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

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

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 deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure. <br /

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

Mechanical behavior of IF 409 ferritic stainless steel

IF 409 stainless steel is a common industrial steel grade used in automotive exhaust systems. It combines a high formability with the corrosion resistance associated with stainless steels. Two particular grades of IF 409 were used in these experiments: a titanium stabilized and a titanium-niobium stabilized. The current work explores the mechanical behavior Of this grade with emphasis on the occurrence of dynamic recrystallization. Hot torsion tests were performed over a range of temperatures and strains to determine the behavior. The tests were analyzed using the flow curves, optical microscopy and texture measurements. The optical micrographs indicated that dynamic recrystallization was occurring. The texture results confirmed that the titanium stabilized steel dynamically recrystallized around 1000°C and the titanium-niobium grade recrystallized around 1200°C. In addition, the effects of strain and temperature were quantified. The results allow the feasibility of dynamic recrystallization in industrial applications to be determined