JP2007031796A - Low-yield-ratio high-tensile-strength steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-yield-ratio high-tensile-strength steel sheet which has superior gas-cutting cracking resistance, superior toughness in a high-heat-input weld joint, low acoustic anisotropy, high plastic deformability and a tensile strength of a 590 MPa grade, and can be applied even to a thin steel sheet. <P>SOLUTION: This steel sheet has a chemical composition containing appropriately controlled components, in which a CE<SB>N</SB>value expressed by the following equation is in a range of 0.27 to 0.33%: CE<SB>N</SB>=[C]+A(c)×ä[Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5[B]}, wherein A(c)=0.75+0.25×tanhä20([C]-0.12)}. The steel sheet also has a structure which contains a former austenite grain with an average aspect ratio (average grain size in main rolling direction/average grain size in sheet thickness direction) of 1.0 to 1.2 in a cross section in a sheet thickness direction, pseudo-polygonal ferrite of 10 to 40 vol.%, an island martensite phase of 0.5 to 3.5 vol.% and the balance a pseudo-pearlite phase. The structure also has ε-Cu phase clusters of 4×10<SP>20</SP>to 26×10<SP>20</SP>pieces/m<SP>3</SP>dispersed therein. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is excellent in gas cutting crack resistance, joint toughness in super-high heat input welding up to about 100 KJ / mm, low acoustic anisotropy, large plastic deformability, and tensile strength of 590 MPa. The present invention relates to the above-described low yield ratio high-tensile steel sheet for welded structures.

In recent years, with the progress of super-high-rise buildings and fewer main girder bridges, in addition to reducing preheating during welding, application of high heat input welding, application of ultrasonic flaw detection tests to welded joints, and ultimate earthquake Application of proof stress design is being promoted on the design and construction side. Accordingly, the following items (1) to (4) have been studied as the steel material side used for them.
(1) In response to the requirement for preheating reduction, reduction of weld cold cracking sensitive composition (P _CM ),
(2) For the application of high heat input welding, toughness of all parts of the welded joint that can withstand the increase in heat input,
(3) For application to the ultrasonic flaw detection test (UT) for welded joints, the sound velocity ratio between the steel material of the transverse wave used for the oblique angle UT and the STB test piece exceeds, for example, a plate thickness of 25 mm, When flaw detection is performed at a refraction angle of 70 °, the acoustic anisotropy is small by being in the range of V / V _STB 0.995 to 1.015 (ultrasonic inspection of steel structure building welds in the Architectural Institute of Japan). According to the definition of the standard),
(4) For the application of ultimate strength design for earthquakes, it is required that the yield ratio (yield point / tensile strength x 100%) is small (that is, the plastic deformability is high) In the case of 80% or less), the thickness and tensile strength of the steel materials used are also being increased in thickness and strength (maximum thickness is 100 mm, 590 MPa or more for building applications, 570 MPa or more for bridges). .

As one method for producing a 590 MPa class steel that satisfies the low yield ratio required for building steel, for example, a technique as disclosed in Patent Document 1 has been proposed. In this technique, C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.70 to 1.50%, P: 0.010% or less, S: 0.002 % Or less, Nb: 0.005 to 0.030% and Ca: 0.0010 to 0.0030%, respectively, the balance being Fe and inevitable impurities, and the carbon equivalent Ceq (JIS) is 0.45% The steel slab is completely rolled in the recrystallization temperature range, cooled to 400 ° C. or lower at a cooling rate of 5 to 30 ° C./second, reheat-quenched (RQ) consisting of temperatures of Ac ₃ points or higher, and two-phase range ( using Q-Q'-T method for performing tempering (T) at a temperature of quenching (Q ') and Ac less than ₁ point from Ac ₁ point or more Ac less than ₃ points), 80% yield ratio or less, the tensile strength 590 MPa or more is obtained.

  Moreover, as a method of utilizing the extremely low carbon bainite structure to improve acoustic anisotropy and producing a steel material having a tensile strength of 570 MPa class or 590 MPa class, for example, each technique as described in Patent Documents 2 to 5 Has also been proposed.

  Among them, in the technique of Patent Document 2, C: 0.001% or more and less than 0.030%, Si: 0.60% or less, Mn: 0.20 to 3.00%, Ni: 2.0% or less, Cu : After the steel material which becomes a composition which respectively contains 0.7-2.0% and Al: 0.10% or less is heated and cooled to the temperature of 860 degreeC or more, it is in the temperature range of 500 degreeC or more and less than 800 degreeC. A method for producing a high strength steel material of 570 MPa class with little material variation and small acoustic anisotropy by reheating and cooling is disclosed.

  In the technique of Patent Document 3, C: 0.005 to 0.025%, Si: 0.60% or less, Mn: 0.4 to 1.6%, P: 0.025% or less, S: 0.00. 010% or less, Al: 0.1% or less, Cu: 0.6-2%, Ni: 0.25-2.0%, Ti: 0.001-0.050% and B: 0.0002-0 A slab comprising a chemical composition composed of .0030% and a weight ratio of Mn / Cu: 2.0 or less and 117 Mn (%) + 163 Cu (%): 250 to 350%, with the balance being Fe and inevitable impurities , After reheating to 1050 to 1250 ° C., by performing hot rolling with a cumulative rolling reduction in a temperature range of 950 ° C. or less of 50% or less and a finish rolling temperature of 800 ° C. or more, toughness in the thickness direction of the steel sheet as it is rolled 590MPa welding electrode with excellent acoustic anisotropy It discloses a method of manufacturing the steel sheet.

  Furthermore, in Patent Document 4, C: 0.025 to 0.045%, Nb: 0.005 to 0.1%, which does not contain Mo, has low acoustic anisotropy, and has excellent weldability. A method for producing a non-tempered, low yield ratio, high strength steel sheet of 590 MPa or higher is shown.

In Patent Document 5, C: 0.015-0.045%, B: 0.0004-0.003%, Cu: 0.5-0.95%, Ni: 0.7-5.0% (however, , Ratio of Ni content [Ni] and Cu content [Cu] [Ni] / [Cu] ≧ 1), Ti: 0.005 to 0.03%, respectively, and represented by the following formula (2) a CE _N value the chemical compositions in the range of 0.27 to 0.33 percent which, island martensite phase of 0.5 to 3.5 vol% and 4 × 10 ²⁰ ~26 × 10 ²⁰ by epsilon-Cu phase clusters of pieces / m ³ is assumed dispersed in bainite area, a small low yield ratio high-tensile steel sheets and the acoustic anisotropy excellent resistance gas cutting crack resistance and high heat input welded joint toughness It is disclosed.
_{CE N = [C] + A} (c) · {[Si] / 24 + [Mn] / 6 + [Cu] / 15 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 + 5 [B]}
(2)
However, A (c) = 0.75 + 0.25 · tanh {20 ([C] -0.12)}, and [C], [Si], [Mn], [Cu], [Ni], [Cr], [ Mo], [Nb], [V] and [B] indicate the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B, respectively.

  However, the following problems are pointed out in the high-tensile steel sheets having a very low carbon bainite structure proposed so far. First, in the technique of Patent Document 1, the proposed medium carbon steel 590 MPa class steel plate is used for construction, but by generating polygonal ferrite, a predetermined strength is secured in the base material and the joint portion. Therefore, since the carbon equivalent is higher than that of the QT type 590Pa grade steel plate, the applied welding heat input is also reduced to 17 KJ / mm, and the toughness at a large heat input welded joint of 100 KJ / mm is low. Moreover, with the technique of the said patent document 2, although the acoustic anisotropy of the steel material obtained is small, the yield ratio exceeds 80% and cannot be used for a building use.

In the technique of Patent Document 3, although the acoustic anisotropy of the obtained steel sheet is small and the yield ratio satisfies the required value (80% or less) for the steel sheet for construction, the effect of increasing the strength due to precipitation of Cu is reduced by cooling after rolling. Depending on the process, when the cooling rate is fast, the above effect is not always obtained stably, and such an effect depends on the plate thickness. The high heat input HAZ toughness is also about 52 to 71 J (absorption energy vE ₀ at 0 ° C.) with a heat input of 50 KJ / mm (Table 2 in the examples), and the average 70 J targeted in the present invention. The above cannot be achieved stably. Further, in this technique, Cu and Ni are included as essential components, but no consideration is given to their appropriate blending ratio (Ni / Cu is 0.47 to 0.95 in Table 1 of the Examples). It becomes susceptible to cracking due to Cu liquefaction in a direction parallel to the surface of the gas cut surface.

  The technique of Patent Document 4 satisfies the required characteristics as a steel sheet for construction by realizing small acoustic anisotropy and low yield ratio while being air-cooled, but the heat input amount of the large heat input welded joint is 20 KJ. / Mm, which is relatively small (for example, Table 3 of Examples), there is no guarantee that high HAZ toughness can be secured at the welding heat input up to about 100 KJ / mm which is the target of the present invention.

  In the technique of Patent Document 5, by applying DQ (direct quenching) -Q′-T and RQ-Q′-T, island-like martensite is dispersed in the bainite ground, and thereby the thickness ranges from 45 to 100. Discloses a high-strength steel sheet that satisfies a target yield ratio of 590 MPa class steel for construction, which is 80% or less. This technique can be said to be an extremely useful technique for a relatively thick steel sheet. However, if the plate thickness is less than 45 mm, the yield ratio may increase rapidly and exceed 80% as the plate thickness decreases. There was a limit to the plate thickness to be applied.

  On the other hand, techniques such as Patent Documents 6 to 8 have been proposed as steel materials having a tensile strength of 590 MPa and excellent in toughness of large heat input weld joints at a heat input of 20 KJ / mm or more.

Among these, Patent Document 6 includes C: 0.001 to 0.03%, Mn: 0.8 to 3.0%, B: 0.0003 to 0.0050%, and Ti / Al is 5. 0 or more, and Ti oxide: 20 to 90%, Al ₂ O ₃ : 70% or less, one or two of Ca oxide and REM oxide: 5 to 50%, MnO: A method for producing a tensile strength 570 MPa grade steel material in which oxide inclusions of 15% or less are dispersed is disclosed.

  In Patent Document 7, C: 0.02% or less, Mn: 0.5 to 2.0%, Nb: 0.010 to 0.10%, B: 0.0003 to 0.0040%, and Non-tempered steel for low temperature use with a tensile strength of 552 to 605 MPa having a composition of B / N of 0.3 to 1.0 is disclosed.

  Further, in Patent Document 8, C: 0.01 to 0.06%, Mn: 1.25 to 2.5%, Cr: 0.1 to 2.0%, Mo: 1.5% or less (0 %), Ti: 0.005-0.03%, B: 0.0006-0.005%, O: 0.0025-0.015%, ([Mn] + 1.5 × [ A high-strength steel sheet having a parameter KP defined by Cr] + 2 × [Mo]) of 2.4% by mass or more is disclosed. Moreover, as a process for producing such a high-tensile steel plate, rolling is completed at 850 to 950 ° C., then cooled, then re-heated to 750 to 800 ° C. and then water-quenched, and finally tempered at 550 to 600 ° C. (Eg, Tables 3 and 4 in the Examples).

  However, in the technique of Patent Document 6 described above, Nb is included in an amount of about 0.005 to 0.10% (0.04 to 0.05% in the example), and the rolling end temperature is 800 ° C. or more (Example). 820 to 850 ° C.), and cannot stably satisfy the acoustic anisotropy required for a steel material for welded structures for architectural purposes (this point will be described later).

  In the technique of Patent Document 7, Nb: 0.010 to 0.10% is contained in the same manner as in Patent Document 6 described above. Rolling of such steel material is finished at 750 ° C. or higher, and air cooling or accelerated cooling is performed. As a result, the tensile strength of the 590 MPa class is satisfied, but the yield ratio is 82% or more and cannot be applied to architectural purposes.

In the technique of Patent Document 8, although low carbon steel (based on a C content of 0.03), Cu, Nb and Mo are not added (No. 1 in Table 1 of Examples). The amount of Mn in this steel exceeds the specified amount (1.6% or less) of the amount of Mn in SM570 of JIS G 3106 and high performance steel SA440 certified by the Minister of Construction. Moreover, although what made Mn amount 1.60% or less is also shown (No. 12 of Table 1), in this steel material, the strength reduction by the addition of Cu and Nb is compensated by addition of Mo, Such a steel material has a toughness as low as 71 J in vE _-40 (absorbed energy at -40 ° C), and is inferior in gas cutting property. That is, in such a steel material, it is difficult for the surface roughness of the gas cut surface to satisfy WES2801, and a notch is generated and becomes a starting point of fracture, so that it is not suitable as a structural member.

By the way, the technique shown by patent document 9 is also known as a technique which made the steel material containing Cu suppress the surface crack resulting from Cu, and was excellent also in the toughness of a welding heat affected zone. This technology is a Cu-containing steel for welded structures containing Si: 0.05 to 0.5%, Cr: 0.1 to 0.6%, B: 0.0005% or less, and cracks during rolling It is said that the toughness vE ₀ of the weld heat affected zone in CO ₂ welding at 12 KJ / mm is good. Moreover, when adding Ni, it has shown that it is desirable to prescribe | regulate to less than 1/3 of the amount of Cu in steel.

However, even if cracking during rolling can be prevented with this technique, when the Cu or Cu alloy in the steel remains in a molten state up to a low temperature in the process of slagging the steel component during gas cutting, Compared to the fact that the amount of heat input is so large that the steel is once melted in comparison, it is easy to easily enter the grain boundary of the gas cut surface of the molten Cu, and may not prevent gas cut cracks. In this state, the starting point of fracture is inherent, and it cannot be applied to a site where axial stress acts in the direction in which the crack opens.
Japanese Patent Publication No. 7-47774 Patent Claims etc. Japanese Patent Laid-Open No. 9-256042 Claims etc. Japanese Patent Application Laid-Open No. 11-193445 Patent Claims, Examples Table 1, 2 etc. JP, 2002-53912, A Claims, Table 3 of an example, etc. JP, 2005-36295, A Claims JP, 2000-345239, A Claims etc. JP, 2001-20034, A Claims etc. JP, 2001-335883, A Claims, Tables 3 and 4 of an example, etc. JP, 2002-371337, A Claims

  The present invention has been made in order to solve such problems in the prior art, and its purpose is excellent in gas cut cracking resistance and high heat input weld joint toughness, and has low acoustic anisotropy and plasticity. An object of the present invention is to provide a low-yield-ratio high-tensile steel plate having a tensile strength of 590 MPa that can be applied even to a thin steel plate having a large deformability.

The low-yield-ratio high-tensile steel sheet of the present invention that has achieved the above-mentioned object is: C: 0.015-0.045% (meaning mass%, the same shall apply hereinafter), Si: 0.4% or less (0% Mn: 0.8 to 1.6%, Cr: 0.5 to 1.3%, sol. Al: 0.08% or less (including 0%), B: 0.0004 to 0.003%, Cu: 0.5 to 0.95%, Ni: 0.7 to 5.0% (however, Ni The ratio [Ni] / [Cu] ≧ 1) of the content [Ni] and the Cu content [Cu], Ti: 0.005 to 0.03% and N satisfying the following formula (1) , substantially free of Nb and Mo, and CE _N value represented by the following formula (2) is in the range of 0.27 to 0.33%, the chemical composition and the balance being Fe and unavoidable impurities The average aspect ratio of the prior austenite grain size in the cross section in the thickness direction (average grain size in the main rolling direction / average grain size in the thickness direction) is 1.0 to 1.2, and 10 to 40 volumes. % Pseudo-polygonal ferrite, 0.5 to 3.5% by volume of an island-like martensite phase, the balance having a pseudo-pearlite phase structure, Of the tissue and has a gist in that 4 × 10 ²⁰ ~26 × 10 ²⁰ atoms / m ³ of epsilon-Cu phase clusters is obtained by dispersing.
[Ti] × 14.0 / 47.9−0.001 ≦ [N] ≦ [Ti] × 14.0 / 47.9 + [B] × 14.0 / 10.8 (1)
However, [Ti], [N], and [B] indicate the contents (mass%) of Ti, Ni, and B, respectively.
_{CE N = [C] + A} (c) · {[Si] / 24 + [Mn] / 6 + [Cu] / 15 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 + 5 [B]}
(2)
However, A (c) = 0.75 + 0.25 · tanh {20 ([C] -0.12)}, and [C], [Si], [Mn], [Cu], [Ni], [Cr], [ Mo], [Nb], [V] and [B] indicate the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B, respectively.

  In the low yield ratio high tensile strength steel sheet of the present invention, (a) V: 0.005 to 0.10%, (b) Ca: 0.0005 to 0.01%, (c) La: 0.0. One or more selected from the group consisting of 002 to 0.02%, Ce: 0.0003 to 0.0050%, Mg: 0.0005 to 0.0030%, and Zr: 0.002 to 0.02. , Etc. are also effective, and the characteristics of the high-tensile steel sheet can be further improved depending on the components contained.

  The high-strength steel sheet of the present invention does not require preheating at a low yield ratio, and can ensure high HAZ toughness of an average of 70 J or higher even when subjected to large heat input welding up to about 100 KJ / mm, and is suitable for earthquakes with large strain rates. For this reason, it is possible to prevent brittle fracture of welded joints, and it has no gas cutting crack defect and has low acoustic anisotropy, so it is a highly reliable thin steel plate as a main welded structural member for high-rise buildings. However, it is a high-tensile steel plate with a tensile strength of 590 MPa that can be applied.

  In addition to the conventional plastic deformability with a yield ratio of 80% or less, the weld heat input of 50 for the 590 MPa class high strength steel plate used for super high-rise buildings, corresponding to the thickening of welded joints. In order to realize an extremely large heat input of up to 100 KJ / mm and to resist an external force having a large strain rate such as a large earthquake, an average of 15 J shown in the welding construction guideline for the diaphragm welded portion in all parts of the welded joint A demanded toughness of 70 J or more, which is much higher than the above, has been demanded.

  On the other hand, in the case of bridges, in addition to the large heat input welding, since there is a tendency to switch the welded joint part from the conventional radiation transmission test to the ultrasonic oblique flaw detection test, as in the case of high-rise buildings, The acoustic anisotropy with respect to the transverse wave is required to be small.

  In the strength member subjected to electroslag welding of up to about 100 KJ / mm in addition to the low yield ratio and small acoustic anisotropy in the 590 MPa class high-tensile steel sheet for construction, We have intensively studied to provide an average toughness of 70 J or more.

  As a result, after heating to 1350-1400 ° C., in addition to the effect of suppressing grain coarsening by TiN for the heat affected zone with a very low cooling rate, (i) crystals in the heat affected zone in the vicinity of the melting line It has been found that it is effective to suppress the precipitation of ferrite at the grain boundaries and the formation of ferrite side plates from the grain boundaries, and (ii) to make the inside of the crystal grains a fine bainite structure.

For (i) above, it has been found that it is effective to segregate the solid solution B at the grain boundaries even when the cooling rate is extremely low. For that purpose, in order to suppress the formation of BN, free N is fixed with Ti, Mo which raises the Ar ₃ transformation point is not added, and an appropriate amount of Cr which lowers the Ar ₃ transformation point is positively added. Was found to be effective.

In addition to the above (ii), the carbon equivalent CE _N defined by the above formula (2) is controlled to 0.27 to 0.33%, and the constituent C is made extremely low, and Nb is not added. It has been proved that it is effective to be an aggregate of fine bainite blocks in which the microstructure in the prior austenite crystal grains in the heat-affected zone is increased in inclination by the addition. It was also found that when the CE _N is in the range of 0.27 to 0.33%, preheating free at the time of small heat input welding can be realized. In addition, in order to compensate for the strength reduction of the base material due to these measures, it is necessary to actively utilize precipitation strengthening due to the ε-Cu phase.

  By the way, when Cu that exhibits precipitation strengthening is excessively added, a low melting point Cu or Cu alloy melted at the time of gas cutting selectively penetrates into the crystal grain boundary, so that there are many hair cracks extending to a depth of about 0.2 mm. Therefore, the Cu content is limited to 0.5 to 0.95%, and the ratio [Ni] / [Cu] of the Ni content [Ni] and the Cu content [Cu] is set to 1 or more to melt. Sometimes it is necessary to reduce the susceptibility to cracking by eutecticizing and increasing the solidification temperature.

  In addition, the acoustic anisotropy of the base material satisfies the range in which it is determined that there is no sound velocity ratio with STB in Appendix Table 1 of “Ultrasonic Inspection Standards for Steel Structure Building Welds” of the Architectural Institute of Japan. Can be realized by controlling the average aspect ratio of the prior austenite grain size of the base material (average grain size in the rolling direction / average grain size in the plate thickness direction) to a range of 1 to 1.2.

  Furthermore, as a measure to reduce (80% or less) the yield ratio that increases as the thickness is reduced, in a microstructure in which island-like martensite phases are dispersed in a bainite base as disclosed in Patent Document 5, the target yield is obtained. Ratio could not be achieved, but by dispersing island martensite in the base that promoted the two-phase separation of pseudopolygonal ferrite, pseudopearlite and C (carbon), It was found that a low yield ratio that can absorb the increase in yield ratio due to the refinement of austenite grains can be realized. Pseudopolygonal ferrite is a phase in which ferrite transformation is completed with incomplete polygonalization, and pseudopearlite means a phase in which pearlite transformation is completed in a non-equilibrium state.

By applying the above measures in a comprehensive manner, even if a large heat input welding of 50 to 100 KJ / mm is applied, it has an average of 70 J or higher in vE ₀ and is not susceptible to gas-cutting cracking. The present inventors have found that a steel plate suitable for steel (SA440) can be obtained, and have completed the present invention.

  The reason for limiting the chemical composition and the microstructure for obtaining the characteristics of the present invention will be described below along the background.

The inventors of the present invention have five steel types (0.035C-1.45Mn-0.95Cu-1Ni-0.0) adjusted with the Mn content variable so that the CE _N value is constant at about 0.30%. 7Cr-0.015Ti-0.0012B-0.0040N-based basic steel, 0.4Mo-added steel, 0.01Nb-added steel, 0.4Mo-0.01Nb-added steel, and 0.05C-based steel) skin plate material And a SN490B-TMC (0.14C-0.3Si-1.25Mn-0.008Nb-0.012Ti system, 60 mm thick) diaphragm material, with a gap of 25 mm and an electroslag with a heat input of 100 KJ / mm Welding and comparing the toughness of the heat affected zone (HAZ) on the skin plate side of the welded joint (notch position: bond showing the lowest toughness in high heat input welding HAZ + 0.5 mm) Was 査 (see experiment No.2~6 of the following Table 4, 7).

From this result, even if the chemical component composition shows the same level as the CE _N value of about 0.30%, the large heat input HAZ toughness is greatly increased by adding no Mo, no Nb, adding Cr, and reducing C. It can be seen that, by combining these requirements and combining these requirements, vE ₀ 70J or more can be guaranteed for the first time even in the vicinity of the bond +0.5 mm position where the HAZ toughness is lowest. It can also be seen that the strength reduction due to the absence of addition of Mo and Nb can be compensated by precipitation strengthening of Cu and solid solution strengthening by Mn and Cr.

  In addition, steel grade with high heat input welding HAZ toughness (steel grade B in Table 1 below) is used as a basic component, and the content of Cu and Ni is changed to LP gas (LPG) cutting from the gas cut surface to the steel plate surface. The maximum depth of cracks extending in the parallel direction was comparatively investigated (see Experiment Nos. 2 and 7 to 16 in Table 7 below).

  From this result, it has been found that by controlling the Cu content to 0.95% or less and the ratio [Ni] / [Cu] to 1.0 or more, the sensitivity to gas cutting cracks is reduced.

  Further, the present inventor has found that the basic component of the 0.035C-1.45Mn-0.95Cu-1Ni-0.7Cr-0.015Ti-0.0012B-0.0040N system (steel type B in Table 1 below) is C. Using steel grades with different contents, the continuous cast slab was heated to 1050-1100 ° C, hot-rolled to 44mm thickness at 900 ° C, heated to 930 ° C and then air-cooled (normalized), and subsequently at 840 ° C. Quenching treatment (Q ′ treatment) and tempering treatment (T treatment) at 500 ° C. were conducted to investigate the effect of C content on strength, yield ratio and microstructure and HAZ toughness at 100 KJ / mm ( (See Experiment No. 2, 6, 17-21 in Table 7 below).

From this result, it was found that when C: 0.015% or more, the yield ratio is 80% or less and the strength is compatible. Moreover, from the investigation results of the high heat input HAZ toughness in these steel types, it was found that the upper limit of the C content needs to be 0.045% in order to ensure an average of 70 J or more at vE ₀ .

  Next, the reason for limiting the chemical component composition in the high-tensile steel sheet of the present invention will be described. First, in the present invention, as described above, C: 0.015 to 0.045%, Si: 0.4% or less (including 0%), Mn: 0.8 to 1.6%, Cr: 0.5 ~ 1.3%, sol. Al: 0.08% or less (including 0%), B: 0.0004 to 0.003%, Cu: 0.5 to 0.95%, Ni: 0.7 to 5.0% (however, Ni The ratio [Ni] / [Cu] ≧ 1) of the content [Ni] and the Cu content [Cu], Ti: 0.005 to 0.03% and N satisfying the above formula (1) Although it is necessary to substantially not contain Nb and Mo, the reasons for limiting the ranges of these elements are as follows.

C: 0.015-0.045%
C is an element effective for ensuring the strength and low yield ratio of high-tensile steel by forming a pseudo-polygonal ferrite phase and a pseudo-pearlite phase, and needs to be contained in an amount of 0.015% or more. However, if C is contained excessively, a bainite phase is formed to increase the yield ratio and the weld cold cracking resistance is deteriorated, and the island-like martensite phase is increased by high heat input welding HAZ to deteriorate toughness. I will let you. For these reasons, the upper limit needs to be 0.045%. In addition, from a viewpoint of coexistence of base material strength and high heat input welding HAZ toughness, a preferable lower limit is 0.02%, and a preferable upper limit is 0.04%.

Si: 0.4% or less (including 0%)
Si is an effective element as a deoxidizer and strengthening element. However, if excessively contained, it increases the island-like martensite phase in the high heat input welding HAZ and deteriorates toughness. For these reasons, the upper limit is set to 0.4%, and the lower content is preferably as low as possible, so the lower limit is set to 0%. When Si is not included, deoxidation can be arbitrarily replaced with Mn, Al, Ti, or the like. In addition, the preferable upper limit of Si content is 0.3%.

Mn: 0.8 to 1.6%
Mn is an element effective for strengthening the steel sheet by transferring the ferrite transformation to a low temperature for a long time to form a pseudopolygonal ferrite phase. For that purpose, it is necessary to contain 0.8% or more of Mn. However, if Mn is contained excessively, the toughness of the base metal and the high heat input weld HAZ and the deterioration of the weld cold crack resistance are caused, so the upper limit is made 1.6%. The minimum with preferable Mn content is 1.0%, and a preferable upper limit is 1.5%.

Cr: 0.5 to 1.3%
Cr is an element effective for forming a pseudo-polygonal ferrite phase and reducing the yield ratio compared with the same strength level by improving hardenability. Moreover, precipitation of the polygonal ferrite phase at the grain boundaries of the high heat input welding HAZ is suppressed, and a bainite phase is easily formed at a low temperature, which is effective in improving toughness. In order to exert such an effect, it is necessary to contain 0.5% or more of Cr. However, if Cr is excessively contained, the island-like martensite phase is increased in the high heat input welding HAZ, so the upper limit needs to be 1.3%. In addition, the minimum with preferable Cr content is 0.6%, and a preferable upper limit is 1.1%.

sol. Al: 0.08% (including 0%)
sol. Al (soluble ability Al) is an element effective for deoxidation, but in order to improve toughness by forming intragranular bainite with Ti oxide as the core in high heat input welding HAZ, the content can be as much as possible. Less is better and the lower limit is 0%. In this case, deoxidation can be arbitrarily replaced with Mn, Si, Ti, or the like. Also, sol. Al effectively works to secure the hardenability of the base material by supplementing N fixation with Ti, but if it is excessively contained, it increases nonmetallic inclusions and causes toughness deterioration. Therefore, sol. When Al is contained, the upper limit needs to be 0.08%. In addition, sol. A preferable upper limit of Al is about 0.06%.

B: 0.0004 to 0.003%
B improves the hardenability and suppresses the ferrite transformation of polygonal form even at a low cooling rate during normalization, and promotes the formation of pseudopolygonal ferrite, and is therefore effective in improving the strength of the base material. In addition, B combines with N to form BN, and this BN acts as an intragranular bainite transformation nucleus. Therefore, toughness due to the suppression of the formation of intergranular ferrite phase and the formation of fine intragranular bainite phase in high heat input HAZ. It works effectively for improvement. For that purpose, B needs to be contained in an amount of 0.0004% or more. However, when B is contained excessively, the hardenability of the steel becomes too high, the island-like martensite phase is increased, the toughness of the base metal and the high heat input welding HAZ is deteriorated, and the resistance to welding cold cracking is reduced. Deteriorate. For these reasons, the upper limit of the B content needs to be 0.003%. In addition, the minimum with preferable B content is 0.006%, and a preferable upper limit is 0.002%.

Cu: 0.5 to 0.95%
Cu is an element effective for improving the strength of the base material by solid solution strengthening and precipitation of clusters of ε-Cu phase. In order to exhibit these effects, it is necessary to contain Cu 0.5% or more. However, if Cu is excessively contained, a Cu-concentrated phase is formed on the gas cut surface and enters the prior austenite grain boundary during thermal expansion to induce cracking, so the upper limit is made 0.95%. The minimum with preferable Cu content is 0.7%, and a preferable upper limit is 0.9%.

Ni: 0.7-5.0%
Ni is an element that improves the hardenability and improves the toughness of the base material and the base of the high heat input welding HAZ, and in order to exert these effects, it is necessary to contain 0.7% or more. However, if Ni is contained excessively, the hardenability becomes too high and the island-like martensite phase increases, leading to deterioration of toughness and also being uneconomical, so the upper limit is made 5.0%. In addition, the minimum with preferable Ni content is 0.9%, and a preferable upper limit is 3%.

However, [Ni] / [Cu] ≧ 1
In the high-tensile steel of the present invention, the ratio [Ni] / [Cu] of the Ni content [Ni] and the Cu content [Cu] needs to be 1 or more. By satisfying these requirements, the melting point of the Cu—Ni alloy phase concentrated on the gas cut surface can be increased, and hot cracking can be prevented.

Ti: 0.005 to 0.03%
Ti is an element effective for fixing solid solution N as TiN, increasing the amount of solid solution B, and improving the hardenability of the base material. Further, when Ti oxide is generated by Ti deoxidation, it acts as a nucleus for formation of an intragranular bainite phase in high heat input welding HAZ and improves toughness. In order to exert such effects, the Ti content needs to be 0.005% or more. However, when Ti is excessively contained, the toughness of the base metal and the HAZ is deteriorated by precipitation of TiC, so the upper limit is made 0.03%. In addition, the minimum with preferable Ti content is 0.008%, and a preferable upper limit is 0.02%.

N: In order to ensure high toughness in the high heat input welding HAZ that satisfies the above formula (1) , fine precipitation of TiN in the prior austenite grains and composite precipitation of BN, It is effective to form the nuclei of intragranular bainite. From such a viewpoint, the lower limit of the N content was (Ti stoichiometric equivalent) −0.001%, and the upper limit was the total amount of Ti and B stoichiometric equivalents. Exceeding this will cause toughness degradation of the HAZ due to the solute N and a decrease in the hardenability of the base metal.

Nb: Nb not substantially contained improves the hardenability by solid solution, but forms a plate-like coarse upper bainite phase in the prior austenite grains in the high heat input welding HAZ, and the crystal orientation is aligned. Therefore, it does not become a barrier for the fracture path and greatly deteriorates the toughness. From such a viewpoint, it is necessary that the high-tensile steel of the present invention is not substantially contained. Note that “substantially free” means that it is allowed to be mixed as an impurity (for example, 0.005% or less).

Mo: Mo which is substantially not contained is an element effective for improving the hardenability and improving the strength. However, the Ar ₃ transformation point is raised to increase the high-temperature bainite phase or the island martensite phase in the high heat input welding HAZ. Promotes formation and degrades toughness. From such a viewpoint, it is necessary that the high-tensile steel of the present invention is not substantially contained. Note that “substantially does not contain” means that it is allowed to be mixed as an impurity (for example, 0.05% or less).

  In the low yield ratio high tensile strength steel sheet of the present invention, (a) V: 0.005 to 0.10%, (b) Ca: 0.0005 to 0.01%, (c) La: 0.0. One or two selected from the group consisting of 002 to 0.02%, Ce: 0.0003 to 0.0050%, Mg: 0.0005 to 0.0030% and Zr: 0.002 to 0.02% Although it is effective to contain the above, the reasons for limiting the range when these components are contained are as follows.

V: 0.005-0.10%
V is an element effective for improving the base material strength. In order to exert such an effect, it is preferable to contain V in an amount of 0.005% or more. However, if it is contained in excess of 0.10%, the high heat input welding HAZ toughness is lowered. In addition, the more preferable minimum of V content is 0.03, and a more preferable upper limit is 0.06%.

Ca: 0.0005 to 0.01%
When Ca fixes S as CaS and controls the form as granular non-metallic inclusions, the Z-direction tensile stress generated during column angle joint welding acts on the S segregated portion in the center of the plate thickness Is effective in improving drawing and toughness and preventing breakage from the segregated portion. Moreover, it combines with [O] to form CaO, and the core of bainite transformation after high heat input welding is dispersed in the prior austenite grains, and the bainite block size is refined to improve the high heat input HAZ toughness. Demonstrate. In order to exert these effects, Ca is preferably contained in an amount of 0.0005% or more. However, if the Ca content exceeds 0.01%, these effects are not only saturated but also the toughness of the base material. However, it deteriorates. In addition, the more preferable minimum of Ca content is 0.01%, and a more preferable upper limit is 0.05%.

1 selected from the group consisting of La: 0.002-0.02%, Ce: 0.0003-0.0050%, Mg: 0.0005-0.0030% and Zr: 0.002-0.02% The seeds or two or more kinds of La and Ce are one kind of rare earth elements (REM), and effectively act to fix S as a sulfide and improve the drawing and toughness of the segregation part. In addition, Ce, Mg and Zr precipitate CeO ₂ , MgO and ZrO ₂ low melting point oxides in the prior austenite grains during cooling after high heat input welding, and bainite transformation is performed using the bainite block. The high heat input HAZ toughness is improved by refining and complicating the fracture path. When La, Ce, Mg, and Zr are less than the above lower limits, these effects are not exhibited. When the La, Ce, Mg, and Zr are more than the upper limits, the presence of excessive nonmetallic inclusions deteriorates the base metal toughness. . More preferable lower limits are La: 0.005%, Ce: 0.0005%, Mg: 0.001%, and Zr 0.005%, respectively, and more preferable upper limits are La: 0.01% and Ce: 0.002. %, Mg: 0.0020%, Zr 0.01%.

  In the high-tensile steel sheet of the present invention, in addition to the above components, it consists of Fe and inevitable impurities, but it can also contain a trace amount component (allowable component) to the extent that it does not impede its properties. Are also included in the scope of the present invention.

In high-tensile steel sheet of the present invention, CE _N value defined by equation (2) above, the average aspect ratio (major rolling direction / thickness direction of the average particle size ratio), pseudo polygonal ferrite and island martensite phase The proportion of the ε-Cu phase in the structure and the dispersion ratio of the clusters in the structure need to be controlled within an appropriate range. The reasons for limiting these ranges are as follows.

CE _N : 0.27 to 0.33%
CE _N defined by the above formula (2) is a carbon equivalent expressing the curability of the welded HAZ. When the CE _N value is less than 0.27%, the thick material cannot satisfy the tensile strength of 590 MPa class. Also the value of CE _N exceeds 0.33%, the resistance to weld cold cracking resistance deteriorates, preheating not only necessary, toughness high heat-input welding HAZ island martensite phase is increased It becomes low and it becomes difficult to stably secure the target average vE ₀ : 70 J at a heat input of 100 KJ / mm. Therefore, in the high-tensile steel of the present invention, the CE _N value defined by the above equation (2) needs to be in the range of 0.27 to 0.33%. The preferable lower limit of the CE _N value is 0.28%, and the preferable upper limit is 0.32%.

Average aspect ratio of prior austenite grain size in the cross section in the thickness direction (main rolling direction / thickness direction)
Average particle size ratio): 1.0 to 1.2
When the aspect ratio of the prior austenite grain size in the cross section in the thickness direction (average grain size in the main rolling direction / average grain size in the thickness direction) exceeds 1.2, the so-called texture in which the crystal orientation is oriented in a specific direction Therefore, the range in which the acoustic anisotropy is determined to have no sound speed ratio with the STB in Appendix Table 1 of “The Ultrasonic Inspection Standard for Steel Structure Building Welds of the Architectural Institute of Japan” When flaw detection is performed with a probe having a nominal refraction angle of 70 ° with a thickness exceeding 25 mm, it exceeds 0.995 ≦ V / V _STB ≦ 1.015), and the defect position cannot be correctly displayed in the ultrasonic flaw detection test. It becomes a construction problem. Therefore, it is necessary to control the average aspect ratio of the prior austenite (average particle size in the main rolling direction / average particle size in the plate thickness direction) in the range of 1.0 to 1.2. Preferably, the average aspect is in the range of 1.0 to 1.1.

It has a structure composed of 10 to 40% by volume of pseudopolygonal ferrite, 0.5 to 3.5% by volume of island-like martensite phase, and the balance consisting of pseudo-pearlite phase, and this structure contains 4 × 10 ²⁰ to in high-tensile steel 26 × 10 ²⁰ atoms / m ³ of epsilon-Cu phase clusters according to the present invention that is obtained by dispersing, thin material yield ratio at a low CE _N (yield strength / tensile strength × 100%) ( For example, in order to provide a yield ratio of 80% or less, which is required for main building components from the viewpoint of seismic design, the plate thickness is less than 45 mm. In addition to being based on a matrix composed of a pseudo-polygonal ferrite phase and a pseudo-pearlite phase that are manifested in the matrix, it is necessary to finely disperse the hard island martensite phase in the matrix. When the dispersion ratio of the island-like martensite phase is less than 0.5% by volume, the yield ratio exceeds 80%. On the other hand, when the dispersion ratio of the island-like martensite phase exceeds 3.5% by volume, the toughness of the base material is low although the yield ratio is 80% or less. Therefore, it is necessary to make island-like martensite 0.5 to 3.5 volume%. Preferably it is 1 to 3 volume%.

  If the pseudopolygonal ferrite phase in the matrix is less than 10% by volume, the yield ratio increases, and if it exceeds 40% by volume, the tensile strength decreases. For these reasons, the ratio of the pseudo-polygonal ferrite phase in the matrix needs to be in the range of 10 to 40% by volume.

The structure of the steel sheet of the present invention is composed of a pseudopolygonal ferrite phase, a pseudopearlite phase and an island-like martensite phase as described above (therefore, the abundance of the pseudopearlite phase is 56.5 to 89.5 volumes). % Range). In such a steel sheet, in order to secure a tensile strength of 590 MPa or more with low CE _N , it is necessary to add precipitation strengthening of the ε-Cu phase by aging in the base. If the ε-Cu phase clusters are dispersed at less than 4 × 10 ²⁰ / m ³ , the tensile strength is insufficient. On the other hand, if the number of ε-Cu phase clusters exceeds 26 × 10 ²⁰ / m ³ , the toughness tends to deteriorate, and a large amount of low melting point Cu alloy is formed during gas cutting. Causing intergranular cracking on the gas cut surface. Therefore, it is necessary to disperse 4 × 10 ^{20 to} 26 × 10 ²⁰ clusters / m ³ of ε-Cu phase clusters in the organization (base). Preferably from 7 × 10 ²⁰ ~22 × 10 ²⁰ atoms / m ^3.

  In order to produce the steel sheet of the present invention, it can basically be produced by a normal method of hot rolling-cooling-heat treatment using slag produced by a continuous casting method or an ingot-making method. In order to satisfy the above, the rolling temperature range should be higher than the austenite (γ) non-recrystallization temperature, and then the normalizing treatment (N treatment) may be performed, followed by two-phase quenching and tempering heat treatment. That is, the average aspect ratio of the prior austenite grain size can be controlled by the rolling temperature range, and the fraction of pseudopolygonal ferrite can be controlled by normalization after rolling and subsequent Q ′ treatment, and the island-like martensite phase and ε− The dispersion rate of the Cu phase clusters can be controlled by the cooling rate or tempering temperature of rolling or two-phase region heat treatment.

  The high-tensile steel plate of the present invention is assumed to be a relatively thin steel plate having a thickness of less than 45 mm. Even in such a thin steel plate, a low yield ratio (80% or less) can be realized. The thickness of the target high-tensile steel plate is not limited to a thickness of less than 45 mm, and can be effectively applied to a thick steel plate having a thickness of 45 mm or more. It satisfies the characteristics.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design modifications may be made in accordance with the gist of the present invention. It is included in the range.

Steel sheets having chemical composition compositions shown in Tables 1 to 3 below were used to manufacture steel sheets under the manufacturing conditions shown in Tables 4 to 6 below. Incidentally, in Table 1-3, the specified range is (1) in the present invention, also shown for the values of the values and [Ni] / [Cu] of CE _N. In Tables 4 to 6, Q is reheating and quenching at a temperature of Ac ₃ point or higher, N is a heating temperature during normalization (temperature of Ac ₃ point or higher), and Q ′ is a two-phase region (Ac ₁ quenching from Ac less than ₃ points) or more points, T is respectively show tempering at temperatures below Ac ₁ point.

  About each obtained steel plate, the aspect ratio of prior austenite (γ) grain size, the volume ratio of island martensite, the base structure (second phase type and ratio, base), the number of ε-Cu phase clusters, etc. are as follows: Measured by method.

[Aspect ratio of former austenite (γ) particle size]
The prior austenite grain boundaries in the plate thickness section in the main rolling direction are revealed using a corrosive solution consisting of (5 ml hydrochloric acid + 1 g picric acid + 100 ml ethanol), and the average grain size in the main rolling direction and the average grain size in the plate thickness direction are determined. Measurements were taken to determine their ratio as the average aspect ratio.

[Volume ratio of island martensite]
The island-like martensite phase was photographed by exposing the main rolling direction and the plate thickness section perpendicular to the main rolling direction using a repeller reagent, and the fraction (volume ratio) was calculated by an image analyzer.

[Base organization]
The base phase and the second phase type were determined by an optical microscope, and the ratio of the precipitated phase was calculated by image analysis processing.

[Number of clusters of ε-Cu phase]
For ε-Cu phase clusters, a thin film is taken from the cross section of the plate thickness, the ε-Cu phase is fixed and the distribution of this cluster is photographed using an analytical electron microscope, and the number per unit area is measured by an image analyzer. Was calculated.

  In addition, for each steel plate obtained, gas cutting crack sensitivity, acoustic anisotropy, base material tensile properties (yield strength, tensile strength, yield ratio), base material toughness, weld cold crack resistance and high heat input weld HAZ toughness Each was evaluated by the following methods.

[Gas cutting crack sensitivity]
Oxygen pressure: 0.6 MPa (6 kgf / cm ² ), LP gas pressure under appropriate cutting conditions (for example, # 5 crater in the case of 100 mm thickness) according to the plate thickness of LP gas widely used for steel plate cutting in the steel frame manufacturing process : 0.06 MPa (0.6 kgf / cm ² ), cutting speed: 210 mm / min. After cutting, the cross section perpendicular to the cut surface was observed with an optical microscope, and the maximum crack depth from the cut surface was measured. . “No crack” was accepted.

[Acoustic anisotropy]
STB sound velocity ratio (V / V _STB ) defined in the ultrasonic inspection standard for steel structure building welds of the Architectural Institute of Japan was measured in the main rolling direction (L direction) and the direction perpendicular to the main rolling direction (C direction). The presence or absence of a difference in sound velocity from STB was determined according to Appendix Table 1. The range of V / V _{STB in} Appendix Table 1 was considered acceptable. For example, when flaw detection is performed with a flaw having a plate thickness of more than 25 mm and a nominal refraction angle of 70 °, 0.995 ≦ V / V _STB ≦ 1.015 is determined to have no acoustic anisotropy.

[Tensile properties of base material]
A specimen of JIS Z 22014 was taken from t / 4 (t is the thickness) of the steel sheet in the C direction (perpendicular to the rolling direction) and subjected to a tensile test in accordance with JIS Z 2241, yield strength (0 _0.2 % proof stress: σ _0.2 ), tensile strength (TS), yield ratio (yield strength / tensile strength × 100%: YR) were measured. Yield strength σ _0.2 : 440 to 540 MPa, tensile strength TS: 590 to 740 MPa, and yield ratio YR: 80% or less were accepted.

[Base material toughness]
A test piece of JIS Z 2202 No. 4 was collected in the L direction (rolling direction) from t / 4 of the steel sheet, and an impact test was performed according to JIS Z 2242 to measure the fracture surface transition temperature (vTrs). The target was vTrs of −20 ° C. or less.

[Weld cold crack resistance]
According to the JIS Z 3158 y-type weld crack test method, covered arc welding was performed at a heat input of 1.7 KJ / mm, and the root crack prevention preheating temperature was measured. 25 degrees C or less was set as the pass.

[Large heat input welding HAZ toughness]
A column / diaphragm welded joint is manufactured by electroslag welding with a heat input of 100 KJ / mm. From the column (skin plate) side, the absorbed energy is said to be the lowest in most cases. A Charpy impact test piece (JIS Z 2204 No. 4) with a notch in the T direction was taken at n = 3, and an average impact absorption energy vE ₀ at 0 ° C. was determined. An average of 70 J or more was accepted.

These results are shown in Tables 7 to 9 below, and can be considered as follows from these results. First, experiment no. 1 is a conventional C-based SA440 steel in, for CE _N is high, weld cold cracking prevention preheating temperature as high as 100 ° C., HAZ toughness of electroslag welding heat input 100 kJ / mm is also low.

  Experiment No. 2 is the basic component system of the present invention, the toughness of the base material satisfies the target, the weldability is good as it does not require preheating, and the HAZ toughness of electroslag welding with a heat input of 100 KJ / mm also exceeds the target average of 70 J or more. Satisfied enough.

Experiment No. 3 to 6 are those of Experiment No. 2 and CE _N, but 0.4Mo, 0.01Nb, 0.4Mo-0.01Nb, and 0.05C, respectively, and high heat input HAZ toughness is low.

  Experiment No. Nos. 7 to 12 are obtained by changing the Ni / Cu ratio by changing the amount of Cu while keeping Ni constant at 1% (steel types H to L in Table 1). In addition, Experiment No. Nos. 13 to 16 are obtained by changing the Ni amount after setting Cu: 0.95%. It can be seen that Cu: exceeding 0.95% (Experiment Nos. 7 and 8) has gas cut cracking sensitivity even when the Ni / Cu ratio is 1 or less. In the case where Cu is less than 0.5% (Experiment No. 11), the yield strength and the tensile strength are lower than the target values.

  Experiment No. Nos. 17 to 20 are constant in the composition of the present invention except for the C amount, and only the C amount is changed (steel types R to U in Table 1). It can be seen that when the C content is less than 0.015% (Experiment No. 17), both the yield strength and the tensile strength are below the target values.

  Experiment No. No. 21 shows that the volume ratio of the island-like martensite constituting the microstructure of the base material falls below the range defined in the present invention, and the yield ratio does not satisfy the target value (80% or less).

  Experiment No. 22 and 23 are obtained by changing the aspect ratio (grain size in the main rolling direction / grain size in the plate thickness direction) of the prior austenite grains of the base material. When the aspect ratio exceeds 1.2, the acoustic anisotropy is obtained. Will have.

  Experiment No. 24 to 26 are within the range defined by the present invention except the Si amount (steel types V to X in Table 1), and only the Si amount is changed. When Si exceeds 0.4% (Experiment No. 26), the volume ratio of island martensite increases, the yield ratio exceeds the target value, and the base material toughness does not satisfy the target value.

Experiment No. Nos. 27, 28, 30, and 31 are within the range defined by the present invention except for the amount of Mn, and only the amount of Mn is changed (Steel types Y and Z in Table 1 and Steel types A1 and B1 in Table 2). Experiment No. with Mn exceeding 1.6% and CE _N exceeding 0.33%. In the case of No. 31 (steel type B1), it can be seen that the yield ratio is high and both the weld cold crack resistance and the high heat input HAZ toughness are below the target values.

  Experiment No. No. 29 is obtained by Q-Q'-T treatment using steel type B, and the yield ratio does not satisfy 80% or less even when the plate thickness is 44 mm.

Experiment No. 32 to 36 are within the range defined by the present invention except for the Cr amount, and only the Cr amount is changed (steel types C1 to G1 in Table 2). Although Cr acts effectively to reduce the yield ratio by the same tensile strength compared, experiment Cr content exceeds 1.3%, CE _N also exceed 0.33% No. In the case of 36 (steel type G1), the base metal toughness, the weld cold crack resistance and the high heat input HAZ toughness do not satisfy the target values.

  Experiment No. 37 to 40 are within the range defined in the present invention except the amount of Al, and only the amount of Al is changed (steel types H1 to K1 in Table 2). Experiment No. with Al over 0.08%. In the case of 40 (steel type K1), it can be seen that the base metal toughness is lower than the target value.

  Experiment No. Nos. 41 to 44 are within the range defined in the present invention except for the B amount, and the B content is changed (steel types L1 to O1 in Table 2). Experiment No. B amount exceeding 0.003% In the case of No. 44 (steel type O1), it can be seen that the yield strength and yield ratio exceed the target values, and the base metal toughness, welding cold crack resistance, and high heat input weld HAZ toughness do not satisfy the target values.

Experiment No. Nos. 45 to 47 are within the range defined in the present invention excluding the Ni amount, and the Ni amount is changed to the upper limit side (steel types P1 to R1 in Table 2). Experiment No. with Ni amount exceeding 5%. In 47 (steel type R1), the base metal toughness and the resistance to welding cold cracking are inferior.

  Experiment No. Nos. 48 to 51 are obtained by changing the TiN amount balance (steel types S1 to V1 in Table 2). Experiment No. In the case where 48 Ti is not added, the yield strength and the high heat input HAZ toughness are lower than the target values. In addition, Experiment No. In No. 51, the N content exceeds the upper limit of the formula (1) defined in the present invention (steel type V1), and the base metal toughness and the high heat input welding HAZ toughness are lower than the target values.

  Experiment No. 52 to 55 are within the range defined by the present invention except for the V amount, and the V amount is changed (steel types W1 to Z1 in Table 2). Experiment No. In No. 55, the amount of V exceeded 0.10% (steel type Z1), and it can be seen that the high heat input HAZ toughness is below the target value.

  Experiment No. Nos. 56 to 59 are obtained by changing the Ca content based on the basic components of the present invention (steel types A2 to D2 in Table 3). When the Ca content exceeds 0.01% (Experiment No. 59), it can be seen that the high heat input welding HAZ toughness tends to deteriorate and does not satisfy the target value.

  Experiment No. 60 to 63 are those in which the basic component of the present invention contains the amount of La, which is a rare earth element, changed (steel types E2 to H2 in Table 3). When the La content exceeds 0.02% (Experiment No. 63), it can be seen that the high heat input HAZ toughness deteriorates and falls below the target value.

  Experiment No. Nos. 64 to 67 are obtained by changing the Mg content based on the basic components of the present invention (steel types I2 to L2 in Table 3). If Mg exceeds 0.0030% (Experiment No. 67), the high heat input welding HAZ toughness deteriorates and falls below the target value.

  Experiment No. Nos. 68 to 71 are obtained by changing the Ce amount in addition to the addition of Ca (steel types M2 to P2 in Table 3). Experiment No. with Ce over 0.0050%. In the case of No. 71, it can be seen that the high heat input welding HAZ toughness deteriorates and falls below the target value.

  Experiment No. Nos. 72 and 73 are added within the scope of the present invention by changing the amount of Zr based on the addition of Ca, and satisfy the target values for the toughness of the base metal, resistance to cold cracking at welding, and high heat input welding HAZ toughness. I understand that

  Experiment No. 74 and 75 are examples of the present invention of an NQ'-T material having a thickness of 40 mm or 20 mm, and the electroslag diaphragm material is SN490B (0.16% C-0.34% Si-1.34) having a thickness of 60 mm. % Mn-0.034% V system) and satisfy the target values in all characteristics.

  Experiment No. 76 is an experiment no. A 20 mm thick material having the same chemical composition as 75 is subjected to a Q-Q'-T treatment after rolling. The base becomes a bainite phase and the yield ratio YR exceeds 80%.

  Experiment No. 77 is Experiment No. A 20 mm thick material having the same chemical composition as that of No. 75 was subjected to Q'-T heat treatment after rolling. Although the yield ratio is slightly increased and the base material toughness is slightly deteriorated as compared with that obtained by applying N-Q'-T heat treatment after 75 rolling, the target value is sufficiently satisfied.

As is clear from these results, in the high-tensile steel sheet that satisfies the requirements specified in the present invention, the toughness and yield ratio of the base material satisfy the target values, and there is no gas cutting cracking susceptibility and acoustic anisotropy. . In addition, it can be seen that the weld cracking resistance is good, and the high heat input weld HAZ toughness has an average vE ₀ of 70 J or more in all the parts. It should be noted that, even in a 1RUN submerged welding corner joint between columns, since it is a high heat input welding equivalent to electroslag welding, it is needless to say that the average HAZ toughness can be secured at 70J or higher in any part. The characteristics of the invention steel can be satisfied. In addition, although it does not show in the Example about the gas cutting property of the high-tensile steel plate of this invention, all had the quality of the gas cutting surface which fully satisfied the 2nd grade or more of WES2801 (1980).

Claims (4)

C: 0.015-0.045% (meaning of mass%, the same applies hereinafter), Si: 0.4% or less (including 0%), Mn: 0.8-1.6%, Cr: 0.5 ~ 1.3%, sol. Al: 0.08% or less (including 0%), B: 0.0004 to 0.003%, Cu: 0.5 to 0.95%, Ni: 0.7 to 5.0% (however, Ni The ratio [Ni] / [Cu] ≧ 1) of the content [Ni] and the Cu content [Cu], Ti: 0.005 to 0.03% and N satisfying the following formula (1) , substantially free of Nb and Mo, and CE _N value represented by the following formula (2) is in the range of 0.27 to 0.33%, the chemical composition and the balance being Fe and unavoidable impurities The average aspect ratio of the prior austenite grain size in the cross section in the thickness direction (average grain size in the main rolling direction / average grain size in the thickness direction) is 1.0 to 1.2, and 10 to 40 volumes. % Pseudo-polygonal ferrite, 0.5 to 3.5% by volume of an island-like martensite phase, the balance having a pseudo-pearlite phase structure, Low yield ratio high-strength steel sheet, characterized in that the in the tissue in which 4 × 10 ²⁰ ~26 × 10 ²⁰ atoms / m ³ of epsilon-Cu phase clusters are dispersed.
[Ti] × 14.0 / 47.9−0.001 ≦ [N] ≦ [Ti] × 14.0 / 47.9 + [B] × 14.0 / 10.8 (1)
However, [Ti], [N], and [B] indicate the contents (mass%) of Ti, Ni, and B, respectively.
_{CE N = [C] + A} (c) · {[Si] / 24 + [Mn] / 6 + [Cu] / 15 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 + 5 [B]}
(2)
However, A (c) = 0.75 + 0.25 · tanh {20 ([C] -0.12)}, and [C], [Si], [Mn], [Cu], [Ni], [Cr], [ Mo], [Nb], [V] and [B] indicate the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B, respectively.   Furthermore, the low yield ratio high-tensile steel sheet according to claim 1, which contains V: 0.005 to 0.10%.   Furthermore, the low yield ratio high-tensile steel sheet according to claim 1 or 2, which contains Ca: 0.0005 to 0.01%.   Further, selected from the group consisting of La: 0.002-0.02%, Ce: 0.0003-0.0050%, Mg: 0.0005-0.0030% and Zr: 0.002-0.02% The low yield ratio high-tensile steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains one or more kinds.