JP2005187853A - Method for producing high strength thick steel plate excellent in toughness in extra-high heat input welded-heat affected part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength thick steel plate excellent in the toughness in extra-high heat input welded-heat affected part, such inlet as to exceed 400 KJ/cm, and its producing method. <P>SOLUTION: This producing method is performed, by which a steel blank having composition composed of 0.05-0.15% C, suitable contents of Si, Mn, P, S, Al and further, 0.1-1.0% Cu, 0.1-2.0% Ni, 0.005-0.030% Ti, 0.0030-0.0060% N, 0.0005-0.0050% Ca, 0.0003-0.0050% B, 0.0010-0.0030% O and 0.35-0.47% carbon equivalent Ceq under satisfying 0.2-0.8 ACR, and the balance Fe with inevitable impurities, is heated to the temperature range of 1,000-1,300°C, and after applying hot-rolling having ≥900° rolling finish temperature, the accelerated cooling treatment for cooling to ≤600°C at 1-20°C/s average cooling speed, is applied and successively, the tempering treatment at not higher than the Ac<SB>1</SB>transformation point, is applied. In this way, the thick steel plate having ≥590 MPa TS, ≤80% YR and showing excellent HAZ toughness even under the extra-high heat input welded-heat affected part , such welding inlet as to exceed 400 KJ/cm, is obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a thick steel plate suitable for a building structure, and in particular, a super high heat input welding having a tensile strength (TS) of 590 MPa or more, a yield ratio of 80% or less, and a welding heat input exceeding 400 kJ / cm. The present invention relates to a high-strength thick steel plate excellent in heat-affected zone toughness. The thick steel plate in the present invention refers to a steel plate having a thickness of 30 mm or more.

  In recent years, with the increase in size and span of building structures, there has been a demand for thicker and higher strength steel used. On the other hand, from the viewpoint of the safety of steel structures, reduction of the yield ratio of the steel material used is required. By reducing the yield ratio, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

  Such a building structure is finished into a desired shape structure by welding. However, as pointed out during the Hyogoken-Nanbu Earthquake, if a welded structure is subjected to an abrupt and large load such as during an earthquake, brittle fracture occurs mainly in the weld area before sufficient plastic deformation occurs. May occur. For this reason, in recent years, steel materials for welded structures are required to have good toughness including the welded portion.

  On the other hand, from the viewpoint of improving the efficiency of construction and reducing the cost of implementation, improvement in welding efficiency is required, and the application range of high-efficiency large heat input welding is expanding. For example, for box columns for building structures, supermerged heat input submerged arc welding, electroslag welding, or the like with a welding heat input exceeding 400 kJ / cm is applied. In such a super-high heat input welded joint, the residence time exposed to high temperatures during welding increases and the cooling rate thereof decreases. For this reason, the structure of the weld heat-affected zone is easily coarsened, and it is difficult to obtain excellent weld heat-affected zone toughness.

  When high heat input welding is applied to a steel material, the most serious problem is toughness deterioration at the bond portion of the weld heat affected zone (hereinafter also referred to as HAZ). The bond portion is exposed to a high temperature just below the melting point during high heat input welding, and the austenite crystal grains are most likely to be coarsened, and are easily transformed into an embrittled upper bainite structure by subsequent cooling. Further, in this bond portion, a brittle structure such as a woodman-stetten structure or island martensite is easily generated, which also causes a decrease in toughness. In particular, in a high-strength thick steel plate having a tensile strength exceeding 590 MPa, it is common to add a large amount of alloy to ensure the strength, so the yield ratio tends to increase and the HAZ toughness also decreases. For this reason, development of the high strength thick steel plate which combines the low yield ratio and the outstanding HAZ toughness is desired.

  In response to such a demand, for example, Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 propose a method for producing a low-yield ratio high-strength steel. The techniques described in Patent Document 1 and Patent Document 2 are both direct quenching methods in which quenching is performed immediately after rolling, the cooling start after rolling is delayed, and about 5 to 60% of ferrite is precipitated, followed by rapid cooling. Thus, it has a two-phase structure of ferrite phase + hard phase. As a result, high strength and low yield ratio are realized. On the other hand, the technique described in Patent Document 3 achieves high strength and low yield ratio by quenching after being kept in the ferrite precipitation temperature range and by forming a two-phase structure of ferrite + hard phase. . Moreover, in the technique described in Patent Document 4, after hot-rolling the steel sheet, it is heated again to the two-phase region of ferrite + austenite, and after quenching, tempering is performed to increase the strength and lower the yield ratio. Has achieved. However, with the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, it is possible to achieve sufficient HAZ toughness even if high strength and low yield ratio can be achieved. There is a problem that has not been reached.

  For such problems, for example, Patent Literature 5, Patent Literature 6, Patent Literature 7, and Patent Literature 8 propose techniques for improving the HAZ toughness of high heat input welding. In Patent Document 5, aiming to improve the weld bond toughness of 100 kJ / cm, a rare earth element and Ti are added in combination, and fine particles are dispersed in the steel to suppress austenite grain growth. Techniques for improving the toughness of steel have been proposed. Patent Document 6 proposes a technique for finely dispersing Ti oxide to increase the toughness of the high heat input HAZ. Patent Document 7 proposes a technique for improving the toughness of high heat input HAZ by finely dispersing Ti oxide and using it as a nucleation site for ferrite transformation. In Patent Document 8, in order to thoroughly reduce solid solution N, Ti and a sufficient Al amount are contained, and further, Ca oxide is used as a fine oxide, and HAZ toughness in super-high heat input welding. High-strength steel sheets that improve the strength have been proposed.

However, even with the techniques described in Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8, the tensile strength is a high strength of 590 MPa or more and the base material yield ratio is a low yield ratio of 80% or less. Furthermore, it has been difficult to stably maintain excellent HAZ toughness even in super-high heat input welding in which the heat input of welding exceeds 400 kJ / cm.
Japanese Patent Publication No.58-10442 JP-A 62-77419 JP-A-2-34721 Japanese Patent Laid-Open No. 4-99817 JP-A-60-184663 JP-A-57-51243 JP-A-60-245768 Japanese Patent Laid-Open No. 2001-107177

  Recently, for example, in column-beam super-high heat input welds, it is required to have high toughness such that Charpy absorbed energy at 0 ° C. exceeds 70 J. Moreover, the same request | requirement also exists in the welding part of a box pillar.

The present invention solves the above-mentioned problems of the prior art, has a tensile strength of 590 MPa or more suitable for a building structure, a low yield ratio of 80% or less, and a welding heat input exceeding 400 kJ / cm. It aims at proposing the manufacturing method of the high intensity | strength thick steel plate excellent in the super large heat input welding heat affected zone toughness which has the outstanding HAZ toughness also in super large heat input welding. In the present invention, “excellent toughness of the super-high heat input welding heat-affected zone” means that the Charpy absorbed energy vE _{0 at} 0 ° C. of the super-high heat input welding heat-affected zone having a welding heat input exceeding 400 kJ / cm is 70 J or more. The case where it has.

  In order to achieve the above-mentioned problems, the present inventors have intensively studied various factors affecting strength, yield ratio, and HAZ toughness. As a result, in order to ensure high toughness in the super-high heat input HAZ with a welding heat input exceeding 400 kJ / cm, the austenite grain coarsening is suppressed in the region heated to a high temperature during welding, and ferrite transformation is promoted during cooling We found that the dispersion of transformation nuclei is important. For this purpose, fine dispersion of TiN by strict component adjustment and adjustment of the amount of dissolved oxygen at the time of Ca addition to 0.0010 to 0.0030%, the addition amount of Ca, S, O is 0.2 to 0.8 ACR. % Was found to be important to adjust so as to satisfy%. It has also been found that it is important to set the carbon equivalent Ceq in the range of 0.35 to 0.47% in order to make the tensile strength of the base material 590 MPa or more.

  Furthermore, after performing hot rolling on the steel material whose components have been adjusted as described above, by combining accelerated cooling processing in which the cooling rate and the cooling stop temperature are optimized, and further tempering heat treatment, the above-described excellent super-large insertion is achieved. It has been found that the thermal HAZ toughness can be combined with a base material characteristic having a tensile strength of 590 MPa or more and a low yield ratio of 80% or less.

The present invention has been completed based on the above findings and further studies. That is, the present invention is, in mass%, C: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.6 to 3.0%, P: 0.018% or less, S: 0.0050% or less, Al: 0.1% or less, Cu : 0.1-1.0%, Ni: 0.1-2.0%, Ti: 0.005-0.030%, N: 0.0030-0.0060%, Ca: 0.0005-0.0050%, B: 0.0003-0.0050%, O: 0.0010-0.0030% , Next formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.35-0.47% and the following formula (2)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… （2）
(Where Ca, O, S: content of each element (mass%))
A steel material having a composition satisfying 0.2 to 0.8 defined by ACR with the balance consisting of Fe and inevitable impurities is heated in the range of 1000 ° C to 1300 ° C, and the heat at which the rolling finish temperature is 900 ° C or higher After hot rolling, it is subjected to accelerated cooling treatment that cools to a temperature of 600 ° C. or less at an average cooling rate of 1 to 20 ° C./s, and then tempering treatment at a temperature below the Ac ₁ transformation point. Is a method for producing a high-strength thick steel plate excellent in toughness of the super-high heat input weld affected zone having a tensile strength of 590 MPa or more and a yield ratio of 80% or less. In the present invention, in addition to the above composition, the mass %, Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less are preferably contained. In addition to each of the above-mentioned compositions, it is further selected by mass%, REM: 0.02% or less, Mg: 0.005% or less Preferably contains one or two.

  According to this invention, a high strength thick steel plate having a tensile strength of 590 MPa or more, a yield ratio of 80% or less, and a super high heat input HAZ toughness exceeding heat input 400 kJ / cm is manufactured. It can greatly contribute to the enlargement of steel structures, the improvement of earthquake resistance of steel structures and the improvement of construction efficiency, and has a remarkable industrial effect.

  First, the reasons for limiting the components of the steel material used in the present invention will be specifically described. In addition, unless otherwise indicated, the% display regarding a component shall mean the mass%.

C: 0.05-0.15%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In the present invention, the content of 0.05% or more is required to obtain the above-described effects. On the other hand, the content exceeding 0.15% deteriorates the HAZ toughness and weld crack resistance. For this reason, C was limited to the range of 0.05 to 0.15%. In addition, Preferably, it is 0.05 to 0.12%.

Si: 0.05-0.50%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. However, if it exceeds 0.50%, the toughness of the base material deteriorates and island martensite is generated in the super-high heat input HAZ. , HAZ toughness is significantly deteriorated. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 0.6-3.0%
Mn has an effect of increasing the strength of steel. In the present invention, Mn needs to be contained in an amount of 0.6% or more in order to ensure a tensile strength of 590 MPa or more. On the other hand, when the content exceeds 3.0%, the toughness and the HAZ toughness of the base material are remarkably deteriorated. For this reason, Mn was limited to the range of 0.6 to 3.0%. In addition, Preferably, it is 0.6 to 2.0%.

P: 0.018% or less P is an element that increases the strength of steel and deteriorates toughness. In particular, P is desirably reduced as much as possible in order to deteriorate the toughness of welds. This tendency becomes significant when P is contained in excess of 0.018%, so 0.018% was made the upper limit. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.005% or more.

S: 0.0050% or less S is finely crystallized at the solidification stage as CaS particles by combining with Ca. These fine particles act as ferrite transformation nuclei during cooling after hot rolling, and contribute to lowering the yield ratio of the base material. Furthermore, at the time of welding, it precipitates as MnS on CaS particles, acts as a ferrite transformation nucleus, and has the effect of improving the weld zone toughness. Such an effect is recognized when the content is 0.0005% or more. On the other hand, if the content exceeds 0.0050%, the toughness of the base metal and the welded portion is deteriorated. For this reason, S was limited to 0.0050% or less.

Al: 0.1% or less
Al acts as a deoxidizer and is most commonly used in the high-strength steel deoxidation process. Moreover, N in steel is fixed as AlN, and it has the effect of maintaining the quenching of B. Such an effect is recognized when the content is 0.005% or more. On the other hand, when the content exceeds 0.1%, the toughness of the base material is lowered, and the toughness is deteriorated by being mixed into the weld metal part during welding. For this reason, Al was limited to 0.1% or less. In addition, Preferably, it is 0.01 to 0.07%.

Cu: 0.1-1.0%
Cu is an element that can increase the strength without deteriorating the toughness, and has a small influence on the HAZ toughness, and is therefore an element useful for increasing the strength. In order to acquire such an effect, it is necessary to contain 0.1% or more. However, the content exceeding 1.0% causes hot brittleness and deteriorates the surface properties of the steel sheet. For this reason, Cu was limited to the range of 0.1 to 1.0%. In addition, Preferably, it is 0.2 to 0.7%.

Ni: 0.1-2.0%
Ni, like Cu, is an element that can increase the strength without deteriorating the toughness, and has a small effect on the HAZ toughness, and is therefore a useful element for increasing the strength. In order to acquire such an effect, it is necessary to contain 0.1% or more. On the other hand, if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni was limited to 0.1 to 2.0%. In addition, Preferably it is 0.2 to 1.7%.

Ti: 0.005-0.030%
Ti has a strong affinity for N and precipitates as TiN during solidification, and contributes to the suppression of coarsening of austenite grains in HAZ, or to strengthening HAZ as a ferrite transformation nucleus. Also, during accelerated cooling after hot rolling, TiN acts as a transformation nucleus that generates a ferrite phase, contributing to a lower yield ratio of the base material. In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.030%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.030% of range. In addition, Preferably, it is 0.010 to 0.030%.

N: 0.0030-0.0060%
N combines with Ti and precipitates as TiN to suppress the coarsening of the austenite grains in the HAZ or contribute to the toughening of the HAZ as a ferrite transformation nucleus. Further, during accelerated cooling after hot rolling, TiN produced in the melting stage acts as a transformation nucleus that produces a ferrite phase, and contributes to a lower yield ratio of the base material. In order to secure the necessary amount of TiN having such an effect, N needs to be contained by 0.0030% or more. On the other hand, when the content exceeds 0.0060%, in the HAZ heated to a temperature at which TiN is dissolved, the amount of solid solution N increases and the HAZ toughness is remarkably lowered. For this reason, N is limited to 0.0030 to 0.0060%.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to improving the ductility of steel through the form control of sulfide. In order to exert such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0050%, the effect is saturated. For this reason, in this invention, Ca was limited to 0.0005 to 0.0050% of range.

  In the present invention, as will be described later, after adjusting the dissolved oxygen content immediately before the addition of Ca to 0.0050% or less, Ca is added to suppress the formation of Ca oxide and to crystallize CaS. CaS crystallizes in molten steel at a temperature lower than that of oxides, so that fine and uniform dispersion is possible in the steel. Such fine CaS particles are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness.

B: 0.0003-0.0050%
B has the effect of increasing the strength of the steel in a small amount through the improvement of hardenability, and forms BN in the coarse-grained area HAZ near the weld bond where it is exposed to a high temperature at which TiN is dissolved. Thus, it contributes to the reduction of solid solution N and the improvement of HAZ toughness as a ferrite transformation nucleus. In addition, BN formed in the melting stage acts as a transformation nucleus that generates a ferrite phase during accelerated cooling after hot rolling, and contributes to a lower yield ratio of the base material. On the other hand, if the content exceeds 0.0050%, the hardenability is remarkably increased, the toughness and ductility of the base metal are deteriorated, and the yield ratio is difficult to control. For this reason, B was limited to a temperature range of 0.0003 to 0.0050%. In addition, Preferably, it is 0.0003 to 0.0020%.

O: 0.0010 to 0.0030%
O is contained as an unavoidable impurity and is present as an oxide in the steel, reducing the cleanliness. For this reason, in this invention, it is preferable to reduce as much as possible. On the other hand, when the oxygen content exceeds 0.0030%, CaO inclusions are coarsened and adversely affect toughness. Further, in the present invention, in order to crystallize Ca as CaO, O having a strong binding force with Ca is strengthened by degassing or adding a deoxidizing agent before addition to 0.0050 O in molten steel. It is preferable to reduce it to less than%. In addition, in order to make O content less than 0.0010%, since refining cost becomes large, it is preferable that O content shall be 0.0010% or more.

Ceq: 0.35-0.47%
In the present invention, the content of each component is adjusted so that the carbon equivalent Ceq defined by the following formula (1) is 0.35 to 0.47% within the above component range.

Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)
When Ceq is less than 0.35%, the hardenability at the time of rolling and quenching is insufficient, and a ferrite main structure is formed, so that a desired tensile strength of 590 MPa or more cannot be secured. On the other hand, if Ceq exceeds 0.47%, the base metal structure after rolling and quenching becomes a bainite single phase, so that a yield ratio of 80% or less cannot be reduced, and the HAZ toughness is remarkably deteriorated. For this reason, Ceq was limited to the range of 0.35-0.47%.

ACR: 0.2 to 0.8
In this invention, after adjusting the amount of dissolved oxygen in molten steel at 0.0010 to 0.0030% at the time of Ca addition, Ca, S and O should satisfy ACR defined by the following formula (2) of 0.2 to 0.8. adjust.

ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
When ACR is less than 0.2, Ca does not crystallize, so S precipitates in the form of MnS alone. Since this MnS is elongated by rolling during the production of the steel sheet and does not disperse uniformly and finely, it causes a decrease in the toughness of the base material and does not contribute to an improvement in the HAZ toughness. On the other hand, when the ACR exceeds 0.8, S is fixed by Ca and S which becomes MnS is insufficient, and MnS does not precipitate on CaS acting as a ferrite formation nucleus, so that improvement of HAZ toughness cannot be expected. Only when the ACR satisfies 0.2 to 0.8 is a composite sulfide in which MnS is deposited on CaS. The presence of this composite sulfide functions as a ferrite transformation nucleus, the HAZ structure is refined, and the HAZ toughness is improved.

  In the present invention, in addition to the basic components described above, one or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, V: 0.2% or less, and / Or REM: 0.02% or less and Mg: 0.005% or less can be included.

One or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.005% or less, V: 0.2% or less
Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel, and can be selected and contained as necessary.

  Cr is preferably contained in an amount of 0.005% or more, but the content exceeding 0.7% deteriorates the HAZ toughness. For this reason, it is desirable to limit Cr to 0.7% or less. Mo is preferably contained in an amount of 0.05% or more, but the content exceeding 0.7% adversely affects the base material toughness and the HAZ toughness. For this reason, it is desirable to limit Mo to 0.7% or less. Nb is preferably contained in an amount of 0.005% or more, but the content exceeding 0.05% deteriorates the base metal toughness and the HAZ toughness. For this reason, it is desirable to limit Nb to 0.05% or less. V is preferably contained in an amount of 0.01% or more, but the content exceeding 0.2% deteriorates the HAZ toughness. For this reason, it is desirable to limit V to 0.2% or less.

One or two selected from REM: 0.02% or less and Mg: 0.005% or less
REM and Mg are both elements that contribute to toughness improvement, and can be selected and contained as necessary.

  REM is preferably contained in an amount of 0.002% or more, but the effect is saturated even if it exceeds 0.02%, so 0.02% was made the upper limit. Mg is a useful element that improves toughness through refinement of crystal grains and is preferably contained in an amount of 0.001% or more, but the effect is saturated even if contained over 0.005%, so 0.005% is the upper limit. It was.

  In addition to the above components, Fe and unavoidable impurities.

  The method for producing the steel material used in the present invention is not particularly limited, and any of the usual melting methods and casting methods can be applied. The molten steel having the above composition can be converted into a converter, electric furnace, vacuum melting furnace. Steel material (slab) by continuous casting method after controlling inclusions by adding CaSi wire after controlling gas components using deoxidation treatment or degassing process It is preferable that To crystallize Ca as CaS during melting. It is preferable to reduce the O in molten steel to 0.0050% or less by strengthening the degassing or adding a deoxidizing agent before adding Ca with strong Ca binding strength. Moreover, in this invention, after adjusting the amount of molten steel oxygen at the time of Ca addition to 0.0050% or less, it is preferable to add and adjust Ca and S so that ACR may satisfy 0.2-0.8.

  Next, these steel materials are reheated to 1000 ° C to 1300 ° C. If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high and the rolling reduction per pass cannot be made large. Therefore, the number of rolling passes increases, and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped. On the other hand, when the reheating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, the reheating temperature of the steel material was limited to a range of 1000 to 1300 ° C.

  The reheated steel material is hot-rolled so that the rolling end temperature is 900 ° C. or higher, and is a thick steel plate having a predetermined thickness. The hot rolling conditions are not particularly limited as long as a predetermined plate thickness and shape can be satisfied except that the rolling end temperature is 900 ° C. or higher. In the case of a very thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for zaku pressure bonding.

  When the rolling end temperature is less than 900 ° C., the deformation resistance increases, the rolling load increases, and the load on the rolling mill increases. Further, in order to lower the rolling end temperature of the thick material to less than 900 ° C., it is necessary to wait in the middle of rolling, which greatly hinders productivity. For this reason, in this invention, the rolling end temperature was limited to 900 ° C. or higher.

  After the hot rolling is finished, the thick steel plate is immediately accelerated and cooled to 600 ° C. or less at an average cooling rate of 1 to 20 ° C./s. If the cooling rate of accelerated cooling is less than 1 ° C./s, the microstructure after accelerated cooling becomes a ferrite main structure, so that the target tensile strength of 590 MPa or more cannot be satisfied. On the other hand, if the cooling rate exceeds 20 ° C./s, the microstructure after accelerated cooling becomes a bainite single-phase structure, and the target yield ratio of 80% or less cannot be satisfied. Only after the cooling rate after rolling satisfies the range of 1 to 20 ° C./s, the microstructure after accelerated cooling becomes a bainite + ferrite two-phase structure. A bainite + ferrite two-phase structure in which a ferrite phase is formed with a fine inclusion such as TiN as a nucleus in the bainite main structure effectively acts on increasing the strength and reducing the yield ratio. In addition, the cooling rate as used in this invention shall mean the average cooling rate from the start of cooling to 600 degreeC in the board thickness 1/4 position of a thick steel plate.

  In this invention, the cooling stop temperature for accelerated cooling is set to 600 ° C. or lower. If the cooling stop temperature exceeds 600 ° C, the target tensile strength of 590 MPa or more cannot be secured. The cooling stop temperature is preferably room temperature or higher.

  The thick steel plate that has been subjected to accelerated cooling and then allowed to cool to room temperature is then tempered.

In the present invention, the tempering process is a process of heating to a temperature below the Ac ₁ transformation point. Thereby, the brittle hardening phase produced | generated by the accelerated cooling process is tempered, and toughness can be improved. The tempering temperature is determined according to the plate thickness, that is, the difference in the cooling rate of the accelerated cooling after hot rolling, and is preferably performed at an Ac ₁ transformation point or less, preferably 600 ° C. or less and 350 ° C. or more. When the tempering temperature exceeds the Ac ₁ transformation point, γ newly generated by the α → γ transformation becomes a hard phase by subsequent cooling, and the toughness deteriorates. When the tempering temperature exceeds 600 ° C., the strength is remarkably lowered and it is difficult to satisfy the target strength. The holding time at the tempering temperature is not particularly limited, and is preferably 60 min or less so as to ensure desired strength and toughness.

  By using the steel material having the above composition, hot rolling under the above conditions, accelerated cooling after hot rolling, and tempering treatment, high strength with a tensile strength of 590 MPa or more and low yield ratio of 80% or less. It is possible to easily produce a thick steel plate that has both a yield ratio and excellent HAZ toughness during super-high heat input welding.

  Thickness of the plate thickness shown in Table 2 by hot rolling under the conditions shown in Table 2 using a steel material (slab: plate thickness 310 mm) prepared in the composition shown in Table 1 by the converter-ladder refining-continuous casting method. A steel plate was used. These thick steel plates were tempered under the conditions shown in Table 2. Some thick steel plates were not tempered.

JIS No. 4 tensile test specimens were collected from 1/4 position of the thickness of each thick steel plate obtained, and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to investigate the tensile properties. In addition, V-notch test specimens were collected from 1/4 position of each thick steel plate obtained in accordance with JIS Z 2202, and Charpy impact test was conducted in accordance with JIS Z 2242. Absorbed energy (vE ₀ ) at 0 ° C. was determined to evaluate the base material toughness.

  In addition, a test plate for joints (size: 400 × 600 mm) was collected from each thick steel plate obtained, and electroslag welding (welding heat input: 1000 kJ / cm) in a groove shape as shown in FIG. A welded joint was prepared. The supply wire was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

From the obtained welded joint, a V-notch test piece having a notch position as a bond portion as shown in FIG. 2 is collected and subjected to a Charpy impact test at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 2242. Then, the absorbed energy (vE ₀ ) at 0 ° C. of the joint bond part was determined, and the joint toughness was evaluated.

  The obtained results are shown in Table 3.

Each of the present invention has a base material characteristic of a tensile strength of 590 MPa or more, a yield strength of 80% or less, a low yield ratio, and a high toughness of absorbed energy (vE ₀ ) of 100 J or more at 0 ° C. Weld heat input: Even when super-high heat input welding is performed at 1000 kJ / cm, vE _{0 of} the bond portion is 100 J or more, indicating excellent HAZ toughness. On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the base material strength, yield ratio, base material toughness, and HAZ toughness do not satisfy the target value.

It is explanatory drawing which shows the groove shape of the electroslag welded joint used in the Example. It is explanatory drawing which shows the extraction | collection point of the Charpy impact test piece from the electroslag welded joint part in an Example.

Claims (3)

% By mass
C: 0.05 to 0.15%, Si: 0.05 to 0.50%,
Mn: 0.6 to 3.0%, P: 0.018% or less,
S: 0.0050% or less, Al: 0.1% or less,
Cu: 0.1-1.0%, Ni: 0.1-2.0%,
Ti: 0.005-0.030%, N: 0.0030-0.0060%,
Ca: 0.0005 to 0.0050%, B: 0.0003 to 0.0050%,
O: 0.0010 to 0.0030%
The carbon equivalent Ceq defined by the following formula (1) is 0.35 to 0.47%, the ACR defined by the following formula (2) satisfies 0.2 to 0.8, and the balance is Fe and inevitable impurities. After heating the steel material having a composition of 1000 ° C to 1300 ° C and performing hot rolling with a rolling end temperature of 900 ° C or higher, the average cooling rate of 1 to 20 ° C / s is 600 ° C or lower. Super-high heat input welding heat with tensile strength: 590MPa or more and yield ratio: 80% or less, characterized in that it is subjected to accelerated cooling treatment that cools to a temperature of ₁ , and then tempering treatment at a temperature below the Ac ₁ transformation point A method for producing high-strength thick steel plates with excellent affected zone toughness.
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… （2）
Here, Ca, O, S: Content of each element (mass%)   In addition to the above composition, the composition further contains one or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less in terms of mass%. 2. The method for producing a high-strength thick steel plate according to claim 1, wherein:   The high content according to claim 1 or 2, further comprising one or two kinds selected from REM: 0.02% or less and Mg: 0.005% or less in mass% in addition to the composition. A manufacturing method of high strength steel plate.

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