JP6766642B2 - Steel sheet with excellent brittle crack propagation stop characteristics and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet having excellent brittle crack propagation stopping characteristics (hereinafter, also referred to as arrest property) (hereinafter, also referred to as arrest steel sheet).

In particular, the present invention is a thick steel material having a yield strength of 460 MPa class (yield strength of 460 to 530 MPa) and a plate thickness of 50 mm or more (hereinafter, also referred to as a thick material), and Kca = 6000 N / mm ^1.5. The present invention relates to a steel plate having an excellent brittle crack propagation stopping property having a temperature (hereinafter, also referred to as an arrest property index TKca = 6000) of −10 ° C. or lower, and a method for producing the same.

The steel plate according to the present invention is widely applied to welded structures such as shipbuilding, construction, bridges, tanks, and marine structures. Further, the steel sheet of the present invention may be distributed not only as a steel sheet but also as a secondary processed product processed into a steel pipe, a column or the like.

With the increase in size of steel structures in recent years, the steel materials used have become thicker (especially thicker than 50 mm) and stronger, and from the viewpoint of ensuring safety, requirements for brittle crack propagation stop characteristics are required. Is getting tougher. However, in general, when the strength and the plate thickness increase, it becomes difficult to secure the arrest property rapidly, which is a factor that hinders the application of the thick high-strength steel sheet to the steel structure. At the same time, the demands of consumers for shorter delivery times are increasing year by year, and there is a strong demand for improved productivity in the steel sheet manufacturing process.

Known metallurgical factors for improving the arrest property of steel materials include (i) grain refinement, (ii) Ni addition, (iii) embrittlement second phase control, and (iv) texture control. There is.

As a method for refining the crystal grains of (i), there is a technique described in Patent Document 1. This is a method in which rolling is performed in an unrecrystallized region of Ar 3 points or more with a rolling reduction of 50% or more, and then two-phase rolling with a rolling reduction of 30 to 50% is performed in the range of 700 to 750 ° C. In addition, as a special method for refining the crystal grains of a steel sheet, the surface of the steel piece is cooled before rolling or after the rough rolling is completed, and rolling is started with a temperature difference from the inside to reheat the surface layer portion. Patent Documents 2 and 3 describe methods for producing fine-grained ferrite.

It is said that the addition of Ni in (ii) suppresses the propagation of brittle cracks by promoting cross-slip in a low temperature range (see Non-Patent Document 1) and improves the arrest property of the matrix (Non-Patent Document). 2).

As a method for controlling the embrittlement second phase of (iii), there is a technique described in Patent Document 4. This is a technique for finely dispersing martensite, which is an embrittlement phase, in ferrite, which is a matrix phase.

Regarding the texture control of (iv), Patent Document 5 describes a method in which low-temperature and large-pressure rolling is performed on ultra-low carbon bainite steel to develop the (211) plane in parallel with the rolled plane.

Japanese Patent Application Laid-Open No. 02-129318 Special Fairness 06-004903 Gazette Japanese Unexamined Patent Publication No. 2003-221619 JP-A-59-047323 JP-A-2002-241891 Japanese Unexamined Patent Publication No. 2008-248382 Japanese Unexamined Patent Publication No. 2013-151743 JP-A-2010-202938

Imao Tamura, "Steel Material Strength Studies", published by Nikkan Kogyo Shimbun, July 5, 1969, p. 125 Hasebe, Kawaguchi, "Brittle fracture propagation stopping characteristics of Ni-added steel sheets by tapered DCB test", Iron and Steel, vol. 61 (1975), p. 875

The method described in Patent Document 1 is intended for low-temperature steel having a microstructure mainly made of ferrite, having a relatively low strength, and having a plate thickness of about 20 mm. Therefore, when applied to a thick material having a plate thickness of 50 mm or more, which is the object of the present invention, it is difficult to secure the reduction rate from the viewpoint of the slab thickness, the temperature waiting time becomes long, and the productivity is significantly lowered. There is a problem that it ends up. Further, it is difficult to secure a yield strength of 460 MPa or more by the method described in Patent Document 1.

When the inventions described in Patent Documents 2 and 3 are to be applied to a thick material as the subject of the present invention, it is difficult to ensure arrestability even if the structure is the same, and the surface layer ferrite is miniaturized. There is a problem that the effect of is relatively small. Further, also in the manufacturing process, there is a problem that the temperature control in the plate thickness direction becomes more difficult and the reduction rate in the reheating process has to be increased, which greatly hinders the productivity.

As described in (ii) above, there is a problem that the alloy cost is too high to produce a steel sheet having a desired arrest property only by adding Ni. Therefore, in order to reduce the amount of Ni added, even if it is attempted to secure the arrest property by using Ni addition and microstructure miniaturization in combination, the effects of other factors used in combination with Ni addition on the arrest property are separated and quantified. Attempts have not been made sufficiently, and it is difficult to say that the manufacturing guidelines for Ni-added high-arrest steel sheets have been clarified.

It is difficult to finely disperse martensite with a thick material as in the invention described in Patent Document 4. Further, in a thick high-strength steel sheet, this kind of embrittlement phase may deteriorate the brittle fracture occurrence property.

The invention described in Patent Document 5 has a problem that when it is applied to a thick material having a plate thickness of 50 mm or more, the rolling efficiency is extremely lowered and it is not suitable for industrial production.

Patent Document 6 defines a boundary in Ni-containing steel in which the angle (crack propagation deflection angle) formed by the <001> axes closest to the crack propagation direction (TD) is 20 ° or more as a grain boundary. , A highly arrestable steel sheet that defines the particle size, Ni content, pearlite fraction, and cementite diameter has been proposed. However, although the present invention allows rotation around the TD, it is calculated to be smaller than the actual particle size because it does not allow rotation around the rolling direction (RD) and the crack propagation deflection angle is small. , There is a problem that the estimation accuracy is poor.

Patent Document 7 describes that the cumulative reduction rate in two-phase region rolling is 40% or more, the reduction rate of one pass on average is 6% or more, and the XRD surface strength ratio and bainite fraction of the texture in the central portion of the sheet thickness. A high arrestable steel sheet that defines vTrs in a quarter of the plate thickness has been proposed. However, in order to ensure arrestability from the viewpoint of aggregate structure, there is a problem that the anisotropy of the material is large and the productivity is low.

Patent Document 8 describes the average area ratio of pseudopolygonal ferrite, the grain size of crystal grains surrounded by large-angle grain boundaries with adjacent crystal orientation differences of 15 ° or more, the average circle-equivalent diameter of island-shaped martensite, and the yield stress. Proposes that a highly arrestable steel sheet can be obtained by satisfying a certain relationship. However, since it is a structure mainly composed of pseudopolygonal ferrite and the reduction rate in the temperature range directly above Ar3 is low, the actual situation is that sufficient arrest property is not obtained.

As described above, it can be applied to a large structure which is a thick material having a plate thickness of 50 mm or more and has an arrest property index TKca = 6000 of -10 ° C or less even if the yield strength is 460 MPa class, which is the target of the present invention. , The technology for stably and efficiently producing high-arrest steel sheets has not yet been established.

The present invention has been made in view of the above circumstances, and has a brittle crack propagation stopping property that has sufficient arrest property as a large-scale structural steel and can be industrially stable and efficiently manufactured. An object of the present invention is to provide an excellent thick high-strength steel sheet. Specifically, the plate thickness is 50 mm or more, the yield strength YS is 460 MPa class, the arrest property index TKca = 6000 is -10 ° C or less, and the HAZ toughness vE-40 ° C of the joint is 100 J or more. An object of the present invention is to obtain a steel sheet having excellent crack propagation stopping characteristics.

In order to solve the above problems, the present inventors have diligently studied the arresting property controlling factor, and obtained the following findings for stably ensuring the arresting property even in a thick material having a plate thickness of 50 mm or more.

(A) The unit of fracture when the brittle crack propagates (refers to the size of the region fractured almost planarly on the fracture surface. Hereinafter, it is also referred to as the fracture surface unit or the basic structure unit) is apparent. It was found that it agrees very well with the particle size obtained by the crystal orientation analysis using EBSD, not the crystal particle size of. Specifically, when the vector closest to the rolling direction (RD) is projected onto the plate surface (ND surface) among the normal vectors of the three {100} planes that can be the crack propagation planes of a certain crystal grain. The average grain size when the boundary between the above ND plane projection vectors in adjacent crystal grains (hereinafter referred to as the crack propagation deflection angle) is 25 ° or more is defined as the grain boundary (hereinafter referred to as the grain boundary). It was found that there is a close correlation between the crack propagation effective crystal grain size) and the arrest property.

(B) It was found that the effect of improving arrestability and the effect of granulation by Ni are independent, and the addition rule is almost established. That is, if the effective crystal grain size for crack propagation can be sufficiently refined, the addition of Ni is not essential, but if Ni is added, the same arrestability can be ensured even if the structure is rough, and the finishing rolling temperature can be increased. It was found that it is possible to reduce the manufacturing load of.

(C) It was found that even if the effective crystal grain size for crack propagation is fine, when the pearlite area ratio exceeds 5%, coarse pearlite tends to be the starting point of brittle fracture and the arrest property is also lowered. In order to avoid this, it is necessary to control the cooling rate and the stop temperature in the accelerated cooling process.

(D) When the area ratio of the coarse ferrite generated in the region (surface layer region) from the surface of the steel plate to a depth of 5% of the plate thickness exceeds 10%, the crack propagation effective crystal grain size of the average plate thickness is fine. Even so, it was found that the arrest property was reduced. In order to avoid this, it is necessary to control so that the finish rolling temperature and the cooling start temperature do not drop too much.

(E) Precipitates and non-metallic inclusions such as Nb and Ti carbonitrides and cementites (hereinafter, for convenience of explanation, the precipitates and inclusions are collectively referred to as precipitates in the present specification. It was found that if it is fine, it does not affect the arrest property, but if it is coarse, it reduces the arrest property. In order to keep the size of the precipitates fine, it is necessary to appropriately control the slab heating, accelerated cooling, and subsequent heat treatment conditions.
The present invention has been made based on these findings, and the gist thereof is as follows.

[1]
By mass%
C: 0.050 to 0.140%,
Si: 0.03 to 0.50%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.005 to 0.040%,
Ti: 0.005-0.030%,
Al: 0.001 to 0.100%,
N: Contains 0.0010 to 0.0080%,
The balance consists of Fe and unavoidable impurities, and the Ps value defined by the following formulas 1 to 3 is 0.40 to 0.54%.
The area ratio of the microstructure is ferrite: 20 to 50%,
Pearlite: 5% or less,
Bainite: Contains 40-80% and
In the surface layer region from the surface of the steel sheet to a depth of 5% of the plate thickness, ferrite having a diameter equivalent to a circle of crystal grains of 25 μm or more is 10% or less in area ratio, and precipitates and inclusions having a diameter equivalent to a circle of 50 nm or more. The average circle-equivalent diameter of those with a large circle-equivalent diameter up to 20% in terms of number is 0.40 μm or less.
Further, in the crystal orientation analysis using EBSD in the vertical cross section in the rolling direction, the crack propagation effective crystal grain size is d (μm) or less of the following formula 4, and the plate thickness t (mm) is 50 mm or more. A steel sheet with excellent brittle crack propagation stop characteristics.
Ps = Ceq + 3.6 × [Nb] + 79 × Bsol… Equation 1
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... Equation 2
Bsol = [B]-([N]-[Ti] x 14/48) x 11/14 ... Equation 3
d = (5.4 × [Ni] +16) × (1.2-t / 300)… Equation 4
Here, [element symbol] such as [C] indicates the content (mass%) of the element, and if it is not contained, 0 is substituted. Further, when B is not added or Bsol <0 in the formula 3, Bsol = 0 in the formula 1 is set.
[2]
In addition, in% by mass,
Cu: 0.05 to 1.50%,
Ni: 0.05 to 2.00%,
Cr: 0.05 to 1.00%,
Mo: 0.02 to 0.50%,
V: 0.005 to 0.100%,
B: It is characterized by containing one or more of 0.0002 to 0.0030%.
The steel sheet having excellent brittle crack propagation stopping characteristics according to the above [1].
[3]
In addition, in% by mass,
Mg: 0.0003 to 0.0050%,
Ca: 0.0005 to 0.0030%,
Zr: 0.0005 to 0.0050%,
REM: Containing one or more of 0.0005 to 0.0100%.
The steel sheet having excellent brittle crack propagation stopping characteristics according to the above [1] or [2].
[4]
When heating a steel piece having the composition according to any one of the above [1] to [3], the lower limit heating temperature TL is set to the following formula 9, and the upper limit heating temperature TU is set to the following formula 10. When it is assumed, the temperature of the steel piece changes from the lower limit temperature TL to the upper limit temperature TU, assuming that the holding starts when the temperature of the piece reaches the lower limit temperature TL after the steel piece is charged into the heating furnace. The temperature of the heating furnace is controlled so as to be maintained in the range of, and the time from the start of holding to the extraction of the steel piece from the heating furnace is defined as the holding time tm (minutes), and the holding time tm (minutes). When the time average temperature of the temperature of the steel pieces in the above is the heating temperature T (° C.), the heating temperature T (° C.) and the holding time tm (minutes) satisfy the following equations 5 to 7.
When rolling in a temperature range of 900 ° C. or higher, rolling is performed so that the rolling reduction of at least the final 3 passes is 10% or more, the time between passes is 15 seconds or less, and the rolling ratio is not lower than the rolling ratio of the previous pass.
Further, when the thickness of the steel piece before each rolling is tb, the temperature of the portion from the steel plate surface to tb / 4 is in the range of Ar3 + 30 ° C. to Ar3 + 80 ° C. using Ar3 represented by the following formula 8. In a plurality of passes to be rolled, after rolling under the conditions that the average reduction rate of each pass is 5.0% or less, the cumulative reduction rate is 40% or more, and the average interval time between passes is 25 seconds or less.
When the plate thickness of the obtained steel plate is t, the temperature of the portion from the surface of the steel plate to t / 4 continues to be from the temperature of Ar3 or higher to the temperature of 400 ° C or lower, and the cooling rate is 5 ° C / sec or more on average. A method for producing a steel sheet having excellent brittle crack propagation stopping characteristics, which is characterized by cooling with.
57000 / {1.2-0.16 × log ([C] [Nb])} ≦ PH ≦ 84000 / (1.9-0.18 × log [Ti])… Equation 5
PH = (T + 273) × {log (tm) +25}… Equation 6
tm ≧ 30… Equation 7
Ar3 = 910-310 [C] +65 [Si] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] ... Equation 8
TL = 57000 / {1.2-0.16 × log ([C] [Nb])} / {log (30) +25} -273 ... Equation 9
TU = 84000 / (1.9-0.18 × log [Ti]) / {log (30) +25} -273 ... Equation 10
Here, [element symbol] such as [C] indicates the content (mass%) of the element, and if it is not contained, 0 is substituted.
[5]
Further, the method for producing a steel sheet having excellent brittle crack propagation stopping characteristics according to [4], which comprises performing a heat treatment at a temperature of 300 to 600 ° C.

By applying the present invention, even if the plate thickness is 50 mm or more and the yield strength is 460 MPa class, the arrest property index TKca = 6000 (temperature at which Kca = 6000 N / mm ^1.5 is ^1.5 ) becomes −10 ° C. or less. It becomes possible to provide a high-arrest steel sheet applicable to a large structure by a stable and efficient manufacturing method.

This is an example of the result of analyzing the boundary between the crystal orientation map and the equal crack propagation resistance region by EBSD. It is a figure which shows an example of the relationship between a crystal orientation difference, a crack propagation deflection angle and a propagation resistance boundary. It is a figure which shows an example of the relationship between the amount of Ni which affects arrest property and the effective crystal grain size of crack propagation. It is a figure which shows an example of the relationship between the amount of Ni required for imparting a predetermined arrest property, and the crack propagation effective crystal grain size. It is a figure which shows an example of the relationship between the average circle equivalent diameter of the top 20% of precipitates / inclusions, and arrest property.

Hereinafter, each requirement constituting the present invention will be described in detail.

[Micro tissue]
In general, the basic structural unit that governs toughness is the ferrite grain size in ferritic steel and the size of regions called packets and blocks in bainite steel. The smaller these sizes, the better the toughness. However, steel with a yield strength of 460 MPa has a structure in which ferrite and bainite coexist, and it is very difficult to objectively define the basic structure unit and measure its size by observing the structure with a normal optical microscope. ..

Therefore, the present inventors first produced a steel sheet having a thickness of 60 mm under various conditions using a steel piece containing no Ni, and based on the method described in WES 2815 for evaluation of arrestability, the width was 500 mm. A temperature gradient type ESSO test was performed using the test piece. After that, the fracture surface of the test piece is observed with a scanning electron microscope (SEM), and the size (unit of fracture surface) of the cleavage surface surrounded by the ductile fracture portion called tier ridge is measured, and between this and the arrest property. It was confirmed that there is a good correlation with.

Next, small pieces were cut out so that the ND surface near the fracture surface could be observed, and EBSD measurement was performed. The crystal orientation analysis result of the grains near the fracture surface was compared with the fracture surface photograph obtained by observing the fracture surface by SEM. We compared and analyzed. By observing the fracture surface photograph, it is considered that it is a resistance to crack propagation at the boundary where the orientation of the fracture surface changes significantly, or the boundary where a large tier ridge is confirmed on the fracture surface. It was set as a unit boundary. Then, the analysis conditions in which the crystal orientation analysis results of the grains in the vicinity of the fracture surface match the fracture surface unit boundaries observed from the fracture surface photograph were examined in detail. An example thereof is shown in FIG.

Usually, based on a map obtained by drawing a boundary with a crystal orientation difference of 15 ° or more, which is defined as a large-angle grain boundary, the inside of the region delimited by the boundary (hereinafter referred to as an equidirectional region). Analyze the representative points of. A cube composed of {100} planes considered to be cleavage planes is shown in FIG. The numerical value of the crystal orientation difference in FIG. 1 is a rotation angle required to match the orientations of adjacent equidirectional regions. Further, the number of the crack propagation deflection angle is the angle formed by the vectors obtained by projecting the <001> axis closest to the RD onto the ND plane in the adjacent equidirectional region, and determines the RD and the rolling width direction (TD). This is the angle required to allow and align the rotation around the axis. The arrows in the figure are large on the boundary where the orientation of the fracture surface changes significantly, which is considered to be the resistance of crack propagation from the fracture surface photograph of the fracture surface observed by SEM, or on the fracture surface. The fracture surface unit boundary, which is the boundary where the tier ridge was confirmed, is shown.

From this, it can be seen that the crack propagation resistance may not be obtained at the boundary when the crystal orientation difference of 15 ° or more used in the usual analysis is set as the threshold value. On the other hand, it generally corresponds to the boundary where the crack propagation deflection angle is large. FIG. 2 summarizes the correspondence between the crystal orientation difference, the crack propagation deflection angle, and the propagation resistance boundary, including the analysis results in other regions.

From FIG. 2, it can be seen that the crystal orientation difference and the presence / absence of propagation resistance do not correspond to each other, whereas the crack propagation deflection angle of 25 ° or more has propagation resistance, that is, it corresponds to the fracture surface unit boundary. .. As described above, from the EBSD analysis result, a boundary having a crack propagation deflection angle of 25 ° or more can be determined, and the average circle equivalent diameter (crack propagation effective crystal grain size) of the region surrounded by this boundary can be calculated.
That is, the crystal orientation analysis using EBSD was performed on the vertical cross section (RD plane) in the rolling direction, and the cutting method conforming to JIS G 0551 was applied to the structure divided into the regions (isodirectional regions) within the orientation difference of 15 °. Of the vectors representing the three <001> axes in a plurality of equidirectional regions that are consecutively adjacent to each other on the measurement line, the vectors that have the smallest angle with the RD are projected onto the steel plate surface (ND surface). When a plurality of equidirectional regions that are continuously adjacent to each other on the measurement line and have an angle (crack propagation deflection angle) of less than 25 ° are regarded as one region (equal crack propagation resistance region), etc. The average circle-equivalent diameter calculated by the cutting method in the crack propagation resistance region is the crack propagation effective crystal grain size.

When the relationship between the crack propagation effective crystal grain size and the arrest property measured in this way was investigated in detail, in order to impart a level of arrest property applicable to large structural steel and a yield strength of 460 MPa class, heating was performed. , Rolling and cooling conditions need to be precisely controlled, which has been found to be a factor that hinders productivity.

Therefore, as a means for solving the above problems, the effect of adding Ni was examined in detail. Using steel pieces cast with various balances of Ni and Mn so that the microstructure and strength are almost the same, steel sheets with a thickness of 60 mm and 90 mm are manufactured under the same manufacturing conditions, and arrested by ESSO test. I investigated the sex.

As a result, it was confirmed that the arrest property tends to improve as the amount of Ni increases, although the effective crystal grain size for crack propagation hardly changes. When observing the fracture surface of the ESSO test piece, it was observed that the three-dimensional unevenness became remarkable as the amount of Ni increased. It is considered that this is because the solid solution Ni promoted the cross slip and the crack growth direction became more random.

Next, the arrest property of the steel sheet obtained by rolling the above Ni-containing steel pieces under various conditions was investigated for the purpose of separating and quantifying the effect of refining the effective crystal grain size for crack propagation when Ni was added. As a result, as shown in FIG. 3, it was found that the effect of improving the arrest property by the granulation showed almost the same tendency regardless of the amount of Ni.

That is, by utilizing an appropriate amount of Ni, the arrest property can be ensured without miniaturizing the crack propagation effective crystal grain size. Therefore, when steel production efficiency is required rather than the alloy cost of Ni, the finish rolling temperature can be raised by adding Ni, and the temperature waiting time is shortened, so that the productivity of thick materials can be significantly increased. It becomes.

Furthermore, using the various steel sheets described above, the effects of crack propagation effective crystal grain size, Ni amount, and plate thickness on arrest properties were analyzed in more detail. As a result, it was found that the condition of the effective crystal grain size for crack propagation needs to be equal to or less than the limit effective crystal grain size for crack propagation.
d = (5.4 × [Ni] +16) × (1.2-t / 300)… Equation 4
Here, [Ni] represents the Ni content (mass%), and t represents the plate thickness (mm).

The first term (5.4 × [Ni] +16) of the formula 4 was derived from FIG. 4 based on the effect of the crack propagation effective crystal grain size and Ni on the arrest property of the 60 mm thick material. Further, the second term (1.2-t / 300) of Equation 4 is a coefficient of the plate thickness effect derived from the difference in arrest property of steel plates having the same crack propagation effective crystal grain size and Ni amount but different plate thicknesses. Is. The limit crack propagation effective crystal grain size d is determined by the product of these. When the crack propagation effective crystal grain size is larger than the limit crack propagation effective crystal grain size d, the frequency of tear ridges formed when the brittle crack propagates from one crystal grain to another crystal grain is sufficient. Therefore, the effect of suppressing crack propagation is reduced, and the arrest property is reduced.

The present inventors also investigated the effects of tissue factors other than the effective crystal grain size for crack propagation on arrest properties. This is because it was confirmed that the arrest property was not sufficient even though the crack propagation effective crystal grain size was fine.
One of them is pearlite, which is mixed in bainite-based organizations. It was found that as the area ratio of the pearlite structure increases, large pearlite increases, which becomes the starting point of brittle fracture, and the arrest property also tends to deteriorate. Therefore, the pearlite area ratio needs to be 5% or less.

It was also confirmed that the size of cementite, which is mainly contained in bainite, or precipitates such as Nb and Ti nitrides, also affects the arrest property. However, when arranged by the average diameter of these precipitates, there is not much correlation with arrestability, and the size of the relatively coarse precipitate, for example, as shown in FIG. 5, from the one having a large circle-equivalent diameter of the precipitate and inclusions. It was found that when the average circle-equivalent diameter (diameter) of the top 20% in terms of the number ratio was more than 0.40 μm, the arrest property was lowered.

Fine precipitates may relieve stress by forming microcracks at the interface with the matrix prior to main crack propagation, which may contribute to improved arrestability, while coarse precipitates are pearlite. Similarly, it becomes a factor that induces brittle fracture and is considered to reduce the arrest property.

Furthermore, it was found that coarse ferrite (hereinafter referred to as surface coarse ferrite) generated in a region (surface layer region) from the surface of the steel sheet to a depth of 5% of the plate thickness also reduces the arrest property. This surface coarse ferrite is produced when a steel having a relatively low hardenability is rolled at a temperature lower than Ar3, or when the start of accelerated cooling falls below Ar3 even if the rolling is completed at Ar3 or higher. When the area ratio of ferrite having a crystal grain equivalent diameter of 25 μm or more in the surface layer region is 10% or less, a remarkable decrease in arrest property can be avoided. From the viewpoint of ensuring arrestability, the area ratio of ferrite having a crystal grain equivalent diameter of 25 μm or more in the surface layer region is preferably small, preferably 5% or less, and more preferably 3% or less.

The remaining structure is ferrite and bainite, which may contain some pearlite. In the steel having a yield stress of 460 MPa class according to the present invention, bainite is mainly used, and the area ratio may be 40% or more. From the viewpoint of ensuring toughness, ferrite should be contained at least in an area ratio of 20%. On the other hand, if the bainite content exceeds 80% in terms of area ratio, the amount of ferrite decreases accordingly and the toughness deteriorates. Therefore, the upper limit of the bainite content is preferably 80%. The upper limit of the area ratio of ferrite is 50% in terms of area ratio from the viewpoint of ensuring strength. Pearlite should be 5% or less.

[component]
Next, the reason for limiting the components of the present invention will be described.

C is an element that contributes to the prevention of tissue coarsening and is an element that is indispensable for increasing the strength, and therefore contains 0.050% or more. In order to ensure these effects, it is preferably 0.055% or more, more preferably 0.060% or more. On the other hand, if the content increases, it becomes difficult to secure the toughness of the heat-affected zone (HAZ) of the weld, and cementite also tends to become coarse, so the content is set to 0.140% or less. In order to ensure these effects, it is preferably 0.120% or less, more preferably 0.110% or less.

Si is a deoxidizing element and contains 0.03% or more in order to solidify and strengthen the matrix. In order to ensure these effects, it is preferably 0.07% or more, more preferably 0.10% or more. On the other hand, if the content exceeds 0.50%, the weldability and HAZ toughness deteriorate, so the content is set to 0.50% or less. In order to secure these effects, it is preferably 0.40% or less, more preferably 0.30% or less.

Mn is contained in an amount of 0.30% or more because it is effective as an element for improving the strength and toughness of the base material. In order to secure these effects, it is preferably 0.50% or more, more preferably 0.80% or more. On the other hand, if it is excessively contained, HAZ toughness and weld crackability are deteriorated, so the content is set to 2.00% or less. In order to ensure these effects, it is preferably 1.90% or less, more preferably 1.80% or less.

P and S are impurities, and the smaller the content, the more desirable. However, since it costs a lot to reduce this industrially, it is preferable to limit P to 0.020% or less and S to 0.010% or less.

Nb is an element that contributes to microstructure miniaturization, transformation strengthening, and precipitation strengthening by containing a small amount, and is an element effective for ensuring the strength of the base metal. Therefore, it is contained in an amount of 0.005% or more. It is set to 0.040% or less in order to deteriorate.

Ti is contained in an amount of 0.005% or more because it is effective in improving the strength and toughness of the base material and the HAZ toughness by finening the structure, strengthening precipitation, and forming fine TiN by containing a small amount. In order to ensure these effects, it is preferably 0.007% or more, more preferably 0.010% or more. On the other hand, if it is contained in excess, the HAZ toughness is significantly deteriorated, so the content is 0.030% or less. In order to ensure these effects, it is preferably 0.025% or less, more preferably 0.020% or less.

Since Al is an important deoxidizing element, it contains 0.001% or more. In order to ensure these effects, it is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if it is excessively contained, the surface quality of the steel piece is impaired and inclusions harmful to toughness are formed, so the content is set to 0.100% or less. In order to ensure these effects, it is preferably 0.080% or less, more preferably 0.060% or less.

N is contained in an amount of 0.0010% or more in order to form a nitride together with Ti and improve HAZ toughness. In order to ensure these effects, it is preferably 0.0015% or more, more preferably 0.0020% or more. On the other hand, if it is contained in excess, embrittlement due to solid solution N occurs, so the content is limited to 0.0080% or less. In order to ensure these effects, it is preferably 0.0070% or less, more preferably 0.0060% or less.

In addition to the above elements, the balance is Fe and unavoidable impurities, but if necessary, selective additive elements can be contained. Selective additive elements are limited for the following reasons. These selective additive elements can be contained in place of a part of Fe.

Since Cu, Cr and Mo are all effective for improving hardenability and increasing strength, Cu and Cr may be contained in an amount of 0.05% or more and Mo may be contained in an amount of 0.02% or more. On the other hand, since excessive content lowers HAZ toughness, it is preferable to limit Cu to 1.50% or less, Cr to 1.00% or less, and Mo to 0.50% or less.

Ni may be contained in an amount of 0.05% or more in order to contribute to ensuring strength and improving arrestability without significantly reducing HAZ toughness. On the other hand, it is preferable to limit the increase in the amount of Ni to 2.00% or less in order to increase the cost of steel pieces.

V is preferably contained in an amount of 0.005% or more because it contributes to an increase in strength by strengthening precipitation. On the other hand, if the content exceeds 0.100%, the HAZ toughness is lowered, so this may be the upper limit.

B is an element that improves hardenability and is effective in increasing the strength of steel by containing an appropriate amount. However, since excessive content impairs weldability, it is preferable to limit the content to 0.0002 to 0.0030%.

Mg, Ca, Zr, and REM form fine oxides and sulfides and contribute to the improvement of HAZ toughness, but excessive content coarsens inclusions and lowers toughness, so Mg is 0.0003 to 0. It is preferable that the content is 0050%, Ca is 0.0005 to 0.0030%, Zr is 0.0005 to 0.0050%, and REM is 0.0005 to 0.0100%. REM is a rare earth element such as La and Ce.

In addition to the above-mentioned definition of the range of each element, it is necessary to contain Ps defined by the following formulas 1 to 3 so as to be 0.40 to 0.54%.
Ps = Ceq + 3.6 × [Nb] + 79 × Bsol… Equation 1
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... Equation 2
Bsol = [B]-([N]-[Ti] x 14/48) x 11/14 ... Equation 3
Here, the term indicated by the [element symbol] such as [C] indicates the content (mass%) of the element. (For example, [C] indicates the content (mass%) of carbon (C).) The coefficient of Equation 1 was experimentally determined from the contribution to hardenability. Bsol corresponds to the estimated value of solid solution B, and when B is not added or Bsol <0 in the formula 3, Bsol = 0 in the formula 1 is set. If the Ps value is less than 0.40%, it is difficult to secure a yield strength of 460 MPa, and if it is more than 0.54%, the strength may be excessive and the toughness and arrestability may be lowered.

[Production method]
Subsequently, the means for producing the steel of the present invention will be described.
First, when the steel piece containing the predetermined component described above is heated, the heating temperature and the holding time are controlled as shown below. Substitute the contents of C and Nb into the left side of the above formula 5, substitute tm into the formula 6 with the shortest 30 (minutes) from the formula 7, and calculate the heating lower limit temperature TL assuming that the left side of the formula 5 is equal to PH. To do.
TL = 57000 / {1.2-0.16 × log ([C] [Nb])} / {log (30) +25} -273 ... Equation 9
Next, the Ti content is substituted into the right side of the equation 5, tm is substituted into the equation 6 as the shortest 30 (minutes) from the equation 7, and the heating upper limit temperature TU is calculated assuming that the PH is equal to the right side of the equation 5. ..
TU = 84000 / (1.9-0.18 × log [Ti]) / {log (30) +25} -273 ... Equation 10
Here, [element symbol] indicates the content (mass%) of the element.
For example, in the case of C: 0.10%, Nb: 0.010%, Ti: 0.010%, the lower limit heating temperature TL is 1008 ° C. and the upper limit heating temperature TU is 1131 ° C.
The temperature of the steel piece is a cross section in the plate thickness direction of the steel piece, which is sequentially calculated by heat transfer calculation from the surface temperature of the steel piece, the atmospheric temperature in the heating furnace, the elapsed heating time in the heating furnace of the steel piece, etc. The average temperature of.
After the steel pieces were charged into the heating furnace, the temperature of the steel pieces was kept within the range from the lower limit heating temperature TL to the upper limit heating temperature TU, starting from the time when the temperature of the steel pieces reached the lower limit heating temperature TL. The temperature of the heating furnace is controlled so that the steel pieces are extracted from the heating furnace. The time from the start of holding described above to the extraction of the steel pieces from the heating furnace is defined as the holding time tm (minutes). Then, the time average temperature of the temperature of the steel pieces during the holding time tm is defined as the heating temperature T (° C.). It is necessary to heat T and tm under the conditions satisfying the following equations 5 to 7.
57000 / {1.2-0.16 × log ([C] [Nb])} ≦ PH ≦ 84000 / (1.9-0.18 × log [Ti])… Equation 5
PH = (T + 273) × {log (tm) +25}… Equation 6
tm ≧ 30… Equation 7
Here, [element symbol] indicates the content (mass%) of the element as described above.
Each coefficient and constant of Equation 5 was experimentally determined from the limit condition for producing coarse austenite (γ) and the limit condition for securing the amount of solid solution Nb. In Table 2 (Tables 2-1 and 2-2 are collectively referred to as Table 2), the left side of the equation 5 is described as ML and the right side is described as MU.
ML = 57000 / {1.2-0.16 × log ([C] [Nb])}
MU = 84000 / (1.9-0.18 x log [Ti])
The holding time was set to 30 minutes or more in order to uniformly dissolve trace alloy elements such as Nb.
The pH shown in Equation 6 was defined based on the tempering parameters used to convert the tempering temperature and holding time. The left side of Equation 5 is the lower limit of the heating conditions that change according to the amounts of C and Nb, and the right side is the upper limit of the heating conditions that change according to the amount of Ti.

Next, rough rolling including rolling of at least 3 passes is performed in order to effectively refine the γ grains generated by heating by recrystallization. Rough rolling is performed in a temperature range of 900 ° C. or higher, and at least in the final 3-pass rolling, the rolling reduction ratio in each rolling pass is 10% or more, the inter-pass time is 15 seconds or less, and the pre-pass in each rolling pass. It is advisable to perform rolling so as not to fall below the rolling reduction ratio.

If the temperature is less than 900 ° C., recrystallization does not proceed sufficiently between the passes, and γ may be mixed.
If the reduction rate of each of the final three passes is less than 10%, the recrystallized γ cannot be sufficiently refined.
If the pressure of the latter pass is lighter in the final three passes, the recrystallized γ may become coarse.
On the other hand, if the time between passes is too long, the recrystallized γ grows as grains, making it difficult to miniaturize the final structure.
The rolling reduction ratio is (inside plate thickness-outside plate thickness) / entry side plate thickness x 100 (%), and the inter-pass time is the time from the end of this pass rolling to the start of the next pass rolling.

Continue to perform finish rolling. Finish rolling is the most important process from the viewpoint of refining the effective crystal grain size for crack propagation, which controls the arrest property, and is a tb / 4 portion from the surface of the steel sheet (tb: thickness of the steel piece before each rolling). ) Is in the range of Ar3 + 30 ° C. to Ar3 + 80 ° C., and in a plurality of passes for finish rolling, the average reduction rate of each pass is 5.0% or less, the cumulative reduction rate is 40% or more, and the average inter-pass time. Rolling may be performed under the condition of 25 seconds or less.
Ar3 is represented by the following formula 8.
Ar3 = 910-310 [C] +65 [Si] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] ... Equation 8
Here, [element symbol] indicates the content (mass%) of the element as described above.
If these conditions are not satisfied, the effective crystal grain size for crack propagation cannot be sufficiently refined, or coarse ferrite is formed on the surface layer portion, so that the arrest property is deteriorated.
The cumulative reduction rate is defined as (inside plate thickness of the first pass-outside plate thickness of the last pass) / entry of the first pass in a plurality of passes in which the tb / 4 portion from the surface of the steel plate is in a predetermined temperature range. It is the side plate thickness x 100 (%). Further, the average reduction rate of the relevant pass is the average of the reduction rate of each pass determined by (entry side plate thickness-outside plate thickness) / entry side plate thickness × 100 (%) in each of the above-mentioned plurality of passes. ..

After the finish rolling is completed, the temperature of the t / 4 portion (t: plate thickness) from the surface of the steel plate may be cooled from Ar3 or more to 400 ° C. or less at a cooling rate of 5 ° C./sec or more on average.

When the cooling start temperature falls below Ar3, the coarse ferrite area ratio of the surface layer portion exceeds 10%, and the arrest property deteriorates. If the cooling rate is less than 5 ° C / sec or the cooling stop temperature is higher than 400 ° C, not only the strength is insufficient, but also the crack propagation effective grain size is insufficiently refined, or pearlite is 5%. It is super-generated and the arrest property is reduced. By the above manufacturing method, a steel sheet having desired properties and excellent brittle crack propagation stopping characteristics can be obtained.

Further, after accelerated cooling, tempering heat treatment may be performed at a temperature of 300 to 600 ° C. in order to adjust the strength and toughness. If the heat treatment temperature is less than 300 ° C., the ductility and toughness are not sufficiently improved, and if the heat treatment temperature exceeds 600 ° C., cementite becomes coarse and the arrest property deteriorates.
The above manufacturing method is one aspect of the manufacturing method for obtaining the steel sheet according to the present invention, and the steel sheet according to the present invention is not limited to the manufacturing method according to this aspect.

Examples are shown below. The present invention is not limited to the following examples, and can be appropriately modified and implemented without significantly deviating from the gist thereof.

Using the steel pieces having the chemical components shown in Table 1, a steel sheet having a thickness of 50 to 100 mm was prototyped according to the manufacturing conditions of Table 2 (Tables 2-1 and 2-2 are collectively referred to as Table 2). .. Table 3 (Table 3-1 and Table 3-2 are collectively referred to as Table 3) shows the structure, base material strength, toughness, arrest property, and HAZ toughness.

The coarse ferrite area ratio of the surface layer region is an optical micrograph of the vertical cross section (RD plane) in the rolling direction for each depth of 1%, 2%, 3%, 4%, and 5% of the plate thickness from the surface of the steel plate. Calculated by analysis. Here, the coarse ferrite has a crystal grain equivalent diameter (diameter) of 25 μm or more, and the area ratio of the coarse ferrite was measured at each location. Then, the area ratios of the coarse ferrites at these five locations were averaged to obtain the coarse ferrite area ratio in the surface layer region.
The area ratios of ferrite, pearlite, and bainite were measured from optical micrographs of 5 mm below the surface of the steel sheet, 1/4 of the plate thickness, and the RD surface at the center of the plate thickness, and average values were calculated.
The diameter of the precipitate was 0.002 μm ² (0.002 × ^10-12) using a photograph taken at 20,000 times with a transmission electron microscope by preparing extraction replicas from three plate thickness positions similar to the above. 100 or more of the precipitates of m ² ) or more were randomly selected, and image analysis was performed on them. The average diameter of the circles of the precipitates corresponding to within 20% in terms of the number from the largest. The value was calculated.
That is, 100 or more of the precipitates having a circle-equivalent diameter (diameter) of 50 nm (50 × ^10-9 m) or more are randomly extracted, and the number of the precipitates corresponding to 20% or less is calculated from the largest. The average value of the circle equivalent diameter of was calculated.
For the effective crystal grain size for crack propagation, EBSD samples are taken from three plate thickness positions similar to the above so that the RD surface is the measurement surface, and a region of 500 × 500 μm is measured at a pitch of 1 μm, and then the crystal orientation. Grain boundaries were determined by performing orientation analysis over a total length of 2 mm based on the map, and calculated by a cutting method based on JIS G 0551 (2013).
For the strength of the base metal, the yield strength (YS) and the tensile strength (TS) were evaluated using a No. 4 tensile test piece of JIS Z 2241 (2011) collected from the center of the plate thickness in the TD direction.
For the toughness of the base metal, the fracture surface transition temperature (vTrs) was evaluated using a 2 mm V notch impact test piece of JIS Z 2242 (2005) collected in the RD direction from the center of the plate thickness.
For arrestability, based on the method described in WES 2815 (2014), a test piece having a total thickness of 500 mm width was collected and subjected to a temperature gradient type ESSO test, and the temperature showing Kca = 6000 N / mm ^1.5. Evaluated at.
For HAZ toughness, a submerged arc welded joint was prepared under the conditions of a reshaped groove with an angle of 30 °, a root gap of 10 mm, and a heat input of about 5 kJ / mm, and a notch was formed along the fusion line of the Charpy test piece collected from the back surface. It was put in and evaluated by the average absorbed energy when three tests were performed at −40 ° C.

No. of the example of the present invention. 1-12 , No. 15 to 22 have a microstructure within the range of the present invention because the chemical components are within a predetermined range and are manufactured under appropriate conditions, and all of them have sufficient strength as a 460 MPa class steel with a yield strength and are a base material. The toughness, arrest property, and HAZ toughness were also good.

On the other hand, No. 23-31 , No. In Nos. 34 to 50, any of strength, arrest property, and HAZ toughness could not be ensured because the chemical composition was out of the scope of the present invention or the production conditions were not appropriate.

No. 26, 30, and 39 are examples in which the heating conditions were not appropriate. No. In Nos. 26 and 39, the heating γ was coarsened because the pH exceeded the upper limit, and the crack propagation effective crystal grain size could not be miniaturized, so that the arrest property was lowered. No. In No. 30, since the pH was less than the lower limit, Nb was not sufficiently dissolved, the bainite area ratio was insufficient, the precipitate diameter was coarse, and the strength and arrestability were lowered.

No. 25 , 27 , 34 , and 35 are examples in which the rough rolling conditions were not appropriate. No. In No. 34, since the rolling end temperature was low, the recrystallization of γ became non-uniform, the crack propagation effective crystal grain size became coarse, and the arrest property deteriorated. No. In 25 and 35, since the reduction rate of the final 3 passes was small, the recrystallized γ could not be sufficiently refined, the crack propagation effective crystal grain size was increased, and the arrest property was lowered. No. In 27, the latter pass was under light pressure .

No. 23 , 29 , 31 , 37 are examples in which the conditions for finish rolling were not appropriate. No. In No. 23, the finish rolling was two-phase rolling, and the cumulative rolling reduction in the range of Ar3 + 30 ° C. to Ar3 + 80 ° C. was insufficient. Therefore, a large amount of coarse ferrite was generated on the surface layer portion, and the arrest property was deteriorated. No. 3 1, although was not a two-phase region rolling, in order to cumulative rolling reduction in the temperature range is not enough, arrestability was insufficient. No. In No. 29, since the average reduction rate in one pass was large, processing heat was generated and the CR effect was weakened, the crack propagation effective crystal grain size could not be miniaturized, and the arrest property was lowered. No. In 37, since the average pass time was long, γ was partially recrystallized, the crack propagation effective crystal grain size was coarsened, and the arrest property was lowered.

No. 24, 36, and 38 are examples in which the accelerated cooling conditions were not appropriate. No. Since the cooling stop temperature of No. 24 was high, the pearlite area ratio was large and the arrest property was lowered. No. Since the cooling rate of No. 36 was low, the effective crystal grain size for crack propagation was not refined, and the arrest property was lowered. No. Since 38 was air-cooled without accelerated cooling, the area ratios of ferrite and pearlite were excessive, the bainite area ratio was too small, the crack propagation effective crystal grain size was not refined, and the strength and arrestability were lowered.

No. In No. 28, since the heat treatment temperature was high, the cementite diameter became large, and sufficient arrestability could not be obtained.

No. 40 to 50 are examples in which the chemical composition deviates from a predetermined range. No. In No. 40, since the Ps value did not reach the lower limit, the crack propagation effective crystal grain size could not be sufficiently refined, and the yield strength and arrest property were low. No. Since the amount of C in 41 was excessive, cementite was coarsened, and arrestability and HAZ toughness were lowered. No. 42 is Si, No. 43 is Mn, No. 44 is P, No. 45 is S, No. 47 is Ti, No. 48 is Al, No. HAZ toughness decreased in 49 because N was excessive. No. No. 46 has an excess of Nb. Since the Ps value of 50 was excessive, the strength became too high and the arrest property and HAZ toughness decreased.

The steel plate according to the present invention can be widely applied to welded structures such as shipbuilding, construction, bridges, tanks, and marine structures.

Claims (5)

By mass%
C: 0.050 to 0.140%,
Si: 0.03 to 0.50%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.005 to 0.040%,
Ti: 0.005-0.030%,
Al: 0.001 to 0.100%,
N: Contains 0.0010 to 0.0080%,
The balance consists of Fe and unavoidable impurities, and the Ps value defined by the following formulas 1 to 3 is 0.40 to 0.54%.
The area ratio of the microstructure is ferrite: 20 to 50%,
Pearlite: 5% or less,
Bainite: Contains 40-80% and
In the surface layer region from the surface of the steel sheet to a depth of 5% of the plate thickness, ferrite having a crystal grain equivalent circle diameter of 25 μm or more has an area ratio of 10% or less.
Among the precipitates and inclusions having a circle equivalent diameter of 50 nm or more, those having a large circle equivalent diameter up to 20% in terms of number have an average circle equivalent diameter of 0.40 μm or less.
Further, in the crystal orientation analysis using EBSD in the vertical cross section in the rolling direction, the crack propagation effective crystal grain size is d (μm) or less of the following formula 4, and the plate thickness t (mm) is 50 mm or more. A steel sheet with excellent brittle crack propagation stopping characteristics.
Ps = Ceq + 3.6 × [Nb] + 79 × Bsol… Equation 1
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... Equation 2
Bsol = [B]-([N]-[Ti] x 14/48) x 11/14 ... Equation 3
d = (5.4 × [Ni] +16) × (1.2-t / 300)… Equation 4
Here, [element symbol] such as [C] indicates the content (mass%) of the element, and if it is not contained, 0 is substituted. If B is not contained or Bsol <0 in the formula 3, Bsol = 0 in the formula 1.
In addition, in% by mass,
Cu: 0.05 to 1.50%,
Ni: 0.05 to 2.00%,
Cr: 0.05 to 1.00%,
Mo: 0.02 to 0.50%,
V: 0.005 to 0.100%,
B: The steel sheet having excellent brittle crack propagation stopping characteristics according to claim 1, which contains one or more of 0.0002 to 0.0030%.
In addition, in% by mass,
Mg: 0.0003 to 0.0050%,
Ca: 0.0005 to 0.0030%,
Zr: 0.0005 to 0.0050%,
REM: The brittle crack propagation stopping property according to any one of claims 1 or 2, which is characterized by containing one or more of 0.0005 to 0.0100%. Steel plate.
The method for producing a steel sheet having excellent brittle crack propagation stopping characteristics according to any one of claims 1 to 3.
When heating a steel piece having the composition according to any one of claims 1 to 3, the lower limit heating temperature TL is based on the following formula 9, and the upper limit heating temperature TU is based on the following formula 10. Then, after the steel pieces are charged into the heating furnace, the holding starts when the temperature of the steel pieces reaches the heating lower limit temperature TL, and the temperature of the steel pieces changes from the heating lower limit temperature TL to the heating upper limit temperature TU. The temperature of the heating furnace is controlled so as to be maintained within the range, and the time from the start of holding to the extraction of the steel piece from the heating furnace is defined as the holding time tm (minutes), and the holding time tm (minutes). When the time average temperature of the temperature of the steel pieces is the heating temperature T (° C.), the heating temperature T (° C.) and the holding time tm (minutes) satisfy the following equations 5 to 7.
When rolling in a temperature range of 900 ° C. or higher, rolling is performed so that the rolling reduction of at least the final 3 passes is 10% or more, the time between passes is 15 seconds or less, and the rolling ratio is not lower than the rolling ratio of the previous pass.
Further, when the thickness of the steel piece before each rolling is tb, the temperature of the portion from the steel plate surface to tb / 4 is in the range of Ar3 + 30 ° C. to Ar3 + 80 ° C. using Ar3 represented by the following formula 8. In a plurality of passes to be rolled, after rolling under the conditions that the average reduction rate of each pass is 5.0% or less, the cumulative reduction rate is 40% or more, and the average interval time between passes is 25 seconds or less.
When the plate thickness of the obtained steel plate is t, the temperature of the portion t / 4 from the surface of the steel plate continues to be from the temperature of Ar3 or higher to the temperature of 400 ° C or lower, and the cooling rate of the average plate thickness is 5 ° C. A method for producing a steel sheet having excellent brittle crack propagation stopping characteristics, which is characterized by cooling with.
57000 / {1.2-0.16 × log ([C] [Nb])}
≤ PH
≤84000 / (1.9-0.18 x log [Ti]) ... Equation 5
PH = (T + 273) × {log (tm) +25}… Equation 6
tm ≧ 30… Equation 7
Ar3 = 910-310 [C] +65 [Si] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] ... Equation 8
TL = 57000 / {1.2-0.16 × log ([C] [Nb])} / {log (30) +25} -273 ... Equation 9
TU = 84000 / (1.9-0.18 × log [Ti]) / {log (30) +25} -273 ... Equation 10
Here, [element symbol] such as [C] indicates the content (mass%) of the element, and if it is not contained, 0 is substituted.
The method for producing a steel sheet having excellent brittle crack propagation stopping characteristics according to claim 4, further comprising heat treatment at a temperature of 300 to 600 ° C.