JP2012188750A - High toughness steel for high heat input welding and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high toughness steel for high heat input welding suitably used for a ship, a marine structure or the like, and a manufacturing method thereof.SOLUTION: The steel comprises, by mass%, C: 0.001-0.015%, Si: 0.01-0.80%, Mn: 1.0-2.0%, P, S, Al: 0.005-0.10%, Mo: 0.30-1.5%, B: 0.0003-0.0050%, Ti: 0.005-0.050%, N: 0.0010-0.0060%, Nb: 0.01% or less (including 0), as needed, further contains one or more out of Cu, Ni, Cr, V, W, Ca, Mg, Zr, REM. The steel material having a composition is heated to 950-1,250°C, and then hot-rolled at a cumulative draft of 50% or more in an austenitic un-recrystallizing temperature zone and a rolling completion temperature of 680-830°C, and thereafter cooled to 580°C or below at the cooling speed of 1.0°C/s or more.

Description

  The present invention is suitable for use in welding structures such as ships, offshore structures, low-temperature storage tanks, line pipes, and civil engineering / architecture fields. High toughness high heat input welding steel having excellent low temperature toughness and strength characteristics of weld heat affected zone, tensile strength of base metal of 590 MPa or more, brittle fracture surface transition temperature (vTrs) of −45 ° C. or less, and production thereof Regarding the method.

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 large heat input welding with welding heat input of 300 kJ / cm or more, such as electrogas welding, and ensuring the toughness of the weld heat affected zone (also referred to as HAZ) is a problem. Yes.

  Conventionally, many countermeasures have been proposed for reducing the HAZ toughness due to high heat input welding. For example, the coarsening of austenite grains in the heat-affected zone is suppressed by the fine dispersion of TiN in the steel to improve the HAZ toughness. The technology 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 high-tensile steel for welding, the microstructure of the weld heat-affected zone is greatly changed from the base metal structure by being heated to a temperature exceeding the Ac ₁ transformation point by welding, and the heating temperature and the temperature after welding are changed. Depending on the cooling rate, the HAZ softens.

  Increase in welding heat input expands the area where such strength decreases, and in order to facilitate the formation of ferrite transformation after welding, to ensure HAZ toughness by making the HAZ structure a ferrite-based structure. In steels for high heat input welding described in Patent Documents 1 and 2, which are characterized by the above, there is a concern that the strength of the welded joint may be reduced.

  Patent Document 3 describes that the HAZ structure is controlled to a high-strength bainite as a method for simultaneously achieving the problems of HAZ toughness and HAZ softening. To improve the strength and toughness of the weld heat-affected zone. The main focus was on TS 490 MPa or more and a base material with a plate thickness of 40 mm or more, and vTrs was about −20 to −40 ° C. and toughness was insufficient.

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

Therefore, the present invention is excellent in HAZ strength and HAZ toughness when subjected to high heat input welding with a heat input of 300 kJ / cm or more, high toughness with a tensile strength of 590 N / mm ² or more and a vTrs of −45 ° C. or less. An object of the present invention is to provide a steel for high heat input welding and a method for producing the same.

The inventors of the present invention are excellent in strength characteristics and low-temperature toughness of the weld heat-affected zone when high heat input welding with a heat input of 300 kJ / cm or more is performed, and a high strength steel having a tensile strength of 590 N / mm ² or more. We conducted intensive studies to improve the base material toughness and obtained the following findings. In the following description, “%” means “mass%”.

  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, by reducing C to 0.015% or less, formation of island martensite (MA) that inhibits toughness is hardly recognized, and HAZ toughness is improved.

  2. The HAZ softening at the time of performing high heat input welding is caused by the recovery and recrystallization phenomenon of bainite. When 0.3% or more of Mo is contained, the recovery and recrystallization are suppressed and the HAZ strength is improved.

3. In order to improve the base metal toughness of the steel having excellent heat input of high heat input welding and tensile strength of 590 N / mm ² or more, P: 0.020% or less and S: 0.0050 or less in the steel composition Is effective.

  4). Further, after heating the steel material having the above characteristics to 950 ° C. to 1250 ° C., hot rolling was performed under the conditions of the cumulative reduction ratio in the austenite non-recrystallized region: 50% or more and the rolling end temperature: 680 to 830 ° C. 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 based on the above findings and further studies, that is, the present invention
1. C: 0.001 to 0.015% by mass%,
Si: 0.01-0.80%,
Mn: 1.0-2.0%,
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: 0.005 to 0.050%,
N: 0.0010 to 0.0060%,
Nb: 0.01% or less (including 0),
A high toughness high heat input welding steel characterized by comprising a balance Fe and inevitable impurities.
2. 2. The steel for high toughness high heat input welding according to 1, wherein the brittle fracture surface transition temperature (vTrs) is −45 ° C. or lower.
3. 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 The high toughness high heat input welding steel according to 1 or 2, comprising one or more selected from 0.05 to 0.50%.
4). 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 for high toughness high heat input welding according to any one of 1 to 3, which contains one or more selected from the above.
After the steel material having the composition according to any one of 5.1, 3, and 4 is heated to 950 ° C. to 1250 ° C., the cumulative rolling reduction in the austenite non-recrystallized region: 50% or more, rolling end temperature : Hot rolling under conditions of 680 to 830 ° C, followed by cooling to 580 ° C or less at a cooling rate of 1.0 ° C / s or more, a method for producing high toughness high heat input welding steel.

According to the present invention, the strength and toughness balance of the weld heat-affected zone is 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. A steel having 590 N / mm ² or more and vTrs of −45 ° C. or less is obtained, which is extremely useful industrially.

In the following description, “%” means “mass%”.
C: 0.001 to 0.015%
In order to secure excellent toughness as a bainite structure in which the base material and the HAZ structure hardly recognize MA, it is necessary to suppress the C content to 0.015% or less. Further, reducing C to less than 0.001% causes a significant decrease in steelmaking productivity, so 0.001 to 0.015%. Preferably, it is 0.001 to 0.012%.

Si: 0.01-0.80%
Si is an element that increases the strength of steel by solid solution strengthening, and contains 0.01% or more 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 a decrease in the base metal and HAZ toughness occur, so 0.01 to 0.80%. Preferably, it is 0.03 to 0.60%.

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 converting the steel structure to bainite. Even at the time of high heat input welding where the heat input of welding exceeds 300 kJ / cm, in order to suppress the ferrite transformation of HAZ and obtain a bainite single phase 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 the content is made 1.0 to 2.0%. Preferably, it is 1.0 to 1.8%.

P: 0.020% or less, S: 0.0050% or less P and S are impurity elements inevitably mixed in. P content exceeds 0.020% and S content exceeds 0.0050%. Significantly reduces the toughness of the base metal. That is, 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, P: 0.020% or less, S: 0.0050% or less. Preferably, P: 0.015% or less, S: 0.0040% or less.

Mo: 0.30 to 1.5%
Mo is an important element in the present invention as well as extremely low carbon. Mo is an element that promotes bainite of the steel structure, and is essential in the steel of the present invention that bainites the base metal and the HAZ structure while reducing Nb.

  In order to exhibit such an effect, it is necessary to contain at least 0.30% of Mo, but when it exceeds 1.5%, the effect becomes saturated.

  Mo has an effect of suppressing recovery and recrystallization of bainite during high heat input welding, and effectively acts to improve HAZ strength by suppressing HAZ softening. Such an effect cannot be sufficiently obtained when the content is less than 0.30%. For these reasons, the Mo content is set to 0.30 to 1.5%. Preferably, it is 0.50 to 1.5%.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer for molten steel, and needs to be contained in an amount of 0.005% or more in order 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, so 0.005 to 0.10%.

B: 0.0003 to 0.0050%
B has the effect of suppressing ferrite transformation and baiting the structure. This effect is manifested at a content of 0.0003% or more. However, when the content exceeds 0.0050%, the effect is saturated. On the contrary, precipitation of BN during cooling promotes ferrite transformation and lowers the HAZ strength. Therefore, the B content is set to 0.0003 to 0.0050%. Preferably, it is 0.0003 to 0.0040%.

Ti: 0.005 to 0.050%
Ti is dispersed as fine TiN in the steel, and has the effect of suppressing the growth of austenite grains of HAZ during high heat input welding by the pinning effect and improving toughness. Further, fixing N in the steel as TiN has an effect of suppressing the precipitation of BN and promoting the action of B described above.

  In order to obtain such an effect, the content of 0.005% or more is required. However, if it exceeds 0.050%, the toughness is reduced due to the coarsening of TiN. 0.005 to 0.050%. Preferably, it is 0.005 to 0.040%.

N: 0.0010 to 0.0060%
N is an element that is inevitably mixed in the steel during the steel making process, but if it exists in a large amount as a solid solution element in the steel, the toughness is remarkably lowered. On the other hand, when TiN is formed by combining with Ti, the HAZ structure can be refined by the pinning effect of austenite grains. In order to form a sufficient amount of TiN, it is necessary to contain 0.0010% or more. However, if it exceeds 0.0060%, the toughness is reduced, so 0.0010 to 0.0060% And

Nb: 0.01% or less (including 0)
Nb, like Mn, has the effect of converting the steel structure in the extremely low carbon region to a bainite single-phase structure, but if contained excessively, Nb (C, N) is reduced during the cooling process during high heat input welding. In the present invention, the Nb content is limited to 0.01% or less (including 0) because it may precipitate and cause a decrease in toughness. Preferably, it is 0.007% or less (including 0).

  The above is the basic component composition of the present invention. In order to further improve the characteristics, one or more selected from Cu, Ni, Cr, V, W, Ca, Mg, Zr, and REM are added. It is possible.

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: 0.05 to One or more selected from 0.50% Cu, Ni, Cr, V, and W are all elements that increase the strength of steel mainly by solid solution strengthening. However, if the content is less than the lower limit, the effect is not sufficient. On the other hand, if the content exceeds the upper limit, the weldability is reduced and the alloy addition cost is increased. It is preferable to be in the range.

One selected from Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, Zr: 0.001-0.02%, REM: 0.001-0.02% Or two or more types of Ca, Mg, Zr, and REM all form oxides and sulfides and disperse in the steel, and the pinning effect refines the austenite grain size of the high heat input weld HAZ, contributing to improved toughness. To do. However, if the content is less than the lower limit, the effect is poor, while if the content exceeds the upper limit, coarse oxides and sulfides increase and the toughness decreases. It is preferable that

  Next, the manufacturing conditions of the present invention will be described. A steel material adjusted to the above component composition is manufactured, heated, hot-rolled, and then cooled. The manufacturing method of a steel raw material is not specifically limited, For example, the molten steel melted with the converter can be continuously cast and a slab can be manufactured. In addition, the following temperature represents the average temperature of the steel plate thickness direction unless otherwise indicated. The average temperature in the plate thickness direction is obtained by simulation calculation from the plate thickness, surface temperature, cooling conditions, and the like.

Heating temperature: 950-1250 ° C
In order to obtain a uniform sized austenite structure before rolling, it is necessary to heat to a temperature of 950 ° C. or higher. However, when the heating temperature exceeds 1250 ° C., the structure becomes extremely coarse and finally obtained steel. Since the structure becomes coarse and toughness decreases, the heating temperature is set to 950 to 1250 ° C.

Cumulative rolling reduction in the austenite non-recrystallization temperature range: 50% or more Increasing the rolling amount in the austenite non-recrystallization temperature range refines the packet size of bainite transformed from austenite grains and improves the toughness of the bainite structure .

  Further, the increase in the amount of reduction in the austenite non-recrystallization temperature region increases the density of dislocations accumulated in the austenite grains. Thereby, a part of the dislocation is inherited by the transformed bainite structure, and the strength is further increased. Such an effect becomes more significant as the cumulative reduction amount in the austenite non-recrystallization temperature range of 950 ° C. or lower is larger, so that it is 50% or more.

Rolling end temperature: 680-830 ° C
When the rolling end temperature is lowered, the strength and toughness of the steel material are improved by the effect of refining the bainite structure due to transformation from recrystallized fine austenite grains and increasing the dislocation density of the bainite structure. However, when the rolling end temperature is lowered to less than 680 ° C., the austenite → ferrite transformation starts during rolling, and the generated ferrite is processed, resulting in problems such as a decrease in toughness and an increase in anisotropy. On the other hand, when the rolling end temperature exceeds 830 ° C., it is difficult to obtain the above effect. Therefore, rolling end temperature shall be 680-830 degreeC.

Cooling rate after rolling: 1.0 ° C./s or more In order to ensure the strength of the base material having a tensile strength of 590 N / mm ² or more, an accelerated cooling process is applied after rolling, and cooling is performed at 1.0 ° C./s or more. Cool at speed. 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 concentration of alloy elements occurs in the untransformed austenite, and a hardened phase is easily generated, resulting in a decrease in toughness. Is 580 ° C. or lower.

  In the present invention, a tempering treatment may be further performed after cooling. The tempering treatment is performed in order to adjust the strength and toughness of the bainite generated during cooling and to improve the toughness by decomposing the MA generated between the bainite laths. If it is not satisfied, the above-mentioned effect is not sufficient. On the other hand, when the temperature exceeds 650 ° C., the strength is remarkably lowered.

  Molten steel having various component compositions shown in Table 1 was melted in a converter and made into a slab (plate thickness 300 mm) by a continuous casting method, and then subjected to heat treatment, rolling treatment and cooling treatment under the conditions shown in Table 2. To give a thick steel plate having a thickness of 50 to 60 mm. The tensile properties, base metal toughness, strength and toughness of the weld heat affected zone were evaluated by the methods described below.

(1) Tensile properties A round bar tensile test piece having a parallel part of 14φ x 85 mm and a distance between gauge points of 70 mm is taken and pulled from the central part 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 A 2 mm V notch Charpy test piece is taken from the center of the thickness of each thick steel plate so that the longitudinal direction of the test piece coincides with the rolling direction, and the brittle fracture surface transition temperature (vTrs) of the base material is obtained. It was.

(3) Strength and toughness of weld heat affected zone A joint was prepared by electrogas welding (EGW) (heat input: 350 to 400 kJ / cm), and a hardness test piece and a joint Charpy test piece were collected. For the evaluation of the HAZ strength, the minimum value of the HAZ hardness obtained by performing the Vickers hardness measurement (load: 98 N) at a pitch of 1 mm so as to include the HAZ from the center of the weld metal toward the base metal at the center of the plate thickness. Using.

The HAZ toughness was evaluated by an average value of three absorbed energies (vE- ₂₀ ) by performing a Charpy impact test at a test temperature of -20 [deg.] C. using a Charpy test piece having a notch at a position 1 mm from the bond part. .

Table 3 shows the results of these tests. No. which is an example of the present invention. 1 to 9 were confirmed to have excellent base material properties such as tensile strength of 590 N / mm ² or more and brittle fracture surface transition temperature of −45 ° C. or less. Also, Charpy impact absorption energy value of the welding heat affected zone at (test temperature -20 ° C., 3 times of the average value) of more than 100 J, and the minimum value of the HAZ hardness 175H _V or more, the strength of the welding heat affected zone, It was confirmed that the toughness was also excellent.

On the other hand, no. 10-25 are comparative examples in which the chemical components or production conditions are out of the scope of the present invention. Base material strength: tensile strength is 590 N / mm ² or more, base material toughness: vTrs ≦ −45 ° C., HAZ toughness: vE ₋₂₀ ≧ 100 J (3 present average value), HAZ hardness: minimum 175H _V or more, at least one of but was not achieved.

Claims (5)

C: 0.001 to 0.015% by mass%,
Si: 0.01-0.80%,
Mn: 1.0-2.0%,
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: 0.005 to 0.050%,
N: 0.0010 to 0.0060%,
Nb: 0.01% or less (including 0),
A high toughness high heat input welding steel characterized by comprising a balance Fe and inevitable impurities.   The brittle fracture surface transition temperature (vTrs) is -45 ° C or lower, and the steel for high toughness and high heat input welding according to claim 1 characterized by the above-mentioned.   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 The high toughness high heat input welding steel according to claim 1 or 2, comprising one or more selected from 0.05 to 0.50%.   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 for high toughness high heat input welding according to any one of claims 1 to 3, characterized by containing one or more selected from the above.   A steel material having the composition according to any one of claims 1, 3, and 4 is heated to 950 ° C to 1250 ° C, and then the cumulative reduction ratio in the austenite non-recrystallized region: 50% or more, rolling end temperature : Hot rolling under conditions of 680 to 830 ° C, followed by cooling to 580 ° C or less at a cooling rate of 1.0 ° C / s or more, a method for producing high toughness high heat input welding steel.