Thick-walled, high tensile strength steel with excellent CTOD characteristics of the weld heat-affected zone, and manufacturing method thereof

Abstract

A thick-walled high-strength steel plate with excellent low-temperature toughness (Charpy impact and CTOD properties of a weld bond) in a multilayer weld zone, and a method for manufacturing the steel plate.

Claims

1. A thick-walled high-strength steel plate with an excellent low-temperature toughness property (“CTOD property”) of a heat-affected zone, the steel plate having a chemical composition comprising: C: 0.020% or more and 0.080% or less by mass %; Si: 0.01% or more and 0.35% or less by mass %; Mn: 1.20% or more and 2.30% or less by mass %; P: 0.008% or less by mass %; S: 0.0035% or less by mass %; Al: 0.010% or more and 0.060% or less by mass %; Cu: 0.70% or more and 1.50% or less by mass %; Ni: 0.40% or more and 2.00% or less by mass %; Nb: 0.005% or more and 0.040% or less by mass %; Ti: 0.005% or more and 0.025% or less by mass %; N: 0.0020% or more and 0.0050% or less by mass %; O: 0.0030% or less by mass %; and the balance being Fe and inevitable impurities, wherein (a) Ceq defined by the relational expression (1) is 0.520% or less,
Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5,  (1) where [M] represents the content (mass %) of the chemical element represented by M, (b) Ti/N (mass ratio) is 1.50 or more and 4.00 or less, (c) relational expression (2) is satisfied,
5.5[C].sup.4/3+15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb].sup.1/2+0.53[Mo]≦3.50  (2), where [M] represents the content (mass %) of the chemical element represented by M, and (d) the hardness of a center segregation part satisfies relational expression (3)
H.sub.Vmax/H.sub.Vave≦1.35+0.006/[C]−t/500,  (3) where H.sub.Vmax represents a maximum value of the Vickers hardness of a center segregation part, H.sub.Vave represents an average value of the Vickers hardness of portions other than portions within ¼ of a thickness from upper and lower surfaces of the steel plate and the center segregation part, [C] represents C content (mass %), and t represents the thickness of the steel plate (mm).

2. The thick-walled high-strength steel plate according to claim 1, the chemical composition further comprising at least one of the group (e) or (f), wherein (e) the chemical composition further comprising at least one of Cr: 0.10% or more and 1.00% or less by mass %, Mo: 0.05% or more and 0.50% or less by mass %, and V: 0.005% or more and 0.050% or less by mass %, and (f) Ca: 0.0005% or more and 0.0050% or less by mass %, wherein relational expression (4) is satisfied,
0<{[Ca]−(0.18+130×[Ca])×[O]}/1.25/[S]<1.00,  (4) where [M] represents the content (mass %) of the chemical element represented by M.

3. A method for manufacturing a thick-walled high-strength steel plate with an excellent CTOD property of a heat-affected zone, the method comprising: heating steel having the chemical composition according to claim 1 at a temperature of 1030° C. or higher and 1200° C. or lower; performing hot rolling on the heated steel such that the cumulative rolling reduction in a temperature range of 950° C. or higher is 30% or more and that the cumulative rolling reduction in a temperature range of lower than 950° C. is 30% or more and 70% or less; performing accelerated cooling on the hot-rolled steel plate down to a temperature of 600° C. or lower at a cooling rate of 1.0° C./s or more; and performing a tempering treatment on the cooled steel plate at a temperature of 450° C. or higher and 650° C. or lower.

4. The thick-walled high-strength steel plate according to claim 1, wherein the steel plate has a yield stress of 420 MPa or more.

5. The thick-walled high-strength steel plate according to claim 1, wherein the steel plate has a tensile strength of 520 MPa or more.

6. The thick-walled high-strength steel plate according to claim 1, wherein the steel plate has a thickness in the range of 50 to 100 mm.

7. The thick-walled high-strength steel plate according to claim 1, wherein the steel plate exhibits an absorbed energy at a temperature of −40° C. (vE.sub.−40° C.) of 200 J or more.

8. A method for manufacturing a thick-walled high-strength steel plate with an excellent CTOD property of a heat-affected zone, the method comprising: heating steel having the chemical composition according to claim 2 at a temperature of 1030° C. or higher and 1200° C. or lower; performing hot rolling on the heated steel such that the cumulative rolling reduction in a temperature range of 950° C. or higher is 30% or more and that the cumulative rolling reduction in a temperature range of lower than 950° C. is 30% or more and 70% or less; performing accelerated cooling on the hot-rolled steel plate down to a temperature of 600° C. or lower at a cooling rate of 1.0° C./s or more; and performing a tempering treatment on the cooled steel plate at a temperature of 450° C. or higher and 650° C. or lower.

9. The thick-walled high-strength steel plate according to claim 2, wherein the steel plate has a yield stress of 420 MPa or more.

10. The thick-walled high-strength steel plate according to claim 2, wherein the steel plate has a tensile strength of 520 MPa or more.

11. The thick-walled high-strength steel plate according to claim 2, wherein the steel plate has a thickness in the range of 50 to 100 mm.

12. The thick-walled high-strength steel plate according to claim 2, wherein the steel plate exhibits an absorbed energy at a temperature of −40° C. (vE.sub.−40° C.) of 200 J or more.

13. The thick-walled high-strength steel plate according to claim 1, wherein the chemical composition comprises Cu: 0.80% or more and 1.50% or less by mass %.

14. The thick-walled high-strength steel plate according to claim 2, wherein the chemical composition comprises Cu: 0.80% or more and 1.50% or less by mass %.

Description

DETAILED DESCRIPTION OF EMBODIMENTS

(1) In embodiments, a chemical composition and hardness distribution in the thickness direction are specified.

(2) 1. Chemical composition

(3) The reason for limitations on the chemical composition will be described. In the description, % represents mass %.

(4) C: 0.020% or more and 0.080% or less

(5) C is a chemical element which is necessary to achieve sufficient strength for the base metal of a high-strength steel plate. In the case where the C content is less than 0.020%, there is a decrease in hardenability. In addition, when sufficient strength is intended to be achieved for a base metal in the case where the C content is less than 0.020%, it is necessary to add large amounts of chemical elements such as Cu, Ni, Cr, and Mo which increase hardenability in order to achieve sufficient strength. Accordingly, there is an increase in cost in the case where the C content is less than 0.020%. In addition, in the case where the C content is more than 0.080%, there is a decrease in weldability and there is a significant decrease in the toughness of a weld zone. Therefore, the C content is set to be 0.020% or more and 0.080% or less, preferably 0.020% or more and 0.070% or less, more preferably 0.020% or more and 0.060% or less, or most preferably 0.020% or more and less than 0.050%.

(6) Si: 0.01% or more and 0.35% or less

(7) Si is a component which is added as a deoxidizing agent in order to achieve sufficient strength for a base metal. Therefore, the Si content is set to be 0.01% or more. However, in the case where the Si content is more than 0.35%, there is a decrease in weldability, and in addition, there is also a decrease in the toughness of a welded joint. It is necessary that the Si content be 0.01% or more and 0.35% or less, preferably 0.23% or less.

(8) Mn: 1.20% or more and 2.30% or less

(9) Mn is a chemical element which is used to achieve sufficient strength for a base metal and a welded joint, and the Mn content is set to be 1.20% or more. However, in the case where the Mn content is more than 2.30%, there is a decrease in weldability, and in addition, there is a decrease in the toughness of a base metal and a welded joint due to hardenability becoming excessively large. Therefore, the Mn content is set to be 1.20% or more and 2.30% or less, preferably more than 1.50% and 2.30% or less.

(10) P: 0.008% or less

(11) P, which is an impurity element, decreases the toughness of a base metal and a weld zone. In particular, in the case where the P content is more than 0.008% in a weld zone, there is a significant decrease in a CTOD property. Therefore, the P content is set to be 0.008% or less, preferably 0.005% or less, or more preferably 0.004% or less. In order to decrease the P content like this, it is necessary to perform an operation for purposefully decreasing the P content, such as one using light reduction rolling performed in a continuous casting process or electromagnetic stirring performed on the downstream side of a continuous casting machine, for example.

(12) S: 0.0035% or less

(13) S is an impurity which is inevitably mixed in. In the case where the S content is more than 0.0035%, there is a decrease in the toughness of a base metal and a weld zone. Therefore, the S content is set to be 0.0035% or less, preferably 0.0030% or less.

(14) Al: 0.010% or more and 0.060% or less

(15) Al is a chemical element which is added in order to dioxidize molten steel, and it is necessary that the Al content be 0.010% or more. On the other hand, in the case where the Al content is more than 0.060%, there is a decrease in the toughness of a base metal and a weld zone, and in addition, since Al is mixed into a weld metal through dilution when welding is performed, there is a decrease in toughness. Therefore, the Al content is set to be 0.060% or less, preferably 0.017% or more and 0.055% or less, more preferably more than 0.015% and 0.055% or less, or most preferably more than 0.020% and 0.055% or less. Here, in embodiments, the Al content is specified in terms of acid-soluble Al (also referred to as, for example, Sol.Al).

(16) Cu: 0.70% or more and 1.50% or less

(17) Cu can increase the strength of a base metal as a result of being finely precipitated. In order to realize this effect, the Cu content is set to be 0.70% or more. On the other hand, since there is a decrease in hot ductility in the case where the Cu content is more than 1.50%, the Cu content is limited to 1.50% or less, preferably 0.80% or more and 1.30% or less.

(18) Ni: 0.40% or more and 2.00% or less

(19) Ni is a chemical element which is effective for increasing the strength and toughness of steel and for improving a CTOD property of a weld zone. In order to realize these effects, it is necessary that the Ni content be 0.40% or more. However, Ni is an expensive chemical element, and defects tend to occur on the surface of a slab when casting is performed in the case where the Ni content is excessively large. Therefore, the upper limit of the Ni content is set to be 2.00%.

(20) Nb: 0.005% or more and 0.040% or less

(21) Since Nb forms a non-recrystallization region in a low temperature range for forming an austenite phase, when performing rolling in this temperature range, Nb contributes to a decrease in the grain diameter of the microstructure of a base metal and to an increase in toughness. In addition, in the case where Nb is added, an effect of precipitation strengthening can be realized by performing air cooling after rolling and cooling have been performed or by performing a tempering treatment thereafter. In order to realize these effects, it is necessary that the Nb content be 0.005% or more, preferably more than 0.013%. However, since there is a decrease in toughness in the case where the Nb content is more than 0.040%, the upper limit of the Nb content is set to be 0.040%, preferably 0.035%.

(22) Ti: 0.005% or more and 0.025% or less

(23) Ti is precipitated in the form of TiN when molten steel is solidified and contributes to an increase in the toughness of a weld zone by suppressing an increase in the austenite grain diameter in a weld zone. However, there is only a small effect in the case where the Ti content is less than 0.005%, and on the other hand, an effect of increasing the toughness of a base metal and a weld zone cannot be realized in the case where the Ti content is more than 0.025% due to an excessive increase in the grain diameter of TiN. Therefore, the Ti content is set to be 0.005% or more and 0.025% or less.

(24) N: 0.0020% or more and 0.0050% or less

(25) N increases the toughness of a base metal by decreasing the crystal grain diameter as a result of forming precipitates in combination with Ti and Al. In addition, N is a chemical element which is necessary to form TiN which suppresses an increase in the grain diameter in the microstructure of a weld zone. In order to realize these effects, it is necessary that the N content be 0.0020% or more. On the other hand, since there is a significant decrease in the toughness of a base metal and a weld zone due to solid solute N in the case where the N content is more than 0.0050%, the upper limit of the N content is set to be 0.0050%.

(26) O: 0.0030% or less

(27) Since there is a decrease in the toughness of a base metal in the case where the O content is more than 0.0030%, the O content is set to be 0.0030% or less, preferably 0.0020% or less.

(28) Ceq: 0.520% or less

(29) In the case where Ceq which is defined by relational expression (1) is more than 0.520%, there is a decrease in weldability and the toughness of a weld zone, and therefore, Ceq is set to be 0.520% or less, preferably 0.500% or less.
Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])5  (1),
where symbol [M] represents the content (mass %) of the chemical element represented by symbol M, and where symbol [M] is assigned a value of 0 in the case where the chemical element represented by symbol M is not added.

(30) Ti/N: 1.50 or more and 4.00 or less

(31) In the case where Ti/N is less than 1.50, there is a decrease in the amount of TiN formed, and therefore, solid solute N which is retained without forming TiN decreases the toughness of a weld zone. In addition, in the case where Ti/N is more than 4.00, there is a decrease in the toughness of a weld zone due to an increase in the grain diameter of TiN. Therefore, Ti/N is set to be 1.50 or more and 4.00 or less, preferably 1.80 or more and 3.50 or less. In the expression Ti/N, symbols Ti and N respectively represent the contents (mass %) of the corresponding chemical elements.
5.5[C].sup.4/3+15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb].sup.1/2+0.53[Mo]≦3.50  (2),
where symbol [M] represents the content (mass %) of the chemical element represented by symbol M.

(32) The left-hand side value of relational expression (2) is an index of the hardness of a center segregation part which is composed of components which tend to be concentrated in the center segregation part, and will be referred to as a Ceq* value in the description below. Since a CTOD test is performed using a test piece including the whole thickness of a steel plate, the test piece includes a center segregation part. Therefore, in the case where the components are significantly concentrated in the center segregation part, a hardened region is formed in a heat-affected zone, and it is not possible to achieve a good CTOD property. By controlling a Ceq* value to be within an appropriate range, it is possible to suppress an excessive increase in hardness in a center segregation part, and an excellent CTOD property can be obtained even in the weld zone of a steel material having a large thickness. The appropriate range of a Ceq* value was empirically obtained, and, since there is a decrease in CTOD property in the case where a Ceq* value is more than 3.50, the Ceq* value is set to be 3.50 or less, preferably 3.20 or less.

(33) Although the basic chemical composition of the heavy wall thickness high-strength steel plate according to embodiments is as described above and the balance consists of Fe and inevitable impurities, the heavy wall thickness high-strength steel plate may further contain one, two or more selected from among Cr: 0.10% or more and 1.00% or less, Mo: 0.05% or more and 0.50% or less, and V: 0.005% or more and 0.050% or less in order to further improve properties.

(34) Cr: 0.10% or more and 1.00% or less

(35) Cr is a chemical element which is effective for increasing the strength of a base metal, and it is preferable that the Cr content be 0.10% or more in order to realize this effect. However, in the case where the Cr content is excessively large, there is a negative effect on toughness, and therefore, it is preferable that the Cr content be 0.10% or more and 1.00% or less in the case where Cr is added, more preferably 0.20% or more and 0.80% or less.

(36) Mo: 0.05% or more and 0.50% or less

(37) Mo is a chemical element which is effective for increasing the strength of a base metal, and it is preferable that the Mo content be 0.05% or more in order to realize this effect. However, in the case where the Mo content is excessively large, there is a negative effect on toughness, and therefore, it is preferable that the Mo content be 0.05% or more and 0.50% or less in the case where Mo is added, more preferably 0.08% or more and 0.40% or less.

(38) V: 0.005% or more and 0.050% or less

(39) V is a chemical element which is effective for increasing the strength and toughness of a base metal in the case where the V content is 0.005% or more. Since there is a decrease in toughness in the case where the V content is more than 0.050%, it is preferable that the V content be 0.005% or more and 0.050% or less in the case where V is added.

(40) In addition to the components described above, the chemical composition according to embodiments may further contain Ca: 0.0005% or more and 0.0050% or less.

(41) Ca: 0.0005% or more and 0.0050% or less

(42) Ca is a chemical element which increases toughness by fixing S. In order to realize this effect, it is necessary that the Ca content be at least 0.0005%. However, in the case where the Ca content is more than 0.0050%, since the effect described above corresponding to the amount of Ca added becomes saturated, it is preferable that the Ca content be 0.0005% or more and 0.0050% or less.
0<{[Ca]−(0.18+130×[Ca])×[O]}/1.25/[S]<1.00  (4),
where symbol [M] represents the content (mass %) of the chemical element represented by symbol M.

(43) {[Ca]−(0.18+130×[Ca])×[O]}/1.25[S] is a value which indicates a ratio of the atomic concentration of Ca which is effective for controlling the form of sulfides to the atomic concentration of S and is also referred to as an ACR (Atomic Concentration Ratio). Using this value, it is possible to estimate the form of sulfides, and the range of an ACR is specified in order to finely disperse ferrite transformation nucleation sites, that is, CaS which does not dissolve even at a high temperature. [Ca], [S], and [O] respectively represent the content (mass %) of the corresponding chemical elements in relational expression (4).

(44) In the case where the ACR value is 0 or less, CaS does not crystallize. Therefore, since S is precipitated in the form of MnS only, a ferrite nucleation site cannot be formed in a heat-affected zone. In addition, since MnS which has been solely precipitated is elongated when rolling is performed, there is a decrease in the toughness of a base metal.

(45) On the other hand, in the case where the ACR value is 1.0 or more, S is completely fixed by Ca, and MnS which functions as a ferrite nucleation site is not precipitated on CaS. Therefore, complex sulfides cannot realize the fine dispersion of ferrite nucleation sites, which results in an effect of an increase in toughness not being realized.

(46) In the case where the ACR value is more than 0 and less than 1.0, since a complex sulfide is formed as a result of MnS being precipitated on CaS, the complex sulfide can effectively function as a ferrite nucleation site. Here, it is preferable that the ACR value be 0.20 to 0.80.

(47) 2. Hardness Distribution
H.sub.Vmax/H.sub.Vave≦1.35+0.006/[C]−t/500  (3)

(48) H.sub.Vmax represents a maximum value of the Vickers hardness of a center segregation part, H.sub.Vave represents an average value of the Vickers hardness of the portions other than portions within ¼ of the thickness from the upper and lower surfaces of the steel plate and the center segregation part, [C] represents C content (mass %), and t represents the thickness of the steel plate (mm). H.sub.Vmax/H.sub.Vave is a dimensionless parameter which indicates the hardness of a center segregation part, and, since there is a decrease in a CTOD value in the case where H.sub.Vmax/H.sub.Vave is more than a value derived by 1.35+0.006/[C]−t/500, H.sub.Vmax/H.sub.Vave is set to be equal to or less than 1.35+0.006/[C]−t/500, preferably equal to or less than 1.25+0.006/[C]−t/500.

(49) H.sub.Vmax is the hardness of a center segregation part and is defined as the maximum value among the values obtained by determining the hardness at intervals of 0.25 mm in the thickness direction in an area including the center segregation part having a length of (thickness/40) mm in the thickness direction using a Vickers hardness testing machine (with a load of 10 kgf). In addition, H.sub.Vave is an average value of hardness and is defined as the average value of the values obtained by determining hardness at certain intervals (for example, 1 to 2 mm) in the thickness direction in an area between a position located at ¼ of the thickness on the upper surface side and a position located at ¼ of the thickness on the lower surface side excluding a center segregation part using a Vickers hardness testing machine with a load of 98 N (10 kgf).

(50) The condition expressed by relational expression (3) may be satisfied without difficulty by selecting casting conditions in order to decrease the degree of center segregation, by controlling the contents of chemical elements, which tend to occur segregation, to be as small as possible, and, regarding rolling conditions, by performing heating for rolling at a low temperature and performing finishing rolling at a low temperature in order to prevent the grain diameter of a bainite structure from increasing in the central part of the thickness.

(51) Subsequently, the microstructure of the heavy wall thickness high-strength steel plate according to embodiments will be described. The microstructure of the heavy wall thickness high-strength steel plate according to embodiments mainly includes 10 vol % or more of an acicular ferrite phase, 5 vol % or more and 50 vol % or less of a bainite phase, and 10 vol % or less of a polygonal ferrite phase.

(52) Acicular ferrite phase: 10 vol % or more

(53) It is preferable that the amount of an acicular ferrite phase be 10 vol % or more, because sufficient strength and toughness for a base metal can be achieved.

(54) Bainite phase: 5 vol % or more and 50 vol % or less

(55) It is preferable that the amount of a bainite phase be 5 vol % or more, because high strength can be achieved. It is preferable that the amount of a bainite phase be 50 vol % or less, because sufficient toughness for a base metal can be achieved.

(56) Polygonal ferrite phase: 10 vol % or less

(57) It is preferable that the amount of a polygonal ferrite phase be 10 vol % or less, because high strength can be achieved.

(58) Examples of microstructures other than those described above include a martensite-austenite constituent, a perlite phase, and a cementite phase, and it is preferable that the total amount of these microstructures be 10 vol % or less.

(59) In addition, the amount of each of the microstructures described above is defined as the amount (vol %) determined by performing image analysis on a photograph taken at a position located at ¼ of the thickness of a heavy wall thickness high-strength steel plate using a scanning electron microscope.

(60) It is preferable that the steel plate according to embodiments be manufactured using the manufacturing method described below. When using steel having the chemical composition described above as a raw material and manufacturing the steel plate using the preferable manufacturing method described below, there is a tendency for relational expression (3) to be satisfied.

(61) Molten steel having a chemical composition within the range according to embodiments is smelted using a common method such as one using a converter furnace, an electric furnace, or a vacuum melting furnace, the smelted steel is made into a slab using a continuous casting process, the slab is made into a steel plate having a desired thickness by performing hot rolling, the hot-rolled steel plate is cooled, and a tempering treatment is performed thereafter. Regarding hot rolling, a slab heating temperature, a rolling reduction, a finishing temperature, a cooling rate after hot rolling has been performed, and a tempering temperature are specified.

(62) Here, in embodiments, the temperature condition of a steel plate is specified in terms of the temperature of the central part of the steel plate, unless otherwise noted. The temperature of the central part in the thickness direction can be derived from, for example, the thickness, the surface temperature, and the cooling conditions using, for example, simulation calculation. For example, by calculating the temperature distribution in the thickness direction using a difference method, the temperature of the central portion in the thickness direction of the steel plate can be derived.

(63) Slab heating temperature: 1030° C. or higher and 1200° C. or lower

(64) The slab heating temperature is set to be 1030° C. or higher in order to certainly bond casting defects inside a slab by pressure by performing hot rolling. In addition, since there is a decrease in the toughness of a base metal and a weld zone due to an increase in the grain size of TiN which has been precipitated at the time of solidification in the case where the slab heating temperature is higher than 1200° C., the upper limit of the slab heating temperature is set to be 1200° C.

(65) Cumulative rolling reduction of hot rolling in a temperature range of 950° C. or higher: 30% or more

(66) The cumulative rolling reduction of hot rolling in a temperature range of 950° C. or higher is set to be 30% or more in order to form a fine microstructure through the use of recrystallization of austenite grains. In the case where the cumulative rolling reduction described above is less than 30%, since abnormally large grains which are formed when heating is performed are retained, there is a negative effect on the toughness of a base metal.

(67) Cumulative rolling reduction of hot rolling in a temperature range of lower than 950° C.: 30% or more and 70% or less

(68) Since austenite grains which are rolled in this temperature range do not recrystallize sufficiently, the austenite grains after hot rolling has been performed remain deformed in a flattened shape, and there is a large amount of internal strain including a large amount of defects such as a deformation zone. These defects function as a driving force of ferrite transformation so as to promote ferrite transformation.

(69) However, in the case where the cumulative rolling reduction of hot rolling in a temperature range of lower than 950° C. is less than 30%, since there is insufficient accumulation of internal energy caused by internal strain, ferrite transformation is less likely to occur, which results in a decrease in the toughness of a base metal. On the other hand, in the case where the cumulative rolling reduction described above is more than 70%, since formation of polygonal ferrite is promoted, high strength and high toughness cannot be achieved at the same time.

(70) Finishing temperature: 650° C. or higher and 790° C. or lower

(71) It is preferable that the finishing temperature be 650° C. or higher in hot rolling, because sufficient strength and toughness of a base metal can be achieved. It is preferable that the finishing temperature be 790° C. or lower, because there is an increase in the toughness of a base metal. In particular, in embodiments, it is preferable that the finishing temperature be 700° C. or higher and 780° C. or lower.

(72) Cooling rate down to a temperature of 600° C. or lower: 1.0° C./s or more

(73) After hot rolling has been performed, accelerated cooling is performed down to an arbitrary temperature of 600° C. or lower at a cooling rate of 1.0° C./s or more. In the case where the cooling rate is less than 1° C./s, sufficient strength for a base metal cannot be achieved. In addition, in the case where cooling is stopped at a temperature higher than 600° C., since there is an increase in the fraction of a ferrite+pearlite structure (the total of a ferrite fraction (vol %) and a pearlite fraction (vol %)), high strength and high toughness cannot be achieved at the same time. In addition, in embodiments, it is preferable that a cooling stop temperature be lower than 280° C., because there is an increase in the strength of a base metal, more preferably 250° C. or lower in particular. Here, there is no limitation on the lower limit of the cooling stop temperature of accelerated cooling.

(74) Tempering temperature: 450° C. or higher and 650° C. or lower

(75) In the case where the tempering temperature is lower than 450° C., there is insufficient tempering effect. On the other hand, it is not preferable that the tempering temperature be higher than 650° C., because there is a decrease in toughness due to an increase in the grain diameters of carbonitride and Cu precipitations, and because there may be a decrease in strength. In addition, it is more preferable that tempering be performed using induction heating, because an increase in the grain diameter of carbides is prevented in the tempering. In order to realize this effect, the temperature of the central part of a steel plate which is calculated using simulation such as a difference method is controlled to be 450° C. or higher and 650° C. or lower.

(76) In the case of the steel according to embodiments, since there is a decrease in the grain diameter of a microstructure in a heat-affected zone by suppressing an increase in the diameter of austenite grains in a heat-affected zone and by finely dispersing ferrite transformation nucleation sites which are not dissolved even at a high temperature, high toughness can be achieved. Also, in a region which is reheated in a temperature range in which a dual phase is formed due to a thermal history when multilayer welding is performed, since the grain diameter of a microstructure in a heat-affected zone is decreased by a first welding pass, there is an increase in the toughness of an untransformed region in the region reheated in a dual-phase temperature range, and there is a decrease in the diameter of retransformed austenite grains. Therefore, it is possible to decrease the degree of a decrease in toughness.

Examples

(77) By preparing continuously cast slabs Nos. A to A1 having the chemical compositions given in Table 1, and by performing hot rolling and a heat treatment on the slabs, thick steel plates having a thickness of 50 to 100 mm were prepared.

(78) In the case of slabs having a P content of 0.005% or less, the degree of segregation was purposefully decreased by performing light reduction rolling in a continuous casting process and by performing electromagnetic stirring on the downstream side of a continuous casting machine.

(79) Microstructure observation was conducted on all the steel plates. The microstructures of the steel plates of the examples of disclosed embodiments mainly included 10 vol % or more of an acicular ferrite phase, 5 vol % or more and 50 vol % or less of a bainite phase, and 10 vol % or less of a polygonal ferrite phase. In the case of the microstructures of the comparative examples, one or more of an acicular ferrite fraction, a bainite fraction, and a polygonal ferrite fraction were out of the range according to embodiments.

(80) A base metal was evaluated using yield stress (YP), tensile strength (TS), and absorbed energy at a temperature of −40° C., that is, vE.sub.−40° C. Yield stress (YP) and tensile strength (TS) were determined using a JIS No. 4 tensile test piece which was sampled from the position located at ½ of the thickness of the steel plate such that the longitudinal direction of the test piece is at a right angle to the rolling direction of the steel plate. In addition, absorbed energy at a temperature of −40° C., that is, vE.sub.−40° C. was determined by performing a Charpy impact test using a JIS V notch test piece which was sampled from the position located at ½ of the thickness of the steel plate such that the longitudinal direction of the test piece is at a right angle to the rolling direction of the steel plate. In the case where all the conditions YP≧420 MPa, TS≧520 MPa, and vE.sub.−40° C.≧200 J were satisfied, base metal properties were evaluated as satisfactory.

(81) The toughness of a weld zone was evaluated using absorbed energy at a temperature of −40° C., that is, vE.sub.−40° C. and a CTOD value at a temperature of −10° C., that is, δ.sub.−10° C. Absorbed energy at a temperature of −40° C., that is, vE.sub.−40° C. was determined using a test piece which was sampled from a welded joint prepared by performing multilayer welding using submerged arc welding with a welding heat input of 45 to 50 kJ/cm using a K type groove so that a weld bond on the straight side located at ¼ of the thickness of the steel plate corresponded to a notch position of a Charpy impact test. In the case where the average value of three determined values of vE.sub.−40° C. satisfied the condition vE.sub.−40° C.≧150 J, the toughness of the welded joint was evaluated as satisfactory. A CTOD value at a temperature of −10° C., that is, δ.sub.−10° C. was determined using a test piece which was sampled so that a weld bond on the straight side corresponded to a notch position of a three-point bend CTOD test piece. In the case where the minimum value among CTOD values (δ.sub.−10° C.) for three test pieces was 0.70 mm or more, the CTOD property of the welded joint was evaluated as satisfactory.

(82) The evaluation of the toughness of a weld zone (a Charpy impact test of a weld bond and a three-point bend CTOD test of a weld bond) was conducted on the steel plates whose base metal properties were evaluated as satisfactory with some exceptions.

(83) Base metal properties, the results of the Charpy impact test, and the results of the CTOD test described above are given in Table 2 along with hot rolling conditions and heat treatment conditions.

(84) Steel codes A to E are the examples of disclosed embodiments, and steel codes F to Z are comparative examples whose chemical composition regarding any of elements therein is out of the range according to embodiments. In the case of comparative example No. 32 where steel Al was used, although the chemical composition was in the range according to embodiments, relational expression H.sub.Vmax/H.sub.Vave≧1.35+0.006/[C]−t/500 was not satisfied. In the cases of No. 1, 2, 5, 6, 8, and 11 all of which were the examples of embodiments, and the results of a Charpy impact test of a weld bond and the results of a three-point bend CTOD test of a weld bond satisfied the objects.

(85) On the other hand, in the case of comparative examples 3, 4, 7, 9, 10, and 12 to 32, the chemical compositions and/or the manufacturing conditions were out of the ranges according to embodiments, base metal properties, the results of a Charpy impact test of a weld bond, or the results of a three-point bend CTOD test did not satisfy the objects. In the case of comparative example No. 32 where steel A1 was used, where the chemical composition was in the range according to embodiments, and where relational expression H.sub.Vmax/H.sub.Vave≦1.35+0.006/[C]−t/500 was not satisfied, the results of a Charpy impact test of a weld bond and the results of a three-point bend CTOD test did not satisfy the objects.

(86) TABLE-US-00001 TABLE 1 (mass %) Code C Si Mn P S Al Cu Ni Nb Ti N Cr Mo A 0.062 0.08 1.57 0.004 0.0016 0.016 0.98 0.87 0.017 0.012 0.0035 0.23 B 0.045 0.09 1.36 0.002 0.0015 0.014 0.94 0.64 0.029 0.009 0.0029 0.15 C 0.035 0.20 1.53 0.003 0.0016 0.027 0.91 0.86 0.031 0.008 0.0032 0.31 D 0.030 0.18 1.78 0.007 0.0015 0.021 1.07 0.89 0.015 0.009 0.0033 E 0.033 0.13 1.35 0.004 0.0018 0.039 0.92 0.53 0.027 0.011 0.0035 0.21 F 0.097 0.18 1.45 0.002 0.0012 0.036 0.91 0.68 0.015 0.007 0.0023 G 0.066 0.44 1.33 0.006 0.0021 0.022 0.95 0.56 0.031 0.010 0.0037 H 0.035 0.12 1.07 0.002 0.0013 0.022 0.85 0.56 0.022 0.009 0.0026 0.19 I 0.022 0.09 2.58 0.004 0.0018 0.021 0.84 0.75 0.017 0.008 0.0032 J 0.052 0.05 1.41 0.021 0.0015 0.029 0.97 1.13 0.017 0.008 0.0034 K 0.040 0.20 1.45 0.003 0.0018 0.089 1.04 0.69 0.025 0.007 0.0020 L 0.026 0.20 1.36 0.006 0.0024 0.040 0.58 0.56 0.019 0.008 0.0037 0.23 M 0.074 0.09 1.59 0.006 0.0020 0.022 1.79 0.61 0.024 0.009 0.0035 N 0.040 0.14 1.68 0.003 0.0017 0.025 0.92 0.58 0.052 0.011 0.0033 P 0.060 0.19 1.61 0.006 0.0017 0.020 0.93 1.02 0.028 0.029 0.0029 Q 0.065 0.09 1.53 0.004 0.0007 0.013 1.10 1.45 0.023 0.013 0.0064 R 0.041 0.17 1.65 0.005 0.0021 0.012 1.04 1.46 0.013 0.008 0.0023 1.12 S 0.034 0.05 1.47 0.005 0.0015 0.033 1.05 1.43 0.020 0.011 0.0033 0.82 T 0.050 0.21 1.55 0.004 0.0011 0.012 1.01 0.66 0.029 0.008 0.0025 U 0.050 0.24 1.62 0.002 0.0008 0.027 1.14 0.74 0.027 0.010 0.0031 V 0.067 0.18 1.47 0.006 0.0009 0.031 1.06 1.26 0.017 0.008 0.0035 W 0.068 0.29 2.09 0.004 0.0020 0.023 1.01 1.41 0.014 0.007 0.0021 0.32 0.16 X 0.045 0.07 1.44 0.005 0.0026 0.021 0.90 1.13 0.014 0.006 0.0048 Y 0.070 0.23 1.58 0.004 0.0025 0.033 0.98 0.91 0.020 0.018 0.0022 Z 0.078 0.22 2.01 0.008 0.0019 0.017 1.11 0.99 0.032 0.011 0.0039 0.32 A1 0.074 0.24 1.51 0.004 0.0019 0.017 1.11 1.15 0.031 0.009 0.0027 Ceq Expression Expression (4) Code V Ca O (1) Ti/N (2) ACR Note A 0.0021 0.0017 0.493 3.43 2.74 0.66 Example B 0.0010 0.407 3.10 2.84 Example C 0.0011 0.0018 0.470 2.50 2.98 0.26 Example D 0.015 0.0012 0.460 2.73 2.83 Example E 0.017 0.0024 0.0015 0.400 3.14 2.69 0.74 Example F 0.0012 0.445 3.04 2.63 Comparative Example G 0.0014 0.0021 0.388 2.70 2.89 0.24 Comparative Example H 0.0011 0.345 3.46 2.29 Comparative Example I 0.0017 0.0021 0.558 2.50 3.54 0.38 Comparative Example J 0.0013 0.427 2.35 2.86 Comparative Example K 0.026 0.0011 0.0013 0.402 3.50 2.76 0.30 Comparative Example L 0.0024 0.375 2.16 2.51 Comparative Example M 0.0022 0.499 2.57 2.99 Comparative Example N 0.0021 0.0011 0.420 3.33 2.57 0.75 Comparative Example P 0.0014 0.458 10.00  3.11 Comparative Example Q 0.0014 0.0020 0.490 2.03 2.95 0.77 Comparative Example R 0.0024 0.707 3.48 2.71 Comparative Example S 0.0021 0.608 3.33 3.18 Comparative Example T 0.069 0.0022 0.433 3.20 2.98 Comparative Example U 0.0007 0.0032 0.445 3.23 2.98 −0.17  Comparative Example V 0.0032 0.0019 0.467 2.29 2.74 1.84 Comparative Example W 0.0010 0.674 3.33 3.28 Comparative Example X 0.0014 0.420 1.25 2.53 Comparative Example Y 0.0014 0.0024 0.459 8.18 2.87 0.17 Comparative Example Z 0.0023 0.617 2.82 3.81 Comparative Example A1 0.0017 0.476 3.33 3.12 Comparative Example Annotation 1 An underlined portion printed in bold indicates a value out of the range according to embodiments. Annotation 2 Expression (1) . . . Ceq = [C] + [Mn]/6 + ([Cu]+ [Ni])/15 + ([Cr] + [Mo] + [V])/5 Annotation 3 Expression (2) . . . 5.5[C]4/3 + 15 [P] + 0.90[Mn] + 0.12[Ni] + 7.9[Nb]1/2 + 0.53[Mo] Annotation 4 Expression (4) . . . {[Ca] − (0.18 + 130 × [Ca]) × [O]}/1.25/[S]

(87) TABLE-US-00002 TABLE 2 Rolling Condition Cumulative Cumulative Rolling Cooling Condition Rolling Reduction at Cooling Heating Reduction Temperature Finishing Finishing Cooling Stop Tempering Steel Temperature at 950° C. or below 950° C. Thickness Temperature Rate Temperature Temperature No. No. (° C.) Higher (%) (%) (mm) (° C.) (° C./s) (° C.) (° C.) 1 A 1039 50 50 75 780 5 220 560 2 A 1117 50 53 70 720 5 220 550 3 A 1235 62 56 50 760 10  260 580 4 B 1055 22 79 50 750 10  210 600 5 B 1073 40 44 100  700 2 110 590 6 C 1123 35 64 70 760 5 100 590 7 C 1052 30 31 150  730   0.8 160 560 8 D 1099 46 57 70 760 5 290 560 9 D 1042 38 62 70 790 5 620 550 10 E 1033 47 69 50 770 10  220 690 11 E 1118 48 68 50 790 10  250 590 12 F 1040 42 60 70 740 5 250 590 13 G 1047 50 53 70 760 5 450 645 14 H 1116 52 51 70 700 5 190 560 15 I 1090 47 56 70 760 5 140 610 16 J 1104 55 48 70 750 5 260 560 17 K 1042 49 67 50 790 10  220 570 18 L 1080 65 52 50 740 10  110 550 19 M 1072 64 54 50 740 10  190 560 20 N 1110 60 58 50 790 10  290 590 21 P 1088 39 45 100  780 2 130 560 22 Q 1031 46 38 100  730 2 150 590 23 R 1073 46 38 100  750 2 140 590 24 S 1036 48 55 70 750 5 220 570 25 T 1031 31 66 70 730 5 240 550 26 U 1127 37 63 70 710 5 200 600 27 V 1111 41 60 70 780 5 210 610 28 W 1041 40 61 70 780 5 100 600 29 X 1054 33 65 70 740 5 180 570 30 Y 1088 33 65 70 720 5 270 610 31 Z 1110 49 54 70 740 5 290 580 32 A1 1121 48 55 70 710 5 280 550 Base Metal Property Left-hand Right-hand Weld Zone Property vE Side of Side of vE CTOD Steel YP TS −40° C. Expression Expression −40° C. δ −10° C. No. No. (MPa) (MPa) (J) (3) (3) (J) (mm) Note 1 A 524 624 227 1.25 1.30 178  1.350 Example 2 A 531 627 254 1.27 1.31 165  0.987 Example 3 A 541 639  46 1.26 1.35 Comparative Example 4 B 410 531  89 1.21 1.38 Comparative Example 5 B 533 617 205 1.22 1.28 165  0.846 Example 6 C 515 641 241 1.06 1.38 191  1.546 Example 7 C 408 556  35 1.14 1.22 Comparative Example 8 D 527 614 283 1.23 1.41 159  0.937 Example 9 D 375 508 142 1.26 1.41 Comparative Example 10 E 411 546 250 1.21 1.43 Comparative Example 11 E 510 618 264 1.20 1.43 165  1.356 Example 12 F 528 675 127 1.53 1.27 34 0.256 Comparative Example 13 G 401 471 148 1.13 1.30 29 0.125 Comparative Example 14 H 412 512 271 1.26 1.38 Comparative Example 15 I 541 627  49 1.45 1.48 Comparative Example 16 J 501 605 281 1.23 1.33 35 0.126 Comparative Example 17 K 546 659 128 1.03 1.40 Comparative Example 18 L 378 515 277 1.01 1.48 Comparative Example 19 M 588 693 110 1.05 1.33 Comparative Example 20 N 497 587 249 1.13 1.40 34 0.131 Comparative Example 21 P 503 587  61 1.13 1.25 Comparative Example 22 Q 522 633 235 1.04 1.24 46 0.137 Comparative Example 23 R 546 654 268 1.01 1.30 22 0.223 Comparative Example 24 S 593 704  78 1.06 1.39 Comparative Example 25 T 589 727  34 1.06 1.33 Comparative Example 26 U 534 648 233 1.04 1.33 56 0.235 Comparative Example 27 V 506 653 216 1.05 1.30 141  0.482 Comparative Example 28 W 574 755  56 1.45 1.30 Comparative Example 29 X 452 594 254 1.15 1.34 89 0.211 Comparative Example 30 Y 471 635 276 1.29 1.30 74 0.356 Comparative Example 31 Z 619 781  56 1.46 1.31 47 0.097 Comparative Example 32 A1 531 627 211 1.47 1.31 41 0.119 Comparative Example Annotation 1 An underlined portion printed in bold indicates a value out of the range according to embodiments. Annotation 2 Expression (3) . . . HVmax/HVave ≦1.35 + 0.006/[C] − t/500