JP6237808B2 - Continuously cast slab, method for producing the same, and method for producing high-tensile steel plate excellent in workability - Google Patents

Description

  The present invention relates to a continuous cast slab that is a material of a high-strength steel plate used for inner plate parts such as structural members and reinforcing members of automobiles, a method for manufacturing the same, and a high-tensile steel plate having excellent workability from the continuous cast slab It relates to a method of manufacturing.

From the viewpoint of protecting the global environment, it is required to reduce CO ₂ emissions. In the automobile industry, reducing the weight while maintaining the strength of automobiles and improving the fuel efficiency of automobiles. It has become a very important issue. In order to reduce the weight while maintaining the strength of automobiles, it is effective to make the steel sheet thinner by increasing the strength of the steel sheet used as a material for automobile parts. Therefore, in recent years, high-tensile steel plates have been actively used for automobile parts.

  As one of means for increasing the strength of a steel sheet, a technique for dispersing fine precipitates in the steel sheet is known. By dispersing fine precipitates uniformly in the steel sheet, the movement of dislocations that cause plastic deformation is hindered, and high strength of the steel sheet is realized. Precipitates that contribute to increasing the strength of the steel sheet are mainly carbides, nitrides, and carbonitrides. These precipitates precipitate in the steel sheet during the cooling process after hot rolling.

  However, in order for the precipitates to contribute to increasing the strength of the steel sheet, the average particle size of the precipitates is preferably a nanometer size. Therefore, the manufacturing conditions and chemical components of the steel sheet are optimized so that the average particle size of the precipitates is nanometer size.

  Since many automotive parts made of steel plates are molded by pressing or burring, etc., steel plates for automotive parts are required to stably exhibit excellent workability (elongation and stretch flangeability). ing. In addition, when steel plates having partially different strengths are press-molded, the amount of springback changes in proportion to the strength of the steel plates, causing a phenomenon that the parts are twisted. That is, in order to obtain a part having desired strength and dimensional / shape accuracy, it is required to satisfy both strength and workability of a steel plate as a material. In general, the workability of steel materials decreases with increasing strength.

  Therefore, many studies have been made on steel sheets having high strength and high workability. For example, Patent Literature 1 and Patent Literature 2 listed below have been proposed as techniques for improving stretch properties and stretch flange properties, which are indexes of workability of steel plates.

  Patent Document 1 discloses a high-tensile steel plate having a workability with a tensile strength of 590 MPa or more, in which carbides containing Ti and Mo having an average particle diameter of less than 10 nm are dispersed and precipitated, which is substantially a ferrite single-phase structure. It is disclosed.

  In Patent Document 2, in mass%, C: 0.05 to 0.2%, Si: 0.001 to 3.0%, Mn: 0.5 to 3.0%, P: 0.001 to 0 .2%, Al: 0.001 to 3%, V: more than 0.1% up to 1.0%, the balance being Fe and inevitable impurities, the structure of ferrite having an average grain size of 1 to 5 μm A hot-rolled steel sheet having a tensile strength exceeding 800 MPa, in which V carbonitride having an average particle diameter of 50 nm or less exists in ferrite grains, is disclosed.

Japanese Patent No. 3591502 JP 2004-143518 A

  By the way, in order to precipitate carbide or nitride having an average particle size of nanometer size in the steel sheet after hot rolling, carbide or nitride that has been precipitated in the continuous cast slab before hot rolling, It is necessary to form a solid solution completely in the heating stage of a continuous cast slab (hereinafter also simply referred to as “slab”) before hot rolling. If the heating time of the slab is lengthened or the heating temperature is increased, the carbides and nitrides precipitated in the continuously cast slab can be dissolved. However, the heating time and the heating temperature are naturally limited due to restrictions on productivity and equipment. In addition, this is disadvantageous in terms of cost.

  The above prior art does not give any consideration to the fact that carbides and nitrides precipitated in the continuous cast slab before hot rolling are completely dissolved before hot rolling, and workability (elongation) There is still a point to be improved from the viewpoint of stably producing a high-tensile steel sheet having excellent characteristics and stretch flange characteristics) and increasing productivity.

  The present invention has been made in view of the above circumstances, and its object is to produce a high-tensile steel plate excellent in workability using fine precipitates, and continuous casting which is a material of the high-tensile steel plate. Carbides and nitrides that have precipitated in the slab can be dissolved in the continuous casting slab in a short time without increasing the heating temperature at the stage of slab heating before hot rolling. The present invention provides a continuous cast slab capable of stably obtaining a high-strength steel sheet excellent in quality and a method for producing the same, and also provides a method for producing a high-tensile steel sheet having excellent workability from the continuous cast slab. That is.

  In order to solve the above-mentioned problems, the present inventors have made it possible to dissolve precipitates such as carbides and nitrides in a continuously cast slab in a short time during hot slab heating before hot rolling. A simulation based on the phase field method, which has a reputation for reproducing the temporal change of the material microstructure in the irreversible process, was performed.

  That is, the initial setting of the TiC distribution shown in FIG. 1 (A) and the austenite shown in FIG. 1 (B) are targeted in the austenite phase matrix of the Fe—C—Mn—Ti component steel with a large amount of TiC precipitated. Under the initial setting conditions shown in FIG. 1 (C) in combination with the initial setting of the grain boundaries, the heating pattern shown in FIG. In the publication 1 (I. Steinbach et al., Phys. D94 (1996). P135-147), the dissolution behavior of TiC and the growth behavior of austenite grains during heating at 1250 ° C. are described. The phase field method simulation was performed by the calculation code Micress based on the Multi Phase Field method. In FIG. 1, what is shown by a circle is TiC, and hereinafter, austenite is also expressed as “γ”.

  FIG. 3 shows the simulation results from the start of calculation until 52 seconds have passed since the dissolution of TiC. As shown in FIG. 3, it can be confirmed that a phenomenon (pinning effect) in which the movement of the austenite grain boundary is inhibited by the remaining TiC in the middle of dissolution is observed from the time when 4 seconds have passed.

  FIG. 4 is a diagram showing the growth behavior of austenite grains, and it can be seen that the austenite grain growth behavior is affected by the movement of the austenite grain boundaries being hindered by TiC. Further, TiC related to the “pinning effect” is changed from the initial setting state (spherical shape), and has a spindle shape with a sharp tip and a swelled center instead of a perfect circle. For this reason, the curvature radius of the TiC / γ phase interface is increased. This is considered to affect the complete solid solution time of TiC, as will be described later.

  FIG. 5 is a graph showing the relationship between the TiC mole fraction and the dissolution time of TiC. The time when the vertical axis becomes 0 (zero) is the time for complete dissolution of TiC. As shown in FIG. 5, the time until the TiC completely dissolves (complete solution time) is longer when the growth behavior of austenite grains is coupled than when only TiC solid solution is present. I understand. Moreover, it was recognized that the time when the difference starts to occur corresponds to the time when the “pinning effect” of the austenite grain boundary due to the residual TiC becomes clear (see FIG. 4). That is, it became clear that the “pinning effect” affects not only the growth behavior of austenite grains but also the dissolution behavior of TiC. The following were considered as the cause.

In general, the unit of interface energy is represented by [J / m ² ], but it can be transformed as [J / m ² ] = [Nm / m ² ] = [N / m]. Can be reinterpreted as “minimization of interface length”. That is, it can be said that the interface is always in a state where a tension is applied to minimize the length.

  Here, paying attention to the triple point of the austenite grain boundary and the TiC / γ interface, as shown in FIG. As a result, TiC has a spindle-like shape as described above. Therefore, as the melting of TiC proceeds, the radius of curvature increases as compared to the perfect circle state. FIG. 6 is a schematic diagram showing the relationship between the surface tension and the austenite interface during dissolution of TiC.

  As the curvature radius (R) of TiC increases, the Gibbs free energy (G) of TiC decreases (Gibbs-Thomson effect; G∝1 / R). The driving force when TiC dissolves is the Gibbs free energy (G), and the melting driving force decreases as the Gibbs free energy (G) decreases. That is, when the curvature radius (R) of TiC is increased, the Gibbs free energy (G) is decreased, so that the progress of dissolution of TiC is delayed, and as a result, the complete solid solution time is considered to be increased. Therefore, the higher the density of austenite grain boundaries, the higher the proportion of TiC involved in the “pinning effect”, and the longer the complete solid solution time of TiC.

  From these results of the phase field method simulation, the following findings (a), (b), and (c) were obtained.

  (B) The carbides and nitrides precipitated in the continuous cast slab before the slab heating prevent the austenite grain boundaries from moving due to ferrite transformation by the slab heating (so-called “pinning effect”). .

  (B) The form of carbides and nitrides changes due to the interaction with the austenite grain boundaries described in (a) above, and the carbides and nitrides are dissolved as the radius of curvature increases.

  (C) In relation to (b) above, the time during which the carbides and nitrides are completely dissolved becomes longer due to the interaction with the austenite grain boundaries. That is, the higher the density of austenite grain boundaries, the longer the time during which carbides and nitrides are completely dissolved.

  From these findings, it can be seen that in order to shorten the time for completely dissolving the precipitate during the slab heating before hot rolling, it is necessary to set the condition that the “pinning effect” due to the precipitate does not easily occur.

  As a measure for that, the first thing to be considered is to reduce the precipitates in the slab, that is, to reduce the chemical components that are the basis of the precipitates that are deposited during and after the casting of the slab. However, this measure cannot be adopted because it directly leads to a decrease in the tensile strength of the steel plate as the final product. Therefore, as another measure for making the “pinning effect” due to precipitates less likely to occur, it is conceivable to reduce the austenite grain boundary density, that is, to increase the austenite grain size generated during slab heating.

  The grain size of austenite produced during slab heating depends on the ferrite grain size before slab heating, that is, at the stage of the cast slab. This is because when the ferrite transforms into austenite during slab heating, the austenite grains are first generated from the ferrite grain boundaries, so if the ferrite grain size increases, thereby reducing the ferrite grain boundary density, the austenite grains This is because the frequency of grain generation is also reduced.

  That is, in order to shorten the time for completely dissolving the precipitate during slab heating, in order to make the “pinning effect” caused by the precipitate less likely to occur, the ferrite grain size should be increased at the stage of the cast slab. And gained knowledge.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.02 to 0.15%, Si: 2.0% or less, Mn: 0.5 to 2.0%, P: 0.08% or less, S: 0.0060 %: Al: 0.005 to 0.1%, N: 0.006% or less, Ti: 0.05 to 0.3%, the balance being Fe and inevitable impurities, hot rolling step A continuous cast slab characterized in that the average grain size of ferrite grains is 70 μm or more at the stage before heating in.
[2] Furthermore, the composition further contains at least one element selected from Nb: 0.005 to 0.1% and V: 0.005 to 0.1% by mass%. The continuous cast slab according to [1].
[3] Furthermore, Cr: 0.005-0.3%, Mo: 0.005-0.3%, Cu: 0.005-0.5%, Ni: 0.005-0. The continuous cast slab according to [1] or [2] above, which contains at least one element selected from 5%.
[4] The continuous casting slab according to any one of [1] to [3] above, further containing B: 0.0002 to 0.005% by mass%.
[5] Furthermore, it contains at least one element selected from Ca: 0.0005 to 0.02% and REM (rare earth element): 0.0005 to 0.02% by mass%. The continuous cast slab according to any one of [1] to [4] above.
[6] [1] to [5] above method for manufacturing a continuous casting slab according to any one of, after casting in a continuous casting machine, the surface temperature of the continuous casting slab of at least A _r3 point A method for producing a continuous cast slab, characterized in that cooling is performed at a cooling rate of 35 ° C./hr or less from the time to 100 ° C.
[7] Continuous casting slab according to any one of the above [1] to [5] above, or a continuous cast slab produced by the production method of the continuous casting slab according to the above [6], A _c3 A method for producing a high-tensile steel sheet having excellent workability, characterized by heating at a temperature equal to or higher than a point, and hot rolling after heating.

  According to the present invention, since the average grain size of the ferrite grains of the continuous cast slab before heating in the hot rolling process is 70 μm or more, precipitates such as carbides and nitrides precipitated in the continuous cast slab are removed. In the slab heat treatment in the hot rolling process, it is possible to completely dissolve in austenite in a short time without increasing the heating temperature. As a result, it is possible not only to stably produce high-tensile steel sheets with excellent workability (elongation characteristics and stretch flange characteristics) using the precipitation of fine precipitates, but also slabs in the hot rolling process. It is possible to shorten the heating time.

It is the schematic which shows the initial setting of each phase in a phase field method simulation. It is a figure which shows the heating pattern of the material in a phase field method simulation. It is the schematic which shows the simulation result from the calculation start time to 52 seconds after the TiC has almost completely dissolved. It is the schematic which shows the growth behavior of an austenite grain. It is a figure which shows the relationship between a TiC molar fraction and the melt | dissolution time of TiC. It is the schematic which shows the relationship between the surface tension at the time of TiC melt | dissolution, and an austenite interface. It is a figure which shows the relationship between the average grain size of the ferrite grain in the slab before a heating, and the complete solid solution time of Ti precipitate.

  Hereinafter, the present invention will be specifically described. “%” Indicating the content of each element means “% by mass” unless otherwise specified.

(1) Chemical component of continuous casting slab C (carbon): 0.02-0.15%
C is an element that increases the strength of the steel sheet mainly by solid solution strengthening. In order to obtain such an effect, the C content needs to be 0.02% or more. On the other hand, if the C content exceeds 0.15%, the stretch flange characteristic is greatly deteriorated. Therefore, the C content needs to be 0.02 to 0.15%, preferably 0.05 to 0.12%, and more preferably 0.07 to 0.11%.

Si (silicon): 2.0% or less Si is an element that stabilizes the strength of the steel sheet by solid solution strengthening and contributes to improving ductility. On the other hand, when the Si content exceeds 2.0%, not only the surface properties are deteriorated, but also the segregation of Mn in the central portion of the plate thickness is promoted, and Si itself is segregated. Therefore, the Si content needs to be 2.0%, preferably 0.05 to 1.0%, and more preferably 0.05 to 0.8%.

Mn (manganese): 0.5 to 2.0%
Mn is an element that increases the strength of the steel sheet mainly by solid solution strengthening. In order to acquire such an effect, it is necessary to make Mn content 0.5% or more. On the other hand, if the Mn content exceeds 2.0%, the center segregation of Mn becomes remarkable, which causes various properties such as stretch flangeability of the steel sheet to deteriorate. Therefore, the Mn content needs to be 0.5 to 2.0%, preferably 0.5 to 1.8%, and more preferably 1.0 to 1.6%.

P (phosphorus): 0.08% or less P is an element that segregates at the grain boundary to lower the elongation. Therefore, the P content is 0.08% or less, but is preferably reduced as much as possible. Preferably it is 0.04% or less, More preferably, it is 0.020% or less, More preferably, it is 0.010% or less. There is no problem even if the content of P is 0 (zero).

S (sulfur): 0.0060% or less S forms sulfides and decreases workability. Therefore, the S content is 0.0060% or less, but it is preferable to reduce it as much as possible. Preferably it is 0.0030% or less, More preferably, it is 0.0020% or less, More preferably, it is 0.0010% or less. There is no problem even if the S content is 0 (zero).

Al (aluminum): 0.005 to 0.1%
Al is an element that acts as a deoxidizer. In order to obtain such an effect, the Al content needs to be 0.005% or more. On the other hand, when the Al content exceeds 0.1%, it remains as an Al oxide in the steel sheet, and the Al oxide aggregates and becomes easy to be coarsened, which causes deterioration of stretch flangeability. Therefore, the Al content needs to be 0.005 to 0.1%, preferably 0.005 to 0.07%, and more preferably 0.015 to 0.05%.

N (nitrogen): 0.006% or less N forms coarse nitrides and lowers workability. Therefore, the N content is 0.006% or less, but it is desirable to reduce it as much as possible, and the N content is preferably 0.005% or less. There is no problem even if the content of N is 0 (zero).

Ti (titanium): 0.05 to 0.3%
Ti is the most important element in the present invention, and has a significant effect on increasing the strength of the steel sheet. In order to obtain such an effect, the Ti content needs to be 0.05% or more. On the other hand, if the Ti content exceeds 0.3%, Ti carbide tends to be coarsened, making it difficult to obtain a desired tensile strength in the steel sheet. Therefore, the Ti content must be 0.05 to 0.3%, and preferably 0.06 to 0.15%.

  The balance is Fe (iron) and inevitable impurities, but for the following reasons, it is preferable to further contain the elements shown in the following (a) to (d) individually or simultaneously.

(A) Nb (niobium): 0.005 to 0.1%, V (vanadium): at least one element selected from 0.005 to 0.1% (b) Cr (chromium): 0. 005-0.3%, Mo (molybdenum): 0.005-0.3%, Cu (copper): 0.005-0.5%, Ni (nickel): 0.005-0.5% At least one element selected from (c) B (boron): 0.0002 to 0.005%
(D) Ca (calcium): 0.0005 to 0.02%, REM (rare earth element): at least one element selected from 0.0005 to 0.02% Each of these will be described below.

Nb: at least one selected from 0.005 to 0.1%, V: 0.005 to 0.1% Nb and V are both carbonitride-forming elements and increase the strength of steel. It is an important element. In order to acquire such an effect, it is preferable to make each content 0.005% or more. On the other hand, if the respective contents exceed 0.1%, these effects are saturated and cost increases. Therefore, the Nb content and the V content are preferably 0.005 to 0.1%.

At least selected from Cr: 0.005-0.3%, Mo: 0.005-0.3%, Cu: 0.005-0.5%, Ni: 0.005-0.5% Type 1 These elements are elements that have an effect of improving hardenability and contribute to improvement of workability. In order to acquire such an effect, it is preferable to make each content 0.005% or more. On the other hand, when the Cr content exceeds 0.3% and the Mo content exceeds 0.3%, such an effect is saturated and the cost is increased. Further, if the Cu content or Ni content exceeds 0.5%, surface flaws are likely to occur during hot rolling. Therefore, Cr content is 0.005-0.3%, Mo content is 0.005-0.3%, Cu content is 0.005-0.5%, Ni content is 0.005-0. 0.5% is preferable. More preferably, the Cr content is 0.005 to 0.1%, the Mo content is 0.005 to 0.1%, the Cu content is 0.005 to 0.2%, and the Ni content is 0.005. ~ 0.2%.

B: 0.0002 to 0.005%
B is an element that delays the transformation of steel from austenite to ferrite, and suppresses the austenite-ferrite transformation, thereby lowering the precipitation temperature of Ti carbide and contributing to refinement of the carbide. In order to obtain such an effect, the B content is preferably 0.0002% or more. On the other hand, if the B content exceeds 0.005%, the bainite transformation effect due to B becomes strong, and it becomes difficult to obtain a ferrite structure. Therefore, the B content is preferably 0.0002 to 0.005%. More preferably, it is 0.0002 to 0.0025%.

Ca: 0.0005 to 0.02%, REM (rare earth element): at least one selected from 0.0005 to 0.02% Ca and REM are effective elements for controlling the form of sulfide. In order to acquire such an effect, it is preferable to make each amount 0.0005% or more. On the other hand, when each amount exceeds 0.02%, such an effect is saturated and the cost is increased. Therefore, the Ca content is preferably 0.0005 to 0.02%, and the REM content is preferably 0.0005 to 0.02%. More preferably, it is 0.0005 to 0.005%, respectively. Note that REM (rare earth element) is a generic name for a total of 17 elements including Sc (scandium), Y (yttrium), and lanthanoid (15 elements).

(2) At the stage before heating in the hot rolling process, the average grain size of the ferrite grains of the continuous cast slab is 70 μm or more. As described above, the precipitates in the continuous cast slab are subjected to slab heat treatment in the hot rolling process. In order to completely dissolve in austenite in a short heating time without increasing the heating temperature, it is necessary to increase the average crystal grain size of the ferrite grains of the continuously cast slab. As the average grain size of the ferrite grains of the continuously cast slab is larger, the precipitate can be dissolved in austenite in a shorter slab heating time. For that purpose, the average grain size of the ferrite grains of the continuous cast slab needs to be 70 μm or more. When the average grain size of the ferrite grains of the continuous cast slab is less than 70 μm, the heating time required for complete precipitation of the precipitates becomes long.

(3) after casting in a continuous casting machine, the ferrite grains having an average crystal 35 ° C. / hr or less of cooling the continuous cast slab at a cooling rate up to the point the surface temperature of the continuous casting slab comprising in 100 ° C. from the time of at least A _r3 point In order to make the particle size 70 μm or more, it is necessary to slow down the cooling rate of the continuously cast slab during the production of the continuously cast slab. Specifically, it is necessary to control the cooling rate of the continuously cast slab to 35 ° C./hr or less. In the present invention, the overall cooling rate including the inside of the slab is controlled to 35 ° C./hr or less by controlling the cooling rate of the slab surface having the fastest cooling rate to 35 ° C./hr or less.

When the continuously cast slab cooling, transformation from austenite to ferrite A _r3 point (depending on the chemical composition of the slab, a certain value in the range of approximately eight hundred to nine hundred and ten ° C.) from what happens in the surface temperature of the continuously cast slab Is cooled at a cooling rate of 35 ° C./hr or less from at least _{Ar 3} point to 100 ° C. When cooling at a cooling rate of 35 ° C./hr or less, the transformation from austenite to ferrite occurs at the _{Ar 3} point, and on the surface of the slab, A ₁ point (727 ° C.) after the transformation from austenite to ferrite starts. to complete the transformation to ferrite from austenite in a slightly subcooled temperature (referred a _r1 point) than, but cooled at a surface temperature of 35 ° C. until the time is at least 100 ° C. / hr or less in the cooling rate of continuously cast slabs Continue. This is because immediately after the surface temperature of the slab has _fallen below the _Ar3 point, the inside of the slab is at a high temperature of the _Ar3 point or higher, the surface temperature of the slab has _fallen below the _Ar3 point, and the transformation of the slab surface has been completed. This is because if the cooling rate of the slab surface is increased immediately thereafter, the cooling rate inside the slab may exceed 35 ° C./hr. That is, this is because the inside of the slab is cooled at a cooling rate of 35 ° C./hr or less until the entire inside of the slab is transformed from austenite to ferrite.

When casting a slab with a continuous casting machine, the surface temperature of the slab having unsolidified molten steel inside the slab may become less than _{Ar 3} point in the continuous casting machine. There is no problem when the temperature of the surface of the slab rises due to the heat from and the surface temperature of the slab becomes higher than the _Ac3 point in the continuous casting machine.

However, when there is no unsolidified molten steel inside the slab, the temperature of the slab surface does not rise or is extremely small, so it is cut by a cutting machine provided on the outlet side of the continuous casting machine and continuously casted. It is necessary to set the surface temperature of the slab at a point of _Ar3 or higher at the time when the steel is discharged (at this time, there is no unsolidified molten steel inside the slab). After that, it is inserted into the heat retaining pit and gradually cooled, or several slabs are stacked and allowed to cool in the atmosphere, so that the surface temperature of the slab reaches 100 ° C. from at least the _{Ar 3} point. The slab is cooled at a cooling rate of 35 ° C./hr or less until the time point.

In the continuous casting machine, once the surface temperature of the slab is made _lower than the _{Ar 3} point, transformation from austenite to ferrite occurs, and then the transformation from ferrite to austenite occurs by making the surface temperature higher than the _{Ac 3} point. In this case, the austenite grain size becomes smaller than the austenite grain size when the slab surface temperature is kept at _{Ar 3} point or higher in the continuous casting machine, and the ferrite grain size generated in the subsequent transformation may also become smaller. . Therefore, it is preferable to keep the slab surface temperature at _Ar3 or higher even in the continuous casting machine. This can be achieved by adjusting the amount of secondary cooling water and / or the casting speed.

The continuous cast slab manufactured in this way is transported to a hot rolling process after performing surface care as necessary. In the hot rolling process, the continuous cast slab is heated at a temperature of _Ac3 point or higher in a heating furnace, and then hot rolled to a predetermined plate thickness and plate width to obtain a hot rolled steel plate. It can also be used as a hot-rolled steel sheet as it is, and can also be used as a cold-rolled steel sheet or a surface-treated steel sheet that has been subjected to a cold rolling and a surface treatment step after cold rolling.

Note that the A _c3 point, a transformation temperature of ferrite to austenite, and the high-tensile steel plate, a tensile strength of steel sheet of more than 340 MPa. In addition, the high-tensile steel plate that is the subject of the present invention is mainly a thin steel plate because there are many uses for automobile bodies, but if the slab manufactured in the present invention is used, it is a product other than the thin steel plate. Even if it exists, it is applicable.

  As described above, according to the present invention, since the average grain size of the ferrite grains of the continuous cast slab before heating in the hot rolling process is 70 μm or more, carbides and nitrides precipitated in the continuous cast slab. It is realized that a precipitate such as a product is completely dissolved in austenite in a short time without increasing the heating temperature by the slab heat treatment in the hot rolling process. As a result, it is possible not only to stably produce high-tensile steel sheets with excellent workability (elongation characteristics and stretch flange characteristics) using fine precipitates, but also to slab heating time in the hot rolling process. Can be shortened.

C: 0.052%, Si: 0.08%, Mn: 1.13%, P: 0.036%, S: 0.0019%, Al: 0.066%, N: 0.0038%, Ti : 250 tons of molten steel having a composition of 0.123%, the balance being Fe and inevitable impurities, was melted in a converter, and then cast into a continuous cast slab with a continuous casting machine. In the continuous casting machine, the surface temperature of the slab was controlled to 920 ° C. or higher, which is higher than the _{Ar 3} point, and the slab having a surface temperature of 920 ° C. or higher was discharged from the continuous casting machine.

  After that, cast the continuous casting slab by charging it into the heat retaining pit, cooling it slowly, letting it cool in the atmosphere, blowing it with a blower and cooling it, spraying and cooling the mist of air and water, etc. The cooling rate of the subsequent slab was changed to cool to room temperature. Samples were taken from slabs cooled to room temperature, and the average grain size of ferrite grains in each slab was examined at a position 10 mm from the slab surface. The average crystal grain size of the ferrite grains was measured according to JIS G 0551: 2013. By changing the cooling rate after casting, the average grain size of ferrite grains in the slab changed. That is, it was confirmed that the average crystal grain size of the ferrite grains was increased by slowing the cooling rate after casting.

Thereafter, a plurality of samples collected from the respective slabs are heated in a soaking furnace at 1250 ° C., the samples are taken out from the soaking furnace every predetermined time, cooled immediately in water immediately after being taken out, and the samples are dissolved. Ti amount was measured. The time when the value of the amount of solid solution Ti hardly changed was defined as the complete solid solution time. In addition, the amount of solid solution Ti was evaluated according to the method described in publication 2 (quantification of solid solution microalloy in steel, iron and steel, Vol.99 (2013) .No.5, p.362). . Moreover, the solid solution state of the solute element at the time of soaking is frozen to room temperature by rapidly cooling the sample soaked at a temperature of the _{Ac 3} point or higher in water from the temperature of the _{Ar 3} point or higher.

  The investigation results are shown in Table 1, and the relationship between the average grain size of ferrite grains in the pre-heating slab and the complete solid solution time of Ti precipitates is shown in FIG.

  As shown in Table 1 and FIG. 7, the larger the ferrite grain size in the slab stage after continuous casting, the shorter the time until the precipitate is completely dissolved. In order to increase the ferrite grain size in the slab stage, the slab cooling rate may be slowed down. In particular, by setting the cooling rate to 35 ° C./hr or less, the average grain size of the ferrite grains may be set to 70 μm or more. it can.

Claims (7)

  In mass%, C: 0.02-0.15%, Si: 2.0% or less, Mn: 0.5-2.0%, P: 0.08% or less, S: 0.0060% or less, Al: 0.005 to 0.1%, N: 0.006% or less, Ti: 0.05 to 0.3%, with the balance being Fe and inevitable impurities, before heating in the hot rolling step A continuous cast slab characterized in that the average grain size of ferrite grains is 70 μm or more at the stage.   Furthermore, it contains at least one element selected from Nb: 0.005 to 0.1% and V: 0.005 to 0.1% by mass%. Continuous slab as described.   Furthermore, by mass%, Cr: 0.005-0.3%, Mo: 0.005-0.3%, Cu: 0.005-0.5%, Ni: 0.005-0.5% The continuous cast slab according to claim 1 or 2, which contains at least one element selected from among them.   The continuous cast slab according to any one of claims 1 to 3, further comprising B: 0.0002 to 0.005% in mass%.   Furthermore, it is characterized by containing at least one element selected from Ca: 0.0005 to 0.02% and REM (rare earth element): 0.0005 to 0.02% by mass%. The continuous cast slab according to any one of claims 1 to 4. The method for manufacturing a continuous cast slab according to any one of claims 1 to 5, wherein after the casting in the continuous casting machine, the surface temperature of the continuous cast slab is at least 100 ° C from the point of _{Ar 3} point. The manufacturing method of the continuous casting slab characterized by cooling at a cooling rate of 35 degrees C / hr or less until it becomes. The continuous cast slab according to any one of claims 1 to 5 or the continuous cast slab manufactured by the method for manufacturing a continuous cast slab according to claim 6 is heated at a temperature _{equal to} or higher than _Ac3 point. And hot rolling after heating, a method for producing a high-strength steel sheet having excellent workability.