An investigation into the corrosion resistance of steels with various contents of carbon and microalloying elements was carried out. It was shown that the presence of a large amount of nanosized (2–3 nm and less) precipitates of the interphase type, particularly niobium carbonitride and vanadium carbonitride, leads to a decrease in the corrosion resistance of hot-rolled sheet products. It was found that, after heat treatment of rolled products at 710 °C, the corrosion resistance of the metal is improved. One of the reasons for this is a decrease in the amount of interphase precipitates, which negatively affect the corrosion resistance of steel, while particles formed in austenite and ferrite do not have such an effect. To ensure high corrosion resistance of steels for oil-field pipelines, microalloying with niobium instead of vanadium is advisable, as well as heat treatment at temperatures above 710 °C.

Effect of Carbonitride Precipitates on the Corrosion Resistance of Low-Alloy Steels under Operating Conditions of Oil-Field Pipelines

Heterogeneity of Structure and Mechanical Properties Studied in Thickness up to 100 mm of Low-Alloy Shipbuilding Steel Sheets with a Yield Strength Not Less Than 420 MPa

Abstract The thermo-mechanical processing of microalloyed steels has been studied widely. However most of the work has been focused on the finishing stages of hot rolling where pancaking of the austenite is desired for strength and toughness. During roughing, full recrystallization is desired for a uniform, through-thickness austenite microstructure. In this work the influence of the strain sequence during roughing on the evolution of austenite in plain carbon, C-Mn-V, C-Mn-Nb-V and C-Mn-Nb-Ti-V steels, was investigated during conventional hot rolling. This was done because strain is one of the most controlling parameters of recrystallization during hot rolling which in turn controls the austenite microstructure. Reheating and roughing simulations were conducted in a Bähr 805D deformation dilatometer using a constant austenitising temperature of 1220 °C. Different heating cycles were adopted in the reheating simulations to see the influence of heating rate and soaking time on the austenite grain size. Heating rate of 7.5 °C/min and constant soaking time of 30 minutes were used for roughing simulations as this closely simulated the slab’s centre thermal profile. The roughing strain sequence was varied from small (0.07 ε/ pass), intermediate (0.1 ε / pass) to large (0.15 ε / pass) at a temperature range of 1150-1050 °C and strain rates of 0.3-2.9 s. Application of a non-standard 0.4 strain in the last pass of each strain schedule was introduced to study its effect on the austenite grain size. Double stroke tests were used to quantify the austenite softening behaviour. The prior austenite grain size was measured from quenched specimens. A FEM study was commissioned to study the localized strain distribution in axisymmetric compression and flat plate rolling simulations. Reheat austenite grain sizes increased with increasing soaking time and decreasing heating rate. The C-Mn-Nb-Ti-V steel had the finest reheat grain size and the C-Mn steel was the coarsest. This was attributed to the presence of the stable TiN in the former. The austenite grain size after roughing simulations decreased with increasing strain per pass. The C-Mn had the coarsest austenite grain size after roughing followed by the C-

The influence of rough rolling conditions on austenite evolution in peritectic microalloyed plate steels

The kinetics of austenite grains' growth upon heating has been investigated, and the processes of dynamic and static recrystallization occurring under different modes of plastic deformation (reduction pattern, deformation temperature) of high-strength steels with various microalloying complexes have been studied. The research made it possible to reveal the thermal deformation conditions for the formation of a finely dispersed homogeneous structure of steel. Technological recommendations have been developed for the production of high-strength steels depending on their microalloying (vanadium, niobium).

Влияние микролегирования на особенности структурообразующих процессов при горячей пластической деформации

Mathematical modelling of hot plastic deformation of Ti-V and Ti-Nb-V microalloyed steels

Purpose: The work presents research results of impact of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure of newly elaborated steels assigned for production of forged machine parts, using the method of thermo-mechanical treatment. Design/methodology/approach: The study was performed with the use of Gleeble 3800 simulator. Stressstrain curves were determined during continuous compression test in a temperature range from 900 to 1100°C and at a strain rate of 1, 10 and 50 s-1. In order to determine recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of specimens were done with a given strain at the rate of 10 s-1, in a temperature range from 900 to 1100°C, with isothermal holding of samples between successive stages of deformation for 2 to 100 s. Recrystallization kinetics of plastically deformed austenite was described using the Johnson-Mehl-Avrami equation. The observations of microstructures of thin foils were done using JEOL JEM 3010 transmission electron microscope. Findings: Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steels is caused by the process of continuous dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Performed two-stages compression tests revealed that microadditions introduced into steel considerably influence the kinetics of static recrystallization. Research limitations/implications: It was found that the time necessary for a total course of recrystallization of austenite is too long to be accepted in the production process of forgings. Practical implications: Executed hot compression tests will contribute to establishing conditions of forging with the method of thermo-mechanical treatment. Originality/value: Strain-stress curves and recrystallization kinetics curves of newly elaborated microalloyed steels have been determined.

W. Ozgowicz

Papers

Effects of Nb, Ti and V on recrystallization kinetics of austenite in microalloyed steels