Development of microalloyed steel for pipeline applications

Abstract

An experimental HSLA steel was produced by the electric arc furnace, vacuum degassing, ladle treatment and continuous casting route. The experimental steel was then rolled in a laboratory using a hot rolling schedule to simulate an industrial controlled hot rolling procedure for the production of plates as closely as possible to investigate the effect of a thermomechanical processing schedule plus the use of water quench, accelerated cooling followed by forced nitrogen gas or air as cooling media, on the mechanical properties of plates. The results showed that the controlled thermomechanical hot rolling schedule of slabs followed by the cooling of plates in either forced nitrogen gas or by accelerated cooling exhibited target properties equivalent to a steel grade API X-80.

Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel

“…The average dislocation density in ferrite increased with a decrease in the finish deformation temperature from 4.5 ± 0.5 9 10 13 m À2 for 1348 K (1075°C) to 4.6 ± 0.5 9 10 13 m À2 for 1248 K (975°C) and to 9.5 ± 1.1 9 10 13 m À2 for 1098 K (825°C). The measured values of dislocation density correspond to those reported earlier for ferrite in carbon [23,24] and Nb-microalloyed steels [9,12,19,25] processed in the same temperature range, i.e., above A r3 temperature of austenite-to-ferrite transformation; however, they are several times lower than those after finish rolling in the a + c phase field (2.3 À 4.0 9 10 14 m À2 [26] ) and more than 10 times lower than after finish rolling in ferrite (7 9 10 14 m À2 [9] ).…”

“…This may lead to >50 MPa of the work hardening contribution to the yield stress (Figure 8(b)), which would account for about 10 pct of the total strengthening effect (Table IV). With an increase in the cooling rate up to 40°C s À1 , a further increase in dislocation density may lead to >100 MPa of the work hardening contribution [55] (about 20 pct of the yield stress, Table IV), and the bainite formation may additionally give up to 300 MPa [12] (about 50 pct of the yield stress). For the data analyzed here, the joint contribution to the yield stress from grain refinement and solid solution strengthening slightly decreased with an increase in the Nb and Ti contents in steel composition (Figure 9(a)).…”

“…Fig. 8-Effect of the finish deformation temperature on (a) dislocation density [9,12,19,25,26] and (b) the work hardening contribution to the yield stress estimated using Eq. [5].…”

“…For example, in a steel containing 0.044C-0.25Mo-0.0554Nb-0.014Ti, the ferrite grain refinement, solid solution strengthening, and work hardening together contributed up to 65 pct to the yield stress, and the precipitation and transformation (presence of bainite) strengthening contributed 35 pct. [12] In another example, in a steel containing 0.064C-0.0625Nb-0.0435Ti the grain refinement, solid solution strengthening and work hardening together contributed up to 96 pct to the yield stress, and only 4 pct was the contribution from precipitation strengthening, [13] which could be explained by a lower microalloying element content compared to that in. [12] However, in a steel containing 0.06C-0.08Nb-0.07Ti the grain refinement, solid solution strengthening and work hardening together contributed 67 pct to the yield stress, and the remaining 33 pct was the contribution from precipitation strengthening, [14] which is more than 8 times higher compared to that reported in, [13] although the microalloying element content increased by only 0.044 wt pct.…”

Effect of the Charging Temperature on the Hot Ductility of Nb‐Containing Steel in the Simulated Hot Charge Process

“…High strength low alloy (HSLA) steels provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels, and offer the advantages of weight and cost savings compared to mild steel. They are widely used in many industrial sectors 3–5. Introduction of the HCR process into the HSLA steel production process should be a useful method for the steel industry to save energy and cost.…”