JP2012077340A - High strength steel sheet having tensile strength of 980 mpa or more and excellent in low temperature toughness at multilayer welding joint portion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is excellent in low temperature toughness at the HAZ of a joint portion even while having the tensile strength of 980 MPa or more.SOLUTION: The steel sheet contains 0.12-0.16% (meaning mass%, and the same hereinafter) of C, 0.05-0.5% of Si, 0.8-1.2% of Mn, 0.01-0.07% of Al, 0.5-0.8% of Mo, 0.025-0.08% of Nb, 0.0004-0.002% of B, and 0.8% or less of Cr and/or 0.06% or less of V, respectively, and the balance comprising iron and inevitable impurities. An MP value specified by equation (1) is 0.550 or larger, and a carbon equivalent Ceq specified by equation (2) is 0.63% or less, wherein equation (1) : MP value=(12/[C])×(([Mo]/96)+([Nb]/93)) and equation (2): Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15.

Description

  The present invention has excellent properties in low temperature toughness of the heat affected zone (hereinafter sometimes referred to as “HAZ”) of the joint when a multi-layer welded joint is formed, and has a tensile strength of 980 MPa or more. It relates to high-strength steel sheets.

  Examples of the use of such a steel sheet include various construction materials and various construction machines. Specific examples include penstock (hydraulic iron pipes), building materials for undamaged buildings, construction machines used at construction sites such as excavators, cranes, and scrapers.

  In the following, the construction machine will be mainly described, but the present invention is not limited to this application.

  Construction machines such as large cranes used at construction sites are loaded on vehicles and transported to the site, but large construction machines are difficult to transport as they are. Yes. Therefore, there is a demand for weight reduction of construction machines. On the other hand, construction machines need to be strong enough to withstand harsh field work. In addition, since construction machines are often used in cold regions, it is required that the base material and the welded joint have good low temperature toughness. In order to meet these demands, a construction machine is required to have a steel plate that is thin but has a strength of 100 kg and has excellent low-temperature toughness.

  A certain amount or more (for example, 0.12% by mass or more) of carbon is necessary for increasing the strength of a steel sheet applied to the above-described uses. In addition, when a steel sheet is welded to form a welded joint, multilayer prime welding is performed (multilayer prime welded joint). In order to ensure the toughness of such a welded joint, the smaller the carbon content, the better. ing.

  In the multi-layer welded joint, the structure is complicated due to the relationship with the joint structure. That is, in the multi-layer welded joint, there are a tempered coarse grain HAZ (CG-HAZ) structure, a tempered fine grain HAZ (FG-HAZ) structure, and a two-phase region heated HAZ (IR-CGHAZ) structure depending on the portion. Among these structures, the IR-CGHAZ structure is known to be the most brittle part. That is, in improving the low temperature HAZ toughness of the welded joint, it is considered to be the most effective means to improve the toughness of the IR-CGHAZ structure.

  The cause of the IR-CGHAZ structure becoming low toughness (most brittle part) is the presence of a hard phase called MA (martensite-austenite mixed structure). That is, since cracks are generated due to MA cracks existing at the grain boundaries of the structure, the more MA there are, the more cracks are generated, which is considered to be the cause of low toughness.

Various techniques have been proposed for realizing a steel sheet excellent in low temperature toughness of a welded joint. As such a technique, for example, in Patent Document 1, MgAl ₂ O ₄ —TiN composite particles having MgN ₂ O ₄ as a core and TiN in the periphery thereof are included to improve the toughness of the heat affected zone of super large heat input welding. A thick steel plate has been proposed. In this technique, MgAl ₂ O ₄ —TiN composite particles are dispersed in steel and the HAZ toughness is improved by suppressing the γ grain growth of HAZ by its pinning action.

  In Patent Document 2, the chemical composition of the steel sheet is controlled so as to satisfy the carbon equivalent (Ceq), the weld cracking susceptibility index (PcM), and the solid solution amount of the alloy element, and the Nb compound and austenite grains in the steel are controlled. By controlling the size and the like within a predetermined range, a high-strength steel sheet having excellent uniformity and toughness in the thickness direction and low anisotropy has been proposed. This technique aims to strengthen the matrix by dissolving the additive alloy element in a supersaturated state, suppress coarsening of the austenite crystal grains by the fine Nb compound, and strengthen the matrix, thereby improving the center of the plate thickness. In addition to improving the strength and toughness of the steel, the high toughness of the surface layer portion is ensured, and furthermore, a high-tensile steel sheet having low anisotropy is provided.

  However, even in such a technique, MA still remains, and from the viewpoint of reducing the MA in the IR-CGHAZ structure and improving the low temperature toughness, a sufficient improvement effect is exhibited. The situation is hard to say.

Japanese Patent Laid-Open No. 11-236645 Japanese Patent Laid-Open No. 10-265893

  This invention is made | formed in view of such a condition, Comprising: The objective is to provide the high strength steel plate which is excellent in the low temperature toughness of HAZ of a joint part, although it has the tensile strength of 980 Mpa or more. .

The steel sheet of the present invention that has achieved the above-mentioned problems is: C: 0.12-0.16% (meaning mass%; the same applies hereinafter), Si: 0.05-0.5%, Mn: 0.8- 1.2%, Al: 0.01-0.07%, Mo: 0.5-0.8%, Nb: 0.025-0.08%, B: 0.0004-0.002%, and Cr: 0.8% or less and / or V: 0.06% or less, respectively, the balance is made of iron and inevitable impurities, and the MP value defined by the following formula (1) is 0.550 or more. And the carbon equivalent Ceq defined by the following formula (2) is 0.63% or less.
MP value = (12 / [C]) × (([Mo] / 96) + ([Nb] / 93)) (1)
However, [C], [Mo], and [Nb] indicate the contents (mass%) of C, Mo, and Nb, respectively.
Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (2)
However, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%) is shown.

  The steel plate of the present invention preferably contains Cu: 0.5% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%) as other elements. This is an embodiment.

  Further, the steel sheet of the present invention further contains, as other elements, Ca: 0.006% or less (not including 0%) and / or Ti: 0.025% or less (not including 0%). Is also a preferred embodiment.

  According to the present invention, the contents of C, Mo, and Nb are controlled so as to satisfy the relationship of the above formula (1), and the chemical composition of the steel sheet is kept within an appropriate range, so that multilayer welding is performed. The production of MA in the IR-CGHAZ structure of the joint can be reduced, and the joint part HAZ can be excellent in low-temperature toughness, and a high-strength steel sheet having a tensile strength of 980 MPa or more can be realized. Such a steel sheet is extremely useful as a material for various building materials and construction machines.

FIG. 1 is a graph showing the relationship between the MP value and the MA area ratio (%). FIG. 2 is a graph showing the relationship between the MP value and vE-20 min.

  In order to reduce the formation of MA in the IR-CGHAZ structure, which is the most brittle part of a multi-layer welded joint, the present inventors have made extensive and detailed studies on the influence of chemical components particularly on the precipitation of MA. As a result, even by C content (0.12% or more) that can ensure high strength of 980 MPa or more and stability of base metal toughness, by finding a component system that can suppress the amount of MA produced in the IR-CGHAZ structure The present invention was completed by finding that a steel sheet capable of ensuring good low temperature toughness of the HAZ of the welded joint can be realized. The operational effects of the present invention will be described along the background of the completion of the present invention.

  At the time of welding the steel plate (welding heating temperature: 1350 ° C. or higher), all precipitates are dissolved, and precipitates are sequentially formed in the subsequent cooling process. Specifically, first, when the steel material of the welding heating part is heated to just below the melting point by welding heat, austenite (hereinafter abbreviated as γ) grains are coarsened to form a CGHAZ structure. At that time, C in the γ grains moves (diffuses) and concentrates at the γ grain boundaries. When the welded heating part is cooled, the concentrated C is fixed at the grain boundary, and a C enriched zone is formed at the grain boundary. Further, the IR-CGHAZ structure is formed by subsequent welding (heating), but the C enriched zone is heated in a two-phase region and transformed into γ grains in the cooling process after welding. Site transformation occurs and MA is formed.

  In view of the MA formation process, as a measure for suppressing MA, when γ grains are coarsened to form a CGHAZ structure, C is prevented from concentrating at the γ grain boundary, that is, C is γ. It is considered effective to suppress the movement to the grain boundary.

  As a method for this purpose, the present inventors have found that an element that easily attracts C, that is, an element that easily forms carbide with C, is contained in the γ grain when the welded heating part is heated to just below the melting point and the γ grain becomes coarse. It was thought that C can be prevented from moving to the γ grain boundary.

Therefore, when the relationship between each element and C was investigated, Ti, Nb, V, and Mo are elements (strong carbides) that easily form C and carbides (TiC, NbC, VC, Mo ₂ C) compared to other elements. It was thought that it was effective for trapping C.

  However, in order to capture C during the γ grain coarsening, a strong carbide forming element needs to be present in the γ grain, and for this purpose, the strong carbide forming element exists in the parent phase (Fe) as a solid solution. Must be. Specifically, if the atomic radius and electronegativity of the strong carbide-forming element are the same as those of Fe (matrix) and the atomic radius and electronegativity are approximately the same (within ± 15%), the element is solid-solved. Therefore, the present inventors examined the atomic radii and electronegativity of Ti, Nb, V, and Mo, and Nb, V, and Mo have the same atomic radius and electronegativity as Fe. , It was found to dissolve.

  However, in order to prevent these elements from moving to the γ grain boundary at high temperatures during welding (molten state), they must be heavier than Fe. Specifically, Nb (atomic weight 93) and Mo (atomic weight 96) are heavier than Fe (atomic weight 56) and can suppress movement to the γ grain boundary, whereas V (atomic weight 51) is lighter than Fe and carbide. It is considered that the movement of (VC) cannot be suppressed.

  From the above examination results, Nb and Mo form carbides with C, and the ratio of suppressing the migration of C is the amount of Mo ([Mo] / 96: [Mo] is the Mo content (mass%), 96 is the sum of Mo atomic weight) and the amount of Nb ([Nb] / 93: [Nb] is Nb content (% by mass), 93 is Nb atomic weight) ([Mo] / 96 + [Nb] / 93) It will be proportional. That is, the higher the ([Mo] / 96 + [Nb] / 93), the higher the C movement suppression effect, and the higher the MA precipitation suppression effect.

The above effect is the sum of the amounts of Mo and Nb ([Mo] / 96 + [Nb]) relative to the amount of C (12 / [C]: [C] is the C content (mass%), 12 is the C atomic weight). As the ratio of / 93) is higher, the C concentration suppression effect is higher, and therefore, the MP value determined by the following equation (1) is determined as the MA formation suppression parameter.
MP value = (12 / [C]) × (([Mo] / 96) + ([Nb] / 93)) (1)
However, [C], [Mo], and [Nb] indicate the contents (mass%) of C, Mo, and Nb, respectively.

  The larger the MP value defined by the C, Mo, and Nb contents, the smaller the amount of precipitated MA. However, the amount of precipitated MA is suppressed to a predetermined amount or less, and excellent low temperature toughness is exhibited. In order to achieve this, the MP value must be 0.550 or more, preferably 0.570 or more, and more preferably 0.590 or more.

  The upper limit of the MP value is not particularly limited because it can take values determined by the ranges of Mo, Nb, and C described later.

  Furthermore, in the steel plate of the present invention, in order to maintain the low temperature joint toughness satisfactorily, it is also necessary that the carbon equivalent Ceq defined by the following formula (2) is 0.63% or less. When the carbon equivalent exceeds 0.63%, the substrate structure of the IR-CGHAZ part is cured, and the toughness is deteriorated. A preferable carbon equivalent is 0.62% or less.

This carbon equivalent Ceq is obtained by converting the influence of each element on the low temperature joint toughness into a carbon amount, and is used in various fields (ASTM standard). In the present invention, such a carbon equivalent Ceq is used as a criterion for determining low temperature joint toughness. In addition, the following formula (2) includes Cu and Ni contained as necessary in addition to the basic components (C, Mn, and Mo) of the steel material of the present invention. For example, the calculation may be performed in consideration of the amount.
Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (2)
However, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%) is shown.

  In the steel sheet of the present invention, the MP value defined by the above formula (1) is 0.550 or more, and the carbon equivalent Ceq defined by the above formula (2) is 0.63% or less, thereby increasing the strength. In addition, the low temperature joint toughness is good, but it is necessary not only to satisfy these relational expressions but also to consider the relationship with the chemical composition of the steel sheet.

  Hereinafter, other component compositions in the steel material (base material) of the present invention will be described. As described above, the steel composition of the present invention has an appropriate content of each chemical component (element) even if the chemical component composition satisfies the predetermined range of the MP value defined by the above formula (1). If it is not within the range, excellent low temperature toughness and tensile strength cannot be achieved. Therefore, in the steel plate of the present invention, in addition to controlling the MP values [formula (1)] defined by appropriate amounts of C, Mo, and Nb within a predetermined range, the amount of each chemical component is It is also necessary to be within an appropriate range as described below. The reasons for limiting the ranges of these components are as follows.

[C: 0.12-0.16%]
C is an important element for improving the hardenability of the steel sheet and ensuring the strength. However, if its content is excessive, the joint toughness is deteriorated, so it is necessary to be 0.16% or less. The upper limit with preferable C content is 0.15%. From the standpoint of ensuring weldability, the smaller the C content, the better. However, when it is less than 0.12%, the hardenability deteriorates and the strength cannot be secured. A preferable lower limit of the C content is 0.13%.

[Si: 0.05 to 0.5%]
Si acts as a deoxidizer when melting steel, and exhibits the effect of increasing the strength of the steel. In order to exhibit such an effect effectively, the Si content needs to be 0.05% or more. However, since the joint toughness deteriorates when the Si content is excessive, it is necessary to make it 0.5% or less. In addition, the minimum with preferable Si content is 0.15%, and a preferable upper limit is 0.4%.

[Mn: 0.8 to 1.2%]
Mn is an element that exhibits the effect of increasing the strength of the steel sheet. In order to exhibit such an effect effectively, it is necessary to contain 0.8% or more of Mn. Preferably it is 0.9% or more. However, when the Mn content exceeds 1.2% and becomes excessive, the joint toughness deteriorates. Preferably it is 1.1% or less.

[Al: 0.01 to 0.07%]
Al is added as a deoxidizer, but if its content is less than 0.01%, sufficient effects are not exhibited, and if it exceeds 0.07% and excessively contained, cleanliness in the steel sheet is hindered. Will be. The preferable lower limit of the Al content is 0.015%, and the preferable upper limit is 0.065%.

[Mo: 0.5-0.8%, Nb: 0.025-0.08%]
Mo and Nb are elements that have a strong affinity for C as described above, are solid-solved in the parent phase (Fe), and are highly effective in reducing low-temperature toughness. In order to exhibit such an effect, Mo needs to be contained by 0.5% or more, preferably 0.55% or more. Moreover, Nb needs to be contained by 0.025% or more, and preferably 0.035% or more.

  However, if Mo and Nb are contained excessively, huge precipitates are formed and weldability is hindered, so the Mo content is 0.8% or less, preferably 0.75% or less. . Further, the Nb content is 0.08% or less, preferably 0.07% or less.

[B: 0.0004 to 0.002%]
B is an element that fixes solute N harmful to the HAZ toughness by generating BN and further improves the low temperature toughness. In order to sufficiently exert such effects, it is necessary to contain B in an amount of 0.0004% or more, and a preferable content is 0.0005% 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 0.002% or less. A preferable B content is 0.0018% or less.

[Cr: 0.8% or less and / or V: 0.06% or less]
Cr and V are both effective elements for increasing the strength by precipitation strengthening. These may be added alone or in combination. Since these effects increase as their content increases, it is desirable that Cr is contained in an amount of 0.2% or more, and V is preferably contained in an amount of 0.025% or more. On the other hand, if excessively contained, the HAZ toughness is deteriorated, so it is preferable to suppress Cr to 0.8% or less and V to 0.06% or less, respectively. More preferably, Cr is 0.78% or less, and V is 0.055% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and the elements (for example, P, S) brought in as raw materials, materials, production facilities, etc. as the unavoidable impurities. , N, Sn, As, Pb, etc.) can be permitted. Of these impurities, P, S, and N are preferably suppressed as follows.

[P: 0.02% or less (excluding 0%)]
Since P which is an impurity element causes temper embrittlement, the amount is preferably as small as possible. From the viewpoint of securing toughness, the P content is preferably suppressed to 0.02% or less, and more preferably 0.015% or less. However, industrially, it is difficult to make P in steel 0%.

[S: 0.01% or less (excluding 0%)]
S is an impurity that causes temper embrittlement, and its amount is preferably as small as possible. From the viewpoint of ensuring toughness, the S content is preferably suppressed to 0.01% or less, and more preferably 0.005% or less. However, industrially, it is difficult to reduce S in steel to 0%.

[N: 0.01% or less (excluding 0%)]
N is an impurity that causes curing, and the amount thereof is preferably as small as possible. From the viewpoint of ensuring toughness, the N content is preferably suppressed to 0.01% or less, more preferably 0.006% or less. However, industrially, it is difficult to reduce N in steel to 0%.

  Further, in the steel sheet of the present invention, if necessary, (a) Cu: 0.5% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%), (B) Ca: 0.006% or less (not including 0%), and / or Ti: 0.025% or less (not including 0%), and the like can be included. Depending on the type, the properties of the steel sheet are improved.

(A) [Cu: 0.5% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%)]
Cu and Ni are both effective elements for increasing the strength by solid solution strengthening. These may be added alone or in combination.

  Specifically, Cu is an element effective for increasing the strength, and is preferably contained in an amount of 0.2% or more. On the other hand, if the content is excessive, cracking is likely to occur during hot working, and joint toughness is also deteriorated.

  Ni is an element effective in improving the strength and toughness of the steel sheet, and is preferably contained in an amount of 0.2% or more. However, when the Ni content is excessive, the joint toughness is also deteriorated.

(B) [Ca: 0.006% or less (not including 0%) and / or Ti: 0.025% or less (not including 0%)]
Ca and Ti are both effective elements for improving HAZ toughness. These may be added alone or in combination.

  Specifically, Ca is an element effective for improving the material in the Z direction (plate thickness direction) by controlling the form of sulfide in steel. However, if the Ca content is excessive, inclusions in the steel increase and the toughness and joint toughness of the steel sheet are impaired, so 0.006% or less is preferable. In addition, a preferable lower limit for effectively exhibiting the effect of Ca is 0.001%.

  Ti is an element effective in forming precipitates with N in the HAZ of the welded joint and suppressing the coarsening of the structure by pinning. Such an effect increases as the content increases, but if it is excessively contained, joint toughness deteriorates, so it is preferable to suppress it to 0.025% or less. A preferable lower limit for effectively exhibiting the effect of Ti is 0.005%.

  As described above, in the present invention, not only the above formulas (1) and (2) but also the chemical composition of the steel sheet is satisfied, so that it has a tensile strength of 980 MPa or more and the base material and the HAZ part low-temperature joint. The toughness is good.

  In particular, the steel sheet of the present invention has a high strength of 980 MPa or more, but can exhibit excellent low temperature joint toughness even when it has a tensile strength of preferably 990 MPa or more, more preferably 1000 MPa or more.

  The structure of the steel sheet (base material) of the present invention is mainly composed of (tempered) bainite (90% or more is bainite). From the viewpoint of improving strength and toughness, a bainite-based structure is preferably 92% or more, more preferably 94% or more.

  In order to produce the steel sheet of the present invention, a molten steel satisfying the above component composition is used, and a steel sheet (quenched / tempered steel sheet: QT steel sheet) may be used according to normal conditions (rolling temperature, rolling reduction, quenching temperature, tempering temperature). . The present invention relates to a thick steel plate. In this field, a thick steel plate generally refers to one having a plate thickness of 3.0 mm or more as defined by JIS. In the present invention, even when a welded joint is formed by multi-layer welding of steel plates, high strength and good HAZ toughness are exhibited. The steel sheet of the present invention can be used as a material for structures requiring low temperature joint toughness, for example, to prevent low temperature toughness deterioration of weld heat affected zone not only in small to medium heat input welding but also in large heat input welding and strength. Can do.

  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. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  A steel ingot having the composition shown in Table 1 below is melted by a normal vacuum melting method, hot rolled to the steel ingot to form a hot rolled plate having a thickness of 25 mm, and heated to 930 ° C. and quenched ( Q) and heated to 500 ° C. and tempered (T) to produce a steel plate (QT steel plate). In addition, when the structure of the steel sheet was examined, all of them were bainite-based structures including 90% or more of bainite.

  Using each steel plate obtained as described above, the strength (TS) and toughness (vE-20 min) of the base material, the toughness (vE-20 min) in IR-CGHAZ, and the MA area ratio were evaluated by the following methods. . In the following measurement method, for each steel plate, three test pieces were used for each steel plate, and the minimum value was obtained.

[Evaluation of base material strength (TS)]
Sample No. 4 of JIS Z2201 was taken from the t (plate thickness) / 4 part of each steel plate in the direction perpendicular to the rolling direction, and subjected to a tensile test according to the procedure of JIS Z2241, and the tensile strength (TS) was measured. It was measured. And the thing with TS of 980 MPa or more was evaluated as a pass.

[Evaluation of toughness of base metal (vE-20 min)]
Test pieces of JIS Z2242 were taken from the t (plate thickness) / 4 part of each steel plate (base material) in the rolling direction, and the toughness of the base material was evaluated. Based on JIS Z2242, a Charpy impact test was performed at -20 ° C., and the absorbed energy (vE-20) was measured. And it evaluated that the minimum value (vE-20min) of vE-20 was 47J or more as being excellent in toughness.

[Evaluation of toughness (vE-20min) in IR-CGHAZ]
In order to obtain an IR-CGHAZ structure, 12.5 mm (length in the plate thickness direction) × 55 mm (length in the width direction) × 32 mm (length in the rolling direction) from t (plate thickness) / 4 portion of each steel plate (base material) The test piece was taken and subjected to a thermal cycle test under the following conditions to evaluate toughness in IR-CGHAZ. At this time, in the thermal cycle test, the test piece was heated to 1350 ° C. and held for 5 seconds, then the temperature range of 800 to 500 ° C. was cooled over about 7 seconds, and then the thermal effect of the subsequent pass was simulated 830. After heating to ° C. and holding for 5 seconds, the temperature range of 800 to 500 ° C. was cooled over about 7 seconds to give a heat cycle corresponding to a welding heat input of 17 kJ / cm. Based on JIS Z2242, the Charpy impact test was performed at -20 degreeC and the absorbed energy (vE-20) was measured. And the minimum value (vE-20min) of vE-20 evaluated 47J or more as being excellent in the toughness of IR-CGHAZ.

[Measurement of MA area ratio in IR-CGHAZ]
The center portion of each test piece subjected to the thermal cycle test described above is subjected to repeller corrosion, and after observing four fields of view of 60 × 80 (μm ² ) at a magnification of 1000 × with an optical microscope, this image data is imaged. The area ratio of MA was calculated by analysis.

  These results are shown in Table 2 below together with the quenching temperature and the tempering temperature.

  It can be considered as follows from Tables 1 and 2 (note that the following No. indicates the experiment No. in Tables 1 and 2). No. 7 to 14 are examples that satisfy the requirements defined in the present invention, the chemical composition and MP value are within the range defined in the present invention, and precipitation of grain boundary cementite is suppressed in IR-CGHAZ. (Area ratio is less than 4%), it can be seen that a low-temperature toughness is good and a steel sheet having a high tensile strength of 980 MPa or more is obtained.

  In contrast, no. 1 to 6 do not satisfy the requirements (chemical component composition, MP value, Ceq value) defined in the present invention.

  Specifically, no. 1-4 are examples in which the chemical composition (Mo and / or Nb content) is outside the specified range of the present invention and does not satisfy the MP value. No. 5 is an example in which the carbon equivalent (Ceq) is outside the specified range of the present invention. No. 6 is an example in which the MP value is outside the specified range of the present invention.

  Of these, No. As for 1-4, the precipitation amount of MA has increased in IR-CGHAZ (area ratio is 4% or more), and the toughness (vE-20 min) in IR-CGHAZ is inferior. No. No. 5 satisfied the MP value, and the formation of MA was suppressed, but the carbon equivalent was poor, so the structure of the welded portion was hardened and the toughness in IR-CGHAZ deteriorated.

  Based on these results, the relationship between the MP value and the MA area ratio (%) is shown in FIG. 1, and the relationship between the MP value and vE-20 min is shown in FIG. As is clear from this result, it can be seen that by controlling the MP value to 0.550 or more, the production of MA can be reduced and good low temperature toughness can be ensured.

Claims (3)

C: 0.12 to 0.16% (meaning mass%, the same shall apply hereinafter)
Si: 0.05 to 0.5%,
Mn: 0.8 to 1.2%
Al: 0.01 to 0.07%,
Mo: 0.5 to 0.8%,
Nb: 0.025 to 0.08%,
B: 0.0004 to 0.002%, and Cr: 0.8% or less, and / or V: 0.06% or less,
The balance is composed of iron and inevitable impurities, the MP value defined by the following formula (1) is 0.550 or more, and the carbon equivalent Ceq defined by the following formula (2) is 0. A high-strength steel sheet having a tensile strength of 980 MPa or more, which is 63% or less, and excellent in low-temperature toughness of a multilayer prime joint.
MP value = (12 / [C]) × (([Mo] / 96) + ([Nb] / 93)) (1)
However, [C], [Mo], and [Nb] indicate the contents (mass%) of C, Mo, and Nb, respectively.
Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (2)
However, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%) is shown.   The element according to claim 1, further comprising Cu: 0.5% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%) as other elements. High strength steel plate.   Further, as another element, Ca: 0.006% or less (not including 0%) and / or Ti: 0.025% or less (not including 0%) are included. High strength steel sheet as described.

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