JP4200854B2 - Method for preventing breakage of high Cr high carbon steel strip in continuous treatment and continuous treatment equipment for steel strip - Google Patents

Description

  The present invention relates to a high Cr-based high carbon steel strip, and particularly relates to prevention of strip breakage that occurs during continuous processing in a continuous processing facility.

  A metal strip such as a steel strip may be passed through a pickling line 100 as shown in FIG. 4, for example, after being hot-rolled and cooled to about 50 ° C. to room temperature by water cooling or air cooling.

  In various types of continuous processing equipment including such pickling lines, a number of metal strips are processed continuously, so the leading end of the preceding metal strip is on the equipment entry side prior to continuous processing such as pickling. The leading end of the following metal strip is rectangularly cut by a cutting device such as a shear 5 and then the leading end of the preceding metal strip and the leading end of the following metal strip are welded by a welding machine using laser, plasma or the like.

  For example, in the pickling line 100 shown in FIG. 4, the metal strip 1 unwound from a payoff reel (unwinding reel) 20 is rectangularly cut by a shear 5 at its leading end and welded and joined by a welding machine 10. After that, it is processed in a pickling tank 40 (process equipment) and taken up by a take-up reel (tension reel) 30. In the middle of the series of steel strip passages, the metal strip 1 passes through rolls that bend the metal strips such as a number of bridle rolls 50 and a plurality of looper rolls 60.

  When the metal strip is a special steel in terms of composition, for example, high carbon steel such as SK4, SK5, etc. with a C content of 0.3% or more and 1.5% or less as specified by JIS G 4401 or JIS G 3311 The welded joint that has been welded for continuous processing tends to become brittle by forming a martensite structure by natural cooling in the atmosphere immediately after the completion of welding. For this reason, for example, when the metal strip passes through the bridle roll 50 and the looper roll 60 in the pickling line 100 shown in FIG. There is a possibility of breaking. Thus, when a fracture occurs in the metal strip welded joint, the operation of the continuous processing facility is stopped for a long time.

  As a means for preventing such martensitic transformation of the welded joint and preventing cracks in the welded joint, a plate joint welding method has been proposed in which the weld is preheated to 200 to 250 ° C. and welded. As a device for preheating a welded joint in joint welding in a continuous processing facility, for example, Patent Document 1 discloses a back bar that is brought into contact with a butt weld from below through an insulating member, and an insulating member and a back bar. A steel plate welding apparatus having a coil that generates a magnetic flux that surrounds and penetrates a steel plate to be welded in a plate thickness direction is disclosed. Using this device, the temperature of the part that requires preheating can be raised efficiently to prevent the weld joint from becoming martensitic, and continuous welding of high carbon steel using laser (or plasma) etc. And it is possible.

  In FIG. 5, the principal part (part of a preheating apparatus) of the steel plate joining apparatus described in Patent Document 1 is shown.

The butt welds of the steel plates (metal strips) 1 and 1 whose end faces are opposed to each other are irradiated with a laser beam (or plasma) beam 70 for welding. On the other hand, when a high-frequency current flows through the coil 80, a magnetic flux penetrating the steel plates (metal strips) 1 and 1 in the thickness direction is formed and applied to the butt welds. As a result, the steel plate (metal strip) 1 of the butt welds. , Inductive currents are intensively generated at the ends facing each other, and the ends are preheated by the Joule heat generation. In general, this preheated portion has a length of several tens of millimeters in the longitudinal direction of the steel sheet with the butt position interposed therebetween. Reference numeral 90 denotes a back bar, which is a backing metal brought into contact with the butt weld from below through an insulating plate 91. Reference numeral 95 denotes a clamp that fixes and holds the steel plates (metal strips) 1 and 1 during welding.
JP-A-6-312285

  However, a special component such as Cr is used to improve the hardenability even if the steel strip is welded in a continuous processing facility while preheating is performed using the steel plate joint welding device described in Patent Document 1. In steel grades (steel strips) such as SUJ2 specified in JIS G 4805 containing steel, strip breakage occurs when the steel strip passes through a place where bending is applied by a bridle roll, looper roll, etc. in a continuous processing facility There was a problem that there might be.

  The present invention solves the above-mentioned problems of the prior art, and can stably and reliably prevent strip breakage when continuously processing a high Cr-based high carbon steel strip with a continuous processing facility. It aims at proposing the fracture | rupture prevention processing method and continuous processing equipment of a band welding junction.

  In order to achieve the above-mentioned object, the present inventors have earnestly investigated the cause of strip breakage that occurs when the SUJ 2 steel strip, which is a high Cr-based high carbon steel, is continuously processed in the pickling line, which is a continuous processing line. investigated.

  First, the experimental results that are the basis of the present invention conducted by the present inventors will be described.

  Service tail (material: SPHC steel) (steel B) (plate thickness: 2.3 mm, plate) at the tip of SUJ 2 steel strip (steel A) (plate thickness: 2.2 mm, plate width: 830 mm) with the composition shown in Table 1 The tail end of the width (900 mm) was butted, and the butted portion was plate-welded using a laser welding machine while preheating to 250 ° C., and continuously processed in the pickling line shown in FIG. The service tail is different from the continuous processing line that is the subject here, and the line that performs batch processing (in units of one) such as a reversing rolling mill is different from the continuous processing line that is going to pass here. In the case where there is a plan to pass later, the end plate that is welded to the tip and tail ends in advance in the previous pass line (in this case, the pickling line that is a continuous processing line) is reversible. In the case of a line that the rolling mill is scheduled to pass later, neither the leading end nor the trailing end of the machine length from the rolling mill to the winding (unwinding) reel and one or more turns on those reels will not slip. It is necessary only beyond the necessary minimum length. In this way, the intermediate steel strip sandwiched between the service tails at the tip and tail ends can be reversibly rolled at full length, and if the service tail is cut off at the final passing line, it will fall within the range of the full length order thickness. Because it is good, it adheres. In the pickling line used, a bridle roll having a diameter of 1400 mmφ was used.

  However, in the steel strip in which the leading end was welded in this way, the strip was broken at the welded joint by bending with a bridle roll in the pickling line. Then, the Vickers hardness measurement and cross-sectional structure | tissue observation by the optical microscope were performed about the fracture | rupture part vicinity.

  The Vickers hardness was measured by so-called micro Vickers hardness, which is often used when examining the hardness of a very surface layer such as a very small steel ingot. At that time, an indentation was applied with a force of 2.94 N using a weight of 300 g (expressed as HMV 300 g).

  FIG. 6 shows a distribution of Vickers hardness (micro Vickers hardness) in the vicinity of the welded joint (broken portion). The “welded joint” in the present invention includes a melt-solidified portion that has been melted and solidified during welding and a weld heat-affected zone that is not melted but affected by welding heat. The melt-solidified part is an area having an average composition in which each butt part of the steel strip to be joined is melted, mixed and solidified because no welding material is used.

  From Fig. 6, the heat affected zone on the side of the SUJ 2 steel strip shows a high value of 850 at a maximum hardness of HMV300g, despite being preheated at 250 ° C, and is extremely hardened. On the other hand, the heat impact department on the service tail side is 200 at HMV300g. In the melt-solidified part, an intermediate value of both hardnesses is shown. Further, from the observation of the cross-sectional structure in the vicinity of the fracture part, the fracture occurred in the heat-affected zone on the SUJ 2 steel strip side, and the structure in the vicinity of the fracture part was a martensitic structure.

  Based on such experimental results, the present inventors have studied various methods for preventing strip breakage in continuous processing of high Cr high carbon steel strip. As a result, in continuous processing of high-Cr high-carbon steel strips, strip breakage prevention is promoted not by preventing the formation of martensite in the welded joints of plate-welding, but by increasing the toughness of the generated martensite. I thought it was effective.

  Based on the above-mentioned idea, the present inventors further examined a method for increasing the toughness of the generated martensite.

  Laser welder with steel A steel strip shown in Table 1 (plate thickness: 2.2 mm, plate width: 830 mm) at the tail end of service tail (steel B, plate thickness: 2.3 mm, plate width: 900 mm) The plate was welded and joined together to perform continuous (pickling) treatment with the continuous treatment equipment shown in FIG. In plate joint welding, the relationship between the time from the completion of welding and the temperature of the welded joint lowered by natural cooling was measured. The temperature of the welded joint was measured with a thermoviewer and the maximum value within the screen field of view was adopted.

  In addition, after the plate-welding is completed, after natural cooling for various times shown in Table 2, the welded joint is heated to various reheating temperatures shown in Table 2 by the heating means 11 by induction heating, and the temperature is expressed at that temperature. The process (levels 1-6) hold | maintained for the various time shown in 2 was performed, the continuous process was performed with the continuous processing equipment, and the presence or absence of the strip fracture | rupture was investigated.

  The obtained results are shown in Table 2. In addition, specimens were collected from the welded joints of these welded steel strips, and the hardness measurement and the cross-sectional structure were observed.

  Moreover, about the welded joint part processed at the level 4, the hardness distribution of a steel strip longitudinal direction is shown in FIG.

  From Table 2, it was found that the temperature of the welded joint dropped to 400 ° C. or less where martensite is believed to be generated reliably after 30 seconds have passed since the completion of welding. Moreover, in order to prevent strip breakage from Table 2, after cooling the weld joint temperature to 400 ° C or less, which is believed to produce martensite, it is reheated to a reheating temperature of 500 ° C or more. It was found that it was necessary to maintain the temperature for 5 seconds or longer (levels 4, 5, and 6). If any one of these conditions is removed, the strip breaks. Level 1 has a high cooling temperature, level 2 has a low reheating temperature, and level 3 has a short holding time.

  Fig. 2 shows that, in the case of Level 4 (after the completion of welding, the weld joint is naturally cooled to 400 ° C at its maximum temperature, then reheated and heated to 500 ° C, and held at that temperature for 5 seconds). It can be seen that the joint hardness is as low as 500 at HMV300g. Moreover, from the structure observation, the structure of the weld joint (steel A side heat affected zone) was tempered martensite.

From such knowledge, the present inventors continuously treat not only SUJ2 but also high Cr-based high carbon steel strips containing C: 0.8% or more and Cr: 1.0% or more in a continuous treatment facility. In order to prevent strip breakage at the weld joint,
After the welding for plate joining is completed, the welded joint is cooled to a predetermined cooling temperature or lower, and then the welded joint is heated to a reheating temperature higher than the predetermined temperature. Holding the reheating temperature above the predetermined temperature for a predetermined time,
The conclusion that is necessary.

  This is thought to be because strip breakage due to mechanical bending in continuous processing could be prevented because martensite generated by natural cooling after welding was tempered to increase the toughness.

  The reheating temperature mentioned here may be a constant temperature or may vary somewhat. In the latter case, if the temperature is equal to or higher than the predetermined temperature, the temperature is increased, the temperature is decreased, and the temperature is increased or decreased. It means that there may be.

  The present invention has been completed based on the above findings and further studies.

That is, the present invention joins the leading end of a high Cr-based high carbon steel strip defined as SUJ2 to JIS G 4805 containing C: 0.8% or more and Cr: 1.0% or more by mass welding. In addition, when the high Cr-based high carbon steel strip is continuously processed by a continuous processing facility, after the welding is completed, the joint formed by the welding of the high Cr-based high carbon steel strip is 200 ° C. or higher and 400 ° C. or lower. The high temperature of the high Cr-based high carbon steel strip in continuous processing is characterized in that the joint is heated to 500 ° C and maintained at a reheating temperature not lower than Ac ₁ transformation point for not less than 5 seconds and not more than 30 seconds. This is a fracture prevention treatment method.

The present invention also includes a payoff reel, a cutting device for cutting a leading end of a steel strip paid out from the payoff reel, and a leading end of the steel strip cut by the cutting device. A welding device, a process facility for continuously processing the steel strip welded and joined by the welding device, and a reel for winding the processed steel strip, and the steel strip to be treated is passed through the series. In the continuous treatment equipment of the steel strip having a roll that causes the steel strip to bend in the passage of the steel strip, the steel strip is a high Cr-based high carbon steel strip, in the welding apparatus or on the outlet side thereof , A continuous treatment facility for a high Cr-based high carbon steel strip, characterized in that a temperature measuring means for measuring the temperature of a welded joint and a heating means are provided. In the present invention, the heating means is preferably an induction heating device.

  According to the present invention, even if a high Cr-based high carbon steel strip is continuously processed in a continuous processing facility, strip breakage can be prevented, long-term operation stoppage in the continuous processing facility can be suppressed, and productivity can be improved. It has a remarkable industrial effect.

The steel strip targeted by the present invention is a high Cr-based high carbon steel strip defined as SUJ2 in JIS G 4805 containing C: 0.8% or more and Cr: 1.0% or more by mass%. In addition, it is preferable that the plate | board thickness of the steel strip made into object by this invention shall be 1-8 mm normally processed continuously.

  When C is less than 0.8% by mass, martensite generated in the welded joint by plate joining is less likely to become embrittled martensite, and strip breakage is less likely to occur due to mechanical bending with a bridle roll or the like in a continuous processing facility. . The upper limit of the C content is not particularly limited, but is the upper limit of the C content of steel materials defined in JIS G 4401, JIS G 3311, JIS G 4805, etc., which are currently widely used. It is preferable to be about mass%.

  On the other hand, if Cr is less than 1.0% by mass, embrittled martensite is less likely to be generated in the welded joint of plate joint welding, and strip breakage is less likely to occur due to mechanical bending in a continuous processing facility. In the case of a general high carbon (for example, SK4 as defined in JIS G 4401) steel strip having a Cr content of less than 0.3% by mass, the hardness of the heat affected zone after joint welding is Usually less than 500 for HMV300g.

  Moreover, it is not necessary to specifically limit the upper limit of the Cr content. The upper limit of the Cr content is preferably about 1.6% by mass, which is the upper limit of the Cr content of the steel grade specified in JIS G 4805, which is currently widely used, but is not limited to this in the present invention. .

  In addition, the high Cr type | system | group high carbon steel strip made into object by this invention is not specifically limited about the alloy element contained besides the above-mentioned component, and its content.

  The high Cr-based high carbon steel strip (hereinafter also simply referred to as a steel strip) containing the above-described components is preferably continuously processed using, for example, a continuous processing facility suitable for the method of the present invention shown in FIG.

  The continuous processing equipment suitable for the method of the present invention is not limited to the form shown in FIG. 1, and may be other pickling lines, continuous annealing lines, cold rolling lines, and the like. A cutting device 5 for rectangularly cutting the leading end of the steel strip 1 paid out from the reel 20, a welding device 10 for welding and joining the leading ends of the steel strip 1 cut by the cutting device 5, and a welding device 10 A heating means 11 disposed on the outlet side of the steel strip, a process facility 40 for continuously processing the welded steel strip 1, and a take-up reel 30 for winding the treated steel strip 1. For the purpose of applying a predetermined tension to the steel strip 1 and absorbing the speed difference of the steel strip in order to ensure continuity of operation, there are a plurality of bridle rolls 50, A looper roll 60 is provided as appropriate.

  The high Cr type high carbon steel strip 1 paid out from the payoff reel 20 is rectangularly cut at the leading end by a cutting device 5. The cutting device is preferably a shear. The steel strip 1 whose front end is cut into a rectangle is then welded to each other with the welding device 10. In the present invention, the welding apparatus 10 is not particularly limited, but a welding machine using laser or plasma is preferable.

The welded steel strip 1 is then cooled until the temperature (maximum temperature) of the welded joint is below a predetermined cooling temperature. Cooling is allowed to cool (natural cooling). The predetermined cooling temperature refers to a temperature at which martensite is reliably generated in a high Cr type high carbon steel strip. In the case of SUJ2, the temperature is 400 ° C. The cooling temperature is 200 ° C. or higher. When the welded joint is cooled only to a high temperature exceeding the predetermined cooling temperature, the formation of martensite is not sufficient. Is likely to occur.

  The steel strip 1 cooled to a temperature equal to or lower than the predetermined cooling temperature is then reheated by the heating means 11, and the welded joint is held at a reheating temperature equal to or higher than a predetermined temperature for a predetermined time. The The predetermined temperature and the predetermined time in reheating mean a temperature and a time at which the generated martensite is surely changed to tempered martensite and sufficiently toughened in the high Cr steel strip. The heating means 11 is preferably induction heating in order to ensure uniform temperature rise in the width direction of each strip. The heating means 11 is fixedly installed on the outlet side of the welding apparatus 10, and the steel strip is conveyed to the installation position of the heating means 11 and stopped while tracking the weld joint after welding.

  In addition, preferably, unlike the above, the welding apparatus 10 and the heating means 11 are integrated, and after welding is completed, the steel strip is heated without moving from that position.

If the reheating temperature is less than a predetermined temperature or the holding is less than a predetermined time, the martensite cannot be sufficiently toughened. The predetermined temperature at reheating For SUJ2, shall be the 500 ° C.. Incidentally, the reheating temperature shall be the following Ac ₁ transformation point. In addition, the holding time is 5 seconds for SUJ2. In view of the processing efficiency in the continuous processing line, the holding time at a predetermined temperature or higher in reheating is preferably 30 seconds or shorter.

  After the welding is completed, the steel strip 1 subjected to the above-described reheating and holding at the welded joint is sent to a process facility 40 such as a pickling tank through a plurality of bridle rolls 50 and looper rolls 60 for continuous processing. Is done. After the processing, it is wound up on the take-up reel 30 via a plurality of bridle rolls 50.

  Using the continuous treatment equipment (pickling line) shown in Fig. 1, payoff the steel A steel strip (high Cr high carbon steel strip) (sheet thickness: 2.2 mm, sheet width: 830 mm) shown in Table 1. Discharge from reel 20, cut the rectangular tail of the tail end of the steel B service tail shown in the preceding table 1 and the end of the following steel A steel strip with a cutting device 5, and butt the tail ends of these steel strips together Then, welding was performed with a laser welding machine (welding apparatus 10). After welding was completed, it was naturally cooled. During natural cooling, the temperature of the welded joint is continuously measured with a thermoviewer, and when the maximum temperature reaches 400 ° C or less, the welded joint is heated with a gas heating steel strip heating device (heating means 11). Was reheated (temperature rising rate: 0.3 to 0.5 ° C./sec), heated to 500 ° C., held at that temperature for 8 seconds, and then naturally cooled.

  The steel strip 1 subjected to the above-described treatment was then passed through the bridle roll 50 and the looper roll 60 and sent to the process equipment 40, where it was subjected to continuous treatment (pickling). In addition, the case where reheating and holding | maintenance of a welding joint part is not performed after welding completion was made into the conventional method (conventional example).

  FIG. 3 shows monthly changes in the ratio (%) of the number of strip breaks to the total number of treatments. January to April is the case where the conventional method is applied. On the other hand, from May to September is the case where the method of the present invention in which reheating and holding at a predetermined heating temperature is applied after welding is completed. After May, when the method of the present invention was applied, no strip breakage occurred. On the other hand, in April to April when the conventional method was applied, strip breakage occurred frequently, and the ratio of the number of strip breaks reached 4% / month at the maximum.

It is a schematic diagram which shows an example of the continuous processing equipment suitable for implementation of the method of this invention. It is a graph which shows the hardness distribution of the high Cr type high carbon steel strip welded joint to which the method of the present invention is applied. It is a graph which shows the monthly change of strip fracture occurrence ratio. It is a schematic diagram which shows an example of the conventional continuous processing equipment. It is a schematic explanatory drawing which shows typically the principal part (part of a preheating apparatus) of the welding apparatus in the conventional continuous processing equipment. It is a graph which shows the hardness distribution of the high Cr type | system | group high carbon steel strip welded joint which applied the conventional method.

Explanation of symbols

1 Steel strip (metal strip, high Cr high carbon steel strip)
5 Cutting device
10 Welding equipment (welding machine)
11 Heating means
20 Payoff reel (unwinding reel)
30 Take-up reel (tension reel)
40 Process equipment (pickling tank)
50 Bridle roll
60 Looper roll
70 beam
80 coils
90 Backbar
91 Insulation plate
95 Clamp
100 continuous treatment equipment (pickling line)

Claims (2)

By joining the leading end of the high Cr high carbon steel strip specified as SUJ2 to JIS G 4805 containing at least C: 0.8% and Cr: 1.0% in mass% by welding. When the carbon steel strip is continuously processed with a continuous processing facility, after the welding is completed, the joint formed by the welding of the high Cr-based high carbon steel strip is allowed to cool to 200 ° C. or more and 400 ° C. or less, A method for preventing breakage of a high Cr high carbon steel strip in continuous processing, characterized in that the temperature of the joint is raised and maintained at a reheating temperature of 500 ° C. or higher and Ac ₁ transformation point or lower for 5 seconds to 30 seconds. A payoff reel, a cutting device that cuts the leading end of a steel strip paid out from the payoff reel into a rectangle, a welding device that welds and joins the leading ends of steel strips cut by the cutting device, and the welding A process facility for continuously processing steel strips welded and joined by an apparatus, and a take-up reel for winding up the treated steel strips. In a continuous treatment facility for a steel strip having a roll that causes the steel strip to bend in the passage route, the steel strip is a high Cr-based high carbon steel strip, and a welded joint is formed in the welding apparatus or on the outlet side thereof. A continuous processing facility for high-Cr high-carbon steel strip, characterized by a temperature measuring means for measuring the temperature of the steel and a heating means.

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