JP3390234B2 - Steel with excellent heat input welding characteristics - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded structural steel material used for construction, bridges, shipbuilding, pressure vessels, etc.
80 when performing 0kJ / cm or more high heat input welding, to a soluble Se' granulated steel material which can ensure low-temperature toughness of the heat affected zone.

[0002]

2. Description of the Related Art Generally, the toughness of a welded part is mainly determined by the toughness of a heat-affected zone of a base metal, particularly a weld bond. That is, since the bond part is heated to a high temperature just below the melting point, the crystal grains become extremely coarse and hardenability increases, and subsequent cooling makes it difficult for ferrite transformation to occur, so that a fragile upper bainite structure is formed. This is because the island-like martensite is generated and the notch toughness is reduced, and this tendency is particularly remarkable in the welding heat affected zone by so-called large heat input welding such as electroslag welding and multi-electrode submerged arc welding.

As measures for improving the toughness of such welded joints, in addition to lowering the C equivalent and reducing the impurity elements such as P and S, the fine precipitates of nitrides such as TiN and AlN are precipitated to fix the solid solution N 2. Along with the measurement, a method of preventing coarsening of austenite grains has been adopted. For example,
Japanese Patent Publication No. 52-8246 discloses a technique for improving the toughness of a high heat input welded bond by dispersing a fine Ti compound in a steel material. Further, as a recent technical tendency in the use of the same Ti compound, Japanese Patent Laid-Open No.
As shown in Japanese Patent Publication No. 3, the excessive presence of N adversely affects toughness,
The amount of N is kept low without increasing it more than necessary, and the amount of Ti is adjusted accordingly. Further, the above publication further includes REM
Also disclosed is a technology for making REM compounds finer by utilizing the compounds and reducing the Si content.

[0004]

[Problems to be Solved by the Invention] In recent years, there is an increasing demand for higher efficiency in welding work and cost reduction, and welding with an even higher heat input of 300 kJ / cm or more, which has not been applied so far, has been applied. I have a desire to increase. However, in this case, the weld bond is exposed to high temperature for a long time, the austenite grains become coarser, and the subsequent cooling becomes slower, resulting in the formation of a fragile upper bainite structure and island martensite. Will be even easier. Therefore, the effect of improving the toughness of the heat-affected zone of the weld is not sufficient by the above-mentioned conventional method that targets heat input up to about 300 kJ / cm.

The present invention solves such a problem, and even in the large heat input welding of 800 kJ / cm or more, which has a larger heat input than the conventional large heat input welding, the toughness of the heat affected zone is excellent. The present invention provides a hot rolled steel material for a welded structure.

[0006]

[Means for Solving the Problems] As a result of various studies of steel materials having a high toughness in the heat-affected zone of the welding with a high heat input of 800 kJ / cm or more, the inventors have found that the heat input with such a large heat input is very large. It was found that the appropriate range of REM and B content for the steel material used for heat welding is different from that of conventional steel, which targets heat input up to about 100 kJ / cm. In addition, it is not possible to improve the toughness of the weld heat affected zone only by optimizing the contents of these two elements, REM and B, and it is necessary to further reduce the Si content to 0.10 wt% or less. The inventors have found that even if any one of the optimization of the content is lacking, the sufficient effect cannot be expected, and the present invention has been completed.

That is, according to the present invention, C: 0.05 to 0.20 wt%
, Si: 0.10 wt% or less, Mn: 0.4 to 2.0 wt%, Al: more than 0.005 to
0.05wt%, Ti: 0.003-0.030wt%, B: 0.002-0.005wt
%, REM: 0.010 to 0.030 wt%, N: 0.002 to 0.005 wt%, the balance being Fe and unavoidable impurities composition, large heat input welding with heat input of 80 kJ / cm or more It is a hot-rolled steel material for welded structure with excellent characteristics, and the second invention is C:
0.05 to 0.20 wt%, Si: 0.10 wt% or less, Mn: 0.4 to 2.0 wt%
, Al: 0.005 ultra ~0.05wt%, Ti: 0.003~0.030 wt% , B:
0.002-0.005 wt%, REM: 0.010-0.030 wt%, N: 0.00
2 to 0.005 wt%, and further Cu: 0.05 to 0.5 wt%
, Ni: 0.05 to 1.0 wt%, Cr: 0.05 to 0.5 wt%, Mo: 0.05
~ 0.5 wt%, Nb: 0.005-0.10 wt%, V: 0.005-0.15 wt%
, Ca: 0.0005 to 0.005 wt% of 1 type or 2 types or more, with the balance being composition of Fe and unavoidable impurities, for large heat input welding characteristics of heat input of 800 kJ / cm or more. Excellent melting
It is a hot rolled steel material for contact structures .

[0008]

[Function] First, if C is less than 0.05 wt%, it is difficult to secure the desired base metal strength, and if it exceeds 0.20 wt%, ferrite transformation becomes difficult due to cooling after welding, and the weld heat affected zone It becomes difficult to refine the structure, and island martensite is generated, so that the toughness of the welded portion cannot be secured.

Mn, like C, is an element that contributes to the improvement of strength. To secure the strength required for welded structural steel, Mn is required to be 0.4 wt% or more, but if added in excess of 2.0 wt%. The crack susceptibility at the time of welding is increased and the toughness of the welded part is adversely affected, so the range was set to 0.4 to 2.0 wt%. Al needs to contain at least 0.005 wt% or more for deoxidation in the ordinary steelmaking process.
Since content causes also degrade toughness of the weld metal not adversely HAZ only more than 0.05 wt%, 0.005 super ~0.04wt
The range is%.

Ti and N form TiN, prevent coarsening of austenite crystal grains during heating by welding, and act as ferrite transformation nuclei to divide and miniaturize the weld heat-affected zone structure. Here, in order to exert the toughness improving effect, Ti and N must be 0.003 wt% or more and 0.002 wt% or more, respectively. However, if 0.005 wt% or more of N is contained, it is considered that the solid solution N adversely affects the toughness of the steel, so the upper limit of N content is 0.005 wt%, and in this case, Ti containing more than 0.0030 wt% is contained. However, not only does it have no effect, but rather an unbalanced excess of Ti with respect to the N content impairs toughness, so the upper limit of Ti content is 0.003.
wt%

B suppresses the precipitation of coarse grain boundary pro-eutectoid ferrite which segregates at the austenite grain boundaries during heating by welding and adversely affects toughness, and promotes the precipitation of intragranular ferrite that divides and refines the structure, Increase the effect of Ti and N. It also has the effect of precipitating as BN during cooling after welding and fixing the solid solution N 2 to improve toughness. In particular, in order to improve the toughness of the weld heat affected zone in welding with a larger heat input than the range of conventional large heat input welding, which is the subject of the present invention, 0.002 wt% or more, which will be described later
It is necessary to add it together with M. However, even if added in excess of 0.005 wt%, the effect saturates and adversely affects toughness, so the range was made 0.002 to 0.005 wt%.

[0012] REM precipitates as sulfur oxide and has an effect of preventing coarsening of austenite crystal grains by itself, and promotes the effects of Ti, B and N as precipitation nuclei of TiN and BN. 80 Big further heat input than conventional high heat input welding 0k
Large heat input of J / cm or more is 0 for improving the toughness of the heat affected zone.
It is necessary to add 010 wt% or more together with B. However, the addition of more than 0.030 wt% impairs the cleanliness of the steel, so the range was 0.010 to 0.030 wt%.

Si is usually added because it increases the strength, but in order to improve the toughness of the large heat input welding heat affected zone of 800 kJ / cm or more targeted by the present invention, Si is 0.10 wt% or less. It is necessary to reduce. Even if the structure of the weld heat affected zone is adjusted by adding the appropriate amount of REM and B, if the Si content exceeds 0.10 wt%, the heat input will be 80
High heat input of 0 kJ / cm or more It is difficult to improve the toughness of the heat affected zone. Fig. 1 shows the heat input of 800 kJ / when the structure of the heat affected zone of welding is adjusted by containing an appropriate amount of REM and B.
The relationship between vE ₀ of the simulated weld heat affected zone simulating a large heat input welded bond area equivalent to cm and the Si content in steel is shown.

The decrease in strength due to the reduction of Si can be compensated by adding another strength improving element or by a manufacturing method such as controlled rolling or accelerated cooling. In addition to these basic components, Cu: 0.05-0.5 wt%, Ni: 0.05-1.0 wt%, Cr: 0.0
5 to 0.5 wt%, Mo: 0.05 to 0.5 wt%, Nb: 0.005 to 0.10 wt
%, V: 0.005 to 0.15 wt% of the elements that increase the strength and Ca: 0.0005 to 0.005 wt%, and one or more of them can be contained, but if Cu is added excessively, Ni is an expensive element because it impairs workability and increases the weld cracking susceptibility, excessive addition impairs economic efficiency, and excessive addition of Cr increases weld cracking sensitivity. The upper limits are 0.5 wt% and 1.0 w, respectively.
t% and 0.5 wt%. Further, Mo is an expensive element and, if added in excess, it enhances the hardenability of the weld heat affected zone and increases the susceptibility to weld cracking, so the upper limit was made 0.5 wt%. The upper limits of Nb and V were set to 0.10 wt% and 0.15 wt%, respectively, because when added excessively, a large amount of precipitates are generated in the base metal and the weld heat affected zone, causing precipitation embrittlement.

Ca is effective in controlling the morphology of MnS and improving the low temperature toughness of the base material and the weld heat affected zone. However, if less than 0.0005 wt%, practical effects cannot be expected, and if more than 0.005 wt% is added, the cleanliness of the steel is impaired, so the addition range was made 0.0005 to 0.005 wt%.

[0016]

EXAMPLES Table 1 shows the chemical composition of trial steels. The strength level of the prototype steel is 400 to 490 MPa class steel. These steels were cast according to a conventional manufacturing process and then hot rolled to a plate thickness of 80 mm.

[0017]

[Table 1]

Similarly, Table 1 shows the tensile strength (TS) of steel and heat input.
The Charpy absorbed energy (vE ₀ ) value at 0 ° C of the weld pond by a simulated welding heat cycle equivalent to 800 kJ / cm is shown. Note that vE ₀ is the average of the values obtained by carrying out the 3 main tests. Steels A to H are examples, low Si-Ti-low N-high RE
Ferrite precipitates in the microstructure of the heat-affected zone due to M-high B treatment, the microstructure is refined, and the formation of island martensite is also suppressed, indicating good toughness.

Steels I to K are comparative steels. Steel I does not have sufficient REM and B contents, and Steel J has insufficient REM contents.Therefore, precipitation of ferrite was insufficient in the heat-affected zone and the microstructure was not sufficiently refined. , Its toughness is lower than that of the example. Since steel K has too much Si content, the toughness of the weld heat affected zone is
It is lower than that of the example. It is considered that this is because the island martensite generated in the weld heat affected zone adversely affects the toughness.

[0020]

EFFECTS OF THE INVENTION According to the present invention, large heat input welding with a heat input of 800 kJ / cm or more can be used in the production of structures such as buildings, bridges, shipbuilding and pressure vessels, thus greatly improving welding efficiency. And the cost reduction is achieved.

[Brief description of drawings]

[Fig.1] 0.14C-1.3Mn-0.025Al-0.015REM-0.01Ti-0.0025
3 is a graph showing the relationship between the Si content of B-0.004N steel and vE _{0 of} a reproduced HAZ simulating a large heat input welded bond portion having a heat input of 800 kJ / cm.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Works Ltd. Technical Research Division (56) Reference JP-A-58-58253 (JP, A) Kai 61-253344 (JP, A) JP-A-1-150453 (JP, A) JP-A 63-5805 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38 / 00-38/60

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.20 wt%, Si: 0.10 wt% or less,
Mn: 0.4~2.0 wt%, Al: 0.005 ultra-~0.05wt%, Ti: 0.003~
0.030 wt%, B: 0.002-0.005 wt%, REM: 0.010-0.03
Heat input 80 0k, characterized by containing 0 wt% and N: 0.002 to 0.005 wt% with the balance being Fe and inevitable impurities.
Hot-rolled steel for welded structure with excellent heat input welding characteristics of J / cm or more. <br/> 2. C: 0.05 to 0.20 wt%, Si: 0.10 wt% or less,
Mn: 0.4~2.0 wt%, Al: 0.005 ultra-~0.05wt%, Ti: 0.003~
0.030 wt%, B: 0.002-0.005 wt%, REM: 0.010-0.03
0 wt%, N: 0.002-0.005 wt%, and Cu: 0.0
5 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, Cr: 0.05 to 0.5 wt%
%, Mo: 0.05-0.5 wt%, Nb: 0.005-0.10 wt%, V: 0.005
~ 0.15wt%, Ca: 0.0005 ~ 0.005wt% 1 or 2
A hot-rolled steel material for welded structures having a large heat input of 800 kJ / cm or more and excellent heat input welding characteristics, characterized in that it contains Fe or more and the balance is Fe and inevitable impurities.