Influence of isothermal treatment on MnS and hot ductility in low carbon, low Mn steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 x 10-4 s -1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The highercarbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility

Effect of mo, nb and v on hot deformation behaviour, microstructure and hardness of microalloyed steels

Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V-to NbV-to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000°C) than the High V steel (975°C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (\u3e20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV steel (293 HV) compared to the NbV (265 HV) and High V (285 HV) steels

Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

The effect of Nb on the continuous cooling transformation curves of ultra-thin strip CASTRIP© steels

The effect of Nb on the hardenability of ultra-thin cast strip (UCS) steels produced via the unique regime of rapid solidification, large austenite grain size, and inclusion engineering of the CASTRIP© process was investigated. Continuous cooling transformation (CCT) diagrams were constructed for 0, 0.014, 0.024, 0.04, 0.06 and 0.08 wt% Nb containing UCS steels. Phase nomenclature for the identification of lower transformation product in low carbon steels was reviewed. Even a small addition of 0.014 wt% Nb showed a potent effect on hardenability, shifting the ferrite C-curve to the right and expanding the bainitic ferrite and acicular ferrite phase fields. Higher Nb additions increased hardenability further, suppressed the formation of ferrite to even lower cooling rates, progressively lowered the transformation start and finish temperatures and promoted the transformation of bainite instead of acicular ferrite. The latter was due to Nb suppressing the formation of allotriomorphic ferrite and allowing bainite to nucleate at prior austenite grain boundaries, a lower energy site than that for the intragranular nucleation of acicular ferrite at inclusions. Strength and hardness increased with increasing Nb additions, largely due to microstructural strengthening and solid solution hardening, but not from precipitation hardening

The effect of cooling rate and coiling temperature on the niobium retention in Ultra-Thin Cast Strip steel

This laboratory study utilised a dilatometer to simulate the run-out table cooling rate and the coiling temperature to investigate the effect of the cooling rate and simulated coiling conditions on the age hardening response of a niobium microalloyed Ultra-thin Cast Strip (UCS®) steel, produced by the CASTRIP® Process. Three cooling rates of 1, 5 and 40 °C/s, covering very slow (1 °C/s) to typical run-out table cooling rates (40 °C/s), down to two coiling temperatures of 500 and 675 °C were used. Dilatation curves were used to determine the temperature range over which the ¿-¿ phase transformations occurred and the final microstructures were characterized using an optical microscope equipped with an image analysis software. The subsequent age hardening response, which previous studies have shown, results from the retention of Nb in solid solution, was assessed by the hardness changes after a post heat treatment at 700 °C for 60 s. A range of age hardening responses were obtained, depending on cooling rates and cooling stop (coiling) temperatures, which indicate a different degree of Nb retention. At the same cooling rate, the lower coiling temperature of 500 °C resulted in higher Nb retention compared to the higher coiling temperature of 675 °C. As the coiling temperature of 675 °C was within the austenite to ferrite transformation range, the simulated slow cooling of the coil impacted the precipitation behaviour of Nb rendering the interpretation more complex and this will be discussed in this paper. For the 500 °C simulated coiling temperature, the higher cooling rate resulted in a higher age hardening increment thus more Nb retention

The development and manufacture of Ultra-Thin Cast Strip steel products with high residual element levels via the CASTRIP® process

A range of Ultra-Thin Cast Strip (UCS) sheet steels with elevated residual levels was produced via the CASTRIP twin drum casting method at Nucor Steel, Indiana. This paper examines the influence of elevated levels of copper, chromium, nickel, and phosphorous on mechanical properties, surface quality, processing, and weldability of UCS sheet steel products produced by the CASTRIP process. Increased levels of copper and phosphorous were found to strengthen UCS sheet steel due to solid solution strengthening but chromium and nickel did not. At lower coiling temperatures and low hot reductions, where processing conditions promote microstructural strengthening, copper and chromium further enhanced strength via increased hardenability. The presence of elevated residual levels of copper is known to potentially lead to hot shortness, unless expensive counter measures are employed, such as nickel additions. Due to the unique solidification and thermal history conditions of the CASTRIP process, higher levels of copper can be tolerated without hot shortness or loss of surface quality. The CASTRIP process is capable of utilizing increased levels of scrap containing higher residual levels, such as post-consumer shredded material. These elevated levels of residual elements can be utilized as a strengthening agent in the finished sheet. Alternatively, this influence on strength can be mitigated when desired through the choice of processing parameters. Additionally, the elevated residual content did not influence either the surface quality or the weldability of the steel

Edge microstructure and strength gradient in thermally cut ti‐alloyed martensitic steels

Recently developed Ti‐alloyed martensitic steels are believed to exhibit higher wear resistance than traditionally quenched and tempered medium carbon steels. However, their properties may deteriorate during thermal cutting and welding as a result of microstructure tempering. This would present significant challenges for the metal fabrication industries. A decrease in strength and wear resistance associated with tempering should vary with steel composition, initial steel microstructure and properties, and cutting method. In this work, we investigated the effect of thermal cutting on the edge microstructure and properties in two alloyed plate steels containing 0.27C‐0.40Ti and 0.39C‐0.60Ti (wt.%) commercially rolled to 12 mm thickness. Three cutting methods were applied to each of the two plates: oxy‐fuel, plasma and water‐jet. Microstructure characterisation was carried out using optical and scanning electron microscopy. With an increase in thermal effect, from water‐jet to plasma to oxy‐fuel, the heat affected zone width increased and hardness decreased in both steels. However, the hardness profile from the cut edge to the base metal significantly varied with steel composition, particularly C and Ti contents. The dependence of grain structure and precipitation kinetics on steel composition, and cutting method, were thoroughly investigated and linked to the hardness profile variation. The obtained results will be used to optimise the technological parameters for cutting and welding of Ti‐alloyed martensitic steels

Development of a family of high strength low carbon microalloyed ultra-thin cast strip products produced by the CASTRIP® process

The manufacture of high strength strip products in strip thicknesses less than 1.5mm is very challenging for the conventional hot and cold rolled processing routes. A range of Nb and Nb/V microalloyed steels have been successfully strip cast by the CASTRIP process enabling the development of a range of high strength Ultra-Thin Cast Strip (UCS) products. It was found that the CASTRIP process fully exploits the strengthening potential of the low C-Mn-Nb-(V) alloy design system. Substantial strengthening by microstructural hardening was provided by Nb. Retention of Nb and V in solid solution in hot rolled coils enabled further strengthening by a subsequent age hardening heat treatment. This paper describes the development of a range of high strength low alloy (HSLA) UCS products produced by the CASTRIP process, utilising a low C-Mn-Nb-(V) alloy system, covering yield strengths in the range of 350-550 MPa, in strip thicknesses of 0.9-1.5mm, for both the as rolled and hot dip galvanised coated conditions

Recent developments with ultrathin cast strip products produced by the Castrip® process

Recent product developments at the Castrip facility at Nucor\u27s Crawfordsville, Ind., plant have focused on expanding its range of light-gauge hot rolled products. This paper presents an overview of these recent product development experiences