JP2704810B2 - High-strength steel for large heat input welding excellent in on-site weldability and jig crack resistance and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a storage tank, a pressure vessel,
As a low-temperature or room-temperature steel material for bridges, offshore structures, etc., it has a large heat input and weldability in fillet welding of hanging tools and on-site welding of blocks (small heat input welding characteristics, welding characteristics without preheating,
High hydrogen atmosphere welding characteristics).

[0002]

2. Description of the Related Art In recent years, the application of large heat input welding has become common, as the size of a structure is increased and the welding efficiency is required to be improved. On the other hand, cracks accompanying the increase in hardness of the welding heat affected zone (HAZ) inside the tank support portion and cracks due to jig trace cracks in the fillet welded portions of hanging tools such as tanks and bridges may be found by release inspection and the like. Often there. In the bridge field, large heat input welding was used for processing at the factory and bolting was generally used for on-site assembly. However, recently, application of welding has been studied to shorten the construction period and preserve the landscape, and preheating has been omitted. On-site weldability such as welding and welding from a primer or a welding rod in a high hydrogen atmosphere has also been required.

[0003] The large heat input weldability has been put to practical use by various steel companies, and is disclosed in JP-A-61-270354 and JP-A-62-18.
No. 42 and Japanese Unexamined Patent Publication No. Sho 62-56518, various improvements are basically made on the basis of TiN, but all of them are steel materials exclusively for large heat input welding, such as fillet welding. None of them have low heat input weldability (crack-free) and local weldability at the same time. On the other hand, as prior arts having excellent small heat input characteristics, there are Japanese Patent Publication No. 58-2570 and Japanese Patent Application Laid-Open No. 1-96329 by the present inventors, but their stability and large heat input welding characteristics are limited. Was.

[0004]

SUMMARY OF THE INVENTION In addition to the large heat input weldability for improving the welding efficiency in the processing of large structures in a factory, cracks in the jig traces and cracks inside the tank support portion in the fillet welds of the hanging tools are eliminated. It is an object of the present invention to significantly improve the weldability at the time of on-site assembly and to provide these combined characteristics. Japanese Patent Publication No. 58-2570 described above as a prior art by the present inventors is a method for producing a B-free, low C-low Si based non-heat treated toughened high tensile strength steel, in which dispersion of a striped structure and high HA by B-free, on the assumption that carbon martensite (M ^* ) formation is suppressed
At the same time with the reduction of Z hardness, it has high strength and high welding energy by high absorption energy (vEs), high toughness (vTrs) and low carbon equivalent (Ceq), and new quality needs (anti-HAZ cracking) Resistance, SR cracking resistance, tempering brittleness).

Japanese Patent Application Laid-Open No. 1-96329 discloses a tempered HT which is B-free and low in C type and has excellent resistance to sulfide stress corrosion cracking.
A method for producing 60 kg steel, which is disclosed in JP-B-58-2570.
In order to eliminate the restriction of low Si by lowering the temperature and heat-treating heat treatment to reduce the HAZ hardness in small heat input welding of about 10 to 30 KJ / cm, stress corrosion cracking resistance in a hydrogen sulfide or ammonia atmosphere is reduced. It was excellent. However, the above-mentioned inventions (Japanese Patent Publication No. 58-2570 and Japanese Patent Application Laid-Open No. 1-96329) have a limitation in large heat input weldability, and lack new stability in their properties to meet new quality needs. Had the disadvantage that The present inventors conducted a detailed investigation on this problem, and found that the content of B as an impurity varied in spite of the fact that B was not added as a B-free steel, and that the stability of quality characteristics was influenced by the present invention. I came to know what had been done.
Further, in order to improve the large heat input weldability, Nb and Ti are based on the invention of Japanese Patent Publication No. 58-2570 of low C-low Si system.
(Tieq) is controlled to a very small and narrow range and MnS
It has been found that it is necessary to actively utilize S existing as an impurity, not as an impurity, but to improve large heat input weldability.

On the other hand, Japanese Patent Application Laid-Open No.
-270354 discloses C: 0.03 to 0.20%, S
i: 0.01 to 0.50%, Mn: 0.30 to 2.0
%, P: 0.02% or less, S: 0.015% or less, B:
0.0003-0.0030%, N: 0.0080% or less as a basic component, Ni, Cu, Cr, M
o, Nb, contain one or two or more and V, furthermore 0.03% or less of Ti, REM, and contains from 0.003 to 0.03% in total one or two or more of Ca all It is a high toughness welded steel with an Al content of 0.003% or less. However, although this invention has improved large heat input characteristics, it does not have Ceq regulation and B is positively added, so it has low heat input characteristics, especially jig trace cracking resistance and on-site weldability. Is inferior, and Al
Since the deoxidation is unstable because of no addition of Ti, the yield of Ti, REM, and Ca, which have a strong affinity for oxygen, is poor, and the added amount varies, resulting in unstable large heat input characteristics. Was.

JP-A-62-1842 discloses C:
0.02 to 0.18%, Si: 0.5% or less, Mn:
0.4-2.0%, S: 0.0007-0.0050
%, Ti; containing 0.030% or less, P: 0.015
% Or less , N: limited to 0.004% or less and N as necessary
containing one or more of i, Cu, Nb, V, Ta, Cr, Mo, and B, and substantially free of a temporary deoxidation product;
The secondary deoxidation product having a particle diameter of 0.1 to 3.0 μm and a composite of Ti nitride and MnS are each mixed with 5 × 10 ⁴ to 1 × 10 ⁴
It is a tough high strength steel with excellent weld toughness characterized by containing ⁵ / mm ³ . However, although the invention is excellent in large heat input characteristics, it does not contain Al and has a weak deoxidizing element T
Since secondary deoxidation is performed only with i, Si, Nb, V, and Ta, deoxidation becomes unstable, making it difficult to produce stable TiN + MnS, and the yield of Ti, a strongly deoxidizing element, becomes unstable. N and Ti are not balanced and Tieq varies, and stable large heat input characteristics cannot be obtained. On the other hand, B is added or Ce is added.
Since there was no regulation of q, it had the drawback that it was inferior in small heat input characteristics, especially in jig trace resistance and in-situ weldability.

Further, Japanese Patent Application Laid-Open No. Sho 62-56518 discloses that C: 0.
03-0.12%, Si: 0.05-0.40%, M
n: 0.7 to 1.6%, P: 0.015% or less, S:
0.010% or less, Sol. Al: 0.001 to 0.0
10%, Ti: 0.005 to 0.020%, B: 0.0
003 to 0.0020%, N: 0.0040 to 0.00
60%, if necessary, contains one or more of Cu, Ni, V, and Ca, and Ti / N: 1.5 to 3.4, Ceq:
A high-strength steel for high heat input welding of 50 kg steel, characterized in that a predetermined hot rolling of steel of 0.34% or less is carried out, and immediately quenched to room temperature and then subjected to low-temperature tempering at 200 to 450 ° C. It is a manufacturing method.

However, the invention is disclosed in Sol. Al is 0.01
0% or less, N was set to 0.0040 to 0.0060% to stably precipitate TiN in molten steel and to precipitate BN in HAZ to improve large heat input characteristics, but deoxidation due to low Al and high N Is unstable, and stable TiN cannot be obtained. On the other hand, as a result of the active addition of B, there is a drawback that the heat input characteristics, especially the jig trace cracking resistance and the field weldability are inferior. Had. To meet the requirements from on-site weldability (welding without preheating, welding under high hydrogen atmosphere) and jig trace resistance (small heat input welding, hydrogen-induced cracking) to large heat input welding, It turned out that the technology could not cope.

[0010]

SUMMARY OF THE INVENTION In order to solve the various problems, the present invention specifies the amounts of B and S as impurities of a low C-low Si high strength steel, optimizes Ceq, and optimizes Ti.
By providing a method of manufacturing high-strength steel for large heat input welding that is excellent in on-site weldability and jig trace resistance in combination with optimization of eq, economical construction of large structures becomes possible and It will dramatically enhance the competitiveness of both the economy and the economy. The gist of the present invention is as follows: (1) C: 0.10% or less by weight%, Si: 0.10%
Hereinafter, Mn: 0.50 to 2.20%, P: 0.015%
Hereinafter, S: 0.0008 to 0.0025%, Sol. A
l: 0.010 to 0.10%, (Si + Sol.A
l): 0.13% or less, Cu: 0.05 to 0.50%,
Ni: 0.05 to 0.80%, Nb: 0.005 to 0.
024%, Ti: 0.005 to 0.018%, N: 0.
0020-0.0050%, the balance being iron and unavoidable impurities, and B as an impurity: B: 0.000
2% or less, and Ceq, Tieq defined by the following equation
Ceq: 0.38% or less, Teq: -0.007 or more
High-strength steel for large heat input welding excellent in on-site weldability and jig trace cracking resistance, characterized in that MnS forms a composite precipitate with TiON or TiN with 0.005%. Ceq: C + ／ Mn + ／ 24Si + 1 / 40Ni
+ ／ Cr + ／ Mo + ／ 14 V Tieq: Ti-3.4 × N

(2) Cr: 0.50% or less by weight%, M
o: 0.30% or less, V: 0.10% or less, one or two or more kinds are contained in steel, which is excellent in on-site weldability and jig trace crack resistance as described in (1) above. High strength steel for large heat input welding. (3) Immediately after casting, a steel in which TiON or TiN is formed by adding Ti and then completely deoxidizing with Al at the time of vacuum degassing, followed by weak deoxidation by Si at the time of tapping when forming a MnS composite precipitate, or Ac After reheating to ₃ points or more and 1170 ° C. or less, subsequent to general plate rolling or unrecrystallized area rolling of 60% or less, cooling at a cooling rate of 1 to 60 ° C./sec.
Perform accelerated cooling to an arbitrary temperature of 0 ° C. or less, and
Or the manufacturing method of the high tensile strength steel for large heat input welding excellent in field weldability and jig trace crack resistance characterized by manufacturing the steel of (2) . (4) Tempering after accelerated cooling is performed.
(3) Large with excellent on-site weldability and jig trace crack resistance as described
This is a method for producing high-strength steel for heat input welding.

[0012]

The present invention will be described below in detail. C is the most effective element for increasing the strength, but increases Ceq, which degrades the on-site weldability, etc., and at the same time, increases the bainite structure during accelerated cooling and the upper bainite (Bu) in the welded HAZ.
Alternatively, the content is limited to 0.10% or less because it is a direct main element that produces high carbon martensite (M ^* ). In view of the technical concept of the present invention, C is desirably as low as possible in terms of strength design, and is preferably 0.06% or less. Although Si is a useful element for deoxidation, it has a repulsive force with C in steel and promotes sweeping of C from ferrite (α) to retained austenite (γ) during transformation in the base material or HAZ during accelerated cooling. As a result, 0.10% to promote the formation of Bu or M ^*
Limited to the following.

Mn is a useful element for increasing the strength inexpensively in the present invention, and is set to 0.50% or more from its necessary lower limit. If added at 2.20% or more, Mn is set to 0% because it impairs base material toughness and weldability. It was limited to .50 to 2.20%. P is limited to 0.015% or less from the viewpoint of weldability and low-temperature toughness. The upper limit of S is preferably set to 0.0025% or less as the impurity is lower, but in the present invention, from the viewpoint of large heat input weldability, 0.0 is preferably set.
008% or more of S is not used as an impurity.
(Ti-Oxynitride) or a composite precipitate of TiN and MnS is formed, and α in γ grains of HAZ is formed.
0.0008 to 0.002 to utilize as a transformation nucleus
Limited to 5%.

Sol. Al is an element necessary for deoxidation like Si, and is selected to be 0.01% or more in order to select a low Si-based component from the technical idea of the present invention. On the other hand, addition of 0.10% or more is similar to Si. In order to promote C enrichment from α to residual γ during transformation and from the viewpoint of weldability, from 0.010 to 0.1
Limited to 0%. In order to form stable TiON or TiN, it is desirable to completely deoxidize with 0.010% or more of Al at the time of vacuum degassing after weak deoxidation with Si at the time of tapping. Further, Sol. Al has an interaction (repulsion) with C in steel in exactly the same way as Si, and Sol. A
Since the single constraint of l and Si is not enough, (Si + So
l. Al) was limited to 0.13% or less.

Cu is added in place of C, Si, and Mn for the purpose of reducing Ceq to improve low-temperature toughness, and is added to 0.05% or more to secure the strength. For the prevention, it is necessary to add an equal amount of Ni, which is not preferable from the viewpoint of cost, and is limited to 0.05 to 0.50%. Ni is added in place of C, Si, and Mn to reduce Ceq for the purpose of improving low-temperature toughness, and is added at 0.05% or more to ensure strength. Addition of 0.80% or more is preferable from the viewpoint of cost. And limited to 0.05 to 0.80%. Nb is the most useful element together with Ti in the thermomechanical process (TMCP). As NbC, it suppresses the growth of γ grains during heating, expands the non-recrystallization temperature range, strengthens NbC precipitation in the deformation zone during rolling, HA during heat welding
In order to prevent Z softening, 0.005% or more is essential. On the other hand, excessive addition of 0.024% or more also impairs large heat input weldability as in the case of excessive Tieq.
%.

Although Ti is necessary for controlling ν grains as in the case of Nb described above, it is essentially required to suppress γ grain growth by TiON in HAZ during large heat input welding and at the same time as α transformation nuclei in γ grains. 0.005% or more is essential for forming a composite precipitate of TiON or TiN and MnS, and excessive addition of 0.018% or more is limited to 0.005 to 0.018% to impair weldability. did. In addition, Ti
Is not preferable in terms of quality if the balance with N is lost even in this range. That is, if N is excessive, slab cracks are induced, and if Ti is excessive, low-temperature toughness and large heat input weldability of the base material are impaired.
4) is desirable. N is required to be 0.0020% or more for TiON or TiN generation as described above,
When the content is 0.0050% or more, large heat input weldability, particularly HAZ toughness is impaired, and cracks in the slab are induced.
It is limited to 0.0050%. It is desirable to balance N as much as possible with Ti (Ti / 3.4) within this range.

B is an important element together with C, which affects the jig trace cracking resistance, fillet weldability and on-site weldability of the present invention, and it is not sufficient to simply use B-free steel. That is, since Fe-Si alloys and Si-Mn alloys sometimes contain B at a high concentration, B as an impurity is limited to 0.0002% or less by selective selection of steelmaking auxiliary materials. It is extremely important. Ceq is used in order to improve the so-called on-site weldability in which welding cracks are small when welding pre-heating is omitted or when invasion hydrogen from a primer, a welding rod, or the like is high.
Limited to 38% or less. Tieq indicates that when Ti becomes excessive and becomes 0.005% or more, the interaction between Ti and C (Sol
ute Drag Effect)
The complex precipitate of nS or TiN + MnS cannot be used as α transformation nucleus in the γ grains of HAZ, and when N becomes excessive and becomes −0.007% or less, TiON or T
Since iN coarsened and the number of composite precipitates as α-transformed nuclei was insufficient, the content was limited to −0.007 to 0.005%. Therefore, it is necessary to aim the components of Ti and N in a narrow range shown in FIG.

Elements other than the above basic components (Cr, M
o, V) does not impair the effect of the present invention even if one or more of them are added for improving the strength and toughness, but other elements (Ca, REM, etc.) are more likely to form sulfides than MnS. And must not be added to inhibit MnS formation essential for the present invention. Cr is added to secure the strength by improving the hardenability. Excessive addition is limited to 0.50% or less to deteriorate the base material toughness and the HAZ toughness during accelerated cooling. Mo is added for securing the strength by improving the hardenability. Excessive addition is limited to 0.30% or less to deteriorate the base material toughness and the HAZ toughness during accelerated cooling. V is added to improve the strength, and is limited to 0.10% or less because weldability and low-temperature toughness deteriorate.

Next, conditions for manufacturing a thick plate will be described. It is desirable to roll the plate immediately after casting the steel, but in general, the productivity of the plate rolling is 3 to the productivity at the time of casting of 1000 T / H.
Since the production capacity does not match 00T / H, the slab temperature often drops to a temperature that requires reheating before thick plate rolling in the current facility structure. Therefore, in order to γ-form, re-heating is performed to the Ac ₃ point or more, and a small amount of Nb, T as in the present invention is obtained.
In the i-system, the temperature was set to 1170 ° C. or lower in order to prevent coarsening of γ. According to the present invention, a high absorption energy (vEs) and a high toughness (vTrs) can be obtained by improving the metal structure at the time of accelerated cooling such as dispersion of the base material stripe structure and suppression of generation of Bu and M ^* in the base material or HAZ by specifying the component system. At the same time, and there is basically no need for rolling restrictions that reduce productivity. In addition, when rolling in a non-recrystallized region of 60% or less in a temperature range where separation does not occur, vTrs can be further improved without impairing vEs, but in this case, there is almost no effect on productivity.

If the accelerated cooling rate is less than 1 ° C./sec, the cooling becomes equivalent to air cooling depending on the sheet thickness, and does not have the meaning of accelerated cooling. Since the meaning of the limitation of the components was impaired without being performed, the temperature was limited to 1 to 60 ° C./sec.
The effect is most remarkable in ec. The reason for setting the accelerated cooling temperature range to 540 ° C. or lower is to secure the strength of the low C-low Si-based steel. Depending on the thickness of the sheet, tempering may be performed when necessary to reduce residual stress when cooling to 300 ° C. or less in terms of strength.

[0021]

EXAMPLES Examples of the present invention are shown in Tables 1 and 2 together with Comparative Examples. Table 1 shows the chemical components of the present invention examples (steel A, B, C, D, E) and comparative examples (steel F, G). In Comparative Examples, B, C and Ceq as impurities were all higher than the range of the present invention, and steel F was made of S, Cu, Ni, Nb, Ti, T
ieq is out of the scope of the present invention, and steel G is made of S, Nb, Ti,
Tieq is outside the scope of the present invention. However, Cu, N
i is limited for the purpose of compensating the strength of the low C-low Si type component in the present invention, and even if the comparative example is out of the range, the metallurgical condition may be satisfied as long as the Ceq of the present invention is satisfied. Absent. Note that the present invention steel also P _CM .16 to 0.18%
Whereas, in the comparative example, it is 0.22 to 0.24%.

[0022]

[Table 1]

The production results of the present invention and the examples are shown in Table 2 together with the results of rolling. The heating temperatures of the examples of the present invention are all within the scope of the claims, whereas the comparative examples are all high C-
Heating temperature is 1190 ° C due to high Nb or high C-high Ti
Is outside the scope of the present invention. On the other hand, Examples B and D of the present invention had a CR cumulative reduction at the non-recrystallization region temperature of 40%, which is within the range of the present invention without occurrence of separation, whereas Comparative Example G had a CR cumulative reduction of 40%. 80%, which is outside the scope of the present invention. In the invention examples D and E, the cooling stop temperature is low because the product thickness is as thick as 32 mm, and tempering is performed to alleviate the residual stress of the steel sheet and to improve the strength toughness balance. Regarding the strength characteristics, both the invention example and the comparative example satisfy the HT490 and HT590 class standards.

On the other hand, the base materials of the present invention all have high toughness (vTrs) and high absorption energy (vTs, here vTs).
_-20 ), while Comparative Example F has vTrs, vT
_-20, which is low, and Comparative Example G has the same level of vTrs as that of the inventive example, despite the fact that a CR reduction of 80% was performed to cause separation at the expense of absorbed energy. The JIS maximum hardness (Hv10) test showing the jig trace cracking resistance (small heat input weldability and also indirectly hydrogen-induced cracking resistance) shows that the present invention examples are all 250 or less,
Comparative examples of component systems in which B as an impurity is high and Bu and M ^* are easily produced are all as extremely high as 300 or more. Under high hydrogen atmosphere (28cc / 100
g) Restraint cracking test by horizontal-layer fillet welding (0 ° C)
In Examples of the present invention, no cracks were observed, whereas in Comparative Examples where Ceq or B as an impurity was high, cracks (low-temperature cracks) were observed in all of the Comparative Examples, and most of the cracks occurred in the root portion. And propagated to the HAZ side.

HAZ toughness by high heat input FCB welding (F
MIN values of vE _-20 average values in L, H1 and H3), all of the examples of the present invention were high, whereas Bu
Component system (C, Si, Nb, Ti) where M ^* easily appears and HA
Insufficient α transformation in γ grains of Z (S, Tie
The comparative example of q) has extremely poor toughness. Further, FIG. 2 shows a hardness distribution of a bead-on-plate test showing the jig trace crack resistance (also indirectly the hydrogen-induced crack resistance and the field weldability). Inventive steel A has a good hardness distribution, whereas Comparative Example F shows H
In AZ, B as an impurity clearly increases the hardness.

[0026]

[Table 2]

[0027]

As described above, the steel of the present invention has a composition system and TMCP.
By specifying the conditions, it was possible to stabilize the transformed structure of high strength steel and improve it to a structure with good toughness. As a result, large heat input weldability, jig trace crack resistance (small heat input weldability, hydrogen-induced crack resistance), and on-site weldability (preheating omitted, high hydrogen atmosphere welding) were also improved. On the other hand, the present invention achieves high toughness of high-tensile steel by improving the metallographic structure, so that CR in the non-recrystallization temperature range as in the prior art can be eliminated and the reduction in productivity during rolling can be eliminated. Things. As a result, not only can the productivity of the thick plate be increased, but also the construction period of the large-scale structure can be shortened and the landscape can be preserved. Therefore, the present invention has enormous economical benefits as well as economic benefits to the industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing target ranges of Ti and N components according to the present invention;

FIG. 2 is a diagram showing a hardness distribution in a bead-on-plate test showing the jig trace crack resistance.

Claims (4)

(57) [Claims] 1. In weight%, C: 0.10% or less, Si: 0.10% or less, Mn: 0.50 to 2.20%, P: 0.015% or less, S: 0.0008 to 0 .0025%, Sol. Al: 0.010 to 0.10%, (Si + Sol. Al): 0.13% or less, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.80%, Nb: 0.005 ０．０0.024%, Ti: 0.005 to 0.018%, N: 0.0020 to 0.0050%, the balance being iron and unavoidable impurities. And Ceq and Tieq determined by the following formulas are Ceq: 0.38% or less, Tieq: −0.007 to 0.005%, and Meq.
High-strength steel for large heat input welding having excellent on-site weldability and jig trace resistance, characterized in that nS forms a composite precipitate with TiON or TiN. Ceq: C + ／ Mn + ／ 24Si + ／ 40Ni + ／ Cr + Mo + ／ 14V Tieq: Ti-3.4 × N 2. Cr: 0.50% or less by weight%, Mo:
3. Oiri having excellent on-site weldability and jig trace crack resistance according to claim 1, wherein one or more types of 0.30% or less and V: 0.10% or less are contained in steel. High strength steel for heat welding. 3. Immediately after casting a steel in which TiON or TiN is formed by adding Ti and then completely deoxidizing with Al at the time of vacuum degassing, followed by weak deoxidation by Si at the time of tapping when forming a composite precipitate of MnS. , Or AC ₃ points or more 1170
After re-heating to below ℃, general plate rolling or 6
0% or less unrecrystallized zone rolling, followed by 1 to 60 ° C / s
The steel according to claim 1 or 2, wherein accelerated cooling is performed to an arbitrary temperature of 540 ° C. or lower at a cooling rate of ec .
A method for producing high-strength steel for large heat input welding which is excellent in on- site weldability and jig trace crack resistance. 4. Tempering is performed after accelerated cooling.
Excellent in field weldability and jig trace crack resistance according to claim 3.
Of high strength steel for high heat input welding.