JP7076325B2 - Thick steel plate and its manufacturing method and welded structure - Google Patents

Description

The present invention relates to a thick steel sheet, a method for manufacturing the same, and a welded structure.

Thick steel sheets are used for welded structures in many fields such as ships, marine structures, plants, bridges, civil engineering, and construction. As a steel material constituting the thick steel sheet, for example, Patent Document 1 discloses a steel material having ensured corrosion resistance. Patent Document 1 optimizes the scale generation conditions in the manufacturing process of a base steel material to which one or more of Ti, Nb or Zr is appropriately added in addition to a predetermined amount of Cu and Cr. As a result, a scale layer with high adhesion and denseness can be formed on the surface of the base steel material, and an excellent temporary rust prevention effect can be obtained for a long period of time. It has been shown that by optimizing the content, it is possible to suppress the corrosion of the base steel material in the scale defects that inevitably occur.

Further, in Patent Document 2, alloy elements such as C, Si, Mo, and Nb are adjusted within an appropriate range, and hot rolling and accelerated cooling conditions are adjusted within an appropriate range to form a structure and bainite as the main phase. The structure can be composed of a second phase having an area ratio of 30% or less, and the second phase can have a structure containing an MA phase having an area ratio of 5% or more. It was shown that a thick steel plate having a low yield ratio of 80% or less and excellent toughness and weld cracking resistance of the heat-affected portion of medium heat welding of about 400 kJ / cm or less was obtained. ing.

Japanese Unexamined Patent Publication No. 2016-199778 Japanese Unexamined Patent Publication No. 2016-180171

In order to realize the safety and maintenance-free of the welded structure, the thick steel plate used for the welded structure has the following characteristics (a) excellent corrosion resistance, (b) high yield strength, and (c). ) High toughness of welded joints is required.

(A) Excellent corrosion resistance Steel materials used to ensure the safety of structures are either increased in thickness in advance in anticipation of the amount of corrosion in the future, or painted to prevent corrosion. Will be done. However, as the plate thickness increases, the welding load increases. Further, in the case of the above-mentioned painting, repainting is required as regular maintenance, which is economically unfavorable. Therefore, it is required to impart corrosion resistance to the steel material itself.

(B) High yield strength There is an idea of absorbing energy such as an earthquake and ensuring safety by yielding (plastic deformation) at an early stage when a load is applied to the structure due to an earthquake or the like. However, once it surrenders, it is economically unfavorable because the relevant part must be replaced. Therefore, there is an idea to ensure safety and economy by using steel materials that do not deform even in the event of an earthquake. In this case, the steel material is required to have a high yield strength.

(C) High toughness of welded joints In welded structures, the welded joints are shaped so that stress concentration is likely to occur, so cracks are likely to occur. From the viewpoint of ensuring safety, in order to prevent the growth of cracks, it is required that the welded joint has excellent low temperature toughness, especially the heat-affected zone (hereinafter referred to as "HAZ"). .. In addition, it is necessary to stably suppress welding cracks from the welding construction surface.

The above-mentioned Patent Document 1 discloses a steel material having corrosion resistance, but does not specify any means for controlling the amount of alloying elements added to ensure HAZ toughness and manufacturing conditions for achieving high yield strength. do not have. Further, Patent Document 2 aims at a low yield ratio in order to secure the plastic deformability from the viewpoint of earthquake resistance. Specifically, MA (island martensite) is generated in a certain fraction to reduce the yield strength. That is, Patent Document 2 is not a technique for achieving the above (b) high yield strength.

As described above, the one that satisfies all the required characteristics of the above (a) to (c) has not been realized. The present invention has been made in view of such circumstances, and provides a thick steel sheet that satisfies all of the above-mentioned required characteristics (a) to (c) and contributes to ensuring the safety and maintenance-free of the welded structure. The purpose is. Another object of the present invention is to provide a method for manufacturing the thick steel plate and a welded structure obtained by using the thick steel plate.

Aspect 1 has a component composition of
C: 0.03 to 0.05% by mass,
Si: 0.15 to 0.55% by mass,
Mn: 1.40 to 1.90% by mass,
P: More than 0% by mass, 0.020% by mass or less,
S: More than 0% by mass, 0.006% by mass or less,
Al: 0.01-0.07% by mass,
Cu: 0.30 to 0.50% by mass,
Ni: 0.10 to 0.30% by mass,
Cr: 0.45 to 0.75% by mass,
Nb: 0.015 to 0.050% by mass,
Ti: 0.003 to 0.030% by mass,
B: 0% by mass or more, 0.0007% by mass or less,
N: 0.0010 to 0.0100% by mass, and Ca: 0.0003 to 0.0060% by mass,
The balance consists of iron and unavoidable impurities, the yield strength is 500 MPa or more, the fraction of island-shaped martensite (MA) in the entire structure of the base metal is 4.0 area% or less, and the following formula (1) is used. It is a thick steel sheet having a Pm of 1.0 or more and 2.3 or less and a Pcm of 0.200 mass% or less represented by the following formula (2).
Pm = (50 x [C]) ² x (1.2 x [Si] +1) x {0.3 x ([Mn] -1.4)} x (0.3 x [Cu] +1) x ( 0.25 x [Ni] +1) x (1.2 x [Cr] +1) x (30 x [Mo] +1) x (2.75 x [V] +1) x (100 x [B] +1) x (3 x [Nb] + 1) ... (1)
Pcm [mass%] = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] and [Nb] are shown in% by mass, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, B and Nb are shown, and the elements not included are zero.

Aspect 2 further comprises the thickness according to the aspect 1 containing at least one element of Mo: more than 0% by mass and 0.30% by mass or less and V: more than 0% by mass and 0.10% by mass or less. It is a steel plate.

In the third aspect, when a welded joint portion is formed by performing submerged arc welding with a welding heat input of 10 kJ / mm, the fraction of MA in the entire structure of the weld heat affected zone in the welded joint portion is 4.4 area. % Is the thick steel plate according to aspect 1 or 2.

Aspect 4 is a welded structure having a welded joint portion and a non-welded portion, wherein the non-welded portion is made of the thick steel plate according to any one of aspects 1 to 3, and is a weld heat affected zone in the welded joint portion. It is a welded structure in which the fraction of MA in the total structure of is 4.4 area% or less.

Aspect 5 is a method for manufacturing a thick steel sheet according to any one of Aspects 1 to 3.
A step of heating a steel piece having the composition according to the first or second aspect to 900 to 1250 ° C.
The hot rolling process after heating and
Including the tempering step after the hot rolling
In the hot rolling step, rolling is performed in a temperature range of Ar ₃ transformation point to 950 ° C. where the surface temperature is obtained by the following formula (3), the cumulative rolling reduction rate is 20% or more, and the surface temperature is 650 to 900. Finish rolling is performed in the temperature range of ° C, and after hot rolling, the average cooling rate is 0.5 from the cooling start temperature of the finish rolling temperature to (finish rolling temperature -150 ° C) to the cooling stop temperature of room temperature to 680 ° C. Cool at ° C / s or higher,
The tempering step is a method for manufacturing a thick steel sheet in which tempering is performed in a temperature range from 400 ° C. to the Ac ₁ transformation point determined by the following formula (4).
Ar ₃ transformation point = 910-310 x [C] -80 x [Mn] -20 x [Cu] -15 x [Cr] -55 x [Ni] -80 x [Mo] ... (3)
Ac ₁ transformation point = 723-14 × [Mn] +22 × [Si] -14.4 × [Ni] +23.3 × [Cr]… (4)
However, [C], [Mn], [Cu], [Cr], [Ni], [Mo] and [Si] are C, Mn, Cu, Cr, Ni, Mo and Mo in mass%, respectively. The Si content is shown, and the elements that do not contain it are set to zero.

Aspect 6 is a method for manufacturing the thick steel sheet according to any one of Aspects 1 to 3.
A step of heating a steel piece having the component composition according to the first or second aspect to 900 to 1250 ° C. and a hot rolling step after the heating are included.
In the hot rolling step, rolling is performed in a temperature range of Ar ₃ transformation point to 950 ° C. where the surface temperature is obtained by the following formula (3), the cumulative rolling reduction rate is 20% or more, and the surface temperature is 650 to 900. Finish rolling is performed in the temperature range of ° C, and after hot rolling, the average cooling is from the cooling start temperature below the finish rolling temperature (finish rolling temperature -150 ° C) to the quenching stop temperature of 200 to 480 ° C. This is a method for manufacturing a thick steel plate that is cooled at a speed of 12 ° C./s or higher.
Ar ₃ transformation point = 910-310 x [C] -80 x [Mn] -20 x [Cu] -15 x [Cr] -55 x [Ni] -80 x [Mo] ... (3)
However, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] indicate the contents of C, Mn, Cu, Cr, Ni and Mo shown in% by mass, respectively. , Elements not included are zero.

According to the present invention, a thick steel plate having excellent corrosion resistance and high yield strength, and having a high toughness in the heat-affected zone of the welded joint when welded, a method for manufacturing the thick steel plate, and the thick steel plate. It is possible to provide a welded structure in which the weld heat-affected zone of the welded joint portion is highly tough, which is obtained by using.

FIG. 1 is a graph showing the relationship between the MA fraction of the base metal and the yield strength. FIG. 2 is a graph showing the relationship between the MA fraction of Pm and HAZ. FIG. 3 is a graph showing the relationship between Pm and yield strength. FIG. 4 is a graph showing the relationship between the quenching stop temperature and the MA fraction of the base metal. FIG. 5 is a graph showing the relationship between the MA fraction of HAZ and the Charpy absorption energy (vE _-5 ) at a test temperature of −5 ° C., which is an index of HAZ toughness.

As described above, the thick steel sheet used for the welded structure is required to have (a) excellent corrosion resistance, (b) high yield strength, and (c) high toughness of the welded joint. Although there are findings to improve individual characteristics, it is difficult to realize these (a) to (c) together because the characteristics of (a) to (c) are in a trade-off relationship. Therefore, the present inventors have diligently studied to satisfy all the required characteristics of (a) to (c).

First, in the component composition, the excellent corrosion resistance (a) can be realized by containing Cu, Ni and Cr. It is known that the addition of Cu, Ni and Cr produces protective rust on the surface of the steel material and has the effect of suppressing the subsequent progress of corrosion. Therefore, the JIS standard (JIS G 3114) stipulates that Cu, Ni, and Cr should be added in predetermined amounts.

Next, the means for achieving the above (b) high yield strength will be examined. It is considered that the hardenability is improved and the strength of the base metal is increased by adding other hardenability improving elements (C, Si, Mn, Mo, V, B, Nb, etc.) together with the above Cu, Ni and Cr. Be done. However, by adding these hardenable elements, a hard second phase such as MA (island martensite) is likely to be formed. When a movable dislocation is introduced into the structure around the hard second phase and an external force is applied, the movable dislocation easily moves, so that plastic deformation is likely to occur at an early stage. That is, there is a problem that the yield strength tends to decrease and the above (b) high yield strength cannot be achieved. Further, the addition of these elements promotes the formation of MA even in the heat-affected zone of the welded joint portion, which causes brittle fracture, and (c) there is a problem that high toughness of the welded joint portion cannot be achieved.

Therefore, when other hardenability improving elements (C, Si, Mn, Mo, V, B, Nb, etc.) are added together with Cu, Ni and Cr, (b) high yield strength and (c) welded joint portion. In order to achieve high toughness, it is necessary to suppress the formation of MA in the HAZ of the thick steel plate (base material) and the welded joint, specifically, the fraction of MA in the entire structure of the thick steel plate (hereinafter referred to as First, it was found that it is necessary to set the percentage of MA to 4.0 area% or less, and the percentage of MA in the entire tissue of HAZ to be 4.4 area% or less.

Then, in order to realize these MA reductions, the Pm calculated by the formula (1) described later is controlled to 1.0 or more and 2.3 or less in the component composition, and especially in the manufacturing process of the thick steel sheet, rolling. Later, it was found that it is necessary to perform tempering under predetermined conditions or control the cooling conditions after rolling. Hereinafter, the steel structure of the thick steel sheet of the present invention, the composition including the predetermined formula (1), the method for manufacturing the thick steel sheet, and the welded structure will be described in order.

1. 1. The steel structure MA of a thick steel sheet has a hard structure as described above, and by reducing MA, movable dislocations are reduced. As a result, when an external force is applied, plastic deformation is less likely to occur and the yield strength increases. In the present invention, the fraction of MA in the entire structure of the thick steel plate (base material) is 4.0 area% or less. The fraction of MA is preferably 3.8 area% or less, more preferably 3.6 area% or less. The lower the fraction of MA, the more preferable, and most preferably 0 area%.

FIG. 1 is a graph showing the relationship between the MA fraction of a thick steel sheet (base material) and the yield strength (YS, YP) obtained using the data of the examples described later. From FIG. 1, it can be seen that the MA fraction of the base metal should be 4.0 area% or less in order to obtain the desired yield strength.

Organizations other than the above MA are not particularly limited. Examples of the structure other than MA include ferrite, pearlite, bainite and the like. As one form of the thick steel sheet of the present invention, the total ratio of ferrite, pearlite, and bainite to the entire structure is 90 area% or more.

2. 2. Component composition In the component composition of the thick steel sheet of the present invention, Pm calculated by the following formula (1) is controlled to 1.0 or more and 2.3 or less.
Pm = (50 x [C]) ² x (1.2 x [Si] +1) x {0.3 x ([Mn] -1.4)} x (0.3 x [Cu] +1) x ( 0.25 x [Ni] +1) x (1.2 x [Cr] +1) x (30 x [Mo] +1) x (2.75 x [V] +1) x (100 x [B] +1) x (3 x [Nb] + 1) ... (1)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] and [Nb] are shown in% by mass, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, B and Nb are shown, and the elements not included are zero.

The above equation (1) was found as follows. From the viewpoint of ensuring corrosion resistance, it is necessary to control the contents of the weathering resistant elements Cu, Ni and Cr as described above. Further, it is necessary to control not only Cu, Ni, Cr but also C, Si, Mn, Mo, V, B, and Nb as elements that affect the improvement of yield strength and MA formation in HAZ. The present inventors have determined the coefficients of each element in consideration of the yield strength improving ability of these elements and the MA producing ability in HAZ, and have set the above formula (1).

The larger the Pm, the higher the yield strength tends to be. In the present invention, the Pm is set to 1.0 or more, and the tempering condition or the cooling condition after hot rolling is controlled according to the manufacturing conditions described later, and the MA fraction of the thick steel sheet is suppressed to 4.0 area% or less. This ensures the above (b) high yield strength. On the other hand, if Pm becomes too large, the MA fraction of HAZ increases in the welded structure and the HAZ toughness deteriorates. Therefore, in the present invention, the Pm is set to 2.3 or less, and the MA fraction of HAZ is suppressed to 4.4 area% or less as described later, thereby ensuring the high toughness of the welded joint portion (c).

FIG. 2 is a graph showing the relationship between the MA fraction of Pm and HAZ obtained using the data of the examples described later. From FIG. 2, it can be seen that in order to suppress the MA fraction of HAZ to 4.4 area% or less, the Pm should be 2.3 or less.

Further, FIG. 3 is a graph showing the relationship between Pm and the yield strength (YS, YP) obtained by using the data of the examples described later. In FIG. 3, the black circles are examples in which the MA fraction of the base metal is suppressed to 4.0 area% or less, and the white squares control the tempering conditions or the cooling conditions after rolling according to the manufacturing conditions described later. This is an example in which the MA fraction of the base metal exceeds 4.0 area%. From FIG. 3, it can be seen that in order to obtain a yield strength of 500 MPa or more, it is necessary to set the Pm to 1.0 or more and to control the tempering condition or the cooling condition after rolling according to the manufacturing conditions described later. ..

The Pm is preferably 1.2 or more, more preferably 1.4 or more, preferably 2.1 or less, and more preferably 1.9 or less.

Further, in the present invention, by setting the Pcm represented by the following formula (2) to 0.200% by mass or less, welding cracks are stably suppressed.
Pcm [mass%] = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, respectively shown in mass%. The contents of Mn, Cu, Ni, Cr, Mo, V and B are shown, and the elements not contained are set to zero.

The Pcm is called a weld crack sensitive composition, and the Pcm is set to 0.200% by mass or less in order to stably suppress weld cracks even in a steel plate having a thick plate thickness of, for example, 100 mm and a large degree of restraint. The Pcm is preferably 0.190% by mass or less. The smaller the Pcm, the more preferable, but considering the component composition defined in the present invention, the lower limit of the Pcm is approximately 0.14% by mass.

In order to secure (a) excellent corrosion resistance, (b) high yield strength and (c) high toughness of the welded joint by controlling the above Pm and manufacturing method, the range of each element is as follows. The street.

[C: 0.03 to 0.05% by mass]
C has the effect of increasing the strength of the steel sheet, but is also an element that increases the hard phase and deteriorates ductility. In order to secure high strength, the amount of C is 0.03% by mass or more. The amount of C is preferably 0.032% by mass or more, more preferably 0.034% by mass or more. On the other hand, in order to suppress the increase in the hard phase and secure ductility, the amount of C is set to 0.05% by mass or less. The amount of C is preferably 0.048% by mass or less, more preferably 0.046% by mass or less.

[Si: 0.15 to 0.55% by mass]
Si is an element necessary to secure high strength by strengthening solid solution. In order to effectively exert this action, the amount of Si is 0.15% by mass or more. The amount of Si is preferably 0.16% by mass or more, more preferably 0.17% by mass or more. However, when the amount of Si becomes excessive, MA is likely to be generated. In order to suppress the formation of MA and secure the above-mentioned characteristics (b) and (c), the amount of Si is 0.55% by mass or less. The amount of Si is preferably 0.40% by mass or less, more preferably 0.30% by mass or less.

[Mn: 1.40 to 1.90% by mass]
Mn is an effective element for improving hardenability and ensuring strength and toughness. In order to exert such an effect, Mn is contained in an amount of 1.40% by mass or more. The amount of Mn is preferably 1.45% by mass or more, more preferably 1.50% by mass or more. However, if Mn is excessively contained, MA is likely to be generated in HAZ, which causes deterioration such as weldability. Therefore, in the present invention, the amount of Mn is 1.90% by mass or less. The amount of Mn is preferably 1.85% by mass or less, more preferably 1.80% by mass or less.

[P: More than 0% by mass, 0.020% by mass or less]
P is an unavoidable impurity that adversely affects the toughness of the base metal and the weld. The content is suppressed to 0.020% by mass or less so as not to cause such inconvenience. The amount of P is preferably 0.019% by mass or less, more preferably 0.018% by mass or less. It is industrially difficult to make it 0% by mass, and the lower limit is about 0.002% by mass.

[S: More than 0% by mass, 0.006% by mass or less]
S is an unavoidable impurity that adversely affects the toughness and ductility of the steel sheet in the plate thickness direction, and is preferably less. From this point of view, the amount of S is suppressed to 0.006% by mass or less. The amount of S is more preferably 0.003% by mass or less, still more preferably 0.001% by mass or less.

[Al: 0.01 to 0.07% by mass]
Al is an element necessary for deoxidation, and also has an effect of fixing N in the steel and preventing deterioration of the toughness of the base metal due to the solid solution N. In order to exert such an effect, the Al content is 0.01% by mass or more. The amount of Al is preferably 0.015% by mass or more, more preferably 0.020% by mass or more. On the other hand, when the amount of Al is excessive, coarse alumina-based inclusions are likely to be formed, and it becomes difficult to secure the toughness of the base metal. Therefore, in the present invention, the amount of Al is 0.07% by mass or less. The amount of Al is preferably 0.065% by mass or less, more preferably 0.050% by mass or less.

[Cu: 0.30 to 0.50% by mass]
Cu is an element that improves weather resistance by densifying the scale layer and exerting an action of enhancing corrosion resistance. It is also an element necessary for ensuring strength. In order to effectively exert these effects, the amount of Cu needs to be 0.30% by mass or more. The amount of Cu is preferably 0.31% by mass or more, more preferably 0.32% by mass or more. However, if the amount of Cu is excessive, not only the ductility is lowered due to precipitation, but also the hardenability becomes excessive and cracks and the like are likely to occur during hot working. Therefore, the amount of Cu needs to be 0.50% by mass or less. The amount of Cu is preferably 0.45% by mass or less, more preferably 0.44% by mass or less, still more preferably 0.43% by mass or less.

[Ni: 0.10 to 0.30% by mass]
Ni has the effect of improving hardenability and at the same time has the effect of suppressing cracking during hot working, which tends to occur due to the addition of Cu. In order to exert such an effect, it is necessary to contain the amount of Ni in an amount of 0.10% by mass or more. The Ni content is preferably 0.11% by mass or more, more preferably 0.12% by mass or more. However, if Ni is excessively contained, the hardenability becomes excessive, and the above-mentioned characteristics (b) and (c) cannot be obtained. Therefore, the amount of Ni is set to 0.30% by mass or less. The amount of Ni is preferably 0.29% by mass or less, more preferably 0.28% by mass or less.

[Cr: 0.45 to 0.75% by mass]
Cr is an element that improves weather resistance by exerting an action of densifying the scale layer and enhancing corrosion resistance. It is also an element necessary for ensuring strength. In order to effectively exert these effects, the amount of Cr needs to be 0.45% by mass or more. The amount of Cr is preferably 0.46% by mass or more, more preferably 0.47% by mass or more. However, if Cr is excessively contained, the hardenability becomes excessive, and the desired properties (b) and (c) cannot be obtained. Therefore, the amount of Cr is set to 0.75% by mass or less. The amount of Cr is preferably 0.70% by mass or less, more preferably 0.55% by mass or less.

[Nb: 0.015 to 0.050% by mass]
Nb is an element effective for forming carbides and carbonitrides and improving the strength. In order to obtain such an effect, it is necessary to contain the amount of Nb in an amount of 0.015% by mass or more. The amount of Nb is preferably 0.016% by mass or more, more preferably 0.017% by mass or more. On the other hand, if Nb is excessively contained, the HAZ toughness deteriorates, so the amount of Nb needs to be 0.050% by mass or less. The Nb content is preferably 0.049% by mass or less, more preferably 0.048% by mass or less.

[Ti: 0.003 to 0.030% by mass]
Ti is an element that combines with N to form TiN, prevents the coarsening of austenite grains, that is, γ grains, during heating before hot rolling, and contributes to the improvement of the toughness of the base metal. It also has the effect of fixing N in the steel and preventing deterioration of the toughness of the base metal due to the solid solution N. In order to exert these effects, it is necessary to contain Ti in an amount of 0.003% by mass or more. The Ti content is preferably 0.004% by mass or more, more preferably 0.005% by mass or more. On the other hand, if the amount of Ti becomes excessive, TiN becomes coarse and the toughness of the base metal deteriorates, so it is necessary to make it 0.030% by mass or less. The amount of Ti is preferably 0.020% by mass or less, more preferably 0.015% by mass or less.

[B: 0% by mass or more, 0.0007% by mass or less]
If the content of B is excessive, the hardenability becomes excessive, so that the above-mentioned characteristics (b) and (c) cannot be obtained. Therefore, the amount of B is set to 0.0007% by mass or less. The amount of B is preferably 0.0005% by mass or less, more preferably 0.0003% by mass or less.

[N: 0.0010 to 0.0100% by mass]
N is an element that produces TiN and AlN, prevents coarsening of γ grains during heating before hot rolling and during welding, and is effective in improving base metal toughness and HAZ toughness. When the content of N is less than 0.0010% by mass, the TiN and the like are insufficient, the γ grains become coarse, and the toughness of the base metal deteriorates. Therefore, the amount of N needs to be 0.0010% by mass or more. The amount of N is preferably 0.0012% by mass or more, and more preferably 0.0014% by mass or more. On the other hand, if the amount of N exceeds 0.0100% by mass and becomes excessive, the increase in solid solution N adversely affects the toughness of the base metal and the toughness of HAZ. Therefore, the amount of N is set to 0.0100% by mass or less. The amount of N is preferably 0.0080% by mass or less, more preferably 0.0070% by mass or less. The amount of N can be further set to 0.0060% by mass or less, and further to 0.0050% by mass or less.

[Ca: 0.0003 to 0.0060% by mass]
Ca contributes to the spheroidization of MnS and is an effective element for improving the toughness of the base metal and the ductility in the plate thickness direction. In order to exert such an effect, the amount of Ca is set to 0.0003% by mass or more. The amount of Ca is preferably 0.0005% by mass or more, more preferably 0.0007% by mass or more, and further preferably 0.009% by mass or more. On the other hand, the amount of Ca is set to 0.0060% by mass or less in order to suppress the coarsening of inclusions and secure the toughness of the base metal. It is preferably 0.0050% by mass or less, more preferably 0.0040% by mass or less. The amount of Ca can be further set to 0.0030% by mass or less, and further to 0.0020% by mass or less.

The basic components of the thick steel sheet of the present invention are as described above, and the balance is iron and unavoidable impurities. The unavoidable impurities are elements brought in depending on the conditions of raw materials, materials, manufacturing equipment and the like, and examples thereof include O and Sb. In addition, for example, there are elements such as P and S, which are usually preferable as the content is smaller and are therefore unavoidable impurities, but the composition range thereof is separately specified as described above. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance is a concept excluding the elements whose composition range is separately defined.

The thick steel sheet of the present invention may further contain the following elements, if necessary.

[Mo: more than 0% by mass, 0.30% by mass or less, V: more than 0% by mass, 0.10% by mass or less, at least one element]
Mo is an element that enhances hardenability and easily forms carbides in steel. In order to obtain this effect, the amount of Mo is preferably more than 0% by mass, more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. However, if Mo is contained in excess, the hardenability becomes excessive, and as a result, the weld cracking resistance deteriorates. Therefore, when Mo is contained, the amount of Mo is preferably 0.30% by mass or less. The amount of Mo is more preferably 0.25% by mass or less.

V is an element effective for forming carbides and nitrides to improve strength and hardenability. In order to obtain this effect, the amount of V is preferably more than 0% by mass, more preferably 0.01% by mass or more, and further preferably 0.05% by mass or more. However, if V is excessively contained, the hardenability becomes excessive, so that the above-mentioned characteristics (b) and (c) cannot be obtained. Therefore, when V is contained, the amount of V is preferably 0.10% by mass or less.

The thick steel plate of the present invention has a plate thickness of, for example, 6 to 100 mm.

The thick steel plate of the present invention having the above characteristics is suitably used for welded structures such as shipbuilding, construction, bridges, and offshore structures. If the thick steel plate of the present invention is used for the welded structure, the safety of the welded structure can be ensured and maintenance-free can be realized.

3. 3. Manufacturing Method Next, a manufacturing method for a thick steel sheet according to the present invention will be described.
The present inventors perform hot rolling described later on a steel piece having a predetermined composition, and then tempering described later, or control the cooling conditions after the hot rolling to obtain the above-mentioned desired conditions. It has been found that a thick steel sheet having the above-mentioned desired properties can be obtained as a result of having the above-mentioned steel structure.

The thick steel sheet according to the present invention is according to the following manufacturing method I or manufacturing method II.
(Manufacturing method I)
A step of heating a steel piece having the component composition to 900 to 1250 ° C., a hot rolling step after the heating, and a tempering step after the hot rolling are included. In the hot rolling step, the surface temperature is high. Rolling is performed so that the cumulative rolling reduction is 20% or more in the temperature range of Ar ₃ transformation point to 950 ° C. determined by the following formula (3), and finish rolling is performed in the temperature range of surface temperature of 650 to 900 ° C. After hot rolling, the temperature from the cooling start temperature of the finish rolling temperature to (finish rolling temperature -150 ° C) to the cooling stop temperature of room temperature to 680 ° C is cooled at an average cooling rate of 0.5 ° C / s or more, and the tempering is performed. In the step, rolling is performed in the temperature range from 400 ° C. to the Ac ₁ transformation point obtained by the following formula (4).
(Manufacturing method II)
Ar ₃ includes a step of heating a steel piece having the component composition to 900 to 1250 ° C. and a hot rolling step after the heating, and in the hot rolling step, the surface temperature is determined by the following formula (3). Rolling with a cumulative rolling reduction of 20% or more in the temperature range of transformation point to 950 ° C, finish rolling in the temperature range of surface temperature of 650 to 900 ° C, hot rolling, and then below the finish rolling temperature. From the cooling start temperature of (finish rolling temperature −150 ° C.) or higher to the quenching stop temperature of 200 to 480 ° C., the temperature is cooled at an average cooling rate of 12 ° C./s or higher.
Ar ₃ transformation point (° C.) = 910-310 x [C] -80 x [Mn] -20 x [Cu] -15 x [Cr] -55 x [Ni] -80 x [Mo] ... (3)
Ac ₁ transformation point (° C.) = 723-14 × [Mn] +22 × [Si] -14.4 × [Ni] +23.3 × [Cr]… (4)
However, [C], [Mn], [Cu], [Cr], [Ni], [Mo] and [Si] are C, Mn, Cu, Cr, Ni, Mo and Mo in mass%, respectively. The Si content is shown, and the elements that do not contain it are set to zero.

Hereinafter, each step of the manufacturing method I will be described first.
(I) Heating step of steel pieces The steel pieces having the above-mentioned composition are heated to 900 to 1250 ° C. By setting the heating temperature of the steel pieces to 1250 ° C. or lower, coarsening of austenite grains can be suppressed. As a result, it is easy to obtain fine crystals and to secure high strength. On the other hand, the heating temperature is set to 900 ° C. or higher in order to facilitate hot rolling by suppressing an excessive increase in deformation resistance during rolling. The heating temperature is preferably 1000 ° C. or higher, more preferably 1050 ° C. or higher, preferably 1200 ° C. or lower, and more preferably 1150 ° C. or lower.

(Ii) Hot rolling process In the hot rolling process, rolling is performed so that the cumulative reduction rate is 20% or more in the temperature range from the Ar ₃ transformation point to 950 ° C. where the surface temperature is obtained by the above formula (3), and the surface is surfaced. Finish rolling is performed in a temperature range of 650 to 900 ° C, and after hot rolling, average cooling is performed from the cooling start temperature of the finish rolling temperature to (finish rolling temperature -150 ° C) to the cooling stop temperature of room temperature to 680 ° C. Cool at a rate of 0.5 ° C./s or higher.

The temperature range from the Ar ₃ transformation point to 950 ° C. is the so-called unrecrystallized region where austenite does not recrystallize during hot rolling. In order to secure a predetermined base metal strength and base material toughness, it is necessary to secure a cumulative reduction rate of 20% or more in this unrecrystallized region. As a result, strain can be accumulated in austenite, the number of transformed nuclei in the cooling process after hot rolling can be increased, and the final structure after transformation can be miniaturized. The cumulative reduction rate in this temperature range is preferably 25% or more, more preferably 30% or more. From the viewpoint of grain refinement, it is preferable that the cumulative reduction rate is larger, but from the viewpoint of productivity in the rolling process, the cumulative reduction rate in this temperature range is preferably 80% or less.

The finish rolling temperature shall be within the range of 650 to 900 ° C. in terms of surface temperature in order to secure a predetermined base metal strength. The finish rolling temperature is preferably 660 ° C. or higher, more preferably 670 ° C. or higher, preferably 890 ° C. or lower, and more preferably 880 ° C. or lower.

After hot rolling, the temperature from the cooling start temperature of the finish rolling temperature to (finish rolling temperature −150 ° C.) to the cooling stop temperature of room temperature to 680 ° C. is cooled at an average cooling rate of 0.5 ° C./s or more. High yield strength can be ensured by setting the cooling shutdown temperature to 680 ° C. or lower. The cooling shutdown temperature is preferably 650 ° C. or lower, more preferably 630 ° C. or lower. Further, by setting the average cooling rate to 0.5 ° C./s or more, high yield strength can be ensured. The average cooling rate is preferably 1.0 ° C./s or higher. On the other hand, when the average cooling rate exceeds 100 ° C./s, the structure of the surface portion of the steel sheet becomes mainly martensite due to shear transformation, and the surface hardness becomes large. Therefore, the upper limit of the average cooling rate is preferably 100 ° C./s or less, and more preferably 80 ° C./s or less.

When the cooling is, for example, air cooling having an average cooling rate of 2.0 ° C./s or less, the cooling start temperature can be set as the finish rolling temperature. In the case of water cooling with an average cooling rate of more than 2.0 ° C./s, the cooling start temperature can be set to a temperature lower than the finish rolling temperature and within the range of (finish rolling temperature −150 ° C.) or higher. ..

(Iii) Tempering step Tempering is performed at a tempering temperature within the range of the Ac ₁ transformation point obtained by the above formula (4) from 400 ° C.
By tempering in the above temperature range, particularly at a tempering temperature of 400 ° C. or higher, the MA of the base metal can be reduced to 4.0% or less in terms of the area ratio of the entire structure. As a result, the yield strength is improved. The tempering temperature is preferably 500 ° C. or higher. On the other hand, when the tempering temperature exceeds the Ac ₁ transformation point, a part of the structure is reverse-transformed and then air-cooled, so that polygonal ferrite is mixed. As a result, the strength is lowered. In addition, since the structure of the reverse metamorphosis part is coarse, the toughness is also lowered. Therefore, the tempering temperature is set to be equal to or lower than the Ac ₁ transformation point. The tempering temperature is preferably 700 ° C. or lower.

The tempering temperature is the temperature inside the steel sheet as shown in Examples described later. The holding time after the inside of the steel sheet reaches the tempering temperature is not particularly limited. As described above, the retention time is preferably long from the viewpoint of eliminating MA, but the dislocation density in the matrix structure is maintained in order to increase productivity and secure strength, and excessive carbide precipitation occurs. From the viewpoint of suppressing coarsening, it is preferable to set it to 15 minutes or less, for example. Cooling from the tempering temperature to room temperature is not particularly limited, and air cooling can be performed.

Next, the manufacturing method II will be described. In the following description of the manufacturing method II, the description of the same process as the manufacturing method I will be omitted. This manufacturing method II does not include a tempering step. In the production method II, the formation of MA is suppressed by cooling after hot rolling.

In the manufacturing method II, the finishing rolling is performed by the same method as in the manufacturing method I. Then, after hot rolling, the temperature from the cooling start temperature lower than the finish rolling temperature (finish rolling temperature −150 ° C.) to the quenching stop temperature of 200 to 480 ° C. is cooled at an average cooling rate of 12 ° C./s or more. ..

MA formation is affected by the degree of C enrichment to untransformed γ. By increasing the cooling rate in the above temperature range, C concentration in untransformed γ is suppressed and MA formation is suppressed. When the quenching stop temperature exceeds 480 ° C., C concentration in untransformed γ is likely to occur and the MA fraction increases. The quenching stop temperature is preferably 450 ° C. or lower. On the other hand, when the quenching stop temperature is less than 200 ° C., the self-tempering effect is weakened, and the MA fraction increases in this case as well. The quenching stop temperature is preferably 250 ° C. or higher.

Further, after hot rolling, the temperature from the cooling start temperature lower than the finish rolling temperature (finish rolling temperature −150 ° C.) to the quenching stop temperature is cooled at an average cooling rate of 12 ° C./s or more. The average cooling rate is preferably 15 ° C./s or higher, more preferably 20 ° C./s or higher, and even more preferably 25 ° C./s or higher. On the other hand, if the average cooling rate is too fast, the structure of the surface portion of the steel sheet becomes mainly martensite due to shear transformation, and the surface hardness becomes large. Therefore, the average cooling rate is preferably 100 ° C./s or less.

FIG. 4 is a graph showing the relationship between the quenching stop temperature and the MA fraction of the base metal obtained by using the data of the examples described later. In FIG. 4, the black circle is an example in which the specified average cooling rate is 12 ° C./s or more, and the white square is an example in which the specified average cooling rate is less than 12 ° C./s. From FIG. 4, it is averaged from the cooling start temperature which is lower than the finish rolling temperature (finish rolling temperature −150 ° C.) or higher in the cooling step after hot rolling even if the tempering step is not provided as in the manufacturing method I. It can be seen that the MA fraction of the base metal can be suppressed to 4.0 area% or less by cooling to a quenching stop temperature of 200 to 480 ° C. at a cooling rate of 12 ° C./s or more.

As the above-mentioned quenching method, for example, water cooling can be mentioned. After the quenching, the room temperature may be cooled to room temperature by non-forced cooling, for example, air cooling. In the air cooling, the average cooling rate from the quenching stop temperature to room temperature can be set to less than 1.0 ° C./s.

4. Welded Structure and Method for Manufacturing The present invention also includes a welded structure obtained by welding the thick steel plate. The welded structure has a welded joint and a non-welded portion, and the welded joint has a weld metal and a weld heat affected zone. In the welded structure of the present invention, the non-welded portion is made of a thick steel plate according to the present invention, and the weld heat-affected zone in the welded joint portion is suppressed in the proportion of island-shaped martensite (MA) in the structure. There is. HAZ toughness can be improved by reducing MA, which is the starting point of fracture. In the present invention, the MA fraction of HAZ is suppressed to 4.4 area% or less.

FIG. 5 is a graph showing the relationship between the MA fraction of HAZ and the toughness of HAZ, specifically, the Charpy absorption energy (vE _-5 ) at a test temperature of −5 ° C., obtained using the data of the examples described later. be. From FIG. 5, it can be seen that in order to increase the low temperature toughness of HAZ, particularly to make vE _-5 47J or more, it is better to suppress the MA fraction of HAZ to 4.4 area% or less.

The MA fraction of the HAZ is preferably 4.2 area% or less, more preferably 4.0 area% or less. Most preferably, it is 0 area%. The structure other than MA is not particularly limited, and examples thereof include ferrite, pearlite, bainite and the like in a total of 90 area% or more.

The method for manufacturing the welded structure is not particularly limited, and the thick steel plate of the present invention may be used for welding by a conventionally known method. For example, various welding methods such as a shielded metal arc welding method, a mag welding method, a tig welding method, a self-shield welding method, a submerged arc welding method, an electroslag welding method, and an electrogas arc welding method can be used.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited by the following examples, and can be carried out with appropriate modifications to the extent that it can meet the above-mentioned and later-described intent, and both of them are the technical scope of the present invention. Included in.

150 kg of a steel material satisfying the composition shown in Table 1 below was vacuum-melted and cast to obtain a slab as a steel piece. Then, after heating the slab at the temperatures shown in Tables 2-1 and 2-2, hot rolling is performed under the conditions shown in Tables 2-1 and 2-2, and then, in some examples, the table. Tempering was performed at the tempering temperatures shown in 2-1 and Table 2-2 to obtain thick steel sheets having a finished thickness of 9 to 100 mm. The Pm, Pcm, Ac ₁ transformation points, and Ar ₃ transformation points in Table 1 were obtained from the above formulas (1) to (4) using the component compositions shown in Table 1, respectively.

In the hot rolling, the cumulative rolling reduction and the finish rolling temperature from the surface temperature of 950 ° C. to the Ar ₃ transformation point are as shown in Tables 2-1 and 2-2. The cooling after finish rolling was as shown in Table 2-1 and Table 2-2. In Tables 2-1 and 2-2, in all the examples where the quenching stop temperature was 300 ° C. or higher, the temperature from the quenching stop temperature to room temperature was defined as air cooling (less than 1.0 ° C./s). In Tables 2-1 and 2-2, the example in which the cooling start temperature is "-" indicates that air cooling was performed from the finish rolling temperature. Further, the example in which the quenching stop temperature is "-" indicates that air cooling (the average cooling rate is as shown in Table 2-1 and Table 2-2) was performed to room temperature. When the tempering was performed, the tempering time was 15 minutes as the holding time after reaching the tempering temperature, and the cooling to room temperature after the tempering was air cooling.

In addition, the cumulative reduction rate in a certain temperature range was calculated by the following formula.
Cumulative reduction rate [%] = (H1-H2) / H1 × 100 in a certain temperature range
Here, H1: Temperature control start thickness [mm] in a certain temperature range
H2: Temperature control completion thickness [mm] in a certain temperature range

The tempering temperature is the temperature inside the steel sheet. Except for the tempering temperature, it is the temperature of the surface of the steel sheet. The temperature inside the steel sheet was determined as follows. That is, at the time of tempering, a dummy material having a thermocouple built in the steel sheet was charged together with the target material, and the measured temperature of the dummy material was set to the temperature inside the steel sheet of the target material.

The obtained thick steel sheet was subjected to a tensile test of the base metal in the following manner. Moreover, the MA fraction of the base material of the thick steel plate was measured. Further, in some examples, as described below, a thermal cycle under the following conditions simulating welding is applied, or a welded joint is actually manufactured as described later to obtain a sample corresponding to a welded structure. rice field. Then, using a sample corresponding to the welded structure, the MA fraction of HAZ and the toughness of HAZ were evaluated. The MA fraction of the base material of the thick steel plate can also be measured in the non-welded portion of the sample corresponding to the welded structure.

(Thermal cycle conditions or welding conditions)
Test No. In 23, 25, 27 to 30, a sample of 12.5 mm × 33 mm × 55 mm is taken from a thick steel plate, and a heat cycle is applied to the sample under the condition that the welding heat input is equivalent to 10 kJ / mm. A sample corresponding to the HAZ of the thing was obtained. This welding heat input corresponds to the maximum amount of heat input practically applied in submerged arc welding (SAW, Submerged-Arc metal-welding method) used in, for example, bridge construction.

In addition, the test No. 1 and test No. In No. 38, two pieces of steel having a size of 50 mm × 250 mm × 900 mm were collected from a thick steel plate, and a welded joint sample was prepared by SAW by the above heat input. Since the toughness of HAZ tends to deteriorate as the amount of heat input to welding increases, the heat cycle was applied under these conditions, which is the safest evaluation. In these samples, the rolling direction of the steel sheet is the direction of the steel sheet length of 900 mm. The weld line is also in the length direction of this steel plate.

[Measurement of MA fraction of base material]
In the non-welded portion of the thick steel plate or the welded joint sample that has not been subjected to the thermal cycle or welding, the range of ± 3 mm in the plate thickness direction at the depth of the plate thickness t / 4 from the surface was defined as the “measurement region”. .. Each steel sheet was cut on a plane perpendicular to the rolling direction, its cross section was corroded with a repera reagent, and the inside of the measurement area was observed in one field with an optical microscope at 1000 times. The area of the MA portion was obtained by image analysis, and the MA fraction was calculated. In each example, the total ratio of ferrite, pearlite, and bainite to the entire structure was 90 area% or more.

[Measurement of MA fraction of HAZ]
The heat-cycled sample is at a depth of plate thickness t / 2, and the welded joint sample is at a depth of plate thickness t / 4 from the surface of the HAZ portion, each in the range of ± 3 mm in the plate thickness direction. The inside was used as the measurement area. Each steel sheet was cut on a plane perpendicular to the rolling direction, its cross section was corroded with a repera reagent, and the inside of the measurement area was observed in one field with an optical microscope at 1000 times. Then, the area of the MA portion was obtained by image analysis, and the MA fraction was calculated.

[Measurement of base metal strength]
JIS Z 2201 No. 4 test piece was prepared from each thick steel plate. A JIS Z 2201 No. 5 test piece was prepared only for a steel material having a finishing thickness of 9 mm. The No. 4 test piece was cut out from the surface of the thick steel plate at a depth of t / 4 in the plate thickness in the direction perpendicular to the rolling direction. The No. 5 test piece was cut out in the direction perpendicular to the rolling direction with the total thickness of the thick steel plate. Using the test piece, a tensile test was performed once for each according to JIS Z 2241, and yield strengths YS, YP and tensile strength TS were measured. Then, when the yield strength (YS, YP) was 500 MPa or more and the tensile strength (TS) was 570 MPa or more, it was evaluated as high strength.

[Evaluation of HAZ toughness]
Three V-notch Charpy impact test pieces of JIS Z 2242 were taken from the sample to which the heat cycle was applied or from the HAZ of the welded joint. Since the entire test piece of the sample subjected to the heat cycle has a structure corresponding to HAZ, the sampling position is not particularly limited. Absorption energy at a test temperature of −5 ° C. was determined using three test pieces. When the minimum value of the absorbed energy was 47 J or more, it was evaluated that the toughness of HAZ was excellent.

Consider the following from Tables 1 to 3.

Test No. 1, 2, 5, 6, 10 to 15, 17, 19, 21 to 25, 36 to 41 satisfy the component composition specified in the present invention, that is, the content of each element, Pm and Pcm, and the present invention. Since it was manufactured under the conditions specified in the above, it had high yield strength, and as confirmed in some examples, it also had excellent toughness of HAZ of the welded structure. Further, since it contains a predetermined amount of Cu, Ni and Cr, it can be said that it also has excellent corrosion resistance.

On the other hand, the examples other than the above did not satisfy at least one of the component composition and the production conditions, and the result was that at least one of the yield strength, the toughness of HAZ of the welded structure, and the corrosion resistance was inferior.

Test No. In 3, 9, 16, 18 and 20, no tempering was performed in the manufacturing process, and quenching to the quenching stop temperature was not performed after hot rolling, so that a large amount of MA structure was present in the base metal. As a result, high yield strength could not be obtained.

Test No. In Nos. 4, 7 and 8, the quenching after hot rolling was completed at a temperature higher than the predetermined quenching stop temperature range, and tempering was not performed, so that a large amount of MA structure was present in the base metal. As a result, high yield strength could not be obtained.

Test No. In No. 26, quenching after hot rolling was completed at a temperature lower than the quenching stop temperature range, and tempering was not performed, so that a large amount of MA structure was present in the base metal. As a result, high yield strength could not be obtained.

Test No. In 27 and 28, since Pm exceeded the upper limit of the specified range, the MA fraction of HAZ was high, and the toughness of HAZ in the welded structure was inferior.

Test No. In 29 to 35, the yield strength was low because Pm was below the lower limit of the specified range.

Claims (6)

Ingredient composition,
C: 0.03 to 0.05% by mass,
Si: 0.15 to 0.55% by mass,
Mn: 1.40 to 1.90% by mass,
P: More than 0% by mass, 0.020% by mass or less,
S: More than 0% by mass, 0.006% by mass or less,
Al: 0.01-0.07% by mass,
Cu: 0.30 to 0.50% by mass,
Ni: 0.10 to 0.30% by mass,
Cr: 0.45 to 0.75% by mass,
Nb: 0.015 to 0.050% by mass,
Ti: 0.003 to 0.030% by mass,
B: 0% by mass or more, 0.0007% by mass or less,
N: 0.0010 to 0.0100% by mass, and Ca: 0.0003 to 0.0060% by mass,
3. The balance consists of iron and unavoidable impurities, the yield strength is 500 MPa or more, and the fraction of island-shaped martensite (MA) in the entire structure is at a depth of t / 4 from the surface of the thick steel sheet . It is 0 area% or less (including 0 area%) , the total ratio of ferrite, pearlite, and bainite to the entire structure is 90 area% or more, and Pm represented by the following formula (1) is 1. A thick steel sheet having a Pcm of 0 or more and 2.3 or less and having a Pcm represented by the following formula (2) of 0.200 mass% or less.
Pm = (50 x [C]) ² x (1.2 x [Si] +1) x {0.3 x ([Mn] -1.4)} x (0.3 x [Cu] +1) x ( 0.25 x [Ni] +1) x (1.2 x [Cr] +1) x (30 x [Mo] +1) x (2.75 x [V] +1) x (100 x [B] +1) x (3 x [Nb] + 1) ... (1)
Pcm [mass%] = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] and [Nb] are shown in% by mass, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, B and Nb are shown, and the elements not included are zero. The thick steel sheet according to claim 1, further comprising at least one element of Mo: more than 0% by mass and 0.30% by mass or less and V: more than 0% by mass and 0.10% by mass or less. When a welded joint is formed by performing submerged arc welding with a welding heat input of 10 kJ / mm, the fraction of MA in the entire structure of the weld heat affected zone in the welded joint is 4.4 area% or less. The thick steel plate according to claim 1 or 2. A welded structure having a welded joint and a non-welded portion.
The non-welded portion is made of the thick steel plate according to any one of claims 1 to 3.
A welded structure in which the fraction of MA in the entire structure of the weld heat affected zone in the welded joint is 4.4 area% or less. The method for manufacturing a thick steel sheet according to any one of claims 1 to 3.
A step of heating a steel piece having the component composition according to claim 1 or 2 to 900 to 1250 ° C.
The hot rolling process after heating and
Including the tempering step after the hot rolling
In the hot rolling step, rolling is performed in a temperature range of Ar ₃ transformation point to 950 ° C. where the surface temperature is obtained by the following formula (3), the cumulative rolling reduction rate is 20% or more, and the surface temperature is 650 to 900. Finish rolling is performed in the temperature range of ° C, and after hot rolling, the average cooling rate is 0.5 from the cooling start temperature of the finish rolling temperature to (finish rolling temperature -150 ° C) to the cooling stop temperature of room temperature to 680 ° C. Cool at ° C / s or higher,
In the tempering step, a method for manufacturing a thick steel sheet in which tempering is performed in a temperature range from 400 ° C. to the Ac ₁ transformation point determined by the following formula (4).
Ar ₃ transformation point = 910-310 x [C] -80 x [Mn] -20 x [Cu] -15 x [Cr] -55 x [Ni] -80 x [Mo] ... (3)
Ac ₁ transformation point = 723-14 × [Mn] +22 × [Si] -14.4 × [Ni] +23.3 × [Cr]… (4)
However, [C], [Mn], [Cu], [Cr], [Ni], [Mo] and [Si] are C, Mn, Cu, Cr, Ni, Mo and Mo in mass%, respectively. The Si content is shown, and the elements that do not contain it are set to zero. The method for manufacturing a thick steel sheet according to any one of claims 1 to 3.
A step of heating a steel piece having the component composition according to claim 1 or 2 to 900 to 1250 ° C.
Including the hot rolling step after heating,
In the hot rolling step, rolling is performed in a temperature range of Ar ₃ transformation point to 950 ° C. where the surface temperature is obtained by the following formula (3), the cumulative rolling reduction rate is 20% or more, and the surface temperature is 650 to 900. Finish rolling is performed in the temperature range of ° C, and after hot rolling, the average cooling is from the cooling start temperature below the finish rolling temperature (finish rolling temperature -150 ° C) to the quenching stop temperature of 200 to 480 ° C. A method for manufacturing a thick steel plate that is cooled at a speed of 12 ° C./s or higher.
Ar ₃ transformation point = 910-310 x [C] -80 x [Mn] -20 x [Cu] -15 x [Cr] -55 x [Ni] -80 x [Mo] ... (3)
However, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] indicate the contents of C, Mn, Cu, Cr, Ni and Mo shown in% by mass, respectively. , Elements not included are zero.

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