JPS62109948A - High-toughness steel for welding - Google Patents

Abstract

PURPOSE:To improve the low-temp. toughness of a weld heat affected zone by decreasing the content of Al as far as possible, adding Ti to a molten steel after the deoxidation treatment thereof to disperse the precipitate uniform and fine nitride, oxide, etc. of Ti in the stage of producing a steel for welding. CONSTITUTION:The molten steel consisting, by wt%, of <0.19% C, <0.80% Si, 0.40-2.0% Mn, <0.020% P, <0.020% S, <0.0050% N, <0.0050% O or <0.020% Al as basic components, or further contg. a specific ratio of 1 or >=2 kinds among Ni, Cr, Mo, Nb, Cu, V, B, REM, Ca, Mg, and Zr is subjected to the deoxidation treatment and thereafter an adequate ratio of Ti is added thereto. The content of Al having the high affinity to N is not so low and therefore, the N in the molten steel does not form AlN and the pulverized particles of TiO, TiN and TiON series having 100-500Angstrom particle size and 5X10<7>-10<9>/mm<3> number of particles are uniformly deposited to provide the ferrite precipitation nuclei of the weld heat affected zone. The low temp. toughness of the weld heat affected zone is thus improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to welding steel used in shipbuilding, bridges, pressure vessels, architecture, etc.
This relates to a welding steel that is effective when low-temperature toughness is required for steel (abbreviated as AZ).

(Prior art) Conventionally, it has been common for the toughness of the welded part, especially the bond part and the HAZ part on the base metal side within a few mm from the bond part, to decrease due to welding, and this decrease in toughness is mainly caused by austenite crystal grains. This is thought to be due to the coarsening of the γ-grain structure and the coarsening of the γ-grain structure. Therefore, in order to improve the toughness of the weld bond and HAZ part,
Conventionally, measures have been taken to refine the grains, prevent coarsening, and refine the structure within the γ grains. The means to achieve this are to refine a single grain with precipitates such as TiN and ZrN, and to divide REM (rare earth metal element, qa, sulfide, uniform oxide) into two parts, and ferrite around it. A method has been proposed to generate nuclei and refine ferrite.

(Problem to be solved by the invention) However, although T+N precipitates are collected and uniformly dispersed, the amount of dispersion is small, and when welding is performed, TiN precipitates near the molten part of the weld HAZ. Most of the particles are dissolved, and the effect of preventing grain coarsening is lost.''7) The structure within the T grains is not sufficiently refined, making it difficult to ensure improvement in low-temperature toughness by improving the structure.

(Means for Solving the Problems) The present invention enables a large amount of Tl-based nitrides and oxides to be uniformly and finely precipitated and dispersed, and to act as ferrite precipitation nuclei without dissolving the precipitates near the weld fusion zone. We conducted research on ways to make it work effectively. the result,
It was discovered that after deoxidizing molten steel by reducing A+, which has a strong affinity for N, as much as possible, adding Ti can control the precipitation of uniformly fine and large amounts of Tl precipitates during the solidification process of the steel. Furthermore, it has been revealed that fine, uniform, and large amounts of TI-based precipitates can be generated by adding Ti after deoxidizing molten steel with C,';In,Si. In this case, it has been found that it is effective to add TI in a temperature range where the temperature of molten steel without TI does not interfere with the casting operation.

Further, the present inventor rolled a steel ingot in which a large amount of uniformly fine TI precipitates were controlled to precipitate, thereby creating a steel plate having a thickness of 25+ nm. Using these steel plates, the toughness of the welded HAZ portion was investigated.

Table 1 shows the chemical composition of steel sheets cast with Ti added after changing the deoxidizing treatment of molten steel, and the average particle size and number of particles of Ti-based precipitates.

Table 2 Single side - welding conditions In Table 1, No. 1 is the molten steel deoxidized with AI and then Ti is added in the high temperature range of the molten steel;
2 was deoxidized with c, st, and Mn, and 11 was added in a state where the molten steel temperature range was high, similar to N081.

No, 3, No, 4 is No, 1, No,
After deoxidizing in the same manner as in 2, T
i is added.

Using a steel plate with a thickness of 25 mm shown in Table 1, one layer of 5A on one side.
III welding was performed. The single-sided, single-layer SAW welding conditions were a heat input of 140 KJ/cm as shown in Table 2, welding was performed at a groove angle of 45° as shown in Figure 1, and the above-mentioned plate thickness was 25 ml.
After forming the weld metal 3 between the base materials 1 and 2 of 1, a notch 5 was made in the bond portion 4, three Charpy test pieces 6 were taken, and a test was conducted at -60°C. Table 3 shows the Charpy test results and 131 tentative) global properties.

The microstructure of the suspected bond area was confirmed to contain large ferrite and island-shaped martenzite.As shown in Table 1, Ti-based precipitates: iX
According to the results of investigation of o, I, and ＼・02, the average particle size is 1000 to 6000, and the number of particles is 1.8 to 2.
There was a m3 for 5 XIO'/. No, 1, No
, The results of the 60Ic l/Alpy test for the suspected bond part are as shown in Table 3, ]l uniformity 3
．． 6 = 1.8 kg f-m, which is a little bad.

The reason why the result of 7 YLP absorbed energy was poor was because the grain size of the Ti-based precipitates was large rather than the number of particles, and the structure was not improved.

The average value of the Charpy absorbed energy of the weld bond parts of No. 3, No. 3, No. 4, and No. 4 was 13 to 18 kgf-m, indicating good toughness. In this case, the average particle size of the Tl-based mass is 3
[)0~200 8 and the number of particles is 8゜O~8.2 XID
8/mm3, and a large amount of fine precipitates are present.

Further, when the microstructures of the weld bond parts No. 3, No. 3, and No. 4 were observed, they exhibited fine ferrite and pearlite structures. This structure improvement resulted in good Charpy toughness. The reason for the effect on the structure is that fine T+ precipitates existed without being dissolved during welding. As a result of examining the Tl-based canter near the bond by the extraction replica method, it was confirmed that the particle size was 300 to 1100, and the number of particles was 2 to 8 Xl04 mm3 remaining. EDX analysis of the remaining Ti-based precipitates revealed that they were Ti-N, Ti-N, and Ti-0-N. Therefore, in order to improve the structure of the bond part, it is necessary to add T i-N, T i -0, T + 0- to the base material.
The particle size of N is 300 to 200, and the number of particles is I
O''/mm3, which is uniformly dispersed in a large amount in the base metal state, allows the structure improvement effect to be lost due to dissolution near the welding bond to remain sufficiently present even under high-temperature welding thermal cycles. It has been confirmed that this makes it possible to improve toughness.

The present invention was made based on the above recognition and the results of various experiments, and the high toughness steel for welding according to the present invention has C: 0.19% or less, S: 0.80% or less, Mn :
0.40-2.0%, P: 0.020% or less, S: 0
．． 0.020% or less, N: 0.0050% or less, 0:
The basic component is 0.0050% or less, and the particle size is 100%.
~500 Å, particle number 5 x 107 - 5 x 109/mm
It is characterized by containing 3 Ti-O, Ti-N, and Ti-N-0 composite precipitates.

Further, according to the present invention, C: 0.18% or less by weight, Si
: 0180% or less, Mn: 0.40-2.0%, P
: 0.020% or less, S: 0.020% or less, Al:
0.020% or less, N:O, 0.050% or less, ○:
The basic component is 0.0050% or less, and Ni:
5% or less, Cr: 2% or less, Mo: 0.5% or less,
Nb: 0.15% or less, V: 0.15% or less, Cu
: 2% or less, B: 0.0010% or less, 86M
: 0.020% or less, Ca: 0.010% or less, Mg:
0.10% or less, Zr: 0.04% or less, one or two
Ti-0°Ti-N containing at least one species, a particle diameter of 100 to 500 Å, and a particle number of 5 x 107 to 5 x 109/mm3
, is characterized by containing a Ti-N-0-based composite precipitate.

Reason for limitation The reason for limiting each component of the steel of the present invention will be explained below.

The reason why C is set to 0.18% or less is that, considering the weldability and toughness of the steel, it is better to have a low C, but in order to ensure the strength of the steel and reduce manufacturing costs, C: up to 0.18%. It is allowed and the upper limit is set to 0.18%.

Si is added to ensure strength and deoxidize molten steel, but 0.
If it exceeds 80%, the toughness of the steel will decrease, so the upper limit should be set at 0.
It was set at 80%.

Mn is added to improve strength and toughness.

If it is less than 0.40%, it will be difficult to secure strength, and 2.0%
Addition of more than
Mn was set in a range of 0.40 to 2.0%.

It is desirable that S be as low as possible in order to improve the toughness of the steel and reduce directional properties, but since lower S content increases manufacturing costs, the upper limit was set at 0.020%.

In order to improve the toughness and weldability of steel, P is
It is desirable that it be as low as possible, and the upper limit was set to 0.020%.

The reason why N is set to 0.0050% or less is that a lower value is preferable in order to ensure the toughness of the weld bond, but in order to effectively utilize Ti nitride precipitates, the upper limit O, 0050%
%. If N exceeds 0.0050%, the particle size of TiN precipitates increases and the number of particles decreases, the effect of improving the structure of the weld bond is lost, and the toughness deteriorates. % or less.

A lower value of 0 is more effective in improving the toughness of steel, but the upper limit was set to 0.0050% in order to precipitate fine and large amounts of Tl-based oxides. If it exceeds 0.0050%, the toughness and ductility of the base metal will decrease, so ○ is 0.00%.
It was set to 50% or less.

N1 is added to improve the toughness and strength of steel, but
If it exceeds 5%, it will increase the hardenability of the weld HAZ part.
% or less.

Cr is added because it improves the strength and hardenability of steel, but if it exceeds 2% it deteriorates weldability, so the upper limit was set at 2%.

3.10 is added to improve hardenability and strength,
If it exceeds 0.5%, the hardenability of the welded HAZ part increases and cracking sensitivity increases, so the upper limit was set at 0.5%.

Both Nb and ■ form carbonitrides to improve the strength of the base metal and have the effect of making grains finer.
%, the toughness of the welded HAZ portion deteriorates, so the upper limit was set at 0.15%.

Cu is added to improve the strength of steel, but if it exceeds 2%, it increases the toughness and cracking susceptibility of the welded HAZ, so it was set to 2% or less.

If A1 exceeds 0.020%, it may exist as AIN, and this AIN dissolves into solid solution due to welding thermal cycles, increases solid solution N, and reduces the toughness of the weld HAZ. Therefore, the upper limit of At is 0.02%. 020%, and the lower limit is 0
．． 020%.

B is added to ensure the hardenability and strength of the base material. If it exceeds 0.003%, the toughness of the welded HAZ part will deteriorate and the cracking susceptibility of the HAZ part will increase, so the upper limit should be set to 0.003%.
003%.

REV (rare earth metal element) is added because it is effective in improving the toughness of the weld HAZ part, but if it exceeds 0.020%, it deteriorates the toughness of the steel, so the upper limit was set at 0.020%.

Ca and iAg are added to reduce the directionality of steel, but 0. Exceeding the old % will reduce the cleanliness of the steel and reduce its toughness,
Since it deteriorates ductility, the upper limit was set at 0.01%.

forming nitrides and reducing solid solution N by 1 degree,
￥JJ improves life, but if it exceeds 0.04%, 2
．． The upper limit was set at 0011% because it would worsen Oka's c'R purity.

Particle diameter is 100-500 Å, number of particles is 5X1.07-5
xlo9/mm' T+-0, t+-0-N, L+-
When one or more N-based precipitates are included, the structure of the weld bond becomes fine ferrite,
It becomes pearlite and has high toughness. Particle size is 500
If it exceeds a person, the strain field disappears and it cannot become a ferrite precipitation nucleus, so the effect is lost. In this case, there is no effect even if the number of particles is large. In addition, if the amount of Ti-based precipitates increases with more than 500 particles, the toughness and ductility of the steel will decrease.

Ti precipitates with a grain size of less than 100 grains are easily dissolved by the welding thermal cycle, which has little effect, and it is difficult to distribute a large amount of precipitates with grain sizes of less than 100 grains using the current technology.

The particle size is 100-500 and the number of particles is 5x107-5x.
109/mm3 is limited to 5 XIO'/m
m' (most of it melts due to the welding thermal cycle,
The fiber composition improvement effect of the weld bond part is small, and the number of particles is 1 x 1.
This is because if it exceeds 0-9/mm', the toughness and ductility of the steel will decrease.

Table 11 of Examples shows the chemical composition of the prototype steel. The strength level of the prototype steel is 40-60kg steel. A to I are steels of the present invention, and J-0 is a comparative steel. A-C is 40 kg class steel, D-F is 50 kg class steel, and G-1 is 60 kg class steel. Both the inventive steel and comparative steel had a plate thickness of 25 mm by rolling.
mm. Each steel plate had the groove shape shown in FIG. 1, and single-sided, single-layer SAW welding was performed under the welding conditions shown in Table 2. Collect a Charpy test piece 6 at the position shown in Figure 1,
A notch 5 was made in the bond portion 4, and a test was conducted at -60°C. Welding materials with corresponding strength levels were used.

Table 5 shows the mechanical performance of the base material, the average particle size and number of Ti-based precipitates, the morphology analysis results of the precipitates, and the single-sided SAI'l
The Charpy test results of the bond part are shown. As is clear from Table 5, the bond part toughness of the steels of the present invention is superior to that of the comparative steels (17t:::"j
If you can do it, go to I (the output is T+-N, T +-O
, Ti-N-0 series, and the number of particles is 130 to 300, which is very important.
lo' ~7.8 XIO"/1m3, which is contained in the pre-1, so even though it will melt in one area during the heat cycle during welding, there will still be enough left to make further weaving. ,
1 blade at 1-60℃ at the bond part has a straightness of 101 to 1.
It is good at 9.3kg f・m.

On the other hand, the form of the precipitates in the comparative steel j~○ is the same as that of the present steel J, but the grain size is 3000~7.
The number of particles for 000 people is 2. Oxl, o' ~6. OXlO
4/mm' and is particularly effective for improving fiber composition. However, the average toughness of the bonded portion at 160°C is 2.3 to 33 kgf·m, which is 1°.

(Effects of the Invention) According to the present invention, it is possible to apply adult heat welding to marine structures in frozen areas or low-temperature LPG storage tanks that require high toughness, which greatly contributes to a significant reduction in welding construction costs. can do.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the welding groove shape, the sampling position of the Nyarupee test piece, and the notch position of the Nyarupee test piece. 1.2...Base metal 3...Weld metal 4...Bond portion 5...Notch 6...Charpy test piece 6

Claims (1)

[Claims] 1. C: 0.19% or less, S: 0.80% or less, by weight;
Mn: 0.40-2.0%, P: 0.020% or less, S
: 0.020% or less, N: 0.0050% or less, O: 0
．． The basic component is 0.050% or less, and the particle size is 100 to 5.
00 Å, the number of particles is 5 x 10^7 to 5 x 10^9/mm^
3. A high-toughness steel for welding characterized by containing Ti-O, Ti-N, and Ti-N-O based composite precipitates. 2. By weight, C: 0.18% or less, Si: 0.80% or less, Mn: 0.40 to 2.0%, P: 0.020% or less,
S: 0.020% or less, Al: 0.020% or less, N:
The basic components are 0.0050% or less, O: 0.0050% or less, and further Ni: 5% or less, Cr: 2% or less, Mo:
0.05% or less, Nb: 0.15% or less, V: 0.15
% or less, Cu: 2% or less, B: 0.0010% or less, R
EM: 0.020% or less, Ca: 0.010% or less, M
Contains one or more of g: 0.10% or less and Zr: 0.04% or less, particle diameter is 100 to 500 Å, and number of particles is 5 x 10^7 to 5 x 10^9/mm^. 3 Ti-O
, Ti-N, and Ti-N-O based composite precipitates.