JPH08232043A - Steel for large heat input welding at heat input of 500kj/ cm or more and its production - Google Patents

Abstract

PURPOSE: To produce a steel for welded structure, excellent in toughness in a weld heat-affected zone even if subjected to large heat input welding. CONSTITUTION: This steel has a composition consisting of, by weight, 0.10-0.20% C, <0.10% Si, 0.4-2.0% Mn, 0.01 to <0.04% Al, 0.002-0.020% Ti, 0.003-0.030% REM, <0.0040% N, 0.0003-<0.0020% B, and the balance essentially Fe with inevitable impurities. Moreover, This steel has a structure in which precipitated B is regulated to <0.0015% and also the oxides and sulfides of REM or Ca and TiN are separately dispersed in the steel, respectively, and further, this steel can contain one or >=2 kinds among 0.05-0.5% Cu, 0.05-1.0% Ni, 0.05-0.5% Mo, 0.005-0.10% Nb, 0.005-0.15% V, and 0.05-0.5% Cr.

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., and particularly to a large heat input welding steel having a heat input of 500 kJ / cm or more and a method for producing the same. Is.

[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 welded 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, a low C equivalent and reduction of impurity elements such as P and S, as well as precipitation of fine precipitates of nitrides such as TiN and BN to fix solid solution N. A method of measuring and precipitating fine ferrite is adopted. For example, JP-A-51-41621
In JP and JP-A-51-43309, Ca or La,
By adding Ce and Ti in combination, fine TiN is dispersed in the steel material, and by using this as a transformation nucleus of ferrite, the high heat input welded bond part has a (ferrite + pearlite) structure to improve toughness. Furthermore, it is said that the addition of Nb, V, and B nitrides stabilizes the effect.

However, in a weld bond portion with a heat input of 500 kJ / cm or more, the time of exposure to high temperature becomes longer, the austenite crystal grains become coarser, and the subsequent cooling becomes slower, so that it becomes difficult to form fine ferrite. Coarse grain boundary ferrite precipitates and the toughness improving effect disappears. Although a method of adding a large amount of Si, which is a ferrite-forming element, can be considered, island martensite is formed and the effect of improving toughness is lost.

Japanese Patent Laid-Open No. 61-190016 also discloses fine ferrite precipitation by using TiN and BN and improvement of toughness of weld bond part by fixing N. Heat input is 500k
The same problem as above occurs at the weld bond area of J / cm or more. In particular, BN forms a solid solution due to heating during welding, and γ
Since it re-precipitates at the grain boundaries, it acts as precipitation nuclei for coarse grain boundary ferrite and has an adverse effect.

In Japanese Patent Laid-Open No. 59-159968, REM acid
In sulfide and TiN dispersed steel, it is said that precipitation of network-like coarse grain boundary ferrite is prevented by grain boundary solid solution B, but normally the presence of solid solution B is 800 to 1000 Island-like martensite is generated in the heating portion at ℃, which impairs toughness.

[0007]

[Problems to be Solved by the Invention] In recent years, there has been an increasing demand for higher efficiency of welding work and cost reduction thereof. Adoption of welding with a larger heat input of 500 kJ / cm or more, which was hardly applied in the past. There is a desire to increase. However, in this case, the time for the weld bond to be exposed to high temperatures becomes longer, the austenite grains become coarser, and the subsequent cooling becomes slower, resulting in the fragile upper bainite structure and island martensite. Generation is also easier. For this reason, the effect of improving the toughness of the weld heat affected zone cannot be said to be sufficient by the above-mentioned conventional method, which targets heat input of less than about 500 kJ / cm.

The present invention solves such a problem, and even in the large heat input welding of 500 kJ / cm or more, which has a larger heat input than the conventional large heat input welding, the toughness of the welding heat affected zone is excellent. It is an object of the present invention to provide a welded structural steel and a manufacturing method thereof.

[0009]

[Means for Solving the Problems] The present inventors have found that 500 kJ / cm
As a result of various examinations of steel materials having good toughness in the above-mentioned large heat input welding heat affected zone, the toughness improvement mechanism to be used is
It was concluded that it is different from the conventional steel that has heat input up to about 300 kJ / cm. That is, it is necessary to reduce the old γ grain size of the weld bond to 250 μm or less by dispersing TiN and REM acid / sulfide, but if these dispersed grains act as ferrite transformation nuclei,
When the cooling rate is slow, such as high heat input welding of 0 kJ / cm or more,
Coarse ferrite precipitates, which adversely affects toughness. Therefore, in order to prevent this, TiN and REM acid / sulfide are not compounded but dispersed separately, and the amount of Si, which is a ferrite-forming element, is 0.10% by weight (hereinafter simply referred to as%). ) It was found necessary to reduce the following.

Further, the present inventors can use a trace amount of B to prevent coarse grain boundary ferrite, but 0.0015
% Or more of the precipitation B becomes a precipitation nucleus of coarse grain boundary ferrite on the contrary, the precipitation B is set to less than 0.0015%, and in this case as well, due to the effect of low Si, the island shape of the heating part at 800 to 1000 ° C is formed. It has been found that the precipitation of martensite is prevented and it is effective in preventing the deterioration of toughness.

As described above, the old γ grain size of the weld bond portion after high heat input welding is 250 μm or less, and bainite containing no island martensite is mainly contained, and grain boundary ferrite is suppressed to 20% or less. It was possible to improve the toughness of the heat-affected zone with a large heat input of 500 kJ / cm or more. That is, the present invention, by weight%, C: 0.10% more than 0.20% or less, Si: 0.10%, Mn: 0.4-2.0%, Al: 0.010% more than 0.04%, Ti: 0.0
02 to 0.020%, REM: 0.003 to 0.030%, N: less than 0.0040%, B: 0.0003% to less than 0.0020%, the balance consisting essentially of Fe and unavoidable impurities, except for the precipitation B
Is less than 0.0015%, and REM acid / sulfide and TiN are contained in the steel.
Is a large heat input welding steel having a heat input of 500 kJ / cm or more, each having a structure in which they are separately dispersed, and the present invention is, by weight%, C: more than 0.10% and 0.20% or less. , Si: 0.10%
Less than, Mn: 0.4 to 2.0%, Al: more than 0.010% and less than 0.04%, Ti:
0.002 to 0.020%, Ca: 0.001 to 0.005%, N: less than 0.0040%, B: 0.0003% to less than 0.0020%, the balance consisting essentially of Fe and unavoidable impurities, but with the precipitation B
Is less than 0.0015%, is a steel for large heat input welding with a heat input of 500 kJ / cm or more, characterized by having a structure in which Ca acid / sulfide and TiN are separately dispersed in the steel, In addition, the present invention provides the steel having the composition and structure described in the above invention, further comprising:
, Cu: 0.05-0.5%, Ni: 0.05-1.0%, Mo: 0.05-
0.5%, Nb: 0.005-0.10%, V: 0.005-0.15%, Cr: 0.0
A steel for high heat input welding having a heat input of 500 kJ / cm or more, characterized by containing 5 to 0.5% of one kind or two or more kinds, and the present invention is to cast a steel having the composition described in the above invention. After that, Ac ₃
After heating to the transformation point or higher, hot rolling is performed at a rolling finishing temperature of 800 to 1000 ° C and a cumulative reduction of 50% or more, and then air cooling or acceleration is performed at a cooling rate at which the steel sheet structure becomes (ferrite + pearlite) and / or bainite This is a method for producing a steel for high heat input welding with a heat input of 500 kJ / cm or more, which is characterized by cooling.

[0012]

First, the reasons for limiting the composition of the steel for high heat input welding of the present invention will be described. C: more than 0.10% and 0.20% or less When C is 0.10% or less, it is difficult to secure the desired base metal strength due to the influence of low Si, and when it exceeds 0.20%,
It is not possible to secure the toughness of the welded part by producing island martensite by cooling after welding. In addition, it causes remarkable hardening during welding with small heat input, increasing the susceptibility to welding cracks.

Mn: 0.4 to 2.0% Mn is an element that contributes to the improvement of strength, similar to C. It is required to be 0.4% or more to secure the strength required for welded structural steel, but 2.0% is required. If added in excess, the crack susceptibility during welding will be increased and the toughness of the weld will be adversely affected, so the content was made 0.4 to 2.0%.

Al: more than 0.010% and less than 0.04% Al is at least 0 due to deoxidation in the ordinary steelmaking process.
Although it is necessary to add more than 010%, on the other hand, the content of 0.04% or more rather deteriorates not only the weld heat affected zone but also the toughness of the weld metal. Therefore, the range is set to more than 0.010% and less than 0.04%. Ti: 0.002 to 0.020%, N: less than 0.0040% Ti and N form TiN, which complements the effect of sulfur oxides of REM or Ca and prevents coarsening of austenite grains during heating by welding. Here, in order to exert the effect of improving toughness, Ti is 0.002% or more, and N is preferably 0.002% or more.
% Or more is required. However, if 0.0040% or more of N is contained, the solid solution N will adversely affect the toughness of the steel, and the BN will be present at grain boundaries.
As it causes the precipitation of coarse grain boundary ferrite, the upper limit of the N content is less than 0.0040%, and in this case, even if Ti exceeding 0.020% is contained, the effect is not only obtained, but rather Excessive Ti that is unbalanced with respect to the amount of N is considered to impair toughness, and also causes the formation of coarse ferrite precipitates due to the formation of a composite of TiN and REM or Ca sulfur oxides, which causes precipitation of Ti content. The upper limit was 0.020%.

B: 0.0003% or more and less than 0.0020% B segregates at the austenite grain boundaries during heating by welding and suppresses precipitation of coarse grain boundary ferrite which adversely affects toughness. In order to improve the toughness of the heat-affected zone in welding with a large heat input, which exceeds the conventional category as specifically targeted by the present invention, in order to utilize γ grain refinement by REM and Ti, It is necessary to add them together. For this purpose, 0.0003% or more must be added. However, even if added in an amount of 0.0020% or more, the effect saturates and there is the adverse effect of precipitating coarse grain boundary ferrite due to BN precipitated at the grain boundaries, so the range was made less than 0.0020%.

REM: 0.003 to 0.030% REM precipitates as sulfur oxides and also has an effect of preventing γ grains from coarsening. It is necessary to add 0.003% or more together with Ti in order to improve the toughness of the heat-affected zone with a large heat input of 500 kJ / cm or more, which has a larger heat input than the conventional large heat input welding. However, the addition of more than 0.030% impairs the cleanliness of the steel, so the range was made 0.003 to 0.030%.

Ca: 0.001 to 0.005% Ca has the same effect as REM, but it must be contained in an amount of 0.001% or more, and if added in excess of 0.005%, the cleanliness of steel is impaired. It was set to 0.001 to 0.005%. Si: less than 0.10% Si is usually added because it increases the strength, but in the present invention, to prevent coarse ferrite precipitation which is harmful to toughness, and to prevent island martensite precipitation,
It is essential to reduce it to less than 0.10%. Without this, no matter how the effects of other components are utilized, the heat input will be 500 kJ / due to the precipitation of coarse ferrite and the precipitation of island martensite.
Improvement of toughness in heat-affected zone with large heat input of cm or more cannot be achieved.

In FIG. 1, 0.14C-1.3Mn-0.025Al-0.015REM-0.01Ti-0.00 containing appropriate amounts of REM, Ti, and B.
In the case of 25B-0.003N steel, a simulated welding heat affected zone (HAZ) simulating a large heat input weld bond equivalent to a heat input of 800 kJ / cm.
The relationship between vE ₀ and the amount of Si 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 the basic components described above, Cu: 0.05-0.5%, Ni: 0.05-
1.0%, Mo: 0.05-0.5%, Nb: 0.005-0.10%, V: 0.00
5 to 0.15% and Cr: 0.05 to 0.5% may be contained in one kind or two or more kinds. However, if Cu is added excessively, the hot workability is impaired and the weld cracking sensitivity is increased. In addition, Ni and Mo are expensive elements, and excessive addition impairs economic efficiency.
It was set to 1.0% and 0.5%. Further, Mo is added to an excessive amount to enhance the hardenability of the heat-affected zone of the weld and the susceptibility to weld cracking. From that point as well, the upper limit was made 0.5%. Nb and V
In order to cause precipitation embrittlement by forming a large amount of precipitates in the base metal and the weld heat affected zone when added excessively, the upper limits were made 0.10% and 0.15%, respectively. Further, Cr has an upper limit of 0.5%, because excessive addition of Cr impairs weldability.

The lower limit of each of these strengthening elements indicates the minimum amount desired to exert a strengthening effect.
Next, the reasons for limiting the structure of the steel of the present invention will be described. Precipitated B is less than 0.0015% Precipitated B as BN or Fe ₂₃ (C, B) ₆ mainly precipitates at austenite grain boundaries. These precipitates B have the adverse effect of precipitating coarse grain boundary ferrite, and when the amount of precipitates B is 0.0015% or more, the toughness of the heat-affected zone of the weld is significantly reduced.
The range was set to less than 0.0015%.

Dispersion of REM Acid / Sulfide and TiN Separately When the REM acid / sulfide and TiN are present in a complex form, the dispersed particles become coarse and the austenite grain coarsening preventing effect is reduced. Further, the composite dispersed particles, in the large heat input welding heat affected zone of 50 kJ / cm or more, does not show the intragranular ferrite precipitation promoting effect even if it exists in the austenite grains, but if it exists in the austenite grain boundaries, precipitation B Similarly, causes coarse grain boundary ferrite precipitation and reduces the toughness of the heat affected zone. Therefore, REM acid / sulfide and TiN were separately dispersed.

Dispersion of Ca Acid / Sulfide and TiN Separately When the Ca acid / sulfide and TiN are present in a complex state, the dispersed particles become coarse and the austenite grain coarsening preventing effect is reduced. Further, the composite dispersed particles, in the large heat input welding heat affected zone of 50 kJ / cm or more, does not show an intragranular ferrite precipitation promoting effect even if present in the austenite grains, but when present in the austenite grain boundaries, precipitation B Similarly, causes coarse grain boundary ferrite precipitation and reduces the toughness of the heat affected zone. Therefore, the Ca acid / sulfide and TiN were separately dispersed.

Next, the reasons for limitation in the method of manufacturing steel for high heat input welding according to the present invention will be described. Heating above the Ac ₃ transformation point Coarse B precipitates that exist in the as-cast state as they are cannot be sufficiently solid-solved during welding heating as they are, and may adversely affect the toughness of the heat-affected zone. In order to dissolve them, and when strengthening elements such as Nb and V are added, they are heated to a temperature higher than the Ac ₃ transformation point after casting in order to sufficiently dissolve them.

Rolling finishing temperature 800-1000 ℃ Steel plate to which large heat input welding of 500 kJ / cm or more is applied has a plate thickness of 40 mm
The thickness is above the above, but in this case the rolling finish temperature is 1000 ° C.
If it exceeds, the grain refinement of the base material becomes insufficient and it becomes difficult to secure the toughness of the base material. The rolling finish temperature is 800 ℃.
If the amount is less than the above, the precipitation of B may be promoted by the introduction of rolling strain or dislocation, and the amount of precipitated B may be 0.0015% or more. For this reason, the rolling finishing temperature was set to 800 to 1000 ° C.

Cumulative reduction of hot rolling of 50% or more In the case of manufacturing a thick steel plate, the cumulative reduction of 50% or more is used to refine the crystal grains up to the center of the plate thickness and to secure the toughness of the base material. did. To set the cooling rate after hot rolling to air cooling or accelerated cooling where the steel sheet structure is (ferrite + pearlite) and / or bainite, to secure the necessary base metal strength as a structural material to which high heat input welding is applied. In addition, the cooling rate after hot rolling is such that the steel sheet structure is (ferrite + pearlite) and / or bainite.

[0026]

EXAMPLES Table 1 shows the chemical composition of trial steels. The strength level of the prototype steel is 400 to 490 MPa class steel. After casting these steels, they are reheated to 1150 ° C and rolled at a finishing temperature of 900 ° C and a plate thickness of 80.
mm, and hot rolling at a cumulative rolling reduction of 62% at 1000 ° C or less was performed, followed by accelerated cooling at 1.5 ° C / s. The Ac ₃ transformation point of each of the test steels was 890 ° C or lower.

All of Examples A to H and Comparative Examples I to L were deposited.
B is less than 0.0015%, but Comparative Example M is precipitated B
There is an excessive amount. Examples A to E have a structure in which REM acid / sulfide and TiN are separately dispersed, and Examples F to H are Ca acid / sulfide and TiN separately dispersed. Comparative Example J, while having a texture
Was complexed with REM acid / sulfide and TiN.

[0028]

[Table 1]

Table 2 shows the tensile strength (TS) of the steels in Table 1 and the Charpy absorbed energy (vE ₀ ) value at 0 ° C. of the weld pond and the metal of the bond by a simulated weld heat cycle corresponding to a heat input of 800 kJ / cm. Indicates an organization. Note that vE ₀ is the average value of the values obtained by carrying out the three tests.

[0030]

[Table 2]

Steels A to H are examples, low Si-low N-RE.
The M-Ti-B treatment or low Si-low N-Ca-Ti-B treatment refined the old γ grains in the heat-affected zone of the weld, and also suppressed the formation of coarse ferrite and island martensite. result,
It shows good toughness. Steels I to L are comparative steels. Steel I has a small amount of B added, so that coarse ferrite is formed, and Steel J has a lower toughness in the weld heat-affected zone as compared with the examples because the Ti content is excessive. ing. It is considered that since the steel K contains too much Al, the internal quality of the steel deteriorates and the toughness of the weld heat affected zone deteriorates due to the adverse effect of excess N 2. Since the steel L has an excessively high Si content, the toughness of the weld heat affected zone 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.

[0032]

According to the present invention, large heat input welding with a heat input of 500 kJ / cm or more can be used for manufacturing structures such as buildings, bridges, shipbuilding and pressure vessels. The cost can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Si content and vE _{0 of} a reproduced HAZ simulating a large heat input welded bond portion having a heat input of 1000 kJ / cm 2.

Front page continuation (72) Inventor Akihiro Matsuzaki 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside the Technical Research Laboratory, Kawasaki Steel Co., Ltd. (72) Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Kawasaki Steel Technical Research Institute Co., Ltd.

Claims (4)

[Claims] 1. By weight%, C: more than 0.10% and 0.20% or less, Si:
Less than 0.10%, Mn: 0.4 to 2.0%, Al: more than 0.010% and less than 0.04%, Ti: 0.002 to 0.020%, REM: 0.003 to 0.030%, N: 0.0
040% or less, B: 0.0003% or more and less than 0.0020%, the balance consisting essentially of Fe and inevitable impurities, except that the precipitation B is less than 0.0015%, and REM acid / sulfide in steel is A steel for large heat input welding with a heat input of 500 kJ / cm or more, which has a structure in which TiN and TiN are dispersed separately. 2. By weight%, C: more than 0.10% and 0.20% or less, Si:
Less than 0.10%, Mn: 0.4 to 2.0%, Al: more than 0.010% and less than 0.04%, Ti: 0.002 to 0.020%, Ca: 0.001 to 0.005%, N: 0.00
Less than 40%, B: 0.0003% or more and less than 0.0020%, the balance consists essentially of Fe and unavoidable impurities, but the precipitation B is less than 0.0015%, and Ca acid / sulfide in steel When
Steel for large heat input welding with a heat input of 500 kJ / cm or more, characterized by having a structure in which TiN is dispersed separately. 3. A steel having the composition and structure according to claim 1 or 2, further comprising Cu: 0.05-0.5% and Ni: 0.05-1.
0%, Mo: 0.05-0.5%, Nb: 0.005-0.10%, V: 0.005
~ 0.15%, Cr: 0.05-0.5% 1 type or 2 types or more, The high heat input welding steel of 500 kJ / cm or more of heat input characterized by the above-mentioned. 4. A steel having the composition according to claim 1, 2 or 3 is cast and then heated to an Ac ₃ transformation point or higher, and a rolling finishing temperature of 800 to
Perform hot rolling at 1000 ° C and cumulative reduction of 50% or more,
After that, the steel sheet structure is (ferrite + pearlite) and /
Alternatively, a method for producing steel for high heat input welding having a heat input of 500 kJ / cm or more, characterized by performing air cooling or accelerated cooling at a bainite cooling rate.