JP7249730B2 - Steel plates, tubular moldings, and stampings - Google Patents

Description

TECHNICAL FIELD The present invention relates to steel sheets , tubular molded products, and press-formed products.

In recent years, in the fields of automobiles, aircraft, ships, building materials, home electric appliances, etc., design has come to be emphasized in order to meet the needs of users. Therefore, in particular, the shape of the exterior member tends to be complicated. However, in order to mold a metal plate with a complicated shape, it is necessary to apply a large amount of strain to the metal plate. Therefore, there is a problem that the external appearance is spoiled.

For example, Patent Literature 1 discloses that uneven striped patterns appear parallel to the rolling direction (riding). Specifically, Patent Document 1 discloses the following. An aluminum alloy rolled sheet for forming with excellent ridging resistance can be obtained by controlling the average Taylor factor when forming is considered to be plane strain deformation with the rolling width direction as the main strain direction. The average Taylor factor calculated from all crystal orientations present in the texture is closely related to the ridging resistance. By controlling the texture so that the value of the average Taylor factor satisfies a specific condition, the ridging resistance can be reliably and stably improved.

Further, in Patent Document 2, in a steel sheet (DP (Dual Phase) steel sheet) having a dual phase metal structure containing a ferrite phase and a martensite phase, by controlling the hardness of the ferrite phase, a dual phase metal structure steel sheet is obtained. Suppression of the occurrence of surface defects is disclosed.

: Patent No. 5683193 : Patent No. 4867336

By the way, Patent Literature 1 discloses that ridging is suppressed in forming a metal plate in which uniaxial tensile deformation occurs in which the main strain direction is the rolling width direction. Therefore, no consideration is given to metal plate forming processes that cause plane strain tensile deformation and biaxial tensile deformation, such as deep drawing and stretch forming.

On the other hand, in recent years, it has been required to manufacture molded products with complicated shapes even in forming processes that cause plane strain tensile deformation and biaxial tensile deformation, such as deep drawing and stretch forming.

However, for example, DP (Dual Phase) steel plate, TRIP (steel (Transformation Induced Plasticity)) steel plate, two-phase stainless steel plate, etc., for metal plates having a multi-phase metal structure, plane strain tensile deformation, or plane strain tensile deformation and If a molding process that causes biaxial tensile deformation is used to form a complicated shape, the surface of the molded product develops depressions, roughening the surface, and detracting from the beauty of the appearance.

Therefore, the object of the present invention is to obtain a molded product with excellent design properties by suppressing the occurrence of surface roughness even when subjected to a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation. An object of the present invention is to provide a steel sheet having a phase metal structure, and a tubular molded article and a press-formed article which suppress the occurrence of surface roughness and are excellent in design.

The above problems are solved by the following means. Namely

<1>
a base layer having a dual-phase metallographic structure;
It is provided on at least one side of the base layer and has a single-phase metal structure. A surface layer in which the Vickers hardness of 70% or more of the hardness measurement points is in the range of 90% to 100% of the maximum Vickers hardness in a cross-sectional area up to twice as large;
steel plate with
<2>
Of the cross-sectional area cut along the thickness direction of the base layer, the surface layer has a hardness measurement point of 70% in a cross-sectional area up to five times the average crystal grain size of the surface layer in the direction from the surface to the base layer. % or more Vickers hardness is in the range of 90% to 100% of the maximum Vickers hardness, the steel sheet according to <1>.
<3>
The steel sheet according to <1> or <2>, wherein the base layer is a steel layer having a dual-phase metal structure of at least two of martensite, ferrite, bainite, and retained austenite.
<4>
The steel sheet according to any one of <1> to <3>, wherein the surface layer is a steel layer having a ferrite single-phase metal structure.
<5>
A tubular molded product obtained by tubularly molding the steel plate according to any one of <1> to <4>.
<6>
A press-formed product obtained by press-forming the steel plate according to any one of <1> to claim 4>.

According to the present invention, even if a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation is performed, the occurrence of rough surface is suppressed and a molded product with excellent design is obtained. It is possible to provide a steel sheet having a structure, and a tubular molded article and a press-molded article which are suppressed in occurrence of surface roughness and excellent in design.

FIG. 1 is a schematic cross-sectional view showing an example of the metal plate according to this embodiment. FIG. 2 is a diagram showing an example of the relationship between the depth direction of the surface layer and the Vickers hardness in the surface layer region of the surface layer. FIG. 3 is a diagram showing an example of the relationship between the plane direction of the surface layer and the Vickers hardness in the surface layer region of the surface layer. It is a schematic diagram for demonstrating the measuring method of the average crystal grain size of a surface layer. FIG. 5A is a schematic diagram showing an example of the stretch forming process. FIG. 5B is a schematic diagram showing an example of a molded product obtained by the stretch forming process shown in FIG. 5A. FIG. 6A is a schematic diagram showing an example of drawing and stretch forming. FIG. 6B is a schematic diagram showing an example of a molded product obtained by the draw stretch forming process shown in FIG. 8A. FIG. 7 is a schematic diagram for explaining plane strain tensile deformation, biaxial tensile deformation, and uniaxial tensile deformation. FIG. 8 is a schematic diagram for explaining a molded article produced in the evaluation of molding processing in Examples.

An embodiment of the present invention will be described below. The same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

<Metal plate>
A metal plate according to this embodiment includes a base layer having a dual-phase metal structure and a surface layer having a single-phase metal structure provided on at least one side of the base layer (see FIG. 1). In addition, the metal plate shown in FIG. 1 has shown the aspect which provided the surface layer on both surfaces of the base layer.
However, after molding the metal plate, the surface layer is preferably provided at least on the side that will be the outer surface of the molded product (for example, in the case of a panel-shaped molded product having a convex portion, the outer surface on the convex side). .

Then, the surface age layer is, of the cross-sectional area cut along the thickness direction of the base layer, in the cross-sectional area up to three times the average crystal grain size of the surface layer in the direction from the surface to the base layer, the hardness measurement point 70 % Vickers hardness is in the range of 90% to 100% of the maximum Vickers hardness. Here, the surface of the surface layer means, of the two surfaces facing each other in the thickness direction, the surface opposite to the side facing the base layer.

In FIG. 1, 10 indicates a metal plate, 12 indicates a base layer, and 14A and 14B indicate surface layers.

Even if the metal plate according to the present embodiment is subjected to a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation, the occurrence of rough skin is suppressed and a molded product with excellent design can be obtained. A metal plate having a multiphase metal structure is obtained. And the metal plate which concerns on this embodiment was discovered by the knowledge shown below.

In recent years, the relationship between the metal structure and mechanical properties of metal plates has been studied. Therefore, the inventors conducted the following study. We observed the change of multi-phase metallographic structure during the forming process in which plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation occurred. Then, the relationship between the surface properties of the processed product after processing and the change in the dual-phase metal structure was examined. As a result, the inventors have found that, in the multiphase metal structure, the metal phase with low hardness is preferentially deformed to form recesses, which causes rough surface and spoils the aesthetic appearance. Specifically, the inventors obtained the following findings.

The dual-phase metallographic structure of a metal plate has hardness differences among a plurality of metal phases. Therefore, among the plurality of metal phases, the metal phase having a lower hardness is preferentially deformed. On the other hand, when the deformed metal phase is observed, the metal phase that is preferentially deformed is the metal phase present in the surface layer of the metal plate.

Therefore, the inventors further focused on the thickness of the metal phase on the surface layer of the metal plate and the in-plane strength difference of the metal phase, and studied. As a result, the inventors obtained the following findings.

Uniform deformation of the metal phase can be promoted by reducing the difference in hardness of the metal phase present in the surface layer of the metal plate among the metal structures on the surface of the metal plate. That is, it is possible to suppress the development of depressions due to partial preferential deformation of the metal phase on the surface layer of the metal plate that appears as rough skin.

Therefore, a base layer having a dual-phase metal structure is provided, and a surface layer having a single-phase metal structure is provided on at least one side of the base layer. Then, of the cross-sectional area cut along the thickness direction of the base layer, in the cross-sectional area up to three times the average crystal grain size of the surface layer in the direction from the surface to the base layer, the Vickers hardness of 70% or more of the hardness measurement point The hardness should be in the range of 90% to 100% of the maximum Vickers hardness. In other words, the surface layer of the metal plate has a single-phase metal structure to reduce the difference in hardness of the metal phase.

As a result, even if a metal plate having a multiphase metal structure is subjected to a forming process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation, partial preferential deformation of the metal phase in the surface layer is performed. It can suppress and promote uniform deformation of the metal phase. As a result, it is possible to suppress the development of concave portions on the surface of the metal plate, that is, the occurrence of rough skin.

From the above findings, the metal plate according to the present embodiment can suppress the occurrence of rough skin even when subjected to a forming process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation. It has been found to result in a metal sheet having a dual-phase metallographic structure which results in a high quality product.

Details of the metal plate according to the present embodiment will be described below.

(Surface layer)
The surface layer has a single-phase metallographic structure. Then, the surface layer has three times the average crystal grain size of the surface layer in the direction from the surface to the base layer (hereinafter also referred to as “surface layer depth direction”) in the cross-sectional area cut along the thickness direction of the base layer. The Vickers hardness of 70% or more of the hardness measurement points is in the range of 90% to 100% of the maximum Vickers hardness.
In particular, from the viewpoint of effectively suppressing the occurrence of rough skin, of the cross-sectional area cut along the thickness direction of the base layer, the cross-sectional area is up to five times the average crystal grain size of the surface layer in the direction from the surface to the base layer. In , the Vickers hardness of 70% or more of the hardness measurement point is preferably in the range of 90% to 100% of the maximum Vickers hardness.
These cross-sectional regions are also referred to as "surface layer regions of the surface layer".

And, when the Vickers hardness of 70% or more of the hardness measurement point is in the range of 90% to 100% of the maximum Vickers hardness, for example, as shown in FIGS. , means that the difference in Vickers hardness is small between the surface layer depth direction and the surface direction of the surface layer (for example, the direction orthogonal to the rolling direction).
FIG. 2 is a diagram showing an example of the relationship between the depth direction of the surface layer and the Vickers hardness in the surface layer region of the surface layer. FIG. 3 is a diagram showing an example of the relationship between the plane direction of the surface layer and the Vickers hardness in the surface layer region of the surface layer. 2 and 3, HV indicates Vickers hardness.

The proportion of Vickers hardness measurement points in the range of 90% to 100% of the maximum Vickers hardness is preferably 80% or more, more preferably 90% or more. The larger this ratio, the smaller the difference in hardness, and the more effectively the occurrence of rough skin can be suppressed.

The Vickers hardness characteristics in the surface region of these surface layers are realized by 1) providing a single-phase metal structure, and 2) aligning the crystal orientation of crystal grains in the single-phase metal structure.

Here, the average grain size of the surface layer is measured by the following method.
A sample having a cross section (hereinafter also referred to as a “T cross section”) cut along the direction perpendicular to the rolling direction and the plate thickness direction is taken from the surface layer of the metal plate to be measured.
Next, the T-section of the sample is polished and etched with nital to corrode and reveal the grain boundaries of the T-section.
Next, with an optical microscope, an area corresponding to 200 μm in the depth direction of the surface layer and 200 μm in width from the surface of the surface layer in the T section of the sample (that is, a 200 μm × 200 μm area with the surface of the surface layer as one side). Observe at 500 magnification.
Next, the average crystal grain size is determined by the line segment method according to JIS G 0551 (2013). Specifically, as shown in FIG. 4, a test line with a length of 0.2 mm is drawn from the surface of the surface layer in the depth direction of the surface layer in the observed image. Five test lines having a length of 0.2 mm are drawn in the surface direction (rolling direction) at intervals of 50 μm or more. Then, the average value of the division lengths of the five test lines (test lines with a total length of 1 mm) dividing the crystal grains is obtained, and the average value is taken as the average grain size of the surface layer.

Next, Vickers hardness is measured as follows.
The Vickers hardness is measured at intervals of 10 μm in the direction perpendicular to the rolling direction and the plate thickness direction with respect to the surface layer region of the surface layer in the T section of the surface layer of the metal plate. Hardness measurement points shall be 100 or more.
In addition, Vickers hardness measures HV10 based on JISZ2244 (2009). Specific measurement conditions are: indenter = Vickers quadrangular pyramid diamond indenter with facing angle of 136°, indentation load = 10 gf, indentation time = 20 s.

Then, from the Vickers hardness at each hardness measurement point, the ratio of Vickers hardness measurement points in the range of 90% to 100% of the maximum Vickers hardness is obtained.

The single-phase metal structure of the surface layer is exemplified by single-phase metal structures such as ferrite and austenite. Among these, the surface layer is preferably a steel layer having a ferrite single-phase metal structure.
Here, the single-phase metal structure means a metal structure in which the main metal phase such as ferrite has an area ratio of 95 (preferably 99%, ideally 100%).

When the surface layer is a steel layer having a ferrite single-phase metal structure, the chemical composition of the steel layer is, for example, C: 0.00050 to 0.0060%, Si: 0.005 to 1.0% by mass. 0%, Mn: 0.05% to 1.50%, P: 0.0010% to 0.100%, S: 0.00030% to 0.010%, Al: 0.00050 to 0.10%, N: 0.00030 to 0.0040%, Ti: 0.0010 to 0.10%, Nb: 0.0010 to 0.10%, and B: 0 to 0.0030%, balance: Fe and impurities The chemical composition contained is exemplified.

If the thickness of the surface layer is too thin, it will be difficult to effectively suppress the occurrence of rough skin. On the other hand, if the thickness of the surface layer is too thick, the strength of the material may not be ensured.
Therefore, the thickness of the surface layer should be at least 3 times the average crystal grain size, and preferably at least 5 times the average crystal grain size. Also, the ratio of the thickness of the surface layer to the total plate thickness is preferably 50% or less.

(base layer)
The base layer has a dual phase metallographic structure. The dual-phase metallographic structure may be a two-phase metallographic structure, or may be a three-phase or higher-phase metallographic structure.
Examples of the base layer include a steel layer, a copper layer, an aluminum alloy layer, a magnesium alloy, a stainless alloy layer, and the like. Among these, a steel layer is preferred, and more specifically, a steel layer having a multi-phase metal structure of at least two of martensite, ferrite, bainite, and retained austenite is more preferred, and includes ferrite, martensite, bainite, and Further preferred is a steel layer having a dual-phase metallographic structure comprising at least one of retained austenite.

Specifically, as the base layer, ferrite: 20 to 80% (preferably 30 to 70%) in terms of area ratio, the remainder: a steel layer having a dual-phase metal structure of at least one of martensite, bainite and retained austenite. are preferably mentioned.

More specifically, as the base layer, DP (Dual Phase) steel having a dual phase metal structure containing ferrite and martensite, TRIP (Transformation Induced Plasticity) having a dual phase metal structure containing ferrite, bainite and retained austenite. ) steel, duplex stainless steel having a dual phase metallographic structure containing ferrite and austenite, and the like.

Here, the area ratio of each phase of the base layer is measured as follows.
A sample having a cross section (hereinafter also referred to as “T cross section”) cut along the direction perpendicular to the rolling direction and the plate thickness direction is taken from the base layer of the metal plate to be measured.
Next, the T-section of the sample is polished and etched with nital to corrode and reveal the grain boundaries of the T-section.
Next, a region (200 μm×200 μm) located in the center of the thickness direction of the base layer in the T cross section of the sample is observed at a magnification of 500 using a scanning electron microscope equipped with a backscattered electron diffraction pattern analysis device (EBSD device). Then, in the observation screen, the area ratio of each phase with respect to the observation screen is obtained.

In the observation screen, each phase is identified as follows.
Ferrite, bainite, and martensite are identified using EBSD measurement result analysis software OIM Analysis version 7.2.1, displaying measurement points having a BCC crystal structure, and crystal orientation distribution and Image Quality Map values. Austenite (including retained austenite) is identified by a similar method, indicating a measurement point with an FCC crystal structure.

When the base layer is a steel layer, the chemical composition of the steel layer is, for example, in mass%, C: 0.00050 to 0.60%, Si: 0.005 to 2.0%, Mn: 0.05% to 3.00%, P: 0.0010% to 0.100%, S: 0.00030% to 0.020%, Al: 0.00050 to 0.10%, N: 0.00030 to 0.050% , Ti: 0.0010 to 0.10%, Nb: 0.0010 to 0.10%, B: 0 to 0.0030%, and the balance: Fe and impurities.

The thickness of the base layer is preferably 0.1 to 4.0 mm.

(Method for manufacturing metal plate)
The method for manufacturing the metal plate according to this embodiment is not particularly limited, but for example, the following method is exemplified.

A first metal plate serving as a base layer and a second metal plate serving as a surface layer are prepared. The first metal plate is a metal plate that becomes a base layer (that is, a layer of, for example, DP steel, TRIP steel, duplex stainless steel, etc.) having a dual-phase metal structure by heat treatment (annealing, etc.) described below. Similarly, the second metal plate is a metal plate that becomes a surface layer having a single-phase metal structure (for example, a steel layer having a ferrite single-phase metal structure) by the following heat treatment (annealing, etc.).

Next, the first metal plate serving as the base layer and the second metal plate serving as the surface layer are overlaid. In this state, the first metal plate and the second metal plate are welded together. After the welded laminated metal sheets are heated by a heat source such as a laser, they are hot-rolled. The heating temperature is, for example, 900-1200.degree. Further, for example, the rolling reduction of hot rolling is set to 50 to 95%.
Next, the laminated hot-rolled sheet is hydrogen-annealed to decarburize the carbon on the surface of the sheet. For example, the hydrogen annealing temperature is 700-1000.degree. Also, the hydrogen annealing time is set to 1 to 200 minutes. When hydrogen annealing is performed under these conditions, the carbon concentration on the surface of the sheet can be reduced more than nitrogen annealing. Become a small organization.
Next, the laminated hot-rolled sheet is cold-rolled. The rolling reduction of cold rolling is, for example, 70% or more. When the rolling reduction of cold rolling is increased to 70% or more, the crystal orientation of the crystal grains in the surface layer region of the surface layer tends to be aligned in the rolling direction.
Next, after annealing at a temperature at which the metallographic structure of the base layer becomes a dual-phase metallographic structure and the metallographic structure of the surface layer becomes a single-phase metallographic structure, quenching is performed. For example, when forming a surface layer having a single phase metal structure of ferrite, the annealing temperature is 750 ° C. or higher, which is the recrystallization temperature of ferrite, and the metal structure of the base layer corresponds to a multiphase region in which the metal structure is a double phase metal structure. temperature.

Through the above steps, a metal plate having a base layer having a dual-phase metallographic structure and a surface layer having a single-phase metallographic structure and having a small Vickers hardness difference in the surface layer region is obtained.

<Molded product>
(tubular molded product)
A tubular molded product according to the present embodiment is a molded product obtained by molding the metal plate according to the present embodiment into a tubular shape. Specifically, the tubular molded article according to this embodiment may be obtained, for example, by the following method.
A metal plate is tubularly formed into an open tube. The butted portions are welded while the circumferential ends of the obtained open tubes are butted against each other. Cold working such as bending, hydroforming (a forming process in which water is poured into the pipe and the pipe is expanded by water pressure) is performed on the welded pipe according to the purpose. A tubular molded article is thus obtained.
In addition, for example, bending in the longitudinal direction of a blank tube is a forming process that causes plane strain tensile deformation. Also, hydroforming is a forming process in which plane strain tensile deformation and biaxial tensile deformation (in particular, unequal biaxial tensile deformation relatively close to equibiaxial deformation) occur.
Then, even if a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation is performed, the outer peripheral surface of the tubular molded product is composed of the surface layer of the metal plate according to the present embodiment. Therefore, the occurrence of rough skin is suppressed, and a tubular molded product excellent in design is obtained.

(Press molded product)
A press-formed product according to the present embodiment is a formed product obtained by press-molding the metal plate according to the present embodiment. Specifically, the press-formed product according to the present embodiment may be obtained by, for example, the following forming process.
And even if it is subjected to a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation, the outer surface of the press-formed product is composed of the surface layer of the metal plate according to the present embodiment. , the occurrence of rough skin is suppressed, resulting in a press-molded product with excellent design.

-Molding-
The metal plate is subjected to a forming process that causes plane strain tensile deformation, or plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation. The forming process includes deep drawing, stretch forming, draw stretch forming, and bending. Specifically, as the forming process, for example, there is a method of protruding the metal plate 10 as shown in FIG. 5A. In this forming process, the edge of the metal plate 10 is sandwiched between the die 11 and the holder 12 on which the draw bead 12A is arranged. As a result, the drawbead 12A is bitten into the surface of the edge of the metal plate 10, and the metal plate 10 is fixed. Then, in this state, a punch 13 having a flat top surface is pressed against the metal plate 10, and the metal plate 10 is stretched. Here, FIG. 5B shows an example of a molded product obtained by the stretch molding process shown in FIG. 5A.
In the stretch forming process shown in FIG. 5A, plane strain deformation occurs in, for example, the metal plate 10 located on the side surface side of the punch 10 (the portion that will become the side surface of the molded product). On the other hand, the metal plate 10 positioned on the top surface of the punch 10 (the top surface of the molded product) undergoes equibiaxial deformation or unequal biaxial tensile deformation relatively close to equibiaxial deformation.

As the forming process, for example, there is a method of drawing and stretching the metal plate 10 as shown in FIG. 6A. In this forming process, the edge of the metal plate 10 is sandwiched between the die 11 and the holder 12 on which the draw bead 12A is arranged. As a result, the drawbead 12A is bitten into the surface of the edge of the metal plate 10, and the metal plate 10 is fixed. Then, in this state, the metal plate 10 is pressed against the metal plate 10 by a punch 13 whose top surface protrudes in a substantially V-shape, and the metal plate 10 is drawn and stretched. Here, FIG. 6B shows an example of a molded product obtained by the draw stretch forming process shown in FIG. 6A.
In the draw stretch forming process shown in FIG. 6A, for example, plane strain deformation occurs in the metal plate 10 located on the side surface side of the punch 10 (the portion that will become the side surface of the molded product). On the other hand, the metal plate 10 located on the top surface of the punch 10 (the top surface of the molded article) undergoes unequal biaxial tensile deformation relatively close to plane strain deformation.

Here, in the molded product (tubular molded product, press-molded product) according to the present embodiment, as shown in FIG. 7, the plane strain tensile deformation is a deformation that extends in the ε1 direction and does not cause deformation in the ε2 direction. . Biaxial tensile deformation is deformation in which elongation occurs in the ε1 direction and elongation also occurs in the ε2 direction. Specifically, plane strain tensile deformation is deformation in which the strain ratio β (= ε2/ε1) is β = 0 when the strain in the biaxial direction is the maximum principal strain ε1 and the minimum principal strain ε2. Biaxial tensile deformation is deformation in which the strain ratio β (=ε2/ε1) is 0<β≦1. In addition, deformation in which the strain ratio β (= ε2/ε1) is 0 < β < 1 is unequal biaxial deformation, and deformation in which the strain ratio β (= ε2/ε1) is β = 1 is equibiaxial deformation is. Incidentally, the uniaxial tensile deformation is deformation in which elongation occurs in the ε1 direction and contraction occurs in the ε2 direction, and the strain ratio β (=ε2/ε1) satisfies −0.5≦β<0.

However, the range of the strain ratio β is a theoretical value. The range of the strain ratio β for each deformation calculated from the strain is as follows.
・ Uniaxial tensile deformation: -0.5 < β ≤ -0.1
・ Plane strain tensile deformation: -0.1 < β ≤ 0.1
・Unequal biaxial deformation: 0.1<β≦0.8
・ Equibiaxial deformation: 0.8 < β ≤ 1.0

On the other hand, in the forming process, it is preferable that at least a part of the metal plate is processed with a processing amount such that the plate thickness reduction rate is 10% or more and 30% or less. If the thickness reduction rate is less than 10%, strain concentration is small, and there is a tendency that recesses are less likely to develop during forming. Therefore, roughening of the surface itself of the press-molded product is less likely to occur. On the other hand, if the plate thickness reduction rate exceeds 30%, the metal plate (molded product) tends to break during molding. Therefore, it is preferable that the processing amount of the molding process is within the above range.

In particular, the molding of the press-formed product may be performed with a processing amount such that the thickness reduction rate of the entire metal plate excluding the edge portion (the portion sandwiched between the die and the holder) is 10% or more and 30% or less. Depending on the shape of the press-formed product to be formed, in particular, the forming process is performed so that the portion of the metal plate located on the top surface of the punch (the portion where the metal plate undergoes biaxial tensile deformation) has a plate thickness reduction rate of 10% or more and 30%. It is preferable to carry out with the following amount of processing. The portion of the metal plate positioned on the top surface of the punch is often the portion most likely to be exposed to the line of sight when the press-formed product is applied as an exterior member. Therefore, when the portion of the metal plate is formed with a large amount of processing, such as a plate thickness reduction rate of 10% or more and 30% or less, the effect of suppressing rough surface becomes remarkable by suppressing the development of concave portions.

That is, the molded product (especially press-molded product) according to the present embodiment has a maximum thickness of the molded product D1 and a minimum thickness of the molded product D2. Even a processed product that satisfies the relationship of D1×100≦30 is a molded product with suppressed surface roughness.

In addition, the plate thickness reduction rate is expressed by the formula: Plate thickness reduction rate = (Ti -Ta)/Ti.

Here, the molded product (tubular molded product, press-molded product) according to the present embodiment is subjected to a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation.
A method for confirming that a molded product has been subjected to a molding process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation is as follows, for example.

Measure the three-dimensional shape of the molded product, create a mesh for numerical analysis, derive the process from the plate material to the three-dimensional shape by inverse analysis by computer, and calculate the maximum principal strain and minimum principal strain in each mesh. and the ratio (the above β) is calculated. By this calculation, it can be confirmed that the forming process is performed to cause plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation.
For example, the three-dimensional shape of the molded product is measured using a three-dimensional measuring machine such as Comet L3D (Tokyo Boeki Techno System Co., Ltd.), and mesh shape data of the molded product is obtained based on the obtained measurement data. Next, using the obtained mesh shape data, numerical analysis of the one-step method (work hardening calculation tool "HYCRASH (JSOL Co., Ltd.)" etc.) is performed, based on the shape of the molded product. Then, from the shape information such as elongation and bending state of the molded product, change in plate thickness, residual strain, etc. of the molded product are calculated. From this calculation, it can be confirmed that a forming process that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation is performed.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, these examples do not limit the present invention.

(Test Example 1): Example where the HV difference in the cross-sectional area up to three times the average grain size is small (Invention Example)
% by mass, C: 0.0029%, Si: 0.012%, Mn: 0.09%, P: 0.02%, S: 0.003%, Al: 0.041%, N: 0.04% 003%, Ti: 0.013%, Nb: 0.023%, B: 0.0007%, and the balance: Fe and impurities, and having a thickness of 10 mm. prepared.
On the other hand, in mass%, C: 0.2%, Si: 0.2%, Mn: 1.2%, P: 0.02%, S: 0.003%, Al: 0.03%, N: 0.003%, Ti: 0.2%, Nb: 0.003%, and B: 0.0018%, and the balance: Fe and impurities. prepared the plate.

Next, the surface layer hot-rolled sheet was welded to both surfaces of the base layer hot-rolled sheet. This welded laminated hot-rolled sheet was heated to 1100° C. and then hot-rolled at a rolling reduction of 92%. Thereafter, the laminated hot-rolled sheet after hot rolling was annealed in hydrogen to decarburize the surface carbon.
Next, the laminated hot-rolled sheet was cold-rolled at a rolling reduction of 81%.
Next, the laminated cold-rolled sheet was annealed at a heating rate of 10° C./s, a soaking temperature of 780° C. for a soaking time of 10 minutes, and then quenched at a cooling rate of 50° C./s.

Through the above steps, a steel sheet having surface layers provided on both sides of the base layer was obtained. The details of the base layer and the surface layer are as follows.
Base layer: A 0.4 mm thick DP steel layer having a dual-phase metallographic structure with an area ratio of ferrite: 40% and martensite: 60% Surface layer: A single-phase metallographic structure with an area ratio of ferrite: 100% and a steel layer with a thickness of 0.1 mm

(Test Example 2) Example of a small HV difference in a cross-sectional area up to five times the average crystal grain size (Invention Example)
A steel plate was obtained in the same manner as in Test Example 1, except that the hydrogen annealing time was lengthened. The details of the base layer and the surface layer are as follows.
Base layer: A 0.4 mm thick DP steel layer having a dual-phase metallographic structure with an area ratio of ferrite: 40% and martensite: 60% Surface layer: A single-phase metallographic structure with an area ratio of ferrite: 100% and a steel layer with a thickness of 0.1 mm

(Test Example 3) An example of a large HV difference in a cross-sectional area up to three times the average crystal grain size (Comparative example: single layer type)
In the same manner as in Test Example 1, except that the hot-rolled sheet for the base layer was cold-rolled and annealed, the area ratio was ferrite: 40%, martensite: 60%. A DP steel plate with a thickness of 0.4 mm was obtained.

(Test Example 4) Example of large HV in the cross-sectional area up to three times the average grain size (Comparative example: laminated type)
A steel plate was obtained in the same manner as in Test Example 1, except that the soaking temperature for annealing was changed to 740°C. The details of the base layer and the surface layer are as follows.
Base layer: DP steel layer with a thickness of 0.4 mm having a dual phase metal structure of 75% ferrite and 25% martensite in area ratio Surface layer: Single phase metal structure of 100% ferrite in area ratio and a steel layer with a thickness of 0.1 mm

(measurement)
For the obtained steel sheets of each example, the Vickers hardness was measured in a cross-sectional area up to 3 or 5 times the average grain size of the surface layer in the direction from the surface toward the base layer according to the method described above. The percentage of Vickers hardness in the range of 90% to 100% of the maximum Vickers hardness was determined.

However, for the DP steel sheet of Test Example 3, Vickers hardness was measured in a cross-sectional area up to 3 or 5 times the average grain size of the steel sheet in the thickness direction from the surface according to the method described above. Then, the percentage of Vickers hardness in the range of 90% to 100% of the maximum Vickers hardness was determined.

In the table, the Vickers hardness ratio in the cross-sectional area up to three times the average crystal grain size is expressed as "Vickers hardness ratio (three times the average crystal grain size). The ratio of the Vickers hardness in the cross-sectional area up to 5 times is expressed as "the ratio of Vickers hardness (three times the average crystal grain size)".

(evaluation)
-Molding processing evaluation A-
The obtained steel plate of each example was stretched to form a press-formed product.
Specifically, as shown in FIG. 8, the diameter R of the top plate portion 20A of the molded product 20 is 150 mm, the height H of the molded product 20 is 18 mm, and the angle θ of the vertical wall portion 20B of the molded product 20 is 90 °. A dish-shaped press-molded product was molded.
In addition, this forming was carried out with a processing amount such that the plate thickness reduction rate of the steel plate serving as the evaluation portion A (center portion of the top plate portion 20A) of the top plate portion 20A was 30%. The deformation ratio β at the evaluation portion A of the top plate portion 20A of this molded product is 1.0.

Then, in the evaluation section A of the molded product, the "arithmetic mean height Pa of the cross-sectional curve" was measured before and after molding, and the increase in Pa (Pa after molding - Pa before molding) was calculated.
The "arithmetic mean height Pa of cross-sectional curves" is the arithmetic mean height defined in JIS B0601 (2001). The measurement conditions were an evaluation length of 1 mm and a reference length of 1 mm.

-Molding processing evaluation B-
In FIG. 8, the evaluation part B (the central part between the center and the edge of the top plate part 20A) of the top plate part plate 20A of the molded product 20 is molded so that the plate thickness reduction rate is 25%. A press-formed product was formed in the same manner as in the evaluation of forming process A, except that the height H of the product 20 was adjusted. The deformation ratio β of this molded product at the evaluation portion B is 0.5. In the evaluation section B of the molded product, the increase in Pa was obtained.

-Molding processing evaluation C-
In FIG. 8, the height H of the molded product 20 is such that the evaluation portion C (the edge of the top plate portion 20A) of the top plate portion plate 20A of the molded product 20 has a processing amount at which the plate thickness reduction rate is 20%. A press-molded product was molded in the same manner as in the molding process evaluation A, except that the was adjusted. The deformation ratio β of this molded product at the evaluation portion C is 0.0. Then, in the evaluation section C of the molded product, the increase in Pa was obtained.

In the molding of the above molding process evaluation A to C, the scribed circle is transferred to the surface of the steel plate corresponding to the evaluation part of the molded product, and the shape change of the scribed circle before and after molding (before and after deformation) is measured. Thus, the maximum principal strain and the minimum principal strain were measured. From these values, the deformation ratio β at the evaluation part of the molded product was calculated.

From the above results, at least one side of the base layer having a dual phase metal structure has a single phase metal structure, and in the surface layer region of the surface layer, the Vickers hardness of 70% or more of the hardness measurement points is 90 of the maximum Vickers hardness. The steel sheets of Test Examples 1 and 2, which have a surface layer in the range of % to 100%, do not cause surface roughness even when subjected to forming processing that causes plane strain tensile deformation, or plane strain tensile deformation and biaxial tensile deformation. It can be seen that a molded article with excellent design property can be obtained by suppressing it.
On the other hand, the steel plates of Test Examples 3 and 4, in which the Vickers hardness of less than 70% of the hardness measurement point is in the range of 90% to 100% of the maximum Vickers hardness, are plane strain tensile deformation, or plane strain tensile deformation and two It can be seen that the surface roughening occurs in the molded product when subjected to a molding process that causes axial tensile deformation.

10 metal plate 12 base layer 14A surface layer 14B surface layer

Claims (4)

a base layer, which is a steel layer having a dual-phase metallographic structure containing ferrite and at least one of martensite, bainite and retained austenite;
A surface layer that is provided on at least one side of the base layer and is a steel layer having a ferrite single-phase metallographic structure, in a cross-sectional area cut along the thickness direction of the base layer, in a direction from the surface toward the base layer a surface layer in which the ratio of Vickers hardness measurement points in the range of 90% to 100% of the maximum Vickers hardness is 70% or more in a cross-sectional area up to three times the average crystal grain size of the surface layer;
steel plate with The surface layer has a maximum Vickers hardness in a cross-sectional area up to five times the average crystal grain size of the surface layer in a direction from the surface toward the base layer, out of the cross-sectional area cut along the thickness direction of the base layer. The steel sheet according to claim 1 , wherein the percentage of Vickers hardness measurement points in the range of 90% to 100% is 70% or more . A tubular molded product obtained by tubularly molding the steel plate according to claim 1 or 2. A press-formed product obtained by press-forming the steel plate according to claim 1 or claim 2.

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