JP2011036898A - Method of preventing fracture of welded joint part of steel strip in manufacture of electric resistance welded steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make continuous tube making possible without the fracture of a welded joint even in a steel grade having high sensibility of weld crack, in the manufacturing process of the electric resistance welded steel pipe which is manufactured by performing electric resistance welding of the edge parts of steel sheets facing each other after performing weld joining after unwinding a steel strip by welding and roll-forming it into a tubular shape continuously. <P>SOLUTION: After performing the weld joining of the rear end of the steel strip unwound previously and the nose of the next unwound steel strip, the welded joint part of the steel strip is once transformed into austenite by performing heat treatment before roll forming and next, ductility of the weld joint part of the steel strip is improved by cooling the joint part gradually. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a heat treatment method for suppressing breakage of a welded seam portion of a steel strip welded in a pipe making line in continuous pipe making of an electric resistance welded steel pipe (electrically welded steel pipe).

  As shown in Fig. 1, the method of manufacturing ERW steel pipes is to pass the uncoiled steel strip, form it into a tube with a forming roll, and weld both opposite edges of the formed steel strip by ERW welding. is there. The method of supplying steel strips to the line consists of a method of supplying steel strips one by one (single hoop pipe construction method), connecting the rear end of the preceding steel strip and the tip of the next steel strip by a welded joint by arc welding, It can be divided into two types: continuous material supply method (continuous pipe making method).

  The method of supplying steel strip continuously by welding is also used for cold rolling mills and plating mills, but in rare cases, the weld seam of the steel strip (weld seam and heat affected zone) may break. is there. After the breakage of the strip steel joint part, the processing of the preceding strip steel and the re-passing plate of the next steel strip are required, so that the work efficiency is remarkably reduced, and the quality defective part before and after the joint is lengthened, and the yield is lowered. For this reason, it is important to prevent the rupture of the steel strip joint in the manufacture of a continuous pipe-forming ERW steel pipe.

  The cause of such fracture is the effect of welding heat due to the local heating of the weld seam by arc welding and the cooling effect due to the heat absorption of the steel strip at normal temperature without being affected by the heat in the vicinity of the weld. This is because the part is rapidly cooled to become a brittle structure. This effect is particularly noticeable in materials that are highly susceptible to weld cracking.

  As a measure for preventing breakage of the steel strip joint, a method of improving toughness by carrying out preheating before welding in a continuous rolling mill or the like, and further maintaining the temperature at a constant temperature after welding so that the weld is not rapidly cooled ( (See Patent Document 1), a method of strengthening the quenching transformation of the welded portion by heating with a laser beam after welding (see Patent Document 2), and an induction heating coil in the backing device, and after welding at about 800 ° C. A technique for performing heat treatment (see Patent Document 3) has been proposed.

  However, in ERW steel pipe mills of continuous pipe making method, not only pulling and bending during strip steel transport, but also shear strength is generated when roll forming into a tube, so the toughness required for the welded seam of the strip steel Is more severe than the toughness of the rolled steel strip.

JP-A-5-50276 JP-A-8-276207 Japanese Patent Publication No. 2-17277

  An object of the present invention is to perform continuous heat treatment without breaking the weld seam even in a steel type having high weld cracking susceptibility by performing an appropriate heat treatment on the steel strip joint to obtain a steel strip weld seam having excellent toughness. It is to provide a method for preventing breakage of a steel strip weld seam that enables a pipe.

  In the present invention, the welded steel strip joint is heated to the austenite transformation temperature or higher, and then air-cooled (slow cooling) to ferrite transform the structure of the heat-affected zone, thereby increasing the toughness of the steel strip joint. The basic idea is to improve. The summary is as follows.

(1) In a manufacturing process of an electric resistance welded steel pipe, which is produced by rewinding the steel strip and continuously roll-forming it into a tubular shape and then electrically resistance-welding the edges of the steel plates facing each other, After joining the rear end of the steel strip and the tip of the steel strip to be rewound next, the weld seam of the steel strip is heat-treated before the roll forming to transform it into austenite and then gradually cool to improve the ductility. A method for suppressing breakage of a welded seam portion of a steel strip, characterized by that.
(2) The steel strip is a steel strip having a high cracking sensitivity with a carbon equivalent Ceq calculated by the following formula using the chemical content (% by mass) of the steel strip. (1) The fracture | rupture suppression method of the steel strip welded seam part as described in (1).
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(3) The steel strip welding according to (1) or (2), wherein arc welding is used as a welding joint method of the steel strip, and gas burner heating is used as the heat treatment method of the weld seam. A method for suppressing breakage of a joint portion.

  According to the present invention, even in the case of a steel strip having a high weld cracking susceptibility that is likely to break, the rate of breakage can be significantly reduced by ferrite transformation of the weld seam. Thereby, improvement in yield and work rate can be achieved.

It is explanatory drawing of the outline | summary of the pipe making line based on a continuous pipe making system. It is a figure which shows the change of the structure | tissue of the steel strip heat affected zone by heating temperature.

  The present inventor said that only the steel strip butt portion is rapidly heated and rapidly cooled by arc welding, so that the toughness at the boundary between the weld metal and the steel strip base material portion is deteriorated. Many of the fractures in the steel strip joints of highly sensitive steel grades are caused by cracking due to shear stress generated at the end of the steel plate during steel pipe forming, and the heat affected zone that has become brittle due to heating and quenching during welding. In consideration of the fact that cracks propagate, a method for suppressing fracture was considered.

Therefore, in order to improve the ductility of the boundary between the embrittled weld metal and the steel strip base material and the weld heat affected zone, the seam of the welded steel plate is heat-treated at various temperatures, The steel sheet structure was examined, and the presence or absence of breakage of the seam portion after forming into the pipe was examined.
As a result, when the heating temperature is less than the austenite transformation temperature, the steel sheet structure after the treatment is the same structure as that of the steel sheet without being heat-treated while being welded. However, it was not reduced compared to the steel sheet without heat treatment. On the other hand, when the heating temperature was higher than the austenite transformation temperature, it became a structure mainly composed of ferrite, and no fracture was observed when it was formed into a tube.

In FIG. 2, an example of the structure | tissue after the heat processing of the steel plate used in the below-mentioned Example is shown.
When the heating temperature is 600 ° C., which is lower than the austenite transformation temperature, it is a bainite-based structure as shown in FIG. 2 (b), and is the same structure as the as-welded structure shown in FIG. 2 (a). When the heat treatment is performed at 750 ° C. which is a temperature higher than the austenite transformation temperature, it can be seen that the structure is mainly composed of ferrite as shown in FIG.

  From the above, before roll forming the weld seam of the steel strip (ie, weld seam and weld heat affected zone) to a pipe, the weld seam is once transformed into austenite by heating to a temperature above the austenite transformation temperature. Then, by gradually cooling and making the cooled structure a ferrite-based structure, the ductility of the weld seam can be improved, thereby suppressing the occurrence of breakage when roll-forming the welded band steel I understood that I could do it.

Next, the case where the present invention is applied to a continuous pipe making type electric resistance welded pipe production line will be further described as an example.
As shown in FIG. 1, the equipment layout of the production line includes an uncoiler for discharging the steel strip wound around the coil, a coil welding machine for cutting and seam welding the end of the discharged steel strip, and a seam welded portion. The heat treatment apparatus, the looper, the molding machine, the electric seam welding machine, and the shaping machine are configured in this order.

The looper has a function of accumulating a buffer for the line stop due to the heat treatment in order to prevent the line stop due to the heat treatment of the joint weld.
As for the installation position of the heat treatment device, it is necessary to heat-treat before the steel strip is formed, so it is necessary to be in front of the molding machine, and it is also possible to break the seam weld due to bending or tension in the looper. Installed in front of.

The steel strip wound on the coil is first discharged from the uncoiler, and then the rear end of the previously used steel strip and the front end of the steel strip to be used next are excised by shear cutting with a coil welder. Then, the end faces of the steel strips face each other to make a butt, and seam welding is performed by arc welding.
The used steel strip is selected from a range of 360 mm to 1300 mm in width and 2 mm to 19 mm in thickness depending on the outer diameter and thickness of the steel pipe.
Welding of steel strips is performed by one-layer welding by DC arc welding for steel strips with a thickness of less than 8 mm, and thick strip steel is welded by DC + AC two-layer arc welding for plate thicknesses of 8 mm or more. The cycle time is shortened.

After the welded steel strip is automatically determined whether or not heat treatment is necessary based on the Ceq value of the steel strip represented by the above formula, the steel strip that is determined to require heat treatment is transported to the position of the heat treatment apparatus. Once the conveyance is stopped, the heat treatment is started in a state where the conveyance is stopped. The steel strip that is determined not to require heat treatment passes through the heat treatment apparatus without stopping.
In addition, in the steel strip having a Ceq value of less than 0.41, there is no problem with fracture of the weld seam.

The heating method of the heat treatment is a COG gas burner method, and the burners are arranged at a pitch of 100 mm from the center to the left and right in the width direction of the steel strip, and arranged at positions 50 mm apart on both sides of the steel strip, and the full width of the steel strip. Can be heated without a temperature difference between the upper and lower surfaces.
The heating range of the strip steel by this burner is the range of the weld seam consisting of the weld seam and the weld heat affected zone, and depends on the plate thickness of the strip and the heat input of the weld, but ± 15 mm or less from the center of the weld seam It is a range.

The heating temperature of the heat treatment is measured by a radiation thermometer arranged in three places in the width direction (center, ± 150 mm from the center), and after confirming that the heating is performed at the austenite transformation temperature or more, the conveyance of the strip steel is started. . Note that no heating and holding time is provided, and cooling after heating is slow cooling by air cooling. As a result, the structure after cooling becomes mainly ferrite.
In addition, since ferrite transformation is performed by gradual cooling from the austenite transformation temperature or higher, the heating temperature is not particularly high even if heated to a very high temperature, so it is sufficient in the range of austenite transformation temperature + 100 ° C. or less. It is.

  After that, according to the usual method, it is formed into a tubular shape by a forming roll, and both opposite edges of the formed steel strip are welded to a pipe by a high-frequency electric seam welder, shaped into a predetermined meter, and then cut between the runs Machined to product length.

Ceq0.35, Ceq0.41 and Ceq0.46, a steel strip having a plate thickness of 12.4 mm and a plate width of 680 mm is prepared, the ends of the steel strips are butted together and arc welded with a coil welder, Heating was performed at 600 ° C., which is lower than the austenite transformation temperature, and 750 ° C., which is higher than the austenite transformation temperature, using a COG gas burner of a heat treatment apparatus. After heating, it was cooled by slow air cooling. In addition, about the steel strip of Ceq0.35, heat processing was not performed.
The chemical composition of the steel used was Ceq 0.41 steel: C 0.16%, Si 0.20%, Mn 1.45%, V 0.05%, Ceq 0.46 steel: C 0.20%, Si 0.20%, Mn 1. Ceq 0.35 steel used as an example without heat treatment: 50%, V 0.05%: C 0.1%, Si 0.20%, Mn 1.45%, V 0.025%.

The steel plate structure of the welded seam after the heat treatment of the steel strip was examined, and the presence or absence of fracture of the welded seam after forming the steel strip into a pipe was examined.
The results are shown in Table 1, and a structural photograph of Ceq0.46 steel is shown in FIG.
At the heating temperature of 600 ° C., the Ceq 0.41 steel and the Ceq 0.46 steel both broke. The structure at that time was also a structure mainly composed of bainite similar to the structure as it was welded without heating as shown in FIG. 2 (a), as shown in FIG. 2 (b).
On the other hand, at a heating temperature of 750 ° C., as shown in FIG. 2 (c), the structure was mainly composed of ferrite, and no breakage occurred during the steel pipe forming.

Claims (3)

  In the manufacturing process of ERW welded steel pipe, which is produced by rewinding the steel strip and continuously rolling it into a tubular shape, then electrically resistance welding the edges of the steel plates facing each other, the rear end of the steel strip that has been rewound first And the end of the steel strip to be rewound next, and the weld seam of the steel strip is heat-treated before the roll forming to transform it into austenite and then gradually cool to improve ductility. A method for suppressing breakage of a welded seam portion of a steel strip. The steel strip is a steel strip having a high cracking sensitivity with a carbon equivalent Ceq calculated by the following formula using the chemical content (mass%) of the steel strip: 0.41 or more. Item 2. The method for suppressing breakage of a steel strip welded seam according to Item 1.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14   The band steel weld seam according to claim 1 or 2, wherein arc welding is used as a welding joint method of the steel strip, and gas burner heating is used as a heat treatment method of the weld seam portion. Breaking suppression method.