JPH0578729A - Method for making ingot of low nitrogen steel containing rare earth element - Google Patents

Abstract

PURPOSE:To produce low nitrogen steel contg. a rare earth metal effective for improving the grindability of high V high-speed tool steel, etc. CONSTITUTION:A consumable electrode optionally contg. a rare earth metal and having <1,000ppm nitrogen content is refined in a vacuum induction furnace on the 1st stage and subjected to electro-slag remelting with a flux contg. a rare earth metal on the 2nd stage.

Description

Detailed Description of the Invention

[0001]

The present invention contains a rare earth element,
Concerning the method for ingot tool steel with low nitrogen content, it is especially suitable for high V high speed tool steel that is used for blades that cut high hardness materials such as taps and end mills, and has good formability by grinding the blades themselves. The present invention relates to a simple ingot making method.

[0002]

2. Description of the Related Art In recent years, as the performance of various machines has increased, the hardness of members used has been increased. High-performance tool steel, which is a tool for cutting such members, is also required to have high performance. On the other hand, the V content has been increased in order to improve the strength and wear resistance of the high speed tool steel, but in this case, the grindability of the cutting tool itself tends to be deteriorated due to the coarsening of VC carbide. Therefore, in order to obtain a fine grindability by finely dividing the VC carbide, N, Ti, Nb, T
Regulation of a content (JP-A-62-21354), addition of rare earth elements (hereinafter abbreviated as REM), Zr, and Hf (JP-A-62-62354)
-2111354, JP-A-57-3742, JP-A-6
1-213350), Al alone or jointly added with REM (JP-A-63-213641), Ca alone or jointly added with Al, REM (JP-A-1-252),
Regulation of Ti and N contents for effectively utilizing the effect of REM addition (Japanese Patent Laid-Open No. 62-21354, Japanese Patent Laid-Open No. 1-14)
2055), Al alone or in order to effectively utilize the effect of joint addition with REM, the content of Ti and the content of N are regulated (Japanese Patent Laid-Open No. 142056), and many proposals for chemical components have been made. To summarize these, in order to improve the grindability by refining the VC carbide of the high-speed tool steel that imparts strength and wear resistance by adding V, and to facilitate the forming into a tool, N, Ti, REM, A by controlling Nb, Ta, etc.
It is effective to add 1, Ca, Zr, Hf and the like. However, according to the experience of the present inventors, N is particularly effective as the element to be regulated, and REM and Al are particularly effective as the elements to be added, and the combination of the regulation and the addition significantly improves the performance. Can be planned.

On the other hand, as a melting facility for high speed tool steel,
Generally, an arc furnace and a high-frequency induction furnace are used, but since both of these melt in the atmosphere, N
I can't avoid picking up. Especially in the case of high-speed tool steel with a high V content, the amount of N picked up is large because the affinity between V and N is large. Further, the adverse effect of melting in the atmosphere also extends to the control of the REM and Al contents. Especially, since REM has a large affinity with oxygen, in melting in the atmosphere and casting, unless some other measures are taken, If the addition yield of REM is extremely low, and if it is significant, the residual amount of REM yield in the steel ingot will be about a flaw. Therefore, many proposals have been made for a method of reducing N and a method of adding REM. Speaking only of the melting method of tool steel having a high V content, for example, the use of a vacuum induction / degassing melting furnace is disclosed in JP-A-61-213350 and JP-A-62.
-2111354 and the like.

However, according to the knowledge of the present inventors, since the vacuum induction furnace has a large degassing action, although N can be easily reduced to 100 ppm or less by using this furnace, REM was added. If so, there is a problem in the following points.
That is, since REM is extremely reactive, it not only reacts with a small amount of oxygen in the melting atmosphere even in a vacuum induction furnace, but also the refractory of the lining refractory of the melting furnace and the equipment used for the casting system ( Normally, oxides of active elements) are reduced, and they themselves become oxides and are mixed in the molten metal. Moreover, mixed R
The oxide of EM has a large specific gravity (6 or more), and the floating separation from the molten metal is extremely slow. Therefore, although the aggregation of REM oxide progresses from the addition of REM to the completion of casting into the mold, since it is in a state of being suspended in the molten metal, REM produced by vacuum induction melting is added, for example, The V-based high speed tool steel is clean as compared with the air-melted product, but the cleanliness is remarkably inferior as compared with the case where REM is not added. In particular, when a large inclusion of REM oxide is present on the outer peripheral portion of the steel ingot, a cutting tool such as a tap or an end mill, which is a main application of the high V type high speed tool steel, covers the outer peripheral portion. Since it is used, it deteriorates the tool life and also becomes the starting point of surface defects during hot working of steel ingots.
It also significantly deteriorates the manufacturing yield. Therefore, improvement of cleanliness becomes an indispensable subject in the production of REM-added high V type high speed tool steel and the like.

[0005]

The present invention is based on the above-mentioned high V
An object of the present invention is to provide a method for ingoting a rare earth element-containing low-nitrogen steel having high grindability such as a high-speed system tool steel and having high cleanability. The high V which is the main goal of the present invention
The high-speed tool steels have, for example, the following chemical components. That is, by weight ratio, C: 0.9 to 1.5%, Si: 2% or less, Cr: 3 to 6%, one or two of Mo and W at 2Mo + W.
14-25%, V2-5%, N: 100ppm or less, rare earth element: 0.005
.About.0.2%, Al 0.002 to 0.3%, or further Co: 15% or less, with the balance being Fe and inevitable impurities.

[0006]

According to the present invention, a vacuum induction furnace is used to melt a consumable electrode having a nitrogen content of less than 100 ppm, and the consumable electrode contains a compound of a rare earth element. It is a method for ingoting rare-earth element-containing low-nitrogen steel, characterized by comprising a second step of remelting electroslag using flux. Here, it is desirable that the rare earth element compound contained in the electroslag remelting flux is at least one of an oxide and a fluoride.

[0007]

In the present invention, the degassing effect of nitrogen, oxygen, etc. by the vacuum induction furnace and the electroslag remelting method (hereinafter referred to as E
(Hereinafter referred to as SR method), and by combining the REM addition effect by making the flux used for the cleaning and the flux used for it a compound of REM, or the addition yield improvement effect, easily containing N of 100 ppm or less and 0.005 to 0.2% of REM, Further, it becomes possible to ingot REM-containing low nitrogen steel such as high V type high speed tool steel which is clean and has excellent hot workability. It has already been stated that the vacuum induction furnace can easily obtain low nitrogen steel. According to the present invention, the REM can be added from the flux in the ESR without performing the REM addition in the melting process in the vacuum induction furnace. However, in order to obtain an accurate REM addition amount, the REM is added at the time of manufacturing the electrode. It is better to keep it.

The ESR method is known as one of the methods for improving the cleanliness of a steel ingot. In the ESR method, the consumable electrode produced by melting in another furnace is immersed in the molten slag in the water-cooled metal crucible, and a direct current or an alternating current is applied to a circuit composed of the consumable electrode, the molten slag, and the steel ingot. By energizing, the consumable electrode is melted from its lower side by the Joule heat of the molten slag to form a molten droplet, and the slag in the crucible is dropped to produce a steel ingot. One of the characteristics of the ESR steel ingot is that a uniform and dense solidified structure can be obtained by layered solidification. Another feature is the refining effect of molten slag. The molten droplets formed by melting the lower portion of the consumable electrode settles in the molten slag leaving the tip of the consumable electrode and drops on the molten metal portion (pool) of the growing steel ingot head.
During this time, the molten metal is protected from the atmosphere by the molten slag, is subjected to refining effects such as deoxidation and desulfurization by contact with the molten slag, and inclusions are also absorbed by the slag. In ESR melting, it is considered that the surface tension accelerates the discharge of inclusions to the surface of the molten metal when the molten metal film is formed at the tip of the consumable electrode, and thus the inclusions are extremely easily removed. Further, even if the inclusions settle in the molten slag while being contained in the melt droplets and reach the molten metal pool, they do not reach the lower part of the molten metal pool, and therefore separation by levitation is easy. Therefore, the adoption of the ESR method is expected to improve the cleanliness of REM-added high V type high speed tool steel and the like.

However, the ESR method is applied to REM-added high V
Upon applying the system high-speed tool steel, a large problem, CaF _2, Al ₂ O ₃ for use in a normal ESR, than with a slag blended in like CaO, uniformly in ESR ingot REM
Is not easy to add. This is because even if the consumable electrode contains REM, when the above-mentioned ordinary slag is used, the oxygen potential in the molten slag is high, and the REM addition yield is significantly reduced due to the REM oxidation consumption. Is. On the other hand, a method in which a REM compound (oxide, fluoride, etc.) is added to the slag, ESR dissolution is performed by direct current electricity, and REM is added to the molten metal by electrolytic reduction of the compound (Japanese Patent Publication No. No. 3606083) or a method of performing ESR by adding one or more kinds of REM to the slag or the consumable electrode (JP-A-59-23811). In any of these, REM compounds are mixed in slag, and the REM in the slag or consumable electrode is effectively alloyed in the steel ingot by electrolytic reduction thereof or by further lowering the oxygen potential in the slag. In the former known example, DC melting is recommended, but as seen in the examples in the specification, REM is added to the steel ingot to the extent that the effect of REM addition can be obtained even with AC melting. If it is desired to further increase the REM in the steel ingot, it is sufficient to add REM to the consumable electrode in advance as in the latter known example.
This is an effective method regardless of whether it is direct current or alternating current.

However, the purpose of the above proposal is to effectively add REM to the steel ingot in the ESR method, and as described above, the REM-added high V type high speed tool steel and the like exert high performance. There is no solution to the other requirement, N regulation. This is ESR
Since the method involves melting in the atmosphere, if no measures are taken, N in the steel ingot will never increase or decrease, and as suggested by JP-A-59-23811, Even if the slag surface is sealed with an inert gas such as argon in order to improve the REM addition efficiency, it is clear that it has only an effect of suppressing the increase of N. Moreover, when the consumable electrode is melted in an arc furnace or a high-frequency induction furnace that is usually used for melting high-speed tool steel, no matter how carefully the raw materials used are selected, as described above, an N content of 200 ppm or less is obtained. That is a difficult technique.

As can be seen from the above description, the present invention provides the degassing effect by the vacuum induction furnace, the cleaning by the ESR method, and the REM addition action by making the flux used therefor contain the compound of REM, Alternatively, it also has the effect of improving the yield of addition, which makes it easy to add N 1
RE that is 0.00ppm or less and contains 0.005 to 0.2% REM to improve machinability, and is clean and has excellent tool life and hot workability.
It is possible to ingot M-containing low N steel. When the means of the ingot making method of the present invention is used, it is possible to obtain operational effects that cannot be obtained by a combination of the operations of the conventionally known vacuum induction furnace application and ESR method application. This is because, first of all, no new steel ingot method that contains REM and has a low N of 100 ppm or less of nitrogen has not been known until now. Like
At best, it was limited to the chemical composition measures or to devise it in the single ingot making method. Secondly, in order to achieve the new steel ingot-making method targeted by the present invention, vacuum induction melting is performed with REM with a low N which cannot be achieved by the ESR method.
Can be added, which compensates for the risk of N increase in the ESR method and the yield instability of REM. On the other hand, ES
The R method cannot be achieved by vacuum induction melting alone.
In order to obtain a uniform distribution of
A special means is adopted in which redissolution is performed using a flux containing a REM compound which is not normally used. In this way, the means of the ingot making method of the present invention are such that the respective elements are mutually complementary, whereby N content is 100 ppm or less and REM content is 0.005 to 0.2% for the first time to improve machinability and cleanliness. It is possible to ingot REM-containing low N steel with excellent hot workability, and the tool produced by applying existing processes such as rolling has a dramatically improved life. is there.

[0012]

Next, the recommended alloy composition range of the REM-containing low N steel suitable for the ingot making method of the present invention will be described. C is Cr, W,
It combines with carbide-forming elements such as Mo and V to form carbides, imparts quenching-tempering hardness, and contributes to wear resistance, heat resistance, and seizure resistance. If the amount is too large, the toughness is lowered and a huge carbide is generated. Therefore, it is desirable to contain the Cr in a balanced manner with the amounts of Cr, W, Mo and V, and to limit the content to 0.9 to 1.5%. Si is preferably added in an amount of 2% or less mainly for the purpose of deoxidation. Cr improves hardenability, wear resistance, oxidation resistance, and improves high temperature strength and temper softening resistance by setting an appropriate content. For the above purpose, it is set to 3% or more, but if it is too large, the high temperature strength and the tempering softening resistance are rather lowered, and the toughness is also lowered. W and M
O combines with C to form a special carbide, which contributes to the improvement of wear resistance and seizure resistance. In addition, the secondary hardening effect by tempering is large, which contributes to high temperature strength. In order to obtain the above effects, 2Mo + W is added so as to satisfy 14 to 25%.
If the amount of W + 2Mo is less than 14%, the above effect is not sufficiently obtained, and if it is too large, the toughness and hot workability are impaired, so 2Mo + W
It is desirable to be 25% or less.

V combines with C to form a hard carbide,
Contributes to improved wear resistance. However, since this carbide is harder than the abrasive grains, it causes the grinding wheel to wear early. In particular,
If a large number of coarse carbides are generated and the distribution is not uniform, the grindability is significantly reduced. For this reason, in the past, when importance was placed on grindability, it was limited to about 1.2% or less. However, according to the present invention, when REM is added and the N content is lowered, it is possible to prevent the generation of MC type carbide mainly composed of coarse V even if a large amount of V is contained. An appropriate amount is contained within the range of up to 5%. If it exceeds 5%, the effect of the present invention becomes small, so it is set to 5% or less, and if it is too small, it does not sufficiently contribute to wear resistance, so it is preferable to set it to 2.0% or more. N is an impurity in the steel of the present invention. When the amount of N exceeds 100 ppm, the effect of refining VC due to addition of REM is impaired, so the amount is set to 100 ppm or less. REM has the effect of increasing the absolute amount of MC type carbides and the effect of finely crystallizing VC carbides. If less than 0.005%, these effects are less,
If it exceeds 0.2%, it will combine with S and O to form inclusions and cause casting defects. Therefore, it is preferable to limit the content to 0.02 to 0.2%. Al has a function of lowering the activity of oxygen in molten steel, and R
It suppresses the generation of EM oxide and promotes the VC miniaturization effect of REM. If it is less than 0.002%, this effect is small, and if it exceeds 0.3%, Al oxides increase and the cleanliness deteriorates. Co dissolves in the matrix to improve the strength and heat resistance of the steel of the present invention and does not directly contribute to the control of the carbide morphology and the improvement of the cast structure according to the present invention, but is added in an amount of 15% or less if necessary.

[0014]

[Example] Raw materials were charged into a vacuum induction furnace having a nominal volume of 6 tons and melted in a vacuum (however, No. 4 is an atmospheric induction furnace). In the vacuum induction furnace, the pressure in the tank was maintained so that the N solubility of the molten metal would be 100 ppm or less in equilibrium theory. After all the components other than REM were adjusted and it was confirmed that the N content of the molten metal was 100 ppm or less, the required amount of REM was added to the furnace and the steel was tapped. Further, in a vacuum (excluding No. 5), a consumable electrode was produced by casting in a mold of a predetermined shape (however, No. 5 was cast in an ingot). The chemical components of the melted consumable electrodes are shown in Table 1, Nos. 1,2,3,4,6,7 above "electrodes". Here, the vacuum induction furnace is represented by VIF. The REM in the consumable electrode by VIF is 0.04 to 0.06% as shown in the REM (electrode) column of the table, and the addition rate of M is 59 to 73% as shown in the REM yield (electrode) column. , N was in the range of 29 to 39 ppm. The consumable electrode was subjected to AC ESR to obtain a steel ingot. The amount of flux used in ESR is approximately 50 kg per ton of charged consumable electrode. Further, in order to prevent the mixing of [N] from the atmosphere during ESR, the inside of the mold was replaced with Ar gas for dissolution. In addition, No. 1 and 2 have almost the same chemical composition as an electrode, No. 6 is an example in which Co is increased with respect to No. 1 and 2, No. 7 is against No. 1 and 2. Is an example in which Co is removed and W and V are increased, and Mo is decreased. In these examples of the present invention, R in the ESR steel ingot is
EM is 0.03 to 0.05%, the yield for REM in the consumable electrode is 67 to 83%, and N in the ESR steel ingot is 42.
It was a sufficiently low range of ~ 53ppm.

On the other hand, Nos. 3 to 5 in Table 1 are comparative examples.
o.3 is the same basic chemical composition as No.1 and 2, when a consumable electrode is melted in a vacuum induction furnace and ESR is carried out without compounding the REM compound in the flux, No.4 is normal high frequency induction in the atmosphere. Melt the consumable electrode in the furnace and use REM as the ESR flux.
When the compound was compounded, No. 5 is a case where a steel ingot was directly formed in a vacuum induction furnace without performing ESR. Comparing the chemical components, in No.3, the RE of the consumable electrode was
Although M is 0.06%, which is almost the same value, after ESR,
It was extremely low at 0.01%, and the REM yield at ESR was about 17%, while the REM yield of the present invention was 67 to 83% as described above. In No. 4, the REM was 0.02%, which was a value relatively close to that of the present invention, but the value of N was close to 200 ppm for both the consumable electrode and the ESR steel ingot. No. 5 is equivalent to the example of the present invention in terms of chemical composition.

The steel ingots of the above 7 examples were subjected to hot slab forging to obtain 10
It was molded into a 0 mm square billet. At this time, the weight of the steel ingot is set to W _I k
g, riser like ingot wastepaper weight W _R kg, when the billet weight after the removal of surface defects by a grinder and W _B kg,
The hot working yield A (%) is expressed by the following equation. _{A = {W B / (W} I -W R)} × 100 is shown in conjunction with this hot working yield in Table 2 below, ESR
All the steel ingots (other than No. 5) were 90% or slightly lower than this, whereas the ingots directly made in the vacuum induction furnace (VIF) (No. 5) were 71%. When the scratched portion of No. 5 was investigated in detail, a large amount of non-metallic inclusions mainly composed of REM oxide, oxysulfide, etc. were observed.

After each of the above billets was annealed, hot wire rod rolling and subsequent annealing were carried out to obtain a wire rod. Furthermore, these materials were cold drawn and ground into bar steel with a diameter of 6.23 mm, and various material properties were evaluated. As the evaluation items, the intervening article position, the amount of VC carbide, the heat treatment hardness, the micro-part bending strength, and the grindability related to the formability of the tool itself were evaluated.
In addition, it was actually manufactured into a uniform diameter hand tap for Japanese Industrial Standard coarse thread and the cutting life was evaluated. The heat treatment conditions were 1220 ° C. salt bath quenching and then tempering three times at 560 ° C. × 1 hr. The results of these evaluations are summarized in Table 2 together with the above-mentioned hot working yield. The intervening article position of each sample is B + C based on the Japanese Industrial Standard for the center part of the flat surface of the steel bar that has been subjected to the above-mentioned heat treatment from the surface layer parallel to its axis to a depth of about 1/8 of the diameter of the material with a diamond grindstone. Inclusions were measured. Among these, examples of inclusion microscopy of samples No. 1 and 3 to 5 are shown in FIG. In addition, the center portion of the plane formed in the same manner was polished with chromium oxide, and VC carbides were observed. An example of this structure is similarly shown in FIG.

[0018]

[Table 1]

[0019]

[Table 2]

The quantification of the VC area ratio of each sample was carried out in the same manner as in the above-mentioned inclusion measurement, after polishing with a diamond wheel, corroded with 10% nitric acid alcohol, and further corroded with Murakami reagent. VC carbides in μm ² were measured. The heat treatment hardness of each sample was measured with a Vickers hardness tester under a load of 30 kg on a plane obtained in the same manner as the inclusion measurement. Further, the micro-part bending strength is measured by grinding and cutting a 0.8 mm-thick thin plate, including the center axis of a steel bar, and measuring about 1/8 of the material diameter from the surface layer part of the thin plate. The load was applied so that the part became a fracture surface. The machinability was evaluated by taking a thin plate with a thickness of 2 mm from the heat-treated steel bar, including the shaft center, and measuring P from the surface part of the thin plate to about 1/8 of the material diameter.
Wet plunge grinding was performed with a surface grinder using the A grindstone, and the ratio was obtained as the ratio between the grindstone wear amount after the grinding test and the grind removal amount of the sample. The grinding conditions are shown in Table 3.

[0021] ↓

[Table 3] ↑

The inclusion test results of Examples 1, 2, 6 and 7 of the present invention are sufficiently lower than those of No. 5 by VIF. In addition, as shown in FIG. 2, the VC carbides had very few VC carbides of 5 μm (2 mm on a × 400 photograph) or more and were finely distributed. Although illustration is omitted, in No. 7, the VC carbides were the most in the examples of the present invention and were finely distributed. On the other hand, No. 3 has almost no REM remaining of 0.01%, and therefore the VC carbides are not sufficiently refined. No.4 is N
Since the amount is high, VC carbides are coarsely crystallized. No.
For No. 5, a fine structure was obtained for VC carbide. However, as described above, a large number of REM oxysulfides and the like having a poor intervening article position were observed as described above. Regarding the heat treatment hardness, a hardness of HV880 or higher was obtained in all cases, and a high hardness of about HV910 was obtained in No. 6 to which a large amount of Co was added.
With respect to micro bending strength, No. 1,2,6,7 of the present invention in which both VC carbides and inclusions had a fine structure were 250 kgf except that the VC area ratio was high and No. 7 was slightly low. A high strength of about / mm ² or more was obtained. In Nos. 3 to 5, the VC carbides or inclusions were rather coarse, and the strength was slightly lower than that of the examples of the present invention. Regarding grindability, No.3,4 with a large amount of VC carbide of 5 μm or more has a large grindstone wear amount and a small grinding ratio, while other samples have a grinding ratio of about 10 The value indicates that the grindability is almost equal to that of SKH55. In the tap cutting test of each sample, S
A 45 C plate was used to make a pilot hole with an inner diameter of 5.0 mm, and a cutting test was performed. Table 4 shows tap cutting test conditions. The cutting life was evaluated by the number of cutting holes until the tap was broken or the internal thread precision exceeded the tolerance defined by Japanese Industrial Standards.

[0023] ↓

[Table 4] ↑

In order to surpass the cutting life of taps on the market, a VC content of 4% or more and a hardness of HV880 or more can be obtained, and 220 kgf / mm ² or more for improving the chipping resistance. It seems necessary to have a micro-section bending strength. All of the examples of the present invention satisfied this condition sufficiently, and a longer life was obtained as compared with Nos. 3 to 5 of the comparative examples.

[0025]

As described above, according to the present invention, it becomes possible to manufacture a steel containing REM and having a low N, which has been difficult by the conventional ingot-making method, with good cleanliness. As a result, the grindability and tool life have been greatly improved. Also,
Compared with the vacuum induction melting method, the hot working yield is significantly improved,
The production cost more than compensates for the cost increase due to double melting.

[Brief description of drawings]

FIG. 1 is a metallographic micrograph showing inclusion microstructure of a representative sample.

FIG. 2 is a metallographic micrograph showing a VC carbide of a representative sample.

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[Procedure amendment]

[Submission date] November 5, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

However, the ESR method is applied to REM-added high V
Upon applying the system high-speed tool steel, a large problem, CaF _2, Al ₂ O ₃ for use in a normal ESR, than with a slag blended in like CaO, uniformly in ESR ingot REM
Is not easy to add. This is because even if the consumable electrode contains REM, when the above-mentioned ordinary slag is used, the oxygen potential in the molten slag is high, and the REM addition yield is significantly reduced due to the REM oxidation consumption. Is. On the other hand, a method in which a REM compound (oxide, fluoride, etc.) is added to the slag, ESR dissolution is performed by a direct current, and REM is added to the molten metal by electrolytic reduction of the compound (Japanese Patent Publication No. No. 3606083) or a method of performing ESR by adding one or more kinds of REM to the slag or the consumable electrode (JP-A-59-23811). In any of these, REM compounds are mixed in slag, and the REM in the slag or consumable electrode is effectively alloyed in the steel ingot by electrolytic reduction thereof or by further lowering the oxygen potential in the slag. In the former known example, DC melting is recommended, but as seen in the examples in the specification, REM is added to the steel ingot to the extent that the effect of REM addition can be obtained even with AC melting. If it is desired to further increase the REM in the steel ingot, it is sufficient to add REM to the consumable electrode in advance as in the latter known example.
This is an effective method regardless of whether it is direct current or alternating current.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

Next, the recommended alloy composition range of the REM-containing low N steel suitable for the ingot making method of the present invention will be described. C is Cr,
It combines with carbide-forming elements such as W, Mo, and V to form carbides, imparts quenching-tempering hardness, and contributes to wear resistance, heat resistance, and seizure resistance. If the amount is too large, the toughness is lowered and a huge carbide is generated. Therefore, it is desirable to contain the Cr in a balanced manner with the amounts of Cr, W, Mo and V, and to limit the content to 0.9 to 1.5%. Si is preferably added in an amount of 2% or less mainly for the purpose of deoxidation. Cr improves hardenability, wear resistance, oxidation resistance, and improves high temperature strength and temper softening resistance by setting an appropriate content. For the above purpose, it is set to 3% or more, but if it is too large, the high temperature strength and the tempering softening resistance are rather lowered, and the toughness is also lowered. W and Mo combine with C to form a special carbide, which contributes to the improvement of wear resistance and seizure resistance. In addition, the secondary hardening effect by tempering is large, which contributes to high temperature strength. In order to obtain the above effects, 2Mo + W is added so as to satisfy 14 to 25%. If the amount of 2Mo + W is less than 14%, the above effect is not sufficiently obtained, and if it is too much, the toughness and hot workability are impaired.
+ W 25% or less is desirable.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

V combines with C to form a hard carbide,
Contributes to improved wear resistance. However, since this carbide is harder than the abrasive grains, it causes the grinding wheel to wear early. In particular,
If a large number of coarse carbides are generated and the distribution is not uniform, the grindability is significantly reduced. For this reason, in the past, when importance was placed on grindability, it was limited to about 1.2% or less. However, according to the present invention, when REM is added and the N content is lowered, it is possible to prevent the generation of MC type carbide mainly composed of coarse V even if a large amount of V is contained. An appropriate amount is contained within the range of up to 5%. If it exceeds 5%, the effect of the present invention becomes small, so it is set to 5% or less, and if it is too small, it does not sufficiently contribute to wear resistance, so it is preferable to set it to 2.0% or more. N is an important impurity in the method of the present invention. N amount
If it exceeds 100 ppm, it is set to 100 ppm or less in order to impair the VC miniaturization effect by the addition of REM. REM has the effect of increasing the absolute amount of MC type carbides and the effect of finely crystallizing VC carbides. If it is less than 0.005%, these effects are small, and if it exceeds 0.2%, it binds to S and O to form inclusions and causes a casting defect. Therefore, it is preferable to limit the content to 0.005 to 0.2%. Al has a function of lowering the activity of oxygen in molten steel, suppresses the formation of oxides of REM, and promotes the VC miniaturization effect of REM. If it is less than 0.002%, this effect is small, and if it exceeds 0.3%, the oxide of Al increases and the cleanliness deteriorates. Co is a solid solution in the matrix and improves the strength and heat resistance described above. It does not directly contribute to the control of the morphology of the carbide and the improvement of the cast structure according to the present invention, but if necessary, it is added in an amount of 15% or less.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

[0014]

[Example] A raw material was charged into a vacuum induction furnace having a nominal capacity of 6 tons and melted in a vacuum (however, Table 1, No. 4 is an atmospheric induction furnace). In the vacuum induction furnace, the pressure in the tank is equilibrium N
The solubility was maintained at 100 ppm or less. After all the components other than REM were adjusted and it was confirmed that the N content of the molten metal was 100 ppm or less, the required amount of REM was added to the furnace and the steel was tapped. Furthermore, in a vacuum (excluding No. 4), a consumable electrode was manufactured by casting in a mold of a predetermined shape (however, No. 5).
Is cast in the ingot). The chemical components of the melted consumable electrodes are shown in Table 1, Nos. 1,2,3,4,6,7 above "electrodes". Here, the vacuum induction furnace is represented by VIF. VIF
REM in the consumable electrode according to, as shown in the REM (electrode) column of the table, is 0.04 to 0.06%, the addition yield of REM is 59 to 73% as shown in the REM yield (electrode) column, N is 2
It was in the range of 9 to 39 ppm. AC electrode ESR for the consumable electrode
Was carried out to obtain a steel ingot. The amount of flux used in ESR is approximately 50 kg per ton of charged consumable electrode. Also, ES
In order to prevent the mixing of [N] from the atmosphere during R,
Dissolution was performed by replacing with r gas. Nos. 1 and 2 have almost the same chemical composition as electrodes.
Is an example in which the amount of Co is increased with respect to No. 1 and 2, and No. 7 is an example in which the amount of W and V is increased and the amount of Mo is decreased while removing Co with respect to No. 1 and 2. In these embodiments of the invention, ESR
REM in steel ingot is 0.03-0.05%, and R in consumable electrode is
The yield for EM was 67 to 83%, and the N in the ESR steel ingot was 42 to 53 ppm, which was a sufficiently low range.

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[0019]

[Table 2]

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In order to exceed the cutting life of a tap on the market, a VC amount of 4% or more in area ratio, a hardness of HV880 or more can be obtained, and a minute amount of 220 kgf / mm ² or more can be obtained to improve the resistance to chipping. It seems necessary to have partial bending strength. All of the examples of the present invention satisfied this condition sufficiently, and a longer life was obtained as compared with Nos. 3 to 5 of the comparative examples.

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[0025]

As described above, according to the present invention, it becomes possible to manufacture a steel containing REM and having a low N, which has been difficult by the conventional ingot-making method, with good cleanliness. As a result, the grindability and tool life have been greatly improved. Also,
Compared with the vacuum induction melting method, the hot working yield is significantly improved,
The reduction in production cost more than compensates for the increase in cost due to double melting.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location C21C 7/00 B 7412-4K E 7412-4K C22B 9/18 9/187 // C22C 38/00 302 E 7217-4K 38/22 38/30 38/34 (72) Inventor Yasufumi Yamane 2107-2 Yasugi-cho, Yasugi-shi, Shimane 2 Inside Hitachi Yasugi Factory (72) Inventor Kensei Uchida Yasugi-shi, Shimane Prefecture 2 2107 Yasugi-machi, Yasugi-machi, Yasugi Plant (72) Inventor Hideki Nakamura 2-2-2 Yasugi-cho, Yasugi-cho, Shimane Prefecture

Claims (3)

[Claims] 1. Nitrogen content of 100ppm by vacuum induction furnace
Rare earth element containing, characterized in that it comprises a first step of melting the consumable electrode which is less than 1 and a second step of remelting the consumable electrode with a flux containing a compound of a rare earth element. Method for making low nitrogen steel. 2. The rare earth element-containing low nitrogen steel according to claim 1, wherein the rare earth element compound contained in the electroslag remelting flux is at least one of an oxide and a fluoride of the rare earth element. Ingot method. 3. A rare-earth element-containing low nitrogen steel in a weight ratio,
C: 0.9-1.5%, Si: 2% or less, Cr: 3-6%, 1 or 2 kinds of Mo and W are 2-25 Mo + W, 14-25%, V 2-5%, N: 100ppm
It is characterized by being a high-speed tool steel with excellent grindability, containing rare earth elements: 0.005 to 0.2%, Al 0.002 to 0.3%, or Co: 15% or less with the balance being Fe and inevitable impurities. The method for ingot of the rare earth element-containing low nitrogen steel according to claim 1 or 2.