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

Reducing the Variability of HSLA Sheet Steels (TRP 9807)

The sensitivity of the yield strength of a 70 ksi HSLA steel to changes in processing variables was investigated using a laboratory hot-rolling mill. Along with a detailed examination of the hot-rolled microstructures, auxiliary experiments were conducted to determine how the decomposition of the austenite phase and the occurrence of ultra-fine precipitate formation could account for the yield strength variability. A set of guidelines was recommended for the reduction of the yield strength variability

The role of new ferrite on retained austenite stabilization in Al-TRIP steels

Microstructure and mechanical properties of two high Al, low-Si TRIP steels with different Cr and Mo contents were studied using continuous galvanizing line (CGL) laboratory simulation. Combined use of specific etching methods, optical and electron microscopy observations and EBSD characterization led to verify the epitaxial growth of ferrite during cooling at a moderate rate from the intercritical annealing to the isothermal holding temperature. The amounts of “new” ferrite formed during cooling and retained austenite obtained after processing are much higher in the steel with lower content of hardenability-promoting elements. Measured tensile properties and mechanical behavior of the steel strongly depend on the amount of new ferrite and retained austenite. It is found that the formation of new epitaxial ferrite from intercritical austenite can effectively contribute to the chemical and particle size stabilization of untransformed austenite as well as to obtain the desired TRIP effect under processing conditions highly compatible with industrial practice, i.e. cooling rates near 15°C/s and isothermal holding times at 460°C shorter than 60 s.Peer reviewe

Design of composition in (Al/Si)-alloyed TRIP Steels

here is an increasing interest in the progressive substitution of Si by Al in TRIP steels in order to obtain alloys with excellent mechanical properties and improved coatability. In this paper, thermodynamic calculations have been carried out with the help of JMatProTM software in order to assess and compare the effects that Si and Al additions exert on the phase transformation, carbon enrichment and alloying element content of phases during continuous galvanizing of multiphase steels. These simulations have provided important implications regarding the optimal combination of Si and Al. It has been found that Al causes a more pronounced increase of A3 temperature and a wider extension of the intercritical range than Si. For a constant volume fraction of phases, the carbon content in austenite is similar for Al and Si-alloyed steels. However, ferrite in Al-alloyed is richer in carbon and consequently an increase in its strength could be expected. The hardenability of intercritically annealed austenite has been estimated for alloys with different combinations of Mn, Al and Si. Finally, simulated CCT diagrams predict for Al-alloyed steels a higher amount of new ferrite formed during cooling from intercritical annealing and the need of shorter isothermal holding times at 460°C. However, Si-TRIP steels would need faster cooling rates to prevent pearlite formation and longer isothermal holding times to complete the bainitic transformation and to obtain a microstructure with high retained austenite.Peer reviewe

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

Conservation Research and Development/ New Ultra-Low Carbon High Strength Steels with Improved Bake Hardenability for Enhanced Stretch Formability and Dent Resistance

Conservation Research and Development/New Ultra-Low Carbon High Strength Steels with Improved Bake Hardenability for Enhanced Stretch Formability and Dent Resistance. The experimental work can be divided into four phases. In each phase, the materials were received or designed, processed and tested, to evaluate the BH increment or response, as a function of compositions and processing conditions. Microstructural characterization by various techniques was performed in order to gain insights into the mechanisms of flow stress increment by bake hardening

Investigation of the critical factors controlling sheared edge stretching of ultra-high strength dual-phase steels

Abstract The present study aimed to explore dual-phase (DP) steels with a good combination of high strength and reasonable global ductility (i.e., total elongation and general stretch formability) and local ductility (i.e., sheared edge ductility or hole expansion ratio). Therefore, a series of ultra-high strength dual-phase steels were designed, melted, rolled, annealed and formed. These steels contained various aluminum additions and vanadium contents and were processed with different coiling temperatures and continuous galvanizing line (CGL) thermal path simulations conducted using a Gleeble 3800 system. The microstructures, tensile properties and hole expansion behaviors of all candidate DP steels were determined and compared. The microstructural and damage evolutions in the process of both hole punching and hole expansion were examined. The results indicated that hole expansion ratios of DP steels could be correlated well with (i) the burnished-to-fracture zone ratios in shear surfaces after hole punching, (ii) the values of reduction in area of tensile specimens after fracturing, and (iii) nanohardness difference between soft ferrite and hard constituents. The micro-voids and micro-cracks introduced by hole punching acted as crack initiation sites, which severely affected the subsequent hole expansion process. Therefore, better sheared edge ductility may benefit from microstructures that retard the crack propagation or void growth and coalescence during hole expansion

Investigation of effects of processing on stretch-flangeability of the ultra-high strength, vanadium-bearing dual-phase steels

Abstract The purpose of this paper was to gain a better insight into the sheared-edge ductility of high strength dual-phase steels for the purpose of helping improve the hole expansion behavior of such steels. A candidate dual-phase (DP) steel in this study was designed and processed with distinct coiling temperatures after conventional finish rolling, and then further processed with two continuous galvanizing line (CGL) simulations. Two CGL simulations with slightly different thermal paths were conducted on a Gleeble 3800 machine; one was to replicate standard galvanizing (GI), where the intercritically annealed steels were cooled to the zinc pot temperature, and the other one was a supercooling process (SC) where the intercritically annealed steels were first cooled to the M90 temperature of the intercritically formed austenite, then up-quenched to the zinc pot temperature. The findings indicated that the combination of a low coiling temperature and GI anneal can obtain the microstructures characterized by a high-volume-percentage of martensite and fine-grained ferrite with a high tensile strength (UTS) of 1092.8 MPa and a good total elongation (TE) of 20.8%. A large amount of fresh martensite was replaced by tempered martensite caused by the changes in CGL thermal paths from GI to SC anneals, resulting in the considerable increase in sheared-edge ductility properties, although with a loss of UTS. Additionally, the damages caused by hole punching were examined by electron backscattered diffraction (EBSD) — kernel average misorientation (KAM) and nanoindentation technologies. These results showed that the micro-voids or micro-cracks in the initial punched hole surfaces and plastic internal strains near the initial punched hole sheared edges introduced by hole punching will severely influence stretch-flangeability

The development and characterization of ultra high GIGA-strength ferritic hot band steels

Abstract The correlations existing among the hot mill processing, as-coiled microstructure and mechanical properties of a Mo–Ti–V microalloyed hot rolled high strength steel were investigated in this current study. Discrete processing parameters, i.e., finish rolling temperatures (FRT) and coiling temperatures (CT), were applied and the corresponding microstructures and mechanical properties were analyzed. It was found that the FRT had only a very minor influence on either microstructures or mechanical properties. However, the CTs strongly affected both the microstructures and mechanical properties. The microstructure in the matrix was observed to change from mainly polygonal ferrite to quasi-polygonal ferrite to granular bainite and upper bainite with falling CTs, accompanied by the formation of martensite/austenite (M/A) constituents at the lower CTs. The strength appeared to be increased by the dislocations originating by the shear component of the displacive phase transformation, and by fine (Ti, Mo)C precipitates, both formed during the coiling process. Strengths reaching values as high as 1166 MPa in yield strength and 1225 MPa in tensile strength were observed in specimens after coiling at 610 °C, and the steels still had a reasonable total elongation of around 20%. However, the hole expansion ratios of these steel conditions were rather low in this study, especially for those with higher strength. Several factors appeared to contribute to the poor hole expansion ratios found in these steel conditions: the presence of coarse TiN inclusions, a large amount of strengthening precipitates and a high dislocation density as well as the presence of M/A constituents