JP7323090B1 - Steel plate and steel plate manufacturing method - Google Patents

Abstract

The present invention provides a steel sheet which is excellent in ammonia stress corrosion cracking resistance, low-temperature toughness of the base metal, HAZ toughness, and strength properties, and which suppresses a decrease in flatness, and a method for producing the same. The steel sheet of the present invention has a predetermined chemical composition, in particular, Ti and N have a Ti/N ratio of 2.00 or more and 4.00 or less, and "169 ≤ 5158 x Ti + 25563 x N ≤ 360". In addition, the carbon equivalent Ceq is set to 0.340 or more and 0.390 or less, and the microstructure at the position of 1/4 of the plate thickness is defined by the total volume fraction of ferrite and pearlite 90% or more, the average grain size of ferrite is 5 μm or more and 20 μm or less, the yield strength is 440 MPa or less, the tensile strength is 490 MPa or more, and when a long length of 2 m is applied to the steel plate surface along the rolling direction 3, the maximum value of the gap between the steel plate surface and the long length is 14 mm or less, and the plate thickness is less than 13 mm.

Description

The present invention provides a steel plate with excellent toughness and corrosion resistance that can be applied to high heat input welding, especially a multi-purpose tank for mixed loading of liquefied petroleum gas (hereinafter referred to as LPG) and liquefied ammonia (hereinafter referred to as a liquefied gas storage tank. The present invention relates to a steel sheet excellent in low-temperature toughness and ammonia stress corrosion cracking resistance to be used in ) and a method for producing the same.

With the recent increase in energy demand, liquefied gas is being actively transported by energy transport ships. For efficient operation of energy carriers, liquefied gas storage tanks may carry not only LPG but also liquefied ammonia.

Since these liquefied gases are transported at low temperatures, steel sheets used for liquefied gas storage tanks are required to have high low-temperature toughness.

In recent years, tanks have become larger, and steel sheets are required to have a high tensile strength (TS) of 490 MPa or more as one of their mechanical properties. On the other hand, liquefied ammonia is known to cause stress corrosion cracking of steel sheets, and the IMO gas code and ship classification regulations require that the yield strength (YS) be 440 MPa or less in order to minimize the risk of ammonia stress corrosion cracking. regulated by Therefore, the liquefied gas storage tank is required to have a high tensile strength of 490 MPa or more and a low yield strength of 440 MPa or less.

Patent Literatures 1 and 2 describe techniques for producing steel that has the low-temperature toughness required for the liquefied gas storage tanks described above and that satisfies the strength range. In Patent Documents 1 and 2, high low-temperature toughness and predetermined strength characteristics are achieved by a method of tempering after quenching a hot-rolled thick steel plate several times. However, these methods have the problem of reducing the flatness of the steel sheet. The flatness that is a problem here means flatness such that the maximum value of the gap between the surface of the steel sheet and the long length exceeds 14 mm in a steel sheet having a thickness of less than 13 mm.

On the other hand, when building a liquefied gas storage tank, steel plates must be joined together, so welding is performed. In this case, from the viewpoint of welding work efficiency, high heat input welding such as submerged arc welding and electrogas arc welding is often adopted. However, when performing these high heat input welding, the heat affected portion near the welded portion of the steel plate base material (welding heat affected zone: Heat Affected Zone, hereinafter abbreviated as HAZ) is transmitted due to the large heat. , the HAZ characteristics may be impaired. As one of the factors that impair the characteristics of the HAZ, for example, in the HAZ exposed to a high temperature just below the melting point during high heat input welding, the austenite grains tend to coarsen, and such coarsened austenite grains are reduced by subsequent cooling. Transformation into an upper bainite structure containing island-shaped martensite, which is inferior in toughness, can be mentioned. As a result, the toughness of the HAZ is lowered.

Patent Document 3 discloses a technique for suppressing coarsening of the austenite grain size in the HAZ by dispersing a large amount of TiN.

Patent Document 4 discloses a technique for reducing the island martensite (MA) in the HAZ by reducing the content of P as well as the content of C and Si.

Patent Document 5 discloses a technique for achieving both brittle crack arrestability and HAZ toughness during high heat input welding by dispersing particles such as TiN in a ferrite-based microstructure with a refined grain size. ing.

JP-A-10-140235 JP-A-10-168516 WO2011/148754 JP 2008-163446 A JP 2015-098642 A

In the techniques described in Patent Documents 1 and 2, quenching is performed after hot rolling, so quenching is necessary. Especially when applied to a thin steel sheet having a thickness of less than 13 mm, quenching causes strain inside the steel sheet, There is a problem that the flatness of the steel plate is lowered.

In addition, the technology described in Patent Documents 3 and 5, which utilizes TiN to improve HAZ toughness during high heat input welding, heats the weld heat affected zone to the melting temperature range of TiN when subjected to high heat input welding. Therefore, there is a problem that TiN decomposes and the effect of TiN dispersion is reduced, and the solid solution Ti and solid solution N generated by the decomposition of TiN embrittle the ground structure of the steel, and the problem that the toughness of the HAZ is significantly reduced. was there.

In addition, in the technique of Patent Document 4, in which the amount of MA is reduced by reducing the content of P, P tends to segregate at grain boundaries and the like. It was not sufficient to uniformly reduce the amount of MA in the HAZ tissue.

Therefore, in view of the above-mentioned circumstances, the present invention has a steel plate with a suitable strength range and the low temperature toughness of the base material (absorbed energy at -55 ° C. of 100 J or more) necessary for a liquefied gas storage tank. An object of the present invention is to provide a steel plate having excellent toughness (absorbed energy of 100 J or more at -55°C) in the HAZ of a joint after heat welding. Another object of the present invention is to provide a method for suitably manufacturing such a steel plate.

In order to achieve the above objects, the present inventors have developed low-temperature toughness and strength properties in the base metal portion of a steel plate, and toughness in the HAZ when welding with a large heat input (hereinafter, also referred to as HAZ toughness by welding with a large heat input. (1) to (3) below. In this specification, "high heat input welding" refers to welding with a heat input of 10 kJ/cm or more and 20 kJ/cm or less.

(1) In order to improve the HAZ toughness after high heat input welding, it is important to suppress the coarsening of the austenite grain size in the HAZ. In order to suppress the coarsening of the austenite grain size of the HAZ, it is important to disperse a large amount of TiN. On the other hand, when the steel plate receives a large amount of heat input, the HAZ is heated to the melting temperature range of TiN, which may cause TiN to dissolve in the steel and the dispersion effect to disappear. In order to suppress the decomposition of TiN in the HAZ, the amount of Ti and N in the steel sheet is such that the mass% ratio (Ti/N) of Ti and N is 2.00 or more and 4.00 or less, and formula (1) described later It was found that the dispersed state of TiN during high heat input welding is maintained by making the chemical composition that satisfies the condition of .

(2) In order to improve the HAZ toughness by high heat input welding, it is important to reduce island martensite (hereinafter referred to as MA) in the HAZ. Then, in order to hardly generate MA in the HAZ, the carbon equivalent (Ceq) represented by the formula (2) described later is controlled to 0.390 or less, and the content of C contained in the steel sheet is 0.090% or less. , it is important that the Si content be 0.10% or less. Furthermore, it is also necessary to set the Mn content to 1.800% or less and the Al content to 0.100% or less in order not to lower the HAZ toughness. In the present invention, the phrase "almost no MA is generated" means that the volume fraction of MA in the microstructure of the HAZ is 10% or less.

(3) While exhibiting the excellent properties of the HAZ described above, in order to further achieve the strength properties of the steel sheet, C, Si, Mn, and Al are added in a predetermined amount or more, and the Ceq is controlled to 0.340 or more. However, it is important to roll by a predetermined hot rolling method. As a result, the volume fraction of ferrite and pearlite at the position of 1/4 of the plate thickness of the steel plate is 90% or more, the type of the residual structure is not particularly limited, and the grain size of the ferrite is 5 μm or more and 20 μm or less. Furthermore, it has been found that even in a steel sheet having a thickness of less than 13 mm, it is possible to suppress the deterioration of the flatness of the steel sheet, and it is possible to omit the straightening process.

The present invention was completed based on these findings and further studies. That is, the gist of the present invention is as follows.
[1] % by mass,
C: 0.060% or more and 0.090% or less,
Si: 0.02% or more and 0.10% or less,
Mn: 1.200% or more and 1.800% or less,
P: 0.010% or less,
S: 0.010% or less,
Al: 0.020% or more and 0.100% or less,
O: 0.0100% or less,
Ti: 0.010% or more and 0.031% or less,
N: 0.0038% or more and 0.0100% or less,
Furthermore, Ti and N are contained in a range where Ti/N, which is the ratio of Ti and N, is 2.00 or more and 4.00 or less and satisfies the following formula (1),
Furthermore, the carbon equivalent Ceq defined by the following formula (2) is 0.340 or more and 0.390 or less,
Having a component composition in which the balance is Fe and unavoidable impurities,
The total volume fraction of ferrite and pearlite at a position 1/4 from the steel plate surface in the plate thickness direction is 90% or more, and the average grain size of the ferrite is 5 μm or more and 20 μm or less, having a microstructure,
Yield strength is 440 MPa or less and tensile strength is 490 MPa or more,
The maximum value of the gap between the steel plate surface and the long length when the long length of 2 m is applied to the steel plate surface along the rolling direction is 14 mm or less,
A steel plate having a thickness of less than 13 mm.
169≦5158×Ti+25563×N≦360 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (2) where each element symbol represents the content (% by mass) of each component, and is 0 when not contained.
[2] The component composition further includes, in % by mass,
Cu: 0.010% or more and 1.000% or less,
Ni: 0.010% or more and 1.000% or less,
Cr: 0.010% or more and 0.500% or less,
Mo: 0.010% or more and 0.500% or less,
V: 0.010% or more and 0.500% or less,
W: 0.01% or more and 0.50% or less,
Co: 0.01% or more and 0.50% or less,
Nb: 0.005% or more and 0.050% or less,
B: The steel sheet according to [1], containing one or more selected from 0.0001% or more and 0.0100% or less.
[3] The component composition further includes, in % by mass,
Ca: 0.0005% or more and 0.0100% or less,
Mg: 0.0005% or more and 0.0100% or less and REM: 0.0005% or more and 0.0200% or less containing one or more selected from [1] or [2] steel plate. [4] A method for manufacturing a steel plate according to any one of [1] to [3],
The steel material is heated to a temperature of 950° C. or more and 1250° C. or less, the rolling start temperature is set to Ar ₃ transformation point + 200° C. or more, and the rolling reduction rate per pass in the non-recrystallization temperature region is 10% or more and the cumulative rolling reduction. A method for producing a steel sheet, comprising hot rolling with a ratio of 65% or more and a rolling end temperature of Ar ₃ transformation point or higher, followed by air cooling.

According to the present invention, it has a tensile strength of 490 MPa or more and a yield strength of 440 MPa or less, so that it has excellent ammonia stress corrosion cracking resistance, and the absorbed energy at -55 ° C. is 100 J or more. It is possible to provide a steel plate having excellent toughness in the HAZ of the joint after high heat input welding, and excellent flatness even if the plate thickness is less than 13 mm, and a method for producing the same, which is industrially remarkable. effect.

First, the steel sheet of the present invention will be specifically described. In the present invention, it is important that the steel plate and the steel material used to manufacture the steel plate have the above strength. In addition, "%" regarding a component composition shall mean "mass %" unless otherwise indicated. Also, the thickness of the steel sheet manufactured by the present invention is less than 13 mm.

[Strength]
The strength characteristics in the present invention are yield strength (YS (yield point YP when there is a yield point, 0.2% yield strength σ _0.2 when there is no yield point)): 440 MPa or less and tensile strength (TS): 490 MPa or more. . Yield strength (YS) is closely related to ammonia stress corrosion cracking resistance. The yield point is defined as 440 MPa or less in order to minimize the Therefore, it can be said empirically that if YS is 440 MPa or less, excellent ammonia stress corrosion cracking resistance is obtained.

Basically, the higher the tensile strength (TS) of a steel sheet, the more suitable it is for a large structural member. Alternatively, a large amount of alloying elements is added, which is likely to increase the cost. In addition, when the tensile strength (TS): exceeds 620 MPa, the yield strength YS for ensuring ammonia stress corrosion cracking resistance (yield point YP when there is a yield point, 0.2% proof stress σ _0.2 when there is no yield point) : Since 440 MPa or less cannot be achieved at the same time, the tensile strength (TS) of the steel sheet is preferably 620 MPa or less. In addition, the tensile strength (TS) of the steel sheet obtained by the present invention is substantially 620 MPa or less.

[Component composition]
Next, in the present invention, it is important that the steel plate and the steel material used for its manufacture have the above chemical composition. Therefore, first, the reasons for limiting the chemical composition of steel in the present invention as described above will be explained. In addition, "%" regarding a component composition shall mean "mass %" unless otherwise indicated.

C: 0.060% or more and 0.090% or less C is an element that has the effect of increasing the strength of steel, and needs to be added to achieve high strength. In order to obtain the above effects, the C content is made 0.060% or more. Furthermore, in order to reduce the content of other alloying elements and manufacture at a lower cost, the C content is preferably 0.065% or more, more preferably 0.070% or more. On the other hand, when the C content is high, not only does the strength of the base material increase, but also the HAZ toughness is greatly reduced due to the formation of MA due to high heat input welding. From these points of view, the C content should be 0.090% or less. Moreover, in order to further suppress deterioration of HAZ toughness and weldability, the C content is preferably 0.085% or less, more preferably 0.080% or less.

Si: 0.02% or more and 0.10% or less,
Si is a component necessary for deoxidation and the like, and is an element that has the effect of increasing the strength of steel by forming a solid solution in ferrite. Since Si is necessary for achieving the desired strength in the base material, the Si content is made 0.02% or more. The Si content is preferably 0.03% or more, more preferably 0.04% or more, in order to reduce the content of other alloying elements and manufacture the steel sheet at a lower cost. On the other hand, if the Si content is high, the toughness of the base material is deteriorated. Moreover, when the Si content is large, the HAZ toughness is significantly reduced due to the generation of MA due to high heat input welding. Therefore, in order to ensure high HAZ weldability, the Si content is made 0.10% or less. In order to improve HAZ toughness, the Si content is preferably 0.09% or less, more preferably 0.08% or less.

Mn: 1.200% or more and 1.800% or less,
Mn is an element that has the effect of increasing the hardenability of steel, and must be added to achieve high strength. In order to obtain the above effect, the Mn content is set to 1.200% or more. Furthermore, the Mn content is preferably 1.300% or more, more preferably 1.400% or more, in order to reduce the content of other alloying elements and to manufacture at a lower cost. On the other hand, if the Mn content exceeds 1.800%, the strength becomes excessively high, the toughness and weldability deteriorate, and the alloy cost becomes excessively high. Therefore, the Mn content is set to 1.800% or less. Furthermore, in order to suppress deterioration of toughness and weldability, the Mn content is preferably 1.700% or less, more preferably 1.600% or less.

P: 0.010% or less P is an element contained in steel components as an unavoidable impurity, and when it segregates at grain boundaries, it has adverse effects such as lowering toughness and weldability. Therefore, it is desirable to reduce the P content as much as possible, but a P content of 0.010% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%, but usually P is an element that is unavoidably contained in steel as an impurity, so industrially it is more than 0%. you can Moreover, since an excessive reduction causes a rise in refining costs, the P content is preferably 0.0005% or more.

S: 0.010% or less S is an element contained in steel components as an unavoidable impurity. It is an element that adversely affects the toughness of the material and HAZ. Therefore, it is desirable to reduce the S content as much as possible, but 0.010% or less is permissible. The lower limit of the S content is not particularly limited, and may be 0%. However, since S is an element that is unavoidably contained in steel as an impurity, industrially it is not more than 0%. may That is, since an excessive reduction causes an increase in refining cost, it is preferable to set the S content to 0.0005% or more.

Al: 0.020% or more and 0.100% or less Al is an element that functions as a deoxidizing agent and has the effect of refining crystal grains and improving the toughness of the base material. In order to obtain these functions and effects, the Al content should be 0.020% or more. On the other hand, if the Al content exceeds 0.100%, oxide inclusions increase and the cleanliness deteriorates, adversely affecting the toughness of the base metal and HAZ. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less.

O: 0.0100% or less O is an element contained in steel components as an unavoidable impurity. is an element. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, and may be 0%, but usually O is an element that is unavoidably contained in steel as an impurity, so industrially it is more than 0%. you can That is, since excessive reduction causes a rise in refining cost, it is preferable to set the O content to 0.0020% or more from the viewpoint of cost.

Ti and N precipitate and disperse as TiN during solidification of steel, suppress coarsening of austenite in the HAZ, and become ferrite transformation nuclei to contribute to high toughness. Elements that play an important role in the present invention. and is contained within the following range.

Ti/N: 2.00 or more and 4.00 or less When Ti/N is less than 2.00, the amount of TiN generated decreases, and dissolved N that does not form TiN lowers the HAZ toughness. Therefore, Ti/N is set to 2.00 or more. Note that Ti/N is preferably 2.10 or more, more preferably 2.20 or more. Moreover, when Ti/N exceeds 4.00, TiN becomes coarse and the HAZ toughness is lowered. Therefore, the upper limit of Ti/N is set to 4.00. From the viewpoint of improving HAZ toughness, it is preferably 3.90 or less, more preferably 3.80 or less. In Ti/N, each element is the content (% by mass) in the steel.

169≦5158×Ti+25563×N≦360 (1)
In the conventional technology for improving toughness during high heat input welding using TiN, when the HAZ is heated under the influence of high heat input welding, TiN decomposes and the above-mentioned dispersion effect decreases or disappears. Since the ground structure of the steel becomes embrittled by solid solution Ti and solid solution N generated by the decomposition, there is a problem that the HAZ toughness is remarkably lowered. In order to solve this problem, it is important to set the calculated value of 5158×Ti+25563×N, that is, the calculated value of the middle side of Equation (1) to 169 or more in order to suppress the decomposition of TiN. From the viewpoint of further improving HAZ toughness, it is preferably more than 169, more preferably 175 or more, and even more preferably 180 or more.
On the other hand, when the calculated value of the middle side of the above formula (1) exceeds 360, a large amount of TiN is generated, which rather reduces the HAZ toughness. Therefore, the calculated value of the middle side of the above formula (1) should be 360 or less. In order to further improve the HAZ toughness, it is preferably less than 360, more preferably 330 or less, and even more preferably 300 or less. In addition, in the formula (1), each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained.

In the present invention, the range of the content of Ti and N is as specified above. It is as follows.

Ti: 0.010% or more and 0.031% or less Ti precipitates as TiN when the steel solidifies, suppresses coarsening of austenite in the HAZ, and serves as ferrite transformation nuclei to contribute to high toughness. It is one of the important elements in invention. In order to secure the necessary amount of TiN, the Ti content should be 0.010% or more. The Ti content is preferably 0.012% or more, more preferably 0.014% or more. On the other hand, if the addition exceeds 0.031%, a large amount of TiN is generated or the TiN particles are coarsened, and the expected effect cannot be obtained, and instead the toughness of the base metal and the toughness of the weld zone are lowered. Therefore, the Ti content should be 0.031% or less. In order to improve toughness, it is preferably 0.028% or less, more preferably 0.025% or less, and even more preferably 0.0022% or less.

N: 0.0038% or more and 0.0100% or less N is an element necessary for the formation of TiN described above, and the N content is made 0.0038% or more in order to secure the necessary amount of TiN. The N content is preferably 0.0040% or more, more preferably 0.0042% or more. On the other hand, when TiN is added in excess of 0.0100%, a large amount of TiN is generated, which rather reduces the toughness of the base material and the toughness of the weld zone. Therefore, the N content is set to 0.0100% or less. In order to improve toughness, the content is preferably 0.0090% or less, more preferably 0.0080% or less, even more preferably 0.0070% or less.

Ceq: 0.340 or more and 0.390 or less
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (2)
It is important that the carbon equivalent defined by is 0.340 or more and 0.390 or less.
If the carbon equivalent is less than 0.340, it is not possible to achieve a high tensile strength (TS) of 490 MPa or more, which is necessary for enlarging the liquefied gas storage tank. In order to achieve higher tensile strength (TS), Ceq is preferably 0.350 or more, more preferably 0.360 or more. On the other hand, if it exceeds 0.390, it is not possible to satisfy the yield strength (YS) of 440 MPa or less required to avoid stress corrosion cracking due to ammonia, and MA generation occurs in the HAZ generated during high heat input welding, HAZ toughness is reduced. Therefore, Ceq is set to 0.390 or less. Preferably, Ceq is 0.380 or less. From the viewpoint of cost, Ceq is more preferably 0.370 or less, and even more preferably 0.360 or less. In addition, in the formula (2), each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained.

The basic chemical composition of the steel sheet of the present invention contains each element in the content described above, and the balance is Fe and unavoidable impurities. For the purpose of further improving properties, particularly improving strength or base material toughness and HAZ toughness, this basic composition can optionally include Cu: 1.00% or less, Ni: 1.00% or less, Cr : 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, B: 0.0100% or less It can further contain one or two or more.

Cu: 0.010% or more and 1.000% or less Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the base material of the steel sheet, and can be optionally added. When Cu is added, the Cu content is preferably 0.010% or more in order to obtain the above effect. On the other hand, when the Cu content exceeds 1.000%, deterioration of toughness and an increase in alloy cost are caused. Therefore, when Cu is added, the Cu content is set to 1.000% or less. In addition, Cu content is more preferably 0.750% or less, and further preferably 0.500% or less.

Ni: 0.010% or more and 1.000% or less Ni, like Cu, is an element that has the effect of improving the strength of the base metal of the steel sheet, and can be optionally added. When Ni is added, the Ni content is preferably 0.010% or more in order to obtain the above effect. On the other hand, when the Ni content exceeds 1.000%, the weldability deteriorates and the alloy cost increases. Therefore, when Ni is added, the Ni content is made 1.00% or less. Note that the Ni content is more preferably 0.750% or less, and even more preferably 0.500% or less.

Cr: 0.010% or more and 0.500% or less Cr, like Cu, is an element that has the effect of improving the strength of the base material of the steel sheet, and can be optionally added. In order to obtain the above effects, the Cr content is preferably 0.010% or more. On the other hand, when the Cr content exceeds 0.500%, the weldability deteriorates and the alloy cost increases. Therefore, when Cr is added, the Cr content is made 0.500% or less. The Cr content is more preferably 0.050% or more, and more preferably 0.250% or less.

Mo: 0.010% or more and 0.500% or less Mo is an element that has the effect of improving the strength of the base material of the steel sheet, like Cu, and can be optionally added. In order to obtain the above effects, the Mo content is preferably 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, the weldability deteriorates and the alloy cost increases. Therefore, when adding Mo, Mo content shall be 0.500% or less. Note that the Mo content is more preferably 0.050% or more, and more preferably 0.250% or less.

V: 0.010% or more and 0.500% or less V is an element having the effect of improving the strength of the base material of the steel sheet, like Cu, and can be optionally added. In order to obtain the above effects, the V content is preferably 0.010% or more. On the other hand, if the V content exceeds 0.500%, the weldability deteriorates and the alloy cost increases. Therefore, when V is added, the V content is made 0.500% or less. The V content is more preferably 0.050% or more, and more preferably 0.250% or less.

W: 0.01% or more and 0.50% or less W is an element having the effect of improving the strength of the base material of the steel sheet, like Cu, and can be optionally added. In order to obtain the above effects, the W content is preferably 0.01% or more. On the other hand, if the W content exceeds 0.50%, the weldability deteriorates and the alloy cost increases. Therefore, when adding W, the W content is made 0.50% or less. The W content is more preferably 0.05% or more, and more preferably 0.25% or less.

Co: 0.01% or more and 0.50% or less Co is an element that has the effect of improving the strength of the steel sheet like Cu, and can be optionally added. In order to obtain the above effects, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 0.50%, the weldability deteriorates and the alloy cost increases. Therefore, when Co is added, the Co content is made 0.50% or less. Note that the Co content is more preferably 0.05% or more, and more preferably 0.25% or less.

Nb: 0.005% or more and 0.050% or less Nb is an element that has the effect of reducing the grain size of prior austenite by precipitating as carbonitrides and improving the toughness of steel. When Nb is added, the Nb content is made 0.005% or more to obtain the above effect. Furthermore, the Nb content is more preferably 0.007% or more. On the other hand, if the Nb content exceeds 0.050%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is made 0.050% or less. The Nb content is more preferably 0.040% or less, even more preferably 0.030% or less, and even more preferably 0.020% or less.

B: 0.0001% or more and 0.0100% or less B is an element that has the effect of significantly improving the hardenability of steel even when added in a very small amount. Therefore, the strength of the base material of the steel plate can be improved. In order to obtain the above effect, when B is added, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0100%, coarse Fe—B-based carbides are formed. Such coarse Fe—B-based carbides act as starting points for fracture and significantly lower the toughness of the base metal and HAZ. Therefore, when B is added, the B content is made 0.0100% or less. Also, the B content is more preferably 0.0050% or less, more preferably 0.0030% or less. Furthermore, in order to avoid high alloying and reduce costs, when B is added, it is preferable to set the upper limit of the B content as described above.

In order to ensure good strength in a steel sheet, it is common to use B as an essential component, but in the present invention, good strength can be obtained in the HAZ even without B.

Furthermore, for the purpose of controlling the morphology of the sulfide-based inclusions so as to exhibit a spherical shape and improving the HAZ toughness, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.010% or less. 1 or 2 or more selected from 0200% or less can be further contained.

Ca: 0.0005% or more and 0.0100% or less Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like that elongates in the rolling direction of the steel sheet. Therefore, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of welded joints and the like. In order to obtain the above effect, when Ca is added, the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, when the Ca content exceeds 0.0100%, the cleanliness of the steel is lowered and the HAZ toughness is lowered. Therefore, when Ca is added, the Ca content is set to 0.0100% or less. Also, the Ca content is more preferably 0.0075% or less, and even more preferably 0.0050% or less.

Mg: 0.0005% to 0.0100% Like Ca, Mg is an element that binds to S and has the effect of suppressing the formation of MnS or the like that elongates in the rolling direction of the steel sheet. Therefore, by adding Mg, the morphology of the sulfide-based inclusions can be controlled so as to exhibit a spherical shape, and the toughness of welded joints and the like can be improved. In order to obtain the above effect, when Mg is added, the Mg content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, when the Mg content exceeds 0.0100%, the cleanliness of the steel is lowered, and the HAZ toughness is lowered. Therefore, when Mg is added, the Mg content is made 0.0100% or less. Moreover, the Mg content is more preferably 0.0075% or less, and even more preferably 0.0050% or less.

REM: 0.0005% or more and 0.0200% or less REM (rare earth metal), like Ca and Mg, is an element that binds to S and has the effect of suppressing the formation of MnS or the like that elongates in the rolling direction of the steel sheet. . Therefore, by adding REM, it is possible to control the morphology of the sulfide-based inclusions so as to exhibit a spherical shape and improve the toughness of welded joints and the like. In order to obtain the above effect, when REM is added, the REM content is preferably 0.0005% or more, more preferably 0.0015% or more. On the other hand, when the REM content exceeds 0.0200%, the cleanliness of the steel is lowered and the HAZ toughness is lowered. Therefore, when REM is added, the REM content is made 0.0200% or less. The REM content is more preferably 0.0100% or less, even more preferably 0.0080% or less, and even more preferably 0.0050% or less. Here, REM is scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. refers to The REM content is the total content of one or more elements selected from the above REMs.

[Microstructure]
The microstructure of the steel sheet of the present invention will be explained.

In addition to having the above chemical composition, the steel sheet of the present invention has a total volume fraction of 90% or more of ferrite and pearlite at a quarter position in the plate thickness direction, and the grain size of the ferrite is 5 μm or more. It has a microstructure that is 20 μm or less. The reason for limiting the microstructure of steel as described above will be explained below.

[The total volume fraction of ferrite and pearlite at a position 1/4 of the plate thickness from the steel plate surface is 90% or more]
Normally, in a thin steel sheet with a thickness of less than 13 mm, when cooling is continued after hot rolling, the structure at the position of 1/4 of the thickness, which represents the structure of the steel plate, is the yield strength (YS) of bainite, martensite, etc. become a high-ranking organization. In the present invention, hot rolling is controlled, and air cooling (radiation cooling) is performed without cooling after hot rolling, as in the steel sheet manufacturing method described later. As a result, the steel sheet satisfies predetermined strength characteristics and has improved low-temperature toughness. At this time, the total volume fraction of ferrite and pearlite is 90% or more in the structure at the position of 1/4 of the plate thickness. In addition, when hot rolling is performed at a temperature at which ferrite below the Ar ₃ transformation point is formed, the ferrite is deformed and the yield strength (YS) is increased. Ferrites that have not been subjected (hereinafter ferrites) are clearly distinguished in the present invention. That is, ferrite as used in the present invention excludes processed ferrite.

[Measurement of microstructure fraction at a position 1/4 of the plate thickness from the steel plate surface]
A sample is taken from each of the obtained steel sheets so that the position of the depth of the position of 1/4 of the thickness of the steel sheet becomes the observation surface. After mirror-polishing the surface of the sample and nital-corrosion, a scanning electron microscope (SEM) is used to photograph a range of 10 mm×10 mm. The area fraction of the microstructure is determined by analyzing the photographed image using an image analyzer. Since the area fraction corresponds to the volume fraction when the anisotropy of the microstructure is small, the area fraction is defined as the volume fraction in the present invention.
In any case, each structure was determined as follows when determining the fraction of the microstructure. The steel material was mirror-polished, nital-etched to expose the structure, and observed with an SEM at a magnification of 500 to 3000 times. Ferrite is a structure that is isotropically grown and does not contain carbide, and the inside of the grain looks black. Pearlite is observed as a structure in which ferrite (white) and carbide (black) look like striped patterns (stripes). Bainite was defined as a structure having a lath-like ferrite structure grown elongated and containing carbides having an equivalent circle diameter of 0.05 μm or more. Martensite was defined as a structure having an elongated lath-like structure similar to bainite and containing carbides having an equivalent circle diameter of 0.05 μm or less. Note that the carbide looks like black dots. Austenite is defined as a non-carbide structure with an equivalent circle diameter of 0.50 μm or more, which exists between bainite or martensite structures.

Note that it is difficult to distinguish between ferrite and deformed ferrite by SEM. Then, the electron beam reflection diffraction (Electron Backscatter Diffraction Patterns: EBSD) method was used. More specifically, first, a test piece is taken from the steel plate, polished, and a colloidal silica solution is used so that the surface parallel to the rolling direction (surface at the position of 1/4 of the plate thickness) is the observation surface. The pieces were polished again. After that, by the EBSD method (electron beam acceleration voltage: 20 keV, measurement interval: 0.1 μm step), five 500 μm×500 μm regions on the observation surface of the test piece were measured. The area fraction of ferrite was calculated as a region where the grain average misorientation (GAM) was less than 1.0.

[Average grain size of ferrite is 5 μm or more and 20 μm or less]
The ferrite in the present invention has an average grain size of 20 μm or less in order to improve toughness at low temperatures. On the other hand, if the grain size of the ferrite is extremely small, the yield strength (YS) rather increases, so the average grain size of the ferrite should be 5 μm or more. In order to improve low-temperature toughness, the average grain size of ferrite is preferably 15 μm or less, more preferably 10 μm or less.

On the other hand, the residual structure is not particularly limited in type, but structures such as deformed ferrite, austenite, bainite, and martensite may be mixed, but the total volume fraction thereof should be less than 10%. The fraction of each structure in the residual structure does not need to be particularly limited, but from the viewpoint of toughness, it is preferable to avoid martensite, which is a microstructure with a large hardness difference from ferrite and pearlite. Bainite is preferred.

[Method for measuring average grain size of ferrite]
For the measurement of the grain size of ferrite, the threshold value of 15°, which is the threshold value for large-angle grain boundaries that are generally recognized as grain boundaries, is used, and the region tightly surrounded by grain boundaries with a crystal misorientation of 15° or more is visualized. Thus, the area average (Area fraction average) of ferrite was calculated. OIM Analysis software manufactured by TSL was used for the calculation. The obtained area-average grain size of ferrite was used as the average grain size of ferrite.

Next, the method for manufacturing the steel sheet of the present invention will be described.

A steel material having the chemical composition described above is heated and hot-rolled to form a hot-rolled steel sheet, which is then cooled to a starting temperature equal to or higher than the Ar ₃ transformation point to form a steel sheet. The manufacturing conditions are described in detail below.

First, the manufacturing conditions of the steel material are not particularly limited, but the molten steel having the above-described chemical composition is melted by a known melting method such as a converter, and is subjected to a known casting method such as a continuous casting method. Therefore, it is preferable to use a steel material such as a slab having a predetermined size. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.

The obtained steel material is directly hot rolled without cooling, or once heated and then hot rolled. Hot rolling is performed at a temperature equal to or higher than the Ar ₃ transformation point, and then air cooling (standing to cool) is started at a temperature equal to or higher than the Ar ₃ transformation point.

(a) Heating temperature of steel material: 950° C. or higher and 1250° C. or lower The heating temperature of the steel material is not particularly limited. The load on the steel increases, and hot rolling becomes difficult. On the other hand, when the temperature exceeds 1250° C., the oxidation becomes significant, the oxidation loss increases, and the yield decreases. For this reason, the heating temperature is preferably 950° C. or higher and 1250° C. or lower. In addition, it is more preferably 1000° C. or higher and 1150° C. or lower.

(b) Hot rolling start temperature: Ar ₃ transformation point + 200°C or higher After heating to the above temperature, hot rolling is performed.
If the temperature at which rolling is started is less than the Ar ₃ transformation point +200°C, it becomes difficult to achieve the hot rolling end temperature: the Ar ₃ transformation point or higher, which will be described later, in a steel sheet having a thickness of less than 13 mm as the final product. Desired strength characteristics cannot be obtained. Therefore, the rolling start temperature is set to Ar ₃ transformation point +200°C or higher. From the viewpoint of securing the time for rolling in the non-recrystallization temperature range described later, the rolling start temperature is preferably set to the Ar ₃ transformation point +250°C or higher. Also, the upper limit of the rolling start temperature should generally follow the above-described heating temperature of the steel material.
Here, the Ar ₃ transformation point can be obtained by, for example, the following equation.
Ar ₃ (° C.)=910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained.

(c) Rolling reduction per pass in the non-recrystallization temperature range: 10% or more and cumulative rolling reduction: 65% or more Ar ₃ transformation point + 150 ° C.), the rolling reduction per pass (hereinafter also referred to as rolling reduction / pass) is less than 10%, or the cumulative rolling reduction is less than 65% , a sufficient working effect on austenite cannot be obtained. If the austenite is not worked sufficiently, the ferrite will not be sufficiently finely grained by air cooling (standing to cool), which will be described later, and the desired low temperature toughness will not be obtained. Therefore, in the non-recrystallization temperature range, the rolling reduction per pass is set to 10% or more and the cumulative rolling reduction is set to 65% or more. Here, the rolling reduction per pass is defined as a value obtained by dividing the cumulative rolling reduction by the total number of passes. In order to further refine the ferrite and further improve the toughness at low temperatures, it is preferable to set the cumulative rolling reduction/pass in the non-recrystallization temperature range to 15% or more.

On the other hand, in order to prevent the load on the rolling mill from becoming too large, it is preferable to set the cumulative draft/pass in the non-recrystallized region to 20% or less.

In addition, the cumulative rolling reduction in the non-recrystallized region is preferably 70% or more, more preferably 75% or more, in order to further improve toughness at low temperatures. On the other hand, if the cumulative rolling reduction in the non-recrystallized region exceeds 80%, the load on the rolling mill becomes too large, so the cumulative rolling reduction in the non-recrystallized region is preferably 80% or less.

(d) Rolling End Temperature: Ar ₃ Transformation Point or Higher The hot rolling process must be completed at a temperature higher than the Ar ₃ transformation point (° C.). When the temperature is lower than the Ar ₃ transformation point (° C.) during hot rolling, a large amount of ferrite is formed in the steel, and deformed ferrite is formed, resulting in an increase in yield strength (YS). Furthermore, since the deformation resistance increases as the temperature decreases, there arises a problem that the load on the hot rolling mill increases.

(e) Cooling start temperature: Ar ₃ transformation point or higher Next, the hot-rolled steel sheet is cooled from the Ar ₃ transformation point or higher. When the cooling start temperature is lower than the Ar ₃ transformation point, ferrite is generated in the surface layer of the steel sheet and coexists with the martensite or bainite structure having a large difference in strength, resulting in a decrease in toughness. Therefore, the cooling start temperature should be the Ar ₃ transformation point or higher.

(f) Cooling method: air cooling After hot rolling, air cooling (standing to cool) to room temperature is performed. This produces a predetermined microstructure. At this time, if cooling such as water cooling is performed, a predetermined microstructure cannot be obtained, and the desired strength characteristics cannot be satisfied. It is not possible to obtain a steel plate with good flatness in which the maximum value of the gap between the steel plate surface and the long length is 14 mm or less when they are brought into contact with each other.

In the case of air cooling (standing to cool), the cooling rate is 1.0° C./s to 5.0° C./s at the position of 1/4 of the plate thickness. In addition, when water cooling is performed, it becomes 50 degrees C/s or more.
A steel sheet having the above-described structure can be obtained by manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing method. The steel plate thus obtained has excellent strength characteristics, toughness and flatness, and is suitable for high heat input welding. The cooling rate can be calculated by (700 (°C) - 500 (°C))/cooling time (s) from 700°C to 500°C. The cooling end temperature is room temperature (not particularly limited, but for example -5 to 50°C).

Molten steel having the chemical composition shown in Table 1 was melted and used as a steel material (slab). These steel materials (slabs) were subjected to hot rolling under the conditions shown in Table 2. The values in the column of formula (1) in Table 1 are values calculated by substituting the content values of Ti and N into the middle side of formula (1).

The obtained steel sheets were evaluated for chemical composition, strength characteristics, microstructure, toughness of the base material and flatness. Further, the obtained steel sheets were subjected to submerged arc welding as described below, and HAZ toughness was evaluated.

[Strength characteristics]
From the full thickness of each steel plate, a JIS Z 2201 No. 1B test piece is taken in the direction perpendicular to the rolling direction, a tensile test is performed according to JIS Z 2241, and the yield strength YS (if there is a yield point, the yield point YP, 0.2% proof stress σ _0.2 when absent) and tensile strength (TS) were measured.

A yield strength of 440 MPa or less was evaluated as a steel sheet excellent in ammonia stress corrosion cracking resistance, and a tensile strength of 490 MPa or more was evaluated as a steel sheet excellent in tensile strength. Yield strength YS is closely related to ammonia stress corrosion cracking resistance, and as a structural member of a liquefied gas bulk carrier, the risk of ammonia stress corrosion cracking resistance is minimized according to the IMO gas code and ship classification regulations. Therefore, the yield point is defined as 440 MPa or less, and steel sheets with YS of 440 MPa or less were judged to be excellent in ammonia stress corrosion cracking resistance.

[Toughness]
In addition, regarding the low-temperature toughness of the steel plate base material, a JIS Z 2202 V-notch test piece was taken in the rolling direction from a portion cut 1 mm from the surface side of each steel plate, and a Charpy impact test was performed in accordance with JIS Z 2242. The absorbed energy at 55°C was measured.

[Flatness]
A gap of 2 m (2000 mm) was applied to the surface of each steel plate along the rolling direction, and the gap between the surface of the steel plate and the long length was measured with a skimming gauge to determine the maximum value. The measurement was performed at a total of three locations, the central portion in the width direction and both ends of the steel plate, and the average value of the three maximum values (the sum of the maximum values at each location/3) was evaluated. In the above evaluation, the long ruler was made of stainless steel, the gap gauge (skimmer gauge) was made of carbon tool steel (SK5), and the target steel plate had a length of 3 m or more in the longitudinal direction.

[Submerged arc welding]
Furthermore, joint test plates sampled from each of the steel plates were subjected to V-groove processing at an angle of 50°, and submerged arc welding was performed using a commercially available low-temperature steel welding wire with a welding heat input of 60 kJ/cm. , joints were produced by high heat input welding. The welding current was 900 A, the voltage was 28 V, and the welding speed was 25 cm/min. Yield strength and toughness were measured using the obtained joints.

[HAZ toughness]
A 1 mm depth from the surface of the joint obtained by high heat input welding is used as the surface layer of the test piece, and FL (fusion line, fusion line), which is the boundary between the weld metal and HAZ, is included in the notch position. took a piece. A Charpy impact test was performed on the sampled test pieces at a test temperature of -55 ° C., and the average value vE-55 ° C. (unit: J) of the absorbed energy of three test pieces performed under the same conditions was obtained as the HAZ toughness. .
The evaluation results thus obtained are also shown in Table 2.

As can be seen from Tables 1 and 2, all of the invention examples have a yield strength YS of 440 MPa or less and a tensile strength TS of 490 MPa or more, the absorbed energy of the steel plate base material and HAZ at -55 ° C. is 100 J or more, and the flat A steel sheet with a flatness of 14 mm/2000 mm or less, excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, and high flatness was obtained.

On the other hand, steel plate No. 1 corresponding to the comparative example. 3, 4, 5, 6, and 7 differ from the invention examples in the microstructure or plate thickness of 1/4 of the plate thickness, and the yield strength YS, tensile strength TS, toughness of the base material at low temperature, or flatness are different. It is inferior to the invention examples. In addition, the steel plate No. corresponding to the comparative example. In No. 19, the carbon content is low and the tensile strength TS is inferior to the invention examples. Steel plate no. In No. 20, the carbon content is high, the yield strength YS is high compared to the invention examples, the ammonia stress corrosion cracking resistance is inferior, and the HAZ toughness is also inferior. Steel plate no. In Nos. 21 to 37, any of the amount of addition of various elements, the stipulations for Ti and N, and Ceq are outside the upper or lower limits stipulated in the present invention, and the strength characteristics of the base material, the toughness of the base material, or the HAZ Inferior to either toughness.

Claims (5)

in % by mass,
C: 0.060% or more and 0.090% or less,
Si: 0.02% or more and 0.10% or less,
Mn: 1.200% or more and 1.800% or less,
P: 0.010% or less,
S: 0.010% or less,
Al: 0.020% or more and 0.100% or less,
O: 0.0100% or less,
Ti: 0.010% or more and 0.031% or less,
N: 0.0038% or more and 0.0100% or less,
Furthermore, Ti and N are contained in a range where Ti/N, which is the ratio of Ti and N, is 2.00 or more and 4.00 or less and satisfies the following formula (1),
Furthermore, the carbon equivalent Ceq defined by the following formula (2) is 0.340 or more and 0.390 or less,
Having a component composition in which the balance is Fe and unavoidable impurities,
The total volume fraction of ferrite and pearlite at a position 1/4 from the steel plate surface in the plate thickness direction is 90% or more, and the average grain size of the ferrite is 5 μm or more and 20 μm or less, having a microstructure,
Yield strength is 440 MPa or less and tensile strength is 490 MPa or more,
The maximum value of the gap between the steel plate surface and the long length when the long length of 2 m is applied to the steel plate surface along the rolling direction is 14 mm or less,
A steel plate having a thickness of less than 13 mm.
169≦5158×Ti+25563×N≦360 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (2)
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained. The component composition further comprises, in mass %,
Cu: 0.010% or more and 1.000% or less,
Ni: 0.010% or more and 1.000% or less,
Cr: 0.010% or more and 0.500% or less,
Mo: 0.010% or more and 0.500% or less,
V: 0.010% or more and 0.500% or less,
W: 0.01% or more and 0.50% or less,
Co: 0.01% or more and 0.50% or less,
Nb: 0.005% or more and 0.050% or less,
The steel sheet according to claim 1, containing one or more selected from B: 0.0001% or more and 0.0100% or less. The component composition further comprises, in mass %,
Ca: 0.0005% or more and 0.0100% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from Mg: 0.0005% or more and 0.0100% or less and REM: 0.0005% or more and 0.0200% or less. A method for manufacturing a steel plate according to claim 1 or 2 ,
The steel material is heated to a temperature of 950° C. or more and 1250° C. or less, the rolling start temperature is set to Ar ₃ transformation point + 200° C. or more, and the rolling reduction rate per pass in the non-recrystallization temperature region is 10% or more and the cumulative rolling reduction. A method for producing a steel sheet, comprising hot rolling with a ratio of 65% or more and a rolling end temperature of Ar ₃ transformation point or higher, followed by air cooling. A method for manufacturing a steel plate according to claim 3,
The steel material is heated to a temperature of 950° C. or more and 1250° C. or less, and the rolling start temperature is set to Ar ₃ The transformation point is +200° C. or higher, the rolling reduction per pass in the non-recrystallization temperature range is 10% or higher and the cumulative rolling reduction is 65% or higher, and the rolling end temperature is Ar. ₃ A method for producing a steel sheet, comprising hot rolling to a temperature higher than the transformation point, followed by air cooling.