KR101186557B1 - Method for evaluation of dynamic deformation property of steel at low temperature using dynamic torsional bar test - Google Patents

Abstract

The present invention provides a method for evaluating the dynamic deformation characteristics of steel. Method for evaluating the dynamic deformation characteristics of the steel according to the present invention, preparing a steel specimen; A low temperature dynamic torsion test of the steel specimens at a lower temperature than room temperature using a dynamic torsion test apparatus having a refrigerant injection device; Obtaining physical property parameters of the steel specimen through the low temperature dynamic torsion test; And analyzing the physical property parameters to evaluate the low temperature dynamic deformation characteristics of the steel specimens, wherein the physical property parameters are fracture shear strains of the steel specimens, and the fracture shear strains of the steel specimens are determined by the adiabatic shear bands of the steel specimens. It depends on the thickness or the presence or absence, and the insulating shear band depends on the number of cavities in the steel specimen.

Description

{Method for evaluation of dynamic deformation property of steel at low temperature using dynamic torsional bar test}

The present invention relates to a method for evaluating dynamic deformation characteristics of steel, and more particularly, to a method for evaluating dynamic deformation characteristics of steel using a dynamic torsion test apparatus at low temperature.

Increasingly severe resource depletion is increasing the demand for the use of structures made of steel in extreme environments such as the polar regions and deep seabeds. If the steel structure is exposed to a sudden impact environment under a low temperature environment, the steel structure is very likely to be damaged and the degree of breakage is high, which can lead to enormous economic loss and casualties. Therefore, there is an urgent need for a method for reliably evaluating the dynamic deformation characteristics (dynamic fracture resistance or operating failure resistance) of the steel under low temperature environment.

The low temperature Charpy impact test is a method of evaluating the dynamic deformation characteristics of steel under low temperature environment. The low temperature Charpy impact test is an easy and simple method of measuring the impact absorption energy of steel specimens with notches, not based on fracture mechanics. The low temperature Charpy impact test has a disadvantage in that the impact absorption energy is changed by the shape, length and thickness of the steel specimen and the notch. In addition, the low temperature Charpy impact test is difficult to analyze the mechanism of how cracks or local deformations start under dynamic load and reach final failure.

On the other hand, the dynamic deformation characteristics (dynamic fracture resistance or dynamic fracture resistance) of the steel in the low temperature atmosphere where the steel is actually used may be completely different from room temperature. Therefore, it is necessary to reliably evaluate the dynamic deformation characteristics of the steel in the low temperature atmosphere actually used.

The problem to be solved by the present invention is to provide a method for reliably evaluating the low temperature dynamic deformation characteristics of the steel at low temperature using a low temperature dynamic torsion testing apparatus.

In order to solve the above problems, the method for evaluating the dynamic deformation characteristics of the steel according to an embodiment of the present invention, preparing a steel specimen; A low temperature dynamic torsion test of the steel specimens at a lower temperature than room temperature using a dynamic torsion test apparatus having a refrigerant injection device; Obtaining physical property parameters of the steel specimen through the low temperature dynamic torsion test; And analyzing the physical property parameters to evaluate the low temperature dynamic deformation characteristics of the steel specimens, wherein the physical property parameters are fracture shear strains of the steel specimens, and the fracture shear strains of the steel specimens are determined by the adiabatic shear bands of the steel specimens. It depends on the thickness or the presence or absence, and the insulating shear band depends on the number of cavities in the steel specimen.

The low temperature dynamic torsion test may be performed by injecting a refrigerant into the steel specimen using a refrigerant injection device. The refrigerant injection device includes a refrigerant supply tank, a refrigerant injection nozzle connected to the refrigerant supply tank for injecting refrigerant into steel, a refrigerant collection tank for collecting refrigerant injected from the refrigerant injection nozzle, and a temperature of the refrigerant or steel specimen. It can be configured with a thermocouple. The coolant sprayed through the coolant spray nozzle allows the steel specimens to be tested for low-temperature dynamic torsion at temperatures below room temperature.

The fracture shear strain of the steel specimens is lowered as the test temperature of the dynamic torsion tester is lowered, but the rate lowered depending on the effective grain size of the steel specimens.

The number of cavities can be determined by the effective grain size and secondary phase fraction of the steel specimen.

The steel specimen may be composed of line pipe steel and the steel specimen may be single phase region rolled steel or abnormal region rolled steel. The fracture shear strain of the steel specimen of the abnormal zone rolled steel may be higher than that of the steel specimen of the single phase zone rolled steel. The steel specimen of the abnormal region rolled steel may not have an insulating shear band, and the steel specimen of the single phase region rolled steel may have an insulating shear band. Steel specimens of the single-phase region rolled steel may vary in width of the insulating shear band depending on the rolling conditions.

The method for evaluating the dynamic deformation characteristics of the steel of the present invention uses a low temperature dynamic torsion test apparatus having a refrigerant injection device. The low temperature dynamic torsion test apparatus performs a low temperature dynamic torsion test at a low temperature below room temperature using a refrigerant spray device.

Steel specimens use line pipe steel, and single-phase region rolled steel or abnormal region rolled steel. The low temperature dynamic torsion test of the steel specimens is carried out to evaluate the dynamic deformation characteristics of the steel by obtaining physical parameters such as maximum shear stress, fracture shear strain, adiabatic shear band, and effective grain size.

Evaluate the dynamic deformation characteristics of steel specimens by fracture shear strain of steel specimens, thickness or formation of adiabatic shear bands of steel specimens, number of cavity formations of steel specimens, or effective grain size and secondary phase fraction of steel specimens. do.

The present invention can reliably evaluate the dynamic deformation characteristics (dynamic fracture resistance or dynamic fracture resistance) of steel in a low temperature atmosphere where steel is actually used. The present invention can evaluate the low temperature dynamic deformation characteristics of the material by investigating the deformation and fracture behavior occurring under the dynamic load at a low temperature for the material used at low temperatures such as polar regions or deep seas. The present invention can be used as a criterion for determining the safety of a product used at low temperatures, and can be used as a base technology for the development of high value-added products through the development of higher temperature properties.

1 is a conceptual diagram showing a low temperature dynamic torsion testing apparatus used in the present invention,
FIG. 2 is a view showing steel specimens (torsion specimens) mounted to the low temperature dynamic torsion test apparatus of FIG. 1,
3 is a view for explaining that the refrigerant is injected into the steel specimen (torsion specimen) mounted on the low temperature dynamic torsion test apparatus of FIG.
4 is a view showing the rolling conditions of the steel used in the present invention,
5 is a view showing the microstructure of the steel specimens used in the present invention by optical and SEM pictures,
6 is a view showing a photograph measuring the effective grain of the steel specimen according to the present invention,
7 is a flowchart illustrating a method for evaluating low temperature dynamic deformation characteristics of steel specimens according to the present invention;
8 is a view showing a shear stress and shear strain curve for evaluating the room temperature dynamic deformation characteristics of steel specimens for comparison with the present invention,
FIG. 9 is a view showing an SEM image of the deformed area under the fracture surface of the steel specimen after the dynamic torsion test at room temperature for comparison with the present invention.
10 and 11 illustrate shear stress and shear strain curves for evaluating low temperature dynamic deformation characteristics of steel specimens according to the present invention;
12 and 13 are views showing the shear strain and the maximum shear stress according to the test temperature after the low temperature dynamic torsion test of the steel specimens according to the present invention,
FIG. 14 is a view showing an SEM image of the deformed area under the fracture surface of the steel specimen after the dynamic torsion test at low temperature according to the present invention;
15 is a view showing the number of cavities according to the test temperature after the low temperature dynamic torsion test of the steel specimen according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art. The present invention is capable of various modifications and various forms, and specific embodiments are illustrated and described in detail in the drawings. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like reference numerals are used for similar elements in describing each drawing. In the accompanying drawings, the dimensions of the structures are enlarged or reduced from the actual dimensions for the sake of clarity of the present invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprises", "having", and the like are used to specify that a feature, a number, a step, an operation, an element, a part or a combination thereof is described in the specification, But do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the contextual meaning of the related art and are to be interpreted as either ideal or overly formal in the sense of the present application Do not.

tester

The present invention uses a low temperature dynamic torsion test apparatus as a test apparatus for evaluating the dynamic deformation characteristics of steel at low temperature. In particular, the inventors have invented a low temperature dynamic torsion test apparatus including a refrigerant injection device to evaluate the dynamic deformation characteristics of the steel at low temperature. The low temperature dynamic torsion tester used a dynamic torsional Kolsky Bar test rig.

1 is a conceptual diagram showing a low temperature dynamic torsion test apparatus used in the present invention, FIG. 2 is a view showing a steel specimen (torsion specimen) mounted to the low temperature dynamic torsion test apparatus of FIG. 1, FIG. A diagram for explaining that the refrigerant is injected into the steel specimen (torsion specimen) mounted on the low temperature dynamic torsion test apparatus of the present invention.

Specifically, the low temperature dynamic torsion testing apparatus 100 may include a data collection system 100b and a dynamic load system 100c, and may also include a quasi-static (or static) load system 100a. The dynamic torsion test apparatus 100 accumulates a certain amount of torque between the clamp 2 and the dynamic loading pulley 4, and then destroys the clamp 2, thereby causing an elastic shear wave. A shear wave is instantaneously transmitted to the steel specimen 10 to deform the steel specimen 10, in which incident, reflected and transmitted waves are sensed by the incident and transmitted strain gages 12, 14, Devices recorded on the oscilloscope 32 are provided.

In FIG. 1, reference numeral 6 denotes a gauge for measuring the amount of torque accumulated between the fastener 2 and the dynamic loading pulley 4, 16 is a power supply, and 36 is a personal computer. Reference numeral 41 is a bar for transmitting a torsional force to the steel specimen 10, 42 is a hydraulic pump, 43 and 44 are compressors, 45 is a static loading pulley, 46 is an electric motor, 47 is a reducer. Reference numeral 20 is for measuring the deformation amount (rotation amount) of the rod 41 in the quasi-static test with a linear variable differential transformer (LVDT), and the measured deformation amount of the rod 41 is recorded in the recorder (recorder) 34. do.

Among the signals recorded on the oscilloscope 32, the average shear strain γ (t) expressed as a function of time from the reflected wave is measured, and the shear stress τ (t) expressed as a function of time from the transmitted wave is measured. From this γ (t) and τ (t), time is eliminated to obtain a dynamic shear stress-shear curve.

The steel specimen (torsional specimen 10) shown in FIG. 2 is machined so that the longitudinal direction of a specimen may be perpendicular to a rolling direction. The steel specimen 10 has a hexagonal column shape having a gauge portion 11 in the form of a thin-walled tube having a gage distance of 2.5 mm and a thickness of 280 μm, as shown in FIG. 2. When machining steel specimens, it is recommended to use an electric discharge machine, and it is advantageous in terms of reproducibility of the experiment not to leave maximal residual stress on the steel specimens by spraying lubrication and cooling water well. In addition, the steel specimen 10 requires precise machining so that thickness variation does not occur in the circumferential direction.

The low temperature dynamic torsion test apparatus 100 of the present invention is equipped with a refrigerant injection device (52, 54, 56, 60) as shown in Figs. 1 and 3 to perform a dynamic torsion test under low temperature. The coolant injection devices 52, 54, 56, and 60 may be configured of a coolant supply tank 52, a coolant injection nozzle 54, a coolant collection tank 56, and a thermocouple 60. The refrigerant injection devices 52, 54, 56, and 60 include a refrigerant supply tank 52, a refrigerant injection nozzle 54 connected to the refrigerant supply tank 52, and a refrigerant injection nozzle 54 to inject refrigerant into the steel 10. A refrigerant collection tank 56 for collecting the refrigerant injected from 54 and a thermocouple 60 for measuring the temperature of the refrigerant or the steel specimen.

A coolant supply tank 52 having a coolant injection nozzle 54 is positioned above the gauge part 11 of the steel specimen 10 to continuously inject a predetermined amount of coolant, and to the lower reference 58. As shown, a coolant collection tank 56 for collecting coolant flowing down the steel specimen 10 was installed to perform a low temperature dynamic experiment. In other words, the present invention may test the dynamic torsion of the steel specimen 10 at a temperature lower than room temperature through the refrigerant injected through the refrigerant injection nozzle 54.

The coolant supply tank 52 is a container made of stainless steel that can withstand low temperatures sufficiently, and uses a heat-insulating container filled with a vacuum between the inner and outer walls of the container so as to sufficiently preserve the temperature of the coolant. As a refrigerant, a mixture of isopentane and liquid nitrogen was used, and a desired temperature can be obtained through proper mixing. The experimental range of temperatures is from room temperature to liquid nitrogen temperature, for example from 25 to -196 ℃. The refrigerant injectors 52, 54, 56, 60 include thermocouples 60 to monitor that the temperature of the refrigerant or steel specimen 10 is sufficiently maintained for the duration of the experiment, the thermocouple being connected to the PC 36. Can connect

Hereinafter, a series of processes for evaluating the low temperature dynamic deformation characteristics of steel using the low temperature dynamic torsion test apparatus invented by the present inventors will be described. The low temperature at which the dynamic torsion is tested in the present invention is room temperature, ie, a temperature lower than 25 ° C. For example, when using liquid nitrogen as the refrigerant is 25 to -196 ℃. In this detailed description, a low temperature at which the dynamic torsion test is performed is selected as -50 ° C or -100 ° C for convenience, and a room temperature is selected as 25 ° C for comparison. Of course, when using liquid helium as the refrigerant may be performed a dynamic torsion test at 25 ℃ to -268 ℃.

Type of steel

Linepipe steel is used as an example of the steel used to evaluate the dynamic deformation characteristics of the steel at low temperatures. Linepipe steels are used to transport natural gas and crude oil. In line pipe steel, the area where refining and consumption takes place is quite far from the production site, and high pressure is applied inside the line pipe steel for rapid transportation. Therefore, when a crack occurs due to a defect or an impact, the line pipe steel may be destroyed in an instant.

When propagating cracks in line pipe steel, the speed reaches 1500 m / s. If there is no arrest point that prevents crack propagation, the crack may progress up to several kilometers. Linepipe steels often transport crude oil in the deep sea and in the polar regions and are exposed to low temperature environments.

When line pipe steel is destroyed by dynamic loads such as high pressure or impact at such low temperatures, it can cause enormous economic loss, environmental pollution, and casualty damage, so it is reliable for low temperature dynamic deformation characteristics (dynamic fracture resistance or dynamic fracture resistance). There is an urgent need for evaluation methods. In this example, low temperature dynamic torsion tests were performed using American Petroleum Institute (API) X70 and X80 grade linepipe steels.

Fabrication of Steel Specimen

4 is a diagram showing the rolling conditions of the steel used in the present invention, Table 1 shows the chemical composition of the steel used in the present invention.

The steel specimens used in this example were API X70 and X-80 grade line pipe steel produced by rolling, and were used as steel specimens by cutting heads of long plates. All steel specimens were subjected to solution treatment at 1091 ℃ ~ 1130 ㅀ C, and then rolled with a large reduction ratio in the non-recrystallization region of austenite. Finish rolling was carried out at two temperatures: an austenite single phase region of Ar 3 and above, and an austenitic and ferrite two phase region of Ar 3 and below.

After the finish rolling, cooling was started in each of the single phase and the abnormal region, and accelerated cooling to the finish cooling temperature (FCT) of 441 ~ 501 ° C. The cooling rate was 7-14 ° C / sec (sec). The steel finish-rolled in the single phase region is a single phase region rolled steel, and the steel rolled in the abnormal region is an abnormal region rolled steel. The X-70 grade steel, which has been finish-rolled in the single-phase and abnormal regions, respectively, is referred to as 'A' and 'B' steel, and the X-80 grade steel, which has been finish-rolled in the single-phase region, is referred to as the 'C' steel.

Furtherance
(wt%)

River
Kinds    C   Si   Mn  Ni + Mo  Nb + Ti  Al   P + S    A   0.062  0.311  1.56   0.20   0.06   0.023   <0.01    B   0.045  0.297  1.56   0.37   0.06   0.031   <0.01    C   0.073  0.230  1.76   0.56   0.05   0.033   <0.01

As shown in Table 1, the steel specimens used in the present invention are carbon (C), silicon (Si), manganese (Mn), nickel (Ni), niobium (Nb), aluminum (A), phosphorus (P). , Sulfur (S), and the remaining amount is iron (Fe). The amount of carbon and the total amount of nickel and molybdenum in the C steel are higher than those in the other steels, and the amount of silicon in the A steel is slightly higher than that of other steels.

Microstructure of Steel Specimen

5 is a view showing the microstructure of the steel specimen used in the present invention by optical and SEM photographs.

Specifically, FIG. 5 is a microstructure of the LT (vertical and longitudinal transverse) planes of the thickness centers of steel specimens of A, B, and C steels, respectively, observed by optical and scanning electron microscopy (scanning electron microscopy), respectively. The volume fraction was measured and shown in Table 2. Microstructure is observed after polishing the top of the rolled sheet and etching with a 2% nital solution. (a), (b) and (c) are optical photographs, and (d,), (e) and (f) are SEM photographs. As shown in FIG. 5, high strength and high toughness steel such as line pipe steel exhibits various microstructures according to alloying elements and rolling conditions.

Polygonal ferrite (PF) has a low stress but high elongation because polygonal grains are relatively large and several secondary phases are not distributed at grain boundaries. Acicular ferrite (AF) has very fine microstructures of about 1 μm and has a high stress and excellent toughness due to the distribution of fine secondary phases at grain boundaries. Upper bainite (UB: upper bainite) is a tissue formed under faster cooling conditions than AF. Stress is better than AF, but the toughness is low. Granular bainite (GB) is formed under cooling conditions that are almost similar to AF, with lower stress and toughness than UB. Second phases, such as smartensite-austenite (MA), retained austenite (RA), and cementite, are formed under fast cooling conditions and have hard, fragile properties.

volume
Fraction
River
Kinds    AF   PF   GB  UB  Second prizes    A   Bal. *   20.6   3.1    B   13.3     Bal. *   12.9   1.8    C   Bal. *   9.7   4.9

In Table 2, * represents the primary matrix phase and Bal. Represents the remaining amount.

As shown in FIG. 5 and Table 2, the A steel finished and rolled in the single phase region is mainly composed of fine AF of less than 1 μm, with about 20% of coarse GB having a size of 10 to 30 μm and about 3% of secondary phases. do. In the above-mentioned area, B steel finished in the above-mentioned region has 70% of PF of 5-20 μm, and 13% of AF and UB transformed from residual austenite are accelerated by accelerated cooling, and the second phase is less than 2%. Like A steel, C steel, which has been finished and rolled in a single phase region, is mainly composed of fine AF, with about 10% UB and about 5% secondary phase. The secondary phase fraction of the three rolled steels (rolled material) is proportional to the content of C and Mo, Ni and other hardenability enhancing elements. Decreases.

Figure 6 is a view showing a photograph measuring the effective grain of the steel specimen according to the present invention.

Specifically, an electron back scatter diffraction (EBSD) analysis of three steels was performed to show a grain-color map. EBSD analysis utilizes steel specimens that are mechanically polished on the surface of the steel specimens and then polished with silica. EBSD analysis was performed in a field emission electron microscope (FE-SEM).

(A), (b), (c) of FIG. 6 indicate A steel, B steel, and C steel, respectively. Table 3 shows the effective grain size and its fraction, which is a high-angle grain having a grain boundary of 15 DEG or more measured by EBSD analysis. Grain-color maps (grain-color maps) appear in a variety of colors depending on the orientation of the grains, the area indicated in a single color can be used as one crystallographic domain factor representing the effective grain size of the specimen.

In FIG. 6, grains having a high inclination of 15 ° or more are represented by different colors, and effective grain sizes decrease in the order of GB, PF, UB, and AF. Table 3 shows that A and B steels have slightly larger average effective grain sizes than C steels, because GB has a significantly larger effective grain size, and PF has a larger effective grain size.

River
Kinds Fine structure
(microstructure) Volume fraction of secondary phase (%) Effective grain size
(Μm)  High angle (≥15≥) grain boundary fraction (%)    A   AF, GB (MA)      3      5.8          80    B PF, AF, UB
(MA, cementite)      2      5.9          84    C   AF, UB (MA)      5      5.1          87

Evaluation of Dynamic Deformation Characteristics of Steel Specimen at Low Temperature

7 is a flowchart illustrating a method for evaluating low temperature dynamic deformation characteristics of a steel specimen according to the present invention.

Specifically, the steel specimen is prepared as described above with reference to FIG. 4 (step S1), the preparation process of the steel specimen is omitted since it was described in FIG. 4, and thus the composition and microstructure of the steel specimen are described in Tables 1 to 3. Omit.

Next, a dynamic torsion test of the steel specimen is performed at a low temperature lower than room temperature using a dynamic torsion test apparatus having a refrigerant injection device (step S2). The low temperature dynamic torsion test of the steel specimen is performed using the low temperature dynamic torsion test apparatus invented by the inventors as described above with reference to FIGS. 1 to 3. Low temperature dynamic torsion testing of steel specimens can be performed at temperatures below room temperature, such as 25 to -196 ° C, as described above. Of course, when using liquid helium as the refrigerant may be performed a dynamic torsion test at 25 ℃ to -268 ℃.

Subsequently, the physical properties parameters of the steel specimens are obtained through the low temperature dynamic torsion test (step S3). Next, the properties parameters are analyzed to evaluate the low temperature dynamic deformation characteristics of the steel specimen (step S4). The property parameter acquisition step and the low temperature dynamic deformation characteristic evaluation of the steel specimen will be described later.

Property Parameter Acquisition of Steel Specimen and Its Dynamic Deformation Evaluation

FIG. 8 is a diagram illustrating shear stress and shear strain curves for evaluating room temperature dynamic strain characteristics of steel specimens for comparison with the present invention.

Specifically, the evaluation of low temperature dynamic properties of steel specimens can be performed by observing shear stress and shear strain curves and microstructure changes that can be obtained after low temperature dynamic torsion tests. 8 is a shear stress and shear strain curve of the steel specimen obtained by using a dynamic torsion test apparatus at room temperature for comparison with the present invention.

Table 4 shows the maximum shear stress, the shear strain at the maximum shear stress point, and the fracture shear strain obtained from FIG. 8. In addition, Table 4 also shows the maximum shear stress, the shear strain at the maximum shear stress point, and the fracture shear strain using the low temperature dynamic torsion test apparatus at low temperature as described later.

River
Kinds Temperature
(℃) Shear stress
(Mpa)  Shear strain at maximum shear stress   Fracture Shear Strain    A      25    465      0.43      0.50    B    439      0.51      0.51    C    481      0.43      0.46    A     -50    474      0.28      0.39    B    456      0.44      0.48    C    495      0.32      0.40    A    -100    462      0.23      0.27    B    461      0.23      0.38    C    508      0.22      0.32

As a result of the dynamic torsion test using the dynamic torsion test apparatus at room temperature, the maximum shear stress is lowered in the order of C, A, B steel, and the fracture shear strain is lowered in the order of B, A, C steel. Since A and C steels include low temperature transformation phases such as AF, GB, and UB, the maximum shear stress is higher than that of B steel, which is an abnormal region rolling steel containing a lot of PF. Steel C contains the largest amount of hard phase UB and secondary phase, and has the largest maximum shear stress.

FIG. 9 is a SEM photograph of the strained area under the fracture surface of the steel specimen after dynamic torsion test at room temperature for comparison with the present invention.

Specifically, FIG. 9 is a photograph observed by SEM after polishing and etching the deformed area under the fracture surface of the steel specimen after the dynamic torsion test at room temperature. The top view shows the fracture surface and the viewing area of the steel specimen. (a), (b) and (c) are SEM images of A steel, B steel and C steel, respectively, and (d,), (e) and (f) are (a), (b) and (c), respectively. Is an enlarged view of the dotted line. In dynamic torsional steel specimens, the tissue near the wavefront is stretched more severely than the tissue inside the specimen. In this area, a significant amount of voids are formed, as indicated by the white arrows.

10 and 11 illustrate shear stress and shear strain curves for evaluating low temperature dynamic deformation characteristics of steel specimens according to the present invention, and FIGS. 12 and 13 are low temperature dynamic torsion tests of steel specimens according to the present invention. It is a diagram showing the shear strain and the maximum shear stress according to the post-test temperature.

Specifically, FIGS. 10 and 11 are shear stress and shear strain curves of the steel specimens obtained by using a low temperature dynamic torsion test apparatus at low temperature, and in Table 4, the maximum shear stress and maximum shear obtained from FIGS. 10 and 11 are shown. Shear strain and fracture shear strain at stress points are shown. 12 and 13 show the fracture shear strain and maximum shear stress as the test temperature is changed to room temperature, -50 ° C and -100 ° C.

As a result of the low temperature dynamic torsion test using the dynamic torsion test apparatus at low temperature, the maximum shear stress is lowered in the order of the C, A, and B steels, and the fracture shear strain is lowered in the order of the B, C, and A steels. In particular, as shown in Table 4. FIGS. 10 to 12, the fracture shear strains decrease as the temperature decreases for all three steel species. B steel, which is an abnormal zone rolled steel, exhibits higher fracture shear strains than A, C steel, which is a single-phase zone rolled steel, and maintains relatively high fracture shear strains, particularly at low temperatures.

As shown by the dotted line region of FIG. 12, steel A of the three steel species has a large decrease in fracture shear strain as temperature decreases, and thus has a fracture shear strain lower than that of C steel at -100 ° C. In other words, steel A has a high rate at which the fracture shear strain decreases as the temperature decreases, and thus has a fracture shear strain lower than that of C steel at -100 ° C.

In metal materials, the stress increases and the elongation decreases as the temperature decreases. In the experiment of the present invention, the maximum shear stress generally increases and the fracture shear strain decreases as the test temperature decreases. However, as indicated by the dotted line in FIG. 13, the steel A has a maximum shear stress at −100 ° C., which is lower than at room temperature or −50 ° C. FIG.

FIG. 14 is a view showing an SEM image of the deformed area under the fracture surface of the steel specimen after the dynamic torsion test at low temperature according to the present invention.

Specifically, FIG. 14 is a photograph taken by SEM after polishing and etching the deformed area under the fracture surface of the steel specimen after the dynamic torsion test at low temperature. The top view shows the fracture surface and the viewing area of the steel specimen. (a), (b) and (c) are SEM images of steels A, B and C, respectively, which were subjected to a low temperature dynamic torsion test at -50 ° C, and (d,), (e) and (f) were respectively SEM photographs of steels A, B, and C subjected to low temperature dynamic torsion tests at -100 ° C.

The steel specimens tested at -50 ° C at low temperature dynamic torsion tests had the plastic flow line similar to the steel specimens at room temperature dynamic torsion test, but the plastic flows in the near wavefront area where the cavities were more deformed than the steel specimens at room temperature dynamic torsion test. More lines are found.

In the low temperature dynamic torsion test at -100 ° C, the single-phase zone rolled steels A and C have a localized plastic zone, ie, adiabatic shear bands in a direction parallel to the shear stress direction, with the plastic flow line concentrated near the fracture surface (destructive surface). Is observed. The thicknesses of the adiabatic shear bands formed in steels A and C are about 8 μm and 15 μm, as indicated by the arrows, corresponding to one-half the thickness of the adiabatic shear bands formed in the gauge region of the actual specimen.

In the steel specimens in which the abnormally-rolled steel B was dynamically deformed at -100 ° C, the plastic flow line became more severe, but no adiabatic shear bands such as steel A and C were formed. This is because a lower yield ratio, i.e., a high work hardening rate, does not cause much concentration of plastic deformation.

15 is a view showing the number of cavities according to the test temperature after the low temperature dynamic torsion test of the steel specimen according to the present invention.

Specifically, Figure 15 shows the number of vacancy according to the test temperature of the steel specimen obtained by using a low temperature dynamic torsion test apparatus. FIG. 15 shows the measurement of the number of cavities per unit area in the area under the dynamic torsional specimen wavefront.

As a result of the low temperature dynamic torsion test using a dynamic torsion test apparatus at low temperature, the number of cavities tends to increase as the test temperature decreases. For steel A, the dynamic torsion test at -100 ° C is somewhat lower than for the dynamic torsion test at -50 ° C. At -100 ° C, the number of cavities in steel C is five times higher than that of steel A.

After the low temperature dynamic torsion test of the present invention as described above, the dynamic deformation characteristics of the steel specimen can be evaluated as follows.

Specifically, as a result of the low temperature dynamic torsion test, it can be seen that B steel, which is an abnormal region rolled steel, has higher fracture shear strain at room temperature as well as low temperature than other single-phase region rolled steels A and C steels. In other words, it can be seen that B steel, which is an abnormal region rolled steel, is superior in dynamic deformation characteristics (dynamic fracture resistance or operating fracture resistance) than other single-phase region rolled steels, A and C steels.

When a load is applied to B steel, which includes PF and about 25% of AF and UB, due to the hardness difference of the tissue, a movable potential is formed at the interface between the tissues, resulting in an even deformation of the material, thereby lowering the yield ratio. The yield ratio indicates resistance in the uniform plastic deformation region, and the lower the yield ratio, the higher the work hardening rate.

When dynamic load is applied to the material, the plastic deformation is concentrated and work hardening and thermal softening are competitive in the area.At this time, the softening progresses rapidly from the time when the thermal softening rate exceeds the work hardening rate, resulting in an insulating shear band and loading. The ability to handle is drastically lowered, leading to destruction. B steel, which has a high work hardening rate, continuously undergoes work hardening during plastic deformation, so that plastic deformation is not easily concentrated, thereby increasing resistance to fracture.

Referring to (a) to (f) of Figure 14, it can be seen that the B steel is less concentrated in the plastic deformation than the other two steel at -50 ℃ and -100 ℃ temperature. In particular, when comparing the photograph of the -100 ℃ portion as described above, the A, C steel is a thermal shear band is formed by the plastic deformation is concentrated near the wave surface, but the B steel is a specimen that is separated from the wave surface without the adiabatic shear band observed Deformation occurs well inside. From this, B steel mainly composed of PF shows the highest fracture shear strain but lower maximum shear stress than A and C steel composed of low temperature transformation phases.

On the other hand, comparing A and C steels, C steels, including UB, which contain the most secondary phases of about 5% and are harder than GB, show higher maximum shear stresses. However, as the temperature decreases, the effect of microstructure on the mechanical properties under dynamic loading is different. Depending on the steel grade, the rate at which other fracture shear strains decrease with temperature decreases. As described above, the region indicated by the dotted line in FIG. 12 shows that A steel has a higher fracture shear strain than C steel at room temperature, but decreases rapidly as temperature decreases, and lower than C steel at -100 ° C. The strain is shown.

The reason why the difference of fracture shear strain occurs in different steel grades when dynamic load is applied at low temperature can be explained by the effective grain size obtained by EBSD analysis. The effective grain size correlates with the linear progression of the cleavage crack, which means how effectively the high-angle grain boundary acts as an obstacle to the cleavage crack progression. The smaller the effective grain size, the better the resistance to fracture, and the lower the temperature, the greater the role, which directly affects mechanical properties.

6 and Table 3, A steel has a larger effective grain size than C steel under the influence of coarse GB of about 40 μm. Because of the large effective grain size GB present in steel A, the fracture progresses linearly long and rapidly at low temperature fracture, so steel A exhibits lower fracture shear strain than dynamic steel in dynamic torsion tests conducted at -100 ° C. As a result, the fracture shear strain of the steel specimen may be lowered as the test temperature of the dynamic torsion test apparatus is lowered, but the ratio lowered depending on the effective grain size of the steel specimen.

The difference in fracture shear strains of A and C steels at low temperatures can be explained by the formation of adiabatic shear bands. That is, the fracture shear strain may vary depending on the thickness or formation of the insulating shear band. The large thickness (width) of the adiabatic shear band means that the deformation is even and easy throughout the specimen, resulting in less strain concentration due to plastic imbalance. Since adiabatic shear bands mean areas where strain is concentrated, less strain concentration occurs in wide bands. 14 (d) and (f), the width (thickness) of the heat insulating shear band of steel C is about twice as wide as that of steel A, which is more severe than the heat insulating shear band, that is, local plastic deformation in steel A. It means that the resistance to breakdown is lowered.

As described above, the steel A exhibits a sharp decrease in fracture shear strain at -100 ° C. As a result, the shear stress is not sufficiently increased as the shear strain increases, so that the maximum shear stress is also reduced as shown in the dotted line region of FIG. 13. . Therefore, when the steel A is deformed under low temperature dynamic load, the adiabatic shear band is well formed and the fracture resistance is lowered so that the shear stress along with the shear strain is reduced.

The reason for the difference in the degree of thermal shear band formation in A and C steels can be explained by the degree of cavity formation. The adiabatic shear band may vary depending on the number of cavity formations in the steel specimen. When dynamic loads are applied to the material, the work energy is the driving force that causes ductile failure through the creation and growth of cavities during deformation, some of which can be used to form adiabatic shear bands. If part of the strain energy applied therein is used in the part related to the creation and growth of the cavity, the driving force for forming the adiabatic shear band is reduced by the strain energy used therein.

Comparing (d) and (f) of Figure 14, it can be seen that the difference in the degree of cavity formation between the A, C steel is large, in order to quantitatively interpret the number of cavities per unit area in the area under the dynamic torsional specimen wavefront 15 is shown in FIG. As shown in FIG. 15, at -100 ° C, the number of cavities is about five times larger than that of A steel.

Therefore, steel C consumes more strain energy than the steel A to form and grow a cavity, and thus, the possibility of forming a heat insulating shear band is lower than that of the steel A, and the width of the heat insulating shear band is also thick. That is, the width of the insulating shear band of the steel specimen of the single-phase region rolled steel may vary depending on the rolling conditions.

The cavity is created non-uniformly in a site where shear strain compatibility is difficult. Preferred sites for cavity formation are inclusions or fine particles, and in metals without particles, voids may form at the grain boundary triple point. The reason why steel C has more cavities than steel A is because of its small and even effective grain size.

6 (a)-(c) and Table 3, it can be seen that the effective grain size of C steel is the smallest and the fraction of the high-angle grain boundary is the highest. Steel A contains coarse GB of about 40 μm and has a large effective grain size and few grain triple points, but steel C has a large size of 15 μm fine AF, so the effective grain size is small and the fraction of the grain triple point has a high co-forming position. There are many.

In addition, C steel has a high volume fraction of the secondary phase due to the addition of hardenable elements such as Ni, Mo, and C, and the secondary phases also serve to provide a co-forming site. Therefore, C steel having a large number of cavity forming sites easily forms a cavity during deformation and consumes energy to form a heat insulating shear band, thereby reducing the concentration of plastic deformation, thereby lowering the possibility of forming a heat insulating shear band than A steel. The number of cavities can be determined by the effective grain size and secondary phase fraction of the steel specimen.

As described above, the single phase region rolled steel in which many AFs are formed has higher shear stress under dynamic load than the abnormal region rolled steel in which PF is mainly formed.

In particular, steel A contains a large number of coarse GBs, so that the effective grain size is large, whereby a heat insulating shear band is easily formed. This is because when coarse GB is formed in steel A, which is mostly composed of AF, this GB degrades the overall dynamic deformation properties. Therefore, care should be taken not to coarsen the GB by appropriately controlling hot rolling conditions.

On the contrary, C steel is composed of fine UB together with AF, so it has a small effective grain size and excellent low temperature dynamic mechanical properties than A steel. This suggests that not only shear stress but also low temperature dynamic elongation can be improved simultaneously by proper alloy design and hot rolling conditions.

100: low temperature dynamic torsion test apparatus, 100a: quasi-static load system, 100b: data collection system, 100c: dynamic load system, 2: fasteners. 4: dynamic loading pulley, 10: steel specimen, 11: gauge portion of steel specimen 6, 12, 14: gauge. 32: oscilloscope, 16: power supply, 36: personal computer (PC), 41: bar, 42: hydraulic pump, 43 and 44: compressor, 45: static loading pulley, 46: electric motor, 47: reducer, 20 : LVDT (Linear variable differential transformer), 52, 54, 56, 60: refrigerant injection device, 52: refrigerant supply tank, 54: refrigerant injection nozzle, 56: refrigerant collection tank, 60: thermocouple

Claims (12)

Preparing steel specimens;
A low temperature dynamic torsion test of the steel specimens at a lower temperature than room temperature using a dynamic torsion test apparatus having a refrigerant injection device;
Obtaining physical property parameters of the steel specimen through the low temperature dynamic torsion test;
Analyzing the physical property parameters to evaluate low temperature dynamic deformation characteristics of the steel specimen,
The physical property parameter is the fracture shear strain of the steel specimen,
Fracture shear strain of the steel specimen is dependent on the thickness or formation of the insulating shear band of the steel specimen,
And wherein said thermally insulating shear band is dependent upon the number of cavity formations of said steel specimen. The method of claim 1, wherein the coolant is sprayed on the steel specimen using the coolant spray device during the low temperature dynamic torsion test. According to claim 2, wherein the refrigerant injection device is a refrigerant supply tank, a refrigerant injection nozzle connected to the refrigerant supply tank for injecting the refrigerant to the steel, a refrigerant collection tank for collecting the refrigerant injected from the refrigerant injection nozzle, And a thermocouple for measuring the temperature of the refrigerant or the steel specimen,
A method of evaluating dynamic deformation characteristics of steel, characterized in that for testing the low temperature dynamic twisting of the steel specimen at a temperature lower than room temperature through the refrigerant injected through the refrigerant injection nozzle. delete The method of claim 1, wherein the fracture shear strain of the steel specimen is lowered as the test temperature of the dynamic torsion test device is lowered, but the rate of lowering according to the effective grain size of the steel specimen is changed. Characteristic evaluation method. delete delete The method of claim 1, wherein the number of cavities is determined by the effective grain size and the secondary phase fraction of the steel specimen. The method according to claim 1, wherein the steel specimen is made of line pipe steel and the steel specimen is single phase region rolled steel or abnormal region rolled steel. 10. The method of claim 9, wherein the fracture shear strain of the steel specimen of the abnormal zone rolled steel is higher than that of the steel specimen of the single-phase zone rolled steel. 10. The method of claim 9, wherein the steel specimen of the abnormal region rolled steel does not have an insulating shear band, and the steel specimen of the single phase region rolled steel has an insulating shear band. 10. The method of claim 9, wherein the steel specimen of the single-phase region rolled steel varies in width of the insulating shear band according to rolling conditions.

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