Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D11/00—Process control or regulation for heat treatments
  • C21D11/005—Process control or regulation for heat treatments for cooling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite

Definitions

• This invention relates generally to the fabrication of articles from ferrous metals having controlled metallurgical properties. More specifically, the invention relates to the fabrication of articles from ferrous alloys having high concentrations of a bainite phase. 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 a bainite phase.
• Ultra high strength steel is used to fabricate various components of motor vehicles, building structures, and other articles of fabrication.
• 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.
• steel products produced thereby have tensile strengths up to and over 200 ksi.
• 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.
• 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.
• the steel alloys 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
• 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.
• the present invention provides a process whereby ferrous articles having precisely controlled metallurgical properties 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 selected metallic phases such as upper, lower or mixed bainite phases. 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.
• a method for preparing a body of a ferrous alloy containing a high percentage of a desired metallic phase or phases is described with particular reference to the formation of bainite phases; but, it may be utilized to form articles having other desirable metallurgical phases therein.
• a workpiece comprised of a ferrous alloy is heated to a temperature above an austenizing temperature of the alloy.
• 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 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 desired phase therein and thereafter the workpiece is cooled to ambient temperature.
• the combination of the holding temperature and time are selected so that a bainite phase is formed in the workpiece.
• the thickness of the workpiece is no greater than 5 millimeters, and in some certain instances it is no greater than 3 millimeters.
• 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.
• 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.
• Figure 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
• Figure 2 is a diagram of a temperature control circuit which may be utilized in the practice of the present invention.
• the present invention is a metallurgical process which greatly simplifies the formation of bainite, or other desirable metallurgical phases, 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 desired phase.
• 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 or other desired phase.
• 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.
• a 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.
• 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.
• the input of heat working in combination with the quench medium establishes a dynamic equilibrium which maintains the workpiece at a selected temperature.
• the workpiece may be very accurately maintained at a holding temperature selected so as to promote the formation of the desired 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 desired phase.
• this process is highly controllable.
• 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. In some instances the alloy will be held at temperatures above 35O 0 C, depending on the phase being formed.
• 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.
• 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.
• 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.
• 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.
• control may be carried out in a feedback mode or in an indirect mode.
• 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 Figure 1 plots time along the horizontal axis and temperature along the vertical axis.
• 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 5O 0 C.
• 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.
• 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.
• 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.
• 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.
• 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. And as noted above, the hold temperature may be above 350 0 C when other phases are being formed.
• the present invention allows for very good spatial control of the metallurgical properties of a workpiece.
• 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.
• physical parameters of a workpiece such as deformability may be optimized for particular applications.
• 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.
• 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.
• the quench controller 14 controls a quench fluid delivery system 18 which supplies a quench fluid to the workpiece 10.
• the heater controller 16 controls a heater 20 which inputs heat to the workpiece.
• the controllers 14, 16 can cooperate to heat and cool the workpiece 10 in accord with a preselected profile.
• controllers 14, 16 can cooperate to heat and cool the workpiece 10 in accord with a preselected profile.
• 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.
• 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.
• a bainite formation step may be readily incorporated into hot forming operations such as die forming, heat treating and the like.
• the present invention operates so as to balance heating and cooling and thereby allow a workpiece to be processed according to a precisely controlled temperature profile. This allows for very precise and accurate selection and control of the article's metallurgical properties.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Articles (AREA)
• Heat Treatments In General, Especially Conveying And Cooling (AREA)
• Control Of Heat Treatment Processes (AREA)
• Heat Treatment Of Steel (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=36072659

Family Applications (1)

Country Status (6)

Families Citing this family (5)

Family Cites Families (13)

• 2005
  • 2005-09-15 US US11/227,354 patent/US20060060268A1/en not_active Abandoned
  • 2005-09-19 WO PCT/US2005/033336 patent/WO2007001374A2/en active Application Filing
  • 2005-09-19 KR KR1020077007958A patent/KR20070099539A/ko not_active Application Discontinuation
  • 2005-09-19 US US11/575,343 patent/US20080053579A1/en not_active Abandoned
  • 2005-09-19 JP JP2007532551A patent/JP2008519154A/ja active Pending
  • 2005-09-19 EP EP05858151A patent/EP1797205A2/en not_active Withdrawn
  • 2005-09-19 BR BRPI0515674-2A patent/BRPI0515674A/pt not_active IP Right Cessation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events