JPH0776408B2 - High chromium cast iron roll material with excellent crack resistance and slip resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a roll material mainly applied to a rolling use layer of a composite roll for rolling which is subjected to hot rolling.

(Prior Art) Conventionally, an adamite roll was used for a hot rolling roll, especially for a work roll before the hot strip mill finishing, but in recent years, the hot rolling steel plate has been improved in quality, thin gauge and special steel. In response to an increase in the rolling ratio and the like, a composite roll is used in which a high chromium roll material having excellent wear resistance is applied to the outer layer, which is the layer used for rolling.

(Problems to be solved by the invention) A high chromium roll material contains C: 2.0 to 3.2 wt% Cr: 10 to 30 wt%, and a large amount of high hardness M ₇ C ₃ type chromium carbide was generated in the structure. Although it is excellent in wear resistance, it has a problem of poor slip resistance and crack resistance. Especially in serious accidents due to insufficient crack resistance, so-called
There is a chipping accident. The chipping accident is a peeling of a small piece having a width and length of several mm and a depth of about 1 mm from the roll surface during rolling. When such an accident occurs, the peeled portion is transferred to the surface of the material to be rolled (steel sheet) as convex portions at regular intervals and is pressed down by the next roll to deteriorate the surface quality of the steel sheet, and at the time of coiling. And steel plate processing,
The raised rolling portion serves as a starting point of crack generation and causes defective products.

The present invention has been made in view of such problems, and an object thereof is to provide a high chromium cast iron roll material having excellent slip resistance and crack resistance.

(Means for Solving the Problems) The high chromium cast iron roll material of the present invention made to achieve the above object has a chemical composition of C: 1.3 to 2.0% by weight and Cr: 11 to 19% Si: 0.3. ~ 1.5%, Mo: 0.8 ~ 3.0% Mn: 0.3 ~ 1.5%, Ti: 0.1 ~ 1.0% Ni: 0.5 ~ 2.4%, balance consisting essentially of Fe, mainly having M ₂₃ C ₆ type crystallized carbide This is what constitutes the invention.

Further, instead of part of Fe of the roll material having the above composition, Ti:
It is preferable to contain 0.1 to 1.0% by weight or Nb: 0.1 to 1.0% by weight.

(Operation) The chemical composition of the high chromium cast iron roll material of the present invention is limited for the following reasons. The unit is% by weight.

C: 1.3 to 2.0% C combines with Cr or Mo to form a carbide.
In the present invention, the C content is reduced as compared with the conventional high chromium cast iron material,
M ₂₃ C with toughness while suppressing the amount of carbide in the structure
It is contained in the above range in order to stably crystallize the _6- type carbide. That is, if it is less than 1.3%, the amount of carbide is too small and wear resistance is insufficient, and if it exceeds 2.0%, the amount of carbide is too much and high hardness M ₇ C ₃ type carbide is crystallized to cause crack resistance and slip resistance. to degrade.

Si: 0.3-1.5% Si is an element necessary for deoxidation of the molten metal, and 0.3% or more is necessary. However, if it exceeds 1.5%, the base becomes brittle.

Mn: 0.3-1.5% Mn is required to be 0.3% or more for deoxidation and desulfurization of molten metal. However, if it exceeds 1.5%, the mechanical properties, particularly the toughness, are significantly deteriorated.

Ni: 0.5 to 2.4% Ni forms a solid solution in the matrix without forming carbides and improves the strength of the matrix and also improves quench hardening. If it is less than 0.5%, the base is not sufficiently strengthened. On the other hand, even if it exceeds 2.4%, it is 5
%, The effect of strengthening the base is recognized, but there is a drawback that the manufacturing cost is high. Further, when added in a large amount, the retained austenite becomes stable, and it becomes easy to adjust the hardness in the subsequent heat treatment.

Cr: 11 to 19% Cr is mainly added to form an appropriate amount of M ₂₃ C ₆ type carbide. If it is less than 11%, the amount of Cr carbide generated is too small and wear resistance deteriorates. On the other hand, if the content exceeds 19%, the amount of Cr carbide produced becomes excessive,
It hinders the improvement of crack resistance, which is the object of the present invention.

Mo: 0.8 to 3.0% Mo forms a carbide like Fe and Cr, and is effective in increasing the temper softening resistance. If it is less than 0.8%, the above effect is not obtained, while if it exceeds 3.0%, retained austenite is stabilized and it becomes difficult to obtain sufficient hardness.

Ti: 0.1 to 1.0% Ti forms a high hardness carbide having a hardness about twice that of Mo carbide, and has an effect of improving wear resistance. Further, like Mo, it is also effective in increasing the temper softening resistance. If it is less than 0.1%, the above effect is not obtained, while if it exceeds 1.0%, segregation is caused due to the difference in specific gravity when the roll outer layer is produced by centrifugal casting, which is not preferable. The high chromium cast iron roll material of the present invention can contain Nb in the following range in addition to the above components.

Nb: 0.1-1.0% Nb makes the cast structure finer and finer, strengthens the matrix, and improves temper softening resistance. If it is less than 0.1%, the above effect is not observed, while if it exceeds 1%, the manufacturing cost increases.

The high chromium cast iron roll material of the present invention is formed of Fe and impurities in addition to the above components. Both P and S are desirable because they make the material brittle, so it is preferable to keep P: less than 0.08% and S: less than 0.06%.

The high chromium cast iron roll material of the present invention suppresses the amount of carbide produced to a low level within the range not impairing the wear resistance and crystallizes the M ₂₃ C ₆ type carbide having toughness, particularly by suppressing the C content. A characteristic is that the carbide is formed into a structure supported by a matrix structure solid-solution strengthened by Ni, and thereby, the crack resistance and the slip resistance are improved while maintaining the wear resistance.

Furthermore, Ti is positively added to the above structure to have toughness.
In addition to the M ₂₃ C ₆ type carbide, a Ti carbide having a high hardness is formed and both carbides are supported by the above-mentioned Ni solution strengthened structure, so that the wear resistance can be particularly improved. Furthermore, if necessary, Nb can be positively added to the above-mentioned structure to make the base structure finer and finer and further strengthen the base structure. As a result, a structure in which the M ₂₃ C ₆ type carbide is supported by a stronger matrix structure can be obtained, and in particular, crack resistance can be improved while maintaining wear resistance.

(Example) The high-chromium cast iron roll material described above is mainly used as a cast material for the outer layer, which is the layer used in the hot rolling composite roll, but as the inner layer (axial core) material, high-grade cast iron or ductile is used. A tough cast iron material such as cast iron or a cast steel material such as graphite cast steel is appropriately used.

Further, as the method for producing the composite roll, after casting the outer layer by the centrifugal force casting method, the centrifugal force casting mold having the outer layer is erected to form a stationary mold, and the inner layer material molten metal is contained therein. There is a method in which the outer layer and the inner layer are welded and integrated by pouring, and it is generally applied because it is simple.

When welding the outer layer and the inner layer, Cr from the outer layer to the inner surface
In order to prevent the mixture of the above, an intermediate layer may be provided between the outer layer and the inner layer. By providing the intermediate layer, it is possible to prevent mixing and diffusion of Cr from the outer layer to the inner layer, and it is possible to effectively prevent embrittlement of the boundary portion between the outer layer and the inner layer.

After casting, the composite roll is usually subjected to the following heat treatment. First, by heating and holding at a high temperature of ₃ or more points of A, Cr and C solid-dissolved in austenite are precipitated as secondary carbides to lower the Cr and C concentrations in austenite, and then cooling (quenching).
In the process, so-called destabilizing heat treatment of austenite is performed, which changes to austenite that easily transforms. Then, in order to transform the austenite into martensite by quenching treatment and subsequently to form a thermally stable structure and reduce residual stress generated during transformation, tempering heat treatment is performed at a temperature of 400 to 600 ° C. By performing the above heat treatment, a hardness of Hs75 to 85 can be given to the outer layer of the roll.

The roll material referred to in the present invention is not limited to the above-described hot rolling rolls, but also includes outer layer materials such as cold rolling rolls, hot skin pass rolls, and H-shaped steel rolls, as well as rollers in rolling auxiliary equipment. Is also applicable material. For example, it can be applied as an outer layer material of a hollow cylindrical roller such as a hot run table roller.

Next, specific production examples of the composite roll in which the high chromium cast iron roll material of the present invention is applied to the outer layer which is the layer used for rolling will be described. For comparison, a composite roll using a conventional high chromium cast iron material as the outer layer was also manufactured as a conventional example.

<Example A> (1) As the molten metal for the outer layer material, high chromium cast iron shown in Table 1 was used, and this was cast into a cylindrical mold rotating on a centrifugal casting machine for a casting thickness of 100 mm. At this time, a coating agent made of resin sand is applied to the inner surface of the die with a thickness of 3 mm,
The number of rotations of the mold was GNo. 140, and the pouring temperature was 1510 ° C in all cases.

(2) The outer layer completely solidified 20 to 22 minutes after the start of the outer layer casting. After that, a centrifugal casting mold having an outer layer inside was erected vertically, and an upper die and a lower die for roll shaft portion casting were connected to both ends thereof to form a stationary casting mold. The ductile cast iron melt shown in Table 2 was cast into the inside as a shaft core melt and completely filled, and then the upper portion was covered with a feeder heat insulator.

(3) Three days after casting, the mold was disassembled, the roll material was taken out, machined, and then heat-treated as shown in Table 3.

(4) After finishing, the outer layers of the product were as shown in Table 4.

(5) Each roll was placed on an actual hot strip mill finishing front stand and subjected to rolling. The rolling result of each roll is calculated by passing the rolled material through T while the outer layer of the roll is worn by 1 mm.
Table 5 shows the on numbers.

From Table 5, it was confirmed that the wear resistance of each roll of this example was improved as compared with the conventional example.

Further, in the roll of the conventional example, the pitch of the fine cracks that cause chipping accidents on the roll surface was 0.5 mm on average, whereas in the roll of the present example, the pitch of the fine cracks is Example A-1, Example A-2 and Example A-
3 The average was 1 mm. This indicates that the roll is insensitive to heat cycle, which proves to be advantageous for crack resistance.

It goes without saying that there was no chipping accident as a result of using this roll. Further, the slip resistance (bite resistance) and the surface roughening resistance were also improved as compared with the roll of the conventional example.

<Example B> (1) Example B shown in Table 6 under the same conditions as Example A.
-1, Example B-2 and Example B-3 were cast into a mold for centrifugal force casting with an outer layer material melt having a casting thickness of 100 mm.
Approximately 14 minutes after the start of casting the outer layer, when part of the inner surface of the outer layer is in a non-solidified state, the molten intermediate material shown in Table 6 is cast on the inner surface of the outer layer to a thickness of 25 mm. I cast it.

(2) About 30 minutes after the start of casting of the outer layer, the outer layer and the intermediate layer were completely solidified. Then, a mold for centrifugal force casting having an outer layer and an intermediate layer was erected vertically, and an upper die and a lower die for roll shaft portion casting were connected to both ends thereof to form a stationary casting mold. The ductile cast iron melts shown in Table 6 were cast into the inside of each as core shaft melts and completely filled, and then the upper portion was covered with a feeder heat insulating material.

(3) Three days after casting, the mold was disassembled and subjected to heat treatment shown in Table 7.

(4) Table 8 shows the thicknesses of the outer layer and the intermediate layer of the finished product roll after finishing and the surface hardness of the outer layer.

In addition, in the middle layer and inner layer of each roll, the Cr content from the outer layer was increased and the Cr content increased due to diffusion.
This is shown in Table 9.

(5) From each of the rolls, a round bar test piece was sampled so that the boundary surface of each layer was 45 degrees with respect to the axial direction, and the compressive strength was examined. The results are shown in Table 10.

For comparison, each outer layer melt and inner layer melt were used to cast a two-layer composite roll in which both were directly welded. In this case, the compression strength in the 45 ° direction at the boundary between the outer layer and the inner layer was It was about 120 kg / mm ² , which was a considerably lower value than the compressive strength of the above-mentioned examples.

Further, in the strength test, in both of the examples, no reduction in slippage at the boundary portion due to the embrittlement of the boundary, which was observed by direct welding of the outer layer and the inner layer, was observed.

(Effects of the Invention) The high chromium cast iron roll material of the present invention suppresses the C content and adds Ni to suppress the amount of carbide produced and at the same time crystallize a tough M ₂₃ C ₆ piece carbide. It was possible to obtain a structure in which the carbide is supported by a strong matrix structure. Therefore, the crack resistance was improved while maintaining the wear resistance, and the occurrence of chipping accidents was prevented. Moreover, since the amount of carbide was suppressed, the coefficient of friction between the roll and the rolled material was increased, and the slip resistance was improved. Further, since the matrix structure is strengthened, the plastic flow resistance is also improved, and the rough skin resistance is improved.

Further, by positively adding Ti, a high hardness Ti carbide is crystallized together with a toughness M ₂₃ C ₆ type carbide,
It was possible to create a structure in which both carbides were supported by a strong base structure. Therefore, in addition to the above effects, the wear resistance can be particularly improved.

Alternatively, by positively adding Nb, the base structure is made finer and finer, and the base structure is further strengthened,
It was possible to form a structure in which the M ₂₃ C ₆ type carbide was supported by a stronger matrix structure. Therefore, in addition to the above effects,
In particular, it was possible to improve crack resistance and rough skin resistance.

Therefore, the high-chromium cast iron roll material of the present invention was able to have excellent crack resistance and slip resistance while maintaining or improving the conventional excellent wear resistance.

Front page continuation (72) Inventor Shinro Maeya 64 Nishimukojima-cho, Amagasaki City, Hyogo Prefecture Kubota Iron Works Co., Ltd., Amagasaki Plant (56) Reference JP 62-23964 (JP, A) JP 62-144807 ( JP, A) JP 62-148005 (JP, A)

Claims (2)

[Claims] 1. Chemical composition by weight% C: 1.3 to 2.0%, Cr: 11 to 19% Si: 0.3 to 1.5%, Mo: 0.8 to 3.0% Mn: 0.3 to 1.5%, Ti: 0.1 to 1.0% Ni: 0.5 to 2.4%, the balance being substantially Fe, and having mainly M ₂₃ C ₆ type crystallized carbide, a high chromium cast iron roll material excellent in crack resistance and slip resistance. 2. A high chromium cast iron excellent in crack resistance and slip resistance, characterized by containing Nb: 0.1 to 1.0 wt% in place of a part of Fe of the roll material according to claim 1. Roll material.