KR100723203B1 - Linepipe steel sheet with superior low temperature toughness and high hic resistance and method for manufacturing the steel sheet - Google Patents

Abstract

The present invention relates to a line pipe steel sheet and a method for manufacturing the same, which are used for transporting gas or crude oil containing wet hydrogen sulfide, and to provide a line pipe steel sheet having excellent low temperature toughness and high hydrogen organic crack resistance, and a method of manufacturing the same. Its purpose is.

In the present invention, the weight%, C: 0.03-0.05%, Mn: 1.0-1.5%, Si: 0.1-0.3%, Al: 0.02-0.50%, Ti: 0.01-0.02%, Nb: 0.03-0.05%, Mo : 0.01-0.10%, S: 0.002% or less, P: 0.01% or less, N: 20-60ppm, Ca: 20-50ppm, Cu: 0.1 ~ 0.2%, Ni: 0.05 ~ 0.3%, Cr: 0.1 ~ 0.2% And a line pipe steel plate having excellent low temperature toughness and hydrogen organic crack resistance, consisting of remaining Fe and other unavoidable impurities, and composed of 20-40% granular bainite, 3% or less martensite, and remaining ferrite. The manufacturing method is taken as the summary.

According to the present invention, it is possible to provide a steel sheet that satisfies API X65 grade and is excellent in low temperature toughness and hydrogen organic crack resistance.

Low temperature toughness, hydrogen organic crack, line pipe, granulator

Description

Linepipe Steel Sheet with Superior Low Temperature Toughness and High HIC Resistance and Method for Manufacturing the Steel Sheet}

1 is a graph showing the change in yield strength according to granular bainite fraction

2 is a graph showing the change in the crack length ratio (%) and the Charpy impact energy value according to the martensite fraction;

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing line pipe steel used for transporting wet hydrogen sulfide gas or crude oil. The present invention relates to a line pipe steel sheet having excellent low temperature toughness and high hydrogen organic crack resistance, and a method of manufacturing the same.

Recently, with the development of a poor oil field or gas field, it is required to increase the resistance to hydrogen induced crack (HIC) of gas or crude oil transport steel containing wet hydrogen sulfide.

In addition, thickening is increasingly required to increase transportation economics.

The mechanism of generating hydrogen organic cracks is that hydrogen generated by the corrosion reaction between steel and wet hydrogen sulphide atmosphere enters and diffuses into the river in an atomic state, and molecular hydrogen is generated in the inclusions in the river. Gas pressure is known to generate hydrogen organic cracks.

Conventional techniques for improving the hydrogen cracking resistance generated as described above are due to the addition of Cu, reducing the inclusions and shape control, or to inhibit the intrusion or diffusion of hydrogen by fine dispersion of carbonitride, or segregation of steel Techniques for improving the hydrogen-organic crack resistance have been proposed by controlling the hardness of the hardened portion, controlling the hardness of the hardened portion due to segregation, and controlling the length of inclusions.

In addition, another method for increasing the hydrogen-organic crack resistance is a method of inhibiting and finely dispersing the object of pearlite tissue through controlled rolling.

The prior art can provide a steel material having relatively excellent hydrogen organic crack resistance, but there is a problem such that the fine pearlite is the starting point of hydrogen organic crack generation depending on the strength.

The present invention satisfies the API X65 grade by forming a composite structure of ferrite and granular bainite by reducing the content of C, which is a formation element of pearlite, and controlling the controlled rolling and accelerated cooling process. An object of the present invention is to provide a line pipe steel sheet having improved toughness and a method of manufacturing the same.

In the present invention, the weight%, C: 0.03-0.05%, Mn: 1.0-1.5%, Si: 0.1-0.3%, Al: 0.02-0.06%, Ti: 0.01-0.02%, Nb: 0.03-0.05%, Mo : 0.01-0.10%, S: 0.002% or less, P: 0.01% or less, N: 20-60ppm, Ca: 20-50ppm, Cu: 0.1 ~ 0.2%, Ni: 0.05 ~ 0.3%, Cr: 0.1 ~ 0.2% Low temperature toughness, consisting of the remaining Fe and other unavoidable impurities, and if necessary, up to 0.05% of V, and the texture of 20-40% granular bainite, up to 3% martensite and remaining ferrite And a line pipe steel sheet excellent in hydrogen organic cracking resistance.

In addition, the present invention is in weight%, C: 0.03-0.05%, Mn: 1.0-1.5%, Si: 0.1-0.3%, Al: 0.02-0.06%, Ti: 0.01-0.02%, Nb: 0.03-0.05% , Mo: 0.01-0.10%, S: 0.002% or less, P: 0.01% or less, N: 20-60 ppm, Ca: 20-50 ppm, Cu: 0.1-0.2%, Ni: 0.05-0.3%, Cr: 0.1- Steel, consisting of 0.2%, remaining Fe and other unavoidable impurities, and containing 0.05% or less of V, if necessary, was manufactured through slab under reduced pressure, and heated in a temperature range of 1100 to 1200 ° C. Hot rolling at 50% or more in the unrecrystallized region and 70% or more in the unrecrystallized region, and then started cooling at Ar ₃ or more to obtain a temperature of 400 to 500 ° C. at a cooling rate of 10 to 15 ° C./sec. The present invention relates to a method for producing a line pipe steel sheet having excellent low temperature toughness and hydrogen organic crack resistance by accelerated cooling to temperature and air cooling to room temperature.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, the carbon content is less than 0.05% and controlled rolling and accelerated cooling processes, in particular, accelerated cooling rate and cooling end temperature to control the matrix structure throughout the thickness of the ferrite and granular bainite ) Linen pipe with a thickness of 25mm or more with excellent hydrogen organic cracking resistance and low temperature toughness by maintaining the strength with the granular bainite fraction of 20-40% and the martensite fraction of 3% or less of the low temperature transformation tissue. It is to provide a steel sheet and a method of manufacturing the same.

It is preferable to limit the C to 0.03 to 0.05%, because if the content is less than 0.03%, the strength of the matrix on the matrix is lowered, and if it exceeds 0.05%, segregation occurs and the hydrogen organic cracking resistance is lowered. .

Si is added as a deoxidizer and a solid solution strengthening element and a component added to increase the impact transition temperature. To achieve this effect, Si is added by 0.1% or more, but when added in excess of 0.3%, the weldability decreases and an oxide film is formed on the surface of the steel sheet. It is severely formed, so its content is limited to 0.1-0.3%.

Since Mn forms MnS, which is a non-metallic inclusion drawn together with S, to lower room temperature elongation and low temperature toughness, it is preferable to manage the Mn less than 1.5%. However, Mn is less than 1.0% due to the low carbon content of the present invention. Since hardenability cannot be secured and it is difficult to form bainite, it is difficult to secure strength, so the amount of manganese added is limited to 1.0 to 1.5%.

The Al is added at least 0.02% as a deoxidizer during steelmaking, but when it is added in excess of 0.06%, Al forms a non-metal oxide Al ₂ O ₃ to lower impact toughness, so the amount of Al is limited to 0.02 to 0.06%.

The Ti is a major element that forms a TiN precipitate during the solidification process of steel to suppress grain growth during heating of the ingot, and inhibits the growth of recrystallized grains during the hot rolling process, thereby playing a major role in grain refinement of steel.

The appropriate amount of Ti is changed according to the content of N. When the amount of Ti is relatively small compared to the amount of nitrogen, the amount of TiN formed is small, which is disadvantageous to refine the grains, whereas when excessively added, TiN is coarse during heating. In addition, the effect of inhibiting grain growth is reduced.

Therefore, the amount of Ti added is usually limited to 0.01-0.02% in consideration of the content of N contained (20-60 ppm).

The Nb is an important element that increases the strength of the steel by solidifying the austenite to increase the hardenability of the austenite and to precipitate as carbonitrides (Nb (C, N)) matching with the matrix (Matrix).

However, when it is contained in a large amount, it is present as a coarse precipitate in the process of playing, and thus becomes a site of hydrogen organic cracking, so its content is limited to 0.05% or less.

 The Mo is an element essential to make the base structure into bainite structure by improving the hardenability and suppressing ferrite transformation, but it has a disadvantage of being expensive, so it is kept below 0.1% and Cr is added to 0.1 to 0.2% to maintain impact toughness. .

The P is an element particularly bad for impact toughness, the lower the content is better, but is inevitable in the steelmaking process, so the content is limited to 0.01% or less so as not to adversely affect the physical properties.

S is present as a non-metallic inclusion of MnS and is elongated by hot rolling to promote anisotropy of steel sheet properties and lower impact toughness, so the content thereof is limited to 0.002% or less.

The N serves to refine the grains of the steel by forming a TiN precipitate with Ti, the addition amount of N to 20-60ppm considering the content of Ti.

Ca is produced by CaS is added to suppress the non-metallic inclusions of MnS, for this purpose is added 20ppm or more.

If the amount is large, the upper limit thereof is 50 ppm because it reacts with O contained in the steel to form CaO, which is a nonmetallic inclusion.

Copper (Cu) is an oxide film forming element of steel, and if its content is less than 0.10%, it has little effect of inhibiting hydrogen penetration under corrosive environment such as hydrogen sulfide, and improves strength when added in excess of 0.20%. Impairs impact toughness.

Nickel (Ni) is added to form an oxide film of steel like copper, but when the content is less than 0.05%, the above effect decreases, and when the content exceeds 0.3%, the strength improvement effect is increased, but during the impact test after welding with pipe It is not preferable because brittleness between the base material and the weld is generated.

Vanadium (V) is a V (C, N) precipitation element and contributes to the increase in tensile strength rather than the increase in yield strength as the dilution increases. Therefore, in the present invention, it is preferable to limit the content to 0.05% in consideration of the balance of toughness in addition to the strength aspect, the impact strength toughness of the base material and the welded part with an increase in the carbon equivalent (Ceq) when containing a large amount exceeding 0.05% Significantly inhibits.

In addition to the alloying component O may be contained as in the case of a general structural steel, in which case the content is preferably limited to O: 0.01-0.05%.

Hereinafter, the manufacturing method of this invention is demonstrated.

First, heating before hot rolling is performed in the temperature range of 1100-1200 degreeC, for the following reason.

In the present invention, Nb is present in a dissolved state in austenite, thereby securing strength and fine recrystallization by fine Nb precipitation.

For this purpose, the slab should be heated to more than 1100 ℃ to allow NbC to dissolve and exist in the atomic state.However, if the heating temperature is more than 1200 ℃, the slab heating temperature ranges from 1100 to 1200 ℃ because the austenite particles are too coarse. It is to be.

After heating as above, hot rolling, where hot rolling is 50% or more in the recrystallization zone and 70% or more in the non-recrystallization zone.

The rolling reduction in the unrecrystallized zone is closely related to low temperature toughness as well as strength, and it is an indicator that should be managed especially under strictness. If the rolling reduction is less than 70%, there is a risk of coarse grain size after transformation due to lack of nucleation sites. .

After rolling and accelerated cooling as described above, and then air-cooled to room temperature, accelerated cooling starts cooling before the Ar ₃ temperature and cooled to a temperature of 400 ~ 500 ℃ at a cooling rate of 10 ~ 15 ℃ / sec.

Here, the cooling start temperature is limited to the Ar ₃ temperature or more because it is necessary to start cooling before the transformation starts to prevent the ferrite fraction from increasing and to obtain granular bainite structure.

In addition, the cooling rate is limited to 10 to 15 ° C / sec. If the cooling rate is too slow, a pearlite band may be formed, resulting in poor low temperature toughness, easy propagation of hydrogen organic cracks, and a cooling rate exceeding 15 ° C / sec. The lower bainite or martensite is formed to have a higher hardness, so that the occurrence of hydrogen organic cracking is high. In the case of thick plates, it is difficult to uniformly cool the entire thickness.

In addition, when the cooling end temperature is lower than 400 ° C, when granular bainite is formed, the MA fraction is increased due to insufficient carbon diffusion, resulting in poor low temperature toughness and hydrogen organic crack resistance.

If the cooling end temperature is higher than 500 ° C., Mn and C are concentrated, and the part not converted to bainite becomes pearlite.

The steel sheet for line pipe of the present invention is composed of 20 to 40% granular bainite, 3% or less martensite, and the remaining ferrite.

If the granular bainite ratio is too low, sufficient strength cannot be secured, and if it is too high, the toughness falls.

It is preferable to limit the fraction of martensite to 3% or less, because when it exceeds 3%, hydrogen organic crack resistance is inferior.

The present invention is preferably applied to a line pipe steel sheet having a thickness of 25 mm or more.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

The steel slab formed as shown in Table 1 was heated at 1150 ℃, hot-rolled 55% in the recrystallization zone, 75% in the non-recrystallization zone, and then cooled to the cooling conditions as shown in Table 2, the microstructure, yield The strength, low temperature toughness and crack length ratio (%) were investigated and the results are shown in Table 2 below.

In Table 1, the comparative material represents a yield strength of 45kg / mm ² for the general low temperature toughness.

In Table 2, F represents ferrite, G.B represents granular bainite, and P represents perlite.

Low temperature toughness is evaluated by the Charpy impact energy value obtained by Charpy impact test on a specimen having a V notch at -40 ° C.

In Table 2, the crack length ratio (%) is measured according to the NACE TM0277 standard.

Composition (% by weight) C Mn Si Al Cu Ni Cr Mo Ti V Ca N (ppm) Nb Invention 0.03 1.3 0.25 0.03 0.2 0.3 0.15 0.1 0.01 0.05 20 ppm 40 0.047 Bar textbook 0.07 1.4 0.2 0.04 - - - - 0.015 0.025 - 50 0.04

Cooling start temperature (℃) Cooling end temperature (℃) Cooling rate (℃ / sec) Microstructure and Fraction (%) Yield strength (Mpa) Low temperature toughness (J) CLR (%) Invention 800 450 12 F (65) + G.B (35) 467 443 0.1 Comparative material 750 540 8 F (74) + P (26) 497 370 25

As shown in Table 2, the yield strength is high, but the CLR (Crack length ratio,%), which exhibits low temperature toughness and H ₂ S (sour) gas resistance, can be seen that the invention is superior.

As described above, the reason why the invention material is superior to the CLR is that the HIC easily propagates along the perlite band in the existing ferrite matrix, whereas in the case of the present invention, the matrix is less granular than the ferrite and the pearlite. Because it is formed as bainite.

(Example 2)

The composition of the steel sheet is the same as the invention material of Table 1 above, Yield strength was measured while changing the granular bainite ratio, and the results were investigated in FIG. 1 and the CLR and Charpy impact energy while changing the fraction of martensite, and the results are shown in FIG. 2.

As shown in FIG. 1, when the ratio of granular bainite is 30% or more, it can be seen that the yield strength satisfies API X65 grade (above 450Mpa), and as shown in FIG. 2, the martensite fraction is 3%. If it is abnormal, the CLR value increases and the Charpy impact energy value decreases.

As described above, according to the present invention, a line pipe steel sheet which satisfies the PI X65 grade and has improved hydrogen organic crack resistance and low temperature toughness can be provided.

Claims (4)

By weight%, C: 0.03-0.05%, Mn: 1.0-1.5%, Si: 0.1-0.3%, Al: 0.02-0.06%, Ti: 0.01-0.02%, Nb: 0.03-0.05%, Mo: 0.01- 0.10%, S: 0.002% or less, P: 0.01% or less, N: 20-60 ppm, Ca: 20-50 ppm, Cu: 0.1-0.2%, Ni: 0.05-0.3%, Cr: 0.1-0.2%, remaining Fe And a line pipe steel sheet having excellent low temperature toughness and hydrogen organic crack resistance, which are made of other unavoidable impurities, and its structure is made of 20-40% granular bainite, 3% or less martensite, and remaining ferrite. The line pipe steel sheet having excellent low temperature toughness and hydrogen organic crack resistance according to claim 1, wherein V is added in an amount of 0.05% or less. By weight%, C: 0.03-0.05%, Mn: 1.0-1.5%, Si: 0.1-0.3%, Al: 0.02-0.06%, Ti: 0.01-0.02%, Nb: 0.03-0.05%, Mo: 0.01- 0.10%, V: 0.05% or less, S: 0.002 or less, P: 0.01% or less, N: 20-60 ppm, Ca: 20-50 ppm, Cu: 0.1-0.2%, Ni: 0.05-0.3%, Cr: 0.1- The slab is made of 0.2%, the remaining Fe and other unavoidable impurity elements under reduced pressure to produce slabs, heated at a temperature in the range of 1100 to 1200 ° C, and at least 50% of the reduction in the recrystallization zone. After hot rolling under the condition that the amount is 70% or more, the cooling is started at Ar ₃ or more, accelerated cooling to a temperature of 400 to 500 ° C at a cooling rate of 10 to 15 ° C / sec, and air cooled to room temperature. Method for manufacturing line pipe steel sheet having excellent organic crack resistance The method of manufacturing a line pipe steel sheet having excellent low temperature toughness and hydrogen organic crack resistance according to claim 3, wherein V is added in an amount of 0.05% or less.

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