JPS635805A - Production of thick steel plate for line pipe having excellent flash butt weldability - Google Patents

Abstract

PURPOSE:To produce a thick steel pipe having excellent flash butt weldability at a low cost by increasing the rolling ratio in the direction perpendicular to the casting direction of an ingot so that the longitudinal direction of the resultant thick steel plate is perpendicular to the casting direction. CONSTITUTION:The ingot is cross rolled in the casting direction and is then subjected to the final rolling so that the final rolling direction is perpendicular to the casting direction at the time of rolling the continuously cast slab. The central segregation part of the hatched part shown in the figure is thereby prevented from coming at the end of the steel pipe. The toughness in the flash butt weld zone of the steel pipe is largely affected by the presence or absence of the central segregation part. The formation of the central segregation part at the pipe end is thereupon obviated by the above-mentioned rolling method, by which the thick steel pipe having the excellent flash butt weldability is produced at a low cost. The welding in places of extremely cold weather or site welding is thus considerably simplified.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing thick steel plates for line pipes with excellent flash butt weldability (especially weld heat-affected zone toughness). It is most desirable to apply it in a mill. Additionally, thick steel plates manufactured using this manufacturing method are used to manufacture large-diameter line pipes for natural gas and crude oil transportation.

[Conventional technology]

Currently, manual welding, automatic submerged arc welding, semi-automatic welding, automatic carbon dioxide shield welding, etc. are used for on-site welding of large diameter line pipes, but flash butt welding is currently being used mainly in the Soviet Union. It is being adopted. The reasons for this are (1) the efficiency of on-site welding work is extremely high, and (2) welding can be performed even in extremely cold regions where humans cannot weld. However, in this welding method, the ends of the steel pipe are heated to a molten state and then pressure welded, resulting in coarse grains in the joint, resulting in a significant deterioration of low-temperature toughness. Welding defects at joints also pose a major problem. (If the center segregation is large in slabs manufactured by continuous casting method,
Since the metal flow at the joint is as shown in Figure 1, low-temperature toughness is poor and welding defects are likely to occur. This is because, in addition to the elements C, Mn, and P, there are many nonmetallic inclusions such as MnS in the center segregation area of the slab, which collects at the joint.However, in the current continuous casting method, It is impossible to completely eliminate segregation, and additional steps such as high-temperature, long-time soaking and diffusion treatment are essential in order to reduce segregation.

For this reason, there is a strong desire to develop a technology to inexpensively manufacture line pipes that are free from center segregation and have good flash butt weldability.

[Problem that the invention seeks to solve]

The method of the present invention provides a method for manufacturing thick steel plates necessary for inexpensively manufacturing line pipes with excellent flash butt weldability. A steel pipe made of a thick steel plate manufactured by the method of the present invention has no center segregation at both ends of the pipe, and the flash butt welded part welded on-site exhibits extremely excellent low-temperature toughness.

[Means for solving the problems and their effects] The gist of the present invention is as follows: C: 0.005 to 0.15. Si: 0
゜5% or less, Mn: 0.6-2.5%, p: o, o
z% or less.

s: 0.005% or less, Aj! : 0.06% or less,
Neo.

005% or less as the basic component, and further Nb:Q, Ql~0
゜1%, Ti: 0.005-0.1%, V
jo, 01-0.1%.

Ni: 0.05-1%, Cu: 0.05-1%,
Cr: 0.05-0.5%, Mo: 0.05
~0.4%, B: 0.0005~0.003%,
Ca: 0.001~O, OO5%, RBM:
Contains 0.01 to 0.05% of one or more types,
When rolling slabs manufactured by the continuous casting method, the remainder of which is iron and unavoidable impurities, the rolling ratio in the direction perpendicular to the casting direction of the slab is increased, and the final rolling direction (longitudinal direction) and casting direction of the resulting thick steel plate are The present invention provides a method for manufacturing a thick steel plate for line pipes with excellent flash butt weldability, characterized in that the casting direction of the pieces is perpendicular.

The present invention will be explained below. As a result of the research conducted by the present inventors, the toughness of flash butt welded joints of steel pipes is as follows: 1)
It was found that the influence of center segregation is the largest among these, and that the effect of flash butt welds is significantly dependent on the chemical composition of the steel, 2) the presence or absence of center segregation, and 3) the post-heat treatment conditions of the weld. Removal of segregation was considered essential for improving toughness.

For this reason, we have found a method for manufacturing thick steel plates that does not cause segregation at the ends of steel pipes, based on the premise that slabs are manufactured using the current continuous casting method. Segregation of slabs exists in the shaded area shown in Figure 2, but in the case of boat-type plate rolling, the segregation of slabs occurs in the casting direction (L direction).
After width ratio rolling in the direction perpendicular to (C direction), the final rolling is L
Since the welding is carried out in the same direction, the end of the steel pipe and the segregated area overlap, resulting in poor flash butt weldability. Therefore, in the present invention, after the slab is width-ratio rolled in the L direction, the final rolling is carried out in the C direction to prevent center segregation from occurring at the end of the steel pipe.

For example, a slab with a thickness of 200 mm has a non-segregating area of 10 m (approximately 1 m) at both ends in the L direction, so 20 m
A non-segregating portion of about 1000 mm exists at the end of the m-thick steel plate (wA pipe).

Next, the reason for limiting the steel components will be explained.

The lower limit of 0.005% of C is the minimum amount to ensure the strength of the base metal and the welded part and to exhibit these effects when adding Nb, V, etc. However, if there are too many Cils,
The upper limit is set at 0.15 because it not only adversely affects the low-temperature toughness of the base metal and weld zone, but also deteriorates weldability and HAZ toughness.
It was determined to be %.

Si is an element contained in deoxidized steel, but it improves weldability, HA
Since Z is an element that deteriorates toughness, the upper limit was set at 0.5% (Al or Ti alone is sufficient for deoxidizing w4,
(Si is preferably 0.1% or less).

Mn is an essential element for ensuring strength and toughness.
Its lower limit is 0.6%. However, too much Mn not only increases the hardenability of the steel and deteriorates weldability and HAZ toughness, but also promotes center segregation of slabs and deteriorates flash butt weldability, so the upper limit is set at 2.5%. did.

In the present invention, the impurities P and S are each 0.02
% or less and 0.005% or less is to improve the low-temperature toughness of the base metal and welded part. Reduction of these elements improves the center segregation of slabs and improves flash butt weldability. Most desirable P.

sl is 0.01% and 0.002% or less, respectively.

AI is an element contained in deoxidized steel, but /1 is 0.0
If it exceeds 6%, the cleanliness and HAZ toughness of the steel will deteriorate, so the upper limit was set at 0.06%. There is no particular lower limit for Al, but AI! When deoxidizing with
05%. However, deoxidation is also possible with Ti. In this case, the less AI the better.

N inevitably mixes into steel and reduces the low-temperature toughness of the steel. In particular, a large amount of solid solution N tends to produce high-carbon island martensite in IAZ, which significantly deteriorates toughness. For this reason, the upper limit of N was limited to 0.005%.

Next, Nb, Ti, V, Ni, Cr, Mo, B, Ca,
The reason for adding REM will be explained. The main purpose of adding these elements to the basic composition system is to improve properties such as strength, toughness, and hydrogen-induced cracking resistance without impairing the characteristics of the steel of the present invention, and the amount of addition is subject to self-limitation. It is of the nature that it should be done.

Nb is an important element that simultaneously improves the strength and toughness of steel through grain refinement and precipitation hardening through controlled rolling.
If it is less than 0.01%, no significant effect will be obtained, and if it is too much, weldability and HAZ toughness will deteriorate. The upper limit is 0.
It is 1%.

Ti is added in a small amount range (0,005-0,02%)
In this case, fine TiN is formed to refine the structure of the base material, HAZ, and improve low-temperature toughness, and when added in an amount exceeding the stoichiometric ratio as TiN, strength is increased by precipitation hardening. It is also used as a deoxidizing agent in place of AI.

The lower limit of Ti is 0.005%, which is effective in terms of material quality, and the upper limit is 0.1%, taking into consideration weldability and HAZ toughness.

■ is an element that has almost the same effect as Nb, but at 0.01%
If it is below, the effect is small, and an upper limit of 0.1% is permissible.

Ni improves the strength and toughness of the base metal without adversely affecting weldability and HAZ toughness, but if it exceeds 1%, it is unfavorable for weldability, so the upper limit was set at 1%.

Cu has almost the same effects as NL, as well as corrosion resistance, hydrogen-induced cracking resistance, etc., but if it exceeds 1%, Cu-cracks will occur during hot rolling, making manufacturing difficult. For this reason, the upper limit was set at 1%.

Cr increases the strength of the base metal and the welded part, but too much Cr deteriorates weldability and HAZ toughness. Its upper limit is 0.5%.

Although Mo is an element that improves both the strength and toughness of the base metal, too much Mo is not preferable because it causes deterioration of the base metal, weld zone toughness, and weldability like Cr. Its upper limit is 0.4%.

The lower limit of the amount of these elements added should be the minimum amount to achieve the desired effect on the material, and the lower limit is 0.05%.
It is.

Ca and REM control the morphology of sulfide (MnS), increase Charpy absorption energy, and are also effective in improving aging C resistance. However, Ca, il is 0.01%
If added below 0.05%, Ca-0 and Ca-5 will be produced in large quantities and become large inclusions, impairing not only the toughness but also the cleanliness of the steel, and also the weldability. It also has a negative impact. Therefore, the upper limit was set at 0.05%.

REM also has the same effect as Ca, and its effective range is 0.001 to 0.05%.

〔Example〕

Next, embodiments of the present invention will be described.

Steel plates (thickness: 19 mm) with various chemical compositions were manufactured using a converter-continuous casting-thick plate process, and flash butt welding was performed to investigate the toughness and soundness of the joints. The toughness is determined by normalizing by high-frequency heating (900℃ x 5IIlin) after welding.
L, carried out by 2**V shear pie test with a notch in the center of the joint.

Table 1 shows the results.

The steel plate manufactured by the method of the present invention has good properties, whereas the comparative steel not according to the present invention has insufficient toughness or soundness of the joint, making it unsuitable for flash butt welding.
stomach.

In steel 1, the final rolling direction is the L direction, and center segregation occurs at the joint, resulting in poor toughness and soundness. Steel 2゜3 are each P
, the toughness and soundness of the joint are poor due to the high S content. In addition, rope 4 has high cl and high hardenability, so it has poor toughness.

〔Effect of the invention〕

The present invention makes it possible to manufacture thick steel plates with excellent flash butt weldability at low cost, and it is possible to realize welding in cold regions and to greatly increase the number of cylinders in on-site welding.

[Brief explanation of the drawing]

Figure 1 is a photograph of metal weave showing the metal flow at a pipe joint by flash butt welding when a slab manufactured by the continuous casting method has large center segregation, and Figure 2 shows the metal flow of a thick steel plate manufactured by the present invention and the conventional method. It is an explanatory view showing a manufacturing method. Figure 1 combination ↓ 4th consecutive 4 completed grandfather slough ゛ wide juice

Claims (1)

[Claims] C: 0.005 to 0.15, Si: 0.5% or less, Mn
: 0.6 to 2.5%, P: 0.02% or less, S: 0.0
05% or less, Al: 0.06% or less, N: 0.005%
The following are the basic components, and further Nb: 0.01 to 0.1%,
Ti: 0.005-0.1%, V: 0.01-0.1%
, Ni: 0.05-1%. Cu: 0.05-1%, Cr: 0.05-0.5%, M
o: 0.05-0.4%, B: 0.0005-0.00
3%, Ca: 0.001-0.005%, REM: 0.
When rolling a slab manufactured by a continuous casting method containing one or more of 01 to 0.05% and the remainder consisting of iron and unavoidable impurities, the rolling ratio in the direction perpendicular to the casting direction of the slab is increased. A method for producing a thick steel plate for line pipes with excellent flash butt weldability, characterized in that the final rolling direction (longitudinal direction) of the resulting thick steel plate is perpendicular to the casting direction of the slab.