JP5842314B2 - High heat input welding steel - Google Patents

Description

  The present invention relates to a steel for welding used in various structures such as shipbuilding, construction, and civil engineering, and particularly has a high heat input welding having excellent heat-affected zone toughness in high heat input with a heat input exceeding 300 kJ / cm. Related to steel.

  As the welded structure becomes larger and the strength and thickness of the steel used increases, the application of highly efficient high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding has increased. Securing the toughness of the weld heat affected zone is an issue.

  A boundary portion between a weld metal and a heat affected zone (HAZ) is generally referred to as a “bond portion”. The heat affected zone (HAZ) in the vicinity of the bond portion is heated to a high temperature particularly in the vicinity of the melting point among the heat affected zone, and then rapidly cooled, so that the hardness often exhibits the highest hardness. Further, it is known that the weld heat affected zone (HAZ) has a large crystal grain size and a significant decrease in toughness when the heat input during welding increases.

  Many countermeasures have been proposed for the reduction of toughness due to large heat input welding. In Patent Document 1, the technique for improving the toughness of large heat input welds is as follows: (1) Particles dispersed in steel (intervening) Prevention of crystal grain coarsening based on the pinning effect of the product) (2) refinement of the transformation structure and effective crystal grain based on the ferrite transformation promotion in the austenite grains, (3) MA It is roughly divided into four points, such as suppression of formation of local embrittlement phase represented by (Martensite-Austenite constituent), and (4) improvement of toughness of geological structure, and thorough reduction of solid solution N as a combination of these. And toughness improvement by the effect of grain refinement by oxides.

  In the case of high heat input welding exceeding 400 kJ / cm, which has been frequently applied in recent years, a steel material capable of ensuring high toughness with a heat input up to about 300 kJ / cm, disclosed in Patent Document 2 It is difficult to secure toughness even with steel materials based on the technology of adding Ca and the technology of adding REM described in Patent Document 3.

  Patent Document 4 relates to a steel material that ensures good weld heat-affected zone toughness even with high heat input welding exceeding 400 kJ / cm, and suppresses austenite coarsening in a high temperature region and promotes ferrite transformation in the subsequent cooling process. As described above, the transformation nucleus is finely dispersed by appropriately containing Ca necessary for the shape control of the sulfide serving as the transformation nucleus.

  Patent Document 5 relates to a steel material that ensures good weld heat affected zone toughness even with a high heat input welding exceeding 500 kJ / cm applied in a BOX column for building, and Mn containing TiN and Ca containing Mg-containing oxides. By dispersing sulfides, coarsening of austenite in a high temperature region is suppressed, and intragranular ferrite transformation in the subsequent cooling process is also promoted.

JP 2001-107177 A JP 60-204863 A Japanese Patent Publication No. 4-14180 Japanese Patent No. 3546308 JP 2003-321728 A

  However, even in the invention steels described in Patent Document 4 and Patent Document 5, in a steel component to which a relatively large amount of C or alloy is added, that is, in a component system having a relatively high strength, the amount of welding heat input. However, a hard embrittlement structure called island martensite is formed in the bond portion structure when large heat input welding exceeding 400 kJ / cm is formed, making it difficult to further improve toughness. In addition, in the invention steel described in Patent Document 5, when Mg is added, Mn sulfide that contributes to intragranular ferrite transformation tends to be difficult to precipitate, and when Charpy impact characteristics at −40 ° C. are required, such as for shipbuilding, further Organizational improvement is required.

  Therefore, an object of the present invention is to provide a steel for high heat input welding in which, in addition to the toughening technique of Patent Document 4, the island-like martensite in the bond structure is reduced to improve the toughness.

The inventors diligently studied effective measures for reducing island martensite formed in the bond structure when high heat input welding was performed, and obtained the following knowledge.
1. It is extremely effective to use acicular ferrite as the intragranular structure of the prior austenite grains in the bond portion.

  As known conventionally, acicular ferrite is also effective in reducing the effective crystal grain size. Acicular ferrite is so-called bainite nucleated from within the grains. When intragranular acicular ferrite is formed, the number of nucleation sites is greater than when bainite is formed from the prior austenite grain boundaries, and the concentration of C in the untransformed austenite is reduced, resulting in island martensite. Is considered to be difficult to form.

  2. In order to promote the formation of intragranular acicular ferrite, it is effective to increase the Mn addition amount to 1.5 to 2.6% by mass in addition to an appropriate amount of Ca addition. Mn + Ca): sulfide contained in the range of 0.1 to 0.7: (CaMn) S is crystallized in the solidification stage when melting the steel sheet.

  In many cases, (CaMn) S is deposited in the form of adhering to a part of the periphery of the oxide, and the particles are spherical. In addition, MnS partially dissolves at high temperature during high heat input welding and reprecipitates on the surface during cooling. MnS itself has the ability to form ferrite nuclei, and MnS formed around The dilute band promotes ferrite transformation and bainite transformation. Furthermore, ferrite transformation nuclei such as TiN, BN, AlN, and VN are deposited on MnS, thereby further promoting ferrite transformation.

  On the other hand, when the amount of Mn added is small, CaS is formed in the solidification stage when melting, but CaS is stable at high temperatures during high heat input welding, and dissolution / re-generation of sulfides containing Mn. Since no precipitation occurs, it does not contribute to the formation of intragranular acicular ferrite.

  3. In order to obtain an acicular ferrite structure in the grains, it is also effective to precipitate pro-eutectoid ferrite along the prior austenite grain boundaries and suppress bainite formation from the prior austenite grain boundaries having a high production rate as much as possible.

  4). In the steel composition, the amount of island martensite generated in the weld heat affected zone is roughly arranged by the value of Ceq (IIW), Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, Each element symbol is content (mass%)): 0.33 to 0.45, and the generation amount of island martensite can be suppressed.

  5. By taking the above measures, it is possible to promote intragranular acicular ferrite transformation in the high heat input welding heat-affected zone, suppress formation of island martensite, and achieve high toughness.

The present invention has been made based on the above findings and further studies, that is, the present invention
1. Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, each element symbol is content (mass%)): steel satisfying 0.33 to 0.45, in mass% C: 0.03-0.08%, Si: 0.01-0.15%, Mn: 1.5-2.6%, P: 0.03% or less, S: 0.0005-0. 0040%, Al: 0.005-0.1%, Nb: 0.003-0.05%, Ti: 0.003-0.03%, N: 0.0025-0.0070%, B: 0 .0003-0.0025%, Ca: 0.0005-0.0030%, the component composition consisting of the balance Fe and inevitable impurities, and Mn / Ca in the mass ratio of Mn / (Mn + Ca): 0 .1 to 0.7 m in size of sulfide or oxysulfide contained in the range of 1 to 0.7 High heat input welding steel 50 to 1000 in ^2, characterized in that it is present dispersed.
2. Further, in terms of mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.4% or less, Mo: 0.4% or less, V: 0.2% or less are selected. The steel for high heat input welding according to 1, which contains one kind or two or more kinds.
3. Furthermore, by mass%, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.02%, REM: 0.001 to 0.02%, or one or more selected from The steel for high heat input welding according to 1 or 2, characterized by containing.

  According to the present invention, steel having excellent weld heat affected zone toughness can be obtained by high heat input welding exceeding 300 kJ / cm such as submerged arc welding, electrogas welding, electroslag welding, and the like, which is extremely useful industrially.

In the present invention, the component composition and the microstructure are defined.
[Component Composition] In the following description, “%” means “mass%”.

Ceq (IIW): 0.33 to 0.45
If Ceq (IIW) is less than 0.33, the required base material strength cannot be obtained. If the area ratio is 0.45 or less, the area fraction of island martensite can be suppressed to less than about 3%, but if it exceeds 0.45, the formation of island martensite is remarkable in the heat affected zone, particularly in the vicinity of the bond. Since the toughness deteriorates, the range of 0.33 to 0.45 is set. Ceq (IIW) is C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol is a content (mass%).

Hereinafter, the reason for limitation of each component is demonstrated.
C: 0.03-0.08%
C is not less than 0.03% in order to obtain the strength required for structural steel, and on the other hand, if it exceeds 0.08%, the formation of island martensite becomes significant, so 0.03 to 0.08% And

Si: 0.01 to 0.15%
Si needs to be 0.01% or more in terms of steelmaking. On the other hand, if it exceeds 0.15%, it degrades the toughness of the base metal and also generates island martensite in the heat-affected zone of high heat input welding. The toughness of the part is deteriorated, so 0.01 to 0.15%.

Mn: 1.5 to 2.6%
Mn needs to be 1.5% or more in order to secure the strength of the base material and promote the formation of acicular ferrite. On the other hand, if it exceeds 2.6%, the toughness of the weld will deteriorate. Therefore, the content is set to 1.5 to 2.6%.

P: 0.03% or less, S: 0.0005 to 0.0040%
In the present invention, P and S are inevitable impurities. If P exceeds 0.03%, the toughness of the welded portion is deteriorated. S needs to be 0.0005% or more in order to generate (Ca, Mn) S necessary for the generation of acicular ferrite. On the other hand, if it exceeds 0.0040%, the toughness of the base material deteriorates. Therefore, the content is set to 0.0005 to 0.0040%.

Al: 0.005 to 0.1%
Al needs to be 0.005% or more, preferably 0.01% or more in terms of deoxidation of steel. On the other hand, if it exceeds 0.1%, the toughness of the base metal is lowered and the toughness of the weld metal is reduced. In order to deteriorate, it is made 0.005 to 0.1%.

Nb: 0.003 to 0.05% or less Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint, but if less than 0.003%, the effect is small. If the content exceeds 50%, the toughness of the weld heat-affected zone will deteriorate, so 0.003 to 0.05% or less.

Ti: 0.003-0.03%
Ti precipitates as TiN during solidification and contributes to the increase in toughness by suppressing the coarsening of austenite and ferrite transformation nuclei in the weld heat-affected zone. On the other hand, if it exceeds 0.03%, the expected effect cannot be obtained due to the coarsening of TiN particles, so 0.003 to 0.03%.

N: 0.0025 to 0.0070%
N is an element necessary to suppress the austenite coarsening in the weld heat affected zone and to secure the necessary amount of TiN that contributes to high toughness by becoming a ferrite transformation nucleus. If less than 0.0025%, TiN is sufficient. On the other hand, if the amount exceeds 0.0070%, the amount of solid solution N increases in the region where TiN is dissolved by the welding heat cycle, so the toughness is remarkably reduced, so 0.0025 to 0.0070%. .

B: 0.0003 to 0.0025%
B needs to be 0.0003% or more in order to generate BN in the weld heat affected zone to reduce the solid solution N and to act as a ferrite transformation nucleus. On the other hand, when added over 0.0025%, hardenability is required. Increases and the toughness of the welded portion deteriorates, so 0.0003 to 0.0025%.

Ca: 0.0005 to 0.0030%
Ca needs 0.0005% or more in order to generate (Ca, Mn) S necessary for the generation of acicular ferrite, and on the other hand, even if contained over 0.0030%, the effect is saturated, 0.0005 to 0.0030%.

The above is the basic component composition of the present invention, but when further improving the characteristics, at least one or more selected from Cu, Ni, Cr, Mo and V can be contained.
Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.4% or less, and Mo: 0.4% or less Cu, Ni, Cr and Mo are effective elements for increasing the strength of the base material. However, in order to obtain the effect, it is necessary to add 0.05% or more of Cu and Ni and 0.02% or more of Cr and Mo. However, if any of these elements is added too much, the toughness is adversely affected. Therefore, when added, it is desirable that Cu and Ni be 1.0% or less and Cr and Mo be 0.4% or less. .
V: 0.2% or less V increases the strength and toughness of the base metal and forms VN in the heat affected zone of the base metal to act as ferrite nuclei. However, if it exceeds 0.2%, the toughness is reduced. Therefore, when it is contained, the content is made 0.2% or less.

  In this invention, when improving the toughness of a welding heat affected zone further, at least 1 sort (s) or 2 or more types chosen from Mg, Zr, and REM can be contained.

Mg: 0.0005-0.0050%, Zr: 0.001-0.02%, REM: 0.001-0.02%
Mg, Zr, and REM have an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, when contained, Mg: 0.0005 to 0.0050%, Zr: 0.001 0.02%, REM: 0.001 to 0.02%. The upper limit is specified because the effect is saturated.
[Sulphides or oxysulfides]
In the steel according to the present invention, the sulfide or oxysulfide containing Mn and Ca in a mass ratio of Mn / (Mn + Ca): 0.1 to 0.7 is 0.1 to 5 μm in the steel. 50 to 1000 particles are dispersed in 1 mm ² in size, and the formation of acicular ferrite is promoted in the vicinity of the bond portion that is exposed to the highest temperature during high heat input welding. The sulfide is (Ca, Mn) S. In addition, when the sulfide is present as an oxysulfide that is a substance combined with an oxide, the formation of the above-mentioned acicular ferrite is promoted as in the case of the sulfide alone.

  If Ceq (IIW) is 0.45 or less, the area fraction of island martensite can be suppressed to less than 3%, but from the viewpoint of improving toughness, it is less than 2%, and further less than 1%. Since it is desirable to suppress, the microstructure is mainly composed of the acicular ferrite structure, and the reduction of island martensite is achieved.

  The amount of island-like martensite produced is mainly due to Ceq (IIW) as described above, but it depends on the bainite transformation mechanism. Is reduced, so that the amount of island martensite produced is also reduced.

  According to the present invention which defines Ceq (IIW) of 0.45 or less and further defines the composition and dispersion state of the sulfide or oxysulfide, the prior austenite grains excluding the grain boundary ferrite precipitated from the prior austenite grain boundaries While the size of the inner structure is 10 μm or less, the formation of island martensite is suppressed to less than 2%, and the toughness of the weld heat affected zone is improved.

  The size of the prior austenite intragranular structure is an evaluation of the size of intragranular ferrite excluding proeutectoid ferrite existing in the prior austenite grain boundaries, and is measured by EBSD (electron beam backscatter diffraction). The average intercept length measured by the line segment method in the grain boundary structure having the above inclination.

  The steel material according to the present invention can be manufactured under manufacturing conditions according to a conventional method for structural steel. For example, the hot metal is first refined in a converter to form steel, and then RH degassing is performed to obtain a steel slab through a continuous casting or ingot-bundling process. This can be reheated and allowed to cool after hot rolling, or alternatively, can be produced by such processes as accelerated cooling, direct quenching-tempering, reheating quenching-tempering, reheating normalization-tempering after the hot rolling. The Hereinafter, the operation and effect of the present invention will be described based on examples.

  In a 150 kg high-frequency melting furnace, steel having the composition shown in Table 1 was melted and formed into a slab having a thickness of 70 mm by hot rolling. The obtained slab was heated to 1150 ° C. for 2 hours, finished to a steel plate having a thickness of 30 mm at a center thickness of 850 ° C., and then accelerated and cooled at a cooling rate of 7 ° C./s. The cooling rate is a simulation of the cooling rate at the position of 1/4 of the thickness of a steel plate having a thickness of 60 mm at the center of the thickness of 30 mm.

  After rolling and rolling a 30 mm steel plate at 500 ° C. for 10 minutes, a round bar tensile test piece with a parallel part 14φ × 85 mm and a distance between gauge points of 70 mm so that the longitudinal direction of the test piece coincides with the plate width direction. The base material strength (yield stress YS, tensile strength TS) was measured. Further, a 2 mm V notch Charpy test piece was taken from the thick steel plate so that the longitudinal direction of the test piece coincided with the rolling direction, and an appropriate Charpy impact test was conducted in the range of −100 to 40 ° C., and the brittle fracture surface ratio was 50%. The fracture surface transition temperature vTrs to be obtained was determined and the toughness was evaluated.

  The obtained steel sheet was examined for the number density and sulfide composition of oxysulfides having a size of 0.1 to 5 μm. The number density of oxysulfides was measured by observing a sample for observing a microstructure using an optical microscope with a magnification of 500 times or 1000 times. Mass ratio in sulfide: Mn / (Mn + Ca) pays attention to sulfide contained in dispersed oxysulfide particles, SEM-EDX (SEM: scanning electron microscope, EDX: energy dispersive X-ray analyzer) It was determined by analysis with an apparatus.

  Furthermore, in order to evaluate the toughness of the high heat input welding heat-affected zone of these steel plates, a reproducible welding heat cycle test was conducted. A reproducible heat cycle test piece of width 80 mm x length 80 mm x thickness 15 mm was collected, heated to 1450 ° C and then cooled to 800-500 ° C in 270 s (recycled welding heat cycle (input in electrogas welding of 30 mm thick steel plate) (Corresponding to a welding heat-affected zone having a heat quantity of 400 kJ / cm) was applied, and vTrs (° C.) was obtained and evaluated by a 2 mmV notch Charpy test.

  The average grain length of the intragranular structure was determined as an index for the formation of acicular ferrite in the reproducible welding heat cycle application part (corresponding to the notch bottom of 2 mmV notch). Intragranular structure average section length is the average section length evaluated by the line segment method with the exception of proeutectoid ferrite present in the prior austenite grain boundaries in grain boundary structures with an inclination of 15 ° or more measured by EBSD. is there.

  The area fraction of island martensite (hereinafter referred to as island martensite fraction (%)) in the reproducible welding heat cycle imparted part (corresponding to the notch bottom of the 2 mmV notch) is expressed as island martensite by a two-step etching method. After taking out, it was calculated by image analysis after tracing a photograph of 2000 times SEM.

Table 2 shows the oxysulfide number density (pieces / mm ² ), sulfide composition (Mn / (Mn + Ca), each element symbol is content (% by mass)) and mechanical properties (YS (MPa)) of the base steel sheet. , TS (MPa)), toughness (Charpy impact test result, vTrs (° C.)), average grain length of the intragranular structure (μm) and repetitive welding heat cycle imparted part (corresponding notch bottom of 2 mmV notch) The site fraction (%) and reproducible welding thermal cycle Charpy impact test results (vTrs (° C.)) are shown.

In Table 2, steel numbers 1, 2 and 8 to 10 are all examples of the present invention that satisfy the chemical composition and microstructure specifications within the scope of the present invention. In part), the intragranular tissue average section length was less than 10 μm, the island-like martensite area fraction was less than 2%, and the reproduced welding thermal cycle Charpy impact test result (vTrs (° C.)) was −55 ° C. or less. .

  On the other hand, Steel Nos. 11 to 23 are comparative examples in which the composition of the components and / or the microstructure is out of the scope of the present invention. Compared to the case of the present invention example, the average intra-granular length (μm) and / or the island-like martensite fraction (%) are large, so that the results of the repeated welding thermal cycle Charpy impact test (vTrs (° C.) was inferior to −45 ° C. or higher.

Claims (3)

Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, each element symbol is content (mass%)): steel satisfying 0.33 to 0.45, in mass% C: 0.03-0.08%, Si: 0.01-0.12%, Mn: 1.63-2.6%, P: 0.03% or less, S: 0.0005-0. 0040%, Al: 0.005-0.1%, Nb: 0.003-0.05%, Ti: 0.003-0.03%, N: 0.0040-0.0060%, B: 0 .0008 to 0.0025%, Ca: 0.0005 to 0.0030%, the component composition consisting of the balance Fe and inevitable impurities, and Mn / Ca in the mass ratio of Mn / (Mn + Ca): 0 The sulfide or oxysulfide contained in the range of 1 to 0.7 is 0.1 to 5 μm in size. heat input 50 to 1000 in m ² which is characterized in that is present in dispersed high heat input welding steels of more than 300 kJ / cm. Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, each element symbol is content (mass%)): steel satisfying 0.33 to 0.45, in mass% C: 0.03-0.08%, Si: 0.01-0.12%, Mn: 1.63-2.6%, P: 0.03% or less, S: 0.0005-0. 0040%, Al: 0.005-0.1%, Nb: 0.003-0.05%, Ti: 0.003-0.03%, N: 0.0040-0.0060%, B: 0 0008 to 0.0025%, Ca: 0.0005 to 0.0030%, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.02%, REM: 0.001 to 0 Contains one or more selected from 0.02%, the balance being Fe and inevitable impurities In the steel composition, a sulfide or oxysulfide containing Mn and Ca in a mass ratio of Mn / (Mn + Ca): 0.1 to 0.7 is 0.1 to 5 μm. A large heat input welding steel having a heat input exceeding 300 kJ / cm, characterized in that 50 to 1000 are dispersed in 1 mm ² . Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, each element symbol is content (mass%)): steel satisfying 0.33 to 0.45, in mass% C: 0.03-0.08%, Si: 0.01-0.12%, Mn: 1.72-2.6%, P: 0.03% or less, S: 0.0005-0. 0040%, Al: 0.005-0.1%, Nb: 0.003-0.05%, Ti: 0.003-0.03%, N: 0.0040-0.0060%, B: 0 0008 to 0.0025%, Ca: 0.0005 to 0.0030%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.4% or less, Mo: 0.4 % Or less, V: one or more selected from 0.2% or less, Mg: 0.0005 to 0.0050%, Z : 0.001 to 0.02%, REM: One or two or more selected from 0.001 to 0.02%, and the component composition consisting of the remaining Fe and unavoidable impurities, and steel The sulfide or oxysulfide containing Mn and Ca in a mass ratio of Mn / (Mn + Ca): 0.1 to 0.7 has a size of 0.1 to 5 μm and 50 to 1000 in 1 mm ^2. Steel for welding with high heat input having an amount of heat input exceeding 300 kJ / cm, characterized by being dispersed.