11 07, 2018

Introduction to Press-Hardened Steels

2018-07-11T17:06:31+00:00 July 11th, 2018|News Blog, NMC Media|

What are Press-Hardened Steels (PHS)?

Press-Hardened Steels are Boron added (.001%-.005%) Carbon /Manganese steels that have been in use since the mid 1980’s for automotive body in white construction.

These steels are also known as HF or Hot Formed Steel.

Press Hardened Steel or Hot Formed Steels undergo a unique manufacturing process where the steel is heated to a minimum temperature of 850*C then formed to a final shape in water cooled dies that control the rates of cooling / quench to insure desired properties are met. The high temperature combined with rapid cooling transforms the microstructure to nearly 100% Martensite and very high finished tensile strengths up to 2000 Mpa.  This process is commonly referred to as “Hot Stamping”. The finished product qualifies as advanced high-strength steel, and is three times as stable as the material was prior to undergoing the process. It’s strength-to-weight ratio is also vastly increased.

The implementation of press hardening applications and the utilization of hardened steels are promising alternatives for optimizing part geometries with complex shapes with no spring back issues. The Hot Forming process allows for the formation of shapes that cannot be cold stamped with Ultra High Strength steels.

The unique properties of this material combine both complexity and strength – and components made with press-hardened steel can accomplish in one piece what would usually require heavier, thicker parts that are welded together.

The Press-Hardening / Hot Forming process was first put in to practice in the mid 1980’s and is primarily used in the creation of crash-sensitive safety components in automobiles. The parts created with this process meet several of the evolving requirements of the automobile industry, such as weight and cost reduction, as well as reduced environmental impact in its creation.

Since being introduced, Hot Forming has steadily increased in popularity.  Recent model year vehicles have seen the percentage of these steels rise to 25% of body in white weight.

PHS / HF Steels can be supplied with coatings that prevent the formation of oxide during the process. The most common of these coatings is Aluminum Silicon (AS) coating, other coatings may include Hot Dipped (GI), Galvannealed (GA) or Zinc Nickle (GP).

These parts are typically used where part stiffness and low deformation are important, typical parts may include.

  • A-Pillar
  • B-Pillar
  • Front / Rear Side Member
  • Crossmembers
  • Door Ring

Properties of Press-Hardened Steels

The strength of press-hardened steels can reach as high as 2000 MPa (215ksi), with a 1000MPa (145 ksi) yield stress total.

The Press Hardening Process

The two different processes for press-hardening are as follows.

Direct, in which the steel is first superheated in a furnace, formed while it is still hot, and then quenched while within the die. The process goes like this:

  • The steel with a ferritic-pearlitic matrix is austenized in a furnace. The blanks are heated up to a high temperature, nearly reaching 1000*C.
  • The material is then removed from the furnace and immediately transported to the stamping machinery within 10 seconds of removal
  • The die is closed, and the part is formed
  • The die is then rapidly quenched. […]