JP5493557B2 - Steel material for large heat input welding - Google Patents

Description

  The present invention is suitable for steel materials used for various steel structures in the fields of ships, construction, civil engineering, etc., especially for high heat input welding with high strength of yield stress of 460 MPa or more and welding heat input exceeding 300 kJ / cm. This is related to steel materials.

  Steel materials used in the fields of ships, construction, civil engineering, etc. are usually finished in a desired shape by welding. Therefore, these structures are required to be excellent in the toughness of the welded portion as well as the base material toughness of the steel material used from the viewpoint of ensuring safety.

  Furthermore, in recent years, the ships and steel structures have become increasingly larger, and the steel materials used have been actively promoted to increase strength and thickness. Accordingly, high-efficiency and high-efficiency welding methods such as submerged arc welding, electrogas welding, and electroslag welding have come to be applied to welding construction. Therefore, even when welding is performed by high heat input welding, a steel material that is excellent in the toughness of the welded portion has become necessary.

  Here, FIG. 1 is a macro-structure photograph of a cross section of the welded portion, and in the center of the welded portion, both the molten base material and the weld metal generated from the welding material are almost uniformly mixed and solidified. There are weld metal parts, and on both sides there are heat affected zones (HAZ) that are affected by the heat input during welding, and the structure and properties of the base metal are altered. On both sides, the base material is present. The boundary portion (broken line portion in the figure) between the weld metal and the heat affected zone is generally referred to as a “bond portion”.

  The heat affected zone (HAZ) located in the vicinity of the bond portion is heated to a temperature near the highest melting point in the heat affected zone, and then subjected to a thermal cycle of rapid cooling, so that the heat input during welding is low. It is known that when it becomes large, the crystal grains become coarse and the toughness is remarkably lowered. Many countermeasures have been studied for the reduction in the toughness of HAZ accompanying such high heat input welding. For example, a technique for finely dispersing TiN in steel to suppress coarsening of austenite grains or to use it as a ferrite transformation nucleus has already been put into practical use. Further, a technique aiming at the same effect as described above by dispersing an oxide of Ti has been developed (for example, see Patent Document 1).

  However, the above-described technology using TiN loses the dispersion effect because the heat affected zone is heated to the melting temperature range of TiN and TiN decomposes and re-dissolves when subjected to high heat input welding. In addition, there is a problem that steel is embrittled due to an increase in solid solution Ti and solid solution N generated by decomposition of TiN, and the toughness is significantly reduced. Moreover, the technique using Ti oxide has a problem that it is difficult to disperse the oxide uniformly and finely.

  As a technique for such a problem, for example, in Patent Document 2, in order to improve the toughness of a weld heat affected zone subjected to high heat input welding exceeding 300 kJ / cm, Ca added for controlling the form of sulfide is added. A technique is disclosed in which CaS is crystallized by optimizing the amount of Cu and effectively used as ferrite transformation nuclei. Since this CaS crystallizes at a lower temperature than the oxide, it can be finely dispersed in the steel. Furthermore, during cooling, this is used as a nucleus for ferrite transformation formation nuclei such as MnS, TiN, and BN. Is finely dispersed and precipitated, the structure of the weld heat-affected zone can be a fine ferrite pearlite structure, and high toughness can be achieved.

  However, even with the technique of Patent Document 2, the steel with a yield stress of 460 MPa or more and a relatively large amount of C and alloy elements added was subjected to high heat input welding with a heat input of welding exceeding 300 kJ / cm. Sometimes, a hard embrittlement structure called island martensite (MA) is formed in the heat affected zone near the bond part by several percent in area fraction, which prevents further improvement of the toughness of the weld. It has become. Therefore, in order to improve the toughness of the high heat input weld in such high strength steel, in addition to the fine dispersion of ferrite transformation nuclei and the reduction of solute N and solute B, in the heat affected zone in the vicinity of the bond part. It is necessary to suppress the formation of island martensite.

  Regarding the technique for reducing the island-shaped martensite, for example, Patent Document 3 discloses that distribution of C into untransformed austenite is achieved by decreasing the amount of C and simultaneously decreasing the transformation start temperature by increasing the amount of Mn. A technology for reducing the generation of island-like martensite is disclosed. Further, in Patent Document 4, in addition to the reduction of the contents of C and Si, the reduction of the content of P is effective for the reduction of the island-like martensite in the HAZ part subjected to high heat input welding. It is disclosed.

Japanese Patent Publication No.57-051243 Japanese Patent No. 3546308 JP 2007-049112 A JP 2008-163446 A

  According to the technique of Patent Document 3, the amount of island martensite can be reduced. However, it is necessary to add 0.03 mass% or more of Nb in order to compensate for the strength reduction due to C reduction, and there is a concern about the generation of island martensite due to this. Furthermore, since this technique uses Ti oxide as the transformation nuclei, the technical problem of finely dispersing it remains. Further, according to the technique of Patent Document 4, it is possible to reduce the amount of island martensite and finely disperse ferrite transformation nuclei by adding an appropriate amount of Ca. However, since it is essential to add expensive Ni, there is a problem that the alloy cost is increased.

  Accordingly, an object of the present invention is to provide a steel material for high heat input welding that suppresses the formation of island martensite in the weld heat affected zone without adding an expensive alloy element and has high strength and excellent weld toughness. It is to provide.

  The inventors of the present invention have developed an island-like martensite produced in a heat-affected zone in the vicinity of a bond portion when a high heat input welding with a heat input exceeding 300 kJ / cm is applied to a high strength steel material having a yield stress of 460 MPa or more. We have intensively studied to reduce the amount. As a result, Mn, which is an element that can increase the strength without generating island martensite as much as possible, is positively added, and the content of P, which is an impurity element, is reduced to 0.008 mass% or less. It has been found that the island-like untransformed austenite generated during cooling after high heat input welding is easily decomposed into cementite, and the formation of island martensite can be substantially suppressed, and the present invention has been completed. .

That is, the present invention is C: 0.01-0.03 mass%, Si: 0.01-0.15 mass%, Mn: 2.6-3.5 mass%, P: 0.008 mass% or less, S: 0 .0005 to 0.0040 mass%, Al: more than 0.005 mass% and less than 0.1 mass%, Ti: 0.003 to 0.03 mass%, N: 0.0025 to 0.0070 mass%, B: 0.0003 to 0 .0025 mass%, with the balance being a component composition composed of Fe and inevitable impurities, and island shape in the heat-affected zone structure near the bond when performing high heat input welding with a heat input of welding exceeding 300 kJ / cm It is a steel material for high heat input welding characterized by having a martensite area fraction of 1% or less.

  The steel material for large heat input welding of the present invention is characterized by further containing V: 0.2 mass% or less in addition to the above component composition.

  Moreover, in addition to the said component composition, the steel material for large heat input welding of this invention is further Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Cr: 0.4 mass% or less, Mo: 0.4 mass % Or less and Nb: 0.04 mass% or less.

  Moreover, in addition to the said component composition, the steel materials for large heat input welding of this invention are further Ca: 0.0005-0.0050mass%, Mg: 0.0005-0.0050mass%, Zr: 0.001-0. 0.02 mass%, REM: It contains 1 type, or 2 or more types chosen from 0.001-0.02 mass%, It is characterized by the above-mentioned.

  According to the present invention, even if high heat input welding exceeding 300 kJ / cm is performed, a high-strength steel material having excellent weld heat affected zone toughness can be provided at low cost. Therefore, the present invention greatly contributes to quality improvement and cost reduction of large steel structures constructed by high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding.

It is a photograph explaining the macro structure of a large heat input weld.

First, the structure of the weld heat affected zone (HAZ) in the steel material of the present invention will be described.
The present invention suppresses the generation of island martensite in the heat affected zone (HAZ) of the weld zone, particularly the heat affected zone in the vicinity of the bond zone that is exposed to the highest temperature and austenite is likely to be coarsened. There is a technical feature in improving the toughness of the part. In order to obtain such an effect, it is necessary to reduce the island-like martensite in the heat affected zone near the bond portion to 1% or less in terms of area fraction.

  Here, the heat-affected zone in the vicinity of the bond portion refers to a heat-affected zone in a range within 500 μm from the bond portion. Moreover, the island-like martensite in the heat affected zone near the bond portion can be confirmed by polishing the cross section of the welded portion, etching, and observing with a SEM. The structure of the heat-affected zone in the vicinity of the bond portion is mainly composed of acicular ferrite or bainite, and includes ferrite, pearlite, etc., outside the island martensite.

Next, the component composition necessary for the steel material of the present invention to reduce the island-like martensite and increase the toughness of the welded portion and to increase the strength of the base material will be described.
C: 0.01-0.03 mass%
C is an element that increases the strength of the steel material, and in order to ensure the strength necessary for structural steel, it is necessary to contain 0.01% by mass or more. However, if C exceeds 0.03 mass%, island-shaped martensite is likely to be generated, so the upper limit is made 0.03 mass%.

Si: 0.01-0.15 mass%
Si is an element added as a deoxidizer, and it is necessary to add 0.01 mass% or more. However, if it exceeds 0.15 mass%, the toughness of the base material is reduced, and island martensite is generated in the heat-affected zone subjected to high heat input welding, which tends to cause a reduction in toughness. Therefore, Si is set to a range of 0.01 to 0.15 mass%.

Mn: 2.6-3.5 mass%
Mn has the effect of making the island-like untransformed austenite produced in the HAZ near the bond part during cooling after high heat input welding easier to decompose into cementite and detoxify it, so that it does not produce island martensite. It is an important element that can increase strength. In order to acquire the said effect, addition of 2.6 mass% or more is required. However, when it is added exceeding 3.5 mass%, a large amount of island martensite is generated, and the toughness of the welded portion is lowered. Therefore, Mn is set to a range of 2.6 to 3.5 mass%. Preferably, it is in the range of 2.7 to 3.0 mass%.

P: 0.008 mass% or less P is not easily decomposed into cementite of the island-like untransformed austenite generated in the HAZ near the bond portion during cooling after high heat input welding, and decreases toughness. Is an important element to be restricted. In particular, when the content exceeds 0.008 mass%, the above-described adverse effect becomes remarkable. Therefore, in the present invention, P is limited to 0.008 mass% or less in order to suppress the above-described adverse effects. Preferably, it is 0.006 mass% or less.

S: 0.0005-0.0040 mass%
S is an element necessary for producing MnS or CaS that forms a nucleation site of ferrite. In order to obtain such an effect, it is necessary to contain 0.0005 mass% or more. However, if it exceeds 0.0040 mass%, the toughness of the base material is lowered. Therefore, S is set to a range of 0.0005 to 0.0040 mass%.

Al: more than 0.005 mass% and less than 0.1 mass% Al is an element added for deoxidation of steel, and needs to be contained in excess of 0.005 mass%. However, when 0.1 mass% or more is added, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, Al is taken as the range of more than 0.005 mass% and less than 0.1 mass%.

Ti: 0.003-0.03 mass%
Ti precipitates as TiN during solidification, suppresses austenite coarsening in the weld heat affected zone, and contributes to improving the toughness of the weld heat affected zone as a ferrite transformation nucleus. In order to obtain such an effect, addition of 0.003 mass% or more is necessary. On the other hand, if added over 0.03 mass%, the precipitated TiN becomes coarse and the above effect cannot be obtained. Therefore, Ti is set to a range of 0.003 to 0.03 mass%.

N: 0.0025 to 0.0070 mass%
N is an element necessary for the generation of TiN, and in order to secure a necessary amount of TiN, it is necessary to contain 0.0025 mass% or more. However, if added over 0.0070 mass%, in the region where TiN is re-dissolved by the welding heat cycle, the amount of solute N increases, and the toughness of the welded portion significantly decreases. Therefore, N is set to a range of 0.0025 to 0.0070 mass%.

B: 0.0003 to 0.0025 mass%
B is a useful element for increasing the toughness of the weld heat affected zone because it forms BN in the weld heat affected zone to reduce solid solution N and acts as a ferrite transformation nucleus. In order to obtain such an effect, it is necessary to add 0.0003 mass% or more. However, if added over 0.0025 mass%, the hardenability is increased and the toughness is reduced. Therefore, B is in the range of 0.0003 to 0.0025 mass%.

In the steel material of the present invention, in addition to the above components, V acting as a ferrite forming nucleus can be added in the following range.
V: 0.2 mass% or less V precipitates as VN, contributes to the improvement of the strength and toughness of the base material, and also acts as a ferrite nuclei. Therefore, V can be added as necessary. In order to acquire these effects, it is preferable to make it contain 0.01 mass% or more. However, excessive addition causes a decrease in toughness, so the upper limit is preferably 0.2 mass%.

In addition to the above-mentioned components, the steel material of the present invention should further contain one or more selected from Cu, Ni, Cr, Mo and Nb effective for increasing the strength of the base material in the following range. Can do.
Cu: 1.0 mass% or less Cu is an element effective for increasing the strength of the base material, and in order to obtain the effect, it is preferable to contain 0.05 mass% or more. However, if too much is added, the toughness is adversely affected. Therefore, when added, it is desirable that the amount be 1.0 mass% or less.

Ni: 1.0 mass% or less Ni is an element effective for increasing the strength of the base material. In order to obtain the effect, Ni is preferably contained in an amount of 0.05 mass% or more. However, if too much is added, the toughness is adversely affected. Therefore, when added, it is desirable that the amount be 1.0 mass% or less.

Cr: 0.4 mass% or less Cr is an element effective for increasing the strength of the base material, and it is preferable to contain 0.02 mass% or more in order to obtain the effect. However, if added in a large amount, the toughness is adversely affected. Therefore, when added, the upper limit is preferably set to 0.4 mass%.

Mo: 0.4 mass% or less Mo is an element effective for increasing the strength of the base material, and in order to obtain the effect, it is preferable to contain 0.02 mass% or more. However, if added in a large amount, the toughness is adversely affected. Therefore, when added, the upper limit is preferably set to 0.4 mass%. From the viewpoint of securing the high heat input weld toughness more stably, it is more preferably less than 0.1 mass%.

Nb: 0.04 mass% or less Nb is an element effective for securing the strength and toughness of the base metal and the strength of the welded joint. In order to obtain the effect, Nb is preferably contained in an amount of 0.004 mass% or more. However, if added in excess of 0.04 mass%, island martensite is generated in the weld heat affected zone and the toughness is lowered. Therefore, when adding, it is preferable to make an upper limit into 0.04 mass%.

Moreover, in addition to the said component, the steel material of this invention may add 1 type (s) or 2 or more types chosen from Ca, Mg, Zr, and REM in the following range.
Ca: 0.0005 to 0.0050 mass%
Ca is an element having an effect of improving toughness by fixing S or dispersing oxides and sulfides, and can be added as necessary. In order to acquire the said effect, it is preferable to contain at least 0.0005 mass%. However, even if added over 0.0050 mass%, the above effect is saturated. Therefore, when adding Ca, it is preferable to set it as the range of 0.0005-0.0050 mass%.

Mg: 0.0005-0.0050 mass%, Zr: 0.001-0.02 mass%, REM: 0.001-0.02 mass%
Mg, Zr, and REM are all elements having an effect of improving toughness due to oxide dispersion. In order to obtain such an effect, it is preferable that Mg is contained in an amount of 0.0005 mass% or more, and Zr and REM are contained in an amount of 0.001 mass% or more. On the other hand, even if Mg exceeds 0.0050 mass% and Zr and REM exceed 0.02 mass%, the effect is only saturated. Therefore, when adding these elements, it is preferable to set it as the said range.

  The balance other than the above components in the steel material of the present invention is Fe and inevitable impurities. However, the content of elements other than those described above is not rejected as long as the effects of the present invention are not impaired.

  In addition, the steel material of this invention can be manufactured by a conventionally well-known method, and there is no restriction | limiting in particular in manufacturing conditions. For example, after the hot metal is made into molten steel with a converter or the like, the steel components are adjusted to the appropriate range by RH degassing or the like, and then the steel piece is made through a continuous casting or ingot-bundling process. Next, the steel slab is reheated and hot-rolled to obtain a steel material having a desired size and then allowed to cool, or after the hot-rolling, accelerated cooling, direct quenching-tempering, reheating quenching-tempering It can be manufactured through a process such as reheating, normalizing and tempering.

  No. having the component composition shown in Table 1. Steels 1 to 22 were melted in a 150 kg high-frequency melting furnace and hot-rolled to obtain steel pieces having a thickness of 70 mm. This steel slab was heated at 1150 ° C. for 2 hours, and then further hot-rolled to obtain a steel plate having a thickness of 18 mm, which was allowed to cool. Next, in order to evaluate the toughness after welding heat cycle for these steel plates, test pieces of width 80 mm × length 80 mm × thickness 15 mm were collected from each of the steel plates, heated to 1450 ° C., and then 800 to 500 ° C. A heat treatment was performed to cool the range of 270 seconds at 270 seconds. This heat treatment corresponds to a welding heat cycle that the welding heat affected zone receives when electrogas welding with a heat input of 400 kJ / cm is performed.

Next, a specimen for structure observation was collected from the steel plate after the heat treatment, and after polishing the cross section, island-like martensite was revealed by a two-step etching method, and then a structure photograph of 5 fields of view using a SEM at 2000 times magnification. The five structural photographs obtained were traced and image analysis was performed to determine the average area fraction of the island martensite, which was taken as the MA fraction of the heat affected zone (HAZ) near the bond.
Further, a 2 mm V notch Charpy test piece was collected from the steel plate after the heat treatment so that the longitudinal direction of the test piece coincided with the rolling direction, and a Charpy impact test was conducted in a temperature range of −100 to 40 ° C. The fracture surface transition temperature vTrs, which is 50%, was determined, and toughness was evaluated.

  Table 2 shows the measurement results of the area fraction of island martensite and the toughness of the weld heat affected zone of each steel plate. From Table 2, No. of the present invention example. It can be seen that all of the steel sheets 1 to 12 have an island martensite area fraction of 1% or less and a good weld heat affected zone toughness of −50 ° C. or less in vTrs. On the other hand, any of chemical components such as C, Mn, and P deviates from the scope of the present invention. In all the steel plates of Comparative Examples 13 to 22, the toughness of the weld heat affected zone is lowered to −10 ° C. or more by vTrs when the area fraction of island martensite exceeds 1%. No. In the steel plate of Comparative Example No. 23, since the N content was too low, the solid solution B in the vicinity of the bond portion was excessive, the hardenability was increased, and the fraction of island martensite was increased. On the other hand, the steel sheet of Comparative Example 24 was an example in which the N content was too high, so that the fraction of island martensite was less than 1%, but the solid solution N increased and the toughness decreased.

Claims (4)

C: 0.01-0.03 mass%, Si: 0.01-0.15 mass%, Mn: 2.6-3.5 mass%, P: 0.008 mass% or less, S: 0.0005-0.0040 mass %, Al: more than 0.005 mass% and less than 0.1 mass%, Ti: 0.003 to 0.03 mass%, N: 0.0025 to 0.0070 mass%, B: 0.0003 to 0.0025 mass% In addition, the island martensite area fraction in the heat-affected zone structure in the vicinity of the bond has a component composition consisting of Fe and unavoidable impurities and the heat input with a welding heat input exceeding 300 kJ / cm. Steel material for high heat input welding of 1% or less. The steel material for high heat input welding according to claim 1, further comprising V: 0.2 mass% or less in addition to the above component composition. In addition to the above component composition, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Cr: 0.4 mass% or less, Mo: 0.4 mass% or less, and Nb: 0.04 mass% or less The steel material for large heat input welding according to claim 1 or 2, comprising one or more selected. In addition to the above component composition, Ca: 0.0005-0.0050 mass%, Mg: 0.0005-0.0050 mass%, Zr: 0.001-0.02 mass%, REM: 0.001-0.02 mass The steel material for high heat input welding according to any one of claims 1 to 3, wherein the steel material contains one or more selected from%.

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