EFFECT OF CARBON CONTENT ON THE PHASE TRANSFORMATION CHARACTERISTICS, MICROSTRUCTURE AND PROPERTIES OF 500 MPa GRADE MICROALLOYED STEELS WITH NONPOLYGONAL FERRITE MICROSTRUCTURES

The influence of C in the range of 0.011-0.043 wt-% on the phase transformation characteristics, mechanical properties andmicrostructure of Fe-2.0Mn-0.25Mo-0.8Ni-0.05Nb-0.03Ti steel was investigated. In the dilatometric experiments, it wasfound that a reduction in the C content increased the phase transformation temperatures, decreased the hardness andpromoted quasi-polygonal ferrite (QF) formation over granular bainitic ferrite (GBF) and bainitic ferrite (BF), but at the sametime the sensitivity of the phase transformation temperatures and hardness to cooling rates was reduced. Mechanical testingof laboratory hot rolled plates revealed that the targeted yield strength of 500 MPa was reached even in the steel withthe lowest C content (0.011wt-%). An increase in C content did not considerably increase the yield strength, although thetensile strength was more significantly increased. Impact toughness properties, in turn, were markedly deteriorated due to thisC content increment. Microstructural analysis of the hot rolled plates showed that an increase in C content decreased thefraction of QF and consequently increased the fraction of GBF and BF, as well as the size and fraction of C-enriched secondarymicroconstituents. In addition, the size of the coarsest crystallographic packets seemed to be finer in the low C steelwith QF dominated microstructure than in its higher C counterparts with higher fractions of GBF-BF, even thought theaverage crystallographic packet size was slightly finer in these higher C steels.Mechanical testing of the simulated CGHAZ’s showed that their toughness properties are not strongly dependenton C content, although there exists a general trend for toughness to slightly weaken with increasing C content. Itcould be concluded that HAZ toughness properties of these types of steels are acceptable. On the basis of dilatometricexperiments, mechanical testing and microstructural analysis it can be stated that a good combination of strength,toughness and weldability as well as microstructural stability can be reached in very low C steels with QF dominatedmicrostructures. Finally, an example of this type of microstuctural concept, which has been successfull

The effects of composition and thermal path on hot ductility of forging steels

This work examines the effects of composition and thermal handling path on the hot ductility of as-cast steelforging ingots. Poor ductility of the as-cast structure can lead to cracking of the ingot prior to forging or theformation of tears early during the forging process. The as-cast structure is particularly susceptible to crackingdue to the large grain size and high degree of microsegregation present.Experiments were conducted to evaluate the ductility of the as-cast steel with varying levels aluminum andnitrogen. Multiple thermal handling paths were followed in order to approximate the different thermal conditionsexperienced approximately six inches below the surface of a large (~40 MT) steel ingot following solidification.Hot tension testing after in-situ melting and solidification was used for quantitative measurements of thematerial ductility. The majority of testing was carried out on a modified P20 mild tool steel. The experimentsindicate a significant loss of ductility for materials with high aluminum and nitrogen contents(AlxN = 5.2x10-4) in the temperature range of 950 °C - 1050 °C upon solidification and direct cooling to thetest temperature. This behavior is not present in material with AlxN products below 1.3x10-4. All materialstested exhibited a loss of ductility when the sample was cooled to 900 °C, immediately reheated to 1000°C andtested. With increasing hold times at 900 °C prior to reheating to 1000 °C, the material with high aluminum andnitrogen contents recovers ductility much more quickly than the low aluminum and nitrogen materials.Funding in part by the Forging Industry Educational & Research Foundation and Ellwood Group, Inc

Crystallographic Analysis of Martensite in 0.2C-2.0Mn-1.5Si-0.6Cr Steel by EBSD

The crystallography of martensite formed in 0.2C-2.0Mn-1.5Si-0.6Cr steel was studied using the EBSDtechnique. The results showed that the observed orientation relationship was closer to the Nishiyama-Wassermann (N-W) than to the Kurdjumov-Sachs (K-S) orientation relationship (OR). The microstructure ofmartensite consisted of parallel laths forming morphological packet-like structures. Typically, there were threedifferent lath orientations in a morphological packet consisting of three specific N-W OR variants sharing thesame {111} austenite plane. A packet of martensite laths with common {111} austenite plane was termed as acrystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packetsize, but occasionally the morphological packet was found to consist of two or more crystallographic packets.Therefore, the crystallographic packet size appeared to be finer than the morphological packet size. Therelative orientation between the variants in crystallographic packets was found to be near 60°/<110>. Thisappears to explain the strong peak observed near 60° in the grain boundary misorientation distribution.Martensite also contained a high fraction of boundaries with their misorientation in the range 2.5-8°.Typically these boundaries were found to be located inside the martensite laths forming lath-like sub-grains,whose long axes were parallel with the long axis of the martensite laths

On the strength of microalloyed steels - An interpretive review

In the mid-1950s, hot rolled carbon steels exhibited high carbon contents, coarse ferrite-pearlite microstructures, and yield strengths near 300 MPa. Their ductility, toughness and weldability were poor. Today, a half-century later, hot rolled steels can exhibit microstructures consisting of mixtures of ferrite, bainite and martensite in various proportions. These structures are very fine and can show yield strengths over 900 MPa, with acceptable levels of ductility, toughness and weldability. This advancement was made possible by the combination of improved steelmaking, microalloying technology and better rolling and cooling practices. The purpose of this paper is to chronicle some of the remarkable progress in steel alloy and process design that has resulted in this impressive

Metallurgy and continuous galvanizing line processing of high-strength dual-phase steels microalloyed with Niobium and Vanadium

It is well-known that the automobile industry continues to search for stronger, more cost-effective steels to lowerthe mass of the vehicle for better fuel consumption and to provide better crash worthiness for safety. Thismovement to higher UTS strength requirements, from the 590-780 range to over 980 MPa, has led to morecomplex alloy design. In the processing of these steels on continuous, hot-dipped, galvanizing lines (CGL), twomajor changes in composition have been the addition of hardenability elements and microalloying. Forexample, very-high strength DP steels, containing high Mn, Cr and Mo along with Nb and V have shown UTSlevels in excess of 1100MPa. This paper will present recent research conducted on four experimental steelscontaining these additions. It will be shown that the choice of intercritical annealing temperature is importantwhen processing microalloyed DP steels, as are the rates of cooling throughout CGL processing. The physicalmetallurgy of producing ultra-high strength DP steels on CG lines will be presented and discussed

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures

Rational Alloy Design of Niobium-Bearing HSLA Steels

In the 61 years that niobium has been used in commercial steels, it has proven to be beneficial via several properties, such as strength and toughness. Over this time, numerous studies have been performed and papers published showing that both the strength and toughness can be improved with higher Nb additions. Earlier studies have verified this trend for steels containing up to about 0.04 wt.% Nb. Basic studies have shown that the addition of Nb increases the recrystallization-stop temperature, T5% or Tnr. These same studies have shown that with up to about 0.05 wt.% of Nb, the T5% temperature increases in the range of finish rolling, which is the basis of controlled rolling. These studies also have shown that at very high Nb levels, exceeding approximately 0.06 wt.% Nb, the recrystallization-stop temperature or T5% can increase into the temperature range of rough rolling, and this could result in insufficient grain refinement and recrystallization during rough rolling. However, the question remains as to how much Nb can be added before the detriments outweigh the benefits. While the benefits are easily observed and discussed, the detriments are not. These detriments at high Nb levels include cost, undissolved Nb particles, weldability issues, higher mill loads and roll wear and the lessening of grain refinement that might otherwise occur during plate rough rolling. This loss of grain refinement is important, since coarse grained microstructures often result in failure in the drop weight tear testing of the plate and pipe. The purpose of this paper is to discuss the practical limits of Nb microalloying in controlled rolled low carbon linepipe steels of gauges ranging from 12 to 25 mm in thickness

Microalloyed steels for high-strength forgings

In the past thirty-five years, two families of microalloyed (MA) steels have been developed for high strength barand forging applications. The first family was introduced in 1974 and represented the medium carbon steelsto which were added small amounts of niobium or vanadium. These early medium carbon contents steelsexhibited pearlite-ferrite microstructures and showed good strength and high-cycle fatigue resistance.About 15 years later, microalloyed multiphase steels were introduced, which had microstructures comprisedof mixtures of ferrite, bainite, martensite, and retained austenite, depending on the compositionand processing. These steels were capable of reaching very high strengths, with good fatigue resistanceand high fracture resistance. Prior to the early 1970s, high strength forgings could be obtained only by finalheat treatment, involving reheating, quenching and tempering (QT). It has been shown repeatedly that the aircooled forgings made from MA pearlite-ferrite steels can exhibit strengths and fatigue resistances similarto those of the more expensive heat treated forgings. This paper will follow the developmentof the microalloyed pearlite-ferrite steels over the past 35 years

Memorie Microalloyed steels for high-strength forgings

In the past thirty-five years, two families of microalloyed (MA) INTRODUCTION Today, there are essentially three methods of manufacturing high strength forgings, i.e., forgings with yield strengths in excess of~600MPa and UTS levels above~850 MPa.: (i) heat treated low alloy steels, (ii) microalloyed medium carbon steels and (iii) microalloyed multi-phase steels. This paper will focus on the second group. As it is well-known that producing forgings using the QT heat treatment process is inefficient, expensive and deleterious to the environment, alternative routes to high strength forgings have been studied for decades. From an engineering perspective, having high strengths in forgings is very attractive, since this enables the possibilities of higher static and dynamic loads, smaller components, and particularly in rotating parts, improved high cycle fatigue resistance. From about the mid-1950s in the USA, the flat rolled steel industry has shown that the small addition of elements such as niobium and vanadium, which together with Ti define the microalloying elelments, to simple C-Mn-Si steels could impressively increase the strength when the steels were rolled and cooled correctly In the medium carbon steels intended for forging applications, V is normally preferred over Nb because of the solubility behavior which permits the dissolution of VCN particles at lower reheat temperatures. The strengthening effect of V can be further improved when used with higher N levels. In today's technology, N in BOF steels is in the range of 40-60ppm, while in EAF steels i