JPS6167755A - Aluminum killed steel for low-temperature service for large heat input welding - Google Patents

Abstract

PURPOSE:To provide excellent low-temp. toughness and to lessen the brittleness in a weld heat affected zone even if an aluminum killed steel is subjected to large heat input welding by specifying the compsn. consisting of C, Si, Mn, Al, Nb, Ti, N and Fe and compsn. relation. CONSTITUTION:This aluminum killed steel for low-temp. service for large heat input welding contains 0.03-0.12wt% C, 0.1-0.50% Si, 0.7-1.70% Mn, 0.01-0.08% Al, 0.005-0.025% Nb, 0.003-0.025% Ti and 0.0040% N, satisfies the relation 3<=Ti/N<=6, contains further at least 1 kind of <=0.005% Ca and <=0.01% Ce and/or at least one kind of <=0.50% Cu, <=1.5% Ni, <=0.50% Cr, <=0.50% Mo and <=0.10% V and consists of the balance Fe and unavoidable impurities. This steel has the excellent low-temp. toughness of the steel plate and weld joint part.

Description

[Detailed description of the invention] The present invention relates to low-temperature aluminum kil+S steel for large heat input welding,
More specifically, the present invention relates to a low-temperature aluminum killed steel for high heat input welding that exhibits little deoxidization in the weld heat affected zone (hereinafter referred to as HAZ), especially near the bond area, even when high heat input welding is performed.

In recent years, in order to improve welding work efficiency and reduce welding construction costs, welding of various structures such as ships,
High-efficiency welding methods such as single-sided, single-layer Bumage arc welding with high heat input and electrogas arc welding are being adopted.

On the other hand, as the development of oil resources in extremely cold regions becomes more active, steel materials used in offshore structures for oil drilling and icebreakers are not only used in TM plates but also in welded joints. Low-temperature toughness is required. However, in general, when high heat input welding is applied to conventional welded structural steel materials, the H
AZ force" is at risk, and this tendency becomes more pronounced as the strength level of @(4) increases.

In general, embrittlement in the HAZ due to large heat input and displacement, especially near the bond area, is caused by welding heat that causes the area to become at least 12
In addition to being heated to a high temperature of 00'C or higher,
After that, it is slowly cooled, and as a result, the crystal grains in the vicinity of the bond area become significantly coarsened, resulting in a coarse upper inite structure with extremely poor toughness, and nitrides such as AIN dissociate due to welding heat. It is thought that this occurs because solid solution N increases and deteriorates the toughness of the soil.

For this reason, various methods have been proposed to improve the toughness of high heat input welds, but these can be roughly divided into the following two methods. The first is to suppress the enlargement of austenite crystal grains near the bond area by using carbonitrides such as Ti, Zr, If, B, etc., and oxides and sulfides such as REM, and to shape the structure into fine ferrite grains. The second method is to reduce N in the steel and at the same time add Ti, pearlite to the steel.
This is a method of adding nitride-forming elements such as Al1 to capture solid solution N dissociated due to high temperatures during welding and improving the toughness of the base. Various practical steels have already been developed based on these methods. has been done.

However, in the method of -h 1, in order to disperse finely and in large numbers nitrides such as TiN, the amount of N must be adjusted to about 0°0040
% or more, while according to the second method,
It is necessary to suppress the amount of N to about 0.0040% or less. As described above, since the conventional methods have conflicting effects on the amount of N in the steel, it has been considered difficult to effectively utilize these methods at the same time to improve the toughness of high heat input welds. .

However, as a result of intensive research by the present inventors in order to solve the above-mentioned problems, the amount of N in the steel is suppressed to a predetermined critical value or less, and the Ti of the predetermined b1 regulated by this amount of N is
and each time a predetermined amount of Nb is added to the steel,
With L, we succeeded in simultaneously achieving the effect of improving the toughness of the base by reducing the amount of N, suppressing the formation of austenite grains by fine carbonitrides of Ti and Nb, and changing the structure to ferrite and pearlite. Even if heat welded, II
It has been found that it is possible to obtain low-temperature aluminum kiln steel for high heat input welding with little AZ embrittlement.

That is, since Nb is a strong carbonitride-forming element, the present inventors focused on the possibility that the addition of Nb to steel would not affect the fine dispersion of Ti carbonitrides. of Nb
For low N steel with addition of
Thermal cycle test equivalent to cm, i.e. 1350°C
After heating, the cooling time from 800°C to 500°C was set to 180 seconds, and the influence of the amount of Ti on the IAZ toughness of the steel was studied, and the following new findings were obtained.

First, Figure 1 shows low N steel (C0,
07%, Si0.25%, Mn1.25%, A10.0
35%, Nb 0.01%) is shown.

For comparison, Nb-free steel (jt/U) is also shown.

As is conventionally known, Nb-free steel contains Ti
The toughness is best when the ratio of N and N is chemically combined, whereas in the case of N l) addition steel, L soil,
The optimal amount of Ti that is effective in the direction of HAZ toughness changes, and when the amount of Ti is excessive compared to Nb-added steel, νTr
s is the lowest.

In this case, the hen weaving of HAZ is shown in Figure 2 (al~(d
As shown in Figure i, as the Ti content increases, the crystal grains become finer, and the amount of ferrite precipitated within the grains increases. In addition to -, the effect of improving the structure due to the precipitates is clear. As described above, the range of Nb, Ti, and N amounts that can be tolerated to improve the toughness of HAZ is completely different from the range of components conventionally known for high heat input welding steels.

Therefore, we further developed various 3.11 composition steels (C0.07%, S
i0.25%, Mn1.25%, Δ7!0.035%,
Nb0.010%, 1” i 0. (l O3%, N
0.0017-0.0072%), welding heat input]
Double-sided submerged arc welding was performed at 0.02 KJ/cm, and the pound part was cut out to determine vE-, and the amounts of Tj and N that affect the toughness were investigated in this way, and the results are shown in Figure 3. As shown, the toughness can be maximized by suppressing the N content to 0.0040% or less, keeping the Ti content in the range of 0.005 to 0.025%, and satisfying the relationship 3≦Ti/N≦6. We have found that improvements can be made to a certain extent.

The present invention has been made based on the knowledge as described above, and an object of the present invention is to provide a low-temperature killed aluminum steel for high heat input welding that has excellent low-temperature toughness.

The low-temperature aluminum kill F steel for high heat input welding according to the present invention has, in weight percent, C 0.03-0.12%, Si 0.1-0.50%, Mn 0.7-1.70%, Aβ 0.01-0. 0.08%, Nb 0.005-0.025%, Ti 0.003-0.025%, N 0.004 (1% or less, consisting of iron and inevitable impurities, and 3≦T
It is characterized by satisfying the relationship: i/N13.

In the steel of the present invention as described above, Nb, 'Fj and N
Although it has not been fully elucidated what metallurgical effect contributes to the improvement of I(AZ toughness), it is thought that it is due to the following effect.

■ Steel plate ij with composite addition of Nb and Ti, (Ti, Nh
)N and ('Nh, Ti)C, and among these, (Ti content, Nb)N is effective in preventing austenite grain phase enlargement like TiN, and , effectively acts as a ferrite transformation nucleus.

(Ti, Nb)N precipitates in the largest amount when heated to a high temperature around 1200°C, and significantly improves the toughness near the 1'' portion of the welding bond.

■ As shown by the relationship 3≦T i /N13 defined in the present invention, the amount of Ti in excess of the amount of Ti that combines with N as TiN in chemical terms, together with the presence of Nb,
In the cooling process after welding, it re-precipitates as (Nb, Ti)C,
Acts as a modified form of ferrite.

That is, it does not remain in a solid solution state that deteriorates toughness.

These effects of the steel of the present invention can be obtained not only when used as rolled, but also when subjected to accelerated cooling treatment after rolling (including direct quenching), normalizing, quenching and tempering, and further tempering treatment after accelerated cooling. Don't be surprised at the quality of the products manufactured by the manufacturer.

Next, the reason for limiting the chemical composition of the low-temperature aluminized moon steel for high heat input welding according to the present invention will be explained.

The lower the C content, the better the weldability and T-T of the steel.
AZ toughness will be good, or if C is less than 0.03%, m material strength will decrease, and if it exceeds 0.12%, deterioration of 7 weldability and HAZ toughness will be noticeable. Become.

Therefore, in the present invention, the amount of C added is 0.03 to 0.
It shall be 12%.

Si needs to be added in an amount of 0.1% or more for deoxidation, and on the other hand, when added in excess, it impairs G's weldability, so its limit is set at 0.5%. .

Mn is at least 0.7 in order to increase the tensile strength of steel.
%, or if it exceeds 1.7%, weldability deteriorates. Therefore, in the present invention, the addition amount of &;

A 7!4; I is essential as a deoxidizing element, but 0
When it is less than 0.01%, the effect cannot be fully exhibited, while on the other hand, when it is added in excess of 0.08%, weldability deteriorates. Therefore, in the present invention, the amount of 1'' added is 0.01 to 0.08%.

Addition of a small amount of Nb is effective for precipitation strengthening and fine grain strengthening, but in addition to these effects, in the present invention, by combining with Ti and N, which will be described later, cooling during the welding thermal cycle is achieved. In the process, it transforms into ferrite (Nb,
Ti) It is added as an essential element to form a C double carbonitride. In order to effectively utilize this effect,
It is necessary to add at least 0.00.5%.

However, when adding too much, even if the effect described in -1- is effectively expressed, a large amount of *■ large haynite is generated at the same time, and good toughness cannot be obtained.
It shall be 25%.

Ti forms TiN and precipitates finely dispersed in the steel.
In order to promote the formation of ferrite/pearlite and refinement of Ili and to reduce solid solution N in the steel to improve toughness, the steel of the present invention, which is an Nb-added steel, is heated at high temperature. and relatively stable (Ti.

Nb) It is added as an essential element to generate N and suppress the coarsening of austenite grains, and to refine N as a ferrite transformation, and to refine the weave.

In order to effectively express such an effect, at least 0.
However, if too much is added, not only will the above effects not be achieved, but Tii in the solid solution state will increase, producing inite that deteriorates the toughness of the steel. -, so the -1-, limit is 0.025
%.

By reducing the amount of N, it is possible to increase the toughness of the base and improve the toughness of the HAZ part, especially the weld pound part. Therefore, in the present invention, Cat is allowed.
The eye is 0.004%.

In the present invention, among the above-mentioned elements, Ti and N
It is necessary that the amount of each addition be within the range indicated by -L, but furthermore, in order to ensure the desired bond toughness, the amount of Ti and the amount of N must not satisfy the following relationship at the same time. is necessary.

3≦T i /N amount 6 As described above, when the relationship between Ti and N is not satisfied, as shown in FIG. 3, especially the welded bond portion has excellent impact characteristics.

The low temperature aluminum killed steel for high heat input welding according to the present invention includes:
In addition to the above elements, at least one element selected from Ca and Ce can be added. Such elements are effective in spheroidizing sulfide-based nonmetallic inclusions and improving anisotropy. However, when these elements are added in excess, they actually impede the cleanliness of the steel and deteriorate its toughness, so the second limit for Ca is 0.005.
%, Ce is 0.01%.

Furthermore, in the present invention, Ca and C described in 112 are added to the steel.
Apart from or together with e, Cu, NiCr, Mo
I-(A
It can be added to the extent that it does not impair the Z-toughness or to further improve the toughness 11.

Cu is effective in improving the strength and corrosion resistance of steel, but when added in excess, it deteriorates T-+A-Z toughness, so its -1 limit is set to 0.50%.

Although Ni is an effective element for improving toughness, the upper limit is set at 1.5% in consideration of economic efficiency.

Cr also improves the strength and toughness of steel, but excessive addition actually deteriorates toughness, so the -1-eye ratio is 0.50.
%.

Mo is effective in increasing the strength of steel, but when added in excess, it deteriorates the toughness, so the limit is set to 0.
．． It shall be 50%.

Like Mo, V is also effective in increasing the strength of the forceps 1, but when added in excess, the toughness deteriorates, so the amount is limited to 0.10%.

As described above, according to the present invention, it is possible to suppress Nb1 in steel to a predetermined value or less, add Ti in a predetermined portion regulated by this N amount, and further add a predetermined amount of Nb to copper. Accordingly, the toughness of the soil due to low N2 and Ti,
Nb, etc. (by precipitation of several pairs of carbonitrides!+1+1 weave improvement, i.e., ferrite I... Pearlitization is simultaneously achieved, and thus vE-H is stable over a wide heat input range at 5 kgf-m or more. Some high toughness pond parts and HAZ
It is possible to obtain a low-temperature aluminum steel for high heat input welding having the following properties.

It should be noted that the high heat input welded structural steel according to the present invention does not lose any of its outstanding characteristics through heat treatment, as described above, and therefore can be manufactured by any method.

The present invention will be explained below with reference to Examples.

Example 1 A steel plate of a predetermined thickness was obtained by the method shown in the table from the present invention @A-H and comparative steels 1 to M having chemical compositions 3 and 1 shown in the table, and this was welded with a predetermined heat input. did. The table shows the impact test results for the welded bond.

The absorbed energy (vE3..) at 60°C of the steel of the present invention with adjusted N, Ti, and Nb is 5 kg.
−m or more, has low impact properties, and has a comparative steel of 9 to
Compared to No. 14, the AZ toughness is significantly better.

That is, the comparison fliiliK and I is 1” i/N
Since the ratio does not satisfy the conditions specified in the present invention, J
Since the amount of N is further high, and although the T i / N ratio of M satisfies the conditions specified in the present invention, the toughness of the weld bond is deteriorated due to the high amount of N. That is clear.

[Brief explanation of drawings]

Figure 1 shows that HAZ toughness cannot be achieved in low N steel.
A graph showing the influence of N on Nb-added steel and Nb-free steel, and FIG. 2 is a micrograph showing the microstructure of the steel according to the present invention.
0. OO9%, (CIo, 014% and (dl 0.
It is 018%. Figure 3 shows TiZN in Nb-added steel.
It is a graph showing the relationship between the ratio and absorbed energy at -60°C (VE-60). Figure 1 Ti amount (10) - %) Engraving of drawings (no change in content) Figure 2 rfI document of drawings (no change in content) Figure 2 Figure 3 TVN Procedural amendment (method) % formula % 1 , Indication of the case 1982 Patent Office No. 190340 2 Title of the invention Low-temperature aluminum kil 1' steel for high heat input welding 3 Person making the amendment Relationship to the case Patent applicant Address 1 Wakihama-cho, Chuo-ku, Kobe City Chome 3-18 Name
Kobe Steel, Ltd. Representative: Fuyuhiko Maki 4, Agent address: Shinmachi Shichifuku Building, 1-8-3 Shinmachi, Nishi-ku, Osaka

Claims (4)

[Claims] (1) C 0.03-0.12%, Si 0.1-0.50%, Mn 0.7-1.70%, Al 0.01-0.08%, Nb 0.005 in weight% ~0.025%, Ti 0.003~0.025%, N 0.0040% or less, the balance consisting of iron and inevitable impurities, and 3≦Ti/
A low-temperature killed aluminum steel for high heat input welding, which satisfies the relationship N≦6. (2) In weight% (a) C 0.03-0.12%, Si 0.1-0.50%, Mn 0.7-1.70%, Al 0.01-0.08%, Nb A group containing 0.005 to 0.025%, Ti 0.003 to 0.025%, N 0.0040% or less, and further consisting of (b) Ca 0.005% or less and Ce 0.01% or less Contains at least one selected from
A low-temperature killed aluminum steel for high heat input welding, which satisfies the relationship N≦6. (3) In weight% (a) C 0.03-0.12%, Si 0.1-0.50%, Mn 0.7-1.70%, Al 0.01-0.08%, Nb 0.005 to 0.025%, Ti 0.003 to 0.025%, N 0.0040% or less, and further, (b) Cu 0.50% or less, Ni 1.5% or less, Cr 0 .50% or less, Mo 0.50% or less, and V 0.10% or less, with the balance consisting of iron and inevitable impurities, and 3≦Ti/
A low-temperature killed aluminum steel for high heat input welding, which satisfies the relationship N≦6. (4) In weight% (a) C 0.03-0.12%, Si 0.1-0.50%, Mn 0.7-1.70%, Al 0.01-0.08%, Nb A group containing 0.005 to 0.025%, Ti 0.003 to 0.025%, N 0.0040% or less, and further consisting of (b) Ca 0.005% or less and Ce 0.01% or less (c) selected from the group consisting of Cu 0.50% or less, Ni 1.5% or less, Cr 0.50% or less, Mo 0.50% or less, and V 0.10% or less 3≦Ti/
A low-temperature killed aluminum steel for high heat input welding, which satisfies the relationship N≦6.