JPWO2012026591A1 - Heat treatment method for structural material and heat treated structural material - Google Patents

Abstract

The heat treatment method for a structural material is a heat treatment method for a structural material that includes a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction. A width e is determined; a region including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region; When the ratio of the cured region is defined as the curing rate fM, the range of the curing rate fM is determined based on the rate of change of the yield stress σY with respect to the curing rate fM; Heat treatment is performed on the effective width region of the structural material.

Description

The present invention relates to a heat treatment method for a structural material and a heat treated structural material.
This application claims priority on August 27, 2010 based on Japanese Patent Application No. 2010-190741 for which it applied to Japan, and uses the content here.

  As structural materials for automobiles and the like, tubular press-formed products having a polygonal cross section are often used. Such a structural material is roughly used for two purposes. One is a structural material that constitutes, for example, an engine compartment, a trunk room, and the like, and is a structural material that acts to crush and absorb impact energy when an automobile or the like collides. The other is a structural material that constitutes, for example, a cabin or the like, and is a structural material that can be prevented from being deformed from the viewpoint of securing a living space for a passenger even when an automobile or the like collides.

  In such a structural material, it is necessary to increase the strength of the structural material in order to absorb impact energy and to suppress deformation at the time of collision. One example is to increase the wall thickness. However, in this case, not only will the volume and weight of the structural material increase, but fuel consumption will deteriorate, and damage to the opponent vehicle in the event of a collision between vehicles will increase.

  On the other hand, as a method for increasing the strength of a structural material without increasing the cross-sectional dimension or thickness of the structural material, various methods for partially performing laser heat treatment on the structural material such as a press-formed product have been proposed ( For example, Patent Documents 1 to 4). Here, the laser heat treatment means that an untreated structural material is irradiated with a laser beam having a high energy density to locally heat the structural material to a temperature equal to or higher than the transformation temperature or the melting point, and then quench hardening is performed by a self-cooling action. Means to do.

  For example, Patent Document 1 discloses a technique for increasing the strength of a press-formed product by performing a local heat treatment on the press-formed product with a laser. Specifically, in Patent Document 1, after cold forming a steel sheet, a laser beam is rapidly heated to a predetermined temperature or higher in a stripe shape or a lattice shape, and then cooled to strengthen the cold-formed press-formed product. doing. By adopting such a method, the occurrence of distortion after heat treatment is suppressed as compared with the case where the entire press-formed product is uniformly heat-treated. In particular, in the technique disclosed in Patent Document 1, laser heat treatment is performed in the form of stripes in the longitudinal direction on the outer surface of the press-molded product or in a grid pattern on the entire outer surface of the press-molded product.

  Also, the technique disclosed in Patent Document 2 discloses that a local heat treatment is performed on the press-formed product for the purpose of increasing the strength of the press-formed product while suppressing the occurrence of distortion. In particular, in the method disclosed in Patent Document 2, heat treatment is performed on a portion where the strength of the press-formed product is required, for example, a high stress portion analyzed by a vehicle crash test, a finite element method, or the like. Specifically, laser heat treatment is performed in a stripe shape or a lattice shape so as to extend over the entire length in the longitudinal direction of the press-formed product.

  Further, Patent Document 3 discloses a method of performing laser heat treatment after controlling the components contained in the steel plate to be subjected to laser heat treatment to a specific component, whereby the laser heat treatment was performed while maintaining the workability of the steel plate. Increase the strength of the location. Also in the method disclosed in Patent Document 3, laser heat treatment is performed on a portion where the strength needs to be increased. Specifically, the laser heat treatment is performed in a linear shape extending over the entire length in the longitudinal direction of the press-formed product. Is going.

  Patent Document 4 discloses a technique of performing a laser heat treatment linearly along the load direction of the compression load on the outer peripheral surface of the press-formed product for the purpose of increasing the impact energy absorption capability of the press-formed product. According to such a method, since the laser heat treatment is performed in the same direction as the input direction of the impact load, the resistance to deformation can be increased and the crushing mode can be made regular. In particular, in the technique disclosed in Patent Document 4, laser heat treatment is continuously performed over the entire length in the longitudinal direction of the press-formed product along the direction of the compression load.

  In any case, in any of the methods disclosed in Patent Documents 1 to 4, laser heat treatment is performed on a portion of the outer surface of the press-formed product that requires strength. Specifically, the laser heat treatment is performed in a linear shape continuously extending over the entire length in the longitudinal direction of the press-formed product, or the laser heat treatment is performed in a lattice shape or the like over the entire outer surface of the press-formed product.

Japanese Unexamined Patent Publication No. Sho 61-99629 Japanese Laid-Open Patent Publication No. 4-72010 Japanese Unexamined Patent Publication No. 6-73439 Japanese Unexamined Patent Publication No. 2004-108541

FIG. 1 shows the axial compressive stress σ _x and compressive strain ε _x (longitudinal length of the cylindrical structural material when the cylindrical structural material receives a compressive load in the axial direction (x direction). The relationship with the amount of deformation in the longitudinal direction) is schematically shown. Among these, σ ₁ , σ _2, and σ ₃ in the figure indicate peak stresses, and a region indicated by diagonal lines W indicates the amount of energy absorbed by the structural material. In particular, σ ₁ indicates the initial peak stress.

Here, as described above, structural materials used in automobiles and the like include a structural material that absorbs impact energy in the event of a collision (hereinafter referred to as “impact absorbing structural material”), and its deformation is suppressed in the event of a collision. There are structural materials (hereinafter referred to as “structural materials for suppressing deformation”). Among these, in the structural material for absorbing shock, it is necessary to make the absorbed energy amount W as large as possible and to make the initial peak stress σ ₁ relatively small.

On the other hand, unlike the structure for shock absorption, in the structural material for suppressing deformation, it is necessary to make the initial peak stress σ ₁ as large as possible. This is because if the initial peak stress σ ₁ is increased, the structural material is less likely to buckle even when a large stress is applied to the structural material. Therefore, it is necessary to perform laser heat treatment on the deformation-suppressing structural material so that the initial peak stress σ ₁ becomes large.

However, in the methods disclosed in Patent Documents 1 to 4 described above, laser heat treatment is performed without considering the initial peak stress σ ₁ at all, and it is said that the ability to suppress deformation of the structural material is sufficiently enhanced. hard.

  Therefore, in view of the above problems, the object of the present invention is to sufficiently improve the deformation suppressing ability by locally curing the structural material by performing a heat treatment on an untreated structural material. It is to provide a structural material.

  Regarding the structural material having at least one bent portion extending in one direction (for example, the longitudinal direction), the inventors of the present invention have a region (location or amount) for performing heat treatment on the untreated structural material, The relationship between the ability to suppress deformation of structural materials, especially the initial peak stress, was investigated.

  As a result, by appropriately controlling the proportion of the cured region cured by heat treatment in the effective width region where the distance in the width direction from each bent portion is within the effective width, the ability to suppress deformation of the structural material, In particular, it has been found that the initial peak stress can be increased.

This invention was made | formed based on the said knowledge, and the summary is as follows.
(1) A heat treatment method for a structural material according to one aspect of the present invention includes a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction. An effective width e of the bent portion is determined; and an area including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region. , determining the scope of the case where the ratio of the region that is hardened defined as hardening rate f _M, cure rate f _M based on the rate of change of the yield stress sigma _Y for curing ratio f _M by heat treatment of the effective width region teeth; performing heat treatment on the effective width region of the structural member so as to satisfy the range of the curing ratio f _M.

(2) In the heat treatment method of the structural material according to the above (1), wherein the rate of change, the value of the hardening rate f _M may be a value when it is zero.

(3) In the heat treatment method of the structural material according to the above (2), the work hardening coefficient E _h calculated based on the change ratio to be equal to or greater than the predetermined value, it determines the range of the hardening rate f _M May be.

(4) In the heat treatment method for a structural material according to (3) above, the predetermined value may be a work hardening coefficient E _h when the hardening rate f _M is 1.

(5) In the heat treatment method of the structural material according to the above (2), the difference of .DELTA..sigma _h between flow stress when the curing rate f _M and the flow stress when the curing rate f _M is 1 is 0 When the difference between the yield stress when the curing rate f _M is 1 and the yield stress when the curing rate f _M is 0 is defined as Δσ _Y , and the rate of change is defined as b, the curing rate f The range of _M may be f _M-min or more and less than 1 represented by the following formula (1).

(6) In the heat treatment method for a structural material according to (5), the range of the curing rate f _M may be f _M-max or less represented by the following formula (2).

(7) In the heat treatment method of the structural material according to the above (1), the equal boundary hardening rate f _M and the rate of change of flow stress sigma _h determined in f _M-max the rate of change with respect to the curing rate f _M, it may determine the extent of the curing rate _{f M} on the basis of the _{f M-max.}

(8) In the heat treatment method of a structural material according to (7), the range of the hardening rate f _M, may determine the range satisfying the following formula (3).

The heat treatment method of the structural material according to (9) above (7), the range of the hardening rate f _M, may determine the f _M-min or more and less than 1 satisfies the following equation (4).

(10) In the heat treatment method for a structural material described in (1) above, the difference between the flow stress when the curing rate f _M is 1 and the flow stress when the cure rate f _M is 0 is defined as Δσ _h. when, as the difference between the change rate this .DELTA..sigma _h is equal to or less than a predetermined value, it may determine the extent of the curing ratio f _M.

(11) In the structural material heat treatment method described in (1) above, with respect to the chemical components contained in the structural material, the mass percentage of carbon is C, the mass percentage of silicon is Si, the mass percentage of manganese is Mn, nickel When the mass percentage of Ni is defined as Ni, the mass percentage of chromium as Cr, the mass percentage of molybdenum as Mo, the mass percentage of niobium as Nb, and the mass percentage of vanadium as V, the region cured by the heat treatment is represented by the following formula: It may be a region equal to or higher than the Vickers hardness calculated by (5) and (6).

  (12) In the structural material heat treatment method according to (1), the heat treatment may be performed by a laser.

  (13) In the heat treatment method for a structural material according to (1) above, one pass of the heat treatment may be continuously performed over the entire length in the one direction.

(14) The heat-treated structural material according to one aspect of the present invention is a structural material including a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction. A region including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region, and a ratio of the effective width region occupied by a region cured by heat treatment the when defined as hardening rate f _M, the curing rate f _M is less than 1, and, in the range of curing rate f _M which is determined based on the rate of change of the yield stress sigma _Y for hardening rate f _M included.

(15) In the heat-treated structural material according to (14), wherein the rate of change, the value of the hardening rate f _M may be a value when it is zero.

The heat-treated structural material according to (16) above (15), the range of the hardening rate f _M is the rate of change work hardening coefficient E _h calculated based on are determined to be equal to or greater than the predetermined value Range may be used.

(17) In the heat-treated structural material according to (16), the predetermined value may be a work hardening coefficient E _h when the hardening rate f _M is 1.

(18) In the heat-treated structural material according to (14), the difference between the flow stress when the curing rate f _M is 1 and the flow stress when the curing rate f _M is 0 is Δσ _h When the difference between the yield stress when the curing rate f _M is 1 and the yield stress when the curing rate f _M is 0 is defined as Δσ _Y , and the rate of change is defined as b, the curing rate f The range of _M may be f _M-min or more represented by the following formula (7).

The heat-treated structural material according to (19) above (18), the range of the hardening rate _{f M} may be not less _{f M-max} represented by the following formula (8).

  (20) In the heat-treated structural material according to (18), each flow stress may be defined as a proof stress when 5% plastic strain occurs.

(21) In the heat-treated structural material according to (19), the width dimension perpendicular to the one direction is w, the yield stress when the hardening rate f _M is 0 is σ _Y0 , and the one direction of the structural material When the stress at each position in the width direction perpendicular to the one direction when the stress that gives the maximum stress of σ _Y0 toward the one direction is defined as σ _x , the effective width e May be defined by the following formula (9).

(22) In the heat-treated structural material described in (14) above, the yield stress when the thickness dimension is t, the Poisson's ratio is ν, the elastic modulus is E, and the hardening rate f _M is 0 is defined as σ _Y0 In this case, the effective width e may be defined by the following formula (10).

(23) In the heat-treated structural material described in (14) above, the yield when the thickness dimension is t, the width dimension perpendicular to the one direction is w, the elastic modulus is E, and the curing rate f _M is 0. When the stress is defined as σ _Y0 , the effective width e may be defined by the following formula (11).

(24) In the heat-treated structural material described in (14) above, with respect to the chemical components contained in the structural material, the mass percentage of carbon is C, the mass percentage of silicon is Si, the mass percentage of manganese is Mn, nickel When the mass percentage of Ni is defined as Ni, the mass percentage of chromium as Cr, the mass percentage of molybdenum as Mo, the mass percentage of niobium as Nb, and the mass percentage of vanadium as V, the region cured by the heat treatment is represented by the following formula: The area | region more than the Vickers hardness computed by (12) and (13) may be sufficient.

  (25) In the heat-treated structural material according to (14), the heat treatment may be performed by a laser.

According to the present invention, compared to the conventional method in which the structure material is locally hardened by performing a heat treatment on an untreated structure material, the structure material is locally hardened. The value of the elastic-plastic buckling stress σ _{p, Cr} corresponding to the initial peak stress σ ₁ of bending can be obtained, and the volume fraction of the hardened region in the effective width region that maximizes the elastic-plastic buckling stress σ _{p, Cr} The rate can be presented properly. Thereby, in the deformation suppression structure, an appropriate deformation suppression guideline can be given.
Further, according to the present invention, it is possible to optimize (reduce) the heat treatment cost necessary for enhancing the deformation suppressing ability of the structural material.
In addition, according to the present invention, by measuring the characteristics of the steel material using the test piece, the volume fraction of the hardened region in the effective width is appropriately presented from the characteristic value of the test piece without evaluating the structure. be able to. In particular, in the case of (2) above, the volume fraction of the hardened region in the effective width can be appropriately presented with as few evaluation pieces as possible.

It is the figure which showed typically the relationship between the compressive stress and compressive strain of an axial direction when a cylindrical structural material receives the compressive load in the axial direction. It is a perspective view which shows an example of the structural material to which the heat processing method of the structural material which concerns on one Embodiment of this invention is applied. It is a cross-sectional view of the structural material shown in FIG. It is a cross-sectional view of the structural material of another example. It is a cross-sectional view of the structural material of another example. It is a cross-sectional view of the structural material of another example. It is a perspective view of the structural material of another example. It is a figure for demonstrating an effective width | variety. It is a figure for demonstrating an effective width | variety. It is a true stress-plastic strain diagram of a steel plate. It is a true stress-true strain diagram of a steel plate. It is a true stress-true strain diagram of a steel plate. It is a true stress-true strain diagram of a steel plate. It is a figure which shows the relationship between the volume fraction of a hardening area | region, the yield strength of a steel plate, and a yield stress. It is a figure which shows the relationship between the volume fraction of a hardening area | region, the yield strength of a steel plate, and a yield stress. It is a figure which shows the relationship between the volume fraction of a hardening area | region, and a work hardening coefficient. It is a figure which shows the manufacturing process of the structural material assembly used in the Example. It is a figure which shows the manufacturing process of the structural material assembly used in the Example. It is a figure which shows the manufacturing process of the structural material assembly used in the Example. It is a side view of the structural material assembly used in the Example. It is a flowchart of the heat processing method of the structural material which concerns on this embodiment. The volume fraction of the hardened zone in the heat treatment method of a structural material according to the present embodiment is a flow chart illustrating an example of a method of determining the range (hardening rate) f _M. The volume fraction of the hardened zone in the heat treatment method of a structural material according to the present embodiment is a flow chart illustrating an example of a method of determining the range (hardening rate) f _M. The volume fraction of the hardened zone in the heat treatment method of a structural material according to the present embodiment is a flow chart illustrating an example of a method of determining the range (hardening rate) f _M.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are assigned to similar components.

Below, the heat processing method of the structural material which concerns on one Embodiment of this invention is demonstrated.
In the heat treatment method for a structural material according to this embodiment, the heat treatment is performed on the structural material including a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the extending direction. In this heat treatment, the distance in the direction perpendicular to the extending direction of the bent portion is within the effective width, which corresponds to a predetermined ratio (that is, effective rate) of the region in the structural material including the bent portion (that is, the effective width region). Part) is cured.
As will be described later, the rate of change of yield stress (yield strength) with respect to the proportion of the effective width region that is cured by heat treatment (ie, the rate of cure) varies depending on the rate of cure, and the amount of change (change) Is greater than the amount of change (the degree of change) in the rate of change of the flow stress relative to the cure rate. Therefore, the work hardening rate of the effective width region necessary for increasing the initial peak stress (deformation suppressing ability) of the structural material is affected by the rate of change of the yield stress with respect to the hardening rate. Therefore, the heat treatment cost is reduced by performing heat treatment on the effective width region that mainly bears the load applied to the structural material so as to satisfy the range of the hardening rate determined based on the rate of change of the yield stress with respect to the hardening rate. While being reduced, the ability to suppress deformation of the structural material can be increased.
The flow stress is a stress generated at the time when the elastic limit is exceeded and the fluid deformation is shifted and after that time. Moreover, a hardening rate may be described as a volume fraction.

In the structural material heat treatment method according to the present embodiment, as shown in FIG. 15, necessary data is input (used) (S1), the effective width for the bent portion is determined (S2), and the yield stress with respect to the hardening rate. A curing rate range is determined based on the change rate (S3), and heat treatment is performed on the effective width region of the structural material so as to satisfy the curing rate range (S4). Here, the effective width can be determined from the definition formula of the effective width of Expression (14) described later or various expressions derived from this definition expression. Further, the range of the curing rate can be determined using the rate of change of yield stress with respect to at least one curing rate. For example, it can be determined from a predetermined correlation (for example, an equation) using the rate of change in yield stress with respect to a predetermined hardening rate as a parameter. Further, for example, the range of the curing rate can be determined based on the curing rate when the change rate of the yield stress with respect to the curing rate satisfies a predetermined condition.
Below, the heat processing method of the structural material which concerns on this embodiment is demonstrated in detail.

  FIG. 2 is a perspective view showing an example of a structural material to which the structural material heat treatment method according to the present embodiment is applied. 3 is a cross-sectional view of the structural material in a cross section perpendicular to the longitudinal direction of the structural material shown in FIG. As shown in FIG. 2, the structural material 10 includes a flat plate-like flat portion 11 (11 a to 11 e) extending in the longitudinal direction and a plurality of bent portions 12 (12 a to 12 d) extending in the longitudinal direction between the flat portions 11. ). That is, as shown in FIG. 3, the structural member 10 includes five flat portions 11a to 11e and four bent portions 12a to 12d provided between the flat portions 11a to 11e.

  The structural member 10 is used for a part of a frame of a vehicle such as an automobile, for example, and is particularly used in a place where it is necessary to suppress the deformation when the automobile collides. Therefore, for example, taking an automobile frame as an example, the structural member 10 is preferably used for a frame constituting a cabin or the like.

  In particular, when the structural material 10 is used for a part of a frame of a vehicle such as an automobile, the structural material 10 is welded to another flat structural material 20 as shown by a one-dot chain line in FIGS. Are used. Therefore, of the five flat portions 11a to 11e of the structural material 10, the flat portions 11a and 11e provided at both edges of the structural material 10 are formed in a flange shape. When the structural material 10 is welded to another structural material 20, the flat portions 11 a and 11 e are welded to the other structural material 20.

  2 and 3, the structural member 10 includes five flat portions 11a to 11e and four bent portions 12a to 12d provided between the flat portions 11a to 11e. . However, the structural material has any shape as long as it has at least one bent portion that extends in one direction (for example, the longitudinal direction) and is bent in a direction perpendicular to the extending direction. For example, you may have cross-sectional shape as shown to FIG. 4A-FIG. 4C.

  In the example shown in FIG. 4A, the structural member 10 ′ includes four flat portions 11 and three bent portions 12 provided between the flat portions 11, and among these, the cross-sectional shape is located on both edges. The flat portion 11 that functions serves as a flange for connecting the structural member 10 ′ to another flat plate-shaped structural member (not shown). In the example shown in FIG. 4B, the structural member 10 '' includes five flat portions 11 and four bent portions 12 provided between the flat portions 11, and among these, the cross-sectional shape is on both edges. The positioned flat portion 11 functions as a flange for connecting the structural member 10 '' with another flat plate-shaped structural member (not shown). In the example shown in FIG. 4C, the structural member 10 ′ ″ includes four flat portions 11 and four bent portions 12 provided between the flat portions 11 so that the cross-sectional shape thereof is a quadrangle. To do.

  In addition, the structural material 10 does not necessarily extend linearly in the longitudinal direction, and may be curved or bent as shown in FIG. 5, for example. When the structural member 10 is curved or bent as described above, a direction along the curve and the bend is referred to as a longitudinal direction. Therefore, in the example shown in FIG. 5, the alternate long and short dash line Z in the figure indicates the longitudinal direction of the structural material 10. Further, the flat portion means a portion of the structural material whose cross section is a straight line (strip shape). Moreover, a bending part means the part of the linear structural material formed by the intersection of the extension direction of two flat parts adjacent in the cross section of a structural material. Accordingly, cases where the flat portion and the bent portion are curved or bent in the longitudinal direction of the structural material, such as the flat portions 11a to 11e and the bent portions 12a to 12d shown in FIG. 5, are included in the flat portion and the bent portion, respectively. .

  In the structural material heat treatment method according to the present embodiment, heat treatment (here, laser heat treatment is performed as an example) is performed on a specific portion of the untreated structural material 10 formed into the shape as described above. As the laser heat treatment means, a laser heat treatment apparatus such as a carbonic acid laser, a YAG laser, or a fiber laser is used. Further, the depth in the plate thickness direction of the region to be hardened by laser heat treatment is hardened to a depth of at least 10% of the plate thickness from the laser light irradiation surface. Further, it is desirable to control the depth in the plate thickness direction of the region to be hardened by laser heat treatment to be less than 90% of the plate thickness from the laser light irradiation surface. Below, the site | part to which laser heat processing is performed is demonstrated.

When the thin plate is buckled by receiving a compressive load, the stress acting on the thin plate is non-uniformly distributed in the cross-section (plate width direction) of the thin plate perpendicular to the direction in which the compressive load acts. For example, when a thin plate having a width w as shown in FIG. 6A receives a compressive load as indicated by an arrow and the thin plate is deformed out of plane by elastic buckling, the longitudinal direction (x direction) applied to the cross section a stress sigma _x of) are distributed as shown in Figure 6B. As shown in FIG. 6B, since the stress acting at the end in the width direction (y direction, ie, w direction) of the thin plate is maximized, plastic buckling tends to occur from the end in the width direction of the thin plate. Therefore, at the initial stage of buckling (for example, in the case of a structural material, it corresponds to deformation until reaching the initial peak stress), it is considered that a portion having a width of a predetermined dimension from the end face in the width direction of the thin plate bears a compressive load. be able to. Therefore, the stress equal to the stress σ _max (corresponding to σ _Y0 described later in the structural material) applied to the end portion in the width direction of the thin plate is uniform in the virtual width 2 × e portion as shown by the broken line in FIG. 6B. It is assumed that this virtual width 2 × e portion carries the entire load. This width e is called an effective width, and this effective width e is defined by the following equation (14), that is, equation (15).

This effective width e can be expressed by the following equation (16) using the elastic modulus E, Poisson's ratio ν, and thickness t of the thin plate, particularly when the yield stress σ _{Y0 of the} thin plate is uniformly distributed. The effective width e can be expressed as in the following formula (17).

The effective width e represented by the above formulas (16) and (17) is a theoretical value, and it has been proved that the experimental result and the yield phenomenon differ greatly depending on the conditions when this theoretical value is used. Therefore, considering the experimental result, the effective width e is defined as in the following formulas (18A) and (19), for example. In Equation (19), λ is a slenderness factor, and is determined as Equation (20) when the yield stress σ _{Y0 of the} thin plate is uniformly distributed in the portion of the effective width e. In the equation (20), k means a flat plate buckling coefficient.

  As for the definition of the effective width e, there are various definitions other than the above formula (18A) as in the following formula (18B). In the heat treatment method for a structural material according to the present embodiment, these various types are defined. Any of the definitions may be used. Also, the stress distribution in the thin plate width direction (that is, the stress distribution shown in FIG. 6B) when the thin plate is buckled by receiving a compressive load is calculated by numerical analysis (for example, numerical integration such as the finite element method). The effective width e that satisfies the above equation (14) may be calculated from the stress distribution thus calculated.

  In consideration of the effective width e as described above, also in the structural material 10 shown in FIG. 2 and the like, the region mainly responsible for the compressive load in each flat portion 11 extends from the bent portion 12 in the width direction (that is, the structural material 10 This is a region whose distance in the direction perpendicular to the longitudinal direction is within the effective width e. Hereinafter, such a region, that is, a region including a bent portion whose distance in the width direction from a certain bent portion is within the effective width e is referred to as an effective width region. This effective width area (effective width area 15 in FIGS. 2 and 3) is indicated by hatching in FIG. 2, and is filled in FIG.

  Thus, in the structural material heat treatment method according to the present embodiment, an untreated structural material (a bent portion of the structural material) having at least one bent portion as shown in the bent portion 12 (12a to 12d) in FIG. The effective width is determined for.

  In the structural material heat treatment method according to the present embodiment, heat treatment (here, laser heat treatment as an example) is performed on a part of the effective width region determined as described above. Hereinafter, the proportion of the effective width region occupied by the region where the laser heat treatment is performed will be described.

FIG. 7 shows a true stress-true plastic strain diagram of a steel sheet having a tensile strength of 440 MPa. When the linear hardening rule as shown in FIG. 7 is used as the work hardening characteristic immediately after yielding of a steel sheet having such stress-strain characteristics, the work hardening coefficient E _h is expressed by the following formula (21). In equation (21), ε _p represents the strain (plastic strain) after the yield of the steel sheet, and σ _h represents the stress when the plastic strain is ε _p . In FIG. 7 and FIGS. 9A and 9B described later, σ _h is described as the stress when the plastic strain ε _p is 1%. As shown in these figures, σ _h may be determined from the stress when the plastic strain ε _p is 1%.

Elastic-plastic buckling phenomenon such steel have been proposed are theoretical formula representing the elastic-plastic buckling stress sigma _{p, Cr} as a function of work hardening coefficient E _h, elastoplastic buckling stress sigma _{p, Cr} Is represented, for example, by the following formula (22). In the following formula (22), w is the width of the steel plate, t is the thickness of the steel plate, and k is a coefficient corresponding to the plate shape and the like. As can be seen from the equation (22), the elastoplastic buckling stress σ _{p, Cr} increases in proportion to the work hardening coefficient E _h .

Here, since the initial peak stress σ ₁ shown in FIG. 1 is considered to have the same tendency as the elastoplastic buckling stress σ _{p, Cr} , the initial peak stress σ ₁ is also proportional to the work hardening coefficient E _h. Will increase. In addition, the said Formula (22) represents the elastic-plastic buckling stress (sigma) _{p, Cr} in the steel plate as shown to FIG. 6A, and the elastic-plastic seat regarding the structural material which has a polygonal cross section as shown in FIG. It does not represent the bending stress σp _{, Cr} . However, when the cross-sectional shape of the structural material is diversified, the cross-sectional shape of the structural material approaches a cylindrical shape, and the elastic-plastic buckling stress σ _{p, Cr} of the cylindrical shell is expressed by the following equation (23). I can express. In formula (23), R is the diameter of the cylinder.

As can be seen from the equation (23), also in the cylindrical shell, the elastic-plastic buckling stress σ _{p, Cr} increases in proportion to the work hardening coefficient E _h . Accordingly, it is considered that the initial peak stress σ ₁ increases in proportion to the work hardening coefficient E _h also in the cylindrical shell.

  FIG. 8 shows a true stress-true strain diagram of an untreated steel sheet having a tensile strength of 440 MPa class and a material heat-treated (quenched) on the entire steel sheet having a tensile strength of 440 MPa. The solid line in FIG. 8 shows the true stress-true strain diagram of the untreated steel sheet, and the broken line shows the true stress-true strain diagram of the steel sheet after heat treatment.

When the work hardening coefficient E _h immediately after yielding is calculated for the steel sheet before and after the heat treatment shown in FIG. 8 by applying the linear hardening rule as shown in FIG. 7, the work hardening coefficient E _h0 of the untreated steel sheet is It can be expressed as in equation (24) (see FIG. 9A). In equation (24), σ _Y0 is the yield stress of the untreated steel plate, ε _Y0 is the true strain of the untreated steel plate when the yield stress is reached, ε _h0 is a predetermined true strain greater than ε _Y0 , σ _h0 Indicates the stress (corresponding to the flow stress described later) of the untreated steel plate when the true strain is ε _h0 . On the other hand, the work hardening coefficient E _hM of the steel plate after the heat treatment can be expressed as the following formula (25) (see FIG. 9B). In equation (25), σ _YM is the yield stress of the steel plate after heat treatment, ε _YM is the true strain of the steel plate after heat treatment when the yield stress is reached, ε _hM is a predetermined true strain greater than ε _YM , σ _{hM Indicates} the stress (corresponding to the flow stress described later) of the steel sheet after the heat treatment when the true strain is ε _hM .

As can be seen from FIG. 8, FIG. 9A and FIG. 9B, when heat treatment is performed on the entire steel plate, the work hardening coefficient E _hM of the steel plate after the heat treatment is larger than the work hardening coefficient E _h0 of the steel plate before the heat treatment. Therefore, when the heat treatment is performed on the entire steel plate, it can be seen that the steel plate after the heat treatment has a larger initial peak stress σ ₁ than the steel plate before the heat treatment.

Thus, it has been found that the initial peak stress σ ₁ is larger between the untreated steel sheet and the steel sheet that has been heat-treated as a whole. However, when the steel sheet is partially heat-treated, the ratio of heat-treating the steel sheet, that is, the ratio of the region hardened to a predetermined hardness or higher by the heat treatment (hereinafter referred to as “cured region”) with respect to the entire steel plate The relationship with the initial peak stress was unclear.

Therefore, the present inventors changed the volume fraction f _M and the work hardening coefficient of the steel sheet after partial hardening when the volume fraction (hardening ratio) f _M of the hardening region with respect to the whole steel sheet was changed from 0 to 100%. As a result of investigating the relationship between E _h and the initial peak stress σ ₁ , the following findings were obtained. Hereinafter, the obtained knowledge will be described in detail.

First, when the volume fraction f _M of the hardened region with respect to the entire steel sheet is changed from 0 to 100%, the proof stress σ _h and the yield stress σ _Y of the steel sheet when 5% plastic strain occurs are shown in FIG. It is thought that it will change as shown.

That is, as shown in FIG. 10, the proof stress σ _h of the steel sheet when a plastic strain of 5% occurs can be approximated by a straight line with respect to the volume fraction f _M. This is because when a certain degree of finite plastic strain is applied to the entire steel sheet, the plastic strain acts almost equally in both the hardened area and the non-hardened area (the area of the steel sheet other than the hardened area, ie, the untreated area). It is to do.

Therefore, yield strength sigma _h after the plastic strain 5% impart to the volume fraction f _M of the hardened region can be expressed as the following equation (26) as a function of the volume fraction f _M.

As described above, even if an approximation that the yield strength σ _h of the steel plate is proportional to the volume fraction f _M of the hardening region (the change amount of the flow stress with respect to the hardening rate is approximately 0), the yield strength of the steel plate is achieved. The relationship between σ _h and the volume fraction f _M of the hardened region can be expressed sufficiently accurately.

On the other hand, as shown in FIG. 10, when the yield stress σ _Y is approximated not by a straight line but by a downwardly convex curve (for example, a quadratic function), it is expressed more accurately by using the volume fraction f _M of the hardened region. can do. When the volume fraction f _M of the hardened region is small, the characteristics of the non-hardened region having a relatively low yield stress become dominant with respect to the yield phenomenon, and the overall yield stress is close to the yield stress of the non-hardened region. (See equation (27)). In contrast, when the volume fraction f _M of the hardened area is large to some extent, when a breakdown phenomenon occurs, the influence of the properties of the cured area is increased. In particular, when the volume fraction f _M of the hardened region is 1, the yield stress as a whole becomes equal to the yield stress of the hardened region (see formula (28)).

Therefore, for example, when the yield stress σ _Y is approximated by a quadratic function of the volume fraction f _M of the hardening region, the yield stress σ _Y (σ _Y (f _M )) is expressed as a function of the volume fraction f _M as follows. It can be expressed as equation (29). In Expression (29), a, b, and c are constants.

Here, by substituting 0 into the volume fraction f _M and first derivative equation (29) for the volume fraction f _M, the constant b in the equation (29), it is expressed by the following equation (30) Can do. That is, the constant b can be approximated by the change gradient of yield stress σ _{Y (f M)} with respect to the volume fraction f _M when the volume fraction f _M of the hardened zone is zero.

When the equations (26) to (30) thus determined are substituted into the equation (21), the work hardening coefficient E _h is a function of the volume fraction f _M of the hardening region, that is, the following equation (31) ).

Here, for example, the plastic strain ε _p is 0.05, the yield stress σ _YM of the hardened region is 794 MPa, the yield stress σ _Y0 of the non-hardened region is 301 MPa, and the yield strength σ _hM of the hardened region when the plastic strain ε _p is _applied . Assuming that the proof stress σ _{h0 of the} uncured region when applying the plastic strain ε _p is 447 MPa and b is 350 MPa, σ _h (f _M ) calculated by the equation (26) and σ calculated by the equation (29) _Y (f _M ) can be expressed as shown in FIG. At this time, the work hardening coefficient E _h (f _M ) calculated by the equation (31) can be expressed as shown in FIG.

For example, as can be seen from the equation (31), the yield stress σ _Y is approximated by a quadratic function of the volume fraction f _M of the hardened region (a function convex downward in the range of the volume fraction f _M of 0 to 1). The work hardening coefficient E _h (f _M ) can also be expressed as a quadratic function of the volume fraction f _M of the hardened region (a function convex upward in the range of the volume fraction f _M of 0 to 1). . For this reason, as can be seen from FIG. 12, the work hardening coefficient E _h (f _M ) becomes maximum at a certain specific volume fraction f _M-max . Therefore, by the volume fraction f _M of the hardened area, may work hardening coefficient E _{h (f M)} is higher than the work hardening coefficient when the volume fraction f _M of the hardened zone is 1 (100%) Exists. In the example shown in FIG. 12, when the volume fraction _{f M} of the hardened region is _{f M-min} to 1, work hardening coefficient _{E h} is the volume fraction _{f M} of the hardened regions is 1 (100%) When the work hardening coefficient E _h (f _M = 1) or more. In other words, in the example shown in FIG. 12, the initial peak stress when the volume fraction f _M of the hardened region is f _M−min −1 is 1 (100%) when the volume fraction f _M of the hardened region is 1 (100%). In some cases (that is, when the entire effective width is heat-treated), the initial peak stress is exceeded.

  Incidentally, as described above, for example, laser heat treatment is used as the heat treatment for locally hardening a part of the steel sheet. In such laser heat treatment, the larger the treatment area, the greater the amount of energy consumed, thus increasing the manufacturing cost. For this reason, from the viewpoint of reducing the manufacturing cost, it is preferable that the region where the laser heat treatment is performed is as narrow as possible.

Here, as described above, when the volume fraction f _M of the hardened region is set to f _M-min or more, the work hardening coefficient E _h is obtained when the volume fraction f _M of the hardened region is 1 (100%). The work hardening coefficient E _h (f _M = 1) or more can be increased. As a result, the initial peak stress, the volume fraction f _M of the hardened region can be increased to more than the initial peak stress at 1 (100%). Therefore, the work hardening coefficient E _h (f _M = 1) and the work hardening coefficient E _h (f _M = f _M−min ) when the volume fraction f _M of the hardening region is 1 (100%) are equal. the volume fraction f _{M-min (hereinafter,} "minimum volume fraction" hereinafter) of the time above, it is preferable to control the volume fraction f _M of the hardened region.

For example, when the yield stress σ _Y is approximated by a quadratic function of the volume fraction f _M of the hardening region, the minimum volume fraction f _M-min is expressed by the following equation (32). In Expression (32), Δσ _h is a difference between σ _hM and σ _h0 (Δσ _h = σ _hM −σ _h0 ), and Δσ _Y is a difference between σ _YM and σ _Y0 (Δσ _Y = σ _YM − σ _Y0 ). In particular, in the case of the conditions as described above (that is, in the case of the conditions shown in FIGS. 11 and 12), the minimum volume fraction f _M-min is 53.3%. Since the minimum volume fraction f _M-min needs to satisfy 0 <f _M-min <1, the constants b and Δσ _h are 0 <b <2Δσ _Y −Δσ _h and Δσ _Y <Δσ _h <2Δσ. It is necessary to satisfy _Y.

Further, as described above, the work hardening coefficient E _h (f _M ), that is, the initial peak stress becomes maximum at a specific volume fraction f _M-max . For this reason, from the viewpoint of increasing the initial peak stress while narrowing the region where laser heat treatment is performed, the volume fraction f _M of the cured region is set to the volume fraction when the work hardening coefficient E _h (f _M ) is maximized. It is preferable to control to a rate f _M-max or less.

Alternatively, the steel sheet peak stress (structural member) from the viewpoint of maximizing the volume fraction f _M of the hardened region, the work hardening coefficient E _{h (f M)} the volume fraction of the time that maximizes f _M- It is preferable to control to _max . Therefore, the volume fraction f _M of the hardened region, the work hardening coefficient E _{h (f M)} the volume fraction of the time that maximizes f _{M-max (hereinafter,} referred to as "the maximum volume fraction") is also controlled to Good.

For example, when the yield stress σ _Y is approximated by a quadratic function of the volume fraction f _M of the hardened region, the maximum volume fraction f _M-max is expressed by the following formula (33). In particular, in the case of the conditions as described above (that is, in the case of the conditions shown in FIGS. 11 and 12), the maximum volume fraction f _M-max is 76.6%. In this case also, since the maximum volume fraction f _M-max needs to satisfy 0 <f _M-max <1, the constants b and Δσ _h are 0 <b <Δσ _h and 0 <b <Δσ _Y It is necessary to satisfy.

Incidentally, the relationship between the volume fraction f _M and the initial peak stress or work hardening coefficient E _h of the hardened region described above, a relationship obtained for steel sheets, for example, structural materials having a shape as shown in FIG. 2 It is not the relationship obtained for 10. Here, in the structural material 10 as shown in FIG. 2, as described above, the region that mainly bears the compressive load is the effective width region 15, and each effective width region 15 is regarded as a steel plate having a width of 2 × e. be able to. Therefore, the volume fraction f _M of the hardened region in such an effective width region, that is, the ratio of the effective width region to the region where the hardening process (for example, laser heat treatment) is performed is set by the method described above. Can do.

For example, the volume fraction f _M of the cured region in each effective width region 15 is not less than f _M-min represented by the above equation (32) and not more than f _M-max represented by the above equation (33). Next, laser heat treatment is performed. In this case, σ _hM , σ _h0 , σ _YM and σ _Y0 related to Δσ _h (= σ _hM −σ _h0 ) and Δσ _Y (= σ _YM −σ _Y0 ) in the equations (32) and (33) are , The yield strength of the heat treatment region (hardened region) at the time of applying the predetermined strain, the yield strength of the untreated region (non-hardened region) at the time of the given strain, the yield stress of the heat treated region (hardened region), and the untreated region ( It shows the yield stress in the non-hardened region. Further, σ _hM , σ _h0 , σ _YM and σ _Y0 are parameters related to a material (steel plate) used for the structural material.

The volume fraction f _M of the hardened areas in the effective width region 15 by setting in this manner, while reducing the area to be laser heat treatment, it is possible to increase the initial peak stress of the structural member 10.

In the above description, the volume fraction f _M of the hardened areas in the effective width region 15 is controlled below f _M-min or more and f _M-max, the volume fraction f _M of the hardened zone, as described above _May be controlled to f _M-min or more and 1 (100%) or less or less than 1. In this case, the volume fraction f _M of the hardened region in each effective width region 15 is greater than the work hardening coefficient when the work hardening coefficient E _h of the effective width region 15 is hardened by laser heat treatment over the entire effective width region 15. It can be determined that the setting is made. Alternatively, the volume fraction f _M of the hardened areas in the effective width region 15 may be controlled to f _M-max as described above.
In summary of the above, the volume fraction f _M based on the rate of change of the yield stress sigma _Y for the volume fraction f _M when the volume fraction f _M of the hardened area is 0 (constant) b as shown in FIG. 17 determines the minimum value of (S311), the maximum value of the range of the volume fraction f _M by determining (S312) that the 1 or less or less than 1, determining the range of the volume fraction f _M of the hardened region it can. Also, determine the minimum of the range of the volume fraction f _M (S311) after the rate of change of the yield stress sigma _Y for the volume fraction f _M when the volume fraction f _M of the hardened area is 0 (constant) b It may be used to determine the maximum value of the range of the volume fraction f _M based on (S313).

Here, an example of a method of determining the constant b to determine the range of the volume fraction f _M of the hardened area above. As a first method, a tensile test is performed on three samples in which the volume fraction f _M of the hardened region of the steel sheet is 0, 1, and any value greater than 0 and less than 1 (for example, 0.5), The constants a, b, and c can be determined by obtaining the yield stress σ _Y of these samples and performing the least square method. Further, as a second method, a tensile test is performed on two samples in which the volume fraction f _M of the hardened region of the steel sheet is 0 and an arbitrary value (for example, 0.1) sufficiently close to 0 exceeding 0. The yield stress σ _Y of these samples can be obtained, and the rate of increase of the yield stress σ _Y with respect to the volume fraction f _M of the hardened region can be determined as a constant b. Here, the method for determining the constant b with the minimum number of data (the number of data of the yield stress σ _Y ) as a simple method has been described, but the upper limit of the number of data is not particularly limited. As the number of data is large, the range of the volume fraction f _M can be determined with higher accuracy.

Furthermore, the yield stress σ _Y and the proof stress σ _h are measured by performing a tensile test according to JIS Z2241 on a JIS No. 5 test piece (test piece) taken from a steel plate (no heat treatment and bending) used for the structural material. can do. In particular, for the measurement of the yield stress σ _YM and the proof stress σ _hM when the volume fraction f _M of the hardened region is 1, a test piece obtained by subjecting the above test piece to a predetermined heat treatment may be used. As this predetermined heat treatment, for example, after the test piece is heated to _Ae3 point ( _Ae3 temperature) or higher, a cooling rate of 10 ° C / s or higher, preferably 30 ° C / s or higher, by a cooling method such as water cooling or air cooling. _May be cooled to the point Ms ( _Ms temperature) or less.

In the measurement of the yield stress σ _YM and the proof stress σ _hM when the volume fraction f _M of the hardened region is more than 0 and 1 or less, the predetermined heat treatment is performed in the longitudinal direction of the test piece. The above-described tensile test may be performed by performing laser heat treatment under corresponding conditions. In this case, pulling the volume fraction f _M of the hardened region was measured after the test, may be determined corresponding relationship between the volume fraction f _M and yield stress sigma _YM and yield strength sigma _hM. To control the volume fraction f _M of the hardened region by the laser heat treatment, the width direction of the test piece laser heat treatment in the longitudinal direction (direction perpendicular to the longitudinal direction) position shift while test piece (1 pass) test piece Repeat on one or both sides.
Moreover, you may use the test piece to which the distortion log | history equivalent to the bending process part (bending part) of the structural material before heat processing was added to the steel plate used for a test piece.

Further, the volume fraction f _M of the hardened region of the can be determined by the following method. For example, the area of the hardened region in a cross section perpendicular to the longitudinal direction of the test piece is measured, and the volume of the hardened region is obtained by multiplying this area by the length (total distance) after laser heat treatment. it can be determined the volume fraction f _M of the hardened region by dividing the total volume of the test piece. The area of the hardened region may be determined from the quenched structure observed with an optical microscope for the cross section perpendicular to the longitudinal direction of the test piece, and determined by obtaining the Vickers hardness using a Vickers hardness meter as described later. You may do it.

Further, in the method of determining the range of the volume fraction f _M of the hardened zone described above, the relationship between the yield strength sigma _h of the steel sheet and the volume fraction f _M of the hardened region was expressed by a linear function, and the yield stress of the steel sheet sigma _Y It expressed in relation to a quadratic function of the volume fraction f _M of the hardened zone, but it is not always necessary to use these functions.
To determine the range of the volume fraction f _M of the hardened region, the rate of change of the yield stress versus the volume fraction f _M of the hardened area, changes according to the volume fraction f _M of the hardened region, the amount of change (degree of change) may be utilized larger than the variation of the rate of change of flow stress versus the volume fraction f _M of the hardened zone (degree of change).
Therefore, for example, the relationship between the yield stress σ _{Y of the} steel sheet and the volume fraction f _M of the hardening region is expressed by an arbitrary function σ _Y (f _M ), and the rate of change of the yield stress with respect to at least one hardening rate (2 If the next function, it is possible to determine the range of the volume fraction f _M of the hardened region by using the equivalent) to the above-described constant b. When the above-mentioned quadratic function is expanded to a general function, the minimum volume fraction f _M-min (other than 1) and the maximum volume fraction f _M-max are determined so as to satisfy the following expressions (34) and (35). can do. Here, σ _Y (f _M ) can also be expressed by a function including the constant b described above.

Further, the relationship between the proof stress σ _{h of} the steel sheet and the volume fraction f _M of the hardened region may be expressed by an arbitrary function σ _h (f _M ). Here, when the above-described linear function and quadratic function are extended to general functions, the maximum volume fraction f _M-max can be determined so as to satisfy the following formula (36).

Furthermore, in addition to the above-described range (for example, f _M-min or more and 1 or less (less than 1) or the range of the following formula (41)), for example, the maximum volume fraction (boundary hardening rate) f _M-max is used. equation (37) may determine the range of the volume fraction f _M of the hardened zone in any of a range of - (40).

By determining the range of the volume fraction f _M of the hardened zone, as in the above formula (37) to (41), stable with excellent balance between improved deformation suppressing capability of reducing the structural material cost of the heat treatment Heat treatment can be performed. Moreover, the range of the volume fraction f _M of the hardened region, the correction term, including the cost and heat treatment conditions and the like may be included as appropriate upper and lower limits.

Further, in addition to the above range, as shown in FIG. 16, the work hardening coefficient E _h (volume fraction f _M and work hardening coefficient E is based on the rate of change of the yield stress σ _Y with respect to the volume fraction f _M of the hardening region. estimating or calculating the relationship between _h) (S301), so the estimated or calculated work hardening coefficient E _h is equal to or greater than a predetermined value may be determined the range of the volume fraction f _M (S302) . For example, the difference between the work hardening coefficient E _h when the volume fraction f _M is 1 and the work hardening coefficient E _h when the volume fraction f _M is f _M-max is ΔE _h , 0 or more and 1 or less. May be defined as the improvement coefficient n, and a value obtained by adding n × ΔE _h to the work hardening coefficient E _h when the volume fraction f _M is 1 may be determined as a predetermined value. Therefore, this predetermined value may be the work hardening coefficient E _h when the volume fraction f _M of the hardening region is 1. Instead of the work hardening coefficient E _h represented by the above formula (21), another work hardening index including at least the yield stress σ _Y as a variable may be used.

The relationship between the yield strength σ _{h of} the steel sheet and the volume fraction f _M of the hardened region is expressed by a linear function, and the relationship between the yield stress σ _{Y of the} steel plate and the volume fraction f _M of the hardened region is expressed by a quadratic function. Then, it is possible to determine the range of the volume fraction f _M of the most conveniently cured area. In this case, instead of the constant b, it can be determined the range of the volume fraction f _M of the hardened region by using the constant a, the constant a as shown in the following formula (42) with a constant b Since it can be expressed (constant a is a dependent variable of constant b), the use of constant a is considered the same as the use of constant b. Similarly, using a dependent variable that can be the rate of change of the yield stress sigma _Y (e.g., rate of change of the yield stress sigma _Y volume fraction f _M is for the volume fraction f _M in the case of 1) to the volume fraction f _M In this case, it is considered that the rate of change of the yield stress σ _Y with respect to the volume fraction f _M is used. That is, as shown in the following equation (43) obtained by substituting the following equation (42) into the primary differential equation of the above equation (29), the change rate of the yield stress σ _Y at an arbitrary volume fraction f _M is expressed as follows. Even if it uses, b which is the rate of change of the yield stress σ _Y when the volume fraction f _M is 0 can be obtained. For example, even if the rate of change of the yield stress σ _Y when the volume fraction f _M is 1 is defined as d as shown in the following formula (44), d is used from the following formula (45). b can be obtained.

Furthermore, it is possible to change rates of yield stress sigma _Y for the volume fraction f _M of the hardened region to determine the range of the volume fraction f _M, based on the volume fraction f _M when a predetermined condition is satisfied. For example, considering that the work hardening coefficient E _h draws a downward convex curve with respect to the volume fraction f _M of the hardening region, the volume fraction of the work hardening coefficient E _h represented by the above formula (21). f _M first derivative is zero becomes like the volume fraction _{f M} for, i.e., the volume fraction that satisfies the above formula (36) to (boundary hardening rate) _{f M} determined the maximum volume fraction _{f M-max} Also good. In this case, the range of the volume fraction f _M of the hardened zone, for example, can be determined in the range that satisfies the above formula (37) to Formula (41). Here, regarding the equations (37) to (41), the minimum volume fraction f _M-min (other than 1) can be determined using the above equation (34). Further, the above _- described ΔE _h is determined from the minimum volume fraction f _M-min and the maximum volume fraction f _M-max determined by Expression (34) and Expression (36), and processing is performed using the above-described improvement coefficient n. as hardening coefficient E _h is the volume fraction f _M is equal to or greater than the value obtained by adding the n × Delta] E _h to work-hardening coefficient E _h when it is 1, may determine the range of the volume fraction f _M.
That is, as shown in FIG. 18, determines the maximum volume fraction _{f M-max} volume fraction _{f M} based on the rate of change of the yield stress sigma _Y for the volume fraction _{f M} of the hardened region (S321), the volume the minimum value of the range of the fraction f _M determined to a predetermined value smaller than the maximum volume fraction f _M-max (S322), if determined the maximum value of the range of the volume fraction f _M to 1 or less, or less than 1 Good (S323). Further, after determining the minimum value of the range of the volume fraction f _M of the hardened region (S322), the maximum value of the range of the volume fraction f _M determined to a predetermined value greater than the maximum volume fraction f _M-max It is good (S324).

In addition, regarding the relationship between the yield stress σ _{Y of the} steel sheet and the volume fraction f _M of the hardened region, the same function (for example, a linear function such as a quadratic function) within a range of the volume fraction f _M of 0 to 1 is used. The range may be divided into a plurality of ranges, and different functions may be used for each of these ranges. However, in order to utilize the change of the rate of change of the yield stress sigma _Y with respect to the volume fraction f _M, when the volume fraction f _M uses the same function within the 0 to 1, the function within this range it is necessary to be able to differentiation upstairs by the volume fraction f _M. In addition, as a method of dividing the range of 0 to 1 into a plurality of ranges and using different functions for each of these ranges, for example, interpolation functions by various interpolation methods (for example, spline interpolation) (interpolation function is linear (line graph)) Can be used). In this case, measured data (for example, 5 points or more) can be directly used as data in the database.
Similarly, regarding the relationship between the flow stress σ _h of the steel sheet and the volume fraction f _M of the hardened region, the same function (for example, a linear function such as a linear function) within a range of the volume fraction f _M of 0 to 1 is used. ) May be used, or this range may be divided into a plurality of ranges, and different functions may be used for each of these ranges.

Here, as described above, to determine the scope of the volume fraction f _M of the hardened region with a small number of measurements as possible (number of tests prepared number and tensile strength of the test piece) has a yield strength sigma _h of steel the relationship between the volume fraction f _M of the hardened region was expressed by a linear function, it is preferred to express the relationship between the volume fraction f _M of the yield stress sigma _Y and curing area of the steel sheet by a quadratic function.

In the above embodiment, σ _h (f _M ) is defined as a yield strength when a plastic strain of 5% occurs, but the plastic strain corresponding to the yield strength is not necessarily limited to 5%. If it is larger than 0%, it may not be 5%. For example, as shown in FIG. 7, FIG. 9A, and FIG. 9B, σ _h (f _M ) can be defined as a yield strength when 1% plastic strain occurs. Therefore, when a yield strength when a predetermined plastic strain occurs (or a stress necessary to cause plastic deformation from a state where the predetermined plastic strain occurs) is defined as a flow stress, σ _h (f _M ) _{Represents the} flow stress, σ _hM represents the flow stress in the hardened region, and σ _h0 represents the flow stress in the non-hardened region (untreated structural material).
Here, the flow stress is greater than the strain corresponding to the yield stress (that is, the plastic strain is greater than 0) and is less than the uniform elongation strain (for example, the maximum nominal strain). The stress at is used. As a general evaluation, this flow stress is preferably 5%.

  In the above, the structural material 10 is locally heated and cured by laser heat treatment. However, local hardening of the structural material 10 is not necessarily performed by laser heat treatment, and may be performed by other heat treatment. In any case, the hardness of the region hardened by the heat treatment is C for the carbon content of the structural material 10 as a steel material, Si for the silicon content, Mn for the manganese content, Ni for the nickel content, and Cr for the chromium content. When the molybdenum content is defined as Mo, the niobium content is defined as Nb, and the vanadium content is defined as V, the hardness is preferably equal to or higher than the reference hardness (Vickers hardness) Hv calculated by the following formulas (45) and (46).

Further, in the embodiment shown in FIGS. 2 and 3, the laser heat treatment is performed on the effective width region 15 around the two bent portions 12b and 12c, and the effective width region 15 around the other two bent portions 12a and 12d. No laser heat treatment is performed. However, the laser heat treatment may be performed also on the effective width region around the other two bent portions, or the laser heat treatment is performed only on the effective width region 15 around one of the two bent portions 12b and 12c. May be performed. In other words, in the present invention, when the structural material has a plurality of bent portions, the effective width region including at least one bent portion may be heat-treated at the volume fraction f _M as described above.

In the following, a heat-treated structural material according to an embodiment of the present invention will be described.
The heat-treated structural material according to this embodiment includes at least one bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction, as in the above-described embodiment. Yes. Therefore, the heat-treated structural material according to the present embodiment includes a structural material having a shape as illustrated in FIGS. Further, for the above-mentioned effective width region, the volume fraction f _M of the above-described hardening region is less than 1, and the volume fraction determined based on the rate of change of the yield stress σ _Y with respect to the volume fraction f _M f is included in the range of _M.
Therefore, the heat-treated structural material according to the present embodiment can exhibit a higher deformation suppressing ability than the conventional one while maintaining the lowest possible cost.
Moreover, the scope of the volume fraction f _M of the hardened region, be determined based on the rate of change of the yield stress sigma _Y for the volume fraction f _M when the value of the volume fraction f _M as described above is 0 Can do. The scope of the volume fraction f _M is a range in which the work hardening coefficient E _h calculated based on the rate of change of the yield stress sigma _Y for the volume fraction f _M is determined to be equal to or greater than the predetermined value. In particular, this predetermined value is preferably a value of work-hardening coefficient E _h when the volume fraction f _M is 1, greater than work hardening coefficient E _h when the volume fraction f _M is 1 More preferably it is a value. Further, the range of the volume fraction f _M of the hardened zone (lower) is preferably at above formula (32) with minimum volume fraction f _M-min or more represented. Similarly, the range of the volume fraction f _M of the hardened zone (upper limit) is preferably at the maximum volume fraction f _M-max or less represented by the above formula (33). In addition, three JIS No. 5 test pieces were sampled from the flat part of the structural material, and two test pieces were set so that the volume fractions f _M of the cured regions of these test pieces were 0, 1, and 0.5, respectively. After performing heat treatment on the three specimens, a tensile test of these three specimens is performed to obtain the required mechanical strength, and the relationship between the yield stress σ _Y and the volume fraction f _M is calculated by the least square method (30 ) Constant b may be determined.

  Moreover, what is necessary is just to define a flow stress as a yield strength when 5% of plastic strain arises. Furthermore, in order to determine the effective width region, the effective width e may be defined by the above formula (15), the above formula (17), the above formula (18B), or the following formula (47). In the case where the effective width e is defined by Expression (15), the finite element method may be used. Further, equation (47) is derived from the above equations (18A) to (20) assuming that the plate buckling coefficient k is 4.

  In addition, the cured region (the region cured by the heat treatment) can be obtained by the same method as in the above embodiment. That is, the hardened region can be determined as a region having a Vickers hardness or higher calculated by the above formulas (45) and (46). The heat treatment is preferably performed by a laser. The history of heat treatment by this laser can be confirmed by observing the structure of the cross section of the structural material.

  The thickness is 1.0 mm, the yield stress is 301 MPa, the tensile strength is 457 MPa, the elongation is 39%, the carbon content is 0.09%, the silicon content is 0.02%, and the manganese content is 1.24%. Eleven JIS5 test pieces were collected from one 440 MPa class steel plate BP. Of these test pieces, ten test pieces were subjected to laser heat treatment in a plurality of passes so as to have a predetermined volume fraction in the longitudinal direction (tensile direction) of the test piece, and the hardening region with respect to the effective width region. Test pieces having a volume fraction of 0.1 to 1 (increments of 0.1) were prepared. For the laser heat treatment, a carbon dioxide laser was used, the laser output was controlled to 5 kW, and the heat treatment speed was controlled to 12 m / min. Further, a tensile test was performed on these 11 test pieces to evaluate yield stress and tensile stress.

As a result, determined from the untreated specimen, the yield stress sigma _Y0 of uncured regions 301 MPa, the plastic strain epsilon _p Strength of uncured regions of the time of grant of 0.05 (0.0537) a sigma _h0 to 447MPa did. Similarly, the test piece volume fraction 1 (100%), 794MPa yield stress sigma _YM curing area, 0.05 (0.0537) plastic strain ε strength of hardened zone during application of _p sigma _hM of Was determined to be 1017 MPa. Furthermore, each yield stress obtained from 11 specimens is plotted against the volume fraction, and the constant b is set to 350 MPa by applying the least square method using the above equation (29) as a regression equation to this plot. Decided. Here, with respect to 3 plots of the yield stress of an untreated specimen, the yield stress of a specimen having a volume fraction of 0.5 (50%), and the yield stress of a specimen having a volume fraction of 1 (100%) Thus, it was confirmed that the same constant b was obtained even when the least square method was performed.
The value of the b (b = 350 MPa), and was determined by tensile test △ sigma _h and △ sigma _Y value _{(△ σ h = 569.2MPa, △} σ Y = 493.0MPa) and into equation (32) As a result, f _M-min = 53.3%.
Further, as a result of substituting the values of b, Δσ _h and Δσ _Y into the equation (33), f _M-max (f _M-max = 76.6%) was obtained.

In addition, as a result of calculating the effective width e using the above formulas (18A) to (20) (or the above formula (47)), 19.2 mm was obtained as the effective width e. Here, the plate buckling coefficient k, which is a coefficient corresponding to the plate shape or the like, is 4, the plate width w is 60 mm, the plate thickness t is 1.0 mm, the yield stress σ _Y0 is 301 MPa, and the elasticity The rate E is 180 GPa. For the plate width w, an average value (60 mm) of the height (50 mm) and the top width (70 mm) of the structural material shown in FIG. 14 was used as a representative value.

  Further, the steel plate BP (FIG. 13A) was bent to produce an untreated structural material 10 having a shape as shown in FIG. 13B. The untreated structural member 10 includes five flat portions arranged so that the cross section has a hat shape as shown in FIG. 14, and among these, the vertical cross section of each side including the three flat portions 11 at the center. The side length was 50 mm, 70 mm, and 50 mm.

  Another structural member 20 having a flat plate shape was spot-welded to the untreated structural member 10 thus manufactured, and a structural member assembly as shown in FIG. 13C was manufactured. The spot welding S was performed at an interval of 30 mm in the longitudinal direction at the center in the width direction of the flat portion constituting the flange portion. Further, the distance from the longitudinal end portion (end portion on the impact applying side, hereinafter referred to as “impact application side end portion”) to the first spot welding was 15 mm.

The structural material assembly thus manufactured was subjected to a plurality of laser heat treatments in the longitudinal direction (tensile direction) of the test piece with a carbon dioxide laser. The laser output was controlled to 5 kW, and the heat treatment speed was controlled to 12 m / min. The laser output and the heat treatment rate in the laser heat treatment were controlled in the same manner in the following examples. Test No. 1, laser heat treatment was performed over the entire region of 19.2 mm from the bent portion shown in black in FIG. 14, that is, over the entire effective width region. Therefore, in this case, the volume fraction of the hardened area with respect to the effective width area was 100%. At this time, the work hardening coefficient E _h calculated by the above formula (31) using the above data was 4155.8 MPa (where ε _p = 0.05).

  Vickers hardness was measured for the places where the laser heat treatment was performed. The Vickers hardness of the untreated structural material was 140, whereas the Vickers hardness after the laser heat treatment was 306, and it was confirmed that the hardened region was sufficiently quenched and hardened.

  The structural material assembly is installed so that the longitudinal direction of the structural material assembly subjected to the laser heat treatment is aligned with the vertical direction, and the impact application side end portion is upward, and immediately above the structural material assembly. An impact test was performed by dropping a 300 kg falling weight from a height of 2 m.

When performing the impact test, a load meter (load cell) was installed immediately below the structural material assembly, and the load history after the falling weight contacted the structural material assembly was measured. At the same time, the displacement history of the falling weight after the falling weight contacted the structural material assembly with the laser displacement meter (the time history of the falling weight of the falling weight after the falling weight contacted the structural material assembly) was also measured. . A load-strain diagram was created based on the load history and displacement history thus measured. The initial peak reaction force was calculated from this load-strain diagram, and the initial peak reaction force was calculated by dividing the initial peak reaction force by the cross-sectional area (340 mm ² ) of the structural material assembly. The initial peak reaction force at this time was 146.9 kN, and the initial peak stress was 432.0 MPa.

Test No. In No. 2, the above test no. In the same manner as in Example 1, an untreated structural material assembly was manufactured, and this structural material assembly was subjected to laser heat treatment. Laser heat treatment was performed so that the volume fraction of the hardened region with respect to the effective width region was 76.6%. At this time, the work hardening coefficient E _h calculated by the above formula (31) using the above data was 4301.6 MPa (where ε _p = 0.05).

  For the structural material assembly thus subjected to the laser heat treatment, the above test No. The impact test was conducted in the same manner as in Example 1, and the initial peak reaction force and the initial peak stress were calculated based on the test results. The initial peak reaction force at this time was 150.6 kN, and the initial peak stress was 443.0 MPa.

Test No. No. 3, the above test no. In the same manner as in Example 1, an untreated structural material assembly was manufactured, and this structural material assembly was subjected to laser heat treatment. Laser heat treatment was performed so that the volume fraction of the hardened region with respect to the effective width region was 53.3%. At this time, the work hardening coefficient E _h calculated by the above formula (31) using the above data was 4155.8 MPa (where ε _p = 0.05).

  For the structural material assembly thus subjected to the laser heat treatment, the above test No. The impact test was conducted in the same manner as in Example 1, and the initial peak reaction force and the initial peak stress were calculated based on the test results. The initial peak reaction force at this time was 146.3 kN, and the initial peak stress was 430.1 MPa.

  The above results are summarized in Table 1 below.

From Table 1, the initial peak stress when the volume fraction (f _M ) of the hardened area with respect to the effective width area is 53.3% (= f _M-min ) is obtained when this volume fraction is 100%. It can be seen that the initial peak stress is almost the same. The initial peak stress when the volume fraction of the hardened area with respect to the effective width area is 76.6% (= f _M-max ) is the initial peak stress when the volume fraction is 53.3% and 100%. It can be seen that it is higher than the peak stress. Thus, test no. 3, test no. Test No. 1 at a lower cost compared to 1. 1 can be obtained. In addition, Test No. 2, test no. Test No. 1 at a lower cost compared to 1. A deformation suppression capability higher than 1 can be obtained.

  By subjecting an untreated structural material to heat treatment at an appropriate location and locally curing the structural material, it is possible to provide a structural material with sufficiently improved deformation suppressing ability.

DESCRIPTION OF SYMBOLS 10 Structural material 11 Flat part 12 Bending part 15 Effective width area | region 20 Structural material

This invention was made | formed based on the said knowledge, and the summary is as follows.
(1) A heat treatment method for a structural material according to one aspect of the present invention includes a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction. An effective width e of the bent portion is determined; and an area including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region. , when the ratio of the hardened region was defined as the curing rate f _M by heat treatment of the effective width region, a rate of change of the yield stress sigma _Y for hardening rate f _M, the yield stress for the hardening rate f _M sigma primary differential value of _Y, or, based on the rate of increase in yield stress sigma _Y for hardening rate f _M to determine the scope of the curing rate f _M; the effective of the structural member so as to satisfy the range of the hardening rate f _M Heat treatment is performed on the width region.

(2) In the heat treatment method for a structural material according to (1) above, the change rate is a first derivative value of yield stress σ _Y with respect to the hardening rate f _M when the value of the hardening rate f _M is 0 , or, the measured value of the yield stress sigma _Y when the value of the measured value and the hardening rate f _M of the yield stress sigma _Y when the value of the hardening rate f _M is 0 is greater than 0 and 0.1 or less The increase rate of the yield stress σ _Y with respect to the hardening rate f _M obtained from

(3) In the heat treatment method for a structural material according to (2) above, the work hardening coefficient E _h calculated based on the rate of change is equal to or higher than the work hardening coefficient E _h when the hardening rate f _M is 1. so that, it may determine the extent of the curing ratio f _M.

(4) In the heat treatment method of the structural material according to the above (2), the difference of .DELTA..sigma _h between flow stress when the curing rate f _M and the flow stress when the curing rate f _M is 1 is 0 When the difference between the yield stress when the curing rate f _M is 1 and the yield stress when the curing rate f _M is 0 is defined as Δσ _Y , and the rate of change is defined as b, the curing rate f The range of _M may be f _M-min or more and less than 1 represented by the following formula (1).

(5) In the heat treatment method of the structural material according to the above (4), the range of the hardening rate _{f M} may be not less _{f M-max} represented by the following formula (2).

(6) In the heat treatment method of the structural material according to the above (1), the equal boundary hardening rate f _M and the rate of change of flow stress sigma _h determined in f _M-max the rate of change with respect to the curing rate f _M, it may determine the extent of the curing rate _{f M} on the basis of the _{f M-max.}

(7) In the heat treatment method of a structural material according to (6), the range of the hardening rate f _M, may determine the range satisfying the following formula (3).

(8) In the heat treatment method of a structural material according to (6), the range of the hardening rate f _M, work hardening coefficient E _h is, the work hardening coefficient E _h when the curing rate f _M is 1 It may be determined that the curing rate f _{M-min is} equal to or greater than 1 and less than 1 .

( 9 ) In the structural material heat treatment method described in (1) above, regarding the chemical components contained in the structural material, the mass percentage of carbon is C, the mass percentage of silicon is Si, the mass percentage of manganese is Mn, nickel When the mass percentage of Ni is defined as Ni, the mass percentage of chromium as Cr, the mass percentage of molybdenum as Mo, the mass percentage of niobium as Nb, and the mass percentage of vanadium as V, the region cured by the heat treatment is represented by the following formula: It may be a region equal to or higher than the Vickers hardness calculated by (5) and (6).

( 10 ) In the structural material heat treatment method according to (1), the heat treatment may be performed by a laser.

( 11 ) In the heat treatment method for a structural material according to (1), one pass of the heat treatment may be continuously performed over the entire length in the one direction.

( 12 ) The heat-treated structural material according to one aspect of the present invention is a structural material including a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction. A region including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region, and a ratio of the effective width region occupied by a region cured by heat treatment If you define a hardening rate f _M and the curing ratio f _M is less than 1, and a rate of change of the yield stress sigma _Y for hardening rate f _M, the yield stress sigma _Y for hardening rate f _M first-order derivative value, or, within the scope of the determined curing ratio f _M based on the rate of increase in yield stress sigma _Y for curing ratio f _M.

( 13 ) In the heat-treated structural material according to ( 12 ), the rate of change is a first derivative value of yield stress σ _Y with respect to the curing rate f _M when the value of the curing rate f _M is 0 , or, the measured value of the yield stress sigma _Y when the value of the measured value and the hardening rate f _M of the yield stress sigma _Y when the value of the hardening rate f _M is 0 is greater than 0 and 0.1 or less The increase rate of the yield stress σ _Y with respect to the hardening rate f _M obtained from

(14) In the heat-treated structural material according to (13), the range of the hardening rate f _M is, work hardening coefficient E _h is in the curing rate f _M 1 calculated based on the rate of change may be determined range such that the work hardening coefficient more than E _h when.

(15) In the heat-treated structural material according to (12), the difference of .DELTA..sigma _h between flow stress when the curing rate f _M and the flow stress when the curing rate f _M is 1 is 0 When the difference between the yield stress when the curing rate f _M is 1 and the yield stress when the curing rate f _M is 0 is defined as Δσ _Y , and the rate of change is defined as b, the curing rate f The range of _M may be f _M-min or more represented by the following formula (7).

The heat-treated structural material according to (16) above (15), the range of the hardening rate _{f M} may be not less _{f M-max} represented by the following formula (8).

( 17 ) In the heat-treated structural material according to ( 15 ), each flow stress may be defined as a proof stress when a 5% plastic strain occurs.

(18) In the heat-treated structural material according to (12), w perpendicular width in the one direction, the curing rate f _M is the yield stress when it is 0 sigma _Y0, the structural member one When the stress at each position in the width direction perpendicular to the one direction when the stress that gives the maximum stress in the direction of σ _Y0 is applied in the one direction is defined as σ _x , the effective width e may be defined by the following formula (9).

(19) In the heat-treated structural material according to (12), the thickness t, the Poisson's ratio [nu, the elastic modulus E, yield stress when the curing rate f _M is 0 and sigma _Y0 When defined, the effective width e may be defined by the following formula (10).

(20) (12) In the heat-treated structural material according to the thickness t, the w perpendicular width in one direction, the elastic modulus E, when the curing rate f _M is 0 When the yield stress is defined as σ _Y0 , the effective width e may be defined by the following formula (11).

( 21 ) In the heat-treated structural material described in ( 12 ) above, with respect to the chemical components contained in the structural material, the carbon mass percentage is C, the silicon mass percentage is Si, the manganese mass percentage is Mn, nickel. When the mass percentage of Ni is defined as Ni, the mass percentage of chromium as Cr, the mass percentage of molybdenum as Mo, the mass percentage of niobium as Nb, and the mass percentage of vanadium as V, the region cured by the heat treatment is represented by the following formula: The area | region more than the Vickers hardness computed by (12) and (13) may be sufficient.

( 22 ) In the heat-treated structural material according to ( 12 ), the heat treatment may be performed by a laser.

Claims (25)

A structural material heat treatment method comprising a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction,
Determining an effective width e of the bend;
A region including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region, and a region cured by heat treatment occupies the effective width region. when the ratio was defined as the curing rate f _M, to determine the scope of the curing rate f _M based on the rate of change of the yield stress sigma _Y for hardening rate f _M;
Performing heat treatment on the effective width region of the hardening rate f _M the structural member so as to satisfy the range of;
A method for heat-treating a structural material. The rate of change, the heat treatment method of structural material of claim 1, wherein the value of the hardening rate f _M is a value when it is zero. The rate of change work hardening coefficient E _h calculated based on so becomes a predetermined value or more, the heat treatment method of the structural member according to claim 2, characterized in that in determining the scope of the curing ratio f _M. The heat treatment method for a structural material according to claim 3, wherein the predetermined value is a work hardening coefficient E _h when the hardening rate f _M is 1. 5. Said curing yield stress when .DELTA..sigma _h, the cure rate f _M is 1 and flow stress when the curing rate f _M and the flow stress when the curing rate f _M is 1 is 0 f difference the .DELTA..sigma _Y and yield stress when rate f _M is _0, the rate of change when defined is b, the range of the hardening rate f _M is represented by the following formula (1) _M- The heat treatment method for a structural material according to claim 2, wherein the heat treatment method is at least _min and less than 1.

The scope of the hardening rate f _M is, the heat treatment method of a structural material according to claim 5, characterized in that at most f _M-max represented by the following formula (2).

The rate of change is determined equal boundary hardening rate _{f M} and the rate of change of flow stress sigma _h for hardening rate _{f M} to _{f M-max,} determines the range of the curing rate _{f M} on the basis of the _{f M-max} The method for heat-treating a structural material according to claim 1, wherein: Heat treatment method of the structural member according to claim 7, characterized in that the range of the hardening rate f _M, is determined in a range satisfying the following formula (3).

Heat treatment method of structural material of claim 7, wherein the range of curing rate f _M, is determined to f _M-min or more and less than 1 satisfies the following formula (4).

If the cure rate f _M is defined as the difference between the .DELTA..sigma _h between flow stress when curing ratio f _M and the flow stress of the case 1 is 0, the difference between the change rate this .DELTA..sigma _h predetermined value so that the following heat treatment method of the structural member according to claim 1, characterized in that in determining the scope of the curing ratio f _M. Regarding the chemical components contained in the structural material, the mass percentage of carbon is C, the mass percentage of silicon is Si, the mass percentage of manganese is Mn, the mass percentage of nickel is Ni, the mass percentage of chromium is Cr, the mass percentage of molybdenum. When the mass percentage of Ni, the mass percentage of niobium is defined as Nb, and the mass percentage of vanadium is defined as V, the area cured by the heat treatment is an area greater than or equal to the Vickers hardness calculated by the following formulas (5) and (6) The method for heat-treating a structural material according to claim 1, wherein:

  The heat treatment method for a structural material according to claim 1, wherein the heat treatment is performed by a laser.   The heat treatment method for a structural material according to claim 1, wherein one pass of the heat treatment is continuously performed over the entire length in the one direction. A structural material comprising a bent portion that extends in one direction of the structural material and is bent in a direction perpendicular to the one direction,
A region including the bent portion whose distance from the bent portion in a direction perpendicular to the one direction is within the effective width e is defined as an effective width region, and a ratio of the effective width region occupied by a region cured by heat treatment the when defined as hardening rate f _M, the curing rate f _M is less than 1, and, in the range of curing rate f _M which is determined based on the rate of change of the yield stress sigma _Y for hardening rate f _M Heat treated structural material characterized in that it is included. The rate of change, heat-treated structural material according to claim 14 wherein the value of the hardening rate f _M is a value when it is zero. Range of the hardening rate f _M is heat-treated according to claim 15, wherein the rate of change work hardening coefficient E _h calculated based on the characterized in that it is a range determined to be equal to or greater than the predetermined value Structural material. The heat-treated structural material according to claim 16, wherein the predetermined value is a work hardening coefficient E _h when the hardening rate f _M is 1. Said curing yield stress when .DELTA..sigma _h, the cure rate f _M is 1 and flow stress when the curing rate f _M and the flow stress when the curing rate f _M is 1 is 0 f difference the .DELTA..sigma _Y and yield stress when rate f _M is _0, the rate of change when defined is b, the range of the hardening rate f _M is represented by the following formula (7) _M- The heat-treated structural material according to claim 14, wherein the heat-treated structural material is _min or more.

The scope of the hardening rate f _M is heat-treated structural material according to claim 18, characterized in that at most f _M-max represented by the following formula (8).

  The heat-treated structural material according to claim 18, wherein each flow stress is defined as a yield strength when a plastic strain of 5% occurs. When the width dimension perpendicular to the one direction is w and the hardening rate f _M is 0, the yield stress is σ _Y0 , and the stress that causes the maximum stress in the one direction of the structural material to be σ _Y0 is the one direction. When the stress at each position in the width direction perpendicular to the one direction when defined as σ _x is defined as σ _x , the effective width e is defined by the following formula (9): Item 15. The heat-treated structural material according to Item 14.

When the thickness dimension is defined as t, the Poisson's ratio as ν, the elastic modulus as E, and the yield stress when the curing rate f _M is 0 as σ _Y0 , the effective width e is defined by the following formula (10). The heat-treated structural material according to claim 14, wherein:

The thickness t, w a vertical width dimension in the one direction, the elastic modulus E, when the yield stress of the case hardening rate f _M is 0 is defined as sigma _Y0, the effective width e is the following The heat-treated structural material according to claim 14, characterized in that it is defined by equation (11).

Regarding the chemical components contained in the structural material, the mass percentage of carbon is C, the mass percentage of silicon is Si, the mass percentage of manganese is Mn, the mass percentage of nickel is Ni, the mass percentage of chromium is Cr, the mass percentage of molybdenum. When the mass percentage of niobium is defined as Nb and the mass percentage of vanadium is defined as V, the area cured by the heat treatment is an area equal to or higher than the Vickers hardness calculated by the following formulas (12) and (13). 15. A heat treated structural material according to claim 14, characterized in that it is.

  The heat-treated structural material according to claim 14, wherein the heat treatment is performed by a laser.

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