JP6252291B2 - Steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a large thickness and excellent arrestability, and a method for producing the same.

  For steel plates used in structures such as shipbuilding, architecture, tanks, offshore structures, line pipes, arrestability (the ability to suppress the propagation of brittle fracture in order to suppress brittle fracture of structures) Brittle fracture propagation stopping performance) is required. In recent years, with the increase in size of structures, there are increasing cases of using high-strength thick steel plates having a yield stress of 390 MPa to 500 MPa and a plate thickness of 40 mm to 100 mm. However, the above-described arrestability generally tends to conflict with strength and thickness. For this reason, the technique which improves arrestability in a high intensity | strength thick steel plate is desired.

Patent Document 1 discloses that the ferrite grains are recrystallized by cooling in the middle of rolling and the subsequent reheating process, and the temperature is raised to the Ac3 transformation point or higher, using α → γ reverse transformation, and near the surface layer portion of the steel sheet. A technique for refining crystal grains and improving arrestability is disclosed. However, such a technique has a problem in that productivity is lowered because recuperation beyond the Ac3 transformation point is an essential process.
In Patent Document 2, the steel sheet surface is water-cooled during rolling, and after ferrite transformation is performed by setting it to the Ar1 transformation point or less, the ferrite structure of the surface layer part is heated using sensible heat at the center of the sheet thickness. Further, a technique is disclosed in which rolling is further performed to refine only the surface layer portion and improve arrestability. However, in this technique, since only the surface layer portion is made fine, it is difficult to obtain sufficient arrestability when the plate thickness is thick.
Therefore, Patent Document 3 has a microstructure with bainite as a parent phase by setting the Ni addition amount to an appropriate value, the average diameter of crystal grains in the surface layer portion is 15 μm or less, A high-strength thick steel plate excellent in arrestability is disclosed in which the average diameter of crystal grains is 40 μm or less.

JP 61-235534 A JP-A-5-148542 JP 2007-302993 A

  The steel sheet of Patent Document 3 is required to add 0.15% or more of Ni, which is an expensive alloy element, so that it is difficult to produce a steel sheet with excellent strength and arrestability at low cost. is there.

  The present invention has been made in consideration of the above circumstances, and its purpose is to provide a thick high-strength steel sheet excellent in toughness and arrestability, which can be produced industrially and stably, and a method for producing the same. It is to provide.

According to the present invention, by mass, C: 0.05 to 0.10%, Si: 0.01 to 0.30%, Mn: 1.40 to 2.00%, Mo: 0.01 to 0 20%, Nb: 0.005-0.030%, Ti: 0.005-0.030%, B: 0.0005-0.0030%, N: 0.0005-0.0050%, Al: 0.002 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to less than 0.15%, Cr: 0 to 0.50%, V: 0 to 0.10%, Ca: 0 to 0 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, P: 0.015% or less, S: 0.010% or less, balance: iron and impurities. -1 or Bsol defined by the following formula 1-2 is 0.0005% or more, and Ar3 defined by the following two formulas is 690. A boundary containing 40% or more and less than 70% bainite and 30% or more and 60% or less of ferrite with an area ratio of not less than 760 ° C. and defining a boundary having a crystal orientation difference of 15 ° or more as a crystal grain boundary, When the region surrounded by the grain boundary is defined as a crystal grain, the average grain diameter of the crystal grain at a position 5 mm deep from the surface is 10 μm or less, and the average grain diameter of the crystal grain at a quarter position of the plate thickness JIS Z2242 is a test piece sampled at ¼ position of the plate thickness, the average grain size of the crystal grains in the central portion of the plate thickness is 30 μm or less, the yield strength is 460 to 580 MPa. 2005: A steel sheet having a fracture surface transition temperature specified in Annex D of −80 ° C. or lower, a specimen taken on the surface, and an NDT temperature specified in ASTM E208-06 of −85 ° C. or lower. Offer Provided.
Bsol = [B] (when [Ti] / [N] is 3.4 or more) Formula 1-1
Bsol = [B] +0.226 [Ti] −0.722 [N] (when [Ti] / [N] is less than 3.4) Formula 1-2
Ar3 = 940−310 × [C] + 40 × [Si] −90 × [Mn] −40 × [Cu] −60 × [Ni] −15 × [Cr] −40000 × [Mo] × Bsol Formula 2
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [B], [Ti] and [N] are C, Si, Mn, respectively. , Cu, Ni, Cr, Mo, B, Ti, and the content expressed by mass%.

The steel plate of the present invention has a thickness of 40 to 100 mm, for example. Moreover, tensile strength is 570-720 MPa, for example. Moreover, Pcm defined by the following formula 3 is, for example, 0.22% or less.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 3
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu and Ni, respectively. , Cr, Mo, V and B mean the content expressed in mass%.

Moreover, according to this invention, it is the method of manufacturing the said steel plate, Comprising: By mass%, C: 0.05-0.10%, Si: 0.01-0.30%, Mn: 1.40-2 0.00%, Mo: 0.01-0.20%, Nb: 0.005-0.030%, Ti: 0.005-0.030%, B: 0.0005-0.0030%, N: 0.0005 to 0.0050%, Al: 0.002 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to less than 0.15%, Cr: 0 to 0.50%, V: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, P: 0.015% or less, S: 0.010% or less, The balance: iron and impurities, and Bsol defined by the following formula 1-1 or the following formula 1-2 is 0.0005% or more. A steel slab in which Ar3 defined by the formula is 690 ° C. or higher and 760 ° C. or lower is heated to 950 to 1100 ° C. to perform hot rolling, the reduction rate in finish rolling of the hot rolling is set to 50% or more, The start temperature of the finish rolling is Ar3-30 ° C to Ar3 + 50 ° C, the start temperature of the finish rolling in the central portion of the sheet thickness is Ar3 ° C to 850 ° C, and cooling is started at a surface temperature of Ar3-30 ° C or higher, A method for producing a steel sheet is provided in which the cooling rate at the central portion of the plate thickness is 3 ° C./s or higher, and cooling is terminated at a surface temperature of 400 ° C. or lower.
Bsol = [B] (when [Ti] / [N] is 3.4 or more) Formula 1-1
Bsol = [B] +0.226 [Ti] −0.722 [N] (when [Ti] / [N] is less than 3.4) Formula 1-2
Ar3 = 940−310 × [C] + 40 × [Si] −90 × [Mn] −40 × [Cu] −60 × [Ni] −15 × [Cr] −40000 × [Mo] × Bsol Formula 2
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [B], [Ti] and [N] are C, Si, Mn, respectively. , Cu, Ni, Cr, Mo, B, Ti, and the content expressed by mass%.

  The manufacturing method of the present invention may be tempered at 500 to 600 ° C. after cooling.

  According to the present invention, it is possible to obtain a steel plate having a large plate thickness, high strength, and excellent arrestability. According to the present invention, it is possible to reduce the cost and improve the safety of a welded steel structure.

It is explanatory drawing of the state by which B carbide | carbonized_material is produced | generated by a grain boundary. It is a photograph which shows the microstructure of a steel plate surface layer part when the start temperature of finish rolling is less than Ar3-30 degreeC (a), and the start temperature of finish rolling is Ar3-30 degreeC or more (b).

  Hereinafter, embodiments of the present invention will be described. The steel plate of the present invention is intended for a high-strength steel plate having a large plate thickness (for example, 40 to 100 mm) used for structures such as shipbuilding, architecture, tanks, offshore structures, and line pipes.

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

(Essential ingredients)
C: 0.05-0.10%
C is required to be 0.05% or more in order to ensure the strength and toughness of the steel sheet by improving hardenability, and 0.05% is the lower limit. On the other hand, if C exceeds 0.10%, weldability and joint toughness (HAZ toughness) are lowered, so 0.10% is the upper limit. Preferably, C: 0.06 to 0.09%.

Si: 0.01-0.30%
Since Si is effective as a deoxidizing element and strengthening element, it needs to be 0.01% or more. However, if it exceeds 0.30%, joint toughness decreases, and Ar3 increases, so 0.30% It is an upper limit. Preferably, Si: 0.01 to 0.10%.

Mn: 1.40 to 2.00%
Mn is required to be 1.40% or more in order to secure the strength of the steel sheet and to lower Ar3. However, if Mn is added in excess of 2.0%, weldability and joint toughness decrease, so 2.0% is the upper limit. Preferably, Mn is 1.50 to 1.80%.

Mo: 0.01-0.20%
Mo improves the hardenability by the combined effect with B, increases the strength of the steel sheet, and decreases the Ar3 content to 0.01% or more. On the other hand, if it exceeds 0.20%, the toughness and joint toughness of the steel sheet are lowered, so the content is made 0.20% or less. Preferably, Mo: 0.03 to 0.10%.

Nb: 0.005 to 0.030%
Nb suppresses the recrystallization temperature, contributes to the refinement of the structure, and increases the strength of the steel sheet, so is 0.005% or more. On the other hand, if it exceeds 0.030%, weldability deteriorates, so 0.030% is made the upper limit. Preferably, Nb is 0.008 to 0.015%.

Ti: 0.005-0.030%
Ti forms TiN and needs to be 0.005% or more in order to improve the toughness and joint toughness of the steel sheet by finely dispersing TiN. On the other hand, if it exceeds 0.030%, the toughness and joint toughness of the steel sheet are lowered, so 0.030% is made the upper limit. Preferably, Ti is 0.008 to 0.015%.

B: 0.0005 to 0.0030%
B increases the strength of the steel sheet by improving the hardenability, so is 0.0005% or more. On the other hand, if it exceeds 0.0030%, toughness is deteriorated and weldability is lowered, so 0.0030% is made the upper limit. Preferably, B: 0.0010 to 0.0020%.

N: 0.0005 to 0.0050%
N makes 0.0005% the lower limit in order to improve the toughness and joint toughness of the steel sheet by forming TiN in the steel material. On the other hand, in order to suppress slab wrinkles, the upper limit is set to 0.0050%. Preferably, N is 0.0020 to 0.0040%.

Al: 0.002 to 0.050%
Al is necessary for deoxidizing and reducing O which is an impurity element. Moreover, the free N in steel is made harmless with AlN. Therefore, it is made 0.002% or more. On the other hand, if Al exceeds 0.050%, joint toughness decreases, so 0.050% is the upper limit. Preferably, it is Al: 0.010-0.040%.

(Optional component)
In addition to the above essential components, the steel sheet of the present invention may further contain the following optional components.

Cu: 0 to 0.50%
Cu has the effect of improving strength and reducing Ar3. In order to acquire the effect, it is desirable to contain Cu 0.1% or more. On the other hand, if it exceeds 0.50%, weldability and joint toughness deteriorate, so 0.50% is made the upper limit. A preferable upper limit is 0.40%.

Ni: 0 to less than 0.15% Ni has the effect of improving strength and reducing Ar3. In order to acquire the effect, it is desirable to contain Ni 0.05% or more. On the other hand, since Ni is expensive and excessive addition increases the cost, it is made less than 0.15%. A preferable upper limit is 0.10%.

Cr: 0 to 0.50%
Cr has the effect of increasing the strength of the steel sheet. In order to acquire the effect, it is desirable to contain Cr 0.1% or more. On the other hand, if it exceeds 0.50%, weldability and joint toughness deteriorate, so 0.50% is made the upper limit. A preferable upper limit is 0.20%.

V: 0 to 0.10%
V has an effect of increasing strength by precipitation strengthening. In order to acquire the effect, it is desirable to contain V 0.02% or more. On the other hand, if it exceeds 0.10%, joint toughness decreases, so 0.10% is made the upper limit. A preferable upper limit is 0.05%.

Ca: 0 to 0.0050%
Ca is preferably contained in an amount of 0.0003% or more in order to improve joint toughness. On the other hand, if it exceeds 0.0050%, the joint toughness decreases, so 0.0050% is made the upper limit. A preferable upper limit is 0.0030%.

Mg: 0 to 0.0050%
Mg is desirably contained in an amount of 0.0003% or more in order to improve joint toughness. On the other hand, if it exceeds 0.0050%, the joint toughness decreases, so 0.0050% is made the upper limit. A preferable upper limit is 0.0030%.

REM: 0 to 0.0050%,
In order to improve REM (rare earth element) joint toughness, it is desirable to contain 0.0003% or more. On the other hand, if it exceeds 0.0050%, the joint toughness decreases, so 0.0050% is made the upper limit. A preferable upper limit is 0.0030%.

(impurities)
The steel sheet of the present invention contains the above essential components and necessary optional components, and the balance is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the production process. However, it is allowed to contain other components as long as the properties of the steel material of the present invention are not impaired. For example, although P and S are contained as impurities in steel, they are preferably limited as follows.

P: 0.015% or less P is an impurity element and needs to be reduced to 0.015% or less in order to reduce weldability and joint toughness. Preferably it is 0.010% or less.

S: 0.010% or less S is an impurity element, and it causes a decrease in toughness and weldability due to MnS generation, so it is necessary to reduce it to 0.010% or less. Preferably it is 0.005% or less.

(Bsol)
Moreover, in the steel plate of this invention, Bsol defined by the following formula 1-1 or the following formula 1-2 is 0.0005% or more. By setting the free B in the steel indicated by Bsol within this range, B segregates at the grain boundaries and the hardenability is improved, and the bainite fraction is increased and the strength and toughness are improved. On the other hand, if it exceeds 0.0030%, the effect of improving hardenability is saturated, so 0.0030% or less is preferable.
Bsol = [B] (when [Ti] / [N] is 3.4 or more) Formula 1-1
Bsol = [B] +0.226 [Ti] −0.722 [N] (when [Ti] / [N] is less than 3.4) Formula 1-2
Here, [B] and [Ti] are contents represented by mass% of B and Ti, respectively.

In the present invention, since ausformed bainite is used, cold rolling is performed as much as possible. Therefore, Ar3 defined by the following two formulas is set to 690 ° C. or higher and 760 ° C. or lower.
Ar3 = 940−310 × [C] + 40 × [Si] −90 × [Mn] −40 × [Cu] −60 × [Ni] −15 × [Cr] −40000 × [Mo] × Bsol Formula 2
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the mass% of C, Si, Mn, Cu, Ni, Cr and Mo, respectively. Means the expressed content.

  When Ar3 is less than 690 ° C., the hardenability becomes excessive, the strength is exceeded, and the toughness is deteriorated. In particular, the toughness at the center of the steel sheet tends to deteriorate. On the other hand, when it exceeds 760 ° C., the surface layer portion of the steel sheet becomes two-phase region rolling at the time of rolling at a low temperature, and coarse processed ferrite is generated, and the arrestability deteriorates.

(Pcm)
The alloying elements contained in the steel generate a hard structure due to the heat effect during welding, and the ductility of the heat-affected zone is reduced and low temperature cracking is likely to occur. Pcm is a value indicating weld cracking sensitivity. In the steel sheet of the present invention, Pcm defined by the following formula 1 is 0.22% or less. If Pcm is 0.22% or less, it is preferable because preheating can be omitted (preheating free) when welding. On the other hand, even if it is less than 0.18%, further improvement such as relaxation of preheating conditions cannot be expected, so 0.18% or more is preferable.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 1
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu and Ni, respectively. , Cr, Mo, V, and B content in mass%.

  It is known that when Mo and B are added in combination, an excellent strength improvement effect can be obtained. B segregates as a solid solution B at the austenite grain boundary, thereby suppressing the formation of ferrite and improving the hardenability. However, as shown in FIG. The effect is reduced. When Mo is added in combination, the formation of B carbide can be suppressed, and the effect of improving hardenability by B is further enhanced. In order to obtain the effect of improving hardenability by B, 5 ppm or more of solid solution B (Bsol) is necessary. In order to prevent B from precipitating as BN (B nitride), Ti is added and N is fixed as TiN (Ti nitride).

(Micro structure)
The microstructure of the steel sheet of the present invention contains 40% or more and less than 70% bainite and 30% or more and 60% or less ferrite by area ratio. The reason why the bainite fraction is 40% or more is that the steel sheet has a large thickness and high strength. On the other hand, when the bainite fraction is 70% or more, particularly the toughness of the central portion of the plate thickness is deteriorated. Therefore, the deterioration of toughness is suppressed by setting the ferrite fraction to 30% or more and 60% or less. Further, if the ferrite fraction is less than 30%, a sufficient toughness deterioration suppressing effect cannot be obtained, and if the ferrite fraction exceeds 60%, it becomes difficult to obtain a steel sheet having a large plate thickness and high strength.
The present invention may contain other microstructures such as pearlite and martensite as long as the bainite and ferrite are within the above range.

(Average particle size)
In the steel sheet of the present invention, the boundary at a crystal orientation difference of 15 ° or more is defined as a crystal grain boundary, and the region surrounded by the crystal grain boundary is defined as a crystal grain. The average grain size of the grains is 10 μm or less, the average grain size of the crystal grains at a quarter position of the plate thickness is 20 μm or less, and the average grain diameter of the crystal grains in the center part of the plate thickness is 30 μm or less.

  The finer the crystal grain size, the better the arrestability, but considering the productivity, the average grain size of the crystal grains at a depth of 5 mm from the surface is 3 μm or more, 1/4 of the plate thickness, the center of the plate thickness The portion may be 10 μm or more.

  The reason why the arrestability is improved by reducing the crystal grain size is as follows. At the crystal grain boundary, since the crystal orientation differs between adjacent crystal grains, the direction in which the crack propagates changes in this portion. Since a small ductile fracture region (tear ridge) is formed in the portion where the crack propagation direction has changed, the energy required for crack propagation increases. Accordingly, the resistance to crack propagation of the material is increased, and the arrestability is improved.

  In general, brittle fracture hardly occurs on the surface layer of a thick steel plate, and a ductile fracture region (shear lip) is easily formed. When the surface layer is refined and the thickness of the refined layer is increased, the shear lip region is expanded. In the unbroken region before shear lip formation, a stress that closes the crack works, and the driving force for crack propagation is reduced. In addition, since the unbroken region eventually undergoes ductile fracture, the energy required for crack propagation increases. For this reason, arrestability improves.

  The reason why the crystal orientation difference is set to 15 ° or more is that if it is less than 15 °, the crystal grain boundary is unlikely to be an obstacle to brittle crack propagation, and the above-described effect of improving arrestability is reduced. The reason why the crystal grain size of the surface layer portion of the steel sheet is 10 μm or less is that if it exceeds 10 μm, the toughness necessary for forming the shear lip cannot be obtained. Further, when the average grain size of the crystal grains at the 1/4 position of the plate thickness exceeds 20 μm, or the average grain size of the crystal grains at the central portion of the plate thickness exceeds 30 μm, the toughness decreases and the brittleness inside the plate thickness. The propagation of cracks becomes dominant and the arrestability is reduced.

  Note that the average grain size of the crystal grains is obtained by an average value of the equivalent circle diameters of the area of the region surrounded by the boundary having a crystal orientation difference of 15 ° or more when evaluated by the EBSP method.

(Production method)
Next, a method for producing the steel plate of the present invention will be described. First, the molten steel adjusted to the above-described appropriate chemical composition is melted by a generally known melting method such as a converter, and is made into a steel slab that is a steel material by a generally known casting method such as continuous casting.

(Heating temperature)
First, the steel slab is heated to 950 to 1100 ° C. during or after cooling at the time of casting to make austenite single phase. This is because if the heating extraction temperature is less than 950 ° C., austenitization becomes insufficient, a coarse structure is formed, and strength and toughness deteriorate. On the other hand, if it exceeds 1100 ° C., the austenite grains become coarse and the toughness decreases.

  In the present invention, since ausformed bainite is used, cold rolling is performed as much as possible. In order to lower the Ar3 point, by adding Ni, Mn, Mo, B, etc., the Ar3 point is lowered to ensure toughness.

(Hot rolling)
Then, hot-rolled steel slabs are hot-rolled to produce a steel sheet having a plate thickness of, for example, 40 to 100 mm with a reduction rate in finish rolling of 50% or more. At this time, the start temperature of finish rolling on the surface is Ar3-30 ° C to Ar3 + 50 ° C, and the start temperature of finish rolling in the center portion of the sheet thickness is Ar3 ° C to 850 ° C. If the cumulative rolling reduction in finish rolling is less than 50%, dislocations and deformation bands introduced into austenite are insufficient and toughness is deteriorated. Moreover, if the start temperature of the finish rolling on the surface is less than Ar3-30 ° C., coarse processed ferrite is generated in the vicinity of the surface layer portion, and the surface layer portion cannot be refined and the arrestability is lowered. On the other hand, when Ar3 + 50 ° C is exceeded, dislocations and deformation bands introduced into austenite are insufficient, and the toughness deteriorates. Further, if the finish rolling start temperature at the center of the plate thickness is less than Ar3 ° C., coarse processed ferrite is generated inside the plate thickness, and the strength and toughness deteriorate. On the other hand, when it exceeds 850 ° C., dislocations and deformation bands introduced into austenite are insufficient, and toughness is deteriorated. In addition, when the cumulative rolling reduction exceeds 75%, the number of rolling passes increases and the productivity decreases, so the cumulative rolling reduction is preferably 75% or less.

  When the start temperature of finish rolling on the surface is less than Ar3-30 ° C., as shown in FIG. 2A, the vicinity of the surface layer portion of the steel sheet is two-phase rolling (rolling in a temperature range where two phases of α + γ exist) Thus, coarse processed ferrite (ferrite formed during rolling) is generated. On the other hand, if the start temperature of finish rolling is Ar3-30 ° C. or higher, it is possible to suppress the formation of coarsely processed ferrite and to rapidly cool (in CLC) with sufficient introduction of dislocations and deformation bands that become ferrite nucleation sites in austenite. 2), a fine structure of ferrite and bainite as shown in FIG. 2B can be formed.

(cooling)
After the hot rolling is finished, cooling is started at a surface temperature of Ar 3-30 ° C. or higher, the cooling rate at the central portion of the plate thickness is set to 3 ° C./s or higher, and cooling is finished at a surface temperature of 400 ° C. or lower. If the cooling start temperature is less than Ar3-30 ° C., coarse ferrite is generated before the cooling starts, and the strength and toughness deteriorate. In order to transform the bainite, a cooling rate of 3 ° C./s or more is required. A cooling rate exceeding 15 ° C./s is difficult to achieve with thick materials. In order to obtain a sufficient bainite structure, the cooling stop temperature is set to 400 ° C. or lower.

(Tempering)
Further, it is desirable to adjust the strength and toughness of the steel sheet by performing a tempering heat treatment at a temperature of 500 to 600 ° C. after cooling. When the tempering temperature exceeds 600 ° C., the strength decreases. On the other hand, if it is less than 500 ° C., the toughness improvement by removing the strain is insufficient.

  By implementing the above manufacturing method, a high-strength steel sheet having a large thickness and excellent arrestability can be manufactured at low cost without requiring a complicated heat treatment step. The steel plate of the present invention thus manufactured has a yield strength YP of 460 to 580 MPa, and is a test piece taken at a 1/4 position of the plate thickness. The fracture surface transition temperature defined in JIS Z2242: 2005 Annex D Is −80 ° C. or lower, and the NDT temperature defined in ASTM E208-06 is −85 ° C. or lower in the test piece collected on the surface. Further, the tensile strength TS can be set to 570 to 720 MPa.

  After adjusting the chemical composition of the molten steel in the steel making process, a steel slab is made by continuous casting, this steel slab is reheated to 950-1100 ° C., and a steel plate having a thickness of 40-100 mm is made by hot rolling, Cooled down. Thereafter, a tempering heat treatment was performed to produce a thick steel plate. Table 1 shows chemical components, Bsol, Pcm, and Ar3 of each steel. Table 2 also shows the slab thickness before hot rolling of the steel plates of each test number, the plate thickness after hot rolling (product thickness), the heat extraction temperature, the cumulative rolling reduction (CR rate) of finish rolling, and the rolling start surface temperature. , Rolling start sheet thickness center (t / 2) temperature, cooling start temperature and end temperature after hot rolling, sheet thickness center (t / 2) cooling rate, and tempering temperature.

  Area ratio of each phase (ferrite, pearlite, bainite) in the microstructure of the steel plate of each manufactured test number, position 5 mm deep from the surface, 1/4 position of the plate thickness, average particle diameter at the center of the plate thickness, Yield strength YP, tensile strength TS, fracture surface transition temperature vTrs, and NDT temperature in the NRL drop load test were measured. Table 3 shows the measurement results. Microstructure phase fractions were measured by optical analysis of the microstructure taken at 400x magnification at a position 5mm deep from the surface of the plate thickness, a quarter of the plate thickness, and a half of the plate thickness by an optical microscope. The average value of the area ratio of each phase with respect to the entire visual field region measured at the position of was obtained. The average crystal grain size was measured by measuring 1 μm pitch of a 500 μm × 500 μm region by EBSP (Electron Back Scattering Pattern) method, and a grain boundary map with a crystal orientation difference of 15 ° or more from adjacent grains was created. The equivalent circle diameter of the crystal grains was determined by image analysis. Moreover, the yield stress (YS) and the tensile stress (TS) were measured by collecting two test pieces in the direction perpendicular to the rolling direction from the position of the sheet thickness t / 4, and obtaining an average value thereof. The tensile test method is based on No. 4 tensile test piece described in JIS Z2241. Further, the fracture surface transition temperature (vTrs) was obtained by collecting a plurality of test pieces in the direction parallel to the rolling direction from the position of the plate thickness t / 4, and performing a Charpy impact test at a plurality of temperatures including −80 ° C., JIS Z2242: 2005 Determined by the method defined in Appendix D. The NDT temperature was determined according to ASTM E208-06 by collecting a plurality of test pieces from the surface layer portion in a direction parallel to the rolling direction.

  The steel sheet of the inventive example in which the components and production conditions are appropriate satisfied the scope of the present invention in terms of the fraction of each phase, average grain size, yield strength YP, fracture surface transition temperature vTrs, and NDT temperature. On the other hand, the steel sheet of the comparative example in which any of the components and production conditions is out of the scope of the invention has a bainite fraction, a ferrite fraction, an average grain size, a yield strength YP, a fracture surface transition temperature vTrs, and an NDT temperature. Any one was out of the scope of the present invention. From the above examples, it was confirmed that a high-strength steel sheet having excellent arrestability can be provided by applying the present invention.

  The present invention is useful for a high-strength steel sheet having a large thickness and excellent arrestability.

Claims (7)

% By mass
C: 0.05-0.10%
Si: 0.01-0.30%,
Mn: 1.40 to 2.00%
Mo: 0.01-0.20%,
Nb: 0.005 to 0.030%,
Ti: 0.005 to 0.030%,
B: 0.0005 to 0.0030%,
N: 0.0005 to 0.0050%,
Al: 0.002 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to less than 0.15%,
Cr: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0050%,
P: 0.015% or less,
S: 0.010% or less,
The rest: iron and impurities
Bsol defined by the following formula 1-1 or the following formula 1-2 is 0.0005% or more,
Ar3 defined by the following two formulas is 690 ° C. or higher and 760 ° C. or lower,
Containing 40% or more and less than 70% bainite, 30% or more and 60% or less ferrite in area ratio,
When a boundary having a crystal orientation difference of 15 ° or more is defined as a crystal grain boundary, and a region surrounded by the crystal grain boundary is defined as a crystal grain, the average grain size of the crystal grains at a position 5 mm deep from the surface Is 10 μm or less, the average grain size of the crystal grains at a 1/4 position of the plate thickness is 20 μm or less, the average grain size of the crystal grains in the center portion of the plate thickness is 30 μm or less,
Yield strength is 460-580 MPa,
The specimen taken at 1/4 position of the plate thickness has a fracture surface transition temperature defined in JIS Z2242: 2005 Annex D of −80 ° C. or lower,
A steel sheet having a NDT temperature of −85 ° C. or less as defined in ASTM E208-06, which is a test piece collected on the surface.
Bsol = [B] (when [Ti] / [N] is 3.4 or more) Formula 1-1
Bsol = [B] +0.226 [Ti] −0.722 [N] (when [Ti] / [N] is less than 3.4) Formula 1-2
Ar3 = 940−310 × [C] + 40 × [Si] −90 × [Mn] −40 × [Cu] −60 × [Ni] −15 × [Cr] −40000 × [Mo] × Bsol Formula 2
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [B], [Ti] and [N] are C, Si, Mn, respectively. , Cu, Ni, Cr, Mo, B, Ti, and the content expressed by mass%.   The steel plate according to claim 1, wherein the plate thickness is 40 to 100 mm.   The steel plate according to claim 1 or 2, wherein the tensile strength is 570 to 720 MPa. The steel plate in any one of Claims 1-3 whose Pcm defined by following formula 3 is 0.22% or less.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 3
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu and Ni, respectively. , Cr, Mo, V and B mean the content expressed in mass%. A method for producing the steel sheet according to any one of claims 1 to 4,
% By mass
C: 0.05-0.10%
Si: 0.01-0.30%,
Mn: 1.40 to 2.00%
Mo: 0.01-0.20%,
Nb: 0.005 to 0.030%,
Ti: 0.005 to 0.030%,
B: 0.0005 to 0.0030%,
N: 0.0005 to 0.0050%,
Al: 0.002 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to less than 0.15%,
Cr: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0050%,
P: 0.015% or less,
S: 0.010% or less,
The rest: iron and impurities
Bsol defined by the following formula 1-1 or the following formula 1-2 is 0.0005% or more,
A steel slab in which Ar3 defined by the following two formulas is 690 ° C. or higher and 760 ° C. or lower is heated to 950 to 1100 ° C. and hot-rolled,
The rolling reduction in the finish rolling of the hot rolling is 50% or more,
The start temperature of the finish rolling on the surface is Ar3-30 ° C to Ar3 + 50 ° C, the start temperature of the finish rolling in the center portion of the sheet thickness is Ar3 ° C to 850 ° C,
A method for producing a steel sheet, wherein cooling is started at a surface temperature of Ar 3-30 ° C. or higher, the cooling rate at the center of the plate thickness is 3 ° C./s or higher, and cooling is terminated at a surface temperature of 400 ° C. or lower.
Bsol = [B] (when [Ti] / [N] is 3.4 or more) Formula 1-1
Bsol = [B] +0.226 [Ti] −0.722 [N] (when [Ti] / [N] is less than 3.4) Formula 1-2
Ar3 = 940−310 × [C] + 40 × [Si] −90 × [Mn] −40 × [Cu] −60 × [Ni] −15 × [Cr] −40000 × [Mo] × Bsol Formula 2
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [B], [Ti] and [N] are C, Si, Mn, respectively. , Cu, Ni, Cr, Mo, B, Ti, and the content expressed by mass%. The said steel slab is a manufacturing method of the steel plate of Claim 5 whose Pcm defined by following formula 3 is 0.22% or less.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 3
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu and Ni, respectively. , Cr, Mo, V and B mean the content expressed in mass%.   The manufacturing method of the steel plate of Claim 5 or 6 which tempers at 500-600 degreeC after cooling.