BLANK AND STRUCTURAL MEMBER

A blank (100) includes a main portion (110) that is made of steel having a tensile strength of 1450 MPa or more and a softened portion (120), the ratio of Vickers hardness of the softened portion (120) to the Vickers hardness of the main portion (110) is 0.7 or more and 0.95 or less, and the softened portion (120) is disposed at a position different from a position of the main portion (110) in an in-plane direction. A structural member (200) includes a first member (210), a second member (220), and a weld (W) at which the first member (210) and the second member (220) are welded to each other.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a blank and a structural member.

Priority is claimed on Japanese Patent Application No. 2019-074620, filed Apr. 10, 2019, the content of which is incorporated herein by reference.

RELATED ART

In a case where a blank is worked to form a formed article having a predetermined shape, the strength may be partially changed by hot forming and subsequent quenching.

Patent Document 1 discloses a technique that performs hot forming and press hardening on a blank to manufacture a B-pillar including a portion having different fracture strength.

PRIOR ART DOCUMENT

[Patent Document 1] Published Japanese Translation No. 2013-513514 of the PCT International Publication

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, the influence of hot forming on the surface properties of the blank is not considered in the technique disclosed in Patent Document 1. Further, the blank is often welded to another member after forming. The blank is also not considered in Patent Document 1 to be welded to another member after forming. Particularly, the influence of heat, which is input during welding, on the strength of the member is not considered in the welding of the blank to another member.

Accordingly, the invention has been made in consideration of the above-mentioned problems, and an object of the invention is to provide a new and improved blank and structural member that can suppress a change in surface properties after forming and the influence of welding.

Means for Solving the Problem

The gist of the invention is as follows.

(1) A blank according to an aspect of the invention includes a main portion that is made of steel having a tensile strength of 1450 MPa or more and a softened portion, a ratio of the Vickers hardness of the softened portion to the Vickers hardness of the main portion is 0.7 or more and 0.95 or less, and the softened portion is disposed at a position different from the position of the main portion in an in-plane direction.

(2) In (1), the softened portion may be formed over a distance of 50% or more of a sheet thickness from one surface of the blank.

(3) In (1) or (2), the softened portion may be formed in a shape of a band.

(4) In any one of (1) to (3), a plated coating may be formed on the main portion, and the plated coating may not be formed on at least a part of a surface of a portion of the blank where the softened portion is formed.

(5) In any one of (1) to (4), at least two or more softened portions may be formed.

(6) In any one of (1) to (5), at least two or more softened portions may be formed on one surface of the blank.

(7) In any one of (1) to (6), the softened portion may be disposed within 100 mm from an end portion in the in-plane direction.

(8) A structural member according to another aspect of the invention includes a first member, a second member, and a weld at which the first member and the second member are welded to each other; the first member includes a main portion that is made of steel having a tensile strength of 1450 MPa or more, and a softened portion that is provided at a portion including the weld; the ratio of the Vickers hardness of the softened portion to the Vickers hardness of the main portion is 0.7 or more and 0.95 or less; and the softened portion is disposed at a position different from a position of the main portion in an in-plane direction of the first member.

(9) In (8), the first member may have a surface roughness of 0.5 μm or less in terms of Ra.

(10) In (8), the first member may include a plated coating of which an Fe concentration is 20% or less.

(11) In (10), the plated coating may be formed on the main portion, and the plated coating may not be formed on at least a part of a surface of a portion of the first member where the softened portion is formed.

(12) In any one of (8) to (11), the softened portion may be formed over a distance of 50% or more of a sheet thickness from a surface of the first member facing the second member.

(13) In any one of (8) to (12), at least two or more softened portions may be formed.

(14) In any one of (8) to (13), the softened portion may be formed in a shape of a band.

Effects of the Invention

As described above, according to the invention, it is possible to provide a new and improved blank and structural member that can suppress a change in surface properties after forming and an influence on welding.

EMBODIMENTS OF THE INVENTION

Preferred embodiments of the invention will be described in detail below with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configuration will be denoted by the same reference numeral and a repeated description thereof will be omitted.

1. First Embodiment

[Example of External Appearance of Blank]

First, the schematic configuration of a blank100according to a first embodiment of the invention will be described with reference toFIG. 1A.FIG. 1Ais a perspective view showing an example of the blank100according to this embodiment. The blank100is a flat steel sheet, and is formed in a predetermined shape by cold forming. Here, the blank100includes a continuous sheet-like steel sheet and a steel sheet that is cut out from a continuous sheet-like steel sheet so as to have a predetermined size.

The blank100has at least two surfaces that face each other and have the largest area.

The blank100has a predetermined width dimension in a plan view (as viewed in a direction perpendicular to the surfaces facing each other and having the largest area, or as viewed in a sheet thickness direction).

The blank100has a sheet thickness in a direction perpendicular to the two surfaces (a surface and a bottom surface) that face each other and have the largest area. The sheet thickness of the blank100is substantially uniform in the in-plane direction of the surfaces that face each other and have the largest area. The blank100has a sheet thickness of, for example, about 1.0 mm to 4.0 mm. The sheet thickness of the blank100is smaller than the width dimension of the blank100.

The blank100may be a steel sheet on which a plated coating is not formed (which is unplated or bare), or may be a plated steel sheet that includes galvanized steel sheets, such as a galvannealed steel sheet and a hot-dip galvanized steel sheet.

Hereinafter, a direction perpendicular to the surfaces of the blank100, which face each other and have the largest area, may be referred to as a sheet thickness direction (Z direction), a direction perpendicular to the width direction of the blank100(X direction) and the sheet thickness direction (Z direction) may be referred to as a longitudinal direction (Y direction), and a direction (a direction along the surfaces facing each other) perpendicular to the sheet thickness direction (Z direction) of the blank100may be referred to as an in-plane direction (X-Y direction).

As shown inFIG. 1A, the blank100includes a main portion110, softened portions120, and joint portion-corresponding regions130that are regions forming regions to be welded (corresponding to regions215to be welded to be described later inFIG. 3A). The main portion110is a region that mainly forms the blank100, and has the same properties as a steel sheet forming the blank100. The main portion110is made of steel having a tensile strength of 1450 MPa or more, for example, steel having a tensile strength of 1470 MPa.

The softened portions120are disposed at positions different from the position of the main portion110in the in-plane direction (X-Y direction). Accordingly, regions of the blank100, which are to be formed into heat affected zones due to welding and joining between a first member (see a substantially hat-shaped member210shown inFIG. 3Aand the like) and a separate second member (see a plate-like member220shown inFIG. 3Aand the like) after the blank100is formed into the first member by a press or the like, can be softened in advance. Further, regions of the blank100other than the regions, which are to be formed into the heat affected zones caused by welding, can be in a high-strength (high-hardness) state without being softened. Accordingly, a difference in hardness between the softened portion120and the heat affected zone, which is caused by welding, of a structural member, which uses a formed member based on the blank100, is reduced, so that the influence of welding can be suppressed and the blank100can be suitably used for a structural member having a high strength and high impact resistance performance.

Further, the softened portions120are disposed within 100 mm from end portions in the in-plane direction. For example, as shown inFIG. 1A, the softened portions120are formed in the joint portion-corresponding regions130within 100 mm from both end portions of the blank100in the width direction of the blank100(the X direction inFIG. 1A). The main portion110may be disposed a range other than the regions in which the softened portions120are disposed. Furthermore, the main portion110may be disposed in a range exceeding 100 mm from the end portions in the in-plane direction. In a case where the softened portions120are disposed within 100 mm from the end portions in the in-plane direction, the heat affected zones caused by welding can be included in the softened portions120even if the end portions of the blank100are cut about 20 mm in, for example, press forming, the finishing of a structural member, or the like to be performed later.

In this way, the regions of the blank100, which are to be formed into the heat affected zones caused by welding, can be softened in advance and regions other than those regions can be in a high-strength (high-hardness) state. Accordingly, a difference in hardness between the softened portion120and the heat affected zone, which is caused by welding, of a structural member, which uses a formed member based on the blank100, is reduced, so that the influence of welding can be suppressed and the blank100can be suitably used for a structural member having a high strength and high impact resistance performance.

The softened portions120are regions that are softer than the main portion110and are partially formed on the blank100. Further, the softened portions120are formed so as to be included in the joint portion-corresponding regions130. The details of the softened portions120will be described later. Each joint portion-corresponding region130is a part of the blank100, and is a region that forms a portion (for example, a flange) to be welded to another member after the blank100is formed in a predetermined shape. For example, the joint portion-corresponding regions130are ranges having a predetermined length in the width direction from both ends of the blank100in the width direction of the blank100(the X direction inFIG. 1A). Further, the joint portion-corresponding regions130may be provided not at the end portions but substantially in the middle of the blank100in the width direction.

The blank100according to this embodiment is formed into a formed article having a predetermined shape by cold forming, and is then welded to another member through welds W.

[Problems of Welding Process and Problems of Hot Forming]

Here, it is known in the related art that the characteristics and structure of a peripheral region of a region (welding nugget) where a base material is melted and solidified due to welding are changed from those of the base material due to heat input during welding. The peripheral region is called a heat affected zone (HAZ). In a case where a member to be welded is a steel including a martensitic structure, temper softening partially occurs due to a temperature rise caused by heat input in the heat affected zone. As a result, the hardness of the heat affected zone may be reduced with respect to the hardness of the base material. There is a possibility that such a reduction in hardness in the heat affected zone causes fracture in a case where a member subjected to welding receives a load, and significantly affects the strength of the entire member.

In addition, in a case where a member to be welded is a high tensile strength steel sheet, the influence of a reduction in hardness in the heat affected zone is increased. That is, since the hardness of an original base material is sufficiently high in the case of steel having a relatively high strength, hardness in the heat affected zone tends to be significantly reduced. Particularly, in a case where welding is performed on steel having a tensile strength of 1450 MPa or more, the influence of a reduction in hardness in the heat affected zone is significant. Further, even in a case where a high tensile strength steel sheet is formed by hot stamping forming, the influence of a reduction in hardness in the heat affected zone is significant in welding to be performed later.

On the other hand, softening a welded portion by partially heating the welded portion after the start of forming is also considered in order to suppress the influence of a reduction in hardness in the heat affected zone. Specifically, treatment for preheating and softening a portion near a portion to be welded before welding to suppress the concentration of strain on the heat affected zone, and the like may be performed. However, since the strength of a member is increased in a case where forming is hot stamping forming, the above-mentioned softening treatment needs to be performed after the start of forming. For this reason, an influence on the shape accuracy of a formed article may be generated due to thermal strain in a method of softening a welded portion by heating the welded portion after the start of forming.

Further, the use of a technique partially changing the fracture strength of a member by press hardening using hot forming as in the technique disclosed in Patent Document 1 is also considered.

However, strength can be partially changed in the technique, but there is a concern about the influence of hot forming on the surface properties of a blank. That is, an oxide film (scale) of a base material or a coating provided on the surface of the base material may be formed on the surface of the blank due to heating. Since the generation of significant scale causes poor external appearance or the like, there is a possibility that characteristics required for a product are not satisfied. For this reason, since it is necessary to take measures, such as separately providing a process of removing scale after hot forming, it is supposed that manufacturing cost and man-hours are increased. Further, in a case where the blank is plated, there is a possibility that plating is modified due to hot forming and corrosion resistance is lowered.

Accordingly, as a result of diligent studies, the inventors have supposed that not a steel for hot forming but a high tensile strength steel sheet having a tensile strength of 1450 MPa or more is used as the blank100being in a stage before forming and the softened portions120are provided in the joint portion-corresponding regions130on the blank100. Therefore, the inventors have found that the influence of a reduction in hardness caused by the heat affected zones can be suppressed in a case where the blank100is welded to another member while being formed in a predetermined shape by not hot forming but cold forming and the degradation of surface properties, which is difficult to be avoided in hot forming, can be suppressed. That is, in a case where such a blank100is used, it is possible to form a member, in which cracks and the like generated due to a reduction in hardness caused by the heat affected zones generated due to welding are less likely to be generated, by cold forming at a low cost while avoiding the degradation of surface properties caused by hot forming. The softened portions120according to this embodiment will be described below.

The softened portion120is a region that is formed in at least a part of the joint portion-corresponding region130on the blank100and is softer than the main portion110. At least two or more softened portions120are formed on the blank100. The ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110is 0.7 or more and 0.95 or less.

Further, at least two or more softened portions120may be formed on one surface of the blank100.

Since the Vickers hardness of the softened portion120is 0.95 or less with respect to the Vickers hardness of the main portion110, it is possible to suppress the influence of a reduction in hardness that causes a problem in the case of a high tensile strength steel sheet having a tensile strength of 1450 MPa or more and is significant in the heat affected zone in a case where welding is performed in the joint portion-corresponding region130. That is, a difference between the hardness of the softened portion120and the hardness of the heat affected zone is sufficiently smaller than a difference between the hardness of the main portion110and the hardness of the heat affected zone. For this reason, problems, such as cracks and the like caused by a difference in hardness, are less likely to be generated in a case where a member is deformed after the forming of the member. Hereinafter, the heat affected zone and the weld W may be used for the same meaning.

On the other hand, since the Vickers hardness of the softened portion120is 0.7 or more with respect to the Vickers hardness of the main portion110, the high strength of the blank100can be maintained. In addition, since the Vickers hardness of the softened portion120is 0.7 or more with respect to the tensile strength of the main portion110, the concentration of deformation on the softened portion120in a case where the blank100is used for forming is suppressed. As a result, the formability of the blank100is ensured.

A method of measuring the Vickers hardness of the main portion and the softened portion is as follows. Portions corresponding to the main portion and the softened portion are taken from the blank100as samples, the surface of the sheet is polished by a depth of 100 to 200 μm in a case where a plated coating is not formed on the surface of the sheet, and the surface of the sheet is then polished by a depth of 100 to 200 μm after a plated coating is removed by grinding in a case where the plated coating is formed on the surface of the sheet, so that mirror sample samples are obtained. Then, a Vickers indenter is pushed against the surface of the sheet in a direction substantially perpendicular to the surface of the sheet to measure hardness. A hardness test is performed by a method disclosed in JIS Z 2244:2009. Hardness is measured using a micro-Vickers hardness tester at 10 points that are arranged at intervals of three times or more the indentation under a load of 1 kgf, and the average value of the measured values is used as Vickers hardness.

The softened portion120may be formed to include a region, in which where there is a concern about the influence of a reduction in hardness caused by welding, in the sheet thickness direction of the blank100(the Z direction inFIG. 1A). For example, as shown inFIG. 1B, the softened portion120may be provided over a distance of 50% or more of a sheet thickness in the sheet thickness direction from the surface facing a mating member during welding. Accordingly, in a case where a formed article and the mating member are pulled in directions where the formed article and the mating member are separated from each other, the influence of a reduction in hardness of the heat affected zone is suppressed. As a result, cracks and the like near the welds W can be suppressed. Further, the softened portion120may be provided over a distance of 80% or more of a sheet thickness in the sheet thickness direction from the surface facing the mating member during welding.

Furthermore, the softened portion120may be formed over the entire area of the blank100in the sheet thickness direction as shown inFIG. 1C. In a case where a part in which a formed member and the mating member are assembled by welding is deformed so as to be bent, forces in a tensile direction are generated near the welds W (seeFIG. 3Band the like). Since the softened portion120is formed over the entire area in the sheet thickness direction, strain near the welds W is reduced. As a result, cracks and the like can be suppressed.

Further, as shown inFIG. 1A, the softened portion120may be formed in the shape of a band of which a longitudinal direction is parallel to the longitudinal direction of the blank100(the Y direction inFIG. 1A) and a width direction is parallel to a direction (the X direction inFIG. 1A) orthogonal to the longitudinal direction in the plan view of the blank100. In this case, the length of the softened portion120in the width direction may be 40 mm or less. Particularly, the length of the softened portion120in the width direction may be 30 mm or less. Further, the length of the softened portion120in the width direction may not be constant. Since the softened portion120has a predetermined length in the width direction, the softened portion120is included in the range of the joint portion-corresponding region130. Accordingly, it is possible to suppress the influence of a reduction in hardness that is caused by heat input during welding in a region to be welded after the forming of the blank100. Further, since the softened portion120has a predetermined length in the width direction, it is possible to cope with a change in a welding position.

The softened portion120may be provided not only in the shape of a band but also in a zigzag shape as shown inFIG. 2A. Further, the softened portion120may be provided in the shape of a closed curve (a circular shape, an oval shape, or the like) as shown inFIG. 2B. Furthermore, the softened portion120may be provided in a C shape as shown inFIG. 2C. In addition, the softened portion120may be provided in the shape of a dot or a U shape. Further, the softened portion120may be formed in a region that includes a region in which a formed welding nugget is provided.

Examples of a method of forming the softened portion120include a method of softening the softened portion120by partially tempering the softened portion120using a publicly known partial heating technique, such as laser heating or high-frequency heating, to reduce a tensile strength. Any other method, which can reduce a tensile strength by partially reducing hardness, may be used as a method of forming the softened portion120and a method other than tempering using heating may also be used. For example, a method of partially decarbonizing the softened portion120, and the like may be used.

[Configuration of Structural Member Using Blank]

Next, the configuration of a structural member200using the blank100according to this embodiment will be described with reference toFIGS. 3A to 3C.FIG. 3Ais a perspective view showing an example of a structural member200according to this embodiment.FIG. 3Bis a schematic diagram of a cross section taken along line I-I′ ofFIG. 3A. As shown inFIG. 3A, the structural member200according to this embodiment includes, for example, a substantially hat-shaped member210as a first member and a plate-like member220as a second member. The blank100is formed so as to have a substantially hat-shaped cross section by cold forming, so that the substantially hat-shaped member210is formed. Examples of cold forming include publicly known cold forming techniques, such as press bending and press drawing, and are not particularly limited.

The first member may be a formed article obtained from the cold forming of the blank100that is a bare steel on which a plated coating is not formed. Specifically, the first member, which is a formed article obtained from the cold forming of the blank100that is a bare steel, has a surface roughness of 0.5 μm or less in terms of Ra (JIS B0601:2001). In contrast, oxidized scale having a thickness of about 7 μm is formed on the surface of a formed article, which is obtained from the hot forming, such as hot stamping, of the blank that is a bare steel. However, this oxidized scale is removed by shot blasting or the like, so that the formed article has a surface roughness of 0.5 μm or more in terms of Ra. Accordingly, the first member, which is a formed article obtained from the cold forming of the blank100that is a bare steel, can be distinguished from the formed article which is obtained from the hot forming of the blank that is a bare steel.

As shown inFIG. 3A, the substantially hat-shaped member210includes a top sheet portion211, standing wall portions213that are bent from the top sheet portion211, and flange-like regions215to be welded that are bent from the sides of the standing wall portions213opposite to the top sheet portion211.

Here, the softened portions120are disposed at positions different from the position of the main portion110in the in-plane direction of the substantially hat-shaped member210(first member). As described above, the softened portions120can be disposed in the regions to be welded in the in-plane direction and the main portion110having a relatively high strength can be disposed in a region away from the regions215to be welded in the in-plane direction. Accordingly, it is possible to suppress a reduction in weld strength at the welds W of the structural member200and to improve the impact resistance performance (crush resistance performance) of the structural member200. The in-plane direction is a direction along the sheet surface of the substantially hat-shaped member210(first member), and means a direction perpendicular to the sheet thickness direction of the substantially hat-shaped member210(first member).

Each softened portion120is formed in at least a part of the region215to be welded. That is, the blank100is formed into the substantially hat-shaped member210by cold forming, so that the softened portions120formed in the joint portion-corresponding regions130of the blank100are formed in the regions215to be welded. Portions where the softened portions120are formed include the welds W. Here, as in the example of 50% shown inFIG. 3B, the softened portion120is formed over a distance of 50% or more of a sheet thickness in the sheet thickness direction from the surface of the region215to be welded facing the plate-like member220.

The width of the softened portion120may be 20 mm or more so as to be capable of including the entire width of the weld W that is 20 mm or less, and may be in a range of, for example, 30 mm to 80 mm.

As shown inFIG. 3C, the softened portion120may be formed over the entire area in the sheet thickness direction from the surface of the region215to be welded facing the plate-like member220, that is, over a distance of 100% of the sheet thickness of the substantially hat-shaped member210(first member), or may be formed over an arbitrary distance in a range equal to or larger than 50% and less than 100% of the sheet thickness of the substantially hat-shaped member210. In a case where the softened portion120is formed over an arbitrary distance in a range equal to or larger than 50% and less than 100% of the sheet thickness of the substantially hat-shaped member210(first member) from the surface of the region215to be welded facing the plate-like member220, the softened portion120may not be formed at a remaining portion opposite to the surface of the region215to be welded facing the plate-like member220. That is, the ratio of the Vickers hardness of a remaining portion, which is opposite to the surface of the region215to be welded facing the plate-like member220, to the Vickers hardness of the main portion110may not be 0.7 or more and 0.95 or less, such 1.0.

As shown inFIG. 3B, the substantially hat-shaped member210is welded to the plate-like member220that is a closing plate. At least a part of the softened portion120is formed to be softened in the region215to be welded.

Welding nuggets as the welds W are formed in the interface between the substantially hat-shaped member210and the plate-like member220and join the substantially hat-shaped member210and the plate-like member220. The welding nuggets may be formed in a linear shape in the longitudinal direction of the structural member200(the Y direction inFIG. 3A). Further, the welding nuggets may be provided not only in a linear shape but also in the shape of a dot, in a C shape, in a U shape, in an oval shape, or in a zigzag shape in a top view (as viewed in the Z direction inFIG. 3B).

The width (a dimension in the width direction of the first member) of the weld W is usually within 20 mm.

Various joining techniques, which are publicly known techniques, can be applied to form the welding nugget. Examples of a method of forming the welding nugget include spot welding, laser welding, and the combined use of spot welding and laser welding.

Next, an example of a method of manufacturing the blank100and the structural member200according to this embodiment will be described with reference toFIG. 4.FIG. 4is a diagram illustrating an example of a method of manufacturing the blank100and the structural member200according to this embodiment. As shown inFIG. 4, a high tensile strength steel sheet1is prepared first. Subsequently, as the result of partial heat treatment, such as laser heating, performed on the steel sheet, the softened portions120are formed in the joint portion-corresponding regions130, so that the blank100is formed. After that, cold forming is performed on the blank100, so that a formed article having a predetermined shape is formed. For example, the blank100is subjected to press forming by an upper die A and a lower die B that are a pair of upper and lower dies, so that the substantially hat-shaped member210having a substantially hat-shaped cross section is formed as shown inFIG. 4. The substantially hat-shaped member210is welded to another member, such as the plate-like member220, through the regions215to be welded including the softened portions120. As a result, the structural member200is formed. The method of manufacturing the blank100and the structural member200according to this embodiment has been described above.

According to this embodiment, the blank100includes the main portion110that is made of steel having a tensile strength of 1450 MPa or more and the softened portions120, and a ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110is 0.7 or more and 0.95 or less. Accordingly, it is possible to suppress the influence of a reduction in hardness in the heat affected zone during welding while suppressing an influence on surface properties in a case where the high-strength blank100is formed. Further, the Vickers hardness of the softened portion120is set to be in a predetermined range with respect to the Vickers hardness of the main portion110. Accordingly, it is possible to ensure the high strength and formability of the blank100while suppressing the influence of a reduction in hardness in the heat affected zone.

Furthermore, according to this embodiment, the blank100is subjected to not hot forming but cold forming and is formed into a formed article that is to be welded to another member. Accordingly, the generation of scale on the surface of the blank100in the case of hot forming is suppressed. As a result, since a process for removing scale in the case of hot forming is not required or can be simplified, manufacturing costs are reduced.

In addition, since cold forming is performed, a change in the tensile strength of the softened portion120during forming is suppressed as compared to hot forming. Accordingly, an effect of reducing the influence of a reduction in hardness during welding, which is obtained from the softened portion120, is ensured even after cold forming. Further, in a case where scale is generated in the joint portion-corresponding region130due to heating during hot forming, a reduction in weld strength may occur in the welding to be performed after hot forming. However, according to this embodiment, a reduction in weld strength can be suppressed since the generation of scale is suppressed due to cold forming.

Furthermore, according to this embodiment, it is not necessary to perform softening treatment accompanied by heating to be performed after the start of forming since cold forming is performed on the blank100provided with the softened portions120. As a result, the influence of thermal strain on shape accuracy is suppressed.

Moreover, according to this embodiment, forming is accurately performed by cold forming from the viewpoint that the influence of thermal strain on shape accuracy does not occur. Accordingly, the joint portion-corresponding regions130provided with the softened portions120are accurately formed as regions215to be welded after forming. Therefore, the softened portions120can be disposed at predetermined positions in the regions215to be welded of the structural member200.

Modification Example

Subsequently, a modification example of the structural member according to the first embodiment of the invention will be described with reference toFIG. 5.FIG. 5is a schematic diagram of a cross section showing another example of the cross-sectional structure of the structural member according to this embodiment. The description of contents common to the description of the first embodiment may be omitted in the description of this modification example.

In this modification example, a first member is a substantially U-shaped member230having a U shape in a cross-sectional view taken along an X-Z plane as shown inFIG. 5. The substantially U-shaped member230includes a top sheet portion231, standing wall portions233, and regions235to be welded. The regions235to be welded are provided at end portions of the substantially U-shaped standing wall portions233opposite to the top sheet portion231, and are welded to a second member. A softened portion120including a weld W is formed at each of the regions235to be welded. Further, the second member is a mating member240that has a U shape in a cross-sectional view taken along an X-Z plane.

According to this modification example, it is easy to work the first member since the first member is formed in a shape without a flange portion. Further, according to this modification example, the accuracy of a welding position is not required since the regions235to be welded are provided at the end portions of the standing wall portions233.

2. Second Embodiment

Subsequently, a blank100according to a second embodiment of the invention will be described with reference toFIGS. 6A and 6B.FIG. 6Ais a perspective view of the blank100according to the second embodiment of the invention.FIG. 6Bis a partial cross-sectional view of the blank100according to the second embodiment. The blank100according to this embodiment is different from the blank according to the first embodiment in that the blank100according to this embodiment includes a plated coating140on the surface thereof. The description of configuration common to the first embodiment will be omitted in the description of this embodiment.

As shown inFIG. 6A, the plated coating140is formed on a main portion110of the blank100to coat the surface of the main portion110. Examples of the plated coating140include a zinc plated coating that is provided to improve corrosion resistance and includes a zinc (Zn)-based alloy as a main component. On the other hand, as shown inFIG. 6B, the plated coating140is not formed on at least a part of the surface of the blank100where the softened portions120are formed. That is, a part or all of the surfaces of the softened portions120are exposed to the outside. The invention is not limited to the example shown inFIG. 6B. In a case where the softened portions120are formed on, for example, one surface of the blank100, the plated coating140may not be formed on at least a part of the surface of the blank100on which the softened portions120are formed. Further, in a case where the softened portions120are formed on one surface of the blank100, the plated coating140may not be formed in regions of both surfaces of the blank100corresponding to portions where the softened portions120are formed.

The blank100where the plated coating140is formed on the surface of the main portion110is formed into a formed article having a predetermined shape by cold forming. The formed article (first member) of the blank100, which is formed by cold forming, includes a plated coating of which the Fe concentration is 20% or less and is lower than the Fe concentration of a plated coating of a formed article which is formed by hot forming, such as hot stamping, and of which the Fe concentration of a plated coating may become 50% or more due to the diffusion of an iron component of a base material in the plated coating caused by heating. Since the content rate of plated metal, such as zinc, contained in the plated coating can be made relatively high in the formed article (first member) that is formed from the cold forming of the blank100including a plated coating, the formed article can have high corrosion resistance. Further, the formed article (first member) of the blank100, which is formed by cold forming, can be distinguished from the formed article of the blank that is formed by hot forming.

Furthermore, such a formed article is welded to another member. In this case, welding is performed at the softened portions120including portions of the surface on which the plated coating140is not formed, so that the formed article having a predetermined shape is welded to another member.

According to this embodiment, since the blank100is formed into a formed article by cold forming even in a case where the plated coating140is formed on the surface of the main portion110, the deterioration of surface properties, such as the degradation or damage of the plated coating caused by heating, is suppressed as compared to the case of hot forming. Particularly, the deterioration of antirust properties, which occurs due to the diffusion of a component of the main portion110in the plating caused by heating, is suppressed as compared to hot forming.

Further, according to this embodiment, the plated coating140is not formed on at a part or all of the surfaces of the softened portions120. Accordingly, in a case where welding is performed in the regions215to be welded, a reduction in weld strength caused by a Zn component contained in the plated coating140is suppressed.

Example of Application of Frame Member According to Embodiment of Invention

The preferred embodiments of the invention have been described in detail above. Here, an example of application of the structural member200, which is formed using the blank100according to the embodiment of the invention, will be described with reference toFIG. 7.FIG. 7is a diagram showing a vehicle frame300as an example to which the structural member200formed using the blank100according to the embodiment of the invention is applied. The structural member formed using the blank100may form the vehicle frame300as a cabin frame or a shock-absorbing frame. Examples of application as a cabin frame include a roof center reinforcement301, a roof side rail303, a B-pillar307, a side sill309, a tunnel311, an A-pillar lower313, an A-pillar upper315, a kick reinforcement327, a floor cross-member329, an under reinforcement331, a front header333, and the like.

Further, examples of application of a frame member as a shock-absorbing frame include a rear side member305, an apron upper member317, a bumper reinforcement319, a crash box321, a front side member323, and the like.

A change in the surface properties of the structural member200, which is formed by cold forming using the blank100according to the embodiment of the invention, after forming is suppressed. Further, high strength is maintained while a reduction in the hardness of the heat affected zone of the region215to be welded is suppressed. Accordingly, since the influence of the surface properties of a vehicle body frame is reduced in a case where the structural member200formed using the blank100according to this embodiment is applied as a member forming the vehicle frame300, strength can be improved.

Examples

In order to evaluate the characteristics of the blank100according to this embodiment, the blank100according to this embodiment was worked to actually produce the structural member200shown inFIG. 3Aand the characteristics thereof were evaluated. The structural member200had the cross-sectional configuration shown inFIG. 3A, the height (the distance in the Z direction inFIG. 3A) of the structural member200was set to 60 mm, the width (the distance in the X direction inFIG. 3A) of the structural member200was set to 80 mm, and the length (the distance in the Y direction inFIG. 3A) of the structural member200was set to 800 mm.

In Examples, a blank, which used a cold-rolled steel sheet having a tensile strength of 1470 MPa and in which partial tempering was performed at predetermined positions on the cold-rolled steel sheet to form the softened portions120and a remaining region of the cold-rolled steel sheet formed the main portion110, was used as the blank100. Cold forming was performed using such a blank100to produce a substantially hat-shaped member210. In this case, the substantially hat-shaped member210was formed so that the softened portions120including welds W were provided at regions215to be welded. Further, a steel sheet having a tensile strength of 780 MPa was used as a plate-like member220. The substantially hat-shaped member210was welded to the plate-like member220at the regions215to be welded. The structural member200was produced according to such a procedure.

The ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110(the Vickers hardness of the softened portion120/the Vickers hardness of the main portion110) was as shown in Table 1 to be described below.

As Comparative Example 1, a structural member was produced according to the same procedure as Example. Further, as Comparative Example 2, a structural member was produced without the softened portions120provided at the blank. That is, Comparative Example 2 means a case where the entire blank100is a cold-rolled steel sheet having a tensile strength of 1470 MPa.

A crushing test for applying bending moment to both ends of the structural member in a longitudinal direction was performed, and a bending strength ratio and a fracture position were evaluated. Here, the bending strength ratio is a ratio that is obtained in a case where the value of bending strength at the time of occurrence of the fracture of the structural member is standardized as the value of the bending strength of Comparative Example 2. Evaluation results were shown together in Table 1.

As shown in Table 1, in Example 1 and Example 2, the ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110was 0.7 or more and a bending strength ratio had a value larger than 0.9. That is, it was shown that sufficiently strength is maintained in Example 1 and Example 2 as compared to Comparative Example 2 in which the softened portions120are not provided. Further, with regard to the evaluation results of a fracture position, fracture occurred at the base material and did not occur in the heat affected zone (HAZ) in both Example 1 and Example 2. That is, as the result of the appropriate adjustment of the Vickers hardness of the softened portion120in Example 1 and Example 2, it was shown that the influence of a reduction in hardness caused by heat input during welding was suppressed.

On the other hand, in Comparative Example 1, a ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110was lower than 0.7 and fracture did not occur in the heat affected zone. However, the bending strength ratio of Comparative Example 1 did not have a value larger than 0.9 and sufficient strength could not be ensured. Further, since softened portions120were not provided in Comparative Example 2, bending strength was sufficiently high but fracture occurred in the heat affected zone due to the influence of a reduction in hardness caused by heat input during welding.

As described above, it was shown that the influence of a reduction in hardness caused by the heat affected zone was suppressed and high strength was maintained by the structural member200of which the ratio of the Vickers hardness of the softened portion120to the Vickers hardness of the main portion110was appropriately controlled.

The preferred embodiments of the invention have been described in detail above with reference to the accompanying drawings, but the invention is not limited to the embodiments. Since it is clear that various modification examples or alteration examples can be thought up by those skilled in the art to which the invention pertains without departing from the technical scope described in claims, it is naturally understood that these examples are also included in the technical scope of the invention.

For example, the substantially hat-shaped member210has been used as the first member in the embodiments, but the invention is not limited to such embodiments. The first member has only to have a predetermined shape required to form the structural member200, and may have a U shape, an arc shape, or the like in a cross-sectional view. Further, the first member may be partially bent in a cross-sectional view or may have irregularities, such as a bead shape.

Furthermore, the plate-like member220has been used as the second member in the embodiments, but the invention is not limited to such embodiments. The second member may have a substantially hat-shaped cross section, or may have a U shape, an arc shape, or the like in a cross-sectional view. Moreover, the second member may be partially bent in a cross-sectional view or may have irregularities, such as a bead shape.

Brief Description of the Reference Symbols