High Speed Steel Roll Research Articles

The Effect of Long-Time Austenization on the Wear Resistance and Thermal Fatigue Properties of a High-Speed Steel Roll

The effects of a long-time austenization treatment on a high-speed steel (HSS) roll have been investigated. Several interesting phenomena were observed, including the decomposition of the primary bulky M3C carbides on grain boundaries and the precipitation of a large number of MC carbides of size comparable to the primary MC carbides in the grains. As a consequence of these changes, the overall carbide size decreased and the homogeneity of the carbide distribution increased. The wear resistance and thermal fatigue properties of the HSS roll were also investigated, and it was found that the long-time austenization treatment resulted in improvements to both properties.

Investigations on Heat Treatment of a High-Speed Steel Roll

High-carbon high-speed steels (HSS) are very abrasion-resistant materials primarily due to their high hardness MC-type carbide and high hardness martensitic matrix. The effects of quenching and tempering treatment on the microstructure, mechanical properties, and abrasion resistance of centrifugal casting high-carbon HSS roll were studied. Different microstructures and mechanical properties were obtained after the quenching and tempering temperatures of HSS roll were changed. With air-cooling and sodium silicate solution cooling, when the austenitizing temperature reaches 1273 K, the metallic matrix all transforms into the martensite. Afterwards, the eutectic carbides dissolve into the metallic matrix and their continuous network distribution changes into the broken network. The second hardening temperature of high-carbon HSS roll is around 793 K. No significant changes in tensile strength and elongation percentage are observed unless the tempering temperature is beyond 753 K. The tensile strength increases obviously and the elongation percentage decreases slightly beyond 753 K. However, the tensile strength decreases and the elongation percentage increases when the tempering temperature exceeds 813 K. When the tempering temperature excels 773 K, the impact toughness has a slight decrease. Tempering at 793-813 K, high-carbon HSS roll presents excellent abrasion resistance.

New Method for Evaluating Thermal Wear of Rolls in Rolling Process

A new method was developed by a thermal wear machine to evaluate the thermal wear of rolls in steel rolling process. The steel strip and rolls were simulated by upper and lower heating disks. The upper heating disk could be kept at a temperature of over 900 °C by induction heating. The pressure between the disks as high as 323. 2 MPa could be achieved and the slipping rate could be 12. 7%. The thermal wear of high speed steel (HSS) roll material, the wear rate of the HSS roll, and the SEM morphology of a worn HSS roll surface were investigated. This method was useful and could be employed to simulate friction and wear between strip and roll during the strip rolling process.

Correlation of microstructure and fracture properties of five centrifugal cast high speed steel rolls

The present study is concerned with effects of microstructural factors such as distribution and fraction of coarse carbides located along solidification cell boundaries and characteristics of tempered martensitic matrix on fracture properties of five high speed steel (HSS) rolls manufactured by a centrifugal casting method. In situ microfracture observation, fracture toughness measurement and fractographic observation were conducted on these rolls to clarify fracture mechanisms. The in situ observation results indicated that coarse carbides located along cell boundaries provided easy intercellular fracture sites under a low stress intensity factor level. In the rolls whose intercellular carbide fraction and matrix hardness were high, fracture easily occurred under a low stress intensity factor. On the contrary, in the rolls where a small amount of intercellular carbides was distributed on the relatively ductile matrix of lath tempered martensite, the fracture path was accompanied by a considerable amount of plastic deformation including shear band formation, thereby resulting in high fracture toughness. In order to obtain better microstructure, hardness and fracture toughness of the HSS rolls, the minimisation of intercellular carbides, the refinement of carbides and the improvement of the matrix characteristics by controlling alloying elements and heat treatment conditions were suggested.

Influence of Rare Earth Elements on Microstructure and Mechanical Properties of Cast High-Speed Steel Rolls

Abstract The influence of rare earth (RE) elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry (DSC) and scanning electron microscopy energy dispersive spectrometry (SEM-EDS). The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing long-pole MC carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.

A study of segregation mechanism in centrifugal cast high speed steel rolls

Segregation influences the microstructure and performance of high speed steel (HSS) roll. The main reason why segregation occurs in centrifugal cast HSS roll is that atom clusters are formed in HSS melt and such atom clusters have different densities. The high-density atom clusters move to the outer periphery and the low-density atom clusters move to the inner periphery of the roll under centrifugal force. Changing the movement law of atom clusters in the centrifugal force field and increasing the solidification rate of HSS melt can lighten the segregation in HSS roll and improve its performance.

Effect of RE-Mg Complex Modifier on Structure and Performance of High Speed Steel Roll

The effects of rare earth (RE) and magnesium complex modifier on the structures and performance of high speed steel (HSS) roll were investigated and the mechanism of modification was analyzed by means of optical microscope (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) analysis, tensile testing machine and impact tester. The results showed that RE-Mg complex modifier could refine the grain, change carbide morphology and distribution, and increase mechanical properties. After heat treatment at 1000 °C, many networked carbides in HSS roll were broken and a number of granular carbides appeared. Most of eutectic carbides were spheroidized and distributed uniformly. The networked carbides disappeared completely after heated to 1050 °C. The reason that RE-Mg improves the microstructure of HSS roll was analyzed.

A study on the crack control of a high-speed steel roll fabricated by a centrifugal casting technique

The effects of roll materials, mould parameter, pouring parameter and cooling parameter on the crack of a high-speed steel (HSS) roll, which is manufactured by means of centrifugal casting, are investigated. The improvement of the HSS roll is effectively achieved through the addition of suitable amounts of potassium and rare earth (RE). The hot tearing force (i.e. the resistance to hot tearing) of the modified HSS roll is increased by 32.77% and reaches 158 N, while the line constriction (i.e. the solidification constriction of the HSS in unit distance) is decreased. In addition, the temperature field and stress field of the roll can be improved by adopting variable-speed centrifugal casting, variable-flux pouring and variable-speed solidification cooling techniques, which help to improve the filling and solidification of the molten steel and eliminate the cracks.

Artificial neural network prediction on wear properties of high vanadium high speed steel (HVHSS) rolls

The present paper is dedicated to the application of artificial neural networks in the prediction of the wear properties of high vanadium high speed steel (HVHSS) rolls, including predictions of wear weight loss W according to carbon content C and number of revolutions N. Multilayer backpropagation networks were created and trained using comprehensive datasets tested by the authors. Very good performances of the neural networks were achieved. The prediction results show that the wear weight loss nearly linear increases with increasing number of revolutions at constant carbon content. The relative wear resistance of roll reaches the optimal value when the carbon content is ∼2·55 wt-%. The prediction values have sufficiently mined the basic domain knowledge of wear process of HVHSS rolls. A convenient and powerful method of optimising the process parameters of abrasive resistant materials has been provided by the authors.

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M3C carbides can be postulated as M3C + γ-Fe → M2C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M2C carbide decomposes as M2C + γ-Fe → MC + M7C3 + M6C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.

熱間圧延用ハイスロールの圧延特性

A high-speed steel roll has been developed for use in hot-rolling employing tool steel compositions, which exhibits good abrasion resistance and good resistance against roughened surfaces. To apply the high-speed steel roll in an existing hot strip mill, it was necessary to develop supportive technologies for preventing roll failure, for accommodating thermal expansion and for stabilizing the friction coefficient. We have improved the flow-detecting technology, and established a control technology to accommodate changes in shape and friction coefficient, and also quantified the aforementioned basic characteristics. The high-speed steel roll development has enabled not only the extension of the roll life, but also the improvement of productivity and quality, and the reduction of production cost.

複層鋳込みクラッド法による<br>熱間圧延用ハイスロールの基本特性

In an attempt to accommodate a smaller-diameter roll in hot rolling, a high-speed steel roll has been developed using tool-steel material on the outer surface, since tool steel has good abrasion resistance and mechanical strength. To employ such a roll in the existing hot-rolling process, it was necessary to develop a roll manufacturing technology to ensure a uniform structure for each large-size roll, and the same time, it was necessary to carry out an analysis of the properties required of rolls for hot rolling and to develop other supporting technology. We have optimized the chemical composition and size of the carbide, and improved the matrix structure, and thereby improved the roll's resistance to abrasion, roughened surfaces, and roll failure. At the same time, we developed a good lubricant suitable for the high-speed steel roll. As a result, we have successfully developed a high-speed steel roll with good abrasion resistance and mechanical strength.

Research on the Crack Forming and Control of Centrifugal Cast High Speed Steel Rolls

The effects of roll materials, mould parameter, pouring parameter and cooling parameter on the crack of high speed steel (HSS) roll fabricated by a centrifugal casting have been analyzed. The modification of HSS roll was researched. The structures and properties of HSS roll modified by potassium and rare earth were improved. The hot tearing force of modified HSS roll increased by 32.77% and reached 158N. Modification was able to decrease the line constriction. The adoption of variable speed centrifugal casting technique, variable flux pouring technique and variable speed solidification cooling technique was able to adjust the temperature field and stress field of the roll, which benefited the filling and solidification of molten steel and eliminated the crack of HSS roll.

Effect of Chromium Content on the Oxidation Behaviour of High-Speed Steels under Dry and Moist Air Environments

Development and use of high-speed steels for manufacturing the roll outer shell of hot strip mills represented a major recent technological advancement in the hot rolling field. However, it was observed that the oxidation behavior of these steels was different from that of conventional rolls. The high-speed steel oxidation rate is about four times higher than the alloys previously used. Furthermore, the rolling conditions are quite aggressive. Contact of the roll with the hot strip, air and water of the cooling system is expected to increase the oxidation of the roll surface in this wet atmosphere. Therefore, it is necessary to study their oxidation behavior in order to achieve the full potential of high-speed steel rolls for hot strip mills. In the present work, the oxidation behavior of three high-speed steels with differences in chromium content was studied. Corrosion tests were carried out in a thermobalance under dry and moist (12.5 % H2O) atmospheres at 765oC for 240 minutes. The corroded samples were examined by X-ray diffraction, scanning electron microscopy and energy dispersive micro-analysis. It was found that the variation of chromium content of the high-speed steels studied was sufficient to influence the oxidation behaviour. Samples with high chromium contents presented smallest final mass gain. The presence of humidity had a significant effect on the oxidation behavior.

Fatigue Behavior of High Speed Steel Roll Materials for Hot Rolling by Laser Impacting

The fatigue behavior of high speed steel (HSS) roll materials for hot rolling was researched under water-cooling conditions by laser impacting. The microstructure of HSS sample and the morphologies of fatigue samples were observed by scanning electron microscope. The phase structure was detected by XRD. The morphology of situ oxide scale was observed by optical microscope, and the expansion coefficient was measured by TGA. The experiment results indicate that the cracks come into being at the carbide-matrix interface, but there are no cracks in the matrix after many times of laser impacting treatment, for the situ sample taken from the fractured roll surface, big carbides are more sensitive to the fatigue, and peel off prior to small ones. The relevant fatigue mechanisms are also discussed.

Development of industrial production of ultrafine grained steel in tandem hot strip mill

Based on a model of microstructure evolution, an appropriate rolling schedule has been worked out to produce ultrafine grained steel (2 to 5 μm) on the hot strip mill. This rolling schedule is characterized by a large reduction in the finishers followed by strong cooling. Implementing such a rolling schedule on a conventional HSM raises major problems: increased rolling torque and force, lowered crown and shape control, disturbed looper control, accelerated roll damage, increased slip between strip and roll, insufficient cooling efficiency. These problems have been solved on the new Nakayama HSM by introducing innovative processes such as: single roll drive with different diameter and a new CVC, curtain wall cooling in the finisher, low inertia hydraulic looper, new high-speed steel rolls with solid lubrication.

The Effects of RE-Mg on Thermal Fatigue Resistance of Cast High Speed Steel (HSS) Rolls

The influence of rare earths(RE) and Mg compound modification on eutectic carbide morphology and matrix of high speed steel(HSS) roll was investigated. The results show that the matrix can be refined by the RE-Mg compound modifier, the eutectic carbides are inclined to turn into small lump, whose distribution is homogeneous. After modification, the heat conductivity coefficient of HSS is increased and heat expansion coefficient is decreased. Moreover, RE-Mg modification is able to increase the cycles of thermal fatigue crack initiation and decrease the propagation rate. After cycling 700 times at the condition of heating at 600oC and cooling in 25oC water, the length of thermal fatigue crack is lessened by 41.12%.

Effects of alloying elements on microstructure, hardness, and fracture toughness of centrifugally cast high-speed steel rolls

A study was made of the effects of alloying elements on the microstructure, hardness, and fracture toughness of centrifugally cast high-speed steel (HSS) rolls. Particular emphasis was placed on the role of hard carbides located along solidification cell boundaries and the type of the tempered martensitic matrix. Microstructural observation, X-ray diffraction analysis, hardness and fracture-toughness measurements, and fractographic observations were conducted on the rolls. The constitution and morphology of carbides observed within the intercellular boundaries varied depending upon the predominant alloying elements that comprised them. These massive carbide formations strongly influenced the bulk material hardness and fracture toughness due to their high hardness. The effects of alloying elements were analyzed on the basis of the liquidus-surface diagram which and indicated that the proper contents of the carbon equivalent (CE), tungsten equivalent, and vanadium were 1.9 to 2.0, 10 to 11, and 5 to 6 pct, respectively. The roll material, containing a small amount of intercellular carbides and lath-type tempered martensitic matrix, had excellent fracture toughness, since carbides were well spaced. Therefore, it was suggested that the optimization of alloying elements was required to achieve the homogeneous distribution of carbides.

Introduction of enhanced indefinite chill and high speed steel rolls in European hot strip mills

During the last 10 years, a new generation of work rolls has been introduced in hot strip mills. High speed steel (HSS) and enhanced indefinite chill (IC) rolls were introduced to replace high chromium (HiCr) rolls and IC rolls, respectively. Enhanced IC rolls have successfully replaced IC rolls in most mills, while HSS rolls are still in the 'trial phase' in many mills. The present status of enhanced IC and HSS rolls in various European mills is reviewed, and relevant experience presented. Outstanding problems requiring solution if the technology is to advance are considered.

Manufacture of centrifugal cast high speed steel rolls for wire rod mills

By means of a centrifugal casting method, high speed steel (HSS) rolls with excellent wear resistance have been manufactured. The hardness of the HSS rolls was 65–67 HRC, the range of hardness variation was smaller than 2 HRC and the impact toughness exceeded 15 J. The wear rate of the rolls was 2.5 × 10−4 mm t−1 of steel passing through the prefinishing stands of the high speed hot wire rod rolling mill. Casting cracking of the HSS rolls was definitely decreased and the rejection rate owing to the presence of cracks was less than 3%, as a result of the use of a two layer paint and varying the rotation rate. In addition, the segregation of alloying elements could be decreased by adding a suitable amount of niobium (0.6–2.0%).