In modern industry, the developing tendency and prospect for productions of the oil and gas pipeline steels is to further improve the strength and toughness by advanced manufacture process of the thermo-mechanical control process (TMCP) to refine microstructure. In this work, the hot deformation behavior as well as its effect on the phase transformation of the clean acicular ferrite pipeline steel with simple chemical composition has been investigated. According to the result, the optimum TMCP parameters were designed. Furthermore, the rolling test was carried out on the experimental rolling mill. The results show that, the high strength and excellent toughness of the clean acicular ferrite pipeline steel can be obtained by controlling the TMCP parameters of the production process appropriately. (c) 2006 Elsevier B.V. All rights reserved.

Challenge of mechanical properties of an acicular ferrite pipeline steel

The effect of Mo- and W-addition on the precipitation hardening of Ti- and Nb-microalloyed high-strength low-alloy steels was investigated. The variation in simulated hot-coiling temperature caused a significant difference in strength, which is attributed mainly to different precipitation-hardening behavior. Among the four alloys with different combination of the microalloying elements: Ti–Mo, Nb–Mo, Ti–W, and Nb–W, the steel with Nb–Mo turned out to be the most effective in precipitation hardening. The strengthening mechanism was discussed based on the microstructural evolution observed by electron microscopy and the theoretical considerations combining the different hardening models.

Strengthening mechanism of hot rolled Ti and Nb microalloyed HSLA steels containing Mo and W with various coiling temperature

A new Ti and Mo micro-alloyed hot-rolled high strength sheet steel was developed by controlling thermomechanical controlled processing (TMCP) schedule, in particular focusing on rolling and coiling temperatures. The results revealed that the steels developed were strengthened mostly by a combined effect of ferrite (α) grain refinement and precipitation hardening. The rolling at austenite (γ) non-recrystallization region provided desirable features by introducing an accumulation of local strains (i.e., dislocations), which served as nucleation sites for γ→α transformation during coiling process, making ferrite grain more fine and homogeneous, although the precipitation hardening was somewhat weakened due to the precipitation of large (Ti, Mo)C carbides (i.e., γ region precipitation). Moreover, these fine and homogeneous ferrite grain, and well developed dislocation structure improved impact properties. The TMCP schedule of the rolling at γ non-recrystallization region (final rolling temperature of 880 °C) and coiling at 620 °C was found to provide an attractive combination of tensile and impact properties.

Development of Ti and Mo micro-alloyed hot-rolled high strength sheet steel by controlling thermomechanical controlled processing schedule

Thermo-mechanical-metallurgical modeling for hot-press forming in consideration of the prior austenite deformation effect

The paper presents the results of thermomechanical treatment via forging on the microstructure and mechanical properties of newly obtained microalloyed steel containing 0.28% C, 1.41% Mn, 0.027% Nb, 0.028% Ti, and 0.019% V. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Continuous and double-hit compression tests were conducted using the Gleeble 3800 thermomechanical simulator. The samples were investigated in a temperature range from 900 to 1100 °C and a strain rate of 1 and 10 s−1. To determine the recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of samples using the applied deformation were conducted in a temperature range from 900 to 1100 °C with isothermal holding of the specimens between successive deformations for 2-100 s. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The applied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and successive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650 °C, reveal values of YS0.2 which equal from 994 to 892 MPa, UTS from 1084 to 958 MPa, KV from 69 to 109 J, KV−40 from 55 to 83 J, and a hardness ranging from 360 to 300 HBW.

Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of Nb-Ti-V Microalloyed Steel

Influence of plastic deformation conditions on structure evolution in Nb–Ti microalloyed steel

Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.

P. Sricharoenchai

Papers

Influence of plastic deformation conditions on structure evolution in Nb–Ti microalloyed steel

Effect of thermomechanical processing on the microstructure and mechanical properties of Nb–Ti microalloyed steel