Steel material with excellent hydrogen-induced cracking resistance and low-temperature impact toughness, and method for manufacturing the same.

Abstract

Provided is a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, and a method for producing the same. [Solution] The steel material contains, by weight, C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 0.8-1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.005-0.025%, Ni: 0.01-0.5%, Mo: 0.01-0.12%, V: 0.005-0.03%, Ti: 0.003% or less (excluding 0), N: 0.002-0.01%, Ca: 0.0005-0.004%, and the remainder is Fe and unavoidable impurities. The steel material contains Al-O-based, Ca-O-based and Al-Ca-O-based oxidizing inclusions having a size of 10 μm or more, and the number of inclusions is 1 mm or more. 2 A steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, characterized in that the number of cracks per unit area is 50 or less and that satisfies [Relationship 1] and [Relationship 2].

Description

Technical Field

[0001] The present invention relates to a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, and a method for manufacturing the same. More specifically, the present invention relates to a steel material having excellent hydrogen-induced cracking (HIC) resistance and low-temperature impact toughness by controlling the relationship between the component composition and some components in alloy design and optimizing the manufacturing conditions, and a method for manufacturing the same.

Background Art

[0002] Recently, pressure vessels used in industries such as energy resource extraction, production, transportation, storage, refining, and power generation have an increasing demand for extremely thick steel materials due to the enlargement of equipment accompanying the increase in usage time. Such steel materials are required to have a low carbon equivalent (Ceq) in order to ensure the structural stability of the welded part. In addition, as the production of crude oil containing a large amount of H2S increases, steel materials for pressure vessels as described above are required to have hydrogen-induced cracking (HIC) resistance. The usage environment of such structures has expanded to extreme areas, and excellent low-temperature impact toughness is also required.

[0003] The cause of the above hydrogen-induced cracking (HIC) is that corrosion occurs when the steel material comes into contact with wet hydrogen sulfide contained in crude oil, and hydrogen atoms generated by the above corrosion penetrate and diffuse into the steel and exist in a molecular state in inclusions or the like inside the steel. When hydrogen atoms are molecularized inside the steel material in this way, hydrogen gas is formed and gas pressure is generated, and brittle cracks are generated and grow along the vulnerable tissues inside the steel due to the pressure, resulting in fracture.

[0004] Therefore, as a solution for improving the hydrogen-induced cracking resistance of steel materials used in a hydrogen sulfide atmosphere, there are methods such as adding elements such as copper (Cu), minimizing or controlling the shape of hardened tissues where cracks are likely to occur and propagate, and controlling internal defects such as inclusions and voids inside the steel material that can act as the accumulation point of hydrogen and the starting point of cracks.

[0005] Patent Document 1 presents a method for increasing hydrogen-induced cracking resistance by appropriately controlling the shape of voids inside steel. Specifically, it involves ensuring that the voids formed in the center of the steel are as spherical as possible, and controlling the ratio of the length of the long side to the short side of the void to 0.7 or more. However, the shape of the voids formed during continuous casting is not constant, and there are limitations to uniformly controlling these shapes throughout the rolling process. As a result, variations in the hydrogen-induced cracking resistance of steel may occur, and improvement measures are needed.

[0006] On the other hand, steel materials for pressure vessels exhibit reduced impact toughness and stability problems as the operating temperature decreases. In particular, steel materials of the same strength exhibit a more significant decrease in internal toughness as their thickness increases. Therefore, for steel materials used in pressure vessels in low-temperature environments, it is necessary to appropriately manage the composition and microstructure to prevent deterioration of impact toughness even at low temperatures.

[0007] The rolling process is one of the typical methods for grain refinement. When rolling is performed at a temperature at which recrystallization is possible, the internal stress generated by the reduction force acts as a driving force, creating new austenite fine grains.

[0008] However, as the thickness of the steel increases, the reduction force that can be applied by rolling becomes limited, making it more difficult to form fine grains through rolling, especially as one approaches the center of the steel. Austenite grains tend to grow at higher temperatures and longer heating times above Ae3, but some alloying elements have the effect of suppressing the growth of austenite grains. These alloying elements dissolve in the steel and act as obstacles to grain growth. Therefore, in the case of extremely thick steel materials where grain refinement by rolling is difficult, the addition of such alloying elements must also be considered in order to refine the grains.

[0009] Furthermore, in the case of quenched and tempered (QT) materials, it is common practice to reheat them to the austenite single-phase region after rolling and air cooling, followed by water cooling and tempering heat treatment. However, if the reheating temperature is too high or the furnace time is too long, the austenite grains grow significantly, reducing the low-temperature impact toughness. Therefore, there is a need for a technology that can solve this problem and ensure excellent low-temperature impact toughness. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Korean Registered Patent Publication No. 10-2164116 [Overview of the project] [Problems that the invention aims to solve]

[0011] This invention relates to steel materials used in a hydrogen sulfide atmosphere and aims to provide a steel material with excellent resistance to hydrogen-induced cracking and low-temperature impact toughness, as well as a method for producing the same.

[0012] The problems that the present invention will address are not limited to those described above. Further problems that the present invention will address are described in the overall content of the specification, and any person with ordinary skill in the art to which the present invention belongs will have no difficulty in understanding these further problems from the content of the specification. [Means for solving the problem]

[0013] The present invention contains, by weight percent, C: 0.12~0.18%, Si: 0.2~0.5%, Mn: 0.8~1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015~0.045%, Nb: 0.005~0.025%, Ni: 0.01~0.5%, Mo: 0.01~0.12%, V: 0.005~0.03%, Ti: 0.003% or less (excluding 0), N: 0.002~0.01%, Ca: 0.0005~0.004%, with the remainder being Fe and unavoidable impurities. Within the steel material, there are one or more inclusions of Al-O, Ca-O, and Al-Ca-O type oxidizing inclusions with a size of 10 μm or larger, and the number of inclusions is 1 mm. 2 The number of winning items is 50 or less. This relates to a steel material that exhibits excellent hydrogen-induced cracking resistance and low-temperature impact toughness, satisfying the following [Relationship 1] and [Relationship 2].

[0014] [Relationship 1] Ceq≦0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the weight percentages of each component.)

[0015] [Relationship 2] 1.2 ≤ Ca / S ≤ 4.0 (The above values ​​for Ca and S represent the content (by weight) of each component.)

[0016] The present invention relates to a steel slab containing, by weight %, C: 0.12~0.18%, Si: 0.2~0.5%, Mn: 0.8~1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015~0.045%, Nb: 0.005~0.025%, Ni: 0.01~0.5%, Mo: 0.01~0.12%, V: 0.005~0.03%, Ti: 0.003% or less (excluding 0), N: 0.002~0.01%, Ca: 0.0005~0.004%, with the remainder being Fe and unavoidable impurities, and satisfying the following [Relationship Formula 1] and [Relationship Formula 2], and is heated to a temperature range of 1100~1200°C. The above heated steel slab is roughly rolled at a temperature of 1050°C or higher, and then finish hot-rolled at a temperature of Ar3 or higher to produce a hot-rolled steel sheet. The above hot-rolled steel sheet is air-cooled, The above air-cooled hot-rolled steel sheet is reheated to a temperature of Ac3 or higher and held for (2.3t+30) minutes or more (where t represents the thickness of the steel (mm)), in a reheating step, The above reheated hot-rolled steel sheet is then quenched to room temperature at a cooling rate of 0.4°C / s or higher. The method for manufacturing a steel material excellent in hydrogen-induced cracking resistance and low-temperature impact toughness includes the step of performing tempering heat treatment on the above-mentioned cooled hot-rolled steel sheet at a temperature range of 600 to 700 °C for (3.4t + 30) minutes (where t represents the thickness (mm) of the steel).

[0017] [Relational Expression 1] Ceq ≦ 0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight%) values of each component)

[0018] [Relational Expression 2] 1.2 ≦ Ca / S ≦ 4.0 (where Ca and S are the content (weight%) values of each component) [Advantages of the Invention]

[0019] According to the present invention, it is possible to provide a steel material for pressure vessels that is excellent in hydrogen-induced cracking resistance after quenching & tempering (QT) and post-weld heat treatment (PWHT) and is excellent in low-temperature impact toughness.

[0020] The various and beneficial advantages and effects of the present invention are not limited to the above-described content and can be more easily understood in the process of explaining the specific embodiments of the present invention. [Brief Description of the Drawings]

[0021] [Figure 1] In the present invention, it is a diagram showing the ultrasonic flaw detection results of the hydrogen-induced cracking evaluation in Invention Example 1. [Figure 2] In an embodiment of the present invention, it is a diagram showing the ultrasonic flaw detection results of the hydrogen-induced cracking evaluation in Comparative Example 1. [Modes for Carrying Out the Invention]

[0022] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Furthermore, the singular form used herein also includes the plural form unless the relevant definition clearly indicates otherwise.

[0023] As used herein, "includes" refers to a specific configuration and does not exclude the existence or addition of other configurations.

[0024] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries shall be interpreted as having the meaning consistent with the relevant technical literature and the content currently disclosed.

[0025] The inventors of this invention recognized that as pressure vessels used in petrochemical industry facilities, storage tanks, and the like become larger, are used in hydrogen sulfide atmospheres, and their operating environments expand to extreme conditions, it is necessary to develop a method to ensure the required physical properties of the materials. In particular, they diligently researched a method to ensure both hydrogen-induced cracking resistance and low-temperature impact toughness in pressure vessel steel materials with a certain thickness or more. As a result, they confirmed that it is possible to provide pressure vessel steel materials with the target physical properties by controlling the relationship between the component composition and some components in alloy design, and by optimizing the manufacturing conditions, thus completing the present invention.

[0026] The steel material of the present invention will be described in detail. First, the alloy composition of the steel material of the present invention will be described in detail. The steel material of the present invention contains, by weight %, C: 0.12~0.18%, Si: 0.2~0.5%, Mn: 0.8~1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015~0.045%, Nb: 0.005~0.025%, Ni: 0.01~0.5%, Mo: 0.01~0.12%, V: 0.005~0.03%, Ti: 0.003% or less (excluding 0), N: 0.002~0.01%, Ca: 0.0005~0.004%, with the remainder consisting of Fe and unavoidable impurities.

[0027] Furthermore, it may contain one or more of the following: Cu: 0.5% or less and Cr: 0.35% or less.

[0028] Carbon (C): 0.12-0.18% by weight (Hereafter, unless otherwise specified in this invention, the content of each element is expressed in weight percent.) The above-mentioned C is an element effective in improving the strength of steel. To obtain such an effect to the fullest extent, it is preferable to include 0.12% or more of C. However, if the content exceeds 0.18%, the degree of segregation in the center of the steel increases, island-like martensite (MA) structure is formed, and hydrogen-induced cracking resistance and low-temperature impact toughness are greatly impaired. Therefore, it is preferable not to exceed 0.18%. More advantageously, it can be included at 0.15% or less.

[0029] Silicon (Si): 0.2-0.5% The above-mentioned Si is not only used as a deoxidizing agent, but is also an element that is advantageous for improving the strength and toughness of steel. To fully obtain these effects, it is preferable that the Si content be 0.2% or more. However, if the content exceeds 0.5%, it may lead to excessive formation of MA, which may reduce hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, it is preferable that the Si content be between 0.2% and 0.5%.

[0030] Manganese (Mn): 0.8~1.5% The above-mentioned Mn is an advantageous element for improving the strength of steel through its solid solution strengthening effect. To fully obtain this effect, it is preferable that the steel contains 0.8% or more of the above-mentioned Mn. However, if the content exceeds 1.5%, it combines with sulfur (S) in the steel to form MnS, which significantly impairs hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, the above-mentioned Mn content is preferably 0.8 to 1.5%, and more preferably 1.0 to 1.5%.

[0031] Phosphorus (P): 0.15% or less (excluding 0%) While the above-mentioned P is advantageous for improving the strength and ensuring corrosion resistance of steel, it can significantly impair the impact toughness of steel, so it is preferable to limit its content to the lowest possible level. In this invention, even with a maximum of 0.015% of the above-mentioned P, it is not difficult to ensure the target physical properties, so it is preferable to limit its content to 0.015% or less. However, considering the level that is inevitably added, 0% can be excluded.

[0032] Sulfur (S): 0.003% or less (excluding 0%) The above-mentioned sulfur (S) is an element that significantly inhibits the hydrogen-induced cracking resistance and impact toughness of steel by bonding with manganese (Mn) in the steel to form MnS, etc. Therefore, it is preferable to control the content of S to the lowest possible level. In the present invention, even with a maximum content of 0.003% of S, it is not difficult to secure the target physical properties, so its content can be limited to 0.003% or less. However, considering the level that is inevitably added, 0% can be excluded.

[0033] Aluminum (Al): 0.015~0.045% The above-mentioned Al is an element that can deoxidize molten steel inexpensively. To obtain the above-mentioned effects sufficiently, it is preferable to include 0.015% or more of the above-mentioned Al. However, if the content is excessive and exceeds 0.045%, it is undesirable because it can not only induce nozzle clogging during continuous casting but also significantly reduce impact toughness due to the formation of Al-based oxidative inclusions. Therefore, it is preferable to include the above-mentioned Al in an amount of 0.015 to 0.045%.

[0034] Niobium (Nb): 0.005~0.025% The above-mentioned Nb precipitates in the form of NbC or Nb(C,N), significantly improving the strength of the base material. When reheated at high temperatures, the dissolved Nb suppresses austenite recrystallization and ferrite or bainite transformation, thereby achieving a microstructure refinement effect. For this purpose, it is preferable to include 0.005% or more. However, if the content is excessive, undissolved Nb may form in the form of TiNb(C,N), which can lead to UT defects, hydrogen-induced cracking resistance, and low-temperature impact toughness. Therefore, it is preferable not to exceed 0.025%. More advantageously, it can be included in a quantity of 0.007 to 0.02%.

[0035] Nickel (Ni): 0.01-0.5% The above-mentioned Ni is an element that can simultaneously improve the strength and low-temperature impact toughness of the base material, and in order to fully obtain such effects, it is preferable that the Ni content be 0.01% or more. However, since Ni is an expensive element, there is a problem that economic efficiency decreases significantly when its content exceeds 0.5%, so it is preferable that the Ni content be between 0.01% and 0.5%.

[0036] Molybdenum (Mo): 0.01~0.12% The above-mentioned Mo is an advantageous element for significantly improving the hardening ability of steel and thus greatly increasing its strength. To fully obtain such effects, it is preferable to include 0.01% or more of the above-mentioned Mo. However, since the above-mentioned Mo is an expensive element, adding it in excess may suppress the formation of ferrite and cause the formation of bainite, thereby impairing low-temperature impact toughness. Therefore, taking this into consideration, it is preferable that the amount does not exceed 0.12%.

[0037] Vanadium (V): 0.005-0.03% The above-mentioned V has a lower solid solution temperature compared to other alloying elements and has the effect of precipitating in the heat-affected zone during welding, thereby preventing a decrease in strength. When sufficient strength cannot be ensured after post-weld heat treatment (PWHT) of a steel material such as the present invention, the strength improvement effect can be obtained by including 0.005% or more of the above-mentioned V. However, if the content exceeds 0.03%, the fraction of hard phases such as MA increases, which may significantly reduce hydrogen-induced cracking resistance and low-temperature impact toughness.

[0038] Titanium (Ti): 0.003% or less (excluding 0%) When added together with N, the above-mentioned Ti forms TiN, thereby reducing the occurrence of surface cracks caused by the formation of AlN precipitates. However, if its content exceeds 0.003%, coarse TiN may be formed during the reheating of the steel slab or during the QT heat treatment or PWHT process, which may act as a factor that inhibits low-temperature impact toughness. Therefore, it is preferable that the above-mentioned Ti is included in an amount of 0.003% or less.

[0039] Nitrogen (N): 0.002~0.01% The above-mentioned N, when added together with Ti, forms TiN, which is advantageous in suppressing grain growth due to heat effects during welding. When adding Ti, it is preferable to include 0.002% or more of the above-mentioned N in order to fully obtain the above-mentioned effect. However, if the content exceeds 0.01%, coarse TiN is formed, which is undesirable because it inhibits low-temperature impact toughness. Therefore, it is preferable that the content of the above-mentioned N be between 0.002% and 0.01%.

[0040] Calcium (Ca): 0.0005~0.004% When the above-mentioned Ca is added to molten steel, it combines with S, which forms MnS inclusions, to suppress the formation of MnS, and also forms spherical CaS, which can suppress the occurrence of cracks due to hydrogen-induced cracking. To obtain the above effects, it is preferable to include 0.0005% or more of Ca, but if the content exceeds 0.004%, the Ca remaining after CaS formation combines with oxygen (O) to form coarse oxidizing inclusions, which are stretched and fractured during rolling, playing a role in promoting hydrogen-induced cracking. Therefore, it is preferable to include the above-mentioned Ca in an amount of 0.0005 to 0.004%.

[0041] Furthermore, in addition to the above composition, it may contain one or more of the following: copper (Cu): 0.5% or less and chromium (Cr: 0.35% or less).

[0042] Copper (Cu): 0.5% or less The above-mentioned Cu is an element that can significantly improve strength through solid solution strengthening and effectively suppresses corrosion of the base material in a humid hydrogen sulfide atmosphere. However, the above effect is not significant under a strong acid atmosphere, and if the Cu content is excessive, it not only increases the carbon equivalent and impairs weldability, but also significantly deteriorates the surface quality of the product. Therefore, when adding the above-mentioned Cu, it can be included in a maximum of 0.5%. However, in this invention, it is made clear that the above-mentioned Cu is not essential, as the target physical properties can be secured even without adding it.

[0043] Chromium (Cr): 0.35% or less The above-mentioned Cr is an element that can prevent a decrease in strength by slowing the decomposition rate of cementite during tempering or post-weld heat treatment (PWHT). However, if its content exceeds 0.35%, the amount of coarse carbides may increase, potentially significantly reducing impact toughness. Therefore, it is preferable that the above-mentioned Cr be included at a maximum of 0.35%. However, in this invention, it is clarified that the above-mentioned Cr is not essential, as the target physical properties can be secured even without adding it.

[0044] The remainder consists of iron (Fe) and unavoidable impurities. Since unavoidable impurities can be unintentionally introduced during the normal steel manufacturing process, it is impossible to completely eliminate them, and the implications of this can be easily understood by engineers in the field of normal steel manufacturing. Furthermore, this invention does not completely exclude the addition of compositions other than the steel composition described above.

[0045] In order to ensure hydrogen-induced cracking resistance and low-temperature impact toughness along with the target level of strength, the steel material of the present invention preferably has its content appropriately adjusted when adding a certain amount of elements that are advantageous for improving these physical properties. Therefore, it is preferable that the carbon equivalent (Ceq) in the following [Relationship Formula 1] is 0.45 or less. If the carbon equivalent (Ceq) exceeds 0.45, although it may be advantageous for ensuring strength, it may greatly impair the physical properties after welding. Furthermore, if a large amount of alloying elements are included, the cost will increase and economic viability will be impaired, so it is preferable that the carbon equivalent (Ceq) is 0.45 or less.

[0046] [Relationship 1] Ceq≦0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the weight percentages of each component.)

[0047] Furthermore, it is preferable that the steel material of the present invention satisfies the following [Relational Formula 2].

[0048] [Relationship 2] 1.2 ≤ Ca / S ≤ 4.0 (The above values ​​for Ca and S represent the content (by weight) of each component.)

[0049] If the Ca / S ratio is less than 1.2, MnS may be formed instead of CaS, potentially significantly increasing the impact toughness and hydrogen-induced cracking of the central region. If the Ca / S ratio is greater than 4, inclusions in which CaO-Al2O3 and CaS are mixed together may be formed, which in this case may also lead to a decrease in impact toughness and the induction of hydrogen-induced cracking. Therefore, the Ca / S ratio is preferably between 1.2 and 4.0.

[0050] The microstructure of the above-mentioned steel material preferably has a polygonal ferrite area fraction of 70% or more, a pearlite area fraction of 20-30%, and the remainder being bainite (including 0%). If the polygonal ferrite area fraction is less than 70%, the impact toughness may decrease significantly, and if the pearlite area fraction deviates from 20-30%, it may induce a decrease or an excess of strength.

[0051] The average grain size of the polygonal ferrite is preferably 25 μm or less. If the average grain size of the polygonal ferrite exceeds 25 μm, the impact toughness may decrease significantly.

[0052] On the other hand, the interior of the above steel material contains oxidizing inclusions such as Al-O, Ca-O, and Al-Ca-O systems with a size of 10 μm or larger, up to 1 mm in diameter. 2 It is preferable that the number of oxidizing inclusions per unit area be 50 or less. If the size of the above oxidizing inclusions is less than 10 μm, it does not significantly affect the physical properties and does not have much technical significance. 2 Exceeding this level could significantly increase the probability of hydrogen-induced cracking occurring.

[0053] The microstructure characteristics of the steel material described above do not differ significantly before and after post-weld heat treatment (PWHT), which will be discussed later.

[0054] The above steel material has an average Crack Length Ratio (CLR) of 10% or less in experiments conducted under the conditions of the relevant international standard, NACE TM0284 Solution A (strong acid), from the surface to the center of the width, with the center of the width as the reference point.

[0055] The above-mentioned steel material exhibits excellent strength and low-temperature impact toughness, with a yield strength of 260 MPa or higher, a tensile strength of 485 MPa or higher, and an average Charpy impact absorption energy (CVN, -46°C) of 150 J or higher, as evaluated perpendicular to the rolling direction at the t / 4 point in the thickness direction (where t represents the thickness of the steel material in mm).

[0056] The physical properties of the steel material described above may be those of steel material that has undergone post-weld heat treatment (PWHT).

[0057] Next, an example of the steel manufacturing method of the present invention will be described in detail. The above manufacturing method involves heating a steel slab that satisfies the alloy composition described above, followed by hot rolling, cooling, reheating, quenching, and tempering.

[0058] Heating of steel slabs It is preferable to heat and homogenize the steel slab that satisfies the alloy composition described above. At this time, it is preferable to heat it to a temperature range of 1100 to 1200°C. If the heating temperature of the steel slab is less than 1100°C, the precipitates (carbon and nitrides) formed in the steel slab will not be sufficiently redissolved, and the formation of precipitates will decrease in the process after hot rolling. On the other hand, if the temperature exceeds 1200°C, the austenite grains will coarseen, which may impair the physical properties of the steel.

[0059] Hot rolling Hot-rolled steel sheets are manufactured by hot-rolling the heated steel slab as described above. It is preferable to roughly roll the heated steel slab at a temperature of 1050°C or higher, and then perform finish hot-rolling at a temperature of Ar3 or higher.

[0060] If the temperature during the rough rolling process is below 1050°C, there is a problem that the temperature will be too low during the subsequent finish hot rolling. In this case, it is important to prevent grain coarsening by applying sufficient reduction force during the rough rolling process, so it is preferable to add a reduction ratio of 10% or more in the final pass of rough rolling. If the reduction force during rough rolling is insufficient, there is a high possibility that grain coarsening will occur after rough rolling.

[0061] Furthermore, if the above-mentioned finish hot rolling temperature is below Ar3, the rolling load will increase, which may lead to quality defects such as surface cracks.

[0062] The above Ar3 can be expressed as follows:

[0063] Ar3=910-310C-80Mn-20Cu-55Ni-80Mo+119V+124Ti-18Nb+179Al (Here, each element is represented by its content (meaning weight conversion))

[0064] Cooling and reheating It is preferable to air-cool the hot-rolled steel sheet manufactured as described above to room temperature, and then reheat it to a temperature of Ac3 or higher and hold it for a certain period of time. This reheating step induces the formation of a fine austenite structure and can also contribute to the refinement of ferrite after water cooling. Although an austenite structure can be formed by reheating the hot-rolled steel sheet, if the reheating temperature is below Ac3, the structure of the hot-rolled steel sheet may become a two-phase structure of ferrite and austenite. Therefore, it is preferable to reheat at a temperature of Ac3 or higher, preferably in the range of 830 to 930°C, and to hold it at the above temperature for (2.3t + 30) minutes or more (where t means the thickness of the steel (mm)) so that 100% austenite phase is sufficiently formed to the center of the hot-rolled steel sheet. If the holding time is less than (2.3t + 30) minutes, 100% austenizing will not occur due to insufficient heat absorption, resulting in a two-phase heat treatment, which may significantly reduce tensile and impact toughness. On the other hand, the above-mentioned upper limit on the holding time has no physical significance and is therefore not particularly limited; it can be easily determined by an ordinary engineer considering the limitations of the equipment, etc.

[0065] The above Ac3 can be expressed as follows:

[0066] Ac3=93.2-436.5C+56Si-19.7Mn-26.6Ni+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al (Here, each element is represented by its content (meaning weight conversion))

[0067] Quenching and tempering The reheated hot-rolled steel sheet described above is preferably quenched to room temperature at a cooling rate of 0.4°C / s or higher. If the cooling rate is less than 0.4°C / s, the microstructure will contain coarse ferrite and pearlite phases, which may impair strength and low-temperature impact toughness.

[0068] It is preferable to perform tempering heat treatment on the cooled hot-rolled steel sheet at a temperature range of 600 to 700°C for (3.4t + 30) minutes or more (where t represents the thickness of the steel in mm). If the cooled hot-rolled steel sheet is heat-treated at a temperature below 600°C, it is difficult to form fine precipitates and secure strength. If the temperature exceeds 700°C, the formation of coarse precipitates may significantly reduce hydrogen-induced cracking resistance and low-temperature impact toughness. If the tempering heat treatment time is less than (3.4t + 30) minutes, the heat treatment will be performed at a temperature lower than the target temperature due to insufficient heating, and although strength may be secured, impact toughness may be significantly impaired. On the other hand, the upper limit of the tempering heat treatment time is not particularly limited as it has no technical significance and can be easily determined by an ordinary engineer considering the limitations of the equipment, etc.

[0069] The cooling after the tempering heat treatment described above is not particularly limited, but can be carried out by air cooling.

[0070] The steel material manufactured as described above can be welded and then subjected to post-weld heat treatment (PWHT). Generally, since steel materials for pressure vessels are used after welding, it is common to perform PWHT heat treatment to overcome the deterioration of toughness at the weld joint. In the present invention, the welding and PWHT processes are not particularly limited. As an example, it is necessary to stabilize the toughness after welding by performing PWHT (post-weld heat treatment) heat treatment on the steel material at a temperature range of 550 to 650°C for at least 1 hour per inch of thickness of the steel material.

[0071] If the PWHT heat treatment temperature is below 550°C, a long heat treatment period will be required, potentially reducing economic efficiency. On the other hand, if the temperature exceeds 650°C, not only will the strength reduction effect become excessively large, but the carbides may coarseen, potentially reducing impact toughness.

[0072] The steel material after the above PWHT heat treatment is cooled to room temperature by air cooling to obtain a steel material composed of ferrite, pearlite, and the remainder being bainite phase. [Examples]

[0073] Next, embodiments of the present invention will be described.

[0074] It goes without saying that the following embodiments can be modified in various ways without departing from the scope of the invention, by anyone with ordinary skill in the art to which the present invention pertains. The following embodiments are for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be defined not only by the claims described below, but also by equivalents thereof.

[0075] (Examples) Slabs were manufactured by continuous casting of molten steel having the alloy composition shown in Table 1 below (weight %), with the remainder being Fe and unavoidable impurities. The slabs were manufactured with a thickness of 700 mm.

[0076] [Table 1]

[0077] In Table 1 above, relational equations 1 and 2 are calculated as follows.

[0078] [Relationship 1] Ceq≦0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the weight percentages of each component.)

[0079] [Relationship 2] 1.2 ≤ Ca / S ≤ 4.0 (The above values ​​for Ca and S represent the content (by weight) of each component.)

[0080] The above-mentioned continuous-cast slabs were reheated to over 1000°C, forged to a thickness of approximately 400 mm, and then air-cooled.

[0081] The forged slab described above was heated to approximately 1100°C, then roughly rolled at approximately 1050°C or higher, and finally hot-rolled at approximately 980°C to obtain a hot-rolled steel sheet with a thickness of approximately 200 mm.

[0082] After the above hot-rolled steel sheet was air-cooled to room temperature, it was reheated to approximately 890°C and held for approximately 480 minutes, then water-cooled (quenched) again to room temperature, reheated to approximately 650°C and held for approximately 710 minutes (tempered), and then air-cooled in a QT heat treatment. Subsequently, the air-cooled hot-rolled steel sheet was heated to approximately 635°C and held for approximately 1200 minutes for PWHT (post-weld heat treatment) heat treatment, and then air-cooled to room temperature to produce the final steel material. The detailed conditions are shown in Table 2.

[0083] [Table 2]

[0084] The microstructure and mechanical properties of the steel materials manufactured as described above were evaluated. The microstructure was observed using an optical microscope, and then the fraction of the microstructure, the ferrite diameter, and the number of inclusions were measured using an analysis program. The microstructure was measured at a point t / 4 (where t is the thickness of the steel material, mm) in the thickness direction of each steel material, and the results are shown in Table 3 below.

[0085] The mechanical properties of each steel material were evaluated at the 1 / 4th t point in the thickness direction. Tensile test specimens were taken at each point in the thickness direction perpendicular to the rolling direction, and the tensile strength (TS), yield strength (YS), and elongation (El) were measured. Impact test specimens conforming to JIS No. 4 standard were taken in the rolling direction at the 1 / 4th t point in the thickness direction, and the average impact toughness (CVN) at -46°C was measured. The results are shown in Table 3 below.

[0086] The above-mentioned inclusions refer to oxidizing inclusions such as Al-O, Ca-O, and Al-Ca-O systems that have a size of 10 μm or larger.

[0087] On the other hand, the ratio of hydrogen-induced crack lengths (CLR, %) in the longitudinal direction of the steel plate, which was used as an indicator of the steel plate's resistance to hydrogen-induced cracking, was evaluated by immersing the test specimen in a 5% NaCl + 0.5% CH3COOH solution saturated with H2S gas at 1 atmosphere for 96 hours, based on the relevant international standard NACE TM0284. The length of the cracks was then measured by ultrasonic testing, and the value was calculated by dividing the sum of the lengths of the individual cracks in the longitudinal direction of the test specimen by the total length of the test specimen. The results are shown in Table 3.

[0088] [Table 3]

[0089] As shown in Table 3 above, in this invention, Examples 1 to 5, manufactured according to the proposed alloy composition, component relationships, and manufacturing conditions, satisfy the microstructure, tensile properties, low-temperature impact toughness, and hydrogen-induced cracking resistance values ​​presented in this invention.

[0090] On the other hand, in the case of Comparative Example 1, the Nb and Ca content was outside the range proposed in the present invention. Not only was the tensile strength low due to insufficient Nb content, but the Ca / S ratio deviated from the value presented in the present invention, making it impossible to adequately control MnS, and resulting in a CLR value that deviated from the value presented in the present invention. In Comparative Example 2, the C content was outside the range presented in the present invention. Although the tensile properties were sufficiently secured, the low-temperature impact toughness deviated from the value presented in the present invention, and it was found that the CLR value also increased significantly due to the increase in the hard phase. In the case of Comparative Example 3, most of the components met the values ​​presented in the present invention, but the Al content was excessively high. Although the tensile and low-temperature impact toughness were satisfied, the presence of Al-based oxides acted as an initiation point for hydrogen-induced cracking, and it was confirmed that the CLR value deviated significantly from the value presented in the present invention.

[0091] On the other hand, Figures 1 and 2 show the ultrasonic testing results of the steel materials of Invention Example 1 and Comparative Example 1 after a hydrogen-induced cracking test at the 1 / 2t point in the center of the width, respectively. In Invention Example 1 in Figure 1, no hydrogen-induced cracking occurred at all, whereas in Comparative Example 1 in Figure 2, where Nb and Ca deviated from the values ​​presented in the present invention, hydrogen-induced cracking was confirmed to have occurred.

Claims

1. In weight percent, it contains C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 0.8-1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.005-0.025%, Ni: 0.01-0.5%, Mo: 0.01-0.12%, V: 0.005-0.03%, Ti: 0.003% or less (excluding 0), N: 0.002-0.01%, Ca: 0.0005-0.004%, with the remainder consisting of Fe and unavoidable impurities. The microstructure consists of a polygonal ferrite area fraction of 70% or more, a pearlite area fraction of 20-30%, and the remainder being bainite (including 0%). After observation with an optical microscope, the fraction of microstructure, ferrite diameter, and number of inclusions were measured using an analysis program, and the average grain size of the polygonal ferrite was found to be 25 μm or less. When evaluated at a point 1 / 4th of the thickness direction of the steel material, the number of inclusions within the steel material that are 10 μm or larger and consist of one or more Al-O, Ca-O, and Al-Ca-O oxidizing inclusions is 1 mm². ２ The number of winning items is 50 or less. A steel material exhibiting excellent hydrogen-induced cracking resistance and low-temperature impact toughness, characterized by satisfying the following [Relationship Formula 1] and [Relationship Formula 2]. [Relationship 1] Ceq ≤ 0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.) [Relationship Equation 2] 1.2 ≤ Ca / S ≤ 4.0 (The above values ​​of Ca and S are the content (by weight) of each component.)

2. The steel material is characterized by containing one or more of Cu: 0.5% or less and Cr: 0.35% or less, and is a steel material with excellent hydrogen-induced cracking resistance and low-temperature impact toughness as described in claim 1.

3. The steel material described in claim 1 is characterized in that, after welding the steel material, a post-weld heat treatment (PWHT) is performed in which heat treatment is performed at a temperature range of 550 to 650°C for at least one hour per inch of the thickness of the steel material, and the yield strength evaluated perpendicular to the rolling direction at a point t / 4 in the thickness direction of the steel material (where t means the thickness of the steel material (mm)) is 260 MPa or more, the tensile strength is 485 MPa or more, and the Charpy impact absorption energy (CVN, -46°C) value at -46°C is an average of 150 J or more, thereby exhibiting excellent hydrogen-induced cracking resistance and low-temperature impact toughness.

4. A method for manufacturing steel materials that exhibit excellent hydrogen-induced cracking resistance and low-temperature impact toughness, In weight percent, it contains C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 0.8-1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.005-0.025%, Ni: 0.01-0.5%, Mo: 0.01-0.12%, V: 0.005-0.03%, Ti: 0.003% or less (excluding 0), N: 0.002-0.01%, Ca: 0.0005-0.004%, with the remainder consisting of Fe and unavoidable impurities. The process involves heating a steel slab that satisfies the following [Relationship 1] and [Relationship 2] to a temperature range of 1100 to 1200°C, The steps include: roughly rolling the heated steel slab at a temperature of 1050°C or higher, and then finish hot rolling it at a temperature of Ar3 or higher to produce a hot-rolled steel sheet; The steps include: cooling the hot-rolled steel sheet by air, The air-cooled hot-rolled steel sheet is reheated to a temperature of Ac3 or higher and held for (2.3t + 30) minutes or longer (where t represents the thickness of the steel in mm), in a reheating step. The reheated hot-rolled steel sheet is then quenched to room temperature at a cooling rate of 0.4°C / s or higher. The process includes the step of tempering the aforementioned quenched hot-rolled steel sheet at a temperature range of 600 to 700°C for (3.4t + 30) minutes or more (where t represents the thickness of the steel in mm), The microstructure of the aforementioned steel material consists of a polygonal ferrite area fraction of 70% or more, a pearlite area fraction of 20-30%, and the remainder being bainite (including 0%). After observation with an optical microscope, the fraction of microstructure, ferrite diameter, and number of inclusions were measured using an analysis program, and the average grain size of the polygonal ferrite was found to be 25 μm or less. A method for producing steel with excellent hydrogen-induced cracking resistance and low-temperature impact toughness, characterized in that, when evaluated at a point 1 / 4 t in the thickness direction of the steel material, the number of inclusions within the steel material is 50 or less per 1 mm², consisting of one or more Al-O, Ca-O, and Al-Ca-O oxidizing inclusions having a size of 10 μm or more. [Relationship 1] Ceq ≤ 0.45 (Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.) [Relationship Equation 2] 1.2 ≤ Ca / S ≤ 4.0 (The above values ​​of Ca and S are the content (by weight) of each component.)

5. The method for producing a steel material with excellent hydrogen-induced cracking resistance and low-temperature impact toughness, as described in claim 4, characterized in that the steel slab contains one or more of Cu: 0.5% or less and Cr: 0.35% or less.

6. A method for producing a steel material with excellent hydrogen-induced cracking resistance and low-temperature impact toughness, as described in 4 or 5, characterized by including a step of welding the steel material and then performing PWHT (post-weld heat treatment) heat treatment at a temperature range of 550 to 650°C for at least one hour per inch of thickness of the steel material.

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