JP5716376B2 - Method for determining surface cracks in slabs in continuous casting - Google Patents

Description

  The present invention proposes an effective method for determining a surface crack of a slab caused by a stress state applied to a slab and a ductility decrease at a high temperature in a straightening band (upper and lower) of a continuous casting machine.

  Transverse cracks (hereinafter referred to as “surface cracks”) generated on the surface of continuous cast slabs are one of the typical defects of continuous cast slabs and contribute to the deterioration of steel sheet quality. Surface cracks must be prevented. As for the cause of the surface cracking of the slab, generally, when the slab surface temperature in the straightening zone of the continuous casting machine is in the ductility lowering temperature (hereinafter referred to as “embrittlement temperature”) region of the cast slab. , Is considered to occur.

  Regarding a method for preventing the occurrence of surface cracks in a continuous cast slab, Patent Document 1 discloses that the surface temperature distribution of a slab in a correction area where a large deformation is applied to the slab is controlled by adjusting the amount of cooling water in the secondary cooling zone. And the method of preventing a surface crack is disclosed by guide | inducing the slab surface temperature in this area | region outside an embrittlement temperature range.

Patent Document 2 discloses a continuous cast slab in which cooling is stopped in a temperature range where the transformation of the austenite phase below Ar ₃ points is not completed, and then the surface temperature of the slab is reheated to 950 to 1200 ° C. By reducing the straightening strain applied to the cast slab from the limit strain of surface cracking determined according to the thickness of the slab, the occurrence of slab surface cracks is prevented. A continuous casting method is disclosed.

  Patent Document 3 discloses a method for predicting the surface crack of a slab by measuring the temperature distribution in the width direction of the slab surface in the secondary cooling zone of a continuous casting machine, When the surface thermal stress of the steel to be determined exceeds the critical stress of the surface crack and at the same time there is a temperature range where the hot ductility decreases in the height difference (Yamatani difference) section, surface cracks occur in the slab. A crack prediction method is disclosed.

JP-A-6-246411 Japanese Patent No. 3239808 JP 2009-50913 A

  However, methods such as those disclosed in Patent Documents 1 and 2 in which the slab surface temperature is reheated to the ductile temperature range have a problem that the efficiency of continuous casting is remarkably lowered, and are not suitable for improving productivity. It is.

  Moreover, in patent document 3, although the magnitude | size of the temperature difference of the high and low mountain valley area of the width direction surface temperature of the slab in an orthodontic belt and the time when an embrittlement temperature is included in the mountain valley area, surface cracks are determined. Other than temperature deviations such as the surface temperature distribution in the width direction of the slab, for example, surface cracks resulting from temperature deviation in the thickness direction of the slab are not considered.

  The object of the present invention is not only the temperature deviation of the slab surface until reaching the straightening zone of the continuous casting machine, but also the thickness direction (cross-section) temperature distribution of the slab and its temperature distribution in the straightening zone. Another object is to propose a method for determining surface cracks in a continuous cast slab in which the occurrence of surface cracks is estimated in consideration of the state of thermal stress caused by the above.

As a result of earnestly researching how to solve the above-mentioned problems of the prior art and achieving the above object, the inventors perform continuous casting of steel by a continuous casting machine having at least a lower correction band. In this case, the linear expansion coefficient ratio αt / αs between the linear expansion coefficient αs of the continuous cast slab surface in the lower straightening zone and the linear expansion coefficient αt at the average temperature in the cross section direction of the slab is the critical linear expansion coefficient ratio αt / A method for determining cracks on the slab surface in continuous casting has been developed, characterized in that it is determined that transverse cracks have occurred on the slab surface when αs (cri) is exceeded.

In the present invention, the critical linear expansion ratio αt / αs (cri) H is applied to the slab when cracks start to occur on the surface of the continuous cast slab in the lower straightening zone of the continuous casting machine. The linear expansion coefficient ratio corresponding to the value at which the critical stress ratio σm / σo (cri) indicated by the ratio between the hydrostatic stress σm and the yield stress σo at that time is a minimum value, It is preferable to estimate the occurrence of surface cracks in the slab depending on whether or not the linear expansion coefficient ratio αt / αs exceeds the limit linear expansion coefficient ratio αt / αs (cri).

  Furthermore, in the present invention, the continuous cast slab is preferably treated with steel containing 0.01 to 36 mass% Ni.

  According to the present invention having the above-described configuration, a line calculated based on a stress ratio obtained from a critical stress ratio (estimated value of slab surface and cross-section average temperature) generated in a continuous cast slab in a straightening band. By comparing the expansion coefficients αt and σs with the predetermined critical linear expansion coefficient, it is a method for determining the surface crack of the slab, so that it takes into account the stress state of the cross section as well as the surface of the slab, Generation | occurrence | production of the surface crack of a slab can be determined correctly.

It is sectional drawing of a continuous casting machine. It is a figure which shows the relationship between the depth of the surface crack which generate | occur | produced in the slab surface, and the critical stress ratio at that time. It is a figure which shows the relationship between the thermal expansion coefficient of Ni containing steel slab, and slab temperature. It is a figure which shows the relationship between the linear expansion coefficient ratio (alpha) t / (alpha) s calculated from the stress ratio of the slab surface, and the temperature distribution of slab. It is a figure which shows the relationship between the critical stress ratio of the test piece surface obtained by the hot three-point bending test, and the test piece (cast piece) temperature, and the broken line has shown the approximate line of the criteria of a surface crack. It is a figure which shows the width direction temperature distribution of the slab surface in a lower correction belt. The solid line indicates the temperature distribution of the continuous cast slab with the slow cooling pattern and the broken line indicates the strong cooling pattern.

Details of the configuration of the present invention will be described below.
FIG. 1 shows a cross-sectional view of a continuous casting machine suitable for applying the method of the present invention. In this figure, the code | symbol 1 shows a continuous casting machine. In the continuous casting machine 1, the ladle 2 in molten steel through the tundish 3, by being a pouring the continuous casting mold 4 cooling, produces a solidified shell. The solidified shell exits the mold 4 and is supported by the slab support roll 5 while being cooled by the secondary cooling zone (secondary cooling device 6), and continuously pulled out while gradually increasing the thickness of the solidified shell. . Meanwhile, in the vertical bending type continuous casting machine as shown in the drawing, the slab 10 formed by thickening the solidified shell is gradually bent from the vertical direction in the upper correction band 7 (bending band), while the lower correction band. It reaches 8 (bending correction belt) and is corrected from the curved state to the horizontal state. The vertical bending type continuous casting machine shown in the figure has few

  According to the experiences of the inventors, generally, in a continuous cast slab of steel containing Ni (hereinafter referred to as “Ni-containing steel”), surface cracks frequently occur in the lower straightening zone 8 of the continuous casting machine 1. I know that. As the cause of this surface crack, in the lower straightening band 8, particularly when a hot ductility decreased portion is present on the surface of the slab, tensile stress or strain is applied to the portion, and it appears as a surface crack. It is considered. Moreover, Ni-containing steel is a steel type in which Ni is easily segregated at grain boundaries, and the grain boundaries become extremely brittle due to grain boundary oxidation or precipitation of inclusions such as AlN from the segregated portion, and surface cracks are likely to occur. .

  Further, according to the knowledge of the inventors, when the secondary cooling pattern during continuous casting is strong cooling, many surface cracks of the continuous cast slab occurred. This is because the slab surface temperature temporarily decreases greatly due to strong cooling, and the temperature difference from the inside of the slab increases, so the thermal stress due to the difference in thermal expansion coefficient generated on the surface of the slab increases. It is considered that this fact and the influence of the above-mentioned embrittled grain boundaries are in a state where surface cracks are likely to occur.

  Such slab surface cracking is caused by, for example, 0.01 mass% Ni steel, which is a large heat input material, 9 mass% Ni steel, which is an LNG tank material, or 36 mass% Ni steel. It is often found in steel types containing 36 mass%. Table 1 below shows typical chemical components of Ni-containing steel types that are prone to surface cracking.

  By the way, according to the research of the inventors, the surface cracking phenomenon of the slab often seen in Ni-containing steel can be shown by the relationship between the hydrostatic stress σm on the slab surface and the yield stress σo at that temperature. I found out. That is, it can be seen that the surface cracking phenomenon of the slab can be expressed by the ratio σm / σo (hereinafter referred to as “stress ratio”) of these two stresses on the slab surface.

  FIG. 2 shows the relationship between the surface crack depth generated on the surface of the test piece during the hot three-point bending test and the stress ratio of the slab surface. In this figure, the intersection of the approximate line indicated by the solid line and the broken line and the horizontal axis is the critical stress ratio. As can be seen from this figure, the surface crack occurs when the stress ratio σm / σo exceeds a certain value (in this case, the stress ratio: 1.8 to 1.9). The limit at which such cracking occurs is defined as a critical stress ratio σm / σo (cri).

The inventors also investigated the temperature distribution, thermal stress, and strain state of the cast slab in the continuous casting machine. For this purpose, first, heat transfer / solidification calculation and thermal stress analysis of the slab were performed. As a result, the ratio αt / αs between the linear expansion coefficient αt at the average temperature in the cross-section direction of the slab and the linear expansion coefficient αs of the slab surface, and the stress ratio σm / σo are the thicknesses of the continuous casting machine and the slab. It was found that there is a certain relationship under the condition that is fixed. From this, it can be seen that the stress ratio σm / σo can be estimated to the linear expansion coefficient ratio αt / αs. Α is a linear expansion coefficient based on normal temperature. For example, as shown in FIG. 3, the thermal linear expansion coefficient α (−) at each slab temperature is thermomechanical analysis (TMA) [JIS G 0202]. It can ask for. In the following description, the thermal expansion coefficient is simply abbreviated as “linear expansion coefficient”.

  Next, the inventors assumed a secondary cooling water amount and a heat transfer coefficient in a secondary cooling zone of a continuous casting machine for a Ni-containing steel continuous cast slab having a thickness of 258 mm and a width of 2100 mm. By performing heat transfer / solidification calculations, the cross-sectional average temperature of the slab and the surface temperature of the slab in the straightening zone were calculated, and the linear expansion coefficient ratio αt / αs was calculated from these values.

  That is, FIG. 4 shows the relationship between the linear expansion coefficient ratio αt / αs calculated based on the calculation result and the stress ratio σm / σo of the slab surface at that time. In the figure, the broken line indicates the approximate line, and the dotted line indicates the critical stress ratio σm / σo (cri) when the surface crack occurs and the linear expansion coefficient ratio αt / αs at that time.

From FIG. 4, the stress ratio σm / σo has a strong correlation substantially proportional to the linear expansion coefficient ratio αt / αs , and the linear expansion coefficient ratio αt / αs can be estimated from the stress ratio of the slab surface. Recognize. This also means that the limit linear expansion coefficient ratio αt / αs (cri) can be obtained from the limit stress ratio σm / σo (cri). Therefore, this limit linear expansion coefficient ratio αt / αs (cri) is set as a limit value (specified value), and the linear expansion coefficient ratio αt / αs of the continuous cast slab in the straightening zone exceeds the above limit linear expansion coefficient ratio (cri). Sometimes it can be estimated (determined) that a surface crack has occurred in the slab, and this makes it possible to determine the crack on the surface of the slab. In this example, it can be estimated from FIG. 4 that the linear expansion coefficient ratio αt / αs is 2.4 when the minimum value of the critical stress ratio σm / σo (cri) is 1.7.

  As described above, according to the present invention, the linear expansion coefficient ratio αt / αs is obtained from the temperature distribution of the continuous cast slab surface and cross section without performing stress analysis. It is possible to determine the presence or absence of cracks on the surface of the slab based on the stress state of the continuous cast slab.

  In the application of the present invention, the stress ratio σm / σo is obtained by measuring the hydrostatic stress σm on the surface of the slab when surface cracks occur in the slab, and hot-testing the test piece cut out from the slab. It can be determined by performing a crack reproduction test such as a point bending test or a hot tensile test. For example, in the crack reproduction test, the hydrostatic pressure stress σm at each opening crack position generated on the surface of the test piece is calculated by stress analysis, and the yield stress σo of the slab (steel type) is subjected to a hot tensile test. can get.

FIG. 5 shows the relationship between the critical stress ratio σm / σo (cri) thus obtained and the crack reproduction test piece temperature. As shown in this figure, by performing a crack reproduction test, the critical stress ratio σm / σo (cri) (shown by a broken line in the figure) of the slab surface according to the slab surface temperature can be obtained, This can be adopted as a reference value (criteria) as to whether or not cracking occurs on the surface of the slab.
Criteria is a criterion for slab cracking. When σm / σo (cri) exceeding the standard value is generated on the surface of the slab, it indicates that the possibility of surface cracking at that location is extremely high. .

  Regarding the heat transfer and solidification calculation of continuous cast slabs, the heat transfer calculation in the solidification process of slabs that is generally used is performed, and the temperature distribution of the cross section of the slab is based on the difference method. Calculate.

  In this example, the Ni-containing steel slab shown in Table 1 having a thickness of 258 mm and a width of 2100 mm is shown in FIG. This is an evaluation of the surface cracks of the cast slab. In this figure, 0 on the horizontal axis at the width position indicates the center in the width direction, and 1000 mm indicates one end. Table 2 shows the casting conditions at this time. One is a continuous casting slab with a strong cooling pattern in which the amount of cooling water up to the lower bending straightening zone of the secondary cooling zone is increased. The continuous casting slab of the slow cooling pattern which reduced the water quantity of the secondary cooling zone is shown. The average temperature of the slab section of each cooling pattern was found to be 1219 ° C. for the strong cooling pattern and 1234 ° C. for the slow cooling pattern by heat transfer and solidification calculations. As a result of calculating the linear expansion coefficient ratio αt / αs at the center of the slab width from these temperatures, the value was 2.6 for the strong cooling pattern and 2.0 for the slow cooling pattern. At this time, the limiting linear expansion coefficient ratio αt / αs (cri) described in the present invention is 2.48, respectively.

  Therefore, according to the method of the present invention, the linear expansion coefficient ratio αt / αs and the limiting linear expansion coefficient ratio αt / αs (cri) are compared, and as shown in Table 2, continuous casting with the cooling pattern is performed. When the surface crack of the cast slab was investigated, the surface crack occurred at the center of the width in the strong cooling pattern. On the other hand, in the slow cooling pattern, the surface temperature of the slab in the lower straightening zone is about 800 ° C., and the surface crack occurs even though the hot ductility lowering temperature range of the slab is 700 to 900 ° C. I did not.

  As described above, if the determination method suitable for the present invention is adopted, the temperature distribution of the slab cross section near the straightening zone of the continuous cast slab can be calculated by heat transfer / solidification calculation without performing stress analysis of the slab in continuous casting. Calculate the ratio of thermal expansion coefficient αt / αs of the slab and compare it with the critical thermal expansion coefficient ratio αt / αs (cri) when surface cracks occur, so that the accurate determination of surface cracks Is possible.

  The above-described technique according to the present invention can be applied not only to a continuous casting machine having a straightening band at the bottom but also to a crack determination method in a wide range of equipment such as a curved continuous casting machine having a straightening band at the top. .

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Ladle 3 Tundish 4 Mold 5 Slab support roll 6 Secondary cooling device 7 Upper straightening zone 8 Lower straightening zone 9 Surface thermometer 10 Slab

Claims (3)

In performing continuous casting of steel by a continuous casting machine having at least a lower straightening zone, the linear expansion coefficient αs of the surface of the continuous cast slab in the lower straightening band and the linear expansion coefficient αt at the average temperature in the cross-section direction of the slab When the linear expansion coefficient ratio αt / αs of the steel sheet exceeds the critical linear expansion coefficient ratio αt / αs (cri), it is determined that transverse cracks have occurred on the surface of the slab. A method for judging cracks on the surface of a slab. The limiting linear expansion coefficient ratio αt / αs (cri) is the hydrostatic pressure stress σm applied to the slab when cracks begin to occur on the surface of the continuous cast slab in the lower straightening zone of the continuous casting machine, and at that time The critical stress ratio σm / σo (cri) indicated by the ratio to the yield stress σo of the steel sheet is a linear expansion coefficient ratio corresponding to a value indicating the minimum value, and the linear expansion coefficient ratio αt / αs of the slab in the straightening zone The presence or absence of surface cracks in the slab is estimated based on whether or not the ratio exceeds the limit linear expansion coefficient ratio αt / αs (cri). Crack determination method.   The method for determining cracks on the surface of a slab in continuous casting according to claim 1 or 2, wherein the continuous cast slab is made of steel containing 0.01 to 36 mass% of Ni.

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