Fundamentals of Automotive Steel Sheets - High-Strength Steel Sheets
1. High-Strength Steel Sheets

High-strength steelhas higher tensile strength than normal, or “mild” steel. Generally, high strength steel is defined as steel with a 340MPa or higher tensile strength. However, this definition may vary depending on vehicle manufacturer, suppliers, and countries. Steel with a tensile strength of 980 MPa and higher is called “Ultra High-Strength Steel” to differentiate it during repairs because the repair methods are restricted.
In addition, there are several types of high-strength steel sheets available. Solid solution strengthened and precipitation strengthened steel adding elements such as nickel (Ni), silicone (Si) manganese (Mn) etc to increase strength. Composite structure strengthened steel is hardened after press forming.

Type
Strengthening Mechanism
Solid Solution Strengthened
The very basic method. To form a solid solution of the alloy elements to crystal lattice. There are two type of solution; one is called substitutional solid solution which forms solid solution by replacing with iron atoms such as silicone (Si), manganese (Mn) and phosphorus (P), another one is called interstitial solid solution which forms solid solution by intruding such as carbon and nitrogen.
Precipitation Strengthened
To interrupt the rearrangement by precipitate through precipitation of carbide or nitride such as titan (Ti) or niobium (Nb).
Composite
Structure
Strengthened
DP Steel
(Dual Phase)
To change the internal composition through heat treatment (strengthened by martensite with a high hardness obtained by quickly cooled down austenite generated in heated steel at a high temperature).
TRIP Steel
(Transformation
Induced Plasticity)
To change the internal composition through heat treatment (strengthened by bainite or residual austenite obtained by quickly cooled down austenite generated in heated steel at a high temperature).


Characteristics:
High tensile strength
High yield point (the point where material changes from elastic to plastic)
High yield rate (yield point/tensile strength)