KR100317950B1 - Controlling method of an adiabatic shear band formed on low carbon steel when cold-forged - Google Patents

Abstract

The present invention relates to a method for evaluating insulation shear band formation and a method for inhibiting insulation shear band formation during cold forging for low carbon steel, and the insulation shear during cold forging for low carbon steels including the process of converter steel, ingot and hot rolling. A method of suppressing band formation, the method comprising: performing solution treatment on the low carbon steel; And cold forging for the solution-treated low carbon steel, and providing a method for suppressing thermal insulation shear band formation during cold forging for low carbon steel.

Increasing the cooling rate not only reduces the fraction of pearlite acting as a deformation resistance, but also prevents the yield point phenomenon by preventing the diffusion of the invasive solid atoms causing the yield point phenomenon, thereby improving the efficiency of cold forging. .

Description

Controlling method of an adiabatic shear band formed on low carbon steel when cold-forged}

The present invention relates to a method for evaluating insulation shear band formation and a method for inhibiting insulation shear band formation during cold forging of low carbon steel, and in detail, a cause of insulation shear band formation while performing a cold forging process while changing the microstructure in the low carbon steel. The present invention relates to a method for analyzing and evaluating, and also to a method for suppressing formation of an adiabatic shear band.

As a conventional method for forging carbon steel, a precision forging method for processing carbon steel into a net-shape or near-net-shape by using a single forging process is generally used. In particular, the cold forging method that proceeds at room temperature without heating the carbon steel is a forging method having a lot of interest in terms of economics because it does not involve a high temperature thermal process.

In general, the cold forging method performed at room temperature uses a high process material such as low carbon steel having a small deformation resistance, but has a problem of forming an insulating shear band by high speed deformation. Since the heat insulating shear band can easily act as a propagation path for cracking due to its high hardness, the conventional cold forging method has a certain limitation in being adopted as a forging method for a general low carbon steel.

On the other hand, cracks generated during the conventional cold forging are known to occur frequently in the corner portion, which is a site where deformation is uneven mainly in the shape of the part. This is because strain discontinuity is caused because the degree of metal flow is partially different at the edges, and as a result, adiabatic shear band is generated as the strain concentration occurs. Conventionally, in order to solve such a problem, a countermeasure in the mold structural aspect of improving the physical shape such as smoothly forming the curvature of the corner portion of the low carbon steel has been proposed.

However, such a conventional countermeasure cannot be seen as a fundamental preventive measure against the formation of a heat insulating shear band of low carbon steel. This would be a way of minimizing the amount of insulating shear bands while accepting an unstoppable limit. In addition, such conventional countermeasures are not always applicable at any time, and are limited in that they can be applied only when the formation of the insulating shear band is directly related to the curvature of the corner.

The technical problem to be achieved by the present invention in the cold forging process performed at room temperature by accurately identifying the essential causes, rather than post-healing the heat-insulating shear band that may occur when the forging process for low carbon steel as described above In order to suppress the formation of the thermal insulation shear band of the present invention, by analyzing / assessing the cause of the insulation shear band formation during cold forging of low carbon steel, which can achieve the technical problem, it provides a method of suppressing the formation of the thermal insulation shear band. The object of the present invention.

1 and 2 are photographs of the A and B steels containing about 0.1 wt% (wt.%) Of carbon, respectively, according to an embodiment of the present invention, observed with an optical microscope.

Figure 3 is a graph showing the relationship between the stress and strain measured after the tensile test of each of the steel A, B steel specimens quasi-static strain rate (10 ^-3 / sec) according to an embodiment of the present invention.

4 and 5 are photographs obtained after the tensile test of the two steel specimens in accordance with an embodiment of the present invention, the tensile surface is observed by a scanning electron microscope (SEM).

6 is a photograph observing a crack formed on a substantial corner portion of the forging when cold forged the steel B according to the embodiment of the present invention with an optical microscope.

7 and 8 are observed by scanning electron microscopy around the crack formed in the A and B steel after cold forging according to an embodiment of the present invention.

FIG. 9 is a dynamic shear stress-shear strain curve obtained after a torsion test at a dynamic strain rate using a conventional Kohlski bar for two specimens of A and B steel according to an embodiment of the present invention.

10 and 11 are photographs obtained by scanning electron microscopy of the wavefront after dynamic torsion tests for each of the specimens A and B steel according to the embodiment of the present invention.

12 and 13 are photographs obtained by scanning electron microscopy of the center of the broken gage portion after the dynamic twist test for each of the specimens A and B steel according to an embodiment of the present invention.

14 to 16 are photographs of three kinds of specimens subjected to air cooling, furnace cooling, and forced air cooling, respectively, after heat treatment with B steel at 870 ° C. for 1 hour according to an embodiment of the present invention.

17 is a stress obtained after the tensile test at the quasi-static strain rate for three kinds of specimens subjected to air and furnace cooling and forced air cooling, respectively, after the heat treatment and heat treatment of steel B at 870 ℃ for 1 hour according to an embodiment of the present invention -Deformation curve.

FIG. 18 is a dynamical process using a conventional Kohlski bar for three types of specimens in which B steel is subjected to air cooling, furnace cooling, and forced air cooling after the solution heat treatment at 870 ° C. for 1 hour according to an embodiment of the present invention. This is the dynamic shear stress-shear curve obtained after the torsion test at strain rate.

19 to 21 are fractures after dynamic torsion tests for three kinds of specimens subjected to air cooling, furnace cooling, and forced air cooling, respectively, after solution heat treatment for 1 hour at 870 ° C. according to an embodiment of the present invention. The center of the specimen gage section was taken with a scanning electron microscope.

The method for evaluating the insulation shear band formation during cold forging for low carbon steel to achieve the technical problem to be achieved by the present invention described above, (a) a plurality of specimens of low carbon steel under the same forging conditions irrespective of whether cracking occurs during cold forging (B) dynamic torsion testing for investigation of shear stress versus shear strain and failure mode for each specimen of cracks in the low carbon steel; c) solution treatment for each of the specimens of the eutectic low carbon steel; (d) cooling with varying cooling conditions for each of the specimens of the eutectic low carbon steel; Analyzing the possibility of thermal insulation shear band formation from the tensile test and the dynamic torsion test results. It features.

In this case, the method for evaluating the insulation shear band formation during cold forging for the low carbon steel is preferably carried out as follows. That is, the step (a) is preferably carried out including the step of analyzing the possibility of the formation of the insulating shear band from the occurrence of the yield point phenomenon in the room temperature quasi-static tensile test, and the possibility of crack formation during cold forging, Step (b) is preferably carried out using a torsional Kohlski bar, and step (b) is based on the presence or absence of thermal softening sections from the maximum shear stress to the failure mode from the dynamic shear stress versus shear strain curve. It is preferable to proceed with the step of analyzing the possibility of the formation of the insulating shear band and the possibility of crack formation during cold forging from the degree of the strain concentration area near the wavefront.

On the other hand, the method of inhibiting the formation of a thermally insulating shear band during cold forging for low carbon steel to achieve the technical problem to be achieved by the present invention described above is a low carbon steel or electric furnace steel, hot-rolled steel, including the process of converter steel, ingot and hot rolling The low carbon steel, including the rolling process, was subjected to a heat treatment for solution treatment for 1 to 2 hours at a temperature of 870 to 900 ° C. before cold forging, and then 6 to 8 ° C./by cooling by forced air cooling. Cold forging is performed at a cooling rate of seconds. The temperature in the solution treatment step is 870 ° C., which is the lowest temperature in this region in terms of the state of carbon content. There is a problem that the mechanical properties are bad because it is coarsened. In addition, the time range of 1 to 2 hours in the solution treatment step indicates the ratio of 1 hour / inch thickness, which is usually used in ASTM to indicate the heat treatment time, and is applicable to this ratio when the thickness of the specimen changes. On the other hand, the forced air cooling rate of 6 to 8 ℃ / sec in the forging step is the optimum cooling rate necessary to eliminate the yield point phenomenon, and when the cooling rate is less than 6 ℃ / second, the cooling rate is to suppress the yield point phenomenon Insufficient deterioration, if more than 8 ℃ / sec has a problem that it is quenched more than necessary to adversely affect the mechanical properties.

The conventional technique is to secure the uniformity of the metal flow applied to the material by improving the mold structure during forging, but at the same time, a microstructure solution to reduce the deformation resistance of the material must be sought. Suppression of crack formation due to can be more effectively achieved. Therefore, the present invention is to investigate the case that the heat insulating shear band is not easily formed under the same forging conditions by changing the microstructure of the low carbon steel, and to determine the formation factor. Based on the correlation between the microstructure and the formation of the insulating shear band, the method of manufacturing low carbon steel for cold forging having better workability can be found.

In accordance with an embodiment of the present invention by adjusting only the manufacturing process under a constant composition to determine the tendency of the insulating shear band formation by changing the microstructure of the low carbon steel for cold forging. First of all, for more direct observation, the relationship between the adiabatic shear band and the crack formation is investigated by investigating the crack sites of low carbon steel where cracking occurs during cold forging. Tensile behavior is analyzed by tensile tests on low carbon steels that form cracks and low carbon steels that do not form cracks under the same forging conditions. In addition, after performing a dynamic torsion test using a torsional Kolsky bar that can systematically analyze adiabatic shear band formation behavior, the shear stress-shear deformation curve and failure mode are investigated. After the low carbon steel in which the crack is formed is subjected to solution treatment at 870 ° C. for 1 hour, the cooling rate is changed to three different types to induce a change in the microstructure. Tensile and dynamic torsion tests are performed again on these three specimens, and then the shear stress-shear curves and failure modes are investigated. After the low carbon steel in which the crack is formed is subjected to solution treatment at 870 ° C. for 1 hour, the cooling rate is changed to three to induce the microstructure change. These three specimens are analyzed again through the tensile test and the dynamic torsion test for the possibility of adiabatic shear band formation. Thus, by finding the microstructure with the lowest possibility of forming the adiabatic shear band under the same composition and forging conditions, and by identifying the microstructure factor that leads the trend, it is possible to provide basic data for the development of low carbon steel with excellent forging workability in the future.

Example

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, embodiments of the present invention may be modified in many different forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, exemplary embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art related to the present invention. In the drawings, the thicknesses of layers or regions are exaggerated for clarity.

The materials used in the examples according to the present invention are two types of low carbon steel having a ferrite-perlite structure having a carbon content of 0.1 wt% or less, and the chemical compositions thereof are shown in Table 1 together with the AISI 1010 steel composition criteria.

River C N Mn Si P S Al Cu Ni Cr AISI 1010 Details 0.08 ~ 0.13 . 0.30-0.60 0.10 ~ 0.15 0.040 max. 0.050 max. . . . . A steel 0.094 0.003 0.41 0.23 0.011 0.031 0.088 0.065 0.026 0.038 B steel 0.071 0.008 0.49 0.30 0.019 0.013 0.012 0.151 0.120 0.058

One of the two steels is made of converter steel, ingot, hot rolling process, and the other is made of rod of 120mm diameter through electric furnace steel and hot rolling process. It will be written. The two steel bars are cut at intervals of 24 mm in length to form a disk, and in order to remove the scale of the surface, the shot film is subjected to a shot blast process, and then the holding film is coated for the purpose of stress dispersion and friction reduction. The coating was lubricated. Subsequently, the disc was placed on a mold and cold forged to form a cylindrical pulley part. At this time, two or three cracks were observed on the upper end of the pulley part made of steel B. Even when made of steel A, no cracks were found. The location of the crack was mainly in the bent corner area of the top of the pulley, the crack did not occur in other areas.

In order to investigate the cause of cracking, the microstructure of the crack formed during cold forging was observed by optical microscope and scanning electron microscope (SEM). The cracks at the top of the pulley were cut parallel to the radial direction of the pulley, and the specimens were collected, and then etched and observed by optical microscope. Scanning electron microscopes (SEM) Was observed. In addition, the microstructure of the steel bar before cold forging was observed by an optical microscope, and the grain size and the pearlite fraction were measured by an image analyzer.

On the other hand, a rod-like tensile test piece with a gauge diameter of 6 mm and a gauge length of 30 mm was processed in a direction parallel to the rolling direction from the center of the steel bar. Tensile tests were carried out using a 10 ton capacity Instron at a cross-head speed of 10 mm / min at room temperature. After the tensile test, the fracture surfaces of the fractured specimens were observed by scanning electron microscopy (SEM) to investigate the fracture pattern.

In addition, dynamic torsion tests were performed using a conventional Kohlski bar to investigate the formation behavior and the possibility of crack formation of the adiabatic shear bands involved in cold forging of two low carbon steels. The specimen used in the dynamic torsion test was processed parallel to the rolling direction of the steel bar, and consisted of a gauge section and a hexagonal flange section with a thin walled tube form with a gauge distance of 2.5 mm and a thickness of 240 µm. . The Standard Kohlski Bar consists of a pair of 2124-T6 aluminum rods 25.4 mm in diameter and 2 meters in length. Dynamic shear stress-shear strain curves were obtained by testing at an average shear strain rate of about 1700 / sec at room temperature. After the test, the fracture surface of the fractured specimen at the center of the gauge and the region where the deformation was severely below the fracture surface were observed by scanning electron microscopy (SEM).

Microstructure and Mechanical Properties

1 is an optical microscope microstructure photograph of steel A, Figure 2 is steel B. Each figure shows a typical ferrite-perlite structure. The band structure in which layered pearlite grains elongate in the rolling direction is not severe, but many non-metallic inclusions such as manganese sulfide (MnS) have been confirmed by energy dispersive spectroscopy (EDS). It is observed. In particular, since steel A in FIG. 1 contains more manganese (Mn) and sulfur (S) than steel B, as a result, more manganese sulfide exists than steel B in FIG. 2. Of course. On the other hand, when quantitatively analyzed by an image analyzer, the average grain size of steel A is about 42 μm, which is coarse than about 30 μm of steel B, and the fraction of pearlite grains is higher than steel B (0.2%) than steel B (9.5%). appear.

3 is a stress strain curve of the A steel 31 and the B steel 33. Yield strength, tensile strength, and elongation were calculated from this and are shown in Table 2 below.

River Obtained Strength (MPa) Stress strength (MPa) Elongation (%) A steel 191 371 44 B steel 247 380 41

In steel A, there is no yield point phenomenon, in steel B, the yield point phenomenon is more pronounced, and yield strength is higher than steel A. The yield point phenomenon is due to the carbon and nitrogen atoms in the ferrite. The yield point phenomenon is high in the B steel with high nitrogen content, and the yield strength is high.

Figure 4 is a wavefront picture of the tensile test specimen at room temperature for steel A, Figure 5 B. The shape of the wavefront is ductile fracture, consisting entirely of dimples. In the steel A, a long band-shaped region is also observed, and this region is formed by breaking along a band structure formed of a pearlite tissue (FIG. 4). Manganese sulfides are observed in both rivers as indicated by arrows in the large dimples (Figs. 4 and 5).

Crack Microstructure Observation

6 is an optical micrograph of cracks formed in the upper inner edge of the pulley cold forged with B steel. It can be observed that the grains are elongated by the plastic deformation by cold forging, and the grains are combed from each other between the cracks formed long. This type of crack is typical of cracks propagating along the band after an insulating shear band is formed. The adiabatic shear band started at the surface portion gradually weakens as it enters the interior, and the crack breaks out of the band and progresses along the elongated pearlite grains as indicated by the arrows and stops. A weakly formed adiabatic shear band can be found near the arrow, and microcracks are formed in the pearlite near the tip of the crack. Unlike the surrounding microstructures of uniform deformation, the adiabatic shear bands are sharply sheared within a very narrow width so that the shape looks like a band, and no phase transformation or melting ground is observed inside.

7 and 8 are photographs obtained by scanning electron microscopy (SEM) of microcracks formed around large cracks in A and B steels after cold forging. It can be seen that the microcracks are formed at an angle of about 45 degrees with the plastic deformation direction by the shear cracking mechanism at the inside of the ferrite or at the perlite / ferrite interface, and the microcracks are also generated in the carbides of the ferrite grain boundary.

Dynamic Torsion Test Results

9 is a dynamic shear stress-strain curve obtained from a dynamic torsion test for two steels. The maximum shear stress, shear strain at maximum shear stress point and fracture shear strain at break were calculated from the curves, and are shown in Table 3 below.

River Shear stress (MPa) Shear strain at peak shear stress Shear strain at break A steel 347 0.47 0.58 B steel 403 0.20 0.54

Referring to FIG. 9, the dynamic shear stress-shear strain curve 91 of the steel A is very gentle compared to the shear stress-strain strain 93 of the steel B after the yield shear stress. After 0.2, the shear stress was maintained at about the same level, and sudden stress reduction appeared near the shear strain of 0.58. However, in steel B, it has the typical curvature behavior of ductile metal materials in which an insulating shear band is formed. In other words, the flow stress gradually decreases after the yield shear stress point, and the shear stress decreases sharply near the shear strain of 0.54, just before the specimen is destroyed.

10 and 11 are photographs of fracture surfaces of specimens subjected to torsion tests of steel A and steel B. FIG. This is a photograph of the fracture surface of a fractured specimen after scanning to examine the deformation and fracture behavior during dynamic torsion test by scanning electron microscope (SEM). In general, it shows a ductile fracture form consisting of dimples drawn in the shear direction during the torsion test of ductile materials. Since the grain size of steel A of FIG. 10 is larger than that of steel B of FIG. 11, the dimple size of steel A tends to be larger than that of B steel. In addition, in the B steel, as indicated by the arrow in Fig. 11, the region where the wavefront is rubbed in the shear direction is observed, which is due to the rapid process from deformation to fracture.

12 and 13 are photographs of the gauge portion of the fractured dynamic torsion specimens of steel A and steel B by scanning electron microscope (SEM). As shown in FIG. 12, deformation in steel A occurs relatively uniformly throughout the specimen. Observing FIG. 13, the uniform deformation occurred in the steel B, but as indicated by the arrow area of FIG. 13, it can be seen that the deformation tends to be concentrated only at the center of the specimen gauge, that is, the area adjacent to the wavefront, rather than the steel A. FIG. .

Insulation Shear Band

In order to prevent cracking during cold forging of low carbon steel, the metal flow of the material should be good and no insulating shear band should be formed during dynamic deformation. However, in low carbon steels made of ferrite-perlite structure, microcracking is likely to occur due to shear cracking of the ferrite, and insulated shear bands are particularly easy in areas where plastic flow is concentrated during extreme deformation such as cold forging. Can occur.

In the embodiment according to the present invention, the cracks formed during cold forging were observed, and the shear deformation characteristics were evaluated by dynamic torsion test and the deformation and fracture patterns were investigated in order to evaluate the degree of formation of the adiabatic shear band experimentally. Important factors affecting the shear deformation characteristics include mechanical factors such as strength, elongation and yield point phenomena, microstructural factors such as ferrite, pearlite, grain boundary carbide, and manganese sulfide inclusions, deformation and fracture behavior during torsion tests. The following is a detailed discussion of each of these factors.

In general, as the shear strain rate increases, the mechanical-thermal instability effect increases, so that the width of the region where the shear strain occurs is narrow, and the insulating band is easily formed. When the normal carbon steel is torsionally tested under quasi-static load, the area where shear deformation occurs is relatively wide and the deformation is uniform, but the shear deformation area becomes narrow and the deformation tends to be concentrated under dynamic load.

The dynamic torsional deformation process can be divided into three stages as follows. That is, (1) the stage of uniform strain before reaching the maximum shear stress, (2) the stage of non-uniform deformation starting at the point of maximum shear stress, and (3) the generation and development of adiabatic shear bands by local concentration of shear strain. Can be distinguished. At this time, the last step is accompanied by the formation of an adiabatic shear band and a local sharp rise in temperature and a decrease in the vertical shear stress.

In dynamic deformation, where the deformation is completed in a very short time (tens of hundreds to hundreds of microseconds), the heat generated with the onset of plastic deformation does not have time to heat transfer from the edge of the specimen to the flange, resulting in an increase in temperature in the center of the specimen. The resulting softening of the material increases the amount of plastic deformation of this part, resulting in non-uniform deformation. The non-uniform deformation is accompanied by mechanical instability due to the different amount of deformation between the center and the edge of the specimen, and thermal instability due to rapid temperature rise with the progress of plastic deformation causes a synergistic effect, which gradually intensifies the non-uniform deformation. Shear plastic deformation is concentrated in the center of the chamber. This local concentration of plastic deformation reduces the ability to withstand loads in this region, resulting in a sharp decrease in shear stress, as shown in the shear stress-shear strain curve of FIG. The possibility of forming can be evaluated.

Considering the formation of a thermal shear band as a result of thermal mechanical instability due to the combined action of plastic instability and local temperature rise, the maximum shear stress point at which the non-uniform deformation stage begins can be regarded as the starting point of the thermal shear band formation. Therefore, when the maximum shear stress and the shear strain at that time are high, it is difficult to form an insulating shear band due to plastic instability under a load and a strain rate given from the outside. From this point of view, steel B has a higher maximum shear stress than steel A but has a shear strain of only 0.2 at the maximum shear stress point, so that an insulating shear band can be easily formed. On the other hand, steel A has a very high shear strain at the maximum shear stress point, and as shown in FIG. 9, it is hardened to some extent by a shear strain of about 0.2, but above that, relatively hard work hardening is not achieved. It shows that uniform deformation can be made under flow stress.

Due to these factors, under the same cold forging condition, steel B may have a thermally insulated shear band and be easily cracked in a specific area where plastic flow is concentrated.However, steel A may have more uniform deformation under lower flow stress than steel B. Since forging cracks are not generated, forging can be easily performed. Indeed, when observing the deformed area of the broken gauge part after the dynamic torsion test, the shear deformation occurs relatively uniformly in the steel A (Fig. 12), but in the B steel, the shear deformation is concentrated in the center of the specimen gauge part. (FIG. 13) Therefore, in the wavefront, there is an area rubbed by the sharp shear deformation and fracture (FIG. 11).

In addition to mechanical-thermal instability, other important factors affecting cold workability to be examined are microhistological factors. If the microstructure is changed, the difficulty of the formation of the shear band is also changed, and in order to prevent the formation of the adiabatic shear band, the microstructure must be finely controlled through the manufacturing process such as alloy design, steelmaking, and rolling. Microhistological factors present in low carbon steels are grain size, fraction and shape of pearlite, grain boundary carbide, manganese sulfide (MnS) inclusions, and the like. Perlite is the most vulnerable part of carbon steel, which acts as a site of breakage by forming microcracks under low load, thereby degrading plastic flow of carbon steel. Therefore, considering that pearlite may degrade the plastic flow of metal and cause cracking and propagation, it may be expected that the lower the fraction of the pearlite, the better the cold forging performance. In addition, grain carbide and manganese sulfide inclusions contained in steel A and steel B are fragile phases, so they can easily develop microcracks even under a lower load than pearlite grains, which may adversely affect plastic flow. However, since their amount is relatively small, they do not affect the overall cold forging performance, and both A and B steels have almost the same shape, size, fraction, grain boundary carbide, and manganese sulfide (MnS) inclusions. Therefore, it is difficult to see that the cold forging performance is changed by the difference in microstructure.

As described above, in case of cold forging of A steel and B steel material, although the basic microstructure and mechanical properties of both steels are almost similar, the metal plastic flow characteristics are different so that there is a difference in crack initiation due to the formation of an insulating shear band. Able to know. This plastic flow difference can be deduced from the presence or absence of the yield point phenomenon of FIG. The yield point phenomenon occurs in low carbon steels, aluminum alloys, titanium alloys, and the like, and is explained in the case of low carbon steels due to difficulty in movement of dislocations caused by invasive solute atoms such as carbon and nitrogen. Carbon or nitrogen atoms are easily diffused to dislocations, and they strongly adhere to the dislocations. When dislocations are displaced from the molten atom due to high stress, slippage occurs at low stresses, causing a yield point. Therefore, the yield point phenomenon can be eliminated if the amount of invasive solute atoms dissolved in the ferrite is reduced or if the solute atoms are not easily diffused into the dislocations. As can be seen in Table 1, the nitrogen content of A steel is much lower than that of B steel, so the yield point phenomenon may not occur. In the B steel, nitrogen atoms exhibit a yield point phenomenon by fixing potentials in ferrite. To prevent this, the content of nitrogen in the steel must be greatly reduced and hot rolling can be applied to reduce the diffusion of carbon and nitrogen atoms to the potential by applying a fast cooling rate. do. However, steel B is difficult to reduce the nitrogen content because it is manufactured by the steelmaking method with electricity. Therefore, if aluminum is added in the steelmaking process to consume nitrogen atoms as aluminum nitride (AlN) compound particles, effective dissolved nitrogen atoms can be reduced. have.

Microstructure control according to the heat treatment cooling rate

As discussed above, the yield point phenomenon of B steel shows that the number of nitrogen atoms to fix the dislocation has a great influence on the plastic flow of the material, and an adiabatic shear band is formed in the region where the plastic flow is concentrated. Therefore, the conditions for eliminating the yield point phenomenon and reducing the formation of the adiabatic shear band were investigated by changing the cooling rate for the B steel exhibiting the yield point phenomenon. After cooling the steel B at 870 ℃ for 1 hour, the cooling conditions were changed by air cooling, furnace cooling, and air-blast cooling, and the cooling rate measured by thermocouple was 0.07, 1.00, 6.23 ° C / sec, respectively.

14, 15 and 16 is a microstructure photograph of each of the air-cooled, furnace-cooled and forced air-cooled specimens with an optical microscope. As the cooling rate increases, it can be seen that the size of ferrite and pearlite grains and the volume fraction of pearlite decrease. In the uncooled specimens, the band structure in which the ferrites are arranged at regular intervals is clearly seen (FIG. 14), but almost disappeared in the forced air-cooled specimens (FIG. 15).

FIG. 17 is a tensile stress-strain curve for each of the specimens at different cooling rates, ie air cooled, furnace cooled and forced air cooled. On the basis of the tensile curve 171 of the air-cooled specimen, a distinct yield point phenomenon is observed in the tensile curve 173 of the furnace-cooled specimen, but as the cooling rate increases, the phenomenon is alleviated and the tensile curve of the forced-air cooled specimen (175). The yield point is almost eliminated at. It can be seen that due to the fast cooling rate, intrusion-type atoms such as carbon or nitrogen are suppressed from dispersing to the potential, thereby reducing the potential fixation atmosphere. On the other hand, the tensile test and Brinell (Brinell) hardness test (load: 100kg) results for B steel are shown in Table 4 below.

B steel Yield strength (MPa) Stress strength (MPa) Elongation (%) Brinell Hardness (HB) Noin 293 395 38.0 66.4 Air cooling 314 402 36.5 68.0 Forced air cooling 242 373 37.0 65.4

Referring to Table 4, the air-cooled specimens have higher yield strength and tensile strength than the uncooled specimens, and the forced air-cooled specimens have fine grains but have a lower fraction of perlite, resulting in the lowest yield strength and tensile strength. This tensile behavior is also consistent with the hardness test results.

18 is a dynamic torsion curve for each of the air cooled, furnace cooled, and forced air cooled specimens. When compared based on the dynamic torsion curve 181 of the air-cooled specimen, the maximum shear stress is the lowest dynamic torsion curve 185 of the forced air-cooled specimen in the same order of the tensile stress. Similar to the dynamic torsional behavior of B steel before heat treatment in the case of the uncooled specimen (183), the plastic shear instability behavior shows a maximum shear stress at a shear strain of about 0.2 and then gradually decreases until it breaks at a shear strain of about 0.6. Seems. The decrease in shear stress after this maximum shear stress point is evidence of the thermal softening effect by strain concentration. On the other hand, the air-cooled and forced air-cooled specimens showed a uniform strain region showing a maximum shear stress at a shear strain of about 0.3 and then a fracture at a strain of about 0.6. It means that it is excellent.

19, 20 and 21 are photographs of a gauge region observed by scanning electron microscopy (SEM) after a dynamic torsion test, in order to determine the concentration tendency of shear deformation for each of the specimens having different cooling rates. As shown in FIG. 19, it can be seen that localized deformation regions having a width of about 250 μm exist in the furnace-cooled specimen where local deformation is concentrated around the fracture surface, and the contrast and the contrast of the portion far away from the fracture surface are different. have. The local deformation region is wider in the air-cooled and forced air-cooled specimens (FIGS. 20, 21), especially in the forced-air-cooled specimens, which is greatly increased to 400 µm or more, indicating a nearly uniform deformation shape and the gauge completely at the dynamic strain rate The behavior is not destroyed.

Embodiments of the present invention described with reference to the accompanying drawings are optimal embodiments. Although specific terms have been used herein, they are used only for the purpose of describing the present invention in detail and are not used to limit the scope of the present invention as defined in the meaning or claims.

These results show that deformation can occur even under dynamic loads by increasing the cooling rate after solution treatment or hot rolling. This is because the faster the cooling rate, the lower the fraction of the pearlite acting as a deformation resistance, and also prevents the yield point phenomenon by inhibiting the diffusion of the invasive solid atoms causing the yield point phenomenon to the potential. In the present invention, the microstructure, mechanical properties, and dynamic torsional properties of low carbon steels with a carbon content of 0.1 wt% or less were analyzed by the formation of adiabatic shear band and the degree of crack initiation. We predicted and suggested a method to prevent the occurrence of cracks. In particular, the fact that the plastic flow of the metal is changed by the difference in the yield point phenomenon to prevent the occurrence of cracks in a way to eliminate or reduce the yield point phenomenon can improve the performance of cold forging.

Claims (4)

In the method of inhibiting the formation of the insulating shear band during cold forging for low carbon steel, including the process of converter steel, ingot and hot rolling, Solution treatment for the low carbon steel; And Including the step of cold forging for the solution-treated low carbon steel, the method of inhibiting the formation of a heat insulating shear band during cold forging for low carbon steel, characterized in that proceeding. The method of claim 1, The solution treatment step is carried out for 1 to 2 hours in the temperature range of 870 to 900 ℃, the cold forging step is a low carbon steel, characterized in that proceeds at a cooling rate of 6 to 8 ℃ / second in the manner of forced air cooling Method of suppressing the formation of adiabatic shear band in cold forging. In the method of suppressing the formation of the insulating shear band during cold forging for low carbon steel, including the furnace steel, hot rolling process, Solution treatment for the low carbon steel; And Including the step of cold forging for the solution-treated low carbon steel, the method of inhibiting the formation of a heat insulating shear band during cold forging for low carbon steel, characterized in that proceeding. The method of claim 3, The solution treatment step is carried out for 1 to 2 hours in the temperature range of 870 to 900 ℃, the cold forging step is a low carbon steel, characterized in that proceeds at a cooling rate of 6 to 8 ℃ / second in the manner of forced air cooling Method of suppressing the formation of adiabatic shear band in cold forging.