JP4881773B2 - Low yield ratio high strength steel plate with excellent low temperature toughness of weld heat affected zone - Google Patents

Description

  The present invention relates to a low-yield-ratio high-tensile steel sheet having excellent low-temperature toughness in the heat-affected zone of a weld. 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 toughness of a base material. 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 the welding can realize 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, and it is due to low heat input. In many cases, it is necessary to apply multilayer welding. Therefore, the high heat input welding method capable of high-efficiency construction is adopted for the production of the low temperature tank and the like, and the HAZ toughness (low temperature toughness) is excellent even at a low temperature of about −60 ° C. There is a need for steel sheets.

  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 530 MPa or less to reduce the total weight of the steel material. It is required that In the case of a tank in which liquefied propane gas is mixed with liquefied ammonium, it is required that the low temperature toughness is excellent as a characteristic of the steel sheet used. It is known that liquefied ammonia causes stress corrosion cracking (SCC) of steel materials, and it is specified as a characteristic of a steel sheet that the yield strength YS is suppressed to 440 MPa or less (Non-Patent Document 1).

  However, in the multi-purpose use in which the above liquefied ammonium and liquefied propane gas are mixed, it is necessary to satisfy the characteristics required for both, and it is mounted on a ship as the marine structure such as a ship becomes larger. As the capacity of steel tanks increases, the steel plate is required to have higher tensile strength, and simultaneously achieving a lower yield ratio (yield ratio YR = YS / TS) associated with the upper limit of yield strength YS is a major issue. It has become.

  Until now, various methods have been proposed to improve the low temperature toughness of the HAZ. For example, Patent Literature 1 and Patent Literature 2 propose a method for improving HAZ toughness by suppressing the austenite grain coarsening by pinning particles such as TiN and Al oxide. Patent Document 3 and Patent Document 4 disclose a technique for miniaturizing crystal grains by making many ferrite transformation nuclei exist in austenite grains. Specifically, the use of TiN, MnS, BN, Ti oxide or the like as ferrite transformation nuclei achieves refinement of crystal grains and improves the low temperature toughness of HAZ.

  However, in any of the above methods, when performing single-sided submerged arc welding with high heat input, precipitates such as TiN are considerably dissolved, and it is difficult to suppress subsequent grain coarsening, Further improvement is required to ensure excellent HAZ toughness (hereinafter sometimes referred to as “HAZ toughness” or simply “HAZ toughness”) at a low temperature of about −60 ° C.

In addition, the HAZ toughness improvement technologies proposed so far have not been considered to have a low yield ratio (for example, 75% or less) required as a multipurpose tank in which liquefied ammonium and liquefied propane gas are mixed. Is the actual situation.
IGC CODE 17.13 (International Code for the Construction and Equipment of Shipping Carrying Liquidated Gases in Bulk) 2002 Edition Japanese Patent Publication No. 55-026164 Japanese Patent No. 2950076 Japanese Patent Publication No. 07-068577 Japanese Patent Publication No. 05-017300

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

The high-tensile steel sheet of the present invention that can achieve the above-mentioned object is C: 0.03 to 0.09% (meaning “mass%”, the same applies to chemical components), Si: 0.01 to 0.00. 25%, Mn: 1.20 to 1.60%, P: 0.010% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.02 to 0.04%, Nb: 0.005 to 0.016%, B: 0.0006 to 0.0020%, N: 0.0045 to 0.0090%, Ti: 0.008 to 0.020%, respectively. And containing the following formula (1), the balance iron and inevitable impurities, and the ferrite fraction in the entire structure is 45 to 85 area%, the balance consists of a bainite structure and / or a martensite structure, The gist is that the average crystal grain size of the ferrite is 19 μm or less. It is intended to.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0}

In the high-tensile steel sheet of the present invention, if necessary, Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05 % Or less (not including 0%) may be included so as to satisfy the following formula (2), and further Ca: 0.003% or less (including 0%) Not included).
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

  According to the present invention, when the yield strength YS of a steel plate (base material) is 440 MPa or less, the tensile strength TS is 530 MPa or more, the steel plate is excellent in toughness, and the steel plate is subjected to high heat input welding. However, since HAZ exhibits excellent toughness at about −60 ° C., it contributes to an increase in the size of a welded structure such as a multipurpose tank in which liquefied ammonia and liquefied propane gas are mixed. A welding method can be employed, and the welded structure can be manufactured in a shorter period of time.

The present inventor has intensively studied to obtain a high-strength steel sheet that is particularly excellent in low temperature toughness of HAZ when welding with high heat input is performed.
as a result,
(A) After setting C to be 0.09% or less and Si to be 0.25% or less, the balance between the prescribed amounts of B, N and Ti is optimized, and a certain amount of Nb is added. For example, the generation of coarse ferrite from the austenite grain boundary (hereinafter sometimes simply referred to as “grain boundary ferrite”) is sufficiently suppressed, and the grain refinement within the austenite grain can be achieved.
(B) Furthermore, when Cu, Ni, and V are added to further increase the strength, if the contents of Cu, Ni, V, and Nb are comprehensively controlled, deterioration of the HAZ toughness can be suppressed.
The specific method was found based on the idea.

  First, in the present invention, by optimizing the balance of individual prescribed amounts of B, N, and Ti and rigorously optimizing the amount of dissolved B, the crystal grains in the austenite grains can be refined, and as a result, HAZ It is characterized in that the low-temperature toughness of can be greatly improved.

FIG. 1 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.010% Nb is a basic component, and B, N, and Ti are within the specified ranges described later. (B-NT / 1.3) {B is B content (mass ppm), NT is N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) Relationship
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
When (N-Ti / 3.4) <0, NT = 0 is indicated.
The same applies to equation (1) below}
A heat cycle test was performed using steel sheets having various values of and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as described in the examples described later, and these results were organized. In the thermal cycle test, welding heat input: 60 kJ / cm (plate thickness 12 mm) was assumed, and after heating and holding at 1400 ° C. × 5 seconds, cooling from 800 ° C. to 500 ° C. was performed in 150 seconds.

From FIG. 1, in order to achieve a low temperature toughness of HAZ of vE- ₆₀ : 100 J or more, as shown in the following formula (1), the value of (B-NT / 1.3) is -20 ppm or more and 10 ppm or less. It is understood that it is necessary to make it fall within the range.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)

  By optimizing the balance of B, N and Ti as in the above formula (1), the coarsening of the grain boundary ferrite due to the solid solution B existing at the grain boundary in the austenite grain is suppressed, and from the grain boundary. It is considered that the effect of suppressing the formation of the ferrite side plate and the effect of BN as the ferrite transformation nucleus could be maximized.

  As described above, to optimize the balance of B, N, and Ti to reliably increase the low temperature toughness of HAZ and to ensure the strength of the base material (steel material), the contents of B, N, and Ti are as follows. Must be within range.

  On the other hand, in order to achieve a low yield ratio YR of a yield strength YS of 440 MPa or less in a high-strength steel sheet having a tensile strength TS of 530 MPa or more without impairing the above HAZ toughness and base material properties, a hard bainite structure It has also been found that an appropriate amount of a soft ferrite phase may be present in the martensite structure and the particle size of the soft phase may be reduced.

FIG. 2 is a graph showing the relationship between the ferrite fraction, the yield strength YS (the yield point YP or 0.2% yield strength σ _0.2 ), and the tensile strength TS. Is. As is clear from this result, it can be seen that, when the ferrite fraction is in the range of 45 to 85 area%, both properties of tensile strength TS: 530 MPa or more and yield strength YS: 440 MPa or less can be satisfied. A preferable range of this ferrite fraction is 50 to 80 area%.

  In the present invention, “the remainder is a bainite structure and / or martensite structure” basically means that other than ferrite is a bainite structure and / or a martensite structure (that is, a bainite structure and / or a martensite structure). The martensite fraction is 15 to 55 area%), and includes other elements inevitably formed in the manufacturing process (cementite and island martensite).

FIG. 3 is a graph showing the relationship between the ferrite grain size and the base material toughness (Charpy impact absorption energy vE- _{60 at} −60 ° C.). As is apparent from this result, it is understood that good base material toughness (vJ- ₆₀ of 100 J or more) is achieved by setting the average grain size of ferrite to 19 μm or less.

  In the high-tensile steel plate of the present invention, in addition to basic components such as C, Si, Mn, P, S, and Al, in order to satisfy the basic characteristics of the steel plate, it is a component involved in the above formula (1). B, N, Ti and the like need to be adjusted appropriately. First, the reasons for limiting the ranges of B, N, Ti, etc. are as follows.

[B: 0.0006 to 0.0020%]
B has the effect of fixing solute N which is harmful to the HAZ toughness by generating BN and promoting the formation of intragranular ferrite. 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 0.0006% 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 0.0020% or less. In addition, the minimum with preferable B content is 0.0008%, and a preferable upper limit is 0.0018%.

[N: 0.0045 to 0.0090%]
N is an element that forms nitrides with elements such as Ti and Al to improve the HAZ toughness, and therefore may be contained in an amount of 0.0045% or more (preferably 0.0060% or more). In addition, the solid solution N causes the HAZ toughness to deteriorate. By increasing the total nitrogen amount, the above-mentioned nitride increases, but solid solution N also becomes excessive. Therefore, in the present invention, the N content is suppressed to 0.0090% or less.

[Ti: 0.008 to 0.020%]
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 exert such an action effectively, it is necessary to contain Ti by 0.008% or more, and preferably 0.012% or more. However, if Ti is excessively contained, the HAZ toughness is reduced instead, so the content is made 0.020% or less.

  In the present invention, as described above, the balance of individual prescribed amounts of B, Ti, and N is optimized, and a certain amount of Nb is added. Nb is an element useful for sufficiently suppressing the formation of coarse grain boundary ferrite and achieving crystal grain refinement in the austenite grains. In the present invention, Nb is contained in an amount of 0.005% or more in order to sufficiently exhibit such an effect. However, if it is excessively contained, island-like martensite (MA), which is a hard phase, is easily generated, and crystals are formed in a certain direction, resulting in deterioration of HAZ toughness. % Or less.

  In order to increase the low temperature toughness of HAZ more reliably, it is effective to further reduce C and Si. In this invention, in order to suppress the production | generation in the HAZ part of MA and to ensure the HAZ toughness at about -60 degreeC, C amount is suppressed to 0.09% or less. On the other hand, C is an element essential for securing the strength of the steel sheet, so 0.03% or more is contained.

  Furthermore, 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 acts to improve the strength, it may be contained in an amount of 0.01% or more, preferably 0.05% or more.

  In addition, as described above, in order to reliably increase the HAZ toughness and to provide other characteristics such as strength and toughness of the steel plate (base material), it is necessary to set the content of components other than the above within the following ranges.

[Mn: 1.20 to 1.60%]
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 (high tensile strength TS) of the steel sheet by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.20% or more of Mn. However, if the amount of Mn becomes excessive, the HAZ toughness deteriorates on the contrary, so it is suppressed to 1.60% or less. Preferably, it is 1.50% or less.

[P: 0.010% 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 0.010% or less.

[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.

[Al: 0.02-0.04%]
Al is an element that is used as a deoxidizer and that also generates AlN-based precipitates to improve the HAZ toughness during high heat input welding. In the present invention, Al is contained in an amount of 0.02% or more. However, when the Al content is excessive, oxide inclusions such as alumina increase and MA formation is promoted to deteriorate the HAZ toughness. Therefore, the Al content is suppressed to 0.04% 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, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. Further, it is possible to further contain the following elements.

[From the group consisting of Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05% or less (not including 0%) 1 or more types selected (however, within the range of the following formula (2))]
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb and V represent the content (mass%) of each element}

  Cu, Ni, and V are all elements useful for ensuring strength. Cu is an element effective for increasing the strength (tensile strength TS) by solid solution strengthening and precipitation strengthening. However, if excessively contained, the hot workability is inhibited, so the content is suppressed to 0.5% or less.

  Ni is an element that simultaneously improves the strength and toughness of the base material. In order to exhibit such an action effectively, it is preferable to contain 0.2% or more. However, excessive addition increases the cost and may induce SCC in liquefied ammonia, so it is suppressed to 0.8% or less.

  V is an element useful for increasing the hardenability and ensuring high strength, and for increasing the temper softening resistance. However, if excessively contained, the HAZ toughness deteriorates, so the content is limited to 0.05% or less.

  In the present invention, as described above, Nb is suppressed to 0.016% or less, and the contents of Cu, Ni, Nb, and V are limited as shown in the following formula (2), thereby comprising Cu, Ni, and V. Even when one or more selected from the group is contained, excellent HAZ toughness can be ensured.

FIG. 4 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.014Ti-0.0014% B-0.0065% N as a basic component, Cu: One or more selected from the group consisting of 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less, and a specified amount of Nb, including (Cu + Ni + 60Nb + 20V) having various values. A heat cycle test was conducted using a steel plate, and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as described in Examples below, and these results were arranged. The heat cycle test was conducted by heating at 1400 ° C. × 5 seconds and then cooling from 800 ° C. to 500 ° C. in 150 seconds assuming welding heat input: 60 kJ / cm (plate thickness 12 mm).

From FIG. 4, when one or more selected from the group consisting of Cu: 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less is contained, vE ₋ as the low temperature toughness of HAZ. It can be seen that in order to achieve ₆₀ : 100 J or more, the value of (Cu + Ni + 60Nb + 20V) needs to be 1.4% or less as shown in the following formula (2). While suppressing Nb to 0.016% or less and comprehensively limiting the contents of Cu, Ni, Nb, and V as described above, the formation of MA, which is a hard phase, is suppressed and excellent HAZ toughness is achieved. Can be secured.
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (%) (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

[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 material satisfying the above-described component composition is heated to 1050 to 1200 ° C. or lower, hot-rolled to a predetermined plate thickness, and after hot rolling at 880 to 720 ° C., cooling at 10 ° C./second or more. Cool to 600 to 700 ° C. at the rate (first cooling), stop the cooling in the middle, air cool for 30 seconds or more, and subsequently cool from the temperature range to 550 to 350 ° C. at a cooling rate of 10 ° C./s or more. . The reason for setting the range of each condition in this method is as follows.

[Heating temperature: 1050 to 1200 ° C.]
The steel material is heated to 1050 ° C. or higher in order to crimp the casting defect. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains become coarse and the base material toughness deteriorates. Therefore, it is necessary to heat at 1050 to 1200 ° C.

[Hot rolling temperature: 880-720 ° C]
After heating to the above temperature, rolling is started, hot rolling is performed at 880 ° C. or lower, and rolling is finished at 720 ° C. or higher. When the rolling temperature is less than 880 ° C., the austenite is recrystallized by rolling, or defects in the deformation band are introduced into the austenite grains without recrystallization, thereby increasing the number of nucleation sites of ferrite transformation and Refinement and base material toughness are improved. For these reasons, rolling needs to be performed at 880 ° C. or lower. However, the hot rolling finish temperature (FRT) is 720 ° C. or higher. When the hot rolling end temperature is less than 720 ° C., the pro-eutectoid ferrite is processed, and the yield strength YS and the yield ratio YR are increased.

[Cooling conditions]
(A) First cooling rate: 10 ° C./second or more After completion of rolling at the above temperature, the material is cooled to a temperature range of 600 to 700 ° C. at a cooling rate of 10 ° C./second or more (preferably 15 ° C./second or more). By stopping the cooling in the middle, fine ferrite precipitates from the overcooled austenite. Thereafter, the ferrite fraction can be controlled within an appropriate range by holding at that temperature (first cooling stop temperature) for 30 seconds or more. If the holding time at this time is less than 30 seconds, the ferrite fraction is insufficient, and even if the cooling stop temperature is less than 600 ° C. or exceeds 700 ° C., the ferrite fraction is lowered. However, if this holding time exceeds 150 seconds, a pearlite structure tends to be formed, so it is preferable to set it to 150 seconds or less.
(B) Second cooling rate: 10 ° C./second or more From the above cooling stop temperature, by cooling to a cooling rate of 10 ° C./second or more to 550 ° C. or less (second cooling stop temperature), a hard second A phase can be generated. When the cooling rate is less than 10 ° C./second or the cooling stop temperature is higher than 550 ° C., the second phase becomes a pearlite-based structure.

  After cooling to 550 ° C. or lower as described above, it is preferable to perform air cooling (AC) to room temperature regardless of the cooling method. Tempering can also be performed at 500 to 600 ° C., and by adding such a process, the strength can be adjusted.

  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.

  Steel pieces having the chemical composition shown in Tables 1 and 2 below are heated to 1050 to 1200 ° C., hot-rolled to a predetermined plate thickness (12 mm or 30 mm), and hot-rolled in a temperature range of 880 to 720 ° C. After finishing the above, it was cooled to 700 to 600 ° C. at a cooling rate of 10 ° C./second or more, the cooling was stopped in the middle, and air cooling was performed for 30 seconds or more. Then, it cooled to 550 degrees C or less (cooling stop temperature) with the cooling rate of 10 degrees C / second or more from the said temperature range. The production conditions at this time are shown in Tables 3 and 4.

For each steel plate obtained as described above, the base material structure (ferrite fraction, ferrite grain size), base material properties [plate thickness, yield strength YS, tensile strength TS, toughness (vE- ₆₀ )], And HAZ toughness (vE- ₆₀ ) were evaluated in the following manner.

[Measurement of ferrite fraction and ferrite particle size]
For the fraction of ferrite (polygonal ferrite), t / 4 parts (t: plate thickness) of each steel plate were observed with an optical microscope at a magnification of 200 times and a field of view: 300 μm × 300 μm. The average value of 5 fields of view was obtained. Further, the average crystal grain size of ferrite was measured by a comparison method defined in JIS G 0551 by observing 10 visual fields at 400 times at a position of t / 4 part (t: plate thickness) of each steel plate.

[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 with yield strength: 440 MPa or less, tensile strength: 530 MPa or more and a yield ratio (YP / TS) of 75% or less was evaluated as a low yield ratio high-tensile steel plate.

In addition, a V-notch test piece of JIS Z 2202 was taken in the rolling direction from a portion cut by 1 mm from the surface side of each steel plate, and a Charpy impact test was performed in the same manner as in JIS Z 2242. The energy (vE- ₆₀ ) was measured. And it was evaluated that the absorbed energy (vE- ₆₀ ) is 100 J or more and has excellent base material toughness.

[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. 5 [(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.
[Welding material]
・ Wire; US-255
・ Front flux; PFI-50LT
・ Backing flux; MF-1R

Then, 3 V-notch test pieces of JIS Z 2202 each cut by 1 mm from the surface side and cut into the HAZ (bond part, bond part + 1 mm [HAZ 1 mm]) position perpendicular to the plate surface were sampled. A Charpy impact test was conducted in the manner of Z2242. 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 ) absorbed energy was 100J or more was evaluated that it was excellent in the low temperature toughness of HAZ.

  These results are shown in Tables 6 and 7 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 17 are excellent in low temperature toughness of HAZ, and are excellent in base material properties (toughness, yield strength YS: 440 MPa or less, tensile strength TS: 530 MPa or more, yield ratio YR: 75% or less). It is a high-strength steel plate, and exhibits excellent characteristics even when the steel plate is welded by a high heat input single-sided submerged arc welding method and used for low temperature conditions.

  On the other hand, No. which does not satisfy the provisions of the present invention. Each of 18 to 40 has the following problems. That is, no. Nos. 18 to 20 have excellent low-temperature toughness of HAZ, but the ferrite fraction of the base material is low, and the desired base material characteristics (yield strength YS: 440 MPa or less, yield ratio YR: 75% or less) are obtained. Absent.

  No. No. 21 has a C content exceeding the upper limit. No. 22 is inferior in HAZ toughness because the Si content exceeds the upper limit. No. In No. 22, the ferrite particle size is large and the base material toughness is also inferior. Furthermore, in all cases, the ferrite fraction of the base material is low, and the desired base material characteristics (yield strength YS: 440 MPa or less, tensile strength TS: 530 MPa or more, yield ratio YR: 75% or less) are obtained. Absent.

  No. No. 23 is inferior in HAZ toughness because the Mn content is insufficient. Moreover, the desired base material characteristics (tensile strength TS: 530 MPa or more, yield ratio YR: 75% or less) are not obtained. On the other hand, no. In No. 24, since the Mn content is excessive, excellent HAZ toughness cannot be secured.

  No. No. 25 has an excessive P content. No. 26 is inferior in HAZ toughness because the S content is excessive. In either case, the ferrite fraction of the base material is low, and the desired base material characteristics (yield strength YS: 440 MPa or less, yield ratio YR: 75% or less) are not obtained.

  No. No. 27 lacks the Al content. No. 28 is inferior in HAZ toughness because the Al content is excessive. No. No. 29 lacks the Nb content. No. 30 is inferior in HAZ toughness because the Nb content is excessive. Of these, No. In No. 29, the ferrite grain size is also large, and the base material toughness is deteriorated.

  No. No. 31 lacks Ti content. No. 32 is inferior in HAZ toughness because the Ti content is excessive. No. No. 33 has insufficient B content. No. 34 is inferior in HAZ toughness because the B content is excessive. No. No. 35 lacks the N content. No. 36 is inferior in HAZ toughness because the N content is excessive.

  No. No. 37 (B-NT / 1.3) exceeds the lower limit of the formula (1). No. 38 is inferior in HAZ toughness because (B-NT / 1.3) is below the upper limit of formula (1).

  No. Although 39 and 40 contain 1 or more types selected from the group which consists of Cu, Ni, and V, since it exceeds the upper limit of Formula (2), it is inferior to HAZ toughness.

It is a graph which shows the relationship between (B-NT / 1.3) and vE- _{60 of} HAZ. It is a graph which shows the relationship between a ferrite fraction and yield strength YS. It is a graph which shows the relationship between a ferrite particle size and the absorbed energy vE- ₆₀ of a base material. It is a graph which shows the relationship between (Cu + Ni + 60Nb + 20V) and vE- _{60 of} HAZ. 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.

Claims (3)

C: 0.03 to 0.09% (meaning “mass%”, chemical components are the same hereinafter), Si: 0.01 to 0.25%, Mn: 1.20 to 1.60%, P: 0.010% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.02 to 0.04%, Nb: 0.005 to 0.016%, B: 0.0006 to 0.0020%, N: 0.0045 to 0.0090%, Ti: 0.008 to 0.020%, respectively, satisfy the following formula (1), the remaining iron and inevitable It is an impurity, the ferrite fraction occupying the whole structure is 45 to 85 area%, the balance is a bainite structure and / or a martensite structure, and the average crystal grain size of the ferrite is 19 μm or less. Low yield ratio with excellent low temperature toughness of weld heat affected zone Tension steel plate.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0} Further, Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05% or less (not including 0%) The low-yield-ratio high-tensile steel sheet according to claim 1, comprising at least one selected from the group consisting of:
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}   Furthermore, the low yield ratio high-tensile steel sheet according to claim 1 or 2, comprising Ca: 0.003% or less (not including 0%).

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