JP6226542B2 - Steel with excellent toughness in weld heat affected zone - Google Patents

Description

  The present invention relates to a thick steel plate suitable for use in welded structures such as shipbuilding, construction, and bridges, and more particularly to a steel material that realizes excellent HAZ toughness even in high heat input welding.

In recent years, structures using steel materials, such as bridges, high-rise buildings, and large ships, have tended to increase in size. To realize such large structures, it is desirable to increase the strength and thickness of steel materials. ing. At the same time, for the purpose of improving the construction efficiency of large structures and reducing the construction cost, there is a demand for improvement in welding efficiency when welding high-strength and thick steel materials.
By the way, in order to improve the welding efficiency of steel materials, it is effective to reduce the number of times of welding to the same location, and a large amount of heat is applied to the steel material rather than repeating the welding to the steel material several times with a small amount of heat (welding heat). High-efficiency welding with high heat input that completes welding at a time by performing welding with high heat input to add (welding heat) is directed.

  However, in general, a weld heat affected zone (hereinafter also referred to as HAZ) that is exposed to welding heat regardless of the amount of heat becomes high temperature during welding, and the crystal grains of the steel material tend to be coarse. In addition, as the amount of heat input to the steel material increases, the weld heat affected zone becomes higher in temperature and the cooling time becomes longer. A high temperature and a long cooling time are conditions that promote the formation of a brittle upper bainite structure in the heat affected zone and the formation of an embrittled structure such as island martensite, and thus reduce the HAZ toughness of the steel material. That is already known.

In order to cope with the decrease in the HAZ toughness caused by the welding heat as described above, techniques disclosed in Patent Documents 1 to 5 have been proposed.
Patent document 1 aims at providing the steel material excellent in the welding heat affected zone toughness (HAZ toughness), and a manufacturing method. Specifically, the manufacturing method disclosed in Patent Document 1 disperses fine sulfides by adding Mg and / or REM in addition to Ca having strong sulfide-forming ability in molten steel to produce fine oxides. The HAZ structure heated to 1400 ° C. or higher is refined and good HAZ toughness is achieved even with high heat input welding of 200 kJ / cm or higher.

  Patent Document 2 aims to provide a high-strength welded structural steel excellent in base metal toughness and welded portion HAZ toughness and a method for producing the same. Specifically, the manufacturing method disclosed in Patent Document 2 includes adding one or more of Mg, Ca, and REM after adding Ti, or adding one or more of Ti, Mg, Ca, and REM. Is intended to refine the heated γ grain size of the base material by finely dispersing oxides and sulfides at the same time, and to further refine the heated γ grain size of HAZ regardless of welding heat input. Yes. In Patent Document 2, as an effect of these two miniaturizations, high strength welded structural steel having good base metal toughness and welded portion HAZ toughness can be manufactured.

  Patent document 3 aims at providing the thick steel plate excellent in the welding heat affected zone toughness of super-high heat input welding, and its manufacturing method. Specifically, the production method of Patent Document 3 controls the form of dendrites formed in the solidification process in addition to adjusting the particle composition of oxides, sulfides, etc. in the molten steel. As a result, Patent Document 3 disperses dispersed particles in a steel sheet more uniformly and finely than in the past, and refines austenite grains even in HAZ during super-high heat input welding where the heat input is 300 kJ / cm or more. It is said that the HAZ toughness can be significantly improved.

  Patent Document 4 aims to provide a high-strength steel sheet of API standard X100 or higher having good HAZ toughness. Specifically, the high-strength steel sheet of Patent Document 4 is limited to the amount of Ti, Mg, REM, Al, S, and N, (1) TiN-based fine precipitation containing Mg-based oxides of 0.1 μm or less. The inclusion of the material suppresses the coarsening of γ grains even in the vicinity of the melting line. Furthermore, this high-strength steel sheet (2) contains a complex of an oxide mainly composed of Ti, Mg, and REM of 0.1 μm or more and MnS to generate IGF from a relatively small γ grain. It is said that the HAZ toughness can be improved by refining the structure throughout the entire HAZ.

Patent Document 5 aims to propose a non-tempered high-tensile steel material having both good base material toughness and weld heat-affected zone toughness. Specifically, the non-tempered high-tensile steel material of Patent Document 5 has an optimal composition range of the oxide-based inclusions as follows: Ti oxide: 20 to 90 wt%, CaO and REM oxide total: 5 to 50 Weight%, Al ₂ O ₃ : Controlled to 70% by weight or less. As a result, non-tempered high-tensile steel can effectively utilize the grain coarsening suppression ability (pinning effect) of inclusions without causing nozzle clogging or generation of harmful inclusion clusters. It is said that the toughness can be improved, and further, the toughness and strength of the base material can be improved by optimally dispersing TiN or VN.

Japanese Patent No. 4261968 Japanese Patent No. 4762450 Japanese Patent No. 4039223 Japanese Patent Laid-Open No. 11-264048 Japanese Patent No. 4144121

As described above, each of Patent Documents 1 to 5 discloses that it is possible to cope with a decrease in the HAZ toughness caused by welding heat. It is difficult to improve the HAZ toughness.
The techniques disclosed in Patent Documents 1 to 3 are intended to refine the HAZ structure by the pinning effect of oxysulfides, but do not mention the structure refinement effect due to intragranular transformation caused by oxides. It cannot be said that it is a technology that can cope with further increase in heat input.

In addition, Patent Document 4 mentions a structural transformation starting from an oxide, but does not show a countermeasure against the coarse oxide and the like, and eliminates the possibility that the HAZ toughness is lowered due to the formation of the coarse oxide. It cannot be said that it is a technology that can cope with further increase in heat input.
Furthermore, the technique disclosed in Patent Document 5 is a technique for refining the HAZ structure by the pinning effect of oxysulfide, but it is not a technique that takes account of the structure transformation control starting from oxysulfide. It cannot be said that this technology can cope with large heat input.

  This invention is made | formed in view of the above-mentioned problem, and it aims at providing the steel material excellent in the toughness (HAZ toughness) of the welding heat affected zone at the time of high heat input welding.

In order to achieve the above object, the present invention has taken the following measures.
That is, technical means for solving the problems in the present invention are: C: 0.02 to 0.13% (meaning mass%), Si: 0.05 to 0.00. 5% or less, Mn: 1.0 to 2.5%, P: 0.03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.002 ~ 0.040% or less, Ti: 0.005 to 0.040%, Zr: 0.0003 to 0.020%, REM: 0.0003 to 0.020%, Ca: 0.0003 to 0.0080% , N: 0.0003 to 0.010% or less (excluding 0%), O: 0.0003 to 0.0050%, the balance being iron and inevitable impurities, wherein the steel is , REM, Zr, Ti, Al, Ca, and a composite oxide containing S, and for the composite oxide in the steel material, the equivalent circle diameter A is 3μm greater oxides of 1 mm ² per 5.0 or less, and all equivalent circle diameter of the composite oxide of 0.1~3μm along with satisfying the following formula (1), the circle equivalent diameter 0 The number of composite oxides of 0.1 to 3 μm is 100 / mm ² or more, and the average composition of all the composite oxides of 0.1 to 3 μm satisfying the following formula (1) is Al ₂ O ₃ : than _{20%, TiO 2: 3~20%, ZrO} 2: 5~50%, REM oxides: 5~50%, CaO: 5~50% , S: characterized in that 1 to 15%.

0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM ₂ O ₃ )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
Here, at least among Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50% It is good to contain 1 type.

Moreover, it is good to contain at least any one among Nb: 0.002-0.10% and V: 0.002-0.10%.
Furthermore, it is good to contain B: 0.0005-0.0050%.

  ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in the toughness (HAZ toughness) of the welding heat affected zone at the time of high heat input welding can be obtained.

It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by embodiment of this invention have. It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by this embodiment have. It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by this embodiment have.

Hereinafter, a steel material (hereinafter simply referred to as a steel material) excellent in toughness of a weld heat affected zone according to an embodiment of the present invention will be described in detail with reference to the drawings.
The steel material according to the present embodiment is a steel material that exhibits excellent toughness in a weld heat affected zone (HAZ, Heat Affected Zone) affected by a very large welding energy, for example, a welding heat input exceeding 60 kJ / mm. is there. In the following description, the welding heat-affected zone of the steel material according to the present embodiment is denoted as HAZ, and the toughness in the HAZ is denoted as HAZ toughness.

  The steel material according to the present embodiment is a composite oxide (an oxysulfide containing Al, Ti, Zr, REM, Ca, and S) that serves as a nucleus of intragranular transformation, with its size and S concentration being appropriately controlled. By generating a constant amount, good HAZ toughness can be stably realized even in high heat input welding. Specifically, the number of coarse composite oxides having a circle equivalent diameter exceeding 3 μm that adversely affects the improvement of HAZ toughness is significantly suppressed, and the circle equivalent diameter useful for improving HAZ toughness is 0.1 to 3 μm. And a predetermined amount or more of complex oxides whose particle sizes are appropriately controlled. With this feature, the steel material according to the present embodiment can stably exhibit excellent HAZ toughness even when welding is performed with a large heat input.

The steel material by this embodiment which has the above-mentioned characteristic is obtained by adding each element so that it may become the chemical composition composition explained below in the secondary refining of molten steel, for example.
The steel material according to the present embodiment (hereinafter simply referred to as the present steel material) has carbon C of 0.02 to 0.13%, silicon Si of 0.05 to 0.5% and manganese Mn of 1.0 to 2.5. %, Phosphorus P 0.03% or less (excluding 0%), sulfur S 0.01% or less (excluding 0%), aluminum Al 0.002 to 0.040% or less, titanium Ti 0.005 to 0.040%, zirconium Zr 0.0003 to 0.020%, rare earth metal REM 0.0003 to 0.020%, calcium Ca 0.0003 to 0.0008%, nitrogen N 0 .0030 to 0.010% or less (excluding 0%), oxygen O is contained in 0.0003 to 0.0050%, and the balance is composed of iron and inevitable impurities. Further, the steel material includes a composite oxide containing REM, Zr, Ti, Al, Ca, and S. As for the composite oxide in the steel material, an oxide having an equivalent circle diameter of more than 3 μm is 5 per 1 mm ² . For the composite oxide having 0 or less and an equivalent circle diameter of 0.1 to 3 μm, the number of composite oxides satisfying the following formula (1) is 100 / mm ² or more.

0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM ₂ O ₃ )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
Furthermore, the average composition of the composite oxide of 0.1 to 3 μm satisfying the above formula (1) is 20% or less for Al ₂ O _{3, 3} to 20% for TiO ₂ , 5 to 50% for ZrO ₂ , REM oxidation The product is 5 to 50%, CaO is 5 to 50%, and S is 1 to 15%.

In the present embodiment, the contents of elements and components are simply described using the percentage “%”, but this is a simplified description of the mass percentage “mass%”. Please keep in mind.
Then, the structure of this steel material mentioned above is demonstrated in detail.
[Carbon C: 0.02 to 0.13%]
Carbon C is an element indispensable for securing the strength of the steel material (base material). However, if the content of C exceeds 0.13%, not only does island martensite (MA) form in the HAZ, leading to a reduction in the toughness of the HAZ, but also weldability due to the generation of CO gas, etc. It also has an adverse effect. Therefore, the C content is specified to be 0.02 mass% or more and 0.13 mass% or less. Preferably they are 0.04 mass% or more and 0.1 mass% or less.

[Silicon Si: 0.05-0.5%]
Silicon Si is an element that has a deoxidizing action and contributes to improving the strength of the base material by solid solution strengthening. However, if the Si content exceeds 0.5%, the weldability and toughness of the steel material deteriorate, so the upper limit of the Si content is set to 0.5%. In particular, in order to increase the HAZ toughness, it is recommended that the Si content be 0.3% or less. However, as the Si content is reduced, the HAZ toughness is improved, while the strength of the steel material may be reduced. Therefore, the lower limit of the Si content is set to 0.05%, and the Si content is specified to be 0.05% by mass or more and 0.5% by mass or less. Preferably they are 0.07 mass% or more and 0.35 mass% or less, More preferably, they are 0.1 mass% or more and 0.25 mass% or less.

[Manganese Mn: 1.0 to 2.5%]
Manganese Mn is an element that contributes to improving the strength of the base material. However, if the Mn content exceeds 2.5%, the weldability of the base material is degraded. Furthermore, since intensity | strength will fall when content of Mn is less than 1.0%, content of Mn is prescribed | regulated to 1.0 mass% or more and 2.5 mass% or less. Preferably they are 1.3 mass% or more and 2.0 mass% or less.

[Phosphorus P: 0.03% or less]
Phosphorus P is an element that easily segregates, and is an element that segregates at a grain boundary in a steel material and lowers the HAZ toughness. Usually, P is unavoidably contained in the base material in an amount of about 0.001%, so the content of P is specified to be 0.03% by mass or less. Preferably it is 0.02 mass% or less, More preferably, it is 0.01 mass% or less. However, this embodiment does not include the case where the P content is 0% by mass.

[Sulfur S: 0.01% or less]
Sulfur S is an element that combines with Mn to produce sulfide (MnS) and lowers the toughness of the base material and the ductility in the plate thickness direction. For example, when S is combined with REM such as lanthanum La or cerium Ce to generate a REM sulfide (for example, LaS or CeS), generation of REM oxide is inhibited, so that HAZ toughness is reduced. However, since S is normally unavoidably contained in the base material in an amount of about 0.0005%, the S content is specified to be 0.01% by mass or less. Preferably it is 0.008 mass% or less, More preferably, it is 0.006 mass% or less. However, this embodiment does not include the case where the S content is 0% by mass.

[Aluminum Al: 0.002 to 0.040%]
Aluminum Al is an element that acts as a deoxidizer. However, when Al is added excessively with respect to the base material, the added Al reduces oxides in the base material to form coarse Al oxide, so that the HAZ toughness decreases. On the other hand, when the Al content is small, the molten steel is easily contaminated with oxygen, so the lower limit of the Al content is 0.002%. Therefore, the Al content is specified to be 0.002 mass% or more and 0.040 mass% or less. Preferably it is 0.004 mass% or more and 0.025 mass% or less, More preferably, it is 0.005 mass% or more and 0.015 mass% or less.

[Titanium Ti: 0.005 to 0.040%]
Titanium Ti is an element that contributes to the improvement of HAZ toughness by generating a nitride such as TiN or an oxide containing Ti in the base material. However, if Ti is excessively added to the base material, the base material itself is hardened by solid solution strengthening of Ti and leads to a decrease in HAZ toughness. Therefore, the Ti content must be suppressed to 0.04% or less. . Therefore, the Ti content is specified to be 0.005% by mass or more and 0.040% by mass or less. Preferably it is 0.010 mass% or more and 0.030 mass% or less, More preferably, it is 0.014 mass% or more and 0.025 mass% or less.

[Zirconium Zr: 0.0003-0.020%]
Zirconium Zr is an element that contributes to the improvement of HAZ toughness by generating a complex oxide containing Zr. However, when Zr is added excessively with respect to the base material, coarse Zr oxide (ZrO ₂ ) is generated and HAZ toughness is lowered. Further, the formation of coarse Zr carbide (ZrC) reduces the toughness of the base material itself. Therefore, the Zr content is specified to be 0.0003 mass% or more and 0.020 mass% or less. Preferably it is 0.0005 mass% or more and 0.015 mass% or less, More preferably, it is 0.0009 mass% or more and 0.010 mass% or less.

[Rare earth metal REM: 0.0003-0.020%]
REM (rare earth element) is an element necessary for generation of oxides. By containing these oxides by REM, the oxides are easily finely dispersed in the steel material. This finely dispersed oxide serves as a production nucleus for intra-grain α of HAZ and contributes to improvement of HAZ toughness. However, when REM is added excessively, solid solution REM is generated and segregates in the base material, so that the toughness of the base material itself deteriorates. Therefore, the content of REM is specified to be 0.0003 mass% or more and 0.020 mass% or less. Preferably it is 0.0005 mass% or more and 0.015 mass% or less, More preferably, it is 0.0009 mass% or more and 0.010 mass% or less.

Specifically, REM means an element containing a lanthanoid element (15 elements from La to Ln) and Sc (scandium) and Y (yttrium). In this embodiment, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y among these elements, and more preferably La and / or Ce.
[Calcium Ca: 0.0003-0.0080%]
Calcium Ca is an element necessary for the production of oxides. Ca also becomes a production nucleus of intra-grain α of HAZ and contributes to the improvement of HAZ toughness. However, when Ca is added excessively, coarse Ca sulfide is generated and the toughness of the base material is deteriorated. Therefore, the Ca content is specified to be 0.0003 mass% or more and 0.0008 mass% or less. Preferably it is 0.0005 mass% or more and 0.0006 mass% or less, More preferably, it is 0.0007 mass% or more and 0.0003 mass% or less.

[Nitrogen N: 0.0003 to 0.010% or less]
Nitrogen N is an element that precipitates nitrides (eg, ZrN, TiN, etc.). Nitride contributes to the improvement of HAZ toughness by suppressing the austenite grain coarsening during welding by the pinning effect. Since N forms a nitride and promotes refinement of austenite grains as the content increases, it effectively works to improve HAZ toughness. However, if the N content exceeds 0.010%, the amount of solute N increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, the N content is specified to be 0.0030% by mass or more and 0.010% by mass or less. Preferably it is 0.0040 mass% or more and 0.0009 mass% or less, More preferably, it is 0.0050 mass% or more and 0.0008 mass% or less.

[Oxygen O: 0.0003 to 0.0050%]
Oxygen O is an essential element for the formation of oxides. If the content is less than 0.0003%, a sufficient amount of oxide cannot be obtained in the base material. On the other hand, when the content is more than 0.0050%, the HAZ toughness is lowered due to the coarsening of the oxide. Therefore, the O content is specified to be 0.0003 mass% or more and 0.0050 mass% or less. Preferably it is 0.0010 mass% or more and 0.0040 mass% or less, More preferably, it is 0.0015 mass% or more and 0.0033 mass% or less.

Here, the content of O represents the total amount of oxygen, and means the total amount of O forming an oxide in the base material and free O dissolved in the base material.
This steel material contains the above-mentioned elements, and the balance consists of iron and inevitable impurities.
The remaining components other than the above-described elements are iron and inevitable impurities (for example, Mg, As, Se, etc.).

Now, the steel material containing the above-described element includes a composite oxide (oxide and / or oxysulfide) containing REM, Zr, Ti, Al, Ca, and S. The composite oxides contained in this steel are Al, Ti, Zr, REM, and Al / Ti / Zr / REM / Ca / S based composite oxides containing oxides and sulfides of Ca. In addition, for example, elements such as Mn and Si and other component elements may be included. Al, Ti, Zr, REM, and Ca-based oxides have good lattice matching with steel materials, and promote intragranular structure transformation (intragranular transformation) in HAZ, so they are effective in refining the HAZ structure. It is.

In the present steel material, the number of oxides having an equivalent circle diameter of more than 3 μm in the above-described composite oxide is 5.0 or less per 1 mm ^{2 in} the cross section of the steel material. Since the complex oxide having an equivalent circle diameter exceeding 3 μm is coarse, in high heat input welding in which the heat input reaches 60 kJ / mm, the HAZ toughness is reduced. Therefore, the number of composite oxides exceeding 3 μm needs to be suppressed to 5.0 pieces / mm ² or less.

On the other hand, in the present steel material, the above-mentioned composite oxide has a circle equivalent diameter of 0.1 μm to 3 μm (hereinafter referred to as 0.1 to 3 μm), satisfies the following formula (1), and It is contained so that the number thereof is 100 / mm ² or more in the cross section of the steel material.
0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM ₂ O ₃ )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
This composite oxide having an equivalent circle diameter of 0.1 to 3 μm is for promoting intragranular structure transformation (intragranular transformation) in HAZ and improving HAZ toughness. A composite oxide having a thickness of 0.1 to 3 μm will be examined. Note that a complex oxide having an equivalent circle diameter of less than 0.1 μm hardly contributes to the improvement of HAZ toughness, and thus is not included in the number of complex oxides.

Hereinafter, the reason why a complex oxide having an equivalent circle diameter of 0.1 to 3 μm should satisfy the formula (1) will be described.
First, REM and Ca are oxysulfide-forming elements that can form oxides and sulfides. Therefore, if the S concentration (mass% S) with respect to the concentration of the oxysulfide forming elements (REM ₂ O ₃ and CaO) is too high, the excessively generated sulfide inhibits the matching between the oxide and the matrix. Therefore, the ability (intragranular transformation ability) of the composite oxide to contribute to the structure control is reduced. Also, if the S concentration (mass% S) relative to the concentration of oxysulfide forming elements (REM ₂ O ₃ and CaO) is too low, strain energy associated with sulfide formation cannot be obtained, which is disadvantageous for intragranular transformation. The intragranular transformation ability decreases. Furthermore, the strain energy resulting from the size of the composite oxide itself (equivalent circle diameter d of the composite oxide) affects the intragranular transformation.

In Formula (1), the 2nd term which considered the conditions considered to influence these intragranular transformation is shown. By the way, in the second term of the formula (1), the S concentration (mass% S) with respect to the oxysulfide forming elements (REM ₂ O ₃ and CaO), and the size of the composite oxide (the equivalent circle diameter d of the composite oxide) ) Has an optimum range for producing strain energy that contributes to intragranular transformation, and it is presumed that an upper limit value and a lower limit value exist in the second term of Equation (1). Therefore, the upper limit value and the lower limit value of the second term of Expression (1) were experimentally obtained.

A method for obtaining the upper limit value and the lower limit value of the second term of Expression (1) will be described.
First, a heat input test simulating welding HAZ with a heat input of 60 kJ / mm was performed on the prototype material. Thereafter, the prototype material after the heat input test was mirror-polished and corroded, and the presence of intragranular transformation caused by the composite oxide was investigated by revealing the structure by the corrosion.
Subsequently, the composition and size of the composite oxide in the prototype were measured by EPMA (Electron Probe MicroAnalyzer), and the value of the second term of the formula (1) was calculated for the composite oxide of 0.1 to 3 μm.

  The presence or absence of intragranular transformation and the calculated value of the second term are summarized as the results shown in Table 1, and the presence or absence of intragranular transformation is indicated by a circle (the value of the second term of the steel material having intragranular transformation). Based on the above, the range of the second term was set to 0.008 or more and 0.289 or less.

The complex oxide having an equivalent circle diameter of 0.1 to 3 μm is contained at 100 pieces / mm ² or more after satisfying the formula (1). Furthermore, the average composition is 20% or less of Al ₂ O ₃ , TiO 2. ₂ is 3% to 20% (3 to 20%), ZrO ₂ is 5% to 50% (5 to 50%), REM oxide is 5% to 50% (5 to 50%), CaO is It is necessary to be 5% or more and 50% or less (5 to 50%) and S is 1% or more and 15% or less (1 to 15%).

This is because the oxide composition affects the lattice matching between the oxide and the steel material in the HAZ. Therefore, if the oxide composition is not controlled to the content in the above range, a composite oxide having a circle equivalent diameter of 0.1 to 3 μm. This is because it cannot contribute to the intragranular transformation in HAZ, that is, it cannot contribute to the refinement of the HAZ structure.
This steel material having the chemical composition as described above is intended for thick steel plates having a plate thickness of about 3.0 mm or more. In addition to small to medium heat input welding, the heat input is 50 kJ / mm or more. Since the HAZ toughness can be prevented from being lowered even in thermal welding, it can be used as a material for large structures such as bridges, high-rise buildings, and ships.

[Nickel Ni, Copper Cu, Chromium Cr, Molybdenum Mo: 0.05 to 1.50%]
In the steel material according to the present embodiment, in addition to the above-described component elements, at least one of nickel Ni, copper Cu, chromium Cr, and molybdenum Mo is 0.05% to 1.50% (0.05 to 1.50). 50%) may be contained.
Cu, Ni, Cr, and Mo are all elements that contribute to increasing the toughness and strength of the steel material, and can be added alone or in combination. For example, in order to effectively improve toughness and strength by adding Cu, it is preferable to contain 0.05% or more of Cu. However, if the Cu content exceeds 1.50%, the strength of the base material is excessively increased and the toughness of the base material is decreased, so that the HAZ toughness is also decreased. Therefore, the Cu content is specified to be 0.05% by mass or more and 1.50% by mass or less.

Like Cu, Ni, Cr, and Mo are preferably contained in an amount of 0.05% or more. However, if the content exceeds 1.50%, the strength of the base material is excessively increased and the toughness of the base material is decreased. Therefore, the HAZ toughness is also lowered. Therefore, the contents of Ni, Cr, and Mo are also specified to be 0.05% by mass or more and 1.50% by mass or less.
[Niobium Nb, Vanadium V: 0.002 to 0.10%]
Furthermore, the steel material according to the present embodiment may contain at least one of niobium Nb and vanadium V in a range of 0.002% to 0.10% (0.002 to 0.10%). Nb and V are both precipitated as carbonitrides. Since this carbonitride exhibits a pinning effect, coarsening of austenite grains at the time of welding is suppressed, contributing to improvement of HAZ toughness. Therefore, in order to effectively improve the HAZ toughness by adding Nb, it is preferable to contain Nb of 0.002% or more. However, if the Nb content exceeds 0.10%, the precipitated carbonitrides become coarse, and on the contrary, the HAZ toughness is lowered. Therefore, the Nb content is specified to be 0.002% by mass or more and 0.10% by mass or less.

V, like Nb, is preferably contained in an amount of 0.002% or more. However, when the content of V exceeds 0.10%, the precipitated carbonitrides are coarsened, and on the contrary, the HAZ toughness is lowered. Therefore, the V content is specified to be 0.002% by mass or more and 0.10% by mass or less.
[Boron B: 0.0005 to 0.0050%]
In addition, the steel material according to this embodiment may contain 0.0005% or more and 0.0050% or less (0.0005 to 0.0050%) of boron B. B is an element that suppresses the formation of grain boundary ferrite and improves toughness. Therefore, in order to improve the toughness of the present steel material by adding B, it is preferable to contain 0.0005% or more of B. However, if the B content exceeds 0.0050%, it precipitates as BN at the austenite grain boundaries, leading to a decrease in toughness. Therefore, the B content is specified to be 0.0005 mass% or more and 0.0050 mass% or less. Preferably it is 0.0010 mass% or more and 0.0040 mass% or less, More preferably, it is 0.0015 mass% or more and 0.0003 mass% or less.

[Manufacture of steel materials according to this embodiment]
The steel material according to the present embodiment can be obtained by adding each element so as to have the above-described chemical component composition, for example, in the secondary refining of molten steel. The steel manufacturing method (manufacturing conditions) shown, that is, the method of adding each element will be described.
In the following description, steel was melted using a vacuum melting furnace (capacity 150 kg), cast into a 150 kg ingot, and cooled to obtain steels shown in examples and comparative examples described later.

[Adjustment of dissolved oxygen content]
First, in a molten steel melted in a vacuum melting furnace, the amount of dissolved oxygen and the S concentration are adjusted before adding an element for forming a complex oxide (a complex oxide forming element).
First, the amount of dissolved oxygen (mass% Of) before the addition of the complex oxide-forming element was adjusted to 0.005% or less. Then, the ratio (mass% Of / mass% S) of dissolved oxygen concentration (mass% Of) and S concentration (mass% S) in molten steel is 0.2 ≦ mass% Of / mass% S ≦ 9.6. S concentration (mass% S) was adjusted so that

Here, the desulfurization method for adjusting the S concentration (mass% S) is not particularly limited, but molten steel having a low S concentration may be used in advance.
The grounds for the dissolved oxygen amount and the S concentration are as follows. First, when the amount of dissolved oxygen exceeds 0.005%, the oxide generated in the molten steel becomes coarse. In addition, when the value of (mass% Of / mass% S) is large, the sulfide necessary for the oxide is not sufficiently generated. Moreover, when the value of (mass% Of / mass% S) is small, not only the desired oxide cannot be obtained, but also the sulfide is generated to a level that inhibits intragranular transformation because the S concentration is too high. End up.

Therefore, there is an appropriate balance between mass% Of and mass% S, and an appropriate range exists for the value of (mass% Of / mass% S). The range was experimentally determined, and 0.2 ≦ mass% Of / mass% S ≦ 9.6.
[Addition of Al]
Next, Al is one of oxysulfide constituent elements, and was added to the molten steel before Ti in order to secure Ti oxide.

[Addition of Ti]
Following the addition of Al, Ti was added to the molten steel before REM and Zr. If Ti is added before Al, all Ti oxides are reduced by Al in the subsequent steps. Therefore, Ti must be added after Al is added. After the addition of Ti, the molten steel was held for 2 to 15 minutes without adding other elements.

Even if it is added in the order of Al → Ti, if the holding time of the molten steel thereafter is less than 2 minutes, a complex oxide of Al and Ti is not sufficiently formed. This is because the reduction proceeds too much. That is, the order of addition of Al and Ti affects the above formula (1).
[Addition of REM / Zr]
After holding the molten steel for 2 to 15 minutes, REM and Zr were added. The order of addition of REM and Zr is not particularly limited. That is, REM → Zr, Zr → REM, or REM and Zr may be added simultaneously.

For the addition amount and the addition amount of REM and Zr, if an excessive even either one of the Zr and REM, it is because coarse complex oxides of more than 3μm in equivalent circle diameter will be formed, also, any If one of these elements is too small, a fine complex oxysulfide having an equivalent circle diameter of 0.1 to 3 μm is insufficient. That is, the amount of Zr and REM added affects the particle size distribution of the composite oxide.

In addition to this, REM has the property of easily forming oxides and sulfides, while Zr has the property of forming oxides but not sulfides. Therefore, in order to optimize the balance between oxide and sulfide, it is necessary to add Zr and REM according to mass% Of and mass% S.
Therefore, the ratio (add [Zr] / add [REM]) of the Zr addition amount and the REM addition amount is determined so as to satisfy the following expression (2).

0.27 × (mass% Of / mass% S) + 0.21 ≦ add [Zr] / add [REM] ≦ 0.41 × (mass% Of / mass% S) +0.77 (2)
Based on the formula (2), when the value of (mass% Of / mass% S) is large, that is, when oxide is easily generated and sulfide is difficult to be generated, Zr is added more than REM (add [ Increase the value of Zr] / add [REM]. In addition, when the value of (mass% Of / mass% S) is small, that is, when sulfides are more easily generated than oxide, REM is added more than Zr (add [Zr] / add [REM] Decrease the value). Based on this concept, the upper and lower limits of the value of add [Zr] / add [REM] were experimentally determined to obtain Equation (2).

[Ca addition and forging]
After REM and Zr were added, Ca was added and cast. Ca also forms oxides and sulfides. However, since the form of these oxides and sulfides basically depends on the form of inclusions already present, special attention should be paid to the form of inclusions before the addition of Ca. is there.
The deoxidizing elements Al, REM, Zr, and Ca are preferably not added all at once to the molten steel, but divided into two or more times or continuously in small amounts.

  In addition, the form of REM, Ca, Zr, Ti added to molten steel is not specifically limited, For example, as REM, pure La, pure Ce, and pure Y, pure Ca, pure Zr, pure Ti, and also Fe -Si-La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Ni-Ca alloy, etc. may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the range of the Ca content defined in the present embodiment.

The composition (content) of the component elements described so far, the relational expression regarding the content of the component elements, the manufacturing conditions, and the like are referred to as “conditions defined in the present embodiment”.
[Casting / Rolling]
Molten steel whose components were adjusted as described above was cast into an ingot. The cast ingot was hot-rolled and processed to produce a thick steel plate having a thickness of 30 mm to 80 mm. In actual operation, the molten steel obtained by adjusting the components may be continuously cast according to a conventional method to form a slab and then hot rolled according to a conventional method.

[Measurement of HAZ toughness value]
In order to evaluate the toughness of the HAZ affected by the welding heat for the obtained thick steel plate, a specimen for a welded joint was sampled from the thick steel plate and subjected to V pre-processing, followed by high heat input welding. Electrogas arc welding was performed at a heat input of 60 kJ / mm. Charpy impact test piece (V of JIS Z 2202) in which a notch is machined in the HAZ near the weld line (bond) at a depth t / 4 (t: thickness of the test piece) from the surface of the welded test piece. Three notch specimens) were collected. For each of these three V-notch test pieces, a Charpy impact test is performed at −40 ° C., the absorbed energy (vE-40) is measured, and the average value and the minimum value of the measurement results of the three V-notch test pieces are measured. The value was determined.

In this measurement result, a test piece (thick steel plate) having an average value of vE-40 exceeding 140 J was evaluated as a steel plate excellent in HAZ toughness.
[Method for measuring composite oxide composition of 0.1 to 3 μm]
A test piece is cut out from the surface of the thick steel plate at a depth t / 4 (t: thickness of the thick steel plate) (taken so that the axis of the test piece passes through the position of the depth t / 4), and the rolling direction and The cross section parallel to the plate thickness direction is mirror-polished and 0.1-3 μm using an electron probe X-ray Microanalyzer (EPMA, trade name JXA-8500F) manufactured by JEOL Datum. The composite oxide composition of was measured. The observation conditions at this time were an acceleration voltage of 20 kV, a sample current of 0.01 μA, a magnification of 5000 times, an observation area of 0.4 mm ² or more, and the component composition at the center of the composite oxide with wavelength dispersion of characteristic X-rays. Quantitative analysis was performed by spectroscopy.

  In other words, the elements to be quantified are Si, Mn, S, Al, Ti, Zr, La, Ce, Ca, and O (oxygen), and the relationship between the X-ray intensity and element concentration of each element using a known substance. Was previously obtained as a calibration curve, and the amount of elements contained in the composite oxide was quantified based on the X-ray intensity and calibration curve obtained from the composite oxide to be analyzed. From the X-ray intensity ratio indicating the presence of these elements, each element other than S was converted to a single oxide, and the composition of the oxide was calculated. S was calculated with a single concentration. In this embodiment, mass conversion is performed as the single oxide and S single element concentration in this way, and the average of a plurality of composite oxides is used as the composite oxide composition.

In addition, when the REM is represented by the symbol M, the REM oxide is present in the steel material in the form of M ₂ O ₃ , M ₃ O ₅ , MO _2, etc., but all the REM oxide is converted to M ₂ O ₃ . Converted. Similarly, all Ti was converted to TiO ₂ .
[Measurement method of equivalent circle diameter and number of composite oxide]
In the composition measurement of the composite oxide using the above-described EPMA, the area of the composite oxide was measured and the composite oxide was assumed to be a circle, and the diameter of the circle corresponding to the measured area was calculated as the equivalent circle diameter. When measuring the number of composite oxides whose equivalent circle diameter exceeds 5 μm, the magnification is 200 times, the observation area is 50 mm ² or more, and the other conditions are the number of composite oxides whose equivalent circle diameter is 5 μm or less. The measurement was performed under the same conditions as in the measurement.

Next, examples of the steel material according to the present embodiment will be specifically described.
Table 2 below shows the chemical composition of steel materials No. 1 to No. 31 which are examples of the steel material according to the present embodiment. All the component compositions of the steel materials No. 1 to No. 31 satisfy the conditions defined in this embodiment.

  Table 3 below shows the manufacturing conditions of steel materials No. 1 to No. 31 which are examples of this steel material. All the manufacturing conditions of the steel materials No. 1 to No. 31 also satisfy the conditions specified in this embodiment. When there are a plurality of addition orders and addition methods of the component elements, the selected addition order and addition method are shown in the remarks column.

Table 4 below shows the test results of the composite oxide particle size and number distribution, the composite oxide average composition, and the HAZ toughness for steel materials No. 1 to No. 31, which are examples of this steel material. Yes. In all the steel materials No. 1 to No. 31, which are examples of the present steel material, the number of complex oxides having a circle equivalent diameter exceeding 3 μm is 5.0 pieces / mm ² or less, and the circle equivalent diameter is 0.1 to 0.1. The number of composite oxides of 3 μm is 100 / mm ² or more. Furthermore, in all the steel materials No. 1 to No. 31, the average composition of the composite oxide having an equivalent circle diameter of 0.1 to 3 μm satisfies the conditions defined in this embodiment. As a result, all the steel materials No. 1 to No. 31 have a HAZ toughness test result of 140 J or more, and can be evaluated as exhibiting excellent HAZ toughness.

  Here, Table 5 below shows component compositions of steel materials No. 32 to No. 67 as comparative examples that do not satisfy the conditions defined in the present embodiment. Steel material No. 32 does not satisfy the conditions defined by the present embodiment for the content of Al. Steel materials No. 34 and 35 do not satisfy the conditions defined by the present embodiment for the Ti content. Steel materials No. 40 and 41 do not satisfy the conditions specified in the present embodiment by the REM content. In the steel materials No. 44 and 45, the content of Zr does not satisfy the conditions specified in this embodiment. Steel materials No. 48 and 49 do not satisfy the conditions defined by the present embodiment for the Ca content. Steel materials No. 52 and 53 do not satisfy the conditions defined by the present embodiment for the S content. About other steel materials, the component composition mentioned above is satisfy | filled.

Table 6 below shows the manufacturing conditions of steel materials No. 32 to No. 67 that do not satisfy the conditions specified in the present embodiment. In the steel materials No. 33, 36, 37, 42, 43, 46, 47, 50, and 51, “addition order of complex oxide forming elements” is marked with “x”, and complex oxide forming elements (Al
And Ti) are added in an order different from that described above. Steel materials No. 38 and 39 show that the holding time of the molten steel after addition of Ti does not satisfy the conditions defined in this embodiment. Steel material No. 52 has shown that the value of (mass% Of / mass% S) does not satisfy the conditions specified in this embodiment. In Steel No. 53, the value of mass% Of, the value of (mass% Of / mass% S), and the results of (add [Zr] / add [REM]) satisfy the conditions specified in this embodiment. It shows no. Steel materials Nos. 54 and 55 both indicate that the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials No. 56 and 57 indicate that the value of mass% Of does not satisfy the conditions defined in the present embodiment. Steel Nos. 58 and 59 indicate that the addition amount of add [Zr] and the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials No. 60 and 61 indicate that the addition amount of add [REM] and the result of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials Nos. 62 to 67 indicate that the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment.

  Thus, steel materials No. 32 to No. 67 do not satisfy the conditions defined in the present embodiment in one or both of the component composition shown in Table 5 and the manufacturing conditions shown in Table 6.

Table 7 shows the particle size and number distribution of the composite oxide, the average composition of the composite oxide, and the HAZ toughness test of steel materials No. 32 to No. 67 as comparative examples that do not satisfy the conditions specified in this embodiment. Results are shown. Steel materials No. 32 to No. 55 do not satisfy the conditions defined by the average composition of the composite product in this embodiment. In the steel materials Nos. 56 and 57, the number of complex oxides having a circle equivalent diameter exceeding 3 μm exceeds 5.0 pieces / mm ² . Steel materials No. 58 to No. 67 have a condition that one or both of the number of complex oxides having an equivalent circle diameter exceeding 3 μm and the number of complex oxides having an equivalent circle diameter of 0.1 to 3 μm are defined in this embodiment. not filled. About each of steel materials No.32-No.55, the conditions which are not satisfy | filled among the conditions prescribed | regulated by this embodiment are shown by "remarks."

  As a result, all of the steel materials No. 32 to No. 67 have a HAZ toughness test result of less than 140 J, and in a comparative example that does not satisfy one or more of the conditions specified in this embodiment, obtain a steel material having excellent HAZ toughness. I could not.

The HAZ toughness of the steel material according to this embodiment shown in Table 4 and the HAZ toughness of the comparative example shown in Table 7 will be compared with reference to FIGS.
FIG. 1 is a graph showing the HAZ toughness of steel materials according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 59 and 61 to 67 of comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 1, the number of complex oxides having an equivalent circle diameter of more than 3 μm is less than 5.0 pieces / mm ² . This is an example in which the number of composite oxides having an equivalent circle diameter of 0.1 to 3 μm is less than 100, and in any case, the HAZ toughness test result is significantly lower than 140J.

FIG. 2 is a graph showing the HAZ toughness of the steel materials according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 32-55 of the comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 1, the number of complex oxides having a circle equivalent diameter of more than 3 μm is less than 5.0 / mm ² , and the number of complex oxides having a circle equivalent diameter of 0.1 to 3 μm. However, the steel materials Nos. 32-55 of the comparative examples are examples in which the average composition of the composite oxide did not satisfy the conditions specified in the present embodiment, and in any case, the test results of the HAZ toughness Is far below 140J.

FIG. 3 is a graph showing the HAZ toughness of the steel material according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 56 and 57 of the comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 3, the number of complex oxides having a circle equivalent diameter of 0.1 to 3 μm is 100 or more, but the steel materials No. 56 and 57 of the comparative example have a circle equivalent diameter of 3 μm. This is an example in which the number of composite oxides exceeding 5.0 is 5.0 pieces / mm ² or more, and in any case, the HAZ toughness test result is significantly lower than 140J.

As described above, if the steel material has a configuration that satisfies the conditions defined in the present embodiment, it can exhibit excellent HAZ toughness even in high heat input welding.
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. In particular, in the embodiment disclosed this time, matters that are not explicitly disclosed, for example, operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component deviate from a range that a person skilled in the art normally performs. Instead, values that can be easily assumed by those skilled in the art are employed.

  For example, although it has been described that the present steel material is manufactured by secondary refining, a steel material that exhibits the same HAZ toughness can be obtained even when manufactured using a converter or an electric furnace. Therefore, the production of the present steel material using a converter or an electric furnace is also included in the technical scope of the present invention.

Claims (4)

C: 0.02 to 0.13% (meaning mass% (mass%); the same applies to the following components)),
Si: 0.05-0.5% or less,
Mn: 1.0-2.5%,
P: 0.03% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.002 to 0.040% or less,
Ti: 0.005 to 0.040%,
Zr: 0.0003-0.020%,
REM: 0.0003-0.020%,
Ca: 0.0003-0.0080%,
N: 0.0003 to 0.010% or less (excluding 0%),
O: a steel material containing 0.0003 to 0.0050%, the balance being iron and inevitable impurities,
The steel material includes a composite oxide containing REM, Zr, Ti, Al, Ca and S,
About the complex oxide in the steel material,
There are no more than 5.0 oxides with an equivalent circle diameter of more than 3 μm per mm ² ,
In addition , all the complex oxides having an equivalent circle diameter of 0.1 to 3 μm satisfy the following formula (1), and the number of complex oxides having an equivalent circle diameter of 0.1 to 3 μm is 100 / mm ² or more. And
Furthermore, the average composition of all composite oxides of 0.1 to 3 μm satisfying the following formula (1) is Al ₂ O ₃ : 20% or less, TiO ₂ : 3 to 20%, ZrO ₂ : 5 to 50%, REM oxide: 5 to 50%, CaO: 5 to 50%, S: 1 to 15%, a steel material excellent in toughness of a weld heat affected zone.
0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM ₂ O ₃ )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm) Ni: 0.05 to 1.50%,
Cu: 0.05 to 1.50%,
Cr: 0.05 to 1.50%,
Mo: 0.05 to 1.50%
The steel material excellent in toughness of the weld heat affected zone according to claim 1, comprising at least one of them. Nb: 0.002 to 0.10%,
V: 0.002 to 0.10%
The steel material excellent in the toughness of the welding heat affected zone according to claim 1 or 2, characterized by containing at least one of the above. B: 0.0005 to 0.0050%
The steel material excellent in the toughness of the weld heat affected zone according to any one of claims 1 to 3, characterized by comprising: