JP2018009239A - Steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet excellent in base material strength and HAZ (Heat Affected Zone) toughness.SOLUTION: The steel sheet has a composition containing C:0.005 mass% to 0.07 mass%, Si:0 mass% to 0.04 mass%, Mn:1.4 mass% to 2.0 mass%, P:over 0 mass% and 0.010 mass% or less, S:over 0 mass% and 0.007 mass% or less, Al:0.010 mass% to 0.040 mass%, Ni:0.1 mass% to 1.50 mass%, Cu:0.1 mass% to 0.8 mass%, Nb:0.004 mass% to 0.025 mass%, Ti:0.010 mass% to 0.025 mass%, N:0.0040 mass% to 0.0080 mass%, Ca:0.0005 mass% to 0.0030 mass% and the balance:Fe with inevitable impurities, and a ratio of content of acid insoluble Ti to content of Ti based on whole composition is 0.80 or less.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate and a manufacturing method thereof.

  With the increase in size of welded structures in the shipbuilding field, the application of high-strength steel sheets having a plate thickness of 50 mm or more and a yield strength of 490 MPa or more is expanding. In such a steel plate, since the amount of heat input increases, the demand for large heat input welding is increased in combination with the viewpoint of improving welding construction efficiency. In large heat input welding, in a heat affected zone (HAZ), a coarse austenite structure is formed by being kept at the austenite temperature for a long time by heating, and along the former austenite grain boundary in the subsequent cooling process Coarse grain boundary ferrite and coarse grain boundary bainite are produced. As a result, the toughness in HAZ (hereinafter also referred to as “HAZ toughness”) cannot be obtained stably.

  For such inconvenience, methods for improving the HAZ toughness in high heat input welding by controlling the form of the Ti-containing nitride have been proposed (Japanese Patent Application Laid-Open Nos. 2010-95781 and 2011-21263). See the official gazette). However, in these methods, there is a limitation in the casting process for controlling the Ti-containing nitride, which may increase the production cost. Further, the strength of the base material is not taken into consideration.

  In addition, a method for improving the HAZ toughness in high heat input welding by controlling the size of oxide inclusions has been proposed (see Japanese Patent Application Laid-Open No. 2010-222652). However, this method requires a precise steelmaking process for controlling the oxide, which may increase the production cost. In this method, the strength of the base material is not taken into consideration.

  Furthermore, a method has been proposed in which the amount of Ca, S, O, etc. is controlled, and the HAZ structure is refined by introducing intragranular transformation nuclei to improve the HAZ toughness (see JP 2013-147740 A). . However, the yield strength of the base material obtained by this method is mostly insufficient at less than 490 MPa, while it is difficult to say that the HAZ toughness is sufficient when the yield strength is 490 MPa or more.

  In addition, a method has been proposed for realizing high base material strength and good HAZ toughness by adding Ti—Ca composite and optimizing the amount of CaO and CaS (Japanese Patent Laid-Open No. 2002-317243). However, in this method, the holding temperature before rolling is as high as 1150 to 1250 ° C., and there is room for improvement in productivity. Moreover, the characteristic in the thick steel plate of 50 mm or more thickness is not considered.

  Furthermore, a method for improving the HAZ toughness by controlling TiN precipitates of 0.1 μm or less has been proposed (see JP 2001-98340 A). However, the amount of heat input being studied is 450 kJ / cm at the maximum, and it cannot be said that the improvement in high heat input welding is sufficient. Further, the strength of the base material is not taken into consideration.

JP 2010-95781 A JP 2011-21263 A JP 2010-222652 A JP 2013-147740 A JP 2002-317243 A JP 2001-98340 A

  This invention is made | formed based on the above situations, and it aims at providing the steel plate excellent in base material strength and HAZ toughness, and its manufacturing method.

  As a result of intensive studies, the inventors have increased the amount of Ti added to the steel sheet, while increasing the fine TiN effective in improving the HAZ toughness, while reducing the HAZ toughness, for example, a coarse particle size of 2.0 μm or more. We found that TiN also increases. Therefore, the present inventors have reached the present invention that can improve the HAZ toughness during high heat input welding by reducing the amount of coarse TiN produced when the Ti addition amount is increased.

That is, the invention made in order to solve the above problems is as follows: C: 0.005 mass% to 0.07 mass%, Si: 0 mass% to 0.04 mass%, Mn: 1.4 mass% or more 2.0% by mass or less, P: more than 0% by mass and 0.010% by mass or less, S: more than 0% by mass and 0.007% by mass or less, Al: 0.010% by mass or more and 0.040% by mass or less, Ni: 0.1% by mass to 1.50% by mass, Cu: 0.1% by mass to 0.8% by mass, Nb: 0.004% by mass to 0.025% by mass, Ti: 0.010% by mass 0.025% by mass or less, N: 0.0040% by mass or more and 0.0080% by mass or less, Ca: 0.0005% by mass or more and 0.0030% by mass or less, and the balance: Fe and inevitable impurities. Having an acid-insoluble Ti content [mass%] [ins] l. Ti], a steel sheet that satisfies the following formula (1) when the Ti content [% by mass] based on the whole composition is [Ti].
[Insol. Ti] / [Ti] ≦ 0.80 (1)

  In the steel sheet, the ratio of the total Ti content and the content of acid-insoluble Ti mainly present as TiN is adjusted to a value satisfying the above formula (1), whereby fine TiN is coarse TiN. Therefore, the brittle fracture due to coarse TiN is suppressed, and the HAZ toughness is excellent. Moreover, the said steel plate is excellent also in base material strength by having the said composition.

When the content [% by mass] of N is [N], [Ti] / [N] is 2.0 or more and 5.0 or less, and the equivalent circle diameter of the TiN-containing precipitate is 0.040 μm or more and 1 μm or less. The number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 to 1 μm in a TiN-containing precipitate having a cross-sectional density of 2.0 × 10 ⁵ pieces / mm ² or more and an equivalent circle diameter of 0.040 μm to 1 μm is 15 % Or less. Thus, the [Ti] / [N], the cross-sectional density of the TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm, and the number ratio of the TiN-containing precipitates having an equivalent circle diameter of 0.1 μm to 1 μm are within the above range. By adjusting, the Ostwald growth of TiN at the time of high heat input welding is suppressed, and the coarsening of the prior austenite grain size is suppressed by TiN that remains undissolved after heat input. As a result, since the formation of coarse grain boundary ferrite and coarse grain boundary bainite is suppressed, the improvement of HAZ toughness can be promoted.

The contents [% by mass] of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are [C], [Si], [Mn], [Cu], [Ni], [Cr], In the case of [Mo], [V], and [B], the following formula (2) may be satisfied.
([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × (1 + 200 × [B]) × 1.115 ≧ 0.72 (2)

  By satisfy | filling the said Formula (2), base material intensity | strength can further be improved, maintaining HAZ toughness.

The contents [% by mass] of C, Mn, Cu, Ni, Cr, Mo and V are respectively [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V]. In this case, the following formula (3) should be satisfied.
110 × [C] + 7 × [Mn] + 4 × [Cu] + 5 × [Ni] + 2.8 × [Cr] + 5 × [Mo] + 7.2 × [V] ≦ 21.5 (3)

  By satisfy | filling said Formula (3), since the temperature of a solidus line rises and solidification completes early at the time of casting, reduction of coarse TiN can be achieved easily. As a result, the HAZ toughness can be further improved.

  Cr: more than 0% by mass and 1.00% by mass or less, Mo: more than 0% by mass and 0.50% by mass or less, V: more than 0% by mass and 0.50% by mass or less, B: more than 0% by mass and 0.0009% by mass Hereinafter, at least one of rare earth metals: more than 0% by mass and 0.0050% by mass or less and Zr: more than 0% by mass and 0.0050% by mass or less may be further contained. By further containing such an element, the strength of the base material can be further improved.

  Another invention made to solve the above problems is C: 0.005 mass% or more and 0.07 mass% or less, Si: 0 mass% or more and 0.04 mass% or less, Mn: 1.4 mass% or more 2.0% by mass or less, P: more than 0% by mass and 0.010% by mass or less, S: more than 0% by mass and 0.007% by mass or less, Al: 0.010% by mass or more and 0.040% by mass or less, Ni: 0.1% by mass to 1.50% by mass, Cu: 0.1% by mass to 0.8% by mass, Nb: 0.004% by mass to 0.025% by mass, Ti: 0.010% by mass 0.025% by mass or less, N: 0.0040% by mass or more and 0.0080% by mass or less, Ca: 0.0005% by mass or more and 0.0030% by mass or less, and the balance: Fe and inevitable impurities. A process of casting molten steel, and an ingot obtained by the casting process A step of hot rolling at a final rolling temperature of 750 ° C. or higher and 820 ° C. or lower; and a cooling step of cooling the steel material after the hot rolling step at a cooling rate of 5 ° C./second or more, A method for producing a steel sheet, wherein cooling from 500 ° C. to 1,450 ° C. is performed in less than 300 seconds.

  In the manufacturing method of the steel sheet, fine TiN is relatively increased with respect to coarse TiN by a casting process in which molten steel having the above composition is cooled from 1,500 ° C. to 1,450 ° C. in less than 300 seconds. A brittle fracture due to TiN is suppressed, and a steel sheet having excellent HAZ toughness can be produced. Moreover, in the manufacturing method of the said steel plate, the steel plate which is excellent also in base material strength can be obtained by performing hot rolling and cooling on the above-mentioned conditions.

  The molten steel is Cr: more than 0% by mass to 1.00% by mass or less, Mo: more than 0% by mass to 0.50% by mass or less, V: more than 0% by mass to 0.50% by mass or less, B: more than 0% by mass to 0% It is preferable to further contain at least one selected from the following: .0009 mass% or less, rare earth metal: more than 0 mass% to 0.0050 mass% or less, and Zr: more than 0 mass% to 0.0050 mass% or less. When the molten steel further contains such an element, the strength of the base material of the obtained steel plate can be further improved.

Still another invention made to solve the above problems is C: 0.005 mass% or more and 0.07 mass% or less, Si: 0 mass% or more and 0.04 mass% or less, Mn: 1.4 mass% 2.0% by mass or less, P: more than 0% by mass and 0.010% by mass or less, S: more than 0% by mass and 0.007% by mass or less, Al: 0.010% by mass to 0.040% by mass, Ni : 0.1 mass% or more and 1.50 mass% or less, Cu: 0.1 mass% or more and 0.8 mass% or less, Nb: 0.004 mass% or more and 0.025 mass% or less, Ti: 0.010 mass %: 0.025 mass% or less, N: 0.0040 mass% or more and 0.0080 mass% or less, Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: Fe and inevitable impurities N content based on the whole composition [mass% [N] and Ti content [% by mass] is [Ti], [Ti] / [N] is 2.0 or more and 5.0 or less, and the equivalent circle diameter is 0.040 μm or more and 1 μm or less. TiN-containing precipitates having a cross-sectional density of TiN-containing precipitates of 2.0 × 10 ⁵ pieces / mm ² or more and TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm. Is a steel sheet having a number ratio of 15% or less.

  The steel sheet has [Ti] / [N], the cross-sectional density of TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm, and the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm to 1 μm within the above range. By adjusting to, the Ostwald growth of TiN at the time of large heat input welding is suppressed, and the coarsening of the prior austenite grain size is suppressed by TiN that remains undissolved after heat input. As a result, the steel sheet is suppressed in the formation of coarse grain boundary ferrite and coarse grain boundary bainite during welding, and is excellent in HAZ toughness. Moreover, the said steel plate is excellent also in base material strength by having the said composition.

  Cr: more than 0% by mass and 1.00% by mass or less, Mo: more than 0% by mass and 0.50% by mass or less, V: more than 0% by mass and 0.50% by mass or less, B: more than 0% by mass and 0.0009% by mass Hereinafter, at least one of rare earth metals: more than 0% by mass and 0.0050% by mass or less and Zr: more than 0% by mass and 0.0050% by mass or less may be further contained. By further containing such an element, the strength of the base material can be further improved.

  Still another invention made to solve the above problems is C: 0.005 mass% or more and 0.07 mass% or less, Si: 0 mass% or more and 0.04 mass% or less, Mn: 1.4 mass% 2.0% by mass or less, P: more than 0% by mass and 0.010% by mass or less, S: more than 0% by mass and 0.007% by mass or less, Al: 0.010% by mass to 0.040% by mass, Ni : 0.1 mass% or more and 1.50 mass% or less, Cu: 0.1 mass% or more and 0.8 mass% or less, Nb: 0.004 mass% or more and 0.025 mass% or less, Ti: 0.010 mass %: 0.025 mass% or less, N: 0.0040 mass% or more and 0.0080 mass% or less, Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: Fe and inevitable impurities Obtained by the process of casting the molten steel having A step of hot rolling the ingot, and a cooling step of cooling the steel material after the hot rolling step at a cooling rate of 5 ° C./second or more. In the casting step, 1,500 ° C. to 1,450 ° C. Is cooled for less than 300 seconds, and is cooled from 1,300 ° C. to 1,200 ° C. for 450 seconds or more and 680 seconds or less. In the hot rolling step, the ingot before rolling is 1,050 ° C. or more and 1, The steel sheet is characterized in that it is held at 200 ° C. or lower for 20 minutes or longer and 5 hours or shorter, the cumulative rolling amount at 900 ° C. or higher is 30% or higher, and the cumulative rolling amount at 820 ° C. or higher and lower than 900 ° C. is 15% or higher. It is a manufacturing method.

  The method for producing the steel sheet is a constant that remains melted after high heat input welding by a casting process in which the molten steel having the above composition is cooled under the above conditions and a hot rolling process in which the steel sheet is rolled after maintaining the temperature of the ingot under the above conditions. Increase the number of TiN of the size. Thereby, the coarsening of the prior austenite grain boundaries is suppressed, and as a result, the formation of coarse grain boundary ferrite and coarse grain boundary bainite is suppressed. Can do. Moreover, in the manufacturing method of the said steel plate, the steel plate which is excellent also in base material strength can be obtained by performing hot rolling and cooling on the above-mentioned conditions.

  The molten steel is Cr: more than 0% by mass to 1.00% by mass or less, Mo: more than 0% by mass to 0.50% by mass or less, V: more than 0% by mass to 0.50% by mass or less, B: more than 0% by mass to 0% It is preferable to further contain at least one selected from the following: .0009 mass% or less, rare earth metal: more than 0 mass% to 0.0050 mass% or less, and Zr: more than 0 mass% to 0.0050 mass% or less. When the molten steel further contains such an element, the strength of the base material of the obtained steel plate can be further improved.

  Since the steel plate of the present invention is excellent in base material strength and HAZ toughness, it can be suitably used for large-sized welded structures. Moreover, the manufacturing method of the steel plate of this invention can obtain the steel plate which is excellent in base material strength and HAZ toughness.

  Hereinafter, an embodiment of a steel plate and a manufacturing method thereof according to the present invention will be described.

[First embodiment]
First, a first embodiment of the present invention will be described.

<Steel plate>
The steel sheet is C (carbon): 0.005 mass% to 0.07 mass%, Si (silicon): 0 mass% to 0.04 mass%, Mn (manganese): 1.4 mass% to 2 0.0 mass% or less, P (phosphorus): more than 0 mass% to 0.010 mass% or less, S (sulfur): more than 0 mass% to 0.007 mass% or less, Al (aluminum): 0.010 mass% or more 0 0.040 mass% or less, Ni (nickel): 0.1 mass% or more and 1.50 mass% or less, Cu (copper): 0.1 mass% or more and 0.8 mass% or less, Nb (niobium): 0.004 % By mass to 0.025% by mass, Ti (titanium): 0.010% by mass to 0.025% by mass, N (nitrogen): 0.0040% by mass to 0.0080% by mass, Ca (calcium) : 0.0005 mass% or more and 0.0030 mass% or less, and the balance Having a composition which is Fe (iron) and unavoidable impurities.

  Although it does not specifically limit as a minimum of the average thickness of the said steel plate, For example, it is 50 mm, and 60 mm is more preferable. On the other hand, although it does not specifically limit as an upper limit of the average thickness of the said steel plate, For example, it is 100 mm. If the average thickness of the steel sheet is smaller than the lower limit, it may be unsuitable for applications such as ships. Conversely, when the average thickness of the steel sheet exceeds the upper limit, processing or the like may be difficult.

[C (carbon)]
Hereinafter, each component of the steel sheet will be described. C is an element necessary for ensuring the strength of the steel sheet. The lower limit of the C content is 0.005% by mass, preferably 0.01% by mass, and more preferably 0.02% by mass. On the other hand, the upper limit of the C content is 0.07% by mass, preferably 0.06% by mass, and more preferably 0.05% by mass. When the C content is smaller than the lower limit, the strength of the steel sheet may be insufficient. On the other hand, when the C content exceeds the upper limit, the solidus temperature of the steel sheet is lowered, which promotes the formation of coarse TiN and may reduce the HAZ toughness.

[Si (silicon)]
Si is an element useful for deoxidation of the steel sheet. The lower limit of the Si content is 0% by mass. On the other hand, the upper limit of the Si content is 0.04% by mass, preferably 0.03% by mass, and more preferably 0.02% by mass. When the Si content exceeds the above upper limit, the Ti activity increases, so that the formation of coarse TiN is promoted, and the HAZ toughness may be reduced.

[Mn (manganese)]
Mn is an element necessary for ensuring the strength of the steel sheet. The lower limit of the Mn content is 1.4% by mass, preferably 1.50% by mass, and more preferably 1.60% by mass. On the other hand, the upper limit of the Mn content is 2.0% by mass, preferably 1.95% by mass, and more preferably 1.92% by mass. If the Mn content is smaller than the lower limit, the strength of the steel sheet may be insufficient. Conversely, when the Mn content exceeds the above upper limit, island-shaped martensite is generated in the HAZ of high heat input welding, and the hardness may be excessively increased and the toughness may be decreased.

[P (phosphorus)]
P is an element that is inevitably contained in the steel sheet and reduces the HAZ toughness. The content of P is more than 0% by mass. The smaller the content of P, the better, but it is difficult to make it 0% by mass industrially. On the other hand, the upper limit of the P content is 0.010% by mass, preferably 0.009% by mass, and more preferably 0.008% by mass. When content of P exceeds the said upper limit, there exists a possibility that the HAZ toughness of the said steel plate may fall.

[S (sulfur)]
S is an element that is inevitably contained in the steel sheet and reduces the HAZ toughness. The S content is more than 0% by mass. The smaller the S content, the better. However, it is difficult to make it 0% by mass industrially. On the other hand, the upper limit of the S content is 0.007% by mass, preferably 0.005% by mass, and more preferably 0.003% by mass. When content of S exceeds the said upper limit, there exists a possibility that the HAZ toughness of the said steel plate may fall.

[Al (aluminum)]
Al is an element necessary for deoxidation of the steel sheet. As a minimum of content of Al, it is 0.010 mass%, 0.015 mass% is preferable, and 0.020 mass% is more preferable. On the other hand, the upper limit of the Al content is 0.040% by mass, preferably 0.038% by mass, and more preferably 0.036% by mass. When the Al content is less than the lower limit, the oxygen concentration in the steel sheet increases, and the HAZ toughness may decrease due to an increase in oxide. On the other hand, when the Al content exceeds the above upper limit, the coarse oxide increases and the HAZ toughness may decrease.

[Ni (nickel)]
Ni is an element that contributes to improving the strength of the steel sheet. The lower limit of the Ni content is 0.1% by mass, preferably 0.15% by mass, and more preferably 0.20% by mass. On the other hand, the upper limit of the Ni content is 1.50% by mass, preferably 1.00% by mass, and more preferably 0.80% by mass. When the Ni content is smaller than the lower limit, the strength of the steel sheet may be reduced. Conversely, if the Ni content exceeds the above upper limit, the hardness may increase excessively and the toughness may decrease.

[Cu (copper)]
Cu is an element that contributes to improving the strength of the steel sheet. The lower limit of the Cu content is 0.1% by mass, preferably 0.12% by mass, and more preferably 0.15% by mass. On the other hand, the upper limit of the Cu content is 0.8 mass%, preferably 0.60 mass%, and more preferably 0.50 mass%. If the Cu content is less than the lower limit, the strength of the steel sheet may be reduced. Conversely, if the Cu content exceeds the upper limit, the hardness may increase excessively and the toughness may decrease.

[Nb (Niobium)]
Nb is an element necessary for ensuring the strength of the steel sheet. The lower limit of the Nb content is 0.004% by mass, preferably 0.006% by mass, and more preferably 0.007% by mass. On the other hand, the upper limit of the Nb content is 0.025% by mass, preferably 0.022% by mass, and more preferably 0.020% by mass. When content of Nb is smaller than the said minimum, there exists a possibility that the intensity | strength of the said steel plate may become inadequate. On the other hand, when the Nb content exceeds the above upper limit, island-shaped martensite is generated in the HAZ of the high heat input welding, and the hardness is excessively increased and the toughness may be decreased.

[Ti (titanium)]
Ti is an element that precipitates as TiN together with N, refines the HAZ structure of high heat input welding, and improves toughness. As a minimum of content of Ti, it is 0.010 mass%, 0.012 mass% is preferable, and 0.013 mass% is more preferable. On the other hand, the upper limit of the Ti content is 0.025% by mass, preferably 0.022% by mass, and more preferably 0.020% by mass. If the Ti content is smaller than the lower limit, the absolute amount of fine TiN is insufficient, and the HAZ toughness improving effect may be insufficient. On the other hand, when the Ti content exceeds the upper limit, solid solution Ti increases in HAZ, and a coarse bainite structure may be formed, making it impossible to ensure HAZ toughness.

[N (nitrogen)]
N is an element that precipitates as TiN together with Ti, refines the HAZ structure of high heat input welding, and improves toughness. The lower limit of the N content is 0.0040% by mass, preferably 0.0045% by mass, and more preferably 0.0050% by mass. On the other hand, the upper limit of the N content is 0.0080% by mass, preferably 0.0075% by mass, and more preferably 0.0070% by mass. When the N content is smaller than the lower limit, the effect of improving the HAZ toughness by fine TiN may be insufficient. On the other hand, when the N content exceeds the above upper limit, the solid solution N in the high heat input welding HAZ may increase, and the toughness may decrease.

[Ca (calcium)]
Ca is an element necessary for deoxidation of the steel sheet. The lower limit of the Ca content is 0.0005% by mass, preferably 0.0008% by mass, and more preferably 0.0010% by mass. On the other hand, the upper limit of the Ca content is 0.0030% by mass, preferably 0.0025% by mass, and more preferably 0.0022% by mass. When the content of Ca is smaller than the lower limit, generation of coarse TiN starting from oxide particles is promoted, and HAZ toughness may be reduced. On the other hand, when the Ca content exceeds the upper limit, the HAZ toughness may decrease due to an increase in coarse oxide.

  In addition to the above-mentioned composition, the steel sheet has Cr (chromium): more than 0% by mass and 1.00% by mass or less, Mo (molybdenum): more than 0% by mass and 0.50% by mass or less, and V (vanadium): 0% by mass. More than 0.50% by mass, B (boron): more than 0% by mass, 0.0009% by mass or less, rare earth metal: more than 0% by mass and 0.0050% by mass or less, and Zr (zirconium): more than 0% by mass. It is preferable to further contain at least one of 0050% by mass or less.

[Cr (chrome)]
Cr is an element that contributes to improving the strength of the steel sheet. In order to improve the strength, Cr is preferably contained in an amount of 0.01% by mass or more, and more preferably 0.05% by mass or more. On the other hand, the addition of Cr may excessively increase the HAZ hardness of high heat input welding and reduce toughness. Therefore, the upper limit of the Cr content is preferably 1.00% by mass, more preferably 0.50% by mass, and still more preferably 0.10% by mass.

[Mo (molybdenum)]
Mo is an element that contributes to improving the strength of the steel sheet. In order to improve the strength, it is preferable to contain Mo by 0.01% by mass or more, more preferably by 0.03% by mass or more, and further preferably by 0.05% by mass or more. On the other hand, the addition of Mo may increase the HAZ hardness of high heat input welding excessively and reduce toughness. Therefore, the upper limit of the Mo content is preferably 0.50% by mass, more preferably 0.30% by mass, and still more preferably 0.20% by mass.

[V (Vanadium)]
V is an element that contributes to improving the strength of the steel sheet. In order to improve the strength, V is preferably contained in an amount of 0.003% by mass or more, more preferably 0.02% by mass or more, and further preferably 0.05% by mass or more. On the other hand, the upper limit of the V content is preferably 0.50% by mass, more preferably 0.35% by mass, and still more preferably 0.15% by mass. When the content of V exceeds the above upper limit, the HAZ hardness of high heat input welding may excessively increase and the toughness may decrease.

[B (boron)]
B is an element that contributes to improving the strength and HAZ toughness of the steel sheet. In order to improve the strength, B is preferably contained in an amount of 0.0002% by mass or more, more preferably 0.0004% by mass or more, and further preferably 0.0005% by mass or more. On the other hand, the upper limit of the B content is preferably 0.0009% by mass, more preferably 0.0008% by mass, and still more preferably 0.0007% by mass. When the content of B exceeds the above upper limit, the toughness of the steel sheet may become unstable.

[Rare earth metal]
Rare earth metal is an element that contributes to deoxidation of the steel sheet, preferably 0.0003% by mass or more, more preferably 0.0010% by mass or more, and more preferably 0.0015% by mass or more. Is more preferable. On the other hand, the upper limit of the rare earth metal content is preferably 0.0050 mass%, more preferably 0.0040 mass%, and still more preferably 0.0030 mass%. When the content of the rare earth metal exceeds the above upper limit, the HAZ toughness may decrease due to an increase in the coarse oxide. Here, the “rare earth metal” means 15 lanthanoid elements from La (lanthanum) having an atomic number of 57 to Lu (lutetium) having an atomic number of 71, Sc (scandium), and Y (yttrium).

[Zr (zirconium)]
Zr is an element that contributes to deoxidation of the steel sheet, preferably 0.0003 mass% or more, more preferably 0.0008 mass% or more, and even more preferably 0.0010 mass% or more. preferable. On the other hand, the upper limit of the Zr content is preferably 0.0050% by mass, more preferably 0.0040% by mass, and still more preferably 0.0030% by mass. When the Zr content exceeds the above upper limit, the HAZ toughness may decrease due to an increase in coarse oxide.

[Remainder]
The steel sheet contains Fe (iron) and unavoidable impurities as the balance in addition to the elements described above. Examples of the inevitable impurities include Sn (tin), As (arsenic), Pb (lead), and the like.

The steel sheet has an acid-insoluble Ti content [mass%] [insol. When the Ti content [% by mass] based on the entire composition is [Ti], the following formula (1) is satisfied.
[Insol. Ti] / [Ti] ≦ 0.80 (1)

  As a right side of the said Formula (1), 0.77 is preferable and 0.75 is more preferable. [Insol. If Ti] / [Ti] exceeds these values, the HAZ toughness tends to decrease due to coarse TiN. [Insol. When steel materials having the same Ti] / [Ti] and different Ti addition amounts are compared, the larger the Ti addition amount, the larger the amount of coarse TiN produced. However, since the amount of Ti added is large, the amount of fine TiN that contributes to the refinement of the HAZ structure is also increasing [insol. If Ti] / [Ti] are equal, the HAZ toughness obtained will be substantially equivalent.

  Here, the content of acid-insoluble Ti was determined by conducting an electrolytic extraction method using an electrolytic solution on a test piece collected at a position of ¼ of the plate thickness in the thickness direction of the steel plate, The compound is extracted by filtration, and this compound is obtained by measuring the Ti content by, for example, ICP emission spectroscopy.

  The acid-insoluble Ti in the present invention is mainly present as TiN, but includes those present as other compounds such as Ti oxide. In addition, the acid-insoluble Ti contains most of the crystallized product generated in the molten steel in the casting process described later, but also includes a part of the precipitate generated in the solid iron.

  When the N content [% by mass] is [N], the lower limit of [Ti] / [N] is preferably 2.0, more preferably 2.5. On the other hand, the upper limit of [Ti] / [N] is preferably 5.0, and more preferably 4.5. If [Ti] / [N] is smaller than the above lower limit, the number of TiNs increases, but the size of TiN becomes small, and there is a possibility that the cross-sectional density of TiN-containing precipitates with a circle-equivalent diameter described later in a certain range is insufficient. is there. On the other hand, when [Ti] / [N] exceeds the above upper limit, the size of TiN, which is Ti diffusion-controlled growth, increases, and coarse TiN may increase.

In the steel sheet, the cross-sectional density of the TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm is 2.0 × 10 ⁵ pieces / mm ² or more, and the TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm. The number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is preferably 15% or less.

Further, the lower limit of the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm to 1 μm is more preferably 2.5 × 10 ⁵ pieces / mm ² , further 3.0 × 10 ⁵ pieces / mm ^2. preferable. If the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is smaller than the above lower limit, the fine TiN contributing to the suppression of coarsening of the prior austenite grain size is reduced, and the HAZ toughness is likely to be lowered. . On the other hand, the upper limit of the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is not particularly limited, but is, for example, 1.0 × 10 ⁶ pieces / mm ² .

  Furthermore, the upper limit of the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is more preferably 10%, and even more preferably 6%. When the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less exceeds the above upper limit, Ostwald growth at high temperature holding during welding is promoted, TiN disappears, and the prior austenite grain size becomes coarse. , HAZ toughness tends to decrease. On the other hand, the lower limit of the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is not particularly limited, and is substantially 0%.

Here, the cross-sectional density of TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm and the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm to 1 μm are values measured by the following methods. . First, an arbitrary part of the steel sheet is cut, and the obtained cut surface is observed with an electron microscope such as a transmission electron microscope (TEM). In the above observation, a Ti-containing precipitate is discriminated using an energy dispersive X-ray spectroscopy (EDX) apparatus or the like, and this is used as a TiN-containing precipitate. Next, the area of each TiN-containing precipitate in the observation field is measured by image analysis and converted into a circle-equivalent diameter. The number of TiN-containing precipitates having a circle-equivalent diameter of 0.040 μm to 1 μm and the circle-equivalent diameter 0 The number of TiN-containing precipitates of 1 μm or more and 1 μm or less is measured, the number per 1 mm ² is calculated, the cross-sectional density is obtained, and the number ratio is obtained from the ratio of the above numbers.

In the steel sheet, the contents [mass%] of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are respectively [C], [Si], [Mn], [Cu], [Ni], In the case of [Cr], [Mo], [V] and [B], it is preferable to satisfy the following formula (2).
Di = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni ]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × (1 + 200 × [B]) × 1.115 ≧ 0.72 ( 2)

  As a right side of the said Formula (2), 0.75 is more preferable and 0.77 is further more preferable. If Di is less than these values, the base material strength may be insufficient.

The contents [% by mass] of C, Mn, Cu, Ni, Cr, Mo and V are respectively [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V]. In this case, it is preferable to satisfy the following formula (3).
A = 110 × [C] + 7 × [Mn] + 4 × [Cu] + 5 × [Ni] + 2.8 × [Cr] + 5 × [Mo] + 7.2 × [V] ≦ 21.5 (3 )

  As a right side of the said Formula (3), 21.2 is more preferable and 21.0 is still more preferable. When A exceeds these values, coarse TiN tends to be generated due to a decrease in the solidus temperature, and the HAZ toughness may be insufficient.

[Usage]
Since the steel sheet is excellent in strength and HAZ toughness during high heat input welding, it can be suitably used for large-sized welded structures such as ships.

<Manufacturing method of steel plate>
As a method for producing the steel sheet of the first embodiment, for example, a casting process for casting molten steel, a hot rolling process for hot rolling the obtained ingot, and a process for cooling the steel material after hot rolling are performed. The method of providing etc. is mentioned. Hereinafter, each step will be described.

[Casting process]
In this step, molten steel having the above composition is cast into a slab shape or the like to obtain an ingot. The molten steel having the above composition can be obtained by appropriately combining conventionally known methods such as desulfurization treatment, deoxidation treatment, and addition of each element.

  In the casting process, the cooling process in the temperature range from 1,500 ° C. to 1,450 ° C. is performed with a cooling time of less than 300 seconds. Coarse TiN is generated in a temperature range (solid-liquid coexistence temperature range) in which the steel partially solidifies during the casting process. That is, in the process where the liquid phase iron is solidified to become solid, Ti is discharged from the solid iron to the liquid phase iron, and the Ti concentration in the liquid phase iron increases. TiN is likely to be produced in liquid phase iron with an increased Ti concentration, and in addition, TiN produced in the liquid phase is easily coarsened. Therefore, in order to reduce coarse TiN, it is important to quickly pass through the solid-liquid coexisting temperature region and suppress the formation and coarsening of TiN. Therefore, if the cooling time from 1,500 ° C. to 1,450 ° C. is 300 seconds or more, coarse TiN is produced during casting, and [insol. Ti] / [Ti] does not satisfy the above formula (1), and consequently the HAZ toughness decreases. The cooling time from 1,500 ° C. to 1,450 ° C. is more preferably less than 285 seconds. When the ingot is plate-shaped with a thickness of t [mm], the cooling temperature is a measurement temperature at a position of t / 4 in the thickness direction from the surface of the ingot.

[Hot rolling process]
In this step, a steel plate is obtained by hot rolling the ingot obtained in the casting step. The final rolling temperature of the ingot during the hot rolling is 750 ° C. or higher and 820 ° C. or lower. If the final rolling temperature is smaller than the above lower limit, the austenite grains become finer and ferrite precipitation in the subsequent cooling step is promoted, so that it may be difficult to obtain a predetermined strength. Conversely, if the final rolling temperature exceeds the above upper limit, the toughness of the steel material may be reduced.

[Cooling process]
After hot rolling, the steel material is cooled. The cooling rate is 5 ° C./second or more. If the cooling rate is lower than the lower limit, ferrite precipitates and it may be difficult to obtain a predetermined strength.

[Second Embodiment]
Next, a second embodiment of the present invention will be described.

<Steel plate>
The steel sheet is C (carbon): 0.005 mass% to 0.07 mass%, Si (silicon): 0 mass% to 0.04 mass%, Mn (manganese): 1.4 mass% to 2 0.0 mass% or less, P (phosphorus): more than 0 mass% to 0.010 mass% or less, S (sulfur): more than 0 mass% to 0.007 mass% or less, Al (aluminum): 0.010 mass% or more 0 0.040 mass% or less, Ni (nickel): 0.1 mass% or more and 1.50 mass% or less, Cu (copper): 0.1 mass% or more and 0.8 mass% or less, Nb (niobium): 0.004 % By mass to 0.025% by mass, Ti (titanium): 0.010% by mass to 0.025% by mass, N (nitrogen): 0.0040% by mass to 0.0080% by mass, Ca (calcium) : 0.0005 mass% or more and 0.0030 mass% or less, and the balance Having a composition which is Fe (iron) and unavoidable impurities.

  The average thickness of the steel plate can be the same as that of the steel plate of the first embodiment. Moreover, the preferable content of C, Si, Mn, P, S, Al, Ni, Cu, Nb, Ti, N, and Ca and the balance of the steel sheet can be the same as those of the steel sheet of the first embodiment. .

  In addition to the above-mentioned composition, the steel sheet has Cr (chromium): more than 0% by mass and 1.00% by mass or less, Mo (molybdenum): more than 0% by mass and 0.50% by mass or less, and V (vanadium): 0% by mass. More than 0.50% by mass, B (boron): more than 0% by mass, 0.0009% by mass or less, rare earth metal: more than 0% by mass and 0.0050% by mass or less, and Zr (zirconium): more than 0% by mass. It is preferable to further contain at least one of 0050% by mass or less. Moreover, preferable content of these compositions of the said steel plate can be made the same as that of the steel plate of said 1st embodiment.

  Moreover, as a minimum of [Ti] / [N] of the said steel plate, it is 2.0 and 2.5 is preferable. On the other hand, the upper limit of [Ti] / [N] is 5.0, and 4.5 is preferable. When [Ti] / [N] is less than the above lower limit, the number of TiNs increases, but the size of TiN decreases, and the cross-sectional density of TiN-containing precipitates having a circle-equivalent diameter in a certain range is likely to be insufficient. . On the other hand, when [Ti] / [N] exceeds the above upper limit, the size of TiN, which is Ti diffusion-controlled growth, increases, and coarse TiN tends to increase.

In the steel sheet, the cross-sectional density of the TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm is 2.0 × 10 ⁵ pieces / mm ² or more, and the TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm. The number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is 15% or less.

The lower limit of the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is preferably 2.5 × 10 ⁵ pieces / mm ^2, more preferably 3.0 × 10 ⁵ pieces / mm ^2. . If the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is smaller than the above lower limit, the fine TiN contributing to the suppression of the prior austenite grain size is reduced, and the HAZ toughness is likely to be lowered. On the other hand, the upper limit of the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is not particularly limited, but is, for example, 5.0 × 10 ⁵ pieces / mm ² .

  Furthermore, the upper limit of the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is preferably 10%, more preferably 6%. When the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less exceeds the above upper limit, the TiN Ostwald growth at high temperature holding during welding is promoted, and the TiN number contributing to the suppression of the prior austenite grain size is small. Thus, the HAZ toughness tends to decrease. On the other hand, the lower limit of the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is not particularly limited, and is substantially 0%.

[Usage]
Since the steel sheet is excellent in strength and HAZ toughness during high heat input welding, it can be suitably used for large-sized welded structures such as ships.

<Manufacturing method of steel plate>
As a manufacturing method of the steel sheet of the second embodiment, for example, a casting process for casting molten steel, a hot rolling process for hot rolling the obtained ingot, and a process for cooling the steel material after hot rolling are performed. The method of providing etc. is mentioned. Hereinafter, each step will be described.

[Casting process]
In this step, molten steel having the above composition is cast into a slab shape or the like to obtain an ingot. The molten steel having the above composition can be obtained by appropriately combining conventionally known methods such as desulfurization treatment, deoxidation treatment, and addition of each element.

  In the casting process, the cooling process in the temperature range from 1,500 ° C. to 1,450 ° C. is performed with a cooling time of less than 300 seconds. Coarse TiN is generated in a temperature range (solid-liquid coexistence temperature range) in which the steel partially solidifies during the casting process. That is, in the process where the liquid phase iron is solidified to become solid, Ti is discharged from the solid iron to the liquid phase iron, and the Ti concentration in the liquid phase iron increases. TiN is likely to be produced in liquid phase iron with an increased Ti concentration, and in addition, TiN produced in the liquid phase is easily coarsened. Therefore, in order to reduce coarse TiN, it is important to quickly pass through the solid-liquid coexisting temperature region and suppress the formation and coarsening of TiN. Therefore, if the cooling time from 1,500 ° C. to 1,450 ° C. is 300 seconds or more, coarse TiN is generated during casting, and the cross-sectional density of the TiN-containing precipitate having a circle-equivalent diameter of 0.040 μm to 1 μm is reduced. As a result, the HAZ toughness decreases. The cooling time from 1,500 ° C. to 1,450 ° C. is more preferably less than 285 seconds. When the ingot is plate-shaped with a thickness of t [mm], the cooling temperature is a measurement temperature at a position of t / 4 in the thickness direction from the surface of the ingot.

  Further, in the casting process, the cooling process in the temperature range from 1,300 ° C. to 1,200 ° C. is performed for 450 seconds or more and 680 seconds or less. The lower limit of the cooling treatment time in the above temperature range is preferably 500 seconds, and the upper limit is preferably 600 seconds. When the cooling treatment time is less than the lower limit, TiN-containing precipitates having an equivalent circle diameter of 0.040 μm or more and 1 μm or less are reduced, and HAZ toughness is lowered. On the other hand, when the cooling treatment time exceeds the upper limit, the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less increases due to an increase in TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more. , HAZ toughness decreases.

[Hot rolling process]
In this step, a steel plate is obtained by hot rolling the ingot obtained in the casting step. In this step, the ingot before rolling is held at 1,050 ° C. or more and 1,200 ° C. or less for 20 minutes or more and 5 hours or less before hot rolling. The lower limit of the holding time is preferably 2 hours. When the holding temperature or time is less than the above lower limit, TiN-containing precipitates having an equivalent circle diameter of 0.040 μm or more and 1 μm or less are reduced and HAZ toughness is lowered because minute TiN of less than 0.040 μm does not grow. On the other hand, when the holding temperature or time exceeds the upper limit, Ostwald growth proceeds excessively, TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm are reduced, and HAZ toughness is lowered.

In the hot rolling step, rolling is performed so that the cumulative reduction amount at 900 ° C. or more is 30% or more, and the cumulative reduction amount at 820 ° C. or more and less than 900 ° C. is 15% or more. Thereby, fine TiN grows by diffusion of Ti induced by strain, the number of TiN-containing precipitates having an equivalent circle diameter of 0.040 μm to 1 μm increases, and HAZ toughness can be improved. On the other hand, when rolling is performed out of the above range, the number of TiN-containing precipitates having an equivalent circle diameter of 0.040 μm or more and 1 μm or less decreases, and the HAZ toughness decreases. In addition, although the upper limit of the cumulative reduction amount in each temperature range is not specifically limited, For example, it is 50%. The “cumulative reduction amount” is the total sum of the reduction amounts per pass, and the “reduction amount” is a value calculated by the following equation (4).
Reduction amount = (t ₀ −t ₁ ) / t ₀ × 100 (4)
Wherein (4), t ₀ is start rolling the thickness of the steel strip when the temperature of the surface is in the rolling temperature range [mm], t ₁ is the rolling of the steel strip when the temperature of the surface is in the rolling temperature range The end thickness [mm] is shown respectively.

[Cooling process]
After hot rolling, the steel material is cooled. The cooling rate is 5 ° C./second or more, and preferably 6 ° C./second or more. If the cooling rate is lower than the lower limit, ferrite precipitates and it may be difficult to obtain a predetermined strength.

[Other Embodiments]
The steel plate and the manufacturing method thereof of the present invention are not limited to the above embodiment.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

<Production of steel sheet (1)>
Using a 150 kg vacuum induction furnace, molten steel having the composition shown in Table 1 was melted, and this molten steel was cast to produce a slab. Here, the cooling time from 1,500 ° C. to 1,450 ° C. was set to the time shown in Table 2. This slab was held at 1,100 ° C. for 3 hours, then hot-rolled at a final finishing temperature of 780 ° C., and water-cooled at a cooling rate of 7.5 ° C./second, so that Examples 1 to 10 having an average thickness of 65 mm and comparative examples 1 to 6 steel plates were obtained.

<[Insol. Measurement of Ti]>
By collecting a test piece at a position of 1/4 of the plate thickness in the thickness direction of the obtained steel sheet, and measuring the concentration of acid-insoluble Ti extracted from the test piece by an electrolytic extraction method using an electrolytic solution. [Insol. Ti] was measured. As the electrolytic solution, an electrolyte solution containing 2 cc of triethanolamine and 1 g of tetramethylammonium chloride in 100 cc of methanol was used. In the measurement, a solution obtained by electrolytic extraction with the above electrolytic solution was filtered through a filter having a pore size of 2.0 μm to obtain a residue, and then the residue was analyzed for chemical components by ICP emission spectroscopy. . Ti] was determined. It should be noted that Ti that is insoluble in the electrolytic extraction with the electrolytic solution can be determined to be acid-insoluble Ti as defined in the present invention. The measurement results are shown in Table 2.

<Evaluation of steel sheet (1)>
The yield strength and HAZ toughness of each steel plate were evaluated by the following methods. The evaluation results are shown in Table 2. In Table 2, “Di” indicates the left side of the formula (2), and “A” indicates the left side of the formula (3).

[Yield strength]
A bar-shaped No. 4 test piece defined in JIS-Z2241: 2011 was cut out from each steel plate. In this cutting, the axial direction of the test piece coincided with the width direction of the steel sheet, and the distance between the central axis of the test piece and one surface of the steel sheet was set to ¼ of the plate thickness of the steel sheet. Next, a tensile test was performed by the method described in JIS-Z2241: 2011, and the yield strength YS [MPa] was measured. The yield strength indicates that the larger the value is, the better the strength is. It can be judged that 490 MPa or more is “good” and less than 490 MPa is “not good”.

[HAZ toughness]
A test piece of 12.5 mm × 32 mm × 55 mm was cut out from a position ¼ of the plate thickness in the thickness direction of each steel plate, held at 1400 ° C. for 60 seconds, and then the cooling time from 800 ° C. to 500 ° C. was 400. The cooling was performed by controlling the speed so as to be 2 seconds. This is a thermal cycle that simulates large heat input welding with a heat input of 55 kJ / mm. Next, in accordance with JIS-Z2242: 2005, three specified Charpy impact test pieces were sampled, subjected to a Charpy impact test at -20 ° C., and the absorbed energy vE [J] was measured. With regard to the HAZ toughness, it can be judged that the case where vE exceeds 100 J is “good”, and the case where 100 J or less is “not good”.

  As is clear from Tables 1 and 2, the steel sheets of Examples 1 to 10 both had good yield strength and HAZ toughness.

  In addition, Examples 2 and 6 to 9 having Di of 0.72 or more were superior in yield strength to Examples 1, 3 to 5, and 10. From this, it is determined that the base material strength can be improved while maintaining the HAZ toughness by setting the value of Di to 0.72 or more.

  Furthermore, Examples 3 to 5 and 10 having A of 21.5 or less were superior in HAZ toughness compared to Examples 1 and 2. From this, it is determined that the HAZ toughness can be further improved by setting the value of A to 21.5 or less.

  Furthermore, Examples 3 to 5 having a Cr content of 0.10% by mass or less and a Mo content of 0.20% by mass or less were superior in HAZ toughness as compared with Examples 6 to 9. From this, it is judged that HAZ toughness can be improved more by making Cr content 0.10 mass% or less and Mo content 0.20 mass% or less.

  On the other hand, Examples 6-9 whose Cr content exceeds 0.10 mass% or Mo content exceeds 0.20 mass% were excellent in yield strength compared with Examples 3-5. From this, it is judged that the yield strength can be further improved by setting the Cr content to over 0.10 mass% or the Mo content to over 0.20 mass%.

  On the other hand, Comparative Examples 1 to 4 are [insol. Since Ti] / [Ti] is more than 0.8, coarse TiN increases and the HAZ toughness is not good. Moreover, Comparative Example 5 is [insol. Ti] / [Ti] is 0.8 or less, but the content of Ti is too large, so that the solid solution Ti increases and the HAZ toughness is not good. Furthermore, Comparative Example 6 is also [insol. Ti] / [Ti] is 0.8 or less, but since the contents of Ti and N are small, the absolute amount of fine TiN is insufficient and the HAZ toughness is not good.

<Production of steel plate (2)>
Next, using a 150 kg vacuum induction furnace, molten steel having the composition shown in Table 3 was melted, and this molten steel was cast to produce a slab. Here, the cooling time from 1,500 ° C. to 1,450 ° C. and the cooling time from 1,300 ° C. to 1,200 ° C. were the times shown in Table 4. After holding this slab at the temperature and time shown in Table 4, it was hot-rolled at 900 ° C. or more and 820 ° C. or more and less than 900 ° C. to obtain the cumulative reduction amount shown in Table 4, respectively, and further water-cooled at the cooling rate shown in Table 4 As a result, steel plates of Examples 11 to 20 and Comparative Examples 7 to 15 having an average thickness of 65 mm were obtained. In addition, “REM” in Table 3 represents rare earth metal.

<Measurement of TiN-containing precipitate>
A test piece was collected at a position of ¼ of the plate thickness in the thickness direction of the obtained steel plate, and a columnar test piece was cut out from each steel plate. In this cutting, the axial direction of the test piece coincides with the rolling direction of the steel sheet, the distance between the central axis of the test piece and one surface of the steel sheet is 1/4 of the average thickness of the steel sheet, and one of the test pieces The bottom surface of the steel plate was a longitudinal section of the steel plate. Next, a replica TEM test piece was created from the bottom surface corresponding to the longitudinal section of the steel sheet of the test piece, and observed with a transmission electron microscope (TEM). The observation conditions were an observation magnification of 15,000 times, an observation visual field of 52.7 μm ^2, and two visual fields were observed. In the observation, a precipitate containing Ti was determined by an energy dispersive X-ray fluorescence (EDX) apparatus, and this precipitate was used as a TiN-containing precipitate. Next, the area of each TiN-containing precipitate in the observation field is measured by image analysis and converted into a circle-equivalent diameter. The number of TiN-containing precipitates having a circle-equivalent diameter of 0.040 μm to 1 μm and the circle-equivalent diameter 0 The number of TiN-containing precipitates of 1 μm or more and 1 μm or less is measured, and the number of TiN-containing precipitates having a circle equivalent diameter of 0.1 μm or more and 1 μm or less is obtained by calculating the number per 1 mm ^2. Asked. Table 4 shows the measurement results.

<Evaluation of steel sheet (2)>
By the above method, the yield strength and the HAZ toughness of each steel plate were evaluated. The evaluation results are shown in Table 4.

  As is clear from Tables 3 and 4, the steel sheets of Examples 11 to 20 had good yield strength and HAZ toughness.

  Further, when the examples are compared, the smaller the number ratio of the TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less, the more excellent the HAZ toughness tends to be, and in particular, Examples 11 and 13 in which the number ratio is 6% or less. 14 and 18 have better HAZ toughness than the other examples.

  On the other hand, since the compositions of Comparative Examples 7 to 10 do not satisfy the scope of the present invention, either yield strength or HAZ toughness is not good. In Comparative Examples 11, 13, and 14, the cross-sectional density of the TiN-containing precipitate having an equivalent circle diameter of 0.040 μm or more and 1 μm or less is insufficient, so that the HAZ toughness is not good. Comparative Example 11 is that the cooling time from 1,500 ° C. to 1,450 ° C. is too long, Comparative Example 13 is that the cumulative reduction amount is 900 ° C. or higher, and Comparative Example 14 is 820 ° C. or higher and lower than 900 ° C. It is considered that the cross-sectional density is insufficient because the cumulative reduction amount of is too small. Furthermore, in Comparative Example 12, the number ratio of TiN-containing precipitates having an equivalent circle diameter of 0.1 μm or more and 1 μm or less is too large, so that the HAZ toughness is not good. In Comparative Example 12, it is considered that the number ratio increased because the cooling time from 1,300 ° C. to 1,200 ° C. was too long.

  The steel plate of the present invention is excellent in base material strength and HAZ toughness. Moreover, the manufacturing method of the steel plate of this invention can obtain the steel plate which is excellent in base material strength and HAZ toughness.

Claims (11)

C: 0.005 mass% or more and 0.07 mass% or less,
Si: 0% by mass or more and 0.04% by mass or less,
Mn: 1.4 mass% or more and 2.0 mass% or less,
P: more than 0% by mass and 0.010% by mass or less,
S: more than 0% by mass and 0.007% by mass or less,
Al: 0.010 mass% or more and 0.040 mass% or less,
Ni: 0.1% by mass or more and 1.50% by mass or less,
Cu: 0.1 mass% or more and 0.8 mass% or less,
Nb: 0.004 mass% or more and 0.025 mass% or less,
Ti: 0.010 mass% or more and 0.025 mass% or less,
N: 0.0040 mass% or more and 0.0080 mass% or less,
Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: Fe and a composition that is an inevitable impurity,
The content [mass%] of the acid-insoluble Ti is changed to [insol. Ti], a steel sheet that satisfies the following formula (1) when the Ti content [% by mass] based on the entire composition is [Ti].
[Insol. Ti] / [Ti] ≦ 0.80 (1) When the content [% by mass] of N is [N], [Ti] / [N] is 2.0 or more and 5.0 or less,
The equivalent circle diameter of a TiN-containing precipitate having a cross-sectional density of 2.0 × 10 ⁵ pieces / mm ² or more and an equivalent circle diameter of 0.040 μm to 1 μm is 0. The steel sheet according to claim 1, wherein the number ratio of TiN-containing precipitates of 1 µm or more and 1 µm or less is 15% or less. The contents [% by mass] of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are [C], [Si], [Mn], [Cu], [Ni], [Cr], The steel plate according to claim 1 or 2, wherein the following formula (2) is satisfied when [Mo], [V], and [B] are set.
([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × (1 + 200 × [B]) × 1.115 ≧ 0.72 (2) The contents [% by mass] of C, Mn, Cu, Ni, Cr, Mo and V are respectively [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V]. The steel plate of Claim 1, Claim 2, or Claim 3 which satisfy | fills following formula (3) when it does.
110 × [C] + 7 × [Mn] + 4 × [Cu] + 5 × [Ni] + 2.8 × [Cr] + 5 × [Mo] + 7.2 × [V] ≦ 21.5 (3) Cr: more than 0% by mass and 1.00% by mass or less,
Mo: more than 0% by mass and 0.50% by mass or less,
V: more than 0% by mass and 0.50% by mass or less,
B: more than 0% by mass and 0.0009% by mass or less,
The rare earth metal: more than 0% by mass and 0.0050% by mass or less, and Zr: more than 0% by mass and 0.0050% by mass or less, further containing at least one kind. Steel plate. C: 0.005 mass% or more and 0.07 mass% or less,
Si: 0% by mass or more and 0.04% by mass or less,
Mn: 1.4 mass% or more and 2.0 mass% or less,
P: more than 0% by mass and 0.010% by mass or less,
S: more than 0% by mass and 0.007% by mass or less,
Al: 0.010 mass% or more and 0.040 mass% or less,
Ni: 0.1% by mass or more and 1.50% by mass or less,
Cu: 0.1% by mass or more and 0.8% by mass or less,
Nb: 0.004 mass% or more and 0.025 mass% or less,
Ti: 0.010 mass% or more and 0.025 mass% or less,
N: 0.0040 mass% or more and 0.0080 mass% or less,
Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: a step of casting molten steel having a composition that is Fe and inevitable impurities;
Hot rolling the ingot obtained in the casting step at a final rolling temperature of 750 ° C. or higher and 820 ° C. or lower;
A cooling step of cooling the steel material after the hot rolling step at a cooling rate of 5 ° C / second or more,
A method for producing a steel sheet, wherein cooling from 1,500 ° C. to 1,450 ° C. is performed in less than 300 seconds in the casting step. The molten steel is
Cr: more than 0% by mass and 1.00% by mass or less,
Mo: more than 0% by mass and 0.50% by mass or less,
V: more than 0% by mass and 0.50% by mass or less,
B: more than 0% by mass and 0.0009% by mass or less,
The method for producing a steel sheet according to claim 6, further comprising at least one of rare earth metals: more than 0% by mass and 0.0050% by mass or less, and Zr: more than 0% by mass and 0.0050% by mass or less. C: 0.005 mass% or more and 0.07 mass% or less,
Si: 0% by mass or more and 0.04% by mass or less,
Mn: 1.4 mass% or more and 2.0 mass% or less,
P: more than 0% by mass and 0.010% by mass or less,
S: more than 0% by mass and 0.007% by mass or less,
Al: 0.010 mass% or more and 0.040 mass% or less,
Ni: 0.1% by mass or more and 1.50% by mass or less,
Cu: 0.1% by mass or more and 0.8% by mass or less,
Nb: 0.004 mass% or more and 0.025 mass% or less,
Ti: 0.010 mass% or more and 0.025 mass% or less,
N: 0.0040 mass% or more and 0.0080 mass% or less,
Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: Fe and a composition that is an inevitable impurity,
[Ti] / [N] is 2.0 or more when the content [% by mass] of N based on the whole composition is [N] and the content [% by mass] of Ti is [Ti]. 0 or less,
The equivalent circle diameter of a TiN-containing precipitate having a cross-sectional density of 2.0 × 10 ⁵ pieces / mm ² or more and an equivalent circle diameter of 0.040 μm to 1 μm is 0. A steel sheet in which the number ratio of TiN-containing precipitates of 1 μm or more and 1 μm or less is 15% or less. Cr: more than 0% by mass and 1.00% by mass or less,
Mo: more than 0% by mass and 0.50% by mass or less,
V: more than 0% by mass and 0.50% by mass or less,
B: more than 0% by mass and 0.0009% by mass or less,
The steel sheet according to claim 8, further comprising at least one of rare earth metals: more than 0% by mass and 0.0050% by mass or less, and Zr: more than 0% by mass and 0.0050% by mass or less. C: 0.005 mass% or more and 0.07 mass% or less,
Si: 0% by mass or more and 0.04% by mass or less,
Mn: 1.4 mass% or more and 2.0 mass% or less,
P: more than 0% by mass and 0.010% by mass or less,
S: more than 0% by mass and 0.007% by mass or less,
Al: 0.010 mass% or more and 0.040 mass% or less,
Ni: 0.1% by mass or more and 1.50% by mass or less,
Cu: 0.1% by mass or more and 0.8% by mass or less,
Nb: 0.004 mass% or more and 0.025 mass% or less,
Ti: 0.010 mass% or more and 0.025 mass% or less,
N: 0.0040 mass% or more and 0.0080 mass% or less,
Ca: 0.0005 mass% or more and 0.0030 mass% or less, and the balance: a step of casting molten steel having a composition that is Fe and inevitable impurities;
Hot rolling the ingot obtained in the casting process,
A cooling step of cooling the steel material after the hot rolling step at a cooling rate of 5 ° C / second or more,
In the casting step, cooling from 1,500 ° C. to 1,450 ° C. is performed in less than 300 seconds, cooling from 1,300 ° C. to 1,200 ° C. is performed in 450 seconds to 680 seconds,
In the hot rolling step, the ingot before rolling is held at 1,050 ° C. or more and 1,200 ° C. or less for 20 minutes or more and 5 hours or less, and the cumulative reduction amount at 900 ° C. or more is 30% or more, 820 ° C. or more and 900 The manufacturing method of the steel plate characterized by making the cumulative reduction amount below 15 degreeC into 15% or more. The molten steel is
Cr: more than 0% by mass and 1.00% by mass or less,
Mo: more than 0% by mass and 0.50% by mass or less,
V: more than 0% by mass and 0.50% by mass or less,
B: more than 0% by mass and 0.0009% by mass or less,
The method for producing a steel sheet according to claim 10, further comprising at least one of rare earth metals: more than 0% by mass and 0.0050% by mass or less, and Zr: more than 0% by mass and 0.0050% by mass or less.