JP2007002302A - High strength-high ductility carbon steel, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon steel in which high strength and high ductility are balanced. <P>SOLUTION: Each powder of pure iron and carbon or the powder of a carbon steel is subjected to mechanical milling treatment. The obtained powder is subjected to discharge plasma sintering, so as to be a steel having a structure in which the average crystal grain size of a mother phase is <1 μm, and cementite grains with a size of ≤100 nm are uniformly dispersed, exhibiting a yield strength of ≥1,500 MPa and a true strain of ≥0.2, and also exhibiting work hardening. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention has a balance between high strength and high ductility, which is formed of nano-order crystal grains, and is useful for application to microstructural members such as micromachines as well as general structural uses such as aircraft and automobiles. The present invention relates to a carbon steel material and a manufacturing method thereof.

  Conventionally, investigations have been made from various viewpoints on increasing the strength of carbon steel and improving its ductility. As a measure for increasing the strength of steel materials, refinement of the grain structure is considered as an effective means, and a device for that purpose has also been proposed.

For example, regarding medium and low carbon steels, proposals have been made for steel wires, wire rods and steel bars that have achieved high strength, high ductility, and high toughness with a homogeneous structure. (Patent Document 1)
These are materials having ultrafine grains characterized in that ferrite is the main phase, the average ferrite grain size is less than 2 μm, and the ferrite grain aspect ratio is less than 1.5, and the carbon concentration range is 0.05 ˜0.35 wt%. In addition, in this material, the grain structure is refined using a bulk material as a starting material by using dynamic recrystallization by performing wire drawing or the like hot.

  As an example of a material having high strength and high ductility, an average particle size of 1.26 μm was obtained for a material having a carbon concentration of 0.25% by weight. As a result, a yield strength of 1290 MPa and a tensile elongation value of 8.6% were obtained. It has gained.

  However, this material does not necessarily have sufficient strength characteristics, ductility level and balance.

Therefore, after austenite crystal grains of a normal low carbon steel with a carbon content of 0.2 wt% or less or a normal low carbon steel with 0.1% or less and an effective amount of B added to promote martensitic transformation are coarsened, The steel material with a martensite phase of 90% or more obtained by performing low strain processing has a high strength and high ductility with a tensile strength of 800 MPa or more, a uniform elongation of 5% or more, and a breaking elongation of 20% or more. Proposed to be carbon steel. (Patent Document 2)
Here, the average crystal grain size is characterized by an ultrafine grain ferrite structure of 1.0 μm or less.

  In this steel material, the uniform dispersion of cementite particles is pointed out as a measure for improving the strength-ductility balance.

  However, in the case of this proposed steel material, combined with a relatively low carbon concentration, crystal grain growth at 600 ° C. occurs easily, and as a result, the strength-ductility balance is not necessarily satisfactory. The situation is not.

Conventionally, strong strain processing is known as a means for grain refinement, and examples include ECAE or ECAP (Equal-Channel-Angular-Extrusion / Equal-Channel-Angular-Pressing) methods (for example, non- Patent Document 1-2). A large strain has been introduced into a material by shear extrusion or mechanical milling, and the crystal grain structure has been refined by recrystallization. However, although the alloy subjected to ECAP exhibits ductility of about 0.2 to 0.3 in terms of tensile strain, the yield strength is only about 700 to 800 MPa. Moreover, in the steel material created by the mechanical milling method, the hot press has been used until now as a method of solidifying the powder into a bulk material. (For example, nonpatent literature 3-4).

In this solidification using a hot press, an extremely high yield stress of about 2500 MPa is obtained, but on the other hand, only a very low ductility of 6% or less is obtained.
Japanese Patent Laid-Open No. 11-124655 JP 2002-28527 A Acta Mater 2004, 52, 1859 Metal Mater Trans 2003,34A, 71 Acta Mater 2003, 51, 3490 Appl Phys Lett 2002,81,1240

  From the background as described above, the present invention has an object to provide a new steel material and a method for manufacturing the same, which solves the problems of the prior art and balances the strength and the large ductility as compared with the prior art. .

  In order to solve the above problems, the present invention provides a carbon steel material characterized by the following and a method for producing the same.

  A carbon steel material comprising iron and carbon containing carbon in the range of 1: 0.1 to 2.1% by mass and unavoidable impurities, wherein the average crystal grain size of the parent phase is less than 1 μm and 100 nm or less A high-strength, high-ductility carbon steel material having a structure in which cementite particles of a size are dispersed, having a yield strength of 1500 MPa or more, exhibiting a ductility with a true strain of 0.2 or more, and exhibiting work hardening.

  Second: A high-strength and high-ductility carbon steel material having a structure in which cementite particles having a size of 50 nm or less are dispersed in an average crystal grain size of the parent phase in the range of 50 to 500 nm.

  Third: A high strength and high ductility carbon steel material having a true stress at break of 3 GPa or more and a hardness of 6 GPa or more.

  Fourth: A method for producing any one of the above carbon steel materials, wherein each powder of pure iron and carbon, or carbon steel powder is subjected to mechanical milling, and the obtained powder is subjected to spark plasma sintering. A method for producing high-strength, high-ductility carbon steel that is solidified by heating.

  5: The method for producing a high-strength, high-ductility carbon steel material according to claim 4, wherein the powder after mechanical milling has an average internal crystal grain size of 500 nm or less and the constituent phase is a single ferrite phase or a two-phase ferrite and cementite. .

  6th: The manufacturing method of the high strength and high ductility carbon steel material which performs mechanical milling in inert gas atmosphere or a vacuum atmosphere.

  Seventh: A method for producing a high-strength, high-ductility carbon steel having a relative density of 95% or more after spark plasma sintering.

  Eighth: A method for producing a high-strength, high-ductility carbon steel material in which discharge plasma sintering is performed for 1 minute or more in a temperature range of 520 to 700 ° C.

  According to the manufacturing method of the steel material of the present invention as described above, (1) the nano-order of the crystal grain structure is performed by mechanical milling, and (2) the obtained powder is sintered at a relatively low temperature by discharge plasma. As a result of the sintering, a uniform dispersion of the cementite phase and a nano-order ultrafine grain structure are maintained.

  For this reason, according to the present invention, a high strength-high ductility carbon steel is provided which exhibits a high ductility with a very high yield strength and a high work hardening until rupture.

  In the production method of the present invention, carbon powder and iron powder can be uniformly mixed by performing strong strain processing by mechanical milling for a predetermined time. In this case, carbon steel having a predetermined concentration in advance can be used as a starting material. In both of the above starting materials, a new interface is introduced during mechanical milling, thereby enabling ultrafine grain structure. In addition, in order to maintain the ultrafine grain structure of the powder produced by mechanical milling, compared with the conventional hot press method, the discharge plasma sintering enables sintering at a temperature of about 2/3. The combined use is essential.

  In order to form cementite particles that are indispensable for the formation and maintenance of an ultrafine crystal grain structure, the carbon concentration is set in the range of 0.1 to 2.1% by mass. More preferably, it is in the range of 0.3 to 1.5% by mass.

  By applying mechanical milling, the crystal grain structure inside the powder is refined over time. In order to reach a certain crystal grain size, the rotational speed and temperature control during the process are indispensable, but here the final grain size after the end of the process is an important factor for improving the strength-ductility balance of the final bulk material. Therefore, the selection of the rotation speed and time of the process is performed so that the ultimate crystal grain size after the mechanical milling process is 1 μm or less. More preferably, the average crystal grain size of the powder matrix is set to 100 nm or less.

  When oxygen is mixed during mechanical milling, a brittle oxide phase is formed, and ductility is significantly reduced. Therefore, it is important to create an atmosphere in which oxygen is excluded as much as possible from the start of mechanical milling.

  As a temperature at the time of spark plasma sintering, an upper limit temperature for preventing the crystal grains from coarsening is set to 700 ° C., and a lower limit temperature for obtaining a high-density parc body after solidification is set to 520 ° C. The time required for sintering is set to 1 minute or longer, and the holding time is set in a range where the crystal grain size after holding does not exceed 1 μm.

  In the present invention, by performing mechanical milling using pure iron and carbon powder as starting materials, the crystal grains are made ultrafine to the nano order, and a high-strength steel powder material is produced. After that, by carrying out warm press solidification by spark plasma sintering at a relatively low temperature of about 2/3 of the conventional hot press method, while suppressing the coarsening of the cementite phase, the uniform dispersion is achieved. The coarsening of the ultrafine crystal grain structure is effectively suppressed, and for example, as shown in the examples, an average crystal grain size of 150 nm is obtained. While this exemplary material exhibits a very high yield strength of 1.9 GPa, it exhibits a compressive strain of 0.35 ductility and work hardening to failure.

In the carbon steel material of the present invention, the composition is 0.1 to 2.1% by mass as described above, more preferably iron and carbon containing carbon in the range of 0.3 to 1.5% by mass, and unavoidable. It is made up of mechanical impurities. The average crystal grain size of the parent phase is less than 1 μm, more preferably in the range of 50 to 500 nm, and cementite particles having a size of 100 nm or less, more preferably 50 nm or less, are uniformly dispersed and have a structure. ing.

  The steel material of the present invention exhibits ductility with a yield strength of 1500 MPa or more and a true strain of 0.2 or more. More preferably, a material having a true stress at break of 3 GPa or more, a hardness of 6 GPa or more, and a relative density of 95% or more is provided.

  Then, an Example and a reference example are shown below. Of course, the present invention is not limited by the following examples.

The following examples and reference examples are carbon steel of Fe-0.8 wt% C.
<Reference Example 1>
Mechanical milling was performed using iron powder with a purity of 99.99% and carbon powder with a purity of 99.999% as starting materials. A mechanical planetary ball mill was used for mechanical milling, and the milling pot and balls were made of stainless steel. Milling was started after the mixed powder was weighed so that the weight ratio of the stainless ball to the mixed powder was 10: 1 and the container was sealed in an argon atmosphere. Milling conditions were set at 250 revolutions per minute for a total of 100 hours. The operation was stopped for 2 hours every 20 hours to prevent the temperature from rising.

After the mechanical milling, the powder was put into a carbon mold having an inner diameter of 10 mm and solidified using a commercially available spark plasma sintering apparatus (Sumitomo Coal Mining Co., Ltd.). Solidification was performed in a vacuum of 10 ⁻³ Pa or less, with an applied load of 5.5 kN (corresponding to 70 MPa as a solidification stress), a holding time of 10 minutes, and a temperature of 400 ° C.

When the parc material obtained after solidification was observed by X-ray diffraction, a cementite peak that was not found in the powder immediately after mechanical milling was confirmed (see FIG. 1). When the average crystal grain size, lattice strain, hardness, and relative density of the pulverized material obtained after solidification were measured, an ultrafine grain structure of 40 nm or less was formed as shown in Table 1, but the relative density was The hardness was 78% and the hardness was about 2.1 GPa. When a compression test was performed on the parc test piece, it was broken during elastic deformation, and high strength-high ductility was not obtained. <Reference Example 2>
The material process was carried out by changing only the discharge plasma sintering temperature to 500 ° C. as the above process conditions.

When the parc material obtained after solidification was observed by X-ray diffraction, a cementite peak that was not found in the powder immediately after mechanical milling was confirmed as in the case of 400 ° C. (see FIG. 1). ). When the average crystal grain size, lattice strain, hardness, and relative density of the bulk material obtained after solidification were measured, an ultrafine grain structure of 60 nm or less was formed as shown in Table 1, but the relative density was 89%. Moreover, the hardness increased to about 6 GPa. When the compression test was performed on the parc specimen, an extremely high yield strength of about 2 GPa was obtained, but the plastic strain was about 0.05, and high ductility was not obtained (see FIG. 3).
<Example 1>
The material process was carried out by changing only the discharge plasma sintering temperature to 600 ° C. as the above process conditions.

When the pulverized material obtained after solidification was observed by X-ray diffraction, a peak of cementite that was not found in the powder immediately after mechanical milling was confirmed as in the case of 400 and 500 ° C. (FIG. 1). When the average crystal grain size, lattice strain, hardness, and relative density of the bulk material obtained after solidification were measured, as shown in Table 1, uniform dispersion of ultrafine crystal grains of about 150 nm and fine cementite particles of 30 nm or less A structure was formed (see FIG. 2), the relative density was as high as 99%, and the hardness increased to about 6.2 GPa. When a compression test was performed on the parc specimen, an extremely high yield strength of about 1.9 GPa was obtained, but a high ductility of about 0.35 was obtained as the plastic strain (see FIG. 3). Moreover, the true stress at the time of the fracture was a value close to 3.5 GPa, and it was confirmed that it had sufficient work hardening ability.

It is the figure which showed the X-ray diffraction result of the development alloy (Fe-0.8wt% C), and in the powder (As-milled Fe-C) just after mechanical milling, there is only the peak of ferrite, carbon is inside the ferrite After the discharge plasma sintering (SPS) is performed, the presence of a peak corresponding to cementite (Fe ₃ C) is confirmed. It is the figure which showed distribution of the crystal grain structure and crystal grain size after solidification shaping | molding of a development alloy (Fe-0.8 wt% C), Comprising: Here, it obtained at the sintering temperature of 600 degreeC of Example 1. The result shows that the electron diffraction pattern shown in the upper left corner of the left figure shows a polycrystalline structure, and the center figure shows the distribution of cementite in white, and fine cementite particles are uniformly dispersed. From the grain size distribution in the right figure, it can be seen that most of the crystal grains in the range of 50 to 200 nm occupy about 150 nm. This shows the true stress-true strain relationship obtained when the developed alloy (Fe-0.8 wt% C) is compressed and deformed at room temperature. Here, 500 shown in Reference Example 2 and Example 1 is shown. The results are shown for materials that were spark plasma sintered at ℃ and 600 ℃.

Claims (8)

  A carbon steel material comprising iron and carbon containing carbon in the range of 0.1 to 2.1% by mass and inevitable impurities, and having an average crystal grain size of a parent phase of less than 1 μm and a size of 100 nm or less. A high-strength and high-ductility carbon steel material having a structure in which particles are dispersed, having a yield strength of 1500 MPa or more, exhibiting ductility with a true strain of 0.2 or more, and exhibiting work hardening.   2. The high-strength and high-ductility carbon steel material according to claim 1, which has a structure in which cementite particles having a size of 50 nm or less are dispersed in an average crystal grain size of the parent phase in the range of 50 to 500 nm.   The high-strength and high-ductility carbon steel material according to claim 1 or 2, wherein the true stress at break is 3 GPa or more and the hardness is 6 GPa or more.   A method for producing a carbon steel material according to any one of claims 1 to 3, wherein mechanical milling is performed using pure iron and carbon powder, or carbon steel powder as a starting material, and the obtained powder is subjected to discharge plasma sintering. A method for producing a high-strength, high-ductility carbon steel material characterized by being consolidated and solidified.   5. The high strength and high ductility carbon steel material according to claim 4, wherein the powder after mechanical milling has an average internal crystal grain size of 500 nm or less and a constituent phase is a single phase of ferrite or two phases of ferrite and cementite. Production method.   6. The method for producing a high strength / high ductility carbon steel material according to claim 4 or 5, wherein mechanical milling is performed in an inert gas atmosphere or a vacuum atmosphere.   The method for producing a high strength / high ductility carbon steel material according to any one of claims 4 to 6, wherein a relative density after spark plasma sintering is 95% or more. 8. The method for producing a high-strength and high-ductility carbon steel material according to any one of claims 4 to 7, wherein the discharge plasma sintering is performed for 1 minute or more in a temperature range of 520 to 700 ° C.