JP2004218069A - High rigidity steel producible by melting method, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high rigidity steel having a high Young's modulus which can be produced not by sintering but by a melting method, and to provide a production method therefor. <P>SOLUTION: In the steel, the boride of the group 4A, 5A or 6A elements consisting essentially of Ti or the compounded substance thereof is dispersed by 5 to 25 vol%. The matrix phase has a composition comprising, by mass% after the dispersion of the boride, ≤0.20% C, 0.10 to 1.00% Si, 0.20 to 8.00% Mn, ≤0.045% P, ≤0.030% S and ≤1.00% Al, and, if required, comprising one or more kinds of metals selected from ≤14.00% Ni, ≤28.00% Cr, ≤4.00% Mo, ≤1.00% V, ≤1.00% Nb and 0.50 to 3.50% Cu, and the balance Fe with inevitable impurities. Dissolution is performed in a closed space of ≤250 Pa. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a high-rigidity steel having a high Young's modulus and capable of being produced by a melting method, and a method for producing the same.

  Since steel materials are the cheapest and can be easily mass-produced, they are most often used for mechanical structural members. These structural members have various required characteristics depending on the parts used, but the major requirement in common has not changed before, that is, weight reduction. This is because, for example, it has been objectively proved that the reduction in the weight of a product such as an automobile and a truck reduces the energy consumption when the product is used and that a great effect on energy saving can be obtained. However, achieving weight savings often involves significant difficulties. In other words, when studying the achievement of weight reduction, it is necessary to determine whether the required strength can be ensured even if the component dimensions are reduced, and the amount of elastic deformation (deflection, This is because it is inevitable to solve the problem that the twisting or the like is suppressed to a level that does not cause a problem.

  Of these, regarding the former problem, iron and steel materials can be added with an appropriate amount of a large number of additional elements, and by the effect of the addition, solid solution strengthening, precipitation strengthening, or by performing optimal heat treatment, Higher strength of the material can be achieved, and many problems have been solved to date.

  However, no definitive solution has been found for the latter. That is, no matter how the solid solution strengthening, precipitation strengthening, or strengthening by heat treatment strengthens the steel, the Young's modulus of the steel material does not change significantly, and the higher the strength and weight, the smaller the component size. This is because the rigidity of the component itself decreases, and conversely, the amount of deformation increases. Also, it is not inconceivable to change to a material other than steel material, but other alloys with a lower density than iron alloys such as Mg alloy, Ti alloy, Al alloy, all have significantly lower Young's modulus than steel material, Changing the alloy used does not solve the problem. Therefore, in the conventional technique, even if the strength capable of withstanding the load stress during use is obtained, if there is a problem in terms of the amount of elastic deformation, there is no other way but to increase the dimensions of the parts and to reduce the amount of elastic deformation. Was. However, this naturally results in an increase in mass, and as a result, the initial object of reducing the weight cannot be achieved.

  Increasing the mass of parts is another problem. That is, the load stress applied to the structural member is reduced by the use of a product (such as an automobile) made of the structural member, the load naturally applied from the outside (for example, the load applied to each part by loading a heavy load on a truck). ) As well as forces naturally applied by the movement of each member irrespective of external force (for example, caused by inertial force acting on a reciprocating member such as a piston pin or by gravity or centrifugal force acting on a rotating shaft). Bending moment, etc.). There is a problem that the inertial force, centrifugal force, gravity, and the like all increase as the mass of each component of the structural member increases. Therefore, there is no other way to reduce the amount of deformation such as bending of each part without increasing the part dimensions but increasing the rigidity of the part. For that purpose, the development of a material having a high Young's modulus is indispensable. Met.

  New attempts to develop materials with a high Young's modulus have recently been made to allow for an appropriate response to this problem, and patent applications resulting from development have been gradually increasing. For example, patent documents 1 to 3 have been filed. These three patents have some differences in their contents, but the three patents have in common that they show a high Young's modulus in a matrix containing iron as a major component. A high Young's modulus can be obtained by dispersing a boride of an element and a group 5A element.

JP-A-7-188874 JP-A-7-252609 JP-A-10-68048

However, the above-mentioned prior art has the following problems.
Of the three patents described above, the inventions described in Patent Documents 1 and 2 have been completed on the premise that they are manufactured by sintering using iron powder or iron alloy powder as a raw material. However, in the production method by sintering, it is needless to say that an expensive mold is required in the molding process, but since it requires a large amount of force, it is not suitable for producing relatively large parts such as crankshafts, and If the parts are large, it is difficult to make the degree of densification of the powder uniform and the productivity is inferior to the melting method, so there is a limit to cost reduction.

  Patent Document 3 describes a high-rigidity and high-toughness steel obtained by a smelting method.However, when dispersing boride by a smelting method, the optimum range of an iron alloy used as a matrix is described above. No new findings are shown for the two patents. That is, although many steel materials such as carbon steel, nickel chromium steel, chromium steel, chromium molybdenum steel, spring steel, and bearing steel are described, this is a problem that has not occurred in a manufacturing method based on powder sintering. The problem which arises in the case of manufacturing by the method is not sufficiently studied. The present inventors have conducted repeated trial production tests and found that, when manufacturing a high-rigidity steel by applying the melting method, the range that can be manufactured is considerably narrowed. It was found that a high-rigidity steel with good quality could not be obtained.

  The present invention clarifies the range of easy production when obtaining high-rigidity steel by the smelting method, enables the production of high-rigidity steel by the smelting method, and reliably achieves weight reduction of structural members. The purpose is to make it possible.

The high-rigidity steel that can be produced by the smelting method according to claim 1 is selected from the group containing a group 4A element, a group 5A element, a group 6A element, and Fe in a matrix phase composed of pure iron or an iron alloy. A steel in which a boride containing at least one or more elements and / or a compound thereof is dispersed in 5 to 25 vol%, and 80% or more of the dispersed boride or / and the compound thereof is TiB ₂ by volume ratio of 80% or more. Further, the present invention provides a high-rigidity steel that can be produced by a melting method, wherein the content of C in the entire steel is 0.20% by mass or less.

  What should be noted in the present invention is that the range of the C content and the range of the volume ratio of boride contained in the matrix have been clarified so that a high-rigidity steel material can be manufactured by the smelting method.

  When produced by the smelting method, as a matter of course, the raw material is heated and melted at a high temperature, so that an active reaction occurs in the molten steel. Some of these reactions are difficult to occur in the powder method, and the reactions may hinder the production of high-performance high-rigidity steel.

  The present inventors have investigated in detail whether a new problem which did not need to be considered in the production by the powder method does not occur when the production is performed by the melting method. As a result, the following findings have been obtained, thereby completing the present invention.

  That is, Ti, Nb, V, Ta, Zr, Hf, Cr, Mo, etc., which are elements belonging to the 4A group, 5A group and 6A group, are all elements having a property of easily becoming a carbide, and the molten steel containing C When an alloy containing the aforementioned 4A group, 5A group, or 6A element and B are added, a part of these elements is bonded not to B but to C, and a carbide is formed. In particular, it was found that when the C content in the molten steel was high, the number of elements that turned into carbides increased, and it was not possible to achieve a state in which the target boride was sufficiently generated.

In particular, TiB ₂ , a Ti boride, has a lower density than steel, and its use is very effective for achieving the object of the present invention for improving the specific Young's modulus (the ratio of the Young's modulus to the density). However, if a large amount of TiC is generated without generating TiB ₂ , the effect of improving the specific Young's modulus is reduced, and at the same time, the castability at the time of melting is lowered, and a good steel ingot cannot be obtained.
However, the generation of a small amount of TiC does not greatly affect the decrease in Young's modulus, while suppressing the growth of TiB ₂ into a long hexagonal column and contributing to the improvement of machinability. It is preferable to generate TiC within a range in which the adverse effect of the decrease is not caused.

It has been known that carbides of Group 4A, 5A, and 6A elements have high Young's modulus for some time. Patents for high-rigidity steel using carbides have been filed, but carbides are liable to be coarsened. Therefore, it was found that there was a tendency that castability was deteriorated. In addition, in the case of high-rigidity smelted steel, it is necessary to roll the steel ingot, further forging, machining, etc., and process it to the desired component shape. Since stricter conditions are required than in the case of manufacturing, special consideration is required. Then, as a result of investigating the influence on hot workability by adding a raw material to be a boride to the matrix phase, after optimizing each component range as described later, the volume ratio of the boride was 25% or less. Has been found to be essential to enable hot working such as rolling and forging.
The invention of claim 1 is based on the knowledge obtained above.

  The reason that the scope of the claims includes a boride containing Fe is that a part of the added B binds to a large amount of Fe without binding to Ti or other 4A group, 5A group, or 6A group element. This is because, in some cases, the production may be difficult if the presence of a boride containing Fe is not recognized at all.

Next, the reason for limiting each condition defined in claim 1 will be described below.
The upper limit of the C content is 0.20% by mass (the component range is not limited to the matrix phase alone in order to clarify whether or not the product is within the scope of the present invention. The content is shown by the content of the total amount of the elements contained in the dispersed boride and the matrix phase, and also includes C which is TiC). However, the added 4A group, 5A group, and 6A group elements are combined with carbon in the molten steel to form carbides, and borides are not generated in a desired amount, and the effect of improving the Young's modulus cannot be sufficiently obtained. That's why. Further, if the amount of carbide is small, it is effective for improving the strength and there is little adverse effect. However, if the amount of carbide is large, the fluidity and hot workability at the time of casting are reduced. Therefore, the upper limit is limited to 0.20%.

However, if C is added in a small amount, even if TiC is formed, there is no significant adverse effect on the Young's modulus, and as described later, in order to finely crystallize TiB ₂ , TiC, which is a nucleus thereof, is used. Is required, and a small amount is a necessary element. In addition, when TiC is generated, TiB ₂ is prevented from growing in an elongated hexagonal column shape, and TiB ₂ and TiC are finely dispersed in the same position, and Ti char boride (EP, Ti, B from the same position in EPMA). , And C are detected.), The hard and elongated TiB2 is shortly crushed during machining, thereby reducing the damage to the tool and improving the tool life. Therefore, considering the workability, the addition amount of C is preferably at least 0.06% by mass, more preferably 0.08% or more.

The reason that the boride to be produced was limited to elements of the 4A group, 5A group, and 6A group other than Fe (Fe was included as described above) is described in the above-mentioned patent document. This is because the boride of the element has a high Young's modulus. In the present invention, among them, the main component is particularly limited to TiB ₂ which is a boride containing Ti. This is because TiB ₂ shows a high Young's modulus, but has a much lower density than steel, and is therefore effective in obtaining a material having a high Young's modulus while reducing the weight. Therefore, in order to improve the specific Young's modulus, it is necessary that 80% or more (volume ratio) of the boride to be formed is a boride containing Ti (TiB ₂ ). This is because when borides other than Ti increase, for example, when the amount of (Mo, Fe) B ₂ , (Cr, Fe) B, etc. increases, the Young's modulus increases as compared with steel before dispersing the borides. However, this is because the effect of improving the specific Young's modulus is smaller than when boride is only Ti. In addition, when these borides increase, there arises a problem that the hot workability and the castability also decrease, and the production by the melting method becomes difficult. Therefore, by mainly dispersing the boride mainly composed of Ti, it is possible to obtain a high-rigidity steel which has a high Young's modulus in spite of its small specific gravity and is easily manufactured by the melting method.

In order to prevent the formation of a boride containing Fe, the ratio of the element to be added, which is the group 4A, 5A, or 6A, and B is adjusted so that the amount of B is not excessive. It is good. Since B binds preferentially to the 4A group, 5A group, and 6A group elements, when B is excessively present, it is combined with a large amount of Fe to form a boride containing Fe. It is. For example, when producing TiB ₂ , the compounding ratio of Ti and B may be set to be close to 1: 2 in atomic ratio (Ti / B = 2.215 in mass ratio). Thereby, the formation of boride containing Fe can be prevented.

  The reason why the volume ratio of the boride or its complex compound dispersed in steel is limited to 5 to 25% is that if it is less than 5%, the effect of improving the Young's modulus cannot be sufficiently obtained. When the amount is too large, the obtained Young's modulus is higher than when the addition amount is small, but it becomes difficult to disperse uniformly, and the Young's modulus becomes uneven depending on the location of the product, and the stable Young's modulus is high. Is difficult to guarantee. On the other hand, in the case of the powder method, since the raw material powder can be made considerably uniform by mixing well, it is more advantageous than the melting method. Further, in the smelting method, when a large amount of boride is added, there is a problem that hot workability is reduced, and it is difficult to form a predetermined shape by rolling and forging. Therefore, the upper limit of the amount of boride to be dispersed is strictly limited as compared with the upper limit specified in the prior application and the like. Also, the larger the amount added, the higher both the Young's modulus and the strength, but the elongation and the drawing are reduced and the hot workability is reduced, so care must be taken.

Next, a feature of the invention according to claim 2 is a steel containing C, Si, Mn, P, S, and Al, and having a matrix phase composed of a balance of Fe and inevitable impurity elements,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, and Al: 1.00% or less. .
The reasons for limiting the range of components excluding C will be described below. In addition, the present invention is intended to provide steel suitable for production by a melting method, and is not limited to a specific use, and is intended to provide steel that can be used for various kinds of uses. Needless to say, a range in which the characteristics are largely different is included.
The content of each element is calculated by dividing the content of only the matrix phase by the mass of the entire steel, and the content in boride is taken into account in the calculation of the content. does not enter.

Si: 0.10-1.00%,
It is necessary to add a small amount of Si in order to ensure the fluidity of molten steel at the time of casting, and it is necessary to contain at least 0.10% or more. Desirably, it is better to be 0.30% or more. On the other hand, when it is contained in a large amount, the boride of the 4A group, 5A group, or 6A group element mainly composed of Ti tends to be coarsened at the time of solidification, and the matrix is hardened more than necessary. Therefore, the production by rolling becomes difficult, so the upper limit was made 1.00%.

Mn: 0.20 to 8.00%,
Mn is an element that contributes to strength improvement by solid solution strengthening, and must be contained at least 0.20% or more. Further, when Mn is added in a large amount, the matrix becomes austenite and the steel becomes non-magnetic, so that it is possible to provide a non-magnetic material having a high Young's modulus. However, if it is contained in a large amount, the hot workability is reduced and the production by hot rolling becomes difficult, so the upper limit was limited to 8.00%.

P: 0.045% or less,
P is present in steel as an unavoidable impurity in production, but if present in large amounts, the toughness of the manufactured steel decreases and the hot workability also decreases, so the upper limit is limited to 0.045%. did.

S: 0.030% or less,
S is an element that is effective in improving machinability. However, when the amount is increased, hot workability is significantly reduced. Therefore, the upper limit is set to 0.030%.

Al: 1.00% or less,
Al is an element necessary for deoxidation, and is also an element effective in refining crystal grains. In addition, when the nitriding treatment is performed, a small amount can be contained because it is effective in improving the hardness of the nitrided layer. However, when contained in a large amount, the boride of the 4A group, 5A group, or 6A group element mainly composed of Ti tends to be coarsened during solidification, and the matrix is hardened more than necessary and the mechanical properties are reduced, so that the upper limit is set. Was set to 1.00%.

Next, the invention of claim 3 clarifies the range of components that can be added as needed and that can be produced by a melting method in accordance with the characteristics required by various uses to be used. Specifically, it contains C, Si, Mn, P, S and Al, and further contains one or more optional elements of Ni, Cr, Mo, V and Al, and the balance Fe and unavoidable Steel having a matrix phase composed of a periodic impurity element,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, and the content when the above-mentioned optional element is contained is represented by mass%. And Ni: 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, and Nb: 1.00% or less. High rigidity steel that can be manufactured.
Hereinafter, the reason for limiting each component of the optional additive element will be described. The reasons for limiting the other components are as described above.

Ni: 14.00% or less,
When a small amount of Ni (approximately 5% or less) is added, it is effective in improving the hardenability and toughness of low alloy steel, and when the matrix is stainless steel, corrosion resistance is a basic element of stainless steel. It is an element that is effective for improvement. When a large amount of Ni is added, the structure becomes austenite, so that the resulting steel can be made non-magnetic. However, if added in a large amount, the hot workability decreases and hot rolling becomes difficult, so the upper limit was made 14.00%.

Cr: 28.00% or less,
Cr is an element necessary for improving the nitriding property in the case of nitriding in low-alloy steel, and is a basic element in stainless steel to ensure excellent corrosion resistance. Can be added. The required amount of addition varies greatly depending on the intended purpose, and a small amount close to the impurity level may be sufficient depending on the purpose. Therefore, the lower limit is not particularly limited. However, when the addition amount is increased, cracks are likely to occur during cooling after rolling, and the adverse effects on hot workability due to the presence of P and S contained as impurity elements become remarkable. It was 28.00%.

Mo: 4.00% or less,
Mo is generally an element that is effective for solid solution strengthening of the matrix and is necessary for improving the corrosion resistance of stainless steel. The necessary amount of addition greatly differs depending on the intended purpose as in the case of Cr, and in some cases, a small amount close to impurities may be used. Therefore, the lower limit is not particularly limited. However, even if it is contained in a large amount, the obtained effect is saturated and only the cost is increased. Therefore, the upper limit is set to 4.00%.

V: 1.00% or less,
V is an element capable of obtaining the effect of improving the hardness of the nitrided layer in the case of performing the nitriding treatment, and can be added in a small amount as necessary. However, even if it is contained in a large amount, the effect is saturated and the cost increases, so the upper limit was made 1.00%.

Nb: 1.00% or less,
Nb generally has the effect of solid solution strengthening of the matrix. Therefore, a small amount can be added if necessary, but if a large amount is added, the effect is saturated and the cost increases, so the upper limit was made 1.00%.

  In addition, the fact that the lower limits of many elements described above are not clarified is that, unlike many other alloy patents for which the present invention has already been published, the invention has been developed on the assumption that it is used for a specific application. The purpose of the invention is not to clarify the range of high-rigidity steels that can be produced by the smelting method, but also to the invention. That is, as described in the section on the reason for limiting Cr, the optimum amount of addition varies greatly depending on the application, and depending on the application, a small amount close to the amount of impurities may be required.

Next, in the invention according to claims 4 and 5, Cu is added to the components of the matrix phase which can be applied to the high-rigidity steel that can be produced by the smelting method described in the inventions of claims 2 and 3 by 0.5 to 3. It is intended to improve the strength by adding 5%.
Specifically, the invention of claim 4 is a steel containing C, Si, Mn, P, S, Al and Cu, and having a matrix phase composed of the balance Fe and unavoidable impurity elements,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, Cu: 0.5 to 3.5%. High rigidity steel that can be manufactured by the smelting method.

The invention according to claim 5 contains C, Si, Mn, P, S, Al and Cu, and further contains one or more optional elements of Ni, Cr, Mo, V, and Al. And a steel comprising, as a matrix phase, a balance of Fe and an unavoidable impurity element,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, Cu: 0.5 to 3.5%, and the above-mentioned arbitrary element When Ni is contained, the Ni content is 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, and Nb: 1.00% by mass%. A high-rigidity steel that can be manufactured by a melting method characterized by the following.

  As described above, the present invention secures a high Young's modulus by dispersing the boride, but when the element added for the purpose of boride generation bonds with carbon in the molten steel, the effect of improving the Young's modulus is reduced. . Therefore, in the present invention, the upper limit of the carbon content of the matrix phase is limited to 0.20% so that the adverse effect on the Young's modulus improving effect is not increased.

  However, C forms a solid solution in steel as an interstitial element and is the most effective element for improving the strength of steel by solid solution strengthening. If it is not possible to improve the strength by solid solution strengthening of C, depending on the parts to be applied, May not be able to secure sufficient strength. The steel of the present invention can achieve considerably high strength compared to before dispersing by dispersing the boride, but if that effect alone is insufficient, by adding Cu to precipitate strengthening, It has been found that further strengthening is possible. Hereinafter, the reasons for limiting the component range of Cu will be described. The reasons for limiting the other components are as described above.

Cu: 0.5 to 3.5%;
Cu has the effect of improving the strength by precipitation strengthening, and is an element necessary for solving the problem of insufficient strength of the present invention in which solid solution strengthening by C cannot be expected. Then, a necessary amount can be added in accordance with the strength required for each part. The reason why the lower limit is set to 0.5% is that a clear effect cannot be obtained as compared with the case where the addition is not performed at less than 0.5%. On the other hand, if it is added in a large amount, the hot workability is reduced and the surface flaws are liable to be generated on the rolled material and the like, and the production becomes difficult. Therefore, the upper limit is set to 3.5%.

  In order to sufficiently obtain the effect of improving the strength by precipitation hardening of Cu, heat treatment for precipitation hardening is required. Specifically, the strength can be improved by heating and holding at 450 to 650 ° C. for 2 to 10 hours.

Next, the invention of claim 6 relates to a method for producing a high-rigidity steel according to the present invention. That is, the material of the steel to be a matrix phase is dissolved in a closed space having an atmospheric pressure of 250 Pa or less, and contains at least one element selected from the group including Group 4A, Group 5A, and Group 6A elements. Dissolving an alloy containing B, which is the first raw material for dispersing the boride or / and the composite thereof in the finally obtained matrix phase,
Lastly, it is selected from the group including a group 4A element, a group 5A element, and a group 6A element, which are the second raw materials for dispersing the boride or / and the composite thereof in the finally obtained matrix phase. A method for producing high-rigidity steel, comprising adding an alloy containing one or more elements, melting the alloy, and then casting the alloy in a mold.

  The high-rigidity steel according to the present invention is produced by melting a raw material using a furnace capable of melting in a closed space under reduced pressure such as a vacuum induction melting furnace. Specifically, the pressure in the closed space is set to 250 Pa (about 1.9 torr) or less. The upper limit of the atmospheric pressure is set to 250 Pa because when the surface of the molten steel comes into contact with the atmosphere, an oxide film is generated on the surface in contact with the atmosphere, and it becomes difficult to pour the molten metal into the mold.

  That is, the molten metal composed of the component of the present invention naturally contains a large amount of Ti, and when it comes into contact with the atmosphere, an oxide of Ti is generated. Since this Ti oxide has a higher melting point than steel, a solid-state oxide film is generated. Even if the ladle containing the molten steel is tilted to pour the molten steel in the presence of such a solid film, there is a problem that the molten steel does not flow into the mold as expected. On the other hand, if this oxide film is to be melted, it must be raised to a temperature much higher than the melting temperature of normal steel, wasting energy, and even if the steel could be melted by raising the temperature, the steel produced would be Since a large amount of oxide-based inclusions is contained, various characteristics are deteriorated as a result.

  In addition, even when pouring from the lower part of the ladle without tilting, the existence of the oxide film becomes an obstacle, so that the casting efficiency is significantly reduced and the temperature drop until the pouring is completed is increased. There is. Therefore, in order to prevent such an oxide film from forming during casting, the pressure of the melting atmosphere is limited to 250 Pa or less.

  In addition, a steel material serving as a matrix and an alloy containing a large amount of B, which is the first material of boride, are dissolved, and finally, a second material, that is, a group 4A element, a group 5A element, and a group 6A element that are to be dispersed is increased. The addition of alloys containing the two elements is based on the fact that the relatively low-temperature coexistence of the iron-boron system exists in two parts, in order to enable the production of steel of stable quality by first producing the steel that will become the matrix. This is for dissolving easily and with low energy by utilizing the crystallization reaction. Furthermore, especially when an alloy containing a large amount of Ti is added at the end, the time from the final addition to tapping may be shortened to suppress the generation of Ti oxide as much as possible.

As described above, in order to suppress the generation of boride containing Fe, it is necessary to adjust the mixing ratio of B and the elements of 4A, 5A, and 6A to be formed so that B does not become excessive. is necessary.
Ferroboron is preferably used as an alloy containing a large amount of B as the first raw material. Further, in the present invention, as the main component of the boride to be dispersed is TiB ₂ in the present invention, it is preferable to use titanium sponge, titanium scrap or the like. In the case of dispersing other Group 4A, 5A and 6A elements, pure metals such as metallic molybdenum, ferroalloys containing a large amount of elements to be dispersed, and the like can be used.
Further, in order to finely crystallize TiB ₂ , the nucleus TiC must be preferentially crystallized. For this reason, it is necessary to appropriately adjust the cooling rate in the mold, and specifically, it is preferable that the cooling rate be lower than 10 K / min.

Finally, the heat treatment method for high rigidity steel according to the present invention will be described.
As described above, in the high-rigidity steel of the present invention, the components of the matrix phase can be adjusted over a wide range according to the intended use. The heat treatment may be a conventional heat treatment (for example, a solution heat treatment at about 1000 ° C. when the matrix phase is austenitic stainless steel) according to the type of the matrix phase steel. Thereby, desired characteristics can be obtained.

  In the case where Cu is added, a heat treatment for sufficiently dissolving Cu (solution heat treatment usually performed in stainless steel, or, in the case of quenching and tempering steel, by controlling the temperature during quenching). ), And the above-mentioned precipitation hardening heat treatment can be performed to improve the strength.

  In the case of performing a heat treatment having a temperature range overlapping with the above-described precipitation hardening treatment of Cu for tempering of a low alloy steel or the like, the heat treatment obtains a normal heat treatment effect and also serves as a precipitation hardening treatment of Cu. Can be.

(Example 1)
Next, the effects obtained by the present invention will be clarified by examples. Tables 1 to 4 show the chemical components of the test materials used in this example. Among them, 1 to 19 steels satisfy the conditions of the component range and the atmosphere pressure at the time of melting specified in the present invention, and 20 to 32 steels have some components or the atmospheric pressure at the time of melting. It is a comparative example which does not satisfy the conditions of the present invention. O in Tables 1 to 4 is not described in the claims but is included as an impurity, and thus shows the actual value.

Melting was performed in a closed space adjusted to an atmospheric pressure of 250 Pa or less using a vacuum induction melting furnace, except for some comparative examples. First, ferroboron is added and melted as an alloy (first raw material) containing a steel material and a B-rich material as a matrix, and finally an alloy containing a large amount of a 4A group, 5A group, or 6A group element to be dispersed ( as the second starting material), the titanium sponge for boride dispersing the TiB ₂ only, if the boride other than TiB ₂ is also dispersed at the same time, pure metal comprising group 4A dispersing, 5A group, and 6A group elements Is added in a required amount to disperse a target amount of boride.

  The test steel consisting of the components shown in Tables 1 to 4 is obtained by forging the obtained steel ingot to a diameter that allows easy preparation of a test piece, and is suitable for each matrix phase component among the heat treatment conditions shown in Table 5. Heat treatment was performed under the above conditions (selected conditions are shown in Table 6 described later), and a test was performed by machining into a predetermined shape. Hereinafter, the test methods performed will be described.

  The Young's modulus was measured by a composite vibrator method using a quartz vibrator by forming a 1 × 2 × 11.2 mm square bar from the center of the test material forged to φ15. The resonance frequency of the crystal unit is 110.25 kHz.

  The castability was evaluated by observing the situation when each steel was actually melted in a vacuum induction melting furnace and then cast into a mold, and based on the results. Specifically, it is equivalent to ordinary steel not dispersing boride, and the case where there is no problem is ◎, the fluidity is slightly inferior to the case of ◎, but the case where there is no significant influence on the casting operation is ○. , Casting was completed, but the fluidity was greatly reduced, a large amount of steel adhered to the furnace body, and the casting work was considerably hindered. The case where the normal casting operation could not be performed even when the furnace body was tilted is indicated by X in Table 6 below. If the casting operation could not be performed, the solidified material was taken out of the furnace and another evaluation was performed.

  For hot workability, a test piece with a parallel part size of φ8 x 50 mm was prepared from the forged material, heated to a predetermined temperature in 100 seconds, maintained at a temperature of 60 seconds, and then pulled at a speed of 50 mm / sec to break. It was evaluated by measuring the squeezing rate of the broken part. In the temperature range of 1000 to 1100 ° C., the case where the drawing ratio is 90% or more is ◎, the case where the drawing ratio is less than 80 to 90% is ○, the case where the drawing ratio is less than 70 to 80% is △, and the case where the drawing ratio is less than 70% is ×. The results are shown in Table 6 below.

  As is clear from Table 6, the steels of the present invention, 1 to 19, have a Young's modulus of 235 to 273 GPa, which is higher than that of ordinary iron alloys of about 200 GPa, and have a castability that is not problematic in actual production. It can be seen that the steel shows hot workability. On the other hand, the 20 to 32 steels of the comparative examples are those in which any of the Young's modulus, castability, and hot workability are reduced because some conditions do not satisfy the conditions specified in the present invention. It is. Specifically, since steels 20 and 30 have a high C content, TiC is generated and the effect of improving the Young's modulus is reduced, and castability and hot workability are reduced. The pressure at the time of melting is too high, so that an oxide film is formed and castability is reduced, and the 22 and 23 steels are those having reduced hot workability due to high Cu or Si content, respectively. Steel No. 24 tried to disperse a large amount of Ti boride, but could not produce a uniform steel ingot, resulting in a steel ingot whose Young's modulus was not constant depending on the portion. The results are not shown.) In addition, the castability and the hot workability were significantly reduced, and the effect of improving the Young's modulus was not sufficiently obtained because the amount of boride added to 25 steel was small. And 26-29 steels are each Mn Ni, since P, the content of S higher, in which hot workability is lowered. The 31-33 steels contain more boride other than Ti than the 15 and 16 steels. However, the increased Young's modulus is less than that of the 15 and 16 steels, and the castability and hot workability are lower. The workability was reduced.

  As described above, there are considerable restrictions on enabling the production by the smelting method, but by manufacturing within the range shown in the present invention, the production by the smelting method is enabled, and the conventional sintering method is used. It is possible to achieve a significant reduction in cost as compared with high rigidity steel.

  Next, another example illustrating the strength obtained by the high-rigidity steel according to the present invention will be described. Of the test materials used in the above examples, tensile test pieces having a parallel portion of φ8 × 55 mm were prepared for steels 1, 2, 4 to 8, 11, 13, 14, and 19 at a tensile speed of 2 mm / min. A tensile test was performed to measure 0.2% proof stress and tensile strength. In order to accurately grasp the change in strength due to the dispersion of the boride, the base steel (ie, only the matrix phase) before the addition of the boride was simultaneously tensioned for 1, 4, 6, 11, and 19 steels. The strength was measured. Table 7 shows the results.

  As is clear from Table 7, dispersing the boride not only improves the Young's modulus but also increases the strength. When the aging treatment is performed by adding Cu, the strength further increases. It can be seen that the strength is also increased by increasing the amount of Mn. As described above, the high-rigidity steel according to the present invention can increase both the Young's modulus and the strength by dispersing a boride or adding Cu, Mo, Mn, or the like. A great lightening effect can be expected.

  As described above, the evaluation results of the static strength are shown. However, many parts such as automobiles are repeatedly subjected to a variable stress for a long period of time, and are required to have excellent fatigue strength. Therefore, among the test materials of the steel of the present invention described above, the base steel (i.e., only the matrix phase) before adding the boride of 1, 2, 4 to 6, 8 steel and 1, 6 steel has a parallel portion. A rotating bending fatigue test (smoothness) was performed on a φ8 test piece, and the fatigue strength at 107 rotations was determined to evaluate the fatigue characteristics. The steels 4, 5, and 8 correspond to the steels obtained by adding Cu to the steels 1, 2, and 6, and from this result, it is possible to accurately grasp the effect of improving the fatigue strength by adding Cu to strengthen the precipitation. can do. Table 8 shows the results.

  It is obvious that the dispersing of the boride not only improves the Young's modulus as described above, but also makes it possible to simultaneously improve the fatigue strength, as is clear from Table 8. Further, similarly to the result of the tensile strength, the strength can be further improved by adding Cu and performing aging treatment. Therefore, a significant reduction in weight can be expected by using the steel of the present invention even for a part where fatigue characteristics are important.

As described above, the conditions under which the present invention can produce high-rigidity steel by the smelting method have been clarified. As a result, it has become possible to manufacture at a significantly lower cost than in the case of manufacturing by sintering, and also to manufacture a large part as compared with sintering.
In addition, the steel of the present invention can increase both Young's modulus and strength by dispersing boride, and if necessary, can further improve the strength by adding Cu and aging treatment, so that a significant weight reduction can be realized. It is assumed that. Therefore, the industrial effect is extremely remarkable.

Claims (6)

In a matrix phase composed of pure iron or an iron alloy, a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe, and / or a complex thereof is contained in a matrix phase. It is a steel in which 25 vol% is dispersed, and 80% or more by volume of the boride or / and a composite thereof dispersed therein is TiB ₂ , and the content of C in the entire steel is 0.20% by mass or less. A high-rigidity steel that can be produced by a melting method, A steel containing C, Si, Mn, P, S, and Al, and having a matrix phase composed of a balance of Fe and inevitable impurity elements,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: A high-rigidity steel that can be produced by a melting method, characterized in that it is 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, and Al: 1.00% or less. It contains C, Si, Mn, P, S, and Al, further contains one or more of Ni, Cr, Mo, V, and Al, and the balance is Fe and inevitable impurity elements. A steel having a matrix phase,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, and the content when the above-mentioned optional element is contained is represented by mass%. And Ni: 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, and Nb: 1.00% or less. High rigidity steel that can be manufactured. A steel containing C, Si, Mn, P, S, Al and Cu, and having a matrix phase consisting of a balance of Fe and inevitable impurity elements,
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, Cu: 0.5 to 3.5%. High rigidity steel that can be manufactured by the smelting method. It contains C, Si, Mn, P, S, Al and Cu, further contains one or more optional elements of Ni, Cr, Mo, V and Al, and the balance Fe and unavoidable impurity elements Steel using a steel consisting of
In the matrix phase, 5 to 25 vol% of a boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, a group 6A element and Fe or / and a complex thereof is dispersed. 80% or more by volume of the dispersed boride or / and composite thereof is TiB ₂ ,
In addition, when the entire steel is 100%, the content of the element contained only in the matrix phase is C: 0.20% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, Cu: 0.5 to 3.5%, and the above-mentioned arbitrary element When Ni is contained, the Ni content is 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, and Nb: 1.00% by mass%. High-rigidity steel that can be produced by a melting method characterized by the following. A boride containing at least one element selected from the group consisting of a group 4A element, a group 5A element, and a group 6A element while dissolving a raw material of steel to be a matrix phase in a closed space having an atmospheric pressure of 250 Pa or less. And / or dissolving an alloy containing B, which is a first raw material for dispersing the composite in the finally obtained matrix phase,
Lastly, it is selected from the group including a group 4A element, a group 5A element, and a group 6A element, which are the second raw materials for dispersing the boride or / and the composite thereof in the finally obtained matrix phase. A method for producing a high-rigidity steel, comprising adding an alloy containing one or more elements, melting the alloy, and then casting the alloy in a mold.