KR20080042296A - Hot-rolled steel having excellent hydrogen induced crack resistance and low temperature toughness and the method for manufacturing the same - Google Patents

Abstract

Hot-rolled steel having excellent hydrogen induced crack resistance and low temperature toughness and a method for manufacturing the same are provided to be employed for a line pipe for crude oil or natural gas. A hot-rolled steel includes 0.02% to 0.05%C, 0.05% to 0.5% Si, 0.5 % to 1.5% Mn, P of less than 0.01%, S of less than 0.01%, 0.02% to 0.05% Al, 0.04% to 0.6% V, 0.005% to 0.2% Ti, 0.1% to 0.3% Cr, 0.0015% to 0.003% Ca, and Fe. The Ca and S are greater than or equal to 1.5 and less than or equal to 4. The occupation area of non-metal is 1000mum^2 in the area of 10mm x 20mm. A steel slab is reheated under the temperature in the range of 1150°C to 1250°C. The steel slab is subject to hot rolling. The steel slab is cooled to the temperature of 450°C to 600°C at the speed of 10°C/sec to 30°C/sec.

Description

Hot-rolled steel having excellent hydrogen induced crack resistance and low temperature toughness and the method for manufacturing the same}

Japanese Laid-Open Patent Publication No. 2003-226915

Japanese Patent Laid-Open No. 2-290947

Japanese Laid-Open Patent Publication No. 58-138724

The present invention relates to hot rolled steels mainly used in line pipes for transporting crude oil or natural gas. More specifically, the present invention relates to a hot rolled steel having excellent hydrogen-organic crack resistance and low temperature toughness by controlling the area and rolling control of nonmetallic inclusions, and a method of manufacturing the same.

In recent years, as the demand for energy increases, oilfields or gas fields in harsh environments are being developed. Especially, as crude oil or natural gas with high H ₂ S gas content in the polar regions is developed, the toughness at low temperatures and the H ₂ S gas are reduced. There is a high demand for development of steel materials with little damage.

Particularly, in the gas containing H ₂ S (hydrogen sulfide) or steel for crude oil transportation, hydrogen induced crack (HIC) caused by H ₂ S gas is problematic, and the hydrogen organic crack is elongated by rolling like MnS. Hydrogen molecules gathered in steel inclusions as hydrogen is generated on the surface of steel materials by the corrosion reaction between steel materials and H ₂ S (hydrogen sulfide) atmosphere as it invades and diffuses into the steel in an atomic state. Cracks are known to occur due to the pressure of.

In the prior art for improving the hydrogen organic cracks, a method of suppressing hydrogen intrusion or diffusion due to Cu addition, MnS reduction and shape control, fine dispersion of carbonitrides, and the like, and a means for reducing central segregation during continuous casting are proposed. However, the prior arts provide a means for securing steels having relatively excellent hydrogen organic crack resistance, but the hydrogen organic cracks are not completely suppressed in a high acidic wet hydrogen sulfide atmosphere.

Accordingly, Japanese Laid-Open Patent Publication No. 2003-226915 discloses a method for effectively controlling hydrogen organic crack, and means for controlling the length of the nonmetallic inclusions known as the hydrogen crack generation point and the hardness of the segregation portion through which the hydrogen organic crack is propagated. Presented.

In addition, Japanese Patent Application Laid-Open No. 2-290947 discloses a method of improving hydrogen-organic crack resistance and low temperature toughness by controlling the composition of nonmetallic inclusions, particularly Al-Ca-O based nonmetallic inclusions. 58-138724 proposes a method for producing steels having excellent hydrogen organic crack resistance and low temperature toughness through inclusion control and rolling control through Ca treatment.

However, the prior art one would like to prevent hydrogen induced crack by controlling non-metallic inclusions, the steel material during the _{Al 2 O 3, Al 2 O} 3 -CaO -based, Al ₂ O ₃ -MgO-CaO-based, Al ₂ O ₃ -CaO Non-metallic inclusions such as -CaS and Nb-Ti-based materials are inevitably present everywhere, and the problems of generation of hydrogen organic cracks and lowering of low temperature toughness due to such nonmetallic inclusions are not completely solved.

The present invention is to improve the above-mentioned conventional problems, and to provide a hot-rolled steel material and a method of manufacturing the same excellent in hydrogen cracking resistance and low temperature toughness through the area and rolling control of non-metallic inclusions.

The present invention for achieving the above object, in weight%, C: 0.02-0.05%, Si: 0.05-0.5%, Mn: 0.5-1.5%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 ~ 0.05%, Nb: 0.04-0.06%, V: 0.04-0.06%, Ti: 0.005-0.02%, Cr: 0.1-0.3%, Ca: 0.0015-0.003%, remaining Fe and other unavoidable impurities, and The present invention relates to a hot-rolled steel material having excellent hydrogen-organic crack resistance and low temperature toughness in which Ca and S satisfy 1.5≤Ca / S≤4 and the area of nonmetallic inclusions is 1000 μm ² or less within an area of 10 mm × 20 mm around the t / 4 position. .

In addition, the present invention is in weight%, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 to 0.05%, Nb : 0.04 to 0.06%, V: 0.04 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.3%, Ca: 0.0015 to 0.003%, remaining Fe and other inevitable impurities, and Ca and S are 1.5 ≤Ca / satisfies S≤4 and, in the central area of 10mm × 20mm as the t / 4 position of the non-metallic inclusions and the area re-heating a steel slab 1000㎛ ² or less at 1150 ℃ ~ 1250 ℃, 70% or less from the non-recrystallized temperature After rolling hot with the above reduction amount, finish hot rolling at the Ar ₃ transformation point or more, and after completion of hot rolling, cooling to 450 ~ 600 ℃ at the rate of 10 ~ 30 ℃ / sec, and winding up the hydrogen-organic crack and low temperature toughness It relates to a method for producing an excellent hot rolled steel.

Where the recrystallization temperature = 887 + 464 x [C] + (6445 x [Nb]-644 x √ [Nb]) + (732 x [V]-230 x √ [V]) + 890 x [Ti ] + 363 × [Al]-357 × [Si]

Ar ₃ transformation point = 910-310 x [C]-80 x [Mn]-20 x [Cu]-15 x [Cr]-80 x [Mo]-55 x [Ni]-0.35 x (t-8) , Wherein the content is weight percent.)

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

As a result of examining the crack origin and structure of the steel material in which the hydrogen-organic crack has occurred, the present inventors have different areas of the nonmetallic inclusions that are the crack origin, depending on the structure and hardness around the crack occurrence, and the area is t / 4 position of the steel. It was confirmed that the cause of generation of hydrogen organic cracks can be eliminated if the thickness is 1000 μm ² or less within an area of 10 mm × 20 mm. In addition, by securing a uniform microstructure in the hot rolled steel by rolling control together with the area control of the non-metallic inclusions, it is possible to secure a hot rolled steel having excellent hydrogen organic crack resistance and low temperature toughness.

First, the composition range of the steel component of the present invention will be described.

C: 0.02 to 0.05% is preferable.

The C is the most economical and effective alloying component to strengthen the steel, when the content is less than 0.02%, the effect of strengthening the steel by combining with Nb, V or Ti is very small. On the other hand, if the content exceeds 0.05%, the central segregation that decreases the HIC resistance is increased, so the content of C is preferably limited to 0.02 to 0.05%.

Si: 0.05-0.5% is preferable.

The Si is an effective component for deoxidation and solid solution strengthening, and when the content is less than 0.05%, it is difficult to obtain a deoxidation effect, and when it exceeds 0.5%, the possibility of deterioration of weldability and brittleness increases, so the content is 0.05 to 0.5%. It is desirable to limit.

Mn: 0.5 to 1.5% is preferable.

The Mn is an essential component to secure strength and toughness. If the content is less than 0.5%, it is difficult to secure strength and toughness. If the content exceeds 1.5%, Mn promotes central segregation during performance, thereby lowering impact toughness and HIC resistance. It is preferable to limit the content to 0.5 to 1.5% since the likelihood is increased.

P: 0.01% or less is preferable.

When the content of P is added in excess of 0.01%, since it increases the possibility of lowering the impact toughness and emulsion stress cracking resistance by promoting the central segregation with Mn when playing, it is preferable to limit the content to 0.01% or less.

S: 0.001% or less is preferable.

S is a component that greatly reduces brittleness by forming MnS together with Mn in steel, and when it exceeds 0.001%, it is highly likely to significantly reduce HIC resistance, and therefore, the content is preferably limited to 0.001% or less. .

Al: 0.02-0.05% is preferable.

Al is a component which deoxidizes with Si, and when it is less than 0.02%, it is difficult to secure a deoxidation effect. When Al is exceeded 0.05%, the alumina aggregate is increased to increase the possibility of lowering the HIC resistance. It is desirable to limit it to -0.05%.

Nb and V: 0.02-0.06% are preferable.

Nb and V are the components exhibiting the precipitation strengthening effect by the addition of a small amount, in the carbon range of the present invention, since the strength increase due to precipitation strengthening is more than 0.06%, respectively, the content is limited to 0.06% or less, respectively, 0.02% If less, it is difficult to secure the above effects. Therefore, the content of Nb and V is preferably limited to 0.02 ~ 0.06%.

Ti: 0.005 to 0.02% is preferable.

The Ti precipitates as TiN in the steel, thereby inhibiting grain growth of austenite when reheating, thereby securing high strength and excellent impact toughness, and precipitated with TiC to strengthen the steel. However, when the content of Ti in the carbon range of the present invention is less than 0.005%, it is difficult to secure the above effects, and when the content of Ti exceeds 0.02%, the effect is not large. Therefore, the content is preferably limited to 0.005 to 0.02%. Do.

Cr: 0.1 to 0.5% is preferable.

The Cr is added to increase strength and to secure corrosion resistance, and the addition of Cr is a component that easily induces transformation into low-temperature metamorphic tissues, when less than 0.1%, it is difficult to secure the effect, and when it exceeds 0.5%, local corrosion occurs. As the risk is increased, it is desirable to limit the content to 0.1 to 0.5%.

Ca: 0.0015 to 0.003% is preferable.

The Ca is a component that serves to suppress the hydrogen organic cracking origin by spheroidizing the shape of the emulsion inclusions, when the content is less than 0.0015% it is difficult to secure the effect. On the other hand, if the content exceeds 0.003%, the amount of inclusions is rather increased, which lowers the HIC resistance. Therefore, it is preferable to limit the content to 0.0015 to 0.003%.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

The present invention is characterized in controlling the component ratios of Ca and S in terms of controlling MnS and Ca-based nonmetallic inclusions in the hot rolled steel.

1.5 ≦ Ca / S ≦ 4 is preferred.

The relationship is an empirical equation obtained through a number of experiments, when the relationship value is less than 1.5, MnS is easily formed, so that the hydrogen cracking resistance may be lowered, while the amount of Ca-based nonmetallic inclusions is increased when it exceeds 4 This can lead to a decrease in hydrogen organic crack resistance and toughness. Therefore, it is preferable to limit the relationship between Ca and S to 1.5-4.

In the present invention, the nonmetallic inclusions are appropriately controlled based on the fact that the size of the nonmetallic inclusions, which is the starting point of cracking, varies depending on the structure and hardness around the cracking occurrence. In other words, the area of nonmetallic inclusions within an area of 10 mm x 20 mm centered on the t / 4 position of the steel to have a thickness of 1000 μm ² or less, thereby improving hydrogen organic cracking resistance and low temperature toughness. In the present invention, the area of the nonmetallic inclusion means an average value through measurement of 10 positions. When the area of the nonmetallic inclusion exceeds 1000 μm ² , the nonmetallic inclusion serves as an initiation point of the hydrogen organic crack in the hot rolling step, which may cause a decrease in hydrogen organic cracking resistance. Therefore, it is preferable to limit the area of a nonmetallic inclusion to 1000 micrometer <^2> or less in area 10mm x 20mm centering on t / 4 position.

Hereinafter, the manufacturing method of the hot rolled steel material which has the steel comprised as mentioned above is demonstrated in detail.

Hot rolled steel is usually manufactured by primary refining in converters, followed by tapping the molten steel of converters by ladle, followed by secondary refining, followed by continuous casting. In secondary refining, inclusions are controlled by bubbling with an inert gas such as Ar.

In the present invention, the area of the non-metallic inclusions within 10mm x 20mm around the 1 / 4t position of the steel to have a thickness of less than 1000㎛ ² to improve the hydrogen-organic crack and low temperature toughness. The control of the non-metallic inclusions according to the present invention can be obtained through the control of the process conditions in the conventional secondary refining process, for example, the secondary refining process is carried out in the degassing process such as Ar bubbling in LF and VTD or RH. The inclusions are controlled by Ar bubbling. The present invention is not necessarily limited to these process conditions, and the inclusions may be controlled by any method.

In the present invention, the control of the non-metallic inclusions at the t / 4 position of the steel is, in the case of a curved continuous casting machine, the most non-metallic inclusions are accumulated at the t / 4 position. Therefore, when the inclusion is controlled at that position, the inclusion is controlled throughout the steel.

According to the present invention, a slab having an area of a nonmetallic inclusion of 1000 μm ² or less within an area of 10 mm × 20 mm around a t / 4 position of steel is reheated and hot rolled, and then wound up after cooling, which will be described in detail.

First, the steel slab formed as described above is reheated at 1150 ~ 1250 ℃. The reheating temperature is determined by the solid solution temperature of the Nb-based precipitate, in the component range of the present invention can be dissolved in more than 1150 ℃, and if the temperature exceeds 1250 ℃ the grain size of the steel is very large, the toughness may be lowered, so the reheating temperature 1150-1250 It is preferable to limit to ° C.

Hot rolling is performed after the reheating, but in the present invention, it is important to control the rolling conditions in terms of securing hydrogen organic crack resistance and low temperature toughness.

After the re-heating at a temperature of 1150 ~ 1250 ℃, hot-rolled at less than the non-recrystallized temperature of at least 70% rolling reduction is carried out, and then finish hot rolled at more than Ar ₃ transformation point.

Below the unrecrystallized temperature, the amount of reduction greatly affects the grain size and uniformity of the hot rolled steel microstructure, and the grain size and uniformity have a great influence on the hydrogen organic crack resistance and low temperature toughness. In addition, when the amount of the reduction is less than 70%, the homogeneity of the grain size may be lowered and thus the low-temperature toughness may be lowered.

In addition, since the ferrite transformation is initiated at a temperature below the Ar ₃ transformation point and the hydrogen-organic crack resistance is very low, the hot finishing rolling temperature is preferably limited to the Ar ₃ transformation point or more.

Here, the undetermined temperature and the Ar ₃ transformation point temperature by using a general relationship,

The recrystallization temperature = 887 + 464 x [C] + (6445 x [Nb]-644 x √ [Nb]) + (732 x [V]-230 x √ [V]) + 890 x [Ti] + 363 × [Al]-357 × [Si]

Ar ₃ transformation temperature = 910-310 × [C]-80 × [Mn]-20 × [Cu]-15 × [Cr]-80 × [Mo]-55 × [Ni]-0.35 × (t-8) Wherein the content is weight percent.

After the end of the hot rolling, after cooling to 450 ~ 600 ℃ at a rate of 10 ~ 30 ℃ / sec is wound up. The cooling should be initiated above the Ar ₃ transformation point temperature, and if initiated at a temperature below that, coarse ferrite may form prior to cooling, which may cause degradation of toughness. In addition, when the cooling rate is less than 10 ° C / sec it is easy to form a pearlite structure to lower the hydrogen organic crack resistance, and if it exceeds 30 ° C / sec bainite may be easily formed, the cooling rate is 110 It is preferable to limit to ˜30 ° C./sec.

In addition, when the coiling temperature is less than 450 ℃ the rigidity of the steel may be difficult to wind the winding, and if it exceeds 600 ℃ the transformation is unstable to facilitate the formation of pearlite structure, the coiling temperature is limited to 450 ~ 600 ℃ It is desirable to.

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

EXAMPLE

Inventive steel (1-2) and comparative steel (1-6) to be prepared as shown in Table 1, after reheating for 2 to 3 hours in the range of 1150 ~ 1250 ℃, followed by hot rolling in the rolling conditions as shown in Table 2 below 12mm Rolled and wound. Hydrogen organic crack resistance and low temperature toughness characteristics of the steels prepared as described above were examined, and the results are shown in Table 2 below.

The HIC resistance of the steel was carried out in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas in accordance with NACE TM 0284, and the degree of cracking was observed by ultrasonic inspection. Low temperature toughness was tested by Charpy impact test, and when the impact energy value at -80 ℃ was over 400J, it was set as acceptance criteria.

division C Si Mn P S Al Nb V Ti Cr Ca Ca / S Nonmetallic Inclusion Area (㎛ ² ) Inventive Steel 1 0.035 0.2 1.4 0.010 0.001 0.03 0.053 0.051 0.02 0.15 0.0025 2.5 985 Inventive Steel 2 0.045 0.2 1.3 0.0089 0.001 0.03 0.051 0.048 0.02 0.48 0.0020 2.0 852 Comparative Steel 1 0.038 0.2 1.3 0.0055 0.0015 0.03 0.05 0.050 0.02 0.23 0.0015 1.0 695 Comparative Steel 2 0.030 0.2 1.3 0.0075 0.007 0.03 0.042 0.032 0.010 0.12 0.0032 4.57 1523 Comparative Steel 3 0.030 0.2 1.3 0.0065 0.008 0.03 0.042 0.032 0.015 0.12 0.0020 2.5 1195 Comparative Steel 4 0.048 0.2 1.5 0.0045 0.001 0.03 0.036 0.042 0.02 0.31 0.0020 2 916 Comparative Steel 5 0.041 0.2 1.6 0.0085 0.001 0.03 0.051 0.041 0.012 0.42 0.0025 2.5 1602 Comparative Steel 6 0.032 0.2 1.6 0.0096 0.001 0.03 0.052 0.051 0.02 0.32 0.0030 3 503

division Steel grade Uncrystallized rolling reduction rate (%) Rolling Finish Temperature (℃) Cooling rate (℃ / sec) Winding temperature (℃) HIC (Car,%) Impact energy (J) Invention 1 Inventive Steel 1 72 850 18 540 0 486 Invention 2 Inventive Steel 2 80 845 22 560 0 469 Comparative Material 1 Inventive Steel 1 75 775 21 545 2.5 413 Comparative Material 2 Inventive Steel 1 72 845 3 520 4.5 386 Comparative Material 3 Inventive Steel 2 63 855 15 650 3.2 365 Comparative Material 4 Inventive Steel 2 64 850 17 520 0 372 Comparative Material 5 Comparative Steel 1 75 842 16 545 5.2 281 Comparative Material 6 Comparative Steel 2 75 850 18 500 6.2 204 Comparative Material7 Comparative Steel 3 83 855 19 520 2.1 402 Comparative Material 8 Comparative Steel 4 76 851 15 520 3.6 412 Comparative Material 9 Comparative Steel 5 76 855 16 550 3.2 214 Comparative Material 10 Comparative Steel 6 72 860 15 540 0 385

As shown in Table 2, in the case of the invention material (1,2) manufactured according to the production method of the present invention using the invention steel (1,2) satisfying the component range of the present invention, HIC 0, impact energy 469J and 486J were able to secure excellent hydrogen organic cracking resistance and low temperature toughness.

However, even when using the inventive steel (1, 2) that satisfies the component range of the present invention, in the case of the comparative material (1-4) not manufactured according to the production method of the present invention, hydrogen organic cracking or low temperature toughness Could not secure excellent steels.

In addition, in the case of the comparative materials (1 to 10) that do not satisfy the component range of the present invention, it was not possible to secure the hydrogen cracking resistance and low temperature toughness targeted by the present invention, in particular the non-metallic inclusions targeted by the present invention In the case of comparative materials 6,7 and 9 which do not satisfy the area of, it is found that the HIC characteristics and low-temperature toughness are inferior even though the rolling conditions are satisfied, and the comparative material 5 having a low Ca / S ratio and the comparative material 6 having a high Ca / S ratio Both HIC characteristics and low temperature toughness were inferior.

As described above, according to the present invention, it is possible to provide a hot rolled steel having excellent hydrogen-organic crack resistance and low temperature toughness by controlling the area and rolling control of the nonmetallic inclusion.

Claims (2)

By weight%, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 to 0.05%, Nb: 0.04 to 0.06% , V: 0.04% to 0.06%, Ti: 0.005% to 0.02%, Cr: 0.1% to 0.3%, Ca: 0.0015% to 0.003%, remaining Fe and other inevitable impurities, and Ca and S are 1.5≤Ca / S≤ The hot rolled steel material which satisfies 4 and is excellent in hydrogen-organic cracking resistance and low temperature toughness whose base metal inclusion area is 1000 micrometers ² or less in the area of 10 mm x 20 mm centering on t / 4 position. By weight%, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 to 0.05%, Nb: 0.04 to 0.06% , V: 0.04% to 0.06%, Ti: 0.005% to 0.02%, Cr: 0.1% to 0.3%, Ca: 0.0015% to 0.003%, remaining Fe and other inevitable impurities, and Ca and S are 1.5≤Ca / S≤ satisfies 4, and re-heating the steel slab 1000㎛ ² or less in the non-metallic inclusions in an area of t / 4 in the center area of 10mm × 20mm a location in 1150 ℃ ~ 1250 ℃ and hot below the non-recrystallized temperature of at least 70% rolling reduction After rolling, finish hot rolling at the Ar ₃ transformation point or more, and after the end of hot rolling, manufacture the hot rolled steel having excellent resistance to hydrogen organic cracking and low temperature toughness after cooling to 450-600 ° C at a rate of 10-30 ° C / sec. Way.