JP4960289B2 - Double layer steel - Google Patents

Description

  The present invention relates to a multilayer steel having high strength and at the same time having a high ductility and a good balance between strength and ductility.

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 steel for automobiles, various properties derived from increasing the strength of steel (ductility, toughness, rigidity, workability, embrittlement characteristics, It is necessary to eliminate the problem of deterioration in corrosion resistance, environmental resistance, etc.), but in particular, increasing strength and increasing ductility are generally contradictory properties, so having high strength At the same time, there has been a strong demand for a steel material having a high ductility and a so-called strength-ductility balance.

As one measure for this, an attempt is made to improve the strength-ductility balance of a material by forming a plurality of materials having different properties. Some of the inventors have made multiple layers of steel (carbon steel, alloy steel, stainless steel, high manganese steel, etc.) with different structures and mechanical properties as a multilayered material of steel and other materials. A multi-layer steel is proposed in which both strength and ductility are improved by rolling them together and heat-treating them as necessary (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% can be obtained. Further, 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, both the first layer and the second layer are a multilayer steel made of a combination of stainless steels, or the second layer is a multilayer steel made of high manganese steel. In other words, a combination of special steel materials achieves high strength and high ductility. These special steel materials require a precise adjustment and management of the manufacturing process, and the price of the additive component elements is high, so that the manufacturing cost is high.

On the other hand, in the conventional technology described in Non-Patent Document 1 (hereinafter referred to as Conventional Technology 2), as a multi-layer material composed of a combination of high carbon steel and brass, it certainly has strength (about 700 MPa) and ductility (60% ) Is obtained, but when considering whether or not it satisfies the characteristics required for automobile materials, even if the ductility is sufficient, a tensile strength of about 700 MPa Then, it must be said that the strength characteristics are extremely insufficient, and it is very difficult to use this as an automobile material. Moreover, the prior art 2 is not a multilayer steel made of a combination of steel materials, and the material cost is also increased.
Therefore, in order to simplify the manufacturing process and at the same time reduce the material cost, cheaper and more general carbon steel is used, which has high strength and high ductility, and has a good balance between strength and ductility. Development of layer steel is strongly desired.

The multi-layer steel of the present invention was developed to meet the above requirements,
“(1) Tensile strength TS _{2 in which} the ferrite structure occupies the maximum volume fraction, and a tensile strength TS _{2 in} which the martensite structure occupies the maximum volume fraction. Is formed by laminating a second layer made of carbon steel or low alloy steel of 1200 MPa or more, and further laminating and integrating a total of three or more layers with the first layer as a surface layer. The ratio (t ₁ / t ₂ ) of the layer thickness t ₁ and the layer thickness t ₂ of the second layer exceeds 1.2, and the tensile strength TS ₂ of the _second layer and the tensile strength of the first layer The ratio of TS ₁ (TS ₂ / TS ₁ ) is 1.2 or more and 6 or less, the tensile strength of the multilayer steel as a whole is 1050 MPa or more, and the product of tensile strength and total elongation is 21000 MPa ·% or more. Multi-layer steel characterized by
(2) The above-mentioned (1), wherein each layer thickness t ₁ of the first layer is 0.04 mm or more, and each layer thickness t _{2 of the second} layer is 0.1 mm or less. Multi-layer steel.
(3) The multilayer steel according to (1) and (2) above, wherein the total thickness of the multilayer steel is 3.0 mm or less.
C: 0.0001 to 0.20%,
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%
Is a carbon steel composed of the balance iron and inevitable impurities,
The second layer is, by weight,
C: 0.05-0.4%
Si: 0.05-3.0%,
Mn: 0.05 to 3.0%
The multilayer steel according to any one of (1) to (3) above, characterized in that it is a carbon steel comprising the balance iron and inevitable impurities.
(5) The first layer is mass%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.001 to 0.01%,
Ni: 0.001 to 0.01%
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
The multi-layer steel according to (4) above, which is a low alloy steel further containing one or more of the above.
(6) The second layer is mass%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.001 to 2.0%,
Ni: 0.001 to 2.5%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 1.0%,
The multilayer steel according to (4) and (5) above, which is a low alloy steel further containing one or more of the above. "
It is characterized by.

  Hereinafter, the present invention will be described in detail.

In the multilayer steel of the present invention, the first layer and the second layer are made of carbon steel (also called ordinary steel or ordinary carbon steel) or low alloy steel (Nb, Ti, V, Cr, Ni, Mo, Cu, respectively). The total content of one or more alloy elements is 5% or less), and the first layer and the second layer are laminated and integrated in a total of three or more layers. It is configured.
The first layer is made of carbon steel or low alloy steel (hereinafter referred to as “α steel”) in which the ferrite structure occupies the maximum volume fraction, while the second layer has the largest martensite structure. It consists of carbon steel or low alloy steel (hereinafter referred to as “Mar steel”) occupying a volume fraction.
Furthermore, in order to increase the strength of the multilayer steel as a whole, not only Mar steel having high tensile strength is used as the constituent material of the second layer, but also the first layer and the surface layer of the multilayer steel are constructed. The α steel to be used must have at least a predetermined tensile strength. Here, the tensile strength of the α steel shown in TS _1, if the tensile strength of the Mar steel shown in TS _2, TS ₂ in the above 1200 MPa, and the Mar steel tensile strength TS ₂ and the α steel The ratio of the tensile strength TS ₁ (TS ₂ / TS ₁ ) must be a value of TS ₁ that satisfies 1.2 or more and 6 or less.
On the other hand, the multi-layer steel of the present invention not only simply improves the strength, but at the same time it is necessary to increase the ductility, the layer thickness of the α steel constituting the first layer and the surface layer is t ₁ , in addition, when the thickness of the Mar steel constituting the second layer was _{t 2,} the thickness _{t 1} of the first layer (alpha steel) thickness _{t 2} of the second layer (Mar steel) is
t ₁ / t ₂ > 1.2
Must be satisfied with the relationship.
That is, as long as the layer thickness t1 of the _first layer (α steel) does not exceed 1.2 times the layer thickness t2 of the _second layer (Mar steel), the strength of the multilayer steel can be increased. Even so, the elongation value (EL value) of the multi-layer steel as a whole is remarkably reduced, and the product of tensile strength and total elongation is less than 21000 MPa ·%, so that the multi-layer steel has an excellent strength-ductility balance. This is because they cannot be squeezed.
As described above, the multi-layer steel of the present invention includes a first layer made of α-steel having a layer thickness t _{1, and} a second layer made of Mar steel having a layer thickness t ₂ , wherein t ₁ / t _When the thicknesses of the first layer and the second layer are determined so that ₂ > 1.2, and the tensile strength of α steel is TS ₁ and the tensile strength of Mar steel is TS ₂ , 1.2 ≦ TS ₂ / TS ₁ ≦ 6 By adjusting the combination of steel types, etc., the tensile strength of the multilayer steel as a whole is 1050 MPa or more, and the product of tensile strength and total elongation is 21000 MPa ·% or more. It is possible to obtain a multilayer steel having a certain high strength, high ductility and excellent strength-ductility balance.

And the said multilayer steel can be manufactured with the following manufacturing methods, for example.
First, a second layer having a layer thickness t ₂ made of Mar steel having a relatively high strength TS ₂ but low ductility is used, and a second layer having a layer thickness t ₁ made of α steel having a relatively excellent ductility but low strength TS ₁ Sandwiched between the first layers, the second layer is stacked thereon, the first layer is further stacked thereon, and the stacking process is sequentially repeated until the total number of layers is N (however, N ≧ 3) and a laminate composed of the first layer (α steel) whose surface layer is rich in ductility is formed (note that t ₁ / t ₂ > 1.2 and 1.2 ≦ TS ₂ / TS ₁ ≦ 6 must be satisfied).
Thereafter, the laminated body is integrated by rolling such as hot rolling and cold rolling to obtain the multilayer steel. About rolling conditions, such as rolling temperature and rolling reduction, what is necessary is just the conditions which can integrate the said laminated body by rolling, Although it does not specifically limit, From the point of making the adhesive strength between each layer strong. The hot rolling temperature is preferably 650 to 1150 ° C., and the rolling reduction (the reduction rate of the layer thickness after rolling with respect to the layer thickness before rolling) is preferably 40 to 60%.

As for the layer thickness of the multilayer steel of the present invention, each layer thickness t _{1 of} the first layer (α steel) is 0.04 mm or more from the viewpoint of high strength, high ductility and weight reduction. , it is desirable each of the thickness _{t 2} of the second layer (Mar steel) is 0.1mm or less, the total layer thickness of the multilayered steel is preferably set to 3.0mm or less.

As for the α steel constituting the first layer of the multi-layer steel of the present invention or the Mar steel constituting the second layer, carbon steel that is easy to manufacture and inexpensive (both ordinary steel and ordinary carbon steel) Or low alloy steel.
As the α steel constituting the first layer, it is necessary to use a steel which is not sufficient in tensile strength but has excellent ductility. Specifically, C: 0.0001 to 0.10%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0% (both mass%), carbon steel having a composition composed of the balance iron and inevitable impurities, the main phase of the structure By using a ferrite structure, for example, the steel has a tensile strength of approximately 400 to 1000 MPa. As will be described later, a total amount of 5% or less of Nb, Ti, V, Cr, Ni, Mo, and Cu may be included as an alloy element.
On the other hand, as Mar steel constituting the second layer, it is necessary to use steel having high tensile strength even if ductility is low. Specifically, C: 0.12 to 0.4%, Si: 0.05-3.0%, Mn: 0.05-3.0% (both mass%), carbon steel having a composition composed of the balance iron and unavoidable impurities, Carbon steel with a higher C content than that of α steel, and further, by relatively increasing the content of Si and Mn components as compared with α steel, the main phase of the structure is a martensitic structure. Steel whose strength is adjusted to 1200 MPa or more. As will be described later, a total amount of 5% or less of Nb, Ti, V, Cr, Ni, Mo, and Cu may be included as an alloy element.
In both the α steel and the Mar steel, the ductility (elongation) is influenced by the component composition, heat treatment conditions, etc., but the elongation is generally 20 to 45% for the α steel, and the Mar steel. The elongation is between 3 and 10%.
In the present invention, an α steel having an excellent tensile strength (TS ₁ ) but excellent ductility, and a Mar steel having an excellent tensile strength (TS ₂ ) even if ductility is low (1.2 ≦ _TS 2 / TS of ₁ ≦ 6) and, by a layer thickness _{t 2} of the thickness _{t 1} and Mar steel α _steel, laminated so that t ₁ / t 2> 1.2, integrated Thus, it is possible to obtain a multi-layer steel having excellent characteristics that combine the characteristics of α steel and Mar steel, that is, excellent high strength, high ductility and strength-ductility balance.

Further, the α steel constituting the first layer of the multilayer steel of the present invention has a total amount of alloy components (Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1). %, V: 0.001 to 0.5%, Cr: 0.001 to 0.01%, Ni: 0.001 to 0.01%, Mo: 0.01 to 3.0%, Cu: 0.00. 01-1.0% of 1 type or 2 types or more.
By containing these alloy components in predetermined amounts as defined above, mechanical properties such as strength, ductility and toughness of the α steel itself can be adjusted. However, it is necessary for the tensile strength TS ₁ to satisfy 1.2 ≦ TS ₂ / TS ₁ ≦ 6. If this value exceeds 6, the ductility and toughness are excellent. Even if a multilayer steel is obtained, it is necessary to be careful because an undesirable multilayer steel lacking the strength-ductility balance is formed. On the other hand, in order to ensure good ductility, TS ₂ / TS ₁ ≧ 1.2 is required.

Moreover, as Mar steel which comprises the 2nd layer of the multilayer steel of this invention, a total amount is 5% or less of an alloy component (Nb: 0.001-0.1%, Ti: 0.001-0. 1%, V: 0.001 to 0.5%, Cr: 0.001 to 2.0%, Ni: 0.001 to 2.5%, Mo: 0.01 to 1.0%, Cu: 0 0.01% to 1.0% or two or more, both of which are added by mass%).
The mechanical properties such as strength, ductility, and toughness of Mar steel can be adjusted by containing these alloy components in the prescribed amounts determined as described above. Since the steel type falls into the category of alloy steel and the manufacturing cost increases, the alloy components to be added and the content thereof are determined as described above.

In the multi-layer steel of the present invention, the first layer is composed of α-steel having a layer thickness t ₁ , and the second layer is composed of Mar steel having a layer thickness t ₂ , where t ₁ / t ₂ > 1. When the layer thicknesses of the first layer and the second layer are respectively determined to be 2, and the tensile strength of the α steel is TS ₁ and the tensile strength of the Mar steel is TS ₂ , By adjusting the strength ratio by combining steel types so that ≦ TS ₂ / TS ₁ ≦ 6, excellent characteristics that each of the first layer and the second layer have (that is, excellent characteristics of the first layer) In addition, it is possible to obtain a multilayer steel having a good balance between strength and ductility and a high strength of the second layer. And as a whole multi-layer steel, since the tensile strength is 1050 MPa or more, and the product of tensile strength and total elongation is 21000 MPa ·% or more, for example, high strength and high ductility are required, and the weight reduction is required. It can be said that the material is suitable as a material.

  The present invention is a multi-layer steel formed by combining α steel and Mar steel having different structures and mechanical properties, and has a proper strength ratio and layer thickness ratio of α steel and Mar steel. By doing so, the strength-ductility balance is excellent, and the strength and ductility of the multilayer steel as a whole can be increased.

The inventors first examined the relationship between the strength ratio TS ₂ / TS _{1 of} α steel and Mar steel and the product TS × El of tensile strength and total elongation. As a result, as shown in FIG. 1, it was found that when TS ₂ / TS ₁ was 6 or less, TS × El was 21000 MPa ·% or more. This means that the strength-ductility balance decreases if the strength of the α steel is too low relative to the strength of the Mar steel.
Next, the present inventors examined the relationship between the layer thickness ratio t ₁ / t _{2 of} α steel and Mar steel and the product TS × El of tensile strength and total elongation. As a result, as shown in FIG. 2, it was found that when t ₁ / t ₂ exceeds 1.2, TS × El is 21000 MPa ·% or more. This means that the strength-ductility balance is improved by increasing the thickness of the α steel compared to the thickness of the Mar steel.
Therefore, in order to constrain Mar steel with α steel and avoid necking of Mar steel at the time of deformation, it is necessary that the difference in strength of each layer is not too large and that the thickness of soft α steel is increased. It is.

The multi-layer steel of the present invention has a layer thickness t ₂ made of Mar steel having high strength TS ₂ but low ductility, and a layer thickness made of α steel having relatively high ductility but low strength TS _1. sandwiched between the first layers of t ₁ , the second layer is laminated thereon, the first layer is further laminated thereon, and such a laminating step is sequentially repeated, for a total number of layers of 3 It is a multi-layer steel formed by forming a laminate composed of the above-mentioned first layer (α steel) having 21 layers and having a high ductility, and then integrating these by rolling.

In the present invention, in order to produce a multi-layer steel that is laminated and integrated, after forming a laminate composed of a first layer and a second layer, for example, hot rolling, cold rolling, warm rolling The laminated body is integrated by an appropriate rolling method such as rolling using both hot rolling and cold rolling, rolling using both hot rolling and warm rolling. In particular, when hot rolling is used, diffusion occurs between the first layer and the second layer during rolling, and as a result, the interlayer adhesion strength between the layers is improved.
Even when cold rolling or warm rolling is performed, by performing a predetermined heat treatment after rolling, as described above, diffusion occurs between the layers. The strength and ductility of the steel can be improved, but the lower limit of the heat treatment temperature is preferably 600 ° C. or higher. In order to increase the strength, it is preferable that after rolling, the second layer is heated to a temperature range or higher, for example, 800 ° C. or higher, where the second layer structure becomes austenite, and then water-cooled.

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

  In the present invention, three kinds of α steel shown in Table 1 were used as the first layer, and four kinds of Mar steel shown in Table 1 were used as the second layer. 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 the combinations shown in Table 3 (continued in Table 2), the thickness of each layer, the number of laminated layers, and the total layer thickness to form a laminate. Furthermore, these laminated bodies were rolled and / or heat-treated under the conditions shown in Table 2 to integrate them, thereby producing multilayer steels of the present invention shown as Examples 1 to 12.

For example, in Example 1, α steel having a thickness of 1.3 mm is used as the first layer shown in Table 1, and Mar steel having a thickness of 1.0 mm is used as the second layer. Nine layers and eight second layers were alternately laminated so that the surface layer was the first layer, and a laminate having a total number of layers of 17 and a total layer thickness of 19.7 mm was formed.
Next, the laminate is hot-rolled under the conditions shown in Table 2 so as to have each layer thickness, the number of laminated layers, and the total layer thickness shown in Table 3, and further heat-treated under the conditions shown in Table 2. Thus, the multilayer steel of Example 1 was produced.
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 α steel in which the ferrite structure occupies almost 100% by volume, It was confirmed that the second layer was Mar steel in which the martensite structure accounted for almost 100% by volume.

Regarding the obtained multilayer steels of Examples 1 to 12, the value of t ₁ / t ₂ , the value of TS ₂ / TS ₁ , and the tensile strength (MPa) and elongation as a whole of the multilayer steel ( %) And strength-ductility balance index values (value of tensile strength (MPa) × elongation (%)) are shown in Table 3.
As is clear from the results in Table 3, it is confirmed that the present invention multilayer steels of Examples 1 to 12 are multilayer steels having high strength and high ductility and excellent strength-ductility balance. It was.

For comparison, steels having the composition, metallographic structure, and tensile strength shown in Tables 1 and 4 were used as the first layer and the second layer in the combinations shown in Table 5, and this is shown in Table 6 (continued in Table 5). By forming a laminate so that each layer layer thickness, the number of laminated layers, and the total layer thickness shown in FIG. 5 are obtained, and by integrating the laminate by rolling and / or heat treatment under the conditions shown in Table 5, Comparative Examples 21 to 21 A comparative multilayer steel shown as 25 was produced.
Obtained for Comparative Example multilayered steel of Comparative Example 21 to 25 was _a value of _{t 1 / t 2, TS 2} / TS 1 value, further, the tensile strength of the entire multilayered steel (MPa), elongation ( %) And strength-ductility balance index values (value of tensile strength (MPa) × elongation (%)) are shown in Table 6.

When comparing the results of Examples 1 to 12 with the results of Comparative Examples 21 to 25,
Comparative Example 21, although Example 1 and _t 1 / _{t 2} is different _only if t 1 / _{t 2} is 1.2 or less, at a low intensity strength - it can be seen that ductility balance is inferior. In Comparative Example 22, the strength of the first layer is lower than that of Example 1, and TS ₂ / TS ₁ is more than 6, so that the ductility is greatly decreased with respect to the strength, and the strength-ductility balance is decreased. is doing. In Comparative Example 23, since t ₁ / t ₂ is lower than that in Example 4, although the strength is high, the ductility is lowered and the strength-ductility balance is lowered.

  As can be seen from the above results, the multi-layer steel of the present invention has excellent high strength and excellent ductility, and also has an excellent balance between strength and ductility. Therefore, it can be said that it is suitable as a material that is lightweight, requires high strength and good workability.

It is a graph which shows the correlation with the strength ratio of a 1st layer and a 2nd layer, and the strength-ductility balance of multilayer steel. It is a graph which shows the correlation with the layer thickness ratio of a 1st layer and a 2nd layer, and the strength-ductility balance of multilayer steel.

Claims (6)

A first layer made of carbon steel or low alloy steel having a tensile strength TS _{1 in which} the ferrite structure occupies the maximum volume fraction, and a tensile strength TS _{2 in} which the martensite structure occupies the maximum volume fraction is 1200 MPa or more. laminating a second layer made of carbon steel or low alloy steel with each other, further, formed by laminating integrally a total of three or more layers the first layer as a surface layer, the thickness t ₁ of the first layer second The ratio (t ₁ / t ₂ ) of the layer thickness t ₂ of the layers exceeds 1.2, and the ratio of the tensile strength TS ₂ of the _second layer to the tensile strength TS ₁ of the first layer (TS ₂ / TS ₁ ) is 1.2 or more and 6 or less, the tensile strength of the multilayer steel as a whole is 1050 MPa or more, and the product of tensile strength and total elongation is 21000 MPa ·% or more. . The multilayer steel according to claim 1, wherein each layer thickness t1 of the _first layer is 0.04 mm or more, and each layer thickness t2 of the _second layer is 0.1 mm or less. .   The multi-layer steel according to claim 1 or 2, wherein a total layer thickness of the multi-layer steel is 3.0 mm or less. The first layer is, by weight,
C: 0.0001 to 0.10%,
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%
Is a carbon steel composed of the balance iron and inevitable impurities,
The second layer is, by weight,
C: 0.12-0.4%
Si: 0.05-3.0%,
Mn: 0.05 to 3.0%
The multi-layer steel according to any one of claims 1 to 3, wherein the steel is a carbon steel made of iron and inevitable impurities. The first layer is, by weight,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.001 to 0.01%,
Ni: 0.001 to 0.01%
Mo: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
The multi-layer steel according to claim 4, which is a low alloy steel further containing one or more of the following. The second layer is, by weight,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.001 to 0.5%,
Cr: 0.001 to 2.0%,
Ni: 0.001 to 2.5%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 1.0%,
The multilayer steel according to claim 4 or 5, wherein the steel is a low alloy steel further containing one or more of the following.

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