JPH11246931A - Steel having ultrafine ferritic structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine ferritic grains, to randomize the orientation and to form a steel structure having high strength by specifying the volume ratio of ferrite specified in the average grain size and surrounded by large-angle boundaries and the accumulating degree of ferrite. SOLUTION: A steel structure in which ferrite having <=3.0 μm average grain size and surrounded by large-angle boundaries of >=15 deg. is contained by >=60% volume ratio, and the accumulating degree of the specified orientations 332} <113> and 113} <110> is made to <=4 is formed. This steel is subjected to compression working of >=30% draft at less than the recrystallization temp. into austenite. As for the austenite grain boundaries or the twin crystals in the austenite grains before the transformation, on the part of >=70% per boundary unit length in the linear boundaries on a view from the surface vertical to the boundary face, ruggedness of <=8 μm cycloe and >=200 nm amplitude is formed. After that, cooling is executed at the velocity of >=3 K/s. In the transforming stage, the ferrite composed of large-angle boundaries is not easily combined nor grow to grains, and it remains fine up to a room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrafine ferritic steel. For more information,
The invention of this application relates to an ultrafine ferrite structure steel useful as a high-strength structural steel, a high-strength general welded structural steel, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, the controlled rolling-accelerated cooling technique has been an effective method for obtaining fine ferrite which can contribute to the improvement of the strength of a low alloy steel. That is, a fine structure is obtained by controlling the cumulative rolling reduction in the austenite unrecrystallized region and the subsequent cooling rate. However, the grain size of the obtained ferrite was at most 10 microns for Si-Mn steel and 5 microns for Nb steel. Furthermore, Japanese Patent Publication No. 62-3922
8. As described in JP-B-62-7247, 2
By applying a reduction having a total area reduction of 75% or more in a temperature range of Ar1 to Ar3 + 100 ° C. including a phase region, and then cooling at 20 K / s or more, ferrite grains of about 3.4 μm are obtained. As described in Japanese Patent Publication No. H5-65564, when the diameter becomes less than 3 microns, an extremely large reduction amount and a cooling rate of 40 K / s or more are required. However, quenching of 20 K / s or more is a means that can be realized only when the sheet thickness is small, and is not widely applicable as a method for manufacturing general welding structural steel.

[0003] Under these circumstances, it has been extremely difficult to reduce the ferrite grain size to less than 3 µm in the past, and it has not been actually realized to reduce the size of ferrite, which contributes to strength improvement. It is. and again,
If the amount of reduction in the unrecrystallized region is increased by controlled rolling, the following problems occur. That is, for example, as shown in FIG. 6 (iron and steel 65 (1979) 1747-1755), as the amount of processing increases, the specific orientation {332} <11
3> and {113} <110>, and the ratio of small-angle grain boundaries increases. Even if it can be miniaturized to about 3 microns, the increase in strength and the increase in fatigue strength do not become as large as expected by the miniaturization. In addition, since the probability that the ferrite grains have the same orientation is increased, the ferrite grains are likely to unite and grow, and it is difficult to make the ferrite grains finer. From this point, 5 μm was the limit for miniaturization of ferrite.

Conventionally, there is no technique for controlling the orientation of the ferrite to be formed, and it has not been possible to make the orientation of the ferrite random at the same time as miniaturizing the ferrite grains. Therefore, this application has overcome the limitations of the prior art as described above, realized ultra-fine ferrite grains and its large-angle grain boundaries, and realized randomization of orientation, useful as grooves for general welding structures, etc. It is an object of the present invention to provide a new ultrafine ferritic steel and a method for producing the same.

[0005]

In order to solve the above-mentioned problems, the present invention is directed to a first invention, in which an average particle size is 3.0 μm or less and is surrounded by a large-angle grain boundary of 15 ° or more. Provided is an ultrafine ferrite structure steel characterized by containing ferrite at a volume ratio of 60% or more and having a specific orientation of ferrite of 4 or less.

[0006] The present invention also provides, as a second invention, a method for producing the steel of the first invention by processing austenitic steel, wherein the austenite grain boundary before transformation is perpendicular to the grain interface. In a linear grain boundary as viewed from above, 70% or more per grain boundary unit length has a period of 8 μm or less and an amplitude of 20 μm or less.
Provided is a method for manufacturing an ultrafine ferritic steel, characterized by having undulations of 0 nm or more.

[0007] Further, as a third invention, this application is a method of manufacturing an austenitic steel to produce the steel of the first invention, wherein the annealing twins in the austenite grains before transformation are formed with respect to the boundary. 70% or more per unit length of a grain boundary at a linear boundary as viewed from above a vertical plane has a period of 8 μm or less,
Provided is a method for producing an ultrafine ferritic steel, characterized by having undulations with an amplitude of 200 nm or more.

[0008] The present application provides, as a fourth invention,
A method for producing a steel according to the first invention by processing austenitic steel, wherein compression is performed at a non-recrystallization temperature of austenite at a rolling reduction of 30% or more, and then cooled at a rate of 3 K / s or more after the processing. Also provided is a method for producing an ultrafine ferritic structure steel characterized by the above. That is, according to the invention of this application as described above, an ultrafine ferrite structure steel which has not been known at all is provided.

This ultrafine ferritic steel has the following features: 1) 60% by volume or more of ferrite surrounded by large-angle grain boundaries of 15 ° or more and having an average grain size of 3.0 μm or less; It is required that the degree of integration of the specific orientation of ferrite be 4 or less.

According to the study of the inventor of the present application, for such a new ultrafine ferrite microstructured steel, in order to achieve ultrafine ferrite, its large-angle grain boundaries, and random orientation, the austenite grain boundaries before transformation are used. And that the annealing twins in the austenite grains before transformation have undulations,
It is based on the fact that non-linearity is necessary for ultrafine-graining, randomization of intragranular ferrite and grain boundary ferrite, and large-angle grain boundary formation. For example, FIG. 1 shows a schematic diagram of a grain boundary. The ferrite to be formed nucleates with a relationship of KS with respect to austenite. Nucleation to have Then, for example, as shown in FIG.
When the austenite grain interface faces various directions, the ferrite to be formed also faces various directions. That is, the orientation randomization of the grain boundary ferrite proceeds. Further, the deformed zone and the annealing twin in the processed austenite grain boundary can be nucleation sites comparable to the grain boundary, but they are formed when they have the same irregularities as the grain boundary in FIG. Ferrite, like grain boundary ferrite, is oriented in various directions. Therefore, the orientation of the so-called intragranular ferrite is also randomized.

Making the annealing twins in the austenite grain boundaries and in the austenite grains undulate is nothing more than imparting a micro strain gradient in the material. As a specific method of imparting a strain gradient, there is a method such as a compression process or a roll rolling process using an anvil. Due to the presence of the above undulations, a ferrite fine grain having an average grain size of 3.0 μm or less, a misorientation angle of adjacent ferrite grains of 15 ° or more, and a specific orientation with a degree of integration of 4 or less is provided. Texture steel is made possible.

In general, fine ferrites are extremely easy to coalesce and grow during the transformation process and thereafter, but ferrites composed of large-angle grain boundaries do not easily coalesce and grow and reach fine temperature at room temperature. Have also been found by the inventor of this application.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be further described below. As described above, the first invention of this application is made of a high-strength ultrafine ferritic steel, which is useful as a general welded structural steel. When the austenite is manufactured by processing, <A> the austenite grain boundary before transformation as in the second invention, and <B> the austenitic grain boundary before transformation as in the third invention. In at least one of the deformation zone and the annealing twin, a linear grain boundary or boundary viewed from a plane perpendicular to the grain boundary or the boundary, in which 70% or more per unit length of the grain boundary has a period of 8 μm. Hereinafter, it is assumed that undulations having an amplitude of 200 nm or more exist.

In this case, as shown in FIG. 3, for example, the undulation at the grain boundary or grain boundary (α) has a period (L) of 8 μm or less and an amplitude (W) of 200 n.
m or more. The above-mentioned requirements are satisfied, for example, by performing austenitization and then performing plane strain compression processing with a rolling reduction of 30% or more at a non-recrystallization temperature equal to or lower than the recrystallization temperature of austenite as in the fourth invention of this application. It becomes possible. And after processing, 3K / s
By cooling as described above, the ultrafine ferritic structure steel as described above is realized.

The period (L) and the amplitude (W) are set to 8 μm or less and 200 nm or more in this process, respectively. When the period (L) exceeds 8 μm and when the amplitude (W) is less than 200 nm, it is difficult to obtain the ultrafine ferrite structure steel of the present invention. The rolling reduction of the compression working is 30% or more, and more preferably 50% or more. As a means for the processing, anvil processing illustrated in FIG. 4 is exemplified as one of suitable means.

In plane distortion compression using this anvil,
Strong machining exceeding 90% in one pass with a reduction in area is also possible. Then, in the anvil processing, as shown in FIG. 4, the processed portion receives a large deformation including a shear deformation even at the same area reduction ratio as compared with the roll rolling. There is no particular limitation on the chemical composition of the ferritic steel of the invention of this application, and Si, Mn, C, P, S, N, Nb, Ti, V, A
1 and the like may be contained at an appropriate ratio. However, when considering weldability, C (carbon) is 0.3 ma.
It is appropriate to set it to ss% or less.

As described above, the fact that the structural steel having an average ferrite grain size of 3.0 μm or less having a random orientation and having a random orientation can be produced by the invention of the present application means that a completely new method for producing a high-strength steel. Will give. Moreover, it is extremely practically significant that an ultrafine structure can be obtained without using expensive elements such as Ni, Cr, Mo, and Cu, and high-strength steel can be manufactured at low cost. That is.

The present invention will be described in more detail with reference to the following examples.

[0019]

EXAMPLES In the following Examples 1 to 3 and Comparative Examples, steel of steel type number 1 (composition 1) in Table 1 was used.

[0020]

[Table 1]

(Example 1) A steel having a composition 1 was austenitized and its grain size was adjusted to 15 microns.
Anvil compression processing was performed at 750 ° C. at a strain rate of 10 / s and a reduction in area of 73% at a time. In order to freeze the austenite grain boundaries during processing, water cooling was performed immediately after processing to cause martensitic transformation, thereby producing a martensitic structure. Observation of the former austenite grain boundary of this martensite structure revealed that there were clear irregularities at 85% per unit length of the grain boundary, the period was 5.5 μm or less, and the amplitude was 3 μm.
It was 50 nm or more. Next, processing was carried out under the same conditions to give the above-described irregularities to the austenite grain boundaries.
Cooling was performed at K / s. The resulting structure was ferrite-pearlite. The average particle size of the ferrite structure was 2.0 microns as measured by a linear cutting method. When the orientation information of the structure of the plane perpendicular to the rolling direction (TD plane) was measured by three-dimensional crystal structure analysis by electron backscatter diffraction (EBSD), the orientation of the ferrite was random as shown in FIG. As shown in FIG. 5, the degree of integration of the {001} // ND orientation was only 1.9 at most. The ratio of the large-angle grain boundaries having the misorientation angle of 15 degrees or more between adjacent ferrite grains is 9% more than the ratio of the grain boundary length on the measurement surface.
5%. The volume ratio of ferrite specified in the present invention was 75%. (Example 2) For a steel of composition 1 having an austenite grain size of 300 microns, 75
Anvil compression processing was performed at a time at 0 ° C. at a strain rate of 10 / s and a reduction in area of 73%. In order to freeze the austenite grain boundaries during processing, water cooling was performed immediately after processing to cause martensitic transformation, thereby producing a martensitic structure. Observation of the former austenite grain boundary of this martensi structure revealed clear irregularities, and the period was 6.1.
Submicron and amplitude greater than 300 nm. Also,
Observation of old annealing twins revealed that 80% per grain boundary unit length
Had a period of 6.2 μm or less and an amplitude of 300 nm or more. Next, processing was performed under the same conditions, and after giving the above irregularities to the austenite grain boundaries and the annealing twins in the grains, cooling was performed at 10 K / s. The resulting structure was ferrite-pearlite.
The average grain size of the ferrite structure was 2.6 microns as measured by a linear cutting method. Electron backscatter diffraction (EB
When the orientation information of the structure of the plane (TD plane) perpendicular to the rolling direction was measured by three-dimensional crystal structure analysis by SD), the orientation of the ferrite was random, and as shown in FIG. The integration degree of｝ // ND orientation was only 2.1. The ratio of the large-angle grain boundaries having the misorientation angle of 15 ° or more between adjacent ferrite grains was 94% from the ratio of the length of the grain boundaries on the measurement surface. The volume ratio of ferrite specified in the present invention was 75%. (Example 3) A steel of composition 1 having an austenite grain size of 15 microns was subjected to a strain rate of 10 / s at 750 ° C.
Then, an anvil compression process with a reduction of area of 50% was performed at one time.
Immediately after the reduction, water cooling was performed, and old austenite was observed.
Further, after rolling, it was cooled at a cooling rate of 10 K / s to produce a ferrite-pearlite structure. The average particle size of the ferrite structure was 2.4 microns as measured by a linear cutting method. When the orientation information of the tissue was measured by a three-dimensional crystal structure analysis method (ODF method) using electron beam backscatter diffraction (EDSD), the degree of integration of the orientation was 3.8. The ratio of the large-angle grain boundaries having a misorientation angle of 15 degrees or more to the ferrite grain boundaries was 95% from the ratio of the length on the measurement surface. In the former austenite grain boundary, irregularities exist at 75% per unit length of the grain boundary, the period is 6.9 microns or less, and the amplitude is 30.
It was 0 nm or more. Using electron backscatter diffraction,
When the orientation of the ferrite grains generated from the grain boundaries was measured, the orientation of the ferrite grains generated from the grain boundaries was random. The volume ratio of ferrite specified in the present invention was 75%. (Comparative Example) A steel of composition 1 having an austenite grain size of 30 microns was subjected to water cooling without any processing to cause martensitic transformation to produce a martensitic structure.
Observation of the former austenite grain boundary of this martensite structure revealed that the former austenite grain boundary was straight, no periodic unevenness was observed, and the amplitude of the occasional unevenness was 200 n.
m or less.

[0022]

As described in detail above, the invention of this application provides a novel ultrafine ferritic steel which is useful as a high strength steel for general welded structures.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing ferrite growth at austenite grain boundaries.

FIG. 2 is a diagram schematically showing the orientation of ferrite at an undulating austenite grain boundary.

FIG. 3 is a diagram schematically showing the undulation cycle and amplitude.

FIG. 4 is a conceptual diagram illustrating anvil processing.

FIG. 5 is a diagram illustrating the orientation and the degree of integration in Example 1.

FIG. 6 is a diagram illustrating the orientation and the degree of integration in Example 2.

FIG. 7 is a diagram showing conventional knowledge about the relationship between the processing amount and the degree of integration.

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[0006] The present invention also provides, as a second invention , a method for producing the steel of the first invention, which comprises a method of producing austenitic steel.
At a non-recrystallization temperature at a compression rate of 30% or more.
In addition, the austenite grain boundaries before transformation have a period of 8 μm or less and an amplitude of 200 nm in a linear grain boundary as viewed from above on a plane perpendicular to the grain interface, at least 70% per unit length of the grain boundary.
The above undulations are formed, and then cooled at a speed of 3 K / s or more.
To provide a method of manufacturing ultra-fine ferrite structure steel, characterized by retirement.

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The present invention further provides, as a third invention , a method for producing the steel according to the first invention, which comprises a method of producing austenitic steel.
At a non-recrystallization temperature at a compression rate of 30% or more.
In addition, the annealing twins in the austenite grains before transformation have a period of 8 μm or less and an amplitude of 20% or more per grain boundary unit length at a linear boundary viewed from a plane perpendicular to the boundary.
The undulation of 0 nm or more is formed, and then the speed is 3 K / s or more.
The present invention provides a method for producing an ultrafine ferritic structure steel characterized by cooling at a temperature .

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[0008] ie, the invention of this application as described above, this ultra fine ferrite structure steel is not known at all so far is provided. ────────────────────────────────────────────────── ───

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FIG. 7

Continuation of the front page (72) Inventor Osamu Umezawa 1-2-1 Sengen, Tsukuba, Ibaraki Pref.

Claims (4)

[Claims] An average particle size is 3.0 μm or less, ferrite surrounded by a large-angle grain boundary of 15 ° or more contains 60% or more by volume, and the degree of integration of a specific orientation of the ferrite is 4 or less. Ultra fine ferrite structure steel characterized by the following. 2. The method for producing an austenitic steel according to claim 1, wherein the austenite grain boundary before transformation is a linear grain as viewed from a plane perpendicular to the grain interface. In the grain boundary, a period of at least 70% per unit length of the grain boundary is 8 μm.
Hereinafter, a method for producing an ultrafine ferritic structure steel characterized by having undulations with an amplitude of 200 nm or more. 3. A method for producing an austenitic steel by processing an austenitic steel according to claim 1, wherein the annealing twins in the austenite grains before transformation have a line viewed from a plane perpendicular to the boundary thereof. A method for producing an ultrafine ferritic steel, characterized in that 70% or more per unit length of a grain boundary has undulations with a period of 8 µm or less and an amplitude of 200 nm or more at a grain boundary. 4. A method for producing a steel according to claim 1, wherein said austenitic steel is worked by applying a compression working at a non-recrystallization temperature of austenite at a rolling reduction of 30% or more, and after working at a rate of 3 K / s or more. A method for producing ultrafine ferritic steel, characterized by cooling at a rate.