JP5352766B2 - Multi-layer steel and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer steel in which boundary peeling strength among layers is high, and a balance of strength-ductility is excellent, and to provide a method for producing the same. <P>SOLUTION: In the multilayer steel in which a first layer composed of a steel essentially composed of an austenitic structure or a ferritic structure and having a tensile strength TS<SB>1</SB>of &le;1,200 MPa and the second layer composed of a steel essentially composed of a martensitic structure and having a tensile strength TS<SB>2</SB>of &ge;1,200 MPa are laminated each other, and three or more layers are laminated and integrated with the first layer as the surface layer, on the boundary between the first layer and the second layer, a boundary crystal structure where oxides such as Si based oxides and Cr based oxides with the major axis of &le;1 &mu;m are arranged in a line shape is formed within crystal grains newly formed by diffusion of the first layer and the second layer, by hot rolling, cold rolling and a heating treatment. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a multilayer steel having high interlaminar interface peel strength and excellent strength-ductility balance, and a method for producing the same.

In recent years, automotive materials have been required to be lighter from the viewpoint of energy saving, high performance, environmental measures, etc. The main measures are to reduce the weight by using non-ferrous materials with low specific gravity or to increase the steel materials for automobiles. Research and development has been conducted to reduce the thickness and weight by increasing strength.
However, if a non-ferrous material with a low specific gravity is used instead of steel, for example, an alloy such as an Al alloy or Mg alloy, the mechanical properties such as strength and rigidity are not sufficient compared to steel, so it is equivalent to steel. In order to obtain mechanical properties, the plate thickness must be increased, and the cross-sectional shape must be complicated, resulting in problems such as reduced formability, embrittlement in dissimilar material joining, and corrosion. It was not possible to obtain a light weight effect sufficient to meet expectations.
On the other hand, when thinning is achieved by increasing the strength of automotive steel materials, various properties (ductility, toughness, rigidity, workability, safety, It is necessary to eliminate the problems of reduced embrittlement, fatigue resistance, corrosion resistance, environmental resistance, etc.), but in particular, high strength and high ductility are generally contradictory characteristics. For this reason, there has been a strong demand for a steel material having a high strength and at the same time having a high ductility and a so-called strength-ductility balance.

  One measure for this is to improve the balance between strength and ductility of the material by layering multiple materials with different properties. As a layered material of steel and other materials, clad material, laminate There are materials.

As a clad material, some of the inventors have rolled a plurality of layers of steel (carbon steel, alloy steel, stainless steel, high manganese steel, etc.) with different structures and mechanical properties. A method of obtaining a multilayer steel having excellent strength and ductility by heat treatment in response is proposed (for example, Patent Document 1).
In addition, for example, even in a multilayer material obtained by pressing and rolling a plurality of layers of high carbon steel and brass, a multilayer material exhibiting high strength and high ductility can be obtained (for example, Non-Patent Document 1). )It has been known.
Japanese Patent Application No. 2006-205283 “METALLURGICAL TRANSACTIONS A” Vol. 24A, July 1993, p. 1647-1653

In the prior art described in Patent Document 1 (hereinafter referred to as Prior Art 1), it is shown that a multilayer steel having high strength and high ductility can be obtained by devising a combination of steel materials, the number of laminated layers, and a lamination process. For example, austenitic stainless steel is used as the first layer, martensitic stainless steel is used as the second layer, and a total of 11 layers of the first layer and the second layer are laminated. By performing hot rolling and heat treatment, it is shown that a multi-layer steel having a strength (tensile strength TS) of 1220 MPa and a ductility (elongation EL) of 25% is obtained, and a high manganese steel is used as the first layer. In addition, carbon steel is used as the second layer, and the first layer and the second layer are laminated in total 11 layers, and by performing hot rolling and heat treatment, the strength (tensile strength TS) is 1150 MPa, Ductility (elongation EL) 34% of the multilayered steel is indicated to be obtained.
However, in the above prior art 1, if the bonding strength at the interface between the first layer and the second layer is not sufficient, the first layer is excellent when it is subjected to processing that requires a large load. In some cases, peeling at the interface between layers could occur without fully exhibiting the ductility and the high strength of the second layer.

On the other hand, in the conventional technology described in Non-Patent Document 1 (hereinafter referred to as Conventional Technology 2), as a multilayer material composed of a combination of high carbon steel and brass, it is certainly strength (about 800 MPa) and ductility (60% ) Is obtained, but when considering whether or not it satisfies the properties required for automobile materials, even if it can be satisfied with the ductility, the tensile strength of about 800 MPa Therefore, it must be said that the strength characteristics are extremely insufficient, and this cannot be used as a material for automobiles.
Accordingly, there is a strong demand for the development of a multilayer steel that has a high bonding strength between layers, does not cause interfacial peeling, and has an excellent balance between strength and ductility even under conditions where a high load is applied.

The multi-layer steel of the present invention and the manufacturing method thereof have been developed to meet the above requirements,
“(1) The austenite structure or ferrite structure occupies the maximum volume fraction, the first layer made of steel having a tensile strength TS ₁ of 1200 MPa or less, and the martensite structure occupies the maximum volume fraction, The second layer made of steel having a thickness TS ₂ of 1200 MPa or more is laminated to each other, and further, a total of three or more layers are laminated and integrated with the first layer as a surface layer. The initial laminated interface with the second layer is located in the crystal grain or the grain boundary of the multilayer steel, and the initial laminated interface is a kind of Si-based oxide, Cr-based oxide, SiCr composite oxide. Alternatively, two or more types are dispersed, and the oxide has an interface crystal structure in which metal bonds are continuously formed between the oxides. The tensile strength and total elongation of the multilayer steel as a whole are 900 MPa or more. Product is 20000 MPa ·% or more Multilayered steel interfacial peel strength and wherein the at 10 N / mm or more.
(2) The oxide is SiO ₂ having a major axis of 1 μm or less and has an interface crystal structure in which metal bonds are formed over at least 1 nm between SiO ₂ grains arranged in a line. The multilayer steel according to (1) above.
(3) The second layer is mass% (hereinafter the same),
C: 0.05-0.4%
Si: 0.05-3.0%,
Mn: 0.05 to 3.0%
Made of steel A comprising the balance iron and inevitable impurities,
The first layer
C: 0.01 to 0.15%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%,
Cr: 12.0 to 24.0%,
Ni: 4.0 to 14.0%,
N: 0.001 to 0.3%
Steel C consisting of the balance iron and inevitable impurities, or
C: 0.001 to 0.15%,
Si: 0.05-3.0%,
Mn: 15.0-32.0%
Steel E consisting of the balance iron and inevitable impurities, or
C: 0.0001 to 0.4%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%
Steel G comprising the balance iron and inevitable impurities,
The multilayer steel according to (1) or (2) above, comprising one or more of any of the above.
(4) Steel A constituting the second layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 16.0%,
Ni: 0.01 to 12.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
1 type or 2 types or more of these are further contained, The multilayer steel as described in said (3) characterized by the above-mentioned.
(5) Steel C constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
The multilayer steel according to (3) or (4), further comprising one or more of them.
(6) Steel E constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 12.0%,
Ni: 0.01-40.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
N: 0.001 to 0.3%,
B: 0.0003 to 0.0100%
The multilayer steel according to any one of (3) to (5) above, further comprising one or more of them.
(7) Steel G constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 12.0%,
Ni: 0.01-40.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
The multilayer steel as set forth in any one of (3) to (6), further including one or more of them.
(8) It is a manufacturing method of the multilayer steel in any one of said (1)-(7), Comprising: The 1st layer and 2nd layer which degreased the surface are laminated | stacked by using the surface layer as a 1st layer Then, the periphery is welded to form a laminate, and the laminate is heated and rolled at 400 to 1100 ° C., then heated at 750 to 1100 ° C. for 1 to 120 minutes, and cooled to room temperature. A method for producing a multilayer steel.
(9) The method for producing a multilayer steel according to (8), wherein the first layer and the second layer are laminated in an inert atmosphere, and the periphery is welded to form a laminate. .
(10) The multilayer steel according to (8) or (9) above, wherein a through hole is formed in at least one place of the laminate before hot rolling, and an oxygen getter material is inserted into the through hole. Manufacturing method.
(11) The method for producing a multilayer steel according to (10), wherein the oxygen getter material is a Ti wire.
(12) After rolling at 400 to 1100 ° C., further cold rolling, heating at 750 to 1100 ° C. for 1 to 120 minutes, and cooling to room temperature, any of the above (8) to (11) The manufacturing method of the multilayer steel as described in one term.
(13) The method for producing a multilayer steel according to any one of (1) to (7), wherein the first layer and the second layer whose surfaces are degreased are laminated with the surface layer as the first layer. Then, the periphery is welded to form a laminate, and cold rolling with a one-pass reduction rate of 35% or more is performed at least once, heated at 750 to 1100 ° C. for 1 to 120 minutes, and cooled to room temperature. A method for producing a multilayer steel characterized by the above. "
It is characterized by.

  Hereinafter, the present invention will be described in detail.

The multi-layer steel of the present invention is a first layer made of steel (hereinafter referred to as “γ / α steel”) having a tensile strength TS ₁ of 1200 MPa or less and an austenite structure or a ferrite structure occupying the largest volume fraction. And a laminated structure integrated with a second layer made of steel (hereinafter referred to as “Mar steel”) having a tensile strength TS ₂ of 1200 MPa or more and a martensite structure occupying the largest volume fraction. Further, the total number of layers is three or more, and the surface layer is composed of the first layer.
In order to increase the strength and ductility of the multi-layer steel as a whole, Mar steel needs to have high tensile strength, while γ / α steel requires excellent ductility. Is done. Since strength and ductility are generally incompatible properties, in order for γ / α steel to have excellent ductility, a certain degree of strength reduction must be allowed. The tensile strength was set to 1200 MPa or less to provide high ductility. On the other hand, Mar steel is required to have a tensile strength of 1200 MPa or more in order to ensure the strength of the multilayer steel as a whole.

Furthermore, in the multilayer steel of the present invention, in order to improve the interfacial peel strength (peel strength) between each layer, as shown schematically in FIG. 1, the initial lamination of the first layer and the second layer is performed. At the interface, the processed structure is recrystallized, and atoms in the first layer and the second layer diffuse to each other, so that new crystal grains are formed. In the crystal grains or in the crystal grain boundaries, oxidation occurs. A product was formed, and an interfacial crystal structure was formed in which metal bonds were continuously formed between the oxides. As a result, an excellent interfacial peel strength (peel strength) of 10 N / mm or more is obtained, and so-called interfacial peel does not occur, and breakage occurs due to necking of the second layer, so that the break elongation is significantly increased. Note that continuous metal bonding means that the metal bonding structure has no gap over 1 nm or more. Moreover, a metal connective structure of about 1 nm can be confirmed by high-resolution TEM observation.
Specifically, the oxides include Si-based oxides such as SiO ₂ , Cr-based oxides such as Cr ₂ O ₃ and CrO _2, and SiCr composite oxides such as (SiCr) xOy and a mixture of SiO ₂ and CrO ₂ . 1 type or 2 types or more. Among these, it is particularly preferable to form an interfacial crystal structure in which SiO ₂ having a major axis of 1 μm or less is dispersed and at least 1 nm or more between SiO ₂ grains arranged in a row is metal-bonded. .
The initial lamination interface is an interface when the first layer and the second layer are laminated. In the laminated steel, the first layer and the second layer diffuse to each other. , It is mainly observed as a site where oxide particles are formed in a row or cluster in the crystal grains.
The interfacial crystal structure can be formed by the following method.
When forming the above-mentioned interface crystal structure by hot rolling, first, the first layer and the second layer are degreased to remove oil on the surface, and the first layer is laminated as a surface layer. The periphery is welded to form a laminate. This laminated body is heated and rolled at 400-1100 ° C.
When hot rolling at a temperature of 750 ° C. or higher to form the above-mentioned interface crystal structure, it is preferable to reduce the oxygen partial pressure at the interface between the first layer and the second layer of the laminate. Therefore, it is preferable to perform the formation of the laminate, particularly the surrounding welding in an inert atmosphere. Further, when a hole penetrating in the thickness direction of the laminate is provided and an oxygen getter material such as a Ti wire is inserted, the oxygen getter material is preferentially oxidized during hot hot rolling, and the first layer and the first layer The oxygen partial pressure at the interface between the two layers can be reduced. An effect can be obtained by providing the oxygen getter material at least at one end or corner of the laminate.
After warm rolling and hot rolling, cold rolling may be further performed as necessary. After cold rolling, it is heated at 750 to 1100 ° C. for 1 to 120 minutes and cooled to room temperature.
In addition, the said interface crystal structure can also be formed by cold rolling and heat processing, without performing hot rolling. In this case, the first layer and the second layer that have been degreased to remove surface oil and the like are laminated so that the surface layer becomes the first layer, and the periphery is welded to form a laminated body. Then, cold rolling is performed as it is. In this case, it is necessary to perform at least one cold rolling at a rolling reduction of 35% or more in one pass of the cold rolling. The interface between the first layer and the second layer becomes a new surface due to the cold rolling under the large pressure, and the interface crystal structure is formed when heating is performed at 750 to 1100 ° C. for 1 to 120 minutes.

In FIG. 2 (a), SUS304 (Steel C in Table 1) is used as the first layer, and SCM415 (Steel A-2 in Table 1) is used as the second layer. Interfacial crystals formed at the interface between the first layer and the second layer observed with a transmission electron microscope (TEM) for the multilayer steel formed by laminating and integrating by 1100 ° C., holding time 2 minutes, water cooling) A tissue photograph is shown.
From this figure, it is confirmed that particles having a major axis of 1 μm or less (white dots scattered in the initial interface in the drawing) are dispersed at the interface between the first layer and the second layer. The Furthermore, this interface crystal structure was analyzed using an energy dispersive X-ray analyzer (EDS). As a result, it was found that the particles arranged in a row were Si-based oxides, that is, SiO ₂ particles, and metal-bonded over at least 1 nm between the particles.
Note that FIG. 2B is a TEM structure photograph in a cold rolled state before the heat treatment, and no interface crystal structure is observed at the interface between the first layer and the second layer.

The influence of the heat treatment temperature and the heat treatment time on the interfacial peel strength (peel strength) of the multilayer steel having the interface crystal structure shown in FIG. The peel strength was measured by performing an interface peeling tensile test as schematically shown in FIG. Note that the peel strength is a value obtained by dividing the tensile load necessary to peel off the interface (adopting a load in a stable load region with respect to the peeling stroke) by the width of the test piece.
The results are shown in FIG. 4. The higher the heat treatment temperature, the higher the interfacial peel strength (peel strength), and the interfacial peel strength (peel strength) increases with the heat treatment time up to 2 minutes. When the time is 2 minutes or more, a significant increase in strength is not observed.
Further, the correlation between the interfacial peel strength (peel strength) and the elongation at break was investigated for the multilayer steel having the interfacial crystal structure shown in FIG.
The results are shown in FIG. 6. As the interfacial peel strength (peel strength) increases, the elongation at break increases, and when the interfacial peel strength (peel strength) is less than 10 N / mm, the interfacial peel breaks. However, when the interfacial peel strength (peel strength) is 10 N / mm or more, it has been found that the fracture occurs not by so-called interfacial peel but by necking of the second layer.
Accordingly, SiO ₂ grains having a major axis of 1 μm or less are dispersed in crystal grains at the interface between the first layer and the second layer constituting the multi-layer steel, and the SiO ₂ grains arranged in a row are mutually aligned. It can be seen that by forming an interfacial crystal structure that is metal-bonded over at least 1 nm in between, the occurrence of interfacial debonding of the multilayer steel can be suppressed.
Even if manufactured in the same manner, if the oxygen partial pressure during rolling is slightly high, Cr-based oxides and SiCr-based composite oxides are formed. An example is shown in FIG. 5, as in FIG. 2, SUS304 (steel C in Table 1) was used as the first layer, and SCM415 (steel A-2 in Table 1) was used as the second layer. It is the optical microscope structure | tissue photograph of the cross section of the multilayer steel formed by carrying out lamination | stacking integration by heating temperature 1100 degreeC, holding time 2 minutes, and water cooling. As a result of analyzing this interface crystal structure using a scanning electron microscope (SEM) and an attached electron dispersive X-ray analyzer (EDS), the oxide is a complex oxide containing Cr and Si. I understood.

For the γ / α steel constituting the first layer of the multilayer steel of the present invention or the Mar steel constituting the second layer, various steels described in claim 3 can be used.
As the Mar steel constituting the second layer, it is necessary to use a steel having a high tensile strength even if the ductility is low.
C: 0.05-0.4%
Si: 0.05-3.0%,
Mn: 0.05 to 3.0%
And steel A composed of the remaining iron and inevitable impurities can be used.
As the γ / α steel constituting the first layer, it is necessary to use a steel that is not sufficient in tensile strength but excellent in ductility. Specifically,
C: 0.01 to 0.15%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%,
Cr: 12.0 to 24.0%,
Ni: 4.0 to 14.0%,
N: 0.001 to 0.3%
Steel C consisting of the balance iron and inevitable impurities, or
C: 0.001 to 0.15%,
Si: 0.05-3.0%,
Mn: 15.0-32.0%
Steel E consisting of the balance iron and inevitable impurities, or
C: 0.0001 to 0.4%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%
Steel G comprising the balance iron and inevitable impurities,
Any one or two or more of these can be used.

Further, the Mar steel constituting the second layer of the multi-layer steel of the present invention has a small amount of alloy components (Nb: 0.001 to 0.1 to the steel A) as described in claim 4. %, Ti: 0.001 to 0.1%, V: 0.001 to 0.5%, Cr: 0.01 to 16.0%, Ni: 0.01 to 12.0%, Mo: 0.00. 1 to 3.0%, Cu: 0.01 to 1.0%, B: 0.0003 to 0.0100%, or two or more. .
By containing a predetermined amount of each of these alloy components as described above, mechanical properties such as strength, ductility and toughness of the Mar steel (steel A) itself can be adjusted.

  Further, the γ / α steel constituting the first layer of the multi-layer steel of the present invention has a small amount of alloy components (Nb: 0.001 to 0.1%, Ti: 0.001) with respect to the steel C. -0.1%, V: 0.001-0.5%, Mo: 0.01-3.0%, Cu: 0.01-1.0%, B: 0.0003-0.0100% 1 type or 2 types or more of them, all of which can be contained by mass%).

  Further, the γ / α steel constituting the first layer of the multi-layer steel of the present invention has a small amount of alloy components (Nb: 0.001 to 0.1%, Ti: 0.001) relative to the steel E. -0.1%, V: 0.001-0.5%, Cr: 0.01-12.0%, Ni: 0.01-40.0%, Mo: 0.01-3.0%, Cu: 0.01 to 1.0%, N: 0.001 to 0.3%, B: 0.0003 to 0.0100%, or two or more. can do.

  Further, the γ / α steel constituting the first layer of the multi-layer steel of the present invention has a small amount of alloy components (Nb: 0.001 to 0.1%, Ti: 0.001) with respect to the steel G. -0.1%, V: 0.001-0.5%, Cr: 0.01-12.0%, Ni: 0.01-40.0%, Mo: 0.01-3.0%, Cu: 0.01 to 1.0%, B: 0.0003 to 0.0100%, or two or more.

The interfacial crystal structure of the laminated interface between the first layer and the second layer of the multilayer steel of the present invention can be formed by the following method.
First, the first layer and the second layer whose surfaces are degreased are laminated so that the surface layer becomes the first layer, and the periphery is welded to form a laminate. The reason why the periphery between the laminated bodies is welded is to prevent oxygen from entering and oxidizing the surface during heating when the laminated bodies are hot-rolled. Therefore, the lamination and welding of the first layer and the second layer may be performed in the air, but in an inert atmosphere, for example, Ar gas, in order to reduce the oxygen partial pressure in the stack. Is preferred. Further, a hole penetrating in the thickness direction of the laminate may be provided, and an oxygen getter material that absorbs oxygen existing between layers may be inserted. The oxygen getter material is preferably a metal that is easily oxidized, such as a Ti wire.
Next, the laminate is rolled at 400 to 1100 ° C. When the rolling temperature is less than 400 ° C., the deformation resistance is high, and when the rolling temperature exceeds 1100 ° C., the oxide grows and the metal bond in the interface crystal structure of the laminated interface between the first layer and the second layer. Becomes insufficient. In addition, in order to raise an intensity | strength after rolling, it hold | maintains at 750-1100 degreeC for 1 to 120 minutes, and cools to room temperature. The heating temperature for the heat treatment is preferably 750 ° C. or higher in order to make the metal structure of the second layer austenite, and the holding temperature is preferably 1 minute or longer in order to sufficiently transform. On the other hand, in order to avoid the coarsening of the particle size, it is preferable to set the heating temperature to 1100 ° C. or less and the holding time to 120 minutes or less. The cooling is preferably water cooling in order to increase the strength.
After rolling at 400 to 1100 ° C., it may be further cold-rolled to a target thickness, then heated at 750 to 1100 ° C. for 1 to 120 minutes, and cooled to room temperature. In this case, similarly, the heating temperature of the heat treatment is preferably 750 to 1100 ° C., the holding temperature is preferably 1 to 120 minutes, and the cooling is preferably water cooling.
The laminated body can be cold-rolled and heat-treated without hot rolling to form the interfacial crystal structure at the interface between the layers.
In this case, the first layer and the second layer are laminated, the periphery is welded to form a laminated body, and cold-rolled as it is to generate a new surface at the interface between the first layer and the second layer. . As before hot rolling, the lamination and welding of the first layer and the second layer may be performed in the air, but it is preferably performed in an inert atmosphere. In the cold rolling, in order to generate a new surface at the interface between the first layer and the second layer, the cold rolling with a reduction rate of one pass of 35% or more is performed at least once.
The subsequent heat treatment is performed at 750 to 1100 ° C. for 1 to 120 minutes. In this case, the heating temperature is preferably 750 ° C. or higher and the holding temperature is 1 in order to cause the diffusion, recrystallization, grain growth and phase transformation of the atoms in each layer at the laminated interface between the first layer and the second layer. Minutes or more are preferred. On the other hand, in order to avoid the coarsening of the particle size, it is preferable to set the heating temperature to 1100 ° C. or less and the holding time to 120 minutes or less. The cooling is preferably water cooling in order to increase the strength.
As shown in FIG. 4, the higher the heating temperature and the longer the heating time, the better the interfacial peel strength (peel strength).

In the multi-layer steel of the present invention, the first layer is composed of γ / α steel having a tensile strength TS ₁ of 1200 MPa or less, and the second layer is composed of Mar steel having a tensile strength TS ₂ of 1200 MPa or more. Then, a total of three or more layers are laminated and integrated with the first layer as a surface layer, and at the initial lamination interface between the first layer and the second layer, Si-based oxidation is performed in the crystal grains or in the crystal grain boundaries. One, two or more kinds of oxides, Cr-based oxides, SiCr composite oxides are dispersed, and preferably have an interface crystal structure in which metal bonds are formed over at least 1 nm between the oxides. Has a high strength by forming an interfacial crystal structure in which SiO ₂ grains having a major axis of 1 μm or less are dispersed and metal bonds are formed over at least 1 nm between the SiO ₂ grains arranged in a line. High ductility, excellent strength-ductility balance It has a good interfacial peel strength (peel strength).
Therefore, as a whole multilayer steel, the tensile strength is 1000 MPa or more and the product of the tensile strength and the total elongation is 20000 MPa ·% or more. Further, when evaluated by a peel test, the interfacial peel strength is 10 N / mm or more. From the above, it can be said that the material is suitable as an automotive material that is required to have high strength, high ductility, good workability, and light weight.

  The present invention combines at least two kinds of steels having different structures and mechanical properties into multiple layers, and forms an interfacial crystal structure between the layers, whereby the tensile strength, ductility, and interfacial peel strength of the multi-layer steels. (Peel strength) can be improved.

In the present invention, the surface layer of the first layer made of γ / α steel having TS ₁ of 1200 MPa or less and the second layer made of Mar steel having tensile strength TS ₂ of 1200 MPa or more is degreased so that the surface layer becomes After laminating to be the first layer and welding the surroundings, after forming a laminate (total number of laminates is 3 or more and the surface layer is composed of the first layer), the laminate is heated, Rolling is performed at 400 to 1100 ° C., and further heat treatment is performed at 750 to 1100 ° C. for 1 to 120 minutes, followed by cooling to room temperature, thereby stacking and integrating. Alternatively, the laminated body may be laminated and integrated by performing cold rolling at a rolling rate of 35% or more in one pass at least once, heating at 750 to 1100 ° C. for 1 to 120 minutes, and cooling to room temperature. it can.
And by performing the above-mentioned warm rolling or hot rolling and heat treatment, or cold rolling and heat treatment, the initial lamination interface between the first layer and the second layer is in the crystal grains or in the crystal grain boundaries, One or more of Si-based oxides, Cr-based oxides, and SiCr composite oxides are dispersed and have an interface crystal structure in which metal bonds are formed over at least 1 nm between the oxides. Preferably, one or both of a Si-based oxide and a Cr-based oxide having a major axis of 1 μm or less are dispersed, and metal bonds are formed over at least 1 nm between the oxides arranged in a line. As a result, a multi-layer steel having an interfacial peel strength (peel strength) of 10 N / mm or more is obtained.

  Hereinafter, the present invention will be described in more detail with reference to specific examples.

  In the present invention, four types of γ / α steel (steel C, steel E, steel G-1, and steel G-2) shown in Table 1 are used as the first layer, and Table 1 is also used as the second layer. Two types of Mar steel (steel A-1 and steel A-2) shown in FIG. Table 1 also shows the tensile strength of each steel.

  The first layer and the second layer shown in Table 1 are laminated so as to have a total layer thickness with the combinations shown in Table 2, each layer thickness, and the number of laminated layers, and the surroundings are welded in the air to obtain a laminated body Then, these laminates were heated and hot-rolled at 400 to 1100 ° C. to produce a multilayer steel. A part thereof was further cold-rolled to a target thickness, heated at 750 to 1100 ° C. for 1 to 120 minutes, and cooled to room temperature. Also, some of the multi-layer steel is hot rolled before being heated by hot rolling, by inserting a Ti wire through the laminate and inserting a Ti wire, and absorbing oxygen between the layers. did. Further, some of the multi-layer steels are not hot-rolled of the laminate, and after cold rolling in which the rolling reduction of the first pass is 35% or more is performed at least once and then 1 to 120 at 750 to 1100 ° C. Prepared by heating for minutes and cooling to room temperature. Detailed conditions of each multilayer steel are shown in Table 2.

For example, in Example 1, γ / α steel having a thickness of 0.63 mm and a tensile strength of 694 MPa as the first layer shown in Table 1, and a thickness of 0.28 mm and a tensile strength as the second layer. Using Mar steel of 2100 MPa, as shown in Table 2, the surface layer is the first layer, 9 layers of the first layer and 8 layers of the second layer are alternately stacked, and the surroundings in the atmosphere Were welded to form a laminate with a total number of layers of 17 and a total layer thickness of 0.9 mm.
Next, the laminate is heated, hot-rolled under the conditions shown in Table 2, and further cold-rolled until the target thickness is reached under the conditions shown in Table 2, and further heat-treated under the conditions shown in Table 2. And the multilayer steel of Example 1 was produced by water-cooling to room temperature.

And when the layer thickness direction center position of each layer of the produced multilayer steel of Example 1 was observed with an optical microscope, the first layer is γ steel in which the austenite structure occupies almost 100% by volume, while the second layer This layer was confirmed to be Mar steel in which the martensite structure occupies almost 100% by volume.
Further, when the interface between the first layer and the second layer of the multilayer steel of Example 1 was observed with TEM using EDS in combination, an interface crystal structure was formed at the interface, and the interface In the crystal grains, SiO ₂ grains having a major axis of 0.01 to 0.1 μm are dispersed, and metal bonds are formed over at least 1 nm between the SiO ₂ grains arranged in a row. It was confirmed.

About the obtained multilayer steels of Examples 1 to 8, the tensile strength (MPa), the elongation (%), and the strength-ductility balance index value (tensile strength (MPa) x elongation ( %)) And interfacial peel strength (peel strength).
The results are shown in Table 3.
In addition, each said measurement is measured by the following test methods. That is, the tensile test was performed based on JIS Z 2241 using a JIS Z 2201 No. 5 test piece. For peel strength, the interface peeling tensile test schematically shown in FIG. 3 was performed, and the tensile load necessary to peel the interface (adopting a load in a stable load region with respect to the peeling stroke) was used as a test piece. It was obtained by dividing by the width of.
Furthermore, since the structure of the interface between the first layer and the second layer was observed by TEM using EDS together, the results are also shown in Table 3.
As is clear from the results in Table 3, the inventive multilayer steels of Examples 1 to 8 have high strength and high ductility, excellent strength-ductility balance, and interfacial peel strength (peel strength). Was 10 N / mm or more, and it was confirmed that it had excellent interface strength.

For comparison, the first layer and the second layer are combined in the combinations shown in Table 4, and the laminate is formed so that the layer thickness, the number of stacked layers, and the total layer thickness are also shown in Table 4, The laminated body is hot-rolled under the same conditions as shown in Table 4 (or not hot-rolled), and then cold-rolled and integrated until the target thickness is reached, so that a multilayer steel of a comparative example (comparison) Examples 21-23) were prepared.
About the obtained multilayer steels of Comparative Examples 21 to 23, tensile strength (MPa), elongation (%), strength-ductility balance index value as a whole multilayer steel (tensile strength (MPa) × elongation (%)) Value) and interfacial peel strength (peel strength) were measured in the same manner as in Examples 1-8.
The results are shown in Table 5.
Furthermore, since the structure of the interface between the first layer and the second layer was observed with TEM using EDS together, the results are also shown in Table 5.

When comparing the results of Examples 1 to 8 with the results of Comparative Examples 21 to 23,
Comparative Example 21 is different from Example 2 only in the heat treatment conditions after cold rolling, but when the heat treatment time at 1000 ° C. is too short as 20 seconds, the interface peel strength is lowered, and the ductility is significantly lowered. I understand that.
Further, Comparative Example 22 is different from Example 4 only in the heat treatment conditions after the cold pressure, but it can be seen that when the temperature is lowered and the time is shortened, the interface peel strength is lowered and the ductility is remarkably lowered.
Comparative Example 23 is different from Example 6 only in the cold rolling conditions. However, since the maximum rolling reduction of one pass is low, the interface peel strength is lowered, and similarly, the ductility is significantly lowered.

  As can be seen from the above results, the multilayer steel of the present invention has high strength, high ductility, strength-ductility balance, and excellent interfacial peel strength (peel strength). It can be said that it is suitable as a material for which weight reduction is desired and high strength and good workability are required, such as a material for use.

It is a schematic diagram which shows the relationship between the structure | tissue of the multilayer steel of this invention, and an initial lamination interface. FIG. 2 is an electron micrograph of an interlayer interface of a multilayer steel composed of SUS304 for the first layer and SCM415 for the second layer, and (a) is for the multilayer steel of the present invention (Example 2). In the initial lamination interface, there is an interface crystal structure in which SiO ₂ grains having a major axis of 1 μm or less are dispersed and metal bonds are formed over at least 1 nm between the SiO ₂ grains arranged in a line. On the other hand, (b) is the one before the heat treatment of (a) (as cold-rolled), and shows that there is no special interface crystal structure at the interface. It is a schematic diagram of the measuring method of peel strength. It is a graph which shows the influence of the heat processing temperature and heat processing time which have on interfacial peel strength (peel strength). It is an optical micrograph of the interlayer interface of the multilayer steel of the present invention in which the first layer is made of SUS304 and the second layer is made of SCM415, and an oxide containing Si and Cr is generated at the initial lamination interface, It shows that there is an interfacial crystal structure that is metal-bonded over at least 1 nm or more between oxides. It is a graph which shows the influence of the peel strength which acts on the ductility of a multilayer material.

Claims (13)

The austenite structure or ferrite structure occupies the maximum volume fraction, the first layer made of steel having a tensile strength TS ₁ of less than 1200 MPa, the martensite structure occupies the maximum volume fraction, and the tensile strength TS ₂ is A second layer made of steel of 1200 MPa or more is laminated to each other, and further, a total of three or more layers are laminated and integrated with the first layer as a surface layer, and an initial of the first layer and the second layer The lamination interface is located in the crystal grain or grain boundary of the multilayer steel, and one or more of Si-based oxide, Cr-based oxide and SiCr composite oxide are dispersed in the initial lamination interface. In addition, the oxide has an interface crystal structure in which metal bonds are continuously formed between the oxides, the tensile strength of the multilayer steel as a whole is 900 MPa or more, and the product of tensile strength and total elongation is 20000 MPa ·% or more. And the interfacial peel strength is Multilayered steel, characterized in that at 0N / mm or more. The oxide is SiO ₂ having a major axis of 1 μm or less, and has an interface crystal structure in which metal bonds are formed over at least 1 nm between SiO ₂ grains arranged in a line. Item 2. The multilayer steel according to item 1. The second layer is mass% (hereinafter the same),
C: 0.05-0.4%
Si: 0.05-3.0%,
Mn: 0.05 to 3.0%
Made of steel A comprising the balance iron and inevitable impurities,
The first layer
C: 0.01 to 0.15%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%,
Cr: 12.0 to 24.0%,
Ni: 4.0 to 14.0%,
N: 0.001 to 0.3%
Steel C consisting of the balance iron and inevitable impurities, or
C: 0.001 to 0.15%,
Si: 0.05-3.0%,
Mn: 15.0-32.0%
Steel E consisting of the balance iron and inevitable impurities, or
C: 0.0001 to 0.4%,
Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%
Steel G comprising the balance iron and inevitable impurities,
The multilayer steel according to claim 1 or 2, comprising any one or more of the following. Steel A constituting the second layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 16.0%,
Ni: 0.01 to 12.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
The multilayer steel according to claim 3, further comprising one or more of them. Steel C constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
The multilayer steel according to claim 3 or 4, further comprising one or more of them. Steel E constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 12.0%,
Ni: 0.01-40.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
N: 0.001 to 0.3%,
B: 0.0003 to 0.0100%
The multilayer steel according to any one of claims 3 to 5 , further comprising one or more of them.
Steel G constituting the first layer is
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.01 to 12.0%,
Ni: 0.01-40.0%,
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
B: 0.0003 to 0.0100%
The multilayer steel according to any one of claims 3 to 6, further comprising one or more of them.   It is a manufacturing method of the multilayer steel as described in any one of Claims 1-7, Comprising: The 1st layer and 2nd layer which degreased the surface are laminated | stacked by using the surface layer as a 1st layer, The circumference | surroundings Welding to form a laminate, heating the laminate, rolling at 400 to 1100 ° C., then heating at 750 to 1100 ° C. for 1 to 120 minutes, and cooling to room temperature Steel manufacturing method. The method for producing a multilayer steel according to claim 8, wherein the first layer and the second layer are laminated in an inert atmosphere, and the periphery is welded to form a laminated body. The method for producing a multilayer steel according to claim 8 or 9, wherein a through hole is formed in at least one place of the laminate before hot rolling, and an oxygen getter material is inserted into the through hole. The method for producing a multilayer steel according to claim 10, wherein the oxygen getter material is a Ti wire. The rolling according to any one of claims 8 to 11 , wherein after rolling at 400 to 1100 ° C, further cold rolling, heating at 750 to 1100 ° C for 1 to 120 minutes, and cooling to room temperature. Manufacturing method of layer steel.
It is a manufacturing method of the multilayer steel as described in any one of Claims 1-7, Comprising: The 1st layer and 2nd layer which degreased the surface are laminated | stacked by using the surface layer as a 1st layer, The circumference | surroundings A laminate is formed by welding, and cold rolling with a reduction rate of 35% or more in one pass is performed at least once, heated at 750 to 1100 ° C. for 1 to 120 minutes, and cooled to room temperature. A method for producing multilayer steel.

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