G.D. Wang

Experimental investigation on thermal wear of high speed steel rolls in hot strip rolling

Abstract The thermal wear ratio of a high speed steel roll was investigated experimentally in hot strip rolling with a DTW- 166 thermal wear testing machine developed by the authors. It is clear that the wear ratio increased with number of cycles. Some of the increase in wear was because of the black oxide layer generated on the roll surface at the beginning. The wear ratio also increased as slippage ratio and loads increased. Loads played a more important role than slippage ratio for thermal wear. The appearance of the roll surface was observed by SEM under different conditions. The mechanism of thermal wear was composed of adhesive, microploughing, microcutting, oxidation, and plastic slippage wear.

Three-dimensional FEM analysis of work roll temperature field in hot strip rolling

Abstract Based on the thermal conduction equation, the three-dimensional (3D) temperature field of a work roll has been analysed by FEM, and the temperature variations on the roll surface have also been described in hot strip rolling. During rolling, the highest roll surface temperature was 593°C, and the maximum temperature difference of the roll surface was 536 K. At the end of the rolling interval, the corresponding values were 63°C and 31 K, respectively. The calculated results were found to be in good agreement with the measured values. Such analysis is effective and beneficial for investigation of the work roll thermal stress and thermal fatigue in hot strip rolling.

Epsilon carbide precipitation and wear behaviour of low alloy wear resistant steels

In this study, we obtained different morphology and size of epsilon carbides (ϵ-carbides) via low temperature tempering of 0.32 wt-% C low alloy wear resistance steel. The objective is to elucidate the determining role of size and morphology of carbides on mechanical properties and three-body impact wear resistance. The formation of small needle-like ϵ-carbides was responsible for increase in yield strength, low temperature impact toughness and three-body impact wear resistance. However, when the ϵ-carbides were large and rod-like, hardness, toughness and three-body impact wear resistance were significantly reduced. The wear mechanism of steel containing needle-like ϵ-carbides primarily involved plastic deformation associated with fatigue and small degree of abrasive wear and furrow, while steels containing rod-like ϵ-carbides were predominantly characterised by furrow.

Black oxide layer formation and banding in high chromium rolls

Abstract In order to improve the surface quality of strip steels it is essential that surface oxide adheres to the roll surface. To understand the mechanism of banding (the detachment of oxide from the roll surface), black oxide layers on a roll surface and on a slab surface were analysed using SEM, optical microscopy, microhardness testing, energy dispersive spectrometry, and X-ray diffraction. A mathematical model for black oxide layer thickness of a high chromium iron roll was established on the basis of the oxidation mechanism and oxide layer structure. It is shown that the black oxide layers, formed on the roll surface, induce the formation of cracks which propagate easily along M3 C and M7 C3 carbides resulting in their final separation from the roll surface during the rolling process.

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

Abstract The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.

Properties assessment of the first industrial coils of low-density duplex δ-TRIP steel

A novel low-density steel with high aluminium content was, for the first time, manufactured as coils by the conventional industry process consisting of continuous casting followed by hot-rolling and cold-rolling. The duplex δ-TRIP microstructure, comprising a mixture of blocky retained austenite and both δ- and α-ferrite, was then produced by laboratory-scale heat treatments. The microstructure generation was achieved by an intercritical annealing followed by direct quenching, which was the same process as that for producing conventional dual-phase steels. The alloy exhibits quite interesting mechanical properties with a combination of 930 MPa tensile strength and of 21% total elongation, both in rolling and transverse directions. The formability of this novel alloy was assessed and discussed.

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

Abstract By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.

Analysis of change in microstructure and properties during high temperature processing of ultralow C and high Nb microalloyed steel

Abstract To further improve the strength and toughness, the advanced thermomechanical controlled processing has been applied in the development of an ultralow C and high Nb bearing steel. In the present investigation, the effects of processing parameters, consisting of the coiling and starting temperatures in non-recrystallisation region, on the final microstructure and mechanical properties of this steel have been studied by tensile, Charpy impact tests, optical microscopy and transmission electron microscopy. Results indicate that the acicular ferrite dominated microstructure can be greatly refined in grain size with decreasing the starting temperature of finishing rolling. However, for high Nb steels, the too low starting temperature would promote the formation of high temperature transformation products and consequently make against the improvement of mechanical properties. In addition, the optimum temperature window of finishing rolling is found to be also related to alloying levels of austenite stabilising elements. At the high starting temperature of finishing rolling, the precipitation strength contribution increases with increasing coiling temperature. However, the increase in strain accumulation associated with low temperature processing greatly reduces the sensitivity of the precipitation strength contribution to coiling temperature.

Design method and verification for long life hot forging die

Abstract During the forging process, hot forging dies can easily sustain damage, including thermal softening, crack extension, wear and so on. The existing homogeneous hot forging die steel cannot meet the service performance requirements. However, the strengthening technology based on functionally gradient material (FGM) technology and surface cladding together can relieve the thermal stress. Based on the zoning of the temperature field distribution during continuous working, the structural design method and preparation process for the long life hot forging die are proposed in this paper; meanwhile, multilayer metal coating based on FGM was carried out and the temperature field, the stress field distribution for the homogeneous materials and the multilayer metal material hot forging die were analysed and compared. The results prove that using long service life hot forging die technology combined with FGM and surface cladding can relieve the overall stress value.

Conversion between non-isothermal and isothermal transformation kinetics of γ to α for C–Mn and Nb microalloyed steels

Abstract In the present paper, austenite γ to ferrite α transformation kinetics of steels at different cooling rates has been measured by thermal dilatation. The relationship between lnln[1/(1 − X)] and ln(|C R|) (C R: cooling rate) was plotted using the data at early stage of cooling, which well fitted a linear relationship for the calculation of exponential values of n in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation for isothermal kinetics using the method developed by Rios. The values of k in the JMAK equation obtained with the Rios method, however, have led to big discrepancies when the isothermal equations were used to predict the transformation kinetics during cooling. By assuming a Gaussian function of temperature, k was calculated using an optimisation method based on the rule of additivity. The isothermal transformation model was used to predict the transformation kinetics during cooling, showing good agreement with the measured data. It has been proved that even though the rule of additivity has to be relaxed to take into account the effects of cooling rates, precise conversion between non-isothermal and isothermal kinetics can still be realised.