JP2009179868A - High tensile strength steel plate having excellent weldability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bainitic high tensile strength steel plate in a class of >590 MPa having excellent weldability in such a manner that satisfactory HAZ toughness can be stably secured even if welding is performed in a wide heat input volume range from small heat input welding to high heat input welding. <P>SOLUTION: The high tensile strength steel plate has a prescribed chemical componential composition, and in which BP value and Pcm prescribed by prescribed relational formulae satisfy 120 to 300 (mass%) and ≤0.20 (mass%), respectively, also, Ti-containing nitrides with the circle equivalent diameter of ≤0.05 μm are present by ≥5.0×10<SP>6</SP>pieces per 1 mm<SP>2</SP>, and, therein, the number of the Ti-containing nitrides with the circle equivalent diameter of 0.01 to 0.03 μm occupies ≥75% to the number of all the Ti-containing nitrides. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-tensile steel plate applied to a welded structure such as a bridge, a high-rise building, and a ship, and in particular, a heat-affected zone (hereinafter simply referred to as “HAZ”) after a small heat input weld to a large heat input weld. It may be referred to as a high-tensile steel plate having excellent toughness.

  In recent years, with the increase in size of the above various welded structures, it is inevitable to weld thick steel plates having a plate thickness of 50 mm or more. For this reason, in all fields, high heat input welding of 25 kJ / mm or more is directed from the viewpoint of improving welding construction efficiency.

  However, if high heat input welding is performed, the HAZ is heated to a high temperature austenite region and then gradually cooled, so the structure of the HAZ part (particularly near the bond part of the HAZ part) becomes coarse and the toughness of that part deteriorates. There is a problem that it is easy to do. It has been a long-standing problem to ensure such good toughness in the HAZ portion (hereinafter sometimes referred to as “HAZ toughness”).

  Various techniques for preventing the deterioration of the HAZ toughness during high heat input welding have been proposed so far. As a technique for improving HAZ toughness, it is known that it is effective to disperse a Ti-containing nitride in a steel material. As such a technique, for example, as shown in Patent Document 1, in bainite steel having a strength of over 590 MPa, it is proposed to optimize the alloy elements and to secure good HAZ toughness by controlling Ti-containing nitrides. Has been.

  The present inventors have previously proposed a steel material in which the HAZ toughness does not deteriorate even when it is affected by high-temperature heat during welding. In this technology, a large amount of N is added to the steel, and the balance of addition of Ti and B is appropriately controlled to increase the amount of TiN that remains in an insoluble state after welding, thereby improving the HAZ toughness. It is.

  On the other hand, high-strength steel sheets are not limited to high heat input welding as described above, but may be welded with a small heat input of about 2 kJ / mm. , And there is a tendency for weld cracks to occur and HAZ toughness to deteriorate. It is known that such problems can be improved by reducing the Ceq value and Pcm value determined by the content of each alloy element (for example, Patent Document 3).

In the 490 MPa class steel sheet having a relatively low strength, the present inventors actively contain Nb in TiN-based inclusions present in the weld steel, control the Ti / Nb ratio, and reduce the particle size. We have proposed a technology to ensure HAZ toughness in a wide heat input range by setting the number of inclusions of 0.01 to 0.25 μm to 1.0 × 10 ⁴ or more per 1 mm ² (patent) Reference 4).
Japanese Patent No. 3746707 JP-A-2005-200716 JP-A-57-143430 JP 2004-2181010 A

  However, in the technology proposed so far, in a bainite steel sheet having a strength exceeding 590 MPa, it is possible to stably ensure good HAZ toughness over a wide heat input range from small heat input welding to large heat input welding. The situation is not reached.

  The present invention has been made in view of such circumstances, and its purpose is to stably provide good HAZ toughness even when welding is performed in a wide heat input range from small heat input welding to large heat input welding. Is to provide a bainite high-strength steel sheet having a weldability exceeding 590 MPa.

The high-tensile steel plate according to the present invention that has solved the above problems is C: 0.01 to 0.08% (meaning “mass%”; the same applies hereinafter), Si: 0.30% or less (0% Mn: 0.5 to 2.0%, Al: 0.01 to 0.05%, Cr: 0.5 to 2.0%, Nb: 0.005 to 0.050%, Ti: 0.010 to 0.040%, B: 0.0010 to 0.0050%, N: 0.0020 to 0.0100%, Ca: 0.0050% or less (excluding 0%), respectively. The balance consists of iron and inevitable impurities, the BP value defined by the following formula (1) is in the range of 120 to 300 (mass%), and the Pcm value defined by the following formula (2) is 0.20. (at mass%) or less, and the following Ti-containing nitride 0.05μm circle equivalent diameter 1 mm ² per 5.0 × 10 ⁶ There above, Ti-containing nitride number of 0.01~0.03μm Among equivalent circle diameter and has a gist in that those occupying more than 75% of the total Ti-containing nitride number.
BP value = 414 [C] +78 [Si] +31 [Mn] +79 [Cr] −14 [Cu] −26 [Ni] +2280 [Nb] +218 [Mo] (1)
Pcm value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] (2)
However, [C], [Si], [Mn], [Cr], [Cu], [Ni], [Nb], [Mo], [V] and [B] are respectively C, Si, Mn, The contents (% by mass) of Cr, Cu, Ni, Nb, Mo, V and B are shown.

  The “equivalent circle diameter” refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of the Ti-containing nitride, and is a transmission electron microscope (TEM) observation surface. Of the nitrides identified above. Further, the Ti-containing nitride targeted in the present invention includes not only TiN but also a part of Ti (at an atomic ratio of about 50% or less) to other nitride-forming elements (for example, Nb, Zr, V, etc.). This also includes the nitride substituted with ().

  In the thick steel plate of the present invention, if necessary, (a) Cu and / or Ni: 2.0% or less (not including 0%), (b) Mo: 0.5% or less (not including 0%) And / or V: 0.1 to 0.5%, (c) one or more elements selected from the group consisting of Mg, Zr and REM: 0.010% or less in total (excluding 0%), etc. It is also useful to contain, and the characteristics of the high-tensile steel sheet will be further improved according to the type of element to be contained.

  According to the present invention, while controlling the BP value defined by the above formula (1) and the Pcm value defined by the formula (2) within an appropriate range, the chemical composition of the steel sheet is within an appropriate range, Furthermore, by controlling the dispersion state (number / density) of fine Ti-containing nitride appropriately, even if welding is performed in a wide heat input range from small heat input welding to large heat input welding, it is stable and good. A bainite high-tensile steel plate of 590 MPa and superior in weldability that can ensure HAZ toughness could be realized.

  In a steel sheet having a strength exceeding 590 MPa, a bainite structure is formed in the HAZ, but there are many unclear points regarding the influence of alloy elements on the bainite structure morphology. In order to grasp the influence of the alloy elements on the HAZ toughness of the bainite steel sheet, the present inventors examined the bainite structure morphology and the alloy design guidelines.

  As a result, the idea was obtained that good HAZ toughness could be secured if the HAZ bainite structure (block size) was refined to 10 μm or less. Next, the refinement of the bainite block was considered to have a correlation with the transformation driving force of the bainite structure, and it was considered that the component design for increasing the transformation driving force should be carried out, and the influence of various alloys was examined.

Considering the formation process of the bainite transformation, the transformation driving force includes the temperature at which the driving force of the bainite transformation is generated (hereinafter referred to as “T ₀ temperature”) and the temperature at which the bainite transformation is actually generated (hereinafter referred to as “Bs”). It was thought that this could be explained by the difference between this point and the point.

Therefore, the influence of alloy elements on each temperature (T ₀ temperature, Bs point) was further examined. Since the above T ₀ temperature can be calculated by thermodynamic calculation, the influence of each alloy element is examined using thermodynamic calculation software (Thermo-calc, available from CRC Research Institute), and the influence of each element. Formulated. On the other hand, since the Bs point cannot be calculated theoretically at present, experimental values are used. That is, Bs points of steel types having different alloying elements were obtained by experiments, and the influence of each element was formulated by regression analysis. By taking the difference between the two formulas and obtaining the formula (T ₀ temperature−Bs point), the BP value defined by the following formula (1) was obtained. And when this BP value exists in the range of 120-300 (mass%), even if it carries out large heat input welding, HAZ will become a suitable bainite structure form and favorable HAZ toughness will be achieved. The preferable lower limit of the BP value is 140, and the preferable upper limit is 260.

BP value = 414 [C] +78 [Si] +31 [Mn] +79 [Cr] −14 [Cu] −26 [Ni] +2280 [Nb] +218 [Mo] (1)
However, [C], [Si], [Mn], [Cr], [Cu], [Ni], [Nb] and [Mo] are C, Si, Mn, Cr, Cu, Ni, Nb and Mo content (mass%) is shown.

  In addition to the basic components (C, Si, Mn, Cr, Nb) of the thick steel plate of the present invention, the elements that define the BP value include those contained if necessary, but (Cu , Ni, Mo, etc.) When these elements are not included, the BP value is calculated assuming that the item is not present. When these elements are included, the BP value may be calculated from the above equation (1).

  In the high-strength steel sheet of the present invention, the Pcm value defined by the following equation (2) is also controlled within an appropriate range in order to ensure a stable HAZ toughness from small heat input welding to large heat input welding. It is necessary (Patent Document 3).

Pcm value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are respectively C, Si, Mn, Cu, Ni, Content (mass%) of Cr, Mo, V, and B is shown.

  The Pcm value defined by the above formula (2) is an important requirement in order not to cause deterioration of HAZ toughness due to excessive strength, and is obtained by converting the hardening ability of each element into C content, and the carbon equivalent Ceq It is equivalent to. By controlling this Pcm value to 0.20 (mass%) or less, it is possible to stably ensure good HAZ toughness from small heat input welding to large heat input welding.

  As in the above formula (1), among the elements that define the Pcm value, other than the basic components (C, Si, Mn, Cr, B) of the thick steel plate of the present invention, it is contained as necessary. Although those are not included (Cu, Ni, Mo, V, etc.), when these elements are not included, the Pcm value is calculated assuming that there is no such item. When these elements are included, Pcm is calculated from the above equation (2). Calculate the value.

  By the way, the present inventors have succeeded in increasing the Ti-containing nitride (hereinafter, sometimes represented by TiN) that remains undissolved even at high temperatures during welding (Patent Document 2). Based on these technologies, studies were made to further improve the HAZ toughness.

At the time of welding, fine TiN dissolves and coarse TiN exhibits a behavior that causes grain growth (Ostwald growth). The present inventors pay attention to such a behavior, and in order to make the TiN distribution fine and uniform even after grain growth by dispersing as much TiN as possible as much as possible, the equivalent circle diameter is 0. It has been found that the TiN of 0.05 μm or less may be controlled to be 5.0 × 10 ⁶ or more per 1 mm ² .

  In addition, the Ostwald growth as described above is promoted when the size distribution (variation) of TiN is large, and attention is also paid to the fact that the structure after welding tends to be non-uniform. The idea was obtained that the fine TiN content should be uniformly dispersed so that the proportion of the fine TiN becomes a certain amount or more. Specifically, if the number of fine TiN having an equivalent circle diameter of 0.01 to 0.03 μm accounts for 75% or more of the total number of TiN, it is possible to prevent the structure after welding from becoming uneven. It turned out.

  In the steel plate of the present invention, fine TiN is mainly dispersed by the control described later. Therefore, even if partially coarse TiN (for example, TiN larger than 0.05 μm in equivalent circle diameter) is included, such coarse TiN does not significantly affect the properties of the steel sheet, so “total TiN” is such The purpose is to include coarse TiN. The ratio of the fine TiN number of 0.01 to 0.03 μm to the total TiN number (hereinafter sometimes referred to as “occupancy ratio”) is preferably 77% or more, more preferably 80%. That's it.

  Next, the component composition in the steel material (base material) of the present invention will be described. As described above, even if the chemical composition of the steel sheet of the present invention satisfies the relational expressions of the above formulas (1) and (2), the content of each chemical component (element) is within an appropriate range. Otherwise, excellent HAZ toughness cannot be achieved. Therefore, in the thick steel plate of the present invention, the distribution of TiN (Ti-containing nitride) is good and the chemical components satisfy the above formulas (1) and (2). It is also necessary that the amount be in the proper range as described below. The reasons for limiting the ranges of these components are as follows.

[C: 0.01 to 0.08%]
C is an element indispensable for ensuring the strength of the steel sheet. If the C content is less than 0.01%, the strength of the steel sheet cannot be ensured. Preferably it is 0.02% or more. However, when the C content is excessive, good HAZ toughness in a wide heat input range cannot be secured. Therefore, C must be suppressed to 0.08% or less, preferably 0.06% or less.

[Si: 0.30% or less (including 0%)]
Si is an element useful for securing the strength of the steel sheet by solid solution strengthening, but if contained excessively, a large amount of MA phase is generated in HAZ, or the Ti-containing nitride is coarsened. It becomes impossible to ensure good HAZ toughness in a wide heat input range. From such a viewpoint, the Si content needs to be 0.30% or less, and is preferably suppressed to 0.25% or less. From the viewpoint of ensuring good HAZ toughness, the Si content may be 0%.

[Mn: 0.5 to 2.0%]
Mn is an element useful for enhancing the hardenability of the steel sheet and ensuring the strength and the HAZ toughness. In order to effectively exhibit such effects, it is necessary to contain 0.5% or more. Preferably it is 0.8% or more. However, if the content exceeds 2.0%, the HAZ is remarkably hardened and the HAZ toughness deteriorates. Therefore, the Mn content is set to 2.0% or less. Preferably it is 1.6% or less.

[Al: 0.01 to 0.05%]
Al is useful as a deoxidizing element. In order to exhibit such an effect, it is necessary to contain 0.01% or more, preferably 0.02% or more. However, if the Al content is excessive, a large amount of MA phase is formed in the HAZ and the HAZ toughness is deteriorated. Therefore, it is necessary to suppress the content to 0.05% or less, and preferably 0.04% or less.

[Cr: 0.5 to 2.0%]
Cr is an element that effectively acts to lower the Bs point than to lower the temperature To, to secure the driving force for bainite transformation and to refine the HAZ structure. In order to exhibit such an effect, Cr needs to be contained by 0.5% or more, and preferably 0.7% or more. However, if the Cr content is excessive, the HAZ toughness at the time of high heat input welding deteriorates, so it is necessary to suppress it to 2.0% or less. Preferably it is 1.6% or less.

[Nb: 0.005 to 0.050%]
Nb is an element that exhibits an effect that contributes to increasing the strength of a steel sheet in a small amount. In order to exhibit such an effect, it is necessary to contain 0.005% or more, preferably 0.012% or more. However, when the Nb content is excessive, the bainite structure is coarsened, and the HAZ toughness during high heat input welding is deteriorated. For these reasons, the Nb content needs to be 0.050% or less, preferably 0.030% or less.

[Ti: 0.010 to 0.040%]
Ti reacts with N to form fine Ti-containing nitrides (for example, TiN), suppresses HAZ austenite grain (γ grain) coarsening, and improves HAZ toughness during high heat input welding. Is a useful element. In order to exhibit such an effect effectively, Ti needs to be contained in an amount of 0.010% or more, preferably 0.012% or more (more preferably 0.015% or more). However, if the Ti content is excessive, the Ti-containing nitride becomes coarse and the number thereof decreases, so that the HAZ toughness during high heat input welding deteriorates. For these reasons, the Ti content should be suppressed to 0.040% or less. Preferably it is 0.035% or less (more preferably 0.030% or less).

[B: 0.0010 to 0.0050%]
B exhibits the effect of making the HAZ structure uniform by precipitating TiN remaining undissolved at high temperatures as BN in the nucleus. In order to exhibit such an effect effectively, it is necessary to make it contain 0.0010% or more. Preferably it is 0.0015% or more. However, if the B content becomes excessive, the HAZ toughness in a wide heat input range deteriorates, so it is necessary to make it 0.0050% or less. Preferably it is 0.0040% or less.

[N: 0.0020 to 0.0100%]
N is an element useful for finely dispersing the Ti-containing nitride to uniformly refine the old γ grain size of the HAZ. In order to exert such effects, the N content needs to be 0.0020% or more. Preferably it is 0.0035% or more. However, when the N content is excessive, the solid solution N amount is increased and the HAZ toughness in a wide heat input range is deteriorated. Therefore, N must be suppressed to 0.0100% or less, preferably 0.0070% or less.

[Ca: 0.0050% or less (excluding 0%)]
Ca is an element that has an effect of reducing coarse Ti-containing nitrides (reducing coarse nitrides that are crystallized in combination with oxide inclusions) and contributes to improvement of HAZ toughness. Such an effect increases as the Ca content increases, but it is preferable to contain 0.0010% or more. However, if the Ca content is excessive, inclusions become coarse and the HAZ toughness deteriorates, so it is necessary to keep the content to 0.0050% or less. Preferably it is 0.0030% 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. , Sn, As, Pb, etc.) can be permitted. Further, it is also effective to positively contain the following elements, and the characteristics of the steel sheet are further improved according to the types of components contained.

[Cu and / or Ni: 2.0% or less (excluding 0%)]
Cu and Ni are elements that exhibit the effect of improving the toughness of the matrix and are effective in improving the HAZ toughness. When the content of these elements is excessive, HAZ hardening becomes remarkable and the driving force of the bainite transformation is lowered, so that the HAZ toughness at the time of high heat input welding is deteriorated. For these reasons, when these elements are contained, the content (one or two kinds) needs to be suppressed to 2.0% or less, and preferably 1.5% or less. In order to exert the above effect, it is preferable to contain 0.1% or more of one or two of these elements (total). More preferably, it is 0.4% or more.

[Mo: 0.4% or less (not including 0%) and / or V: 0.1-0.5%]
Mo and V are elements that effectively act to raise the temperature T ₀ but lower the Bs point, secure a driving force for bainite transformation, and refine the HAZ structure. Such an effect is effectively exhibited by containing 0.1% or more of V as the content of Mo increases. However, if these contents are excessive, the HAZ toughness at the time of high heat input welding deteriorates, so it is preferable to keep Mo to be 0.4% or less and V to 0.5% or less.

[One or more elements selected from the group consisting of Mg, Zr and REM: 0.010% or less in total]
Mg, Zr and REM (rare earth elements) are elements that contribute to improvement of HAZ toughness during high heat input welding by refining oxide inclusions in the steel sheet. These effects increase as their content increases, but if the content becomes excessive, the inclusions become coarse and the HAZ toughness during high heat input welding deteriorates, so the total is 0.010% or less. It is preferable to suppress. In the present invention, REM (rare earth element) means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

  In the present invention, in order to control the fine dispersion of the Ti-containing nitride as described above, the component composition is adjusted so that the X value defined by the following formula (3) is 10 or more, and the heating time before rolling It is recommended that the slab be formed so that the cooling rate during casting is cooled at a temperature range of 1500 to 1300 ° C. at 10 ° C./min or more. In order to control the cooling rate in this way, means for reducing the slab thickness or increasing the amount of cooling water can be used. These manufacturing conditions will be described.

X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb] +14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V] (3)
However, [C], [Si], [Mn], [Nb], [Cu], [Ni], [Cr], [Mo] and [V] are respectively C, Si, Mn, Nb, Cu, Content (mass%) of Ni, Cr, Mo, and V is shown.

  In addition to the basic components (C, Si, Mn, Nb, Cr) of the steel sheet of the present invention, among the elements that define the X value, as in the formulas (1) and (2), Although it is included if necessary (Cu, Ni, Mo, V, etc.), when this element is not included, the X value is calculated assuming that there is no such item, and when this element is included, (3) The X value may be calculated from the equation.

  The present inventors have clarified that the Ti-containing nitride precipitates during the casting of the steel ingot, but the precipitation state is affected by the alloy elements (for example, Japanese Patent Application No. 2006-163852). The X value that defines the relationship of the above expression (3) is a function related to the temperature range in the δ region. The “δ region” means a region including δ iron in the steel phase diagram. The “region including δ iron” includes not only a region including δ iron but also a region including δ iron and other states such as a two-phase region of δ + γ. The “temperature range in the δ range” refers to a temperature range including δ iron (difference between the upper limit temperature and the lower limit temperature in the δ range). Here, in the steel having a specific composition, for example, when there is a temperature range of only δ iron and a temperature range of δ + γ iron, the sum of these temperature ranges is the temperature range of the δ region. The temperature range of the δ region can be calculated by inputting the chemical composition of the steel sheet into thermodynamic calculation software (Thermo-calc, available from CRC Research Institute).

  Since the diffusion rate of Ti is fast in this δ iron, it is considered that when the temperature range in the δ region is wide, the time during which δ iron is present becomes longer, and coarse Ti-containing nitrides are easily formed. Therefore, the refinement of the Ti-containing nitride was studied by adjusting the chemical composition and reducing the temperature range in the δ region. For this purpose, in Thermo-calc calculation, the influence of each chemical component on the temperature range in the δ region was examined by changing only one of the chemical component amounts based on the specific component. As a result of such studies, the above-mentioned X value, which is correlated with the temperature range of the δ region and expressed as a function of the chemical composition, was determined.

  The coefficient in the above formula of the X value corresponds to the amount of change in the temperature range in the δ region when each chemical component is changed from the specific component steel. Specifically, for example, when the coefficient of [C] is “500”, when the C content is increased by 0.01%, the temperature range in the δ region decreases by about 5 ° C. in the calculation of Thermo-calc. Means. The X value and the temperature range in the δ region are in an inversely proportional relationship (the relationship that the temperature range in the δ region decreases as the X value increases).

  Based on such an idea, steel sheets having various X values were manufactured and examined. By increasing the X value, the average particle diameter of the Ti-containing nitride can be refined and the HAZ toughness can be improved. It has been found. If the amount of each chemical component is within an appropriate range, the average particle diameter, the HAZ toughness, and the base material toughness of the Ti-containing nitride improve as the X value increases. The lower limit of the X value is 10 (preferably 15, more preferably 20). The upper limit of the X value is determined from the appropriate amount of each chemical component and is 128.

  On the other hand, when the heating time before rolling exceeds 4 hours, TiN becomes coarse, the number of 0.05 μm or less decreases, and the cooling rate during casting (temperature range of 1500 to 1300 ° C.) is 10 ° C. / Even if it is less than min, the coarsening of TiN progresses, and the number of Ti-containing nitrides having an equivalent circle diameter of 0.05 μm or less is reduced.

  The thickness of the high-tensile steel plate of the present invention is not limited, but basically a thick steel plate having a thickness of 3.0 mm or more is assumed. The plate thickness of the thick steel plate of the present invention is preferably 50 mm or more, more preferably 60 mm or more. In other words, the thick steel plate of the present invention exhibits good HAZ toughness even in a wide range of heat input welding with a heat input of 5 to 100 kJ / mm. Therefore, even if the plate thickness is large, the heat input is increased. It can be efficiently welded.

  The high-strength steel sheet of the present invention thus obtained can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and the toughness of the weld heat-affected zone not only in small to medium heat input welding but also in high heat input welding Deterioration can be prevented.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  An Al killed steel sheet having the composition shown in Tables 1 and 2 below is melted, and this molten steel is cooled while controlling the cooling rate at the time of casting (a temperature range of 1500 to 1300 ° C.) and a slab (cross-sectional shape: 150 mm × 250 mm) Then, while controlling the heating time, it was heated to 1100 ° C. and hot-rolled to obtain a hot-rolled plate having a thickness of 50 mm, and air-cooled after rolling to obtain a test plate. In Table 1, REM was added in the form of a misch metal containing about 50% Ce and about 25% La. In Table 1, "-" indicates that no element is added.

  For each test plate produced as described above, the number density of Ti-containing nitrides (the number of equivalent-circle diameters of 0.05 μm or less and the equivalent-circle diameter of 0.01 to 0.03 μm) was as follows. Occupancy ratio), tensile strength TS and HAZ toughness of the thick steel plate were measured. These results are represented by the X value [= 500 [C] +32 [Si] +8 [Mn] -9 [Nb] +14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]]. Tables 3 and 4 below show the cooling rate during casting and the heating time before rolling.

[Measurement of number density of Ti-containing nitride]
The t (plate thickness) / 4 portion of each steel plate was observed with a transmission electron microscope (TEM) under the conditions of an observation magnification of 60,000, an observation field of view 2 × 2 (μm), and five observation locations. Then, the area of each Ti-containing nitride in the field of view was measured by image analysis, and the equivalent circle diameter of each nitride was calculated from this area. In addition, it was discriminate | determined by EDX (energy dispersive X-ray detector) that it was Ti containing nitride.

The number of Ti-containing nitrides having an equivalent circle diameter of 0.05 μm or less is calculated per 1 mm ² , and the total Ti content of the fine Ti-containing nitride having an equivalent circle diameter of 0.01 to 0.03 μm The number ratio (occupancy:%) with respect to nitrides (including those with an equivalent circle diameter exceeding 0.05 μm) was calculated.

[Tensile test]
Sample No. 4 of JIS Z 2201 was taken from t (plate thickness) / 4 part of each steel plate in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z 2241. Tensile strength (TS) Was measured. And the thing with TS of 590 Mpa or more was evaluated as a pass.

[Evaluation of HAZ toughness]
Sample No. 4 of JIS Z 2201 was sampled from the t (plate thickness) / 4 part of each steel plate in a direction perpendicular to the rolling direction, and a heat cycle test simulating small heat input welding and large heat input welding was performed. And HAZ toughness was evaluated. At this time, the thermal cycle test was performed under the following two conditions (A) and (B).
(A) After heating a test piece to 1400 degreeC (maximum temperature) and hold | maintaining for 5 second, the temperature range of 800-500 degreeC is cooled over 40 second.
(B) After heating a test piece to 1400 degreeC (maximum temperature) and hold | maintaining for 5 second, the temperature range of 800-500 degreeC is cooled over 200 second.

  The above condition (A) is a heat cycle corresponding to a welding heat input of 4 kJ / mm, and (B) is a heat cycle corresponding to a heat input of 25 kJ / mm. is there.

Each of the obtained test pieces was subjected to a Charpy impact test at −10 ° C. in accordance with JIS Z 2242, and the absorbed energy (vE ₋₁₀ ) was measured. At this time, the absorbed energy (vE _-10 ) was measured for three test pieces, and the average value was obtained. And it evaluated that the value of vE- ₁₀ was more than 150 J under the condition (A) and more than 120 J under the condition (B) as excellent in HAZ toughness.

  From these results, it can consider as follows (In addition, the following No. shows steel No. of Tables 1-4.). No. Nos. 1 to 16 are examples satisfying the requirements defined in the present invention, in which chemical component composition, BP value, Pcm value, X value, and fine dispersion of Ti-containing nitride are appropriately performed, and in a wide heat input range. It can be seen that a steel sheet having good HAZ toughness is obtained.

  In contrast, no. 17 to 33 are examples that deviate from any of the requirements defined in the present invention, and the toughness of the weld heat affected zone is inferior. Details are as follows.

  No. No. 17 is such that the C content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is deteriorated even when the form of the Ti-containing nitride is good.

  No. No. 18 is that the Si content in the steel sheet exceeds the range specified in the present invention, and the form of the Ti-containing nitride is inferior (fine Ti-containing nitride is not obtained). Good HAZ toughness is not obtained in both heat and large heat input. No. No. 19 is the one in which the Mn content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness at the time of high heat input welding is deteriorated even if the Ti-containing nitride has a good form.

  No. No. 20 is that the content of Al in the steel sheet exceeds the range specified in the present invention, and even when the form of the Ti-containing nitride is good, either at the time of small heat input welding or at the time of large heat input welding HAZ toughness is also deteriorated. No. No. 21 is the one in which the Cr content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness at the time of high heat input welding is deteriorated even if the form of the Ti-containing nitride is good.

  No. No. 22 is that the Nb content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness at the time of high heat input welding is deteriorated even if the form of the Ti-containing nitride is good. No. No. 23 has a Ni content in the steel sheet exceeding the range specified in the present invention, and the HAZ toughness at the time of high heat input welding is deteriorated even if the Ti-containing nitride has a good form.

  No. 24 and 25 are those in which the Ti content in the steel sheet is outside the range defined in the present invention, and the form of the Ti-containing nitride is poor (a fine Ti-containing nitride has not been obtained), At least the HAZ toughness during high heat input welding is degraded.

  No. 26, the B content in the steel sheet exceeds the range specified in the present invention, and even when the Ti-containing nitride is in good form, both in the case of small heat input welding and large heat input welding HAZ toughness is degraded. No. In No. 27, the N content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness during large heat input welding is deteriorated even if the Ti-containing nitride has a good form.

  No. No. 28 does not contain Ca in the steel sheet, the form of the Ti-containing nitride is poor (fine Ti-containing nitride is not obtained), and good HAZ toughness during high heat input welding Is not obtained.

  No. No. 29 does not contain Nb in the steel sheet, and the Cu content exceeds the range specified in the present invention (Pcm value is also increased), and even if the form of the Ti-containing nitride is good, The HAZ toughness is deteriorated both in the small heat input welding and the large heat input welding.

  No. No. 30 is the content of Cr in the steel sheet is less than the range specified in the present invention (BP value is also small), and even when the Ti-containing nitride is in good form, at the time of small heat input welding In addition, the HAZ toughness is deteriorated both in the high heat input welding. No. No. 31 has a Mo content in the steel sheet exceeding the preferred range of the present invention, and the form of the Ti-containing nitride is poor (a fine Ti-containing nitride has not been obtained), and a small heat input. The HAZ toughness is deteriorated in both welding and high heat input welding.

  No. In Nos. 32 and 33, the production conditions deviate from the appropriate conditions, the Ti-containing nitride is coarsened, the sufficient number density of the Ti-containing nitride has not been achieved, and the HAZ toughness during high heat input welding Has deteriorated.

Claims (4)

C: 0.01 to 0.08% (meaning “mass%”; the same applies hereinafter), Si: 0.30% or less (including 0%), Mn: 0.5 to 2.0%, Al: 0 0.01 to 0.05%, Cr: 0.5 to 2.0%, Nb: 0.005 to 0.050%, Ti: 0.010 to 0.040%, B: 0.0010 to 0.0050 %, N: 0.0020 to 0.0100%, Ca: 0.0050% or less (excluding 0%), respectively, the balance is made of iron and inevitable impurities, and is defined by the following formula (1) The BP value is in the range of 120 to 300 (mass%), the Pcm value defined by the following formula (2) is 0.20 (mass%) or less, and the equivalent circle diameter is 0.05 μm or less. There are 5.0 × 10 ⁶ or more Ti-containing nitrides per 1 mm ² , of which a circle equivalent diameter is 0.01 to 0.03 μm T A high-tensile steel sheet excellent in weldability, wherein the number of i-containing nitrides accounts for 75% or more of the total number of Ti-containing nitrides.
BP value = 414 [C] +78 [Si] +31 [Mn] +79 [Cr] −14 [Cu] −26 [Ni] +2280 [Nb] +218 [Mo] (1)
Pcm value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] (2)
However, [C], [Si], [Mn], [Cr], [Cu], [Ni], [Nb], [Mo], [V] and [B] are respectively C, Si, Mn, The contents (% by mass) of Cr, Cu, Ni, Nb, Mo, V and B are shown.   Furthermore, Cu and / or Ni: 2.0% or less (0% is not included), The high-tensile steel plate according to claim 1.   The high-tensile steel sheet according to claim 1 or 2, further comprising Mo: 0.5% or less (excluding 0%) and / or V: 0.1-0.5%.   The element according to any one of claims 1 to 3, further comprising at least one element selected from the group consisting of Mg, Zr and REM: 0.010% or less (excluding 0%) in total. High tensile steel plate.