JPH0364590B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a wear-resistant casting useful as a rolling roll or the like. [Prior Art] The body of a rolling roll is required to have excellent wear resistance from the viewpoint of improving durability and improving the surface quality of the rolled material. This is because if the wear resistance is poor, unevenness is likely to occur on the roll surface due to wear and roughness, and the unevenness will be transferred to the surface of the rolled material as roll marks, resulting in surface defects. In this case, it is necessary to frequently replace the rolls, which increases the frequency of rolling line operation interruptions, significantly reduces productivity, and requires roll repair (surface modification).
This is because the costs required will increase. Traditionally, cast iron materials such as chilled castings and nickel grain cast iron have been mainly used as materials for the bodies of rolling rolls, and measures to improve their wear resistance include high alloying, crystallization of centite and double carbides, etc. Efforts have been made to make it harder. Furthermore, when wear resistance is particularly required, the roll surface is formed of cemented carbide (WC-Co sintered material) instead of the above-mentioned cast iron material. [Problems to be Solved by the Invention] Rolls whose bodies are made of chilled castings or nickel grain cast iron are relatively inexpensive, but do not have sufficient wear resistance. There is a limit to the effect of improving wear resistance due to high alloying, etc. On the other hand, cemented carbide is extremely hard and has a high degree of wear resistance that surpasses that of cast iron rolls, but its production requires large amounts of expensive Co and WC, and its sintering process requires hot isostatic pressing. The drawback is that the manufacturing cost is extremely high because it requires (HIP) equipment and special technology. The present invention has been made in view of the above, and provides a method for producing iron-based alloy castings having a high degree of wear resistance desired as rolling rolls and the like. [Means and effects for solving the problem] The method for manufacturing a wear-resistant casting of the present invention has C: 1.5 to
5.0%, Si: 3.5% or less, W: 25.0-80.0%, Ni/
Fe: 5.5/94.5 to 35/65, the remainder being substantially Fe, or in the casting of iron-based alloys in which a part of the Fe is replaced with a seed or two or more elements, the molten metal has a grain size of 10
By using ~250μm tungsten carbide particles or tungsten titanium double carbide particles as an inoculant and adding 0.05 to 2.0% of the molten metal amount to crystallize primary tungsten carbide, the bulk crystallized tungsten carbide is uniformly dispersed in the matrix. It is characterized by the formation of a two-phase structure. The present invention will be explained in detail below. In iron-based alloys containing a large amount of W, tungsten carbide crystallizes in the molten metal during the cooling and solidification process during casting. The composition of the iron-based alloy molten metal in the present invention is
When applied to the C-W-Fe ternary phase diagram shown in the figure, the liquid phase surface is above the bottom line (1700℃→1200℃) (high carbon side), and tungsten carbide (WC) is crystallized as the primary crystal. I understand that. The crystallized tungsten carbide is extremely hard with a hardness (Hv) of about 2400, and by dispersing and mixing it in the matrix, the wear resistance of the casting can be greatly improved. In casting the above-mentioned iron-based alloy castings, it is desirable to prevent crystallization of iron and tungsten double carbide [(Fe, W) ₆ C]. (Fe, W) ₆ C double carbide has a hardness of approximately 1350 Hv, so it has less effect on improving wear resistance than tungsten carbide (Hv: approximately 2400), and due to the consumption of W and C due to its crystallization, tungsten This is because carbide crystallization is reduced accordingly. Furthermore, in addition to preventing the crystallization of (Fe, W) ₆ C, it is necessary to suppress the growth and coarsening of crystallized tungsten carbide. As the crystallized tungsten carbide grows and coarsens, its dispersibility in the matrix deteriorates, and when the crystallized tungsten carbide becomes connected to each other, the toughness of the casting decreases, and it also becomes difficult to use when cutting the casting, rolling roll, etc. During actual use as a machine, defects and
This is because falling off (accompanied by surface roughening) is likely to occur. In other words, in order to effectively exhibit the dispersion hardening effect of crystallized tungsten carbide, (Fe,
W) It is necessary to prevent crystallization of _6C double carbide and form a two-phase structure in which only crystallized tungsten carbide is uniformly dispersed. According to the method of the present invention, the above-mentioned iron-based alloy casting can be obtained as a casting having a two-layer structure in which massive crystallized tungsten carbide is uniformly dispersed in the matrix. The grain size and area ratio of the crystallized tungsten carbide vary depending on the amount of inoculation during casting, etc., but the grain size (circular conversion value) is approximately 5 to 100 μm.
and the area ratio (according to the 1mm grid intersection measurement method)
is approximately 15-75%. Figure 1 shows the structure of the casting. M is a matrix metal (martensitic phase), and P is a crystallized tungsten carbide. The crystallized tungsten carbide P is dispersed in the matrix as relatively fine lumps having a geometric crystal habit. The reason for limiting the chemical composition of the molten iron-based alloy in the present invention is as follows. C: C is an essential element for the formation of crystallized tungsten carbide (WC). If the content is less than 1.5%, bulk tungsten carbide will not crystallize, but continuous eutectic (Fe, W) ₆ C double carbide will crystallize. On the other hand, if it exceeds 5.0%, graphite crystallizes in the matrix and it becomes brittle. Therefore, it is set at 1.5 to 5.0%. Si: Si has the effect of deoxidizing the molten alloy and improving the flowability during casting, and also has the effect of preventing the crystallization of acicular tungsten carbides during the solidification process (crystallization leads to embrittlement of the casting) has. However, if it exceeds 3.5%, the matrix becomes extremely brittle, so the upper limit is set at 3.5%. W: W is an essential element for the formation of crystallized tungsten carbide (WC). To crystallize a sufficient amount of primary WC carbide, at least
Requires 25.0% content. However, when it exceeds 80.0%, the melting point of the alloy increases, making melting and casting difficult. Therefore, it should be 25.0 to 80.0%. Ni: Ni is an element that suppresses the crystallization of iron carbides such as cementite (Fe ₃ C), and prevents the bonding of C with Fe. By adding an appropriate amount, eutectic (Fe,
W) It is possible to crystallize only massive tungsten carbide without crystallizing ₆ double carbide. To obtain this effect, 5.5% of Fe [Ni/(Ni+
Fe) × 100 (%)] or more is replaced with Ni (Ni/Fe
= 5.5/94.5 or higher) is required. The effect increases as the amount of Ni increases. Preferably Ni/Fe
The ratio should be 22/78 or higher. However, increasing the amount of Ni
On the other hand, the increased amount of retained austenite leads to softening of the matrix. Ni/Fe ratio
If it exceeds 35/65, it becomes difficult to convert the matrix into martensite even if sub-zero treatment is performed.
Therefore, the upper limit is set at the Ni/Fe ratio of 35/65. In addition to the essential elements C, Si, W, and Ni, the above iron-based alloy contains one or more of Mn, Mo, Cr, Nb, V, Co, and other elements for the purpose of improving the material quality of the matrix. It can be made into a component composition containing. The content of each element is determined depending on its properties and purpose of addition, but generally Fe
Up to 10% (total amount) of the molten metal can be replaced with these elements, and as a specific example, 5.0
% Mn below 1.0% for matrix reinforcement
Mo less than 1.0%, Cr less than 1.0%, Nb less than 1.0%, 1.0
% V or less, 1.0% or less to improve high temperature strength
An example of this is the addition of Co or the like. In iron-based alloys containing these elements, 5.5 to 35% of the remaining Fe is
Ni (Ni/Fe=5.5/94.5 to 35/65). The present invention uses tungsten carbide (WC,
The molten metal is inoculated with fine powder of W ₂ C) or tungsten titanium double carbide ((W,Ti)C) as an inoculant. These fine powders added to the molten metal as an inoculant disintegrate and disperse in the molten metal and are uniformly distributed, and the uniformly distributed particles dissolve but do not diffuse, resulting in residual or crystallized nuclei. It acts as a catalyst and promotes uniformly dispersed crystallization of primary tungsten carbide. This inoculation prevents the crystallized tungsten carbide from becoming coarse and its lumps from becoming continuous, and forms a structure in which the crystallized lumps of tungsten carbide are uniformly dispersed in the matrix. The particle size of the inoculant powder was set to 10 to 250 μm because
Fine particles less than 10 μm will completely dissolve and diffuse into the molten metal, while coarse particles exceeding 250 μm will remain in the molten metal, and in either case, sufficient inoculation effect cannot be obtained. It is. Further, the amount of inoculation must be at least 0.05% (by weight) of the amount of molten metal. This is because if the amount of inoculation is smaller than that, the effect of promoting the dispersion and crystallization of crystallized tungsten carbide will be insufficient even if the nucleation effect is observed. The effectiveness increases as the amount of inoculation increases, but approximately
A sufficient inoculum effect can be obtained by adding up to 2.0%, and the effect does not change much even if the amount is increased beyond that. In addition, if the amount of inoculation is too large,
The heat absorption of the administered powder causes the molten metal to rapidly cool down and become viscous, causing casting defects, and if the fine powder remains undissolved, the initial two-phase structure cannot be obtained. Therefore, the upper limit of the amount of inoculation is 2.0% of the amount of molten metal. The inoculant can be added to the molten metal in the furnace immediately before or during tapping from the melting furnace, or in the ladle after tapping.
Further, it is also possible to adopt a method in which the molten metal is added during pouring of the molten metal into the mold, or a method in which the molten metal is sprinkled in advance into the mold. There are no special conditions or restrictions added to the casting of the above-mentioned iron-based alloys, and the target casting can be obtained, for example, as a solid columnar body by static casting, or as a hollow cylindrical body. You can also get it. When intended for rolls, wear resistance is a surface issue, so a hollow cylindrical body is cast, and a core material is cast in the hollow hole using another type of metal.
Alternatively, a concentric two-layer structure consisting of the outer layer 1 and the shaft core 2 may be formed by fitting and fixing a separately manufactured core material by shrink fitting or the like, as shown in FIG. In this way, the required material properties can be met while saving the amount of molten metal component elements and expensive tungsten carbide used as an inoculant. Furthermore, when a centrifugal force casting method is used to cast a hollow cylindrical casting, the crystallized tungsten carbide is centrifugally transferred due to the difference in specific gravity with the molten metal, thereby increasing the distribution density of the crystallized tungsten carbide in the surface layer. In addition, if problems such as casting defects or insufficient crystallization of crystallized tungsten carbide occur due to rapid cooling of the molten metal during casting, use a heat-retaining mold such as a ceramic mold to alleviate the temperature drop. It is also effective to perform casting by preheating to an appropriate temperature, for example, 400 to 900°C. [Example] [] Manufacture of test casting Molten iron-based alloy is melted in a high-frequency melting furnace, and inoculation is applied or omitted to form a ceramic mold (inner diameter
30mm, height 100mm) (casting temperature:
(1550°C), solid cylindrical test castings a to e were obtained. Test castings a to c are invention examples, and test casting d
and e are comparative examples (test casting d: insufficient Ni amount,
Test casting e: Insufficient Ni amount and no inoculation). The molten metal composition and inoculation conditions for each iron-based alloy are as follows. The inoculant is W ₂ C powder (particle size
(10 to 63 μm), and the mold was preheated to 800°C before use. Test casting a (invention example) (1) Molten metal composition (wt%) C: 4.0, Si: 0.58, Mn: 0.7, W: 40.0,
Cr: 0.5, Ni: 8.0, Fe: 46.22. (The amount of Ni is Fe
(equivalent to a 14.8% replacement of (2) Amount of inoculation: 1.0% of the amount of molten metal Inoculation timing: Added in the furnace just before tapping Test casting b (invention example) (1) Molten metal composition Same as test casting a (2) Amount of inoculation: 0.15 of the amount of molten metal % Inoculation time: Addition in ladle Test casting c (invention example) (1) Molten metal composition Same as test casting a (2) Inoculation amount: 0.05% of molten metal amount Inoculation time: Addition in ladle Test casting d (Comparative example) (1) Molten metal composition (wt%) C: 4.1, Si: 0.54, Mn: 0.8, W: 82.0,
Ni: 2.1, Cr: 0.88, Fe: 49.58. (The amount of Ni is Fe
(equivalent to a 4% replacement of (2) Inoculation amount: 1.0% of the molten metal amount (same as test casting a) Inoculation time: added in the furnace just before tapping (same as test casting a) Test casting e (comparative example) (1) Molten metal composition Test Same as test casting d (2) Inoculation None [] Structure of test castings The metallographic structure of each test casting a to e was observed and the following results were obtained. Test casting a (invention example) Fig. 1 shows the metal structure (magnification: 400) (corrosive liquid: Murakami reagent). When etched with Murakami reagent,
(Fe, W) ₆ C double carbide is corroded brown, so
It can be distinguished from tungsten carbide.
There is no (Fe, W) ₆ C double carbide in the matrix M, only massive crystallized tungsten carbide (X
According to line diffraction, a two-phase structure in which WC (all identified as WC) is dispersed is formed. The massive crystallized carbides are uniformly dispersed, with an average particle size of about 50 μm and an area ratio of about 40%. Test casting b (invention example) (Fe, W) It has a two-phase structure with no accompanying ₆ C double carbide, and the average grain size of the massive crystallized tungsten carbide is about 55 μm, the area ratio is about 40%, and the The dispersibility is good. Test casting c (invention example) (Fe, W) It has a two-phase structure with no accompanying ₆ C double carbide, and the average grain size of the massive crystallized tungsten carbide is about 70 μm, the area ratio is about 35%, and the The dispersibility is good. Test casting d (comparative example - insufficient Ni content) Fig. 2 shows the metal structure (magnification: 400) (corrosive liquid: Murakami reagent). In matrix M, in addition to massive crystallized tungsten carbide P, (Fe, W) ₆ C
A three-phase structure in which double carbide D) is mixed is formed. (Fe, W) ₆ C double carbide was determined by X-ray diffraction.
It is identified as Fe ₃ W ₃ C−Fe ₄ W ₂ C. The particle size of the double carbide is approximately 40 to 200 μm, and the area ratio is approximately 10%.
It is. The grain size of the massive crystallized tungsten carbide P is about 50 μm, and the area ratio is about 35%. Test casting e (comparative example - insufficient Ni content, no inoculation) Similar to test casting d, it has a three-phase structure in which (Fe, W) ₆ C double carbides are mixed, and its grain size and area ratio are It is almost the same as casting d. The grain size of the massive crystallized tungsten carbide is approximately 125 μm,
The interview rate is approximately 30%. [] Wear test Test pieces were taken from each of the test castings a to e, and a wear test was conducted using an Okoshi type rapid wear tester under the following conditions. In addition, test castings f and g were chilled castings (C: 3.99%, Si: 0.29%, Mn:
0.23%, Ni: 3.38%, Cr: 0.98%, Mo: 0.22
%, balance Fe) and Ni grain cast iron castings (C:
3.22%, Si: 0.75%, Mn: 0.68%, Ni: 4.38
%, Cr: 1.64%, Mo: 0.35%, balance Fe). Table 1 shows the specific wear amount Ws (mm ² /Kgf) obtained by the above wear test and the surface roughness (Rmax) of the test piece after the test. () Rotating wheel: SUJ2, hardness (H _RC ) 62, rotating wheel width 3.0 mm. () Wear rate: 3.4m/sec () Wear distance: 200m () Final load: 16.8Kg・f

〔Effect of the invention〕

The iron-based alloy casting manufactured by the present invention has a two-phase structure in which massive crystallized tungsten carbide is uniformly dispersed in a matrix. Due to the uniform dispersion effect of the bulk crystallized tungsten carbide, it exhibits good wear resistance and maintains a smooth surface condition for a long time when used as a rolling roll, for example.
Contributes to improving roll life and surface quality of rolled materials. Of course, its use is not limited to rolling rolls, but is useful as a wear-resistant material for various uses.

[Brief explanation of drawings]

Figures 1 and 2 are micrographs (magnification: 400) showing the metal structure of the casting, and Figure 3 is C-
FIG. 4 is an axial cross-sectional view showing an example of the cross-sectional structure of the roll. M: matrix, P: crystallized WC carbide, D:
Eutectic (Fe, W) ₆ C double carbide.

Claims (1)

[Claims] 1 C: 1.5-5.0%, Si: 3.5% or less, W: 25.0-
80.0%, Ni/Fe: 5.5/94.5 to 35/65, the balance is substantially Fe, or when casting iron-based alloys in which part of the Fe is replaced with one or more elements, grains are added to the molten metal. By using tungsten carbide particles or tungsten titanium double carbide particles with a diameter of 10 to 250 μm as an inoculant and adding 0.05 to 2.0% of the amount of the molten metal to crystallize primary tungsten carbide, the bulk crystallized tungsten carbide is uniformly formed in the matrix. A method for producing wear-resistant castings characterized by forming a two-phase structure dispersed in.