JP4213022B2 - High-stiffness steel that can be manufactured by melting method and manufacturing method thereof - Google Patents

Description

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

  Steel materials are most commonly used for machine structural members because they are the cheapest and can be easily mass-produced. These structural members have various required characteristics depending on the parts used, but the common requirement is that there is no change from the past, which is weight reduction. This is because it has been objectively proved that weight reduction reduces the amount of energy consumed when using a product such as an automobile or a truck, and a great effect in energy saving can be obtained. However, achieving weight reduction often involves great difficulties. In other words, when considering the achievement of weight reduction, the required strength can be ensured even if the component dimensions are reduced, and the amount of elastic deformation caused by the force applied during actual use (deflection, This is because it is impossible to avoid the solution of whether or not torsion is suppressed to a level that does not cause a problem.

  Among these, with regard to the former problem, it is possible to add an appropriate amount of a large number of additive elements to the steel material, and by the effect of the addition, solid solution strengthening, precipitation strengthening, or by performing an optimal heat treatment, The strength of the material can be increased, and many problems have been solved so far.

  However, no definitive solution has been found for the latter. In other words, even if steel is strengthened by means of solid solution strengthening, precipitation strengthening or strengthening by heat treatment, the Young's modulus of the steel material does not change greatly, and the higher the strength and weight, the smaller the component dimensions. This is because the rigidity of the component itself decreases, and the amount of deformation increases conversely. In addition, it is not inconceivable to change to materials other than steel materials, but other alloys with lower density than iron alloys such as Mg alloy, Ti alloy, Al alloy, all have Young's modulus significantly lower than steel materials, The problem cannot be solved even by changing the alloy used. Therefore, in the conventional technology, even if the strength that can withstand the load stress during use is obtained, if there is a problem in terms of the amount of elastic deformation, there is only a method for suppressing the amount of elastic deformation by increasing the component dimensions. It was. However, this naturally leads to an increase in mass, and as a result, it is no longer possible to achieve the original weight reduction.

  The increase in the mass of parts is also a problem from another aspect. In other words, the load stress applied to the structural member is the load applied to each component by using a product (such as an automobile) made of the structural member, which is naturally received from the outside (for example, loading a heavy object on a truck). ) As well as the force that is naturally applied by the movement of each member regardless of the external force (for example, the inertial force acting on a reciprocating member such as a piston or a pin, or the gravity or centrifugal force acting on the rotating shaft) Bending moment). This inertial force, centrifugal force, gravity, and the like all have a problem that they increase as the mass of each part of the structural member increases. Therefore, in order to reduce the amount of deformation such as bending of each component, there is no method other than increasing the rigidity of the component without increasing the size of the component. For this purpose, development of a material having a high Young's modulus is indispensable. Met.

  In order to enable an appropriate response to this problem, new attempts for the development of materials having a high Young's modulus have recently been made, and patent applications as development results are gradually increasing. For example, patent documents 1 to 3 have been filed. Although these three patents are slightly different from each other in terms of their contents, the common point of all three patents is that the group 4A is known to exhibit a high Young's modulus in a matrix composed mainly of iron. It is characterized in that a high Young's modulus can be obtained by dispersing a boride of the element, group 5A element.

Japanese Patent Laid-Open No. 7-188874 JP-A-7-252609 JP-A-10-68048

However, the above-described conventional technology has the following problems.
Of the above-mentioned three patents, 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 manufacturing method by sintering, an expensive mold is necessary in the molding process. However, since a large amount of force is required, it is not suitable for manufacturing relatively large parts such as a crankshaft, 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.

  Further, Patent Document 3 describes a high-rigidity and high-toughness steel obtained by a melting method, but the optimum range for an iron alloy used as a matrix when dispersing boride by a melting method is described above. It does not show any new findings for the two patents. In other words, many steel materials such as carbon steel, nickel chrome steel, chrome steel, chrome molybdenum steel, spring steel, bearing steel are described, but this is a problem that did not occur in the manufacturing method by powder sintering, The problem that occurs when manufacturing by the method is not fully studied. As a result of the present inventors repeatedly investigating trial production tests, when manufacturing high-rigidity steel by applying the melting method, the manufacturable range is considerably narrowed, and if it is not manufactured within that range, the quality It was found that high-stiffness steel with good quality could not be obtained.

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

First, as a high-rigidity steel that can be produced by a melting method, one or more elements selected from the group comprising a group 4A element, a group 5A element, a group 6A element, and Fe in a matrix phase made of pure iron or an iron alloy Is a steel in which 5 to 25 vol% of a boride containing and / or a composite thereof is dispersed, and 80% or more by volume of TiB ₂ in the dispersed boride or / and the composite thereof is TiB ₂ , There is a high-rigidity steel that can be produced by a melting method characterized in that the C content in the whole is 0.20% by mass or less.

  What should be noted in the present invention is that the range of the C content contained in the matrix and the range of the volume fraction of boride in order to make it possible to produce a highly rigid steel material by a melting method.

  When manufactured by the melting method, as a matter of course, the raw material is heated to be melted at a high temperature, so that an active reaction occurs in the molten steel. Among these reactions, there is a reaction that is difficult to occur in the powder method, and this reaction may be an obstacle in producing high-performance steel with good performance.

  The inventors of the present invention investigated in detail whether or not a new problem that did not need to be considered in the production by the powder method would occur when produced by the melting method. As a result, the present invention has been completed by obtaining the following knowledge.

  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 carbides, and a molten steel containing C. When an alloy containing the 4A group, 5A group, and 6A group elements and B are added therein, a part of these elements does not bond with B but bonds with C, and a carbide is formed. In particular, it has been found that when the C content in molten steel increases, the number of elements that become carbide increases, and the target boride cannot be sufficiently produced.

In particular, TiB _2, which is a Ti boride, has a lower density than steel, and its use is very effective for achieving the object of the present invention in order to improve the specific Young's modulus (ratio of Young's modulus to the density). However, if TiB ₂ is not produced and a large amount of TiC is produced, the effect of improving the specific Young's modulus is lowered, 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 has no significant effect on the decrease in Young's modulus. On the other hand, it suppresses the growth of TiB ₂ into a long hexagonal column and contributes to the improvement of machinability. It is preferable to generate TiC within a range where there is no adverse effect of the decrease.

In addition, carbides of 4A group, 5A group, and 6A group elements have been known to show high Young's modulus for a long time, and patents for high-rigidity steels using carbides have been filed, but carbides are easy to coarsen, Therefore, it turned out that there exists a tendency which worsens castability. Also, in the case of molten high-rigidity steel, it is necessary to roll the steel ingot and further perform forging, machining, etc. to the desired part shape. Since severer conditions are required compared with the case of manufacturing, special consideration is required. Therefore, 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 is 25% or less. It has been found that it is essential to enable hot working such as rolling and forging .

  In addition, the reason why the boride containing Fe is included in the claims is that a part of the added B does not bind to elements such as Ti, 4A group, 5A group, and 6A group but binds to Fe that exists in a large amount. This is because production may be difficult if the presence of a boride containing Fe is not recognized at all.

Next , the reasons for limiting each condition will be described below.
The upper limit of the C content is 0.20% by mass (in order to clarify whether the component range is within the scope of the present invention in the product or not , the dispersed boride and the matrix phase In addition, the content is limited to the total content of elements contained in C. In addition, the content of C is TiC). This is because the group 6A element and carbon in the molten steel are combined with carbon in the molten steel to form carbides, and borides are not generated in a target amount, and the Young's modulus improvement effect cannot be obtained sufficiently. A small amount of carbide is effective for improving the strength and is less harmful. However, if a large amount of carbide is produced, the flowability and hot workability during casting deteriorate, so the upper limit was limited to 0.20%.

However, if C is added in a small amount, even if TiC is produced, the Young's modulus does not have a significant adverse effect, and in order to finely crystallize TiB ₂ as will be described later, its core TiC Is necessary, and a small amount is a necessary element. Further, when TiC is formed, TiB ₂ is prevented from growing into a long and hexagonal columnar shape, and TiB ₂ and TiC are finely dispersed in the same position with each other. All three elements of C and C are detected), and the damage to the tool when the hard and elongated TiB2 is shortly crushed during machining is reduced, and the tool life is improved. Therefore, in consideration of workability, the amount of C added is at least 0.06% by mass, more preferably 0.08% or more.

Except for Fe, the borides to be generated are limited to elements of Group 4A, 5A, and 6A (including Fe as described above). This is because elemental borides have a high Young's modulus. In the present invention, the main component is particularly limited to TiB ₂ which is a boride containing Ti. This is because TiB ₂ exhibits a high Young's modulus, but has a much lower density than steel, and is therefore effective for 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 to make boride containing Ti (TiB ₂ ) 80% or more (volume ratio) of the boride to be generated. This is because the amount of borides other than Ti increases. For example, when the amount of (Mo, Fe) B ₂ , (Cr, Fe) B, etc. increases, the Young's modulus increases compared to the steel before the boride is dispersed. However, this is because the specific Young's modulus improvement effect is smaller than when the boride is only Ti. Moreover, when these borides increase, the hot workability and castability will also fall and the problem that manufacture by a melting method will become difficult will also arise. Therefore, by mainly dispersing boride mainly composed of Ti, it is possible to obtain a high-rigidity steel having a high Young's modulus for a small specific gravity and easy to manufacture by a melting method.

In order to prevent the formation of borides containing Fe, the ratio of the addition amount of the 4A group, 5A group, 6A group element and B to be generated is adjusted so that no excessive B exists. What should I do? B binds preferentially to 4A group, 5A group, and 6A group elements, so when B is in an excessively existing state, it is combined with a large amount of Fe to form a boride containing Fe. It is. For example, when TiB ₂ is generated, the mixing ratio of Ti and B may be close to an atomic ratio of 1: 2 (mass ratio of Ti / B = 2.215). Thereby, the production | generation of the boride containing Fe can be prevented.

  The reason why the volume ratio of the boride dispersed in the steel or its composite is limited to 5 to 25% is that the Young's modulus improvement effect cannot be obtained sufficiently if it is less than 5%. If it is over, the resulting Young's modulus will be higher than when the amount added is small, but it will be difficult to disperse uniformly, resulting in non-uniform Young's modulus depending on the location of the product, stable high Young's modulus It is because it becomes difficult to guarantee. On the other hand, the powder method is more advantageous than the melting method because it can be made fairly uniform by mixing raw material powders well. In addition, in the melting method, when a large amount of boride is added, there is a problem that hot workability is lowered and it is difficult to process into a predetermined shape by rolling or forging. Therefore, the upper limit of the amount of boride to be dispersed was strictly limited as compared with the upper limit limited in the prior application. Further, although the Young's modulus and the strength are both increased as a large amount is added, it is necessary to pay attention because the elongation and drawing are lowered and the hot workability is lowered.

Next, the feature of the invention described in claim 1 is steel that contains C, Si, Mn, P, S, and Al, and has a matrix phase composed of the balance Fe and unavoidable impurity elements,
In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. , 80% or more by volume of the dispersed boride or / and its composite is TiB ₂ ,
And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, Mn: It is a high-rigidity steel that can be produced by a melting method characterized by 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, and Al: 1.00% or less. .
The reason for limiting the component range excluding C will be described below. The present invention is intended to provide a steel suitable for production by a melting method, and is not limited to a specific application, but is intended to provide a steel that can be used for various types of applications. Of course, it includes a range having greatly different characteristics.
The content of each element is calculated by dividing the mass contained only in the matrix phase by the mass of the entire steel, and the content in the boride should be taken into account in the calculation of the content. does not enter.

Si: 0.10 to 1.00%,
Si needs to be added in a small amount in order to ensure the fluidity of the molten steel at the time of casting, and it must contain at least 0.10%. Desirably, it is good to set it as 0.30% or more. On the other hand, when contained in a large amount, boride of 4A group, 5A group, 6A group element mainly composed of Ti is easily coarsened during solidification, and the matrix is hardened more than necessary, so that hot workability is lowered. Therefore, since the production by rolling becomes difficult, 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 nonmagnetic, so that it is possible to provide a nonmagnetic and high Young's modulus material. However, when it is contained in a large amount, the hot workability is lowered and the production by hot rolling becomes difficult, so the upper limit is limited to 8.00%.

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

S: 0.030% or less,
S is an element effective in improving the machinability, but when the amount is increased, the hot workability is remarkably lowered, so the upper limit was made 0.030%.

Al: 1.00% or less,
Al is an element necessary for deoxidation, and is also an element effective for crystal grain refinement. Further, when nitriding is performed, it is effective for improving the hardness of the nitrided layer, so that it can be contained in a small amount. However, if contained in a large amount, the boride of the 4A group, 5A group, and 6A group elements mainly composed of Ti is easily coarsened during solidification, and the matrix is hardened more than necessary, so that the mechanical properties are lowered. Was 1.00%.

Next, the invention of claim 2 clarifies the component range that can be added as necessary and can be produced by a melting method in accordance with 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, with the balance being Fe and inevitable. Steel having a matrix phase composed of mechanical impurity elements,
In the matrix phase, boride containing one or more elements selected from the group containing 4A group element, 5A group element, 6A group element and Fe, and / or its complex is dispersed in an amount of 5 to 25 vol%. , 80% or more by volume of the dispersed boride or / and its composite is TiB ₂ ,
And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, Mn: When the content is 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, and the above-mentioned optional element is contained, 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. It is in high-stiffness steel that can be manufactured.
Hereinafter, the reason for limitation of each component of an arbitrary addition element is demonstrated. The reasons for limiting the other components are as described above.

Ni: 14.00% or less,
When Ni is added in a small amount (approximately 5% or less), it is effective in improving the hardenability and toughness of the low alloy steel. When the matrix is stainless steel, corrosion resistance is a basic element of stainless steel. It is an effective element for improvement. When a large amount of Ni is added, the structure is austenitized, so that the obtained steel can be made nonmagnetic. However, if it is 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 nitriding properties when nitriding in low-alloy steels. In stainless steel, it is a basic element for ensuring excellent corrosion resistance. Can be added. The required amount of addition varies greatly depending on the intended purpose, and depending on the purpose, a small amount close to the impurity level may be sufficient, so 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 effect on hot workability due to the presence of P and S contained as impurity elements becomes significant, so the upper limit is set. It was 28.00%.

Mo: 4.00% or less,
Mo is generally effective for solid solution strengthening of the matrix, and is an element necessary for improving corrosion resistance in stainless steel. Further, the necessary addition amount varies greatly depending on the intended purpose, similarly to Cr, and in some cases, a small amount close to impurities may be sufficient, so the lower limit value is not particularly limited. However, since the effect obtained even if it is contained in a large amount only saturates and increases the cost, the upper limit was made 4.00%.

V: 1.00% or less,
V is an element that can obtain the effect of improving the hardness of the nitrided layer when nitriding is performed, 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 is increased, so the upper limit was made 1.00%.

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

  The lower limit value is not clarified in many of the elements described above, which is developed on the assumption that the present invention is used for a specific application, unlike many other alloy patents whose applications have already been published. This is because it aims to clarify the range of high-rigidity steel that can be manufactured by the melting method, not just the invention. That is, as explained in the section on the reason for limiting Cr, the optimum addition amount varies greatly depending on the use, and depending on the use, a small amount of addition close to the impurity amount may be required.

Next, according to the third and fourth aspects of the present invention, Cu is further added to the component of the matrix phase applicable to the high-rigidity steel that can be produced by the melting method shown in the first and second aspects of the invention. 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 inevitable impurity elements,
In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. , 80% or more by volume of the dispersed boride or / and its composite is TiB ₂ ,
And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, 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% It is a high-rigidity steel that can be manufactured by melting.

The invention of claim 4 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 steel using the balance Fe and inevitable impurity elements in the matrix phase,
In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. , 80% or more by volume of the dispersed boride or / and its composite is TiB ₂ ,
And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, 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 optional elements In the case of containing Ni, the content is Ni: 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, Nb: 1.00% by mass%. It is a high-rigidity steel that can be produced by a melting method characterized by the following.

  As described above, the present invention secures a high Young's modulus by dispersing boride, but when the element added for the purpose of boride formation is combined with carbon in the molten steel, the Young's modulus improving effect 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 improvement effect is not increased.

  However, C dissolves in steel as an interstitial element and is the most effective element for improving the strength of steel by solid solution strengthening. If the strength cannot be improved by solid solution strengthening of C, May not be able to secure sufficient strength. The steel of the present invention can achieve a considerably higher strength than before being dispersed by dispersing the boride, but when the effect alone is insufficient, by adding Cu to enhance precipitation, It has been found that the strength can be further increased. Hereinafter, the reasons for limiting the Cu component range will be described. The reasons for limiting the other components are as described above.

Cu: 0.5 to 3.5%,
Cu has an 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 due to C cannot be expected. And a required amount can be added according to the intensity | strength requested | required for every components. The reason why the lower limit is set to 0.5% is that if it is less than 0.5%, a clear effect cannot be obtained as compared with the case where it is not added. On the other hand, when too much is added, hot workability is deteriorated and surface flaws are likely to occur in the rolled material and the production becomes difficult, so the upper limit was made 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, invention of Claim 5 is related with the manufacturing method of the highly rigid steel which is this invention. That is, the steel raw material that becomes a matrix phase is melted in a closed space having an atmospheric pressure of 250 Pa or less, and at least one element selected from the group including a group 4A element, a group 5A element, and a group 6A element is included. Melting an alloy containing B as a first raw material for dispersing boride or / and its composite in the finally obtained matrix phase;
Finally, it is selected from the group including the 4A group element, 5A group element, and 6A group element which are the second raw materials for dispersing the boride or / and its composite in the finally obtained matrix phase. An alloy containing one or more elements is added, dissolved, and then cast into a mold.

  The high-rigidity steel according to the present invention is manufactured by melting raw materials using a furnace that can be melted in a closed space such as a vacuum induction melting furnace. Specifically, the atmospheric pressure in the closed space is set to 250 Pa (about 1.9 torr) or less. The reason why the upper limit of the atmospheric pressure is set to 250 Pa is that when the surface of the molten steel comes into contact with the atmosphere, an oxide film is generated on the contact surface with the atmosphere, making it difficult to pour molten metal into the mold.

  That is, the molten metal composed of the components 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 oxide film is produced. Even if the ladle containing the molten steel is tilted in an attempt to pour the molten steel in a state where such a solid film exists, 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 considerably higher than the melting temperature of ordinary steel, and energy is wasted. Since a large amount of oxide inclusions is contained, there arises a problem that various characteristics are deteriorated as a result.

  Also, even when trying to pour from the bottom of the ladle without tilting, the presence of the oxide film becomes an obstacle, the casting efficiency is remarkably lowered, and the temperature drop until the end of pouring is increased. There is. Therefore, in order to prevent the formation of an oxide film that becomes an obstacle during casting, the atmospheric pressure of the melting atmosphere is limited to 250 Pa or less.

  Also, the steel material used as the matrix and the alloy containing a large amount of B which is the first material of boride are dissolved, and finally the 4A group element, 5A group element and 6A group element which are the second material to be dispersed are increased. One of the reasons for adding the alloy is to produce a stable steel by producing the matrix steel first, so that the two are relatively low temperature co-existing in the iron-boron system. This is because the crystallization reaction is used for easy and low energy dissolution. Furthermore, when an alloy containing a large amount of Ti is added at the end, the time from the final addition to the hot water is shortened to suppress the generation of Ti oxide as much as possible.

As described above, in order to suppress the formation of boride containing Fe, it is possible to adjust the compounding ratio of the elements of Group 4A, Group 5A, and Group 6A to be generated so that B does not become excessive. is necessary.
Further, ferroboron may be used as an alloy containing a large amount of B as the first raw material. Further, as the second raw material, TiB ₂ is mainly used for the boride to be dispersed in the present invention, so it is preferable to use sponge titanium, titanium scrap or the like. When other 4A group, 5A group, and 6A group elements are dispersed, pure metal such as metallic molybdenum, alloy iron containing a large amount of elements to be dispersed, or the like can be used.
Further, in order to crystallize TiB ₂ finely, TiC as the nucleus must be crystallized preferentially. For this reason, it is necessary to appropriately adjust the cooling rate in the mold, and specifically, it is preferable to set the rate slower than 10 K / min.

Finally, the heat treatment method for high-rigidity steel according to the present invention will be described.
As described above, the high-rigidity steel of the present invention can adjust the components of the matrix phase in a wide range according to the intended use. Then, the heat treatment may be performed according to the steel type of the matrix phase as conventionally performed (for example, when the matrix phase is austenitic stainless steel, solution heat treatment at about 1000 ° C.). Thereby, the target characteristic can be obtained.

  In addition, when Cu is added, heat treatment for sufficiently dissolving Cu (in the case of a solution heat treatment usually performed in stainless steel, or in steel that is quenched and tempered, the temperature is adjusted at the time of quenching. Can be sufficiently dissolved, and then the above precipitation hardening heat treatment can be performed to improve the strength.

  In addition, when performing heat treatment that overlaps the above-mentioned Cu precipitation hardening treatment with a temperature range by tempering low alloy steel, etc., this heat treatment can obtain a normal heat treatment effect and also serve as Cu precipitation hardening treatment. Can do.

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 these, 1-19 steel is steel which satisfies the conditions of the component range prescribed | regulated by this invention, and the conditions of the atmospheric pressure at the time of melt | dissolution, and 20-32 steel has the atmospheric pressure at the time of a one part component or melt | dissolving. It is a comparative example which does not satisfy the conditions of the present invention. In addition, although O described in Tables 1 to 4 is not described in the claims but is contained as an impurity, it shows the actual value.

Melting was performed in a closed space adjusted to an atmospheric pressure of 250 Pa or less, except for some comparative examples, using a vacuum induction melting furnace. First, an alloy containing a large amount of elements of Group 4A, Group 5A, and Group 6A to be dispersed by adding ferroboron as an alloy containing a large amount of B and a steel material (first material). As the second raw material), if the boride to be dispersed is only TiB ₂ , the sponge titanium is dispersed. If the boride other than TiB ₂ is also dispersed at the same time, the pure metal containing the 4A group, 5A group and 6A group elements to be dispersed is used. Is added, and a desired amount of boride is dispersed.

  The test steels composed of the components shown in Tables 1 to 4 forge the obtained steel ingot to a diameter that makes it easy to create a test piece, and are suitable for each matrix phase component among the heat treatment conditions shown in Table 5. The samples were heat-treated under the conditions (selected conditions are shown in Table 6 below), machined into a predetermined shape, and tested. Hereinafter, the test method performed will be described.

  The Young's modulus was measured by preparing a square bar of 1 × 2 × 11.2 mm from the center portion of the specimen forged to φ15, and measuring it by a composite vibrator method using a crystal vibrator. The resonance frequency of the crystal resonator is 110.25 kHz.

  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 the result was evaluated. Specifically, it is equivalent to ordinary steel without boride dispersed, and when there is no problem, ◎, fluidity is slightly inferior to ◎, but there is no significant influence on casting work ○ Although casting was possible, the fluidity was greatly reduced, and a large amount of steel adhered to the furnace body, which significantly hindered the casting operation, and the productivity dropped significantly. The case where normal casting work could not be carried out even if the furnace body was tilted was shown in Table 6 below. For those that could not carry out the casting operation, the solidified material was taken out into the furnace and subjected to other evaluations.

  For hot workability, a test piece having a parallel part size of φ8 × 50 mm was prepared from a forged material, heated to a predetermined temperature in 100 seconds, held at the temperature for 60 seconds, and then pulled at a speed of 50 mm / second to break. And evaluated by measuring the squeezing rate of the fractured portion. And in the temperature range of 1000 to 1100 ° C., when the drawing ratio is 90% or more, ◎, 80 to less than 90% ○, 70 to less than 80% Δ, and less than 70% × This is shown in Table 6 below.

  As is apparent from Table 6, the steels 1 to 19 of the present invention have a Young's modulus of 235 to 273 GPa and a high Young's modulus as compared with about 200 GPa of a normal iron alloy, and there is no problem in actual production. It can be seen that it exhibits hot workability. On the other hand, steels 20 to 32, which are comparative examples, are those in which any of the Young's modulus, castability, and hot workability is lowered because some conditions do not satisfy the conditions specified in the present invention. It is. Specifically, Steel Nos. 20 and 30 have a high C content, so TiC is generated and the Young's modulus improvement effect is reduced, and castability and hot workability are reduced. Since the atmospheric pressure at the time of dissolution is too high, an oxide film is generated and castability is lowered, and steels 22 and 23 are those in which hot workability is lowered due to high Cu or Si content, Steel No. 24 tried to disperse a large amount of Ti boride, but it could not produce a uniform steel ingot, and the Young's modulus was not constant depending on the part. The rate measurement results are not shown.) And castability and hot workability are greatly reduced, and 25 steel has a small amount of added boride, so the Young's modulus improvement effect could not be sufficiently obtained. 26-29 steel is Mn Ni, since P, the content of S higher, in which hot workability is lowered. In addition, the steels 31 to 33 are borides other than Ti, which are increased compared to the 15 and 16 steels. However, the increased Young's modulus compared to the 15 and 16 steels is small, and the castability and hotness are small. The workability is reduced.

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

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

  As is apparent from Table 7, not only the Young's modulus is improved but also the strength is increased by dispersing the boride, and the strength is further increased when Cu is added to perform aging treatment. It can be seen that the strength is increased by increasing the amount of Mn. As described above, the high-rigidity steel according to the present invention can increase the Young's modulus and strength by a method such as dispersing boride or adding Cu, Mo, Mn, etc. A great lightening effect can be expected.

  Although the evaluation results for static strength have been described above, parts such as automobiles are often subjected to repeated 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 parallel parts of the steels of the base steel (that is, only the matrix phase) before adding boride of steels 1, 2, 4-6, 8 and 1 and 6 A rotating bending fatigue test (smooth) was performed on a φ8 specimen, and the fatigue strength at 10 7 revolutions was determined to evaluate the fatigue characteristics. Steels 4, 5, and 8 correspond to steels with Cu added to steels 1, 2, and 6. From this result, the fatigue strength improvement effect due to precipitation strengthening by adding Cu is accurately grasped. can do. The results are shown in Table 8.

  It goes without saying that the Young's modulus is improved as described above by dispersing the boride, but as is apparent from Table 8, it can be seen that the fatigue strength can also be improved at the same time. Similarly to the result of tensile strength, the strength can be further improved by adding Cu and aging treatment. Therefore, a significant reduction in weight can be expected by using the steel of the present invention even for a portion where fatigue characteristics are important.

As described above, the conditions under which high-rigidity steel can be produced by the melting method have been clarified by the present invention. As a result, it is possible to manufacture at a significantly lower cost compared to the case of manufacturing by sintering, and it is also possible to manufacture large parts compared to sintering.
In addition, the steel of the present invention can increase both Young's modulus and strength due to the dispersion of boride, and if necessary, the strength can be further improved by adding Cu and aging treatment. It is what. Therefore, the industrial effect is extremely remarkable.

Claims (5)

A steel containing C, Si, Mn, P, S, and Al and having a matrix phase composed of the balance Fe and inevitable impurity elements,
  In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. 80% or more by volume of the boride and / or its composites dispersed is TiB. ₂ And
  And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, Mn: A high-rigidity steel that can be manufactured by a melting method, characterized by 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, and further contains one or more optional elements of Ni, Cr, Mo, V and Nb, and consists of the remainder Fe and unavoidable impurity elements. A steel having a matrix phase,
  In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. 80% or more by volume of the boride and / or its composites dispersed is TiB. ₂ And
  And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, Mn: When the content is 0.20 to 8.00%, P: 0.045% or less, S: 0.030% or less, Al: 1.00% or less, and the above-mentioned optional element is contained, 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 composed of the remaining Fe and inevitable impurity elements,
  In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. 80% or more by volume of the boride and / or its composites dispersed is TiB. ₂ And
  And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, 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 melting. Contains C, Si, Mn, P, S, Al, and Cu, and further contains one or more optional elements of Ni, Cr, Mo, V, and Nb, and the balance Fe and inevitable impurity elements A steel using matrix steel for the matrix phase,
  In the matrix phase, boride containing at least one element selected from the group containing 4A group element, 5A group element, 6A group element and Fe and / or its complex is dispersed in an amount of 5 to 25 vol%. 80% or more by volume of the boride and / or its composites dispersed is TiB. ₂ And
  And the content rate of the said element contained only in the said matrix phase when the whole steel is made into 100% is the mass%, C: 0.20% or less, Si: 0.10-1.00%, 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 optional elements In the case of containing Ni, the content is Ni: 14.00% or less, Cr: 28.00% or less, Mo: 4.00% or less, V: 1.00% or less, Nb: 1.00% by mass%. A high-rigidity steel that can be produced by a melting method characterized by the following. A method for producing the high-rigidity steel according to any one of claims 1 to 4,
  A boride containing one or more elements selected from the group comprising a group 4A element, a group 5A element, and a group 6A element, while dissolving a steel raw material that becomes a matrix phase in a closed space having an atmospheric pressure of 250 Pa or less Or / and melting the alloy containing B as the first raw material for dispersing the composite in the finally obtained matrix phase;
  Finally, it is selected from the group including the 4A group element, 5A group element, and 6A group element which are the second raw materials for dispersing the boride or / and its composite in the finally obtained matrix phase. A method for producing high-rigidity steel, wherein an alloy containing one or more elements is added and dissolved, and then cast into a mold.