JP6724452B2 - Quenched steel pipe member and method for manufacturing quenched steel pipe member - Google Patents

Description

The present invention relates to a quenched steel pipe member and a method for manufacturing a quenched steel pipe member.

There is known a three-dimensional hot bending and quenching (3DQ) technique capable of producing a steel pipe member having a complicated shape without using a die. 3DQ is used, for example, in the manufacture of automobile structural materials. The structural material manufactured by 3DQ processing is lightweight and has high strength.

3DQ is the following method. That is, while feeding the steel pipe as the material, the induction heating device locally heats the steel pipe up to the Ac3 transformation point or higher, and the cooling device on the downstream side of the induction heating device in the feeding direction of the steel pipe rapidly cools (quenching). To do. At this time, a local high temperature part is generated between the induction heating device and the cooling device in the steel pipe. The steel pipe is hot-deformed by applying a bending moment in an arbitrary direction to the high temperature portion, and quenching and fixing of the shape are performed by cooling with a cooling device.

As a bending moment imparting means for imparting a bending moment to the high temperature portion, a movable roller die arranged downstream of the cooling means in the steel pipe feeding direction as in Patent Document 1 and a downstream roller side in the steel pipe feeding direction as in Patent Document 2 Examples include a chuck and a manipulator attached to the end of the steel pipe, and a chuck and a manipulator attached to the end of the steel pipe on the upstream side in the steel pipe feeding direction as in Patent Document 3.

In addition to bending, 3DQ can also be twisted. Patent Documents 1 and 3 disclose that twisting can be performed by 3DQ. Patent Document 4 discloses a torsion member based on 3DQ.

International Publication No. 2006/093006 International Publication No. 2010/050460 International Publication No. 2011/007810 International Publication No. 2010/084898

By the way, a steel pipe member quenched by high-frequency induction heating such as 3DQ has high strength, but when a screw or a bolt is used for joining with another member, there is a problem that it is hard and difficult to process such as drilling. .. On the other hand, when a hole is drilled in the steel pipe member before quenching, the edge of the hole is overheated by induction heating during quenching, and cooling water enters the inside of the steel pipe member through the hole, resulting in uneven cooling. , There is a problem. It is difficult to solve the problem that occurs when a hole is drilled in a steel pipe member before quenching.

In view of the above circumstances, a main object of the present invention is to provide a technique for providing a hardened steel pipe member manufactured by quenching by high-frequency induction heating such as 3DQ with a portion that can be easily processed even after quenching.

The quenched steel pipe member of the first aspect of the present invention is a quenched tubular body, and a recessed portion in which the hardness of the bottom portion is lower than the hardness of the other portion of the body, the recessed portion being inwardly recessed from the body. And have.

A quenched steel pipe member according to a second aspect of the present invention is the quenched steel pipe member according to the first aspect, wherein the bottom of the recess is in an unquenched state.

The hardened steel pipe member of the third aspect of the present invention is the hardened steel pipe member of the first aspect or the second aspect, wherein the hardness difference between the bottom of the recess and the other portion of the main body is in the range of 150 to 250 HV. It is said to be inside.

A quenched steel pipe member according to a fourth aspect of the present invention is the quenched steel pipe member according to any one of the first to third aspects, wherein the recessed portion is formed at an end portion in the longitudinal direction of the main body.

In the method for manufacturing a quenched steel pipe member according to a fifth aspect of the present invention, a tubular body before quenching, in which a recess is formed in the outer periphery in a part in the longitudinal direction, is passed through an induction heating coil having a shape corresponding to the tubular body. After heating, quench and quench.

A method for manufacturing a quenched steel pipe member according to a sixth aspect of the present invention is the method for manufacturing a quenched steel pipe member according to the fifth aspect, wherein the recessed portion is formed in the tubular body before quenching by press working.

INDUSTRIAL APPLICABILITY The present invention can provide a technique of providing a steel pipe member manufactured by quenching by high-frequency induction heating such as 3DQ with a portion that can be easily processed even after quenching.

It is a perspective view of the hardening steel pipe member which concerns on one Embodiment of this invention. FIG. 3 is a cross-sectional view of the tubular body showing a state in which the tubular body before quenching is pressed by the pressing device used in the embodiment of the present invention. It is a perspective view showing the whole 3DQ device composition used by one embodiment of the present invention. It is a figure which shows a mode that the induction heating coil of the 3DQ device was seen from the downstream in the tube feeding direction to the upstream, together with the cross section of the tube inserted. It is a perspective view of the hardening steel pipe member which concerns on other embodiment of this invention. It is a graph which shows the heating temperature on the straight line Y and the straight line Z at the time of quenching the quenched steel pipe member of an Example. It is a graph which shows the hardness on the straight line Y and the straight line Z of the quenched steel pipe member of an Example.

Steel pipe members that have been quenched by high-frequency induction heating such as 3DQ (hereinafter referred to as "quenched steel pipe members" as appropriate) have high strength, so when using screws, bolts, etc. for joining with other members, they are hard and drilled. There is a problem that it is difficult to process such as. On the other hand, when welding is used for joining the hardened steel pipe member and another member, there is a problem that a portion of the hardened steel pipe member which receives heat of welding has low strength due to HAZ softening.

Here, quenching a steel material is to transform the steel material into austenite and then rapidly cool it to transform into martensite. The higher the ratio of the martensite structure to the structure of the steel material is, the higher the strength of the steel material is. Therefore, the above-described problem occurs in the quenched steel pipe member formed of the steel material. The present inventor thought that the above problems could be solved if the ratio of the martensite structure in the worked or welded portion of the quenched steel pipe member could be reduced or the martensite structure could be eliminated.

Generally, the martensitic structure of steel is generated by rapidly cooling the austenite phase. The proportion of this austenite phase can be adjusted by the heating temperature. That is, the ratio of the martensite structure can be adjusted by the heating temperature of the steel material. When the steel material is heated to the Ac3 transformation point or higher, the entire structure can be made into an austenite phase, and when it is rapidly cooled, the entire structure of the steel material can be made to have a martensite structure. On the other hand, if the heating temperature of the steel material is lower than the Ac1 transformation point, the austenite phase does not occur, and therefore the martensite structure does not occur even when the steel is rapidly cooled. When the steel material is heated to a temperature between the Ac1 transformation point and the Ac3 transformation point, the proportion of the austenite phase increases as the temperature rises, and the proportion of the martensite structure after quenching increases. That is, the inventor of the present invention considered that if a low heating temperature lower than the Ac3 transformation point can be selectively provided in the steel pipe member, a low martensite structure ratio can be selectively generated.

In order to partially generate a portion having a low heating temperature in the steel pipe member by heating by the high frequency induction heating, it is conceivable to reduce the induction current generated on the surface of the steel pipe member. Further, in order to partially lower the heating temperature in the longitudinal direction of the steel pipe member, it is conceivable to lower the electric power supplied to the induction heating coil when the portion for lowering the heating temperature passes through the induction heating coil. Further, in order to partially lower the heating temperature within the cross section of the steel pipe member, it is conceivable to partially widen the clearance between the portion for lowering the heating temperature and the induction heating coil. However, these methods cannot reduce the heating temperature locally, that is, in an island shape in both the cross section and the longitudinal direction of the steel pipe member.

Therefore, the present inventor, when induction heating the steel pipe member before quenching that is partially recessed, because the clearance with the induction heating coil in the recessed portion becomes wide, the heating temperature becomes lower than other locations. Thought. By using this technology, it is thought that it is possible to provide a steel pipe member that is manufactured by quenching by high-frequency induction heating such as 3DQ with a portion that is easy to process even after quenching and that is difficult to reduce strength by heat such as welding. To be

The embodiments of the present invention described below are based on the above findings.

A quenched steel pipe member 20 (hereinafter, appropriately referred to as "steel pipe member 20") and a method for manufacturing the quenched steel pipe member 20 according to an embodiment of the present invention will be described.

First, the steel pipe member 20 will be described.
As shown in FIG. 1, the steel pipe member 20 has a tubular main body 22 that has been quenched by high-frequency induction heating of a 3DQ device 40 (see FIG. 3) described below, and is in a state of being recessed inward with respect to the main body 22. The bottom portion 24A has a hollow portion 24 having a hardness lower than the hardness of other portions of the main body 22. In addition, in FIG. 1, the longitudinal direction of the steel pipe member 20 is indicated by an arrow X direction, and in FIG. 4, the longitudinal direction is indicated by an arrow A direction when viewed in a cross section of the steel pipe member 20 in a direction orthogonal to the longitudinal direction. Is indicated by the arrow B direction.

As shown in FIG. 1, the main body 22 has a rectangular tubular shape, and is configured to include four flat plate portions 22A and four corner portions 22B connecting the flat plate portions 22A to each other. The angle formed by the adjacent flat plate portions 22A is 90 degrees. As the main body 22, for example, an electric resistance welded pipe (electric resistance welded pipe) is used. The present invention is not limited to the above configuration, and a seamless tube may be used as the main body 22.

The main body 22 has a longitudinal intermediate portion 23 (see FIG. 3) bent by the 3DQ device 40.

The recesses 24 are formed on both ends 22C of the main body 22 in the longitudinal direction. The recessed portion 24 is formed by extruding the flat plate portion 22A of the main body 22 inward, and the bottom portion 24A is flat.

Further, at least the bottom portion 24A of the recess 24 is in an unquenched state. Therefore, the hardness of the bottom portion 24A is set to be lower than the hardness of the other portion (the portion other than the recessed portion) of the quenched main body 22. The term "hardness" as used herein refers to Vickers hardness. In addition, the “non-quenched state” here means a state where the martensite structure is 30% or less.

The hardness difference between the hardness (Vickers hardness) of the bottom portion 24A of the depression 24 and the hardness (Vickers hardness) of the other portion of the main body 22 is set within a range of 150 to 250 HV.

The hardness of the bottom 24A of the recess 24 is set within the range of 200 to 300 HV. The hardness of the bottom portion 24A of the recess 24 is more preferably set within a range of 210 to 230 HV that can secure at least a tensile strength of 660 to 720 MPa.

On the other hand, the hardness of the other parts of the main body 22 is set within the range of 350 to 450 HV. The hardness of the other portions of the main body 22 is more preferably set to 380 HV or more, which can secure at least a tensile strength of 1200 MPa or more.

Next, the press device 30 and the three-dimensional hot bending and quenching device 40 (hereinafter appropriately referred to as “3DQ device 40”) used for manufacturing the steel pipe member 20 will be described.

As shown in FIG. 2, the pressing device 30 is a device for pressing a rectangular tubular body 80 which becomes the main body 22 of the steel pipe member 20 after quenching. The pressing device 30 includes a fixed die 32 and a movable die 34 for pressing the recessed portions 24 at both ends 80C in the longitudinal direction of the tubular body 80. In this pressing apparatus 30, the fixed die 32 inserted into the end portion 80C of the tubular body 80 supports the flat plate portion 80A of the tubular body 80 from the inside, and the movable die 34 guides the flat portion 80A toward the concave portion 32A formed in the fixed die 32. The flat plate portion 80A of the body 80 is pushed out to process the hollow portion 24 in the tubular body 80.

As shown in FIG. 3, the 3DQ device 40 includes a feeding device 42, an induction heating coil 44, a cooling device 46, a movable roller die 50 as a bending moment applying device 48, and a support roller 52. ..

The feeding device 42 includes a chuck 43 that holds a rear end of the tubular body 80 (an upstream end portion in the feeding direction of the tubular body 80 (hereinafter, appropriately referred to as a “pipeline feeding direction”)), The tubular body 80 is sent in the longitudinal direction by pushing out the chuck 43. The chuck 43 has a structure capable of gripping the tubular body 80.

As shown in FIG. 4, the induction heating coil 44 is provided so that the tubular body 80 fed by the feeding device 42 is inserted therethrough. The induction heating coil 44 has a shape corresponding to the tubular body 80, specifically, a rectangular shape substantially similar to the cross section of the tubular body 80. When the tubular body 80 is inserted into the induction heating coil 44, the portion of the tubular body 80 inserted into the induction heating coil 44 is rapidly heated by the induction heating coil 44. Here, the clearance C1 between the hollow portion 24 of the tubular body 80 and the induction heating coil 44 is wider than the clearance C2 between the other portion of the tubular body 80 and the induction heating coil 44.

The cooling device 46 is provided downstream of the induction heating coil 44 in the tube feeding direction and in proximity to the induction heating coil 44. The cooling device 46 rapidly cools the tubular body 80 that has been rapidly heated by the induction heating coil 44. As a result, the tubular body 80 is quenched and the strength is improved.

The bending moment imparting device 48 (hereinafter appropriately referred to as “bending device” 48) is composed of a movable roller die 50 and a support roller 52. The support roller 52 is provided upstream of the induction heating coil 44 in the pipe feeding direction and close to the induction heating coil 44, and supports the pipe 80 movably in the pipe feeding direction. On the other hand, the movable roller die 50 is provided downstream of the cooling device 46 in the tube feeding direction, and is configured to be movable while holding the tube 80. As the movable roller die 50 moves, a bending moment is applied to the tubular body 80, and the supporting roller 52 receives a reaction force against the bending from the tubular body 80, whereby the softened high temperature portion generated in the tubular body 80 is deformed. ..

Next, a method of manufacturing the steel pipe member 20 using the press device 30 and the 3DQ device 40 will be described.

First, a pipe body 80 which will be the main body 22 of the steel pipe member 20 after quenching with the 3DQ device 40 is prepared.

Next, the depressions 24 are formed in both ends 80C of the tubular body 80 using the pressing device 30. Then, the tubular body 80 is set in the 3DQ device 40. Specifically, the tube body 80 is set on the feeding device 42, and the tube body 80 is gripped by the chuck 43.

Next, the tubular body 80 is induction-heated using the 3DQ device 40. At this time, as shown in FIG. 4, the clearance C1 between the hollow portion 24 of the tubular body 80 and the induction heating coil 44 is larger than the clearance C2 between the other portion of the tubular body 80 and the induction heating coil 44. Since it becomes wider, the heating temperature of the hollow portion 24 of the tubular body 80 becomes lower than the heating temperature of other portions of the tubular body 80. In this embodiment, the heating temperature of the other portion of the tubular body 80 is set to the Ac3 transformation point or higher, and the heating temperature of the recess 24 is set to the Ac3 transformation point or lower. The heating temperature of the hollow portion 24 of the tubular body 80 depends on the depth of the hollow portion 24 (that is, the size of the clearance C1), the pipe diameter, the thickness, the material (heat conductivity), and the current flowing through the induction heating coil 44. It can be adjusted by changing the size and frequency.
From the viewpoint of facilitating the drilling of the recess 24 of the steel pipe member 20, it is preferable that the heating temperature of the recess 24 of the tubular body 80 be less than the Ac3 transformation point. The lower the heating temperature of the hollow portion 24 of the tubular body 80, the lower the hardness after quenching, so the workability is improved. However, the lower the heating temperature of the hollow portion 24 of the tubular body 80 is, the lower the strength of the quenched tubular body 80 (that is, the steel pipe member 20) is. Therefore, the heating temperature of the hollow portion 24 of the tubular body 80 is equal to or higher than the Ac1 transformation point. It is preferable that the temperature is lower than the Ac3 transformation point.
On the other hand, from the viewpoint of suppressing the reduction in strength of the hollow portion 24 of the steel pipe member 20 due to welding, the heating temperature of the hollow portion 24 of the tubular body 80 is preferably set to the Ac1 transformation point or lower. However, since a slight amount of martensite structure is allowed, a heating temperature of Ac1 transformation point or higher is also allowed.

The induction-heated tube 80 is rapidly cooled by the cooling device 46. In this way, the pipe body 80 is quenched and the steel pipe member 20 is manufactured.

Next, the function and effect of the steel pipe member 20 of the present embodiment will be described.
In the steel pipe member 20, since the hardness of the bottom portion 24A of the recessed portion 24 is lower than the hardness of other portions of the main body 22, even if a screw or a bolt is used for joining with another member, the bottom portion 24A is drilled. And the like (post-processing) can be easily performed.

Further, in the steel pipe member 20, since the recessed portion 24 is in an unquenched state, even when welding is used for joining with other members, the portion that has received the heat of welding has low strength due to HAZ softening (tempering). Is suppressed.

Further, in the steel pipe member 20, since the bottom portion 24A of the hollow portion 24 is flat, the contact between the other member and the quenched steel pipe member 20 can be made into a surface contact, so that the load from the other member can be received by the surface. You can

Further, in the steel pipe member 20, the hardness difference between the Vickers hardness of the bottom portion 24A of the hollow portion 24 and the Vickers hardness of the other portion of the main body 22 is within the range of 150 to 250 HV. Therefore, the strength of the bottom portion 24A of the recess 24 and the hardness for facilitating the post-processing can be compatible.

In the steel pipe member 20, since the recessed portion 24 is formed at the end portion 22C in the longitudinal direction of the main body 22, for example, the recessed portion 24 is formed as compared with that formed at a bendable portion such as the intermediate portion 23. It is possible to suppress the strength reduction.

[Supplementary explanation of the above embodiment]
In the method for manufacturing the steel pipe member 20 of the above-described embodiment, the depression 24 is formed in the rectangular tubular body 80 using the pressing device 30, but the present invention is not limited to this configuration. For example, a square steel tubular body 80 may be formed by bending a steel plate having the recess 24 formed by pressing into a rectangular closed cross section. With the above configuration, the configuration of the pressing device can be simplified as compared with the case where the recessed portion 24 is formed by pressing the member (tube 80) having the closed cross section.

Further, in the steel pipe member 20 of the above-described embodiment, the recesses 24 are provided at both ends 22C of the main body 22, but the present invention is not limited to this configuration. For example, like the steel pipe member 60 of the modified example shown in FIG. 5, the recessed portions 64 may be provided in addition to the both end portions 62C of the main body 62. With the above-described configuration, the choices of sites for post-processing the steel pipe member 60 are increased. Further, in the steel pipe member 60, since the recessed portion 64 is provided in addition to the both end portions 62C of the main body 62, as described above, the steel plate on which the recessed portion 64 is formed by pressing is bent into a rectangular closed cross section to form a rectangular tubular shape. It is preferable to use a method of forming a tubular body. Note that reference numeral 62A in FIG. 5 indicates a flat plate portion of the steel pipe member 60, and reference numeral 62B indicates a corner portion of the steel pipe member 60.

Further, the feeding device 42 used in the above-mentioned embodiment pushes the rear end of the tubular body 80 by the chuck 43 and inserts the tubular body 80 into the induction heating coil 44 whose position is fixed. The feeding device is not limited to this. The feeding device may be any device that moves (relatively moves) the tubular body 80 in the longitudinal direction with respect to the induction heating coil 44. The coil 44 may be moved to feed the induction heating coil 44 in the longitudinal direction of the tubular body 80.

The bending moment applying device 48 used in the above embodiment is not particularly limited as long as the bending moment can be applied to the tubular body 80. The bending moment imparting device may be a chuck and a manipulator attached to the end portion 80C of the pipe body 80 on the downstream side in the feed direction, or a chuck attached to the end portion 80C of the pipe body 80 on the upstream side in the feed direction. It may be a manipulator.

Moreover, the steel pipe member 20 of one embodiment of the present invention can be used as various structural members for automobiles. For example, it can be used as a bumper reinforcing member provided along the vehicle width direction at a front end portion and a rear end portion of a vehicle, a front pillar, a door beam arranged in a door, and a seat reinforcing material.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above, and various modifications other than the above can be carried out without departing from the gist of the invention. Of course.

<Experimental example>
In order to verify the effect of the present invention, a quenched steel pipe member of an example having the same configuration as the quenched steel pipe member 20 of the embodiment was prepared and the following experiment was conducted. In the steel pipe member of the example, the clearance C1 is 7 mm and the clearance C2 is 3 mm. That is, the clearance C1 is set to a value that is at least twice the clearance C2.

(Experiment 1)
In Experiment 1, the heating temperature of the other portion and the heating temperature of the hollow portion of the steel pipe member of the example during quenching were measured. Specifically, the heating temperature along the straight line Y and the straight line Z passing through the recessed portion of the steel pipe member 20 shown in FIG. 1 was measured. The measured temperature is graphed and shown in FIG.

(Experiment 2)
In Experiment 2, the hardness of the other portion and the hardness of the recessed portion of the steel pipe member of the example after quenching were measured. Specifically, the hardness along the straight line Y and the straight line Z passing through the recessed portion of the steel pipe member 20 shown in FIG. 1 was measured. The measured hardness is shown in a graph in FIG. 7.

As shown in FIG. 6, on the straight line Y of the steel pipe member of the example, it can be seen that the heating temperature is lowered in the region corresponding to the depression. It is presumed that this is because the clearance between the hollow and the induction heating coil in the recessed portion was widened, so that the induction current was lowered and the heating temperature was lowered.

As shown in FIG. 7, on the straight line Z of the steel pipe member of the example, it can be seen that the hardness is reduced in the region corresponding to the depression. As shown in FIG. 6, this is because the heating temperature of the dent portion during quenching is lower than that of the other portion, and the dent portion is in an unbaked state, that is, the martensite structure is less than that of the other portion, or It is presumed that the martensite structure was lost.

From the above, by using the technique disclosed in the present invention, it is possible to provide a hardened steel pipe member manufactured by quenching by high frequency induction heating such as 3DQ with a portion that can be easily processed even after quenching. it is obvious.

20, 60 Steel pipe members (hardened steel pipe members)
22, 62 Main body 22C, 62C End part 24, 64 Dimple part 24A Bottom part 44 Induction heating coil 80 Tubular body 80C End part C1 clearance C2 clearance

Claims (5)

Having a quenched tubular body and a recess,
The recess portion, reaches the longitudinal end faces of the longitudinal is formed only on the end Rutotomoni the body of the main body a shape projecting and inward recesses inwardly relative to the body, bottom flat shape And a hardened steel pipe member in which the hardness of the bottom portion is lower than the hardness of other portions of the main body. The quenched steel pipe member according to claim 1, wherein a bottom portion of the recessed portion is in an unquenched state. The hardened steel pipe member according to claim 1 or 2, wherein the hardness difference between the bottom of the recess and the other portion of the main body is in the range of 150 to 250 HV. In the tubular body before quenching, there is a recessed portion that is recessed inward with respect to the tubular body only at the end portion in the longitudinal direction and protrudes inward and reaches the end face in the longitudinal direction of the tubular body and has a flat bottom. A method for manufacturing a quenched steel pipe member, comprising heating the formed tubular body through an induction heating coil having a shape corresponding to the tubular body, followed by quenching and quenching. The method for manufacturing a quenched steel pipe member according to claim 4 , wherein the recessed portion is formed in the pipe body before quenching by press working.

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