JPH05171341A - Production of thick steel plate excellent in toughness in welding heat-affected zone - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel plate excellent in toughness at low temp. in a welding heat-affected zone by adding Ti, Mg, and Ca to a molten steel containing specific amounts of dissolved oxygen under specific conditions, subjecting a cast slab having the resulting specific composition to reheating at specific temp., and preparing a thick steel plate. CONSTITUTION:Ti and Mg are added by 0.010-0.040% and 0.001-0.010% by weight, respectively, to a molten steel containing 0.003-0.020% dissolved oxygen and then, within 15min, 0.005-0.050% Ca is added, and the resulting molten steel is solidified, by that, a cast slab which has a composition consisting of 0.02-0.18% C, <=0.5% Si, 0.8-2.0% Mn, <=0.015% P, 0.0001-0.01% S, <=0.004% Al, 0.003-0.060% Nb, 0.002-0.060% N, 0.005-0.020% Ti, 0.0001-0.0010% Mg, 0.0005-0.0050% Ca, 0.001-0.006% O, and the balance iron with inevitable impurities and practically free from Al is prepared. This cast slab is reheated at <=1250 deg.C, and the objective thick steel plate is produced.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a welding heat affected zone (hereinafter referred to as HA
(Hereinafter referred to as Z), which is excellent in low temperature toughness, and a method for producing a high-strength steel sheet.

[0002]

2. Description of the Related Art HAZ toughness of low alloy steel is (1) grain size, (2) high carbon island martensite (M ^* ),
It is governed by various metallurgical factors such as the dispersed state of the hardened phase such as carbide, (3) presence or absence of grain boundary embrittlement, and (4) microsegregation of elements. Among them, it is well known that the grain size of HAZ has a great influence on the low temperature toughness.
Many techniques for making the Z structure finer have been developed and put to practical use.

For example, in 50 kgf / mm ² class high-strength steel, means for finely dispersing TiN to improve HAZ toughness has been disclosed (“Iron and Steel”, No. 65, June 1979).
Vol. 8, page 1232.) However, these precipitates are melt lines (Fusion Lin) that are heated to high temperatures during welding.
e: hereinafter referred to as FL).
There is a drawback that coarsening of the structure occurs, and toughness deteriorates in the HAZ in the immediate vicinity of FL.

To solve this problem, a Ti oxide is finely dispersed in the steel, and an intragranular ferrite (hereinafter referred to as IFP) is generated in the HAZ at the time of welding by using the Ti oxide as a transformation nucleus. It has been shown in JP-A-63-21235 and JP-A-1-15321 that the particle size can be further reduced to improve the HAZ toughness.

[0005]

DISCLOSURE OF THE INVENTION The present inventors
Based on the technique of improving the HAZ toughness by finely dispersing the oxide, as a result of diligent examination of the relationship between the HAZ structure and the toughness, even in the steel in which the Ti oxide is finely dispersed in the steel,
It was found that the HAZ toughness of steel sheets used in low temperature environments such as the North Sea and LPG tanks is not sufficient. However, at present, there is no technique capable of stably refining the HAZ structure to near the FL to improve the HAZ toughness. The present invention proposes a method for producing a high-strength steel sheet having excellent weld HAZ toughness. The high-strength steel of the present invention has an excellent low-temperature toughness because the structure is refined in the entire HAZ including the vicinity of FL.

[0006]

Means for Solving the Problems The gist of the present invention is to provide Ti and Mg in molten steel having a dissolved oxygen content of 0.003 to 0.020% at Ti: 0.010 to 0.040% and Mg: 0.
001 to 0.010%, and within 15 minutes, 0.005 to 0.050% of Ca is added, followed by solidification, and C: 0.02 to 0.18% by weight, Si: 0.
5% or less, Mn: 0.8 to 2.0%, P: 0.015%
Hereinafter, S: 0.001 to 0.01%, Al: 0.004
% Or less, Nb: 0.003 to 0.060%, N: 0.0
02-0.0060%, Ti: 0.005-0.020
%, Mg: 0.0001 to 0.001%, Ca: 0.0
005 to 0.0050%, O: 0.001 to 0.006
%, And, if necessary, Cr: 0.05-1.
00%, Ni: 0.05 to 4.00%, Mo: 0.05
~ 0.4%, V: 0.005-0.080%, Cu:
0.05-1.50%, B: 0.0003-0.002
A slab containing 0% of one kind or two or more kinds, the balance of which is iron and unavoidable impurities and which does not substantially contain Al is 1
It is to manufacture a steel sheet after reheating at a temperature of 250 ° C. or lower.

[0007]

The present invention will be described in detail below. According to the research conducted by the inventors, the HAZ toughness is (1) the chemical composition of steel,
(2) It depends on the structure (size of crystal grains and distribution state of hardened phase), and it is considered that optimization of steel components and refinement of crystal grains by this are essential for high toughness. JP 63
As disclosed in JP-A-210235 and JP-A-1-15321, the steel in which Ti oxide is finely dispersed has a fine HAZ structure by generating IFP in HAZ (particularly in the vicinity of FL) during welding. And the HAZ toughness can be remarkably improved.

On the other hand, the inventors of the present invention filed Japanese Patent Application No. 03-22865.
As shown in the specification No. 4, when Ti and Mg are added together, the oxide containing Ti can be dispersed in the steel in a larger amount and finer than in the case of Ti alone deoxidized steel, and the oxide is used as a core to form grains. It was found that excellent HAZ toughness can be achieved because the inner ferrite is generated.

However, as a result of the subsequent examination, Ti and Mg
Since the oxides generated when the alloys are added together tend to agglomerate in molten steel in clusters and tend to float, it is not always easy to disperse them uniformly throughout the slab. It has been found preferable to shorten However, in the actual production of steel materials, for example, even when solidifying by a continuous casting method, Ti, Mg
It is difficult to start the solidification immediately after the addition of.

Then, as a result of studying oxides having little time dependency until solidification, after adding Ti and Mg,
Further, it was found that the addition of Ca reduces the time dependence of the oxide distribution state, and it has been found that it is extremely effective for uniform fine dispersion. That is, Ti, Mg, Ca
By prescribing the addition method of the above, it is possible to uniformly and finely disperse the complex oxide mainly composed of Ti and containing Mg and Ca in the steel, regardless of the time until solidification, in other words, regardless of the position of the slab. It was found that the above was possible, and the present invention was achieved.

The oxide dispersed in the steel according to the present invention does not necessarily have a constant composition, but contains mainly Ti,
It contains a small amount of Mg and Ca. Other traces of Al, Mn, S
In some cases, i can be measured, but both can be ferrite transformation nuclei, and the steel in which a large amount of the above complex oxide is finely dispersed in the steel according to the present invention is used in a region near FL and heated to 1400 ° C. or higher. Also, the oxide existing in the γ grains during the γ-α transformation is used as a nucleus to generate IFP, and the HAZ structure is remarkably refined.

In addition, the area away from FL (5 mm from FL)
The HAZ toughness can be improved by including fine TiN in a region up to a certain degree). This is fine TiN
This is because the coarsening of γ grains is suppressed and the structure is refined.

In order to finely disperse a large amount of Ti-containing oxide having such an effect in steel, first, T
The amount of dissolved oxygen in the molten steel before adding i, Mg, and Ca needs to be 0.003 to 0.020%. This is because if the dissolved oxygen amount is less than 0.003%, the oxygen amount after deoxidation with Ti, Mg, and Ca will decrease, and the final number of fine oxides will decrease.

However, if the amount of dissolved oxygen exceeds 0.020%, even if Ti, Mg, and Ca are added, deoxidation is not sufficiently performed, and the cleanliness decreases and the toughness of the base material deteriorates. Further, a coarse oxide is likely to be formed, and this becomes a starting point of brittle fracture, so that the HAZ toughness also deteriorates. It is necessary to add about twice the amount of Ti that is finally necessary because about 50% of the added amount of Ti is discharged as slag due to deoxidation.

Further, since the yield of Ca is about 10%, it is necessary to add about 10 times the Ca that is finally required. Since Mg has a further poor yield, it is necessary to add about 10 to 20 times the required amount.

The order of addition of Ti, Mg and Ca is T
Either i or Mg may come first. Or even if added at the same time, there is almost no difference in effect.
However, it is necessary to add Ca last. Since Ca has a strong affinity with oxygen, when Ca is added first, the oxide formed in the steel becomes an oxide mainly composed of Ca. Since the oxide containing Ca as a main component has a significantly low IFP forming ability, the effect of refining the HAZ structure cannot be expected.

Mg has a stronger affinity with oxygen than Ti, but since the content of Mg is small in the present invention, the oxide produced is a Ti-based oxide having a strong IFP producing ability regardless of the order of addition. Become.

When Ti and Mg are added and Ca is further added, from the time when Ti or Mg is added later,
Or within 15 minutes from the time when both were added simultaneously, C
It is necessary to add a. This is because, if 15 minutes or more has passed since the addition of Ti and Mg, the aggregation and coalescence of the Ti-containing oxide and the floating progress, so that the final fine oxide cannot be uniformly dispersed in the steel.

When Ti and Mg are added separately, it is not necessary to strictly define the addition interval of Ti and Mg like the addition interval of these and Ca, but the addition interval is within 30 minutes or the method of adding them at the same time. Is preferred for uniform dispersion of the oxide.

Ti, Mg, C as contents in the steel material
The amounts of a, N and O are limited for the following reasons. Ti, M
The lower limit of the amount of g, Ca, N, and O is the minimum amount necessary to form TiN, Mg, and Ca composite oxide, TiN. T
The upper limit of the i amount is to prevent the HAZ toughness from being deteriorated due to the formation of TiC. The upper limit of the amount of Ca is to prevent a large amount of CaO from being produced and impairing the toughness and cleanliness of steel. The upper limit of Mg is to prevent the IFP generation ability of the oxide containing Ti from decreasing. The upper limit of the amount of N is to prevent deterioration of HAZ toughness due to solid solution N. The upper limit of the amount of O is to prevent deterioration of cleanliness and toughness of steel due to the formation of non-metallic inclusions.

As described above, the amounts of Ti, Mg, Ca, N, and O are limited, and the addition conditions of Ti, Mg, and Ca are specified to uniformly finely disperse the oxide containing Ti in the steel, and further Even if TiN is produced, excellent HAZ toughness cannot be obtained unless the basic components are suitable. The reasons for limiting the other basic components will be described below. The lower limit of C2 is 0.02%,
This is the minimum amount for ensuring the strength of the base material and the welded portion and exerting these effects when Nb, V, etc. are added. However, if the C content is too large, not only the HAZ toughness is adversely affected but also the base metal toughness and weldability are deteriorated, so the upper limit was made 0.18%.

As Si content increases, weldability and HA
The Z toughness deteriorates, so the upper limit was made 0.5%.

Since Mn is an essential element for securing strength and toughness, its lower limit is set to 0.8%. But M
If the amount of n is too large, the hardenability becomes excessive and the weldability and HA
Since Z toughness deteriorates, the upper limit was made 2.0%.

Since P adversely affects the toughness of both the base material and the welded portion, it should be reduced as much as possible, and the upper limit was made 0.015%.

S is an element that forms MnS and promotes the formation of intragranular ferrite, and is required to be 0.001% or more in order to exert its effect, but excessive addition exceeding 0.01% is coarse. S forms a range of 0.001 to 0.01% because it forms an A-based inclusion and causes a decrease in ductility and toughness of the base material and an increase in anisotropy of mechanical properties.

Al is an element generally contained in steel due to the necessity of deoxidation, but it is an element which is not preferable in the present invention and should be reduced as much as possible, so it is limited to 0.004% or less.
This is because if Al is contained in the steel, it will combine with oxygen and Ti oxide will not be generated. Deoxidation is Ti and S
It is possible with i alone.

Nb is an element which suppresses ferrite generated at the γ grain boundaries and is effective for making the HAZ structure fine. To obtain this effect, at least 0.003% is necessary. However, if the amount of Nb is too large, the production of IFP is hindered, so the upper limit was made 0.060%.

Next, Cr, Ni, Mo, V, Cu, B
The reason for adding is explained. The main purpose of further adding these elements to the basic composition is to improve the properties such as strength and toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself.

Cr is an element that enhances the strength of the base material and the welded portion, but if it is less than 0.05%, the effect is weak, and if it exceeds 1.0%, the weldability and HAZ toughness are deteriorated.
The range was 5 to 1.0%.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness.
If it is less than 5%, the effect is not clear, and addition of 4.0% or more is unfavorable for weldability, so the range was made 0.05 to 4.0%.

Mo is an element that enhances the strength of the base material and the welded portion, but if it exceeds 0.4%, it has the same weldability and H as Cr.
The upper limit was made 0.4% in order to deteriorate the AZ toughness. If less than 0.05%, the effect is weak, so 0.05% to
It was 0.4%.

V is effective for increasing the strength of the base material due to precipitation strengthening, but at 0.005% or less, the effect is not clear,
If it exceeds 0.08%, the toughness deteriorates, so 0.00
The range was 5 to 0.08%.

Cu has an effect similar to that of Ni as well as an effect of corrosion resistance, resistance to hydrogen-induced cracking, etc.
%, The effect is not recognized, so the lower limit is 0.05%
And However, if it exceeds 1.5%, cracking occurs during hot rolling, which makes manufacturing difficult, so the upper limit is 1.5%.
And

B segregates as a solid solution B in the γ grain boundary in the vicinity of FL after welding and suppresses grain boundary ferrite. To obtain this effect, a minimum B content of 0.0003% is required. However, excessive addition of B causes coarse precipitates such as FE ₂₃ (CB) ₆ to precipitate at grain boundaries and deteriorate toughness, so the upper limit was made 0.0020%.

Even if the composition of the steel is limited as described above, TiN cannot be finely dispersed in the steel before welding unless the manufacturing method is appropriate. Therefore, it is necessary to limit the manufacturing conditions.

When a once solidified slab is manufactured by cooling and then reheating it, it is necessary to prevent the slab reheating temperature from becoming too high for fine dispersion of TiN. To prevent coarsening of TiN, the reheating temperature is set to 12
It needs to be 50 ° C or lower. However, in the present invention, it is not always necessary to reheat the slab, and there is no problem even if hot charge rolling or direct rolling is performed.

In the present invention, the rolling method after reheating the cast slab is not particularly limited, but so-called thermo-mechanical treatment, controlled rolling, quenching after rolling, and normalizing treatment are suitable for securing strength and toughness. is there. This is an excellent HA
This is because if the toughness of the base material is inferior even if Z toughness is obtained, it is insufficient as a steel material. Even if the steel is reheated below the Ac ₁ transformation point for the purpose of dehydrogenation after production, the characteristics of the present invention will not be impaired.

The steel produced by this method can be used for welded steel structures such as offshore structures, pressure vessels, shipbuilding, bridges, constructions and line pipes.

[0039]

EXAMPLES Examples are shown in Table 1. Steel plates of various steel components are manufactured by well-known converter, continuous casting, and thick plate process, submerged arc welding (SAW) is performed, and HAZ toughness is -6.
It was investigated by a 2 mm V notch Charpy test at 0 ° C. The test piece was taken from a position of 1/4 of the plate thickness, and the notch position was FL and the position shifted by 1 mm from FL to the HAZ side (HA
Z1 mm).

[0040]

[Table 1]

[Table 2]

[Table 3]

[Table 4] As is clear from Table 1, all the steels according to the invention of steels 1 to 15 have good HAZ toughness. On the other hand, steel 16
Since the comparative steels of ~ 30 do not satisfy the requirements of the present invention,
In all, the HAZ toughness is inferior to the steel of the present invention.

Among the comparative steels, since steel 16 does not contain Ti and Mg, the HAZ toughness is inferior because the number of oxides capable of forming IFP is extremely small and TiN has no effect of suppressing the austenite grain size. Steel 17 contains Ti and Ca but does not contain Mg, so that the amount of IFP produced is smaller than that of the steels of the present invention, and therefore HAZ toughness is insufficient.

Since the amount of Mg in steel 18 is too large, the number of oxides is small and the oxides become coarse, so that the toughness deteriorates. Since Steel 19 does not contain Ca, the oxide distribution in the slab becomes non-uniform, and HAZ toughness cannot be stably ensured.
Steel 20 has a small amount of dissolved oxygen before addition of Ti and Mg,
As a result, the number of oxides generated cannot be secured, and HAZ
Inferior toughness.

On the contrary, since the steel 21 has an excessive amount of oxygen, it contains a large amount of coarse oxide, and the oxide itself becomes a starting point of brittle fracture.
AZ toughness is not improved. Steel 22 is not only the amount of oxygen,
Since it also contains Mg and Ca in excess, the HAZ toughness is further deteriorated. Steels 23 and 24 were added with Ca first, so
Oxides mainly composed of Ca increase, and conversely, the number of oxides mainly composed of Ti having an IFP forming ability decreases, so that the IFP formation in the HAZ is insufficient and the HAZ toughness deteriorates.

In Steel 25, since the time from the addition of Ti and Mg to the addition of Ca is long, the agglomeration and coalescence of the oxides containing Ti proceed, so that a uniform dispersion of fine oxides cannot be achieved. , HAZ toughness deteriorates. Since Steel 26 has a large amount of Al, IFP is not formed and HAZ toughness is poor. In addition, since the steel 27 has an excessive amount of N, fine dispersion of TiN is not achieved, and the amount of solute N also increases, so that the HAZ toughness deteriorates. Since the amounts of C, B, and V of steels 28 to 30 are not appropriate, the HAZ toughness deteriorates. From the above examples, it is clear that the present invention provides extremely excellent HAZ toughness.

[0045]

EFFECTS OF THE INVENTION The technique of using Ti oxide to generate intragranular ferrite in the HAZ structure to refine the structure is HA
This is an excellent technique for improving Z toughness. The present invention is a technique capable of achieving a large amount and uniform fine dispersion of an oxide containing Ti by additionally adding Mg and Ca to molten steel in addition to Ti, and as a result, the HAZ toughness can be further improved. Therefore, it is possible to provide a welded structural steel having high safety even under more severe use conditions, and the effect is extremely remarkable.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Terada 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Works

Claims (2)

[Claims] 1. The amount of dissolved oxygen is 0.003 to 0.
Ti and Mg in 020% molten steel are respectively Ti: 0.0
10-0.040%, Mg: 0.001-0.010%
Within the range of 0.005% Ca within 15 minutes.
˜0.050% is added and then solidified, C: 0.02 to 0.18% Si: 0.5% or less Mn: 0.8 to 2.0% P: 0.015% or less S: 0. 001-0.01% Al: 0.004% or less Nb: 0.003-0.060% N: 0.002-0.0060% Ti: 0.005-0.020% Mg: 0.0001-0 .0010% Ca: 0.0005 to 0.0050% O: 0.001 to 0.006% Recast a slab containing Al and unavoidable impurities, the balance of which is substantially free of Al, at a temperature of 1250 ° C or lower. After that, a steel plate is manufactured, and a method for manufacturing a thick steel plate having excellent weld heat-affected zone toughness. 2. By weight%, Cr: 0.05 to 1.00% Ni: 0.05 to 4.00% Mo: 0.05 to 0.4% V: 0.005 to 0.080% Cu : 0.05 to 1.50% B: 0.0003 to 0.0020%, or a combination of two or more thereof, to produce a thick steel plate having excellent weld heat affected zone toughness according to claim 1. Method.