Manufacturing method of continuous casting slab

Abstract

To provide a production method of a continuous casting slab which does not generate slab cracking during cooling of the slab even if the slab is a high alloy slab with low toughness.SOLUTION: A production method of a continuous casting slab for a high-strength steel comprises casting a slab containing 0.10-0.40% of C, 0.10-2.50% of Si, and 1.00-5.00% of Mn in mass%, and then cooling the slab at a predetermined cooling rate so that the average former austenite grain size at a position 10 mm from the surface layer of the continuous casting slab is 100 μm or more and 0.5 mm or less, and the microstructure is a three-phase composite structure of ferrite, pearlite, and bainite, and the microstructure is 10% or more of ferrite, 10% or more of pearlite, and 1% or more and 30% or less of bainite in area ratio.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for producing a continuously cast slab which prevents cracks during cooling. [Background technology]

[0002] In recent years, in the field of automobiles, in order to achieve both thinner car bodies and collision safety, high strength steels have been further strengthened and therefore the use of high alloys has been progressing. However, the use of high alloys has significantly reduced the toughness of the slabs.

[0003] As toughness decreases due to high alloying, cracks during cooling of slabs, so-called "rest cracks", have become frequent. When rest cracks occur, the slab breaks during transportation and cannot be used for hot rolling. Even if the slab does not break, the cracks open during hot rolling and the hot rolled steel sheet breaks. Alternatively, if the cracks are small, they appear as surface defects such as scabs and slivers on the steel sheet after hot rolling, cold rolling, annealing, or plating. Usually, cracks on the slab surface are removed with a grinder. However, the toughness of the slab decreases due to high alloying, and the cracks grow due to the stress of the grinder, and they may not be completely removed. If the cracks are small, they may be overlooked and may appear as surface defects on the steel sheet after hot rolling, cold rolling, annealing, or plating. For this reason, it is necessary to suppress cracks in the slab.

[0004] Figure 1 is an image of the fracture surface of a crack in a continuously cast slab observed with a scanning electron microscope (SEM). The fracture surface appeared to be an intergranular fracture surface along the prior austenite grain boundaries. Figure 2 shows a microstructural photograph of the cross section of the crack. The depth of the crack was mainly about 20 mm from the surface of the slab. The crack propagated near the prior austenite grain boundaries, and grain boundary ferrite was present at the tip of the crack. Pearlite, or pearlite and bainite, was also observed within the prior austenite grains.

[0005] Intergranular fracture occurs when the prior austenite grains are coarse and the grain boundaries are embrittled. Precipitates and ferrite are more likely to form at the grain boundaries than within the grains. Precipitates at the grain boundaries reduce the grain boundary strength and cause a decrease in toughness. If the prior austenite grains are coarse, the proportion of the grain boundaries decreases, and the precipitate density increases, making the grain boundaries even more embrittled. In addition, when intergranular ferrite occurs, a difference in strength occurs between the prior austenite grains and the pearlite and bainite within the grains, so stress is concentrated in the grain boundary ferrite, which has low strength, and it progresses to cracks even with lower stress. Again, if the prior austenite grains are coarse, thin, linearly elongated intergranular ferrite precipitates, so the extension of the crack cannot be stopped and the damage increases. On the other hand, when the slab is cooled, stress is generated due to the difference in thermal contraction and transformation expansion between the surface and the interior of the slab. If this stress is high, the slab will crack when cooled to room temperature. In recent years, high-alloy, high-strength steels have low slab toughness, so cracks that occur in this way are deep and difficult to remove by manual maintenance such as grinding, which has been a problem that significantly reduces slab yields.

[0006] Regarding this point, measures against cracking of slabs have been studied, for example, as described in Patent Documents 1 and 2. Patent Document 1 discloses a method of suppressing bainite / martensite transformation and reducing stress caused by the transformation expansion by slowly cooling from 700 to 500°C, which is the temperature range in which austenite transforms into ferrite. Patent Document 2 discloses a method of starting slow cooling immediately after casting, maintaining the temperature at 700°C or higher for 10 hours or more, and then further slowly cooling the temperature to 700 to 500°C to reduce the temperature difference and stress during transformation. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-139209 A [Patent Document 2] JP 2019-167560 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, the conventional techniques have the following problems. The method of cooling a slab of high-tensile steel after casting described in Patent Documents 1 and 2 controls the internal stress generated in the slab to be small. However, in recent highly alloyed high-strength steels, the toughness of the slab is low, so the state of the prior austenite grain boundary where the crack propagates is also very important. The methods described in Patent Documents 1 and 2 do not control the prior austenite grain size or grain boundary ferrite, and do not limit the microstructure of the slab. In addition, as a result of intensive research by the inventors, it was found that the slab containing a large amount of C, Si, and Mn produced by the conventional techniques has a very low toughness, and the occurrence of cracks in the slab cannot be sufficiently suppressed.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a method for manufacturing a continuous cast slab that does not cause slab cracking during cooling, even in the case of a high alloy slab with low toughness. [Means for solving the problem]

[0010] The inventors have conducted extensive research to achieve the above object. As a result, they have analyzed the fracture morphology of slab cracks and found that the fracture surface is at least one of a grain boundary fracture surface along the prior austenite grain boundary and an intragranular fracture surface (cleavage fracture surface) crossing the prior austenite grain boundary. Furthermore, the inventors have conducted detailed research and found that the slab cracks cannot be suppressed only by controlling the cooling rate and reducing the stress by reducing the temperature unevenness, but are greatly influenced by the morphology of the microstructure. Specifically, they have found that the slab cracks during the cooling process of the continuously cast slab can be suppressed by controlling the average prior austenite grain size and microstructure of the continuously cast slab and improving its toughness, and have come up with the present invention.

[0011] The present invention was completed based on the above findings and through further investigation. That is, the gist and configuration of the present invention are as follows. 1. A method for producing continuous casting slabs for high strength steel, comprising: In mass%, it contains C: 0.10 to 0.40%, Si: 0.10 to 2.50%, and Mn: 1.00 to 5.00%, Optionally, at least one element selected from P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, sol.Al: 0.100% or less, and O: 0.0100% or less is contained; Furthermore, optionally, after casting a slab containing at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.020% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the balance being Fe and unavoidable impurities, The temperature of the slab is quenched to below the Bs point in the temperature range of 1200 to 900 ° C. at a position 10 mm below the surface at the center of the slab width, and then the cooling is stopped and the slab is reheated to above the Ac3 point so that the average prior austenite grain size at a position 10 mm from the surface of the continuous casting slab is 100 μm to 0.5 mm, The average cooling rate in the temperature range of 850°C to 700°C and 700°C to 500°C at the center surface of the slab width is determined so that the transformation start lines of ferrite, pearlite, and bainite are passed on the continuous cooling transformation diagram created according to the component composition, and the microstructure is a three-phase composite structure of ferrite, pearlite, and bainite. The microstructure is set so that, in terms of area ratio, ferrite is 10% or more, pearlite is 10% or more, and bainite is 1% or more and 30% or less. Here, the area ratio of the microstructure is determined as ferrite, and the remaining pearlite and bainite are observed using a scanning electron microscope with ferrite removed from the field of view. In the obtained structure image, the area ratio of bainite is determined as the structure of the recesses, and the area ratio of pearlite is determined as the structure of the recesses containing lamellar carbides, and the total area ratio with ferrite is calculated as 100%. A method for producing a continuous cast slab. Effect of the Invention

[0012] According to the present invention, it is possible to provide a continuously cast slab that does not develop cracks during the cooling process, even if the slab has a high alloy component system for high strength steel plate. [Brief description of the drawings]

[0013] [Figure 1] This is an image of the fracture surface of a crack in a continuously cast slab observed by a scanning electron microscope. [Diagram 2] 1 is a cross-sectional micrograph of the cracked portion. [Diagram 3] 1 is a graph showing the relationship between the cooling rate and the microstructure of a continuously cast slab on a continuous cooling transformation diagram (CCT diagram). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments of the present invention will be described in detail. The following embodiments are merely examples of steel compositions and structures for embodying the technical idea of ​​the present invention, and are not intended to limit the configuration to those described below. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0015] First, the suitable range of the microstructure of the continuous cast slab and the reason for limiting it will be explained. In the following explanation, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. In addition, the observation of the microstructure was performed at room temperature.

[0016] [Average prior austenite grain size: 100 μm to 0.5 mm] The average prior austenite grain size is a factor that determines the unit of fracture, and the larger it is, the lower the toughness of the slab becomes, and slab cracks that show grain boundary fracture surfaces occur. In conventional continuously cast slabs, the average prior austenite grain size is very large, at several mm in size. This significantly reduces the toughness of the continuously cast slab. This was not a problem in conventional low alloy steels because the original toughness was also high, but it can be a very serious problem in high alloy high strength steels. Therefore, in this embodiment, the average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is set to 100 μm or more and 0.5 mm or less. The factor that determines the austenite grain size is the cooling temperature. For example, the surface temperature of the slab at the center of the width is in the temperature range of 1200 to 900 ° C., and then the slab surface is quenched to the Bs point or less, and then the cooling is stopped and the slab is reheated to the Ac3 point or more, whereby the average prior austenite grain size at a depth of 10 mm from the surface layer of the continuously cast slab can be refined. In addition, it is preferable to cool the slab so that the residence time at 1450 to 1200°C is 40 to 130 seconds. Since it is difficult to actually measure the above temperatures, the temperature history at a position 10 mm below the surface of the continuously cast slab was calculated by heat transfer analysis. The analysis position was the center of the slab width, where the residence time in the above temperature range is the longest inside the slab. The average prior austenite grain size is preferably 0.4 mm or less.

[0017] [Microstructure] The ratio of internal structures such as ferrite, pearlite, and bainite below the austenite grains is also a factor that determines the unit of fracture, and it is known that an appropriate ratio improves toughness. The microstructure of a slab is greatly affected by the cooling rate below the temperature at which austenite transforms into ferrite (Ar3 temperature). The inventors have found that the toughness of a steel slab can be improved by controlling the cooling rate and setting the area ratio of the microstructure to 10% or more of ferrite, 10% or more of pearlite, and 1% to 30% or less of bainite. Bainite is preferably 5% to 30%.

[0018] The cooling rate for controlling the above-mentioned microstructures varies greatly depending on the steel composition. Therefore, we created a continuous cooling transformation diagram (CCT diagram) for the steel with that composition and determined the cooling rate that provides the optimum microstructure.

[0019] The cooling conditions can be controlled by changing the slab temperature at the exit of the continuous casting machine, the time until the slabs are stacked, the number of slabs stacked, water toughening treatment, etc. The cooling rate was measured using a thermocouple. A thermocouple was installed in the center (longitudinal center and widthwise center) of the top surface of the wide surface (long side x width) of the slab after it came out of the continuous casting machine.

[0020] Continuously cast slabs containing a large amount of C, Si, and Mn have extremely low toughness, and sufficient toughness to prevent the occurrence of thermal cracking cannot be ensured by controlling the requirements of only the average prior austenite grain size and the type of microstructure, and thermal cracking occurs. Therefore, it is important for the continuous cast slab for high strength steel according to this embodiment to simultaneously satisfy the requirements of the average prior austenite grain size and the microstructure.

[0021] Next, the appropriate range of the composition and the reason for limiting it will be explained. In the following explanation, "%" representing the content of the component elements of the steel means "mass %" unless otherwise specified.

[0022] [C: 0.10~0.40%] C is an important element for increasing the strength of a steel sheet. If the C content is less than 0.10%, it becomes difficult to achieve the tensile strength required for the steel sheet. On the other hand, if the C content exceeds 0.40%, it is impossible to obtain a microstructure in which ferrite, pearlite, and bainite are mixed as described above. Therefore, the C content is set to a range of 0.10 to 0.40%, preferably 0.12% or more, preferably 0.35% or less, more preferably 0.15% or more, and more preferably 0.30% or less.

[0023] [Si:0.10~2.50%] Si needs to be added to ensure the retention of austenite in the annealing process. In addition, it is an essential additive element because it contributes to high strength through solid solution strengthening. For this reason, it is necessary to add 0.10% or more. On the other hand, if it is added in excess of 2.50%, not only does the effect saturate, but also strong scale is generated on the hot-rolled sheet. This deteriorates the appearance and pickling properties, so the upper limit is set to 2.50%. Therefore, the Si content is set to a range of 0.10 to 2.50%, preferably 0.50% or more, preferably 2.0% or less, more preferably 1.00% or more, and more preferably 1.80% or less.

[0024] [Mn: 1.00~5.00%] Mn is an element added to increase the strength of steel sheet. Specifically, it is an element added to control the strength of steel sheet through transformation control during hot rolling. At less than 1.00%, sufficient strengthening cannot be achieved, so it is necessary to add 1.00% or more. On the other hand, adding more than 5.00% saturates the effect and is uneconomical. Therefore, the Mn content is set to a range of 1.00 to 5.00%, preferably 1.50% or more, preferably 4.50% or less, more preferably 1.80% or more, and more preferably 4.00% or less.

[0025] The continuous cast slab of this embodiment has the above-mentioned composition, the balance being Fe and inevitable impurities, and has an average prior austenite grain size and microstructure within appropriate ranges. Insofar as this is the case, taking into consideration other properties, it may contain P of 0.100% or less, S of 0.0200% or less, N of 0.0100% or less, sol.Al of 0.100% or less, and O of 0.0100% or less. Here, examples of impurities include Zn, Pb, and As. A total content of 0.100% or less of these inevitable impurities is permitted.

[0026] P segregates at prior austenite grain boundaries and embrittles the grain boundaries, which may cause slab cracks. Therefore, the P content is preferably 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable that the P content be 0.001% or more. Therefore, the P content is preferably 0.100% or less. Preferably, it is 0.001% or more. More preferably, it is 0.070% or less.

[0027] S exists as sulfide and is an element that causes slab embrittlement. Therefore, the S content is preferably 0.0200% or less. Although there is no particular lower limit for the S content, it is preferably 0.0001% or more due to constraints on production technology. Therefore, the S content is preferably 0.0200% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

[0028] Al is an element that inhibits the formation of carbides during slab cooling and promotes the formation of retained austenite, and thus affects the fraction of retained austenite in the slab. It is also preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, there is a risk of causing slab embrittlement. Therefore, the Al content is preferably 0.100% or less, more preferably 0.010% or more, and even more preferably 0.080% or less.

[0029] N exists as a nitride and is an element that causes embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is preferably 0.0100% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

[0030] O exists as an oxide and is an element that causes embrittlement of the slab. Therefore, the O content is preferably 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is preferably 0.0100% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

[0031] The continuous cast slab of this embodiment is for high strength steel and, in addition to the above-mentioned composition, further contains Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Cu: 1.00% or less. In addition, at least one element selected from Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.020% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained alone or in combination of two or more thereof.

[0032] If the contents of Ti, Nb and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the toughness of the slab is not reduced. Therefore, the contents of Ti, Nb and V are preferably 0.200% or less. Although the lower limits of the contents of Ti, Nb and V are not particularly specified, the contents of Ti, Nb and V are more preferably 0.001% or more, since they increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing. Therefore, when Ti, Nb and V are contained, the contents of each are 0.200% or less. More preferably, they are 0.001% or more. Even more preferably, they are 0.100% or less.

[0033] If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and the toughness of the slab is not reduced. Therefore, the content of Ta and W is preferably 0.10% or less. Although the lower limit of the content of Ta and W is not particularly specified, the content of Ta and W is more preferably 0.01% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ta and W are contained, the content of each is 0.10% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.08% or less.

[0034] If B is 0.0100% or less, it does not affect the toughness of the slab. Therefore, the content of B is preferably 0.0100% or less. Although there is no particular lower limit for the content of B, since B is an element that segregates to the austenite grain boundaries during hot rolling and annealing and improves hardenability, the content of B is more preferably 0.0003% or more. Therefore, when B is contained, the content is 0.0100% or less. More preferably, it is 0.0003% or more. Further preferably, it is 0.0080% or less.

[0035] If the content of Cr, Mo, and Ni is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, the content of Cr, Mo, and Ni is preferably 1.00% or less. Although there is no particular lower limit for the content of Cr, Mo, and Ni, since these elements improve hardenability, the content of Cr, Mo, and Ni is more preferably 0.01% or more. Therefore, when Cr, Mo, and Ni are contained, the content of each is 1.00% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.80% or less.

[0036] If Co is 1.00% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Co content is preferably 1.00% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content is 1.00% or less. More preferably, it is 0.001% or more. Further preferably, it is 0.80% or less.

[0037] If Cu is 1.00% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Cu content is preferably 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, it is more preferable that the Cu content be 0.01% or more. Therefore, when Cu is contained, its content is 1.00% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.80% or less.

[0038] Sn does not affect the toughness of the slab if it is 0.200% or less. Therefore, the Sn content is preferably 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content is more preferably 0.001% or more. Therefore, when Sn is contained, its content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

[0039] If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Sb content is preferably 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that suppresses decarburization and enables the strength adjustment of the steel plate, it is more preferable that the Sb content is 0.001% or more. Therefore, when Sb is contained, its content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

[0040] If the content of Ca, Mg and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase and the toughness of the slab will not decrease. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less. Although the lower limit of the content of Ca, Mg and REM is not particularly specified, since these elements make the shape of nitrides and sulfides spheroidal and improve the toughness of the slab, the content of Ca, Mg and REM is more preferably 0.0005% or more. Therefore, when Ca, Mg and REM are contained, the content of each is 0.0100% or less. More preferably, it is 0.0005% or more. Even more preferably, it is 0.0050% or less.

[0041] If Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the contents of Zr and Te are preferably 0.100% or less. Although there is no particular lower limit for the contents of Zr and Te, since they are elements that make the shape of nitrides and sulfides spherical and improve the toughness of the slab, it is more preferable that the contents of Zr and Te are each 0.001% or more. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. More preferably, they are 0.001% or more. Even more preferably, they are 0.080% or less.

[0042] If Hf is 0.10% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab does not decrease. Therefore, the Hf content is preferably 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel plate, the Hf content is more preferably 0.01% or more. Therefore, when Hf is contained, its content is 0.10% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.08% or less.

[0043] If the Bi content is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

[0044] In addition, when the contents of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi are less than the preferable lower limit values, the effects of the present invention are not impaired, and therefore these elements are included as unavoidable impurities. EXAMPLES

[0045] [Measurement of average prior austenite grain size] Here, the method for measuring the average prior austenite grain size is as follows. A sample was cut out from the center of the width of the cooled slab, and the slab thickness cross section parallel to the slab width direction was the observation surface. The observation surface was then mirror-polished using diamond paste, then finish-polished using colloidal silica, and further etched with 3 vol% nital to reveal the structure on the observation surface. Using an optical microscope, five fields of view were observed at a magnification of 10 times at a position 10 mm below the surface of the continuous casting slab to obtain a structure image. The average value of the prior austenite grain size was obtained from the obtained structure image by a cutting method in accordance with JIS G 0551:2020.

[0046] [Method of measuring the area ratio of ferrite] The measurement method of the ferrite area ratio is the same as the measurement method of the average prior austenite grain size, and the observation surface of the slab is prepared. Next, the observation surface is mirror-polished using diamond paste, then finish-polished using colloidal silica, and further etched with 3 vol% nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, 10 fields of view are observed at a position 10 mm below the surface layer of the continuous casting slab at a magnification of 50 times using a SEM (Scanning Electron Microscope), and the obtained structure image is calculated for the area ratio of ferrite for 10 fields of view using PHOTOSHOP (registered trademark) of Adobe, and the area ratio of ferrite is calculated by averaging these values. Note that ferrite has a larger grain size than other structures (pearlite, bainite, tempered martensite, quenched martensite, and retained austenite), and has a smooth surface and dark contrast, so it can be easily distinguished at a magnification of 50 times.

[0047] [Method for measuring the area ratio of pearlite and bainite] The method for measuring the area ratio of these structures is the same as the above-mentioned method for measuring ferrite, where the structures are revealed on the observation surface of the slab. Using an SEM under conditions of an accelerating voltage of 15 kV, 10 fields of view were observed at a magnification of 10,000 times, excluding ferrite from the field of view, at a position 10 mm below the surface layer of the continuously cast slab, and the area ratios of pearlite and bainite were calculated for the obtained structure images for the 10 fields of view using Adobe's PHOTOSHOP (registered trademark), and these values ​​were averaged, and calculated to be 100% in total together with the area ratio of ferrite measured by the above-mentioned method, and the area ratios of each structure were obtained. Bainite is a structure of the recesses, and pearlite is a structure of the recesses that contains lamellar carbides.

[0048] [Method of evaluating slab cracks] The evaluation method for slab cracks was based on the penetrant test specified in JIS Z 2343:2017, and the presence or absence of cracks was evaluated on the wide surface (length x width) and narrow surface (length x thickness) other than the cut surface of the slab. After applying the developer, the appearance of the penetrant was visually checked to check for cracks and defects on the surface.

[0049] The chemical composition of the steel used in the study is shown in Table 1, and the slab cooling conditions, slab microstructure, and slab cracking patterns are shown in Table 2. In the microstructure column, F, P, and B represent ferrite, pearlite, and bainite, respectively.

[0050] Tests No. 1 to 4 were conditions in which the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab was larger than 0.5 mm. In these cases, slab cracking could not be suppressed even if the cooling conditions for the slab after leaving the continuous caster were changed in various ways.

[0051] In Tests No. 5 to 9, the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab was 0.5 mm or less, but the microstructure types or their ratios were not suitable, and slab cracking could not be suppressed.

[0052] Test Nos. 10 to 23 are examples of the invention, in which the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab was 0.5 mm or less, and the microstructure had an area ratio of ferrite of 10% or more, pearlite of 10% or more, and bainite of 1% to 30% or less. In these cases, no slab cracking occurred after cooling.

[0053] In the case of the continuous cast slab according to the present invention, transfer may occur depending on various conditions. When transfer occurs, the cooling rate of the continuously cast slab may temporarily exceed the specified cooling rate. However, since the time required for transformation is very slow, at 10 hours or more, if the handling time is about the same as that for transfer (at most 1 to 2 hours), the occurrence of cracks due to placement will not occur. Therefore, in the present invention, the average cooling rate is specified instead of the maximum cooling rate.

[0054] [Table 1]

[0055] [Table 2]

[0056] In summary, it was found that cracking during cooling of the slab can be suppressed by setting the average prior austenite grain size 10 mm below the surface of a continuously cast slab to between 100 μm and 0.5 mm, and by making the microstructure at the same location a three-phase composite structure of ferrite, pearlite, and bainite.

[0057] To obtain such a slab structure, for example, when the surface temperature at the center of the slab width is in the temperature range of 900°C to 1200°C, the slab surface is quenched to below the Bs point, the cooling is stopped, the surface is reheated to above the Ac3 point, and then the average cooling rates are set to respective predetermined ranges such that the surface temperature at the center of the slab width is 850°C to 700°C and 700°C to 500°C. However, the manufacturing method is not limited to this method.

[0058] Next, FIG. 3 shows a method for determining a cooling rate that provides a suitable microstructure. FIG. 3 is a continuous cooling transformation diagram (CCT diagram) of steel C in Table 1. In FIG. 3, the transformation start lines of ferrite, pearlite, bainite, and martensite, and the transformation end line of martensite are indicated by symbols F, P, B, Ms, and Mf. Furthermore, in FIG. 3, the cooling rate lines are indicated by symbols X, Y, Z, and W in order of increasing cooling rate. Since the three phases of ferrite, pearlite, and bainite precipitate in the range of Y to Z on the cooling rate line, it can be seen that the suitable cooling rate is within this range. However, since it is difficult to predict the fraction of the structure after transformation from the continuous cooling transformation diagram, and the cooling rate of the slab is generally not always constant, it is preferable to treat it as a guideline. The method for creating the continuous cooling transformation diagram to be used is not particularly specified. It may be calculated using general commercial software, or may be created by experiment. [Industrial Applicability]

[0059] A continuous cast slab having a microstructure conforming to the present invention can provide a continuous cast slab for a high alloy, high strength steel plate that is free from slab cracks after casting, and can also prevent problems such as holes during rolling, making it industrially useful.

Claims

[Claim 1] A method for producing continuous casting slabs for high strength steel, comprising the steps of: In mass%, it contains C: 0.10 to 0.40%, Si: 0.10 to 2.50%, and Mn: 1.00 to 5.00%, Optionally, at least one element selected from P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, sol. Al: 0.100% or less, and O: 0.0100% or less is contained; Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, S A slab containing at least one element selected from b: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.020% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the balance being Fe and unavoidable impurities, is cast, The temperature at the 10 mm position below the surface at the center of the slab width is 1200 to 900°C and then quenched to below the Bs point. ３ The average prior austenite grain size at a position 10 mm from the surface of the continuously cast slab is 100 μm or more and 0.5 mm or less. The average cooling rate in the range of 850°C to 700°C and 700°C to 500°C at the surface at the center of the slab width is determined so as to pass through the transformation start lines of ferrite, pearlite, and bainite on a continuous cooling transformation diagram prepared according to the component composition, and the microstructure is a three-phase composite structure of ferrite, pearlite, and bainite. The microstructure is set so that, in terms of area ratio, ferrite is 10% or more, pearlite is 10% or more, and bainite is 1% or more and 30% or less. Here, the area ratio of ferrite is determined for the microstructure, and the pearlite and bainite of the remaining portion are observed using a scanning electron microscope with ferrite removed from the field of view, and in the obtained structure image, the area ratio of bainite is determined as the structure of the recesses, and the area ratio of pearlite is determined as the structure of the recesses containing lamellar carbides, and the total area ratio with ferrite is calculated as 100%. A method for producing a continuous cast slab.

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