JP4815913B2 - Method for preventing shear cracking of thick steel plates - Google Patents

Description

  The present invention relates to a method for preventing shear cracking of a thick steel plate caused by residual hydrogen in steel, and more particularly to a cooling device provided on a transfer line and suitable for a steel plate for high-strength line pipe that performs cooling.

  Thick steel plates for line pipes are an important product that plays a part in profits in the field of thick plates. They are characterized by mass production and are becoming stronger every year. In mass production, establishment of technology aimed at stable production of X70 to X100 grades in the future is required.

  One of the problems in the mass production process of high-strength line pipe materials is cross-sectional cracks that occur during online shearing. Cross-sectional cracks are more likely to occur with higher strength materials, and horizontal cracks and oblique cracks starting from the vicinity of the segregation part with a plate thickness of ½ part (center part) occur in the cross-section after shearing. In particular, when a deep crack of about 2 to 3 mm is generated at the time of shearing, the crack is enlarged at the time of pipe making and becomes a serious defect, and therefore sufficient consideration is required for its suppression.

  It is conventionally known that the steel sheet slow cooling process before shearing is effective as a method for preventing this shear cracking, and it is assumed that the shear cracking is caused by hydrogen embrittlement cracking caused by hydrogen in the steel sheet. .

  Especially in recent years, the manufacturing method using HCR and TMCP has been the mainstream in recent years, and hydrogen in steel tends to be cooled without being sufficiently removed and transported to a shear line in a hydrogen supersaturated state. The environment is prone to cracking.

  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 of improving the cooling efficiency by reducing the pressure after placing the steel plate on is disclosed.

Patent Document 3 discloses a method of calculating the optimum slow cooling time from the residual hydrogen rate by slab slow cooling and product slow cooling by determining the crack critical hydrogen concentration C _th based on the yield stress (YS) prediction value of the steel sheet. Is disclosed.

The above-described method of gradually cooling a steel sheet to remove hydrogen is also implemented in an actual steel sheet manufacturing process as a method for suppressing hydrogen cracking, and is considered effective for suppressing hydrogen cracking after shearing.
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, and the effect of accelerated cooling of the steel sheet and the amount of hydrogen in the steel sheet at a specific temperature, for example, the temperature during shearing (100 to 200 ° C.). It does not specify how to predict.

  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 residual hydrogen rate specified by β defined by the slab residual hydrogen value (ppm) is not given in the form of an accurate evaluation of the actual hydrogen diffusion phenomenon. It cannot be used as a method of evaluating

  Thus, in order to prevent the occurrence of hydrogen cracking after shearing, the method of slowly cooling the steel sheet to remove hydrogen is unstable for the actual operation, and for line pipes where higher strength is expected. It is difficult to say that this technology can be applied to steel, and the productivity is greatly reduced.

  Then, an object of this invention is to provide the method of preventing the shear crack resulting from the hydrogen in steel of the steel for X70-X100 grade high strength line pipe, without reducing productivity.

The object of the present invention is achieved by the following means.
1. In a method of preventing shear cracking by shearing the thick steel plate that has been water-cooled after hot rolling with a shearing machine and further cutting the end after shearing,
The hydrogen concentration C _H in the thick steel plate at the time of shearing is obtained, and when the C _H is 2 ppm or more, the amount of clearance between the upper blade and the lower blade of the shearing machine is cut and removed from the end after the shearing. A method for preventing shear cracking of thick steel plates.

2. Based on the hydrogen concentration C _H0 at the end of rolling predicted from the hydrogen ladle analysis value at the time of manufacturing the slab of the thick steel plate, or the hydrogen ladle analysis value, the hydrogen concentration C _H in the thick steel plate at the time of shearing, 2. The method for preventing shear cracking of a thick steel plate according to 1, wherein the method is obtained by calculation.

  According to the present invention, after shearing, a steel plate that is prone to hydrogen cracking and a steel plate that is unlikely to occur are separated in advance before shearing, and only the steel plate that is prone to hydrogen cracking is cut after shearing. It is possible to perform on-line shearing of high-strength materials while suppressing the above. As a result, it is not necessary to perform a low-efficiency cutting method such as gas cutting, and it is possible to flexibly handle a large amount of orders for high-strength materials, which is extremely useful in the industry.

The present invention predicts the amount of hydrogen in steel during shearing (first step), and when there is a concern about the occurrence of hydrogen cracking, it is sheared in order to reduce residual stress on the shear plane that causes tensile stress. It is characterized by cutting and removing a part of the end (second step).
(First step)
It is possible to obtain the hydrogen concentration at the slab stage from the hydrogen ladle analysis value before slab casting, and when the slab size, slab heating temperature, rolling pass schedule, etc. are clear, the hydrogen concentration in the thick steel plate at the end of rolling Can be obtained in advance in association with the ladle analysis value.
Hydrogen concentration in steel at the slab stage or at the end of rolling (hereinafter referred to as initial hydrogen concentration: C _H0 ) and hydrogen concentration in the thick steel plate at the time of cooling and shearing after completion of rolling (hereinafter referred to as hydrogen concentration at shearing: C The relationship of ( _H ) is obtained in advance by calculation using a diffusion model or by an experimental method using a small sample.

For example, the initial hydrogen concentration is C _H0, and the hydrogen concentration C _H in the thick steel plate at the time of shearing is obtained by diffusion model calculation using the temperature history record in subsequent rolling, water cooling, and air cooling. Here, the time of shearing indicates a time point immediately before the start of shearing, and when the time point of rolling end is time 0, it is given from the actual value or predicted value of the time from the end of rolling to the start of shearing.

Next, there is a correlation between the steel sheet material and the shear cracking limit hydrogen amount, and generally there is a tendency that hydrogen cracking is more likely to occur with higher strength materials, so the allowable value C _limit of shearing hydrogen concentration according to the desired material. Is determined in advance, and compared with the hydrogen concentration C _H during shearing determined by the above calculation, the necessity of processing after shearing is determined.

When C _H ≧ C _limit , there is a concern that hydrogen cracking may occur after shearing. After shearing, treatment for preventing hydrogen cracking is performed. When C _H <C _limit , hydrogen cracking may occur after shearing. However, the treatment for preventing hydrogen cracking is not performed.

  This will be specifically described below. Hydrogen diffusion is expressed by a one-dimensional unsteady diffusion equation expressed by the following equation (1).

Here, [x: plate thickness direction (mm) (plate thickness 2l, x = 0... Plate thickness center, | x | = l ... front and back surfaces), C (x, t): hydrogen concentration (mass ppm), D: diffusion coefficient (m ² / sec), t: time (sec)], the initial condition at time t = 0 is the following equation 2, and the boundary condition is the following equation 3, the solution of the above equation 1. The following equation 4 is obtained. In Equation 4, τ is given by Equation 5.

  Although various literature values have been reported for the hydrogen diffusion coefficient D, it is assumed here that the following equation 6 is obtained.

Next, the initial hydrogen concentration C _H0 is defined as the hydrogen average concentration according to the following equation 7 when the steel plate average temperature T is 730 ° C. (time t = 0).

And C _H0 is given in advance in correlation with the hydrogen ladle value, for _example, a C H0 = 3 (mass ppm) .

When performing the calculation of Equation 5, the temperature T of Equation 6 needs to be given as a function of time t. As an example of the temperature pattern of high-strength material, on the premise of strong cooling during controlled cooling,
“Temperature” Cooling start temperature: 730 ° C. Cooling stop temperature: 200 ° C. Shear temperature: 100 ° C.
“Cooling speed” When given as 730 ° C. to 200 ° C .: 20 ° C./sec 200 ° C. to 100 ° C .: 0.2 ° C./sec (air cooling), the relationship between temperature T and time t is 0 <t ≦ 26.5 In the case of the following equation 8, 26.5 <t ≦ 526.5, the following equation 9 is applied. Substituting the obtained relationship into Equation 6 and calculating Equations 5 and 4.

  FIG. 5 shows the initial hydrogen concentration distribution (t = 0) and the sheared hydrogen concentration distribution (t = 526.5) obtained by the above-described method for a thick steel plate (X100 grade material) having a plate thickness of 25 mm.

  From the figure, the decrease in the amount of hydrogen from the end of rolling (t = 0) to the time of shearing (t = 526.5) is small. This is because, in the case of a high-strength steel sheet, the cooling stop temperature is low and a sufficient hydrogen diffusion time cannot be secured.

The allowable value C _limit of the hydrogen concentration during shearing varies depending on the material. For example, when C _limit = 2 ppm, the hydrogen concentration at the plate thickness can be calculated from the distribution of hydrogen concentration during shearing (t = 526.5) in FIG. Since it is determined that C _H = 2.8 ppm and C _H ≧ C _limit , a treatment for preventing hydrogen cracking is performed after shearing.

In the above description, with hydrogen concentration in the thickness average as the hydrogen concentration, the C _H and C _limit, using the hydrogen concentration in the thickness center portion comprising the highest hydrogen concentration in the steel plate C (0, t) May be.

(Second step)
In the case of C _H ≧ C _limit , as a process for preventing hydrogen cracking after shearing, a clearance greater than or equal to the clearance between the upper and lower blades of the shearing machine is cut and removed from the end portion caused by the cutting of the shearing machine.

  That is, after cutting the thick steel plate to a desired size with a shearing machine, the end portion after the cutting is at least a region where strain during shearing remains, the clearance d between the upper blade and lower blade during shearing (normally 10 mm of the plate thickness). A new cut is made so as to remove the inner region.

  The target thick steel plate here has a general thickness of about 6 mm to 40 mm, although it depends on its strength and equipment specifications.

From the shear surface strain distribution by FEM analysis, the strain remaining in the region to which strong processing is applied during shearing causes the generation of residual stress that promotes hydrogen cracking. The area corresponding to the clearance d between the upper and lower blades (usually around 10% of the plate thickness) is newly cut and removed from the end after shearing to reduce residual stress that promotes the opening of cracks caused by hydrogen To do.
Since the second step can be performed by cutting with a shearing machine or cutting with an edge mirror or the like, the cutting includes cutting.

  FIG. 6 is a photograph of a cross section taken from the side after shearing and cutting the end after shearing when a laboratory shearing experiment was performed using a test piece taken from an actual high-strength line pipe material. (The photographing position is shown in FIG. 7. The horizontal stripe pattern in the photograph is a saw cutting mark before shearing).

  In the shear test, a rectangular test piece of 13.4 mm (plate thickness) × 100 mm (width) × 320 mm (long) of material X100 was used as a laboratory shear (clearance d = 1. After the shearing, the end portion including the shearing surface (cut surface) was cut and removed by the same amount as the clearance: 1.5 mm after the shearing.

  FIG. 6A shows the state of shearing, and FIG. 6B shows the state in which the strain remaining portion at the time of shearing is cut and removed after shearing, and the strain state of the cross section can be visually confirmed from the distortion of the saw cutting trace in the vicinity of the shear surface.

  From FIG. 6, while the strain remains in the vicinity of the cross section of the test piece as it is sheared (FIG. 6A), after the shear, the test piece cut by the same amount as the clearance: 1.5 mm is the original. The distortion portion is completely removed (FIG. 6B).

  FIG. 3 is a cross-sectional view of a test piece that has been sheared (indicated as a normal blade in the figure) and a test piece that has been sheared and removed from the end by 1.5 mm, which is the same amount as the clearance between the upper and lower blades during shearing. The X-ray residual stress measurement at the center position is performed, and the results of arranging the residual stress measurement values into isotropic tensile stress and equivalent stress are shown.

The test piece obtained by cutting and removing the same amount as the clearance between the upper blade and the lower blade at the time of shearing is smaller in both the equivalent stress σ _eq and the isotropic tensile stress σ _s than the unsheared test piece. In particular, it is considered that a significant reduction in the isotropic tensile stress σ _s effectively acts to suppress the occurrence of cracks.

  FIG. 4 shows the results of the hydrogen cracking test in the case of the shearing state and after the shearing, when the clearance amount between the upper blade and the lower blade during shearing is cut and removed.

The test was performed using a test piece according to the above-described residual stress measurement test. First, a strip-shaped sample (13.4 mm × 100 mm × 30 mm) was cut out with the sheared cross-sectional portion and the cross-sectional portion cut after the shearing and the end portion after shearing remaining, and the cross-section of the obtained strip-shaped sample other than parts as hydrogen cracking test specimens with surface polishing, performed cathode hydrogen charging (-1.0 V _[VS SCE]) in _0.2N-H 2 SO _4, was observed the presence of hydrogen cracking after charging. The amount of charged hydrogen was measured using a glycerol substitution method.

  From the figure, cracking is observed in a short charge time of about 2 hours when shearing, and hydrogen cracking is likely to occur even with a small amount of hydrogen. On the other hand, after shearing, the clearance between the upper and lower blades during shearing is reduced. In the case of cutting and removing, cracks did not occur even after long-time (72 h) charging, and the effect of suppressing hydrogen cracking after shearing was confirmed.

  A specific example of the second step is shown in FIG. FIG. 1 shows a shear surface cutting method assuming a shearing machine in an actual machine. In the figure, 1 is a steel plate, 2 is an upper shearing blade, 3 is a lower shearing blade, and 4 is a steel plate presser base constituting a part of the shearing machine. Indicates.

After sheared first stroke the steel plate 1 a clearance of the upper and lower blades at the time of shearing the upper shear blade 2 and the lower shearing blades 3 as d ₁ (FIG. 1 (a)), the cutting amount d _2, The position of the thick steel plate is corrected so that the clearance d1 between the upper blade and the lower blade during shearing is greater than or equal to ₁ (FIG. 1 (b)), and cutting is performed with the upper shear blade 2 as the second stroke (FIG. 1 (c)). . Further, the position of the cutting blade is adjusted so that the clearance d ₁ ′ between the upper blade and the lower blade at the time of shearing in the second stroke is as small as possible.

  In addition, in implementation of a 2nd process, it is good also as two units | sets, the shearing machine which cut | disconnects a thick steel plate to the desired dimension, and the shearing machine which cut | disconnects the edge part after a cutting | disconnection.

FIG. 2 shows another embodiment, in which 21 denotes a notch, 22 denotes a first-stage blade, and 23 denotes a second-stage blade (the other symbols are the same as those in FIG. 1). The shearing machine has an upper blade (stepped shearing blade) 2 having two stages in the shearing direction and having a notch portion 21 therebetween, and after shearing with the first stage blade 22, the second stage Further, the edge portion is cut by an amount equal to the clearance d ₁ between the upper blade 2 and the lower blade 3 during shearing.

  When the shearing machine having the above-described configuration is used, after the first shearing, the position of the thick steel plate is adjusted (when there is one shearing machine) or the thick steel plate is transported (when there are two shearing machines). Is preferable, and productivity is further improved.

  The second step can be performed by a method of cutting the shear plane by a predetermined amount using an edge mirror or the like other than the method described above with reference to FIGS. In addition, when a predetermined amount is cut and removed using a shearing blade after shearing, the main shear deformation region moves to the shavings side and the deformation on the material side is small, and as a result, the newly introduced strain amount is There is little residual stress that promotes hydrogen cracking.

  In the practice of the present invention, it is desirable to arrange a shearing machine together with a cooling device and a heating device on a thick steel plate conveyance line, and to perform shearing and subsequent cutting with the shearing machine, thereby improving productivity. These arrangements are appropriately selected according to the desired material.

  Moreover, even if it is determined that the possibility of hydrogen cracking occurring in the first step is low, the present invention does not hinder the implementation of the second step.

The figure which shows one Example of this invention. The figure which shows the other Example of this invention. The figure which shows the residual stress of what cut | disconnected and removed the same amount as the clearance of the upper blade and the lower blade at the time of shearing after shearing. The figure which shows the hydrogen-cracking test result of what cut | disconnected and removed the clearance amount of the upper blade and lower blade at the time of a shearing after shearing with shearing. The figure which shows the time change of the hydrogen concentration in steel. FIG. 4 is a cross-sectional view perpendicular to the shearing direction after shearing, in which (a) remains sheared and (b) shows a state in which the strain remaining portion at the time of shearing is removed after shearing. The figure explaining the imaging | photography position of the cross section after cut | disconnecting the edge part after shearing after shearing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Upper shear blade 3 Lower shear blade 4 Steel plate presser base 21 Notch part 22 First stage blade 23 Second stage blade

Claims (2)

In a method for preventing shear cracking by shearing a steel plate that has been water-cooled after hot rolling with a shearing machine and then cutting the end portion after shearing, the hydrogen concentration C _{H in the} steel plate during shearing is obtained, and C _H In the case of 2 ppm or more, a shear crack prevention method for a thick steel plate, characterized by cutting and removing a clearance amount or more between the upper blade and the lower blade of the shearing machine from the end after the shearing. Based on the hydrogen concentration C _H0 at the end of rolling predicted from the hydrogen ladle analysis value at the time of manufacturing the slab of the thick steel plate, or the hydrogen ladle analysis value, the hydrogen concentration C _H in the thick steel plate at the time of shearing, The method for preventing shear cracking of a thick steel plate according to claim 1, wherein the method is obtained by calculation.