JP2007175774A - Laser welding method for continuous hot rolling process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser welding method for making rolled stocks in which a low temperature transformed structure is produced after laser welding usable in continuous hot rolling process. <P>SOLUTION: The laser welding method for the continuous hot rolling process includes: a stage where the rolled stocks in which the low temperature transformed structure is produced are brought into contact with each other; and a stage where the contact part between the rolled stocks is subjected to laser welding using a welding material comprising, by weight, ≤0.1% C and 0 to 1.22% Cr in a welding stage. When continuous hot rolling is performed using the laser welding method, the hardened structure in the weld zone can be reduced, and the welding quality of the weld zone can be stably provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser welding method for rolled material that is continuously produced by mutually joining rolled materials in a continuous manufacturing process. More specifically, in a laser welding process in which rapid heating and rapid cooling are performed, a continuous manufacturing process that can stably provide the weld quality of the welded portion by reducing the hardened structure of the welded portion with respect to the steel type in which the low temperature transformation structure occurs. The present invention relates to a laser bonding method.

  In the technical field of producing metal sheet materials, there is an increasing demand to improve productivity and quality by increasing the production process and to increase the size of products that can be manufactured.

  Such a continuous manufacturing process is being applied to high-grade steel types such as electric steel sheets and ferritic stainless steel sheets.

  A typical example of the continuous manufacturing process is a PCM (Pickling & Tandem cold rolling mill) process in which pickling and continuous rolling are connected and proceed simultaneously. Cold rolled coils obtained from hot rolled coils have a method of performing pickling and continuous rolling (Tandem cold rolling mill), respectively, but a pickling and tandem cold rolling mill (PCM) process in which pickling and continuous rolling are connected and proceed simultaneously. But it can be manufactured. The PCM process can greatly improve productivity as compared with the processes of performing pickling and TCM, and has been applied a lot recently.

  What is important in such a continuous rolling field is a joining technique of a rolled sheet material in which the trailing end of the preceding rolled sheet material and the leading end of the following rolled sheet material are mutually joined. Techniques for joining rolled plate materials for continuous rolling include a solid phase joining method and a joining method by welding.

  The joining method by welding is such that, at the entrance side of the continuous rolling line, the rear end of the preceding rolled sheet and the subsequent rolled sheet are mutually welded to form a weld, and then the subsequent rolling line is passed. . In this case, when the quality of the welded portion is not good, the welded portion breaks during passing through the subsequent rolling line, causing a serious problem that the production is completely interrupted. Therefore, for continuous rolling, it is considered to be an important technique to ensure the quality characteristics of the hot and cold rolled coil welds. In particular, the PCM line requires a stricter welding quality than the existing line because the production line is longer than the existing pickling line and the TCM line and the number of loopers in which the coil is inserted is large.

  As a welding method applied to the continuous rolling line, there are flash butt welding in which short-circuiting and flashing are repeatedly generated, and laser welding using a high-density heat source.

  In flash butt welding, the amount of heat input is large, so there is a limit to the selection of materials to be welded. As an example, joint strength is not ensured for electrical steel sheets, ferritic stainless steels, high carbon steels, and the like, and sheet breakage may occur during cold rolling. In particular, high carbon steel is evaluated as a material that is very difficult to flash butt weld because of its high carbon (C) content. In addition, it has been pointed out that there is a problem in reproducibility, such as variations in quality characteristics of individual welds, even when repeated work is performed with the welding schedule and welding conditions set constant.

  Laser welding is known to provide superior quality characteristics compared to existing flash butt welding because of its high energy density and low heat input.

  However, when high carbon steel is welded and continuously rolled by laser welding, there is a problem that pores and pinholes are generated in the weld metal part and cracks are generated in the weld metal and the weld heat affected zone.

  Porosity and pinholes are closely related to the carbon content in the material. It is known that during welding, the carbon in the molten metal reacts with oxygen in the atmosphere, and the gas that could not be released to the outside while producing CO gas remains during solidification and generates pores.

  Therefore, it is important to reduce the carbon content of the molten metal part, and the generation of pores can be reduced by using an appropriate welding material.

  Cracks in the weld are related to the hardened structure, and in high carbon steel, the main cause is a martensite structure generated from the rapid heating and rapid cooling processes during welding. Since the weld metal portion and the weld heat affected zone are generated simultaneously in the hardened structure of the welded portion, the improvement methods are complicated and varied.

  As described above, as a prior art for continuously rolling a steel material in which a hardened structure is generated, there are the following techniques.

  First, Patent Document 1 relates to heat-treating a welded portion, and discloses a method of using a fixed heat source and maintaining a specific heat treatment temperature for a certain time according to the carbon content of the rolled material. Yes. However, this method has a problem in that the overall welding time increases due to the heat treatment maintenance time.

There is Patent Document 2 as another prior art. This technique relates to a method of performing heat treatment in a temperature range of 400 ° C. or more and AC ₁ point or less within 1 minute after welding a welded portion by laser welding. However, this technique must be maintained for a very long time in order to completely remove the hardened structure at 400 ° C. or more and AC ₁ point or less, and when rapidly cooled like laser welding, Since it is necessary to heat and heat the welded part within several seconds after welding, there is a problem that the heat treatment method is very complicated.

  Moreover, there exists patent document 3 in another prior art. This technique relates to a method of inserting and welding low-carbon steel excellent in weldability between joints of high-carbon steel sheets. This technique is not suitable for mass production because the number of welding processes increases more than twice as compared with other welding methods and a leader strip must be prepared every time.

  As another prior art, there is Patent Document 4. This technique relates to a heat treatment method in which a laser weld is cooled while passing through a mixed region of ferrite and pearlite. This technique relates to laser welding, and has a problem that it is difficult to transform into a region where ferrite and pearlite are mixed because it is rapidly heated and rapidly cooled as compared with arc welding. In particular, when welding to high carbon steel, there is a problem that the hardening phenomenon is remarkably generated.

  There is Patent Document 5 as another prior art. This technique relates to a method of heat-treating a rolled material after laser welding is completed on a joint. However, since this technique also applies laser welding, there is a problem that transformation is completed in the martensite structure immediately before the welded portion is rapidly cooled and heat treatment is performed, and microcracks are generated. Therefore, this technology is difficult to apply in production lines that require high quality, such as PCM.

  As another prior art, there is Patent Document 6. This technology relates to a method of using a filler wire at a dissimilar joint between high carbon steel and low carbon steel and preventing cracks in the weld by irradiating the low carbon steel with a laser beam without performing heat treatment. . However, this technique has a problem that the hardened structure generated from the weld heat affected zone on the high carbon steel side that is not melted cannot be removed.

  Still another prior art is Patent Document 7. This technique relates to flash butt welding, and relates to a method of performing heat treatment on a weld. Although the welding methods are different, there are Patent Literature 8, Patent Literature 9, and Patent Literature 10 as techniques similar to the above technique of performing heat treatment on the welded portion. However, all of these techniques have a problem that it is difficult to stably ensure the quality of the welded portion of the high carbon steel sheet containing 0.5% or more of carbon.

  As described above, there are many steel plate joining techniques for continuous rolling, but in most cases, this is a method applied to a high carbon steel having a relatively low carbon content and a production line that does not require much welding quality.

  Therefore, in a laser welding process in which rapid heating and rapid cooling are performed, a steel type in which a low temperature transformation structure is generated after welding, such as high carbon steel, DP, TRIP, and CP steel, can be applied to continuous rolling. There is a demand for technology that can ensure quality characteristics.

Japanese Patent Publication No. Hei 5-50276 Japanese Patent Publication No. Hei 5-132719 Japanese Patent Publication No. Hei 8-57502 Japanese Patent Publication No. 8-215872 Japanese Patent Publication No. 2000-317642 Japanese Patent Publication No. 2001-353587 Japanese Patent Publication No. 2000-317642 Japanese Patent Publication No. Hei 5-132719 Japanese Patent Publication No. 2000-317642 Japanese Patent Publication No. 2004-76159

  Therefore, the present invention is for solving such a conventional problem, and the object of the present invention is to reduce the hardened structure of the laser welded portion and stably ensure the quality of the welded portion. It is an object of the present invention to provide a laser welding method capable of greatly improving productivity for a manufacturing process.

In order to achieve the above object, the present invention includes a step of bringing the rolling materials in which a low temperature transformation structure is generated into contact with each other,
Laser welding using a welding material containing C: 0.1% by weight or less and Cr: 0-1.22% by weight in the welding stage with respect to the contact portion of the rolled material. Provide a laser welding method.

  In the present invention, the welding material is typically carbon steel or Ni alloy, and it is preferable to use any one of wire, powder or thin film.

  In the welding method according to an embodiment of the present invention, it is preferable that the contact portion of the rolled material is preheated in the range of 600 ° C. to 800 ° C. before the welding step. In the welding method according to an embodiment of the present invention, it is preferable that after the welding step, the welded portion is heated in the range of 800 ° C. to 1100 ° C. and post-heat treated. In the present invention, the pre-heat treatment and the post-heat treatment are most preferably applied together.

  In the present invention, examples of the rolled material in which the low temperature transformation structure is generated include high carbon steel, DP steel, TRIP steel, and CP steel having a C content of 0.5% by weight or more. Examples of the high carbon steel include C: 0.5% or more, Si: 0.1-0.5%, Mn: 0.3-0.6%, P: 0.05% or less, S: 0.05% or less, Cu: 0.5% or less, Ni: 3% or less, Cr: 0.05-0.5%, Al: 0.05% or less, other inevitable impurities and the rest It is composed of Fe.

  The laser welding method for continuous manufacturing process according to the present invention includes pickling and cold rolling (PCM, Pickling & Tandem Cold Rolling Mill), pickling and oil coating (POL, Pickling & Oiling Line), annealing and pickling. (APL, Annealing & Pickling Line), a pickling step (PL, Pickling Line), a cold rolling step (TCM, Tandem Cold Rolling Mill), and the like.

  The laser welding method of the continuous manufacturing process according to the present invention provides a welding condition for a continuous manufacturing process that has not been applied so far, and is capable of continuously producing steel in which a low temperature transformation structure is generated. There is.

  In addition, since the present invention continuously produces a rolled material capable of generating a low temperature transformation structure, it is possible to obtain a good laser weld joint having no welding defect. As shown in FIG. It provides a technical effect that enables continuous work without any problems.

  In addition, the present invention has the technical effect of significantly improving the productivity of the steel material in the continuous production process by reducing the welding operation time to the range of 25 seconds of the general steel level regardless of the carbon content of the steel material.

  As described above, when the welding conditions of the present invention are applied, even a steel that can generate a low temperature transformation structure can withstand a strong compressive load applied to a continuous production line and a tensile load applied between the stands. Therefore, it is possible to perform a continuous manufacturing process in which the welded portion does not break even during the continuous production process.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the drawings.
In the present invention, the “rolled material in which a low temperature transformation structure is generated” means a rolling material in which a welded portion is formed by mutual joining by a laser welding method, and then the structure of the welded portion is transformed into a low temperature transformed structure during cooling. The low temperature transformation structure means a structure containing martensite and bainite. Rolled materials that generate a low temperature transformation structure after laser welding include high carbon steel or high strength steel materials.

  High carbon steel means steel having a C content of 0.5% by weight or more. In typical examples of high carbon steel, C: 0.5% or more, Si: 0.1-0.5, in terms of weight% (hereinafter,% means weight% unless otherwise specified) in the present invention. %, Mn: 0.3 to 0.6%, P: 0.05% or less, S: 0.05% or less, Cu: 0.5% or less, Ni: 3% or less, Cr: 0.05-0 .5%, Al: 0.05% or less, and other unavoidable impurities and the remaining Fe. Any steel that contains 0.5% or more of C in terms of carbon content should be interpreted as being included in the high carbon steel meant in the present invention. It does not matter whether an element such as Mo, V, Ti, W, B, Nb, or Sb is added.

  High-strength material means steel designed to have a tensile strength of 450 MPa, for example, steel having a low-temperature transformation structure. Examples of steel having this low-temperature transformation structure steel include DP steel (Dual Phase steel), TRIP (Transformation Induced Plasticity) steel, and CP steel (Composite Phase steel). This is called normal transformation structure steel. DP steel is a steel in which two phases with different properties of soft ferrite and martensite with high strength are mixed, and it is a steel type that can simultaneously ensure excellent workability and high strength with a small number of alloy elements. TRIP steel is composed of a ferrite phase with excellent ductility, a bainite phase with excellent strength, and a metastable austenite phase that exists at high temperatures. When the metastable austenite phase is deformed, the strength is increased. It is a steel grade that changes to an excellent martensite phase. CP steel is steel in which precipitates exist in the structure of martensite or bainite. These transformation structure steels are those whose steel components are designed to have a low temperature transformation structure, and are transformed into a transformation structure after laser welding.

  The steel grades designed to have high-carbon steel and low-temperature transformation structure in this way are jointed by laser welding for continuous rolling, and then have a brittle low-temperature transformation structure such as martensite and bainite in the weld during cooling. A phase transformation will occur. As described above, when the low temperature transformation structure is generated, various problems occur such as cracks in the welded portion and plate breakage in the continuous rolling process.

  Further, in the present invention, the “welded portion” is a joint formed by welding the rear end of the preceding rolled plate for continuous rolling and the front end of the subsequent rolled plate with a laser welding machine. This means that it includes a molten metal portion that is solidified after being melted by the laser, and a weld heat affected zone (HAZ) that is not melted but is thermally influenced by the heat source of the laser.

  Further, in the present invention, the term “continuous manufacturing process” means a process of continuously rolling a rolled material in a hot rolling or cold rolling line, and two or more coils are formed by interconnecting the rolled materials. In the connected state, it is used to mean all continuous lines that pass continuously, such as a polishing process, a hot dip galvanizing process, or an annealing process.

  First, in order to achieve the object according to the present invention, the cause of the hardening phenomenon in the weld zone, which was confirmed through the structural inspection of the laser weld zone where the low temperature transformation structure is generated and the Erichsen test, will be examined.

  According to the results confirmed by the inventors of the present invention, it was confirmed that the hardening phenomenon of the weld occurred in the weld metal and the weld heat affected zone, respectively, and the low temperature transformation structure worked as the main cause.

  Such a low temperature transformation structure can be controlled to a certain level by the chemical composition of the weld. For this reason, the carbon content of a weld metal part can be reduced using the welding material whose carbon content is lower than a rolling material. More preferably, the hardness can be reduced by heat treatment together with the welding material.

  The quality characteristics of the welded part are affected by the hardness of the welded part as described above, but are also affected by the hardness distribution of the entire welded part.

  Accordingly, the present invention further provides a technique for heat-treating the welded portion, while reducing the hardness of the welded portion, while relaxing the hardness distribution of the entire welded portion, It is possible to ensure quality stably.

  Further, in a continuous line such as PCM, if welding and heat treatment are performed separately, the welding operation time becomes long, so that the overall production rate is reduced. And as the carbon content of the rolled material increases, the heat treatment time also increases.

  Therefore, in the present invention, in order to solve such a problem, the heat treatment apparatus is configured integrally with a welding machine as shown in FIG. 1 so that the welded portion of the rolled material is heat treated simultaneously with the welding. Constitute.

  The welding apparatus according to the present invention will be described with reference to FIG. 1. The welding apparatus according to the present invention mainly comprises a welding machine 10 and a heat treatment machine 20. All of these devices are preferably formed integrally, but are not necessarily limited thereto. FIG. 1 schematically shows a welding apparatus according to the present invention, and therefore, a configuration in which these apparatuses are integrally formed is not shown. Since it can be implemented by equipment technology, a detailed description of the technical configuration for integrally configuring these devices will be omitted.

  The welding machine 10 according to the present invention includes a laser generator 12 that oscillates a laser 14 and a filler supplier 16 that supplies a welding material. The heat treatment machine 20 is applicable to any means that can quickly heat as a heat source for heating the moving rolled material, and it is preferable to use a high-frequency induction coil. Such a heat treatment machine 20 includes a preheater 22 that heats the rolled material 40 before being welded and is installed before the welded portion, and a post-heater 24 that heats the rolled material 50 after welding.

  In the case of mutually welding the moving rolled materials using the welding apparatus according to the present invention, first, the front end of the subsequent rolled material 40 and the rear end of the preceding rolled material 50 are brought into contact with each other, and then heated in the preheater 22. The welded portion 60 welded by the machine 10 and joined to each other is post-heated by the post-heater 24.

In FIG. 1, reference numeral 30 denotes a guide roll.
When welding the rolled material which moves using the welding apparatus by this invention, a rolled material also moves, but a welding apparatus will also move with this. At this time, the moving direction of the rolled material and the moving direction of the welding apparatus can be the same or opposite. When the moving direction of the rolled material and the moving direction of the welding apparatus are the same, it is preferable that the moving speed of the rolled material is the same as or faster than the moving speed of the welding apparatus.

  FIG. 2 schematically shows a thermal cycle history for a welded portion of a rolled material and a resulting structure state in the case where welding is performed using the mobile welding apparatus in the embodiment of the present invention.

  The present invention provides a method for controlling the chemical composition of the welded portion and a heat treatment method for the welded portion as a method for controlling the hardening phenomenon of the welded portion for continuous rolling of the rolled material in which a low temperature transformation structure occurs after laser welding. To do.

Below, such a method is demonstrated in order.
First, a method for controlling the chemical composition of the weld will be described.
This method is a method for controlling the welding material that forms almost all the molten metal in the weld zone. The welding material is supplied from the filler supplier 16 to the welded portion 60. Basically, a welding material having a C content of 0.1% or less and a Cr content of 0 to 1.22% is preferable. Examples of such a welding material include carbon steel and Ni alloy of high toughness material. The carbon steel series is preferable because the welding quality can be secured more stably.

  When the optimum welding parameters cannot be derived, the weld material made of stainless steel and Ni alloy has reduced wettability with the high-carbon steel, which is the base material, and the base material components are not completely diluted due to the difference in melting point. Therefore, a phenomenon that the welded portion is intermittently embrittled may occur. However, the Ni alloy is not removed from the welding material of the present invention.

  In the present invention, the composition of the weld metal part is preferably controlled so that the carbon content is 0.4% or less. This is because in the case of laser welding, since a very small amount of welding material is melted to fill the welded portion, the dilution rate of the base metal is very large compared to normal arc welding.

  For example, in the case of a high carbon steel rolled material having a carbon content in the rolled material of 0.85% or more, the dilution rate is set to a maximum of 30%, and the carbon content of the weld metal part is set to 0.4% or less. In order to maintain, the carbon content of the welding material should be about 0.1 wt% or less.

  Chromium (Cr) reacts with carbon in the rolled material and forms chromium carbide in the vicinity of the weld metal and the weld heat affected zone, so the chromium (Cr) content is controlled to 1.22% or less. Is preferred.

  Therefore, in the present invention, the welding material supplied from the welding machine is preferably carbon steel or Ni alloy of C: 0.1% or less and Cr: 0 to 1.22% or less. Carbon steel is a welding material mainly composed of Fe, and Ni alloy is a welding material mainly composed of Ni. These can be applied to normal carbon steel or Ni alloy while C and Cr satisfy the conditions of the present invention. Although the shape of the welding material is preferably a wire, a welding material in the form of powder or thin film can also be used.

Next, the heat processing method of a welding part is demonstrated.
In the present invention, the heat treatment of the welded portion includes a preheat treatment performed before welding for preventing cracks in the welded joint portion, and a post-heat treatment performed to alleviate the hardening phenomenon of the welded joint portion after welding.

  If, for example, laser welding is performed on a rolled material in which a low temperature transformation structure is generated, and only post-heat treatment is performed on the welded joint, the welded portion after welding is rapidly cooled before post-heat treatment, and welding is performed. Cracks may occur in the part.

  Therefore, in order to relieve the rapid cooling heat cycle by laser welding with respect to the welding material in which the low temperature transformation structure is generated, it is preferable to perform pre-heat treatment before welding.

  The temperature of such pre-heat treatment is higher than the martensite transformation temperature (Ms) when a mobile heat treatment machine is applied as in one embodiment of the present invention. Preheating to a high temperature is preferred.

  Therefore, according to one embodiment of the present invention, when preheating a high carbon steel weld, it is preferable that the temperature of the preheat treatment is in the range of 600 ° C to 800 ° C. When preheating is performed at a preheating temperature of 600 ° C or lower, sufficient time for preheating cannot be secured for the moving rolled material, so that sufficient quality characteristics cannot be ensured in the welded portion, and the preheating temperature is 800 ° C or higher. If so, deformation is generated in the weld due to excessive heat input, and a good weld cannot be secured.

In the present invention, the post-heat treatment of the welded portion is performed based on two concepts.
The first concept _is a relatively long time maintained to weld martensitic structure with _{A C1} below is changed to Tenpado (tempered) martensite, a tempered (Tempering) processing method for ensuring ductility.

  The second concept is a method of transforming into a ferrite and pearlite structure by actively controlling the cooling heat cycle during laser welding.

  Here, the tempering method has to be heat-treated for a relatively long time, so it is an advantage that sufficient ductility can be ensured by this, but in a coil production line with a high production speed, if the post-heat treatment takes a long time, the productivity Decreases. Therefore, in the laser welding system using a mobile heat source, the latter method of relaxing the cooling cycle after laser welding is more preferable.

  The post-heat treatment temperature of the welded portion is preferably 800 to 1100 ° C, and more preferably 950 to 1100 ° C. Further, the post-heat treatment of the welded portion is preferably naturally cooled without having a maintenance time after heating.

  When the post-heat treatment temperature of the welded portion is 800 ° C. or lower, the heat input is insufficient, a martensite structure is formed in the structure of the welded portion after cooling, and there is no hardness reduction effect. Further, when the temperature of the post heat treatment is 1100 ° C. or higher, the heat input is too large and the structure of the welded portion is coarsened, and a part of the martensitic structure of the hardened structure is regenerated at the time of cooling, Characteristics deteriorate.

  The laser welding method of the present invention can be applied to all methods for producing a rolled material in a continuous production process. For example, pickling and cold rolling (PCM, Pickling & Tandem Cold Rolling Mill), pickling and oil coating (POL, Pickling & Oiling Line), annealing and pickling (APL, Annealing & Pickling Line), pickling ( PL, Pickling Line), cold rolling process (TCM, Tandem Cold Rolling Mill) and the like.

In the following, preferred embodiments according to the present invention will be described.
In this example, a high-carbon steel hot-rolled material (main component Fe) having a composition as shown in Table 1 was used. The thickness of the rolled material was 2.0 mm. Such rolled materials were welded together using a CO ₂ laser welder with a maximum output of 12 kW.

  The welding material used in this example uses a filler in a wire state (Φ0.9 mm), and its chemical composition is as shown in Table 2 below, and is a carbon steel series of ER70S-G and ER80S-G. And ER308 of stainless steel series, ERNiCrMo03, ERNi, etc. of nickel alloy series were used. Of the chemical components of carbon steel series ER70S-G and ER80S-G, Fe is the main component and is not shown in Table 2.

  The laser welding conditions when the rolled material is welded with a laser welding machine are such that no weld defects such as pore underfill (Underfill) occur in the weld, the laser output is 8.4 kW, and the welding speed is 4. It was 5 m / min, and the interval between the joints was 0.15 mm.

  The heat treatment for the welded portion was performed using a high-frequency induction heating furnace having a heat source having a size of 20 w × 200 lmm and moving along the welding line while changing the output.

  The heat treatment conditions were a heating rate of about 100 ° C./s, heat treatment while changing the temperature of the pre-heat treatment and post-heat treatment, and then natural cooling (air cooling) after the heat treatment.

  When heat-treating the welded part, the temperature of the welded part heated by the high-frequency induction furnace is determined by spot welding an R-type thermocouple to the melting boundary, measuring the temperature history by welding, and reaching the maximum temperature from the temperature history curve. This temperature was determined as the heat treatment temperature.

  According to an embodiment of the present invention, when the Erichsen height is 4 mm or more and the PCM line passing standard is satisfied, the Eriksen height is evaluated using an Erichsen tester (Erichsen), and the Eriksen height is 4 mm as a reference. Decided whether or not. The evaluation method of the quality characteristic of the welded portion was evaluated by measuring the height of plastic deformation up to the time of occurrence of cracks in the welded portion.

  First, the quality evaluation results according to the welding materials and heat treatment conditions for SK85 steel having a carbon content of 0.85% are shown in Table 3 below.

  As can be seen from Table 3, when no welding material is used for the welded part of SK85 steel or heat treatment is not carried out, a welded joint that can pass through a weld crack generated immediately after welding. Was not obtained.

  In addition, even when only the pre-heat treatment or the post-heat treatment was carried out alone, it was not possible to ensure the quality characteristics of the welded part to the extent that the plate could be passed.

  On the other hand, when all the pre-heat treatment and post-heat treatment are carried out as in the present invention, it can be seen that the quality of the welded portion is improved as compared with the other embodiments.

  As can be seen from Table 3, when the pre-heat treatment temperature is set to 600 to 800 ° C. and the post-heat treatment temperature is heated in the range of 950 to 1100 ° C., Erichsen, which is a limit standard for PCM continuous rolling plate passing It shows stable weld quality characteristics exceeding 4.0mm in height.

  In the case of ER70S-G, ER80S-G, and ERNi with low chromium (Cr) content, the welding material exceeded the limit standard of Erichsen height of 4.0 mm, but the chromium (Cr) content was respectively In the case of ER308 and ERNiCrMo-3 contained in 19.86% and 21.17%, satisfactory welding quality could not be obtained even when the same heat treatment technology was applied.

  This is because the chromium (Cr) component contained in the welding material and the carbon component of the rolled material react during welding to generate chromium-carbides at the grain boundaries, and embrittle the grain boundaries of the weld. It is.

  Next, the quality evaluation results according to the welding materials and heat treatment conditions for S50C steel having a carbon content of 0.5% are shown in Table 4 below.

  The examples shown in Table 4 were carried out under the heat treatment temperature conditions showing the highest Erichsen values confirmed from Table 3, that is, the preheat treatment temperature 723 ° C. and the post heat treatment temperature 1005 ° C.

  As can be seen from Table 4, it can be seen that the quality characteristics of the laser welded portion of S50C steel are generally superior to the SK85 steel laser welded portion regardless of the welding material and heat treatment method.

  It can also be seen that the quality characteristics of the laser welded part of S50C steel can be obtained when the post-heat treatment alone is carried out alone and a value exceeding the Eriksen height of 4.0 mm, which is the limit standard, is obtained.

  Such a result seems to be due to the fact that the hardening phenomenon has been reduced by reducing the carbon content in the steel.

  Also, referring to Table 4, it can be seen that the quality characteristics of the weld are further improved when the pre-heat treatment and the post-heat treatment are applied simultaneously.

  Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the welding conditions in the continuous rolling welding of the high carbon steel shown in the above embodiments, but the idea of the present invention is applied. It can be applied to various welding conditions necessary for a continuous manufacturing process.

  Accordingly, the present invention can be variously modified and implemented within the scope of the claims and the detailed description of the invention, and this also belongs to the scope of the present invention.

It is a conceptual diagram which shows the laser welding apparatus by one Example of this invention. It is a graph which shows the thermal cycle of the laser welding part by one Example of this invention. It is a photograph which shows the laser welding part after the PCM continuous rolling regarding the rolling material of SK85 steel in one Example of this invention.

Claims (8)

In a method of laser welding a rolled material for a continuous manufacturing process,
A step of bringing the rolling materials in which a low temperature transformation structure is generated into contact with each other;
And a step of laser welding using a welding material containing C: 0.1 wt% or less and Cr: 0 to 1.22 wt% at the welding stage to the contact portion of the rolled material. Laser welding method.   The laser welding method for a continuous manufacturing process according to claim 1, wherein the welding material is selected from carbon steel or Ni alloy.   The laser welding method for a continuous manufacturing process according to claim 1, wherein the welding material is in the form of a wire, a powder or a thin film.   The laser welding method for a continuous manufacturing process according to claim 1, wherein the contact portion of the rolled material is preheated in a range of 600 ° C to 800 ° C before the welding step.   The laser welding method for a continuous manufacturing process according to claim 1 or 4, wherein after the welding step, the welded portion is heated in a range of 800 to 1100 ° C and post-heat treated.   The continuous material according to claim 1, wherein the rolled material in which the low temperature transformation structure is generated is selected from high carbon steel, DP steel, TRIP steel, and CP steel having a C content of 0.5 wt% or more. Laser welding method for manufacturing process.   The high carbon steel is C: 0.5% or more, Si: 0.1-0.5%, Mn: 0.3-0.6%, P: 0.05% or less, S: 0% by weight. 0.05% or less, Cu: 0.5% or less, Ni: 3% or less, Cr: 0.05 to 0.5%, Al: 0.05% or less, and from other inevitable impurities and the remaining Fe The laser welding method for a continuous manufacturing process according to claim 6, wherein the composition is a composition.   The above continuous production process includes pickling and cold rolling (PCM, Pickling & Tandem Cold Rolling Mill), pickling and oil coating (POL, Pickling & Oiling Line), annealing and pickling (APL, Annealing & Pickling Line), acid The laser welding method for a continuous manufacturing process according to claim 1, wherein the laser welding method is any one of a washing step (PL, Pickling Line) and a cold rolling step (TCM, Tandem Cold Rolling Mill).

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