The effect of tempering temperature on microstructure, mechanical properties and bendability of direct-quenched low-alloy strip steel

Abstract

The tempering of re-austenized, quenched and tempered (RAQT) martensitic steels is an extensively studied and well understood field of metallurgy. However, a similar understanding of the effect of tempering on direct-quenched (DQ) high-strength steels has been lacking. Now, for the first time, the effect of tempering in the range of 250-650 °C on the strength, toughness, bendability, microstructure, crystallography and dislocation density of a DQ steel is reported. In the case of tempering at 570 °C, the effects of having a RAQ or DQ starting condition are compared. For the composition and thermal cycles studied, it was found that a peak tempering temperature in the range of 570-600 °C resulted in a DQT steel with an optimal balance of strength, bendability and toughness, i.e. a yield strength greater than 960 MPa, a minimum usable bending radius of 2 times the sheet thickness and T28J of-50 to-75 °C depending on the test direction. Crystallographic texture, dislocation density and the distribution of carbides are important factors affecting the bendability of DQT strip. Tempering had no effect on texture, but strongly influenced the size and distribution of carbides thereby resulting in differences in bendability and impact toughness transition temperature.

The Effect of Tempering on the Microstructure and Mechanical Properties of a Novel 0.4C Press-Hardening Steel

“…The water quenched variant showed only minor traces of auto-tempering, but transition carbides began to form when tempering at 150 °C and 200 °C. The decrease of dislocation density with increasing tempering temperature followed the results found in the literature [22,25,[38][39][40]. Saastamoinen et al [38] reported dislocation density for both direct quenched (DQ) and reheated and quenched (RAQ) around 4.0 × 10 15 m −2 for martensite containing 0.1% C, which explains some of the difference with the current results.…”

“…The decrease of dislocation density with increasing tempering temperature followed the results found in the literature [22,25,[38][39][40]. Saastamoinen et al [38] reported dislocation density for both direct quenched (DQ) and reheated and quenched (RAQ) around 4.0 × 10 15 m −2 for martensite containing 0.1% C, which explains some of the difference with the current results. Using the same Williamson-Hall method as used here, in the case of 0.3% C martensite, Kennett et al [39] found dislocation densities in the as-quenched state in the range 8-10 × 10 15 m −2 , depending on PAGS, while Takebayashi et al [40] found a dislocation density of 2.0 × 10 16 m −2 .…”

Effects of Cooling Rate during Quenching and Tempering Conditions on Microstructures and Mechanical Properties of Carbon Steel Flange

“…Moreover, tempering drives various phenomena such as the release of residual stress, decline in dislocation density and appearance of fine precipitates. These phenomena combine with the transformation of MA and decomposition of the secondary phase identified to control the mechanical properties of the steels [ 4 , 10 , 19 , 30 , 45 ].…”

“…When dealing with bulk flanges of large dimensional size and complex structure, it should be taken into consideration that heat treatment would not be uniformly applied to each location of the products. However, most previous studies have tended to use standardized specimens [ 18 , 19 , 20 , 21 , 22 ] or sheet plates [ 9 , 16 ] of certain sizes to which uniform heat treatment can be applied within the specimens, so the results cannot be directly used to understand the bulk flanges. It is rather useful, especially from an industrial point of view, to investigate the effect of heat treatment on flanges by directly obtaining local properties of interest from the flanges under heat treatment processes.…”

Laser heat treatment of martensitic steel and dual-phase steel with high martensite content

“…On AHSS grade steel, the softening of the material is often seen as a limiting drawback for joining. However, tempering of martensite can also induce an increase in ductility [15][16][17]. Hence the local softening brought by LHT could grant an increase of ductility of martensitic steels where it's required.…”