JP2013136813A - High toughness steel for high heat input welding and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high toughness steel for high heat input welding having high HAZ strength and toughness and tensile strength of 590 N/mmor more and a method for producing the same.SOLUTION: The steel includes in basic composition 0.001-0.015% of C, 0.01-0.80% of Si, 1.0-2.0% of Mn, 0.020% or less of P, 0.0050% or less of S, 0.005-0.10% of Al, 0.30-1.5% of Mo, 0.0003-0.0050% of B, 0.010-0.050% of Ti, 0.0060-0.0100% of N, and 0.01% or less of Nb, with the balance comprising Fe and unavoidable impurities; and satisfies (Ti+0.5×Nb)/N≤3.40, and 1.0×10pieces/mmof TiN and (Ti, Nb)N precipitates with a particle size of 0.01-0.10 μm exist in the steel when heated to a temperature region of 1,400°C to (the melting point-10°C). In the production method, rolling with cumulative rolling reduction in a γ unrecrystallized region of 50% or more etc. is performed.

Description

  The present invention is suitable for use in ships, marine structures, low-temperature storage tanks, line pipes and welded structures in the field of civil engineering and construction, etc. The present invention relates to a steel for high toughness large heat input welding having excellent low temperature toughness and strength characteristics of a weld heat affected zone and high strength with a tensile strength of 590 MPa or more and a method for producing the same.

Steel materials used in the fields of shipbuilding, construction, civil engineering, etc., as these structures increase in size, high-strength, thick-walled materials are assumed to be easy to manufacture and have good usage performance (workability and weldability) Recently, YP460N / mm grade ² steel with a plate thickness of 50 mm has been developed and used as a steel for shipbuilding. Such steel materials are often subjected to high heat input welding with a heat input of 300 kJ / cm or more, such as electrogas welding, and ensuring the toughness of the weld heat affected zone is a problem.

Many countermeasures have been proposed in the past for reducing the toughness of the heat-affected zone due to high heat input welding. For example, the toughness of the weld zone is suppressed by suppressing the austenite grain coarsening in the heat-affected zone by fine dispersion of TiN in the steel. The technology to improve this has already been put into practical use.
In Patent Document 1, Nb is contained in TiN-based inclusions in steel, and at the time of high heat input welding, Nb is dissolved from the inclusions to suppress austenite grain coarsening in the heat affected zone. In addition, a technique for improving HAZ toughness by suppressing Nb in the inclusions and suppressing bainite during small heat input welding is described.
Patent Document 2 describes a steel for high heat input welding in which Ti oxide is found to be excellent as a ferrite nucleus in a weld heat affected zone, and Ti oxide is uniformly dispersed in the steel.

On the other hand, in the high-tensile steel material for welding, the weld heat-affected zone is softened by welding, which causes a problem of strength reduction as a welded joint. In the case of high heat input welding, the ferrite transformation is likely to occur due to an increase in welding heat input, so that the strength reduction becomes significant. With respect to such a problem, the above-described conventional technique is effective for preventing the reduction of the HAZ toughness, but is not effective for preventing the reduction (softening) of the HAZ strength. This is because HAZ toughness can be ensured by making the HAZ structure a ferrite-based structure, but it facilitates HAZ softening.
For this reason, in Patent Document 3, a technique for controlling the HAZ structure to high strength bainite is used. However, Patent Document 3 focuses on ultra-low carbon bainite as a method for improving HAZ toughness, and there is a problem that the technology cannot be applied to low-temperature specification steels that require higher HAZ toughness. It was.

JP 2004-2181010 A JP 57-51243 A JP 2000-345282 A

The present invention is for high toughness high heat input welding in which high HAZ strength and high HAZ toughness are obtained when high heat input welding with a heat input of 300 kJ / cm or more is performed and tensile strength is 590 N / mm ² or more. It aims at providing steel and its manufacturing method.

The present inventors have investigated the strength characteristics of the heat-affected zone when a high strength steel having a tensile strength of 590 N / mm ² or more is subjected to improvement of base metal toughness and large heat input welding with a heat input of 300 kJ / cm or more. In order to improve the low-temperature toughness, we conducted intensive studies and obtained the following knowledge.
1. In order to improve the high heat input HAZ toughness, as described in Patent Document 3, it is effective to use the HAZ structure as bainite. Furthermore, when C is further reduced to 0.015% or less, formation of island martensite (MA) that inhibits toughness is hardly recognized, and HAZ toughness is improved.
2. Furthermore, the presence of TiN and (Ti, Nb) N precipitates in the weld heat affected zone is effective for pinning austenite grains. The effect is that the particle diameter of these precipitates is 0.01 to 0.10 μm, It is most effective when 1.0 × 10 ⁵ or more / 1 mm ² exists. The dispersion state can be achieved by adjusting the component composition so as to satisfy (Ti + 0.5 × Nb) /N≦3.40 (each element is content and mass%).
3. The HAZ softening at the time of high heat input welding is caused by the bainite recovery / recrystallization phenomenon, and the addition of Mo in an amount of 0.30% or more suppresses recovery / recrystallization and improves the HAZ strength.
4). In order to improve the base metal toughness of the steel having a high heat input weld heat-affected zone property and a tensile strength of 590 N / mm ² or more, P and S of steel components are P: 0.020% or less and S: It is effective to set it to 0.0050% or less. Moreover, after heating said steel raw material to 950 to 1250 degreeC, it hot-rolls on the conditions of the cumulative reduction rate in an austenite non-recrystallized region: 50% or more, rolling completion temperature: 650-800 degreeC, and then The base material toughness can also be improved by cooling to 580 ° C. or lower at a cooling rate of 1.0 ° C./s or higher.

The present invention has been made with further studies based on the above findings, and the gist thereof is as follows [1] to [4].
[1] By mass%, C: 0.001 to 0.015%, Si: 0.01 to 0.80%, Mn: 1.0 to 2.0%, P: 0.020% or less, S: 0.0050% or less, Al: 0.005 to 0.10%, Mo: 0.30 to 1.5%, B: 0.0003 to 0.0050%, Ti: 0.010 to 0.050%, N: 0.0060 to 0.0100%, Nb: 0.01% or less, balance Fe and inevitable impurities, and (Ti + 0.5 × Nb) /N≦3.40 (each element has a content , Mass%), and has a component composition satisfying the following conditions: (melting point−10 ° C.) or less, when heated to a temperature range of 1400 ° C. TiN and (Ti, Nb) high toughness high heat input soluble to N precipitates are characterized by the presence 1.0 × 10 ⁵ cells / mm ² or more Use steel.
[2] Further, Cu: 0.10 to 0.60%, Ni: 0.10 to 1.0%, Cr: 0.10 to 0.80%, V: 0.02 to 0.10 by mass% %, W: Steel for high toughness high heat input welding according to [1], containing one or more selected from 0.05 to 0.50%.
[3] Further, by mass%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.02%, REM: 0.001 to 0.02 The steel having high toughness and high heat input welding according to [1] or [2], containing one or more selected from%.
[4] After heating the steel material having the component composition according to any one of [1] to [3] to 950 to 1250 ° C., the cumulative rolling reduction in the austenite non-recrystallized region: 50% or more, rolling end temperature: High rolling according to any one of [1] to [3], wherein hot rolling is performed under a condition of 650 to 800 ° C., and then cooled to 580 ° C. or less at a cooling rate of 1.0 ° C./s or more. A method for producing steel for toughness high heat input welding.

According to the present invention, the strength and toughness balance of the weld heat-affected zone excellent in large heat input welding with a heat input of 300 kJ / cm or more such as submerged arc welding, electrogas welding, electroslag welding, etc. can be stably secured. Steel with a tensile strength of 590 N / mm ² or more is obtained, which is extremely useful industrially.

Hereinafter, the present invention will be described in detail.
First, the component composition of the steel of the present invention will be described. Unless otherwise specified, “%” for the composition is “% by mass”.
・ C: 0.001 to 0.015%
In order to make the base material and the HAZ structure a bainite structure in which island-like martensite is hardly observed, it is necessary to suppress the C content to 0.015% or less. Further, in the mass production process, reducing C to less than 0.001% causes a significant decrease in productivity, so C is limited to a range of 0.001 to 0.015%.

・ Si: 0.01-0.80%
Si is an element that increases the strength of the steel by solid solution strengthening, and 0.01% or more is added in order to ensure a tensile strength of 590 MPa or more. However, if the content exceeds 0.80%, the weldability is impaired, and adverse effects such as deterioration of the base metal and HAZ toughness occur. Therefore, Si is limited to the range of 0.01 to 0.80%. .

Mn: 1.0-2.0%
Mn has the effect of suppressing the ferrite transformation of steel in the extremely low carbon region and increasing the strength by baiting the structure of the steel material. Even at the time of high heat input welding with a heat input of 300 kJ / cm or higher, in order to suppress the ferrite transformation of HAZ and obtain a bainite single structure, it is necessary to contain 1.0% or more of Mn. On the other hand, if the content exceeds 2.0%, the base material and the HAZ toughness decrease, so Mn is limited to a range of 1.0 to 2.0%.

P: 0.020% or less S: 0.0050% or less P and S are important elements of the present invention as well as extremely low carbon. These impurity elements are inevitably mixed elements, and P content exceeding 0.020% and S content exceeding 0.0050% significantly reduce the toughness of the base material. P segregates at the grain boundaries of Fe, weakens the bonding force between Fe atoms, and promotes brittle fracture of Fe in a low temperature region. S has the same grain boundary segregation as P, and the formation of sulfides reduces the toughness of the base material. From the above, it was limited to the range of P: 0.020% or less and S: 0.0050% or less.

-Al: 0.005-0.10%
Al is an element that acts as a deoxidizer for molten steel, and 0.005% or more is required to obtain a sufficient deoxidation effect. However, if it exceeds 0.10%, the cleanliness of the steel is lowered, and the base metal and the HAZ toughness are lowered. Therefore, Al is limited to the range of 0.005 to 0.10%.

Mo: 0.30 to 1.5%
The addition of Mo is an important element of the present invention as well as extremely low carbon. Mo is an element that promotes bainite or martensite of the structure, and is an essential element for bainite or martensite of the base material and the HAZ structure while reducing Nb. In order to express such an effect, it is necessary to add at least 0.30% of Mo, but when it exceeds 1.5%, the effect is saturated. Further, Mo has an effect of suppressing recovery / recrystallization of bainite or martensite during high heat input welding, and effectively acts to improve HAZ strength by suppressing HAZ softening. Also for this effect, addition of 0.30% or more of Mo is necessary. From the above, Mo was limited to the range of 0.30 to 1.5%.

・ B: 0.0003 to 0.0050%
B has the effect of suppressing ferrite transformation and baiting the structure. This effect is manifested when the B content is 0.0003% or more. However, when the content exceeds 0.0050%, the effect is saturated, and the ferrite transformation may be accelerated by precipitation of BN during cooling. Is limited to the range of 0.0003 to 0.0050%.

・ Ti: 0.010 to 0.050%
Ti is dispersed as fine TiN and (Ti, Nb) N precipitates in the steel, and has the effect of suppressing the austenite grain growth of HAZ during high heat input welding by the pinning effect and improving toughness. Moreover, there exists an effect which accelerates | stimulates the effect | action of B mentioned above by fixing N in steel as TiN. In order to obtain such an effect, 0.010% or more of Ti should be added. However, if it exceeds 0.050%, the toughness decreases due to the coarsening of TiN. For this reason, Ti was limited to a range of 0.010 to 0.050%.

・ N: 0.0060-0.0100%
N precipitates TiN or (Ti, Nb) N precipitates and suppresses coarsening of austenite grains. In order to obtain such an effect, it is contained in an amount of 0.0060% or more. On the other hand, if it exceeds 0.0100%, it is solidified in a temperature region where TiN and (Ti, Nb) N precipitates are dissolved by the welding heat cycle. Since the amount of dissolved N increases and the toughness decreases, it is limited to the range of 0.0060 to 0.0100%.

-Nb: 0.01% or less Nb has the effect | action which makes the structure | tissue of the steel material in an ultra-low-carbon area | region the single structure of bainite like Mn. However, if the Nb content is high, Nb (C, N) precipitates during the cooling process during high heat input welding, resulting in a decrease in toughness. For this reason, Nb is reduced to 0.01% or less as much as possible.

・ (Ti + 0.5 × Nb) /N≦3.40 (each element here is content, mass%)
This parameter defines the particle size and distribution state of TiN and (Ti, Nb) N precipitates. In the present invention, this parameter is defined to be 3.40 or less so as to be effective for pinning austenite grains. When (Ti + 0.5 × Nb) / N exceeds 3.40, the pinning effect of the austenite grains due to these precipitates is lowered and adversely affects the toughness, so that it is 3.40 or less.

Cu: 0.10 to 0.60%, Ni: 0.10 to 1.0%, Cr: 0.10 to 0.8
1% or more selected from 0%, V: 0.02-0.10%, W: 0.05-0.50% Cu, Ni, Cr, V, W are all mainly It is a useful element that increases the strength of steel by solid solution strengthening, and is added as necessary. However, if the content is less than the lower limit, the effect of addition is poor. On the other hand, if the content exceeds the upper limit, the weldability is reduced and the alloy addition cost is increased.

Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr
: One or more selected from 0.001 to 0.02%, REM: 0.001 to 0.02% Ca, Mg, Zr, and REM all form oxides and sulfides. It is dispersed in steel and has the effect of refining the austenite grain size of the high heat input weld HAZ by the pinning effect, and is added as necessary. However, if the content is less than the lower limit, the effect of addition is poor. On the other hand, if the content exceeds the upper limit, coarse oxides and sulfides increase, and instead the toughness is lowered. Is preferred.

Next, the microstructure of the steel of the present invention will be described.
-The microstructure when reheated to a temperature range of 1400 ° C or higher has a particle size of 0.01 to 0.10
In the present invention, the steel having the above component composition is (melting point−10 ° C.) or less, and 1400 ° C. or more, and μm TiN and (Ti, Nb) N precipitates are 1.0 × 10 ⁵ pieces / 1 mm ² or more in the steel. the microstructure when heated to a temperature range, the particle diameter 0.01～0.10Myuemu, TiN and (Ti, Nb) N precipitates is present 1.0 × 10 ⁵ cells / 1 mm ² or more in the steel A microstructure is assumed. When the particle diameter of the TiN and (Ti, Nb) N precipitates is less than 0.01 μm, it undergoes solid solution when subjected to the above heat cycle, whereas when it exceeds 0.10 μm, the dispersion state is coarse and the distance between the particles becomes wide. Therefore, the effective pinning effect cannot be exhibited and coarse austenite grains are formed, so that the range is 0.01 to 0.10 μm. And even if it is a particle diameter within this range, if it is less than 1.0 × 10 ⁵ particles / 1 mm ² , an effective pinning effect cannot be obtained, so TiN and (Ti, Nb) N precipitates are contained. Precipitates are uniformly dispersed in the steel at 1.0 × 10 ⁵ pieces / 1 mm ² or more.

Next, the reasons for limiting the production conditions of the present invention will be described.
In addition, what is necessary is just to manufacture a steel raw material by casting the molten steel adjusted to said suitable component composition by a conventionally well-known method, such as continuous casting, after melt | dissolving by a normally well-known method, such as a converter.

-Heating temperature of steel material: 950 ° C-1250 ° C
In order to obtain a uniform sized austenite structure before rolling, the steel material must be heated to a temperature of 950 ° C. or higher. However, if the heating temperature exceeds 1250 ° C., the structure becomes extremely coarse. The temperature was limited to the range of 950 ° C to 1250 ° C.

・ Cumulative reduction ratio in the austenite non-recrystallized region: 50% or more Increase in the amount of reduction in the austenite non-recrystallized region in hot rolling makes the packet size of bainite transformed from austenite grains finer and reduces the toughness of the bainite structure. Improve. In addition, an increase in the amount of reduction in the non-recrystallized region increases the density of dislocations accumulated in the austenite grains. Thereby, a part of dislocation is inherited by the bainite structure after transformation at the time of transformation, and the strength is further increased. Such an effect becomes remarkable when the cumulative reduction amount in the austenite non-recrystallized region at 950 ° C. or lower is 50% or more.

-Rolling end temperature: 650-800 ° C
The reduction of the rolling end temperature of hot rolling also increases the strength and toughness of the steel due to the refinement of the bainite structure due to transformation from the recrystallized fine austenite grains and the high dislocation density of the bainite structure. The upper limit is 800 ° C. However, when the rolling end temperature is lowered to less than 650 ° C., the austenite → ferrite or austenite → bainite transformation starts, and as a result of processing the produced ferrite or bainite, there is a problem of reduced toughness or increased anisotropy. Therefore, the lower limit of the rolling end temperature is 650 ° C.

-Cooling rate after hot rolling: 1.0 ° C / s or more In order to ensure the strength of the base metal, an accelerated cooling process after rolling is applied and cooling is performed at a cooling rate of 1.0 ° C / s or more. When it is less than 1.0 ° C./s, the strength of the base material is lowered.
Cooling stop temperature: 580 ° C. or less When the cooling stop temperature exceeds 580 ° C., the element is intensified into untransformed austenite and a hardened phase is easily generated. Since this hardening phase reduces toughness, the cooling stop temperature was limited to 580 ° C. or lower.

  In the present invention, a tempering process may be further performed after the cooling process. This tempering treatment is performed to adjust the strength and toughness of the bainite produced during cooling and to improve the toughness by decomposing the island martensite produced between the bainite laths. Otherwise, the above effect is not recognized, and when the temperature exceeds 650 ° C., the strength is remarkably lowered. Therefore, the tempering temperature is preferably about 500 to 650 ° C.

Molten steel having various composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method.
Using these slabs (plate thickness 300 mm) as a raw material, heat treatment, rolling treatment and cooling treatment were performed under the conditions shown in Table 2 to obtain a thick steel plate having a plate thickness of 50 to 60 mm. Table 3 shows the results of investigation on the tensile properties and base metal toughness of the obtained thick steel plates. Table 3 also shows the results of investigations on the number of precipitates having a weld heat-affected zone (HAZ) strength, toughness, and particle size: 0.01 to 0.1 μm when high heat input 1-pass joint welding is performed. Also shown.

Each characteristic and the number of precipitates were evaluated as follows.
(1) Tensile properties A round bar tensile test piece with a parallel part of 14φ x 85mm and a distance between gauge points of 70mm is taken and pulled from the center of the plate thickness of each thick steel plate so that the longitudinal direction of the test piece coincides with the plate width direction. The test was conducted and the yield strength (0.2% proof stress) and tensile strength were measured.
(2) Base material toughness From the plate thickness center of each thick steel plate, a 2 mm V notch shear py test piece was taken so that the longitudinal direction of the test piece coincided with the rolling direction, and the brittle fracture surface transition temperature (vTrs) of the base material was determined. Asked.
(3) Strength and toughness of weld heat affected zone (HAZ) After producing a joint by electrogas welding (EGW) of high heat input welding (350-400 kJ / cm), the HAZ strength is measured by welding the center of the plate thickness. From HAZ to HAZ and the base material, hardness measurement (load: 98 N) was performed at a pitch of 1 mm, and the minimum value of HAZ hardness was evaluated. The HAZ toughness was evaluated by absorption energy vE- ₄₀ (average value of three) at a test temperature of −40 ° C. using a Charpy test piece having a notch at a position 1 mm from the bond part.
(4) Measurement of particle diameter and number of precipitates Resimulating the above welding conditions to reheat to 1450 ° C., followed by rapid cooling, and observation with a transmission electron microscope (TEM). A total of five visual fields were observed with a microscope magnification of 60,000 to 20,000 times, and the precipitates having a particle size of 0.01 to 0.1 μm were measured, and the average particle size and the number per 1 mm ² were calculated. The particle diameter is a diameter obtained by circularly approximating the particles and obtained from the area of each particle.

From Table 3, No. of the present invention example. It was confirmed that all of Nos. 1 to 7 have excellent base material properties such as a tensile strength of 590 N / mm ² or more and a brittle fracture surface transition temperature of −45 ° C. or less. In addition, the steel of the present invention has a Charpy impact absorption energy value (test temperature of −40 ° C., average value of three) of the weld heat-affected zone of 100 J or more, and a minimum HAZ hardness value of 175 HV or more. Excellent in strength and toughness of the part.
On the other hand, at least one of the chemical components and the production conditions is a comparative example No. 1 that is out of the scope of the present invention. 8 to 23 are inferior in any one or more of the above characteristics.

Claims (4)

% By mass
C: 0.001 to 0.015%
Si: 0.01-0.80%
Mn: 1.0-2.0%
P: 0.020% or less S: 0.0050% or less Al: 0.005-0.10%
Mo: 0.30 to 1.5%
B: 0.0003 to 0.0050%
Ti: 0.010 to 0.050%
N: 0.0060 to 0.0100%
Nb: 0.01% or less, balance Fe and inevitable impurities, and satisfies the condition of (Ti + 0.5 × Nb) /N≦3.40 (each element here is content, mass%) TiN and (Ti, Nb) N having a component composition of (melting point−10 ° C.) or less and having a particle diameter of 0.01 to 0.10 μm in steel when heated to a temperature range of 1400 ° C. or more. A steel having high toughness and high heat input welding, wherein precipitates are present at 1.0 × 10 ⁵ pieces / mm ² or more.   Further, Cu: 0.10 to 0.60%, Ni: 0.10 to 1.0%, Cr: 0.10 to 0.80%, V: 0.02 to 0.10%, W in mass% The high toughness high heat input welding steel according to claim 1, comprising one or more selected from 0.05 to 0.50%.   Furthermore, in mass%: Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, Zr: 0.001-0.02%, REM: 0.001-0.02% The steel for high toughness high heat input welding according to claim 1 or 2, which contains one or more selected from the above. After heating the steel raw material which has a component composition as described in any one of Claims 1-3 to 950-1250 degreeC, the cumulative reduction rate in an austenite non-recrystallization area | region: 50% or more, rolling completion temperature: 650-800 The high toughness insertion according to any one of claims 1 to 3, wherein hot rolling is performed under the condition of ° C and then cooled to 580 ° C or less at a cooling rate of 1.0 ° C / s or more. Manufacturing method of steel for heat welding.