Applications of hot stamping Technology in BIW

In recent years, automotive lightweight has led to the increasing demand of high strength steel (HSS) auto parts. Lots of correlative companies and research institutes have invested large amount of energy into the research of advanced automotive HSS forming technology. Though the advantage of HSS is high strength, it still has some disadvantages such as the poor forming performance, the uncontrollable springback, and easy to crack during forming process. Therefore, it is dif?cult to realize the manufacture of complex auto parts by adopting traditional cold stamping technology, where the hot stamping technology is needed.

HSS hot stamping technology is a new manufacturing technology developing in recent years, which combines the traditional hot forging and cold stamping technology. It is a mode of production that integrates the stamping of steel under the condition of high temperature and forming and quenching in dies. It is also known as Hot Forming, Hot Stamping, Hot Press, Press Hardening, or Die Quenching abroad. Hot stamping technology of HSS can be used to manufacture car structural components whose tensile strength is up to 1500 Mpa, such as the front bumper, tail bumper, A&B&C PILLAR, , roof frame, underbody, door panel,and so on.

The proportion of hot stamping boron steel applied in VOLVO vehicles

Hot Stamping Process

Hot stamping process is the necessary means to realize the product forming and strength increasing of hot stamping steel, and it is the indispensable premise for HSS of hot stamping to acquire high strength performance. Its typical technological process is: ?rst, heat the sheet to a speci?c temperature range for austenization and austenitize it completely, and then transfer it into the water cooling hot stamping dies for stamping and quenching, and ?nally realize the microstructure’s phase transformation and the increase of strength. According to the complexity of processing and the forming process of actual parts, hot stamping process can be divided into directly hot stamping and indirectly hot stamping process.

1. Direct hot stamping process:

In the direct hot stamping process, a blank is heated up in a furnace, transferred to the press and subsequently formed and quenched in the closed dies.Afterwards, the product is trimmed by laser and ?nished through other follow-up processes.The advantages of direct hot stamping process are as follows:

(1) The blank is formed and hardened in one mold, which saves the cost of preforming and accelerates the pace of production.

(2) The blank is flat, which not only saves heating area and energy, but also can be heated by a variety of heating methods, for example, induction heating.

The disadvantages of hot stamping process are as follows: it cannot be used for forming automobile parts with complex shapes, and it needs the laser cutting equipment. In addition, the design of cooling system of molds is more complex.For automotive body structure, the parts with simple shape, not necessary for deep drawing, can be manufactured by the direct hot stamping, such as the inside and outside pieces of b-pillar.

Parts with the direct method of hot stamping

2. Indirect hot stamping process

In the indirect hot stamping process, the sheet metal is performed by cold forming before it is heated to austenite temperature in a furnace. After being held for a period of time for full austenization, it will be transferred into the hot stamping tools with cooling system to be stamped and quenched at the same time. The indirect hot stamping process is also named as “multi-step”. Compared to the cold stamping steel, though the flow ability of the heated steel increases, the tensile strength would decrease accordingly, making it easy to crack for complex part during forming process. The indirect hot stamping process is the exact method invented to solve the problem and form complex BIW parts that may not be formed by the direct hot stamping process.

Compared to the direct hot stamping, the HSS sheet is uncoiled and cut according to the shape of the product, then preformed by the traditional process such as cold forming process, flanging, punching and cutting-edge, and so on. After that, the preformed semi-?nished product is transferred to the continuous furnace to be heated and insulated before it is sent to be formed and quenched. Afterwards the product will be trimmed by laser processing or other necessary follow-up processes according to the characteristics of the components, or directly output ?nished product.

Parts produced by indirect hot stamping technique

The advantages of indirect hot stamping process are as follows:

(1) The parts with complex shapes and almost all of the current stamped carrying parts can be formed by indirect hot stamping process.

(2) After the performing of the blank, it is unnecessary to worry about the forming performance of the blank at high temperature in subsequent hot stamping process, which can ensure the martensite microstructure of the blank followed by complete quenching.

(3) The blank can be processed by trimming, flanging, punching and other pro-

cessing after being preformed so that it will be easier for processing after it is quenched. For example, the blank that is quenched must be trimmed by laser cutting equipment, which greatly increases the cost.

Continuous Ring Heating Furnace:

The continuous ring heating furnace used in the hot stamping should ensure that the blank is heated up to the setting temperature for a complete austenitization. At the same time, the high-temperature oxidation and decarburization should be avoided for those blanks without anti-oxidation coating. The continuous ring heating furnace has its unique core technology in comparison with other heating furnaces, and its brief descriptions are as follows:

The hot stamping automatic production line

(1) Most of the automotive HSS blank is sheet. The sheet should be heated up to the speci?ed temperature and then hold heat in order to be fully austenitized. Therefore, the temperature distribution in the heating furnace is required to be homogenous;

(2) For the steel sheet without protective coating from oxidation, its oxidation and decarburization is very serious at elevated temperatures. Therefore, the furnace is needed to take strict measures of gas protection;

(3) All the parts of the heating furnace run under the conditions of long term and continuous operation at a high temperature (over 800 °C), so their expansion property, high temperature fatigue, failure rate and maintainability must be taken into consideration;

(4) Hot stamping production line needs to satisfy the setting cycle time, so it needs the equipment for loading or unloading which can meet the requirements for the high-temperature work and corresponding automatic control system;

(5) One of the purposes of hot stamping technique is to save energy and protect environment, thus it also requires energy-ef?cient heating furnace.