JPS63183152A - High-strength steel excellent in brittle fracture generation characteristic in weld zone - Google Patents

Abstract

PURPOSE:To manufacture a high-strength steel excellent in brittle fracture generation characteristic in a weld zone, by preparing a steel plate which contains specific percentages of C, Si, Mn, Al, and P and in which maximum value of P concentration is controlled. CONSTITUTION:A slab of a steel consisting of, by weight, 0.03-0.15% C, 0.05-0.50% Si, 0.5-2.0% Mn, 0.005-0.05% Al, 0<=0.010% P, and the balance essentially Fe except inevitable impurities is cast. Subsequently, this slab is heated to >=about 1,250 deg.C and subjected to rolling reduction at >=about 1,000 deg.C at a draft of >=about 10% per pass to undergo rolling at a draft of >=about 20% in total, and while held at >=about 1,000 deg.C, the above slab is held again at >=about 1,250 deg.C for >=about 10hr and air-cooled to be formed into a rolling material. Then, the above material is heated to >=about 900 deg.C to undergo hot rolling, so that thick steel plate in which the maximum value of P concentration per 0.5mmX0.5mm in a plate-thickness direction is regulated to <=0.08% can be formed.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to structural materials such as jacket-type platforms for oil production, and has excellent brittle fracture occurrence characteristics in welded parts. This relates to a thick steel material suitable for the node part.

(Prior Art) In recent years, construction of jacket-type platforms for oil production has been carried out, but the jackets of the jacket-type platforms for oil production in the sea are affected by waves and currents. In particular, stress concentration is high at the point where the steel pipes intersect, and since they are joined by welding, particularly severe brittle fracture characteristics are required.

Extra-thick steel plates with a thickness of 50 to 200 mm used for the above-mentioned applications are processed into die grooves, fatigue notches are introduced into the straight bond parts joined by multilayer welding, and COD is performed at a temperature of -10°C. When tested, the COD characteristics deteriorate due to the thick plate thickness and low-temperature transformation products such as island martensite produced by multilayer welding.

(Problem to be solved by the invention) The normal heat input for joining extra-thick steel plates for steel pipes is 5 kJ/m.
Submerged arc welding or shielded metal arc welding is used when the thickness is less than m, and since the plate thickness is thick, multilayer welding is performed. It is known that when multi-layer welding is performed, the COD characteristics are the worst in the area (referred to as ICCGHAZ) where the coarse grains generated by welding heat are heated to a two-phase region of Ar3 point and Ac point in the next pass (referred to as ICCGHAZ). There is. This is because the base material is Arz and Ac
This is because island-like martensite is generated at the prior austenite grain boundaries when heated to the two-phase region of , and this island-like martensite worsens the brittle fracture occurrence characteristics. Island martensite decomposes into ferrite and carbide by heating at a temperature of 350° C. or higher for a long time. but,
In the case of normal welding, the island martensite does not decompose because the time it is heated above 350'C is short.
COD characteristics are not improved.

The present inventors measured the maximum concentration of P in steel sheets using an X-ray microanalyzer (EPMA) with a width of 0.5 mm, and
The relationship between this and the brittle fracture occurrence characteristics of welded parts was investigated using the D test. As a result, the maximum value of P concentration in the steel plate was reduced to 0.2
0.08 per 5mm” (0.5ms+X0.5mm)
% or less, the formation of island-like martensite is suppressed, and even if island-like martensite is formed, the island-like martensite at the welding part becomes ferrite and carbide due to the heat during welding, and C
It was found that the OD characteristics were improved.

(Means for solving the problems) The present invention includes: C: 0.03 to 0.15% by weight, Si: 0.05 to 0.50% by weight, Mn: 0.
5-2.0% by weight1 Al 70.005-0.05% by weight, P: 0.0
10% by weight or less, the remainder is substantially composed of Fe excluding unavoidable impurities, and the maximum value of P concentration in the plate thickness direction per area of 0.5 mm x 0.5 mm is 0.08% A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized by the following: C: 0.03 to 0.15% by weight, Si: 0.05 to 0.50% by weight, Mn: 0.
5-2.0% by weight, Al: 0.005-0.05% by weight, P: 0.
0.010% by weight or less, in addition to 0.03% by weight or less of Nb, 0.1% by weight or less of V, 1.0% by weight or less of Ni, and 1.0% by weight or less of Cu. Including the above, the remainder is steel consisting essentially of Fe excluding unavoidable impurities, and 0.5 mm
A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized in that the maximum value of Pg degree per area of 0.5 mm in the thickness direction is 0.08% or less, C: 0.03 to 0 .15% by weight, St: 0.05-0.50!1%, Mn: 0.
5-2.0% by weight, At: 0.005-0.05% by weight, P: 0.
0.010% by weight or less, in addition to 0.01% by weight or less of Ti, and 0.02% by weight or less of rare earth metals.
The remainder is copper consisting essentially of Fe excluding unavoidable impurities, and the maximum value of P concentration in the plate thickness direction per area of 0.5 mm x 0.5 mm is 0.08% or less. A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, further comprising: C: 0.03 to 0.15% by weight, Si: 0.05 to 0.50% by weight, Mn: 0.
5-2.0% by weight 1^1: 0.005-0.05% by weight, P: 0.
0.010% by weight, in addition to 0.03% by weight or less of Nb, 0.1% by weight
Contains one or more of the following V, 1.0% by weight or less of Ni, 1.0% by weight or less of Cu, and in addition, 0.01% by weight or less of Ti, 0.02% by weight or less A steel containing one or more rare earth metals of 0.0 to 100%, with the remainder consisting essentially of Pe excluding unavoidable impurities, and 0.
5 mm The problem was solved.

(Function) C: The C content is 0.0 in order to obtain the strength required for structural steel such as jacket-type platforms for oil production.
It is necessary to add 3% by weight or more. On the other hand, in consideration of weld hardenability and weld cracking susceptibility, the upper limit is set to 0.15% by weight or less.

St: St is required in an amount of 0.05 weight or more for deoxidation. If the amount of Si added is increased, the strength will increase, but if it exceeds 0.50 wt, the toughness of the base material will deteriorate, so the upper limit is set to 0.50 wt% or less.

Mn: Mn is 0 to give ductility and strength to the base material.
．． It is necessary to add 5% by weight or more. However, if the amount added exceeds 2.0% by weight, the weld hardenability will be significantly increased, so the upper limit is set at 2.0% by weight.

^1: 0.005% by weight or more of Al is required for deoxidizing steel, but if the amount added exceeds 0.050% by weight, the toughness of the weld will deteriorate significantly, so the upper limit should be set to 0.005% by weight or more. 0
50% by weight.

P: P segregates at grain boundaries, causes intergranular fracture, and significantly deteriorates toughness. In addition, the upper limit of the P content is 0.01 because it increases the amount of island-like martensite in the ICCGHAZ part and significantly deteriorates the brittle fracture occurrence characteristics.
0% by weight.

Nb: During hot rolling, Nb not only expands the unrecrystallized region and reduces the size of ferrite grains after transformation into austenite to improve toughness, but also improves the toughness of bainite in the final structure during accelerated cooling after hot rolling. , the amount of low-temperature transformation products such as martensite can be increased, and the strength can be significantly increased.

However, if Nb is added in an amount exceeding 0.03% by weight, it deteriorates the crackability and the toughness of the welded part after stress relief annealing, so the upper limit of the amount of Nb added is set to 0.03% by weight.

v:■ is added to improve strength and toughness like Nb, but if it exceeds 0.1% by weight, it will deteriorate the toughness after stress relief annealing of the weld, so the upper limit is 1.0% by weight. %.

Ni: Ni is added because it can improve the strength and toughness of steel without adversely affecting the hardenability and toughness of the weld heat affected zone, but from a cost perspective, the upper limit is set at 1.
．． 0% by weight.

Cu: Cu improves corrosion resistance in addition to having the same effects as Ni, but if added in an amount exceeding 1.0% Ni, hot brittleness tends to occur, so the upper limit is set to 1.0% by weight. .

Ti: Ti exists as TiN in steel and suppresses the growth of austenite grains in the weld heat affected zone. but,
If the amount added exceeds 0.01% by weight, if TiN decomposes into solid solution Ti in the bond area where it is rapidly heated to near the melting point in the next pass, the hardness of the weld heat affected zone will increase and CO
O value decreases. Therefore, the upper limit of the amount of Ti added is 0.
01% by weight or less.

Rare earth metals (REM): REM is a metal with RUM (0
. However, if it is added in excess of 0.02% by weight, the cleanliness of the steel will decrease and the toughness of the steel will deteriorate, so the upper limit should be set at 0.
．． 02% by weight.

Next, the reason for limiting the maximum value of the P concentration will be described.

The toughness of a multilayer weld of a thick-walled steel plate is determined by the toughness of the ICCGHAZ, and in the case of a normal thick-walled steel plate, there are regions where the island martensite produced in the ICCGHAZ does not decompose, so the toughness of the entire weld is not excellent. The reason is that
This is because when a region with a locally high P concentration overlaps with the ICCGHAZ portion, the shape and diameter of the island-like martensite becomes large, and the island-like martensite becomes difficult to decompose due to heat during welding.

The inventors set C: 0.07% by weight, Si: 0.3
1% by weight, Mn: 1.5% by weight, P: 0.00
2 to 0.005% by weight, S: 0.003% by weight, Al
: 0.025 weight m1Ti: 0.005 weight%
, N: 0.0025% by weight of the composition, thickness 50
The mn+ test steel was made into a mold groove as shown in Fig. 1,
Multilayer welding was performed under the welding conditions shown in Table 1. From this welded joint, a COD test piece with a notch in the bond was collected, and -
The test was conducted at 10°C. The COD specimen after the test was cut as shown in FIG. 2, and P was analyzed on the diagonally lined surface by the following method. In the thickness direction of these test steels and over the entire thickness of the steel plate, an X-ray microanalyzer was used to measure the width of 0.5 mm and the length of 0.5 mm.
In other words, the P concentration was measured for an area of 0.25 mm.The relationship between the maximum P concentration measured for each test steel and the COO value at -10°C when the notch of the weld entered the bond was As shown in the figure, the maximum value of P concentration is 0.08
%, the COO value deteriorates significantly. Due to this, in the present invention, the area is 0.25 m
The maximum value of P concentration per m2 was set to 0.08%. Furthermore, P
The surface on which the analysis was performed was 1 from the bond on which the COD test was performed.
Although they are separated by 0 mm, it is known that there is no difference in the degree of segregation at this distance.

Table 1: The high-strength steel of the present invention undergoes a normal steel manufacturing process, is made into a slab by ingot-blowing or continuous casting, and is made into a thick plate through the following steps.

These slabs were heated to over 1250°C and then heated to 10
After applying a rolling reduction of 10% or more per pass and 20% or more in total at a temperature of 00"C or higher, 1000°C
While maintaining the temperature above, the material is again held at a temperature of 1250° C. or more for 10 hours or more, and then air cooled to make a rolling material. Next, this rolling material is heated to a temperature of 900° C. or higher and hot rolled to form a steel plate.

(Example) Table 2 shows the compositional components of steel, the strength and toughness of the base metal, the maximum value of P concentration, and the C
The results of examining OO are shown.

Perfume numbers 1, 2, 4, 5.8-11 and 17-20 were made into slabs by continuous casting. Among these, No. 1゜2 used a continuously cast slab as a raw material for rolling, and No. 3 No. 3 used a slab that was subjected to blooming rolling and was used as a raw material for rolling.

Slabs with perfume number 4, 5.8-11 and 17-20 are 12
After heating to a temperature of 50°C or higher, rolling is performed at a temperature of 1000'C or higher with a reduction rate of 11% per pass, resulting in a total reduction of 21%.
After rolling and holding at a temperature of 1000°C or higher, heat again to a temperature of 1280°C to 1320°C,
The material was kept at this temperature for 10 to 20 hours and cooled in air to obtain a rolling material.

In addition, steel numbers 6, 7, and 12 to 16 are made into slabs by blooming, heated to a temperature of 1280 to 1320°C, and then rolled at a reduction rate of 11% per pass at 1000°C or higher. After 21% rolling at
The material was heated to a temperature of , maintained in this temperature range for 10 to 20 hours, and air-cooled to obtain a rolling material.

These rolling materials were heated to a temperature of 960 to 1050°C, and hot rolling was completed by 740°C to a plate thickness of 50n+.
After making the steel plate into a steel plate, perfumes 1 to 17 and 20 were heated to 5°C immediately.
Accelerated cooling to 480°C at a cooling rate of /second; Steel plate 1
8.19 was accelerated to room temperature at a cooling rate of 5°C/sec, and then heat-treated at 600''C for 1 hour.

After processing these steel plates into the grooves shown in FIG. 1, they were submerged arc welded under the welding conditions shown in Table 1.

From these welded joints, the cross section is 50mm x 100m
A COD test piece was taken with the fatigue notch at the position shown in FIG. Introduction and testing method of fatigue notch is B55
762: 197! According to J. In addition, a tensile test piece was taken from each of the steel plates in the bending direction, and a V-notch Charpy test piece was taken from the C direction.

Table 2 shows the strength, toughness, and fusion line of the steel plate.
The COD characteristics at 0°C and -30"C are shown.
The test was conducted with three samples per test temperature.

Perfume Nos. 1 to 3 have a P content exceeding the range of the present invention, or even if the P content is within the range of the present invention, the maximum P concentration is high, and both of them are Melting line C
The OD characteristics were poor.

In addition, perfume number 4 has a low P content, and the maximum value of P concentration is 0.
．． Since the COD value was as low as 0.058%, excellent COD characteristics were exhibited.

Furthermore, perfume number 5 is Nb, and perfume number 6 is Cu.

Ni, Nb, perfume number 7 contains Cu, Ni, V-,
Perfume No. 8 is Ni, Perfume No. 9 is Ti, and Perfume No. 10 is T.
i, REM, perfume number 11 is Nb, Ti, perfume number 12 is Cu, Ni, Ti, perfume number 13 is Cu,
Ni+Nb+Ti, fragrance number 14 is Cu, Ni.

V, Ti, perfume number 15 is Cu + Ni, Nb + T
i, REM, perfume number 16 is Cu, Ni, Nb,
V, Ti, REM, fragrance number 17 contains Cu, fragrance number 18 contains Cu, Ni, fragrance number 19 contains Cu.

Ni, Nb, Ti, and REM are added, and perfume number 20 is added with V within the range specified by the present invention. Perfume Nos. 5 to 20 have excellent COD characteristics for all steel plates.

Furthermore, the strength was improved by adding Cu, Ni, Nb, and V.

(Effects of the Invention) As explained above, the steel of the present invention is a high-strength steel that has excellent COD characteristics in the weld zone and excellent brittle fracture occurrence characteristics in the weld zone.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the K-shaped groove shape, and Figure 2 is a diagram showing the shape of the K-shaped groove.
It is a diagram showing a surface analyzed by macro EPMA, and FIG. 3 is a diagram showing the surface of P analyzed by macro EPMA.
n-0,002~0.008%P-0.003%S-0
．． 025%At-0,005%Ti-0,0025%N
FIG. 4 is a diagram showing the relationship between the maximum P concentration of steel and the COO value, and FIG. 4 is a diagram showing the notch position of a COD test piece. Figure 3 Q Q, 02 0.04 0.06 00
9 0. fo 0. f2P Nokei agricultural value (%)

Claims (1)

[Claims] 1. C: 0.03-0.15% by weight, Si: 0.05-0.50% by weight, Mn: 0.5-2.0% by weight, Al: 0.005-0.005% by weight. 0.05% by weight, P: 0.010% by weight or less, the remainder is steel consisting essentially of Fe excluding unavoidable impurities, and P per area of 0.5 mm x 0.5 mm.
A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized in that the maximum concentration in the plate thickness direction is 0.08% or less. 2, C: 0.03-0.15% by weight, Si: 0.05-0.50% by weight, Mn: 0.5-2.0% by weight, Al: 0.005-0.05% by weight, P: 0.010% by weight or less, in addition to 0.03% by weight or less of Nb, 0.1% by weight or less of V, 1.0% by weight or less of Ni, and 1.0% by weight or less of Cu. Containing one or more types, the remainder is steel consisting essentially of Fe excluding unavoidable impurities, and 0.5 mm
A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized in that the maximum value of P concentration in the plate thickness direction per 0.5 mm area is 0.08% or less. 3, C: 0.03-0.15% by weight, Si: 0.05-0.50% by weight, Mn: 0.5-2.0% by weight, Al: 0.005-0.05% by weight, P: 0.010% by weight or less, in addition to 0.01% by weight or less of Ti, 0.02% by weight
A steel containing one or more of the following rare earth metals, with the remainder consisting essentially of Fe excluding unavoidable impurities,
And P in the thickness direction per area of 0.5 mm x 0.5 mm
A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized by having a maximum concentration of 0.08% or less. 4, C: 0.03-0.15% by weight, Si: 0.05-0.50% by weight, Mn: 0.5-2.0% by weight, Al: 0.005-0.05% by weight, P: 0.010% by weight or less, in addition to 0.03% by weight or less of Nb, 0.1% by weight or less of V, 1.0% by weight or less of Ni, and 1.0% by weight or less of Cu. Contains one or more kinds, and further contains 0.
0.01% by weight or less of Ti, 0.02% by weight or less of one or more rare earth metals, and the remainder is substantially composed of Fe excluding unavoidable impurities, and is 0.5mm thick.
A high-strength steel with excellent brittle fracture occurrence characteristics in welded parts, characterized in that the maximum value of P concentration in the plate thickness direction per 0.5 mm area is 0.08% or less.