KR20180070383A - High strength clad steel having good delayed fracture resistance - Google Patents

Abstract

A high strength clad steel sheet excellent in delayed fracture resistance is provided.
The high-strength clad steel sheet according to the present invention comprises at least one core layer made of a high-Mn steel sheet containing Mn in an amount of 8% And an AHSS steel containing 3% or less of Mn and having an area fraction occupied by the retained austenite phase in the microstructure of 3% or less, A plurality of outer shell layers.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high strength clad steel,

The present invention relates to a high strength clad steel sheet excellent in delayed fracture resistance and, more particularly, to a high strength clad steel sheet excellent in delayed fracture characteristics and workability which can be used for outer plates and inner plate structural parts such as doors, hoods, trunks, .

BACKGROUND ART In recent years, in the automobile manufacturing industry, in order to reduce carbon dioxide in exhaust gas in response to environmental regulations, to reduce fuel consumption of fossil fuel cars and to reduce battery consumption of electric vehicles, weight reduction of automobile bodies is continuously being promoted. Particularly, in the case of parts such as doors, hoods, and trunks, which can not be expected to be lighter due to the application of the existing automotive steel sheet, automobile manufacturers can use aluminum, plastic, magnesium There is an increasing tendency to adopt such non-ferrous lightweight materials.

However, these nonferrous lightweight materials are expensive, and paintability and mechanical properties are not enough to be applied to steel sheets for automobile automobiles. Therefore, it is necessary to develop new alternative technologies. Particularly, since aluminum is light and has excellent corrosion resistance, its application range is expanding. However, there is a real problem that it is difficult to secure the workability for parts processing because the strength required for use as automobile material to replace steel is low.

On the other hand, in the case of a steel sheet for automobiles, a method of lighter weight of the above parts by using a thin steel plate includes a P-doped low carbon steel, a Bake Hardenable steel, and a Duel Phase Steel DP Steel) is used to reduce the weight of components by designing to reduce the thickness of parts. In addition, recently, a high Mn product having both high strength and high formability has been developed. However, since the hydrogen breaks due to penetration of defects in the deformed part during severe molding, We need to solve this problem to achieve museum through. In general, the delayed fracture problem of high Mn steel can be improved by adding Al, but when Al is added, the cost increases and the twin phenomenon is suppressed and the strength decreases. Therefore, it is necessary to develop high Mn steel with improved delayed fracture without reducing or adding Al content.

On the other hand, clad steel sheets having various thicknesses and physical properties unlike the high strength steel sheets for panels are also made of heavy plates and thin plates and are used as automobiles, ships, or multi-functional home appliances. Clad steel sheet is a technique to produce a product with new properties and properties by laminating various existing steel or nonferrous light weight materials by various heat treatment and rolling method. When this technology is applied to automotive steel sheets, it is possible to produce products with excellent properties and properties that can not be realized by conventional steel production methods.

Korean Patent Laid-Open Publication No. 2016-0082358 (published on July 20, 2016)

Accordingly, the present invention has been made in order to overcome the above-described problems of the prior art, and it is an object of the present invention to provide a high Mn steel which has high moldability and strength, It is an object of the present invention to provide a high strength clad steel sheet excellent in delayed fracture resistance with excellent general steel or AHSS steel as an sheath.

Further, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems which are not mentioned can be understood from the following description in order to clearly understand those skilled in the art to which the present invention belongs .

According to an aspect of the present invention,

At least one core layer made of a high Mn steel sheet containing Mn in an amount of 8% or more by weight; And

A general steel or an AHSS steel containing 3% or less Mn and having an area fraction occupied by the retained austenite phase in the microstructure of 3% or less as its own weight% The present invention relates to a high strength clad steel sheet excellent in delayed fracture resistance characteristics.

In the present invention, the high Mn steel sheet constituting the core layer contains 0.01 to 1.0% of C, 8 to 30% of Mn, 0.01 to 1.3% of Al, 0.01 to 3.0% of Si, 0.1 to 3.0% of P, 0.02% or less of S, 0.1% or less of N, and the balance of Fe and unavoidable impurities.

The high manganese steel sheet may be a TWIP (twin-organic thermoplastic) steel having a tensile strength of 600 to 1000 MPa and an elongation of 30% or more.

The high Mn steel sheet may further contain 0.005 to 2.0% of Ni or 0.005 to 5.0% of Cr.

The high Mn steel sheet may further include at least one selected from the group consisting of 0.005 to 0.5% of Ti, 0.005 to 0.5% of Nb, 0.005 to 0.5% of V, and 0.005 to 1% of W.

Further, the high Mn steel sheet may further include at least one selected from 0.005 to 0.2% of Sn, 0.005 to 0.2% of Sb and 0.0001 to 0.01% of B, in addition to the above.

In the present invention, the high-Mn steel thin plate used as the core layer of the clad steel sheet has a tensile strength of 600 to 1,400 MPa as well as an elongation of 30% as compared with a tensile strength of 270 to 300 MPa of a steel used in a conventional laminated clad product And has a high elongation. Therefore, when the clad steel sheet is manufactured using the above, the weight of the automobile parts can be further reduced as compared with the existing steel and clad steel sheets, and the weight of the parts made of the aluminum plate can be reduced .

Further, when a clad steel sheet having a general steel or an AHSS steel as an outer layer is formed on the high-Mn steel sheet core layer, there is an advantage that the problem of deterioration of the delayed-failure resistance characteristic of the conventional high-Mn steel sheet can be effectively solved.

Therefore, when the lightweight multi-high strength clad steel sheet of the present invention is used, the mechanical properties of automotive shell panel parts such as doors, hoods, and trunk lid outer panels made of aluminum are remarkably improved .

1 is a perspective view illustrating an example of a multiple clad steel sheet manufactured using the high Mn steel and general steel of the present invention.
FIG. 2 (a) is a photograph of Inventive Example 1 showing a cup in which no delayed fracture occurred after the delay fracture property test in a Salt Spray of a high Mn steel and an AHSS multiple clad steel sheet in the present embodiment, FIG. 2 (b) This is a photograph of Comparative Example 2 showing a cup in which delayed fracture occurred in Mn-monopoly steel.

Hereinafter, the present invention will be described in detail.

1 is a perspective view illustrating an example of a multiple clad steel sheet manufactured using the high Mn steel and general steel of the present invention.

As shown in FIG. 1, the clad steel sheet of the present invention comprises a core layer made of a high-Mn steel sheet and an outer layer made of ordinary steel or AHSS steel at upper and lower portions of the core layer.

Specifically, the clad steel sheet of the present invention comprises at least one core layer made of a high Mn steel sheet containing 8% or more Mn by itself, And an AHSS steel containing 3% or less of Mn and having an area fraction occupied by the retained austenite phase in the microstructure of 3% or less, A plurality of outer shell layers.

First, the clad steel sheet of the present invention uses a high Mn steel sheet as a core layer.

In the present invention, it is preferable that the thickness of the high mild steel thin plate is 0.1 to 10.0 mm. In order to obtain high elongation and workability, it is preferable that the high-manganese steel has a high thickness fraction. However, there is no need to limit the laminate thickness fraction because the delayed fracture characteristics are improved even if the thickness of the general steel clad in the shell is thin.

Since the multi-clad steel sheet of the present invention utilizes a high manganese steel having a TS x El value of 60,000, it is possible to produce a steel having excellent resistance to delayed fracture in addition to high strength and high elongation, have.

On the other hand, in the case of a high manganese steel sheet used as a core layer for a clad steel sheet in the present invention, it is necessary to use the TWIP phenomenon of austenite structure to secure high workability and strength. Therefore, an example of the composition range of high Mn steel that can maintain austenitic metal structure and ensure high elongation through TWIP effect is presented as follows. In other words, the high Mn steel sheet of the present invention comprises 0.01 to 1.0% of C, 8 to 30% of Mn, 0.01 to 1.3% of aluminum, 0.01 to 3.0% of Si, 0.01 to 3.0% of Si, P % Or less, S: 0.02% or less, and N: 0.1% or less, and the remainder including Fe and unavoidable impurities. Therefore, the high manganese steel sheet may be a TWIP (twin-organic thermoplastic) steel having a tensile strength of 600 to 1000 MPa and an elongation of 30% or more.

Hereinafter, the emphasis components of the high Mn steel sheet which can be used as the core layer of the multi-clad steel sheet of the present invention and the reason for limiting the content thereof will be described.

The C is an element that increases austenite stabilization and stacking fault energy. For this purpose, it is preferable to contain 0.01% by weight or less (hereinafter referred to as "%"). On the other hand, when the C content exceeds 1.0% by weight, carbide formation is facilitated in the heat treatment step, thereby providing a place where cracking of hydrogen embrittlement starts, so that the delayed fracture characteristics are degraded and the lamination defect energy is excessively increased, The strength is decreased and the elongation is lowered because the deformation behavior due to the slip deformation occurs. The preferred content of C is 0.01 to 0.1%.

The Mn is an element essential for stably securing the austenite structure, and is an element for raising the stacking fault energy. When the Mn content is less than 8%, martensite deteriorating the formability is formed and the strength is increased. However, since the ductility decreases sharply and the lamination defect energy decreases, a part of the austenite easily transforms into? -Martensite. 8%. On the other hand, when the content of Mn is more than 30% by weight, the production cost due to a large amount of manganese and the increase in the content of phosphorus (P) cause slab cracking. Also, as the content of Mn increases, the internal grain boundary oxidation occurs excessively during the reheating of the slab, causing oxide defects on the surface of the steel sheet, and therefore, it is preferable that the content of Mn is less than 30% because the surface characteristics are deteriorated during hot dip galvanizing.

The aluminum is usually added for deoxidation of steel, and in the present invention, it is preferably contained in an amount of 0.01% or more. However, when the content exceeds 1.3%, the occurrence of twinning is suppressed to weaken the ductility and the material. In addition, the casting becomes worse during continuous casting, . This is in contrast to the addition of Al in an amount of 1.4% or more to improve the delayed fracture property of automotive high-Mn steel. In the present invention, since a general steel excellent in resistance to delayed fracture is clad on the outer surface of the clad steel sheet, The content is not required to be a high content as in the conventional product.

The Si may be added to improve the strength as an element for increasing the yield strength by reducing the crystal grain size by the employment effect as an element to be solid solution strengthened. For this, in the present invention, it is preferable to contain 0.01% or more. However, when the added amount of Si is increased, silicon oxide is formed on the surface of the steel sheet during hot rolling, which deteriorates pickling ability and deteriorates the surface quality of the hot-rolled steel sheet. In addition, the addition of a large amount of silicon greatly reduces the weldability of steel, and therefore, it is preferable that the amount of silicon is 3.0% or less.

On the other hand, P and S are inevitably included in the production of steel. In the present invention, P and S are preferably contained in an amount of 0.1% or less and 0.02% or less, respectively. This is because if P exceeds the upper limit of the above-mentioned content, P segregates to reduce the workability of the steel, and S forms a coarse manganese sulfide (MnS), thereby generating defects such as flange cracks.

N acts on aluminum in the solidification process of the austenite grains to precipitate fine nitrides to accelerate the generation of twin, thereby improving the strength and ductility of the steel sheet. However, when the content exceeds 0.1% It precipitates to deteriorate the hot workability and elongation. Therefore, it is preferable that the content of nitrogen is 0.1 wt% or less.

The high Mn steel sheet may further contain 0.005 to 2.0% of Ni or 0.005 to 5.0% of Cr.

The Ni contributes to the stabilization of the austenite, which is not only advantageous for improving the elongation but also contributes effectively to high temperature ductility. The effect of high Ni content, which is a high temperature toughness improving element, at 0.005% or less is very insignificant for high temperature toughness. As the content of Ni increases, the effect is also great in prevention of delayed fracture and slab crack prevention. , And the content thereof is 0.005 to 2.0%.

The Cr reacts with external oxygen during the hot rolling or annealing operation to preferentially form a Cr-based oxide film (Cr ₂ O ₃ ) with a thickness of 20 to 50 μm on the surface of the steel sheet, so that Mn, Si, Thereby contributing to the stabilization of the surface texture and improving the surface properties of the plating layer. However, when the content is less than 0.005%, the effect is insignificant. When the content exceeds 5.0%, chromium carbide is formed and the workability and delayed fracture characteristics are deteriorated.

Further, the high Mn steel sheet of the present invention may further include at least one selected from the group consisting of 0.005 to 0.5% of Ti, 0.005 to 0.5% of Nb, 0.005 to 0.5% of V, and 0.005 to 1% of W.

The Ti is a strong carbide forming element that bonds with carbon to form a carbide. The carbide formed at this time is an element effective in refining the crystal grains by inhibiting grain growth. Also, when the boron compound is added in combination, a high-temperature compound is formed in the columnar phase boundary to prevent grain boundary cracking. However, when the Ti content is less than 0.005%, such an effect is unlikely to be expected. When the Ti content exceeds 0.5%, excess Ti is segregated in the grain boundaries to cause intergranular embrittlement, or the precipitated phase is excessively coarsened, Is preferably in the range of 0.005 to 0.5%.

The Nb is a carbide-forming element that forms a carbide by binding with carbon in the steel, and can increase strength and fine grain size. Since Nb forms a precipitation phase at a temperature lower than Ti, it can contain 0.005 to 0.5% as an element having a fine crystal grain size and a precipitation strengthening effect by precipitation formation. If the addition amount is less than 0.005%, the above effect is insignificant. If it exceeds 0.5%, excess Nb segregates in grain boundaries to cause intergranular embrittlement or excessively coarsening of the precipitated phase, In the process, the recrystallization is delayed to increase the rolling load, so the amount of Nb added is preferably 0.005 to 0.5%.

V and W are elements which form carbonitride by combining with C and N, such as Ti. In the present invention, since a fine precipitate phase is formed at a low temperature, the precipitation strengthening effect is large and plays an important role in securing austenite. If the contents of V and W are less than 0.005%, the addition effect is insignificant. If V and W are more than 0.5% and 1.0% respectively, the precipitated phase becomes excessively coarse to lower the grain growth effect and cause hot brittleness Therefore, it is preferable that V contains 0.005 to 0.5% and W contains 0.005 to 1.0%.

Further, the high-Mn steel sheet of the present invention may further comprise at least one selected from 0.005 to 0.2% of Sn, 0.005 to 0.2% of Sb and 0.0001 to 0.01% of B,

The above Sn and Sb have a role of improving the plating property in the production of the plating material of the high manganese steel. However, when the plating material is added excessively, the internal oxidation is severe and the plating ability is weakened.

The boron (B) strengthens the grain boundaries to inhibit grain boundary oxidation during the hot rolling process and has a strength increasing effect even in a very small amount. However, in order to obtain such an additive effect, it is required to contain 0.0001% or more, but when it exceeds 0.01%, the surface and quality of material due to formation of large BN deteriorate, so that the upper limit is limited to 0.01%.

Next, the clad steel sheet of the present invention is a general steel or an AHSS steel containing 3% or less of Mn in terms of its own weight percentage and having an area fraction occupied by the retained austenite phase in its microstructure of 3% or less, And one or more shell layers formed on upper and lower portions of the layer.

In the present invention, the general steel or the AHSS steel laminated on the shell layer has a content of Mn of 3% or less by weight in its own weight and a residual austenite fraction of 3% or less by area in the microstructure. If the residual austenite fraction exceeds 3% by area, it is preferable that hydrogen is accumulated in the austenite to easily induce delayed fracture so that the residual austenite fraction is limited to 3% or less. The thickness of the general steel is preferably 0.1 to 10.0 mm.

On the other hand, the production of the high manganese steel and the clad steel sheet to be applied to the multi-clad steel sheet of the present invention is performed by a conventional method and is not particularly limited. As a preferred example, a general steel having a composition of the above formula may be laminated with a triple or more sheath, welded by welding, hot-rolled, and then cold-rolled. More preferably, hot-rolled hot-rolled steel sheet is subjected to primary cold-rolling at a reduction ratio of 30% or more, recrystallization annealing, secondary cold-rolling at a reduction ratio of 30% or more, secondary cold- Rolling process.

Hereinafter, the present invention will be described in detail by way of examples.

(Example)

A general steel specimen (A) of 3% or less Mn and a high Mn steel specimen (B-D) of 3% or more Mn were prepared as shown in Table 1 below. As shown in Table 1 below, a multi-clad steel sheet (E-G) was prepared by using the high-Mn steel sheet as the core and the general steel as the sheath. The thickness, laminating ratio and material of the steel test strip A-D and the clad steel strip E-G were measured and are also shown in Table 1 below. The material of the clad steel sheet is generally proportional to the lever rule. On the other hand, in Table 1, the lamination ratio refers to the ratio of the number of laminated steel sheets, the unit of thickness is m, the unit of YS and TS is MPa, and the unit of EL is%.

River
Bell Lamination
ratio two
To Composition Component (% by weight) material C Si Mn P S S.Al V Nb YS TS EL Comparative Example 1 A Single river 1.8 0.06 2 0.01 0.001 0.04 300 540 18 Comparative Example 2 B Single river 1.8 0.5 2 15 0.01 0.001 0.9 0.4 0.05 756 1354 53.4 Comparative Example 3 C Single river 2.2 0.6 18 0.01 0.001 0.4 0.1 0.05 697 1082 57 Comparative Example 4 D Single river 2.2 0.7 One 21 0.01 0.001 0.3 0.05 648 1089 53.4 Inventory 1 E A: B: A =
1: 1: 1 2.2

568 1015 53.2 Inventory 2 F A: C: A =
1: 2: 1 2.2 535 847 46.5 Inventory 3 G A: D: A =
2: 1: 2 4 496 820 41

Table 2 shows the results of the delayed fracture measurements of Comparative Examples 1-4 and Inventive Examples 1-3. It was determined that the delayed fracture characteristics were excellent when the delayed fracture was not occurred within 480hr under the Salt spray condition.

DR thickness Quantity start 24hr 72hr 120hr 192hr 312hr 504hr Evaluation results of delayed fracture Comparative Example 1 (A) 1.8 1.8 5 ○ ○ ○ ○ ○ ○ ○ Good Comparative Example 2 (B) 1.8 1.8 5 ○ × × × × × × Bad Comparative Example 3 (C) 1.8 2.2 5 ○ × × × × × × Bad Comparative Example 4 (D) 1.8 2.2 5 ○ × × × × × × Bad Inventory 1 (E) 1.8 2.2 5 ○ ○ ○ ○ ○ ○ ○ Good Inventory 2 (F) 1.8 2.2 5 ○ ○ ○ ○ ○ ○ ○ Good Inventory 3 (G) 1.8 4 5 ○ ○ ○ ○ ○ ○ ○ Good

As shown in Table 1-2, a high strength Mn steel B, C, D steel (Comparative Example 2-4) for improving the delayed fracture characteristics of a general steel A steel (Comparative Example 1) It is confirmed that the delayed-destructive capacity of the E, F, and G grades (Inventive Examples 1-3) produced by cladding in the above-mentioned examples is improved.

FIG. 2 (a) is a photograph of Inventive Example 1 showing the delayed fracture of the high manganese steel and the AHSS multi-clad steel sheet after the delayed fracture property test in the salt spray, and FIG. 2 (b) This is a photograph of Comparative Example 2 showing a cup in which delayed fracture occurred in monopoly steel.

While the present invention has been particularly shown and described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of course, this is possible. Therefore, the scope of the present invention should not be limited to the above-described embodiments but should be defined by the following claims as well as equivalents thereof.

Claims (6)

At least one core layer made of a high Mn steel sheet containing Mn in an amount of 8% or more by weight; And
A general steel or an AHSS steel containing 3% or less Mn and having an area fraction occupied by the retained austenite phase in the microstructure of 3% or less as its own weight% And an outer shell layer having an inner diameter less than that of the outer cladding layer.
The high Mn steel sheet according to claim 1, wherein the core layer comprises 0.01 to 1.0% of C, 8 to 30% of Mn, 0.01 to 1.3% of Al, 0.01 to 3.0% of Si, 0.01 to 3.0% of Si, : 0.1% or less, S: 0.02% or less, N: 0.1% or less, and the balance being Fe and unavoidable impurities.
The high-strength clad sheet according to claim 1, wherein the high-Mn steel sheet is a TWIP (twin-organic thermoplastic) steel having a tensile strength of 600 to 1000 MPa and an elongation of 30% or more.
The high-strength clad sheet according to claim 2, wherein the high-Mn steel sheet further contains 0.005 to 2.0% of Ni or 0.005 to 5.0% of Cr.
The high Mn steel sheet according to claim 2, wherein the high Mn steel sheet further comprises at least one selected from the group consisting of 0.005 to 0.5% of Ti, 0.005 to 0.5% of Nb, 0.005 to 0.5% of V, and 0.005 to 1% of W High-strength clad steel sheet with excellent delayed fracture characteristics.
The high Mn steel sheet according to claim 2, wherein the high Mn steel sheet further comprises at least one of 0.005 to 0.2% of Sn, 0.005 to 0.2% of Sb and 0.0001 to 0.01% of B, Excellent high strength clad steel.

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