QStE380TM, QStE420TM, QStE460TM and QStE500TM form a family of high-strength low-alloy (HSLA) hot-rolled steels developed for cold forming of automotive structural parts. They share the same design logic: high yield strength, good cold formability and reliable weldability, obtained through thermo-mechanical rolling rather than through expensive alloying. The difference between the grades is the strength level, and choosing correctly is a matter of matching the load with the forming process.
Decoding the Name
The designation follows Chinese automotive steel naming practice: QStE marks a high-yield-strength steel for cold forming, the number is the minimum yield strength in MPa (380, 420, 460 or 500), and TM stands for thermo-mechanically rolled, meaning the strip is finished by controlled rolling and controlled cooling to refine the grain. The fine-grained microstructure is what allows these grades to combine strength with the ductility needed for cold stamping and roll forming. Note that these grades are defined in Chinese automotive standards such as GB/T 20887.1 and major mill specifications; they are not German standard grades despite the St in the name.
Mechanical Properties
| Grade | Minimum yield strength ReH | Tensile strength Rm | Forming characteristic |
|---|---|---|---|
| QStE380TM | 380 MPa | 450-590 MPa | Best formability of the four; deep profiles possible |
| QStE420TM | 420 MPa | 480-620 MPa | Balanced strength and drawing behaviour |
| QStE460TM | 460 MPa | 520-670 MPa | High strength for load-bearing sections |
| QStE500TM | 500 MPa | 560-720 MPa | Highest strength; more restricted forming |
As the grade number rises, yield and tensile strength increase while ductility and formability decrease slightly, and the elongation requirement is thickness-dependent and defined in the applicable standard. This gradient lets a designer place each grade on a specific structural role.
Composition and Processing
The family is low-carbon micro-alloyed steel, typically with carbon up to about 0.12%, manganese up to about 1.5%, tight phosphorus and sulphur limits, and small additions of niobium or titanium for precipitation strengthening. Strict impurity control and the thermo-mechanical route keep the material weldable without preheat in most shop conditions, while preserving the fine grain that balances strength and cold formability.
Typical Applications
QStE380TM: non-critical structural brackets, floor panels, seats and other components with moderate load.
QStE420TM: chassis frames, cross members, suspension arms and wheel discs.
QStE460TM: truck frame rails, longitudinal beams and heavy brackets.
QStE500TM: high-load core parts, crane and trailer structures, and components where weight reduction drives the design.
Forming and Welding Considerations
All four grades cold form well because the yield-to-tensile ratio is kept under control, but the bend radius and springback increase with strength. Use larger bend radii for QStE460TM and QStE500TM, and account for springback in tooling design. Welding follows normal HSLA practice: clean surfaces, correct heat input and filler matching the strength class; the micro-alloyed composition keeps the heat-affected zone tough when parameters are controlled.
Frequently Asked Questions
1. What does the number in QStE420TM mean?
It is the minimum yield strength in MPa. QStE420TM guarantees at least 420 MPa yield, QStE500TM at least 500 MPa, and so on.
2. Why is TM rolling better than quench and temper?
Thermo-mechanical rolling achieves the strength with lower alloy cost and better weldability, and it produces a finer, more uniform grain. For vehicle structures it is the standard economic route.
3. Are these grades weldable?
Yes. Their low carbon equivalent is designed for arc welding without preheat in most thicknesses; follow the filler-metal recommendation for the strength class.
4. Can QStE500TM be deep drawn?
It can be formed, but not deep drawn like a mild steel. It is intended for bending, roll forming and moderate stamping; when a part needs both high strength and deep drawing, consider a dual-phase grade instead.
5. What standard governs these grades?
They are defined in Chinese automotive steel standards, notably GB/T 20887.1, and in the published specifications of major Chinese mills; the mill certificate should state the governing standard.
6. What thickness range is supplied?
These are hot-rolled products, commonly supplied as pickled and oiled strip from about 1.5 mm to 8 mm, with heavier plate on request for specific applications.