Abstract:
A bainite phase is formed in a body of a ferrous alloy by heating the workpiece to a temperature above an austenizing temperature of the alloy and thereafter contacting the workpiece with a quenching medium. Heat is input to the workpiece during at least a portion of the time it is in contact with the quenching medium. The quenching medium and/or the source of heat are regulated so that they work in combination to maintain the workpiece at a holding temperature which is no more than 350° C. but above the temperature at which the martensite phase forms in the alloy. After maintaining the alloy at the holding temperature for a predetermined period of time, it is cooled to ambient. Also disclosed are systems for implementing the method.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority of U.S. Provisional Patent Application Ser. No. 60/610,720 filed Sep. 17, 2004, entitled “Metal Forming Process with Resistance Heating.” 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to the fabrication of articles from ferrous metals. More specifically, the invention relates to the fabrication of articles from ferrous alloys having high concentrations of lower bainite. Most specifically, the invention relates to the fabrication of structural members for motor vehicles, and other articles of manufacture, fabricated from high strength steel having a high concentration of lower bainite.  
       BACKGROUND OF THE INVENTION  
       [0003]     Ultra high strength steel is used to fabricate various components of motor vehicles, building structures, and other articles of fabrication. In typical processes of the prior art, such articles are fabricated directly from martensite steel or they are fabricated from a low carbon steel which is converted to martensite during the production process. In either instance, steel products produced thereby have tensile strengths up to and over 200 ksi. However, such martensite structures are compromised in elongation properties (typically 4-6%). Bainite steels, particularly lower bainite steels, have very high strength but also have very good elongation properties, typically approaching 15%. The superior elongation properties greatly enhance the utility of such articles and/or simplify their fabrication.  
         [0004]     While bainite steels are advantageous in many applications, the preparation of steels having a significant bainite phase, and in particular a lower bainite phase, is difficult. In order to form the bainite phase, the steel alloys must be first heated to a temperature above their austenizing temperature then quenched to a particular holding temperature which is above the temperature at which the martensite phase forms, and they must be held at this temperature for relatively long periods of time so that the bainite phase may form. Lower bainite is formed when the steel is held at a temperature close to, but above, the temperature at which martensite forms. The holding step requires both the combination of fairly high temperatures and very good temperature control. As a consequence, this step is typically carried out in relatively large volumes of high temperature fluids such as molten salts or heated oils. These high temperature baths present significant hazards, consume relatively large amounts of energy, and occupy relatively large areas.  
         [0005]     As will be explained in detail hereinbelow, the present invention provides a process whereby bainite and in particular lower bainite containing articles may be fabricated in a process having a highly simplified holding step which eliminates the use of high temperature baths and the like, but still promotes the formation of the bainite phase. Accordingly, the process of the present invention may be readily adapted to the large volume, high speed fabrication of high strength steel alloys articles such as intrusion beams, frame members, body members and the like for motor vehicles, as well as components of building structures and other articles of manufacture.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0006]     Disclosed herein is a method for preparing a body of a ferrous alloy containing a high percentage of a bainite phase. In a first step of the method, a workpiece comprised of a ferrous alloy is heated to a temperature above an austenizing temperature of the alloy. Thereafter, the workpiece is contacted with a quenching medium. Heat is input to the workpiece at least during a portion of the time that it is in contact with the quenching medium so that the combination of the input heat and the quenching medium cooperate to maintain the workpiece at a holding temperature which is below 350° C. but above the martensite phase formation temperature of the alloy. The workpiece is maintained at this holding temperature for a period of time sufficient to form a bainite phase therein and thereafter the workpiece is cooled to ambient temperature. In particular embodiments of the present invention, the combination of the holding temperature and time are selected so that a lower bainite phase is formed in the workpiece. In specific instances, the thickness of the workpiece is no greater than 5 millimeters, and in some certain instances it is no greater than 3 millimeters.  
         [0007]     In some instances, the workpiece can be heated by resistance heating wherein an electrical current is flowed therethrough. The quenching medium may comprise a gas or a liquid, and it may be flowed across, sprayed onto, or flowed through the workpiece, while in other instances, the workpiece may be immersed in a bath of the quenchant fluid. In some instances, the workpiece may be subjected to a forming operation which either changes or maintains its shape. This forming operation may be implemented either prior to heating above the austenizing temperature, while the workpiece is at the austenizing temperature, while the workpiece is at the hold temperature, or after the process is complete. Also disclosed herein are articles made by the process of the present invention as well as apparatus for carrying out the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a graph showing one heating profile which may be implemented in accord with the present invention to produce a lower bainite phase in a workpiece; and  
         [0009]      FIG. 2  is a diagram of a temperature control circuit which may be utilized in the practice of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     The present invention is a metallurgical process which greatly simplifies the formation of bainite, and in particular lower bainite, containing articles by decoupling the function of the quench medium from that of the hold medium heretofore employed. As such, the present invention eliminates the need to utilize large, high temperature, liquid baths for holding articles at an elevated temperature during the formation of the bainite phase.  
         [0011]     The present invention relies upon a combination of input heat and a quench medium to dynamically balance the temperature of a workpiece so as to effectively hold that workpiece at an elevated temperature sufficient to promote the formation of a bainite phase.  
         [0012]     In a typical process of the present invention, a workpiece comprised of a ferrous alloy is first heated to a temperature above its austenizing temperature. This heating may be accomplished by any means known in the art. For example, heating may be carried out in a furnace, by an inductive heater or by resistive heating where an electrical current is flowed through an article. Once the article is heated to this initial temperature, it is then contacted with a quench medium. This quench medium may comprise a simple fluid such as water or a water-based liquid, an oil or the like, or in some instances it may be a liquefied or vaporized gas. The quench fluid may be in the form of a bath in which the article is immersed, or it may comprise a sprayed volume of fluid. In any instance, the fluid need not be at a very high temperature since heat will also be input to the workpiece. The quench fluid may comprise a gas including ambient air, an inert gas such as nitrogen, argon or the like, or a reactive gas such as a nitriding or carburizing gas. During at least a part of the time that the workpiece is in contact with the quenching medium, it is also being heated. The input of heat working in combination with the quench medium establishes a dynamic equilibrium which maintains the workpiece at a selected temperature. By controlling the parameters of heating and quenching, the workpiece may be very accurately maintained at a holding temperature selected so as to promote the formation of the bainite phase. The balance of heat and cooling is maintained for a period of time sufficient to form this phase, after which the heat input is terminated and the workpiece cooled below a transition temperature so as to preserve the bainite phase. As will be appreciated, this process is highly controllable. In a typical operation, the workpiece is maintained at a holding temperature which is no greater than 350° C. but above the temperature at which the martensite phase begins to form. The precise temperature ranges will depend upon the specific alloys being treated.  
         [0013]     Heat input during the hold phase may be implemented by various processes known in the art. One particular process which may be advantageously employed in the present invention comprises a resistive heating wherein an electrical current is flowed through the article so as to generate heat. The resistive heating process is economical and simple to implement and control. In some instances, the article may be initially heated to a temperature above the austenizing temperature by a resistive heating process preferably carried out in the absence of a quench medium and thereafter contacted with a quench medium so as to cool it to the hold temperature. Such heating and holding may be advantageously carried out in a single workstation and can be coordinated with further forming processes carried out before, during or after the hold step.  
         [0014]     Other heating processes such as inductive heating, flame heating, radiant energy heating and the like may also be employed in the practice of the present invention. In some instances it may be advantageous to measure the temperature of the workpiece during the hold step, and temperature information obtained thereby may be utilized to control the input of heat and/or parameters of the quench medium such as temperature, velocity, pressure and the like as appropriate, so as to allow for accurate temperature control. Such control may be carried out in a feedback mode or in an indirect mode.  
         [0015]     Referring now to  FIG. 1 , there is shown a time versus temperature profile for one process in accord with the present invention as operable to form a lower bainite phase in a ferrous body. The graph of  FIG. 1  plots time along the horizontal axis and temperature along the vertical axis. At the beginning of the process, the workpiece is at ambient temperature, which is understood to be a normal room temperature encountered in the workplace; and in any instance, an ambient temperature is a temperature which is sufficiently low so that significant metallurgical transitions will not occur in the workpiece. Typically, ambient temperatures are below 50° C. In a first portion of the process, the workpiece is heated to a temperature which is above the austenizing temperature of the alloy comprising the workpiece. This temperature will vary dependent upon the particular alloy employed; however, one of ordinary skill in the art could readily determine what this temperature should be. This first stage heating is typically carried out fairly rapidly, although this is not a requirement of the invention. Heating may be in a furnace, in which instance parts may be maintained therein until further processed. In other instances, heating may be carried out on individual parts immediately before processing. Heating can be by flame, induction, resistance heating or any other method available to those of skill in the art. Once the workpiece is raised to a temperature above the austenizing temperature, it is then cooled to a holding temperature which is typically below 350° C. but above the temperature at which a martensite phase will form in the alloy. The part is cooled from the austenizing temperature to the holding temperature by application of a quenchant fluid thereto. During at least a portion of the time that the workpiece is in the hold phase, heat is applied thereto in conjunction with the quenchant fluid. This heating may be by any means known in the art; however, because of ease of application and control, resistance heating is one preferred heating method. Induction heating is another. The combination of heat input and quenchant will establish a dynamic equilibrium, as noted above, which maintains the workpiece at the hold temperature. The application of heat may be continuous or it may be intermittent as is necessary. The workpiece is held at the hold temperature for a period of time sufficient to form desirable amounts of the bainite phase; and thereafter, it is cooled to an ambient temperature thereby locking in the bainite phase. Cooling is typically accomplished by the quenchant fluid, and additional heat is not input in most instances.  
         [0016]     As will be apparent to those of skill in the art, the aforedescribed heating profile may be modified depending upon the specific metallurgy of the alloy being employed and/or in accordance with desired processing parameters. For example, the rate at which the workpiece is heated or cooled may be varied. Likewise, the hold phase, while shown as being a single horizontal line, may be a stepped line or a gradually sloping line. In some instances, temperature spikes may be selectively incorporated into the profile.  
         [0017]     Also, it should be noted that the present invention allows for very good spatial control of the metallurgical properties of a workpiece. For example, by appropriately configuring the heating and quenching delivery systems, portions of a workpiece may be selectably heated, held and cooled so as to preferentially form a bainite phase in certain portions of a workpiece. In this manner physical parameters of a workpiece such as deformability may be optimized for particular applications.  
         [0018]     Referring now to  FIG. 2 , there is shown a schematic depiction of one temperature control circuit which may be implemented in the present invention. The circuit of  FIG. 2  operates to maintain a workpiece  10  at a preselected temperature. In that regard, a temperature sensor  12  is in communication with the workpiece and is operative to produce an output signal indicative of its temperature. The temperature sensor  12  may comprise a thermocouple, a thermometer, a thermistor or some other such contact device. Also, it may comprise a non-contact device such as an optical pyrometer. In any instance, the temperature sensor  12  produces a control signal which is communicated to a quench controller  14  and/or a heater controller  16 . In the illustrated embodiment, the temperature sensor communicates with both the quench controller and the heater controller; but in some instances, sufficient temperature control can be obtained in the workpiece by controlling only a single one of the controllers. As illustrated, the quench controller  14  controls a quench fluid delivery system  18  which supplies a quench fluid to the workpiece  10 . Similarly, the heater controller  16  controls a heater  20  which inputs heat to the workpiece. As will be appreciated, the controllers  14 ,  16  can cooperate to heat and cool the workpiece  10  in accord with a preselected profile. One of skill in the art can readily incorporate other embodiments of controller in the system of the present invention.  
         [0019]     Most preferably, the methods of the present invention are implemented utilizing workpieces which are relatively thin, and in that regard have a thickness of no more than 5 millimeters; and in some particular instances, the thickness of the articles is no more than 3 millimeters. It has been found that very good temperature control, and uniform metallurgical properties can be obtained utilizing articles of this dimension. However, thicker articles may be fabricated by appropriately adjusting the temperature and heat transfer capacity of the quench medium. The method of the present invention is very well adapted to the fabrication of relatively thin sheet metal articles such as bumper beams, intrusion beams, frame members, body panels and the like for motor vehicles. It may also be utilized to fabricate panels, beams, braces and similar components of building structures. The method of the present invention may be implemented in conjunction with other forming processes such as roll forming, stamping, bending, die forming and the like. As such, stations and systems for the formation of the bainite phase may be incorporated directly into various apparatus, particularly in those instances where processes are carried out at elevated temperatures. For example, a bainite formation step may be readily incorporated into hot forming operations such as die forming, heat treating and the like.  
         [0020]     In view of the disclosure presented herein, numerous modifications and variations of the present invention will be readily apparent to those of skill in the art. The foregoing is illustrative of specific embodiments of the invention, but is not meant to be a limitation upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention.