JP5459166B2 - Steel plate for ice sea structure - Google Patents

Description

  The present invention relates to a steel plate used for a structure in a cold region, and more particularly to a steel plate for an ice sea structure having excellent ductile fracture characteristics of a base material at low temperatures and toughness of a weld heat affected zone.

  In recent years, energy demand has been increasing more and more, and development of offshore oil resources has been activated. Among the marine structures used for these, platforms and jackup rigs are becoming larger year by year, and as a result, steel materials such as steel plates become thicker, and higher reliability against destruction of structures can be secured. It is an important issue.

  The search areas for submarine oil resources have shifted to cold regions and deep waters in recent years, and marine structures operating in these regions or seas are extremely severe in weather compared to marine structures used in temperate regions.・ Exposed to ocean conditions.

  In particular, steel sheets used for ice-sea structures used in cold regions are required to have toughness at a very severe low temperature of, for example, −40 ° C. or less, and also have excellent weldability.

  Furthermore, in order to ensure the reliability against the destruction of the structure, the user's inspection standards are strict, and the toughness of the steel sheet is excellent not only in the conventional Charpy impact value for both the base metal and the welded part but also in the CTOD value at the minimum operating temperature. It is required to be.

  Thus, steel sheets used in ice-sea structures installed in cold regions are required to improve not only the base material but also the low temperature toughness of the weld heat affected zone (hereinafter referred to as “HAZ”).

  Conventionally, it has been known that lowering C is effective in dramatically improving the HAZ toughness of steel sheets. To compensate for the decrease in strength due to lowering C, increasing the strength by adding various alloys and aging precipitation Strengthening using a curing action is attempted. For example, ASTM A710 discloses a Cu precipitated steel that uses the aging precipitation hardening action of Cu, and some reports based on this concept have been made.

  Patent Document 1 discloses a method for producing a Cu precipitation hardening type high-strength steel excellent in low-temperature toughness of a multi-layer weld, and Patent Document 2 discloses a tensile strength in which Cu is added by 0.5 to 4.0% A method for producing a high strength, high toughness steel having a strength of 686 MPa or more and excellent elongation properties is disclosed.

  On the other hand, although Cu is not necessarily added, Patent Document 3 describes a steel material that is excellent in low temperature toughness in HAZ even if high heat input welding is performed by optimizing the amount of solute B. In the invention described in Patent Document 3, fatigue cracks in addition to low-temperature toughness of HAZ are obtained by making the structure a composite structure composed of a soft phase (ferrite, tempered bainite, tempered martensite) and a hard phase (bainite, martensite). Progress resistance is also improved.

Japanese Patent Laid-Open No. 5-179344 JP-A-5-186820 JP 2008-248338 A

  None of Patent Documents 1 to 3 reveals the low-temperature CTOD characteristics of an extremely thick steel plate. However, even when stable characteristics are obtained in a Charpy impact test that evaluates a micro test piece cut and sampled to a size of 10 mm × 10 mm, the required characteristics are required in the CTOD characteristic that is evaluated with a test piece having the same thickness as the structure. There are many cases that cannot be satisfied. Furthermore, higher CTOD values are now being demanded.

  The search area for submarine oil resources is shifting to a region with more severe weather conditions. In particular, in recent years, seafloor resources have been searched for in the sea where ice is stretched all over, or where there are many ice blocks and icebergs (ice sea area), ensuring good low temperature CTOD characteristics that can be used in such an environment. There is a need for a thick steel plate.

  An object of this invention is to provide the steel plate for ice sea structures excellent in base material toughness and HAZ toughness.

  As a result of various studies on the chemical composition and structure of steel, the present inventors have solved the above-mentioned problems and developed steel sheets for ice sea structures having excellent base material toughness and HAZ toughness. (I) to (c) were obtained.

  (B) Structures in the ice sea may be subjected to large-scale deformation due to collisions of ice blocks. Therefore, not only the previously considered brittle crack generation and propagation but also the development of ductile cracks. It is also necessary to prevent destruction. For this reason, it is indispensable to provide a steel sheet having excellent ductile fracture characteristics at low temperatures. In order to improve the ductile fracture characteristics of the base material, it is necessary to have a structure mainly composed of polygonal ferrite. In particular, the ductile fracture characteristics of the base material are remarkably improved when the polygonal ferrite area ratio at the 1/4 thickness of the base material is 80% or more.

(B) In order to improve the reliability against the fracture of the structure, it is necessary to improve not only the ductile fracture characteristics of the base material but also the HAZ toughness. Refinement of the HAZ structure is effective for improving the HAZ toughness, and TiN can be dispersed by containing Ti and N in the steel sheet, and coarsening of austenite grains during welding can be suppressed. However, when N is contained excessively, all of N does not form TiN, so that solid solution N remains and causes a significant deterioration in toughness. When Ti is excessive, TiN dissolves during welding, and the pinning effect for refining austenite grains disappears. Excess Ti forms TiC, which is a hard phase, and deteriorates toughness. For this reason, it is important to contain Ti and N in a specific ratio, and it is necessary to satisfy the following formula (i). In addition, the element symbol in Formula (i) represents content (mass%) of each element contained in a steel plate.
1.8 ≦ Ti / N ≦ 4.0 (i)

(C) Welding work is indispensable for the construction of ice-sea structures, but since extremely thick steel plates are used for ice-sea structures, they are generally joined by multi-layer welding. In the heat-affected zone of the multi-layer weld zone, there is a region that is heated to 1300 ° C. or higher by welding in the preceding pass, and then reheated to a two-phase region of Ac ₁ temperature or higher and Ac ₃ temperature or lower by subsequent passes. In this region, a hard structure called austenite and martensite, which is called island martensite, is likely to be formed. If a large amount of this island martensite is generated, the HAZ toughness is remarkably deteriorated. In order to prevent the formation of island martensite, it is effective to reduce the Si and Al contents in addition to lowering the C content. Increasing these contents has the effect of suppressing the precipitation of cementite and promoting the formation of island martensite. Therefore, the generation amount of island martensite can be reduced by reducing the Si and Al contents. In particular, by satisfying the following equation (ii), the area ratio of island martensite can be made 1% or less, and the influence on toughness deterioration can be ignored. In addition, the element symbol in Formula (ii) represents content (mass%) of each element contained in a steel plate.
C + Si / 7.5 + 2Al ≦ 0.20 (ii)

  This invention is completed based on said knowledge, and makes a summary the steel plate for ice sea structures shown to following (1)-(5).

(1) In mass%,
C: 0.002-0.10%,
Si: 0.02 to 0.6%,
Mn: 0.3-3.0%
P: 0.06% or less,
S: 0.03% or less,
Al: 0.002% or more and less than 0.020%,
In addition to Ti: 0.003-0.03% and N: 0.007% or less,
Cu: 0.01-3.5% and Ni: 0.01-9.5%
1 or 2 selected from the group consisting of Fe and impurities, the chemical composition satisfying both the following formulas (i) and (ii), and a metal at a thickness of 1/4 part A steel sheet for ice sea structures, wherein the area ratio of polygonal ferrite in the structure is 80% or more.
1.8 ≦ Ti / N ≦ 4.0 (i)
C + Si / 7.5 + 2Al ≦ 0.20 (ii)
However, each element symbol shown in Formula (i) and (ii) represents content (mass%) of each element contained in a steel plate.

(2) In place of part of Fe, in mass%,
Cr: 3.5% or less,
Mo: 3.0% or less,
V: 0.2% or less,
The steel plate for ice sea structures according to (1) above, which contains at least one selected from Nb: 0.1% or less and B: 0.01% or less.

(3) In place of part of Fe, in mass%,
The steel plate for ice sea structures according to (1) or (2) above, which contains one or two selected from Ca: 0.02% or less and Mg: 0.02% or less.

(4) In place of part of Fe, in mass%,
The ice sea structure according to any one of (1) to (3) above, which contains one or two selected from REM: 0.01% or less and Zr: 0.01% or less Steel plate for goods.

(5) in the microstructure of the heat affected zone of the joint when it is joined by a multilayer welding, the area ratio of the island martensite is from above (1), characterized in that less than 1% to (4) A steel sheet for ice sea structures according to any one of the above.

  According to the present invention, it is possible to obtain a steel sheet having excellent ductile fracture characteristics and HAZ toughness even at a low temperature of −40 ° C. or lower. Since the steel sheet of the present invention has high reliability against breakage due to large-scale deformation, in particular, an ice-sea structure for excavating and producing marine resources in a cold area, which has been increasingly demanded in recent years, and a structure by drift ice, iceberg, etc. It is most suitable for steel plates used in the outer shell to reduce damage to objects.

(A) Chemical composition First, the reasons for limiting the chemical composition of the steel sheet of the present invention will be described. Hereinafter,% means mass%.

C: 0.002-0.10%
C is an element necessary for ensuring strength. If it is less than 0.002%, the required strength cannot be ensured. On the other hand, if it exceeds 0.10%, the toughness of the base material deteriorates. Moreover, in order to suppress the island-like martensite which generate | occur | produces at the time of welding and to ensure HAZ toughness, a large amount is not preferable. From the viewpoint of suppression of island martensite, it is preferable that the C content is small. The C content is preferably 0.05% or less.

Si: 0.02 to 0.6%
Si has a deoxidizing action and contributes to an increase in the strength of the steel sheet. In order to obtain these effects, Si is contained by 0.02% or more. However, when the content exceeds 0.6%, the toughness is reduced. For this reason, Si content shall be 0.02-0.6%. If the Si content is high, the HAZ toughness is reduced due to the formation of island martensite, so the Si content is preferably 0.4% or less.

Mn: 0.05 to 3.0%
Mn has an effect of enhancing the hardenability of steel and is an effective component for securing strength. If the content is less than 0.05%, strength and toughness cannot be obtained due to insufficient hardenability. On the other hand, if the content exceeds 3.0%, segregation increases and hardenability increases too much, and the toughness of both the HAZ and the base material decreases during welding. Therefore, 0.05 to 3.0% is set. A preferred lower limit is 0.5%, a more preferred lower limit is 0.7%, a preferred upper limit is 2.5%, and a more preferred upper limit is 2.0%.

P: 0.06% or less P is unavoidably present in steel as an impurity. If it exceeds 0.06%, it not only segregates at the grain boundaries and lowers toughness, but also causes hot cracking during welding, so it is necessary to make it 0.06% or less. Preferably it is 0.03% or less.

S: 0.03% or less S is unavoidably present in steel as an impurity. If the amount is too large, the center segregation is promoted or a large amount of stretched MnS is formed, so that the mechanical properties of the base material and the HAZ are deteriorated. Preferably it is 0.01% or less.

Al: 0.002% or more and less than 0.020% Al is an essential element for deoxidation. In order to obtain this effect, a content of 0.002% or more is necessary. However, when the content is 0.020% or more, the toughness is easily deteriorated in the HAZ particularly during welding. This is presumably because coarse cluster-like alumina inclusion particles are easily formed and the generation of island martensite is promoted. Therefore, the Al content is 0.002 or more and less than 0.020%. It is preferable to make it contain exceeding 0.005%, and it is preferable to set it as 0.015% or less.

Ti: 0.003 to 0.03%
Ti reacts with N in the steel to precipitate as TiN, suppresses austenite coarsening in HAZ, and acts as a core of ferrite transformation to refine the intragranular structure, so HAZ toughness To improve. In order to obtain this effect, it is necessary to contain 0.003% or more of Ti. However, if the Ti content is excessively increased, the solid solution Ti is increased and the HAZ toughness is lowered. Therefore, the content is made 0.03% or less. A preferred lower limit is 0.005%, a more preferred lower limit is 0.007%, a preferred upper limit is 0.025%, and a more preferred upper limit is 0.02%.

N: 0.007% or less N is unavoidably present in steel as an impurity. When present in a large amount, the HAZ toughness is deteriorated. Unless N is 0.007% or less, it is inevitable that the toughness of both the base metal and the HAZ deteriorates.

  Cu and Ni contain at least one element.

Cu: 0.01 to 3.5%
Cu increases the strength of both the base material and HAZ without degrading toughness. In order to acquire this effect, Cu is contained 0.01% or more. However, if it exceeds 3.5%, the hardenability of the steel is excessively increased and the tendency to deteriorate the HAZ toughness becomes strong. Therefore, the upper limit of the Cu content is set to 3.5%. Preferably it is 2.5% or less, More preferably, it is 1.5% or less.

Ni: 0.01 to 9.5%
By adding a proper amount of Ni, the strength and toughness of the base material are improved without adversely affecting the weldability and the HAZ toughness. In order to acquire this effect, Ni is contained 0.01% or more. However, if the Ni content exceeds 9.5%, it becomes extremely expensive as a structural steel material and loses economic efficiency. Preferably it is 5.0% or less, More preferably, it is 2.5% or less. The effect of improving hardenability is also obtained when the content is 0.01% or more. In particular, when Cu is contained, it is necessary to contain 0.01% or more of Ni in order to prevent cracking (Cu checking) during rolling.

1.8 ≦ Ti / N ≦ 4.0
As described above, Ti combines with N, precipitates as TiN, and contributes to the refinement of the HAZ structure, thereby improving the HAZ toughness. This effect is determined by the ratio of Ti and N. When Ti / N is less than 1.8, solid solution N that cannot be combined as TiN increases, and the HAZ toughness is deteriorated. On the other hand, when Ti / N exceeds 4.0, Ti that cannot be bonded as TiN forms coarse carbides and deteriorates HAZ toughness. For this reason, Ti / N shall be in the range of 1.8-4.0.

C + Si / 7.5 + 2Al ≦ 0.20
In multilayer welding often used in ice-sea structures, after the first pass is welded, the periphery of the region is welded again in the subsequent pass. For this reason, there exists a region (ICCGHAZ; InterCritically reheated Coarse Grained Heat Affected Zone) that is reheated to the two-phase region. In ICCGHAZ, when heated to a two-phase region, carbon is concentrated in austenite and a large amount of island martensite is formed. Island martensite increases the ductile-brittle transition temperature and degrades HAZ toughness. Therefore, if the generation amount of island martensite in ICCGHAZ is reduced, deterioration of toughness can be suppressed.

  The reduction of island martensite is realized by avoiding the concentration of carbon by reducing the C content, and by promoting the precipitation of cementite by reducing the Si and Al contents.

  The degree of contribution of the C, Si and Al contents to the formation of island martensite differs depending on each element, and various experiments revealed that Al is twice that of C and C is 7.5 times that of Si. When a steel sheet having a chemical composition in which the content of each element contained in the steel sheet is in the above-described range and C + Si / 7.5 + 2Al is 0.20 or less is produced under predetermined manufacturing conditions, multilayer welding is performed. The amount of island martensite produced in the ICCGHAZ is less than 1% in terms of area ratio.

  The steel sheet for ice-sea structure of the present invention contains each of the above elements, and the balance consists of Fe and impurities. However, the steel sheet for ice sea structure of the present invention may contain one or more elements selected from Cr, Mo, V, Nb, B, Ca, Mg, REM and Zr instead of a part of Fe. good.

  Impurities are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes, and mean tolerable within a range that does not adversely affect the present invention.

Cr: 3.5% or less Cr is useful for enhancing carbon dioxide gas corrosion resistance and enhancing hardenability. However, if the content exceeds 3.5%, even if other component conditions are satisfied, it becomes difficult to suppress the hardening of the HAZ, and the carbon dioxide corrosion resistance improving effect is saturated. Therefore, when Cr is contained, the content is set to 3.5% or less. In order to acquire said effect, it is preferable to make it contain 0.1% or more.

Mo: 3.0% or less Mo is useful for improving the strength and toughness of the base material. However, if it exceeds 3.0%, the hardness of the HAZ is increased and the toughness and SSC resistance are impaired. Therefore, when Mo is contained, the content is made 3.0% or less. In order to acquire said effect, it is preferable to make it contain 0.1% or more.

V: 0.2% or less V is useful for improving the strength of the base material mainly by carbonitride precipitation during tempering. However, if it exceeds 0.2%, the performance improvement effect of the base material is saturated and the toughness is deteriorated. Therefore, when V is contained, the content is set to 0.2% or less. In order to acquire said effect, it is preferable to make it contain 0.02% or more.

Nb: 0.1% or less Nb is useful for improving the strength and toughness of the base material by refining and carbide precipitation. However, if it exceeds 0.1%, the performance improvement effect of the base material is saturated and the HAZ toughness is significantly impaired. Therefore, when Nb is contained, the content is made 0.1% or less. In order to acquire said effect, it is preferable to make it contain 0.01% or more.

B: 0.01% or less B has an effect of improving hardenability and increasing strength. However, when the content exceeds 0.01%, the effect of increasing the strength is saturated, and the tendency of deterioration in toughness becomes remarkable in both the base material and HAZ. Therefore, when B is contained, the content is set to 0.01% or less. In order to acquire said effect, it is preferable to make it contain 0.0005% or more.

Ca: 0.02% or less Ca reacts with S in steel to form an acid / sulfide (oxysulfide) in molten steel. Unlike MnS, this acid / sulfide does not extend in the rolling direction during rolling and remains spherical after rolling, so it suppresses weld cracking and hydrogen-induced cracking starting from cracks at the tip of the stretched inclusions. Has the effect of However, if its content exceeds 0.02%, toughness may be deteriorated. Therefore, when Ca is contained, its content is set to 0.02% or less. In order to acquire said effect, it is preferable to make it contain 0.0005% or more.

Mg: 0.02% or less Mg generates an Mg-containing oxide, serves as a TiN generation nucleus, and has an effect of finely dispersing TiN. However, if it exceeds 0.02%, the amount of oxide becomes excessive and ductility is reduced. Therefore, when it contains Mg, the content shall be 0.02% or less. In order to acquire said effect, it is preferable to make it contain 0.0003% or more.

REM: 0.01% or less REM contributes to refinement of the structure of the weld heat affected zone and fixation of S. However, excessive addition of REM leads to a decrease in cleanliness due to inclusions. Since this inclusion has a relatively small influence on toughness deterioration, it is acceptable if it is 0.01% or less. Therefore, when it contains REM, the content shall be 0.01% or less. In order to acquire said effect, it is preferable to make it contain 0.001% or more.

  REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total amount of the above elements.

Zr: 0.01% or less Zr produces Zr oxide in steel, serves as a generation core of fine ferrite in the weld heat affected zone, and has the effect of refining the microstructure. However, if it exceeds 0.01%, the amount of oxide becomes excessive and ductility is reduced. Therefore, when Zr is contained, the content is made 0.01% or less. In order to acquire this effect, it is preferable to make it contain 0.001% or more.

(B) Steel plate structure (polygonal ferrite area ratio of metal structure at 1/4 part of plate thickness is 80% or more)
In steel sheets used in ice seas involving ice mass collisions, it is necessary to consider ductile fracture as well as brittle fracture. In order to satisfy both excellent brittle fracture characteristics and ductile fracture characteristics at the same time, the microstructure is mainly composed of polygonal ferrite. In particular, if the area ratio of polygonal ferrite is 80% or more, the ductile fracture characteristics of the base material are ensured. Therefore, the polygonal ferrite area ratio in the ¼ part thickness of the steel sheet is set to 80% or more. There are no particular restrictions on the remaining structure other than polygonal ferrite. It should be noted that the structure of the ¼ part thickness is observed because the cooling rate at the ¼ part thickness is an intermediate cooling rate between the ½ part thickness and the surface. It is because it can observe.

  In order to mainly use the steel sheet structure as polygonal ferrite, a slab having a composition defined in the present invention is prepared and then manufactured as follows. First, the slab is heated. Although heating temperature should just be 900-1200 degreeC, since rolling at specific temperature becomes important at the next hot rolling process, it is preferable that heating temperature is low.

In the subsequent hot rolling process, the slab is rolled. For the formation of polygonal ferrite, rolling in the temperature region immediately above the Ar ₃ point, which is an unrecrystallized region, is important. At this time, if rolling is performed in the range of Ar ₃ point to Ar ₃ point + 40 ° C., distortion is introduced into the steel sheet and it is likely to be polygonized. Although it is not necessary to perform rolling in the range of Ar ₃ points to Ar ₃ points + 40 ° C. in all rolling passes, it is preferable to roll more than half of all passes in this temperature range. In order to introduce a large amount of distortion, it is also effective to increase the reduction ratio in this temperature range. After rolling, air cooling may be performed to obtain a ferrite structure. Polygonal ferrite can be generated by slow cooling. On the other hand, since air-cooling reduces the production efficiency, accelerated cooling may be performed after air cooling to Ar ₃ point −50 ° C. or lower to generate polygonal ferrite. If accelerated cooling is performed, the coarsening of polygonal ferrite is suppressed, so that a fine base metal structure can be obtained, and improvement in strength and toughness can be expected. After cooling by air cooling or accelerated cooling, tempering treatment can be performed at a temperature of 200 ° C. to Ar ₁ point, and the area ratio of polygonal ferrite does not change within this temperature range.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

A 200 mm-thick slab having the composition shown in Table 1 was prepared, heated to 1000 ° C. in a heating furnace, and then rolled so that the plate thickness after the final pass was 19 to 75 mm. At this time, rolling in the temperature range of Ar ₃ points to Ar ₃ points + 40 ° C. was performed at half or more of the total number of passes, and rolling was completed at Ar ₃ points or more. The calculated values were used for Ar ₃ points.
_{Ar 3 = 910-310C-80Mn-20Cu} -15Cr-55Ni-80Mo + 0.35 (t-8) (t is the plate thickness, mm)

After rolling, the steel sheet was allowed to cool to room temperature, and the steel sheet temperature became Ar ₃ points −50 ° C. or lower, and then cooled to room temperature with water. For comparison, one example (test No. f shown in Table 2) was directly water cooled without air cooling after rolling.

  After producing the test steel, a sample was taken from a part of the test steel, and the structure at the 1/4 thickness was observed with a scanning electron microscope. The area ratio of polygonal ferrite was measured by subjecting an SEM photograph magnified 1000 times to image processing.

  On the other hand, the test steel was subjected to multilayer welding with the heat input shown in Table 2 as a base material. From the produced welded joint, a test piece was prepared so that the notch of the CTOD test piece was at the position of the melt line, and a CTOD test was performed at -40 ° C or -60 ° C to obtain a limit CTOD value.

  Moreover, the microstructure was observed about the weld cross section and the island-like martensite in ICCGHAZ was observed. Table 2 also shows the test results.

  From Table 2, the test No. according to the present invention. In each of the steel sheets 1 to 27, the area ratio of polygonal ferrite is 80% or more, and the critical CTOD value in HAZ is 0.1 mm or more and has good toughness. In addition, no island martensite was observed in the structure of the ICCGHAZ part.

  On the other hand, a test No. deviating from the provisions of the present invention. The steel sheets a to f had a critical CTOD value of 0.1 mm or less even at −40 ° C., where the conditions were not severe.

  Test No. The steel sheet a had a C content higher than the specified amount, and the formula (ii) was not satisfied. Moreover, since Cu and Ni are not contained, the number of island martensites in the ICCGHAZ portion is increased, and it is considered that the limit CTOD value is lowered.

  Test No. The steel sheet of b had a Si content higher than the specified amount, and the formula (ii) was not satisfied. Therefore, it was considered that the number of island martensite in the ICCGHAZ portion increased and the critical CTOD value was lowered.

  Test No. Since the steel plate of c has a Mn content higher than the specified amount, it is considered that the area ratio of polygonal ferrite was lowered and the critical CTOD value was lowered.

  Test No. In the steel sheet d, since the N content was higher than the specified amount and the formula (i) was not satisfied, the solute N increased and the critical CTOD value was considered to be low.

  Test No. Since the steel sheet of e did not satisfy the formula (ii), it is considered that the number of island martensite in the ICCGHAZ portion increased and the critical CTOD value was lowered.

  Although the chemical composition is within the range specified in the present invention, after rolling, the test No. 1 was cooled by water without being left to cool for a long time. Since the steel sheet f was directly water-cooled without air cooling after rolling, it was considered that polygonal ferrite was not sufficiently generated and the critical CTOD value was lowered.

  According to the present invention, it is possible to obtain a steel sheet having excellent ductile fracture characteristics and HAZ toughness even at a low temperature of −40 ° C. or less, and in particular, in the cold region where demands are increasing in recent years, It is most suitable for steel plate used for the outer shell to reduce the damage to the structure caused by ice and iceberg structures and drift ice and icebergs.

Claims (5)

% By mass
C: 0.002-0.10%,
Si: 0.02 to 0.6%,
Mn: 0.3-3.0%
P: 0.06% or less,
S: 0.03% or less,
Al: 0.002% or more and less than 0.020%,
In addition to Ti: 0.003-0.03% and N: 0.007% or less,
Cu: 0.01-3.5% and Ni: 0.01-9.5%
1 or 2 selected from the group consisting of Fe and impurities, the chemical composition satisfying both the following formulas (i) and (ii), and a metal at a thickness of 1/4 part A steel sheet for ice sea structures, wherein the area ratio of polygonal ferrite in the structure is 80% or more.
1.8 ≦ Ti / N ≦ 4.0 (i)
C + Si / 7.5 + 2Al ≦ 0.20 (ii)
However, each element symbol shown in Formula (i) and (ii) represents content (mass%) of each element contained in a steel plate. Instead of a part of Fe, in mass%,
Cr: 3.5% or less,
Mo: 3.0% or less,
V: 0.2% or less,
The steel plate for ice sea structure according to claim 1, comprising at least one selected from Nb: 0.1% or less and B: 0.01% or less. Instead of a part of Fe, in mass%,
The steel sheet for ice sea structure according to claim 1 or 2, comprising one or two selected from Ca: 0.02% or less and Mg: 0.02% or less. Instead of a part of Fe, in mass%,
The ice-sea structure according to any one of claims 1 to 3, comprising one or two selected from REM: 0.01% or less and Zr: 0.01% or less. Steel plate. In the weld heat affected zone of the microstructure of the joint when it is joined by a multilayer welding, according to claim 1, the area ratio of the island martensite is characterized in that less than 1% to claim 4 Steel sheet for ice-sea structures.