JP2007051341A - Steel with high young's modulus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel with a high Young's modulus which exhibits a high Young's modulus, and simultaneously maintains satisfactory ductility and toughness by a relatively simple means. <P>SOLUTION: As dispersed grains, V<SB>3</SB>B<SB>4</SB>type boride of ≥5 mass% and M(C, N) type carbonitride (M is V and/or Ti) of ≥1.5 mass% are incorporated into a steel, and also, the total content of both is controlled to 8 to 30 mass%. The steel with a high Young's modulus can be produced by subjecting a slab comprising, by mass, 0.1 to 0.8% C, 0.05 to 0.5% Si, 0.2 to 1.5% Mn, 1 to 7% Ti, 4 to 8% V, 2 to 3% B, 0.005 to 0.2% Al, 1 to 7% Cr and 0.001 to 0.01% N, and the balance Fe with impurities to hot working in the temperature range of 850 to 1,200°C at the total reduction of area of ≥70%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high Young's modulus steel suitable for members that require rigidity, such as materials for automobiles, materials for robots, sports equipment, and materials for machine structures, and a method for producing the same.

  In the automobile field, the weight of the vehicle body has been reduced to improve fuel efficiency. The performance required of the material for weight reduction includes rigidity as well as strength. This is because if the rigidity is improved, the deflection is suppressed, the friction loss due to the deflection can be reduced, and the thickness can be reduced and the shaft diameter can be reduced, so that the part can be miniaturized.

As a technique to improve the rigidity (generally evaluated by Young's modulus) of steel,
・ Addition of alloy elements (Cr, Co, Re, etc.),
・ Use of collective organization
・ Dispersion of highly rigid compounds,
Is mentioned.

  Of these methods, the addition of alloying elements such as Cr, Co, and Re improves the Young's modulus by only a few percent at most. When texture is used, the Young's modulus anisotropy increases, and there is a limit to application to structural members. In contrast, when a compound having a high Young's modulus is dispersed in an Fe matrix to form a composite material, an isotropically high Young's modulus can be obtained.

The Young's modulus of the composite material is generally determined by the volume ratio of the contained compound as described in Non-Patent Document 1 below. That is, the higher the Young's modulus of the contained compound dispersed in the steel and the greater the content, the greater the Young's modulus of the composite material. According to the composite law of Young's modulus, for example, by including either about 15 vol% TiB ₂ or about 20 vol% TiC in the Fe matrix, a Young's modulus of about 250 GPa can be obtained. Young's modulus: increases from 208 GPa to about 20%.

  However, it is difficult to produce a composite type high Young's modulus steel containing a large amount of a high-rigidity compound in the steel through ordinary melting and hot working processes. Therefore, the following techniques have been proposed so far for high Young's modulus steel containing a large amount of such a highly rigid compound.

(1) Method by mechanical alloying method (Patent Document 1 below);
(2) Method by melting: (a) Method of heating to a temperature at which the element constituting the compound is completely dissolved in iron or an iron alloy, and crystallizing or precipitating a highly rigid compound during cooling and solidification (the following patent) Documents 2) and (b) A method in which alloy raw materials are blended at a predetermined ratio, completely melted in a vacuum or an inert gas atmosphere, and then cast into a mold or a ceramic mold (the following patent documents) 3).
JP-A-5-239504 JP-A-10-68048 JP 2001-59146 A "Elastic modulus of metallic materials" edited by the Japan Society of Mechanical Engineers (Effective date: October 31, 1980, publisher: Japan Society of Mechanical Engineers)

  The mechanical alloying method (1) has a problem that the cost is high and a large part cannot be manufactured. In the method (a) and (b) of the melting method (2) described above, there is a problem that particles crystallized from the molten steel are generally easily coarsened, causing a decrease in ductility, toughness and fatigue strength.

  This invention makes it a subject to provide the high Young's modulus steel which does not have such a problem, and its manufacturing method. Specifically, we provide a composite material type high Young's modulus steel that contains a large amount of a high-rigidity compound in steel without using mechanical alloying and without reducing ductility and toughness, and a method for producing the same. It is to be.

  Based on the premise of melting that can be manufactured at low cost, the present inventors prevent a decrease in ductility, toughness and fatigue strength by finely dispersing a sufficient amount of high-rigidity compound in the steel during melting. Focusing on the above, research was repeated to find a highly rigid compound that can be finely dispersed in steel.

Then, alloy raw materials are blended at a predetermined ratio, and MB ₂ type boride and MB type boride are crystallized in a normal melting and solidification process, thereby making it possible to prevent toughness and its production. The invention of the method was filed as Japanese Patent Application No. 2003-154879.

  As a result of further studies, the present inventors have found that in steels containing C, B, V, and Cr, the compounds that can be crystallized or precipitated are borides, carbonitrides, or carbon borides shown below. Was revealed. The measured value of Young's modulus of each compound is shown in parentheses.

VB type ₂ boride (Young's modulus: 547 GPa),
V ₃ B type ₄ boride (Young's modulus: 620 GPa),
M (C, N) type carbonitride (M is V and / or Ti) (Young's modulus: 460 GPa),
FeB type boride (Young's modulus: 200 GPa or less).

M ₂₃ (C, B) Type ₆ boride (Young's modulus: 200 or less)
Among the above-mentioned compounds, VB ₂ type boride, V ₃ B ₄ type boride, and M (C, N) type carbonitride having high Young's modulus may contribute to the improvement of Young's modulus of steel. It is a thing. VB type ₂ boride is a high melting point compound and crystallizes out by eutectic with the Fe matrix during the solidification process. On the other hand, since V ₃ B ₄ type boride has a lower melting point than VB ₂ type boride, it does not form during normal solidification and precipitation processes. However, it has been found that when hot working is applied to a steel piece from which VB ₂ type boride has crystallized, V ₃ B ₄ type boride is formed due to phase separation of VB ₂ type boride.

The formation process of the V ₃ B ₄ type boride is believed to be as follows:
(I) VB type ₂ boride is crystallized by eutectic with Fe matrix during solidification process;
(Ii) VB type ₂ boride is plastically deformed by hot working the eutectic structure of (i);
(Iii) Due to plastic deformation, phase separation of VB ₂ → VB ₂ + V ₃ B ₄ → V ₃ B ₄ + (Fe, V) B occurs, and a V ₃ B ₄ type boride is formed.

The form of the V ₃ B ₄ type boride formed in the process of plastic deformation in this manner is finer and spherical in shape than the VB ₂ type boride, and is uniformly dispersed in the steel. On the other hand, the VB type ₂ boride crystallized by eutectic shows a lamellar structure with the Fe matrix and is much coarser than the spherical type V ₃ B ₄ type boride.

Therefore, a steel containing a large amount of VB type ₂ boride can exhibit a high Young's modulus because of the high Young's modulus of this boride, but due to the lamellar lamellar structure of this boride, the ductility in the tensile test is low. Low, yield strength, elongation and bending strength deteriorate. On the other hand, a steel containing a high Young's modulus V ₃ B ₄ type boride provides a high Young's modulus while maintaining a high level of proof stress, elongation, and bending strength because the boride is fine spherical. Therefore, it was found that it is optimal for increasing the Young's modulus of steel.

The composition range of steel in which V ₃ B ₄ type boride can be produced is, by mass, C: 0.1% or more, 0.8% or less; Si: 0.05% or more, 0.5% or less; Mn : 0.2% or more, 1.5% or less; Ti: 1% or more, 7% or less; V: 4% or more, 8% or less; B: 2% or more, 3% or less; Al: 0.005% or more 0.2% or less; Cr: 1% or more, 7% or less; N: 0.001% or more and 0.01% or less.

Also, in order to change the phase from VB ₂ type boride to V ₃ B ₄ type boride by plastic deformation during hot working, machining with a total area reduction of 70% or more in the working temperature range of 850 ° C. to 1200 ° C. It has also been found that it is only necessary to provide additional hot working.

On the other hand, the contribution of V ₃ B ₄ type boride is small in terms of increasing the strength of steel. Therefore, another measure to improve the strength is necessary. Among the above-mentioned precipitates, M (C, N) type carbonitride [M is V and / or Ti] does not compete with V ₃ B ₄ and is therefore suitable as precipitation strengthening particles.

The present invention completed based on the above findings,
(A) V ₃ B ₄ type boride 5% by mass or more and (b) M (C, N) type carbonitride (M is V and / or Ti) 1.5% by mass or more, high Young's modulus steel, characterized in that the total amount of a) and (b) is 8-30% by mass,
It is.

The high Young's modulus steel of the present invention preferably contains no VB ₂ type boride. The chemical composition of the high Young's modulus steel of the present invention is mass%,
C: 0.1% to 0.8%; Si: 0.05% to 0.5%;
Mn: 0.2% to 1.5%; Ti: 1% to 7%;
V: 4% to 8%; B: 2% to 3%;
Al: 0.005% to 0.2%; Cr: 1% to 7%;
N: It is preferable to contain 0.001% or more and 0.01% or less, and the remainder consists of Fe and impurities.

The present invention is also in weight percent,
C: 0.1% to 0.8%; Si: 0.05% to 0.5%;
Mn: 0.2% to 1.5%; Ti: 1% to 7%;
V: 4% to 8%; B: 2% to 3%;
Al: 0.005% to 0.2%; Cr: 1% to 7%;
N: 0.001% or more and 0.01% or less, and the remainder is hot-worked with a total area reduction of 70% or more in a temperature range of 850 ° C. to 1200 ° C. in a steel piece made of Fe and impurities. A method for producing a high Young's modulus steel, characterized by:
Also related.

  According to the present invention, a high Young's modulus steel containing a large amount of a high-rigidity compound dispersed in the steel and having good ductility and toughness can be obtained by a relatively simple means. Thereby, for example, high Young's modulus steel suitable for members that require rigidity, such as materials for automobiles, materials for robots, sports equipment, and materials for machine structures, can be stably supplied at a relatively low cost. .

Hereinafter, the present invention will be described in more detail. In the following description, all the percentages (including the content of dispersed compounds in steel) relating to the chemical components of steel are mass%.
[Content of highly rigid compound]
The high Young's modulus steel according to the present invention comprises (a) 5% or more of V ₃ B ₄ type boride and (b) 1.5% or more of M (C, N) type carbonitride as high rigidity compounds. (M represents V and / or Ti). The total amount of (a) and (b) is 8-30%. The Young's modulus of V ₃ B ₄ is 620 GPa, and the Young's modulus of M (C, N) is 460 GPa. The former particularly contributes to increasing the Young's modulus of steel, and the latter contributes to increasing the strength of steel. As described above, V ₃ B ₄ type boride, since it is impossible to deposit in melting process, by adding hot working the billet VB ₂ type boride crystallized out, VB ₂ type borides It is produced in steel by phase separation.

When these high-rigidity compounds are contained in the iron matrix to increase the rigidity of the steel, the desired Young's modulus cannot be obtained if the content of the V ₃ B ₄ type boride is less than 5%. On the other hand, if the amount of M (C, N) carbonitride deposited is less than 1.5%, the strength decreases. Furthermore, if the total content of the V ₃ B ₄ type boride and the M (C, N) type carbonitride is less than 8%, the Young's modulus and the strength are not compatible. On the other hand, the upper limit of the total content of the V ₃ B ₄ type boride and the M (C, N) type carbonitride is 30% by mass that is possible with molten steel.

The preferable range of the amount of these compounds is 10% or more of V ₃ B ₄ type boride, 2.0% or more of M (C, N) type carbonitride, and the total of both is 12 to 20%. As other compounds, FeB type boride, M ₂₃ (C, B) ₆ (M is V and / or Ti) type carbon boride, and the like may precipitate, but the total precipitation amount is less than 7.5%. Is desirable. As described above, (Fe, V) by plastic deformation from VB ₂ results in V ₃ B ₄ since B-type boride is produced as a by-product, the borides are also contained in the steel.

Plastic deformation from VB ₂ to V ₃ B ₄ is preferably allowed to proceed fully into the completely hot worked. If this plastic deformation is incomplete and VB ₂ type boride remains in the steel structure, a coarse lamella composed of VB ₂ type boride and M (C, N) type carbonitride crystallized during melting. The structure remains, the ductility of the steel decreases, and the elongation and bending strength decrease. Therefore, it is preferable that the steel of the present invention does not contain VB type ₂ boride. However, a residue of about 1% or less is allowed.

[Chemical composition of steel]
In order to produce V ₃ B ₄ type boride which plays a major role in increasing the Young's modulus in the high Young's modulus steel of the present invention by plastic deformation by hot working, the chemical composition of the steel is C: 0.1% or more 0.8% or less; Si: 0.05% or more, 0.5% or less; Mn: 0.2% or more, 1.5% or less; Ti: 1% or more, 7% or less; V: 4% or more 8% or less; B: 2% or more, 3% or less; Al: 0.005% or more, 0.2% or less; Cr: 1% or more, 7% or less; N: 0.001% or more, 0.01 % Of the composition (the balance is Fe and impurities).

C: 0.1% or more and 0.8% or less C is an important element that effectively increases the strength of the steel and controls the matrix structure. Further, carbides such as VC and TiC are crystallized at the time of solidification, and the coarsening of V boride and Ti boride that is also crystallized at the time of solidification is suppressed. However, when the content is less than 0.1%, the above effect cannot be obtained. On the other hand, if the content exceeds 0.8%, a coarse and low Young's modulus M ₂₃ (C, B) _6- type carbonitride is likely to be produced, which adversely affects hot workability, ductility, and toughness. And lowers the Young's modulus of the steel. Desirable C content is 0.3% or more and 0.5% or less.

Si: 0.05% or more, 0.5% or less Si is an element effective for deoxidation, but if its content is less than 0.05%, this effect is difficult to obtain. On the other hand, if it exceeds 0.5%, a Laves phase that adversely affects toughness is likely to be formed.

Mn: 0.2% or more, 1.5% or less Mn fixes impurities in steel and suppresses hot cracking. However, if the Mn content is less than 0.2%, the above effect cannot be obtained. On the other hand, if the content exceeds 1.5%, the grain boundaries are weakened and the toughness and ductility are adversely affected.

Ti: 1% or more and 7% or less Ti forms oxides, nitrides, carbides, borides, and the like during the solidification process, and is effective in refining the solidified structure. Furthermore, in order to raise melting temperature, there exists an effect | action which raises the hot working temperature range. However, if the content is less than 1%, the above effect cannot be obtained. On the other hand, if the content exceeds 7%, coarse Ti carbonitrides and Ti borides are formed, which adversely affects hot workability, ductility and toughness. Desirable Ti content is 3% or more and 6% or less.

V: 4% or more, 8% or less V is an element that forms a V ₃ B ₄ type boride and is effective in increasing the Young's modulus of steel. Furthermore, V (C, N) type carbonitrides which are precipitation strengthening particles are generated. However, if the content is less than 4%, the above effect cannot be obtained. On the other hand, if the content exceeds 8%, coarse V borides and carbonitrides are formed, which adversely affects hot workability, ductility and toughness. Desirable V content is 5% or more and 7% or less.

B: 2% or more and 3% or less B is an important element for forming a V ₃ B ₄ type boride. However, when the content is less than 2%, the amount of boride to be formed is small, and a desired Young's modulus (240 GPa) or more cannot be obtained. On the other hand, if the content exceeds 3%, the boride becomes coarse and adversely affects hot workability, ductility, and toughness.

sol Al: 0.005% to 0.2% Al is added from the viewpoint of deoxidation. It also has the effect of miniaturizing the solidified structure. However, if the content is less than 0.005%, these effects are not obtained. On the other hand, when it exceeds 0.2%, a coarse oxide is easily generated. A preferable Al content is 0.05% or more and 0.2% or less.

Cr: 1% or more and 7% or less Cr contributes to stabilization and refinement of the V ₃ B ₄ type boride. However, less than 1% has no effect. On the other hand, when it exceeds 7%, the V ₃ B ₄ type boride is not generated stably, which adversely affects hot workability, ductility, and toughness. Desirable Cr content is 4% or more and 6% or less.

N: 0.001% or more and 0.01% or less N generates nitrides in the solidification process and contributes to the refinement of the solidified structure. However, this effect is not obtained when the content is less than 0.001%. On the other hand, if the content exceeds 0.01%, coarse nitrides are formed, which adversely affects hot workability, toughness, and ductility.

The high Young's modulus steel of the present invention is preferably essentially composed of the above components, the remaining Fe and impurities. However, it contains 5% or more of V ₃ B ₄ type boride and 1.5% or more of M (C, N) type carbonitride (M means V and / or Ti), and these If the total amount of the two kinds of compounds is 8 to 30%, the chemical composition of the steel may be different from the above. For example, as an alloy element other than the above, an alloy element that does not form borides such as Cu, Ni, and Mo can be further contained.

[Production method]
The high Young's modulus steel according to the present invention is manufactured by a method characterized by hot working a steel slab having the above chemical composition at a temperature of 850 ° C. to 1200 ° C. with a total area reduction of 70% or more. be able to. The steel piece may be manufactured by a normal melting method.

Hot working temperature range: 850 ° C to 1200 ° C
Gist of the present invention, the VB ₂ type boride issued in the steel crystallization by providing the hot working is plastically deformed, a phenomenon VB ₂ type boride was plastic deformation phase separation V ₃ B ₄ type boride Is used to uniformly disperse a large amount of V ₃ B ₄ type boride, which has a high Young's modulus, in steel.

As a result of investigating this phenomenon at various hot working temperatures, when the hot working temperature is less than 850 ° C., the VB ₂ type boride is not sufficiently plastically deformed, and from the VB ₂ type boride to the V ₃ B ₄ type boride. It became clear that no phase change occurred. Moreover, since the deformability is insufficient when the hot working temperature is low, cracking is likely to occur. On the other hand, when the hot working temperature exceeds 1200 ° C., a low-melting point compound such as FeB is dissolved, which becomes an impediment to hot working. Therefore, the hot working temperature range is 850 ° C. or more and 1200 ° C. or less. The preferred hot working temperature range is 870-1100 ° C.

Amount of hot work: 70% or more in total area reduction rate When the work rate by hot working in the temperature range of 850 to 1200 ° C. is less than 70% in total area reduction, V ₃ B ₄ type boride is formed. In addition to its small amount, it does not disperse uniformly, which adversely affects ductility and toughness. Therefore, the hot working rate in the above temperature range is 70% or more in terms of the total area reduction. This total area reduction is preferably 80% or more.

  The type of hot working is not particularly limited, but forging, rolling, drawing, etc. are possible. The high Young's modulus steel of the present invention can be provided in various shapes such as a plate material, a pipe material, a bar material, and a mold material.

  In this example, steels A to L having the chemical components shown in Table 1 were dissolved in Fe in the alloy components other than Ti and B in Fe, and then alloy iron (ferroboron) containing B and Ti The alloy iron containing / B was added in this order and completely dissolved to obtain a 50 kg steel ingot. For steel H, only B-containing alloy iron was added. As for the size of the steel ingot, the upper diameter was 180 mm, the lower diameter was 130 mm, and the height was 310 mm.

A part of Steel A was subjected to an evaluation test with the cast material thus obtained (Test No. 1).
Except for the above, after heating to the temperature shown in Table 2, hot forging was performed in the temperature range shown in Table 2, the number of passes, and the total area reduction rate, and processed into a rod shape. When the temperature decreased during hot working, reheating was performed so that the hot forging temperature was within a predetermined range. Cooling after hot forging was air cooling.

  A test piece was collected from the central portion of the steel bar thus obtained, and crystallized substances / precipitates were identified, Young's modulus was measured, tensile properties, and bending strength were measured. These test results are also shown in Table 2. Moreover, about some bar steel, the shape of the precipitation compound was determined by cross-sectional SEM observation of the test piece.

The crystallized substance / precipitate was identified by X-ray diffraction of the residue obtained with 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte.
The content of the crystallized product / precipitate was calculated from the peak intensity of the X-ray diffraction.

The Young's modulus was obtained by measuring the resonance frequency by a normal lateral resonance method using a Young's modulus measurement sample having a thickness of 1.5 mm, a width of 10 mm, and a length of 60 mm.
Tensile properties were determined by using a round bar tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 40 mm at a tensile speed of 10 mm / min and yield strength, tensile strength, and elongation at break.

The bending strength was determined by using a sample for measuring the bending strength having a diameter of 5 mm and a length of 70 mm, applying a load at a fulcrum distance of 50 mm and a bending speed of 1 mm / min to obtain a load (breaking load) until breaking.
When the bending load P [N], the bending force σ [MPa] is given by the following equation.

Folding force σ = 8P · L / π · d ³
L: Distance between fulcrums,
d: Sample diameter.

  In Table 2, test numbers 1 to 5 show examples of steels having the same chemical composition within a specific range suitable for the present invention but having different processing histories.

Test number 1 is a cast material as cast. As a result of X-ray diffraction and cross-sectional SEM observation, VB type ₂ boride and M (C, N) type carbonitride were produced in a layered form (as a lamellar structure). V ₃ B type ₄ boride was not detected. Therefore, although the Young's modulus was high, the ductility by the tensile test was low and the bending strength was also low.

Test No. 2 is a steel that has been subjected to appropriate hot working according to the present invention. 5% or more of V ₃ B ₄ boride was generated by hot working, and no VB ₂ boride was detected. The generated V ₃ B ₄ type boride was fine particles that were spheroidized, and the average aspect ratio of the particles was found to be close to 1 by cross-sectional SEM observation. As a result, this steel exhibited performances of tensile strength> 600 MPa, elongation> 10%, and bending strength> 2,500 MPa or more while having a high Young's modulus.

FIG. 1 shows the X-ray diffraction patterns of the compounds detected for the steels of test numbers 1 and 2. As a diffraction peak of vanadium boride, only a peak attributed to VB ₂ is observed in Test No. 1 and only a peak attributed to V ₃ B ₄ is observed in Test No. 2. Therefore, it can be seen that the phase separation from VB ₂ to V ₃ B ₄ occurred completely due to the plastic deformation by the hot working performed in Test No. 2.

FIG. 2 shows cross-sectional SEM images of the steels of test numbers 1 and 2. In Test No. 1, the dispersed compound is a flat or lamellar lamellar structure, whereas in Test No. 2, which shows a magnification twice that of the dispersed compound, the dispersed compound is nearly spherical (average aspect ratio is 1), much finer and uniform. The lamellar tissue is not found at all. Also from this figure, it can be seen that phase separation from VB ₂ to V ₃ B ₄ occurs completely by applying appropriate hot working.

FIG. 3 shows a TEM image of the steel type B, which is the steel of test number 2, when the reduction in area is changed in the same temperature range as that of test number 2. FIG. 3A is a TEM image when the area reduction rate is 50%, and FIG. 3B is a TEM image when the area reduction rate is 80%. At an area reduction of 50%, VB ₂ + V ₃ B ₄ particles were observed, the phase separation from VB ₂ → V ₃ B ₄ was incomplete, and the particles were coarse. On the other hand, at an area reduction rate of 80%, phase separation from VB ₂ to V ₃ B ₄ occurs completely, VB ₂ disappears, and fine spherical particles of V ₃ B ₄ and (Fe, V) B Of particles were observed. From this figure, it can be seen that vanadium boride undergoes a phase change of VB ₂ → VB ₂ + V ₃ B ₄ → V ₄ B ₅ + (Fe, V) B during the hot working process.

In test number 3, since the hot forging temperature was too high, cracking occurred during hot working, and an evaluation sample could not be obtained.
In test No. 4, the hot working temperature was appropriate, but the total area reduction rate was 46.2%, and the amount of processing was too small. Therefore, the amount of V ₃ B ₄ type boride was insufficient, VB type ₂ boride remained. In the cross-sectional SEM observation, the VB ₂ type borides and M (C, N) had remained layered tissue type carbonitrides. As a result, the elongation and bending strength were unsatisfactory.

In test number 5, since the hot forging temperature was too low, cracks occurred during hot working, and no sample for evaluation was obtained.
Test numbers 6 and 7 also show examples of steels having the same chemical composition suitable for the present invention, but with different processing histories.

In test number 6, 5% or more V ₃ B ₄ type boride was generated by hot working according to the present invention. It was confirmed from cross-sectional SEM observation that the V ₃ B ₄ type boride was uniformly dispersed with spherical fine particles as in Test No. 2. Thereby, although it was steel with high Young's modulus, the performance of tensile strength> 600 MPa, elongation> 10%, and bending strength> 2,500 MPa or more was exhibited.

In test number 7, the hot working temperature was appropriate, but the amount of processing was too small, so the amount of V ₃ B ₄ type boride produced was insufficient, and VB ₂ type boride and M (C, N) A layered structure made of type carbonitride remained. As a result, elongation and bending strength were unsatisfactory.

Test numbers 8-10 all illustrate steel according to the present invention in which both chemical composition and hot working conditions are appropriate. Although the chemical compositions were different from each other, 5% or more of V ₃ B ₄ type boride was formed by appropriate hot working. It was confirmed from cross-sectional SEM observation that all of these V ₃ B ₄ type borides were uniformly dispersed in steel with the same spherical fine particles as in Test No. 2. Therefore, although it is a steel having a high Young's modulus, it exhibits performances of tensile strength> 600 MPa, elongation> 10%, and bending strength> 2500 MPa.

Test number 11 is an example in which the amount of C is too large. Therefore, although the hot working conditions were appropriate, the amount of V ₃ B ₄ type boride in the steel did not reach the lower limit of 5% in the present invention. This is thought to be because a large amount of M ₂₃ (C, B) ₆ type carbon boride was generated because of the large amount of C. The Young's modulus of this steel did not reach 240 GPa, and the elongation and bending strength were poor. This is because the Young's modulus of a large amount of precipitated M ₂₃ (C, B) ₆ type carbon boride is as low as 200 GPa or less, and the steel containing a large amount thereof has a low Young's modulus.

Test number 12 is an example in which the amount of C is too small and Cr is not included. Although the hot working conditions were appropriate, the amount of V ₃ B ₄ type boride produced was insufficient, and a layered structure consisting of VB ₂ type boride and M (C, N) type carbonitride was obtained. The remaining. Further, since the amount of C is small, the amount of M (C, N) type carbonitride is too small, and the total amount of V ₃ B ₄ type boride and M (C, N) type carbonitride is the lower limit in the present invention. It did not reach 8%. As a result, all of tensile strength, elongation, and bending strength were unsatisfactory.

Test No. 13 is an example that does not contain Ti and the amount of N is too large. Since the temperature range in which hot working can be performed is narrow, cracking occurred during hot forging, and an evaluation sample could not be obtained.
Test number 14 is an example in which the Ti amount is too large and the V amount is too small. Since this steel contains a large amount of TiB ₂ type boride (Young's modulus: about 550 GPa), the Young's modulus was not high, but V ₃ B ₄ type boride was not formed at all, so the toughness and ductility were poor. Met.

Test number 15 is an example in which there are too many Cr, V, and B amounts. Although a large amount of the compound was precipitated, all the VB ₂ type borides could be phase-separated by appropriate hot working, and a large amount of V ₃ B ₄ type borides were formed. Therefore, Young's modulus became very high. However, since a large amount of boride was precipitated during melting, the amount of M (C, N) carbonitride deposited was insufficient, and the tensile strength was low. Moreover, although ductility and bending strength were also unsatisfactory, the cause is thought to be because the precipitates were agglomerated and coarsened.

Test number 16 is an example in which the Ti amount, V amount, and B amount are all too small. The desired Young's modulus could not be obtained due to the small amount of high rigidity compound in the steel.
Test number 17 exemplifies steel according to the present invention where both chemical composition and hot working conditions are appropriate. Although the chemical compositions were different from each other, 5% or more of V ₃ B ₄ type boride was formed by appropriate hot working. It was confirmed from cross-sectional SEM observation that all of these V ₃ B ₄ type borides were uniformly dispersed in steel with the same spherical fine particles as in Test No. 2. Therefore, although it is a steel having a high Young's modulus, it exhibits performances of tensile strength> 600 MPa, elongation> 10%, and bending strength> 2500 MPa.

As can be seen from the above, V ₃ B ₄ type boride in an amount in accordance with the present invention and M (C, N) type steel containing carbonitride is simultaneously shown above high Young's modulus 240GPa a (rigid), The ductility and toughness are also high, and the tensile strength> 600 MPa, the elongation> 10%, and the bending strength> 2,500 MPa or more are exhibited.

The high Young's modulus steel of the present invention can be produced by subjecting steel having an appropriate chemical composition to hot working under appropriate conditions. Even if the chemical composition is appropriate, the hot working conditions are inadequate, or conversely, if the hot working conditions are appropriate, the steel chemical composition is inadequate. High Young's modulus steel containing ₃ B type ₄ boride and M (C, N) type carbonitride cannot be obtained.

The X-ray diffraction pattern of the extraction residue extract | collected from the cast material (test number 1) and forging material (test number 2) with the same chemical composition of steel is shown. The cross-sectional SEM image of the cast material (test number 1) and forging material (test number 2) with the same chemical composition of steel is shown. When hot working by changing the reduction rate for steel type B, and a TEM image showing a process of generating V ₃ B ₄ type boride.

Claims (4)

(A) V ₃ B ₄ type boride 5% by mass or more and (b) M (C, N) type carbonitride (M is V and / or Ti) 1.5% by mass or more, A high Young's modulus steel characterized in that the total amount of a) and (b) is 8 to 30% by mass. The high Young's modulus steel according to claim 1, which does not contain VB type ₂ boride. % By mass
C: 0.1% to 0.8%; Si: 0.05% to 0.5%;
Mn: 0.2% to 1.5%; Ti: 1% to 7%;
V: 4% to 8%; B: 2% to 3%;
Al: 0.005% to 0.2%; Cr: 1% to 7%;
N: The high Young's modulus steel of Claim 1 or 2 containing 0.001% or more and 0.01% or less, and remainder consists of Fe and an impurity. % By mass
C: 0.1% to 0.8%; Si: 0.05% to 0.5%;
Mn: 0.2% to 1.5%; Ti: 1% to 7%;
V: 4% to 8%; B: 2% to 3%;
Al: 0.005% to 0.2%; Cr: 1% to 7%;
N: 0.001% or more and 0.01% or less, and the remainder is hot-worked with a total area reduction of 70% or more in a temperature range of 850 ° C. to 1200 ° C. in a steel piece made of Fe and impurities. A method for producing a high Young's modulus steel, characterized by comprising:

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