JPH0468069B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing continuously cast slabs and a method for hot working thereof, particularly a method for preventing hot cracking of slabs during the production of continuously cast slabs, and a so-called direct rolling process or The present invention relates to a method for preventing hot cracking of slabs in a hot charge rolling process. More specifically, the present invention is a medium-low carbon steel containing either or both of Si and Mn, or a low alloy steel containing 1% or less of each of alloying elements such as Al, Nb, Ti, Ta, V, and B. Continuous casting of steel, direct rolling process in which steel is rolled immediately without reheating immediately after continuous casting, or hot charge rolling process in which hot rolling is performed after reheating without cooling to room temperature. This invention relates to a method for preventing cracking of a steel billet during rolling. (Prior art) When producing medium-low carbon steel or low alloy steel as described above by a continuous casting method using, for example, a curved continuous casting machine, the continuously cast slab is mainly subjected to an electric current during slab straightening. Surface cracks often occur due to bending stress caused by bending and thermal stress caused by cooling, and this tendency is particularly noticeable in Nb-containing steel. Such cracks require a cleaning process before proceeding to the next process, so it is necessary to cool the product to around room temperature. Even in the case of the normal rolling process using cold slabs, the need for a care process complicates operations and increases costs.
On the other hand, in direct rolling and hot charge rolling processes that aim to reduce costs through energy and labor savings, the occurrence of such cracks is a significant obstacle to their practical application. Furthermore, even if the slab does not have any flaws, cracks may occur during direct rolling or hot charge rolling, which also poses a significant obstacle to the practical application of these processes. Incidentally, the occurrence of such cracking is particularly remarkable in materials containing a high content of S as an impurity. Therefore, in order to manufacture products stably and inexpensively by the direct rolling or hot charge rolling process, it is necessary to avoid the occurrence of defects in slabs during continuous casting and the subsequent process of direct rolling or hot charge rolling. It is desired to establish a method for completely preventing the occurrence of surface flaws. On the other hand, even when continuous casting slabs are once cooled, reheated, and hot-worked, if there are no defects in the continuous casting slabs themselves, the flaw removal process becomes unnecessary, and the practical benefit is extremely large. Even if there is a case,
It is desired to establish a method that completely prevents the occurrence of defects during the production of continuously cast slabs. First, as a method for preventing surface flaws occurring in such continuously cast slabs, Japanese Patent Application Laid-Open No. 128255/1983 discloses a method for continuous impact of shot balls. However, as stated in p. 290 of the publication, lines 3 to 7, this method is aimed at crimping cracks directly under the mold, removing trapped foreign matter, and preventing oxidation on the surface of the slab. and
Furthermore, as is clear from Figure 4 of the publication,
This is just a processing step before entering the guide rolls directly below the mold. Cracks continue to occur even after this, and as will be described later, this method is not a complete preventive measure against the occurrence of cracks. Furthermore, JP-A-54-155123 discloses a method of applying plastic strain to a slab, which involves adjusting the amount of plastic strain in the surface layer, slab temperature, and austenite grain size within a certain range. According to the findings of the present inventors, these conditions alone cannot completely prevent the occurrence of defects. Furthermore, roll reduction, shot blasting, and laser pulses, which have been proposed as means for imparting plastic strain, do not provide sufficient effects. That is, if a slab including an unsolidified portion is rolled down with a normal roll, the entire thickness of the solidified shell will simply be depressed, and strain cannot be imparted to the surface layer of the slab. In addition, the depth at which strain can be imparted by Shot Plus is too shallow to be effective, and the shot recovery method is impractical due to many problems. Furthermore, the method using laser pulses applies heat to the surface of the slab several tens of micrometers thick, and attempts to impart strain due to the temperature difference between the slab and the inside. This is almost impossible in principle because the temperature difference is small.
Furthermore, since there is cooling water on the surface of the slab, the results are even weaker, making it extremely difficult to apply to actual production lines. In addition, as a means to prevent the occurrence of defects during hot rolling in direct rolling or hot charge rolling processes following continuous casting, as disclosed in Japanese Patent Application Laid-Open No. 58-52442, the cooling rate during continuous casting is Although measures have been proposed such as controlling
Although it is theoretically possible, there are many problems in applying it to actual operations. (Problems to be Solved by the Invention) Thus, an object of the present invention is to eliminate surface flaws that occur during the production of continuously cast slabs and when direct rolling or hot charge rolling them, without impairing productivity at all. The objective is to completely prevent cracking in the process, enable stable operation of the process, and significantly reduce costs. The present inventors discovered that these cracks as surface defects are caused by the heat applied to continuously cast slabs in the low-temperature austenite (γ) region during the cooling process, and in some cases in the coexistence region with ferrite (α). This occurs due to low strain rate deformation such as stress or external stress applied to the slab during straightening of the slab in this temperature range [Mat.Sci.Eng., 62 (1984)]
p.109-119, and Trans.JIM, 25 (1984)
p.160-167], and also discovered and announced that during hot rolling, this occurs due to high strain rate deformation in the relatively low-temperature γ region, and that both are caused by the destruction of γ grain boundaries. . In the present invention, the temperature at which the slab is passed through the straightening rolls after light processing is applied to the surface during straightening (low-temperature γ range or α + γ two-phase coexistence range)
This is because surface flaws occur frequently due to low strain rate deformation in the steel, and the precipitates that cause flaws are coarsened and rendered harmless in order to prevent the formation of fine precipitates that cause cracks. Nothing but. The embrittlement of the material during low strain rate deformation is
Carbonitrides such as NbC, TaC, TiC, and VN precipitate continuously at the γ grain boundaries during deformation, and also precipitate finely within the grains, and even relatively soft ferrite ( α) precipitates in a film shape, and the inside of the grain becomes relatively strengthened, and strain concentrates in the precipitate-free zone along the γ grain boundaries and the soft part of the film α, resulting in interfacial separation between the grain boundary precipitates and the matrix. [Mat.Sci.Eng., 62 (1984) p.109~
119, Trans.JIM, 25 (1984) p.160-167). In addition, embrittlement during high strain rate deformation seen during hot rolling is also caused by (Fe,
This is also caused by continuous precipitation of Mn)S and intragranular strengthening due to fine precipitation within the grains. In this case, if γ-grain boundary continuous precipitation and intragranular precipitation of carbonitrides occur before this high strain rate deformation, (Fe, Mn)
The embrittlement caused by S is significantly accelerated. Therefore, in order to prevent embrittlement due to γ grain boundary cracking in both processes, it is necessary to make the γ grains finer to reduce the susceptibility to grain boundary embrittlement, or to reduce the susceptibility to grain boundary embrittlement by making the γ grains finer, or by reducing the embrittlement by the time of problematic deformation (for example, during slab straightening and rolling). Precipitates may be coarsened to prevent γ grain boundary precipitation and intragranular fine precipitation during deformation. However, the current situation is that sufficient measures are not being taken due to equipment and operational constraints. For example, agglomeration and coarsening of precipitates can be achieved by reducing the cooling rate or maintaining constant temperature during cooling [For carbonitrides, see Mat.Sci.
Eng., 62 (1984) p.109-119; see Japanese Patent Application Laid-Open No. 58-52442 for information on sulfides], but it is not practical because it takes an order of magnitude longer time to cool down and significantly impairs productivity. . There is also a proposal to make grains finer by recrystallizing γ grains (see JP-A-54-155123), but since the original γ grains are extremely coarse,
Since the area of the grain boundaries as recrystallization nuclei is small, it is necessary to apply a large strain, and as stated in JP-A-54-155123, the strain must be at least 40% or more to obtain fine crystal grains with a grain size of 0.1 mm or less. It is virtually impossible to apply a large amount of plastic strain to the slab including the unsolidified portion. Also, considering the embrittlement mechanism mentioned above, it is possible to suppress the occurrence of surface flaws by adjusting the chemical composition of the steel, but the chemical composition of the steel must be added to give the material the desired characteristics. There are many restrictions as there are some things that cannot be obtained, so it is not a fundamental countermeasure. For example, to prevent the precipitation of AlN, it is possible to improve ductility by reducing Al and N, or by adding Ti to fix N in the γ grains as TiN, but these reductions involve increased costs, and Ti addition is There are many harmful effects such as impairing the toughness of the parts. Furthermore, addition of Nb, etc. is essential for ensuring product quality, and it is impossible to take measures by changing it. Reducing S is also effective, but it is accompanied by an increase in cost and does not necessarily lead to a reduction in total cost. Here, let us confirm once again the differences between JP-A-58-52442 and the present invention. JP-A No. 58-52442 and the present invention both result in the use of NbC and the like, which cause cracks during slab straightening.
It is similar in that it attempts to prevent fine precipitation of AlN, etc. However, the former uses ultra-slow cooling to achieve its purpose, which takes an extremely long time.
In contrast, the present invention creates precipitation nuclei through light processing of the slab surface during cooling, causes harmful precipitates to precipitate early, and further coarsens them. It is intended to achieve that purpose. Therefore, it differs from the former in its technical concept and structure, and its effectiveness is also significantly different in that it can achieve its purpose without compromising productivity at all. (Means for Solving the Problems) The present inventors have devised a method for solving the agglomeration and coarsening of carbonitrides and sulfides in a practical and short time without using the above-mentioned ultra-slow cooling of slabs or constant temperature maintenance during cooling. After repeated research into methods to achieve this goal, they discovered that the objective could be achieved if the surface layer of the slab could be processed under specific conditions that were practically applicable while the slab was cooling. As described above, the present invention was made for the first time based on this discovery, and is completely different from the conventional method both metallurgically and essentially in terms of technical concept and effect. Here, the present invention, in the broadest sense, applies a processing strain of 5% or more to a surface layer depth of 2 mm or more of a slab during continuous casting in a temperature range of 700 to 1200°C, and then passes it through straightening rolls. This is a method for producing continuously cast slabs, which is characterized by the following. According to a preferred embodiment of the present invention, a working strain of 5% or more is applied to the surface layer depth of 2 mm or more of a slab during continuous casting, and the strain rate ε〓(s ^-1 ) is set to ε〓≧a・exp[− b/T+273] (where a=1.3614, b=7500, T is the slab surface temperature of 700°C≦T≦1200°C), and then passing through a straightening roll. This is a method for manufacturing slabs. The continuously cast slab thus obtained may be directly hot worked without being reheated, or may be reheated without being cooled to room temperature and then hot worked. Here, hot working means all hot working such as forging in addition to normal rolling. As described above, according to the present invention, under the conditions within the shaded area in FIG. 1 showing the relationship between slab surface temperature and processing strain rate, 5%
By applying the above working strain, hot cracking of the slab during continuous casting, and furthermore, hot cracking of the steel slab during so-called direct rolling and hot charge rolling can be effectively prevented. Here, the principle of the present invention will be further explained based on experimental data as follows. That is, steel having the composition shown in Table 1 was prepared, and tensile test samples were taken from the steel to conduct the following experiment.

[Table] FIG. 2 is an explanatory diagram showing various processing and heat treatment conditions adopted in this experiment. First, in order to simulate the machining applied to continuously cast slabs during the cooling process, the material solution-treated at 1350°C was heated to
We investigated the influence of the aperture value (RA) when tensile deformation was performed at 850°C at 10 ^-3 s ^-1 and the holding time when the specimen was held isothermally at 1100°C as a treatment equivalent to slow cooling prior to this tensile deformation. (Figure 2 case, and reference). Regarding A steel containing Nb, the test results are shown in the third section.
The figures are summarized in a graph. In the case corresponding to the conventional method, RA is extremely small, and ductility does not improve unless the case is held at 1100°C for a long time. However, if a 10% working strain is applied at ε = 10 ^-1 s ^-1 before holding at 1100°C as in case and above, the ductility will be greatly improved by holding at 1100°C for a short time. When the surface layer of the slab is processed at a low temperature, it is preferable to slow down the cooling and reheat afterward, as in the case. A similar graph obtained for steel B is shown in FIG. The case shows the case with 10% pre-processing. Therefore, it can be seen that processing the surface layer of the slab is effective in preventing cracking. Next, the effects of the amount of strain (ε) and the strain rate ε〓 of the processing were investigated. In the case shown in Figure 2, ε (strain amount) when pre-processed at ε 〓 ~ ^{10 -1} s ^-1 and held at 1100℃ for 10 minutes
Figure 5 summarizes the results of investigating the influence of RA on RA values. From the results shown, it was found that if the pre-processing strain was as low as 5%, a reduction of area of 50% or more could be obtained, and cracks in the slab were extremely unlikely to occur.
A similar tendency was observed in both steel A and steel B. In addition, the influence of ε〓 (strain rate) is also large; for example, when steel A is held at 1100°C for 10 minutes in the case shown in Figure 2, the temperature at 1100°C is
Figure 6 shows the relationship between the strain rate and RA value after 10% pre-processing at 900°C. From Figure 6, it can be seen that the larger ε〓 is, the greater the effect is obtained. At the pre-processing temperature of 1100℃, ε〓≧10 ^-1 s ^-1 ,
It can be seen that ε〓≧3×10 ^-1 s ^-1 is required for 900Kg. Next, the relationship between cracking during direct rolling after slab straightening was evaluated. The results are shown graphically in FIG. 7 as a relationship between RA value and isothermal holding time. In the case of the conventional method, A, B, C
Both steels exhibit low RA values, and on the other hand, even in cases where only isothermal holding treatment is performed prior to processing, a large improvement effect is not necessarily obtained. On the other hand, in the case of pre-processing with approximately 10% strain, an RA value of 50% or more was obtained with isothermal holding within 4 minutes after pre-processing, indicating that ductility could be greatly improved in both cases. Next, the reason for limiting the processing conditions in the present invention will be explained. The reason why the area to which processing strain is applied is limited to 2 mm or more of the surface layer of the slab is based on the knowledge that flaws that occur within 2 mm from the surface remain as cracks or streaks in subsequent processes. The reason why the amount of machining strain is limited to 5% or more and the strain rate is limited as shown in the above formula is that the amount of machining strain is limited to 5%.
If the amount of machining is not greater ^than
〓
This is based on the reason that if the condition of ≧a exp [-b/T+273] is not met, nucleation of precipitates may be difficult in some cases. In other words, in order to accumulate strain at such high temperatures, precipitation nuclei must be generated before recovery of the introduced dislocations occurs, and the higher the temperature, the higher ε〓 increases to prevent the accumulation of strain. The condition at that time is ε〓≧a·exp[−b/T+273]. Here, the reason why the conditional expression giving the strain rate takes such a form is as follows. Generally, the flow stress σ under high temperature deformation is expressed with respect to the strain rate ε〓 as follows. σ=K・ε〓・exp [−Q/k(T+273)] Here, K: Constant Q: Activation energy of diffusion In other words, the larger ε〓, the larger σ becomes, and the higher the temperature T, the lower σ becomes. do. This is due to the balance between work hardening due to the accumulation of dislocations during deformation, ie, an increase in dislocation density, and recovery to offset this, ie, a decrease in dislocation density due to the movement of atoms. Now, under the condition that the flow stress is constant, the relationship between ε〓 and T can be rewritten as ε〓=a・exp[−Q/k(T+273)], so ε〓=a・It is organized into the form exp[-b/T+273]. In the above equation, the knowledge obtained from Figure 6, that is, the obtained ε〓 for RA≧50% is ε〓≧5.62×10 ^-3 s ^-1 at 1100℃, and ε〓≧2.276 at 900℃.
×
By using the fact that 10 ³ s ^-1 , the values of coefficients a and b are determined, and a=1.3614 and b=7500 are obtained. In addition, the lower limit of T was set at 700°C because if the slab has already been cooled to a lower temperature, it will be difficult to coarsen the precipitates even with subsequent reheating. On the other hand, if the slab temperature exceeds 1200°C, the introduced dislocations will recover extremely quickly, making it impossible to generate the desired precipitates. In the present invention, the conditions for surface processing of a slab slab are shown as the shaded area in FIG. Furthermore, the optimal conditions for ε〓 and T will be explained using FIG. Figure 8 is a graph summarizing the embrittlement tendency observed in the slab when strain processing performed prior to slab straightening was simulated by tensile deformation under a series of processing conditions. , the shaded area is 50
% or less. The composition of the sample steel at this time was as follows.

[Table] In the figure, in region B, precipitation of carbonitrides occurs at the γ-grain boundaries and within the grains, and there is a risk of cracking in some cases even during machining of the surface layer of the slab. In region D, there is a risk of cracking due to precipitation of sulfides, and it is preferably avoided. Area A,
Areas C and E are safe, but as described in FIG. 1, area A is not preferable in this case due to strain accumulation. Figure 8 shows the phenomenon when tensile deformation is applied to Nb-containing steel as described above. If the target steel type or processing method is different, region D may disappear. For example, in high Mn or low S steel, the D region is less likely to occur, and the D region also disappears when the stress is compressive reduction. In the present invention, as a processing method for imparting processing strain as described above, for example, the use of a roll with protrusions on the surface of a guide roll, an air hammer, a special hydraulic press, etc. can be considered, and the required processing strain and strain rate can be considered. Other methods may be adopted in some cases as long as they can be achieved. The applicable steel types of the present invention are not particularly limited, but continuous casting slabs include AlN, NbC, TaC, TiC, BN, and VN.
This method is particularly effective for steel types that are prone to surface flaws that appear to be caused by precipitation. On the other hand, in a component system in which carbonitrides are difficult to precipitate, a great effect can be obtained in preventing surface defects mainly caused by sulfide precipitation during direct rolling or hot charge rolling. It is also conceivable to further promote the coarsening of precipitates by utilizing recuperation from inside the slab after surface processing is completed, but such a method is of course included in the present invention. This method is particularly effective in cases where the slab must be strongly cooled due to internal cracks or other problems. Example 1 This example is intended to demonstrate, by a simulation method, that by applying processing strain in advance, cracks do not occur in the slab even during subsequent rolling processing. 40mm thick, 220mm wide, with the composition shown in Table 2.
A slab with a length of 600 mm was cast, and half of the slab surface was subjected to processing strain under the conditions shown in Table 3 using a small electric hammer with a flat tip. The strain rate at this time was approximately 50 s ^-1 . The amount of processing strain was approximately 20% on average over the 5 mm surface layer. Next, the thus obtained slab was subjected to hydraulic bending, and the presence or absence of surface cracks was visually observed. For steel type I, length 20~
A deep crack of 50 mm, or a length of 5 to 10 mm for steel types.
Although lateral cracks of mm in diameter occurred, no surface cracks were observed at all in the area where processing strain was applied.

【table】

【table】

[Table] Example 2 Using a curved continuous casting machine with a radius of 12.5 m at a steel factory, slabs with a cross section of 250 mm x 2100 mm were cast under different conditions, and the degree of surface flaws on the slabs after straightening was evaluated. Visual evaluation was performed. Figure 9 shows a casting slab striking device whose power source is a hydraulic cylinder that moves in the width direction of the slab between rolls on the upper surface of the slab, which was used to apply processing strain to the surface layer of the slab at this time. Shown with line. In the illustrated example, machining strain is applied to the slab 1 by the slab striking device 2 at a stage when there is a portion of unsolidified molten steel. The slab striking device is indenter 3
It is composed of a hydraulic cylinder 4 connected thereto, and the amount of impact etc. of these are controlled by a controller 7 via a hydraulic unit 5 and a hydraulic pump unit 6. FIG. 10 shows the condition of the surface layer of a slab subjected to processing strain by an indenter attached to the tip of a slab striking device. Indenter spherical diameter 15mm, depth of depression 3mm,
The number of rolling blows was 180 cycles/min, and the average strain in the 3 mm surface layer of the slab was 7% and the strain rate was 0.3 s ^-1
It was hot. Table 4 shows the composition of the steel used in this example, and Table 5 shows the casting conditions and results. As can be seen from these results, many cracks occurred in the slab produced by the conventional method in which no machining strain was applied prior to straightening the slab, but the surface of the slab treated by the present invention did not undergo straightening. There were no cracks at all after that.

【table】

【table】

【table】

[Table] Next, a test was conducted using the same continuous casting machine and the same striking device, and the surface temperature pattern of the slab was strong cooling and recuperation type. Table 6 shows the composition of the steel used in this example, Table 7 shows the casting conditions and results, and Table 1 shows the casting conditions and results.
Figure 1 shows the temperature pattern at this time. Cracks occurred in the slabs produced by the conventional method in which no surface treatment was performed prior to straightening, whereas no cracks occurred at all on the surface of the slabs subjected to surface treatment by the method of the present invention. Example 3 Using a curved continuous casting machine with a radius of 12.5 m at a steel factory, a slab with a cross section of 250 mm x 2100 mm and the chemical composition shown in Table 8 was cast under different conditions as shown in Table 9, and after straightening. Surface flaws on the slab were visually evaluated. In addition, as a method of imparting processing strain to the surface layer of the slab at this time, as shown in Figure 12, the upper guide roll between 9 and 11 m from the molten metal surface is replaced with a roll with protrusions as shown in the same figure. The temperature pattern shown in Figure 13 was used. At this time, the protrusion with the shape shown in Figure 12 has a thickness of 72 mm.
A static iron pressure of 0.06 to 0.07 Kg/ ^mm2 is applied to the solidified shell surface of ~79 mm as a reaction force, and the strain spreads as shown in Fig. 14. From the following formula, the slab surface layer 5
A strain of at least 7% could be applied to a depth of mm. The strain rate was estimated to be 2×10 ⁻¹ s ⁻¹ . H=(Z+0.5)-1/√2×a S=(1.8~2.2)×a, and to give a minimum strain of 5%, a=7mm, H=3
mm was necessary. As shown in Table 9, according to the continuous casting machine equipped with the roll with projections according to the present invention, impressions of projections remained on the slab surface, but no cracks or defects occurred at all. In the conventional method that does not use a roll with projections, cracks and defects occur frequently, so the effect of the present invention is clear.

【table】

[Table] Since it was found that this method could prevent the occurrence of surface flaws in continuously cast slabs, we further tested the effect of preventing cracks during direct rolling. Steel A and B shown in Table 10 are melted, and the slab is cast using the method described above, with four sets of protruding rolls biting into both the upper and lower surfaces of the slab, and according to the temperature pattern shown in Figure 15. Thereafter, it was cut and rolled in 5 passes to a thickness of 150 mm using a rolling roll with a diameter of 1300 mm, and the degree of occurrence of surface flaws was visually evaluated. The results are summarized in Table 11. The conventional example is an example in which a roll with protrusions is not used. It is clear from the results shown in Table 11 that significant effects can be obtained by the present invention.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a graph showing the range of processing strain rate defined in the present invention with respect to processing temperature; FIG. 2 is a schematic diagram showing various processing heat treatment patterns;
Figures 7 to 7 are graphs showing the results of preliminary tests to consider the influence of pre-processing and strain rate on ductility; Figure 8 shows the embrittlement region as a function of strain rate and deformation temperature. Graph; Figures 9 and 10 are schematic explanatory diagrams of a slab striking device as a means for applying processing strain; Figure 11 shows the temperature pattern of the slab when it is reheated after processing. Graphs shown; Figure 12 is a schematic explanatory diagram when processing strain is applied using a roll with protrusions; Figure 13 is a graph showing the temperature pattern of the slab when processing strain is applied using a roll with protrusions; The figure is a schematic explanatory diagram of the propagation of processing strain when using a roll with protrusions;
The figure is a graph showing a slab surface temperature pattern when the slab is cut after being subjected to processing strain and further hot worked (rolled). 1: Slab, 2: Slab striking device, 3: Indenter, 4:
Hydraulic cylinder, 5: Hydraulic unit, 6: Hydraulic pump unit, 7: Controller.

Claims (1)

[Claims] 1.5 in the surface layer depth of 2 mm or more of the slab during continuous casting.
% or more, the strain rate ε〓(s ^-1 ) is ε〓≧aexp[-b/T+273] (where a=1.3614, b=7500, T is the slab surface temperature 700℃≦
A method for producing a continuously cast slab, characterized by passing the cast slab through straightening rolls after applying it under conditions (T≦1200°C). 2 5 on the surface layer depth of 2 mm or more during continuous casting
% or more, the strain rate ε〓(s ^-1 ) is ε〓≧aexp[-b/T+273] (where a=1.3614, b=7500, T is the slab surface temperature 700℃≦
A method for hot working continuous cast slabs, characterized by directly hot working the continuous cast slabs without reheating the obtained continuous cast slabs by passing them through straightening rolls after applying the conditions (T≦1200°C) . 3. 5 at a depth of 2 mm or more in the surface layer of the slab during continuous casting.
% or more, the strain rate ε〓(s ^-1 ) is ε〓≧aexp[-b/T+273] (where a=1.3614, b=7500, T is the slab surface temperature 700℃≦
A continuously cast slab characterized by being passed through straightening rolls after being applied under conditions of T≦1200°C), reheating the obtained continuous casting slab without cooling to room temperature, and then hot working. hot working method.