Effect of MC Type Carbides on Wear Resistance of High Wear

ISIJ International, Vol. 61 (2021), No. 10

© 2021 ISIJ2597

ISIJ International, Vol. 61 (2021), No. 10, pp. 2597–2604

https://doi.org/10.2355/isijinternational.ISIJINT-2021-127

* Corresponding author: E-mail: [email protected]

© 2021 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

In the hot rolling of steel, several roll problems such as wear, surface deterioration, reduced life and high replace-ment frequency occur owing to the increasing strength of rolled materials and increasing rolling volume per unit time. Therefore, the requirements for improving the qual-ity of hot-rolled roll materials in terms of properties such as wear resistance, surface deterioration resistance, and thermal crack resistance are becoming stricter.1–4) Recently, high-carbon high-speed steel cast-iron rolls (hereinafter referred to as “HSS rolls”) have been developed as highly wear-resistant rolls. HSS rolls substantially contain alloys similar to high-speed steel, and because of carbide-forming elements (e.g., Cr, Mo, and V), its microstructure consists of finely dispersed hard MC-type and M2C-type carbides in the primary austenite interstices. The secondary carbides

Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

Kazunori KAMIMIYADA,1)* Shinya ISHIKAWA,1) Hirofumi MIYAHARA2) and Yuji KONNO1)

1) Nippon Steel Rolls Corporation, 46-59 Nakabaru Tobata-ku Kitakyushu-city Fukuoka, 804-0002 Japan.2) Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, 744 Motooka Nishi-ku Fukuoka, 819-0395 Japan.

(Received on March 30, 2021; accepted on June 23, 2021; originally published in Tetsu-to-Hagané, Vol. 106, 2020, No. 12, pp. 883–891; J-STAGE Advance published date: August 14, 2021)

High-speed steel cast-iron rolls were developed around 1990 and have been widely used for the earlier stand of hot strip mills. However, for the later stand of hot strip mills, the use of high-speed steel cast-iron rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the later stand, enhanced indefinite chilled rolls in which MC-type carbides of high hardness are crystallized in a conventional indefinite chilled roll has been developed. However, since the wear resis-tance of enhanced indefinite chilled rolls is significantly inferior to that of high-speed steel cast-iron roll, the development of a new cast iron roll with superior wear resistance applicable to the later stand of hot strip mills was studied. The present development roll has improved wear resistance by increased amount of the high hardness MC-type carbide-forming elements. In addition, the reduction of the carbon equiva-lent for less amount of eutectic carbide resulted in the reduction of the residual stress down to the same level as the indefinite chilled roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional indefinite chilled roll. In addition, the results suggest that the wear resistance of work rolls in hot strip mills is greatly controlled by the amounts of MC-type carbides, despite the roll hardness being the same.

KEY WORDS: hot strip mill; wear resistance; crack resistance; eutectic carbide; high-nickel grain roll; indefinite chilled roll; roll; high-speed steel; residual stress.

are also finely precipitated in the matrix phase upon heat treatment, which contribute to the balance of mechanical properties.5–7) Therefore, HSS rolls have been widely used for the earlier stands of hot strip mills, and the application rate of HSS rolls has become nearly 100% over the last few decades in Japan.

However, HSS rolls have been used for the later stands of hot strip mills in only a few cases and are rarely used in the final stage of the later stand, because if a rolling accident, such as a cobble incident, occurs, the steel material sticks on the roll surface and deep cracks tend to form on the roll surface. At the same time, damage to the rolls is severe owing to the high crack growth rate, resulting in large vol-ume loss.8,9) Therefore, indefinite chilled rolls (also called high-nickel grain rolls), which belong to Ni-hard cast iron, are still widely used instead of HSS rolls in the later stand of hot strip mills.10,11) In recent years, enhanced indefinite chilled rolls, which contain MC-type carbides and have improved wear resistance, have been developed and widely


© 2021 ISIJ 2598

used in the later stands of hot strip mills.12–14) However, the wear resistance of these enhanced indefinite chilled rolls is still insufficient compared to that of HSS rolls.

Therefore, in this study, the effect of MC-type carbides on wear resistance was clarified from the results of using enhanced indefinite chilled rolls, and the rolling incident resistance (cobble resistance and suppression of surface crack growth) of developed highly wear-resistant cast-iron rolls was evaluated with regard to their use in the later stands of hot strip mills.

2. Development Guidelines for Highly Wear-resistant Cast-iron Rolls

2.1. Target Performance (Roll Performance)To evaluate the performance of hot-rolling rolls, tonnage

of rolled steel per 1 mm of roll consumption in diameter (ton/mm) is usually used as an index. This index is called “roll performance”. Currently, indefinite chilled rolls are widely used in the later stand of hot strip mills. An example of the roll performance of this type of roll is shown in Fig. 1. Figure 1 shows the transition of the roll performance of our indefinite chilled rolls in the finished F5 stand of mill A. The roll performance data were standardized by data from 1999 when only conventional indefinite chilled rolls (NiGr) were applied. In mill A, the first stage of enhanced indefinite chilled rolls (NiGr-E1) were applied in 1999, whereby the roll performance was improved by about 15%. The second stage of indefinite chilled rolls (NiGr-E2), developed in 2002, were applied to some of the rolls in the latter half of 2002, and all the rolls were switched to NiGr-E2 in 2004. Since then, this NiGr-E2 roll has been mainly applied, except for a few minor changes. The roll performance of the NiGr-E2 roll has remained stable at about 180% on average. In other words, the roll performance of the NiGr-E2 roll is about 1.8 times higher than that of conventional indefinite chilled rolls (NiGr). However, the roll performance of the enhanced indefinite chilled rolls have almost reached their limit. Therefore, the target performance of the present devel-oped roll (referred to as PRESENT in figures and tables) was set to be more than three times that of conventional indefinite chilled rolls (NiGr). The roll performance was

evaluated using the test roll of the present developed roll for actual rolling.

2.2. Wear ResistanceAs previously mentioned, the enhanced indefinite chilled

rolls are designed to be have MC-type carbides in the stan-dard indefinite chilled rolls, focusing on the high-hardness MC-type carbides crystallized in HSS rolls.12–14) Hence, the effects of elements such as V, Nb, and Ta on the formation of MC-type carbides in the solidification process have been reported.15,16) It has also been reported that the addition of a small amount of Ti can lead to the formation of TiC and Ti(CN), which become the crystallization nuclei of VC and NbC granules.17,18) Figure 2 shows examples of the micro-structures of the enhanced indefinite chilled rolls (NiGr-E2) developed by utilizing these findings. The microstructure consists of grainy MC-type carbides finely dispersed in the matrix along with grainy graphite crystallization.

As shown in Fig. 1, NiGr-E2 rolls exhibit a roll perfor-mance that is 1.8 times higher than that of NiGr rolls. On the other hand, the surface hardness of the conventional (NiGr) and enhanced (NiGr-E1 and NiGr-E2) rolls is almost the same level, with about 80 in Shore hardness (HS). These results suggest that the wear resistance characteristics of the work rolls in the later stand of hot strip mills are greatly controlled by the amounts of MC-type carbides, despite the

Fig. 1. Transition of roll performance ratio of indefinite chilled roll. *Roll performance: rolled tonnage per 1 mm of roll consumption in diameter (ton/mm).

Fig. 2. Microstructures of the enhanced indefinite chilled roll (NiGr-E2) (Online version in color.)


© 2021 ISIJ2599

roll hardness being the same. Therefore, in this study, we first decided to develop a new material that can maintain the hardness at a conventional level (about HS80) and contains a large amount of MC-type carbide comparable to that in HSS rolls. Furthermore, by applying the present developed material to actual rolling mill, we aimed to improve wear resistance to more than three times higher than that of con-ventional rolls (NiGr). However, MC-type carbide-forming elements inhibit graphitization. Thus, in the alloy design of the present developed material, the amounts of MC-type carbide-forming elements was limited within the range in which enough graphite could be obtained.

2.3. Rolling Incident Resistance (Cobble Resistance and Suppression of Surface Crack Growth)

Work rolls for hot rolling are exposed to thermal cycles during rolling. Therefore, an appropriate compressive residual stress is applied to the outer layer of each roll to suppress the propagation of thermal cracks.19) However, it is considered that on excessive compressive residual stress in the outer layer promotes the propagation of cracks on the surface toward the inside of the roll.8) On the other hand, it has been reported that when the compressive residual stress of the outer layer of the roll is about 200 MPa, crack propagation is not promoted.9) Indefinite chilled rolls are characterized by a low compressive residual stress on the roll surface of 150–250 MPa.20) In addition, indefinite chilled rolls have excellent rolling incident resistance owing to the presence of graphite in the microstructure. Because of the lubricating effect of graphite, indefinite chilled rolls are also considered to have excellent sticking resistance.21) Meanwhile, it is empirically known that thermal crack resistance deteriorates with the increase of eutectic carbide crystallized in a network shape.

Therefore, a graphite crystallization-type material was applied to maintain the sticking resistance of the developed rolls. However, there was concern that the amount of graph-ite would decrease owing to the higher alloy content than that in the NiGr-E2 rolls. For this reason, the amount of crystallized network-shaped eutectic carbide was designed to be reduced to improve the thermal crack resistance. Fur-thermore, in the present developed roll, we aim to make the compressive residual stress generated in the outer layer be at a level equivalent to that in NiGr and NiGr-E2 rolls (150–250 MPa) to suppress the propagation of cracks in order to maintain the rolling incident resistance.

3. Material Evaluation Experiment

3.1. Experimental Procedure3.1.1. Alloying Design of Specimen

The chemical compositions of the specimens used in the experiments are shown in Table 1. The present developed roll was designed according to the policy of improving the thermal crack resistance by reducing the amount of eutectic

carbide. For this reason, the carbon equivalent CE (C+1/3Si mass%) of the test material was reduced to 3.0–3.5 mass% from the actual results (3.5 to 4.0 mass%) of NiGr and NiGr-E2 rolls. On the other hand, macroscale segregation due to centrifugal force is a problem in the manufacture of centrifugal casting rolls. To achieve a uniform fine disper-sion of MC-type carbides in the roll material, it is necessary to ensure that the composition does not exceed the hypereu-tectic composition of the L → γ + MC eutectic reaction.5) Therefore, the amount of MC-type carbide-forming ele-ments were set to hypoeutectic compositions in which the primary crystal is γ phase. The specimens were melted in a 50 kg high-frequency induction melting furnace and cast into sand molds of ϕ100 mm × 250 L. The centrifugally cast materials used in actual operating equipment were also evaluated. These materials were subjected to tempering heat treatment at 430 to 470°C.

3.1.2. Hardness and MicrostructureThe purpose of using the present roll is to improve the

rolling incident resistance by suppressing the growth of cracks on the surface of the roll. Therefore, only low tem-perature tempering was performed on the specimen to keep the same low residual stress as in indefinite chilled rolls, and the CE of the specimen was reduced to 3.0–3.5 mass% to improve the thermal crack resistance. As a result, the hardness may also decrease. Therefore, in order to main-tain the conventional level of hardness, it is necessary to understand the relationship between the amount of alloying element added and the hardness. At first, the Shore hard-ness of these test materials were measured using a diamond hammer of 36 g dropped from a height of 19 mm compliant with JIS B 7727 and JIS Z 2246 and microstructure was also investigated. In addition, the results from the former study of enhanced indefinite chilled rolls suggest that the amount of MC-type carbide-forming element added has a significant effect on wear resistance properties, but such ele-ments also inhibit graphitization. Therefore, we investigated the relationship between chemical composition, hardness, and microstructure, and determined the maximum MC-type carbide-forming element composition range in which enough graphite crystallization is possible.

3.1.3. Wear ResistanceThe wear resistance of the present developed mate-

rial (referred to as PRESENT in figures and tables) was evaluated by a hot wear test using the hot rolling wear tester shown in Fig. 3.22) Two types of HSS material for centrifugal casting rolls (HSS-1 and HSS-2), the enhanced indefinite chilled roll material (NiGr-E2), and the present developed material were set in the test disk at four locations in the circumferential direction, as shown in Fig. 3; rolling friction was applied under the same conditions for each test material. The wear test was performed at a specimen tem-perature of 510 to 520°C, a slip ratio of 4.4%, applied load of 392 N, peripheral speed of the test piece of 3.1 m/s and 2 500 rotations against mating disk (S45C) at 840 to 850°C. The wear resistance was evaluated from the average wear depth measured from the profiles of the specimens obtained before and after the test.

Table 1. Chemical composition of specimens.

CE (mass%) Mn (at%) Ni + Cr + Mo (at%) V + Nb (at%)

2.7–3.7 0.5–1.0 5.0–10.0 4.0–6.0


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3.1.4. Rolling Incident Resistance (Cobble Resistance and Suppression of Surface Crack Growth)

We believe that the rolling incident resistance (cobble resistance and suppression of surface crack growth) can be evaluated by the thermal crack and sticking resistances, and we therefore conducted the friction heat quenching and drop-weight friction thermal shock tests. The fric-tion heat quenching test was carried out using the fric-tion heat quenching test machine with a test piece size of 30×30×60 mm, a load of 3.92 kN, a disk rotation speed of 1 500 ppm, and a pressing time of 5 s, as shown in Fig. 4. After the quenching test, the thermal crack initiation on the disk contact surface was examined, and the crack depth was measured. The drop-weight friction thermal shock test was carried out in the same manner as that reported by Ohhashi et al.23) under the test conditions described by Inoue et al.,22) that is, a rapid thermal shock due to friction was applied to the surface of each roll test piece with a pin made of mild steel, as shown in Fig. 5. The size of each roll test piece was 20 mm×20 mm×30 mm, the size of the mild steel pin was 5 mm in diameter × 40 mm in length, and the amount of reduction of mild steel pin was 1 mm. After the test, the profile and roughness of the roll test piece in the lateral direction were measured at a position 5 mm from the lower end of the indentation in accordance with JIS’82 standards.

3.2. Results and Discussion3.2.1. Evaluation of Hardness and Microstructure

The relationship between the amounts of alloying ele-ments (Ni, Cr, and Mo) added and the shore hardness (HS) is shown in Fig. 6. The Shore hardness increased from 70 HS to 82 HS with increasing amount of Ni+2 (Cr+Mo) from 6.8 at% (hereinafter, at% is abbreviated as%) to 8.4% in the composition as shown in Table 1. However, no clear correlation between the amount of MC-type carbide-forming element (V+Nb), ranged from 4.0% to 6.0%, and Shore hardness was found. It was also confirmed that the same relationship was found in specimens prepared by centrifugal casting. In the present study, it is necessary to determine the composition range of the MC-type carbide-forming elements within which graphite crystallization is possible. Thus, the microstructures of the samples were investigated to check the graphite crystallization situation in them. Figure 7 shows the effects of the composition of (V+Nb) and CE (mass%) on the graphite crystallization. Graphite can be crystallized up to 5.2% of V+Nb, which indicates that the amount of MC-type carbide-forming ele-ment added could be increased to about twice as much as

Fig. 3. Schematic drawing of apparatus for the hot rolling wear test. (Online version in color.)

Fig. 4. Schematic drawing of apparatus for friction heat quench-ing test. (Online version in color.)

Fig. 5. Schematic drawing of apparatus for drop weight type fric-tion thermal shock test.


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3.2.3. Evaluation of Rolling Incident Resistance (Cobble Resistance and Suppression of Surface Crack Growth)

The results of the friction heat quenching test are shown in Fig. 8, the observation of the thermal cracking surface in (a), the cross-sectional view of thermal crack of the specimen and the measurement of the crack depth in (b), respectively. For comparison, the NiGr-E2 material was also evaluated under the same conditions. The number of cracks generated on the surface of the developed specimen is less than that on the NiGr-E2 sample, and the crack depth was about 3.2 mm, similar to that in the NiGr-E2 material. The results of the drop-weight friction thermal shock test are shown in Fig. 9. Since the stuck steel make the specimen surface rough and the degree of sticking could be evaluated by surface roughness, we decided to use the profile and roughness Rmax in a relative comparative evaluation of the sticking resistance of each material. The average roughness Rz and maximum roughness Rmax of our developed mate-rial were relatively small, about 10.07 μm and 13.89 μm, respectively. On the other hand, the profile was very uneven and the maximum roughness was as large as 26.77 μm in the HSS-roll materials. The Rz of the NiGr-E2 material was 7.98 μm, which was slightly smaller than that of the developed material. The establishment of a quantitative evaluation method for sticking resistance is an issue for the future, but it is considered that the estimated sticking

Fig. 6. Relationship between hardness and chemical compositions (Ni, Cr, Mo).

Fig. 7. Influence of V, Nb content and carbon equivalent on the amount of graphite.

that in NiGr-E2 rolls. Depending on the density of each ele-ment, there is a possibility of macrosegregation due to cen-trifugal force applied during the manufacture of actual rolls. The composition of the outer layer of the present developed roll was eventually determined to be CE: 3.3 mass%, Mn: 0.8%, Ni+Cr+Mo: 7.4%, and V+Nb: 4.4%, in which the amount of MC-type carbide-forming element was about 1.9 times that in NiGr-E2 rolls.

3.2.2. Evaluation of Wear ResistanceThe average wear depth was measured from the profiles

of the specimens obtained before and after the hot rolling wear test, and the results are shown in Table 2. The wear resistance of the present developed material is superior to that of NiGr-E2 and is intermediate between those of NiGr-E2 and HSS compositions. In addition, some research results have been reported on the evaluation of wear resis-tance of hot rolling rolls using the same type of hot rolling wear tester,22,24–26) and it is also reported that further optimi-zation of testing conditions26) are necessary for quantitative evaluation of the wear resistance properties in actual rolling. However, the wear resistance of the roll with the present composition can be predicted to be intermediate between those of actual NiGr-E2 and HSS rolls.

Table 2. Average wear depth from the profile of the specimen after hot rolling wear test.

Roll type Average wear depth (μm)

PRESENT 1.65

NiGr-E2 1.72

HSS-1 1.35

HSS-2 1.23

Fig. 8. Results of friction heat quenching test, (a) surface of speci-men, (b) depth of crack observed in cross section. (Online version in color.)


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Fig. 9. Evaluation result of drop weight type friction thermal shock test, (a) specimen after test, (b) the profile of the steel rod contact face on the test piece, (c) measurement result of roughness. (Online version in color.)

resistance of the developed material is almost the same as that of the NiGr-E2 material.

4. Experiment with Actual-size Rolls

4.1. Experimental ProcedureIn the present developed specimens, low residual stresses

comparable to those of conventional NiGr and NiGr-E2 rolls have been achieved. As a result, roll problems such as spalling, can be expected to be reduced by suppressing the growth of cracks on the roll surface during rolling incidents. Therefore, the present developed material was manufac-tured into rolls for the later stand of hot strip mill B. The Shore hardness of the roll surface was determined, and the residual stress was measured using X-rays. The mechanical properties (compressive strength and fracture toughness) of the rolls were also investigated using test specimens taken from the outer layer near the barrel center of this test roll. Furthermore, to evaluate the performance of the developed rolls, the rolling performance of the test rolls was evaluated in F5 stand in the finishing section of hot strip mill B and the finishing stand work section of plate mill C, in which indefinite chilled rolls are normally used.

4.2. Experimental Results and Discussion4.2.1. Evaluation of Hardness, Residual Stress and

Mechanical PropertiesFigure 10 shows the measurement results of the hard-

ness distribution in the radial direction after cutting and investigating the present developed roll (diameter: 625 mm; rolling outer layer thickness: 50 mm/piece). The Shore hard-ness was determined to be about 82 HS to 84 HS immedi-ately below the surface layer and about 80 HS at a depth of 70 mm, upon comparing these results with the results

of the investigation of NiGr-E2 rolls of the same size, the hardness distributions of the two rolls were found to be almost the same. Then, the barrel surface of the developed roll was observed under the un-etched and etched condi-tions, as shown in Fig. 11. Compared with the NiGr-E2 material (Fig. 2), graphite was crystallized in the matrix, although its amount was slightly reduced. Also, MC-type carbides, which contribute to the improvement of wear resistance, were finely dispersed. Furthermore, the amount of eutectic carbide crystallized in the network shape was reduced, as shown by the white phase in Fig. 11. This is because the CE (C+1/3Si mass%) of 3.0 to 3.5 mass% of the developed material was reduced compared with the actual results (3.5 to 4.0 mass%) for NiGr and NiGr-E2 materials, but the hardness of this sample was comparable to that of the NiGr-E2 roll. Table 3 shows the Shore hard-ness of the center of the test roll barrel, the residual stress values measured by X-ray diffraction, and the measurement

Fig. 10. Distribution of hardness in radial direction of test roll.


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results of mechanical properties. The surface hardness of the present developed rolls is comparable (HS82) to that of NiGr-E2 rolls (79 HS to 83 HS). Moreover, the residual stress is within the actual range (150 to 250 MPa) of NiGr and NiGr-E2 rolls. The compressive strength and fracture toughness of the developed material are 2 800 N/mm2 and 26.2 MPa·m1/2, respectively, which are higher than those of the NiGr-E2 material. This is presumably the result of the contribution of the increased amount of finely dispersed MC-type carbide, the decreased amount of graphite, and the divided crack growth path because of the reduced amount of network-shaped eutectic carbide. However, the type of eutectic carbide cannot be determined because detailed analysis has not been performed; Considering the presence of Graphite, the eutectic carbides may be consist of mainly M3C-type which can easily co-exist with Graphite and pos-sibly M7C3 or M2C-type carbides partly.

4.2.2. Evaluation of Roll PerformanceThe roll performance of the developed rolls is evaluated

on the later stand F5 of hot strip mill B, and the obtained results are shown in Fig. 12. Here, the roll performance is defined by the weight of the rolled product when the roll diameter is reduced by 1 mm. The roll performance of the present developed rolls is about 3.1 times higher than that of the conventional NiGr rolls and about 1.7 times higher than that of the NiGr-E2 rolls. In addition, we found rolling incident frequency of the developed roll was same level as NiGr-E2 rolls and no significantly deeper cracks compared with NiGr-E2 rolls were observed in the actual rolling. Therefore, we have concluded that the rolling incident resis-tance of the developed roll is same level as NiGr-E2 rolls. This result suggests that one of the conditions for making rolling incident resistance (cobble resistance and suppres-sion of surface crack growth) of the developed roll be same level as NiGr-E2 rolls is to make the compressive residual stress in the outer layer same (150 to 250 MPa) as those of

the NiGr and NiGr-E2 rolls. This result is consistent with the previous research report9) indicating that the compres-sive residual stress in the outer layer of the rolls is not a factor that promotes crack propagation if it is as low as around 200 MPa. However, the amount of eutectic carbide of the developed material is significantly reduced compared with those of NiGr and NiGr-E2 materials. Nevertheless, the wear resistance of each roll material seems to improve as the amounts of MC-type carbide-forming elements (V and Nb) added increases. Therefore, there is a relationship between the amount of MC-type carbide-forming element added and the roll performance ratio. The results are shown in Fig. 13, and the roll performance, which is an indication of the wear resistance of the roll material, becomes 100% to 310% with MC-type carbide-forming elements increase from 0% to 4%. Since the hardnesses of the rolls evaluated in this study were all almost the same (about 80 HS), the wear resistance of the hot rolling roll is considered to be predominantly con-trolled by the amount of MC-type carbide. Therefore, we can conclude the increase in amount of MC-type carbide-forming element added is extremely effective for the wear resistance of the hot rolling roll. Since it has been reported that carbides are more effective in terms of increasing stick-ing resistance,27) the research in future on a new roll material design may be without graphite crystallization for further rolling performance improvement.

The indefinite chilled rolls are widely used in the later stand of hot strip mills as well as in the work rolls of plate mills. Therefore, the performance of the developed rolls in the finishing stand of the plate mill C was evaluated. Figure 14 shows the results. The roll performance of the developed rolls was 1.4 times higher than that of the NiGr-E2 rolls; this is similar to the results of the evaluation used in a hot strip mill. In this study, the results showed a difference in the improvement of roll performance for hot strip mills (170%) in Fig. 12 and that for plate mills (140%) in Fig. 14. The roll performance depends on the amount of wear during rolling and the amount of grinding of the roll surface after each rolling operation. The rolling method (tandem or reverse) and the difference in rolling temperature between the hot strip mill (F5) and the plate mill are also factors of roll performance. Hence, the length of rolled steel between grindings after each rolling campaign is larger in hot strip

Fig. 11. Microstructure of this development roll. (Online version in color.)

Table 3. Mechanical properties of this developed material.

Roll typeCompressive

Strength (N/mm2)

K1C (MPa·m1/2)

Hardness (HS)

Compressive Residual Stress

(MPa)

PRESENT 2 800 26.2 82 202, 220

NiGr-E2 2 400–2 600 20–25 79–83 150–250

HSS 2 700–3 200 25–30 85–90 300–600

Fig. 12. Performance of this development roll in the hot strip mill. *Roll performance: rolled tonnage per 1 mm of roll con-sumption in diameter (ton/mm).


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the amount of eutectic carbide and the suppression of the residual stress on the roll surface to within the range of 150 to 250 MPa.

(3) The Shore hardness of the developed roll showed a correlation with Ni+2(Cr+Mo) (at%) regardless of the content of MC-type carbide-forming elements.

(4) The wear resistance of the work roll for hot rolling showed a correlation with the content of MC-type carbide-forming element (V+Nb) at a similar Shore hardness.

(5) Since further MC-type carbide forming element increase seems to be difficult with Graphite containing material, following research should be required: Materials design for wear resistance improvement other than MC-type carbide increase. Research of better roll incident materials on Graphite free basis.

AcknowledgmentsWe thank Nippon Steel Co., Ltd. Technology Develop-

ment Division for letting us use the hot rolling wear tester and the drop-weight friction thermal shock tester and also thank the Technical Development Division of Nippon Steel Co., Ltd. for useful advice in carrying out the test.

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26) A. Yanagida, R. Ujiie and R. Oi: Tetsu-to-Hagané, 104 (2018), 722 (in Japanese).

27) K. Fujio, A. Yamamoto and S. Nishikawa: Tetsu-to-Hagané, 101 (2015), 372 (in Japanese).

Fig. 14. Performance of this development roll in the plate mill. *Roll performance: rolled tonnage per 1 mm of roll con-sumption in diameter (ton/mm).

Fig. 13. Relationship between chemical compositions (V, Nb) and roll performance ratio (%). *Roll performance: rolled tonnage per 1 mm of roll consumption in diameter (ton/mm).

mills than in plate mills, and the wear resistance of the developed rolls would be more clearly reflected in the roll performance for the hot strip mill.

5. Conclusion

The following results were obtained through the develop-ment of a new high-wear-resistant cast-iron roll with good rolling incident resistance (cobble resistance and suppres-sion of surface crack growth) that can be used in the later stand of hot strip mills.

(1) The present developed roll was designed as a graphite-crystallization high-alloy cast-iron roll, in which the amounts of MC-type carbide-forming elements (V and Nb) are increased. The roll performance of this present developed roll was improved to about three times that of conventional indefinite chilled rolls (NiGr).

(2) The developed roll had a rolling incident resistance (cobble resistance and suppression of surface crack growth) equivalent to that of enhanced indefinite chilled rolls (NiGr-E2). This is considered to be a result of the reduction in

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Effect of MC Type Carbides on Wear Resistance of High Wear