Patent Publication Number: US-2016244855-A1

Title: Method For Trimming A Hot Formed Part

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This U.S. National Stage Patent Application claims the benefit of PCT International Patent Application Ser. No. PCT/US2014/061519 filed Oct. 21, 2014 entitled “Method For Trimming A Hot Formed Part,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/893,318 filed Oct. 21, 2013, entitled “Method For Trimming A Hot Formed Part,” the entire disclosures of the applications being considered part of the disclosure of this application and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to hot formed steel parts, such as automotive body components, and methods for manufacturing the hot formed steel parts. 
     2. Related Art 
     Automotive body components are oftentimes manufactured by hot forming a steel blank. The process includes heating the steel blank in an oven to a temperature of approximately 850° C. to 900° C. until the steel blank obtains an austenite microstructure. Next, the heated blank is transferred from the oven to a hot forming apparatus which includes a pair of dies. The heated blank is then stamped or pressed to a predetermined shape between the dies. The hot forming process also typically includes a quenching step to increase the strength of the hot formed part. During the quenching step, the hot formed part is cooled to a temperature low enough to transform the austenite microstructure to a martensite microstructure. 
     After the hot forming process, the hot formed part is removed from the dies and transferred to a separate location for at least one post-forming operation. The hot formed part is typically trimmed, pierced, sheared, or otherwise cut to achieve a desired shape. However, due to the high strength of the martensite microstructure present in the hot formed part, expensive post-forming processes and equipment are typically required to cut the hot formed part and achieve the desired shape. For example, a costly laser cutting process is oftentimes used to trim the hot formed part. 
     SUMMARY OF THE INVENTION 
     The invention provides a method for manufacturing a hot formed steel part, such as an automotive body component, which is trimmed, pierced, sheared, or otherwise cut to a desired shape, without a costly post-forming operation, such as laser cutting. The method first includes heating a blank formed of steel material to a temperature of 880° C. to 950° C., and maintaining the blank at the temperature of 880° C. to 950° C. until the microstructure of the steel material is substantially austenite. The method then includes disposing the blank on a lower forming surface of a lower die while the blank is at a temperature of at least 400° C. and the microstructure of the blank is still substantially austenite. The heated blank is initially spaced from an upper forming surface of an upper die. The upper die is coupled to a cutting component, and the cutting component is disposed adjacent the upper forming surface. 
     The method next includes bringing the upper die toward the lower die to form and cut the heated blank. The step of bringing the upper die toward the lower die includes bringing the upper forming surface of the upper die into contact with the blank to form the blank between the upper and lower forming surfaces; and moving at least a portion of the upper die and the cutting component together longitudinally until the cutting component cuts at least a portion of the blank. The cutting step is conducted while the blank is at a temperature of at least 400° C. and the microstructure of the blank is substantially austenite. 
     The method further includes cooling the blank at a rate of at least 27 degrees per second. The cooling step is conducted while the upper forming surface and the lower surface remain in contact with the cut blank and until the microstructure of the cut blank includes martensite. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  illustrates a method of manufacturing a hot formed part according to an exemplary embodiment of the invention; 
         FIG. 2A  is a cross-sectional view of a hot forming apparatus according to an exemplary embodiment of the invention immediately before a cutting step; 
         FIG. 2B  is a cross-sectional view of a hot forming apparatus according to an exemplary embodiment of the invention immediately after a cutting step; 
         FIG. 3  is a cross-sectional view of a hot forming apparatus according to another exemplary embodiment of the invention; 
         FIG. 4  is a perspective view of an exemplary hot formed part showing an approximate temperature profile along the hot formed part at the start of a cutting step; and 
         FIG. 5  is a chart illustrating a load force applied to a hot formed part by a cutting component of a hot forming apparatus according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention provides an improved method for manufacturing a hot formed steel part  20 , such as an automotive body component, without a costly post-forming operation. The method includes heating a steel blank  22  to an austenite temperature, and cutting the heated blank  22  while forming the heated blank  22 , or immediately after forming the heated blank  22 , between a pair of dies  24 ,  26  of a hot forming apparatus  28 . The cutting step occurs while the microstructure of the blank  22  is still substantially austenite.  FIG. 1  illustrates steps of the hot forming method according to an exemplary embodiment.  FIGS. 2A, 2B, and 3  illustrate exemplary hot forming apparatuses  28 , and  FIG. 4  illustrates an exemplary hot formed part  20 . 
     The method begins by providing the blank  22  formed of a steel material, which can be any type of steel material. In one embodiment, the steel material used to form the blank  22  comprises 0.18% to 0.28% carbon, 0.7% to 1.0% silicon, 1.0% to 2.0% manganese, 0.12% to 0.7% chromium, 0.1% to 0.45% molybdenum, 0.025% maximum phosphorus, 0.008% to 0.01% sulfur, 0.02% to 0.05% titanium, 0.01% to 0.06% aluminum, and 0.002% to 0.004% boron, based on the total weight of the steel material. In another embodiment, the steel material comprises a mixture of manganese and boron, for example 22MnB5. The size and shape of the blank  22  depends on the desired size, shape, and application of the hot formed part  20  to be manufactured. In one embodiment, the blank  22  is initially provided with a coating formed of aluminum and silicon (AlSi). This coating ultimately forms a diffusion layer along the surface of the hot formed part  20 . 
     Once the blank  22  is provided, the method includes annealing or otherwise heating the blank  22  in an oven or furnace. The blank  22  is heated or annealed for a period of time causing an austenite microstructure to form throughout the steel material. The temperature and duration of the heating step varies depending on the dimensions of the blank  22  and type of steel material used. However, the blank  22  is typically heated to a temperature of 880° C. to 950° C. and is held at that temperature for at least 30 seconds to form the austenite microstructure. In one embodiment, the blank  22  is heated to a temperature of 910° C. for at least 20 seconds. In another embodiment, the blank  22  is heated to a temperature of 930° C. for at least 20 seconds. During the heating step, all carbides in the steel material of the blank  22  should dissolve so that there are no residual carbides. After the heating step, the microstructure of the steel material is substantially austenite, for example at least 75% austenite, or entirely austenite (100% austenite). 
     The heating step is adjusted slightly when the steel blank  22  is coated with the AlSi coating, as additional time is required for the AlSi coating to form a diffusion layer having a sufficient thickness along the surface of the blank  22 . Maintaining the blank  22  at a temperature above 800° C. for at least 150 seconds is typically required for the AlSi coating to form a diffusion layer having a sufficient thickness. Additional heating time is also required due to the reflective nature of the AlSi coating at temperatures of 580° C. to 780° C. 
     Immediately following the heating step, the heated blank  22  is quickly transferred from the oven to the hot forming apparatus  28  while the blank  22  is still above the austenite temperature and thus still includes the substantially austenite microstructure. In one embodiment, the steel material of the blank  22  is entirely austenite when it enters the hot forming apparatus  28 . In another embodiment, the steel material of the blank  22  includes at least 75% austenite, but less than 100% austenite, when it enters the hot forming apparatus  28 . The blank  22  is transferred quickly to the hot forming apparatus  28  so that the temperature of the blank  22  stays above 400° C. 
     The method next includes forming and trimming, piercing, shearing, or otherwise cutting the heated blank  22  to a desired shape in the hot forming apparatus  28 . The forming and cutting steps both occur in the hot forming apparatus  28  and during a single die stroke. In other words, the cutting step occurs simultaneously with the forming step or immediately thereafter. The blank  22  is at a temperature of at least 400° C., such as a temperature of 400° C. to 800° C. during the forming and cutting steps. In addition, the forming and cutting steps are both conducted while the steel material includes a 100% austenite microstructure or at least a substantially austenite microstructure. 
       FIGS. 2A and 2B  illustrate an exemplary hot forming apparatus  28  in a closed position. In this embodiment, the hot forming apparatus  28  includes an upper die  24 , a lower die  26 , a cutting component  30 , a pad  32 , upper springs  34 , and lower springs  36 . The cutting component  30  and upper springs  34  are fixed to a first portion  38  of the upper die  24 , for example by bolts. A second portion  40  of the upper die  24 , referred to as an upper form, presents an upper forming surface  42  and is surrounded by the first portion  38  and the cutting component  30 . The upper springs  34  are disposed on the second portion  40  and bias the first portion  38  away from the second portion  40 . Thus, the first portion  38  and connected cutting component  30  are movable relative to the second portion  40  of the upper die  24 . For example, when the upper springs  34  are compressed, the first portion  38  of the upper die  24  and cutting component  30  move together longitudinally such that the cutting component  30  moves past the upper forming surface  42  and toward the pad  32 . The cutting component  30  is formed of a material capable of cutting the steel material of the blank  22 . In the exemplary embodiments, the cutting component  30  is also formed of a steel material, referred to as trim steel. 
     As shown in  FIGS. 2A and 2B , the lower die  26  includes a third portion  44 , referred to as a lower form, which presents a lower forming surface  46  for supporting the steel blank  22 . The lower springs  36  are fixed to a fourth portion  48  of the lower die  26 , for example by bolts. The pad  32  is disposed on opposite sides of the lower forming surface  46  beneath the cutting component  30 , and the lower springs  36  bias the pad  32  toward the cutting component  30  and the upper die  24 . Although the Figures show the upper die  24  positioned above the lower die  26 , the position of the hot forming apparatus  28  could be reversed such that the upper die  24  is positioned below the lower die  26 . 
     Prior to the forming step, the hot forming apparatus  28  is in an open position, and thus the upper die  24  and cutting component  30  are spaced from the lower die  26  and pad  32 . The geometry of the upper forming surface  42  and the lower forming surface  46  varies depending on the desired shape of the part  20  to be formed. In the embodiment of  FIGS. 2A and 2B , the upper forming surface  42  is recessed, and the lower forming surface  46  is received in the recessed upper forming surface  42  when the apparatus  20  is closed. Also, prior to the forming step, when the hot forming apparatus  28  is open, no pressure is placed on the lower springs  36 , such that the lower springs  36  are extended and the pad  32  is generally aligned with a portion of the lower forming surface  46 . 
     The forming step occurs immediately after transferring the heated blank  22  to the hot forming apparatus  28 , so that the temperature of the blank  22  stays above 400° C. In the embodiment of  FIGS. 2A and 2B , the heated blank  22  is disposed on the uppermost portion of the lower forming surface  46  such that the edges of the heated blank  22  project outwardly of the lower forming surface  46  and are located above the pad  32 . The forming step then includes bringing the first and second portions  38 ,  40  of the upper die  24  together with the cutting component  30  downwardly toward the lower die  26  and the heated blank  22 . While the upper die  24  and cutting component  30  move downward toward the heated blank  22 , the upper springs  34  are not compressed. Thus, the first portion  38  of the upper die  24  and the cutting component  30  do not move relative to the second portion  40  of the upper die  24  during the forming step. 
     As the upper die  24  moves downward, the upper forming surface  42  contacts and presses the heated steel blank  22  around the lower forming surface  46  to form the blank  22  to a predetermined shape, as shown in  FIGS. 2A and 2B . The upper forming surface  42  presses the heated blank  22  until the edges of the heated blank  22  rest on or slightly above the pad  32  on opposite sides of the lower forming surface  46 . The steel material of the blank  22  is still substantially austenite during the forming step, for example at least 75% austenite or 100% austenite. 
     The method further includes cutting the heated blank  22  to provide the desired shape while the blank  22  is still in the hot forming apparatus  28  and includes the substantially austenite microstructure. The cutting step occurs during the same die stroke as the forming step. In the exemplary embodiment of  FIGS. 2A and 2B , the first portion  38  of the upper die  24  compresses the upper springs  34 , and the first portion  38  and the cutting component  30  continue moving downward together while the second portion  40  of the upper die  24  remains in a fixed position. The cutting component  30  then moves longitudinally past the upper forming surface  42  while the upper forming surface  42  remains in contact with the heated blank  22 . During the cutting step, the cutting component  30  cuts at least a portion of the steel blank  22 . In one embodiment, the cutting component  30  moves past the lower forming surface  46  and shears the edges off the blank  22 . In this case, the cutting component  30  presses the edges, referred to as scrap  54 , into the pad  32 , thereby compressing the lower springs  36 . In this embodiment, the cutting component  30  cuts through the entire thickness t of the blank  22 , and the desired final shape of the blank  22  is achieved without any post-forming operation outside of the hot forming apparatus  28 , such as laser trimming. In another embodiment, shown in  FIG. 2B , only a portion of the thickness t of the blank  22  is cut by the cutting component  30  in the hot forming apparatus  28 . For example, the cutting component  30  may cut through not greater than 95%, for example 75% to 95%, or 90% of the thickness t of the steel blank  22 . In this case, the scrap  54  remains attached to the blank  22 , but is easily removed from the part  20  outside of the hot forming apparatus  28 . 
     An alternate embodiment of the hot forming apparatus  128  is shown in  FIG. 3 . The method conducted using the forming apparatus of  FIG. 3  is referred to as a “zero entry” method. In this embodiment, the hot forming apparatus  128  includes the cutting component  130  fixed to the first portion  138  of the upper die  124 , without the upper springs  34 , lower springs  36 , and pad  32 . The second portion  140  of the upper die  124  presents the recessed upper forming surface  142  and the third portion  144  of the lower die  126  presents the lower forming surface  146 . However, unlike the hot forming apparatus  28  of  FIGS. 2A and 2B , the cutting component  130  is fixed to the second portion  140  of the upper die  124 , and the second portion  140  is fixed to the first portion  138 . In addition, the upper forming surface  142  and the cutting component  130  provide an upper ledge  150  therebetween, and the lower forming surface  146  presents a lower ledge  152  aligned with the upper ledge  150  for shearing the heated blank  122 . As in the embodiment of  FIGS. 2A and 2B , the upper die  124  and cutting component  130  move downward, and the upper forming surface  142  presses the heated blank  122  around the lower forming surface  146  to a predetermined shape. 
     As alluded to above, in the embodiment of  FIG. 3 , the cutting component  130  does not move relative to the first portion  138  or the second portion  140  of the upper die  124 . Instead, the upper ledge  150  of the upper die  124  moves toward the lower ledge  152  of the lower die  126  to shear the edges off the heated blank  122 . Alternatively, the cutting component  130  could cut through less than 95% of the thickness t of the blank  122 , such that the scrap  154  remains connected to the blank  122 , but can be easily removed outside of the hot forming apparatus  128 . In either case, the shearing step begins when the distance between the upper ledge  150  and lower ledge  152  is equal to the thickness t of the steel blank  122 . As in the embodiment of  FIGS. 2A and 2B , the forming and cutting steps occur in a single die stroke and while the microstructure of the blank  122  is substantially austenite. 
     In other embodiments, the cutting step can include trimming, piercing, or another type of cutting technique, instead of shearing, or in addition to shearing. Thus, the cutting component  30  of the hot forming apparatus  28  is designed accordingly. Preferably, the hot forming apparatus  28  is designed so that the cutting clearance, also referred to as the die clearance, is between 2% and 15% of the thickness t of the blank  22 . In the embodiments of  FIGS. 2A, 2B, and 3  the cutting clearance is equal to the distance between a cutting edge of the cutting component  30  and a cutting edge of the adjacent lower forming surface  46 , when the hot forming apparatus  28  is closed. 
     As stated above, the step of cutting the blank  22  occurs while the steel material is still at a temperature of at least 400° C., preferably 400° C. to 850° C., and still has a substantially austenite microstructure.  FIG. 4  is a perspective view of an exemplary hot formed part  20 , specifically a B-pillar, showing the approximate temperature profile along the part  20  at the start of the cutting step, which in this case includes trimming and piercing. The temperature profile indicates that the majority of the hot formed part  20  is at a temperature of at least 685° C. and the steel material is still 100% austenite at the start of the cutting step.  FIG. 5  is a chart illustrating the load force applied to the hot formed part  20  by a 16 mm cutting component  30 , such as a punch. The load force is provided for temperatures ranging from 25° C. to 800° C., and for part thicknesses t ranging from 1.0 to 1.8 mm.  FIG. 5  also indicates that the temperature of the cutting step is from 400° C. to 800° C. 
     In order for the microstructure of the blank  22  to remaining substantially austenite during the cutting step, a quick process is required. In one embodiment, when the steel material includes 100% austenite during the cutting step, the amount of time from when the heated blank  22  exits the oven until forming the heated blank  22  between the forming surfaces  42 ,  46 , i.e. the time at which the hot forming apparatus  28  is closed, is only 5 to 15 seconds. In another embodiment, when the steel material includes some retained austenite during the cutting step, but less than 100% austenite, the amount of time from when the heated blank  22  exists through the door of the oven until the hot forming apparatus  28  is closed is 5 to 20 seconds. 
     After the forming and cutting steps, the method includes cooling the blank  22  in the hot forming apparatus  28 , while the hot forming apparatus  28  is still closed. The cooling step typically includes quenching. The hot forming apparatus  28  can include any type of cooling mechanism to cool or quench the hot formed blank  22 . For example, the upper and lower dies  24 ,  26  could include a plurality of cooling channels for conveying a cooling fluid therethrough. 
     The hot formed blank  22  should be cooled or quenched at a rate that causes a martensite microstructure to form in the steel material, and preferably throughout the entire steel material so that the finished hot formed part  20  is 100% martensite. The martensite microstructure provides increased strength which is beneficial when the hot formed part  20  is used as an automotive body component, such as a B-pillar. In one embodiment, the method includes cooling the hot formed blank  22  at a minimum cooling rate of  27  degrees per second to obtain the martensite microstructure throughout the steel material. The method finally includes opening the hot forming apparatus  28  once the temperature of the hot formed part  20  is 200° C. or lower, and allowing the hot formed part  20  to cool to room temperature. Since the cutting step is performed in the hot forming apparatus  28 , the method does not require any costly post-forming operations outside of the hot forming apparatus  28 , such as a separate laser cutting process. If the scrap  54  remains attached to the hot formed part  20 , a simple and inexpensive post-forming operation can be used to remove the scrap  54 . 
     The invention also provides a hot formed part  20  manufactured using the method and hot forming apparatus  28  described above. The hot formed part  20  is manufactured by forming the heated blank  22  to a predetermined shape and then trimming, piercing, shearing, or otherwise cutting the blank  22  in the hot forming apparatus  28  to achieve a desired shape. Thus, there is no need for a costly post-forming operation, such as laser trimming. The hot formed part  20  preferably includes a martensite microstructure throughout the steel material with no residual carbides in the steel material, which could decrease the ultimate tensile strength (UTS) of the part  20 . In addition, the hot formed part  20  can optionally include a diffusion layer comprising AlSi. In one embodiment, the hot formed part  20  has a yield strength of 500 MPa to 1,600 MPa; an ultimate tensile strength (UTS) of 900 MPa to 2,000 MPa; an elongation of 5.0%, minimum; and a hardness (HRV) of 300 to 600. The hot formed part  20  can be designed for use as any type of automotive body component, such as a pillar, rocker, roof rail, bumper, or door intrusion beam of an automotive vehicle. In one embodiment, the hot formed part  20  is a B-pillar having the design shown in  FIG. 4 . Alternatively, the hot formed part  20  can be used in a non-automotive application. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the claims.