JPH0448864B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a composite roll for hot rolling in which an outer layer and an inner layer (core) are welded and integrated, particularly an outer layer material of a composite roll used in a roughing stand of a hot strip mill. This invention relates to a high chromium cast iron material with good slip resistance used as a material. (Prior art) The properties required for the outer layer (rolling layer) of a composite roll used in the roughing stand of a hot strip mill include wear resistance, accident resistance, roughness resistance,
For example, slip resistance (biting resistance) can be mentioned, but in recent years, the trend in thin plate rolling has been to improve quality and save energy. There is a need for improvement. Conventionally, the outer layer materials include special cast steel, adamite material, hardened forged steel material, graphite steel, spheroidal graphite cast iron,
Grain materials are used (some are also used as single roll materials). Furthermore, recently, in some mills, high chromium cast iron rolls used as rolling rolls before the finishing stand are used as they are in the roughing stand. (Problems to be Solved by the Invention) However, the hardness of the adamite material and the steel-based outer layer material is Hs55 or less, and although the slip resistance is good, there are problems in wear resistance and roughness resistance. . In addition, Hs50-75 is used in some places for spheroidal graphite cast iron, but it has poor wear resistance and crack resistance.
Some companies use Hs65-75, but they have problems with crack resistance and rough skin resistance. On the other hand, high chromium cast iron rolls used as rolling rolls in the front stage of the finishing stand have problems in that they tend to slip against the rolled material, have poor bite into the rolled material, and are prone to fatigue cracks. The present invention has been made in view of such problems, and provides an outer layer material for a composite roll for hot rolling that has excellent wear resistance and roughness resistance, and also has excellent slip resistance and crack resistance. The purpose is to (Means for Solving the Problems) In order to achieve the above object, the present invention makes the outer layer material of the composite roll as follows: C: 0.91 to 1.2% Ni: 0.5 to 2.0% Si: 0.5 to 1.5% Cr: 8-16% Mn: 0.5-1.5% Mo: 0.9-2.5% P: 0.08% or less S: 0.06% or less The balance consists of Fe and normal impurities, and the structure is composed of secondary carbide, tempered martensite, and bainite structure. It has a mixed structure of matrix and _M7C3 type eutectic chromium carbide, and has a hardness of Hs60~85 _.
Made of low carbon, high chromium cast iron material. (Example) First, the reason for limiting the components of the outer layer material of the present invention will be described. Hereinafter, the unit is weight %. C: 0.91 to 1.2% C is a high hardness (Fe, Cr) ₇ C It is determined by the desired amount of carbide while maintaining a balance with the Cr content within the range that stabilizes the type ₃ carbide.
If it is less than 0.8%, the amount of carbide will be too low and the wear resistance will be insufficient, so it is better to contain it at 0.91% or more.On the other hand, if it exceeds 1.2%, the amount of carbide will be too large and the slip resistance, roughness resistance, and crack resistance will deteriorate. descend. Si: 0.5-1.5% Si is an element necessary for deoxidizing the molten metal, and must be contained at least 0.5%. However, Si
Since C lowers the solubility of C in austenite, excessive content will result in insufficient carbide formation, making it difficult to obtain hardness and deteriorating mechanical properties, so it should be kept at 1.5% or less. Mn: 0.5-1.5% Mn is required at least 0.5% for deoxidizing the molten metal and removing harmful S. However, if it exceeds 1.5%, mechanical properties, particularly toughness, will deteriorate significantly. P: 0.08 or less P is an element that is preferably as small as possible in the roll material, and should be kept at 0.08% or less to prevent embrittlement of the material. S: 0.06% or less Like P, the lower the S content, the more desirable it is, and the content should be 0.06% or less to prevent embrittlement. Ni: 0.5-2.0% Ni is contained to improve hardenability and actively adjust hardness, and its content is 0.5%.
If the content is less than 2.0%, there will be no sufficient effect, while if the content exceeds 2.0%, it will stabilize austenite and increase residual austenite, making it difficult to adjust the hardness by heat treatment after casting. Cr: 8 to 16% Cr is contained to improve toughness and wear resistance, but in order to obtain high hardness (Fe, Cr) ₇ C ₃ type carbide, it is necessary to balance it with the C content. There is. Regarding the C content of the present invention, if the C content is less than 8%, it is not possible to obtain a sufficient amount of the above-mentioned M ₇ C ₃ type carbide, whereas if the C content exceeds 16%, the amount of M ₂₃ C ₆ type carbide increases. . Since this carbide has a lower hardness than the M ₇ C ₃ type carbide, it becomes difficult to obtain sufficient wear resistance. Mo: 0.9 to 2.5% Mo increases tempering resistance and at the same time enters into the carbide and is effective in increasing carbide hardness, but if the content is less than 0.8%, this effect is small, so if it is contained at 0.9% or more. It is better to let 2.5
If the content exceeds %, austenite becomes stabilized and hardness becomes difficult to obtain. In addition to the above components, the outer layer material according to the present invention contains the remaining components.
Formed with Fe and normal impurities. The outer layer material is cast into a composite roll and then subjected to high-temperature diffusion annealing and quenching and tempering heat treatment, as in the case of conventional high chromium cast iron. That is, in high chromium steel with a high C content, the matrix exhibits an austenitic structure when as cast, but it is not suitable as a roll material in terms of roughness resistance and wear resistance. In order to transform this austenite structure into a martensite or bainite structure, it is necessary to destabilize this austenite. That is, by maintaining the temperature at Ac ₁ point or higher, Cr carbide precipitates in the matrix, and the C and Cr concentrations in the matrix decrease. The form of Cr carbide varies depending on the holding temperature, and it tends to take the M ₇ C ₃ type form at temperatures above 950°C. In order to obtain a hardened structure, the cooling rate is Ps in CCT.
A critical cooling rate or higher that does not cross the line is required.
In the case of high chromium systems, the Ps line does not cross even with relatively slow cooling, but a cooling rate of 250°C/Hr is required. Furthermore, tempering is preferably performed at a temperature of 400 to 600°C in view of the balance between obtaining a thermally stable structure and the hardness of the product. In addition, in order to reduce the residual stress of the roll in balance with the thermal stress generated in the roll, the appropriate temperature for the strain relief heat treatment is 400 to 600°C. As a result of the above heat treatment, the structure of the outer layer material becomes a mixed structure of matrix and eutectic carbide (M ₇ C ₃ type), and the matrix structure consists of precipitated secondary carbide (M ₇ C ₃ type), tempered martensite, and bainite. It has a hardness of Hs60-85. Rolls for rough stands are particularly required to have wear resistance, roughness resistance, and slip resistance, but if Hs is less than 60, the wear resistance is poor, while if Hs exceeds 85, a quenched structure remains, making it difficult to be affected by heat. Roughness resistance becomes an issue when using large rough stands. Furthermore, the inventor has confirmed that slip resistance is influenced by C content rather than hardness;
No problems arise with the low C% of the present invention. The inner layer material (shaft core material) of the composite roll using the outer layer material of the present invention is appropriately selected from ductile cast iron, high grade cast iron, graphite steel, etc., which have excellent toughness.
For example, suitable ductile cast iron includes the following components (wt%): C: 3.0 to 3.8% Ni: 2.0% or less Si: 1.6 to 3.0% Cr: 1.5% or less Mn: 1.0% or less Mo: 1.0% or less P: 0.1% or less Mg: 0.02 to 0.1% S: 0.02% or less The balance is real Among the above components, Cr is desirably as low as possible from the viewpoint of the material quality of the inner layer material, but some degree of mixing and diffusion cannot be avoided in order to melt a part of the inner surface of the outer layer and weld it to the shaft core material. In the above ingredients, the graphitization promoting element Si is 1.6
~3.0%, so in this Si range it is 1.5%
It is allowed up to If it exceeds 1.5%, Si will be contained in a larger amount and cementite will be excessive, resulting in a significant deterioration in toughness. In addition, other reasons for limiting the components are described below. C: 3.0-3.8% If C is less than 3.0%, it will be mixed in from the outer layer
Cr causes the material to chill, which leads to a significant decrease in toughness, and graphitization of more than 3.8% progresses, resulting in insufficient strength of the inner layer material and a decrease in neck hardness, making the neck part more likely to become rough during use. Si: 1.6-3.0% If Si is less than 1.6%, graphitization is poor and a lot of cementite precipitates, leading to deterioration of the strength of the inner layer.
If it exceeds 3.0%, graphitization will be promoted and strength will deteriorate. Mn: 1.0% or less Mn combines with S and forms MnS to eliminate the adverse effects of S, but if it exceeds 1.0%, the material deteriorates significantly. P: 0.1% or less It increases the fluidity of the molten metal, but since it makes the material brittle, it is preferable to have it as low as possible, and from a cost perspective it should be 0.1% or less. S: 0.02% or less Similar to P, the lower the content, the more desirable it is, and since the inner layer material is ductile cast iron, in order to spheroidize graphite, it is necessary to reduce S, which is an element that inhibits spheroidization, to 0.02% or less. Ni: 2.0% or less Added for stabilization and toughness of graphite, but 2.0% or less
There is no noticeable effect even if it exceeds 2.0%, and from a cost perspective, it should be kept at 2.0% or less. Mo: 1.0% or less Mo is undesirable because it inhibits graphite crystallization, but it should be kept at 1.0% or less as long as it does not cause any actual damage. Mg: 0.02 to 0.1% 0.02% is necessary for graphite spheroidization, and if it is less than this, spheroidization will be poor and strong ductile cast iron will not be obtained. However, if it exceeds 0.1%, it is undesirable in terms of Mg's chilling effect and dross. In addition, when using the inner layer material of ductile cast iron, in order to reliably prevent Cr from mixing and diffusing from the outer layer to the inner layer, increasing the Cr content of the inner layer material and deteriorating its toughness, the following specific composition (weight It is effective to interpose an intermediate layer made of cast iron material (%) between the two. C: 1.0 to 2.5% Ni: 1.5% or less Si: 0.5 to 1.5% Cr: 3 to 10% or less Mn: 0.5 to 1.5% Mo: 1.0% or less P: 0.1% or less The balance is substantially Fe S: 0.1% or less The reasons for limiting the components of the intermediate layer are described below. C: 1.0 to 2.5% C: If the outer layer Cr is melted by the middle layer molten metal and mixed completely uniformly, C will be the middle layer molten metal.
Cr will be 3-10% in total. If the C content is less than 1.0%, the casting temperature of the intermediate layer will be high, and the outer layer will be easily melted, causing the Cr% to further increase.The purpose of the intermediate layer is to prevent Cr from diffusing into the shaft core material. Moreover, if the C content exceeds 2.5%, the amount of carbides increases, and the intermediate layer itself lacks toughness, so there is no point in providing the intermediate layer. Therefore, C should be 1.0 to 2.5%. Si: 0.5-1.5% 0.5% is necessary for deoxidizing the molten metal. If it exceeds 1.5%, it becomes brittle and causes deterioration of mechanical properties. Mn: 0.5-1.5% 0.5% is necessary because it has the same effect as Si and converts into MnS to eliminate the negative effects of S.
If the content exceeds 1.5%, the effect will be saturated,
This leads to deterioration of mechanical properties. P: 0.1% or less P increases the fluidity of the molten metal, but in roll materials it reduces the toughness of the material, so it should be kept at 0.1% or less. S: 0.1% or less Like P, it makes the roll material brittle, so the content should be 0.1% or less without causing any actual damage. Ni: 1.5% or less Ni is included to impart hardenability and toughness, and even if it is not actively added, it will be mixed in from the outer layer and become 0.3% or more, but if it is contained up to 1.5%, there will be no problem. It has this effect. However, if the content exceeds 1.5%, the hardenability is good, but the matrix becomes too hard, which is undesirable from the viewpoint of toughness and residual stress. Therefore, the Ni content should be 1.5% or less. Cr: 3 to 10% or less A lower Cr content is desirable from the point of view of providing the intermediate layer, but the chemical composition of the ladle before casting the intermediate layer is preferably less than 1.0%, which is easy to control industrially. .
In this case, the composition after casting is such that it enters from the outer layer.
When Cr is added, the Cr content increases to 3 to 10%.
If it exceeds 10%, the material deterioration of the intermediate layer itself will become significant. Furthermore, in order to suppress the Cr content of the inner layer material to 1.5% or less, the Cr content of the intermediate layer must be adjusted to a range of 3 to 10% under casting conditions. Therefore, the Cr content is set to 3 to 10%. Mo: 1.0% or less Mo has the same effect as Ni, and if it is contained in an amount exceeding 1.0%, the intermediate layer will become too hard, so this content should be 1.0% or less. When the intermediate layer described above is used, not only can mixing and diffusion of Cr from the outer layer to the inner layer be reliably prevented, but also embrittlement at the boundary can be effectively prevented. That is, when no intermediate layer is used, the boundary part has an intermediate composition between the low C% of the outer layer and the high C% of the inner layer (relatively high C%, high C%).
Cr%), carbide crystallizes in a layered manner at the boundary between the outer layer and the inner layer, and the boundary becomes brittle, but the crystallization of this layered carbide can be prevented by the interposition of the intermediate layer. Next, a method for manufacturing the composite roll will be described. Generally, as a method for manufacturing composite rolls, it is easy to use centrifugal casting. That is,
As shown in Fig. 1, first, the outer layer material molten metal is poured into a predetermined centrifugal force casting mold to centrifugally cast the outer layer A, and in some cases, the intermediate layer is cast on the inner surface of the outer layer A by centrifugal force. After that, as shown in Fig. 2, the mold is stood vertically or inclined, and a suitable molten metal of the inner layer material with high toughness is poured into the mold having the outer layer A, and the outer layer A and the inner layer B are welded. An integrated composite roll is cast. Thus, an integral composite roll is obtained in which the inner layer is made of a material with excellent breakage resistance, while the outer layer has excellent slip resistance, abrasion resistance, etc. In Fig. 1, 1 is a centrifugal casting mold;
2 is a sand mold for forming a neck portion, 3 is a rotating roller, 4 is a drive motor, 5 is a pouring gutter, and 6 is a ladle. In addition, in Fig. 2, 7 is a surface plate, and 8 is a weir bowl. ing. Next, specific examples will be listed and explained. Example 1 Manufacturing example of a two-layer composite roll with a body diameter of 900 mm and a body length of 1500 mm (total length 3800 mm) (1) A centrifugal casting die with a 3 mm thick resin side coating formed on the inner surface was made into a G No. As shown in Fig. 1, the molten metal for the outer layer shown in Table 1 was cast at 1530°C to a thickness of 100 mm. (2) After 22 minutes, stop the rotation of the mold and stand it vertically as shown in Figure 2. After 27 minutes from the start of outer layer casting, pour the molten metal of the inner layer material (ductile cast iron) shown in Table 1.
12Ton was cast.

[Table] (3) Three days after casting, the mold was taken apart and after rough processing,
Diffusion annealing was performed at a high temperature above the transformation point, followed by quenching and tempering heat treatment. (4) After being machined to predetermined dimensions, the outer layer thickness of the product roll was 70 to 73 mm, and the surface hardness was Hs76 to 78. In addition, the Cr content in the inner layer increased by 0.6% compared to the molten metal component because the inner surface of the outer layer was melted by an average of about 15 mm, and some of the Cr in the outer layer was mixed and diffused into the inner layer.
Cr: 0.66%. (5) After finishing the composite roll, it was used in an actual roughing stand of a hot strip mill, and results of 7000 to 7200 Ton/mm were obtained. The average rolling performance of conventional Adamite rolls (around Hs52) is 4000Ton/mm, so this roll has an approximately 1.8 times improvement in Ton/mm compared to the conventional roll.
This led to a significant improvement in roll consumption. In addition, there were no problems with slip resistance or biting property compared to Adamitrol, and there were no problems with rough skin or cracks. Furthermore, the roll was good, with no dents caused by scale entrainment that often occur in rough-stand Adamite rolls due to its high hardness. Example 2 Manufacturing example of a three-layer composite roll with a body diameter of 900 mm x body length of 1500 mm (total length 3800 mm) (1) Under the same manufacturing conditions as in Example 1, the molten metal of the outer layer material shown in Table 2 was made to have a thickness of 100 mm. After casting into
When a part of the inner surface of the outer layer was in an unsolidified state (14 minutes after the start of casting of the outer layer), the molten metal of the intermediate layer material shown in Table 2 was continuously cast onto the inner surface of the outer layer to a thickness of 25 mm. (2) After the intermediate layer was completely solidified (30 minutes after the start of casting the outer layer), the rotation of the mold was stopped, the mold was stood vertically, and the molten inner layer material shown in Table 2 was poured.

[Table] (3) Three days after casting, the mold was taken apart and after rough processing,
Diffusion annealing was performed at a high temperature above the transformation point, followed by quenching and tempering heat treatment. (4) After being machined to predetermined dimensions, the outer layer thickness of the product roll was 69 to 73 mm, the intermediate layer thickness was 23 to 26 mm, and the outer layer surface hardness was Hs77. Also,
The Cr content in the intermediate layer was 4.67% due to mixing and diffusion of Cr from the outer layer, and that in the inner layer was 0.33%. (5) A round bar test piece was taken from the roll so that the boundary surface of each layer was at 45 degrees with respect to the axial direction, and the compressive strength was examined. As a result, the compressive strength in the 45 degree direction at the boundary between the outer layer and the intermediate layer in this example was 187.1 Kg/mm ² , and that at the boundary between the intermediate layer and the inner layer was 175.7 Kg/mm ² . For comparison, a two-layer composite roll was cast using the molten metal for the outer layer material and the molten metal for the inner layer material, in which both were directly welded. In this case, the compressive strength in the 45-degree direction at the boundary between the outer layer and the inner layer was approximately 145 kg. / ^mm2 ,
This value was considerably lower than the compressive strength of the previous example. Further, during the strength test, in the examples of the present invention, no slipping phenomenon at the boundary caused by the embrittlement of the boundary observed due to direct welding of the outer layer and the inner layer was observed. (Effects of the Invention) As explained above, the high chromium cast iron according to the present invention has sufficient (Fe, Cr) ₇ C and is stable even at high temperatures of type ₃ , despite having a low C content of 0.8 to 1.2%. Other components are controlled within specific ranges to crystallize high-hardness carbide, and the matrix is formed of secondary carbide, tempered martensite, and bainite structure, so it has the resistance required for hot rolling. Slip resistance can be improved without impairing abrasion resistance and roughness resistance, and since the carbide content is not excessive, crack resistance can also be improved. As described above, the outer layer material of the present invention has excellent abrasion resistance and roughness resistance, as well as slip resistance and
Since it also has good crack resistance, it has great utility in the field of hot rolling where both of these properties are required, for example as an outer layer material for composite rolls of rough stands in hot strip mills.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of a centrifugal casting mold showing the manufacturing process of a composite roll.

Claims (1)

[Claims] 1% by weight: C: 0.91-1.2% Si: 0.5-1.5% Mn: 0.5-1.5% P: 0.08% or less S: 0.06% or less Ni: 0.5-2.0% Cr: 8-16 % Mo: 0.9 to 2.5% The balance consists of Fe and normal impurities, and the structure is a mixed structure of a matrix consisting of secondary carbide, tempered martensite, and bainite structure, and _{M7C3 type} eutectic chromium carbide, and has a hardness of Hs60. ~85
1. An outer layer material for a composite roll for hot rolling, which is a low carbon, high chromium cast iron material having excellent wear resistance, roughness resistance, crack resistance and slip resistance.