JPS5935629A - Manufacture of high-tension electric-welded steel pipe having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To obtain an electric-welded steel pipe having superior toughness at low temp. at the weld zone as well as the base metal part by specifying the composition of a steel plate as a material and conditions during the heating and cooling of a weld zone formed by electric welding. CONSTITUTION:A steel plate consisting of 0.01-0.08% C, <=1.5% Mn, <=0.5% Si, <=0.03% P, <=0.008% S, <=0.04% Ti, 0.001-0.050% Nb, 0.001-0.050% V, <=0.010% N and the balance Fe is electric-welded. The weld zone is heated at 790-1,050 deg.C for >=5sec and rapidly cooled from 750-950 deg.C at 30-150 deg.C/sec cooling rate. By this method, a fine acicular ferrite structure can be provided to the weld zone, and an electric-welded steel pipe having high toughness at low temp. is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength electric resistance welded steel pipe in which the electric resistance welded portion has excellent low-temperature toughness comparable to that of the base metal portion.

In the conventional manufacturing method, fine ferrite is formed in the base metal through controlled trawling and controlled cooling, resulting in a product with excellent low-temperature toughness, but in the electric resistance welded part, the rolled structure disappears and a coarse cast structure is formed. Furthermore, since schifrene ferrite cannot be formed by rapid cooling after welding, the low-temperature toughness is poor.

The present invention advantageously overcomes the above-mentioned drawbacks, and its gist is that c: o, o i ~ 0.08%;
MnS2.5%, SiS2.5%, P≦0.03%, S
≦o, oos%, Ti≦0.04%, Nb: 0.00
1 to 0.050%, ■2 0.001 to 0.050%, N
≦0.010%, the balance being Fe and unavoidable impurities, welded by electric resistance welding, and then the electric resistance welded part was
Heat for 5 seconds or more at ℃~1050℃, then 750℃~950℃
From the temperature of 30℃/sec to 150C/ge
This is a method for manufacturing a high-strength electric resistance welded steel pipe with excellent low-temperature toughness, which is characterized by rapidly cooling the pipe at step c to form a fine Achquilar ferrite structure in the electric resistance welded part.

12+1 According to the present invention, by limiting the composition of the material, the heating conditions for the ERW welded part after ERW welding, and the subsequent cooling conditions, the ERW welded part can be made to have an excellent low temperature toughness comparable to that of the base material. This is extremely advantageous as it makes it possible to manufacture sewn steel pipes.

Next, the present invention will be explained in detail.

First, regarding the ingredients of the material, C is set at 0.01 to 0.08% in order to improve toughness through grain refinement, and other elements are used to compensate for strength.

If the amount of Mn is more than 1.5%, it will have a negative effect on weldability, so it is preferable to keep it at 1.5% or less.9St is necessary to maintain strength, but if it is more than 0.5%, it will reduce the electrical power. 4 netrators are easily generated at the seam weld,
It should be 0.5 inches or less.

Since P has a negative effect on segregation, P should be 0.03 or less.

As for S, the toughness deteriorates due to elongated inclusions of MnS, so it is preferable that the S content be lower than -4L<0.008%.

TI and N precipitate as fine Ti-N, and this imitative fine T
Since i-N refines the structure of the weld zone and improves toughness, TI is contained in an amount of 0.04% or less and N is contained in an amount of 0.010% or less.

For Nb and V, it is necessary to ensure strength, and for Nb it is 0.0
The range is 0.01 to 0.050%, and ■ is 0.001 to 0.
050% o range.

The material is deoxidized with AI, and the normal amount of A remaining at that time is
I! Contains.

Next, the heating and cooling conditions after electric resistance welding will be described. In the present invention, heating is performed in the range of 790°C to 1050°C for 5 seconds or more after electric resistance welding. If the temperature is lower than 9.790°C, it will not be possible to improve the brittle structure of the welded part, and if it exceeds 1050°C, the structure of the welded part will not be improved. This is undesirable because it causes coarsening of the particles.

Moreover, if the heating time is less than 5 seconds, the structure of the welded part cannot be completely improved, which is not preferable.

Regarding cooling conditions, the cooling start temperature is 750℃ to 950℃.
The cooling rate is also 30 Vsee - 15
The reason for this limitation is that the precipitation of acicular ferrite requires a speed of 30°C/BeQ or higher when passing through the transformation point of Ar5), and its upper limit is also 150°C. /5et- is preferred.

Regarding the temperature, a temperature of 750° C. or below, which is near the Ar5 point, is preferable because the grains do not become fine, but if water cooling is started at a temperature higher than 950° C., no improvement in the structure improvement is observed, which is not preferable.

Regarding such heating and cooling conditions, the present inventors conducted various experiments, and FIG.
After heating for 5 seconds or more in the range of 0°C, the low-temperature toughness was investigated by changing the cooling rate to 5 Vsee and 50'C/sec.

As a result, the cooling rate of 50℃7'5ecO is 5℃/s
It has become clear that the transition temperature (vTrs) is lower than that of ee, and the low-temperature toughness is improved.

The 21st gl is the transition temperature (v'rrs
) is shown by changing the C content, C: 0, 0
30℃ at 8% content, /see ”450
In the C/sec range, the low-temperature toughness is improved, but with a C content of 0.15%, good results cannot be obtained.

Figure 3 is an optical micrograph (400x) after electric resistance welding in a component system with C: 0.08%.
After heating at 0℃ for 5 seconds, air cooling at 5℃/se (a)
Welded part, (b) when cooled with water at 50°C/sec with base metal part (
C) Shows the structure of the weld.

Note that (a) is vTrS = -20℃, ferrite grain size N
0 = 10.9, Φ) is vTrs = -80℃, ferrite grain size No = 12.8, (C) is vTrs = -
40° C., ferrite grain size N0 = 12.6.

That is, as is clear from FIG. 3, according to the present invention, which controls the heating and cooling conditions after electric resistance welding, post-normal
Compared to the coarse ferrite grains of (e.e. air cooling), the ferrite structure becomes ultra-fine, the ferrite grain size is comparable to that of the base material, and the low-temperature toughness is good.

As described above, the present invention adjusts the heating and cooling conditions after electric resistance welding to 79.
Heat for 5 seconds or more at 0°C to 1050°C, and then lower the cooling start temperature to 7.
50℃～950℃～301Th/see～150℃
By performing water cooling at a cooling rate of /see, an electric resistance welded steel pipe with good low-temperature toughness can be obtained even at the welded part, and the effect is extremely large.

Next, the manufacturing process of the electric resistance welded steel pipe of the present invention is shown in Fig. 4, in which 1 is the electric resistance welded part, 2 is the welding roll, 3 is the NOI post normalizer, 4 is the NO2yf post normalizer, and 5 is the NO3 This is a post-normalizer, after which six water-cooling zones are installed, and the cooling start temperature and water-cooling cooling rate are arbitrarily set.

Arrows in the figure indicate the direction of travel.

Next, Table 1 shows examples of the present invention.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between heating temperature and transition temperature after electric resistance welding, Figure 2 is a diagram showing the relationship between cooling rate and transition temperature,
Figure 3 (A) l (b), (e) are c: o, os
Micrograph X after electric resistance welding in a component system of % (400
FIG. 4 is an explanatory diagram showing the manufacturing process of the electric resistance welded steel pipe of the present invention. Machining...Erw welding section 2...Welding roll 3...
・NOI ylP normalizer 4...NO2s
= Stonormalizer 5...NO3, f? Stock normal day 6...Water cooling zone nail) 1 Figure 2 Leave 1'JLJL ('C/+ec) Figure 3 (4)

Claims (1)

[Claims] c: o, o i ~ o, o s %, M
nS2.5%, 81≦0.5 ratio, P≦0.03
H, S≦o, oos%, TI≦0.04%, Nb:0.
001-0.050%, V: 0.001-0.050%
, N≦o, oiochi, electric resistance welding is performed using a material steel plate consisting of the remainder Fe and unavoidable impurities, and then the electric resistance welded part is
Heat at 90°C to 1050°C for 5 seconds or more, then heat to 750°C to 95°C.
Temperature of 0℃ to 30℃/geC to 150℃/se
A method for manufacturing a high-tensile resistance-welded steel pipe with excellent low-temperature toughness, characterized by rapidly cooling the pipe at c to form a fine acicular ferrite structure in the electric and pure contact areas.