JP2001150122A - Manufacturing method of stock for cold/warm plastic working and its cold/warm plastic working method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a stock for hot/warm plastic working and its plastic working method to solve a problem of lowering of fatigue strength, etc., and strength reliability due to a carbide crack in cold/ warm plastic working. SOLUTION: A manufacturing method of a stock is so that a molten steel containing C/Cr in term of (Cr+15.5C) of 14.0-42.0 mass % is sprayed to droplets, is collected/solidified to be an ingot, a max length of the carbide in the structure is set to <=5 μm. A plastic working method is so that by using the stock by the manufacturing method, the deformation in elongation or compression having a cross section change rate of >=15 % is applied to at least to part by cold/warm plastic working at <=800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to, in particular, drive or motion parts and pump parts of automobiles and home appliances, for example.
Furthermore, parts that are formed and manufactured by cold or warm plastic working (hereinafter, referred to as cold and warm plastic working) such as structural parts and screw parts, dies for plastic working such as forging, etc.
The present invention relates to a method for manufacturing a cold and warm plastic processed product of a tool and a material used therefor.

[0002]

2. Description of the Related Art In recent years, environmental problems have been increasing, and demands for high heat resistance and high wear resistance have been increasing in drive systems and motion system components used in automobile engines and peripheral parts thereof. Conversion of materials to tool steels and the like with excellent wear resistance has begun. The method of forming and manufacturing parts can be broadly divided into cutting and plastic working. In cutting, it is difficult to reduce costs due to low processing efficiency and material yield. Since the strength cannot be improved by forming a fiber flow, the other plastic working is expected as a molding and manufacturing method capable of improving the strength while reducing the cost. Of these, cold and warm plastic working have both excellent surface properties and high dimensional accuracy. In a conventional structural steel system, a part or its material has been formed and manufactured by forging or drawing in a cold or warm state. However, recently,
A technical report and the like have been published regarding industrially enabling cold forging of JIS SKD11, which was considered impossible.

[0003]

According to the report of Natsume et al., “Transactions of the Japan Society of Mechanical Engineers, Material Mechanics Lecture Collection '90 P.323”, when the alloy tool steel is subjected to cold plastic working, carbides contained in the steel are found. Have been pointed out that they are cracked by cold plastic working, form voids (voids) in the steel, and the fatigue strength increases. This is a critical weakness for engines, peripheral components and molds that place the highest priority on reliability in terms of strength, which requires a high plastic working rate, as well as processing difficulties. This is one of the reasons why tool steel with a large amount of carbide is not popularized, despite its excellent heat resistance and wear resistance, as a cold and warm plastic processed product for the above applications where material strength is required. Was. The object of the present invention is to prevent carbide cracking during cold and warm plastic working,
A cold and warm plastic processed product and a cold and warm plastic processed material used to solve the reduction in reliability such as fatigue strength, which is lower in cost and has wear resistance or heat resistance. Is to provide.

[0004]

Means for Solving the Problems The inventor of the present invention has been studying the most suitable material for a cold and warm plastic processed product, and optimizing the material composition, the size of the carbide, and the manufacturing process. The present inventors have found that the reliability in terms of strength, such as the fatigue strength of a warm plastic processed product, can be greatly improved, and have reached the present invention. That is, in the present invention, C and Cr are converted to (Cr + 15.5)
By spraying molten steel containing 14.0 to 42.0 mass% in (C) on droplets and collecting and solidifying the droplets to form an ingot, the maximum length of carbides in the structure is reduced to 5 μm or less. A method for producing a material for cold or warm plastic working characterized by doing, and using a material for cold or warm plastic working according to the method for producing, by cold or warm plastic working at 800 ° C. or less, at least This is a cold or warm plastic working method characterized by partially applying elongation or compression deformation having a cross-sectional change rate of 15% or more. That is, the material for plastic working of the present invention is particularly effective under cold and warm plastic working conditions in which at least a part of the material is subjected to elongation or compression deformation of 15% or more in cross section at 800 ° C. or less. .

[0005] The material of the present invention may be an ingot in shape, but more preferably, a steel material having a shape suitable for mass production such as a bar, a plate, or a block by a known plastic working method such as rolling and forging. It is also desirable to improve the plastic workability by these plastic working methods. Also,
In the present invention, the composition of the raw material or the processed product is defined as
(Mo + 1 / 2W) to contain 15.0% by mass or less and V 5.0% by mass or less, and, by mass%, Si: 2% or less, Mn: 2 %
Hereinafter, Ni: 5% or less, Nb: 1% or less, Co: 12%
Hereinafter, one or both of those containing one or more of S: 0.2% or less are desirable. In the present invention, elongation or compression deformation with a cross-sectional change rate of 15% or more means that a material expands or deforms in a specific direction, the cross-sectional area perpendicular to that direction decreases by 15% or more, and the material deforms or compresses. Includes deformation in which the cross-sectional area perpendicular to that direction increases by 15% or more. The cold and warm plastic working according to the present invention includes various plastic working methods such as die forging, extrusion, drawing, rolling, and spinning.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, an important feature of the present invention is that the components of cold and warm plastically processed products or their materials, the size of carbides, and the manufacturing process have been optimized.
First, the present inventors set C and Cr as 1 (Cr + 15.5C) as preferable target materials for obtaining the effects of the present invention.
Those containing 4.0 to 42.0 mass%, preferably one or more of Mo and W (Mo + ／ W)
And those containing not more than 15.0% by mass and not more than 5.0% by mass of V were selected. These include various tool steels.

[0007] In order to improve the wear resistance of cold and warm plastic processed products, it is necessary to contain a large amount of carbide. The carbide for improving the wear resistance is mainly of a composition of M ₇ C ₃ and is obtained by containing C and Cr at (Cr + 15.5C) at 14.0% by mass or more. Further, when M ₂ C carbide, MC carbide, and M ₆ C carbide are present in the metal structure in addition to M ₇ C ₃ carbide, the wear resistance can be further improved. The M ₂ C carbide, MC carbide and M ₆ C carbide are formed by adding Mo, W and V. Furthermore, when these elements are added, heat resistance can be improved in addition to wear resistance. However, since the addition of an excessively large amount of alloying elements causes the material to become brittle, C and Cr are
(Cr + 15.5C) needs to be 42.0% by mass or less, and Mo and W are (Mo + 1 / 2W).
It is desirable that the content be 0% by mass or less and V be 5.0% by mass or less. Further, in order to keep the wear resistance by not adding the expensive elements W, Mo, and V or to reduce the amount of addition, C: 1.0% by mass or more, Cr: 6.0% by mass or more. It is desirable to use one or both.

[0008] Further, Si is added for imparting oxidation resistance and hardness, and Mn and Ni are added for imparting hardenability in the case of large products. Further, Ni is also added to improve the essential ductility of the matrix. Nb is easily bonded to C
By forming bC, the crystal grains are refined to improve the toughness, and NbC improves the wear resistance. Nb is added to obtain these effects. Co is added to increase high-temperature strength and hardness. S is added to improve the machinability when a part of the cold or warm plastic processed product is subjected to cutting and grinding. The addition of a large amount of these alloy elements is Ni, Co
Except for the above, the material becomes brittle, and addition of too much Ni or Co does not show the improvement corresponding to the addition, and lowers the cost and merit.

The amount of each additive is expressed in terms of mass%, Si: 2
%, Mn: 2% or less, Ni: 5% or less, Nb: 1%
Hereinafter, the content of Co is preferably 12% or less and the content of S is 0.2% or less, and one or more of these may be appropriately added.
Materials such as tool steels targeted by the present invention, in order to impart various properties depending on the manufacturing process and use of the material,
In addition to the above, one or more of Ti, Cu, N, Al, and REM may be contained in a total of 1% by mass or less, which does not hinder the effects of the present invention. As described above, the effect of the present invention is remarkably achieved in the above-mentioned tool steel or the like in which a large amount of carbide exists in the metal structure. Moreover, unless this large amount of carbide is present, a cold and warm plastically processed product having excellent wear resistance cannot be obtained.

Next, the most important feature of the present invention will be described. The most important feature of the present invention is that the maximum length of the carbide is 5 μm.
It is defined below and that the ingot is to be produced by a spray forming method as a starting material. It has been found that when the maximum length of the carbide exceeds 5 μm, for example, carbide cracks are generated by plastic working at a temperature of 800 ° C. or less and a cross-sectional change rate of 15% or more. As described above, this carbide crack
Voids (voids) are formed in the steel, and the variation in strength, such as the fatigue strength of cold and warm plastic processed products, is increased. This is a fatal weak point in the engine, its peripheral parts, and the mold, which is the problem to be solved by the present invention, which places the highest priority on reliability in terms of strength. Therefore, it is essential that the maximum length of the carbide be 5 μm or less.

In general, tool steel having excellent wear resistance is obtained by melting steel containing the above-mentioned alloy element using a melting facility such as an electric furnace and casting it into a mold to form a steel ingot. Then, the steel ingot is subjected to hot forging or the like and put to practical use. The size of the steel ingot at this time is industrially several hundred kg or more. The size of the carbide is greatly affected by the solidification rate of the steel ingot, and the carbide increases as the solidification rate decreases. However, even in a steel ingot of only 10 kg, carbide having a length of about 10 μm is contained in the metallographic structure of steel containing 14.0% by mass or more of (Cr + 15.5C) necessary for obtaining wear resistance. It is formed. In other words, in general melting and casting methods,
It is not possible to obtain a practical material having excellent wear resistance and preventing cracking of carbides during cold and warm plastic working.

A well-known method for obtaining fine carbides in steel is powder metallurgy. In this method, a molten metal containing a predetermined alloy element is formed into a powder having a diameter of about 1 μm or the like by using an atomizing method or the like, which is sealed in a can, and is subjected to hot isostatic pressing, for example, at 1000 atm. This is a method of producing a steel ingot by consolidating at a temperature of ° C. It is believed that this method can be used to produce starting materials such as tool steel, where the maximum length of carbides targeted by the present invention is 5 μm or less. However, the powder metallurgy method, in which the number of steps increases compared to the general melting method, increases the chances of contamination.Therefore, the steel produced by this powder metallurgy method has a lower degree of cleanliness than general melting materials. It is considered to be. Cold and warm plastic products with low cleanliness are likely to have large variations in strength, and are critical for engines, peripheral components, and molds where reliability is considered first. It is a weak point.

Further, the powder metallurgy method raises the price of starting materials because the process of producing steel ingots is complicated and takes a lot of time as described above. The use of this material reduces the willingness to adopt it in engines and its peripheral parts where cost reduction is the primary consideration, and there is no actual use. An object of the present invention is to provide a low-cost and high-performance product by changing the cutting process to cold and warm plastic working, so that powder metallurgy does not meet the object of the present invention.

The spray forming method is a method in which a molten metal is jetted from a nozzle in a protective atmosphere to form droplets, which are rapidly solidified and accumulated on a rotating base (collector) to produce an ingot. As a material using this manufacturing method, for example, a Si-Al alloy for a semiconductor package (patent WO97037) from OSPREMETALS
75), from SANVIK Corrosion resistant N for waste treatment plant
An i-alloy (patent WO9809751) and the like are introduced. However, there has been no report on a material for a cold or warm plastic work product which is an object of the present invention.

The present inventor has newly found that if this manufacturing method is adopted, it is possible to manufacture an ingot of a tool steel or the like having the above-described composition having a maximum carbide length of 5 μm or less, and has conceived the present invention. The spray forming method does not require a complicated manufacturing process such as the powder metallurgy method, and thus does not cause an increase in manufacturing cost.Moreover, since there is little opportunity for contact with a large amount of gas, powder having an oxide film is inevitably used. Higher cleanliness than compacting powder metallurgy. Therefore, the most significant feature of the present invention is that the starting material is manufactured by the spray forming method.

Further, in the present invention, the cold and warm plastic working to be applied to the raw material starting from the ingot is preferably performed at a working temperature of 800 ° C. or less, and at least a part thereof has a sectional change rate of 15% or more. Cold or warm plastic working to add elongation or compression deformation. It is a well-known phenomenon that, in hot plastic working, a metal phase efficiently flows into voids formed by carbide cracks to eliminate the voids. However, the inflow of this metal phase is 80
It hardly occurs in cold and warm plastic working at 0 ° C or less. In the present invention, since the crack of the carbide is prevented by setting the maximum length of the carbide in the structure to 5 μm or less, there is no need for the inflow of the metal phase, and therefore, the effect of the present invention is particularly within this range. Be demonstrated. This is the reason why the desired plastic working conditions are specified as described above.

Next, the reason for recommending elongation or compression deformation with a deformation cross-sectional change rate of 15% or more will be described. It is considered that carbide cracks occur because large plastic deformation occurs, and a carbide having a low plastic deformation ability cannot follow the deformation amount as compared with the metal phase. Cold and warm plastic working with a cross-sectional change rate of 10% or less hardly causes carbide cracks. However, in the case of a practical material, for example, a 2500 kg steel ingot produced using a general melting method, carbide cracks occur even in cold and warm plastic working with a cross-sectional change rate of 10%.
Therefore, a product having no cross-sectional change rate of 15% or more and having no carbide crack due to cold and warm plastic working has been impossible with conventional materials. Further, the engine, its peripheral parts, and the mold are generally rich in irregularities, and in the case of forming and manufacturing by plastic working, the entire or local strain rate is often 15% or more in terms of cross-sectional change rate. Therefore, the present invention exerts its value in the case where it is manufactured by cold and warm plastic working with a sectional change rate of 15% or more.

[0018]

Embodiments of the present invention will be described below. Table 1
Test materials were prepared from the molten tool steels having the respective compositions shown in Table 2 under the conditions shown in Table 2. That is, from the molten steel of each composition, a steel ingot having the weight indicated in the table was prepared by the spray forming method and the smelting method, and the forging ratio at 1100 ° C. was 4 for sample 1X, 10 for sample 2X, and 3X for sample. Were forged at 16. Here, X is each of the compositions a, b, and c in Table 1. After annealing each forged material at 850 ° C., a bar sample having a diameter of 35 mm and a length of 1 m was taken from each steel slab, and the maximum length of carbide at the center was measured.

[0019]

[Table 1]

[0020]

[Table 2]

Next, at room temperature for each of the above bar samples,
Cold drawing was performed at a reduction in area of 10%, 15%, 20% and 36%, and the presence or absence of voids in the center of each cold drawn product was confirmed. The presence or absence of voids was confirmed by observing a sample finished to a mirror surface by buffing with an optical microscope and an electron microscope. Table 3 shows the results of the above-described maximum length measurement of the carbides and the results of checking for the presence or absence of voids for the samples having composition a in Table 1. The results of composition b and composition c were almost the same as composition a. From Table 3, it can be seen that Samples 2a and 3a, that is, samples made of a steel ingot prepared by using a general melting method, had a maximum carbide length before drawing of 12 μm and 84 μm, respectively.
It can be seen that voids are formed in any of the samples when cold drawing is performed with a surface reduction rate of 5% or more. Sample 1a,
That is, a sample made of a steel ingot prepared by using the spray forming method of the present invention has a maximum carbide length of 5 μm.
m. Therefore, it can be seen that no void is formed even when a maximum of 36% cold drawing is performed.

[0022]

[Table 3]

Further, in order to quantitatively grasp the state of generation of voids and its influence, each of the cold-drawn products having the composition a was subjected to a quenching temperature of 1030 ° C. and a tempering temperature of 530 ° C. × 2.
Samples that had been subjected to heat treatments were prepared and their density and hardness were measured. Table 4 shows the measurement results. From Table 4, it can be seen that the density decreases and the tempering hardness decreases as the voids are formed and increased. In the case of no drawing and cold drawing with a reduction in area of 10%, the density of sample 1a and sample 2a was 7.71 g / cm ² and the hardness was 60H.
Although RC or higher can be obtained, the density becomes low when cold drawing is performed with a reduction in area of 15% or more in sample 2a and 10% or more in sample 3a, and the hardness cannot be obtained in 60HRC or more. I understand. In the sample 1a of the present invention, the density did not change in any processing, and the hardness was 60%.
HRC or better is obtained. Although the fatigue test could not be performed due to the sample, it is easy to imagine that the fatigue strength varies and decreases due to the stress concentration due to the formation of voids and therefore notches.

[0024]

[Table 4]

To summarize the results, sample 2 has a relatively small carbide due to the small ingot, but has a length of more than 5 μm, so that voids are generated by cold drawing with a reduction in area of 15% or more. I do. In sample 3a, since the steel ingot is large, the carbide generated at the time of solidification is large. Even when hot forging is performed at a forging ratio of 16 and a large forging ratio, the carbide is not sufficiently small, and is 5 μm.
, Voids are generated by cold drawing with a reduction in area of 10% or more. Sample 1 of the present invention
a was a relatively large ingot, and even when hot forging was performed with a forging ratio of 4 and a relatively small forging ratio, only carbides having a maximum length of 5 μm or less were found. For this reason, no void was generated even in cold drawing with a reduction in area of 36%. Furthermore, there was a correlation between the state of generation of voids and the density and hardness after the heat treatment, and the effects of the present invention were quantitatively grasped.

[0026]

As described above, according to the present invention, low cost, heat resistance and wear resistance are attained, and a decrease in strength reliability due to carbide cracks during cold and warm plastic working is suppressed. Cold and warm plastic working materials can be obtained,
From the characteristics of cold and warm plastic working, it is possible to commercialize various parts with low cost, high precision, and high mechanical properties.

Claims (5)

[Claims] 1. Spraying molten steel containing 14.0 to 42.0% by mass of C and Cr in (Cr + 15.5C) into droplets and collecting and solidifying the droplets to form an ingot, A method for producing a raw material for cold or warm plastic working, wherein the maximum length of carbides in a structure is 5 μm or less. 2. The method for producing a raw material for cold or warm plastic working according to claim 1, wherein the ingot is formed into a steel material by plastic working. 3. An ingot is made of one or two of Mo and W.
Up to 15.0% by weight of the species at (Mo + 1 / 2W) and V
The method for producing a raw material for cold or warm plastic working according to claim 1 or 2, which contains 5.0% by mass or less. 4. An ingot having a mass% of Si: 2% or less, Mn: 2% or less, Ni: 5% or less, Nb: 1% or less, Co: 12% or less, and S: 0.2% or less. The method for producing a raw material for cold or warm plastic working according to any one of claims 1 to 3, which contains one or more types. 5. The cold or warm plastic working material according to claim 1, wherein at least a part thereof has a sectional change rate of 15% or more by cold or warm plastic working at 800 ° C. or less. Cold or warm plastic working method characterized by applying elongation or compression deformation.