JP2009041057A - Thick steel plate for high-heat-input welding superior in shear cutting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate which shows adequate HAZ toughness even in high-heat-input welding, and shows such superior shear cutting as not to cause shear-cutting crack even when having been cut with a shear cutting machine. <P>SOLUTION: The thick steel plate has a chemical composition including adequately adjusted components while satisfying the following expressions: (1) 1.5≤[Ti]/[N]≤4.0; and (2) 40≤value X≤160, wherein value X=500[C]+32[Si]+8[Mn]-9[Nb]+14[Cu]+17[Ni]-5[Cr]-25[Mo]-34[V], (wherein [ ] means a content (mass%) of each element), and the balance Fe with unavoidable impurities. The thick plate has a structure in which bainite occupies 95 area% by fraction or more, an average block size of the bainite is 40 μm or less by a circle equivalent diameter, and the difference between the largest circle equivalent diameter and the average circle equivalent diameter of the block size of the bainite is 40 μm or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a thick steel plate applied to a welded structure such as a ship or an offshore structure, and is particularly excellent in toughness of a heat affected zone (HAZ) after high heat input welding and shear cutting ability. It is also related to an excellent thick steel plate.

  In recent years, for example, container ships and the like have been increased in size, and a thick steel plate having a thickness of 40 mm or more may be used. In order to efficiently weld such thick steel plates, large heat input or super large heat input welding (hereinafter, may be represented by “high heat input welding”) such that the heat input amount is 40 kJ / mm or more. There is a need to do.

  However, if high heat input welding is performed, the HAZ is heated to a high temperature austenite region and then gradually cooled, so that the structure becomes coarse and the HAZ toughness is remarkably deteriorated. For these reasons, conventionally, the amount of welding heat input has been limited.

  In order to achieve good HAZ toughness by such high heat input welding, for example, Patent Document 1 reduces the C content in the thick steel plate and limits the content of P that is inevitably mixed. In addition, it is proposed to control the contents of Nb and B within an appropriate range. Further, Patent Document 2 suppresses the formation of coarse ferrite by positively containing Nb in TiN-based inclusions present in welding steel. However, in these techniques, when TiN is insufficient or when TiN is sufficient, the TiN is coarsened or the pinning effect due to CaO oxide or the like is not sufficient, so that HAZ toughness is further improved. There is room for. Moreover, it does not consider about the shear cutting property of the base material steel plate mentioned later.

  Patent Document 3 proposes that a steel material contains a relatively large amount of N and appropriately controls the content balance of Ti and B. However, even in such a technique, the precipitation amount of TiN or BN is not sufficient or fine, and the ferrite becomes coarse due to low hardenability without addition of Nb, so that the HAZ toughness is further improved. There is room for. Moreover, it does not consider about the shear cutting property of the base material steel plate mentioned later.

  On the other hand, a thick steel plate rolled to a predetermined thickness by hot rolling is measured after being cooled in a cooling bed, and the measured thick steel plate is cut to a predetermined width and length. Become. This cutting is performed by a shearing machine or gas cutting. Usually, a thick steel plate having a thickness of less than about 50 mm is cut by a shearing machine, and a thick steel plate having a thickness of more than about 50 mm is gas-cut, but the shearing machine has a top portion of the thick steel plate. And there is a clock shear that cuts the bottom part, a side shear that cuts the ear part, a slitter that divides the width direction of the thick steel plate into two parts, an end shear that cuts the length direction to a predetermined dimension, etc., and consists of these shearing machines By passing the cutting line to be made, the thick steel plate is made to have a predetermined dimension.

  As for the defect of the cut surface of the steel sheet generated in the shearing process as described above, sagging, burrs, mechanical cracks, cutting, stepping, etc. are known. It cannot be used, and maintenance such as blinder polishing and recutting are required, leading to a decrease in yield and an increase in manufacturing cost.

  Various proposals have heretofore been made as countermeasures against the above-described cut surface defects. As such a technique, for example, Non-Patent Document 1 proposes that the condition of the shearing machine is managed to an appropriate value, assuming that the degree of the cut surface defect is largely determined by the equipment condition of the shearing machine. Patent Document 4 discloses a method of reducing the sagging by preheating a crop portion including a cutting line and then cutting with a shearing machine.

  According to the study of the prior art by the present inventors, even when the equipment conditions of the shearing machine are properly managed, the tensile strength that can be applied for mechanical structures, civil engineering, and line pipes is sufficient. It was confirmed that cracks along the central portion of the plate thickness (hereinafter referred to as “shear cutting cracks”) occurred on the fracture surface of the high-toughness high-strength thick steel plate of 550 MPa or more, although the frequency of occurrence was low.

  Thick steel plates applied to various uses as described above need to be mass-produced in a short period of time and are cut by a shearing machine without being preheated. When shear cutting cracks occur, re-cutting must be performed by off-line gas cutting to remove the cracked parts, which greatly increases the work period and increases manufacturing costs. However, the actual situation is that a technique capable of effectively preventing such shear cutting cracks has not been established so far.

As a technique for preventing shear cutting cracking, for example, Patent Document 5 proposes a technique for improving shear cutting performance by adjusting segregation in steel by increasing the cooling rate after rolling (accelerated cooling). However, since high-speed cooling is performed, there is a problem that the strength range that can be produced is limited, and this is insufficient as a countermeasure against shear cutting cracks.
Japanese Patent Laid-Open No. 2003-166033 JP 2004-2181010 A JP-A-2005-200716 JP-A-6-190627 JP 2001-26821 A "Iron and Steel Handbook Vol. 3 III (1) Rolling Foundation and Steel Sheet" (Edited by Japan Iron and Steel Institute, 3rd edition, page 285)

  The present invention has been made under such circumstances, and its purpose is to exhibit good HAZ toughness even in high heat input welding and to prevent shear cutting cracks even when cut by a shear shearing machine. It is to provide a thick steel plate excellent in shear cutting ability.

The thick steel plate of the present invention that can achieve the above object is C: 0.030 to 0.15% (meaning mass%, the same shall apply hereinafter), Si: 1.0% or less (not including 0%), Mn: 0.8 to 2.0%, P: 0.03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10 %, Ti: 0.015-0.03%, B: 0.0010-0.0035%, N: 0.0050-0.01%, Ca: 0.005% or less (excluding 0%), O: each containing 0.01% or less (excluding 0%), satisfying the following formulas (1) and (2), the balance being Fe and inevitable impurities, and a bainite fraction of 95 areas %, The average equivalent circle diameter of the bainite block size is 40 μm or less, and the bainite block Largest circle difference equivalent diameter and the average circle equivalent diameter of size is to have the gist to the point at 40μm or less. 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 bainite block.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)

  In the thick steel plate of the present invention, from the viewpoint of good low temperature toughness and HAZ toughness, (a) the temperature range of the δ region is 40 ° C. or less, and (b) the position of the depth t / 4 (t = plate thickness) ), It is preferable to satisfy the requirements such that the average particle diameter of the Ti-based charcoal / nitride is 43 nm or less. The “Ti-based carbon / nitride” is intended to include any of carbides, nitrides, and carbonitrides containing Ti.

  In the thick steel plate of the present invention, if necessary, (a) Nb: 0.035% or less (not including 0%), (b) Cu: 2.0% or less (not including 0%), Ni: One or more selected from the group consisting of 2.0% or less (excluding 0%) and Cr: 2.0% or less (not including 0%), (c) Mo: 1.0% or less (0% (D) V: 0.1% or less (not including 0%), (e) one or more selected from the group consisting of Mg, Sr, and Ba: 0.01% or less in total (0 %), (F) rare earth element: 0.01% or less (not including 0%), (g) one or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (0) not included, (h) Co: not more than 2.5% (not including 0%) and / or W: not more than 2.5% (not including 0%), etc. It is also useful to have, and the properties of the steel sheet are further improved depending on the components contained.

  According to the present invention, the amount and structure of each component fall within an appropriate range, the chemical component composition is adjusted so as to satisfy the above formulas (1) and (2), and the average block size of bainite. By appropriately controlling the difference between the equivalent circle diameter and the maximum equivalent circle diameter of the block size of bainite and the average equivalent circle diameter, it exhibits excellent HAZ toughness even in high heat input welding and excellent shear cutting ability. Thick steel plate was realized.

  The present inventors tried to achieve good HAZ toughness even in high heat input welding by refining Ti-based charcoal / nitride. The dispersion state of conventional Ti-based charcoal / nitride has been considered to be determined only by the balance of addition of Ti and N if the cooling rate during solidification of molten steel is constant. However, as a result of intensive studies by the present inventors, it is possible to finely disperse Ti-based carbon / nitride even with the same Ti and N addition amount by reducing the temperature range in the δ region represented in the phase diagram of steel. I found.

  The X value that defines the relationship of the above equation (2) is a function related to the temperature range in the δ region. The present inventors have tried to improve the HAZ toughness and found the relationship of the above formula (2). First, the background will be described. 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 the comprehensive 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 if the temperature range in the δ region is wide, the time during which the δ iron exists becomes longer, and coarse Ti-based carbon / nitride is likely to be formed. Therefore, the refinement of the Ti-based carbon / 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, an X value correlated with the temperature range of the δ region and expressed as a function of the chemical composition was determined:
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)

  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).

  In addition to the basic components (C, Si, Mn) of the thick steel plate of the present invention, the elements that define the X value include elements that are included as necessary (Nb, Cu, Ni, Cr, Mo, V, etc.) When these elements are not included, the X value is calculated assuming that these items are not present, and when these elements are included, the X value may be calculated from the above formula.

  Based on this idea, steel sheets with various X values were manufactured and investigated, and by increasing the X value, the average particle diameter of Ti-based charcoal / nitride can be refined and the HAZ toughness improved. I found out that I could make it.

  And it discovered that the low temperature toughness of a steel plate was further improved by increasing X value. This phenomenon is presumed to be due to a decrease in the average particle size of the Ti-based carbon / nitride by increasing the X value.

As described above, the thick steel plate of the present invention has a chemical component composition represented by the following formula (2):
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)
There is a major feature in that However, the present invention is limited to the above estimation reasons (decrease in the average particle diameter of charcoal / nitride due to decrease in temperature range of δ region, improvement of HAZ toughness and low temperature toughness due to decrease in average particle diameter, etc.) Rather, the scope of the invention is defined by the claims. That is, a thick steel plate that satisfies the structural requirements defined in the claims is included within the scope of the present invention.

  If the amount of each chemical component is within an appropriate range, the average particle diameter of the Ti-based carbon / nitride, the HAZ toughness, and the base material toughness improve as the X value increases. The lower limit of the X value is 40 (preferably 45, more preferably 50). The upper limit of the X value is determined from an appropriate amount of each chemical component and is 160 (preferably 100 or less, more preferably 75 or less). From the viewpoint of suppressing the formation of the hard phase MA structure (mixed structure of martensite-austenite), the preferable upper limit of the X value is 75.

In the thick steel plate of the present invention, the Ti-based carbon / nitride is made fine by adjusting the chemical composition so that the X value is 40 or more. However, when the balance between the Ti content and the N content is lost, the toughness of the steel sheet, particularly the HAZ toughness, is deteriorated. Specifically, when the ratio of Ti content [Ti] to N content [N] ([Ti] / [N]) exceeds 4.0, the Ti-based carbon / nitride becomes coarse, and the HAZ toughness Decreases. On the other hand, if it is less than 1.5, the low temperature toughness and the HAZ toughness are reduced by the influence of excess N. Therefore, in the thick steel plate of the present invention, in addition to the above formula (2) that defines the X value, the following formula (1):
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
One of the features is that the Ti content [Ti] and the N content [N] are balanced so as to satisfy the above. The preferable lower limit of [Ti] / [N] is 2.0, and the preferable upper limit is 3.5.

  From the viewpoint of toughness, the Ti-based carbon / nitride in the thick steel plate of the present invention is preferably fine. Therefore, the Ti-based carbon / nitride in the thick steel plate of the present invention is preferably 43 nm or less, more preferably 40 nm or less, and still more preferably 35 nm or less.

  The value of the average particle diameter of the Ti-based carbon / nitride in the present invention is a value measured as follows. First, a position at a depth t / 4 (t = plate thickness) as a portion representing the thermal history of a steel plate is observed with a transmission electron microscope (TEM) at an observation magnification of 60,000 times or more and an observation field of view of 2.0 μm × 2. Observation is performed under conditions of 0 μm or more and five or more observation places. Then, the area of each charcoal / nitride in the field of view is measured, and the equivalent circle diameter of each charcoal / nitride is calculated from this area. The value obtained by arithmetic average (arithmetic mean) of the equivalent circle diameters of each charcoal / nitride is defined as the average particle diameter of the Ti-based charcoal / nitride in the present invention.

  In addition, discrimination | determination whether it is Ti type | system | group charcoal / nitride is decided by the component used as the main body of each charcoal / nitride particle | grain. That is, “Ti-based charcoal / nitride” means that the ratio of Ti is 50% by mass or more when the total mass of the remaining elements excluding carbon and nitrogen is 100%. The amount of element can be determined by an energy dispersive X-ray detector (EDX). However, since too fine carbon / nitride cannot be measured, “Ti-based carbon / nitride” in the present invention is limited to 5 nm or more.

  A thick steel plate is required to have excellent shear cutting properties in addition to good HAZ toughness as described above. If there is a defect in the cut surface after shear cutting, it cannot be used as a product as it is, and care or recutting with a grinder or the like is required, leading to a decrease in yield and an increase in manufacturing cost. The present inventors have also studied from the viewpoint of achieving improved shear cutting performance. As a result, if the microstructure control is focused on the bainite block size (hereinafter referred to as “bainite block size”), the structure of the thick steel plate is mainly bainite (bainite fraction is 95 area% or more), It has been found that cutting properties can be improved.

  In the steel plate of the present invention, the average equivalent circle diameter of the bainite block size is set to 40 μm or less, and the difference between the maximum equivalent circle diameter of the bainite block size and the average equivalent circle diameter is controlled to 40 μm or less. By reducing the size, excellent shear cutting performance can be realized. The mechanism by which the shear cutting performance is improved by controlling the bainite structure has not been fully clarified, but the finer and more uniform the structure, the less the local (micro) stress concentration, and the material Since it itself becomes tough, it is estimated that it is less likely to break along the cleavage plane.

  In order to exert the above effects, the bainite block size needs to be 40 μm or less in terms of the average equivalent circle diameter, but the finer the bainite block size, the better, preferably 30 μm or less, more preferably 20 μm or less ( More preferably, it is 10 μm or less. In addition, the difference between the maximum equivalent circle diameter of the bainite block size and the average equivalent circle diameter is controlled to 40 μm or less from the viewpoint of reducing the variation in the structure, but the smaller the difference value, the better. The thickness is preferably 30 μm or less, more preferably 20 μm or less (more preferably 10 μm or less).

  The bainite block size was measured using an EBSP analyzer (Electron Backscatter Pattern Analyzer: “TexSEM” manufactured by Laboratories) and FE-SEM (electrolytic emission scanning electron microscope: “XL30S-FEG” manufactured by Philips). . The boundary (equivalent circle diameter) was measured using a boundary having an inclination angle of 15 ° or more as a bainite block. The measurement conditions at this time are as follows: measurement area: 250 μm × 250 μm, measurement pitch: 0.4 μm interval, and measurement points whose confidence index (Confidence Index) indicating the reliability of the measurement direction is smaller than 0.1 are analyzed. Excluded. In addition, those having a bainite block size of 2.0 mm or less were judged as measurement noise and excluded from the average calculation of the bainite block size.

  In the present invention, it is necessary to have a structure mainly composed of bainite. Further, by defining the bainite block size as described above, good shear cutting ability is exhibited. However, in order to exhibit such an effect, 100 area% does not necessarily need to be a bainite structure, and it is sufficient if the bainite fraction is 95 area% or more. Examples of structures other than bainite include martensite, ferrite, and pearlite. In the present invention, the bainite fraction was measured according to the following method.

[Measurement method of bainite fraction]
A 2 cm square test piece taken from the t / 4 (t: plate thickness) position of each steel plate is mirror-polished, etched with a nital etchant (2% nitric acid-ethanol solution), and the structure is observed with an optical microscope. The bainite fraction was calculated by an image analyzer (manufactured by Media Cybernetics: Imega-Pro Plus) for a photograph taken at (magnification: 100 times) and n = 10 (times). At this time, all lath structures other than ferrite were regarded as bainite.

  The thick steel plate of the present invention has a chemical composition that satisfies the relationship of the above formulas (1) and (2), and by controlling the bainite block size, it has excellent shear cutting properties as well as HAZ toughness. Become. However, even if these requirements are satisfied, the above effects cannot be achieved unless the content of each chemical component (each element) is within an appropriate range. Therefore, in the thick steel plate of the present invention, in addition to the above formulas (1) and (2) and the bainite block size satisfying the specified range, the amount of each chemical component is in an appropriate range as described below. It is also characterized by being inside. Hereinafter, chemical components will be described individually.

[C: 0.030 to 0.15%]
C is an element necessary for ensuring the strength of the steel sheet, and is an effective element for reducing the temperature range in the δ region in the steel phase diagram. If the C content is less than 0.030%, those effects are not exhibited. On the other hand, when the C content exceeds 0.15%, the hard second-phase MA structure is excessively increased, and the base material toughness and the HAZ toughness are lowered. Therefore, the C content is set to 0.030 to 0.15%. The preferable lower limit of the C content is 0.040% (more preferably 0.050% or more), and the preferable upper limit is 0.10% (more preferably 0.070% or less).

[Si: 1.0% or less (excluding 0%)]
Si is an element effective for securing the strength of the steel sheet, and for that purpose, it is preferable to contain 0.01% or more (more preferably 0.10% or more). However, when Si is excessively contained, a large amount of MA structure is generated, and the base material toughness and the HAZ toughness are lowered. Therefore, the upper limit thereof needs to be 1.0%. The upper limit with preferable Si amount is 0.8%, More preferably, it is 0.50%, More preferably, it is 0.40%.

[Mn: 0.8 to 2.0%]
Mn is an element effective for improving the hardenability and ensuring the strength of the steel sheet. When the Mn content is less than 0.8%, the effect of securing the strength is not sufficiently exhibited. On the other hand, if the Mn content exceeds 2.0%, the base metal toughness and the HAZ toughness are lowered. Therefore, the Mn content is set to 0.8 to 2.0%. The minimum with preferable Mn content is 1.00%, More preferably, it is 1.20%, More preferably, it is 1.50%. On the other hand, the preferable upper limit of the amount of Mn is 1.80%, more preferably 1.60%.

[P: 0.03% or less (excluding 0%)]
Since the impurity element P adversely affects the base material toughness and the HAZ toughness, the amount is preferably as small as possible. Therefore, the amount of P is 0.03% or less, preferably 0.010% or less. However, industrially, it is difficult to reduce the P content in steel to 0%.

[S: 0.01% or less (excluding 0%)]
S is an element that forms MnS and lowers the ductility, and the adverse effect is large particularly in high-strength steel. Therefore, the amount is preferably as small as possible. Therefore, the amount of S is 0.01% or less, preferably 0.005% or less. However, industrially, it is difficult to reduce the amount of S in steel to 0%.

[Al: 0.01 to 0.10%]
Al is an element having an effect of improving the base material toughness by deoxidation and refinement of the microstructure. In order to sufficiently exhibit such an effect, Al is contained in an amount of 0.01% or more. However, if Al is excessively contained, the base metal toughness and the HAZ toughness are lowered, so the upper limit is made 0.10%. The minimum with preferable Al content is 0.020%. On the other hand, the preferable upper limit is 0.060%, more preferably 0.040% or less.

[Ti: 0.015-0.03%]
Ti is an effective element for improving HAZ toughness by forming fine nitrides with N and suppressing the coarsening of austenite grains of HAZ during welding (so-called pinning effect). In order to sufficiently exhibit such an effect, Ti is contained by 0.015% or more. However, if the Ti content becomes excessive, the HAZ toughness deteriorates on the contrary, so the upper limit of the Ti content was set to 0.03%. The Ti content is preferably 0.017% or more and 0.020% or less.

[B: 0.0010 to 0.0035%]
B produces HAZ, especially in the vicinity of the bond part, in the vicinity of the bond part, and generates intragranular ferrite with BN as the nucleus, and also has a fixing action of solute N, which is important for improving HAZ toughness. It is an element. In this invention, in order to fully exhibit the effect, B is contained more than the content in a normal thick steel plate, 0.0010% or more. However, if the B content is excessive, a coarse bainite structure is formed during high heat input welding, and the HAZ toughness deteriorates. Therefore, the upper limit of the B content is set to 0.0035%. A preferable lower limit of the B content is 0.0015% (more preferably 0.0020% or more), and a preferable upper limit is 0.0030% (more preferably 0.0025% or less).

[N: 0.0050 to 0.01%]
N is an element that combines with Ti to form fine carbonitrides, suppresses the coarsening of austenite grains during high heat input welding, and improves the HAZ toughness. If the N content is too small, the above effect is not sufficiently exhibited, so the lower limit was set to 0.0050%. On the other hand, if the N content is excessive, the base material toughness and the HAZ toughness are adversely affected, so the upper limit was set to 0.01%. The minimum with preferable N content is 0.0055%, More preferably, it is 0.0060% or more. Moreover, the upper limit with preferable N content is 0.0090%, More preferably, it is 0.0080% or less.

[Ca: 0.005% or less (excluding 0%)]
Ca is an element having an effect of improving HAZ toughness. Specifically, Ca improves HAZ toughness by reducing anisotropy due to inclusion shape control of spheroidizing MnS. On the other hand, HAS toughness is improved by forming CaS and CaO and suppressing the coarsening of HAZ austenite grains. In order to sufficiently exhibit such an effect, it is preferable to contain 0.0005% or more of Ca in the steel sheet. However, if the Ca content is excessive, the base metal toughness and the HAZ toughness are deteriorated. Therefore, the upper limit is set to 0.005%. The upper limit with preferable Ca amount is 0.0030%, More preferably, it is 0.0025%.

[O: 0.01% or less (excluding 0%)]
O is an element that reacts with Al, Ca, Mg and the like to form a stable oxide at a high temperature and effectively works to prevent coarsening of the prior austenite grains of HAZ. Such an effect increases as the content increases. However, when the content is excessive, the cleanliness decreases and the HAZ toughness decreases. Therefore, the upper limit is set to 0.01%.

  The thick steel plate of the present invention basically comprises Fe and inevitable impurities in addition to the above components. However, the present invention does not exclude thick steel plates containing other elements, and within the scope of the present invention is a thick steel plate containing other component elements as long as the effects of the present invention are not impaired. Is also included.

  For example, in the thick steel plate of the present invention, in addition to the above components, if necessary, (a) Nb: 0.035% or less (excluding 0%), (b) Cu: 2.0% or less (0 %), Ni: 2.0% or less (not including 0%) and Cr: 2.0% or less (not including 0%), (c) Mo: 1.0% or less (not including 0%), (d) V: 0.1% or less (not including 0%), (e) one or more selected from the group consisting of Mg, Sr, and Ba 0.01% or less (not including 0%), (f) rare earth element: 0.01% or less (not including 0%), (g) one or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (excluding 0%), (h) Co: 2.5% or less (excluding 0%) and / or W: 2.5% or less (0% It is also effective to contain (not contained), etc., and the characteristics of the steel sheet are further improved according to the type of the ingredient to be contained.

[Nb: 0.035% or less (excluding 0%)]
Nb is an effective element for improving the hardenability of the substrate and increasing the strength of the steel sheet, and is contained as necessary. However, when the Nb content is excessive, the base material toughness and the HAZ toughness are lowered, so the upper limit was set to 0.035%. Nb is preferably contained in an amount of 0.005% or more, more preferably 0.010% or more in order to exert the effect. A more preferable upper limit of the Nb content is 0.025%, and further preferably 0.020% or less.

[Selected from the group consisting of Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) One or more
Cu, Ni, and Cr are all elements that increase the hardenability and contribute to the strength improvement, and can be added as necessary. Among these, Cu is considered to have the effect of reducing the temperature range in the δ region in the same manner as C and miniaturizing the Ti-based carbonitride. Ni is also an effective element for reducing the temperature range in the δ region. In order to sufficiently exhibit such an effect, it is recommended that the content thereof is preferably 0.20% or more, more preferably 0.40% or more. If these amounts are excessive, the base material toughness and the HAZ toughness tend to decrease, so the upper limit was set at 2.0%. Preferably it is 1.0% or less.

[Mo: 1.0% or less (excluding 0%)]
Mo is an element effective for improving hardenability and improving strength and preventing temper embrittlement, and can be added as necessary. In order to sufficiently exhibit such an effect, it is recommended that the Mo content is preferably 0.05% or more, more preferably 0.10% or more. However, when the Mo content is excessive, the base metal toughness and the HAZ toughness deteriorate, so the upper limit was set to 1.0%. The Mo content is more preferably 0.50% or less.

[V: 0.1% or less (excluding 0%)]
V is an element having an effect of enhancing hardenability and temper softening resistance by addition of a small amount, and can be added as necessary. In order to sufficiently exhibit such effects, it is recommended that the V amount is preferably 0.01% or more, more preferably 0.02% or more. However, if the amount of V is excessive, the base metal toughness and the HAZ toughness deteriorate, so the upper limit was set to 0.1%. The amount of V is preferably 0.05% or less.

[One or more selected from the group consisting of Mg, Sr, and Ba: 0.01% or less in total (not including 0%)]
Mg, Sr and Ba are effective elements for improving the HAZ toughness by generating fine oxides in the thick steel plate and suppressing the coarsening of the austenite grains of the HAZ. In order to fully exhibit such an effect, it is preferable to contain 0.0003% or more of these one or more (total). However, if these contents are excessive, the base metal toughness and the HAZ toughness are deteriorated, so the upper limit was made 0.01%. A more preferable upper limit is 0.0040%, still more preferably 0.0020%.

[Rare earth elements: 0.01% or less (excluding 0%)]
Rare earth elements (REM) exist as oxides or sulfides, have the effect of improving HAZ toughness by promoting the austenite grain refinement of HAZ and promoting ferrite transformation, and can be effectively utilized as necessary. However, if REM is excessively contained, the HAZ toughness is deteriorated, so the upper limit is preferably made 0.01%. A more preferable upper limit of REM is 0.003%. In addition, REM used by this invention contains all of the lanthanoid series rare earth elements, and what is necessary is just to contain these 1 or more types.

[One or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (excluding 0%)]
Zr, Ta, and Hf are elements that are effective in improving HAZ toughness because they form carbon and nitrides similarly to Ti and suppress the coarsening of austenite grains of HAZ during welding. In order to exert such effects sufficiently, it is preferable to contain one or more of the above elements in a total amount of 0.001% or more. However, if these contents are excessive, the base material toughness and the HAZ toughness are decreased. Therefore, when these elements are contained, the total amount is preferably 0.05% or less, and more preferably 0.03% or less.

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are elements having an effect of improving hardenability and increasing the strength of the steel sheet. In order to sufficiently exhibit such an effect, it is preferable to contain one or both of these at 0.2% or more. However, if these amounts are excessive, the base material toughness and the HAZ toughness deteriorate, so the upper limit of these amounts was set to 2.5%.

  In producing the thick steel plate of the present invention, the steel satisfying the requirements of the chemical composition, [Ti] / [N] and X value as described above is melted by an ordinary melting method, and the molten steel is cooled. For example, after heating to a range of 950 to 1300 ° C., hot rolling is performed, and the number of rolling passes in the temperature range of 850 ± 50 ° C. is set to four times or more (however, the reduction amount per pass is 5 mm or more), then, rolling may be finished so that the cumulative reduction ratio at 750 to 800 ° C. becomes 10 to 30%, and then the temperature range of 700 to 400 ° C. may be cooled at 5 ° C./second or more. .

  In the above production conditions, the number of rolling passes in the temperature range of 850 ± 50 ° C. is 4 times or more and 5 mm or more per pass from the viewpoint of miniaturization of bainite block size, and the number of rolling passes at this time Is less than 4 times, the bainite block size cannot be reduced to an average equivalent circle diameter of 40 μm or less. The cumulative rolling reduction at 750 to 800 ° C. is set to 10 to 30% from the viewpoint of reducing the bainite block size and the maximum value, and the rolling reduction at this time is less than 10%. The average equivalent circle diameter of bainite block size exceeds 40 μm, and when the rolling reduction exceeds 30%, the maximum equivalent circle diameter of bainite block size increases, and the difference between these (the maximum of bainite block size) The difference between the equivalent circle diameter and the average equivalent circle diameter) exceeds 40 μm.

  The temperature range of 700 to 400 ° C. is cooled at 5 ° C./second or more from the viewpoint of reducing the bainite block size by cooling the temperature range (700 to 400 ° C.) at which bainite is transformed as fast as possible. The average equivalent circle diameter of the bainite block size at which the cooling rate in this temperature range is less than 5 ° C./second exceeds 40 μm.

  Since the thick steel plate of the present invention controls the X value to narrow the temperature range in the δ region, the molten steel is cooled under normal conditions (for example, from 1500 ° C. to 1100 ° C. at 0.1 to 2.0 ° C. / By cooling at a cooling rate of seconds) to form a slab, a sufficiently small average particle diameter of Ti-based carbon / nitride can be formed. However, in order to form finer charcoal / nitrides, it is preferable to change the cooling water amount and cooling method of the casting machine to improve the cooling rate during solidification.

  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. However, the plate thickness of the steel plate of the present invention is preferably 20 mm or more, preferably 40 mm or more, and more preferably 60 mm or more. This is because the thick steel plate of the present invention exhibits good HAZ toughness even with high heat input or super high heat input welding where the heat input is 40 kJ / mm, so even if the plate thickness is thick, the heat input is increased. This is because it can be efficiently welded.

  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 as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having the compositions shown in the following Tables 1 to 4 were melted by a normal melting method, and this molten steel was cooled from 1500 ° C. to 1100 ° C. at a cooling rate of 0.1 to 2.0 ° C./min to form a slab. Thereafter, it was heated to 1100 ° C. and hot-rolled, and optionally tempered to produce a high-tensile steel sheet having a thickness of 40 mm. At this time, the number of rolling passes in the temperature range of 850 ± 50 ° C. (amount of reduction per pass: 4 mm for test No. 78, 10 mm for others), cumulative reduction rate at 750 to 800 ° C., and 700 to The cooling rate in the temperature range of 400 ° C. was controlled. Tables 1 to 4 below show [Ti] / [N] calculated from the chemical composition of the steel sheet, the X value, and the value of the temperature range in the δ region calculated from Thermo-calc (in the table, “δ region”). Description).

  About the steel plate produced as described above, the particle size (average, maximum) of Ti-based charcoal / nitride, the tensile strength of the steel plate, the base material toughness, the HAZ toughness and shear shearability are measured by the following methods, The area ratio of bainite was measured by the method described above. These results, together with the production method (number of rolling passes in the temperature range of 850 ± 50 ° C., cumulative rolling reduction at 750 to 800 ° C., and cooling rate in the temperature range of 700 to 400 ° C.), are shown in Tables 5 to 8 below. Shown in

[Average particle diameter of Ti-based charcoal / nitride]
The position at the depth t / 4 (t = plate thickness) was observed with a transmission electron microscope (TEM) under the conditions of an observation magnification of 60,000, an observation field of view of 2.0 μm × 2.0 μm, and five observation locations. Then, the area of each charcoal / nitride in the field of view was measured, and the equivalent circle diameter of each charcoal / nitride was calculated from this area. The circle equivalent diameter of each carbonitride was arithmetically averaged (arithmetic mean), and the average circle equivalent diameter of the Ti-based carbon / nitride in each steel sheet was calculated. Further, the maximum equivalent circle diameter of the bainite block size was selected, and the difference between the average equivalent circle diameters was obtained. (In Tables 5 to 8 below, “diameter difference (maximum-average)” is indicated.)

[Tensile strength of steel sheet]
At a depth t / 4 (t = plate thickness), a JIS No. 4 test piece was sampled so that the longitudinal direction of the test piece would be the plate width direction (C direction) of the steel plate, and a tensile test was performed to The strength was measured.

[Base material toughness]
At the position of depth t / 4 (t = plate thickness), V-notch standard test pieces defined in JIS Z 2242 are sampled so that the longitudinal direction of the test pieces is the rolling direction (L direction) of the steel sheet, A Charpy impact test (impact blade radius: 2 mm) was performed at the temperature, and the absorbed energy (vE _-40 ) at -40 ° C was measured.

[HAZ toughness]
Welding (electrogas arc welding) with a heat input of 40 kJ / mm was performed, and a JIS No. 4 specimen was taken from the site shown in FIG. 1 (the notch position was 0.5 mm HAZ side from the bond) and Charpy impact test (impact The blade radius was 2 mm), and the absorbed energy (vE- ₄₀ ) was measured. In the present invention, the absorbed energy (vE _-40 ) is 200 J or more.

[Evaluation of shear cutting ability]
The shear cutting performance is checked by examining the cut surface of the thick plate after cutting with magnetic powder deep scratches, and the shear cutting performance is good for those that do not have shear cutting cracks (shown as “O” in the table below). Was recognized as defective (indicated as “x” in the table below).

  From these results, it can be considered as follows. First, test no. 1 to 46 satisfy the requirements defined in the present invention, but it is understood that all the properties (tensile strength of steel sheet, base metal toughness, HAZ toughness and shear cutting property) are good.

  In contrast, test no. 47 to 78 lacks any of the requirements defined in the present invention, and any of the characteristics is deteriorated.

It is the schematic which shows the position which extract | collected the test piece for HAZ toughness (vE- ₄₀ ) measurement.

Claims (11)

C: 0.030 to 0.15% (meaning of mass%, the same shall apply hereinafter), Si: 1.0% or less (excluding 0%), Mn: 0.8 to 2.0%, P: 0.0. 03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, N: 0.0050 to 0.01%, Ca: 0.005% or less (not including 0%), O: 0.01% or less (not including 0%) Each of them is contained, the following formulas (1) and (2) are satisfied, the balance is composed of Fe and inevitable impurities, and the bainite fraction is 95 area% or more of the structure. The diameter is 40 μm or less, the maximum equivalent circle diameter of the bainite block size and the average equivalent circle diameter Steel plate for large heat input welding is excellent in shear cutting properties, wherein the difference is 40μm or less with.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)   The thick steel plate for high heat input welding according to claim 1, wherein the temperature range of the δ region is 40 ° C or lower.   The thick steel plate for high heat input welding according to claim 1 or 2, wherein an average particle diameter of the Ti-based carbon / nitride is 43 nm or less at a position at a depth t / 4 (t = plate thickness).   The thick steel plate for high heat input welding according to any one of claims 1 to 3, further comprising Nb: 0.035% or less (not including 0%).   Further, Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) The thick steel plate for high heat input welding according to any one of claims 1 to 4, comprising one or more selected.   Furthermore, it contains Mo: 1.0% or less (excluding 0%), The thick steel plate for high heat input welding in any one of Claims 1-5.   The thick steel plate for high heat input welding according to any one of claims 1 to 6, further comprising V: 0.1% or less (not including 0%).   Furthermore, 1 type or more chosen from the group which consists of Mg, Sr, and Ba: 0.01% or less in total (excluding 0%) is contained, The large input in any one of Claims 1-7 Thick steel plate for heat welding.   Furthermore, the thick steel plate for high heat input welding in any one of Claims 1-8 which contains rare earth elements: 0.01% or less (0% is not included).   Furthermore, 1 type or more chosen from the group which consists of Zr, Ta, and Hf: 0.05% or less (it does not contain 0%) in total is contained, The large input in any one of Claims 1-9 Thick steel plate for heat welding.   Furthermore, Co: 2.5% or less (0% is not included) and / or W: 2.5% or less (not including 0%) is contained. Heavy steel plate for high heat input welding.

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