JPH03162522A - Manufacture of high tension steel plate having superior toughness of high heat input weld heat-affected zone - Google Patents

Abstract

PURPOSE:To obtain the title steel plate by specifying the amts. of Ti, N, O and B in a steel having a specified compsn. and ensuring Ti oxide, fine TiN, sol. B and a TiN-MnS-BN combined precipitate. CONSTITUTION:A substantially Al-free steel consisting of, by weight, 0.03-0.15% C, <=0.6% Si, 0.8-2.0% Mn, <=0.025% P, <=0.010% S, <=0.004% Al, 0.003-0.060% Nb, 0.005-0.030% Ti, 0.0003-0.0010% B, 0.0040-0.0065% N, 0.001-0.006% O and the balance Fe and contg. 5X10<3>-1X10<8> grains of Ti-based oxide having 0.05-10mum grain size and 5X10<3>-1X10<8> grains of a TiN-MnS-BN combined precipitate is continuously cast into a billet. This billet is reheated at <=1,250 deg.C and a steel plate is manufactured. By this method, steel plates each having superior toughness at low temp. over the entire high heat input weld heat-affected zone as well as in the base metal are obtd. in large quantities at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is particularly applicable to submerged arc welding (SAW), electrogas welding (EGW), electroslag welding (
This invention relates to a method for producing high-tensile steel plates with excellent heat-affected zone low-temperature toughness during large heat input welding such as S E S).

Steel produced using this method can be used in pressure vessels, shipbuilding, bridges, architecture, etc.
Can be used for welded #4 structures such as line pipes.

(Prior art) The HAZ toughness of low alloy steel is determined by (1) I. ! Size of i-product grains, (2) dispersion state of hardened phases such as high carbon island martensite (M town, upper payinite (Bu)), (3) presence or absence of grain boundary embrittlement, (4) micro-segregation of elements, etc. Subject to various metallurgical factors.

In particular, it is known that the grain size of the HAZ has a large effect on low-temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.

For example, a method has been disclosed for finely dispersing TiN to improve the HAZ toughness during adult heat welding of 50 kg f/1st class high tensile strength steel ("Tetsu to Hagane" No. 65, published June 1970).
Vol. 8, No. I, p. 232). However, most of these precipitates are dissolved during high heat input welding, resulting in coarse graining of the HAZ structure and increase in solid solution N, which has the drawback of deteriorating HAZ toughness.

In response to this problem, some of the inventors of the present invention finely dispersed Ti oxide in steel and generated intragranular acicular ferrite (hereinafter referred to as IFP) in the HAZ during welding, thereby making the HAZ structure finer. It was shown in JP-A-63-210235 and JP-A-1-15321 that HAZ toughness can be significantly improved.

However, as a result of intensive investigation into the relationship between high heat input welding HAZ structure and toughness, we found that even in steel with finely dispersed Ti oxides, the cooling rate after welding is slow in adult heat welding HAZ. It is not possible to completely generate an IFP structure, and in order to drastically improve the HAZ toughness of steel plates used in low-temperature environments such as Arctic waters and LPG tanks, it is necessary to introduce new technological ideas. It has been found.

(Problems to be Solved by the Invention) The present invention proposes a method for producing a high-tensile steel plate having excellent heat-welded HAZ toughness. Especially in high heat input welding HAZ! Because the time of exposure to high temperatures of 000° C. or higher is long and the cooling rate after welding is slow, the control of the HAZ structure is incomplete even in steel in which Ti oxides are dispersed. The high tensile strength steel of the present invention has a fine structure throughout the HAZ including the vicinity of the fusion line, and has excellent low-temperature toughness.

(Means for Solving the Problems) The gist of the present invention is that, in weight %, c: o. oa~0.1
5%, SI: 0.6% or less, Mn: 0.8 to 2.0
%, P: 0.025% or less, s: O. OlO% or less, Al: 0.004% or less, Nb: 0.003-0.
060%, T I+0.005-0.030%, B:0
．． [)003-0.0010%, N: 0.0040
~0.0065%, 0:o. oot～o. Ti-based oxide with a particle size of 0.05 to 10 to ooa% and T
iN+MnS+BN composite precipitates each at 5×103~1
A steel containing substantially no Aj7 containing 8 ×lO/h and the balance consisting of iron and unavoidable impurities is made into a slab by continuous casting, and after reheating at a temperature of 1250°C or less, a steel plate is manufactured. and C: 0.03-0
．． 15%, SI: 0.6% or less, Mn: 0. ll~2
．． 0%, P: 0.025% or less, s: 0.010%
Below, Al: 0.004% or less, Nb: 0.003~
0.060%, Tl: 0.005-0.030%, B:
0.0003-0.0010%, N: 0.0040
~0.0065%, Oo. oot～o. In addition to ooe%, Ni: 0.05-4.00%, Cu: 0.05-1
．． 50%, Cr: 0.05-1.00%, ■=0
．． 005 to 0.080%, Mo: 0.0! +～0
．． 40%, Zr: 0.003 to 0.015%, Ca
: (1. Composite precipitates of TiN+MnS+BN and oxide mainly containing Tl with a particle size of 0.05-10 and containing one or more of (1005-0.00596) are each Substantially A containing 108 pieces of I
, 17 is continuously cast into slabs,
After heating this with ilG at a temperature of 1250° C. or lower, a steel plate is manufactured.

(for production) The present invention will be explained in detail below.

According to the inventors' research, HAZ toughness greatly depends on (+) the chemical composition of the steel and (2) the structure (crystal grain size and hardened phase distribution state), and that the optimization of the steel composition and the resulting crystallization It was thought that grain refinement was essential for achieving high toughness. As some of the inventors have shown in the above-mentioned publications, in steel in which Ti oxide is finely dispersed, the HAZ structure is refined by generating IFP in the HAZ (especially near the fusion line) during welding. , HAZ toughness can be significantly improved.

On the other hand, in the HAZ, in order from the vicinity of the fusion line, there are: ■ region of coarse γ grains of 200 or more blood (hereinafter referred to as coarse grain region), ■ region of 80~
A region of slightly coarse γ grains of about 200uA (hereinafter referred to as sub-coarse grain region); ■A region of fine γ grains of about 80 μA or less (fine grain region)
If you perform adult heat welding, you will be exposed to high temperatures of 1000℃ or higher, which is the so-called γ region, for a longer time, and the cooling rate after welding will be slow, so the width of the coarse grain region and sub-coarse grain region will increase. becomes large, so it is important to refine the structure in this region in order to improve HAZ toughness.

In the coarse grain region, as shown in the above publications, HA is produced by finely dispersing Ti oxide to generate IFP.
The Z structure is refined, and the HAZ toughness can be improved.

However, since the cooling rate after welding is slow in high heat input welding, the formation of ferrite from the grain boundaries in the coarse grain region becomes noticeable, making it difficult to form IFP.

Therefore, regarding the method of suppressing ferrite formation from grain boundaries by adding B}A. i1L f:.

As a result, it was found that in the vicinity of the fusion line after welding, B segregates as solid solution B in the ascending γ grains, suppressing grain boundary ferrite. However, this excessive addition of B in n''fS causes the formation of Fe (C B) 6 at the γ grain boundaries, and conversely promotes the formation of ferrite from the γ grain boundaries, resulting in HAZ
Toughness deteriorates.

In the sub-coarse grain region, it is inevitably difficult to generate IFP. This is because the temperature at which ferrite is formed from the γ grain boundary is higher than the temperature at which IFP is formed, so when the γ grain size is less than 200zza, the inside of the γ grain is covered with grain boundary ferrite, and the IF
This is because there is almost no space for P to grow. Therefore, in the sub-coarse grain region, the structure is not sufficiently refined by IFP, and the HAZ toughness deteriorates. Particularly in the case of adult heat welding, the width of this sub-coarse grain region increases, causing a problem of deterioration of toughness.

On the other hand, we conducted a study from the viewpoint of reducing the width of the sub-coarse grain region by using the whitening effect of fine TiN, and set the NQ to 40 to [
By setting i5ppm, the width of the sub-coarse grain region is reduced and H
It was found that AZ toughness was improved.

If Nr:L is less than 40 ppm, the effect of suppressing the sub-coarse grain region width is weak. If it is added in excess of 65 ppm, excess N will precipitate as solid solution B and BN in the HAZ near the melting line, resulting in γ Solid solution B, which is effective in suppressing ferrite from grain boundaries, cannot be secured, and HAZ toughness deteriorates.

However, no matter how much the width of the sub-coarse grain region is reduced by effective use of fine TiN, the sub-coarse grain region cannot be completely eliminated because the size of the γ grains continuously changes in the HAZ. Therefore, the inventors developed an IF with Ti oxide as the core.
In addition to the microstructure effect due to the formation of P, TiN+! v
It has been found that excellent HAZ toughness can be obtained in the sub-coarse grain region by combining the microstructure refinement effects of the lns+BN composite precipitates.

By matching the region where the TiN+MnS+BN composite precipitate precipitates with the sub-coarse grain region, the structure of the sub-coarse grain region is refined and the toughness is improved. At this time, TiN+MnS+BNl
It is necessary to have 5 x 103 to 1 x 108 pieces/q of j2 synthetic precipitates.

T i N+Mn S+BNl synthesis precipitate is 5 x 10
If it is less than 1×io8 particles/1, there is no effect on refining the structure in the sub-coarse grain region.In addition, if it exceeds 1 × io8 particles/1, B in the steel will exist as TiN + MnS + BNm precipitates even in the coarse grain region. In addition, solid solution B, which is effective in suppressing ferrite from γ grain boundaries, cannot be secured, and HAZ toughness deteriorates.

In order to obtain the effect of refining the HAZ structure (especially in the coarse grain region) by Tl oxide, the oxide of an appropriate size must be uniformly dispersed in the steel. The particle size of Ti oxide is 0.05 to 1O, the number is 5 x 1O
~IXIO" pieces/one are required.

The particle size of Ti oxide is less than 0.5, or the number of particles is 5.
When it is less than ×103 pieces/lIjl, the IFP generation ability becomes weak. In addition, if the particle size of Ti oxide exceeds 10Ilff+ or the number exceeds 1 x 108 particles/1, the Ti oxide itself may become a point of initiation of brittle cracks, or the cleanliness of the steel will decrease and only HAZ In addition, the low-temperature toughness of the base material also deteriorates.

Note that T1 oxide refers to an oxide whose main component is T1.
It is an oxide containing 50% or more of i203. Also, the number of oxides and TiN+MnS+BN is E P MA (Ele
ctron Probe Mlcroanalyser
) shall be measured.

That is, the point of the present invention is that Ti, N, O, B
By limiting ffi, ■ Ti oxide necessary to generate IFP, ■ fine TiN necessary to suppress the width of the coarse grain region and sub-coarse grain region, and ■ grain boundary ferrite in the coarse grain region. Solid solution B necessary for suppression, ■TiN+Mn necessary for refining the structure in the sub-coarse grain region
The objective is to secure S+BNiQ precipitates and obtain good low-temperature toughness throughout the HAZ.

In normal steel manufacturing methods, T1 oxide, TiN, and Ti are present in steel.
In order to ensure the N+MnS+BN composite precipitate, it is essential to optimize Ti, N, O, and Bffl. For this reason, Ti, N, O, and Bm are each set to Ti:0.
005-0.030%, N: 0.0040-0.00
85%, O: 0, ro 01-o. ooe%, B
: It is necessary to limit it to 0.0003 to 0.0010%.

The lower limit of Tj, N, O, B amount is Ti oxide, TiNST
This is the minimum amount of melon required to generate an iN+MnS+BN composite precipitate. The upper limit of Ti is HAZ due to the formation of TiC.
This is to prevent deterioration of toughness, and the upper limit of Nm is
This is to prevent deterioration of HAZ toughness due to lack of solid solution B, which is effective in suppressing ferrite from γ grain boundaries in the ferrite itself and in coarse grain regions. The upper limit of On is also set to prevent deterioration of the cleanliness and toughness of the steel due to the formation of nonmetallic inclusions. The upper limit of Bffi is set to prevent coarse precipitates such as Fe (CB) a from precipitating at the γ grain boundaries and deteriorating the low-temperature toughness.

However, even if steel contains Ti oxide, TiN, TiN
Even if a +MnS+BN composite precipitate is formed, excellent HAZ toughness cannot be obtained unless the basic components are appropriate. The reasons for limiting the other basic components will be explained below.

The lower limit of the amount of C, 0.03%, is the minimum amount to ensure the strength of the base metal and welded part and to exhibit these effects when adding Nb, V, etc. However, too much CIi not only adversely affects the HAZ toughness but also the base metal toughness.
Since it deteriorates weldability, the upper limit should be set to 0. It was set at 15%.

S1 is an element contained in deoxidized steel, and as S+ increases, weldability and HAZ toughness deteriorate, so the upper limit is set at 0.6%.
And so. In the steel of the present invention. J deoxidation is sufficient, and T
1 deoxidation may be sufficient. From the viewpoint of HAZ toughness, the content of S1 is preferably about 0.15%.

Mn is an essential element for ensuring strength and toughness.
Its lower limit is 0.8%. However, if there is too much Mnm, not only will the hardenability increase and weldability and HAZ toughness deteriorate, but also it will not be possible to obtain a base material strength that meets the target standards. For this reason, Mn! The upper limit of i was set to 2.0%.

・In the steel of the present invention, P and S are unavoidable impurities.
The reason why they are set to 0.025% or less and 0.010% or less, respectively, is to further improve the low-temperature toughness of the base material and HAZ. A low PR tends to reduce the tendency for grain boundary fracture at the joint, and a reduction in SR tends to suppress the formation of grain boundary ferrite. The most preferable ps amounts are 0 and O, respectively.
l%, 0.005096 or less.

Although AJ7 is an element generally contained in deoxidized steel, it is an undesirable element in the present invention, and is therefore limited to 0.004% or less. This is because when AN is contained in steel, it combines with oxygen and no Ti oxide is produced. Deoxidation is possible with only T1 and Si, and in the present invention A
The smaller the Nfit, the better; o. ooa% or less is desirable.

Nb has the function of suppressing the formation of ferrite in the rise of γ grains and promoting the formation of fine IFPs with T1 oxide as the nucleus. To obtain this effect, the minimum Nbff is 0.003%.
i is required. However, if Nbffi is too large, it will hinder the formation of fine IFP, so the upper limit should be set to 0.
060%.

Next, the reason for adding N1, Cu, Cr, V, Mo, Zr, and Ca will be explained.

The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the characteristics of the steel of the present invention.

Therefore, the amount added should be limited.

Nl has no adverse effect on weldability or HAZ toughness,
Although it improves the strength and toughness of the base metal, if it is less than 0.05%, the effect is weak, and if it is more than 4.0%, it is unfavorable for weldability, so the upper limit was set at 4.0%.

Cu has almost the same effect as Nl, as well as corrosion resistance, hydrogen-induced cracking resistance, etc., but if it exceeds 1.5%, Cu-cracks will occur during hot rolling, making manufacturing difficult.

Therefore, the upper limit was set at 1.5%.

Cr is an element that increases the strength of the base metal and welded part,
If it exceeds 1.0%, the welding properties and HAZ toughness will deteriorate, and if it is less than 0.05%, the effect will be weak.

Therefore, Cr is set to 0.05 to 1.0%.

V is an element that has almost the same effect as Nb, but 0.00
If it is less than 5%, the effect is small, and an upper limit of 0.08% is permissible.

Mo is an element that increases the strength of the base metal and welded part,
If it exceeds 0.4%, like Cr, the toughness of the base material and joints will deteriorate.
This is undesirable as it leads to deterioration of weldability.

Further, if it is less than 0.05%, the effect is weak. Therefore, the amount of Mo is set to 0.05 to 0.4%.

Zr is a deoxidizing element like Ti. When Zr is added to steel, a composite oxide mainly consisting of Ti and Zr is formed and acts as a nucleus for IFP production. To obtain this effect, 0.003% or more is required. Also HAZ
Is the upper limit set to 0.0154 to prevent deterioration of toughness?
It was set at 6. Note that in order to generate a composite oxide of Ti and Zr, Zr must be added after Ti is added.

Ca controls the morphology of sulfides, increases Charpy absorbed energy, improves low-temperature toughness, and is also effective in improving hydrogen-induced cracking resistance. But Caffi is 0
．． If it is less than 0.005%, there is no practical effect;
If it exceeds 0.05%, a large amount of Cab and CaS will be generated and become large inclusions, which will impair not only the toughness but also the cleanliness of the steel, and will also have a negative effect on weldability. It is set to 0.00506.

Even if the content of the steel is determined as described above, unless the manufacturing process is appropriate, fine Ti oxides and TiN cannot be finely dispersed in the steel before welding. For this reason, it is also necessary to limit the manufacturing conditions.

First, industrially, it is essential to manufacture steel by continuous casting. The reason for this is that in the continuous casting method, the solidification rate of molten steel is fast and a large amount of fine Ti oxides and TiN are obtained in the slab. In the ingot making method using large ingots, T
] It is difficult to finely disperse oxides and TiN in slabs.

In the case of wide continuous casting, the cooling rate depends on the thickness of the slab, but from the perspective of HAZ toughness, the thickness is 350 m.
m or less is desirable.

Furthermore, the reheating temperature of the slab needs to be 1250°C or lower. This is because if the steel is reheated at a temperature higher than this, TiN will become coarse and there will be no fine TiN in the steel before welding, making it impossible to refine the structure in the HAZ. In addition,
In the present invention, it is not necessary to reheat the slab, and there is no problem at all even if the slab is subjected to single-charge rolling or direct rolling.

In the present invention, the rolling method after reheating the slab is not particularly limited, but so-called working heat treatment, quenching and tempering after rolling, and normalizing treatment are appropriate for ensuring strength and toughness. This means that even if excellent HAZ toughness is obtained, I
This is because if the toughness of the recycled material is poor, it is insufficient as a steel material.

In order to improve the low-temperature toughness of the base material, it is necessary to refine the viscous grains of steel. As a method of processing heat treatment, (1
) controlled rolling, (2) controlled rolling followed by accelerated cooling, and (3) direct rolling quenching and tempering, but the most preferred method is a combination of controlled rolling and accelerated cooling. Note that even if this steel is reheated to a temperature below the Ac transformation point for the purpose of dehydrogenation or the like after manufacturing, the features of the present invention will not be impaired.

(Example) Examples are shown in Table 1.

Steel plates (thickness 12 to 5
0m), submerged arc welding (SAW),
We carry out adult heat welding of electro gas welding (EGW),
HAZ toughness was investigated by 2 mm V notch Charpy test at -60°C.

The test piece was taken from the 1/4 t position, the notch position was at the fusion line (F L), and the HAZ was 1.3 mm. The FL notch can evaluate I{AZ toughness in the coarse grain region, and HAZ IJmI1 can evaluate the I{AZ toughness in the subgrain region.

As is clear from Table 1, all the steels according to the present invention have good adult heat welding HAZ toughness. On the other hand, the comparative steel deteriorates in HAZ.

Among the comparative steels, steel 19 has a large amount of Allf;t<Ti oxide is not produced, so the amount of IFP produced is small and the HAZ toughness is deteriorated. Steel 20 does not have the effect of suppressing ferrite from the γ grain boundaries in the coarse grain region because no B is added, and it has no effect of refining the structure in the sub-coarse grain region, so it has no effect on HA.
Z toughness deteriorates. Since steel 21 contains a large amount of Bm, Fe (CB) e is generated in the coarse grain region, resulting in deterioration of HAZ toughness.

23 Steel 22 has low Tlffi<There is no IFP generation effect due to Ti oxide and no γ grain suppressing effect due to TiN, so the HAZ toughness deteriorates. Steel 23 has a high Tiffi content (HAZ toughness deteriorates due to Tic precipitation. Steel 24
has less Nffi (γ grain suppression effect of TiN and TiN
+MnS+BN has little effect on refining the structure in the sub-coarse grain region <HAZ toughness deteriorates.

m25 has too much Nu, so γ in the solid solution N and rough ablation region
HAZ toughness deteriorates due to lack of soluble B, which is effective in suppressing ferrite from grain boundaries. In the case of the upper steel 26, the number of T1 oxides is small, and the amount of IFP produced is small, so the HAZ toughness deteriorates. Steel 27 has too many Ti oxides, so the cleanliness of the steel deteriorates and the base metal and HAZ toughness deteriorate. Steel 28 has a small number of TiN + MnS + BN composite precipitates, so the structure is not refined in the sub-coarse grain region, resulting in HA.
Z toughness deteriorates. Steel 29 has too many TiN+MnS+BN coagulated precipitates, so it becomes impossible to secure solid solution B, which is effective in suppressing ferrite from the γ grain boundaries, in the coarse grain region, and the HAZ toughness deteriorates.

(Effects of the Invention) According to the present invention, it has become possible to manufacture steel having excellent low-temperature toughness not only in the base material but also in the entire area of the hot welding HAZ in large quantities and at low cost.

As a result, we were able to significantly improve the construction efficiency of welded structures and their safety.

teenager Reason Man

Claims (1)

[Claims] 1. In weight%, C: 0.03 to 0.15%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.010% or less, Al: 0.004% or less, Nb: 0.003 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0010%, N : 0.0040 to 0.0065%, O: 0.001 to 0.006% A Ti-based oxide with a particle size of 0.05 to 10 μm and a composite precipitate of TiN+MnS+BN were each mixed into 5×10
^3 to 1 x 10^8 pieces/mm^3, with the remainder being iron and unavoidable impurities. Steel that does not contain substantially Al is made into a slab by continuous casting, and this is cast at a temperature of 1250°C or less. A method for producing a high tensile strength steel plate with excellent heat-affected zone toughness by large heat input welding, which is characterized by producing the steel plate after reheating. 2. In weight%, Ni: 0.05-4.00%, Cu: 0.05-1.50%, Cr: 0.05-1.00%, V: 0.005-0.080%, A claim characterized in that the steel contains one or more of Mo: 0.05 to 0.40%, Zr: 0.003 to 0.015%, and Ca: 0.0005 to 0.005%. Item 1. The method for producing a high-tensile steel plate having high heat-input welding and excellent heat-affected zone toughness according to item 1.