JPH0219724B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a spheroidal graphite cast iron Caliba roll with excellent crack resistance and wear resistance, and a method for manufacturing the same. The present invention relates to a base tissue having excellent characteristics for sexual activity. <Prior Art> In a rough rolling stand in a hot rolling facility, the rolled material is at a high temperature and the rolling speed is slow, so the thermal load on the rolls is extremely large. For this reason, rough rolling rolls are prone to cracks due to thermal fatigue, and roll breakage accidents often occur due to the propagation of these cracks under severe rolling conditions. Conventionally, spheroidal graphite cast iron rolls have been used as rolls suitable for such uses. Spheroidal graphite cast iron rolls have a structure in which spheroidal graphite is dispersed in the matrix, and the graphite prevents the development of cracks and disperses the cracks in a hexagonal pattern, so it has relatively good crack resistance. <Problems to be solved by the invention> However, the conventional spheroidal graphite cast iron rolls
It still lacks crack resistance under severe usage conditions of heat and load, such as in the rough rolling stand during hot rolling, and often causes breakage accidents. In particular, as shown in FIG. 1, in the case of a caliber roll whose surface is processed with a caliber for rolling, the stress at the bottom 2 of the caliber is particularly severe, so it can be said that the usage conditions are more severe than that of a plain roll. In the figure, the caliber portion 1a is designated as a box, 1b as an oval, 1c as a square, and 1d as a diamond, 3 as a flange portion, 4 as a journal portion, and 5 as a journal portion.
6 indicates the roll layer (usually 10 to 70 mm thick), and 6 indicates the roll core. Needless to say, the occurrence of roll breakage accidents is not only caused by an increase in the amount of roll wear.
This causes a decrease in rolling efficiency and an accompanying decrease in productivity. In addition to breakage resistance, there is also a strong demand for improvement in abrasion resistance, and it is necessary to improve the roll material including its abrasion resistance. For wear resistance, it is necessary to improve the hardness of the layer used in rolls, but since the rolls are used for hot rolling, martensite and bainite, which have low tempering resistance and whose hardness decreases at high temperatures, can achieve this purpose. It is not possible. Further, as for cementite, the wear resistance improves as the amount increases, but since increasing the amount significantly impairs crack resistance and accident resistance, it is difficult to increase the amount more than the conventional level. It is known that the relationship between cracks and roll material is: ● Occurrence of heat cracks; toughness (elongation) ● Occurrence of headband cracks; Proof strength ● Progress of cracks; fracture toughness value. Among these, the fracture toughness value is related to both toughness and yield strength, and as the toughness and yield strength improve, the fracture toughness value improves. As a measure to improve toughness, it is effective to make the base structure a fine mixed structure of two phases of ferrite and pearlite, as described in Japanese Patent Application No. 55-32070. However, the ferrite/pearlite two-phase structure inevitably has a low yield strength and is prone to headband cracks. Note that the headband crack refers to a crack in which heat cracks are connected in the circumferential direction at the position where the maximum stress occurs. Rapid cooling during heat treatment is effective in improving yield strength, but at the cooling rate (500°C/Hr or less) shown in Japanese Patent Application No. 168649/1980,
It has little effect. <Means for Solving the Problems> The present invention has been made in view of the above problems.
By improving the chemical composition and heat treatment of spheroidal graphite cast iron Kariba roll, we aim to provide a spheroidal graphite cast iron Kariba roll that has unprecedented crack resistance and wear resistance, and a method for producing the same. The means is that the chemical composition is in weight percent: C: 3.0-3.8% Ni: 1.2-3.0% Si: 1.5-2.5% Cr: 0.05-0.6% Mn: 0.2-1.0% Mo: 0.1-0.8% P: 0.2% or less Mg: 0.02 to 0.1% S: 0.06% or less The balance is essentially Fe, and the base structure of the layer used for the roll is a two-phase fine mixed structure of ferrite and dense pearlite, and the area ratio of ferrite is 1-10%,
And the proof stress is 55Kg/mm2 ^or more, and the hardness of the bottom of the caliber is Hs2 higher than the hardness of the flange at the same longitudinal position.
~10 high, and the amount of cementite in the matrix structure at the bottom of Caliba is approximately the same as the amount of cementite at the same diameter position of the flange part, and the manufacturing method of the Caliba roll is as follows. Pre-roll casting material of spheroidal graphite cast iron having the above chemical composition,
Before the heat treatment, rough processing is performed in advance so as to leave 1 to 20 mm of excess thickness on the caliber part, and then at least the roll layer is held at 800 to 860°C for 20 hours or less, and then cooled at 500 to 1500°C/Hr. It is characterized by cooling at a speed of 400 to 600°C and then isothermal transformation at 520 to 620°C. <Function> The spheroidal graphite cast iron Calibarol having a characteristic chemical composition according to the first invention has a base structure of its used layer as a two-phase fine mixed structure of minute ferrite and dense pearlite, and has a ferrite content of 1 to 10% (area ratio). ), it is strong and has a high yield strength of 55 kg/mm ² or more, and has excellent crack resistance. Also,
In addition to making the structure of the layer used the above-mentioned two-phase fine mixed structure, the amount of cementite in the structure at the bottom of the caliber, which requires particularly wear resistance during hot working, is determined by changing the amount of cementite in the core at the same diameter position of the flange. Since it is approximately the same as that, the cementite at the bottom of the caliber is not excessive, so the hardness is improved without deteriorating the crack resistance, and the wear resistance is excellent. In addition, in the manufacturing method of the second invention, before heat treatment, a plain roll cast material of spheroidal graphite cast iron having a specific chemical composition is rough-processed in advance by leaving an extra thickness of 1 to 20 mm at the caliber part, and then Afterwards, at least the layer used in the roll is maintained at 800 to 860° C. for 20 hours or less, so that the base structure of the layer used in the caliber portion and the flange portion can be made into a mixed structure of ferrite and austenite with a small amount of ferrite. Then,
Since it is cooled to 400 to 600℃ at a cooling rate of 500 to 1500℃/Hr, it is possible to prevent cracking defects and the generation of coarse pearlite, and it also prevents the precipitation of ferrite during cooling, reducing the ferrite area ratio in the matrix to 1. It can be ~10%. If the ferrite area ratio is low,
It is possible to prevent a decrease in the yield strength of the roll material and improve crack resistance. And 520~620℃
Since the austenite undergoes isothermal transformation, dense pearlite, which has a hardness at room temperature and hardness at high temperatures that are approximately proportional to each other, and which is the basis of wear resistance, toughness, and high yield strength, is precipitated from austenite. Further, the amount of cementite in the layer used does not increase due to the above-mentioned heat treatment and exists only in dense pearlite, so there is no disadvantage in terms of crack resistance and accident resistance. <Example> Hereinafter, the spheroidal graphite cast iron Caliba roll of the present invention will be explained together with its manufacturing method. First, the range of components of the spheroidal graphite cast iron material of Calibarol of the present invention will be explained below along with the reason for its limitation. In addition, the unit is shown in weight%. C: 3.0-3.8% C is directly involved in the formation of cementite and graphite. If it is less than 3.0%, the amounts of cementite and graphite will be insufficient, the heat crack dispersion effect will be insufficient, and the characteristics of spheroidal graphite cast iron cannot be fully exhibited. However, if the amount is too large, the material becomes brittle, so the upper limit is set at 3.8%. Si: 1.5 to 2.5% Si influences the ratio of graphite to cementite, and if it is less than 1.5, the amount of cementite becomes excessive.
On the other hand, if it exceeds 2.5%, there will be too much graphite, which will cause softening and weaken the base structure.
Undesirable. Mn: 0.2-1.0% It suppresses the damage caused by S and is effective in increasing hardness. 0.2% or more is required for this purpose, but 1.0%
If it exceeds this value, the material becomes hard and brittle. P: 2.0 or less P makes the material brittle, so it should be 0.2% or less.
Note that since P improves the fluidity of the molten metal and increases wear resistance, it may be added in an amount of 0.01% or more, with an upper limit of 0.2%. S: 0.06% or less Similar to P, S makes the material brittle and also inhibits the spheroidization of graphite, so the content should be 0.06% or less. Ni: 1.2-3.0% Has the effect of improving graphitization and hardenability.
If it is less than 1.2%, no hardening effect can be expected on large-capacity objects such as rolls. On the other hand, if it exceeds 3.0%, long-term isothermal transformation will be required to obtain pearlite, which is the object of the present invention, resulting in a cost disadvantage. In addition, it is possible to precipitate bainite and martensite in materials with a content exceeding 3.0%, and the hardness increases greatly, but they are unstable to heat color and are not suitable for use as hot rolling rolls. Cr: 0.05-0.6% Mainly intended for cementite stabilizing effect, and if it is less than 0.05%, this effect cannot be expected. On the other hand, if it exceeds 0.6%, the amount of cementite becomes excessive and the material becomes brittle. Mo: 0.1 to 0.8% It has the effect of stabilizing cementite and improving hardenability, but if it is less than 0.1%, the effect cannot be expected. On the other hand, if it exceeds 0.8%, the amount of cementite becomes excessive and the effect of improving base hardness is saturated, which is economically disadvantageous. Mg: 0.02-0.1% Necessary for graphite spheroidization; less than 0.02% causes poor spheroidization. On the other hand, if it exceeds 0.1%,
In addition to making casting defects more likely to occur, the increased amount of cementite leads to embrittlement of the material. The roll material of the present invention contains the above-mentioned components and the remainder is substantially Fe, but the following components can be included as needed. Al: 0.3% or less In the case of spheroidal graphite cast iron materials, inoculation treatment is generally performed, but in that case, in order to enhance the effect of Al, the quality of the material is improved by containing 0.3% or less. Ca: 0.1% or less Since it has the same effect as Al, the material quality is improved by containing 0.1% or less. Rare earth element: 0.1% or less Effective as a graphite spheroidization stabilizer, containing 0.1% or less improves material quality. The spheroidal graphite cast iron Caliba roll of the present invention made of such material will now be described in detail in the order of its manufacturing steps with reference to the heat treatment diagram shown in FIG. (1) Processing before heat treatment One feature of the present invention is that, prior to heat treatment, the spheroidal graphite cast iron pre-roll material obtained by casting is subjected to rough processing in advance by cutting and forming the caliber part. FIG. 3 shows, as an example, the rough processing state of a Caliba roll having a Diamond Caliba, where the solid line in the figure shows the rough processing state before heat treatment, and the dotted line in the figure shows the product dimensions after heat treatment. In this case, of course, the first part shown corresponds to the caliber part, and the third part corresponds to the flange part. When rough processing this caliber part, leave 1 to 20 mm (on one side) of its extra thickness (dimension d in the figure). In other words, the smaller the extra thickness is, the more effective it is, but if it is less than 1 mm, the surface decarburized layer during heat treatment cannot be completely removed in the final Caliba finishing process, while if it exceeds 20 mm, the surface cutting allowance will be too large, making it difficult to use in the present invention. This is because the effect becomes smaller. Note that there is no need to specify the extra thickness of the flange portion for the purpose of the present invention, but one example is generally about 3 mm of one side thickness. Further, the excess thickness of the journal portion 4 is not an important matter, and depending on the case, the flange portion and the journal portion may be heat treated without any processing. (2) Heat treatment Casting materials that have undergone rough processing as described above are
Heat treatment is performed according to the heat treatment diagram shown in the figure. The effects of each zone in the heat treatment diagram will be described below. A: This is a temperature raising process, and there are no particular restrictions other than adjusting it so as not to cause troubles such as cracking due to thermal strain. B: 800°C to 860°C. In this temperature range, the structure is ferrite + austenite (fine ferrite), and by subsequent cooling, a two-phase structure of ferrite and pearlite is formed, and the toughness can be greatly improved. If the temperature is less than 800°C, the amount of ferrite becomes too large and there is no problem in terms of toughness, but the yield strength decreases. on the other hand
When the temperature exceeds 860°C, all the steel becomes austenite and the toughness deteriorates. Regarding retention time, from a cost perspective,
20 hours or less is appropriate. C: 500-1500°C/Hr. In this part, austenite → ferrite + pearlite changes, but in order to suppress the precipitation of ferrite at temperatures below 800℃ and to achieve a yield strength of 55Kg/mm ² or more, the material with the chemical composition described above must be heated to 500℃/mm 2 or more.
More than hr is required. Even if the temperature exceeds 1500℃/Hr, the degree of improvement in material quality does not increase proportionally, and there are many difficulties in actual production. When treated at this cooling rate, the amount of ferrite is 1 to 10% (area ratio). Note that this cooling rate needs to be achieved in the entire layer used as shown in FIG. In order to achieve a cooling rate of 500 to 1500°C/Hr, methods such as spray water cooling are required. Furthermore, a method of rapidly heating and cooling only the layer to be used is also effective. D: 400°C to 600°C. If the temperature is lower than 400°C, cracking will definitely occur at the above cooling rate. When the temperature exceeds 600℃, the temperature rises further due to recuperation from inside the roll (620℃).
℃ or higher), pearlite becomes coarse. E: 520°C to 620°C. Although it is mainly intended for isothermal transformation, with the chemical composition mentioned above, pearlite transformation hardly occurs at 520°C (it takes a long time for pearlite transformation to occur). 620℃ is roughly the pearlite nose, and unless the pearlite is below 620℃, it will not become dense pearlite and no improvement in yield strength can be expected. Regarding the time, from a cost perspective, 50 hours or less is appropriate. In the present invention, after rough machining of the caliber part in advance,
By performing the heat treatment described above, the hardness of the roll-used layer such as the bottom of the caliber can be improved by Hs2 to 10 relative to the roll core at the same diameter position as the bottom of the caliber. In addition, since the amount of cementite in the caliber portion is approximately the same as the amount of cementite at the same diameter position in the flange portion, there is no increase in the amount of cementite.
There is no need for a decrease in crack resistance. by the way,
In order to improve the wear resistance of the caliber part, caliber casting is sometimes performed, but since this method is designed to increase the amount of cementite in the caliber part from the beginning, it has the disadvantage that deterioration of crack resistance cannot be avoided. be. In addition, before the heat treatment of the present invention, heat treatment for the purpose of decomposing cementite (as generally known,
Treatment at 900 to 1000°C for 1 to 20 hours) is also effective in improving toughness, but the characteristics of cementite decomposition heat treatment are as follows: (i) Cementite near the roll surface decomposes rapidly. (ii) There is little decomposition of cementite inside the roll (including the bottom of the roll). This is the same in the case of heat treatment with caliber (it is assumed that this is not due to temperature distribution but to a problem with the material, for example, the cementite inside the roll has a high Cr content). (iii) It is disadvantageous in terms of cost, and problems such as roll deformation and oxide film formation are likely to occur. (iv) When the Cr content exceeds 0.2%, the amount of cementite decomposition decreases rapidly. Therefore, the requirements for the applicable stand are lower due to the deterioration of the wear resistance of the flange side wall, which normally requires wear resistance, and the degree of improvement in crack resistance deeper than the bottom of the caliber, where crack resistance is required. should be considered accordingly. In addition, after the above prescribed heat treatment, strain relief annealing may be performed (high alloy type, large-shaped roll), but there is no particular special feature in this strain relief annealing, and as usual, 400 to 650 It is carried out at a temperature of °C. The heat treatment of the present invention described above will be explained based on specific data. A. Relationship between cooling rate and mechanical properties (1) Test material A sample was taken at a position 80 mm from the surface of a φ800 mm roll and used as a test material. The chemical composition of the sample material was as shown in Table 1.

[Table] (2) Heat treatment In Figure 2, B: 850°C x 3 hours C: Various cooling rates D: 460°C E: 550°C x 12 hours (3) Results are shown in Figure 4. From this figure, the degree of increase in yield strength and hardness is large at temperatures above 500℃/Hr;
It can be seen that there is almost no change in the material quality even when the temperature exceeds 1500℃/Hr. The pearlite nose of this material is approximately 620℃
However, when it is cooled to 620°C, it is considered to be mostly austenite (with a small amount of ferrite remaining) when it is 300°C/Hr, transformation is completed at 500°C/Hr, and austenite remains at 1500°C/Hr. It will be done. B Relationship between cooling rate and metal structure (1) Test material A sample was taken at a position 50 mm from the surface of a φ780 mm roll and used as a test material. The chemical composition of the sample material was as shown in Table 2.

[Table] (2) Heat treatment In Figure 2, B: 830℃×10Hr C: 160℃/Hr and 800℃/Hr D: 500℃ E: 520℃×40Hr (3) Results Cooling rate of 160℃/Hr Micrographs of the samples (comparative samples) are shown in FIGS. 5a, b, c, and micrographs of the 800° C./Hr samples (example samples) are shown in FIGS. 6a, b, c. The enlargement magnification is x50 for a, x400 for b, and x2000 for c. From the metallographic micrograph, it is clearly seen that the example sample of the present invention has less ferrite than the comparative example sample, and that the pearlite itself is also formed into a dense layer. Next, specific examples and comparative examples will be listed and explained. (1) The spheroidal graphite cast iron Caliba roll materials of Examples 1 to 5 and Comparative Examples 1 to 3 shown in Table 3 were subjected to Caliba rough processing in advance, and each heat treatment zone in Fig. 2 was heat treated under the following conditions. Record B: 850℃×6Hr C: D shown in Table 4: 450℃ E: 580℃×18Hr

【table】

[Table] (2) Mechanical properties of the layer used in the Kariba bottom
Shown in the table.

[Table] (3) Actual Machine Test Comparative Example 1 had high elongation but low yield strength, and a breakout accident occurred due to headband cracks. This is due to the low cooling rate. In Comparative Example 2, the Cr content was too high, resulting in low elongation and a problem of heat cracking. In Comparative Example 3, since the Ni content was too low, the yield strength could not be increased and the problem of headband cracking occurred. As a result of applying Examples 1 to 5 of the present invention according to the required characteristics of the stand, no particular problems occurred in terms of both crack resistance and abrasion resistance, and they were well received. <Effects> As described above, the spheroidal graphite cast iron Caliba roll of the present invention has excellent crack resistance and wear resistance, so even if it is used in a hot rolling rough stand with severe processing conditions, it will not cause cracks due to thermal fatigue. It is possible to reduce the wear associated with rolling and rolling as much as possible, extend the life of the roll, and greatly contribute to improving productivity. In addition, the method for manufacturing a KARIBA roll according to the present invention ensures that the layer used for the roll including the KARIBA part can be reliably and easily formed into a base structure with a dense pearlite structure while preventing the precipitation of ferrite as much as possible. Therefore, it is an excellent method for producing Calibarol.

[Brief explanation of drawings]

FIG. 1 is a partial vertical cross-sectional view of a Caliba roll, FIG. 2 is a heat treatment diagram according to the manufacturing method of the present invention, FIG. 3 is an explanatory diagram showing an example of a rough processing state of a Caliba roll, and FIG.
The figure is a graph showing cooling rate and mechanical properties.
Figures a, b, and c are metallographic micrographs at a cooling rate of 160°C/Hr. Figure 6 a, b, and c are cooling rates.
A metallographic micrograph at 800°C/Hr is shown. 1, 1a, 1c, 1d...Kariba part, 2...Kariba bottom part, 3...Flange part, 5...Used layer, 6
...The core.

Claims (1)

[Claims] 1 Chemical composition in weight%: C: 3.0-3.8% Si: 1.5-2.5% Mn: 0.2-1.0% P: 0.2% or less S: 0.06% or less Ni: 1.2-3.0% Cr: 0.05-0.6% Mo: 0.1-0.8% Mg: 0.02-0.1% The remainder is essentially Fe, and the base structure of the layer used for the roll is a two-phase fine mixed structure of ferrite and dense pearlite, and the area ratio of ferrite is 1-10%,
And the proof stress is 55Kg/mm2 ^or more, and the hardness of the bottom of the caliber is Hs2 higher than the hardness of the flange at the same diameter position.
A spheroidal graphite cast iron Karivar roll with excellent crack resistance and wear resistance, characterized in that the amount of cementite in the matrix structure at the bottom of Kariba is approximately the same as the amount of cementite at the same diameter position of the flange part. 2 Chemical composition in weight%: C: 3.0-3.8% Si: 1.5-2.5% Mn: 0.2-1.0% P: 0.2% or less S: 0.06% or less Ni: 1.2-3.0% Cr: 0.05-0.6% Mo: 0.1 to 0.8% Mg: 0.02 to 0.1% The balance is essentially Fe, and the pre-roll casting material of spheroidal graphite cast iron is rough-processed in advance by leaving an extra thickness of 1 to 20 mm at the caliber part before heat treatment. ,
After that, at least the roll layer is heated to 800~860℃
Spheroidal graphite with excellent crack resistance and abrasion resistance, which is maintained for 20 hours or less, cooled to 400 to 600 degrees Celsius at a cooling rate of 500 to 1,500 degrees Celsius, and then subjected to isothermal transformation at 520 to 620 degrees Celsius. Method of manufacturing cast iron Kaliba rolls.