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WO2016106621A1 - Method of hot forming a component from steel - Google Patents

Method of hot forming a component from steel Download PDF

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Publication number
WO2016106621A1
WO2016106621A1 PCT/CN2014/095753 CN2014095753W WO2016106621A1 WO 2016106621 A1 WO2016106621 A1 WO 2016106621A1 CN 2014095753 W CN2014095753 W CN 2014095753W WO 2016106621 A1 WO2016106621 A1 WO 2016106621A1
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WO
WIPO (PCT)
Prior art keywords
weight
parts
less
amount
sheet blank
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Ceased
Application number
PCT/CN2014/095753
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French (fr)
Inventor
Jeff Wang
Xiaochuan XIONG
Paul J. Belanger
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GM Global Technology Operations LLC
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GM Global Technology Operations LLC
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Priority to PCT/CN2014/095753 priority Critical patent/WO2016106621A1/en
Publication of WO2016106621A1 publication Critical patent/WO2016106621A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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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
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment

Definitions

  • the present disclosure is related to a method of hot forming a component from steel.
  • Press hardened steel also referred to in the art as “boron-steel” or “hot-stamped steel” , is one of the strongest steels used for automotive body structural applications—having tensile strength properties on the order of about 1, 500 mega-Pascal (MPa) .
  • the use of PHS structural components is being uses on many structural components, such as on a vehicle.
  • the structural components may be bumpers, center pillars, hinge pillars, cross members, and the like.
  • Such structural components made of PHS are often produced via a so-called “hot forming” or “hot stamping” process.
  • hot forming As part of the production process, large slabs of steel undergo various metal working processes to obtain sheet stock, which is rolled into spools or coils.
  • one or more blanks are cut from the metal coil.
  • the cut blanks which may or may not be preformed at ambient temperatures, are heated to elevated temperatures, e.g., around 900 degree Celsius (C) , and thereafter transferred to water cooled dies. Subsequently, the hot blanks are formed and quenched in the dies to achieve the final shape.
  • elevated temperatures e.g., around 900 degree Celsius (C)
  • a component of press hardened steel is formed by hot forming a sheet blank formed from transformation induced plasticity (TRIP) steel.
  • the sheet blank is heated in a furnace to no greater than 850° Celsius (C) to obtain a heated sheet blank.
  • the heated sheet blank is transferred to a die to form an intermediate workpiece.
  • the intermediate workpiece is hot stamped in the die.
  • Concurrent to hot stamping the intermediate workpiece the intermediate workpiece is cooled at a cooling rate until a temperature of not greater than a finished temperature is achieved.
  • the cooling rate is not less than a critical cooling rate.
  • Hot stamping and cooling form the component having a martensite microstructure, a ductility of between 8%and 20%, and a tensile strength of at least 1, 400 megapascal (MPa) .
  • FIG. 1 is a schematic illustrative view of a system for making a component from a sheet blank using hot forming.
  • FIG. 2A is a schematic illustrative view of an apparatus including the component.
  • FIG. 2B is a schematic illustrative side view of the sheet blank having a base layer and a coating composition.
  • FIG. 3 is a flow chart describing an example method for hot forming the component from a sheet blank.
  • FIG. 4 is a graphical illustration of time versus temperature during hot forming the component from the heat blank as shown in FIG. 1.
  • FIG. 1 illustrates a system 10 for making a formed component 12 from a sheet blank 14 via hot forming.
  • the formed component 12 may be attached to an apparatus 15.
  • the apparatus 15 may be a vehicle and the component 12 may be a bumper, a cross member, a pillar, a body panel, and the like.
  • the component 12 may also be used for any other desired apparatus 15, including, but not limited to, an aircraft, a building, a bridge, a boat, machinery, and the like.
  • the system 10 may include a furnace 16, a transfer device 18, and a stamping press 20.
  • the sheet blank 14 may be passed through the furnace 16.
  • the sheet blank 14 comprises transformation induced plasticity (TRIP) steel.
  • the furnace 16 is configured to heat and soak the sheet blank 14 at a temperature of at least the austenitization temperature T 1 of the TRIP steel.
  • the transfer device 18 is configured to transfer the sheet blank 14 from the furnace 16 to the stamping press 20, once the sheet blank 14 has been soaked sufficiently long enough to fully austenitized the TRIP steel of the sheet blank 14. Meaning, the TRIP steel of the sheet blank 14 is austenite.
  • the transfer device 18 may be a robotic arm, a conveyor, a slide mechanism, and the like.
  • the stamping press 20 includes a die 22 that provides a cooling mechanism.
  • the die 22 is designed to form a desired final shape of the component 12 from the austenitized sheet blank 14.
  • the die 22 may include a first forming die 22A and a second forming die 22B that are brought together to form the desired final shape do the component 12 therebetween.
  • die 22 is configured to provide cooling to the sheet blank 14 as the first and second forming die 22A, 22B are brought together to form the component 12.
  • the first and second forming die 22A, 22B are selectively brought together to form the component 12.
  • the die 22 may be a water-cooled die 22. More specifically, the first and second forming die 22A, 22B may each define at least one cooling channel (not shown) therein.
  • the cooling channels may be configured to control the cooling provided by the first and second forming die 22A, 22B such that the formed component 12 is quenched in a controlled manner, consistent across surfaces of the formed component 12 to cause a phase transformation from austenite to martensite, as will also be explained in more detail below. Therefore, the first and second die 22A, 22B may cooperate to function as a heat sink to draw heat from, and otherwise quench, the formed component 12.
  • the sheet blank 14 may be formed by a blanking operation (not shown) . During the blanking operation, the sheet blank 14 is cut out of a sheet or roll of material, as is known to those skilled in the art.
  • the sheet blank 14 may be formed using any desired cutting method.
  • the sheet blank 14 may be formed in a blanking press (not shown) by a punch operation, a stamping press, and the like.
  • other forming methods may be used to cut the sheet blank 14 from the material, including, but not limited to via laser cutting (not shown) so long as the desired dimensions and/or shape of the sheet blank 14 is achieved.
  • the sheet blank 14 is a generally flat sheet of material.
  • the sheet blank 14 comprises a base layer 24, including a first side 26 and a second side 28, opposing the first side 26.
  • the base layer 24 comprises TRIP steel.
  • a coating composition 30 may be disposed on the first and/or second side 26, 28 of the base layer 24.
  • the coating composition 30 may comprise zinc. More specifically, the coating composition 30 may be primarily comprised of zinc. It should be appreciated, however, that the coating composition 30 is not limited to comprising zinc, but may also be comprised of other elements.
  • the sheet blank 14 undergoes the hot forming process to provide the component 12 formed from press hardened steel (PHS) .
  • PHS press hardened steel
  • the TRIP steel may be formed from manganese, to yield a medium manganese content having 4-12% manganese, i.e., a Mn-TRIP steel.
  • the Mn-TRIP steel may be, for example, a 7Mn-TRIP steel, a 10-Mn-TRIP steel, and the like.
  • the Mn-TRIP steel may have the following composition, based on 100 parts by weight of the Mn-TRIP steel: carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight; manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight; and with the remainder being iron and impurities inherent in processing.
  • the composition of the Mn-TRIP steel may include: silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight; chromium present in an amount of less than about 1 part by weight; titanium present in an amount of less than about 0.2 parts by weight; aluminum present in an amount of less than about 0.1 part by weight; phosphorus present in an amount of less than about 0.2 parts by weight; and sulfur present in an amount of less than about 0.05 parts by weight.
  • the TRIP steel may be Delta-TRIP steel, where the TRIP steel has a greater concentration of aluminum than silicon.
  • the Delta-TRIP steel may have the following composition, based on 100 parts by weight of the Delta-TRIP steel: carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight; aluminum present in an amount of from greater than about 1 parts by weight to less than about 5 parts by weight; and the remainder being iron and impurities inherent in processing.
  • composition of the Delta-TRIP steel may include: manganese present in an amount of from greater than about 0.1 parts by weight to less than about 1 parts by weight; silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight; chromium present in an amount of less than about 1 part by weight; titanium present in an amount of less than about 0.2 parts by weight; phosphorus present in an amount of less than about 0.2 parts by weight; and sulfur present in an amount of less than about 0.05 parts by weight.
  • the TRIP steel may be a Mn-TRIP steel, a Delta-TRIP steel, and the like.
  • the TRIP steel has an austenitization temperature T 1 of no greater than 782 °C.
  • the TRIP steel has an austenitization temperature T 1 of between 750 and 800 degrees Celsius (°C) . More preferably, the austenitization temperature T 1 may be between 750 and 782 °C.
  • zinc has a melting temperature of 420 °C and, at 782 °C, reacts with iron via a eutectoid reaction and forms a brittle phase.
  • liquid zinc can attack grain boundaries of iron and, hence, form cracks, i.e., a phenomenon called liquid metal embrittlement (LME) .
  • Zinc attacks austenite more than in other iron phases, such as ferrite.
  • the LME in coated-steel happens when the following conditions are met: (a) a liquid metal phase in the coating layer 30; (b) a phase in the base layer 24 that is sensitive to LME; and (c) deformation of the base layer 24. When all of these conditions are met, liquid metal will wet the freshly exposed grain boundary (of the phase in the substrate) and cause de-cohesion/separation along the grain boundary.
  • the austenitization temperature T 1 will always be greater than the melting temperature of the coating composition 30 (mainly zinc) .
  • the key is to hot form the TRIP steel of base layer 24 at a temperature that is below 782 °C to avoid LME by: (a) depletion of liquid zinc in the coating by the reaction of Zinc + Iron (Zn + Fe) at 782 °C as the temperature drops to below 782 °C; and (b) transforming austenite in the base layer 24 to other phases (such as ferrite) that are less sensitive to LME.
  • the temperature of the sheet blank 14 during the hot forming process is conducted below 782 °C, because at 782 °C, a zinc iron (ZnFe) compound is formed, which depletes the liquid zinc.
  • ZnFe zinc iron
  • the LME described above is avoided.
  • providing a zinc coating composition 30 reduces welding complexity when other components already have a zinc coating composition.
  • the TRIP steel has a critical cooling rate that is the slowest rate of cooling to produce a fully hardened martensitic condition within the component 12.
  • the critical cooling rate is no greater than about 10 Kelvin/second (K/s) .
  • TRIP steel may have lower critical cooling rates, such as about 0.5 K/s.
  • the TRIP steel not only greatly reduces the austenitization temperature T 1 , but also significantly shifts the nose of the continuously cooling transformation (CCT) curve to the right, allowing more time so the critical cooling rate can be slower.
  • the lower critical cooling rate improves the hardenability of the TRIP steel.
  • lowering the critical cooling rate required to achieve the fully hardened martensitic condition provides an intermediate workpiece 14B that is “forgiving’ in terms of achieving a full martensite microstructure everywhere on the intermediate workpiece 14B. This means that a lower critical cooling rate is less demanding during cooling of the intermediated workpiece 14B.
  • some of the surface (s) of the first and second forming die 22A, 22B may have dirt or oxide that builds up, thus changing thermal conductivity locally, causing “hot spots” .
  • the hot spots may promote a slower cooling rate locally, as opposed to other areas on the surface (s) that do not have the buildup. Therefore, if the critical cooling rate required is higher than the actual rate of cooling at the hot spots and/or anywhere else along the surface (s) , slow cooling results, thereby producing a non-martensite microstructure with less hardness.
  • the critical cooling rate required for the TRIP steel is less than the actual rate of cooling at the hot spots and anywhere else along the surface (s) of the first and second forming die 22A, 22B, the ability to achieve the fully hardened martensite is not affected.
  • FIG. 3 depicts a method 100 of making the component 12 with the system 10.
  • FIG. 4 depicts a time (t) and temperature (T) curve representing the making of the component 12. The method 100 is described herein with respect to the system 10, component 12, and the time and temperature curve, respectively shown in FIGS. 1, 2A, and 4.
  • the method 100 commences at step 102 where the sheet blank 14 is provided.
  • the sheet blank 14 is formed from steel, e.g., Mn-TRIP steel and Delta-TRIP steel, as described above. Further, the sheet blank 14 may include a coating composition 30 disposed on the first and/or second side 26, 28 of a base layer 24.
  • step 104 the sheet blank 14 is heated in the furnace 16 to the austenitization temperature T 1 of the TRIP steel to provide a heated sheet blank 14A.
  • the sheet blank 14 may be at ambient temperature T 0 , e.g., approximately 25 °C.
  • the austenitization temperature T 1 is preferably no greater than 782 °C. More preferably, the austenitization temperature T 1 is between 750 and 800 °C. Even more preferably, the austenitization temperature is between 750 and 782 °C to obtain the heated sheet blank T 1 .
  • the method 100 proceeds to step 106.
  • the heated sheet blank 14 is soaked in the furnace 16 for a desired amount of time at, or slightly above, the austenitization temperature T 1 . More specifically, the sheet blank 14 is soaked in the furnace 16 until the TRIP steel is fully austenitized, i.e., austenite. By way of a non-limiting example, the sheet blank 14 may be soaked to between 1 and 10 minutes. However, in one embodiment, the combination of steps 104 and 106, i.e., respectively heating and soaking the sheet blank 14, may be completed in about four minutes.
  • the method 100 proceeds to step 108.
  • the heated sheet blank 14A is transferred to the die 22 to form an intermediate workpiece 14B. More specifically, when the heated sheet blank 14A is removed from the furnace 16, the heated sheet blank 14A is at the austenitization temperature T 1 . As described above, the austenitization temperature T 1 of the TRIP steel is most preferably between 750 and 782 °C. However, once the heated sheet blank 14 exits the furnace 16, the heated sheet blank 14 immediately begins to lose temperature (cool) in the ambient temperature surrounding the furnace 16 to provide an intermediate workpiece 14B at an intermediate temperature T 2 . Therefore, the intermediate workpiece 14B is inserted into the die 22 at an intermediate temperature T 2 that is less than the austenitization temperature T 1 of the heated sheet blank 14B that was removed from the furnace 16.
  • the amount of cooling during the transfer to the die 22 is dependent on the time that it takes to complete the transfer to the die 22 and the temperature of the ambient air between the furnace 16 and the die 22.
  • the intermediate temperature T 2 of the intermediate workpiece 14B is between 700 and 750 °C upon insertion into the die 22 and the time to transfer between the furnace 16 and the die 22 is about six seconds.
  • the method concurrently proceeds to steps 110 and 112.
  • the intermediate workpiece 14B is respectively hot stamped and quenched, i.e., cooled, in the die 22. More specifically, at step 110, a first forming die 22A and a second forming die 22B are brought together, with the intermediated workpiece 14B disposed therebetween to form the desired final shape of the component 12.
  • the temperature of the intermediate workpiece 14B at the beginning of step 110 may be between about 700 and 750 °C.
  • the intermediate workpiece 14B is cooled in the die 22 as the first and second forming die 22A, 22B form the desired final shape.
  • the temperature of the intermediate workpiece 14B at the beginning of step 112 may be between about 700 and 750 °C.
  • the intermediate workpiece 14B is cooled via watercooling. More specifically, as described above, each of the first and second forming die 22A, 22B may each define at least one cooling channel (not shown) therein. As is known, the cooling channels may be configured to control the cooling rate provided by the first and second forming die 22A, 22B to quench the component 12.
  • Water is injected to flow through the channels to promote a heat exchange between the intermediate workpiece 14B and the water, via the first and second forming die 22A, 22B, to cool the intermediate workpiece 14B at a desired cooling rate until a temperature of the intermediate workpiece 14B that is not greater than a finished temperature T 3 is achieved.
  • the cooling rate is the time it takes to cool the intermediate workpiece 14B from the intermediate temperature T 2 to the finished temperature T 3 .
  • a lapsed time between completing the transfer of the intermediate workpiece 14B and achieving the formed component 12 at the finished temperature T 3 is no greater than 12 seconds. It should be appreciated that the lapsed time may be greater than 12 seconds, as the lapsed time may be dependent upon a thickness of the sheet blank 14A.
  • the finished temperature T 3 is the temperature corresponding to when the workpiece 14B has sufficiently achieved the microstructure changes to martensite.
  • the finished temperature T 3 any temperature that greater than or equal to least a martensite start temperature T s and less than or equal to a martensite finish temperature T F of the TRIP steel.
  • the martensite start temperature M s is the temperature at which the transformation from austenite to martensite begins on cooling.
  • the martensite finish temperature M f is the temperature at which the transformation from austenite to martensite finishes on cooling.
  • the finished temperature T 3 is between 150 and 200 °C. However, it should be appreciated that the martensite finish point is dependent upon the composition of the TRIP steel.
  • step 110 the shape of the component 12 is formed and the finished temperature is achieved, the method proceeds to step 114.
  • the die 22 may be opened to release the component 12.
  • the component 12 may have a ductility of between 8%and 12%, with a tensile strength of around 1, 800 MPa.
  • the formed component 12 may have a ductility of between 10%and 20%, with a tensile strength of around 1, 400 MPa.

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Abstract

A component of press hardened steel (PHS) is formed by hot forming a sheet blank formed from transformation induced plasticity (TRIP) steel. The sheet blank is heated in a furnace to no greater than 820° Celsius (C) to obtain a heated sheet blank. The heated sheet blank is transferred to a die to form an intermediate workpiece. The intermediate workpiece is hot stamped in the die. Concurrent to hot stamping the intermediate workpiece, the intermediate workpiece is cooled at a cooling rate until a temperature of not greater than a finished temperature is achieved. The cooling rate is not less than a critical cooling rate. Hot stamping and cooling form the component having a martensite microstructure, a ductility of between 8％and 20％, and a tensile strength of at least 1, 400 megapascal (MPa).

Description

METHOD OF HOT FORMING A COMPONENT FROM STEEL TECHNICAL FIELD
The present disclosure is related to a method of hot forming a component from steel.
BACKGROUND
Press hardened steel (PHS) , also referred to in the art as “boron-steel” or “hot-stamped steel” , is one of the strongest steels used for automotive body structural applications—having tensile strength properties on the order of about 1, 500 mega-Pascal (MPa) . The use of PHS structural components is being uses on many structural components, such as on a vehicle. The structural components may be bumpers, center pillars, hinge pillars, cross members, and the like.
Such structural components made of PHS are often produced via a so-called “hot forming” or “hot stamping” process. As part of the production process, large slabs of steel undergo various metal working processes to obtain sheet stock, which is rolled into spools or coils. During hot forming, one or more blanks are cut from the metal coil. The cut blanks, which may or may not be preformed at ambient temperatures, are heated to elevated temperatures, e.g., around 900 degree Celsius (C) , and thereafter transferred to water cooled dies. Subsequently, the hot blanks are formed and quenched in the dies to achieve the final shape.
SUMMARY
A component of press hardened steel (PHS) is formed by hot forming a sheet blank formed from transformation induced plasticity (TRIP) steel. The sheet blank is heated in a furnace to no greater than 850° Celsius (C) to obtain a heated sheet blank. The heated sheet blank is transferred to a die to form an intermediate workpiece. The intermediate workpiece is hot stamped in the die. Concurrent to hot stamping the intermediate workpiece, the intermediate workpiece is cooled at a cooling rate until a temperature of not greater than a finished temperature is achieved. The cooling rate is not less than a critical cooling rate. Hot stamping and cooling form the component  having a martensite microstructure, a ductility of between 8％and 20％, and a tensile strength of at least 1, 400 megapascal (MPa) .
The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the present teachings when taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustrative view of a system for making a component from a sheet blank using hot forming.
FIG. 2A is a schematic illustrative view of an apparatus including the component.
FIG. 2B is a schematic illustrative side view of the sheet blank having a base layer and a coating composition.
FIG. 3 is a flow chart describing an example method for hot forming the component from a sheet blank.
FIG. 4 is a graphical illustration of time versus temperature during hot forming the component from the heat blank as shown in FIG. 1.
DETAILED DESCRIPTION
Referring to the drawings in which like elements refer to like components throughout the several Figures, FIG. 1 illustrates a system 10 for making a formed component 12 from a sheet blank 14 via hot forming. Referring to FIG. 2A, the formed component 12 may be attached to an apparatus 15. The apparatus 15 may be a vehicle and the component 12 may be a bumper, a cross member, a pillar, a body panel, and the like. It should be appreciated that the component 12 may also be used for any other desired apparatus 15, including, but not limited to, an aircraft, a building, a bridge, a boat, machinery, and the like.
Referring again to FIG. 1, the system 10 may include a furnace 16, a transfer device 18, and a stamping press 20. The sheet blank 14 may be passed through the furnace 16. As will be explained in more detail below, the sheet blank 14 comprises  transformation induced plasticity (TRIP) steel. The furnace 16 is configured to heat and soak the sheet blank 14 at a temperature of at least the austenitization temperature T1 of the TRIP steel.
The transfer device 18 is configured to transfer the sheet blank 14 from the furnace 16 to the stamping press 20, once the sheet blank 14 has been soaked sufficiently long enough to fully austenitized the TRIP steel of the sheet blank 14. Meaning, the TRIP steel of the sheet blank 14 is austenite. The transfer device 18 may be a robotic arm, a conveyor, a slide mechanism, and the like.
The stamping press 20 includes a die 22 that provides a cooling mechanism. The die 22 is designed to form a desired final shape of the component 12 from the austenitized sheet blank 14. The die 22 may include a first forming die 22A and a second forming die 22B that are brought together to form the desired final shape do the component 12 therebetween.
Further, die 22 is configured to provide cooling to the sheet blank 14 as the first and second forming die 22A, 22B are brought together to form the component 12. The first and second forming die 22A, 22B are selectively brought together to form the component 12. The die 22 may be a water-cooled die 22. More specifically, the first and second forming die 22A, 22B may each define at least one cooling channel (not shown) therein. As is known, the cooling channels may be configured to control the cooling provided by the first and second forming die 22A, 22B such that the formed component 12 is quenched in a controlled manner, consistent across surfaces of the formed component 12 to cause a phase transformation from austenite to martensite, as will also be explained in more detail below. Therefore, the first and  second die  22A, 22B may cooperate to function as a heat sink to draw heat from, and otherwise quench, the formed component 12.
The sheet blank 14 may be formed by a blanking operation (not shown) . During the blanking operation, the sheet blank 14 is cut out of a sheet or roll of material, as is known to those skilled in the art. The sheet blank 14 may be formed using any desired cutting method. By way of a non-limiting example, the sheet blank 14 may be formed in a blanking press (not shown) by a punch operation, a stamping press, and the like. However, it should be appreciated that other forming methods may be used to cut  the sheet blank 14 from the material, including, but not limited to via laser cutting (not shown) so long as the desired dimensions and/or shape of the sheet blank 14 is achieved.
Referring now to FIG. 2B, typically, the sheet blank 14 is a generally flat sheet of material. The sheet blank 14 comprises a base layer 24, including a first side 26 and a second side 28, opposing the first side 26. The base layer 24 comprises TRIP steel. A coating composition 30 may be disposed on the first and/or  second side  26, 28 of the base layer 24. The coating composition 30 may comprise zinc. More specifically, the coating composition 30 may be primarily comprised of zinc. It should be appreciated, however, that the coating composition 30 is not limited to comprising zinc, but may also be comprised of other elements. The sheet blank 14 undergoes the hot forming process to provide the component 12 formed from press hardened steel (PHS) .
In one embodiment, the TRIP steel may be formed from manganese, to yield a medium manganese content having 4-12％ manganese, i.e., a Mn-TRIP steel. The Mn-TRIP steel may be, for example, a 7Mn-TRIP steel, a 10-Mn-TRIP steel, and the like. As such, the Mn-TRIP steel may have the following composition, based on 100 parts by weight of the Mn-TRIP steel: carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight； manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight； and with the remainder being iron and impurities inherent in processing.
Additionally, the composition of the Mn-TRIP steel may include: silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight； chromium present in an amount of less than about 1 part by weight； titanium present in an amount of less than about 0.2 parts by weight； aluminum present in an amount of less than about 0.1 part by weight； phosphorus present in an amount of less than about 0.2 parts by weight； and sulfur present in an amount of less than about 0.05 parts by weight.
As one possible alternative, the TRIP steel may be Delta-TRIP steel, where the TRIP steel has a greater concentration of aluminum than silicon. As such, the Delta-TRIP steel may have the following composition, based on 100 parts by weight of the Delta-TRIP steel: carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight； aluminum present in an amount of from  greater than about 1 parts by weight to less than about 5 parts by weight； and the remainder being iron and impurities inherent in processing.
Additionally, the composition of the Delta-TRIP steel may include: manganese present in an amount of from greater than about 0.1 parts by weight to less than about 1 parts by weight； silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight； chromium present in an amount of less than about 1 part by weight； titanium present in an amount of less than about 0.2 parts by weight； phosphorus present in an amount of less than about 0.2 parts by weight； and sulfur present in an amount of less than about 0.05 parts by weight.
By way of a non-limiting example, the TRIP steel may be a Mn-TRIP steel, a Delta-TRIP steel, and the like. As will be explained in more detail below, the TRIP steel has an austenitization temperature T1 of no greater than 782 ℃. Preferably, the TRIP steel has an austenitization temperature T1 of between 750 and 800 degrees Celsius (℃) . More preferably, the austenitization temperature T1 may be between 750 and 782 ℃. Likewise, zinc has a melting temperature of 420 ℃ and, at 782 ℃, reacts with iron via a eutectoid reaction and forms a brittle phase. Also deformation, liquid zinc can attack grain boundaries of iron and, hence, form cracks, i.e., a phenomenon called liquid metal embrittlement (LME) . Zinc attacks austenite more than in other iron phases, such as ferrite. The LME in coated-steel happens when the following conditions are met: (a) a liquid metal phase in the coating layer 30； (b) a phase in the base layer 24 that is sensitive to LME； and (c) deformation of the base layer 24. When all of these conditions are met, liquid metal will wet the freshly exposed grain boundary (of the phase in the substrate) and cause de-cohesion/separation along the grain boundary. The austenitization temperature T1 will always be greater than the melting temperature of the coating composition 30 (mainly zinc) . The key is to hot form the TRIP steel of base layer 24 at a temperature that is below 782 ℃ to avoid LME by: (a) depletion of liquid zinc in the coating by the reaction of Zinc + Iron (Zn + Fe) at 782 ℃ as the temperature drops to below 782 ℃； and (b) transforming austenite in the base layer 24 to other phases (such as ferrite) that are less sensitive to LME.
Thus, as explained in more detail below, during the formation of the component 12, the temperature of the sheet blank 14 during the hot forming process is  conducted below 782 ℃, because at 782 ℃, a zinc iron (ZnFe) compound is formed, which depletes the liquid zinc. As such, the LME described above is avoided. As such, an increased zinc concentration on the hot formed component 12, resulting in improved corrosion protection on the component 10. Since welding dissimilar metals is difficult, if not impossible, providing a zinc coating composition 30 reduces welding complexity when other components already have a zinc coating composition.
The TRIP steel has a critical cooling rate that is the slowest rate of cooling to produce a fully hardened martensitic condition within the component 12. In one embodiment, the critical cooling rate is no greater than about 10 Kelvin/second (K/s) . However, it should be appreciated that TRIP steel may have lower critical cooling rates, such as about 0.5 K/s.
The TRIP steel not only greatly reduces the austenitization temperature T1, but also significantly shifts the nose of the continuously cooling transformation (CCT) curve to the right, allowing more time so the critical cooling rate can be slower. The lower critical cooling rate improves the hardenability of the TRIP steel. Further, lowering the critical cooling rate required to achieve the fully hardened martensitic condition provides an intermediate workpiece 14B that is “forgiving’ in terms of achieving a full martensite microstructure everywhere on the intermediate workpiece 14B. This means that a lower critical cooling rate is less demanding during cooling of the intermediated workpiece 14B. By way of a non-limiting example, during the concurrent forming and cooling (steps 110, 112) of the intermediate workpiece 14B some of the surface (s) of the first and second forming  die  22A, 22B may have dirt or oxide that builds up, thus changing thermal conductivity locally, causing “hot spots” . As a result, the hot spots may promote a slower cooling rate locally, as opposed to other areas on the surface (s) that do not have the buildup. Therefore, if the critical cooling rate required is higher than the actual rate of cooling at the hot spots and/or anywhere else along the surface (s) , slow cooling results, thereby producing a non-martensite microstructure with less hardness. However, if the critical cooling rate required for the TRIP steel is less than the actual rate of cooling at the hot spots and anywhere else along the surface (s) of the first and second forming  die  22A, 22B, the ability to achieve the fully hardened martensite is not affected.
FIG. 3 depicts a method 100 of making the component 12 with the system 10. FIG. 4 depicts a time (t) and temperature (T) curve representing the making of the component 12. The method 100 is described herein with respect to the system 10, component 12, and the time and temperature curve, respectively shown in FIGS. 1, 2A, and 4.
The method 100 commences at step 102 where the sheet blank 14 is provided. The sheet blank 14 is formed from steel, e.g., Mn-TRIP steel and Delta-TRIP steel, as described above. Further, the sheet blank 14 may include a coating composition 30 disposed on the first and/or  second side  26, 28 of a base layer 24.
The method proceeds to step 104, where the sheet blank 14 is heated in the furnace 16 to the austenitization temperature T1 of the TRIP steel to provide a heated sheet blank 14A. At the start of heating the sheet blank 14 in the furnace 16, the sheet blank 14 may be at ambient temperature T0, e.g., approximately 25 ℃. As described above, the austenitization temperature T1 is preferably no greater than 782 ℃. More preferably, the austenitization temperature T1 is between 750 and 800 ℃. Even more preferably, the austenitization temperature is between 750 and 782 ℃ to obtain the heated sheet blank T1. Once the austenitization temperature T1 of the sheet blank 14 is achieved, the method 100 proceeds to step 106.
At step 106, the heated sheet blank 14 is soaked in the furnace 16 for a desired amount of time at, or slightly above, the austenitization temperature T1. More specifically, the sheet blank 14 is soaked in the furnace 16 until the TRIP steel is fully austenitized, i.e., austenite. By way of a non-limiting example, the sheet blank 14 may be soaked to between 1 and 10 minutes. However, in one embodiment, the combination of  steps  104 and 106, i.e., respectively heating and soaking the sheet blank 14, may be completed in about four minutes. Next, the method 100 proceeds to step 108.
At step 108, the heated sheet blank 14A is transferred to the die 22 to form an intermediate workpiece 14B. More specifically, when the heated sheet blank 14A is removed from the furnace 16, the heated sheet blank 14A is at the austenitization temperature T1. As described above, the austenitization temperature T1 of the TRIP steel is most preferably between 750 and 782 ℃. However, once the heated sheet blank 14 exits the furnace 16, the heated sheet blank 14 immediately begins to lose temperature  (cool) in the ambient temperature surrounding the furnace 16 to provide an intermediate workpiece 14B at an intermediate temperature T2. Therefore, the intermediate workpiece 14B is inserted into the die 22 at an intermediate temperature T2 that is less than the austenitization temperature T1 of the heated sheet blank 14B that was removed from the furnace 16. The amount of cooling during the transfer to the die 22 is dependent on the time that it takes to complete the transfer to the die 22 and the temperature of the ambient air between the furnace 16 and the die 22. In one embodiment, the intermediate temperature T2 of the intermediate workpiece 14B is between 700 and 750 ℃ upon insertion into the die 22 and the time to transfer between the furnace 16 and the die 22 is about six seconds. Next, the method concurrently proceeds to  steps  110 and 112.
At  steps  110 and 112, the intermediate workpiece 14B is respectively hot stamped and quenched, i.e., cooled, in the die 22. More specifically, at step 110, a first forming die 22A and a second forming die 22B are brought together, with the intermediated workpiece 14B disposed therebetween to form the desired final shape of the component 12. The temperature of the intermediate workpiece 14B at the beginning of step 110 may be between about 700 and 750 ℃.
Concurrently, at step 112, the intermediate workpiece 14B is cooled in the die 22 as the first and second forming  die  22A, 22B form the desired final shape. The temperature of the intermediate workpiece 14B at the beginning of step 112 may be between about 700 and 750 ℃. In one embodiment, the intermediate workpiece 14B is cooled via watercooling. More specifically, as described above, each of the first and second forming  die  22A, 22B may each define at least one cooling channel (not shown) therein. As is known, the cooling channels may be configured to control the cooling rate provided by the first and second forming  die  22A, 22B to quench the component 12. Water is injected to flow through the channels to promote a heat exchange between the intermediate workpiece 14B and the water, via the first and second forming  die  22A, 22B, to cool the intermediate workpiece 14B at a desired cooling rate until a temperature of the intermediate workpiece 14B that is not greater than a finished temperature T3 is achieved. It should be appreciated that the cooling medium is not limited to using water, as other medium may also be used, such as oil and the like. The cooling rate is the time it takes to cool the intermediate workpiece 14B from the intermediate temperature T2 to the finished  temperature T3. In one embodiment, a lapsed time between completing the transfer of the intermediate workpiece 14B and achieving the formed component 12 at the finished temperature T3 is no greater than 12 seconds. It should be appreciated that the lapsed time may be greater than 12 seconds, as the lapsed time may be dependent upon a thickness of the sheet blank 14A.
The finished temperature T3 is the temperature corresponding to when the workpiece 14B has sufficiently achieved the microstructure changes to martensite. The finished temperature T3 any temperature that greater than or equal to least a martensite start temperature Ts and less than or equal to a martensite finish temperature TF of the TRIP steel. The martensite start temperature Ms is the temperature at which the transformation from austenite to martensite begins on cooling. Likewise, the martensite finish temperature Mf is the temperature at which the transformation from austenite to martensite finishes on cooling. In one embodiment, the finished temperature T3 is between 150 and 200 ℃. However, it should be appreciated that the martensite finish point is dependent upon the composition of the TRIP steel. Once the finished temperature T3 of the TRIP steel is achieved, microstructure changes within the TRIP steel are completed.
At the completion of  steps  110 and 112, the shape of the component 12 is formed and the finished temperature is achieved, the method proceeds to step 114.
At step 114, the die 22 may be opened to release the component 12. In one non-limiting example, if the component 12 is formed from Mn-TRIP steel, the component 12 may have a ductility of between 8％and 12％, with a tensile strength of around 1, 800 MPa. In another non-limiting example, if the component 12 is formed from Delta-TRIP steel, the formed component 12 may have a ductility of between 10％and 20％, with a tensile strength of around 1, 400 MPa.
While the best modes for carrying out the many aspects of the present teachings have been described in detail, those familiar with the art to which these teachings relate will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims.

Claims (19)

  1. A method of forming a component, the method comprising:
    providing a sheet blank formed from steel；
    heating the sheet blank in a furnace to no greater than 820° Celsius (C) to obtain a heated sheet blank；
    transferring the heated sheet blank to a die to form an intermediate workpiece；
    hot stamping the intermediate workpiece in the die；
    concurrent to hot stamping the intermediate workpiece, cooling the intermediate workpiece in the die at a cooling rate until a temperature of not greater than a finished temperature is achieved；
    wherein the cooling rate is not less than a critical cooling rate； and
    wherein hot stamping and cooling the intermediate workpiece forms the component.
  2. The method, as set forth in claim 1, wherein the finished temperature is the temperature that is greater than or equal to a martensite start temperature and less than or equal to a martensite finish temperature.
  3. The method, as set forth in claim 2, wherein the finished temperature is between 150 and 200℃.
  4. The method, as set forth in claim 3, wherein heating the sheet blank in the furnace is further defined as heating the sheet blank in the furnace to between 750 and 782℃ to obtain a heated sheet blank.
  5. The method, as set forth in claim 1, wherein the critical cooling rate is not greater than 10 Kelvin per second (K/s) .
  6. The method, as set forth in claim 1, wherein an intermediate temperature of the heated sheet blank when the hot stamping and the cooling of the intermediate workpiece each commences is between 700 and 750℃.
  7. The method, as set forth in claim 1, wherein providing a sheet blank is further defined as:
    disposing a coating composition comprising zinc onto a base layer formed from transformation induced plasticity (TRIP) steel to thereby form a sheet blank；
    wherein the base layer includes a first side and a second side, opposing the first side； and
    further wherein disposing includes applying the coating composition to at least one of the first side and the second side.
  8. The method, as set forth in claim 1, wherein providing a sheet blank is further defined as providing a sheet blank formed from transformation induced plasticity (TRIP) steel comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight； and
    manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight.
  9. The method, as set forth in claim 8, wherein providing a sheet blank is further defined as providing a sheet blank formed from TRIP steel further comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight；
    silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight；
    chromium present in an amount of less than about 1 part by weight；
    titanium present in an amount of less than about 0.2 parts by weight；
    aluminum present in an amount of less than about 0.1 part by weight；
    phosphorus present in an amount of less than about 0.2 parts by weight；
    sulfur present in an amount of less than about 0.05 parts by weight； and
    the remainder being iron and impurities inherent in processing.
  10. The method, as set forth in claim 8, wherein the formed component has a ductility of between 8％ and 12％ , with a tensile strength of at least 1, 800 megapascal (MPa) .
  11. The method, as set forth in claim 1, wherein providing a sheet blank is further defined as providing a sheet blank formed from transformation induced plasticity (TRIP) steel comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    aluminum present in an amount of from greater than about 1 parts by weight to less than about 5 parts by weight.
  12. The method, as set forth in claim 11, wherein the sheet blank comprises transformation induced plasticity (TRIP) steel further comprising the following components by weight based on total weight:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    manganese present in an amount of from greater than about 0.1 parts by weight to less than about 1 parts by weight；
    silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight；
    chromium present in an amount of less than about 1 part by weight；
    titanium present in an amount of less than about 0.2 parts by weight；
    aluminum present in an amount of from greater than about 1 part by weight to less than about 5 parts by weight；
    phosphorus present in an amount of less than about 0.2 parts by weight；
    sulfur present in an amount of less than about 0.05 parts by weight； and
    the remainder being iron and impurities inherent in processing.
  13. The method, as set forth in claim 11, wherein the formed component has a ductility of between 10％ and 20％ , with a tensile strength of at least 1,400 MPa.
  14. A formed component comprising:
    a press hardened steel (PHS) having a martensite microstructure, a ductility of between 8％ and 20％ , and a tensile strength of at least 1, 400 megapascal (MPa) ；
    wherein the martensite microstructure, the ductility of between 8％ and 20％ , and the tensile strength of at least 1, 400 MPa are imparted by:
    providing a sheet blank formed from steel；
    heating the sheet blank in a furnace to no greater than 820° Celsius (C) to obtain a heated sheet blank；
    transferring the heated sheet blank to a die to form an intermediate workpiece；
    hot stamping the intermediate workpiece in the die；
    cooling the intermediate workpiece at a cooling rate until a temperature of not greater than a finished temperature is achieved；
    wherein the finished temperature is the temperature that is greater than or equal to a martensite start temperature and less than or equal to a martensite finish temperature；
    wherein the cooling rate is not less than a critical cooling rate； and
    wherein hot stamping and cooling the intermediate workpiece forms the component.
  15. The formed component, as set forth in claim 14, wherein providing a sheet blank is further defined as:
    disposing a coating composition comprising zinc onto a base layer formed from transformation induced plasticity (TRIP) steel to thereby form a sheet blank；
    wherein the base layer includes a first side and a second side, opposing the first side； and
    further wherein disposing includes applying the coating composition to at least one of the first side and the second side.
  16. The formed component, as set forth in claim 14, wherein providing a sheet blank is further defined as providing a sheet blank formed from transformation induced plasticity (TRIP) steel comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight； and
    the remainder being iron and impurities inherent in processing.
  17. The formed component, as set forth in claim 16, wherein providing a sheet blank is further defined as providing a sheet blank formed from TRIP steel further comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    manganese present in an amount of from greater than about 4 parts by weight to less than about 12 parts by weight；
    silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight；
    chromium present in an amount of less than about 1 part by weight；
    titanium present in an amount of less than about 0.2 parts by weight；
    aluminum present in an amount of less than about 0.1 part by weight；
    phosphorus present in an amount of less than about 0.2 parts by weight；
    sulfur present in an amount of less than about 0.05 parts by weight； and
    the remainder being iron and impurities inherent in processing.
  18. The formed component, as set forth in claim 14, wherein providing a sheet blank is further defined as providing a sheet blank formed from transformation induced plasticity (TRIP) steel comprising, based on 100 parts by weight of the TRIP steel:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    aluminum present in an amount of from greater than about 1 parts by weight to less than about 5 parts by weight； and
    the remainder being iron and impurities inherent in processing.
  19. The formed component, as set forth in claim 18, wherein the sheet blank comprises transformation induced plasticity (TRIP) steel further comprising the following components by weight based on total weight:
    carbon present in an amount of from greater than about 0.1 part by weight to less than about 0.4 parts by weight；
    manganese present in an amount of from greater than about 0.1 parts by weight to less than about 1 parts by weight；
    silicon present in an amount of from greater than about 0.1 part by weight to less than about 0.5 parts by weight；
    chromium present in an amount of less than about 1 part by weight；
    titanium present in an amount of less than about 0.2 parts by weight；
    aluminum present in an amount of from greater than about 1 part by weight to less than about 5 parts by weight；
    phosphorus present in an amount of less than about 0.2 parts by weight；
    sulfur present in an amount of less than about 0.05 parts by weight； and
    the remainder being iron and impurities inherent in processing.
PCT/CN2014/095753 2014-12-31 2014-12-31 Method of hot forming a component from steel Ceased WO2016106621A1 (en)

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US11913085B1 (en) 2022-08-08 2024-02-27 GM Global Technology Operations LLC Methods for preparing high performance press-hardened steel components
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US10385415B2 (en) 2016-04-28 2019-08-20 GM Global Technology Operations LLC Zinc-coated hot formed high strength steel part with through-thickness gradient microstructure
US10619223B2 (en) 2016-04-28 2020-04-14 GM Global Technology Operations LLC Zinc-coated hot formed steel component with tailored property
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US11951522B2 (en) 2018-06-19 2024-04-09 GM Global Technology Operations LLC Low density press-hardening steel having enhanced mechanical properties
US11530469B2 (en) 2019-07-02 2022-12-20 GM Global Technology Operations LLC Press hardened steel with surface layered homogenous oxide after hot forming
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US11400690B2 (en) 2019-12-24 2022-08-02 GM Global Technology Operations LLC High performance press-hardened steel assembly
US11913085B1 (en) 2022-08-08 2024-02-27 GM Global Technology Operations LLC Methods for preparing high performance press-hardened steel components
US12344915B2 (en) 2023-01-10 2025-07-01 GM Global Technology Operations LLC Hot dip galvanized ultra-high strength steel sheets with high flatness

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