JP2006009098A - Cu-CONTAINING STEEL AND ITS PRODUCTION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide Cu-containing steel having fine recrystallized grains, and to provide a method for producing the same utilizing the drag effect of Cu. <P>SOLUTION: The Cu-containing steel comprises, by mass, 0.5 to 5% Cu and also has an austenitic structure of recrystallized grains with a mean grain size of 0.5 to 5 μm. It is produced by subjecting steel comprising 0.5 to 5 μm Cu to high speed cumulative deformation working at a cumulative strain of ≥2 and at a strain rate of ≥1/s in the temperature range of an Ar<SB>3</SB>transformation point to 980°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a Cu-containing steel and a method for producing the same, and more specifically, a Cu-containing steel having austenite structure of fine recrystallized grains and useful as a starting material for various steel products, and a Cu drag described later. (Drag) The present invention relates to a method of manufacturing by effectively exhibiting the (Drag) effect.

For the production of high-toughness and high-strength steel materials, the effectiveness of refining the crystal grains in the structure is known, and studies are being made mainly on simple component steel types.
On the other hand, in the refining operation using scrap, Cu in the scrap is not easily oxidized as compared with Fe, and thus is not removed during refining. Therefore, as the number of refining operations using scrap increases, Cu concentrates in the obtained steel, and in the future, it becomes a considerable concentration from the impurity level concentration, affecting the properties of the steel. It is expected to give.

However, Cu is known to exhibit the behavior described later in the steel structure, and recently, by utilizing this behavior of Cu, the steel structure is controlled and the recrystallized grains are made finer. Development of high-toughness and high-strength steel is under consideration.
For example, after heating a Cu-containing steel having a Cu concentration of 1.0 to 3.0% by mass in the austenite region, the temperature at which the free energy of the austenite structure (γ phase) and the ferrite structure (α phase) become equal (T _0). ) In a temperature range containing 30) or more, and by cooling the obtained material, a steel having a structure mainly composed of an α phase having an average grain size of recrystallization of 10 μm or less is obtained. An obtaining method has been proposed (see Patent Document 1).

Here, the characteristics of conventionally known Cu-containing steel will be described.
The first feature is that when Cu-containing steel is heated in an austenite region, the diffusion coefficient of Cu in the austenite structure is small, so Cu concentrates at the grain boundary of the γ phase (austenite structure), and changes from γ phase to α phase. The transformation point Ar ₃ is lowered. For example, in the case of Fe-0.1% C-2% Cu steel, the Ar ₃ point decreases by about 70 to 140 ° C.

Cu concentrated at the grain boundary exhibits a drag effect that hinders diffusion of Fe atoms, and suppresses grain growth of the γ phase.
Therefore, Cu of the Cu-containing steel suppresses the grain growth of the γ phase at the temperature at which the steel is heated, and at the same time, the α phase (ferrite structure) is supercooled.
Accordingly, the second feature is that when hot-working is performed on the Cu-containing steel in the above-described state, the γ phase becomes a recrystallized grain structure, and the grain boundary has a soft α which is in a supercooled state. The phase is dynamically precipitated and becomes a nucleation site for phase transformation.

Therefore, the deformation resistance during hot working is reduced. For example, in the case of Fe-0.1% C-2% Cu steel, even if the processing temperature decreases by 40 ° C. from just below the Ae ₃ point to just above the Ar ₃ point, the deformation resistance decreases to about 370 MPa → 200 MPa. To do.
Therefore, this Cu-containing steel can be easily processed and can be deformed greatly even in a low temperature range. As a result, the recrystallized grains of the γ phase can be made into a fine grain structure having a large orientation difference.

The third feature is that when an aging treatment is performed on the steel after the deformation processing described above, Cu precipitates in the steel structure and the steel can be age hardened.
For example, in the case of Fe-4% Cu steel, the water-cooled steel has a hardness (Hv) of about 240, so that it can be sufficiently cold worked. Then, after cold working, if the workpiece is subjected to an aging treatment for 1 hour at a temperature of 530 ° C., for example, the hardness (Hv) rises to about 330.

This can be said to be an industrially desirable behavior in that it is soft when processing a desired shape and can be subjected to aging treatment as it is to increase the strength.
JP 2000-226638 A

Although the steel of Patent Document 1 is manufactured using the decrease in Ar ₃ point due to segregation of Cu grain boundaries, the grain size of recrystallized grains is 10 μm or less. As far as the example is concerned, it is about 7 to 9 μm. And the organization is mainly α phase.
However, in recent trends, finer recrystallized steel is required, but considering the requirement, the particle size of the steel of Patent Document 1 is unsatisfactory.

  The present invention is a steel developed based on the above points and a method for producing the same, and an object thereof is to provide a Cu-containing steel having finer recrystallized grains and a method for producing the same.

In order to achieve the above object, the present invention is characterized by comprising an austenite structure of recrystallized grains containing 0.5 to 5% by mass of Cu and having an average grain size of 0.5 to 5 μm. A Cu-containing steel is provided.
In the present invention, the ferrite pearlite structure transformed from the austenite structure described above, a bainite structure, or a single phase structure of a martensite structure, or a mixed structure of single phase structures having two or more of them, Furthermore, a Cu-containing steel comprising an austenite structure remaining untransformed and a structure in a mixed state is provided.

Moreover, in this invention, with respect to steel containing 0.5 to 5% by mass of Cu, high-speed cumulative deformation with a cumulative strain of 2 or more and a strain rate of 1 / sec or more in a temperature range of Ar ₃ transformation point to 980 ° C. There is provided a method for producing a Cu-containing steel characterized by performing processing.

In the development and research of Cu-containing steel having refined crystal grains, the inventors set the target grain size of the crystal grains to 0.5 to 5 μm, particularly about 1 μm. At the same time, on the premise of a hot working process that is actually operating, a search was made for conditions that can be produced by that process.
In the hot working process, in general, a steel material heated to a predetermined temperature is introduced into, for example, a rolling device installed in one or more stages, and the steel material is sequentially deformed. Therefore, in this process, the steel material is deformed and processed by the preceding apparatus, and then introduced into the succeeding apparatus at a certain time interval and subjected to further deforming process.

In other words, in this hot working process, the steel material to be machined is placed in a state where it will not undergo any deformation process for a certain period of time in addition to the deformation process that is actually received.
Based on the above, the present inventors have made the following considerations regarding the refinement of recrystallized grains.
1. First, in order to obtain refined recrystallized grains, it is necessary to simultaneously realize the following two technical matters.

The first matter is that the structure of recrystallized grains is refined in the first place immediately after hot working.
The second matter is that even if the recrystallized grains are fine immediately after processing, the growth of the grains with the passage of time prevents the recrystallized grains from becoming coarse.
The present inventors set this time zone as 100 seconds based on the size of the material and manufacturing conditions, and decided to proceed with the examination of the conditions under which grain growth does not proceed for 100 seconds.

Regarding the first matter described above, the inventors of the present invention based on the dynamic precipitation of α phase during hot working by the action of Cu concentrated in the γ grain boundary, with the Cu-containing steel having a low Ar ₃ point. By utilizing the characteristic that the deformation resistance is reduced, we decided to adopt a design policy of refining the recrystallized grains immediately after processing by performing large deformation processing in a low temperature range.
And when examination was made regarding the above-described deformation processing mode, it was found that deformation processing under conditions described later that induce processing heat generation is effective.

2. The following considerations were made on the second matter.
First, grain growth is a change for lowering the interfacial energy (ΔG) per unit volume, and ΔG as the driving force is represented by the following equation.
ΔG = 2 ・ Vgb ・ V / R (1)
In the formula, Vgb represents the interfacial energy per unit area of the grain boundary, V represents the molar volume, and R represents the average radius of the recrystallized grains.

As is apparent from the equation (1), ΔG and R are in an inversely proportional relationship. As the recrystallized grains are finer, ΔG increases, and as ΔG increases, the faster grains Growth progresses. In other words, the smaller the recrystallized grains, the more difficult it is to suppress the grain growth.
By the way, in the situation where one crystal grain (1) and another crystal grain (2) are adjacent to each other across the grain boundary of three atoms and the growth of the crystal grain (1) is progressing, This is a phenomenon in which atoms move from crystal grains (2) toward crystal grains (1). In that case, the behavior that the atom of the crystal grain (2) jumps into the grain boundary and the reaction causes one atom on the side of the crystal grain (1) to be ejected into the crystal grain (1) continuously occurs. As a result, the grain boundaries move into the crystal grains (2). As a result, the growth of the crystal grains (1) and the erosion of the crystal grains (2) occur, and the grain growth proceeds.

Therefore, the growth rate of crystal grains is expressed as the product of the above-mentioned ΔG and the mobility M of the grain boundary, which is expressed by the following equation.
dR / dt = ΔG · M (2)
In the formula (2), if the structure is a single phase, the mobility M of the grain boundary does not depend on the radius R of the recrystallized grains.

As is clear from equation (2), if ΔG is constant (it is a function of R if the changes in Vgb and V are ignored), the growth rate of recrystallized grains is determined by the mobility M of the grain boundaries. Determined by the size of.
If it is possible to form a state in which the movement of Fe atoms hardly occurs at the grain boundaries between the crystal grains (1) and the crystal grains (2), the growth rate of the recrystallized grains can be reduced.
For this reason, in the case of Cu-containing steel, it is considered that the growth rate of γ grains slows down because Cu concentrated at the grain boundaries of γ grains exhibits a drag effect on Fe atoms.

By the way, the steel structure of interest is a single-phase structure of γ phase. Then, regarding the grain growth of the recrystallized grains that proceeds with ΔG as the driving force in that case, the following formula is derived by integrating the formula (2).
R ² −R ₀ ² = k ₂ · t (3)
Where R ₀ is the average radius (initial radius) of recrystallized grains at the start of grain growth, t is the holding time of steel, and k ₂ is the growth rate constant, k ₂ = 4 · Vgb · V Indicated by / M.

As is clear from Equation (3), the grain growth of recrystallized grains proceeds with time. In that case, the growth rate of the recrystallized grains is regulated by the magnitude of the k ₂ value.
For example, in the case of a steel with a very small k ₂ value, even if it is held at a certain temperature for a long time, the growth of recrystallized grains proceeds very slowly, but on the contrary, in the case of a steel with a large k ₂ value Significant grain growth proceeds in a short time.

At present, the target grain size is set to about 1 μm, and the grain growth of this recrystallized grain is being examined. Therefore, R ₀ = 0.5 (= 1.0 / 2) is substituted in equation (3). Then, when the correlation between the k ₂ value and the t value at each R value is obtained, FIG. 1 is obtained.
As is apparent from FIG. 1, even if the recrystallized grains have an initial grain size of 1 μm, in order for the recrystallized grains to hardly grow for 100 seconds, the k ₂ value of the Cu-containing steel is 1 × 10 ^It should be ^-10 mm ² / sec or less.

Conversely, in the case of Cu-containing steel having a k ₂ value of 1 × 10 ⁻¹⁰ mm ² / sec or less, when a recrystallized grain structure having a grain size of 1 μm is formed by hot working, the structure is 100. Will be maintained for seconds.
Incidentally, iron and steel, 70 (1984), according to FIG. 2, which is published in 1984, pp, in rolling and k ₂ values for each steel type in a normal hot rolling temperature (850 ° C. or higher), the steel k ₂ The value is 1 × 10 ⁻⁷ mm ² / sec or more. In that case, if it returns to FIG. 1, the time which the recrystallized grain with an initial particle diameter of 1 micrometer will maintain the particle size is about 0.1 second.

That is, in the case of each steel type shown in FIG. 2, when it is hot-rolled, grain growth proceeds in a very short time.
Of course, the k ₂ value can be reduced to 1 × 10 ⁻¹⁰ mm ² / sec or less by extrapolating the straight line relating to each steel type in FIG. However, the rolling temperature in that case is as low as about 500 ° C., and at this temperature, as a practical matter, rolling cannot be performed.

Therefore, the design target of the Cu-containing steel to be developed is that hot working is possible, and the k ₂ value is 1 × 10 ⁻¹⁰ mm ² / sec or less at the temperature during the hot working. This means that the fine recrystallized grain structure formed during hot working is maintained for 100 seconds.
Based on the above consideration, the present inventors investigated the relationship between the k ₂ value and the rolling temperature for steel A, which is Fe-1% Cu steel. The results are indicated by the ● marks in FIG.

As apparent from FIG. 2, the Fe-1% Cu steel has a very low k ₂ value compared to other steel types, and the k ₂ value is 1 × 10 ⁻¹⁰ mm if the rolling temperature is 780 ° C. or less. It was found that the value was ² / sec or less.
That is, in the case of this steel, the structure of recrystallized grains having an average grain size of 1 μm can be maintained for 100 seconds. This may be because Cu effectively exerts a drag effect on Fe atoms and suppresses the progress of grain growth.

The steel of the present invention is manufactured as follows.
First, a steel having a Cu content of 0.5 to 5% by mass and having a single-phase structure of γ phase during hot working is prepared.
Here, in the case of a steel having a Cu content of less than 0.5% by mass, the amount of Cu is too small and the drag effect on Fe atoms is not effectively exhibited, so that recrystallization with an average particle size of about 1 μm is assumed. Even if a grain structure is formed, grain growth proceeds within a short time, and the structure cannot be maintained for 100 seconds.

On the other hand, when the Cu content is about 4% by mass, the drag effect of Cu on Fe atoms reaches saturation.
When the Cu content is more than 5% by mass, for example, cracks and the like are generated during the batch rolling of steel, although the grain growth of recrystallized grains does not proceed. For this reason, the upper limit of the Cu content needs to be set to 5% by mass.

The limit of the drag effect indicated by the one-dot broken line in FIG. 2 is a condition when all the γ grain boundaries of Fe are filled with different atoms, and at this time, the rate of grain growth is the smallest. Also, it is impossible to achieve the area below this line with the drag effect.
In the present invention, the above-described steel is subjected to hot working such as hot rolling under the following conditions.

First, the processing temperature is set to Ar ₃ point (726 ° C.) or higher and 980 ° C. or lower. That is, in order to change the steel structure to a single-phase structure of γ phase during hot working, the working temperature is set to Ar ₃ or higher.
In addition, as shown in FIG. 2, the k ₂ value at which a crystal grain having a grain size of 1 μm is not grown for 100 seconds is 1 × 10 ⁻¹⁰ mm ² / sec, but this value agrees with the limit of the drag effect. Since the temperature of 980 ° C is 980 ° C, the upper limit of the processing temperature is set to 980 ° C.

Considering that hot working can proceed smoothly while suppressing grain growth and recrystallization is promoted, it is usually preferable to set the working temperature to 700 to 850 ° C.
And the hot processing in this invention is performed using process heat_generation | fever. That is, dynamic recrystallization of γ grains is realized by performing high-speed cumulative deformation processing.
In that case, if the applied strain is large, the strain rate is low, or conversely, if the strain rate is large but the cumulative strain is small, the heat generated by the processing will not be sufficient, and dynamic recrystallization of fine γ grains will not be possible. It cannot be realized.

For example, even when hot working at a high strain rate, if the accumulated strain at that time is small, the dissipation of heat to the surroundings becomes remarkable and the temperature rise is reduced, resulting in a structure that promotes recrystallization. It becomes difficult.
Considering that the target grain size of the recrystallized grains in the present invention is 0.5 to 5 μm, in order to realize this target grain size, the cumulative strain is 2 or more and the strain rate is 1 / sec or more. It is necessary to adopt certain processing conditions.

Then, the average strain diameter of the recrystallized grains is set to a desired value within the range of 0.5 to 5 μm by performing hot working in which the cumulative strain and strain rate are appropriately selected and combined. be able to.
As is clear from FIG. 2, the drag effect of Cu is the largest, but the drag effect is also exhibited by elements such as Cu, Ni, Mo, and Si.

  The Cu-containing steel of the present invention thus produced has a single-phase structure of γ phase in the hot working process, but by performing the desired controlled cooling on the steel, The austenite structure can be transformed into a ferrite / pearlite single phase structure, a bainite single phase structure or a martensite single phase structure, or a mixed structure of these single phase structures. Further, the austenite structure remaining untransformed can be made into a structure mixed with the above structure.

  For example, a steel having a structure in which an α phase (ferrite) having a crystal grain size of 3 μm or less is contained in an area ratio of 10% or more, for example, a martensite phase or a bainite phase having a block length of 5 μm or less in an area ratio of 10%. It can be made of steel having the structure contained above.

(1) Preparation of steel C: 0.11% by mass, Si: 0.27% by mass, Mn: 0.47% by mass, Cu: 0.99% by mass, Ni: 3.27% by mass, Mo: 0.3% Example steel (1) comprising 28% by mass, Al: 0.94% by mass, N: 0.0074% by mass and the balance consisting of Fe and inevitable impurities was prepared.
As shown in FIG. 2, this steel (1) is a Cu-containing steel A having a k ₂ value of 1 × 10 ⁻¹⁰ mm ² / sec or less at a temperature of 780 ° C. or less.

On the other hand, C: 0.4 mass%, Si: 0.25 mass%, Mn: 0.80 mass%, Cu: 0.18 mass%, Ni: 0.10 mass%, Cr: 1.01 mass%, Comparative Example Steel (2) comprising Mo: 0.16% by mass, Al: 0.017% by mass, N: 0.013% by mass, the balance being Fe and inevitable impurities was prepared.
This steel (2) is a steel corresponding to JIS SCM440 and is a steel containing an impurity level of Cu.

(2) Effect of high-speed cumulative deformation processing Steel (1) was hot-rolled at a temperature of 750 ° C. under conditions of a cumulative strain of 1.5 and a strain rate of 0.72 / sec. A micrograph of the steel structure after processing is shown in FIG.
As is clear from FIG. 3, fine recrystallized grains are scattered, but as a whole, this steel has a large crushed gamma phase texture.

On the other hand, steel (1) was hot-rolled at a temperature of 750 ° C. under the conditions of a cumulative strain of 2.9 and a strain rate of 12.1 / sec. A micrograph of the processed structure is shown in FIG.
As apparent from FIG. 4, this steel has a fine recrystallized austenite structure with an average grain size of about 1.3 μm.
(3) Verification of Cu-drug effect Steel (1) and steel (2) were hot-rolled at a temperature of 750 ° C. under conditions of a cumulative strain of 2.9 and a strain rate of 12.1 / sec. Next, the processed steel was held at a temperature of 750 ° C. for 60 seconds. The structural change of steel (1) is shown in FIG. 5, and the structural change of steel (2) is shown in FIG.

As is clear from the comparison between FIG. 5 and FIG. 6, both the steel (1) and the steel (2) are composed of fine recrystallized grains after hot rolling. In this case, grain growth has progressed after 60 seconds.
However, in the case of steel (1), no progress of grain growth is observed. This is because the drag effect of Cu is effectively exhibited in the steel (1).

Since the Cu-containing steel of the present invention is manufactured by high-speed cumulative deformation processing with processing heat generation, the recrystallized grains are extremely refined to 0.5 to 5 μm. And since the drag effect with respect to the Fe atom of Cu is utilized effectively, grain growth hardly progresses for 100 seconds after hot processing.
Therefore, this steel has high strength and high toughness, and can be tempered into steel of various structures by controlled cooling after hot working, and its industrial use as a starting material for producing steel with desired characteristics. The value is great.

It is a graph which shows the relationship between the growth rate constant in the grain growth of the recrystallized grain whose initial grain diameter is 1 micrometer, and holding time. It is a graph which shows the relationship between the growth rate constant and processing temperature in each steel type. 2 is a photomicrograph showing the structure of Example Steel (1) when hot rolled under conditions deviating from the conditions of the method of the present invention. 2 is a photomicrograph showing the structure of Example Steel (1) when hot rolled under the conditions of the present invention. It is a microscope picture which shows the structure | tissue when the structure of Example steel (1) hot-rolled on the conditions of this invention and it hold | maintained for 60 second at the temperature of 750 degreeC. It is a microscope picture which shows the structure | tissue when the structure of the comparative example steel (2) hot-rolled on the conditions of this invention and it hold | maintained for 60 second at the temperature of 750 degreeC.

Claims (3)

  A Cu-containing steel comprising an austenite structure of recrystallized grains containing 0.5 to 5% by mass of Cu and having an average grain size of 0.5 to 5 μm.   The structure according to claim 1, further comprising at least one structure selected from the group consisting of a ferrite pearlite structure transformed from the austenite structure, a bainite structure, and a martensite structure, and a structure in a mixed state with the austenite structure remaining untransformed. Cu-containing steel. It is characterized by performing high-speed cumulative deformation processing with a cumulative strain of 2 or more and a strain rate of 1 / sec or more on a steel containing 0.5 to 5% by mass of Cu in a temperature range of Ar ₃ transformation point to 980 ° C. A method for producing Cu-containing steel.