less (excluding 0) of Si, 0.1 to 2.0% of Mn, 0.05% : 0.03% or less, the balance being Fe and other unavoidable impurities, and subjecting the steel sheet containing the ferrite and the pearlite to a microstructure by cold rolling and heat treatment. The average dislocation density obtained by XRD measurement of the steel sheet is 7 × 10 ¹⁵ / m ² or more, the microstructure of the thermal processing curing steel sheet, including martensite having a martensite phase and a second hardness, having a first hardness of at least 90 volume%, and martensite having the first hardness site Wherein the martensite has been transformed in the vicinity of the pearlite and its adjacent region before the heat treatment, and the martensite having the second hardness is transformed in the ferrite and its adjacent region before the heat treatment.

In another aspect of the present invention, there is provided a steel sheet comprising, by weight%, 0.12 to 0.25% of C, 0.5% or less of Si (excluding 0), 0.1 to 2.0% of Mn, 0.05% , The remainder Fe and other unavoidable impurities, and cold-rolling the steel sheet containing ferrite and pearlite into a microstructure at a reduction ratio of 30% or more; Heating the cold-rolled steel sheet to a heating temperature (T ^* ) of Ac3 to (Ac3 + 500) 캜; Maintaining the heated steel sheet at a high temperature; And a step of cooling the heated steel sheet, wherein the heating rate (v _r , ° C / sec) during heating satisfies the following relational expression 2 and the high temperature holding time of the heated steel sheet is not more than 3 seconds 0 sec), and the cooling rate (v _c , ° C / sec) during cooling satisfies the following relational expression (3).

[Relation 2]

v _r > (T ^* / 110) ²

[Relation 3]

v _c ? (T ^* / 80) ²

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to one embodiment of the present invention, it is possible to provide a heat-treated curing-type steel sheet which is excellent in strength and ductility without adding expensive alloying elements.

As a result of intensive studies to solve the problems of the prior art described above, the present inventors have found that, by optimizing the carbon content and appropriately controlling the cold rolling and the heat treatment process, the dislocation density of the steel sheet is increased and the hardness of the steel sheet It is possible to simultaneously improve the strength and ductility of a steel sheet without adding expensive alloying elements by forming martensite of the species.

Hereinafter, a heat-curing type steel sheet excellent in strength and ductility, which is one aspect of the present invention, will be described in detail. In the present invention, the term " heat treatment " means a heating and cooling step performed after cold rolling.

First, the alloy composition of the heat-cured steel sheet of the present invention will be described in detail.

Carbon (C): 0.12 to 0.25 wt%

Carbon is an indispensable element for improving the strength of the steel sheet, and it is necessary to appropriately add the martensite structure and the dislocation density to be realized in the present invention. When the content of C is less than 0.12% by weight, it is difficult to secure 90% by volume or more of martensite structure in the microstructure of the steel sheet after the heat treatment. In addition, even if a martensite structure is formed, sufficient dislocation density can not be ensured inside and sufficient strength can not be obtained. On the other hand, if the content of C exceeds 0.25% by weight, the ductility of the steel sheet is lowered. Therefore, in the present invention, the content of C is preferably controlled to 0.12 to 0.25% by weight.

Silicon (Si): 0.5% by weight (excluding 0% by weight)

Si is an element which improves strength of steel by solid solution strengthening and deoxidization of molten steel but is an element which inhibits surface quality of steel sheet by forming scale on the surface of steel sheet during hot rolling and is not intentionally added in the present invention. Therefore, it is preferable to control the content as low as possible, and the upper limit of the silicon content is controlled to 0.5 wt% or less.

Manganese (Mn): 0.1 to 2.0 wt%

Mn not only improves the strength and hardenability of the steel but also functions to inhibit the occurrence of S cracks by forming MnS by binding with S inevitably contained in steel manufacturing processes. In order to obtain such effects in the present invention, the content of Mn is preferably 0.1 wt% or more. On the other hand, if it exceeds 2.0% by weight, there is a problem that the toughness of the steel is deteriorated. Therefore, in the present invention, the content of Mn is preferably controlled to 0.1 to 2.0% by weight.

Phosphorus (P): not more than 0.05% by weight

P is an impurity inevitably contained in the steel, and is an element which is a main cause of deterioration of the ductility of steel formed by grain boundaries. Therefore, it is preferable to control P as low as possible. In theory, it is advantageous to limit the content of P to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the P content is controlled to 0.05 wt%.

Sulfur (S): 0.03 wt% or less

S is an impurity inevitably contained in the steel. It is an element that reacts with Mn to form MnS to increase the content of precipitates and cause brittleness of steel. Therefore, it is preferable to control S as low as possible. In theory, it is advantageous to limit the content of S to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of S is controlled to 0.03% by weight.

The remainder Fe and unavoidable impurities. On the other hand, addition of an effective component other than the above-mentioned composition is not excluded.

Hereinafter, the microstructure of the heat-cured steel sheet according to the present invention will be described in detail.

The heat-cured steel sheet according to the present invention not only satisfies the above-mentioned components but also preferably has an average dislocation density determined by XRD measurement of 7 x 10 ¹⁵ / m ² or more. If the average dislocation density does not fall within the above range, there is a problem that it is difficult to sufficiently secure the required strength. On the other hand, since the higher the average dislocation density is, the higher the strength is, the upper limit is not particularly limited in the present invention.

The heat-cured steel sheet according to the present invention preferably has 90% by volume or more of martensite having a first hardness and martensite having a second hardness as its microstructure. When the above-mentioned two kinds of martensites are less than 90% by volume, there is a problem that it is difficult to sufficiently secure the required strength. On the other hand, the remainder other than the above-described structure may be ferrite, pearlite, cementite, or bainite.

According to an embodiment of the present invention, the heat-curing type steel sheet is a steel sheet produced by cold-rolling and heat-treating a steel sheet containing ferrite and pearlite as a microstructure, wherein the martensite having the first hardness is composed of pearlite And the martensite having the second hardness may be transformed in the ferrite and its adjacent region before the heat treatment. As described later, the inventors of the present invention have found that when the heat treatment conditions of the cold-rolled steel sheet are appropriately controlled, the diffusion of carbon is minimized and the two types of martensite can be formed.

When the above-mentioned structure is secured by the microstructure of the steel sheet, deformation first occurs in the martensite having a low hardness at the beginning of processing, and work hardening occurs as the deformation progresses, so that ductility of the steel sheet is improved. On the other hand, in order to obtain the above-mentioned effects, it is more preferable that the ratio of the difference between the first hardness and the second hardness to the first hardness satisfies the following relational expression (1). If the ratio is less than 5%, the effect of improving the ductility of the steel sheet is insufficient. On the other hand, if the ratio exceeds 30%, deformation may concentrate on the interface of the two types of martensite structure, The ductility of the steel sheet may be lowered.

[Relation 1]

5? (First hardness - second hardness) / (first hardness) * 100? 30

Meanwhile, according to an embodiment of the present invention, the average packet size of the two types of martensite may be 20 μm or less. If the packet size exceeds 20 탆, the block size and the plate size in the martensite structure may increase at the same time, which may lower the strength and ductility of the steel sheet.

Hereinafter, a method of manufacturing a heat-cured steel sheet having excellent strength and ductility, which is another aspect of the present invention, will be described in detail.

The steel sheet containing ferrite and pearlite is cold-rolled by a microstructure while satisfying the above-mentioned composition. As described above, when the ferrite and the pearlite are adequately secured by the microstructure of the steel sheet before the heat treatment and the heat treatment conditions are appropriately controlled, two types of martensite having different hardness after the heat treatment can be formed.

The reduction rate in cold rolling is preferably 30% or more. When the steel sheet is cold-rolled at a reduction ratio of 30% or more as described above, the ferrite structure is stretched in the rolling direction to include a large amount of residual strain therein, and the pearlite structure is also stretched in the rolling direction to form fine carbides do. The cold-rolled ferrite and pearlite structure thus miniaturize the size of the austenite grains during the subsequent heat treatment, facilitate the solidification of the carbides, and further improve the strength and ductility of the steel sheet.

Next, the cold-rolled steel sheet is heated to a heating temperature (T ^* ) of Ac3 to (Ac3 + 500) 占 폚. If the heating temperature (T ^* ) is less than Ac3 DEG C, austenite is not sufficiently formed, and 90% by volume or more of martensite structure after cooling can not be obtained. On the other hand, when the heating temperature (T ^* ) exceeds (Ac3 + 500) ° C, the size of the austenite grains is coarsened, and the diffusion of carbon is accelerated to obtain two types of martensite having different hardness after cooling none. Therefore, the heating temperature is preferably Ac3 to (Ac3 + 500) ° C or lower, more preferably Ac3 to (Ac3 + 300) ° C.

During the heating, the heating rate (v _r , ° C / sec) preferably satisfies the following relational expression (2). If v _r does not satisfy the relational expression (2), the size of the austenite grains is coarsened during heating, and the two types of martensite having a different hardness after cooling due to excessive diffusion of carbon can not be obtained. On the other hand, the upper limit is not particularly limited because coarsening of the austenite grains and diffusion of carbon are prevented as the heating rate is increased.

[Relation 2]

v _r > (T ^* / 110) ²

(Where the unit of T ^* is ° C)

According to an embodiment of the present invention, it is more preferable that the cold-rolled and heated steel sheet has an austenite single-phase structure with an average diameter of 20 μm or less as the microstructure. If the average diameter of the austenite single-phase structure exceeds 20 탆, there is a possibility that the packet size of the martensite structure formed after cooling may be coarsened, and the martensitic transformation temperature may rise to decrease the strength and ductility of the steel sheet .

Next, the heated steel sheet is maintained at a high temperature. At this time, it is preferable that the high-temperature holding time is 3 seconds or less (including 0 seconds). The high-temperature holding time means the time taken to start cooling the steel sheet which has reached the heating temperature. When the high-temperature holding time is 3 seconds or less, dislocation is prevented from being loosened to maintain a high dislocation density and also to prevent excessive diffusion of carbon. Further, before cooling, the average diameter of the austenite grains is controlled to be not more than 20 mu m, and after cooling, martensite having an average packet size of not more than 20 mu m can be secured. On the other hand, the lower limit of the time is not particularly limited because it is advantageous to maintain a high dislocation density, to diffuse carbon, and to prevent coarsening of austenite grains.

Next, the heated steel sheet is cooled. At this time, it is preferable that the cooling rate (v _c , ° C / sec) satisfies the following relational expression (3). If v _c does not satisfy the relational expression (3), the size of the austenite grains during cooling is coarsened, and two types of martensite having a different hardness after cooling due to excessive diffusion of carbon can not be obtained. In addition, there is a problem in that the transformation into ferrite, pearlite or bainite structure occurs during cooling and the target martensite volume fraction can not be secured. On the other hand, as the cooling rate increases, coarsening of the austenite grains and diffusion of carbon are prevented, so the upper limit is not particularly limited.

[Relation 3]

v _c ? (T ^* / 80) ²

(Where the unit of T ^* is ° C)

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

After the steel sheet having the composition shown in Table 1 below was prepared, the steel sheet was subjected to cold rolling, heating and cooling under the conditions shown in Table 2 below. Thereafter, the microstructure of the steel sheet was observed, and the mechanical properties were measured and shown in Table 3 below. At this time, the Vickers hardness test of each microstructure was carried out under the condition that the tensile test was performed at a rate of 5 mm / min for the ASTM subsize test piece, and the hardness test was performed for 10 seconds at a load of 5 g.

On the other hand, an X-ray diffractometer (Bruker XRD D8-Advanced diffractometer) was used for dislocation density measurement. The dislocation densities were measured by comparing the peaks measured in the standard specimen with the peaks measured in the respective steels, since the degree of expansion of the peaks measured according to the dislocation density varied. The standard specimens were prepared by heating the cold-rolled specimen of 100% ferrite structure to 1000 ° C at a rate of 5 ° C / min, holding it for 10 hours, and cooling it to room temperature at a rate of 1 ° C / min. The standard specimens and each steel material were polished on a sand paper surface and then chemically polished with 4% hydrofluoric acid for 20 minutes to remove the thickness of 0.1 mm or more. Copper source and Nickel filter were used for XRD measurement, and monochromated Cu K radiation of 40 kV and 36 mA was used. The diffraction pattern measured by XRD was then calculated by Fourier constants such as fitting using the pseudo Voigt function. Finally, the mean dislocation density was calculated using modified Williamson-Hall and Warren Averbach method.

Steel grade C Mn Si P S Comparative River 1 0.10 1.49 0.003 0.02 0.003 Inventive Steel 1 0.21 0.89 0.005 0.015 0.012

Steel grade Reduction rate (%) T ^* (占 폚) v _r
(° C / sec) v _r ^*
(° C / sec) v _c
(° C / sec) v _c ^*
(° C / sec) Maintain high temperature
Time (sec) Remarks Comparative River 1 60 900 300 67 1000 126 One Comparative Example 1 60 1000 300 83 1000 156 One Comparative Example 2 Inventive Steel 1 70 900 300 67 1000 126 One Inventory 1 70 1000 300 83 1000 156 One Inventory 2 70 1100 300 100 1000 189 One Inventory 3 70 1200 300 119 1000 225 0 Honorable 4 70 1000 200 83 1000 156 One Inventory 5 70 1000 100 83 1000 156 One Inventory 6 70 1000 300 83 200 156 One Honorable 7 70 1000 300 83 1000 156 2 Honors 8 70 700 300 40 1000 76 One Comparative Example 3 70 1000 300 83 1000 156 5 Comparative Example 4 70 1000 300 83 1000 156 20 Comparative Example 5 70 1000 300 83 80 156 One Comparative Example 6 70 1200 300 119 1000 225 One Proposition 9 70 1300 300 140 1000 264 One Inventory 10 v _r ^* means the heating rate ((T ^* / 110) ² ) calculated by the relation 1 and v _c ^* means the cooling rate ((T ^* / 80) ² ) calculated by the relation 2.

Steel grade minuteness
group Dislocation density
(m- ² ) 1st
Hardness
(HV) Second
Hardness
(HV) Relationship 1 Packet size (탆) Seal
burglar
(MPa) Elongation
(%) Remarks Comparative River 1 M1 + M2 4.3 × 10 ¹⁵ 454 372 28.1 8.9 1347 8.2 Comparative Example 1 Comparative River 1 M1 + M2 3.9 × 10 ¹⁵ 437 368 25.8 12.2 1311 9.7 Comparative Example 2 Inventive Steel 1 M1 + M2 11.1 × 10 ¹⁵ 662 513 22.5 6.8 1795 7.4 Inventory 1 Inventive Steel 1 M1 + M2 10.4 × 10 ¹⁵ 650 520 20 8.5 1775 8.1 Inventory 2 Inventive Steel 1 M1 + M2 10.1 × 10 ¹⁵ 627 510 23.7 13.7 1771 7.7 Inventory 3 Inventive Steel 1 M1 + M2 9.1 × 10 ¹⁵ 619 526 25.1 16.7 1702 8.1 Honorable 4 Inventive Steel 1 M1 + M2 10.2 × 10 ¹⁵ 634 513 19.1 11.8 1763 7.3 Inventory 5 Inventive Steel 1 M1 + M2 9.4 × 10 ¹⁵ 607 549 9.6 10.7 1742 7.1 Inventory 6 Inventive Steel 1 M1 + M2 8.9 × 10 ¹⁵ 614 545 11.2 9.1 1711 7.2 Honorable 7 Inventive Steel 1 M1 + M2 9.0 × 10 ¹⁵ 631 560 11.2 9.6 1759 7.2 Honors 8 Inventive Steel 1 F + P 2.1 × 10 ¹⁵ - - - - 1387 3.2 Comparative Example 3 Inventive Steel 1 M1 + M2 6.7 × 10 ¹⁵ 591 563 4.8 19.7 1712 5.9 Comparative Example 4 Inventive Steel 1 M1 + M2 5.9 × 10 ¹⁵ 578 553 4.3 27.7 1699 2.9 Comparative Example 5 Inventive Steel 1 F + P 0.1 × 10 ¹⁵ - - - - 649 20.1 Comparative Example 6 Inventive Steel 1 M1 + M2 7.5 × 10 ¹⁵ 570 543 22.1 4.7 1689 6.7 Proposition 9 Inventive Steel 1 M1 + M2 7.5 × 10 ¹⁵ 559 536 28.9 4.1 1684 6.4 Inventory 10 Where F is ferrite, P is pearlite, M1 is martensite having a first hardness, and M2 is martensite having a second hardness.

The inventive examples 1 to 10, which satisfy the alloy composition and the manufacturing conditions proposed by the present invention, have a dislocation density of 7 × 10 ¹⁵ / m ² or more and a hardness difference of 5 to 30% , And a tensile strength of 1500 MPa or more and an elongation of 6% or more.

On the other hand, in Comparative Examples 1 and 2, the carbon content in the steel was so low that the martensite having a sufficiently high dislocation density was not formed and the strength was inferior. In Comparative Example 3, the heating temperature (T ^* ) was too low to obtain a composite structure of ferrite and pearlite. In Comparative Example 6, the cooling rate was too slow to obtain a composite structure of ferrite and pearlite. Also, in Comparative Examples 4 and 5, since the holding time at a high temperature was too long, dislocation density, hardness difference, and packet size were out of the range controlled by the present invention, resulting in poor strength and ductility.

Claims (9)

And the balance Fe and other unavoidable impurities are contained in an amount of 0.12 to 0.25% of C, 0.5% or less of Si (excluding 0), 0.1 to 2.0% of Mn, 0.05% or less of P, As a steel sheet,
The average dislocation density obtained by XRD measurement of the steel sheet is 7 x 10 ¹⁵ / m ² or more,
Wherein the steel sheet has 90% by volume or more of martensite having a first hardness and martensite having a second hardness,
Wherein the first hardness has a hardness value greater than the second hardness,
Wherein the ratio of the difference between the first hardness and the second hardness to the first hardness satisfies the following relational expression (1): " (1) "
[Relation 1]
5? (First hardness - second hardness) / (first hardness) * 100? 30
The method according to claim 1,
Wherein the average packet size of the martensite having the first hardness and the martensite having the second hardness is 20 占 퐉 or less.
The method according to claim 1,
Wherein the steel sheet has a tensile strength of 1500 MPa or more and an elongation of 6% or more.
0.1% to 2.0%, P: 0.05% or less, S: 0.03% or less, the balance Fe and other unavoidable impurities, in terms of% by weight, of C: 0.12 to 0.25% , A cold-rolled and heat-treated steel sheet comprising ferrite and pearlite as a microstructure,
The average dislocation density obtained by XRD measurement of the steel sheet is 7 x 10 ¹⁵ / m ² or more,
Wherein the microstructure of the heat-cured steel sheet comprises 90 vol% or more of martensite having a first hardness and martensite having a second hardness,
The martensite having the first hardness is transformed in the pearlite and the adjacent region before the heat treatment, and the martensite having the second hardness is transformed in the ferrite and the adjacent region before the heat treatment, and the heat treatment curing type Steel plate.
[Relation 1]
5? (First hardness - second hardness) / (first hardness) * 100? 30
delete 5. The method of claim 4,
Wherein the average packet size of the martensite having the first hardness and the martensite having the second hardness is 20 占 퐉 or less (excluding 0 占 퐉).
5. The method of claim 4,
Wherein the steel sheet has a tensile strength of 1500 MPa or more and an elongation of 6% or more.
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