JP4953172B2 - Method to refine ferrite structure by laser irradiation - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for refining the structure of an iron-based alloy. More specifically, the present invention relates to a method and an apparatus for refining a metal structure of a part where a fatigue crack such as a weld toe part is likely to occur or a part where the strength needs to be locally increased.
[0002]
[Prior art]
Conventionally, a technique for refining an iron-based alloy structure by a thermomechanical control process (TMCP) is known. In recent years, practical use of means for obtaining an ultrafine multiphase structure has been promoted by a mesoscopic structure control technique combining large strain hot working and heat treatment in a magnetic field. With such refinement of the metal structure, iron-based alloys, especially low-carbon steels, have made great progress in terms of high tensile strength and high toughness.
[0003]
[Problems to be solved by the invention]
However, even when iron-base alloys such as low carbon steel having the above-mentioned excellent characteristics are joined together by welding, the tensile strength and fatigue strength of the welded joint may decrease. There is a problem that it is not possible to make full use of the excellent properties such as high tensile strength and high toughness of the alloy. In addition, there is not always an effective means for the need to increase the strength locally.
[0004]
As means for refining the metal structure of the weld joint, there is a heat treatment method for a weld bead disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-301376. In this prior art, a welding torch, a preheating laser, and a post heating laser are aligned in the same plane, and are moved together in a joint portion of two parts to perform preheating, welding, and post heating together. . With this configuration, the temperature difference between the preheating spot by the preheating laser, the postheating spot by the postheating laser, and the welding spot is controlled, so that the microstructure of the weld bead to be welded is determined.
[0005]
However, the crystal grain size that can be reached by the above-mentioned prior art is about several tens of μm, and it is impossible to obtain a fine structure of the order of several μm, to which the present invention is directed. In the present invention, even when high strength steel having a fine structure obtained by TMCP or the like is joined by welding, the structure of the welded joint portion is made finer than the structure of the base material, and the metal material including the welded joint portion Another object of the present invention is to provide a method and apparatus for refining a metallographic structure that can obtain a structure having no fatigue strength even if it has mechanical characteristics.
[0006]
[Means for Solving the Problems]
The invention according to claim 1 for solving the above-mentioned problem is that after the fatigue damaged portion of the low carbon steel material damaged by fatigue is irradiated with a laser beam to be melted and solidified, the molten / solidified portion is obtained. and near the geometry sudden change portion, of the ferrite structure by laser irradiation repeated twice to 20 times is subjected to immediate rapid cooling is performed rapidly heated by laser irradiation to a temperature range of a ₃ transformation point or above the surface of the material does not melt This is a miniaturization method.
[0007]
According to a second aspect of the invention, a low carbon steel material, when repeating it twice to 20 times to perform immediately rapid cooling is performed rapidly heated by laser irradiation to a temperature range of A ₃ transformation point or above the surface of the material does not melt 2. The method for refining a ferrite structure by laser irradiation according to claim 1, wherein the laser irradiation is performed in the presence of a defocus distance in which the focal point of the laser beam is separated from the material surface by a predetermined dimension.
[0008]
[Action]
Since the present invention has the above configuration, the crystal structure of a desired part such as a welded joint of an iron-base alloy can be refined to the order of several μm. Can be fully utilized. Moreover, the metal structure of a desired site can be refined and strengthened.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described according to preferred embodiments thereof.
[0010]
In the present invention, rapid heating and rapid cooling by laser irradiation is performed twice to 20 times on a desired portion in an iron-based alloy such as low carbon steel, for example, a weld metal and a base metal heat-affected zone of a weld joint. This is one of the points. FIG. 1 shows a thickness of 6 mm mild steel sheet (by weight, C: 0.1%, Si: 0.15%, Mn: 0.52%, P: 0.02%, S: 0.02%, (Remainder: Fe and inevitable impurities, crystal grain size: 23 μm) The relationship between the number of laser irradiations and the ferrite crystal grain size d when laser welding is performed on the weld joint is shown. The laser used at this time was a YAG laser processing machine, laser output P: 1.5 kW, irradiation speed v: 0.5 m / min, defocus distance Fd: 40 mm, shield gas: Ar gas (flow rate: 30 l / min) The laser irradiation was performed under the following conditions.
[0011]
As is clear from FIG. 1, the ferrite particle diameter d at 0 mm, 0.1 mm, and 0.2 mm from the lower surface (melting boundary) of the weld metal (WM) corresponds to the increase in the number of laser irradiations. It is getting smaller. When laser irradiation is performed once, the crystal grain size is about 6 μm, which is about 1/4 of the initial crystal grain size. Furthermore, when laser irradiation is repeated, the crystal grain size gradually decreases in accordance with an increase in the number of laser irradiations. When it is 5 to 6 times or more, it converges to a constant size of about 4 μm. The final grain size varies depending on the chemical composition of the steel sheet, the laser output, the moving speed of the laser torch, and the defocus distance Fd. However, under any condition, the final particle size is almost constant by repeating the laser irradiation up to 20 times. A fine particle having a particle size of is reached.
[0012]
In steel materials such as low carbon steel, ferrite grains are refined by the γ → α transformation. The conventional normalization treatment uses this transformation, but since the heating rate and the cooling rate are low, the reachable grain size is 20 μm to 30 μm and there is a limit to the refinement of crystal grains. The inventors believe that the use of a laser beam that can take a very high heating rate and cooling rate can significantly advance the refinement of crystal grains, and a YAG laser processing machine is used to apply laser irradiation to the steel material surface to produce crystals. Attempts were made to refine the grain.
[0013]
As a result, the ferrite grain size of the base material: 23 μm became 6 μm by rapid heating and rapid cooling by one laser irradiation. By repeating the laser irradiation, the ferrite grains were further refined and became 4 μm after 5 or more laser irradiations. Moreover, although the crystal grain in the welded part is larger than that of the base material, it was also refined to 4 μm or less by two laser irradiations.
[0014]
The heating rate and cooling rate of the steel material in the normal normalizing treatment are both 0.1 ° C./s, whereas the heating rate when laser irradiation is performed on the surface of the steel plate is about 300 ° C. / S or more (average heating rate at 500 ° C. to 800 ° C.) is usually about 800 ° C./s, and the cooling rate is about 10 ° C./s to 400 ° C./s (average cooling rate at 800 ° C. to 500 ° C.). . As described above, the heating rate and the cooling rate at the time of laser irradiation are 100 times to 8000 times the heating rate and the cooling rate in the normalizing process. This suppresses the growth of γ (austenite) grains caused by the α → γ transformation, and also suppresses the growth of α grains due to the γ → α transformation in the subsequent cooling process. In addition, since the heating rate is extremely high, the structure of the entire material is not changed to γ (austenite) by one laser irradiation, and it is necessary to repeat laser irradiation to change the entire material to γ (austenite). However, as the number of times of laser irradiation increases, the total length of the fine ferrite grain boundaries increases, so that the number of nucleation sites for γ (austenite) formation by the next laser irradiation increases. And γ grains are also refined, and nucleation sites of α grains in the subsequent cooling process also increase. As a result, ferrite nuclei cannot grow and become fine grains.
[0015]
Next, the defocus distance will be described. FIG. 2 shows the relationship between the laser beam defocusing distance (the distance between the focal point of the laser beam and the irradiation target surface) Fd (mm) and the depth of the miniaturized crystal region. FIG. 3 shows the relationship between the defocus distance Fd (mm) of the laser beam and the width of the miniaturized crystal region. Both FIG. 2 and FIG. 3 are those when the thickness of the steel plate is 6 mm. As apparent from FIGS. 2 and 3, the maximum depth and width are obtained when the defocus distance Fd of the laser beam is 47 mm under the condition that the material surface does not melt. Thus, the defocus distance Fd of the laser beam is an important operating parameter in the control of the crystal grain refinement process.
[0016]
The method of refining a metallographic structure by laser irradiation according to the present invention is to mechanically apply a damaged portion such as a crack caused by fatigue of a steel material by melting and solidifying the damaged portion by laser beam irradiation and applying it to the portion. Properties can be restored.
[0017]
Next, a description will be given of an apparatus used when carrying out the method for refining a metal structure by laser irradiation according to the present invention. FIG. 4 shows an outline of an apparatus for refining a metal structure by laser irradiation according to the present invention. In FIG. 4, 1 is a laser oscillator, 2 is an optical cable, and 3 is a laser torch. Reference numeral 4 denotes a laser beam, and Fd is a defocus distance. Reference numeral 5 denotes a steel plate, which is a laser irradiation target. Reference numeral 6 denotes a laser torch operating robot control mechanism. The laser torch operating robot control mechanism 6 controls the traveling direction and traveling speed of the robot. Defocusing distance Fd control mechanism 7 controls the defocusing distance Fd using the surface position (height) of the steel plate 5 and the laser torch position as operation parameters. The laser beam 4 emitted from the laser oscillator 1 is led out by the optical cable 2 and is irradiated onto the surface of the steel plate 5 by the laser torch 3. Laser irradiation is repeated until the crystal grain size of the steel sheet is refined to a desired size. When the steel plate 5 is not fatigued, laser irradiation is performed under conditions where the laser irradiation portion does not melt. When damage such as a crack due to fatigue is received, the region is melted and solidified by laser beam irradiation, and then laser irradiation is repeated under the condition that re-melting is not performed, thereby refining the surface crystal.
[0018]
【Example】
Low carbon steel consisting of C: 0.1% by weight, Si: 0.15%, Mn: 0.52%, P: 0.02%, S: 0.02%, balance: Fe and inevitable impurities Thickness: 3 mm, 4 mm, 5 mm and 6 mm steel plates were cut into 50 mm × 50 mm, using a YAG laser processing machine, output P: 1.5 kW, irradiation speed v: 0.5 m / min, defocus distance Under the conditions of Fd: 40 mm and shield gas: Ar gas (flow rate: 30 l / min), laser irradiation with a length of 100 mm was repeatedly performed at the center of the test piece.
The region melted and solidified by the laser irradiation was 0.2 mm from the steel plate surface. In order to make the cooling rate constant, the next laser irradiation was performed after the test piece was cooled to room temperature. The number of times of laser irradiation was up to 10 times.
[0019]
For the steel sheets having thicknesses of 3 mm, 4 mm, 5 mm and 6 mm, the thermal cycle of the laser irradiation part was measured. The maximum heating temperature at the measurement position was 930 ° C to 980 ° C. The average rate of temperature increase from 500 ° C. to 800 ° C. was 800 ° C./s, and hardly changed depending on the plate thickness. The cooling rate was 120 ° C./s when the plate thickness was 3 mm, the cooling rate increased as the plate thickness increased, and was 440 ° C./s when the plate thickness was 6 mm. In any case, it was found that the heating and cooling behavior by laser irradiation is rapid heating and rapid cooling.
[0020]
FIG. 5 shows the structure before the laser irradiation and the crystal structure when the number of times of laser irradiation is 1 and 10 for a steel sheet having a thickness of 6 mm. When the ferrite grain size before laser irradiation is 23 μm and the number of times of laser irradiation is one, the ferrite grain size is smaller than the grain size before laser irradiation, but the crystal grain size varies. It can be seen. Moreover, the part which was originally pearlite has changed into the hardened structure. When the number of irradiations is 10, the crystal grains become finer, and the hardened structure seen in the case of one laser irradiation is hardly seen.
[0021]
Using the section method, the size of the ferrite at each depth was measured from the bond portion. The results when the plate thickness is 6 mm are shown in FIG. In FIG. 6, Δ, ○, and ● indicate the size of crystal grains at positions where the distance (depth) from the bond portion is 0 mm, 0.1 mm, and 0.2 mm. The crystal grain size before laser irradiation was 23 μm.
[0022]
At any position, as the number of times of laser irradiation increases, the crystal grain size gradually decreases. At a position where the depth from the bond portion is 0.2 mm, the crystal grain size is about 4 μm by the fourth laser irradiation. Further, since it was heated to a high temperature in the vicinity of the bond portion as Δ (0 mm), the crystal grains were slightly larger than the portion having a depth of 0.1 mm to 0.2 mm.
[0023]
Similarly, the crystal grain size was measured when the plate thickness was 3 mm, 4 mm, and 5 mm. FIG. 7 shows the plate thickness and crystal grain size at a position where the distance from the bond portion is 0.2 mm, using the number of laser irradiations as a parameter. As can be seen from FIG. 7, in the case of one laser irradiation, there was no significant difference in crystal grain size between the plate thicknesses. In the third laser irradiation, the crystal grain size was 4.3 μm when the plate thickness was 6 mm, whereas it was as large as 5.2 μm when the plate thickness was 3 mm. This is considered to be because the cooling rate was lowered as the plate thickness was reduced, and the effect of crystal grain refinement was reduced accordingly. However, when laser irradiation was repeated 7 times and 9 times, there was not much difference in crystal grain size due to the plate thickness.
[0024]
When the laser irradiation was repeated, the crystal grains gradually became finer, and when the plate thickness was 6 mm, the crystal grain size of the base material was about 4 μm by the fourth laser irradiation when the plate thickness was 6 mm. Further, the effect of refining crystal grains slightly decreased as the plate thickness decreased, but when the laser irradiation was 7 times or more, the value became a constant value of about 4.3 μm.
[0025]
【Effect of the invention】
According to the present invention, the crystal structure of a desired local region such as a welded joint portion of an iron-based alloy can be refined to the order of several μm, the fatigue strength can be increased, and the high tensile strength of the steel material can be increased. It is possible to make full use of the results of characteristic improvement technology such as conversion. In addition, local damage such as cracks caused by fatigue is not only remelted to eliminate cracks, but also is generated by reducing dislocation density and renewing, and then by laser beam irradiation with controlled defocus distance. After melting and solidifying, the structure can be refined and fully recovered. Furthermore, the structure of only the surface portion of the suddenly changing portion of the geometric shape can be made finer, so that the fatigue strength can be made higher than that during construction.
[0026]
According to the second aspect of the present invention, it is possible to easily control the depth and width of the structure refinement region of the iron-based alloy and the amount of heat input to the material.
[Brief description of the drawings]
[0027]
FIG. 1 is a graph showing the relationship between the number of repetitions of laser irradiation and the ferrite grain size. FIG. 2 is a graph showing the relationship between the defocus distance Fd of the laser beam and the depth of the crystal refinement region. Graph showing the relationship between the distance Fd and the width of the crystal refinement region. FIG. 4 is a schematic diagram showing the apparatus for refining the metal structure by laser irradiation according to the present invention. FIG. 6 is a graph showing the relationship between the number of repetitions of laser irradiation and the ferrite grain size d for each distance (depth) from the melting boundary in the position example of the present invention. Graph [Fig. 7] Graph showing the relationship between steel sheet thickness and ferrite grain size d, with the number of repetitions of laser irradiation as a parameter [Explanation of symbols]
[0028]
DESCRIPTION OF SYMBOLS 1 Laser oscillator 2 Optical cable 3 Laser torch 4 Laser beam 5 Steel plate 6 Laser torch operating robot control mechanism 7 Defocusing distance Fd control mechanism

Claims (2)

After to the fatigue damage of the low carbon steel material damaged by fatigue by irradiating a laser beam allowed melting and solidification, the molten-solidified portion and the vicinity thereof geometry sudden change portion, A ₃ transformation point or more A method for refining a ferrite structure by laser irradiation, characterized in that rapid heating by laser irradiation in a temperature range where the material surface does not melt and immediate cooling is repeated 2 to 20 times. Low carbon steel material, A ₃ laser irradiation when the transformation point or above the surface of the material is repeated twice to 20 times is subjected to immediate rapid cooling is performed rapidly heated by laser irradiation to a temperature range which does not melt is a focus of the laser beam 2. The method for refining a ferrite structure by laser irradiation according to claim 1, wherein the method is performed in the presence of a defocus distance separated from the material surface by a predetermined dimension.

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