JP7368710B2 - Steel parts for vehicles - Google Patents

Description

The present invention relates to a steel member for a vehicle used in a vehicle such as an automobile.

In products such as automobiles that require high collision performance, collision performance can be improved by increasing the thickness of the members that make up the vehicle body. However, simply increasing the thickness of the members increases the weight of the vehicle, which is disadvantageous in terms of fuel efficiency and driving performance. For this reason, in vehicle components used in vehicles such as automobiles, it is desirable to both ensure superior crash performance and reduce weight by increasing the strength and reducing the thickness of the material. However, since high-strength materials generally have low ductility, they have worse formability than materials with high ductility, and may be difficult to process depending on the shape of the product. Therefore, from the viewpoints of crash performance, weight reduction, and formability, there is a need for vehicle members that are relatively soft when molded and hard when the vehicle body is completed.

As a method for increasing the hardness of a member, a method is known in which a member processed from a blank into a product shape is subjected to heat treatment to harden the surface. As a member whose surface has been hardened by heat treatment, Patent Document 1 describes a molded body in which specific portions have been subjected to nitriding treatment. Patent Document 2 discloses that a reinforced portion strengthened by quenching and a non-reinforced portion are arranged in the thickness direction, and the reinforced portions are provided alternately on the inside in the thickness direction and the outside in the thickness direction along the axial direction. The article describes a shock absorbing member that has been developed.

Japanese Patent Application Publication No. 2002-020854 Japanese Patent Application Publication No. 2011-131842

In the nitriding treatment as described in Patent Document 1, the material is hardened only up to several micrometers from the surface of the material, and there is room for improvement in terms of collision performance. The shock absorbing member described in Patent Document 2 is used as a member (for example, a front side member of an automobile) that is expected to receive input from the axial direction in the event of a collision, and its use as a member constituting a vehicle body is limited. be done. In addition, it is difficult to harden the impact absorbing member alternately on the inside in the thickness direction and the outside in the thickness direction along the axial direction of the shock absorbing member as in Patent Document 2, and it is difficult to achieve this with general hardening equipment. However, there are problems in terms of ease of manufacturing parts.

On the other hand, in order to increase the hardness of the member after molding, it is also conceivable to use a bake-hardenable steel plate (BH steel plate), which is intended to increase the hardness by baking hardening, as the material of the member, for example. However, there is room for improvement in terms of crash performance with the amount of bake hardening achieved by general bake hardenable steel plates.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the collision performance of a steel member for a vehicle without increasing the thickness or increasing the strength of the material.

One aspect of the present invention that solves the above-mentioned problems is a steel member for a vehicle having a work-hardened surface layer , in which a region of 50% or more of the surface area on at least one of the front side and the back side The hardness at a position 1/6 of the plate thickness from the surface in the direction is greater than the hardness at the center of the plate thickness, and the average Vickers hardness at the 1/6 position and the average Vickers hardness at the center of the plate thickness It is characterized by a hardness difference of 20 to 60 HV.
One aspect of the present invention from another perspective is a steel member for a vehicle, in which a region of 50% or more of the surface area on at least one of the front side and the back side is a portion of the plate thickness from the surface in the plate thickness direction. The hardness at the 1/6 position is greater than the hardness at the center of the plate thickness, and the hardness difference between the average Vickers hardness at the 1/6 position and the average Vickers hardness at the center of the plate thickness is 20 to 60 HV. The average Vickers hardness at the center of the plate thickness is 350 HV or more.

In steel parts for vehicles, collision performance can be improved without increasing the thickness or increasing the strength of the material.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the cross section parallel to the board|plate thickness direction of the vehicle member based on one Embodiment of this invention. FIG. 3 is a diagram for explaining the definition of a vehicle member. It is a figure for explaining the measuring method of the average Vickers hardness of a vehicle member. It is a figure for explaining the measuring method of the average Vickers hardness of a vehicle member. FIG. 3 is a diagram showing an analytical model for a four-point bending impact test simulation. It is a figure showing the maximum reaction force of each analytical model in simulation (1). It is a figure which shows the energy absorption amount of each analytical model in simulation (1). FIG. 3 is a diagram showing predicted fracture strains of each analytical model. It is a figure showing the maximum reaction force of each analytical model in simulation (2). It is a figure which shows the energy absorption amount of each analytical model in simulation (2).

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. Note that in this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals and redundant explanation will be omitted.

The term "vehicle steel member" as used herein refers to a member that can be used as a component of a vehicle body, such as an inner steel member that constitutes a floor panel, a roof panel, or a structural member (such as a side member or a cross member) of a vehicle. Examples include parts such as panels and outer panels. In addition, "vehicle steel members" include not only molded parts that have been processed into a product shape, but also intermediate products between a blank and a product-shaped part, and even the blank itself. Note that some vehicle body components are composed of composite members in which multiple members are joined together by spot welding, laser welding, etc., but "vehicle steel components" in this specification refer to such components. It refers to a single member that constitutes a composite member, rather than a composite member joined with other members such as. Further, for example, when a component of a vehicle body is composed of a composite member having a welded part, a "vehicle steel member" in this specification refers to a welded part W around the periphery and an edge 1a as shown in FIG. Refers to the members in the area surrounded by.

FIG. 1 is a diagram showing a cross section parallel to the thickness direction of a vehicle member 1 according to the present embodiment. In the vehicle member 1 of this embodiment, the hardness near the surface 2 is higher than the hardness at the center of the thickness, and the surface layer is harder than the center of the thickness. To be more specific, when the thickness of the vehicle member 1 is t, the average Vickers hardness at "a position 1/6 of the thickness t from the surface 2" in the thickness direction is equal to "the surface 2" in the thickness direction. The average Vickers hardness is larger than the average Vickers hardness at the "position from 1/2 of the plate thickness t", that is, at the "position of the plate thickness center C" of the vehicle member 1, and the difference in hardness is 5 to 60 HV. In addition, in the vehicle member 1 of this embodiment, the hardness of the surface layer portion is higher than the hardness of the center portion of the plate thickness on both the front surface 2 (front surface 2a) and the back surface 2 (back surface 2b). ing. That is, the average Vickers hardness at a position 1/6 of the plate thickness t from the front surface 2 (front surface 2a) in the plate thickness direction is 5 to 60 HV higher than the average Vickers hardness at the position of the plate thickness center C, and , the average Vickers hardness at a position 1/6 of the plate thickness t from the back surface 2 (back surface 2b) in the plate thickness direction is 5 to 60 HV higher than the average Vickers hardness at the position of the plate thickness center C. . In the vehicle member 1, the hardness of the surface layer portion is higher than the hardness of the center portion of the plate thickness on both the front surface 2 (front surface 2a) and the back surface 2 (back surface 2b) as in the present embodiment. However, from the viewpoint of improving collision performance, it is sufficient that the hardness of the surface layer portion on at least one of the front side and the back side is higher than the hardness of the center portion of the plate thickness.

As shown in Examples below, the vehicle member 1 in which the surface layer is 5 to 60 HV harder than the central part of the plate thickness can improve the collision performance without increasing the plate thickness or increasing the strength. That is, according to the vehicle member 1 of the present embodiment, the collision performance can be improved without increasing the thickness of the plate, so that the vehicle member 1 has the same or better collision performance than the conventional vehicle member 1. Also, the plate thickness can be made thinner than that of the conventional vehicle member 1, and the weight of the vehicle can be reduced. In addition, according to the vehicle member 1 of the present embodiment, the collision performance can be improved without increasing the strength of the material, so that the vehicle member 1 has the same or higher collision performance than the conventional vehicle member 1. Also, it is possible to use a material with lower strength than that of the conventional vehicle member 1, and moldability is improved. Therefore, the vehicle member 1 of this embodiment is a member that is well-balanced in crash performance, weight reduction, and formability.

In order to obtain the effect of improving collision performance as described above, the average Vickers hardness at the position of 1/6 of the plate thickness t in the plate thickness direction starting from the surface 2 must be set at the plate thickness center C. The hardness must be greater than the average Vickers hardness at the position, and the difference in hardness must be 5 to 60 HV. If the hardness difference is less than 5 HV, a sufficient effect of improving collision performance cannot be obtained. From the viewpoint of effectively improving collision performance, the hardness difference is preferably 10 HV or more, more preferably 20 HV or more, and even more preferably 30 HV or more. On the other hand, if the hardness difference exceeds 60 HV, the ductility of the surface layer will decrease, and there is a concern that cracks will occur in the surface layer during a collision. In order to further reduce such concerns about cracking, the hardness difference is preferably 50 HV or less, more preferably 40 HV or less. Further, in order to easily obtain the effect of improving collision performance, it is preferable that the average Vickers hardness at the position of the plate thickness center C of the vehicle member 1 is 350 HV or more.

In addition, in the vehicle member 1, the area where the hardness difference between the position of 1/6 of the plate thickness t and the position of the plate thickness center C is 5 to 60 HV is the front surface 2 (front surface) of the vehicle member 1. An area that is 50% or more of the surface area of 2a), or an area that is 50% or more of the surface area of the back surface 2 (back surface 2b), or an area that is 50% or more of the surface area of the front surface 2 (front surface 2a). And it needs to be 50% or more of the surface area of the back surface 2 (back surface 2b). That is, the area where the hardness difference is 5 to 60 HV needs to be 50% or more of the surface area of the surface 2 on at least one of the front side and the back side . If the region having a hardness difference of 5 to 60 HV is less than 50% of the surface area, a sufficient effect of improving collision performance cannot be obtained. From the viewpoint of effectively improving collision performance, it is preferable to have as many regions as possible having the above-mentioned hardness difference. Therefore, it is more preferable that the area having the hardness difference is 75% or more of the surface area of the vehicle member 1, and even more preferably 100%. Note that when the vehicle member 1 is attached to the vehicle body, the region of the vehicle member 1 having the above hardness difference does not necessarily have to be located on the collision side. For example, in the front side member of an automobile, even if the parts of the vehicle member 1 having the hardness difference described above are concentrated in the rear part of the front side member, the impact load will also be applied to the rear part of the front side member in the event of a frontal collision. is input, the collision performance is improved compared to the case where there is no part having the above-mentioned hardness difference.

(Method of calculating average Vickers hardness)
The average Vickers hardness of the vehicle member 1 is calculated as follows. First, as shown in FIG. 3, the vehicle member 1 is cut at an arbitrary position. Note that when the vehicle component 1 is welded to another component, the vehicle component 1 can be welded at any position in the region excluding the heat affected zone, which corresponds to the part where the material changes due to heat input during welding. cut. Next, as shown in Fig. 4, on the cut surface of the vehicle member 1, Vickers hardness according to JIS Z 2244:2009 was applied to 10 points located at 1/6 of the plate thickness t from the surface 2 in the plate thickness direction. The Vickers hardness was measured point by point. However, the 10 hardness measurement points are spaced 3 mm apart from each other. The Vickers hardness of a total of 10 points measured using the above procedure is arithmetic averaged, and the calculated average value of Vickers hardness is defined as the "average Vickers hardness at a position 1/6 of the plate thickness t from surface 2" in this specification. "Hardness" Similarly, the Vickers hardness was measured at each of the 10 points at the center of the plate thickness, and the Vickers hardness was obtained by taking the arithmetic average of the 10 measured Vickers hardnesses. The average value of is defined as "average Vickers hardness at the position of plate thickness center C" in this specification.

Then, the average Vickers hardness was measured at 20 randomly selected locations on the vehicle member 1, and at 15 or more locations, the average Vickers hardness was measured at 1/6 of the plate thickness t and at the center of the plate thickness C. If the hardness difference at the position is 5 to 60 HV, then statistically speaking, "In an area of 50% or more of the surface area, the hardness difference between the position of 1/6 of the plate thickness t and the position of the plate thickness center C is 5 to 60 HV." It is estimated that 60 HV.

(Method of calculating surface area)
The surface area of the vehicle member 1 is calculated from the obtained 3D model by acquiring shape data of the vehicle member 1 using a coordinate measuring machine.

(Example of manufacturing method for vehicle parts)
In order to obtain a vehicle member 1 in which the hardness of the surface layer is 5 to 60 HV higher than the hardness of the center of the plate thickness, it is necessary to plastically deform and work harden the surface layer of the material. According to various analysis results made by the present inventor, the vehicle member 1 having the above-mentioned hardness difference imparts a plastic strain of 2% or more at a position 1/4 of the plate thickness t from the surface 2 in the plate thickness direction. It turns out that you can get it by doing this. Examples of methods for imparting such plastic strain include applying light reduction to a steel plate after rolling, and applying a larger than normal plastic strain to a steel plate using a leveler that corrects warping, waving, etc. of the steel plate. There is a way to give it. Note that the plastic strain applied by a normal leveler at a position of 1/4 of the plate thickness t is about 1%, and at this level of plastic strain, the work hardening of the surface layer where the hardness difference is 5 HV or more occurs. does not occur.

There are methods to increase the hardness of the surface layer by hardening or nitriding the steel plate, but in the case of quenching, the surface layer becomes 70 HV or more harder than the center of the plate thickness, making it difficult for members with such a difference in hardness. In this case, cracks in the surface layer portion are more likely to occur during a collision than in the vehicle member 1 of this embodiment. The same applies to the case of nitriding, and the surface layer becomes 70 HV or more harder than the center of the plate thickness. In addition, in the nitriding treatment, hardening is possible only to a depth of about several micrometers from the surface 2, so the hardness at the position of 1/6 of the plate thickness t and the hardness at the position of the plate thickness center C are approximately equal. The hardness distribution is different from that of the vehicle member 1 shown in FIG. For this reason, a member whose surface layer portion is hardened by nitriding cannot have the same crash performance as the vehicle member 1 of this embodiment.

For example, when a blank made of steel plate is press-formed into a desired product shape, work hardening may occur locally in the molded product, but in the case of parts that make up the body of an automobile, there are areas where work hardening occurs during the forming process. is an area that is approximately 10% of the surface area of the member. Even if the difference in hardness between the surface layer and the center of the plate thickness is 5 to 60 HV in this region, it will hardly contribute to improving the crash performance.

Although an example of the embodiment of the present invention has been described above, the present invention is not limited to this example. It is clear that those skilled in the art can come up with various changes or modifications within the scope of the technical idea described in the claims, and these naturally fall within the technical scope of the present invention. It is understood that it belongs to

<Simulation (1)>
A four-point bending impact test simulation was performed using the analytical model shown in FIG. The four-point bending impact test is one of the tests used to evaluate the collision performance of members. The test specimen 10 in this simulation is a member with a hat-shaped cross section, a height of 65 mm, a width of 100 mm, a length of 900 mm, and a plate thickness of 0.8 mm. As the material for the test specimen 10, Model 1 assumes a steel plate with a tensile strength of 1470 MPa, and Models 2 to 7 assume a steel plate with a tensile strength of 1180 MPa. The steel plates of models 1 and 2 have uniform hardness over the entire material area, and the steel plates of models 3 to 7 have a hardness that is 1/6th of the thickness from the surface in the thickness direction and a hardness at the center of the thickness. It is larger than. The difference in hardness between the hardness at 1/6 of the plate thickness from the surface and the hardness at the center of the plate thickness is 5HV for model 3, 20HV for model 4, 40HV for model 5, 60HV for model 6, and 80HV for model 7. It has become. Further, in the test specimens 10 of Models 3 to 7, the regions having the above-mentioned hardness differences are the entire front surface and the entire back surface of the test specimen 10.

Using these models, a simulation was performed with the bending span set to 700 mm and the impact speed of the impactor 20 set to 36 km/h. The results are shown in FIGS. 6 and 7. In FIG. 6, the maximum reaction force of each analytical model against an impact is compared, and in FIG. 7, the amount of energy absorbed by each analytical model at the time of a 20 mm stroke of the impactor 20 is compared.

As shown in FIGS. 6 and 7, the maximum reaction force and energy absorption of Models 3 to 7 were equal to or higher than Model 1, which has higher tensile strength than Models 3 to 7. That is, the test specimens 10 of Models 3 to 7 have crash performance equivalent to or better than the test specimen 10 of Model 1, which uses a steel plate with one grade higher strength.

Next, a tensile test simulation was performed based on the material properties assumed from the hardness of each model, and the fracture strain predicted from the results of the tensile test simulation was compared with the maximum strain in the four-point bending impact test simulation. The results are shown in FIG.

As shown in Figure 8, if the hardness difference between the average Vickers hardness at 1/6 of the plate thickness and the average Vickers hardness at the center of the plate thickness exceeds 60 HV, the maximum strain in the impact test will be lower than the predicted breaking strain. It was bigger than that. Therefore, in a vehicle member in which the difference in hardness between the surface layer and the center of the plate thickness exceeds 60 HV, it is considered that there is an increased risk of cracking during a collision.

According to the results shown in Figures 6 to 8, the difference in hardness between the average Vickers hardness at a position 1/6 of the thickness from the surface in the thickness direction and the average Vickers hardness at the center of the thickness is 5 to 60 HV. If so, it becomes possible to improve the collision performance of the vehicle member while reducing the risk of cracking during a collision.

<Simulation (2)>
Based on model 5 of simulation (1) above, a model was created in which the ratio of the area where the hardness difference (40 HV) between the surface layer and the center of the plate thickness occurred to the surface area of the member was changed, and a four-point bending impact test simulation was performed. was carried out. In each model, the regions where the hardness difference of 40 HV occurs are randomly distributed. The simulation conditions are the same as those in simulation (1) except that the hardness distribution of the test specimen 10 is different. The results of this simulation are shown in FIGS. 9 and 10. In FIG. 9, the maximum reaction force of each analytical model against an impact is compared, and in FIG. 10, the amount of energy absorbed by each analytical model at the time of a 20 mm stroke of the impactor 20 is compared. 9 and 10, the "ratio A% of the surface area" on the horizontal axis means that a hardness difference of 40 HV occurs between the surface layer part and the thickness center part in the area of A% of the surface area on the front side of the test specimen 10, and This means that in a region of A% of the surface area of the back side of the test specimen 10, there is a hardness difference of 40 HV between the surface layer part and the center part of the plate thickness. Note that the hardness difference in areas other than the area where the hardness difference of 40 HV occurs is less than 5 HV.

As shown in FIGS. 9 and 10, when the area where the hardness difference occurs is 50% or more of the surface area, the performance is approximately the same as when the area is 100% of the surface area. In other words, from the perspective of improving collision performance, the area where the difference in hardness between the average Vickers hardness at the 1/6th position in the plate thickness direction and the average Vickers hardness at the center position of the plate is 5 to 60 HV is 50 HV of the member surface area. % or more is sufficient.

INDUSTRIAL APPLICATION This invention can be utilized as a vehicle member used for an automobile etc.

1 Vehicle member 1a Edge 2 Surface 2a Front surface 2b Back surface 10 Test specimen 20 Impactor C Plate thickness center t Plate thickness

Claims (3)

A steel member for a vehicle having a work-hardened surface layer ,
In an area of 50% or more of the surface area of at least one of the front and back surfaces, the hardness at a position 1/6 of the thickness from the surface in the thickness direction is greater than the hardness at the center of the thickness. ,
A steel member for a vehicle, wherein the difference in hardness between the average Vickers hardness at the ⅙ position and the average Vickers hardness at the center of the plate thickness is 20 to 60 HV. The steel member for a vehicle according to claim 1, wherein the hardness difference is 40 HV or less. A steel member for a vehicle,
In an area of 50% or more of the surface area of at least one of the front and back surfaces, the hardness at a position 1/6 of the thickness from the surface in the thickness direction is greater than the hardness at the center of the thickness. ,
The hardness difference between the average Vickers hardness at the 1/6 position and the average Vickers hardness at the center position of the plate thickness is 20 to 60 HV,
A steel member for a vehicle, wherein the average Vickers hardness at the center of the plate thickness is 350 HV or more.

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