JP2005068519A - Method for producing high strength thick steel plate for building structure, having excellent toughness to super-large heat input welding-affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high strength thick steel plate for building structure having excellent toughness in a super-large heat input welding-affected zone. <P>SOLUTION: This production method employs a steel blank having a composition containing, by mass%: 0.05 to 0.15% C, 0.05 to 0.50% Si, 0.6 to 1.6% Mn, ≤0.005% S, and an appropriate amount of P, Al, Cu, and Ni. The steel blank further contains 0.005 to 0.030% Ti, 0.0003 to 0.0050% B, 0.0005 to 0.0050% Ca, 0.0030 to 0.0060% N, 0.0010 to 0.0030% O, and has 0.2 to 0.8 ACR defined by ACR=äCa-(0.18+130Ca)×O}/(1.25×S) and ≤0.47 Ceq. The steel blank is heated to a temperature in the range of 1,000 to 1,300°C, and subjected to hot-rolling having ≥900°C rolling finish temperature and then to accelerated cooling to ≤600°C at ≥1°C/s cooling speed. Further, this rolled steel plate is heated to a reheating temperature in two-phase range of (Ac1+10°C) to (Ac1+70°C) and desirably held for 5 to 60 min and thereafter rapidly cooled, subjected to reheating quenching and then tempering. Then, this steel blank may contain one or more kinds of Cr, V and Mo. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a thick steel plate suitable for use in a building structure, and particularly suitable for a use in which super-high heat input welding with a welding heat input exceeding 400 kJ / cm, such as columns and beams of a building structure, is performed. The present invention relates to a high-strength steel plate for building structures having a tensile strength exceeding 590 MPa.

  In recent years, with the increase in size and span of building structures, the steel materials used are required to be thicker and stronger. 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. Furthermore, as pointed out in the Hyogoken-Nanbu Earthquake, welded steel structures are subject to brittle fracture mainly in the weld zone before undergoing sufficient plastic deformation when subjected to a sudden and large load load, such as during an earthquake. May occur. For this reason, in recent years, steel materials for welded structures are required to have good toughness including welds.

  On the other hand, from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, improvement in welding efficiency is required, and high-efficiency welding with large heat input is directed. For example, in a box-type four-sided box column (or concrete-filled box-type four-sided box column) mainly applied to high-rise and super-high-rise buildings, a multi-electrode one-pass submerged arc with a welding heat input of 600 kJ / cm at the corner weld. Electroslag welding is applied so that the welding heat input exceeds 1000 kJ / cm at the diaphragm. In such a weld, the residence time exposed to high temperature during welding increases and the cooling rate thereof decreases. For this reason, the structure of the weld heat-affected zone (hereinafter also referred to as HAZ) is likely to be coarse, and it is generally difficult to obtain good HAZ toughness.

  In particular, in a high-strength steel having a tensile strength exceeding 590 MPa, it is common to add a large amount of an alloy for securing the strength, so that the yield ratio tends to increase and the HAZ toughness also decreases. For this reason, a high-tensile steel sheet having both a low yield ratio and excellent 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-tensile steel. The techniques described in Patent Document 1 and Patent Document 2 are direct quenching methods in which quenching is performed directly after rolling, after the start of cooling after rolling is delayed, and about 5 to 60% of ferrite is precipitated. , Rapidly cooled to a two-phase structure of ferrite phase + hardened phase. This achieves a low yield ratio. On the other hand, in the technique described in Patent Document 3, a low yield ratio is achieved by cooling after holding in the ferrite precipitation temperature range to form a two-phase structure of ferrite and hardened phase. Moreover, in the technique described in Patent Document 4, after quenching the hot-rolled steel sheet, the steel is again heated to a ferrite + austenite two-phase region, quenched, and tempered to achieve a low yield ratio. ing. However, the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 have a problem that even if the yield ratio can be reduced in this way, sufficient HAZ toughness is not achieved. is there.

  For such problems, for example, Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8 propose techniques for improving the HAZ toughness of high heat input welding. In Patent Document 5, with the aim of improving the weld bond toughness of 100 kJ / cm, a rare earth element and Ti are added in combination to disperse fine particles in the steel and suppress the grain growth of austenite. Techniques for improving toughness have been proposed. Patent Document 6 proposes a technique for finely dispersing Ti oxide to increase the toughness of the high heat input welding HAZ. Patent Document 7 proposes a technique for improving the toughness of high heat input welding 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 amount of Al are contained, and further, Ca oxide is used as a fine oxide, and HAZ toughness in super large heat input welding. High-tensile steel sheets that improve the strength have been proposed.

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

The present invention solves the above-described problems of the prior art, has a tensile strength of 590 MPa or more, a low yield ratio of 80% or less, and a super high heat input welding in which the welding heat input exceeds 400 kJ / cm. Another object of the present invention is to propose a method for producing a high-strength thick steel sheet for building structures having excellent HAZ toughness and excellent toughness in super-high heat input welds. Here, “excellent HAZ toughness even in super high heat input welding exceeding 400 kJ / cm” refers to a case where the Charpy absorbed energy _V E _{0 at} 0 ° C. is 70 J or more.

In order to achieve the above-mentioned problems, the present inventors have studied and examined various factors affecting strength, yield ratio, and super large heat input welding HAZ toughness exceeding 400 kJ / cm. as a result,
In order to obtain high toughness in super high heat input welding HAZ with a heat input exceeding 400 kJ / cm, it is necessary to suppress coarsening of austenite grains in a region heated to a high temperature and to finely disperse transformation nuclei that promote ferrite transformation during cooling. Therefore, after adding TiN and adjusting the amount of dissolved oxygen at the time of Ca addition to 0.0010 to 0.0030%, the addition amount of Ca, S, and O should satisfy ACR of 0.2 to 0.8%. It was found that it is important to adjust the carbon equivalent Ceq to 0.47% or less by further adding B. Furthermore, after subjecting the steel material whose components are adjusted as described above to hot rolling, an accelerated cooling process in which the cooling rate and the cooling stop temperature are optimized, and a heat treatment in which the two-phase region is reheated, quenched, and tempered. By combining, even with thick steel plates exceeding 50 mm, the above-mentioned excellent super high heat input welding HAZ toughness and the base metal properties with tensile strength: 590 MPa or more and low yield ratio of 0.80% or less It was found that both can be combined.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.6 to 1.6%, P: 0.018% or less, S: 0.005% or less, Al: 0.1% or less, Cu: 0.1 to 1.0%, Ni: 0.1-2.0%, Ti: 0.005-0.030%, B: 0.0003-0.0050%, Ca: 0.0005-0.0050%, N: 0.0030-0.0060%, O: 0.0010-0.0030%, (1) Formula ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
(Where Ca, O, S: content of each element (mass%))
The ACR defined by the formula is 0.2 to 0.8, and the following equation (2): Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
(Where Ceq: carbon equivalent (%), C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
A steel material having a composition including a carbon equivalent Ceq defined by the formula of 0.47% or less and the balance consisting of Fe and inevitable impurities is heated to a temperature in the range of 1000 to 1300 ° C, and the rolling end temperature is 900 ° C. After performing the above hot rolling, accelerated cooling to 600 ° C. or less is performed at a cooling rate of 1 ° C./s or more to form a thick steel plate, and further to the thick steel plate, (Ac1 transformation point + 10 ° C.) to ( Super high heat input welding heat, characterized in that after heating to the reheating temperature in the two-phase region (Ac1 transformation point + 70 ° C), reheating and quenching is performed, followed by tempering. A method for producing a low-yield-ratio high-strength thick steel sheet for building structures having excellent toughness at the affected area.
(2) In (1), in addition to the above composition, it is contained by mass%, Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, Mo: 0.01 to 0.30%. A method for producing a low-yield-ratio, high-strength thick steel sheet for building structures.
(3) In (1) or (2), the reheating temperature is maintained for 5 to 60 minutes, and the method for producing a low yield ratio high strength thick steel sheet for building structures.

  First, the reasons for limiting the composition of the steel material used will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.05-0.15%
C is an element that increases the strength of steel. In the present invention, C is required to be contained in an amount of 0.05% or more in order to ensure the strength (TS590 MPa or more) required for a thick steel plate for building structures. On the other hand, if the content exceeds 0.15%, the toughness of the weld is reduced and the sensitivity to low-temperature weld cracking is increased. For this reason, in the present invention, C is limited to a 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 requires 0.05% or more in steelmaking. However, if it exceeds 0.50%, it lowers the toughness of the base metal and generates island martensite in super high heat input welding HAZ. And reduces the HAZ toughness. 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: 0.6-1.6%
Mn is an element that improves 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 the strength (TS590 MPa or more) required for a thick steel plate for building structures. On the other hand, the content exceeding 1.6% significantly deteriorates the HAZ toughness. For this reason, Mn was limited to the range of 0.6 to 1.6%. In addition, Preferably, it is 0.8 to 1.6%.

P: 0.018% or less P is an element unavoidably contained in steel as an impurity, and is desirably reduced as much as possible in order to deteriorate the toughness of the steel. In particular, the P content exceeding 0.018% significantly reduces the HAZ toughness of high strength steel. For this reason, in this invention, P was limited to 0.018% or less. In addition, Preferably it is 0.015% or less.

S: 0.005% or less In the present invention using a Ca-containing steel material, S combines with Ca to form fine crystals as CaS particles during the solidification process, and further precipitates as MnS on the CaS particles during welding. It acts as a transformation nucleus and has the effect of improving the toughness of the HAZ, particularly the coarse grain region HAZ near the fusion part (bond part). Such an effect is recognized when the content of S is 0.0005% or more. On the other hand, if the content exceeds 0.005%, the toughness of the base metal and the welded portion is deteriorated. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.0005 to 0.0030%.

Al: 0.1% or less
Al acts as a deoxidizer and is most commonly used in the high-strength steel deoxidation process. In addition, N is fixed as AIN during heat treatment, and the effect of maintaining the hardenability of B is also obtained. Such an effect is recognized when Al: 0.005% or more is contained. On the other hand, if the content exceeds 0.1%, the weld metal part is mixed during super-high heat input welding and the toughness of the weld metal part is lowered. For this reason, in this invention, Al was limited to 0.1% or less. In addition, Preferably, it is 0.010 to 0.070%.

Cu: 0.1-1.0%
Cu is an element that can increase the strength while maintaining high toughness, and has a small adverse effect on the HAZ toughness. Therefore, Cu is 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, the content exceeding 1.0% causes hot brittleness and lowers 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 strength while maintaining high toughness, and has a small adverse effect on HAZ toughness, and is therefore an element useful for increasing strength. In order to obtain such an effect, a content of 0.1% or more is required. 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, thereby suppressing the austenite grain coarsening in the HAZ or contributing to the toughening of the HAZ as a ferrite transformation nucleus. In order to acquire such an effect, Ti needs to contain 0.005% or more. On the other hand, if the content exceeds 0.030%, TiN coarsens on the contrary, and the above effect cannot be expected. For this reason, Ti was limited to 0.005 to 0.030%. In addition, Preferably, it is 0.010 to 0.030%.

B: 0.0003-0.0050%
B has the effect of increasing the strength of the steel through the improvement of hardenability, and forms BN in the coarse-grained area HAZ near the weld fusion part (bond part) exposed to a high temperature at which TiN dissolves. Therefore, it contributes to the reduction of solid solution N and the improvement of HAZ toughness as a ferrite transformation nucleus. In order to acquire such an effect, 0.0003% or more needs to be contained. On the other hand, if the content exceeds 0.0050%, the hardenability is remarkably increased, the toughness and ductility of the base material are deteriorated, and the yield ratio is difficult to control. For this reason, in the present invention, B is limited to the range of 0.0003 to 0.0050%. In addition, Preferably, it is 0.0003 to 0.0020%.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to improving the ductility of steel by controlling the form 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%. In the present invention, as will be described later, preferably, the amount of dissolved oxygen immediately before the addition of Ca is adjusted to 0.0030% or less, and then Ca is added to suppress the formation of Ca oxide to crystallize CaS. CaS crystallizes in molten steel at a lower temperature than oxides, so that fine and uniform dispersion is possible in the steel. These CaS fine particles are combined with MnS fine particles and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness.

N: 0.0030-0.0060%
N combines with Ti and precipitates as TiN to suppress coarsening of austenite grains in the HAZ, or contributes to high toughness of the HAZ as a ferrite transformation nucleus. In order to secure the necessary amount of TiN having such an effect, N needs to be contained in an amount of 0.0030% or more. On the other hand, if the content exceeds 0.0.0060%, the toughness of the weld metal is lowered. For this reason, N was limited to 0.0030-0.0.0060%.

O: 0.0010 to 0.0030%
O is contained as an unavoidable impurity and is present as an oxide in the steel to lower the cleanliness. For this reason, although it is preferable to reduce as much as possible in this invention, in order to make it less than 0.0010%, refining cost becomes large. On the other hand, if the content exceeds 0.0030%, CaO inclusions become coarse, which adversely affects toughness. For this reason, O was limited to 0.0010 to 0.0030%.

ACR: 0.2-0.8
In the present invention, the amount of dissolved oxygen in molten steel at the time of Ca addition is adjusted to 0.0010 to 0.0030%, and the ACR defined by the following formula (1) is satisfied for Ca, S and O to satisfy 0.2 to 0.8. Add and adjust.

ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
Here, Ca, O, S: Content of each element (mass%)
When ACR is less than 0.2, CaS does not crystallize, so S precipitates in the form of MnS alone. This MnS is stretched by rolling at the time of manufacturing the steel sheet and does not disperse uniformly and finely, so that the toughness of the base metal is reduced and does not contribute to the improvement of the welded HAZ toughness. On the other hand, when ACR exceeds 0.8, S which is fixed by Ca and becomes MnS is insufficient, and MnS does not precipitate on CaS, so that it does not work as a ferrite formation nucleus, and improvement in 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.

Ceq: 0.47% or less In the present invention, the content of each component is adjusted so that the carbon equivalent Ceq defined by the following formula (2) is 0.47% or less within the above-described component composition range.

Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
Where Ceq: carbon equivalent (%),
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)
Next, the results of experiments conducted by the present inventors on the influence of Ceq on HAZ toughness will be described.

  Ti: 0.005-0.025%, B: 0.0004-0.0025%, ACR was 0.28-0.73, Ceq was changed from 0.353-0.495% Reproduced thermal cycle test piece, welding heat input 1000kJ A reproducible thermal cycle (1400 ° C. heating, 800 to 500 ° C. cooling time: 1000 s) assuming a thermal history of HAZ of / scm electroslab welding was applied. V-notch Charpy impact test specimens were collected from these test specimens, Charpy absorbed energy (vE0) at 0 ° C. was determined, and reproducible HAZ toughness was evaluated. The obtained results are shown in FIG. 1 in the relationship of vE0-Ceq.

From FIG. 1, the reproducible HAZ toughness (vE0) decreases with increasing Ceq. This is presumably because the structure becomes an upper bainite structure due to an increase in Ceq, and island martensite increases. Also, from Fig. 1, in order to ensure excellent HAZ toughness with Charpy absorbed energy _V E ₀ at 0 ° C of 70 J or more in super high heat input welding exceeding 400 kJ / cm, carbon equivalent Ceq is 0.47% or less. It is understood that it is necessary to.

  In addition to the basic composition described above, in the present invention, for the purpose of further increasing the strength, one of Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, Mo: 0.01 to 0.30% or Two or more kinds can be contained. Cr, V, and Mo are all elements that increase the strength of steel, and can be selected and contained as necessary to ensure the strength of the base material and the strength of the welded joint.

  Cr is an element that increases the strength of steel, and such an effect is recognized with a content of 0.05% or more. On the other hand, the content exceeding 0.50% deteriorates the HAZ toughness. For these reasons, Cr is preferably limited to a range of 0.05 to 0.50%.

  V, like Cr, is an element that increases the strength of steel, and such an effect is recognized with a content of 0.005% or more. On the other hand, the content exceeding 0.01% deteriorates the HAZ toughness. For these reasons, V is preferably limited to a range of 0.005 to 0.01%.

  Mo, like Cr and V, is an element that increases the strength of steel, and such an effect is recognized with a content of 0.01% or more. On the other hand, the content exceeding 0.30% adversely affects the HAZ toughness. For this reason, it is preferable to limit Mo to the range of 0.005-0.01%.

  The steel material used in the present invention is a molten steel having the above composition, which is melted by a known melting method using a converter or the like, or further through a refining process such as ladle refining, preferably by a continuous casting method. A slab or the like is preferable.

  The obtained steel material is heated to a temperature in the range of 1000 to 1300 ° C. and subjected to hot rolling at a rolling end temperature of 900 ° C. or higher to obtain a thick steel plate having a predetermined size and shape.

  If the heating temperature of the steel material is less than 1000 ° C., the steel material has a high deformation resistance, so it cannot be strongly pressed, and it is difficult to sufficiently reduce it to the center of the plate thickness. In particular, in the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, UT defects (zaku) may remain. On the other hand, if the heating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. Therefore, the heating temperature of the steel material was limited to a temperature in the range of 1000 to 1300 ° C.

  The hot rolling applied to the steel material is not particularly limited as long as the 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.

  If the rolling end temperature is less than 900 ° C., the deformation resistance becomes too high, the rolling load increases, and the load on the rolling mill increases. Moreover, in order to reduce the rolling temperature to less than 900 ° C., it is necessary to wait in the middle of rolling, which greatly impedes productivity. For this reason, in this invention, rolling completion temperature was 900 degreeC or more.

  After the end of rolling, the thick steel plate is immediately accelerated to 600 ° C. or lower at a cooling rate of 1 ° C./s or higher. If the cooling rate after rolling is less than 1 ° C./s, it is not possible to secure a high strength of a target tensile strength of 590 MPa or more. For this reason, in this invention, the cooling rate after completion | finish of hot rolling was 1 degree-C / s or more. In addition, although the upper limit of the cooling rate after completion | finish of hot rolling is determined by the capability of cooling equipment, it is preferable to set it as about 50 degrees C / s from a viewpoint of a shape. Moreover, the cooling rate as used in the field of this invention shall mean the average cooling rate from the start of cooling to 600 degreeC in the 1/4 thickness position of a thick steel plate.

  In the present invention, the cooling stop temperature of accelerated cooling is set to 600 ° C. or lower. If the cooling stop temperature exceeds 600 ° C, a target tensile strength of 590 MPa or higher cannot be secured. After completion of accelerated cooling, the thick steel plate may be allowed to cool to room temperature in the atmosphere, or may be subjected to reheating quenching and tempering directly and continuously without cooling to room temperature.

  The accelerated steel plate is then subjected to a reheating quenching-tempering treatment.

  The reheating quenching process is a process of heating to a reheating temperature in the range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 70 ° C.), which is a two-phase region of ferrite and austenite, and then quenching (quenching). . This reheating quenching process is an important process in order to give a sufficient strength difference between the hard phase and the soft phase, and to achieve a high strength and a low yield ratio. When the reheating temperature is less than (Ac1 transformation point + 10 ° C.), the amount of austenite generated during reheating is small, and it is difficult to ensure high strength. On the other hand, if it exceeds (Ac1 transformation point + 70 ° C.), the amount of austenite to be generated increases, the concentration of carbon in the generated austenite phase is diluted, and the hardenability is lowered. For this reason, high strength and low yield ratio cannot be obtained. For this reason, the reheating temperature was limited to a range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 70 ° C.). In addition, Preferably, it is the range of (Ac1 transformation point +10 degreeC)-(Ac1 transformation point +50 degreeC).

  Further, the rapid cooling (quenching) from the reheating temperature is preferably performed at a cooling rate of 0.1 ° C./s or more. The austenite phase produced during reheating has high hardenability due to the concentration of carbon, and if it is cooled at a cooling rate of 0.1 ° C./s or more, martensitic transformation can easily occur and the strength can be increased.

  The holding time at the above reheating temperature is preferably between 5 and 60 minutes. If the holding time is less than 5 minutes, carbon cannot be sufficiently concentrated in the austenite phase. On the other hand, if the holding time is longer than 60 min, the concentration of carbon in the austenite phase is diluted, resulting in a problem of strength reduction due to a decrease in hardenability. For this reason, the holding time at the reheating temperature is desirably 5 to 60 minutes.

  After reheating, the rapidly cooled (quenched) thick steel plate is then tempered. The tempering temperature is determined according to the plate thickness, that is, according to the difference in cooling rate during the accelerated cooling after rolling or the reheating quenching process, and is not particularly limited, but is preferably performed at 600 ° C. or less. When the tempering temperature exceeds 600 ° C., the strength is remarkably reduced and the target strength cannot be satisfied.

  By using the steel material having the composition described above, hot rolling under the above-mentioned conditions, reheating quenching-tempering treatment, it is excellent in HAZ toughness at the time of super-high heat input welding even in extra-thick steel plates exceeding 100 mm in thickness. In addition, it is possible to easily produce a high yield thick steel plate having a low yield ratio and a tensile strength: 590 MPa class and a yield ratio: 80% or less.

  Molten steel having the composition shown in Table 1 was melted in a converter, degassed with a ladle, and then made into a slab (steel material: thickness 310 mm) by a continuous casting method. These steel materials were subjected to hot rolling under the conditions shown in Table 2 (heating temperature, rolling end temperature), and then accelerated cooling (cooling speed, cooling stop temperature) under the conditions shown in Table 2 (cooling rate, cooling stop temperature). ) To obtain a thick steel plate having the thickness shown in Table 2. Subsequently, these thick steel plates were subjected to reheating quenching-tempering treatment under the conditions shown in Table 2.

From the 1 / 2T part of the obtained thick steel plate, in the rolling direction, JIS No. 4 tensile test piece according to JIS Z 2201 and V-notch Charpy impact test piece according to JIS Z 2202 were collected. The tensile properties of the material and the absorbed energy (vE ₀ ) (J) at 0 ° C. were determined.

From a part of the obtained thick steel plate, a reproducible thermal cycle test piece was further collected. A thermal cycle (1400 ° C heating, _800-500 ° C cooling time: Δt _800-500 = _{1000 s} ) simulating the thermal history of HAZ in electroslag welding with a heat input of 1000 kJ / cm was applied to these reproduced thermal cycle test pieces. Granted. A V-notch Charpy impact test piece (10 mm thick) conforming to the provisions of JIS Z 2202 is collected from the test piece that has been subjected to such a thermal cycle, and a Charpy impact test is performed at 0 ° C. vE ₀ ) (J) was determined and the reproduced HAZ toughness was examined.

  In addition, a joint test plate (thickness: 60 mm) was sampled from a portion of the resulting thick steel plate, and this joint test plate was placed on the skin plate side, and a 60 mm thick steel plate (60 mm on the diaphragm side). (Kilo steel) was arranged, and electroslag welding (heat input 900 kJ / cm) was performed with a gap of 25 mm to produce a T-type welded joint. The electroslag welding wire was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

Charpy impact test specimens (V-notch test specimens) are taken from each position on the skin plate side of the welded joint obtained (welded metal part weld bond part, HAZ 1 mm, HAZ center part) and subjected to a Charpy impact test at 0 ° C. The joint toughness was evaluated by calculating the absorbed energy (vE ₀ ) (J) at 0 ° C.

  The obtained results are shown in Table 3.

The example of the present invention has excellent base material characteristics having a sufficiently high strength of TS: 590 MPa or more and a low yield ratio of 80% or less, and a reproducible HAZ toughness of vJ ₀ of 70 J or more. It is a thick steel plate having heat input welding HAZ toughness. Furthermore, the joint toughness also shows excellent super-high heat input welded joint toughness with vE ₀ exceeding 70 J in all parts of the weld metal part, bond part and HAZ.

  On the other hand, in the comparative example that departs from the scope of the present invention, the base material performance does not satisfy the target, or the reproduced HAZ toughness and the weld joint toughness are low, and the target super high heat input welding HAZ toughness is not satisfied.

FIG. 1 is a graph of the effect of carbon equivalent (CeqW) on reproducible HAZ toughness.

Claims (3)

% By mass
C: 0.05 to 0.15%, Si: 0.05 to 0.50%,
Mn: 0.6 to 1.6%, P: 0.018% or less,
S: 0.005% or less, Al: 0.1% or less,
Cu: 0.1-1.0%, Ni: 0.1-2.0%,
Ti: 0.005-0.030%, B: 0.0003-0.0050%,
Ca: 0.0005 to 0.0050%, N: 0.0030 to 0.0060%,
O: 0.0010 to 0.0030%
In the range where the ACR defined by the following formula (1) is 0.2 to 0.8% and the carbon equivalent Ceq defined by the following formula (2) is 0.47% or less, and the balance is Fe and inevitable impurities. The steel material with the composition is heated to a temperature in the range of 1000 to 1300 ° C, subjected to hot rolling to a rolling end temperature of 900 ° C or higher, and then accelerated to 600 ° C or lower at a cooling rate of 1 ° C / s or higher. After cooling to a thick steel plate, the steel plate is further heated to a reheating temperature in the two-phase region from (Ac1 transformation point + 10 ° C) to (Ac1 transformation point + 70 ° C), and then rapidly recooled and quenched. A method for producing a low yield ratio, high strength thick steel sheet for building structures, which is excellent in super-high heat input heat affected zone toughness, characterized by performing reheating quenching-tempering treatment that is then performed.
ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
Here, Ca, O, S: Content of each element (mass%)
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
Where Ceq: carbon equivalent (%),
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass) In addition to the above composition, the composition further contains one or more of Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, and Mo: 0.01 to 0.30% in mass%. The manufacturing method of the low yield ratio high-strength thick steel plate for building structures of 1. The method for producing a low yield ratio high strength thick steel sheet for building structure according to claim 1 or 2, wherein the reheating temperature is maintained for 5 to 60 minutes.

Images