JP4332064B2 - High HAZ toughness steel for high heat input welding with heat input of 20-100 kJ / mm - Google Patents

Description

  The present invention relates to a steel material for welded structure excellent in the toughness of a weld heat affected zone (hereinafter referred to as HAZ) used for ships, offshore structures, mid-to-high-rise buildings, bridges and the like.

  In recent years, demands for material properties of steel materials for welding used in large structures such as ships, offshore structures, high-rise buildings, and bridges have increased. Furthermore, when building such a structure, in order to promote the efficiency of welding, it is desired to apply a high heat input welding method represented by an electrogas welding method, an electroslag welding method, Similar to the toughness of the steel itself, the demands on the toughness of the HAZ are becoming stricter.

  Conventionally, many proposals have been made focusing on the HAZ toughness of steel materials to which the high heat input welding method is applied. For example, as disclosed in Patent Document 1 and the like, there is a method of reducing the HAZ austenite grains and improving toughness by securing fine Ti nitrides in the steel. Patent Document 2 proposes a method for improving the toughness of HAZ by utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite. Further, Patent Document 3 proposes a method for improving HAZ toughness by utilizing a composite precipitate of Ti nitride and BN as precipitation nuclei of grain boundary ferrite.

  However, since this Ti nitride is almost dissolved in the vicinity of the boundary (hereinafter also referred to as a weld bond portion) with the weld metal having a maximum ultimate temperature exceeding 1400 ° C. of HAZ, the effect of improving toughness is reduced. There is a problem that it ends up. For this reason, it is difficult for steel materials using Ti nitride as described above to achieve the strict requirements for HAZ toughness in recent years and the required characteristics of HAZ toughness in super-high heat input welding.

  As a method for improving the toughness in the vicinity of the weld bond portion, steel containing Ti oxide is used in various fields such as thick plates and section steel. For example, as exemplified in Patent Document 4 and Patent Document 5 in the thick plate field, steel containing Ti oxide is very effective in improving the toughness of a high heat input weld, and is applicable to high-tensile steel. Promising. This principle is based on Ti oxide, which is stable even at the melting point of steel, and Ti nitride, MnS, etc. are precipitated in the middle of the temperature drop after welding, and fine ferrite is formed using these as sites, resulting in toughness. The generation of coarse ferrite that is harmful to steel is suppressed, and deterioration of toughness can be prevented.

  However, such a Ti oxide has a problem that the number dispersed in steel cannot be increased too much. The cause is considered to be coarsening or aggregation of Ti oxides, and if the number of Ti oxides is increased, coarse Ti oxides of 5 μm or more, so-called inclusions are increased. This inclusion of 5 μm or more should be avoided because it is harmful because it becomes a starting point of destruction of the structure or causes a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it is necessary to utilize an oxide that is less likely to be coarsened or aggregated and is more finely dispersed than Ti oxide.

  Further, as a method of dispersing such Ti oxide in steel, there are many methods by adding Ti to molten steel which does not substantially contain a strong deoxidizing element such as Al. However, it is difficult to control the number of Ti oxides and the degree of dispersion in the steel simply by adding Ti to the molten steel. Further, the number and the degree of dispersion of precipitates such as TiN and MnS are controlled. It is also difficult. Therefore, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, there are problems that the number of Ti oxides is not sufficient or that the toughness variation in the thickness direction of the thick plate occurs.

  Furthermore, in the method disclosed in Patent Document 4 and the like, the upper limit of the Al amount is limited to a very small amount of 0.007% in order to easily generate a Ti oxide. Therefore, when the amount of Al in the steel material is small, the toughness of the base material may be lowered due to a shortage of the amount of AlN precipitates. Further, when a steel sheet having a small Al content is welded using a commonly used welding material, the toughness of the weld metal may be lowered.

  In order to solve such a problem, in Patent Document 6 and Patent Document 7, Ti—Al composite oxide or composite oxide of Ti, Al, and Ca generated by adding Al immediately after adding Ti or adding Al and Ca composite. A technology that utilizes the above has been proposed. With such a technique, it has become possible to greatly improve the high heat input welding HAZ toughness.

Japanese Patent Publication No. 55-026164 Japanese Patent Laid-Open No. 03-264614 Japanese Patent Laid-Open No. 04-143246 JP 61-079745 A JP-A-62-103344 Japanese Patent Laid-Open No. 06-293937 JP-A-10-183295

  However, in the shipbuilding industry and the construction industry, in recent years, application of large heat input welding of 20 kJ / mm or more, and large heat input welding of 100 kJ / mm at large has been studied. Steel materials having dramatically improved weld heat-affected zone toughness than conventional methods such as ˜7 have been required. Furthermore, when the carbon equivalent (Ceq) is increased to 0.35 or more from the viewpoint of securing strength, the conventional method requires a further improvement because the HAZ toughness, particularly the toughness improvement in the vicinity of the weld bond portion, is greatly reduced. It became so.

  Therefore, the present invention is a steel material having a Ceq of 0.35 or more and 0.40 or less, and can realize excellent HAZ toughness even in a large heat input welding with a heat input of 20 to 100 kJ / mm, with a heat input of 20 to 100 kJ / mm. An object of the present invention is to provide a high HAZ toughness steel material for high heat input welding.

  The present inventors have conceived that this can be achieved by controlling the metallographic structure of the HAZ part by optimizing the additive elements when high heat input welding is performed with a steel material having a Ceq of 0.35 or more and 0.40 or less. . The background until the present invention is made will be described below.

  In order to refine the reheated austenite grains, it is necessary to suppress the growth of the austenite grains at a high temperature as described above. The most effective method for that is to pin the austenite grain boundaries with dispersed particles and stop the grain boundary movement. As the dispersed particles having such an action, conventionally, Ti nitrides (see Patent Documents 1 to 3) and Ti oxides stable at a high temperature of 1400 ° C. or higher (see Patent Documents 4 and 5) are utilized as pinning particles. It has been. And, since the pinning effect of the grain boundaries by the dispersed particles is larger as the volume fraction of the dispersed particles is larger, the volume fraction of the oxide is increased by using Al and Ca as occasional deoxidizing elements, and appropriate particles are obtained. A method of making the diameter has been proposed (see Patent Documents 6 and 7). Thereby, the reheated austenite grains of HAZ are refined very effectively by pinning, and the HAZ toughness is improved accordingly.

  However, when the welding heat input is a large heat input of 20 to 100 kJ / mm, the HAZ structure becomes a coarse structure mainly composed of upper bainite, particularly in steel materials having a Ceq of 0.35 or more and 0.40 or less, so that sufficient HAZ toughness is obtained. A new problem has arisen. Therefore, in order to improve the HAZ toughness, it was considered necessary to make the structure in the grains fine. And it discovered that addition of B was effective in order to make the bainite structure in a grain fine.

  However, simply adding B may not make the bainite structure in the grains fine. Therefore, the present inventors have further studied in detail. As a result, it was found that when the EB represented by the following formula (2) satisfies −0.0012 or more and 0 or less, the structure in the grains becomes a fine structure including acicular ferrite.

The present invention has been made for the first time after further studies based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.03 to 0.18%, Si: 0.01 to 0.50%, Mn: 0.40 to 2.00%, Al: 0.005 to 0.017 % Ti: 0.005-0.030%, Ca: 0.0005-0.0050%, N: 0.0010-0.0100%, B: 0.0002-0.0024%, P: 0.020% or less, S: 0.020% or less, the balance is Fe and inevitable impurities, and the carbon equivalent (Ceq) represented by the following formula (1) is 0.35 ≦ Ceq ≦ 0.40 The average grain size of austenite grains with a reproducible HAZ structure when the amount of dissolved B (EB) represented by the following formula (2) satisfies −0.0012 ≦ EB ≦ 0 and a welding heat cycle equivalent to 45 kJ / mm is applied. Heat input 20-100 kJ /, characterized in that the diameter is 250 μm or less mm high HAZ toughness steel for high heat input welding.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
EB = B−0.77 × N + 0.23 × Ti (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, B, N, and Ti are content (mass%) of each element.
(2) Further, as selective elements, in mass%, Cu: 0.10 to 1.00%, Ni: 0.10 to 4.00%, Nb: 0.005 to 0.100%, V: 0.010 -0.100%, Cr: 0.01-0.60%, Mo: 0.01-0.60%, Mg: 0.0002-0.0050%, REM: 0.0002-0.1000% The high HAZ toughness steel material for high heat input welding having a heat input of 20 to 100 kJ / mm as described in the above (1), comprising one or more kinds.

  The present invention supplies steel sheets that satisfy severe toughness requirements for the destruction of ships, offshore structures, mid-to-high-rise buildings, etc., and has an extremely large effect on this type of industrial field. Further, it means the safety of structures. The contribution to society is very large.

  Hereinafter, embodiments of the present invention will be described in detail.

  The present invention can be applied to steel pipe materials, steel bars, strips, hot-rolled steel sheets, etc., in addition to general steel materials for welded structures used in ships, offshore structures, medium-rise buildings, bridges, etc. In this case, the toughness of the welded joint can be greatly improved.

  First, the reasons for limiting the basic component range of the present invention will be described.

  C is an effective component for improving the strength of steel, and the lower limit is set to 0.03%. Excessive addition significantly reduces the weldability and HAZ toughness of the steel material, so the upper limit is set to 0.18%.

  Si is a component necessary for securing the strength of the base material, deoxidation and the like, and is added in an amount of 0.01% or more, but the upper limit was made 0.50% in order to prevent the toughness from being lowered by the hardening of HAZ.

  Mn needs to be added in an amount of 0.40% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is set to 2.00% within an acceptable range of toughness and cracking of the welded portion. .

  The smaller the content of P, the better. However, in order to reduce this industrially, it takes a great deal of cost, so the allowable range was set to 0.020% or less.

  The content of S is preferably as small as possible. However, since it takes a great deal of cost to reduce this industrially, the allowable range is set to 0.020% or less.

Al is an important deoxidizing element, and the lower limit was set to 0.005%. In addition, when Al is present in a large amount, the surface quality of the slab deteriorates, so the upper limit was made 0.070%. Note that the upper limit of the Al content is 0.017% or less based on the examples.

  Ti is added in an amount of 0.005% or more in order to combine with N to form Ti nitride. However, since the HAZ toughness decreases as the amount of dissolved Ti increases, the upper limit is set to 0.030%.

  Ca needs to be added in an amount of 0.0005% or more in order to produce a Ca-based oxide. However, excessive addition generates coarse inclusions, so 0.0050% was made the upper limit.

  N is added in an amount of 0.0010% or more in order to combine with Ti to form Ti nitride. However, as the amount increases, the solid solution N increases and the HAZ toughness is lowered, so 0.0100% is made the upper limit. did.

B is an element effective in suppressing the growth of ferrite generated at the heated austenite grain boundary, and is added at least 0.0002%. However, if added in a large amount, the toughness of the steel material is deteriorated, so the upper limit was made 0.0024 %.

  Cu is added in an amount of 0.10% or more as necessary in order to improve the strength and corrosion resistance of the steel material. However, if it exceeds 1.00%, the HAZ toughness is lowered, so the upper limit was made 1.00%.

  Ni tends to improve strength without significantly reducing the toughness of the base metal and also improve HAZ toughness. Therefore, Ni is added in an amount of 0.10% or more as necessary. In order to increase the cost, the range of Ni is set to 0.10% to 4.00%.

  Nb is an effective element for improving the strength of the base material by improving the hardenability, and is added as necessary. However, if the addition is less than 0.005%, a sufficient strength increase cannot be obtained. Further, excessive addition exceeding 0.100% significantly reduces the toughness of the base metal, so the Nb addition range was set to 0.005% or more and 0.100% or less.

  V is an element effective for improving the strength of the base material, and is added in an amount of 0.010% or more as necessary. However, excessive addition significantly reduces the base material toughness. Was the upper limit.

  Since Cr and Mo are also effective elements for improving the strength of the base material, 0.01% or more is added as necessary. However, excessive addition significantly reduces the base material toughness, respectively. The upper limits were 0.60% and 0.60%.

  Both Mg and REM have deoxidizing power next to Ca in molten steel, and have the function of assisting the formation of fine oxides by Ca. Compared with Ca, the cost increase is large, and coarse inclusions are formed to inhibit the toughness of the steel sheet and the HAZ. Therefore, the upper limits were set to 0.0050% and 0.1000%, respectively.

  Next, the reason for limiting the EB value represented by the above equation (2) will be described.

  As described above, the inventors considered that it is necessary to refine the bainite structure in the grains to improve the HAZ toughness, and for that purpose, the addition of B was found to be effective. The structure formed from the austenite grain boundary in the reproduced HAZ structure when a welding heat cycle equivalent to 45 kJ / mm is applied, which was mainly composed of upper bainite lumps when B was not added, was elongated in a needle shape by addition of B. It was found that the structure changed to a structure with a large proportion of ferrite and the structure became finer. However, if B is simply added, the entire surface becomes an upper bainite structure, and the HAZ toughness may not be improved, but may decrease.

  Therefore, as a result of further intensive studies by the present inventors, it was found that the state in which B is in a solid solution is one of the causes that the HAZ toughness is not improved. And the relationship with HAZ toughness was examined by using as an index the solid solution B amount (EB) represented by the above formula (2) containing Ti and N forming compounds with B.

  In the examination, a reconstructed HAZ structure when a welding heat cycle equivalent to 45 kJ / mm was applied (hereinafter also simply referred to as “reproduced HAZ structure”) was used. Welding heat cycle equivalent to 45 kJ / mm means heat obtained from several parts of HAZ obtained when electrogas welding of heat input of 45 kJ is performed on a steel plate having a thickness of 65 mm using a heat cycle test piece. It means the thermal history at the melting line at the center of the thickness analyzed based on the history. More specifically, as shown in FIG. 1, heating is performed from room temperature to a maximum heating temperature of 1400 ° C. in 40 seconds, held at this maximum heating temperature for 25 seconds, and then cooled from 800 ° C. to 500 ° C. over 5 minutes. It means the heat history to do.

  As a result, as shown in FIG. 2, when the value of EB is in the range of 0% by mass or less, the structure changes to a structure having a high proportion of the needle-shaped ferrite in the reproduced HAZ structure, and the structure becomes finer. I found out. As shown in FIG. 3, when the austenite grain size is 250 μm or less and the EB value is in the range of −0.0012 mass% to 0 mass%, the reproduced HAZ structure is the above-described microstructure. Thus, it was found that the reproduced HAZ toughness was -100 J or more in the Charpy absorbed energy value at -20 ° C, and the target toughness was obtained. In FIG. 3, the average austenite grain size is 250 μm or less for plots other than the plot described as “Austenite coarse”. Further, although the reason why the reproduced HAZ toughness deteriorates when the value of EB is less than −0.0012% by mass is not clear, it is probably because the amount of dissolved N in the HAZ is large.

  In addition, as a method of refining austenite grains having a reproduced HAZ structure to 250 μm or less, the present invention disperses a number of fine oxides in steel by containing Ca in an amount of 0.0005 mass% or more, and pinning the oxides. A method of refining crystal grains can be used. By sequentially deoxidizing the molten steel with Al and Ca during the refining of the molten steel, it becomes possible to more reliably disperse many fine oxides in the steel.

  Test materials were made with the chemical components shown in Table 1. A1 to A19 are invention steels, and B1 to 11 are comparative steels. The test material is melted by vacuum melting. Deoxidation was performed by adjusting the dissolved oxygen of the molten steel with C before adding Ti, and then adding Ti, Al, and Ca in order for the test materials other than B9 and B10. For B9, Ti and Al were added simultaneously for deoxidation, and for B10, Ti, Al, and Ca were added simultaneously for deoxidation. Then, these steel materials were heated to 1200 ° C. to obtain 15 mm rolled materials, and heat cycle test pieces were collected. The obtained test piece was subjected to the thermal cycle shown in FIG. 1 simulating high heat input welding equivalent to 45 kJ / mm, and the toughness by the Charpy impact test was evaluated. Toughness was evaluated based on an average energy value obtained by testing three test materials at -20 ° C. The metal structure of the reproduced HAZ structure of each test material was subjected to nital corrosion and observed with an optical microscope. Specifically, the austenite grains were measured from a photograph (160 mm × 200 mm) taken at a magnification of 50 times by a cutting method.

  Table 2 shows the Ceq value represented by the formula (1), the EB value represented by the formula (2), the austenite grain size of the reproduced HAZ structure, and the HAZ toughness value.

  As is apparent from Table 2, the steels of the present invention A1 to A19 have an excellent HAZ toughness with an absorption energy value of 100 J or more in a Charpy impact test at -20 ° C.

  On the other hand, B1 to 11 of Comparative Examples all have Charpy absorbed energy values at −20 ° C. of 100 J or less and have low HAZ toughness. These causes are all out of the scope of the present invention. That is, B1 to 8 are because the EB values are out of the scope of the present invention. B9 is because the deoxidation condition is different from other steels, and the fine oxide sufficient to suppress the coarsening of the austenite grains of the reproduced HAZ structure cannot be dispersed, so the austenite grain size becomes coarser than 250 μm. . This is because B10 is mainly composed of ferrite having a low Ceq value and a coarse HAZ structure in addition to the same reason as B9. This is because B11 has an upper bainite structure having a high Ceq value and a coarse HAZ structure.

It is a figure which shows the thermal cycle reproduction which reproduces the welding heat affected zone of welding heat input 45kJ / mm equivalency. It is a figure which shows the relationship between the amount of solute B (EB value), and the area fraction of the acicular ferrite which occupies in reproduction | regeneration HAZ structure | tissue. It is a figure which shows the relationship between solid solution B amount (EB value) and HAZ toughness.

Claims (2)

% By mass
C: 0.03-0.18%
Si: 0.01 to 0.50%
Mn: 0.40 to 2.00%
Al: 0.005 to 0.017 %
Ti: 0.005-0.030%
Ca: 0.0005 to 0.0050%
N: 0.0010 to 0.0100%
B: 0.0002 to 0.0024% is contained,
P: 0.020% or less S: 0.020% or less,
The balance is Fe and inevitable impurities, and the carbon equivalent (Ceq) represented by the following formula (1) is 0.35 ≦ Ceq ≦ 0.40, and the solid solution B amount (EB) represented by the following formula (2) The average particle size of the austenite grains of the reproduced HAZ structure when satisfying −0.0012 ≦ EB ≦ 0 and applying a welding heat cycle corresponding to 45 kJ / mm is 250 μm or less,
A high HAZ toughness steel material for large heat input welding with a heat input of 20 to 100 kJ / mm.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
EB = B−0.77 × N + 0.23 × Ti (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, B, N, and Ti are content (mass%) of each element. Furthermore, as a selection element by mass%, Cu: 0.10 to 1.00%
Ni: 0.10 to 4.00%
Nb: 0.005 to 0.100%
V: 0.010-0.100%
Cr: 0.01-0.60%
Mo: 0.01-0.60%
Mg: 0.0002 to 0.0050%
REM: 0.0002 to 0.1000%
The high HAZ toughness steel material for large heat input welding with a heat input of 20 to 100 kJ / mm according to claim 1, characterized by containing one or more of the following.

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