Two copper-bearing high-strength low-alloy (HSLA) steels with chemistry similar to HSLA-100, were made on a laboratory scale, one in an air induction (100 kg) furnace and the other in a vacuum induction (50 kg) furnace. The ingots cast were hot-rolled to 25 mm thick plates which were subsequently austenitized and tempered at different temperatures (400–700°C) for 1 h. Evaluation of mechanical properties and microstructure of as-quenched and tempered plates revealed that substantial improvement in strength (YS-1024 and 1025 MPa; UTS-1079 and 1111 MPa for steels 1 and 2) occurred at the expense of impact toughness on tempering at 500°C owing to profuse Cu precipitation in the matrix. With increase in tempering temperature however, the notch toughness improved considerably, reaching peak values of 53 and 123 Joules (J) at −85°C for steels 1 and 2 at 650 and 700°C tempering temperatures, respectively. The partially recovered matrix and the coarsened Cu precipitates in this temperature range presumably enhanced dislocation movement and notch toughness.

https://www.tf.uni-kiel.de/matwis/amat/iss/kap_9/articles/hsla_steel_1.pdf

Effect of tempering temperatures on the mechanical properties and microstructures of HSLA-100 type copper-bearing steels

The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77, Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1 hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85 °C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068 MPa, with 88 J at −85 °C).

Influence of Tempering on the Microstructure and Mechanical Properties of HSLA-100 Steel Plates

HSLA-115 is a novel high-strength low-alloy structural steel derived from martensitic Cu-bearing HSLA-100. HSLA-100 is typically used in conditions with overaged Cu precipitates, to obtain acceptable impact toughness and ductility. Present work on HSLA-115 demonstrates that incorporating sub-nanometric-sized M2C carbides along with Cu precipitates produces higher strength steels that still meet impact toughness and ductility requirements. Isothermal aging at 823 K (550 °C) precipitates M2C carbides co-located with the Cu precipitates and distributed heterogeneously at lath boundaries and dislocations. 3D atom-probe tomography is used to characterize the evolution of these precipitates at 823 K (550 °C) in terms of mean radii, number densities, and volume fractions. These results are correlated with microhardness, impact toughness, and tensile strength. The optimum combination of mechanical properties, 972 MPa yield strength, 24.8 pct elongation to failure, and 188.0 J impact toughness at 255 K (−18 °C), is attained after 3-hour aging at 823 K (550 °C). Strengthening by M2C precipitates offsets the softening due to overaging of Cu precipitates and tempering of martensitic matrix. It is shown that this extended yield strength plateau can be used as a design principle to optimize strength and toughness at the same time.

Multicomponent High-Strength Low-Alloy Steel Precipitation-Strengthened by Sub-nanometric Cu Precipitates and M 2 C Carbides

Hardening mechanisms and impact toughening of a high-strength steel containing low Ni and Cu additions

HSLA-100 steel with high content of alloying elements (nominally in wt pct, 3.5 Ni, 1.6 Cu, and 0.6Mo) is now used to produce heavy plates for constructing a hull and drilling platform. We proposed here a substantially leaner steel composition (containing 1.7 Ni, 1.1 Cu, and 0.5Mo) to produce a heavy plate to 80 mm thickness with mechanical properties comparable with those of the HSLA-100 grade. A continuous cooling transformation (CCT) diagram of the steel was constructed. Key parameters of thermal treatment and revealing mechanisms of strengthening and toughening were derived based on industrial production trials. The microstructures of the 80-mm-thick plate were lath-like bainite (LB) at near surface of the quarter thickness (t/4), and granular bainite (GB)+LB at center thickness (t/2) after solutionizing and water quenching (Q). The effect of tempering (T) on the microstructures and properties of the plate was investigated. Excellent combination of room temperature strength and low-temperature Charpy V-notch (CVN) toughness approximately equivalent to that of the HSLA 100 grade (YS > 690 MPa, CVN energy >100 J even at 193 K [−80 °C]) was achieved in the plate treated by the QT process with tempering temperature of 898 K (625 °C). The combination of strength and toughness at t/4 is superior to that at t/2 of the plate under both as-quenched and QT conditions. This result is attributed to that the fraction of high-angle grain boundaries (HAGBs) at t/4 is higher than that at t/2.

Strengthening and Toughening of a Heavy Plate Steel for Shipbuilding with Yield Strength of Approximately 690 MPa

The structural steels used in critical construction applications have traditionally been heat-treated low-alloy steels. These normalized and/or quenched and tempered steels derive strength from their carbon contents. Carbon is a very efficient and cost-effective element for increasing strength in ferrite-pearlite or tempered martensitic structures, but it is associated with poor notch toughness. Furthermore, it is well known that both the overall weldability and weldment toughness are inversely related to the carbon equivalent values, especially at high carbon contents. The stringent control needed for the welding of these traditional steels is one of the major causes of high fabrication costs. In order to reduce fabrication cost while simultaneously improving the quality of structural steels, a new family of high-strength low-alloy steels with copper additions (HSLA-100) has been developed. The alloy design philosophy of the new steels includes a reduction in the carbon content, which improves toughness and weldability.

A.J. DeArdo

Papers

HSLA-100 steels: Influence of aging heat treatment on microstructure and properties

Ultra-low carbon bainitic plate steels : processing, microstructure and properties

A New Microalloyed, Multi-Phase Steel for High Strength Forging Applications

The High Strength Cold Forging Applications of a New Multi-Phase Steel

The High Strength Cold Forging Applications of a New Low Carbon Multi-Phase Steel