CENTRIFUGAL CAST CALIBER ROLL FOR HOT ROLLING MILL (AS AMENDED)

Abstract

A centrifugal cast caliber roll for a hot rolling mill has excellent abrasion resistance, fatigue resistance, and slip resistance. The roll has a chemical composition containing, by mass %, C: 1.8% or more and 3.0% or less, Si: 0.2% or more and 1.0% or less, Mn: 0.2% or more and 1.5% or less, Cr: 5% or more and 9% or less, Mo: 4.0% or more and 7.0% or less, V: 4.0% or more and 7.0% or less, Nb: 0.5% or more and 2.0% or less, and the balance being Fe and inevitable impurities, in which the relationship 0.6(C0.24V0.13Nb)1.4 (where C, V, and Nb each denote the content (mass %) of the corresponding chemical element) is satisfied, and has a surface hardness of Hs 67 or higher and Hs 76 or lower in terms of shore hardness.

Claims

1. A centrifugal cast caliber roll for a hot rolling mill, the roll having a chemical composition containing, by mass %, C: 1.8% or more and 3.0% or less, Si: 0.2% or more and 1.0% or less, Mn: 0.2% or more and 1.5% or less, Cr: 5% or more and 9% or less, Mo: 4.0% or more and 7.0% or less, V: 4.0% or more and 7.0% or less, Nb: 0.5% or more and 2.0% or less, and the balance being Fe and inevitable impurities, in which relational expression (1) below is satisfied, and having a surface hardness of Hs 67 or higher and Hs 76 or lower in terms of Shore hardness:
0.6(C0.24V0.13Nb)1.4(1), wherein C, V, and Nb each denote the content (mass %) of the corresponding chemical element.

2. The centrifugal cast caliber roll for a hot rolling mill according to claim 1, the roll being a rolling roll for a seamless steel pipe.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0042] FIG. 1 is a graph illustrating the influence of roll hardness (Shore hardness Hs) on roll life ratio and slip.

[0043] FIG. 2 is a diagram schematically illustrating the shape and dimensions of a sleeve roll used as a test roll.

[0044] FIG. 3 is a graph illustrating the influence of (C0.24V0.13Nb) on fatigue resistance index.

[0045] FIG. 4 is a diagram schematically illustrating the skeleton framework of a testing machine used for a wear test and a hot rolling contact fatigue test.

[0046] FIG. 5 is a diagram schematically illustrating the shape of a hot rolling contact fatigue test piece (fatigue test piece) and the shape and dimensions of notches formed on the outer circumferential surface of the test piece.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0047] First, reasons for the limitations on the chemical composition of the centrifugal cast caliber roll for a hot rolling mill according to the present invention will be described. Hereinafter, mass % used when describing a chemical composition shall be simply referred to as %.

[0048] C: 1.8% or more and 3.0% or less

[0049] C increases hardness as a result of forming a solid solution in the matrix and carbides and influences the wear resistance and fatigue resistance of a roll as a result of forming hard carbides. In the case where the C content is less than 1.8%, there is deterioration in wear resistance is appeared due to a decrease in the amount of hard carbides. On the other hand, in the case where the C content is more than 3.0%, there is deterioration in fatigue resistance and wear resistance due to embrittlement caused by coarsening of carbides. Therefore, the C content is limited to be 1.8% or more and 3.0% or less.

[0050] Si: 0.2% or more and 1.0% or less

[0051] Si is a chemical element which functions as a deoxidizing agent and which is effective for improving the castability of molten iron and steel, and it is necessary that the Si content be 0.2% or more in order to obtain such effects. On the other hand, in the case where the Si content is more than 1.0%, since the effects become saturated, it is not possible to expect an increase in the effects corresponding to an increase in the Si content, and it is difficult to control to achieve the desired hardness due to an increase in the amount of retained austenite. Therefore, the Si content is limited to be 0.2% or more and 1.0% or less.

[0052] Mn: 0.2% or more and 1.5% or less

[0053] Mn is a chemical element which is effective for negating the negative effect of S by fixing S in the form of MnS and which is effective for improving hardenability by forming a solid solution in the matrix. Although it is necessary that the Mn content be 0.2% or more in order to obtain such effects, the effects become saturated in the case where the Mn content is more than 1.0%, and it is not possible to expect an increase in the effects corresponding to the cost for increasing the Mn content in the case where the Mn content is more than 1.5%. Therefore, the Mn content is limited to be 0.2% or more and 1.5% or less, or preferably 0.2% or more and 1.0% or less.

[0054] Cr: 5% or more and 9% or less

[0055] Cr is a chemical element which is effective for improving wear resistance by combining with C to form mainly eutectic carbides and which is effective for stabilizing rolling by decreasing the frictional force between a material to be rolled and the roll when rolling is performed. In order to obtain such effects, it is necessary that the Cr content be 5% or more. On the other hand, in the case where the Cr content is more than 9%, slip or sticking occurs. Therefore, the Cr content is limited to be 5% or more and 9% or less.

[0056] Mo: 4.0% or more and 7.0% or less

[0057] Mo has an important function of improving the fatigue resistance and wear resistance of a roll through solid solution strengthening as a result of forming a solid solution in the matrix and carbides. In order to obtain such an effect, it is necessary that the Mo content be 4.0% or more. On the other hand, in the case where the Mo content is more than 7.0%, there is deterioration in fatigue resistance due to the formation of hard and brittle free carbides mainly containing Mo. Therefore, the Mo content is limited to be 4.0% or more and 7.0% or less, or preferably 4.3% or more and 6.8% or less.

[0058] V: 4.0% or more and 7.0% or less

[0059] V is a chemical element which is important in the present invention for achieving satisfactory wear resistance and fatigue resistance at the same time. V is a chemical element which improves wear resistance by forming very hard carbides (MC-type carbides) and which significantly improves the fatigue resistance of a roll as a result of having an effective function of dividing eutectic carbides in order to allow the eutectic carbides to dispersedly crystallize. Such effects become marked in the case where the V content is 4.0% or more. On the other hand, in the case where the V content is more than 7.0%, since there is coarsening of MC-type carbides, and since the centrifugal casting segregation of MC-type carbides is promoted, various properties of a roll become unstable. Therefore, the V content is limited to be 4.0% or more and 7.0% or less, or preferably 5.5% or more and 6.8% or less.

[0060] Nb: 0.5% or more and 2.0% or less

[0061] Nb is a chemical element which improves the wear resistance and fatigue resistance of a roll by strengthening MC-type carbides as a result of forming a solid solution in the MC-type carbides. In addition, Nb is a chemical element which is effective for improving the fatigue resistance of a roll by inhibiting eutectic carbides from fracturing as a result of promoting the division of eutectic carbides. Also, Nb has a function of inhibiting the segregation of MC-type carbides when centrifugal casting is performed. Such effects become marked in the case where the Nb content is 0.5% or more. On the other hand, in the case where the Nb content is more than 2.0%, since the growth of MC-type carbides in molten iron and steel is excessively promoted, the segregation of carbides is promoted when centrifugal casting is performed. Therefore, the Nb content is limited to be 0.5% or more and 2.0% or less, or preferably 0.6% or more and 1.3% or less.

[0062] In an aspect of the present invention, the contents of C, V, and Nb are controlled to be within the ranges described above so that relational expression (1) below is satisfied.

0.6(C0.24V0.13Nb)1.4(1)

[0063] (Here, C, V, and Nb each denote the content (mass %) of the corresponding chemical element)

[0064] The expression (0.24V+0.13Nb) indicates the amount of C which is expended in the formation of MC-type carbides. (C0.24V0.13Nb) is also referred to as effective carbon content and indicates the amount of C (mass %) which forms a solid solution in the matrix or eutectic carbides. Therefore, this effective C content influences the wear resistance and fatigue resistance of a roll and frictional force between a material to be rolled and a roll as a result of influencing the hardness of the matrix and the amount of eutectic carbides. In particular, in order to achieve excellent fatigue resistance, it is necessary that the effective C content (mass %) be limited to 0.6 or more and 1.4 or less. In the case where the effective C content (mass %) is out of the range of 0.6 or more and 1.4 or less, as FIG. 3 indicates, there is a decrease in the fatigue resistance of a roll. It is more preferable that the effective C content (mass %) be 0.7% or more and 1.3% or less. With this, it is possible to further decrease scatter in fatigue resistance.

[0065] The remainder other than the constituent chemical elements described above is Fe and inevitable impurities.

[0066] Examples of the inevitable impurities include P: 0.05% or less, S: 0.05% or less, N: 0.06% or less, and B: 0.02% or less. Since P deteriorates the properties of iron and steel as a result of being segregated at grain boundaries, it is preferable that the P content be as small as possible in the present invention. It is acceptable that the P content be 0.05% or less in the present invention. In addition, since S deteriorates the properties of iron and steel as a result of existing in the form of sulfide-based inclusions, it is preferable that the S content be as small as possible in the present invention. It is acceptable that the S content be 0.05% or less in the present invention. In addition, N is usually mixed into iron and steel in an amount of about 0.06% or less. Within such a range of N content, there is no influence on the effect of the present invention. In addition, B is an impurity chemical element which is mixed into iron and steel from scrap, which is a raw material to be melted, casting flux, and so forth. It is preferable that the B content be as small as possible in the present invention. It is acceptable that the B content be 0.02% or less in the present invention, because there is no negative influence on the effect of the present invention.

[0067] Hereafter, the reasons for the limitations on the hardness of the centrifugal cast caliber roll for a hot rolling mill according to an aspect of the present invention will be described.

[0068] The centrifugal cast caliber roll for a hot rolling mill according to an aspect of the present invention has the chemical composition described above, a caliber surface hardness of Hs 67 or higher and Hs 76 or lower in terms of Shore hardness. In the case of a rolling roll for a hot-rolled steel sheet, the hardness is generally controlled to be about Hs 79 or more. In the case of a caliber roll for a hot rolling mill such as a roll for manufacturing a seamless steel pipe, for which the present invention is intended, it is difficult to stably perform rolling due to slip occurring when rolling is performed in the case where the hardness is higher than Hs 76. On the other hand, in the case where the hardness is lower than Hs 67, there is a decrease in wear resistance and fatigue resistance, and surface deterioration may occur. Therefore, in the case of the centrifugal cast caliber roll for a hot rolling mill according to an aspect of the present invention, the caliber surface hardness is limited to be Hs 67 or higher and Hs 76 or lower in terms of Shore hardness.

[0069] Hereafter, a preferable method for manufacturing the centrifugal cast caliber roll for a hot rolling mill according to the present invention will be described.

[0070] It is preferable that molten metal having the chemical composition described above be prepared, poured into a mold, and then cast. There is no particular limitation on what method is applied for preparing molten metal, and any of ordinary melting methods such as one in which a high-frequency induction furnace is used may be applied. Here, in an aspect of the present invention, casting is performed by using a centrifugal casting method, which is inexpensive and operated at low energy costs. When casting is performed, it is preferable to use a mold (rotary mold) whose inner surface is covered with a refractory having a thickness of 0.5 mm to 6 mm composed mainly of zircon and the like.

[0071] Although it is preferable that the centrifugal cast caliber roll for a hot rolling mill according to the present invention be a sleeve-type roll having a single layer, the roll may be composed of plural layers. In the case where the roll is composed of plural layers, it is preferable that the roll be an integrated roll which is composed of an integrated combination of an outer layer and an inner layer and which is manufactured by pouring molten metal having the chemical composition of the inner layer during the solidification of the outer layer or after the solidification of the outer layer. In addition, since spalling tends to occur in a cast product (roll) in the case where the molten metal is cast into a caliber shape mold, it is not necessary that the mold have a caliber shape. It is preferable that, for example, a cylindrical shape be formed with no caliber shape being formed in a casting process and that a caliber shape be formed by performing forging and/or, for example, machining after the casting process.

[0072] Here, it is preferable that an inner layer be composed of, for example, spheroidal graphite cast iron, vermicular graphite cast iron (VC cast iron), hypereutectoid steel, adamite steel, or spheroidal graphite steel, which is excellent in terms of casting capability and mechanical properties. In addition, since a part of the outer layer material is melted for integration, embrittlement of the inner layer may occur due to alloy chemical elements (carbide-forming chemical elements) such as Cr and V contained in the outer layer material mixing into the inner layer, which requires attention.

[0073] The roll provided with a caliber is subsequently subjected to a quenching treatment and a tempering treatment in order to obtain a caliber roll having a caliber surface hardness within the range described above. Here, it is preferable that a quenching treatment be performed by charging the roll into a heat treatment furnace, by heating the roll to a temperature of 950 C. to 1100 C., and by then cooling the roll with air. In addition, it is preferable that a tempering treatment be performed by heating the roll to a temperature of 430 C. to 600 C. and by then cooling the roll.

[0074] Hereafter, the present invention will be described more in detail on the basis of examples.

EXAMPLES

Example 1

[0075] Molten metals having the chemical compositions given in Table 1 were prepared by using a high-frequency induction furnace, and then, ring roll materials (having an outer diameter of 250 mm, an inner diameter of 130 mm, and a length of 60 mm) were obtained by casting using a centrifugal casting method. Here, the casting temperature was 1470 C. to 1540 C., and the centrifugal force was 160 G in multiples of gravity. After casting had been performed, by performing a quenching treatment and a tempering treatment, the hardness was controlled.

[0076] Here, the hardness was determined at five positions in the vicinity of the surface of the ring roll material by using a Shore hardness meter, and the average of the five determined values was defined as the average hardness of the corresponding roll material.

[0077] A fatigue test piece and a wear test piece were taken from the obtained ring roll material.

[0078] The fatigue test piece had the shape illustrated in FIG. 5 at panel (a) (having an outer diameter of 60 mm, an inner diameter of 25 mm, and a width of 10 mm), and notches having the dimensions and the shape illustrated in FIG. 5 at panel (b) (having a depth t of 1.3 mm and a length L in the circumferential direction of 1.0 mm) were formed at two positions in the outer circumferential surface of the fatigue test piece by using a wire electric discharge machining method with a wire having a diameter of 0.2 mm. In addition, the edges of the rolling contact surface of the fatigue test piece had chamfered corners.

[0079] A hot rolling contact fatigue test was performed on the fatigue test piece described above in order to evaluate fatigue resistance.

[0080] A hot rolling contact fatigue test was, as illustrated in FIG. 4, performed by using a slip-rolling-type method between two discs, which were the test piece and a counter material. That is, while the test piece (fatigue test piece) was cooled with water and rotated at a rotational speed of 700 rpm, the counter piece (composed of S45C and having an outer diameter of 190 mm, a width of 10 mm, and chamfered corners) was heated at a temperature of 830 C. by using a high-frequency induction heating coil, pressed onto the rotating test piece With a contact load of 980 N, and rotated with a slip ratio of 10%. By rotating the test piece until the two notches machined in the fatigue test piece broke, and by counting the respective rotation numbers until the notches broke, the average of the two rotation numbers was defined as a breaking rotation number. Subsequently, by using the breaking rotation number of test material No. 21, that is, the comparative example (standard) given in Table 2 as a standard value, and by calculating the ratio of the breaking rotation number of each of the ring roll materials to the standard value, that is, (breaking rotation number of ring roll material)/(breaking rotation number of test material No. 21, that is, comparative example (standard)), the calculated ratio was defined as a fatigue resistance index and used as an index of fatigue resistance. Here, a case where the fatigue resistance index was 1.1 or more was judged as a case of excellent fatigue resistance.

[0081] The wear test piece had an outer diameter of 60 mm, an inner diameter of 25 mm, and a width of 10 mm. In addition, the edges of the rolling contact surface of the wear test piece had chamfered corners. A wear test was performed on the wear test piece described above in order to evaluate wear resistance.

[0082] A wear test was, as illustrated in FIG. 4, performed by using a slip-rolling-type method between two discs, which were the test piece and a counter material. That is, while the test piece (wear test piece) was cooled with water and rotated at a rotational speed of 700 rpm, the counter piece (composed of S45C and having an outer diameter of 190 mm, a width of 15 mm, and chamfered corners) was heated at a temperature of 830 C., pressed onto the rotating test piece with a contact load of 980 N, and rotated with a slip ratio of 10% until the cumulative rotation number was 200000. After the wear test had been performed, the amount of decrease in weight due to wear of the wear test piece was determined.

[0083] Subsequently, by using the amount of decrease in weight due to wear of test material No. 21, that is, the comparative example (standard) given in Table 2 as a standard value, and by calculating the ratio of the amount of decrease in weight due to wear of each of the test materials to the standard value, that is, (amount of decrease in weight due to wear of test material)/(amount of decrease in weight due to wear of test material No. 21, that is, comparative example (standard)), the calculated ratio was defined as a wear resistance index and used to evaluate wear resistance. Here, a case where the wear resistance index was 1.1 or more was judged as a case of excellent wear resistance.

[0084] The obtained results are given in Table 2.

TABLE-US-00001 TABLE 1 Relational Expression (1)* Test Chemical Composition (mass %) Effective C Molten Impurity Content Satisfied Metal C Si Mn Cr Mo V Nb P S N B (mass %) or not Note A 1.9 0.8 0.9 7.8 5.3 4.8 0.8 0.025 0.008 0.025 0.003 0.6 Conforming Example B 2.2 0.5 0.4 6.4 5.6 6.0 1.1 0.024 0.011 0.036 0.009 0.6 Conforming Example C 2.0 0.4 0.7 6.3 4.2 5.6 1.0 0.008 0.012 0.041 0.007 0.5 X Comparative Example D 2.4 0.5 0.6 6.2 5.4 6.0 0.8 0.017 0.013 0.034 0.006 0.9 Conforming Example E 2.6 0.4 0.6 6.2 5.3 5.9 0.8 0.014 0.011 0.039 0.009 1.1 Conforming Example F 2.9 0.5 0.5 6.1 4.2 6.1 0.6 0.009 0.014 0.024 0.003 1.4 Conforming Example G 1.7 0.5 0.5 6.2 5.1 5.6 0.7 0.025 0.010 0.026 0.002 0.3 X Comparative Example H 3.1 0.4 0.5 7.2 4.6 6.1 1.0 0.018 0.009 0.033 0.004 1.5 X Comparative Example I 2.2 0.3 0.4 6.1 5.4 6.4 1.4 0.024 0.013 0.045 0.001 0.5 X Comparative Example J 3.3 0.6 0.5 5.2 4.3 6.2 0.9 0.006 0.004 0.037 1.7 X Comparative Example K 2.4 0.2 0.3 9.1 6.8 6.8 0.8 0.016 0.009 0.038 0.7 Conforming Example L 2.4 0.4 0.5 4.8 5.8 5.6 1.8 0.022 0.014 0.016 0.8 Conforming Example M 2.5 0.2 1.3 5.7 4.3 6.0 0.9 0.018 0.012 0.024 0.016 0.9 Conforming Example N 2.5 0.5 0.4 5.8 5.2 5.9 0.8 0.018 0.010 0.029 0.008 1.0 Conforming Example O 2.6 1.6 0.8 4.2 3.8 4.3 0.9 0.022 0.006 0.036 0.007 1.5 X Comparative Example P 2.6 0.4 0.4 5.6 4.9 6.1 1.2 0.023 0.012 0.042 1.0 Conforming Example Q 2.0 0.3 0.5 6.8 6.2 4.4 2.0 0.024 0.007 0.038 0.007 0.6 Conforming Example R 3.3 0.5 0.5 9.6 5.4 5.2 2.4 0.015 0.016 0.028 0.003 1.7 X Comparative Example S 1.8 0.5 0.4 7.2 6.7 4.2 0.9 0.017 0.015 0.028 0.002 0.7 Conforming Example T 2.4 0.5 0.5 5.4 7.7 5.2 0.5 0.025 0.008 0.028 0.003 1.1 Comparative Example Y 2.2 0.3 0.3 6.0 2.5 5.0 1.5 0.018 0.011 0.032 0.048 0.8 Comparative Example *0.6 (C 0.24V 0.13Nb) 1.4 . . . (1) Effective C content: C 0.24V 0.13Nb An underlined portion indicates an item out of the range according to the present invention.

TABLE-US-00002 TABLE 2 Test Material Molten Hardness Test Result No. Metal No. Hs Wear Resistance Fatigue Resistance Note 1 A 67 1.1 1.2 Example 2 B 73 1.2 1.2 Example 3 C 65 0.7 0.7 Comparative Example 4 D 68 1.5 1.5 Example 5 E 73 1.6 1.6 Example 6 F 76 1.2 1.3 Example 7 G 64 0.8 0.4 Comparative Example 8 H 78 0.8 0.6 Comparative Example 9 I 71 0.7 0.8 Comparative Example 10 J 69 0.6 0.4 Comparative Example 11 K 73 1.6 1.6 Example 12 L 73 1.3 1.4 Example 13 M 75 1.3 1.3 Example 14 N 67 1.4 1.5 Example 15 O 77 0.8 0.6 Comparative Example 16 P 69 1.4 1.4 Example 17 Q 75 1.2 1.3 Example 18 R 71 1.1 0.6 Comparative Example 19 S 74 1.2 1.2 Example 20 T 72 1.1 0.7 Comparative Example 21 Y 72 1.0 (Standard) 1.0 (Standard) Comparative Example (Standard) An underlined portion indicates an item out of the range according to the present invention.

[0085] In the case of any of the examples of the present invention, although Shore hardness was within a range of Hs 67 or higher and Hs 76 or lower, which was lower than the hardness of the outer layer material of an ordinary roll for rolling a steel sheet, that is, Hs 81, there was no tendency for wear resistance or fatigue resistance to decrease with decreasing hardness in this range of hardness. With reference to the chemical compositions of molten metal given in Table 1, and on the basis of a comparison between the examples of the present invention and the comparative examples given in Table 2, it is considered that conditions regarding chemical composition such as effective C content has a rather large influence on wear resistance and fatigue resistance.

Example 2

[0086] Molten metal having the same chemical composition as that of test molten metal N (having an effective C content (C0.24V0.13Nb) of 0.98) given in Table 1 was cast into a single-layer sleeve roll having a circular cylindrical shape (having an outer diameter of 575 mm, an inner diameter of 255 mm, and a length of 2.0 m) by using a centrifugal casting method (with a centrifugal force of 195 G). The obtained sleeve roll was subjected to soft annealing and cut into plural pieces (having a length of about 350 mm). By providing a caliber having a desired shape to these sleeve rolls by performing crude processing, by heating the machined sleeve rolls to a temperature of 950 C. to 1100 C. in order to perform quenching, and by performing a tempering treatment including heating the quenched sleeve rolls to a temperature of 430 C. to 600 C. plural times, hardness was controlled.

[0087] Subsequently, by performing finish processing on the obtained sleeve rolls, test rolls (having a length of 305 mm) having the caliber shape (having a caliber bottom diameter of 81 mm) illustrated in FIG. 2 were obtained. By fitting these test rolls to a mandrel mill stand (stand #2) in a seamless steel pipe manufacturing line (172 passes), test. rolling was performed on 1000 pipes or more for each of the sleeve rolls in a rolling cycle in which a 13%-Cr-steel. pipe was mainly rolled in order to investigate whether or not slip occurred.

[0088] The obtained results are given in Table 3.

TABLE-US-00003 TABLE 3 Test Roll No. Hardness Hs Occurrence of Slip Note R1 72 Not occurred Example R2 74 Not occurred Example R3 77 Occurred Comparative Example R4 67 Not occurred Example R5 63 Not occurred (Surface Comparative Example Deterioration) R6 65 Not occurred (Surface Comparative Example Deterioration) R7 79 Occurred Comparative Example R8 82 Occurred Comparative Example An underlined portion indicates an item out of the range according to the present invention.

[0089] In the case of any of the rolls having the high-speed steel chemical composition within the range according to the present invention and a hardness of Hs 67 or higher and Hs 76 or lower in terms of Shore hardness, slip did not occur when used for hot-rolling seamless steel pipes. Surface deterioration of a roll occurred in the case of a hardness of lower than Hs 67, which is out of the range described above, and slip occurred in the case of a hardness of higher than Hs 76. It is clarified that a roll having the chemical composition within the range according to the present invention and a surface hardness of Hs 67 or higher and Hs 76 or lower in terms of Shore hardness is a roll with which it is possible to prevent surface deterioration and slip.

[0090] As described above, a caliber roll having the chemical composition and hardness within the ranges according to the present invention is a roll with which slip does not occur when hot rolling is performed, which is excellent in terms of wear resistance and fatigue resistance, and which is effectively used for rolling a seamless steel pipe.