WO2023218787A1

WO2023218787A1 - Continuous casting slab and method for manufacturing same

Info

Publication number: WO2023218787A1
Application number: PCT/JP2023/012746
Authority: WO
Inventors: 智也小田垣; 大輝川▲崎▼; 健二鼓; 高太郎田中; 真村田; 政志船橋
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: JPWO2023218787A1; TW202346606A; JP7477052B2

Abstract

The present invention provides: a continuous casting slab in which season cracking does not occur while cooling, even when the continuous casting slab has a low toughness; and a method for manufacturing the same. A continuous casting slab for high-strength steel, characterized in that: the average prior austenite grain size at a 10-mm position from the continuous casting slab surface layer is 0.5-2.0 mm inclusive; and the microstructure is such that the total of the area ratio of ferrite and the area ratio of perlite is 90% or more and the area ratio of ferrite is below 5% or 10% or more.

Description

Continuous casting slab and its manufacturing method

The present invention relates to a continuously cast slab that prevents cracking during cooling and a method for manufacturing the same. More specifically, the present invention relates to a continuously cast slab for high-strength steel (high tensile strength steel) that is effective in preventing cracking and does not cause problems with holes during rolling, and a method for producing the same.

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 steel, and in order to achieve this, the use of higher alloys. High alloying greatly reduces the toughness of the slab.

With the decline in slab toughness due to high alloying, cracking during slab cooling, so-called cracking, has become more frequent. If a crack occurs, the slab may break during slab transportation, and the slab may not be able to be subjected to hot rolling. Furthermore, even if the slab does not break, cracks may open during hot rolling of the slab and the hot rolled steel plate may break. Alternatively, if the cracks in the slab are small, they appear as surface defects such as sludge defects and sliver defects on the steel plate after hot rolling, cold rolling, annealing, or plating. Cracks on the slab surface are usually removed using a grinder. However, the toughness of the slab decreases due to high alloying, and the cracks in the slab propagate due to the stress of the grinder, so that it may not be possible to completely remove the cracks in the slab. On the other hand, small cracks in the slab may be overlooked and appear as surface defects on the steel plate after hot rolling, cold rolling, annealing, or plating. For these reasons, it is necessary to suppress cracking of the slab.

FIG. 1 is an enlarged photograph taken using a scanning electron microscope (SEM) of the fracture surface of a cracked part of a high-strength steel slab that was fractured due to a crack. As is clear from FIG. 1, the fracture surface of the slab crack 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 slab crack. The depth of the slab cracks was mainly about 20 mm from the slab surface layer. The slab crack propagated near the prior austenite grain boundaries, and grain boundary ferrite was present at the tip of the slab crack. 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 slab 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. Furthermore, when grain boundary ferrite is formed, there is a difference in strength between pearlite and bainite within the grains, so stress concentration occurs in the grain boundary ferrite portion, which has low strength, and even a lower stress develops into cracks in the slab. Here too, if the prior austenite grains are coarse, grain boundary ferrite that extends thinly in a straight line will precipitate, making it impossible to stop the extension of slab cracks and increasing the damage caused by cracks in the slab. 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 large, slab cracking will occur when the slab is cooled to room temperature. Since the slab toughness of recent high-alloy high-strength steels is low, it is difficult to remove deep cracks in the slab by using a grinder, etc., and this has been a problem that greatly reduces the yield of slabs. .

From this point of view, methods have been proposed to suppress the occurrence of cracks in high-tensile steel slabs. For example, Patent Document 1 proposes a method of suppressing bainite/martensite transformation and reducing stress caused by the transformation expansion by slowly cooling the temperature range of 700 to 500°C, which is the temperature range where austenite transforms to ferrite. has been done. That is, Patent Document 1 discloses a method capable of suppressing the occurrence of cracks even in high-tensile steel, which is a type of steel where cracks are likely to occur. Specifically, the method for cooling a high-tensile steel slab disclosed in Patent Document 1 is based on the knowledge that the internal stress of high-tensile steel depends on its cooling rate. This method suppresses the occurrence of cracks by controlling the cooling rate of the slab.

In addition, Patent Document 2 discloses that slow cooling is started immediately after the slab is cast, 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, thereby reducing temperature differences and transformation. Methods have been proposed to reduce stress. In other words, Patent Document 2 describes a method for cooling a slab for high-strength steel sheets that does not cause not only slab cracking during cooling but also quality defects such as baldness during hot rolling even if the slab has a component containing Si. A method is disclosed. Specifically, the method for cooling a high-strength steel plate slab disclosed in Patent Document 2 is to continuously cast a high-strength hot-rolled steel plate slab with a limited content of chemical components such as C, Si, and Mn at 500 to 700°C. The average cooling rate is 20° C./hr or less.

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

However, the above conventional technology has the following problems. The method of cooling a high-strength steel slab after casting described in Patent Document 1 focuses only on the temperature range from 700°C to 500°C when the slab is cooled after casting. The internal stress generated in the slab is controlled 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. Since the method described in Patent Document 1 does not control the prior austenite grain size or grain boundary ferrite, the carbon content was increased using the method for cooling a slab of high-strength steel described in Patent Document 1. Even if the slab is manufactured, it is not possible to sufficiently suppress the occurrence of cracking in the slab.

Furthermore, the cooling method for high-strength steel plate slabs described in Patent Document 2 is based on the knowledge that the cause of slab cracking is the addition of Si to the steel and the thermal stress generated due to temperature unevenness within the slab. , focuses on reducing thermal stress to suppress cracking of slabs. However, in the method for cooling a high-strength steel plate slab described in Patent Document 2, there are no limitations on the microstructure of the slab. For this reason, even if a slab is manufactured using the cooling method for a high-strength steel plate slab described in Patent Document 2, it is not possible to sufficiently suppress the occurrence of cracking in the slab.
In addition, as a result of intensive studies by the present inventors, it was found that the conventional slabs containing a large amount of C, Si, and Mn had considerably low toughness, making it impossible to completely suppress cracking, and causing problems with holes during rolling. We found that this occurs.

The present invention has been made in view of the above circumstances, and even in continuously cast slabs with low toughness, cracks do not occur during cooling of the slab, and holes are not formed during rolling. The purpose of the present invention is to provide a continuous casting slab that does not cause trouble and a method for manufacturing the same.

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 surfaces include at least two of the following: intergranular fracture surfaces along prior austenite grain boundaries, and intragranular fracture surfaces (cleavage fracture surfaces) that cross prior austenite grain boundaries. It was discovered that one species exists. 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, by controlling the average prior austenite grain size and microstructure in continuously cast slabs and improving their toughness, we are able to suppress slab cracking during the cooling process of continuously cast slabs, and to reduce the problem of holes during rolling. They discovered that it is possible to prevent this from occurring, and came up with the present invention.

That is, the continuous casting slab according to the present invention, which advantageously solves the above problems, is a continuous casting slab for high-strength steel, and the average prior austenite grain size at a position 10 mm from the surface layer of the continuous casting slab is 0.5 mm or more and 2.0 mm. and the microstructure is characterized in that the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of ferrite is less than 5% or 10% or more. .

The continuous casting slab according to the present invention contains (a) in mass %, C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, Mn: 0.40% It is considered that containing 5.00% or less of the above amount may be a more preferable solution.

Furthermore, the method for producing a continuously cast slab according to the present invention is a method for producing a continuously cast slab for high-strength steel in which slab cracking caused by cooling is suppressed,
A continuous casting slab having the composition described in (a),
Cooling conditions in which the cooling temperature of the continuous casting slab at the center in the width direction of the continuous casting slab and at a position 10 mm from the surface layer of the continuous casting slab is 1200°C or more and 1450°C or less, and the residence time of the continuous casting slab is 130 seconds or less. a first cooling step of cooling by;
A second cooling step in which the continuously cast slab is cooled under cooling conditions in which the surface temperature at the center in the width direction is 700° C. or more and 850° C. or less and the average cooling rate is 20° C./hr or less;
The continuous casting slab is characterized by including a third cooling step of cooling under cooling conditions in which the surface temperature at the center in the width direction is from 500° C. to 700° C. and the average cooling rate is 10° C./hr or less.

According to the present invention, it is possible to provide a continuous casting slab that does not cause cracking during the cooling process and does not cause problems with holes during rolling, even with the composition system of continuous casting slabs for high-strength steel. .

This is a photograph taken using a scanning electron microscope (SEM) of a fractured surface of a cracked part of a continuously cast slab for high-strength steel that was fractured due to a crack. It is a photograph of the cross-sectional structure of the above-mentioned cracked part. 1 is an enlarged photograph observed with an optical microscope of a continuous casting slab manufactured in an invention example (Test No. D-9) of a continuous casting slab according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be specifically described. Note that each drawing is schematic and may differ from the actual drawing. Furthermore, the following embodiments are intended to exemplify devices and methods for embodying the technical idea of the present invention, and the configuration 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 embodiment]
A continuous casting slab according to the first embodiment will be described. The continuous casting slab according to the present embodiment is a continuous casting slab for high-strength steel, and (i) the average prior austenite grain size at a position 10 mm from the surface layer of the continuous casting slab is 0.5 mm or more and 2.0 mm or less, and ( ii) The microstructure is characterized in that the total area ratio of ferrite and pearlite is 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, according to the invention according to this embodiment, by providing at least the characteristics (i) to (iii) above, cooling is possible even in recent continuous casting slabs for high-strength steel, which have extremely low toughness. It is possible to provide a continuously cast slab for high-strength steel with a high yield, without causing cracks in the slab during the rolling process, and preventing problems with holes during rolling.

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. Furthermore, the microstructure of the continuously cast slab was observed at room temperature.

As mentioned above, when we observed the fracture morphology of the fracture surface of the cracked part of a continuously cast slab for high-strength steel that had fractured due to over-place cracking, we found that most of the cracks had propagated to about 20 mm below the surface layer of the slab, and that prior austenite It was found that this was a form of "grain boundary fracture" in which cracks developed at grain boundaries. In other words, in continuously cast slabs for high-strength steel, the causes of cracking due to grain boundary fracture are the coarse prior austenite grain size and the grain boundary ferrite, which is a factor that embrittles the grain boundaries. It is the existence of an organization. In high-alloy high-strength steel plates, in addition to the extremely low toughness of the original continuously cast slab, if these embrittlement factors are present, even if the slab is slowly cooled to reduce stress, it is difficult to suppress cracking due to placement. Can not do it. Therefore, in this embodiment, as necessary conditions for a continuous casting slab for high-strength steel that does not cause cracking during the cooling process, (i) the average prior austenite grain size at a predetermined position from the surface layer of the continuous casting slab; We focused on these two phenomena, consisting of ii) to (iii) the microstructure of the continuously cast slab.

<(i) Regarding average prior austenite grain size>
The continuous casting slab for high-strength steel according to the present embodiment is a continuous casting slab for high-strength steel in which cracking caused by cooling is suppressed, and includes (i) average prior austenite grains at a position of 10 mm from the surface layer of the continuous casting slab; It is characterized by having a diameter of 0.5 mm or more and 2.0 mm or less. Here, the average prior austenite grain size is a factor that determines the unit of fracture of the slab. Grain boundaries have the characteristic that precipitates tend to concentrate because solute components tend to concentrate. That is, as the average prior austenite grain size increases, the grain boundary ratio per unit volume decreases, so the precipitate density increases, thereby decreasing the toughness of the continuously cast slab. Here, the average prior austenite grain size refers to the average value of a plurality of prior austenite grain sizes calculated from prior austenite grain sizes measured in a plurality of visual fields.

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. In conventional low alloy steels, the average prior austenite grain size was not a problem because the original continuously cast slab had high toughness, but in high alloy high strength steels, the average prior austenite grain size became a very serious issue. It can be. Therefore, in the continuous casting slab according to the present embodiment, the average prior austenite grain size at a position 10 mm from the surface layer of the continuous casting slab was set to 2.0 mm or less. It is preferable that the average prior austenite grain size is 2.0 mm or less because precipitates concentrated at prior austenite grain boundaries can be dispersed and the toughness of the continuously cast slab is not reduced.
On the other hand, although the lower limit of the average prior austenite grain size is not strictly limited, in order to reduce the average prior austenite grain size to a fine size of less than 0.5 mm, it is necessary to perform strong cooling at the initial stage of solidification, for example. In that case, there is a risk of heterogeneous coagulation breakout occurring. Therefore, the lower limit of the average prior austenite grain size is preferably 0.5 mm or more. Note that the lower limit of the average prior austenite grain size is preferably 0.8 mm or more, more preferably 1.0 mm or more.

The average prior austenite grain size is defined as the grain size of crystal grains constituting the prior austenite structure at a position 10 mm from the surface layer of the continuous casting slab. Here, when setting the average prior austenite grain size, the reason why it was specified to be 10 mm from the surface layer of the continuous casting slab is that most of the cracks in slabs have progressed to about 20 mm below the surface layer of the slab. This is because a position 10 mm from the surface layer of the cast slab is considered to be a necessary position to suppress cracking of the slab.
On the other hand, a region less than 5 mm from the surface of the continuously cast slab is directly quenched by the mold or water spray directly under the mold. For this reason, the continuously cast slab has a microstructure with a fine γ grain size, and the slab has high toughness, and it is difficult to imagine that the starting point of placement cracks occurs from this region. Therefore, the region less than 5 mm from the surface layer of the continuously cast slab can be excluded from the position where control of the slab structure is required. Therefore, the position where the continuous casting slab structure needs to be controlled is the position 10 mm from the depth in the slab thickness direction, for example, based on the position 10 mm from the surface layer of the continuous casting slab, It may be 5 to 20 mm.

In the continuous casting slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuous casting slab is cooled. The temperature at which the continuously cast slab is cooled is particularly in the range of 1450° C. or lower and 1200° C. or higher, and is influenced by the residence time. Furthermore, the longer the residence time of the continuously cast slab, the coarser the average prior austenite grain size. That is, in order for the continuous casting slab according to this embodiment to satisfy the condition that (i) the average prior austenite grain size at a position 10 mm from the surface layer of the continuous casting slab is 0.5 mm or more and 2.0 mm or less, It is important to control the residence time of the continuously cast slab in the above process. Specifically, it is preferable that the residence time of the continuously cast slab at a temperature of 1450° C. or lower and 1200° C. or higher at a position 10 mm in the thickness direction from the surface layer of the slab is 130 seconds or less.
If the residence time of the continuously cast slab at temperatures below 1450°C and above 1200°C is 130 seconds or less, the average prior austenite grain size can be reduced to 2.0 mm or less, and by controlling the average prior austenite grain size small, precipitates and This is preferable because grain boundary ferrite can be dispersed, the toughness of the slab can be improved, and cracks in the slab can be suppressed.
Furthermore, from this point of view, the residence time of the continuously cast slab is preferably 120 seconds or less, more preferably 110 seconds or less, and still more preferably 100 seconds or less.
Note that the lower limit of the residence time of the continuous casting slab is not particularly limited, but if the residence time is too short, the risk of breakout during continuous casting due to uneven solidification increases, so it is set to 40 seconds or more.
That is, if the residence time of the continuously cast slab at temperatures below 1450° C. and above 1200° C. is less than 40 seconds, there is a risk of cracking due to uneven solidification and breakout, so it is preferably set to 40 seconds or more. From this point of view, the residence time of the continuously cast slab at a temperature of 1450° C. or lower and 1200° C. or higher is more preferably 60 seconds or more, and even more preferably 70 seconds or more.

The residence time of continuously cast slabs can be controlled by adjusting the cooling conditions at the initial stage of slab casting. For example, in continuous steel casting, molten steel whose composition has been adjusted is first injected into a water-cooled copper mold to form an initial solidified shell. After that, drawing begins, and after it comes out of the water-cooled copper mold, it is cooled by water spray. The slab surface temperature in the above range is greatly influenced by the cooling inside the mold and directly below the mold, so for example, it is possible to improve the thermal conductivity of the mold powder to lubricate the inside of the mold, or to improve the thermal conductivity of the mold powder used to lubricate the inside of the mold. It can be controlled by increasing the flow rate.

By controlling these cooling conditions, it is possible to control the average prior austenite grain size in the 10 mm below the surface of the continuously cast slab. It is difficult to actually measure the cooling temperature of continuously cast slabs. For this reason, heat transfer analysis is performed to represent the area from a depth of 5 mm in the thickness direction of the continuous casting slab to a depth of 20 mm in the thickness direction of the continuous casting slab. By calculating the temperature history at a position 10 mm in, it is possible to estimate the cooling temperature of the continuous casting slab. In order to maximize the residence time in the temperature range inside the continuous casting slab, the heat transfer analysis position can be set at the center of the width of the continuous casting slab.

<(ii) to (iii) Microstructure of continuous casting slab>
The continuous casting slab according to the present embodiment has (ii) a microstructure in which the total area ratio of ferrite and pearlite is 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10%. % or more. In other words, in addition to the average prior austenite grain size at a position 10 mm from the surface of the continuous casting slab being 2.0 mm or less, the ratio of internal structures such as bainite and ferrite is also a factor that determines the unit of fracture, and it is necessary to It is known to improve the toughness of slabs. Therefore, the inventors controlled the cooling rate so that (ii) the microstructure had a total area ratio of ferrite and pearlite of 90% or more, and (iii) the area ratio of ferrite was less than 5%. It has also been found that when the content is 10% or more, the toughness of the slab is improved. Note that the area ratio of ferrite and the area ratio of pearlite can be calculated based on the observation results of the microstructure of the continuously cast slab using an observation means such as an optical microscope. Then, using an observation means such as an optical microscope, ferrite and pearlite contained in the microstructure of the continuously cast slab can be distinguished.

Based on the identification results of the microstructure of the continuous casting slab, the area S _total of the microstructure of the continuous casting slab and the total area S _{(ferrite+pearlite)} of the area S _ferrite and the area S _pearlite are calculated. Then, the ratio of the area S _{(ferrite+pearlite), which is} the sum of the ferrite area S _ferrite and the pearlite area S _pearlite to the area S _total of the microstructure of the continuous casting slab, is defined as an area ratio (%) and calculated.

The continuous casting slab according to the present embodiment is characterized in that (ii) the microstructure has a total area ratio of ferrite and pearlite of 90% or more. That is, in the continuous casting slab according to the present embodiment, (ii) the ratio of the area S _{(ferrite+pearlite),} which is the sum of the ferrite area S _ferrite and the pearlite area S _pearlite , to the area S _total of the microstructure of the continuous casting slab. If the area ratio (%) is 90% or more, it is possible to reduce thermal stress and transformation stress due to bainite-martensitic transformation during slow cooling of the slab, and these stresses that occur are also sufficiently absorbed within the microstructure. It is preferable because it can improve the toughness of continuously cast slabs by dispersing it into ferrite and pearlite, which are present in large quantities. On the other hand, if the area ratio is less than 90%, the toughness of the continuously cast slab will decrease, which is not preferable.

Furthermore, the continuous casting slab according to the present embodiment is characterized in that (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, in the continuously cast slab according to the present embodiment, when the area ratio of ferrite is 5% or more and less than 10%, thin ferrite exists at the grain boundaries, stress concentrates on the soft ferrite part, and cracks develop. It is not desirable because If the area ratio of ferrite is less than 5%, it is preferable because even if a crack develops, it stops immediately, and when the area ratio of ferrite is 10% or more, it is preferable because stress is difficult to concentrate in the ferrite part and the crack does not develop.

Here, grain boundary ferrite is a factor that determines grain boundary strength. When grain boundary ferrite occurs, it reduces the toughness of continuously cast slabs. Furthermore, since ferrite has lower strength than austenite, pearlite, and bainite, there is a problem in that when stress is applied, stress tends to concentrate on grain boundary ferrite. Based on this point of view, the inventors of the present invention have repeatedly investigated and found that the microstructure types of the continuous casting slab according to the present embodiment can be continuously cast by suppressing the formation of grain boundary ferrite even in a pearlite-based structure. It was discovered that the toughness of cast slabs can be greatly improved.

Note that ferrite contains iron containing up to 0.02% by mass of carbon, and has a structure close to that of pure iron. Ferrite is a ferromagnetic material from room temperature to 780° C., and is the softest and most ductile of all steel structures. Pearlite is a structure obtained when austenite is slowly cooled. Pearlite consists of ferrite layers and cementite layers, and is formed by arranging these layers alternately.

The precipitation of grain boundary ferrite is greatly influenced by the cooling rate in the ferrite transformation region. If the cooling rate is slower than the critical rate, ferrite precipitation will occur, so it is necessary to control the cooling rate to below 850°C and below 700°C. If the cooling rate in the ferrite transformation region is slower than the critical rate but sufficient precipitation time cannot be secured, ferrite will precipitate preferentially at grain boundaries where precipitation is likely to occur. For this reason, stress during pearlite and bainite-martensite transformation, which will be transformed later, will be concentrated in the soft ferrite portion, which will cause slab placement cracks, which is not suitable. As a countermeasure, it is possible to grow ferrite precipitated from grain boundaries and turn it into polygonal ferrite by reducing the cooling rate in the ferrite transformation region and ensuring sufficient time for ferrite precipitation. In the case of polygonal ferrite, the toughness of the slab can be improved because it suppresses excessive stress concentration.

Furthermore, in order to suppress slab cracking, it is important not only to suppress embrittlement of prior austenite grain boundaries, but also to reduce stress during transformation. Therefore, the microstructure of the continuously cast slab can be controlled by variously controlling the cooling rate in the pearlite transformation region (700° C. or lower and 500° C. or higher).

In addition, cooling after the continuous casting slab leaves the continuous casting machine depends on the temperature of the slab at the exit side of the continuous casting machine, the time it takes to stack multiple slabs, the number of slabs to be stacked, the presence or absence of a heat insulation cover, and water toughness treatment. It can be controlled by changing conditions such as. Measuring the cooling rate can be done with a thermocouple. For example, measurement can be performed by installing a thermocouple at the center of the top of the wide side (long side) of the slab after it comes out of the continuous casting machine.

As explained above, according to the invention according to the first embodiment, even in recent continuous casting slabs for high-strength steel where the toughness of continuous casting slabs is extremely low, slab placement cracks do not occur during the cooling process, and Problems such as holes during rolling can be prevented, and a continuously cast slab for high-strength steel with a high yield can be obtained.

[Second embodiment]
A continuous casting slab according to a second embodiment will be described. The continuous casting slab according to the present embodiment is different from the continuous casting slab according to the above embodiment in that the continuous casting slab has C: 0.10% or more and 1.00% or less and Si: 0.10% or more and 2.50% by mass. % or less, Mn: 1.50% or more and 5.00% or less.
In the following description, "%" representing the content of component elements of steel means "mass %" unless otherwise specified.

<C: 0.10% or more and 1.00% or less>
In the continuous casting slab according to this embodiment, the reason for limiting each chemical component contained in the continuous casting slab will be explained. Note that the content of each chemical component contained in the continuous casting slab is expressed in mass %. The reason why the content of C contained in the continuous casting slab is set to 0.10% or more and 1.00% or less is as follows. C contained in continuous casting slabs for high-strength steel is an element necessary for increasing the strength of high-strength steel plates made from continuous casting slabs. If the C content is less than 0.10%, the strength required for a high-strength steel plate cannot be obtained, so the lower limit of the C content is 0.10%. On the other hand, if the C content exceeds 1.00%, it is not preferable because the weldability and workability of the high-strength steel sheet become insufficient.

Therefore, from such a viewpoint, in the continuously cast slab according to the present embodiment, it is preferable that the content of C contained in the continuous cast slab is 0.10% or more and 1.00% or less, and more preferably 0.12% or less. % or more and 0.40% or less, and particularly preferably 0.15% or more and 0.40% or less.

<Si: 0.10% or more and 2.50% or less>
Next, the reason why the Si content contained in the continuous casting slab for high-strength steel is set to 0.10% or more and 2.50% or less is as follows. Si contained in the continuous casting slab is an element necessary for ensuring retained austenite in the steel plate in the annealing process of the high strength steel plate using the continuous casting slab as a raw material. In addition, Si contained in the continuous casting slab is an essential additive element because it contributes to increasing the strength of high-strength steel sheets through solid solution strengthening. If the Si content is less than 0.10%, the strength required for a high-strength steel plate cannot be obtained, so the lower limit of the Si content is 0.10%.

On the other hand, when the Si content exceeds 2.50%, the effect of obtaining the strength required for high-strength steel sheets is saturated, and strong scales are formed in hot-rolled sheets before being processed into high-strength steel sheets. Occur. As a result, the upper limit of the Si content is 2.50% because it deteriorates the appearance and pickling properties of the high-strength steel sheet.

Therefore, from such a viewpoint, in the continuous casting slab according to the present embodiment, it is preferable that the Si content contained in the continuous casting slab is 0.10% or more and 2.50% or less, and further 0.50% or less. % or more and 2.00% or less, more preferably 1.00% or more and 1.80% or less.

<Mn: 0.40% or more and 5.00% or less>
Furthermore, the reason why the content of Mn contained in the continuous casting slab is set to 0.40% or more and 5.00% or less is as follows. Mn contained in the continuous casting slab is an element necessary to further increase the strength of the high-strength steel plate. Specifically, Mn is an element added to control the strength of a high-strength steel plate through transformation control during the hot rolling process of continuously cast slabs. If the Mn content is less than 0.40%, the high-strength steel plate cannot be sufficiently strengthened, so the lower limit of the Mn content is 0.40%. On the other hand, if the Mn content exceeds 5.00%, the degree of sufficient strengthening of the high-strength steel plate becomes saturated, and the manufacturing cost of the high-strength steel plate increases, which is not preferable from an economic standpoint.

Therefore, from such a viewpoint, in the continuous casting slab according to the present embodiment, it is preferable that the Mn content contained in the continuous casting slab is 0.40% or more and 5.00% or less, and 1.20% or less. It is more preferably 4.50% or less, and more preferably 1.40% or more and 4.00% or less.

The continuous casting slab according to 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 with an appropriate composition. To that extent, considering other properties, P is 0.100% or less, S is 0.0200% or less, N is 0.0100% or less, Al is 0.100% or less, and O is 0.0100% or less. , may be contained. Here, unavoidable impurities include Zn, Pb, and As. The total content of these unavoidable impurities is allowed to be 0.100% or less.

Since P segregates in prior austenite grain boundaries and embrittles the grain boundaries, it may cause slab placement cracks. 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 according to the present embodiment is suitable for use in high-strength steel plates, and in addition to the above-mentioned composition, it further contains Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, and Ta. : 0.10% or less, W: 0.10% or less, Cr: 2.00% or less, Mo: 2.00% or less, Ni: 2.00% or less, Cu: 2.00% or less, B: 0 At least one element selected from .0100% or less may be contained alone or in combination of two or more.

If the content of each 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. Although the lower limits of the contents of Ti, Nb, and V are not particularly specified, the strength of the steel sheet can be improved by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. It is more preferable that the contents of Ti, Nb, and V are each 0.001% or more because the content of Ti, Nb, and V increases. 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. Although there is no particular lower limit to the content of Ta and W, the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, 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.

The continuous casting slab according to the present embodiment may contain at least one selected from Cr, Mo, Ni, and Cu, as necessary, within a range that does not impair the object of the present invention. Cr, Mo, Ni, and Cu have the effect of increasing the strength of the steel plate through microstructural control during hot rolling of the continuous casting slab. This effect becomes remarkable by adding 0.01% or more of each of Cr, Mo, Ni, and Cu, so it is preferable to add 0.01% or more. If the amount of each element exceeds the upper limit of each element, the weldability, hot workability, etc. of the steel plate will deteriorate, so the upper limit of the amount of each element of Cr, Mo, Ni, and Cu is set at 1.00%. . Therefore, when the continuous casting slab contains Cr, Mo, Ni, and Cu, the content of each of them is 1.00% or less. Preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

B may be added to control the structural transformation during hot rolling and annealing of the continuously cast slab, since it affects the strength through structural strengthening. B does not affect the toughness of the slab if it is 0.0100% or less. 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 of continuous casting slabs and improves hardenability, the B content should be 0. It is more preferable to set it to 0003% or more. 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 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. Although the lower limit of the Sn content is not particularly defined, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. 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, each content of Ca, Mg, and REM is preferably 0.0100% or less. Note that the lower limits of each content of Ca, Mg, and REM are not specified in particular, 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 It is more preferable for each of these to be 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, each content of Zr and Te is preferably 0.100% or less. Note that the lower limits of each content of Zr and Te are not particularly specified, but since Zr and Te are elements that make the shape of nitrides and sulfides spheroidal and improve the toughness of slabs, the content of Zr and Te is More preferably, each amount is 0.001% or more. 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.

As explained above, according to the invention according to the second embodiment, it is possible to obtain the strength required for high-strength steel, and furthermore, it is possible to produce a continuously cast slab that has excellent weldability, workability, and appearance of high-strength steel. Obtainable.

[Third embodiment]
A method for manufacturing a continuous casting slab according to a third embodiment will be described. The method for manufacturing a continuous casting slab according to the present embodiment is a method for manufacturing a continuous casting slab for high-strength steel in which slab cracking caused by cooling is suppressed, and includes the components of the continuous casting slab described in the above embodiment. A continuous casting slab having a composition is provided such that the cooling temperature of the continuous casting slab at the center in the width direction of the continuous casting slab and at a position 10 mm from the surface layer of the continuous casting slab is 1200°C or more and 1450°C or less, and the continuous casting slab is retained. a first cooling step of cooling under cooling conditions for a time of 130 seconds or less;
A second cooling step in which the continuously cast slab is cooled under cooling conditions in which the surface temperature at the center in the width direction is 700° C. or more and 850° C. or less and the average cooling rate is 20° C./hr or less;
A third cooling step of cooling the continuously cast slab under cooling conditions in which the surface temperature at the center in the width direction is from 500° C. to 700° C. and the average cooling rate is 10° C./hr or less.

Here, the lower limit of the average cooling rate in the second cooling process and the third cooling process is not specified in particular, but when multiple slabs are stacked and a heat insulation cover is used, the lower limit is 700°C or more and 850°C or less, and 500°C or more and 700°C The average cooling rate below .degree. C. is at least 2.degree. C./hr and 1.degree. C./hr, respectively. In the second cooling process and the third cooling process, cooling at an average cooling rate that is slower than these average cooling rates requires, for example, placing the slab in a heating furnace and applying heat, which requires equipment, so it is not recommended from an economic point of view. I also don't like it. Therefore, in the second cooling process, the lower limit of the average cooling rate from 700°C to 850°C is 2°C/hr, and in the third cooling process, the lower limit to the average cooling rate from 500°C to 700°C is 1°C/hr. It is good to say.

Note that in the method for manufacturing a high-strength steel plate slab according to the present embodiment, transshipment may occur depending on the conditions of the manufacturing process. When transshipment occurs, the cooling rate of the 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.
Each process included in the continuous casting slab manufacturing method according to this embodiment will be described below.

(First cooling process)
The method for manufacturing a continuous casting slab according to the present embodiment is a method for manufacturing a continuous casting slab for high-strength steel in which slab cracking caused by cooling is suppressed, and includes the components of the continuous casting slab described in the above embodiment. Continuously cast slabs with the composition
Cooling conditions in which the cooling temperature of the continuous casting slab at the center in the width direction of the continuous casting slab and at a position 10 mm from the surface layer of the continuous casting slab is 1200°C or more and 1450°C or less, and the residence time of the continuous casting slab is 130 seconds or less. It includes a first cooling step of cooling.

The first cooling step is a step for controlling the average prior austenite grain size contained in the continuous casting slab according to the above embodiment to 2.0 mm or less at a predetermined position. By controlling the average prior austenite grain size to 2.0 mm or less, it is possible to reduce the density of precipitates that precipitate at prior austenite grain boundaries, suppress the precipitation of harmful grain boundary ferrite, and improve slab toughness.
In the continuous casting slab manufacturing method according to the present embodiment, the factor that determines the average prior austenite grain size is the temperature at which the slab is cooled. In the first cooling step, the temperature at which the continuous casting slab is cooled is in the range of 1450°C or lower and 1200°C or higher. As described above, the continuous casting slab manufacturing method according to the present embodiment focuses on the cooling temperature of the continuous casting slab in the range of 1450°C or lower and 1200°C or higher, which is a factor that determines the average prior austenite grain size. is under control.

Furthermore, in the first cooling step, the residence time of the continuous casting slab in the above temperature range for cooling the continuous casting slab is 130 seconds or less. It is preferable that the residence time of the continuously cast slab at the above temperature is 130 seconds or less, since the average prior austenite grain size can be made 2.0 mm or less, and cracking in the slab can be suppressed. There is no particular lower limit to the residence time of continuously cast slabs at temperatures above 1,200°C and below 1,450°C, but if the residence time is too short, the risk of breakout during continuous casting due to uneven solidification increases, so it is recommended that the residence time be 40 seconds or more. It is preferable to do so, more preferably 60 seconds or more, and even more preferably 70 seconds or more.

(Second cooling process)
Next, in the method for manufacturing a continuously cast slab according to the present embodiment, the continuously cast slab is cooled under cooling conditions in which the average cooling rate is 20°C/hr or less when the surface temperature at the widthwise center of the continuously cast slab is 700°C or more and 850°C or less. Includes a second cooling step. The second cooling step is a step for suppressing the precipitation of grain boundary ferrite contained in the microstructure of the continuously cast slab according to the above embodiment, and making the area ratio of ferrite less than 5% or more than 10%.

In the second cooling step, the temperature at which the continuous casting slab is further cooled is 700°C or higher and 850°C or lower. In this way, the method for manufacturing a continuously cast slab according to the present embodiment controls the temperature by focusing on the cooling rate in the temperature range in the ferrite transformation region where ferrite precipitation can be controlled.

In the second cooling step, the average cooling rate of the continuously cast slab is 20° C./hr or less in the above temperature range in which the continuously cast slab is cooled. If the average cooling rate exceeds 20° C./hr, thin ferrite precipitation occurs only at prior austenite grain boundaries, which embrittles the grain boundaries, which is not suitable. If the average cooling rate of the continuous casting slab is 20°C/hr or less, sufficient residence time of the continuous casting slab in the ferrite transformation temperature range can be ensured, allowing grain boundary ferrite to grow into polygonal ferrite and forming grain boundary ferrite. This is preferable because it can prevent stress concentration on.
Although the lower limit of the average cooling rate is not strictly limited, it is preferably 2° C./hr or more since a separate energy source is required to control the cooling rate. More preferably, the average cooling rate is 5° C./hr or more and 18° C./hr or less.

(Third cooling process)
Further, in the method for manufacturing a continuously cast slab according to the present embodiment, the continuously cast slab is cooled under cooling conditions in which the average cooling rate is 10°C/hr or less when the surface temperature at the center in the width direction is 500°C or more and 700°C or less. Includes a third cooling step.
The third cooling step is a step for changing the microstructure of the continuously cast slab according to the above embodiment to a pearlite-based structure and reducing internal stress. Specifically, the third cooling step calculates the area ratio (%), which is the ratio of the _total area S _{(ferrite + pearlite)} of the ferrite area S _ferrite and pearlite area S _pearlite to the microstructure area S total of the continuous casting slab. This is a process to make it 90% or more.

In the third cooling step, the temperature at which the continuous casting slab is further cooled is 500°C or more and 700°C or less. In this manner, the continuous casting slab manufacturing method according to the present embodiment controls the temperature by focusing on the cooling rate in the temperature range in the pearlite transformation region.

In the third cooling step, the average cooling rate of the continuously cast slab is 10° C./hr or less in the above temperature range in which the continuously cast slab is cooled. If the average cooling rate of the continuously cast slab exceeds 10° C./hr, bainite/martensite will precipitate from the pearlite-based microstructure, which will cause large stress, which is not suitable. Bainite/martensite has a lower transformation temperature than pearlite, and the transformation stress is applied to the pearlite part that has already undergone transformation, which is a factor that promotes cracking.
From this point of view, it is preferable that the average cooling rate of the continuously cast slab is 10° C./hr or less, since internal stress can be reduced by suppressing bainite transformation and creating a pearlite-based structure.
The lower limit of the average cooling rate is not strictly limited, but since a separate energy source is required to control the cooling rate, it is preferably 1°C/hr or more, more preferably 5°C/hr or more. .

As described above, the continuous casting slab manufacturing method according to the present embodiment employs a three-step cooling process to precisely control the average prior austenite grain size and the microstructure of the continuous casting slab. As a result, it is possible to provide a continuously cast slab for high-strength steel, which suppresses slab cracks that occur due to cooling and prevents problems such as holes during rolling.

As explained above, according to the continuous casting slab manufacturing method according to the third embodiment, even if the composition system of the continuous casting slab for high-strength steel is used, the cooling process is divided into three stages, and each cooling process is By precise control, it is possible to provide a continuously cast slab for high-strength steel that does not cause cracking during the cooling process and prevents problems such as holes during rolling.

[Other embodiments]
Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. The structure and details of the present invention can be modified in various ways within the technical scope of the present invention, which can be understood by those skilled in the art. Furthermore, systems or devices that combine the separate features included in each embodiment in any way are also included within the technical scope of the present invention.

Hereinafter, the effects of the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples. That is, in order to confirm the effects of the present invention, the present inventors conducted comparative examples (Test No. A-1 to A-4, Test No. B-1 to B-8, Test No. C-1 to C -3) and the invention examples (Test Nos. D-1 to D-24), continuous casting slabs were manufactured using each steel type as a raw material. Table 1 shows comparative examples (Test No. A-1 to A-4, Test No. B-1 to B-8, Test No. C-1 to C-3) and invention examples (Test No. D-1). ~ D-24) The steel types A to I that are the raw materials for the continuous casting slabs used are shown below.

Here, the cooling conditions for the continuous casting slab are (I) residence time [s] of 1200°C or more and 1450°C or less, (II) average cooling rate [°C/hr] of 700°C or more and 850°C or less, and (III) A three-stage cooling process consisting of an average cooling rate [° C./hr] of 500° C. or more and 700° C. or less was employed, and cooling was performed by changing the conditions of each of these stages as appropriate.

Tables 2 to 4 show the continuous casting slab cooling conditions (I) to (III), the microstructure of the obtained continuous casting slab, and the evaluation of slab placement cracking.

Measurement of the average prior austenite grain size, calculation of ferrite and pearlite area ratios, and evaluation of cracks in the continuous casting slabs in the continuous casting slabs produced in the comparative examples and inventive examples were performed as follows.

<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 was mirror polished using diamond paste, then finished polished using colloidal silica, and further polished using 3vol. % nital to reveal the structure on the observation surface. Using an optical microscope, 5 fields of view are observed at a magnification of 10 times at a position 10 mm below the surface layer of the slab to obtain a microstructure image of the continuously cast slab. The prior austenite grain size obtained by observing the obtained microstructure image in 5 fields was determined by a cutting method based on JIS G 0551:2020, and the average value of these was calculated as the average prior austenite grain size.

<Measurement method of ferrite area ratio>
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 was mirror polished using diamond paste, then finished polished using colloidal silica, and further polished using 3vol. Etch with % nital to reveal the tissue. A microstructure image of the continuously cast slab obtained by observing 10 fields of view at a magnification of 50 times at a position 10 mm below the surface of the slab using an SEM (Scanning Electron Microscope) at an accelerating voltage of 15 kV. The area ratio of ferrite was calculated for 10 fields of view using Adobe's PHOTOSHOP (registered trademark), 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 for measuring area ratio of pearlite>
The method for measuring the area ratio of the pearlite structure involves making the structure appear on the observation surface of the slab, similar to the method for measuring ferrite described above. Under the condition of an accelerating voltage of 15 kV, using a 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 slab, and the obtained microstructure image of the continuously cast slab was obtained. Using Adobe's PHOTOSHOP (registered trademark), calculate the area ratio of pearlite and the area ratio of bainite for 10 fields, average these values, and calculate the total by combining them with the area ratio of ferrite measured using the method described above. It was calculated to be 100% and determined as the area percentage of each tissue. Pearlite is a eutectoid crystal of ferrite and cementite, and when observed using the above-mentioned scanning electron microscope, the flaky layers of both have a pearl-like luster.

The microstructure of the continuously cast slab according to the present invention is mainly pearlite and has no grain boundary ferrite. However, it is difficult to strictly classify grain boundary ferrite and polygonal ferrite. Therefore, the present inventors conducted a thorough investigation of slabs in which slab placement cracks had occurred, and found that a large amount of ferrite was present at grain boundaries when the area ratio of ferrite was greater than 5% and less than 10%. That is, it is a structure in which the area ratio of ferrite + pearlite is 90% or more, and the area ratio of ferrite is less than 5% or 10% or more.

<Slab crack evaluation>
The evaluation method for slab placement cracks was based on the penetrant test specified in JIS Z 2343:2017, and the presence or absence of cracks on the wide and narrow sides of the slab was evaluated. After applying the developer, the appearance of the penetrating solution was visually observed to visually check for cracks in the slab that had occurred on the surface.
In addition, if there is a crack with a length of 50 mm or more, there is a high risk of the slab breaking during slab handling or in the heating furnace, and there is also a high possibility that it will lead to problems with holes during rolling. The criteria were as follows.
・Slab cracks 〇...No cracks of 50 mm or more in length on the slab surface ・Slab cracks in place ×...Cracks of 50 mm or more in length on the slab surface

<Comparative example (Test No. A-1 to A-4)>
Test No. Condition A is the microstructure of the slab that is satisfied by the continuously cast slabs manufactured in A-1 to A-4. Condition A is a condition for an example in which the average prior austenite grain size at a position 10 mm below the surface layer of the slab is larger than 2.0 mm. In these cases, the toughness of the prior austenite grain boundaries has decreased due to the increased precipitate density at the prior austenite grain boundaries, so even if the conditions for slow cooling of the slab after it exits the continuous slab casting machine are varied, Slab cracking could not be suppressed.

<Comparative example (Test No. B-1 to B-8)>
Test No. Condition B is the microstructure of the slab that is satisfied by the continuously cast slabs manufactured in B-1 to B-8. Condition B is a condition in which the average prior austenite grain size at a position 10 mm below the surface layer of the slab is 2.0 mm or less, but grain boundary ferrite is precipitated at the prior austenite grain boundaries. In these cases, grain boundary ferrite reduces the toughness of prior austenite grain boundaries, so even if the subsequent cooling rate is varied and the microstructure composition is changed, it is difficult to suppress slab cracking. I couldn't.

<Comparative example (Test No. C-1 to C-3)>
Test No. Condition C is the microstructure of the slab that is satisfied by the continuously cast slabs manufactured in C-1 to C-3. Condition C is a condition in which the average prior austenite grain size is 2.0 mm or less and the precipitation of grain boundary ferrite is suppressed, but as 10% or more of bainite precipitates, slab cracking cannot be suppressed. It is a condition. Since bainite transformation occurs at a lower temperature than pearlite transformation, there is a large density difference with austenite, and the transformation stress is also large, so it is thought that slab cracking could not be suppressed.

<Invention Examples (Test Nos. D-1 to D-24)>
Test No. Condition D is the slab microstructure that is satisfied by the continuously cast slabs manufactured in D-1 to D-24. Condition D is the condition of the present invention example, and the continuous casting slab manufactured in the present invention example has an average prior austenite grain size of 2.0 mm or less, and a microstructure with almost no grain boundary ferrite and bainite. There were also few organizations. In other words, in addition to improving toughness by controlling prior austenite grain boundaries and dispersing precipitates, improving toughness by suppressing grain boundary ferrite precipitation, and reducing internal stress by suppressing bainite precipitation, it is possible to improve the toughness of the slab after cooling. No cracks have occurred.

According to Tables 2 to 4, (i) the average prior austenite grain size at a position 10 mm from the slab surface is 0.5 mm or more and 2.0 mm or less, and (ii) the microstructure of the continuously cast slab is composed of ferrite and pearlite. It has been found that slab placement cracking during cooling of the slab can be suppressed by setting the total area ratio of (iii) to 90% or more and (iii) the area ratio of the ferrite to less than 5% or 10% or more.

That is, the continuously cast slab of the present invention has an average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the slab surface layer, and a microstructure with a total area ratio of ferrite and pearlite of 90% or more. Since the area ratio of ferrite is less than 5% or more than 10%, it is possible to provide a slab for high-alloy high-strength steel without slab cracking after casting, and it is also possible to prevent problems such as holes during rolling. Become. That is, according to the invention examples and comparative examples, the average prior austenite grain size at a position 10 mm from the slab surface layer is 0.5 mm or more and 2.0 mm or less, and the microstructure has a total area ratio of ferrite and pearlite of 90 mm. % or more, and (iii) it has been found that cracking during cooling of the slab can be suppressed by setting the area ratio of the ferrite to less than 5% or 10% or more.

FIG. 3 is an enlarged photograph observed with an optical microscope of the continuous casting slab manufactured in the invention example of the continuous casting slab (Test No. D-9). Based _on the enlarged photograph of the continuously cast slab observed with an optical microscope shown in FIG _. The ratio of the area S _{(ferrite+pearlite),} which is the sum of the area S _pearlite and the area S pearlite, was calculated as an area ratio (%). As a result, the continuously cast slab of the present invention example has an average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the slab surface layer, and the microstructure has an area ratio of ferrite and pearlite. The total amount was found to be more than 90%. Furthermore, it was found that the continuously cast slabs of the examples of the present invention had a ferrite area ratio of less than 5% or more than 10%.

In order to obtain such a microstructure of a continuously cast slab, for example, the temperature at a position 10 mm below the surface layer of the slab is 1450°C or lower, and the residence time at 1200°C or higher is 130 seconds or less, and then the temperature at the center surface of the slab width is lowered. Cooling is performed so that the cooling rate at 850°C or lower and 700°C or higher is 20°C/hr or less, and further, the average cooling rate at 700°C or lower and 500°C or higher is 10°C/hr or less. It is preferable to employ three stages of cooling.

Note that if a continuously cast slab is strongly cooled in a temperature range of 1450°C or lower and 1200°C or higher, there is a risk that cracks will occur on the slab surface due to uneven solidification. In such a case, for example, when the temperature at the center of the slab width and at a depth of 10 mm below the surface layer of the slab is between 900°C and above and below 1200°C, the slab surface should be rapidly cooled to below the BS point, and then cooling should be stopped and reheated to above the AC3 point. Therefore, it is possible to refine the prior austenite grain size by utilizing austenite reverse transformation. After that, cooling is performed so that the cooling rate is 20°C/hr or less in the area where the temperature of the central surface of the slab width is 850°C or lower and 700°C or higher, and then the average temperature of the central surface of the slab width is 700°C or lower and 500°C or higher. By setting the cooling rate to 10° C./hr or less, a continuously cast slab having such a microstructure can be obtained. Furthermore, the method for manufacturing a continuous casting slab having the microstructure of the continuous casting slab described above is not limited to this.

The continuously cast slab of the present invention has an average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the surface layer of the slab, and a microstructure with a total area ratio of ferrite and pearlite of 90 mm. % or more, and the area ratio of ferrite is less than 5% or more than 10%, so it is possible to provide high-strength steel slabs 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 continuous casting slab for high strength steel,
The average prior austenite grain size at a position 10 mm from the surface layer of the continuous casting slab is 0.5 mm or more and 2.0 mm or less, and
A continuous casting slab characterized in that the microstructure has a total area ratio of ferrite and pearlite of 90% or more, and the area ratio of ferrite is less than 5% or 10% or more.
The continuous casting slab is
In mass%,
C: 0.10% or more and 1.00% or less,
Si: 0.10% or more and 2.50% or less,
The continuous casting slab according to claim 1, containing Mn: 0.40% or more and 5.00% or less.
A method for manufacturing a continuous casting slab for high-strength steel in which slab cracking caused by cooling is suppressed, the method comprising:
A continuous casting slab having the composition according to claim 2,
Cooling conditions in which the cooling temperature of the continuous casting slab at the center in the width direction of the continuous casting slab and at a position 10 mm from the surface layer of the continuous casting slab is 1200°C or more and 1450°C or less, and the residence time of the continuous casting slab is 130 seconds or less. a first cooling step of cooling by;
A second cooling step in which the continuously cast slab is cooled under cooling conditions in which the surface temperature at the center in the width direction is 700° C. or more and 850° C. or less and the average cooling rate is 20° C./hr or less;
A continuous casting slab characterized by comprising: a third cooling step of cooling under cooling conditions in which the surface temperature at the center in the width direction of the continuous casting slab is 500° C. or more and 700° C. or less, and the average cooling rate is 10° C./hr or less. manufacturing method.