KR100997341B1 - Steel plate excellent in toughness of large heat-input weld heat affected zone - Google Patents

Abstract

The subject of this invention is not only the case where the welding heat input welds, for example, the super heat input welding which becomes 60 kJ / mm or more with respect to the low S steel plate whose S content was reduced to about 0.005% or less, but also the welding heat input amount. To provide a steel sheet that can exhibit excellent HAZ toughness even when welding is performed at a relatively large heat input amount of, for example, 15 kJ / mm or more.

What is necessary is just to set the amount of solid solution N contained in the said steel plate into 0.0010 to 0.0060% with respect to the steel plate which contains C, Mn, P, S, Al, Ti, N, Ca, O as needed, and contains Si as needed.

High heat input welding, HAZ toughness, steel plate, ferrite, austenitic

Description

Steel plate excellent in toughness of welding heat affected zone in high heat input welding {STEEL PLATE EXCELLENT IN TOUGHNESS OF LARGE HEAT-INPUT WELD HEAT AFFECTED ZONE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel sheets applied to welded structures, such as ships, buildings, and offshore structures. More particularly, the present invention relates to steel materials for improving the toughness of steel sheets when high heat input welding with a welding heat input amount of about 15 kJ / mm or more is performed. .

Structures in various fields, such as ships, buildings, and offshore structures, are generally constructed by welding steels together.However, the steels used in such structures are not only for the strength of the steel itself but also for the toughness of the welded portion in view of securing safety. Good thing is also required.

In addition, in recent years, with the increase in the size of the welded structure, in view of the improvement in the construction efficiency of the structure and the reduction in the construction cost, the improvement of the welding construction efficiency has been required, and the increase of the welding heat input is directed. In particular, superheat input welding tends to be performed in which the welding heat input becomes 60 kJ / mm or more.

In performing the above super heat input welding, in the site | part which receives the heat influence of a welding base material (steel plate as a to-be-welded material) (henceforth a "welding heat affected zone" or "HAZ"). Toughness matters. This HAZ is a position of several millimeters on the base material side than the interface (bond portion) of the weld metal and the base material, so that the base material is exposed to high temperature just below the melting point during welding, and the austenite particles in the metal structure tend to coarsen, and The cooling rate is also slowed down by the increase in the heat input of the weld, so that coarse structure is easily formed. For this reason, there was a problem that the HAZ toughness tends to be lowered.

As a technique for improving the HAZ toughness, various techniques have been proposed until now. For example, Nonpatent Document 1 describes that the toughness of the weld bond portion is improved by finely dispersing TiN in a steel sheet, and this Nonpatent Document 1 examines the toughness improving mechanism. In this non-patent document 1, the toughness improvement effect is not obtained due to the decrease of the solid solution N in the steel sheet, but it is evident that the effect of changing the HAZ structure from the upper bainite structure to the fine ferrite + pearlite structure is large. have.

In addition, Non-Patent Document 2 describes that HAZ toughness deteriorates when there is too much solid solution N. In order to improve HAZ toughness, the N content in the steel sheet is reduced to N (specifically, the content of N is reduced to about 0.0015%). It is described what is necessary.

In addition, Patent Document 1 discloses that in order to improve HAZ toughness in super heat input welding, the amount of solid solution N which adversely affects toughness is thoroughly reduced to around 0 ppm, and the particle diameter refinement effect is also observed in the high temperature region near the melting point. It is described that an oxide can be utilized. Specifically, from the viewpoint of thoroughly reducing the amount of solid solution N, it has been described to improve the HAZ toughness in superheat input welding by containing Ti and a sufficient amount of Al and using Ca oxide as a fine oxide. . In addition, in this patent document 1, HAZ toughness is evaluated by measuring the absorption energy of the Charpy impact test in 0 degreeC.

[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-107177

[Non-Patent Document 1] Journal of Welding Society, Vol. 2, No. 1, P. 33-39 (published in 1984)

[Non-Patent Document 2] Korean Welding Society, Vol. 3, No. 4, P. 758-766 (published 1985)

As described above, Non-Patent Literatures 1 and 2 have been studied to improve HAZ toughness by dispersing TiN in a steel sheet or reducing the amount of solid solution N. However, since the desulfurization ability in the steelmaking stage was very low in the era (1984 to 1985) in which the technology of Non-Patent Documents 1 and 2 was disclosed, the S content contained in the steel sheets disclosed in Non-Patent Documents 1 and 2 is It is large (specifically, 0.0060% or more).

On the other hand, it is 2001 that the technique of the said patent document 1 was disclosed, and the desorption capability in the steelmaking stage is improved compared with the time when the said nonpatent literature 1 or 2 was disclosed. Therefore, this patent document 1 has proposed the technique of improving the toughness of a super heat input welding heat affected zone with respect to the steel plate which suppressed S content to 0.002 to 0.006%. However, also in the technique disclosed in this patent document 1, in order to improve HAZ toughness, the amount of solid solution N is reduced to around 0 ppm, and even in the times of non-patent documents 1 and 2, even in the time of patent document 1, which is close to modern times, HAZ In order to improve toughness, it was expected to reduce the amount of N employed.

This invention is made | formed in view of such a situation, The objective is the super heat-heat-weld welding that a welding heat input amount becomes 60 kJ / mm or more, for the low S steel plate which S content was reduced to about 0.005% or less, for example. The present invention provides a steel sheet capable of exhibiting excellent HAZ toughness even in the case where welding is performed at a relatively large heat input amount of, for example, 15 kJ / mm or more. Further, another object of the present invention is to provide a steel sheet exhibiting good HAZ toughness even at low temperatures.

The steel sheet which concerns on this invention which could solve the said subject is C: 0.03 to 0.15% (mean of mass%, the same is hereafter), Si: 0.50% or less (including 0%), Mn: 1 to 2.0 %, P: 0.020% or less (0% not included), S: 0.005% or less (0% not included), Al: 0.005 to 0.06%, Ti: 0.005 to 0.030%, N: 0.005 to 0.015% , Ca: 0.001% to 0.0035% and O: 0.0015% or less (does not contain 0%), respectively, and the point is that the solid solution N contained in the steel sheet is 0.0010% to 0.0060%.

It is preferable that content of the chemical component contained in the said steel plate satisfy | fills the relationship prescribed | regulated by following formula (1) and / or following formula (2). However, [] shows content (%) of each element.

[Equation 1]

1.00 ≤ [Ti] / [N] ≤ 2.5

[Equation 2]

2.00 ≤ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≤ 10.0

The steel sheet of the present invention is based on a steel sheet containing the above chemical components, the balance of which is made of iron and unavoidable impurities. In the steel sheet of the present invention, as another element,

(a) B: 0.0035% or less (not including 0%),

(b) Cu: 2% or less (does not contain 0%), Ni: 2% or less (does not contain 0%) and Cr: 1.5% or less (does not contain 0%) One or more elements,

(c) Mo: 0.5% or less (not including 0%),

(d) Nb: 0.035% or less (does not contain 0%) and / or V: 0.1% or less (does not contain 0%),

(e) Mg: 0.005% or less (not including 0%),

(f) Zr: 0.1% or less (does not contain 0%) and / or Hf: 0.05% or less (does not contain 0%),

(g) Co: 2.5% or less (does not contain 0%) and / or W: 2.5% or less (does not contain 0%),

(h) REM: 0.01% or less (not including 0%)

And the like.

In the present invention, the HAZ toughness is long for not only the conventional steel sheet containing about 0.006% or more of S, but also the chemical composition of the low S steel sheet having an S content of about 0.005% or less. HAZ toughness at the time of high heat input welding can be improved by positively containing (or remaining) solid solution N which was judged to have an adverse effect on the S steel sheet. In particular, the low-sizing steel sheet of the present invention exhibits excellent HAZ toughness even when superheated welding is performed, such as 60 kJ / mm or more of weld heat input, and thus is very useful as a material for various building structures and the like. Moreover, according to this invention, the steel plate which shows favorable HAZ toughness even at low temperature of -55 degreeC level can be provided.

The present inventors studied various factors about the deterioration of HAZ toughness at the time of performing the high heat input welding whose welding heat input amount is about 15 kJ / mm or more about the low S steel plate whose S amount was reduced to about 0.005% or less. Repeated. As a result, suppressing coarsening of austenite in a region heated to a high temperature during welding, and preventing coarsening of ferrite produced during cooling after welding, improves HAZ toughness even for a reduced S steel sheet. I got the knowledge that I could.

Therefore, the present inventors made further studies from the viewpoint of suppressing the coarsening of the ferrite particles. As a result, when the transformation from austenite to ferrite occurs at a high temperature during cooling after welding, it is easy to cause grain growth until the completion of cooling, so that the chemical composition of the steel sheet is reduced so that the start temperature of the ferrite transformation is as low as possible. What is necessary is just to design a composition, and if solid solution N is included in 0.0010 to 0.0060% of range, the initiation temperature of ferrite transformation can be made low, and coarsening of ferrite can be prevented at the time of cooling after welding, and HAZ toughness is improved. I found something that could be done.

In order to make the start temperature of a ferrite transformation low, it is judged that what is necessary is just to solidify the austenite stabilizing element which is easy to stabilize austenite as much as possible. C, Ni, Mn, etc. are known as this austenite stabilizing element. However, since all of these elements significantly increase the carbon equivalent, they are elements that easily cause low temperature cracking and the like during welding, and deteriorate the weldability. This carbon equivalent is a value computed from the amount of alloying elements contained in a steel plate in consideration of weld embrittlement, and is computed by a following formula. As the value of the carbon equivalent (PCM) increases, weld cracking tends to occur. In addition, in formula, [] has shown content (%) of each element.

PCM (%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B]

Therefore, the present inventors have repeatedly studied to delay the start of ferrite transformation and shift the transformation start temperature to the low temperature side without increasing the carbon equivalent and deteriorating weldability. As a result, since the solid solution N hardly affects the carbon equivalent, it became clear that weldability can be maintained in a favorable state. Furthermore, it became clear that good HAZ toughness can be achieved even at low temperatures at -55 ° C.

That is, since many sulfide-based inclusions were produced in the steel sheet during the time when the desorption ability in the steelmaking process was low, austenite is hardly coarsened and also acts as a ferrite metamorphic nucleus. When it contained a lot, only the bad effect by solid solution N was outstanding, and HAZ toughness was deteriorated. However, if the desorption ability in steelmaking process is improved and the steel sheet is lowered as of the present, the amount of sulfide inclusions present in the steel sheet is reduced, so that even if such steel sheet contains solid solution N, there is a problem of deterioration in HAZ toughness. Rather, it is possible to delay the onset of ferrite transformation without increasing the carbon equivalent by actively containing solid solution N.

However, if the amount of solid solution N is less than 0.0010%, the effect of delaying the onset of ferrite transformation cannot be obtained. Therefore, the amount of solid solution N is made 0.0010% or more. The solid solution N amount is preferably 0.0013% or more, more preferably 0.0015% or more, and still more preferably 0.0018% or more. However, when the amount of solid solution N exceeds 0.0060% and becomes excessive, since solid solution N will adhere to the electric potential in steel, HAZ toughness will deteriorate rather. Therefore, the amount of solid solution N is made into 0.0060% or less, Preferably it is 0.0055% or less, More preferably, it is 0.005% or less, More preferably, it is 0.004% or less.

As described above, the steel sheet of the present invention can lower the start temperature of the ferrite transformation during cooling after welding by actively containing the solid solution N, which is conventionally expected to be reduced as much as possible, By suppressing the growth of ferrite particles, the HAZ toughness can be improved, and the chemical composition of the steel sheet needs to be appropriately adjusted. The chemical composition of a steel plate is mentioned later.

By the way, the steel sheet of the present invention is preferably as fine as possible ferrite particles generated during cooling after welding, but in order to make the ferrite particles fine, while pinning not to coarsen the austenite under the heat effect during welding, It is preferable that the inclusions which become transformation nuclei when transforming from austenite to ferrite during cooling after welding are finely dispersed in the steel sheet. While exhibiting such a pinning effect, inclusions serving as ferrite transformation nuclei include, for example, CaS and TiN, and MnS producing these as nuclei also exhibits the same effect. In order to fine-disperse these inclusions, after adjusting the chemical composition of a steel plate suitably as mentioned later, content of Ti and N satisfy | fills following formula 1, or content of Ca, S, O satisfy | fills following formula 2 It is preferable to make it. Of course, it is especially preferable to satisfy following formula 1 and following formula 2 simultaneously.

Equation 1 below shows the balance of the contents of Ti and N. In order to finely disperse TiN and to have a large number of nuclei of ferrite, it is preferable to adjust the components so that the contents of Ti and N satisfy the following formula (1). However, in Formula 1, [Ti] and [N] represent content (mass%) of Ti and N, respectively.

[Equation 1]

1.00 ≤ [Ti] / [N] ≤ 2.5

If the value of [Ti] / [N] (hereinafter, sometimes referred to as "P value") is less than 1.00, the amount of TiN produced is small and the effect of improving the HAZ toughness is not sufficiently exhibited. Therefore, it is preferable to make P value 1.00 or more, More preferably, it is 1.30 or more, More preferably, it is 1.50 or more, Especially preferably, it is 1.60 or more. However, when P value exceeds 2.5, TiN will coarsen and HAZ toughness will tend to worsen rather. Therefore, the P value is preferably 2.5 or less, more preferably 2.30 or less, still more preferably 2.20 or less, and particularly preferably 2.10 or less.

Equation 2 below shows the balance of the contents of Ca, S, and O. In order to finely disperse CaS after CaO is produced in the step of solidifying molten steel in the manufacturing process of the steel sheet, it is preferable to adjust the components so that the content of Ca, S, and O satisfies the following formula (2). The coefficient for each component represents the degree to which each element contributes to the refinement of CaS, and is obtained by experiments performed by the present inventors repeatedly. That is, it shows that the tendency to disperse CaS in high density in order of O, S, and Ca in the range of the chemical component prescribed | regulated by this invention is shown. However, in Formula 2, [Ca], [S], and [O] represent content (mass%) of Ca, S, and O, respectively.

[Equation 2]

2.00 ≤ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≤ 10.0

When the value of 1000 × ([Ca] + 2 × [S] + 3 × [O]) (hereinafter sometimes referred to as "Q value") is less than 2.00, the amount of CaS produced decreases and HAZ toughness improvement effect is reduced. It is not enough. Therefore, it is preferable to make Q value into 2.00 or more, More preferably, it is 3.00 or more, More preferably, it is 4.00 or more. However, when the Q value exceeds 10.0, CaS tends to coarsen and the HAZ toughness tends to be worse. Therefore, the Q value is preferably 10.0 or less, more preferably 9.00 or less, still more preferably 8.00 or less, particularly preferably 7.00 or less.

In the steel sheet of this invention, in order to exhibit the characteristic, also controlling the chemical composition is also an important requirement. The reason for limiting the range including the solid solution N content and the elements (Ti, N, Ca, S and O) involved in the above formulas 1 to 2 is as follows.

[C: 0.03 to 0.15%]

C is an element necessary for securing the strength of the steel sheet (welding base material), and in order to secure the desired strength, it is necessary to contain C at least 0.03%. C content becomes like this. Preferably it is 0.05% or more, More preferably, it is 0.055% or more. However, when C is excessively contained, HAZ toughness will fall rather. From this, the upper limit needs to be 0.15%. C content becomes like this. Preferably it is 0.13% or less, More preferably, it is 0.10% or less, More preferably, it is 0.080% or less, Especially preferably, it is 0.075% or less.

[Si: 0.50% or less (including 0%)]

Si is an effective element for securing the strength of the steel sheet, and is included as necessary. However, when it contains too much, a large amount of island martensite phase (M-A phase) will precipitate in steel materials (base material), and HAZ toughness will deteriorate. From this, the upper limit was made into 0.50%. The upper limit with preferable Si content is 0.40%, and a more preferable upper limit is 0.30%. Moreover, the preferable minimum at the time of positively containing Si is 0.1%. Si may be 0%.

[Mn: 1 to 2.0%]

Mn is an effective element in improving the hardenability and securing the strength of the steel sheet. In order to exert such an effect, it is necessary to contain Mn 1% or more. The minimum with preferable Mn content is 1.3%. However, when Mn is excessively contained, the HAZ toughness of the steel sheet is deteriorated, so the upper limit is made 2.0%. The upper limit with preferable Mn content is 1.8%, and a more preferable upper limit is 1.7%.

[P: 0.020% or less (not including 0%)]

P is an unavoidable impurity, and since it adversely affects the toughness and HAZ toughness of the steel sheet, it is preferable to use as few as possible. From this viewpoint, P is suppressed to 0.020% or less. P content becomes like this. Preferably it is 0.017% or less, More preferably, it is 0.015% or less, More preferably, it is 0.010% or less, Especially preferably, it is 0.0075% or less.

[S: 0.005% or less (not including 0%)]

S is an element that effectively forms CaS when molten steel solidifies during casting, forms MnS using CaS as a nucleus after welding, and effectively forms ferrite in the HAZ portion. In order to effectively exhibit such an effect, it is preferable to contain S 0.0003% or more, and the effect increases as the content increases. However, when it contains more than 0.005%, the toughness and HAZ toughness of a base material will deteriorate. Therefore, S content is made into 0.005% or less. S content becomes like this. Preferably it is 0.0030% or less, More preferably, it is 0.0020% or less, More preferably, it is 0.0010% or less, Most preferably, it is 0.0007% or less. In order to reduce S to a predetermined range, the desulfurization time may be relatively long (for example, 25 minutes or more).

[Al: 0.005% to 0.06%]

Al is an element that effectively acts as a deoxidizer and also exhibits an effect of miniaturizing the microstructure of the steel sheet to improve base material toughness. In order to exert such an effect, Al content is made into 0.005% or more. The minimum with preferable Al content is 0.010%, and a more preferable minimum is 0.020%. However, when it contains too much, a large amount of island martensite phase (M-A phase) will precipitate on a steel plate (base material), and HAZ toughness will deteriorate. From this, the upper limit is made into 0.06%. The upper limit with preferable Al content is 0.040%, and a more preferable upper limit is 0.035%.

[Ti: 0.005% to 0.030%]

Ti is an element that forms a nitride and suppresses coarsening of the old austenite particles at the time of high heat input welding, thereby effectively improving HAZ toughness. In order to exhibit such an effect, Ti content is made into 0.005% or more. The minimum with preferable Ti content is 0.0080%, and a more preferable minimum is 0.010%. However, excessively containing Ti precipitates coarse inclusions and rather deteriorates HAZ toughness. Therefore, the upper limit of Ti content is made into 0.030%. The upper limit with preferable Ti content is 0.025%.

[N: 0.005% to 0.015%]

N is an element required to secure a predetermined amount of solid solution N to improve HAZ toughness. In addition, N is an element that effectively acts to increase the HAZ toughness at the time of heat input welding by preventing TiN from being precipitated finely in the old austenite particles to prevent coarsening of the old austenite particles by the pinning effect. In addition, TiN also functions as a ferrite transformation nucleus and also serves to refine HAZ tissue. In order to exhibit such an effect, N content needs to be 0.005% or more. The minimum with preferable N content is 0.0060%. However, when N content becomes too much and exceeds 0.015%, coarse TiN will precipitate and HAZ toughness will fall. The upper limit with preferable N content is 0.012%, a more preferable upper limit is 0.010%, a more preferable upper limit is 0.0090%, and an especially preferable upper limit is 0.0080%.

[Ca: 0.001% to 0.0035%]

Ca is an element which controls the form of sulfide in a steel plate and contributes to the improvement of HAZ toughness. In order to exhibit such an effect, it is necessary to contain 0.001% or more. Ca content becomes like this. Preferably it is 0.0013% or more, More preferably, it is 0.0015% or more, More preferably, it is 0.0020% or more. However, even when it contains exceeding 0.0035% too much, HAZ toughness deteriorates rather. Therefore, Ca content is made into 0.0035% or less. Ca content becomes like this. Preferably it is 0.0030% or less.

[O (oxygen): 0.0015% or less (does not contain 0%)]

O is incorporated as an unavoidable impurity, but exists as an oxide in steel. However, if the content exceeds 0.0015%, coarse oxides (for example, CaO, etc.) are formed, and HAZ toughness deteriorates. From this, O content is made into 0.0015% or less. The upper limit with preferable O content is 0.0013%.

In the steel sheet of the present invention, in addition to the above chemical components, iron and inevitable impurities (for example, Sb, Se, Te, etc.) may be included, but a trace component (acceptable component) may not contain the properties thereof. will be. The steel plate containing such a trace component is also included in the scope of the present invention.

In addition, the steel sheet of the present invention, if necessary, as another element, at least one element selected from the group consisting of (a) B, (b) Cu, Ni and Cr, (c) Mo, (d) Nb and / Or containing elements such as V, (e) Mg, (f) Zr and / or Hf, (g) Co and / or W, and (h) REM. The range at the time of containing these components and the reason which set the range are as follows.

[(a) B: 0.0035% or less (not including 0%)]

B is an element that effectively acts to improve the HAZ toughness by generating an intraparticle ferrite containing BN as a nucleus in the vicinity of the bond portion of the HAZ when subjected to superheat input welding. However, when the B content is excessive, the structure of the bond portion becomes coarse bainite structure, so that the HAZ toughness deteriorates. From this, when containing B, it is good to make the upper limit into 0.0035%. B content becomes like this. Preferably it is 0.0025% or less. In addition, the minimum with preferable B content is 0.0010%.

[(b) from the group consisting of Cu: 2% or less (does not contain 0%), Ni: 2% or less (does not contain 0%), and Cr: 1.5% or less (does not contain 0%) One or more elements selected]

Cu, Ni, and Cr are all elements that effectively act to increase the hardenability and improve the strength of the steel sheet. However, when the content of these elements is excessive, the HAZ toughness is rather lowered. Therefore, it is preferable to set it as 2% or less with respect to Cu, More preferably, it is 1% or less, More preferably, it is good to set it as 0.5% or less. About Ni, it is preferable to set it as 2% or less, More preferably, it is 1% or less, More preferably, it is good to set it as 0.5% or less. About Cr, it is preferable to set it as 1.5% or less, More preferably, you may set it as 1.0% or less. The minimum with preferable all for effectively exhibiting the said effect is 0.20%, and a more preferable minimum is 0.40%.

[(c) Mo: 0.5% or less (does not contain 0%)]

Mo is an element that effectively acts roughly and roughly to improve the hardenability and secure the strength, and is an element that acts to prevent tempering brittleness. This effect increases as the content increases, but when the Mo content is excessive, the HAZ toughness deteriorates, so it is preferable to make it 0.5% or less. Mo content is more preferably 0.3% or less. The minimum with preferable Mo content for showing the said effect effectively is 0.05%.

((d) Nb: 0.035% or less (does not contain 0%) and / or V: 0.1% or less (does not contain 0%)]

Nb and V are elements that act to improve the hardenability and improve the strength of the base metal. In addition, V also serves to increase the tempering softening resistance. However, since HAZ toughness deteriorates when it contains a large amount, it is preferable to make Nb content into 0.035% or less, More preferably, it is good to set it as 0.030% or less. The V content is preferably 0.1% or less, and more preferably 0.050% or less. In order to exhibit such an effect effectively, it is preferable to make Nb content into 0.005% or more, and to make V content into 0.010% or more.

[(e) Mg: 0.005% or less (not including 0%)]

Mg is an element which forms MgO in a steel plate, and this MgO acts to suppress coarsening of austenite particle in a HAZ part, and has the effect of improving HAZ toughness. However, when the Mg content is excessive, the inclusion MgO is coarsened and the HAZ toughness is rather deteriorated, so the content is preferably 0.005% or less. Mg content becomes like this. More preferably, it is 0.0035% or less. The minimum with preferable Mg content for exhibiting the said effect effectively is 0.001%.

[(f) Zr: 0.1% or less (does not contain 0%) and / or Hf: 0.05% or less (does not contain 0%)]

Zr and Hf, like Ti, combine with N to form a nitride, and the nitride is an element that effectively works to improve the toughness of HAZ by miniaturizing austenite particles of HAZ during welding. However, when it contains too much, HAZ toughness will fall rather. For this reason, when including these elements, it is preferable to make Zr into 0.1% or less, and it is preferable to make Hf into 0.05% or less. The minimum with preferable Zr content for exhibiting the said effect effectively is 0.001%, and the minimum with preferable Hf content is 0.001%.

[(g) Co: 2.5% or less (does not contain 0%) and / or W: 2.5% or less (does not contain 0%)]

Co and W are elements that act to increase the base material strength by improving the hardenability. However, when it contains too much, since HAZ toughness deteriorates, it is good to set both the upper limit of Co and W to 2.5%. The upper limit with more preferable Co content is 2%, and a more preferable upper limit is 1.5%. The upper limit with more preferable W content is 2%, and a more preferable upper limit is 1.5%. The minimum with preferable Co content is 0.1%, and the minimum with preferable W content is 0.1%.

[(h) REM: 0.01% or less (not including 0%)]

REM (rare earth element) is an element which contributes to the toughness improvement of HAZ by making the shape of inclusions (for example, an oxide, a sulfide, etc.) which are inevitably mixed in a steel plate fine and spherical. This effect increases as the content increases, but when the content of REM is excessive, the inclusions of the REM itself coarsen and the HAZ toughness deteriorates rather. Therefore, it is preferable to suppress the content to 0.01% or less. REM is more preferably suppressed to 0.0080% or less. The minimum with preferable REM content is 0.0005%.

In the present invention, REM means a lanthanide element (15 elements from La to Ln) and Sc (scandium) and Y (yttrium).

The method for producing the steel sheet of the present invention is not particularly limited, and according to a conventional method, the chemical component composition, its balance, and the amount of solid solution N may be adjusted in the above range.

In order to adjust the amount of solid solution N in the above range, for example, the slab is heated to a predetermined temperature, and then hot rolling may be performed to perform a quenching treatment as necessary.

The temperature at which the slab is heated is exemplified by a method of calculating the appropriate slab heating temperature by thermodynamic calculation by inputting the chemical composition of the steel sheet to, for example, integrated thermodynamic calculation software (available from Thermo-calc, an integrated laboratory of CRC). That is, the amount of solid solution N is known to be affected by the heating temperature of the slab, the amount of N of the base metal, or the amount of addition of nitride forming elements such as Ti and Nb, and the amount of addition of other elements such as C, Si, and Al, but the thermodynamic calculation The target can be obtained by the relationship between the heating temperature of the slab and the chemical composition of the steel sheet.

Therefore, in the present invention, in order to adjust the amount of solid solution N to the above range, the amount of alloying elements included in the base material is substituted in advance by thermodynamic calculation by integrated thermodynamic calculation software (Thermo-calc) to determine the amount of solid solution N and slab heating temperature. The relational expression was calculated | required, the range of slab heating temperature for adjusting to the range of predetermined | prescribed solid solution N amount is calculated, and the slab is heated so that it may become this temperature range, and it hot-rolled.

In addition, the amount of solid solution N is also affected by the cooling rate after hot rolling in addition to the slab heating temperature, and when the cooling rate is small, the amount of solid solution N is small because the nitride easily precipitates. Therefore, as a factor other than slab heating temperature, even if it controls the average cooling rate in the temperature range of 800 degreeC to 500 degreeC after hot rolling in the range of 7-12000 degreeC / min, for example, you may fine-adjust the solid solution N amount. do.

What is necessary is just to make deoxidation time in a steelmaking step longer than usual in order to control oxygen amount (O amount) so that the balance of content of Ca, S, and O may satisfy said formula (2). That is, although the normal deoxidation time is about 20 minutes, in order to reduce oxygen amount, it is preferable to make deoxidation time relatively long for 30 minutes or more. Although the upper limit of deoxidation time is not specifically limited, For example, it is about 40 minutes.

The steel plate made into object by this invention assumes the thick steel plate which has a plate | board thickness of 60 mm or more basically, and has the same characteristic also in the plate | board thickness below that, and will be included in the subject of this invention. In addition, the heat input amount at the time of welding the steel plate of this invention assumes 60 kJ / mm or more, and when welding is performed with such super high charge amount, it shows favorable HAZ toughness, but it is not limited to this heat input amount, For example, even a heat input amount of 15 kJ / mm or more shows good HAZ toughness.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are not intended to limit the present invention, and all modifications to the design in view of the preceding and later aspects are included in the technical scope of the present invention. will be.

[First Embodiment]

After the steel (residual iron and unavoidable impurities) of the component composition shown in following Table 1 or Table 2 was melt | dissolved by the conventional solvent method, this molten steel was cast into slab, and the said slab is shown in Table 3 or Table 4, It is heated to a temperature (actual value) and hot-rolled to make a hot-rolled sheet having a thickness of 60 mm, followed by water cooling and quenching after the end of hot-rolling to give a tensile strength of 490 to 780 MPa (50 to 80 kg / mm2). Various high tension steel sheets (test plates) were manufactured. In the steelmaking step, the deoxidation time was set to 30 minutes.

In Table 1 and Table 2 below, REM was added in the form of a misch metal containing about 50% La and about 25% Ce.

The heating temperature of the slab was controlled while taking into account the slab heating temperature (calculated value) calculated from Thermo-calc in relation to the amount of solid solution N as described above.

In Table 3 or Table 4, P values ([Ti] / [N]) and Q values [1000 × ([Ca] + 2 × [S]) specified in the present invention are determined from the component compositions shown in Tables 1 or 2 below. ] + 3 × [O])] was also shown.

The actual amount of solid solution N of the various high tensile steel sheets obtained in this way was measured. The amount of solid solution N was extracted by electrolytic extraction, and the amount of nitride was measured by the indophenol absorption spectrophotometry, and the total amount of N contained in the steel sheet was subtracted from the amount of nitride N. The measurement results are shown in Table 3 or Table 4 below.

Solid solution N amount (mass%) = N amount (mass%) which consists of total N amount (mass%)-nitride contained in a steel plate

Next, the obtained high tensile strength steel plate (plate | board thickness is 60 mm) was welded on condition of the following, the welding part was produced, and HAZ toughness was evaluated.

[Welding condition]

Welding Method: Electro Gas Arc Welding

Welding Current: 400 A

Welding voltage: 40 V

Welding speed: 0.6 mm / sec

Heat input amount: 60 kJ / ㎜

Welding Wire: DWS-50GTR, DWS-50GTF

Improved shape: Improvement angle 18 ° (reverse V improvement), root spacing 10 mm

In the welded portion thus obtained, as shown in Fig. 1, the Charpy impact test specimen (height 10 mm × width 10 mm × length 55 mm) specified in JIS Z2202 was selected from the position of t / 4 (t is the plate thickness) of the steel sheet. The sample was taken, a notch was placed at the position of the +0.5 mm base material side from the bond portion, and the V-notch Charpy impact value (vE- ₅₅ ) at -55 ° C was measured to evaluate HAZ toughness. The measurement results are shown in Table 3 or Table 4 below. At this time, the V-notch Charpy impact value (vE- ₅₅ ) in 60 kJ / mm of welding heat input made 100 J or more pass.

From the results shown in the following Tables 1 to 4, the following can be considered. No. 1 to 29 are examples satisfying the requirements specified in the present invention, and since the chemical composition of the steel sheet and the amount of solid solution N are properly controlled, excellent HAZ toughness is obtained even when superheat welding is performed with a welding heat input of 60 kJ / mm. Can be exercised.

In particular, No. 1 to 19 and No. 26-28, since the chemical composition of a steel plate satisfy | fills P value and Q value which are prescribed | regulated by this invention, HAZ toughness becomes favorable especially 300 J or more.

In this regard, 31 to 40 are examples which lack any one of the requirements defined in the present invention.

Among these is No. Although the chemical composition of the steel plate is satisfy | filling the range prescribed | regulated by this invention in 31-34, the solid solution N contained in a steel plate is an example out of the range prescribed | regulated by this invention, and HAZ toughness is bad. No. 35-40 is the amount of solid solution N contained in a steel plate satisfy | fills the range prescribed | regulated by this invention, but the chemical composition of a steel plate is an example out of the range prescribed | regulated by this invention, and HAZ toughness is bad.

No. 11 and No. 33, No. 14 and No. 34, No. 19 and No. The chemical composition (steel grade) of 32 is the same, but since the slab heating temperature is changed, the amount of solid solution N changes. That is, No. 32 to 34 are not suitable for slab heating temperature, and the amount of solid solution N does not fall within an appropriate range, so the HAZ toughness is bad.

Second Embodiment

HAZ toughness at the time of changing welding conditions was evaluated for the steel plate shown in the said 1st Example.

A plate having a thickness of 25 mm was cut out from the various high tensile steel sheets (60 mm in thickness) obtained in the first embodiment, and a welding joint was produced by performing electrogas arc welding with a heat input amount of 15 kJ / mm. Other welding conditions are the same as in the first embodiment.

In the welded portion thus obtained, the V-notch Charpy impact value (vE- ₅₅ ) at -55 ° C was measured under the same conditions as in the first example to evaluate HAZ toughness. The measurement results are shown in Table 3 or Table 4 below. At this time, the V-notch Charpy impact value (vE- ₅₅ ) in 15 kJ / mm of welding heat input made 100 J or more pass.

The steel materials (No. 1 to 29) of the present invention had good HAZ toughness even when welding was performed with a heat input amount of 15 kJ / mm. Therefore, the steel sheets (No. 1 to 29) of the present invention may be subjected to super heat input welding of 60 kJ / mm, as shown in the first embodiment, even if the heat input welding is performed with a heat input amount of 15 kJ / mm. It can be seen that the HAZ toughness.

On the other hand, steel sheets (Nos. 31 to 40) which deviate from the range defined by the present invention have a heat input amount of 60 kJ / mm, as shown in the first embodiment, even if the heat input amount is 15 kJ / mm. It can be seen that even the super heat input welding deteriorates the HAZ toughness.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic explanatory drawing which shows the sampling position of a Charpy impact test piece from a welding part.

Claims (11)

C: 0.03 to 0.15% (the meaning of mass%, hereinafter the same), Si: 0.50% or less (including 0%), Mn: 1 to 2.0%, P: 0.020% or less (not including 0%), S: 0.005% or less (not including 0%), Al: 0.005% to 0.06%, Ti: 0.005 to 0.030%, N: 0.005% to 0.015%, Ca: 0.001-0.0035% and O: 0.0015% or less (not containing 0%), respectively, and the remainder is a steel sheet composed of Fe and unavoidable impurities, Solid solution N amount contained in the said steel plate is 0.0010 to 0.0060%, The steel sheet excellent in the toughness of the weld heat affected zone in high heat input welding characterized by the content of the chemical component contained in the said steel plate satisfying the relationship prescribed | regulated by following formula (1). [Equation 1] 1.00 ≤ [Ti] / [N] ≤ 2.5 However, [] shows content (%) of each element. The steel sheet according to claim 1, wherein the weld heat affected zone has excellent toughness in high heat input welding in which a content of a chemical component contained in the steel sheet satisfies a relationship defined by the following expression (2). [Equation 2] 2.00 ≤ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≤ 10.0 However, [] shows content (%) of each element. The method according to claim 1 or 2, further as another element, B: Steel sheet excellent in the toughness of the weld heat affected zone in high heat input welding containing 0.0035% or less (not including 0%). The method according to claim 1 or 2, further as another element, 1% or more selected from the group consisting of Cu: 2% or less (does not contain 0%), Ni: 2% or less (does not contain 0%), and Cr: 1.5% or less (does not contain 0%) Steel plate excellent in the toughness of the weld heat affected zone in the high heat input welding containing the above. The method according to claim 1 or 2, further as another element, Mo: The steel sheet excellent in the toughness of the weld heat affected zone in the high heat input welding containing 0.5% or less (not containing 0%). The method according to claim 1 or 2, further as another element, Steel plate excellent in the toughness of the weld heat affected zone in the high heat input welding containing at least one selected from Nb: 0.035% or less (not including 0%) and V: 0.1% or less (not including 0%). . The method according to claim 1 or 2, further as another element, Mg: Steel sheet excellent in the toughness of the weld heat affected zone in high heat input welding containing 0.005% or less (not including 0%). The method according to claim 1 or 2, further as another element, Steel plate excellent in the toughness of the weld heat affected zone in the high heat input welding containing at least one selected from Zr: 0.1% or less (not including 0%) and Hf: 0.05% or less (not including 0%). . The method according to claim 1 or 2, further as another element, Steel sheet excellent in the toughness of the weld heat affected zone in the high heat input welding containing at least one selected from Co: 2.5% or less (not including 0%) and W: 2.5% or less (not including 0%). . The method according to claim 1 or 2, further as another element, REM: Steel plate excellent in the toughness of the weld heat affected zone in high heat input welding containing 0.01% or less (not including 0%). delete

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