JPH06172917A - Production of high tensile strength steel for large heat input welding, excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel excellent in toughness at low temp. used for large heat input welding for improving the welding efficiency of a large-size structure. CONSTITUTION:This high tensile strength steel for large heat input welding, excellent in toughness at low temp., can be produced by performing Ti addition prior to complete deoxidation in the secondary refining succeeding to slight- degree deoxidation by Si and Mn at tapping, at the time of the formation of a compound precipitate of MnS, as impurity, with TiON or TiN, in a steel having a composition consisting of 0.05-0.18% C, 0.10-0.5O% Si, 0.80-1.80% Mn, <=0.015% P, 0.0008-0.0044% S, 0.010-0.10% Al, 0.005-0.24% Nb, 0.005-0.018% Ti, 0.0020-0.0060% N, and the balance iron with inevitable impurities. The effects of the steel remain unchanged even if one or >=2 elements among Cu, Ni, Cr, Mo, and V are added, as necessary, to the above composition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel for high heat input welding, which is excellent in low temperature toughness as a steel material used for shipbuilding, pressure vessels, tanks, bridges, constructions, marine structures and the like. .

[0002]

2. Description of the Related Art In recent years, it has been demanded that the welding efficiency be improved as the size of the structure is increased, and the application of high heat input welding is generally used for joining steel structures. The steel for large heat input welding has been put into practical use, and is disclosed in JP-A-61-270354 and 62-5651.
8 and Japanese Patent Laid-Open No. 02-250917, which are basically developed by the present inventors.
Although the below-mentioned improvements have been made based on TiN, which is the technical idea of Japanese Patent No. 088, each has a problem in the overall quality characteristics in consideration of its stability and other steel material characteristics.

That is, in JP-A-61-270354, C: 0.03 to 0.20%, Si: 0.01 to 0.
50%, Mn: 0.30 to 2.0%, P: 0.02% or less, S: 0.015% or less, B: 0.0003 to 0.0
030%, N: 0.0080% or less as a basic component, optionally containing one or more of Ni, Cu, Cr, Mo, Nb, and V, and Ti of 0.03% or less. ，
One or more of REM and Ca in total 0.003-
It is a high toughness welding steel containing 0.03% and total Al of 0.003% or less. Furthermore, JP-A-62-56518
Japanese Patent Publication C: 0.03 to 0.12%, Si: 0.05 to
0.40%, Mn: 0.7 to 1.6%, P: 0.015
% Or less, S: 0.010% or less, Sol. Al: 0.0
01-0.010%, Ti: 0.005-0.020
%, B: 0.0003 to 0.0020%, N: 0.00
40 to 0.0060% as a basic component, optionally containing one or more of Cu, Ni, V and Ca, and T
Steel having i / N: 1.5 to 3.4 and Ceq: 0.34% or less is subjected to predetermined hot rolling, and immediately cooled to room temperature 2
It is a method for producing a high-strength steel for high heat input welding of 50 kg steel, which is characterized by performing low temperature tempering at 00 to 450 ° C.

On the other hand, Japanese Patent Application Laid-Open No. 02-250917 aims to improve the large heat input characteristic only by TiN + MnS without adding B, and C: 0.02 to 0.3%, Si: 0.3
%, Mn: 0.50 to 2.50%, Ni: 0.2 to
4.5%, Nb: 0.003 to 0.015%, Cu:
0.2-2.0%, N: 0.01% or less, Ti / N:
2.0-4.0, Al: 0.005-0.1%, S:
A low temperature characterized by heating steel containing 0.003 to 0.008% as a basic component and optionally one or more of Cr, V, and Mo to a predetermined cooling rate and 1150 ° C or higher. This is a method for producing high strength steel for high heat input welding with excellent toughness.

[0005]

[Patent Document 1] Japanese Patent Application Laid-Open No. 61-270354, for the purpose of improving the large heat input characteristic, has an Al content of 0.0
Since the content is 03% or less, deoxidation becomes unstable, and the yield of Ti, REM, and Ca, which have a strong affinity for oxygen, is poor and the addition amount varies, resulting in unstable large heat input characteristics. It was In addition, the heat-affected zone (HA
Z, Ti, REM, C as precipitation sites of BN
Since an acid / sulfide having a different property of a is used, the instability of the large heat input characteristic is increased, and there is a problem that B is positively used. Further, JP-A-62-56518 discloses Sol. Al is 0.010% or less, N is 0.004
The amount of 0 to 0.0060% is used to achieve stable precipitation of TiN in molten steel and also to precipitate BN in HAZ to improve large heat input characteristics, but deoxidation becomes unstable because of low Al and stable. It has a drawback that TiN and high N cannot be obtained, and on the other hand, there is a problem that B is positively added.

On the other hand, Japanese Patent Laid-Open No. 02-250917 discloses stable TiN because the deoxidation method is not clarified.
It is difficult to form a complex precipitate of + MnS and S
Of 0.003 to 0.008%, the lamella tear resistance is deteriorated and the large heat input characteristic is unstable. The present invention eliminates the instability of large heat input characteristics and improves lamellar tear resistance characteristics,
An object of the present invention is to provide a method for producing a high-strength steel for high heat input welding, which is excellent in low-temperature toughness and which also solves the quality problem associated with the addition of no B.

[0007]

Means for Solving the Problems The present inventor has made a detailed study on the above-mentioned problems of the prior art and found that B and M
It was found that the addition of no sulfide-forming element other than n and the clarification of the deoxidation method are effective in improving the stability of the large heat input characteristic. JP-A-61-270354 and 62-565.
In order to solve the problem of containing B and the problem of instability of large heat input in JP-A No. 18, the addition of B is eliminated, and the use of sulfide-forming elements other than Mn is eliminated, and deoxidation is stabilized. In order to realize the same, Al was restricted to 0.010 to 0.10%.

On the other hand, S is 0.0008-for deterioration of the anti-lamellar tear property disclosed in Japanese Patent Laid-Open No. 02-250917.
It was improved by restricting it to 0.0044%. Furthermore, TiN + M
For unstable precipitation of nS complex precipitate, after deoxidation at the time of tapping is weakly deoxidized by Si + Mn to form a primary deoxidation product of Mn-Si complex oxide (Mn-Silicate) system. In subsequent secondary refining (vacuum degassing or ladle refining, etc.), it is possible to add Ti before complete deoxidation with Al in the intragranular ferrite (IGF) in the HAZ part.
When forming TiON (Oxy-Nitride) or a composite precipitate of TiN and MnS, which is a generation nucleus, it is extremely effective in reducing the size and increasing the number, and 0.004
For the first time, stable formation of the composite precipitate is possible even under low S of 4% or less.

That is, the gist of the present invention is as follows. (1) C: 0.05 to 0.18% by weight%, Si: 0.
10 to 0.50%, Mn: 0.80 to 1.80%, P:
0.015% or less, S: 0.0008 to 0.0044
%, Al: 0.010 to 0.10%, Nb: 0.005
~ 0.024%, Ti: 0.005-0.018%,
N: 0.0020 to 0.0060% of the steel containing the balance iron and unavoidable impurities, and Mn as an impurity
When S forms a composite precipitate with TiON or TiN,
In the secondary refining following weak deoxidation by Si and Mn at the time of tapping, Ti is added before complete deoxidation by Al and T
A method for producing high-strength steel for high heat input welding, which is excellent in low-temperature toughness, characterized by forming a composite precipitate with MnS having iON or TiN as a nucleus.

(2) Cu by weight%: 0.05 to 0.80
%, Ni: 0.05 to 1.50%, Cr: 0.05 to
0.80%, Mo: 0.05 to 0.50%, V: 0.0
1-0.09% of 1 type or 2 types or more were contained in steel, The manufacturing method of the high tensile steel for large heat input welding excellent in the low temperature toughness of the said (1) description characterized by the above-mentioned.

[0011]

The present invention will be described in detail below. C is 0.05% as the cheapest and most effective element for increasing strength
If added over 0.18%, the weldability and joint toughness are deteriorated, so the content is limited to 0.05 to 0.18%. Si
Is added in an amount of 0.10% or more in order to form Mn-Silicate together with Mn at the time of tapping to form a stable MnS precipitation nucleus after solidification. If it exceeds 0.50%, weldability and joint toughness are impaired. , 0.10 to 0.50%. Mn is added as 0.8% or more as a useful element for increasing strength at a low cost next to C, and if it exceeds 1.80%, the weldability deteriorates, so Mn is limited to 0.80 to 1.80%. It should be noted that addition of Mn-Silicate at the time of tapping and formation of MnS after solidification is sufficient in the range of addition,
Sulfide-forming elements such as Ca, REM and Zr should not be added.

From the viewpoint of weldability and low temperature toughness, P is limited to 0.015% or less. From the viewpoint of improving the joint toughness, S is made 0.0008% or more for the formation of MnS which forms a composite precipitate with TiON or TiN which becomes IGF-forming nuclei, and is 0.004.
If it exceeds 4%, the anti-lamellar property is impaired, so that
Although it is limited to 0008 to 0.0044%, 0.0008 to 0.004% when severe lamellar tear resistance is required.
It is preferably limited to 003%.

Al is the most important element for deoxidation and is added in an amount of 0.010% or more to reduce non-metallic inclusions.
%, The weldability and low temperature toughness are impaired, so 0.010
Limited to ~ 0.10%. Furthermore, Al is added after finishing the addition of Ti during ladle refining or vacuum degassing to produce Mn-Silicate during tapping and Ti in the weakly deoxidized state.
Stable precipitation of TiON or TiN by addition is attempted. N
b is added in an amount of 0.005% or more to control the crystal grains during hot rolling and to prevent HAZ softening, and the addition of more than 0.024% hinders the joint toughness, so is limited to 0.0005 to 0.024%. did.

Ti is Ti in the HAZ during high heat input welding.
0.005% or more is added in order to suppress the growth of austenite (γ) grains by ON or TiN and to form a composite precipitate of TiON or TiN and MnS which becomes ferrite (α) transformation nuclei in the γ grains. % Excessively impairs weldability and joint toughness, so 0.005-
It was limited to 0.018%. The addition of Ti was stable T
Limited to performing Ti addition before complete deoxidation by Al in secondary refining such as vacuum degassing following weak deoxidation by Si + Mn during tapping to form a composite precipitate of iON or TiN and MnS . Even if Ti is within this range, Ti is added in an equal amount to N (N × 3.
4) is preferable. N is 0.0020% or more for forming TiON or TiN. If it exceeds 0.0060%, the slab defect is deteriorated and the joint toughness is deteriorated.
It was limited to 0.0020 to 0.0060%.

Elements other than the above basic components (Cu, N
(i, Cr, Mo, V) is added to improve strength and toughness by adding one or more of them, but the effect of the present invention is not impaired, but other elements (Ca, REM, Zr, B, etc.) Must not be added in order to prevent the inhibition of MnS formation and the deterioration of characteristics associated with the addition of B, which are important technical ideas of the present invention. C
u is added to 0.05% or more by substituting C, Si, and Mn for the purpose of reducing Ceq in order to improve weldability and low temperature toughness, but if it exceeds 0.80%, approximately equal amount is added to prevent hot brittleness, Since it is necessary to add at least half of Ni, which is not preferable from the viewpoint of cost, the amount of Ni is limited to 0.05 to 0.80%.

Ni is Ceq for improving weldability and low temperature toughness.
0.05% by replacing with C, Si, Mn for the purpose of reduction
Although it is added as described above, the addition of more than 1.50% is not preferable from the viewpoint of cost and is limited to 0.05 to 1.50%.

Cr is added in an amount of 0.05% or more in order to improve hardenability and ensure strength, but excessive addition of more than 0.80% hinders weldability and low temperature toughness.
It was limited to 05 to 0.80%. Mo is added in an amount of 0.05% or more in order to improve hardenability and ensure strength.
Addition of more than 0.50% is not preferable from the viewpoint of weldability and cost, and is limited to 0.05 to 0.50%.

[0018]

EXAMPLES Examples of the present invention are shown in Tables 1 and 2 together with comparative examples. Table 1 shows the chemical composition of the present invention examples (Steel A to Steel G) and Comparative Examples (Steel H, Steel I). In Steel H, S is higher than the range of the present invention, and in Steel I, the sulfide-forming element Ca, which should not be added in the present invention, is added. In the steelmaking results, in the present invention example, weak deoxidation by Si + Mn at the time of tapping, followed by complete deoxidation by Al after Ti addition during vacuum degassing. On the other hand, in the comparative example, Ti and Ca were added after complete deoxidation with Al + Si at the time of tapping.
Casting was performed by normal continuous casting. The manufacturing results of the example of the present invention and the comparative example are shown in Table 2 together with the results of rolling.

[0019]

[Table 1]

[0020]

[Table 2]

Steel A is a conventional controlled rolling (CR), Steel B, Steel C, Steel H are the recently developed thermomechanical treatment (TMCP), Steels D to F are direct quenching (DQ) and low temperature tempering (TM). T), Steel G was manufactured by normal quenching and tempering (QT). Steels A to C and Steel H are HT490 steels, and Steels D to G and Steel I are HT580 steels, each of which satisfies the strength level of the relevant standard. On the other hand, regarding the low temperature toughness of the base metal, all steels stably satisfy the required values of the relevant standards.

The reduction value (RAz) and HAZ toughness by the Z-direction tensile test are high in the examples of the present invention. TiN
The steel H of the comparative example, which aims to improve the joint toughness by increasing the amount of + MnS, has the highest HAZ toughness value, but the RAz is the lowest, and does not satisfy the lamellar tear resistance standard. Further, the steel I of the comparative example containing Ca for improving the lamellar tear resistance does not form a composite precipitate of TiN and MnS, which becomes α transformation nuclei in the coarsened γ grains of the joint HAZ part, so that H
AZ toughness is extremely poor.

On the other hand, according to the present invention, a stable composite precipitate of TiON or TiN and MnS is formed by specifying the deoxidizing method without repeating, and a large heat input characteristic in the low S region is used using an expensive REM or the like. Not only was it improved remarkably, but also other steel material properties (lame resistance and hardness increase due to B addition) were improved.

[0024]

EFFECT OF THE INVENTION The steel of the present invention has improved heat input characteristics and lame resistance by specifying the component system and deoxidation method, and can avoid quality problems (problems with hardness increase) associated with B addition. I chose As a result, it is possible to stabilize the large heat input characteristics without using expensive elements and to provide a steel with excellent overall steel material characteristics, and to shorten the lead time for manufacturing large structures by large heat input welding. It is a thing. Therefore, the present invention provides enormous economic benefits to industry as well as those skilled in the art.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.18%, Si: 0.10 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S : 0.0008 to 0.0044%, Al: 0.010 to 0.10%, Nb: 0.005 to 0.024%, Ti: 0.005 to 0.018%, N: 0.0020 to 0 .0060% In steel consisting of balance iron and unavoidable impurities, when MnS as an impurity forms a complex precipitate with TiON or TiN, weak deoxidation by Si and Mn at the time of tapping and secondary refining followed by Al Ti before complete deoxidation
A method for producing a high-strength steel for high heat input welding, which is excellent in low-temperature toughness, characterized by forming a composite precipitate with MnS having TiON or TiN as a nucleus by addition. 2. Cu: 0.05 to 0.80%, Ni: 0.05 to 1.50%, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.50 in% by weight. %, V: 0.01 to 0.09% of one or two or more of them are contained in the steel, and the method for producing a high-strength steel for high heat input welding having excellent low temperature toughness according to claim 1. .