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The possibility of applying a new material of fiber-reinforced metal (FRM) comprising high-speed tool steel (HSS) containing alumina fiber to the work rolls for hot rolling has been examined. This type of FRM is expected to exhibit improved tribological properties (wear resistance, etc.) without lowering toughness or other mechanical properties, even at high fiber mixing ratios. FRM specimens of high alumina fiber contents were prepared by sintering, and their wear resistance, rolling performance, and mechanical properties were tested using laboratory facilities. It was found that the wear resistance of the FRM was three times that of cast HSS rolls, its rolling load and friction coefficient were slightly lower, and the thermal shock resistance and tensile strength equal to or better than those of the latter. The new FRM thus proved promising as a new material for hot rolling rolls. NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 111 MARCH 2016 68 wear resistance of the material, and the performance of FRM rolls during hot rolling. As is often the case with new roll materials, at the beginning of the use of cast HSS rolls there were problems peculiar to the material, such as an increase in rolling loads due to MC type carbides (this will be explained in more detail in Sub-section 3.2).10) In light of these observations, the difference in rolling loads and friction coefficients between cast HSS and FRM rolls and their basic mechanical properties were examined through tests using laboratory facilities. This paper reports the results of these tests and studies. 2. Test Method 2.1 Trial production of FRM In the manufacture of the FRM specimens for the present test, commercially available HSS powder and ceramic fiber were used as the raw materials. The HSS powder was that of SKH10 according to JIS G 4403 “HSSs”, and it was atomized to a particle size of 45 μm or less. As easily procurable ceramic fibers, SiC and alumina were selected, and their compatibility with the HSS powder was examined through sintering tests using a uniaxial hot press. Bundles of SiC or alumina fibers were mixed with the HSS powder at a concentration of 10 volume % and sintered into FRM specimens approximately 23 × 35 × 3 mm in size. The sintering conditions were as follows: temperature 1 000°C; pressure roughly 40 MPa; and holding time roughly 2 h. Figure 1 shows sectional photomicrographs of the specimens thus obtained. Whereas in the specimen of HSS and alumina fiber (part (a)), where the fiber shape was well preserved and the adhesion of the metal matrix (C) to the fiber (A) was good, in that of HSS and SiC fiber (part (b)), the fiber was deformed and there were defects presumably caused by some reaction between the metal matrix (C) and the fiber (B); see the voids indicated with white arrows at the metal/fiber interfaces. Based on this finding, alumina fiber was selected for the trial manufacture of the specimens for the tests reported in this work. The alumina fiber used for the test FRM roll to examine the suitability of the material for the hot rolling rolls was defined as follows: purity 95% or more; ratio of α phase roughly 50% or more; and shape 8 to 10 μm in diameter with an aspect ratio of roughly 50, or 3 to 7 μm in diameter with an aspect ratio of 20 to 40 by crushing in a ball mill. The alumina fiber in either of the above two different shapes was added to the HSS powder at a concentration of 20 volume %, and the powders were then thoroughly mixed into a homogeneous mixture. The mixture was then sintered at 1 000°C for about 3 hours under a pressure of 98 MPa using a hot isostatic press (HIP), and the sintered pieces were subjected to the same heat treatment as that for cast HSS rolls. The specimens were machined and set in test rolls. The heat treatment condition was set such that the Shore hardness of the FRM specimens would be 83 to 86, approximately the same as that of the cast HSS rolls. Figure 2 shows the metallographic structures of the FRM specimens: the black dots and streaks in parts (a) and (b) are the alumina fibers, and the rest is the metal matrix. From these views, the alumina fiber is homogenously dispersed in the matrix without specific orientations. Table 1 shows the compositions of the FRM samples thus prepared. To investigate the effects of the mixing amount of the alumina fiber over the wear resistance of the material, two comparative specimens, one containing the alumina fiber at 10 volume % and the other by 25, were also prepared. In addition, for the hot rolling test, sleeve-shaped FRM rolls, 80 mm in outer diameter, were manufactured by preparing cylinders of Cr-Mo steel according to JIS G 4053 (SCM, alloy steels for machine structural use), 60 mm Fig. 1 Microstructure of FRM materials after hot press (A) Al2O3 fiber, (B) SiC fiber, (C) High speed steel matrix Fig. 2 Microstructure of representative FRM materials made in this study on trial NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 111 MARCH 2016 69 in outer diameter, 45 mm in inner diameter and 100 mm in length, and cladding their outer surfaces with a layer, roughly 10 mm in thickness, of the FRM of different compositions by the HIP. 2.2 Test equipment and conditions To examine and evaluate the wear resistance, thermal shock resistance, mechanical properties, and rolling performance of the FRM specimens prepared as above, and to compare the results with corresponding value of cast HSS rolls, the following tests were conducted: hot wear test using a hot rolling wear tester; hot rolling test of steel coils using a laboratory mill; thermal shock test on a friction thermal shock tester; and tensile test. For the hot rolling wear test, a test roll was manufactured by fitting the FRM materials at four positions along the periphery as illustrated in Fig. 3, so that the four FRM specimens of different compositions were subjected to rolling friction in a run under the same condition, and their wear depths compared. The condition of the hot wear test is shown in Table 2. Figure 4 schematically illustrates the laboratory mill used for the hot rolling test of steel coils, and Table 3 shows the condition of the rolling test. In the hot rolling test on the laboratory mill, the wear resistance was evaluated in terms of wear depth defined as the difference in the roll profiles before and after the test, as well as the rolling characteristics such as rolling loads and forward slip. In the friction thermal shock test, in the same manner as that of Ohashi et al.,11) test pieces of the materials in question, 20 × 20 × 30 mm in size, were subjected to impacts of mild steel pins, 5 mm in diameter and 40 mm in length, to impose instantaneous thermal shocks by friction, and the thermal shock resistance was evaluated in terms of the heat crack propagation detected under sectional microscopic observation at the center of the friction area. 3. Test Results and Discussion 3.1 Wear resistance Figure 5 shows the wear resistance of the FRM roll materials obtained through the hot rolling wear test; for comparison purposes, the graph also shows the figures of a cast HSS roll (HSS) and a HSS roll made by the HIP process but without mixing alumina fiber (HSS (HIP)). Note here that FRM-3 and 4 are comparative test pieces of the FRM containing, respectively, 10 and 25 volume % alumina fiber with an aspect ratio of approximately 50. The wear amount along the ordinate is normalized with the value of the cast HSS roll as 1. The wear resistance of the HSS roll by HIP is nearly twice that of the cast HSS roll, and that of the FRM roll containing 20 volume % alumina fiber of an aspect ratio of 50 (FRM-1) proved to be about four times that of the latter. The wear resistance of the FRM containing 20 volume % alumina fiber with an aspect ratio of 20 to 40 (FRM-2) was substantially the same as that of FRM-1 (20 volume % alumina fiber with aspect ratio 50). This indicates that the wear resistance of FRM depends more on the mixing amount of the fiber than its aspect ratio. This is Table 2 Experimental conditions of hot rolling wear test

Characteristics of Wear and Rolling on Fiber Reinforced Metal at High Temperature