JP6007968B2 - High-strength and highly ductile steel plate and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength and highly ductile thick steel plate and a method for manufacturing the same. For example, even when a ship collision such as an oil spill accident caused by a tanker collision occurs, a breakage in a side surface of the ship is suppressed. The present invention relates to a high-strength and highly ductile thick steel plate having a thickness of 5 mm or more and a method for producing the same.

  In recent years, environmental pollution due to oil spills caused by landing of large tankers and collisions has become a problem. Therefore, in the recent shipbuilding field, even if a ship has a collision accident, its destruction (break) is kept to a minimum, and damage such as oil spills from tankers and inundation from damaged parts is minimized. Techniques for doing this are being considered.

  Among them, as an approach from the steel material side for the hull, it has been proposed that the energy at the time of collision is absorbed in the steel material itself to suppress the destruction of the hull.

  For example, as a method for improving the energy absorption capability at the time of collision, Patent Document 1 proposes a technique in which the structure of a steel sheet is mainly composed of ferrite (α) and the α phase is strengthened. This technique is characterized in that the α fraction F is 80% or more and the lower limit (H ≧ 400−2.6 × F) is defined for the hardness H of α.

  Further, Patent Document 2 proposes a technique for including a retained austenite (γ) phase in the front and back layers of a steel plate. This technique contains C, Si, Mn, and Al, and further contains a strengthening element as necessary. The front and back layers of at least 1/8 or more of the plate thickness of the steel plate have an area ratio of 1.0 to 20%. It contains residual γ.

  In addition to these, Patent Document 3 states that the ferrite (α) phase fraction in the steel sheet metal structure should be 70% or more at the plate thickness center portion and 50% or more at the plate thickness surface layer portion to increase uniform elongation. Thus, a technique for improving the collision resistance is disclosed.

  Furthermore, in Patent Document 4, the area fraction of α in the total metal structure of the steel sheet is 90% or more, the average α particle size is 3 to 12 μm, the maximum α particle size is 40 μm or less, and the average equivalent circle diameter of the second phase. Has been proposed that improves the impact absorption by increasing the product of the uniform elongation and the breaking stress to be 0.8 μm or less.

  Patent Document 1 and Patent Document 2 described above disclose means for increasing the product of elongation and strength (EL × (YP + TS) / 2) as an index (impact absorption energy) representing impact resistance. However, from the viewpoint of suppressing breakage when ships collide with each other, it is becoming clear through large-scale collision simulations that the elongation value itself has a greater influence than the above index. In the technique of Patent Document 1, since the α particle size is 5 μm or less and the hardness of α is as high as Hv 160 to 190, the elongation itself is not necessarily high, and the effect of suppressing the breakage at the time of collision cannot be expected so much.

  In addition, in the technique of Patent Document 2, alloy elements are added to make the structure contain residual γ, and steel disclosed as examples has a high carbon equivalent (Ceq) or Si. Is a high steel grade. Therefore, it is difficult to ensure weldability and joint toughness, and application to actual ships is considered to be limited.

  On the other hand, in the technique of Patent Document 3, the alloy element addition amount is kept low, and by controlling the α phase fraction, hardness, and particle size of the central portion of the plate thickness by two-stage cooling, uniform elongation is achieved. Although improvement is aimed at, when manufacturing the wide-width long steel plate for shipbuilding, the material dispersion | variation arises and it is hard to say that it is a practical manufacturing method.

  Patent Document 4 discloses information on chemical components and metal structures of steel materials, but there are many practically uncertain points in the manufacturing method. In other words, the manufacturing method described in the detailed description recommends reheating after hot rolling and cooling. However, in steel sheets for shipbuilding that are indispensable for low-priced and mass production, processes such as reheating are the production costs. And there is concern about practical application from the viewpoint of manufacturing construction period.

  In view of the above, with a high strength and high ductility thick steel plate, for example, about a high strength and high ductility thick steel plate that has both strength and ductility that can suppress the breakage of the side surface of the ship when the ship collides. This technology is not yet established.

Japanese Patent Laid-Open No. 10-306340 JP-A-11-246935 Japanese Patent Application Laid-Open No. 2003-089841 JP 2007-162101 A

  In view of the above circumstances, the present invention is a high-strength, high-ductility thick steel plate, for example, a high-strength, high-ductility thick steel plate that has both strength and ductility capable of suppressing the breakage of the side surface of a ship at the time of a collision and the like. An object is to provide a manufacturing method.

  Consider the mechanism by which a fracturing occurs in the side of the ship when the ship collides. For example, when the tip of another ship collides with the ship side wall, the entire tip of the ship sinks into the flat steel plate on the ship side wall. It is stretched and is pulled greatly. And when a steel plate is destroyed, the steel plate of a ship side wall part will break.

  Therefore, in order to prevent the steel plate on the side of the ship from being broken when the ship collides, when the steel sheet is greatly bent in the initial stage of the collision, it must be able to withstand the bending and bend. The part that is not stretched is greatly stretched and causes tensile deformation, but it is necessary that the part does not stretch and break.

  Increasing the elongation of the steel sheet is essential in order to suppress the fracturing at the side of the ship when the ship collides, but in general, increasing the strength of the steel sheet deteriorates the elongation of the steel sheet. And high-strength and highly ductile steel sheet that achieve both elongation and elongation are desired.

  In order to obtain a high-strength, high-ductile thick steel plate, particularly a high-strength, high-ductile thick steel plate that can suppress the breakage of the ship side surface at the time of a collision, As a result of research focused on, ferrite + pearlite steel, which easily suppresses fluctuations in strength and elongation within the steel sheet, improved ductility due to the presence of α phase as a microstructure and strength improvement due to pearlite being the second phase. And found that a high-strength and highly ductile thick steel plate having both strength and ductility can be obtained by controlling the maximum concentration of P in the thickness and the hardness in the thickness direction. completed.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.050-0.200%
Si: 0.200 to 1.000%
Mn: 0.50 to 2.00%,
P: 0.015% or less,
S: 0.003% or less,
Ti: 0.003-0.020%,
Al: 0.002 to 0.050%,
N: 0.0010 to 0.0060%,
O: 0.0005 to 0.0060%,
One or more of Ca, Mg, and REM are added in a total amount of 0.0005 to 0.0080%,
further,
Nb: 0 to 0.030%,
V: 0 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to 1.00%,
Cr: 0 to 0.50%,
Mo: 0 to 0.500%,
B: 0 to 0.0030%
Containing, and
Ti / N is 0.5-4.0,
The balance is a steel composed of Fe and inevitable impurities,
The microstructure is composed of a ferrite phase area fraction of 1/4 thick part with 80-95%, a pearlite phase area fraction of 1/4 thick part with 5-20%, and the 1/4 thick part ferrite phase and pearlite. phase other tissues in bainite phase, an organization that area fraction of the bainite phase Ru consists of 10% or less,
The average aspect ratio of the ferrite grain of 1/4 thickness part is 1.0 to 1.5,
The average particle diameter of the ferrite grain of 1/4 thickness part is 5 to 20 μm,
The average dislocation density in the ferrite phase of ¼ thick part is 7 × 10 ¹² / m ² or less,
In a 1 mm pitch Vickers hardness test, the average value of Vickers hardness from the surface of the steel sheet to 1/4 thickness or from 3/4 thickness to the back is from 1/4 thickness to 3/4 thickness. 80-105% of the average Vickers hardness of
A high-strength and highly ductile steel plate characterized by

(2) In the high-strength and highly ductile thick steel plate described in (1) above, there are 10 inclusions having a length of 5 μm or more within a range of ½ thickness ± (thickness) 10% in the thickness direction of the plate thickness / A high-strength, high-ductile steel plate characterized by being ² mm2 or less.

  (3) In the high-strength and highly ductile thick steel plate described in (1) or (2) above, the maximum P concentration is 0.1 in the range of 1/2 thickness ± (thickness) in the thickness direction of the thickness. A high-strength and highly ductile steel plate characterized by being from 0.2 to 0.20%.

(4) In the high strength and high ductility thick steel plate according to any one of the above (1) to (3), the ferrite transformation start temperature Ar ₃ during cooling represented by the following formula (1) is 760 to 820. A high-strength, high-ductile, thick steel plate characterized by being at ° C.
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%.

(5) A method for producing the high-strength and highly ductile thick steel plate according to any one of (1) to (4) above, wherein the steel slab is heated in a range of 950 to 1100 ° C, and the cumulative reduction rate Is performed at a surface temperature of the steel slab of Ar ₃ −30 ° C. or higher and a recrystallization start temperature T _rex ° C. or lower at which crystal grain growth begins, and the finished rolled thick steel plate is air-cooled. A method for producing a high-strength, high-ductility, thick steel plate, characterized by cooling to room temperature.
However,
Ar ₃ is represented by the following formula (1), and T _rex is represented by the following formula (2).

_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%.

T _rex = −91900 [Nb *] ² +9400 [Nb *] + 770 (2)
However, [Nb *] is obtained by the following equation (3).
Sol. Nb = (10 ^{(-6770 / (T + 273) +2.26)} ) / (C + ^12/14 × N) (3)
In the formula (3), T is the heating temperature of the steel slab, the unit is Celsius temperature (° C.),
[Nb] ≧ [Sol. Nb], [Nb *] = [Sol. Nb]
[Nb] <[Sol. In the case of Nb], [Nb *] = [Nb]
And Here, [Nb] represents the Nb content (mass%) by [Sol. Nb] represents Sol.Nb (solid solution Nb) (mass%) obtained by the formula (3).

(6) The method for producing a high-strength, high-ductility, thick steel plate according to (5) above, wherein the surface temperature of the steel plate that has been finish-rolled is Ar ₃ −150 ° C. or higher, Ar ₃ −50 ° C. A high-strength and highly ductile thick plate characterized by performing water cooling at a cooling rate of more than 1 ° C./second to 20 ° C./second to the following temperature, followed by air cooling at a cooling rate of 1 ° C./second or less. A method of manufacturing a steel sheet.

  (7) In the method for producing a high-strength, high-ductile, thick steel plate according to (5) or (6) above, when producing molten steel, the amount of dissolved oxygen in the molten steel is adjusted to 40 ppm or less by a vacuum degassing device. Then, Al is added so that the final content of Al is 0.002 to 0.050%, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less, and then one or two of Ca, Mg, and REM A high-strength, high-ductility, thick steel sheet characterized by adding at least one seed so that the total final content of one or more of Ca, Mg, and REM is 0.0005 to 0.0080% Manufacturing method.

  (8) A method for producing a high-strength, high-ductility, thick steel plate according to any one of (5) to (7) above, wherein the slab is the final stage of solidification of the slab when continuously casting molten steel In the range of the central solid phase ratio of 0.2 to 0.7, the gap between the casting rolls is narrowed to 0.2 mm to 3.0 mm per 1 m of the casting traveling direction, and casting is performed while being reduced. A method for producing a high strength and ductile steel plate.

  By using the high-strength and high-ductile steel plate of the present invention for a ship, even if a collision accident between ships occurs, it is possible to prevent the ship's steel sheet from breaking and breaking, so a tanker From the viewpoint of environmental protection and safety, it is possible to reduce marine pollution due to oil spills in the event of a ship accident or the like, or to reduce the amount of inundation from a collision damaged part.

  The present invention will be described in detail below.

  In order to suppress the breakage of the ship side surface when the ship collides, it is essential to increase the elongation of the steel plate on the ship side surface. Elongation can be divided into uniform elongation and local elongation, but these dominating factors are different and are usually difficult to achieve. That is, the uniform elongation can be increased by increasing the hardness of the second phase in addition to improving the ductility of α itself, and it is generally more advantageous to use a composite structure. On the other hand, it is advantageous that the local elongation has a uniform structure such as uniform hardness distribution and fine dispersion of the second phase and inclusions. From the standpoint of preventing destruction when a structure collides, it is desirable to improve both in a balanced manner rather than intensively improving either elongation. It is known that the elongation value varies greatly depending on the shape of the test piece. When a standard JIS No. 1B tensile test piece is used, the uniform elongation is 14 to 24% and the local elongation is 9 to 16%. The total elongation (T.EL) (hereinafter also simply referred to as elongation) 23 to 40%, which is the sum of the local elongations, was set as a target value in the present invention, but the elongation decreases as the plate thickness decreases.

  Specifically, when the steel plate thickness is 5 mm or more and 10 mm or less, the elongation is 23% or more, when the steel plate thickness is 10 mm or more and 15 mm or less, the elongation is 24% or more, and when the steel plate thickness is 15 mm or more and 20 mm or less, the elongation is increased. When the steel plate thickness is more than 20 mm and 30 mm or less, the elongation is 26% or more, and when the steel plate thickness is more than 30 mm and 50 mm or less, the elongation is 27% or more. preferable.

  According to the analysis by the National Maritime Research Institute, the steel sheet stretches to 23%, which is about 1.5 times that of general steel (16%), until a hole is opened when it is hit from the side of the ship. It is known that the impact absorption energy of the steel is about 3 times that of the conventional steel material, and it is known that it has a feature that the hole is not easily opened in the hull. Therefore, in the present invention, the target value of the lower limit of elongation is set to 23%. As other characteristics, the yield stress (YP) was 355 to 500 MPa, the tensile strength (TS) was 490 to 620 MPa, and the steel plate thickness (t) was 5 to 50 mm.

  As a high-strength and highly ductile thick steel plate that can achieve such a target value, the present inventors presuppose that ferrite (pearlite steel) is easy to suppress fluctuations in strength and elongation within the steel plate. Based on the guideline of improving the ductility of the phase and improving the strength by pearlite, which is the second phase, we conducted a detailed investigation on the influence of the chemical composition and manufacturing conditions of the steel sheet and found the following.

  In order to improve the ductility of the α phase, it is necessary to increase the cleanliness of α as much as possible. However, since it is necessary to secure the strength of the steel plate, a certain amount of C forming pearlite and substitutional solid solution elements such as Si and Mn must be added. The elements such as Nb and Ti that form precipitates in α are limited to the minimum necessary amount, and N and impurity elements such as P and S, which increase the yield stress remarkably by interstitial solid solution, are minimized. It is effective to reduce. In addition, by adding sulfides containing Ca, Mg, or REM (rare earth elements such as La and Ce) alone or in combination to suppress the formation of coarse inclusions (stretched MnS, etc.), elongation is improved. Be effective. If the dislocation density in α is increased, it is easily proliferated by plastic deformation, causing α to harden and lower the elongation. Therefore, the dislocation density should be reduced. It has been found that P segregates in the central portion of the plate thickness, forms an embrittlement region and causes cracking, and degrades local elongation, so that it is necessary to reduce the maximum concentration of P.

  Moreover, the strength can be improved by dispersing the pearlite, which is the second phase, but in order to suppress the breakage of the side surface of the ship when the ship collides, the structure in the thickness direction of the steel sheet is made uniform. It has been found that there is an effect in making the hardness distribution in the thickness direction uniform.

  In the present invention, based on these findings, the steel components and the microstructure of the high-strength and high-ductile steel plate were determined.

  First, the reasons for limiting the steel components of the steel sheet of the present invention will be described. In addition, "%" about a component means the mass% altogether.

(C: 0.050-0.200%)
C is an element indispensable for forming pearlite and increasing the strength, so 0.050% or more is added. On the other hand, if the amount of C increases, it becomes difficult to secure weldability and joint toughness, so 0.200% is made the upper limit. The C content is preferably 0.100% or more and 0.160% or less.

(Si: 0.200 to 1.000%)
Si is an inexpensive deoxidizing element, and is effective for solid solution strengthening, and also increases the transformation point and contributes to the reduction of dislocation density in α, so 0.200% or more is added. On the other hand, if the amount of Si exceeds 1.000%, the weldability and joint toughness are deteriorated, so the upper limit is made 1.000%. The amount of Si is preferably 0.300% or more and 0.500% or less.

(Mn: 0.50 to 2.00%)
Since Mn is effective as an element for improving the strength and toughness of the base material, 0.50% or more is added. On the other hand, if Mn is added excessively, joint toughness and weld cracking properties are deteriorated, so the upper limit is made 2.00%. The amount of Mn is preferably 0.80% or more and 1.60% or less, and more preferably 0.90% or more and 1.50% or less.

(P: 0.015% or less, S: 0.003% or less)
P and S are inevitable impurities, and in order to ensure elongation and toughness, the lower the content of P and S, the better. Therefore, P is 0.015%, and S is 0.003%.

(Ti: 0.003-0.020%)
Since Ti contributes to the improvement of toughness through refinement of the structure of the base material and the welded portion by adding a small amount, Ti is added in an amount of 0.003% or more. On the other hand, if added in excess, the weld is hardened and the toughness is remarkably deteriorated, so 0.020% is made the upper limit. The amount of Ti is preferably 0.006 to 0.013%.

(Al: 0.002 to 0.050%)
Since Al is an important deoxidizing element, 0.002% or more is added. On the other hand, if Al is added excessively, the surface quality of the steel slab is impaired, and inclusions harmful to toughness are formed, so 0.050% is made the upper limit. The amount of Al is preferably 0.002 to 0.040%, and more preferably 0.010 to 0.040%.

(N: 0.0010 to 0.0060%)
N forms a nitride with Al and improves joint toughness, so the lower limit of the content is 0.0010% or more, preferably 0.002% or more. On the other hand, if the content of N is excessive, embrittlement and a decrease in elongation occur due to solute N, so the upper limit is made 0.0060% or less. The upper limit with preferable N amount is 0.0050% or less, More preferably, it is 0.0040% or less.

(One or two or more of Ca, Mg, and REM are added in a total amount of 0.0005 to 0.0080%)
Ca, Mg, and REM are all important elements that suppress the formation of coarse inclusions (such as stretched MnS) by forming sulfides. Since these elements have the same effect, the individual addition amount is not limited, but the total addition amount needs to be 0.0005 to 0.0080%. If the total amount added is less than 0.0005%, the effect of improving elongation cannot be obtained stably. On the other hand, even if it exceeds 0.0080% and it adds excessively, an effect will be saturated and a coarse acid and sulfide will be formed and toughness and elongation will be degraded. Therefore, the total addition amount is set to 0.0005 to 0.0080%, preferably 0.0010 to 0.0060%, and more preferably 0.0015 to 0.0040%.

(O: 0.0005 to 0.0060%)
O forms an oxide together with Mg, Ca, and REM. If it exceeds 0.0060%, the oxide becomes coarse and elongation and toughness decrease, so the content is made 0.0060% or less. On the other hand, the smaller the amount of O, the better. However, in order to reduce O, for example, the recirculation work in the RH vacuum degassing apparatus takes a long time and is not practical.

(Ti / N is 0.5 to 4.0)
The reason why Ti / N is set to 0.5 to 4.0 is to fix Ti with N and suppress the generation of TiC that causes deterioration of elongation. Then, the amount of N is increased, so that solid solution N is generated and the elongation is deteriorated, and further, the surface flaws of the slab are generated. On the other hand, if it exceeds 4.0, TiC is generated and the elongation is deteriorated. Therefore, Ti / N is set to 0.5 to 4.0.

  Furthermore, in order to ensure strength, Nb: 0 to 0.030%, V: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 1.00%, Cr: You may add 1 type (s) or 2 or more types in the group of 0-0.50%, Mo: 0-0.50%, B: 0-0.030%.

  Nb contributes to the refinement of the structure by adding a small amount and is an element effective for securing the strength of the base material, so 0.030% or less is added. Addition of more than 0.030% Nb hardens the weld and significantly deteriorates toughness, so 0.030% is made the upper limit. In order to obtain the effect of Nb addition, it is preferable to add 0.003% or more, but even if it is less than that, the effect of the present invention is not inhibited.

  V contributes to an increase in strength by precipitation strengthening, so 0.050% or less is added. Addition of more than 0.050% V may impair joint toughness, so 0.050% is made the upper limit. In order to obtain the effect of V addition, it is preferable to add 0.010% or more, but even if it is less than that, the effect of the present invention is not inhibited.

  Cu, Cr, and Mo all improve the hardenability and are effective for increasing the strength. However, if added excessively, the hardness of the joint may increase and the toughness may decrease. It is preferable to add 50% or less, Cr 0.50% or less, and Mo 0.50% or less. In order to obtain the effect, it is preferable to add 0.05% or more of Cu, 0.05% or more of Cr, and 0.01% or more of Mo. Does not interfere.

  Ni is effective in securing strength and improving toughness, but if added over 1.00%, the cost increases, so the upper limit is made 1.00%. In order to obtain the effect of adding Ni, it is preferable to add 0.05% or more, but even if less than that, the effect of the present invention is not inhibited.

B is added in an amount of 0.003% or less because it can improve the hardenability and improve the strength of the base material by adding a small amount. If added over 0.003%, the elongation and joint toughness deteriorate. In order to obtain the effect of addition of B, it is preferable to add 0.0003% or more, but even if it is less than that, the effect of the present invention is not inhibited. The lower limit of these selective elements may be 0%.
The balance of the components described above is Fe and inevitable impurities.

  Next, the reasons for limiting the microstructure of the steel sheet of the present invention will be described.

(The ferrite phase area fraction of 1/4 thick part is 80 to 95%, the pearlite phase area fraction of 1/4 thick part is 5 to 20%)
As the ferrite (α) phase area fraction increases, the uniform elongation property improves, and when the α phase space factor is 80% or more, the elongation property is drastically improved. Although the structure changes somewhat in the plate thickness direction, the ferrite phase area fraction of the 1/4 thick portion needs to be 80% or more in order to ensure sufficient elongation. On the other hand, if it exceeds 95%, the strength cannot be secured, so the ferrite phase area fraction of the 1/4 thick part was set to 80 to 95%. This plate thickness ¼ thickness portion is a region where the cooling rate is relatively faster than that of the plate thickness center portion during cooling, a hard phase is easily generated, and uniform elongation is likely to deteriorate. When considering the entire plate thickness, it is necessary to consider the characteristic difference from the central portion of the plate thickness, so the ferrite phase area fraction of the 1/4 thick portion was limited to 80-95%, but 85-90% preferable.

  Moreover, the yield point YP and the tensile strength TS, which are strength properties, are incompatible with the elongation properties EL, and it is generally considered difficult to improve both at the same time, and increase the ferrite phase area fraction. The elongation characteristics are improved by the above, but if the elongation is improved, the tensile strength is lowered. Therefore, there is a limit to securing the strength characteristics only by increasing the ferrite phase area fraction.

  Therefore, in the present invention, the pearlite (P) phase area fraction of the ¼ thick part is set to 5% or more in order to ensure the yield point YP and the tensile strength TS, which are the strength characteristics, while ensuring the elongation characteristics. However, if it exceeds 20%, the elongation cannot be secured, so the upper limit was made 20%. Preferably, it is 10 to 15%.

  Note that the total of the ferrite phase area fraction and the pearlite phase area fraction of the 1/4 thick part is preferably 90% or more, and even if less than 10% bainite is present, the effect of the present invention is hindered. It is not a thing. Further, the ferrite phase area fraction and the pearlite phase area fraction are obtained by photographing the microstructure at a magnification of 500 times with an optical microscope and obtaining the area fraction of each phase by image analysis.

(The average aspect ratio of the 1/4 thick ferrite grains is 1.0 to 1.5)
The average aspect ratio of the 1/4 thick part ferrite grains is preferably as small as possible, and when it exceeds 1.5, the dislocation density is high and the elongation deteriorates, so the upper limit was set to 1.5. Moreover, the lower limit was set to 1.0 at which the ferrite grains became spherical.

(The average grain size of the 1/4 thick ferrite grains is 5 to 20 μm)
Since the strength cannot be secured when the average grain size of the 1/4 thick ferrite grains exceeds 20 μm, the upper limit is set to 20 μm. Also, the finer the ferrite grains, the better. However, since it is difficult to realize industrially when the grain size is less than 5 μm, the lower limit is set to 5 μm.

(The average dislocation density in the ferrite phase of ¼ thick part is 7 × 10 ¹² / m ² or less)
In order to ensure elongation, the average dislocation density in the ferrite (α) phase needs to be 7 × 10 ¹² / m ² or less. When the dislocation density exceeds 7 × 10 ¹² / m ² , the dislocations proliferate remarkably due to plastic deformation of the steel sheet, the ferrite (α) becomes hard, and sufficient total elongation (T.EL%) cannot be obtained. The lower the dislocation density, the better. However, it is rarely less than 1 × 10 ¹² / m ² . A preferable upper limit of the average dislocation density is 6 × 10 ¹² / m ² .

(In the 1 mm pitch Vickers hardness test, the average value of Vickers hardness from the surface of the steel sheet to 1/4 thickness or from 3/4 thickness to the back is 1/4 to 3/4 thick. Up to 80 to 105% of the average Vickers hardness)
When cooling a thick steel plate, the front and back layer portions of the plate thickness have a relatively faster cooling rate than the central portion of the plate thickness and are easily hardened. If the hardness in the vicinity of the surface layer portion is too large, the elongation is deteriorated. Considering the elongation characteristics of the entire plate thickness, the effect of the hardening of the plate thickness front and back layers can be tolerated to some extent, but if the hardness difference between the plate thickness front and back layers and the plate thickness center is large, the effect cannot be ignored. Come. Therefore, in the Vickers hardness test of 1 mm pitch, the Vickers hardness (Hv) average value of the plate thickness front and back layer parts (from the steel sheet surface to 1/4 thickness part, or from 3/4 thickness part to the back surface), It is necessary to set it to 80 to 105% of the average value of the Vickers hardness (Hv) at the center portion of the plate thickness (from 1/4 to 3/4 thickness). In order to ensure the elongation, it is better to suppress the hardness of the front and back layer portions of the plate thickness, and 80% of the average value of the Vickers hardness at the center portion of the plate thickness is an industrially lower limit. On the other hand, if it exceeds 105%, it is difficult to ensure elongation. Therefore, (average thickness of Vickers hardness at the front and back layer portions) / (average Vickers hardness value at the center of the thickness) was set to 80 to 105%.

(1/2 thickness in the thickness direction of the plate thickness ± (plate thickness) 10% range, 10 μm / mm ² or less inclusions with a length of 5 μm or more)
Coarse inclusions {MnS, sulfides or oxides such as alumina (aluminum oxide Al ₂ O ₃ ), etc., having a length of 5 μm or more become the starting point of ductile fracture (voids) and may deteriorate local elongation. It is preferred that there be less inclusions with a maximum length of 5 μm or more in the range of ½ thickness ± (thickness) of 10% in the thickness direction, and although it is not particularly limited, its presence form from the viewpoint of deterioration of local elongation Is preferably 10 pieces / mm ² or less. Inclusions are measured by particle analysis using SEM.

(1/2 thickness in the thickness direction of the plate thickness ± (maximum concentration of P in the plate thickness) 10% range is 0.02 to 0.20%)
P is segregated at the center during continuous casting and forms an embrittlement region at the center of the plate thickness, causing cracks and deteriorating local elongation. Therefore, it is preferable that the maximum concentration of P is small. The upper limit of the maximum concentration of P is not particularly specified, but in order to ensure the elongation, it means the thickness center portion (1/2 thickness in the thickness direction of the thickness ± (thickness) 10% range). The maximum concentration of P is preferably 0.20% or less. Moreover, since it is practically difficult to make the maximum concentration of P less than 0.02%, 0.02% is set as the lower limit, and 0.02 to 0.20% is set as a preferable range.
The maximum concentration of P is within the range of ± (plate thickness) 10% of the center portion of the plate thickness where the center segregation is likely to occur. For example, if the plate thickness is 10 mm, the center portion of the plate thickness is 20% (± 10%). With respect to a 2 mm (± 1 mm) angle, an EPMA (Electron Probe MicroAnalyzer: Electron Probe Microanalyzer) is used to measure the measurement range of the above 2 mm angle at an acceleration voltage of 15 kV, a beam diameter of 20 μm, an irradiation time of 20 ms, and a measurement pitch of 20 μm. It is the maximum value of P concentration when measured.

(Ferrite transformation start temperature Ar ₃ when cooling is 760 to 820 ° C.)
The ferrite transformation start temperature Ar ₃ when cooling the steel is such that the higher the Ar ₃ as the steel composition, the higher the temperature, the more the ferrite transformation is performed, so that the dislocation density in the ferrite grains decreases and the elongation improves. Therefore, it is preferable that the Ar _{3 of} the steel is large, but if it exceeds 820 ° C. and is too large, the ferrite becomes coarse and the strength decreases, so the upper limit is preferably made 820 ° C. On the other hand, if Ar ₃ is too small, bainite is formed and elongation deteriorates, so it is preferable to set 760 ° C. as the lower limit.
Incidentally, the ferrite transformation start temperature Ar ₃ at the time of cooling is represented by the known formula (1).

_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%.

  Next, the reasons for limiting the manufacturing conditions in the present invention will be described.

First, as a pre-casting treatment, after performing primary refining that removes carbon from molten steel, when adjusting the components of the molten steel, the dissolved oxygen content of the molten steel is adjusted to 65 ppm or less, preferably 40 ppm or less by vacuum degassing treatment. In order to adjust the dissolved oxygen content of the molten steel to 40 ppm or less, for example, the degree of vacuum of the RH vacuum degassing apparatus is 1 to 5 torr, and the molten steel is adjusted to reflux for 1 to 3 minutes.
After the dissolved oxygen content of the molten steel becomes 40 ppm or less, Al is added to the molten steel so that the final content of Al is 0.002 to 0.050%. When the amount of dissolved oxygen in the molten steel exceeds 40 ppm, even if Al is added as a deoxidizing material and the reflux operation is performed with an RH vacuum degassing apparatus, the final dissolved oxygen amount in the molten steel is 16 ppm or less. This is because it cannot be adjusted to 10 ppm or less. Also, the smaller the amount of dissolved oxygen, the better, and there is no need to set the lower limit of the amount of dissolved oxygen in the molten steel.

Next, after adjusting the dissolved oxygen content of the molten steel to 10 ppm or less, the total final content of one or more of Ca, Mg, and REM is one or more of Ca, Mg, and REM is 0.0005. It is added so as to be ˜0.0080% and is preferentially sulfided to suppress MnS formation.
If the amount of dissolved oxygen exceeds 10 ppm, it may be oxidized when Ca, Mg, or REM is added, and sulfide control may not be sufficient. In order to adjust the dissolved oxygen amount of the molten steel to 10 ppm or less, for example, the degree of vacuum of the RH vacuum degassing apparatus is 1 to 5 torr, the molten steel is refluxed for 10 to 60 minutes, and the dissolved oxygen amount of the molten steel is adjusted to 10 ppm or less. To do. If the degree of vacuum is 1 to 5 torr and the molten steel is not refluxed for 10 to 60 minutes, the amount of dissolved oxygen cannot be reduced to 10 ppm or less. Also, the smaller the amount of dissolved oxygen, the better, and there is no need to set the lower limit of the amount of dissolved oxygen in the molten steel.

  When producing the slab by continuously casting the molten steel whose components are adjusted, in the range where the central solid phase ratio of the slab which is the final stage of solidification of the slab is 0.2 to 0.7%, the gap between the casting rolls, While narrowing down to 0.2 to 3.0 mm per 1 m in the casting direction, preferably 0.5 to 2.0 mm per 1 m in the casting direction, more preferably 0.7 to 1.5 mm per 1 m in the casting direction, Casting and discharging the concentrated molten steel such as P to the upstream side. Thereby, harmful center segregation can be reduced. It is known that the central solid fraction here can be defined as the solid fraction of the melted portion in the center of the slab thickness direction and in the width direction of the slab, and can be determined by heat transfer and solidification calculations. ing. In addition, although it is preferable to lightly reduce, in the case of a component with low P content, it is not necessary to lightly reduce.

  Next, the cast slab (steel slab) is hot-rolled.

  In hot rolling, the cast steel slab is heated at a low temperature in the range of 950 to 1100 ° C, preferably 1000 to 1050 ° C. When heated at a heating temperature of 1100 ° C. or lower, austenite (sometimes referred to as γ) grains can be refined, ferrite can be refined, and the γ → α transformation temperature can be increased to reduce the dislocation density. The upper limit was set to ° C. Further, if it is less than 950 ° C., γ conversion is insufficient and toughness deteriorates, so 950 ° C. was set as the lower limit. After roughly rolling the heated steel slab, finish rolling with a cumulative reduction ratio of 50 to 75% is performed. If the cumulative rolling reduction exceeds 50%, α nucleation sites in γ increase, and α can be refined and the γ → α transformation temperature can be increased. However, if it exceeds 75%, productivity deteriorates. The cumulative rolling reduction is 50 to 75%, preferably 55 to 65%.

Finish rolling is an important step for making α fine, and the surface temperature of the steel slab is the ferrite transformation start temperature Ar ₃ -30 ° C. or higher when cooling as shown by the following formula (1). The recrystallization is started at a recrystallization start temperature T _rex ° C. or less at which the crystal grain growth shown in (2) starts. If the temperature is lower than Ar ₃ -30 ° C., it becomes two-phase rolling, and stretched ferrite is formed and the elongation deteriorates. On the other hand, if it exceeds T _rex, it does not result in non-recrystallized zone rolling, and ferrite becomes coarse and deteriorates elongation.

T _rex is a temperature (recrystallization limit temperature) required to complete recrystallization in the time between passes of normal plate rolling (about 10 to 15 seconds). (2).
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%.

T _rex = −91900 [Nb *] ² +9400 [Nb *] + 770 (2)
Here, [Nb *] is obtained by the following equation (3).

Sol. Nb = 10 ^{(-6770 / (T + 273) +2.26))} / (C + ^12/14 × N) (3)
In addition, T in the formula (3) is the heating temperature of the steel slab, the unit is the Celsius temperature (° C.),
[Nb] ≧ [Sol. Nb], [Nb *] = [Sol. Nb],
[Nb] <[Sol. Nb], [Nb *] = [Nb],
And Here, [Nb] represents the Nb content (mass%) by [Sol. Nb] is Sol. Nb (solid solution Nb) (mass%) is represented.
Also, T _rex formula (2) is an empirical formula, and some Nb is not dissolved by heating at a low temperature, so the amount of solid solution Nb (sol.Nb amount) is the relationship between the solid solution Nb and the recrystallization temperature. This is the formula obtained from

As a cooling step after finish rolling, air-cooling of the finish-rolled thick steel plate at a cooling rate of 1 ° C./second or less, or a cooling rate to a surface temperature of the steel plate of Ar ₃ −150 ° C. or higher and Ar ₃ −50 ° C. or lower. Air-cooled after water-cooling at a temperature of more than 1 ° C / second and not more than 20 ° C / second. The air cooling end temperature is room temperature.

  Air cooling at a cooling rate of 1 ° C./second or less increases the ferrite transformation temperature because the cooling rate is low, so that the dislocation density in the ferrite grains is lowered and the elongation can be improved. There is no need to specifically limit the lower limit of the cooling rate of air cooling.

After completion of rolling, air cooling may be performed, but in order to increase the strength, the surface temperature of the steel sheet is reduced to a temperature of Ar ₃ −150 ° C. or higher and Ar ₃ −50 ° C. or lower with a cooling rate of 1 ° C./second or more and 20 ° C./second or less. Air cooling may be performed after water cooling. If the cooling stop temperature is less than Ar ₃ -150 ° C., the transformation temperature becomes low, dislocation density rises in the ferrite grains and bainite formation occurs, and the elongation deteriorates. On the other hand, if Ar ₃ exceeds -50 ° C., the effect cannot be obtained. If the cooling rate of water cooling exceeds 20 ° C./second, the transformation temperature becomes low and elongation deteriorates, so the upper limit of the cooling rate of water cooling was set to 20 ° C./second. Since water cooling is effective as long as it is equal to or higher than the cooling rate of air cooling, the lower limit of the cooling rate of water cooling is set to exceed 1 ° C./second.

  Examples of the present invention will be described below with reference to Tables 1 to 3.

  A steel plate having a thickness of 6 to 40 mm was made on a trial basis using the steel pieces having chemical components shown in Table 1 under the manufacturing conditions shown in Table 2. In addition, the amount of dissolved oxygen before Ca, Mg, and REM in Table 2 means before adding one or more of Ca, Mg, and REM. The cooling rate (° C./s) in the column of cooling conditions is a cooling rate at a ½ thickness portion obtained from a measured surface temperature by a heat conduction analysis by a known differential method. “Air cooling” described in the cooling pattern column of Table 2 is an example in which air cooling is performed without performing water cooling (accelerated cooling), and “partial water cooling” is air cooling after performing partial water cooling after rolling. This is an example.

  The structural characteristics of each manufactured steel sheet shown in Table 3 were measured as follows.

  First, as for the microstructure of the steel sheet, a sample was taken so that a vertical cross section in the width direction of the steel sheet could be observed, and the optical structure was 1 mm from the surface, the thickness 1/4, and the metal structure at the center of the thickness at a magnification of 500 times. I took a picture. Next, after binarization processing was performed under appropriate conditions using image analysis software, the total area of α and the second phase (mainly pearlite but partly including bainite) was obtained, and the total area of the imaging unit By dividing, the fraction of each phase (area fraction%) was determined.

  From the surface of the steel plate (plate thickness t) to 1/4 thickness (front to t / 4), or from 3/4 thickness to back (back to 3t / 4), and from 1/4 thickness to 3 / Each Vickers hardness average value of up to 4 thickness parts (central part of t / 4-3t / 4 thickness part) is 1-20 pitch Vickers hardness test, measured 1-20 points under the condition of load 98N, The average value was obtained.

  The average dislocation density in the α phase was obtained by taking a thin film sample from each position of the plate thickness, performing bright-field observation photography using a transmission electron microscope (TEM) at a magnification of 40,000, and obtaining from the obtained TEM image. The number of intersections (N) between an arbitrary straight line (length: L) and a dislocation line is measured, and the average dislocation density (ρ) is calculated by the following equation (4) using the value of film thickness: t. Calculated.

    ρ = 2N / Lt (4)

  Table 3 shows the measurement results of the structural characteristics and mechanical properties {yield stress (YP), tensile strength (TS), total elongation (T.EL)}.

  The mechanical properties were evaluated for tensile strength (TS) using a JIS Z 2241 (2011) No. 1B tensile specimen taken in the direction perpendicular to the rolling direction from the center of the plate thickness. Yield stress (YP) means the yield strength of the permanent elongation method when the permanent elongation of JIS Z2241 (2011) is 0.2%, and the total elongation (T.EL) is the total elongation At of the rupture of JIS Z2241 (2011). This means that JIS1B was used as the test piece.

No. of the example of the present invention. Nos. 1 to 20 have chemical components, microstructures, and production conditions within the scope of the present invention, so that all have a total elongation (T.EL) of 23 to 40%, a yield strength (YP) of 355 to 500 MPa, and a tensile strength (TS). 490 to 620 MPa could be secured. In addition, No. No. 16 has an excessive amount of dissolved oxygen, and the amount of light reduction during continuous casting was less than 0.2 mm. Therefore, the maximum concentration of P in the center is high, and the ferrite transformation start temperature Ar ₃ is 820 ° C. This is an example that satisfies the requirement of claim 1 when using a steel with a high component exceeding 825 ° C. No. 17 is an example that satisfies the requirements of claims 1 and 2 in the case of using steel having a high component of ferrite transformation start temperature Ar ₃ of 822 ° C. without performing light reduction during continuous casting. No. No. 18 is an example that satisfies the requirements of claims 1 and 3 in the case where a steel having a high dissolved oxygen content and a ferrite transformation start temperature Ar ₃ as high as 826 ° C. is used. No. 19 is an example satisfying claims 1 and 4 in which the maximum concentration of P in the central portion is high because the degree of vacuum and the reflux time are insufficient, and the light reduction amount during continuous casting is less than 0.2 mm. It is. All of these examples could secure the microstructure and mechanical properties of the present invention.

  On the other hand, no. Nos. 21 to 36 were out of the scope of the present invention because any one of chemical components, microstructures (ferrite phase, etc.) and production conditions deviated from the scope of the present invention. ) Was not obtained.

  That is, no. Since the heating temperature of Nos. 21 and 22 was too high, γ could not be refined by finish rolling, the ferrite phase did not satisfy the requirements of the present invention, and the total elongation (T.EL) was low. No. In No. 23, the cumulative rolling reduction of finish rolling is insufficient, the ferrite phase does not satisfy the requirements of the present invention, and the pearlite fraction is low and does not satisfy the requirements of the present invention. Therefore, yield strength (YP), tensile strength (TS ), The total elongation (T.EL) was low. No. In 24 and 25, the start and finish temperatures of finish rolling were too high, so the ferrite phase did not satisfy the requirements of the present invention and the total elongation (T.EL) was low. No. No. 26, the cooling stop temperature due to water cooling is too low, and the ferrite phase does not satisfy the requirements of the present invention, and the value of front / back surface average Hv / center average Hv × 100% is high, and the total elongation (T.EL) is high. It was low.

No. 27-28, the cooling rate during water cooling is partly too high, the ferrite phase does not satisfy the requirements of the present invention, the front / back surface average Hv / center average Hv × 100% is high, and the total elongation ( T.EL) was low.
No. In No. 29, the start and finish temperatures of finish rolling were outside the range of the present invention, the ferrite phase did not satisfy the requirements of the present invention, and the total elongation (T.EL) was low.
No. No. 30 had a low tensile strength (TS) and total elongation (T.EL) because the amount of Si was insufficient.
No. No. 31, since the amount of S was excessive, there were many stretched inclusions having a maximum length of 5 μm or more in the range of 1/2 thickness ± (plate thickness) 10% (center portion) by particle analysis by SEM, Elongation (T.EL) was decreasing. No. No. 32 had an excessive amount of P, so the maximum concentration of P was high and the total elongation (T.EL) was low in the range of 1/2 thickness ± (thickness) 10% (center). No. No. 33 had an excessive amount of Nb, resulting in a decrease in total elongation (T.EL).

No. No. 34 had a large amount of stretched inclusions having a maximum length of 5 μm or more because the amount of Ca + Mg + REM was insufficient, and the elongation (T.EL) was low. No. No. 35 has a high Ti / N value and TiC is generated, and there are many coarse inclusions having a maximum length of 5 μm or more in the range of 1/2 thickness ± (thickness) of 10% (center portion). The total elongation (T.EL) was low.
No. In No. 36, the content of Si, P, S, and Nb and the value of Ti / N were outside the scope of the present invention, so the ferrite phase defined in the present invention was not obtained, and the total elongation (T.EL) was low. .

Claims (8)

% By mass
C: 0.050-0.200%
Si: 0.200 to 1.000%
Mn: 0.50 to 2.00%,
P: 0.015% or less,
S: 0.003% or less,
Ti: 0.003-0.020%,
Al: 0.002 to 0.050%,
N: 0.0010 to 0.0060%,
O: 0.0005 to 0.0060%,
One or two or more of Ca, Mg, and REM as a total addition amount of 0.0005 to 0.0080%
In addition,
Nb: 0 to 0.030%,
V: 0 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to 1.00%,
Cr: 0 to 0.50%,
Mo: 0 to 0.500%,
B: 0 to 0.0030%
Containing, and
Ti / N is 0.5-4.0,
The balance is a steel composed of Fe and inevitable impurities,
The microstructure is composed of a ferrite phase area fraction of 1/4 thick part with 80-95%, a pearlite phase area fraction of 1/4 thick part with 5-20%, and the 1/4 thick part ferrite phase and pearlite. phase other tissues in bainite phase, an organization that area fraction of the bainite phase Ru consists of 10% or less,
The average aspect ratio of the ferrite grain of 1/4 thickness part is 1.0 to 1.5,
The average particle diameter of the ferrite grain of 1/4 thickness part is 5 to 20 μm,
The average dislocation density in the ferrite phase of ¼ thick part is 7 × 10 ¹² / m ² or less,
In a 1 mm pitch Vickers hardness test, the average value of Vickers hardness from the surface of the steel sheet to 1/4 thickness or from 3/4 thickness to the back is from 1/4 thickness to 3/4 thickness. 80-105% of the average Vickers hardness of
A high-strength and highly ductile steel plate characterized by The high-strength and highly ductile thick steel plate according to claim 1, wherein the number of inclusions having a length of not less than 10 μm / mm ² within a range of ½ thickness ± (thickness) of 10% in the thickness direction of the plate thickness is 5 μm or more, A high-strength and highly ductile steel plate characterized by   3. The high strength and high ductility thick steel plate according to claim 1 or 2, wherein the maximum concentration of P is 0.02 to 0.00 in the range of 1/2 thickness ± (thickness) 10% in the thickness direction of the thickness. A high-strength, high-ductile, thick steel plate characterized by being 20%. In the high strength and high ductility thick steel sheet according to any one of claims 1 to claim 3, a ferrite transformation start temperature Ar ₃ is seven hundred sixty to eight hundred twenty ° C., at the time of cooling is represented by the following formula (1) A high-strength, highly ductile steel plate characterized by
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%. It is a method of manufacturing the high strength and high ductility thick steel plate according to any one of claims 1 to 4, wherein the steel slab is heated in the range of 950 to 1100 ° C, and the cumulative reduction ratio is 50 to 75. % Finish rolling is performed at a steel slab surface temperature of Ar ₃ −30 ° C. or higher and a recrystallization start temperature T _rex ° C or lower at which crystal grain growth starts, and the finished rolled steel plate is cooled to room temperature by air cooling. The manufacturing method of the high intensity | strength highly ductile thick steel plate characterized by the above-mentioned.
However,
Ar ₃ represents the following formula (1),
T _rex is represented by the following formula (2).
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, an element symbol represents content (mass%) of each element. In addition, the element which is not contained shall be 0%.
T _rex = −91900 [Nb *] ² +9400 [Nb *] + 770 (2)
However, [Nb *] is obtained by the following equation (3).
Sol. Nb = (10 ^{(-6770 / (T + 273) +2.26)} ) / (C + ^12/14 × N) (3)
In addition, T in the formula (3) is the heating temperature of the steel slab, the unit is the Celsius temperature (° C.),
[Nb] ≧ [Sol. Nb]
[Nb *] = [Sol. Nb]
[Nb] <[Sol. Nb]
[Nb *] = [Nb]
And Here, [Nb] represents the Nb content (mass%) by [Sol. Nb] is Sol. Nb (solid solution Nb) (mass%) is represented. A method of producing a high strength and high ductility thick steel sheet according to claim 5, the finish rolled thick steel plate, the surface temperature of the steel sheet Ar ₃ -150 ° C. or more, up to Ar ₃ -50 ° C. temperature below A method for producing a high-strength, high-ductility, thick steel sheet, characterized in that water cooling is performed at a cooling rate of more than 1 ° C / second and not more than 20 ° C / second, and then air cooling is performed at a cooling rate of 1 ° C / second or less .   It is a manufacturing method of the high intensity | strength highly ductile thick steel plate of Claim 5 or Claim 6, Comprising: When manufacturing molten steel, the amount of dissolved oxygen of molten steel is adjusted to 40 ppm or less with a vacuum degassing apparatus, Then, Al Is added so that the final content of Al is 0.002 to 0.050%, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less, then one or more of Ca, Mg, and REM are added, A method for producing a high-strength and high-ductility thick steel plate, characterized in that the total content of one or more of Ca, Mg, and REM is added to 0.0005 to 0.0080%.   A method for producing a high-strength, high-ductile, thick steel plate according to any one of claims 5 to 7, wherein when the molten steel is continuously cast, the central solid phase of the slab, which is the final stage of solidification of the slab High strength and high ductility thickness, characterized by narrowing the gap between the casting rolls to 0.2 mm to 3.0 mm per 1 m in the casting traveling direction and casting while reducing the ratio in the range of 0.2 to 0.7 Manufacturing method of sheet steel.