At present, hot forming is divided into direct hot stamping and indirect hot stamping, as shown in Figure 1. In direct hot stamping, the semi-finished product is heated first, and then transferred to a closed mold for forming and quenching; indirect hot stamping mainly uses a pre-formed cold mold, and a cold-punched pre-formed part is first performed for parts with more complex shapes or deep drawing. After that, it is basically the same as the direct hot stamping process, such as the middle channel and other parts.

22MnB5 is the most commonly used steel grade for hot stamping technology. Usually, the structure before hot stamping is ferrite + pearlite, and the tensile strength is about 600 MPa. After hot stamping, the structure is full martensite, and the tensile strength is about 1 500 MPa (see Figure 2a).

As we all know, C content has a greater impact on the strength of the material after quenching, while Mn, Cr and other elements have less impact. After adjusting the elements, the desired phase transformation and hardening effect can be obtained through the cooling rate of flexibility. Among them, the B element has a greater contribution to hardening, and it can slow down the transformation of austenite to soft phase (ferrite), namely Improve hardenability.

Process steps

Speaking of the cost of a process, the first thing we need to know is the process steps of this process, as shown in the figure below, the process steps of hot stamping: uncoiling blanking-heating-hot stamping forming-laser cutting. The high-strength steel plate is heated to above 900°C and quickly transported to the press, and hot stamping is performed when the material has high ductility. The water-cooled mold ensures rapid cooling of the parts and obtains a complete martensite structure with high tensile strength. Due to the ultra-high strength of the parts, laser cutting is used for trimming and hole cutting in the subsequent process.

Heat the billet to 900°C

laser cutting

Production equipment introduction

In hot stamping, heating and stamping are completed in one assembly line. The length of the heating furnace is about 30m~50m, the tonnage of the punching machine is about 800T~1200T, the area is about 500 square meters to 800 square meters, the power is 1600 kilowatts to 2000 kilowatts, and the investment is about 60 million to 100 million yuan.

There are many models of laser cutting machines. For example, the TruLaser Cell 8030 (TRUMPF) in the picture above has an investment of about 6 million yuan (700,000 euros), a power of 3 kilowatts, and an area of about 75 square meters.

Seeing here, have you discovered that this is a money-burning process, and this investment is not affordable for ordinary enterprises, so for thermoforming suppliers, there will not be multiple production lines at the same time. When we evaluate the cost, we don’t need to do too much entanglement and consideration in equipment selection. Basically, we can use what tonnage to see the tonnage of the production line at the part supplier’s home. This is not like ordinary stamping. We can choose a cost-effective The tonnage of the highest equipment.

Labor and production rhythm

The number of operators in the hot stamping line is 3~4, and the production cycle is 16~22 seconds.

The laser cutting machine has 1 worker, and the beat is 25~35m/s.

Case Analysis

The following parts are the A-pillar inner reinforcement plate.

The stamping adopts 1000T stamping machine, 1 punches 4 pieces, the beat is about 20S, and the laser cutting is 80S.

At present, the application of foreign thermoforming technology has gradually developed from traditional hot stamping of equal-thickness plates to advanced thermoforming, and mass-produced vehicles have come out. In addition to domestic joint venture brands, the thermoforming of independent brands is still dominated by traditional hot stamping of thick plates, and the following problems still exist in the application process:

a. The cost of raw materials is high and the source is few. 

The raw materials are mainly non-coated plates, and the corrosion resistance and surface quality of the final parts cannot be guaranteed. Almost all European and American manufacturers use coated hot-formed steel. At this stage, thick steel plates such as Al-Si coatings and laser tailor-welded blanks are protected by patents, and only one foreign supplier provides them, and only one German supplier provides rolled plates.

b. The analysis ability of CAE is poor. 

At present, it is difficult to accurately describe the performance of rolled poor-thickness plates and variable-strength plates, resulting in low accuracy of CAE analysis. c. The quality of soft modules is poor. When processing fast soft mold parts with uncoated steel plates, the surface quality of the parts is very poor due to the absence of protective atmosphere during the heating stage; the mold does not have cooling channels, resulting in the stability of the part performance cannot be guaranteed.

c. The design level is low. 

The design initiative is still in the hands of the thermoforming suppliers, and they do not have complete design and analysis capabilities, including complete simulation capabilities.

d. Acceptance standards can guarantee the quality of the final product.

including evaluation requirements for appearance, size, composition, performance and microstructure.

Link to this article： Hot stamping process cost analysis

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