Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
2001-07-02
2004-04-20
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S230000, C148S212000
Reexamination Certificate
active
06723175
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a formed member made of a steel sheet member, for example, a panel member or structural member for a body of vehicles such as automobiles, and a method for producing the same.
As is commonly known, in recent years, in the field of vehicles such as automobiles and the like, there is a strong request for improving fuel efficiency from the view point of environmental problems, in addition to a request for improving the cost efficiency in vehicle running. In particular, it is requested to further reduce the weight of a body which forms a major part of the vehicle weight. On the other hand, in order to keep and improve the passenger protecting performance in the case of a collision of the vehicle, it is necessary to ensure the body strength and the body rigidity of more than certain levels.
As described above, for a body of automobile, it is required to concurrently achieve ensuring of the body strength and the body rigidity for improving the safety in the case of a collision, and reducing of weight for improving the fuel efficiency.
As is well known in the art, in order to improve the passenger protecting performance in the case of a collision of the vehicle, it is necessary to increase the strength of the steel sheet itself used for forming a panel member and a structural member of the body, or improve the structural strength by increasing the thickness of the steel sheet or by providing an additional reinforcing member.
However, of these approaches, the latter (strengthening the structure) is difficult to be adopted in principle because increase of the thickness of the steel sheet and additional provision of a reinforcing member necessarily lead to increase of the body weight.
On other hand, as for the former approach (strengthening of the steel sheet itself), it is known that only steel sheets of classes having a tensile strength up to 440 MPa (so-called 440 MPa steel sheet, of which minimum value of the tensile strength in specification is 440 MPa, actually having a tensile strength in the range of about 440 to 550 MPa) can be applied in usual, because of the restriction in ensuring plastic formability of the steel sheet by press forming and the like, and that steel sheets of higher strength are extremely difficult to be formed.
In the case of the above range of the tensile strength (approximately 500 MPa or less), however, even if an attempt is made to decrease the thickness by using a steel sheet of high strength, it is difficult to achieve a sufficient effect in regard to reduction of body weight and reduction of production costs.
By the way, as one type of heat treatment for steel materials, it is conventionally known that by subjecting a steel material containing a nitriding element (so-called nitriding steel) to a certain heat treatment (nitriding treatment), it is possible to obtain a steel member with soft inside having extremely high hardness in the surface part and the vicinity.
This nitriding treatment essentially aims at improving the abrasion resistance of the surface part while restricting increase of the inside hardness of the member and ensuring its toughness, and does not intend to improve the strength of the whole member. However, in recent years, from the view of ensuring concurrently both reduced weight and strength/rigidity of the vehicle body, consideration is made to apply surface heat treatment techniques including such a nitriding treatment on structural members and reinforcing members for an automobile. For example, Japanese Unexamined Patent Laid-Open Publication HEI 11-279685 discloses that for the purpose of producing a high-strength press formed member having excellent impact absorptivity with satisfactory dimensional accuracy and a low production cost, a steel sheet of a specific composition is subjected to a nitriding treatment after press work is completed.
Thus, in the above prior art, an effort is made to improve the strength of a steel sheet itself while ensuring press formability by subjecting the nitriding steel sheet to a nitriding treatment after being press formed. In practice, however, the strength (tensile strength) obtained after the nitriding treatment is around 700 MPa maximum which is not satisfactory. Therefore, it is not possible to obtain a significant reduction of weight and production cost solely using the teachings of Japanese Unexamined Patent Laid-Open Publication HEI 11-279685.
To improve the performance of the vehicle in protecting passengers during a collision, the structural strength of the vehicle may be improved by increasing the thickness of the steel sheet used to form body panel member and structural member of the vehicle. However, this method of increasing structural strength by increasing the sheet thickness of such body parts and structural members is not preferred if such method does not significantly increase the rigidity of the whole body since vehicle body weight and the production cost is increased.
Therefore, it is desirable to provide a one-piece press formed member having portions with different properties from other portions of the formed member which may be, for example, a body panel member or a structural member.
With regards to such a request, a so-called tailored blank method is known as a method for forming a part having different properties from other parts in a one-piece member. The tailored blank method is to join, by welding and the like, blank materials (sheet-like members) having different properties for a predetermined specification (such as sheet thickness and/or heat treatment properties) to obtain one-piece sheet member (preform), and thereafter, to perform plastic forming such as press forming to obtain a final formed member.
By employing such a method, it is possible to obtain a formed member having a part therein of which properties is different from other parts for a predetermined specification in accordance with the properties of the blank material, while generally improving yield of the sheet-like material.
Therefore, by applying the tailored blank method to the production of bodies of automobiles, it is possible to improve yield of the steel sheet material, and to form a part having different properties from that of other parts in one-piece member constitutes a portion of vehicle body. Thereby, it is also possible to determine optimum material for the steel sheet material, sheet thickness, heat treatment, surface treatment and the like, in consideration of properties to be possessed (for example, strength, rigidity, corrosion resistance and the like) and the effects of reducing the body weight and reducing the production cost.
However, in the past, in a case where a formed member having a part of higher strength than that of the other parts is attempt to formed by applying the tailored blank method, the application of the method is limited to a certain range since only the steel sheet of which tensile strength is up to about 440 MPa can be utilized in view point of keep the press formability of the steel sheet as explained above. Therefore, it is hard to achieve sufficient effect in reducing the body weight and reducing the production cost.
In the tailored blank method, a plastic forming is conducted after welding the blank materials. Therefore, in particular, in a case where a blank material having high tensile strength is used, since the mechanical properties of the blank material is remarkably different from those of the other blank material, defects such as crack or break are apt to occur at the jointed portion. This may be a major factor to limit the application of the method.
By the way, as one of the methods for reinforcing a formed member, for example, a flame member used to a body of vehicles such as automobiles, it is known to fill foam material (foam urethane resin, for example) as filler material at least a part of inside the closed section of a formed member. Thereby, it is possible to improve the strength, the rigidity and the energy absorbing property for impact load, without large increases of weight as
Fukahori Mitsugi
Hanakawa Katsunori
Ishida Kyoso
King Roy
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
Wilkins, III Harry D.
LandOfFree
Formed member made of steel sheet and method for producing same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Formed member made of steel sheet and method for producing same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Formed member made of steel sheet and method for producing same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3190106