Hot Stamping Steel PHS

Hot Stamping Steel (PHS), also known as press hardening steel, is an advanced material used in the hot stamping process. It is specifically designed to provide high strength and excellent crashworthiness in automotive applications. PHS undergoes a unique manufacturing process 

Firstly it is heated to a high temperature, allowing it to form complex shapes. The formed steel is then rapidly cooled to create a hardened structure with exceptional strength. This heat treatment process enhances the material's ability to withstand severe impacts, making it ideal for manufacturing critical components in vehicle safety systems. Steel PHS offers automakers the advantage of producing lighter-weight components without compromising strength and safety, resulting in improved fuel efficiency and enhanced overall vehicle performance.

The need for high-strength steel plates for automotive mechanisms is becoming increasingly evident in light of the need for lighter, safer, and more impact-resistant vehicles. The Swedish company's patent for hot stamping and its development (1977 patent) was used on several tool blades. 1984 Saab Automobile was the first car manufacturer to use thermoformed steel plates (hardened boron plates) for the Saab 9000. Hot-formed parts are used on the bodywork for: front fascia beams, A-pillar upper and lower reinforcement panels, B-pillar inner and reinforcement panels, center channel, front and rear seat beams, door impact beams, and rear impact beams.

There are two ways of hot stamping: direct and indirect molding processes.

1. Direct moulding process

In the direct hot stamping process, semi-finished products are heated, transferred to a press, and subsequently formed and hardened in a closed mold.

2. Indirect forming process

① Pre-forming: If the size of the part is large and the shape is relatively complex, the indirect forming process should be used to improve the pass rate of the product. The initial strength of the steel plate during pre-forming is about 600MPa. The sheared steel plate is placed in the press to be deformed into the target part shape.

② Heating: The steel plate is heated at 927℃~950℃ for about 5 minutes, which is conducive to the austenitising of the crystal structure of the steel plate. Under austenitising conditions, an oxide film is quickly formed when the steel comes into contact with air. To avoid surface oxidation and decarburization, a large number of metal plates are coated with a protective layer.

③ Hot forming: To avoid cooling the workpiece before forming, the workpiece must be stamped and formed in the furnace as soon as possible. At the same time, the forming must be completed before the martensitic phase change. Rapid closing of the mold box and the forming process is the key to the success of hot stamping. After forming, the workpieces are quenched in the closed mold, and the cooling system is based on heat removal using cooling water in a conduit. To avoid quenching in the part between the die and the workpiece fixture during the forming process, an empty distance is usually left in the hot stamping system.

④ Quenching: Quenching under cooling water for 5~10 s. As the hot workpiece is in contact with the cold die, the hot workpiece is quenched inside the closed die. If the cooling rate exceeds the minimum cooling rate, which is about 27 K/s, at a temperature of about 400°C, it will result in a transformation of the non-diffusive martensitic organization, which will ultimately result in a high strength part. The evolution of the martensitic organization during the quenching process results in a tensile strength of more than 1500 MPa.

3. Material properties

- Tensile strength: ≥1500 MPa

- Yield strength: 950~1250MPa

- Elongation at break: 5 percent

Advantages of hot-forming steels

1/ The classic automotive applications for hot-formed steels are the A- and B-pillars of vehicles, but they are also used for roof edge beams, sidewall reinforcement panels, headliner cross-members, and instrument panel cross-members, as well as door impact beams, windscreen reinforcements, and floor beams. It is worth noting that these thermoformed parts are now often designed with highly complex geometries, which is where PHS steels come in.

2/ The same part can have different strength levels A thermoforming mold can be subdivided into several different hardening processes. For example, creating a thermoforming 'soft zone' does not require the entire part to be hardened. The obscured, unquenched portion of the part will have a lower strength level and therefore absorb high energy during impact. With its higher tensile strength, the quenched part can withstand higher forces. In recent years, significant developments have been made in the field of PHS soft zone processes. For example, the soft zone of the lower part of the B-pillar of a car.

3/ Unequal Thickness Plates Welded Together Unequal thickness plates are another way to achieve 'performance zoning' in thermoformed parts. Cold rolled unequal thickness plates are a type of cold rolled strip with different thicknesses in different width sections. As a result, the thickness of the strip can be varied in different widths, so that it is possible to specify where it should be thicker and where it should be thinner, depending on the final requirements for the part and its performance.

Unequally thick plate spar welding plates can be even more versatile: you can weld PHS strips of different thicknesses together, or even weld PHS strips to non-PHS strips.

4/ Design Freedom Hot Stamping Steel is heated up to 900 °C and stamped into shape making it easy to hot form PHS steels into shapes with very complex and deep-drawn shapes. With this design freedom, automotive designers can therefore be more creative with their components: perhaps they can use the high tensile strength of PHS steels to design lighter parts; perhaps they can reduce the number of parts, i.e., consolidate parts; perhaps they can design 'soft zones' to make parts perform better in crash tests.

5/ Superior Finished Part Accuracy The advantage of heat-treated steels is that they have little or no spring back, which results in greater form accuracy. With the increasing tensile strength of today's high-strength steels, the idea that spring back is likely to increase is firmly entrenched in the minds of many.

On the other hand, we will understand and be able to predict and control springback in cold-formed automotive parts better than ever before, even when using gigapascals.