Abstract
The microalloying with niobium (Nb) and titanium (Ti) is standardly applied in low carbon steel high-strength low-alloy (HSLA) steels and enables austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. In that respect, it is important to better understand the precipitation kinetics as well as the precipitation sequence in a typical Nb-Ti-microalloyed steel. Various characterization methods were utilized in this study for tracing microalloy precipitation after simulating different austenite TMCP conditions in a Gleeble thermo-mechanical simulator. Atom probe tomography (APT), scanning transmission electron microscopy in a focused ion beam equipped scanning electron microscope (STEM-on-FIB), and electrical resistivity measurements provided complementary information on the precipitation status and were correlated with each other. It was demonstrated that accurate electrical resistivity measurements of the bulk steel could monitor the general consumption of solute microalloys (Nb) during hot working and were further complemented by APT measurements of the steel matrix. Precipitates that had formed during cooling or isothermal holding could be distinguished from strain-induced precipitates by corroborating STEM measurements with APT results, because APT specifically allowed obtaining detailed information about the chemical composition of precipitates as well as the elemental distribution. The current paper highlights the complementarity of these methods and shows first results within the framework of a larger study on strain-induced precipitation.
Highlights
The development of weldable low-carbon steels with high mechanical strength and good toughness is the basis for many modern applications in the structural, energy, and automotive sectors
Grain refinement in combination with low carbon content is effective in lowering the ductile-to-brittle transition temperature (DBTT) and increasing the ductile plateau toughness
C and quenched after short holding times up to 1200 s, carbon clusters were observed as shown in Figure 11b, whereas for niobium, the atom map (Figure 11a) more measurements measurements for each each holding holding indicated no obvious clustering in any of the cases
Summary
The development of weldable low-carbon steels with high mechanical strength and good toughness is the basis for many modern applications in the structural, energy, and automotive sectors. Key to this steel development is the use of microalloying in combination with thermo-mechanical controlled processing (TMCP) [1,2,3,4]. Such high-strength low-alloyed (HSLA) steels are being produced as strip and plate products covering a wide range of thicknesses and yield strength levels up to 700 MPa. The involved strengthening mechanisms are in first place grain refinement followed by precipitation and dislocation strengthening. Grain refinement in combination with low carbon content is effective in lowering the ductile-to-brittle transition temperature (DBTT) and increasing the ductile plateau toughness.
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