Abstract:
A processing method is provided integrating hot stamping and controllable subsequent heat treatment in the technical field of hot working for steels. It includes the steps of selecting and preparing a steel blank, hot stamping treatment, controllable temperature quenching, adjusting the temperature of the steel blank to a tempering temperature and fine stamping at the same time. The invention combines the hot stamping and controllable heat treatment to improve the strength of material with precise plastic deformation and save energy as well. The tensile strength of the processed steel is greater than 1900 Ma.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of PCT/CN2011/072806, filed Apr. 14, 2011, incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a process in the technical field of hot stamping for sheet metals, and more particularly, to an integrated processing method for sheet steel hot stamping and heat treatment. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    A series of high-strength sheet steels has been developed successively to meet the demand for products with light weight and high reliability in the modern manufacturing industry. At present, the strength of sheet steel used for manufacturing the car body for high-class car is no more than about 1400 MPa. 
         [0004]    The Chinese patent application with the document number CN101121955 published on Feb. 13, 2008 has been searched out, and it discloses “a heat treatment process for improving the mechanical performance of quenched steel by carbon partitioning and tempering”. In addition, an article, T. Y. Hsu “Quenching-Partitioning-Tempering (Precipitation)(Q-P-T) Process for Ultra-high-strength steel”, Heat treatment, Vol. (No. 2, 2008), reports a new heat treatment technology adopting a Quenching-Partitioning-Tempering heat treatment technology. The technology involves the following steps: 1) The first step is austenization: heating the steel to above austenization temperature. 2) The second step is quenching: controlling the quenching temperature to be amid of the martensitic finish temperature (Mf) and the martensitic start temperature (Ms) to make the austenite partially transform into the martensite. 3) The third step is partitioning: raising the temperature slightly above the Ms point and keeping it for a short time, which forms a thin layer of austenite with high content of carbon around the martensite lath by repartitioning the carbon between the martensite and the austenite 4) The fourth step is changing the temperature of the steel to a tempering temperature to induce precipitation hardening in the martensite, which further improves the hardness and strength of the martensite. 5) The fifth step is cooling: cooling down the steel from the tempering temperature to the room temperature to transform part of retained austenite into martensite while remain a thin layer of retained austenite with high content of carbon around the martensite lath generated previously (in step 2) to attain the high toughness of the steel. After the Quenching-Partitioning-Tempering treatment, the tensile strength of the medium carbon steel containing silicon and niobium reaches 2100 MPa, and the ultimate elongation ratio reaches 11%. Therefore, the Q-P-T process has tremendous promising application in relevant industries. 
         [0005]    However, if the cold stamping is performed after the Q-P-T treatment, the high strength of the sheet material results in short lifespan of the blanking die and cold forming die (such as the drawing die and cold stamping die), and if the cold stamping is performed before the Q-P-T treatment, the dimensional precision of the stamped piece cannot be maintained due to the heat treatment distortion, and the advantages of the cold stamping are eliminated. Therefore, the disadvantages of the above technologies make them unsuitable for mass production with the sheet steels as raw material. It cannot meet the demand for mass production such as vehicle body manufacturing. 
       SUMMARY OF THE INVENTION 
       [0006]    To solve the above problem in the prior art, the invention provides a processing method integrating sheet steel hot stamping and controllable quenching together, which combining the hot stamping and heat treatment to simultaneously improve the strength of material and ensure precise plastic deformation of the steel, as well as to save energy. 
         [0007]    The invention includes the following technical scheme and steps. 
         [0008]    The first step is selecting and preparing a steel blank: blanking or cutting a steel plate, a steel sheet or a section beam in the cold state to make the steel blank. 
         [0009]    The said blanking includes the punching using a cutting die, a blanking die or a knife-edge die. 
         [0010]    The second step is hot stamping process: heating the steel blank to an austenization temperature and hot stamping it. 
         [0011]    The said heating means heating under vacuum or in protective atmosphere using a resistance furnace or a fuel furnace. 
         [0012]    The third step is quenching with a controlled cooling temperature: quenching the hot stamped workpiece. 
         [0013]    The said quenching means setting the cooling temperature T1 as follows: Mf≦T1≦Ms, wherein the range of Ms is given as follows: 200° C.≦Ms≦500° C., and the range of Mf is given as follows: 50° C.≦Mf≦200° C., thus to transform 50% to 90% austenite to martensite. 
         [0014]    The fourth step is partitioning-tempering operation: reheating or maintaining the workpiece at T1, then lowering the temperature to the tempering temperature to perform the tempering heat treatment and performing the fine stamping at the same time. 
         [0015]    The said reheating means positioning the steel blank in a salt-bath furnace, a fluidized bed furnace or an air heating furnace, raising the temperature to a heating temperature T2, 100° C.≦T2≦500° C., and keeping the temperature for 10 to 900 seconds. 
         [0016]    The said tempering heat treatment means positioning the steel blank obtained after the reheating treatment in a salt-bath furnace, a fluidized bed furnace or an air heating furnace, and performing the tempering treatment at a temperature T3, wherein the range of the tempering temperature T3 is given as follows: 100° C.≦T3≦500° C. 
         [0017]    According to the different requirement of products, the above process may be simplified in the actual operation procedure, and the quenching temperature may equal to the temperature of the reheating treatment and the temperature of the tempering heat treatment, that is, T1=T2=T3 or T1≠T2=T3, etc. Besides, the Q-P-T process can be accomplished directly in the die, where the work-piece is cooled down by the die and temperature controlled by controlling the die temperature. 
         [0018]    The advantages of the invention are described as follows. 
         [0019]    1) Since the blanking and punching are performed when the sheet steel is in an unhardened state before the heat treatment, so that the mass production may be achieved by using common blanking and punching method as used for common material. 
         [0020]    2) The hot stamping and the controllable quenching heat treatment are combined. In the austenite state, the deformation resistance is small, and plastic forming with large deformation can be performed. After the forming step, the quenching is performed directly without reheating, which saves energy and avoids distortion due to temperature change. 
         [0021]    3) After the temperature-controlled processes of quenching, partitioning and tempering, the steel still has a certain quantity of austenite, so that it remains plastic deformation ability to some extent. The followed fine stamping takes this advantage and ensures the final precision of the shape. Through the reasonable combination of hot stamping and heat treatment, the technology simultaneously obtains the same material strength as that obtained via the quenching-partitioning-tempering process and the workpiece precision as well. Therefore, the technology is of great promising to application in mass industrial production and achievement of substantial technical progress. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The embodiments of the invention are described in detail hereinbelow, and they are performed on the premise of the technical scheme of the invention. The embodiments provide the detailed implementing steps and specific operating procedures. However, the protection scope of the invention is not limited to the following embodiments. 
       Embodiment 1 
     The Manufacture of Helmet with Super High Strength 
       [0023]    A 1 mm thick cold rolled steel sheet with the thickness made of the medium carbon steel containing silicon and niobium is selected as material. In the state as it is provided, the cold rolled sheet is blanked using the friction press and cut to make the sheet blank of the helmet. The sheet blank is heated via a mesh belt heating furnace, and it lies on the mesh belt at the feed-in end of the mesh belt furnace and conveyed into the furnace by the mesh belt to be heated. The temperature of the furnace is controlled at 840° C. to complete austenitizing the steel sheet blank. The atmosphere nitrogen and methanol is introduced into the furnace to protect it. After the sheet blank is out of the furnace, it is transferred to the hot stamping machine by a manipulator quickly to be stamped. The hot stamping machine has three working stations. The first working station is drawing, the second one is shaping, and the third one is trimming After the hot stamping, the helmet is immediately soaked into the nitrate bath at the temperature of 210° C. After the work piece stays therein for 3 minutes, the temperature of the work piece has been lowered to 210° C. Thus, the moderate-cold quenching is completed, and the partitioning operation is performed quickly. The helmet is transferred to a nitrate furnace at the temperature of 400° C. from the moderate-cold isothermal nitrate tank and kept for 1.5 minutes. Afterwards, it is transferred to a fluidized bed furnace at the temperature of 410° C. to perform the tempering treatment. The tempering treatment lasts for 15 minutes. After the tempering treatment, the fine stamping and shaping is performed immediately when it is still hot. The fine stamping and shaping is performed via the friction press of 100 tons. Thus, the helmet meets the requirement for shape, size and tensile strength of the products. 
       Embodiment 2 
     Car Door 
       [0024]    The cold rolled steel sheet made of the medium carbon steel containing silicon and niobium is selected as material. The sheet with thickness of 1.2 mm is used to make the car door via the following operations. 
         [0025]    Operation 1 is blanking and punching: blanking and punching using the press in the cold state to cut it to make the sheet blank of the car door. 
         [0026]    Operation 2 is austenitizing: heating the sheet blank of the car door to the austenitization temperature of 840° C. by a roller type heating furnace, and introducing the protective atmosphere nitrogen and methanol into the furnace to avoid surface oxidation and decarburization of the sheet blank. 
         [0027]    The operation 3 is hot stamping: after finishing the heating operation and taking the sheet blank out of the roller type heating furnace, transferring the sheet blank to the stamping press immediately by the manipulator to punch the sheet blank in the austenite state to form the shape of the car door. 
         [0028]    The operation 4 is moderate-cold quenching-partitioning: after the hot stamping operation, putting the car door into the fluidized bed at the temperature of 250° C. quickly by the manipulator to quench it, keeping it for 5 minutes to low the temperature of the car door to the temperature of the quenching, and completing the partitioning operation at the same time. 
         [0029]    The operation 5 is tempering: taking the car door out of the fluidized bed at the partitioning temperature and transferring it into a tempering furnace to temper it at the tempering temperature of 250° C., wherein the tempering furnace is an air furnace equipped with a stirring blower and using electricity for heating, and the tempering operation lasts for 20 minutes. 
         [0030]    The operation 6 is fine stamping: after the tempering operation, taking the car door out of the tempering furnace and putting it into the fine-stamping die directly by the manipulator to perform the fine stamping when it is still hot. 
       Embodiment 3 
     Thin-Walled Beam 
       [0031]    The length and width of the beam are respectively 4250 mm and 250 mm, the height of side walls is 120 mm, and a hole with a diameter of 120 mm is punched on each of the two ends. The section beam is made from plate of medium carbon steel containing silicon and niobium and the thickness is 5 mm The beam is produced as follows. 
         [0032]    The operation 1 is blanking and punching: punching to make the plate blank of the beam in the cold state using a knife-edge die or blanking die, and making the circular holes at the two ends by laser cutting process. 
         [0033]    The operation 2 is austenitizing: heating the plate blank of the beam in a through-type heating furnace, wherein the stepping or chain plate type driving mechanism passes through the whole furnace chamber and extends from the feed-in furnace door to the feed-out furnace door at the two ends, and the steel blank is lain on the driving mechanism, driven into the furnace from the feed-in port by the driving mechanism to pass through the furnace, and heated to the austenitization temperature of 850° C. 
         [0034]    The operation 3 is hot stamping: transferring the steel blank to a forming die of a stamping machine immediately by the manipulator to be punched after it is out of the furnace. 
         [0035]    The operation 4 is moderate-cold quenching: after the hot stamping operation, quenching the beam immediately, and cooling it down in the aqueous polymer solutions for 2 seconds firstly, and then transferring it to the fluidized bed at the temperature of 200° C. and remaining for 5 minutes. 
         [0036]    The operation 5 is partitioning: transferring the beam to a partitioning heating fluidized bed at the temperature of 400° C. immediately and remaining for 150 seconds after the beam remains in the moderate-cold fluidized bed for the set time period. 
         [0037]    The operation 6 is tempering: after remaining for 150 seconds, transferring the beam to a tempering furnace at the temperature of 400° C. immediately, wherein the tempering lasts for 30 minutes, and the tempering furnace is an air furnace using electricity for heating and equipped with stirring blower therein. 
         [0038]    The operation 7 is fine stamping: after the tempering operation, taking the beam out of the tempering furnace and putting it on a precision die to punch it immediately when it is still hot, thereby the distortion produced during the quenching is modified, and the fine stamping cools the beam to the room temperature. 
       Embodiment 4 
     Front Fender 
       [0039]    The cold rolled steel sheet made of the medium carbon steel containing boron is selected as material. The sheet with thickness of 1.0 mm is used to make the front fender of a car via the following operations. 
         [0040]    Operation 1 is blanking and punching: blanking using the press in the cold state to cut it to make the sheet blank of the front fender. 
         [0041]    Operation 2 is austenitizing: heating the sheet blank of the front fender to the austenitization temperature of 900° C. using a roller type heating furnace, and introducing the protective atmosphere nitrogen and methanol into the furnace to avoid surface oxidation and decarburization of the sheet blank. 
         [0042]    The operation 3 is hot stamping: after finishing the heating operation and taking the sheet blank out of the roller type heating furnace, transferring the sheet blank to a stamping press immediately by the manipulator to punch the sheet blank in the austenite state to form the shape of the front fender. 
         [0043]    The operation 4 is moderate-cold quenching-partitioning: after the hot stamping operation, putting the front fender into the fluidized bed at the temperature of 300° C. by the manipulator immediately to quench it and remaining for 10 to 60 seconds to finish the partitioning operation at this quenching temperature for the front fender. 
         [0044]    The operation 5 is tempering: taking the front fender out of the fluidized bed at the partitioning temperature and transferring it to a tempering furnace to temper it at the tempering temperature of 350° C., wherein the tempering furnace is an air furnace equipped with a stirring blower and using electricity for heating, and the tempering operation lasts for 120 seconds. 
         [0045]    The operation 6 is fine stamping: after the tempering operation, taking the front fender out of the tempering furnace and putting it into a fine-stamping die directly by the manipulator to perform the fine stamping.