JP2009120876A - Low-yield ratio high-tensile strength steel sheet excellent in welding heat-affected zone and low-temperature toughness in base material, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-yield ratio high-tensile steel sheet excellent in low temperature toughness of an HAZ, even subjected to welding with large heat input and also, excellent in low temperature toughness of the base material (steel sheet). <P>SOLUTION: The low-yield ratio high-tensile steel sheet satisfies a prescribed chemical component composition and also, in a micro-structure at the position of t/4 (t: sheet thickness), a ferrite partial ratio occupying in the total structure is 60-85 area%, and an island-shaped martensite partial ratio is 1-5 area% and the balance composed of a mixed structure of a bainite structure and further, remaining austenite in the island-shaped martensite occupies ≥60 area%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a low-yield-ratio high-tensile steel sheet excellent in low-temperature toughness of a weld heat-affected zone and a base material. When used in applications exposed to low temperatures, for example, liquefied ammonia and liquefied propane gas are mixed. The present invention relates to a low-yield ratio high-tensile steel sheet that is excellent in low-temperature toughness of a weld heat-affected zone and a base material, which can be applied to a multipurpose tank. The present invention is not limited to the above-described high-strength steel plate welding method, and can be applied to submerged arc welding, electrogas arc welding, etc., but in the following, it is said that securing the toughness of the weld heat affected zone is particularly difficult. The case where the single-sided submerged arc welding with large heat input is applied will be described as an example.

  In recent years, single-sided submerged arc welding with large heat input, for example, with a heat input of 50 to 200 kJ / cm, has been widely used to produce marine structures and low-temperature tanks for storing liquefied gas such as LPG in a short period of time. It has been adopted. However, while this welding can achieve high efficiency of construction, it is difficult to stably secure the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) formed by welding. In many cases, it is necessary to apply multilayer welding by heat input. Therefore, the high heat input welding method capable of high-efficiency construction is adopted for manufacturing the low temperature tank and the like, and excellent HAZ toughness (hereinafter referred to as “HAZ low temperature”) even at a low temperature of about −60 ° C. There is a need for a steel sheet that ensures “toughness” or simply “HAZ toughness”.

  On the other hand, the steel sheet used for the liquefied ammonia tank has a low yield strength YS of 440 MPa or less to prevent stress corrosion cracking (SCC) and a tensile strength TS of 510 MPa or more to reduce the total weight of the steel material. It is required that In the case of a tank in which liquefied ammonia and liquefied propane gas are mixed, it is required that the low-temperature toughness is further excellent as a characteristic of the steel sheet (welding base material) to be used. It is known that liquefied ammonia causes stress corrosion cracking (SCC) of a steel sheet, and as a property of the steel sheet, it is specified that the yield strength YS is suppressed to 440 MPa or less (Non-patent Document 1).

  However, in the multipurpose use where the above liquefied ammonia and liquefied propane gas are mixed, it is necessary to satisfy the characteristics required for both of them, and it is installed in ships as the marine structures such as ships increase in size. As the capacity of the tank is increased, it is also required to increase the tensile strength of the steel sheet, and simultaneously achieving a lower yield ratio (yield ratio YR = YS / TS) due to the upper limit of the yield strength YS is a major issue. It has become.

  Generally, a low component system is used to improve the HAZ toughness, and the structure is mainly composed of a bainite structure by accelerated cooling or the like, so that both strength and HAZ / base metal toughness can be achieved, but a low yield ratio is achieved. I can't do that. In order to achieve a low yield ratio of the steel sheet, the steel structure is duplexed (dual phase), that is, the soft phase that controls the yield strength (usually ferrite) and the hard phase (pearlite, to ensure tensile strength). (Bainite, martensite, etc.) is effective, and the presence of the soft phase lowers the strength compared to the bainite.

  It is effective to make the hard phase martensite in order to improve the strength, but it is a high carbon martensite that is produced in a state in which lath martensite or C is concentrated to a high concentration (this is called “lens-like martensite”). Since it is hard and brittle, it is harmful to low-temperature toughness and has not been actively used so far.

  Until now, various techniques for improving HAZ / base metal toughness while ensuring strength have been proposed. For example, Patent Document 1 proposes a technique for improving the HAZ toughness and the base metal toughness while increasing the strength by making the hard phase mainly bainite and containing a large amount of Ni (6.5 to 12%). ing. However, in such a technique, an increase in cost due to the addition of Ni cannot be avoided, and there is a restriction that the Ni content is limited to 5% or less due to stress corrosion cracking (SCC) of the steel sheet (Non-Patent Document 1). The fact is that the effects of the addition cannot be fully utilized.

  Patent Document 2 proposes a technique for improving the base material toughness by decomposing lath martensite by heat treatment such as tempering. However, in this technique, strength reduction due to heat treatment is unavoidable, and as a result, strengthening elements such as Ni and Mo have to be included, and it has not yet been possible to achieve both HAZ toughness.

On the other hand, in Patent Document 3, an alloy element such as Nb or Mo having a refinement effect of prior austenite is added, and island-shaped martensite (MA) which is a hard phase is actively generated, By defining the form (aspect ratio) and size, a technique for minimizing the deterioration of the base metal toughness due to high carbon martensite and achieving both high strength and toughness has been proposed. However, due to the addition of Nb, Mo, etc., the HAZ toughness has not been achieved, and the toughness of the base metal is at a level that can ensure further safety (at the fracture surface transition temperature vTrs of −90 ° C. or lower). Has not reached.
IGC CODE 17.13 (International Code for the Construction and Equipment of Shipping Carrying Liquidated Gases in Bulk) 2002 Edition JP-A 63-290246 JP 58-153730 A JP 2002-3983 A

  The present invention has been made in view of such circumstances, and the purpose thereof is excellent in low temperature toughness of HAZ even when welding is performed with high heat input, and also in low temperature toughness of a base material (steel plate). The object is to provide an excellent low yield ratio high strength steel sheet.

  The high-tensile steel sheet of the present invention that can achieve the above-mentioned object is C: 0.06 to 0.09% (meaning “mass%”, the same applies to chemical components), Si: 0.05 to 0.00. 25%, Mn: 1.2 to 1.6%, P: 0.01% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.06% The following (excluding 0%), B: 0.0007 to 0.0015%, Ti: 0.009 to 0.018%, N: 0.0050 to 0.0080%, and Nb: 0.012 to 0. 020% of each is contained, the amount of solute N in the steel is 0.0003 to 0.0040%, the balance is iron and inevitable impurities, and the micro of t / 4 (t: plate thickness) position In the structure, the ferrite fraction in the whole structure is 60 to 85 area%, and the island-like martensite fraction is 1 to 5 area%. Ri, the balance being mixed structure of bainite, and further retained austenite in the island martensite has a gist in that it is 60 area% or more.

  In the high-tensile steel sheet of the present invention, if necessary, (a) Cu: 0.4% or less (not including 0%) and / or Ni: 0.4% or less (not including 0%), ( b) From the group consisting of Mo: 0.15% or less (not including 0%), Cr: 0.15% or less (not including 0%), and V: 0.04% or less (not including 0%) It is also preferable to include one or more selected, (c) Ca: 0.003% or less (not including 0%), and the like, and the characteristics are improved according to the components to be included.

In producing such a low-yield-ratio high-tensile steel sheet as described above, a steel slab satisfying the chemical composition as described above is heated to a temperature of 1000 to 1250 ° C., and hot at a temperature not lower than the Ar ₃ transformation point. After finishing rolling, cooling is started from a temperature range of 620 to 720 ° C., accelerated cooling is performed at an average cooling rate of 10 ° C./second or more to a temperature range of 350 to 450 ° C., and then left to cool. Good.

  According to the present invention, the yield strength YS of a steel plate (base material) is 440 MPa or less, the tensile strength TS is 510 MPa or more, and the low temperature toughness of the steel plate (base material) is excellent. Even when applied, HAZ exhibits excellent toughness at -60 ° C., which contributes to an increase in the size of a welded structure such as a multi-purpose tank in which liquefied ammonia and liquefied propane gas are mixed. A single-sided submerged arc welding method can be employed, and the welded structure can be manufactured in a shorter period of time.

  This inventor examined from various angles in order to implement | achieve the high-tensile steel plate excellent in the low temperature toughness of HAZ and a base material. As a result, in order to ensure HAZ toughness, the chemical component composition was adjusted while setting C to be 0.09% or less and Si to be 0.25% or less, and then within the austenite grains by TiN and BN. By making a predetermined amount of solid solution N exist in the island martensite (MA) while making the best use of the grain refinement effect of the above, the retained austenite (residual γ) is stabilized, and the It was found that a certain amount of residual γ could be ensured, whereby the strength and toughness of the steel sheet (base material) and the HAZ toughness were compatible, and the present invention was completed.

  In the high-tensile steel plate of the present invention, the ferrite fraction needs to be adjusted to 60 to 85 area% in the microstructure at the t / 4 (t: plate thickness) position. If the ferrite fraction is less than 60 area%, the yield strength YS: 440 MPa or less cannot be achieved, and if it exceeds 85 area%, the tensile strength TS: 510 MPa or more cannot be ensured. A preferable range of this ferrite fraction is 65 area% or more and 80 area% or less.

  The high-strength steel sheet of the present invention is composed of a mixed structure of ferrite, MA, and bainite, but the MA fraction in the structure needs to be adjusted to 1 to 5 area%. MA is necessary for securing the strength of the steel sheet, and from this point of view, it is necessary to set it to 1 area% or more. However, if the MA fraction becomes excessive and exceeds 5 area%, the low temperature toughness of the base material decreases. It will be. Further, the residual γ ratio in MA needs to be 60 area% or more in order to ensure the base material toughness. That is, the residual γ in the MA is necessary to ensure the base material toughness. For that purpose, it is necessary to ensure 60% by area or more (the remainder is high carbon martensite). The ratio of carbon martensite increases and good base material toughness cannot be secured.

  In the high-tensile steel sheet of the present invention, in order to satisfy the basic characteristics as the steel sheet, it is necessary to appropriately adjust the chemical composition while reducing the C and Si contents. The reasons for limiting the ranges of the basic components (C, Si, Mn, P, S, Al, B, N, Ti, Nb) are as follows.

[C: 0.06 to 0.09%]
In order to suppress the formation of MA, which is a hard phase, and to ensure the HAZ toughness at −60 ° C., it is necessary to suppress the C content to 0.09% or less. On the other hand, C is also an element essential for ensuring the strength of the steel sheet, so it is contained in an amount of 0.06% or more.

[Si: 0.05 to 0.25%]
By reducing Si to 0.25% or less, the formation of MA can be sufficiently suppressed, and the low temperature toughness of HAZ can be easily ensured. On the other hand, since Si is an element that is used for deoxidation of molten steel and effectively works to improve strength, it is necessary to contain 0.05% or more.

[Mn: 1.2 to 1.6%]
Mn is an element useful for capturing S as MnS and suppressing degradation of HAZ toughness due to S. It is also an element that contributes to increasing the strength of the steel sheet by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.2% or more of Mn. Preferably it is 1.35% or more. However, if the amount of Mn becomes excessive, the HAZ toughness and the base metal toughness deteriorate instead, so the content is limited to 1.6% or less.

[P: 0.01% or less (excluding 0%)]
P is an element that deteriorates the HAZ toughness, so it is necessary to reduce it as much as possible. In the present invention, P is suppressed to 0.01% or less. However, P is an impurity inevitably mixed in the production of steel, and it is difficult to make the amount 0% industrially.

[S: 0.003% or less (excluding 0%)]
S is an element that generates coarse sulfides and degrades the HAZ toughness. Therefore, it is necessary to reduce as much as possible, and in the present invention, it is suppressed to 0.003% or less. However, S is an impurity inevitably mixed in the production of steel, and it is difficult to make the amount 0% industrially.

[Al: 0.06% or less (excluding 0%)]
Al is used as a deoxidizer, but if the Al content is excessive, oxide inclusions such as alumina increase and the HAZ toughness deteriorates, so the content is suppressed to 0.06% or less.

[B: 0.0007 to 0.0015%]
B has the effect | action which accelerates | stimulates the production | generation of an intragranular ferrite by producing | generating BN, and improves HAZ toughness. Solid solution B also has the effect of suppressing the coarsening of grain boundary ferrite and the formation of ferrite side plates, and making the crystal grains in the austenite grains finer. In order to fully exhibit this effect, it is necessary to contain B 0.0007% or more. On the other hand, when there is too much B, a crystal | crystallization is formed in a fixed direction by the effect | action of excess solute B, and HAZ toughness deteriorates on the contrary. Therefore, the B content is limited to 0.0015% or less.

[Ti: 0.009 to 0.018%]
Ti is an element that generates TiN-based precipitates and promotes the formation of intragranular ferrite, and is also effective in suppressing austenite grain coarsening. It is also an element contributing to high strength. In order to effectively exhibit such an action, it is necessary to contain Ti by 0.009% or more, and preferably 0.010% or more. However, when Ti is excessively contained, not only the HAZ toughness is lowered, but also the amount of free N (solid solution N) is reduced, and the effect of stabilizing the retained austenite (residual amount γ) in rolling is reduced. In order to weaken, it is necessary to make it 0.018% or less.

[N: 0.0050 to 0.0080%]
N is an element that improves the HAZ toughness by forming a nitride with an element such as Ti, B, Al, or Nb. Further, since N dissolved in a solid solution (the amount of which will be described later) precipitates in MA and stabilizes residual γ, 0.0050% or more including this amount (preferably 0) .0060% or more) must be contained. However, if N is excessive, the amount of solute N is increased and the HAZ toughness is deteriorated.

[Nb: 0.012-0.020%]
Nb is an element that generates NbN-based precipitates and is effective in suppressing austenite grain coarsening. It is also an element contributing to high strength. In order to effectively exhibit such an action, the Nb content needs to be 0.012% or more, preferably 0.015% or more. However, if it is excessively contained, not only the HAZ toughness is lowered, but also the amount of free N is lowered, and the effect of stabilizing the residual γ amount in rolling is weakened.

[Solution N: 0.0003 to 0.0040%]
As mentioned above, N is an element that improves the HAZ toughness by forming nitrides. However, N (solid solution N) remaining after forming nitrides forms a solid solution in MA and stabilizes residual γ. It has a function to make it. In order to exhibit such an effect, it is necessary to ensure that the amount of solute N is 0.0003% or more (preferably 0.0004% or more). However, if the solute N is excessive, the HAZ toughness is deteriorated, so it is suppressed to 0.0040% or less (preferably 0.0037% or less).

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, elements brought in depending on the situation of raw materials, materials, production facilities, etc. (for example, Mg, Zr, O, H, rare earth elements, etc.) can be mixed. Further, if necessary, it is also effective to positively contain the following elements, and the characteristics of the steel sheet are further improved according to the type of elements contained. The reason for setting the range when these elements are contained is as follows.

[Cu: 0.4% or less (not including 0%) and / or Ni: 0.4% or less (not including 0%)]
Cu and Ni are both useful elements for securing the base material toughness and the strength. These effects increase as their content increases. However, when excessively contained, Cu inhibits hot workability and Ni deteriorates HAZ toughness. In addition, excessive addition of Ni may induce stress corrosion cracking (SCC) in liquid ammonia. For these reasons, the content when Cu and Ni are contained is determined to be 0.4% or less.

[Mo: 0.15% or less (not including 0%), Cr: 0.15% or less (not including 0%), and V: 0.04% or less (not including 0%) One or more
Mo, Cr, and V are all effective elements for enhancing the hardenability and increasing the strength. These effects increase as the content thereof increases. However, if excessively contained, the HAZ toughness is deteriorated, so that Mo is 0.15% or less, Cr is 0.15% or less, and V is 0.00. It is better to keep it below 04%.

[Ca: 0.003% or less (excluding 0%)]
Ca is an element effective for fixing S, which adversely affects HAZ toughness, as CaS, and for improving the toughness by controlling the form of nonmetallic inclusions in a granular form. In order to exert such effects sufficiently, it is preferable to contain 0.0010% or more of Ca, but even if Ca is contained excessively, these effects are saturated and the HAZ toughness deteriorates. Therefore, the Ca content is preferably 0.003% or less.

  In order to produce the steel material of the present invention having the above-described structure, a low yield ratio high-tensile steel plate excellent in low temperature toughness of HAZ can be obtained by, for example, the following method.

The steel slab satisfying the above-described component composition is heated to a temperature of 1000 to 1250 ° C., and after the hot rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point, cooling is started from a temperature range of 620 to 720 ° C., Accelerated cooling is performed at an average cooling rate of 10 ° C./second or more to a temperature range of 350 to 450 ° C., and then allowed to cool. The reason for setting the range of each condition in this method is as follows.

[Heating temperature of steel slab during hot rolling: 1000 to 1250 ° C.]
The heating temperature of the steel slab at the time of hot rolling needs to be set to an appropriate range in order to keep the austenite grains at the time of heating small and to refine the rolling structure. 1250 ° C. is an upper limit that does not excessively coarsen the austenite grains during heating. If the heating temperature exceeds this, the austenite grains become coarser, the structure after transformation becomes coarser, and the toughness of the steel deteriorates remarkably. On the other hand, if the heating temperature is too low, it is difficult not only to secure the rolling completion temperature (above Ar ₃ transformation point), which will be described later, but also from the viewpoint of solutionization of Nb that has the function of suppressing the coarsening of austenite grains. The lower limit of the heating temperature was 1000 ° C.

[Hot rolling finish temperature: temperature above Ar ₃ transformation point]
The hot rolling end temperature is a condition for ensuring that the steel material is in an austenite state, similarly to the heating temperature. For this purpose, the temperature must be equal to or higher than the Ar ₃ transformation point. If the temperature is lower than the Ar ₃ transformation point temperature, ferrite may be transformed and precipitated non-uniformly during rolling, and the ferrite may be processed (rolled), which is not preferable from the viewpoint of material variation. The upper limit of the rolling end temperature is within the range of the heating temperature. In the present invention, the “Ar ₃ transformation point” is a value determined by the following formula (1).
Ar ₃ transformation point (° C.) = 930−230 · [C] + 25 · [Si] −74 · [Mn] −56 · [Cu] −16 · [Ni] −9 · [Cr] −5 · [Mo] -1620 [Nb]
... (1)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] are respectively C, Si, Mn, Cu, Ni, Cr, Mo and When the content (% by mass) of Nb is shown and no alloy element is added, the calculation is made assuming that the term is not present.

[Accelerated cooling start temperature: 620 to 720 ° C.]
After the hot rolling is finished, it is allowed to cool (corresponding to 1 ° C./second) to reach a predetermined temperature range, and then accelerated cooling is started. When the cooling start temperature exceeds 720 ° C., the growth of nucleation of ferrite transformation is insufficient, resulting in insufficient ferrite fraction and increased yield strength. On the other hand, when the cooling start temperature is less than 620 ° C., the ferrite fraction is excessively increased, and pearlite is generated during the subsequent cooling and the strength and toughness of the base material are lowered.

[Average cooling rate during accelerated cooling: 10 ° C / second or more]
The average cooling rate when cooling from the cooling start temperature (620 to 720 ° C.) to a temperature of 350 to 450 ° C. needs to be 10 ° C./second or more. When this cooling rate is less than 10 ° C./second, pearlite is generated during cooling and a predetermined ferrite + bainite mixed structure cannot be obtained (becomes ferrite + pearlite structure), and the base material toughness and strength are reduced. become. The upper limit of the average cooling rate is not limited, but is preferably about 60 ° C./second in consideration of industrial applicability.

[Accelerated cooling stop temperature: 350 to 450 ° C.]
When the cooling stop temperature is lower than 350 ° C., martensite (lass martensite) is generated, and the toughness and strength of the base material are lowered with a ferrite + martensite structure. On the other hand, when the cooling stop temperature is higher than 450 ° C., a predetermined amount of island martensite cannot be secured, and the toughness and strength of the base material are lowered.

[Cooling after accelerated cooling]
After accelerated cooling, it is necessary to cool. A predetermined ferrite fraction can be secured by this cooling. In the present invention, “cooling” means a state in which the cooling rate is less than 1.0 ° C./second by stopping the cooling and allowing the steel sheet to stand.

  In addition, the temperature shown above was managed by the temperature of the position of t / 4 part (t: board thickness) as a position which exhibits the average performance of a steel plate. The steel material of the present invention can be advantageously applied to so-called thick steel plates. The plate thickness at this time is about 7 mm or more, and the upper limit is not particularly limited, but is usually about 40 mm or less.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  The steel pieces having the chemical composition shown in Table 1 below are heated to 1100 ° C., hot-rolled to a predetermined plate thickness (12 mm or 30 mm), and after the hot rolling is finished, It cooled to the temperature range (cooling stop temperature), and then left to cool. The production conditions at this time are shown in Table 2. In addition, the solid solution N amount shown in Table 1 is a value measured by the following method.

[Measurement method of solid solution N amount]
A 10 × 10 × 50 (mm) sample was cut out from t / 4 part (t: plate thickness) of each steel plate, and this sample was immersed in an electrolytic solution (ethanol solution containing 10% by mass of acetylacetone) to obtain 100 mA. Was applied for 5 hours to electrolyze the base metal Fe, and precipitates (TiN, BN, NbN, etc.) present in the electrolytic liquid were filtered through a filter having a mesh diameter of 0.1 μm to collect the residue. . The N concentration in the residue was determined, and the amount subtracted from the total N amount was defined as the solid solution N amount.

For each steel plate obtained as described above, the matrix structure (mixed structure, ferrite fraction, MA fraction, residual γ ratio), matrix characteristics [yield strength YS, tensile strength TS, yield ratio YR ( YS / TS), toughness (fracture surface transition temperature vTrs)], and HAZ toughness (vE- ₆₀ ) were evaluated in the following manner.

[Measurement of ferrite fraction]
The ferrite fraction was measured for t / 4 parts (t: plate thickness) of each steel plate after etching with a 3% nital solution, taking 10 fields of view at 400 times magnification using an optical microscope, and analyzing the image. Measurement was performed using software, and an average value was obtained. Further, the ferrite fraction was measured not only at the position of t / 4 part (t: thickness) of each steel sheet but also at the position of t / 2 part (t: thickness), and the same structure was obtained. It was confirmed that

[MA fraction]
At the position of t / 4 part (t: thickness) of each steel plate, after repeller corrosion, an optical microscope is used to observe an area of 1 field of view: 50 μm × 50 μm at a magnification of 1000 times, and using image analysis software Measurements were made and the average of 10 fields of view was determined.

[Residual γ ratio]
At the position of t / 4 part (t: thickness) of each steel plate, X-ray diffraction is performed, and the theoretical strength ratio is calculated from the peak strength ratio of the α-Fe (200) plane and the γ-Fe (200) plane by the Liberty method. The ratio of residual γ was determined by calculation. The X-ray diffractometer used at this time used RAD-RU300 (trade name: manufactured by Rigaku Corporation), the target was Co, the target output was 40 kV, and 200 mA.

[Evaluation of base material properties]
JIS Z 2201 No. 1B test piece was taken from the total thickness of each steel plate in the direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z 2241, yield strength YS (if there is a yield point, Yield point YP, 0.2% yield strength (σ _0.2 ) when not present, and tensile strength (TS) were measured. And the thing whose yield strength: 440MPa or less, tensile strength: 510MPa, and yield ratio (YS / TS) was 80% or less was evaluated as the low yield ratio high-tensile steel sheet of the present invention.

  Further, a V-notch test piece of JIS Z 2202 was taken in the rolling direction from a portion of each steel plate cut by 1 mm from the surface side, and a Charpy impact test was performed in the manner of JIS Z 2242 to measure the fracture surface transition temperature vTrs. And it evaluated that the fracture surface transition temperature vTrs had the outstanding base material toughness with -90 degreeC or less.

[Evaluation of HAZ toughness]
Single-sided submerged arc welding using the steel plate was performed by the FCB method. The FCB method is a method of laying a backing flux on a copper plate, pressing it against the back of the groove, and completing the welding while forming a back bead from one side of the surface. Yes. The groove shape is shown in FIG. 1 [(a) when the plate thickness is 12 mm, (b) when the plate thickness is 30 mm]. As the welding material, the following low-temperature steel welding material (manufactured by Kobe Steel) was used, and a welded joint was produced under the welding conditions shown in FIG. 2 and Table 3.
[Welding material]
・ Wire; US-255
・ Front flux; PFI-50LT
・ Backing flux; MF-1R

Then, three V-notch test pieces of JIS Z 2202 each cut by 1 mm from the surface side and notched perpendicularly to the plate surface at the position of HAZ (bond part) were collected, and Charpy impact was performed according to the procedure of JIS Z 2242. A test was conducted. And the absorbed energy (vE- ₆₀ ) in test temperature: -60 degreeC was measured. And the thing whose average value of this absorbed energy (vE- ₆₀ ) is 100J or more was evaluated as excellent in the low temperature toughness of HAZ.

  These results are shown in Table 4 below together with the actual welding conditions (construction method, heat input).

  From these results, it can be considered as follows (note that the following No. indicates the experiment No. in the table).

  No. satisfying the requirements defined in the present invention. The steel sheets 1 to 9 are high-tensile steel sheets having excellent HAZ low-temperature toughness and excellent base material properties (low-temperature toughness, yield strength YS: 440 MPa or less, tensile strength TS: 510 MPa or more), The steel plate is welded by a high heat input single-sided submerged arc welding method, and exhibits excellent characteristics when used for low temperature conditions.

  On the other hand, No. which does not satisfy the provisions of the present invention. 10 to 19 have the following problems. That is, no. In No. 10, the amount of solid solution N is excessive, the toughness of the base material is deteriorated, the Ti content is insufficient, and the low temperature toughness of the HAZ is lowered. No. In No. 11, the amount of dissolved N is insufficient, the toughness of the base material is deteriorated, and the Ti content is excessive, so the low temperature toughness of HAZ is reduced. No. In No. 12, the amount of dissolved N is insufficient, the toughness of the base material is deteriorated, and the content of C and Si is excessive, so the low temperature toughness of HAZ is lowered.

  No. No. 13 has an excessive Mn content and is inferior in HAZ toughness and base metal toughness. No. Although 14 is excellent in base material toughness, the B content is insufficient, and the low temperature toughness of HAZ is lowered.

  No. No. 15 has excellent HAZ toughness, but the cooling start temperature is high and the ferrite fraction is low, so the desired base material properties (yield strength YS: 440 MPa or less) are not obtained. No. No. 16 has excellent HAZ toughness, but the cooling start temperature is low and the ferrite fraction is high, so the desired base material properties (tensile strength TS: 510 MPa or more) are not obtained.

  No. No. 17 has excellent HAZ toughness, but the cooling stop temperature is low and the ferrite fraction is low, so the desired base material characteristics (fracture surface transition temperature vTrs: −90 ° C. or lower) are not obtained. No. No. 18 is excellent in HAZ toughness, but has a low average cooling rate, has a ferrite + martensite structure, deteriorates the base material toughness, and desired base material characteristics (tensile strength TS: 510 MPa or more) ) Is not obtained. No. No. 19 has a high cooling start temperature, does not produce MA, deteriorates the base material toughness, and does not obtain the desired base material characteristics (tensile strength TS: 510 MPa or more).

  Based on the results of Table 4 above, the relationship between the amount of solute N and the base material toughness (fracture surface transition temperature vTrs) is shown in FIG. 3 [in the figure, “●” indicates the steel of the present invention (Test Nos. 1-9). ) And “▲” are comparative steels (test Nos. 10 to 12)], and by controlling the amount of solute N within an appropriate range (0.00030 to 0.0040%), good base metal toughness is exhibited. You can see that.

Sectional drawing of the groove shape in the welding in an Example is shown. The schematic diagram of the electrode arrangement | positioning at the time of FCB welding is shown. It is a graph which shows the relationship between the amount of solute N and the fracture surface transition temperature vTrs of a base material.

Claims (5)

  C: 0.06 to 0.09% (meaning “mass%”, chemical components are the same hereinafter), Si: 0.05 to 0.25%, Mn: 1.2 to 1.6%, P: 0.01% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.06% or less (not including 0%), B: 0.0007 to 0 .0015%, Ti: 0.009 to 0.018%, N: 0.0050 to 0.0080% and Nb: 0.012 to 0.020%, respectively, and the amount of solute N in the steel is 0.0003 to 0.0040%, the balance is iron and inevitable impurities, and in the microstructure at the position of t / 4 (t: plate thickness), the ferrite fraction in the entire structure is 60 to 85 area% The island-like martensite fraction is 1 to 5% by area and the balance is a mixed structure of bainite structure. Low yield ratio high-strength steel sheet excellent in low temperature toughness of the heat affected zone and the base metal, wherein the further residual austenite in the island martensite is 60 area% or more.   Furthermore, Cu: 0.4% or less (not including 0%) and / or Ni: 0.4% or less (not including 0%), low yield ratio high tension according to claim 1 steel sheet.   Furthermore, Mo: 0.15% or less (not including 0%), Cr: 0.15% or less (not including 0%), and V: 0.04% or less (not including 0%) The low-yield-ratio high-tensile steel sheet according to claim 1 or 2, which contains one or more selected.   The low yield ratio high tensile strength steel sheet according to any one of claims 1 to 3, further comprising Ca: 0.003% or less (not including 0%). In producing the low-yield-ratio high-tensile steel sheet according to any one of claims 1 to 4, a steel slab satisfying the chemical composition is heated to a temperature of 1000 to 1250 ° C, and the Ar ₃ transformation point or higher is reached. After finishing the hot rolling at a temperature, cooling is started from a temperature range of 620 to 720 ° C., accelerated cooling is performed at an average cooling rate of 10 ° C./second or more to a temperature range of 350 to 450 ° C., and then allowed to cool A method for producing a low-yield-ratio high-tensile steel sheet, characterized in that

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