JP2017095755A - Continuously cast slab and manufacturing method of high tensile strength steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slab which enables carbide, nitride or the like deposited in the slab to be subjected to solid solution in a short time without increasing a heating temperature in a process of slab heating before hot rolling.SOLUTION: A continuously cast slab contains, by mass%, C:0.05 to 0.12%, Si:0.05 to 1.0%, Mn:0.5 to 1.8%, P:0.04% or less, S:0.0030% or less, Al:0.005 to 0.07%, N:0.0025% or less, Ti:0.05 to 0.15% and the balance Fe with inevitable impurities. When an average crystal grain diameter of ferrite grains of the slab before heating in a hot rolling process is defined as D, the average crystal grain diameter Dof ferrite grains of the slab and nitrogen content of the slab satisfy a relationship of formula (1). In the formula (1), Dis an average crystal grain diameter (mm) of the ferrite grains of the slab before heating and [mass%N] is nitrogen content (mass%) of the slab. D/[mass%N]≥18.0 ...(1).SELECTED DRAWING: Figure 8

Description

  The present invention relates to a continuous cast slab that is a material of a high-strength steel plate used for an inner plate part such as a structural member or a reinforcing member of an automobile, and a method for manufacturing a high-tensile steel plate using the continuous cast slab.

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 completely 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) From the viewpoint of stably producing a high-strength steel sheet having excellent characteristics and stretch flange characteristics) and increasing productivity, there is still a point to be improved.

  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. It is to provide a continuous cast slab capable of stably obtaining a high-strength steel plate excellent in the above, and to provide a method for producing a high-tensile steel plate using the continuous cast slab.

  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. 2, that is, heating from 800 ° C. to 1250 ° C. at a heating rate of 30 ° C./s. 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, TiC is shown in a circle. 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. It is understood that the movement of austenite grain boundaries is prevented by TiC, and the growth behavior of austenite grains is affected 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”. In other words, it can be said that the interface is always in a state where tension is applied to minimize the length.

  Here, paying attention to the triple point between the austenite grain boundary and the TiC / γ interface, as shown in FIG. 6, the TiC / γ interface is shaped like being pulled by the austenite grain boundary due to the balance of tension at each interface. 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.

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

  (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 making the “pinning effect” caused by 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.

  Therefore, 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 for the precipitates that precipitate in the slab during and after casting. However, this measure cannot directly be adopted because it directly leads to a decrease in the tensile strength of the steel plate as the final product. Therefore, the present inventors examined details of the types of precipitates and obtained the following results.

  That is, in a steel sheet containing Ti as a main precipitate forming element, the precipitate related to the tensile strength of the steel sheet is a Ti-based carbide precipitated in a nanometer size, and other precipitates (for example, Ti-based nitrides). Does not contribute to the improvement of tensile strength.

  Further, in a component slab containing 0.05 to 0.15% by mass of Ti as a precipitate-forming element, Ti-based carbide is disclosed in Publication 2 (KJIrvine FBPickering and T. Gladman: JISI, 205 (1967 ), p. 161), based on the solubility product, the solid solution is sufficiently dissolved at the normal slab heating temperature (1200 ° C or higher). Therefore, the pinning effect by the Ti-based carbide is lost during the slab heating. Is called. That is, it is not necessary to consider the Ti-based carbide in the slab as a precipitate that causes the pinning effect.

  On the other hand, when the slab contains about 0.006% by mass of nitrogen, the Ti-based nitride is generally considered to be a solid solution at 1400 ° C. or higher based on the solubility product (see Publication 2). Therefore, in slab heating in a normal temperature range of about 1200 to 1250 ° C. before hot rolling, the Ti-based nitride does not dissolve, and the Ti-based nitride continues to exhibit the pinning effect. Further, the Ti-based nitride in the slab has a coarse precipitate size of micrometer size and is not preferable from the viewpoint of improving the tensile strength.

  In other words, the Ti-based nitride in the slab is preferably absent because it exhibits a pinning effect during the growth of austenite grains in the slab, and the Ti-based nitride in the slab does not exist from the viewpoint of improving tensile strength. Is preferred.

  From these examination results, in order to make the “pinning effect” due to precipitates less likely to occur, it is effective to suppress the generation amount of Ti-based nitride in the slab, that is, nitrogen ( N) The knowledge that it is effective to suppress the content was obtained.

  By the way, the particle size of austenite generated during slab heating depends on the ferrite particle size of the slab before slab heating, that is, at the stage of casting. This is because when ferrite transforms into austenite during slab heating, austenite grains are first generated from the ferrite grain boundaries, so by increasing the ferrite grain size of the slab, the density of the ferrite grain boundaries of the slab is increased. This is because the frequency of austenite grains decreases as the size decreases. Hereinafter, ferrite is also indicated as “α”.

  From these examination results, (1) the nitrogen content of the slab is reduced to suppress the production of Ti-based nitrides in the slab, and the austenite grain growth during slab heating is not hindered. (2) the slab By combining the two points of increasing the ferrite grain size of the slab at the casting stage according to the nitrogen content of the steel, the “pinning effect” due to precipitates can be made less likely to occur. The knowledge that the precipitate in the slab can be completely dissolved in a short time was obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.05 to 0.12%, Si: 0.05 to 1.0%, Mn: 0.5 to 1.8%, P: 0.04% or less, S: 0.0030% or less, Al: 0.005-0.07%, N: 0.0025% or less, Ti: 0.05-0.15%, with the balance being Fe and inevitable impurities, heat when during the average crystal grain size of the ferrite grains of the slab in the stage before the heating in the rolling step was D _alpha, nitrogen content of the average grain size D _alpha and slabs and is of the following formula (1) relationship Continuous casting slab characterized by satisfying
D _α / [mass% N] ≧ 18.0 (1)
However, in the formula (1), _Dα is the average crystal grain size (mm) of ferrite grains of the slab in the stage before heating, and [mass% N] is the nitrogen content (mass%) of the slab.
[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] The high-tension characterized by heating the continuous cast slab according to any one of [1] to [5] above at a temperature of A _c3 point or higher and hot rolling after heating. A method of manufacturing a steel sheet.

According to the present invention, the continuous cast slab has a nitrogen content of 0.0025% by mass or less, and the average grain size D _α of the ferrite grains of the continuous cast slab before heating in the hot rolling step is set to be equal to that of the continuous cast slab. According to the nitrogen content, the size satisfies the relationship of the above formula (1), so that the production of Ti nitride in the continuously cast slab is suppressed, and the “pinning effect” that inhibits the movement of the austenite grain boundary. As a result, the average grain size D _α of the ferrite grains is set to a predetermined value or more, and the density of the austenite grain boundaries is reduced, so that the carbides and nitrides precipitated in the continuous casting slab are reduced. 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. This makes it 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 N content in the slab before a heating, and the complete solution time of Ti precipitate. It is a figure which shows the relationship between D ( _alpha) / [mass% N] 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): 0.05 to 0.12%
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 0.05% or more. On the other hand, if the C content exceeds 0.12%, the stretch flange characteristic is greatly deteriorated. Therefore, the C content needs to be 0.05 to 0.12%, preferably 0.07 to 0.11%.

Si (silicon): 0.05 to 1.0%
Si is an element that stabilizes the strength of the steel sheet by solid solution strengthening and contributes to improving ductility. In order to acquire such an effect, it is necessary to make Si content 0.05% or more. On the other hand, when the Si content exceeds 1.0%, not only the surface properties are lowered, but also the segregation of Mn at the center of the plate thickness is promoted, and Si itself is segregated. Therefore, the Si content needs to be 0.05 to 1.0%, preferably 0.05 to 0.8%.

Mn (manganese): 0.5 to 1.8%
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 1.8%, the central 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 1.8%, preferably 1.0 to 1.6%.

P (phosphorus): 0.04% or less P is an element that segregates at a grain boundary to reduce elongation. Therefore, the P content is 0.04% or less, but it is preferable to reduce it as much as possible. Preferably it is 0.02% or less, More preferably, it is 0.01% or less. There is no problem even if the content of P is 0 (zero).

S (sulfur): 0.0030% or less S forms sulfides and decreases workability. Therefore, although S content is made into 0.0030% or less, it is preferable to reduce as much as possible. 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 0.07%
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 0.07%, it remains as an oxide of Al in the steel sheet, and the Al oxide is aggregated to be easily coarsened, which causes deterioration of stretch flangeability. Therefore, the Al content needs to be 0.005 to 0.07%, preferably 0.015 to 0.05%.

N (nitrogen): 0.0025% or less N forms a coarse Ti-based nitride in the slab, and this Ti-based nitride exhibits a pinning effect during the growth of austenite grains in the slab, and promotes the growth of austenite grains. Inhibit. That is, the growth of austenite grains is inhibited and solid solution of precipitates in the slab is prevented. Therefore, the N content needs to be 0.0025% or less. Preferably, the N content is 0.0020% or less, and even more preferably, the N content is 0.0015% or less. The lower the N content, the better. However, lowering the N content more than necessary causes an increase in manufacturing cost, so the lower limit of the N content is set to 0.0010%.

Ti (titanium): 0.05 to 0.15%
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 0.15%, Ti carbide tends to be coarsened, making it difficult to obtain a desired tensile strength in the steel sheet. Therefore, the Ti content needs to be 0.05 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, it is preferable to make Nb content and V content 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, the Cr content is 0.005-0.3%, the Mo content is 0.005-0.3%, the Cu content is 0.005-0.5%, and the 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, it is preferable that the Ca content is 0.0005 to 0.02% and the REM content is 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 D _α of the ferrite grains of the continuously cast slab satisfies the relationship of the following formula (1) with respect to the nitrogen content of the slab: D _α / [Mass% N] ≧ 18.0 (1)
However, in the formula (1), _Dα is the average crystal grain size (mm) of ferrite grains of the slab in the stage before heating, and [mass% N] is the nitrogen content (mass%) of the slab.

Incidentally, when the nitrogen content of the slab is 0.0025% and the average crystal grain size D _alpha ferrite grains must be at least 0.045 mm, when the nitrogen content of the slab is 0.0020% The ferrite grains must have an average crystal grain size D _α of 0.036 mm or more. The average crystal grain size D _alpha ferrite grains of the slab prior to heating may be used data measured at the position of 10mm from the slab surface.

As described above, the larger the average grain size D _α of the ferrite grains of the continuous cast slab before heating in the hot rolling step, the larger the precipitate in the continuous cast slab, the higher the heating temperature during the slab heat treatment in the hot rolling step. Can be completely dissolved in austenite without increasing the temperature of the steel and with a short heating time.

As described above, in the present invention, in order to promote the growth of austenite grains, the N content of the slab is regulated to 0.0025% or less. Even if the N content of the slab is 0.0025% or less, the lower the N content of the slab, the more the pinning effect due to the Ti-based nitride does not occur, and the solid solution of the precipitate is promoted. . However, if the average grain size D _α of the ferrite grains of the continuous cast slab before heating in the hot rolling process is too small, precipitation in the slab is limited even if the N content of the slab is regulated to 0.0025% or less. The time for complete solid solution will be longer.

In the heating process before hot rolling, the present inventors can efficiently and stably produce a high-tensile steel sheet having excellent workability if the precipitates in the slab can be completely dissolved by heat treatment within 65 minutes. It is confirmed that it can be manufactured. Accordingly, various changes were made to the N content of the slab and the average grain size D _α of ferrite grains of the slab, and precipitation in the slab was performed by heat treatment within 65 minutes under the condition that the N content of the slab was 0.0025% or less. objects were investigated average crystal grain size D _alpha ferrite grains required to complete solid-solution (see example below).

The survey is the heating time required for the complete solid solution, shortening the lower the N content of the slab, also, since the shortening as ferrite grains mean crystal grain size D _alpha is large, the heating time required for a complete solid solution It was considered as being influenced by the ratio (= D _α / [mass% N]) of the ferrite grain average crystal grain size D _α and the N content of the slab.

As a result, when the N content of the slab and the ferrite grain average grain size D _α of the slab before heating satisfy the relationship of the above formula (1), precipitates in the slab can be obtained by heating within 65 minutes. It was confirmed that can be completely dissolved.

The average crystal grain size D _alpha ferrite grains of the slab before heating is consistent with _alpha average crystal grain size D of the ferrite grains of the continuous cast slab after casting in a continuous casting machine. The average grain size D _α of the ferrite grains of the continuous casting slab depends on the cooling rate at the _{Ar 3} point or less of the continuous casting slab after casting in the continuous casting machine, and the slower this cooling rate, the ferrite of the continuous casting slab. The average crystal grain size D _α of the grains increases. However, in the present invention, although depending on the N content of the slab, it is sufficient that the average grain size D _α of the ferrite grains of the continuous cast slab is 0.036 mm or more, and the cooling rate of the continuous cast slab after casting is sufficient. There is no need to slow down too much.

  As a method of adjusting the N content of the slab to 0.0025% or less, in the RH degassing refining process of the molten steel before the continuous casting process, the molten steel is exposed to an atmosphere under reduced pressure, and N (nitrogen) in the molten steel is removed. The removal method can be used.

The continuous cast slab thus obtained is heated at a temperature of the slab surface temperature of _Ac3 or higher for 65 minutes or less, and then hot-rolled to provide a high-tensile steel plate with excellent workability. Can be obtained. Various conditions for hot rolling, that is, rough rolling conditions and finish rolling conditions do not need to be specified, and can be set as appropriate in accordance with the characteristics of the target steel sheet. Moreover, conditions such as cooling and winding after hot rolling can also be set as appropriate according to the characteristics of the target steel sheet.

Note that the A _r3 point, a transformation temperature from austenite to ferrite, whereas, the A _c3 point, a transformation temperature of ferrite to austenite, A _r3 point at low temperature than _C3 point A is there. Further, the high-tensile steel plate is a steel plate having a tensile strength of 340 MPa or more.

As described above, according to the present invention, the nitrogen content of the continuous cast slab is 0.0025% or less, and the average grain size D _α of the ferrite grains of the continuous cast slab before heating in the hot rolling step. Is set to a size that satisfies the relationship of the above formula (1) according to the nitrogen content of the continuously cast slab, so that the formation of Ti-based nitrides in the continuously cast slab is suppressed, and the movement of the austenite grain boundaries is inhibited. The “pinning effect” is reduced, and as a result, the density of the austenite grain boundaries is reduced due to the fact that the average grain size D _α of the ferrite grains is set to a predetermined value or more, and precipitates in the continuous casting slab. The slab heat treatment in the hot rolling process allows the precipitates such as carbides and nitrides to be completely dissolved in austenite in a short time without increasing the heating temperature.

The N content is 0.0013%, 0.0018%, 0.0020%, 0.0025%, 0.0038%, 0.0066%, and the other components are C: 0.07 to 0.11. %, Si: 0.05 to 0.8%, Mn: 1.0 to 1.6%, P: 0.02% or less, S: 0.0020% or less, Al: 0.015 to 0.05% , Ti: 0.11 to 0.15%, with a balance of high-tensile steel composed of Fe and unavoidable impurities, 250 tons of molten steel is melted by a combination of a converter and an RH vacuum degasser Then, it cast into a continuous casting slab with a continuous casting machine. In the continuous casting machine, the surface temperature of the slab in the continuous casting machine was controlled to 920 ° C. or higher, which is higher than the _Ar3 point, and a slab having a surface temperature of 920 ° C. or higher was produced by the continuous casting machine.

  Then, the continuous casting slab is allowed to cool in the atmosphere, cooled by blowing with a blower, or cooled by spraying mist of air and water to change the cooling rate of the slab after casting. Until cooled. A sample was taken from the slab cooled to room temperature. The average grain size of ferrite grains in each slab was investigated 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.

After that, a plurality of samples collected from each slab are heated in a soaking furnace at 1250 ° C., the samples are taken out from the soaking furnace every predetermined heating time, and cooled immediately in water immediately after taking out. The amount of dissolved Ti was measured (test numbers 1 to 18). And the heating time when the value of solid solution Ti amount almost did not change was made into complete solution time. In addition, the analysis method of the amount of solid solution Ti is performed according to the method described in publication 3 (quantification of solid solution microalloy in steel, iron and steel, Vol.99 (2013) .No.5, p.362). It was. Also, the soaking samples in A _c3 point above the temperature, by quenching from the A _r3 point temperature above the water, the solid solution state of the solute elements during soaking is frozen to room temperature.

Table 1 shows the results of the examinations in Test Nos. 1-18. FIG. 7 shows the relationship between the N content in the slab before heating and the complete solid solution time of the Ti precipitate. In FIG. 7, it is displayed apart compared ferrite grains mean crystal grain size D _alpha slab before heating.

  As shown in Table 1 and FIG. 7, the larger the ferrite grain size in the slab stage after continuous casting and the smaller the N content in the slab, the shorter the time until Ti precipitates are completely dissolved. Shorter. In addition, the present inventors have confirmed that a high-tensile steel sheet excellent in workability can be produced efficiently and stably if the time during which the precipitate in the slab is completely dissolved is 65 minutes or less. Yes.

As shown in Table 1 and FIG. 7, a complete solid solution to take the heating time is shortened lower the N content in the slab, also shortens the larger ferrite grains mean crystal grain size D _alpha heating before slab Therefore, it is assumed that the heating time required for complete solid solution is affected by the ratio (= D _α / [mass% N]) of the ferrite grain average crystal grain size D _α and the N content of the slab. The target value of the heating time required for the above was set to 65 minutes or less, and the conditions for achieving the heating time target value of 65 minutes or less were examined.

FIG. 8 shows the ratio (= D _α / [mass% N]) between the average grain size D _α (mm) of the ferrite grains of the slab before heating and the N content (mass%) of the slab, and the Ti precipitates. The relationship with complete solution time is shown. As is clear from Table 1 and FIG. 8, when the N content of the slab and the ferrite grain average grain size D _α of the slab before heating satisfy the relationship of the above formula (1), it is within 65 minutes. It was confirmed that the precipitates in the slab could be completely dissolved by the heat treatment.

Claims (6)

In mass%, C: 0.05 to 0.12%, Si: 0.05 to 1.0%, Mn: 0.5 to 1.8%, P: 0.04% or less, S: 0.0030 %: Al: 0.005 to 0.07%, N: 0.0025% or less, Ti: 0.05 to 0.15%, the balance being Fe and inevitable impurities, hot rolling step the average crystal grain size of the ferrite grains of the slab in the stage before the heating when the D _alpha in the nitrogen content of the average grain size D _alpha and slabs and is, to satisfy the relation (1) below Continuous casting slab characterized by that.
D _α / [mass% N] ≧ 18.0 (1)
However, in the formula (1), _Dα is the average crystal grain size (mm) of ferrite grains of the slab in the stage before heating, and [mass% N] is the nitrogen content (mass%) of the slab.   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. A method for producing a high-strength steel sheet, comprising: heating the continuously cast slab according to any one of claims 1 to 5 at a temperature _{equal to} or higher than an _Ac3 point, and hot rolling after heating.