JP7243916B2 - Steel plate and steel plate manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel sheet, particularly a steel sheet applicable to high heat input welding, and a method for producing the same. Specifically, it relates to a steel plate having excellent toughness in the HAZ after high heat input welding. Moreover, the steel plate of the present invention can be suitably used for large-scale structures such as ships, marine structures, low-temperature storage tanks, and architectural/civil engineering structures.

In recent years, structures such as ships, offshore structures, cryogenic storage tanks, and construction/civil engineering structures are becoming larger, and the steel materials used are being actively promoted to have higher strength and thicker wall thickness. It is

In order to efficiently manufacture the structures described above in a short period of time, it is desired to use a high heat input welding method represented by a submerged arc welding method, an electrogas welding method, an electroslag welding method, and the like. However, when a steel plate is subjected to high heat input welding, the heat affected zone (HAZ) of the steel plate loses its properties due to the large amount of heat transmitted to it. I had a problem.
For example, in the HAZ, which is exposed to a high temperature just below the melting point during high heat input welding, the austenite crystal grains are likely to coarsen, and such coarsened austenite crystal grains are formed by subsequent cooling. It transforms into a bainite structure. Therefore, the toughness of the HAZ tends to decrease.

Many countermeasures have been proposed so far for the problem of deterioration in HAZ toughness due to such high heat input welding.
For example, Patent Literature 1 describes a technique for suppressing coarsening of austenite grains by finely dispersing TiN in steel. Further, Patent Document 2 describes a technique for dispersing Ti oxides that are stable at higher temperatures.
Furthermore, from the viewpoint of reducing island martensite (MA) in the HAZ, Patent Document 3 discloses a technique of reducing the content of P in addition to reducing the content of C and Si. ing.

Japanese Patent Publication No. 55-026164 JP-A-57-051243 JP 2008-163446 A

However, in the above technology that utilizes TiN for refining austenite, when subjected to high heat input welding, the weld heat affected zone is heated to the melting temperature range of TiN, so TiN is decomposed and dispersed. There is a problem that the effect is lost, or the ground structure of the steel is embrittled by solid solution Ti and solid solution N generated by the decomposition of TiN, and the toughness of the weld heat affected zone is significantly reduced.
Moreover, in the above-described technique that utilizes Ti oxides for refining austenite, there is a problem that it is difficult to disperse the predetermined oxides finely and uniformly in the steel sheet.
Furthermore, in the above technique of reducing the content of P for the purpose of reducing the amount of MA, the distribution of P, which tends to segregate at grain boundaries, causes variations in the suppression of the amount of MA, and the amount of MA in the HAZ structure is uniformly reduced. It was insufficient from the viewpoint of
Therefore, in view of the above circumstances, the present invention provides a steel plate having excellent low-temperature toughness, particularly in HAZ generated when high heat input welding is performed (hereinafter referred to as high heat input HAZ), and a method for producing such a steel plate. It is intended to

In order to solve the above problems, the inventors have made intensive studies on techniques for improving the low-temperature toughness of the high heat input HAZ, and as a result, have obtained the following findings.

First, the inventors finely and abundantly disperse TiN and suppress the decomposition of TiN when subjected to high heat input welding, so that the effect of suppressing the coarsening of austenite grains due to TiN can be maintained. thought.

Therefore, as a result of intensive studies by the inventors, the amount of Ti and N added is in a range where Ti/N, which is the mass% ratio of Ti and N, is 2.10 or more and 3.60 or less, and the following formula (1) is satisfied. By adjusting the chemical composition with and setting the average cooling rate when casting molten steel to obtain a steel material, particularly the average cooling rate at a position 1 mm from the surface of the steel material, to 100 ° C./min or more, TiN is finely and It has been found that TiN can be dispersed in a large amount and that the decomposition of TiN is suppressed even during high heat input welding.
169≦5158×Ti+25563×N≦309 (1)

In addition, by containing C, Si, Mn, Al, Nb, Ti, and N in an amount within a predetermined range and controlling the carbon equivalent Ceq within a predetermined range, the generation of martensite islands (MA) in the HAZ is effective. It has been found that the low-temperature toughness of the large heat input HAZ, which is even more excellent than the conventional one, can be obtained because the heat can be effectively suppressed.
The present invention has been completed based on these findings and further studies.

That is, the gist of the present invention is as follows.
1. % by mass, C: 0.045% or more and 0.080% or less, Si: 0.02% or more and less than 0.10%, Mn: 1.60% or more and less than 1.95%, P: 0.010% or less , S: 0.010% or less, Al: 0.010% or more and 0.100% or less, Nb: 0.005% or more and 0.050% or less, O: 0.0100% or less, Cu: 0.50% or less , Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less, V: 0.30% or less, Ti: 0.010% or more and 0.025% or less, and N: 0.5% or less. 0038% or more and 0.0084% or less, Ti/N, which is the mass% ratio of Ti and N, is 2.10 or more and 3.60 or less, and satisfies the following formula (1), Furthermore, the carbon equivalent Ceq represented by the following formula (2) is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities.
A steel sheet, wherein TiN particles present at a depth of 1 mm from the surface of the steel sheet have an average particle diameter of 20 nm or more and 50 nm or less and a density of 5.0×10 ⁸ pieces/cm ² or more.
169≦5158×Ti+25563×N≦309 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (2)
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained.

2. Furthermore, in mass%, W: 0.30% or less, Co: 0.30% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.01% or less. 2. The steel sheet according to 1 above, containing one or more selected from 0200% or less.

3. 3. The steel sheet according to 2 above, wherein the content of B is 0.0002% or more and 0.0012% or less.

4. A method for manufacturing a steel sheet, wherein, in mass%, C: 0.045% or more and 0.080% or less, Si: 0.02% or more and less than 0.10%, Mn: 1.60% or more and less than 1.95% , P: 0.010% or less, S: 0.010% or less, Al: 0.010% or more and 0.100% or less, Nb: 0.005% or more and 0.050% or less, O: 0.0100% or less , Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less, V: 0.30% or less, Ti: 0.010% or more. 025% or less and N: 0.0038% or more and 0.0084% or less, and with respect to Ti and N, Ti/N, which is the mass% ratio of Ti and N, is 2.10 or more and 3.60 or less, and the following Molten steel having a chemical composition that satisfies the formula (1) and further has a carbon equivalent Ceq represented by the following formula (2) of 0.400 or more and 0.500 or less, and the balance being Fe and unavoidable impurities, After casting to obtain a steel material, hot rolling is performed using the steel material, and the average cooling rate in the casting is 100° C./min or more and 500° C./min or less. Production method.
169≦5158×Ti+25563×N≦309 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (2)
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained.

5. The molten steel further contains W: 0.30% or less, Co: 0.30% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and 5. The method for producing a steel sheet according to 4 above, containing one or more selected from REM: 0.0200% or less.

6. 6. The method for producing a steel plate according to 5 above, wherein the content of B in the molten steel is B: 0.0002% or more and 0.0012% or less.

ADVANTAGE OF THE INVENTION According to this invention, the steel plate which has the toughness which was excellent also in the welded joint of a large heat input, and the manufacturing method of such a steel plate can be provided.
The steel plate obtained by the present invention is suitable for, for example, high heat input welding with excellent workability in the construction of container ships, and therefore has a remarkable industrial effect.

Next, embodiments of the present invention will be specifically described.
<Steel plate>
The steel sheet of the present invention has a predetermined chemical composition. In the chemical composition of the steel sheet of the present invention, the content of each element of C, Si, Mn, P, S, Al, Nb, O, Cu, Ni, Cr, Mo, V, Ti and N is specified, Regarding Ti and N, the range of the mass % ratio (Ti/N) of Ti and N and the predetermined formula (1) are defined, and the range of Ceq represented by the predetermined formula (2) is further defined.
The steel plate of the present invention can exhibit excellent toughness in welded joints with a large heat input, so it can be suitably used for large structures such as container ships.
Then, the steel sheet of the present invention can be obtained, for example, by the manufacturing method described below.

[Component composition]
First, the reason for limiting the chemical composition of the steel sheet in the present invention will be explained.
In addition, hereinafter, "%" relating to the chemical composition of the steel sheet means "% by mass" unless otherwise specified.

(C: 0.045% or more and 0.080% or less)
C is an element that has the effect of increasing grain boundary strength that contributes to HAZ toughness, and is necessary to achieve a desired HAZ toughness value. It is also necessary to achieve base material strength. To obtain these effects, the C content should be 0.045% or more. From the viewpoint of increasing the grain boundary strength and the HAZ toughness value, the C content is preferably 0.050% or more, more preferably 0.055% or more. On the other hand, if the C content is too high, the toughness in the HAZ is lowered. In particular, when the C content in the HAZ is too high, austenite coarsens and transforms due to high heat input welding, or MA is generated, thereby significantly reducing the toughness of the HAZ. From the viewpoint of preventing these, the C content is made 0.080% or less. Moreover, from the viewpoint of further suppressing a decrease in HAZ toughness, the C content is preferably 0.075% or less.

(Si: 0.02% or more and less than 0.10%)
Si is an element that has the effect of suppressing the formation of coarse carbides and increasing the HAZ toughness, and is necessary to achieve a desired HAZ toughness value. It is also a necessary component for securing the strength of the base material and for deoxidizing. In order to obtain these effects, the Si content should be 0.02% or more. From the viewpoint of increasing the HAZ toughness value, the Si content is preferably 0.03% or more, more preferably 0.04% or more. On the other hand, if the Si content is too high, MA is generated due to high heat input welding, and the toughness of the HAZ is greatly reduced. Therefore, in order to ensure high HAZ weldability, the Si content is made less than 0.10%. From the viewpoint of improving the toughness of the HAZ, the Si content is preferably 0.09% or less, more preferably 0.08% or less.

(Mn: 1.60% or more and less than 1.95%)
Mn is an element that increases the hardenability of steel, suppresses the formation of coarse carbides, ensures the strength of the base material, and increases the toughness of the HAZ, and is necessary to achieve the desired HAZ toughness value. be. In order to obtain these effects, the Mn content should be 1.60% or more. Moreover, from the viewpoint of increasing the HAZ toughness value, the Mn content is preferably 1.65% or more, more preferably 1.70% or more. On the other hand, if the Mn content is too high, the toughness of the HAZ is lowered and the alloy cost is excessively increased. From these points of view, the Mn content should be less than 1.95%. Moreover, from the viewpoint of further suppressing a decrease in HAZ toughness and from the viewpoint of further suppressing costs, the Mn content is preferably 1.90% or less, more preferably 1.85% or less.

(P: 0.010% or less)
P has an adverse effect of lowering the HAZ toughness by segregating at grain boundaries. Therefore, it is desirable to reduce the P content as much as possible, but a P content of 0.010% or less is acceptable. On the other hand, the lower limit of the P content is not particularly limited, and may be 0%. Generally, P is an element that is unavoidably contained in steel as an impurity, so industrially it may exceed 0%. In addition, excessive reduction of P leads to an increase in refining cost, so from the viewpoint of cost, the P content is preferably 0.005% or more.

(S: 0.010% or less)
S exists in steel as sulfide-based inclusions such as MnS, and has adverse effects such as lowering the toughness of the HAZ and becoming a starting point for brittle fracture. Therefore, it is desirable to reduce the S content as much as possible, but 0.010% or less is permissible. On the other hand, the lower limit of the S content is not particularly limited, and may be 0%. Normally, S is an element that is unavoidably contained in steel as an impurity, so industrially it may exceed 0%. In addition, excessive reduction of S leads to an increase in refining cost, so from the viewpoint of cost, it is preferable to set the S content to 0.005% or more.

(Al: 0.010% or more and 0.100% or less)
Al is an element that has the effect of reducing oxide-based inclusions and improving the toughness of the HAZ by acting as a deoxidizing agent. In addition, it has the effect of improving the strength of the base material. In order to obtain these effects, the Al content should be 0.010% or more. On the other hand, if the Al content exceeds 0.100%, oxide-based inclusions rather increase, resulting in a decrease in cleanliness and a decrease in HAZ toughness. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less.

(Nb: 0.005% or more and 0.050% or less)
Nb is an element that has the effect of increasing the toughness of the HAZ through grain refinement. It also has the effect of improving the strength and toughness of the base material. In order to obtain the above effects, the Nb content is set to 0.005% or more. The Nb content is preferably 0.007% or more, more preferably 0.009% or more. On the other hand, when the Nb content exceeds 0.050%, MA is generated in the HAZ and the toughness is lowered. Therefore, the upper limit of the Nb content is set to 0.050%. From the viewpoint of improving HAZ toughness, the Nb content is preferably 0.045% or less, more preferably 0.040% or less, and even more preferably 0.035% or less.

(O: 0.0100% or less)
O is an element that can be contained as an unavoidable impurity, but in the present invention, it is an element that should be particularly reduced, so its content is defined. O forms an oxide, becomes a starting point of brittle fracture, and has an adverse effect of lowering the toughness of the HAZ. 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%. Normally, O is an element that is unavoidably contained in steel as an impurity, so industrially it may exceed 0%. In addition, excessive reduction of O causes a rise in refining cost, so from the viewpoint of cost, it is preferable to set the O content to 0.0020% or more.

(Cu: 0.50% or less)
Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet (base material), and can be optionally added. When Cu is added, the Cu content is preferably 0.01% or more in order to obtain the above effect. On the other hand, when the Cu content exceeds 0.50%, deterioration in HAZ toughness and an increase in alloy cost are caused. Therefore, when Cu is added, the Cu content is set to 0.50% or less. In addition, as for Cu content, 0.20% or more is more preferable. On the other hand, the Cu content is more preferably 0.40% or less, still more preferably 0.30% or less.

(Ni: 0.50% or less)
Ni, like Cu, is an element that has the effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. When Ni is added, it is preferable to add 0.01% or more of Ni in order to obtain the above effect. On the other hand, if the Ni content exceeds 0.50%, the weldability deteriorates and the alloy cost increases. Therefore, when Ni is added, the Ni content should be 0.50% or less. In addition, as for Ni content, 0.20% or more is more preferable. On the other hand, the Ni content is more preferably 0.40% or less, even more preferably 0.30% or less.

(Cr: 0.50% or less)
Cr, like Cu, is an element that has the effect of improving the strength of the steel sheet (base material) and can be optionally added. In order to obtain the above effects, the Cr content is preferably 0.01% or more. On the other hand, when the Cr content exceeds 0.50%, the weldability deteriorates and the alloy cost increases. Therefore, when Cr is added, the Cr content is made 0.50% or less. Note that the Cr content is more preferably 0.05% or more. On the other hand, the Cr content is more preferably 0.40% or less, still more preferably 0.30% or less.

(Mo: 0.30% or less)
Mo is an element that has the effect of improving the strength of the steel sheet (base material) like Cu, and can be added arbitrarily. In order to obtain the above effects, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.30%, the weldability deteriorates and the alloy cost increases. Therefore, when adding Mo, Mo content shall be 0.30% or less. In addition, as for Mo content, 0.05% or more is more preferable. On the other hand, the Mo content is more preferably 0.20% or less.

(V: 0.30% or less)
V, like Cu, is an element that has the effect of improving the strength of the steel sheet (base material) and can be added arbitrarily. In order to obtain the above effects, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.30%, the weldability deteriorates and the alloy cost increases. Therefore, when V is added, the V content is set to 0.30% or less. Note that the V content is more preferably 0.05% or more. On the other hand, the V content is more preferably 0.20% or less.

(Ti: 0.010% or more and 0.025% or less)
Ti is an element that precipitates as TiN during solidification of steel, suppresses coarsening of austenite in the weld heat-affected zone, and contributes to high toughness as a ferrite transformation nucleus, and is an important element in the present invention. It is one of the elements. In order to secure the necessary amount of TiN from the viewpoint of HAZ toughness, it is preferable to contain 0.010%. The Ti content is more preferably 0.012% or more, and even more preferably 0.014% or more. On the other hand, if it is added in excess of 0.025%, a large amount of TiN is generated or TiN particles are coarsened, and the expected effect cannot be obtained. As a result, the toughness of the weld zone is lowered. Therefore, the upper limit of the Ti content is preferably 0.025%. From the viewpoint of improving toughness, it is more preferably 0.023% or less, even more preferably 0.021% or less, and even more preferably 0.019% or less.

(N: 0.0038% or more and 0.0084% or less)
N is an element necessary for the formation of TiN described above, and it is preferable to contain 0.0038% or more of TiN in order to secure the required amount from the viewpoint of HAZ toughness. The N content is more preferably 0.0040% or more, more preferably 0.0042% or more. On the other hand, when TiN is added in excess of 0.0084%, a large amount of TiN is produced, which rather reduces the toughness of the weld zone. Therefore, the upper limit of the N content is preferably 0.0084%. From the viewpoint of improving toughness, the N content is more preferably 0.0082% or less, even more preferably 0.0080% or less, and even more preferably 0.0078% or less.

Ti and N precipitate as TiN during solidification of the steel, suppress the coarsening of austenite in the weld heat-affected zone, and become ferrite transformation nuclei to contribute to high toughness. It is an element and is contained within the following range.

(Ti/N: 2.10 or more and 3.60 or less)
If the mass % ratio of Ti to N (Ti/N) is less than 2.10, solid solution N that does not become TiN increases, and the toughness of the HAZ is lowered. Therefore, Ti/N is set to 2.10 or more. Note that Ti/N is preferably 2.20 or more, more preferably 2.30 or more. On the other hand, when Ti/N exceeds 3.60, TiN coarsens and lowers the toughness of the weld zone. Therefore, the upper limit of Ti/N is set to 3.60. From the viewpoint of improving the toughness of the HAZ, it is preferably 3.50 or less, more preferably 3.40 or less. Also, in Ti/N, each element is the content in the steel (% by mass).

(169≦5158×Ti+25563×N≦309 (1))
In the conventional technique for improving the toughness of the HAZ during high heat input welding using TiN, as a result of exposing the weld heat affected zone to high heat input welding, TiN decomposes and its dispersion effect disappears. There was a problem that the formed solute Ti and solute N embrittles the ground structure of the steel and markedly lowers the toughness of the HAZ.
In the present invention, it is essential to set the value of 5158×Ti+25563×N (also referred to as the value of formula (1) in the present invention) to 169 or more in order to suppress the decomposition of TiN. From the viewpoint of further improving HAZ toughness, the value of formula (1) is preferably greater than 169, more preferably 175 or more, and even more preferably 180 or more.
On the other hand, when the value of the above formula (1) exceeds 309, a large amount of TiN is produced, which rather reduces the toughness of the HAZ. Therefore, the value of the above formula (1) should be 309 or less. From the viewpoint of further improving the toughness, the value of formula (1) is preferably less than 309, more preferably 280 or less, even more preferably 260 or less.

(Carbon equivalent Ceq: 0.400 or more and 0.500 or less)
In order to achieve excellent strength of the steel plate (base material), the following formula (2):
Ceq=C+Mn/6+(Cu+Ni)/15+(V+Cr+Mo)/5 (2)
It is essential that the defined carbon equivalent Ceq is 0.400 or more. In order to obtain the above effect, Ceq is preferably 0.410 or more, more preferably 0.420 or more, and even more preferably 0.430 or more.
On the other hand, when Ceq exceeds 0.500, MA formation occurs in the large heat input HAZ, and the toughness of the HAZ decreases. Therefore, Ceq is set to 0.500 or less. From the viewpoint of component cost, Ceq is preferably 0.490 or less, more preferably 0.480 or less.

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 other unavoidable impurities. For the purpose of further improving properties, particularly improving strength or base material toughness and HAZ toughness, this basic component composition optionally contains W: 0.30% or less, Co: 0.30% or less, B: One or more selected from 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less can be further contained.

(W: 0.30% or less)
W, like Cu, is an element that has the effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. 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 0.30%, the weldability deteriorates and the alloy cost increases. Therefore, when adding W, the W content is made 0.30% or less. Note that the W content is more preferably 0.05% or more. On the other hand, the W content is more preferably 0.20% or less.

(Co: 0.30% or less)
Co, like Cu, is an element that has the effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. 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 0.30%, the weldability deteriorates and the alloy cost increases. Therefore, when Co is added, the Co content is set to 0.30% or less. Note that the Co content is more preferably 0.05% or more. On the other hand, the Co content is more preferably 0.20% or less.

(B: 0.0100% or less)
B is an element that has the effect of significantly improving hardenability even when added in a very small amount. Therefore, the strength of the steel plate (base material) can be improved. In addition, by contributing to the improvement of hardenability in the HAZ, it suppresses the formation and growth of a coarse ferrite structure, and by forming precipitates with N, it acts as a transformation nucleus and contributes to the refinement of the structure. , the HAZ toughness can also 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, if the B content exceeds 0.0100%, coarse Fe—B-based carbides may form. 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. The B content is more preferably 0.0050% or less, more preferably 0.0030% or less, more preferably 0.0012% or less, and 0.0010% or less. is more preferred. Also, from the viewpoint of avoiding high alloying and suppressing costs, when B is added, it is desirable to set the upper limit of the B content as described above.

(Ca: 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 elongated in the rolling direction. 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 such an 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. A decrease in cleanliness leads to a decrease in HAZ toughness. Therefore, when Ca is added, the Ca content is set to 0.0100% or less. The Ca content is more preferably 0.0050% or less, and even more preferably 0.0025% or less.

(Mg: 0.0100% or less)
Mg, like Ca, is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. 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. A decrease in cleanliness leads to a decrease in HAZ toughness. Therefore, when Mg is added, the Mg content is made 0.0100% or less. The Mg content is more preferably 0.0050% or less, even more preferably 0.0025% or less.

(REM: 0.0200% or less)
Like Ca and Mg, REM (rare earth metal) is an element that binds to S and suppresses the formation of MnS or the like elongated in the rolling direction. 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. When REM is added to obtain the above effect, 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 deteriorates. A decrease in cleanliness leads to a decrease in HAZ toughness. 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.

[Particle size and density of TiN]
Next, TiN in the steel sheet of the present invention, particularly TiN particles present at a depth of 1 mm from the surface of the steel sheet, will be described.

Average grain size of TiN particles at a depth of 1 mm from the surface of the steel sheet: 20 nm or more and 50 nm or less TiN precipitates during solidification of the steel, suppresses coarsening of austenite in the weld heat-affected zone, and acts as ferrite transformation nuclei. It is a precipitate that plays an important role in the present invention, such as contributing to high toughness. If the average grain size of the TiN particles is less than 20 nm, the TiN will decompose during welding and the dispersing effect will disappear, or the solid solution Ti and solid solution N generated by such decomposition will embrittle the ground structure of the steel. The toughness of the HAZ is significantly reduced. Therefore, the average grain size of TiN grains at a depth of 1 mm from the surface of the steel sheet is set to 20 nm or more. From the viewpoint of improving the HAZ toughness, the average grain size is preferably 25 nm or more, more preferably 30 nm or more. On the other hand, if the average grain size exceeds 50 nm, the effect of suppressing coarsening of austenite is reduced, and the HAZ toughness is reduced. Therefore, the average grain size of TiN at a depth of 1 mm from the surface is set to 50 nm or less. From the viewpoint of improving HAZ toughness, the average grain size is preferably 45 nm or less, more preferably 40 nm.

Density of TiN particles at a depth of 1 mm from the steel plate surface: 5.0 × 10 ⁸ /cm ² or more Effect of suppressing coarsening of austenite in the weld heat affected zone and increasing toughness by becoming ferrite transformation nuclei In order to obtain , it is important to disperse a large amount of TiN particles. If the TiN particles at a depth of 1 mm from the surface are dispersed in an amount of 5.0×10 ⁸ or more per 1 cm ² , the above effect can be sufficiently obtained. Therefore, the density of TiN particles at a depth of 1 mm from the surface of the steel sheet is set to 5.0×10 ⁸ particles/cm ² or more. From the viewpoint of improving HAZ toughness, the density is preferably 8.0×10 ⁸ pieces/cm ² or more, more preferably 1.0×10 ⁹ pieces/cm ² or more. On the other hand, the upper limit of the density is not particularly limited, but from the viewpoint of the content of Ti and N specified in the present invention, and if the density is too high, the average particle size of TiN becomes too fine. It is 1.0×10 ¹⁰ pieces/cm ² or less.

In the present invention, TiN particles refer to precipitates each containing 10% or more of Ti and N. In addition, the average particle size and density of the TiN particles described above are determined by taking a sample so that the observation surface is at a depth of 1 mm from the surface of the steel sheet, and observing with a microscope. , the area circle equivalent diameter and the number of TiN grains are specified, and can be calculated from these.

The characteristics of the joints produced by high heat input welding are mainly achieved by the chemical composition design of the steel material and the manufacturing method of the steel material within the chemical composition range of the steel plate described above. It is not affected by the influence and the properties after hot rolling. For example, the plate thickness, strength, toughness, and microstructure of the steel plate (base material) suitable for application to hatch side coamings in container ships are as follows.

[Thickness]
The thickness of the steel sheet of the present invention is not particularly limited. For example, when the steel sheet is applied to the hatch side coaming portion of a container ship, a steel sheet of substantially 50 mm or more is applied, and a steel sheet of 75 mm or more is more preferable.

[Base material strength]
Although the strength of the steel plate (base material) of the present invention is not particularly limited, for example, when applied to the hatch side coaming portion of a container ship, the yield strength at the plate thickness 1/2 position (also referred to as 1/2t) is 390 MPa or more. steel plate is recommended. It is preferably 430 MPa or higher, more preferably 460 MPa or higher.

[Base material toughness]
Although the toughness of the steel sheet of the present invention is not particularly limited, for example, when it is applied to the hatch side coaming portion of a container ship, a steel sheet having a toughness of 53 J or more in terms of absorbed energy at -40°C at the 1/4 position of the plate thickness is recommended. be. Preferably, it is 64J or more, more preferably 75J or more.

The microstructure of the steel sheet of the present invention, which is ideal for obtaining the above thickness and base material strength, will be described below. In the composition of the present invention, the pearlite fraction in the ferrite-pearlite structure is reduced. Therefore, it becomes difficult to secure a predetermined base material strength. Therefore, the volume ratio of bainite in 1/2t, which is the thickness central portion, is preferably 80% or more, more preferably 90% or more, and may be 100%. By setting the volume fraction of bainite within the above range, it is possible to ensure predetermined base material strength and base material toughness.
Further, if the volume fraction of bainite satisfies the above range, the remaining fine structure may include a structure other than bainite such as ferrite, pearlite, etc., which is generally recognized as a steel plate structure.

<Manufacturing method of steel plate>
The grain size and density of TiN particles, which affect joint properties, are affected by the chemical composition of the steel material and the casting process. Therefore, in the steel sheet manufacturing method of the present invention, only the conditions of the casting process for obtaining the steel material to be subjected to hot rolling are specified, in addition to the above-described chemical composition of the steel. Other manufacturing methods and conditions are not particularly limited, but after the casting process, the heating process may be performed before the hot rolling process, or after the hot rolling process, the cooling process may be performed. Furthermore, for example, when applied to the hatch side coaming part of a container ship, the heating temperature in the heating process; the rolling start temperature in the hot rolling process, the cumulative reduction rate in the non-recrystallized region, and the rolling end temperature; the cooling start in the cooling process The temperature, average cooling rate, and cooling stop temperature are preferably adjusted to the following conditions.
A steel plate obtained by a manufacturing method that satisfies these conditions has excellent base material strength and excellent toughness in a large heat input HAZ, so it can be suitably used for manufacturing large structures such as container ships. is.

The conditions for manufacturing the steel material are not particularly limited except for limiting the cooling rate when obtaining the steel material such as a slab. The steel material is produced, for example, by a known melting method such as a converter, and a steel material such as a slab having desired dimensions by a known casting method such as a continuous casting method. It is preferable to obtain as

[Casting process]
In casting, molten steel having the chemical composition described above can be used. Moreover, the molten steel used for casting can be set to 1400° C. or higher.

(Average cooling rate: 100°C/min or more and 500°C/min or less)
When obtaining a steel material by casting, cooling conditions during casting are essential. That is, when casting a steel material, if the average cooling rate is less than 100 ° C./min in the temperature range of 1400 to 1250 ° C. where TiN precipitates at a position of 1 mm from the surface of the steel material, the base material in the product steel plate (steel plate ), the size of TiN becomes coarse. When the TiN size is coarsened, the TiN density of the base material (steel plate) is lowered, the austenite structure in the high heat input HAZ is coarsened, and the toughness of the HAZ may be lowered.
Therefore, in the steel sheet manufacturing method of the present invention, the average cooling rate during casting (the average cooling rate of the steel material) is set to 100° C./min or more. The average cooling rate is preferably 150° C./min or higher, more preferably 200° C./min or higher. On the other hand, when the cooling rate of the steel material exceeds 500° C./min, the density of TiN increases, but the size of TiN becomes finer, TiN melts during high heat input welding, and the austenite grains become coarse. Therefore, the toughness of the HAZ deteriorates. In addition, since cracks occur on the surface of the steel material, there is a risk that the cost of removing the cracks and the yield of the material will decrease. Therefore, the average cooling rate during casting is set to 500° C./min or less. The average cooling rate is preferably 400° C./min or less, more preferably 300° C./min or less.
The temperature range for measuring the average cooling rate is 1400 to 1250°C.

Unless otherwise specified, the temperature in each step described below is the temperature at the thickness center (1/2t) of each steel material.

[Heating process]
(Heating temperature: 950°C or higher and 1250°C or lower)
The heating temperature of the steel material in the heating step is preferably 950° C. or higher and 1250° C. or lower. If the heating temperature is less than 950° C., the heating temperature is too low, the deformation resistance increases, and the load on the hot rolling mill increases, which may make subsequent hot rolling difficult. On the other hand, when the heating temperature exceeds 1250 ° C., the austenite grains become coarse and not only the toughness of the steel plate base material and the high heat input HAZ is lowered, but also the oxidation is remarkable, the oxidation loss increases, and the yield decreases. There is a risk of Note that the heating temperature is more preferably 1000° C. or higher. On the other hand, the heating temperature is more preferably 1150° C. or lower.

[Hot rolling process]
(Rolling start temperature: Ar ₃ points + 100 ° C. or higher)
When hot-rolling the steel material heated as described above, if the temperature at which rolling is started is less than Ar ₃ point + 100 ° C., recrystallization does not occur sufficiently in the hot-rolled hot-rolled sheet, and austenite grains are formed. not fine. If a steel sheet is manufactured using such a hot-rolled sheet in which the austenite grains are not sufficiently refined, the toughness of the steel sheet may be lowered. Therefore, the rolling start temperature is preferably Ar ₃ point + 100°C or higher. From the viewpoint of securing the time for rolling in the non-recrystallized region described later, the rolling start temperature is more preferably Ar ₃ point +150°C or higher, and more preferably Ar ₃ point +200°C or higher. Although the upper limit of the rolling start temperature is not particularly limited, it is about the heating temperature of the steel material described above.
The Ar ₃ point (°C) can be obtained according to the following formula (3).
Ar ₃ points (°C)
=910-273C-74Mn-57Ni-16Cr-9Mo-5Cu (3)
Here, in the formula (3), each element symbol represents the content (% by mass) of the element in the steel, and the element not contained is set to 0.

(Cumulative rolling reduction in non-recrystallized region: 40% or more)
In the non-recrystallized region (in the present invention, the steel material means a region where the temperature is less than Ar ₃ point + 100 ° C.), if the cumulative reduction rate is less than 40%, austenite can be sufficiently worked. However, there is a possibility that the toughness of the base metal in the product steel sheet may be lowered. Therefore, in the non-recrystallized region, it is desirable to set the cumulative rolling reduction to 40% or more. From the viewpoint of further improving the toughness of the base material, it is preferably 45% or more, more preferably 50% or more.

(Rolling end temperature: Ar ₃ points or more)
The hot rolling process is preferably completed at a temperature above the Ar ₃ transformation point (°C). If the temperature is less than the Ar ₃ transformation point (°C) during hot rolling, a large amount of ferrite is generated in the steel, so the volume fraction of bainite cannot be increased, and there is a risk that the desired strength cannot be obtained. In addition, since the deformation resistance increases as the temperature decreases, the load on the hot rolling mill increases. From the viewpoint of securing the cooling start temperature in the post-process, the hot rolling temperature is preferably Ar ₃ +20°C or higher.

[Cooling process]
(Cooling start temperature: Ar ₃ points or more)
It is desirable to start cooling the hot-rolled sheet obtained through hot rolling as described above at a temperature equal to or higher than the Ar ₃ transformation point (°C). If the cooling start temperature is lower than the Ar ₃ transformation point (°C), a large amount of ferrite is generated in the steel, so the volume fraction of bainite cannot be increased, and there is a risk that the desired strength cannot be obtained. Therefore, it is desirable to set the cooling start temperature to Ar ₃ point (°C) or higher.

(Average cooling rate from 600 to 500°C: 2.0°C/s or more)
If the average cooling rate after starting the cooling is less than 2.0° C./s, the steel is slowly cooled and a large amount of ferrite is generated in the steel. Therefore, the volume fraction of bainite cannot be increased, and there is a possibility that the base material cannot obtain a predetermined strength. Therefore, the average cooling rate to 500° C. or lower in the cooling step is preferably 2.0° C./s or higher. On the other hand, the upper limit of the average cooling rate is not particularly limited, but the maximum industrial cooling rate at the plate thickness 1/2 position of 50 mm is 20 ° C / s, which avoids an increase in cooling cost due to excessive rapid cooling. Therefore, it is preferable to set it to 20° C./s or less. The temperature range for measuring the average cooling rate is 600 to 500°C.

(Cooling stop temperature: 500°C or less)
It is desirable that the cooling step for performing the above cooling is performed until the temperature at 1/2t becomes 500° C. or less, that is, at a cooling stop temperature of 500° C. or less. If the cooling stop temperature exceeds 500° C., a large amount of ferrite is generated in the steel, and the volume fraction of bainite cannot be increased, so there is a risk that the base metal will not obtain a predetermined strength. On the other hand, the lower limit of the cooling stop temperature is not limited, but if the cooling stop temperature is too low, the shape of the steel sheet deteriorates.

A base material (steel sheet) having a microstructure according to the present invention can be obtained by subjecting a steel material having the chemical composition described above to the manufacturing process described above. The steel plate thus obtained has excellent toughness in the large heat input HAZ and is suitable for use in hatch side coamings in container ships.

Here, in the present invention, regarding the base material properties detailed in the examples, the yield strength (YS): 390 MPa or more is considered to be excellent strength properties, and the absorbed energy at -40 ° C. (vE - 40 ° C.): A base material toughness of 53 J or more is considered to be excellent. As for the toughness of the HAZ, the absorbed energy at −20° C. (vE−20° C.): 46 J or more is regarded as excellent toughness. In addition, regarding the toughness of the HAZ, when vE-20 ° C. is 53 J or more, the toughness is superior, when it is 64 J or more, the toughness is even more excellent, and when it is 92 J or more, the toughness is particularly excellent.

The steel sheet of the present invention can effectively avoid coarsening of austenite grains in a large heat input HAZ, and can obtain a high vE-20° C. in welded joints including such a HAZ. Thus, the steel plate of the present invention is suitable for use in high heat input welding.

EXAMPLES The present invention will be specifically described below based on examples. In addition, the following examples show a preferable example of the present invention, and do not limit the present invention in any way. In addition, the following examples can be modified within the scope of the present invention, and such aspects are also included in the technical scope of the present invention.

Molten steel having the chemical composition shown in Table 1 was prepared and cast under the conditions shown in Table 2 to form a steel material (slab). A rolling step and a cooling step were sequentially performed to obtain each steel plate.

For each steel plate obtained, the average grain size and density of TiN particles at a depth of 1 mm from the surface, and the depth of 1/2 the plate thickness from the surface (in this example, the center of the plate thickness, also referred to as 1/2t) , the volume fraction of bainite was measured. In addition, yield strength (YS) and base material toughness (vE-40°C) were evaluated as base material properties of the steel sheets.
Furthermore, a joint test plate sampled from each of the above steel plates was subjected to V-groove processing, and a high heat input welding of 200 kJ / cm was performed using a commercially available welding wire for low temperature steel. Joints were produced by heat input welding. Then, the obtained joint was used to evaluate the HAZ toughness. Each test method is as follows. The properties evaluated using the joint obtained in this way were defined as the joint properties.

[Average particle size and density of TiN particles at a depth of 1 mm from the surface]
A sample was taken from the steel plate so that the observation surface was located at a depth of 1 mm from the surface of the steel plate. A thin film sample was prepared from the collected sample by an extraction replica method, and a 10 μm×10 μm range was photographed using a transmission electron microscope (TEM). Furthermore, by EDX analysis, for precipitates containing 10% or more of Ti and N, the area circle equivalent diameter and number of precipitates are analyzed from the photographed image using an image analyzer, and the average grain size and density are calculated. bottom.

[Microstructure at 1/2t]
(Bainite volume ratio)
A sample was taken from the steel plate so that the central portion of the plate thickness was the observation surface. After the surface of the collected sample was mirror-polished and further subjected to nital corrosion, a 10 mm×10 mm range was photographed using a scanning electron microscope (SEM). The photographed image was analyzed using an image analyzer as follows to obtain the fraction of the bainite structure, and the obtained value was defined as the volume fraction of bainite.

In any case, the discrimination of the bainite structure when obtaining the fraction of the fine structure was performed as follows. That is, the photographing was performed by magnifying 500 to 3000 times to obtain a SEM image. In the SEM image, a structure having an elongated lath-shaped ferrite structure and containing carbides having an equivalent circle diameter of 0.05 μm or more was identified as a bainite structure.

[Base material properties]
(base material strength)
A JIS Z 2201 No. 14A test piece was taken from the thickness center of the steel sheet in the direction perpendicular to the rolling direction so that the thickness center (1/2 thickness position) was the center of the test piece. The sampled test piece was subjected to a tensile test in accordance with JIS Z 2241, and the yield strength YS (unit: MPa) was measured as the strength of the base material.

(base material toughness)
An NKU No. 4 impact test piece was taken in a direction parallel to the rolling direction from the 1/4 plate thickness position of the steel plate so that the 1/4 plate thickness position was the center of the test piece and the notch position. A Charpy impact test was performed on the sampled test pieces at a test temperature of -40 ° C., and the average value vE-40 ° C. (unit: J) of the absorbed energy of three test pieces performed under the same conditions was taken as the base material toughness. bottom.

[HAZ characteristics]
(Toughness of HAZ)
An NKU No. 4 impact test piece was taken so that the surface of the joint obtained by high heat input welding to a depth of 1 mm was the surface layer of the test piece, and the HAZ was the notch position. A Charpy impact test was performed on the sampled test pieces at a test temperature of -20 ° C., and the average value vE-20 ° C. (unit: J) of the absorbed energy of the three test pieces performed under the same conditions was used as the HAZ toughness. .
The evaluation results thus obtained are also shown in Table 2.

As shown in Table 2, all the invention examples according to the present invention have a large heat input HAZ vE-20°C of 46 J or more. In addition, the steel sheets manufactured in the suitable range all exhibit high strength with a base material YS of 390 MPa or more and high base material toughness with a vE-40 ° C. of 53 J or more, and both base material strength and base material toughness are achieved. . Thus, it can be seen that the steel sheets of the invention examples are excellent in large heat input weldability.

On the other hand, steel plate No. 1 corresponding to the comparative example. Nos. 18 to 40 and 56 to 64 have low HAZ toughness or low HAZ toughness and low base metal YS because none of the chemical compositions of the steel materials satisfy the conditions of the present invention. I understand.

In addition, the steel plate No. corresponding to the comparative example. 5 and 6, although the chemical composition of the steel material satisfies the conditions of the present invention, the average cooling rate during casting does not satisfy the conditions of the present invention. It can be seen that at least one of the densities falls outside the scope of the present invention, and thus the toughness of the HAZ is low.

As described above, it can be seen that by following the present invention, it is possible to provide a steel plate having excellent toughness in a joint after high heat input welding.

Claims (6)

in % by mass,
C: 0.045% or more and 0.080% or less,
Si: 0.02% or more and less than 0.10%,
Mn: 1.60% or more and less than 1.95%,
P: 0.010% or less,
S: 0.010% or less,
Al: 0.010% or more and 0.100% or less,
Nb: 0.005% or more and 0.050% or less,
O: 0.0100% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
V: 0.30% or less,
Ti: 0.010% or more and 0.025% or less and N: 0.0038% or more and 0.0084% or less, and Ti/N, which is the mass% ratio of Ti and N, is 2.10 3.60 or less and satisfies the following formula (1), further, the carbon equivalent Ceq represented by the following formula (2) is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities have a certain composition,
A steel sheet, wherein TiN particles present at a depth of 1 mm from the surface of the steel sheet have an average particle size of 20 nm or more and 50 nm or less and a density of 5.0×10 ⁸ pieces/cm ² or more.
169≦5158×Ti+25563×N≦309 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (2)
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained. Furthermore, in mass %,
W: 0.30% or less,
Co: 0.30% or less,
B: 0.0100% or less,
Ca: 0.0100% or less,
The steel sheet according to claim 1, containing one or more selected from Mg: 0.0100% or less and REM: 0.0200% or less. The steel sheet according to claim 2, wherein the B content is B: 0.0002% or more and 0.0012% or less. A method for manufacturing a steel plate according to any one of claims 1 to 3 ,
in % by mass,
C: 0.045% or more and 0.080% or less,
Si: 0.02% or more and less than 0.10%,
Mn: 1.60% or more and less than 1.95%,
P: 0.010% or less,
S: 0.010% or less,
Al: 0.010% or more and 0.100% or less,
Nb: 0.005% or more and 0.050% or less,
O: 0.0100% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
V: 0.30% or less,
Ti: 0.010% or more and 0.025% or less and N: 0.0038% or more and 0.0084% or less, and Ti/N, which is the mass% ratio of Ti and N, is 2.10 3.60 or less and satisfies the following formula (1), further, the carbon equivalent Ceq represented by the following formula (2) is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities After casting molten steel having a certain chemical composition to obtain a steel material, hot rolling is performed using the steel material,
A method for producing a steel sheet, wherein the average cooling rate in the casting is 100° C./min or more and 500° C./min or less.
169≦5158×Ti+25563×N≦309 (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (2)
However, each element symbol represents the content (% by mass) of each component, and is set to 0 when not contained. The molten steel is further, in mass%,
W: 0.30% or less,
Co: 0.30% or less,
B: 0.0100% or less,
Ca: 0.0100% or less,
The method for producing a steel sheet according to claim 4, containing one or more selected from Mg: 0.0100% or less and REM: 0.0200% or less. The method for manufacturing a steel plate according to claim 5, wherein the B content of the molten steel is B: 0.0002% or more and 0.0012% or less.