WO2023218784A1

WO2023218784A1 - Continuous cast slab

Info

Publication number: WO2023218784A1
Application number: PCT/JP2023/012738
Authority: WO
Inventors: 大輝川▲崎▼; 智也小田垣; 健二鼓; 和彦山崎
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-05-09
Filing date: 2023-03-29
Publication date: 2023-11-16
Also published as: JPWO2023218784A1; TW202345994A; JP2024069381A

Abstract

Provided is a continuous cast slab that, even if the slab is highly alloyed and has low ductility, is configured not to cause slab fracture during cooling of the slab. The continuous cast slab for high-strength steel that, at a 10 mm position from the surface of the continuous cast slab, has an average former austenite grain size of 100 μm-0.5 mm, and has a micro structure in which, in terms of area ratio, ferrite is at least 10%, pearlite is at least 10%, and bainite is 1-30%. Preferably, the continuous cast slab contains, in terms of mass%, 0.10 to 0.40% of C, 0.10 to 2.50% of Si and 1.00 to 5.00% of Mn.

Description

Continuous casting slab

The present invention relates to a continuously cast slab that prevents cracking during cooling.

In recent years, in the field of automobiles, in order to achieve both further thinning of car bodies and ensuring collision safety, progress has been made in increasing the strength of high-strength steels and using higher alloys for this purpose. High alloying greatly reduces the toughness of the slab.

With the decrease in toughness due to high alloying, cracking during cooling of the slab, so-called cracking, has become more frequent. If cracks occur, the slab will break during slab transportation, making it impossible to use it for hot rolling. Further, even if the slab does not break, cracks open during hot rolling and the hot rolled steel plate breaks. Alternatively, if the cracks are small, they appear as surface defects such as sludge marks and sliver marks on the steel plate after hot rolling, cold rolling, annealing, or plating. Usually, cracks on the slab surface are removed using a grinder. However, the toughness of the slab decreases due to high alloying, and cracks may propagate due to the stress of the grinder, making it impossible to completely remove them. Small cracks 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 cracking of the slab.

Figure 1 is an observation image of the fracture surface of a crack in a continuously cast slab using a scanning electron microscope (SEM). The fracture surface had the appearance of a grain boundary fracture surface along prior austenite grain boundaries. Figure 2 shows a photograph of the cross section of the crack. The depth of the cracks was mainly about 20 mm from the surface layer of the slab. The crack propagated near the prior austenite grain boundary, and grain boundary ferrite was present at the crack tip. In addition, pearlite or pearlite and bainite were observed within the prior austenite grains.

Grain boundary fracture occurs when the prior austenite grains are coarse and the grain boundaries become brittle. Precipitates and ferrite are more likely to form at grain boundaries than inside grains. Precipitates at grain boundaries reduce grain boundary strength and become a factor in reducing toughness. If the prior austenite grains are coarse, the proportion occupied by the grain boundaries will decrease, and the density of precipitates will increase, making the grain boundaries even more brittle. Further, when grain boundary ferrite is generated, there is a difference in strength between pearlite and bainite within the grain, so stress concentration occurs in the grain boundary ferrite portion, which has low strength, and even a lower stress develops into cracks. Again, if the prior austenite grains are coarse, grain boundary ferrite that stretches thinly in a straight line will precipitate, making it impossible to stop the crack from spreading and causing more damage. On the other hand, when the slab is cooled, stress is generated due to differences in thermal contraction and transformation expansion between the surface and the inside of the slab. If this stress is high, slab cracking will occur when the slab is cooled to room temperature. Due to the low toughness of the slabs of recent high-alloy high-strength steels, the cracks that occur are deep and difficult to remove by cleaning with a grinder, etc., and this has been a problem that greatly reduces the yield of slabs.

Regarding this point, countermeasures against slab cracking are being considered, 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 the material to 700 to 500°C, which is the temperature range at which austenite transforms to ferrite. There is. Patent Document 2 discloses that slow cooling is started immediately after casting, and further slow cooling is carried out at a temperature of 700 to 500 °C for 10 hours or more at a temperature of 700 °C or higher to reduce temperature differences and stress during transformation. A method is disclosed.

Japanese Patent Application Publication No. 2020-139209 Japanese Patent Application Publication No. 2019-167560

However, the conventional technology had the following problems. The methods described in Patent Documents 1 and 2 for cooling a high-strength steel slab after casting control the internal stress generated in the slab to be small. However, because the toughness of slabs in recent high-alloyed high-strength steels is low, the state of prior austenite grain boundaries, where cracks propagate, is also extremely important. In the methods described in Patent Documents 1 and 2, the prior austenite grain size and grain boundary ferrite are not controlled, and the microstructure of the slab is not limited in any way. In addition, as a result of intensive studies by the inventors, it was found that slabs containing a large amount of C, Si, and Mn produced using conventional techniques had considerably low toughness and could not sufficiently suppress the occurrence of cracking in slabs. .

The present invention has been made in view of the above circumstances, and an object thereof is to provide a continuously cast slab that does not cause slab cracking during cooling, even if it is a high alloy slab with low toughness. .

The inventors have made extensive studies to achieve the above objective. As a result, we analyzed the fracture morphology of slab cracks, and found that the fracture surface was at least one of the following: intergranular fracture surfaces along prior austenite grain boundaries, and intragranular fracture surfaces (cleavage fracture surfaces) that cross prior austenite grain boundaries. discovered that seeds exist. Furthermore, the inventors conducted detailed studies and found that cracking in slabs cannot be suppressed by stress reduction alone by controlling the cooling rate and reducing temperature unevenness, and that the morphology of the microstructure has a large effect. I made it. Specifically, the inventors discovered that by controlling the average prior austenite grain size and microstructure of continuously cast slabs and improving their toughness, slab cracking during the cooling process of continuously cast slabs can be suppressed, and this led to the invention. did.

The present invention was completed based on the above findings and further studies.
That is, the gist of the present invention is as follows.
1. A continuously cast slab for high-strength steel, wherein the average prior austenite grain size at a position 10 mm from the surface of the continuous cast slab is 100 μm or more and 0.5 mm or less, and the microstructure is 10% or more of ferrite in terms of area ratio. , a continuously cast slab characterized by containing 10% or more of pearlite and 1% or more and 30% or less of bainite.
2. 1. The continuous casting slab contains, in mass %, C: 0.10 to 0.40%, Si: 0.10 to 2.50%, and Mn: 1.00 to 5.00%. Continuous casting slab.

According to the present invention, it is possible to provide a continuously cast slab that does not generate cracks during the cooling process even if the component system is a high-alloy high-strength steel plate.

This is a scanning electron microscope image of the fracture surface of a crack in a continuously cast slab. It is a photograph of the cross-sectional structure of the above-mentioned cracked part. It 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).

Hereinafter, embodiments of the present invention will be specifically described. Furthermore, the following embodiments are intended to illustrate the steel composition and structure for embodying the technical idea of the present invention, and the structure is not limited to the following. That is, the technical idea of the present invention can be modified in various ways within the technical scope described in the claims.

First, the appropriate range of the microstructure of a continuously cast slab and the reason for its limitation will be explained. In the following description, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. In addition, the tissue was observed at room temperature.

[Average prior austenite grain size: 100 μm or more and 0.5 mm or less]
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 is, and slab cracks exhibiting intergranular fracture surfaces occur. In conventional continuously cast slabs, the average prior austenite grain size is very large, several mm in size. For this reason, the toughness of the continuously cast slab is greatly reduced. This did not pose a problem with conventional low-alloy steels due to their high inherent toughness, but it can become a very serious problem with 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 continuous casting 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, after rapidly cooling the slab surface to below the Bs point in a temperature range of 1200 to 900°C at the center of the slab width, cooling is stopped and the temperature returns to the Ac3 point or above. By heating, it is possible to refine the average prior austenite grain size at a depth of 10 mm from the surface layer of the continuous casting slab. In addition, it is preferable to perform cooling so that the residence time at 1450 to 1200° C. is 40 seconds or more and 130 seconds or less. Since it is difficult to actually measure the above-mentioned temperature, the temperature history at a position 10 mm below the surface layer of the continuous casting slab was calculated by heat transfer analysis. The analysis position was set at the center of the slab width, where the residence time in the above temperature range is the longest within the slab. Note that the average prior austenite grain size is preferably 0.4 mm or less.

[Microstructure type]
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 toughness can be improved with an appropriate ratio. The microstructure of the slab is greatly influenced by the cooling rate below the temperature at which austenite transforms into ferrite (Ar ₃ temperature). The inventors improved the toughness of the steel slab by controlling the cooling rate and controlling the microstructure so that the area ratio was 10% or more ferrite, 10% or more pearlite, and 1% or more and 30% or less bainite. I found out what to do. Preferably, the bainite content is 5% or more and 30% or less.

The cooling rate for controlling the microstructure described above varies greatly depending on the steel composition. Therefore, a continuous cooling transformation diagram (CCT diagram) of steel with the above components was prepared, and a cooling rate at which the microstructure becomes suitable was determined.

This cooling condition can be controlled by changing conditions such as the temperature of the slabs at the exit side of the continuous casting machine, the time it takes to stack the slabs, the number of slabs to be stacked, or water toughness treatment. The cooling rate was measured using a thermocouple. After the slab came out of the continuous casting machine, a thermocouple was installed at the center of the upper surface (longitudinal center and widthwise center) of the wide surface (long side x width) of the slab.

Continuously cast slabs containing a large amount of C, Si, and Mn have extremely low toughness, and by controlling only the average prior austenite grain size and microstructure type to meet the requirements, sufficient toughness is ensured to prevent placement cracks. I was unable to do so, and a crack occurred. Therefore, it is important that the continuous casting slab for high-strength steel according to this embodiment simultaneously satisfies the requirements for the average prior austenite grain size and the microstructure.

Next, the appropriate range of component composition and the reason for its limitation will be explained. In the following description, "%" representing the content of component elements of steel means "mass %" unless otherwise specified.

[C: 0.10-0.40%]
C is an important element that increases the strength of steel sheets. If the C content is less than 0.10%, it becomes difficult to achieve the tensile strength required for the steel plate. On the other hand, if the C content exceeds 0.40%, the microstructure in which ferrite, pearlite, and bainite are mixed cannot be obtained as described above. Therefore, the C content is set in the range of 0.10 to 0.40%. Preferably it is 0.12% or more. Preferably it is 0.35% or less. More preferably, it is 0.15% or more. More preferably, it is 0.30% or less.

[Si: 0.10-2.50%]
Si needs to be added in order to ensure retained austenite in the annealing process. In addition, it is an essential additive element because it contributes to increasing strength through solid solution strengthening. From this, it is necessary to add 0.10% or more. On the other hand, addition of more than 2.50% not only saturates the effect, but also causes strong scale to occur in the hot rolled sheet. Since this deteriorates the appearance and pickling properties, the upper limit is set at 2.50%. Therefore, the Si content is set in the range of 0.10 to 2.50%. Preferably it is 0.50% or more. Preferably it is 2.0% or less. More preferably, it is 1.00% or more. More preferably, it is 1.80% or less.

[Mn: 1.00-5.00%]
Mn is an element added to increase the strength of the steel sheet. Specifically, it is an element added to control steel sheet strength through transformation control during hot rolling. If it is less than 1.00%, sufficient reinforcement cannot be achieved, so it is necessary to add 1.00% or more. On the other hand, if it is added in excess of 5.00%, the effect will be saturated and it will be uneconomical. Therefore, the Mn content is set in the range of 1.00 to 5.00%. Preferably it is 1.50% or more. Preferably it is 4.50% or less. More preferably, it is 1.80% or more. More preferably, it is 4.00% or less.

The continuous casting slab of the present embodiment has the above-mentioned component composition, with the remainder consisting of Fe and unavoidable impurities, and has an average prior austenite grain size and microstructure within an appropriate range. To that extent, considering other characteristics, P is 0.100% or less, S is 0.0200% or less, N is 0.0100% or less, sol. It may contain 0.100% or less of Al and 0.0100% or less of O. Here, examples of impurities include Zn, Pb, and As. The total content of these unavoidable impurities is allowed to be 0.100% or less.

Since P segregates at prior austenite grain boundaries and embrittles the grain boundaries, it may cause slab cracking. Therefore, the content of P is preferably 0.100% or less. Although the lower limit of the P content is not particularly defined, it is preferably 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the content of P is preferably 0.100% or less. Preferably it is 0.001% or more. More preferably, it is 0.070% or less.

S is an element that exists as a sulfide and causes slab embrittlement. Therefore, the S content is preferably 0.0200% or less. Although the lower limit of the S content is not particularly specified, it is preferably 0.0001% or more due to production technology constraints. 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.

Al is an element that affects the fraction of retained austenite in the slab because it suppresses the generation of carbides during cooling of the slab and promotes the generation of retained austenite. Further, it is preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, there is a risk of slab embrittlement. Therefore, the Al content is preferably 0.100% or less. More preferably, it is 0.010% or more. More preferably, it is 0.080% or less.

N is an element that exists as a nitride and causes slab embrittlement. Therefore, the N content is preferably 0.0100% or less. Although the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0001% or more due to constraints on production technology. 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.

O is an element that exists as an oxide and causes embrittlement of the slab. Therefore, the content of O is preferably 0.0100% or less. Although the lower limit of the O content is not particularly defined, it is preferable that the O content is 0.0001% or more due to production technology constraints. 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.

The continuous casting slab of this embodiment is for high-strength steel, and in addition to the above-mentioned composition, 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% Below, at least one element selected from Zr: 0.020% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less is used alone or in combination. A combination of the above may be contained.

If the content of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated and the toughness of the slab will not be reduced. Therefore, it is preferable that the contents of Ti, Nb, and V are each 0.200% or less. Note that the lower limits of the content of Ti, Nb, and V are not particularly specified, but the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated and the toughness of the slab will not be reduced. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. Note that there is no particular lower limit to the content of Ta and W, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when Ta and W are contained, their contents are each 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

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. Note that the lower limit of the B content is not particularly specified, but since it is an element that segregates at austenite grain boundaries during hot rolling and annealing and improves hardenability, the B content should be 0.0003% or more. It is more preferable to do so. Therefore, when B is contained, its content should be 0.0100% or less. More preferably, it is 0.0003% or more. More preferably, it is 0.0080% or less.

If each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates and inclusions do not increase and the toughness of the slab does not deteriorate. Therefore, it is preferable that the contents of Cr, Mo, and Ni are each 1.00% or less. Note that the lower limit of the content of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, it is more preferable that the content of Cr, Mo, and Ni is each 0.01% or more. . Therefore, when Cr, Mo, and Ni are contained, their contents are each 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Co is 1.00% or less, coarse precipitates and inclusions do not increase and the toughness of the slab does not deteriorate. Therefore, the Co content is preferably 1.00% or less. Although the lower limit of the Co content is not particularly specified, since it is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content should be 1.00% or less. More preferably, it is 0.001% or more. More preferably, it is 0.80% or less.

If Cu is 1.00% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Cu content is preferably 1.00% or less. Although the lower limit of the Cu content is not particularly specified, since it is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, if Cu is contained, the content should be 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Sn is 0.200% or less, it does not affect the toughness of the slab. Therefore, the content of Sn is preferably 0.200% or less. Note that the lower limit of the Sn content is not particularly specified, but since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content should be 0.001% or more. It is more preferable. Therefore, if Sn is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If Sb is 0.200% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the content of Sb is preferably 0.200% or less. Although the lower limit of the Sb content is not particularly defined, it is more preferable that the Sb content is 0.001% or more, since it is an element that suppresses decarburization and enables the strength adjustment of steel sheets. . Therefore, if Sb is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If each of Ca, Mg and REM is 0.0100% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less. Note that the lower limits of the contents of Ca, Mg, and REM are not particularly stipulated, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of slabs, the contents of Ca, Mg, and REM are More preferably, each content is 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each 0.0100% or less. More preferably, it is 0.0005% or more. More preferably, it is 0.0050% or less.

If Zr and Te are each 0.100% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the contents of Zr and Te are preferably 0.100% or less. Note that the lower limits of the contents of Zr and Te are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, the contents of Zr and Te are each 0.001. % or more is more preferable. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. More preferably, it is 0.001% or more. More preferably, it is 0.080% or less.

If Hf is 0.10% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Hf content is preferably 0.10% or less. Note that there is no particular lower limit to the Hf content, but since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of steel sheets, the Hf content should be 0.01% or more. It is more preferable to do so. Therefore, if Hf is contained, the content should be 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

If Bi is 0.200% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Bi content is preferably 0.200% or less. Although the lower limit of the Bi content is not particularly defined, since it is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

In addition, each content of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is preferable. If it is less than the lower limit, the effect of the present invention will not be impaired, and therefore it is included as an unavoidable impurity.

[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 width center position of the slab after cooling, so that the slab thickness cross section parallel to the slab width direction served as the observation surface. 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 on the observation surface. Using an optical microscope, 5 fields of view are observed at 10x magnification at a position 10 mm below the surface layer of the continuous casting slab to obtain a tissue image. The obtained structure image was cut according to JIS G 0551:2020 to determine the average value of the prior austenite grain size.

[Measurement method of area ratio of ferrite]
To measure the ferrite area ratio, prepare the observation surface of the slab in the same manner as the above-mentioned method for measuring the average prior austenite grain size. Next, the observation surface is mirror-polished using diamond paste, then final polished using colloidal silica, and further etched with 3 vol % nital to reveal the structure. Using an SEM (Scanning Electron Microscope) at an accelerating voltage of 15 kV, 10 fields of view were observed at a magnification of 50x at a position 10 mm below the surface layer of the continuous casting slab, and the obtained tissue images were collected using Adobe. Using PHOTOSHOP (registered trademark), the area ratio of ferrite was calculated for 10 fields of view, and the values were averaged to determine the area ratio of ferrite. In addition, ferrite has a larger grain size than other structures (pearlite, bainite, tempered martensite, hardened martensite, retained austenite), has a smooth surface, and has a dark contrast, so it can be easily seen at 50x magnification. I can tell the difference.

[Method of measuring area ratio of pearlite and bainite]
The method for measuring the area ratio of these structures causes the structure to appear on the observation surface of the slab, similar to the method for measuring ferrite described above. Using an SEM, 10 fields of view were observed at a magnification of 10,000 times with the ferrite removed from the field of view at a position 10 mm below the surface layer of the continuous casting slab using an SEM at an accelerating voltage of 15 kV. (registered trademark), calculate the area ratio of pearlite and bainite for 10 fields of view, average those values, and calculate the total to 100% when combined with the area ratio of ferrite measured using the method described above. , was calculated as the area ratio of each tissue. Bainite is a concave structure, and pearlite is a concave structure containing lamellar carbides.

[Evaluation method of slab cracking]
The evaluation method for slab cracks is based on the penetrant test specified in JIS Z 2343:2017, and cracks on wide areas (length x width) and narrow areas (length x thickness) other than the cut surface of the slab are evaluated. The presence or absence was evaluated. After applying the developer, the appearance of the penetrating solution was visually observed to visually check for cracks and flaws on the surface.

Table 1 shows the chemical composition of the steel used in the study, and Table 2 shows the slab cooling conditions, slab microstructure, and slab cracking mode. F, P, and B in the microstructure column represent ferrite, pearlite, and bainite, respectively.

Test No. Conditions 1 to 4 are conditions in which the average prior austenite grain size at a position 10 mm below the surface layer of the continuous casting slab is larger than 0.5 mm. In these cases, slab cracking could not be suppressed even if various changes were made to the conditions for cooling the slab after it exited the continuous casting machine.

Test No. Examples 5 to 9 are examples in which the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is 0.5 mm or less, but the microstructure types or their ratios were not compatible and slab cracking could not be suppressed.

Test No. Nos. 10 to 23 are invention examples in which the average prior austenite grain size at a position 10 mm below the surface layer of the continuous casting slab is 0.5 mm or less, and the microstructure has an area ratio of 10% or more of ferrite and 10% of pearlite. % or more, and the bainite content was 1% or more and 30% or less. No slab cracking occurred in these after cooling.

Note that the continuously cast slab according to the present invention may be transshipped depending on various conditions. When transshipment occurs, the cooling rate of the continuously cast slab may temporarily exceed the predetermined cooling rate. However, since the time required for transformation is very slow at 10 hr or more, the handling time required for transshipment (1 to 2 hr at most) will not cause cracking. Therefore, in the present invention, the average cooling rate is defined instead of the maximum cooling rate.

In summary, by setting the average prior austenite grain size at a position 10 mm below the surface layer of a continuous casting slab to 100 μm or more and 0.5 mm or less, and making the microstructure at the same position a three-phase composite structure of ferrite, pearlite, and bainite, it is possible to It has been found that cracking can be suppressed.

In order 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 rapidly cooled to below the Bs point, and then the cooling is stopped and the AC ₃ points This can be achieved by recuperating the heat above and then setting the average cooling rate at a temperature of 850° C. to 700° C. and a temperature of 700° C. to 500° C. within a predetermined range, respectively. However, the manufacturing method is not limited to this method.

Next, in FIG. 3, a method for determining the cooling rate at which the microstructure becomes suitable is shown. 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 lines 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 descending order of cooling rate. Since the three phases of ferrite, pearlite, and bainite precipitate in the range Y to Z on the cooling rate line, it can be seen that a 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 a cast slab is generally not always constant, it is preferable to treat it as a guideline. Note that the method of creating the continuous cooling transformation diagram to be used is not particularly specified. It may be calculated using general commercial software or created through experiments.

If the continuous casting slab has a microstructure that conforms to the present invention, it is possible to provide a continuous casting slab for high-alloy high-strength steel plates without slab cracking after casting, and it is also possible to prevent problems such as holes during rolling. Therefore, it is industrially useful.

Claims

A continuously cast slab for high-strength steel, wherein the average prior austenite grain size at a position 10 mm from the surface of the continuous cast slab is 100 μm or more and 0.5 mm or less, and the microstructure is 10% or more of ferrite in terms of area ratio. , a continuously cast slab characterized by containing 10% or more of pearlite and 1% or more and 30% or less of bainite.
The continuous casting slab is
The continuous casting slab according to claim 1, containing, in mass %, C: 0.10 to 0.40%, Si: 0.10 to 2.50%, and Mn: 1.00 to 5.00%.