JP2023160552A - Method for heating continuously cast slab - Google Patents

Abstract

To suppress cracking of a cast slab in a heating process in a heating furnace.SOLUTION: In a method for heating a cast slab cast by a continuous casting method before charging it into a heating furnace, the cast slab contains, in mass%, C: 0.02-0.60%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.005-1.000%, and N: 0.0100% or less. At least a part of the surface of the cast slab is rapidly heated, an area within 10% of the thickness of the cast slab from the surface of the cast slab is heated at 150°C or more to a temperature less than Ae1 point at a heating rate of 150°C/min or more by the rapid heating, and the cast slab is charged into the heating furnace after the rapid heating.SELECTED DRAWING: Figure 1

Description

This application discloses a method for heating continuously cast slabs.

In recent years, high-strength steel has been used in various technical fields. For example, in the field of automobiles, efforts are being made to reduce the weight of automobile bodies by applying high-strength steel in order to improve fuel efficiency. Furthermore, high-tensile steel is increasingly being used in automobile bodies to ensure the safety of passengers.

High-strength steel is made by adding large amounts of C, Si, and Mn to improve strength. Here, it is known that addition of large amounts of C, Si, and Mn makes the steel material brittle. Therefore, when a slab is obtained by continuous casting of steel to which large amounts of C, Si, and Mn are added, and then heated in a heating furnace, the temperature difference between the surface and the inside of the slab results in Cracks tend to occur in slabs due to thermal strain and transformation strain associated with transformation.

In conventional technology, when a slab after continuous casting is heated in a heating furnace, the heating rate in the embrittlement region is controlled, and the temperature deviation between the surface and inside of the slab is controlled by performing slow heating. Measures have been taken to suppress stress and strain by reducing them (Patent Documents 1 to 3). In addition, before charging the slab into the heating furnace, the slab is heated in a predetermined temperature range and pressed down to compress the porosity at the center of the slab, thereby suppressing cracking of the slab when heated in the heating furnace. A technique to do so has also been disclosed (Patent Document 4).

Japanese Patent Application Publication No. 2010-265534 Japanese Patent Application Publication No. 6-328214 Japanese Patent Application Publication No. 2013-011007 Japanese Patent Application Publication No. 2020-066007

As disclosed in Patent Documents 1 to 3, the heating rate of the slab in the heating furnace is finely controlled, and as disclosed in Patent Document 4, the heating rate of the slab in the heating furnace is controlled. However, it may be difficult to apply pressure due to equipment and logistics constraints. In this regard, a new technology is needed that can suppress cracking of slabs during heating in a heating furnace.

This application, as one of the means to solve the above problems,
A method of heating a slab cast by a continuous casting method before charging it into a heating furnace, the method comprising:
The slab, in mass %, C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, Contains S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less,
At least a portion of the surface of the slab is rapidly heated,
By the rapid heating, a region within 10% of the thickness of the slab from the surface of the slab is heated to a temperature below Ae 1 point at a temperature increase rate of 150 ° C / min or more and a temperature increase of 150 ° C or more. is,
After the rapid heating, the slab is charged into the heating furnace.
A method for heating continuously cast slabs is disclosed.

In the method of the present disclosure, at least a portion of the surface of the slab along the width direction may be rapidly heated.

In the method of the present disclosure, at least a portion of each of both sides of the slab in the width direction may be rapidly heated.

In the method of the present disclosure, a part of the surface along the width direction of the slab is rapidly heated, and the width W _H of the rapidly heated surface is 1/8 or more and 3/8 of the total width W of the slab. It may be 4 or less.

According to the method of the present disclosure, when the slab is heated in a heating furnace, cracking of the slab is less likely to occur.

An example of a portion of the surface of a slab to which rapid heating is applied is schematically shown. An example of a region to which rapid heating is applied is schematically shown in a cross section taken along the line II-II in FIG. This shows the results of thermal stress analysis when the heating time on the slab surface and the temperature difference before and after heating were changed. If the tensile stress at the center of the slab decreased by 20% or more, "●" indicates that there was no decrease. The results are plotted as "x". This is an example of thermal stress analysis results immediately after rapid heating of a slab, showing (a) temperature of the slab, (b) stress generated in the slab, and (c) state of plastic strain generated in the slab. This is an example of thermal stress analysis results after rapidly heating a slab and then allowing it to cool. It shows the following: (a) temperature of the slab, (b) stress generated in the slab, and (c) plastic strain generated in the slab. Indicates the condition.

In the conventional technology, the heating rate in the heating furnace is controlled, the slab is heated slowly, and the temperature difference between the surface and the inside of the slab is minimized as much as possible. This suppresses cracking of slabs during the heating process. However, such temperature control may be difficult due to equipment and logistics constraints. The present inventor utilized thermal stress analysis to search for a method that can suppress cracking of slabs during the heating process in a heating furnace. As a result, by rapidly heating at least a portion of the surface of the slab to give it a thermal history before heating in the heating furnace, it is possible to reduce the stress generated in the center of the slab during the heating process in the heating furnace. It has been found that cracking of slabs can be significantly suppressed. Hereinafter, the method of heating continuously cast slabs of the present disclosure will be described in detail.

The method of the present disclosure is a method in which a slab cast by a continuous casting method is heated before being charged into a heating furnace. The slab contains, in mass%, C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, Contains S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less. In the method of the present disclosure, at least a portion of the surface of the slab is rapidly heated. By the rapid heating, the area within 10% of the thickness of the slab from the surface of the slab reaches a temperature of less than 1 point Ae at a temperature increase rate of 150 ° C / min or more and a temperature increase of 150 ° C or more. heated. In the method of the present disclosure, after the rapid heating, the slab is charged into the heating furnace.

1. Continuous Casting Slab In the method of the present disclosure, first, a slab is obtained by a continuous casting method. Continuous casting conditions are not particularly limited, and may be the same as conventional conditions.

1.1 Shape of slab The slab cast by the continuous casting method may be a slab, bloom, or billet. In particular, a slab or a bloom is preferable, and a slab is more preferable. The slab may have a width and a thickness in a cross-sectional shape perpendicular to the continuous casting direction, and may have a length in the continuous casting direction. If the slab is a slab, its width corresponds to the long side (corresponding to the long side of the mold) in the cross-sectional shape perpendicular to the continuous casting direction, and its thickness corresponds to the short side (corresponding to the short side of the mold) in the cross-sectional shape. ). Further, when the slab is a bloom or a billet, the aspect ratio (long side length/short side length) is generally smaller than that of a slab. When the cross-sectional shape is a circle, the thickness and width refer to the diameter of the cross-section. When the slab is a slab, its width may be, for example, 800 mm or more and 1500 mm or less, its thickness may be, for example, 100 mm or more and 300 mm or less, and its length may be, for example, 5 m or more and 9 m or less. It may be the following.

1.2 Chemical composition of slab The slab contains C, Si, and Mn in a predetermined amount or more, and may be used as a material for high-strength steel sheets, for example. In this application, the term "high-strength steel plate" refers to a steel plate with a tensile strength of 500 MPa or more. The tensile strength may be 780 MPa or more, 980 MPa or more, 1180 MPa or more, or 1470 MPa or more, and may be 2100 MPa or less, 2000 MPa or less, or 1900 MPa or less. The tensile test of the steel plate is performed, for example, in accordance with JIS Z 2241, by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the steel plate.

The slab contains C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, S : 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less. When the content of C, Si, and Mn is in such a range, the brittleness of the slab tends to decrease, and cracks are likely to occur during the heating process in a heating furnace. can be suppressed. In this application, unless otherwise specified, "~" indicating a numerical range is used to include the numerical values written before and after it as the lower limit and upper limit.

(C: 0.02-0.60%)
C is the most fundamental element that affects not only the static strength of steel, but also its fatigue strength, toughness, and ductility. If C is too small, the static strength and fatigue strength of the steel may become insufficient. In this regard, the lower limit of the C content may be 0.02% by mass, 0.05% by mass, 0.10% by mass, or 0.15% by mass. Moreover, if there is too much C, the toughness of the steel tends to deteriorate excessively. In this regard, the upper limit of the content of C may be 0.60% by mass, 0.50% by mass, 0.40% by mass, or 0.30% by mass.

(Si: 0.5-3.0%)
Si is an important element that has the highest solid solution strengthening ability next to C. When obtaining high tensile strength steel, the concentration of Si is increased. Specifically, the lower limit of the Si content may be 0.5% by mass, 0.8% by mass, or 1.0% by mass. On the other hand, if the Si content is too high, there is a risk that toughness and workability will be deteriorated. In this regard, the upper limit of the Si content may be 3.0% by mass, 2.5% by mass, or 2.0% by mass.

(Mn: 1.0-3.0%)
Mn is an important element for improving hardenability and ensuring hardness deep into the steel material even when the cooling rate is insufficient. When obtaining high tensile strength steel, the Mn concentration is increased. Specifically, the lower limit of the Mn content may be 1.0% by mass or 1.5% by mass. On the other hand, too much Mn may deteriorate toughness and workability. In this regard, the upper limit of the Mn content may be 3.0% by mass or 2.8% by mass.

(P: 0.100% or less)
P is an element that promotes Mn concentration in the unsolidified portion during the solidification process of molten steel, lowers the Mn concentration in the negatively segregated portion, and promotes an increase in the area ratio of ferrite, and the smaller the amount, the better. Furthermore, while excessive P content may increase steel strength, it may cause brittle fracture of the steel. In this regard, the upper limit of the P content may be 0.100% by mass, 0.050% by mass, or 0.010% by mass. On the other hand, the lower limit of the P content is not particularly limited and may be 0% by mass, but controlling the P content to less than 0.001% by mass increases the refining time and This may lead to an increase in manufacturing costs. In order to prevent an increase in manufacturing costs, the P content may be 0.001% by mass or more.

(S: 0.010% or less)
S is an element that generates nonmetallic inclusions such as MnS in steel and causes a decrease in the ductility of the steel, and the smaller the amount, the better. In this regard, the upper limit of the S content may be 0.010% by mass, 0.008% by mass, or 0.005% by mass. On the other hand, the lower limit of the S content is not particularly limited, and may be 0% by mass or 0.001% by mass.

(Al: 0.005-1.000%)
Al is an element that acts as a deoxidizer for steel and stabilizes ferrite. Such effects are likely to be obtained when the Al content is 0.005% by mass or more. The content of Al may be 0.010% by mass or more. On the other hand, if Al is contained excessively, ferrite transformation and bainite transformation may be excessively promoted during the cooling process during annealing in the lower process, resulting in a decrease in the strength of the steel. Such problems can be easily avoided when the Al content is 1.000% by mass or less. The content of Al may be 0.800% by mass or less.

(N: 0.0100% or less)
N is an element that forms coarse nitrides and reduces the workability of steel, and the smaller the amount, the better. The content of N may be 0% by mass, 0.0001% by mass or more, 0.0010% by mass or more, or 0.0100% by mass or less. It may be 0.0050% by mass or less.

In addition to the above-mentioned basic elements, the slab may contain arbitrary elements other than those mentioned above. Since the optional element does not need to be included, the lower limit is 0%. The slab has, for example, Ti: 0-0.500%, Co: 0-0.500%, Ni: 0-0.500%, Mo: 0-0.500%, Cr: 0-0. 2.000%, O: 0 to 0.0100%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0 .0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0.0500% It may contain one or more types selected from. Note that the types and amounts of the above-mentioned arbitrary elements are merely examples, and the types and amounts of the arbitrary elements that can be included in the slab are not limited to those described above.

1.3 Temperature of slab before rapid heating The temperature of the slab is not particularly limited as long as rapid heating described below can be performed before charging into the heating furnace. The slab may be cooled through cooling or the like. Before performing the rapid heating described below, the surface temperature of the slab may be more than 150°C lower than Ae1, for example, 0°C or more and 600°C or less. Moreover, before performing the rapid heating described below, the center temperature of the slab may be more than 150° C. lower than Ae1, for example, 0° C. or more and 600° C. or less. Further, before performing the rapid heating described below, the absolute value of the difference between the surface temperature of the slab and the center temperature of the slab may be, for example, 0° C. or more and 600° C. or less. Note that the effects of rapid heating, which will be described later, are not substantially affected by the temperature of the slab at the start of rapid heating. That is, no matter what temperature the slab is before rapid heating, the desired effect can be achieved by rapid heating.

2. Rapid Heating In the method of the present disclosure, at least a portion of the surface of the slab is rapidly heated before charging the slab into the heating furnace, thereby causing compressive plastic deformation on at least a portion of the surface of the slab. This reduces the tensile stress generated inside the slab during the heating process in the heating furnace. Usually, during the heating process in a heating furnace, the surface of the slab is heated by radiant heat. As a result, the surface of the slab undergoes thermal expansion, which causes tensile stress to occur inside the slab, leading to cracking. In contrast, by creating compressive plastic deformation on the slab surface in advance using the method of the present disclosure, even if the slab surface thermally expands during the heating process in the heating furnace, the tensile stress generated inside the slab will be large. As a result, cracking of the slab is suppressed. Specifically, in the method of the present disclosure, at least a portion of the surface of the slab is rapidly heated before the slab is charged into the heating furnace, and due to the rapid heating, the surface of the slab is heated. A region within 10% of the slab thickness is heated to a temperature below Ae 1 point at a temperature increase rate of 150° C./min or more and a temperature increase amount of 150° C. or more.

2.1 Portion to be rapidly heated on the surface of the slab In the method of the present disclosure, at least a portion of the surface of the slab may be rapidly heated. The portion of the surface of the slab that is rapidly heated is determined depending on the location where it is desired to suppress the occurrence of cracks in the slab during the heating process in the heating furnace. In the method of the present disclosure, the entire surface of the slab may be rapidly heated, or a portion of the surface of the slab may be rapidly heated.

In the method of the present disclosure, as shown in FIGS. 1 and 2, it is preferable that at least a portion of the surface along the width direction of the slab (the wide surface of the slab) is rapidly heated. When the surface along the width of the slab is rapidly heated, the distance between the rapidly heated part and the center of the slab is close, and the effect of compressive plastic deformation caused by rapid heating is likely to reach the center of the slab. , the tensile stress generated inside the slab during the heating process in the heating furnace is more likely to be reduced.

Further, in the method of the present disclosure, as shown in FIGS. 1 and 2, it is more preferable that at least a portion of each of both sides of the slab along the width direction is rapidly heated. In particular, as shown in FIGS. 1 and 2, in a cross section perpendicular to the casting direction of the slab, at least a portion of each of both sides along the width direction of the slab is rapidly heated, and one side It is preferable that the rapidly heated portion A1 on the other side and the rapidly heated portion A2 on many sides face each other with the slab center O in between. In this way, by rapidly heating both sides of the slab, warpage and deformation of the slab can be suppressed, and the effect of compressive plastic deformation caused by rapid heating can be more easily extended to the center of the slab, making it possible to The tensile stress generated inside the slab during the heating process is particularly likely to be significantly reduced.

In addition, in the method of the present disclosure, as shown in FIGS. 1 and 2, only the surface along the width direction of the slab, rather than the surface along the thickness direction of the slab (the narrow side of the slab), is rapidly Preferably, it is heated. Even if rapid heating is applied to the surface along the thickness direction of the slab, the effect of compressive plastic deformation is difficult to reach the center of the slab. In the method of the present disclosure, a sufficient effect can be obtained by rapidly heating only the surface along the width direction of the slab.

Furthermore, in the method of the present disclosure, as shown in FIGS. 1 and 2, a part of the surface along the width direction of the slab is rapidly heated, and the width W _H of the rapidly heated surface is It is preferable that the total width W is 1/8 or more and 3/4 or less. In particular, as shown in FIG. 2, it is preferable that the width W _H is 1/8 or more and 3/4 or less of the width W, and that the portion to be rapidly heated includes the center in the width direction of the slab. More preferably, the widthwise center of the portion to be heated substantially coincides with the widthwise center of the slab. According to the findings of the present inventor, by heating only a part of the slab width instead of the entire width, energy costs can be reduced and compressive stress in the width direction can be suppressed. This makes it easier to generate compressive plastic strain in the longitudinal direction of the slab, making it easier to generate compressive plastic strain in the longitudinal direction of the slab, making it possible to more effectively suppress the occurrence of cracks in the slab during the heating process in the heating furnace. In addition, slab cracking during the heating process in the heating furnace is particularly likely to occur near the center O of the slab (i.e., near the center in the width direction), but by rapidly heating only the vicinity of the center in the width direction of the slab, The effect of compressive plastic deformation due to rapid heating is more likely to reach the center of the slab, and the tensile stress generated at the center of the slab during the heating process in the heating furnace is more likely to be reduced.

Further, in the method of the present disclosure, as shown in FIG. 1, at least a part of the surface of the slab along the width direction is rapidly heated over the entire length of the slab in the longitudinal direction (length direction). It is preferable that As a result, it is possible to generate strong compressive stress in the longitudinal direction in the rapid heating section, and compressive plastic strain in the longitudinal direction, which more effectively suppresses the occurrence of slab cracking during the heating process in the heating furnace. can do.

2.2 Area that is rapidly heated in the cross section of the slab The compressive stress on the surface layer of the slab due to rapid heating is balanced with the tensile stress inside the slab. If the area heated by rapid heating (areas A1 and A2 in Figure 2) in the slab cross section (cross section perpendicular to the length direction) is too thick, the area where tensile stress is generated to balance it (areas A1 and A2 in Figure 2) is too thick. Since region B) becomes relatively too small, large tensile stress is more likely to be generated at the center of the slab due to compressive plastic deformation due to rapid heating. That is, a large tensile stress is generated at the center of the slab even before it is charged into the heating furnace, making the slab more likely to crack. For example, as far as the present inventor has confirmed, when 10% of the slab thickness is rapidly heated from the surface of the slab, a tensile stress of 25% of the yield stress is generated inside the slab, and 17% of the slab thickness is generated inside the slab. %, a tensile stress of 50% of the yield stress is generated inside the slab. In this regard, in the method of the present disclosure, it is important that the rapid heating heats a region within 10% of the thickness of the slab from the surface of the slab. In other words, in the method of the present disclosure, as shown in FIG. 2, the ratio T _H /T of the thickness T _H of each of rapidly heated regions A1 and A2 to the thickness T of the slab is 0.1 or less. It is. In this way, only the very surface layer of the slab is rapidly heated, and by minimizing compressive deformation due to rapid heating in deeper parts, the tensile stress at the center of the slab is suppressed. . In order to keep the area to be rapidly heated within 10% of the thickness of the slab, for example, as described later, the heating time must be within 1 minute, or the heating means can heat only the very surface layer of the slab. You can use any means.

2.3 Temperature increase rate due to rapid heating If the temperature increase rate due to rapid heating is too low, it will take a long time to heat the slab to the desired temperature, and the rapidly heated area of the slab cross section will be excessively heated. It tends to become thick. As a result, as described above, a large tensile stress is generated at the center of the slab, making the slab susceptible to cracking. The faster the temperature increase rate due to rapid heating, the greater compressive plastic deformation can be caused only in the very surface layer of the slab, and the higher the effect is likely to be ensured. In this regard, in the method of the present disclosure, it is important that the temperature increase rate by rapid heating is 150° C./min or more. The upper limit of the temperature increase rate is not particularly limited. The temperature increase rate may be 150°C/min or more, 155°C/min or more, 160°C/min or more, 165°C/min or more, or 170°C/min or more, and 600°C/min or less, 550°C /min or less, 500°C/min or less, or 450°C/min or less.

2.4 Amount of Temperature Increase Due to Rapid Heating If the amount of temperature increase due to rapid heating is too small, the above-mentioned compressive plastic deformation cannot be produced, and sufficient effects cannot be obtained. The larger the amount of temperature rise due to rapid heating, the more compressive plastic deformation can be caused in the very surface layer of the slab, making it easier to ensure high effects. In this regard, in the method of the present disclosure, by rapid heating, an area within 10% of the thickness of the slab from the surface of the slab reaches a temperature increase rate of 150°C/min or more and a temperature increase amount of 150°C or more. It is important that the material is heated. That is, in a region within 10% of the thickness of the slab from the surface of the slab, the temperature difference before and after rapid heating is 150° C. or more. The temperature increase amount may be 150°C or higher, 155°C or higher, 160°C or higher, 165°C or higher, or 170°C or higher, 600°C or lower, 550°C or lower, 500°C or lower, 450°C or lower, 400°C or lower, The temperature may be 350°C or lower or 300°C or lower.

2.5 Heating temperature by rapid heating From the perspective of suppressing slab cracking during the heating process in a heating furnace, there are no particular restrictions on the heating temperature by rapid heating as long as the above heating range, heating rate, and amount of temperature rise are met. There isn't. However, if the heating temperature is Ae 1 or higher, the rapidly heated portion is likely to undergo transformation, and there is a risk that the mechanical properties of the slab will vary excessively. In this regard, in the method of the present disclosure, by rapid heating, an area within 10% of the thickness of the slab from the surface of the slab reaches a temperature increase rate of 150°C/min or more and a temperature increase amount of 150°C or more. Therefore, it is important that the material be heated to a temperature below the Ae1 point. The heating temperature by rapid heating is a temperature below the Ae 1 point, and may be a temperature of 700°C or lower, 680°C or lower, or 650°C or lower.

Note that a method for specifying the Ae1 point of steel having an arbitrary composition is known. For example, the Ae1 point can be specified based on the following formula described in K.W. Andrews: JISI, vol. 203, 1965, p.721.

Ae1 (℃) = 727-10.7 [Mn] - 16.9 [Ni] + 29.1 [Si] + 16.9 [Cr] + 6.38 [W] + 290 [As]
(Here, [Mn], [Ni], [Si], [Cr], [W], and [As] are the content (mass%) of each element in the slab.)

2.6 Heating time by rapid heating The heating time by rapid heating is not particularly limited as long as a region within 10% of the thickness of the slab from the surface of the slab is rapidly heated under the above conditions. As far as the present inventor has confirmed, if the heating time is too long, it becomes difficult to keep the rapidly heated region within 10% of the thickness of the slab from the surface of the slab. In this regard, the heating time is preferably, for example, 1 minute or less. The lower limit of the heating time is not particularly limited, and may be more than 0 seconds, 1 second or more, 3 seconds or more, 5 seconds or more, or 10 seconds or more.

2.7 Rapid heating means Any rapid heating means may be used as long as the above heating conditions are achieved. In the method of the present disclosure, rapid heating is preferably performed by at least one of high-frequency induction heating, laser heating, and plasma heating, for example. In addition, in the method of the present disclosure, when rapidly heating the entire length of the slab, it is not necessary to rapidly heat the entire length of the slab at the same time. For example, bar heaters may be installed above and/or below the slab conveying path. Alternatively, the entire length of the slab may be rapidly heated by moving the bar heater relatively in the longitudinal direction of the slab while adjusting the bar heater output and conveying speed of the slab. Alternatively, the entire length of the surface of the slab may be rapidly heated by, for example, moving a bar heater having a length comparable to or longer than the length of the slab relatively in the width direction of the slab.

3. Charge to Heating Furnace and Heating in Heating Furnace As described above, the rapidly heated slab is inserted into the heating furnace in a state having the above-mentioned compressive plastic deformation. The heating furnace may be any general heating furnace. The method for inserting the slab into the heating furnace may also be a general method. The slab charged into the heating furnace is heated to a predetermined temperature in the heating furnace. For example, the slab may be heated in a heating furnace to a temperature suitable for hot rolling. The heating temperature in the heating furnace may be, for example, 1000°C or more and 1300°C or less. The heating time in the heating furnace may be, for example, 30 minutes or more and 240 minutes or less.

In the method of the present disclosure, as described above, compressive plastic deformation is applied to the surface layer of the slab in advance by rapid heating, and even if the surface layer of the slab is heated and thermally expanded during the heating process in the heating furnace, Tensile stress generated at the center of the slab is difficult to increase. As a result, cracking of the slab during the heating process can be suppressed. Therefore, in the method of the present disclosure, in the heating process in the heating furnace, there is no need to finely control the heating temperature of the slab unlike in the conventional slow heat, and there is a high degree of freedom in the heating conditions in the heating furnace. As mentioned above, slabs containing large amounts of C, Si, and Mn are naturally prone to cracking during the heating process in a heating furnace, but according to the method of the present disclosure, for example, the average temperature increase rate in the heating furnace is 10°C. Even if the speed is higher than /min, cracking of slabs is difficult to occur. The average temperature increase rate in the heating furnace may be 13° C./min or more or 16° C./min or more.

4. Note: The stress and plastic strain on the compression side caused by rapid heating are larger than the stress and plastic strain on the tension side that occur during the subsequent cooling process, so even if the slab is allowed to cool after rapid heating, The effects of the method of the present disclosure can be obtained. That is, in the method of the present disclosure, the temperature of the slab may be lowered after rapid heating and before charging into the heating furnace. However, from the viewpoint of ensuring a higher effect, in the method of the present disclosure, the amount of temperature change of the slab after rapidly heating the slab until it is charged into the heating furnace is within 300°C, within 250°C, The temperature is preferably within 200°C or within 150°C. Further, the cooling rate (temperature reduction rate) of the slab after rapid heating and before charging into the heating furnace is preferably equal to or lower than the cooling rate, for example, less than 150°C/min, 100°C/min or less. , or preferably 50° C./min or less.

As described above, according to the method of the present disclosure, cracks are unlikely to occur in the slab during the heating process in the heating furnace. Furthermore, in the method of the present disclosure, there is no need to finely control the heating rate of the slab as in the prior art. Moreover, even when the heating rate in the heating furnace is set at a high rate, cracks in the slab are less likely to occur.

Examples according to the present invention are shown below, but the present invention is not limited to this example of one condition. In the present invention, various conditions may be adopted as long as the purpose is achieved without departing from the gist thereof.

1. Thermal Stress Analysis A thermal stress analysis was performed assuming that the slab after continuous casting was charged into a heating furnace at 150° C. without being rapidly heated, and this was used as the evaluation standard. Further, thermal stress analysis was similarly performed on the assumption that the slab after continuous casting was rapidly heated before being charged into the heating furnace and then charged into the heating furnace at 150°C. The rapid heating conditions were variously changed, and those in which the tensile stress at the center of the slab decreased by 20% or more with respect to the evaluation criteria were evaluated as "●", and those in which the tensile stress did not decrease were evaluated as "×". FIG. 3 shows the results of thermal stress analysis when the heating time of the slab surface and the temperature difference before and after heating were varied. As shown in Figure 3, in order to exhibit the tensile stress reduction effect, only the very surface layer of the slab (up to about 10% of the thickness, achieved by heating for less than 1 minute) must be rapidly heated. Therefore, it can be said that it is effective to raise the temperature of the surface layer by 150° C. or more from the temperature at the start of heating.

FIG. 4 shows an example of the thermal stress analysis results when a part of the surface along the width direction of the slab is rapidly heated over the entire length in the longitudinal direction (calculated using a symmetric model in the longitudinal and width directions). FIG. 4(a) shows the temperature of the slab, FIG. 4(b) shows the stress generated in the slab, and FIG. 4(c) shows the state of plastic strain generated in the slab. As shown in FIGS. 4(a) to 4(c), compressive stress and accompanying plastic strain on the compression side occur in the rapidly heated portion. This is thought to be due to the fact that although the heated surface layer thermally expands as the temperature rises, it is constrained to a low-temperature non-heated region.

FIG. 5 shows the results of thermal stress analysis when the slab is cooled to lower the temperature after rapid heating under the conditions shown in FIG. 4. FIG. 5(a) shows the temperature of the slab, FIG. 5(b) shows the stress generated in the slab, and FIG. 5(c) shows the state of plastic strain generated in the slab. As shown in FIG. 5(c), it can be seen that the compressive plastic strain caused by rapid heating remains even when the temperature of the slab decreases.

Usually, in a heating furnace, the surface layer of the slab is heated, and as the surface layer of the slab thermally expands, tensile stress is generated inside the slab, which causes cracking of the slab. On the other hand, from the above thermal stress analysis results, we found that by rapidly heating the very surface layer of the slab before charging it into the heating furnace and applying compressive plastic deformation to the very surface layer of the slab, Even if the surface of the slab undergoes thermal expansion and elongation during the heating process in the heating furnace, it can be said that the tensile stress generated at the center of the slab can be kept low.

2. Actual Machine Test The validity of the thermal stress analysis results was confirmed through an actual machine test. First, molten steel made of predetermined components was continuously cast to produce a slab. Table 1 below shows the chemical composition of the slab manufactured by continuous casting. Continuous casting was performed at a casting speed of 0.7 to 1.5 m/min using a slab casting mold with a thickness of 240 to 280 mm and a width of 1100 mm. Thereafter, the slab was cut into predetermined lengths and cooled.

Thereafter, part or all of both sides of the slab along the width direction were rapidly heated under the conditions shown in Table 2 below. Here, a high frequency induction heating device was used for rapid heating, the frequency was set to 1500 Hz, and the temperature increase rate shown in Table 2 was achieved by adjusting the output. In addition, the temperature of the slab before and after rapid heating is measured using a thermoviewer, and the penetration depth calculated from the measured heating temperature, the specific values of the metal (specific resistance, relative magnetic permeability), and the frequency. Using the three-dimensional heat transfer analysis, the heating depth was calculated, and the temperature at the start of rapid heating and the temperature at the end of rapid heating in an area within 10% of the thickness of the slab was calculated.

After rapid heating, the slab that reached the temperature shown in Table 2 was charged into a heating furnace and heated to 1210 to 1250°C at the average temperature increase rate shown in Table 2, and then roughened in hot rolling. A steel plate was obtained by performing steps up to rolling. After cooling the steel plate, it was visually checked for cracks or streak-like abnormalities. The results are shown in Table 2 below.

In Table 2, "T" is the thickness of the slab and corresponds to T shown in FIG.
“W _H ” is the width of the rapid heating region and corresponds to W _H shown in FIG.
"W" is the total width of the slab and corresponds to W shown in FIG.
In addition, Example 8 is an example in which the entire circumference of the slab was rapidly heated, and by rapidly heating both the surface along the width direction and the surface along the thickness direction of the slab, W _H /W was increased. This is an example where the ratio was 1/1. In Examples 1 to 7, rapid heating was performed only on the surface along the width direction of the slab, and no rapid heating was performed on the surface along the thickness direction of the slab.
"Temperature increase rate" means the temperature increase rate during rapid heating in a region within 10% of the thickness of the slab.
"Rapid heating start temperature" means the temperature at the start of rapid heating in a region within 10% of the slab thickness.
"Rapid heating end temperature" means the temperature immediately after rapid heating ends in a region within 10% of the slab thickness.
The "temperature increase amount" is the difference between the "rapid heating end temperature" and the "rapid heating start temperature" in a region within 10% of the slab thickness.
“Heating depth” means the ratio of the thickness T _H of the rapid heating region to the slab thickness T ((T _H /T)×100), and “T” and “T _H ” are each shown in FIG. Corresponds to T and T _H.
"Charging temperature" means the temperature of the surface of the slab at the time it is charged into the heating furnace (in the case of rapid heating, the temperature of the area within 10% of the slab thickness of the rapidly heated portion).
"Average temperature increase rate" refers to the average temperature increase rate set in the heating furnace.

The following can be seen from the results shown in Table 2.

Regarding Comparative Example 1, cracks and abnormalities were observed in the steel plate after rough rolling. Regarding Comparative Example 1, since the slab was not rapidly heated before being charged into the heating furnace, tensile stress was generated inside the slab due to thermal expansion of the surface layer of the slab during heating in the heating furnace, which caused the rough rolling. This is thought to have led to later cracks and abnormalities.

In Comparative Example 2, cracks and abnormalities were also observed in the steel plate after rough rolling. Regarding Comparative Example 2, although the slab was heated before being charged into the heating furnace, the heating rate was low and the heating time was long, resulting in a heating depth of 15% of the slab thickness. It is thought that compressive plastic deformation occurred deep from the surface of the slab, and to balance this, a large tensile stress was generated at the center of the slab, which led to cracks and abnormalities after rough rolling. it is conceivable that.

Regarding Comparative Example 3, although no cracks or abnormalities were observed in the steel plate after rough rolling, the metal structure of the steel plate was not as desired, and desired mechanical properties could not be secured. Regarding Comparative Example 3, when the slab was rapidly heated before being charged into the heating furnace, the temperature of the surface layer of the slab exceeded the Ae1 point, so there was variation in the metal structure between the surface layer and the inside of the slab due to transformation. It is thought that the mechanical properties deteriorated due to the occurrence of.

Regarding Comparative Examples 4 and 5, cracks and abnormalities were observed in the steel sheets after rough rolling. Regarding Comparative Examples 4 and 5, although the slabs were heated before being charged into the heating furnace, the heating rate was low and the amount of temperature rise was also small, so that sufficient compressive plastic deformation was generated in the surface layer of the slabs. It is thought that this caused a large tensile stress to occur at the center of the slab during the heating process in the heating furnace, which led to cracks and abnormalities after rough rolling.

In contrast, in Examples 1 to 8, no cracks or abnormalities occurred in the steel sheets after rough rolling. From the results of Comparative Examples 1 to 5 and Examples 1 to 8, when the following conditions are met, cracks in the slab can be suppressed during the heating process in the heating furnace, and cracks and abnormalities can be suppressed even when rolling is performed afterwards. This can be said to be something that can be suppressed.

(1) The slab is mass%: C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% Below, S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less are contained.
(2) At least a part of the surface of the slab is rapidly heated, and due to the rapid heating, an area within 10% of the slab thickness from the surface of the slab is heated at a rate of 150°C/min or more, and With a temperature increase of ℃ or more, it is heated to a temperature below 1 point of Ae.
(3) After rapid heating, the slab is charged into a heating furnace.

Claims (4)

A method of heating a slab cast by a continuous casting method before charging it into a heating furnace, the method comprising:
The slab, in mass %, C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, Contains S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less,
At least a portion of the surface of the slab is rapidly heated,
By the rapid heating, a region within 10% of the thickness of the slab from the surface of the slab is heated to a temperature below Ae 1 point at a temperature increase rate of 150 ° C / min or more and a temperature increase of 150 ° C or more. is,
After the rapid heating, the slab is charged into the heating furnace.
Method of heating continuously cast slabs. At least a portion of the surface along the width direction of the slab is rapidly heated;
The method according to claim 1. At least a portion of each of both sides along the width direction of the slab is rapidly heated.
The method according to claim 1. A part of the surface along the width direction of the slab is rapidly heated, and the width W _H of the rapidly heated surface is 1/8 or more and 3/4 or less of the total width W of the slab.
The method according to any one of claims 1 to 3.

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