18 research outputs found
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Evaluation of Toughness of High Strength Low Alloy (HSLA) Steels as a Function of Carbon Content
The influence of carbon content on the microstructure and toughness of HSLA steel at room temperature was investigated based on experimental work and literature. It was revealed that increasing the carbon content in from 0.06 to 0.14 wt-% is detrimental to toughness, giving higher impact transition temperature. The deterioration of toughness was correlated to undesired changes in the microstructure, which showed an increase in pearlite volume fraction at the expense of ferrite. At high carbon content, cementite of pearlite was found to grow more rapidly to form continuous plates which act as preferred sites for crack nucleation and propagation. In addition, the lamellar spacing of the pearlite increased as a function of carbon content, which in turn gave worse toughness. The presence of high carbon content and carbide forming elements in the chemical composition was more detrimental to toughness due to the formation of thick carbides around the grain boundaries. These carbides act as a path for crack propagation, which makes it easy for cracks to cohere, leading to intergranular fracture. Keywords - HSLA steel, Carbon, Brittleness, Toughness, Impact Transition Temperature (ITT)
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The influence of Widmanstätten ferrite, martensite and grain boundary carbides on the strength and impact behaviour of high Al (0.2%) and Nb containing hot rolled steels
The influence of Al and Nb on the strength and impact behaviour of hot rolled 0.06%C, 1.4%Mn steels has been determined after hot rolling to 15 and 30 mm thick plate. When 0.16%Al was added to the plain C-Mn steel, the impact behaviour significantly improved even though Widmanstätten ferrite (WF) was present. This improvement was due to refinement of the grain boundary carbides and removing the N from solution as AlN. The hot rolled steels all contained WF but when Nb was added more WF formed as well as MA giving poor impact behaviour. Reducing the hardenability from that shown in previous work by decreasing C from 0.1 to 0.06%, Nb from 0.03 to 0.02%, and cooling rate from 33 to 17 K/min had no effect in improving the impact performance of hot rolled Nb steels. To ensure optimum properties not only is it necessary to reduce the hardenability, but WF formation must be discouraged by having a high Ar3. This can only be presently achieved by refining the austenite grain size via control rolling the Nb containing steels; the benefit of adding Al can then, readily be seen. Suggestions are made as to how this might be achieved for hot rolling
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Influence of Al content on the corrosion resistance of micro-alloyed hot rolled steel as a function of grain size
High-strength low-alloy steel (HSLA) has been widely used in many applications involving automobiles, aerospace, construction, and oil and gas pipelines due to their enhanced mechanical and chemical properties. One of the most critical elements used to improve these properties is Aluminium. This work will explore the effect of Al content on the corrosion behaviour of hot rolled high-strength low-alloy steel as a function of grain size. The method of investigation employed was weight loss technique. It was obvious that the increase in Al content enhanced corrosion resistance through refinement of grain size obtained through AlN precipitation by pinning grain boundaries and hindering their growth during solidification which was found to be beneficial in reducing corrosion rate
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Hot rolled high Al containing steels as a replacement for the control rolled high strength low alloy (HSLA) steels
The extent to which Al and Nb can be used to improve the properties of hot rolled steels has been investigated with the aim of obtaining mechanical properties similar to those given by the more expensive, control rolled or normalised route, eg. API X52 line pipe. Three steels with 0.02%Al, 0.16%Al and 0.16%Al, 0.018%Nb have been examined and their strength and impact behaviour obtained. The 0.16%Al steel had a similar strength to the 0.02%Al containing steel∼300MPa, but better impact behaviour (30-40°C lower 54J, ITT) with an impact transition temperature (ITT) of −90°C which from previous work will be due to a refinement of the grain boundary carbides. The present work shows that the addition of Nb to this high Al containing steel, although beneficial to strength, giving a lower yield strength (LYS) of 385 MPa, close to that given by some of the control rolled steels gives very poor impact behaviour with a 54J ITT of only −20°C. The improvement of strength is mainly a result of precipitation hardening by NbCN with some benefit from grain refinement while the deterioration of impact behaviour might be due to the presence of lower transformation products or coarser carbides. Further work is required to positively clarify the cause of this deterioration and to explore further options in achieving the aim of obtaining a hot rolled steel with strength in the range 350-400MPa and 54J ITT of −50°C
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Understanding the high temperature side of the hot ductility curve for steels
The study tests the validity of the tensile hot ductility test for assessing cracking during the straightening operation. Steels with a thin film of deformation induced ferrite (DIF) or fully austenitic when straightening were examined. In both cases dynamic recrystallisation (DRX) occurs at high temperatures. However, DRX is not possible on straightening, the grain size being too coarse and strains too low. When, DRX occurs, ductility is overestimated compared to the un-recrystallised condition on bending. For steels with DIF films if the depth of the trough is ≥40%RA (reduction of area) cracking is unlikely. However, for TWIP steels, the estimated RA for unrecrystallised ϒ can be much < 40% causing cracking even though measured ductility is well in excess
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Hot ductility of high Al TWIP steels containing Nb and Nb-V
The hot ductility of B-Ti-Nb-high Al (1.5%Al) containing TWIP steels having Ti/N ratios mainly in excess of 3.4/1 was obtained. After soaking at 1250°C, the tensile specimens were cooled at 12 or 60°C min−1 to the test temperature and then strained to failure at 3 × 10−3 s−1. Ductility was always good (reduction of area >40%), independent of Ti/N ratio or cooling rate. The good ductility is due to B segregation strengthening the grain boundaries and the low S level (0.005%S) limiting the volume fraction of MnS inclusions and restricting AlN precipitation to the matrix. Increasing the cooling rate, higher N levels and Nb resulted in a small improvement in ductility. An addition of V to the Nb-containing steels caused a slight deterioration in the hot ductility
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Investigation of corrosion resistance of high-strength low-alloy (HSLA) steel in fresh and salt water for pipeline application
Corrosion behaviour of HSLA steel immersed in fresh and salt water for a four weeks period was investigated by weight loss method at room temperature. The main applications considered for the current work are underground water pipes and the pipeline systems in desalination plants. The corrosion behaviour in the saltwater medium was more severe but the rate was found to drop in the fourth week, presumably due to the formation of a protective passive film on the surface. The combination of ferrite and cementite phases in the microstructure contributed to promoting the corrosion rate due to the initiation of galvanic cells between both phases
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The influence of grain size and precipitation and a boron addition on the hot ductility of a high Al, V containing TWIP steels
The hot ductility of V containing high Al, TWIP steels was determined at a 1000°C, when no dynamic recrystallisation occurred, and precipitation was too coarse to influence ductility. A change in grain size from ∼1250 to 500 µm caused the reduction of the area to increase by ∼25%, intimating that the improvement in ductility on adding boron is due to its segregation to the boundaries causing grain refinement on solidification. Fine VC precipitation was mainly responsible for deteriorating ductility. In these steels, the ductility of un-recrystallised austenite decreases gradually with increasing temperature from 700°C to 1000°C and this in combination with fine precipitation can markedly change the shape of the hot ductility curve from ductility decreasing to increasing with temperature
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Influence of Cu and Ni on the Hot Ductility of Low C Steels with Respect to the Straightening Operation When Continuous Casting
Cu-containing steels can suffer from hot shortness unless Ni is added to protect them but whether this problem also influences the straightening operation is not clear. Previous work on hot ductility has found that only when the tensile samples from Cu-containing steels are oxidised can any deleterious influence of copper be found. However, this paper shows that oxidation is not essential. It is more complex than that and, as Cu has been suggested for TRIP steels at levels up to 2.5% Cu to increase their strength and ductility, a greater understanding is required, both of hot shortness and cracking during straightening. The present paper explores the hot ductility behaviour of steels alloyed with Cu and Ni in the straightening temperature range, 700–1000 °C, when tested in air and in an argon atmosphere. Segregation of Cu to the sulphides and grain boundaries occurred allowing the formation of fine Cu2S particles at the austenite grain boundaries favouring intergranular failure and this was more pronounced under oxidising conditions and required strain. It was concluded that a Cu addition, as well as causing hot shortness at higher temperatures will also cause cracking problems in the straightening temperature range in the more sensitive to cracking grades of steel and although the problems are different they are nevertheless interrelated and provided there is sufficient Ni, both may be avoided
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The Influence of Small Additions of Alloying Elements on the Hot Ductility of AHSS Steels: A Critical Review Part 2
In this paper, the influence of small additions of Cr, Mo, Cu, Ni, B, Ca, Zr, and Ce on the hot ductility of advanced high-strength steels (AHSS) has been reviewed. Most of these small additions have a positive effect in improving hot ductility on straightening during continuous casting operations and should be considered when problems with cracking in continuous casting are encountered. In many of these cases, the reason for these generally small but important improvements in hot ductility is not known with certainty, but the segregation of these elements to the austenite grain boundaries, strengthening the bonding, is often suggested