JPS6169918A - Production of high-strength extra thick coil having excellent hic resistant characteristic and toughness - Google Patents

Abstract

PURPOSE:To produce a high-strength extra-thick coil having an excellent HIC resistant characteristic and toughness by executing respective stages for rolling roughly a heated steel slab to an intermediate stage, waiting for temp. and subjecting further the same to rough rolling, finish rolling and coiling under specific conditions. CONSTITUTION:The steel slab is heated to about 1,150-1,300 deg.C and is waited for temp. down to <=950 deg.C in the intermediate stage when the slab is rolled down to the thickness of about >=4 times the thickness of the product coil by rough rolling. The slab is then subjected successively to the rough rolling and finish rolling in such a manner that the outlet temp. of a finishing mill in the finish rolling stage in succession to the remaining rough rolling attains >=720 deg.C. The rolled sheet is coiled at 450 deg.C. The component compsn. of the blank steel material contains preferably about <=0.15% C, about 0.05-0.50% Si, about 0.50-1.50% Mn, about 0.10-0.50% Ni, about 0.005-0.100% Nb, about 0.10-0.50% Cu, about <=0.070% Al, about 0.025% P, about <=0.003% S and about 0.0010-0.0060% Ca.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention relates to a coil for line pipes used in pipelines for transporting oil, natural gas, etc. The technical content is a method for manufacturing strong, extra-thick coils.

In recent years, line pipes for sour gas, for example, have tended to have larger diameters and thicker walls to transport sour gas over long distances to consumption areas due to the development of large-scale sour gas wells.
Efforts are being made to improve strength by increasing tensile strength.

In addition, H
, S is contained in a large amount, line pipe hydrogen induced cracking (Hyarogen Induced C
rackxng s (hereinafter referred to as IIG), which often causes leakage and explosion accidents, has become a problem.0 This HIC is a phenomenon different from the conventionally known so-called sulfide corrosion cracking of high-strength steel. This phenomenon is observed even when no stress is applied, and is characterized by its occurrence almost unaffected by the strength and hardness of the steel material.

According to research to date, HIO is caused by hydrogen infiltrating steel from the environment due to corrosion, gathering at the boundary between nonmetallic inclusions and the steel base, and resulting from the gas pressure. So-called A-based sulfide inclusions, such as MnS, are most harmful to HIC because of the stress concentration caused by the shape effect (notch effect) at the tip of the material. Like this H
IOG;! , there is a strong correlation with the amount of MnS and the degree of elongation, and the less elongated MnS is, the lower the cracking susceptibility is.

Furthermore, development of energy resources such as oil and natural gas has become widespread even in cold regions, and as line pipes are installed at low temperatures, requirements for toughness have also increased.

In this way, line pipes are required to have various properties, so coils for line pipes are required to have not only strength but also excellent I (IC) resistance, and in some cases, toughness. There are also strict requirements.

(Prior Art) Conventionally, this type of coil has been manufactured under normal conditions.

(1) Adding grain refining elements such as Nb, (2)
Lowering the slab heating temperature (3) Finishing mill entrance temperature (hereinafter referred to as FET) in finish rolling
(abbreviated as "FDT") is regulated to 950°C or less, and the sheet bar thickness is increased to increase the finishing reduction ratio; (4) the finish mill outlet temperature (hereinafter abbreviated as FDT) in finish rolling is lowered; and (5) Lower the winding temperature (hereinafter abbreviated as OT).

Although good toughness can be obtained under the above conditions, there is the problem of deterioration of HIC resistance as described below.

First, lowering the slab heating temperature improves toughness, but since Nb is not completely dissolved, NbO does not precipitate during the rolling process, resulting in a decrease in strength. Although this decrease in strength can be compensated for by increasing Mn, increasing Mn causes the formation of MnS, which is disadvantageous for HIC resistance.

If the rolling reduction ratio is increased when the thickness of the coil is thick, the resistance during rolling becomes too large and rolling becomes impossible.

When the FDT is lowered, a bainite structure is formed, and the HIC resistance deteriorates.

(When 3T is lowered, it becomes a bainite structure and has high resistance to HI.
C characteristics deteriorate.

(Problems to be Solved by the Invention) Therefore, it is an object of the present invention to provide a method for manufacturing an extremely thick coil having excellent strength and HIC resistance.

A further object of the present invention is to provide a method for manufacturing an extra-thick coil that has excellent toughness in addition to the above properties.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides that when a steel slab is heated and the slab is roughly rolled, the thickness of the steel slab is four times or more greater than the thickness of the product coil in this rough rolling. Wait until the temperature reaches 950°C or less at the intermediate stage where the rolling is completed, and then perform the remaining rough rolling and finish rolling in which the finishing mill exit temperature in the finishing mill following the subsequent rough rolling is 720°C or higher. The process continues and is then wound at a winding temperature of 4506C or higher.

Further, this invention will be explained with reference to FIG.

First, a steel slab is heated in a heating furnace l. The heating temperature is a general range in hot rolling (for example, 1150 to 130
0°C).

Next, the steel slab is subjected to rough rolling. Rough rolling is performed by reverse roll R0, one-pass rolling R2, R8, and tandem rolls R,, R, and rolling is performed at an intermediate stage, for example, reverse roll R and one-pass rolling R,, R,. At least 4 times the product film thickness, tandem roll R1
Before this (indicated by R and FT in the figure), the temperature is waited until the temperature reaches 950°C or less, and then the subsequent rough rolling is performed.

Next, finish rolling is performed using finishing roll rows F 1 to F 7 , and at this time, the FDT is maintained at 720° C. or higher to perform rolling. Although the upper limit of FDT is not particularly set, since it is regulated to 950°C or less in the intermediate stage of rough rolling, the upper limit of FDT is actually about 800°C.

Finally, winding is performed using a coiler, and the GT at this time is set to 450° C. or higher. In this case as well, no upper limit is set.

Next, the material components suitable for use in the present invention and the desirable range of components will be described.
05-0.50%, MnO, 50-1.50%, N
i0110~0.50%, Nb0.005~0.100
%, cu O, 10-0.50%, A10.070%
Below, a steel slab with a composition containing PO, 025% or less, so, ooa% or less, and aaO, 0010 to 0.0060, or MO 0,01 to 0.3091+, Vo, 0
05-0.100% and Bo, 0005-0.0050
It is preferable to use a copper slab further comprising at least one selected from %.

If C exceeds 0.15%, problems will occur in toughness and weldability, so O3i is added as a deoxidizing agent to keep it below 0.15%, but if it exceeds 0.50%, it will cause brittleness. To increase the amount, it is set to 0.05 to 0.50%.

If Mn is less than 0.50%, strength cannot be obtained, but if it exceeds 1.50%, HIG resistance deteriorates, so it is set to 0.50 to 1.50%.

Nb is required in an amount of 0.005% or more to improve toughness by refining maestenite grains and refining ferrite grains after transformation, and to improve strength by strengthening carbide precipitation, but 0.100% Nb is necessary. If it exceeds 0.0, the toughness will deteriorate.
OaU, which is set at 0.005 to 0.100%, is an effective element for HIO and 0.10- or more is necessary, but OaU is set at 0.005 to 0.100%.
If it exceeds 50%, weldability deteriorates, so 0.10 to 0.
．． ONi (50%) is added in the same amount as O (to prevent embrittlement due to U).

AI is added as a deoxidizing agent like Si, but at 0.07
If it exceeds 0.5%, the quality of the steel changes, so OP is set to 0.50% or less. It is an element that is a major factor in II (3), and although it is better to keep it as low as possible, it should be 0.025% or less for industrial manufacturing purposes.

S is an element that is the main factor of RIG, and the lower the value, the higher the HIO
Therefore, it is essential to keep it below o, ooa%.

qa controls the form of sulfides and is effective for RIG, and it is necessary to add it so that Ca/S≧2.0, but if it exceeds 0.0060%, the cleanliness will deteriorate. It is assumed to be 0.0010-0.0060chi.

Each of the above components is necessary for applying the method of the present invention, and
High toughness steel with IO properties can be produced.

Furthermore, when the requirements for toughness are severe, the toughness can be improved by selectively adding the following components.

Mo has the effect of improving toughness, and if it is less than 0.01%, it has no effect on improving toughness, and if it exceeds O, aO%, hot workability deteriorates, so Mo is 0.01 to 0.30%. %. Note that the addition of MO can also improve the strength. O (2) can improve the toughness by making the austenite grains and ferrite grains finer after transformation, but if it exceeds 0.100%, the toughness will deteriorate. , 0.005 to 0.100 chi.

Ti increases the free %i in steel by adding 0.005% or more.
It is effective in fixing IN and improving toughness, but 0.100
If it exceeds 0.005 to 0.100, it becomes brittle.
% O B is effective in improving the toughness of welds when added in an amount of 0.0005% or more, but if it exceeds 0.0050%, the toughness deteriorates, so it is added in an amount of 0.0005 to 0.0050%. Each component of MO1V1Ti and B above exhibits similar effects.

(Function) In this invention, the control temperature at the intermediate stage of rough rolling is set to 95%.
The reason for setting the temperature below 0°C is that 950°C is the upper temperature of the non-recrystallized region, and rolling down below this temperature is necessary to improve toughness. In order to stabilize the I\i transition temperature (vTrs), it is necessary to maintain the rolling reduction at 75 inches or more, as shown in Figure 2, and the thickness at the intermediate stage of rough rolling is If it is made four times the coil thickness, the rolling reduction at this time corresponds to 75%, and the subsequent rough rolling and finishing rolling can be performed at a rolling reduction of 75 inches or more. Note that FIG. 2 shows the influence of the rolling reduction rate on vTrs at temperatures below 950°C.

Next, as shown in Fig. 3, it is possible to improve the stiffness by lowering the FDT, but as shown in Fig. 4,
If the DT falls below 720°C, the H2C crack area ratio increases, so in order to maintain HIG resistance, it is desirable that the FDT be 720°C or higher. The HIO crack area ratio here refers to the NAGE test solution (5% Napl + 0.5
% 0H8COOI (+ H,S saturated aqueous solution, pH 3,
After immersing the test piece cut from the coil in 0 to 4.0) for 96 hours in a stress-free state, it was rolled by scanning the entire surface parallel to the rolling surface using a continuous scanning water immersion ultrasonic flaw detector. This is the percentage of the crack area relative to the scanned area, which is the percentage of the crack area relative to the scanned area, where the cracks projected onto a plane parallel to the plane are automatically plotted.Next, from Figure 5, it can be seen that the toughness improves if the CT is lowered, but at 500°C Below, the effect of improving toughness is not seen, and when CT falls below 450℃, the HIC ratio n area increases (see Figure 6). HT by the same method as in Figure 4.
, which is the result of measuring the C crack area ratio.

(Example) Examples of the present invention will be shown below.

Using a steel slab melted to the chemical composition shown in Table 1 below, and under the rolling conditions shown in Table 2 below, grade API5 was obtained.
Table 2 shows the results of investigating the toughness, strength, and H-work resistance C properties of each coil rolled to LIXX65 and plate thickness 16.0.

As is clear from Table 2, the fill processed by the rolling method of the present invention has a significantly improved vTrs of -180 to -140°C than the comparative material (conventional rolling method).
There was no HICi generation in the AOE test solution.0 In the comparison material, the slab heating temperature (SR'I') was set to 11.
Coil A6, whose temperature was lowered to 50°C, improved its vTrs to 1115°C, but it lacked strength and did not meet the standards for AP 5LXX65.Furthermore, coil B5, whose FDT was lowered to 710°C, had a vTrs of 1110°C. Naru, HIO
will occur.

(Effects) As described above, according to the present invention, it is possible to manufacture a high-strength, extra-thick film with excellent HIO resistance, and to provide coils that are optimal for oil, natural gas, etc. line pipes.
It is also possible to further improve the toughness if necessary.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the rolling method of the present invention, Fig. 2 is a graph showing the influence of rolling reduction and rate on the fracture surface transition temperature (vTrs), and Fig. 8 is a graph showing the influence of FD'l' on vTrs. Figure 4 is a graph showing the effect of FD'I' on HIC, Figure 5 is a graph showing the effect of CjT on v'Frs, and Figure 6 is a graph showing the effect of CT on HIC. be.

Claims (1)

[Claims] 1. When heating a steel slab and rough rolling the slab, the temperature is increased to 950°C or less at an intermediate stage where the rough rolling reduces the thickness to 4 times or more the thickness of the product coil. Then, the remaining rough rolling and finishing rolling are continued one after another in which the finishing mill exit temperature in the finishing mill following the rough rolling is 720°C or higher, and then the coiling temperature is 450°C or higher. A method for manufacturing a high-strength, extra-thick coil with excellent HIO resistance and toughness, characterized by: winding.