JP5050537B2 - Thick steel plate cutting method - Google Patents

Description

  The present invention relates to a method for cutting a thick steel plate by a mechanical cutting method such as shear shearing, and in particular, due to residual hydrogen in steel that occurs during online cutting of a high-strength steel plate that is cooled by a cooling device provided on a transfer line. The present invention relates to a material suitable for preventing occurrence of cross-sectional cracks.

  In recent years, thick steel plates are required to have high strength and thickness, and in order to ensure productivity, online processing is required in each process of heating, rolling, cooling, and refining. In the case of thick steel plates for line pipes, since they are produced in large quantities, there is a strong demand for on-line processing, and there is an urgent need to establish technology for stably producing X120 grade steel plates online.

  One of the problems in the mass production process of online high-strength thick steel plates is a cross-sectional crack that occurs during online cutting. Cross-sectional cracks are more likely to occur with higher strength materials, and horizontal cracks and oblique cracks starting from the vicinity of the segregated portion with a plate thickness of ½ part (center part) occur in the cut section.

  In particular, in the case of a thick steel plate for a line pipe, if a deep crack of about 2 to 3 mm occurs at the time of cutting, the crack expands and becomes a serious defect at the time of pipe making, so it is necessary to prevent a cross-sectional crack.

  Cross-sectional cracks are caused by hydrogen embrittlement cracks caused by hydrogen in the steel sheet, and it has been conventionally known that a steel sheet slow cooling process before cutting is effective as a method for preventing cross-sectional cracks.

  As a technique for suppressing hydrogen cracking by slow cooling of a steel plate, for example, Patent Document 1 discloses a steel plate stacking method for performing efficient slow cooling, particularly at the end of a steel plate, and Patent Document 2 discloses the inside of a slow cooling box. A method for improving the slow cooling efficiency by depressurizing the steel plate after placing the steel plate on is disclosed.

Patent Document 3 discloses a method for calculating the optimum annealing time from the residual hydrogen rate by slab annealing and product annealing by obtaining the crack critical hydrogen concentration Cth from the predicted value of the yield stress (YS) of the steel sheet. Has been.
Japanese Patent Laid-Open No. 10-203212 JP 2001-303127 A JP-A-10-251746

  However, the method of performing slow cooling of steel sheets described in Patent Document 1 and Patent Document 2 requires an off-line process in which the steel sheets on the cooling floor are lifted by a crane and stacked in a slow cooling place, and a long processing time is required. There are also concerns about the occurrence of surface flaws.

  Furthermore, in general, the stacked slow cooling does not always provide a sufficient slow cooling effect because the slow cooling temperature and the slow cooling time vary depending on the plate thickness and the cooling end temperature.

  In addition, the method described in Patent Document 3 considers only the effect of slow cooling on hydrogen cracking, does not consider the effect of accelerated cooling of the steel sheet, and is a specific temperature, for example, the temperature at the time of cutting ( The method for predicting the amount of steel sheet hydrogen at 100 to 200 ° C. is not specified.

  For example, α defined by the slab residual hydrogen value (ppm) immediately before insertion of the heating furnace / slab residual hydrogen value (ppm) immediately after casting, the product residual hydrogen value (ppm) immediately before shipment from the factory / immediately before insertion of the heating furnace The value of residual hydrogen rate defined by β defined by the slab residual hydrogen value (ppm) is not given by accurately evaluating the actual hydrogen diffusion phenomenon. It cannot be used as an evaluation method.

  Thus, in order to prevent the occurrence of hydrogen cracking after cutting described in Patent Documents 1 to 3, the method of gradually cooling the steel sheet to remove hydrogen is unstable in actual operation, and X100 grade. It is hard to say that this technology can be applied to thick steel plates for line pipes, which are expected to have higher strength and thinner walls.

  Therefore, the present invention prevents even a high-strength thick steel plate exceeding the X100 grade manufactured by accelerated cooling without slow cooling of the thick steel plate after water cooling during online cutting. It aims to provide a way to do.

The object of the present invention is achieved by the following means.
1. It is a method of cutting a thick steel plate that is cooled after hot rolling, and the cut surface is welded after cutting. The cut surface of the thick steel plate is 35 ° _C./until the temperature T _{s of the} cut surface becomes _{equal to} or higher than the _Ac1 transformation temperature. A method for cutting a thick steel plate, characterized by heating at a sec .
2. After cutting the steel plate cooling after hot rolling, in the cutting method of steel plate heated to above A _c1 transformation temperature cut surfaces at 35 ° C. / sec or more, the distance in a direction perpendicular to the cutting plane α + β (α : cutting plane preset cutting allowance, beta: fusion allowance of the weld metal during welding, however, the maximum temperature T _max of the base material side to be) more inner containing alpha = 0 is cut into 500 ° C. or less A method for cutting a thick steel plate, wherein the surface is heated at 35 ° C./sec or more .
3. Immediately after the heating of the cut surface, characterized by cooling the cutting surface, 1 or 2 thick steel cutting method according.

  According to the present invention, it is possible to effectively suppress cracking in a non-heating type cutting method that is easy to be on-line such as a saw blade cutting method or a shearing method using a shear, and hydrogen removal can be performed before cutting a thick steel plate. Slow cooling is not required.

  As a result, after cooling by the cooling device provided on the thick steel plate conveyance line, it is possible to continuously cut on-line and manufacture a large amount of high-strength thick steel plate with high productivity, which is extremely useful industrially.

In the present invention, the cut surface of the thick steel plate is locally rapidly heated and heated to the _Ac1 transformation temperature or higher, thereby selectively transforming only the processed structure in the vicinity of the cut surface and softening the structure. This is a method for reducing the residual stress. This will be described below.

  First, when the steel plate 1 is mechanically cut along the cutting line 11 (FIG. 1A), a cut surface 2 is generated in the steel plate 1. The vicinity of the cut surface 2 becomes a region 21 subjected to machining strain and deforms (FIG. 1B).

  The reason why only the cut surface is locally heated in the present invention is that, in mechanical cutting of a thick steel plate such as shear shearing, the portion that undergoes processing strain during cutting is limited to the vicinity of the cut surface.

  The part subjected to processing strain is the area that is inserted inward by the clearance of the upper and lower blades (usually about 10% of the plate thickness) when cutting from the cut surface to the base material side, and the area is work hardened and easily cracked In addition, since a high tensile residual stress remains, cracking occurs even with slight embrittlement factors such as segregation and hydrogen in steel. In the present invention, the term “cut surface” refers to a portion including the cut surface and the region.

In the present invention, in order to reduce the residual strain energy accompanying work hardening (that is, dislocation strain) and residual stress, the cut surface is rapidly heated to heat the processed structure to the _Ac1 transformation temperature or higher. That is, residual strain energy is reduced by transformation.

Unlike the method of reducing the residual stress mainly by recovery and recrystallization below the _Ac1 transformation temperature, the material of the cut surface itself changes from the material before rapid heating, but in the case of a thick steel plate, the cut surface is melted by welding. Therefore, the material change in the cut surface is allowed.

  That is, in the very vicinity of the cut surface, since welding is performed at a high temperature together with the welding material by welding after cutting, a heat history can be freely given before welding.

  The reason why the heating of the cut surface is rapid heating is that the base material side that is not melted by welding and that is away from the cut surface may change the material by heating the cut surface. Heating, that is, raising the temperature of only the local region in the vicinity of the cut surface in a short time, and making the heat input to the base material side as small as possible.

  The rapid heating rate may be selected at a rate of at least 28 ° C./sec or more (within 25 seconds if heated to 700 ° C.), for example, so as not to disturb the cutting pitch before heating.

In the present invention, the rapid heating is performed at a distance α + β (α: cutting plane cutting allowance, β: fusion allowance with weld metal at the time of welding, including α = 0) in a direction perpendicular to the cutting plane. The maximum temperature T _max on the base material side is set to be equal to or lower than the minimum temperature _Tr that affects the base material.

  The present invention will be described for a butt weld joint. The cutting allowance for cutting from the cut surface for groove processing (the cutting surface 4 before the groove processing is the origin, the cutting distance from the cutting surface 4 to the groove line 3) is the distance α, the weld metal 6 in the butt weld joint If the distance β is a distance β, the portion within the distance α + β from the cut surface 4 is melted by cutting or welding after the cut surface is heated, so that a thermal history can be given freely at the stage of cutting surface heating. It is possible (Figs. 2 and 3).

On the other hand, the amount of heat input, that is, the amount of temperature rise at a position that exceeds the distance α + β and approaches the base material side increases, and the maximum temperature T _max at the position exceeds the temperature _Tr that affects the base material. When the temperature rises, the base material changes.

Therefore, in the present invention, in order to obtain only the hardness reduction and residual stress reduction effect in the vicinity of the cut surface while avoiding the influence on the base material, the heating rate of the cut surface is controlled so that T _max ≦ T _r. .

The specific heating conditions were determined in advance by experiments to determine the effect of the heating rate when heating the cut surface on the maximum temperature T _max at the position remaining on the base metal side of the cut surface after welding. Select using the results obtained.
For example, α is 5 to 10 mm, β is 10 mm for a 15 mm-thick plate, and _Tr is, for example, recrystallization temperature (around 500 ° C., change depending on internal strain etc.), etc. Determine according to the material. _Tmax can be easily derived, for example, by giving an end face heat input condition and a general one-dimensional unsteady heat conduction equation if only one-dimensional heat diffusion is taken into account because the heat transfer distance is short.

FIG. 5 shows the results obtained when the maximum temperature at the position of 1 mm from the cut surface to the base metal side is variously changed when the heating speed of the cut surface (in this experiment, the burner crater feed rate) v is varied. Thus, the cutting surface temperature rose most at the heating rate of v = 2 mm / sec, and the highest temperature reached about 740 ° C., which is higher than the _Ac1 transformation temperature.

  At the same time, FIG. 6 shows the result of obtaining the maximum temperature reached at a position of 10 mm from the cut surface to the base material side. Compared with the maximum temperature achieved at a position of 1 mm from the cut surface to the base material side, the temperature rise on the base material side increases under the condition of the heating rate v = 2 mm / sec, and the maximum temperature reached 500 ° C. or higher. Yes.

When steel exceeds 500 ° C, the material generally changes due to recovery or recrystallization. Therefore, if 500 ° C is defined as _Tr , the cutting surface is locally and rapidly changed without causing the material change of the base material from Figs. It is understood that when heating to the _Ac1 transformation temperature or higher, v = 2 mm / sec or higher is necessary as a necessary heating rate.

The heating rate in the vicinity of the cut surface is 35 ° C./sec or higher due to the average slope of the heating curve at the heating rate v = 2 mm / sec when heating to the A _c1 transformation temperature or higher in FIG. Become. T _max is a calculated value obtained by estimating the temperature of the cut surface from the heating condition or a value obtained by actual measurement with a radiation thermometer, and is the maximum temperature at the distance α + β.

  FIGS. 7 and 8 schematically show the results of the above-described experiment. The heating rate (temperature increase rate) at the cut surface is increased (FIG. 7), and the maximum reached temperature at the base material inside the cut surface is defined as Tr. The following (FIG. 8) shows the technical idea of the present invention.

  In the above-described experiment, the test piece (18 thickness × 100 width × 25 length (mm)) shown in FIG. 4 was used, and the cut surface 2 by the mechanical cutting method was set to a burner crater feed speed v of 2 to 10 mm of the acetylene gas burner. Heating was performed at a change rate of / sec.

  At the center of the cut surface 2, the temperature history when the cut surface 2 is heated by attaching a K-sheath thermocouple of φ0.5 mm at positions where the vertical distances a are 1 mm (cut surface) and 10 mm (base material side), respectively. Recorded.

  The burner used a crater for cutting thick steel plates. The heating conditions were an oxygen partial pressure of 0.2 MPa, an acetylene gas partial pressure of 0.03 MPa, and a crater height of 10 mm.

  Here, if the distance a = 10 mm is assumed to be a distance corresponding to α + β, the temperature at a position closer to the base material side than 10 mm from the cut surface is an index representing the base material side thermal effect.

  If it is difficult to set the maximum temperature of the base material to the Tr temperature only by adjusting the heating speed of the cut surface, cooling is performed after heating the cut surface.

  For example, when a heating method using the heat generated by the material itself, such as an induction heating device, is used, the heating rate can be increased. However, if the heat generating area is large, the heat input to the base material side also increases simultaneously.

  In addition, when the thickness of the plate to be heated is large, the amount of heat input from the cross section inevitably increases, so the heat input to the base material side also increases.

  In such a case, by sufficiently raising the temperature of the cut surface and then cooling the cut surface by a method such as blast or water cooling, the heat applied to the cut surface is released to the outside and the heat input to the base material side is reduced. It is possible to suppress.

  In addition, although the part which becomes a base material side from distance (beta) is heated below Tr before welding and a welding heat cycle is further superimposed by welding, material deterioration by the said welding heat cycle is not considered in this invention.

  Two samples of 14 mm thickness x 100 mm width x 320 mm length were cut out from a 14 mm thick X100 grade steel plate (Ac1 transformation point: 723 ° C.), and then cut at a clearance d = 1.5 mm using an experimental shearing machine. It was.

  The heating rate was changed for one of the samples after cutting, the cutting surface was heated, and the residual stress change and hydrogen cracking sensitivity in the vicinity of the cutting surface were compared. The other sheet was cut and the residual stress and hydrogen cracking susceptibility near the cut surface were measured.

  For the cutting surface heating, an acetylene gas burner (oxygen partial pressure 0.2 MPa, acetylene gas partial pressure 0.03 MPa, crater height 10 mm) was used. The burner crater feed rate v is 740 ° C., since the portion that undergoes processing strain at the time of cutting is about 1.5 mm in the depth direction from the cut surface, so that the temperature reached at that portion is 440 ° C. and v = 6 mm / sec. Heating was performed at a heating rate of v = 2 mm / sec.

FIG. 9 shows the X-ray residual stress measurement result and the hardness measurement result of the cross-sectional center position of each sample. Σ _{z on the} vertical axis indicates the residual stress in the plate thickness direction, and σ _y indicates the residual stress in the plate width direction. While the sample as-cut has a very large tensile residual stress due to the influence of strong processing, among the test materials subjected to cross-section burner heating after cutting, when heated at a heating rate of v = 2 mm / sec, In the sample that is rapidly heated to the A _c1 transformation temperature or higher, both σ _z and σ _y are significantly reduced as compared with the sample that has been cut. In addition to the reduction in residual stress, the hardness was greatly reduced.
On the other hand, when heated at a heating rate of v = 6 mm / sec, the ultimate temperature is not higher than the _Ac1 transformation temperature as shown in the results of FIG. Is.

  Next, in order to investigate the sensitivity to hydrogen cracking, the sample as-cut and the sample heated by burner after cutting were charged with hydrogen, and then the presence or absence of cross-sectional cracks was observed by color check.

First, from each sample after the X-ray residual stress measurement, the measurement surface, that is, the cut surface is left, and the sample is cut into a size of 14 mm × 100 mm × 30 mm, and the surface other than the cross section is subjected to surface polishing and 0.1 N—H ₂ SO ₄ The cathode hydrogen charge (-1.2V [vs SCE], 24h) was performed in it.

When the cross section was observed after the color check, it was found that the sample as cut had many cracks in the cross section, and it was found that hydrogen cracking was likely to occur even with a small amount of hydrogen, while the heating rate v = 2 mm after cutting. No cracking occurred in the sample that was burner-heated at / sec, and the effect of suppressing the occurrence of hydrogen cracking by rapid heating after cutting was confirmed.
The sample that was burner-heated at a heating rate of v = 6 mm / sec was cracked but was small, indicating that the heating effect was insufficient, including the residual stress measurement results of FIG. Table 1 shows the test results.

  FIG. 10 shows the measurement of X-ray residual stress on a sample cut surface in which a sample was rapidly heated in the same manner as described above using an induction heating device (50 kHz, Max 150 kW) instead of the burner heating, and further water-cooled immediately after heating. It is the result of having performed.

  The test results of the cut samples are also shown. While a large tensile residual stress is generated at the time of cutting, the residual stress is reduced in the sample after induction heating and water cooling, rather, it becomes a slight compressive residual stress due to the temperature history during cooling. It was confirmed that

  Table 2 shows the results of a hydrogen charge experiment. The crack suppression effect was confirmed in the sample which performed induction heating + water cooling. According to the present invention, even when water cooling is performed after rapid heating of the cut surface, a crack suppressing effect can be obtained.

It is a figure which illustrates the cutting operation | work of a thick steel plate, (a) is a cutting position, (b) is a schematic diagram explaining a cut surface. The schematic diagram explaining groove processing. The figure explaining a butt-welding joint. The figure explaining test piece shape. The figure which shows the influence of the heating rate of a cut surface on the highest ultimate temperature in the position which becomes 1 mm from a cut surface to a base material side. The figure which shows the influence of the heating rate of a cut surface on the highest ultimate temperature in the position which becomes 10 mm from a cut surface to a base material side. The figure which shows the area | region (beta). The figure which shows the test result of FIG. 5 typically. The figure which shows the test result of FIG. 6 typically. Example (a figure showing a cross-sectional center position X-ray residual stress measurement result and a hardness measurement result). Example (The figure which shows the X-ray residual stress measurement result at the time of performing water cooling immediately after a heating with an induction heating apparatus).

Explanation of symbols

1, 1a, 1b Steel plate 11 Cutting line 2 Cutting surface 21 Region 3 Groove line 31 Abutting surface (root face)
4 Cut surface 6 Weld metal 7 Weld heat affected zone 8 Test piece

Claims (3)

It is a method of cutting a thick steel plate that is cooled after hot rolling, and the cut surface is welded after cutting. The cut surface of the thick steel plate is 35 ° _C./until the temperature T _{s of the} cut surface becomes _{equal to} or higher than the _Ac1 transformation temperature. A method for cutting a thick steel plate, characterized by heating at a sec . In the method for cutting a thick steel plate, in which the thick steel plate cooled after hot rolling is cut, and then the cut surface is heated to 35 ° C./sec or higher to the _Ac1 transformation temperature or higher , a distance α + β (α : Cutting plane cutting allowance, β: Fusion allowance with weld metal during welding (including α = 0)) Cutting so that the maximum temperature T _max on the inner side of the base metal is 500 ° C. or less characterized by heating the surface at 35 ° C. / sec or more, the steel plate cutting method. Immediately after the heating of the cut surface, characterized by cooling the cutting surface, the cutting method of steel plate according to claim 1 or 2 wherein.

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