Chapter 3Hot Forming Process

3.1 Direct Hot Forming Process

In the direct hot forming process, a blank is heated up in a furnace, transferred tothe press and subsequently formed and quenched in the closed mold [1] (Fig. 3.1).First the sheet is uncoiled and cut according to the shape of the product, then theblank is transferred to a continuous furnace, in which it is heated and fullyaustenitized. Thus the product is formed and quenched after the blank is trans-ferred to the hot forming mold with cooling system. Afterwards the product istrimmed by laser and finished through other follow-up process.

The advantages of direct hot forming process are as follows:

1. The blank is formed and hardened in one mold which saves the cost of pre-forming and accelerates the pace of production.

2. The blank is flat which not only saves heating area and energy, but also can beheated by a variety of heating methods, such as induction heating.

The disadvantages of hot forming process are as follows: it cannot be used forforming automobile parts with complex shapes, and it needs the laser cuttingequipment. In addition, the design of cooling system of molds is more complex.

For automotive body structure, the parts with simple shape and no need for deepdrawing can be manufactured by the direct hot forming, such as the inside and outsidepieces of b-pillar, the inner plate of side panels, the inner plate of the threshold, thecentral pillar of front bezel and door beam, and so on (Fig. 3.2) [2, 3].

The mold for a brace dase panel of a car and the forming process are shown inFig. 3.3. This brace dase panel is simple and the drawing depth is small, so it canbe manufactured by the direct hot forming: the blank is put into the furnace whereit is heated to 950 �C and fully austenitized for 5 min, and then is transferred to thehot forming mold with cooling system quickly (Fig. 3.3). Hot forming parts areshown in Fig. 3.4. In order to verify the feasibility of the forming process,the microstructure and the mechanical properties of hot forming parts can be

P. Hu et al., Theories, Methods and Numerical Technology of Sheet Metal Coldand Hot Forming, Springer Series in Advanced Manufacturing,DOI: 10.1007/978-1-4471-4099-3_3, � Springer-Verlag London 2013

35

tested. In Fig. 3.5 6 test samples from different regions of the brace dase panel aretested for hardness and metallographical structure. The hardness test results areshown in Table 3.1. We can see that 6 samples’ hardness (HR) is all more than 47(Unit), far above the hardness of raw material, indicating that the microstructure ofthe part is uniform martensite after the process of hot forming. Metallographicresults are shown in Fig. 3.6. The raw material mainly exhibits a ferritic-pearliticmicrostructure with a small amount of carbon before hot forming; the part displaysa uniform martensite microstructure after hot forming, and the content ofmartensite is more than 95 %. Tensile test shows that the yield strength of hotforming parts that are made by hot forming exceeds 1,000 MPa, and the tensilestrength is above 1,600 MPa. These results illustrate that the material properties ofthe central pillar of front bezel produced by direct hot forming meet the technicalrequirements of the pillar [2, 4–7]. These tests also prove the feasibility of thedirect forming process.

The hot forming mold of a reinforced beam is shown in Fig. 3.7, and this beamcan be directly formed through the rational design of mold. The blank is heated to950 �C and fully austenitized for 5 min, and then is sent to the hot forming moldwith cooling system to form the beam and quench it (Fig. 3.7) by the robot. Thehot forming process is shown in Fig. 3.8. The reinforced beam formed is shown inFig. 3.9 The tensile experimental results of the reinforced beam are shown inFig. 3.10.The yield strength of the reinforced beam is over 1,000 MPa, and thetensile strength is more than 1,500 MPa, which also shows the validity of the hotforming mold and hot forming process.

Fig. 3.2 Parts with the directmethod of hot forming

Fig. 3.1 Sketch map of direct hot forming process

36 3 Hot Forming Process

Fig. 3.3 Hot forming die of brace dase panel, CTR , and its process

Fig. 3.4 Brace dase panel, CTR with one-step method of hot forming

Fig. 3.5 Testing samples for microstructure and hardness of the hot forming part

3.1 Direct Hot Forming Process 37

Fig. 3.6 Microstructure of the material before hot forming and after hot forming

Fig. 3.7 Hot forming die with one-step method of the reinforced beam inside the automobiledoor

Table 3.1 Distribution of Rockwell hardness (HRC)

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

47 48.2 49.8 49.5 50 48.7

38 3 Hot Forming Process

3.2 Indirect Hot Forming Process

In the indirect hot forming process, sheet metal is preformed through cold formingat first and then is put into the mold with the cooling system after it is heated toaustenite temperature and held for a time it [2, 8]. The indirect hot forming processis also known as ‘‘multi-step’’ method of hot forming (Fig. 3.11). First the sheet isuncoiled and cut according to the shape of the product, and then it is formed bypreforming process, such as the traditional cold forming process, flanging,punching and cutting edge, and so on. Then the preformed semi-finished productsare transferred to the continuous furnace to be heated and insulated, and sent to thehot forming mold with cooling system after being fully austenitized to be formedand quenched. Afterwards, the products are trimmed by laser and finished throughother follow-up processes according to the characteristics of the necessarycomponents or directly output.

For automotive body structure, the parts that have complex shape, or requiredeep drawing or need punching, trimming, or other complex technology must bemanufactured by the indirect hot forming.

The advantages of indirect hot forming process are:

1. The parts with complex shapes and almost all of the current stamped carryingparts can be formed by indirect hot forming process.

2. After the preforming of the blank, it is unnecessary to worry about the formingperformance of the blank at high temperature in subsequent hot formingprocess, which can ensure the martensite microstructure of the blank followedby complete quenching.

3. The blank can be processed by trimming, flanging, punching, and otherprocessing after being preformed so that it will be easier for processing after itis quenched. For example, the blank that is quenched must be trimmed by lasercutting equipment, which greatly increases the cost.

Fig. 3.8 Direct hot formingprocess of the reinforcedbeam inside the automobiledoor

3.2 Indirect Hot Forming process 39

The forming experiment of a strengthened beam is shown in Fig. 3.12. First, thepart manufactured by direct hot forming is shown in Fig. 3.13, and it is clear to seethe cracks at both ends of the beam. These cracks are due to the strengthenedbeams with 3 U-shaped deep drawing areas which greatly increase the difficulty offorming. For the special structure of the beam, indirect hot forming process isdeveloped on the basis of the numerical simulation [9–11] and the strengthenedbeam is usually formed by two sets of molds. The preforming mold is shown inFig. 3.14a, and the final formed product, the mold for quenching and theproductive process are shown in Fig. 3.14b.

First, according to the shape of the part, the blank size is got by software withinverse forming algorithm, and then the blank is uncoiled and cut, afterwards it issent into the preforming mold which is shown in Fig. 3.14a for the traditional coldforming. The required geometry of the part is obtained through the trimmingprocess; Afterwards, the preforming part is put into the furnance and fullyaustenitized at a temperature of 950 �C for 5 min, and then it is quickly sent intothe mold with cooling system for forming and quenching, as shown in Fig. 3.14b.The hot forming part is shown in Fig. 3.15. In order to verify the feasibility of

Fig. 3.9 Reinforced beam inside automobile door made by the direct hot forming process

Fig. 3.10 Engineeringstress–strain curve of thereinforced beam

40 3 Hot Forming Process

forming process, microstructure and mechanical properties of some parts should betested. The results of tensile test for the samples cut on the strengthened beam areshown in Fig. 3.16. The stress–strain curve shows that the yield strength of the hotforming parts is more than 1,000 MPa, and tensile strength is above 1,600 MPa.The experimental results show that the strengthened door beam formed by indirecthot forming meets the technical requirements for hot forming [2, 4–7], and alsoprove the validity of the indirect hot forming process and the design of hot formingmold.

Fig. 3.11 Sketch map of hot forming process with indirect method of hot forming

Fig. 3.12 Parts produced by indirect hot forming technique

Fig. 3.13 Reinforced beam with direct method of hot forming

3.2 Indirect Hot Forming process 41

3.3 The Key Parameters and Optimal Controlin Hot Forming process

The required process systems for mass production of hot forming automotivestructure of high strength steel includes the temperature system from heating tostamping process, the transmitting time system of steel and parts, the rate andcontrol of hot forming, the control of cooling rate of the mold, heating furnace, thethermal fatigue durability test of the mold with cooling system, and so on. Bymeans of these studies, some defects, such as the nonuniform distribution ofthickness, wrinkling, cracking, and bad quenching, etc., are effectively reduced oreven eliminated to ensure the quality stability of the products on production line.

The mechanical properties of the blanks at different temperatures are tested byhigh-temperature universal material testing system to draw the appropriate con-stitutive relations. The blanks temperature in the process of heating, transferring,and stamping is input in the finite element code, then the basic law of temperatureis obtained. The change in temperature throughout the whole process is monitored

Fig. 3.14 Reinforced beam with direct method of hot forming

Fig. 3.15 Reinforced beam with multistep method of hot forming

42 3 Hot Forming Process

with the thermometer through many small-volume experiments, and the optimaltemperature system is obtained compared to that of the finite element results. Inorder to obtain the required parameters in mold manufacturing process, theformability of the sheets at high temperatures is tested by use of self-made high-temperature forming performance test system. The structural parts formed bystamping need to be tested for tensile properties, collision and other performanceto validate the properties of structural components, and the production process ofthe mold is optimized according to the results.

3.3.1 The Heating Temperature, Holding Timeand Optimization Control

Hot stamping is a new technology of sheet metal forming, which is completelydifferent from that of conventional cold stamping. There are many technicalparameters and the technology process is complicated, including many key pro-cedures, such as heating, forming and cooling, etc. In order to realize the austeniticto martensite transformation, and ensure the mechanical properties of the product,the selection of technical parameters in different procedures is crucial. The heatingtemperature is an important technical parameter, together with austenization, fullyhomogeneous austenization, austenite grain size, etc. All the parameters need to beoptimized.

The heating temperate and holding time is the main process parameters duringthe heating period. The heating temperature should be kept above the recrystal-lization temperature to ensure the austenization of sheet. But the heating tem-perature cannot be too high, or it will result in sheet metal surface overburning andgrain growth, which will influence the quality and performance of quenched parts.The holding time influences the homogeneity of austenization. The sheet should beholding for a period after being heated to a specified temperature to promote theaustenization. But the holding time cannot be too long, or it will lead to grain

Fig. 3.16 Engineeringstress–strain curve of thereinforced beam

3.3 The Key Parameters and Optimal Control in hot forming process 43

growth and other worse mechanical properties. Besides, the long holding time willincrease the production period and reduce the production efficiency.

3.3.2 Transfer Time of High Temperature Sheet

After high temperature austenization, the sheet metal will be transferred to thewater-cooled die by manipulator and it will be cooled in the process. If the transfertime is too long, it will increase the high temperature oxidation of sheet metal. Onthe other hand, bainite transformation and ferrite transformation will be induced inthe sheet metal. The transfer time of high temperature sheet in accordance with theproduction takt time needs to ensure that the sheet temperature in the die should bebetween 700 and 600 �C, to obtain superior hot forming performance [5].

3.3.3 Hot Forming Rate, Cooling Rate in Dieand the Control of Them

During the hot forming period, sheet metal needs to be stamped in austenite state.The workpiece should be stamped quickly to avoid much heat loss and over quicktemperature drop. Therefore, the hot stamping needs high speed hydropress.During the cooling period, the workpiece is quenched by die surface to induceaustenite to martensite transformation and further strengthened. However, thiskind of phase transformation has something to do with cooling speed. Only whenthe cooling speed exceeds a critical value can the austenite transfer into martensite.If not, bainite will appear during the forming period, which influences theimprovement of workpiece quality. For 22MnB5 high strength steel sheet, in hotstamping process, the minimum cooling speed to achieve austenite to martensitetransformation is 30 �C/s [12, 13]. Thus, in order to ensure austenitic to martensitetransformation in hot stamping technology, the cooling speed of workpiece mustbe greater than this value. For this reason, we should improve the circulation of thecooling medium pressure and cycle speed, timely take away the quantity of heat ofdie surface, and make the cooling medium keep in a temperature range at the sametime [14]. But it is not true the higher the cooling speed, the better it is. Too highcooling speed will lead to forming cracks.

References

1. Karbasian H, Tekkaya AE (2010) A review on hot stamping [J]. J Mater Process Technol7:165–207

2. Ma N, Hu P, Zai S, Guo W (2009) Technology and application of hot forming of highstrength steel. Automobile Technol Mater 12:28–30 (In Chinese)

44 3 Hot Forming Process

3. Aspacher J (2008) Forming hardening concepts. 1st international conference on hot sheetmetal forming of high-performance steel, Kassel, Germany, pp 77–81

4. Ma N, Hu P (2010) Hot forming technique and its equipments for ultra high strength steel[C]/. Proceedings of MSEC2010, ASME pp 362–367

5. Ma N, Hu P, Yan K, Guo W, Meng X, Zhai S (2010) Research on boron steel for hot formingand its application. J Mech Eng 46(14):177–181 (In Chinese)

6. Berglund G (2008) The history of hardening of boron steel in northern Sweden. 1stInternational conference on hot sheet metal forming of high-performance steel, Kassel,Germany, pp 175–177

7. Erhardt R, Böke J (2008) Industrial application of hot forming press simulation. 1stInternational conference on hot sheet metal forming of high-performance steel, Kassel,Germany, pp 83–88

8. Yadav A, Altan T (2006) Hot stamping boron-alloyed steels for automotive parts. Part I JStamping J 12:40–43

9. Ma N, Hu P, Guo W (2009) Technology and equipment of hot forming for ultra high strengthsteel [J]. Automobile Parts 45:28–30 (in Chinese)

10. Ma N, Hu P, Shen G et al. (2009) The model of warm forming and the numerical simulation.In: 2009 International seminar on production and application technology of automotive steel,2009, (In Chinese), pp 289–293

11. Ma N, Shen G et al. (2011) Study of hot forming for high strength steel: numericalsimulation-dynamic explicit algorithm, Acta Mechanica Solida Sinica 8(11):143–152(In Chinese)

12. Ma N, Hu P, Guo W (2010) Experiments and analysis of relations among heat, stress andtransformation of boron steel for hot forming [J]. Trans Mater Heat Treat 12(5):33–40(In Chinese)

13. Naderi M (2007) Hot stamping of ultra high strength steels. Doctoral Theses, RWTH Aachen14. Fan DW, Kim HS, Birosca S, De Cooman BC (2007) Critical review of hot stamping

technology for automotive steels[C]. AIST Steel properties and applications conferenceproceedings, combined with Ms and T’07, Materials Science and Technology 2007,pp 99–110

References 45