JPH1192855A - Steel having ultrafine double phase structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the strength of a ferritic steel and to improve the balance of the strength and ductility by providing the steel with an ultrafine double phase structure in which the average grain size is regulated to a specified value or below, the ferrite surrounded with a large-angle grain boundary whose orientation difference angle in the grain boundary is regulated to a specified value or above is used as the mother phase and the pearlite of a specified ratio or above is incorporated. SOLUTION: In this structure, the average grain size is <=4 μm, the ferrite surrounded with a large-angle grain boundary whose orientation difference angle in the boundary is regulated to >=15 deg. is used as the mother phase and the pearlite of >=3 vol.% is incorporated. For the ultrafining and the enlargement of the angle of the grains, anvil compression working is extremely effective, and intensive working of >90% reduction of area per pass is also possible. The worked part is applied with large deformation including shear deformation compared to the case by roll rolling even at the same reduction of area, so that, by executing the anvil compression working in an unrecrystallized region, the objective ferritic grains can be obtd. by the working of >=50% reduction of area. Thus, by regulating the austenitic grain size, the amt. to be worked and the working temp., the objective steel can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrafine double phase steel which is useful as a high strength structural steel.

[0002]

2. Description of the Related Art Conventionally, as a method for strengthening a steel material, solid solution strengthening, strengthening by a second phase by compounding with martensite, precipitation strengthening, and grain refinement are known. Above all, as a method for increasing both strength and toughness and improving the balance between strength and ductility, refinement of crystal grains is the most excellent method. This method does not require the addition of expensive elements such as Ni and Cr that enhance the hardenability, so that it is possible to manufacture high-strength steel at low cost.
From the viewpoint of the refinement of the crystal grains, in structural steel,
When the crystal grain size of ferrite is reduced to 3 μm or less, the strength is expected to increase rapidly. However, with a particle diameter of about 5 μm obtained up to now by general thermomechanical processing technology, although the strength is increased,
The fact is that a large increase in strength has not been obtained.

[0003] Further, as the ferrite structure is refined, both the yield strength and the tensile strength increase, but the uniform elongation significantly decreases, and the yield strength increases more than the tensile strength. Occurs. That is, the yield ratio increases. This means that the n value (work hardening index) decreases. The same applies to ultrafine ferrite single-phase steels having a ferrite grain size of 4 μm or less, although the strength is increased but the elongation is significantly reduced.

[0004] In view of the above, in order to improve the balance between strength and ductility of ferritic steel while increasing the strength, it is completely different from the conventional idea of miniaturizing ferrite grains. There was a need for measures from the perspective of Accordingly, an object of the invention of the present application is to overcome the limitations of the conventional technology as described above, and to provide a new ferrite structure steel in which the strength of a ferrite structural steel is increased and the balance between strength and ductility is also improved. And

[0005]

The present invention solves the above-mentioned problems by providing a ferrite having an average grain size of 4 μm or less and surrounded by large-angle grain boundaries having a misorientation angle of grain boundaries of 15 ° or more. And a parent phase, and contains pearlite in a volume ratio of 3% or more.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the ultrafine dual-phase structure steel of the present invention has been made based on the following background.
That is, according to the results of studies by the inventors, it has been found that when the ferrite grain size is 4 μm or less, the strength and uniform elongation are simultaneously increased as compared with a ferrite single phase due to the formation of a double phase with pearlite. Ferrite grain size is 10μm
In the general ferrite-pearlite duplex stainless steel as described above, although the strength increases as the pearlite volume ratio increases, the elongation and the n value decrease. However, when the ferrite grain size is 4 μm or less, the pearlite volume increases. As the rate increases, strength and elongation increase at the same time. For this reason, according to the present invention, it is possible to improve the strength and the strength-ductility balance by making the ferrite ultrafine and making it into a dual phase with pearlite. In particular, when the ferrite particle size is 2 μm or less, the effect of increasing the strength and ductility by forming a dual phase with pearlite is remarkable. As a result, it is possible to realize an ultrafine structure steel having high strength, ductility, n value, and low yield ratio.

Therefore, according to the present invention, as described above, 1) a ferrite matrix having an average ferrite grain size of 4 μm or less and surrounded by a large-angle grain boundary having a misorientation angle of 15 ° or more at a ferrite grain boundary, 2) Ultrafine dual phase steel is specified by containing 3% or more by volume of pearlite.

[0008] The steel of the present invention can be produced, for example, based on the following findings. That is, first, as a result of the research by the inventors, it has been found that anvil compression processing is extremely effective for ultrafine ferrite and formation of large-angle grain boundaries. FIG. 1 illustrates an aspect of the anvil compression processing. In the anvil compression, it is possible to perform a strong working exceeding 90% in one pass at a reduction in area. The processed part undergoes a large deformation including a shear deformation even at the same area reduction ratio as that of the roll rolling. As a result, in the unrecrystallized area,
By performing this anvil compression working, ferrite grains having an average grain size of 4 μm or less can be obtained by working with a reduction in area of 50% or more. The ferrite has a so-called large angle grain boundary having an adjacent misorientation angle of 15 ° or more. That is, by controlling the austenite grain size, the amount of processing, and the processing temperature before processing, a large-angle grain boundary and 4 μm
The following ferrite fine structure steel can be manufactured.

Generally, fine ferrite is very easy to coalesce and grow during the transformation process and thereafter.
Ferrite composed of large-angle grain boundaries does not easily coalesce and grow, and reaches room temperature in a fine state. As a result, the cooling rate
The above-mentioned fine particles can be obtained even at 3 K / s or more, compared to the conventional 20 K / s or more. Such a slow cooling rate has never been considered before. A strain rate of 1 / s at the time of processing according to the present invention is sufficient. 1-10 / s is a general strain rate of plate rolling.

The relationship between the width of the anvil used for processing and the thickness of the sample can be appropriately adjusted, and a lubricant may be applied between the anvil and the sample. In view of the above, in the present invention, after an austenite is formed by heating to a temperature of 3 or more points, anvil compression processing is performed at a temperature of 3 or more points and an anvil compression rate of 50% or more, and then cooled at a rate of 3 K / s or more. Is appropriate.

The average austenite grain size before processing is 30
It is preferably 0 μm or less, and more preferably 30 μm or less. The amount of processing is required to be 50% or more in cross-sectional reduction ratio, and is preferably 70% or more in order to obtain a particle size of less than 2 microns. The processing temperature requires an austenite unrecrystallized region,
It is desirable to be within 3 + 200 ° C. In order to obtain as fine grains as possible, it is desirable that the temperature be Ar3 + 100 ° C. or less.

The phase other than the ferrite and pearlite phases may have one or more of bainite, martensite, and retained austenite, and may have precipitates such as carbide, nitride, and oxide. You may. However, it is indispensable that ferrite be used as a matrix and that the volume ratio of pearlite be 3% or more. However,
When the volume ratio of pearlite exceeds 40%, toughness and weldability deteriorate. However, it may exceed 40% in applications where these characteristics are not a problem. From such a viewpoint, as a general welded structural steel, the volume ratio of pearlite is 40%.
It is appropriate to use up to.

The average grain size of ferrite as defined in the present invention is measured by, for example, a linear cutting method. The orientation of the ferrite grain boundary is determined by observing several fields of about 0.1 × 0.1 mm typical of the processed part with an SEM, and hundreds of ferrite grains are scattered by electron beam backscatter diffraction (EBSD) per field. Method. The case where the misorientation angle of the ferrite grain boundary is 15 ° or more is defined as a large-angle grain boundary. When the large-angle grain boundaries occupy 70% or more of the entire grain boundaries, it is assumed that the structure is composed of large-angle grain boundaries. The ratio of large-angle grain boundaries is 7
If it is less than 0%, the effect of increasing the strength due to the refinement of the structure cannot be said to be sufficient.

The chemical composition of the steel may vary, but the content of C (carbon) is 0.1% by weight.
It is appropriate that the content be 3% or less. With further additions,
This is because it becomes difficult to set the volume ratio of ferrite to 60% or more. The composition includes expensive elements such as Ni,
It is not always necessary to use Cr, Mo, Cu, or the like. The composition may contain Si, Mn, Al, P, S and N together with C, with the balance being Fe and unavoidable impurities.

As described above, in the present invention, the increase in strength due to ultrafine ferrite of 4 μm or less is achieved, for example, when the tensile strength is 20 μm, which is 480 μm.
What was about MPa, about 600 MPa at 4 μm,
At 2 μm, it is remarkably increased to about 700 MPa, and a decrease in ductility due to ultra-fine ferrite is suppressed, and a balance between strength and strength-ductility is improved.

Actually, when the volume ratio of pearlite is 25%, the uniform elongation is improved by 25% when the average particle diameter of ferrite is 3 μm, and the uniform elongation is doubled when the average particle diameter is 2 μm. On the other hand, surprisingly, in the conventional 20 μm ferrite structure, the inclusion of pearlite further deteriorates the ductility. This phenomenon occurs because the average ferrite grain size is 4
It becomes remarkable as the size exceeds μm.

In the present invention, the average grain size of the ferrite is set to 4 μm or less. A practical effect appears when the volume fraction of pearlite is 3% or more. The upper limit can be considered as an allowable range of expected strength. In this case, for example, the strength shown in FIG. 2 calculated from the stress-strain curve obtained by calculating using the Semant method of micromechanics based on the data obtained from the Swift equation for ferrite single-structure steel -The balance of uniform elongation (change in ferrite grain size) can be used as a guide. The solid line in FIG. 2 shows a pearlite having a volume ratio of 25%.

Hereinafter, the present invention will be described in more detail with reference to examples.

[0019]

EXAMPLE 1 A steel having the composition shown in Table 1 was heated to 900.degree. C., completely austenitized, cooled to 750.degree. C., and immediately annealed as shown in FIG. Was done. After compression, cooling was performed at 10 K / s to 500 ° C. As a result, a ferrite-pearlite dual phase structure steel having an average ferrite grain size of 2.0 μm in the processed portion was obtained. The pearlite volume ratio was 25%. Electron Backscatter Diffraction (EBS
When the tilt angle of the ferrite grain boundary was measured by the method D), the angle was 15 °.
The ratio of the grain boundaries having the above inclination angles to all ferrite grain boundaries was 90%. The tensile strength, yield strength, and uniform elongation of the steel were 710 MPa, 600 MPa, and 0.0 MPa, respectively.
6

[0020]

[Table 1]

Example 2 A steel having the composition shown in Table 1 was heated to 950.degree. C. and completely austenitized, and then cooled to 800.degree. A 3.0 micron, 10% perlite volume fraction steel was obtained. This steel also had ferrite surrounded by large-angle grain boundaries. Tensile strength is 580MP
a, the uniform spread was 0.09. Comparative Example 1 Steel having the same composition as in Example 1 was heated to 900 ° C. and completely austenitized, then cooled to 800 ° C., and roll-rolled at a cumulative draft of 70%. After rolling, 500 ° C
Until then, cooling was performed at 10 K / s. As a result, a ferrite-pearlite steel having an average ferrite grain size of 6 microns was obtained.

The volume ratio of pearlite was 25%. The tensile strength of this is 550 MPa, and the uniform elongation is
0.15. Because the particle size was 6 microns, the strength was significantly reduced. There was no effect of improving uniform elongation due to the presence of pearlite. Comparative Example 2 A powder metallurgy method having the composition shown in Table 1 and having an average particle size of 2
A micron ferrite steel was obtained. The tensile strength and uniform elongation (true strain) of the steel were 630 MPa and 0.03, respectively.
Met.

It was confirmed that strength-ductility was not balanced. Comparative Example 3 A steel having the composition shown in Table 1 was subjected to hot rolling, cold rolling and heat treatment, and as a result, a ferrite-pearlite steel having an average ferrite grain size of 3.2 microns was obtained. As a result of EBSD measurement, the ratio of the grain boundaries having an inclination angle of 15 ° or more to the ferrite grain boundaries was 50%. At that time, the tensile strength and the uniform spread were 530 MPa and 0.12.

[0024]

According to the invention of this application, the strength is improved by the refinement of ferritic steel, the ductility is improved, and the balance between strength and ductility is improved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an anvil compression process.

FIG. 2 is a graph showing a relationship between a tensile strength and a ferrite particle diameter of uniform elongation and a pearlite volume ratio.

Claims (1)

[Claims] 1. A ferrite surrounded by a large-angle grain boundary having an average grain diameter of 4 μm or less and a misorientation angle of a grain boundary of 15 ° or more as a mother phase, and containing pearlite in a volume ratio of 3% or more. Ultra-fine dual phase steel.