JP5233365B2 - Steel material for large heat input welding - Google Patents

Description

  The present invention relates to a steel material used for various steel structures in the fields of shipbuilding, construction, civil engineering, and the like, and more particularly, to a steel material suitable for a purpose of receiving high heat input welding with a heat input of welding exceeding 300 kJ / cm.

  Steel materials used in fields such as shipbuilding, construction, and civil engineering are generally finished into steel structures having a desired shape by welding. Therefore, in these structures, from the viewpoint of safety, it is required that the steel material (base material) to be used is excellent in toughness as well as welded portion toughness. .

  On the other hand, as these steel structures become larger and larger in recent years, the strength and thickness of steel materials used have increased, and submerged arc welding, electrogas welding and electroslag welding have been used for welding. High-efficiency large heat input welding such as is now being applied. Therefore, a steel material having excellent weld toughness when constructed by high heat input welding has become necessary.

  However, it is generally known that when the welding heat input becomes large, the structure of the weld heat affected zone becomes coarse and the toughness decreases. Many countermeasures have been proposed to date against such a decrease in toughness due to high heat input welding. For example, a technique for finely dispersing TiN to suppress austenite grain coarsening or to use it as a ferrite transformation nucleus has already been put into practical use. Patent Document 1 discloses a technique for increasing the toughness of a welded portion by increasing the ferrite nucleation ability by dispersing Ti oxide, and Patent Document 2 by precipitating BN. Yes. Furthermore, Patent Documents 3 and 4 disclose techniques for obtaining high toughness by adding Ca or REM to control the form of sulfide.

  However, in the prior art using TiN, the above-mentioned action of TiN is lost in the weld heat affected zone heated to a temperature range where TiN dissolves, and further, the structure of the ground is caused by solid solution Ti and solid solution N. There was a problem that the toughness was significantly lowered due to embrittlement. Moreover, the technique using the Ti oxide or BN precipitate described in Patent Documents 1 and 2 has a problem that it is difficult to finely disperse oxides and the like uniformly. In order to solve this problem, various studies have been made to improve the dispersibility by a method such as compounding oxides, but in high heat input welding where the heat input exceeds 300 kJ / cm, austenite grains are used. There is a problem that it is difficult to secure the toughness of the weld heat affected zone by suppressing the growth of steel. Moreover, in the technique of adding Ca described in Patent Document 3 or the technique of adding REM described in Patent Document 4, high toughness can be ensured if the heat input amount is about 300 kJ / cm. In high heat input welding exceeding cm, it is difficult to ensure the toughness of the heat affected zone even with these techniques.

On the other hand, the inventors have disclosed a steel material capable of obtaining good weld heat affected zone toughness even in high heat input welding exceeding 400 kJ / cm. The feature of this technique is that an appropriate amount of Ca used for the morphology control of sulfide is contained in order to improve the toughness of the heat-affected zone subjected to high heat input welding exceeding 400 kJ / cm. Specifically, first, CaS is precipitated at the stage of melting and solidifying steel. This is because CaS precipitates at a lower temperature than oxides and can be finely dispersed in steel. At this time, the content of Ca and S and the amount of dissolved oxygen in the steel are controlled within an appropriate range, and after securing the proper amount of solid solution S after crystallization of CaS, MnS is formed on the surface of finely dispersed CaS. Precipitation is particularly important. MnS itself has the ability to form ferrite nuclei, and also has the effect of promoting ferrite transformation by forming a Mn dilute band around it. Further, since ferrite-forming nuclei such as TiN, BN, AlN, and VN are further deposited on the MnS, the ferrite transformation is further promoted. By the above measures, it is possible to finely disperse the ferrite transformation nuclei that do not melt even at high temperatures during high heat input welding, and to make the microstructure of the weld heat affected zone a fine ferrite pearlite structure, thereby achieving high toughness. became.
JP 57-51243 A JP-A-62-170459 JP 60-204863 A Japanese Examined Patent Publication No. 04-14180 Japanese Patent No. 3546308

  However, according to the research by the inventors, the technology described in Patent Document 5 is applied to a component steel to which a relatively large amount of C or an alloy element is added, that is, a relatively high strength steel. Even in the heat-affected zone in the vicinity of the bond when high heat input welding with a heat input greater than 300 kJ / cm is applied, a few percent of hard embrittlement structure called island martensite is generated. It has become clear that the improvement of Here, the heat-affected zone in the vicinity of the bond is a region where the austenite grains are coarsened by being exposed to the highest temperature during welding among the heat-affected zones. For example, the heat-affected zone in the vicinity of the dotted line shown in FIG. That means.

  Therefore, the object of the present invention is to generate island martensite in the heat-affected zone near the bond even when high heat input welding exceeding 300 kJ / cm is performed on high alloy steel containing a large amount of alloying elements. An object of the present invention is to provide a steel material for high heat input welding that can be suppressed and has excellent toughness after welding.

  The inventors diligently studied an effective measure for suppressing the formation of island martensite formed in the heat-affected zone in the vicinity of the weld bond when high heat input welding is performed. As a result, it has been found that it is extremely effective to mainly use the austenite grain structure in the heat-affected zone near the weld bond as the main body of acicular ferrite. This is because acicular ferrite is bainite nucleated from within the grains, and when acicular ferrite is formed in the grains, it nucleates compared to when bainite is formed from the prior austenite grain boundaries. This is probably because the number of sites increases and the C enrichment to untransformed austenite is reduced, making it difficult to form island martensite. When the area fraction of the acicular ferrite is about half or more, the grain boundary ferrite precipitated from the prior austenite grain boundary in the grain boundary structure having an inclination angle of 15 ° or more measured by EBSD (electron beam backscatter diffraction). Since the average segment length obtained by the line segment method is 10 μm or less, this average segment length can be used as an index of the area fraction of acicular ferrite, that is, the area of island martensite It has been found that the average intercept length may be controlled in order to make the fraction not more than a predetermined value.

  In order to promote the formation of intragranular acicular ferrite, it is effective to increase the Mn content to 1.8 to 2.6 mass%. This is because the Mn dilute band formed when MnS precipitates on the surface of CaS promotes the ferrite transformation and bainite transformation, but if the Mn addition amount is high, the Mn content between the parent phase and the dilute band This is considered to be because the difference between the above increases and the driving force of ferrite transformation or bainite transformation increases. 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.

This invention is completed based on the said knowledge, C: 0.03-0.08 mass%, Si: 0.01-0.15 mass%, Mn: 1.8-2.6 mass%, P: 0.03 mass% or less, S: 0.0005 to 0.0040 mass%, Al: 0.005 to 0.1 mass%, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.003 Contains 0.05 mass%, Ti: 0.003-0.03 mass%, N: 0.0025-0.0070 mass%, Ca: 0.0005-0.0030 mass%, B: 0.0003-0.0025 mass% And the following formula (1);
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
However, the above element symbol is the content of each element (mass%)
The carbon equivalent Ceq is in the range of 0.33 to 0.45 , and Ca, O and S are represented by the following formula (2):
0 ≦ (Ca− (0.18 + 130 × Ca) × O) /1.25/S≦1 (2)
However, the above element symbol is the content of each element (mass%)
The composition of the heat-affected zone in the vicinity of the bond when high heat input welding with a heat input exceeding 300 kJ / cm has a component composition consisting of Fe and unavoidable impurities in the balance , Large heat input welding characterized in that the size of the former austenite grain structure excluding the grain boundary ferrite precipitated from the austenite grain boundary is 10 μm or less, and the island-like martensite is 2% or less in area fraction. Steel material. Here, the size of the prior austenite intragranular structure is an average intercept length measured by a line segment method in a grain boundary structure having an inclination angle of 15 ° or more measured by EBSD.

The present invention, in addition to the above chemical composition, C r: 0.4 mass% or less and Mo: characterized in that it contains one or two elements selected from among the following 0.4 mass%.

  According to the present invention, a steel material having excellent toughness after welding can be obtained even when high heat input welding exceeding 300 kJ / cm is performed. Therefore, the present invention greatly contributes to improving the quality and safety of large steel structures constructed by high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding.

First, the component composition of the steel for high heat input welding of the present invention will be described.
C: 0.03-0.08 mass%
C is an element necessary for obtaining the necessary strength as structural steel, and the lower limit is 0.03 mass%. On the other hand, in order to suppress generation of island martensite, the upper limit is set to 0.08 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. On the other hand, if it exceeds 0.15 mass%, the toughness of the base metal is deteriorated, and in addition, island-like martensite is generated in the heat-affected zone near the bond in the high heat input welding to deteriorate the toughness. Therefore, Si is set to a range of 0.01 to 0.15 mass%.

Mn: 1.8 to 2.6 mass%
Mn needs to be added in an amount of 1.8 mass% or more in order to secure the strength of the base material and promote the formation of acicular ferrite. On the other hand, addition exceeding 2.6 mass% deteriorates the toughness of the welded portion. Therefore, Mn is set in the range of 1.8 to 2.6 mass%. Preferably, it is in the range of 1.8 to 2.2 mass%.

P: 0.03 mass% or less P is an impurity inevitably mixed, and inclusion exceeding 0.03 mass% degrades the toughness of the welded portion. Therefore, P is set to 0.03 mass% or less.

S: 0.0005-0.0040 mass%
In the present invention, S is an important element for producing CaS and MnS, and needs to be contained in an amount of 0.0005 mass% or more. On the other hand, if it exceeds 0.0040 mass%, the toughness of the base material decreases. Therefore, S is set to a range of 0.0005 to 0.0040 mass%.

Al: 0.005 to 0.1 mass%
Al is a component necessary for deoxidation of steel, and it is necessary to add 0.005 mass% or more. On the other hand, if added over 0.1 mass%, the toughness of the base metal is lowered and the toughness of the weld metal is lowered. Therefore, Al is made into the range of 0.005-0.1 mass%.

Nb: 0.003-0.05 mass%
Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint. In order to achieve the above effect, Nb needs to be added in an amount of 0.003 mass% or more. On the other hand, if added over 0.05 mass%, the toughness of the weld heat affected zone decreases. Therefore, Nb is set to 0.003 to 0.05 mass%.

Ti: 0.003-0.03 mass%
Ti is an element that precipitates as TiN during solidification and suppresses the austenite coarsening in the weld heat affected zone, or promotes ferrite transformation as a ferrite nucleation and contributes to high toughness. . In order to acquire the said effect, addition of 0.003 mass% or more is required. On the other hand, if it exceeds 0.03 mass%, the TiN particles become coarse and the expected 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 forming TiN, and if it is less than 0.0025 mass%, a sufficient amount of TiN cannot be obtained. On the other hand, if it exceeds 0.0070 mass%, the amount of solid solution N increases in the region where TiN is dissolved by the welding heat cycle, resulting in a decrease in toughness. Therefore, N is set to a range of 0.0025 to 0.0070 mass%.

B: 0.0003 to 0.0025 mass%
B is an element that generates BN at the weld heat affected zone, reduces the 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. On the other hand, when added over 0.0025 mass%, hardenability will increase and toughness will fall. Therefore, B is in the range of 0.0003 to 0.0025 mass%.

Cu: 1.0 mass% or less Cu has an effect of improving strength while ensuring toughness of the base material. In order to acquire the said effect, it is preferable to add 0.1 mass% or more. On the other hand, addition exceeding 1.0 mass% causes hot brittleness and deteriorates the surface properties of the steel material. Therefore, Cu is added in a range of 1.0 mass% or less.

Ni: 1.0 mass% or less Ni, like Cu, has the effect of increasing strength while maintaining the toughness of the base material. In order to acquire the said effect, it is preferable to add 0.1 mass% or more. However, adding more than 1.0 mass% only saturates the effect. Therefore, Ni is set to a range of 1.0 mass% or less.

Ca: 0.0005 to 0.0030 mass%
Ca is an element that improves toughness by fixing S, and the above effect can be obtained by addition of 0.0005 mass% or more. On the other hand, even if added over 0.0030 mass%, the effect is only saturated. Therefore, in the present invention, Ca is in the range of 0.0005 to 0.0030 mass%.

0 ≦ (Ca− (0.18 + 130 × Ca) × O) /1.25/S≦1
Ca, S and O are as follows:
0.2 ≦ (Ca− (0.18 + 130 × Ca) × O) /1.25/S≦1
It is necessary to contain and satisfy the relational expression. Here, Ca, S, and O are content (mass%) of each element.
When the value of (Ca− (0.18 + 130 × Ca) × O) /1.25/S is less than 0, CaS does not crystallize, so S precipitates in the form of MnS alone. This MnS is elongated at the time of rolling the steel sheet to cause toughness reduction of the base material, and MnS melts in the weld heat affected zone, so that it is not possible to achieve the fine dispersion of ferrite forming nuclei that is the main point of the present invention.
On the other hand, when the value of (Ca− (0.18 + 130 × Ca) × O) /1.25/S exceeds 1, most of S is fixed by Ca, so that MnS acting as a ferrite nuclei is formed on CaS. Since it does not precipitate, it cannot function as a ferrite formation nucleus.
Therefore, only when the above formula is satisfied, MnS is deposited on CaS to form a composite sulfide, and the ferrite transformation is promoted.

In addition, O is contained in steel as an unavoidable impurity and reduces the cleanliness of steel. For this reason, it is desirable to reduce as much as possible in the present invention. In particular, when the O content exceeds 0.0030 mass%, CaO inclusions are coarsened and the base material toughness is reduced, so that the content is preferably 0.0030 mass% or less.
Further, in the present invention, in order to crystallize Ca as CaS, it is necessary to reduce the content of O having a strong binding force with Ca before adding Ca, and the residual O amount before adding Ca is It is preferable that it is 0.0030 mass% or less. As a method for reducing the amount of residual O, for example, a method of strengthening degassing or introducing a deoxidizer can be used.

In addition to the above components, the steel material of the present invention can further contain V having a function as a ferrite forming nucleus in the following range.
V: 0.2 mass% or less V is an element effective for improving the strength and toughness of the base material, and is an effective element that precipitates as VN and also functions as a ferrite formation nucleus. However, addition exceeding 0.2 mass% causes a decrease in toughness. Therefore, when adding, it shall be 0.2 mass% or less.

Moreover, in addition to the said component, the steel material of this invention can add Cr and Mo in the following range for the purpose of intensity | strength improvement further.
Cr: 0.4 mass% or less Cr is an element effective for increasing the strength of the base material. However, if added in a large amount, the toughness is adversely affected, so the upper limit is made 0.4 mass%. Therefore, when adding Cr, it is preferable to set it as 0.4 mass% or less.
Mo: 0.4 mass% or less Mo is an element effective for increasing the strength of the base material. However, if added in a large amount, the toughness is adversely affected, so the upper limit is made 0.4 mass%. Therefore, when adding Mo, it is preferable to set it as 0.4 mass% or less.

  In the steel material of the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effect of the present invention is not impaired, the inclusion of other elements is not rejected.

Next, in the present invention, the reason why the carbon equivalent Ceq and the heat affected zone structure near the bond are regulated will be described.
As described above, the present invention adjusts the contents of C, Mn, Ca, S, and O to a limited range, and thus has excellent weld toughness even in high heat input welding exceeding 300 kJ / cm. Can be provided. However, in addition to satisfying the above component composition, the steel material of the present invention needs to have a carbon equivalent Ceq in the range of 0.33 to 0.45 mass%. If the carbon equivalent Ceq is less than 0.33 mass%, the base material strength required as a steel material may not be obtained. On the other hand, if Ceq exceeds 0.45 mass%, the formation of island martensite in the heat-affected zone near the bond becomes significant, and the toughness of the weld may be reduced.

  In the steel material of the present invention, the fraction of island martensite in the heat-affected zone near the bond is required to be 2% or less in terms of area fraction. The area fraction of island martensite can be generally suppressed to less than 3% by setting Ceq to 0.45 mass% or less. However, according to the inventors' research, island-shaped martensite exceeding 2%, even if 3%, adversely affects toughness. Therefore, in order to reliably prevent a decrease in the toughness of the welded portion, it is preferable to suppress the area fraction of island martensite in the heat-affected zone near the bond to 2% or less, desirably less than 1%.

  In order to suppress the area fraction of island martensite to 2% or less, it is preferable to increase the area fraction of acicular ferrite in the heat-affected zone near the bond as much as possible. This is because the formation of island martensite in the heat-affected zone near the bond mainly depends on Ceq, but also depends on the bainite transformation mechanism. As described above, acicular ferrite is generated by bainite transformation in the old austenite grains in the heat-affected zone in the vicinity of the bond. However, as the amount of acicular ferrite increases, the concentration of C into untransformed austenite decreases. The amount of island martensite produced is small. And when the structure excluding the grain boundary ferrite precipitated from the prior austenite grain boundary from the structure of the heat-affected zone near the bond was measured by EBSD, the average intercept measured by the line segment method in the grain boundary structure having an inclination angle of 15 ° or more It was found that when the length was 10 μm or less, the area fraction of the acicular ferrite was approximately half or more.

  Therefore, in the present invention, in order to make the generation amount of island martensite 2% or less in terms of area fraction, in addition to limiting Ceq to the range of 0.33 to 0.45 mass%, further, in the vicinity of the bond The area fraction of acicular ferrite in the heat-affected zone was approximately halved or more, and the average section length measured by the line segment method in the former austenite grain structure of the heat-affected zone in the vicinity of the bond was controlled to 10 μm or less.

Next, the preferable manufacturing method of the steel material of this invention is demonstrated.
The steel material of the present invention is made by refining the hot metal in a converter, electric furnace, or the like, and further performing secondary refining such as vacuum degassing as necessary to smelt steel having a predetermined composition suitable for the present invention. And then, it is preferable to make a steel piece (slab) through a continuous casting process or an ingot-bundling process. The steel slab is then reheated and hot-rolled and allowed to cool, or after the hot-rolling, accelerated cooling as necessary, direct quenching-tempering, reheating quenching-tempering, reheating It is preferable to obtain a product through a process such as normalizing and tempering.

  Using a 150 kg high-frequency melting furnace, steel having the composition shown in Table 1 was melted to form a steel ingot, and then hot rolled to obtain a sheet bar having a thickness of 70 mm. Next, this sheet bar is heated to 1150 ° C. for 2 hours, then subjected to hot rolling at a finish rolling end temperature of 850 ° C. at the center of the plate thickness to obtain a steel plate with a plate thickness of 30 mm, and the cooling rate at the center of the plate thickness is Accelerated cooling was performed to achieve 7 ° C./sec. This cooling rate is a simulation of the cooling rate of a 1 / 4t portion of a steel plate having a thickness of 60 mm with a steel plate having a thickness of 30 mm. Then, after tempering the steel plate at 500 ° C. for 10 minutes, in order to evaluate the base material properties, from the steel plate, a round bar tensile test piece having a parallel portion 14φ × 85 mm and a distance between gauge points of 70 mm, and 2 mmV Notch Charpy impact test specimens were collected and subjected to tensile tests and Charpy impact tests, and yield stress YS, tensile strength TS, and toughness characteristics (vTrs) were measured.

  Furthermore, in order to evaluate the toughness characteristics after the welding heat cycle of the steel sheet, a test piece having a width of 80 mm, a length of 80 mm, and a thickness of 15 mm was collected from these steel sheets, and the test piece was heated to 1450 ° C. and 800-500 after heating. A heat treatment was performed to cool the temperature at 270 seconds. This heat treatment corresponds to a thermal cycle that the heat-affected zone near the bond undergoes when electrogas welding with a heat input of 400 kJ / cm is performed. Thereafter, a 2 mm V notch Charpy impact test piece was sampled from this test piece and subjected to a Charpy impact test to evaluate toughness characteristics (vTrs).

Moreover, about the steel plate after the said heat processing, the area fraction of acicular ferrite (AF) and island-like martensite (MA) was measured. The area fraction of acicular ferrite was obtained by tracing an optical microscopic structure photograph and analyzing the image. Further, the area fraction of island martensite was obtained by tracing island structure martensite photographed with SEM after revealing island martensite by a two-step etching method and analyzing the image.
Moreover, about the steel plate after the said heat processing, the average section length of the prior austenite intragranular structure | tissue and the area fraction of island-like martensite (MA) were measured. Here, the average intercept length of the prior austenite grain structure is obtained by measuring the heat-affected zone in the vicinity of the bond in two fields of 300 μm × 300 μm in EBSD and drawing a boundary map having an inclination angle of 15 ° or more. From the obtained grain boundary structure, grain boundary ferrite precipitated from the prior austenite grain boundary was excluded, and the previous austenite grain structure was specified, and was obtained using a line segment method. In addition, the area fraction of island martensite was obtained by tracing and analyzing the image of 2000 times of tissue photograph taken with SEM (scanning electron microscope) after the island martensite was revealed by the two-step etching method. Asked.

  Table 2 shows the results of the base metal characteristics and weld toughness characteristics measured as described above. From Table 2, the steel sheets of the inventive examples having component compositions suitable for the present invention have a structure in the heat-affected zone near the bond, the average section length is 10 μm or less, and the island-like martensite is 2% or less. As a result, it can be seen that good weld toughness of vTrs of −55 ° C. or lower is obtained. On the other hand, in the steel sheet of the comparative example whose component composition is outside the range of the present invention, one or more of the average segment length in the structure of the heat-affected zone near the bond and the area fraction of island martensite are present in the present invention. By deviating from the above range, only the inferior toughness of the heat-affected zone near the bond is obtained.

It is a schematic diagram explaining the heat affected zone of the bond vicinity in this invention.

Claims (2)

C: 0.03-0.08 mass%, Si: 0.01-0.15 mass%, Mn: 1.8-2.6 mass%, P: 0.03 mass% or less, S: 0.0005-0.0040 mass %, Al: 0.005 to 0.1 mass%, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.003 to 0.05 mass%, Ti: 0.003 to 0.03 mass% N: 0.0025-0.0070 mass%, Ca: 0.0005-0.0030 mass%, B: 0.0003-0.0025 mass%, and the carbon equivalent Ceq shown in the following formula (1) is 0.00 . in the range of from 33 to 0.45, and, Ca, O and S contains satisfies the following expression (2), has a component composition and the balance being Fe and unavoidable impurities, the weld heat input 30 The structure of the heat-affected zone in the vicinity of the bond when high heat input welding exceeding kJ / cm is applied, and the size of the prior austenite intragranular structure excluding the grain boundary ferrite precipitated from the prior austenite grain boundary is 10 μm or less. And the steel material for high heat input welding characterized by island-like martensite being 2% or less by area fraction. Here, the size of the prior austenite intragranular structure is an average intercept length measured by a line segment method in a grain boundary structure having an inclination angle of 15 ° or more measured by EBSD.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
0 ≦ (Ca− (0.18 + 130 × Ca) × O) /1.25/S≦1 (2)
However, the above element symbol is the content of each element (mass%) In addition to the above chemical composition, C r: 0.4mass% or less and Mo: rafters according to claim 1, characterized in that it contains 0.4 mass% 1 kind or two kinds selected from among the following Steel for heat welding.

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